CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/123,833, filed December 10, 2020, titled MULTIFOCAL IMAGING WITH INCREASED WAVELENGTH SEPARATION, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The disclosed subject matter relates to a wavelength selection apparatus for selecting multiple wavelengths of a single pulsed light beam to form multiple aerial images in a single lithography exposure pass.

BACKGROUND

[0003] Photolithography is a process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography optical source provides the deep ultraviolet (DUV) light (a DUV light beam) used to expose a photoresist on the wafer. The DUV light beam for photolithography is generated by an excimer light source. Often, the light source is a laser source and the output of the laser source is a pulsed laser beam. The DUV light beam is passed through a beam delivery unit, a reticle or a mask, and then projected onto a prepared silicon wafer. In this way, a chip design is patterned onto a photoresist that is then developed, etched and cleaned, and then the process repeats.

[0004] Typically, an excimer laser uses a combination of one or more noble gases, which can include argon, krypton, or xenon, and a reactive gas, which can include fluorine or chlorine. The excimer laser can create an excimer, a pseudo-molecule, under appropriate conditions of electrical simulation (energy supplied) and high pressure (of the gas mixture), the excimer only existing in an energized state. The excimer in an energized state gives rise to amplified light in the DUV range. An excimer light source can use a single gas discharge chamber or a plurality of gas discharge chambers. The DUV light beam can have a wavelength in the DUV range, which includes wavelengths from, for example, about 100 nanometers (nm) to about 400 nm.

SUMMARY

[0005] In some general aspects, a wavelength selection apparatus is arranged relative to a pulsed optical source that produces a pulsed light beam. The wavelength selection apparatus includes: a center wavelength selection optic configured to select at least one center wavelength for each pulse of the pulsed light beam in accordance with an angle of incidence of the pulsed light beam on the center wavelength selection optic; a tuning mechanism arranged along a path of the pulsed light beam to the center wavelength selection optic, the tuning mechanism configured to optically interact with the pulsed light beam and to select the angle of incidence of the pulsed light beam on the center wavelength selection optic; and a diffractive optical element that is passive and transmissive and is arranged along the path of the pulsed light beam at a location at which the pulsed light beam is fully magnified or at least mostly magnified. The diffractive optical element is configured to interact with the pulsed light beam and to produce a plurality of pulsed light sub-beams from the pulsed light beam, each pulsed light sub-beam associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a distinct wavelength and the optical spectrum of the pulsed light beam includes a peak at each distinct wavelength.

[0006] Implementations can include one or more of the following features. For example, the diffractive optical element can be a diffractive beam splitter, a diffraction grating, a phase grating, a binary phase grating, or a blazed phased grating.

[0007] The tuning mechanism can include four refractive optical elements. Each refractive optical element can be a right-angled prism. The tuning mechanism can include four right-angled prisms, and the location at which the pulsed light beam is at least mostly magnified is in the optical path between the right-angled prism that is closest to the center wavelength selection optic and the right-angled prism that is second closest to the center wavelength selection optic. The tuning mechanism can include four right-angled prisms arranged along the path of the pulsed light beam to the diffractive optical element, and the pulsed light beam is fully magnified between the four right-angled prisms and the center wavelength selection optic.

[0008] A wavelength separation between the distinct wavelengths of the plurality of pulsed light subbeams can be greater than about 10 picometers (pm), about 30 pm, or about 45 pm. The center wavelength for each pulse of the pulsed light beam can be about 248 nanometers (nm) or about 193 nm. The wavelength separation between the distinct wavelengths of the plurality of pulsed light subbeams can depend on a periodic shape of the diffractive optical element.

[0009] The wavelength selection apparatus can also include an actuator configured to adjust a position of the diffractive optical element relative to the path of the pulsed light beam, such that the diffractive optical element is positioned along the path of the pulsed light beam at some moments and is not positioned along the path of the pulsed light beam at other moments, the diffractive optical element interacting with the pulsed light beam only if the diffractive optical element is positioned along the path of the pulsed light beam. The actuator can be further configured to adjust an angle of the diffractive optical element relative to a direction of the path of the pulsed light beam at the diffractive optical element, such that the distinct angle of incidence of each produced pulsed light subbeam on the center wavelength selection optic is adjusted.

[0010] The plurality of pulsed light sub-beams can include three or more pulsed light sub-beams. [0011] The tuning mechanism and the center wavelength selection optic can be arranged to interact with the pulsed light beam in a Littrow configuration. The center wavelength selection optic can be a reflective optical element.

[0012] An aerial image can be formed for each distinct wavelength of the pulsed light beam. [0013] The wavelength selection apparatus can also include a control system and one or more actuators associated with the tuning mechanism. The control system can be configured to adjust a signal to the one or more actuators to thereby adjust the angle of incidence of the pulsed light beam on the center wavelength selection optic.

[0014] The diffractive optical element can be arranged perpendicular to a direction of propagation of the pulsed light beam along the path. The diffractive optical element can be configured to recombine the plurality of pulsed light sub-beams from the center wavelength selection optic to form the pulsed light beam.

[0015] In other general aspects, an optical system includes: a light source configured to produce a pulsed light beam that is directed along a path toward a lithography exposure apparatus; a lithography exposure apparatus configured to interact with the pulsed light beam; and a wavelength selection apparatus arranged relative to the light source. The wavelength selection apparatus includes: a center wavelength selection optic configured to select at least one center wavelength for each pulse of the pulsed light beam in accordance with an angle of incidence of the pulsed light beam on the center wavelength selection optic; a tuning mechanism arranged along the path of the pulsed light beam to the center wavelength selection optic, the tuning mechanism configured to optically interact with the pulsed light beam and to select the angle of incidence of the pulsed light beam on the center wavelength selection optic; and a diffractive optical element that is passive and transmissive and is arranged along the path of the pulsed light beam at a location at which the pulsed light beam is fully magnified or at least mostly magnified. The diffractive optical element is configured to interact with the pulsed light beam and to produce a plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated. Each pulsed light sub-beam is associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a distinct wavelength and the optical spectrum of the pulsed light beam includes a peak at each distinct wavelength.

[0016] Implementations can include one or more of the following features. For example, the diffractive optical element can be a diffractive beam splitter, a diffraction grating, a phase grating, a binary phase grating, or a blazed phased grating.

[0017] The tuning mechanism can include four refractive optical elements. Each refractive optical element can be a right-angled prism. [0018] A wavelength separation between the distinct wavelengths of the plurality of pulsed light subbeams can be greater than about 10 picometers (pm), about 30 pm, or about 45 pm. The center wavelength for each pulse of the pulsed light beam can be about 248 nanometers (nm) or 193 nm. [0019] The wavelength selection apparatus can include an actuator configured to adjust a position of the diffractive optical element relative to the path of the pulsed light beam such that the diffractive optical element is positioned along the path of the pulsed light beam at some moments and is not positioned along the path of the pulsed light beam at other moments, the diffractive optical element interacting with the pulsed light beam only if the diffractive optical element is positioned along the path of the pulsed light beam. The optical system can also include a control system configured to control the wavelength selection apparatus to adjust the position of the diffractive optical element relative to the path of the pulsed light beam.

[0020] The lithography exposure apparatus can include a mask positioned to interact with the pulsed light beam from the light source and a wafer holder configured to hold a wafer. A plurality of distinct aerial images can be formed on the wafer at the wafer holder, each distinct aerial image based on the distinct wavelength of the associated pulsed light sub-beam that passes through the mask along a direction of propagation.

[0021] The optical system can include a control system and one or more actuators associated with the tuning mechanism. The control system can be configured to adjust a signal to the one or more actuators to thereby adjust the angle of incidence of the pulsed light beam on the center wavelength selection optic.

[0022] In other general aspects, a method is performed for forming a plurality of aerial images with a single pulsed light beam. The method includes: generating the pulsed light beam along a path toward a wafer; selecting an angle of incidence of the pulsed light beam on a center wavelength selection optic to select at least one center wavelength for each pulse of the pulsed light beam by optically interacting the pulsed light beam with a tuning mechanism arranged along the path of the pulsed light beam to the center wavelength selection optic; producing a plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated, including splitting the pulsed light beam into the plurality of pulsed light sub-beams by interacting the pulsed light beam with a diffractive pattern arranged along the path of the pulsed light beam, each pulsed light sub-beam being associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a respective one of the distinct wavelengths that are separated by at least 10 picometers (pm); and forming the plurality of aerial images in the single pulsed light beam on the wafer, wherein each aerial image is formed based on a distinct wavelength. [0023] Implementations can include one or more of the following features. For example, the pulsed light beam can be interacted with the diffractive pattern by transmitting the pulsed light beam through a diffractive optical element. [0024] Each distinct angle of incidence onto the center wavelength selection optic associated with each pulsed light sub-beam can be determined by a periodic shape of the diffractive pattern.

[0025] The angle of incidence of the pulsed light beam on the center wavelength selection optic can be selected by adjusting one or more angles of refractive optical elements within the tuning mechanism.

[0026] The plurality of pulsed light sub-beams can be produced from the pulsed light beam by adjusting a position of the diffractive pattern relative to the path of the pulsed light beam. Adjusting the position of the diffractive pattern can include controlling by moving a diffractive optical element that includes the diffractive pattern.

[0027] The plurality of aerial images can be formed on the wafer by flattening the intensity profile of the pulsed light beam at the wafer.

[0028] The method can also include recombining the plurality of pulsed light sub-beams leaving the center wavelength selection optic by interacting the pulsed light sub-beams with the diffractive pattern arranged along the path of the pulsed light beam, such that the plurality of pulsed light sub-beams are produced when the pulsed light beam interacts with the diffractive pattern travelling along the path to the center wavelength selection optic, and the plurality of pulsed light sub-beams are recombined to form the pulsed light beam when the pulsed light sub-beams interact with the diffractive pattern travelling along the path away from the center wavelength selection optic.

[0029] In other general aspects, a wavelength selection apparatus is associated with a pulsed optical source that produces a pulsed light beam. The wavelength selection apparatus includes: a center wavelength selection optic configured to select at least one center wavelength for each pulse of the pulsed light beam in accordance with an angle of incidence of the pulsed light beam on the center wavelength selection optic; a tuning mechanism arranged along a path of the pulsed light beam to the center wavelength selection optic, the tuning mechanism configured to optically interact with the pulsed light beam and to select the angle of incidence of the pulsed light beam on the center wavelength selection optic, the tuning mechanism including four refractive optical elements; and a passive and transmissive diffractive optical element arranged along the path of the pulsed light beam at a location between the tuning mechanism and the center wavelength selection optic. The diffractive optical element is configured to interact with the pulsed light beam and to produce a plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated. Each pulsed light sub-beam is associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a distinct wavelength and the optical spectrum of the pulsed light beam includes a peak at each distinct wavelength. DESCRIPTION OF DRAWINGS

[0030] Fig. 1 is a block diagram of an optical system that includes a light source configured to produce a pulsed light beam, a lithography exposure apparatus configured to interact with the pulsed light beam, and a wavelength selection apparatus configured to select a plurality of distinct center wavelengths in the pulsed light beam.

[0031] Fig. 2A is a block diagram of an implementation of the wavelength selection apparatus of Fig. 1 including a center wavelength selection optic, a tuning mechanism, and a diffractive optical element.

[0032] Fig. 2B is a block diagram of the center wavelength selection optic of Fig. 2A and an implementation of the diffractive optical element of Fig. 2A that is a phase grating.

[0033] Fig. 2C is a graph of an example of an optical spectrum of the pulsed light beam of Fig. 1 that includes a peak at each distinct center wavelength in the pulsed light beam.

[0034] Fig. 3A is a block diagram of an implementation of the lithography exposure apparatus of Fig. 1 including a projection optical system configured to interact with the pulsed light beam from the light source of Fig. 1, a mask positioned to interact with the pulsed light beam, and a wafer holder configured to hold a wafer.

[0035] Fig. 3B is a block diagram of an implementation of the projection optical system of Fig. 3A including a slit, the mask of Fig. A, and a projection objective that includes a lens.

[0036] Fig. 3C is a schematic diagram of the wafer of Fig. 3A including a plurality of aerial images at different planes along the z axis of the wafer, each of the aerial images being formed by the projection optical system of Fig. 3B in a single exposure pass.

[0037] Fig. 4A is a block diagram of an implementation of the wavelength selection apparatus of Fig. 2A that includes an implementation of the tuning mechanism including a set of optical components arranged to optically interact with the pulsed light beam, an implementation of the diffractive optical element, and an implementation of the center wavelength selection optic.

[0038] Fig. 4B is a block diagram showing a beam magnification and a beam refraction angle through one of the optical components of the wavelength selection apparatus of Fig. 4A.

[0039] Fig. 5 A is a block diagram of a top view along the Z axis of the wavelength selection apparatus of Fig. 4A, where the Z direction is perpendicular to the path of travel of the light beam.

[0040] Fig. 5B is a block diagram of a top view along the Z axis of another implementation of the wavelength selection apparatus of Fig. 2A.

[0041] Fig. 6A is a block diagram of a side view along the Y axis of the wavelength selection apparatus of Fig. 4A including an actuator configured to adjust a position of the diffractive optical element relative to the path of the pulsed light beam, the diffractive optical element being along the path of the pulsed light beam. [0042] Fig. 6B is a block diagram of a side view along the Y axis of the wavelength selection apparatus of Fig. 4A including the actuator of Fig. 6A, the diffractive optical element external to the path of the pulsed light beam.

[0043] Fig. 7A is a block diagram of a side view along the Z axis of the wavelength selection apparatus of Fig. 4A including an actuator configured to adjust an angle of the diffractive optical element relative to a direction of the path of the pulsed light beam at the diffractive optical element, and another actuator configured to adjust an angle of one of the optical components in the tuning mechanism to thereby adjust the angle of incidence of the pulsed light beam on the center wavelength selection optic.

[0044] Fig. 7B is a block diagram of a side view along the Z axis of the wavelength selection apparatus of Fig. 4A including the actuators of Fig. 7A, each of the actuators arranged in an adjusted position.

[0045] Fig. 8 is a flow chart of a procedure for forming the plurality of aerial images of Fig. 3C with the single pulsed light beam of Fig. 1.

[0046] Fig. 9 is a block diagram of an example of an implementation of the optical system of Fig. 1 that includes the wavelength selection apparatus of Fig. 2A.

[0047] Fig. 10A is a block diagram of a top view along the Z axis of the wavelength selection apparatus of Fig. 4A including an implementation of the diffractive optical element that is a blaze grating.

[0048] Fig. 10B is a block diagram of the diffractive optical element of Fig. 10A that is a blaze grating.

[0049] Fig. 10C is a side view of the diffractive optical element of Figs. 10A and 10B, in which the Z axis is up and down along the page.

DESCRIPTION

[0050] Referring to Fig. 1, an optical system 100 includes a light source 105 that is a pulsed optical source configured to produce a light beam 102, a lithography exposure apparatus 107 configured to interact with the pulsed light beam 102, and a wavelength selection apparatus 110 arranged relative to the light source 105. The light beam 102 is directed along a path 104 toward the lithography exposure apparatus 107. The light beam 102 is a pulsed light beam that includes pulses of light separated from each other in time. The pulses of the light beam 102 are centered around a wavelength that is in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The pulsed light beam 102 is used to pattern microelectronic features on a substrate or wafer received in the lithography exposure apparatus 107. The size of the microelectronic features patterned on the wafer depends on the wavelength of the pulsed light beam 102, with a lower wavelength resulting in a small minimum feature size or critical dimension. For example, when the wavelength of the pulsed light beam 102 is 248 nm or 193 nm, the minimum size of the microelectronic features can be, for example, 50 nm or less.

[0051] The wavelength selection apparatus 110 is placed at a first end of the light source 105 to interact with the light beam 102 produced by the light 105 source. The light beam 102 is a beam produced at one end of a resonator within the light source 105. For example, the light beam 102 can be a seed beam produced by a master oscillator. The wavelength selection apparatus 110 is configured to finely tune or adjust the spectral properties of the pulsed light beam 102, including the wavelength of the pulsed light beam 102.

[0052] Specifically, referring also to Fig. 2 A, the wavelength selection apparatus 110 includes a tuning mechanism 112 and a center wavelength selection optic 116. The light beam 102 enters and exits the wavelength selection apparatus 110 through an aperture 211. The center wavelength selection optic 116 is configured to select at least one center wavelength for each pulse of the pulsed light beam 102 in accordance with an angle of incidence at which the pulsed light beam 102 that is directed along the path 104 interacts with the center wavelength selection optic 116. The center wavelength selection optic 116 can be, for example, a reflective optical element such as a reflective grating. The tuning mechanism 112 is arranged along the path 104 of the pulsed light beam 102 to the center wavelength selection optic 116. The tuning mechanism 112 is configured to optically interact with the pulsed light beam 102 and to select the angle of incidence of the center ray of the pulsed light beam 102 on the center wavelength selection optic 116.

[0053] The wavelength selection apparatus 110 is designed to produce a pulsed light beam 102 that can form a plurality of aerial images at the wafer in the lithography exposure apparatus 107, with each aerial image being at a spatially distinct location along a z axis in the wafer, as discussed in more detail below. The location of the aerial image along this wafer z axis depends at least in part on the wavelength of the light beam 102. Thus, by varying or otherwise controlling the wavelength of the light beam 102, the position of the aerial image or images in the wafer can be controlled. Moreover, by providing pulses having different primary wavelengths of light during a single exposure pass, a plurality of aerial images, which are each at a different location along the wafer z axis, can be formed in a single exposure pass without having to move components of the lithography exposure apparatus 107 and the wafer relative to each other along the wafer z axis.

[0054] The wavelength of the pulsed light beam 102 can be adjusted using the tuning mechanism 112, which can include optical components, such as reflective optical components including reflective prisms and right-angled prisms, configured to rotate at a repetition rate in order to alternate or dither the wavelength of the pulsed light beam 102 with each pulse or every integer number of pulses. For example, rotating right-angled prisms within the tuning mechanism 112 can achieve a maximum wavelength separation of 15 picometers (pm). However, a wavelength separation that is larger than this maximum wavelength separation can be desired or required depending on the desired or required microelectronic features. Moreover, there is a desire to produce a plurality of different wavelengths in the pulsed light beam 102 at one moment in time so as to simultaneously produce a plurality of aerial images in the wafer. To this end, the wavelength selection apparatus 110 also includes a diffractive optical element 114. The diffractive optical element 114 is configured to interact with the pulsed light beam 102 and to produce a plurality of pulsed light sub-beams 221, 223, 225 from the pulsed light beam 102. Each pulsed light sub-beam 221, 223, 225 is associated with a respective distinct center wavelength wl, w2, w3 (Fig. 2C). With the diffractive optical element 114 arranged in the wavelength selection apparatus 110 along the path of the light beam 102, a wavelength separation 220s between the distinct center wavelengths wl, w2, w3 of the plurality of pulsed light sub-beams 221, 223, 225 can be greater than about 10 picometers (pm). For example, the wavelength separation 220s can be about 30 pm or about 45 pm. The size of the wavelength separation 220s depends on the properties of the diffractive optical element 114.

[0055] The diffractive optical element 114 is arranged along the path 104 of the pulsed light beam 102 at a location at which the pulsed light beam 102 is fully magnified or at least mostly magnified. The advantage of this arrangement includes the optical peak power of the pulsed light beam 102 being less or smaller at positions along the path 104 in which the beam is fully magnified (compared to other positions along the path 104). In some implementations, such as shown in Fig. 2A, the diffractive optical element 114 is arranged along the path 104 between the tuning mechanism 112 and the center wavelength selection optic 116. In the example of Fig. 2A, the diffractive optical element 114 is arranged perpendicularly to a direction of propagation of the pulsed light beam 102 along the path 104. That is, the surface normal of the diffractive optical element 114 is parallel with the path 104. In other examples, the diffractive optical element 114 can be arranged such that the diffractive optical element 114 is not perpendicular to the direction of propagation of the pulsed light beam 102. Specifically, the diffractive optical element 114 can be arranged, for example, so that its surface normal is within 10 degrees of the direction of propagation of the pulsed light beam 102.

[0056] The diffractive optical element 114 is passive, and therefore operates on the light beam 102 in a passive manner, which means that no additional energy is required for the diffractive optical element 114 to operate. The diffractive optical element 114 operates on the pulsed light beam 102 by separating the pulsed light beam 102 into sub-beams 221, 223, 225 directed along distinct angles. The diffractive optical element 114 is also transmissive to the pulsed light beam 102 and the pulsed light beam 102 interacts with the diffractive optical element 114 by passing through the diffractive optical element 114. In the example of Fig. 2A, the diffractive optical element 114 produces three pulsed light sub-beams 221, 223, 225, each directed along a distinct direction and angle. In other examples, the diffractive optical element 114 can produce two pulsed light sub-beams or more than three pulsed light sub-beams. Moreover, in the example of Fig. 2A, the diffractive optical element 114 is further configured to recombine the plurality of pulsed light sub-beams 221, 223, 225 returning from the center wavelength selection optic 116 to form the pulsed light beam 102 that is directed along the path 104 toward the lithography exposure apparatus 107.

[0057] Each pulsed light sub-beam 221, 223, 225 travels along a respective path 222, 224, 226 to the center wavelength selection optic 116. Each pulsed light sub-beam 221, 223, 225 is associated with a distinct angle of incidence 222A, 224A, 226A (shown with double-sided curved arrows) on the center wavelength selection optic 116 such that each pulsed light sub-beam 221, 223, 225 is associated with the distinct center wavelength wl, w2, w3. The wavelength separation 220s (Fig. 2C) between the distinct center wavelengths wl, w2, w3 of the plurality of pulsed light sub-beams 221, 223, 225 depends at least in part on a periodic spacing 114s between a periodic feature of the diffractive optical element 114. Each of the pulsed light sub-beams 221, 223, 225 maintains its angular offset as it travels along its path. Moreover, if the pulsed light beam 102 is moved angularly or translated, then all sub-beams are thereby angularly moved or translated in unison, without changing the angular separation between each pulsed light sub-beam 221, 223, 225 and also without changing the wavelength separation between each distinct center wavelength. However, in this situation, the center wavelengths wl, w2, w3 would be shifted by an amount determined by how much the pulsed light beam 102 is moved or translated.

[0058] In the example shown in Fig. 2B, the diffractive optical element 114 is a phase grating 214 such as a binary phase grating or a blaze phase grating having a periodic spacing 214s between a periodic surface relief 214g. Light transmitted through the phase grating obtains position-dependent phase changes, which may also result from a surface relief, or alternatively from a holographic (interferometric) pattern. The blaze phase grating has an advantage of 100% efficiency into one order (m = -1). Moreover, the blaze phase grating can be configured to have two different blaze angles (one on each side) and essentially split the light beam 102 into two sub-pulses (each with 50% energy); meaning there is no energy contribution to higher order modes. Additionally, the blaze phase grating 214 can be slid horizontally (for example, in the XY plane as shown in Fig. 5 A) to shift more of the light beam 102 to one order versus the other order. This ability to control or shift optical power between aerial images (thus, from one aerial image to another) is useful in optimizing or improving multifocal imaging at the wafer. A phase grating works by a change of the index of refraction in the medium, that is, by modulation of the index of refraction. The phase grating is designed to work at different wavelengths by adjusting the thickness and the index modulation of the medium. An example of a binary phase grating is the Binary Phase Grating from HOLO/OR of Ness Ziona, Israel.

[0059] In other implementations, the diffractive optical element 114 can be a diffractive beam splitter or a diffraction grating that has grooves to interact with the pulsed light beam 102. An example of a one dimensional diffractive beam splitter is the ID Beam Splitter by HOLO/OR of Ness Ziona, Israel. [0060] An optical spectrum 220 (Fig. 2C) of the pulsed light beam 102 includes a peak at each distinct center wavelength wl, w2, w3. The optical spectrum 220 contains information about how the optical energy or power of the light beam 102 is distributed over different wavelengths (or frequencies). The diffractive optical element 114 (including both the diffractive beam splitters/gratings and the phase gratings 214) is governed by a periodic change in a physical feature. For example, diffractive beam splitters and gratings include grooves while the phase gratings can include a periodic surface relief (such as shown in Fig. 2B) or an interferometric pattern. In both cases, the spacing 114s, 214s between these features determines the spacing between these distinct center wavelengths wl, w2, w3. For example, the difference A (pk2pk) between any two adjacent center wavelengths is directly proportional to a change (AaL) in the incidence angle of the sub-beam at the center wavelength selection optic 116 relative to the incidence angle of the pulsed light beam 102 (without the presence of the diffractive optical element 114). Moreover, the change (AaL) in incidence angle of the sub-beam depends on this feature spacing as well as the order of the sub-beam. Lastly, the difference (AL(pk2pk)) is also proportional to d/./dcxL, which is the variation in wavelength of the pulsed light beam 102 (without the presence of the diffractive optical element 114) relative to the incidence angle of the pulsed light beam 102 (without the presence of the diffractive optical element 114) on the center wavelength selection optic 116. Thus, the design of the diffractive optical element 114 determines the magnitude of the change in the angle of incidence of each sub-beam on the center wavelength selection optic 116.

[0061] Referring also to Figs. 3A-3C, in some implementations, the lithography exposure apparatus 107 includes a projection optical system 327 and a wafer holder 329 configured to hold a wafer 328. The projection optical system 327 includes a mask 336b positioned to interact with the pulsed light beam 102 from the light source 105. The lithography exposure apparatus 107 can be a liquid immersion system or a dry system. The pulsed light beam 102 enters the lithography exposure apparatus 107 through an aperture 311 along the path 104 to interact with the mask 336b in the projection optical system 327 and the wafer 328. Microelectronic features are formed on the wafer 328 by, for example, exposing a layer of radiation-sensitive photoresist material on the wafer 328 with the pulsed light beam 102.

[0062] As shown in Fig. 3B, the projection optical system 327 includes a slit 336a, the mask 336b, and a projection objective, which includes a lens 336c. The pulsed light beam 102 enters the projection optical system 327 and impinges on the slit 336a, and at least some of the pulsed light beam 102 passes through the slit 336a. In the example of Figs. 3A-3C, the slit 336a is rectangular and shapes the pulsed light beam 102 into an elongated rectangular shaped light beam. A pattern is formed on the mask 336b, and the pattern determines which portions of the shaped light beam are transmitted by the mask 336b and which are blocked by the mask 336b. The design of the pattern is determined by the specific microelectronic circuit design that is to be formed on the wafer 328.

[0063] The shaped light beam interacts with the mask 336b. The portions of the shaped light beam that are transmitted by the mask 336b pass through (and can be focused by) the projection lens 336c and expose the wafer 328. The portions of the shaped light beam that are transmitted by the mask 336b form an aerial image in the x-y plane of the wafer 328. The aerial image is the intensity pattern formed by the light that reaches the wafer 328 after interacting with the mask 336b. The aerial image is at the wafer 328 and extends generally in the x-y plane.

[0064] The optical system 100 that includes the wavelength selection apparatus 110 is able to form a plurality of aerial images during a single exposure pass, with each of the aerial images being at a spatially distinct location along the z axis in the wafer 328. In this example, the projection optical system 327 forms three aerial images 331, 333, 335 at different planes along the z axis of the wafer 328 in a single exposure pass. Each of the aerial images 331, 333, 335 is formed from light having a center wavelength different than the center wavelength of the other of the aerial images 331, 333, 335. Specifically, each of the aerial images 331, 333, 335 is formed from a respective one of the pulsed light sub-beams 221, 223, 225, each of the pulsed light sub-beams 221, 223, 225 having the respective distinct center wavelength wl, w2, w3. As such, one aerial image 331, 333, 335 is formed for each distinct center wavelength wl, w2, w3 of the pulsed light beam 102.

[0065] As described above, the location of the aerial image 331, 333, 335 along the z axis depends on the characteristics of the projection optical system 327 (including the projection lens 336c and the mask 336b) and the wavelength of the pulsed light beam 102. In general, light of a single center wavelength that passes through the mask 336b is focused to a focal plane by the projection lens 336c. The focal plane of the projection lens 336c is between the projection lens 336c and the wafer holder 329, with the position of the focal plane along the z axis of the wafer 328 depending on the properties of the projection optical system 327 and the center wavelength of the pulsed light beam 102. Thus, varying or otherwise controlling the center wavelength of the pulsed light beam 102 allows the position of the aerial images 331, 333, 335 to be controlled. The aerial images 331, 333, 335 are formed from the pulsed light beam 102 having different center wavelengths wl, w2, w3. In this way, the aerial images 331, 333, 335 are at different locations in the wafer 328. The aerial images 331, 333 are separated from each other along the z axis of the wafer 328 by a separation distance 330a, and the aerial images 333, 335 are separated from each other along the z axis by a separation distance 330b. The separation distance 330a depends on the difference between the center wavelength wl of the pulsed light beam 102 that forms the aerial image 331 and the center wavelength w2 of the pulsed light beam 102 that forms the aerial image 333. The separation distance 330b depends on the difference between the center wavelength w2 of the pulsed light beam 102 that forms the aerial image 333 and the center wavelength w3 of the pulsed light beam 102 that forms the aerial image 335.

[0066] The wafer holder 329 and the mask 336b (or other parts of the projection optical system 327) generally move relative to each other in the x, y, and z directions during scanning for routine performance corrections and operation, for example, the motion may be used to accomplish basic leveling, compensation of lens distortions, and for compensation of stage positioning error. This relative motion is referred to as incidental operational motion. However, in the system of Fig. 3A, the relative motion of the wafer holder 329 and the projection optical system 327 is not relied upon to form the separation distance 330a, 330b. Instead, the separation distance 330a, 330b is formed due to the ability to control the primary center wavelengths wl, w2, w3 in the pulses of the pulsed light beam 102 that pass through the mask 336b during the exposure pass. Thus, unlike some prior systems, the separation distance 330a, 330b is not created only by moving the projection optical system 327 and the wafer 328 relative to each other along the z direction. Moreover, the aerial images 331, 333, 335 are all present at the wafer 328 during the same exposure pass. In other words, the optical system 100 does not require that the aerial image 331 be formed in a first exposure pass and the aerial images 333, 335 be formed in subsequent exposure passes.

[0067] The light in the first aerial image 331 interacts with the wafer at a plane 331a, the light in the second aerial image 333 interacts with the wafer at a plane 333a, and the light in the third aerial image 335 interacts with the wafer at a plane 335a. In some embodiments, the wafer will have already been patterned at a previous level or levels and will include features at different topographical locations on the wafer, that is, at different planes along the z axis such as, but not limited to, planes 331a, 331b and 331c. The aforementioned interactions can form electronic features or other physical characteristics, such as openings or holes, on the wafer 328. Because the aerial images 331, 333, 335 are at different planes along the z axis, the aerial images 331, 333, 335 can be used to form three-dimensional features on the wafer 328 or they can be used to form features at different topographical levels of the wafer. For example, the aerial image 331 can be used to form a periphery region, the aerial image 333 can be used to form a channel that is at a different location along the z axis than the periphery region, and the aerial image 335 can be used to form a recess that is at a different location along the z axis than the periphery region and the channel. In this way, a plurality of distinct aerial images 331, 333, 335 are formed on the wafer 328, each distinct aerial image 331, 333, 335 being based on the distinct center wavelength wl, w2, w3 of the associated pulsed light sub-beam 221, 223, 225 that passes through the mask 336b along a direction of propagation along the path 104. In this manner, light at different wavelengths can be used to form patterns at different levels of the wafer topography. As such, the techniques discussed herein may be used to form a three-dimensional semiconductor component, such as a three-dimensional NAND flash memory component.

[0068] Referring to Fig. 4 A, an implementation 410 of the wavelength selection apparatus 110 includes an implementation 412 of the tuning mechanism 112, an implementation 414 of the diffractive optical element 114, and an implementation 416 of the center wavelength selection optic 116 (Fig. 1). The tuning mechanism 412 includes a set of optical features or components 440a-440d arranged to optically interact with the pulsed light beam 102 along the path 104. Each of the optical components 440a-440d can be a refractive optical element such as a right-angled prism. In the example of Fig. 4A, the tuning mechanism 412 includes four right-angled prisms 440a-440d. In other examples, the tuning mechanism 412 can include less than four or more than four optical components. Each of the right-angled prisms 440a-440d is arranged along the path 104 of the pulsed light beam 102 to the diffractive optical element 114. Each of the prisms 440a-440d is a transmissive prism that acts to disperse and redirect the pulsed light beam 102 as it passes through the body of the prism 440a-440d. Each of the prisms 440a-440d can be made of a material (such as, for example, calcium fluoride) that permits the transmission of the wavelength of the pulsed light beam 102. In the example of Fig. 4A, the center wavelength selection optic 416 is a reflective grating that is designed to disperse and reflect the pulsed light beam 102; accordingly, the center wavelength selection optic 416 is made of a material that is suitable for interacting with the pulsed light beam 102 having the wavelength in the DUV range.

[0069] As shown in Fig. 5A, the prisms 440a, 440b, 440c, 440d, the center wavelength selection optic 416, and the diffractive optical element 414 are arranged along an XY plane such that the path of the light beam 102 generally travels along the XY plane. From the view of Fig. 5A, it can be seen that the prism 440a is positioned farthest from the center wavelength selection optic 416 while the prism 440d is positioned closest to the center wavelength selection optic 416. The pulsed light beam 102 enters the wavelength selection apparatus 410 through an aperture 411, and then travels through the prism 440a, the prism 440b, the prism 440c, and the prism 440d, in that order, prior to impinging upon a diffractive surface 416s of the center wavelength selection optic 416. With each passing of the pulsed light beam 102 through a consecutive prism 440a-440d, the light beam 102 is optically magnified and redirected (refracted at an angle) toward the next optical component. As such, in the example of Fig. 4A, the pulsed light beam 102 is fully magnified between the four right-angled prisms 440a-440d and the center wavelength selection optic 416. And, the diffractive optical element 414 is placed in this location. Because the pulsed light beam 102 is fully magnified at the diffractive optical element 414, the energy or power of the pulsed light beam 102 is more evenly distributed over the surface area of the diffractive optical element 414.

[0070] Referring to Fig. 5B, in one implementation 510 of the wavelength selection apparatus 410, the location at which the pulsed light beam 102 is at least mostly magnified can be in the optical path 104 between the right-angled prism 440d that is closest to the center wavelength selection optic 416 and the right-angled prism 440c that is second closest to the center wavelength selection optic 416. Thus, in these implementations, the diffractive optical element 414 is arranged between the prism 440d and the prism 440c at the location in which the pulsed light beam 102 is at least mostly magnified.

[0071] Referring again to Fig. 4A, the diffractive optical element 414 interacts with the pulsed light beam 102 and produces the plurality of pulsed light sub-beams 221, 223, 225 (shown in Fig. 2A) that are each directed along the respective paths 222, 224, 226 to the center wavelength selection optic 416 and are each associated with a distinct angle of incidence on the center wavelength selection optic 416. Thus, each of the pulsed light sub-beams 221, 223, 225 is associated with the distinct center wavelength wl, w2, w3 and the optical spectrum 220 (Fig. 2C) of the pulsed light beam 102 includes a peak at each distinct center wavelength wl, w2, w3. The diffractive optical element 414 does not modify an optical magnification of each of the produced pulsed light sub-beams 440a-440d.

[0072] The pulsed light beam 102 is diffracted and reflected from the center wavelength selection optic 416 back through the diffractive optical element 414, the prism 440d, the prism 440c, the prism 440b, and the prism 440a, in that order, prior to passing through the aperture 411 as the pulsed light beam 102 exits the wavelength selection apparatus 410. The diffractive optical element 414 recombines the three pulsed light sub-beams 221, 223, 225 that are traveling from the center wavelength selection optic 416 to reform the pulsed light beam 102 prior to interacting with the tuning mechanism 412. With each passing through the consecutive prisms 440a-440d of the tuning mechanism 412 from the center wavelength selection optic 416, the pulsed light beam 102 is optically compressed as it travels toward the aperture 411.

[0073] In the example of Fig. 4A, each of the prisms 440a-440d is wide enough along the transverse direction of the pulsed light beam 102 so that the light beam 102 is contained within the surface at which it passes. Each prism 440a-440d optically magnifies the light beam 102 on the path toward the center wavelength selection optic 416 from the aperture 411, and therefore each prism 440a-440d is successively larger in size from the prism 440a to the prism 440d. Thus, the prism 440d is larger than the prism 440c, which is larger than the prism 440b, and the prism 440a is the smallest prism.

[0074] Referring to Fig. 4B, the rotation of a prism P (which can be any one of prisms 440a-440d) of the tuning mechanism 412 changes an angle of incidence at which the pulsed light beam 102 impinges upon the entrance surface H(P) of that rotated prism P. Moreover, two local optical qualities, namely, an optical magnification OM(P) and a beam refraction angle 8(P), of the light beam 102 through that rotated prism P are functions of the angle of incidence of the light beam 102 impinging upon the entrance surface H(P) of that rotated prism P. The optical magnification OM(P) of the light beam 102 through the prism P is the ratio of a transverse width Wo(P) of the light beam 102 exiting that prism P to a transverse width Wi(P) of the light beam 102 entering that prism P.

[0075] A change in the local optical magnification OM(P) of the pulsed light beam 102 at one or more of the prisms P within the tuning mechanism 412 causes an overall change in the optical magnification OM 438 of the pulsed light beam 102 through the tuning mechanism 412. The optical magnification OM 438 of the light beam 102 through the tuning mechanism 412 is the ratio of the transverse width Wo of the light beam 102 exiting the tuning mechanism 412 to a transverse width Wi of the light beam 102 entering the tuning mechanism 412.

[0076] Additionally, a change in the local beam refraction angle 8(P) through one or more of the prisms P within the tuning mechanism causes an overall change in an angle of incidence of the pulsed light beam 102 at the surface 416s of the center wavelength selection optic 416. Therefore, the angle of incidence of each of the pulsed light sub-beams 221, 223, 225 at the surface 416s is also changed with a rotation of one of these prisms. In this way, the center wavelength of the pulsed light beam 102 can also be adjusted by changing the angle of incidence at which the pulsed light beam 102 impinges upon the diffractive surface 416s of the center wavelength selection optic 416.

[0077] In some implementations, the center wavelength selection optic 416 is a high blaze angle Echelle grating, and the pulsed light beam 102 incident on the center wavelength selection optic 416 at any angle of incidence that satisfies a grating equation will be reflected (diffracted). Moreover, if the center wavelength selection optic 416 is used such that the angle of incidence of the light beam 102 onto the center wavelength selection optic 416 is equal to the angle of exit of the light beam 102 from the center wavelength selection optic 416, then the center wavelength selection optic 416 and the tuning mechanism 412 (the prisms 440a-440d) are arranged to interact with the pulsed light beam 102 in a Littrow configuration and the wavelength of the light beam 102 reflected from the center wavelength selection optic 416 is the Littrow wavelength. It can be assumed that the vertical divergence of the light beam 102 incident onto the center wavelength selection optic 416 is near zero. To reflect the nominal wavelength, the center wavelength selection optic 416 is aligned, with respect to the light beam 102 incident onto the center wavelength selection optic 416, so that the nominal wavelength is reflected back through the tuning mechanism 412 (the prisms 440a-440d) to be amplified in the optical system 100 (when the tuning mechanism 412 is used in the optical system 100). The Littrow wavelength can then be tuned over the entire gain bandwidth of the resonators within optical system 100 by varying the angle of incidence of the pulsed light beam 102 onto the center wavelength selection optic 416.

[0078] In some implementations, the wavelength selection apparatus 410 is in communication with a control system 450 through a data connection 452. The control system 450 includes electronics in the form of any combination of firmware and software. Moreover, any one or more of the center wavelength selection optic 416, the diffractive optical element 414, and the prisms 440a-440d of the tuning mechanism 412 can be coupled to respective actuation systems that include actuators that are associated with the tuning mechanism 412 and connected to the control module 450. In the example of Fig. 4A, the control module 450 is connected to actuation systems 414A, 441 A including actuators that are physically coupled to the diffractive optical element 414 and the prism 440d, respectively. In other examples, more than one of the prisms 440a-440d can be coupled to respective actuation systems that are connected to the control module 450.

[0079] The control system 450 includes an electronic processor, an electronic storage, and an input/output (I/O) interface. The electronic processor is one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, a processor receives instructions and data from a read-only memory or a random access memory or both. The electronic processor can be any type of electronic processor. The electronic storage can be volatile memory, such as RAM, or non- volatile memory. In some implementations, the electronic storage can include both non-volatile and volatile portions or components. The electronic storage stores instructions, perhaps as a computer program, that, when executed, cause the processor to communicate with other components in the control system 450 or other components of the wavelength selection apparatus 410. The I/O interface is any kind of electronic interface that allows the control system 450 to receive and/or provide data and signals to other components of the wavelength selection apparatus 410, an operator, and/or an automated process running on another electronic device. For example, the I/O interface can include one or more of a touch screen or a communications interface.

[0080] Each of the actuators of the actuation systems 414 A, 441 A is a mechanical device for moving or controlling the respective optical component. The actuators receive energy from the control system 450, and convert that energy into some kind of motion imparted to the respective optical component. For example, the actuators can be any one of force devices and rotation stages for rotating one or more of prisms of a tuning mechanism. The actuators can include, for example, motors such as stepper motors, valves, pressure-controlled devices, piezoelectric devices, linear motors, hydraulic actuators, voice coils, etc.

[0081] With reference to Figs. 6A and 6B, one or more actuators 614A of the actuation system 414A (Fig. 4A) can be configured to adjust a position of the diffractive optical element 414 relative to the path 104 of the pulsed light beam 102. Specifically, the position of the diffractive optical element 414 is adjusted along the Z axis by the actuator 614A, the Z axis being perpendicular to the path (which is in the XY plane) along which the light beam 102 travels. The actuator 614 A can move the diffractive optical element 414 such that the diffractive optical element 414 is positioned along the path 104 of the pulsed light beam 102 at some moments (Fig. 6A) and is not positioned along the path 104 of the pulsed light beam 102 at other moments (Fig. 6B). For example, the actuator 614A can include a linear motor, such as a linear stepper motor. The control system 450 can control the actuator 614A based on, for example, a pre-programmed recipe or user input.

[0082] The diffractive optical element 414 interacts with the pulsed light beam 102 only if the diffractive optical element 414 is positioned along the path 104 of the pulsed light beam 102 (Fig. 6A). Thus, when the diffractive optical element 414 is positioned along the path 104 of the pulsed light beam 102, the plurality of pulsed light sub-beams 221, 223, 225 are produced by the diffractive optical element 414 such that the pulsed light beam 102 has the associated center wavelengths wl, w2, w3 to form a plurality of aerial images 331, 333, 335 at the wafer. When the diffractive optical element 414 is not positioned along the path 104 of the pulsed light beam 102 (Fig. 6B), the diffractive optical element 414 does not interact with the light beam 102, and the pulsed light beam 102 contains only the primary center wavelength of the light to form a single aerial image at the wafer 328. [0083] With reference to Figs. 7A and 7B, one or more actuators 714A of the actuation system 414A (Fig. 4A) can be configured to adjust an angle of the diffractive optical element 414 about the Z axis, so that the surface normal of the diffractive optical element 414 rotates relative to a direction of the path 104 of the pulsed light beam 102. The distinct angle of incidence of each produced pulsed light sub-beam 221, 223, 225 on the center wavelength selection optic 416 is also adjusted when the angle of the diffractive optical element 414 relative to the direction of the path 104 is adjusted.

[0084] In one example, the actuator 714A can be configured to correct the angle of the diffractive optical element 414 relative to the direction of the path 104 when the diffractive optical element 414 becomes grossly misaligned (for example, greater than 10 °) with the direction of the path 104 of the pulsed light beam 102. For example, vibrations or other mechanical disturbances within the wavelength selection apparatus 410 can cause the diffractive optical element 414 to become misaligned, and the actuator 714A can correct these misalignments by adjusting the angle of the diffractive optical element 414 relative to the direction of the path 104 of the pulsed light beam 102. As an example, the diffractive optical element 414 can be considered misaligned if the angle between the diffractive optical element 414 and the direction of the path 104 of the pulsed light beam 102 is not equal to 90 degrees or is not within a threshold range around 90 degrees.

[0085] Moreover, one or more actuators 741A of the actuation system 441A (Fig. 4A) that are associated with the tuning mechanism 412 can be configured to adjust a signal to the one or more actuators 741A to thereby adjust the angle of incidence of the pulsed light beam 102 on the center wavelength selection optic 416. Specifically, in this example, the prism 440d is physically coupled to the actuator 741 A that rotates the prism 440d about the z axis. The distinct angle of incidence of each produced pulsed light sub-beam 221, 223, 225 on the center wavelength selection optic 416 is also adjusted when the angle of incidence of the pulsed light beam 102 on the center wavelength selection optic 416 is adjusted. In the example of Figs. 7A and 7B, the control system 450 can control the actuator 741 A and the actuator 714A based on, for example, a pre-programmed recipe or user input. Each of the actuators 714A, 741A can be, for example, a rotary motor, such as a rotary stepper motor. [0086] Referring to Fig. 8, a procedure 860 is performed for forming a plurality of aerial images with a single pulsed light beam (such as the pulsed light beam 102). The procedure 860 can be performed with respect to the optical system 100 (Fig. 1) that includes the wavelength selection apparatus 110 (Figs. 2A-2C), the light source 105, and the lithography exposure apparatus 107 including the wafer 328 (Fig. 3A-3C). The procedure 860 can also be performed with respect to any one of the wavelength selection apparatus 410 (Figs. 4A and 5 A) and the wavelength selection apparatus 510 (Fig. 5B). In the following, the procedure 860 is described with respect to the optical system 100.

[0087] The procedure 860 includes generating the pulsed light beam 102 along a path toward a wafer (861). For example, the pulsed light beam 102 can be generated by the light source 105 and directed along the path 104 toward the wafer 328 in the lithography exposure apparatus 107. In the example of Fig. 1, the pulsed light beam 102 is directed from the light source 105 (after the light beam 102 is produced by the light source 105) to interact with the wavelength selection apparatus 110. The pulsed light beam 102 is then directed from the wavelength selection apparatus 110 to the lithography exposure apparatus 107 that includes the wafer 328 in which aerial images can be formed.

[0088] An angle of incidence of the pulsed light beam on a center wavelength selection optic (such as 116) is selected to thereby select at least one center wavelength for each pulse of the pulsed light beam (863). To select the at least one center wavelength for each pulse, the pulsed light beam 102 optically interacts with a tuning mechanism arranged along the path of the pulsed light beam to the center wavelength selection optic (863). For example, the angle of incidence of the pulsed light beam 102 on the center wavelength selection optic 116 can be selected by interacting the pulsed light beam 102 with the tuning mechanism 112 of the wavelength selection apparatus 110 arranged along the path 104 of the pulsed light beam 102 to the center wavelength selection optic 116. In the optical system 100 of Fig. 1 (which can be a DUV system), the center wavelength for each pulse of the pulsed light beam 102 can be selected to be about 248 nanometers (nm) or 193 nm.

[0089] In some implementations, the tuning mechanism 112 can be the tuning mechanism 412 (Fig. 4A) that includes the optical elements 440a-440d (that are refractive right-angled prisms), and the angle of incidence of the pulsed light beam 102 on the center wavelength selection optic 416 can be selected (and changed) by changing or adjusting the arrangement of at least one of the optical elements 440a-440d of the tuning mechanism 412 relative to the path 104 of the pulsed light beam 102. In other words, the angle of incidence of the pulsed light beam 102 on the center wavelength selection optic 416 is selected by adjusting one or more angles of the refractive optical elements 440a- 440d within the tuning mechanism 412. In this way, the center wavelength of the pulsed light beam 102 is selected by adjusting the tuning mechanism 112 (or the tuning mechanism 412 of Fig. 4A) and interacting the pulsed light beam 102 with the tuning mechanism 112.

[0090] A plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated are produced, including by splitting the pulsed light beam into the plurality of pulsed light sub-beams by interacting the pulsed light beam with a diffractive pattern arranged along the path of the pulsed light beam (865). Each pulsed light sub-beam is associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light subbeam has a different wavelength, that is, each pulsed light sub-beam is associated with a respective one of the distinct wavelengths that are separated by at least 10 picometers (pm) (865). For example, the plurality of pulsed light sub-beams 221, 223, 225, which are spatially separated and not temporally separated, can be produced by splitting the pulsed light beam 102 into the plurality of pulsed light sub-beams 221, 223, 225. To split the pulsed light beam 102, the pulsed light beam 102 can interact with the diffractive pattern of the diffractive optical element 114 arranged along the path 104 of the pulsed light beam 102. In other words, the pulsed light beam 102 can interact with the diffractive pattern of the diffractive optical element 114 by transmitting the pulsed light beam 102 through the diffractive optical element 114.

[0091] Moreover, the plurality of pulsed light sub-beams 221, 223, 225 can be produced from the pulsed light beam 102 by adjusting a position of the diffractive pattern relative to the path 104 of the pulsed light beam 102. For example, the position of the diffractive optical element 114 can be adjusted by translating and/or rotating the diffractive optical element 114 by controlling, for example, the actuators 614A, 714A (Figs. 6A-7B), such that the position of the diffractive pattern of the diffractive optical element 114 relative to the path 104 of the pulsed light beam 102 is also adjusted. In other words, the position of the diffractive pattern can be adjusted by controlling the movement of the diffractive optical element 114 that includes the diffractive pattern.

[0092] Each of the produced pulsed light sub-beams 221, 223, 225 is associated with the distinct angle of incidence 222A, 224A, 226 A, respectively, on the center wavelength selection optic 116 such that each pulsed light sub-beam 221, 223, 225 is associated with a respective one of the distinct center wavelengths wl, w2, w3 that are separated by at least 10 pm. Specifically, the wavelength separation between the distinct center wavelengths wl, w2, w3 of the plurality of pulsed light sub-beams 221, 223, 225 can be greater than about 10 picometers (pm), or about 30 pm, or about 45 pm. Furthermore, each distinct angle of incidence 222 A, 224 A, 226 A on the center wavelength selection optic 116 associated with each pulsed light sub-beam 221, 223, 225, respectively, is determined by the groove spacing 114s of the diffractive pattern (which is included within the diffractive optical element 114). [0093] In the example of Fig.1, the plurality of pulsed light sub-beams 221, 223, 225 leaving the center wavelength selection optic 116 are recombined by interacting the pulsed light sub-beams 221, 223, 225 with the diffractive pattern arranged along the path of the pulsed light beam 102. In this way, the plurality of pulsed light sub-beams 221, 223, 225 are produced when the pulsed light beam 102 interacts with the diffractive pattern travelling along the path 104 to the center wavelength selection optic 116, and are recombined to form the pulsed light beam 102 when the pulsed light sub-beams 221, 223, 225 interact with the diffractive pattern travelling along the path 104 away from the center wavelength selection optic 116. In the example of Fig. 1, the diffractive optical element 114 recombines the plurality of pulsed light sub-beams 221, 223, 225 after the pulsed light sub-beams 221, 223, 225 interact with the center wavelength selection optic 116 to form the recombined pulsed light beam 102 that is directed along the path 104 toward the wafer 328. As such, the recombined pulsed light beam 102 that includes the plurality of distinct center wavelengths wl, w2, w3 can then be directed to interact with the lithography exposure apparatus 107 to form the plurality of aerial images 331, 333, 335 on the wafer 328.

[0094] The plurality of aerial images are formed in the single pulsed light beam on the wafer such that each aerial image is formed based on a distinct center wavelength (867). For example, the plurality of aerial images 331, 333, 335 are formed in the single pulsed light beam 102 on the wafer 328 such that the aerial images 331, 333, 335 are formed at different locations on the z axis of the wafer and each aerial image 331, 333, 335 is based on one of the distinct center wavelengths wl, w2, w3. Because the single pulsed light beam 102 interacts with the diffractive pattern to select the plurality of center wavelengths wl, w2, w3 of the recombined pulsed light beam 102, each of the plurality of aerial images 331, 333, 335 are formed in the single light beam 102 in a single exposure pass. The intensity profile of the pulsed light beam 102 is flattened at the wafer 328 in the lithography exposure apparatus 107. Intensity is flattened at the wafer 328 because of the increasing number of sub-pulses, each of which has the same optical power. The more sub-pulses (each with a distinct center wavelength), the flatter the power distribution will be through focus at the wafer 328.

[0095] Thus, by interacting the pulsed light beam 102 with the diffractive pattern (or the diffractive optical element 114), the plurality of aerial images 331, 333, 335 that are associated with the distinct center wavelengths wl, w2, w3, respectively, are formed on the wafer 328 in the single pulsed light beam 102 and in a single lithography exposure pass.

[0096] Referring to Fig. 9, a block diagram of an example of an implementation 900 of the optical system 100 is shown. The optical system 100 is a photolithography system 900 that include an optical source 905 as the light source 105. The optical source 905 produces the pulsed light beam 102, which is provided to the lithography exposure apparatus 107. The optical source 905 can be, for example, an excimer optical source that outputs the pulsed light beam 102 (which can be a laser beam). As the pulsed light beam 102 enters the lithography exposure apparatus 107, it is directed through the projection optical system 327 and projected onto the wafer 328, as discussed above with reference to Figs. 3A-3C. In this way, one or more microelectronic features are patterned onto a photoresist on the wafer 328 that is then developed and cleaned prior to subsequent process steps, and the process repeats. The photolithography system 900 also includes the control system 450 (Fig. 4A), which, in the example of Fig. 9, is connected to components of the optical source 905 (including the wavelength selection apparatus 410) as well as to the lithography exposure apparatus 107 to control various operations of the system 900.

[0097] In the implementation shown in Fig. 9, the optical source 905 is a two-stage laser system that includes a master oscillator (MO) 970 that provides a seed light beam 902s to a power amplifier (PA) 972. The MO 970 and the PA 972 can be considered to be subsystems of the optical source 905 or systems that are part of the optical source 905. The power amplifier 972 receives the seed light beam 902s from the master oscillator 970 and amplifies the seed light beam 902s to generate the pulsed light beam 102 for use in the lithography exposure apparatus 107. For example, the master oscillator 970 can emit a pulsed seed light beam, with seed pulse energies of approximately 1 milliJoule (mJ) per pulse, and these seed pulses can be amplified by the power amplifier 972 to about 10 to 15 mJ. [0098] The master oscillator 970 includes a discharge chamber 971 having two elongated electrodes 974, a gain medium 976 that is a gas mixture confined within the discharge chamber 971, and a fan for circulating the gas mixture between the electrodes 974. A resonator is formed between the wavelength selection apparatus 410 (Fig. 4A) on one side of the discharge chamber 971 and an optical output coupler 978 on a second side of the discharge chamber 971. The wavelength selection apparatus 410 finely tunes or adjusts the spectral properties of the pulsed light beam 102, including the wavelength and the bandwidth of the pulsed light beam 102, by tuning or adjusting the seed light beam 902s.

[0099] The master oscillator 970 can also include a line center analysis module 979 that receives an output light beam from the output coupler 978 and a beam coupling optical system 980 that modifies the size or shape of the output light beam as needed to form the seed light beam 902s. The line center analysis module 979 is a measurement system that can be used to measure or monitor the wavelength and/or bandwidth of the seed light beam 902s. The line center analysis module 979 can be placed at other locations in the optical source 905, or it can be placed at the output of the optical source 905. [0100] The gas mixture used in the discharge chamber 971 can be any gas suitable for producing a light beam at the wavelength and bandwidth required for the application. For an excimer source, the gas mixture can contain a noble gas (rare gas) such as, for example, argon or krypton, a halogen, such as, for example, fluorine or chlorine and traces of xenon apart from helium and/or neon as buffer gas. Specific examples of the gas mixture include argon fluoride (ArF), which emits light at a wavelength of about 193 nm, krypton fluoride (KrF), which emits light at a wavelength of about 248 nm, or xenon chloride (XeCl), which emits light at a wavelength of about 351 nm. The excimer gain medium (the gas mixture) is pumped with short (for example, nanosecond) cunent pulses in a high-voltage electric discharge by application of a voltage to the elongated electrodes 974.

[0101] The power amplifier 972 includes a beam coupling optical system 982 that receives the seed light beam 902s from the master oscillator 970 and directs the light beam 902s through a discharge chamber 973, and to a beam turning optical element 981, which modifies or changes the direction of the seed light beam 902s so that it is sent back into the discharge chamber 973 and through the beam coupling optical system 982. The discharge chamber 973 includes a pair of elongated electrodes 975, a gain medium 977 that is a gas mixture, and a fan for circulating the gas mixture between the electrodes 975.

[0102] The output pulsed light beam 102 is directed through a bandwidth analysis module 983, where various parameters (such as the bandwidth or the wavelength) of the beam 102 can be measured. The output light beam 102 can also be directed through a beam preparation system 984. The beam preparation system 984 can include, for example, a pulse stretcher, where each of the pulses of the output light beam 102 is stretched in time, for example, in an optical delay unit, to adjust for performance properties of the light beam that impinges the lithography exposure apparatus 107. The beam preparation system 984 also can include other components that are able to act upon the light beam 102 such as, for example, reflective and/or refractive optical elements (such as, for example, lenses and minors), filters, and optical apertures (including automated shutters).

[0103] The photolithography system 900 also includes the control system 450. In the implementation shown in Fig. 9, the control system 450 is connected to various components of the optical source 905. For example, the control system 450 can control when the optical source 905 emits a pulse of light or a burst of light pulses that includes one or more pulses of light by sending one or more signals to the optical source 905. The control system 450 is also connected to the lithography exposure apparatus 107. Thus, the control system 450 also can receive instructions and/or data from the lithography exposure apparatus 107. The lithography exposure apparatus 107 can include a dedicated controller (that can communicate with the control system 450) that can control the exposure of the wafer 328 and thus can be used to control how electronic features are printed on the wafer 328. In some implementations, the lithography controller can control the scanning of the wafer 328 by controlling the motion of the slit 336a in the x-y plane (Fig. 3B). The lithography exposure apparatus 107 also can include, for example, temperature control devices (such as air conditioning devices and/or heating devices), and/or power supplies for the various electrical components that are controlled by the lithography controller. In some implementations, the lithography controllers is a part of the control system 450, and the control system 450 can include more than one sub-control system.

[0104] Moreover, the control system 450 can control the various components of the wavelength selection apparatus 410. For example, the control system 450 can control the position of each of the prisms 440a-440d, the position of the diffractive optical element 414, and the position of the center wavelength selection optic 416.

[0105] Referring also to Figs. 10A-10C, an implementation 1014 of the diffractive optical element 114 is shown. In this implementation 1014, the diffractive optical element 114 is a blaze grating placed between the prism 440d and the center wavelength selection optic 416. The periodic structure or feature is linearly arranged along the Z axis so that the periodic structure is linearly symmetric about a center line 1014c that is parallel with the Z axis. As discussed above, if the blaze grating 1014 is shifted along a direction Ds that is perpendicular to the direction of travel of the light beam 102 (and also in the XY plane), then the amount of light that goes into one sub-beam versus the other subbeams and therefore impinges on the selection optic 416 can be adjusted. In this way, the amount of optical power in one aerial image at the wafer can be changed versus the other aerial image. Multifocal imaging at the wafer can be controlled.

[0106] The embodiments can be further described using the following clauses:

1. A wavelength selection apparatus for a pulsed optical source that produces a pulsed light beam, the wavelength selection apparatus comprising: a center wavelength selection optic configured to select at least one center wavelength for each pulse of the pulsed light beam in accordance with an angle of incidence of the pulsed light beam on the center wavelength selection optic; a tuning mechanism arranged along a path of the pulsed light beam to the center wavelength selection optic, the tuning mechanism configured to optically interact with the pulsed light beam and to select the angle of incidence of the pulsed light beam on the center wavelength selection optic; and a diffractive optical element that is passive and transmissive and is arranged along the path of the pulsed light beam at a location at which the pulsed light beam is at least mostly magnified, the diffractive optical element configured to interact with the pulsed light beam and to produce a plurality of pulsed light sub-beams from the pulsed light beam, each pulsed light sub-beam associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light subbeam is associated with a distinct wavelength and the optical spectrum of the pulsed light beam includes a peak at each distinct wavelength.

2. The wavelength selection apparatus of clause 1, wherein the diffractive optical element is a diffractive beam splitter, a diffraction grating, a phase grating, a binary phase grating, or a blazed phased grating.

3. The wavelength selection apparatus of clause 1, wherein the tuning mechanism comprises four refractive optical elements.

4. The wavelength selection apparatus of clause 3, wherein each refractive optical element is a right- angled prism.

5. The wavelength selection apparatus of clause 1, wherein a wavelength separation between the distinct wavelengths of the plurality of pulsed light sub-beams is greater than about 10 picometers (pm), about 30 pm, or about 45 pm.

6. The wavelength selection apparatus of clause 1, wherein the center wavelength for each pulse of the pulsed light beam is about 248 nanometers (nm) or about 193 nm.

7. The wavelength selection apparatus of clause 1, wherein the wavelength separation between the distinct wavelengths of the plurality of pulsed light sub-beams depends on a periodic shape of the diffractive optical element.

8. The wavelength selection apparatus of clause 1, wherein the tuning mechanism comprises four right-angled prisms arranged along the path of the pulsed light beam to the diffractive optical element, and the pulsed light beam is fully magnified between the four right-angled prisms and the center wavelength selection optic.

9. The wavelength selection apparatus of clause 1, further comprising an actuator configured to adjust a position of the diffractive optical element relative to the path of the pulsed light beam, such that the diffractive optical element is positioned along the path of the pulsed light beam at some moments and is not positioned along the path of the pulsed light beam at other moments, the diffractive optical element interacting with the pulsed light beam only if the diffractive optical element is positioned along the path of the pulsed light beam.

10. The wavelength selection apparatus of clause 9, wherein the actuator is further configured to adjust an angle of the diffractive optical element relative to a direction of the path of the pulsed light beam at the diffractive optical element, such that the distinct angle of incidence of each produced pulsed light sub-beam on the center wavelength selection optic is adjusted.

11. The wavelength selection apparatus of clause 1, wherein the plurality of pulsed light sub-beams comprises three or more pulsed light sub-beams.

12. The wavelength separation apparatus of clause 1, wherein the tuning mechanism and the center wavelength selection optic are arranged to interact with the pulsed light beam in a Littrow configuration.

13. The wavelength separation apparatus of clause 1, wherein the center wavelength selection optic is a reflective optical element.

14. The wavelength separation apparatus of clause 1, wherein an aerial image is formed for each distinct wavelength of the pulsed light beam.

15. The wavelength selection apparatus of clause 1, further comprising a control system and one or more actuators associated with the tuning mechanism, wherein the control system is configured to adjust a signal to the one or more actuators to thereby adjust the angle of incidence of the pulsed light beam on the center wavelength selection optic.

16. The wavelength selection apparatus of clause 1, wherein the diffractive optical element is arranged perpendicular to a direction of propagation of the pulsed light beam along the path.

17. The wavelength selection apparatus of clause 1, wherein the diffractive optical element is further configured to recombine the plurality of pulsed light sub-beams from the center wavelength selection optic to form the pulsed light beam.

18. The wavelength selection apparatus of clause 1, wherein the tuning mechanism comprises four right-angled prisms, and the location at which the pulsed light beam is at least mostly magnified is in the optical path between the right-angled prism that is closest to the center wavelength selection optic and the right-angled prism that is second closest to the center wavelength selection optic.

19. An optical system comprising: a light source configured to produce a pulsed light beam that is directed along a path toward a lithography exposure apparatus; a lithography exposure apparatus configured to interact with the pulsed light beam; and a wavelength selection apparatus arranged relative to the light source, the wavelength selection apparatus comprising: a center wavelength selection optic configured to select at least one center wavelength for each pulse of the pulsed light beam in accordance with an angle of incidence of the pulsed light beam on the center wavelength selection optic; a tuning mechanism arranged along the path of the pulsed light beam to the center wavelength selection optic, the tuning mechanism configured to optically interact with the pulsed light beam and to select the angle of incidence of the pulsed light beam on the center wavelength selection optic; and a diffractive optical element that is passive and transmissive and is arranged along the path of the pulsed light beam at a location at which the pulsed light beam is fully magnified or at least mostly magnified, the diffractive optical element configured to interact with the pulsed light beam and to produce a plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated, each pulsed light sub-beam associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a distinct wavelength and the optical spectrum of the pulsed light beam includes a peak at each distinct wavelength.

20. The optical system of clause 19, wherein the diffractive optical element is a diffractive beam splitter, a diffraction grating, a phase grating, a binary phase grating, or a blazed phased grating.

21. The optical system of clause 19, wherein the tuning mechanism comprises four refractive optical elements.

22. The optical system of clause 21, wherein each refractive optical element is a right-angled prism.

23. The optical system of clause 19, wherein a wavelength separation between the distinct wavelengths of the plurality of pulsed light sub-beams is greater than about 10 picometers (pm), about 30 pm, or about 45 pm.

24. The optical system of clause 19, wherein the center wavelength for each pulse of the pulsed light beam is about 248 nanometers (nm) or 193 nm.

25. The optical system of clause 19, wherein the wavelength selection apparatus further comprises an actuator configured to adjust a position of the diffractive optical element relative to the path of the pulsed light beam such that the diffractive optical element is positioned along the path of the pulsed light beam at some moments and is not positioned along the path of the pulsed light beam at other moments, the diffractive optical element interacting with the pulsed light beam only if the diffractive optical element is positioned along the path of the pulsed light beam.

26. The optical system of clause 25, further comprising a control system configured to control the wavelength selection apparatus to adjust the position of the diffractive optical element relative to the path of the pulsed light beam.

27. The optical system of clause 19, wherein the lithography exposure apparatus comprises a mask positioned to interact with the pulsed light beam from the light source and a wafer holder configured to hold a wafer.

28. The optical system of clause 27, wherein a plurality of distinct aerial images are formed on the wafer at the wafer holder, each distinct aerial image based on the distinct wavelength of the associated pulsed light sub-beam that passes through the mask along a direction of propagation. 29. The optical system of clause 19, further comprising a control system and one or more actuators associated with the tuning mechanism, wherein the control system is configured to adjust a signal to the one or more actuators to thereby adjust the angle of incidence of the pulsed light beam on the center wavelength selection optic.

30. A method for forming a plurality of aerial images with a single pulsed light beam, the method comprising: generating the pulsed light beam along a path toward a wafer; selecting an angle of incidence of the pulsed light beam on a center wavelength selection optic to select at least one center wavelength for each pulse of the pulsed light beam by optically interacting the pulsed light beam with a tuning mechanism arranged along the path of the pulsed light beam to the center wavelength selection optic; producing a plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated, including splitting the pulsed light beam into the plurality of pulsed light sub-beams by interacting the pulsed light beam with a diffractive pattern arranged along the path of the pulsed light beam, each pulsed light sub-beam being associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a respective one of the distinct wavelengths that are separated by at least 10 picometers (pm); and forming the plurality of aerial images in the single pulsed light beam on the wafer, wherein each aerial image is formed based on a distinct wavelength.

31. The method of clause 30, wherein interacting the pulsed light beam with the diffractive pattern comprises transmitting the pulsed light beam through a diffractive optical element.

32. The method of clause 30, wherein each distinct angle of incidence onto the center wavelength selection optic associated with each pulsed light sub-beam is determined by a periodic shape of the diffractive pattern.

33. The method of clause 30, wherein selecting the angle of incidence of the pulsed light beam on the center wavelength selection optic comprises adjusting one or more angles of refractive optical elements within the tuning mechanism.

34. The method of clause 30, wherein producing the plurality of pulsed light sub-beams from the pulsed light beam comprises adjusting a position of the diffractive pattern relative to the path of the pulsed light beam.

35. The method of clause 34, wherein adjusting the position of the diffractive pattern comprises controlling by moving a diffractive optical element that includes the diffractive pattern.

36. The method of clause 30, wherein forming the plurality of aerial images on the wafer comprises flattening the intensity profile of the pulsed light beam at the wafer.

37. The method of clause 30, further comprising recombining the plurality of pulsed light sub-beams leaving the center wavelength selection optic by interacting the pulsed light sub-beams with the diffractive pattern arranged along the path of the pulsed light beam, such that the plurality of pulsed light sub-beams are produced when the pulsed light beam interacts with the diffractive pattern travelling along the path to the center wavelength selection optic, and the plurality of pulsed light subbeams are recombined to form the pulsed light beam when the pulsed light sub-beams interact with the diffractive pattern travelling along the path away from the center wavelength selection optic.

38. A wavelength selection apparatus for a pulsed optical source that produces a pulsed light beam, the wavelength selection apparatus comprising: a center wavelength selection optic configured to select at least one center wavelength for each pulse of the pulsed light beam in accordance with an angle of incidence of the pulsed light beam on the center wavelength selection optic; a tuning mechanism arranged along a path of the pulsed light beam to the center wavelength selection optic, the tuning mechanism configured to optically interact with the pulsed light beam and to select the angle of incidence of the pulsed light beam on the center wavelength selection optic, the tuning mechanism including four refractive optical elements; and a passive and transmissive diffractive optical element arranged along the path of the pulsed light beam at a location between the tuning mechanism and the center wavelength selection optic, the diffractive optical element configured to interact with the pulsed light beam and to produce a plurality of pulsed light sub-beams from the pulsed light beam that are spatially separated and not temporally separated, each pulsed light sub-beam associated with a distinct angle of incidence on the center wavelength selection optic such that each pulsed light sub-beam is associated with a distinct wavelength and the optical spectrum of the pulsed light beam includes a peak at each distinct wavelength.

39. The wavelength selection apparatus of clause 38, wherein the diffractive optical element is a diffractive beam splitter, a diffraction grating, a phase grating, a binary phase grating, or a blazed phased grating.

40. The wavelength selection apparatus of clause 38, wherein the tuning mechanism comprises four refractive optical elements.

41. The wavelength selection apparatus of clause 38, wherein a wavelength separation between the distinct wavelengths of the plurality of pulsed light sub-beams is greater than about 10 picometers (pm), about 30 pm, or about 45 pm.

[0107] Other implementations are within the scope of the claims.