ULTRAVIOLET LIGHT SOURCE

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 62/827,521, filed April 1, 2019 and titled CONTROLLING CONVERSION EFFICIENCY IN AN EXTREME ULTRAVIOLET

LIGHT SOURCE, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to controlling conversion efficiency (CE) in an extreme ultraviolet

(EUV) light source.

BACKGROUND

[0003] Extreme ultraviolet (“EUV") light, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a material that includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range when in a plasma state. In one such method, often termed laser produced plasma (“LPP"), the required plasma may be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

[0005] In one aspect, an extreme ultraviolet (EUV) light source includes: a vacuum vessel; a target material supply system configured to supply targets to an interior of the vacuum vessel, the targets including at least a first target that has an initial shape at an initial target region in the vacuum vessel; a first optical source configured to provide a first light beam to a first target region in the vacuum vessel, the first light beam being configured to modify the initial shape of the initial target to form a modified target; and a second optical source configured to provide a second light beam to a second target region in the vacuum vessel, the second target region configured to receive the modified target, the second light beam being configured to interact with the modified target and to convert at least some of the target material in the modified target to a plasma that emits EUV light. The initial shape of the first target is controlled to thereby control an amount of plasma produced from the interaction between the second light beam and the modified target.

[0006] Implementations may include one or more of the following features. The target material may include a molten metal, and the supply system may include: a reservoir configured to hold the target material; a nozzle configured to be fluidly coupled to the reservoir and to emit the targets into the interior of the vacuum vessel; and an actuator mechanically connected to the nozzle. The initial shape of the first target at the initial target region may be controlled by causing the actuator to vibrate the nozzle at more than one frequency. A spacing between the first target and a second target may be controlled by adjusting a pressure applied to the target material in the reservoir, and the second target may be supplied by the target supply system before the first target. The initial shape of the first target may be based on the controlled spacing between the first target and a second target.

[0007] In some implementations, the EUV light source also includes a third optical source configured to provide a third light beam to a third target region. In these implementations, the third target region is configured to receive the first target, and the initial shape of the first target at the initial target region is controlled by interacting the first target with the third light beam.

The third target region may be closer to the target material supply system than the first target region and the second target region.

[0008] The initial shape of the first target at the initial target region may be an oblate sphere of molten metal having a first extent along a first direction and a second extent along a second direction that is perpendicular to the first direction, and the ratio of the first extent to the second extent may be between 0.6 and 0.8.

[0009] The initial shape of the first target at the initial target region may be an oblate sphere of molten metal having a first extent along a first direction and a second extent along a second direction that is perpendicular to the first direction, and the ratio of the first extent to the second extent may be between 0.75 and 0.9.

[0010] The initial shape of the first target at the initial target region may be an oblate sphere of molten metal having a first extent along a first direction and a second extent along a second direction that is perpendicular to the first direction, and the ratio of the first extent to the second extent may be about 0.8.

[0011] The modified target may have a morphology that is determined by the initial shape of the first target at the initial target region, the morphology describing a shape of the target and/or a target material density in three dimensions. The modified target may include a lateral extent in one of the three dimensions, the lateral extent may depend on a distance between the first target region and the second target region.

[0012] The initial shape of the first target material droplet being controlled to thereby control an amount of plasma produced from the interaction between the second light beam and the modified target may include the initial shape of the first target material being controlled to thereby control a conversion efficiency (CE) of the EUV light source, the CE being a ratio of energy supplied to the modified target to energy emitted from the plasma as EUV light.

[0013] The initial target region may be between the target material supply system and the first target region.

[0014] In another general aspect, a method of controlling conversion efficiency (CE) in an extreme ultraviolet (EUV) light source includes determining an initial shape of an initial target by controlling a component of the EUV light source; causing a pre-pulse light beam to interact with the initial target to form a modified target; and causing a main optical pulse to interact with the modified target to produce a plasma that emits EUV light. The interaction between the modified target and the main optical pulse is associated with a conversion efficiency (CE), the CE being a ratio of energy supplied to the modified target to energy emitted from the plasma as EUV light, and the CE is controlled based on the determined initial shape of the initial target.

[0015] Implementations may include one or more of the following features. The component of the EUV light source may include a reservoir that is part of a target material supply system, and determining the initial shape of the initial target may include controlling an amount of pressure on molten target material in the reservoir before the initial target is produced by the target supply system. Controlling the amount of pressure on the molten target material in the reservoir may controls a spacing between the initial target and another target, and the initial shape of the initial target may be based on the spacing.

[0016] The component of the EUV light source may be an actuator coupled to a capillary tube of a target material supply system, and determining the initial shape of the initial target may include controlling the actuator such that the actuator vibrates the tube at more than one frequency. Controlling the actuator such that the actuator vibrates the tube at more than one frequency may produces a stream of coalesced targets from a jet of target material, and the method also may include adjusting one of the more than one frequencies such that two of the coalesced targets merge into a merged target, and the initial target is the merged target.

[0017] The component of the EUV light source may include a target material supply system configured to supply the initial target and at least a second target, and determining the initial shape of the initial target may include controlling the target material supply system such that a spacing between the initial target and the second target is adjusted, the second target being supplied by the target supply system before the initial target.

[0018] The component of the EUV light source may include an initial light source configured to provide an initial light beam, and determining the initial shape of the initial target may include controlling the initial light source such that the initial light beam interacts with the initial target, and the initial shape of the initial target may be at least partially determined by interacting the initial target with the initial light beam.

[0019] Implementations of any of the techniques described above may include an EUV light source, a system, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. DRAWING DESCRIPTION

[0020] FIG. 1 is a block diagram of an extreme ultraviolet (EUV) light source.

[0021] FIG. 2 is a block diagram of another extreme ultraviolet (EUV) light source.

[0022] FIG. 3 is a flow chart of an example process to control conversion efficiency (CE) in an EUV light source.

[0023] HG. 4 is a block diagram of another extreme ultraviolet (EUV) light source.

[0024] FIG. 5 is a block diagram of another extreme ultraviolet (EUV) light source. [0025] FIGS. 6A-6D are examples of experimental data.

[0026] FIGS. 7A and 7B are block diagrams, each of a lithography system.

[0027] FIG. 8 is a block diagram of another extreme ultraviolet (EUV) light source. DETAILED DESCRIPTION

[0028] Referring to FIG. 1, a block diagram of an extreme ultraviolet (EUV) light source 100 is shown. The EUV light source 100 is part of an EUV lithography system 101 that includes an output apparatus 199 (such as a lithography apparatus) that receives an exposure beam 198 produced by the EUV light source 100. A stream 121 of targets is produced by a target supply system 110 and travels toward a target region 124_2. Each target in the stream 121 includes target material that emits EUV light in a plasma state. Techniques for controlling a conversion efficiency (CE) of the light source 100 by controlling an initial target shape are disclosed. The initial target shape is the shape of the targets in the stream 121 prior to interaction with a light pulse 104_2 (which is also referred to as a pre-pulse).

[0029] In the example shown in FIG. 1, an initial target 121p, which is one of the targets in the stream 121, is in the target region 124_2. The light pulse 104_2 interacts with the initial target 121p to form a modified target 121m. The modified target 121m may be, for example, a diskshaped distribution of target material that has a larger extent in the x-y plane than the target 121p and a smaller extent along the z axis than the initial target 121p. The modified target 121m may be a cloud or mist of particles that has a larger volume than the initial target 121p in three dimensions. The modified target 121m interacts with a light pulse 104_1 (also referred to as a main pulse) to form a plasma 196 that emits light 197 (which includes EUV light 193). The modified target 121m has a morphology or morphology characteristics that determine or influence how much of the target material in the modified target 121m is converted into the plasma 196. The conversion efficiency (CE) is the ratio of energy supplied to the modified target 121m by the light pulse 104_1 to the amount of energy emitted from the plasma 196 as the EUV light 193. Because the morphology of the modified target 121m influences the amount of target material that is converted to the plasma 196, the moiphology of the modified target 121m influences the amount of the EUV light 193 produced and thus also influences the CE.

[0030] The EUV light source 100 includes a control system 150 that controls the initial target shape to thereby control the morphology of the modified target 121m and, thus, the CE. In the discussion below, a final target is a target that is used in the production of the plasma 196. In the example of FIG. 1, the modified target 121m is the final target. The morphology of the final target describes, for example, a spatial arrangement or shape of the target material in the final target and/or a density of target material in the final target in at least one dimension. In some implementations, the morphology of the final target describes the density of the final target in three dimensions.

[0031] The initial target 121p is a target in the stream 121 that is in the target region 124_2 but has not yet interacted with the light pulse 104_2. The shape of the initial target is also called the initial target shape. The location where the initial target 121p is in the vacuum chamber may be referred to as the initial target region. In the example of FIG. 1, the shape of the initial target 121p is the initial target shape, and the initial target region is the region where the initial target 121p is just prior to interacting with the light pulse 104_2. The various targets in the stream 121 may have different shapes. For example, some of the targets in the stream 121 may be substantially spherical droplets and the initial target 121p may have a non-spherical shape. Thus, the initial target shape may be non-spherical even if some of the targets in the stream 121 are spherical droplets.

[0032] The control system 150 controls the morphology of the final target by controlling the initial target shape. The control system 150 controls the initial target shape by, for example, adjusting a pressure p that is applied to target material in a reservoir 118, controlling the frequencies at which a modulator 132 that is mechanically coupled to the target supply system 110 vibrates to thereby introduce relative motion between individual targets in the stream 121 such that the targets merge to form a larger target with a particular shape before reaching the target region 124_2, and/or interacting the initial target with a third light pulse (such as the light pulse 204_3 of FIG. 2). An overview of the EUV light source 100 is provided prior to discussing these various techniques in more detail.

[0033] The targets in the stream 121 are spatially separated from each other and are spatially distinct from each other. Under expected operating conditions of the light source 100, targets in the stream 121 enter the target region 124_2 one at a time. The target region 124_2 also receives the light pulse 104_2. An interaction between the light pulse 104_2 and the initial target 121p forms the modified target 121m. The interaction between the light pulse 104_2 and the initial target 121p may enhance the ability of the modified target 121m to absorb the light pulse 104_1. For example, the interaction between the light pulse 104_2 and the initial target 121p may change the shape, volume, and/or size of the distribution of the target material and/or may reduce the density gradient of the target material along the direction of propagation of the main pulse 104_1. The changes in spatial characteristics also may cause changes in physical characteristics. For example, if the modified target 121m is larger than the initial target 121p in at least one dimension, the target material is spread out in that dimension and the density of target material is lower in that dimension as compared to the density of the initial target 121p along the same dimension.

[0034] The target region 124_2 is between the target supply system 110 and the target region 124_1. The modified target 121m drifts generally along the x direction to the target region 124_1 and is irradiated by the light pulse 104_1. The interaction between the modified target 121m and the light pulse 104_1 converts at least some of the target material in the modified target 121m into the plasma 196 that emits the light 197. The creation of the plasma 196 through the interaction between the modified target 121m and the light beam 104_1 is referred to as a plasma-creation event.

[0035] The light 197 includes the EUV light 193, which has wavelengths that correspond to the emission lines of the target material. The EUV range may include light having a wavelength of, for example, 5 nanometers (nm), 5nm-20nm, 10nm-120nm, or less than 50nm. The light 197 also may include wavelengths that are not in the EUV range. Light at wavelengths that are not in the EUV range is referred to as out-of-band light. For example, the target material may include tin. In these implementations, the light 197 includes the EUV light and also includes out-of-band light such as deep ultraviolet (DUV), visible, near infrared (NIR), mid-wavelength infrared (MWIR), and/or long-wavelength infrared (LWIR) light. The DUV light can include light having wavelengths between about 120nm-300nm, the visible light can include light having wavelengths between about 390nm-750nm, the NIR light can include light having wavelengths between about 750nm-2500nm, the MWIR light may have a wavelength between about 3000nm- 5000nm, and the LWIR light may have a wavelength between about 8000nm-12000nm.

[0036] The EUV light source 100 includes an optical element 113 in the vacuum chamber 109. The optical element 113 is positioned to collect at least some of the light 193 to form the exposure beam 198. The optical element 113 may be, for example a curved mirror that has a reflective surface 116 that faces the target region 124_1. The optical element 113 also may include an aperture (not shown) that allows pulses of light (such as the light pulse 104_1) to reach the target region 124_1. The reflective surface 116 receives and reflects at least some of the light 193 to form the exposure beam 198. The reflective surface 116 has a coating or other optical mechanism such that the optical element 113 reflects wavelengths in the EUV range but does not reflect out-of-band components of the light 197 or reflects only a nominal amount of the out-of-band components of the light 197. In this way, the exposure beam 198 includes primarily EUV light and includes little or no out-of-band light. The lithography apparatus 199 uses the EUV exposure beam 198 to expose a substrate 195 (for example, a silicon wafer) to thereby form electronic features on the substrate 195.

[0037] The light pulse 104_1 is a single pulse of light that is part of a light beam 106_1. The light beam 106_1 is a train of pulses each of which is separated from adjacent pulses in time.

The pulse 104_1 has a finite temporal duration referred to as the pulse duration. The pulse duration may be the total time during which the light pulse 104_1 has a non-zero optical power. Other metrics may be used to describe the pulse duration. For example, the pulse duration may be less than the time during which the light pulse 104_1 has a non-zero power, such as the full- width at half maximum (FWHM) of the pulse 104_1. The light beam 106_1 is formed by an optical source 108_1 and is delivered to the target region 124_1 by a beam delivery system 105_1.

[0038] The light pulse 104_2 is a single light pulse in a light beam 106_2, which includes a train of pulses separated in time. The light pulse 104_2 has a finite temporal duration. The light pulse

104_2 is formed by an optical source 108_2, propagates along a beam path 107_2, and is delivered to the target region 124_2 via a beam delivery system 105_2.

[0039] The optical sources 108_1 and 108_2 are part of an optical system or light-generation module 108. The optical sources 108_1 and 108_2 may be, for example, two lasers. For example, the optical sources 108_1, 108_2 may be two carbon dioxide (CO2) lasers. In other implementations, the optical sources 108_1, 108_2 may be different types of lasers. For example, the optical source 108_2 may be a solid state laser, and the optical source 108_1 may be a CO2 laser. The first and second light beams 106_1, 106_2 may have different wavelengths. For example, in implementations in which the optical sources 108_1, 108_2 include two CO2 lasers, the wavelength of the first light beam 106_1 may be about 10.26 micrometers (pm) and the wavelength of the second light beam 106_2 may be between 10.18 pm and 10.26 pm. The wavelength of the second light beam 106_2 may be about 10.59 pm. In these implementations, the light beams 106_1 , 106_2 are generated from different lines of the CO2 laser, resulting in the light beams 106_1, 106_2 having different wavelengths even though both beams are generated from the same type of source.

[0040] The light pulse 104_2 may have a duration of 1 picosecond (ps) to 100 nanoseconds (ns), for example, the pulse 104_2 may have a duration of 1-100 ns and a wavelength of about 1 pm or 10.6 pm. In some implementations, the pulse 104_2 is a laser pulse that has energy of about 1-100 ml, a pulse duration of about 1-70 ns, and a wavelength of about 1-10.6 pm. In these implementations, the modified target 121m may be a substantially disk-shaped target. In some implementations, the pulse 104_2 has a duration of less than 1 ns and a wavelength of 1 pm. For example, the pulse 104_2 may have a duration of 300 ps or less, 100 ps or less, between 100-300 ps, or between 10-100 ps. In these implementations, the modified target 121m may be a cloud or mist of particles of target material.

[0041] The beam delivery systems 105_1, 105_2 include respective optical systems 112_1, 112_2. The optical systems 112_1, 112_2 include one or more optical elements or components that are able to interact with the respective light beams 106_1, 106_2. For example, the optical components elements or components may include passive optical devices such as mirrors, lenses, and/or prisms, and any associated mechanical mounting devices and/or electronic drivers. These components may steer and/or focus the light beam 106_1. Additionally, the optical elements or components may include components that modify one or more properties of the optical beam to form and/or modify a light pulse. For example, the optical components may include active optical devices, such as acousto-optic modulators and/or electro-optic modulators, capable of changing the temporal profile of the light beam 106_1 or the light beam 106_2 to form the light pulse 104_1 or the light pulse 104_2, respectively. In the example of FIG. 1, the light beam 106_1 and the light beam 106_2 interact with separate beam delivery systems 105_1, 105_2, respectively and travel on separate optical paths 107_1, 107_2, respectively. However, in other implementations, the light beams 106_1 and 106_2 share all or part of the same optical path and also may share the same beam delivery system.

[0042] The EUV light source 100 also includes the target supply system 110 that emits the stream 121 of targets into a vacuum chamber 109. The target supply system 110 includes a target formation structure 117, which includes a nozzle that defines an orifice 119. In operational use, the orifice 119 is fluidly coupled to the reservoir 118, which contains a target mixture 111 under pressure p. The target supply system 110 also includes a pressure system 170. The pressure system 170 includes, for example, pumps, gas supplies, valves, and/or other devices that are able to increase, decrease, or maintain the pressure p applied to the target mixture 111 in the reservoir 118.

[0043] The target mixture 111 includes the target material, which is any material that has an emission line in the EUV range when in a plasma state. The target material may be, for example, tin, lithium, or xenon. Other materials may be used as the target material. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr ₄ , SnBr ₂ , SnH ₄ ; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture may also include impurities such as non-target particles or inclusion particles, for example, particles of tin oxide (SnOi) or particles of tungsten (W).

[0044] In the implementation shown in FIG. 1, the structure 117 includes a capillary tube 114 that extends generally along the x direction to the orifice 119. The orifice 119 is at an end of the capillary tube 114 and is in the vacuum chamber 109. The capillary tube 114 may be made from, for example, glass in the form of fused silica or quartz. The target mixture 111 is in a form that is able to flow. For example, in implementations in which the target mixture 111 includes a metal (such as tin) that is solid at room temperature, the metal is heated to a temperature at or above the melting point of the metal and is maintained at that temperature such that the target is in liquid form in the target mixture 111. The target mixture 111 flows through the capillary tube 114 and is ejected into the chamber 109 through the orifice 119. The Laplace pressure is the difference in pressure between the inside and the outside of a curved surface that forms the boundary between a gas region and a liquid region. The pressure difference is caused by the surface tension of the interface between the liquid and the gas. When the pressure p is greater than the Laplace pressure, the target mixture 111 exits the orifice 119 as a continuous jet 125.

[0045] The jet 125 breaks up into individual targets according to the Rayleigh-Plateau instability of a liquid jet. In the implementation of FIG. 1, a sidewall 115 of the capillary tube 114 is mechanically coupled to an actuator 132. The actuator 132 may be, for example, a piezo actuator that expands and contracts in response to an applied voltage signal to thereby cause deformations in the sidewall 115. By deforming the sidewall 115, a pressure wave is formed in the target mixture 111 in the supply system 110, and the pressure of the target mixture 111 in the supply system 110 is modulated. The pressure modulations control the break-up of the jet 125 into droplets such that the individual droplets coalesce into larger droplets that arrive at the target region 124_2 at a desired rate and having certain characteristics. For example, the action of the actuator 132 may be controlled in a particular manner to control the initial shape of the targets in the stream 121, as discussed in more detail with respect to FIGS. 3 and 5.

[0046] In FIG. 1 and FIG. 2, dashed lines indicate communication paths or data links along which electrical signals that include data and information flow. The communication paths or data links are any type of connection capable of transmitting data. For example, the data links or communication paths may be a wired and/or wireless connection configured to transmit electronic signals and commands that include data and/or information. The target supply system 110 is coupled to the control system 150 via a data link 152. The control system 150 is configured to control various components of the target supply system 110 by sending command signals 129 to the target supply system 110 via the data link 152.

[0047] For example, in some implementations, the actuator 132 is coupled to the control system

150 via the data link 152. In these implementations, the control system 150 generates command signals 129 that are provided to the actuator 132. When the command signals 129 are applied to the actuator 132 or to an element associated with the actuator 132, the actuator 132 moves in a manner governed by the content of the command signal 129. For example, the actuator 132 may be a piezoelectric ceramic material that changes shape based on an applied voltage. In these implementations, the control system 150 generates voltage waveforms that are delivered to the actuator 132. The magnitude and/or polarity of the waveform applied to the actuator 132 is based on the signals from the control system 150. Due to the mechanical coupling between the capillary tube 114 and the actuator 132, when the actuator 132 moves or vibrates, the sidewall 115 is deformed, and the pressure of the target mixture 111 in the capillary tube 114 is modulated.

[0048] In some implementations, the control system 150 is coupled to the pressure system 170 by the data link 152. The control system 150 controls the pressure p by sending command signals 129 to the pressure system 170. Moreover, the control system 150 may be coupled to both the actuator 132 and the pressure system 170 and/or a component within the supply system 110 that is coupled to the actuator 132 and the pressure system 170 such that the control system 150 is configured to control the actuator 132 and the pressure system 170.

[0049] The control system 150 includes an electronic processor module 154, an electronic storage 156, and an I/O interface 158. The electronic processor module 154 includes 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, an electronic processor receives instructions and data from a read-only memory, a random access memory (RAM), or both. The electronic processor module 154 may include any type of electronic processor. One or more of the electronic processors of the electronic processor module 154 executes command signal instructions that are stored on the electronic storage 156. The command signal instructions govern the formation of the command signals 129.

[0050] The electronic storage 156 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 156 includes non-volatile and volatile portions or components. The electronic storage 156 may store data and information that is used in the operation of the control system 150. For example, the electronic storage 156 may store information that relates the initial target shape to the morphology of final targets.

[0051] The electronic storage 156 also may store instructions, such as the command signal instructions, as a collection of instructions or a computer program, that, when executed, cause the electronic processor module 154 to generate the command signals 129 and communicate with the supply system 110. In another example, the electronic storage 156 may store instructions that, when executed, cause the control system 150 to interact with a separate machine. For example, the control system 150 may interact with other EUV light sources that are located in the same plant or facility.

[0052] The I/O interface 158 is any kind of interface that allows the control system 150 to exchange data and signals with an operator, the optical source 108_1, one or more components of the optical source 108_1, the lithography apparatus 199, and/or an automated process running on another electronic device. The I/O interface 158 may include one or more of a visual display, a keyboard, and a communications interface, such as a parallel port, a Universal Serial Bus (USB) connection, and/or any type of network interface, such as, for example, Ethernet. The I/O interface 158 also may allow communication without physical contact through, for example, an IEEE 802.11, Bluetooth, or a near-field communication (NFC) connection. [0053] The EUV light source 100 also may include a sensor system 130, which provides a signal 157 that includes data related to the light 197 or the EUV light 193 to the control system 150.

The sensor system 130 includes a sensor 135 capable of detecting one or more wavelengths of the light 197. The sensor 135 may be any sensor capable of detecting or sensing the presence of any of the wavelengths in the light 197. Thus, the sensor 135 may be a sensor capable of detecting EUV light or a sensor that is capable of detecting one or more wavelengths of out-of- band light. In implementations that include the sensor system 130, the instructions stored on the electronic storage 156 may include instructions that, when executed, analyze the signal 157 from the sensor system 130 and use information about the light 197 to inform the adjustment of the initial shape of the targets in the stream 121.

[0054] FIG. 2 is a block diagram of an EUV light source 200 that is part of an EUV lithography system 201. The EUV light source 200 is another example of an EUV light source. The EUV light source 200 is the same as the EUV light source 100 (FIG. 1), except the EUV light source 200 uses a third light pulse 204_3 to control the initial shape of the initial target 121p.

[0055] The third light pulse 204_3 is part of a light beam 206_3 that is generated by an optical source 208_3 and is directed a target region 224_3 by a beam delivery system 205_3. The beam delivery system 205_3 is similar to the beam delivery systems 105_1 and 105_2, except the optical elements of the beam delivery system 205_3 have spectral features that allow interaction with the light beam 206_3. The target region 224_3 is between the target region 124_2 and the orifice 119. The interaction between the third light pulse 204_3 and a target 221i modifies the geometric properties of the target 221 i prior to an interaction with the light pulse 104_2. The target 221i is a target in the stream 121 (FIG. 1). After interacting with the pulse 204_3, the target 221i drifts toward the target region 124_2 with other targets in the stream 121. Thus, the third light pulse 204_3 is used to control or modify the initial target shape of the targets in the stream 121.

[0056] The initial target shape may be controlled by controlling characteristics of the light beam 206_3. For example, the initial target shape may be controlled by controlling the intensity and/or temporal duration of the light pulse 204_3 that interacts with the target 22 li. The control system 150 is coupled to the optical source 208_3 and/or the beam delivery system 205_3, and the control system 150 may control the characteristics of the light pulse 204_3 (for example, the intensity of the light pulse 204_3 as a function of time) by controlling the optical source 208_3 and/or the beam delivery system 205_3. The initial target shape may also be controlled by controlling the position overlap between the light pulse 204_3 and the target 221i, for instance, the light pulse 204_3 may be directed to hit one side of the target 22 li instead of hitting the center of the target 22 li.

[0057] The wavelength of the light pulse 204-3 may be, for example, about 200 nm to about 10 pm. The beam delivery system 205_3 is similar to the beam delivery systems 205_1 and 205_2. The optical source 208_3 is similar to the optical source 208_2.

[0058] Referring to FIG. 3, a flow chart of an example process 300 to control CE in an EUV light source is shown. The process 300 may be performed with the EUV light source 100 (FIG. 1) or the EUV light source 200 (FIG. 2).

[0059] An initial shape of an initial target is determined by controlling a component of an EUV light source (310). The initial target shape is the shape of the target when the target is in the target region 124_2 before interaction with the light pulse 104_2. To control the initial target shape, the control system 150 may control the target supply system 110 in the EUV light source 100 or the EUV light source 200. The control system 150 also may control the optical source

208_3 and/or the delivery system 205_3 of the EUV light source 200. These approaches are discussed in turn below.

[0060] In some implementations, the control system 150 controls the target supply system 110 by adjusting the pressure p applied to the target mixture 111 in the reservoir 118. FIG. 4 shows an implementation in which the control system 150 adjusts the pressure p. FIG. 4 is a block diagram of an EUV light source 400 at a time just prior to a pulse 404_2 (which is a pulse of the light beam 106_2) interacting with an initial target 421p and during or shortly after a plasmagenerating event that formed an EUV light-emitting plasma 496. In the example of FIG. 4, the stream 121 includes targets 421_a, 421_b, and the initial target 421p. The targets 421_a, 421_b, and 421p travel from the orifice 119 to the target region 124_2 on a trajectory that is generally in the x direction.

[0061] As discussed above, the target mixture 111 is released from the orifice as the jet 125, and the jet 125 breaks into individual targets that are each separated from an adjacent target by a distance 423 along the direction of travel (the x direction in this example). The distance 423 impacts the initial shape of the targets in the stream 121. The EUV light source 100 produces the exposure beam 198 on a periodic basis such that the substrate 195 may be exposed quickly. Thus, plasma-generating events occur regularly during operation of the EUV light source. The plasma 496 and/or other matter formed by or associated with the plasma 496 is in the vacuum chamber 109 while the targets in the stream 121 travel toward the target region 124_2. The plasma 196 and/or the other matter interact with the targets in the stream 121 and may change the initial shape of the targets. The other matter formed by or associated with the plasma 496 may include, for example, ions emitted from the plasma 496, optical light emitted by the plasma 496, and scattered light (such as scattered light from the main-pulse 104_1 and/or the pre-pulse 104_2).

[0062] The strength of the interaction between the plasma 496 and/or the other matter depends on the distance between the target to be shaped and the target region 124_1 and the amount of time that has passed since the plasma-creating event. Thus, increasing the distance 423 reduces the amount of shaping caused by the interaction with the plasma 496 and decreasing the distance 423 increases the amount of shaping caused by the interaction with the plasma 196. Increasing the pressure p increases the velocity of the targets in the stream 121 and increases the distance 423. Decreasing the pressure p decreases the velocity of the targets in the stream 121 and decreases the distance 423. In a simplified form and without considering a pressure drop at the nozzle he relationship between the pressure p and the distance 423 is shown in Equation (1):

where d is the distance between targets (the distance 423 in the example of FIG. 4), T is the period of the generation of the EUV light 193 (the inverse of the frequency at which pulses of the

EUV light is provided to the output apparatus 199), p is the pressure applied to the reservoir 118, and p is the density of the target material 111.

[0063] To control the pressure p, the control system 150 generates a command signal 129 that is provided to the pressure system 170. In these implementations, the control system 150 generates a command signal 129 that is provided to the pressure controller 170 to control the pressure p. For example, the command signal 129 indicates a desired pressure p’ and provides a command to the pressure system 170 that causes the pressure system to apply the desired pressure p’ to the target material 111 in the reservoir 118. The desired pressure p’ may be a relatively small change to the pressure p. For example, the desired pressure p’ may be a percentage change in the pressure p of 0.1% or less. [0064] The desired pressure p’ may be determined based on, for example, a look-up table or database that is stored on the electronic storage 156. The look-up table may include information related to the conditions under which the EUV light source 100 is being used. The initial target shape that corresponds to the optimal CE depends on characteristics (such as the shape and density) of the modified target 121m. For example, in implementations in which the pulse 404_2 has a wavelength of about 1 pm and a temporal duration of about 10-100 ns, the modified target 121m is generally disk-shaped with the greatest spatial extent in the x-y plane. In

implementations in which the pulse 404_2 has a wavelength of about 1 pm and a temporal duration of about 10-100 ps, the modified target 121m is a cloud or mist of particles and other matter. Thus, the shape and the density of the modified target 121m depends on characteristics of the pulse 404_2. The optimal initial target shape varies depending on characteristics of the pulse 404_2 and the desired morphology of the final target. Therefore, the lookup table may store information that relates characteristics (such as temporal duration) of the pulse 404_2 to an initial target shape and a corresponding distance 423 to achieve that initial shape. The desired pressure p’ may be stored in association with the characteristics of the pulse 404_2, an corresponding initial target shape for those characteristics, and a distance 423.

[0065] In response to receiving the command signal 129, the pressure system 170 activities pumps, valves, and other devices to change the pressure p to the requested value. In some implementations, the pressure system 170 includes a pressure sensor that measures the value of the pressure p, and the measured pressure p is compared to the desired pressure prior to the control system 150 providing the command signal 129. For example, the pressure system 170 may provide the value of the applied pressure p (or an indication of the value applied pressure p) to the control system 150 via the data link 152, or the control system 150 may retrieve the value of the pressure p from the pressure system 170.

[0066] Thus, the initial target shape may be determined by controlling the pressure system 170, which is a component of the EUV light source 100.

[0067] In some implementations, the initial target shape is determined by controlling the actuator 132. The actuator 132 is part of the target supply system 110 and is thus also a component of the EUV light source 100. FIG. 5 is a block diagram of an EUV light source 500. The EUV light source 500 is an example implementation of the EUV light source 100 in which the control system 150 is coupled to the actuator 132 via the data link 152. The control system 150 controls the actuator 132 to determine the initial target shape.

[0068] As discussed above, the motion of the actuator 132 creates pressure waves in the target mixture 111 and causes the jet 125 to break up into the targets that make up the stream 121. The frequency or frequencies at which the actuator 132 is vibrated or otherwise actuated determines various characteristics of the targets in the stream 121. For example, the vibration of the actuator 132 may be used to determine the rate at which the targets in the stream 121 arrive at the target region 124_2 and the shape of the targets.

[0069] To determine the initial shape of the target by controlling the actuator 132, the control system 150 provides a modulation command signal 129 to the actuator 132. The motion of the actuator 132 is used to control the characteristics of the targets in the stream 121. As discussed above, the motion of the actuator 132 modulates the target material 111 in the target supply system 110 such that the jet 125 breaks into individual targets. The frequencies at which the actuator 132 vibrates determine the characteristics of the targets in the stream 121, including the initial shape of the targets.

[0070] The control system 150 provides a command signal 129 to the actuator 132. The command signal includes information that causes an actuation signal that has components at least at a first frequency and a second frequency to be applied to the actuator 132. The actuator 132 may be, for example, a piezo actuator. In these implementations, the command signal 129 is a voltage signal that includes components at two different frequencies or information that causes a device associated with the actuator 132 to generate voltages at the two different frequencies and apply those voltages to the actuator 132. In response to the application of the voltage signal, the actuator 132 vibrates at the first and second frequencies. The control system 150 may include a function generator that generates voltage waveforms having an amplitude that, when applied to the modulator 132, is sufficient to move the modulator 132. The frequency of the voltage waveform is controlled by the operator through the I/O interface 158 and/or by instructions stored on the electronic storage 156.

[0071] The first frequency is a higher frequency than the second frequency. Vibrating the capillary tube 114 at the first frequency causes the jet 125 to break up into relatively small targets of desired sizes and speeds. The second frequency is used to modulate the velocity of the targets in the stream and to encourage droplet coalescence such that larger targets, each formed from a plurality of the relatively small targets, are formed. In any given group of targets, the various targets travel at different velocities. The targets with higher velocities may coalesce with the targets with lower velocities to form larger coalesced targets that make up the stream 121. These larger targets are separated from each other by a larger distance (such as the distance 423 of FIG. 4) than the non-coalesced droplets. After coalescence, the targets in the stream 121 are approximately spherical and have a size that is on the order of 30 micrometer (pm).

[0072] Additional frequencies may be applied to the actuator 132. Introducing additional spectral components into the actuation signal allows a better controlled coalescence process and may be used to determine the initial target shape. For example, in addition to the first and second frequencies, a sine wave having a frequency of, for example, 30-100 kHz, 40-60 kHz, or 50 kHz also may be applied to the actuator 132 and/or one of the first frequency or the second frequency may be adjusted to a different frequency and/or waveform shape such that additional frequency components are applied to the actuator 132. The application of the additional spectral components introduces relative motion between two adjacent coalesced targets such that the two adjacent targets approach each other while traveling toward the target region 124_2. The two adjacent targets merge to form a new, larger target that does not necessarily have a spherical shape. In this way, the initial shape of the targets in the stream 121 that arrive at the target region 124_2 may be determined by controlling the actuator 132.

[0073] Thus, the initial target shape may be determined by controlling the actuator 132.

[0074] Referring also to FIG. 2, the initial target shape also may be determined by controlling the optical source 208_3 and/or the delivery system 205_3. As discussed above, an interaction between the target 221i and the pulse 204_3 may shape the target 221i. For example, the delivery system 205_3 may include an electro-optic modulator (EOM) that is controllable to adjust the temporal duration of the pulse 204_3 to control an extent of the target 221p in the x-y plane and/or in the y-z plane. In another example, the delivery system 205_3 may include optical elements such as, for example, mirrors, that steer the pulse 204_3 relative to the target region 224_3. In these examples, the initial target shape is determined by directing the pulse 204_3 to a particular portion of the target 22 li. For example, the pulse 204_3 may be directed to a portion of the target 221i that is displaced in the X or -X direction relative to the center of the target 221 i. [0075] After the initial target shape is determined, a final target is formed (320). The final target is the collection of target material that interacts with the pulse 104_1 to form the plasma 196. In the example of FIG. 1, the final target is the modified target 121m. The final target is formed by interacting the initial target (the target that has the initial target shape) with a pre-pulse, which is a pulse in the light beam 106_2 (for example, the pulse 104_2 of FIG. 1). The interaction between the pre-pulse and the initial target changes the geometric arrangement of the target material in the target 121p to form the modified target 121m.

[0076] A plasma-generating event is initiated (330). The plasma-generating event occurs when the main pulse (for example, the pulse 104_1 of FIG. 1) interacts with the final target (such as the modified target 121m) and forms the plasma 196. The plasma 196 emits the light 197. The CE associated with the plasma-generating event depends on the amount of energy delivered to the final target by the main pulse and the amount of EUV light emitted from the plasma 196, which depends on the portion of the target material converted into the plasma 196. The morphology of the final target influences the efficiency of the conversion of target material, and the morphology of the final target is controlled by determining the initial target shape as discussed in (310). Thus, determining the initial target shape by controlling a component of the EUV light source 100 or the EUV light source 200 controls the CE.

[0077] FIGS. 6A-6D show experimental data related to the initial target shape.

[0078] FIG. 6A is a matrix of 21 different shadowgrams that relate final target morphology to different initial target shapes. In the examples shown in FIG. 6A, the pre-pulse had an energy of 2-3 ml, a wavelength of 1.064 pm, and duration of 10 ns.

[0079] The shadowgrams are arranged in columns A-G, with each of the columns A-G including one shadowgram in each of rows 1-3. Each shadowgram shows the initial target shape in the lower left portion and the final target. The initial target shape is characterized as ratio of an extent along the x direction to an extent along the z direction. The initial target shape is different in each of the columns A-G. The shadowgrams in column A were created with an initial target shape with a ratio of about 0.6. The ratio of the initial target shape increases from the column A to column G. The shadowgrams in column G were created using an initial target shape with a ratio of 1.8. The initial target shapes are substantially oblate (in the x direction) in column A and are prolate (in the x direction) in column G. The x direction and the z direction are the same in FIGS. 6A and 6B as in FIG. 1. The three shadowgrams in rows 1-3 in each of the columns A-G have the same initial target shape and show data collected at three different times.

[0080] As shown by comparing the data column-wise, the morphology of the final target changes as the initial target shape changes. Moreover, as is apparent by comparing

shadowgrams within a column, the morphology of the final target formed from an initial target with a particular initial target shape is fairly consistent. The data shown in FIG. 6A indicates that the morphology of the final target is dependent upon the initial target shape.

[0081] FIG. 6B is a matrix of shadowgrams that show results for a similar experiment in which the pre-pulse had an energy of 2-3 ml, a wavelength 1.064 pm, and a duration of 12 ps. The results again show that the morphology of the final target created from a particular target shape is fairly consistent, indicating that the morphology of the final target is dependent upon the initial target shape.

[0082] FIG. 6C shows measured CE (%) as a function of the ratio of the x and z extents for a main pulse having 5 different pedestal energies. A pedestal is a portion of the main pulse that precedes the primary portion of the main pulse in time but is still part of the main pulse. The CE data shown in FIG. 6C is for a plasma-generating event initiated by irradiating a final target with a 1 pm pre-pulse that had a 1-100 ns duration.

[0083] The data shown in FIG. 6C includes plots 681, 682, 683, 684, 695 that represent CE (%) as a function of initial target shape ratio for pedestal energies of 0 millijoule (mJ), 0.5 ml, 1 mJ, 1.5 ml, and 2 mJ, respectively. Reviewing the plots 681 -685 together reveals that the CE as a function of initial target shape ratio has a similar profile for all of the pedestal energies. This indicates that the CE is influenced by the initial target shape independently of the pedestal energy. The plots 681-685 indicate that, for the tested conditions, the ratio of about 1 produces the best CE regardless of the pedestal energy.

[0084] FIG. 6D shows measured CE (%) as a function of initial target size (pm) for six different initial target shapes. In FIG. 6D, the six different initial target shapes are six different initial target shape ratios, and the CE (%) is plotted as a function of the size of the initial target in the direction in which the targets travel (for example, the X direction of FIG. 1 ).

[0085] The CE is for a plasma-generating event initiated by irradiating a final target with a 1.064 pm pre-pulse that had a 12 ps duration. FIG. 6D includes plots 691, 692, 693, 694, 695, 696 which represent CE for a final target created from an initial target having an initial target shape ratio of 0.6, 0.8, 1.0, 1.2, 1.4, and 1.6 respectively. An initial target that has a target shape ratio of 1.0 is target that is substantially spherical and is thus an undistorted target or a target with an initial shape that was not controlled by a process such as the process 300. As shown in FIG. 6D, the CE depends on the initial target shape. In particular, for relatively large targets (for example, greater than about 650 pm), undistorted initial target does not produce the highest CD. Thus, by determining an initial target shape by the process 300, a performance improvement may be achieved.

[0086] FIG. 7 A is a block diagram of a lithography system 700 that includes a source collector module SO. The lithography system 700 is an example of the lithography system 101. The lithography system 700 also includes: an illumination system IL configured to condition a radiation beam B. The radiation beam B may be an EUV light beam emitted from the source collector module SO. The lithography system 700 also includes a support structure MT constructed to support a patterning device MA. The support structure MT may be, for example, a mask table, and the patterning device MA may be, for example, a mask or reticle. When the radiation beam B interacts with the patterning device MA, a spatial pattern associated with the patterning device MA is imparted onto the radiation beam B. The support structure MT is coupled to a first positioner PM that is configured to position the patterning device MA. Further, the system 700 includes a substrate table WT constructed to hold a substrate W, which may be, for example, a resist-coated wafer. The substrate table WT is connected to a second positioner PW that is configured to position the substrate W. The system 700 also includes a projection system PS that is configured to project the patterned radiation beam E (also referred to as exposure light E or an exposure beam E) onto a target portion C of the substrate W. The target portion C may be any portion of the substrate W. In the example of FIG. 7 A, the substrate W includes a plurality of dies D, and the target portion C includes more than one of the dies D.

[0087] The illumination system IL includes optical components for directing, shaping, and/or controlling the radiation beam B and the exposure light E. The optical components may include refractive, reflective, magnetic, electromagnetic, electrostatic, or any other type of optical components.

[0088] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithography system 700, and/or other conditions, such as, for example, whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic and/or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0089] The patterning device MA is any device that may be used to impart a pattern onto the radiation beam B. The patterning device MA may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. In implementations in which the patterning device MA is a mask, the patterning device MA may be, for example, binary mask, an alternating phase-shift mask, or an attenuated phase-shift, or a hybrid mask type. In implementations in which the patterning device MA is a programmable mirror array, the patterning device MA includes a matrix arrangement of mirrors, each of which may be individually tilted so that each of the mirrors is capable of reflecting the radiation beam B in a different direction that does not depend on the direction in which the radiation beam B is reflected by the other mirrors in the matrix. The pattern that is imparted onto incident light is determined by the position of the various mirrors in the matrix. The pattern may correspond to a particular functional layer in a device being created in the target portion C of the substrate W. For example, the pattern may correspond to electronic features that together form an integrated circuit.

[0090] The projection system PS includes optical components that direct the exposure light E to the target portion C. The optical components of the projection system PS may be refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. Furthermore, it may be desired to use a vacuum for EUV radiation because gases may absorb EUV radiation. A vacuum environment may therefore be provided with the aid of a vacuum wall and vacuum pumps.

[0091] In the example of FIGS. 7A and 7B, the system 700 is a reflective type that includes reflective optical components and a reflective patterning device MA. The lithography system 700 may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such multiple stage machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0092] The illumination system IL receives an extreme ultraviolet radiation beam B from the source collector module SO. The EUV light sources 100 (FIG. 1 ), 200A (FIG. 2 A), and 200B (FIG. 2B), and 800 (FIG. 8) are examples of the source collector module SO.

[0093] The illumination system IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illumination system IL may include various other components, such as facetted field and pupil minor devices. The illumination system IL may be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross-section.

[0094] The radiation beam B interacts with the patterning device MA such that the pattern is imparted onto the radiation beam B. The radiation beam B is reflected from the patterning device MA with the pattern imparted as the exposure light E. The exposure light E passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. With the aid of the second positioner PW and a second position sensor PS2, the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. The positioning sensors PS1 and PS2 may be, for example, interferometric devices, linear encoders, and/or capacitive sensors. The patterning device MA and the substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0095] The lithography system 700 may be used in at least one of the following modes: (1) a step mode, (2) a scan mode, or (3) a third or other mode. In the step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the

(de-)magnification and image reversal characteristics of the projection system PS. In the third or other mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan.

This mode of operation can be readily applied to maskless lithography that utilizes

programmable patterning device, such as a programmable mirror array of a type as referred to above. Combinations and/or variations on these three modes of use and/or entirely different modes of use may also be employed.

[0096] FIG. 7B shows an implementation of the lithography system 700 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO includes a vacuum environment. Each of the systems IL and PS also include a vacuum environment. An EUV radiation emitting plasma is formed within the source collector module SO. The source collector module SO focuses the EUV radiation emitted from a plasma to an intermediate focus IF such that the radiation beam B (760) is provided to the illumination system IL.

[0097] The radiation beam B traverses the illumination system IL, which in the example of FIG.

7B includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21 at the patterning device MA and maintains a uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam B at the patterning device MA, the exposure light E (the patterned beam B) is formed and the exposure light E (26) is imaged by the projection system PS via reflective elements 28, 30 onto the substrate W.

Additionally, the exposure light E interacts with a slit that shapes the exposure light E such that the exposure light E has a rectangular cross-section in a plane that is perpendicular to the direction of propagation. To expose the target portion C on substrate W, the source collector module SO generates pulses of radiation to form the radiation beam B while the substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on the patterning device MA through the rectangular exposure light E.

[0098] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in FIG. 7B.

[0099] Numerous additional components used in the operation of the source collector module and the lithography system 700 as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 3 and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithography system 700.

[0100] Referring to FIG. 8, an implementation of an LPP EUV light source 800 is shown. The light source 800 may be used as the source collector module SO in the lithography system 700. Furthermore, the optical source 108_2 ofFIGS. 1 and 2 may be part of the drive laser 815.

[0101] The LPP EUV light source 800 is formed by irradiating a target mixture 814 at a plasma formation region 805 with an amplified light beam 810 that travels along a beam path toward the target mixture 814. The target material of the targets in the stream 121 may be or include the target mixture 814. The plasma formation region 805 is within an interior 807 of a vacuum chamber 830. When the amplified light beam 810 strikes the target mixture 814, a target material within the target mixture 814 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 814. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0102] The light source 800 includes a drive laser system 815 that produces the amplified light beam 810 due to a population inversion within the gain medium or mediums of the laser system 815. The light source 800 includes a beam delivery system between the laser system 815 and the plasma formation region 805, the beam delivery system including a beam transport system 820 and a focus assembly 822. The beam transport system 820 receives the amplified light beam 810 from the laser system 815, and steers and modifies the amplified light beam 810 as needed and outputs the amplified light beam 810 to the focus assembly 822. The focus assembly 822 receives the amplified light beam 810 and focuses the beam 810 to the plasma formation region 805.

[0103] In some implementations, the laser system 815 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 815 produces an amplified light beam 810 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 815 may produce an amplified light beam 810 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 815. The term“amplified light beam” encompasses one or more of: light from the laser system 815 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 815 that is amplified and is also a coherent laser oscillation.

[0104] The optical amplifiers in the laser system 815 may include as a gain medium a filling gas that includes CO2 and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system 815 may include a pulsed laser device, for example, a pulsed, gas-discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 815 may also include a cooling system such as water that may be used when operating the laser system 815 at higher powers.

[0105] The light source 800 includes a collector mirror 835 having an aperture 840 to allow the amplified light beam 810 to pass through and reach the plasma formation region 805. The collector mirror 835 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation region 805 and a secondary focus at an intermediate location 845 (also called an intermediate focus) where the EUV light may be output from the light source 800 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 800 may also include an open-ended, hollow conical shroud 850 (for example, a gas cone) that tapers toward the plasma formation region 805 from the collector mirror 835 to reduce the amount of plasma-generated debris that enters the focus assembly 822 and/or the beam transport system

820 while allowing the amplified light beam 810 to reach the plasma formation region 805. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation region 805.

[0106] The light source 800 may also include a master controller 855 that is connected to a droplet position detection feedback system 856, a laser control system 857, and a beam control system 858. The light source 800 may include one or more target or droplet imagers 860 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation region 805 and provide this output to the droplet position detection feedback system 856, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 856 thus provides the droplet position error as an input to the master controller 855. The master controller 855 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 857 that may be used, for example, to control the laser timing circuit and/or to the beam control system 858 to control an amplified light beam position and shaping of the beam transport system 820 to change the location and/or focal power of the beam focal spot within the chamber 830.

[0107] The supply system 825 includes a target material delivery control system 826 that is operable, in response to a signal from the master controller 855, for example, to modify the release point of the droplets as released by a target material supply apparatus 827 to correct for errors in the droplets arriving at the desired plasma formation region 805.

[0108] Additionally, the light source 800 may include light source detectors 865 and 870 that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 865 generates a feedback signal for use by the master controller 855. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0109] The light source 800 may also include a guide laser 875 that may be used to align various sections of the light source 800 or to assist in steering the amplified light beam 810 to the plasma formation region 705. In connection with the guide laser 875, the light source 800 includes a metrology system 824 that is placed within the focus assembly 822 to sample a portion of light from the guide laser 875 and the amplified light beam 810. In other implementations, the metrology system 824 is placed within the beam transport system 820. The metrology system 824 may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam 810. A beam analysis system is formed from the metrology system 824 and the master controller 855 since the master controller 855 analyzes the sampled light from the guide laser 875 and uses this information to adjust components within the focus assembly 822 through the beam control system 858.

[0110] Thus, in summary, the light source 800 produces an amplified light beam 810 that is directed along the beam path to irradiate the target mixture 814 at the plasma formation region 805 to convert the target material within the mixture 814 into plasma that emits light in the EUV range. The amplified light beam 810 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 815. Additionally, the amplified light beam 810 may be a laser beam when the target material provides enough feedback back into the laser system 815 to produce coherent laser light or if the drive laser system 815 includes suitable optical feedback to form a laser cavity.

[0111] Other implementations are within the scope of the claims.

[0112] Other aspects of the invention are set out in the following numbered clauses.

1. An extreme ultraviolet (EUV) light source comprising:

a vacuum vessel;

a target material supply system configured to supply targets to an interior of the vacuum vessel, the targets comprising at least a first target, wherein the first target has an initial shape at an initial target region in the vacuum vessel; a first optical source configured to provide a first light beam to a first target region in the vacuum vessel, the first light beam being configured to modify the initial shape of the initial target to form a modified target; and

a second optical source configured to provide a second light beam to a second target region in the vacuum vessel, the second target region configured to receive the modified target, the second light beam being configured to interact with the modified target and to convert at least some of the target material in the modified target to a plasma that emits EUV light, wherein

the initial shape of the first target is controlled to thereby control an amount of plasma produced from the interaction between the second light beam and the modified target.

2. The EUV light source of clause 1, wherein the target material comprises a molten metal, and the supply system comprises:

a reservoir configured to hold the target material;

a nozzle configured to be fluidly coupled to the reservoir and to emit the targets into the interior of the vacuum vessel; and

an actuator mechanically connected to the nozzle.

3. The EUV light source of clause 2, wherein the initial shape of the first target at the initial target region is controlled by causing the actuator to vibrate the nozzle at more than one frequency.

4. The EUV light source of clause 2, wherein a spacing between the first target and a second target is controlled by adjusting a pressure applied to the target material in the reservoir, and the second target is supplied by the target supply system before the first target.

5. The EUV light source of clause 4, wherein the initial shape of the first target is based on the controlled spacing between the first target and a second target.

6. The EUV light source of clause 1, further comprising a third optical source configured to provide a third light beam to a third target region, and wherein the third target region is configured to receive the first target, and the initial shape of the first target at the initial target region is controlled by interacting the first target with the third light beam.

7. The EUV light source of clause 6, wherein the third target region is closer to the target material supply system than the first target region and the second target region.

8. The EUV light source of clause 1, wherein the initial shape of the first target at the initial target region comprises an oblate sphere of molten metal having a first extent along a first direction and a second extent along a second direction that is perpendicular to the first direction, and the ratio of the first extent to the second extent is between 0.6 and 0.8.

9. The EUV light source of clause 1, wherein the initial shape of the first target at the initial target region comprises an oblate sphere of molten metal having a first extent along a first direction and a second extent along a second direction that is perpendicular to the first direction, and the ratio of the first extent to the second extent is between 0.75 and 0.9.

10. The EUV light source of clause 1, wherein the initial shape of the first target at the initial target region comprises an oblate sphere of molten metal having a first extent along a first direction and a second extent along a second direction that is perpendicular to the first direction, and the ratio of the first extent to the second extent is about 0.8.

11. The EUV light source of clause 1, wherein the modified target has a morphology that is determined by the initial shape of the first target at the initial target region, the morphology describing a shape of the target and/or a target material density in three dimensions.

12. The EUV light source of clause 11, wherein the modified target comprises a lateral extent in one of the three dimensions, the lateral extent depending on a distance between the first target region and the second target region.

13. The EUV light source of clause 1, wherein the initial shape of the first target material droplet being controlled to thereby control an amount of plasma produced from the interaction between the second light beam and the modified target comprises the initial shape of the first target material being controlled to thereby control a conversion efficiency (CE) of the EUV light source, the CE being a ratio of energy supplied to the modified target to energy emitted from the plasma as EUV light.

14. The EUV light source of clause 1, wherein the initial target region is between the target material supply system and the first target region.

15. A method of controlling conversion efficiency (CE) in an extreme ultraviolet (EUV) light source, the method comprising:

determining an initial shape of an initial target by controlling a component of the EUV light source;

causing a pre-pulse light beam to interact with the initial target to form a modified target; and causing a main optical pulse to interact with the modified target to produce a plasma that emits EUV light, wherein the interaction between the modified target and the main optical pulse is associated with a conversion efficiency (CE), the CE being a ratio of energy supplied to the modified target to energy emitted from the plasma as EUV light, and the CE is controlled based on the determined initial shape of the initial target.

16. The method of clause 15, wherein the component of the EUV light source comprises a reservoir that is part of a target material supply system, and

determining the initial shape of the initial target comprises controlling an amount of pressure on molten target material in the reservoir before the initial target is produced by the target supply system.

17. The method of clause 16, wherein controlling the amount of pressure on the molten target material in the reservoir controls a spacing between the initial target and another target, and the initial shape of the initial target is based on the spacing.

18. The method of clause 15, wherein the component of the EUV light source comprises an actuator coupled to a capillary tube of a target material supply system, and

determining the initial shape of the initial target comprises controlling the actuator such that the actuator vibrates the tube at more than one frequency.

19. The method of clause 18, wherein controlling the actuator such that the actuator vibrates the tube at more than one frequency produces a stream of coalesced targets from a jet of target material, and further comprising adjusting one of the more than one frequencies such that two of the coalesced targets merge into a merged target, and the initial target is the merged target.

20. The method of clause 15, wherein component of the EUV light source comprises a target material supply system configured to supply the initial target and at least a second target, and determining the initial shape of the initial target comprises controlling the target material supply system such that a spacing between the initial target and the second target is adjusted, the second target being supplied by the target supply system before the initial target.

21. The method of clause 15, wherein the component of the EUV light source comprises an initial light source configured to provide an initial light beam, and

determining the initial shape of the initial target comprises controlling the initial light source such that the initial light beam interacts with the initial target, and wherein the initial shape of the initial target is at least partially determined by interacting the initial target with the initial light beam.