CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. Application No. 15/295,280, filed on October 17, 2016, which is incorporated herein b reference i its entirety.

TECHNICAL FIELD

The disclosed subject matter relates to controlling a spectral feature, such as, for example, bandwidth or wavelength, of a tight beam output from an optical source that supplies light to a lithography exposure apparatus.

BACKGROUND

In semiconductor lithography (or photolithography}, the fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes pertoraied on a semiconductor (for example, silicon) substrate (which is also referred to as a wafer). A photolithograph exposure apparatus or scanner is a machine that, applies a desired pattern onto a target, portion of the substrate. The wafer is fixed to a stage so that the wafer generally extends along a plane defined by orthogonal Xi. and Yr, directions of the scanner. The wafer is irradiated by a light beam, which has a wavelength in the deep ultraviolet (DUV) range. The light beam travels along an axial direction, which corresponds with the scanner. The Zv. direction of the scanner is orthogonal to the lateral XL~YL plane.

An accurate knowledge of spectral features or properties (for example, a bandwidth) of a light beam output from an optical source such as a laser is important in many scientific and industrial applicaiions. For example, accurate know ledge of the optical source bandwidth is used to enable control of a minimum feature size or critical dimension (CD) i deep ultraviolet (DUV) optical lithography. The critical dimension is the feature size that is printed on a semiconductor substrate (also referred to as a wafer) and therefore the CD can require fine size control. 1» optical lithography, the substrate is irradiated by a light beam produced by an optical source. Often, the optica! source is a laser source and the light beam is a laser beam. SUMMARY

In some general aspects, a spectral feature of a pulsed light beam produced by an optical source is controlled by a method. The method includes producing, from the optical, source, a pulsed light beam at a pulse repetition rate: directing the pulsed light beam toward a substrate recei ved in a lithography exposure apparatus to expose the substrate to the pulsed light beam; modifying a pulse repetition rate of the pulsed light beam as it is exposing the substrate. The method includes determining an amount of adjustment to a spectral feature of the pulsed light beam, the adjustment amount compensating for a variation in the spectral feature of die pulsed light beam that correlates to the modification of the pulse repetition rate of the pulsed light beam, The method includes changing the spectral feature of the pulsed light beam by the determined adjustment amount as the pulsed light beam is exposing the substrate to thereby compensate for the variation in the spectral feature.

Implementations can include one or more of the following features. For example, the amount of adjustment to the spectral feature can be determined by: accessing a correlation recipe, the correlation recipe defining a relationship between the repetiiion rate and the spectral feature for the optical source; determining the spectral feature that correlates to the modified pulse repetition rate in the recipe; and calculating the adjustment amount of the spectral feature that offsets the spectral feature correlated to the modified pulse repetition rate.

The method can include creating the correlation recipe for the optical source prior to directing the pulsed light beam toward the substrate. The method can include creating the correlation -recipe · for the pulsed optical source tit between a pair of bursts of poises of the pulsed light beam.

The spectral feature of the pulsed light, beam, can be changed by adjusting one or more components of the optical source. The one or more components of the optical source can be adjusted by adjusting one or more optical components of a spectral feature selection apparatus that interact with the pulsed light beam. The one or more optical components of the spectral feature selection apparatus can be adjusted by rotating a prism of the spec tral feature selection apparatus. The prism of the spectral fea ture selection apparatus can be rotated to thereby change the spectral feature by rotating the prism from a first stable equilibrium position-to a second stable equilibrium position in a time that is less than or equal to 50 milliseconds. The prism of the spectra! feature selection: apparatus ca be rotated by rotating the prism from a first angle to a second angle, wherein the first and second angles can be any angle in a 360° rotation.

The spectral feature of the pulsed light beam can be changed by changing the spectrai feature in between bursts of pulses of the pulsed light beam.

The method can include receiving an instruction to modify the pulse repetition rate of the pulsed light beam as it is exposing the substrate by a particular value, wherein modifying the pulse .repetition rate of the pulsed light beam includes modifying the repetition rate of th pulsed light beam by the particular value.

The spectrai feature of the pulsed light beam can be changed by the determined adjustment amount as the pulsed light beam is exposing the substrate to thereby compensate for the spectral feature variation causes the spectral feature of the pulsed light beam to maintained within a predetermined stable range.

The spectral feature can be maintained in the predetermined, stable range and a critical dimension of a feature formed in the substrate can be■maintained to within a predetermined acceptable range.

The spectral feature ca be the bandwidth of the pul sed light beam.

In other general aspects, a system includes an illumination system, a spectral feature selection apparatus, and a control system. The illumination system produces and directs a pulsed light beam toward a photolithography exposure apparatus. The illumination system includes ao optical source that produces the pulsed light beam at a pulse repetition rate that is capable of being changed. The spectral feature selection apparatus is configured to select a spectral feature of the pulsed light beam. The spectral feature selection apparatus includes a set of optical components arranged in the path of the pulsed light beam. The control system is operariveiy connected to the optical source and to tlie spectra! feature selection apparatus. The control system is configured to: control the repetition rate at which the pulsed light beam is produced including modifying the repetition rate of the pulsed Ught beam as it is exposing a substrate in. the photolithography exposure apparatus; determine an amount of adjustment to a spectral feature of the pulsed light beam, the adjustment amount compensating for a variation in the spectral feature of the pulsed l ight beam that correlates to the modification of the pulse repetition rate of the pulsed light beam; and send a signal to the spectral feature selection apparatus to move at least one optical component to thereby change the spectral feature of the pulsed light beam by the determined adjustment amount as the pulsed light beam is exposing the substrate to thereby compensate for the spectral feature variation.

Implementations can include one or more of the following features. For example, the set of optical components of the spectral feature selection apparatus can include at least one prism. The control system can be configured to send a signal, to a rapid actuator associated with the at least one prism to cause the prism to rotate to thereby change the spectral feature. The set of optical components of the spectral feature selection apparatus can include: a dispersive optical element arranged to interact with the pulsed light beam, and a pluralit of prisms arranged in the path of the pulsed light beam between the dispersive optical element and the optical source.

The spectral feature selection apparatus can include an actuation system having at least one actuator associated with a prism and configured to rotate the associated prism to thereby adjust a spectral feature of the pulsed light beam.

The rapid actuator can include a rotation stage that rotates about a rotation axis and includes a region that is mechanically linked to the prism. The rotation stage can be configured to rotate about the rotation axis along a full 360° of angle of rotation.

The illumination system can include a beam preparation system configured to receive the pulsed light beam produced from the optical source and to direct the pulsed ligh beam toward the photolithography exposure apparatus. DESCRIPTION OF DRAWINGS

Fig. i is a block diagram of photolithography sy stem producing a pulsed light beam that is directed to a photolithography exposure apparatus;

Fig. 2 is a graph of an exemplary optical spectrum of the pulsed light beam produced by the photolithography system of Fig. 1 ;

Fig. 3 is a block diagram of an exemplar}' optical source that can be used in the photolithography system of Fig. 1;

Fig. 4 is a block diagram of an exemplary spectral feature selection apparatus that can be used in the photolithography system of Fig. 1 ;

Fig. 5 is a block diagram of an exemplar}' control system that can be used in the.

photolithography system of Fig. 1 ; Fig. 6 Is a block diagram of an exemplary -spectral, feature selection apparatus that can be used in the photolithography system of Fig, 1 ;

Fig. 7 is a flow chart of an exemplary procedure performed by-fhe photolithography system of Fig. 1 to rapidly control a bandwidth of the pulsed light beam to compen sate for a vari ation in a repetition rate of the pulsed light beam;

Fig. 8 is a flow chart of an exemplary procedure performed by the photolithography system of Fig. 1 to determine an adjustment to the bandwidth of the pulsed light beam;

fig. 9 is an exemplary graph showing the relationship between a bandwidth of the pulsed light beam and a repetition rate for two different optical sources; and

Fig. 10 is a flow chart of an exemplary procedure for operating the photolithography system of Fig. 1.

DESCRIPTIO

Referring to Fig, I, a photolithography system 100 includes an illumination system 150 that produces a pulsed light beam 1 10 having a wavelength that is nominally at a center

wavelength and is directed to a photolithography exposure apparatus or scanner 1 15, The pulsed light beam 1 1.0 is used to pattem microelectronic features on. a subsiTaie or wafer 120 received in the scanner 1 15. The ilhimmation system 150 includes an optical source 105 that produces the pulsed light beam 1 10 at a pulse repetition rate that is capable of being changed.

The illumination system 150 includes a spectrai feature selection apparatus 13.0. The spectrai feature selection apparatus 130 interacts with the light beam .1 10 produced by the optica! source 105 and is configured to select one or more spectral features (such as the bandwidth or wavelength) of the pulsed light beam 1 10. The spectral feature selection apparatus 130 includes a set of optical components (shown in Fig. 4, for example) arranged in the path of the pulsed light beam 1 10. The illumination system 150 includes a control system 185 operatively connected to the pulsed optical source 105 and to the spectral feature selection apparatus 130. And, the scanner 1 15 includes a lithography controller 140 operatively connected to the control system 185 and components, within the scanner 1 15.

The pulse repeti tion rate of the pulsed light beam 1 50 is the rate at which pulses of the light beam 1 10 are produced by the optical source 105. Thus, for example, the repetition rate of the pulsed light beam 1 10 is Ι/Δί, where At is the time between the pulses. The control system 85 is generally configured to control the repetition rate at which the pulsed light beam. .1 10 is produced including modifying the repetition rate of the pulsed light beam as it is exposing the wafer 120 in the photolithography exposure apparatus 1 1 .

In some implementations, the scanner 115 triggers the optical source 105 (through the communication between the controller 140 and the control system 185) to produce the pulsed light beam 1 10, so the scanner 1 15 controls the repetition rate by way of the controller 1 0 and the control system 185. For example, the controller 140 sends a signal to the control system 185 to maintain the repetition rate of the light beam 1 10 withi a particular range of acceptable rates. The scanner 1 15 generally maintains the repetition rate constant for each burst of puises of the light beam 1 10. A. burst of pulses of the light beam 1 10 can correspond to an exposure field on the wafer 120. The exposure field is the area of the wafer 120 that is exposed in one scan of an exposure slit or window within the scanner 1 1.5. A burst of pulses can include anywhere from 10 to 500 pulses, for example.

Customers who .manage the scanner 115 wish to be able to modify the pulse repetition rate of the light beam 1 10 as it is being scanned across the wafer 120. Accordingly, the scanner 1 15 can also request (by way of the controller 140 and the control system 185) a change or modification to the repetition rate of the light beam 1 1 and such a change equest can occur in between bursts of pulses. For example, the customer may prefer to operate at lower repetition rates to al low the customer to use fewer pulses per wa fer 120, instead of simply attenuating the light beam Π0 within the scanner 1 15.

Several performance characteristics of the illumination system 150 (such as the parameters of the light beam 1 1 produced by the illumination system 150) are sensitive to changes in the repetition rate. For example, one or more spectral features (such as the bandwidth or wavelength) of the light beam 1 10 can fluctuate or vary when the repetition rate of the light beam 1 1 is changed . For example, the bandwi dth, of the li ght beam 1.10 depends on the

wave front of the light beam 1 10, and the wav errant of the light beam 1 10 can become distorted when the repeti tion rate of th e pulses of the li ght beam 110 are adjus ted. The destabilization of the bandwidth leads to unacceptable variations in the critical dimension (CD) at the wafer 120 and therefore leads to unreliable performance from the illumination system ISO. Moreover, the variations of the performance characteristics of the illumination system 150 can be different from one design of the illumination system 150 to another design of the illumination system 150. Thus, a single solution to stabilizing the performance characteristics of the illumination system- ISO due to the adjustments of the repetition rate of the light beam 1 10 is not feasible.

Specifically, the critical dimension (CD) is the smallest feature size that can be printed on the wafer 120 by the system 100. The CD depends on the wa velength of the light beam i 10. To maintain a uni form CD of the microelectronic features printed on the wafer 120, and on other wafers exposed by the system 100, the center wavelength of the light beam 1 10 should remain at an expected or target center wavelength or within a range of wavelengths around t he target wa velength. Thus, in addition to maintaining the center wa velength at the target center wavelength or within a range of acceptable wavelengths about the target center wavelength, it is desirable to maintain the bandwidth of the ligh t beam 1 10 (the range of wa velengths in. the light beam 1 10) to within an acceptable range of bandwidths.

i n order to maintain the bandwidth of the light beam 110 to a acceptable range, the control system 185 is configured to determine an amount of adjustment to the bandwidth of the pulsed light beam 110, where the adjustment to the bandwidth of the pulsed light beam 110 compensates for a change or variation in the bandwidth of the pulsed light beam 1 10 that is caused by a modification of the pulse repetition rate of the pulsed light beam 1 10. Additionally, the control system 185 is configured to send a signal to the spectral feature selection apparatus 130 to move at least one optical component of the apparatus 130 to thereby change the bandwidth of the pulsed light beam 110 by the determined adjustment amount as the pulsed light beam 110 is exposing the wafer 120 to thereb compensate for the bandwidth variation caused b the modification of the puls repetition rate of the pulsed light beam 1 10.

The bandwidth of the pulsed light beam 1 10 can be changed in between any two bursts of pulses. Moreover, the time thai it takes for the bandwidth to be changed from a first value to a second value and also to stabilize at the second value should be less than the time between the bursts of pulses. For example, if the period of time between bursts is 50 milliseconds (ms), then the total time to change the bandwidth from a first value to a second value and stabilize at the second value should be less than 50 ms. The contiOl system 185 and the spectral feature selection apparatus 30 are designed to enabl such a rapid change of the band width, as discussed in detail below.

In some implementations, the scanner 1 15 does not know the value of the repetition rate of the light beam 1 10; rather, the scanner 1 15 merely provides a trigger to the pulsed optical source 105 (by way of the control system 185) to produce the pulses at a specific repetition rate. In other implementations; the scanner 1 15 or the illumination system 150 can monitor the pulse repetition rate by measuring a time between consecutive pulses of the light beam 10 and use this information to control or modify the repetition rate of the light beam 1 10. These

measurements can be performed, for example, by a measurement (metrology) system. 170.

The controller 140 of the scanner 1 15 sends a si gnal to the control system 185 to adjust or modify the repetition rate of the pulsed light beam 1 1.0 that is being scanned across the wafer 120. The signal sent to the control system 185 can cause the control system 185 to modify an electrical signal sent to the pulsed optical source 105. For example, if the pulsed optical source 105 includes a gas laser amplifier then the electrical signal provides a pulsed current to electrodes within one or more gas discharge chambers of the pulsed optical source 105.

Details about the photolithography system 100 are provided next. Specifically, with reference again to Fig. 1 , the wafer 120 is placed on water table constructed to hold the wafer 120 and connected to a positioner configured to accurately position the wafer 120 in accordance with certain parameters and under control of the controller 140.

The light beam 1 10 is directed through a beam preparation system 1 12, which can include optical elements that modify aspects of the light beam 1 10. For example, the beam preparation system 1 12 can include reflective and/or refractive optical elements, optical pulse stretchers, and optical apertures (including automated shatters).

The pulses of the light beam 1 1 are centered around a wa velength that is in the deep ultraviolet (DUV) range, for example, with wavelengths of ^' 248 nanometers (am) or 1 3 nm. The size of the microelectronic features patterned on the wafer 320 depends on the wavelength of the pulsed light beam 110, with a lower wavelength resulting i a small minimum feature size or critical dimension . When the wavelength of the pulsed light beam 1 10 is 248 nm or 193 nm, the minimum size of the microelectronic features can he, for example, 50 nm or less. The bandwidth that is used for analysis and control of the pulsed light beam 1 1 can be the actual, instantaneous bandwidth of its optical spectrum 200 (or emission spectrum), which contains information on liow the opti cal energy or po wer of the light beam 1 10 i s di stributed o ver different wa velengths (or frequencies), as shown in Fig. 2.

The photolithography system 100 can also include the metrolog system 170, which can include a sub-system that measures one or more spectral features (such as the bandwidth or wavelength) of the light beam. 1 .0. Because of various disturbances applied to the photolithography system 100 during operation, the value of the spectral feature (such as the bandwidth or the wavelength) of the light beam 1 1.0 at the wafer 120 may not correspond to or match with the desired spectral feature (that is, the spectral feature that the scanner 1 15 expects). Thus, the spectral feature (such as a characteristic bandwidth) of light beam 1 10 is measured or estimated during operation by estimating a value of a metric from, the optical spectrum so that an operator or an automated system (for example, a feedback controller) can use the measured or estimated bandwidth to adj ust the properties of the optical source 105 and to adjust, the optical spectrum of the light beam 1 10, The sub-system of the metrology system 170 measures the spectral feature (such as the bandwidth and/or the wa velength) of the light beam 110 based on this optical spectrum.

The metrology system 170 recei ves a portion of the light beam 1 10 that is redirected from a beam separation device thai is placed in a path between the optical, source 105 and the scanner 1 15. The beam separation device directs a first portion or percentage of the light beam 1 10 into the metrology system 170 and directs a second portion or percentage of the l ight beam 1 10 toward the scanner 115, in some implementations, the majority of the light beam i 10 is directed in the second portion toward the scanner 1 15. For example, the beam separation device directs a fraction (for example, 1-2%) of the light beam 110 into the metrology system 170. The beam separation device can be, for example, a beam splitter.

The scanner 1 15 includes an optical arrangement having, for example, one or more condenser lenses, a mask, and an objective arrangement The mask is movable along one or more directions, such as along an optical axis of the light beam 1 1 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables the image transfer to occur from the mask to the photoresist on the wafer 120. The illuminator system adjusts the range of angles for the light bea 1 10 impinging o the mask. The illuminator system also homogenizes (makes uniform) the intensity distribution of the light beam 1 10 across the mask.

The scanner 115 can include, among other features, the lithography controller 140, air conditioning devices, and power supplies for various electrical components, in addition to controlling the repetition rate of the pulses of the light beam 1 10 (discussed above), the lithography controller 140 controls how layers are printed on the wafer 120. The lithography controller 140 includes memory tliat stores information, such, as process recipes and also may store information about which repetition rates may be used or are preferable as described more fully below.

The wafer 120 is irradiated by the light beam 1 10, A process program or recipe determines the length of the exposure on the wafer 120, the mask, used, as well as other factors that affect the exposure. During lithography, as discussed above, a plurality of pulses of the light beam 1 10 illuminates the same area of the wafer 120 to constitute an illumination dose. The number N of pulses of the light beam 1 10 that illuminate the same area can. be referred to as the exposure window or sli t and the size of the slit c an be controlled by an exposure slit placed before the mask. In some implementations, the value of N is in. the tens, for example, from 10- 100 pulses. la other implementations, the value of N is greater than 100 pulses, for example, from 100-500 poises.

One or more of the mask, the objective arrangement, and the wafer 120 can be moved relative to each other during the exposure to scan the exposure window across an exposure field, The exposure field is the area of the wafer 120 that is exposed in one scan of the exposure slit or window.

Referring to Fig. 3, an exemplary optical source 305 is a pulsed laser source that produces a pulsed laser beam as the light beam 3 10. The opticai source 305 is a two-stage laser system that includes a master oscillator (MO) 300 that provides a seed light beam 61 1. to a power amplifier (PA) 310, The master oscillator 300 typically includes a gain medium in which amplification occurs and an optical feedback mechanism such as an opticai resonator. The power amplifier 310 typically includes a gain medium in which amplification occurs when seeded with the seed laser beam from the master oscillator 300. If the power amplifier 310 is designed as a regenerati ve ring resonator then it is described as a power ring amplifier (PRA) and i this case, enough optical feedback can be provided from the ring design. The master oscillator 300 enables fine tuning of spectral parameters such as the center wavelength and the bandwidth at relatively low output pulse energies. The power amplifier 310 receives the output from the master

oscillator 300 and amplifies this output to attain the necessary power for output to use in photo lithography .

The master oscillator 300 includes a discharge chamber having two elongated electrodes, a laser gas that serves as the sain medium, and a fan circulating the gas between the electrodes. A laser resonator is formed between die spectral feature selection apparatus 130 on one side of the discharge chamber and receiving the seed light beam 61 1 , and an output coupler 315 on a second side of the discharge chamber to output the seed light beam 61 1 to the power amplifier 310,

The optical source 305 can also include a line enter analysis module (LAM) 320 that receives an output .from the output coupler 315, and one or more beam modification optical systems 325 that modify the size and/or shape of the beam as needed. The line center analysis module 320 is an example of one type of measurement system that can be used to measure the wa velength (for example, the center wavelength) of the seed l ight beam.

The power amplifier 310 includes a power amplifier discharge chamber, and if It is regenerative ring amplifier, the power amplifier also mcludes a beam reflector or beam turning device 330 that reflects the light beam back into the discharge chamber to form a circulating path. The power amplifier discharge chamber includes a pair of elongated electrodes, a laser gas that serves as the gain medium, and a fan for circulating the gas between the electrodes. The seed light beam is amplified by repeatedly passing through the power amplifier 310. The beam modification optical system 325 provides a way (for example, a partially-reflecting mirror) to in- couple the seed light beam and to out-couple a portion of the ampli fied radiation from the power amplifier to form the output light beam 11 .

The laser gas used in the discharge chambers of the master oscillator 300 and the power amplifier 310 can be any suitable gas for producing a lase beam around the required

wavelengths and bandwidth. For example, the laser gas can be argon fluoride (ArF), which emits light at a wavelength of about 193 nm, or krypton fluoride ( rF), which emits light at a

wavelength of about 248 nm.

The line center analysis module 320 monitors the wavelength of the output of the master oscillator 300. The line center analysis module 320 can be placed at other locations within the optical source 305, or it can be placed at the output of the optical source 305.

The spectral feature selection apparatus 130 receives the seed light beam 61 1 front the optical source 105 (or 305) and finely tones the spectral output of the optical source 105, 305 based on the input from the control system 185. Referring to Fig. 4, an exemplary spectral feature selection apparatus 430 is shown that couples to light from the optical source 105, 305. in some implementations, the spectral feature selection apparatus 130 receives the light from the master oscillator 300 to enable the fine inning of the spectral features such as wavelength and bandwidth within the master oscillator 300.

The spectra! feature selection apparatus 430 can include a control module 452 that includes electronics in the fo m of any combination of firmware aid software. The module 452 is connected to one or more actuation systems 454, 456, 458. Each of the actuation systems 454, 456, 58 can include one or more actuators that are connected to respective optical features 460, 462, 464 of an optical system 466. The optical features 460, 462, 464 are configured to adjust particular characteristics of the generated light beam 1 10 to thereby adjust the spectral feature of the light beam 1 10. The control module 452 receives a control signal from the control system 1 5, the control signal including specific commands to operate or control one or more of the actuation systems 454, 456. 458. The actuation systems 454, 456, 458 can be selected and designed to work cooperatively.

Each optical feature 460, 462, 464 is optically coupled to the light beam 1 10 produced b the optical source 1 5, Each of the actuators of the actuation systems 454, 456, 458 is a mechanical device for moving or controlling the respective optical features 460, 462, 464 of the optical system 466. The actuators receive energy from the module 452, and convert that energy into some kind of motion imparted to the optical .features 460, 462, 64 of the optical system 466. For example, the actuation systems can be any one of feree devices and rotation stages for rotating one or more of prisms of a beam expander. The actuation systems 454, 456, 458 can include, for example, motors such as stepper motors, valves, pressure-controlled devices, piezoelectric devices, linear motors, hydraulic actuators, voice coils, etc.

The spectral feature selection apparatus 130 can be designed like the apparatuses 130, 430, 530, 630, 730 shown in and described with respect to Figs. 3 A, 3B, 4A-4C, 5A~5C, 6A.-&D, and 7 of U.S. Application No. 15/295,280, filed on October 17, 2016, which is incorporated herein by reference in its entirety.

Referring to Fig. 5, details about the control system. 185 are provided that relate to the aspects of the system and method described herein. The control system 185 can include other .features not show in Fig. 5, In. general , the control system .185 includes one or more of digital electronic circuitry, computer hardware, firmware, and software.

The control system 185 incl udes memory 500, which can be read-only memory and or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data includ all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EP O , EEP OM, and flash memory devices;

magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD- ROM disks. The control system 185 can also include one or more input devices 505 (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices 510 (such as a speaker or a monitor).

The control system 185 includes one or more programmable processors 515, and one or more computer program products 520 tangibly embodied in a machine-readable storage device for execution by a programmable processor (such as the processors 515). The one or more programmable processors 515 can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processor 5.15 receives instructions and data from memory 500, Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits).

The control system 185 includes, among other components, a spectral feature analysis module 525, a lithography analysis module 530, a decision module 535, a light source actuation module 550, a lithography actuation module 555, and a beam preparation actuation module 560. Each of these modules can be a set of computer program products executed by one or more processors such as the processors 515. Moreover, any of the modules 525, 530, 535 550, 555, 560 can access data stored within the memory 500,

The spectral feature analysis module 525 receives the output from the. metrology system ί 70. The lithography analysis module 530 receives information from the lithography controller 140 of the scanner 1 15. The decision module 535 receives the outputs from the analyses modules (such as the modules 525 and 530) and determines which actuation module or modules need to be activated based on the outputs from the analyses modules. The light source actuation module 550 is connected to one or more of the optical source 105 and the spectral feature selection apparatus 130. The lithography actuation module 555 is connected to the scanner 115, and specifically to the lithography controller 140. The beam preparation actuation module 560 is connected to one or more components of the beam preparation system 1 12.

While only a few modules are shown in Fig. 5, it is possible for the control system 185 to include other modules. Additionally, although the control system 185 is represented as a box in which ail of the components appear to be co-located, it is possible for the control system 185 to be made u of components that are physical ly remote from each other. For example, the light source actuation module 550 can be physically co- located with the optical source 105 or the spectral feature selection apparatus 130.

In general, the control system 185 receives at least some information about the light beam 1 10 from the metrology system 170, and the spectral feature analysis module 525 performs an analysis on the information to determine how to adj ust one or more spectral features (for example, the bandwidth) of the light beam 1 10 supplied to the scanner 115. Based on this determination, the control system 185 sends signals to the spectral feature selection apparatus 130 and/or the optical source 105 to control operation of the optical source 105.

In general, the spectral feature analysis module 525 performs all of the analysis needed to estimate one or more spectra! features (for example, the wavelength and/or the bandwidth) of the light beam 1 1 , The output of the spectral feature analysis module 525 is an estimated value of the spectral feature.

The spectral feature analysis module 525 includes a comparison block connected to receive the estimated spectral feature and also connected to receive a spectral feature target value. In general, the comparison block outputs a spectral feature error value that represents a difference between the spectral feature target value and the esiimated value. The decision modiue 535 receives the spectral feature error value and determines how best to effect a correction to the system 1.00 in order to adjust the spectral feature. Thus, the decision module 535 sends a signal to the light source actuation module 550, which determines how t adjust the spectral feature selection apparatus 130 (or the optical source 105) based on the spectral feature error value. The output of the light source actuation module 550 includes a set of actuator commands that are sent to the spectral feature selection apparatus 1.30. For example, light source actuation module 550 sends the commands to the spectral feature control module 452, which is connected to the spectral feature actuation, systems 454, 456. 458.

The control, system. 1.85 causes the optical source 1.05 to operate at a given .repetition rate. M ore specifically, the scanner 1 15 sends a trigger signal to. the optical source 105 for every pulse (that is, on a pulse-to-pulse basis) and the time interval bet een those trigger si nals can be arbitrary, but when the scanner 1 15 sends trigger signals at regular Intervals then the rate of those signals is a repetition rate. The repetition rate can be a rate requested by the scanner 115. The repetition rate of the pulses-produced by the power amplifier 310 is determined by the repetition rate at which the master oscillator 300 is controlled by the control system 185, under the instructions from the controller 140 in the scanner 1 15. The repeiition rate of the pulses output from the power amplifier 310 is the repetition rate seen by the se mier Ϊ 15.

The photolithography system 100 can provide the user or customer (who operates the seamier 1 15) with the ability to choose any one of .many repetition rates depending on the needs of a particular application. As discussed above, performance characteristics {for example, the spectra! features such as the bandwidth of the light beam 11.0) ma vary wi th the repetition rate.

Referring to Fig. 6, an exemplary optical system 666 is a line narrowing module that includes a set of dispersive optical elements, such as a beam expander 600 made of four

refractive optics (prisms 660, 662, 664, 668) and a diffractive optic (grating 670). The seed Sight beam 61 1 passes through an aperture 605 as it enters the line narrowing module 666 and also passes through the aperture 605 as it exits the line narrowing module 666.

The line narrowing module 666 is designed to adjust the wavelength of the seed light beam 61 that is produced within the resonator of the master oscillator 300 by adjusting an angle of incidence of the seed light beam 61 1 impinging on a diffractive surface 671 of the grating 670. Specifically, this can be done by adjusting an angular dispersion provided by the grating 670. One or more of the prisms 660, 662, 664, 668 and the grating 670 can be rotated to adjust the angle of incidence of the seed light beam 6.1 1 and therefore adjust the wavelength of the seed light beam 61 1 produced b the master oscillator 300,

The wavelength of the seed light, beam 63 1 is selected by adjusting the angle at which the grating 670 reflects the seed light beam 61 1, The grating 670 reflects different spectral components of the light beam 61 1 within the emission band of the gain mediu of the m aster oscillator 300. Those wavelength components that are reflected at larger angles from the grating 670 to the optical axis of the resonator of the master oscillator 300 suffer greater losses on subsequent round trips, and therefore the narrowing of the bandwidth is provi ded. Bandwidth narrowing occurs because those wavelength components of the light beam 61 Ϊ emerging from the prisms at angles larger than a fixed acceptance angle of the resonator of the master oscillator 300 are eliminated from the light beam 61 1 as it resonates. Thus, the bandwidth of the light beam 6.11 is determined by the dispersion of the grating 670 as well as the magnification provided by the beam expander 600 (the four prisms 660, 662, 664, 668) because a smaller range of wavelengths emanate from the beam expander 600 at angles within the acceptance angle of the resonator of the master oscillator 300.

The orating 670 can be a high blaze angle Echelle aratiivg. and the light beam 6.1 1 incident on the grating 670 at any angle that satisfies a grating equation will be reflected

(diffracted). The grating equa tion provides the rel a ti onship between the spectral order of the grating 670, the diffracted wavelength, the angle of incidence of the light beam 61 1 onto the grating 670. the angle of exit of the light beam 61 1 diffracted off the grating 670, the vertica divergence of the light beam 61 1 inciden t onto the grating 670, and the groove spacing of the grating 670. Moreover, if the grating 670 is used such that the angle of incidence of the light beam 61 1 onto the grating 670 is equal to the angle of exit of the light beam 61 .1 from the grating 670, then the grating 670 and the beam expander 600 are arranged in a Littrow configuration and the wavelength of the light beam 6.1 1 reflected from the grating 670 is the Littrow wavelength. It- can be assumed that the vertical divergence of the light beam 61 1 incident onto the grating 670 is near zero. To reflect the nominal wavelength, the grating 670 is aligned, wi th respect to the light beam 610 incident onto the grating 670, so that the nominal wavelength is reflected back through the beam expander 600 to be amplified in the chamber of the master oscillator 300, The Littrow wavelength can then be tuned over the entire gain bandwidth of the master oscillator 300 by varying the angle of i ncidence of the light beam 611 onto the grating 670.

The prism 660 that is the farthest from the grating 670, and is also the smallest in size, is mounted on an actuator 660 that causes the prism 660 to rotate, and such rotation changes the optical magnification of the f ight beam 61 1 impinging upon the grating 670. The actuator 660a is a rotary stage that enables rapid control of the position of the prism 660 to enable a rapid adjustment to the bandwidth of the light beam 61 1 (and therefore the light beam 1 1 ). The rotary stage 660a can include a mounting surface (such as a plate 660p) on which the prism 660 is secured and a motor (not shown but mounted within a housing 660h) that is mechanically coupled to the mounting surface to enable the rotation of the mounting surface. The rotary stage 660a i s able to rotate the prism 660 at a speed that enables the bandwidth of the light beam 61 1 and therefore the light beam 1 10 to be changed, from a first bandwidth to a second bandwidth and also to stabilize at. the second band width wi thin a time frame between bursts of pulses of the light beam 1 10. The rotary design of the actuator 660a imparts a purel rotational motion to mounting surface on which the prism 660 is mounted without the use of any linear motion or .flexure motion that are found on prior actuators for the prism 660. Moreover , the use of a rotary stage 660a enables the prism 660 to be rotated about full 360°, unlike the prior actuator that uses a linear stepper motor pins a flexure design .in which the prism 660 could only be rotated about the angle determined from the flexure).

In some implementations, the rotary stage 660a can use a direct drive stepper motor as the motor to rotate the mounting surface. A direct drive stepper motor is a conventional electromagnetic motor that uses a built-in step motor functionality for position control, lit other implementations in which a higher resolution ra motion may he needed, the .rotary stage 660a can us a piezoelectric motor technology.

The rotary stage 660a ca be a rotary stage mat is controlled with a motor controller using a variable- frequency drive control methodic provide the rapid rotation of the prism 660.

The adjustment to the bandwidth of the light beam 61 1 and light beam 1 10 using the rotation of the prism 660 can be considered to be a coarse adjustment; which means that it is able to adjust the bandwidth o ver a wider range of bandwidths, for example, a range of about 250 nm.

The next prism 662 thai is closer to the grating 670, and has a size that is either larger than or equal to the size of the prism 660, can be fixed in space in some implementations. The next prism 664 that is closer to the grating 670 has a size that is either larger tha or equal to the size of the prism 662.

The prism 664 can be mounted to an actuator 664a that causes the prism 664 to rotate, and such rotation of the prism 664 can provide For fine control of the wavelengt of the seed light beam 611. The actuator 664a can be a rotary stage that is controlled with a piezoelectric motor. The piezoelectric motor operates by making use of the converse pi ezoelectric effect in which a material produces acoustic or ultrasonic vibrations in order to produce a linear or rotary motion.

The prism 668 that is closest to the grating 670 has a size that is either larger than or equal to the size of the prism 664 (the prism 668 is the largest prism of the beam expander 600). The prism 668 can be mounted to an actuator 668a that causes the prism 668 to rotate and such rotation of the pri sm 668 can provide for coarse control of the wavelength of the seed light beam 611 , In some implementations, the actuator 668a is a rotary stage that includes a mounting surface to which the prism 668 is fixed and a motor that rotates the mounting surface. The motor of the actua tor 668a can be a piezoelectric motor that is fifty times faster than a prior linea stepper motor and flexure combination design. The actuator 66Sa can also include a rotary encoder that provides angular position feedback for the control system 185.

Referring to Fig. 7, the photolithography system 100 (under control, of the control system 185 and optionally the controller 140) performs a procedure 700 for controlling the bandwidth of the light beam .1 10 produced by the optical source 105.

The procedure 700 includes producing, from the pulsed optical source 105, the pulsed light beam 1 1 at a pulse repetition rate (705). For example, the control system 185 can send signals to the optical source 105 to produce the pulsed light beam. 1 1.0, and moreover, the controller 140 can provide the desired pulse repetition rate to the control system 185.

The pulsed light beam 1 10 is directed toward the substrate 120 received in the scanner

115 to expose the substrate 120 to the pulsed light beam 1 10 (710). For example, the light beam 1 .10 emitted from the optical source 105 is directed to the scanner 1 15 by way of the beam preparation system 1 12,

The pulse repetition rate of the pulsed light beam 110 is modified as it is exposing the substrate 120 (715). For example, as the light beam 1 1 is exposing the substrate 120, the controller 140 sends a signal to the control system 185 to cause the optical source 105 to change the repetition rate of the pulses of the light beam 1.10, as discussed above. Thus, the control system 185 may receiye an instruction from the controller 140 to modify the pulse repetition rate of the pulsed light beam 10 as it is exposing the substrate .120 by a particular value. In this way, the control system 185 determines how to adjust the signal outpu from the light source actuation module 550 based on the requested.

Next, an amount of adjustment to a bandwidth of the pulsed light beam 1.10 is determined (720), where such adjustment amount compensates for a variation in the bandwidth of the pulsed light beam 1 1 that is caused by the modification, of the pulse repetition rate of the pulsed light beam 1 10. The bandwidth of the pulsed light beam 1 1 is changed by this determined adjustment amount, as the pulsed light beam 11.0 is exposing the substrate 120 to thereby compensate for the bandwidth variation (770).

in some implementations, the control system 185 determines the amount of adjusmieut to the bandwidth of the pulsed light beam 1 10 (720) by performing an exemplary procedure 820. The procedure 820 includes accessing a correlation recipe between the repetition rate and the bandwidth (822), determining the bandwidth that correlates to die modified pulse repetition rate in the recip (824), and calculating the adjustment amount of the bandwidth that offsets the bandwidth correlated to the modified pulse repetition rate (826).

The control system 185 accesses the correlation recipe (822) that can be stored within memory 500. The correlation recipe defines correlation between the repetition rate and the bandwidth for that, optical source 105. For exam le, the correlatioii recipe could indicate that the light beam 1 10 has a particular bandwidth for a specific repetition rate, and the correlation recipe ca also indicate how the bandwidth of the light beam 10 changes as the repetition rate is modified. Tor example, with reference to Fig. , a graph 900 (dashed Sine) shows a first exemplary correlation recipe, which is the relationship between the bandwidth of the light beam 1.1 and the repetition rate of the light beam 110 for a first optical source 105a. A graph 950 (dot- dashed line) shows a second distinct exemplary correlation recipe, which is the relationship between the bandwidth of the light beam 1 10 and the repetition rate of the light beam 1.10 for a second optical source 105b.

In some implementations, the correlation recipe such as 900 or 950 is created for the respective pulsed optical source 105a, 1 5b prior to directing the pulsed light beam 1 10 that is produced by that optical source toward the substrate 120, In other implementations, the correlation recipe such as 900 or 950 is created for the respective optical source 105a, 105b during the time that lapses in between a pair of bursts of pulses of the pulsed light beam 1 10. The correlation recipe can be pre-loaded into memory 500 at the time that the optical, source 1.05 is manufactured and/or updated during maintenance, or while the optical source 105 is operating.

The correlation recipe is determined by .measuring the values of the bandwidth of the light beam 1 10 produced by the optical source 105 as the repetition rate of the light beam 1 10 is modified. The bandwidth of the light beam 1 10 can be measured by the metrology system 170.

For example, with reference to Fig. 10, the correlation recipe can be determined or created by performing a procedure 1000. Initially, the i31.iraiinati.on system 150 is operated at a repetition rate R while i is online but not while its output (the pulsed light beam 1 10) is being used by the scanner 1 15 (1 02). Next, one or more performance parameters (such as spectral features, for example, the bandwidth) of the optical source 105 are measured by the illumination system 150 while the illumination system 150 is operating at the repetition rate R ( 1004). The measured performance parameters and the repetition rate E at which the performance parameters are measured are stored, for example, within memory 500 (1006). If the ilhuaination system 150 has been operated at all of the repetition rates R. of interest (1008) then the procedure 1000 is completed for thai illumination system 150. Otherwise the repetition rate R is changed to a new repetition rate (1010) and the one or more performance parameters of the optical source 105, for example, the bandwidth of the light beam 1 10, are measured by the illumination system 150 (1004). The new repetition rate R can be obtained by incrementing the immediately prior value by a fixed amount, or the value of the repetition rate R can be changed using other methods including decreasing the repetition rate R by a fixed amount, increasing or decreasing the repetitio rate R by a variable or random amount, or by testing values of the repetition rate R that are expected, to be of greatest interest,

The illumination system 150 can perform this procedure 1000 in a relatively short period of time, for example, on the order of minute. Moreover, the illumination system 150 can be configured to perform this procedure 1000 for varying resolutions in order to control the overall duration of the procedure 1000 or to provide more refined correlation between the bandwidth and the repetition rate R, For example, the repetition rate R could be incremented by 10 Hz at 1010 for a high resolution analysis or correlation, or the repetition rate R could be incremented by 20 Hz at 1010 for a lower resolution analysis or correlation.

The bandwidth of the pulsed light beam 110 can be changed by the determined adjustment amount (770) at least in part by adjusting one or more optical components of the spectral feature selection, apparatus 130. For example, the control system 185 can determine an adjustment signal to send to the light source actuation module 550, such signal instructing. the module 452 to send specific signals to one or more of the actuation systems 454, 456, 468 to thereby modify one or more optical features 460, 462, 464. The bandwidth can be rapidly changed (770) by rotating the prism 660 with the rapid actuator 660a. Moreover, the bandwidth can be rapidly changed (770) in a time thai is less than or equal to the time between bursts of pulses of the pulsed l ight beam 1 10. For example, the prism 660 i s rotated from a first stable equilibrium position to a second stable equilibrium position in a time that is less than or equal to 50 milliseconds. Because the rapid actuator 660a is a rotation stage, it is possible for the prism 660 to be rotated to an angle within a iu!l 360° rotation range.

By changing the bandwidth of the pulsed light beam. 1 1.0 by the determ ined adjustment amount as the pulsed light beam is exposing the substrate, the photolithography system 100 is able to compensate for the variation in the bandwidth of the pulsed light beam 1 10 that is caused by the modificatio of the pulse repetition rate of the pulsed light beam 1 10 and the bandwidth of the pulsed light beam 1 10 can therefore be maintained within a predetermined stable range even if the repetition rate of the light beam 1 10 is changed dur ing scanning. Moreover, by maintaining the bandwidth in the predetermined stable range, the critical dimensio of a feature formed in the substrate 120 can also be maintained to within a predetermined acceptable range.

Other implementations are within the scope of the following claims.

For example, in other implementations, the prism 662 is mounted to its own actuator 662a that causes the prism 662 to rotate, and such rotation changes the angle of incidence of the light beam 61 1 impinging upon the grating 670 and can be used to provide for fine control of the wavelength of the light beam 61 1. The actuator 662a can be a piezoelectric rotation stage, in these other implementations, the prism 664 can be mounted to an actuator 664a that provides for fine control of the bandwidth of the light beam 61 1. Such an actuator 664a could be a stepper motor rotary stage.

In other implementations, the prism 660 can be mounted, so that its moment axis does not align with the rotation axis of the actuator 662a. in these implementations, the prism axis is offset from the axis of the actuator 662a along a direction perpendicular to the rotation axis of the actuator 662a. An extension arm can be mounted at one end to the rotation axis of the actuator 662a and at a second end to the moment axis of the prism 660.