Systems and methods for controlling center wavelengths

Abstract

To provide systems and methods for controlling a center wavelength of a laser system.SOLUTION: A method for controlling a center wavelength includes estimating a center wavelength error. The method also includes determining, based on the estimated center wavelength error, a first actuation amount for a first actuator controlling movement of a first prism. The method also includes actuating the first actuator based on the actuation amount. The method also includes determining whether the first prism is off-center. The method also includes, in response to determining that the first prism is off-center, determining a second actuation amount for the first actuator and determining a third actuation amount for a second actuator for controlling movement of a second prism. The method also includes actuating the first actuator and the second actuator based on the second and third actuation amounts, respectively. The method finds application in multi-focal imaging operations.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63 / 036,700, filed June 9, 2020, and U.S. Application No. 63 / 079,191, filed September 16, 2020, both entitled SYSTEMS AND METHODS FOR CONTROLLING A CENTER WAVELENGTH, each of which is incorporated herein by reference in its entirety.

[0002] FIELD OF THE DISCLOSURE

[0002] This disclosure relates to laser systems, such as excimer lasers, that generate light and systems and methods for controlling the center wavelength thereof. [Background technology]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern in a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

[0004] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Lithographic apparatus may use extreme ultraviolet (EUV) radiation, which has a wavelength in the range of 4 to 20 nm, for example 6.7 nm or 13.5 nm, or deep ultraviolet (DUV) radiation, which has a wavelength in the range of about 120 to about 400 nm, for example 193 or 248 nm.

[0005]

[0005] A master oscillator power amplifier (MOPA) is a two-stage optical resonator device that generates a highly coherent amplified light beam. The performance of a MOPA can be highly dependent on the alignment of the master oscillator (MO). The alignment of the MO can include the alignment of the gas discharge chamber, the alignment of the input and output optics, and the alignment of the spectral feature adjuster.

[0006] However, MO alignment can be time consuming and require hours of manual maintenance. Furthermore, monitoring and adjusting MO alignment can inhibit or block the output light beam to, for example, a DUV lithography tool.

[0007]

[0007] Wavelength stability is also affected when the device experiences thermal and other transients. In monochromatic mode, two actuators, a stepper motor and a piezoelectric transducer (PZT), work in conjunction with each other to stabilize the center wavelength. In operation, the stepper motor has limited resolution, so the PZT is used as the primary actuator. On the other hand, in bichromatic mode, wavelength stability is based on the center wavelength, i.e., the average of two AC spectra; in this mode, the PZT is responsible for generating the waveform that generates the AC wavelength. Summary of the Invention

[0008]

[0008] Therefore, it is necessary to control the central wavelength.

[0009] In some embodiments, the present disclosure is directed to a system and method for controlling a central wavelength for an imaging operation. The system may include a first actuator configured to control movement of a first prism, a second actuator configured to control movement of a second prism, and a controller configured to estimate a central wavelength error, determine a first actuation amount of the first actuator based on the estimated central wavelength error, actuate the first actuator based on the first actuation amount, determine whether the first prism is off-center, and in response to determining that the first prism is off-center, determine a second actuation amount of the first actuator, determine a third actuation amount of the second actuator, and actuate the first and second actuators based on the second and third actuation amounts, respectively.

[0010] The method may include estimating a central wavelength error. The method may also include determining a first actuation amount of a first actuator controlling movement of the first prism based on the estimated central wavelength error. The method may also include actuating the first actuator based on the first actuation amount. The method may also include determining whether the first prism is off-center. The method may also include determining a second actuation amount of the first actuator and a third actuation amount of the second actuator for controlling movement of the second prism in response to determining that the first prism is off-center. The method may also include actuating the first actuator and the second actuator based on the second and third actuation amounts, respectively. In some embodiments, the method may be performed using the system described above.

[0011]

[0011] In some embodiments, estimating the center wavelength error may include calculating a first average value of the center wavelengths in the odd bursts and a second average value of the center wavelengths in the even bursts, and determining an average value of the first and second average values, wherein the center wavelength error is based on the average value of the first and second average values.

[0012]

[0012] In some embodiments, determining the first actuation amount may include determining a difference between the target central wavelength and an estimated central wavelength, and determining the first actuation amount based on the difference between the target central wavelength and the estimated central wavelength.

[0013]

[0013] In some embodiments, determining the difference between the target central wavelength and the estimated central wavelength may include determining the difference using a digital filter.

[0014]

[0014] In some embodiments, determining the third actuation amount of the second actuator may be based on the position of the first prism after actuating the first actuator based on the second actuation amount.

[0015]

[0015] In some embodiments, determining the third actuation amount may further include determining the third actuation amount so as to reduce the difference between the target center wavelength and the estimated wavelength.

[0016] In some embodiments, the imaging operation includes a multi-focal imaging operation, and the method may further include operating the light source in a two-color mode. In some embodiments, operating the light source in the two-color mode may include generating a first laser radiation beam at a first wavelength using a first laser chamber module, generating a second laser radiation beam at a second wavelength using a second laser chamber module, and combining the first and second laser radiation beams along a common output beam path using a beam combiner. In some embodiments, estimating the central wavelength error may include estimating a central wavelength error of the first laser radiation beam. In some embodiments, in the two-color mode, a wavelength target may move back and forth between two known setpoints within a burst (e.g., pulse by pulse), and a PZT may be used to track the rapidly changing target, leaving little room for control over the central wavelength.

[0017] In some embodiments, the present disclosure is directed to a system and method for controlling a central wavelength. The system may include a light source configured to generate a light beam, a first actuator configured to control movement of a first prism, a second actuator configured to control movement of a second prism, and a controller. The controller may be configured to determine a wavelength error of the light beam generated by the light source, determine whether the wavelength error is greater than a first threshold, and in response to determining that the wavelength error is greater than the first threshold, move the first actuator a first step size, and in response to determining that the wavelength error is less than the first threshold, determine an average wavelength error, determine whether the average wavelength error is greater than a second threshold different from the first threshold, and in response to determining that the average wavelength error is greater than the second threshold, move the first actuator a second step size and enable the low pass filter, and in response to determining that the average wavelength error is less than the second threshold, enable the low pass filter, update the voltage applied to the second actuator, and move the first actuator a third step size.

[0018] The method may include determining a wavelength error of a light beam generated by the light source. The method may also include determining whether the wavelength error is greater than a first threshold. The method may also include, in response to determining that the wavelength error is greater than the first threshold, moving a first actuator configured to control movement of the first prism by a first step size. In response to determining that the wavelength error is less than the first threshold, the method may also include determining an average wavelength error, determining whether the average wavelength error is greater than a second threshold different from the first threshold, moving the first actuator by the second step size and enabling a low-pass filter in response to determining that the average wavelength error is greater than the second threshold, and enabling the low-pass filter, updating a voltage applied to a second actuator configured to control movement of the second prism, and moving the first actuator by a third step size in response to determining that the average wavelength error is less than the second threshold. In some embodiments, the method may be performed using the system described above.

[0019]

[0019] In some embodiments, determining the wavelength error may include measuring the center wavelength of a light beam generated by the light source and determining the difference between the center wavelength and the target center wavelength.

[0020]

[0020] In some embodiments, the method may further include determining whether the number of shots of the light source pulses is a multiple of the update interval, and updating the voltage applied to the second actuator in response to determining that the number of shots is equal to the update interval.

[0021]

[0021] In some embodiments, the method may further include disabling the low pass filter and movement of the second actuator in response to determining that the wavelength error is greater than the first threshold.

[0022] In some embodiments, the first step size is a fixed step size of the actuator.

[0023] In some embodiments, the second step size is a function of the wavelength error.

[0024] In some embodiments, the third step size is a function of the voltage applied to the second actuator.

[0025] In some embodiments, moving the first actuator by the second step size includes moving the first actuator every n pulses, where n is greater than 1.

[0026]

[0026] In some embodiments, the average wavelength error is based on the wavelength error and an average value of multiple wavelength errors over a number of pulses.

[0027] In some embodiments, the method includes controlling the central wavelength in a multi-focal imaging operation, and the method may further include operating the light source in a two-color mode. In some embodiments, operating the light source in a two-color mode may include generating a first laser radiation beam at a first wavelength using a first laser chamber module, generating a second laser radiation beam at a second wavelength using a second laser chamber module, and combining the first and second laser radiation beams along a common output beam path using a beam combiner. In some embodiments, determining the wavelength error of the light beam generated by the light source includes determining a central wavelength error of the first laser radiation beam. In some embodiments, in the two-color mode, a wavelength target may move back and forth between two known setpoints within a burst (e.g., pulse by pulse), and a PZT may be used to track the rapidly changing target, leaving little room for control of the central wavelength.

[0028] In some embodiments, the present disclosure is directed to a system and method for controlling a central wavelength for multifocal imaging operations. The system may include an actuator configured to control movement of a prism, and a controller configured to combine a dither waveform with an offset value to move the actuator, generate a pulse-to-pulse wavelength based on the dither waveform and the offset value, generate a rolling average of the central wavelength based on the pulse-to-pulse wavelengths of a plurality of pulses, estimate a drift rate to predict the central wavelength of future pulses, and update the offset rate based on the estimated drift rate.

[0029]

[0029] The method may include combining the dither waveform with an offset value to move an actuator that controls movement of the prism. The method may also include generating an inter-pulse wavelength based on the dither waveform and the offset value. The method may also include generating a rolling average of the center wavelength based on the inter-pulse wavelengths of the plurality of pulses. The method may also include estimating a drift velocity to predict the center wavelength of future pulses. The method may also include updating the offset value based on the estimated drift velocity. In some embodiments, the method may be performed using the above system.

[0030] In some embodiments, the offset value is based on a direct current (DC) voltage.

[0031] In some embodiments, the initial value of the DC voltage is zero volts.

[0032]

[0032] In some embodiments, the offset value may include a first offset value, and estimating the drift velocity may include estimating the drift velocity based on a rolling average value of the center wavelength, the first offset value, and a second offset value that moves a second actuator that controls movement of the second prism.

[0033] In some embodiments, estimating the drift velocity may include estimating the accumulated center wavelength drift velocity using a Kalman filter framework.

[0034] In some embodiments, estimating the drift velocity may include predicting the center wavelength N pulses ahead of the current pulse.

[0035] In some embodiments, estimating the drift velocity may include converting a Kalman filter framework into a Kalman predictor to predict the center wavelength N pulses ahead of the current pulse.

[0036]

[0036] In some embodiments, the inter-pulse wavelength of the multiple pulses includes the wavelength of the current pulse.

[0037]

[0037] In some embodiments, updating the offset value may include updating the offset value based on a rolling average value of the center wavelength at the end of the burst.

[0038] In some embodiments, the multi-focal imaging operation includes a bi-color mode, and the method may further include operating the light source in the bi-color mode. In some embodiments, operating the light source in the bi-color mode may include generating a first beam of laser radiation at a first wavelength using a first laser chamber module, generating a second beam of laser radiation at a second wavelength using a second laser chamber module, and combining the first and second laser radiation along a common output beam path using a beam combiner.

[0039]

[0039] Further features and exemplary aspects of the embodiments, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It should be noted that the embodiments are not limited to the specific embodiments described herein. Such embodiments are shown herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein. [Brief explanation of the drawings]

[0040]

[0040] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments and, together with the description, serve to further explain the principles of the embodiments and to enable those skilled in the art to make and use the embodiments.

[0041] [Figure 1]

[0041] FIG. 1 is a schematic diagram of a lithographic apparatus according to an exemplary embodiment. [Figure 2]

[0042] 1 is a schematic top view of a light source device according to an exemplary embodiment. [Figure 3]

[0043] 3 is a schematic partial cross-sectional view of a gas discharge stage of the light source apparatus shown in FIG. 2, according to an exemplary embodiment. [Figure 4]

[0044] 3 is a schematic partial cross-sectional view of a gas discharge stage of the light source apparatus shown in FIG. 2, according to an exemplary embodiment. [Figure 5]

[0045] 1 illustrates a method for tuning the central wavelength for multi-focal imaging, according to an embodiment. [Figure 6A]

[0046] 10 illustrates a method for tuning a central wavelength for multi-focal imaging, according to some embodiments. [Figure 6B]

[0046] A method for tuning the central wavelength for multi-focal imaging according to some embodiments is shown. [Figure 7]

[0046] A method for tuning the central wavelength for multi-focal imaging according to some embodiments is shown. [Figure 8]

[0046] A method for tuning the central wavelength for multi-focal imaging according to some embodiments is shown. [Figure 9]

[0047] 1 illustrates a flowchart for aligning a gas discharge stage, according to an exemplary embodiment. [Figure 10]

[0048] 1 is an exemplary computer system useful for implementing various embodiments of this disclosure.

[0042]

[0049] Features and exemplary aspects of the embodiments will become more apparent from the following detailed description when read with reference to the drawings, in which like reference numerals identify corresponding elements throughout. In the drawings, like reference numerals generally indicate identical, functionally similar, and / or structurally similar elements. Furthermore, the leftmost digit(s) of a reference number generally identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout this disclosure should not be construed as drawings to scale. DETAILED DESCRIPTION OF THE INVENTION

[0043]

[0050] This specification discloses one or more embodiments incorporating features of the present invention. The disclosed embodiment or embodiments are merely exemplary of the invention. The scope of the invention is not limited to the disclosed embodiment or embodiments. The invention is defined by the claims appended hereto.

[0044]

[0051] References to described embodiments, and to "one embodiment," "an embodiment," "an exemplary embodiment," etc., herein indicate that the described embodiments may include a particular feature, structure, or characteristic, but that each embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Furthermore, when a particular feature, structure, or characteristic is described in connection with one embodiment, it is understood that it is within the knowledge of one of ordinary skill in the art to implement such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described.

[0045]

[0052] Spatially relative terms such as "beneath," "below," "lower," "above," "on," "upper," and the like may be used herein to facilitate describing the relationship of one element or feature to another element or features, as shown in the figures. Spatially relative terms are intended to encompass various orientations of the device in use or operation in addition to the orientation shown in the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0046]

[0053] As used herein, the terms "about," "substantially," or "approximately" refer to a given quantitative value that may vary based on a particular technique. Based on a particular technique, the terms "about," "substantially," or "approximately" may refer to a given quantitative value that varies within, for example, 1-15% of that value (e.g., ±1%, ±2%, ±5%, ±10%, or ±15% of that value).

[0047]

[0054] Embodiments of the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present disclosure may also be implemented as instructions stored on a tangible, machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, machine-readable media may include read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and / or instructions may be described herein as performing certain operations. However, it should be understood that such description is merely for convenience and that such operations actually result from a computing device, processor, controller, or other device that executes the firmware, software, routines, and / or instructions.

[0048]

[0055] Before describing such embodiments in detail, it will be useful to present an exemplary environment in which embodiments of the present invention may be practiced.

[0049]

[0056] Exemplary Lithography System

[0057] Figure 1 shows a lithography system comprising a radiation source SO and a lithographic apparatus LA. The source SO is configured to generate a beam of EUV and / or DUV radiation B and to provide this beam of EUV and / or DUV radiation B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0050]

[0058] The illumination system IL is configured to condition the EUV and / or DUV radiation beam B before it is incident on the patterning device MA. To that end, the illumination system IL may comprise a facetted field mirror device 10 and a facetted pupil mirror device 11. Together, the facetted field mirror device 10 and the facetted pupil mirror device 11 impart a desired cross-sectional shape and a desired intensity distribution to the EUV and / or DUV radiation beam B. In addition to or instead of the facetted field mirror device 10 and the facetted pupil mirror device 11, the illumination system IL may comprise other mirrors or devices.

[0051]

[0059] After being so conditioned, the EUV and / or DUV radiation beam B interacts with the patterning device MA (e.g. a DUV transmissive mask or an EUV reflective mask). This interaction results in a patterned EUV and / or DUV radiation beam B'. The projection system PS is configured to project the patterned EUV and / or DUV radiation beam B' onto the substrate W. To this end, the projection system PS may comprise a number of mirrors 13, 14 configured to project the patterned EUV and / or DUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV and / or DUV radiation beam B' to form images of features that are smaller than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although the projection system PS is shown in Figure 1 as having only two mirrors 13, 14, the projection system PS may comprise a different number of mirrors (e.g. 6 or 8 mirrors).

[0052]

[0060] The substrate W may include a pre-formed pattern, in which case the lithographic apparatus LA aligns the image formed by the patterned EUV and / or DUV radiation beam B' with the pre-formed pattern on the substrate W.

[0053]

[0061] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure significantly below atmospheric pressure, may be provided within the source SO, illumination system IL and / or projection system PS.

[0054]

[0062] Exemplary Light Source Device

[0063] As discussed above, a master oscillator power amplifier (MOPA) is a two-stage optical resonator device. The master oscillator (MO) (e.g., the first optical resonator stage) generates a highly coherent optical beam (e.g., from a seed laser). The power amplifier (PA) (e.g., the second optical resonator stage) increases the optical power of the optical beam while maintaining the beam characteristics. The MO may include a gas discharge chamber, input and output optics (e.g., an optical coupler (OC)), and a spectral feature adjuster (e.g., a line narrowing module (LNM)). The input and output optics and the spectral feature adjuster may surround the gas discharge chamber to form an optical resonator.

[0055]

[0064] The performance of the MOPA is highly dependent on the alignment of the MO. The alignment of the MO may include the alignment of the gas discharge chamber, the alignment of the OC, and the alignment of the LNM. Each alignment (e.g., chamber, OC, LNM, etc.) may contribute to alignment errors and variations in the MO over time. However, aligning the MO is time-consuming and may require several hours of manual maintenance (e.g., synchronized performance maintenance (SPM)). Additionally, initial alignment may be difficult (e.g., by trial and error) if the chamber, OC, and LNM are significantly misaligned (e.g., lack of an initial reference point). Furthermore, monitoring and adjusting the MO alignment may inhibit (e.g., block) the output light beam (e.g., DUV light beam) to, for example, a DUV lithography tool.

[0056]

[0065] Imaging light (e.g., a visible laser beam) may be projected (e.g., sequentially or simultaneously) onto the chamber, OC, and LNM to illuminate and guide the alignment of the OC and / or LNM along the chamber's optical axis (e.g., first and second optical ports). Amplified spontaneous emission (ASE) from the gas discharge chamber can serve as a beacon (e.g., a reference point) to facilitate aiming (e.g., laser aiming) of the imaging light along the MO cavity's optical axis (e.g., along the chamber, OC, and LNM optical axes). The ASE may also be used to initially align (e.g., coarsely align) the chamber with the MO cavity's optical axis. Additionally, sensing devices (e.g., cameras) may be used to visually probe various object planes within the MO (e.g., chamber ports, OC apertures, LNM apertures, etc.) and quantify alignment errors (e.g., image comparison). For example, the sensing device can examine the near-field (NF) and far-field (FF) regions of the imaged light on various object planes and apply adjustments (e.g., fine alignment) by, for example, beam profiling (e.g., horizontal symmetry, vertical symmetry, etc.).

[0057]

[0066] The light source devices and systems discussed below can reduce master oscillator alignment times (e.g., SPM), reduce master oscillator alignment variations over time, and monitor and dynamically control quantifiable master oscillator alignment errors to provide a highly coherent light beam, for example, in a DUV lithography tool.

[0058]

[0067] Figures 2 to 4 show light source apparatus 200 according to various exemplary embodiments. Figure 2 is a schematic top view of light source apparatus 200 according to an exemplary embodiment. Figures 3 and 4 are schematic partial cross-sectional views of gas discharge stage 220 of light source apparatus 200 shown in Figure 2 according to exemplary embodiments.

[0059]

[0068] FIG. 2 illustrates a light source apparatus 200 according to various exemplary embodiments. The light source apparatus 200 may be configured to monitor and dynamically control quantifiable alignment errors of a gas discharge stage 220 (e.g., MO) to provide a highly coherent and aligned light beam (e.g., light beam 202, amplified light beam 204) to, for example, a DUV lithography apparatus (e.g., LA). The light source apparatus 200 may further be configured to reduce alignment time of the gas discharge stage 220 (e.g., MO) and reduce alignment variations of the gas discharge stage 220 (e.g., MO) over time. While the light source apparatus 200 is illustrated in FIG. 2 as a standalone apparatus and / or system, embodiments of the present disclosure may be used with other optical systems, such as, but not limited to, a radiation source SO, a lithography apparatus LA, and / or other optical systems. In some embodiments, the light source apparatus 200 may be the radiation source SO of the lithography apparatus LA. For example, the DUV radiation beam B may be the light beam 202 and / or the amplified light beam 204.

[0060]

[0069] The light source device 200 may be a MOPA formed by a gas discharge stage 220 (e.g., MO) and a power ring amplifier (PRA) stage 280 (e.g., PA). The light source device 200 may include the gas discharge stage 220, a line analysis module (LAM) 230, a master oscillator wavefront engineering box (MoWEB) 240, the power ring amplifier (PRA) stage 280, and a controller 290. In some embodiments, all of the above components may be housed within a three-dimensional (3D) frame 210. In some embodiments, the 3D frame 210 may comprise a metal (e.g., aluminum, steel, etc.), ceramic, and / or any other suitable rigid material.

[0061]

[0070] Gas discharge stage 220 may be configured to output a highly coherent optical beam (e.g., optical beam 202). Gas discharge stage 220 may comprise a first optical resonator element 254, a second optical resonator element 224, an input / output optical element 250 (e.g., an OC), an optical amplifier 260, and a spectral feature adjuster 270 (e.g., an LNM). In some embodiments, input / output optical element 250 may comprise first optical resonator element 254, and spectral feature adjuster 270 may comprise second optical resonator element 224. First optical resonator 228 may be defined by input / output optical element 250 (e.g., via first optical resonator element 254) and spectral feature adjuster 270 (e.g., via second optical resonator element 224). To form first optical resonator 228, first optical resonator element 254 may be partially reflective (e.g., a partial mirror), and second optical resonator element 224 may be reflective (e.g., a mirror or a grating). The first optical cavity 228 can direct light generated by the optical amplifier 260 (e.g., amplified spontaneous emission (ASE) 201) into the optical amplifier 260 for a number of passes to form the optical beam 202. In some embodiments, as shown in FIG. 2, the gas discharge stage 220 can output the optical beam 202 to a PRA stage 280 that is part of the MOPA device.

[0062]

[0071] The PRA stage 280 may be configured to amplify the optical beam 202 from the gas discharge stage 220 in a multi-pass configuration and output the amplified optical beam 204. The PRA stage 280 may include a third optical resonator element 282, a power ring amplifier (PRA) 286, and a fourth optical resonator element 284. The third optical resonator element 282 and the fourth optical resonator element 284 may define a second optical resonator 288. To form the second optical resonator 288, the third optical resonator element 282 may be partially reflective (e.g., a partial beam splitter), and the fourth optical resonator element 284 may be reflective (e.g., a mirror, prism, or beam reverser). The second optical resonator 288 may direct the optical beam 202 from the gas discharge stage 220 into the PRA 286 for a number of passes to form the amplified optical beam 204. In some embodiments, the PRA stage 280 may output the amplified light beam 204 to a lithography apparatus, such as a lithography apparatus (LA). For example, the amplified light beam 204 may be an EUV and / or DUV radiation beam B from a radiation source SO of the lithography apparatus LA.

[0063]

[0072] 2-4 , optical amplifier 260 may be optically coupled to input / output optics 250 and spectral feature adjuster 270. Optical amplifier 260 may be configured to output ASE 201 and / or light beam 202. In some embodiments, optical amplifier 260 may utilize ASE 201 as a beacon to guide alignment of the optical axis of chamber 261 and / or the optical axis of gas discharge stage 220 (e.g., MO cavity). Optical amplifier 260 may include chamber 261, gas discharge medium 263, and chamber adjuster 265. Gas discharge medium 263 may be disposed within chamber 261, and chamber 261 may be disposed on chamber adjuster 265.

[0064]

[0073] The chamber 261 may be configured to hold a gas discharge medium 263 within first and second chamber optical ports 262 a, 262 b. The chamber 261 may include a first chamber optical port 262 a and a second chamber optical port 262 b opposite the first chamber optical port 262 a. In some embodiments, the first and second chamber optical ports 262 a, 262 b may form an optical axis of the chamber 261.

[0065]

[0074] 3, first chamber light port 262a may be in optical communication with input / output optics 250. First chamber light port 262a may comprise first chamber wall 261a, first chamber window 266a, and first chamber opening 264a. In some embodiments, first chamber opening 264a may be a rectangular opening, as shown in FIG.

[0066]

[0075] As shown in Figure 4, the second chamber light port 262b can be in optical communication with the spectral feature adjuster 270. The second chamber light port 262b can comprise a second chamber wall 261b, a second chamber window 266b, and a second chamber opening 264b. In some embodiments, the second chamber opening 264b can be a rectangular opening, as shown in Figure 4. In some embodiments, the optical axis of the chamber 261 passes through the first and second chamber openings 264a, 264b.

[0067]

[0076] The gas discharge medium 263 may be configured to output the ASE 201 (e.g., 193 nm) and / or the light beam 202 (e.g., 193 nm). In some embodiments, the gas discharge medium 263 may include an excimer laser gas (e.g., Ar2, Kr2, F2, Xe2, ArF, KrCl, KrF, XeBr, XeCl, XeF, etc.). For example, the gas discharge medium 263 may include ArF or KrF, and upon excitation (e.g., applied voltage) from surrounding electrodes (not shown) in the chamber 261, the ASE 201 (e.g., 193 nm) and / or the light beam 202 (e.g., 193 nm) may be output from the first and second chamber optical ports 262a, 262b. In some embodiments, the gas discharge stage 220 may include a voltage power supply (not shown) configured to apply high-voltage electrical pulses between electrodes (not shown) in the chamber 261.

[0068]

[0077] The chamber adjuster 265 may be configured to spatially (e.g., laterally, angularly) adjust the optical axis of the chamber 261 (e.g., along the first and second chamber optical ports 262a, 262b). As shown in FIG. 2 , the chamber adjuster 265 may be coupled to the chamber 261 and the first and second chamber optical ports 262a, 262b. In some embodiments, the chamber adjuster 265 may have six degrees of freedom (e.g., six axes). For example, the chamber adjuster 265 may comprise one or more linear motors and / or actuators that provide six degrees of freedom (e.g., forward / backward, up / down, left / right, yaw, pitch, and roll) adjustment of the optical axis of the chamber 261. In some embodiments, the chamber adjuster 265 may adjust the chamber 261 laterally and angularly to align the optical axis of the chamber 261 (e.g., along the first and second chamber optical ports 262a, 262b) with the optical axis of the gas discharge stage 220 (e.g., the MO cavity). For example, as shown in FIG. 2, the optical axis of the gas discharge stage 220 (e.g., MO cavity) may be defined by the optical axes of the chamber 261 (e.g., along the first and second chamber optical ports 262a, 262b), the input / output optical element 250 (e.g., OC aperture 252), and the spectral feature adjuster 270 (e.g., LNM aperture 272).

[0069]

[0078] The input / output optical element 250 may be configured to be in optical communication with the first chamber optical port 262a. In some embodiments, the input / output optical element 250 may be an optical coupler (OC) configured to partially reflect the light beam and form the first optical resonator 228. For example, OCs are previously described in U.S. Patent No. 7,885,309, issued February 8, 2011, which is incorporated by reference herein in its entirety. As shown in FIG. 2 , the input / output optical element 250 may comprise a first optical resonator element 254 for directing (e.g., reflecting) light into the optical amplifier 260 and transmitting light (e.g., light beam 202, ASE 201) from the optical amplifier 260 out of the gas discharge stage 220 (e.g., MO cavity).

[0070]

[0079] As shown in FIG. 3 , input / output optical element 250 may include OC aperture 252 and first optical resonator element 254. First optical resonator element 254 may be configured to angularly adjust (e.g., tilt and / or tilt) light passing through OC aperture 252 vertically and / or horizontally relative to chamber 261 (e.g., first chamber optical port 262 a). In some embodiments, OC aperture 252 may be a rectangular aperture. In some embodiments, alignment of gas discharge stage 220 may be based on alignment of first chamber aperture 264 a and OC aperture 252. In some embodiments, first optical resonator element 254 may angularly adjust (e.g., tilt and / or tilt) input / output optical element 250 such that reflections from input / output optical element 250 are parallel to the optical axis of gas discharge stage 220 (e.g., MO cavity). In some embodiments, first optical resonator element 254 can be an adjustable mirror (e.g., a partially reflecting mirror, a beam splitter, etc.) that can be angularly adjusted (e.g., tilted and / or tilted). In some embodiments, OC aperture 252 can be fixed and first optical resonator element 254 can be adjustable. In some embodiments, OC aperture 252 can be adjustable. For example, OC aperture 252 can be spatially adjusted vertically and / or horizontally relative to chamber 261.

[0071]

[0080] The spectral feature conditioner 270 (e.g., an LNM) can be configured in optical communication with the second chamber light port 262b. In some embodiments, the spectral feature conditioner 270 can be a line narrowing module (LNM) configured to perform spectral line narrowing on the light beam. For example, LNMs are previously described in U.S. Patent No. 8,126,027, issued February 28, 2012, which is incorporated herein by reference in its entirety.

[0072]

[0081] As shown in FIG. 2, the spectral feature adjuster 270 may include a second optical resonator element 224 for redirecting (e.g., reflecting) light from the optical amplifier 260 (e.g., light beam 202, ASE 201) back into the optical amplifier 260 toward the input / output optical element 250.

[0073]

[0082] 4, spectral feature adjuster 270 may comprise an LNM aperture 272 and a tilt angle modulator (TAM) 274. TAM 274 may be configured to angularly adjust light passing through LNM aperture 272 vertically and / or horizontally relative to chamber 261 (e.g., second chamber light port 262b). In some embodiments, LNM aperture 272 may be a rectangular aperture. In some embodiments, alignment of gas discharge stage 220 may be based on the alignment of second chamber aperture 264b and LNM aperture 272. In some embodiments, TAM 274 may angularly adjust (e.g., tilt and / or tilt) spectral feature adjuster 270 so that reflections from spectral feature adjuster 270 are parallel to the optical axis of gas discharge stage 220 (e.g., MO cavity). In some embodiments, TAM 274 may comprise an adjustable mirror (e.g., a partially reflecting mirror, a beam splitter, etc.) and / or an adjustable prism that may be angularly adjusted (e.g., tilted and / or tilted). In some embodiments, LNM aperture 272 may be fixed and TAM 274 may be adjustable. In some embodiments, LNM aperture 272 may be adjustable. For example, LNM aperture 272 may be spatially adjusted vertically and / or horizontally relative to chamber 261.

[0074]

[0083] In some embodiments, the adjustable mirror (e.g., a partially reflecting mirror, a beam splitter, etc.) and / or adjustable prism of TAM 274 may include multiple prisms 276a-d. Prisms 276a-d may be actuated to manipulate the angle of incidence of light entering second optical resonator element 224, which may serve to select a narrow band of wavelengths that are reflected back along the optical path. In some embodiments, prism 276a may be attached to a stepper motor with limited step resolution and may be used for coarse wavelength control. Prism 276b, which offers improved resolution and bandwidth compared to prism 276a, may be actuated using a piezoelectric transducer (PZT) actuator. In operation, controller 290 may use prisms 276a, 276b in a dual-stage configuration.

[0075]

[0084] The LAM 230 may be configured to monitor the line center (e.g., center wavelength) of the light beam (e.g., light beam 202, imaging light 206). The LAM 230 may be further configured to monitor the energy of the light beam (e.g., ASE 201, light beam 202, imaging light 206) for metrology wavelength measurements. For example, LAMs are previously described in U.S. Patent No. 7,885,309, issued February 8, 2011, which is incorporated herein by reference in its entirety.

[0076]

[0085] As shown in FIG. 2, LAM 230 may be optically coupled to gas discharge stage 220 and / or MoWEB 240. In some embodiments, LAM 230 may be located between gas discharge stage 220 and MoWEB 240. For example, as shown in FIG. 2, LAM 230 may be optically coupled directly to MoWEB 240 and optically coupled to gas discharge stage 220. In some embodiments, as shown in FIG. 2, beam splitter 212 may be configured to direct ASE 201 and / or light beam 202 to PRA stage 280 and direct ASE 201 and / or light beam 202 to an imaging device. In some embodiments, as shown in FIG. 2, beam splitter 212 may be located within MoWEB 240.

[0077]

[0086] MoWEB 240 may be configured to perform beam shaping on the light beam (e.g., light beam 202, imaging light 206). MoWEB 240 may further be configured to monitor forward and / or backward propagation of the light beam (e.g., ASE 201, light beam 202, imaging light 206). For example, MoWEB has been previously described in U.S. Patent No. 7,885,309, issued February 8, 2011, which is incorporated herein by reference in its entirety. As shown in FIG. 2 , MoWEB 240 may be optically coupled to LAM 230. In some embodiments, LAM 230, MoWEB 240, and / or imaging device may be optically coupled to gas discharge stage 220 via a single optical configuration.

[0078]

[0087] The controller 290 may be configured to communicate with the input / output optics 250, the chamber conditioner 265, and / or the spectral feature conditioner 270. In some embodiments, the controller 290 may be configured to provide a first signal 292 to the input / output optics 250, a second signal 294 to the spectral feature conditioner 270, and a third signal 296 to the chamber conditioner 265. In some embodiments, the controller 290 may be configured to provide signals (e.g., the first signal 292 and / or the second signal 294) to the input / output optics 250 and / or the spectral feature conditioner 270, and to adjust the input / output optics 250 (e.g., adjust the first optical resonator element 254) and / or adjust the spectral feature conditioner 270 (e.g., adjust the TAM 274) based on an output from the imaging device 400 (e.g., a two-dimensional (2D) image comparison).

[0079]

[0088] In some embodiments, first optical resonator element 254, chamber adjuster 265, and / or TAM 274 can be in physical and / or electronic communication with controller 290 (e.g., first signal 292, second signal 294, and / or third signal 296). For example, first optical resonator element 254, chamber adjuster 265, and / or TAM 274 can be adjusted (e.g., laterally and / or angularly) by controller 290 to align an optical axis of chamber 261 (e.g., along first and second chamber optical ports 262 a, 262 b) with an optical axis of gas discharge stage 220 (e.g., MO cavity) defined by input / output optics 250 (e.g., OC aperture 252) and spectral feature adjuster 270 (e.g., LNM aperture 272).

[0080]

[0089] During normal operation, the laser wavelength may be subject to disturbances and drift when the optical components undergo thermal transients and when the laser duty cycle changes. The primary wavelength actuator is the LNM. As discussed above, the LNM may comprise a plurality of prisms 276a-d and a second optical resonator element 224 (e.g., a grating). The plurality of prisms 276a-d may be actuated to manipulate the angle of incidence of light entering the second optical resonator element 224, which serves to select a narrow band of wavelengths that are reflected back along the optical path. In some embodiments, the magnitude of the angle of incidence may control the selected wavelength.

[0081]

[0090] In some embodiments, multiple prisms 276a-d may be used to adjust the final angle of incidence to control the magnitude of the angle of incidence and, therefore, the selected wavelength. For example, prism 276a may control the final angle of incidence more than prism 276b. That is, in some embodiments, controller 290 uses prisms 276a-b in a dual-stage configuration, with prism 276a used for large jumps and desaturating prism 276b, which is used for finer changes in the final angle of incidence. Controlling prisms 276a-b is particularly important for MFI operations, which require not only adjustment around the setpoint, but also precise tracking of a sine wave at the Nyquist frequency and precise control of the center point (i.e., center wavelength) of the sine wave. The processes described with respect to FIGS. 5, 6A, 6B, and 7-9 provide a method for controlling the center wavelength for imaging operations, such as MFI operations.

[0082]

[0091] The multi-focal imaging operation may include a two-color mode. Operating the light source in the two-color mode may include generating a first laser radiation beam at a first wavelength using a first laser chamber module, generating a second laser radiation beam at a second wavelength using a second laser chamber module, and combining the first and second laser radiation beams along a common output beam path using a beam combiner. In the two-color mode, the wavelength target may move back and forth between two known setpoints within a burst (e.g., pulse by pulse), and a PZT may be used to track the rapidly changing target, leaving little room for control over the center wavelength.

[0083]

[0092] FIG. 5 illustrates a method 500 for adjusting a central wavelength for multifocal or other imaging, according to one embodiment. It should be understood that not all steps in FIG. 5 are required to practice the disclosures set forth herein. Furthermore, some of the steps may be performed simultaneously, sequentially, and / or in a different order than shown in FIG. 5. Method 500 will be described with reference to FIGS. 1 through 4, although method 500 is not limited to these exemplary embodiments.

[0084]

[0093] In some embodiments, method 500 is directed to developing a feedback loop to adjust the center wavelength of the laser radiation beam by moving actuators that control the movement of prisms 276a and 276b, respectively, based on the average center wavelength error estimated from LAM 230. To accomplish this, the center wavelength of the most recent pulse may be estimated using LAM data. In some embodiments, the difference between the target center wavelength and the estimated center wavelength may be provided to controller 290 to determine the desired actuation of prism 276b to compensate for disturbances to the center wavelength. Because prism 276b has a limited range of movement, controller 290 may also ensure that prism 276b is centered by actuating prism 276b as needed.

[0085]

[0094] At 510, method 500 may include estimating a center wavelength error. For example, the center wavelength error may be estimated based on a first average value of the center wavelengths in the odd bursts and a second average value of the center wavelengths in the even bursts, and a third average value is determined based on the first and second average values. In some embodiments, the center wavelength error may be based on a difference between the center wavelength and the third average value.

[0086]

[0095] At 520, method 500 may include determining an actuation amount of a first actuator that controls movement of prism 276b based on the estimated center wavelength. For example, controller 290 of FIG. 2 may determine a difference between the target center wavelength and the estimated wavelength and determine how much to actuate the actuator that controls movement of prism 276b to compensate for the difference. At 530, method 500 may include actuating the actuator that controls movement of prism 276b based on the actuation amount.

[0087]

[0096] At 540, method 500 may include determining whether prism 276b is off-center. In response to determining that prism 276b is centered, method 500 ends at 550. In response to determining that prism 276b is off-center, method 500 may include, at 560, determining a second amount of actuation of an actuator controlling the movement of prism 276b and determining a third amount of actuation of a second actuator controlling the movement of prism 276a based on the second actuation of the first actuator. That is, controller 290 can determine how much actuation is required for both prisms 276a, 276b to correct the center wavelength error.

[0088]

[0097] 6A-6B, 7, and 8 illustrate methods for adjusting a central wavelength for imaging operations, such as multi-focal imaging, according to some embodiments. It should be understood that not all steps in FIGS. 6-8 are required to practice the disclosures presented herein. Furthermore, some of the steps may be performed simultaneously, sequentially, and / or in a different order than shown in FIGS. 6A-6B, 7, and 8. These methods shall be described with reference to FIGS. 1-4, although they are not limited to these exemplary embodiments.

[0089]

[0098] 6A-6B, 7, and 8 are directed to methods for adjusting the center wavelength of a laser radiation beam, for example, in two-color MFI mode. Two-color MFI mode can face challenges, such as prism 276b having little room for center wavelength control in two-color mode, step disturbances from mode transitions, and / or peak separation changes that can generate transient events when handled using pure feedback, and the center wavelength controller can interact with other controllers, such as the peak separation controller, leading to performance degradation or instability. To address these challenges, in some embodiments, prism 276a can be moved in bursts to compensate for large center wavelength errors while limiting the movement of prism 276b to compensate for small low-pass filtered errors. Furthermore, in some embodiments, prism 276a can be moved to desaturate prism 276b. In some embodiments, prism 276a can be moved outside of bursts upon detecting a two-color mode transition or peak separation target change. In some embodiments, the control bandwidth between the center wavelength controller and another controller, such as a peak isolation controller, may be separated from one another.

[0090]

[0099] 6A-6B , at 610, method 600 may include exciting a light source, such as a laser chamber in an MFI system. At 620, method 600 may include determining a wavelength error of the light source, which may be a first laser radiation beam at a first wavelength from a first laser chamber module or a second laser radiation beam at a second wavelength generated using a second laser chamber module. In some embodiments, determining the wavelength error may include measuring a center wavelength of the light beam generated by the light source and determining a difference between the center wavelength and a target center wavelength.

[0091]

[0100] At 630, the method 600 may include determining whether the wavelength error is greater than a first threshold. For example, the threshold may be 200 femtometers. It should be understood by one of ordinary skill in the art that this is merely an exemplary threshold and that other thresholds are also contemplated in accordance with aspects of the present disclosure.

[0092]

[0101] In some embodiments, in response to determining 640 that the wavelength error is greater than a threshold, method 600 may include moving a first actuator to control movement of prism 276a. For example, the first actuator may be moved a first step size per pulse while a filter, such as a low-pass filter, and movement of a second actuator for controlling movement of prism 276b are disabled. For example, the first actuator may be moved in a direction that reduces the wavelength error. The first step size may be a fixed step size, such as one full step of the first actuator. By moving the first actuator while the filter and second actuator are disabled, method 600 imparts a visible change in the wavelength error and desaturates prism 276b. In some embodiments, after the first actuator is moved the first step, method 600 ends in 698 by waiting for the next pulse of the light source.

[0093]

[0102] In some embodiments, in response to determining that the wavelength error is less than the first threshold, at 650, the method 600 may include determining an average wavelength error. In some embodiments, the average wavelength error may be a moving average based on low-pass filtering techniques, as would be understood by one of ordinary skill in the art. At 660, the method 600 may include determining whether the average wavelength error is greater than a second threshold. In some embodiments, the second threshold may be different from the first threshold. For example, the second threshold may be 100 femtometers. It should be understood by one of ordinary skill in the art that this is merely an exemplary threshold, and other thresholds are also contemplated in accordance with aspects of the present disclosure. In some embodiments, the average wavelength error may be based on the wavelength error and an average of multiple wavelength errors over a number n of pulses, where n is a number of pulses greater than 1. That is, the average wavelength error may be a moving average of the wavelength error.

[0094]

[0103] In some embodiments, in response to determining 670 that the average wavelength error is greater than a second threshold, method 600 may include moving the first actuator by a second step size, enabling a low-pass filter, and disabling movement of the second actuator. For example, the first actuator may be moved in a direction that reduces the wavelength error. In some embodiments, the second step size may be proportional to the wavelength error, e.g., the smaller the average wavelength error, the smaller the step size of the first actuator, and vice versa. In some embodiments, the second step size may be smaller than the full step size. In some embodiments, the second step size may be larger than the full step size. By moving the first actuator by a step size proportional to the average wavelength error, method 600 prevents overshooting the desired position of prism 276a. In some embodiments, after the first actuator has moved the second step, method 600 ends in 698 by waiting for the next pulse of the light source.

[0095]

[0104] In some embodiments, in response to determining 680 that the average wavelength error is less than the second threshold, method 600 may include moving the first actuator by a third step size. In some embodiments, the third step size may be proportional to the voltage applied to the second actuator, resetting the voltage applied to the second actuator. Thus, in some embodiments, the third step size may be based on the voltage applied to the second actuator rather than the average wavelength error.

[0096]

[0105] In some embodiments, at 690, method 600 may include determining whether the shot number of pulses is a multiple of the update interval. The shot number may be, for example, the number of pulses of the light beam. In some embodiments, the update interval may be, for example, every 5 or 10 pulses. It should be understood by those skilled in the art that these are merely exemplary update intervals, and other update intervals are also contemplated in accordance with aspects of the present disclosure. That is, in some embodiments, method 600 may include determining whether the pulse is, for example, the 5th or 10th pulse. In some embodiments, if the shot number is not equal to the update interval, method 600 ends at 698 by waiting for the next pulse of the light source.

[0097]

[0106] In some embodiments, if the number of shots equals the update interval, then method 600 may include updating the voltage applied to the second actuator at 695. For example, the voltage applied to the second actuator may be based on the average wavelength error, such that movement of prism 276b adjusts for the average wavelength error in subsequent pulses. In some embodiments, after updating the voltage applied to the second actuator, method 600 ends at 698 by waiting for the next pulse of the light source.

[0098]

[0107] In some embodiments, method 700 of FIG. 7 may be performed between pulses of the light source. During this time, the light source may transition between operating modes, for example, between monochromatic and bichromatic modes, and as a result, the center wavelength may change due to the change in operating conditions. To address this, as shown in FIG. 7, method 700 may also include detecting a change in the operating conditions of the light source at 710. In response to detecting the change in the operating conditions of the light source at 720, method 700 may include determining a center wavelength change. For example, determining the center wavelength change may include determining a midpoint of the target peak separation. At 730, method 700 may include moving the first actuator by a step size based on the center wavelength change. In some embodiments, the process described with respect to FIG. 7 may be performed between bursts of the light source. By doing so, method 700 provides for a reduced wavelength error the next time the light source is activated.

[0099]

[0108] In some embodiments, method 800 of FIG. 8 may be performed between pulses of the light source. During this time, the target peak separation may change. To address this, as shown in FIG. 8, method 800 may include detecting a change in peak separation at 810. In response to detecting the change in peak separation at 820, method 800 may also include determining a center wavelength change. For example, determining the center wavelength change may include determining an average value between a previous peak separation target and a new peak separation target. At 830, method 800 may include moving the first actuator by a step size based on the center wavelength change. In some embodiments, the processes described with respect to FIGS. 7 and 8 may be performed between bursts of the light source. By doing so, methods 700 and 800 provide for a smaller wavelength error the next time the light source is activated. Additionally, using the processes described in FIGS. 7 and 8, the present disclosure reduces the number of bursts required to complete a transition between different operating modes.

[0100]

[0109] 9 illustrates a method 900 for adjusting a central wavelength, which may be used, for example, for multi-focal imaging, according to an embodiment. It should be understood that not all steps in FIG. 9 are required to practice the disclosures set forth herein. Furthermore, some of the steps may be performed simultaneously, sequentially, and / or in a different order than shown in FIG. 9. Method 900 will be described with reference to FIGS. 1 through 4, although method 900 is not limited to these exemplary embodiments.

[0101]

[0110] In some embodiments, the process discussed with respect to Figure 9 provides for moving an actuator that controls the movement of prism 276b during a burst. That is, the process discussed with respect to Figure 9 provides an instar-burst solution for addressing changes in center wavelength, which may be from, for example, a first laser radiation beam at a first wavelength from a first laser chamber module in MFI mode, or a second laser radiation beam at a second wavelength generated using a second laser chamber module. To accomplish this, in some embodiments, the process described with respect to Figure 9 estimates the drift rate of the center wavelength to compensate for the measurement delay of the center wavelength.

[0102]

[0111] In some embodiments, a dither waveform (or sequence) may be combined with an offset for moving the actuator of prism 276b. For example, the dither waveform may be a noise application used to randomize quantization. The offset may be updated at the end of burst (EOB) and / or at a set pulse interval. In some embodiments, the EOB update may move the actuator of prism 276b to zero out an estimated center wavelength drift obtained by averaging wavelength measurements across the burst. In some embodiments, the interval update may be based on an estimation process described herein. In some embodiments, the estimation process described herein may be based on a rolling average estimate of the center wavelength up to the current pulse, and access may be provided to both an offset for the actuator of prism 276b and a second offset for the actuator of prism 276a. In other words, in some embodiments, the process for estimating the drift rate may be based on the current positions of prisms 276a, 276b, as well as the respective offsets of each actuator, the rolling average of the center wavelengths, and estimating the total accumulated center wavelength drift using a Kalman filter framework. In some embodiments, to compensate for the delay of the LAM 230, the two-shot-ahead drift may be predicted by converting the Kalman filter into a Kalman predictor, i.e., by using known inputs and disturbances, the drift rate may be estimated using open-loop propagation to predict the drift rate two steps ahead of the current burst.

[0103]

[0112] In some embodiments, the Kalman filter may be modeled using Equations 1 and 2. In some embodiments, at any given point, the center wavelength relative to the center wavelength target may be based on the sum of the positions of prisms 276a, 276b scaled by an appropriate gain and the accumulated wavelength drift D(k) at time k. In some embodiments, the accumulated wavelength drift may be modeled as a linear drift, where the rate at time k is defined as DSR(k), but the rate is unknown. As a result, the drift rate may change successfully over time and may be incorporated into the state vector, allowing for estimation of the drift rate.

[0104]

number

[0105]

[0113] With the model thus constructed, a steady-state Kalman filter may be implemented as in Equation 3, where A, B, C, and D are defined in Equation 1 and Equation 2, Q and R are tuning parameters, and S is the solution to the algebraic Riccati equation given in Equation 4.

[0106]

number

[0107]

[0114] In some embodiments, a controller, such as controller 290, may be provided with the total accumulated drift and estimated drift rate so that it can compensate for changes in the center wavelength.

[0108]

[0115] In some embodiments, the offset P3 offset may be defined as in Equation 5. By using known inputs and disturbances built into the model, the drift velocity may be estimated using open-loop propagation of the model two steps ahead.

[0109]

number

[0110]

[0116] Based on the above, the drift rate may be estimated based on real-time wavelength measurements. The drift rate may be used to predict the magnitude of wavelength drift and compensate on a shot-by-shot basis. In some embodiments, the drift rate may be modeled as an accumulator with a variable accumulation rate, and a Kalman filter may be used to estimate the accumulation rate based on an estimate of the center wavelength (e.g., the arithmetic mean of all wavelength measurements in the current burst). In some embodiments, to compensate for the measurement delay of LAM 230, the center wavelength N pulses, e.g., two pulses ahead, may be predicted and used to determine the offset applied to the actuator of prism 276b. For example, in some embodiments, N pulses may be two pulses, although it should be understood by those skilled in the art that this is merely an exemplary number of pulses and that more or fewer pulses are contemplated in accordance with aspects of the present disclosure. In some embodiments, this offset may be updated on a shot-by-shot basis with sub-femtometer resolution.

[0111]

[0117] At 910, method 900 may include combining the dither waveform with an offset value for actuating the prism. In some embodiments, the offset value may be used to move an actuator for controlling the movement of prism 276b. In some embodiments, the offset value is based on a direct current (DC) voltage applied to the actuator for controlling the movement of prism 276b. In some embodiments, the DC voltage has an initial value of zero volts.

[0112]

[0118] At 920, the method 900 may include generating an inter-pulse wavelength based on the dither waveform and the offset value. For example, the inter-pulse wavelength may be generated using the LAM 230. In some embodiments, the inter-pulse wavelength may also be based on other disturbances from within the lithographic apparatus LA.

[0113]

[0119] At 930, the method 900 may include generating a rolling average of the center wavelength based on the inter-pulse wavelengths of the plurality of pulses. In some embodiments, the inter-pulse wavelengths of the plurality of pulses include the wavelength of the current pulse.

[0114]

[0120] At 940, method 900 may include estimating a drift velocity to predict a center wavelength of a future pulse. In some embodiments, the offset value for moving an actuator associated with prism 276b may be a first offset value, and estimating the drift velocity may include estimating the drift velocity based on a rolling average value of the center wavelength, the first offset value, and a second offset value for moving a second actuator controlling the movement of second prism 276a. In some embodiments, estimating the drift velocity includes estimating the accumulated center wavelength drift velocity using a Kalman filter framework. For example, the Kalman filter framework may estimate the accumulated center wavelength drift velocity based on the rolling average value of the center wavelength, the first offset value, and the second offset value. Also, estimating the drift velocity may include predicting a center wavelength N pulses ahead of the current pulse, e.g., two pulses ahead. To accomplish this, the Kalman filter framework may be converted into a Kalman predictor for predicting a center wavelength N pulses ahead of the current pulse, e.g., two pulses ahead.

[0115]

[0121] At 950, the method 900 may include updating the offset value based on the estimated drift rate. In some embodiments, updating the offset value may also be based on a rolling average value of the center wavelength at the end of the burst in addition to the estimated drift rate.

[0116]

[0122] Exemplary Computer System

[0123] Various embodiments and components thereof may be implemented using one or more well-known computer systems, such as the exemplary embodiments, systems, and / or devices shown in the figures or otherwise discussed. Computer system 1000 may be any well-known computer capable of performing the functions described herein.

[0117]

[0124] Computer system 1000 includes one or more processors (also referred to as central processing units or CPUs), such as processor 1004. Processor 1004 is connected to a communication infrastructure or bus 1006.

[0118]

[0125] One or more of the processors 1004 may each be a graphics processing unit (GPU). In one embodiment, a GPU is a processor that is a specialized electronic circuit designed to process mathematically intensive applications. A GPU may have a parallel structure that is efficient for processing large blocks of data in parallel, such as mathematically intensive data common in computer graphics applications, images, videos, etc.

[0119]

[0126] The computer system 1000 also includes user input / output devices 1003 such as a monitor, keyboard, pointing device, etc. that communicate with a communications infrastructure 1006 via a user input / output interface 1002 .

[0120]

[0127] The computer system 1000 also includes a main or primary memory 1008, such as random access memory (RAM). The main memory 1008 may include one or more levels of cache. The main memory 1008 stores control logic (i.e., computer software) and / or data.

[0121]

[0128] The computer system 1000 may also include one or more secondary storage devices or memories 1010. The secondary memory 1010 may include, for example, a hard disk drive 1012 and / or a removable storage device or drive 1014. The removable storage drive 1014 may be a floppy disk drive, a magnetic tape drive, a compact disk drive, an optical storage device, a tape backup device, and / or any other storage device / drive.

[0122]

[0129] The removable storage drive 1014 may transfer information to and from a removable storage unit 1018. The removable storage unit 1018 includes a computer-usable or readable storage device that stores computer software (control logic) and / or data. The removable storage unit 1018 may be a floppy disk, magnetic tape, compact disk, DVD, optical storage disk, or any other computer data storage device. The removable storage drive 1014 reads from and / or writes to the removable storage unit 1018 in well-known manner.

[0123]

[0130] According to an exemplary embodiment, secondary memory 1010 may include other methods, means, or approaches for making computer programs and / or other instructions and / or data accessible to computer system 1000. Such methods, means, or approaches may include, for example, removable storage unit 1022 and interface 1020. Examples of removable storage unit 1022 and interface 1020 may include a program cartridge and cartridge interface (such as found in a video game device), a removable memory chip (such as an EPROM or PROM) and associated socket, a memory stick and USB port, a memory card and associated memory card slot, and / or any other removable storage unit and associated interface.

[0124]

[0131] Computer system 1000 may further comprise a communications or network interface 1024. Communications interface 1024 enables computer system 1000 to communicate with and exchange information with any combination of remote devices, remote networks, remote entities, etc. (individually and collectively referred to by reference numeral 1028). For example, communications interface 1024 may enable computer system 1000 to communicate with remote devices 1028 via communications path 1026, which may be wired and / or wireless and may include any combination of a LAN, a WAN, the Internet, etc. Control logic and / or data may be transferred to and from computer system 1000 via communications path 1026.

[0125]

[0132] In one embodiment, a non-transitory, tangible apparatus or article of manufacture having a non-transitory, tangible, computer-usable or readable medium having control logic (software) stored thereon is also referred to herein as a computer program product or program storage device, including, but not limited to, computer system 1000, main memory 1008, secondary memory 1010, removable storage units 1018 and 1022, and tangible articles of manufacture embodying any combination thereof. Such control logic, when executed by one or more data processing devices (e.g., computer system 1000), causes such data processing devices to perform the operations described herein.

[0126]

[0133] Based on the teachings contained in this disclosure, it will be apparent to one skilled in the art how to make and use embodiments of the present disclosure using data processing devices, computer systems and / or computer architectures other than those shown in Figure 10. In particular, embodiments may operate with software, hardware, and / or operating system implementations other than those described herein.

[0127]

[0134] Although particular reference has been made to the use of embodiments in the field of optical lithography, it should be understood that embodiments may also be used in other fields, depending on the context, for example in imprint lithography, and are not limited to optical lithography. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device is imprinted into a layer of resist supplied to the substrate and the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is then removed from the resist leaving a pattern in it when the resist is cured.

[0128]

[0135] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, and thus should be interpreted by one of ordinary skill in the art in light of the teachings herein.

[0129]

[0136] As used herein, the term "substrate" describes a material onto which a layer of material is added. In some embodiments, the substrate itself may be patterned, and the material added onto it may also be patterned or may remain unpatterned.

[0130]

[0137] The following examples are illustrative, but not limiting, of embodiments of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the art and obvious to those skilled in the art are within the spirit and scope of the present disclosure.

[0131]

[0138] Although specific reference may be made herein to the use of the apparatus and / or system in the manufacture of ICs, it should be explicitly understood that such an apparatus and / or system has many other possible applications, such as in integrated optical systems, guidance and detection patterns for magnetic domain memories, LCD panels, thin film magnetic heads, etc. In light of these alternative applications, those skilled in the art will recognize that any use of the terms "reticle," "wafer," or "die" herein may be considered synonymous with (or interchangeable with) the more general terms "mask," "substrate," and "target portion," respectively.

[0132]

[0139] While specific embodiments have been described above, it will be understood that embodiments may be practiced otherwise than as described, and this description is not intended to limit the scope of the claims.

[0133]

[0140] It is understood that the "Description of the Invention" section, and not the "Summary" and "Abstract" sections, are intended to be used to interpret the claims. The "Summary" and "Abstract" sections may describe one or more exemplary embodiments as envisioned by the inventors, but cannot describe all exemplary embodiments, and therefore are not intended to limit the scope of the embodiments and appended claims in any way.

[0134]

[0141] The embodiments have been described above using functional components and their relationships that illustrate implementation of specific functions. The boundaries of these functional components have been arbitrarily defined herein for the convenience of description. Alternative boundaries may be defined as long as the specific functions and their relationships are appropriately performed.

[0135]

[0142] The foregoing description of specific embodiments sufficiently reveals the general nature of the embodiments, such that those skilled in the art can readily modify and / or adapt such specific embodiments to various applications without undue experimentation and without departing from the general concept of the embodiments. Accordingly, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

[0136]

[0143] Other aspects of the invention are described in the following numbered clauses: 1. A method for controlling a central wavelength for imaging operation, comprising: estimating the central wavelength error; determining a first actuation amount of a first actuator that controls the movement of the first prism based on the estimated central wavelength error; actuating the first actuator based on a first actuation amount; determining whether the first prism is off-center; determining a second amount of actuation of the first actuator in response to determining that the first prism is off-center, and determining a third amount of actuation of the second actuator to control movement of the second prism; and The method includes actuating a first actuator and a second actuator based on second and third actuation amounts, respectively. 2. Estimating the central wavelength error calculating a first average value of the center wavelengths in the odd bursts and a second average value of the center wavelengths in the even bursts; and 10. The method of claim 1, including determining an average of the first and second average values, wherein the center wavelength error is based on the average of the first and second average values. 3. Determining a first actuation amount determining the difference between the target center wavelength and the estimated center wavelength; and 10. The method of claim 1, including determining a first actuation amount based on a difference between the target center wavelength and the estimated center wavelength. 4. The method of clause 3, wherein determining the difference between the target central wavelength and the estimated central wavelength includes determining the difference using a digital filter. 5. The method of clause 1, wherein determining the third actuation amount of the second actuator is based on the position of the first prism after actuating the first actuator based on the second actuation amount. 6. The method of clause 5, wherein determining the third actuation amount further comprises determining the third actuation amount to reduce the difference between the target center wavelength and the estimated wavelength. 7. The imaging operation includes a multi-focal imaging operation, and the method further includes operating the light source in a two-color mode, and operating the light source in the two-color mode includes: generating a first laser radiation beam at a first wavelength using a first laser chamber module; generating a second laser radiation beam at a second wavelength using a second laser chamber module; and combining the first and second laser emissions along a common output beam path using a beam combiner; 10. The method of claim 1, wherein estimating the central wavelength error includes estimating a central wavelength error of the first laser radiation beam. 8. A method for controlling a central wavelength, comprising: determining a wavelength error of a light beam generated by the light source; determining whether the wavelength error is greater than a first threshold; moving a first actuator controlling movement of the first prism by a first step size in response to determining that the wavelength error is greater than a first threshold; In response to determining that the wavelength error is less than the first threshold, determining the mean wavelength error; determining whether the average wavelength error is greater than a second threshold different from the first threshold; In response to determining that the average wavelength error is greater than a second threshold, moving the first actuator by a second step size and enabling the low pass filter; and The method includes, in response to determining that the average wavelength error is less than a second threshold, enabling the low pass filter, updating a voltage applied to a second actuator controlling movement of the second prism, and moving the first actuator by a third step size. 9. Determining wavelength error measuring the central wavelength of a light beam generated by the light source; and 9. The method of clause 8, comprising determining a difference between the center wavelength and the target center wavelength. 10. Determining whether the number of pulses of the light source is a multiple of the update interval; and 9. The method of clause 8, further comprising updating a voltage applied to the second actuator in response to determining that the number of shots is equal to the update interval. 11. The method of clause 8, further comprising disabling the low pass filter and movement of the second actuator in response to determining that the wavelength error is greater than the first threshold. 12. The method of clause 8, wherein the first step size is a fixed step size of the actuator. 13. The method of clause 8, wherein the second step size is a function of wavelength error. 14. The method of clause 8, wherein the third step size is a function of a voltage applied to the second actuator. 15. The method of clause 8, wherein moving the first actuator by a second step size includes moving the first actuator every n pulses, where n is greater than 1. 16. The method of clause 8, wherein the average wavelength error is based on the wavelength error and an average value of multiple wavelength errors over a number of pulses. 17. A method includes controlling a central wavelength in a multi-focal imaging operation, the method further includes operating the light source in a two-color mode, and operating the light source in the two-color mode includes: generating a first laser radiation beam at a first wavelength using a first laser chamber module; generating a second laser radiation beam at a second wavelength using a second laser chamber module; and combining the first and second laser emissions along a common output beam path using a beam combiner; 9. The method of clause 8, wherein determining a wavelength error of the light beam generated by the light source includes determining a central wavelength error of the first laser radiation beam. 18. A method for controlling a central wavelength for multi-focal imaging operation, comprising: combining the dither waveform with an offset value for moving an actuator that controls the movement of the prism; generating an inter-pulse wavelength based on a dither waveform and an offset value; generating a rolling average of the center wavelength based on inter-pulse wavelengths of the plurality of pulses; Estimating the drift velocity to predict the central wavelength of future pulses; and The method includes updating the offset value based on the estimated drift rate. 19. The method of clause 18, wherein the offset value is based on a direct current (DC) voltage. 20. The method of clause 19, wherein the initial value of the DC voltage is zero volts. 21. The offset value includes a first offset value; 19. The method of clause 18, wherein estimating the drift velocity includes estimating the drift velocity based on a rolling average value of the center wavelength, a first offset value, and a second offset value that moves a second actuator that controls movement of the second prism. 22. The method of clause 21, wherein estimating the drift rate includes estimating the accumulated center wavelength drift rate using a Kalman filter framework. 23. The method of clause 22, wherein estimating the drift velocity includes predicting the center wavelength N pulses ahead of the current pulse. 24. The method of clause 23, wherein estimating the drift velocity includes converting a Kalman filter framework into a Kalman predictor to predict the center wavelength N pulses ahead of the current pulse. 25. The method of clause 18, wherein the interpulse wavelength of the plurality of pulses includes the wavelength of the current pulse. 26. The method of clause 18, wherein updating the offset value further comprises updating the offset value based on a rolling average value of the center wavelength at the end of the burst. 27. A first actuator that controls the movement of the first prism; a second actuator for controlling the movement of the second prism; Estimate the central wavelength error; determining a first actuation amount of the first actuator based on the estimated central wavelength error; activating the first actuator based on a first actuation amount; determining whether the first prism is off-center; determining a second amount of actuation of the first actuator and a third amount of actuation of the second actuator in response to determining that the first prism is off-center; a controller that operates the first and second actuators based on the second and third actuation amounts, respectively. 28. To estimate the central wavelength error, the controller further comprises: calculating a first average value of the center wavelengths of the odd bursts and a second average value of the center wavelengths of the even bursts; 28. The system of clause 27, wherein an average value of the first and second average values ​​is determined, and the central wavelength error is based on the average value of the first and second average values. 29. To determine the first actuation amount, the controller further determining a difference between the target central wavelength and the estimated central wavelength; 28. The system of clause 27, wherein the first actuation amount is determined based on a difference between the target center wavelength and the estimated center wavelength. 30. The system of clause 29, wherein the controller further determines the difference between the target center wavelength and the estimated center wavelength using a digital filter. 31. The system of clause 27, wherein the third actuation amount of the second actuator is based on a position of the first prism after actuating the first actuator based on the second actuation amount. 32. The system of clause 31, wherein the controller further determines the third actuation amount so as to reduce the difference between the target center wavelength and the estimated wavelength to determine the third actuation amount. 33. The imaging operation includes a multi-focal imaging operation; the system further comprising a light source operating in a two-color mode; The controller also generating a first beam of laser radiation at a first wavelength using a first laser chamber module; generating a second beam of laser radiation at a second wavelength using a second laser chamber module; operating the light source in a two-color mode by combining the first and second laser emissions along a common output beam path using a beam combiner; 28. The system of clause 27, wherein estimating the central wavelength error includes estimating a central wavelength error of the first laser radiation beam. 34. A light source for generating a light beam; a first actuator that controls the movement of the first prism; a second actuator for controlling the movement of the second prism; determining a wavelength error of a light beam generated by the light source; determining whether the wavelength error is greater than a first threshold; moving the first actuator by a first step size in response to determining that the wavelength error is greater than a first threshold; In response to determining that the wavelength error is less than the first threshold, Determine the mean wavelength error; determining whether the average wavelength error is greater than a second threshold different from the first threshold; a controller that, in response to determining that the average wavelength error is greater than a second threshold, moves the first actuator by a second step size and enables a low pass filter, and, in response to determining that the average wavelength error is less than the second threshold, enables the low pass filter, updates a voltage applied to the second actuator, and moves the first actuator by a third step size. 35. To determine the wavelength error, the controller further comprises: measuring the central wavelength of a light beam generated by the light source; The system of clause 34, determining the difference between the central wavelength and the target central wavelength. 36. The controller further determining whether the number of pulses of the light source is a multiple of the update interval; 35. The system of clause 34, wherein in response to determining that the number of shots is equal to the update interval, the system updates the voltage applied to the second actuator. 37. The system of clause 34, wherein the controller further disables movement of the low pass filter and the second actuator in response to determining that the wavelength error is greater than the first threshold. 38. The system of clause 34, wherein the first step size is a fixed step size of the actuator. 39. The system of clause 34, wherein the second step size is a function of wavelength error. 40. The system of clause 34, wherein the third step size is a function of the voltage applied to the second actuator. 41. The system of clause 34, wherein the controller further moves the first actuator every n pulses (n greater than 1) to move the first actuator by the second step size. 42. The system of clause 34, wherein the average wavelength error is based on the wavelength error and an average value of multiple wavelength errors over a number of pulses. 43. The system performs multi-focal imaging operations; The controller also generating a first beam of laser radiation at a first wavelength using a first laser chamber module; generating a second beam of laser radiation at a second wavelength using a second laser chamber module; operating the light source in a two-color mode by combining the first and second laser emissions along a common output beam path using a beam combiner; 35. The system of clause 34, wherein determining a wavelength error of the light beam generated by the light source includes determining a central wavelength error of the first laser radiation beam. 44. A system for controlling central wavelength for multi-focal imaging operation, comprising: an actuator for controlling the movement of the prism; The dither waveform is combined with an offset value to move the actuator. generating an interpulse wavelength based on the dither waveform and the offset value; generating a rolling average of the central wavelength based on the inter-pulse wavelengths of the plurality of pulses; Estimating the drift velocity to predict the central wavelength of future pulses; and a controller that updates the offset value based on the estimated drift rate. 45. A system of clause 44 in which the offset value is based on direct current (DC) voltage. 46. ​​The system of clause 45, in which the initial value of the DC voltage is zero volts. 47. The offset value includes a first offset value; 45. The system of clause 44, wherein estimating the drift velocity includes estimating the drift velocity based on a rolling average value of the center wavelength, a first offset value, and a second offset value that moves a second actuator that controls movement of the second prism. 48. The system of clause 47, wherein the controller further estimates the accumulated center wavelength drift rate using a Kalman filter framework to estimate the drift rate. 49. The system of clause 48, wherein the controller further predicts the center wavelength N pulses ahead of the current pulse to estimate the drift velocity. 50. The system of clause 49, wherein the controller further converts the Kalman filter framework into a Kalman predictor to predict the center wavelength N pulses ahead of the current pulse to estimate the drift velocity. 51. The system of clause 44, wherein the interpulse wavelength of the plurality of pulses includes the wavelength of the current pulse. 52. The system of clause 44, wherein to update the offset value, the controller further updates the offset value based on a rolling average value of the center wavelength at the end of the burst.

[0137]

[0144] The breadth and scope of the embodiments should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. 1. A method for central wavelength control in an optical resonator, comprising: estimating a first wavelength error of the light beam; determining whether the wavelength error is greater than a first threshold; performing a first adjustment of a first prism with a first actuator in response to determining that the first wavelength error is greater than the first threshold; and in response to determining that the first wavelength error is not greater than the first threshold value; estimating a second wavelength error of the light beam; determining whether the second wavelength error is greater than a second threshold; performing a second adjustment of the first prism with the first actuator in response to determining that the second wavelength error is greater than the second threshold; and In response to determining that the second wavelength error is not greater than the second threshold, determining whether a shot number of pulses of the light beam is a multiple of an update interval.

2. estimating the first wavelength error measuring a central wavelength of the light beam; and 2. The method of claim 1, further comprising determining a difference between the measured center wavelength and a target center wavelength. method.

3. 10. The method of claim 1, further comprising disabling a filter and disabling a second actuator for controlling a second prism in response to determining that the first wavelength error is greater than the first threshold.

4. 10. The method of claim 1, further comprising enabling a filter and disabling a second actuator for controlling a second prism in response to determining that the second wavelength error is greater than the second threshold.

5. 2. The method of claim 1, further comprising: performing a third adjustment to move the first prism in response to determining that the second wavelength error is not greater than the second threshold, wherein a step size of the movement of the first prism by the third adjustment is proportional to a voltage applied to a second actuator.

6. The method of claim 1 , wherein the first adjustment is a fixed step size of the actuator.

7. The method of claim 1 , wherein the second adjustment is based on a function of the second wavelength error.

8. The method of claim 1 , wherein the second wavelength error is based on the first wavelength error and an average value of a plurality of wavelength errors over a number of pulses.

9. and performing the central wavelength control in a multi-focal imaging operation performed in a dichromatic mode, the dichromatic mode comprising: generating a first beam of laser radiation at a first wavelength using a first laser chamber module; generating a second laser radiation beam at a second wavelength using a second laser chamber module; and combining the first and second laser radiations along a common output beam path using a beam combiner; The method of claim 1 , wherein estimating the first wavelength error of the light beam comprises determining a central wavelength error of the first laser radiation beam.

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