Reducing speckle in excimer light sources

Abstract

A method of reducing speckle in an excimer light source comprises: generating a light beam consisting of pulses having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length, and each pulse being defined by a pulse duration; for one or more pulses, modulating an optical phase over the pulse duration of the pulse to generate a modified pulse having a second temporal coherence defined by a second temporal coherence length, the second temporal coherence length being less than the first temporal coherence length of the pulse; forming a pulsed light beam from at least the modified pulses; and directing the formed pulsed light beam toward a substrate within a lithography exposure apparatus.

Description

[0001] This application is a divisional application of a patent application with an international application date of December 4, 2017, an international application number of PCT / US2017 / 064556, a national application number of 201780083474.9, and an invention name of “REDUCING SPECTREL IN EXCIMER LIGHT SOURCE”.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Application No. 15 / 407,153, filed on January 16, 2017, which is incorporated herein by reference in its entirety. Technical Field

[0004] The disclosed subject matter relates to apparatus and methods for reducing speckle in excimer light sources, such as deep ultraviolet (DUV) light sources used in semiconductor lithography. Background Art

[0005] In semiconductor lithography (or photolithography), the manufacture of integrated circuits (ICs) includes performing various physical and chemical processes on a semiconductor (e.g., silicon) substrate (which is also referred to as a wafer). A photolithography exposure device or scanner is a machine that applies a desired pattern to a target portion of a substrate. The wafer is irradiated by a beam extending in an axial direction, and the wafer is fixed to a stage so that the wafer generally extends along a transverse plane that is substantially orthogonal to the axial direction. The beam has a wavelength in the deep ultraviolet (DUV) range, for example, from about 10 nanometers (nm) to about 400 nm. Summary of the invention

[0006] In some general aspects, a method includes generating a beam of pulses having wavelengths in a deep ultraviolet range, wherein each pulse has a first temporal coherence defined by a first temporal coherence length, and each pulse is defined by a pulse duration. For one or more pulses, an optical phase is modulated over the pulse duration of the pulse to generate a modified pulse having a second temporal coherence defined by a second temporal coherence length, the second temporal coherence length being less than the first temporal coherence length of the pulse. The method includes forming a pulsed beam from at least the modified pulses; and directing the formed pulsed beam toward a substrate within a lithographic exposure apparatus.

[0007] Embodiments may include one or more of the following features. For example, a beam of pulses may be generated by generating a seed beam of pulses; and optically amplifying the pulses of the seed beam by repeatedly passing the pulses of the seed beam through a resonator to generate a beam of amplified pulses. By modulating the optical phase on the pulse duration of the amplified pulses, the optical phase may be modulated on the pulse duration of the pulses to generate the modified pulses. By modulating the optical phase on the pulse duration of the pulses of the seed beam, the optical phase may be modulated on the pulse duration of the pulses to generate modified pulses; and the beam of amplified pulses may be generated by optically amplifying the modified pulses. By directing the beam of amplified pulses toward the substrate, the pulsed beam formed by the modified pulses may be directed toward the substrate.

[0008] The method may further comprise reducing the bandwidth of the pulses of the optical beam prior to modulating the optical phase over the pulse duration of the pulses to produce the modified pulses. The optical phase may be modulated over the pulse duration of the pulses to cause the bandwidth of the pulses to increase but remain within a target bandwidth range.

[0009] The optical phase may be modulated over the pulse duration of the pulse by convolving the spectrum of the electric field of the pulse with a Fourier transform associated with modulating the optical phase over the pulse duration of the pulse. The optical phase may be modulated over the pulse duration of the pulse to reduce dynamic speckle contrast of the pulsed light beam directed toward the substrate.

[0010] The method may further include increasing the duration of the pulses in the light beam directed toward the substrate. The duration of the pulses in the light beam may be increased by splitting the amplitude of each pulse of the light beam into split parts, introducing a time delay between the split parts to produce time-delayed parts of the pulses, and recombining the time-delayed parts of the pulses to provide a time-stretched pulse of the light beam. The optical phase may be modulated over the pulse duration of a pulse by modulating the optical phase over the pulse duration of one or more split parts of the pulse.

[0011] The optical phase may be modulated over the pulse duration of the pulses to cause an increase in the pulse bandwidth of the optical beam.

[0012] The method may further comprise selecting a frequency range over which the optical phase is modulated over the pulse duration of the pulse.

[0013] The frequency range may be selected by determining a target frequency range that will produce a target bandwidth for the modified pulse; and maintaining the frequency range within the determined target frequency range to maintain the bandwidth of the modified pulse within the target bandwidth. The method may include measuring the bandwidth of the pulse before modulating the optical phase on the pulse duration of the pulse to determine whether the modified pulse will have a bandwidth within the target bandwidth. The method may include measuring the bandwidth of the modified pulse before modulating the optical phase on the pulse duration of a next pulse to determine whether the modified pulse has a bandwidth within the target bandwidth. The method may include calculating a target bandwidth for a particular next pulse based on the measured bandwidths of a plurality of previously modified pulses.

[0014] The frequency range may be selected by selecting a frequency range at which the optical phase is modulated over the pulse duration of the pulse for each pulse in the optical beam.

[0015] By modulating the refractive index of the material through which the pulse is guided, the optical phase can be modulated over the pulse duration of the pulse.

[0016] The method may include adjusting the bandwidth of pulses directed toward the substrate by adjusting the rate at which the optical phase is modulated.

[0017] The method may include: estimating the bandwidth of the modified pulse; determining whether the estimated bandwidth is within a target bandwidth; and if it is determined that the estimated bandwidth is outside the target bandwidth, adjusting the frequency range over which the optical phase is modulated, thereby adjusting the bandwidth of the next modified pulse.

[0018] By randomizing the optical phase over the pulse duration of the pulses, the optical phase may be modulated over the pulse duration of the pulses.

[0019] An optical pulse can be associated with a waveform, the waveform being represented by points in time, and by applying different time delays to different points of the waveform, the optical phase can be modulated over the pulse duration of the pulse. Different time delays can be applied to different points of the waveform by passing the optical pulse through a medium and changing the refractive index of the medium as the pulse passes through the medium.

[0020] The amplitude of the modulation may vary randomly over the pulse duration of the pulses.The amplitude of the modulation may vary in a manner to reduce one or more of dynamic speckle and bandwidth of the pulsed light beam directed towards the substrate.

[0021] Each pulse may have a first spatial coherence defined by a first spatial coherence length, and the method may include reducing the spatial coherence of the pulse when the pulse is modulated so that the modified pulse has a second spatial coherence defined by a second spatial coherence length, which is less than the first spatial coherence length of the pulse.

[0022] In other general aspects, an apparatus includes a light source, a phase modulator system, a measurement device, and a control system. The light source is configured to generate a light beam consisting of pulses having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length, and each pulse is defined by a pulse duration. The phase modulator system is in the path of the pulsed light beam and is configured to, for at least one pulse, modulate an optical phase over the pulse duration of the pulse to generate a modified pulse having a second temporal coherence defined by a second temporal coherence length, the second temporal coherence length being less than the first temporal coherence length. The measurement device is configured to measure a characteristic of a test pulse. The test pulse is a pulse having the first temporal coherence or a modified pulse having the second temporal coherence. The control system communicates with the measurement device and the phase modulator system. The control system is configured to: receive measurement characteristics of the test pulse from the measurement device; determine whether the bandwidth of the modified pulse is within a target bandwidth based on the received measurement characteristics; and, if it is determined that the bandwidth of the modified pulse is outside the range of the target bandwidth, adjust the frequency of modulating the optical phase on the pulse duration of the pulse that produces the modified pulse.

[0023] Embodiments may include one or more of the following features. For example, the apparatus may include a beam guiding device in the path of a pulsed beam formed by the modified pulses. The beam guiding device may be configured to guide the pulsed beam formed by the modified pulses toward a substrate within a lithographic exposure apparatus. The phase modulator system may be within the lithographic exposure apparatus. The phase modulator system may include a two-dimensional array of phase modulators. The two-dimensional array of phase modulators may be located within a beam homogenizer. The two-dimensional array of phase modulators may also be configured to reduce the spatial coherence of the pulses for each pulse so that the modified pulses have a second spatial coherence that is less than the spatial coherence of the pulses.

[0024] The light source may include: a first-stage light source, the first-stage light source is configured to generate a seed beam consisting of pulses and includes a spectral tuning device for tuning one or more spectral features of the seed beam, and a second-stage optical amplifier having a resonator with a gain medium, the optical amplifier is configured to receive the pulses of the seed beam and generate a beam consisting of amplified pulses. The first-stage light source may be configured to generate a seed beam consisting of pulses, including a solid-state gain medium. The phase modulator system may be between the first-stage light source and the second-stage optical amplifier.

[0025] The test pulse may be a modified pulse.

[0026] The control system may be in communication with the light source, and the control system may be configured to send a signal to the light source to adjust the bandwidth of the pulse if it is determined that the bandwidth of the modified pulse is outside the target bandwidth range.

[0027] The characteristic of the test pulse measured by the measurement device may be the bandwidth of the test pulse.

[0028] The apparatus may comprise an optical temporal pulse stretcher configured to increase the duration of the modified pulse.The optical temporal pulse stretcher may be a passive optical element.

[0029] The phase modulator system may comprise a Pockels cell comprising a medium through which the pulsed light beam passes. By modulating the refractive index of the medium of the Pockels cell, the optical phase may be modulated over the pulse duration of the pulse.

[0030] The phase modulator system may include a single phase modulator or a plurality of phase modulators. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a block diagram of a photolithography system that generates a pulsed light beam directed toward a photolithography exposure device;

[0032] Figure 2 Is Figure 1 A graph of an exemplary spectrum of a pulsed light beam generated by a photolithography system;

[0033] Figure 3 is a graph of an exemplary spectrum of a pulsed beam, wherein the spectrum includes unwanted structures;

[0034] Figure 4 It is for Figure 3 A graph of exemplary interference visibility of a spectrum of;

[0035] Figure 5 yes Figure 1 A block diagram of an exemplary phase modulator system;

[0036] Figure 6 is from Figure 5 a graph of an exemplary spectrum of a modified pulsed light beam output by a phase modulator system;

[0037] Figure 7 It is for Figure 6 A graph of exemplary interference visibility of a spectrum of;

[0038] Fig. 8A is a graph of an exemplary phase modulation applied to a pulse of a pulsed light beam as a function of time;

[0039] Figure 8B yes Fig. 8A A graph of the frequency content of an exemplary phase modulation;

[0040] Fig. 9 is a block diagram of an exemplary phase modulator system including one or more phase modulators;

[0041] Fig.10 is a block diagram of an exemplary phase modulator system in which one or more phase modulators are optically arranged in series;

[0042] Fig.11 is a block diagram of an exemplary phase modulator system in which one or more phase modulators are optically arranged in parallel;

[0043] Fig.12 is a block diagram of an exemplary phase modulator of a phase modulator system, wherein the exemplary phase modulator is designed as a Pockels cell;

[0044] Fig.13A is a block diagram of an exemplary photolithography system in which a phase modulator system is placed within a light source;

[0045] Fig. 13B is a block diagram of an exemplary light source into which a phase modulator system is placed;

[0046] Fig.14 is a block diagram of an exemplary photolithography system in which a phase modulator system is positioned within a beam preparation system between a light source and a photolithography exposure apparatus;

[0047] Fig.15A is a block diagram of an exemplary photolithography system in which a phase modulator system is placed within an optical temporal pulse stretcher of a beam preparation system between a light source and a photolithography exposure apparatus;

[0048] Fig. 15Bis a block diagram of an exemplary optical temporal pulse stretcher in which a phase modulator system is placed;

[0049] Fig.16A is a block diagram of an exemplary photolithography system in which a phase modulator system is placed within an optical arrangement of a photolithography exposure apparatus;

[0050] Fig. 16B is a block diagram of an exemplary optical arrangement in which a phase modulator system is placed;

[0051] Fig.17A is a block diagram of an exemplary photolithography exposure apparatus;

[0052] Fig. 17B yes Fig.17A A block diagram of an exemplary illumination module within a photolithography exposure apparatus;

[0053] Fig.18 yes Figure 1 A block diagram of an exemplary spectral signature measurement system within a photolithography system;

[0054] Fig.19 yes Figure 1 A block diagram of an exemplary light source within a photolithography system;

[0055] Fig. 20 yes Figure 1 A block diagram of an exemplary spectral feature selection system within a photolithography system;

[0056] Fig.21 yes Figure 1 A block diagram of an exemplary control system within a photolithography system;

[0057] Fig. 22 is a flow chart of an exemplary process performed by a photolithography system for reducing the temporal coherence of a beam pulse; and

[0058] Fig.23 is a flow chart of an exemplary process performed by a photolithography system for selecting a frequency range within which the optical phase over the pulse duration of a pulse is modulated to reduce the temporal coherence of the pulse. DETAILED DESCRIPTION

[0059] Reference Figure 1, the photolithography system 100 includes a light source 105 (such as an excimer light source) that generates a pulsed light beam 110 under the control of a control system 185. The pulses of the light beam 110 have a wavelength that is nominally at a center wavelength in the deep ultraviolet (DUV) range, for example, between about 10 nanometers (nm) and about 400nm. In some embodiments, the wavelength of the pulse is about 248nm, while in other embodiments, the wavelength of the pulse is about 193nm. The light source 105 emits light in the form of light pulses rather than continuous waves. Therefore, the light source 105 emits energy pulses with short durations. These periodic pulses can be considered as pulse trains and form a light beam 110. The duration of a pulse (also referred to as a pulse width or length) can be defined as the time during which the pulse power continues to remain above a certain percentage (e.g., half) of its maximum value.

[0060] The light beam 110 is directed to a photolithography exposure device (or scanner) 115 by a beam preparation system 112, which may include beam guidance and beam modification optics. Specifically, within the photolithography exposure device 115, the pulsed light beam 110 is directed through an optical arrangement 113, which is configured to prepare and modify the light beam 110 as needed before directing the light beam 110 toward a semiconductor substrate (wafer) 120 on a wafer stage 121. Under the control of a lithography controller 140, the light beam 110 and the wafer 120 are scanned (moved) relative to each other, thereby patterning microelectronic features on the wafer 120. The size 120 of the microelectronic features patterned on the wafer depends on the wavelength of the light beam 110, and a lower wavelength results in a smaller minimum size of the microelectronic features. When the wavelength of the light beam 110 is 248nm or 193nm, the minimum size of the microelectronic features can be, for example, 50nm or less. The focal position of the pulse of the light beam 110 at the wafer 120 is related to the wavelength of the light beam 110. Additionally, the bandwidth of the beam 110 can affect the critical dimension (CD) of these features.

[0061] Various disturbances (such as, for example, temperature gradients, pressure gradients, optical distortions) act on the light source 105 and the light beam 110 to modify characteristics such as spectral properties or characteristics (such as bandwidth and wavelength) or energy of the light beam 110. For example, chromatic aberration caused by optical components that interact with the light beam 110 can result in an increase in the bandwidth of the light beam 110. Therefore, the photolithography system 100 includes other components, such as, for example, a spectral feature selection system 130 (which is configured to adjust one or more spectral features of the light beam 110) and a metrology system 170 (which is configured to measure one or more characteristics of the light beam 110). Together with the control system 185, these components are used in combination to determine the effects of disturbances on the light beam 110 and to correct for the effects of such disturbances on the light beam 110.

[0062] Due to the design of light source 105, the pulses of light beam 110 have coherence - both temporal and spatial coherence, and this coherence (either or both temporal and spatial) can result in a speckle pattern at wafer 120. The speckle pattern is an intensity pattern resulting from the mutual interference of a collection of wavefronts of light beam 110, and the interference is higher with higher levels of coherence (both temporal and spatial). The speckle patterns generated at wafer 120 are undesirable because they result in a reduction in CD uniformity (CDU) and, therefore, result in blurring of microelectronic features produced on wafer 120.

[0063] Static speckle is caused by spatial coherence in the beam 110, and dynamic speckle is caused by temporal coherence in the beam 110. The photolithography system 100 includes a phase modulator system 145 placed in the path of the beam. The phase modulator system 145 is configured to, for each pulse, generate a modified pulse having a temporal coherence that is less than the temporal coherence of the pulse entering the phase modulator system 145, thereby reducing dynamic speckle induced at the wafer. The phase modulator system 145 can be placed in the path of the beam 110 output from the light source 105, or it can be placed in the path of a beam such as a seed beam generated within the light source 105, as discussed below. A pulsed beam 110 is formed from these modified pulses (which have reduced temporal coherence); the formed pulsed beam 110 is directed toward a wafer 120 within the photolithography exposure apparatus 115.

[0064] refer to Figure 2, each pulse of the light beam 110 exhibits a spectrum 200. The spectrum 200 may be referred to as the spectral shape or spectrum of the light beam 110. The spectrum 200 contains information about how the optical power or optical energy of the light beam 110 is distributed at different wavelengths (or frequencies) 202. The spectrum 200 of the light beam 110 is depicted in the form of a graph, where the spectral intensity 201 is plotted as a function of the wavelength or optical frequency 202. The spectral properties or characteristics of the light beam 110 include any aspect or representation of the spectrum 200. For example, bandwidth is a spectral feature. The bandwidth of the light beam 110 is a measure of the width W of the spectrum 200, and the width W can be given according to the wavelength or frequency of the laser. Any suitable mathematical structure (e.g., metric) related to the details of the spectrum 200 can be used to estimate the value of the bandwidth that characterizes the light beam. For example, the full width (referred to as FWXM) of the spectrum 200 at a fraction (X) of the maximum peak intensity of the spectral shape can be used to characterize the beam bandwidth. As an example, in a commonly used spectral shape characterization, the fraction X is 50%, and the corresponding metric is called the full width at half maximum (FWHM). As another example, the width of the spectrum 200 (referred to as EY) containing the fraction (Y) of the integrated spectral intensity can be used to characterize the beam bandwidth. In one example, in common use for characterizing the spectral properties of the beam 110, the fraction Y is 95%.

[0065] The pulses of the optical beam 110 are temporally coherent because longitudinal cavity modes are generated by the geometry and configuration of the one or more resonators within the light source 105, and these longitudinal cavity modes interfere with each other and produce unwanted structure in the spectrum of each pulse. Figure 3 An exemplary spectrum 300 with unwanted structures is schematically depicted in FIG. Moreover, each pulse of the light beam 110 may produce a different unwanted structure. The pulse bandwidth of the light beam 110 may be measured (e.g., using the metrology system 170) by averaging the number of pulses used by the photolithography exposure device 115 to illuminate a single location on the wafer 120. The average value of the bandwidth is the effective bandwidth, and it may be a smooth function of the wavelength.

[0066] The amount of temporal coherence of a pulse is a measure of the average correlation between the value of the electric field waveform of the pulse at any pair of times and itself delayed by an amount τ. The delay at which the phase of the electric field waveform drifts by a significant amount (and therefore the correlation decreases by a significant amount) is defined as the coherence time T C When the delay τ is 0, the coherence is perfect, but as the delay τ approaches T C , the degree of coherence drops significantly. Temporal coherence can be measured in an interferometer such as a Michelson interferometer, where the electric field waveform of a pulse is combined with a copy of itself delayed by a time τ. A detector measures the time-averaged intensity of the light leaving the interferometer, and the resulting interference visibility gives the temporal coherence at the delay τ. Figure 4 The Figure 3 300. The interference visibility is calculated as the Fourier transform of the normalized power spectral density of the pulse, and the coherence time T C (from which the coherence length can be calculated) is calculated as

[0067] T c =∫|visibility| 2 dτ.

[0068] From the coherence time T C Calculate the coherence length L C Specifically, the coherence length L C is the pulse wave at the coherence time T C The distance traveled. Figure 3 and Figure 4 In the example, the coherence length L C is unacceptably high. The spectrum 300 ( Figure 3 ) leads to sidebands in visibility ( Figure 4 ), and thus leads to a larger temporal coherence length L than would be expected based on the average slit bandwidth. C By passing the pulse through the pulse modulator system 145, the coherence length L of the pulse C Reduced to acceptable levels, as referenced Figure 5-Figure 7 discussed.

[0069] refer to Figure 5 In particular, the pulse 505 entering the phase modulator system 145 has a first temporal coherence length L C1 For example, pulse 505 entering phase modulator system 145 may be a pulse that exhibits an unwanted structure in spectrum 300 (and also has a temporal coherence length L greater than the desired C , such as Figure 4 ). The phase modulator system 145 reduces the temporal coherence of the pulse 505 by modulating the optical phase over the pulse duration of the pulse 505 to produce a pulse having a second temporal coherence length L C2 The second temporal coherence of the modified pulse 510 is defined as the second temporal coherence length L C2 Less than the first temporal coherence length L C1 The electric field of the modified pulse 510 can be described as the electric field of the incoming pulse 505 multiplied by the phase rotation or delay introduced by the phase modulator system 145. The spectrum of the electric field of the modified pulse 510 is convolved or averaged by the Fourier transform of the phase modulation provided by the phase modulator system 145.

[0070] The second temporal coherence length L C2 It can be the first time coherence length L C1 For example, the second temporal coherence length L C2 The first time coherence length L C1 Such a reduction in temporal coherence length may result in an overall reduction in speckle at the wafer 120 of between 5-30%, for example, about 18%. As discussed in more detail below, the amount of reduction in temporal coherence is limited by other characteristics of the pulses that are affected by performing phase modulation on the pulses, such as bandwidth.

[0071] Figure 6 An exemplary modified spectrum 600 of the modified pulse 510 is shown in FIG. Figure 7 The interference visibility 700 of the modified spectrum 600 is shown in FIG. The structure present in the spectrum 300 is mostly removed or greatly reduced in the modified spectrum 600. In addition, the coherence length L relative to the incoming pulse 505 is C , the temporal coherence length L of the modified spectrum 600 (and the modified pulse 510) C Specifically, based on the temporal coherence time T C Calculate the temporal coherence length L of the modified spectrum 600 C , the time coherence time T C is based on Figure 7 The interference visibility 700 shown in FIG.

[0072] In general, the temporal coherence length of a pulse is inversely proportional to the instantaneous bandwidth of the pulse. Thus, in general, if the temporal coherence length is reduced (by performing the phase modulation discussed herein), the instantaneous bandwidth of the pulse 510 at the output of the phase modulator system 145 increases relative to the instantaneous bandwidth of the pulse 505 at the input of the phase modulator system 145. However, as discussed herein, the overall slot average bandwidth (the bandwidth seen by the wafer 120) remains constant or within a range of acceptable bandwidths for two reasons. First, the bandwidth of the incoming pulse 505 is reduced to offset some of the shift toward the instantaneous bandwidth of the outgoing pulse 510. Second, the wafer 120 experiences the slot average bandwidth, which is not affected by the shift in instantaneous bandwidth on a pulse-to-pulse basis because the shift value is averaged over the slots.

[0073] refer to Fig. 8A , an exemplary phase modulation 800 is shown. Fig. 8A, the phase modulation shows the amplitude of the phase applied to the pulse 505 as a function of time. The frequency of the phase modulation is the rate at which the optical phase of the pulse is modified or delayed. Thus, the frequency f can be considered as 1 / Δt, where Δt is the time between the peaks of the phase modulation. In practice, a range of frequencies are applied to the phase modulator system 145. Figure 8B The frequency content of the phase modulation 800 applied to the pulse 505 is shown. Specifically, Figure 8B can be considered as the Fourier transform of the phase modulation 800. In this example, as shown, a number of frequencies are applied and the limit value of the frequency is 1 in arbitrary units (e.g., 1 GHz). The limit value of the frequency (which is the maximum allowed frequency allowed) can be changed by filtering in order to change the frequency content of the phase modulation 800. As the maximum allowed frequency of the phase modulation increases and therefore a larger frequency range is applied, the bandwidth of the modified pulse 510 increases. Therefore, the maximum allowed frequency of the phase modulation is selected (e.g., by signal filtering) so as not to increase the bandwidth of the modified pulse 510 beyond the target bandwidth.

[0074] refer to Fig. 9 , the exemplary phase modulator system 945 includes one or more phase modulators 945A, 945B, 945C, etc. In embodiments where the phase modulator system 945 includes a plurality of phase modulators 945A, 945B, 945C, etc., the phase modulators 945A, 945B, 945C, etc. can then be optically arranged in series so that each pulse passes through each phase modulator 945A, 945B, 945C, etc. in series (or sequentially). Such an arrangement is as Fig.10 In this arrangement, other optical elements may be arranged between two or more phase modulators 945A, 945B, 945C, etc.

[0075] On the other hand, in embodiments where the phase modulator system 945 includes a plurality of phase modulators 945A, 945B, 945C, etc., the phase modulators 945A, 945B, 945C, etc. can then be optically arranged in parallel so that a portion of each pulse passes through one of the phase modulators 945A, 945B, 945C, etc. and each portion passes through the phase modulators 945A, 945B, 945C, etc. simultaneously. This arrangement Fig.11 , which shows six phase modulators 945A, 945B, 945C, 945D, 945E, 945F arranged in parallel. Such an arrangement may be a two-dimensional array of phase modulators, where the array extends in a direction transverse to the direction of the pulse 505.

[0076] refer to Fig.12, an exemplary phase modulator 1245i can be designed as a Pockels cell 1246. The Pockels cell 1246 includes an electro-optical crystal 1247 through which a pulse 1205 (such as pulse 505) propagates. The phase delay in the crystal 1247 can be modulated by applying a variable voltage to electrodes 1248, 1249 connected to the crystal 1247. The electrodes 1248, 1249 are connected to a voltage source 1257, which is controlled by a control system 185. An exemplary voltage applied to the crystal 1247 ranges from kilovolts to several kilovolts or tens of kilovolts. The refractive index of the electro-optical crystal 1247 changes in proportion to the electric field applied by the electrodes 1248, 1249, and the refractive index changes with the refractive index. Therefore, by changing or modulating the refractive index in the crystal 1247, the phase delay can be modulated.

[0077] The geometric arrangement of the electrodes 1248, 1249 within the Pockels cell 1246 determines the arrangement of the electric field relative to the direction of travel of the pulse 505. For example, the Pockels cell 1246 can be a longitudinal device, where the electrodes 1248, 1249 are arranged at the input face 1251 and the output face 1252 of the crystal 1247, and in this case, the electric field generated is along the longitudinal direction, which is parallel to the direction of the pulse 505. As another example, the Pockels cell 1246 can be a transverse device, where the electrodes 1248, 1249 are arranged at one or more side faces 1253, 1254 of the crystal 1247, and in this case, the electric field generated is along the transverse direction, which is perpendicular to the direction of the pulse 505.

[0078] Crystal 1247 is made of a material whose refractive index is changed by controlling the electric field through crystal 1247 using electrodes 1248, 1249. The material of crystal 1247 should be transmissive at the wavelength of pulse 1205, and therefore, it should transmit DUV light. In addition, the material of crystal 1247 should be made of a material that has a high threshold for optical damage because pulse 1205 passing through crystal 1247 has high optical power. For example, referring to Fig.19 , the pulses in the seed beam 1910A output by the master oscillator (MO) 1900 may be about 1-6 watts (W) (e.g., at a pulse repetition rate of 6000 Hz, each pulse has an instantaneous energy of 1 millijoule (mJ)), while the pulses in the beam 110 at the output of the power amplifier 1925 may be about 10-200 W. For example, the crystal 1247 may be made of potassium dihydrogen phosphate (KDP), deuterated KDP (DKDP), ammonium dihydrogen phosphate (ADP), beta-barium borate (BBO), or quartz.

[0079] refer to Fig.13A and Fig. 13BIn some embodiments, the phase modulator system 145 is placed within the light source 105. Specifically, the light source 105 can be designed as a dual-stage light source 1305, and in this case, it may be beneficial to place the phase modulator system 145 between the first-stage light source 1300 and the second-stage optical amplifier 1310 and in the path of the pulsed seed beam 1310A generated by the first-stage light source 1300. This dual-stage design separates the power generator (second-stage optical amplifier 1310) from the bandwidth and wavelength control generator (first-stage light source 1300). The power of the seed beam 1310A is much lower than the power of the beam 110 output from the second-stage optical amplifier 1310. Therefore, by placing the phase modulator system 145 in the path of the seed beam 1310A, the flux and power level of the pulses entering the modulator system 145 are small (relative to the power level of the output beam 110) and the possibility of material damage within the phase modulator system 145 is reduced.

[0080] refer to Fig.14 In other embodiments, the phase modulator system 145 is placed within the beam preparation system 112 and in the path of the pulsed light beam 110 as it travels through the beam preparation system 112 .

[0081] For example, refer to Fig.15A and Fig. 15B, the phase modulator system 145 is placed within an optical temporal pulse stretcher 114 within the beam preparation system 112. The temporal pulse stretcher 114 is configured to increase the duration of pulses passing through it. The temporal pulse stretcher 114 optically acts on the pulses of the beam 110 to increase the duration of the pulses without introducing significant losses, thereby reducing the peak power of the beam 110 without reducing its average power. The temporal pulse stretcher 114 is an optical and passive configuration of optical elements that splits the amplitude of the pulses of the beam 110 into split parts 1503 with one or more beam splitters 1501, introduces optical delays 1502 among these split parts, and then recombine these time-delayed parts of the pulse (e.g., using the beam splitters 1501) to provide a temporally stretched pulse of the beam 110 at the output. Therefore, the temporal pulse stretcher 114 can include optical components such as a beam splitter (for the beam splitter 1501) and a reflective optical device (which forms the optical delay 1502). The reflective optical device can be a plane mirror or a curved (e.g., concave or convex) mirror that can be confocal. The delay introduced in the split portion of the pulse generated by the temporal pulse stretcher 114 is equal to or longer than the fast time component of the light beam 110. The phase modulator system 145 can be placed in the split portion of the pulse of the light beam 110. By applying optical phase modulation to the pulse of the light beam 110 within the split portion, the flux level on the phase modulator system 145 can be lower (e.g., 50% of the flux level of the light beam 110 that has not been split). In addition, by placing the phase modulator system 145 in the split portion of the pulse of the light beam 110, the recombined portions of the pulse within the temporal pulse stretcher 114 can be independent of each other and thereby reduce the temporal coherence of the pulse. Specifically, the phase modulator system 145 acts on the split and delayed portions of the pulse and ensures that these split portions are incoherent with the undelayed portions before recombination.

[0082] refer to Fig.16A and Fig. 16B In other embodiments, the phase modulator system 145 is placed inside the optical arrangement 113 within the photolithography exposure apparatus 115. Next, before discussing the placement and configuration of the phase modulator system 145 within the optical arrangement 113, details about the photolithography exposure apparatus 115 are provided.

[0083] refer to Fig.17A, the optical arrangement 113 includes, among other features, an illumination module 1702, a reticle stage or mask 1734 including a reticle, a projection stage 1704, which includes, among other optical devices, one or more projection lenses 1730. The mask 1734 can be moved in one or more directions, such as along an optical axis 1738 of the pulsed beam 110 or in a plane perpendicular to the optical axis 1738. The projection stage 1704 includes the projection lens 1730 and enables image transfer from the mask 1734 to the photoresist on the wafer 120.

[0084] The illumination module 1702 creates a pupil shape, i.e., the state and shape of the light beam 110, before the light beam 110 interacts with the mask 1734. For example, the illumination module 1702 adjusts the angular range of the light beam 110 incident on the mask. The illumination module 1702 includes a device 1732 that adjusts the light beam 110 and causes the light beam 110 to present a prescribed shape before it interacts with the mask 1734. For example, the device 1732 may include an array of individually adjustable micromirrors that can create a pupil shape in a fast manner (e.g., in seconds or minutes). The array may include hundreds or thousands of micromirrors and may be programmable. In addition, the device 1732 may include a field definition element (FDE). The micromirrors of the array may be used to illuminate certain positions of the incident plane of the FDE. The FDE converts each individual position in its incident plane into an angle on the mask layer while producing a uniform illumination spot. Therefore, by changing the micromirrors, any angular distribution (or pupil) at the mask level can have a uniform intensity distribution.

[0085] The illumination module 1702 further includes a beam homogenizer 1736 that homogenizes (makes uniform) the intensity distribution of the pulsed light beam 110 that passes through the mask 1734. Fig. 17B , the beam homogenizer 1736 can include a pair of two-dimensional lenslet arrays 1742, 1744 and a condenser lens 1746 to provide uniform irradiance at an illumination plane 1748 (which can be at or within the device 1732). The first lenslet array 1742 that interacts with the beam 110 can be considered as an objective lens array, and the second lenslet array 1744 that interacts with the beam 110 can be considered as a field array. The objective lens array images the source at the field array, and the field array re-images all fields using a condenser lens so that they overlap at the illumination plane and produce uniform irradiance. The two-dimensional lenslet arrays 1742, 1744 include a plurality of lenslets (small lenses) arranged in a plane extending perpendicular to the optical axis 1738.

[0086] The lithographic apparatus 115 may include, among other features, a lithographic controller 140 , air conditioning, and power supplies for various electrical components. The lithographic controller 140 controls how layers are printed on the wafer 120 .

[0087] The wafer 120 is irradiated by the beam 110. The process or recipe determines the exposure length on the wafer 120, the mask 1734 used, and other factors that affect the exposure. During lithography, multiple pulses of the beam 110 irradiate the same area of ​​the wafer 120 to form an exposure dose. The number N of pulses of the beam 110 that irradiate the same area can be called an exposure window or slit, and the size of the slit can be controlled by an exposure slit (not shown) placed in front of the mask 1734. The slit can be designed as a baffle and can include multiple blades that can be opened and closed; and the size of the exposure area is determined by the distance between the blades in the non-scanning direction and also by the length (distance) of the scan in the scanning direction. In some embodiments, the value of N is tens, for example, from 10-100 pulses. In other embodiments, the value of N is greater than 100 pulses, for example, from 100-500 pulses. One or more of the mask 1734, the projection stage 1704, and the wafer 120 can be moved relative to each other during exposure to scan the exposure window over the exposure field. The exposure region is the area of ​​the wafer 120 that is exposed in one scan of the exposure slit or window.

[0088] As mentioned above, the phase modulator system 145 can be placed inside the optical arrangement 113 within the photolithography exposure device 115. The phase modulator system 145 can be placed in the illumination module 1702. For example, Fig. 16B As shown in , the phase modulator system 145 is placed inside the beam homogenizer 1736.

[0089] In some implementations, the phase modulator system 145 is placed near one of the two-dimensional lenslet arrays 1742, 1744 or between the two-dimensional lenslet arrays 1742, 1744. In this example, the phase modulator system 145 can be a two-dimensional array 1745 of phase modulators optically arranged in parallel with each other (for clarity, only one of which is shown in FIG. 1 ). Fig. 16B This design is similar to Fig.11 The phase modulator 1745 is arranged in a plane extending in a direction perpendicular to the optical axis 1738, and the pulses of the light beam 110 travel through the phase modulator 1745 along the direction of the optical axis 1738. In addition, the number of phase modulators in the array of the phase modulator system 145 can correspond to the number of lenslets in the arrays 1742, 1744, and each phase modulator 1745 is aligned with a pair of lenslets from the corresponding array 1742, 1744. By placing the phase modulator system 145 within the beam homogenizer 1736, the speckle can be further reduced because the correlation between the lenslets in the arrays 1742, 1744 can be reduced or removed.

[0090] In other embodiments, the phase modulator system 145 can be placed between the lenslet array 1744 and the focusing lens 1746 or before the lenslet array 1742. In other embodiments, the phase modulator system 145 is placed in the device 1732 or near an individually adjustable micromirror array. The phase modulator system 145 can be placed on top of or near a focusing lens array (not shown) that focuses light from the beam homogenizer 1736 on each individual micromirror of the device 1732 to ensure that there is no light leakage on the micromirrors. For example, the phase modulator system 145 can be placed between the beam homogenizer 1736 and the lens focusing array in the device 1732.

[0091] Due to interference, the actual spectral characteristics (such as bandwidth or wavelength) of the pulses of the light beam 110 at the wafer 120 may not correspond to or match the expected spectral characteristics. Therefore, the metrology system 170 measures or senses the actual spectral characteristics (such as bandwidth or wavelength) of the light beam 110 during operation of the light source 105 by estimating a metric value from the measured spectrum 200 (e.g., Figure 2 ). An operator or an automated system (e.g., control system 185) can use the measured or sensed bandwidth of light beam 110 to adjust properties of light source 105 (e.g., by sending a signal to spectral signature selection system 130) and thereby adjust the spectrum (and spectral signature) of light beam 110. Control system 185 receives the output of metrology system 170 and analyzes the sensed spectral profile and estimates one or more spectral signatures of light beam 110 based on this analysis.

[0092] refer to Fig.18 , the metrology system 170 includes a spectral feature measurement system 1801 having a beam splitter 1860 and a diagnostic device 1865. The diagnostic device 1865 receives a pulsed beam 110' separated from the beam 110 by the beam splitter 1860. The beam splitter 1860 is placed in a path between the light source 105 and the photolithography exposure device 115. The beam splitter 1860 directs the pulsed beam 110' (which is a first portion or percentage of the pulsed beam 110) into the diagnostic device 1865 and directs a second portion or percentage of the pulsed beam 110 toward the photolithography exposure device 115. In some embodiments, most of the pulsed beam 110 is directed toward the photolithography exposure device 115 in the second portion. For example, the beam splitter 1860 directs a portion (e.g., 1-2%) of the pulsed beam 110 into the diagnostic device 1865 and therefore, the pulsed beam 110' has approximately 1-2% of the power of the pulsed beam 110. The beam splitter 1860 can be, for example, a beam splitter.

[0093] Diagnostic device 1865 includes spectral detection system 1810 that measures one or more spectral characteristics (such as bandwidth and / or wavelength) of pulsed light beam 110 based on information about spectrum 200 of pulsed light beam 110'. As discussed herein, spectral detection system 1810 includes: spectrometer 1848 (such as an etalon spectrometer) that interacts with pulsed light beam 110' and outputs spatial components corresponding to spectral components of light beam 110'; and sensor 1850 that estimates one or more spectral characteristics based on the spatial components of the output.

[0094] To uniformly sample the spectral content of the light beam 110' at the sensor 1850, to uniformly distribute the intensity of the light beam 110' at the sensor 1850, and to provide more accurate spectral feature measurements from the sensor 1850, the diagnostic device 1865 includes a beam preparation system 1800 that includes, among other features, a beam homogenizer 1805. The beam homogenizer 1805 reduces speckle noise and improves beam homogenization of the pulsed light beam 110' incident on the sensor 1850 of the spectral detection system 1010.

[0095] The beam preparation system 1800 may include other elements or components for modifying aspects of the pulsed beam 110'. For example, the beam preparation system 1800 may also include one or more pulse stretcher systems, one or more diffuser systems, and one or more spatial conditioning systems. The pulse stretcher system is a pulse stretcher that optically acts on the pulsed beam 110' to increase the duration of the pulses in the pulsed beam 110" without introducing significant losses, thereby reducing the peak power of the beam 110' without reducing its average power. The diffuser system includes one or more optical elements that are configured to uniformly diffuse the pulsed beam 110'. The diffuser system causes the beam 110' to be uniformly dispersed in a plane transverse to the direction of travel of the beam 110', thereby minimizing or removing high intensity bright spots. The diffuser system can change the angular divergence of the pulsed beam 110'. The diffuser system smoothes or otherwise mitigates diffraction spikes that sometimes occur within a beam homogenizer. The diffuser system can be a microlens array or a diffractive optical device (which can be transmissive or reflective). The spatial conditioning system is used to refract the pulsed beam 110' to spread out the spacing between diffraction spikes that can be generated within the beam homogenizer. The spatial conditioning system can be a lens that is positioned so that its focal plane overlaps the beam homogenization plane generated by the beam homogenizer.

[0096] The spectral detection system 1810 includes an aperture 1849 through which the pulsed beam 110' is directed, as well as an input lens 1862, an optical frequency separation device (such as an etalon) 1863, and an output lens 1864. The aperture 1849 can be placed on the focal plane of the input lens 1862 so that each point from the focal plane acts as a point light source, and thus, the input lens 1862 is used to collimate the pulsed beam 110' before entering the etalon 1863. The output lens 1864 is positioned at the exit of the etalon 1863 so that the focal plane of the output lens 1864 overlaps the active area of ​​the sensor 1850.

[0097] In some embodiments, the etalon 1863 includes a pair of partially reflective glass or optical plates 1863A, 1863B, which can be spaced a short distance (e.g., millimeters to centimeters) apart, with the reflective surfaces facing each other. In other embodiments, the etalon 1863 includes a single plate with two parallel reflective surfaces. The plates 1863A, 1863B can be wedge-shaped to prevent interference fringes from the rear surface; the rear surface is also typically anti-reflective. When the pulsed light beam 110' passes through the pair of plates 1863A, 1863B, it is reflected multiple times and produces multiple transmitted light rays, which are collected by the output lens 1864 and brought to the active area of ​​the sensor 1850. The spectral detection system 1810 can also include an optical relay between the output lens 1864 and the sensor 1850 as needed to ensure that the sensor 1850 is located at the focal plane of the output lens 1864.

[0098] Etalon 1863 interacts with beam 110' and outputs a plurality of spatial components 1874 (which are Fig.18 18), which corresponds to the spectral components of the pulsed light beam 110'. The spectral components of the light beam 110' are in the spectrum 1872 of the pulsed light beam 110'; therefore, they correspond to how the light energy or power values ​​(spectral intensities) of the pulsed light beam 110' are distributed at different wavelengths. The spatial components 1874 correspond to these intensities mapped to a two-dimensional space. Therefore, the etalon 1863 transforms the spectral information (such as wavelength) of the pulsed light beam 110' into spatial information that can be sensed or detected by the sensor 1850. The transformation maps the spectral information (such as wavelength) to different locations in space so that the sensor 1850 can observe the spectral information.

[0099] The etalon 1863 produces an interference pattern as a spatial component 1874 that presents the appearance of a set of concentric rings. If the intensity distribution of the pulsed beam 110' on the aperture 1849 is more uniform, the interference pattern presents a more uniform intensity distribution. In particular, the sharpness of the rings depends on the reflectivity of the plates 1863A, 1863B of the etalon 1863. Thus, if the reflectivity of the plates 1863A, 1863B is high (so that the etalon has a high quality (Q) factor) when the pulsed beam 110' is a monochromatic beam, the etalon 1863 produces a set of narrow bright rings against a dark background. The transmission of the etalon 1863 as a function of wavelength is shown in the resulting fringe pattern 1871, which produces a spectrum 1872 that is directed to the control system 185. The complete interference pattern is not required to perform the calculation or estimation; alternatively, only the fringes can be generated in an area slightly larger than the active area of ​​the sensor 1850. The sensor 1850 receives and senses the output spatial component 1874. The sensor 1850 can be defined by a plane that generally indicates the active area of ​​its sensing region. The plane of the sensing region can be perpendicular to the direction of propagation of the spatial component 1874.

[0100] Sensor 1850 can be a detector that receives and senses output spatial components 1874. For example, one type of suitable detector that can be used to measure along one dimension is a linear photodiode array. A linear photodiode array is composed of multiple elements of the same size, formed by an equally spaced linear arrangement in a package. The photodiode array is sensitive to the wavelength of the light beam 110', and if the light beam 110' has a wavelength in the deep ultraviolet range, then the photodiode array is sensitive to light with a wavelength in the deep ultraviolet range. As another example, sensor 1850 can be a two-dimensional sensor, such as a two-dimensional charge coupled device (CCD) or a two-dimensional complementary metal oxide semiconductor (CMOS) sensor. Sensor 1850 should be able to read out data at a sufficiently fast rate, for example, at about 6kHz.

[0101] The control system 185 is connected to the output of the sensor 1850 as well as the optical source 105 and the optical characteristic selection system 130, which is optically coupled to the pulsed light beam 110. The control system 185 measures properties of the spatial component 1874 and analyzes these measured properties to calculate an estimate of the spectral characteristics of the pulsed light beam 110. The control system 185 can perform the measurements, analyses, and calculations on each pulse of the light beam 110 or on a group of pulses of the light beam 110.

[0102] The properties of the measured spatial component 1874 can be a single scalar (which is completely described by the magnitude or value) or a vector (which is completely described by both the magnitude and direction). An example of a scalar property is a metric such as the width W of the spectrum 1872. In this example, the entire shape of the spectrum 1872 may be unknown but the metric is known and this is used to estimate the shape of the spectrum 1872. An example of a vector property is the entire waveform describing the spectrum 1872. In this example, one can calculate any metric from the entire spectrum, and by having the entire spectrum, one can make more accurate calculations. The sensed spatial component can be measured for a range of one or more pulses of the pulsed beam 110'.

[0103] The control system 185 can measure the width W of the spectrum 1872 as a property. The width W of the spectrum 1872 can provide an estimate of the bandwidth (spectral characteristics) of the light beam 110'. In some embodiments, the width W of the spectrum 1872 is determined using a metric such as FWXM (the full width of the spectrum 1872 at a fraction X of the maximum peak intensity). In other embodiments, the width W of the spectrum 1872 is determined using a metric such as EY (the width of the spectrum containing a fraction Y of the integrated spectral intensity). Other metrics are suitable for measuring properties of the spectrum 1872.

[0104] refer to Fig.19 In some embodiments, the light source 105 is an exemplary light source 1905. The light source 1905 is a pulsed laser source that produces a pulsed laser beam as the light beam 110. The light source 1905 is a two-stage laser system that includes a master oscillator (MO) 1900 that provides a seed beam 1910A to a power amplifier (PA) 1910. The master oscillator 1900 typically includes a gain medium in which amplification occurs and an optical feedback mechanism such as an optical resonator. The power amplifier 1910 typically includes a gain medium in which amplification occurs when seeded with a seed laser beam from the master oscillator 1900. The power amplifier 1910 can be a power ring amplifier (PRA) that is designed as a regenerative ring resonator. In this case, sufficient optical feedback can be provided from the ring design. The spectral feature selection device 130 receives the light beam 110A from the master oscillator 1900 to enable fine tuning of spectral parameters such as the central wavelength and bandwidth of the light beam 110A at relatively low output pulse energy. The power amplifier 1910 receives the light beam 1910A from the master oscillator 1900 and amplifies the output to obtain the power required for the output used in photolithography.

[0105] In some embodiments, the master oscillator 1900 includes a discharge chamber having two elongated electrodes, a laser gas used as a gain medium, and a fan to circulate the gas between the electrodes. A laser resonator is formed between the spectral feature selection device 130 on one side of the discharge chamber and an output coupler 1915 on a second side of the discharge chamber to output a seed beam 1910A to a power amplifier 1910.

[0106] In other embodiments, the master oscillator 1900 includes a solid state material as a gain medium. Solid state media that can be used include crystals or glasses doped with rare earth or transition metal ions, or semiconductor lasers. The master oscillator 1900 using a solid state gain medium generates a seed beam 1910A. The solid state gain medium can be optically pumped with a flash lamp or an arc lamp, or using a laser diode or a titanium (Ti) sapphire (Ti: sapphire) laser.

[0107] Exemplary solid-state gain media may be neodymium-doped yttrium aluminum garnet (Nd:YAG), neodymium-doped yttrium lithium fluoride (Nd:YLF), or Ti:sapphire. Solid-state gain media are capable of producing a single-mode output that is highly temporally (and spatially) coherent and also has a narrow bandwidth. The seed beam 1910A output from the solid-state gain medium of the master oscillator 1900 may be at a wavelength that is not the desired wavelength (e.g., it may be outside the DUV wavelength range). In this case, the seed beam 1910A may be directed through one or more wavelength conversion elements to ensure that the wavelength of the seed beam 1910A directed to the power amplifier 1910 is at the desired DUV wavelength. For example, if the seed beam 1910A output from the solid-state gain medium within the master oscillator 1900 is at a wavelength of approximately 773.6 nm (e.g., as is the case for a Ti:sapphire gain medium), the seed beam may be directed through two wavelength conversion elements 1910A to convert the wavelength to approximately 193.4 nm. The one or more wavelength conversion elements may use nonlinear optical techniques such as sum frequency generation to convert the wavelength to a desired wavelength.

[0108] The highly coherent properties of the solid-state gain medium of the master oscillator 1900 may be addressed using a phase modulator system 145, which may be used to reduce the coherence (temporal and spatial) of a seed beam 1910A produced by such a master oscillator 1900. In addition, the phase modulator system 145 also has the effect of increasing the bandwidth of the seed beam 1910A; as the temporal coherence of the seed beam 1910A decreases, the bandwidth of the seed beam 1910A increases. Thus, the phase modulator system 145 may be used to increase the bandwidth of the seed beam 1910A and also to reduce the temporal coherence of the seed beam 1910A.

[0109] The light source 1905 may also include another spectral signature metrology module (such as a line center analysis module or LAM) 1920 that receives the output from the output coupler 1915, and one or more beam modification optical systems 1925 that modify the size and / or shape of the beam as desired. The spectral signature metrology module 1920 is part of the metrology system 170 and is an example of one type of measurement system that may be used to measure the wavelength (e.g., center wavelength) of the seed beam.

[0110] The power amplifier 1910 includes a power amplifier discharge chamber and, if it is a regenerative ring amplifier, a beam reflector or beam steering device 1930 that reflects the beam back into the discharge chamber to form a circulation path. The power amplifier discharge chamber includes a pair of elongated electrodes, a laser gas that serves as a gain medium, and a fan for circulating the gas between the electrodes. The seed beam 1910A is amplified by repeatedly passing through the power amplifier 1910. The beam modification optical system 1925 provides a means (e.g., a partial reflector) to couple the seed beam 1910A inwardly and outwardly couple a portion of the amplified radiation from the power amplifier to form the output beam 110.

[0111] The laser gas used in the discharge chamber of the master oscillator 1900 and the power amplifier 1910 can be any suitable gas for generating a laser beam around the desired wavelength and bandwidth. For example, the laser gas can be argon fluoride (ArF) emitting light at a wavelength of about 193 nm, or krypton fluoride (KrF) emitting light at a wavelength of about 248 nm.

[0112] The spectral signature metrology module 1920 monitors the wavelength of the output (light beam 1910A) of the master oscillator 1900. The spectral signature metrology module 1920 may be placed elsewhere within the light source 1905, or it may be placed at the output of the light source 1905.

[0113] The repetition rate of the master oscillator 1900 is controlled by the control system 185 according to instructions from the controller 140 in the photolithography exposure device 115, thereby determining the repetition rate of the pulses generated by the power amplifier 1910. The repetition rate of the pulses output from the power amplifier 1910 is the repetition rate seen by the photolithography exposure device 115.

[0114] As discussed above, the bandwidth can be coarsely and finely controlled using only optical elements. On the other hand, by controlling the differential timing between the activation of the electrodes within the master oscillator 1900 and the power amplifier 1910, the bandwidth can be quickly controlled in a fine and narrow range, while the bandwidth can be controlled in a coarse and wide range by adjusting the angle of the prism within the spectral feature selection system 130.

[0115] refer to Fig. 20 In some embodiments, the spectral signature selection device 130 includes a set of optical features or components 2000, 2005, 2010, 2015, 2020 arranged to optically interact with the pulsed light beam 110A and a control module 2050 of electronics including any combination of firmware and software. The optical components 2000, 2005, 2010, 2015, 2020 can be configured to provide a coarse spectral signature adjustment system; and, if the adjustment of such components is fast enough, they can be configured to provide a fine spectral signature adjustment system. Although not described in detail in the literature, the spectral signature selection device 130 includes a set of optical features or components 2000, 2005, 2010, 2015, 2020 arranged to optically interact with the pulsed light beam 110A and a control module 2050 of electronics including any combination of firmware and software. The optical components 2000, 2005, 2010, 2015, 2020 can be configured to provide a coarse spectral signature adjustment system; and, if the adjustment of such components is fast enough, they can be configured to provide a fine spectral signature adjustment system. Fig. 20 , but the spectral signature selection device 130 may include other optical or other non-optical features for providing fine spectral signature control.

[0116] The control module 2050 is connected to one or more actuation systems 2000A, 2005A, 2010A, 2015A, 2020A, which are physically coupled to the corresponding optical components 2000, 2005, 2010, 2015, 2020. The optical components of the device 130 include a dispersive optical element 2000, which may be a grating, and a beam expander 2001 made of a set of refractive optical elements 2005, 2010, 2015, 2020, which may be prisms. The grating 2000 may be a reflective grating designed to disperse and reflect the light beam 110A; therefore, the grating 2000 is made of a material suitable for interacting with the pulsed light beam 110A having a wavelength in the DUV range. Each of the prisms 2005, 2010, 2015, 2020 is a transmissive prism, which is used to disperse and redirect the light beam 110A as it passes through the prism body. Each prism may be made of a material that allows the wavelength of the light beam 110A to be transmitted, such as, for example, calcium fluoride.Although four refractive optical elements 2005, 2010, 2015, 2020 are shown, fewer than four or more than four may be used in the beam expander 2001.

[0117] The pulsed light beam 110A enters the device 130 through the aperture 2055 and then travels through the prism 2020, prism 2010 and prism 2005 in sequence before impinging on the diffractive surface 2002 of the grating 2000. Each time the light beam 110A passes through the successive prisms 2020, 2015, 2010, 2005, the light beam 110A is optically magnified and redirected (refracted at a certain angle) toward the next optical component. When the light beam 110A leaves the device 130, the light beam 110A is diffracted and reflected from the grating 2000 before passing through the aperture 2055, and then passes through the prism 2005, prism 2010, prism 2015 and prism 2020 in sequence. With each passage from the grating 2000 through the successive prisms 2005, 2010, 2015, 2020, the light beam 110A is optically compressed as it travels toward the aperture 2055.

[0118] The rotation of the prism of the beam expander 2001 (which may be any of the prisms 2005, 2010, 2015, 2020) changes the angle of incidence of the beam 110A on the incident surface of the rotating prism. In addition, two local optical properties of the beam 110A passing through the rotating prism, namely the optical magnification and the beam refraction angle, are functions of the angle of incidence of the beam 110A on the incident surface of the rotating prism. The optical magnification of the beam 110A passing through the prism is the ratio of the lateral width of the beam 110A leaving the prism to the lateral width of the beam 110A entering the prism.

[0119] The change in the local optical magnification of the beam 110A at the one or more prisms within the beam expander 2001 causes an overall change in the optical magnification OM 2065 of the beam 110A passing through the beam expander 2001. The angle of the optical magnification OM 2065 of the beam 110A passing through the beam expander 2001 is the ratio of the lateral width Wo of the beam 110A leaving the beam expander 2001 to the lateral width Wi of the beam 110A entering the beam expander 2001. In addition, the change in the local beam refraction angle by the one or more prisms within the beam expander 2001 causes an overall change in the angle of incidence 2062 of the beam 110A at the surface 2002 of the grating 2000.

[0120] The wavelength of the light beam 110A can be adjusted by changing the angle of incidence 2062 of the light beam 110A onto the diffractive surface 2002 of the grating 2000. The bandwidth of the light beam 110A can be adjusted by changing the optical magnification 2065 of the light beam 110.

[0121] The device 130 is designed to adjust the wavelength of the light beam 110A generated in one or more resonators of the light source 105 by adjusting the incident angle 2062 of the light beam 110A onto the diffractive surface 2002 of the grating 2000. In particular, this can be accomplished by rotating one or more prisms 2005, 2010, 2015, 2020 and the grating 2000, thereby adjusting the incident angle 2062 of the light beam 110A.

[0122] In addition, the bandwidth of the light beam 110A produced by the light source 105 is adjusted by adjusting the optical magnification OM 2065 of the light beam 110A. Thus, the bandwidth of the light beam 110A can be adjusted by rotating one or more of the prisms 2005, 2010, 2015, 2020, which causes the optical magnification 2065 of the light beam 110A to change. Because rotation of a particular prism causes a change in the local beam refraction angle and the local optical magnification at that prism, control of wavelength and bandwidth is coupled in this design.

[0123] Additionally, the bandwidth of the light beam 110A is relatively sensitive to the rotation of the prism 2020, and relatively insensitive to the rotation of the prism 2005. This is because any change in the local optical magnification of the light beam 110A due to the rotation of the prism 2020 is multiplied by the product of the changes in the optical magnifications in the other prisms 2015, 2010, and 2005, because those prisms are located between the rotating prism 2020 and the grating 2000, and the light beam 110A must travel through these other prisms 2015, 2010, and 2005 after passing through the prism 2020. On the other hand, the wavelength of the light beam 110A is relatively sensitive to the rotation of the prism 2005, and relatively insensitive to the rotation of the prism 2020.

[0124] For example, to change the bandwidth without changing the wavelength, the optical magnification 2065 should be changed without changing the incident angle 2062, which can be achieved by rotating prism 2020 a larger amount and by rotating prism 2005 a smaller amount.

[0125] The control module 2050 is connected to one or more actuation systems 2000A, 2005A, 2010A, 2015A, 2020A that are physically coupled to the respective optical components 2000, 2005, 2010, 2015, 2020. Although an actuation system is shown for each optical component, some optical components in the device 130 may remain stationary or not be physically coupled to an actuation system. For example, in some embodiments, the grating 2000 may remain stationary and the prism 2015 may remain stationary and not be physically coupled to an actuation system.

[0126] Each of the actuation systems 2000A, 2005A, 2010A, 2015A, 2020A includes one or more actuators connected to their respective optical components. Adjustment of the optical components results in adjustment of specific spectral characteristics (wavelength and / or bandwidth) of the light beam 110A. The control module 2050 receives control signals from the control system 185 that include specific commands to operate or control one or more actuation systems. The actuation systems can be selected and designed to work in coordination.

[0127] Each actuator of the actuation system 2000A, 2005A, 2010A, 2015A, 2020A is a mechanical device for moving or controlling a corresponding optical component. The actuator receives energy from the module 2050 and converts the energy into a certain motion imparted to the corresponding optical component. For example, the actuation system can be any one of a force device and a rotation stage for rotating one or more prisms of a beam expander. The actuation system can include, for example, a motor such as a stepper motor, a valve, a pressure control device, a piezoelectric device, a linear motor, a hydraulic actuator, a voice coil, etc.

[0128] The grating 2000 may be a high blaze angle step grating, and the light beam 110A incident on the grating 2000 will be reflected (diffracted) at any incident angle 2062 that satisfies the grating equation. The grating equation provides a relationship between the spectral order of the grating 2000, the diffracted wavelength (the wavelength of the diffracted light beam), the incident angle 2062 of the light beam 110A on the grating 2000, the exit angle at which the light beam 110A is diffracted off the grating 2000, the vertical divergence of the light beam 110A incident on the grating 2000, and the groove pitch of the diffractive surface of the grating 2000. Furthermore, if the grating 2000 is used so that the incident angle 2062 of the light beam 110A onto the grating 2000 is equal to the exit angle of the light beam 110A from the grating 2000, then the grating 2000 and the beam expander (prisms 2005, 2010, 2015, 2020) are arranged in a Littrow configuration and the wavelength of the light beam 110A reflected from the grating 2000 is the Littrow wavelength. It can be assumed that the vertical divergence of the light beam 110A incident on the grating 2000 is close to zero. In order to reflect the nominal wavelength, the grating 2000 is aligned relative to the light beam 110A incident on the grating 2000 so that the nominal wavelength is reflected back through the beam expander (prisms 2005, 2010, 2015, 2020) to be amplified in the light source 105. Then, by changing the incident angle 2062 of the light beam 110A onto the grating 2000, the Littrow wavelength can be tuned across the entire gain bandwidth of the resonator within the light source 105.

[0129] Each of the prisms 2005, 2010, 2015, 2020 is sufficiently wide in a transverse direction of the light beam 110A so that the light beam 110A is contained within the surface through which it passes. Each prism optically magnifies the light beam 110A on its way from the aperture 2055 toward the grating 2000, and thus the size of each prism increases successively from prism 2020 to prism 2005. Thus, prism 2005 is larger than prism 2010, which is larger than prism 2015, and prism 2020 is the smallest prism.

[0130] As discussed above, the bandwidth of the light beam 110A is relatively sensitive to the rotation of the prism 2020, and relatively insensitive to the rotation of the prism 2005. This is because any change in the local optical magnification of the light beam 110A due to the rotation of the prism 2020 is multiplied by the product of the changes in the optical magnification in the other prisms 2015, 2010, and 2005, because those prisms are located between the rotated prism 2020 and the grating 2000, and the light beam 110A must pass through these other prisms 2015, 2010, and 2005 after passing through the prism 2020. On the other hand, the wavelength of the light beam 110A is relatively sensitive to the rotation of the prism 2005, and relatively insensitive to the rotation of the prism 2020. Therefore, the wavelength can be roughly changed by rotating the prism 2005, and the prism 2020 can be rotated (in a rough manner). The incident angle 2062 of the light beam 110A changes due to the rotation of the prism 2005, and the rotation of the prism 2020 counteracts the change in magnification caused by the rotation of the prism 2005. The prism 2020 can be used for coarse, large range, slow bandwidth control. In contrast, by controlling the prism 2010, the bandwidth can be controlled in a fine and narrow range and even faster.

[0131] refer to Fig.21 , provides details about the control system 185, which is related to various aspects of the systems and methods described herein. The control system 185 may include Fig.21 Other features not shown. Typically, control system 185 includes one or more of digital electronic circuitry, computer hardware, firmware, and software.

[0132] The control system 185 includes a memory 2100, which may be a read-only memory and / or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, for example, semiconductor memory devices such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. The control system 185 may also include one or more input devices 2105 (such as a keyboard, touch screen, microphone, mouse, handheld input device, etc.) and one or more output devices 2110 (such as a speaker or monitor).

[0133] The control system 185 includes one or more programmable processors 2115, and one or more computer program products 2120 tangibly embodied in a machine-readable storage device for execution by a programmable processor (such as the processor 2115). The one or more programmable processors 2115 can each execute a program of instructions to perform a desired function by operating on input data and generating appropriate output. Typically, the processor 2115 receives instructions and data from the memory 2100. Any of the foregoing may be supplemented or incorporated by a specially designed ASIC (application-specific integrated circuit).

[0134] The control system 185 includes, among other components, a spectral signature analysis module 2125, a lithography analysis module 2130, a decision module 2135, a phase modulation module 2140, a light source actuation module 2150, a lithography actuation module 2155, and a beam preparation actuation module 2160. Each of these modules may be a set of computer program products executed by one or more processors, such as the processor 2115. In addition, any of the modules 2125, 2130, 2135, 2140, 2150, 2155, 2160 may access data stored in the memory 2100.

[0135] The spectral signature analysis module 2125 receives output from the metrology system 170. The phase modulation module 2140 interfaces with the phase modulator system 145 to control the signal provided to the voltage source 1257 for controlling the electrodes 1248, 1249 of the phase modulator system 145. The lithography analysis module 2130 receives information from the lithography controller 140 of the photolithography exposure apparatus 115. The decision module 2135 receives output from analysis modules (such as modules 2125 and 2130) and determines which actuation module or modules (such as the phase modulation module 2140 or the light source actuation module 2150) need to be activated based on the output from the analysis modules.

[0136] The light source actuator module 2150 is connected to one or more of the light source 105 and the spectral feature selection device 130. The photolithography actuator module 2155 is connected to the photolithography exposure device 115, and specifically to the photolithography controller 140. The beam preparation actuator module 2160 is connected to one or more components of the beam preparation system 112. The connections between modules within the control system 185 and between modules within the control system 185 and other components of the photolithography system 100 can be wired or wireless.

[0137] Although Fig.21Only a few modules are shown in the figure, but the control system 185 may include other modules. In addition, although the control system 185 is shown as a box in which all components appear to be located together, the control system 185 can be composed of components that are physically remote from each other. For example, the light source actuation module 2150 can be physically co-located with the light source 105 or the spectral feature selection device 130.

[0138] Typically, the control system 185 receives at least some information about the light beam 110 from the metrology system 170, and the spectral signature analysis module 2125 performs analysis on the information to determine how to adjust one or more spectral signatures (e.g., bandwidth) of the light beam 110 supplied to the photolithography exposure device 115. Based on the determination, the control system 185 sends a signal to the spectral signature selection device 130 and / or the light source 105 to control the operation of the light source 105 via the control module 2050. Typically, the spectral signature analysis module 2125 performs the analysis required to estimate one or more spectral signatures (e.g., wavelength and / or bandwidth) of the light beam 110. The output of the spectral signature analysis module 2125 is an estimate of the spectral signature that is sent to the decision module 2135.

[0139] The spectral signature analysis module 2125 includes a comparison block that is connected to receive the estimated spectral signature and is also connected to receive the spectral signature target value. Typically, the comparison block outputs a spectral signature error value that represents the difference between the spectral signature target value and the estimated value. The decision module 2135 receives the spectral signature error value and determines how to best correct the system 100 in order to adjust the spectral signature. Therefore, the decision module 2135 sends a signal to the light source actuator module 2150, which determines how to adjust the spectral signature selection device 130 (or light source 105) based on the spectral signature error value. The output of the light source actuator module 2150 includes a set of actuator commands that are sent to the spectral signature selection device 130. For example, the light source actuator module 2150 sends commands to the control module 2050, which is connected to Fig. 20 An actuation system within an exemplary device 130 is shown in FIG.

[0140] In addition, the lithography analysis module 2130 can receive instructions from the lithography controller 140 of the photolithography exposure device 115, for example, to change one or more spectral characteristics of the pulsed light beam 110 or to change the pulse repetition rate of the light beam 110. The lithography analysis module 2130 performs analysis on these instructions to determine how to adjust the spectral characteristics and sends the analysis results to the decision module 2135. The control system 185 operates the light source 105 at a given repetition rate, which is the rate at which pulses are generated. More specifically, the photolithography exposure device 115 sends a trigger signal to the light source 105 for each pulse (i.e., on a pulse-to-pulse basis) (through the control system (through the lithography analysis module 2130)), and the time intervals between these trigger signals can be arbitrary, but when the photolithography exposure device 115 sends trigger signals at regular intervals, then the rate of those signals is the repetition rate. The repetition rate can be the rate requested by the photolithography exposure device 115.

[0141] The phase modulation module 2140 may receive instructions (e.g., from the decision module 2135) on how to modify the signal provided to the voltage source 1257 for controlling the electrodes 1248, 1249 of the phase modulator system 145. By adjusting the signal provided to the voltage source 1257, the modulation frequency of the optical phase of the pulses is adjusted.

[0142] refer to Fig. 22 , process 2200 is performed by the photolithography system 100. A light beam 110 consisting of pulses is generated (2205). Each pulse has a wavelength in the deep ultraviolet range, and each pulse has a first temporal coherence defined by a first temporal coherence length. Each pulse is defined by a pulse duration. The pulse duration can be defined as the time that the pulse power remains continuously above a certain percentage (e.g., half) of its maximum value.

[0143] For one or more of the pulses, an optical phase of the pulse is modulated over a pulse duration of the pulse to produce a modified pulse (2210). The modified pulse has a second temporal coherence defined by a second temporal coherence length that is less than a first temporal coherence length of the pulse (i.e., the unmodified pulse). For example, the optical phase is modulated over the pulse duration of the pulse to produce the modified pulse (2210) by convolving a spectrum of the electric field of the pulse by a Fourier transform that is related to modulating the optical phase over the pulse duration of the pulse.

[0144] The optical phase of the pulse may be modulated 2210 by modulating the refractive index of the material through which the pulse is directed. For example, the optical phase of pulse 1205 may be modulated by modulating the refractive index of an electro-optical crystal 1247 through which pulse 1205 propagates. The optical phase may be modulated at a frequency within a frequency range.

[0145] The optical phase of the pulse may be modulated (2210) by randomizing the optical phase over the pulse duration of the pulse. The optical phase of the pulse may be modulated (2210) by changing the position of a time point on the electrical waveform of the pulse.

[0146] The optical pulse is associated with a waveform, the waveform being represented by points in time, and the optical phase of the pulse can be modulated 2210 by applying different time delays to different points of the waveform. Different time delays are applied to different points of the waveform by passing the optical pulse through a medium (or material such as an electro-optic crystal 1247) and changing the refractive index of the medium as the pulse passes through the medium. The amplitude of the modulation of the optical phase can vary randomly over the pulse duration of the pulse.

[0147] A beam is formed from the one or more modified pulses (2215), and the formed pulsed beam is directed toward the wafer 120 within the photolithography exposure apparatus 115 (2220).

[0148] An optical phase is modulated (2210) over a pulse duration of the pulses to produce modified pulses to reduce dynamic speckle (also referred to as dynamic speckle contrast) of a pulsed light beam directed toward a substrate.

[0149] The beam of pulses may be generated by generating a seed beam 1910A of pulses. The beam of amplified pulses 110 may be generated by optically amplifying pulses of the seed beam 1910A by passing the pulses of the seed beam repeatedly through a resonator (e.g., within a power amplifier 1910). The optical phase may be modulated on the pulse duration of the pulses by modulating the optical phase on the pulse duration of the amplified pulses to generate modified pulses. The optical phase may be modulated on the pulse duration of the pulses by modulating the optical phase on the pulse duration of the pulses of the seed beam 1910A to generate modified pulses. The beam of amplified pulses 110 may be generated by optically amplifying the modified pulses.

[0150] The process 2200 may also include reducing the bandwidth of the beam pulses before modulating the optical phase over the pulse duration of each pulse to produce modified pulses. Modulating the optical phase over the pulse duration of the pulses causes the bandwidth of the pulses to increase but still be below the target bandwidth. This phenomenon, in which the bandwidth of the modified pulses increases as the frequency of the optical phase modulation increases, can be used to adjust the bandwidth of the pulses directed toward the wafer 120 by adjusting the rate or frequency at which the optical phase of the pulses is modulated.

[0151] The process 2200 may also include increasing the duration of pulses in the light beam directed toward the wafer 120. The duration of pulses in the light beam directed toward the wafer 120 may be increased by dividing the amplitude of each pulse of the light beam into split portions, introducing a time delay between the split portions to produce time-delayed portions of the pulse, and then recombining the time-delayed portions of the pulse to provide a time-stretched pulse of the light beam. In addition, an optical phase may be modulated (2210) on the pulse duration of the pulse to produce a modified pulse by modulating the optical phase on the pulse duration of one or more split portions of the pulse.

[0152] The bandwidth of the modified pulses of the optical beam may be greater than the bandwidth of the pulses of the optical beam before modulation. Fig.23 , process 2200 may also include an additional process 2300 for selecting a frequency range for modulating the optical phase over the pulse duration of the pulse. Basically, process 2300 operates based on the principle that a modulation frequency (or a maximum allowed modulation frequency) can be selected by determining a target modulation frequency that will produce a bandwidth within a target bandwidth range of the modified pulse; and then the maximum allowed modulation frequency is kept below the determined target modulation frequency, thereby ensuring that the modulation frequency is within the target modulation frequency, and thereby keeping the bandwidth of the modified pulse within the target bandwidth range. Process 2300 includes measuring a characteristic of a test pulse (2305). The test pulse is a pulse having a first temporal coherence (before being modulated) or a modified pulse having a second temporal coherence. The characteristic of the test pulse measured may be the bandwidth of the test pulse. The measured characteristic is received (2310) and the control system 185 determines whether the bandwidth of the modified pulse is within the target bandwidth range based on the received characteristic (2315). If the control system 185 determines that the bandwidth of the modified pulse is outside the target bandwidth range, then the maximum allowed frequency at which the optical phase is modulated is adjusted (2320). By adjusting the maximum allowed frequency at which the optical phase is modulated, the bandwidth of the modified pulse can be adjusted. For example, the phase modulation module 2140 can adjust the control signal provided to the voltage source 1257 of the phase modulator system 145 to adjust the maximum allowed frequency of modulation. Also, the control signal can be adjusted by adjusting the signal filter through which the control signal is directed.

[0153] Process 2300 may be performed for each pulse of light beam 110 .

[0154] Other implementations are within the scope of the following claims.

Claims

1. An apparatus for reducing speckle in an excimer light source, comprising: a phase modulator system in a path of a light beam having pulses of wavelengths in the deep ultraviolet range, the phase modulator system being configured to, for each of a plurality of pulses of the light beam, modulate an optical phase over a pulse duration of the pulse to produce a modified pulse having a second temporal coherence length that is less than a first temporal coherence length of the pulse prior to being modulated; a measuring device configured to measure a characteristic of a test pulse, wherein the test pulse is a pulse having the first temporal coherence or the modified pulse having the second temporal coherence; and a control system in communication with the measurement device and the phase modulator system, the control system being configured to: receiving a measured characteristic of the test pulse from the measurement device; determining whether a bandwidth of the modified pulse is within a target bandwidth based on the received measured characteristic; and If it is determined that the bandwidth of the modified pulse is outside the range of the target bandwidth, the frequency at which the optical phase is modulated over the pulse duration of the pulse producing the modified pulse is adjusted.

2. A light source, comprising: a first-stage light source configured to generate a seed beam comprised of pulses and comprising a spectral tuning device configured to tune one or more spectral features of the seed beam, each pulse having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length, and each pulse being defined by a pulse duration; a coherence reduction system in a path of the seed beam pulses and configured to, for each pulse of the seed beam, generate a modified seed beam pulse having a second temporal coherence defined by a second temporal coherence length, the second temporal coherence length being less than the first temporal coherence length; as well as A second-stage optical amplifier includes a resonator and a gain medium, and is configured to receive the modified seed beam pulses output from the coherence reduction system and generate a beam consisting of amplified pulses.

3. The light source according to claim 2, wherein: The coherence reduction system includes a phase modulator system, which is configured to, for each pulse of the seed beam, modulate an optical phase over the pulse duration of the pulse to produce the modified seed beam pulse having the second temporal coherence defined by the second temporal coherence length, which is less than the first temporal coherence length.

4. The light source according to claim 3, wherein The phase modulator system includes a two-dimensional array of phase modulators.

5. The light source according to claim 3, wherein: The phase modulator system is also configured to, for each pulse of the seed beam, reduce a spatial coherence of the pulse such that the modified pulse of the seed beam has a second spatial coherence that is less than the spatial coherence of the seed beam pulse.

6. The light source according to claim 3, wherein: The phase modulator system comprises a Pockels cell including a medium through which the pulsed seed beam passes.

7. The light source according to claim 6, wherein The phase modulator system is configured such that, for each pulse of the seed beam, modulating the optical phase over the pulse duration of the pulse to produce the modified seed beam pulse includes modulating a refractive index of the medium of the Pockels cell.

8. The light source of claim 3, wherein the phase modulator system comprises a single phase modulator.

9. The light source according to claim 2, wherein: The first-stage light source includes a master oscillator, which includes a gain medium and an optical feedback mechanism, and amplification is configured to occur in the gain medium of the master oscillator, and the second-stage optical amplifier includes a power amplifier, which includes a gain medium, and when seeded with the seed beam from the master oscillator, amplification is configured to occur in the gain medium of the power amplifier.

10. The light source according to claim 9, wherein The master oscillator comprises a gas discharge chamber having two electrodes, and the gas discharge chamber is configured to contain a laser gas used as a gain medium for the master oscillator, wherein a laser resonator is formed between the spectral tuning device on one side of the gas discharge chamber and an output coupler on a second side of the gas discharge chamber, and the seed beam leaves the first-stage light source through the output coupler.

11. The light source according to claim 2, wherein: The coherence reduction system is configured to, for each seed beam pulse, generate the modified seed beam pulse having a bandwidth greater than a bandwidth of the seed beam pulse.

12. An apparatus for reducing speckle in an excimer light source, comprising: a light source configured to generate a light beam consisting of pulses and having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length, and each pulse being defined by a pulse duration; a coherence reduction system in a path of said beam of pulses, and said coherence reduction system being configured to, for each of said pulses in said beam, modulate an optical phase during said pulse duration of said pulse to produce a modified pulse having a second temporal coherence defined by a second temporal coherence length, said second temporal coherence length being less than said first temporal coherence length of said pulse; as well as A beam guiding device, in the path of the pulsed beam formed from the modified pulses, is configured to guide the pulsed beam formed from the modified pulses towards a substrate within a lithographic exposure apparatus.

13. The apparatus of claim 12, wherein the second temporal coherence length is between 50-95% of the first temporal coherence length.

14. The apparatus of claim 12, wherein the light source comprises: A first light source is configured to generate a seed beam consisting of seed beam pulses; and a second-stage optical amplifier configured to receive the seed beam pulses and generate a beam consisting of amplified pulses.

15. The apparatus of claim 14, wherein the first-order light source comprises a solid-state gain medium.

16. The apparatus of claim 12, wherein the light source comprises a solid-state gain medium.

17. The apparatus according to claim 12, wherein: The coherence reduction system includes a two-dimensional array of phase modulators located within a beam homogenizer.

18. The apparatus according to claim 17, wherein: The two-dimensional array of phase modulators is also configured to, for each pulse, reduce the spatial coherence of the pulse such that the modified pulse has a second spatial coherence that is less than the spatial coherence of the pulse.

19. The apparatus of claim 12, further comprising an optical temporal pulse stretcher configured to increase the duration of the modified pulse.

20. The apparatus of claim 12, wherein: The coherence reducing system comprises a Pockels cell comprising a medium through which the light beam of the pulse passes, and modulating the optical phase during the pulse duration of the pulse comprises modulating a refractive index of the medium of the Pockels cell.

21. The apparatus of claim 12, wherein: The coherence reduction system includes a single phase modulator.

22. The apparatus of claim 12, wherein: The coherence reduction system is configured to, for each pulse of the optical beam, generate the modified optical beam pulse having a bandwidth greater than a bandwidth of the optical beam pulse.

23. The apparatus of claim 12, wherein: The coherence reduction system comprises a two-dimensional array of phase modulators optically arranged parallel to each other.

24. The apparatus of claim 23, wherein the coherence reduction system is located between two lenslet arrays of a beam homogenizer, wherein each phase modulator is aligned with a pair of lenslets.

25. The apparatus of claim 23, wherein the coherence reduction system is adjacent a pair of lenslet arrays of a beam homogenizer, wherein each phase modulator is aligned with a pair of lenslets.

26. The apparatus of claim 12, further comprising a control system in communication with the coherence reduction system, the control system being configured to: determining whether the bandwidth of the modified pulse is within a target bandwidth range; and If it is determined that the bandwidth of the modified pulse is outside the range of the target bandwidth, the frequency at which the optical phase is modulated over the pulse duration of the pulse producing the modified pulse is adjusted.

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