WO2020036736A1

WO2020036736A1 - Reducing speckle in a pulsed light beam

Info

Publication number: WO2020036736A1
Application number: PCT/US2019/044327
Authority: WO
Inventors: Donald Harrison BARNHART; Eric Anders MASON; Thomas Patrick DUFFEY; Rajasekhar Madhava RAO
Original assignee: Cymer, Llc
Priority date: 2018-08-17
Filing date: 2019-07-31
Publication date: 2020-02-20
Also published as: TWI704429B; TW202013088A

Abstract

An optical apparatus includes: a pulse stretcher apparatus configured to receive an optical pulse of a pulsed light beam from a light source and to output a pulse cluster having a cluster width that is greater than the width of the received pulse, the pulse cluster being defined by a plurality of child pulses produced from the received pulse. The pulse stretcher apparatus comprises two or more optical circuits. The pulse stretcher apparatus is configured to produce each of the child pulses substantially displaced temporally from the other child pulses in the pulse cluster by at least a coherence time of the received pulse. The pulse cluster includes at least a set of three or more child pulses having peak intensities greater than a threshold level that is at least 80% of a peak intensity of a highest-intensity child pulse in the pulse cluster.

Description

REDUCING SPECKLE IN A PULSED LIGHT BEAM

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 62/719,554, filed August 17, 2018 and titled Reducing Speckle in a Pulsed Light Beam, and U.S. Application No. 62/720,675, filed August 21, 2018 and titled Reducing Speckle in a Pulsed Light Beam, both of which are incorporated herein in their entireties by reference.

TECHNICAL FIELD

[0002] The disclosed subject matter relates to a pulse stretcher apparatus for reducing speckle in a light beam supplied to a substrate of a lithography exposure apparatus.

BACKGROUND

[0003] In semiconductor lithography (or photolithography), the fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes performed on a semiconductor (for example, silicon) substrate (which is also referred to as a wafer). A lithography exposure apparatus (which is also referred to as a scanner) is a machine that applies a desired pattern onto a target region of the substrate. The wafer is fixed to a stage so that the wafer generally extends along an image plane defined by orthogonal XL and YL directions of the scanner. The wafer is irradiated by a light beam, which has a wavelength in the ultraviolet range, somewhere between visible light and x-rays, and thus has a wavelength between about 10 nanometers (nm) to about 400 nm. Thus, the light beam can have a wavelength in the deep ultraviolet (DUV) range, for example, with a wavelength that can fall from about 100 nm to about 400 nm or a wavelength in the extreme ultraviolet (EUV) range, with a wavelength between about 10 nm and about 100 nm. These wavelength ranges are not exact, and there can be overlap between whether light is considered as being DUV or EUV.

[0004] The light beam travels along an axial direction, which corresponds with the ZL direction of the scanner. The ZL direction of the scanner is orthogonal to the image plane (XL-YL). The light beam is passed through a beam delivery unit, filtered through a reticle (or mask), and then projected onto a prepared wafer. The relative position between the wafer and the light beam is moved in the image plane and the process is repeated at each target region of the wafer. In this way, a chip design is patterned onto a photoresist that is then etched and cleaned, and then the process repeats.

SUMMARY

[0005] In some general aspects, an optical apparatus includes: a pulse stretcher apparatus configured to receive an optical pulse of a pulsed light beam from a light source and to output a pulse cluster having a cluster width that is greater than the width of the received pulse, the pulse cluster being defined by a plurality of child pulses produced from the received pulse. The pulse stretcher apparatus comprises two or more optical circuits. The pulse stretcher apparatus is configured to produce each of the child pulses substantially displaced temporally from the other child pulses in the pulse cluster by at least a coherence time of the received pulse. The pulse cluster includes at least a set of three or more child pulses having peak intensities greater than a threshold level that is at least 80% of a peak intensity of a highest-intensity child pulse in the pulse cluster.

[0006] In other general aspects, a method of reducing speckle in a pulse of a pulsed light beam includes receiving a pulse of the pulsed light beam, the pulse having a pulse duration; creating a plurality of child pulses from the received pulse, wherein each child pulse comprises a plurality of sub-pulses, each child pulse having the same shape as received pulse; separating each of the child pulses from another child pulse in time by at least a coherence time of the received pulse; and outputting a pulse cluster defined by the time- separated child pulses, the pulse cluster having a cluster duration that is larger than the pulse duration of the received pulse and including at least a set of three or more child pulses having peak intensities greater than a threshold level that is at least 80% of a peak intensity of a highest-intensity child pulse in the pulse cluster.

[0007] In further general aspects, an optical apparatus includes a pulse stretcher apparatus configured to receive an optical pulse and to output, for the received pulse, a pulse cluster having a cluster width that is greater than a width of the received pulse, the pulse cluster being defined by a plurality of child pulses produced from the received pulse. The pulse stretcher apparatus includes a plurality of mirrors arranged relative to a plurality of beam splitters. The number of beam splitters in the plurality is independent of the temporal duration of the pulse cluster. The number of beam splitters in the plurality is based on a temporal duration of the received pulse and an effective number of child pulses that define the pulse cluster. DESCRIPTION OF DRAWINGS

[0008] Fig. 1 is a block diagram showing speckle in a light beam pulse as well as a temporal pulse profile of the light beam pulse;

[0009] Fig. 2 is a block diagram of an optical apparatus configured to reduce speckle and speckle contrast in the light beam pulse of Fig. 1, and also including the temporal pulse profiles of the light beam pulse input into the optical apparatus and output from the optical apparatus;

[0010] Fig. 3 is a schematic diagram showing an output pulse cluster from the optical apparatus of Fig. 2 as well as a temporal pulse profile of the pulse cluster;

[0011] Fig. 4 is a temporal pulse profile of a single child pulse in the pulse cluster showing the sub-pulses that make up the child pulse;

[0012] Fig. 5 is a block diagram of a lithography apparatus in which an optical apparatus of Fig.

2 is incorporated, the optical apparatus receiving an amplified light beam of optical pulses (or light beam pulses and outputting an amplified light beam of pulse clusters, in which each pulse cluster is formed from a single light beam pulse and has a cluster width that is greater than the width of the received light beam pulse that forms the pulse cluster;

[0013] Fig. 6 is a block diagram of an optical apparatus that includes a pulse stretcher apparatus configured to receive a light beam pulse and to output, for the received light beam pulse, a pulse cluster having a cluster width that is greater than a width of the received light beam pulse;

[0014] Fig. 7 is block diagram of an implementation of a pulse stretcher apparatus that can be used in the optical apparatus of Fig. 6, the pulse stretcher apparatus having three optical circuits;

[0015] Fig. 8 is a schematic illustration of an implementation of a pulse stretcher apparatus having three optical circuits;

[0016] Fig. 9 is a graph of the temporal pulse shape of a pulse cluster output from the pulse stretcher apparatus, in which a threshold level is at least 90% of a peak intensity of the highest- intensity child pulses;

[0017] Fig. 10 is a graph of the temporal pulse shape of a pulse cluster output from the pulse stretcher apparatus, in which a threshold level is at least 95% of a peak intensity of the highest- intensity child pulses;

[0018] Fig. 11 is a flow chart of a procedure performed for reducing speckle in a light beam pulse; [0019] Fig. 12 is a block diagram of an implementation of a light source designed as a pulsed optical source that produces an amplified light beam of optical pulses; and

[0020] Fig. 13 is a schematic illustration of an implementation of a pulse stretcher apparatus having three optical circuits.

DESCRIPTION

[0021] Referring to Fig. 1, speckle in an optical pulse 100 of a light beam occurs because the pulse 100 is highly coherent, narrow, and uniform. Speckle is a random granular pattern 102 that can be observed at a surface 104 on which the light beam pulse 100 diffusely strikes. Generally, speckle causes non-uniformity in the light intensity across the area of the surface 104. Speckle contrast can be measured and is quantified as the fractional standard deviation of the intensity map at any location on that surface 104. For example, a light beam pulse 100 having a bandwidth of 300 femtometers (fm) can exhibit speckle contrast as has as 20% in some cases.

[0022] Moreover, speckle contrast is generally inversely proportional to a width W(i) 106 of a temporal pulse profile or shape 108 of the light beam pulse 100. The width W(i) 106 of the temporal pulse profile 108 of the light beam pulse 100 is a measure of the pulse duration of the light beam pulse 100. Thus, by increasing the width of the temporal pulse profile 108 of the light beam pulse 100, the speckle contrast can be reduced. The temporal pulse profile 108 contains information on how the optical energy, spectral intensity, or power is distributed over time and is depicted in the form of a diagram or graph in which the spectral intensity (not necessarily with an absolute calibration) is plotted as a function of the time.

[0023] Referring to Fig. 2, an optical apparatus 210 is configured to reduce speckle and speckle contrast in the light beam pulse 100. The optical apparatus 210 includes a pulse stretcher apparatus 214 having two or more optical circuits 218_l , 2l8_2. The pulse stretcher apparatus 214 is configured to receive the light beam pulse 100 of the light beam from a light source 220. The pulse stretcher apparatus 214 increases the width of a temporal pulse profile 108 of the light beam pulse 100 in order to reduce speckle and speckle contrast. The pulse stretcher apparatus 214 increases the width of the temporal pulse profile 108 of the light beam pulse 100 by adding optical time delay into the light beam pulse 100. Moreover, the pulse stretcher apparatus 214 increases a number of spatially and temporally shifted child modes (or child pulses) emitted for each light beam pulse 100 and also increases or maximizes an angular (and therefore spatial) separation between each of these child pulses, as discussed in greater detail below.

[0024] Specifically, the pulse stretcher apparatus 214 outputs a pulse cluster 224 for each light beam pulse 100 that is received. The pulse cluster 224 has a combined speckle contrast that is less than that for the light beam pulse 100. In particular, the light intensity pattern 202 observed at the surface 104 on which the pulse cluster 224 diffusely strikes lacks the random granular pattern 102 observed for the light beam pulse 100.

[0025] The pulse cluster 224 is defined by a plurality of child pulses 226 produced from the received light beam pulse 100. A temporal pulse profile 238 of the pulse cluster 224 exhibits an overall width W(o) 236 that is greater than the width W(i) 106 of the temporal pulse profile 108 of the light beam pulse 100. This means that the duration of the pulse cluster 224 is greater than the duration of the light beam pulse 100. As shown in greater detail in Fig. 3, the temporal pulse profile 238 of the pulse cluster 224 exhibits a plurality of pulse shapes 246 (labeled as 246_l, 246_2, ... 246_l4), with each pulse shape 246 corresponding to a child pulse 226 (labeled as 226_l, 226_2, ... 226_l4, respectively) in the pulse cluster 224. Each of the child pulses 226 has substantially the same temporal profile or pulse shape 246 as the temporal pulse profile 108 of the light beam pulse 100. The depiction of the child pulses 226 and the pulse cluster 224 is merely provided to facilitate an understanding of the concepts and is not to scale. There may be fewer than fourteen or more than fourteen child pulses 226 and corresponding pulse shapes 246.

[0026] The pulse stretcher apparatus 214 is configured to produce each of the child pulses 226 substantially displaced temporally from the other child pulses 226 in the pulse cluster 224 by at least a coherence time Tc of the received light beam pulse 100. This is shown more clearly in the temporal pulse profile 238, in which the pulse shape 246 of each child pulse 226 is temporally displaced by an amount Dp from the pulse shape 246 of the other child pulses 226 in the pulse cluster 224, where the amount Dp is at least the coherence time Tc. The measurement of the displacement Dp is taken, for example, between the peak intensity values of the pulse shapes 246. The temporal displacement between the pulse shapes 246 of two particular child pulses 226 may or may not be distinct from the temporal displacement between the pulse shapes of two other child pulses 226. For example, as shown in Fig. 3, the amount Dpl may be different from the amount Dp2, but both Dpl and Dp2 are at least as large as the coherence time Tc. [0027] The amount of temporal coherence of the light beam pulse 100 is a measure of the average correlation between the value of the electric field waveform of the light beam pulse 100 and itself delayed by an amount t. The delay over which the phase of the electric field waveform of the light beam pulse 100 wanders by a significant amount (and hence the correlation decreases by a significant amount) is defined as the coherence time Tc of the light beam pulse 100. At a delay t of 0, the degree of coherence is perfect but the degree of coherence drops significantly as the delay t approaches Tc. The temporal coherence of the light beam pulse 100 can be measured in an interferometer such as a Michelson interferometer, in which the electric field waveform of the light beam pulse 100 is combined with a copy of itself that is delayed by a time t. A detector measures the time-averaged intensity of the light exiting the interferometers, and the resulting interference visibility gives the temporal coherence at the delay t. The interference visibility is determined by a Fourier transform of the normalized power spectral density of the light beam pulse 100, and the coherence time Tc can be calculated based on the interference visibility and the delay t.

[0028] As also shown in the temporal pulse profile 238, the pulse cluster 224 includes at least a set of three or more child pulses 226 having peak intensities I(P) that are greater than a threshold level 240. The fourteen corresponding pulse shapes 246 that have peak intensities I(P) that are greater than the threshold level 240 are labeled in Fig. 3. There may be other pulse shapes 246 that have peak intensities I(P) that are less than the threshold level 240, but for clarity, these low- intensity pulse shapes 246 (and their counterpart child pulses 226) are not labeled in Fig. 3. The threshold level 240 is at least 80% of a peak intensity 242 of a highest-intensity child pulse (or highest-intensity child pulses) 250 in the pulse cluster 224. For example, as shown in Fig. 3, child pulses 226_4, 226_9, and 226_l 1 can be considered as highest-intensity child pulses 250_4, 250_9, 250_l 1, respectively, as evident from their respective pulse shapes 246_4, 246_9, 246_l l.

[0029] As discussed above, the pulse stretcher apparatus 214 includes two or more optical circuits 218_l , 2l8_2. While only two optical circuits 218_l , 2l8_2 are shown in Fig. 2, it is possible for there to be more than two optical circuits (examples are provided below). Each optical circuit 218_l , 2l8_2 receives an input pulse (which can be the light beam pulse 100 for the optical circuit 218_l or a sub-pulse formed at the output of another optical circuit). Each optical circuit 218_l , 2l8_2 outputs a plurality of sub-pulses. Moreover, each child pulse 226 includes one or more of these sub-pulses. Thus, each child pulse 226 that is output in the pulse cluster 224 is either one sub-pulse or is a summation or combination of a plurality of sub-pulses.

[0030] For example, as shown in Fig. 4, child pulse 226_2 is a combination or summation of three sub-pulses 226_2i, 226_2ii, 226_2iii, and the pulse shape 246_2 is a summation of the respective pulse shapes 246_2i, 246_2ii, and 246_2iii of these sub-pulses. In this case, each of the sub-pulses 226_2i, 226_2ii, 226_2iii have been delayed by traveling respective paths through the pulse stretcher apparatus 214 relative to the sub-pulses in the pulse cluster 224 (and other sub-pulses in the pulse cluster 224 are delayed relative to the sub-pulses 226_2i, 226_2ii, 226_2iii). However, the sub-pulses 226_2i, 226_2ii, 226_2iii are added up in a coherent manner to form the child pulse 226_2.

[0031] The optical circuits 218_l , 2l8_2 optically act on the light beam pulse 100 to increase the duration of the light beam pulse 100 without introducing significant losses so that even if the peak power of the pulse cluster 224 is reduced its average power is maintained. The optical circuits 218_l , 2l8_2 are each optical and passive configurations of optical elements that split the amplitude of the light beam pulse (which can be the light beam pulse 100 into optical circuit 218_l or an intermediary or sub-pulse into a later optical circuit 2l8_2) into split portions. The optical circuits 218_l , 2l8_2 introduce optical delays among these split portions, and then recombine these temporally-delayed portions of the light beam pulse to provide a temporally stretched pulse (that is, the pulse cluster 224) at the output. In this way, different temporal portions (these are the child pulses 226) of the light beam pulse 100 that are not coherent are output, and the speckle noise of the pulse cluster 224 is further reduced and therefore the spatial uniformity of the pulse cluster 224 is improved.

[0032] As discussed in greater detail below, the optical circuits 218_l , 2l8_2 therefore include optical components such as beam splitters and reflective optics. The reflective optics can be flat mirrors or curved (for example, concave or convex) mirrors that could be confocal.

[0033] The beam splitters can be plate beam splitters, which are made of a thin, flat glass plate that has been coated on the first (splitting) surface of the substrate. The plate can include an anti- reflection coating on the second surface to remove unwanted Fresnel reflections. The beam splitters can dielectric mirrors (which are formed as plates). Any partially reflecting mirror can be used for splitting light beams. The angle of incidence between the surface of a beam splitter and the pulse can be 45°, but it can also have other values. A wide range of power splitting ratios can be achieved via different designs, such as, for example, dielectric coatings, thin metal coatings, polarizers, beam-splitter cubes, pellicles, and beam-splitter optics not in the shape of cubes (for example, polygonal shapes or prisms).

[0034] As discussed, the delay introduced in the child pulse 226 produced by the pulse stretcher apparatus 214 is equal to or longer than the coherence time Tc of the light beam pulse 100 from which that child pulse 226 is formed. The coherence time Tc can be on the order of a nanosecond (ns) or few nanoseconds. For example, a pulse duration of the light beam pulse 100 can be about 40 ns. Moreover, in some implementations, at any given moment, the light beam pulse 100 is temporally coherent with other moments in the light beam pulse 100 that fall within 2.5 ns of that given moment, but the light beam pulse 100 has significantly reduced coherence with moments in the light beam pulse 100 that are delayed by more than 2.5 ns. Thus, the coherence time Tc (which is the delay over which the phase or amplitude of the pulse wanders by a significant amount as discussed above) is about 2.5 ns in this example. In this example, the delay introduced in each of the child pulses 226 can be at least about 2.5 ns. The total path length that a sub-pulse takes through the pulse stretcher apparatus 214 on one pass can be anywhere between tens of centimeters (cm) to several meters (m). An example of a pulse stretcher apparatus 214 is discussed below with reference to Figs. 7 and 8.

[0035] Referring to Fig. 5, in some implementations, an optical apparatus 510 is incorporated into a lithography apparatus 554. The optical apparatus 510 receives an amplified light beam 501 of optical pulses 500 (or light beam pulses 500). The amplified light beam 501 is produced by a light source 520. The output of the optical apparatus 510 is an amplified light beam 560 of pulse clusters 524, in which each pulse cluster 524 is formed from a single light beam pulse 500 and has a cluster width 536 that is greater than the width 506 of the received light beam pulse 500 that forms the pulse cluster 524. The amplified light beam 560 is supplied to a lithography exposure apparatus 564, which uses the amplified light beam 560 for patterning a substrate 568. The optical apparatus 510 is therefore used to reduce the speckle in the amplified light beam that is used to pattern the substrate 568. In particular, speckle in the amplified light beam 501 causes an increase in speckle contrast, which is a non-uniformity in the light intensity across the illumination area on the substrate 568. This can lead to non-visual defects on the substrate 568 and lead to a reduction in yield of the process for fabricating integrated circuits with the substrate 568. The amplified light beam 560 has reduced speckle, and accordingly there is a reduction in speckle contrast at the substrate 568.

[0036] Referring to Fig. 6, an optical apparatus 610 includes a pulse stretcher apparatus 614 configured to receive the light beam pulse 600 and to output, for the received light beam pulse 600, a pulse cluster 624 having a cluster width 636 that is greater than a width 606 of the received light beam pulse 600. The light beam pulse 600 has a temporal pulse profile 608 and the pulse cluster 624 has a temporal pulse profile 238. The optical apparatus 610 is similar to the optical apparatus 210 in that the pulse cluster 624 is defined by a plurality of child pulses 626 produced from the received light beam pulse 600.

[0037] The pulse stretcher apparatus 614 includes a plurality 670 of mirrors 672 [672_l, 672_2,

... 672_M] arranged relative to a plurality 674 of beam splitters 676 [676_l, 676_2, ... 676_B]. The number B of beam splitters 676 in the plurality 674 is independent of the width 636 of the pulse cluster 624. Moreover, the number B of beam splitters 676 in the plurality 674 is based on the duration 606 of the received light beam pulse 600 and on an effective number N(eff) of child pulses 626 that define the pulse cluster 624.

[0038] Specifically, the design of the pulse stretcher apparatus 614 is based on a metric or model for estimating a speckle contrast at the substrate 568 from the pulse cluster 624. The metric is discussed next and is based on the assumption that the pulse cluster 624 has several modes (that is, it is multi-modal), and is characterized by both spatial coherence and temporal coherence. In an implementation, the speckle contrast SC at the substrate 568 is given by:

where l is the wavelength of the child pulses 626, Tc is coherence time of received light beam pulse 600, N(eff) is the effective number of child pulses 626 in the pulse cluster 624, A_beam is an effective size of the child pulse 626 (an extent taken along the plane perpendicular to the direction of travel of the child pulse 626), W^_n is an effective divergence of the light beam formed by the pulse clusters 624, and Ko is a time duration of the light beam pulse 600 input to the pulse stretcher apparatus 614. The value of N(eff) is determined based on the number of child pulses 626 and can be dependent on other characteristics of the child pulses 626. For example, the value of N(eff) can be based on the relative amplitudes of the child pulses 626. The value of Ko can be determined using any suitable metric. One example of a metric is something referred to as a total integrated square (or Tis). Tis is given by:

(/ P(t)dt)²

Tis f P(t)²dt ’ [2] where P(t) is a function of the temporal pulse profile 608 of the light beam pulse 600.

[0039] Based on the metric for the speckle contrast SC given in Equation 1, it becomes apparent that in order to reduce the value of the speckle contrast SC of the pulse cluster 624, the effective number N(eff) of child pulses 626 should be increased. Moreover, the speckle contrast SC is independent of the temporal duration of the pulse cluster 624. This is evident because the width 636 (which is a measure of the temporal duration of the pulse cluster 624) is not found in Equation 1. Lastly, the speckle contrast SC depends inversely on the time duration KO of the received light beam pulse 600 (which is exhibited as the width 606 of the temporal pulse profile 608 of the received light beam pulse 600).

[0040] Accordingly, the design of the pulse stretcher apparatus 614 takes into account the number of child pulses 626 with the factor N(eff), and that speckle contrast SC can be lower by increasing the number of child pulses 626. The number of child pulses 626 can be increased by increasing the number B of beam splitters 676 in the pulse stretcher apparatus 614.

[0041] Thus, number B of beam splitters 676 in the plurality 674 is based on the coherence time Tc of the received light beam pulse 600, the effective beam size A_beam of the child pulse 626, and the effective divergence W^_n of the light beam formed by the pulse cluster 624.

[0042] The number M of mirrors 672 in the plurality 670 can be based on a desired temporal duration 636 of the pulse cluster 624. The design of the pulse stretcher apparatus 614 does not seek to merely increase a number M of mirrors 672 in order to reduce speckle contrast, or to increase a pulse delay length, but also reduces or minimizes optical losses by using as few mirrors as possible to achieve the desired pulse delay length. As indicated in the Equation 1, the speckle contrast SC can be reduced independently of an increase in the temporal duration 636 of the pulse cluster 624 (and therefore without dependence on the number M of mirrors 672). The number M of mirrors 672 in the plurality 670 can be based on the number B of beam splitter 676 in the plurality 674. For example, the number of mirrors can be determined as the number of mirrors needed for each beam splitter to form a delay circuit (from and to the beam splitter). [0043] Using the metric in Equation 1, the effective number N(eff) of child pulses 626 that forms the pulse cluster 624 is at least five.

[0044] Referring to Fig. 7, an example of a pulse stretcher apparatus 714 having three optical circuits 718_l , 7l8_2, 718_3 is shown. Each optical circuit 718_l, 7l8_2, 718_3 includes a respective set 780_l, 780_2, 780_3 of mirrors. Moreover, each optical circuit 718_l, 7l8_2,

718_3 includes at least one beam splitter 784_l, 784_2, 784_3.

[0045] Each optical circuit 718_l , 7l8_2, 718_3 is configured to receive at least one input pulse. For the optical circuit 718_l , the input pulse is the light beam pulse 700. For the subsequent optical circuit 718_2, the input pulse is a set of sub-pulses 786_2 output by the optical circuit 718_l . The sub-pulses 786_2 are formed from the input light beam pulse 700 after being optically processed by the optical circuit 718_l . For the subsequent optical circuit 718_3 , the input pulse is a set of sub-pulses 786_3 output by the optical circuit 7l8_2. The sub-pulses 786_3 are formed from the input light beam pulse 700 after being optically processed by the optical circuits 718_l and 7l8_2. The sub-pulses 786_2 may be spaced apart from each other by different temporal separations and may have different peak intensities. Similarly, the sub-pulses 786_3 may be spaced apart from each other by different temporal separations and may have different peak intensities. The last optical circuit 718_3 outputs a plurality of sub-pulses.

Because some of the sub-pulses substantially overlap with each other temporally, two or more sub-pulses that are output from the optical circuit 718_3 combine together (are coherently summed) to form each child pulse 726 in the pulse cluster 724. Moreover, each of the child pulses 726 are separated in time from each other by an amount that is greater than the coherence time Tc, as discussed above.

[0046] In the example of Fig. 7, the optical circuits 718_l , 7l8_2, 718_3 of the pulse stretcher apparatus 714 are arranged in series with each other. It is possible for at least one of the optical circuits 718_l , 7l8_2, 718_3 of the pulse stretcher apparatus 714 to include a plurality of beam splitters.

[0047] The direction (or angular separation) of the sub-pulses as they travel through each of the optical circuits 718_l , 7l8_2, 718_3 can be adjusted by the steering of each of the sub-pulses, and this creates distinct propagation directions for each of the child pulses 726, without exceeding a maximum allowed beam divergence for the pulse cluster 724. [0048] In some implementations, each optical circuit 718_1, 7l8_2, 718_3 of the pulse stretcher apparatus 714 is designed to have a slightly different path length (and therefore a different delay time) than the other optical circuits of the pulse stretcher apparatus 714. For example, the minimum different in delay time between each optical circuit 718_l, 7l8_2, 718_3 is at least the coherence time Tc of the received light beam pulse 100.

[0049] Referring to Fig. 8, an implementation of a pulse stretcher apparatus 814 having three optical circuits 818_l , 818_2, 818_3 is shown. In this implementation, the first optical circuit 818_l includes one beam splitter 884_la; the second optical circuit 818_2, includes two beam splitters 884_2a and 884_2f; and the third optical circuit 818_3 includes one beam splitter 884_3a. The first optical circuit 818_l includes four mirrors 880_lb, 880_lc, 880_ld, 880_le arranged relative to the beam splitter 884_la to define a first closed optical path. The second optical circuit 818_2 includes six mirrors 880_2b, 880_2c, 880_2d, 880_2e, 880_ 2g, 880_2h arranged relative to the two beam splitters 884_2a and 884_2f of the second optical circuit 818_2 to define a second closed optical path. The third optical circuit 818_3 includes four mirrors 880_3b, 880_3c, 880_3d, 880_3e arranged relative to the beam splitter 884_3a to define a third closed optical path. In particular, each optical circuit 818_l , 818_2, 818_3 defines a closed optical path (or loop) in which at least one beam splitter provides both an input into the closed optical path and an output from the closed optical path. For example, in the optical circuit 818_l , the beam splitter 884_la provides both a method for inputting into and a method for outputting from the first closed optical path. In the optical circuit 818_2, the beam splitter 884_2a provides a method for inputting into and a method for outputting from the second closed optical path. In the optical circuit 818_3 , the beam splitter 884_3a provides a method for inputting into and a method for outputting from the third closed optical path.

[0050] The beam splitter 884_la is arranged and configured to divert a portion of an input pulse into at least a portion of the first closed optical path. A second portion of the input pulse passes through the beam splitter 884_la, while a third portion of the input pulse is lost as energy or heat at the beam splitter 884_la. Similarly, the beam splitter 884_2a is arranged and configured to divert a portion of an input pulse into at least a portion of the second closed optical path. A second portion of the input pulse passes through the beam splitter 884_2a, while a third portion of the input pulse is lost as energy or heat at the beam splitter 884_2a. The beam splitter 8842f is also in the second closed optical path and it also provides a way in which to divert a portion of its input pulse through a last part of the second closed optical path as well.

[0051] In some implementations, each of the mirrors in each optical circuit 818_l , 818_2, 818_3 has a reflective surface that interacts with the input pulse or sub-pulses. This reflective surface is the surface in Fig. 8 that interacts with the dashed line (which represents the input pulse or sub pulses). Each reflective surface of each of the mirrors of the optical circuits 818_l , 818_2, 818_3 can have a reflectivity greater than 98 % or greater than 99%. For example, the reflectivity of each of the mirrors of the optical circuits can be greater than or equal to 98%, 99%, 99.5%, 99.9%, or 99.92%, or can be other suitable values.

[0052] Each beam splitter 884_la, 884_2a, 884_2f, and 884_3a has a first splitting surface that receives the input pulse and splits or divides that pulse into two or more sub-pulses. In general, each of the beam splitters in an optical circuit has a first splitting surface that has a reflectivity between 25-75%. For example, if the reflectivity of the beam splitter 884_la is 25% then 25% of the power of the input pulse is split and directed to the mirror 880_lb (and through the first closed optical path) while 75% of the power of the input pulse passes through the beam splitter 884_la and exits the optical circuit 818_l as a sub-pulse. As another example, if the reflectivity of the beam splitter 884_2a is 50% then 50% of the power of the input pulse is split and directed to the mirror 880_2b (and through the second closed optical path) while 50% of the power of the input pulse passes through the beam splitter 884_2a and is directed toward the beam splitter 884_2f as a sub-pulse.

[0053] In some implementations, the non-splitting second surface of a beam splitter has a reflectivity less than 5%. For example, the non-splitting second surface of the beam splitter 884_la can be coated with an anti-reflection coating.

[0054] In some implementations, the reflectivity of the splitting surface of the beam splitter 884_la is about 60-65% (or about 62-63%); the reflectivity of the splitting surface of the beam splitter 884_2a is about 65-72% (or about 68-69%); the reflectivity of the splitting surface of the beam splitter 884_2f is about 28-35% (or about 31-33%); and the reflectivity of the splitting surface of the beam splitter 884_3a is about 60-65% (or about 62-63%) The precise reflectivity of each of the beam splitters can be controlled to within 0.5% or a target value.

[0055] The mirrors can be arranged in each optical circuit 818_1 , 818_2, 818_3 such that each sub-pulse that is output from the optical circuit 818_1 , 818_2, 818_3 is spatially separated from the other sub-pulses at the splitting surface of the beam splitter. In this way, the mirrors can be used to separate the sub-pulses from each other in space (spatially).

[0056] In some implementations, each of the mirrors of one or more of the optical circuits 818_l , 818_2, 818_3 is a concave mirror having a curved surface that is a segment of a sphere. Such a confocal imaging design is used in order to retain the optical pulse of the light beam within the optical circuits as it is propagated through the optical circuits.

[0057] As shown in this implementation, each optical circuit 818_l , 818_2, 818_3 has an even number (four, six, and four, respectively) of mirrors.

[0058] As discussed above, each of the beam splitters 884_la, 884_2a, 884_2f, 884_3a includes at least one splitting surface. In various implementations, at least one of the beam splitters 884_la, 884_2a, 884_2f, 884_3a includes two splitting surfaces that face each other. For example, one or more of the beam splitters can have partially-transmissive and partially- reflective coatings on both an entry surface and an exit surface. In various implementations, each of the two splitting surfaces of the beam splitter has a reflectivity that is between 25-75%. For some configurations of the apparatus, the pulses are split in both directions going through a particular beam splitter.

[0059] Referring to Fig. 9, in other implementations, the threshold level 940 is at least 90% of the peak intensity 242 of the highest-intensity child pulses 226_4, 226_9, and 226_l 1 in the pulse cluster 224. Referring to Fig. 10, in other implementations, the threshold level 1040 is at least 95% of the peak intensity 242 of the highest-intensity child pulses 226_4, 226_9, and 226_l 1 in the pulse cluster 224.

[0060] Referring to Fig. 11, a procedure 1182 is performed for reducing speckle in a light beam pulse. Initially, a pulse 100 of the pulsed light beam is received (1183), for example, from the light source 220. The light beam pulse 100 has a pulse duration 106 (as shown in the temporal pulse profile 108 of Fig. 2). A plurality of child pulses 226 are created from the received light beam pulse 100 (1184). For example, the light beam pulse 100 is directed through the pulse stretcher apparatus 214, which produces the pulse cluster 224 including the child pulses 226. As discussed above, each child pulse 226 is made up of one or more sub-pulses. And, each child pulse 226 has the same temporal pulse profile shape as that of the received light beam pulse 100. Additionally, each of the child pulses 226 is separated from other child pulses 226 in time by at least the coherence time Tc of the received light beam pulse 100 (1185). In particular, the mirrors and beam splitters within the optical circuits 218_1, 2l8_2 are arranged and designed to enable this separation. The pulse cluster 224 defined by the time-separated child pulses 226 is outputted (1186). The pulse cluster 224 has a cluster duration 236 that is larger than the pulse duration 106 of the received light beam pulse 100. The pulse cluster 224 includes at least a set of three or more child pulses 226 having peak intensities greater than a threshold level 240, the threshold level being at least 80% of the peak intensity 242 of a highest-intensity child pulse (or pulses) in the pulse cluster 224.

[0061] The child pulses 226 can be created from the received light beam pulse 100 (1184) by splitting the received light beam pulse 100 having the pulse duration 106 into a plurality of sub pulses by directing the received light beam pulse 100 through the plurality of optical circuits 218_l , 2l8_2.

[0062] The child pulses 226 can be separated in time from the other child pulses by at least a coherence time Tc of the received light beam pulse 100 (1185) by splitting the received light beam pulse 100 into the sub-pulses and directing at least some of the sub-pulses onto paths having distinct lengths. For example, the first optical path defined by the optical circuit 818_l has a different length than the second optical path defined by the optical circuit 818_2.

Moreover, it is possible for there to be closed optical paths within the optical circuit 818_2 that have distinct lengths.

[0063] The procedure 1182 can also include separating each of the child pulses 226 from the other child pulses 226 in space (in additional to temporally), by, for example, mis-aligning the mirrors in each of the optical circuits in the pulse stretcher apparatus. In this way, the pulse cluster 224 is defined by the time- separated and spatially-separated child pulses 226.

[0064] The cluster duration 236 of the pulse cluster 224 can be at least five times greater than the pulse duration 106 of the received light beam pulse 100.

[0065] Referring to Fig. 12, in some implementations, the light source 520 is designed as a pulsed optical source 1220 that produces an amplified light beam 1201 of optical pulses 1200. The optical source 1220 is a two- stage system that includes a first gas discharge stage 1290 and a second gas discharge stage 1291. In general, the first stage 1290 includes a first gas discharge chamber housing an energy source and containing a gas mixture that includes a first gain medium the second gas discharge stage 1291 includes a second gas discharge chamber housing an energy source and containing a gas mixture that includes a second gain medium. [0066] The first stage 1290 includes a master oscillator (MO) and the second stage 1291 includes a power amplifier (PA). The MO provides a seed light beam 1292 to the PA. The master oscillator typically includes a gain medium in which amplification occurs and an optical feedback mechanism such as an optical resonator. The power amplifier typically includes a gain medium in which amplification occurs when seeded with the seed laser beam 1292 from the master oscillator. If the power amplifier is designed as a regenerative ring resonator then it is described as a power ring amplifier (PRA) and in this case, enough optical feedback can be provided from the ring design. A spectral feature adjuster 1293 receives a pre-cursor light beam from the master oscillator of the first stage 1290 to enable fine tuning of spectral parameters such as the center wavelength and the bandwidth of the light beam 1292 at relatively low output pulse energies. The power amplifier receives the light beam 1292 from the master oscillator and amplifies this output to attain the necessary power for output to use in photolithography by the lithography exposure apparatus (such as apparatus 564).

[0067] The master oscillator includes a discharge chamber having two elongated electrodes, a laser gas that serves as the gain medium, and a fan circulating the gas between the electrodes. A laser resonator is formed between the spectral feature adjuster 1293 on one side of the discharge chamber, and an output coupler 1294 on a second side of the discharge chamber to output the seed light beam 1292 to the power amplifier.

[0068] The power amplifier includes a power amplifier discharge chamber, and if it is a regenerative ring amplifier, the power amplifier also includes a beam reflector or beam turning device that reflects the light beam back into the discharge chamber to form a circulating path.

The power amplifier discharge chamber includes a pair of elongated electrodes, a laser gas that serves as the gain medium, and a fan for circulating the gas between the electrodes. The seed light beam 1292 is amplified by repeatedly passing through the power amplifier. The second stage 1291 can include a beam modification optical system that provides a way (for example, a partially-reflecting mirror) to in-couple the seed light beam 1292 and to out-couple a portion of the amplified radiation from the power amplifier to form the amplified light beam 501.

[0069] The laser gas used in the discharge chambers of the master oscillator and the power amplifier can be any suitable gas for producing a laser beam around the required wavelengths and bandwidth. For example, the laser gas can be argon fluoride (ArF), which emits light at a wavelength of about 193 nm, or krypton fluoride (KrF), which emits light at a wavelength of about 248 nm.

[0070] In general, the light source 1220 can also include a control apparatus 1295 in

communication with the first stage 1290 and the second stage 1291. The control apparatus 1295 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control apparatus 1295 includes memory, which can be read-only memory and/or random- access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of 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 apparatus 180 can also include one or more input devices (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices (such as a speaker or a monitor).

[0071] The control apparatus 1295 includes one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by a programmable processor. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processor receives instructions and data from memory. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application- specific integrated circuits).

[0072] The control apparatus 1295 includes a set of modules, with each module including a set of computer program products executed by one or more processors such as the processors.

Moreover, any of the modules can access data stored within the memory. Each module can receive data from other components and then analyze such data as needed. Each module can be in communication with one or more other modules.

[0073] Although the control apparatus 1295 is represented as a box (in which all of its components can be co-located), it is possible for the control apparatus 1295 to be made up of components that are physically remote from each other. For example, a particular module can be physically co-located with the optical source 1220 or a particular module can be physically co located with the spectral feature adjuster 1293. [0074] Referring to Fig. 13, another implementation of a pulse stretcher apparatus 1314 includes four optical circuits 13 l8_l, 1318_2, 1318_3 , 1318_4. In this implementation, each of the optical circuits 13 l8_l, 1318_2, 1318_3 , 1318_4 includes a single respective beam splitter l384_la, l384_2a, l384_3a, l384_4a. Each optical circuit 13 l8_l, 1318_2, 1318_3 , 1318_4 includes four mirrors arranged relative to the respective beam splitter to define a closed optical path. In particular, the first optical circuit 13 l8_l includes four mirrors l380_lb, l380_lc, l380_ld, and l380_le; the second optical circuit 1318_2 includes four mirrors l380_2b, l380_2c, l380_2d, and l380_2e; the third optical circuit 1318_3 includes four mirrors l380_3b, l380_3c, l380_3d, and l380_3e; and the fourth optical circuit 1318_4 includes four mirrors l380_4b, l380_4c, l380_4d, and l380_4e. In particular, each optical circuit 13 l8_l, 1318_2, 1318_3 , 1318_4 defines a closed optical path (or loop) in which at least one beam splitter provides both an input into the closed optical path and an output from the closed optical path.

[0075] Each beam splitter l384_la, l384_2a, l384_3a, l384_4a is arranged and configured to divert a portion of an input pulse into at least a portion of the respective closed optical path. A second portion of the input pulse passes through each beam splitter l384_la, l384_2a, l384_3a, l384_4a, while a third portion of the input pulse is lost as energy or heat at each beam splitter l384_la, l384_2a, l384_3a, l384_4a.

[0076] In some implementations, each of the mirrors in each optical circuit 13 l8_l, 1318_2,

1318_3 , 1318_4 has a reflective surface that interacts with the input pulse or sub-pulses. This reflective surface is the surface in Fig. 13 that interacts with the dashed line (which represents the input pulse or sub-pulses). Each reflective surface of each of the mirrors of the optical circuits 13 l8_l, 1318_2, 1318_3 , 1318_4 can have a reflectivity greater than 90%, greater than 95%, greater than 98%, or greater than 99%.

[0077] Each beam splitter l384_la, l384_2a, l384_3a, l384_4a has a first splitting surface that receives the input pulse and splits or divides that pulse into two or more sub-pulses. In general, each of the beam splitters in an optical circuit has a first splitting surface that has a reflectivity between 25-75%. For example, if the reflectivity of the beam splitter 1384_ la, l384_2a, l384_3a, l384_4a is 25% then 25% of the power of the input pulse is split and directed to the first mirror in the circuit while 75% of the power of the input pulse passes through the beam splitter l384_la, l384_2a, l384_3a, l384_4a and exits the optical circuit 13 l8_l, 1318_2,

1318_3 , 1318_4 as a sub-pulse. As another example, if the reflectivity of the beam splitter l384_la, l384_2a, l384_3a, l384_4a is 50% then 50% of the power of the input pulse is split and directed to the first mirror in the optical circuit while 50% of the power of the input pulse passes through the beam splitter.

[0078] In some implementations, the non-splitting second surface of a beam splitter has a reflectivity less than 5%. For example, the non-splitting second surface of the beam splitter can be coated with an anti-reflection coating.

[0079] In some implementations, the reflectivity of the splitting surface of each beam splitter l384_la, l384_2a, l384_3a, l384_4a is about 60-66% (or about 62-64%). For example, the reflectivity of the splitting surface of each beam splitter l384_la, l384_2a, l384_3a, l384_4a can be about 64%. The precise reflectivity of each of the beam splitters can be controlled to within 0.5% or a target value.

[0080] The mirrors can be arranged in each optical circuit 13 l8_l, 1318_2, 1318_3 , 1318_4 such that each sub-pulse that is output from the optical circuit 13 l8_l, 1318_2, 1318_3 , 1318_4 is spatially separated from the other sub-pulses at the splitting surface of the beam splitter. In this way, the mirrors can be used to separate the sub-pulses from each other in space (spatially).

[0081] In some implementations, each of the mirrors of one or more of the optical circuits 13 l8_l, 1318_2, 1318_3 , 1318_4 is a concave mirror having a curved surface that is a segment of a sphere. Such a confocal imaging design is used in order to retain the optical pulse of the light beam within the optical circuits as it is propagated through the optical circuits.

[0082] As shown in this implementation, each optical circuit 13 l8_l, 1318_2, 1318_3 , 1318_4 has an even number (four, six, and four, respectively) of mirrors.

[0083] As discussed above, each of the beam splitters includes at least one splitting surface. In other implementations, each of the beam splitters includes two splitting surfaces that face each other. Moreover, each of the two splitting surfaces of the beam splitter has a reflectivity that is between 25-75%. In this way, the pulses are split in both directions going through a particular beam splitter.

[0084] The embodiments may further be described using the following clauses:

1. An optical apparatus comprising:

a pulse stretcher apparatus configured to receive an optical pulse of a pulsed light beam from a light source and to output a pulse cluster having a cluster width that is greater than the width of the received pulse, the pulse cluster being defined by a plurality of child pulses produced from the received pulse;

wherein the pulse stretcher apparatus comprises two or more optical circuits;

wherein the pulse stretcher apparatus is configured to produce each of the child pulses substantially displaced temporally from the other child pulses in the pulse cluster by at least a coherence time of the received pulse; and

wherein the pulse cluster includes at least a set of three or more child pulses having peak intensities greater than a threshold level that is at least 80% of a peak intensity of a highest- intensity child pulse in the pulse cluster.

2. The optical apparatus of clause 1, wherein each optical circuit is configured to receive an input pulse that is the received pulse or a sub-pulse formed from the input pulse and to output a plurality of sub-pulses, wherein each child pulse includes one or more sub-pulses.

3. The optical apparatus of clause 1, wherein the optical circuits of the pulse stretcher apparatus are arranged in series with each other.

4. The optical apparatus of clause 1, wherein at least one of the optical circuits of the pulse stretcher apparatus includes a plurality of beam splitters.

5. The optical apparatus of clause 1, wherein:

a first optical circuit of the pulse stretcher apparatus includes one beam splitter;

a second optical circuit of the pulse stretcher apparatus includes two beam splitters; and a third optical circuit of the pulse stretcher apparatus includes one beam splitter.

6. The optical apparatus of clause 5, wherein:

the first optical circuit of the pulse stretcher apparatus includes four mirrors arranged relative to the beam splitter of the first optical circuit to define a first closed optical path;

the second optical circuit of the pulse stretcher apparatus includes six mirrors arranged relative to the two beam splitters of the second optical circuit to define a second closed optical path; and

the third optical circuit of the pulse stretcher apparatus includes four mirrors arranged relative to the beam splitter of the third optical circuit to define a third closed optical path.

7. The optical apparatus of clause 1, wherein each optical circuit defines a closed optical path in which at least one beam splitter provides both an input into the closed optical path and an output from the closed optical path. 8. The optical apparatus of clause 1, wherein each of the child pulses has substantially the same temporal profile as the received pulse.

9. The optical apparatus of clause 1, wherein each optical circuit comprises:

an optical arrangement including a plurality of mirrors, and

at least one beam splitter configured to divert a portion of an input pulse into at least a portion of the optical arrangement.

10. The optical apparatus of clause 9, wherein each of the mirrors of an optical circuit has a reflectivity greater than 99%.

11. The optical apparatus of clause 9, wherein each of the beam splitters of an optical circuit has a splitting surface that has a reflectivity between 25-75%.

12. The optical apparatus of clause 11, wherein each of the beam splitters of an optical circuit has a non-splitting surface that has a reflectivity less than 5%.

13. The optical apparatus of clause 9, wherein the mirrors are arranged in an optical circuit such that each sub-pulse is spatially separated from the other sub-pulses at the splitting surface of the beam splitter.

14. The optical apparatus of clause 9, wherein each of the mirrors of an optical circuit is a concave mirror having a curved surface that is a segment of a sphere.

15. The optical apparatus of clause 9, wherein each optical circuit comprises an even number of mirrors.

16. The optical apparatus of clause 9, wherein at least one of the beam splitters of an optical circuit includes two splitting surfaces, and each of the two splitting surfaces of the beam splitter has a reflectivity that is between 25-75%.

17. The optical apparatus of clause 1, wherein each child pulse is a coherent summation of sub-pulses that separated in time by substantially less than the coherence time of the received pulse.

18. The optical apparatus of clause 1, wherein the threshold level is at least 90% of the peak intensity of the highest-intensity child pulse in the pulse cluster.

19. The optical apparatus of clause 1, wherein the threshold level is at least 95% of the peak intensity of the highest-intensity child pulse in the pulse cluster.

20. A method of reducing speckle in a pulse of a pulsed light beam, the method comprising: receiving a pulse of the pulsed light beam, the pulse having a pulse duration; creating a plurality of child pulses from the received pulse, wherein each child pulse comprises a plurality of sub-pulses, each child pulse having the same shape as received pulse; separating each of the child pulses from another child pulse in time by at least a coherence time of the received pulse; and

outputting a pulse cluster defined by the time- separated child pulses, the pulse cluster having a cluster duration that is larger than the pulse duration of the received pulse and including at least a set of three or more child pulses having peak intensities greater than a threshold level that is at least 80% of a peak intensity of a highest-intensity child pulse in the pulse cluster.

21. The method of clause 20, wherein creating the child pulses from the received pulse comprises splitting the received pulse having the pulse duration into a plurality of sub-pulses by directing the received pulse through a plurality of optical circuits.

22. The method of clause 20, wherein separating each of the child pulses from another child pulse in time by at least a coherence time of the received pulse comprises splitting the received pulse into the sub-pulses and directing at least some of the sub-pulses onto paths having distinct lengths.

23. The method of clause 20, further comprising separating each of the child pulses from the other child pulses in space,

wherein outputting the pulse cluster defined by the time-separated child pulses comprises outputting the pulse cluster defined by the spatially- separated child pulses.

24. The method of clause 20, wherein the coherence time is on the order of a nanosecond

(ns).

25. The method of clause 20, wherein outputting the pulse cluster defined by the time- separated child pulses comprises outputting the pulse cluster having a cluster duration that is at least five times greater than the pulse duration of the received pulse.

26. An optical apparatus comprising:

a pulse stretcher apparatus configured to receive an optical pulse and to output, for the received pulse, a pulse cluster having a cluster width that is greater than a width of the received pulse, the pulse cluster being defined by a plurality of child pulses produced from the received pulse;

wherein: the pulse stretcher apparatus comprises a plurality of mirrors arranged relative to a plurality of beam splitters,

the number of beam splitters in the plurality is independent of the temporal duration of the pulse cluster, and

the number of beam splitters in the plurality is based on a temporal duration of the received pulse and an effective number of child pulses that define the pulse cluster.

27. The optical apparatus of clause 26, wherein the number of mirrors in the pulse stretching apparatus is based on a desired duration of the pulse cluster.

28. The optical apparatus of clause 26, wherein the number of beam splitters in the plurality is based on a coherence time of the received pulse, an effective beam size of the received pulse, and an effective beam divergence of the received pulse.

29. The optical apparatus of clause 26, wherein a number of child pulses that forms the pulse cluster is at least five.

[0085] Other implementations are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. An optical apparatus comprising:

wherein the pulse stretcher apparatus comprises two or more optical circuits;

2. The optical apparatus of claim 1, wherein each optical circuit is configured to receive an input pulse that is the received pulse or a sub-pulse formed from the input pulse and to output a plurality of sub-pulses, wherein each child pulse includes one or more sub-pulses.

3. The optical apparatus of claim 1, wherein the optical circuits of the pulse stretcher apparatus are arranged in series with each other.

4. The optical apparatus of claim 1, wherein at least one of the optical circuits of the pulse stretcher apparatus includes a plurality of beam splitters.

5. The optical apparatus of claim 1, wherein:

a first optical circuit of the pulse stretcher apparatus includes at least one beam splitter; a second optical circuit of the pulse stretcher apparatus includes at least one beam splitter; and

a third optical circuit of the pulse stretcher apparatus includes at least one beam splitter.

6. The optical apparatus of claim 1, wherein each optical circuit defines a closed optical path in which at least one beam splitter provides both an input into the closed optical path and an output from the closed optical path.

7. The optical apparatus of claim 1, wherein each of the child pulses has substantially the same temporal profile as the received pulse.

8. The optical apparatus of claim 1, wherein each optical circuit comprises:

an optical arrangement including a plurality of mirrors, and

9. The optical apparatus of claim 8, wherein each of the mirrors of an optical circuit has a reflectivity greater than or equal to 98% or greater than or equal to 99%.

10. The optical apparatus of claim 8, wherein each of the beam splitters of an optical circuit has a splitting surface that has a reflectivity between 25-75%.

11. The optical apparatus of claim 10, wherein each of the beam splitters of an optical circuit has a non-splitting surface that has a reflectivity less than 5%.

12. The optical apparatus of claim 8, wherein the mirrors are arranged in an optical circuit such that each sub-pulse is spatially separated from the other sub-pulses at the splitting surface of the beam splitter.

13. The optical apparatus of claim 8, wherein each of the mirrors of an optical circuit is a concave mirror having a curved surface that is a segment of a sphere.

14. The optical apparatus of claim 8, wherein at least one of the beam splitters of an optical circuit includes two splitting surfaces, and each of the two splitting surfaces of the beam splitter has a reflectivity that is between 25-75%.

15. The optical apparatus of claim 1, wherein each child pulse is a coherent summation of sub-pulses that separated in time by substantially less than the coherence time of the received pulse.

16. The optical apparatus of claim 1, wherein the threshold level is at least 90% or at least 95% of the peak intensity of the highest- intensity child pulse in the pulse cluster.

17. A method of reducing speckle in a pulse of a pulsed light beam, the method comprising:

receiving a pulse of the pulsed light beam, the pulse having a pulse duration;

creating a plurality of child pulses from the received pulse, wherein each child pulse comprises a plurality of sub-pulses, each child pulse having the same shape as received pulse; separating each of the child pulses from another child pulse in time by at least a coherence time of the received pulse; and

18. The method of claim 17, wherein creating the child pulses from the received pulse comprises splitting the received pulse having the pulse duration into a plurality of sub-pulses by directing the received pulse through a plurality of optical circuits.

19. The method of claim 17, wherein separating each of the child pulses from another child pulse in time by at least a coherence time of the received pulse comprises splitting the received pulse into the sub-pulses and directing at least some of the sub-pulses onto paths having distinct lengths.

20. The method of claim 17, further comprising separating each of the child pulses from the other child pulses in space, wherein outputting the pulse cluster defined by the time-separated child pulses comprises outputting the pulse cluster defined by the spatially- separated child pulses.

21. The method of claim 17, wherein outputting the pulse cluster defined by the time- separated child pulses comprises outputting the pulse cluster having a cluster duration that is at least five times greater than the pulse duration of the received pulse.

22. An optical apparatus comprising:

wherein:

the pulse stretcher apparatus comprises a plurality of mirrors arranged relative to a plurality of beam splitters,

23. The optical apparatus of claim 22, wherein the number of mirrors in the pulse stretching apparatus is based on a desired duration of the pulse cluster.

24. The optical apparatus of claim 22, wherein the number of beam splitters in the plurality is based on a coherence time of the received pulse, an effective beam size of the received pulse, and an effective beam divergence of the received pulse.

25. The optical apparatus of claim 22, wherein a number of child pulses that forms the pulse cluster is at least five.