Pulse stretcher system and method

Abstract

The invention discloses a pulse stretcher unit and a corresponding inspection system. The pulse stretcher unit includes a selected number of beam splitters arranged along a selected main path and including an input beam splitter, an output beam splitter, and one or more intermediate beam splitters; and a light reflective surface arrangement defining a selected number n of secondary paths extending between the selected number of beam splitters. Wherein the selected number of beam splitters direct an input beam between the selected primary path and the selected number of secondary paths for splitting an input pulse into a series of pulses having substantially equal amplitudes. And wherein the length of the secondary path approximately follows a series of forms L / 2k, where k = 0, 1, 2, where L is a selected length of a secondary path.

Description

Technical Field

[0001] The present disclosure relates to the manipulation of electromagnetic radiation pulses, and in particular to techniques for stretching a pulse into multiple similar pulses. Background Art

[0002] Short laser pulses provide high instantaneous amplitude with low average power. Therefore, short laser pulses are often applied to study nonlinear interactions in light.

[0003] US 5,309,456 provides a device (20) for increasing the length of a laser pulse to reduce its peak power without substantially losing the average power of the pulse. The device (20) uses a white unit (10) having a plurality of optical delay paths (18a-18d) which continuously increase the number of round trips between a field lens (13) and an objective lens (11 and 12). A pulse (26) from a laser (27) travels to a laser output end (37) via a multi-branch reflection path (28) between a beam splitter (21) and a total reflection mirror (24). The laser pulse (26) is also injected into the input mirrors (14a-14d) of the optical delay paths (18a-18d) via the beam splitter (21). Pulses from the output mirrors (16a-16d) of the optical delay path (18a-18d) go simultaneously to the laser output (37) and the input mirrors (14b-14d) of the longer optical delay path. The beam splitter (21) is 50% reflective and 50% transmissive, providing equal attenuation of all pulses at the laser output (37).

[0004] US 5,337,333 describes a method of formatting a laser beam pulse (20) using one or more delay loops (10). The delay loop (10) has a partially reflecting beam splitter (12) and a plurality of highly reflective mirrors (14) arranged so that the laser beam pulse (20) enters the delay loop (10) through the beam splitter (12) and circulates therein along a delay loop length (24) defined by the mirrors (14). As the laser beam pulse (20) circulates within the delay loop (10), a portion of the laser beam pulse (20) is emitted onto each complete loop when it reaches the beam splitter (12). The laser beam pulse (20) is thereby formatted into a plurality of sub-pulses (50, 52, 54 and 56). The delay loops (10) are used in combination to produce a complex waveform by combining the sub-pulses (50, 52, 54 and 56) using additive waveform synthesis.

[0005] US 7,567,343 describes that as circuit patterns have become more refined in recent years, it is necessary to improve the detection sensitivity of defects. In order to solve this problem, a laser with a UV band wavelength is used as an irradiation laser to increase the sensitivity. Pulse oscillation lasers are generally used as UV lasers. However, the peak value (maximum output) of pulse oscillation lasers is too large for the required average output power. For example, when the average output power of the laser is 2W, the pulse interval is 10ns, and the pulse width is 10ps, the peak value (maximum output) becomes as high as 2kW, and there is a possibility of damage to the sample. Therefore, it is necessary to reduce the peak value (maximum output) while maintaining the average output power so that the sample is not damaged. In the present invention, the device is configured so that the pulse light is optically divided into several pulses, and these pulses are given corresponding paths, and the lengths of these paths are set to be different from each other, thereby reducing the peak value (maximum output) while maintaining the average output value. Summary of the invention

[0006] Laser irradiation provides efficient and high intensity irradiation of a selected wavelength range. Laser irradiation is generally capable of achieving selected spatial and / or temporal coherence of the output light, thereby providing various advantages over other light sources. Therefore, the use of laser irradiation, and in particular pulsed laser irradiation, provides various advantages in irradiation applications.

[0007] Pulsed laser irradiation provides higher instantaneous irradiation intensity while using lower average power over time. However, some applications may require increasing the pulse duration, as well as tailoring the length and amplitude of the irradiation pulses.

[0008] The present invention provides a pulse stretcher unit, which is suitable for manipulating an input illumination pulse beam to generate an output beam formed by a pulse train, the pulse train comprising a selected number of illumination pulses with substantially equal pulse amplitudes. The pulse stretcher of the present disclosure is configured to increase the time and / or longitudinal length of the output pulse, so that the pulse etendue can be increased and the pulse temporal coherence can be reduced.

[0009] The present disclosure provides a pulse stretcher unit formed by a plurality of beam splitters configured to receive an input light beam and split the input light beam into propagation along a selected common propagation axis and along a selected number of auxiliary paths defined by an arrangement of reflective surfaces. The arrangement of the beam splitters directs a plurality of light portions to a selected number of auxiliary paths having optical lengths selected according to a predetermined length ratio, thereby providing a selected separation between sub-pulses of an output light beam to provide an output light beam carrying a pulse train formed by a selected number of pulses having substantially equal pulse amplitudes.

[0010] The pulse stretcher can be configured to selectively change the spatial arrangement of the output beam, thereby enabling the spread of the output beam to be increased. It can be used to adjust selected illumination conditions for spatial illumination and spatial inspection / imaging of semiconductor wafers having selected patterns and / or for inspection / imaging of selected mask patterns used in lithography or other manufacturing processes.

[0011] Thus, according to a broad aspect, the present disclosure provides a pulse stretcher unit comprising:

[0012] a selected number of beam splitters arranged along a selected primary path and comprising an input beam splitter, an output beam splitter, and one or more intermediate beam splitters, and

[0013] an arrangement of light reflecting surfaces defining a selected number n of secondary paths extending between the selected number of beam splitters; wherein

[0014] (a) the input beam splitter is configured to receive an input light beam and direct a first portion of the light beam along the selected primary path and a second portion of the light beam along a first secondary path;

[0015] (b) the output beam splitter is configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the nth secondary path, and to provide at least one output beam propagating along the selected primary path; and

[0016] (c) each one or more intermediate beam splitters are configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the i-th secondary path, and to provide a first output beam portion propagating along the selected primary path and a second output beam portion propagating along the corresponding i+1-th secondary path; and wherein

[0017] The length of the secondary path follows approximately the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path.

[0018] Typically, a beam splitter before the last beam splitter may direct the second beam portion to propagate along the nth auxiliary path.

[0019] According to some embodiments, the length of the secondary path follows the form L / 2 k , k = 0, 1, 2, where L is a selected length of a secondary path with a length variation of at most 25%. In some embodiments, the length of the secondary path can be from the form L / 2 k Up to 10% variation, k = 0, 1, 2, where L is the selected length of a secondary path. In some embodiments, the allowed variation is up to 5%.

[0020] According to some embodiments, the length of the secondary path can be greater than the length of the portion of the selected primary path between adjacent beam splitters. For example, the length of the portion of the selected primary path between adjacent beam splitters can be on the order of 1-50 centimeters, and the length of the secondary path can vary between 0.5 meters and 10 meters.

[0021] According to some embodiments, the pulse stretcher unit may also include an output reflecting surface, wherein the output beam splitter provides a portion of the output beam to propagate along the selected main path and provides a second output beam portion to propagate along an output auxiliary path, and the output reflecting surface is positioned to guide the second output beam portion to propagate along the selected auxiliary output path.

[0022] According to some embodiments, the selected number of beam splitters includes one or more beam splitters mounted on a movable platform, which can be moved along at least one axis perpendicular to the selected main path so that the output beam can be in the form of two parallel output beams.

[0023] According to some embodiments, the output beam splitter is mounted on a movable platform that is movable along at least one axis perpendicular to the selected main path, so that the output beam can be in the form of two parallel output beams.

[0024] According to some embodiments, the light reflecting surface arrangement includes a selected number of curved reflecting surfaces configured to focus light beams reaching the curved reflecting surfaces to maintain at least one of a beam divergence angle and a beam width.

[0025] According to some embodiments, the selected number of beam splitters consists of polarization independent beam splitters.

[0026] According to some embodiments, the input beam splitter may be a polarizing beam splitter, and the output beam splitter and one or more intermediate beam splitters are polarization-independent beam splitters.

[0027] According to some embodiments, the pulse stretcher unit may further comprise one or more polarization rotation plates located in the selected main path between the input beam splitter and an adjacent beam splitter.

[0028] According to some embodiments, the pulse stretcher unit may further include a polarization rotation plate located in the auxiliary path between the input beam splitter and an adjacent beam splitter.

[0029] According to some embodiments, the selected number of beam splitters includes polarization-independent beam splitters.

[0030] According to some embodiments, the selected number of beamsplitters includes or consists of beamsplitters configured to split the input light beam at a power ratio between 0.4:0.6 and 0.6:0.4.

[0031] According to some embodiments, the arrangement of beam splitters defines one or more secondary paths, and wherein the one or more secondary paths are folded by one or more reflective surfaces, the one or more reflective surfaces being arranged to cause radiation to reflect back and forth between the reflective surfaces while passing through the secondary paths.

[0032] According to some embodiments, at least one secondary path comprises or is formed by a plurality of spherical mirrors and a common plane mirror arranged such that the radiation beam is reflected a selected number of times between the spherical mirrors and the plane mirror.

[0033] According to some embodiments, the pulse stretcher unit may operate in the ultraviolet wavelength range.

[0034] According to another broad aspect, the present disclosure provides a pulse stretching system, comprising at least one first pulse stretcher unit and at least one second pulse stretcher unit; the at least one first pulse stretcher unit comprising:

[0035] a selected number of beam splitters arranged along a selected primary path and comprising an input beam splitter, an output beam splitter, and one or more intermediate beam splitters, and

[0036] an arrangement of light reflecting surfaces defining a selected number n of secondary paths extending between the selected number of beam splitters; wherein

[0037] (a) the input beam splitter is configured to receive an input light beam and direct a first portion of the light beam along the selected primary path and a second portion of the light beam along a first secondary path;

[0038] (b) the output beam splitter is configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the nth secondary path, and to provide at least one output beam propagating along the selected primary path; and

[0039] (c) each one or more intermediate beam splitters are configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the i-th secondary path, and to provide a first output beam portion propagating along the selected primary path and a second output beam portion propagating along the corresponding i+1-th secondary path; wherein

[0040] The length of the secondary path follows approximately the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path; and where

[0041] The at least one second pulse stretcher unit is placed downstream of the at least one first pulse stretcher unit such that an output beam of the at least one first pulse stretcher unit is an input beam of the at least one second pulse stretcher unit.

[0042] The term about refers to a variation in length of at most 25%, or at most 10% or at most 5%.

[0043] According to some embodiments, the at least one second pulse stretcher may be configured to stretch the input light beam at least partially in the spatial domain.

[0044] According to some embodiments, the pulse stretcher unit may be configured using any one of the variations or embodiments described herein.

[0045] According to another broad aspect, the present disclosure provides an inspection system comprising a light source unit, a pulse stretcher unit, and a collection unit for collecting an illumination signal from a sample, wherein the pulse stretcher unit is configured to receive an input illumination beam from the light source unit and split one or more pulses in the input illumination beam into a plurality of pulses, and direct an output beam comprising the plurality of pulses to illuminate the sample, wherein the plurality of pulses have selected time separations therebetween and have substantially equal amplitudes.

[0046] According to some embodiments, the pulse stretcher unit comprises a selected number of beam splitters arranged along a selected main path and comprising an input beam splitter, an output beam splitter and one or more intermediate beam splitters, and

[0047] an arrangement of light reflecting surfaces defining a selected number n of secondary paths extending between the selected number of beam splitters; wherein

[0048] (a) the input beam splitter is configured to receive an input light beam and direct a first portion of the light beam along the selected primary path and a second portion of the light beam along a first secondary path;

[0049] (b) the output beam splitter is configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the nth secondary path, and to provide at least one output beam propagating along the selected primary path; and

[0050] (c) each one or more intermediate beam splitters are configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the i-th secondary path, and to provide a first output beam portion propagating along the selected primary path and a second output beam portion propagating along the corresponding i+1-th secondary path; wherein

[0051] The length of the secondary path follows approximately the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path.

[0052] The term about refers to a variation in length of at most 25%, or at most 10% or at most 5%.

[0053] According to some embodiments, the at least one second pulse stretcher may be configured to stretch the input light beam at least partially in the spatial domain.

[0054] According to some embodiments, the pulse stretcher unit may be configured using any one of the variations or embodiments described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to better understand the subject matter disclosed herein and to illustrate how it may be implemented in practice, embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

[0056] Figure 1 schematically illustrates a pulse stretcher unit according to some embodiments of the present disclosure;

[0057] Figure 2 illustrates additional configurations of a pulse stretcher unit according to some embodiments of the present disclosure;

[0058] Figure 3 illustrates another configuration of a pulse stretcher unit employing a polarization rotator according to some embodiments of the present disclosure;

[0059] Figure 4 shows a pulse stretcher unit employing a movable output beam splitter according to some embodiments of the present disclosure;

[0060] Figure 5 shows an output pulse train and formation of a pulse train within a pulse stretcher according to some embodiments of the present disclosure;

[0061] Figure 6 shows a pulse stretcher configuration employing a common reflection surface for multiple secondary paths according to some embodiments of the present disclosure;

[0062] Figure 7illustrates a cascaded pulse stretcher arrangement according to some embodiments of the present disclosure; and

[0063] Figure 8 An inspection system employing a pulse stretcher unit according to some embodiments of the present disclosure is illustrated. DETAILED DESCRIPTION

[0064] As described above, the present disclosure provides a pulse stretcher unit configured to receive an input radiation beam formed of one or more short pulses, and stretch the one or more short pulses to provide an output beam comprising a plurality of radiation pulses having substantially equal amplitudes therebetween. Figure 1 , which schematically illustrates a pulse stretcher unit 100 according to some embodiments of the present disclosure. System 100 includes a selected number of beam splitter units, including beam splitters 10, 12, 14 and 16 in this example, and an arrangement of reflective surfaces 200 to 212 in this example. Beam splitters 10 to 16 include an input beam splitter 10, an output beam splitter 16 and one or more intermediate beam splitters (12 and 14 in this example). Beam splitters 10 to 16 are configured to guide an input radiation beam through the pulse stretcher unit along a main path. In addition, each beam splitter is configured to guide a portion of the input beam received by it along a corresponding auxiliary path. Therefore, the pulse stretcher unit 100 defines a main path for beam propagation, and a selected number N-1 auxiliary paths defined by the number N beam splitters used in the pulse stretcher.

[0065] The output pulses are aligned and an output beam is provided having a series of pulses having equal amplitudes and being equally spaced from each other. The optical paths of the auxiliary paths follow the conditions selected so that the longest auxiliary path has a length of L, the second longest auxiliary path has a length of L / 2, the next auxiliary path has a length of L / 4, L / 8, and so on. Typically, the length of an auxiliary path is defined as L / 2 k , where k = [0, 1, 2 ...] It should be noted that the optical path / length of the secondary path may be approximate, and a variation of at most 25%, or at most 10%, or at most 5% may be appropriate.

[0066] The beam splitters 10 to 16 are typically configured as 50 / 50 beam splitters, splitting each input pulse into two output beam portions, one output beam portion propagating along a primary path and one output beam portion propagating along a corresponding secondary path. This configuration allows each pulse to be split into two pulse portions of approximately equal intensity. In addition, the optical path length difference between the secondary paths and the distance between the beam splitters 10 to 16 along the primary path can typically be selected to prevent temporal overlap between the divided pulses, thereby splitting the input pulse into a pulse train with approximately equal distances between pulses. In addition, the beam splitter can typically be a polarization-independent beam splitter, splitting the input beam into a first beam portion and a second beam portion that are independent of the polarization component of the beam. Typically, if the length of the secondary path is different from the desired length relationship, the output beam may produce a delay variation between its pulse portions. This may be advantageous in some applications where a precise delay between pulse portions is not required.

[0067] In order to maintain the form factor of the pulse stretcher unit 100, a selected number of reflective surfaces may be used to fold a selected number of secondary paths. Figure 2 Shown in. Figure 2 Additional configurations of the pulse stretcher 100 according to some embodiments of the present disclosure are schematically illustrated. In this example, the system 100 illustrates a reflective surface arrangement at approximately 200 that defines a selected number of secondary paths for radiation propagation. Thus, the first input beam splitter 10 is positioned to receive an input beam IB and split the input beam into a first portion that propagates along a primary path MP of the pulse stretcher unit 100 and a second portion that propagates along a first secondary path AP1. The second beam splitter 12 is configured to receive a first input beam arriving along the primary path MP and split it into a first portion that propagates along the primary path and a second portion that propagates along a second secondary path AP2. Additionally, the second beam splitter is configured to receive a second input beam arriving from the first secondary path and split the second input beam into a first portion that propagates along the primary path MP and a second portion that propagates along the second secondary path AP2. In general, the nth beam splitter of the pulse stretcher 100 receives a first input beam portion arriving along the main path and guides it into a first and a second beam propagating along the main path and the nth auxiliary path, and receives a second input beam portion arriving along the n-1 auxiliary path and guides it into a first and a second beam propagating along the main path and the nth auxiliary path.

[0068] The last beam splitter 16 of the pulse stretcher unit 100 receives a first input beam portion arriving along the main path and a second input beam portion arriving along the auxiliary path, and splits each input beam portion into a first output beam portion propagating along the main path MP and a second output beam portion propagating along the auxiliary output path APn. The output beam splitter 16 is generally used to combine the beam portions propagating along the main path and the respective auxiliary paths. Therefore, the beam splitter 16 may include a layered structure that prevents light from being transmitted through the beam splitter to the last auxiliary path APn and allows the light to propagate only along the main path to provide a single output beam OB.

[0069] In some other embodiments, the output beam splitter 16 is a 50 / 50 beam splitter that directs a portion of the light to the auxiliary path APn. In these embodiments, the auxiliary path APn includes one or more light-guiding optical elements that direct the light along the path to provide a second output beam portion that is generally aligned parallel to the first output beam. For example, the light deflection element 19 can be positioned to align the light component propagating along the auxiliary path APn to be parallel to the output beam OB.

[0070] In some embodiments, the output beam splitter 16 can be configured as a beam combiner that receives input beam portions arriving along the primary path MP and the secondary path and transmits only a single output beam portion OB, thereby not directing the light portion along the secondary path APn. The output beam splitter 16 can be provided with a suitable coating to produce a destructive interference condition for the light directed along the secondary path APn, while providing a constructive interference condition for the light propagating along the output beam path OB.

[0071] Generally, a pulse stretcher according to the present disclosure may utilize a polarization-independent beam splitter. However, in some embodiments, the input polarization beam splitter 10 may be a polarization beam splitter, while the intermediate beam splitters 12 and 14 and the output beam splitter 16 may be polarization-independent beam splitters. Figure 3 , which illustrates an additional example of a pulse stretcher according to some embodiments of the present disclosure. In this example, the pulse stretcher includes an input beam splitter 10 configured as a polarization beam splitter having a selected polarization axis and a polarization rotator 22, such as a half-wave plate. The pulse stretcher in this example can generally be configured to receive an input light beam having a selected polarization state, such as a linear polarization aligned at 45° to the polarization axis of the input beam splitter 10, a circular polarization that can be split into equal power in a first and a second orthogonal linear polarization orientation, or other polarization states.

[0072] Figure 3The pulse stretcher configuration illustrated in is configured to provide an output pulse train having a selected polarization orientation, and may be associated with one or more polarization shifting elements (eg, half-wave plates, quarter-wave plates, or others) to selectively produce an output beam of arbitrarily selected polarization.

[0073] In addition, if Figure 2 and Figure 3 As shown, the selected number of auxiliary paths AP1 to APn may include a selected number of reflective surfaces 32 and 34, such as mirrors, for reflecting the beam portions along the auxiliary paths to provide an optical path of selected length and a selected time delay between pulse components. The reflective surfaces may include one or more spherical reflective surfaces 32 having a selected radius of curvature, the radius of curvature being selected to maintain at least one of the divergence angle and width of the beam to provide substantially the same beam profile for different pulses of the output beam. Figure 3 As shown, the secondary path may use a combination of a flat reflective surface 34 and a curved reflective surface 32 according to the optical design to maintain the divergence angle and / or width of the output light beam OB.

[0074] Figure 4 , an additional configuration of a pulse stretcher according to some embodiments of the present disclosure is illustrated in FIG. Here, the pulse stretcher is illustrated as having Figure 3 The output beam splitter 16 is mounted on a movable platform 15. The movable platform 15 can be used to change the position of the output beam splitter 16 along at least one axis, and can be selectively used to adjust the position of the output beam splitter 16 along two or three axes. In addition, Figure 4 An additional reflective surface 17 is illustrated, which is configured as a retroreflector, for example, and is positioned to align with the second beam portion transmitted by the output beam splitter 16. The additional reflective surface 17 is configured to guide the second beam portion generated by the input beam portion that reaches the output beam splitter 16 from the auxiliary path AP3 and is transmitted through the beam splitter 16, and the second beam portion generated by the input beam portion that reaches the output beam splitter 16 along the main path and is reflected by the beam splitter 16. Due to the change in the position of the beam splitter 16, these output beam portions do not overlap. Therefore, the additional reflective surface 17 can be configured as a retroreflector to direct the second output beam portion to the auxiliary path APn regardless of the exact lateral position of the second output beam portion.

[0075] The situation is similar for the first output beam portion output from the output beam splitter 16. Therefore, due to the shift in the position of the output beam splitter 16, the beam portions propagating along the primary path MP and the secondary path AP3 are impinged on the spatially separated points of the beam splitter 16, and the output beam OB can be formed by two to four substantially parallel output beams. Such a configuration can be used to enhance the illumination etendue, but increase the output beam cross-section while substantially maintaining the divergence angle and / or width of the output beam.

[0076] It should be noted that, in general, any one of a selected number of beam splitters can be placed on the movable platform such that lateral displacement of the position of the beam splitter causes a displacement of the beam portions to produce one or more non-overlapping output beams. The selection of one or more beam splitters located on the movable platform can determine the power ratio between the non-overlapping beam portions depending on the number of beam splitters upstream or downstream of the movable platform.

[0077] Figure 5 1 shows the basic effect of the pulse stretcher 100 according to some embodiments of the present disclosure. Typically, an input short pulse enters the pulse stretcher and is initially separated by the first input beam splitter 10. The input pulse is split into two parts, a first part propagating along the main path MP, and a second part propagating along a first auxiliary path AP1 having a selected optical path / length L. Due to the difference in length between the main path MP and the auxiliary path AP1, the first part arrives at the second beam splitter 12 before the second part propagating along the auxiliary path, as illustrated in row a.

[0078] The second beam splitter 12 receives two input pulses, the first input pulse arrives from the main path MP and is split into a first portion propagating along the main path MP and a second portion propagating along the second auxiliary path AP2. After a selected time of the order of L / c, the second portion arrives at the second beam splitter 12 and is also split into a first portion propagating along the main path MP and a second portion propagating along the second auxiliary path AP3, as illustrated in row b.

[0079] Figure 5 Rows c, d and e in further illustrate the operation of subsequent beam splitters, showing that each beam splitter receives a first input pulse propagating along the main path MP and splits it into a first portion propagating along the main path and a second portion propagating along the corresponding auxiliary path. After the first pulse, the beam splitter receives a second pulse propagating along the auxiliary path, which is again split into a first portion propagating along the main path and a second portion propagating along the corresponding auxiliary path. The next beam splitter similarly receives additional beam portions associated with the respective paths available for the beam portions, thereby separating each beam portion and increasing the number of pulses.

[0080] As mentioned above, the beam splitter is typically selected to be a 50 / 50 beam splitter. However, to overcome the potential loss of illumination power due to multiple reflections of the beam portion propagating along the secondary path, the beam splitter can direct more illumination power to the secondary path. For example, the beam splitter can be, for example, a 60 / 40 beam splitter, or other ratio selected based on the loss level along the secondary path, with higher power being transmitted to the secondary path.

[0081] Figure 6 Additional exemplary configurations of pulse stretchers according to some embodiments are shown in . Figure 6 A pulse stretcher is shown, which is configured to fold multiple auxiliary paths between the arrangement of spherical reflectors 201 to 216 and the flat reflector 200 shared by the auxiliary paths. More specifically, an input pulse IB can be input to the pulse stretcher 100. The input pulse IB is separated in a cascade manner by beam splitters 10 to 16 to generate multiple pulse portions that propagate along the main path and / or a selected number of auxiliary paths. Spherical mirrors 201 to 216 are positioned for multiple auxiliary paths. More specifically, in this example, mirrors 201 to 208 are associated with the first auxiliary path AP1 and define the length L=32m of the auxiliary path AP1. Mirrors 209 to 212 are associated with the second auxiliary path AP2 and define its length as 16m=L / 2. The third auxiliary path is defined by mirrors 213 and 214 and has a length of 8m=L / 4. Mirror 215 defines a fourth auxiliary path with a length of 4m=L / 8. Finally, mirrors 216 and 19 define a fifth secondary path with a length of 2m=L / 16.

[0082] The pulse stretcher may use an input polarizing beam splitter 10 and a polarization rotator 22 (such as a half-wavelength plate) located between the first beam splitter 10 and the second beam splitter 12 to provide a polarized output beam based on an input pulse having a suitable polarization orientation at the input.

[0083] As indicated above, the position of the output reflective element 19 allows for adjustment of the spatial arrangement of the output beam OB, thereby enabling the extent of the output beam to be determined according to desired illumination requirements. Similarly, the output beam splitter, in this example the output beam splitter 18, may be arranged on a mobile mount that is configured to be selectively displaced along one or more directions. Figure 4 Position adjustment of the output beam splitter is shown in , and can be used to provide selected customization of output beam parameters (e.g., beam width, spread, etc.).

[0084] In general, the pulse stretcher of the present disclosure provides conversion of input pulses into output pulse trains having selected time delays between pulses and having substantially equal pulse amplitudes. The pulse stretcher of the present disclosure may be used in combination with one or more conventional pulse stretcher units. Figure 7 A pulse stretcher system 500 is illustrated, formed using a first pulse stretcher 100 configured as described above and a second pulse stretcher 120 configured to receive an input optical signal and generate an output signal formed of a plurality of spatially separated light beams. The first pulse stretcher 100 of the system 500 is configured as described herein to receive an input signal having one or more short pulses and to generate an output signal formed of a plurality of pulses separated in time or slightly overlapping in time. The output beam of the first pulse stretcher 100 has a temporal structure formed of a plurality of pulses and is used as an input beam of the second pulse stretcher unit 120. The second pulse stretcher unit 120 is configured to receive an input beam and generate a spatial arrangement of an output beam OB. In this example, the second pulse stretcher 120 is configured using an arrangement of a plurality of reflective surfaces or mirrors and one or more partially reflective surfaces. With each reflection of the electromagnetic radiation beam through the second pulse stretcher, a portion of the reflected beam is coupled out to generate an output beam OB. This configuration can be used to stretch the input short pulse in the time domain as well as in the spatial domain. More specifically, the input short pulse can be stretched into multiple pulses with selected time separation and approximately the same intensity using the first pulse stretcher unit 100. The output beam with multiple pulses (e.g., pulse train) is further separated using the second pulse stretcher 120 to output a beam stretched in space, thereby providing a wide output beam OB. The spatial stretching can provide a selected spatial distance between multiple parts of the output beam OB, thereby providing different spatial parts that are completely separated or partially overlap each other.

[0085] The pulse stretcher unit 100 of the present disclosure may be used for various illumination and imaging purposes. Figure 8 An optical inspection system configured for inspecting a sample 1010 is illustrated. The system includes a light source, such as a laser 150, that provides a radiation beam of a selected wavelength range. Typically, due to the selected wavelength range and the construction requirements of the laser system for emitting the selected wavelength range, the laser output beam is formed by a plurality of short pulses. The output of the laser unit 150 is directed to the input port of the pulse stretcher 100 using an optical arrangement (illustrated by a reflective surface 155). The pulse stretcher 100 is configured as described above and is operable to receive an input short pulse and temporally stretch the input pulse to produce an output beam formed by a plurality of pulses having approximately the same amplitude. The output beam of the first pulse stretcher 100 can be directed to a second pulse stretcher 120 (if used) to provide spatial broadening of the output beam. The second pulse stretcher can be used according to the desired beam characteristics, and in some embodiments, the system can use only the first pulse stretcher 100.

[0086] The broadened output beam may be directed using an optical arrangement 140 to illuminate a sample 1010. The optical arrangement 140 may include one or more lenses and may be configured to provide a desired illumination pattern onto the sample 1010. The radiation impinging on the sample is further directed to a collection / imaging unit 180 for generating output data indicative of the radiation interaction with the sample 1010. The collected data is further used to determine the structure of the sample 1010. In some inspection systems, the collection / imaging unit 180 may share one or more common optical elements with the optical arrangement 140. Such one or more common optical elements may include one or more lenses, gratings, filters, and / or one or more beam splitting optical elements configured to direct the illumination onto the sample 1010 and to direct the collected illumination to one or more detectors of the collection / imaging unit 180.

[0087] In general, the inspection system 1000 can be used to inspect semiconductor wafers and / or masks or reticles suitable for manufacturing semiconductor devices. The inspection system can use a selected wavelength range and, for example, can use short ultraviolet radiation. Existing typical laser units operating at such short wavelengths provide short pulse outputs. Therefore, the use of a pulse stretcher enables the illumination beam characteristics to be customized within the limitations of the available laser units. The pulse stretcher described herein can extend the pulse duration by generating multiple pulses having substantially the same amplitude and a desired delay between each other, wherein the delay can be determined by the minimum relative length of the auxiliary path relative to the corresponding main path. It should be understood that the various auxiliary paths of the pulse stretcher can have any desired order. Therefore, although illustrated in order according to length in the above, the order of the auxiliary paths based on length can be random, or any other selected order.

[0088] Therefore, the present disclosure provides a pulse stretcher configuration that provides temporal stretching of pulses and, optionally, also spatial stretching of pulses. The pulse stretcher unit is formed by a selected number of beam splitters arranged along a main path of light propagation. The selected number of pulse beam splitters includes an input beam splitter, an output beam splitter, and one or more intermediate beam splitters. Each beam splitter is configured to receive an input light beam and split the input light beam into a first portion propagating along the main path and a second portion propagating along a corresponding auxiliary path, thereby changing the optical path of the beam components to produce a relative delay between the beam portions. More specifically, the intermediate beam splitter receives input light beam portions propagating along the main path as well as along the auxiliary path, and separates each received light beam portion, thereby doubling the number of pulses in the output pulse train.

[0089] It should be noted that the various features described in the various embodiments can be combined according to all possible technical combinations.

[0090] It should be understood that the application of the present invention is not limited to the details set forth in the description contained herein or shown in the accompanying drawings. The present invention can have other embodiments and can be practiced and executed in various ways. Therefore, it should be understood that the wording and terminology adopted herein are for illustrative purposes and should not be considered as limiting. Therefore, it will be understood by those skilled in the art that the concepts on which the present disclosure is based can be easily used as a basis for designing other structures, methods and systems to achieve several purposes of the subject matter currently disclosed.

[0091] It will be readily appreciated by those skilled in the art that various modifications and changes may be made to the embodiments of the invention described above without departing from the scope of the present disclosure as defined by the appended claims.

Claims

1. A pulse stretcher unit, comprising: a selected number of beam splitters arranged along a selected primary path and comprising an input beam splitter, an output beam splitter, and one or more intermediate beam splitters, and an arrangement of light reflecting surfaces defining a selected number n of secondary paths extending between the selected number of beam splitters; in (a) the input beam splitter is configured to receive an input light beam and direct a first portion of the light beam along the selected primary path and a second portion of the light beam along a first secondary path; (b) the output beam splitter is configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the nth secondary path, and to provide at least one output beam propagating along the selected primary path; as well as (c) each one or more intermediate beam splitters are configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the i-th secondary path, and to provide a first output beam portion propagating along the selected primary path and a second output beam portion propagating along the corresponding i+1-th secondary path; and wherein The length of the secondary path follows approximately the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path.

2. The pulse stretcher unit according to claim 1, wherein the length of the secondary path follows the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path with a length variation of at most 10%.

3. The pulse stretcher of claim 1, wherein the length of the secondary path is greater than the length of the portion of the selected primary path between adjacent beam splitters.

4. The pulse stretcher unit according to claim 1 further includes an output reflective surface, wherein the output beam splitter provides a portion of the output beam to propagate along the selected main path and provides a second output beam portion to propagate along an output auxiliary path, and the output reflective surface is positioned to guide the second output beam portion to propagate along the selected auxiliary output path.

5. The pulse stretcher unit of claim 1 , wherein the selected number of beam splitters comprises one or more beam splitters mounted on a movable platform, the movable platform being movable along at least one axis perpendicular to the selected main path, thereby enabling the output beam to be in the form of two parallel output beams.

6. The pulse stretcher unit according to claim 1, wherein the output beam splitter is mounted on a movable platform, which can be moved along at least one axis perpendicular to the selected main path, so that the output beam can be in the form of two parallel output beams.

7. The pulse stretcher unit of claim 1, wherein the light reflective surface arrangement comprises a selected number of curved reflective surfaces configured to focus light beams arriving at the curved reflective surfaces to maintain at least one of a beam divergence angle and a beam width.

8. The pulse stretcher unit of claim 1, wherein the selected number of beam splitters consists of polarization-independent beam splitters.

9. The pulse stretcher of claim 1, wherein the input beam splitter is a polarization beam splitter, and the output beam splitter and one or more intermediate beam splitters are polarization-independent beam splitters.

10. The pulse stretcher unit of claim 9, further comprising a polarization rotation plate located in the selected main path between the input beam splitter and an adjacent beam splitter.

11. The pulse stretcher unit of claim 9, further comprising a polarization rotation plate located in the secondary path between the input beam splitter and an adjacent beam splitter.

12. The pulse stretcher unit of claim 1, wherein the selected number of beam splitters comprises polarization-independent beam splitters.

13. The pulse stretcher unit of claim 1, wherein the selected number of beam splitters consists of beam splitters configured to split an input beam at a power ratio between 0.4:0.6 and 0.6:0.

4.

14. A pulse stretcher unit according to claim 1, wherein the arrangement of the beam splitters defines one or more auxiliary paths, and wherein the one or more auxiliary paths are folded by one or more reflective surfaces, and the one or more reflective surfaces are arranged so that radiation is reflected back and forth between the reflective surfaces and passes through the auxiliary paths.

15. The pulse stretcher unit according to claim 1, wherein at least one secondary path is formed by a plurality of spherical mirrors and a common plane mirror, the plurality of spherical mirrors and the common plane mirror being arranged such that the radiation beam is reflected a selected number of times between the spherical mirrors and the plane mirror.

16. The pulse stretcher unit of claim 1, capable of operating in the ultraviolet wavelength range.

17. A pulse stretching system comprising at least one first pulse stretcher unit and at least one second pulse stretcher unit; The at least one first pulse stretcher unit comprises: a selected number of beam splitters arranged along a selected primary path and comprising an input beam splitter, an output beam splitter, and one or more intermediate beam splitters, and an arrangement of light reflecting surfaces defining a selected number n of secondary paths extending between the selected number of beam splitters; wherein (a) the input beam splitter is configured to receive an input light beam and direct a first portion of the light beam along the selected primary path and a second portion of the light beam along a first secondary path; (b) the output beam splitter is configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the nth secondary path, and to provide at least one output beam propagating along the selected primary path; and (c) each one or more intermediate beam splitters are configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the i-th auxiliary path, and to provide a first output beam portion propagating along the selected primary path and a second output beam portion propagating along the corresponding i+1-th auxiliary path; wherein The length of the secondary path follows approximately the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path; and where The at least one second pulse stretcher unit is placed downstream of the at least one first pulse stretcher unit such that an output beam of the at least one first pulse stretcher unit is an input beam of the at least one second pulse stretcher unit.

18. The pulse stretcher system of claim 17, wherein the at least one second pulse stretcher is configured to stretch the input light beam at least partially in the spatial domain.

19. An inspection system, comprising a light source unit, a pulse stretcher unit and a collection unit for collecting an illumination signal from a sample, wherein the pulse stretcher unit is configured to receive an input illumination beam from the light source unit and separate one or more pulses of the input illumination beam into a plurality of pulses having selected time separations and substantially equal amplitudes, and guide an output beam comprising the plurality of pulses to illuminate the sample.

20. The inspection system of claim 19, wherein the pulse stretcher unit comprises a selected number of beam splitters arranged along a selected main path and comprising an input beam splitter, an output beam splitter, and one or more intermediate beam splitters, and an arrangement of light reflecting surfaces defining a selected number n of secondary paths extending between the selected number of beam splitters; in (a) the input beam splitter is configured to receive an input light beam and direct a first portion of the light beam along the selected primary path and a second portion of the light beam along the selected first secondary path; (b) the output beam splitter is configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along the nth secondary path, and to provide at least one output beam propagating along the selected primary path; as well as (c) each one or more intermediate beam splitters are configured to receive a first input beam portion propagating along the selected primary path and a second input beam portion propagating along an i-th secondary path, and to provide a first output beam portion propagating along the selected primary path and a second output beam portion propagating along a corresponding i+1-th secondary path; in The length of the secondary path follows approximately the form L / 2 k , k = 0, 1, 2, where L is the selected length of an auxiliary path.

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