Apparatus for and method of laser speckle reduction

Abstract

Apparatus for and method of controlling a pattern variation in a semiconductor exposure tool in which the divergence of a beam of laser radiation generated by a laser radiation source is caused to assume a value greater than the smallest divergence the laser radiation source can achieve but less than a specified maximum for beam divergence for an exposure device using the beam of laser radiation.

Description

APPARATUS FOR AND METHOD OF LASER SPECKLE REDUCTIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 735,464, filed December 18, 2024, titled APPARATUS FOR AND METHOD OF LASER SPECKLE REDUCTION, which is incorporated herein by reference in its entirety.FIELD

[0002] The present disclosure relates to systems and methods for aligning optical components for use in an exposure apparatus, for example, in a lithographic apparatus, an inspection apparatus, or a metrology apparatus, and particularly to components in optical pulse stretchers useful for lengthening the pulse of the output of a laser source.BACKGROUND

[0003] A lithographic apparatus applies a desired pattern onto a substrate such as a wafer of semiconductor material, usually onto a target portion of the substrate. A patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the wafer. Transfer of the pattern is typically accomplished by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain adjacent target portions that are successively patterned.

[0004] Lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Herein, for the sake of simplicity, both steppers and scanners will be referred to as exposure devices. The following description uses a scanner as an example of an exposure device with the understanding that the principles elucidated herein apply equally to other types of exposure devices including steppers.

[0005] During a semiconductor manufacturing process, specifically during various stages such as lithography, etching, deposition, and chemical mechanical polishing, an inspection or a metrology apparatus leverages radiation to illuminate a surface of the substrate, pattern defects, or any anomalies that may impact yield. For patterning device inspection, the inspection or the metrology apparatus examines the patterning device for defects such as missing features, extra features, or contamination, which could be transferred to the substrate during the lithography process. The metrology can even provide overlay measurement, ensuring that different layers of the devices are aligned with each otherduring the manufacturing process. Moreover, the inspection or the metrology apparatus offers in-line process control by providing real-time feedback, allowing users to adjust process parameters on the fly.

[0006] The light source used to illuminate the pattern and project it onto the substrate can be of any one of a number of configurations. Deep ultraviolet excimer lasers commonly used in lithography systems include the krypton fluoride (KrF) laser at 248 nm wavelength and the argon fluoride (ArF) laser at 193 nm wavelength. The laser source can include an optical pulse stretcher for lengthening the pulse of the output of a high power gas discharge laser system.

[0007] A stochastic process is one in which the results of the process are randomly determined. For semiconductor patterning the dimensions of interest are sufficiently small that stochastic effects become an important part of the variations that affect the dimensions, shapes, and placement of the patterns being fabricated.

[0008] A manifestation of stochastic variations in lithography (as well as etch and other parts of the patterning process) is that the patterns being produced are rough rather than smooth. The roughness of the edge of a feature is called line-edge roughness (LER), and the roughness of the width of a feature is called linewidth roughness (LWR). The roughness of the centerline of the feature (the midpoint between left and right edges) is called pattern placement roughness (PPR). Stochastic effects in patterning can reduce the yield and performance of semiconductor devices in several ways. For example feature-to- feature size variation caused by stochastics (also called local CD uniformity, LCDU) adds to the total budget of CD variation. Feature-to-feature pattern placement variation caused by stochastics (also called local pattern placement error, LPPE) adds to the total budget of pattern placement error, PPE.

[0009] Thus, LCDU refers to stochastic-induced variation in CD. Conventionally, CDU looks at the variation of CD across a chip, exposure field, or wafer, having causes such as mask CD variation, film variations across the wafer, focus control across the exposure field, hotplate temperature uniformity, and many other factors. As feature sizes continue to shrink for more advanced nodes, LCDU control becomes more critical for improving defectivity, edge placement error (EPE), and yield enhancement.

[0010] Control and reduction of EPE will be necessary to meet future nodes. There are four main components to EPE (ranked from the greatest contribution to the least contribution): linewidth roughness errors, overlay errors, optical proximity correction (OPC) CD errors, and global CD uniformity (CDU) errors.Where:

[0011] EPE is edge placement error;

[0012] HRQPC is the half-range of the critical dimension (CD) error due to optical proximity (OPC) residuals;

[0013] PBA is the proximity bias average = field average CD by feature caused by scanner tool-to-tool variation;

[0014] OVL is the overlay error;

[0015] CDU is the CD uniformity which includes reticle error, scanner error, and error from etch and deposition processes; and

[0016] LWR is the linewidth roughness.

[0017] The first two terms of this equation relate to systematic errors, the terms under the radical relate to global errors, and the last term relates to local errors. Additional information about EPE and on the types of errors that affect EPE and their sources is available from Mulkens, M. Hanna, B. Slachter, W. Tel, M. Kubis, M. Maslow, C. Spence, and V. Timoshkov, “Patterning control strategies for minimum edge placement error in logic devices,” Proc. SPIE 10145, 1014505(2017).https: / / doi.org / 10.1117 / 12.2260155.

[0018] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0019] One cause of local errors is dose variation during wafer exposure. The emitted laser beam is at least partially spatially and temporally coherent. In the illumination system and / or the projection system, radiation from different parts of the laser beam emitted from the radiation source may intermix. The spatial coherence of the laser beam may cause different parts of the radiation beam which are mixed together to interfere with each other, thereby forming an interference pattern. In particular an interference effect commonly known as speckle may occur. Similarly, a speckle noise, agranular pattern that arises from the interference of coherent radiation of the laser beam, may degrade the quality of measurement in metrology or inspection applications.

[0020] Speckle is a positional variation in the intensity of a radiation beam which results from mutual interference of a set of wavefronts. It constitutes a dosing error contributing to stochastic variations. It would be advantageous for some applications to reduce laser speckle to in turn reduce stochastic variations such as LER, LWR, and LCDU. It is in this context that the need for the disclosed subject matter arises.SUMMARY

[0021] The following presents a concise summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify any elements as key or critical nor delineatethe scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0022] According to an aspect of an embodiment there is disclosed a method of controlling a variation in a photolithographic process in the variation depends at least in part on speckle produced by a laser radiation source used in the used in the photolithographic process, the method comprising ascertaining a beam divergence specification for an exposure device and causing the laser radiation source to generate a beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification.

[0023] The variation may comprise one or more variation selected from line edge roughness, line width roughness, local critical dimension uniformity, hole local critical dimension uniformity, circle edge roughness, and reduction in line placement error and improvements in overlay. Causing the laser radiation source to generate a beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification may comprise passing the beam through diverging optics. The diverging optics may comprise a beam reducer.

[0024] The laser radiation source may include an optical pulse stretcher and causing the laser radiation source to generate a beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification may comprise detuning one or more mirrors in the optical pulse stretcher. The optical pulse stretcher may comprise at least one image relay mirror and detuning the optical pulse stretcher may comprise changing an orientation, a position, or both the orientation and the position of the image relay mirror.

[0025] According to another aspect of an embodiment there is disclosed a laser radiation source comprising a seed pulse subsystem configured to generate a seed pulse, an amplification subsystem arranged to receive the seed pulse and to amplify the seed pulse to obtain an amplified pulse, and an output subsystem arranged to receive the amplified pulse and to condition the amplified pulse, the output subsystem including an optical pulse stretcher and divergent optics configured to increase a beam divergence of the amplified pulse. The divergent optics may comprise a beam reducer.

[0026] According to another aspect of an embodiment there is disclosed a method of reducing speckle contrast, the method comprising generating a laser beam pulse, separating the laser beam pulse into a plurality of sub-pulses each entering a respective temporally shifted path respectively defined by a plurality of imaging relay mirrors, adjusting at least one of the plurality of imaging relay mirrors to increase a spatial shift and a temporal shift of the plurality of sub -pulses, and combining the plurality of sub-pulses into an output pulse. Adjusting the at least one of the plurality of imaging relay mirrors may include changing an orientation, a position, or both the orientation and position of the at least one of the plurality of imaging relay mirrors to increase a spatial divergence value of at least one of the plurality of sub-pulses.

[0027] According to another aspect of an embodiment there is disclosed a method of increasing a beam divergence in a DUV light source comprising generating a beam of laser radiation, separating the beam of laser radiation into a plurality of sub-beams pulse of laser radiation which are temporally shifted with respect to each other, and combining the plurality of sub-beams into an output pulse while increasing a divergence of at least one of the sub-beams.

[0028] According to another aspect of an embodiment there is disclosed a method of controlling stochastic variation in a semiconductor device fabrication process by decreasing speckle contrast in laser radiation from a laser radiation source, the method comprising causing the laser radiation source to generate a beam of pulses of laser radiation, using an optical pulse stretcher to separate the beam into a plurality of derivative beams, modifying respective wavefronts of at least some of the derivative beams, and combining the derivative beams into an output beam.

[0029] The respective wavefronts may be modified to increase a divergence of the output beam. The stochastic variation may comprise one or more of line edge roughness, line width roughness, local critical dimension uniformity, hole local critical dimension uniformity, circle edge roughness, and reduction in line placement error and improvements in overlay. Modifying respective wavefronts of at least some of the derivative beams may be performed by using detuned mirrors in respective optical paths traversed by the derivative beams in the optical pulse stretcher. Modifying respective wavefronts of at least some of the derivative beams may be performed by using mirrors having mismatched curvatures in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher. Modifying respective wavefronts of at least some of the derivative beams may be performed by one or more optical elements in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher. Modifying respective wavefronts of at least some of the derivative beams is performed by a beam splitter with a modified surface in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher. The modified surface may comprise a surface with an attached optical wedge. The modified surface may comprise a surface with a modified curvature.

[0030] Modifying respective wavefronts of at least some of the derivative beams may be performed by a beam combiner with a modified surface in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher. The modified surface may comprise a surface with an attached optical wedge. The modified surface may comprise a surface with a modified curvature.

[0031] Further features and advantages of the present subject matter, as well as the structure and operation of various embodiments of the present subject matter, are described in detail below with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the subject matter by way of example rather than limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. None of the drawings is to scale unless otherwise indicated.

[0033] FIG. 1 is a schematic diagram of an overall broad conception of an exposure system according to an aspect of the disclosed subject matter.

[0034] FIG. 2 is a schematic diagram of an overall broad conception of a laser system used in a lithography system according to an aspect of the disclosed subject matter.

[0035] FIGS. 3A, 3B, and 3C are qualitative graphs of beam divergence versus time for various types of stretched laser pulses.

[0036] FIG. 4 is a schematic diagram of an output subsystem for a laser radiation source according to an aspect of an embodiment.

[0037] FIG. 5 is a schematic diagram showing various elements and light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

[0038] FIGS . 6A and 6B are schematic diagrams of features of the optical pulse stretcher of FIG. 5.

[0039] FIG. 7 is a flow chart of a method for reducing line edge roughness in a semiconductor photolithography process according to an aspect of an embodiment.

[0040] FIG. 8 is a flow chart of a method for reducing line edge roughness in a semiconductor photolithography process according to an aspect of an embodiment.

[0041] Further features and advantages of the subject matter, as well as the structure and operation of various embodiments of the subject matter, are described in detail below with reference to the accompanying drawings.DETAILED DESCRIPTION

[0042] This specification discloses one or more embodiments that incorporate the features of this subject matter. The disclosed embodiments merely exemplify the present subject matter. The scope of the present subject matter is not limited to the disclosed embodiments. The present subject matter is defined by the claims appended hereto.

[0043] The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, “an exemplary embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0044] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper,” “left,” “right” and the like, may be used herein for ease of description to describe the relationship of one element or feature to one or more other element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0045] Before describing specific embodiments in more detail, it is instructive to present an example environment in which embodiments of the present subject matter may be implemented. Referring to FIG. 1, an exposure system 100 includes an illumination system 105. As described more fully below, the illumination system 105 includes a light source that produces a pulsed light beam 110 and directs it to an optical processing apparatus or scanner 115 that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold wafer 120 and connected to a positioner (not shown) configured to accurately position the wafer 120 in accordance with certain parameters. In some embodiments, the exposure system 100 is a photolithography system for patterning, an inspection system for quality control, or a metrology system for measurement.

[0046] The exposure system 100 uses a light beam 110 having a wavelength in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The minimum size of the microelectronic features that can be patterned on the wafer 120 depends on the wavelength of the light beam 110, with a lower wavelength permitting a smaller minimum feature size. The bandwidth of the light beam 110 can be the actual, instantaneous bandwidth of its optical spectrum (or emission spectrum), which contains information on how the optical energy of the light beam 110 is distributed over different wavelengths. The scanner 115 includes an optical arrangement having, for example, one or more condenser lenses, a mask, and an objective arrangement. In some embodiments, the mask is stationary while the wafer moves. In some embodiments, the mask is movable along one or more directions, such as along an optical axis of the light beam 110 or in a plane that is perpendicular to the optical axis. In some embodiments, both the mask and the waver move synchronously for faster pattern transfer. The objective arrangement includes a projection lens and enables the image transfer to occur from the mask to the photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for the light beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the light beam 110 across the mask.

[0047] The scanner 115 can include, among other features, a controller 130, air conditioning devices, and power supplies for the various electrical components. The controller 130 controls how layers are printed on the wafer 120. The controller 130 includes a memory that stores information such as process recipes. A process program or recipe determines the length of the exposure on the wafer 120 based on,for example, the mask used as well as other factors that affect the exposure. During exposure process, a plurality of pulses of the light beam 110 illuminate the same area of the wafer 120 to constitute an illumination dose.

[0048] The exposure system 100 also includes a control system 135. In general, the control system 135 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 135 also 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.

[0049] The control system 135 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). The control system 135 may also include one or more programmable processors, and one or more computer program products tangibly embodied in a machine -readable storage device for execution by one or more programmable processors. 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 processors receive instructions and data from the memory. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The control system 135 can be centralized or be partially or wholly distributed throughout the exposure system 100.

[0050] Referring to FIG. 2, an exemplary laser source system within the illumination system 105 is a pulsed laser source that produces a pulsed laser beam as the light beam 110. FIG. 2 shows illustratively and in block diagram a gas discharge laser system according to an embodiment of certain aspects of the disclosed subject matter. The gas discharge laser system may include, e.g., a seed laser system 140, a power amplification system 145, e.g., a single-pass power amplifier chamber, a power ring amplifier chamber, or an oscillator chamber, relay optics 150 and laser system output subsystem 160. The seed laser system 140 may include, e.g., a master oscillator (“MO”) chamber 165.

[0051] The seed laser system 140 includes a master oscillator output coupler 175, which may comprise a partially reflective mirror, forming with a reflective grating (not shown) in a line narrowing module (“LNM”) 170, an oscillator cavity in which the seed laser system 140 oscillates to form the seed laser output pulse, i.e., forming a master oscillator (“MO”). The system may also include a laser quality analysis module 180. The laser quality analysis module 180 may include an etalon spectrometer for fine wavelength measurement and a coarser resolution grating spectrometer. A master oscillator wavefront engineering box (“WEB”) 185 may serve to redirect the output of the seed laser system 140 toward the power amplification system 145, and may include, e.g., beam expansion with, e.g., a multi prism beam expander (not shown) and coherence busting, e.g., in the form of an optical delay path (not shown).

[0052] The power amplification system 145 may include, e.g., a power amplifier lasing chamber 200, which may be formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a power amplifier WEB 210 and may be redirected back through the gain medium in the chamber 200 by a beam reverser 220. The power amplifier WEB 210 may incorporate a partially reflective input / output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms.

[0053] Another laser quality analysis module 230 at the output of the power amplification system 145 may receive the output laser light beam of pulses from the power amplification system 145 and pick off a portion of the light beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output light beam of pulses then passes through an optical pulse stretcher (“OPuS”) 240 and an output combined autoshutter metrology module (“CASMM”) 250, which may also be the location of a pulse energy meter. One purpose of the OPuS 240 may be, e.g., to convert a single output laser pulse into a pulse train. Derivative pulses created from the original single output pulse are delayed with respect to each other. By distributing the original laser pulse energy into a train of derivative pulses, the effective pulse length of the laser can be expanded and at the same time the peak pulse intensity reduced. In operation, the OPuS 240 stretches the excimer or other gas discharge laser, e.g., a molecular fluorine gas discharge laser, having a given effective pulse duration (time integral square) to a longer effective pulse duration having several peaks.

[0054] The OPuS 240 thus receives the laser beam from the power amplifier WEB 210 via the laser quality analysis module 230 and direct the output of the OPuS 240 to the CASMM 250. Other suitable arrangements may be used in other embodiments. One of ordinary skill in the art will appreciate that this ordering of components is not critical and that a different ordering can be used.

[0055] The power amplifier lasing chamber 200 and the MO chamber 165 are configured as chambers in which electrical discharges between electrodes may cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules, including, e.g., Ar, Kr, and / or Xe, to produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth and center wavelength selected in a the LNM 170, as is known in the art.

[0056] As mentioned, speckle may cause different parts of a transferred pattern such as different positions along a line feature to receive different doses of radiation. Variation in the received dose may lead to stochastic variations such as LER, LWR LCDU. Controlling these variations becomes increasingly important as the critical dimension decreases.

[0057] For example, the contribution of speckle to the LER depends on the speckle contrast. Laser speckle contrast (SC) can be estimated by the following empirical formula:where X is the wavelength, A is the beam size (diameter), Q is the beam divergence, and T is the intrinsic laser beam coherent time. TIS is the time integral square of pulse waveform P(t), which measures effective pulse duration:

[0058] The first term under the radical represents the spatial coherence of the laser beam while the second term under the radical represents the temporal coherence.

[0059] Increasing the beam divergence thus reduces spatial coherence and improves the overall speckle contrast. As explained in more detail below, in an OPuS which stretches pulses by breaking a primary input beam down into a number of derivative beams which are recombined into an output beam, the beam divergence may be increased by modifying the wavefronts of the derivative beams such that each derivative beam has slightly different wavefront This can be accomplished, for example, by changing the beam pointing of the derivative beams.

[0060] In some embodiments, after broadening an angular spread of the laser beam, the resulting beam profile projects an oval or substantially fusiform shape on a beam analysis tool. A longest straight line passing through a center of the resulting beam profile defines a longer axis of the profile, and a straight line passing through the center and substantially perpendicular to the longer axis defines a shorter axis. The longer axis defines the direction of a vertical divergence and the shorter axis defines the direction of a horizontal divergence. In some embodiments, the horizontal divergence refers to one or a combination of derivative beam profiles. Thus, according to an aspect of an embodiment, beam divergence, for example, vertical divergence or horizontal divergence is increased to reduce speckle contrast. As explained in more detail below, this is permissible because the exposure device can in fact accept a laser radiation beam with more divergence than the lowest amount of divergence of which the laser is capable.

[0061] A qualitative representation of the amount of divergence of the output of an OPuS aligned to minimize divergence is shown in FIG. 3A. As can be seen, the OPuS breaks the primary input beam down into a number of derivative beams, four in the example represented in FIG. 3A. Pulses of those beams are represented by the triangles that are staggered in time to result in a longer pulse with multiple peaks. The derivative beams have a divergence given by the height of the triangles. In the scenario represent in FIG. 3 A the OPuS has been tuned in a manner to the lowest beam divergence of which the source is capable. This lowest possible (minimum) beam divergence value is indicated by the dashed line labeled A. This minimum divergence value is below the exposure device, such as a scanner’s specification for the highest divergence value the scanner can accept, a value indicated by the dashed line labeled B. This establishes a margin or headroom range labeled by bracket C. In at least oneembodiments, a ratio of labeled B to labeled A ranges from about 1.5 to about 3. A ratio greater than 3 increases beam spot, resulting in a reduced resolution, in some instances. A ratio smaller than 1.5 is insufficient to reduce spatial coherence, in some instances.

[0062] One way to increase the divergence of the beam reaching the scanner is to increase the beam divergence after the beam exits the OPuS. This resultant stretched pulse is shown in FIG. 3B. There, after adjusting a profile and symmetry of vertical and horizontal divergences, for example, centering the derivative beams along a first axis (e.g. the vertical axis) of the primary beam, the OPuS is again aligned for minimum beam divergence but a divergent optical element down beam of the OPuS adds beam divergence to the beam. FIG. 4 shows a nonlimiting example of a possible arrangement for achieving this result. In the arrangement of FIG. 4 elements are arranged as in the output subsystem 160 of the arrangement shown in FIG. 2. Thus, there is a laser quality analysis module 330, an OPuS 340, and a CASMM 350. The arrangement of FIG. 4 also includes divergent optics 360. The divergent objects 360 can be any suitable optical element or combination of optical elements that can increase the divergence of the beam from the OPuS 340. For example, the divergent optics could be a beam reducer. A beam reducer (m < 1) provides an output beam with a smaller waist diameter (Wo’ < mW0) and a larger divergence angle (0out > 0in / m ) than the input beam. One of ordinary skill in the art will appreciate that these elements may be in a different order and that the divergent optics 360 does not necessarily have to be positioned immediately down beam from the OPuS 340. The divergent optical element increases the beam divergence by, for example, a factor of two or even more as long as the resultant beam divergence is within specifications for the scanner.

[0063] Another way to increase beam divergence is by controlling the alignment (position, orientation, or both) of optical elements within the OPuS such as the OPuS image relay mirror alignment to increase derivative pulse divergence. In some embodiments, at least one of the vertical divergence and the horizontal divergence is increased after the symmetry of vertical and horizontal beam profile is verified. For example, the horizontal divergence is increased by adjusting a first image relay mirror to enlarge a first divergence on a first side (e.g., right) and adjusting a second image relay mirror to enlarge a second divergence on a second side (e.g., left). In some embodiments, the adjustment of the two imaging relay mirrors is repeated until a target divergence value is achieved, while the symmetry of the divergence is maintained. In some embodiments, the divergence is increased by adjusting a separation of related image relay mirrors. A qualitative representation of the divergence of the output of a detuned OPuS which is not aligned to minimize divergence is shown in FIG. 3C. In some embodiments, each derivative pulse divergence increases from the lowest possible beam divergence value A to the highest possible beam divergence value B. In some embodiments, each derivative pulse decreases from the highest possible beam divergence value B to the lowest possible beam divergence value A.

[0064] FIG. 5 is a schematic diagram of front view of an example of an optical pulse stretcher 401 having first optical pulse stretcher 401a and second optical pulse stretcher 401b. The optical pulse stretcher 401 receives input primary beam pulse 411 and stretches it to output a stretched output beampulse 413. In some embodiments, the optical pulse stretcher 401 receives input primary beam pulse 411 and stretches it to output the stretched output beam pulse 413 via a same optical pulse stretcher, such as the first pulse stretcher 401a or the second optical pulse stretcher 401b. In some embodiments, the optical pulse stretcher 401 receives input primary beam pulse 411 and stretches it to output the stretched output beam pulse 413 via different optical pulse stretchers. For example, the first optical pulse stretcher 401a receives the input primary beam pulse 411 and the second optical pulse stretcher 401b outputs the stretched output beam pulse 413, or vice versa.

[0065] In FIG. 5, the beam 411 enters the second optical pulse stretcher 401b where it encounters one of the beam splitters 503. The beam splitter splits the beam into a beam that continues to propagate in the same direction as the primary beam and a derivative beam which in the arrangement shown propagates horizontally to be reflected at the ends of its path by a respective of one of image relay mirrors 501 and 502. The second optical pulse stretcher 401b can include two or more (in the example shown, four) stages 402a, 402b, 402c, and 402d of confocal optical pulse stretchers.

[0066] As shown in FIG. 5 and in an expanded view in FIG. 6A and 6B, each confocal pulse stretcher includes a pair of image relay mirrors such as that labeled image mirror 501a in FIG. 6A. These four stages of confocal optical pulse stretchers can be positioned approximately parallel to each other in second optical pulse stretcher 401b. The second optical pulse stretcher 401b can be positioned perpendicular or approximately perpendicular to first optical pulse stretcher 401a. In other words, in some embodiments, first optical pulse stretcher 401a (e.g., an orthogonal optical pulse stretcher that may be positioned vertically) is positioned perpendicular or approximately perpendicular to the four stages of confocal optical pulse stretchers of second optical pulse stretcher 40 lb which is positioned horizontally in the figure.

[0067] The second optical pulse stretcher 401b can include four stages of confocal optical pulse stretchers. However, the embodiments of this disclosure are not limited to these examples, and second optical pulse stretcher 401b can include other numbers of stages of confocal optical pulse stretchers. In some embodiments, for various optical duration requirements, the first optical pulse stretcher 401a includes from one to three stages of optical pulse stretchers and the second optical pulse stretcher 401b includes from one to three stages of optical pulse stretchers.

[0068] According to some embodiments, the image relay mirrors can include circular or rectangular concave mirrors. The mirrors can be designed and positioned based on atelecentric design.

[0069] As shown in FIG. 6B, the image relay mirrors such as image relay mirror 501a can be provided with an adjuster 510 that can control the orientation (tip and tilt) of the image relay mirror 501A as well as the beam wise (along X-axis in the figure) position of the mirror 501a. The adjuster 510 can be electro-actuatable and operated automatically under the control of a control signal applied on line 511. The adjuster 510 can also be operated manually using through the-wall-adjusters. The orientation and position of the image relay mirrors can be adjusted to affect the divergence of the beam 413 exiting the optical pulse stretcher 401.

[0070] According to an aspect of an embodiment the optical pulse stretcher is tuned so that the laser radiation source provides a beam having a divergence in the margin between (1) the lowest divergence the laser radiation source is capable of and (2) the maximum divergence the scanner can accept by purposefully adjusting the optical pulse stretcher alignment so that the actual divergence of the beam provided to the stepper is larger than the laser radiation source’s lowest achievable value, i.e., by detuning the optical pulse stretcher. The higher divergence reduces spatial coherence and so laser beam speckle.

[0071] Thus an increase in beam divergence can be achieved by detuning derivative beam horizontal / vertical pointing by changing the tip and / or tilt of one or more of the OPuS image relay mirrors and by detuning derivative beam horizontal divergence by changing the separation of two or more OPuS image relay mirrors.

[0072] Detuning provides a useful additional mechanism for speckle reduction. Further pulse stretching yields diminishing returns as regards speckle reduction because at 450ns TIS and above speckle contrast is dominated by the spatial coherence component of the expression provided above. Also, detuning operates within the horizontal divergence margin allowed by scanner and so can be performed without incurring a penalty in scanner operation.

[0073] Increasing the beam divergence by detuning the optical pulse stretcher has several advantages. It exhibits superior repeatability and sensitivity in divergence detuning control. Also, already deployed sources are field upgradable to be able to implement this method. It requires only a minimal hardware change. It would have no impact on other beam parameters including the efficiency of the optical pulse stretcher. It also makes use of the full divergence margin allowed by scanner.

[0074] FIG. 7 is a flow chart describing a method of reducing line edge roughness by reducing speckle contrast in accordance with an aspect of an embodiment. As shown in FIG. 7, in a first step S10, a specification value for the maximum divergence the scanner can accept is determined. Then, in a step S20, the source beam divergence is set to a value between its lowest achievable divergence and the scanner divergence specification value.

[0075] FIG. 8 is a flow chart describing a method of reducing LER by reducing speckle contrast in accordance with another aspect of an embodiment. As shown in FIG. 8, in a first step S10, a specification value for the maximum divergence the scanner can accept is determined. Then, in a step S30, an optical pulse stretcher included in the beam source is detuned so that the source beam divergence assumes a value between lowest achievable divergence and scanner divergence specification value.

[0076] Thus, according to an aspect of an embodiment, speckle is reduced by modifying the wavefront of the derivative beams in an OPuS. This can be accomplished, for example, by detuning the mirrors in the OPuS as described above. It can also be accomplished by modifying the specifications of the existing image relay mirrors to allow the derivative beams to accumulate additional wavefront modification, for example, by using different / mismatched curvatures (including anamorphiccurvatures) of the OPuS image relay mirrors. It can also be accomplished by inserting one or more additional optical element(s) such as a waveplate in some of the optical paths traversed by the derivative beams. It can also be accomplished by modifying the surface of the beam splitters / combiners in the OPuS to introduce additional wavefront change. In the case of modifying the surface of the a beam splitter / combiner, a small wedge or spherical / cylindrical surface change produce the desired effect of accumulating additional wavefront modification for subsequent derivative beams.

[0077] As mentioned, LER is only one example of a stochastic variation that can be controlled by controlling the laser speckle. Other variations include LWR, LCDU, hole local critical dimension uniformity, and circle edge roughness (CER).

[0078] The present subject matter has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0079] Some of the above description is in terms of functional block diagrams with some functions allocated to some blocks and other functions allocated to other blocks. It will be understood that the division between blocks and the allocations are arbitrary and that different divisions and allocations are possible so long as the overall functions are carried out as described above.

[0080] The above description includes examples of multiple embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for each of these embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of elements of the various embodiments are possible based on the disclosure. Accordingly, the described embodiments are intended to be representative of and encompass all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0081] Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Also, although elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and / or embodiment may be utilized with all or a portion of any other aspect and / or embodiment, unless stated otherwise.

[0082] Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The protection is not restricted to the details of anyforegoing embodiments. The protection extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0083] Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a subcombination or variation of a subcombination.

[0084] Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.

[0085] For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

[0086] Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and / or steps. Thus, such conditional language is not generally intended to imply that features, elements, and / or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and / or steps are included or are to be performed in any particular embodiment.

[0087] Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z . Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0088] Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5 % of, within less than 1% of, within less than 0.1 % of, andwithin less than 0.01 % of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.

[0089] The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as non -exclusive.

[0090] Aspects and implementations of the present disclosure can be further described using the following numbered clauses:1. A method of controlling a variation in an exposure process in the variation depends at least in part on speckle produced by a laser radiation source used in the used in the exposure process, the method comprising: ascertaining a beam divergence specification for an exposure device; and causing the laser radiation source to generate a beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification.2. The method of clause 1 wherein the variation comprises one or more variation selected from line edge roughness, line width roughness, local critical dimension uniformity, hole local critical dimension uniformity, circle edge roughness, and reduction in line placement error and improvements in overlay.3. The method of clause 1 wherein causing the laser radiation source to generate the beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification comprises passing the beam through diverging optics.4. The method of clause 3 wherein the diverging optics comprise a beam reducer.5. The method of clause 1 wherein the laser radiation source includes an optical pulse stretcher and wherein causing the laser radiation source to generate the beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification comprises detuning one or more mirrors in the optical pulse stretcher.6. The method of clause 5 wherein the optical pulse stretcher comprises at least one image relay mirror and wherein detuning the one or more mirrors in the optical pulse stretcher comprises changing an orientation, a position, or both the orientation and the position of the image relay mirror.7. A laser radiation source comprising: a seed pulse subsystem configured to generate a seed pulse; an amplification subsystem arranged to receive the seed pulse and to amplify the seed pulse to obtain an amplified pulse; andan output subsystem arranged to receive the amplified pulse and to condition the amplified pulse, the output subsystem including an optical pulse stretcher and divergent optics configured to increase a beam divergence of the amplified pulse.8. The laser radiation source of clause 7 wherein the divergent optics comprises a beam reducer.9. A method of reducing speckle contrast, the method comprising: generating a laser beam pulse; separating the laser beam pulse into a plurality of sub-pulses each entering a respective temporally shifted path respectively defined by a plurality of imaging relay mirrors; adjusting at least one of the plurality of imaging relay mirrors to increase a spatial shift and a temporal shift of the plurality of sub-pulses; and combining the plurality of sub-pulses into an output pulse.10. The method of clause 9, wherein adjusting the at least one of the plurality of imaging relay mirrors includes changing an orientation, a position, or both the orientation and position of the at least one of the plurality of imaging relay mirrors to increase a spatial divergence value of at least one of the plurality of sub-pulses.11. A method of increasing of a beam from a divergence in a DUV light source comprising: generating a beam of laser radiation; separating the beam of laser radiation into a plurality of sub -beams pulse of laser radiation which are temporally shifted with respect to each other; and combining the plurality of sub-beams into an output pulse while increasing a divergence of at least one of the sub-beams.12. A method of controlling stochastic variation in a semiconductor device fabrication process by decreasing speckle contrast in laser radiation from a laser radiation source, the method comprising: causing the laser radiation source to generate a beam of pulses of laser radiation; using an optical pulse stretcher to separate the beam into a plurality of derivative beams; modifying respective wavefronts of at least some of the derivative beams; and combining the derivative beams into an output beam.13. The method of clause 12 wherein the respective wavefronts are modified to increase a divergence of the output beam.14. The method of clause 12 wherein the stochastic variation comprises one or more of line edge roughness, line width roughness, local critical dimension uniformity, hole local critical dimension uniformity, circle edge roughness, and reduction in line placement error and improvements in overlay.15. The method of clause 12 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by using detuned mirrors in respective optical paths traversed by the derivative beams in the optical pulse stretcher.16. The method of clause 12 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by using mirrors having mismatched curvatures in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.17. The method of clause 12 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by one or more optical elements in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.18. The method of clause 12 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by a beam splitter with a modified surface in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.19. The method of clause 18 wherein the modified surface comprises a surface with an attached optical wedge.20. The method of clause 18 wherein the modified surface comprises a surface with a modified curvature.21. The method of clause 12 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by a beam combiner with a modified surface in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.22. The method of clause 21 wherein the modified surface comprises a surface with an attached optical wedge.23. The method of clause 21 wherein the modified surface comprises a surface with a modified curvature.

[0091] The above-described aspects and implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A method of controlling a variation in an exposure process in the variation depends at least in part on speckle produced by a laser radiation source, the method comprising: ascertaining a beam divergence specification for an exposure device; and causing the laser radiation source to generate a beam of laser radiation having a beam divergence between the smallest divergence achievable by the laser radiation source and the beam divergence specification.

2. The method of claim 1 wherein the variation comprises one or more variation selected from line edge roughness, line width roughness, local critical dimension uniformity, hole local critical dimension uniformity, circle edge roughness, and reduction in line placement error and improvements in overlay.

3. The method of claim 1 wherein causing the laser radiation source to generate the beam of laser radiation comprises passing the beam through diverging optics.

4. The method of claim 3 wherein the diverging optics comprise a beam reducer.

5. The method of claim 1 wherein the laser radiation source includes an optical pulse stretcher and wherein causing the laser radiation source to generate the beam of laser radiation comprises detuning one or more mirrors in the optical pulse stretcher.

6. The method of claim 5 wherein the optical pulse stretcher comprises at least one image relay mirror and wherein detuning the one or more mirrors in the optical pulse stretcher comprises changing an orientation, a position, or both the orientation and the position of the image relay mirror.

7. A laser radiation source comprising: a seed pulse subsystem configured to generate a seed pulse; an amplification subsystem arranged to receive the seed pulse and to amplify the seed pulse to obtain an amplified pulse; and an output subsystem arranged to receive the amplified pulse and to condition the amplified pulse, the output subsystem including an optical pulse stretcher and divergent optics configured to increase a beam divergence of the amplified pulse.

8. The laser radiation source of claim 7 wherein the divergent optics comprises a beam reducer.

9. A method of controlling stochastic variation in a semiconductor device fabrication process by decreasing speckle contrast in laser radiation from a laser radiation source, the method comprising: causing the laser radiation source to generate a beam of pulses of laser radiation; using an optical pulse stretcher to separate the beam into a plurality of derivative beams; modifying respective wavefronts of at least some of the derivative beams; and combining the derivative beams into an output beam.

10. The method of claim 9 wherein the respective wavefronts are modified to increase a spatial divergence of the output beam.

11. The method of claim 9 wherein the stochastic variation comprises one or more of line edge roughness, line width roughness, local critical dimension uniformity, hole local critical dimension uniformity, circle edge roughness, and reduction in line placement error and improvements in overlay.

12. The method of claim 9 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by using detuned mirrors in respective optical paths traversed by the derivative beams in the optical pulse stretcher.

13. The method of claim 9 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by using mirrors having mismatched curvatures in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.

14. The method of claim 9 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by one or more optical elements in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.

15. The method of claim 9 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by a beam splitter with a modified surface in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.

16. The method of claim 15 wherein the modified surface comprises a surface with an attached optical wedge.

17. The method of claim 15 wherein the modified surface comprises a surface with a modified curvature.

18. The method of claim 9 wherein modifying the respective wavefronts of at least some of the derivative beams is performed by a beam combiner with a modified surface in at least some of the respective optical paths traversed by the derivative beams in the optical pulse stretcher.

19. The method of claim 18 wherein the modified surface comprises a surface with an attached optical wedge.

20. The method of claim 18 wherein the modified surface comprises a surface with a modified curvature.

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