Laser system and lithographic apparatus

Abstract

A laser system includes: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; and an output terminal through which the laser beam exits the laser system; the laser, the optical system, and the output terminal are configured such that the laser beam travels sequentially to the first optical element, the second optical element, and the output terminal; wherein the first optical element has a first focal length, the second optical element has a second focal length equal to the first focal length, and the second optical element is separated from the first optical element by a distance twice the first focal length.

Description

[Technical Field]

[0001] This invention relates to a laser system. This laser system is applicable in the field of lithography and can provide a laser beam to a lithography device. The laser may, for example, comprise an excimer laser. [Previous Technology]

[0002] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. Lithography apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). A lithography apparatus can, for example, project a pattern (also often referred to as a "design layout" or "design") of patterned devices (e.g., masks) onto a radiation-sensitive material (resist) layer provided on a substrate (e.g., a wafer).

[0003] As semiconductor manufacturing processes continue to advance, the size of circuit components has been shrinking for decades, while the number of functional elements, such as transistors, per device has been steadily increasing, following a trend commonly known as "Moore's Law." To keep pace with Moore's Law, the semiconductor industry is pursuing technologies that enable the production of increasingly smaller features. To project patterns onto a substrate, lithography devices can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature patterned onto the resist on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lasers (such as excimer lasers) can be used to provide electromagnetic radiation in the form of laser beams to the lithography device. Laser beams have characteristics such as beam shape and beam size.

[0004] There may be a need for a system and method for providing electromagnetic radiation having controlled characteristics such as beam shape and beam size, which at least partially solves one or more problems associated with known systems and methods, whether identified herein or otherwise. [Summary of the Invention]

[0005] According to a first aspect of the disclosed subject matter, a laser system is provided, comprising: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; and an output terminal through which the laser beam exits the laser system; the laser, the optical system, and the output terminal are configured such that the laser beam propagates sequentially to the first optical element, the second optical element, and the output terminal; wherein the first optical element has a first focal length, the second optical element has a second focal length equal to the first focal length, and the second optical element is spaced from the first optical element by a distance equal to twice the first focal length.

[0006] The laser beam generated by this laser will generally have a non-zero divergence. Therefore, the cross-sectional size of the laser beam will be larger at the output end than at the laser. The divergence can change over time (e.g., between different pulses for a pulsed laser beam). Therefore, the cross-sectional size of the laser beam at the output end can fluctuate over time. This size of the laser beam at the output end is determined by the divergence of the laser beam and the distance the laser beam travels between the laser and the output end. Furthermore, the pointing direction of the laser beam generated by the laser can generally change over time (e.g., between different pulses for a pulsed laser beam). As a result, the position of the laser beam at the output end will change. This change in the position of the laser beam is determined by the pointing direction of the laser beam at the laser and the distance the laser beam travels between the laser and the output end.

[0007] An optical system can be used to form an image of a first plane (located between the first optical element and the laser) in a second plane. The first plane and the second plane are separated by a distance four times the first focal length (in the direction of laser beam propagation). This optical system reduces the effective propagation length of the laser beam emitted by the laser by a distance equal to four times the focal length. By reducing the effective propagation length, one or more characteristics of the laser beam (e.g., at the output end) are improved for a given actual propagation length of the laser system (e.g., relative to a configuration of the same physical size but omitting the optical system). The improvement of one or more characteristics may include a desired beam size (e.g., a reduced beam size) and / or a desired beam shape. By reducing the effective propagation length, the stability of the laser beam (e.g., at the output end) is improved for a given actual propagation length of the laser system (e.g., relative to a configuration of the same physical size but omitting the optical system). Stability may represent the change (i.e., time-varying) of one or more laser beam characteristics over time. Improved stability can include: more stable beam shape (i.e., the time-varying nature of beam shape reduction), more stable beam position (i.e., the time-varying nature of beam position reduction), and / or more stable beam size (i.e., the time-varying nature of beam size reduction). Advantageously, the optical system allows the laser beam to propagate over a longer path length between the laser and the output of the laser system while maintaining the desired set of characteristics and / or improved stability. Furthermore, this allows for more physical space for positioning the optical components between the laser and the output for a given beam stability and / or given characteristics at the output. The optical system can be referred to as a laser beam stabilization system and / or an effective propagation length reduction system.

[0008] The first focal length and the second focal length may be referred to as focal length f. The focal length may be a positive focal length (for example, the first optical element and the second optical element may include a converging optical element).

[0009] As now discussed, the optical system of the laser system according to the first aspect of the disclosed subject matter is advantageous. The inventors have recognized that this optical system can form an image of any plane (which may be called the object plane) in a conjugate plane (which may be referred to as the image plane) located at a distance 4f from the object plane. As used herein, the plane in front of the optical system can be understood as the plane upstream of the optical system, i.e., any plane positioned between the laser and the first optical element. This imaging occurs regardless of whether such planes coincide with the focal plane of the first or second optical element. In fact, the optical system can simultaneously image multiple conjugate planes, for example, image the first object plane onto the first image plane (at a distance 4f from the first object plane) and image the second object plane onto the second image plane (at a distance 4f from the second object plane). Advantageously, this allows for greater flexibility in positioning the optical elements of the optical system between the laser and the output end. Advantageously, this allows optical elements to be placed at as far a distance as permitted by other constraints of the laser system, thereby further reducing the effective propagation length of the laser beam. Furthermore, this allows the optical elements to be positioned around other components that can be positioned within the laser system.

[0010] Specifically, the optical system offers greater flexibility in positioning compared to known optical imaging systems. In known optical imaging systems, optical elements are typically positioned at a specific distance from both the object plane and the image plane of interest. The specific location on the object plane is typically the position of the front focal plane of the optical system or the focal length of the optical element. Moving these optical elements from this specific distance typically results in a defocused image and / or a diverging laser beam. Advantageously, using the optical system described herein removes such positioning constraints.

[0011] Since the optical system has two optical elements with equal focal lengths and spaced apart by two focal lengths, the optical system has an effective magnification of 1 or -1. The length of the optical system can be considered as twice the focal lengths of the first and second optical elements. Given the space requirements of the laser system, the focal lengths of the first and second optical elements can be selected to maximize the length of the optical system.

[0012] It should be understood that the laser beam propagating sequentially to the first optical element, the second optical element, and the output terminal means that the laser beam propagates to the first optical element, the second optical element, and the output terminal in that order. That is, the first optical element receives the laser beam from the laser, the second optical element receives the laser beam from the first optical element, and the output terminal receives the laser beam from the second optical element. Each element (the first optical element, the second optical element, and the output terminal) may receive the laser beam directly from the preceding element or indirectly (e.g., via an intervention component). Preferably, but not necessarily, the second optical element may receive the laser beam directly from the first optical element.

[0013] It should be understood that as the laser beam propagates sequentially to the first optical element, the second optical element, and the output end, and as the laser beam exits the laser system through the output end, the laser beam propagates through the optical system only once. That is, the laser beam propagates to each of the first optical element, the second optical element, and the output end only once (in sequence). In other words, the laser beam (or any significant portion thereof) does not propagate to the first optical element, the second optical element, or the output end twice (e.g., there is no recirculation of the laser beam or any significant portion thereof through the optical system). Furthermore, the laser beam is not split into multiple components that travel through the optical system via different paths. This can also be referred to as the laser beam propagating through the laser system in a linear manner (or linearly). This can also be referred to as the laser beam propagating through each of the first optical element, the second optical element, and the output end one by one. A laser beam propagating in a linear manner is different from a laser beam that propagates through portions of the laser system in a circular or repetitive manner.

[0014] The first optical element may be configured to receive substantially all of the laser beam. Similarly, the second optical element may be configured to receive substantially all of the laser beam. It should be understood that some losses are experienced in any laser system, so substantially all losses can be considered to represent "all" of the laser beam when taking losses into account, but does not include significant or planned removal of a portion of the beam. That is, a substantial portion of the laser beam is not removed (e.g., using a beam splitter or otherwise) before the laser beam is received by the first optical element. In other words, there is no beam splitter directly positioned before the first optical element.

[0015] The laser beam contains radiation. The laser beam may contain pulsed or continuous radiation. The optical elements may include lenses and / or mirrors.

[0016] The laser may include an excimer laser. Excimer lasers are known to produce laser beams with pointing errors and / or fluctuations and high divergence. The aforementioned optical system is particularly advantageous when used with an excimer laser, due to the optical system's ability to improve the characteristics and stability of the laser beam (e.g., improve and / or stabilize the beam position, size, and shape).

[0017] The laser system may further include a pulse stretcher for increasing the pulse length of pulses in the laser beam. The pulse stretcher may include one or more beam splitters and one or more delay lines. The pulse stretcher may be configured to receive input pulses to convert the input pulses into pulse trains. Increasing the pulse length can reduce the effect of the spot, for example, because different time portions of the pulse can have different spot patterns. The pulse stretcher can be particularly advantageous when used in conjunction with a lithography apparatus, because lithography exposure can be adversely affected by the spot. In some cases, due to spatial constraints in the laser system, it is not possible to physically place the pulse stretcher between the laser and the output of the laser system. Therefore, it may be necessary to increase the laser propagation distance to guide the laser beam to the pulse stretcher, for example, due to spatial constraints in the laser system, located elsewhere. The use of the optical system according to the first state can advantageously reduce the effective propagation length of the laser beam, thereby enabling the use of additional components such as pulse stretchers, while still providing improved beam stability and / or improved beam characteristics compared to a laser system without an optical system. Furthermore, it provides greater flexibility in the positioning of additional components such as pulse stretchers, while maintaining the desired beam stability and / or laser beam characteristics at the output of the laser system.

[0018] As discussed above, the optical system reduces the effective propagation distance between the laser and the output. This allows for a larger physical distance between the laser and the output (for a given beam stability / characteristics at the output). Furthermore, this allows for more physical space to be accommodated for the pulse stretcher. Alternatively, it allows for greater freedom in positioning the pulse stretcher. For example, this allows steering optics to guide the laser beam to and from the pulse stretcher, which is not physically located between the laser and the output, without sacrificing the stability or beam characteristics of the laser beam at the output due to this increased physical optical path.

[0019] A pulse stretcher may be configured between the laser and the first optical element. This configuration should be interpreted relative to the propagation path of the laser beam. That is, the laser beam propagates from the laser to the pulse stretcher, and then from the pulse stretcher to the first optical element. In this arrangement, the first optical element indirectly receives the laser beam from the laser. The output of the pulse stretcher contains substantially all pulses of the laser beam (minus any unintentional losses, such as those attributable to specular absorption) such that the first optical element receives substantially all pulses. That is, substantially all (total) power of the input pulses is transmitted into the pulse train such that the first optical element receives substantially all (total) power of the input pulses. The pulse stretcher may not have a total focusing magnification. The pulse stretcher may have an effective propagation length of zero. The output of the pulse stretcher may contain substantially collimated laser beams.

[0020] The pulse stretcher reduces the peak power of the pulse. In this configuration, the lower peak power is incident on the optical elements of the optical system. Advantageously, this configuration can reduce damage and thus increase the lifespan of the optical system.

[0021] The pulse stretcher may include a pulse stretcher output terminal through which a pulse with an increased pulse length can exit the pulse stretcher. A first optical element may be positioned close to the pulse stretcher output terminal. By positioning the first optical element close to the pulse stretcher output terminal, the propagation distance of the laser beam outside the optical system is reduced. By reducing the propagation distance of the laser beam outside the optical system, the stability and / or characteristics of the laser beam are improved. This reduction is possible due to the imaging properties of the optical system.

[0022] It should be understood that by positioning the first optical element close to the output end of the pulse stretcher, the output end of the pulse stretcher can be positioned between the focal plane in front of the first optical element and the first optical element. This is because the inventors recognized that it is possible for an optical system to form an image of any plane in front of the optical system in a conjugate plane behind the optical system.

[0023] The distance between the output of the pulse stretcher and the first optical element can be less than the first focal length. This configuration is possible due to the imaging properties of the optical system (the optical system can image any plane in front of the optical system onto a conjugate plane, regardless of whether such plane coincides with the focal plane of the first or second optical element). The optical system provides greater flexibility in configuring the laser system.

[0024] The second optical element can directly receive the laser beam from the first optical element. That is, there may be no intervening component between the first and second optical elements. When the laser beam is focused between the first and second optical elements, the energy flux (energy per unit area) between them is high. The absence of an intervening component can advantageously reduce component damage and / or reduce laser beam absorption.

[0025] The laser system may further include a housing in which the laser and optical system are housed. The housing may include an exit aperture located at or near the output end of the laser system. A second optical element may be positioned close to the exit aperture. The housing can be considered to substantially surround the laser and optical system. By positioning the second optical element close to the exit aperture, the distance the laser beam travels outside the optical system before exiting the laser system is reduced. By reducing the distance the laser beam travels outside the optical system, the stability and / or beam characteristics of the laser beam at the exit aperture are improved. This reduction is possible due to the imaging properties of the optical system.

[0026] The distance between the exit aperture and the second optical element may be less than the first focal length. This configuration is possible due to the imaging properties of the optical system (the optical system can image any plane in front of the optical system onto a conjugate plane, regardless of whether such plane coincides with the focal plane of the first or second optical element).

[0027] The laser system may further include a second optical system comprising a third optical element and a fourth optical element. The third optical element may have a third focal length. The fourth optical element may have a fourth focal length equal to the third focal length. The fourth optical element may be spaced from the third optical element along the optical axis by a distance twice the third focal length. The third focal length may be equal to or different from the first focal length. The second optical system may be located in one of the following positions: between the laser and the (first) optical system, between the laser and the pulse stretcher, or between the first optical system and the output of the laser system. The laser system may include other optical systems, such as a third optical system having a fifth focal length.

[0028] Using more than one such optical system can advantageously allow for a further reduction in the effective propagation distance, while still allowing for additional components (e.g., pulse stretchers) without requiring additional components to be positioned between the first and second optical elements of the optical system (where such components are susceptible to damage due to high energy flux).

[0029] The laser system may further include a second pulse stretcher. This second pulse stretcher can provide a further increase in pulse length and / or otherwise process the pulse. The second pulse stretcher may be configured between the (first) optical system and the output. That is, the second optical element can guide the laser beam to the second pulse stretcher, and the second pulse stretcher can output the laser beam to the output of the laser system. This configuration advantageously reduces the effective propagation distance of the pulse between the first and second pulse stretchers.

[0030] According to a second aspect of the disclosed subject matter, a lithography apparatus is provided, comprising a laser system as described in any of the foregoing technical solutions. Improved stability and / or improved characteristics of the laser beam provided by the laser system can result in improved lithography performance.

Implementation Method

[0039] In this invention document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet (UV) radiation or deep ultraviolet (DUV) radiation (e.g., having wavelengths of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm) and extreme ultraviolet (EUV, e.g., having wavelengths in the range of about 5 nm to 100 nm).

[0040] As used herein, the terms "reducing mask," "mask," or "patterning device" can be broadly interpreted as referring to a general patterning device that can impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be generated in a target portion of a substrate. In this context, the term "light valve" may also be used. Examples of such patterning devices, besides classic masks (transmission or reflection; binary, phase-shift, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

[0041] FIG1A schematically depicts a lithography apparatus LA. The lithography apparatus LA includes: an illumination system (also called an illuminator) IL configured to adjust a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the patterning device MA to the radiation beam B onto a target portion C (e.g., containing one or more dies) of the substrate W.

[0042] In operation, the illumination system IL receives a radiated beam from the radiation source SO, for example, via a beam delivery system BD. The radiation source SO includes a laser system. The illumination system IL may include various types of optical components for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL can be used to adjust the radiated beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterned device MA.

[0043] The term “projection system” PS as used herein should be broadly interpreted to encompass all types of projection systems suitable for the exposure radiation used and / or other factors suitable for use with wetted liquids or in a vacuum, including refractive, reflective, reflective-refractive, composite, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof. Any use of the term “projection lens” herein may be considered synonymous with the more general term “projection system” PS.

[0044] A lithography apparatus LA may be of the type in which at least a portion of the substrate may be covered by a liquid (e.g., water) having a relatively high refractive index to fill the space between the projection system PS and the substrate W – this is also known as immersion lithography. Further information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0045] The lithography apparatus LA may also be of the type having two or more substrate supports WT (also known as "dual stages"). In such "multi-stage" machines, substrate supports WT can be used in parallel, and / or a substrate W located on one of the substrate supports WT can be used to prepare the substrate W for subsequent exposure, while another substrate W on the other substrate support WT is used to expose a pattern on that other substrate W.

[0046] In addition to the substrate support WT, the lithography apparatus LA may also include a measurement stage. The measurement stage is configured to hold sensors and / or cleaning devices. The sensors may be configured to measure properties of the projection system PS or the properties of the radiated beam B. The measurement stage may hold multiple sensors. The cleaning devices may be configured to clean parts of the lithography apparatus, such as parts of the projection system PS or parts of the system providing the wetting liquid. The measurement stage may move below the projection system PS when the substrate support WT is away from the projection system PS.

[0047] In operation, a radiation beam B is incident on a patterned device (e.g., a mask) MA held on a mask support MT, and is patterned by a pattern (design layout) existing on the patterned device MA. Having traversed the mask MA, the radiation beam B is passed through a projection system PS, which focuses the beam onto a target portion C of the substrate W. The substrate support WT can be accurately moved, for example, to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, a first locator PM and possibly another position sensor (not explicitly depicted in FIG. 1) can be used to accurately position the patterned device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterned device MA and the substrate W. Although the substrate alignment marks P1, P2, as described, occupy dedicated target portions, these marks can be located in the space between the target portions. When substrate alignment marks P1 and P2 are located between target portions C, these substrate alignment marks P1 and P2 are referred to as cut track alignment marks.

[0048] For illustrative purposes, a Cartesian coordinate system is used. A Cartesian coordinate system has three axes: the x-axis, the y-axis, and the z-axis. Each of the three axes is orthogonal to the other two axes. Rotation about the x-axis is called Rx rotation. Rotation about the y-axis is called Ry rotation. Rotation about the z-axis is called Rz rotation. The z-axis may generally coincide with the optical axis of the lithography device (e.g., in the vertical direction in FIG1), while the x-axis and y-axis may define planes perpendicular to the optical axis (e.g., the horizontal plane in FIG1). The Cartesian coordinate system is not limiting of the invention but is used for illustrative purposes only. Alternatively, another coordinate system, such as a cylindrical coordinate system, may be used for illustrative purposes. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0049] FIG1B schematically depicts a view of a known laser system as one aspect of the disclosed subject matter. FIG1B shows an example of a known laser system 100 operable to generate a pulsed laser beam 110. Specifically, FIG1B illustratively and in block diagram shows a gas discharge laser system 100. The laser beam 110 may be referred to as a beam.

[0050] The gas discharge laser system 100 includes a solid-state or gas discharge seed laser system 115, an amplification stage, such as a power ring amplifier ("PRA") stage 150, a relay optics 140, and a laser system output subsystem 170.

[0051] The seed laser system 115 includes: a master oscillator ("MO") chamber 125; a master oscillator output coupler ("MO OC") 130; and a line narrowing module ("LNM") 120.

[0052] The master oscillator output coupler ("MO OC") 130 may include a partial mirror, and the line narrowing module ("LNM") 120 may include a reflective grating. The master oscillator output coupler ("MO OC") 130 and the line narrowing module ("LNM") 120 together define an oscillator cavity in which a seed laser oscillates to form a seed laser output pulse. This oscillator cavity may be referred to as the master oscillator ("MO").

[0053] The laser system 100 also includes a line-center analysis module ("LAM") 135. The LAM 135 may include an etalon spectrometer for fine wavelength measurements and a coarser resolution grating spectrometer. The MO wavefront engineering box ("WEB") 145 can be used to redirect the output of the MO seed laser system 115 to the amplification stage 150, and may include, for example, a beam expander having, for example, a multi-prism beam expander (not shown) and, for example, a coherence disruptor in the form of an optical delay path (not shown).

[0054] The magnifying stage 150 includes a PRA laser-acting chamber 160, which may also be an oscillator, for example, formed by a seed beam injection and output coupling optics (not shown), which is incorporated into a PRA WEB 165 and redirected back through the gain medium in the chamber 160 by a beam reverser 155. The PRA WEB 165 may include a partially reflective input / output coupler (not shown) for the nominal operating wavelength (e.g., approximately 193 nm for an ArF system), a maximum reflector, and one or more prisms.

[0055] The solid-state or gas discharge seed laser system 115, the magnifying stage 150 and the relay optics 140 can be considered together as a laser operable to generate a laser beam.

[0056] A bandwidth analysis module ("BAM") 175 at the output end of the magnifying stage 150 receives the output laser pulse beam from the magnifying stage 150 and picks up a portion of the beam for metrological purposes, such as measuring the output bandwidth and pulse energy. The output pulse beam is then passed through an optical pulse stretcher ("OPuS") 180. One purpose of the OPUS 180 may be, for example, to convert a single output laser pulse into a pulse train. Secondary pulses generated from the original single output pulse may be delayed relative to each other. By distributing the original laser pulse energy into secondary pulse trains, the effective pulse length of the laser can be extended while the peak pulse intensity is reduced. The output pulse beam is then passed through an automatic shading meter module ("CASMM") 185 of the output combination, which may also be the position of a pulse energy meter. OPUS 180 can therefore receive the laser beam from PRA WEB 165 via BAM 175 and direct the output of OPUS 180 to CASMM 185. Other suitable configurations may be used in other embodiments.

[0057] As is known in the art, the PRA laser-acting chamber 160 and MO 125 are configured as chambers in which the discharge between the electrodes can cause the laser-acting gas in the laser-acting gas to discharge to generate a reverse population of high-energy molecules, including, for example, Ar, Kr and / or Xe, to generate relatively wide-band radiation, which can be selected via a relatively narrow bandwidth and center wavelength from the line-narrowing module ("LNM") 120.

[0058] Typically, tuning occurs within the LNM 120. A typical technique for line narrowing and laser tuning involves providing a window at the back of the laser's discharge cavity through which a portion of the laser beam is transmitted into the LNM. Here, a portion of the beam is broadened by a prism beam expander and guided to an optical element, such as a grating that reflects a narrower portion of the laser's broader spectrum back into the discharge chamber, where the narrower portion of the laser's broader spectrum is amplified. Typically, the laser is tuned by using an actuator, such as a piezoelectric actuator, to change the angle of the beam illumination grating. Alternatively, a transmission optical element, such as a prism, can be used to transmit a narrower portion of the laser's broader spectrum back into the discharge chamber, where the narrower portion of the laser's broader spectrum is amplified. The laser can be tuned by using an actuator, such as a piezoelectric actuator, to change the angle of the beam illumination prism. The laser system 100 can be operable to generate bursts having one or more pulses of one wavelength and then switchable to generate bursts having one or more pulses of different wavelengths.

[0059] The radiation source SO shown in Figure 1A and described above may include the laser system 100 shown in Figure 1B.

[0060] Embodiments of the present invention relate to novel laser systems. Such novel laser systems may generally belong to the type of laser system 100 shown in FIG. 1B and may include one or more of the features of laser system 100 shown in FIG. 1B. Examples of such novel laser systems will now be described with reference to FIG. 2 through FIG. 5.

[0061] FIG2 schematically depicts the new laser system 200. Laser system 200 may form part or all of the radiation source SO of FIG1A. Laser system 200 may form part or all of the type of laser system shown in FIG1B and described above. As described more fully below, laser system 200 includes a laser 210 operable to generate a laser beam 220 and an optical system 230 including a first lens 231 and a second lens 232. Laser 210 may belong to the type depicted in FIG1B, for example including a solid-state or gas discharge seed laser system 115, a magnifying stage 150, and a relay optics 140.

[0062] The laser beam 220 propagates to (and through) the first lens 231 and the second lens 232 before propagating to the output end 240, where the laser beam exits the laser system 200.

[0063] The laser beam 220 has a beam size, beam shape, and beam profile. The beam profile is a spatial intensity profile in a plane perpendicular to the propagation direction of the laser beam 220. Examples of beam profile types are Gaussian and top-hat. The beam shape is the shape of the laser beam 220 in a plane perpendicular to the propagation direction of the laser beam 220. Examples of beam shapes are circular and rectangular. The beam size is the size or dimension of the laser beam 220 in a plane perpendicular to the propagation direction of the laser beam. The beam size can be characterized, for example, by a radius (e.g., for rotationally symmetric beam shapes, such as circular beams) or by distances along the major and minor axes (e.g., for beam shapes with decreasing rotational symmetry, such as rectangular beams). The beam size may be referred to as the beam width. Those skilled in the art will understand that laser beams typically do not have clearly defined edges, and the edges are defined as areas containing a certain amount of intensity contained in the laser beam. Various conventions are used to measure and represent beam width, such as 1 / e2 (the distance between two completely opposite points of the beam, where the intensity is 1 / e2 times the maximum intensity of the beam), FWHM (the distance to the completely opposite points of the beam, where the intensity is 50% of the maximum intensity of the beam), and D4σ (four times the standard deviation of the intensity distribution).

[0064] The laser beam generally propagates along the optical axis. That is, the optical axis can be defined as the nominal or target path along which the laser beam propagates. Small deviations, namely divergence and pointing errors, can occur, causing a portion of the laser beam to propagate in a direction not aligned with the optical axis, as described in more detail below.

[0065] The laser beam 220 has a divergence, the quantization of which increases in beam size as the laser beam 220 propagates (e.g., through free space). Isotropic divergence causes the beam size of the laser beam to increase isotropically in all directions perpendicular to the propagation direction of the laser beam, resulting in a larger beam size as the laser beam propagates through free space. The divergence effect generally depends on the distance the laser beam 220 propagates (propagation distance) combined with the amount of divergence of the laser beam 220. For a given divergence, the laser beam 220 will have a larger beam size as its distance from the laser 210 increases. Anisotropic divergence can cause the beam size of the laser beam to increase by a first amount in a first direction perpendicular to the propagation direction of the laser beam, and by a second amount in a second direction perpendicular to the propagation direction of the laser beam, thus causing the laser beam to change its beam shape (e.g., a circular beam may become elliptical as it propagates) in addition to its beam size as it propagates through free space. The divergence of the laser beam 220 may be fixed, or it may vary over time (e.g., between different pulses in the case of a pulsed laser). This may be referred to as wave divergence. Wave divergence typically causes the beam size and / or shape to vary over time (i.e., wave beam size and / or beam shape).

[0066] The laser beam 220 may experience pointing errors, wherein a portion of the laser beam 200 is emitted at a non-zero angle relative to the optical axis. The term optical axis is known in this art and can be defined for the nominal or target pointing direction of the laser beam 220. Over a sufficiently long period of time (e.g., a sufficient number of pulses), the laser beam 220 may be substantially rotationally symmetric about the optical axis. However, over a shorter period of time (e.g., between different pulses), the pointing direction of the laser beam 220 may vary relative to the optical axis. Pointing errors affect the position of the beam relative to the optical axis. In the absence of pointing errors, each portion of the laser beam 220 is guided substantially along (or rotationally symmetric about) the optical axis such that the center of the laser beam 220 may substantially coincide with the optical axis. Thus, in the absence of pointing errors, as the laser beam 220 propagates (e.g., through free space), it will strike any virtual target positioned along the optical axis, i.e., the beam position coincides with the optical axis. With pointing error, as the laser beam propagates (e.g., through free space), the center of the laser beam does not propagate along the optical axis but at a non-zero angle to it. As a result, as the propagation distance of the laser beam 220 increases, the distance between the center of the laser beam 220 and the optical axis increases. Consequently, the laser beam 220 may miss (or partially miss) virtual targets positioned along the optical axis (centered on the optical axis). That is, due to pointing error, the beam position may be misaligned relative to the optical axis. The pointing of a portion of the laser beam (i.e., the emission direction relative to the normal optical axis) can change over time, resulting in pointing fluctuations, which are called pointing fluctuations. Specifically, when laser 210 is a pulsed laser, generally, the pointing can change with each pulse. Pointing fluctuations can cause the beam position to change over time. Similar to divergence, the effect of pointing error depends on the laser propagation distance combined with the magnitude of the pointing error. Therefore, for a given pointing error, the beam position will become increasingly misaligned with the optical axis as the laser beam propagates further from laser 210.

[0067] The laser system 200 includes an optical system 230 that reduces the effects of divergence and / or pointing error. Specifically, the optical system 230 reduces the effective propagation distance of the laser beam 220, thereby reducing the effects of divergence and pointing error that depend on the propagation distance.

[0068] The optical system 230 includes a first lens 231 and a second lens 232. The first lens 231 and the second lens 232 have equal focal lengths f and are separated by a distance of 2f. The first lens 231 and the second lens 232 have positive focal lengths (i.e., they are converging rather than diverging lenses). In this configuration, the optical system 230 forms an imaging system with a total magnification of 1 (or -1).

[0069] The laser beam 220 is fairly well collimated, although it has a non-zero divergence as described above. When the laser beam 220 is incident on the first lens 231, it is focused onto the plane between the first lens 231 and the second lens 232. The diverging laser beam beyond this plane is converted back into a fairly well collimated laser beam 220 (with substantially the same divergence as the incident laser beam) by the second lens 232.

[0070] The inventors have recognized that this optical system 230 forms an image of any plane (which may be called the object plane) in front of the optical system 230 (i.e., between the laser 210 and the first lens 231) in a conjugate plane (which may be called the image plane). The distance between the object plane and the image plane is equal to four times the focal length, i.e., 4f. Therefore, although the laser beam 220 propagates a distance of 4f between each pair of object planes and image planes, the size and position of the laser beam 220 will be the same in these two planes. Thus, when the laser beam 220 propagates between each pair of object planes and image planes, the effects of divergence and pointing error are practically zero. Therefore, the imaging performed by the optical system 230 reduces the effective propagation length of the laser beam 220 propagating from the laser 210 to the output end 240 by four times the focal length, i.e., 4f.

[0071] This imaging occurs regardless of whether the object plane and / or image plane coincide with the focal plane of the first lens 231 or the second lens 232. This allows for a high degree of flexibility in the configuration of, for example, lenses 231 and 232, because these lenses can be placed anywhere between the laser 210 and the output terminal 240, as long as they are spaced 2f apart from each other. The focal length f of the first lens 231 and the second lens 232 can be selected to maximize the distance between the object plane and the image plane for proper imaging, and thus maximize the reduction of the effective propagation length of the laser beam 220. The effective propagation length of the laser beam 220 within the laser system 200 is equal to the actual path length of the laser beam 220 between the laser 210 and the output terminal 240 minus 4f.

[0072] Optical systems comprising one or more lenses are known to be used as imaging systems to form an image of a plane (which may be called the object plane) in another plane (which may be called the image plane). Typically, when imaging is performed using such optical imaging systems, the image loses contrast and becomes out of focus (i.e., the beam diverges) as the distance from the image plane increases. Therefore, when passing a beam through a standard optical imaging system known in this art, additional optics are typically required after the image plane to periodically refocus the beam.

[0073] As those skilled in the art will understand, laser beams are generally well collimated (although in practice, as discussed elsewhere, they have non-zero divergence). Such laser beams can be referred to as nearly collimated or substantially collimated. The optical system 230 described herein is configured to form an image of the laser beam 220 such that the substantially collimated beam is imaged as a substantially collimated beam. Therefore, no additional optics are required to refocus the imaged beam. In fact, the optical system 230 can be used without additional optics to obtain the advantages of improved beam characteristics and / or improved stability.

[0074] The first lens 231 receives substantially all of the laser beam 220 from the laser 210. The optical system 230 is configured such that the laser beam 210 travels sequentially (i.e., in order) to the first lens 231, the second lens 232, and the output terminal 240. That is, the first lens 231 receives the laser beam 220 from the laser 210, the second lens 232 receives the laser beam 220 from the first lens 231, and the output terminal 240 receives the laser beam 220 from the second lens 232, at which point the laser beam 220 (e.g., through an aperture (not shown) at or near the output terminal 240) exits the laser system 200. As the laser beam 220 travels sequentially through the first lens 231, the second lens 232, and the output terminal 240, and as it exits the laser system 200 through the output terminal 240, the laser beam 220 travels through the optical system 200 only once. This can be described as the laser beam 220 traveling linearly through the optical system (i.e., without recirculation) and / or the laser beam 220 traveling sequentially through the first lens 231, the second lens 232, and the output terminal 240 (i.e., without multiple passes). In this way, substantially all of the laser beam 220 is imaged by the optical system 230 without any unnecessary loss (e.g., unnecessary loss due to absorption in the optical elements and / or removal of a portion of the beam).

[0075] The first lens 231 and the second lens 232 may be referred to as optical elements. In practice, focusing optical elements other than lenses may be used instead of either or both of the first lens 231 and the second lens 232. For example, a focusing mirror may be used instead of a lens as an optical element. It should be understood that when a mirror is used as an optical element, the laser beam 220 will propagate to and interact with the optical element (e.g., scatter from the optical element) rather than propagate through the optical element. Optical elements typically converge rather than diverge.

[0076] Laser systems typically have spatial constraints that at least partially define the size, shape, and / or configuration of the laser system and its components. Spatial constraints may arise due to the laser system being housed within an enclosure, for example, for ease of delivery and / or for shielding the laser beam from a user near the system. Such enclosures may have a fixed size / shape, thus defining the placement of the components housed therein. Alternatively, spatial constraints may arise due to user requirements, such as the area and / or volume in which the user can place the laser system.

[0077] Laser systems also typically have additional components for processing the laser beam. For example, a laser system may include one or more beam expanders, pulse stretchers, beam shapers, etc. Figure 1B, as described above, is an example of a laser system 100, which includes additional components such as a bandwidth analysis module 175, a pulse stretcher 180, and a measurement module 185.

[0078] Such additional components can be placed between the laser and the output of the laser system (i.e., along the path of the laser beam between the laser and the output of the laser system). Physically placing such components between the laser and the system output (e.g., in a straight line, with little or no laser beam redirection) can be difficult or impossible, especially given the spatial constraints discussed above. Therefore, it may be necessary to space the components considerably apart from the laser, and to guide the laser beam to the components (e.g., using a mirror) before it is redirected to the output of the laser system. However, with this configuration, the laser beam propagates over a greater distance (e.g., several meters or tens of meters) compared to a system without the additional components, and thus experiences greater effects due to divergence and / or pointing errors. For example, due to the greater propagation distance, such laser systems may output laser beams with one or more of the following characteristics: larger beam size, undulating beam size, undesirable beam shape, undulating beam shape, and undulating beam position.

[0079] Figure 3 schematically depicts a laser system 300 with additional components. The laser system 300 includes a laser 310 that emits a laser beam 320. In this example, laser 310 is a pulsed laser that emits pulsed radiation. That is, the laser beam 320 contains pulses of laser radiation. A mirror 315 guides the laser beam 320 to a pulse stretcher 360. Pulse stretchers are known in the art and are used to increase the pulse length of the radiation pulse. For example, the pulse duration can be increased from approximately tens of nanoseconds to approximately hundreds of nanoseconds. The pulse stretcher 360 may be of the type described above with reference to Figure 1B (i.e., an OPoS 180 operable to convert a single output laser pulse into a pulse train). After passing through the pulse stretcher 360, the laser beam 320 propagates to a first lens 331 and a second lens 332, which form an optical system equivalent to the optical system described above. After propagating through the second lens 332, the laser beam 320 exits the laser system 300 through the output end 340, which coincides with the exit aperture of the housing 350. The laser 310, the pulse stretcher 360, and the lenses 331 and 332 are housed inside the housing 350.

[0080] The optical system 330 is configured such that the laser beam 310 travels sequentially (i.e., in order) to the first lens 331, the second lens 332 and the output terminal 340.

[0081] The pulse stretcher 360 is located in a position not directly between the laser beam 310 and the output terminal 340. For example, and as described above, this can be attributed to spatial constraints. Therefore, the laser beam 320 propagates an additional distance away from the output terminal 340 for processing by the pulse stretcher 360, before propagating towards and through the output terminal 340. The optical system 330, including the first lens 331 and the second lens 332, is positioned to minimize the effective propagation length of the laser beam 320, although the actual propagation length of the laser beam is increased.

[0082] The first lens 331 is positioned close to the output end of the pulse stretcher 360. Because the optical system can image any conjugate plane, the first lens 331 does not need to be positioned at a focal length f from the output end of the pulse stretcher 360 (or the focal plane of the pulse stretcher 360). In fact, in this configuration, the object plane imaged by the optical system is located between the laser 310 and the pulse stretcher 360, for example, as indicated by the dotted line 370 in Figure 3. This is possible due to the imaging properties of the optical system comprising the first lens 331 and the second lens 332 separated by 2f. This allows the first lens 331 to be placed very close to the output end of the pulse stretcher 360, for example, within less than 1 focal length f.

[0083] The second lens 332 is positioned close to the output terminal 340 of the laser system 300 . Because the optical system can image any pair of conjugated planes, the second lens 332 need not be positioned one focal length from the output end 340 of the laser system. In fact, in this configuration, the plane conjugated with the object plane represented by the dot line 370 is located outside the laser system 300 , as indicated by the dot line 380 . It should be noted that Figure 3 is illustrative in nature and not drawn to scale.

[0084] Fully flexible configurations (i.e., wherein the first optical element 331 may be positioned at any distance from the output end of the pulse stretcher 360 or other additional components) is possible as long as the pulse stretcher 360 (or other additional components in its position) does not affect the beam parameters of the laser beam 320 . For example, this can be achieved in the case where the pulse stretcher 360 has an effective propagation length of zero. In this way, any beam parameter of the laser beam prior to the pulse stretcher 360 (e.g., a single laser pulse received by the pulse stretcher 360 from the laser 310) is replicated in the pulse chain, regardless of where the optical system 330 is located (relative to the pulse stretcher 360 ).

[0085] Alternatively, in some embodiments, an optical element that does affect the parameters of the laser beam 320 may be introduced (e.g., between the laser 310 and the optical system 330 ). For such embodiments, the optical system 330 may be considered to image the real object plane before the optical system 330 onto the image plane after the optical system 330 if a sufficient distance exists between the optical element and the first lens 331 . Alternatively, the optical system 330 may be considered to enable the virtual object plane before the optical system 330 to be imaged onto the image plane after the optical system 330 .

[0086] The first lens 331 and the second lens 332 are positioned so that their focal length f permits them to be 2f away from each other while being positioned close to the output end of the pulse stretcher 360 and the output end 340 of the laser system 300 , respectively. Thus, the 4f distance reduced by the effective propagation length of the laser beam 320 is maximized. Thus, the effect of divergence and directional error is reduced. For example, the laser system 300 can output a laser beam having one or more of the following when compared to a laser system without an optical system as described herein: a smaller beam size, a reduced beam size fluctuation, a more desirable beam shape, a reduced beam shape fluctuation, and a reduced beam position fluctuation.

[0087] In laser systems that do not have the optical systems described herein, suboptimal laser beam characteristics and / or low beam stability can cause the laser beam to at least partially miss the exit aperture of the housing. This can result in a significant portion of the laser beam being blocked from exiting the laser system, for example, because a portion of the laser beam may hit the housing instead of exiting through the exit aperture. Such losses can affect the efficiency of the laser system, for example, by approximately 10%. When the laser system is used with a lithography apparatus, any variation in such losses can affect the dose control of the lithography apparatus, which is undesirable as described below. Therefore, a laser system 300 of the type depicted in Figure 3 can significantly improve the efficiency of the laser system 300 because the laser beam characteristics and / or stability are improved, and thus the laser beam 220 can exit the exit aperture better without being blocked by the housing 350. Furthermore, in laser systems where suboptimal laser beam characteristics / stability cause the laser beam to at least partially miss the exit aperture of the housing, higher laser power can be used to offset the loss. Higher laser power typically leads to increased wear and / or damage to any components in the laser beam path, resulting in a reduced optical lifetime of the components. Therefore, a laser system 300 having the optical system 330 described herein can advantageously increase the lifetime of the components in the laser system.

[0088] Figures 4A and 4B illustrate simulations of the output laser beams of laser systems having and not having the optical systems 230 and 330 shown in Figures 2 and 3 and as described above, respectively. In both laser systems, the laser emits a laser beam with a top cap profile and a rectangular shape, achieved by selecting the laser and various beam processing components. The only significant difference between the output laser beams of the two laser systems is the inclusion of optical systems 230 and 330 as described herein. The output laser beams are shown in a plane perpendicular to the propagation direction of the laser beam.

[0089] Figure 4A shows a contour plot 410 and an image plot 420 illustrating the beam profile of a simulated laser beam (hereinafter referred to as the first simulated beam) at the output of a laser system without an optical system. Both the contour plot 410 and the image plot 420 show that the first simulated beam is generally elliptical in shape and has a substantially Gaussian profile. The first simulated beam has a height H1 and a width D1.

[0090] Figure 4B shows a contour plot 430 and an image plot 440 illustrating the beam profile of a simulated laser beam (hereinafter referred to as the second simulated beam) at the output of a laser system with an optical system. Both the contour plot 430 and the image plot 420 show that the shape of the second simulated beam is more rectangular than that of the first simulated beam. The "corners" of this rectangle have some curvature, but the shape is significantly more rectangular compared to the first simulated beam. The second simulated beam has a height H2 that is less than the height H1 of the first simulated beam. The second simulated beam has a width D2 that is less than the width D1 of the first simulated beam. Therefore, the second simulated beam has a smaller beam size than the first simulated beam. In this example, this demonstrates how a laser system with optical systems 230 and 330 reduces divergence effects, resulting in a smaller beam size and a more desirable beam shape.

[0091] Although not shown in the image, the stability of the second simulated beam is improved compared to the first simulated beam. That is, compared to the first simulated beam, the time-varying characteristics of the beam size, beam profile, beam position, and beam shape of the second simulated beam are reduced. This demonstrates that the effects of divergence and directional fluctuations are reduced.

[0092] FIG. 5 schematically depicts the laser system of FIG. 3 with an additional optical system. Similar components are numbered accordingly. In this configuration, the laser system 300 includes a second optical system 530, which includes a third lens 531 and a fourth lens 532. The third lens 531 and the fourth lens 532 have equal focal lengths f2, which in this configuration are different from the focal lengths f of the first lens 331 and the second lens 332. In other configurations, the second optical system 530 may be used, which includes lenses having focal lengths equal to those of the lenses in the first optical system.

[0093] The optical system 330 is configured such that the laser beam 310 travels sequentially (i.e., in order) to the third lens 531, the fourth lens 532 and the output terminal 340.

[0094] By providing a third lens 531 and a fourth lens 532 for this second optical system 530, the effective propagation distance of the laser beam 320 is further reduced by 4f2. Providing this second optical system 530 will further reduce the effects of divergence and pointing error.

[0095] The optical systems described herein (e.g., 230, 330, 530) can be used with a range of lasers in applications where lasers are used. The optical systems described herein are particularly suitable for lithography, for example, for lithography apparatuses such as those described with reference to FIG. 1A. For optimal lithography exposure, accurate control of the radiation dose supplied to the substrate is required. Furthermore, for optimal lithography exposure, the radiation supplied to the patterning device and subsequently the substrate can be selected to have a specific profile (e.g., the angular distribution of light). This profile may be referred to as the illumination mode or pupil mode. Suboptimal laser beam characteristics can lead to suboptimal dose control and / or suboptimal illumination mode, and thus suboptimal lithography exposure. To achieve high-quality exposure in lithography, it is generally necessary to provide radiation with known characteristics (e.g., in a laser beam), such as known beam size, known beam shape, and known beam position. Furthermore, to achieve high-quality exposure in lithography, it is generally necessary to provide radiation with high stability of fluctuations in, for example, beam size, beam shape, and beam position. Therefore, by combining a lithography device with an optical system as described herein, the laser system can be optimized and the stability of the laser beam supplied to the lithography device can be improved.

[0096] In a lithography apparatus, the laser beam output from the laser system typically travels through one or more illumination systems (e.g., illuminator IL of FIG. 1A) used to propagate and / or modulate the laser beam around the lithography apparatus. The instability of the laser beam can affect the imaging characteristics of this illumination system. In known lithography apparatuses, standard illumination systems may be unable to effectively propagate and / or modulate a laser beam with low stability. For example, given an unstable laser beam, the illumination system may fail to achieve the desired illumination pattern. Therefore, additional components, including increasing the actual propagation distance of the laser beam in the laser system (and thus increasing the instability of the laser beam at the output of the laser system), may require adjustment or replacement of the illumination system. Such adjustments or replacements can be costly and / or time-consuming. Providing a laser system as described herein allows for an increased actual propagation distance while maintaining sufficiently high stability so that the original illumination system can still be used to obtain optimal lithography exposure.

[0097] Some lasers exhibit beam patterns (e.g., random variations in beam intensity) that can be seen in the beam profile. These beam patterns can be superimposed on the nominal beam profile as perturbations; for example, the beam profile may have a generally Gaussian nominal profile, with the beam variations superimposed on a Gaussian distribution of intensity. These beam patterns are particularly common in lasers with high coherence and / or lasers emitting radiation with narrow bandwidths (e.g., less than one picometer).

[0098] Such narrow-bandwidth lasers are ideally suited for use in lithography apparatuses. In lithography, the spot size in the laser beam can cause significant variations in the dose of radiation supplied to the substrate (e.g., a change of several percent relative to the desired dose). In lithography, the spot size in the laser can negatively affect the ability of the illumination system to function effectively, for example, causing suboptimal illumination patterns and / or suboptimal dose control. This dose variation and / or suboptimal illumination pattern can lead to suboptimal lithography exposure. For example, such dose errors and suboptimal illumination patterns can affect critical dimensional uniformity (CDU).

[0099] A pulse stretcher can be used to reduce the spot effect when using a pulsed laser. The pulse stretcher converts a single laser pulse into a pulse train of secondary pulses. Each secondary pulse has a different spot pattern; therefore, the total spot size of the entire pulse is at least partially averaged. That is, the time-averaged spot pattern of the entire pulse train is smaller than that of the initial pulse.

[0100] As described above, the use of additional components such as pulse stretchers may require an increase in the path length (i.e., actual propagation distance) of the laser beam in the laser system due to constraints in positioning the pulse stretcher. This increased propagation distance will result in suboptimal laser beam characteristics at the output of the laser system and / or the input of the lithography device. Therefore, the optical system described herein is particularly advantageous when used in laser systems with pulse stretchers.

[0101] An example pulse stretcher includes a beam splitter and a delay line, the delay line including a configuration of a beam steering element (e.g., a mirror) defining a closed-loop propagation path from and back to the beam splitter. Specifically, the delay line is configured to receive a portion of a radiated pulse and, after a delay time, return that portion of the radiated pulse to the beam splitter. In this way, the delay line can be considered to form a closed loop or circular path to and from the beam splitter. The beam splitter receives an initial pulse and separates it into a first portion (i.e., a first-stage pulse) and a second portion (i.e., a second-stage pulse). The first-stage pulse is passed to the output of the pulse stretcher. The second-stage pulse is passed to the delay line. The delay line guides the second-stage pulse along the delay path and then returns to the beam splitter. When the beam splitter receives the second-stage pulse, it separates the second-stage pulse into third and fourth-stage pulses. The third-stage pulse is delivered to the output of the pulse stretcher, arriving at the output with a time delay relative to the first-stage pulse (the delay time is determined by the length of the delay path of the delay line). The fourth-stage pulse is delivered to the delay line and subsequently propagates along the delay path in the same manner as the second-stage pulse. Each time a secondary pulse propagates along the delay path and returns to the beam splitter, the secondary pulse is delayed by a time relative to the previous primary pulse. By using this configuration, the secondary pulses cycle along the delay path multiple times, with each cycle adding an additional time delay relative to the first smaller pulse. The subsequent output of the pulse stretcher is a chain of secondary pulses, each primary pulse delayed in time from the previous primary pulse by the time delay determined by the delay line.

[0102] It should be understood that although a secondary pulse train itself contains multiple pulses, it is considered as a single pulse with an extended pulse length (relative to the initial pulse). A single pulse can be considered to be temporally separated from adjacent single pulses for a time specified by the laser (i.e., repetition rate). Gaps (temporal separations) may occur between secondary pulses within a single pulse, but the gaps between secondary pulses are significantly smaller than the laser's repetition rate.

[0103] The delay line is described as a closed loop or circular configuration containing beam steering elements. The terms circular or loop used in this context are to indicate that a portion of the laser beam (e.g., a secondary pulse) circulates around elements in the pulse stretcher one or more times. It will be apparent to those skilled in the art that the light propagates linearly, and therefore the circular configuration comprises a series of linear paths that cooperate to guide the laser beam around the closed loop. For example, if the delay line includes first, second, third, and fourth mirrors, a portion of the laser beam can be propagated through the pulse stretcher to the following components in the following order: beam splitter, first mirror, second mirror, third mirror, fourth mirror, beam splitter (second time), first mirror (second time), second mirror (second time), third mirror (second time), fourth mirror (second time), beam splitter (third time), pulse stretcher output. The beam splitting and circular configuration of the pulse stretcher are important for the purpose of the delayed pulse portion of the pulse stretcher. This contrasts with an optical system as described herein, which has a linear configuration such that the laser beam propagates to each component of the optical system (i.e., the first and second optical elements) only once before exiting the laser system. That is, the laser beam travels sequentially (in order) and linearly (non-cyclically) to the first optical element, the second optical element, and the output. This linear travel of the laser beam can also be described as sequential travel, because the laser beam does not travel to / through the first optical element, the second optical element, or the output more than once.

[0104] The optical system described herein does not have a beam splitter directly preceding the first optical element, such that substantially all laser beams (received from the laser or from components preceding the optical system, such as pulse stretchers) are received by the first optical element of the optical system. It should be understood that the term "substantially" is used because there will always be some loss in the optical system, such as the optical element absorbing a small portion (e.g., 0 to 2%) of the laser beam.

[0105] A pulse stretcher may include a focusing element positioned, for example, along a delay line. However, the total focusing magnification of a pulse stretcher is typically zero, that is, it has a magnification of 1 or -1. Using this configuration, the pulse stretcher can have an effective propagation length of zero.

[0106] When using a pulse stretcher, it is particularly advantageous to position the optical system after the pulse stretcher (i.e., between the pulse stretcher and the output, rather than between the pulse stretcher and the laser). This configuration is illustrated in Figure 3, in which the optical system 320 is positioned after the pulse stretcher 360 (i.e., positioned such that it receives the laser beam 320 after it has propagated through the pulse stretcher 360). This configuration can result in reduced damage to optical components (e.g., lenses 331, 332) because the peak power (and peak energy) of the stretched pulse is less than the peak power / energy of the initial (unstretched) pulse. Therefore, by positioning the optical system 320 after the pulse stretcher 360, a lower radiation peak power is incident on the optical elements 331, 332 of the optical system 320.

[0107] In another example laser system, a second pulse stretcher is used. In this laser system, a first pulse stretcher is positioned between the laser and the optical system, and a second pulse stretcher is positioned after the optical system (i.e., between the output of the optical system and the laser system). In this configuration, the second optical element of the optical system is positioned close to the input of the second pulse stretcher. Because the optical system can image a plane in front of the optical system onto a conjugate plane behind the optical system (these mutually conjugate planes are separated by a distance four times the focal length of the two optical elements), the second optical element does not need to be positioned one focal length from the input of the second pulse stretcher. In fact, the second optical element can be positioned less than one focal length from the input of the second pulse stretcher, thereby allowing the 4f distance to be maximized. In this configuration, the laser beam travels indirectly between the second optical element and the output of the laser system, that is, the laser beam travels to the second optical element, travels to the second pulse stretcher, and then travels to the output.

[0108] In another example laser system, a pulse stretcher is positioned between the first and second optical elements of the laser system. This configuration can be advantageous for certain requirements, such as specific space constraints. If additional components do not provide total focusing magnification to the laser beam, for example, if the total magnification of the additional components is 1 (or -1), then the pulse stretcher or other additional components may be positioned between the first and second optical elements.

[0109] Distances, such as the distance between an image plane and an object plane, are referenced herein. Unless otherwise specifically described, such references may be considered as distances in the direction of propagation of the laser beam. It should be understood that a mirror may be used, for example, to redirect the laser beam, thereby changing the direction of propagation of the laser beam. Distances measured in the direction of propagation of the laser beam can therefore be used to trace the path of propagation by the laser beam, including any redirection.

[0110] In some configurations described herein, the second optical element is allegedly located close to the output of the laser system. In such configurations, the second optical element may be located between the first optical element and the output of the laser system. Alternatively, the second optical element may coincide with the output of the laser system. In some configurations, the laser system may have a housing and an exit aperture through which the laser beam exits the laser system. In this configuration, the second optical element may be located outside the housing such that the exit aperture is located between the first and second optical elements. In this case, the output of the laser system may be considered to coincide with the output of the laser system, rather than with the exit aperture corresponding to the output of the laser system.

[0111] The optical elements mentioned herein may include lenses and / or mirrors. Spherical lenses may be used. In some configurations, for example, if only one dimension of the laser beam (e.g., the vertical portion) requires an effective reduction in propagation distance, cylindrical lenses may be used. Spherical lenses are preferred due to their low cost, ease of implementation, and / or symmetry effect on the laser beam.

[0112] The laser system described herein allows the output laser beam of the laser system to have one or more desired characteristics. The benefits of providing a reduced beam size at the output of the laser system have been specifically mentioned herein. However, it should be understood that in some applications, a larger beam size may be desirable. The laser system described herein also provides increased stability of the laser beam at the output of the laser system. A stable output from the laser system is typically required.

[0113] The laser system described herein may include a laser that produces radiation having wavelengths in the deep ultraviolet (DUV) range (e.g., having wavelengths of 248 nanometers (nm) or 193 nm) (i.e., in laser beams 230, 330, 530). The laser may include an excimer laser, such as an argon fluoride ArF laser or a krypton fluoride KrF laser.

[0114] The use of the pulse stretcher has been referenced herein (e.g., to Figures 3 and 5). It should be understood that this component is used for illustrative purposes, and other components, in lieu of or other than the pulse stretcher, for shaping, controlling, and / or otherwise processing the laser beam may also be used in combination with the optical systems 230, 330, and 530 described herein. Examples of such other components that may be used in laser systems are described with reference to Figure 1B, such as the bandwidth analysis module 175, the pulse stretcher 180, and the measurement module 185.

[0115] The characteristics of laser beams have been referenced in this document. The stability of the characteristics of laser beams can also be considered as a characteristic of laser beams.

[0116] The best or optimized characteristics have been referenced herein. Those familiar with this technique will understand that best can have different meanings depending on the application. For example, best in lithography may mean sufficiently high quality given the requirements of lithography exposure. In some applications, best can be interpreted as an improvement over a second-best example.

[0117] It should be understood that, in the context of laser beams or radiation beams, the terms travel and propagate are synonymous and these two terms (and their derivatives, such as traveling and propagating) may be used interchangeably herein.

[0118] While reference may be specifically made herein to the use of lithography devices in IC manufacturing, it should be understood that the lithography devices described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guiding and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0119] Although reference is specifically made herein to the use of a laser system within the context of lithography apparatus, such a laser system can be used in other apparatuses. For example, a laser system may form part of a mask detection apparatus, a measuring apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). Such apparatuses are generally referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions. Alternatively, embodiments of the invention may be used in any apparatus that uses laser radiation (whether related to lithography or otherwise).

[0120] The embodiments may be further described using the following clauses: 1. A laser system comprising: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; and an output through which the laser beam exits the laser system; the laser, the optical system, and the output being configured such that the laser beam propagates sequentially to the first optical element, the second optical element, and the output; wherein the first optical element has a first focal length, the second optical element has a second focal length equal to the first focal length, and the second optical element is spaced from the first optical element by a distance twice the first focal length. 2. The laser system of clause 1, wherein the laser comprises an excimer laser. 3. The laser system of clause 1 or 2, further comprising a pulse stretcher for increasing the pulse length of a pulse in the laser beam. 4. The laser system of claim 3, wherein the pulse stretcher is disposed between the laser and the first optical element. 5. The laser system of claim 3 or 4, wherein the pulse stretcher includes a pulse stretcher output terminal, a pulse having an increased pulse length can exit the pulse stretcher through the pulse stretcher output terminal, and wherein the first optical element is positioned close to the pulse stretcher output terminal. 6. The laser system of claim 5, wherein the distance between the pulse stretcher output terminal and the first optical element is less than the first focal length. 7. The laser system of any of the preceding claims, wherein the second optical element receives the laser beam directly from the first optical element. 8. The laser system of any of the preceding claims, further comprising a housing in which the laser and the optical system are disposed, wherein: the housing has an exit aperture located at or near the output terminal of the laser system; and the second optical element is positioned close to the exit aperture. 9. The laser system of claim 8, wherein the distance between the exit aperture and the second optical element is less than the first focal length. 10. The laser system of any of the preceding claims, further comprising: a second optical system including a third optical element and a fourth optical element; wherein the third optical element has a third focal length, the fourth optical element has a fourth focal length equal to the third focal length, and the fourth optical element is spaced from the third optical element along the optical axis by a distance twice the third focal length. 11. The laser system of any of the preceding claims, further comprising a second pulse stretcher. 12. A lithography apparatus comprising the laser system of any of the preceding claims.13. A laser system comprising: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; and an output terminal through which the laser beam exits the laser system; the laser, the optical system, and the output terminal being configured such that the laser beam propagates sequentially to the first optical element, the second optical element, and the output terminal; wherein the first optical element has a first focal length, the second optical element has a second focal length, and the second optical element is spaced from the first optical element by a distance substantially equal to the sum of the first focal length and the second focal length.

[0121] Although specific embodiments of the present invention have been described above, it should be understood that the invention may be practiced in other ways different from those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below. [Simplified Explanation of the Diagram]

[0031] Various versions of the system and method will now be described with reference to the accompanying drawings, which are provided as examples only, in which:

[0032] Figure 1A schematically depicts the lithography device;

[0033] Figure 1B schematically depicts a view of a known laser system according to one of the revealed subjects;

[0034] Figure 2 schematically depicts the new laser system;

[0035] Figure 3 schematically depicts a second new laser system with additional components;

[0036] Figure 4A illustrates a simulation of the output laser beam of a laser system without an optical system as described herein;

[0037] Figure 4B illustrates a simulation of the output laser beam of a laser system having the optical system described herein;

[0038] Figure 5 schematically depicts the laser system of Figure 3 with an additional optical system.

Claims

1. A laser system comprising: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; an output terminal through which the laser beam exits the laser system; and a housing containing the laser and the optical system, wherein the housing has an exit aperture located at or near the output terminal of the laser system and the second optical element is positioned near the exit aperture; the laser, the optical system, and the output terminal are configured such that the laser beam propagates sequentially to the first optical element, the second optical element, and the output terminal; wherein the first optical element has a first focal length, the second optical element has a second focal length equal to the first focal length, and the second optical element is spaced from the first optical element by a distance equal to twice the first focal length, wherein the distance between the exit aperture and the second optical element is less than the first focal length.

2. The laser system of claim 1, wherein the laser comprises an excimer laser.

3. The laser system of claim 2, further comprising a pulse stretcher for increasing the pulse length of one of the pulses in the laser beam.

4. The laser system of claim 3, wherein the pulse stretcher is disposed between the laser and the first optical element.

5. The laser system of claim 4, wherein the pulse stretcher includes a pulse stretcher output, a pulse having an increased pulse length can be emitted from the pulse stretcher through the pulse stretcher output, and wherein the first optical element is positioned close to the pulse stretcher output.

6. The laser system of claim 1, further comprising a pulse stretcher for increasing the pulse length of one of the pulses in the laser beam.

7. The laser system of claim 6, wherein the pulse stretcher is disposed between the laser and the first optical element.

8. The laser system of claim 7, wherein the pulse stretcher includes a pulse stretcher output terminal, a pulse having an increased pulse length can be emitted from the pulse stretcher through the pulse stretcher output terminal, and wherein the first optical element is positioned close to the pulse stretcher output terminal.

9. The laser system of claim 8, wherein a distance between the pulse stretcher output and the first optical element is less than the first focal length.

10. The laser system of claim 1, wherein the second optical element receives the laser beam directly from the first optical element.

11. The laser system of claim 1, further comprising: a second optical system including a third optical element and a fourth optical element; wherein the third optical element has a third focal length, the fourth optical element has a fourth focal length equal to the third focal length, and the fourth optical element is spaced from the third optical element along the optical axis by a distance twice the third focal length.

12. The laser system of claim 8, further comprising a second pulse stretcher.

13. A lithography apparatus comprising: a laser system including: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; an output terminal through which the laser beam exits the laser system; and a housing containing the laser and the optical system, wherein the housing has an exit aperture located at or near the output terminal of the laser system and the second optical element is positioned near the exit aperture; the laser, the optical system, and the output terminal are configured such that the laser beam propagates sequentially to the first optical element, the second optical element, and the output terminal; wherein the first optical element has a first focal length, the second optical element has a second focal length equal to the first focal length, and the second optical element is spaced from the first optical element by a distance equal to twice the first focal length, wherein the distance between the exit aperture and the second optical element is less than the first focal length.

14. The lithography apparatus of claim 13, further comprising a pulse stretcher for increasing the pulse length of one of the pulses in the laser beam, wherein the pulse stretcher is disposed between the laser and the first optical element, and the pulse stretcher includes a pulse stretcher output end through which a pulse with the increased pulse length can exit the pulse stretcher, and wherein the first optical element is positioned close to the pulse stretcher output end.

15. A laser system comprising: a laser operable to generate a laser beam; an optical system including a first optical element and a second optical element; an output terminal through which the laser beam exits the laser system; and a housing containing the laser and the optical system, wherein the housing has an exit aperture located at or near the output terminal of the laser system and the second optical element is positioned near the exit aperture; the laser, the optical system, and the output terminal are configured such that the laser beam propagates sequentially to the first optical element, the second optical element, and the output terminal; wherein the first optical element has a first focal length, the second optical element has a second focal length, and the second optical element is spaced from the first optical element by a distance substantially equal to the sum of the first focal length and the second focal length, wherein the distance between the exit aperture and the second optical element is less than the second focal length.

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