Patent Publication Number: US-2022224069-A1

Title: Output light beam formation apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Application No. 62/863,980, filed Jun. 20, 2019 and titled OUTPUT LIGHT BEAM FORMATION APPARATUS, and U.S. Application No. 62/916,462, filed Oct. 17, 2019 and titled OUTPUT LIGHT BEAM FORMATION APPARATUS, both of which are incorporated herein in their entireties by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an output light beam formation apparatus. The output light beam formation apparatus may be used with, for example, a deep ultraviolet light (DUV) source. 
     BACKGROUND 
     Photolithography is the process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography optical source provides the deep ultraviolet (DUV) light used to expose a photoresist on the wafer. One type of gas discharge light source used in photolithography is known as an excimer light source or laser. An excimer light source typically uses a gas mixture that is a combination of one or more noble gases, such as argon, krypton, or xenon, and a reactive species such as fluorine or chlorine. The excimer light source derives its name from the fact that under the appropriate condition of electrical stimulation (energy supplied) and high pressure (of the gas mixture), a pseudo-molecule called an excimer is created, which only exists in an energized state and gives rise to amplified light in the ultraviolet range. An excimer light source produces a light beam that has a wavelength in the deep ultraviolet (DUV) range and this light beam is used to pattern semiconductor substrates (or wafers) in a photolithography apparatus. The excimer light source can be built using a single gas discharge chamber or using a plurality of gas discharge chambers. The gas mixture in the gas discharge chamber may be exhausted from the gas discharge chamber or chambers. 
     SUMMARY 
     In one general aspect, an apparatus includes: a beam splitter on a beam path, the beam splitter configured to receive light from a deep ultraviolet (DUV) light source; and a first plurality of reflective optical elements on the beam path. The beam splitter is configured to direct a portion of the light received from the DUV light source toward the first plurality of reflective optical elements; the first plurality of reflective optical elements is configured to rotate a divergence of the portion of the light to produce rotated light; and the beam splitter is configured to direct the rotated light and a portion of the light received from the DUV light source onto an output beam path. 
     Implementations may include one or more of the following features. The apparatus also may include a polarization element on the beam path. The polarization element may be configured to change a polarization state of the rotated light such that, on the output beam path, the rotated light has the same polarization state as light received from the DUV light source. The apparatus also may include a second plurality of reflective optical elements on the beam path between the first plurality of optical elements and the polarization element. The first plurality of reflective optical elements may include four mirrors. The first plurality of reflective optical elements may be configured to rotate the divergence of the portion of the light by 90 degrees. 
     The light received from the DUV light source may have a vertical divergence having an initial vertical divergence value and a horizontal divergence having an initial horizontal divergence value, and, on the output beam path, the rotated light may have a horizontal divergence equal to the initial vertical divergence value and a vertical divergence equal to the initial horizontal divergence value. The rotated light and another portion of light from the DUV light source may be combined at the beam splitter to form an output beam that propagates on the output beam path, the output beam may have a vertical divergence value that is based on the vertical divergence of the rotated light and the vertical divergence of the other portion of light from the DUV light source, and a horizontal divergence value that is based on the horizontal divergence of the rotated light and the vertical divergence of the other portion of the light from the DUV light source. The horizontal divergence value of the output beam may be greater than the initial horizontal divergence value, and the vertical divergence value of the output beam may be less than the initial vertical divergence value. The light from the DUV light source may include at least a first pulse of light, the rotated light comprises a rotated pulse of light formed from reflection of a first portion of the first pulse of light, the other portion of light from the DUV light source comprises a transmitted second portion of the first pulse of light, and the output beam may be based on a reflection of the rotated pulse of light and the other portion. 
     The second plurality of reflective optical elements may include two or more mirrors. The polarization element may include a half waveplate or a phase retarding mirror. The beam path may be a closed path defined by the beam splitter, the first plurality of reflective optical elements, the second plurality of reflective optical elements, and the polarization element. 
     In another aspect, a deep ultraviolet (DUV) light source includes a light generation apparatus comprising at least one discharge chamber configured to hold a gaseous gain medium and to produce a DUV light beam; and an output beam formation apparatus including: a beam splitter on a beam path, the beam splitter configured to receive the DUV light beam; a polarization element on the beam path; and a first plurality of reflective optical elements on the beam path. The beam splitter is configured to direct a portion of the DUV light beam toward the first plurality of reflective optical elements; the first plurality of reflective optical elements is configured to rotate a divergence of the portion of light to produce rotated light; the waveplate is configured to change a polarization state of the rotated light such that, on the output beam path, the rotated light has the same polarization state as the DUV light beam; and the beam splitter is configured to form an output beam based on the rotated light and a portion of the DUV light beam. 
     Implementations may include one or more of the following features. The light generation apparatus may include: a first discharge chamber configured to hold a first gaseous gain medium and to produce a DUV seed light beam; and a second discharge chamber configured to hold a second gaseous gain medium and to amplify the seed light beam to form an amplified DUV light beam. The output beam formation apparatus may be configured to be positioned between the first discharge chamber and the second discharge chamber, and the DUV light may be the DUV seed light beam. 
     The DUV light source also may include a beam stretching apparatus configured to receive the output beam. The output beam formation apparatus may be between the light generation apparatus and the beam stretching apparatus. 
     The polarization element may be between a second plurality of reflective optical elements and the beam splitter. 
     In another aspect, a method for a deep ultraviolet (DUV) light source includes: receiving a portion of a DUV light beam, the portion of the DUV light beam having a polarization state, a vertical divergence of an initial vertical divergence value, and a horizontal divergence of an initial horizontal divergence value; rotating the portion of the DUV light beam to form a rotated light beam, the rotated light beam having a vertical divergence of the initial horizontal divergence value and a horizontal divergence of the initial vertical divergence value; passing the rotated light beam through an optical delay; forming an output light beam that propagates on an output beam path based on the rotated light beam and another portion of the DUV light beam. 
     Implementations may include one or more of the following features. In some implementations, before combining the rotated light beam and another portion of the DUV light beam, the rotated light beam is passed through a polarization element such that the rotated light beam and the other portion of the DUV light beam have the same polarization state on the output beam path. 
     The DUV light beam may include a pulsed DUV light beam, the rotated light beam comprises a reflection of a first one of the pulses in the pulsed DUV light beam, and the other portion of the DUV light beam may include part of the first one of the pulses. 
     Implementations of any of the techniques described above and herein may include a process, an apparatus, a system, and/or a method. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of a system that includes an output formation apparatus. 
         FIGS. 1B-1D  show various views of an input light beam. 
         FIG. 2  is a block diagram of an output beam formation apparatus. 
         FIG. 3A  is a block diagram of another output beam formation apparatus. 
         FIG. 3B  is a block diagram of an optical element. 
         FIG. 4A  is a block diagram of a photolithography system. 
         FIG. 4B  is a block diagram of a projection optical system that may be used in the photolithography system of  FIG. 4A . 
         FIG. 5  is a block diagram of another photolithography system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , a block diagram of a system  100  is shown. The system  100  includes a light source  101  that produces an input light beam  102 , an output beam formation apparatus  110  that produces an output light beam  111 , and an optical assembly  190  that receives the output light beam  111 . The input light beam  102  propagates on an input path  105 . The output beam propagates on an output path  112 . The output beam formation apparatus  110  reduces the spatial coherence of the input light beam  102  by modulating a divergence of the input light beam  102 . Modulating the divergence means rotating or otherwise changing the axis along which the least, minimum, or smallest divergence extends and/or the axis along which the greatest, highest, or maximum divergence extends by bending, folding, and/or redirecting the input light beam  102 . In some implementations, the output beam formation apparatus  110  also maintains the polarization state of the input light beam  102  such that output light beam  111  has the same polarization state as the input light beam  102 . 
     The optical assembly  190  is a deep ultraviolet (DUV) scanner apparatus (such as the lithography exposure apparatus  469  shown in  FIG. 4A ) that exposes a wafer with the output light beam  111  to form electronic features, or the optical assembly  190  is an intermediate optical element that directs and/or modifies the output light beam  111  before the output light beam  111  reaches the DUV scanner apparatus. For example, the optical assembly  190  may be an optical oscillator (such as the discharge chamber  565 _ 2  shown in  FIG. 5 ), a mirror, a lens, or a pulse stretcher between the output beam formation apparatus  110  and a DUV scanner apparatus. The input light beam  102  and the output light beam  111  are electromagnetic fields that may exhibit spatial and/or temporal coherence. An electromagnetic field has spatial coherence if the phase of the electromagnetic field is the same at different locations along the wavefront. The beam  102  has temporal coherence if the phase of the electromagnetic field at a single location is periodically the same at different times. When a light beam has spatial and/or temporal coherence, the wavefronts in the beam may randomly interfere with each other to produce speckle. The speckle causes spatial and/or temporal noise in the light beam, and the speckle may cause a speckle pattern that has a non-uniform intensity profile in the X-Y plane at the apparatus receiving the light beam (the optical assembly  190  in this example). The presence of speckle may cause the light at the optical assembly  190  to have a non-uniform intensity in the X-Y plane. This non-uniformity can result in uneven exposure of the wafer. For example, speckle may result in variation of the sizes of the exposed regions of photoresist that define the formation of microelectronic features, causing the features to be improperly formed and defective. Accordingly, for some applications, it is desirable to reduce spatial coherence. 
     As discussed in greater detail below, the output beam formation apparatus  110  reduces the spatial coherence of the input light beam  102  by modulating the divergence of the input light beam  102 . Divergence (or beam divergence) is an angular measure of the increase in beam diameter as a function of distance from a beam waist or from an aperture or source that emits the beam. The input light beam  102  propagates in the Z direction. The divergence of the input light beam  102  is different along two orthogonal axis in the X-Y plane, which is the plane that is orthogonal to the direction of propagation Z.  FIGS. 1B-1D  show various views of an example of the input light beam  102 . The divergence of the input light beam  102  in two orthogonal planes is illustrated in  FIGS. 1B and 1C .  FIG. 1B  is a cross-sectional view of the input light beam  102  in the X-Z plane.  FIG. 1C  is a cross-sectional view of the input light beam  102  in the Y-Z plane. The divergence of the input light beam  102  in X-Z plane is also called the vertical divergence and is labeled Dv in  FIG. 1B . The divergence of the input light beam  102  in the Y-Z plane is also called the horizontal divergence and is labeled Dh in  FIG. 1C .  FIG. 1D  shows the input light beam  102  in the X-Y plane. The spatial coherence of the input light beam  102  may be reduced by increasing the divergence of the input light beam  102 . For example, increasing the divergence of the input light beam  102  along the Y axis reduces the horizontal spatial coherence (the spatial coherence along the Y axis). 
     In the example of  FIGS. 1B-1D , the vertical divergence (Dv) of the input light beam  102  is greater than the horizontal divergence (Dh), and the input light beam  102  has an elliptical shape or profile in the X-Y plane. The input light beam  102  may have other profiles in the X-Y plane. For example, the input light beam  102  may have a rectangular shape in the X-Y plane. 
     The output beam formation apparatus  110  modulates the divergence of the input light beam  102  by rotating the input light beam  102  such that the axes along which the minimum and maximum divergence extend are modulated. For example, the output beam formation apparatus  110  may modulate the divergence of the input light beam  102  by rotating the axes of minimum and maximum divergence of each of a plurality of successive reflected portions of the input light beam  102  by 90°. 
     A light beam has divergence in all directions in the plane perpendicular to the direction of propagation. The divergence of the input light beam  102  has a greatest or maximum extent along one axis and a smallest or least extent along another axis. Referring to  FIG. 1D , the maximum extent of the divergence or the maximum divergence of the input light beam  102  in the plane X-Y is along the X axis. The minimum, least, or smallest divergence of the input light beam  102  is along the Y axis. The output beam formation apparatus  110  rotates the divergence of the input light beam  102  by rotating the axes of the minimum and maximum divergence in the plane perpendicular to propagation. For example, in implementations in which the output beam formation apparatus  102  is configured to rotate the divergence of the input light beam  102  by 90°, the output beam formation apparatus  110  folds and changes the direction of the input light beam  102  such that the divergence of the light beam  102  is rotated by 90°. In this example, the output light beam  111  has an elliptical shape in the X-Y plane, but a maximum divergence along the Y axis and the minimum divergence along the X axis (thus, 90° rotated as compared to the divergence of the input beam). In this way, the divergence of the output light beam  111  at the optical assembly  190  is modulated as compared to the divergence in the input beam  102 . By modulating the divergence, the spatial coherence is reduced in at least one direction. 
     The input light beam  102  may have any polarization state or may be unpolarized. Polarization is a parameter that describes the direction of oscillation of the electric field of a light beam. A type of polarization and a direction of polarization define a polarization state. The type of polarization may be linear, circular, elliptical, or random, or the light beam may be unpolarized. A light beam that is linearly polarized has an electric field that oscillates in a single plane that is constant over time, with the polarization state indicating the plane of oscillation. For linearly polarized light, the oscillations are confined to a single plane. Circularly polarized light has an electric field that describes a helix along the direction of propagation. Circularly polarized light may have different, orthogonal states. For example, circularly polarized light may be right-handed polarized light, in which the electric field rotates clockwise (as viewed from a point that receives the light), or left-hand polarized light, in which the electric field rotates counterclockwise (as viewed from a point that receives the light). In the examples discussed below, the input light beam  102  is linearly polarized along the Y axis. 
     Referring to  FIG. 2 , a block diagram of an output beam formation apparatus  210  is shown. The output beam formation apparatus  210  is an implementation of the apparatus  110  ( FIG. 1 ). The output beam formation apparatus  210  modulates the divergence of an input light beam  202  to produce an output light beam  211 . The output light beam  211  may be provided to an optical assembly, such as the optical assembly  190  ( FIG. 1 ). The input light beam  202  and the output light beam  211  are pulsed light beams. 
     The output beam formation apparatus  210  includes a beam splitting apparatus  213 , a beam rotation apparatus  215 , and a beam delay apparatus  217 . The beam splitting apparatus  213 , the beam rotation apparatus  215 , and the beam delay apparatus  217  are on a beam path  216 . The beam path  216  is a closed path. Light enters and exits the path  216  at the beam splitting apparatus  213 . Light that enters the beam path  216  may traverse the beam path  216  more than once. 
     The beam splitting apparatus  213  is any optical element that is capable of splitting or dividing the input light beam  202 . The beam splitting apparatus  213  may be, for example, a beam splitter. The beam splitting apparatus  213  includes an interface  214  that reflects some of the input light beam  202  onto an output beam path  212  and transmits the remainder onto the beam path  216 . The beam splitting apparatus  213  divides the input light beam  202  into a first portion  203  and a second portion  204 . The first portion  203  is the transmitted portion of the input light beam  202  and the second portion  204  is the reflected portion of the input light beam  202 . In this example, the input light beam  202  is a pulse of light. The first portion  203  and the second portion  204  are also pulses of light, and each has an intensity that is determined by the beam splitting apparatus  213 . For example, in implementations in which the beam splitting apparatus  213  reflects half of the incident light and transmits the remaining incident light, the first portion  203  and the second portion  204  have equal pulse energies. 
     The beam splitting apparatus  213  may be made of two prisms that are joined together at the interface  214 . The amount of the input light beam  202  that is reflected depends on the material properties and construction of the beam splitting apparatus  213 . For example, the beam splitting apparatus  213  may be configured to reflect half of incident light onto the output beam path  212  and to transmit the remainder onto the beam path  216 . Although a negligible amount of light is absorbed or scattered by the beam splitting apparatus  213 , the combined intensity of light transmitted and reflected by the beam splitting apparatus  213  at any given time is substantially equal to the intensity of the light incident on the beam splitting apparatus  213  at that time. 
     The beam rotation apparatus  215  is a plurality of optical elements that are arranged to rotate the beam divergence of the first portion  203 . For example, the beam rotation apparatus  215  may include three or four reflective optical elements (such as mirrors) that fold and/or redirect the first portion  203  to form a rotated portion  203 ′. The greatest and least (or smallest) amounts of divergence of the rotated portion  203 ′ are rotated relative to those of the first portion  203 . For example, if the beam rotation apparatus  215  is configured to rotate the first portion  203  by 90°, then the rotated first portion  203 ′ has a horizontal divergence that is equal to the vertical divergence of the first portion  203 , and the rotated first portion  203 ′ has a vertical divergence that is equal to the horizontal divergence of the first portion  203 . The beam rotation apparatus  215  may direct the portion  203  in more than one direction to form the rotated portion  203 ′. For example, the beam rotation apparatus  215  may direct the first portion  203  out of the plane of the page. An implementation of the beam rotation apparatus  215  is shown in  FIG. 3A . 
     The output beam formation apparatus  210  also includes the beam delay apparatus  217 . The beam delay apparatus  217  determines the length of the beam path  216  and thus determines when the rotated portion  203 ′ arrives at the beam splitting apparatus  213 . The beam delay apparatus  217  is one or more optical elements on the beam path  216 . For example, the beam delay apparatus  217  may include a plurality of reflective optical elements, such as mirrors. If the delay introduced by all of the optical elements that define the beam path  216  (including the beam delay apparatus  217 ) is less than the temporal duration of the input beam pulse  202 , then the rotated portion  203 ′ will partially overlap with the input beam pulse  202  in time. The beam delay apparatus  217  also steers the rotated portion  203 ′ toward the beam splitter apparatus  215 . 
     After passing through the beam delay apparatus  217 , the rotated portion  203 ′ is incident on the beam splitting apparatus  213 . Some of the light in the rotated portion  203 ′ is transmitted through the beam splitting apparatus  213  and onto the output beam path  212 . The remainder of the rotated portion  203 ′ is reflected onto the beam path  216  as portion  203 _ 1 . The portion  203 _ 1  interacts with the beam rotation apparatus  215  and the beam delay apparatus  217  again to produce another rotated portion  203 _ 1 ′ that is returned to the beam splitter apparatus  213 . 
     Some of the rotated portion  203 _ 1 ′ is transmitted onto the output beam path  212 . Because the rotated portion  203 _ 1 ′ is based on a reflection of the rotated portion  203 ′ and the rotated portion  203 ′ is based on the input light beam  202 , the divergence of the rotated portion  203 ′ and the divergence of the rotated portion  203 _ 1 ′ are not the same. For example, if the direction of greatest divergence of the rotated portion  203 ′ is 90° different than the direction of greatest divergence of the input light beam  202 , the direction of greatest divergence of the rotated portion  203 _ 1 ′ is the same as the direction of greatest divergence of the input light beam  202 , and so on. The intensity of each subsequent portion decreases depending on the reflection and transmission characteristics of the beam splitting apparatus  213 . The output light beam  211  is formed from the light that is reflected or transmitted by the interface  214 . Thus, the output light beam  211  includes the second portion  204  and the transmitted portion of the rotated portions  203 ′,  203 _ 1 ′ . . .  203 _n′. The divergence (and intensity) of these components varies, and the effect is that an assembly on the output beam path  212  (such as, for example, the optical assembly  190 ) receives the output light beam  211  with a modulated divergence. The modulated divergence results in the output light beam  211  having a lower spatial coherence along at least one direction. 
     Other implementations of the output beam formation apparatus  210  are possible. For example, the beam delay apparatus  217  may be between the beam splitting apparatus  213  and the beam rotation apparatus  215 . In some implementations, the output beam formation apparatus  210  includes a polarization element on the beam path  216  that ensures that the polarization of the rotated portion  203 ′ is the same as the input pulse  205 . In another example, the beam splitting apparatus  213  may have a configuration other than the configuration shown in  FIG. 2 . For example, the beam splitting apparatus  214  may be arranged such that the portion  203  is formed by the beam splitting apparatus  213  reflecting part of the input light beam  202 , and the second portion  204  is formed by the beam splitting apparatus  213  transmitting part of the input light beam  202 . 
     Referring to  FIG. 3A , a block diagram of another output beam formation apparatus  310  is shown. The output beam formation apparatus  310  is another example of an implementation of the output beam formation apparatus  110  ( FIG. 1 ). The output beam formation apparatus  310  includes a beam splitting apparatus  313 , a beam rotation apparatus  315 , a beam delay apparatus  317 , and a waveplate  318  that define an optical path  316 . The waveplate  318  may be, for example, a half-waveplate or phase retarding mirror. The waveplate  318  is configured to change the polarization state of incident light into an orthogonal polarization state. For example, if the incident light on the waveplate  318  is horizontally linearly polarized, the light that exits the waveplate  318  is vertically linearly polarized. Light enters and exits the optical path  316  at the beam splitting apparatus  313 . The optical path  316  is a closed path, and light may traverse the optical path  316  more than once. The output beam formation apparatus  310  forms an output light beam  311  based on an input light beam  302 . 
     For the example of  FIG. 3A , the X, Y, and Z axes are defined by a coordinate system  391 . When discussing the orientation of various components and elements, the term axis encompasses directions that are away from and toward the origin. For example, an element that extends along the X axis may extend in the +X and −X directions. In the example discussed below, the input light beam  302  has a horizontal linear polarization state (the polarization vector is not along the X axis) and a greater beam divergence along the X axis than the Y axis. In  FIG. 3A , the double-headed arrows indicate the polarization state. 
     An input light beam  302  propagates in the +Z direction and is incident on the beam splitting apparatus  313 . The beam splitting apparatus  313  reflects and transmits light at an interface  314  based on the optical properties of the material or materials used in the beam splitting apparatus  313 . The beam splitting apparatus  313  is any device that is capable of dividing incident light into a reflected portion and a transmitted portion. The beam splitting apparatus  313  is configured to transmit 50% of incident light and reflect 50% of incident light at an interface  314 . The beam splitting apparatus  313  may be, for example, a non-polarizing cube beam splitter that includes two triangular prisms that are joined together. In this example, the plane at which the two triangular prisms are joined is the interface  314 . 
     The beam splitting apparatus  313  and the optical elements that make up the beam rotation apparatus  315  and the beam delay apparatus  317  do not all have the same angle of incidence and may be rotated about the X, Y, or Z axis. In the discussion below, a nominal plane of an optical element is a plane near the active surface of that optical element, but from which the active surface of the optical element is rotated. Referring also to  FIG. 3B , a block diagram of an optical element  322  is shown. The optical elements that make up the beam rotation apparatus  315 , the beam delay apparatus  317 , and the beam splitting apparatus  313  are similar to the optical element  322 . The optical element includes an active surface  323  and a base  324 . Together, the active surface  323  and the base  324  form the optical element  322 . The optical element  322  may be implemented without the base  324 . The active surface  323  is the portion of the optical element  322  that interacts with incident light. In the example of  FIG. 3B , the active surface  323  is nominally in the X-Z plane and is rotated an angle α about the Z axis in the counterclockwise direction. In  FIG. 3B , the +Z direction is into the page. In  FIG. 3B  and the discussion below, the rotation about an axis is given from the perspective of looking toward the origin. 
     The beam splitting apparatus  313  and the interface  314  are nominally in the X-Z plane and are rotated clockwise 45° about the X axis. A portion  302 ″ of the input beam  302  is transmitted through the interface  314  onto the output beam path  312 . The portion  302 ″ forms part of the output light beam  311 . The portion  302 ″ has an intensity that is 50% of the intensity of the input light beam  302 . The beam splitting apparatus  313  is non-polarizing. Thus, the polarization state of the portion  302 ″ is the same as the polarization state of the input light beam  302 . The divergence of the portion  302 ″ is also the same as the divergence of the input beam  302 . The interface  314  also reflects part of the input light beam as a portion  302 ′. The portion  302 ′ is polarized along the Z axis and the greatest divergence is along the X axis. The portion  302 ′ propagates along the +Y direction to the beam rotation apparatus  315 . 
     The beam rotation apparatus  315  rotates the divergence of the portion  302 ′. The beam rotation apparatus  315  includes four reflective optical elements  322   a ,  322   b ,  322   c , and  322   d . Referring also to  FIG. 3B , each of the reflective optical elements  322   a ,  322   b ,  322   c ,  322   d  includes a reflective surface that is similar to the active surface  323 . Each reflective optical element  322   a - 322   d  may be, for example, a plane mirror that has a flat reflective surface that interacts with incident light. The reflective surface  323  is made of a material that reflects the wavelength or wavelengths of light in the input beam  302 . The reflective surface  323  may be, for example, coated calcium fluoride (CaF) or fused silica. For simplicity, the individual reflective surfaces are not shown on the optical elements  322   a .  322   b ,  322   c ,  322   d  in  FIG. 3A . However, the reflective surfaces are the portions of the optical elements  322   a ,  322   b ,  322   c ,  322   d  that interact with light and define the portion of the beam path  316  that passes through the beam rotation apparatus  315 . The optical elements  322   a ,  322   c , and  322   d  and the beam splitting apparatus  313  are organized in a coplanar arrangement: each of the optical elements  322   a ,  322   c ,  322   d  has a reflective surface in the same X-Y plane as the beam splitting apparatus  313 . The reflective surface of the optical element  322   b  is displaced from that X-Y plane in the +Z direction. The reflective surface of the optical elements  322   a - 322   d  are rotated relative to one of the axes in the coordinate system  391 . 
     The reflective surface of the optical element  322   a  nominally extends in the X-Z plane and is rotated 45° clockwise about the X axis. The reflective surface of the optical element  322   a  is parallel to the interface  314  of the beam splitting apparatus  313 . The center of the reflective surface of the optical element  322   a  and the center of the interface  314  are in the same X-Y plane. The reflective surface of the optical element  322   b  nominally extends in an X-Y plane that is displaced in the +Z direction relative to the X-Y plane that includes the centers of the reflective surface of the optical elements  322   a  and  322   c . The reflective surface of the optical element  322   b  is rotated slightly about the X axis such that light incident on the optical element  322   b  is reflected toward the optical element  322   c . The optical element  322   c  is close to the optical element  322   a . The center of the reflective surface of the optical element  322   c  is in the same X-Y plane as the center of the reflective surface of the optical element  322   a . The reflective surface of the optical element  322   c  nominally extends in the X-Y plane and is rotated 45° clockwise about the Y axis. The center of the reflective surface of the optical element  322   d  is in the same X-Y plane as the center of the reflective optical elements  322   a  and  322   c . The reflective surface of the optical element  322   d  nominally extends in the X-Z plane and is rotated 45° clockwise about the Z axis. 
     An example of the portion  302 ′ propagating through the beam rotating apparatus  315  is discussed next. In this example, the input beam  302  is linearly polarized along the Y axis and has a divergence that is greater along the X axis than the Y axis. 
     The interaction between the input beam  302  and the interface  314  produces the portion  302 ′, which is a reflection of the input beam  302 . The portion  302 ′ propagates in the +Y direction. The portion  302 ′ is linearly polarized along the Z axis. At the input to the beam rotation apparatus  315 , the portion  302 ′ and the light beam  302  have the same divergence along the X axis. The divergence of the portion  302 ′ along the Z axis is the same as the divergence of the input light beam  302  along the Y axis. In other words, the greatest divergence of the portion  302 ′ is along the X axis. 
     Of the optical elements  322   a - 322   d  that make up the beam rotation apparatus, the optical element  322   a  is closest to the beam splitting apparatus  313 . Thus, the portion  302 ′ interacts with the optical element  322   a  before interacting with the optical elements  322   b - 322   d . The optical element  322   a  reflects the portion  302 ′ in the +Z direction toward the optical element  322   b . After interacting with the optical element  322   a , the portion  302 ′ is linearly polarized along the Y direction. The greatest divergence of the portion  302 ′ is along the X axis immediately after interacting with the optical element  322   a . The optical element  322   b  reflects the portion  302 ′ in the −Z direction (or at a small angle relative to the Z direction) toward the optical element  322   c . The greatest divergence of the portion  302 ′ is along the X axis immediately after interacting with the optical element  322   b . The optical element  322   c  reflects the portion  302 ′ to the optical element  322   d  along the +X direction. After interacting with the optical element  322   c , the direction of the greatest divergence of the portion  302 ′ is along the Z axis. Thus, the optical element  322   c  rotates the divergence of the portion  302 ′. The portion  302 ′ is reflected from the optical element  322   d  along the −Y direction. After interacting with the optical element  322   d , the greatest divergence of the portion  302 ′ is along the Z axis. 
     Thus, the beam rotating apparatus  315  rotates the divergence of the portion  302 ′ by 90°. In this example, at the input of the beam rotating apparatus  315  (the optical element  322   a ), the portion  302 ′ is propagating in the +Y direction, the greatest divergence is along the X axis, and the least (or smallest) divergence is along the Z axis. At the output of the beam rotating apparatus  315  (immediately after the optical element  322   d ), the portion  302 ′ propagates in the −Y direction, the greatest divergence is along the Z axis, and the least (or smallest) divergence is along the X axis. 
     The optical elements  322   a - 322   d  are not polarizing elements, and the portion  30 T remains linearly polarized while propagating through the beam rotation apparatus  315 . The polarization of the portion  302 ′ is defined relative to a surface on which the portion  302 ′ is incident. Because the reflective surfaces of the optical elements  322   a - 322   d  do not all have the same angle of incidence, the axis of polarization of the portion  302 ′ rotates in the beam rotation apparatus  315 . The portion  302 ′ is linearly polarized along the Z axis at the optical element  322   a . The interaction with the optical element  322   a  rotates the polarization to the Y axis. The optical element  322   b  reflects the portion  302 ′ back along the −Z direction and the portion  302 ′ remains linearly polarized along the Y axis. The optical element  332   c  does not rotate the polarization of the portion  302 ′. However, the optical element  322   d  rotates the polarization from along the Y axis to along the X axis. Thus, the beam rotating apparatus  315  also changes the axis polarization of the portion  302 ′. In this example, the beam rotating apparatus  315  changes the polarization from along the Z axis at the input of the beam rotating apparatus  315  to along the X axis at the output of the beam rotating apparatus  315 . 
     The output beam formation apparatus  310  also includes the delay apparatus  317 . The delay apparatus  317  directs the portion  302 ′ back to the beam splitting apparatus  313 . The delay apparatus  317  is also used to determine the length of the optical path  316  and thereby controls when the portion  302 ′ arrives at the beam splitting apparatus  313 . For example, if the portion  302 ′ (which is formed from a reflection of an input light pulse) is to be superimposed with a later-occurring portion of that same light pulse, then the delay apparatus  317  is configured such that the portion  302 ′ reaches the beam splitting apparatus  313  before the end of the input light pulse. The output light beam  311  received by the optical assembly  190  is a pulse of light formed by the light that interacts with the beam splitting apparatus  313 . 
     The delay apparatus  317  includes a reflective optical element  325   a  and a reflective optical element  325   b . The reflective optical elements  325   a ,  325   b  are similar to the reflective optical elements  322   a - 322   d . Each reflective optical element  325   a ,  325   b  includes a reflective surface similar to the reflective surface  323  ( FIG. 3B ). For simplicity, the reflective surfaces of the optical elements  325   a ,  325   b  are not shown in  FIG. 3B . The reflective surface of the optical element  325   a  extends nominally in the X-Z plane and is rotated 45° counterclockwise about the Z axis. The reflective surface of the optical element  325   b  extends nominally in the X-Z plane and is rotated 45° clockwise about the Z axis. The reflective surface of the optical element  325   a  is orthogonal to the reflective surface of the optical element  322   d . The reflective surface of the optical element  325   b  is parallel to the reflective surface of the optical element  322   d . The centers of the reflective surfaces of the optical elements  322   d ,  325   a , and  325   h  are in the same X-Y plane. The length of the optical path  316  may be adjusted by adjusting the distance between the optical elements  325   a  and  322   d , and/or the distance between the optical elements  325   a  and  325   b.    
     The optical element  325   a  is between the optical element  322   d  and the optical element  325   b . The optical element  325   a  is displaced in the −Y direction relative to the optical element  322   d  and in the +X direction relative to the optical element  325   h . The reflective surface of the optical element  325   a  receives the portion  302 ′ from the optical element  322   d . Prior to interacting with the optical element  352   a , the portion  302 ′ is linearly polarized along the X axis and the greatest divergence is along the Z axis. The optical element  325   a  reflects the portion  302 ′ in the −X direction to the optical element  325   b . The interaction with the optical element  325   a  rotates the polarization of the portion  302 ′ to be along the Y axis but does not rotate the divergence. The portion  302 ′ then interacts with the optical element  325   b . The optical element  325   b  reflects the portion  302 ′ along the +Y direction toward the beam splitting apparatus  313 . The interaction with the optical element  325   b  rotates the polarization of the portion  302 ′ to be along the X axis but does not rotate the divergence. 
     Thus, the portion  302 ′ propagates in the +Y direction toward the beam splitting apparatus  313  with the greatest divergence along the Z axis and the polarization along the X axis. When the portion  302 ′ reaches the interface  314 , 50% of the portion  302 ′ is reflected in the +Z direction and onto the output beam path  312 . The reflected part of the portion  302 ′ is labeled as  311 ′. The reflected portion  311 ′ has the greatest divergence along the Y axis and the least divergence along the X axis. The output portion  311 ′ and the portion  302 ″ form the output light beam  311 . The portion  302 ″ has the greatest divergence along the X axis and the least (or smallest) divergence along the Y axis. On the other hand, the output portion  311 ′ has the greatest divergence along the Y axis and the least (or smallest) divergence along the X axis. The output portion  311 ′ adds divergence along the Y axis such that the output light beam  311  has a greater divergence along the Y axis and a smaller divergence along the X axis than the input beam  302 . As a result, the output light beam  311  has a lower spatial coherence along the Y axis than the input beam  302  and thus has less noise along the Y axis than the input beam  302 . In this way, the output beam formation apparatus  310  reduces speckle and improves the performance of the system  300 . 
     In the discussion of the above example, the input light beam  302  propagates in the +Z direction, is linearly polarized along the Y axis, has a greatest divergence is along the X axis, and has a least divergence is along the Y axis. In other examples, an input light beam may be linearly polarized along some other axis perpendicular to the direction of propagation (or may have other polarization or lack of polarization), may have a greatest divergence along some other axis perpendicular to the direction of propagation, and may have a least divergence along some other axis that may or may not be perpendicular to the axis of greatest divergence. 
     The remaining light in the portion  302 ′ is transmitted through the interface  314  and remains on the beam path  316 . Light continues to circulate in the beam path  316  and is manipulated by the beam rotation apparatus  315  and the beam delay apparatus  317  as discussed above, and additional contributions to the output light beam  311  are made. For the example of  FIG. 3  in which the beam splitting apparatus reflects 50% of incident light and transmits 50% of incident light, the divergence of the output light beam  311  is represented by Equations (1) and (2): 
     
       
         
           
             
               
                 
                   
                     Horizontal 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     beam 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     divergence 
                   
                   = 
                   
                     
                       
                         2 
                         3 
                       
                       ⁢ 
                       HDi 
                     
                     + 
                     
                       
                         1 
                         3 
                       
                       ⁢ 
                       VDi 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     Vertical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     beam 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     divergence 
                   
                   = 
                   
                     
                       
                         2 
                         3 
                       
                       ⁢ 
                       VDi 
                     
                     + 
                     
                       
                         1 
                         3 
                       
                       ⁢ 
                       
                         HDi 
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In Equations (1) and (2), HDi is the horizontal beam divergence of the input beam  302  in radians, and VDi is the vertical beam divergence of the input beam  302  in radians. The vertical beam divergence is the beam divergence along the X axis in the coordinate system  391 . The horizontal beam divergence is the beam divergence along the Y axis in the coordinate system  391 . For implementations in which the input beam  302  has the greatest divergence along the X axis, the output beam formation apparatus  310  reduces the divergence along the X axis and increases the divergence along the Y axis. Thus, in these implementations, the output beam formation apparatus  310  reduces the spatial coherence along the Y axis and the output light beam  311  has a lower spatial coherence along the Y axis (and less speckle) as compared to the input beam  302 . For implementations in which the input beam  302  has the greatest divergence along the Y axis and the least divergence along the X axis, the output beam formation apparatus  310  reduces the divergence along the Y axis and increases the divergence along the X axis. In these implementations, the output beam formation apparatus  310  reduces the spatial coherence along the X axis. 
     The modulation of the divergence of the output light beam  311  occurs without regard to the polarization of the light that circulates on the path  316 . In other words, the modulation of the divergence occurs regardless of the polarization state of the light that circulates on the path  216  and may be achieved with unpolarized light. Nonetheless, for some applications, it is desirable that the output light beam  311  have a particular polarization state. In these implementations, the output beam formation apparatus  310  may include the waveplate  318 . 
     As discussed above, after interacting with the optical element  325   b , the polarization of the portion  302 ′ is along the X axis. The polarization of the input beam  302  is along the Y axis. The output beam formation apparatus  310  changes the divergence of the input beam  302  even if the portion  302 ′ and the input beam  302  do not have the same polarization. However, for some applications, it is important that the output light beam  311  have the same polarization as the input beam  302 . Thus, the output beam formation apparatus  310  may include a polarizing element that changes the polarization of the portion  302 ′ without rotating or otherwise changing the divergence of the portion  302 ′. In the example of  FIG. 3A , the output beam formation apparatus  310  includes the waveplate  318  to change the polarization of the portion  302 ′ before the portion  302 ′ reaches the beam splitting apparatus  313 . The waveplate  318  is a half-wave plate that is positioned on the beam path  316 . The portion  302 ′ passes through the half-wave plate and the polarization is rotated by 90°. After passing through the waveplate  318 , the portion  302 ′ is polarized along the Z axis but the divergence is not changed. When the portion  302 ′ reaches the beam splitting apparatus  313 , the output portion  311 ′ is reflected from the interface  314  and the polarization is rotated to be along the Y axis. Thus, the waveplate  318  changes the polarization of the portion  302 ′ such that the polarization of the output portion  311 ′ and the input beam  302  are the same, and the output light beam  311  has a constant polarization. 
       FIGS. 4A and 5  provide examples of DUV light sources  460  and  560 , respectively, that may use the output beam formation apparatus  110 ,  210 , and/or  310 . The output beam formation apparatus  110 ,  210 , or  310  may be placed at any one of the locations shown by the box element labeled  110 . 
     Referring to  FIGS. 4A and 4B , a photolithography system  450  includes the DUV light source  460  that provides a light beam  441  to a lithography exposure apparatus  469 , which processes a wafer  470  received by a wafer holder or stage  471 . The DUV light source  460  includes a discharge chamber  465 , which encloses a gain medium  461 , a cathode  462   a , and an anode  462   b . The gain medium  461  is a gaseous gain medium. The discharge chamber  465  is sealed such that the gain medium  461  remains in and is contained by the discharge chamber  465 . Only one gas discharge chamber  465  is shown in  FIG. 4A . However, the light source  460  may include more than one discharge chamber, such as shown in  FIG. 5 . 
     The DUV light source  460  also includes a gas management system  479 . The gas management system  479  is in fluid communication with an interior  478  of the DUV light source  460 . The gas management system  479  may include devices that manage the pressure and/or the fluid substances in the interior  478 . For example, the gas management system  479  may include pumps, fans, filters, and/or other devices capable of managing gases and debris. The gas management system  479  may remove unwanted chemical substances, elements, or mixtures from the interior  478 . For example, the gas management system  479  may purge out oxygen from the interior  478  using another chemical (in the form of a gas) such as, for example, nitrogen (N 2 ) or helium (He). The gas used by the gas management system  479  to remove the unwanted substances is referred to as a purge gas  412 . Although the purge gas  412  is in the interior  478  and may surround the discharge chamber  465 , the purge gas  412  does not penetrate the discharge chamber  465  and does not disturb or change the chemical composition of the gain medium  461 . The light beam  441  propagates in the interior  478  and thus propagates in the purge gas  412 . 
     The light beam  441  may be a pulsed light beam that includes pulses of light separated from each other in time. The lithography exposure apparatus  469  includes a projection optical system  475  through which the light beam  441  passes prior to reaching the wafer  470 , and a metrology system  472 . The metrology system  472  may include, for example, a camera or other device that is able to capture an image of the wafer  470  and/or the light beam  441  at the wafer  470 , or an optical detector that is able to capture data that describes characteristics of the light beam  441 , such as intensity of the light beam  441  at the wafer  470  in the x-y plane. The lithography exposure apparatus  469  may be a liquid immersion system or a dry system. The photolithography system  450  also includes a control system  480  to control the light source  460  and/or the lithography exposure apparatus  469 . 
     Microelectronic features are formed on the wafer  470  by, for example, exposing a layer of radiation-sensitive photoresist material on the wafer  470  with the light beam  441 . Referring also to  FIG. 4B , the projection optical system  475  includes a slit  476 , a mask  474 , and a projection objective, which includes a lens system  477 . The lens system  477  includes one or more reflective or refractive optical elements that are capable of interacting with light in the DUV range. The light beam  441  enters the optical system  475  and impinges on the slit  476 , and at least some of the beam  441  passes through the slit  476 . In the example of  FIGS. 4A and 4B , the slit  476  is rectangular and shapes the light beam  441  into an elongated rectangular shaped light beam. The mask  474  includes a pattern, and this pattern determines which portions of the shaped light beam are transmitted by the mask  474  and which are blocked by the mask  474 . The design of the pattern is determined by the specific microelectronic circuit design that is to be formed on the wafer  470 . 
     In the example of  FIG. 4 , the output beam formation apparatus  110  is between the light source  460  and the lithography exposure apparatus  469 . The output beam formation apparatus  110  receives the beam  441  and reduces the spatial coherence of the beam  441  before it is provided to the lithography exposure apparatus  469 . 
     Referring to  FIG. 5 , a block diagram of a photolithography system  550  is shown. The system  550  is an example of an implementation of the system  450  ( FIG. 4A ). For example, in the photolithography system  550 , a light source  560  is used as the light source  460  ( FIG. 4A ). The light source  560  produces a pulsed light beam  541 , which is provided to the lithography exposure apparatus  469 . The photolithography system  550  also includes a control system  580 , which, in the example of  FIG. 5 , is connected to components of the optical source  560  as well as to the lithography exposure apparatus  469  to control various operations of the system  550 . In other implementations, the control system  580  may be implemented as two separate control systems, one to control various aspects of the light source  560  and another to control the lithography exposure apparatus  469 . In still other implementations, various other control systems  580  may be implemented. 
     In the example shown in  FIG. 5 , the light source  560  is a two-stage laser system that includes a master oscillator (MO)  567  that provides a seed light beam  542  to a power amplifier (PA)  568 . The MO  567  and the PA  568  may be considered to be subsystems of the light source  560  or systems that are part of the light source  560 . The PA  568  receives the seed light beam  542  from the MO  567  and amplifies the seed light beam  542  to generate the light beam  541  for use in the lithography exposure apparatus  469 . For example, in some implementations, the MO  567  may emit a pulsed seed light beam, with seed pulse energies of approximately 1 milliJoule (mJ) per pulse, and these seed pulses may be amplified by the PA  568  to about 10 to 15 mJ. 
     The MO  567  includes a discharge chamber  565 _ 1  having two elongated electrodes  562   a _ 1  and  562   b _ 1 , a gain medium  561 _ 1  that is a gas mixture, and a fan (not shown) for circulating the gas mixture between the electrodes  562   a _ 1 ,  562   b _ 1 . A resonator is formed between a line narrowing module  586  on one side of the discharge chamber  565 _ 1  and an output coupler  581  on a second side of the discharge chamber  565 _ 1 . 
     The discharge chamber  565 _ 1  includes a first chamber window  563 _ 1  and a second chamber window  564 _ 1 . The first and second chamber windows  563 _ 1  and  564 _ 1  are on opposite sides of the discharge chamber  565 _ 1 . The first and second chamber windows  563 _ 1  and  564 _ 1  transmit light in the DUV range and allow DUV light to enter and exit the discharge chamber  565 _ 1 . 
     The line narrowing module  586  may include a diffractive optic such as a grating that finely tunes the spectral output of the discharge chamber  565 _ 1 . The light source  560  also includes a line center analysis module  584  that receives an output light beam from the output coupler  581  and a beam coupling optical system  583 . The line center analysis module  584  is a measurement system that may be used to measure or monitor the wavelength of the seed light beam  542 . The line center analysis module  584  may be placed at other locations in the light source  560 , or it may be placed at the output of the light source  560 . 
     The gas mixture that is the gain medium  561 _ 1  may be any gas suitable for producing a light beam at the wavelength and bandwidth required for the application. For an excimer source, the gas mixture  561 _ 1  may contain a noble gas (rare gas) such as, for example, argon or krypton, a halogen, such as, for example, fluorine or chlorine and traces of xenon apart from a buffer gas, such as helium. Specific examples of the gas mixture include argon fluoride (ArF), which emits light at a wavelength of about 193 nm, krypton fluoride (KrF), which emits light at a wavelength of about 248 nm, or xenon chloride (XeCl), which emits light at a wavelength of about 351 nm. Thus, the light beams  541  and  542  include wavelengths in the DUV range in this implementation. The excimer gain medium (the gas mixture) is pumped with short (for example, nanosecond) current pulses in a high-voltage electric discharge by application of a voltage to the elongated electrodes  562   a _ 1 ,  562   b _ 1 . 
     The PA  568  includes a beam coupling optical system  583  that receives the seed light beam  542  from the MO  567  and directs the seed light beam  542  through a discharge chamber  565 _ 2 , and to a beam turning optical element  582 , which modifies or changes the direction of the seed light beam  542  so that it is sent back into the discharge chamber  565 _ 2 . The beam turning optical element  582  and the beam coupling optical system  583  form a circulating and closed loop optical path in which the input into a ring amplifier intersects the output of the ring amplifier at the beam coupling optical system  583 . 
     The discharge chamber  565 _ 2  includes a pair of elongated electrodes  562   a _ 2 ,  562   b _ 2 , a gain medium  561 _ 2 , and a fan (not shown) for circulating the gain medium  561 _ 2  between the electrodes  562   a _ 2 ,  562   b _ 2 . The gas mixture that forms the gain medium  561 _ 2  may be the same as the gas mixture that forms gain medium  561 _ 1 . 
     The discharge chamber  565 _ 2  includes a first chamber window  563 _ 2  and a second chamber window  564 _ 2 . The first and second chamber windows  563 _ 2  and  564 _ 2  are on opposite sides of the discharge chamber  565 _ 2 . The first and second chamber windows  563 _ 2  and  564 _ 2  transmit light in the DUV range and allow DUV light to enter and exit the discharge chamber  565 _ 2 . 
     The output light beam  541  may be directed through a beam preparation system  585  prior to reaching the lithography exposure apparatus  469 . The beam preparation system  585  may include a bandwidth analysis module that measures various parameters (such as the bandwidth or the wavelength) of the beam  541 . The beam preparation system  585  also may include a pulse stretcher (not shown) that stretches each pulse of the output light beam  541  in time. The beam preparation system  585  also may include other components that are able to act upon the beam  541  such as, for example, reflective and/or refractive optical elements (such as, for example, lenses and mirrors), filters, and optical apertures (including automated shutters). 
     The DUV light source  560  also includes the gas management system  479 , which is in fluid communication with an interior  578  of the DUV light source  560 . As discussed above, the gas management system  479  provides the purge gas  412  to the interior  578 . In the example of  FIG. 5 , the purge gas  412  surrounds the chambers  565 _ 1  and  565 _ 2  and also surrounds optical components of some of the subsystems of the DUV light source  560 . For example, the purge gas  412  surrounds the optical components in the line narrowing module  586 , output coupler  581 , the line center analysis module  584 , the beam coupling optical system  583 , and the beam turning optical element  582 . Although the purge gas  412  is in the interior  578  and surrounds the discharge chambers  565 _ 1  and  565 _ 2  and various other optical components, the purge gas  412  does not penetrate the discharge chambers  565 _ 1  and  565 _ 2  and does not disturb or change the chemical composition of the gain mediums  561 _ 1  and  561 _ 2 . 
     The photolithography system  550  also includes the control system  580 . The control system  580  may control when the light source  560  emits a pulse of light or a burst of light pulses that includes one or more pulses of light by sending one or more signals to the light source  560 . The control system  580  is also connected to the lithography exposure apparatus  469 . Thus, the control system  580  also may control the various aspects of the lithography exposure apparatus  469 . For example, the control system  580  may control the exposure of the wafer  470  ( FIG. 4A ) and thus may be used to control how electronic features are printed on the wafer  470 . In some implementations, the control system  580  may control the scanning of the wafer  470  by controlling the motion of the slit  476  in the x-y plane ( FIG. 4B ). Moreover, the control system  580  may exchange data with the metrology system  472  and/or the optical system  475  ( FIG. 4B ). 
     The lithography exposure apparatus  469  also may include, for example, temperature control devices (such as air conditioning devices and/or heating devices), and/or power supplies for the various electrical components. The control system  580  also may control these components. In some implementations, the control system  580  is implemented to include more than one sub-control system, with at least one sub-control system (a lithography controller) dedicated to controlling aspects of the lithography exposure apparatus  469 . In these implementations, the control system  580  may be used to control aspects of the lithography exposure apparatus  469  instead of, or in addition to, using the lithography controller. 
     When the gain medium  561 _ 1  or  561 _ 2  is pumped by applying voltage to the electrodes  562   a _ 1 ,  562   b _ 1  or  562   a _ 2 ,  562   b _ 2 , respectively, the gain medium  561 _ 1  and/or  561 _ 2  emits light. When voltage is applied to the electrodes at regular temporal intervals, the light beam  541  is pulsed. Thus, the repetition rate of the pulsed light beam  541  is determined by the rate at which voltage is applied to the electrodes. The repetition rate of the pulses may range between about 500 and 6,000 Hz for various applications. In some implementations, the repetition rate may be greater than 6,000 Hz, and may be, for example, 12,000 Hz or greater, but other repetition rates may be used in other implementations. 
     The output beam formation apparatus  110  (or  210  or  310 ) may be placed between the discharge chambers  565 _ 1  and  565 _ 2  or between the discharge chamber  565 _ 2  and the lithography apparatus  469 . In implementations that include the beam preparation system  585  the output beam formation apparatus  110  (or  210  or  310 ) may be placed between the discharge chamber  565 _ 2  and the beam preparation system  585 . 
     Other aspects of the invention are set out in the following numbered clauses. 
     1. An apparatus comprising:
 
a beam splitter on a beam path, the beam splitter configured to receive light from a deep ultraviolet (DUV) light source; and
 
a first plurality of reflective optical elements on the beam path, wherein the beam splitter is configured to direct a portion of the light received from the DUV light source toward the first plurality of reflective optical elements;
 
the first plurality of reflective optical elements is configured to rotate a divergence of the portion of the light to produce rotated light; and
 
the beam splitter is configured to direct the rotated light and a portion of the light received from the DUV light source onto an output beam path.
 
2. The apparatus of clause 1, further comprising a polarization element on the beam path, wherein the polarization element is configured to change a polarization state of the rotated light such that, on the output beam path, the rotated light has the same polarization state as light received from the DUV light source.
 
3. The apparatus of clause 2, further comprising a second plurality of reflective optical elements on the beam path between the first plurality of optical elements and the polarization element.
 
4. The apparatus of clause 3, wherein the first plurality of reflective optical elements comprises four mirrors.
 
5. The apparatus of clause 1, wherein the first plurality of reflective optical elements is configured to rotate the divergence of the portion of the light by 90 degrees.
 
6. The apparatus of clause 1, wherein the light received from the DUV light source has a vertical divergence having an initial vertical divergence value and a horizontal divergence having an initial horizontal divergence value, and, on the output beam path, the rotated light has a horizontal divergence equal to the initial vertical divergence value and a vertical divergence equal to the initial horizontal divergence value.
 
7. The apparatus of clause 6, wherein the rotated light and another portion of light from the DUV light source are combined at the beam splitter to form an output beam that propagates on the output beam path, the output beam has a vertical divergence value that is based on the vertical divergence of the rotated light and the vertical divergence of the other portion of light from the DUV light source, and a horizontal divergence value that is based on the horizontal divergence of the rotated light and the vertical divergence of the other portion of the light from the DUV light source.
 
8. The apparatus of clause 7, wherein the horizontal divergence value of the output beam is greater than the initial horizontal divergence value, and the vertical divergence value of the output beam is less than the initial vertical divergence value.
 
9. The apparatus of clause 8, wherein the light from the DUV light source comprises at least a first pulse of light, the rotated light comprises a rotated pulse of light formed from reflection of a first portion of the first pulse of light, the other portion of light from the DUV light source comprises a transmitted second portion of the first pulse of light, and the output beam is based on a reflection of the rotated pulse of light and the other portion.
 
10. The apparatus of clause 3, wherein the second plurality of reflective optical elements comprises two or more mirrors.
 
11. The apparatus of clause 2, wherein the polarization element comprises a half waveplate or a phase retarding mirror.
 
12. The apparatus of clause 2, wherein the beam path is a closed path defined by the beam splitter, the first plurality of reflective optical elements, the second plurality of reflective optical elements, and the polarization element.
 
13. A deep ultraviolet (DUV) light source comprising:
 
a light generation apparatus comprising at least one discharge chamber configured to hold a gaseous gain medium and to produce a DUV light beam; and
 
an output beam formation apparatus comprising:
 
a beam splitter on a beam path, the beam splitter configured to receive the DUV light beam;
 
a polarization element on the beam path; and
 
a first plurality of reflective optical elements on the beam path, wherein
 
the beam splitter is configured to direct a portion of the DUV light beam toward the first plurality of reflective optical elements;
 
the first plurality of reflective optical elements is configured to rotate a divergence of the portion of light to produce rotated light;
 
the waveplate is configured to change a polarization state of the rotated light such that, on the output beam path, the rotated light has the same polarization state as the DUV light beam;
 
and
 
the beam splitter is configured to form an output beam based on the rotated light and a portion of the DUV light beam.
 
14. The DUV light source of clause 13, wherein the light generation apparatus comprises: a first discharge chamber configured to hold a first gaseous gain medium and to produce a DUV seed light beam; and
 
a second discharge chamber configured to hold a second gaseous gain medium and to amplify the seed light beam to form an amplified DUV light beam.
 
15. The DUV light source of clause 14, wherein the output beam formation apparatus is configured to be positioned between the first discharge chamber and the second discharge chamber, and the DUV light comprises the DUV seed light beam.
 
16. The DUV light source of clause 13, further comprising a beam stretching apparatus configured to receive the output beam.
 
17. The DUV light source of clause 16, wherein the output beam formation apparatus is between the light generation apparatus and the beam stretching apparatus.
 
18. The DUV light source of clause 13, wherein the polarization element is between a second plurality of reflective optical elements and the beam splitter.
 
19. A method for a deep ultraviolet (DUV) light source, the method comprising:
 
receiving a portion of a DUV light beam, the portion of the DUV light beam having a polarization state, a vertical divergence of an initial vertical divergence value, and a horizontal divergence of an initial horizontal divergence value;
 
rotating the portion of the DUV light beam to form a rotated light beam, the rotated light beam having a vertical divergence of the initial horizontal divergence value and a horizontal divergence of the initial vertical divergence value;
 
passing the rotated light beam through an optical delay;
 
forming an output light beam that propagates on an output beam path based on the rotated light beam and another portion of the DUV light beam.
 
20. The method of clause 19, further comprising, before combining the rotated light beam and another portion of the DUV light beam, passing the rotated light beam through a polarization element such that the rotated light beam and the other portion of the DUV light beam have the same polarization state on the output beam path.
 
21. The method of clause 19, wherein the DUV light beam comprises a pulsed DUV light beam, the rotated light beam comprises a reflection of a first one of the pulses in the pulsed DUV light beam, and the other portion of the DUV light beam comprises part of the first one of the pulses.
 
     Still other implementations are within the scope of the claims.