Patent Publication Number: US-9835959-B1

Title: Controlling for wafer stage vibration

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. application Ser. No. 15/295,280, filed on Oct. 17, 2016, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The disclosed subject matter relates to an apparatus for compensating for variations in the vibration of a wafer stage during scanning of the wafer by adjusting a characteristic of a pulsed light beam directed toward the wafer. 
     BACKGROUND 
     In semiconductor lithography (or photolithography), the fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes performed on a semiconductor (for example, silicon) substrate (which is also referred to as a wafer). A photolithography exposure apparatus or scanner is a machine that applies a desired pattern onto a target portion of the substrate. The wafer is fixed to a stage so that the wafer generally extends along a plane defined by orthogonal X L  and Y L  directions of the scanner. The wafer is irradiated by a light beam, which has a wavelength in the deep ultraviolet (DUV) range. The light beam travels along an axial direction, which corresponds with the Z L  direction of the scanner. The Z L  direction of the scanner is orthogonal to the lateral X L -Y L  plane. The critical dimension (CD) is the smallest feature size of a pattern that can be printed on the wafer by the exposure apparatus. It is important to maintain a uniform or controlled CD to enable better control over the microelectronic features printed on the wafer. 
     SUMMARY 
     In some general aspects, a method includes: producing, from the pulsed optical source, a pulsed light beam; directing the pulsed light beam toward a substrate mounted to a stage of a lithography exposure apparatus; performing a scanning operation between the pulsed light beam and the substrate, wherein the scanning operation comprises projecting the pulsed light beam onto each sub-area of the substrate and moving one or more of the pulsed light beam and the substrate relative to each other; determining a value of a vibration of the stage for each sub-area of a substrate; for each sub-area of the substrate, determining an amount of adjustment to a bandwidth of the pulsed light beam, the adjustment amount compensating for a variation in the stage vibration so as to maintain a focus blur within a predetermined range of values across the substrate; and changing the bandwidth of the pulsed light beam by the determined adjustment amount as the pulsed light beam is exposing a substrate to thereby compensate for the stage vibration variations. 
     Implementations can include one or more of the following features. For example, the amount of adjustment to the bandwidth in a particular sub-area can be determined by accessing a lookup table to determine a value of the bandwidth that maintains an effective stage vibration constant. The method can include creating the lookup table prior to directing the pulsed light beam toward the substrate. 
     The value of the stage vibration for each sub-area of a substrate can be determined by determining the value of the stage vibration for each sub-area of the substrate after a scanning operation has been performed between the pulsed light beam and the substrate. The bandwidth of the pulsed light beam can be changed by the determined adjustment amount as the pulsed light beam is exposing a substrate by changing the bandwidth of the pulsed light beam by the determined adjustment amount as the pulsed light beam is exposing a substrate that has not yet been scanned by the pulsed light beam. 
     The amount of adjustment to the bandwidth of the pulsed light beam can be determined prior to directing the pulsed light beam toward the substrate. The amount of adjustment to the bandwidth of the pulsed light beam can be determined while the pulsed light beam is directed toward the substrate and at each sub-area of the substrate. 
     The bandwidth of the pulsed light beam can be changed by adjusting one or more optical components of a spectral feature selection apparatus. The one or more optical components of the spectral feature selection apparatus can be adjusted by rotating and translating a prism of the spectral feature selection apparatus. 
     The focus blur can be maintained within the predetermined range to maintain a critical dimension of a feature formed in the substrate to within a predetermined range. 
     The bandwidth of the pulsed light beam can be changed by changing the bandwidth in between bursts of pulses of the pulsed light beam. 
     The method can include measuring a bandwidth of the pulsed light beam that is directed toward the substrate, and adjusting the bandwidth of the pulsed light beam if the measured bandwidth is outside an acceptable range of bandwidths. 
     In other general aspects, an apparatus includes: an optical source producing a pulsed light beam; a beam directing system directing the pulsed light beam toward a substrate mounted to a stage of a lithography exposure apparatus; a scanning system configured to project the pulsed light beam onto each sub-area of the substrate and move one or more of the pulsed light beam and the substrate relative to each other; a metrology apparatus configured to determine a value of a vibration of the stage for each sub-area of a substrate; and a control system connected to the optical source, the scanning system, and the metrology apparatus. The control system is configured to: receive the determined values of the stage vibration for each sub-area from the metrology apparatus; for each sub-area, determine an amount of adjustment to a bandwidth of the pulsed light beam, the adjustment amount compensating for a variation in the stage vibration so as to maintain a focus blur within a predetermined range of values across the substrate; and send a signal to the optical source to modify the bandwidth of the pulsed light beam by the determined adjustment amount as the pulsed light beam is exposing a substrate to thereby compensate for the stage vibration variation. 
     Implementations can include one or more of the following features. For example, scanning system can be configured to move one or more of the pulsed light beam and the substrate relative to each other along a lateral plane. The lateral plane is perpendicular to an axial direction along which the pulsed light beam is directed, and the metrology apparatus can be configured to determine the value of the vibration of the stage along the axial direction. 
     The apparatus can include a spectral feature selection apparatus configured to select a spectral feature of the pulsed light beam, the spectral feature selection apparatus including a set of optical components arranged in the path of the pulsed light beam, wherein the control system is connected to the spectral feature selection apparatus. The control system can send a signal to the optical source to modify the bandwidth of the pulsed light beam by sending a signal to the spectral feature selection apparatus to move at least one optical component to thereby change the bandwidth of the pulsed light beam. The set of optical components of the spectral feature selection apparatus can include at least one prism, and the control system can send the signal to the spectral feature selection apparatus to move the at least one optical component to thereby change the bandwidth of the pulsed light beam by sending a signal to a rapid actuator associated with the at least one prism to cause the prism to rotate to thereby change the bandwidth. 
     The set of optical components of the spectral feature selection apparatus can include a dispersive optical element arranged to interact with the pulsed light beam, and a plurality of prisms arranged in the path of the pulsed light beam between the dispersive optical element and the optical source. 
     The spectral feature selection apparatus can include an actuation system including at least one rapid actuator associated with a prism and configured to rotate the associated prism to thereby adjust a spectral feature of the pulsed light beam. 
     The rapid actuator can include a rotation stage that rotates about a rotation axis and includes a region that is mechanically linked to the prism. The rotation stage can be configured to rotate about the rotation axis along a full 360° of angle of rotation. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a photolithography system producing a pulsed light beam that is directed to a photolithography exposure apparatus; 
         FIG. 2  is a schematic drawing depicting a map of a wafer that is imaged within the photolithography exposure apparatus of  FIG. 1 , the map showing the sub-areas of the wafer; 
         FIG. 3  is a graph of an exemplary optical spectrum of the pulsed light beam produced by the photolithography system of  FIG. 1 ; 
         FIG. 4  is a block diagram of an exemplary photolithography exposure apparatus that can be used in the photolithography system of  FIG. 1 ; 
         FIG. 5A  is a block diagram of an exemplary spectral feature selection apparatus that can be used in the photolithography system of  FIG. 1 ; 
         FIG. 5B  is a block diagram of an exemplary prism within the spectral feature selection apparatus of  FIG. 5A , and showing a beam magnification and beam refraction angle through the prism; 
         FIG. 6A  is a block diagram of exemplary spectral feature selection apparatus that includes a rapid actuator associated with at least one of the prisms and can be used in the photolithography system of  FIG. 1 ; 
         FIG. 6B  is a view taken along the  6 B- 6 B section of one of the prisms of the apparatus of  FIG. 6A ; 
         FIG. 6C  is a view along the Z SF  direction of the prism of  FIG. 6B  showing rotation of the prism; 
         FIG. 7A  is a block diagram of exemplary spectral feature selection apparatus that includes a rapid actuator associated with at least one of the prisms and can be used in the photolithography system of  FIG. 1 ; 
         FIG. 7B  is a view taken along the  7 B- 7 B section of one of the prisms of the apparatus of  FIG. 7A ; 
         FIG. 7C  is a view along the Z SF  direction of the prism of  FIG. 7B  showing rotation of the prism; 
         FIG. 8  is a block diagram of an exemplary optical source that can be used in the photolithography system of  FIG. 1 ; 
         FIG. 9  is a block diagram of an exemplary control system that can be used in the photolithography system of  FIG. 1 ; 
         FIG. 10  is a flow chart of an exemplary procedure performed by the photolithography system of  FIG. 1  to rapidly adjust a bandwidth of the pulsed light beam to compensate for a variation in stage vibration in each sub-area of the wafer; 
         FIG. 11  is a flow chart of an exemplary procedure performed by the photolithography system of  FIG. 1  to determine an adjustment to the bandwidth of the pulsed light beam for each sub-area of the wafer; and 
         FIG. 12  are graphs of exemplary relationships between stage vibrations measured for all of the sub-areas of the wafer, and bandwidth of the pulsed light beam that compensate for unwanted variations in stage vibrations. 
     
    
    
     DESCRIPTION 
     Referring to  FIG. 1 , a photolithography system  100  includes an illumination system  150  that produces a pulsed light beam  110  having a wavelength that is nominally at a center wavelength and is directed to a photolithography exposure apparatus or scanner  115 . The pulsed light beam  110  is used to pattern microelectronic features on a substrate or wafer  120  that is mounted to a stage  122  in the scanner  115 . These microelectronic features patterned on the wafer  120  are limited in size by the critical dimension (CD), and the critical dimension is influenced by focus blur, which is a blur in the focus of the light beam  110  at the wafer  120 . Focus blur is caused at least in part by the effects of chromatic aberration due to the bandwidth of the light beam  110 , the stage vibration or oscillation along the Z L  direction, and a tilt of the stage  122  from the X L -Y L  plane. 
     The stage vibration is the vibration of the stage  122  that happens along the Z L  direction. The stage vibration varies across the lateral direction (across the X L  and Y L  directions) of the wafer  120 . For example, as shown in  FIG. 2 , an exemplary map  200  of a wafer  220  is shown in which the stage vibration along the Z L  direction is characterized by a moving standard deviation (MSD) value that is derived from a stage interferometer error signal. Higher values of stage vibration blur the image and thus cause non-uniformity in the CD. The map  200  of the wafer  220  shows how the stage vibration varies across exposure fields  223  of the wafer  220 . The exposure field  223  of the wafer  220  is the area of the wafer  220  that is exposed in one scan of an exposure slit or window. 
     Because the focus blur depends at least in part on the bandwidth of the light beam  110  and the stage vibration along the Z L  direction, it is possible to maintain the focus blur at a constant value by adjusting the bandwidth of the light beam as it is scanned across the wafer  220  to compensate for the variation in the stage vibration across the surface of the wafer  220 . The photolithography system  100  and associated method described herein is designed to determine the bandwidth adjustment or value at each location or sub-area (for example, each exposure field) of the wafer  220  that offsets the change in the focus blur otherwise caused by the variation in the stage vibration at that wafer sub-area to thereby maintain the focus blur and control the CD across the wafer  220 . The adjustment to the bandwidth occurs while the light beam  110  is being scanned across the wafer  220 . For example, under control of a spectral feature selection apparatus  130 , the bandwidth of the light beam  110  can be adjusted for each sub-area (such as each exposure field  223 ) of the wafer  220 . 
     Moreover, in order to enable the rapid adjustment to the bandwidth for each sub-area of the wafer  120 ,  220 , the spectral feature selection apparatus  130  has been redesigned to provide for more rapid adjustment of the bandwidth of the pulsed light beam  110  while the light beam  110  is being scanned across the wafer  120  to enable the adjustment of the bandwidth for each sub-area of the wafer  120 . 
     Specifically, the spectral feature selection apparatus  130  can include a coarse spectral feature adjustment system  130 A and a fine spectral feature adjustment system  130 B. The coarse spectral feature adjustment system  130 A is used for coarse, large range, and slow control of the spectral feature (such as the bandwidth), and is collection of optical components that interact with the pulsed light beam  110 A produced by the optical source  104 . The fine spectral feature adjustment system  130 B is used for fine, narrow range, and fast control of the spectral feature such as the bandwidth. The fine spectral feature adjustment system  130 B can include an optical system that interacts optically with the pulsed light beam  110 A to control one or more spectral features. The fine bandwidth adjustment system  130 C can include a non-optical system that interacts with other aspects of the optical source  105  in a rapid manner to control one or more spectral features such as the bandwidth. For example, the fine spectral feature adjustment system  130 C can be configured to adjust aspects of the timing associated with the gas discharge chamber or chambers within the optical source  105  to thereby adjust the bandwidth of the pulsed light beam  110 . 
     Details about the photolithography system  100  are described next. With reference again to  FIG. 1 , the illumination system  150  includes an optical source  105  that produces the pulsed light beam  110  at a pulse repetition rate that is capable of being changed. The illumination system  150  includes a control system  185  that communicates with the optical source  105  and other features within the illumination system  150 . The illumination system  150  also communicates with the scanner  115  to control the operation of the illumination system  150  and aspects of the pulsed light beam  110 . 
     The control system  185  is operatively connected to the pulsed optical source  105  and to the spectral feature selection apparatus  130 . And, the scanner  115  includes a lithography controller  140  operatively connected to the control system  185  and components within the scanner  115 . 
     The pulse repetition rate of the pulsed light beam  110  is the rate at which pulses of the light beam  110  are produced by the optical source  105 . Thus, for example, the repetition rate of the pulsed light beam  110  is 1/Δt, where Δt is the time between the pulses. The control system  185  is generally configured to control the repetition rate at which the pulsed light beam  110  is produced including modifying the repetition rate of the pulsed light beam as it is exposing the wafer  120  in the scanner  115 . 
     In some implementations, the scanner  115  triggers the optical source  105  (through the communication between the controller  140  and the control system  185 ) to produce the pulsed light beam  110 , so the scanner  115  controls the repetition rate, spectral features such as the bandwidth or wavelength, and/or the dose by way of the controller  140  and the control system  185 . For example, the controller  140  sends a signal to the control system  185  to maintain the repetition rate of the light beam  110  within a particular range of acceptable rates. The scanner  115  generally maintains the repetition rate constant for each burst of pulses of the light beam  110 . A burst of pulses of the light beam  110  can correspond to an exposure field on the wafer  120 . The exposure field is the area of the wafer  120  that is exposed in one scan of an exposure slit or window within the scanner  115 . A burst of pulses can include anywhere from 10 to 500 pulses, for example. 
     As discussed, the critical dimension (CD) is the smallest feature size that can be printed on the wafer  120  by the system  100 . The CD depends on the wavelength of the light beam  110 . Thus, in order to maintain a uniform CD of the microelectronic features printed on the wafer  120 , and on other wafers exposed by the system  100 , the center wavelength of the light beam  110  should remain at an expected or target center wavelength or within a range of wavelengths around the target wavelength. Thus, in addition to maintaining the center wavelength at the target center wavelength or within a range of acceptable wavelengths about the target center wavelength, it is desirable to maintain the bandwidth of the light beam  110  (the range of wavelengths in the light beam  110 ) to within an acceptable range of bandwidths. 
     In order to maintain the bandwidth of the light beam  110  to an acceptable range, or to adjust the bandwidth of the light beam  110 , the control system  185  is configured to determine an amount of adjustment to the bandwidth of the pulsed light beam  110 . Additionally, the control system  185  is configured to send a signal to the spectral feature selection apparatus  130  to move at least one optical component of the apparatus  130  (for example, the prism  520 ) to thereby change the bandwidth of the pulsed light beam  110  by the determined adjustment amount as the pulsed light beam  110  is exposing the wafer  120  to thereby compensate for the variation in stage vibration along the Z L  direction across the surface of the wafer  220 . 
     In some implementations, the bandwidth of the pulsed light beam  110  can be changed in between any two bursts of pulses. In this example, the time that it takes for the bandwidth to be changed from a first value to a second value and also to stabilize at the second value should be less than the time between the bursts of pulses. For example, if the period of time between bursts is 50 ms, then the total time to change the bandwidth from a first value to a second value and stabilize at the second value should be less than 50 ms. The time that it takes for the bandwidth to be changed from a first value to a second value and also to stabilize at the second value can be as low as 50 milliseconds (ms) in this example. In other implementations, it is possible to change the bandwidth of the pulsed light beam  110  during a burst of pulses or between exposure fields (and not necessarily between bursts). In some implementations, the bandwidth of the pulsed light beam  110  can be changed in between pulses of the light beam  110 . For example, the bandwidth can be changed by 10 femtometers (fm) in about tens of microseconds (μs), for example, within 100 to 200 μs. In other implementations, the bandwidth of the pulsed light beam  110  is changed field to field (in between exposure fields); for example, the bandwidth can be changed by 100 fm in about 10 to 15 ms. 
     The control system  185  and the spectral feature selection apparatus  130  are designed to enable such a rapid change of the bandwidth, as discussed in detail below. 
     The controller  140  of the scanner  115  sends a signal to the control system  185  to adjust or modify an aspect (such as the bandwidth or the repetition rate) of the pulsed light beam  110  that is being scanned across the wafer  120 . The signal sent to the control system  185  can cause the control system  185  to modify an electrical signal sent to the pulsed optical source  105  or an electrical signal sent to the apparatus  130 . For example, if the pulsed optical source  105  includes a gas laser amplifier then the electrical signal provides a pulsed current to electrodes within one or more gas discharge chambers of the pulsed optical source  105 . 
     The wafer  120  is placed on the wafer stage  122  (also referred to as a table) and the stage  122  is connected to a positioner configured to accurately position the wafer  120  in accordance with certain parameters and under control of the controller  140 . 
     The photolithography system  100  can also include the measurement system  170 , which can include a sub-system that measures one or more spectral features (such as the bandwidth or wavelength) of the light beam  110 . Because of various disturbances applied to the photolithography system  100  during operation, the value of the spectral feature (such as the bandwidth or the wavelength) of the light beam  110  at the wafer  120  may not correspond to or match with the desired spectral feature (that is, the spectral feature that the scanner  115  expects). Thus, the spectral feature (such as a characteristic bandwidth) of light beam  110  is measured or estimated during operation by estimating a value of a metric from the optical spectrum so that an operator or an automated system (for example, a feedback controller) can use the measured or estimated bandwidth to adjust the properties of the optical source  105  and to adjust the optical spectrum of the light beam  110 . The sub-system of the measurement system  170  measures the spectral feature (such as the bandwidth and/or the wavelength) of the light beam  110  based on this optical spectrum. 
     The measurement system  170  receives a portion of the light beam  110  that is redirected from a beam separation device that is placed in a path between the optical source  105  and the scanner  115 . The beam separation device directs a first portion or percentage of the light beam  110  into the measurement system  170  and directs a second portion or percentage of the light beam  110  toward the scanner  115 . In some implementations, the majority of the light beam  110  is directed in the second portion toward the scanner  115 . For example, the beam separation device directs a fraction (for example, 1-2%) of the light beam  110  into the measurement system  170 . The beam separation device can be, for example, a beam splitter. 
     The pulses of the light beam  110  are centered around a wavelength that is in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The size of the microelectronic features patterned on the wafer  120  depends, among other things, on the wavelength of the pulsed light beam  110 , with a lower wavelength resulting in a small minimum feature size or critical dimension. When the wavelength of the pulsed light beam  110  is 248 nm or 193 nm, the minimum size of the microelectronic features can be, for example, 50 nm or less. The bandwidth that is used for analysis and control of the pulsed light beam  110  can be the actual, instantaneous bandwidth of its optical spectrum  300  (or emission spectrum), as shown in  FIG. 3 . The optical spectrum  300  contains information about how the optical energy or power of the light beam  110  is distributed over different wavelengths (or frequencies). 
     The light beam  110  is directed through a beam preparation system  112 , which can include optical elements that modify aspects of the light beam  110 . For example, the beam preparation system  112  can include reflective and/or refractive optical elements, optical pulse stretchers, and optical apertures (including automated shutters). 
     The illumination system  150  includes a spectral feature selection apparatus  130 . The spectral feature selection apparatus  130  is placed at a first end of the optical source  105  to interact with a light beam  110 A produced by the optical source  105 . The light beam  110 A is a beam produced at one end of the resonators within the optical source  105  and can be a seed beam produced by a master oscillator, as discussed below. The spectral feature selection apparatus  130  is configured to finely tune the spectral properties of the pulsed light beam  110  by tuning or adjusting one or more spectral features (such as the bandwidth or wavelength) of the pulsed light beam  110 A. 
     Referring also to  FIG. 4 , the wafer  120 ,  220  is irradiated by the light beam  110 . The lithography exposure apparatus  115  includes an optical arrangement that includes an illuminator system  129  having, for example, one or more condenser lenses, a mask  134 , and an objective arrangement  132 . The mask  134  is movable along one or more directions, such as along a Z L  direction (which generally corresponds to the axial direction of the light beam  110 ) or in an X L -Y L  plane that is perpendicular to the Z L  direction. The objective arrangement  132  includes a projection lens and enables the image transfer to occur from the mask  134  to the photoresist on the wafer  120 . The illuminator system  129  adjusts the range of angles for the light beam  110  impinging on the mask  134 . The illuminator system  129  also homogenizes (makes uniform) the intensity distribution of the light beam  110  across the mask  134 . 
     The lithography apparatus  115  can include, among other features, a lithography controller  140 , air conditioning devices, and power supplies for the various electrical components. The lithography controller  140  controls how layers are printed on the wafer  120 . 
     In some implementations, an immersion medium can be supplied to cover the wafer  120 . The immersion medium can be a liquid (such as water) for liquid immersion lithography. In other implementations in which the lithography is a dry system, the immersion medium can be a gas such as dry nitrogen, dry air, or clean air. In other implementations, the wafer  120  can be exposed within a pressure-controlled environment (such as a vacuum or partial vacuum). 
     Referring again to  FIG. 4 , a process program or recipe determines the length of the exposure on the wafer  120 , the mask  134  used, as well as other factors that affect the exposure. During lithography, a plurality of pulses of the light beam  110  illuminate the same area of the wafer  120  to form an illumination dose. The number of pulses N of the light beam  110  that illuminate the same area can be referred to as an exposure window  400  and the size of the window  400  can be controlled by an exposure slit  405  placed before the mask  134 . The slit  405  can be designed like a shutter and can include a plurality of blades that can be opened and closed. And, the size of the exposed area is determined by the distance between the blades in the non-scanning direction and also by the length (distance) of the scan in the scanning direction. In some implementations, the value of N is in the tens, for example, from 10-100 pulses. In other implementations, the value of N is greater than 100 pulses, for example, from 100-500 pulses. 
     One or more of the wafer stage  122 , the mask  134 , and the objective arrangement  132  are fixed to associated actuation systems to thereby form a scanning arrangement. In the scanning arrangement, one or more of the mask  134 , the objective arrangement  132 , and the wafer  120  (via the stage  122 ) are moved relative to each other during the exposure to scan the exposure window  400  across an exposure field  223 . 
     Referring again to  FIG. 1 , the photolithography system  100  also includes a wafer metrology apparatus  145  that is configured to determine a value of the stage vibration in the Z direction for each sub-area (for example, for each exposure field  223 ) of the wafer  120 ,  220 . The metrology apparatus  145  is connected to the control system  185  so that the control system  185  receives the values of the stage vibration in the Z L  direction for each wafer sub-area. The control system  185  can store the values of the stage vibration in the Z L  direction for each wafer sub-area. 
     In some implementations, the metrology apparatus  145  is configured to be used in an offline mode in which the wafer  120 ,  220  is analyzed after or before the wafer  120 ,  220  has been patterned by the light beam  110 . The data obtained by such a scan can be used by the control system  185  for one or more wafers that will be scanned in the future. The data obtained by such a scan can be used to create a correction map of the wafer  120 ,  220  that can be used for the next wafer to be scanned. 
     In other implementations, the metrology apparatus  145  is used in an online mode in which the wafer  120 ,  220  is analyzed while the wafer  120 ,  220  is being patterned by the light beam  110 . For example, the exposure field of the wafer  120 ,  220  could be probed in between bursts of the light beam  110  or another aspect of the scanner  115  that indicates the stage vibration in the Z L  direction can be probed during scanning of the wafer  120 ,  220 . 
     The metrology apparatus  145  can be any apparatus that can probe the stage vibrations. In some implementations, the metrology apparatus  145  includes an interferometer that measures small displacements or and surface irregularities on the surface of the wafer  120  at each wafer sub-area (such as each wafer exposure field  223 ). From this data, a moving standard deviation (MSD) value along the Z L  direction at each wafer sub-area (such as each wafer exposure field  223 ) can be derived and the value of the MSD ZL  can be considered to represent the stage vibration along the Z L  direction. 
     As another example, the metrology apparatus  145  can be the scanner  115 , which can perform probe the wafer  120  before exposure or between scans of the wafer  120 . 
     The metrology apparatus  145  can be a self-contained system such as a high resolution scanning electron microscope (SEM) that is designed for high resolution imaging, to be able to display feature sizes of less than, for example 1 nm. The SEM is a type of electron microscope that produces images of a sample (in this case, the wafer  120 ) by scanning the wafer  120  with a focused beam of electrons. The SEM can achieve resolution better than 1 nanometer (nm). 
     The metrology apparatus  145  can employs scanning white light interferometry, which provides quantitative noncontact, three dimensional measurements of the wafer  120 . In this technique, a white light beam passes through a filter and then a microscope objective lens to the surface of the wafer  120 . The light reflecting back from the surface of the wafer  120  is combined with a reference beam and captured for software analysis within the apparatus  145 . After obtaining data for each point, the apparatus  145  can generate a three dimensional image (topography) of the surface of the wafer  120 . Such a topographical map of the wafer  120  enables the measurement of stage vibration. 
     In other implementations, the metrology apparatus  145  is a scatterometer that transmits a pulse of energy toward the wafer  120  and measures the reflected or diffracted energy from the wafer  120 . 
     Referring to  FIG. 5A , in some implementations, the spectral feature selection apparatus  130  includes a set of optical features or components  500 ,  505 ,  510 ,  515 ,  520  arranged to optically interact with the pulsed light beam  110 A and a control module  550  that includes electronics in the form of any combination of firmware and software. The optical components  500 ,  505 ,  510 ,  515 ,  520  can be configured to provide a coarse spectral feature adjustment system  130 A (shown in  FIG. 1 ); and, if the adjustment of such components is rapid enough, it can be configured to provide a fine spectral feature adjustment system  130 B (shown in  FIG. 1 ). Although not shown in  FIG. 5A , it is possible for the spectral feature selection apparatus  130  to include other optical features or other non-optical features for providing fine spectral feature control. 
     The control module  550  is connected to one or more actuation systems  500 A,  505 A,  510 A,  515 A,  520 A physically coupled to respective optical components  500 ,  505 ,  510 ,  515 ,  520 . The optical components of the apparatus  130  include a dispersive optical element  500 , which can be a grating, and a beam expander  501  made of a set of refractive optical elements  505 ,  510 ,  515 ,  520 , which can be prisms. The grating  500  can be a reflective grating that is designed to disperse and reflect the light beam  110 A; accordingly, the grating  500  is made of a material that is suitable for interacting with a pulsed light beam  110 A having a wavelength in the DUV range. Each of the prisms  505 ,  510 ,  515 ,  520  is a transmissive prism that acts to disperse and redirect the light beam  110 A as it passes through the body of the prism. Each of the prisms can be made of a material (such as, for example, calcium fluoride) that permits the transmission of the wavelength of the light beam  110 A. 
     The prism  520  is positioned farthest from the grating  500  while the prism  505  is positioned closest to the grating  500 . The pulsed light beam  110 A enters the apparatus  130  through an aperture  555 , and then travels through the prism  520 , the prism  510 , and the prism  505 , in that order, prior to impinging upon a diffractive surface  502  of the grating  500 . With each passing of the beam  110 A through a consecutive prism  520 ,  515 ,  510 ,  505 , the light beam  110 A is optically magnified and redirected (refracted at an angle) toward the next optical component. The light beam  110 A is diffracted and reflected from the grating  500  back through the prism  505 , the prism  510 , the prism  515 , and the prism  520 , in that order, prior to passing through the aperture  555  as the light beam  110 A exits the apparatus  130 . With each passing through the consecutive prisms  505 ,  510 ,  515 ,  520  from the grating  300 , the light beam  110 A is optically compressed as it travels toward the aperture  555 . 
     Referring to  FIG. 5B , the rotation of a prism P (which can be any one of prisms  505 ,  510 ,  515 , or  520 ) of the beam expander  501  changes an angle of incidence at which the light beam  110 A impinges upon the entrance surface H(P) of that rotated prism P. Moreover, two local optical qualities, namely, an optical magnification OM(P) and a beam refraction angle δ(P), of the light beam  110 A through that rotated prism P are functions of the angle of incidence of the light beam  110 A impinging upon the entrance surface H(P) of that rotated prism P. The optical magnification OM(P) of the light beam  110 A through the prism P is the ratio of a transverse wide Wo(P) of the light beam  110 A exiting that prism P to a transverse width Wi(P) of the light beam  110 A entering that prism P. 
     A change in the local optical magnification OM(P) of the light beam  110 A at one or more of the prisms P within the beam expander  501  causes an overall change in the optical magnification OM  565  of the light beam  110 A through the beam expander  501 . The optical magnification OM  565  of the light beam  110 A through the beam expander  501  is the ratio of the transverse width Wo of the light beam  110 A exiting the beam expander  501  to a transverse width Wi of the light beam  110 A entering the beam expander  501 . 
     Additionally, a change in the local beam refraction angle δ(P) through one or more of the prisms P within the beam expander  501  causes an overall change in an angle of incidence of  562  of the light beam  110 A at the surface  502  of the grating  500 . 
     The wavelength of the light beam  110 A can be adjusted by changing the angle of incidence  562  at which the light beam  110 A impinges upon the diffractive surface  502  of the grating  500 . The bandwidth of the light beam  110 A can be adjusted by changing the optical magnification  565  of the light beam  110 . 
     The spectral feature selection apparatus  130  is redesigned to provide for more rapid adjustment of the bandwidth of the pulsed light beam  110  while the light beam  110  is being scanned across the wafer  120  by the scanner  115 . The spectral feature selection apparatus  130  can be redesigned with one or more new actuation systems for more effectively and more rapidly rotating one or more of the optical components  500 ,  505 ,  510 ,  515 ,  520 . 
     For example, the spectral feature selection apparatus  130  includes a new actuation system  520 A for more effectively and more rapidly rotating the prism  520 . The new actuation system  520 A can be designed in a manner that increases the speed with which the prism  520  is rotated. Specifically, the axis of rotation of the prism  520  mounted to the new actuation system  520 A is parallel with a rotatable motor shaft  522 A of the new actuation system  520 A. In other implementations, the new actuation system  520 A can be designed to include an arm that is physically linked to the motor shaft  522 A at one end and physically linked to the prism  520  at the other end to provide additional leverage for rotating the prism  520 . In this way, the optical magnification OM of the light beam  110 A is made to be more sensitive to rotation of the prism  520 . 
     In some implementations, the prism  505  is flipped relative to the prior design of the beam expander to provide for more rapid adjustment of the bandwidth. In these cases, the bandwidth change becomes relatively faster (when compared with prior designs of the apparatus  130 ) with a relatively smaller rotation of the prism  520 . The change in optical magnification per unit rotation of the prism  520  is increased in the redesigned spectral feature selection apparatus  130  when compared with prior spectral feature selection apparatuses. 
     The apparatus  130  is designed to adjust the wavelength of the light beam  110 A that is produced within the resonator or resonators of the optical source  105  by adjusting an angle  562  of incidence of at which the light beam  110 A impinges upon the diffractive surface  502  of the grating  500 . Specifically, this can be done by rotating one or more of the prisms  505 ,  510 ,  515 ,  520  and the grating  500  to thereby adjust the angle of incidence  562  of the light beam  110 A. 
     Moreover, the bandwidth of the light beam  110 A that is produced by the optical source  105  is adjusted by adjusting the optical magnification OM  565  of the light beam  110 A. Thus, the bandwidth of the light beam  110 A can be adjusted by rotating one or more of the prisms  505 ,  510 ,  515 ,  520 , which causes the optical magnification  565  of the light beam  110 A to change. 
     Because the rotation of a particular prism P causes a change in both the local beam refraction angle δ(P) and the local optical magnification OM(P) at that prism P, the control of wavelength and bandwidth are coupled in this design. 
     Additionally, the bandwidth of the light beam  110 A is relatively sensitive to the rotation of the prism  520  and relatively insensitive to rotation of the prism  505 . This is because any change in the local optical magnification OM( 520 ) of the light beam  110 A due to the rotation of the prism  520  is multiplied by the product of the change in the optical magnification OM( 515 ), OM( 510 ), OM( 505 ), respectively, in the other prisms  515 ,  510 , and  505  because those prisms are between the rotated prism  520  and the grating  500 , and the light beam  110 A must travel through these other prisms  515 ,  510 ,  505  after passing through the prism  520 . On the other hand, the wavelength of the light beam  110 A is relatively sensitive to the rotation of the prism  505  and relatively insensitive to the rotation of the prism  520 . 
     For example, in order to change the bandwidth without changing the wavelength, the optical magnification  565  should be changed without changing the angle of incidence  562 , and this can be achieved by rotating the prism  520  by a large amount and rotating the prism  505  by a small amount. 
     The control module  550  is connected to one or more actuation systems  500 A,  505 A,  510 A,  515 A,  520 A that are physically coupled to respective optical components  500 ,  505 ,  510 ,  515 ,  520 . Although an actuation system is shown for each of the optical components it is possible that some of the optical components in the apparatus  130  are either kept stationary or are not physically coupled to an actuation system. For example, in some implementations, the grating  500  can be kept stationary and the prism  515  can be kept stationary and not physically coupled to an actuation system. 
     Each of the actuation systems  500 A,  505 A,  510 A,  515 A,  520 A includes one or more actuators that are connected to their respective optical components. The adjustment of the optical components causes the adjustment in the particular spectral features (the wavelength and/or bandwidth) of the light beam  110 A. The control module  550  receives a control signal from the control system  185 , the control signal including specific commands to operate or control one or more of the actuation systems. The actuation systems can be selected and designed to work cooperatively. 
     Each of the actuators of the actuation systems  500 A,  505 A,  510 A,  515 A,  520 A is a mechanical device for moving or controlling the respective optical component. The actuators receive energy from the module  550 , and convert that energy into some kind of motion imparted to the respective optical component. For example, the actuation systems can be any one of force devices and rotation stages for rotating one or more of prisms of a beam expander. The actuation systems can include, for example, motors such as stepper motors, valves, pressure-controlled devices, piezoelectric devices, linear motors, hydraulic actuators, voice coils, etc. 
     The grating  500  can be a high blaze angle Echelle grating, and the light beam  110 A incident on the grating  500  at any angle of incidence  562  that satisfies a grating equation will be reflected (diffracted). The grating equation provides the relationship between the spectral order of the grating  500 , the diffracted wavelength (the wavelength of the diffracted beam), the angle of incidence  562  of the light beam  110 A onto the grating  500 , the angle of exit of the light beam  110 A diffracted off the grating  500 , the vertical divergence of the light beam  110 A incident onto the grating  500 , and the groove spacing of the diffractive surface of the grating  500 . Moreover, if the grating  500  is used such that the angle of incidence  562  of the light beam  110 A onto the grating  500  is equal to the angle of exit of the light beam  110 A from the grating  500 , then the grating  500  and the beam expander (the prisms  505 ,  510 ,  515 ,  520 ) are arranged in a Littrow configuration and the wavelength of the light beam  110 A reflected from the grating  500  is the Littrow wavelength. It can be assumed that the vertical divergence of the light beam  110 A incident onto the grating  500  is near zero. To reflect the nominal wavelength, the grating  500  is aligned, with respect to the light beam  110 A incident onto the grating  500 , so that the nominal wavelength is reflected back through the beam expander (the prisms  505 ,  510 ,  515 ,  520 ) to be amplified in the optical source  105 . The Littrow wavelength can then be tuned over the entire gain bandwidth of the resonators within optical source  105  by varying the angle of incidence  562  of the light beam  110 A onto the grating  500 . 
     Each of the prisms  505 ,  510 ,  515 ,  520  is wide enough along the transverse direction of the light beam  110 A so that the light beam  110 A is contained within the surface at which it passes. Each prism optically magnifies the light beam  110 A on the path toward the grating  500  from the aperture  555 , and therefore each prism is successively larger in size from the prism  520  to the prism  505 . Thus, the prism  505  is larger than the prism  510 , which is larger than the prism  515 , and the prism  520  is the smallest prism. 
     The prism  520  that is the farthest from the grating  500 , and is also the smallest in size, is mounted on the actuation system  520 A, and specifically to the rotation shaft  522 A, which causes the prism  520  to rotate, and such rotation changes the optical magnification of the light beam  110 A impinging upon the grating  500  to thereby modify the bandwidth of the light beam  110 A output from the apparatus  130 . The actuation system  520 A is designed as a rapid actuation system  520 A because it includes a rotary stepper motor that includes the rotation shaft  522 A to which the prism  520  is fixed. The rotation shaft  522 A rotates about its shaft axis, which is parallel with the rotation axis of the prism  520 . Moreover, because the actuation system  520 A includes the rotary stepper motor, it lacks any mechanical memory and also lacks an energy ground state. Each location of the rotation shaft  522 A is at the same energy as each of the other locations of the rotation shaft  522 A and the rotation shaft  522 A lacks a preferred resting location with a low potential energy. 
     Referring to  FIGS. 6A and 6B , in a first implementation, a spectral feature selection apparatus  630  is designed with a grating  600  and four prisms  605 ,  610 ,  615 ,  620 . The grating  600  and the four prisms  605 ,  610 ,  615 ,  620  are configured to interact with the light beam  110 A produced by the optical source  105  after the light beam  110 A passes through an aperture  655  of the apparatus  630 . The light beam  110 A travels along a path in an X SF -Y SF  plane of the apparatus  630  from the aperture  655 , through the prism  620 , the prism  615 , the prism  610 , the prism  605 , and then is reflected from the grating  600 , and back through the prisms  605 ,  610 ,  615 ,  620  before exiting the apparatus through the aperture  655 . 
     The prisms  605 ,  610 ,  615 ,  620  are right-angled prisms through which the pulsed light beam  110 A is transmitted so that the pulsed light beam  110 A changes its optical magnification as it passes through each right-angled prism. The right-angled prism  620  that is farthest from the dispersive optical element  600  has the smallest hypotenuse of the plurality, and each consecutive right-angled prism closer to the dispersive optical element  600  has a larger or same size hypotenuse than the adjacent right-angled prism that is farther from the dispersive optical element. 
     For example, the prism  605  that is closest to the grating  600  is also the largest in size, for example, its hypotenuse has the largest extent of the four prisms  605 ,  610 ,  615 ,  620 . The prism  620  that is farthest from the grating  600  is also the smallest in size, for example, its hypotenuse has the smallest extent of the four prisms  605 ,  610 ,  615 ,  620 . It is possible for adjacent prisms to be the same size. But, each prism that is closer to the grating  600  should be at least as large or greater than in size its adjacent prism because the light beam  110 A is optically magnified as travels through the prism  620 , the prism  615 , the prism  610 , and the prism  605 , and thus the transverse extent of the light beam  110 A enlarges as the light beam  110 A gets closer to the grating  600 . The transverse extent of the light beam  110 A is the extent along the plane that is perpendicular to the propagation direction of the light beam  110 A. And, the propagation direction of the light beam  110 A is in the X SF -Y SF  plane of the apparatus  630 . 
     The prism  605  is physically coupled to an actuation system  605 A that rotates the prism  605  about an axis that is parallel with the Z SF  axis of the apparatus  630 , the prism  610  is physically coupled to an actuation system  610 A that rotates the prism  610  about an axis that is parallel with the Z SF  axis, and the prism  620  is physically coupled to a rapid actuation system  620 A. The rapid actuation system  620 A is configured to rotate the prism  605  about an axis that is parallel with the Z SF  axis of the apparatus  630 . 
     The rapid actuation system  620 A includes a rotary stepper motor  621 A that has a rotation shaft  622 A and rotation plate  623 A fixed to the rotation shaft  622 A. The rotation shaft  622 A and therefore the rotation plate  623 A rotate about a shaft axis AR that is parallel with a center of mass (that corresponds to a rotation axis AP) of the prism  620  and is also parallel with the Z SF  axis of the apparatus  630 . Although not necessary, the shaft axis AR of the prism  620  can correspond with or align with the center of mass (the rotation axis AP) of the prism  620  along the X SF -Y SF  plane. In some implementations, the center of mass (or rotation axis AP) of the prism  620  is offset from the shaft axis AR along the X SF -Y SF  plane. By offsetting the shaft axis AR from the prism  620  center of mass, the position of the light beam  110 A can be adjusted to be at a particular position on the surface of the grating  600  whenever the prism  620  is rotated. 
     By mounting the prism  620  to the rotation plate  623 A, the prism  620  is directly rotated about its rotation axis AP as the shaft  622 A and rotation plate  623 A are rotated about their shaft axis AR. In this way, rapid rotation or control of the prism  620  is enabled when compared with a system that uses a linear stepper motor having a linearly translatable shaft (that is converted into a rotational motion using a flexure). Because a rotational step of the shaft  622 A (and plate  623 A) directly correlates to a rotational step of the prism  620  (without the imparting any a linear motion), the rotary stepper motor  621 A is able to rotate the prism  620  at a speed that enables more rapid adjustment of spectral features (such as the bandwidth) of the light beam  110 A and therefore the light beam  110 . The rotary design of the stepper motor  621 A imparts a purely rotational motion to the prism  620 , which is mounted without the use of any linear motion or flexure motion that are found on prior actuators for the prism  620 . Moreover, the use of a rotary shaft  622 A enables the prism  620  to be rotated about a full 360°, unlike the prior actuator that used a linear stepper motor plus a flexure design (in which the prism  620  could only be rotated about the angle determined from the flexure). In some implementations, in order to achieve a tuning of the bandwidth of the light beam  110 A in an acceptable range, the prism  620  is capable of being rotated by 15 degrees. The prism  620  can be rotated by larger than 15 degrees though it is not necessary with the current bandwidth range requirements. 
     In some implementations, the stepper motor  621 A can by a direct drive stepper motor. A direct drive stepper motor is a conventional electromagnetic motor that uses a built-in step motor functionality for position control. In other implementations in which a higher resolution in motion may be needed, the stepper motor  621 A can use a piezoelectric motor technology. 
     The stepper motor  621 A can be a rotary stage that is controlled with a motor controller using a variable-frequency drive control method to provide the rapid rotation of the prism  620 . 
     As discussed above, the advantage of using a rotary stepper motor  621 A is to obtain more rapid rotation of the prism  620  because the rotation axis AP of the prism  620  is parallel with the rotational shaft  622 A and also the shaft axis AR. Thus, for every unit rotation of the shaft  622 A, the prism  620  rotates by an incremental unit and the prism  620  rotates as fast as the rotational shaft  622 A can rotate. In some implementations, in order to increase the stability of this configuration, and increase the stability of the prism  620 , the rapid actuation system  620 A includes a position monitor  624 A that is configured to detect a position of the rotational shaft  622 A of the rotary stepper motor  621 A. The error between the measured position of the rotational shaft  622 A and the expected or target position of the rotational shaft  622 A correlates directly with the error in the position of the prism  620  and thus, this measurement can be used to determine the rotational error of the prism  620  (that is, the difference between actual rotation and commanded rotation) and to correct for this error during operation. 
     The control module  550  is connected to the position monitor  624 A to receive the value of the position of the rotational shaft  622 A and the control module  550  is also able to access a stored or current value of the commanded position of the rotational shaft  622 A so that the control module  550  can perform the calculation to determine the difference between the measured value of the position and the commanded position of the rotational shaft  622 A and also determine how to adjust the rotational shaft  622 A to reduce this error. For example, the control module  550  can determine a size of rotation as well as a direction of rotation of the rotational shaft  622 A to offset the error. Alternatively, it is possible for the control system  185  to perform this analysis. 
     The position monitor  624 A can be a very high resolution optical rotary encoder that is built integrally with the rotational plate  623 A. The optical rotary encoder uses optical sensing technology and on the rotation of an internal code disc that has opaque lines and patterns on it. For example, the plate  623 A is rotated (hence the name rotary encoder) in a beam of light such as a light emitting diode and the markings on the plate  623 A act as shutters blocking and unblocking the light. An internal photodiode detector senses the alternating light beam and the encoder&#39;s electronics convert the pattern into an electrical signal that is then passed on to the control module  550  through the output of the encoder  624 A. 
     In some implementations, the control module  550  can be designed with a rapid internal dedicated controller solely for operating the rotary stepper motor  621 A. For example, the rapid internal dedicated controller can receive the high resolution position data from the encoder  624 A and can send a signal directly to the rotary stepper motor  621 A to adjust the position of the shaft  622 A and thereby adjust the position of the prism  620 . 
     Referring also to  FIG. 6C , the illumination system  150  changes a spectral feature such as the bandwidth of the light beam  110 A under control of the control system  185 , which interfaces with the control module  550 . For example, in order to coarsely and broadly control the bandwidth of the light beam  110 A and the light beam  110 , the control module  550  sends a signal to the rotary stepper motor  621 A of the rapid actuation system  620 A to rotate the rotational shaft  622 A from a first angle θ 1  (on the left side of  FIG. 6C ) to a second angle θ 2  (where Δθ=θ 2 −θ 1 ) (on the right side of  FIG. 6C ). And, this change of angle of the shaft  622 A is directly imparted to the plate  623 A, which is fixed to the shaft  622 A, and thereby also imparted to the prism  620 , which is fixed to the plate  623 A. The rotation of the prism  620  from θ 1  to θ 2  causes a corresponding change in the optical magnification OM  565  of the pulsed light beam  110 A that interacts with the grating  600  from OM 1  to OM 2 , and the change in the optical magnification  565  of the pulsed light beam  110 A causes a change in the bandwidth of the pulsed light beam  110 A (and the light beam  110  as well). The range of the bandwidth that can be achieved by rotating the prism  620  using this rapid actuation system  620 A can be a broad range and can be from about 100 femtometers (fm) to about 450 fm. The overall bandwidth range achievable can be at least 250 fm. 
     The rotation of the prism  620  associated with the rapid actuation system  620 A by one unit of rotation of the rotational shaft  622 A causes the bandwidth of the pulsed light beam  110 A to change by an amount that is less than a resolution of a bandwidth measurement device (for example, as a part of the measurement system  170 , which is discussed below) that measures the bandwidth of the pulsed light beam  110 . The prism  620  can be rotated by up to 15 degrees to achieve such a change in bandwidth. In practice, the amount of rotation of the prism  620  is constrained only by the optical layout of the other components of the apparatus  630 . For example, a rotation that is too large could cause the light beam  110 A to be displaced by an amount that is so large that the light beam  110 A does not impinge upon the next prism  615 . In some implementations, in order to achieve a tuning of the bandwidth of the light beam  110 A in an acceptable range, the prism  620  is capable of being rotated by 15 degrees, without risk of the light beam  110 A walking off any of the other prisms  605 ,  610 , or  615 . The prism  620  can be rotated by larger than 15 degrees though it is not necessary with the current bandwidth range requirements. 
     Referring again to  FIG. 6A , the prism  610  can be mounted to an actuation system  610 A that causes the prism  410  to rotate, and such rotation of the prism  610  can provide for fine control of the wavelength of the light beam  110 A. The actuation system  610 A can include a rotary stepper motor that is controlled with a piezoelectric motor. The piezoelectric motor operates by making use of the converse piezoelectric effect in which a material produces acoustic or ultrasonic vibrations in order to produce a linear or rotary motion. 
     The next prism  615  that is closer to the grating  600 , and has a size that is either larger than or equal to the size of the prism  620 , can be fixed in space in some implementations. The next prism  610  that is closer to the grating  600  has a size that is either larger than or equal to the size of the prism  615 . 
     The prism  605  that is closest to the grating  610  has a size that is either larger than or equal to the size of the prism  610  (the prism  605  is the largest prism of the beam expander). The prism  605  can be mounted to an actuation system  605 A that causes the prism  605  to rotate and such rotation of the prism  605  can provide for coarse control of the wavelength of the light beam  110 A. For example, the prism  605  can be rotated by 1-2 degrees to tune the wavelength of the light beam  110 A (and thus the light beam  110 ) from about 193.2 nanometers (nm) to about 193.5 nm. In some implementations, the actuation system  605 A includes a rotary stepper motor that includes a mounting surface (such as the plate  623 A) to which the prism  605  is fixed and a motor shaft that rotates the mounting surface. The motor of the actuation system  605 A can be a piezoelectric motor that is fifty times faster than a prior linear stepper motor and flexure combination design. Like the actuation system  620 A, the actuation system  605 A can include an optical rotary encoder that provides angular position feedback for the control system  185  or the control module  650 . 
     Referring to  FIGS. 7A and 7B , in another implementation of the spectral feature selection apparatus  730 , a rapid actuation system  720 A is designed to rotate R the prism  720  of a beam expander that is farthest from the grating  700  about the shaft axis AR. Optionally or additionally, the actuation system  710 A associated with the prism  710  can also be a rapid actuation system that is designed like the rapid actuation system  720 A or  620 A. 
     The apparatus  730  includes an extending arm  725 A that has a first region  740 A that is mechanically linked to the rotational plate  723 A at the location of the shaft axis AR. The extending arm  725 A has a second region  745 A that is offset from the shaft axis AR along a direction in the X SF -Y SF  plane (and thus along a direction that is perpendicular to the shaft axis AR) so that the second region  745 A is not intersected by the shaft axis AR. The prism  720  is mechanically linked to the second region  745 A. 
     Both the center of mass (the prism axis AP) of the prism  720  and the shaft axis AR remain parallel with the Z SF  axis of the apparatus  730 ; however, the center of mass of the prism  720  is offset from the shaft axis AR. A rotation of the extending arm  725 A about the shaft axis AR by an angle Δθ imparts a combined movement to the prism  720 : a rotation R of the prism  720  about the shaft axis AR by an angle Δθ (see  FIG. 5C ) within the X SF -Y SF  plane, and a linear translation T to the prism  720  along a direction that lies within the X SF -Y SF  plane of the apparatus  730 . In the example of  FIG. 7C , the prism  720  is rotated R from a first angle θ 1  to a second angle θ 2  and is translated T from a first position Pos 1  in the X SF -Y SF  plane to a second position Pos 2  in the X SF -Y SF  plane. 
     The linear translation T to the prism  720  thereby translates the light beam  110 A along a direction that is parallel with the longer axis  701  of the surface  702  of the grating  700 . The longer axis  701  also lies along the X SF -Y SF  plane of the apparatus  730 . By performing this translation of the light beam  110 A, it is possible to control which area or region of the grating  700  is illuminated at the lower end of the range of possible optical magnifications OM. Moreover, the grating  700  and the surface  702  of the grating is non-uniform; namely, some regions of the surface  702  of the grating  700  impart a different change to the wavefront of the light beam  110 A than other regions of the surface  702  of the grating  700  and some regions of the surface  702  impart more distortion to the wavefront of the light beam  110 A than other regions of the surface  702 . The control system  185  (or control module  550 ) can control the rapid actuation system  720 A to thereby adjust the linear translation T to the prism  720  and adjust the translation of the light beam  110 A along the longer axis  701  to take advantage of the non-uniformity of the grating  700  surface  702  and illuminate a higher distortion region of the grating surface  702  near one end of the grating surface  702  to raise the spectral bandwidth even more than the effect of simply lowering the optical magnification would achieve. 
     Additionally, the linear translation T to the prism  720  also translates the hypotenuse H (see  FIG. 7C ) of the prism  720  during rotation of the prism  720  relative to the location of the light beam  110 A. The translation to the hypotenuse H therefore exposes new regions of the hypotenuse H to the light beam  110 A during operation of the apparatus  730 . Over the lifetime of the apparatus  730 , the prism  720  is rotated from one end of its rotation range to the other end and also more regions are exposed to the light beam  110 A, which reduces the amount of damage imparted to the prism  720  by the light beam  110 A. 
     Similar to the apparatus  630 , the spectral feature selection apparatus  730  also includes a grating  600 , and the beam expander includes the prisms  705 ,  710 ,  715 , which are positioned along the path of the light beam  110 A between the prism  720  and the grating  700 . The grating  700  and the four prisms  705 ,  710 ,  715 ,  720  are configured to interact with the light beam  110 A produced by the optical source  105  after the light beam  110 A passes through an aperture  755  of the apparatus  730 . The light beam  110 A travels along a path in the X SF -Y SF  plane of the apparatus  730  from the aperture  755 , through the prism  720 , the prism  715 , the prism  710 , the prism  705 , and then is reflected from the grating  700 , and back through the consecutive prisms  705 ,  710 ,  715 ,  720  before exiting the apparatus  730  through the aperture  755 . 
     Referring to  FIG. 8 , an exemplary optical source  805  is a pulsed laser source that produces a pulsed laser beam as the light beam  110 . The optical source  805  is a two-stage laser system that includes a master oscillator (MO)  800  that provides the seed light beam  110 A to a power amplifier (PA)  810 . The master oscillator  800  typically includes a gain medium in which amplification occurs and an optical feedback mechanism such as an optical resonator. The power amplifier  810  typically includes a gain medium in which amplification occurs when seeded with the seed laser beam from the master oscillator  800 . If the power amplifier  810  is designed as a regenerative ring resonator then it is described as a power ring amplifier (PRA) and in this case, enough optical feedback can be provided from the ring design. The spectral feature selection apparatus  130  receives the light beam  110 A from the master oscillator  800  to enable fine tuning of spectral parameters such as the center wavelength and the bandwidth of the light beam  110 A at relatively low output pulse energies. The power amplifier  810  receives the light beam  110 A from the master oscillator  800  and amplifies this output to attain the necessary power for output to use in photolithography. 
     The master oscillator  800  includes a discharge chamber having two elongated electrodes, a laser gas that serves as the gain medium, and a fan circulating the gas between the electrodes. A laser resonator is formed between the spectral feature selection apparatus  130  on one side of the discharge chamber, and an output coupler  815  on a second side of the discharge chamber to output the seed light beam  110 A to the power amplifier  810 . 
     The optical source  805  can also include a line center analysis module (LAM)  820  that receives an output from the output coupler  815 , and one or more beam modification optical systems  825  that modify the size and/or shape of the beam as needed. The line center analysis module  820  is an example of one type of measurement system within the measurement system  170  that can be used to measure the wavelength (for example, the center wavelength) of the seed light beam. 
     The power amplifier  810  includes a power amplifier discharge chamber, and if it is a regenerative ring amplifier, the power amplifier also includes a beam reflector or beam turning device  830  that reflects the light beam back into the discharge chamber to form a circulating path. The power amplifier discharge chamber includes a pair of elongated electrodes, a laser gas that serves as the gain medium, and a fan for circulating the gas between the electrodes. The seed light beam  110 A is amplified by repeatedly passing through the power amplifier  810 . The beam modification optical system  825  provides a way (for example, a partially-reflecting mirror) to in-couple the seed light beam  110 A and to out-couple a portion of the amplified radiation from the power amplifier to form the output light beam  110 . 
     The laser gas used in the discharge chambers of the master oscillator  800  and the power amplifier  810  can be any suitable gas for producing a laser beam around the required wavelengths and bandwidth. For example, the laser gas can be argon fluoride (ArF), which emits light at a wavelength of about 193 nm, or krypton fluoride (KrF), which emits light at a wavelength of about 248 nm. 
     The line center analysis module  820  monitors the wavelength of the output (the light beam  110 A) of the master oscillator  800 . The line center analysis module  820  can be placed at other locations within the optical source  805 , or it can be placed at the output of the optical source  805 . 
     Referring to  FIG. 9 , details about the control system  185  are provided that relate to the aspects of the system and method described herein. The control system  185  can include other features not shown in  FIG. 9 . In general, the control system  185  includes one or more of digital electronic circuitry, computer hardware, firmware, and software. 
     The control system  185  includes memory  900 , which can be read-only memory and/or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. The control system  185  can also include one or more input devices  905  (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices  910  (such as a speaker or a monitor). 
     The control system  185  includes one or more programmable processors  915 , and one or more computer program products  920  tangibly embodied in a machine-readable storage device for execution by a programmable processor (such as the processors  915 ). The one or more programmable processors  915  can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processor  915  receives instructions and data from memory  900 . Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). 
     The control system  185  includes, among other components, a spectral feature analysis module  925 , a metrology module  927 , a lithography analysis module  930 , a decision module  935 , a light source actuation module  950 , a lithography actuation module  955 , and a beam preparation actuation module  960 . Each of these modules can be a set of computer program products executed by one or more processors such as the processors  915 . Moreover, any of the modules  925 ,  930 ,  935 ,  950 ,  955 ,  960  can access data stored within the memory  900 . 
     The spectral feature analysis module  925  receives the output from the measurement system  170 . The metrology module  927  receives the data from the metrology apparatus  145 . The lithography analysis module  930  receives information from the lithography controller  140  of the scanner  115 . The decision module  935  receives the outputs from the analyses modules (such as the modules  925 ,  927 , and  930 ) and determines which actuation module or modules need to be activated based on the outputs from the analyses modules. The light source actuation module  950  is connected to one or more of the optical source  105  and the spectral feature selection apparatus  130 . The lithography actuation module  955  is connected to the scanner  115 , and specifically to the lithography controller  140 . The beam preparation actuation module  960  is connected to one or more components of the beam preparation system  112 . 
     While only a few modules are shown in  FIG. 9 , it is possible for the control system  185  to include other modules. Additionally, although the control system  185  is represented as a box in which all of the components appear to be co-located, it is possible for the control system  185  to be made up of components that are physically remote from each other. For example, the light source actuation module  950  can be physically co-located with the optical source  105  or the spectral feature selection apparatus  130 . 
     In general, the control system  185  receives at least some information about the light beam  110  from the measurement system  170 , and the spectral feature analysis module  925  performs an analysis on the information to determine how to adjust one or more spectral features (for example, the bandwidth) of the light beam  110  supplied to the scanner  115 . Based on this determination, the control system  185  sends signals to the spectral feature selection apparatus  130  and/or the optical source  105  to control operation of the optical source  105  via the control module  550 . 
     In general, the spectral feature analysis module  925  performs all of the analysis needed to estimate one or more spectral features (for example, the wavelength and/or the bandwidth) of the light beam  110 . The output of the spectral feature analysis module  925  is an estimated value of the spectral feature. 
     The spectral feature analysis module  925  includes a comparison block connected to receive the estimated spectral feature and also connected to receive a spectral feature target value. In general, the comparison block outputs a spectral feature error value that represents a difference between the spectral feature target value and the estimated value. The decision module  935  receives the spectral feature error value and determines how best to effect a correction to the system  100  in order to adjust the spectral feature. Thus, the decision module  935  sends a signal to the light source actuation module  950 , which determines how to adjust the spectral feature selection apparatus  130  (or the optical source  105 ) based on the spectral feature error value. The output of the light source actuation module  950  includes a set of actuator commands that are sent to the spectral feature selection apparatus  130 . For example, light source actuation module  950  sends the commands to the control module  550 , which is connected to the actuation systems within the apparatus  530 . 
     The control system  185  causes the optical source  105  to operate at a given repetition rate. More specifically, the scanner  115  sends a trigger signal to the optical source  105  for every pulse (that is, on a pulse-to-pulse basis) and the time interval between those trigger signals can be arbitrary, but when the scanner  115  sends trigger signals at regular intervals then the rate of those signals is a repetition rate. The repetition rate can be a rate requested by the scanner  115 . 
     The repetition rate of the pulses produced by the power amplifier  810  is determined by the repetition rate at which the master oscillator  800  is controlled by the control system  185 , under the instructions from the controller  140  in the scanner  115 . The repetition rate of the pulses output from the power amplifier  810  is the repetition rate seen by the scanner  115 . 
     The prism  520  (or prism  620 ,  720 ) can be used for coarse, large range, slow bandwidth control. By contrast, the bandwidth can be controlled in a fine and narrow range and even more rapidly by controlling a differential timing between the activation of the electrodes within the MO  800  and the PRA  810 . 
     Referring to  FIG. 10 , a procedure  1000  is performed by the photolithography system  100  to reduce the impact that stage vibrations (that is, vibrations of the stage  122 ) have on the CD on the wafer  120  to thereby improve the critical dimension uniformity (CDU). 
     The pulsed light beam  110  is produced ( 1005 ) for example, by the optical source  105 . The pulsed light beam  110  is directed toward the wafer  120 , which is mounted to the stage  122  of the scanner  115  ( 1010 ). For example, the pulsed light beam  110  produced by the optical source  105  is modified, as needed, and redirected by the beam preparation system  112  toward the scanner  115 . 
     The pulsed light beam  110  is scanned across the wafer  120  ( 1015 ), for example, by moving the pulsed light beam  110  and the wafer  120  relative to each other along the lateral plane (the X L -Y L  plane). Specifically, the lithography controller  140  can send one or more signals to the actuation systems associated with the wafer stage  122 , the mask  134 , and the objective arrangement  132  to thereby move one or more of the mask  134 , the objective arrangement  132 , and the wafer  120  (via the stage  122 ) relative to each other during the exposure to scan the exposure window  400  across an exposure field  223  of the wafer  220 . 
     The value of the vibration of the stage  122  for each exposure field of a wafer is determined ( 1020 ). This determination can be performed at least in part by the metrology apparatus  145 , and the output sent to the control system  185 . This determination can be performed prior to scanning the wafer  120  with the pulsed light beam  110 . 
     For each exposure field of the wafer  120 , the control system  185  determines an amount of adjustment to a bandwidth of a pulsed light beam  110  directed toward the wafer  120  ( 1025 ). The adjustment amount compensates for a variation in the stage vibration in that exposure field outside of an acceptable range and maintains the focus blur within a predetermined range of values across the entire wafer  120 . Next, and as the pulsed light beam  110  is being scanned across the wafer  120 , the control system  185  sends a signal to the spectral feature selection apparatus  130  to change the bandwidth of the pulsed light beam  110  by the determined adjustment amount for each exposure field of the wafer  120  ( 1035 ). 
     Additionally, during the procedure  1000 , the control system  185  also performs a parallel procedure  1040  for controlling one or more spectral features of the pulsed light beam  110  while the light beam  110  is being scanned across the wafer  120 . The procedure  1040  includes measuring one or more spectral features of the pulsed light beam  110  ( 1045 ) and determining whether any of the measured spectral features are outside of an acceptable range of values ( 1050 ). For example, the spectral feature analysis module  925  of the control system  185  can receive the spectral feature measurement from the measurement system  170  ( 1045 ). The spectral feature analysis module  925  can determine whether any of the spectral features are outside an acceptable range of values ( 1050 ). If any of the spectral features are outside an acceptable range of values, then those spectral features are adjusted ( 1055 ). For example, the decision module  935  can send a signal to the light source actuation module  950 , which sends a signal to the spectral feature selection apparatus  130  to adjust one or more spectral features of the light beam  110  ( 1055 ). 
     The procedure  1040  can be performed in parallel with the procedure  1000 , for example, during the scanning or at regular intervals, for example, for each exposure field  223 . Moreover, it is possible for the control system  185  to coordinate the adjustment to the bandwidth needed to compensate for the stage vibration variation ( 1030 ) with any required adjustment to the bandwidth to ensure that the bandwidth is within an acceptable range of values ( 1055 ). 
     Referring to  FIG. 11 , the photolithography system  100  can perform a procedure  1025  for determining an amount of adjustment to the bandwidth of the pulsed light beam  110  directed toward the wafer  120  for a particular sub-area (such as an exposure field) of the wafer  120 . Such adjustment amount compensates for a variation in the stage vibration in that exposure field outside of an acceptable range and maintains the focus blur within a predetermined range of values across the entire wafer  120 . The procedure  1025  can be performed by the control system  185 . The control system  185  accesses a lookup table ( 1126 ). The lookup table is a table or correction map that is created based on previously determined stage vibration for the scanner  115  and it defines a relationship between the bandwidth of the light beam  110  for each sub-area of the wafer  120 , the value of the bandwidth being selected to offset the measured value of the stage vibration in that sub-area. The stage vibration values depend on the scanner  115  and wafer stage  122  being used. Thus, the relationship between stage vibration and the sub-area of a wafer  120  for one particular scanner  115  and stage  122  may be different than the relationship for another particular scanner  115  and stage  122 . 
       FIG. 12  shows an example of how the lookup table can be created. Initially, a wafer  120  is placed in the scanner  115 , and the metrology apparatus  145  probes the wafer  120  at each sub-area to determine the stage vibration in the direction Z L  for each sub-area of the wafer  120 . Graph  1200  shows an exemplary relationship between the number of occurrences (frequencies) of various stage vibrations in Z L  for all sub-areas of the wafer  120 . In this example, the stage vibration in Z L  is the moving standard deviation (MSD) value along the Z L  direction at each wafer sub-area (which can correspond to a wafer exposure field  223 ). This graph  1200  shows that, in this example, the nominal MSD(Z L ) value occurs at a much higher frequency than a higher MSD(Z L ) value from the center of the wafer  120  to the edge of the wafer  120 . The control system  185  seeks to maintain an effective stage vibration SVE at a constant value. The effective stage vibration is related to the measured stage vibration (SVM) (such as that shown in graph  1200 ) and the bandwidth (BW) of the pulsed light beam  110 ) as follows:
 
SVE=√{square root over (SVM 2 +(BW·CA) 2 )},
 
where CA is the chromatic aberration experienced by the pulsed light beam  110  as it is directed toward the wafer  120 . Thus, the control system  185  can calculate the BW after the graph  1200  is obtained so as to maintain a constant value of the effective stage vibration. Graph  1250  shows the values of the BW of the light beam  110  that can be used to offset the measured stage vibration SVM and maintain the effective stage vibration SVE constant.
 
     The control system  185  reviews the lookup table (such as the graph  1500 ) and determines the bandwidth that compensates for the measured stage vibration in that wafer sub-area so as to maintain an effective stage vibration constant in that sub-area ( 1028 ) and then outputs this value of the determined bandwidth ( 1030 ). 
     In some implementations, the lookup table can be created by the metrology apparatus  145  prior to directing the pulsed light beam  110  toward the wafer  120  that is currently mounted to the stage  122 . Thus, the lookup table can be created based on a prior scanned wafer  120 . 
     Other implementations are within the scope of the following claims.