Patent Publication Number: US-7720120-B2

Title: Method and apparatus for laser control in a two chamber gas discharge laser

Description:
FIELD 
   The disclosed subject matter is generally related to laser systems and, more particularly, is related to a laser control system for a two chamber gas discharge laser. 
   BACKGROUND 
     FIG. 1  is an illustration of a block diagram of a MOPA (Master Oscillator/Power Amplifier) laser system  10  as is known in the prior art. The MOPA laser system  10  is used, for instance, in the area of integrated circuit lithography. In one embodiment of the MOPA laser system  10 , a 193 nm ultraviolet laser beam is provided at the input port of a lithography machine/scanner  2  such as stepper or scanner machines supplied by Canon or Nikon with facilities in Japan or ASML with facilities in the Netherlands. The MOPA laser system  10  includes a laser energy control system  4  for controlling both pulse energy and accumulated dose energy output of the system at pulse repetition rates, for instance, of 4,000 Hz or greater. The MOPA laser system  10  provides extremely accurate triggering of the discharges in the two laser chambers relative to each other with both feedback and feed-forward control of the pulse and dose energy. 
   The main components of the laser system  4  are often installed below the deck/floor  5  on which the scanner  2  is installed. However, the MOPA laser system  10  includes a beam delivery unit  6 , which provides an enclosed beam path for delivering the laser beam to an input port of scanner  2 . The light source includes a seed laser generator, e.g., a master oscillator  11  and an amplifier laser portion, e.g., a power amplifier  12 , described in more detail below, and which may also be an oscillator, e.g., a power ring oscillator (“PRA”), also described in more detail below. For convenience sake throughout this application the seed laser may be referred to as an MO and the amplifier laser may be referred to as a power amplifier or simply a PA, with the intent to cover other forms of seed laser arrangements and amplifier laser arrangements, such as a power ring amplifier (“PRA”), which is in fact an oscillator, i.e., a power oscillator (“PO”), together forming a MOPO, and unless expressly stated otherwise these terms are meant to be so broadly defined. The light source also includes a pulse stretcher  22 . 
   The master oscillator  11  and the power amplifier/power oscillator  12  each include a discharge chamber  11 A,  12 A similar to the discharge chamber of single chamber lithography laser systems. These chambers  11 A,  12 A contain two electrodes, a laser gas, a tangential for circulating the gas between the electrodes and water-cooled finned heat exchangers. The master oscillator  11  produces a first laser beam  14 A which is amplified, in a PA configuration by two passes through the power amplifier  12 , or in the case of a PO/PRA configuration, by oscillation in the PO/PRA, to produce a second laser beam  14 B as shown in  FIG. 1 . The master oscillator  11  includes a resonant cavity formed by an output coupler  11 C and a line narrowing package  11 B. The gain medium for the master oscillator  11  is produced between two elongated electrodes contained within the master oscillator discharge chamber  11 A. The power amplifier  12  is basically a discharge chamber  12 A and in this preferred embodiment is almost exactly the same as the master oscillator discharge chamber  11 A providing a gain medium between two electrodes, but the power amplifier  12  may have no resonant cavity, unlike a PO/PRA. This MOPA laser system  10  configuration permits the master oscillator  11  to be designed and operated to maximize beam quality parameters such as wavelength stability and very narrow bandwidth; whereas the power amplifier  12  is designed and operated to maximize power output. For this reason the MOPA laser system  10  represents a much higher quality and much higher power laser light source than single chamber systems. 
   As noted above the amplifier portion may be configured, e.g., for two beam passages through the discharge region of the amplifier discharge chamber, or for oscillation in the cavity containing the amplifier discharge chamber, as shown in  FIG. 1 . The beam oscillates within the cavity containing the master oscillation chamber  11 A between LNP  11 B and output coupler  11 C (with 30 percent reflectance) of the MO  11  and is severely line narrowed on its passages through LNP  10 C. A wavelength of a laser beam emitted from the output coupler  11 C is measured by a line center analysis module  7 . The line narrowed seed beam is reflected downward by a mirror in the MO wavelength engineering box (MO WEB)  24  and reflected horizontally at an angle slightly skewed (with respect to the electrodes orientation) through the PA wavelength engineering box (PA WEB)  26  to the amplifier chamber  12 . At the back end of the amplifier, a beam reverser  28  reflects the beam back for a second pass through PA chamber  12 , or for oscillation in the PO/PRA chamber, horizontally in line with the electrodes orientation. A bandwidth of a laser emitted from the discharge chamber  12 A is measured by a spectral analysis module  9 . 
   The laser system output beam pulses  14 B pass from the PA/PO chamber  12 A to a beam splitter  16 . The beam splitter  16  reflects about 60 percent of the power amplifier output beam  14 B into a delay path created by four focusing mirrors  20 A,  20 B,  20 C and  20 D. The 40 percent transmitted portion of each pulse of beam  14 B becomes a first hump of a corresponding stretched pulse of an output beam pulse  14 C. The output beam  14 C is directed by beam splitter  16  to a mirror  20 A which focuses the reflected portion to point  22 . The beam then expands and is reflected from mirror  20 B, which converts the expanding beam into a parallel beam and directs it to a mirror  20 C which again focuses the beam again at point  22 . This beam is then reflected by mirror  20 D which like the  20 B mirror changes the expanding beam to a light parallel beam and directs it back to beam splitter  16  where 60 percent of the first reflected light is reflected perfectly in line with the first transmitted portion of this pulse in output beam  14 C to become most of a second hump in the laser system output beam pulse. The 40 percent of the reflected beam transmits beam splitter  16  and follows exactly the path of the first reflected beam producing additional smaller humps in the stretched pulse. The result is the completed output beam  14 C which is stretched in pulse length from about 20 ns to about 70 ns. A beam delivery unit (BDU) delivers the output beam  14 C. The BDU may include two beam-pointing mirrors  40 A,  40 B one or both of which may be controlled to provide tip and tilt correction for variations beam pointing. 
     FIG. 2  is an illustration of an energy control block diagram  50  for the MOPA/MOPO Laser System of  FIG. 1 , in accordance with the prior art.  FIG. 2  illustrates various control elements that control a voltage supply  52  to the MOPA laser system  10 . The energy control block diagram  50  includes a static control  54 , which provides a basically determined voltage anticipated to achieve an energy target  56  (if there are no other influences for which account need be made). A feed forward block  58  provides a voltage adjustment based on a trigger interval  60 . Trigger interval  60  is used to compute repetition rate, shot number and duty cycle, which impact the ‘voltage input—energy output’ relationship. The voltage adjustment is computed as a function of these values. An energy servo  62  adjusts the voltage input  52  based on a calculated voltage error  64  of the previous shot. A dither cancellation  66  adjusts voltage to cancel energy changes caused by a timing dither  68 . Finally, an energy dither  70  provides a periodic signal added to the voltage input  52  used to estimate the effects of voltage on MO energy, output energy, and MOPA timing. These five voltage signals are added together to generate the voltage input  52 . As the laser fires, the energy  72  is measured. The energy target is subtracted from the measured energy  72  to create an energy error signal  74 , which is scaled by dV/dE, the laser estimate  76  of the derivative of voltage with respect to energy. The resulting voltage error  64  is used to drive adaptation algorithms  78  which adjust some of the voltage signals in a way that minimizes either energy errors, dose errors, energy sigma, or some combination thereof. 
   The MOPA laser system  10  shown in  FIG. 1  is an improvement on the single chamber systems, providing greater beam control, beam power, and stability than the single chamber systems. However, resolving tonal disturbances and further sharpening timing and energy control of the system can significantly improve operation. 
   SUMMARY 
   Aspects of embodiments of the disclosed subject matter provide a system and method for controlling a laser system. Briefly described, in architecture, aspects of one possible embodiment of the system, among others, can be implemented as follows. The system contains an oscillator gas chamber and an amplifier gas chamber. A first voltage input is operatively connected to deliver electrical pulses to a first pair of electrodes within the oscillator gas chamber and a second pair of electrodes within the amplifier gas chamber. An output of the gas chambers is an energy dose calculated by a trapezoidal window. A control circuit connects to the first voltage input for modifying the first voltage input. A feedback control loop communicates an output of the gas chambers to the control circuit for modifying the first voltage input. 
   Aspects of the disclosed subject matter can also be viewed as providing methods for controlling a laser system. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: delivering a first voltage input operatively in the form of electrical pulses to a first pair of electrodes within a oscillator gas chamber and a second pair of electrodes within a amplifier gas chamber; calculating an energy dose of an output of the gas chambers with a trapezoidal window; modifying the first voltage input with a control circuit; and communicating an output of the gas chambers to the control circuit with a feedback control loop for modifying the first voltage input. 
   Other systems, methods, features, and advantages of the disclosed subject matter will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the disclosed subject matter, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the disclosed subject matter. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is an illustration of a block diagram of a MOPA/MOPRA Laser System. 
       FIG. 2  is an illustration of an energy control block diagram for the MOPA/MOPRA Laser System of  FIG. 1 . 
       FIG. 3  is an illustration of a graph representing trapezoidal windows of varying repetition rates, in accordance with a first exemplary embodiment of the disclosed subject matter. 
       FIG. 4  is an illustration of a graph of a frequency response of the dose operator for the windows illustrated in  FIG. 3 , in accordance with a first exemplary embodiment of the disclosed subject matter. 
       FIG. 5  is an illustration of an energy control block diagram for the MOPA/MOPRA Laser System of  FIG. 1 , in accordance a first exemplary embodiment of the disclosed subject matter. 
       FIG. 6  is a flowchart illustrating a method of providing the laser control system of  FIG. 5 , in accordance with the first exemplary embodiment of the disclosed subject matter. 
   

   DETAILED DESCRIPTION 
   Elements of the disclosed subject matter are based upon the recognition that while square windows have been used in the past for energy dose calculation, some benefits may be realized by adapting to alternative shaped windows.  FIG. 3  is an illustration of a graph representing trapezoidal windows of varying repetition rates, in accordance with a first exemplary embodiment of the disclosed subject matter.  FIG. 4  is an illustration of a graph of a frequency response of the dose operator for the windows illustrated in  FIG. 3 , in accordance with aspects of a first exemplary embodiment of the disclosed subject matter. Note that while there are a set of zeros that vary for varying window widths, there are clearly zeros for all windows at 20% and 40% of the sample rate. These zeros correspond to the zeros of a 5 pulse moving average. It can be shown that a trapezoidal window is the convolution of a rectangular window having a length equal to the window size, less the trailing edge and a 5 pulse rectangular window. 
     FIG. 5  is an illustration of an energy control block diagram  150  for the MOPA/MOPRA laser system  10  of  FIG. 1 , in accordance a first exemplary embodiment of the disclosed subject matter.  FIG. 5  illustrates various control elements that control a voltage input  152  to the MOPA/MOPRA laser system  10 . The energy control block diagram  150  includes a static control  154 , which provides a basically determined voltage anticipated to achieve an energy target  156  (if there are no other influences for which account need be made). Part of a purpose of the static control  154  is to make the energy controller responsive to changes in the energy target  156 . If a user adjusts an energy target, the first voltage input  152  for the first shot should be computed to meet the new energy setpoint. The static control  154  may provide the following voltage signal:
   V =( dV/dE ) ref *( E   target   −E   ref +ε( E   target   −E   ref ) 2   +V   ref    
where E ref  is approximately set to a nominal energy of the laser system  10  and V ref  is the voltage approximately required to fire the laser at E ref .
 
   A feed forward block  158  provides a voltage adjustment based on a trigger interval  160 . Trigger interval  160  is used to compute repetition rate, shot number and duty cycle, which impact the ‘voltage input—energy output’ relationship. The voltage adjustment for the trigger interval  160  is computed as a function of these values. More specifically, the voltage signal provided by the feed forward block  158  may be given by:
 
 V=f   0 ( D )+ f   1 ( R,n )
 
where D is the duty cycle, R is the repetition rate, and n is the shot number. Note that the feed forward voltage has two terms. One term, f 0 , depends on duty cycle and/or burst interval, and another term, f 1 , depends on shot number and repetition rate. By design, f 1  is identically zero on the first shot of each burst. Therefore, f 0  alone determines the feed forward voltage for the first shot of the burst. This term is intended to adjust the laser for changes in efficiency, which typically persists throughout a burst. The f 1  term captures the shape of any transients. This law assumes that duty cycle or interburst interval effect just moves the energy vs. shot number up or down equal amounts for all shots in a burst. The shape of the energy transient is assumed to depend only on repetition rate. The f 1 (R,n) function compensates for the shape of the energy transient.
 
   The f 1 (R,n) function is maintained as a table versus repetition rate and shot number. A simple integrator is used to adapt the bins. In the past, the bins were initialized to zero, requiring several bursts before the laser control was correctly inverting the transient at the repetition rate. Instead of initializing these bins with zero, feed forward bins may be initialized with values of trained bins that are nearest in frequency. Initializing the bins with a value nearer the correct value for the shape of the energy transient allows the laser control more quickly, and with greater accuracy, to invert the transient at the repetition rate. 
   An energy servo  162  adjusts the voltage input  152  based on a calculated voltage error  164  of the previous shot within the same burst. The adjustment from the energy servo  162  may be calculated in at least a couple of different modes. First, IISquared feedback is a feedback law known to those having ordinary skill in the art. This feedback law feeds back one voltage proportional to an integral of the voltage error (an integral gain) and another proportional to the voltage error integrated twice (an I squared gain). Several sets of gains are provided for the IISquared filter: soft; hard; and MO. Soft gains are used in operational modes when the objective is to minimize shot to shot energy error. Soft gains are selected to minimize energy errors. Hard gains are used in dose and sigma modes. The hard gains are intended to minimize dose (integrated energy error) and tend to be larger than the soft gains. The MO gains are used in MO energy control modes and are also intended to minimize energy errors. 
   An alternative to IISquared feedback is dose feedback. Like the IISquared controller with hard gains, the dose feedback controller is intended to minimize dose, however, it uses a control law that gives better performance with non-rectangular dose windows (e.g., trapezoidal dose windows). Dose feedback is controlled by a dimensioning parameter and a vector of gains. The dose feedback controller is available only in dose and sigma modes and may utilize a Linear Quadratic Regulator to minimize the quadratic sum of energy dose and energy error  172 . Utilization of the Linear Quadratic Regulator instead of 100% integral feedback has been shown in tests to reduce energy dose error by approximately 25%. 
   The effect of the laser system  4 , as shown in  FIG. 5 , is to translate the voltage input  152  to energy (evaluated by the energy measurement  172 ) through a static gain. In addition, there is a set of disturbances added to the energy signal. Thus, the state of the system can be equated to the disturbance dynamics and the dose operator. The energy servo  162  is directed to providing voltage adjustments responsive to the behavior of the dose operator. The energy dose feedback may be calculated as the inner product of a state feedback vector, K and a vector characterizing the state of the dose operator, xd.
 
 V dose=− K xd  
 
where K is computed as a solution of a linear quadratic regulator which minimizes a weighted sum of the square of the energy error and the square of the energy dose error.
 
   A dither cancellation  166  adjusts voltage to cancel energy changes caused by a timing dither  168 . A side effect of this cancellation is that it estimates the derivative of voltage with respect to MOPA/MOPRA timing at fixed energy, a value used to compute MopaOpPoint  180  (operating point of the MOPA laser system, u, which may be defined as:
 
 u =1 /E*dV/dt  at constant energy,
 
where E is laser energy, V is voltage and t is MOPA timing, the difference in firing times between the MO and PA chambers). For certain aspects of timing control, the local slope of the timing versus energy curve is needed. This information is obtained by applying a dither signal to the differential timing commanded to the MO and PA commutator triggers. Because timing couples into energy, this dither signal produces a matching dither in energy. The dither cancellation algorithm adaptively finds a voltage signal which when applied to the laser exactly cancels the dither in energy produced by the timing dither signal. Thus, the timing dither no longer appears in the energy signal and therefore has no impact on energy sigma or energy dose.
 
   A by-product of this cancellation algorithm is the derivative of voltage with respect to timing at fixed energy. This by-product is the slope information that the timing dither was applied to identify in the first place. A parameter used in the laser control system for gas control, dMpopdMopa (the derivative of MopaOpPoint with respect to the difference between MO and PA chamber firing times), may be used to make the dither cancellation approximately instantly responsive to changes in MOPA timing (the difference in MO and PA chamber firing times). If this parameter is off, then on “‘large’” (1-2 ns) changes in MOPA/MOPRA timing, MopaOpPoint  180  (“Mpop”) will jump to a new value and then over then next several thousand shots, drift to a different value. During the time while the MopaOpPoint  180  estimate is converging, some of the timing dither signal will bleed through into energy. If the aforementioned gas control parameter is set correctly, MopaOpPoint  180  should jump to a new value on a “large” MOPA/MOPRA timing change and then remain at the new value with materially diminished drift. 
   As noted, with respect to  FIG. 4 , utilizing trapezoidal windows, there are clearly zeros for all windows at 20% and 40% of the sample rate (where the leading and trailing edges of the trapezoidal windows are 5 pulses). The amplitude of a dither may be set low normally to reduce the energy dither, but the low amplitude delays calculation of the derivative of the energy dose verses voltage estimate. If the dither is moved under the zero of one of the trapezoidal windows, the amplitude can be raised with diminished negative impact. 
   Mpop compensation  182  adjusts the voltage input  152  to compensate for changes in MOPA timing. This adjustment is primarily to stabilize energy for changes of a DtMopaTarget in excess of approximately 1 nanosecond. If a laser is running with bandwidth control enabled (ASC) and is currently operating away from a resonance, to keep bandwidth up, the control system has decreased the delay between MO and PA triggers. At this point, MopaOpPoint will be a low, negative value because DtMopaTarget is several ns below the value for peak efficiency. Then, the scanner  2  switches repetition rate to one which lies on a bandwidth resonance. Bandwidth goes up and the bandwidth controller advances DtMopaTarget by a few nanoseconds to compensate. This advancement moves the laser several nanoseconds closer to peak efficiency and energy increases in a stepwise manner. This step change in energy will affect energy dose until the energy servo  162  has a chance to compensate. 
   In the meantime, MopaOpPoint compensation  182  combats this effect. Using the same value used to adjust dither cancellation for changes in Mopa timing, dMpopDMopa, it is possible to compute the amount voltage will need to change for a given change in Mopa timing. When Mopa timing is changed quickly, the MopaOpPoint compensation  182  can predict what voltage change that is also needed and provide an appropriate voltage signal to the voltage input  152  without having to wait for an energy error  174  to appear. The MopaOpPoint compensation  182  may be described as:
 
 V=Eu   2 /2 k  
 
where E is the laser energy, u is the MopaOpPoint, and k is dMpopDMopa or the derivative of MopaOpPoint with respect to Mopa timing, the difference in firing time between the MO and PA chambers.
 
   A disturbance prediction  184  adjusts the voltage input  152  based on a prediction of the voltage error, assuming that the disturbance acting on energy is a DC offset plus several tones. The tones are at frequencies that are multiples of the MO and PA/PO blower speeds. This predicted voltage error is subtracted from the voltage input  152 , thus removing the effects due to DC offset or blower blade passage. 
   Finally, an energy dither  170  provides a periodic signal added to the voltage input  152  used to estimate the effects of voltage on MO energy, output energy, and MOPA timing. The periodic signal from the energy dither  170  is n shots long and may be described by the equation:
 
 V [k]=A  cos(2  πn   d   k/n )  k =0,  . . . , n− 1
 
where A is the dither amplitude and n d  is the number of cosine periods within one full cycle of the dither.
 
   The dither signal is held off for a fixed number of shots before being started in each burst. The delay is provided so that the dither cannot combine with beginning of burst effects to push the laser 4 out of spec. In order to prevent other signal at or near the dither frequency from interfering with derivative estimates, the phase of the dither signal may be randomized. Randomization is done by randomizing the value of k in the above equation to start the dither. For the windows shown in  FIG. 3 , the dither frequency is 1 fifth of the rep rate (5 shots leading and trailing each trapezoidal window). This is a key frequency for lasers using trapezoidal windows to calculate dose. For windows which have four pulse leading and trailing edges, 20% of the rep rate lies inside a zero of the dose operator. Hence, dithering at this frequency will have no effect on dose. 
   These voltage signals are added together to generate the voltage input  152 . As the laser fires, the energy is measured  172 . The energy target is subtracted from the measured energy to create an energy error signal  174 , which is scaled by dV/dE, the laser estimate  176  of the derivative of voltage with respect to energy. The resulting voltage error  164  is used to drive adaptation algorithms  178  which adjust some of the voltage signals in a way that minimizes energy errors, dose errors, or energy sigma, or some combination thereof. 
     FIG. 6  is a flowchart  200  illustrating a method of providing the laser control system  150  of  FIG. 5 , in accordance with the first exemplary embodiment of the disclosed subject matter. It should be noted that any process descriptions or blocks in flow charts should be understood as representing modules, segments, portions of code, or steps that include one or more instructions for implementing specific logical functions in the process, and alternate implementations are included within the scope of the disclosed subject matter in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the disclosed subject matter. 
   As is shown by block  202 , a first voltage input is delivered operatively in the form of electrical pulses to a first pair of electrodes within an oscillator gas chamber and a second pair of electrodes within a amplifier gas chamber. An energy dose of an output of the gas chambers is calculated with a trapezoidal window (block  204 ). The first voltage input is modified with a control circuit by adding to the first voltage input an energy dose feedback calculated as V dose =−K xd, where K is a state feedback vector computed as a solution of a linear quadratic regulator that minimizes a weighted sum of the square of an energy error and the square of an energy dose error and xd is a vector characterizing a state of a dose operator (block  206 ). An output of the gas chambers is communicated to the control circuit with a feedback control loop for modifying the first voltage input (block  208 ). 
   It should be emphasized that the above-described embodiments of the disclosed subject matter, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the disclosed subject matter and protected by the following claims.