Abstract:
Methods and systems for generating pulses of laser radiation at higher repetition rates than those of available excimer lasers are disclosed that use multiple electronic triggers for multiple laser units and arrange the timings of the different triggers with successive delays, each delay being a fraction of the interval between two successive pulses of a single laser unit. Methods and systems for exposing nanoscale patterns using such high-repetition-rate lasers are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 13/135,290, filed on Jun. 30, 2011, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to pulsed laser source systems, and particularly relates to methods and apparatus for ultraviolet, pulsed, excimer laser source systems with high repetition rates. Such laser systems are useful as light sources in nanolithography systems for production of electronic devices. 
     2. Description of Related Art 
     The manufacture of modern electronic devices, commonly referred to as integrated circuits (ICs) or chips, requires a number of fabrication technologies. One of the most critical of such fabrication technologies is lithography, the process of patterning the billions of structures that form the individual components of the devices on the semiconductor wafers. Advances in the manufacture of electronic devices have required the patterning of ever smaller structures on the wafers, which, for the process of lithography, is referred to as requiring higher (i.e., finer) patterning resolution. 
     A key element in a lithography system that enables it to achieve a fine patterning resolution is its light source, which in modern lithography systems is an ultraviolet excimer laser due to its short wavelength. Typically, modern lithography systems use an Argon Fluoride (ArF) excimer laser source that emits radiation of 193 nanometer (nm) wavelength. Due to the fundamental physical operating mechanism of such a laser, it operates only as a pulsed source, with a typical pulse repetition rate of a few hundred to a few thousand pulses per second. 
     A modern lithography system with an excimer laser source also comprises a high-resolution, large-field projection lens that creates an image of a master pattern present on a mask onto the semiconductor wafer. The overall performance of the lithography system is determined by the projection lens, the light source, the mask, and several other factors. Current state-of-the-art lithography systems are capable of producing device structures in high volumes with a minimum feature size in the vicinity of 22-45 nm. With such small feature sizes, electronic chips with several billion transistors can be produced. 
     The demands on electronic systems to operate at ever greater speeds and have ever greater storage capacities are requiring more advanced chips with minimum feature sizes smaller than 22 nm. Modern lithography systems are incapable of patterning electronic structures with such small features with sufficiently high production throughputs for required cost efficiencies. There is thus a need to develop advanced lithography systems that can provide a patterning resolution significantly finer than 22 nm and patterning throughput of, for example, 100 or more wafers per hour. Such lithography systems are currently not available. To meet these objectives, many new lithography approaches are being investigated in the semiconductor industry and at research institutions, including extreme ultraviolet lithography, maskless lithography, immersion lithography, and other. 
     Of these new approaches, maskless lithography holds particularly strong promise due to its many advantages, including high resolution and elimination of the mask as a requirement in the lithography process. (That the latter is significant can be recognized by noting that the cost of the mask set for patterning the layers of a modern chip exceeds five million dollars.) Examples of methods and apparatus for maskless lithography are disclosed in U.S. Pat. No. 6,312,134, Seamless, Maskless Lithography System Using Spatial Light Modulator, 2001; U.S. Pat. No. 6,707,534, Maskless Conformable Lithography, 2004; U.S. Pat. No. 6,870,554, Maskless Lithography with Multiplexed Spatial Light Modulators, 2005; and U.S. Pat. No. 7,164,465, Maskless Lithography with Sub-Pixel Resolution, 2004. 
     In a maskless lithography system, the conventional hard mask as used in a typical optical projection lithography system is replaced by a spatial light modulator (SLM) array. Each element (i.e., individual element) in the SLM array can be programmed to be “On” or “Off”, i.e., reflective or nonreflective for a reflective-type SLM (or transmissive or nontransmissive for a transmissive-type SLM), so that the collection of all the beams emerging from an SLM array can be programmed to represent any desired pattern of light pixels that can then expose a photosensitive medium to create the corresponding pattern therein. 
     State-of-the-art SLMs have modulator elements of size in the vicinity of 10 micrometer×10 micrometer. In a maskless lithography system, by using a projection lens with a reduction ratio of approximately 200:1, an image pixel size of (10 micrometer)/(200)=50 nm can be produced. Thus, in order to improve the resolution of a maskless lithography system, the modulator element size must be reduced or the projection lens reduction ratio must be increased, both of which avenues are difficult. 
     It will therefore be beneficial to devise a technique that provides higher resolution for a maskless lithography system than the minimum feature size (“pixel size”) printed on the basis of the SLM element size and the projection lens reduction ratio. 
     Methods and apparatus for maskless lithography for providing a resolution finer than a pixel size, i.e., sub-pixel resolution, have been developed and are disclosed in U.S. Pat. No. 6,717,650, Maskless Lithography with Sub-Pixel Resolution, 2004, and U.S. Pat. No. 7,170,669, Spatial Modulator with Minimized Heat Absorption and Enhanced Resolution Features, 2007. These methods and apparatus define sub-pixel-size features by partial overlap between pixel-size features, exploit nonlinear photoresponse characteristics of the imaging media, and effectively use massively parallel bit addressing for full-pattern definition and high throughput. 
     In addition to the above considerations, the performance achievable by maskless lithography systems is dependent not only upon the ability of the SLM to rapidly transfer the pattern information from the data file to the imaging medium, but also upon the ability of the light source to illuminate the SLM with a new pulse every time the SLM frame (i.e., the array of all the modulator elements) is refreshed (i.e., provided a new set of pattern data). Modern SLMs can have frame refresh rates as high as 25 kHz, i.e., all the modulator elements can be provided with new “On” or “Off” information 25,000 times per second. In order to utilize such a high frame refresh rate capability, the light source must also be able to provide the same number of pulses per second. Modern excimer laser light sources are available with pulse repetition rates that are limited to approximately 6 kHz. Available light sources are therefore inadequate for implementation in maskless lithography systems with the highest refresh rate SLM arrays. 
     Therefore, there is a need to develop an ultraviolet excimer laser light source capable of providing pulses at repetition rates in the vicinity of 25 kHz and preferably even higher. 
     It is an object of this invention to provide a method for producing pulsed ultraviolet laser radiation at high repetition rates. 
     It is another object of this invention to provide an apparatus for producing pulsed ultraviolet excimer laser radiation with repetition rates of tens of thousands of pulses per second. 
     It is yet another object of this invention to provide a high-resolution maskless lithography method and apparatus utilizing a high-repetition-rate laser light source for illuminating a spatial light modulator array. 
     With the above examples of objects, other objects of this invention will be evident to those skilled in the art of semiconductor manufacturing, lithography, and related fields. 
     An advantage of the invention is that it enables effective utilization of high-refresh-rate spatial light modulators in maskless lithography systems for achieving high throughputs and high resolutions. 
     Another advantage of the invention is that it provides the ability to produce high-repetition-rate laser pulses using lower-repetition-rate laser sources. 
     Yet another advantage of this invention is that it enables the optimization of the combined operation of the illumination source and the spatial light modulator array in a maskless lithography system to achieve optimum throughput and resolution. 
     With the above examples of the advantages, other advantages of this invention will be evident to those skilled in the art of semiconductor manufacturing, lithography, and related fields. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the basic concept of the invention, showing multiple excimer laser units, each unit having a master-oscillator sub-unit and a power-amplifier sub-unit, each laser unit triggered by well defined pulse triggers, such that specified delays introduced between the trigger pulses produce a final laser pulse train that has a substantially higher effective repetition rate than possible with a single laser unit. 
         FIG. 2  shows the basic concept of maskless lithography and identifies the key components of such a lithography system, including a light source, a spatial light modulator array, a projection lens, and a scanning stage. 
         FIG. 3  illustrates the operation of a spatial light modulator array, showing how individual modulators can be turned on or off as required for exposure of the desired pattern directly on the semiconductor wafer. 
         FIG. 4  shows a representative excimer laser system configuration called “master-oscillator-power-amplifier” (MOPA) that comprises two sub-units, called a master oscillator (MO) and power amplifier (PA). 
         FIG. 5  illustrates the arrival times of the pulses from the different MOPA units with the relative delays between them of T 2 , T 3 , and T 4 , each of which is a fraction of the interval T between successive pulses from a MOPA unit. 
         FIG. 6  illustrates the arrival times of the pulses from four different MOPA units, each of which has an interval 160 μs between successive pulses, with the relative delays between them of 40, 80, and 120 μs, which are one-fourth, half, and three-fourths of the interval between successive pulses from a single MOPA unit. 
         FIG. 7  illustrates an embodiment of the invention in which the arrival times of the pulses from the different MOPA units are made variable by making the relative delays between them of T b , T c , and T d  variable. 
         FIG. 8  shows an embodiment of the invention in which the arrival times of the pulses from the different MOPA units are dynamically optimized by utilizing information about the pattern to be exposed and the spatial light modulator elements to be illuminated. 
         FIG. 9  illustrates an embodiment of the invention in which the arrival times of the pulses from the different MOPA units are made variable by making the relative delays between them of T b , T c , and T d  variable directly by signals from a computer. 
         FIG. 10  illustrates a maskless lithography system comprising a high-repetition-rate laser source, a spatial light modulator array, a projection lens, other optical components, a scanning stage, and control computer that utilizes detailed pattern information to optimize and interactively control all subsystems. 
     
    
    
     SUMMARY OF THE INVENTION 
     In this invention, it is disclosed how an excimer laser source system can be constructed that has a higher pulse repetition rate than that of available excimer lasers. The fundamental concept of the invention, illustrated in  FIG. 1 , is to use multiple electronic triggers for multiple excimer laser units and arrange the timings of the different triggers with successive delays, each delay being a fraction of the interval between two successive pulses of a single laser unit. 
     The basic concept of maskless lithography is well known (prior art). Conceptually, a representative optical maskless lithography system ( FIG. 2 ) uses an ultraviolet laser beam, e.g., from an excimer laser, to illuminate a spatial light modulator, e.g., a digital micromirror device (DMD), which is a 2-D array of micromodulators. Acting as reflectors, these hundreds of thousands (or millions) of modulators direct a desired set of beamlets into a high-reduction-ratio projection lens, which images the modulators on a photoresist-coated wafer, which can be stepped or scanned. In brief, the key features and attributes of such a maskless lithography are the following: (i) The conventional hard mask used in optical projection exposure tools is replaced by a spatial light modulator (SLM). (ii) The SLM is a 2-D array of reflective or transmissive light modulators which can independently control 10 6  or even &gt;10 7  light beams. (iii) A desired set of beamlets reflected from the SLM (or transmitted beamlets if it is a transmissive SLM) are directed into a projection lens ( FIG. 3 ). These “On” beams correspond to locations where exposure in the resist is desired. The other (“Off”) beams are not captured by the projection lens. 
     Currently, the most advanced types of SLMs are the digital micromirror device (DMD) chips, among which a leading device has approximately two million modulator elements and a “frame refresh rate” of approximately 23 kHz, which is the rate at which the DMD chip frame, i.e., all the mirror elements, can be sent new bit information. In order to achieve as high a wafer exposure rate as possible, the lithography system must utilize the maximum frame refresh, which requires that the light source be able to illuminate the SLM with at least one new pulse for each frame. Thus, the illumination source, i.e., the ultraviolet laser, must be able to emit pulses at the rate of at least 23 kHz. Often, it is desirable that each SLM frame be illuminated by two or more pulses to achieve more precise exposure control. In such a case, the illumination source must be able to provide pulses at a rate that is a multiple of the SLM frame refresh rate. 
     The most desirable light source for high-resolution lithography is an ultraviolet excimer laser due to its short wavelength, high power, and other favorable characteristics. For example, many modern lithography systems for commercial production of semiconductor devices use an Argon Fluoride (ArF) excimer laser emitting pulsed radiation at a wavelength of 193 nm. It will be therefore desirable also to develop a maskless lithography system that can use an ArF excimer laser as the light source. In the ArF excimer laser active medium which contains a mixture of fluorine, argon and a buffer gas, the laser photon is emitted when an electron falls from an excited metastable state of the rare gas halide (ArF) to an unstable state of that halide. Due to the nature of this fundamental operating mechanism of an excimer laser, it can emit laser radiation only in a pulsed manner. The highest rate at which laser pulses can be emitted by modern excimer lasers is limited to a few thousand pulses per second, with a maximum rate of approximately 3-6 kHz. Since modern SLMs can have frame refresh rates of several times that value, it is desirable to develop methods that can significantly increase the pulse repetition rates of excimer lasers, as described in this invention. 
     A typical laser comprises an optical resonator with two mirrors between which the active medium is contained and between which the light rays bounce back and forth, enabling the laser beam to build up by stimulated emission of radiation. A representative excimer laser system, due to high gain in its active lasing medium, can be constructed with an optical resonator that has only one mirror, the other mirror being replaced by transmitting window. While such a configuration provides high laser power output, it may also broaden the frequency bandwidth of the laser radiation, which is sometimes not desirable. In order to produce a laser beam with better spectral characteristics, an alternate configuration for excimer laser systems comprises two sub-units, each being essentially a separate excimer laser, called a master oscillator (MO) and a power amplifier (PA), as shown in  FIG. 4 . In such a configuration, called master-oscillator-power-amplifier (MOPA), the MO unit has an optical resonator with two mirrors and produces a laser beam with a very narrow spectral bandwidth, typically less than 0.5 picometer, and low pulse energy. The beam from the MO is then used to “seed” a PA unit, which has no resonator mirrors (both mirrors being replaced by transmitting windows) and whose function is to amplify the beam from the MO while maintaining its spectral characteristics. The resulting laser beam emerging from the MOPA therefore has both the narrow bandwidth produced by the MO and the high pulse energy produced by the PA. However, as explained in the preceding paragraph, it does not have the desired high pulse repetition rate needed to utilize the high frame refresh rates of modern SLMs. 
     In this invention, an excimer laser source system is disclosed that has a higher pulse repetition rate than that of available excimer lasers. In its key concept, the invention uses multiple electronic triggers for multiple excimer laser units and arranges the arrival times of the different triggers with successive delays such that each delay is a fraction of the interval between two successive pulses of a single laser unit. For example, consider an excimer laser with a pulse repetition rate of 2 kHz. The interval between two successive pulses from such a laser is 500 microseconds. If now one uses five lasers and triggers them by pulses that are successively delayed by 100 microseconds, then the five lasers taken together will produce 10,000 laser pulses per second that are emitted at intervals of 100 microseconds, i.e., effectively, providing an excimer laser source with a pulse repetition rate of 10 kHz. 
     In the excimer laser active medium, a high-voltage electric discharge is produced by a high-voltage pulse generator, such as a thyratron, that is triggered by low-voltage pulses from a conventional electronic pulse generator. In this invention, such a pulse generator serves as a master pulse generator. Using the above example, each pulse from the generator is split into five pulses. Of these, pulse no. 1 triggers a MOPA unit directly; pulse no. 2 passes through an electronic delay unit that delays its arrival by 100 microseconds and triggers the discharge of a MOPA system no. 2; pulse no. 3 passes through an electronic delay unit that delays its arrival by 200 microseconds and triggers the discharge of a MOPA system no. 3; and so on, until pulse no. 5 which passes through an electronic delay unit that delays its arrival by 400 microseconds and triggers the discharge of a MOPA system no. 5. The pulses emerging from the five MOPA units, considered together, then effectively provide a train of excimer laser pulses arriving every 100 microseconds, i.e., at the rate of 10 kHz. As will be immediately clear, this configuration can be varied in many different ways to provide an excimer laser source system with many different desirable pulse repetition schemes. 
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention is shown in  FIG. 1 . The system illustrated in  FIG. 1  is an Argon Fluoride excimer laser system that produces laser pulses of 193 nm wavelength at a high repetition rate, for example, 25 kHz. The system can similarly be a KrF or other excimer laser. It comprises multiple sets (for example, four) of master-oscillator-power-amplifier (MOPA) units ( 100 ,  200 ,  300 ,  400 ). Each MOPA unit (prior art) comprises a master-oscillator (MO) sub-unit and a power-amplifier (PA) sub-unit. For example, MOPA  1 , denoted as  100 , comprises an MO  101  and a PA  102 . Similarly, MOPA  2  ( 200 ) comprises an MO  201  and a PA  202 , etc. Each of the MOs is an excimer laser having an optical resonator. For example, MO  101  is an excimer laser comprising active lasing medium  103 , electrodes  104  and  105 , and resonator mirrors  106  and  107 . 
     Each MO produces an excimer laser “seed” beam with narrow spectral bandwidth and low pulse energy. For example, MO  101  produces an excimer laser seed beam  120  with a spectral bandwidth of a fraction of a picometer (pm) and a pulse energy of a few microjoules (μJ). The resonator mirror  106  is a high-reflectivity mirror with a flat or concave surface. The mirror  107  has a lower reflectivity than that of mirror  106 , and may have a flat or concave surface. The active medium  103  is a mixture of various gases, such as argon, fluorine, and a buffer gas, such as helium. The electrodes  104  and  105  produce a high-voltage electric discharge in the active lasing medium  103 . The electrode  104  receives a high-voltage pulse  108  from a high-voltage pulse generator  109 . Electrode  105  is typically grounded. Pulse  108  may be, for example, a 30 kilovolt (kV) pulse and may have a pulsewidth on the order of a microsecond (μs). The high-voltage pulse generator  109  comprises, for example, a thyratron, and is triggered by a trigger  110  from a typical low-voltage electronic pulse generator  10 . The pulse rate provided by the pulse generator  10  may be, for example, 6.25 kHz, which then will also be the repetition rate at which laser pulses are emitted by MO  101  as seed beam  120 . 
     The power amplifier  102  comprises active lasing medium  111 , electrodes  112  and  113 , and transmissive windows  114  and  115  which have flat surfaces. The active medium  111  is a mixture of various gases, such as argon, fluorine, and a buffer gas, such as helium. The electrodes  112  and  113  produce a high-voltage electric discharge in the active lasing medium  111 . The electrode  112  receives a high-voltage pulse  116  from a high-voltage pulse generator  117 . Electrode  113  is typically grounded. Pulse  116  may be, for example, a 30 kV pulse and may have a pulsewidth on the order of a μs. The high-voltage pulse generator  117  comprises, for example, a thyratron, and is triggered by a trigger  110  from a typical low-voltage electronic pulse generator  10  in synchronism with the triggering of the pulse generator  109 . The excimer laser seed beam  120  emitted by MO  101  is amplified by PA  102 , resulting in the final laser output beam from MOPA  100 , denoted as beam  121 . The spectral bandwidth of beam  121  is substantially the same as the spectral bandwidth of beam  120  and may be, for example, a fraction of a picometer. The energy of each pulse in beam  121  is significantly greater than the energy of each pulse in beam  120  and may be, for example, a few millijoules (mJ). 
     In an alternate configuration, the master oscillator (MO  101 ) may be an ultraviolet laser source other than an excimer laser. For example, a pulsed rare gas ion laser, or a pulsed tunable dye laser, or a pulsed tunable solid-state laser may be frequency-multiplied in a nonlinear optical medium to generate narrow-bandwidth ultraviolet laser radiation at the same wavelength as an excimer laser and may thus serve as the seed laser for the power amplifier (PA  102 ) which is an excimer laser. 
     MOPA  200  is nearly identical (but not entirely) to MOPA  100  and comprises MO  201  and PA  202  which are triggered by, respectively, pulses  208  and  216  from high-voltage pulse generators  209  and  217 , both of which in turn are triggered synchronously by pulse  210 . The difference between MOPA  100  and MOPA  200  is that the low-voltage trigger pulse  210  is not synchronous with trigger pulse  110 . Rather, trigger pulse  210  is delayed after  110  by a certain delay interval T 2 . The pulse  210  is produced by splitting the output signal  150  from the low-voltage pulse generator  10  into as many signals as there are MOPA units. For the example of  FIG. 1 , the output signal  150  is split into four signals,  110 ,  250 ,  350 , and  450 . As already described in the preceding paragraphs, the signal  110  triggers the high-voltage pulse generators  109  and  117  of MOPA  100 . The signal  250  goes through an electronic delay unit  260  which produces the pulse  210  that is delayed after pulse  110  by a time interval T 2 . Since pulse T 2  triggers the high-voltage pulse generators  209  and  217 , pulses in the output beam  221  of MOPA  200  are delayed from pulses in the output beam  121  of MOPA  100  by the same time interval T 2 . We remark that electronic delay units such as  260 ,  360 , and  460  are readily available as electronic instruments that are commonly used. 
     In a like manner, as illustrated in  FIG. 1 , the signal  350  goes through an electronic delay unit  360  which produces the pulse  310  that is delayed after pulse  110  by a time interval T 3 . Since pulse T 3  triggers the high-voltage pulse generators  309  and  317 , pulses in the output beam  321  of MOPA  300  are delayed from pulses in the output beam  121  of MOPA  100  by the same time interval T 3 . Similarly, the signal  450  goes through an electronic delay unit  460  which produces the pulse  410  that is delayed after pulse  110  by a time interval T 4 . Since pulse T 4  triggers the high-voltage pulse generators  409  and  417 , pulses in the output beam  421  of MOPA  400  are delayed from pulses in the output beam  121  of MOPA  100  by the same time interval T 4 . The output beams  121 ,  221 ,  321 , and  421  from MOPA units  100 ,  200 ,  300 , and  400 , respectively, and their relative arrival times and delays are further illustrated in  FIG. 5 . Note that each of the delays T 2 , T 3 , and T 4  is a fraction of the interval T between successive pulses of a MOPA, and T 4 &gt;T 3 &gt;T 2 . 
     As an example, illustrated in  FIG. 6 , the pulse repetition rate of the pulse generator  10  may be 6.25 kHz, which provides an interval of 160 μs between successive pulses. Thus, pulses  110  arrive at the rate of 6.25 kHz and so do the output pulses  121  from MOPA  100  which therefore also have an interval of 160 μs between successive pulses. For this example, the delay intervals T 2 , T 3 , and T 4  may be 40, 80, and 120 μs, respectively. Therefore, the trigger pulses  210 ,  310 , and  410 , while having the same repetition rate of 6.25 kHz as trigger  110 , arrive with delays of 40, 80, and 120 μs, respectively, after triggers  110 . As a result, the laser pulses  221 ,  321 , and  421  from MOPAs  200 ,  300 , and  400  also arrive with delays of 40, 80, and 120 μs, respectively, after the laser pulses  121  from MOPA  100 . Therefore, when the output laser pulses from all four MOPAs are considered in combination, they effectively provide a train of laser pulses that arrive every 40 μs. The combined system is therefore equivalent to a pulsed laser source that has a repetition rate of 1/40 μs=25 kHz. 
     The embodiment illustrated in  FIG. 6  is a specific example in which the pulse repetition rate of the composite laser system (25 kHz) is a multiple (four in this case) of the pulse repetition rate of each of the individual laser units (6.25 kHz). Thus, the delays produced by electronic delay units  260 ,  360 , and  460  (40, 80, and 120 μs, respectively) cause the pulses of the combined MOPA units to be equally spaced with a time interval of 40 μs between successive pulses. Note that in the illustration of  FIG. 5 , the delays produced by electronic delay units  260 ,  360 , and  460  have been denoted as generalized quantities T 2 , T 3 , and T 4 , respectively. In a different embodiment, the delays T 2 , T 3 , and T 4  can be made variable, which will enable MOPA units  200 ,  300 , and  400  to emit their laser pulses at any desired time instants. Such an embodiment is illustrated in  FIG. 7 , in which the variable delays for the arrival times of the pulses from MOPA units  200 ,  300 , and  400  with respect to the arrival times of the pulses from MOPA unit  100  are denoted as T b , T c , and T d , respectively. Note that T b &lt;T c &lt;T d &lt;T. 
     In another embodiment, the timings of all the laser pulses are controlled dynamically by signals that are related to the pattern locations on the semiconductor wafer that are intended to be exposed by the laser pulses. More specifically, as illustrated in  FIG. 8 , the pattern information  550 , which is in the form of a bit map stored in a data file  500 , is converted by a control computer  600  into timing signals  610 ,  620 ,  630 , and  640  using a suitable algorithm. These timing signals are sent to the pulse generator  10  and the variable delay units  260 ,  360 , and  460 . The result is that the laser pulses  121 ,  221 ,  321 , and  421  from MOPA units  100 ,  200 ,  300 , and  400 , respectively, arrive at the spatial light modulator array which directs them to the semiconductor wafer at timing instants that are determined for optimized exposure of the desired pattern. 
     In another embodiment, shown in  FIG. 9 , the control computer  600  may provide the desired trigger signals directly (i.e., without the need for separate delay units  260 ,  360 , and  460 ) with the proper time delays between them. These trigger signals  615 ,  625 ,  635 , and  645 , may be sent to the MOPA units  100 ,  200 ,  300 , and  400 , respectively, to trigger their firing which results in output pulses  121 ,  221 ,  321 , and  421 . 
     An embodiment illustrating the implementation of the high-repetition-rate laser source in a maskless lithography system is shown in  FIG. 10 . The high-repetition-rate laser source, as discussed with reference to  FIGS. 4-9 , is denotes as  700  in  FIG. 10 . As described in the preceding paragraph, the pattern information  550  is sent from data file  500  to control computer  600 , which converts the bit-map-format pattern information into timing signals  610 ,  620 ,  630 , and  640 , which are sent to the provides high-repetition-rate laser source  700 , which provides laser pulses  121 ,  221 ,  321 , and  421 , which are sent to the spatial light modulator array  800  through illumination optics  750 . The SLM, receiving signals  650  from control computer  600 , directs light rays  810  into projection lens  850 , which directs them onto semiconductor wafer  900  mounted on scanning stage  950 . The control computer  600 , in addition to processing the pattern information for providing signals to laser source  700  and SLM array  800 , also optimally controls projection lens  850  and scanning stage  950  for their desired operation. 
     The above embodiments are just a few examples to illustrate the disclosed invention. Numerous other variations that fall within the scope of the invention are possible and will be evident to those skilled in arts of semiconductor manufacturing, lithography, signal processing, and related fields.