Abstract:
A laser writing system includes a high reflector and an output coupler that define an oscillator cavity. A gain medium and a mode locking device are positioned in the oscillator cavity. A diode pump source produces a pump beam that is incident on the gain medium and an output beam is produced. A support holds a workpiece. Means are provided for directing the output beam across the workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application entitled “Method and Apparatus to Detect a Flaw in a Surface of an Article”, identified as 09/321,499, filed concurrent herewith; and U.S. patent application entitled “Quasi-Continuous Wave Lithography Apparatus and Method”, identified as 09/322,121, filed concurrent herewith, which applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to laser writing, and more particularly to a method and apparatus for laser writing using a diode pumped, mode-locked laser and/or laser/amplifier system. 
     2. Description of Related Art 
     The production of an integrated circuit normally begins with a photomask, which is a photographic negative of a layer of the circuit. The photomask for the first layer of the integrated circuit is projected onto a wafer of silicon which is coated with a photosensitive material. The latent image of the circuit pattern for the first layer is then developed, and the silicon uncovered in this process is appropriately treated to change its electrical characteristics. The steps are repeated for each circuit layer using an appropriate photomask. 
     Two masking techniques used are contact printing, in which the mask is in contact or in extremely close proximity to the photoresist layer, and projection printing, in which the mask is imaged onto the photoresist. Projection printing offers the advantage that the mask is out of contact with the photoresist, thereby avoiding the hazard of accidental abrasion of the photoresist coating or the mask. A disadvantage is that, generally speaking, increases in the resolution of the image lens are accompanied by reductions of the image field; that is, reductions in the mask area that can be imaged onto the wafer. 
     In the fabrication of semiconductor devices by photolithographic techniques, a semiconductor wafer is coated with a photoresist, and exposed to actinic light projected through a mask. Development and etching of the selectively exposed photoresist defines a pattern on the wafer surface which may be used for establishing diffusion areas, conductor patterns, and the like. Modern integrated circuit fabrication requires several printing steps to be performed successively, with each mask exposure being in precisely controlled registration with previously formed patterns. 
     Contact printing was later supplanted by one-to-one projection printing of the circuit onto the photoresist material. Reticles have also been used. These reticles are photomasks of one layer of an integrated circuit pattern enlarged ten times and produced on a glass plate. 
     Photomasks are produced by photographic reduction of computer generated artwork with the use of a raster-scanned beam. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to provide a method and apparatus for laser writing. 
     Another object of the invention is to provide a method and apparatus for laser writing using a diode pumped laser or laser/amplifier system. 
     Yet another object of the invention is to provide a method and apparatus for laser writing using a diode pumped, mode locked, laser or laser/amplifier system. 
     These and other objects of the invention are achieved in a laser writing system including a high reflector and an output coupler that difine an oscillator cavity. A gain medium and a mode locking device are positioned in the oscillator cavity. A diode pump source produces a pump beam that is incident on the gain medium and an output beam is produced. A support holds a workpiece. Means are provided for directing the output beam across the workpiece. 
     In another embodiment, the laser writing system includes a high reflector and an output coupler that define an oscillator cavity. A gain medium and a mode locking device are positioned in the oscillator cavity. A first amplifier is coupled to the oscillator cavity. A diode pump source produces a pump beam that is incident on the gain medium and an output beam is produced and amplified by the first amplifier. A support holds a workpiece. Means are provided for directing the amplified output beam across the workpiece. 
     In another embodiment, a method of laser writing provides a diode pumped laser system including an oscillator cavity, a gain medium and a mode locking device positioned in the oscillator cavity. An output beam is produced from the laser system. The output beam is directed to a workpiece surface. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a block diagram of a laser, laser/amplifier system useful with the present invention. 
     FIG. 2 is a block diagram of a laser writing system using the laser, laser/amplifier system of FIG.  1 . 
     FIG. 3 is a block diagram illustrating the use of a beam expander with the output beam generated from the laser, laser/amplifier system of FIG.  1 . 
     FIG. 4 is a block diagram of a laser writing system with the beam expander of FIG.  3 . 
     FIG. 5 illustrates an embodiment of the laser writing system of the present invention using orthogonal movement tables. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a laser writing system that includes a laser system. The laser system includes an oscillator system or an oscillator/amplifier system. The oscillator/amplifier system is similar to the oscillator system but includes one or more amplifiers. The oscillator and oscillator/amplifier systems can be coupled with second, third, fourth and fifth harmonic generators. A second harmonic generator can be used alone with the oscillator and oscillator/amplifier systems and in various combinations with third, fourth and fifth harmonic generators. Additionally, the harmonic generators can be coupled with an OPO. The OPO can be pumped by a fundamental beam from an oscillator or from the harmonic generators. An output of the OPO can be mixed with the harmonic generators to generate a variable wavelength source. 
     In one embodiment, the oscillator system includes an Nd:YVO 4  gain media and is mode locked by a multiple quantum well absorber. In a specific embodiment of this oscillator system, the oscillator is pumped by a single fiber-coupled diode bar that provides 13 watts of pump power incident on the Nd:YVO 4  gain media, and typically produces 5-6 watts of 5-15 picosecond pulses at 80 MHz repetition rate. 
     In another embodiment, an oscillator/amplifier system includes an Nd:YVO 4  gain media mode locked by a multiple quantum well absorber, a double pass amplifier and two single pass amplifiers. Each of the amplifiers has an Nd:YVO 4  gain media and is pumped by two fiber-coupled diode pump sources. This oscillator/amplifier system produces 25-30 watts of 5-15 picosecond pulses at 80 MHz repetition rate. 
     The oscillator and oscillator/amplifier systems can be mode locked with a multiple quantum well saturable absorber, a non-linear mirror mode locking method, a polarization coupled mode locking method or other mode locking techniques, including but not limited to use of an AO modulator. An example of a quantum well saturable absorber is disclosed in U.S. Pat. No. 5,627,854, incorporated herein by reference. An example of a non-linear mirror mode locking method is disclosed in U.S. Pat. No. 4,914,658, incorporated herein by reference. An example of a polarization coupled mode locking method is disclosed in Ser. No. 09/062,057, filed Apr. 17, 1998, assigned to the same assignee as this application and incorporated herein by reference. In order to producer shorter pulses and a single output beam the gain media is positioned adjacent to a fold mirror as described in U.S. Pat. No. 5,812,308, incorporated herein by reference. 
     A high power oscillator system with the performance of an oscillator/amplifier system is achieved by using multiple fiber-coupled diodes and either a non-linear mirror mode locking technique or a polarization coupled mode locking method. This high power oscillator system produces 10-20 watts of output power with 4-10 picosecond pulses at a repetition rate of 80-120 MHz. High repetition rates are desirable for applications where the laser system is used as a quasi-CW source. For some applications, 80 MHz repetition rate is sufficiency high to be consider to be quasi-CW. This repetition rate is achieved with an oscillator cavity length of 1.8 meters. When the cavity length is shorted to 0.4 meters the repetition rate increases to 350 MHz. 
     Referring now to FIG. 1, one embodiment of an oscillator system  10  has a resonator cavity  12  defined by a high reflector  14  and an output coupler  16 . A gain media  18  is positioned in resonator cavity  12 . Suitable gain media  18  include but are not limited to, Nd:YVO 4 , Nd:YAG, Nd:YLF, Nd:Glass, Ti:sapphire, Cr:YAG, Cr:Forsterite, Yb:YAG, Yb:glass and the like. A preferred gain media  18  is Nd:YVO 4 . A mode locking device  19  is positioned in oscillator cavity  12 . In the embodiment, oscillator system  10  is mode locked and pumped by a fiber-coupled bar  20  that produces 13 watts of power. Oscillator cavity  12  can produce 1 to 6 watts of power nominally at a 80 MHz repetition rate with pulse widths of 5 to 15 picoseconds. 
     Optionally included is one or more amplifiers, generally denoted as  23 . An output beam  22  from resonator cavity  12  can be amplified by a first amplifier  24 . A second amplifier  26  can be included. Additional amplifiers may also be included to increase power. Typically, amplifiers  24  and  26  have the same gain media used in resonator cavity  12 . Nd:YVO 4  is a suitable gain media material because it provides high gain in an amplifier. The higher gain of Nd:YVO 4  provides a simplified amplifier design requiring fewer passes through the gain media. Amplifiers  24  and  26  produce output beams  28  and  30  respectively. Amplifiers  24  and  26  can be single pass, double pass and four pass. A four pass amplifier is disclosed in U.S. Pat. No. 5,812,308, assigned to the same assignee as this application and incorporated herein by reference. Oscillator/amplifier system  10  using an oscillator, a double pass amplifier and two single pass amplifiers can provide 30 watts of average power. 
     Output beams  22 ,  28  or  30  can be incident on a harmonic generator generally denoted as  31  and can include a second harmonic generator  32 . An output  34  from second harmonic generator  32  can be incident on a third harmonic generator  36  to produce an output beam  40 . Output  34  can be incident on a fourth harmonic generator  42  to produce an output beam  44 . It will be appreciated that oscillator system  10  can include various combinations of harmonic generators  32 ,  36 ,  42  as well as a fifth harmonic generator or an OPO. Second harmonic generator  32  can use non-critically phase matched LBO, third harmonic generator  36  can employ type II LBO and fourth harmonic generator  42  can use type IBBO. 
     In a specific embodiment, oscillator system  10  includes oscillator cavity  12  with harmonic generation. Output beam  22  is incident on second harmonic generator  32 . In this specific embodiment, oscillator system  10  may also include third and fourth harmonic generators  36  and  42 . The output power of this oscillator system  10  is 5 watts at 1064 nm. A harmonic generation system produces 2 watts at 532 nm or 1 watt at 355 nm or 200 milliwatts at 266 nm. 
     In another specific embodiment, Nd:YVO 4  is the gain media of oscillator/amplifier system  10 , and 29 watts of 7 picosecond pulses at 1064 nm is produced. The harmonic generation system can generate 22 watts at 532 nm or 11 watts at 355 nm or 4.7 watts at 266 nm. 
     In another specific embodiment, oscillator/amplifier system  10  includes oscillator cavity  12 , a four pass amplifier  24  and second harmonic generator  32  to produce 2 watts at 532 nm. This oscillator/amplifier system can pump an OPO that utilizes non-critically phase matched LBO as described in Kafka, et al., J. Opt. Soc. Am. B 12, 2147-2157 (1995) incorporated herein by reference. 
     In another specific embodiment, oscillator/amplifier system  10  includes oscillator cavity  12 , a double pass amplifier  24  and three single pass amplifiers  26  that produces 42 watts of 7 picosecond pulses at 1064 nm. This oscillator/amplifier system can pump an OPO using non-critically phase-matched KTA and produce an output beam at 1535 nm. The output beam at 1535 nm can be mixed with a 1064 nm beam to provide 11.6 watts at 629 nm, as described in Nebel, et al., in  Conference on Lasers and Electro - Optics,  Vol. 6 of 1998 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1998) postdeadline paper CPD3. 
     40 watts fiber-coupled bars, commercially available from Opto-Power, Tucson, Ariz. can be used to increase the output power of oscillator or oscillator/amplifier systems  10 . The use of an Nd:YVO 4  gain media  18  with a doping level of less than 0.5% can also be used to increase the output power of oscillator or oscillator/amplifier systems  10 . The combination of the 40 watt fiber-coupled bars with the low doped Nd:YVO 4  gain media greatly increases the output power of oscillator and oscillator/amplifier systems  10 . Use of low doped Nd:YVO 4  gain media  18  can also reduce the sensitivity of oscillator cavity  12  to misalignment as well as improve the output beam quality from an amplifier  24  or  26 . The use of low doped Nd:YVO 4  gain media, a longer Nd:YVO 4  gain media as well as a larger pump volume in Nd:YVO 4  gain media is disclosed in commonly owned application Ser. No. 09/199,031, filed Nov. 24, 1998, incorporated herein by reference. 
     Referring now to FIG. 2, output beams  22 ,  28 ,  30 ,  34 ,  40  or  44  (hereafter collectively referred to as output beam  110 ) can be passed through an acousto-optic modulator  112 , which may be controlled by driver  114 , including but not limited to a radio frequency (RF) driver  114 . Hereafter, oscillator system and/or oscillator/amplifier system  10 , shall collectively be referred to as oscillator  10 . A controller  116  is coupled to driver  114 . Controller  116  may include a computer work station that permits the creation and design of different integrated circuits. Output beam  110  is directed to a scanning mirror  118 , which directs the beam into a spot  120  on a photoresist coated integrated circuit wafer  122 . In one embodiment, integrated circuit wafer  122  is mounted on a motorized X-Y axis movable table  124  that is also controlled by controller  116  and output beam  110  is passed through a lens  125  including but not limited to a microscope objective. Focussed laser spot  120  can be on the order of 0.7 microns. Both the motorized X-Y table  124  and output beam  110  are under computer control to expose the photoresist selectively according to information stored in controller  116 . An interconnect pattern of an integrated circuit is transferred from data stored in controller  116  directly to the photoresist layer of integrated circuit wafer  122 . Scanning mirror  118  improves the throughput of the system by enabling output beam  110  and focussed spot  120  to be moved rapidly along both the X and Y axis. Table  124  has a certain amount of inertia and this restricts the ability of table  124  to be moved rapidly along both the X and Y axis and increases the overall speed of production by orders of magnitude. Scanning mirror  118  is used to deflect output beam  110  in order to produce a selected size spot on the Y axis scan line. In one embodiment, scanning mirror  118  is motor driven and moved mechanically. 
     Output beam  110  can be switched on and off by acousto-optic modulator  112 . The modulated output beam is then deflected by scanning mirror  118 . This same type of scanning function can be provided by a resonant scanner commercially available from General Scanning of Watertown, Mass. 
     Referring now to FIGS. 3 and 4, the deflected output beam  110  then enters a beam expander  126 . In one embodiment, beam expander  126  includes at least two lenses  128  and  130 . Output beam  110  becomes an expanded beam  132 . Expanded beam  132  can be focussed with lens  125  according to the size, shape, degree of pattern accuracy and level of actinic light exposure. 
     In another embodiment illustrated in FIG. 5, expanded beam  132  is directed at a photoresist-coated workpiece  134  to effect its exposure. Relative motion between workpiece  134  and expanded output beam  132  is controlled to produce a desired pattern. The relative motion can be produced in a number of ways including but not limited to, (i) movement of oscillator  10 , (ii) use of a movable light reflector or refractor or (iii) movement of workpiece  134 . 
     In the embodiment illustrated in FIG. 5, a solid surface table  136  carries a mount  138  for oscillator  10 . Output beam  110  is directed towards workpiece  134  that is supported on table  136 . Three movement tables  140  can be provided for independent translational movement along orthogonal X, Y and Z axes. Two movement tables  142  may be provided for independent rotational movement about vertical and horizontal axes. A fixed lens  144  is used to focus or partially focus output beam  110  into an image on workpiece  134 , which is shown for purposes of illustration only, as spherical. 
     Exposure of the photoresist on workpiece  134  is achieved by appropriate computer controlled movement of workpiece  134  through movement of tables  140  and  142  to effect the desired pattern shape, resolution and level of exposure. A controller (not shown) governs the distance of workpiece  134  from lens  144 , the angle of incidence of output beam  110  on workpiece  134 . output beam  110  image size and intensity as well as its surface velocity. A beam image on workpiece  134  can be formed in front of or behind a focal point of lens  144 , depending on the size of the spot desired. In some applications lens  144  may be removed entirely. Compound lenses and systems of lenses of any geometry may be used for focussing or beam shaping. When required, attenuating filters and irises may also be used in various combinations. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.