Abstract:
A Q-switched solid state laser system ( 24 ) operates in a pulse processing control system ( 10 ) that employs an autopulse mode and a pulse-on-position mode to stabilize the pulse-to-pulse laser energy delivered to a workpiece ( 20 ) that is moved by an X-Y positioner ( 18 ). In the autopulse mode, laser pulses are emitted at a near maximum PRF, but the pulses are blocked from reaching the workpiece by an external modulator ( 28, 32 ), such as an acousto-optic modulator (“AOM”) or electro-optic modulator (also referred to as a Pockels cell). In the pulse-on-position mode, the laser emits a pulse in response to the positioner moving to a location on the workpiece that coincides with a commanded coordinate location. The processing control system delivers a stream of coordinate locations, some requiring processing, at a rate that moves the positioner and triggers the laser at about the near maximum PRF. The pulse processing control system sets the AOM to a transmissive state whenever a coordinate location requiring processing is commanded and otherwise sets the AOM to a blocking state. This pulse timing technique provides a nearly constant interpulse period for the laser, thereby improving its pulse-to-pulse energy stability at the near maximum PRF.

Description:
TECHNICAL FIELD 
     This invention relates to lasers and more particularly to a method and an apparatus for providing uniform energy laser pulses at a high pulse repetition frequency (“PRF”) in on-the-fly specimen processing applications. 
     BACKGROUND OF THE INVENTION 
     Lasers are typically employed in a variety of industrial operations including inspecting, processing, and micro-machining substrates, such as electronic materials. For example, to repair a dynamic random access memory (“DRAM”), a first laser pulse is used to remove an electrically conductive link to a faulty memory cell of a DRAM device, and then a second laser pulse is used to remove a resistive link to a redundant memory cell to replace the faulty memory cell. Because faulty memory cells needing link removals are randomly located, workpiece positioning delay times typically require that such laser repair processes be performed over a wide range of PRFs, rather than at a constant PRF. This production technique is referred to in the industry as on-the-fly (“OTF”) link processing and allows for greater efficiency in the rate at which links on a given wafer can be repaired, thereby improving the efficiency of the entire DRAM production process. 
     However, it is well known that the laser energy per pulse typically decreases with increasing PRF, a characteristic that is particularly true for Q-switched, solid-state lasers. This energy per pulse roll-off limits the upper PRF range for many laser memory repair processes. Moreover, memories and other electronic components are manufactured with various processes each requiring processing by a particular range of pulse energies, often referred to as a “process window.” For many memory devices, the “process window” requires that laser pulse energy vary by less than 5% from a selected pulse energy value. 
     Prior workers have taken various approaches for ensuring operation within a process window or for opening up the process window. For example, U.S. Pat. No. 5,590,141 for METHOD AND APPARATUS FOR GENERATING AND EMPLOYING A HIGH DENSITY OF EXCITED IONS IN A LASANT, which is assigned to the assignee of this application, describes solid-state lasers having lasants exhibiting a reduced pulse energy drop off as a function of PRF and, therefore, a higher usable PRF. Such lasers are, therefore, capable of generating more stable pulse energy levels when operated below their maximum PRF. 
     It is also known that laser processing applications typically employ positioners to rapidly move target locations on a workpiece through a sequence of programmed processing positions. The movements of the positioner and the laser pulse timing are asynchronous, requiring lasers in such applications to operate in an OTF mode having an inherently wide range of PRFs. The resulting wide range of interpulse periods causes corresponding pulse to pulse energy variations and indefinite pulse firing timing, which leads to inaccurate laser pulse positioning on a workpiece. 
     Accordingly, U.S. Pat. No. 5,453,594 for RADIATION BEAM POSITION AND EMISSION COORDINATION SYSTEM, which is assigned to the assignee of this application, describes a technique for synchronizing a clock that controls the positioner with a variable clock that controls OTF laser pulse emission. The synchronized clocks allow the laser to emit pulses in synchronism with positioner movements across target locations on the workpiece, thereby improving laser pulse positioning accuracy. 
     The above-described laser processing applications typically employ infrared (“IR”) lasers having 1,047 nanometer (“nm”) or 1,064 nm fundamental wavelengths. Applicants have discovered that many laser processing applications are improved by employing ultraviolet (“UV”) energy wavelengths, which are typically less than about 500 nm. Such UV wavelengths may be generated by subjecting an IR laser to a harmonic generation process that stimulates the second, third, or fourth harmonics of the IR laser. Unfortunately, the pulse to pulse energy levels of such UV lasers are particularly sensitive to PRF and interpulse period variations. 
     What is needed, therefore, is an apparatus and a method for generating stable UV laser processing pulse energies at a high PRF in high-accuracy OTF laser processing applications. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide an apparatus and a method for generating stable laser processing pulse energy at a high PRF in high-accuracy OTF laser processing applications. 
     Another object of this invention is to satisfy the above object for OTF. specimen processing applications employing UV wavelengths. 
     A Q-switched solid state laser operates in cooperation with a pulse processing control system that employs an autopulse mode and a pulse-on-position mode to stabilize the pulse-to-pulse laser energy delivered to target locations on a workpiece that is moved by a positioner. In the autopulse mode, laser pulses are emitted at a near maximum PRF, but the pulses are blocked from reaching the workpiece by an external modulator, such as an acousto-optic modulator (“AOM”) or electro-optic modulator (also referred to as a Pockels cell). In the pulse-on-position mode, the laser emits a pulse each time the positioner moves a workpiece location through coordinates that coincide with a commanded laser beam coordinate. The processing control system moves the positioner at a near constant velocity that causes triggering of the laser at about the maximum PRF in response to the workpiece passing through a regularly spaced apart set of commanded laser beam coordinates. The pulse processing control system sets the AOM to a transmissive state whenever a location to be processed is commanded and sets the AOM to a blocking state whenever a location not to be processed is commanded. This pulse timing technique provides a nearly constant interpulse period for the laser, thereby stabilizing its pulse-to-pulse energy level at the near maximum PRF. 
     This invention is advantageous for generating stable pulse-to-pulse laser pulse energy when processing features that ordinarily require near random interpulse periods. This invention is also advantageous for stabilizing the pulse-to-pulse energy of a Q-switched solid state laser generating frequency-doubled, -tripled, or quadrupled laser pulses by employing a nonlinear harmonic generation process. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing a workpiece positioner and a laser processing control system interconnected according to this invention. 
     FIG. 2 is a process flow diagram showing the functional steps carried out by this invention to provide stable pulse-to-pulse UV laser energy in an autopulse mode and a pulse-on-position processing mode. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a laser pulse processing control system  10  of this invention in which a system control computer  12  and an embedded control computer  14  coact to control a beam position controller  16  that receives position information from an X-Y positioner  18  that positions a workpiece  20  relative to a UV laser beam  22 . UV laser beam  22  may propagate through various optical elements (not shown) in addition to the fold mirror that is shown. X-Y positioner  18  may also include a Z positioner  23  that may be coupled to either the X or Y stage. X-Y positioner  18  is preferably based on a positioning system described in U.S. Pat. No. 5,751,585 for HIGH SPEED, HIGH ACCURACY MULTI-STAGE TOOL POSITIONING SYSTEM, which is assigned to the assignee of this application. A commercial example of a laser pulse processing control system suitable for use with this invention is the Model 9300 Memory Yield Improvement System manufactured by Electro Scientific Industries, Inc. of Portland, Oreg., the assignee of this application. 
     A UV laser system  24  preferably includes a Q-switched solid state IR laser  26 , such as a diode-pumped, acousto-optically Q-switched Nd:YV 0   4  laser; an AOM  28  for modulating the pulse amplitude of IR laser  26 ; and a frequency multiplier  30  for converting the infrared wavelength emissions from IR laser  26  into green and/or UV wavelengths by employing well-known second, third, or fourth harmonic conversion processes. AOM  28  may be alternatively positioned after frequency multiplier  30  as indicated by the position of an AOM  32  shown in phantom lines. In either embodiment, a laser controller  34  controls the transmissivity of AOM  28  or  32  to transmit or block UV laser beam  22  directed toward workpiece  20 . 
     System control computer  12  conveys across a bus  36  into embedded control computer  14  position coordinates of workpiece  20  locations requiring laser processing. In a typical specimen processing application, workpiece  20  includes regularly spaced apart device structures, such as fusible links, only some of which require processing. The locations requiring processing are referred to as target locations, and the locations not requiring processing are referred to as intermediate locations. Embedded control computer  14  adds to the target location coordinates intermediate location coordinates that are spaced apart to trigger IR laser  26  at nearly equal time intervals. Embedded control computer  14  conveys the target and intermediate position coordinates one at a time at a predetermined rate across a bus  38  to registers  40  in beam position controller  16  and simultaneously loads control data across a bus  42  to registers  44  in laser controller  34 . The predetermined rate controls the movement velocity of X-Y controller  18 , and the control data indicate whether the coordinate location is a target location to be processed and further includes mode and timing information. 
     Laser controller  34  operates timers  46  in either an autopulse mode or a pulse-on-position mode. In autopulse mode, timers  46  start in response to the control data in registers  44 ; and, in the pulse-on-position mode, timers  46  start in response to receiving a position coincidence signal  48  from a comparator  50  in beam position controller  16 . Position encoders  52  in beam position controller  16  indicate to comparator  50  the current position of X-Y positioner  18 , and when the current position matches the position coordinates stored in registers  40 , position coincidence signal  48  is generated indicating that workpiece  20  is properly positioned over a target position or an intermediate position. Accordingly, if workpiece  20  has been positioned over a target position, timers  46  simultaneously operate the Q-switch in IR laser  26  and set AOM  28  to a transmissive state until a cycle done interrupt  54  is conveyed from timers  46  to embedded control computer  14 . The transmissivity of AOM  28  is preferably controllable as either a laser pulse gating device or as a pulse amplitude modulator. 
     The pulse-to-pulse energy level stability of UV laser system  24  directly depends on the pulse-to-pulse energy level stability of IR laser  26 . To meet this requirement, the interpulse period between emitted laser pulses is made substantially equal. This reduces thermal and radiant effects that would otherwise be caused by laser duty cycle variations arising from firing IR laser  26  at varying interpulse periods. Such thermal and radiant effects can include changes to the refractive indices of the nonlinear crystals, thereby modifying the phase-matching conditions for harmonic generation, which causes large variations in the harmonic output energy. Such thermal and radiant effects can also cause the energy per pulse of IR laser  26  to vary which will then cause the output of UV laser system  24  to fluctuate. 
     This invention provides nearly equal interpulse timing periods for IR laser  26  in either the autopulse or pulse-on-position modes. During the autopulse mode, AOM  28  prevents UV laser beam  22  from processing workpiece  20 . During the pulse-on-position mode, position coincidence signal  48  and the target location data in registers  44  determine the transmissive state of AOM  28  and, therefore, which UV laser beam  22  pulses process workpiece  20 . 
     Further referring to FIG. 2, a workpiece process  56  carried out by this invention provides stable pulse-to-pulse UV laser system  24  processing energy to target locations on workpiece  20 . 
     A start block  58  represents the start of workpiece process  56 . 
     An autopulse mode block  60  represents embedded control computer  14  setting control system  10  to a default autopulse mode in which timers  46  trigger IR laser  26  at a user programmable PRF. 
     In autopulse mode, logic in timers  46  set a pulse block signal line  62  true to gate AOM  28  off, thereby preventing a usable amount of energy emitted by UV laser system  24  from reaching workpiece  20 . 
     When control system  10  prepares to initiate a pulse-on-position processing run, a coordinate receiving block  64  represents embedded control computer  14  receiving from system control computer  12  target location coordinates on workpiece  20  requiring laser processing. 
     An intermediate coordinate computing block  66  represents embedded control computer  14  adding to the target location coordinates intermediate location coordinates not requiring laser processing. The intermediate location coordinates are spaced apart to cause triggering of IR laser  26  at a uniform PRF. Embedded control computer  14  switches control system  10  from autopulse mode to a pulse-on-position mode. 
     A coordinate loading block  68  represents embedded control computer  14  conveying across bus  38  to registers  40  a location coordinate and conveying across bus  42  to laser controller  34  pulse-on-position mode enabling data. Pulse block signal line  62  continues to cause AOM  28  to block UV laser system  24  pulse energy from being transmitted to work piece  20 . 
     A last coordinate decision block  70  represents checking to determine whether the location coordinate is a run terminating location coordinate in the current OTF processing run. If it is, the process returns to autopulse mode block  60 . 
     Otherwise, the process continues to a positioner moving block  72  that represents moving beam positioner  18  in response to the location coordinate. 
     A position comparison decision block  74  represents comparing data in position encoders  52  to the location coordinate stored in registers  40 , and when the data and coordinate match, causing comparator  50  to issue position coincidence signal  48 . 
     A timer starting block  76  represents starting timers  46  in response to receiving position coincidence signal  48 . After a predetermined time period, timers  46  will drive a Q-switch gating line  78  to trigger a laser pulse from IR laser  26  for a predetermined time period (user programmable to meet process window requirements). 
     Before timers  46  time out, a pulse transmitting decision block  80  represents setting AOM  28  to a laser pulse blocking state  81  or a laser pulse transmitting state  82  in response to the states of position coincidence signal  48  and pulse block signal line  62 . 
     A laser triggering block  84  represents triggering IR laser  26  in response to the Q-switch gating line signal generated in response to timer starting block  76 . This timing sequence allows AOM  28  to settle to a programmed transmissivity level before Q-switch gating line  78  causes IR laser  26  to emit a pulse. 
     After emission of the pulse, a pulse blocking block  85  represents setting AOM  28  to a transmissivity level that blocks laser pulses. 
     An end of cycle block  86  represents generating cycle done interrupt  54  in response to Q-switch gating line  78  returning to the pulse inhibiting state. Cycle done interrupt  54  causes workpiece processing process  56  to return to coordinate loading block  68 , which represents causing embedded control computer  14  to load next coordinate location into registers  40 . 
     As indicated above, last coordinate decision block  70  represents checking to determine whether the location coordinate is the last location coordinate in the current OTF processing run. If it is, the OTF processing run is complete and workpiece processing process  56  returns to autopulse mode block  60 . Embedded control computer  14  returns laser controller  34  to the autopulse mode, any further position coincidence signals  48  are ignored, and pulse block signal line  60  is set to inhibit UV laser system  24  pulses from reaching workpiece  20 . 
     In a typical OTF processing run, a linear row of workpiece locations is selectively processed in the pulse-on-position mode, after which control system  10  switches to autopulse mode, loads a next linear row of workpiece locations, returns to pulse-on-position mode, and repeats until the workpiece is completely processed. 
     Timers  46  are set up according to the following laser operating guidelines. A period τ p  represents a targeted interpulse period and therefore the maximum PRF at which laser pulses may be emitted by UV laser system  24  for delivery to workpiece  20 . The PRF value is user-selectable up to a maximum value that is typically limited by the time required by X-Y positioner  18  to move between target locations. A period τ represents the actual interpulse period between adjacent commanded pulses. Timers  46  produce values of τ that equal τ p  in autopulse mode and approximately equal τ p  when in pulse-on-position feedback mode. A period τ I  represents a transition interpulse period, which is the period between pulses while control system  10  is switching modes. Transition interpulse period, τ I  may have an arbitrary value between 0 and τ p  that causes IR laser  26  to emit a pulse having an indefinite pulse energy. Accordingly, during transition interpulse period τ I , pulse block signal line  62  is set true to block transmission of any pulses through AOM  28  to workpiece  20 . 
     In autopulse mode, IR laser  26  fires repetitively at the predetermined PRF because registers  44  set timers  46  to time out, fire IR laser  26 , reset, and start over; whereas during the pulse-on-position mode, whenever IR laser  26  fires, timers  46  are reset. 
     Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. For example, system control computer  12  and embedded control computer  14  may be combined into a single processor and beam positioner controller  16  and laser controller  34  may be implemented as some combination of hard-wired digital logic, programs executing in a signal processor, microprocessor, state machine, and analog circuitry. IR laser  26  may alternatively be a Nd:YLF laser having a 1,047 nm fundamental wavelength or a Nd:YAG laser having a 1,064 nm fundamental wavelength, and frequency multiplier  30  may accordingly generate the second, third, and fourth harmonics (524 nm, 349 nm, and 262 nm) of the Nd:YLF laser or the second, third, and fourth harmonics (532 nm, 355 nm, and 266 nm) of the Nd:YAG laser. Of course, the workpiece may be virtually any material or component requiring laser processing. Similarly, skilled workers will recognize that it may be possible that the workpiece  20  could be most effectively processed by an infrared wavelength, in which case output from IR laser  26  would be directed to workpiece  20  without frequency conversion to any of the above described harmonic wavelengths. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. Accordingly, it will be appreciated that this invention is also applicable to laser applications other than those found in processing electronic components. The scope of the present invention should, therefore, be determined only by the following claims.