US20050224469A1 - Efficient micro-machining apparatus and method employing multiple laser beams - Google Patents

Abstract

A laser beam switching system employs a laser coupled to a beam switching device that causes a laser beam to switch between first and second beam positioning heads such that while the first beam positioning head is directing the laser beam to process a workpiece target location, the second beam positioning head is moving to another target location and vice versa. A preferred beam switching device includes first and second AOMs positioned such that the laser beam passes through the AOMs without being deflected. When RF is applied to the first AOM, the laser beam is diffracted toward the first beam positioning head, and when RF is applied to the second AOM, the laser beam is diffracted toward the second beam positioning head.

Description

RELATED APPLICATION
• This is a continuation-in-part of U.S. patent application Ser. No. 10/611,798, filed Jun. 30, 2003.
FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
• Not applicable
TECHNICAL FIELD
• This invention relates to lasers and, more particularly, to a method and an apparatus for increasing workpiece machining throughput by alternately switching a single laser beam among two or more beam paths such that one of the beam paths is employed for machining one workpiece while another the beam path is positioned for machining another workpiece.
BACKGROUND OF THE INVENTION
• Lasers are widely employed in a variety of research, development, and industrial operations including inspecting, processing, and micromachining a variety of electronic materials and substrates. For example, to repair a dynamic random access memory (“DRAM”), laser pulses are used to sever electrically conductive links to disconnect faulty memory cells from a DRAM device and then to activate redundant memory cells to replace the faulty memory cells. Because faulty memory cells needing link removals are randomly located, the links that need to be severed are randomly located. Thus, during the laser link repairing process, the laser pulses are fired at random pulse intervals. In another words, the laser pulses are running at a wide variable range of pulse repetition frequencies (“PRFs”), rather than at a constant PRF. For industrial processes to achieve greater production throughput, the laser pulse is fired at the target link without stopping the laser beam scanning mechanism. This production technique is referred to in the industry as “on-the-fly” (“OTF”) link processing. Other common laser applications employ laser pulses that are fired only when they are needed at random times.
• However, the laser energy per pulse typically decreases with increasing PRF while laser pulse width increases with increasing PRF, characteristics that are particularly true for Q-switched, solid-state lasers. While many laser applications require randomly time-displaced laser pulses on the demand, these applications also require that the laser energy per pulse and the pulse width be kept substantially constant. For link processing on memory or other IC chips, inadequate laser energy will result in incomplete link severing, while excessive laser energy will cause unacceptable damage to the passivation structure or the silicon substrate. The acceptable range of laser pulse energies is often referred to as a “process window.” For many practical IC devices, the process window requires that laser pulse energy vary by less than 5 percent from a selected pulse energy value.
• Various approaches have been implemented to ensure operation within a process window or to expand 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 patent application, describes solid-state lasers having lasants exhibiting a reduced pulse energy drop off as a function of increasing PRF and, therefore, a higher usable PRF. Such lasers are, therefore, capable of generating more stable pulse energy levels when operated below their maximum PRFs.
• U.S. Pat. No. 5,265,114 for SYSTEM AND METHOD FOR SELECTIVELY LASER PROCESSING A TARGET STRUCTURE OF ONE OR MORE MATERIALS OF A MULTIMATERIAL, MULTILAYER DEVICE, which is also assigned to the assignee of this patent application, describes using a longer laser wavelength such as 1,320 nanometers (“nm”) to expand the link process window to permit a wider variation of the laser pulse energy during the process.
• U.S. Pat. No. 5,226,051 for LASER PUMP CONTROL FOR OUTPUT POWER STABILIZATION describes a technique of equalizing the laser pulse energy by controlling the electrical current of the pumping diodes. The technique works well in practical applications employing a laser PRF below about 25 KHz or 30 KHz.
• The above-described laser processing applications typically employ infrared (“IR”) lasers having wavelengths from 1,047 nm to 1,324 nm, running at PRFs not over about 25 to 30 KHz. However, production needs are demanding much higher throughput, so lasers should be capable of operating at PRFs much higher than about 25 KHz, such as 50 KHz to 60 KHz or higher. In addition, many laser processing applications are improved by employing ultraviolet (“UV”) energy wavelengths, which are typically less than about 400 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, due to the nature of the harmonic generation, the pulse-to-pulse energy levels of such UV lasers are particularly sensitive to time variations in PRF and laser pulse interval.
• U.S. Pat. No. 6,172,325 for LASER PROCESSING POWER OUTPUT STABILIZATION APPARATUS AND METHOD EMPLOYING PROCESSING POSITION FEEDBACK, which is also assigned to the assignee of this patent application, describes a technique of operating the laser at a constant high repetition rate in conjunction with a position feedback-controlled laser pulse picking or gating device to provide laser pulse picking on demand at a random time interval that is a multiple of the laser pulse interval. This technique affords good laser pulse energy stability and high throughput.
• A typical laser pulse picking or gating device is an acousto-optic modulator (“AOM”) or electro-optic modulator (“EOM”, also referred to as a Pockels cell). Typical EOM material such as KD*P or KDP suffers from relatively strong absorption at the UV wavelengths, which results in a lower damage threshold of the material at the wavelength used and local heating along the laser beam path within the device, causing changes of the half wave-plate voltage of the device. Another disadvantage of the EOM is its questionable ability to perform well at a repetition rate over 50 KHz.
• AOM material is, on the other hand, quite transparent to UV light of 250 nm up to IR light of 2,000 nm, which allows the AOM to perform well throughout typical laser wavelengths within the range. An AOM can also easily accommodate the desirable gating of pulses at a repetition rate of up to a few hundred KHz. One disadvantage of the AOM is its limited diffraction efficiency of about 75-90 percent.
• FIG. 1 shows a typicalprior art AOM10 driven by a radio frequency (“RF”)driver12 and employed for a laser pulse picking or gating application, andFIGS. 2A to2D (collectively,FIG. 2) show corresponding prior art timing graphs for incominglaser pulses14,AOM RF pulses15, andAOM output pulses16 and20.FIG. 2A shows constant repetitionrate laser pulses14a-14kthat are emitted by a laser (not shown) and propagated toAOM10.FIG. 2B demonstrates two exemplary schemes for applyingRF pulses15 toAOM10 to select which ones oflaser pulses14a-14k, occurring at corresponding time periods22a-22k, are propagated toward a target. In a first scheme, asingle RF pulse15cde(shown in dashed lines) is extended to covertime periods22c-22ecorresponding tolaser pulses14c,14d, and14e; and, in a second scheme, separatedRF pulses15c,15d, and15eare generated to individually cover therespective time periods22c,22d, and22eforlaser pulses14c,14d, and14e.FIGS. 2C and 2D show the respectivefirst order beam20 and zeroorder beam16 propagated fromAOM10, as determined by the presence or absence ofRF pulses15 applied toAOM10.
• Referring toFIGS. 1 and 2, AOM10 is driven byRF driver12. When noRF pulses15 are applied toAOM10, incominglaser pulses14 pass throughAOM10 substantially along their original beam path and exit asbeam16, typically referred to as the zeroorder beam16. WhenRF pulses15 are applied toAOM10, part of the energy of incominglaser pulses14 is diffracted from the path of the zeroorder beam16 to a path of afirst order beam20. AOM10 has a diffraction efficiency that is defined as the ratio of the laser energy infirst order beam20 to the laser energy in incominglaser pulses14. Eitherfirst order beam20 or zeroorder beam16 can be used as a working beam, depending on different application considerations. For simplicity,laser pulses14 entering AOM10 will hereafter be referred as “laser pulses” or “laser output,” and pulses delivered to the target, because they are picked by AOM10, will be referred to as “working laser pulses” or “working laser output.”
• Whenfirst order beam20 is used as the working beam, the energy of the working laser pulses can be dynamically controlled from 100 percent of its maximum value down to substantially zero, as the power of RF pulses15 changes from their maximum power to substantially zero, respectively. Because the practical limited diffraction efficiency of an AOM under an allowed maximum RF power load is about 75 percent to 90 percent, the maximum energy value of the working laser pulses is about 75 percent to 90 percent of the energy value inlaser pulses14. However, when zeroorder beam16 is used as the working beam, the energy of the working laser pulses can be dynamically controlled from 100 percent of the maximum energy inlaser pulses14 down to 15 percent to 20 percent of the maximum value, as the power of RF pulses15 changes from substantially zero to its maximum power, respectively. For memory link processing, for example, when no working laser pulse is demanded, no leakage of system laser pulse energy is permitted, i.e., the working laser pulse energy should be zero, so firstorder laser beam20 is preferably used as the working beam.
• With reference again toFIG. 2,RF pulses15 are applied toAOM10 at random time intervals and only when working laser pulses are demanded, in this case, at random integral multiples of the laser pulse interval. The random output of working laser pulses results in random variable thermal loading onAOM10. Variable thermal loading causes geometric distortion and temperature gradients inAOM10, which cause gradients in its refractive index. The consequences of thermal loading distort a laser beam passing throughAOM10, resulting in deteriorated laser beam quality and instability in the laser beam path or poor beam positioning accuracy. These distortions could be corrected to some degree if they could be kept constant. However, when the system laser pulses are demanded randomly, such as in laser link processing, these distortions will have the same random nature and cannot be practically corrected.
• Test results on an AOM device, such as a Model N23080-2-1.06-LTD, made by NEOS Technologies, Melbourne, Fla., showed that with only two watts of RF power, the laser beam pointing accuracy can deviate as much as one milliradian when the RF is applied on and off randomly to the AOM. This deviation is a few hundred times greater than the maximum deviation allowed for the typical memory link processing system. Laser beam quality distortion resulting from the random thermal loading on theAOM10 will also deteriorate the focusability of the laser beam, resulting in a larger laser beam spot size at the focusing point. For applications such as the memory link processing that require the laser beam spot size to be as small as possible, this distortion is very undesirable.
• What is needed, therefore, is an apparatus and a method for randomly picking working laser pulses from a high repetition rate laser pulse train without causing distortion of the laser beam and adversely affecting positioning accuracy caused by random thermal loading variation on the AOM. What is also needed is an apparatus and a method of generating working laser pulses having constant laser energy per pulse and constant pulse width on demand and/or on-the-fly at a high PRF and with high accuracy at different pulse time intervals for variety of laser applications such as laser link processing on memory chips. Moreover, what is needed is an efficient, high-throughput apparatus and method for utilizing the working laser pulses.
SUMMARY OF THE INVENTION
• An object of this invention is, therefore, to provide an apparatus and a method for picking laser pulses on demand from a high repetition rate pulsed laser.
• The following are several of the advantages of the invention. Embodiments of this invention perform such pulse picking with minimal thermal loading variation on the AOM to minimize distortion of the laser beam and positioning accuracy. They include an apparatus and a method for generating system on demand laser pulses having stable pulse energies and stable pulse widths at selected wavelengths from the UV to near IR and at high PRFs for high-accuracy laser processing applications, such as memory link severing. The embodiments of this invention provide an efficient, high-throughput apparatus and method for utilizing the working laser pulses.
• A workpiece processing system of this invention employs a laser coupled to a beam switching device that causes a laser beam or laser pulses to switch between first and second beam positioning heads such that when the first beam positioning head directs the laser beam to process a first workpiece, the second beam positioning head moves to a next target location on a second workpiece or a second set of locations on the first workpiece. When the first beam positioning head completes processing of the first workpiece and the second beam positioning head reaches its target position, the beam switching device causes the beam to switch to the second beam positioning head and then the second beam positioning head directs the laser beam to target locations on the second workpiece while the first beam positioning head moves to its next target position.
• An advantage of the present laser beam switching system is that the first and second workpieces receive almost the full power of the laser beam for processing. The total time utilization of the laser beam is increased by almost a factor of two, depending on the processing-to-moving time ratio. This greatly increases system throughput without significantly increasing system cost.
• A preferred beam switching device includes first and second AOMs that are positioned adjacent to each other so that the laser beam (or laser pulses) normally pass undeflected through the AOMs and terminate on a beam blocker. When RF energy is applied to the first AOM, about 90 percent of the laser beam is diffracted as a first laser beam and 10 percent remains as a residual laser beam that terminates in the beam blocker. Likewise, when RF energy is applied to the second AOM, about 90 percent of the laser beam is diffracted as a second laser beam and 10 percent remains as a residual laser beam that terminates in the beam blocker. In this embodiment, the laser generating the laser beam is constantly running at its desired pulse repetition rate.
• Employing the beam switching device is advantageous because constant operation of the laser eliminates thermal drifting of the laser output. Moreover, by operating the first and second AOMs with pulse picking methods of this invention, thermal loading variations in the AOMs will be minimized, thereby increasing laser beam positioning accuracy.
• Another advantage of employing the first and second AOMs as a beam switching device is that they can operate as a laser power control device, eliminating a need for a separate laser power controller in a typical laser-based workpiece processing system. Power control is possible because the response times of the AOMs are sufficiently fast for programming laser pulse amplitudes of the switched laser beam during processing of individual target locations on the workpieces. A typical laser processing application is blind via formation in etched-circuit boards, in which it is often necessary to reduce the laser pulse energy when the laser beam reaches the bottom of the via being formed.
• Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a simplified schematic view of a prior art AOM device and an RF driver, transmitting a zero order beam, a first order beam, or both of them.
• FIGS. 2A-2D are corresponding prior art timing graphs of, respectively, laser pulses, RF pulses, and first and zero order AOM output laser pulses.
• FIGS. 3A-3C are corresponding exemplary timing graphs of, respectively, laser outputs, RF pulses, and working laser outputs as employed in a preferred embodiment.
• FIGS. 4A-4C are alternative corresponding exemplary timing graphs of, respectively, laser outputs, RF pulses, and working laser outputs that demonstrate the use of the AOM for energy control of the working laser outputs.
• FIG. 5 is a simplified schematic block diagram of a laser beam switching system of this invention.
• FIG. 6 is a waveform timing diagram representing operational timing relationships among various components of the laser beam switching system ofFIG. 5.
• FIG. 7 is a simplified schematic block diagram representing a preferred dual AOM laser beam switching device for use with this invention.
• FIG. 8 is a waveform timing diagram representing operational timing relationships among various components of a laser beam switching system employing the dual AOM switching device ofFIG. 7.
• FIG. 9 is a simplified schematic block diagram of a typical workpiece processing system employing the laser beam switching device ofFIG. 7.
• FIG. 10 is a waveform timing diagram representing operational timing relationships among various components of the workpiece processing system ofFIG. 9.
• FIGS. 11A and 11B are simplified block diagrams representing workpiece processing systems of this invention employing a common optical processing path for multiple laser beams propagating from one and two laser sources, respectively.
• FIG. 12 is a simplified schematic block diagram representing an alternative workpiece processing system of this invention employing a fast EOM and a polarizing beam splitter to implement a laser beam switching device of this invention.
• FIG. 13 is a simplified pictorial block diagram representing an alternative laser beam switching system employing a fast steering mirror for switching a laser beam along alternate first and second pathways.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
• Thermal loading variations in AOMs, such asprior art AOM10, can be mitigated by employing pulse picking and laser power control methods shown with reference toFIGS. 3A-3C and4A4C, respectively.FIGS. 3A-3C (collectively,FIG. 3) show corresponding timing graphs of laser outputs24a-24k(collectively, laser outputs24),RF pulses38a-38k(collectively, RF pulses38) applied toprior art AOM10, and workinglaser outputs40a,40c,40d,40e, and40i(collectively, working laser outputs40). In particular,FIG. 3A shows laser outputs24a-24kthat are emitted by a laser (not shown) at a constant repetition rate and separated by substantially identicallaser output intervals41. In typical embodiments, the laser output repetition rate may range from about 1 KHz up to about 500 KHz. Exemplary laser output repetition rates range from about 25 KHz to greater than about 100 KHz. For link processing embodiments, each of working laser outputs40 preferably includes a single laser pulse having a multiple nanosecond pulse width. However, skilled persons will recognize that each of working laser outputs40 may include a burst of one or more laser pulses, such as disclosed in U.S. Pat. No. 6,574,250 for LASER SYSTEM AND METHOD FOR PROCESSING A MEMORY LINK WITH A BURST OF LASER PULSES HAVING ULTRASHORT PULSE WIDTHS, which is assigned to assignee of this patent application, -or bursts of one or more pulses having pulse widths ranging from about 10 picoseconds to about 1,000 picoseconds.
• FIG. 3B shows a preferred RF pulse picking scheme employingRF pulses38 having pulse durations, such as42aand42b(collectively, RF pulse durations42) separated byRF pulse intervals43a-43j(collectively, RF pulse intervals43) that are substantially regular or uniform to keep thermal loading variations onAOM10 within a preassigned operational tolerance. Such tolerance may be a specific thermal load window, but the preassigned tolerance may also or alternatively be windows of spot size or beam position accuracy. In one embodiment, the thermal loading variation is maintained within 5 percent and/or the beam pointing accuracy is maintained within 0.005 milliradian. In a preferred embodiment, at least one ofRF pulses38 is generated to correspond with each of laser outputs24.
• Whenever one of working laser outputs40 is demanded to impinge on a target such as an electrically conductive link, one ofRF pulses38 is applied toAOM10 in coincidence with one of laser outputs24 such that it is transmitted throughAOM10 and becomes the demanded one of working laser outputs40.
• InFIG. 3B, thecoincident RF pulses38 areRF pulses38a,38c,38d,38e, and38i.FIG. 3C shows the resulting corresponding workinglaser outputs40a,40c,40d,40e, and40i. When no working laser output is demanded to correspond with laser outputs24,RF pulses38 are applied toAOM10 in noncoincidence with corresponding ones of laser outputs24. InFIG. 3B, thenoncoincident RF pulses38 areRF pulses38b,38f,38g,38h,38j, and38k.FIG. 3C shows that no working laser outputs40 correspond withnoncoincident RF pulses38.
• Thenoncoincident RF pulses38 are preferably offset from the initiations of respective laser outputs24 by time offsets44 that are longer than about 0.5 microsecond. Skilled persons will appreciate that while time offsets44 are shown to follow laser outputs24, time offsets44 could alternatively precede laser outputs24 by a sufficient time to prevent targeting of laser working outputs40. Thus,RF pulse intervals43 surrounding one ofnoncoincident RF pulses38 may be shorter (such asRF pulse intervals43band43h) than the overall average RF pulse interval43 (such as43c,43d,43f,43g, and43j) or longer (such asRF pulse intervals43a,43e, and43i) than the averageRF pulse intervals43.
• With reference again toFIG. 3C,nonimpingement intervals46band46c between workinglaser outputs40cand40dand between workinglaser outputs40dand40e, respectively, are about the same as thelaser output interval41. Thenonimpingement intervals46aand46dbetween workinglaser outputs40aand40cand between workinglaser outputs40eand40i, respectively, are roughly integer multiples of thelaser output interval41.
• Skilled persons will appreciate that even though working laser outputs40 are preferablyfirst order beam20 for most applications, such as link processing, working laser outputs40 may be zeroorder beam16 where leakage is tolerable and higher working laser output power is desirable.
• In a preferred embodiment, the coincident andnoncoincident RF pulses38 not only employ about the same RF energy, which is the product of an RF power value and an RF duration, but also employ about the same RF power value and about the same RF duration.
• FIGS. 4A-4C (collectively,FIG. 4) show corresponding timing graphs of laser outputs24,RF pulses38 applied toAOM10, and working laser outputs40 that demonstrate howAOM10 can be additionally employed to control the output power of working laser outputs40.FIG. 4A is identical toFIG. 3A and is shown for convenience only.FIGS. 4B and 4C showRF pulses38′ and working laser outputs40′, with thecorresponding RF pulses38 and working laser outputs40 shown superimposed on them in dashed lines for convenience. The energy values of working laser outputs40′ are attenuated by applying less RF power toAOM10 forRF pulses38′ than forRF pulses38; however, theRF pulse durations42′ are increased forRF pulses38′ over theRF durations42 employed forRF pulses38 to maintain a substantially constant product of RF power value and RF duration in order to maintain substantially constant thermal loading onAOM10. This technique permits on-demand selection for a continuum of output powers between working laser outputs40 or40′ without substantial variance in thermal loading onAOM10. Skilled persons will appreciate that the RF power values andRF durations42 of thenoncoincident RF pulses38 can be kept as original or can be altered to be within a specified tolerance of the RF loading variation of thecoincident RF pulses38′.
• RF pulse duration42′ is preferably selected from about one microsecond to about one-half oflaser output interval41, more preferably shorter than 30 percent oflaser output interval41. For example, if the laser repetition rate is 50 KHz andlaser output interval41 is 20 microseconds,RF pulse duration42′ can be anywhere between one microsecond and ten microseconds. The minimumRF pulse duration42 or42′ is determined by the laser pulse jittering time and the response time ofAOM10. It is preferable to initiate corresponding ones ofRF pulses38 and38′ surrounding the midpoints of laser outputs24. Likewise, it is preferable forRF pulses38 and38′ to be delayed or offset about half of the minimum RF pulse duration from the initiation of corresponding laser outputs24.
• It will be appreciated that the RF power ofRF pulses38 applied toAOM10 can be adjusted to control the energy of working laser outputs40 and40′ to meet target processing needs, whileRF pulse durations42 and42′ ofRF pulses38 and38′ can be controlled accordingly to maintain a substantially constant RF energy or arithmetic product of the RF powers and durations ofRF pulses38 and38′.
• The above-described techniques for employing an AOM in a workpiece processing application address beam steering accuracy and process window requirements, but do not address workpiece processing throughput and efficiency concerns. Employing a single laser for workpiece processing is time-inefficient because significant time and laser power is wasted while moving the laser output and workpiece target location relative to one another. Using a laser beam for an application, such as etched-circuit board via formation, typically results in only 50 percent laser beam utilization time because of the time needed to move the beam between target locations. Beam splitting does not correct this low time utilization problem. Prior workers have employed multiple laser beams to improve processing throughput, but the additional cost and wasted laser power is still a concern.
• This invention provides apparatus and methods for improving the throughput and efficiency of a single laser workpiece processing system. In this invention, AOMs employing pulse picking techniques are used in combination with a laser beam switching, or multiplexing, technique to improve workpiece processing and efficiency.
• FIGS. 5 and 6 represent a laserbeam switching system50 and associated timing aspects of this invention in which a laser emitslaser pulses54 that are reflected by anoptional fold mirror56 to abeam switching device58.Beam switching device58 causeslaser pulses54 to switch between first and second beam positioning heads60 and62 such that when firstbeam positioning head60 is causinglaser pulses54 to process a target location on afirst workpiece64, secondbeam positioning head62 is moving to a target location on asecond workpiece66.Laser pulses54 are directed frombeam switching device58 tobeam positioning head62 by anoptional fold mirror68. When firstbeam positioning head60 completes processing ofworkpiece64, either an optional shutter (not shown), such as a Q-switch, turnslaser52 off, as shown inFIG. 6, orlaser pulses54 are dumped. to a beam blocker (not shown). When secondbeam positioning head62 reaches its target position,laser pulses54 are switched on by the shutter and secondbeam positioning head62 directslaser pulses54 to target locations onworkpiece66 while firstbeam positioning head60 moves to its next target position.FIG. 6 represents workpiece processing times as intervals P and positioner move times between target positions as intervals M.
• An advantage of laserbeam switching system50 is that first andsecond workpieces64 and66 alternately receive almost the full power oflaser pulses54 for processing. The total time utilization oflaser pulses54 is increased by almost a factor of two, depending on the processing-to-moving time ratio. This greatly increases system throughput without significantly increasing system cost.
• FIGS. 7 and 8 show a preferredbeam switching device70 and related timing relationships.Beam switching device70 includes first and second AOMs72 and74 positioned in optical series relation so that a laser beam orlaser pulses76 normally pass undeflected throughAOMs72 and74 and terminate aslaser beam76A on abeam blocker78. However, when afirst RF driver80 applies about 6 Watts of 85 MHz RF signal tofirst AOM72, about 90 percent oflaser beam76 is diffracted aslaser beam76B and 10 percent remains aslaser beam76A. Likewise, when asecond RF driver82 applies about 6 Watts of 85 MHz RF signal tosecond AOM74, about 90 percent oflaser beam76 is diffracted aslaser beam76C and 10 percent remains aslaser beam76A. In this embodiment, the laser generatinglaser beam76 is constantly running at its desired pulse repetition rate.
• When employingbeam switching device70, no shutter or Q-switch is needed if time intervals are required when switching betweenlaser beams76B and76C because it is necessary only to shut off the RF signals applied to both first and second AOMs72 and74, thereby dumping all oflaser beam76 onbeam blocker78.
• Beam switching device70 is advantageous because constant operation of the laser eliminates thermal drifting of the laser output. Moreover, by operating AOMs72 and74 with the pulse picking methods described with reference toFIGS. 3 and 4, thermal loading variations will be minimized, thereby increasing laser beam positioning accuracy. Each of first and second AOMs72 and74 is preferably a Model N30085, manufactured by NEOS Technologies, Inc., of Melbourne, Florida. The N30085 AOM has a specified 90 percent diffraction efficiency when driven with two Watts of 85 MHz RF power.
• Another advantage ofbeam switching device70 is that it can operate as a laser power control device, eliminating a need for a separate laser power controller in a typical laser-based workpiece processing system. Power control is possible because the response times ofAOMs72 and74 are sufficiently fast for programming laser pulse amplitudes oflaser beams76B and76C during processing of single target locations in workpieces. A typical laser processing application is blind via formation in etched-circuit boards, in which it is often necessary to reduce the laser pulse energy when the laser beam reaches the bottom of the via being formed.
• FIGS. 9 and 10 show, respectively, a typicalworkpiece processing system90 employingbeam switching device70 and related operational timing relationships. Alaser92 and avariable beam expander94 cooperate to producelaser beam76 that propagates throughbeam switching device70, which operates as described with reference toFIGS. 7 and 8 to producelaser beams76A,76B, and76C.Laser beam76A terminates inbeam blocker78.Laser beam76B is reflected by anoptional fold mirror96 and directed by afirst XY scanner98 to targetlocations1,2,3, and4 on afirst workpiece100. Likewise,laser beam76C is reflected by anoptional fold mirror102 and directed by asecond XY scanner104 to targetlocations1,2,3, and4 on asecond workpiece106. First andsecond XY scanners98 and104 are mounted on respective first and second X positioning stages108 and110, and first andsecond workpieces100 and106 are mounted on aY positioning stage112. Skilled workers will understand that the scanners and workpieces are mounted on a split axis configured positioner system but that planar and stacked configurations may alternatively be employed. Skilled workers will also understand that the target locations on the first and second workpieces may be on a common substrate and/or may not share corresponding target locations.
• FIG. 10 showslaser beam76B processing (drilling) target location1 onworkpiece100 whilesecond XY scanner104 is moving the position oflaser beam76C to target location1 onworkpiece106. Whenlaser beam76C is processing target location1 onworkpiece106,first XY scanner98 is moving the position oflaser beam76B to targetlocation2 onworkpiece100. This process continues fortarget locations2,3, and4 until processing of target location4 onworkpiece106 is complete, at which time first and second X positioning stages108 and110 andY positioning stage112 execute a long move to position first andsecond XY scanners98 and104 overtarget locations5,6,7, and8 ofrespective workpieces100 and106. The X and Y linear positioning stages operate in constant motion in cooperation with the XY scanners. Positioning systems suitable for use with this invention are 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 patent application.
• FIG. 11 A shows aworkpiece processing system120 of this invention that employs a common modular imagedoptics assembly122 andvariable beam expander94 for optically processing bothlaser beams76B and76C. In this embodiment,laser92 and an optional fixedbeam expander124 cooperate to producelaser beam76 that propagates throughbeam switching device70, which operates as described with reference toFIGS. 7 and 8 to producelaser beams76A,76B, and76C.Laser beams76B and76C propagate along separate propagation path portions. Afirst turn mirror126 directslaser beam76B through a half-wave plate128, which changes the polarization state oflaser beam76B by 90 degrees relative to the polarization state oflaser beam76C. The 90 degree phase-displacedlaser beam76B is directed by asecond turn mirror130 to apolarizing beam combiner132.Laser beam76C is directed by athird turn mirror134 topolarizing beam combiner132, which combines into a common propagation path portion the separate path portions along whichlaser beams76B and76C propagate.Laser beams76B and76C merge into acommon laser beam76D, which propagates along the common path portion through imagedoptics assembly122 and optionalvariable expander94 and into apolarizing beam splitter136. Secondpolarizing beam splitter136 separatescommon laser beam76D intolaser beams76B and76C.Laser beam76B is directed by afourth turn mirror138 into, for example,first XY scanner98; andlaser beam76C is directed into, for example,second XY scanner104.
• Beam expander124 sets the shape oflaser beams76B and76C in the form of a Gaussian spatial distribution of light energy.Imaged optics assembly122 shapes the Gaussian spatial distribution oflasers76B and76C to form output beams of uniform spatial distribution for delivery toXY scanners98 and104. A preferred imaged optics assembly is of a diffractive beam shaper type such as that described in U.S. Pat. No. 5,864,430.
• FIG. 11B shows an alternativeworkpiece processing system120′, in whichbeam switching device70 is removed andlaser beams76B and76C propagate fromseparate laser sources92band92c, respectively. The size oflaser beam76B is set by abeam expander124b, and the size oflaser beam76C is set by abeam expander124c. The use ofseparate laser sources92band92cfacilitates optical component configurations in which one or more of turn mirrors126,130, and134 can be eliminated, as shown inFIG. 11B.
• Each ofworkpiece processing systems120 and120′ is advantageous because only one set of expensive beam imaging optics is required. Moreover, forworkpiece processing system120, employingbeam switching device70 permits implementation with smaller optical components because switching is accomplished with a smaller beam width than that which would be found with downstream switching components.
• FIG. 12 shows another alternativeworkpiece processing system140 of this invention employing afast EOM142 and apolarizing beam splitter144 to implement switching of alaser beam146 between first and second XY beam scanning heads98 and104. Inworkpiece processing system140,laser92 emitslaser beam146, which propagates through and is optically processed by anoptics module148 and alaser power controller150.Laser beam146 exitslaser power controller150 and entersfast EOM142, which alternately polarizeslaser beam146 into respective unrotated-polarization and rotated-polarization laser beams146U and146R.Polarizing beam splitter144 receivesunrotated laser beam146U and directs it to aturn mirror152 to firstXY scanning head98.Polarizing beam splitter144 receives rotatedlaser beam146R and directs it to secondXY scanning head104.
• A disadvantage ofworkpiece processing system140 is that current practical EOMs are limited in laser pulse repetition rates and are unable to withstand high amounts of ultraviolet laser beam power. Another limitation is that dumping unneeded laser beam energy requires shuttering or turning offlaser92, such as by a Q-switch positioned inside the cavity oflaser92.
• On the other hand,workpiece processing system140 is advantageous because it is simpler than the dual AOMbeam switching device70 described with reference toFIG. 7 and has a high extinguishing ratio that allows practically all of the power inlaser beam146 to impinge on target locations aslaser beams146U and146R.
• FIG. 13 shows an alternative embodiment of a laserbeam switching system210 in which alaser212 emits alaser beam214 that is deflected by a fast steering mirror (“FSM”)216 along alternate first andsecond paths218 and220.FSM216 preferably employs a mirror having a deflection angle that is controlled by materials that translate voltages into angular displacements.FSM216 operates similar to a galvanometer driven rotating mirror but at angular speeds up to 10 times faster than galvanometers and over anangular deflection range222 of up to about 5 milliradians. Deflecting a typical laser beam diameter with such a limited angular deflection range requires apath length224 that is sufficiently long, preferably about one meter, to separate first andsecond beam paths218 and220 by asufficient distance226, preferably about 10 millimeters, for inserting between them an HR coatedright angle prism228 that further separates and directs first andsecond beams baths218 and220 for reflection by respective first and second turning mirrors230 and232 to associated laser beam scanning heads (not shown).Switching laser beam214 at a location where it is smallest in diameter, such as before any beam expander, would minimizepath length224 required to sufficiently separate first andsecond paths218 and220 where they are reflected byright angle prism228.
• FSM216 may be a two-axis device that could further provide switching oflaser beam214 to more than two positions. For example,laser beam214 could be directed to a beam blocker during long moves as described with reference toFIGS. 9 and 10 to maintain constant thermal conditions inlaser212 and minimize duty cycle related laser beam power stability problems.
• Laserbeam switching system210 allows implementing a single laser workpiece processing system having the same workpiece processing throughput as a two laser system, provided the move times are over 3 ms and the workpiece processing time and laser beam switching time are less than 1.0 ms.
• Laserbeam switching system210 is advantageous because the use of a single laser and associated optics reduces cost by 20 percent to 40 percent, depending on the type of laser required, as compared with a two laser system.
• Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. For example, galvonometer and rotating mirror devices may also be used as laser beam switching devices; IR, visible, and UV lasers may be employed; target locations may be on single or multiple workpieces; laser beam switching may be effected to more than two or three beam paths; multiple lasers may be employed and each of their respective laser outputs switched among multiple paths; AOMs may be switched by single or multiple RF sources; and the scanning heads employed may further include galvanometers, FSMs, and other than XY coordinate positioning techniques.
• It will be obvious to skilled workers that many other changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.

Claims (6)

1. A system configured to direct a laser beam selectively in multiple beam propagation directions in a coordinated manner to achieve high-speed processing of material in different regions of a target specimen, comprising:

a laser source emitting a laser beam that includes a series of laser pulses;

a beam switching device receiving the series of laser pulses and, in response to a beam switching signal, directing first and second groups of the laser beam pulses to propagate along respective first and second beam axes;

a first positioning mechanism responding to a first control signal to provide relative movement of the first beam axis and the target specimen to selectively position the first beam axis at different first target regions of the target specimen and to process material in the first target regions of the target specimen;

a second positioning mechanism responding to a second control signal to provide relative movement of the second beam axis and the target specimen to selectively position the second beam axis at different second target regions of the target specimen to process material in the second target regions of the target specimen;

a controller producing the beam switching signal and the first and second control signals to effect coordinated system operation in first and second operational sequences;

the first operational sequence including the beam switching device directing the first group of laser beam pulses for incidence on a selected one of the first target regions, the first positioning mechanism providing the relative movement to enable the first group of laser pulses to process material in the selected first target region, and, during the material processing by the first group of laser pulses, the second positioning mechanism providing the relative movement to position the second beam axis to a selected one of the second target regions; and

the second operational sequence including the beam switching device directing the second group of laser beam pulses for incidence on the selected second target region, the second positioning mechanism providing the relative movement to enable the second group of laser pulses to process material in the selected second target region, and, during the material processing by the second group of laser pulses, the first positioning mechanism providing the relative movement to position the first beam axis from the selected first target region to a next selected one of the first target regions.

2. A beam switching device that receives a laser beam and provides beam outputs that propagate selectively along different beam axes, comprising:

a controller producing a control drive signal in first and second states;

first and second optically associated acousto-optic modulators, the first acousto-optic modulator receiving an incoming laser beam, and the first and second acousto-optic modulators cooperating in response to the first and second states of the control device signal to produce respective first and second laser beam outputs propagating from the second acousto-optic modulator; and

the first laser beam output including a major component propagating along a first beam axis and a minor component propagating along a first minor component axis, and the second laser output including a major component propagating along a second beam axis that is angularly offset from the first beam axis and a minor component propagating along a second minor component axis that is substantially coincident to the first minor component axis.

3. The beam switching device ofclaim 2, further comprising a beam blocker positioned to terminate the minor components propagating along the first and second minor component axes.

4. The beam switching device ofclaim 2, in which:

the controller includes first and second RF drivers that are operationally associated with the respective first and second acousto-optic modulators; and

in the first state of the control drive signal, the first, RF driver causes the first acousto-optic modulator to pass the incoming laser beam as an undeflected beam incident on the second acousto-optic modulator and the second RF driver causes the second acousto-optic modulator to diffract the incident undeflected beam to form the major component to propagating along the first beam axis and the minor component propagating along the first minor component axis.

5. The beam switching device ofclaim 2, in which:

the controller includes first and second RF drivers that are operationally associated with the respective first and second acousto-optic modulators; and

in the second state of the control drive signal, the second RF driver causes the second acousto-optic modulator to pass incident light as an undeflected beam and the first RF driver causes the first acousto-optic modulator to diffract the incoming laser beam to form the major component propagating along the second beam axis and the minor component to propagating along the second minor component axis.

6. The beam switching device ofclaim 2, in which the first and second optically associated acousto-optic modulators are positioned in optical series.

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