Abstract:
A quasi-CW diode- or lamp-pumped, A-O Q-switched solid-state UV laser system ( 10 ) synchronizes timing of the quasi-CW pumping with movement of the positioning system ( 36 ) to reduce pumping while the positioning system ( 36 ) is moving from one target area ( 31 ) to the next target area ( 31 ) to form multiple vias in a substrate at a high throughput. Thus, the available UV power for via formation is higher even though the average pumping power to the laser medium ( 16 ), and thermal loading of the laser pumping diodes ( 14 ), remains the same as that currently available through conventional CW pumping with conventionally available laser pumping diodes ( 14 ). The quasi-CW pumping current profile can be further modified to realize a preferred UV pulse amplitude profile.

Description:
RELATED APPLICATIONS 
     This patent application derives priority from U.S. Provisional Patent Application No. 60/275,246, filed Mar. 12, 2001. 
    
    
     COPYRIGHT NOTICE 
     ©2001 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR § 1.71(d). 
     TECHNICAL FIELD 
     This invention relates to diode-pumped, solid-state lasers and, in particular, to quasi-CW diode-pumped UV laser systems and processing methods employing them, such as for forming vias in circuit boards. 
     BACKGROUND OF THE INVENTION 
     Different types of lasers systems have been employed to drill vias at point-to-point target areas on electronic devices or work pieces such as printed circuit boards (PCBs). The following discussion is presented herein only by way of example to diode-pumped, solid-state ultraviolet (UV) laser systems and work piece targets and should not be considered limiting to the scope of invention. 
     When an acousto-optically (A-O) Q-switched, continuous-wave (CW) diode-pumped (DP), solid-state (SS) laser system, such as Electro Scientific Industries, Inc.&#39;s (ESI) Model 5200 which includes a Light Wave Electronics&#39; (LWE) Model 210 laser, is employed to create vias, the pumping diode or diodes remain active continuously. Laser emission is prevented by closing the Q-switch whenever the positioning system is directed to a new target area on the work piece. After the positioning system is aligned to the new target area, the laser system delivers a laser output containing one or more laser pulses by opening the Q-switch at a predetermined repetition rate. 
     The LWE Model 210 employs two 20 Watt (W) CW-diodes for pumping and generates 3 W of UV output power at 10 kHz repetition rate. The CW pumping current to the diodes is limited by thermal loading of the diodes. If an application warrants greater UV output power, then either more diodes or diodes with higher current/power must be employed, such as two 30 W diode laser bars or four 20 W diode laser bars. About 8 W of UV output power can be expected from such designs. However, if higher pumping power is employed, thermal loading on the solid-state laser medium is increased. Thermally overloading the laser medium can permanently damage it or cause significant degradation of the laser beam quality and limit the power available. This limitation imposes a critical engineering challenge to the laser system design and manufacturing. 
     Other pumping schemes are, however, available for a laser design, such as pulse pumping and quasi-CW pumping. An electro-optically (E-O) Q-switched pulsed DPSS UV laser, such as early versions of Lambda Physics&#39; UV “Gator” Model, provide higher laser pulse power but at low pulse repetition rates. For each pumping pulse, only one UV laser pulse is generated. The pumping duration time is limited to a few hundred microseconds (μs) so the laser output pulse repetition rate is typically limited to below 2 kHz. This pumping scheme is not preferred for drilling vias because it adversely affects drilling throughput. 
     Traditional quasi-CW pumping resembles pulse pumping but exhibits longer pumping duration time at a lower peak pumping power. The pumping scheme can exhibit a pumping repetition rate of about 1-2 kHz, and the pumping duration time can be from a few hundred μs to a few milliseconds (ms), based on the repetition rate and the duty cycle of the diodes used. This pumping scheme allows pumping to a higher level than does CW pumping because the diode “rests” (and thermal loading reduces or stops) whenever the pumping is off. Therefore, the laser output power can be higher during the pumping time period compared to that of a comparable CW pumped laser. The laser output is controlled by regulating the current to the diode(s). The pumping repetition rate of this pumping scheme is, however, still a serious drawback. Typical applications for quasi-CW pumping include those that utilize a long laser pulse width and a modest peak power, such as laser bonding and welding. 
     A laser system that includes a pumping scheme that facilitates both higher power and a faster repetition rate to increase drilling throughput is therefore desirable. 
     SUMMARY OF THE INVENTION 
     Conventional UV laser via drilling systems employ a standard frequency conversion scheme to convert the laser&#39;s fundamental wavelength in the IR region to the UV. Such systems preferably employ high UV power and a high pulse repetition rate to achieve high throughput via formation, hence A-O, Q-switched DPSS laser systems have heretofore been preferred for drilling vias. 
     A commercially desirable system would prefer higher UV power for reducing the via drill time, or to make acceptable vias on some “hard to drill” materials, such as copper and FR4. Thus, a high UV output power (5 to 15 W) at a high pulse repetition rate (a few kHz to a few tens kHz) would be preferred. 
     Also to be commercially useful, via formation on PCBs, for example, demands a laser system to be capable of making 300 to 400 vias per second. Thus, the laser positioning system has to move to 300 to 400 new locations every second. Typically, it takes the laser system less than one ms to drill one via, but in some cases longer than one ms to move to a new location for a next via. Hence, the time for the laser being ON is actually less than the time the laser is being OFF, which makes the use of the laser quite inefficient. 
     The present invention provides a quasi-CW diode- or lamp-pumped, A-O, Q-switched solid-state UV laser that synchronizes the timing of the quasi-CW pumping to avoid or reduce pumping while the positioning system is moving from one target area to the next target area and to increase the pumping level beyond the CW pumped level while drilling vias. Thus, the available UV power for via formation is higher even though the average pumping power to the laser medium, and thermal loading of the pumping diodes, remains the same as for conventional CW pumping with conventionally available laser diodes. The quasi-CW pumping current profile can be further modified to realize a preferred UV pulse amplitude profile. 
     Such a quasi-CW diode- or lamp-pumped, A-O Q-switched, solid-state UV laser is new; the synchronization of the quasi-CW pumping with the beam scanning is new; and the usage of such a laser system for via formation is new. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram of one embodiment of a quasi-CW diode-pumped, A-O Q-switched laser with intra-cavity tripling frequency conversion. 
     FIG. 2A is a simplified graphical depiction of an exemplary waveform of quasi-CW pumping diode current. 
     FIG. 2B is a simplified graphical depiction of exemplary A-O Q-switched laser pulses superimposed on the quasi-pumping diode current shown in FIG.  2 A. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a simplified schematic diagram of a preferred embodiment of a quasi-CW, diode-pumped, A-O Q-switched, solid-state UV laser system  10  with synchronized targeting, pumping, and firing to form vias at a high throughput rate. With reference to FIG. 1, laser resonator  12  of laser system  10  is shown with diodes  14  pumping laser medium  16  from the side. Skilled persons will appreciate, however, that the resonator  12  can fold and that the pumping scheme can be “end pumping” or that laser system  10  could employ other possible well-known configurations. Exemplary diodes  14  include, but are not limited to, Models SDL-3200 series 100 W quasi-CW arrays and 960 W high-duty factor stacked arrays sold by SDL, Inc. of San Jose, Calif. Exemplary solid-state laser mediums ( 16 ) include laser mediums having YAG, YLF, and YVO 4  compositions. Between an IR-reflective mirror  18  and UV (third harmonic)-transmissive output coupler  20 , resonator  12  also includes, along its optic axis  22 , an acousto-optic (A-O) Q-switch  24 , a frequency doubler  26 , and a frequency tripler  28  for intra-cavity frequency conversion. Skilled persons will appreciate that frequency conversion can be accomplished externally to resonator  12 . 
     FIGS. 2A and 2B (collectively FIG. 2) are respective simplified graphical depictions of an exemplary waveform of quasi-CW-pumping diode current pulses or intervals  50   a ,  50   b , and  50   c  generically current intervals  50 ) and of exemplary A-O Q-switched laser pulses  60   a ,  60   b , and  60   c  (generically laser pulses  60 ) superimposed on the quasi-CW-pumping diode current waveform shown in FIG.  2 A. With reference to FIGS. 1 and 2, laser system operation is synchronized such that when the laser system  10  finishes making one via at a first target area  31  on work piece  30 , the central processing unit (CPU)  32  stops the diode pumping (turns the diode current level I a  to zero) or reduces the diode pumping to a pre-determined low current level I b  by controlling power supply  34 . Exemplary power supplies  34  include, but are not limited to, Model SDL-820, for 10-15 amp CW laser diode driver with typical 10 μs current transition time; Model SDL-830 for about 50 amp CW laser driver; or Model SDL-928, for about 150 amp peak quasi-CW laser diode array drivers. All sold by SDL, Inc. of San Jose, Calif. 
     Then, positioning system  36  moves the beam output position to a new target area  31 . The beam positioning system  36  preferably includes a translation stage positioner that employs at least two transverse stages permitting quick movement between target areas  31  on the same or different work pieces  30 . In a preferred embodiment, the translation stage positioner is a split-axis system where a Y stage moves work piece  30 , and an X stage moves a fast beam positioner and associated focusing lens(es). The Z dimension between the X stage and Y stage may also be adjustable. The positioning mirrors align the optical path  22  through any turns between laser resonator  12  and the fast beam positioner. The fast beam positioner may for example employ high resolution linear motors and/or a pair of galvanometer mirrors that can conduct unique or repetitive processing operations based on provided test or design data. The stages and positioner can be controlled and moved independently or coordinated to move together. 
     Beam positioning system  36  can employ conventional vision or beam to work alignment systems that work through an objective lens or off axis with a separate camera and that are well known to skilled practitioners. In one embodiment, an HRVX vision box employing Freedom Library software in a positioning system  36  sold by Electro Scientific Industries, Inc. is employed to perform alignment between the laser resonator  12  and the target areas  31  on the work piece  30 . Other suitable alignment systems are commercially available. 
     In addition, beam positioning system  36  also preferably employs non-contact, small-displacement sensors to determine Abbe errors due to the pitch, yaw, or roll of the stages that are not indicated by an on-axis position indicator, such as a linear scale encoder or laser interferometer. The Abbe error correction system can be calibrated against a precise reference standard so the corrections depend only on sensing small changes in the sensor readings and not on absolute accuracy of the sensor readings. Such an Abbe error correction system is described in detail in International Publication No. WO 01/52004 A1 published on Jul. 19, 2001 and U.S. Publication No. 2001-0029674 A1 published on Oct. 18, 2001. The relevant portions of the disclosure of the corresponding U.S. patent application Ser. No. 09/755,950 of Cutler are herein incorporated by reference. 
     Many variations of positioning systems  36  are well known to skilled practitioners and some embodiments of positioning system  36  are described in detail in U.S. Pat. No. 5,751,585 of Cutler et al. The ESI Model 5320 microvia drilling system available from Electro Scientific Industries, Inc. of Portland, Oreg. is a preferred implementation of positioning system  36  and has been used for laser drilling of resin coated copper packages for the electronics industry. Other preferred positioning systems such as Model series numbers 27xx, 43xx, 44xx, or 53xx, manufactured by Electro Scientific Industries, Inc. in Portland, Oreg., can also be employed. Skilled persons will also appreciate that a system with a single X-Y stage for work piece positioning with a fixed beam position and/or stationary galvanometer for beam positioning may alternatively be employed. Those skilled in the art will recognize that such a system can be programmed to utilize toolpath files that will dynamically position at high speeds the focused UV laser system output pulses  40  to produce a wide variety of useful patterns, which may be either periodic or non-periodic. 
     The CPU  32  causes current to be applied or increased to the diodes  14  either when the positioning system  36  reaches, or is about to reach, a new or second target area  31  or at a predetermined time interval following and inhibition or reduction in diode pumping. The CPU  32  instructs the Q-switch control  38  to open the Q-switch  24  to deliver the laser pulses  60  at a predetermined repetition rate until the second via is made. 
     The profile of the pumping current intervals  50  can be modulated to control the shape of the peak power profile of the laser pulses  60  during the quasi-CW pumping, such as flat, from low to high (shown in FIG. 2A) or from high to low during the period. Furthermore, the current profiles can be modulated to have different amplitudes so for example a high peak power can be used for drilling metal layers and lower peak power can be used for drilling dielectric layers, if desired. Similarly, the time periods for current pumping intervals  50  can be adjusted to suit the size, depth, and material of the via to be processed, such as longer current intervals  50  for larger diameter vias. FIGS. 2A and 2B demonstrate that the laser system  10  permits, but does not require, variable periods of current pumping intervals  50  and variable periods between current pumping intervals  50 , while the duty cycle can be kept the same. The duty cycle could, however, be varied as well, if desirable for laser output profiling. 
     The quasi-CW pumping repetition rate can easily be made as high as 2 kHz. The interval time between the quasi-CW pumping doesn&#39;t have to be constant as long as the average thermal loading to the laser pumping diode  14  and/or laser medium  16  remains relatively constant or below thermal damage levels. 
     In one embodiment, the diodes  14  and power supply  34  of a CW pumped 5 W UV laser system  10  are changed to be conducive to variable current pumping. The resulting laser system  10  is able to run at a duty cycle of 2 to 1 at 500 Hz. The diodes  14  pump the laser medium  16  for 1 ms before they stop for another 1 ms. Thus, during the pumping period, about twice as much current can be put into the diodes  14  (without adversely affecting the average thermal loading on the diodes  14  or the laser medium  16 ). Thus, the laser power during that 1 ms pumping period can be more than twice as much as that from a comparable CW pumped laser (especially after the nonlinear frequency conversion). The A-O Q-switch  24  used in the laser resonator  12  repetitively switches to deliver the laser pulses  60  at a pre-determined repetition rate of, for instance, 10 kHz or up to 50 kHz. 
     Work piece  30  that may, for example, be an IC chip package, MCM, capacitor, circuit board, resistor, or hybrid or semiconductor microcircuit. For convenience, work piece  30  is described below as only having four layers. Top and bottom conductive layers may contain, for example, standard metals such as, aluminum, copper, gold, molybdenum, nickel, palladium, platinum, silver, titanium, tungsten, metal nitrides, or combinations thereof. Conventional metal layers vary in thickness, typically between 9-36 μm, but may be thinner or thicker. The conductive layers are typically made of the same material. 
     A dielectric matrix or layer is sandwiched between the conductive layers and may, for example, contain a standard organic dielectric material such as benzocyclobutane (BCB), bismaleimide triazine (BT), cardboard, cyanate esters, epoxies, phenolics, polyimides, polytetrafluorethylene (PTFE), various polymer alloys, or combinations thereof. Conventional organic dielectric layers vary considerably in thickness, but are typically much thicker than the metal layers. An exemplary thickness range for organic dielectric layers is about 30-400 μm. 
     The dielectric layer may also contain a standard reinforcement component that may include a fiber matte or dispersed particles of, for example, aramid fibers, ceramics, or glass woven or dispersed throughout the organic dielectric. Conventional reinforcement components are typically individual filaments or particles of about 1-10 μm in size and/or woven bundles of 10 μm to several hundreds of microns. Skilled persons will appreciate that reinforcement components may be introduced as powders into the organic dielectrics and can be noncontiguous and nonuniform. Such composite or reinforced dielectric layers typically require laser processing at a higher fluence than is needed to ablate unreinforced dielectric layers. Skilled persons will also appreciate that these various layers may also be internally noncontiguous, nonuniform, and nonlevel. Stacks, having several layers of metal, dielectric, and reinforcement material, may be thicker than 2 mm. 
     Via diameters preferably range from 25-300 μm, but laser system  10  may produce vias that have diameters as small as about 5-25 μm or greater than 1 mm. Because the preferred ablated spot size of laser pulses  60  is about 25-75 μm in diameter, vias larger than 25 μm may be produced by trepanning, concentric circle processing, or spiral processing. Skilled persons will appreciate that vias may be noncircular, such as square, rectangular, oval, slot-like, or other surface geometries. 
     Through-hole vias cleanly and evenly penetrate all layers and materials of work piece  30  and preferably exhibit negligible taper from via top to via bottom. Blind vias do not penetrate all layers and/or materials, typically stopping at a lower or bottom conductive layer. Proper selection of the laser parameters permits the lower or bottom conductive layer to remain unaffected even if it comprises the same metal component(s) as the top metal layer. 
     The parameters of laser output  40  are selected to facilitate substantially clean, sequential drilling, i.e., via formation, in a wide variety of metallic, dielectric, and other material targets that may exhibit different optical absorption, ablation threshold, or other characteristics in response to UV or visible light. The parameters of laser system output  40  include an average energy per pulse greater than about 120 μJ measured at the work surface, preferably greater than 200 μJ; spot size diameters or spatial major axes of less than about 50 μm, and preferably from about 1-50 μm; a repetition rate of greater than about 1 kHz, preferably greater than about 5 kHz, and most preferably even higher than 20 kHz; and a wavelength preferably between about 190-532 nm, and most preferably between about 250 nm and 400 nm. Specific preferred wavelengths include, but are not limited to, 1064 nm, 532 nm, 355 nm, 349 nm, or 266 nm. 
     The preferred parameters of laser output  40  are selected in an attempt to circumvent certain thermal damage effects by utilizing temporal pulse widths that are shorter than about 150 ns, and preferably from about 40-90 ns or lower. Skilled persons will also appreciate that the spot area of laser pulses  60  is generally circular, but may be slightly elliptical. Preferred UV laser drilling parameters are disclosed in U.S. Pat. Nos. 5,593,606 and 5,841,099. 
     Blind vias, and particularly blind vias with large diameters, are preferably created by a two pass process in which the conductive layer for all target areas is removed in the first pass and then the dielectric layer for all the target areas is removed during the second pass with the fluence of the laser output being below the conductive layer ablation threshold. After the top conductive layer of all the target areas is removed, the fluence for the laser output during the second pass can be reduced by defocusing the laser spot and/or by increasing the repetition rate, as well as by decreasing the current to the laser pumping diode  14 . 
     Skilled persons will appreciate that blind vias can also be created in a single pass process where both the conductive and dielectric layers of each target are is removed before the positioning system  36  moves to a subsequent target area  31 . Single pass processing is preferred for creating smaller diameter vias. In a single pass process, it would be more efficient to maintain a fairly high fluence as the laser pulses  60  begin to remove the dielectric layer, but as the laser pulses  60  clear away the dielectric layer and expose the bottom conductive layer such that it begins to absorb heat from laser output  40 , damage to the bottom conductive layer would be reduced by using a lower fluence. Thus, a gradual defocus of the laser spot or decrease in the pumping current during dielectric removal would be faster, more efficient, and protect bottom metal layer better than using a single fluence for dielectric removal. These and other laser output profiling techniques for via drilling processes are described in detail in U.S. patent application Ser. No. 09/823,922 and U.S. Patent Publication No. US2001-0045419, published on Nov. 29, 2001. The detailed description and drawings of U.S. patent application Ser. No. 09/823,922 are herein incorporated by reference. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiment of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.