Bonding Metal Electrical Members With a Frequency Doubled Pulsed Laser Beam

1. Field of the invention

The present invention is directed to light with short wavelengths (300-600 nm) for pulsed laser bonding of metal electrical members in microelectronic applications. In one embodiment a frequency doubled pulsed Nd:YAG laser beam at 1064 nm wavelength is directed to a nonlinear crystal to produce a laser beam with 533 nm and 1064 nm wavelengths.

2. Description of Related Art

Lasers used in microelectronic bonding provide concentrated localized heating. This is especially useful for soldering or welding bonds on electrical interconnects such as customizable copper/polyimide substrates with fine pitch dimensions, low thermal stress tolerance or heat sensitive components.

Yttrium-aluminum-garnet (YAG) crystal doped with neodymium (Nd) can produce a laser beam with a fundamental wavelength of about 1064 nm (1.064 microns) which is infrared radiation (IR) and invisible to the human eye. It has been found that Nd:YAG lasers provide a desirable balance between maximizing the absorption of metals and minimizing the absorption of polymer substrates. The use of 1064 nm Nd:YAG lasers for bonding electrical members in microelectronics is well known in the art; see, for instance, U.S. Pat. No. 4,697,061 to Spater et al, U.S. Pat. No. 4,845,355 to Andrews et al, and U.S. Ser. Nos. 07/405,377 and 07/558,127 to Spletter et al, each of which is hereby incorporated by reference.

Nd:YAG lasers at high frequencies in the range of 300-600 nm wavelengths, in particular the frequency doubled 533 nm wavelength (green light), can provide a substantial increase in the amount of laser energy absorbed by appropriate metal electrical members as they are heated and bonded. M. Greenstein in "Optical Absorption Aspects of Laser Soldering for High Density Interconnects," Applied Optics, Vol. 28, No. 21 (Nov. 1, 1989) points out that for both gold and copper metallurgies, the 533 nm wavelength provides significantly more absorption of laser energy than 1064 nm wavelength. Furthermore, as the temperature increases the predicted optical absorption of gold and copper increases as well. For example, Greenstein found that gold plated copper surfaces of TAB lead frames absorb about 40% energy from a pulsed Nd:YAG laser beam at 533 nm wavelength versus about 1%-5% energy absorption at 1064 nm wavelength.

533 nm wavelength radiation can be generated by directing a 1064 nm wavelength pulsed Nd:YAG laser at a nonlinear doubling crystal such as potassium titanyl phosphate. The crystal is affected by the electrical field of the laser beam to produce light with a frequency in the second harmonic at 533 nm wavelength. Upon applying 1064 nm wavelength radiation to the crystal typically 5%-10% is converted to 533 nm radiation and 5%-10% is lost as heat, reflections, etc. Unfortunately the crystal is highly sensitive to orientation and impinged power, and is prone to damage unless used in a very controlled environment. Water cooling is often provided to keep the crystal temperature approximately 30.degree.-40.degree. C.

While recently developed of GaAlAs diode lasers produce efficient and stable 533 nm wavelengths and look promising, current laser powers are in the tens-of milliwatts range which is not sufficient for microelectronic bonding.

Nd:YAG laser outputs can be continuous wave (CW), shuttered with an acousto-optic or electro-optic device (Q-switched), or pulsed. While these outputs are each at 1064 nm wavelength, the peak output powers differ widely and these differences can have a profound effect on the suitability of any Nd:YAG laser for a particular application.

Most currently available frequency doubled Nd:YAG lasers are either Q-switched or CW and as such are not suitable for microelectronic bonding. CW frequency doubled Nd:YAG lasers produce an energy flux that is difficult to control and tend to thermally shock and damage a bond site. Q-switched frequency doubled Nd:YAG lasers produce extremely high peak power for short pulse widths, e.g. kilowatts for nanoseconds, and as such drill or cut through the electrical members instead of bonding them by welding or solder reflow.

Frequency doubled pulsed (FDP) Nd:YAG lasers are typically used for range finding and night vision applications. Commercial vendors include Lumonics Corp. and Kigre Corp. The 533 nm wavelength creates green light which is well absorbed by the human eye. However the lasers employed for these applications have low output power and high repetition rate and therefore are not suitable for microelectronic bonding.

An object of the present invention is to provide laser bonding for electrical members by contacting first and second metal electrical members to form a bond interface therebetween, and directing a pulsed laser beam with at least 3% of the total beam energy between 300-600 nm wavelength to couple to at least one of the members, thereby bonding the first and second members at the bond interface.

Preferably the 300-600 nm wavelength energy is at least 3 millijoules, at least one of the electrical members comprises copper or gold, and the pulsed laser beam includes a pulsed solid state laser beam with 1064 nm wavelength radiation.

Another object of the present invention is to bond first and second contacting metallic electrical members by directing a FDP solid state laser to couple to at least one of the members. The FDP solid state laser can comprise a fundamental wavelength of about 1064 nm and a frequency doubled wavelength of about 533 nm.

Still a further object is wherein a 1064 nm pulsed solid state laser is directed at a nonlinear crystal to produce frequency doubled radiation at 533 nm wavelength. Suitable nonlinear crystals include lithium iodate (LiIO.sub.3), lithium niobate (LiNbO.sub.3), potassium titanyl phosphate (KTP) and beta barium borate (BBO).

Yet a further object of the present invention is wherein the solid state laser can be generated by various crystals such as yttrium-aluminum-garnet (YAG), yttrium-lithium-flouride (YLF) and yttrium-aluminum-phosphide (YAP), each of which is doped with neodymium (Nd).

A still further object is the frequency doubled solid state laser pulses are at least 30 millijoules and the 533 nm wavelength therein is at least 3 millijoules.

Still another object is to provide laser bonding for gold, gold plated copper, and copper electrical members.

A further object is wherein the first member is a bonding pad on an integrated circuit and the second member is a TAB tape lead. In one embodiment the bonding pad is gold and the TAB tape lead is gold plated copper.

Yet a further object is an apparatus for generating a FDP solid state laser beam wherein the percentage of 533 nm and 1064 nm wavelengths is adjustable by conventional attenuators.

A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is read in conjunction with the following drawings, in which:

FIG. 1 is a schematic view of an apparatus for generating a FDP Nd:YAG laser beam,

FIG. 1A is a cross-sectional view taken along line 1A--1A of FIG. 1 showing a housing containing a YAG rod and flashlamp,

FIG. 2 is a graph showing the waveform of a pulsed Nd;YAG laser beam,

FIG. 3a-3c are a cross-sectional views of a microelectronic bond formed by directing a frequency doubled pulsed Nd:YAG laser beam,

FIG. 4a-4c are photomicrographs of microelectronic bonds formed in accordance with the present invention, and

FIG. 5 is a schematic view of another apparatus for generating a FDP Nd:YAG laser beam wherein the fundamental and doubled frequencies can be attenuated separately.

Referring now to FIGS. 1 and 1A, an apparatus for generating a frequency doubled pulsed (FDP) solid state Nd:YAG laser beam is designated generally as 10. Apparatus 10 contains a cavity 12 comprising elliptical housing 17, which as seen in FIG. 1A contains YAG rod 14 and flashlamp 16 positioned adjacent to one another at the focii of the elliptical chamber inside housing 17. Inside wall 15 of housing 17 is covered with highly reflective gold. YAG rod 14 is pumped with one krypton flashlamp 16 to produce a laser beam in rod 14. YAG rod 14 is positioned and aligned between highly reflective reflector mirror 18 and output reflector mirror 20. Mirror 20 is coated to pass a portion of the light therethough and out cavity 12. Suitable coatings for mirror 20 are conventional and known to those with skill in the art. Flashlamp 16 generates a 1064 nm wavelength pulsed Nd:YAG laser beam. Mirrors 18 and 20 are positioned to reflect the laser beam from rod 14 generated by flashlamp 16 back-and-forth in cavity 12 as well as allowing a portion 23 of the radiation, such as 5%-10%, to exit cavity 12 through output mirror 20.

Inserted into the laser beam path is nonlinear potassium titanyl phosphate (KTP) crystal 22 for frequency doubling. Crystal 22 may be placed either inside or outside the laser cavity 12, referred to as "intracavity" or "extracavity" doubling, respectively. Intracavity frequency doubling produces the most green light because the electric field of the laser is most strong inside the cavity, especially with a multimode laser, and crystal 22 will produce the maximum green light when high power densities are impinged thereon. For this reason an intracavity crystal 22 is shown herein and preferred. It should be noted however that intracavity crystals are more prone to damage from high power densities than extracavity crystals. To overcome this difficulty, as opposed to conventional intracavity frequency doubled lasers, the present invention does not excite the crystal with extremely high power levels in an attempt to generate a high level of green light.

Instead the present invention employs low power 1064 nm wavelength IR, for instance 100 millijoules. As a result laser output 23 is mostly 1064 nm wavelength radiation and only a small percentage of 533 nm radiation is generated (e.g. 5%-15%).

To increase the percentage of 533 nm wavelength radiation, an optional infrared reflector 24 can be placed outside cavity 12 in the path of laser beam 23 to reflect 1064 nm wavelength component 25. Different amounts of IR 25 can be reflected away depending on the coating applied to IR reflector 24.

In another embodiment output mirror 20 was replaced with a similar mirror having a different coating. Depending on the coating applied to output mirror 20, final laser beam 38 could be varied and contain up to 33% of its power from 533 nm wavelength; although 3%-15% 533 nm wavelength with correspondingly 85%-97% 1064 nm wavelength is considered optimal since, within this range, stress is reduced on KTP crystal 22 while sufficient 533 nm wavelength radiation remains to couple into workpiece 28. In one experiment the total amount of green and IR energy was about 5-10 and 50-60 millijoules, respectively, which was considered suitable for bonding gold microelectronic members. At least 3 millijoules of green 533 nm light is preferred to assure reliable bonds.

Workpiece reflector 26 can divert the FDP Nd:YAG laser beam 34 toward workpiece 28 such as copper or gold electrical members to be bonded. Objective lens 30 can focus the FDP Nd:YAG laser beam 36 into laser beam 38 and upon workpiece 28. Video camera 32 can be mounted coaxially behind workpiece reflector 26 to permit viewing workpiece 28 through objective lens 30 as it is irradiated and coupled by the FDP Nd:YAG laser beam 38.

Mirror 18, housing 17, crystal 22 and output mirror 20 can be secured to rail 38 by rail supports 40, 42, 44 and 46 respectively.

A power versus time curve for the 1064 wavelength pulsed Nd:YAG laser of the present invention is shown in FIG. 2. Suitable pulse times for microelectronic bonding are typically between 0.5-5.0 milliseconds, with 0.6-1.75 milliseconds preferred. Flashlamp 16 is turned on and off for each pulse. As a result there is an exponential increase in power output until a maximum level is reached, shown as at 0.6 milliseconds, at which point the power exponentially decreases, shown as zero power at 0.8 milliseconds. With a pulsed laser the metals to be bonded are heated to a maximum temperature at which time the energy input exponentially decreases. The pulsed laser provides a higher power level at the middle of the pulse than the Q-switched laser. However, the peak power is orders of magnitude less than the spike at the beginning of the Q-switch pulse. This exponentially increasing power from the pulsed laser preheats the bond site and provides the bond site with the most power when it can use it most efficiently. It also provides decreasing power which tends to allow less internal stress buildup than the instantaneous drop-off of energy that is characteristic of the Q-switch or CW laser. The pulsed Nd:YAG laser pulse width should be short enough to allow bonding without damaging the surrounding integrated circuit. A 50 watt average power pulse Nd:YAG laser made by Carl Haas Gmbh and Co. was found satisfactory.

Referring now to FIGS. 3a-3c, workpiece 28 can comprise an integrated circuit die 60 with 1 mil thick gold pads or bumps 62 with each side 4.4 mils long. TAB tape lead 64 is gold plated copper 1.3 mils thick and 3 mils wide. In FIG. 3a lead 64 is aligned with and contacted to bump 62 to form bond site 66 and bond interface 68. In FIG. 3b frequency doubled pulsed Nd:YAG laser beam 70 is generated by apparatus 10 and directed to couple to at least one metal electrical member, shown as lead 64 at bond site 66. Preferably 85% of the power in laser beam 70 is at 1064 nm wavelength and 15% is at 533 nm wavelength. A 10 Hz repetition rate can be used with 100 millijoules per pulse and a 1.75 millisecond pulse width. In FIG. 3c bump 62 is welded and bonded to lead 64 and laser beam 70 is discontinued.

Referring now to FIGS. 4a-4c, scanning electron microscope (SEM) photographs are shown of test bonds fabricated in accordance with the present invention. A high magnification video microscope and tape/die fixturing were set up to enable precise alignment of the laser beam with the target bond site. In particular the tape/die fixture was mounted directly on a rail on which the laser was built to eliminate misalignment caused by movement of the die and/or pedestal when the fixturing was being installed in the laser beam path after alignment. Also, a different output mirror 20 with a different coating was used to allow more green light to come directly from the laser cavity, thus boosting the final laser beam composition to 33% green and 67% IR, thereby eliminating the need to reflect extraneous IR via an infrared reflector. The bonds were formed with approximately 25 millijoules of green light and 50 millijoules of IR. All of the leads shown herein broke in span during pull testing, which indicates the bonds were stronger than the tensile strengths of the leads. FIGS. 4a-4c show the bonds after they were pull tested. FIG. 4a shows bonds with well controlled melting. FIG. 4b shows a closer look at the bonds in which the melt zone is localized to within a 3 mil region. Melting and bonding under the lead is seen at the back of the bond. FIG. 4c is a cross-section of a bond showing complete void-free bonds. The copper lead (the dark grey trapezoid at the center of the lead) is not melted.

In the embodiments previously described, the relative percentages of green light and IR in apparatus 10 that coupled to workpiece 28 were either fixed or only partially adjustable. That is, without IR reflector 24 the percentages of IR and green light were both fixed, whereas with IR reflector 24 the percentages of IR and green light were only partially adjustable in that the percentage of IR could only be decreased and consequently the percentage of green light could only be increased.

Shown in FIG. 5 is an apparatus which could allow the percentages of green light and IR to be fully adjustable before the laser beam couples to a bond site. In FIG. 5 like parts to FIG. 1 are similarly numbered with the addition of a suffix "a". An apparatus 10a not currently reduced to practice is proposed. Apparatus 10a could allow a FDP Nd:YAG laser beam to be split so that the 1064 nm and 533 nm wavelengths could be separately and independently reduced or attenuated. As previously described FDP Nd:YAG laser beam 23a exits from mirror 20a in cavity 12a. Laser beam 23a could be directed to first dichroic mirror 72 which would split laser beam 23a into 533 nm wavelength component 74 and 1064 nm wavelength component 76 by reflecting predominantly 533 nm component 74. 1064 nm component 76 could be directed to reflector mirror 78 which would reflect 1064 nm component 76 toward IR attenuator 82. Likewise green attenuator 80 could be placed in the path of 533 nm component 74. Attenuators 80, 82 could be adjusted to filter or reduce the desired amounts of components 74, 76 respectively. For example, a stack of neutral density filters could be used so that the number of filters in the stack determines the amount of attenuation. After passing through 1064 nm attenuator 82, 1064 nm component 76 could be directed at second dichroic mirror 84 which like first dichroic mirror 72 would not alter the course of 1064 nm component 76. Returning to 533 nm component 74, after passing through 533 nm attenuator 80, 533 nm component 74 could strike another reflector mirror 86 and be directed toward second dichroic mirror 84. Upon striking second dichroic mirror 84, 533 nm component 74 could be reflected thereby combining components 74 and 76 into a single FDP Nd:YAG laser beam 36a, Laser beam 36a could be directed at objective lens 30a to focus beam 38a on workpiece 28a. Thus by adjusting attenuators 80, 82 the percentage of 1064 nm and 533 nm wavelengths in laser beam 38a could be varied and optimized for a particular workpiece 28a.

The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the invention have been described for the purpose of disclosure, numerous changes in the steps of the process and the parts of the apparatus will be readily apparent to those skilled in the art, and are encompassed within the spirit of the present invention and the scope of the appended claims. Such changes include Nd:YLF or Nd:YAP laser beams; tin, tin plated, tin-lead, gold alloy or copper alloy electrical members; slight variations in laser beam wavelengths; a coverglass which can be tin coated pressed against the electrical members to hold them in place while permitting the laser beam to couple to at least one of the electrical members; and various coatings and finishes (e.g. bright or matte) on the electrical members.