For Au/Sn solder applications
BY MIKE GORSKI AND BRET HALPERN
Gold/tin solders have low melting points, high-yield strengths and good oxidation resistance. They are essential for laser die bond and other packaging applications where Au/Sn preforms are commonly used. But as electronic and optical-electronic devices get smaller, solder preforms become difficult to handle1, and there is growing interest in replacing them with vapor deposited Au/Sn layers. One approach to this is the new Jet Vapor Deposition (JVD) * technology.2,3
An effective process for vapor deposition of Au/Sn solder layers must satisfy several requirements: 1) high deposition rates because of the thick layers typically required, 2) good composition control because the melting point varies sharply with composition near the eutectic, 3) minimal material waste, 4) adaptability to large and small substrates, and 5) processing that is damage-free on resist layers.
These requirements are met by JVD's unique “sonic jet sources in low vacuum,” whose high deposition rates can coat a 4″ Si wafer with 5 µm of Au/Sn in less than 10 minutes. Substrates range from sub-millimeter parts to 8 and 12″ wafers, of almost any material. Virtually no Au/Sn is wasted: the capture efficiency is ~95 percent, and escaped metal can be easily trapped and recycled. Deposition takes place at near-room temperature, with no damage to patterned resist.
JVD Principles
JVD uses a sonic jet in low vacuum, produced by a source generally shaped like a nozzle, whose exit can be simply a hole in a plate. A fast flow of pure argon or helium carrier gas is supplied to the nozzle and sustained by a mechanical pump downstream. The flow rate, orifice size and pump speed are chosen to give “critical flow,” in which the upstream pressure is greater than twice the downstream pressure. In critical flow, gas moving slowly inside the nozzle emerges as a collimated jet, typically 1 cm in diameter, moving at the speed of sound,
105 cm/second for He. This sonic jet is aimed at a substrate a few inches downstream. Atoms or molecules are injected into the jet, travel downstream, diffuse to the substrate and deposit. The Au/Sn production sources described below illustrate the general approach.
In the JVD “hot filament, wire feed” source, a fine wire of Au/Sn alloy is fed continuously by a computer-controlled mechanism against a hot W filament, just upstream of the nozzle exit, as seen in Figure 1. On contact, the Au/Sn wire melts, wets and vaporizes. Vaporized Au and Sn atoms are entrained in the accelerating carrier gas and swept downstream with the jet to the substrate. The Au/Sn deposit is concentrated in a circular zone about the size of the jet cross section. Inside the jet, the mean free path is ~102 cm; Au and Sn frequently collide with carrier gas atoms, so that radial diffusion of Au and Sn gives the deposit a Gaussian profile. Because the Au/Sn wire is fed at a constant speed, the flux of Au and Sn atoms is constant, and deposition rate is determined by wire feed rate.
A variant of this source is the JVD “electron jet” or “e-jet”.4 To the hot filament and wire feed, the e-jet adds an intense low-voltage, high-current plasma, initiated by electron emission from a second hot filament, as in Figure 1. The jet then sweeps this plasma to the substrate. Ar+ ions in the plasma, at extremely high density, >1015 ions/cc, are used for surface pre-cleaning and for film ion bombardment at high currents, ~1 ampere, and low energies, ~20 – 60 e.v. This e-jet source thus combines substrate cleaning, a constant high rate of deposition and film property control by ion bombardment.
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Relative Motion of the Jet and Substrate
Most solder applications need a uniform Au/Sn film over a larger area than the jet cross-section; this requires relative motion in two dimensions between jet and substrate. Several possibilities exist since one can move the source, the substrate or even the gas jet itself; the appropriate combination depends on the size, shape and number of substrates. Typical substrates are: ceramic squares and rectangles, with sides between 1 and 4″; silicon wafers of 2, 4, 6 and 8″ diameter; smaller components of regular or irregular 3-D form, and, occasionally, optical fibers.
A batch of 20 ceramic squares, 2 x 2″, is mounted on a spinning, oscillating carousel (Figure 2), while the Au/Sn jet is aimed at the carousel. The shape of the deposit depends on the carousel motion: a spot if motionless, a band if spinning, and a uniformly thick layer if spinning and oscillating. This carousel approach is ideal for batches of small or moderate sized substrates, in single or repeated runs. Due to high deposition rates, short pump down times and the absence of high-vacuum equipment, JVD's turnaround time is measured in minutes.
Larger substrates, such as 4 or 6″ wafers, often are better processed individually. Here a “spin-scan” approach is used, as in the right side of Figure 3. The wafer is mounted on a fast spinning platen, providing one degree of motion; the second degree is provided by the jet source, mounted on a sliding flange. The jet “scans” the wafer along a diameter, following a computer-controlled velocity profile: to compensate for different residence times, the source must move more rapidly through the wafer's center than the wafer's edge. Although determining the correct velocity profile takes experimental effort, this approach gives economical use of Au/Sn for circular wafers. Rectangular substrates also can be coated by spin-scan, although some Au/Sn is unavoidably wasted.
For single rectangular substrates an “X-Y scan” is used, as in the left side of Figure 3. Here, the jet is stationary, and the substrate is mounted on a moveable X-Y stage, which is scanned back and forth rapidly in one direction, and slowly in the other. Constant speeds are appropriate for rectangular substrates because there is no residence time difference as on a spinning disc.
 Figure 2. A spinning, oscillating carousel for JVD batch coating of small wafers with excellent thickness uniformity. |
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Multiple Jets, Multicomponents and Multilayers
Because the jet moves at the speed of sound, no downstream information, such as pressure changes or material fluxes, can propagate upstream. In “critical flow,” nozzle conditions remain independent of chamber conditions. Therefore, a jet's deposition rate can be calibrated in terms of upstream conditions alone. Several jets can be used, either in sequence to form multilayers or together to synthesize multicomponents and alloys5, as shown in Figure 4. This “source independence” also opens two useful aids to Au/Sn deposition: predeposition of bond layers, and post-deposition of Au “caps.”
Generally, micron thick Au/Sn coatings must be deposited on an adhesion-diffusion barrier multilayer such as Ti/Pt/Au, several hundred nanometers thick.
 Figure 3. Relative motion schemes for coating single substrates. Many combinations of source and substrate motion are possible. |
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Au/Sn Composition Accuracy
Film composition may be verified as that of the original wire by measuring the film melting point. For wire of eutectic composition, the film and wire melting points are as expected, 278°C, within an experimental error of a few degrees. Au and Sn atoms evidently have similar diffusive behavior in a jet and diffuse to the substrate in their original proportions. EDS measurements confirm this conclusion.
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Cleanliness in JVD
JVD “cleanliness” is sometimes questioned because of the high operating pressure, ~1 torr, and use of mechanical pumps. Nonetheless, JVD's deposition environment is clean. Carrier gas enters through getters that purify to better than 1 ppm; at 1 torr, the impurity level is ~107 torr, similar to a good high vacuum. At high carrier flow rate, the chamber's natural leaks contribute little to the impurity pressure. Furthermore, the high local deposition rate of Au and Sn atoms overwhelms any impurity flux. In effect, with normal precautions on initial pump down and maintenance, a JVD system performs like a high-vacuum system.
Substrate Cleaning in JVD
Prior to Au/Sn deposition, substrates must be cleaned, particularly if they have patterned resist layers, where organic residue can affect Au/Sn adhesion on exposed metal pads. In JVD, the substrate can be cleaned with a light ion etch. Even without bias, the plasma sheath at the wafer surface provides a natural potential of ~10 V for bombarding ions, and experience shows this treatment to be effective. An alternative is to use a JVD “microwave discharge” jet source, which produces atomic oxygen in high concentrations and removes organic residue by swift conversion to CO or CO2 at room temperature.
Room Temperature Deposition
Many substrates, particularly if resist coated, are thermally sensitive. An advantage of JVD is that these substrates experience negligible temperature rise, largely because of source-substrate relative motion. Any point on a substrate spends little time in the jet exposed to heat and most of its time out of the jet. Measurements of temperature rise on a 4″ wafer in spin-scan show that ion etching produces a final temperature <65°C. Similar results are found from measurements on a spinning carousel. While it is possible to deliberately raise substrate temperature, for example by exposure to high-power e-jet plasma, JVD generally is a low-temperature process.
Au/Sn Material Cost
Au/Sn alloy wire is several times more expensive than the equivalent amount of bulk Au. To reduce materials cost in production, a very high rate jet source that vaporizes bulk Au/Sn also has been successfully tested. Some Au and Sn atoms in the jet are not captured on the substrate, but are swept away in the “wall jet,” flowing radially outward along the substrate. Weighing experiments show that these can be recovered by efficient trapping on surfaces positioned near the jet. For the “bulk Au/Sn” source noted above, recycling is straightforward.
Conclusion
JVD's “sonic jets in low vacuum” are an appealing alternative to preforms for Au/Sn solder layer deposition. JVD deposits thick layers at high rates, with no material waste and with accurate composition control. JVD provides intense plasmas for substrate cleaning and ion bombardment. Multiple, independent jets can synthesize multilayers, alloys and multicomponents, on substrates from millimeter dimensions to 12″ wafers, of many shapes and all materials, using a variety of motion systems. Bond-barrier and cap layers can be deposited with no vacuum break. JVD provides an effectively “high vacuum” environment at ~1 torr, using only low-cost mechanical pumps. JVD offers fast turnaround times and small system footprints, with low vacuum systems and versatile sources adaptable to individual manufacturing needs.
*Jet Vapor Deposition is trademark of Jet Process Corp.
REFERENCES
- J. Wills, “Die Bonding for High Power Devices,” Advanced Packaging, May 2002.
- J. Schmitt and B.L. Halpern, US Patents 4,788,082, 11/29/88 and 5,725,672, 3/10/98.
- Z. Zhang, et al., “Jet Vapor Deposition: A New, Low Cost, Metallization Process,” IMAPS Proceedings, 1997 International Symposium on Microelectronics, P. 144.
- L. Halpern, U.S. Patent 5,571, 332, November 15, 1996.
- J. Schmitt and B.L. Halpern, “Multiple Jets and Moving Substrates: Jet Vapor Deposition of Multicomponent Thin Films,” Journal of Vac. Sci. Technology, A, 12, 1623 (1994).
Bret Halpern, director of research, and Mike Gorski, engineer, may be contacted at Jet Process Corp., 24 Science Park, New Haven, CT 06511; (203) 777-6000; Fax: (203) 777-6007; E-mail: [email protected]; [email protected].