Solder Deposition in JVD: Solder alloys of precise composition are available commercially as wire or ribbon, and these form the starting materials for the "wire feed" jet source of Figure 5. The wire-feed is JPC's mainstay for commercial production of solder coatings; as an example, consider AuSn. A fine wire or ribbon of AuSn alloy is fed against a hot W coil located upstream of the nozzle exit. On contact, the AuSn wire melts, wets, and vaporizes; individual Au and Sn atoms are swept downstream with the jet and deposit on the substrate, concentrated within a circle ~ 1 cm in diameter; radial diffusion of Au and Sn in the jet gives the AuSn deposit a Gaussian profile. At constant AuSn wire feed rate the flux of Au and Sn atoms is also constant, and the AuSn deposition rate is proportional to wire feed rate; this considerably simplifies thickness calibration. The Au to Sn ratio of the wire is preserved in the growing film, a critical consideration for solder bumping.
Pre-cleaning is possible by including a second hot filament for thermionic emission of electrons; this "e-jet" configuration ignites a low voltage, high-current Argon plasma jet. Low energy Ar+ ions in the jet then bombard and gently clean the substrate.
The wire feed can be used for metals that do not alloy with and degrade the hot tungsten coil structure. This includes all metals and alloys commonly used for solder deposition, such as AuSn, InSn, and SnAg. Very high rates are possible with this wire feed jet source; even a relatively slow wire feed can give deposition rates approaching microns per minute over a 4" wafer. Most of JPC's commercial deposition of solders is carried out with the hot filament wire feed / e-jet combination, both in atomistic and cluster modes
A few solders, such as Indium, are too soft for wire feeding, and some alloys (of AuSn, e.g.) cannot be drawn to wire or ribbon. In such cases we use the "wick jet" shown in Figure 6, which has three components in series: a reservoir of molten solder, at a temperature somewhat above the melting point; a "wick" made from tungsten wires, at temperature greater than that of the reservoir; and a "hot zone" vaporizer consisting of a tungsten filament at very high temperature. The wick jet works by capillarity. As metal atoms vaporize from the hot zone, surface tension draws more liquid metal from the reservoir, through the wick, to the hot zone. In steady state the rate of vaporization and replenishment are equal. The wick jet is universal for all solder metals. Wick transport can maintain an extremely high vaporization rate at the hot zone, and therefore a high deposition rate downstream. A major economic advantage of the wick jet is its use of bulk material as the supply for the reservoir; the material cost is thus as low as possible, with impurity level the only variable. Surface oxides float to the top of the reservoir, but are not drawn up the wick. Alloy compositions that do not lend themselves to wire drawing are easily handled as a melt, so there is no constraint on solder composition.
For the much thinner adhesion/diffusion barrier layers, such as Ti and Pt, we have used for many years a jet source based on hollow cathode sputtering at "high" pressures of ~ 1 torr. A hollow cathode provides a target for Argon glow discharge sputtering of metal atoms, and also plays the role of a cylindrical nozzle. Argon is let in at one end, flows through the hollow cathode, and forms the sonic jet which sweeps sputtered atoms to the substrate. JVD sputter jet sources give much lower deposition rate than the wire feed, but they can generate a vapor of virtually any metal, and are appropriate for thin films of Ti, Pt, and Au required as adhesion, diffusion barrier, and protective layers prior to solder bumping. The deposition rate can be calibrated against Ar+ sputter current; when the gas flow conditions are fixed, this calibration is stable and accurate.
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