Tutorial #62
George A. Riley, PhD
April 2006
For years, ultrasonic vapor jet deposition has been an elegant solution in search of a problem. Now, with our growing 21st-century needs for die and wafer stacking, post-processing, lead-free solder deposition, smaller feature sizes, higher power densities, increased electrical and thermal conductivity, and smaller, higher quality bumps ― its time has come.
Vapor jet deposition produces uniform, low-stress layers of metals, alloys, lead-free solders, and other materials, deposited in carefully controlled patterns onto a wide variety of surfaces at high throughput, low temperature, and low cost.[1] The system has produced solder bumps, including gold-tin, indium-tin, pure indium, and other lead-free alloys, with feature sizes below ten microns, including four micron bumps on eight micron pitch.
Beyond solder bumping, eutectic gold-tin solder pre-forms, a packaging mainstay for many years, can’t cope with today’s tiny dimensions. Vapor-deposited gold-tin can pattern an entire wafer or substrate with micro-preforms in minutes. Gold-tin is also the low-creep mounting of choice for many optical components, another growing requirement.[2]
One approach to interconnecting stacked wafers depends on AuSn bumps of minimal size and thickness, with uniform composition [3]. The low temperature of vapor deposition is compatible with most photoresists, allowing fine-patterned bumps. Homogenous AuSn is deposited, not alternating stacked layers, insuring an exact melting point. Another wafer-stacking favorite, eutectic copper-tin, is also possible with jet deposition.
The equipment required to do all of this is relatively simple and inexpensive, compared to common evaporation, sputtering, or plating systems. Figure 1 shows the basic concept. A hot filament vaporizes source material in a stream of argon or other carrier gas. A nozzle accelerates the gas to sonic speed, transporting a collimated flow of material onto the substrate. Mechanical motion of the substrate creates a uniform-thickness coating over the entire area.
Figure 1. Sonic jet vapor deposition, conceptual view.
The system operates in a low-vacuum (~ 1 Torr) chamber, maintained by mechanical pumps. Vapors are created by computer-controlled feeding of a wire of the desired material composition against a hot tungsten filament. The deposition rate is controlled by the wire feed rate. Little material is wasted; the capture rate of the substrate is above 90%, and escaped material can be easily trapped and recycled.
An extension of the basic concept, shown in Figure 2, adds a plasma component to the vapor stream. A hot thermionic filament upstream of the vapor source supplies electrons to the argon flow, creating a plasma. The argon plasma provides both pre-cleaning of the substrate and low-energy high-current ion bombardment of the deposited film, to further control deposited film properties.
Figure 2. Schematic view showing plasma jet vapor operation.
A variety of mechanical motions converts the jet to a uniform deposit of material over the entire target surface. Figure 3 shows the rotating-substrate approach, for wafers up to 300 mm. While the wafer spins at a steady rate, the jet is scanned linearly across its diameter at a computer-varied rate to create a uniform deposit. For batch processing, a large rotating platen carries multiple wafers or substrates arranged around its periphery through the moving jet stream.
A more efficient method for smaller substrates is a spinning carousel, as shown in Figure 3. The carousel, which can hold multiple substrates or wafers up to 50 mm diameter, allows a fixed spot if stationary, a band if rotating, and a uniform area deposit if oscillating along its axis while spinning.
Figure 3. Spinning carousel arrangement for smaller substrates and wafers.
Carousel mounting also provides a simple method to deposit multilayers, including gradient layers. The chamber allows multiple vapor sources to be positioned around the carousel. Each may be independently controlled to deposit different materials, singly or in any ratio, to provide multilayer or multicomponent compounds. As an example, multicomponent jet vapor deposition allows depositing the required Ti-Pt-Au bond and barrier layers, followed by depositing AuSn, in one operation without breaking vacuum.
Figure 4. Added nozzles for multilayer multimetal deposition.
In addition to the solder depositions described here, the process can deposit metals such as Au, Sn, Cu, Cr, Al and Ag, as well as various alloys. It can produce a wide range of simple and compound metal oxides and nitrides useful in electronics fabrication and as hard coatings, thermal barriers, and component encapsulation.
In summary, the vapor jet deposition system offers a low cost, high throughput solution to many 21st century packaging problems, including depositing lead-free solders. Fortunately for us, it was developed late in the 20th century, and is here now to solve today’s problems.
REFERENCES
[1] M. Gorski and B. Halpern, “Jet Vapor Deposition,” Advanced Packaging, February 2003.
[2] E. J. Wills, “Die Bonding For High Power Devices,” Advanced Packaging, May 2002.
[3] P.Pandojirao-Sunkojirao et al., “Electrical Interconnection for 3D Wafer Stacks,” IMAPS Device Packaging Conference, Phoenix AZ, March 2006.
FOR MORE INFORMATION
Jet Process Corporation WWW.JETPROCESS.COM