Gallium alloy breakthrough for via-filling application
09/01/2000
A low-temperature processable ternary gallium alloy proves useful for microelectronic packaging interconnects.
BY SWAPAN K. BHATTACHARYA AND DANIEL F. BALDWIN
 |
The application of metallic amalgams as an interconnect material in the microelectronics packaging industry is well-established.1-5 The amalgams are mechanically alloyed mixtures of a liquid metal and other metallic powders formed at room temperature. Step one of amalgamation, the wetting of the powdered metal particles by the liquid metal, is achieved by either tumbling or mixing with a mortar and pestle. The second step is the mechanical alloying process, where the liquid metal penetrates through the skin of the metal particles, thereby forming intermetallic compounds.3-4 Metallic amalgams, which consist of several compositions of gallium (Ga), indium and mercury, can be tailored to a wide range of applications. These applications include die attach, hermetic seal, flip chip, chip on glass, lead-free solders, optical interconnect, heat sink, via-filling and structural bonding.1 The compositions of these amalgams vary depending on the specific application. The primary processing requirements ensure reasonably long shelf life and good fluidity to allow wetting of the bonding materials.2 Among these metallurgical systems, gallium was selected for via-filling application because of its low toxicity, low vapor pressure (even at elevated temperatures), high thermal and electrical conductivity, and reactivity to copper (Cu) and nickel (Ni) to form amalgams that can be hardened within a wide range of time and temperatures. Gallium alloys also can be classified as metallic adhesives because they process similarly to organic adhesives at room temperatures. Upon cure, however, these alloys develop electrical and thermal properties comparable to solder, making them ideally suitable for interconnect materials. Gallium alloys' inherent characteristic of wetting most metals and oxides without the use of flux can be a very attractive feature for the microelectronic packaging industry. These alloys provide low via-contact resistance when compared to the conductive polymer-based composites. Also, bulk gallium alloys are mechanically stronger than standard solder alloys.1-4
 Figure 1. Schematic of the mixing procedure. |
The binary compositions of gallium with nickel and gallium with copper have been studied for several applications.1,5 The copper/gallium amalgam has a very short shelf life, is difficult to wet and demonstrates low bond strength when joined to ceramics. Nickel/gallium undergoes significant volumetric expansion when the system is cured at temperatures of approximately 120° C, which may disrupt the interfacial bonding.1 A combination of nickel and copper powders in gallium matrix produces a ternary system with better wettability and low volumetric change during hardening. From a processing standpoint, one disadvantage of current alloy compositions is the shelf life of only a few hours at ambient temperature. Storage below 20°C, however, can enhance the shelf life of the alloy.6 The processability of the ternary gallium alloy is demonstrated on a laser-drilled stainless steel substrate using a simple stencil-printing process.
Stainless Steel Substrates
Because of its superior strength-to-weight ratio, flexibility, corrosion resistance, impact strength, thickness, thermal conductivity, fire resistance, high temperature stability, reliability, cost and aesthetic appearance, stainless steel offers several advantages that traditional electronic materials do not. Some of these key advantages can be utilized for its application as a niche substrate material for multichip module-deposited (MCM-D) and multichip module-laminate (MCM-L) packaging, especially when substrate flexibility is highly desired.7,8 Currently, most flexible substrates are high-temperature polymer-based materials, such as polyimides and liquid crystalline polymers. In this study, via-filling was demonstrated on a flexible stainless steel substrate with laser-drilled 0.25-mm diameter vias. Via-filling was performed using a stencil-type printing process suitable for large-area manufacturing. Process-induced warpage of the flexible stainless steel substrates for MCM-D packaging has been discussed in recent publications.7,8
 Figure 2. Schematic of the via-filling process. |
The selected stainless steel test vehicle had a grade of 301, thickness of 0.2 mm and dimensions of 300 x 300 mm. For final assembly, 0.25-mm diameter holes with 1-mm pitch were selected. A template of nine via-grids (arranged in a 3 x 3 array) was patterned using an Nd:YAG laser. Each via-grid contained an array of 10 x 10 vias in an area approximately 1 x 1 cm. These grids were spaced at 5.75 cm from the outer edges of the panel. The distance between the consecutive via-grid patterns in x and y directions is approximately 7.5 cm. In total, 900 holes were patterned on the 300 x 300-mm substrate. Parylene N was vapor-deposited on top and bottom stainless steel surfaces and around via walls. Copper and nickel powders were -325 mesh size. Metallic gallium had a purity of 99.9 percent.
Fabrication Process
Gallium metal was placed on a hot plate at approximately 80°C - well above its melting temperature of 30°C. The sample temperature was controlled with a thermocouple to ±2°C. A pestle was cleaned with methanol to remove organic contaminants. The capsule and pestle (Figure 1) also were preheated on the hot plate. The male end of the screw-type capsule was wrapped with a Teflon tape to prevent gallium leakage. After preheating, a measured amount of liquid gallium was placed into the capsule using an adjustable liquid pipette. Copper and nickel particles were weighed with a balance to an accuracy of 0.0001 grams and were placed in the capsule for amalgamation.
 Table 1. Materials composition for gallium alloy. |
Table 1 provides the amounts of gallium, nickel and copper required for mechanical alloying. The total charge was limited by the size of the capsule. The capsule was screwed together tightly and placed in an automix. The automix was run at 3,000 cpm for 30 seconds, followed by a 30-second dwell and finally 30 seconds at 3,900 cpm. After high-speed tumbling, the material was ready for printing. A small spatula was used to transfer gallium alloy to the stainless steel substrate. The substrate was preheated in an oven at 100°C. Via-filling, which was performed with a rubber squeegee (Figure 2), averaged approximately one minute per grid of 10 x 10 holes.
After via-filling was completed, additional material was removed from the stainless steel substrates using the rubber squeegee. Final cleaning was done with a warm 10-percent hydrochloric (HCl) solution in deionized water, after which the surface was wiped off with a wet paper towel. Without the final HCl wash, the vias could not be isolated electrically. The panel was cured in a convection oven at 130°C for 16 hours. The heating and cooling rates were maintained at approximately 5°C/min. Samples were cooled to temperatures below 70°C (to prevent cracking of the dielectric coating) before they were removed from the oven.7,8
Characterization
The thickness of the dielectric coating was measured by optical spectrophotometry and surface profilometry; the nominal thickness was 0.035 mm. The resistance of the vias and their electrical isolation from other vias were checked using a standard DC multimeter. The gallium alloy produced good electrical contacts, as was evident from the via interconnect test. The parylene N coating on stainless steel showed good electrical isolation at the side walls of the filled vias after stencil-printing.
The stainless-steel surface with filled vias was examined using a scanning electron microscope. Some voids and cracks were observed on the surface of the gallium alloy (Figure 3a). To determine the dispersion of the copper and nickel particles, energy dispersive X-ray microanalysis was conducted (Figures 3b and d). The X-ray maps indicate good dispersion of the metal particles in the gallium matrix (Figure 3c). Bright dots in each figure represent the X-ray contour of the metal particles. Cross-sections of the filled vias also were examined and the alloys showed good adhesion to the via side walls.
Summary
Conductive polymer pastes frequently are used as via-filling materials for microelectronic interconnections. These polymer-based paints have low electrical and thermal conductivity and produce shrinkage voids during the cure of the host polymer. In this study, an alternative low-temperature processable metallic gallium alloy (analogous to amalgams in the dental industry) has been formulated for via-filling application. The gallium alloy can be hardened or cured at or near room temperature to form a hard metallic moiety with high melting temperatures. The ternary gallium alloy is formulated using an appropriate mixture of liquid gallium metal with nickel and copper powders. The processability of this alloy for room-temperature via-filling application is demonstrated using a stencil-printing process. A test vehicle is fabricated using a 0.2-mm thick and 300 x 300-mm stainless steel panel with laser-drilled vias. Parylene N is deposited on the stainless steel panel and around the inside via walls to electrically isolate via-filling material from the body of the stainless steel substrate and make the surface nonconductive. Filled vias were examined for electrical isolation from neighboring vias and for electrical continuity in the thickness direction. Results show that the ternary gallium alloy is an effective via-filling material that can be applied to vias as small as 0.25 mm. It is believed that this novel alloy also can be used for thin-film processes on related MCM-L substrates.
Acknowledgements:
The DARPA MCM-D Research and Development Contract MDA927-94-3-0035-000104 supported this work. The authors wish to thank Dr. Peter Ludlow and members of the participating companies (Boeing, Raytheon, MicroModule Systems and Intarsia Corporation) for many helpful discussions.
References:
- G. Schuldt and C. MacKay, "Amalgams for Electronics Interconnect," Proceedings of the 7th Electronics Materials and Processing Congress, Cambridge, Mass., August 1992, pp. 141-147.
- T. Dolbear, "Liquid Metal Pastes for Thermal Connections," Proceedings of the 7th Electronics Materials and Processing Congress, Cambridge, Mass., August 1992, pp. 133-139.
- D. Baldwin, R. Deshmukh and C. Hau, "Preparation and Properties of Gallium Alloy for Use as Microelectronic Interconnect Materials," The International Journal of Microcircuits and Electronic Packaging, Vol. 19, No. 1, 1996, pp. 37-45.
- C. MacKay, "Amalgams as Alternative Bonding Materials," International Electronic Packaging Society Conference, 1989, p. 1259.
- C. MacKay, "Bonding Amalgams and Method of Making," US Patent 5,053,195, October 1, 1991.
- G. Rodriguez, "Analysis of Solder Paste Release in Fine Pitch Stencil Printing Processes," Master's Thesis, School of Mechanical Engineering, Georgia Institute of Technology, 1998.
- A. Dang, I. C. Ume and S. Bhattacharya, "A Study on Warpage of Flexible SS Substrates for Large Area MCM-D Packaging," ASME J. Electronic Packaging, June 2000.
- A. Dang, I. C. Ume and S. Bhattacharya, "In-situ Warpage Measurement During Thermal Cycling of Dielectric Coated SS Substrates," ASME J. Electronic Packaging, June 2000.
SWAPAN K. BHATTACHARYA, senior research scientist, and DANIEL F. BALDWIN, assistant professor, can be contacted at Georgia Institute of Technology, Packaging Research Center, 813 Ferst Drive, Atlanta, GA 30332-0560. For more information, contact Swapan Bhattacharya at 404-894-7891; Fax: 404-894-0957; E-mail: [email protected].