Tutorial #70
George Riley
January 2007
Based upon the paper “Lithography-Grade Controlled Expansion Substrates for Wafer Level Packaging” by Greg Rudd and Bob Cronk, presented at SMTA’s IWLPC 2006.
The continuing drive towards more power in less space has increased the demand for substrates such as aluminum nitride, with relatively low coefficients of thermal expansion (CTE) and high thermal conductivity (TC). A parallel drive, towards the cost savings of wafer-to-wafer packaging, has placed stringent mechanical requirements on wafer-bondable substrates for power applications. In addition to high TC and low CTE, wafer-bondable substrates must also meet the stringent size and mechanical specifications of the wafers.
Tungsten-copper (W/Cu) metal composites have long been used for heat sinks and high power component packaging, because they combine high thermal conductivity with ease of machining, plating, and brazing. Now W/Cu wafers are available as substrates for wafer-level bonding. They meet lithography-grade mechanical specifications with the added benefit of a CTE precisely tailorable over a range from 6.4 to 8.1 ppm, depending upon the proportions of each constituent. Table 1 gives the material properties for different ratios of tungsten to copper.
|
Table 1. Typical Material Properties |
||||
| Composition (W/Cu weight %) |
90/10 |
85/15 |
80/20 |
75/25 |
| Thermal Expansion (ppm/K) |
6.4 |
7.0 |
7.6 |
8.1 |
| Thermal Conductivity (W/m-K) |
201 |
210 |
219 |
229 |
| Density |
17.2 |
16.6 |
16.2 |
15.7 |
Tungsten-copper is neither an alloy nor a compound, it is a metal-matrix composite. As shown in figures 1A and 1B, the copper and tungsten particles retain their identity in the mix. These SEM images of tungsten-copper fracture surfaces show the matrix, with copper “necks” surrounding the angular tungsten grains. This dispersed copper increases the ductility of the composite.
Figure 1A. Standard 90/10 W/Cu using 4 – 7 micron average particle size tungsten powder, showing the copper surrounding each tungsten particle.
Figure 1B. Fine grain 90/10 W/Cu using 1 – 3 micron average particle size tungsten powder.
The composite is formed by powder metallurgy. Pure tungsten powder is compressed under high pressure in rubber molds. No binders, lubricants, or additives are used in the process. The compacted power has a porous, chalklike consistency. It can readily be can be handled and sawn to dimensions approximating the finished wafer.
The compacted powder is sintered at 2000°C in a hydrogen furnace to achieve the desired degree of porosity. High purity oxygen-free strips of copper metal are then placed in contact with the tungsten, and the combination is reheated above 1,083°C, melting the copper. The molten copper is drawn into the tungsten matrix by capillary action, forming the metal composite.
The composite is mechanically processed by grinding, annealing, lapping, and polishing into substrate wafers. Annealing and double-sided lapping relieve any residual stresses, insuring that the wafer maintains flatness during any subsequent high-temperature processing.
Lithographic processing requirements limit the allowable amounts of distortion in the substrate. Typical substrate limits, based upon wafer specifications, are:
- Bow: Less than 0.015″ (40 micron)
- Warp: Less than 0.015″ (40 micron)
- TTV: Less than 0.0006″ (15 micron)
- Surface, Ra: < 2 μinch (0.05 micron)
The wafers can be metallized by plating and physical vapor deposition as desired. Since clean W/Cu is difficult to attach to, an adhesion layer of electroplated or electroless nickel may be deposited, and sintered for stronger bonding to the W/Cu. Alternatively, reactive metal adhesion layers can be vacuum deposited. Copper, gold, or other metals may be plated or deposited over the adhesion layer. Silver or tin/silver solder is often used for attachment. Direct bonding of composite substrates to gallium arsenide wafers for improved RF device performance is being offered by one manufacturer at www.epitactix.com
In summary, tight dimensional control, excellent thermal performance, and selectable CTE make W/Cu wafers an attractive substrate for wafer to wafer packaging of high power devices.
FOR MORE INFORMATION
The full paper may be found in the proceedings of SMTA International Wafer-Level Packaging Conference, November 2006, available from SMTA.
Company Contacts
Spectra-Mat,Inc – Watsonville, CA 95076
Phone: 831 – 722 – 4116