High-power RF and microwave transistors have long used various materials with high thermal conductivity as substrates and heat spreaders, including copper whose thermal conductivity is 400 W/mK, copper-tungsten (200 W/mK), copper molybdenum (250 W/mK) and copper-molybdenum-copper (350 W/mK). For GaN devices, SiC with thermal conductivity up to 400 W/mK, is the substrate material used for most high performance devices.
However, industrial-grade synthetic diamond has much higher thermal conductivity, which ranges from 1200 to 2000 W/mK, which allows heat to be removed far more effectively than with any other material. In recent years, GaN device manufacturers have become much more interested in its potential to significantly increase the performance of GaN discrete and MMIC devices both as a substrate and heat spreader. There are currently a few devices using GaN on Diamond substrates, but aluminum-diamond metal-matrix composites (MMC) fabricated by Nano Materials International Corp. (NMIC) are increasingly used as heat spreaders for GaN devices with very high RF output powers (see Figure 7).
Aluminum-diamond MMCs heat spreaders are located below the die or package and can be employed regardless of the substrate material with a thermal conductivity of about 500 W/mK. Aluminum-diamond can work efficiently with die-attach processes and has yields equal to or even better than traditional heat spreader materials. NMIC’s aluminum-diamond MMC’s employ an aluminum alloy composition that is infiltrated into a packing of industrial-grade diamond particles.
The diamond provides exceptional thermal conductivity and the aluminum provides structure and CTE matching, as well as a very smooth surface on the top and bottom that serves as the attach face. The process flow employed in producing NMIC aluminum-diamond MMCs is shown in Figure 8. The infiltration process produces aluminum-diamond plates called “mother plates,” and multiple parts are cut from the mother plates with their size optimized to meet part dimensions. This reduces the amount of waste material produced by the cutting process. These parts are then nickel- and gold-plated to produce the final product. One of the challenges during development was related to the interface between the two materials. To solve it, NMIC developed technology to convert the surface of the diamond to SiC, a technique the company has patented.
In the last few years, the process has become very well-characterized, and aluminum-diamond MMCs are now both cost-effective and available in high volumes in sizes up to 45 mm x 45 mm—larger than other diamond-based alternatives that are limited in size or thickness. Customers are using the MMCs in both hermetic and non-hermetic packages in high-reliability and space-qualified applications, both having stringent thermal cycling requirements.
In addition to high thermal conductivity, a stable CTE is essential when a material is attached to a transistor package. Aluminum-diamond’s CTE has always been the equal of other engineered materials, but its stability in larger sizes sets it apart, with a CTE of 6.5 to 7.5 ppm/K, which is close to that of GaN die.
The improvements made to aluminum-diamond MMCs have in a few short years transformed this technology into one that is proving to be a viable alternative to other heat spreader materials. It exploits the inherently high thermal conductivity of diamond while also being manufacturable in large quantities and delivering better CTE performance than diamond alone.