Raytheon's Diamond TechnologyProviding Thermal Management for Power Semiconductors
Next-generation radar, communications and electronic warfare systems, especially those employing high-power gallium nitride (GaN) based radio frequency (RF) devices, will benefit from advanced methods of thermal management to remove the large quantities of heat generated in these systems. Figure 1 compares the thermal conductivity of materials commonly used in the semiconductor industry. Diamond, especially the high optical quality material produced at Raytheon using the chemical vapor deposition (CVD) process, has considerably higher conductivity than the other conventional materials.
Diamond Manufacturing Process
Raytheon is an acknowledged leader in the growth and fabrication of diamond in sheet form. At Raytheon, diamond is synthesized by the CVD process from methane and hydrogen gases in the presence of a microwave plasma. The microwave plasma CVD process (MPCVD) produces the highest quality diamond; other methods, such as hot filament CVD, tend to produce lower quality, less thermally conductive material due to the incorporation of impurities from the hot filament.
Figure 2 illustrates a microwave plasma CVD reactor employed at Raytheon to produce high quality diamond. The microwave plasma, created by exciting hydrogen gas with microwave radiation, decomposes the methane gas into several different carbon species that react/combine with hydrogen in the gas. Diamond growth takes place on a metallic substrate onto which diamond particles have been added. These particles or diamond “seeds” act as nucleating sites for the diamond wafer growth. The atomic hydrogen formed in the plasma plays a critical role by etching away any graphitic or non-diamond material that might deposit along with diamond. Rotation of the metal disk ensures diamond thickness uniformity.
Raytheon’s diamond deposition reactors are capable of growing up to 5-inch diameter diamond parts. In Figure 3, one can see the various sizes and shapes of diamond material produced at Raytheon.
Diamond Heat Transfer Technology
The primary use for diamond at Raytheon is thermal management, specifically the dissipation of the large heat flux that is generated by high-power devices such as GaN HEMTs (high electron mobility transistors).
High junction temperatures degrade RF performance and decrease reliability. A common approach to reduce temperatures is to space transistor gate fingers further apart and reduce the operating power density. However, this increases monolithic microwave integrated circuit (MMIC) size and cost and reduces output power.
A novel solution that is being developed at Raytheon to remove heat from high-power devices and enable them to reliably achieve full performance, is to integrate diamond directly into the device structure as illustrated in Figure 4. Thermal modeling has shown that this technology can improve thermal transport through the substrate so that power handling can be significantly increased relative to the current state-of-the-art of GaN-on-SiC (silicon carbide) technology. Diamond attached to the bottom of the GaN efficiently spreads the extraordinarily high heat fluxes (0.1–1 kW/mm2) typical of GaN device junctions with minimal substrate temperature drop. Although additional heat spreading is expected by incorporating thin film diamond on top of the device, thermal simulations indicate that the majority of the benefit is achieved with the bottom diamond; hence there is little additional benefit to be gained by incorporating diamond coatings in these device structures.
Raytheon’s CVD diamond technology produces very high thermal conductivity material. By combining the extraordinary thermal conductivity of this material with a fully integrated GaN-diamond device architecture, Raytheon seeks to advance our industry-leading GaN device technology to even higher power and performance levels.