Selective Area Deposition of Hot Filament CVD Diamond on 100mm MOCVD Grown AlGaN/GaN Wafers

Aluminum gallium nitride (AlGaN)/Gallium nitride (GaN)-based electronic devices demonstrates high power density and operating frequencies. These materials are however susceptible to ‘self-heating’ with increasing power density; an effect that limits high electron-mobility in the carrier channel region where current is restricted to a two-dimensional electron gas. The most obvious solution to this shortcoming is to dissipate the unwanted heat, a corrective measure that is unfortunately limited by the thermal resistance between the source and the heat sink. Fortunately, diamond thin films can resolve this issue since they are ideal passive heat spreaders, provided sufficiently low thermal resistance can be achieved.

Recent publications have highlighted the potential for using chemical vapor deposition (CVD) of polycrystalline diamond, where considerable improvements in power density and operating frequency have already been reported following the integration of a diamond heat spreader with high electron mobility transistors (HEMT). Intensive research efforts are devoted to obtain further improvements in diamond and integration approaches with semiconducting materials, like GaN, for improving thermal management in electronics and photonics.

To this note, a Texas State University research team proposed and demonstrated selective deposition of thin diamond using hot filament CVD (HFCVD) on 100mm AlGaN/GaN HEMT wafers without damaging the thin AlGaN barrier layer or GaN channel layer. Their work is published in the American Chemical Society research journal Crystal Growth & Design.

In brief, the novel technique entailed selective diamond growth on AlGaN/GaN wafers. There are two main steps. First, the selective distribution of the nano-diamond seeds is accomplished by dispersal in photoresist, spin coating, and lithographic patterning to produce areas with seed-laden resist surrounded by bare AlGaN/GaN wafer. Using this stage entailed all the steps from selection to growth, then to deposition of the nano-diamonds. Second, the wafer with patterned resist intact are transferred to the HFCVD chamber for diamond growth. The researchers employed various characterization techniques to investigate the resulting physical properties of the diamond film.

The authors were able to achieve diamond features measuring as small as one micrometer in lateral resolution with excellent uniformity across the 100-mm wafer. In addition, significant etching of the GaN layer was observed from morphological and structural characterization when diamond was grown on GaN without protective processes. A thin layer of plasma enhanced CVD silicon nitride, deposited prior to seeding and diamond deposition, was found to be essential to protect the AlGaN/GaN wafer including the thin (25 nm) AlGaN barrier layer at the surface.

In summary, the Texas State University research team presented a novel technique for selectively depositing polycrystalline diamond on an AlGaN/GaN HEMT structure using HFCVD. The approach can be transferable to large-scale GaN wafer production, for achieving integrated diamond heat spreaders for thermal management of high power GaN-based electronic and photonic devices. It is also compatible with many other semiconductor devices where self-heating limits performance.