Diamond wafer - The passive way to manage the heat and keep the VSWR low at high frequencies

RF resistors able to operate above 8 GHz while handing >100 W are critical for successful operation of phased array radar and 5G wireless infrastructure. This requires a substrate material that maximizes heat removal whilst keeping parasitic reactance to a minimum. In this work it is demonstrated that this cannot be achieved with traditional substrates (AlN and BeO), but would be achievable by using CVD diamond.

Diamond’s permittivity is ~15-35% lower than those of BeO and AlN respectively and is stable to changes in frequency and temperature, varying by only 5% from low frequencies up to tens of GHz, and only shifting by 730 ppm/°C from room temperature up to few hundreds of °C (Figure 1-a) [7]. Stability of thermal conductivity as a function of temperature is also an important factor for successful operation of resistives at elevated frequencies. At 125°C the thermal conductivity for AlN and BeO are reduced by 30-40% from room temperature values (Figure 1-b). For the same span of temperature change the thermal conductivity of CVD diamond is changed by 10-15%.

Stability of RF and thermal properties enables a resistor made from purest single crystal diamond to potentially outperform AlN and BeO by factors of 10-15, as demonstrated through the ability of diamond to continue to dissipate higher level of the reflected power with increasing temperature. Figure 1-b shows the thermal conductivity of two diamond grades with bulk thermal conductivity of 1000 W/mK and 1800 W/mK, demonstrating significant improvement and stability over the thermal conductivities of AlN and BeO. In combination with its lower permittivity (Figure 1-a), CVD diamond resistives enable a 4-8x improvement in performance over that of resistives built on AlN and BeO substrates.

Figure1: a) Real part of the diamond permittivity vs frequency. Note that at high frequency (>108 Hz) it is almost independent of frequency. Inset, change of diamond permittivity vs temperature. b) Thermal conductivity of the different materials used as a dielectric substrate in high power resistors.

It was demonstrated that by switching the substrate of high power RF resistors from AlN and BeO to CVD diamond it is possible to extend their operative frequency range well above X-band whilst handling powers above 100 W. This could offer a step change improvement towards minimizing distortion and complexity of high power electronics in 5G communications and military millimeter-wave devices operating in X-band and above.

CSMH focuses on the research and production of diamond wafers. At present, it has diamond wafers, diamond heat sinks, GaN on diamond, AlN on diamond and other products，providing professional and leading diamond products and solutions for domestic and foreign customers.