Introduction

Semiconductor devices are being used today more and more often to either provide power, such as in RF power amplifiers, or control power, such as in power inverters, in electronic systems. These electronic systems range from smart phones to cellular base stations to radar systems and from electrical vehicles to alternative energy. In all cases, the heat dissipation of these semiconductor devices can often be a significant thermal management, and therefore power management, challenge.

Synthetic diamond heat spreaders and GaN-on-Diamond wafers have emerged as a leading technology for thermal management of these new high-power semiconductor devices. This is due to diamond's excellent thermal conductivity, the highest of any material at room temperature and as much as five times that of copper. Diamond heat spreaders can be used with any semiconductor material and can lower the temperature of the gate junction of semiconductor devices from 10 to 30%. GaN-on-diamond wafers of course specifically cool GaN devices, but lower gate-junction temperatures by as much as 40 to 50% and enable more than three times the power density compared to similar GaN-on-SiC devices.

Diamond heat spreaders

Element Six synthesizes the diamond for heat spreaders using plasma-assisted microwave CVD (chemical vapor deposition). Synthetic diamond grown with this method can generate free-standing diamond wafers up to 140 mm in diameter and up to 1mm thick with thermal conductivities greater than 2000W/mK, five times that of copper as Figure 1 indicates. Also as shown in Figure 1, the use of microwave CVD enables precise engineering of the diamond properties, including a range of thermal conductivities that provide different cost-performance ratios to match any application’s specific needs.

Diamond heat spreaders are metalized and then attached to the bottom of semiconductor die, usually with as thin a layer of solder as possible. This attachment method brings the diamond to within 100 to 300 microns of the gate junctions of the device. Figure 2 shows the typical configuration used with diamond heat spreaders, and importantly indicates that diamond spreads heat equally effectively in both lateral and vertical directions; spreading the heat laterally is particularly important for RF power amplifiers which typically have hot spots of less than 1 micron in diameter with intense heat density. The attachment method at TIM1, again as shown in Figure 2, is key to the effectiveness of the diamond heat spreader.

The metallization of the heat spreader and die must be very thin, on the order of a few 100’s of nanometers; and the metallization of the diamond must be done carefully with a carbide-forming metal as the first layer. The attachment solder layer should also be as thin as possible, ideally less than 10-microns. If optimally integrated into a package, a diamond heat spreader can reduce gate junction temperatures by as much as 20 to 30% more than ceramic packages without diamond heat spreaders included.

GaN-on-diamond wafers

GaN-on-diamond wafer substrates, available today for beta shipments, will offer for the first time a new thermal management tool for GaN semiconductor devices. Today’s GaN-on-SiC semiconductor devices are thermally limited from reaching their intrinsic power capabilities, which can be as high as 40W/mm2. The new GaN-on-diamond wafers bring diamond less than 1 micron away from the GaN epitaxial layer, as shown in Figure 3, by removing the original substrate and any interface layers, depositing a new 35-nm dielectric interface layer, and then growing diamond on this new interface layer.

The close proximity of the GaN epi layer to diamond reduces gate junction temperatures of GaN devices by as much as 50% more than similar GaN-on-SiC devices and because diamond spreads heat laterally as well as vertically, also enables more than 3 times the power density of GaN-on-SiC devices. This 3-times power density means that either GaN devices can be 3 times smaller with the same power output or the same size as GaN-on-SiC devices with 3 times more power output, particularly for RF power amplifier devices with their very small hot spots.

System benefits of CVD diamond cooling

Diamond heat spreaders and GaN-on-diamond substrates essentially offer a portfolio of thermal-management solutions with increasing effectiveness for GaN R&F and power devices as indicated in Figure 4. Diamond heat spreaders have approximately 25% lower thermal resistance than ceramic packages without diamond while GaN-on-diamond devices have up to 50% lower thermal resistance relative to similar GaN-on-SiC devices. Thus in the design of RF power amplifiers for radar and communications systems and of power converters and RF power amplifiers for commercial cellular base stations and satellite systems, diamond heat spreaders and GaN-on-diamond devices can be used in varying degrees to reduce cooling complexity and cost or increase lifetimes. In addition, GaN-on-diamond can be used to achieve a three-fold increase in the areal power density of a GaN transistor.

Impact on cooling complexity and costs – A semiconductor’s thermal resistance is an important parameter in the design of a microelectronic module and its associated thermal management system. The thermal resistance and desired lifetime drives the entire system design and sets the requirements for the ultimate coolant temperature. The reduced thermal resistance of diamond heat spreaders and GaN-on-diamond substrates can allow simpler, less expensive thermal management systems by enabling higher coolant temperatures because the temperature rise from the coolant to the gate is lower.

The savings is not only in the reduced cost of the cooling sub-system, but also in reduced on-going cost through energy savings. Alternatively, these diamond thermal solutions can enable significantly longer system lifetimes using the same cooling mechanism and thus operating the gate junctions of power devices at lower temperatures. It is estimated that running these power devices at 10oC lower temperatures will double their lifetimes.

Impact of power density on costs – The reduced thermal resistance of GaN-on-Diamond devices enables higher areal power densities. Various groups have recently shown that the GaN HEMT gate fingers on diamond can be brought threefold closer together on diamond than on SiC. This means devices can be 1/3 the size resulting in smaller and less expensive GaN-on-diamond devices. To the power amplifier merchant seller, processing 3 times fewer GaN-on-Diamond wafers than GaN-on-SiC to achieve the same RF output power means significant reductions in fab costs assuming that commercial GaN-on-diamond wafers are competitively priced compared to GaN-on-SiC wafers.

If the GaN-on-Diamond wafer price is low enough, then some of the power amplifier seller’s savings could be passed on to the system maker in a reduced power amplifier price per watt. In addition, system designers may be able to use fewer power amplifier devices by taking advantage of the higher power density to generate more power per device of the same size. Since each power amplifier requires peripheral circuitry to support it, fewer power amplifiers would mean a reduction in peripheral circuitry, thereby lowering system cost.

Conclusion

As high-power semiconductor devices get smaller and hotter, their increased heat flux presents more challenging thermal management. CVD diamond, with its highest thermal conductivity of any material at room temperature, offers solutions to these thermal management challenges in the form of either heat spreaders or GaN-on-diamond substrates. Both diamond solutions decrease the temperature delta between device gate junction and package substrate or heat sink, to varying degrees, thus lowering system costs, increasing lifetimes, or both.

Heat spreaders typically are 100 to 300 microns away from device gate junctions; the closer the heat spreader is to the device gate and the higher the thermal conductivity of the material in between, the more effective the diamond. GaN-on-diamond substrates can bring GaN gate junctions to within one micron of the diamond, thereby dramatically increasing the effectiveness of the diamond. Research in academia and other Government institutions around the World continue to look at the Holy Grail of active semiconductor devices made in diamond, to obtain not only the benefits of ultimate cooling, but also leveraging some of diamond’s other intrinsic properties such as high combined mobilities and breakdown strength.�