Increasing power densities and decreasing transistor dimensions are hallmarks of many of today’s semiconductor devices, including high-voltage power transistors, laser diodes, and RF power amplifiers. GaN devices, such as power converters for photovoltaic cells and cellular base-station power amplifiers, are further enabling higher power densities in smaller die sizes. Both trends are significantly increasing the thermal management challenge within the chip and surrounding package or module.
Thermal-management solutions for these high-power-density devices must go beyond today’s commonly used materials to prevent forcing designers to compromise on performance, or reduce their design thermal margins, impacting product reliability. Chip lifetimes are heavily dependent upon operating temperature, with every 10°C increase in junction temperature representing a 2x decrease in device lifetime. In fact more than half of the failures in today’s electronic systems are due to temperature.
Synthetic diamond is a good material for thermal management in semiconductor packaging for advanced electronic systems driving towards higher power density as it combines exceptionally high thermal conductivity with electrical isolation. Room temperature thermal conductivity values greater than 2000 W/m K have been reported for the highest quality polycrystalline synthetic diamond grown by microwave plasma assisted chemical vapor deposition (CVD) techniques. This is the highest value of all known materials and exceeds that of copper, which is commonly regarded as an excellent thermal conductor, by a factor of more than five and by over a factor of 10 higher than commonly used high K electrical insulators, such as aluminium nitride or beryllium oxide (Figure 1). The commercial availability of large CVD diamond plates (Figure 2) has opened a host of possible options in which CVD diamond can be used in the heat management of electronic and opto-electronic devices. Control of diamond grown by CVD has allowed the synthesis of material with optimized cost/performance for various thermal-management challenges in semiconductor packages and modules.
How Thermal Conductivity Works in Diamond
In contrast to a metal, where thermal conductivity is provided by the mobility of conduction band electrons, heat transfer in the insulator diamond is solely carried by lattice vibrations, i.e. phonons. The reason for the outstanding thermal conductivity is the stiffness of the sp3 bonds forming its rigid structure together with the low mass of the carbon atoms.
Using a microwave chemical vapor deposition (CVD) synthesis process, polycrystalline diamond can be engineered to optimize the cost/performance ratio for various thermal-management applications. Contributors to increasing the phonon scattering and thermal resistance, and thus decreasing the thermal conductivity, in polycrystalline diamond are a sample’s impurities, vacancies, grain boundaries, dislocations and extended defects. Common impurities that can impact the thermal conductivity in diamond are nitrogen, hydrogen, and the 13C isotope of carbon. The highest thermal conductivity is expected for isotopically pure 12C diamond with very low nitrogen content. By tailoring the synthesis process, compromising between cost and performance, it has been possible to produce a range of thermal grades with thermal conductivities varying by a factor of two between 1000 W/mK and 2000 W/mK (Figure 1).
Integrating Diamond into Semiconductor Packages
The effectiveness of CVD diamond as a heat spreader in power-converter or RF electronic packages depends very much on how it is integrated into the package. CVD diamond can be integrated in the following two different ways:
- Prefabricated wafers to hold multiple devices allowing wafer scale processing at device manufacturers (including metallisation & mounting), followed by singulation of wafers to produce individual subcomponents. For example, Figure 3 shows a GaN-on-diamond wafer;
- Free-standing, individual CVD diamond units bonded using conventional metallization and soldering techniques; singulated heat spreaders (Figure 4) can be laser cut to any size with typical thicknesses ranging from 200 to 2000 microns.
Of course, the thermal conductivity alone is not the whole story. For a truly compelling case for CVD diamond in electronic packaging thermal management solutions, the thermal engineer must consider carefully:
- Device requirements. What are the current requirements (i.e. a laser diode array may require 50-100 A)? Is the heat uniformly distributed across the device (such as in an IGBT), or are there local hot-spots?
- Thermal barrier resistance of the thermal interface materials, which contributes to the overall thermal resistance in the heat path.
- Required heat spreader characteristics including electrical conductivity or isolation, coefficient of thermal expansion, density, and flatness.
The thermal barrier resistance should be minimized. In a metallized bonded approach, for example, the thickness of the metal, its thermal conductivity, and interfacial thermal properties all contribute to an overall thermal barrier resistance. Additional factors that contribute to the overall thermal barrier resistance are the flatness and roughness of the interface. Flatness and roughness are important for minimizing thermal resistance and improving adhesion. Any deviation of the surfaces must be accommodated by greater thickness of the bonding material, thus increasing the thermal resistance. CVD diamond is processed using a combination of lapidary and polishing techniques to planarize, control thickness, and apply a surface finish to the material. The average surface roughness can be controlled to Rα <20 nm. Depending on the wafer diameter and thickness, the flatness can be controlled to as low as <1 μm.
CVD diamond is a chemically inert material, therefore common metals do not adhere well to it. To metalize CVD diamond, carbide forming materials must be used. A commonly used metalization scheme is Ti/Pt/Au. The Ti layer is key to adhesion by the formation of a carbide layer at the diamond interface. The thickness of the Au layer depends on whether it is a base layer for soldering or wire-bonding. The Pt layer serves as a barrier to Au diffusion. The best results are achieved where the sputtering energy can be controlled to ensure carbide formation at the Ti/diamond interface. A metallic bond to the die is therefore preferable, such as solder (joining temperature 100-300°C), or a thermal compression joint or diffusion bond (joining temperature typically >450°C). A common joint for mounting electronic devices for example is Au0.8Sn0.2 eutectic solder.
The coefficient of thermal expansion (CTE) must be considered for a number of crucial reasons. The use of CVD diamond as a heat spreader can either lower the operating temperature of electronic and opto-electronic devices or it can enable a higher operating output power for the same junction temperature. In either case, changes in temperature result in changes in length of materials bonded in a stack, which results in thermo- mechanical stress in the device. The magnitude of these stresses depends on the size/geometry of the device, the change in temperature during attachment and during operation, and the mismatch in TCEs within the stack. Unless properly managed, these stresses can significantly impact the lifetime of the device. A common approach to managing these stresses is using compliant layers such as pure indium or indium compounds to attach the CVD diamond heat spreader and the device, with careful control of the thickness of such a layer to avoid increasing the overall thermal resistance.
Rapidly increasing power densities are making the thermal management of semiconductor devices significantly more challenging. CVD synthetic diamond, with the highest thermal conductivity of any material, is therefore increasingly being used in heat spreaders for high-power die in such applications as high-voltage power switching for photovoltaic cells, planar optical ICs, and even high-power LEDs.