Diamond Thermal Spreaders and Heat Sinks

Diamond has the highest thermal conductivity of any material, which makes it an excellent choice for thermal management in electronic devices. Once a semiconducting device is fabricated, the chip must be packaged and built into an electrical system or electronic product. The superior thermal properties of diamond materials - it has the highest thermal conductivity and a low thermal expansion coefficient - can be leveraged for thermal management in electronic packaging and electrical power systems.Diamond’s thermal conductivity is five times higher than copper's. While copper is a great thermal conductor, copper also conducts electricity. Metals conduct heat and electricity via free electrons. Circuits need to be electrically isolated from the copper to prevent shorting. Diamond, however, has high dielectric strength.

Diamond, ceramics, glasses and other electrical insulators conduct heat through phonons or lattice vibrations. Diamond has a rigid lattice due to extremely strong atomic bonding. Diamond’s rigid lattice provides high vibrational frequencies and therefore a high Debye temperature of 2,220 K resulting in limited impedance from phonon-phonon scattering.

Figure 1. Diafilm heat spreaders compared to copper, beryllium oxide and aluminum nitride materials.

The combination of high thermal conductivity and high dielectric strength of diamond heat sinks is valuable for high-power laser diode arrays, RF modules and high-power transistors. Beryllium oxide (BeO) substrates are used in some of the electronic packaging applications due to its combination of high thermal conductivity and dielectric strength. Diamond heat spreaders have replaced toxic BeO spreaders while providing improved heat dissipation. Element 6 provides ranges of chemical vapor deposition (CVD) diamond heat spreaders with varying thermal conductivities under the Diafilm brand name. Electronic heat sinks and packaging substrates made from diamond are excellent at dissipating heat, while also electrically insulating the microelectronics from other devices and circuit components. Efficient heat dissipation prolongs the lifetime of those electronic devices, and the devices' high replacement costs justify the use of efficient, though relatively expensive diamond heat sinks. Synthetic diamond heat spreaders prevent silicon and other semiconducting materials from overheating.

Figure 2. Thermal properties of diamond and various heat spreader materials.

CVD diamond has a much lower coefficient of thermal expansion (about 1 ppm/° C) compared to silicon (2.6 ppm/° C), GaAs (5.7 ppm/° C), GaN (3.2 – 5.6 ppm/° C) and copper (16.6 ppm/° C). The thermal expansion differences must be accounted for and modeled early in the design process to prevent stresses during thermal cycling from reducing device lifetime and reliability. In some designs, electronic devices are sandwiched between two diamond layers to balance stresses.

Diamond substrates can be used as a packaging material through metallization and soldering to provide a thermal solution. Direct deposition of CVD diamond onto devices is another method for integrating a heat spreader into a product. Another approach is to use prefabricated diamond wafers on which multiple conventional semiconductor devices or diamond semiconducting devices are fabricated.

In experiments to develop for high electron mobility transistors (HEMTs) (GaN-on-diamond substrates for HEMT applications), thermal conductivity measurements of the gallium nitride (GaN)-on-diamond and GaN-on-silicon carbide (SiC) indicated that diamond could drive down thermal impedance (° C/W) by as much as 58% compared to SiC, along with three-fold higher power density. As a result, a GaN-on-diamond field-effect transistors could have a power density three times higher. GaN-on-diamond wafers would be an ideal platform for high-power GaN transistors deployed in cellular base stations and military communications applications.