Synthetic Diamond for Thermal Management of Electronic Devices

Q&A with Thomas Obeloer, Business Development Manager
Element Six Technologiesí†

Why is Diamond Considered a Supermaterial?
Supermaterials are called such due to their unrivaled ability to deliver extreme performance across a variety of applications. Diamond has a range of physical and chemical properties that make it one of the most versatile supermaterials on the planet. †For example, a few of these properties include the highest known thermal conductivity, widest optical transparency window, high electrical carrier mobility, chemical inertness, wide bandgap, electric insulation and even long quantum decoherence times for spin qubits. Many of these properties stem from its molecular structure, also key to its exceptional mechanical properties linked to its hardness. This combination of properties combined with new manufacturing techniques means diamond is finding new technological applications across precision machining, optics, electronics, wastewater treatment, sensors and semiconductor industries.

Figure 1: Single crystal chemical vapor deposition (CVD) diamond

Figure 1: Single crystal chemical vapor deposition (CVD) diamond

How is Synthetic Diamond Different from Natural Diamond?
The atomic structures of natural and synthetic diamond are identical, however they differ in their production method. Mined diamond is formed deep underground in molten rock when carbon material is subjected to extremely high pressure and temperatures over long periods of time. Natural diamonds were first classified by their optical properties, with most having an absorption edge of around 330 nm (Type I) and a small percentage having an absorption edge of around 220 nm (Type II) as a consequence of nitrogen impurities incorporated during the natural diamond growth process. It is as a consequence of these impurities and that no two natural diamonds have experienced exactly the same conditions, which make them unique. While this might be desirable in gems, for industry the variation, scarcity and cost negatively impacts their use.

In contrast, techniques developed in the lab for growing synthetic diamond address these challenges making synthetic diamond a viable engineering material. Today two techniques are mostly used to produce the bulk of industrial diamonds, the first to some extent mimics the conditions natural diamond are made at high temperatures and pressures (HPHT) only in a repeatable environment. These diamonds are often yellow in color as a consequence of this growth process and a specific nitrogen impurity. These diamonds can then be sintered into disks that can be used for rock and drilling applications and have the appearance of black disks.

A second synthetic diamond growth technique, called chemical vapor deposition (CVD) exploits the relatively small difference in stability between the two allotropes (sp2 and sp3) of carbon at elevated temperatures. Heating a gas containing typically methane and hydrogen gas to temperatures in excess of 2000įC produces a plasma of carbon and atomic hydrogen species. These carbon-containing species precipitate out like rain and form a mixture of graphite and diamond forms on a much cooler substrate typically held at <1,000įC. However, in the presence of the hydrogen atoms the graphite is etched away leaving behind the diamond.

The CVD process enables growth of both polycrystalline and single crystal diamond. By controlling the properties of the plasma and using dopants itís possible to tailor these grades for considerations of performance and cost, e.g. optical transparency or thermal conductivity. CVD enables all of diamondís properties to be unlocked and is the preferred process for certain applications such as thermal management due to the ability to control the diamond purity and create high quality materialsóto routinely achieve values >5x of copper or 10x that of other ceramic thermal conductors such as aluminum nitride (AlN) or beryllium oxide (BeO).

What Factors Have Caused Thermal Management to Become an Issue in the Electronics Industry?
As electronic devices become smaller and more powerful, it becomes more and more difficult to manage heat. Considering that 50 percent of electronic failures occur due to heat-related issues, there is a large demand for advanced thermal management solutions. This is particularly crucial for gallium nitride (GaN)-based RF semiconductor devices, such as might be used in mobile base stations or RADAR, where thermal obstacles have limited the ability to achieve GaNís intrinsic performance potential. GaN-based devices generate heat in extremely small and concentrated areas and create high operating temperatures which can negatively affect device performance and endurance. In some cases, when a GaN device delivers very high RF power, its lifetime can be degraded if heat is not removed effectively from the device channel.

What Properties of Synthetic Diamond have made it Ideal for Thermal Management, Specifically?
Most materials with high thermal conductivity are also electrically conductive, such as copper, however synthetic diamond has thermal conductivity that is more than five times higher than the best metal values. In addition to being an insulator, this is critical in applications where dielectric loss is important or where electrical isolation is needed between the device and ground. In contrasting applications where a conducting substrate is needed, we have developed metallization techniques that encapsulate the diamond but without significant reduction in the bulk thermal conductivity. This is invaluable for electronics, where diamond-enabled heat dissipation prolongs the lifetime of those electronic devices without impacting performance.

Diamondís other properties such as mechanical stiffness, chemical inertness and low density are also very desirable in applications in robust environments.

CVD diamond in particular has become most valuable in the development of heat spreaders due to its status as the highest known thermally conductive material, low density and low thermal expansion coefficient. Since diamondís thermal conductivity is four times higher than copper, these heat spreaders enable support of a larger mechanical and thermal load in thermal management applications, enabling system operation at elevated temperatures, enhanced heat removal capability and, crucially, reduced overall system cost.

Figure 2: Various grades of polycrystalline diamond grown by chemical vapor deposition (CVD)

Figure 2: Various grades of polycrystalline diamond grown by chemical vapor deposition (CVD)

Specifically, CVD diamond heat spreaders can reduce temperatures in GaN devices by more than 20 percent. They are applicable in all types of electronic and electrical applications including telecommunications and microelectronic devices, where the build-up of heat can destroy circuitry or severely impair performance. Properly spreading and dissipating heat is essential to enabling smaller, more powerful electronic devices, and synthetic diamond has proved to be the most effective thermal management material available.

In What Ways can Synthetic Diamonds Unique Properties be Applied to Electronic Devices?
Diamondís exceptional thermal conductivity and insulating properties make it the material of choice to significantly reduce thermal resistance in a variety of electronic applications. CVD diamond is available in different grades with thermal conductivities ranging from Tc>1000 W/mK to Tc>2000 W/mK. Also, CVD diamond has isotropic characteristics, enabling enhanced heat spreading in all directions.

Element Six has also developed a range of electronic grade CVD diamond that can be used in a variety of demanding radiation detection applications, including high energy physics, neutron and other high radiation detection, quantum information processing and electrochemical sensing applications.

Figure 3: A CVD diamond heat spreader

Figure 3: A CVD diamond heat spreader

This proprietary CVD process to grow diamonds enables material engineers to tightly control growth conditions, eliminate chemical impurities and manifest various properties into diamond material. This ensures production of a highly consistent material with predictable properties and behavior that enables a diverse range of applications, including heat spreaders for high power and high frequency electronics.

Has Diamond Made any Significant Advances in the Thermal Management of Electronic Devices?
A recent collaboration between Element Six and the Institute of Microelectronics in Singapore applied a diamond heat spreader with a hybrid Silicon micro-cooler to cool GaN devices. The heat dissipation capabilities were compared through experimental tests and fluid-solid coupling simulations, both showing consistent results. In one configuration, using a diamond heat spreader 400 μm thick with a thermal conductivity of Tc†> 2000 W/mK, to dissipate 70 W heating power, the maximum chip temperature could be reduced by 40.4 percent, for test chips 100 μm thick. At 10 kW/cm2, hotspot heat flux could be dissipated while maintaining the maximum hotspot temperature under 160ļC. The concentrated heat flux was effectively reduced by the diamond heat spreader, and much better cooling capability of the Silicon micro-cooler had been achieved for high power GaN devices.

How Can These Advancements Enable Further Innovation in the Electronics Industry?
As power densities continue to increase in application areas as diverse as base station telecommunications to laser diodes for car head lamps, the challenges for improved thermal management are likely to only increase. Diamond is well positioned to capitalize and help enable some of these transitioning new technologies, offering significantly reduced system costs with simultaneous increased performance.

There is need in the technical community to continue to develop the processes and integrations schemes consistent with volume, cost and performance. Further investment in packaging and novel metallization methodologies is likely to reap a return.

Going forward, the bottle neck in improved thermal management remains the interface between the diamond heat spreaders and electrical devices. It is encouraging to see research institutes begin to ask the questions what is the next wide bandgap semiconductor materialóbeyond SiC and GaN and look at device geometries whereby diamond is also the active part of the electrical deviceónot just the thermal heat spreader to cool the device. This is perhaps the ultimate hope for diamond in electronic devices.

About Thomas Obeloer
Thomas Obeloer is a business development manager at Element Six Technologiesí Santa Clara, CA branch.† He works on thermal management and optical applications of CVD diamond. He holds a masterís degree in mechanical engineering from Hanover University (Germany) and has more than 20 yearsí experience in manufacturing, process engineering and business development. For the last 15 years, he has been involved in sales and marketing activities for advanced thermal management, mainly in the field of advanced materials, such as high-performance ceramics, CVD diamond and diamond composites.

For more information please visit†www.e6.com

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