Conduction in Diamond Films

2. Thermal Properties of Diamond Passivation
Maxat N. Touzelbaev

At room temperature, chemical-vapor-deposited (CVD) diamond is an excellent thermal conductor and an electrical insulator, making it ideal for passive use in high-power electronic systems. The impact on heat-transfer is greatest if the diamond is within micrometers of active semiconducting regions, which motivates the use of diamond passivation in direct contact with active regions. Figure 1 illustrates diamond step coverage on a micromachined silicon structure, which represents important progress towards passivation of operating devices. Because the thermal-conduction properties of diamond within micrometers of its interfaces with other materials are far poorer than those of bulk diamond, diamond passivation may not achieve its targeted cooling capabilities.

This project provides data and analytical tools needed to optimize the design and deposition of microscale passive diamond regions. The effective thermal resistance at diamond boundaries is related to the diamond microstructure near the interface and to the details of the deposition process using phonon transport theory. Electron micrographs are used to subdivide the near-interface region into a series of layers distinguished by the volume fraction of amorphous carbon and the grain orientation. The phonon scattering rate and local thermal conductivity are related to the minimum diamond grain size and its variation within the film. Measurements of the thermal resistance are performed using rapid heating and thermometry at the surface of a metal overlayer, as depicted in Figure 2. Parallel research on diamond deposition relates the diamond microstructural details, including the nucleation density and grain orientation, to the details of the growth process, including the nucleation pretreatment. Figure 3 compares data and predictions for the total thermal resistance for conduction normal to thin diamond layers.

This study yields specific recommendations for the diamond deposition process that result in a lower effective thermal resistance for conduction into diamond passivation. This project also determines the benefit in performance and reliability that can be expected for classes of power devices through the use of diamond passivation.

Figure 1: Cross-sectional scanning electron micrograph of a polycrystalline silicon microbridge onto which diamond has been deposited (courtesy of H. Güttler, Daimler-Benz AG). This sample represents progress towards the use of diamond as passivation for power integrated circuits, with the motivation of improving conduction cooling. The small grains shown in this image impede thermal conduction by scattering phonon heat carriers.

Figure 2: Schematic of one technique used to measure the vertical thermal resistance of deposited diamond films. A heating pulse of duration near 10 ns from the Nd:YAG laser yields a rapid temperature rise and decay which are captured using laser-reflectance thermometry. See Journal of Applied Physics, Vol. 77, p. 1385 (1995).

Figure 3: Thermal resistance (10-8 m2 K W-1) for conduction normal to diamond layers grown at varying temperatures. The grain size at the substrate interface is d_G₀. The rate of change of the grain size with the vertical coordinate is g. Diamond and Related Materials Vol. 7, pp. 1-14 (1998).

Collaboration
Materials Research Group, Daimler-Chrysler.

Recent Publications

Touzelbaev, M.N., and Goodson K.E., 1997, "Impact of Nucleation Density on Thermal Resistance Near Diamond-Substrate Boundaries," AIAA Journal of Thermophysics and Heat Transfer, Vol. 11, pp. 506-512.

Touzelbaev, M.N., and Goodson, K.E., 1998, "Applications of Micron-Scale Passive Diamond Layers for the IC and MEMS Industries," Diamond and Related Materials, Vol. 7, pp. 1-14. Invited paper at DIAMOND 1997, Edinburgh, Scotland, August 3-8, 1997.

Goodson, K.E., 1996, "Thermal Conduction in Nonhomogeneous CVD Diamond Layers in Electronic Microstructures," ASME Journal of Heat Transfer, Vol. 118, pp. 279-286.

Goodson, K.E., 1995, "Impact of CVD Diamond Layers on the Thermal Engineering of Electronic Systems," Annual Review of Heat Transfer, Vol. 6, Begell, New York, pp. 323-353.

Goodson, K.E., Käding, O.W., Rösler, M. and Zachai, R., 1995, "Experimental Investigation of Thermal Conduction normal to Diamond-Silicon Boundaries, " Journal of Applied Physics, Vol. 77, pp. 1385-1392.

Major Review Articles and Book Chapters

Touzelbaev, M.N., and Goodson, K.E., 1998, "Applications of Micron-Scale Passive Diamond Layers for the IC and
MEMS Industries," Diamond and Related Materials, Vol. 7, pp. 1 - 14. 7.

Goodson, K.E., 1995, "Impact of CVD Diamond Layers on the Thermal Engineering of Electronic Systems," Annual
Review of Heat Transfer, Vol. 6, Begell, New York, pp. 323-353.

Sponsorship Daimler-Benz AG. Defense Advanced Research Projects Agency

Research Projects
New! Micromachined Two-Phase Heat Exchangers
1. Thermal Engineering of IC Devicesand Metallization
2. Thermal Conduction in Diamond Films
3. Thermal Conduction in Organic Films and Devices
4. Thermal Conduction in Silicon Films and Devices
5. Thermal Engineering of IC Packaging
6. Near-Field Photothermal Diagnostics
7. Novel Thermal MEMS
8. Microdevices in the Heat Transfer Teaching Laboratory

Last Update: February 25, 1999.
Comments: Bill King