Chemical Vapor Deposition of Diamond
Applications to Thermal Management

Introduction
Passive electronic applications of chemical vapor deposited (CVD) diamond films to thermal management are now readily available. In the last few years, CVD diamond films have been deposited which have achieved thermal conductivity values similar to natural diamond (20 W/cm °K). For most heat sink applications, thermal conductivities greater than 8 W/cm °K are adequate, and films which meet or exceed this specification are routinely deposited. These films are commercially available from all the major players in the CVD diamond market: DeBeers, Diamonex, Norton, and Sumitomo.

Natural and high pressure high temperature (HPHT) synthetic diamond heat sinks have been used for years in specialized applications. Their cost increases dramatically with size, and they are not available in sizes beyond a few millimeters. Thick CVD diamond films can be deposited over relatively large areas and, therefore, offer the possibility for new, unique applications such as in laser diode mounts, multi-chip modules (MCMs), surface acoustic wave (SAW) devices, and high power GaAs-based circuits. These films are polished, metallized, and laser cut to the desired size and shape.

Technical Issues Thermal conductivity is the most important requirement in thermal management applications. Values in excess of 8 W/cm °K (about twice the value of copper) are adequate and routine in most applications. Higher thermal conductivities are possible but, to achieve them, films must be grown at lower rates. Therefore, production costs increase to the point where film prices can be prohibitive. Films must also be reasonably thick in order to spread the heat properly. As the typical thickness required is 300 to 500 µm, deposition techniques must have high enough rates to keep deposition times within practical limits.

For many applications, as-grown CVD diamond has too rough a surface for metallization. The substrate side of the film is often smooth but is typically finer grain diamond material with poor thermal conductivity. Thus, films require lapping or polishing after deposition (usually on both sides). Polishing significantly impacts production costs, due to the hardness of diamond, and new polishing techniques are constantly being developed in an effort to reduce these costs.

Specifications are quite stringent for flatness and bow of the films. These specifications become increasingly difficult to satisfy as the area of the film increases. Since film deposition takes place at substrate temperatures near 1000 °C, and at high heat fluxes through the substrate, the substrate bows slightly. Even in the absence of film internal stress, as the substrate is separated from the film, the film can bow and typically takes a "potato chip" shape. The proper design and choice of substrate, in combination with an optimized deposition temperature, can eliminate these problems. Cracks can also develop in the films leading to destruction during the polishing process. This characteristic would have a deleterious impact on yield and, therefore, process techniques which eliminate cracks must be developed.

General Dynamics(1*) reported using CVD diamond substrates for thermal management in an avionics module for the U.S. Air Force. The thermal properties of diamond permitted a cost reduction of the overall device package even though the price of the diamond substrate was higher than the previous substrate. Similar applications will become more commonplace as engineers become acquainted with the availability of diamond as a substrate material.

Sumitomo of Japan has been examining applications of CVD diamond substrates for SAW filters in wireless telephones. This application represents another example wherein the unique properties of diamond make it the material of choice. In this case, a diamond substrate would be part of every wireless telephone built, representing only one example of the potentially large market for CVD diamond substrates.

Thermal Management Market and Players
AT&T announced(2*) that thermal management comprised the largest active CVD diamond market today. AT&T estimated their own consumption for 1993 to be on the order of $10M in finished laser diode heat sinks, a significant increase from 1992, estimated at approximately $5M.

For the past several years, the U.S. government, through the Defense Advanced Research Projects Agency (DARPA), has funded the development of CVD diamond for MCMs. DARPA has made a multi-million dollar investment to develop low cost diamond substrates for MCMs, and ASTeX continues to participate in this program.

Diamond Material and CVD Equipment Requirements
In addition to the necessary thermal conductiivities, other CVD diamond requirements are:

Each material requirement translates into a performance specification for equipment in this application area:

The following table shows a sample analysis of costs. We investigated the costs of CVD diamond on an annual basis, assuming a conservative 85% uptime and operation 310 days of the year for the reactor. Thus, a total of 7,500 hours of reactor operation is possible each year. The system uptime estimate takes several factors into account:

For this cost analysis, we have assumed typical thermal management quality deposition processes which are capable of 4 µm/hour and 7µm/hour rates, respectively.

Building costs have not been included in this simplified model. We have also assumed that a diamond production infrastructure would already exist at the customer site to absorb the rest of the FTE labor. Microwave-based CVD diamond deposition technology is a definite economical alternative to both filament- and torch-based deposition systems.

Equipment capabilities and status of process knowledge at Seki Technotron.
Seki offers a variety of diamond deposition systems which includes the AX5200, AX5250, AX6500 and AX6600, all capable of producing quality diamond materials.
The AX6500 and AX6600 have already proven the processing solutions of CVD diamond materials for the thermal management market in a R&D and production environment.

The plot above shows a typical Raman Spectrum* (taken with a Seki Technotron STR250 Raman Spectrometer) for a high quality diamond film grown in a Seki Microwave Plasma CVD System. There is no significant contribution to the spectrum from non-diamond carbon phases usually present at 1550cm^-1.

The AX6500 demonstrated diamond growth rates which were crucial to Seki’s ability to introduce equipment(AX6600) to this market area in a timely and cost effective manner. Seki has already demonstrated that material from the AX6600 satisfies the thermal conductivity criteria stated above.

The greatest impact on the price of bulk diamond is still to come. The prospect for lower costs in future advanced systems is excellent, with diamond production costs predicted to drop to less than $10/carat before polishing. Production costs at this level will open up new markets which are price sensitive.

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