The Road to Commercialization of Vapor-Phase-Grown Diamond
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The Road to Commercialization of Vapor-PhaseGrown Diamond Shinichi Shikata Introduction Considering all materials, diamond has many superior properties such as elastic constant (hardness), thermal conductivity, transparency to optical visible light and x-rays, a wide-bandgap semiconductor, and negative electron affinity. These properties lead to various applications in many industries. Because of the recent successful development of vaporphase-growth technology, large-area diamond is available at a low manufacturing cost. However the commercialization of diamond at this stage is limited to tools, speaker diaphragms, heatsinks, and optical windows. For each application, diamond utilization is limited. In this article, three avenues on the road to commercialization of vapor-phasegrown diamond will be discussed. The categories appear in Table I with the properties of diamond and its applications. First, applications already commercialized are briefly reviewed, and the requirements for expanding their market are discussed. Second, the technologies and applications coming in the near term are introduced. Last, future candidate applications for diamond are introduced along with technical obstacles to be overcome. Commercialized Applications and Requirements for Market Expansion Tools Through the utilization of vapor-phase deposition of diamond, tools that require coating on complicated structures or large areas have been introduced. Diamond tools have specific applications in the machining of aluminum-based al-
MRS BULLETIN/SEPTEMBER 1998
loys, which are now replacing iron-based metals in many industries. Diamond coatings have been studied on cemented carbides, which consist of WC, Co, and a small amount of other carbides. However the adhesion problem has not been completely solved. Thus commercialization is limited to applications in drills and end mills, which are used in processing of printing boards.1 The freestanding thick diamond films are also used on tools through attachment of the diamond to the tool by a soldering technique. The key issue in expanding applications in this field is the enhancement of adhesion, especially through studies on intermediate layers using sophisticated growth techniques and/or other surfacemodification techniques such as ion implantation. There is a growing market in the field of tape-automated-bonding (TAB) tools, which are used in assembly of integrated
Speaker Diaphragm The speaker diaphragm is a first application of vapor-phase-grown diamond. The sound velocity of diamond can be as high as 18.2 km/s because of its high elastic constant, and it exceeds the velocity—by two to three times—of conventional diaphragm materials such as Ti, Be, B, and alumina.4 There are two types of diaphragms: diamond-coated and the freestanding films. For the former, 2-/xm-thick-diamond film is coated by microwave plasma-assisted CVD on an alumina diaphragm having a thickness of 30 /u,m. To improve the adhesion between alumina and diamond, an intermediate layer of SiC is employed. For the latter type
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