Controlled planar interface synthesis by ultrahigh vacuum diffusion bonding/deposition
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An ultrahigh vacuum (UHV) diffusion bonding/deposition instrument was designed and constructed, which can produce homophase and heterophase planar interfaces from a wide array of materials. The interfaces are synthesized in situ by diffusion bonding of two substrates with or without various interfacial layers, at temperatures up to about 1500 °C. Substrate surfaces can be heat treated, ion-beam sputter cleaned, and chemically characterized in situ by Auger electron spectroscopy prior to deposition and/or bonding. Bicrystals can be synthesized by bonding two single-crystal substrates at a specified orientation. Interfacial layers can be deposited by electron beam evaporation and/or sputter deposition in any layered or alloyed combination on the substrates before bonding. The instrument can accommodate cylindrical and/or wafer type specimens whose sizes are sufficient for fracture mechanical testing to measure interface bond strength. A variety of planar interfaces of metals, semiconductors, and ceramics were synthesized. Examples of bonded stainless steel/Ti/stainless steel, Si/Si, and sapphire/sapphire interfaces are presented.
I. INTRODUCTION
Understanding the effect on properties of materials of interface structures and the effects that chemical impurities have on interfaces has become the subject of many extensive research efforts in recent years.1–7 Controlling surface chemistry and roughness for adhesion of two semiconductor wafers without an adhesive is also a critical issue for successful fabrication of microelectronic devices.8,9 Of the many types of interfaces examined to date, the majority of them were found in engineering composites with processing-induced impurity distributions of varying magnitude that could not be easily controlled. In addition, many of the interfaces had complex curved shapes that rendered difficult most types of structure or properties analysis. To make planar interfaces under conditions of very high purity or with closely controlled impurity contents, and to enable analysis of the effects of impurities on structure and properties, we have selected the solid-state diffusion bonding method in ultrahigh vacuum (UHV). This method has proven to be excellent for producing planar high-purity bonded interfaces reproducibly.10,11 We have designed and constructed an interface synthesis unit, to fabricate bonded interfaces suitable for fracture mechanical testing, electronic properties measurements, and structural characterization by electron microscopy. In this unit, substrate surfaces which are to form the interface are thoroughly cleaned either by sputtering, 1008
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J. Mater. Res., Vol. 15, No. 4, Apr 2000 Downloaded: 01 Apr 2015
thermal desorption, or a combination of these methods. In addition, an important and unique design feature is the inclusion of two thin-film deposition methods, to control doping of the surfaces prior to bonding, for systematic studies of impurity effects upon the structure and properties of interfaces. In this paper, details of th
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