Processing and characterization of alumina thin films on chemically vapor deposited diamond substrates for producing adh
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Processing and characterization of alumina thin films on chemically vapor deposited diamond substrates for producing adherent metallizations E. S. K. Menon and I. Dutta Department of Mechanical Engineering, Center for Materials Science and Engineering, Naval Postgraduate School, Monterey, California 93943 (Received 11 August 1997; accepted 27 April 1998)
In order to make the surface of chemically vapor deposited diamond (CVDD) substrates amenable to metallization by both thin and thick film approaches currently utilized in electronic packaging, a thin, adherent, insulating aluminum oxide film was grown on diamond at low temperatures (,675 K). The film was produced by reactive thermal evaporation of Al and O in an oxygen atmosphere, followed by low-temperature annealing in oxygen. A Cr intermediate layer was deposited on diamond prior to the deposition of aluminum oxide in order to enhance adhesion between the oxide and diamond. The chemistry, crystal structure, and microstructure of the film were characterized in detail via scanning and transmission electron microscopy, as well as Auger electron spectroscopy. Particular attention was given to the mechanisms of bonding across the CVDD-Cr and Cr-alumina interfaces, as well as the stability of the surface treatment following metallization by fritted pastes requiring firing at elevated temperatures. The Cr was found to be bonded with CVDD by Cr23 C6 formation, while the bonding between the Cr and alumina layers was provided by the formation of a compositionally modulated solid solution with Al2 O3 -rich and Cr2 O3 -rich regions. I. INTRODUCTION
Chemically vapor deposited (CVD) diamond is very attractive as a substrate material for electronic packaging1 because of its excellent thermal conductivity (1500–2000 Wym-K), high electrical resistivity (1013 – 1014 V-cm), low dielectric constant (5.6), and low loss tangent (0.0005). However, typical conductive metallizations, such as gold (Au), copper (Cu), and aluminum (Al), display very poor adhesion to diamond, thereby causing problems associated with the reliability and stability of metallized diamond packages. Additionally, the large difference between the thermal expansion coefficient of diamond (,1–2 3 1026yK) and most metals (15 –20 3 1026yK) results in the generation of large residual stresses in most metallizations, leading to problems associated with peeling and/or cracking of the metallization. As a result, standard thin or thick film techniques utilized in metallizing ceramic substrates are not suitable for use on diamond. Several processes for metallizing diamond have been proposed recently.2–7 All these processes involve the application of a transition metal, either as the final metallization or as an intermediate layer between CVD diamond and the conductive metallization. Typical metals used for this purpose include Ti, W, Cr, Nb, Ta, or an alloy (e.g., W-Ti). In each case, the selected metal is a strong carbide former, and is believed to bond with diamond via metal
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