Sol Gel Derived Aluminum Nitride Thin Films Via the Nitridation of Alumina
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SOL GEL DERIVED ALUMINUM NITRIDE THIN FILMS VIA THE NITRIDATION OF ALUMINA M.W. RUSSELL AND E.P. GIANNELIS Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT Crystalline aluminum nitride thin films (- 1200 A) were prepared via nitridation of amorphous aluminum oxide and characterized by XRD, SEM, TEM, oxygen resonance, Auger depth-profile analysis and ellipsometry. Precursor chemistry, heating rate, nitridation temperature and nitridation time were varied to explore the competition between densification, crystallization and nitridation. Oxygen content in the films decreased with increasing nitriding temperature or nitriding time. Heating rate was determined to have minimal effect on the oxygen content for the parameters evaluated. A modified Kissinger analysis of DTA data for the crystallization of ct-A12 0 3 yielded an effective activation energy of -104 kcal/mol. INTRODUCTION Aluminum nitride is challenging the performance of traditional A12 0 3 and BeO substrates because of its high thermal conductivity (> 140 W/m K), high refractory and chemical resistance, large band gap (-6 eV) and coefficient of thermal expansion closely matching those of silicon and gallium arsenide. These properties make aluminum nitride an attractive material for a number of applications in electronics, including device components and Si-based ICs. Until recently, AIN has not been used for substrates because its predicted thermal conductivity of 320 W/m K had not been realized. With the development of suitable powder processing technologies, AIN with a thermal conductivity five to six times that of alumina has been achieved [1-2]. AIN has been prepared via several synthesis techniques, and both single-crystal and powder methods are discussed in the literature. Popular routes to commercial quantities of powder are carbothermic reduction and direct nitriding of aluminum. Carbothermic reduction involves heating alumina and carbon in nitrogen; the alumina is reduced by the carbon and then nitrided. Direct-nitriding involves exposing molten aluminum to a reactive nitriding atmosphere [3]. These methods have the disadvantage of the high processing temperatures, often in excess of 1700 'C, required to achieve sintering and uniform composition [4]. High-purity AIN has also been prepared by CVD [3] or via pyrolysis of organometallic precursors [5]. In addition, Hoch and Nair report on the reduction/nitridation reactions of amorphous alumina and aluminum hydroxide gels as precursors to ultra-fine ceramic powders at temperatures 51350 0C [6]. The successful implementation of AIN substrates in hybrid circuitry requires facile planarization and metallization. An additional barrier to the widespread use of AIN substrates is the large expense incurred in surface preparation. We report on the synthesis of aluminum nitride thinfilms by the nitridation of sol-gel derived alumina as a potential method for surface modification of bulk aluminum nitride substrates. Nuclear resonance techniques are particularly attrac
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