Growth of Large Crystalline Grain Al Thin Films on Amorphous Substrates
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G. SBERVEGLIERI, V. CANEVARI, N. ROMEO, and C. SPAGGIARI GNSM-CISM, Dipartimento di Fisica, Via M.D'Azeglio 85 - 43100 Parma,
Italy
ABSTRACT (111) uniquely oriented large crystalline grain Al thin films have been grown on amorphous substrates such as glass or fused quartz. Al has been evaporated by means of an electron beam in a vacuum of 10-7 mbar which was obtained by a conventional oil diffusion pump in conjunction with a titanium sublimation pump and a series of shields cooled at the liquid nitrogen temperature. By studying the variation of the grain size as a function of the growth temperature, a large increase in the grain size has been found at a substrate temperature 100IC below the Al melting point. This has been interpreted as due to the beginning of the metal surface melting and, as a consequence, to the quasi rheotaxial growth of the metal on itself. When the growth has been carried out at a substrate temperature close to the Al melting point (625*C), the grain size has been found out to increase exponentially as a function of the film thickness with a slope which slows down at a thickness of about 1 Um. (111) oriented columnar grains with a size of 50 - 100 pm, hitherto unreported, have been obtained on glass substrates kept at a maximum temperature of 6550C. The surface morphology of the Al films has been studied by SEM microscopy while the film structural properties have been studied by an X-ray powder-diffractometer and by computer - controlled pole figure goniometer.
INTRODUCTION Quasi-Rheotaxy, which consists in the growth of thin films on metal layers kept close to their melting point, has been used in the past for the growth of some photovoltaic materials like CuInSe 2 and CdTe on Pb or Bi [1,2]. But for the growth of Si it is necessary to use, as a substrate, metals with a higher melting point like Al since Si has an autoepitaxy temperature much larger than the Pb or Bi melting point [3,4]. The influence of the growth parameters on the Al structural properties between R.T. and 4000C substrate temperature was studied by Curran et al. [5] but these authors reported a grain size not larger than 3 pm which is not sufficient for the growth of Si films for photovoltaic applications. Thin Al films find an important application in the conducting interconnections of integrated circuits, where as it has been demonstrated by Vaidya and Sinha [6], the texture has to be compact and must exhibit the (111) orientation in order for the electromigration to be avoided. In this work we have studied the growth of thin Al films on amorphous substrates as a function of growth temperature, film thickness and evaporation rate with the aim of obtaining uniquely oriented large grain thin films.
EXPERIMENTAL DETAILS Since the presence of 02 or H2 0 vapor negatively affects the growth of Al films [7] we have used a vacuum system in which, in conjunction with a conventional oil diffusion pump, a titanium pump was installed close to the substrate holder. Besides liquid nitrogen shields were settled close to the Al sourc
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