Deposition of Thin Insulating Films by Plasma Enhanced CVD
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G. LUCOVSKY, P.D. RICHARD, D.V. TSU and Department of Physics, North Carolina State University Raleigh, North Carolina 27695-8202 R.J. MARKUNAS Research Triangle Institute Research Triangle Park, North Carolina 27709
ABSTRACT We discuss a new low temperature process for the deposition of electronic In contrast to conventional quality thin films of silicon oxide and nitride. plasma enhanced chemical vapor deposition [PECVD), this process involves the remote excitation of one of the gas reactants followed by the extraction of the active species out of the plasma region where they react to generate precursor molecules. The precursors undergo a CVD reaction at a heated substrate to form the desired thin film. The process is called remote PECVD Insulators produced in this way show significant reductions in the [RPECVD]. incorporation of impurity groups such as SiH and SiOH relative to films grown by the PECVD process at the same substrate temperatures.
I INTRODUCTION There is considerable interest in the low temperature [100-300*C] deposition of electronic quality films of silicon dioxide, SiO 2 , and silicon nitride, Si N4 These are of interest in device structures such as insulated gate field e~fect transitors, xerographic photoreceptors and imaging devices. Conventional CVD processes require elevated temperatures in the vicinity of 700 to 1OOO*C for the deposition of SiO2 and Si3N4 films. These temperatures are far in excess of what can be used in the processing of devices using compound semiconductors or hydrogenated amorphous silicon. Alternative approaches have reduced the deposition temperature by supplying an additional source of energy in the CVD process. The two most commonly used are plasmaenhanced CVD [1] and photo-assisted CVD [2]. Both processes suffer from similar problems related to chemical purity. Many photo-CVD processes are based on schemes which employ Hg vapor as an active constituent of the gas mixture [2], and PECVD processes for SiO2 and Si 3 N4 generally result in 'the incorporation of high concentrations of bonded H, of the order of 5 to 10 at. % for the oxides and in excess of 20 at. % for the nitrides [1]. There is some evidence that H is associated with trapping centers in dielectric materials [3]. Using an empirical tight-binding Hamiltonian and a cluster Bethe Lattice structural model, we have shown that the SiH bonding group introduces localized occupied states in the gap of both SiO2 and Si 3 N4 , and that the SiH, states just below the SiOH and SiNH groups introduce empty anti-bonding conduction band minimum in the same two materials [4]. The major goal of our research has been to develop a low temperature process which would minimize the incorporation of H in the deposited films. We have achieved a significant degree of progress using a deposition process in which we preferentially excite only one of the gas phase reactants and allow the deposition to occur outside the plasma region [5-7]. This process is similar to deposition schemes that have been previously used by other groups [
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