Relation between Plasma Parameters and Material Properties

Relation between Plasma Parameters and Material Properties 

Luft and Tsuo have presented a qualitative summary of the effects of various plasma parameters on the properties of the deposited a-Si H [6]. These generalized trends are very useful in designing deposition systems. It should be borne in mind, however, that for each individual deposition system the optimum conditions for obtaining device quality material have to be determined by empirical fine tuning. The most important external controls that are available for tuning the deposition processs are the power (or power density), the total pressure, the gas flow(s), and the substrate temperature. In the following the effects of each parameter on material properties will be discussed. 

Schematic representation of the intluence of power density and flow rate on the deposition rate. [After A. Matsuda. J. Vac. Sci. Tecimol. A 16., t6, i (1998).) 

The electrical properties (dark conductivity and photoconductivity) are reported to first decrease and then increase upon increasing power [361]. The optical bandgap increases with increasing power, due to the increase of the hydrogen content [63, 82, 362, 363]. However, at very high power levels, microcrystalline silicon is formed [364], which causes the hydrogen content (and, consequently, the bandgap) to decrease. 

As a result of the deterioration of film properties with increasing power levels, to obtain device quality material the use of low power is required, albeit with a concomitant low deposition rate. Increasing the deposition rate without altering the device quality material properties is a large research challenge. 

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