Impact of High-Precision RF-Plasma Control on Very-Low-Temperature Silicon Epitaxy

A technique for formation of abrupt and arbitrary carrier depth profile in epitaxial silicon film deposited at temperatures as low as 300° C has been developed utilizing an RF-DC coupled-mode bias sputtering system. The effect of ion bombardment energy and ion flux density on the quality of silicon film has been investigated. It was found that the activation level of dopants in the grown silicon film can be widely varied by appropriately controlling the ion bombardment energy while maintaining excellent crystallinity. For controlling the carrier concentrations in the deposited silicon film with an appropriate degree of accuracy, the ion bombardment energy must be precisely controlled with a degree of accuracy higher than 0.5 V, since the properties of the deposited films are very sensitive to the ion bombardment energy. For this purpose, a technique for measurement of the plasma potential is required. While the Langmuir probe technique has been applied to the measurements in DC and also RF discharges, its use in RF discharges has been hindered by the problem of RF interference. Therefore, we have developed advanced Langmuir probes for plasma diagnoses in RF discharges at the plasma excitation frequency of 200 MHz.