Reversing the Temperature Gradient in a PECVD Deposition Chamber to Alter the Radicals Generated in the Plasma

The temperature gradient in a typical parallel plate plasma chemical vapor deposition (PECVD) reactor decreases from the heated anode to an unheated cathode and chamber walls. The film growth properties on the substrate attached to the anode are sensitively dependent and hence controlled by the anode temperature. However, the plasma properties are sensitive to the gas temperature near the cathode and therefore, reversing the temperature gradient by heating the earth shield around the cathode, allows reduction of the electron temperature and therefore the reactive radical species produced in the plasma. We show that in a silane-hydrogen gas plasma, the light induced degradation in solar cells is significantly reduced when the earth-shield is heated but the anode is not heated, while maintaining the substrate temperature. We suggest that light induced degradation can be further reduced or even eliminated by cooling the anode while further raising the earth-shield temperature to maintain the substrate temperature. At lower substrate temperatures, necessary for some substrate materials, usually poorer material quality is obtained. Using the reverse temperature gradient approach, with a cooled anode and heated earth-shield, high quality material with no light induced degradation can also be obtained at lower substrate temperatures.