High-rate deposition of silicon thin film layers using linear plasma sources operated at very high excitation frequencies (80-140 MHz)

Abstract

The plasma enhanced chemical vapor deposition technology (PECVD) is one of the commonly used deposition techniques for amorphous and microcrystalline silicon thin film and solar cell fabrication. The VHF-PECVD process developed at Dresden University of Technology enables the homogenous dynamic deposition of a-Si:H and μc-Si:H layers at high deposition rates. The most important features of this deposition system are linear plasma sources (500x100mm) operated at very high excitation frequencies (81.36 MHz – 140 MHz) in combination with moving substrates. The advantage of such plasma concept is the homogeneous deposition on large area substrates. The higher excitation frequency leads to an increase in electron density and to an enhancement of silane dissociation in the bulk plasma. Furthermore, the lower ion-bombardment energy reduces defect creation in the layers. In this work the influence of excitation frequencies (81.36 – 140 MHz) on the amorphous silicon deposition process will be analyzed. First, a study of the plasma homogeneity of a-Si:H was carried out. At the 140 MHz excitation frequency a homogeneous deposition of amorphous silicon with deposition rates of 1.56 nm/s has been achieved, compared to 0.66 nm/s at 81.36 MHz and the same process pressure. Furthermore, the influence of higher excitation frequencies on the material and structural properties of intrinsic layers has been analyzed and will be discussed. Finally, p-i-n solar cell structures with successful implemented intrinsic absorber layers deposited at higher excitation frequencies at different deposition parameters were fabricated and will be presented.