Problems of Power Feeding in Large Area PECVD of Amorphous Silicon

Abstract

The production of amorphous silicon, e.g. for solar cells, requires large area, high-deposition rate plasma reactors. Increasing the radio frequency from the conventional 13.56MHz up to VHF has demonstrated higher deposition and etch rates and lower particle generation, a reduced ion bombardement and lower breakdown, process and bias voltages.But otherwise the use of VHF leads to some problems. The non-uniformity of deposition rate increase due to the generation of standing waves (TEM wave) and evanescent waveguide modes (TE waves) at the electrode surface.Increasing the frequency and/or the deposition area the plasma impedance, the capacitic stray impedance of the RF electrode and other parasitic capacitive impedances decrease. Increasing the frequency and/or the RF power, the phase angle of the discharge and of the impedance at every point at the lines between the RF matching network an the RF electrode tends more and more towards -90°. This results in increasing currents and standing waves with extremly high local current maximas. Increasing resistances of lines and contacts due to the skin effect and loss-caused heating up of the lines the power losses increase extremely, up to 90% and more. In spite of the increasing of the coupled power, the plasma power does not increase. Thermal destructions of the lines due to extreme expansion or melting are possible.Some solutions to reduce the non-uniformity of the deposition rate like multipower feeding, central backside power feeding, electrode segmentation, use of load impedances, published in former publications, will be discussed in connection with several reactor types (coaxial, large area, long plasma source) in view of the efficiency of power coupling and the practical realization. Solutions to minimize the power losses at the lines will be presented.

Research Progress of the Linear Plasma Source Used in the Films Deposition over Large Areas

“…GHz)激励源 [7] 等；其二，采用更加优越的放电位型， 例如用射频做等离子体激励源，可以采用有电极电容 /电感耦合射频放电 [8,9] ，也可以采用无电极电感耦合 放电和螺旋波放电 [10,11] 等；其三，采用磁场增强/约束 放电 [11][12][13][14][15][16] ，例如用微波激励源，可以采用发散磁场位 [17] 。 近年来，一种线形等离子体源引起了国内外专家 广泛的关注 [7,[20][21][22][23][24][25][26][27][28] 线形微波 [14] ~10 11~1 0 12 L > 1 ~5%…”

Multiscale modeling and run-to-run control of PECVD of thin film solar cells

“…Specifically, during the initial operation of a clean PECVD reactor the plasma phase exhibits transient behavior as the interior surfaces become coated by the deposition species causing drift in the electron density profiles and in the film thickness. Second, at the reactor length scale (for example, a 20 cm wafer is used in this work) consumption and transport of deposition species across the wafer surface have been shown to cause growth rate differences greater than 19% (Stephan et al, 1999;Sansonnens et al, 2003).…”

Multiscale Computational Fluid Dynamics: Methodology and Application to PECVD of Thin Film Solar Cells

“…One such phenomenon, which remains a persistent issue during the manufacture of amorphous silicon thin films, is non-uniformities which develop in the thickness and morphology of deposited layers across the radius of the wafer. Spatially non-uniform deposition has been well characterized [14][15][16] and shown to affect the efficiency of solar cell products [17], resulting in poor device quality and increased costs [18]. As such, there exists a need for accurate PECVD reactor models which are capable of predicting the codependent behavior of the macroscopic gas phase and microscopic thin film growth.…”