Fast growth of amorphous silicon layers by amplitude modulation PECVD

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Fast growth of amorphous silicon layers by amplitude modulation PECVD A. C. W. Biebericher, J. Bezemer, W. F. van der Weg, W. J. Goedheer* Debye Institute, Interface Physics, Utrecht University, P.O. Box 80 000, NL-3508 TA Utrecht, the Netherlands * FOM Institute for Plasmaphysics 'Rijnhuizen', P.O. Box 1207, NL-3430 BE Nieuwegein, the Netherlands ABSTRACT Hydrogenated amorphous silicon has been deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) with a square wave amplitude modulated rf signal of 50 MHz. We studied the dependence of the deposition rate on the modulation frequency, using optical emission spectroscopy and plasma modeling. We observed an enhancement in deposition rate by modulating the plasma. This behavior is explained by the characteristics of the electronenergy distribution during the periodical onset of the plasma. According to a one-dimensional fluid model, high-energy electrons cause a large production of radicals at the onset. The heating occurs over the whole plasma volume, leading to an increase of the homogeneity of the layers. The discharge structure is changed completely. A comparison is made between results obtained at 13.56 MHz and at 50 MHz deposition voltages. INTRODUCTION Plasma Enhanced Chemical Vapor Deposition (PECVD) is a widely applied method to produce amorphous silicon. However, device quality material can only be obtained using a rather low deposition rate. The deposition rate can, for example, be enhanced by increasing the power or the gas pressure. But in general, a higher deposition rate may lead to a decrease of the density. Furthermore, when the α-γ′ transition [1] is passed by increasing the gas pressure, powder is formed, which usually leads to deterioration of the material quality. The formation of powder can be reduced or prevented by modulating the amplitude of the rf signal by a square wave. Anandan et al. observed a considerable decrease both in the deposition rate and in the powder formation, when the 13.56 MHz excitation was modulated by a square wave (SQWM) of 2 Hz [2,3]. The samples became more homogeneous. In the γ′ regime, we observed a decrease in deposition rate from 1.0 nm/s in a continuous-wave (cw) SiH4/H2 plasma a value to 0.2 nm/s in a similar plasma modulated by a frequency of 100 Hz, with a carrier frequency of 50 MHz, and an average power of 10 W. Nevertheless, we found an increase in deposition rate when modulating the power in the α regime. A maximum appeared at a frequency of about 100 kHz [4,5]. A sharp peak in optical emission is detected at the start of each modulation pulse. Scarsbrook et al. [6] and Howling et al. [7] already observed a similar SiH* emission peak in pulsed plasmas. The latter group inferred that the peak was caused by a pulse of high-energy electrons. Another study on SQWM was performed by Overzet and Verdeyen [8], who measured electron densities, using a carrier frequency of 2.9 MHz and modulation frequencies up to 20 kHz. Booth et al. [9] measured the optical emission of an SQWM argon plasma and performed partic