Power Feeding in Large Area PECVD of Amorphous Silicon

Abstract

Plasma processes are usually worked out in a small-scale environment (electrode area maximum 121 cm2, rf- and VHF- excitation frequencies). In order to meet the requirements of large area device applications they have to be upscaled. The investigations of glow discharge systems for different PECVD reactors (parallel plate- and coaxial electrodes) have shown, that the reactor design (power supply, line connection) sharply influences the large area deposition process. The voltage distribution on the driven electrode especially determines the uniformity of the deposited layer thickness. Possibilities which influence the voltage distribution on large areas will be discussed. The results of large area electrode description as an electrical line will be discussed in comparison with different reactor configurations and the optimization of the behavior of the deposition process. The experimental results of a coaxial reactor (electrode area 5000 cm2, substrate length 120 cm) show that a homogenous deposition of amorphous silicon (layer uniformity of thickness over the length better ± 7 %) by connecting the driven electrode with additional electrical devices is possible.

Calculation of the electrode shape for suppression of the standing wave effect in large area rectangular capacitively coupled reactors

“…1 Generally, these reactors are driven at the standard 13.56 MHz excitation frequency, but there is a strong interest in using higher frequencies because of advantages such as high deposition rate, 2 and reduced sheath voltage and ion energy bombardment. 3,4 However, it has been shown that when higher frequency is combined with large area reactor, standing wave effects become the main source of nonuniformity in conventional capacitively coupled parallel plate reactors, 1,[5][6][7][8][9][10] or in reactors using a ladder electrode. 11 Taking into account the wavelength reduction or worsening effect due to the presence of the plasma, 1,7,9 the standing wave nonuniformity already becomes important when the reactor size is about one tenth of the free space wavelength 0 at the excitation frequency ͑ 0 / 10= 2.2 m at 13.56 MHz, but only 0.3 m at 100 MHz͒.…”

Experimental and numerical studies on voltage distribution in capacitively coupled very high-frequency plasmas

“…A very high-frequency (VHF; 30-300 MHz) plasma has the advantage of a higher deposition rate and lower ion damage; however, it is difficult to generate a uniform plasma in a large area due to the standing wave and high stray impedance [1]. Silicon films fabricated through PECVD using a VHF plasma have demonstrated a deposition uniformity of ±6.1% over a 50 cm × 60 cm substrate at a frequency of 54.24 MHz using multiple power supplies and a load impedance at the end of the VHF electrode [2] and one of ±18% over a 35 cm × 45 cm substrate at 70 MHz [3]. Takeuchi and co-workers [4] developed a ladder electrode consisting of multiple stainless steel rods and demonstrated hydrogenated amorphous silicon (a-Si : H) deposition at a frequency of 13.56 MHz.…”

Methods of deposition of hydrogenated amorphous silicon for device applications

“…Due to the fact that the wavelength of the radio-frequency signal is of the order of the substrate dimensions (3 m at 100 MHz), it can be expected that uniform deposition is more difficult at these high frequencies [477]. In fact, a practical optimum frequency is used around 60-70 MHz [478,479], which provides a good compromise between high deposition rate and attainability of uniform deposition.…”