A. Kottwitz

Investigation of growth mechanisms of microcrystalline silicon in the very high frequency range

Abstract Very high frequency low pressure plasma was applied to deposit intrinsic microcrystalline silicon layers using a resonance plasma source operating at 43, 70, 116 and 165 MHz. Silane concentration, substrate temperature, plasma power and excitation frequency were varied. Samples were investigated by Raman scattering, infrared spectroscopy and X-ray diffraction. Growth rates of up to 0.5 A/s for predominantly crystalline films were attained. For substrate temperatures between 80°C and 510°C microcrystalline layers were obtained. All microcrystalline layers had a 〈1 1 0〉 -texture in the growth direction. Surface processes, such as diffusion and etching, proposed for the growth of microcrystalline silicon, will be discussed in context with the results achieved here. The results of a simple 2D-Monte-Carlo-simulation of the growth process will be presented.

Deposition Conditions for Large Area PECVD of Amorphous Silicon

The production of amorphous silicon devices usually requires large area, high-deposition-rate plasma reactors. Non-uniformity of the film thickness at high power and deposition rate is found to be an important factor for large area deposition. Increasing the radio frequency from the conventional 13.56 MHz up to VHF has demonstrated advantages for the deposition of a-Si:H films, including higher deposition rates and lower particle generation. The use of VHF for large area deposition leads to the generation of standing waves and evanescent waveguide modes at the electrode surface and on the power feeding lines. Thereby increasing the non-uniformity of the film thickness. The uniformity of the film thickness for an excitation frequency strongly depends on the deposition parameters e.g. pressure, input power, silane flow and the value of load impedances. With increasing exciting frequencies the range of deposition parameters for obtaining uniform films narrows. Subsequently it is shown that for a large-area plasma-box reactor (500 × 600 mm 2 plate size) with a double-sided RF electrode, the non-uniformity of the film decreases due to a homoge-neization of the electrode voltage distribution by using multiple power supplies and load impedances on the end of the RF electrode. The uniformity errors decrease from ±20% to ±2.4% (27.12MHz) and from ±40% to ±5.9% (54.24MHz). Experimental results of the film uniformity will be discussed in dependence on excitation frequencies and the deposition parameters.

High rate PECVD of a-Si alloys on large areas

Abstract The deposition of a-Si alloys on large areas was investigated in the frequency region from 3 to 30 MHz. With increasing frequency an increasing deposition rate (5–30 μm h −1 ) and a decreasing powder formation result. An upscaling from small areas (100 cm 2 –1000 cm 2 ) to larger areas (2500 cm 2 for the parallel plate and 5000 cm 2 for the coaxial reactor) was realized. With a homogeneous r.f. voltage we obtained a homogeneous r.f. power dissipation and a homogeneous layer thickness (±5%). An additional stabilization for the discharge over the length was obtained by use of a permanent magnetic field. We have calculated the deposition rate as a function of r.f. power, pressure, gas flow rate, frequency, magnetic field, discharge length and gas temperature on the basis of a semiquantitative plasma chemistry model.

High rate deposition of microcrystalline silicon using resonance plasma source (Helix) -- Plasma properties and deposition results

Microcrystalline silicon layers have been deposited by PECVD using a resonance plasma source (Helix) operating at frequencies of 46, 68, 113 and 163 MHz. The plasma discharges in hydrogen and various hydrogen/silane mixtures were investigated by optical emission spectroscopy (OES) and mass spectroscopy (MS). Growth rate, crystalline fraction and hydrogen content of the layers were studied for different gas compositions, excitation frequencies and plasma powers. Plasma monitoring revealed preferably the formation of positive ions. The density of positive hydrogen ions increased steadily 15 times by raising the frequency from 46 MHz to 163 MHz, whereas the ion energy was reduced from 75 eV to 23 eV and the radiation from hydrogen decreased to 50%. Growth rates up to 1.5 {micro}m/h have been achieved for microcrystalline layers at 230 C deposition temperature. The content of hydrogen was below 15 at.%. Raman spectroscopy measurements reveal that 20 to 70% of the silicon was crystalline depending on the silane concentration.

VHF Large Area Plasma Processing on Moving Substrats

The production of amorphous and microcrystalline silicon, e.g. for solar cells, requires large area, high-deposition rate plasma reactors. Increasing the 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 the use of VHF for large area systems leads to some problems. The non-uniformity of deposition rate increases due to the generation of standing waves and evanescent waveguide modes at the electrode surface. One possibility to process large area substrates is the use of a one-dimensional extended, homogeneous plasma source in combination with a moving substrate. The requirements, which result from the deposition process and from the RF-engineering, corresponds with the developed plasma source, using deposition frequencies in the VHF-range (50-100 MHz), almost perfectly. Using a source of 550mm length experiments were done with 81.36MHz at RF power densities of 70-180mW/cm 2 , silane/ hydrogen pressures of 5-30Pa and flow rates of 10-300sccm. The measured potential distribution error was ±2%. Optical emission spectroscopy delivered discharge intensity errors of ±3-10%. Deposition rates up to 20µm/h for amorphous silicon (60A/s) and film thickness inhomogenities less than ±5% were achieved (with an area of the moved substrate of 30cm–30cm). Experimental results of the film properties will be discussed in relation to the deposition parameters and compared with complementary experiments, carried out on a small scale equipment with excitation frequencies up to 165 MHz.

A model for high-rate film deposition from dusty RF discharges

Abstract In this work, a model for the discharge kinetics and plasma chemistry of radio frequency discharges with dust particle generation is presented. Spatially dependent densities of radicals condensing with nonunity sticking coefficients on the electrode and wall surfaces were calculated for a parallel plate deposition reactor with a plug gas flow parallel to the electrode surface. Previously, the electron density and the fractions of power dissipation in the discharge bulk in the sheaths and due to the oscillating sheath boundaries were calculated by the electron kinetics submodel for given deposition parameters. Particle generation was considered as a multi-step process: generation of negative ions by electron attachment; clustering of negative ions; and growth of clusters by parent molecule addition. Predictions of deposition rates and dust-particle formation were made for gas mixtures containing SiH4 and electronegative gases (SiF4 and SiH2Cl2).

Improved quality of high deposition rate a-Si:H films prepared at usual substrate temperature

Abstract In order to optimize the deposition conditions for the growth of a-Si:H deposited at high rates, a numerical model of the plasma process was developed which takes into account the generation of powder at frequencies between 3.6–27.12 MHz. In accordance with the results of the simulation, high deposition rates, 8 nm/s, were obtained by using the magnetically enhanced radio frequency glow discharge decomposition of SiH 4 in a coaxial-type diode system. The effect of the frequency-change above the ion sheath transition frequency limit was studied in combination with the variation of substrate temperature and deposition rate. By increasing the frequency from 3.6 to 27.12 MHz at substrate temperature of 250°C, the material quality of the a-Si:H films was improved (photo- to dark-conductivity ratio 10 5 , defect densities 10 16 cm −3 , and an optical gap of 1.9 eV) and applications in photoelectric devices at deposition rates up to 4.5 nm/s were realized.

High-Rate Rpecvd of a-Si:H Films by Means of a VHF Resonant Plasma Source

Thin film polycrystalline silicon on large-area glass substrate is a promising material for low-cost high efficiency solar cells. High deposition rate (up to 5 nm/s) a-Si:H films suitable for recrystallization were deposited using a {lambda}/4 helical resonator source. Refractive index, Tauc-gap, photo- and dark conductivities were measured for film characterization. The metastable behavior was characterized by the light-induced degradation of the photoconductivity. Hydrogen content and bonding configuration were analyzed by IR absorption and mass separated thermal fusion transients, film microstructure was studied by intentionally incorporating carbon and oxygen. Most of the hydrogen is located on internal surfaces in the otherwise dense material. Differences between the deposition from the highly excited plasma and the conventional remote PECVD process are discussed.