J. Bezemer

A self-consistent fluid model for radio-frequency discharges in SiH4–H2 compared to experiments

A one-dimensional fluid model for radio-frequency glow discharges is presented which describes silane/hydrogen discharges that are used for the deposition of amorphous silicon (a-Si:H). The model is used to investigate the relation between the external settings (such as pressure, gas inlet, applied power, and frequency) and the resulting composition of the gas and the deposition rate. In the model, discharge quantities such as the electric field, densities, and fluxes of the particles are calculated self-consistently. Look-up tables of the rates of the electron impact collisions as a function of the average electron energy are obtained by solving the Boltzmann equation in a two term approximation for a sequence of values of the reduced electric field. These tables are updated as the composition of the background neutral gas evolves under the influence of chemical reactions and pumping. Pumping configuration and gas inlet are taken into account by adding source terms in the density balance equations. The e...

Local structure and bonding states in a‐Si1−xCx:H

Infrared spectra of a‐Si1−xCx:H deposited in a glow discharge of a silane/methane mixture have been measured. Comparison with elastic recoil detection and Rutherford backscattering spectrometry shows that the mean number of hydrogen atoms attached to silicon per silicon atom ([HSi]/[Si]) increases with higher carbon content and that more Si—H2 bonding configurations are formed. Hydrogen is preferentially bonded in (Si—H2)n clusters, which partly explains the observed apparent shift of the Si—H stretching mode to higher energy. The remaining contribution to this shift is believed to result from Si—H on surfaces of voids instead of an inductive effect. From composition measurements we observe that for each carbon atom, three hydrogen atoms are incorporated in the material, suggesting that during deposition carbon is initially incorporated in CH3 groups. However, the mean number of C—H bonds per carbon atom decreases from about 2.2±0.4 to 1.4±0.3 with increasing carbon content, indicating that the majority o...

Structural properties of a-Si:H related to ion energy distributions in VHF silane deposition plasmas

Abstract We present measurements on typical silane-hydrogen RF/VHF deposition plasmas and the corresponding a-Si:H films deposited from these plasmas. A range of process settings was used, covering both the α and the γ′ regime of the discharge. Mass resolved ion energy distributions were measured at the grounded electrode to determine the ion flux at the growing surface. Although the main precursors are radicals, in the lower pressure α regime the flux of ions towards the surface can account for at least 10% of the observed growth rate. In the γ′ regime this contribution to the growth of the film is less. We measured internal stress, hydrogen concentration, hydrogen bonding configuration, and refractive index to determine the effects of the ion bombardment on the structure of the deposited a-Si:H films. Good structural properties, i.e. a refractive index of about 4.25 at 600 nm and a minimum number of SiH2 bonds, are found above a threshold energy of 5 eV per deposited atom. This observation is explained in terms of knock-on processes of the deposited atoms by ions and an increased mobility of the growth precursors at the surface. Both these processes promote the formation of a dense film.

On the transmission function of an ion-energy and mass spectrometer

Abstract The operation of a mass spectrometer system with an electrostatic energy analyser, designed for measurements of mass-resolved ion-energy distributions, is discussed. We show how the electric fields in the different electrostatic lenses present in the system can be optimized. These lenses direct the ions entering the system into the energy filter and the quadrupole mass filter. These lenses can exhibit chromatic aberration. The conditions without chromatic aberration have been found by simulating the ion trajectories in the part of the system up to the energy filter. Also, an experimental method is presented to find these settings. We show that the energy-dependent transmission of ions through the system is mainly determined by its acceptance angle. Ionenergy spectra from an argon plasma have been measured and corrected for the transmission of the ions through the system. Published by Elsevier Science B.V.

Structural, compositional and optical properties of hydrogenated amorphous silicon-carbon alloys

Abstract Steady-state optical modulation spectroscopy (OMS) measurements have been carried out on well characterized hydrogenated amorphous silicon-carbon alloy layers. A series of samples with methane-to-silane gas flow ratios varying from 0.1 to 0.7 was deposited at 250°C using the conventional r.f. glow-discharge deposition method. In order to characterize the material, we investigated the structural, compositional and optical properties of these alloys by means of Fourier-transform infrared spectroscopy, Raman spectroscopy, elastic recoil detection, Rutherford back-scattering spectrometry and optical transmission and reflection spectroscopy. OMS measurements were performed at room temperature and at T 50 K. From room-temperature data the energy positions of the dangling-bond states were deduced. Transition energies involving D° states are found to shift to higher values with increasing carbon content, while transition energies involving D− states remain almost constant. At lower temperatures, band-ta...

Deposition‐rate reduction through improper substrate‐to‐electrode attachment in very‐high‐frequency deposition of a‐Si:H

We have tracked down one of the major causes for nonuniformities in film thickness in large‐area deposition of hydrogenated amorphous silicon, a‐Si:H. To simulate improper substrate‐to‐electrode attachment we deliberately introduced a gap behind the substrate. The rf‐excitation‐frequency dependence of the influence of this gap on the deposition rate is presented. We show that a local small gap behind the glass has a detrimental effect on the local deposition rate, and therefore on the uniformity of the films. For example, at a frequency of 60 MHz typically the reduction of the deposition rate amounts to 25% when a gap of 1 mm is present. To explain the observed effects the plasma‐sheath dynamics are considered. The relations between the dc self‐bias voltage, the amplitude of the applied rf voltage, and the deposition rate are determined experimentally. A theoretical model that explains the reduction of the deposition rate is presented. We conclude from the model that the ion density in the sheath is indep...

Deposition rate in modulated radio-frequency silane plasmas

Plasma-enhanced chemical-vapor deposition of amorphous silicon by a square-wave amplitude-modulated radio-frequency excitation has been studied by optical emission spectroscopy and plasma modeling. By the modulation, the deposition rate is increased or reduced, depending on the plasma parameters. The increase in the deposition rate in powder-free (α-regime) plasmas is explained by the behavior of the electrons. High-energy electrons cause a large production of radicals at the onset of the plasma, as evidenced by an overshoot in optical emission. This is confirmed by a one-dimensional fluid model. An optimum in the deposition rate at a modulation frequency of about 100 kHz is determined by the decay time of the electron density.

Frequency effects in capacitively coupled radio-frequency glow discharges: a comparison between a 2-D fluid model and experiments

The results of a 2-D fluid model for argon radiofrequency (RF) discharges in a closed cylindrical vacuum chamber are compared with experimental data from an amorphous silicon deposition reactor operated in argon. Good agreement is obtained for the relation between the DC autobias voltage and the dissipated power in the frequency range 40-100 MHz at pressures between 10 and 60 Pa. Scaling laws are presented for the dissipated power and for the ion fluxes toward the electrodes. These quantities are expressed in the DC bias voltage, the RF excitation frequency and the background pressure. Also the uniformity of the ion fluxes is studied. The model yields a linear relation between the applied RF voltage and the DC bias voltage. This relation depends only on the geometry of the discharge chamber and shows an offset. >

Ion bombardment in silane VHF deposition plasmas

The measurement of mass resolved ion energy distributions at the grounded substrate in an RF glow discharge allows to determine the ion flux and the ion energy flux towards the surface of a growing hydrogenated amorphous silicon (a-Si:H) layer. This provides the means to study the influence of ions on the structural properties of a-Si:H. Here the authors focus on the {alpha}-{gamma}{prime} transition as occurs in silane-hydrogen plasmas at an RF frequency of 50 MHz and a substrate temperature of 250 C. The structural properties of the layers appear to depend on the kinetic energy of the arriving ions. This is supported by measurements of ion fluxes under other deposition conditions and by characterization of the corresponding layers. The influence of ions on the growth is discussed in terms of their flux, and the amount of delivered kinetic and potential energy to the growing film. The measurements suggest that a threshold energy of about 5 eV per deposited atom is needed for the construction of a dense amorphous silicon network.

Incorporation of phosphorus and boron in amorphous silicon measured with (p, γ) resonant reactions

Abstract Phosphorus and boron concentrations in thin films of hydrogenated amorphous silicon prepared in a glow discharge were determined by means of the resonant nuclear reactions 31 P(p, γ) 32 S and 11 B(p, γ) 12 C. These reactions are free of interference from other elements in the film or the substrate and offer a good depth resolution: better than 50 nm in the case of boron and better than 10 nm in the case of phosphorus. Concentrations of dopants down to 100 ppm were measured. Calibration was achieved by measuring the γ-ray yield of the resonant reaction 30 Si(p, γ) 31 P in the same film. In this way it was possible to determine the phosphorus (or boron) to silicon ratio without an external standard. Results of a study of the incorporation of phosphorus as a function of the rf power in the glow discharge are presented.