Atm Wilbers

The continuum emission of an arc plasma

We present free-bound Biberman factors (12,000, 13,500 and 14,500 K) for the wavelength range of 250–320 nm and 380–800 nm. These values were determined by measuring the absolute continuum intensity of a thermal argon plasma in a cascade arc (2mm dia) for a pressure range of 2–6×105 Pa and a current range of 20–60 A. The continuum emission is corrected for free-free contributions. Two highly accurate experimental reference values of the free-bound factors were used to check the electron density. Agreement with experiments in the u.v. is good. Comparison with theoretical values also shows good agreement below 430 nm. Above this wavelength, theory predicts an edge structure which is not apparent in the available experimental values. The agreement between our results and recent experimental values (for which the electron density was obtained with two-wavelength interferometry) is good. The influence of non-equilibrium has been found to be negligible. We conclude that absolute continuum measurements are well suited to determine electron densities (visible spectroscopy) and electron temperatures (u.v. spectroscopy) by using the known free-bound Biberman factors. Prediction of the absolute intensity as a function of wavelength is possible within 10%.

Characteristic quantities of a cascade arc used as a light source for spectroscopic techniques

The authors have determined the characteristic quantities (the electron temperature and the electron density) of an argon plasma in a cascade arc (diameter of 2 and 4 mm) for a pressure range of 1*105-8*105 Pa and a current range of 20-70 A. The absolute continuum intensity was also determined in the wavelength ranges from 250-320 nm and 380-800 nm for pressures of 2, 4 and 6 bar and 20, 40 and 60 A in the case of a 2 mm arc. The plasma is close to local thermal equilibrium (LTE). Using the mentioned quantities, prediction of the absolute intensity as a function of the wavelength is possible from 140 nm to the infrared within 10%. The use of the arc as a light source in photon induced chemical vapour deposition, spectroscopic ellipsometry and infrared absorption spectroscopy is discussed.

Radiative energy loss in a two-temperature argon plasma

We have calculated the total radiative loss in an argon plasma at wavelengths from 100 nm to 100 μm (zero absorption) as a function of temperature (3000–15,000 K) for several pressures (10-1 × 106Pa) under LTE and non-LTE conditions. The investigated non-equilibrium aspects are deviations of the neutral ground state population with respect to the equilibrium population (partial LTE). A difference between heavy particle and electron temperature is included. When the calculated total radiative loss is divided by the square of the electron density, a curve is obtained which gives the total radiative loss as a function of temperature. The influences of pressure and deviations from LTE on this curve are small and in many cases negligible. Almost all influences of pressure and deviations from equilibrium are incorporated in the electron density. Absolute measurements in an inductively-coupled plasma can be simulated with realistic values of the b factor (Boltzmann decrement).

Amorphous hydrogenated silicon films produced by an expanding argon-silane plasma investigated with spectroscopic IR ellipsometry

Abstract We have produced amorphous hydrogenated silicon (a-Si:H) films from silane with an unconventional deposition technique, a supersonically expanding d.c. arc plasma. The deposited films are analysed mainly by using spectroscopic IR ellipsometry. Further analysis has been performed with scanning electron microscopy, IR absorption spectroscopy and in situ He-Ne ellipsometry. The film structure appears to be strongly linked to the degree of ionization of the expanding beam, the injection location of silane gas, the degree of dissociation and the percentage of the injected hydrogen gas. Deposition at low arc power results in films of polysilane, which are very sensitive to oxidation during air exposure. Without hydrogen injection, films with a high refractive index and low hydrogen content are obtained (below the detection limit of the IR transmission spectrometer). Hydrogen injected in the middle of the plasma arc results in a-Si:H films with a refractive index of 3.75 at 632 nm; this value is close to the indices of the best films obtained with plasma-enhanced chemical vapour deposition (PECVD). In these films, the strength of the vibrational absorption at 2000 cm -1 , which can be assigned to SiH stretch bonds, is equal to the strength of a vibration at 2085 cm -1 . Because the bending absorptions of SiH 2 at 860 and 890 cm -1 are not detected in the films produced, it is concluded that this 2085 cm -1 absorption peak in our films is caused by bond stretching of SiH rather than by that of SiH 2 . As in PECVD, the optimum substrate temperature at which films of good quality are obtained is in the range from 525 to 575 K. The deposition rate is of the order of several nanometers per second.

Fast silicon etching using an expanding cascade arc plasma in a SF6/argon mixture

An expanding cascaded arc is used as a fluorine atom source for fast etching of silicon. Extremely high etch rates up to 1.3 μm/s have been obtained. A reactor parameter study has been performed. The obtained selectivity Si/SiO2 is ∼11 for substrate temperatures of 600 °C, increasing to ∼20 at 100 °C. The etching proces is fully isotropic.

The VUV emissivity of a high-pressure cascade argon arc from 125 to 200 nm

We have investigated the vacuum ultraviolet (VUV) emissivity of a cascade arc in argon from 125 to 200 nm. The temperature and pressure dependences of this emissivity are the subject of this paper. Enhancement of the emissivity has been obtained by adding nitrogen to the argon. In some wavelength ranges, the increase of the emissivity due to broadened lines is more than a factor of 10 in a spectral range of several nanometers. These lines reach the blackbody limit, corresponding to the electron temperature of the plasma inside the arc. The electron temperature has been varied between 12,200 and 14,500 K. The relative temperature dependence of the free-bound Biberman factor as a function of wavelength in this temperature range has been determined from continuum measurements and compared to the theoretical temperature dependence of the free-bound Biberman factors according to Hofsaess. The agreement is good.

Deposition of a-Si:H using a supersonically expanding argon plasma

Amorphous Hydrogenated Silicon (a-Si:H) is a material that is widely used in the field of solar cells and other optoelectronics. The only method available to produce high quality a-Si:H is by means of plasma enhanced chemical vapor deposition (PECVD). Radicals responsible for deposition diffuse from a glow discharge toward a substrate that is heated up to 600 K where a layer grows with a speed of typically 0.1 nm/s. The deposition rate is limited because of transport is diffusion determined. An increase of this deposition rate and material efficiency can be expected if the radicals are transported toward the substrate using another transport mechanism.