Gj Gijs Meeusen

Emission spectroscopy on a supersonically expanding argon/silane plasma

Results from emission spectroscopy measurements on an Ar/SiH4 plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A, V=80 V, Ar flow=60 scc/s and pressure 4 × 104 Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4 is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018 m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+ particles. The emission spectrum of the SiH radical, the A 2Δ–X 2Π electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombi...

Photodetachment effect in a radio frequency plasma in CF4

Experiments to study negative ion densities have been carried out using the photodetachment effect in a rf plasma in CF4. Electrons are detached from the negative ions under the influence of the pulse of a Nd:YAG laser. The induced increase of the electron density is measured as a function of time using the shift of the resonance frequency of a microwave cavity containing the plasma. The negative ion density [F−] is found to be about (4±1)×1015 m−3, a factor 4±1 higher than the electron density.

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.

Negative ions in a radio-frequency plasma in CF4

Experiments to study negative ion densities in a radio-frequency CF4 plasma have been carried out using a photodetachment technique. Electrons are photodetached from the negative ions using the pulse of a Nd-YAG laser at the tripled (355 nm) or the quadrupled (266 nm) frequency. The photodetached electrons are detected by a microwave method as a sudden increase of the electron density in the plasma. The negative ion density, which consists mainly of F- is found to be typically four times higher than the stationary electron density at a pressure of 13 Pa, an RF power of 15 W and a CF4 flow of 15 SCCM. The measured decay of the detached electrons after the laser pulse has been interpreted in terms of electron attachment and ambipolar diffusion. The results demonstrate the possibilities for use of this technique to evaluate attachment coefficients in active plasmas. The attachment rate constant for CF4 is found to be (7+or-1)*10-17 m3 S-1 at RF powers of 15 W. The electron diffusion coefficient is 0.13+or-0.12 m2 s-1 at standard conditions of 1 Torr and 300 K.

On expanding recombining plasma for fast deposition of a-Si:H thin films

A high-density expanding recombining plasma is investigated for deposition of a-Si:H thin films. The deposition method allows high growth rates and it relies on separation of plasma production in a high-pressure thermal arc, and transport of fragments of injected SiH4 monomer to the substrate. Some characteristics of the plasma are discussed together with an explanation of the dominant chemical kinetics, which proceed mainly through heavy-particle interactions. The deposition results indeed show very high growth rates from 2-30 nm s-1 on areas of 30 cm2. The properties of the layers are characterized by measuring their refractive index (in the range 3.1-3.8) and bandgap 1.2-1.5 eV). Analysis of the oxygen content in the deposited films shows oxidation of the samples in air, which is probably associated with the microstructure of the layers.

Optical Diagnostics for High Electron Density Plasmas

Nowadays high electron density plasmas are, beside their fundamental interest, widely used for many applications, e.g., light sources and plasma processing. The well known examples of high electron density plasmas can be found among the class of thermal plasmas as, e.g., the Inductively Coupled Plasma (ICP) and the Wall Stabilized Cascaded Arc (WSCA). Usually the pressure of the plasma is high, i.e., sub atmospheric to atmospheric. Other examples are the plasmas generated in tokamaks for fusion purposes and the recently exploited plasmas for etching and deposition devices such as the Electron Cyclotron Resonance plasmas. For the plasmas mentioned, the electron density is typical in the range of 1018 to 1023 m−3, and the electron velocity distribution is close to a Maxwellian velocity distribution.

Fast deposition of a-C:H and a-Si:H using an expanding thermal plasma beam

Summary form only given. A fast deposition method, utilizing a thermal plasma which expands into a vacuum vessel, has been used to deposit amorphous hydrogenated silicon and carbon layers (a-Si:H and a-C:H, respectively). The deposited layers are produced by admixing silane and methane (or acetylene) to the argon carrier plasma. In contrast to the conventional plasma enhanced chemical vapour deposition where the deposition is diffusion limited, in this deposition device the deposition mechanism is flow determined. As a result, the deposition rates are large, typically 100 nm/s for a-C:H and 10 nm/s for a-Si:H. The a-Si:H layers are deposited on crystaline silicon and Corning glass substrates, and the a-C:H layers are deposited on either steel, zinc or silicon substrates.

Thermal discharges: experiments and diagnostics

In plasma processing commonly a distinction is made between low pressure (or low (ion) temperature) plasmas and thermal plasmas1. The transition between these two classes is gradual. Plasmas cover a wide spectrum in electron density (1015/m3–1023/m3) and ionization degree (10−7–1). In low pressure plasmas2 as RF—discharges the ionization degree is usually small and these plasmas are characterized by an abundance of molecular fragments and large ambipolar fields. The high electron density thermal plasmas have a high ionization degree and nearly full dissociation and a high heavy particle temperature. In this paper we will focuss on these plasmas.

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.