Silane discharges

The primary process of SiH decomposition is electron impact which produces a large number of different neutral and ionic species as shown in Table 1. The density of S1H2 and SiH neutral species produced has been found to be much larger than the density of the ions. For example, mass spectrometric data for silane discharges indicate that the density of ionic species is lower by 10 compared with the density of neutral species. Further, mass spectrometer signals of ionic species, such as SiH SiH 25 SiH", SiH", and Si2H , increase by more than two orders of magnitude as the r-f power is increased, eg, from 2 to 20 W. A rapid rise in the population of ions, with power, implicitly means an increase in electron density. 

Optical emission is a result of electron impact excitation or dissociation, or ion impact. As an example, the SiH radical is formed by electron impact on silane, which yields an excited or superexcited silane molecule (e + SiHa SiH -t-e ). The excess energy in SiH is released into the fragments SiH SiH -I-H2 + H. The excited SiH fragments spontaneously release their excess energy by emitting a photon at a wavelength around 414 nm. the bluish color of the silane discharge. In addition, the emission lines from Si. H, and H have also been observed at 288, 656, and 602 nm, respectively. 

Probe measurements in silane discharges have been reported [296,297]. Apparently, no difficulties were experienced, as the deposited amorphous silicon layer on the tip was sufficiently photoconductive. For typical silane discharge conditions values for are found to be between 2 and 2.5 eV. Electron densities are around 1 x 10 cm - [296]. Probe measurement in the ASTER system failed due to strong distortions of the probe current, even after following cleaning procedures. 

In silane discharges, one observes the following when the discharge is off, the mass spectrometric signal at m/e = 31 amu/e (SiH ) as a function of electron energy is due to dissociative ionization of SiHa in the ionizer of the QMS, with an ionization potential of 12.2 eV [312]. The signal with the discharge on is due to ionization of the radical SiHa plus the contribution from dissociative ionization... 

In silane discharges several ions are observed to be involved in a charge exchange process, and therefore maxima in their ion energy distribution at distinct energies are observed. The charge carrier density and the plasma potential that result from the fit of the lED allow for the quantification of the related parameters sheath thickness and ion flux. This method has been be used to relate the material quality of a-Si H to the ion bombardment [301. 332] see also Section 1.6.2.3. 

The influence of power variation on the material properties is in agreement with the trends observed by Andujar et al. [246], who studied the a-y transition in pure silane discharges at 13.56 MHz. Further, this has also been observed for pure silane discharges at 50 MHz by Meiling et al. [423],... 

Surface bombardment by positive ions (particularly SiH ) plays a critical role in the growth of amorphous silicon films. The silane radicals SiHs and SiH2 have a positive electron affinity therefore, the silane discharge is essentially electronegative and dissociative attachment processes make a significant contribution in the balance of charged particles and production of negative silane ions ... 

The non-dissociated SiELr molecules also participate in the surface reactions, but mostly as deactivating chemical agents. Detailed kinetics of the gas-phase processes in the silane discharges is much more comphcated (Kushner, 1988). In particular, the kinetics of silicon clusterization in the silane discharges will be discussed later in this chapter in connection with the formation of nanoparticles. 

Nanoparticles in Plasma Kinetics of Dusty Plasma Formation in Low-Pressure Silane Discharges... 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

Electrical Discharge, Irradiation, and Photolysis. Early reports of the decomposition of SiH in an electrical discharge indicated that the main products were hydrogen, soHd sHicon subhydrides of composition SiH 2 i smaH quantities of higher silanes (37). However, more recent... 

One measure to improve the devolatilization of the compound is to work in an open mixer during the silanization step. In the experiments described in the following paragraphs, silanization was done pressure-less in an open mixer for 150 s at a constant temperature (145°C). The temperamre was held constant during the silanization period by adjustment of the rotor speed. After 150 s of silanization the compound was discharged and further analyzed. 

This section treats the plasma physics and plasma chemistry of the typical silane-hydrogen RF discharge, with occasional examples that employ a somewhat higher excitation frequency. Electrical characterization of the discharge is followed by an analysis of the silane chemistry. An appropriate set of gas phase species is presented, which are then used in the modeling of the plasma. A comparison is made between modeling results and experimental work in ASTER. Extension to 2D modeling is presented as well. 

In a silane-hydrogen discharge the feedstock gases SiHa and H2 take part in all the processes that occur. A large number of reactions have been proposed (see e.g. Kushner [190]). Nienhuis et al. [191] have performed a sensitivity analysis in their self-consistent fluid model, from which a minimum set of reactions have been extracted for a typical low-pressure RF discharge. Tables II and III list these reactions. They will be used in the plasma models described in subsequent sections. The review articles on silane chemistry by Perrin et al. [192] and on hydrogen by Phelps [193] and Tawara et al. [194] have been used. The electron collision data are compiled in Figure 13 [189]. 

The ID fluid discharge model has been applied to the ASTER deposition system (see Section 1.2.4). The deposition reactor has an inner volume of 10 1 and an inner diameter of 20 cm. The upper electrode is grounded (see Fig. 4a), and the powered electrode is located 2.7 cm lower. Other typical silane-hydrogen discharge parameters are summarized in Table IV. 

RF frequency variation. In Figure 18 are shown the effects of RF frequency on the partial pressures of silane, hydrogen, and disilane (Fig. 18a) and on the deposition rate (Fig. 18b). The total pressure is 16 Pa, and the plasma power is 5 W. The discharge is in the a-regime. 

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