Film growth precursors in a remote SiH4 plasma used for high-rate deposition of hydrogenated amorphous silicon

Abstract

The SiH4 dissociation products and their contribution to hydrogenated amorphous silicon (a-Si:H) film growth have been investigated in a remote Ar–H2–SiH4 plasma which is capable of depositing device-quality a-Si:H at 10 nm/s. SiH3 radicals have been detected by means of threshold ionization mass spectrometry for different fractions of H2 in the Ar–H2-operated plasma source. It is shown that at high-H2 flows, SiH4 dissociation is dominated by hydrogen abstraction and that SiH3 contributes dominantly to film growth. At low-H2 flows, a significant amount of very reactive silane radicals, SiHx(x⩽2), is produced, as concluded from threshold ionization mass spectrometry on SiH2 and optical emission spectroscopy on excited SiH and Si. These radicals are created by dissociative recombination reactions of silane ions with electrons and they, or their products after reacting with SiH4, make a large contribution to film growth at low-H2 flows. This is corroborated by the overall surface reaction probability which decreases from ∼0.5 to ∼0.3 with increasing H2 fraction. The film properties improve with increasing H2 flow and device-quality a-Si:H is obtained at high H2 fractions where SiH3 dominates film growth. Furthermore, it is shown that at high-H2 flows the contribution of SiH3 is independent of the SiH4 flow while the deposition rate varies over one order of magnitude.

Temperature dependence of the surface roughness evolution during hydrogenated amorphous silicon film growth

“…The films have been deposited by the remote expanding thermal plasma ͑ETP͒ under conditions in which film growth is dominated by SiH 3 and in which ion bombardment is absent. 14 The surface reactions are, therefore, purely chemical.…”

Cavity ring down study of the densities and kinetics of Si and SiH in a remote Ar-H2-SiH4 plasma

“…In previous articles, it has been demonstrated that the dissociation of SiH 4 in the ETP is either governed by ion-induced reactions for a purely Ar operated plasma source or by atomic hydrogen reactions when relatively high H 2 flows are added to the Ar. 3,24,29 In Fig. 1, an overview is given of the contribution to film growth of species that have been investigated in detail so far as a function of this H 2 flow.…”

“…Figure 1 shows furthermore that at high H 2 flows ͑Ͼ7.5 sccs͒, the deposition rate can almost completely be explained by the contribution of SiH 3 radicals to film growth ͑estimated contribution ϳ90%͒. 3,24 The formation of SiH 3 is due to the hydrogen abstraction reaction HϩSiH 4 →SiH 3 ϩH 2 ͑1͒…”

“…[25][26][27] Additionally, Fig. 1 reveals that when going to the ion dominated region by reducing the H 2 flow in the cascaded arc, the contribution of SiH 3 to film growth decreases 3,24 while under these conditions also relatively poor a-Si:H film properties are obtained. 25,27 This change has been attributed to the fact that dissociation of SiH 4 leads primarily to the production of very reactive radicals such as SiH 2 , SiH, and Si.…”

“…In addition to mass spectrometry, [1][2][3] laser-induced fluorescence ͑LIF͒ based techniques, [4][5][6][7][8][9][10][11][12][13] ultraviolet absorption spectroscopy, [14][15][16] and intracavity 17,18 and infrared laser 19,20 absorption techniques, recently also cavity ring down absorption spectroscopy ͑CRDS͒ has been introduced in the field of SiH 4 plasmas. This sensitive, and relatively easy-to-apply diagnostic technique 21 has already been used for the detection of SiH 3 , [22][23][24] SiH 2 , 18 and nanometric Si-based particles.…”