G. Turban

Mass spectrometry of a silane glow discharge during plasma deposition of a-Si: H films

Abstract We undertook a mass spectrometric investigation of the ionic and neutral species present during the deposition of a-Si: H using an r.f. glow discharge in silane (mixed with helium or hydrogen). A correlation between the neutral composition of the plasma and the nature of the IR vibrational modes in the deposited film is proposed. The ionic species extracted from the silane discharge are not characteristic of the direct ionization of SiH4. The predominance of the SiH3+ ion is attributed to the ion-molecule reaction SiH 2 + + SiH 4 → SiH 3 + + SiH 3 Secondary ions Si2Hn+ (n = 1−7) are also observed. Mass spectrometry of the ionic species resulting from the interaction of a hydrogen plasma with the a-Si: film suggests that atomic hydrogen plays an active role during the growth of the film.

A comparative study of oxygen/organosilicon plasmas and thin SiOxCyHz films deposited in a helicon reactor

Thin SiOxCyHz films have been prepared by plasma enhanced chemical vapor deposition (PECVD) on silicon substrates at low pressure (2 mTorr) and 300 W rf power, using tetraethoxysilane (TEOS) or hexamethyldisiloxane (HMDSO) as a monomer and oxygen as a reactive gas. The plasma composition, the structure and properties of the deposited films are studied as a function of the organosilicon fraction (Xorg). Optical emission spectroscopy is carried out in order to identify the species in the plasma. The layers are characterized by in situ spectroscopic ellipsometry and by several ex situ diagnostics including infra-red spectroscopy, X-ray photoelectron spectroscopy, gravimetry and chemical etching. At low values of Xorg, the structure and properties of the films and optical emission spectra are very similar whatever the organosilicon precursor. At high values of Xorg, the structure and properties of the deposited films and emitting species significantly depend on the organosilicon precursor.

Reactive plasma deposited SixCyHz films

Abstract SixCyHz films have been prepared at 200°C by reactive plasma deposition from SiH4 and CH4 diluted in helium in a tubular reactor. These films have a ratio s (equal to Si ( Si+C )) ranging from 0.2 to 0.8, a refractive index ranging from 1.96 to 2.6 and an optical energy band gap in the range 2.7-2.2 eV. The total quantity of hydrogen in the film is 40% when s=0.5. Infrared analysis shows that these films have large fractions of homonuclear bonds and that this material is best described as a polymer. Mass spectrometric measurements of the gaseous products formed in the SiH4-CH4-He plasma have been performed and the results are related to the composition of the deposited layers.

Infrared absorption of hydrogenated amorphous SiC and GeC films

Abstract We have observed and assigned the main IR absorption bands of Si x C 1− x H z and Ge x C 1− x H z films produced with an r.f. glow discharge. The influence of neighbouring atoms on the frequency of SiH and CH bond stretching vibrations was determined. Agreement between observed and calculated frequency shifts was obtained for Si x C 1− x H z films.

Reaction mechanisms of the radio frequency glow discharged deposition process in silane-helium

Abstract The kinetics of a low pressure (0.2 Torr) r.f. discharge in SiH 4 -He has been studied. The measured mass transport rate towards the wall is compared with the transport rate derived from a simplified mono-dimensional kinetic model. The silane decomposition is assumed to proceed mainly through electron impact, and only neutral radicals SiH n have been considered for the diffusional mass transport. A good agreement is found between the experimental and theoretical rates. The measurement of the mean electron density means that an activation constant for silane can be determined. The neutral species evolving from the plasma have been studied by mass spectrometry. The formation of disilane has been observed for pressures above about 0.5 Torr.

Ion and radical reactions in the silane glow discharge deposition of a-Si:H films

A mass spectrometric analysis of the positive ions and neutral products in a silane glow discharge has been performed. The active species, created by dissociation, disproportionation, and ion-molecule reactions are mainly SiH2, SiH3, and H. A calculation of the distribution of the SiHn+ ions shows that the silane concentration monitors the abundance of SiH3+. The diffusional transport of radicals toward the discharge-tube walls can explain the observed deposition rates. The study of SiH4-SiD4 and SiH4-D2 plasmas emphasizes several reactions which modify the free-radical populations depending on the discharge conditions: disproportionation, termination, recombination, and abstraction. Heterogeneous reactions have also been observed: etching of the film by H atoms and direct incorporation of hydrogen in the growing film. A general scheme for the plasma deposition mechanism is proposed.

Effect of negative bias voltage on a-C:H films deposited in electron cyclotron resonance plasma

a-C:H films were prepared by chemical vapour deposition assisted by electron cyclotron resonance (ECR) plasma from CH4 at low pressure (p ≤ 0.35 Pa) with radio frequency (RF) power applied to the substrate. The effects of negative bias voltage Vb on deposition rate R and structure of the films were examined. As a consequence of RF polarisation, good adhesive diamond-like carbon (DLC) films with R ≤ 200 A/min were obtained. The microstructure of the films has been studied by ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and electron energy loss spectroscopy (EELS). Films deposited from ECR plasma at 0.35 Pa were compared to films obtained from an RF plasma of CH4 in a diode reactor at 7 Pa. The FTIR and EELS results suggest that with increasing Vb the amount of hydrogen and C sp2 decreases and the network becomes highly sp3 bonded. HRTEM studies reveal the existence of nanocrystalline diamond clusters in the amorphous matrix. It appears that the deposition from a dual ECR-RF plasma at low pressure can be a powerful and flexible technique to obtain a-C:H films with an important amount of C sp3 bonding and a low hydrogen content.

A study of the silane glow discharge deposition by isotopic labelling

Abstract Neutral and ionic species in SiH4-SiD4-He and SiH4-H2-D2 r.f. glow discharges at pressures of 0.15-1 Torr were measured by mass spectrometry. We found partially deuterated monosilanes (SiH2D2 and SiHD3). The disilane formed, Si2H6−nDn, exhibits all the degrees of deuteration (n = 0−6). These results are consistent with the presence of SiH2 and SiH3 radicals in the discharge. Reaction pathways are proposed to account for these results. The observed plasma etching of an a-Si:H film in a D2 plasma and the IR analysis of the Si-D bonds in the film showed that the formation of these bonds is the result of both silane deuteration and of deuterium incorporation in the growing film.

Diagnostics in helicon plasmas for deposition

and helicon plasmas used for plasma enhanced chemical vapour deposition of films are investigated in the 1 - 10 mTorr pressure and 0 - 800 W rf power ranges. The positive oxygen ions are analysed by energy selective mass spectrometry and Langmuir probes. The oxygen atom concentration is monitored by actinometry and ionization threshold mass spectrometry. In oxygen plasmas it is shown that is the major positive ion, and that the oxygen molecules are far from being completely dissociated, due to a very high oxygen atom recombination frequency on the reactor walls. The dissociation degree increases with the rf power reaching 10% at 500 W. In plasmas, the plasma density and electron temperature decrease as the TEOS fraction increases. In contrast, the degree of oxygen dissociation increases sharply with the addition of a few per cent TEOS, is maximum for about 5% TEOS and decreases as TEOS fraction is further increased. In a 95:5 plasma (5 mTorr, 300 W) the fluxes of oxygen positive ions and atoms impinging onto a floating substrate are estimated to be and respectively. Under these plasma conditions, near-stoichiometric films, with low OH content, are deposited at ambient temperature. The corresponding atom to ion flux ratio is about 250, which suggests the dominant role of oxygen atoms in the deposition kinetics. The comparison of the compositions of layers grown in a 5 mTorr 95:5 plasma at two rf powers confirms the major role of oxygen atoms.

Plasma etching of refractory metals (W, Mo, Ta) and silicon in SF6 and SF6-O2. An analysis of the reaction products

The etching rates and reaction products of refractory metals (W, Mo, and Ta) and silicon have been studied in a SF6-O2 r.f. plasma at 0.2 torr. The relative concentrations of WF6 and WOF4 and the intensities of the WFn+ (n=3−5), WOFm+ (m=1−3), MoFn+, and MoFm+ ions have been measured by mass spectroscopy. An analysis of the neutral composition of the plasma during etching of these metals and a comparison with the results obtained for silicon show that at least two species are involved for W and Mo etching: fluorine and oxygen atoms. A reaction scheme is proposed.