J. A. Mucha

The reaction of fluorine atoms with silicon

Fluorine atoms etch silicon with a rate, RF(Si) = 2.91±0.20×10−12T1/2nFe−0.108 eV/kT A/min, where nF (cm−3) is the atom concentration. This etching is accompanied by a chemiluminescent continuum in the gas phase which exhibits the same activation energy. These phenomena are described by the kinetics: (1) F(g)+Sisurf→SiF2(g), (2) SiF2(g) +F(g) →SiF*3(g), (3) SiF2(g) +F2(g) →SiF*3(g) +F(g), (4) SiF*3(g) →SiF3(g) +hνcontinuum where formation of SiF2 is the rate‐limiting step. A detailed model of silicon gasification is presented which accounts for the low atomic fluorine reaction probability (0.00168 at room temperature) and formation of SiF2 as a direct product. Previously reported etch rates of SiO2 by atomic fluorine are high by a constant factor. The etch rate of SiO2 is RF(SiO2) = (6.14±0.49)×10−13nF T1/2e−0.163/kT A/min and the ratio of Si to SiO2 etching by F atoms is (4.74±0.49)e−0.055/kT.

Luminescence and structural study of porous silicon films

The luminescence properties of 3 μm thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6–7 nm size clusters (instead of the postulated columns). In the as‐prepared films, there is a significant concentration of Si—H bonds which is gradually replaced by Si—O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at ≊690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50 μs. The correlation of the spectral and structural information suggest...

On the role of oxygen and hydrogen in diamond‐forming discharges

Plasma emission actinometry has been used to study the mechanism by which small additions of oxygen (∼0.5%) enhance the rate of diamond deposition in a dilute (4%) CH4/H2 discharge at high temperature (900–1300 K). Increasing amounts of CH4 in the feed depress [H], while increasing the O2 concentration, up to ∼5%, produces a fivefold increase in atomic hydrogen in the discharge zone. Invoking a mechanism where diamond growth competes with the formation of an amorphous/graphitic inhibiting layer, these results and earlier studies suggest that oxygen (1) increases [H] which selectively etches amorphous/graphitic carbon, (2) accelerates reaction of this layer with molecular hydrogen, and (3) may itself act as a selective etchant of nondiamond carbon. As a result, the number of active diamond growth sites is increased and enhanced growth rates are observed. We also have grown diamond by alternating a CH4/He discharge with a H2/O2/He discharge and results are consistent with this mechanism. Instantaneous growt...

Comparison of XeF2 and F‐atom reactions with Si and SiO2

Silicon gasification by XeF2 is compared with F‐atom etching under conditions typical of those used in plasma etching. Temperatures ranged from −17 to 360 °C and XeF2 pressures were between 0.05 and 2 Torr. Silicon etching by XeF2 shows a sharply different etch rate/temperature dependence than the Si/F or Si/F2 reaction systems; there is no detectable reaction between XeF2 and SiO2 in contrast to the F‐atom/SiO2 system. These data indicate that physisorption can limit silicon etching by XeF2 and show that basic studies which use XeF2 as a model compound for the etching of silicon and SiO2 by F atoms should be interpreted with caution.

Diamond crystal growth by plasma chemical vapor deposition

We have grown diamond crystals and polycrystalline diamond films from CH4/H2/O2 gas feeds in a simple, high‐power density, 2450‐MHz discharge tube reactor. Single‐crystal growth rates over 20 μm/h have been achieved. The material has been analyzed using Raman spectroscopy, Auger spectroscopy, and x‐ray diffraction. Control of nucleation is a major problem for growing sound films, and the high temperatures currently required for growth will limit applications. Oxygen additions were necessary to deposit diamonds over the range of feed composition we studied.

Plasmaless dry etching of silicon with fluorine‐containing compounds

Silicon is rapidly etched by the gas‐phase halogen fluorides ClF3, BrF3, BrF5, and IF5, in analogy to XeF2 etching silicon. Nearly complete selectivity over SiO2 is achieved in all cases. By contrast, ClF and Groups III and V fluorides such as NF3, BF3, PF3, and PF5 do not spontaneously etch either Si or SiO2 under the same experimental conditions. These relatively inexpensive interhalogens can be applied to pattern silicon and more generally to remove silicon or polysilicon layers without a plasma. Low‐temperature plasmaless gasification of substrates by these fluorine‐containing interhalogens is an economically attractive alternative to fluorine‐based plasma etching.

Silicon oxide deposition from tetraethoxysilane in a radio frequency downstream reactor: Mechanisms and step coverage

Isotopic labeling and step coverage studies of silicon oxide deposited from tetraethoxysilane (TEOS) have been carried out by introducing TEOS(16O) downstream from an 18O2 discharge. Rutherford backscattering (RBS) data on films deposited near 440 °C show that, on average, one Si–O bond in the original TEOS molecule is preserved in the process, while mass spectrometric results indicate only H216O and C18O16O as gaseous products of the cleavage of the remaining three Si–O bonds. Infrared analyses of films deposited at room temperature show large amounts of Si–OH in a gel‐like material, and the presence of a C■O species. The results suggest a mechanism dominated by diffusion and condensation of Si–OH species that form extensive chains and preserve an Si–16O bond from the original precursor. This is followed by cross‐linking to form the final silicate network; however, Si–O bond cleavage is apparently occurring at potential cross‐linking sites via a carbonate intermediate that promotes isotopic scrambling. S...

Frequency effects and properties of plasma deposited fluorinated silicon nitride

The properties of low‐hydrogen, fluorinated plasma‐enhanced chemical vapor deposition (PECVD) silicon nitride films grown using NF3/SiH4/N2 feed mixtures in 200 kHz and 14 MHz discharges were compared. High‐energy ion bombardment at 200 kHz is expected to enhance surface diffusion and chemical reconstruction. Compared to fluorinated silicon nitride deposited at 14 MHz under otherwise comparable conditions, the 200 kHz films had a lower Si–H bond concentration (≲1×1021 cm−3), lower total hydrogen content (5–8×1021 cm−3), better resistance to oxidation, lower compressive stress (−0.7 to −1.5 Gdyne/cm), and higher density (3.1 g/cm3). The dielectric constant of better low‐frequency Class I films was constant to 500 MHz, while that of high‐frequency films fell up to 15% between 100 Hz and 10 MHz. The absorption edges of low‐frequency PECVD fluorinated silicon nitride films were between 5.0 and 6.1 eV, which compare with 4.4 to 5.6 eV for the high‐excitation frequency fluorinated material and 3 to 4 eV for con...

The thermal conductivity of chemical‐vapor‐deposited diamond films on silicon

The thermal conductivity of chemical‐vapor‐deposited diamond films on silicon is measured for the case of heat flow parallel to the plane of the film. A new technique uses thin‐film heaters and thermometers on a portion of the film which is made to be free standing by etching away the substrate. Effects of thermal radiation are carefully avoided by choosing the length scale properly. Data for several films yield thermal conductivities in the range 2–6 W/cm °C. This is comparable to copper (4 W/cm °C) and is in a range that would be useful as a thin‐film dielectric material, provided that the interface thermal resistance can be minimized. The conductivity varies inversely with the growth rate and the Raman linewidth.

Doping and crystallographic effects in Cl‐atom etching of silicon

Absolute rates for the intrinsic reaction between Cl atoms and surfaces of P‐doped polycrystalline silicon, P‐doped Si(100) and As, Sb‐doped Si(111) substrates were measured for the first time as a function of dopant concentration (Ne) and substrate temperature in a downstream reaction system. This study clearly shows that when there is no ion bombardment, increasing Ne increases the Si‐Cl reaction rate even when silicon is lightly doped (∼1015 cm−3), in contrast to in‐discharge studies. Moreover, results showed that crystal orientation influences the Cl‐Si reaction more than Ne, for Ne<1020 cm−3. The data are fitted to a modified Arrhenius expression, R=νNγenClT1/2e−E/kT, with R the etch rate and nCl the gas phase Cl concentration. The calculated values of the activation energy E are 4.1–4.7 kcal/mole for all doping levels and crystallographic orientations. Therefore, the doping effect is manifested solely in the preexponential (νNγe) of the Arrhenius expression, and the data qualitatively agree with a c...