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MAS6 encodes an essential inner membrane protein required for mitochondrial protein import in the yeast Saccharomyces cerevisiae (Emtage and Jensen, 1993). To identify new inner membrane import components, we isolated a high-copy suppressor (SMS1) of the mas6-1 mutant. SMS1 encodes a 16.5-kDa protein that contains several potential membrane-spanning domains. The Sms1 protein is homologous to the carboxyl-terminal domain of the Mas6 protein. Like Mas6p, Sms1p is located in the mitochondrial inner membrane and is an essential protein. Depletion of Sms1p from cells causes defects in the import of several mitochondrial precursor proteins, suggesting that Sms1p is a new inner membrane import component. Our observations raise the possibility that Sms1p and Mas6p act together to translocate proteins across the inner membrane.

The production ofSOF4 initiated by the reaction of F atoms withSOF2 has been studied in a gas-flow reactor at 295 K for helium bath gas number densities in the range (3.0–27.0)×1016 cm-3. The effect of O atoms on the formation ofSOF4 has been analyzed in terms of the competing reactionsSOF3+F?SOF4 andSOF3+O?SO2F2+F. This analysis leads to the conclusion that the rate coefficients for these two processes are equal within an uncertainty of about 50%. Furthermore, both experiment and calculations indicate that the rate coefficient for reactions between F atoms andSOF3 is close to its high-pressure limit under the conditions employed. The experiments set a lower limit on this rate coefficient of 5×10-11 cm3 s-1, while calculations based on unimolecular rate theory suggest that it may be greater than 1×10-10 cm3 s-1. These results make it clear that the two reactions shown above cannot explain the relative abundances ofSOF4 andSO2F2 which are observed inSF6/O2 plasmas. This suggests thatSF2 is a major precursor in the sequence of reactions following the dissociation ofSF6.

The plasma chemistry of SF6/O2 mixtures is particularly complicated because of the large number of possible reactions. Over a wide range of conditions, products including SF4, SOF4, SOF2, and SO2F2 can be formed but thre is considerable uncertainty about the major reactions which contribute to the formation of these species. In this work reactions of oxygen atoms with SOF2 and fluorine atoms with SOF2 and SO2 have been studied in order to determine the principal sources of SO2F2 in these plasmas. Reactions were studied at 295 K in a gas flow reactor sampled by a mass spectrometer. No reaction could be detected between oxygen atoms and SOF2, which for the conditions employed, means that the upper limit for the reaction rate coefficient is 1×10-14 cm3 sec-1. The reaction of fluorine atoms with SOF2 was studied with the helium bath gas number density ranging from 3.1×1016 to 2.0×1017 cm-3. Within this range the rate coefficient increased with increasing [He] from (4.1 to 10.8)×10-14 cm3 sec-1. SO2 was found to react with fluorine atoms with a rate coefficient which appeared to be independent of the helium bath gas number density over the range given above. The possibility that this reaction occurred entirely on the walls of the reactor is discussed.

Reactions of both SF4 and SF5 with F have been studied at 295 K in a gas-flow reactor sampled by a mass spectrometer. The rate coefficient for the combination reaction of F with SF4 to produce SF5 was found to increase from (0.9 to 3.0)×10-12 cm3 s-1 when the helium bath gas number density was increased from (2 to 26)×1016 cm-3. The values obtained here are three orders of magnitude higher than a recent estimate of the high-pressure value based on the modelling of photochemical studies. The experimental results have been compared with RRKM and master equation calculations in which a simplified Gorin model has been used to determine the structure of the transition state. These calculations show that reasonable agreement can be obtained between the experimental data and the calculation if a small (~2 KJ/mol) activation energy is assumed. The rate coefficient for the reaction between SF5 and F to produce SF6 was found to be independent of helium bath gas number density within the range given above. The value obtained for the rate coefficient was 9×10-12 cm3 s-1 with an uncertainty of a factor of 2. This value is close to that of 1×10-11 cm3 s-1 computed from the simplified Gorin model and to the value of 1.7×10-11 cm3 s-1 deduced from modelling of photochemical experiments.

Processes which occur in microwave discharges of dilute mixtures of SF6 and O2 in He have been examined using a flow reactor sampled by a mass spectrometer. Two classes of experiments were performed. In the first set of experiments, mixtures containing 6×1011 cm-3 SF6, 6×1016 cm-3 He, and O2 in the range (0–3.6)×1013 cm-3 were passed through a 20-W 2450-MHz microwave discharge. The gas mixtures arriving at a sample point downstream from the discharge were examined for SF6, SF4, SOF2, SOF4, SO2F2, SO2, F, and O. In the second class of experiments, rate coefficients were measured for the reactions of SF4 with O and O2 and for the reaction of SF with O. The rate coefficient for the reaction of SF with O was found to be (4.2±1.5)×10-11 cm-3 s-1. SF4 was found to react so slowly with both oxygen atoms and oxygen molecules that only upper limits could be placed on the rate coefficients for these reactions. These values were 2×10-14 cm3 s-1 and 5×10-15 cm3 s-1 for reactions with O and O2 respectively. The observed distribution of products from the discharged mixtures is discussed in terms of the measured rate coefficients.

A model has been developed in an attempt to explain the chemistry which occurs in plasmas produced in mixtures of CF4 and O2. Emphasis is placed on gas-phase free radical reactions, and the predictions of the model are compared with experimental results. Dissociation rates following electron impact are deduced mainly from experimental observations although relative dissociation rates have been calculated. An important assumption of the model is that CF2 can be produced as a primary dissociation product following electron impact. Furthermore, this process is favored over that producing CF3 by more than a factor of 2. Experimental evidence is presented to support this assumption. Although the model agrees well with experiment on the total amount of fluorine produced, some discrepancy exists between the predicted and measured values of [F2]. It is suggested that the higher concentrations detected in the experiments resulted from recombination of F atoms in the sampling region. The agreement for concentrations of CO2, CO, and COF2 is generally better than a factor of 2 over a wide range of experimental conditions.

Reactions of both SF5 and SF2 with O(3P) and molecular oxygen have been studied at 295 K in a gas flow reactor sampled by a mass spectrometer. For reactions with O(3P), rate coefficients of (2.0±0.5)×10-11 cm3 s-1 and (10.8±2.0)×10-11 cm3 s-1 were obtained for SF5 and SF2 respectively. The rate coefficients for reactions with O2 are orders of magnitude lower, with an estimated upper limit of 5×10-16 cm3 s-1 for both SF5 and SF2. Reaction of SF2 with O(3P) leads to the production of SOF which then reacts with O(3P) with a rate coefficient of (7.9±2.0)×10-11 cm3 s-1. Both SO and SO2 are products in the reaction sequence initiated by reaction between SF2 and O(3P). Although considerable uncertainty exists for the heat of formation of SOF, it appears that SO arises only from reaction between SOF and O atoms which is also the source of SO2. These results are discussed in terms of a reaction scheme proposed earlier to explain processes occurring during the plasma etching of Si in SF6/O2 plasmas. A comparison between the results obtained here and those reported earlier for reactions of both CF3 and CF2 with O and O2 shows that there is a marked similarity in the free radical chemistry which occurs in SF6/O2 and CF4/O2 plasmas.

A model has been developed to describe the chemistry which occurs in CF4 plasmas and the etching of Si both in the plasma and downstream. One very important feature of this model is that for discharge residence times which vary by more than an order of magnitude, the amount of CF4 consumed is low and relatively constant. This is because the gas-phase combination reactions between F and both CF3 and CF2 lead to the rapid reforming of CF4. The model predicts that CF2 is a major species in the gas phase and that the [F] detected as a sample point downstream is a very sensitive function of [CF2]/[F] in the discharge. Even though the calculations show that [F] in the discharge varies only slightly over the wide range of experimental conditions considered, large variations in [F] at the sample point occur because the [CF2]/[F] ratio in the discharge changes. The concentrations of C2F6 and SiF4 are predicted to within a factor of 2 over a very wide range of experimental conditions. This confirms the importance of gas-phase free radical reactions in the etching of Si.

Reaction rate coefficients have been measured at 295 K for both CF3 and CF2 with atomic and molecular fluorine. The reaction between CF3 and F was studied over a gas number density range of (2.4–23)×1016 cm-3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10-11 cm3 s-1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF2+F increased from (0.4–2.3)×10-12 cm3 s-1 as the helium gas number density increased from (3.4–28.4)×1016 cm-3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.

A model has been developed to describe the chemistry which occurs in SF6/O2 plasmas and the etching of silicon in these plasmas. Emphasis is placed nn the gas-phase free radical reactions, and the predictions n( the model are compared with experimental results. Forty-seven reactions are included, although a subset of 18 reactions describes the chemistry equally well. Agreement between the calculated and measured concentrations of stable products downstream of the plasma is better than a factor of 2. The need for additional kinetic data and fàr well-characterized diagnostic studies of SF6/O2 plasmas is discussed.