A continuum model was used to analyze charged particle transport and potential distribution in low-pressure radio frequency (rf) glow discharges. The method of lines with orthogonal collocation on finite elements for the spatial discretization was found to be an effective numerical technique for solving the model equations. An argonlike (electropositive) discharge was compared to a pure chlorine (electronegative) discharge. The electronegative discharge was found to have much thinner sheaths, much greater potential drop and electric field strength in the bulk plasma, and severe modulation by the applied rf (10 MHz frequency) of the electron temperature, ionization, and excitation rate, even in the bulk. The effect of varying excitation frequency was also examined. The results showed that continuum models can capture the essential features of both kinds of discharges. Integration of these models with neutral species transport and reaction can result in powerful tools for the modeling and design of plasma reactors.

A mathematical model for plasma etching of silicon using tetrafluoromethane in a radial flow reactor was developed. Finite element methods were employed to calculate the two-dimensional flow, temperature, and species concentration fields. Etching rate and uniformity were studied as a function of reactor operating conditions, including the effect of flow direction. For the parameter values examined, the etching rate increased monotonically with flow rate. For low substrate temperature (298 K), inward flow resulted in higher etching rate as compared to outward flow, but the trend reversed at higher temperature. For flow rates greater than 200 sccm, outward flow with a uniform electron density distribution gave the best uniformity results. A one-dimensional radial dispersion approximation was used to study the effect of squarewave power modulation (pulsed-plasma reactor). The etching rate increased with decreasing pu]ise period and with increasing duty cycle. Under conditions which would result in high depletion of the precursor gas in a continuous-wave reactor (e.g., for low flow rates), the pulsed-plasma reactor can offer substantial improvement in uniformity without sacrificing the etching rate.

A two~dimensional transport and reaction model of a high pressure (-1 torr) high frequency (13.56 MHz) single-wafer parallel plate plasma reactor was developed. The chemical etching uniformity was studied as a function of reactor operating conditions. The ratio of the reactivity of the surrounding electrode surface as compared to that of the wafer surface, S, critically affected uniformity. A bullseye clearing pattern was predicted for S < 1, and the reverse pattern for S > 1, while etching was uniform for S = 1. In the case of S ~ 1, and for the parameter range studied, the absolute uniformity was found to improve by surrounding the wafer with a material of similar reactivity, by increasing the flow rate, or by decreasing the reactor pressure or power. However, such actions also served to decrease the etch rate. The oxygen plasma was used as a model experimental system to test the theoretical predictions. An experimental technique based on spatially resolved optical emission spectroscopy in concert with actinometry and the Abel transform was developed to obtain a three-dimensional mapping of the reactant (O atom) concentration profile in the plasma reactor. When a reactive film was covering part of the substrate electrode, a profound decrease in the reactant concentration was observed over the film. At the same time, large concentration gradients developed, especially at the boundary of the reactive film with the surrounding electrode. Good quantitative agreement was found between the model predictions and the experimental reactant concentration data for the range of pressure, power, flow rate, and reactive film radius examined.

A two-dimensional finite element simulation of gas flow and reactive species distribution in a parallel-plate singlewafer isothermal reactor was conducted. The oxygen plasma etching of polymer under high pressure (-1 torr), high frequency (13.56 MHz) conditions was chosen as a model system for analysis with emphasis on chemical etching. Etching rate and uniformity were examined as a function of reactor geometry and operating conditions. A maximum in etching rate with flow rate was observed, and this maximum shifted to higher flow rates as the power increased. The pressure dependence of the etching rate was complex, but in genera] a broad maximum in etching rate with pressure existed, which shifted to higher pressures as the flow rate increased. Etching rate increased but etching uniformity degraded as the wafer reactivity increased. A shower radius at least equal to the wafer radius, and a plasma radius slightly greater than the wafer radius were found to give the best etching rate and uniformity results under the conditions examined. Two novel reactor designs were also studied, namely, a reactor with a graded gas velocity at the shower, and a pulsed-plasma reactor. When compared to the conventional reactor, the new designs can yield improved uniformity and etching rate (the etching rate of the pulsed-plasma reactor was prorated by the duty cycle). ABSTRACTSilicon surface contamination and near-surface damage of the Si(100) substrate caused by reactive ion etching (RIE) of SiO2 overlayer in C2FdCHF3 plasma were characterized by x-ray photoelectron spectroscopy (XPS), He-ion channeling technique and high-resolution transmission electron microscopy (HRTEM). XPS study indicates that a thin (<20A thick) C,F polymeric layer was formed on the Si surface and underneath the polymer film a thin oxide layer (-10A) was produced due to air exposure after RIE. These films were also characterized in terms of chemical composition and bonding

A simplified model for a time-dependent plasma-assisted chemical vapor deposition reactor was developed based on transport and reaction principles. The model equations were solved by the method of lines using collocation on finite elements for the spatial discretization. Emphasis was placed on the deposition rate and uniformity as a function of reactor operating conditions. A pulsed-plasma reactor was analyzed, and compared to a continuous-wave (CW) plasma reactor. Under conditions which would result in high depletion of the precursor gas in the CW reactor, the pulsed-plasma reactor yielded improved uniformity, albeit the deposition rate was reduced. The effect of a recycle stream on both the CW and pulsed-plasma reactors was also studied. For the CW reactor, recycle was most beneficial under conditions of low depletion of the precursor gas. For cases of intermediate depletion of the precursor gas, a CW reactor with recycle or a combination of pulsed-plasma and recycle can result in nearly uniform deposit without sacrificing the deposition rate. Analytic solutions were derived for the CW reactor with recycle, and for a well-mixed pulsed-plasma reactor. The results apply equally well to pulsed-plasma etching reactors conforming to the model assumptions and operating under corresponding conditions.