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.

A model of the chemical processes occurring in CF4/O2 discharges used in plasma etching

Measurements of monosilicon (SiHn) and disilicon (Si2Hn) radicals at the cathode surface of dc discharges in silane and silane‐argon mixtures are reported. Silyl radical density per decomposed silane was constant for fixed flow conditions over a range of powers and silane‐argon ratios. The relative densities for other monosilicon radicals SiHn/SiH3 decreased with increased fraction of silane in silane‐argon mixtures. The density of disilicon radicals was observed to be comparable to some of the monosilicon radicals, with Si2H2 and Si2H4 the dominant Si2Hn species. Formation and destruction reactions are discussed for these radicals, disilane, and the deposited film. We deduce that disilane is formed primarily on surfaces and that sputtering is a significant source for radicals near the cathode.

Mono‐ and disilicon radicals in silane and silane‐argon dc discharges

Polymerization reactions in radio frequency fluorocarbon plasmas of CF4, C2F6, and C4F8 have been studied by electron attachment mass spectrometry (EAMS). In these plasmas polymerization occurs readily and molecules containing up to ten carbon atoms (the mass limit of the mass spectrometer) have been found. The densities of large polymers increase with increasing size of the parent gas. In a fluorine-rich environment like a CF4 plasma the detected polymers are mainly fully saturated with F (CnF2n+2). As the amount of fluorine in the parent gas decreases, also the degree of saturation of the polymers decreases, which is clearly seen in C2F6 and C4F8 plasmas. The unsaturated polymers are more reactive, so they can stick more easily to surfaces and possibly create thick polymer films, which are often observed after discharge operation. The polymerization rate depends on the chemical activity of the plasma, which can be easily enhanced by increasing the radio frequency power. The positive ions, extracted from...

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Polymerization of fluorocarbons in reactive ion etching plasmas

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 1356 MHz plasmas in pure CF4 at 50 and 200 mTorr Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces Fluorine-poor conditions will be considered in a following paper Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions This mechanism often predominates over the gas-phase

CFx radical production and loss in a CF4 reactive ion etching plasma: Fluorine rich conditions

A model has been constructed to describe the electrical characteristics of the central bulk plasma region in a 13.56-MHz parallel-plate discharge in chlorine at pressures of about 1 torr. This region is modeled as a volume-controlled plasma with the electron balance dominated by single-step electron-impact ionization and attachment and with the electron energy distribution function in equilibrium with the local instantaneous electric field. Relationships between the ionization frequency, the attachment frequency, the electron drift velocity, and the electric field are provided by solutions of the Boltzmann equation for mixtures of Cl2 and Cl which result from Cl2 dissociation. From a measured current waveform and Cl2/Cl density ratio, the model generates the local electric-field waveform, the time-varying electron density, and the power density in the central portion of the bulk plasma. The calculated time-averaged power input per unit discharge length compares well with experimentally determined values.

A Model for the Bulk Plasma in an RF Chlorne Discharge

A CF4 plasma etching silicon has been simulated to identify dominant chemical processes and to quantify the effects of various reaction and transport parameters. The model was a one‐dimensional plug‐flow reactor in which a packet of gas is followed through the plasma and into the afterglow region, allowing the simulation to be performed as an initial value problem in ordinary differential equations. Two temperature zones were used with all known significant reactions incorporated into the chemical mechanism with the best available rate constants. Adjustable parameters were included only for certain sticking coefficients, surface recombination rates, and surface polymerization rates. Appropriate adjustment of these parameters gives satisfactory agreement between the simulations and experimental measurements of downstream gas‐phase composition. The model unambiguously shows that fluorine atoms are the main reactive species in the plasma, that gas phase chemistry is clearly dominated by neutral reactions, an...

David Edelson

Papers

Computer simulation of a CF4 plasma etching silicon

Radial Diffusion in Drift‐Tube Experiments