Electron and chemical kinetics in the low-pressure RF discharge etching of silicon in SF/sub 6/
Monte Carlo simulation and Boltzmann equation solutions have been used to study the electron kinetics. All electronic excitation of SF/sub 6/ is assumed to be dissociative in analogy with the known product channels in ionization and multiphoton dissociation. The electric-field-to-gas-density ratios are high (E/n greater than or equal to 1000 Td, where 1 Td (Townsend) = 1 x 10/sup -17/ V . cm/sup 2/) in low-pressure (p < 0.3 torr) radiofrequency (RF) discharges. At these high E/n values, the electron energy relaxation time is much shorter than the 74-ns period at 13.56 MHz. Furthermore, the time scale of the chemical kinetics is much longer than the period of the applied RF voltage. Therefore the electron energy distribution can ''track'' the time-varying electric field, and time- and space-averaged rate coefficients can be used in chemical kinetics models. A rate equation model has been used to study the chemical kinetic processes. Electron-impact dissociation and ionization are the dominant sources of chemically active species. An electron density of 1 x 10/sup 8/ cm/sup -3/ is estimated from the known average values of E/n and the discharge input power. Two limiting cases are studied for the positive and negative ion diffusion losses: a) trapped negative ions and positive ion loss at the ambipolar diffusion rate; and b) positive and negative ion losses at the free diffusion rates. Neutral particle diffusion losses are estimated by using an effective diffusion length which takes surface reflection into account and increases as the surface reflection probability increases. The predicted particle flux to the silicon wafer surface is dominated by neutral F and SF/subX/ and the predicted etching rate due to these species is at the low end of the experimentally observed range.
- Research Organization:
- Westinghouse R and D Center, Pittsburgh, PA 15235
- OSTI ID:
- 5897497
- Journal Information:
- IEEE Trans. Plasma Sci.; (United States), Vol. PS-14:2
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
RF SYSTEMS
ELECTRIC DISCHARGES
SILICON
ETCHING
AMBIPOLAR DIFFUSION
BOLTZMANN EQUATION
CHEMICAL REACTION KINETICS
ELECTRIC FIELDS
ELECTRON DENSITY
ELECTRONS
EQUATIONS
EXCITATION
IONIZATION
MEDIUM VACUUM
MONTE CARLO METHOD
MULTI-PHOTON PROCESSES
SILICON FLUORIDES
SPACE DEPENDENCE
TIME DEPENDENCE
DIFFERENTIAL EQUATIONS
DIFFUSION
ELEMENTARY PARTICLES
ELEMENTS
ENERGY-LEVEL TRANSITIONS
FERMIONS
FLUORIDES
FLUORINE COMPOUNDS
HALIDES
HALOGEN COMPOUNDS
KINETICS
LEPTONS
PARTIAL DIFFERENTIAL EQUATIONS
REACTION KINETICS
SEMIMETALS
SILICON COMPOUNDS
SURFACE FINISHING
360601* - Other Materials- Preparation & Manufacture