A Diagnostic Approach to Plasma‐Etching Kinetics: Determination of Atom Concentrations

Abstract

The concentrations of titanium atoms and fluorine atoms in an SF6 plasma were measured as a function of pressure during plasma etching of a titanium sheet. Absorption of emission from a hollow cathode lamp at 399.86 nm was used to determine the titanium atom concentration. F-atom concentrations were determined by argon actinometry. A kinetic model was also developed. According to the model, the etching is due to fluorine atoms produced in the plasma, resulting in the formation of a TiF radical. Both physical and chemical processes were important in the etching, with the diffusion of gaseous constituents dominating at lower pressures and reactions in the gas phase dominating at higher pressures. The etching rate was found to be proportional to the fluorine atom concentration. The model, established by use of steady-state approximations, was fitted to the experimental data for titanium atom concentrations. From the model, the concentration of fluorine atoms was determined to be proportional to the square of the pressure, as determined experimentally. The titanium atom concentrations were related to the total pressure by the expression[Ti] = aP4/(1 + bP3) 2The rate constant for the reaction of SF~ with Ti or TiF was estimated to be 6.0 • 103 Pa ' s-' (2.6 • 10 -11 cm 3 s-l). Absorption measurements on atoms are simply made, and provide one approach to understanding the kinetics of plasma etching.

New Interpretation of the Mechanism for Etching Ti in a SF 6 Plasma

“…On the basis of the above arguments it is evident that the model proposed by Reeves et al, 3 with the generalizations suggested here, can provide a good representation of their data on the plasma etching of Ti with SFs, in particular describing well the pressure dependence of [F], [Ti], and the etch rate.…”

Inductively Coupled Plasma Etching of Bulk Titanium for MEMS Applications

“…[4][5][6][7][8] Reported gases suitable for titanium etching include: CCl 4 /O 2 with additions of fluorinecontaining gases; 4 CCl 4 /CCl 2 F 2 with admixtures of O 2 ; 5 Cl 2 /BCl 3 ; 6 Cl 2 /N 2 ; 7 CF 4 , CF 4 /O 2 , SiCl 4 , SiCl 4 /CF 4 , and CHF 3 ; 8 CF 4 /O 2 ; 9 and SF 6 . 10 Although titanium thin films are commonly used in microelectronics, micromechanical structures dry etched into titanium thin films have only recently been demonstrated. 11 However, because thin films limit aspect ratio and often contain residual stresses, bulk titanium may be a more suitable option for certain MEMS applications.…”