Reactive sputter etching of Si, SiO2, Cr, Al, and other materials with gas mixtures based on CF4 and Cl2

Abstract

All etching processes require control of the relative etch rates of the mask and substrate materials. This is especially important in vacuum-etching processes where directional etching is often obtained at the expense of increased mask etch rate. Etch rate control requires good control of the etching parameters, and here at a constant gas flow of 2×10−2 Pa m3/s we use target voltage (peak-to-peak) and partial pressure of reactant species as controlled variables rather than power density and total pressure, as is common practice. SiO2 and Si etch rates in CF4 with a Si target were found to vary as P1/2V2 where V is the peak-to-peak target voltage (varied between 0.6 kV and 2.9 kV) and P is the reactant gas pressure varied between 0.35 Pa (2.7 μm) and 14.3 Pa (110 μm). Cr and Al rates varied as V3, but were almost independent of pressure. These differences enable accurate selection of relative etch rates and appropriate masks. For instance, SiO2/Cr etch rate ratios as high as 50:1 may be achieved for 3.7 Pa of CF4 at 0.8 kV peak-to-peak target voltage. Chlorine mixed with CF4 enables the properties of the two gases to be combined. Thus, by adjusting only the gas mixture from pure CF4 to pure Cl2, the Si/SiO2 etch rate ratio can be continuously varied between 0.33 and 6, respectively, at 1.3 kV peak-to-peak target voltage, 3.7 Pa pressure, and with substrates on a Si target.