Dopant-dependent Ion Assisted Etching Kinetics in Highly Doped Polysilicon Reactive Ion Etching

The effects of dopant on ion-assisted etching of phosphorus-doped, boron-doped, and undoped polycrystalline silicon (polysilicon) are clarified in low-temperature magnetron reactive ion etching with Cl2/Ar gas at -30°C, where spontaneous etching by chlorine radicals is negligible. The etching product and the relative Cl2 concentration in the discharge are measured using quadrupole mass spectrometers with different configurations. The etching product is SiCl4, and does not exhibit dopant dependence. Parameters of dopant-dependent etching kinetics are derived using the linear dependence of the ion-induced chemical sputtering yield on surface site coverage, Langmuir's adsorption formula, and the equation of etchant continuity. The etch rates of all types of polysilicon at a constant chlorine concentration decrease with increasing etching area because chlorine adsorption is inhibited by the etching product returning to the etching surface. The etch rates with no such inhibition by the etching product returning to the etching surface increase and saturate with increasing Cl2/Ar ratio, and the saturated etch rates of P-doped and B-doped polysilicon are, respectively, 1.37 and 0.67-fold higher than that of undoped polysilicon. This implies that the enhancement and retardation of etch rates for n-type and p-type polysilicon are mainly due to increases and decreases in the chemical sputtering yield. On the other hand, the etch rate is restricted by the etchant flux, and this shows little dependence on the dopant type. This suggests the sticking coefficient of chlorine has little dependence on the type of dopant.