DOPANT-DEPENDENT ION-ASSISTED ETCHING KINETICS IN HIGHLY DOPED POLYSILICON REACTIVE ION ETCHING

The effects of dopant on ion-assisted etching of phosphorus-doped, bor on-doped, and undoped polycrystalline silicon (polysilicon) are clarif ied in low-temperature magnetron reactive ion etching with Cl-2/Ar gas at -30 degrees C, where spontaneous etching by chlorine radicals is n egligible. The etching product and the relative Cl-2 concentration in the discharge are measured using quadrupole mass spectrometers with di fferent configurations. The etching product is SiCl4, and does not exh ibit dopant dependence. Parameters of dopant-dependent etching kinetic s are derived using the linear dependence of the ion-induced chemical sputtering yield on surface site coverage, Langmuir's adsorption formu la, and the equation of etchant continuity. The etch rates of all type s of polysilicon at a constant chlorine concentration decrease with in creasing 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 sur face increase and saturate with increasing Cl-2/Ar ratio, and the satu rated etch rates of P-doped and B-doped polysilicon are: respectively, 1.37 and 0.67-fold higher than that of undoped polysilicon. This impl ies that the enhancement and retardation of etch rates for n-type and p-type polysilicon are mainly due to increases and decreases in the ch emical sputtering yield. On the other hand, the etch rate is restricte d by the etchant flux, and this shows little dependence on the dopant type. This suggests the sticking coefficient of chlorine has little de pendence on the type of dopant.