On the constant composition and thickness of the chlorinated silicon surface layer subjected to increasing etching product concentrations during chlorine plasma etching

The influence of etching products on the surface layer formed during chlorine (Cl2) plasma etching of unmasked crystalline p-type Si(100) was investigated using vacuum sample-transfer and angle-resolved x-ray photoelectron spectroscopy (XPS). Varying the Cl2 flow rate from 10.0 to 0.4 sccm at a constant pressure of 4 mTorr controlled the etching product concentration. Gas-phase Cl, Cl2, and SiCly (y=0–3) were monitored (∼1 cm above the wafer) by optical emission spectroscopy. For a positive ion density of 7×1010 cm−3 and an average ion energy of ∼140 eV, the Si etching rate decreased linearly with Cl2 flow from 2850 Å/min at 10.0 sccm to 1920 Å/min at 0.4 sccm. From these rates, mass balance, and the Si area, the ratio of product-to-etchant (SiCly-to-Cl) flux to the wafer varied from 0.078 to 11 at 10.0 and 0.4 sccm, respectively. After etching, Cl was present in the Si(100) surface layer as SiClx (x=1–3) at XPS Si (2p3/2) binding energies of 99.9, 101.0, and 102.0 eV, respectively, relative to Si at 99.1 eV. The amounts of the three silicon chlorides and the total Cl (derived from its 2p peak) were nearly independent of the product-to-etchant flux ratio. Depth profiles were obtained from an inversion of the observed take-off angle dependences of the XPS signals. For the Cl2 flow rates investigated, the chlorinated surface layer was ∼16 Å thick, with Cl falling off in a graded fashion. The Cl areal density, integrated throughout the layer, was similar for all experimental conditions and averaged 2.63±0.15×1015 Cl/cm2. The stoichiometry of the chlorosilyl layers was also independent of Cl2 flow rate and averaged [SiCl]:[SiCl2]:[SiCl3]=[1.0]:[0.45±0.09]:[0.33±0.02]. Reaction pathways are presented to interpert both the constancy of the chlorinated surface layer and the decrease in etching rate as the Cl2 flow rate was decreased from 10.0 to 0.4 sccm.