Surface analysis of polysilicon gate etching by pulsed-microwave plasma

The mechanism of highly selective etching by a pulsed-microwave electron-cyclotron-resonance plasma was investigated by analyzing surface-reaction layers formed on nonpatterned poly-Si and SiO2 samples and gate-patterned samples with a gate width of 32 nm. The samples were etched by using an HBr/O2/Ar/CH4 gas chemistry and varying the duty cycle of the pulsed microwave. The reaction layers, which were revealed as a hydrocarbon layer on a SiBrxOy layer, were analyzed by X-ray photoelectron spectroscopy. The upper layer was a hydrocarbon layer, which protected SiO2 from ion bombardment and also prevented Br flux from being supplied to the SiO2. The lower layer was a SiBrxOy layer, which suppressed the etching of the underlying Si substrate. The formation of the hydrocarbon layer was controlled by the duty cycle of the microwave plasma. Etch stop, which occurred at a low peak-to-peak voltage (wafer bias) of the continuous microwave plasma, was prevented by controlling the thickness of the hydrocarbon layer in the pulsed-microwave plasma. Gate-oxide punch-through, which occurred at a high peak-to-peak voltage of wafer bias in the case of the continuous microwave plasma, was also prevented in the case of the pulsed microwave plasma by forming reaction layers with high C/Br ratio.