Isotropic dry etching of Si selectively to Si0.7Ge0.3 for CMOS sub-10 nm applications

The fabrication of Si0.7Ge0.3 sub-10 nm nanochannels in gate-all-around devices requires a highly selective Si isotropic etching process. The etching of Si selectively to Si0.7Ge0.3 with CF4/N-2/O-2 downstream plasma has been investigated using various morphological and surface characterization techniques. Conditions such as 400 W microwave power, 700 mTorr pressure, 25 degrees C chuck temperature, and 22% CF4:22% N-2:56% O-2 feed gas mixture were found to be optimum for selectivity and etch rates. X-ray photoelectron spectroscopy showed that, during the etching process, a highly reactive 8 nm thick SiOxFy layer is formed on Si. Meanwhile, a 2 nm thick passivation layer is observed on SiGe. The latter is a mixture of SiOxFy and GeOxFy species that protected the alloy from etching. The process selectivity was improved by investigating different wet and dry oxidant treatments prior to etching. The dry oxidant treatment gives the best results in terms of selectivity. These results obtained on blanket wafers have been validated on pattern wafers. Scanning electron microscopy demonstrated that SiGe nanowires were fully released with a high selectivity after dry oxidation followed by the etching process.