Floating low-temperature radio-frequency plasma oxidation of polycrystalline silicon-germanium

Low temperature oxide formation is an important process in the fabrication of thin-film transistors (TFT) used in active-matrix liquid crystal displays. However, low temperature oxide is prone to have defects at the SiO2/polycrystalline–SiGe interfaces. We have recently developed a novel rf (radio frequency) plasma oxidation method for polycrystalline SiGe (poly-SiGe) materials. The poly-SiGe wafers are oxidized in an oxygen rf plasma with the samples electrically floating. That is, the sample voltage is the same as the sheath potential of the floating wall and is always negative with respect to the bulk of the plasma since electrons have higher mobility than ions. The slightly negative potential on the wafers attracts low energy oxygen ions from the plasma and the resulting damage on the wafers is thus lower than that induced by the more commonly used and energetic electron cyclotron resonance (ECR) source. No deliberate heating is applied during oxidation since the samples are heated spontaneously by the plasma, but the temperature is measured to be below 100 °C throughout the entire process. The oxidation rate is comparable to that of ECR plasma oxidation. Depth profiles are acquired by Auger electron spectroscopy and the interfaces are examined by x-ray photoelectron spectroscopy. The n-channel metal oxide silicon device fabricated on the as-grown gate oxide shows good electrical characteristics. The process is thus compatible with inexpensive large-area, low-temperature fabrication of TFTs on glass substrates.