Negative Ion Silicon Oxidation in Oxygen Plasma with High Rate and Low Damage.

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For one application of negative ions in plasma, silicon oxidation at low temperature was studied by employing oxygen negative ion. In downstream region of microwave oxygen plasma, the silicon oxidation was examined under DC bias. The oxidation depth was much higher under the positive bias than the negative. It was found that the oxidation depth became the highest at a certain distance in downstream, and this was consistent with that at which the probe saturation current ratio showed a local minimum, suggesting a maximum negative ion density there. An in-depth analysis by XPS showed that the oxidation proceeded almost linearly with the substrate bias voltage, and this voltage dependence was also found on the Si substrate heated at 200℃. In order to find an effective condition for ion irradiations, the silicon oxidation was examined under various radio frequencies of the substrate bias. The oxidation depth showed a strong frequency dependence and had a maximum around the ion plasma frequency and thus, it was concluded that the oxidation was due to the negative ions. An X-ray photoelectron spectroscopy analysis revealed less suboxide in the negative-ion irradiation compared with the positive-ion oxidation.For one practical application of this technique, the silicon trench oxidation was also studied with the sample of 0.18 μm width and 0.35 μm depth, and the oxidation characteristics for each portion of trench were examined. The directionality, hence the ratio of oxidation speed in bottom to that on the side wall of the trench, could be improved more than three times by applying the radio frequency bias. With no bias, however, the exidation in bottom was found to be quite difficult and the speed was just less than 50% of the side wall. This concludes that the negative ion silicon oxidation is an innovative technique and its application to the shallow trench cell isolation is highly expected.