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

作为负离子在等离子体中的应用之一，利用氧负离子研究了硅的低温氧化。在微波氧等离子体下游区，研究了直流偏压下硅的氧化。正偏压下的氧化深度远大于负偏压下的氧化深度。结果表明，氧化深度在下游一定距离处最大，这与探针饱和电流比出现局部极小值的位置相一致，表明负离子浓度在该处最大。XPS分析表明，氧化过程与衬底偏压几乎呈线性关系，在200℃加热的Si衬底上也发现了这种电压依赖性。为了找到一个有效的离子辐照条件，在不同的射频衬底偏压下的硅氧化进行了研究。氧化深度表现出很强的频率依赖性，并在离子等离子体频率附近具有最大值，因此，得出的结论是，氧化是由于负离子。X射线光电子能谱分析表明，负离子辐照产生的亚氧化物比正离子氧化少;作为该技术的一个实际应用，对宽0.18 μm、深0.35 μm的硅沟槽氧化进行了研究，考察了沟槽各部分的氧化特性。通过施加射频偏压，可以将方向性提高三倍以上，从而使沟槽底部的氧化速度与侧壁上的氧化速度之比提高。但在无偏压条件下，底部氧化很困难，其速度仅为侧壁的50%以下。因此，负离子硅氧化技术是一种创新技术，在浅槽单元隔离中的应用前景广阔。

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.