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Study of SiO2 Modifications Induced by Oxygen Plasmas and Their Effect on Wet Processes

In typical CMOS process flows, critical oxides are often exposed to damaging treatments such as ion implantation and oxygen plasma ashing, commonly used during resist removal or pre-etch treatments. These processes can induce major modifications in SiO2 bulk and surface structure. As a consequence wet etch rate of the material is impacted and wet cleaning steps have to be tuned. In this work we have studied the effect of oxygen plasmas on an ion implanted oxide layers using surface charge, HF etch rate profiling [1] and thermo-luminescence techniques. A thermally grown silicon dioxide, about 250A thick, was boron implanted with energies from 40 to 300KeV using a dose in the 10 at/cm2 range. After ion implantation, wafers were treated with a resist removal sequence (dry and wet steps) to reproduce a common CMOS process flow. Some samples were then processed in oxygen plasma in RF, microwave and TCP (Transformer Coupled Plasma) reactors. Etch rate of a thermal oxide in hydrofluoric acid is proportional to the damaging level in the layer: the higher is the number of oxygen vacancies, the faster is the oxide etching. The depth of the damage from RF and TCP plasmas was found to be proportional to the RF power because of the higher energy of the oxygen plasma ions induced. In case of 400W RF power, plasma damage affects 30÷40A oxide from the top surface and the etch rate is about 4.5 times respect to reference oxide. Using a power of 200W and 300W or using the TCP process, surface damage is limited to the topmost 20÷30A with a relative etch rate of 3÷3.5. We also observed a damage recovery effect of RF and TCP plasmas in the bulk of the oxide, up to 200A depth. When 100W RF plasma power is used we observed by surface charge measurement a large spread in charging potential, because of the poor plasma uniformity at such low power [Figure 1]: surface charging varies from 2 to 8V. As a consequence of the poor plasma uniformity even the etch rate was found to be strongly non-uniform across the wafer: in low charging areas (e.g. point 8 in Figure 2) the ion implantation damage is almost completely recovered while in other areas (e.g. point 11 in Figure 2) the damage is increased. Thermo-luminescence measurements show the presence of different kind of point defects whose quantity and nature is modulated by the plasma. In case of the sample treated with 100W RF plasma, we obtained different thermo-luminescence glow curves in high charging areas and low charging areas [Figure 3] after 30KeV X-ray irradiation of the samples. TCP oxygen plasma was also confirmed by this technique to have the lowest amount of defects among the measured samples. Figure 1: Charging map of 250A thermal oxide treated with 100W RF plasma