Platinum etching characteristics in aqua regia have been studied. It was found that prior exposure to an oxygen plasma inhibits the dissolution of platinum in aqua regia. Oxygen, far more abundant in the exposed platinum than in the unexposed platinum, plays a key role in forming an inhibition layer, such as PtO2, which prevents chlorine ion attack. This inhibition layer appears to retard platinum etching effectively in chlorine-based etch solutions. The layer has been observed to form at a fast rate, and it is insensitive to the oxygen partial pressure in the plasma chamber. The insoluble characteristics of both the inhibited platinum and the platinum silicide in aqua regia make it feasible to form an unframed contact interconnection for applications of very large scale integration.
The degree to which deposited metals cover steps over topography is important to the yield and reliability of devices in very large scale integrations. In evaporated and sputtered films the most difficult steps to cover are those with straight walls. This is especially true with aluminum and its alloys. Via step coverage has been studied as a function of bias sputtering parameters. It was found that 1.3 μm diam, straight-walled vias 1 μm deep, are planarized (100% step coverage) under the same conditions that 60% coverage is achieved in 2.8 μm vias. Unbiased deposition results in only 20% coverage. The effects of target power, bias voltage, and anode voltage on bias current and step coverage were studied. The low power delivered to the substrate under conditions for planarization is not consistent with a mechanism involving backsputtering of aluminum; no significant change in the measured deposition rate was found when bias was introduced. Cross-sectional microscopy suggests a mechanism involving forward scattering of surface atoms at the via edge under bombardment by bias ions.
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