An array-type atmospheric-pressure radio-frequency (RF) plasma generator is proposed for high-precision and high-throughput numerically controlled (NC) processes. We propose the use of a metal-oxide-semiconductor field-effect transistor (MOSFET) circuit for direct RF switching to achieve plasma on–off control. We confirmed that this type of circuit works correctly using a MOSFET with a small parasitic capacitance between its source and gate. We examined the design method for the distance between adjacent electrodes, which corresponds to the parasitic capacitance between adjacent electrodes and is very important in the individual on–off control of each electrode. We developed a prototype array-type plasma generator apparatus with 19 electrodes and the same number of MOSFET circuits; we then confirmed that each electrode could control its plasma on–off state individually. We also demonstrated that the thickness uniformity of the surface Si layer of a silicon-on-insulator wafer could be processed to less than 1 nm peak to valley by the NC sacrificial oxidation method using the apparatus.
Modern surface processing of semiconductors or oxide materials requires highly precise temporal control of each processing step. In addition, large wafers must be processed quickly for high throughput. We have developed a numerically controlled sacrificial oxidation method with atmospheric-pressure plasma using electrode arrays. In this method, we oxidized the surface of a wafer with atmospheric-pressure plasma applied precisely by an electrode array, and then dipped the wafer in HF solution to remove the surface oxide layer. The plasma process time can be controlled independently at each electrode area. The oxidation rate and surface profile of the treated wafer are crucial for precision processing. We investigated the oxidation rate of atmospheric-pressure plasma oxidation by spectroscopic ellipsometry and examined the surface morphologies of untreated and treated wafers by atomic force microscopy. The surface profile smoothness correlated with the plasma oxidation time and electrode voltage during oxidation. The surface roughness tended to increase when the sample was oxidized for longer times with higher electrode voltage. This correlation between surface roughness and oxidation time resembled the results of Si/SiO2 interfacial roughness in the case of thermal oxidation. In the plasma sacrificial oxidation process, the increase of surface roughness at the Si/SiO2 interface by plasma oxidation must be considered.
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