Self-limiting etch depths using simultaneous sputter etching/deposition technique

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Abstract
During sputter etching in argon, time is most commonly used as the parameter determining the total etch depth. If the argon is replaced by CH4 during substrate etching the effect will be that etching stops, CH4 decomposes in the plasma, and a carbon film starts to deposit on the substrate. This effect will also be obtained if a mixture of Ar/CH4 is used during processing. Depending on the concentration of CH4 in the gas mixture it will take different times before a monolayer of carbon is formed at the substrate surface. During this time interval the substrate material is exposed to energetic ion sputter etching. The total amount of original substrate material etched away before the substrate is completely covered by carbon is dependent of Ar/CH4 ratio and on substrate material. The carbon layer can easily be removed by an O2 plasma when the process is completed. A process has thus been obtained that can create a self-limited etch depth (SLED) into a surface material independent of processing time as long as the time exceeds a minimum value. This SLED effect is presented for different substrates like Pt, Al, Au, Zr, Si, and SiO2. The etch depth can be varied by changing the CH4 concentration and the bias voltage. We also show how the technique can be used to obtain a high selectivity etch between Au and Si and between Au and SiO2.

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