Direct pattern etching for micromachining applications without the use of a resist mask

The method of direct pattern etching without the use of a conventional resist mask has been developed. This method takes advantage of the field-shielding effect of a Faraday cage, inside which the substrate is located. A stainless-steel stencil mask, constituting the upper plane of the cage fixed on a cathode in a plasma etcher, was used as a pattern mask in reactive ion etching. A CF4 plasma at 5 mTorr was used to etch the initially bare substrate of a Si wafer covered with a 1-μm-thick blanket SiO2 film. The mask patterns with the minimal dimension of 40 μm were accurately transferred to the substrate with the etch profiles vertical to the substrate surface. When the gap distance between the stencil mask and the substrate surface was as small as 0.5 mm, the ratio of the etch rate below an opening to that below a blocking portion of the mask was over 5600. On the other hand, the etch rate ratio fell virtually to unity when the substrate was apart from the mask by 10.5 mm. The simulation study of ion trajectories showed that the ion beams were increasingly diverged as ions traveled away from the mask inside the cage. That is, the beams were well separated from each other in close proximity to the stencil mask, resulting in fine pattern etching. At positions sufficiently far from the mask, however, the deviated ion beams extensively overlapped with each other to yield the uniform ion flux.