Simulation of silicon KOH and dry etching methods for micromould fabrication

The purpose of this project is to investigate the suitability of wet etching methods such as KOH etching for silicon micromoulds fabrication. Those moulds are intended for preceramic polymer (PCP) casting and specific requirements regarding the profile of the etched structures should be achieved. A process allowing to create deep structures with a high aspect ratio and smooth sidewalls would be the most appropriate. Moreover, obtaining slanted sidewalls could be useful as it would allow the resulting ceramic part to unmould itself during the shrinkage induced by pyrolysis. This is why KOH etching has to be considered, as it is an anisotropic process based on the etching along specific crystalline planes. Indeed, the etching rate along the planes of silicon is the slowest one, allowing therefore to etch structures with atomically smooth tilted walls. A mask with many different openings was then designed and fabricated in order to define the limits of KOH etching in peculiar situations. The consequences of using detailed structures containing reflex angles or the effects of tilting them on the mask with respect to the flat of the wafer will be then determined. The etching simulation was then realised thanks to the etching simulation software IntelliEtch, provided by IntelliSense. This tool allows to simulate both KOH and dry etching methods before visualising the results in 3D. It was therefore possible to firstly show that KOH is indeed a simple method to obtain smooth slanted and vertical walls by playing with the alignment of the mask openings. However, underetching of the mask can also easily occur, leading therefore to the loss of details contained in the original shapes design. Solutions can be found to overcome those issues by modifying the masks or by playing with other wafer orientations, but the problem of etching detailed structures nevertheless remains. An idea is therefore to use deep reactive ion etching methods such as Bosch process, as its advantages are mainly the ability to obtain high aspect ratio structures without underetching and therefore to etch detailed mask openings. Many unwanted effects can however occur with Bosch process, but by comparing both KOH etching and DRIE, it can be noticed that the drawbacks linked to both methods can compensate each other. It is therefore interesting to study the extent to which a process combining both techniques could be appropriate for micromoulds fabrication. Through a literature study, it will finally be shown that it is possible to obtain high aspect ratio structures containing many details with straight smooth sidewalls and which could be easily unmoulded. Different ways to combine KOH and dry etching methods that allow reaching all the requirements for an optimal micromould fabrication will also be presented.