MEMS device fabrication requires a broad range of reactions and chemistries to create the structures and functions of these complicated electro-mechanical systems. MEMS etching processes in particular, merit attention for material compatibilities because, of course, the function of etching is to remove material. You need to make sure that you remove what you want to remove without removing anything else.
Oxide and Nitride Selectivity for MEMS Etching Processes
Silicon dioxide and silicon nitride layers are very common in MEMS devices, and the removal of sacrificial oxide layers by etching is a cornerstone of MEMS processing. So, it is critical that anoxide etching process does not detrimentally affect any silicon nitride layers that are present for passivation or other purposes. This desired material selectivity can be enhanced with the use of a catalyst. Oxide etches faster in the presence of a specific catalyst, but silicon nitride does not. This approach etches oxide faster with minimal impact on any nitride layers that are exposed to MEMS etching processes.
Even when only oxide is present, it is possible to create a selective etching process by taking advantage of the difference between oxide that is thermally grown and oxide that is deposited with plasma-enhanced chemical vapor deposition (PECVD). Specifically, thermal oxide has a higher density than PECVD oxide, so it is harder to etch. This allows for differential etching using thermal oxide as an etch stop layer when deposited oxide is the sacrificial layer in a MEMS structure. With this control over the PECVD material set and the processes to etch them selectively, it is possible to create MEMS structures with just oxide layers. This can simplify some of the processing, while avoiding the challenges of having a greater mix of materials.
The Whole Gamut
The vapour HF etching system produced by memsstar is highly compatible with a wide range of materials, with the list expanding as we continue to work with our customers and support ongoing research in the field.
Table 1 shows a summary of the most common materials known to work as either a sacrificial layer or a functional layer, along with the compatibility of memsstar’s tools.
In the oxide section, only doped oxides such as phospho-silicate glass (PSG) and borophosphosilicate glass (BPSG) are incompatible with memsstar’s HF etching systems. For structural layers, all of the identified materials are compatible, although PECVD silicon nitride needs extra attention to ensure compatibility. (Contact memsstar for expert advice.) For other functional layers, only tantalum and photoresist layers are incompatible, and titanium nitride may or may not be compatible, depending on the TiN deposition process.
Table 1. Compatibility of HF vapour-phase MEMS etching processes using a variety of materials. (*TiN deposition methods differ, depending on the system used, as described by Kumar et al. .)
By understanding the fundamentals of these material issues involved in MEMS processing, it is possible to develop a more efficient route to setting up processes for a range of different devices. We describe the range of commonly utilised materials and their compatibility with the HF process, which can inform the material choices of future designs and can reduce development times. Often there is flexibility in choosing materials for interconnect and other features, so understanding how they interact with HF MEMS etching processes can help you optimise these material choices.
Bottom Line: Material compatibility issues are critical for fabricating MEMS devices, especially with such a wide variety of materials and processes involved. Be sure to check out our new white paper with a deeper dive on these topics and other factors in MEMS fabrication.
 ‘Failure Mechanisms of TiN Thin Film Diffusion Barriers’, Kumar, N. Purrezaei, K., Lee, B., Douglas, E.C., Thin Solid Films, vol 164, 1988, pp. 417-428.