Fabricating MEMS devices presents unique challenges because of the three-dimensional (3D) and mechanical nature of the products. Specific mechanical structures are needed to perform the MEMS functions that involve vibration, rotation, or other types of motion (Figure 1). To accomplish this, the MEMS process flow calls for literally adding a dimension to the process flow of conventional semiconductor device fabrication, where the typical goal is to create planar electronic devices.
Figure 1. MEMS devices create unique process challenges – literally adding a dimension to conventional planar device processing.
Vapour-phase etching is often the process of choice because it is more highly controlled than wet etching. This advantage relies on deep knowledge of MEMS process variables, which is where memsstar can help. Let’s take a look.
Water vs. Alcohol Catalyst
Etching SiO2 with HF is common in MEMS device creation. The etching reaction requires the presence of a catalyst, which can be either water or alcohol-based. When working with either water or alcohol (ethanol being the most commonly used process catalyst for alcohol-based systems), memsstar researchers found that etching conditions achieving a stable etching rate utilising water as the catalyst were less aggressive than those where an alcohol catalyst was used. Alcohol naturally tries to absorb the water generated as part of the reaction, thus requiring more aggressive etch conditions to give low level etch performance. Using water as the process catalyst, however, is recommended for higher etch rates and greater process performance.
For the etch reaction mechanism, one of the reaction by-products, H2O in a gas phase, can also be utilised within the process chamber to further feed the process and maximise etching. The best way to utilise this is to increase the residence time of the H2O vapour generated by the reaction. Pressure control is the primary method of doing so. Since the etching initiation of certain types of oxide films occurs at differing levels of process pressure, this is a primary technique for controlling the etching process.
Process pressures also come into play for selectivity, since processing at much higher pressures will result in a significant loss in selectivity between oxide and nitride films. This loss is attributed to the increase in residence time of the reaction gasses and also their resultant by-products, such as the SiFx molecule. The increased time within the chamber of these by-products will cause a greater etching rate of the nitride compared to oxide. By keeping the pressure low, the etching rate is more controlled and the residence time of reactive species is kept low. This reduces the impact on nitride films while oxide films are etching.
When increasing the HF flow, there is relatively less H2O available in relation to HF, so the dissociation of HF is reduced. This results in counter-intuitive behaviour: Adding more of the etching species, HF, will actually slow down the etch process. Many users are accustomed to etch processes where adding more of the etch species accelerates the process, but the HF process works in the opposite way.
Increasing the flow of H2O follows a more typical behaviour. Adding more process catalyst dissociates the HF faster, making it possible to initiate and sustain the etch more readily. If too high a flow is provided, the etching rate will eventually be dictated by the H2O generated by the process itself. As such, the amount of H2O generated from the etch is also significant. This results in feedback that can influence the etch initiation and its ongoing etch performance. In memsstar oxide etching systems, the pressure control of the HF flow and the H2O flows work together. By considering them as an interlinked series of factors, a user can easily manipulate the conditions to find a suitable stable process for their specific device with the benefit of continuing repeatability in this process.
Bottom Line: Catalyst selection, process pressure, and gas flow are some of the process variables that memsstar can help you understand to get the most out of your MEMS vHF processes. Be sure to check out our new white paper with a deeper dive on these topics and other factors in the MEMS process equation and stay tuned for Part 2 and 3 of this blog topic.