A Technique for Detecting the Presence of Water Under micro-Devices working in a Liquid Environment
Journal Title: Biomedical Journal of Scientific & Technical Research (BJSTR) - Year 2018, Vol 6, Issue 1
Abstract
This study addressesissues specifically related to the underwater dynamic actuation of Micro Electro Mechanical Systems (MEMS), such as micro-cantilevers or comb-drive microstructures. As a practical application, cantilevers made using MUMPs (Multi-User MEMS Processes) foundry are considered. Since the gap between the cantilevers and the underlying surface is very narrow (about 2 μm), water does not always flow easily underneath the cantilevers, especially when the surface is hydrophobic with respect to the liquid environment. As long as the MEMS surface is hydrophobic, the lower the gap, the higher the pressure required to flow into micro-channels or gaps .As a consequence, this issue significantly modifies cantilevers’ performances. To gain an insight of the phenomenon, a new measurement technique was developed, referred to as MFTEC (MEMS Freezing by Thermo Electric Cooling), which can verify the presence of water under cantilevers or other micro-devices. There is a large and emerging demand for the use of Micro Electro Mechanical Systems (MEMS) working in aqueous environments, especially for biosensors and biotechnology applications. Applications span from general purpose devices, like Atomic Force Microscopy (AFM) probes used in liquid environment [1] to highly specialized devices exploiting horizontal cantilevers or vertical micropillars [2-3] used for the detection of the biomolecule of interest in the analyte solution [4]. In all these cases, the devices are obtained from silicon or silicon derivatives (e.g. silicon dioxide or silicon nitride) using standard or custom-designed MEMS processes. One possibility for the fabrication of cantilevers is the multi-user MEMS processes (MUMPs), relatively inexpensive and therefore suitable for low-cost prototyping. MUMPs provide three poly-silicon layers (0.5, 2 and 1.5 μm thick respectively) and a metal layer, typically used to provide electrodes or as a surface for a subsequent functionalization. The minimum feature spacing using MUMPs technology is around 2 μm see Figure 1. MUMPs provide two structural poly-silicon layers for moving structures, respectively called: POLY1 and POLY2.If POLY1 is used, alone or in combination with POLY2, a nominal gap of 2 m can be achieved between these structures and the base plane, using a PSG (Phospho Silicate Glass) sacrificial layer. If only POLY2 is used, the designer has the choice of a 2m or 3.5 m gap, since one or two PSG layers can be added during manufacturing. Since the gap is, in any case, very narrow, the behaviour and the role of the liquid environment around the MEMS are not easily predictable. One of the relevant issues is whether the gap between the moving structure and the base is completely filled with liquid once the MEMS are submersed in the liquid environment. Obtaining the answer is unfortunately not always simple, since this phenomenon is affected by many parameters, such as liquid surface tension, MEMS shape, temperature, presence of micro-bubbles in the liquid, and the liquid filling procedure. To the best of our knowledge, in actual fact, a specific instrument for measuring the presence of water under MEMS structures with sufficient accuracy does not exist at present.
Authors and Affiliations
D Borin, P Gallina, O Sbaizero
Keywords
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- EP ID EP592296
- DOI 10.26717/BJSTR.2018.06.001304
- Views 213
- Downloads 0
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