Biological and environmental sensors offer unique manufacturing challenges beyond those of other MEMS and sensor-based devices. Fundamentally, sensors incorporating merged technologies of this kind can still be considered as two distinct components – a mechanical device with an added bio component. The mechanical section may translate an electrical signal, which is due to the change in the material of the bio component being activated. The actuation of some devices, however, is unrelated to electrical response and due instead to a chemical change. The biggest emerging source of bio and environmental sensors involves specific biological materials that trigger the sensor due to a specific chemical exposure.
The biological aspect of these devices relates more specifically to, but not exclusively, medicine and the new wave of lab on a chip type devices. Looking first at the mechanical aspect of a medical device, we have found there to be two common starting points: microfluidic structures, which funnel and direct a biological sample through a fluidic channel and advanced MEMS designs. This allows interaction with a bespoke chemical compound and advanced MEMS devices coated with bioactive film. The channels used to transport the biological sample to the active sites can be pre-treated in some designs with a hydrophilic coating to allow the sample to flow through the device more efficiently. By making the surface reject the build-up of liquid, the material is pushed through to the important reactive sites.
More advanced examples of biosensor technology are beginning to utilise active, moving MEMS devices, such as cantilevers and bridges, which have been treated with bio-active coatings. The major difference is the actuation mechanism, where the specific molecule targeted binds to a cantilever. This binding causes a deflection in the device. Depending on whether the device induces a change in capacitance or is a piezoelectric material that creates a charge due to the deflection, it results in a signal that can be translated into a positive result.
Biosensor Technology – Impact on the manufacturing process
The MEMS device will first have to be released using standard manufacturing processes. However, for the binding of these molecules to influence the structure, the devices are typically in the very low microns or nanometres in terms of critical dimensions. Wet etching techniques have become a less viable manufacturing route due to the size and sensitive nature of the devices and the ease of which stiction can cause device failure. Vapor phase etch techniques enable a path to manufacture due to the controlled dry etch environment in which the devices can be released.
Adding the capability to deposit a wide range of these bio-active coatings to the MEMS device under vacuum, this leads to a device in which the mechanical MEMS could be released via vapor phase etching and activated with the bio-active coating without breaking vacuum. This results in increased manufacturing yield and reduced costs as the available systems can process 200 mm wafers for volume development.
The merging of vapor phase release processing with hydrophobic anti-stiction coatings is rapidly becoming a critical need in consumer electronics for microphones, motion sensing and pressure sensing devices but the future need for volume manufacturing of bio-active MEMS sensors appears to be following in these footsteps.
Vapor Phase Etching for MEMS manufacturing
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