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2014
Journal Article
Titel
Development of an implantable integrated MEMS pressure sensor using polyimide epoxy composite and Atomic Layer Deposition for encapsulation
Abstract
Introduction The futher miniaturization of MEMS sensors (micro-electro-mechanical system) opens up a wide range of medical implants applications. Typical medically approved encapsulation materials are titanium or ceramic, which have thick layer (~100µm) and hence are relatively large. For MEMS pressure sensors the membranes must be free to move and in contact with tissues or body fluids. A thin and hermetically sealing encapsulation adapting to the shape of the MEMS sensor is needed. Flexible encapsulation based on silicone is no hermetic sealing. Here, a novel encapsulation method is demonstrated for a capacitive MEMS pressure sensor system. A high temperature resistant polyimide-epoxy composite was used as die attach material and as sealing compound for bond wires and parts of the chip surface (figure 1). This was followed by an atomic-layer-deposition (ALD) of aluminium oxide (Al2O3) and tantalum pentoxide (Ta2O5) (figure 2) providing a hermetically sealed encapsulation. The polyimide-epoxy is compatible with the ALD deposition temperature of 275°C for several hours. Methods To locate even the smallest defects in the passivation a destructive amperometric measurement method in phosphate buffered saline (PBS) was chosen. To test the performance and repeatability, the pressure measurement system was operated and calibrated in a 0.9% saline solution (figure 3). Accelerated life testing at 60°C and 120°C in saline solution was also carried out. Results The maximal calibration error of the MEMS pressure sensor was below 2hPa in 0.9% saline solution. During continuous operation the sensor module survived an accelerated life testing at 60°C in saline solution for 1500 hours. During an offline accelerated life testing in saline solution at 120°C the sensor module survived for 116 hours. Conclusion With an polyimide-epoxy composite as sealing and a final ALD deposition a new encapsulation techncology for implantable MEMS pressure sensors was developed.