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Development of an implantable integrated MEMS pressure sensor using polyimide epoxy composite and Atomic Layer Deposition for encapsulation

: Gembaczka, Pierre

Biomedizinische Technik 59 (2014), Nr.s1, S.S1053-S1054
ISSN: 0013-5585
ISSN: 1862-278X
Deutsche Gesellschaft für Biomedizinische Technik (DGBMT Jahrestagung) <48, 2014, Hannover>
Zeitschriftenaufsatz, Konferenzbeitrag
Fraunhofer IMS ()
MEMS; pressure sensor; ALD passivation; polyimide-epoxy composite

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.
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.
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.
With an polyimide-epoxy composite as sealing and a final ALD deposition a new encapsulation techncology for implantable MEMS pressure sensors was developed.