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A Study On Nonhermetic Micropackage For Implantable MEMS Systems
Keywords: micropackaging , implantable MEMS system, nonhermetic
With recent progresses in implantable MEMS systems for sensory prosthesis, medical monitoring and control, there is the pressing need of new micropackage techniques for these sensors, actuators and integrated electronic systems. The requirements of new micropackage are: light weight, small added volume, and short turn-around time and low cost. Non-hermetic micropackage technology for MEMS Systems with life time in months and years and meet all the requirements has being studied at Case Western Reserve University for several years. The theory, approaches and technology development as well as the experimental results will be presented with tables and characteristic curves and discussions. Through observations on polymeric packages of implant telemetry systems since 1969, Ko formulated the hypothesis that: the short life times observed are due to coating defects not the materials; Nonhermetic chronic package with longer life time can be developed. The requirements are:-clean system, near zero defect and cavity in coating, as well as coating materials retain high resistivity when saturated with vapor. Guided by the hypothesis, an approach for nonhermetic chronic micropackage technology was developed. Which include: i). Cleaning the substrate and components from ionic contaminants; ii). Develop near zero density of defect and cavity coating by using Thin film multiple layer repeated roller coating with pressure techniques; and iii). Use multiple layers of coating materials for mechanical support, vapor barrier, and biocompatibility. These approaches are based on reasoning that: 1). multiple layers of thin coating of a material will greatly reduce the through coating defects; 2). the repeated roller coating may reduce the cavity in the package; 3). Multiple materials can optimize the package performance. We used planar 1 or ½ mm spaced Interdigitated Electrodes (IDE) on printed circuit board as evaluation device, and 40oC and 85oC saline baths for life time evaluation; and defined package life time as the time in the saline bath when the IDE resistance decreased from original value of 10^13 to 108 ohms. (the equivalent sheet resistivity of the PCB drop to 1.37e10 Ohms-cm). Several batches of IDE were evaluated. One to five layers of thin (25-50um) PDMS coating was evaluated. The life time of 3 to 5 layers coated samples are 10 to 100 times better than the single layer coating with same total thickness. However the improvement diminishes beyond 5 layers. Repeated roller stroke in one coating layer also improved life time greatly. For actual packages, Hysol epoxy was used for mechanical support, Parylene-C for vapor barrier and PDMS for tissue biocompatibility. Multiple thin layers of each material were used for long life time micropackage. Several generations of micropackage technology of IDE and piezoresistive pressure sensors were developed. The life time improved from 59 days, 25 % yield, to 343 days, 100% yield in 40 centigrade bath, and 130 days, 100% yield in 85 centigrade bath. Micropackage with Iife time in years can be developed. The results, Problems and remanding challenges will be presented and discussed. The partial support by NIH R-21-EB014442 is gratefully acknowledged.
Case Western Reserve University
Cleveland, Ohio

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