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Non-hermetic Micropackage for Chronic Implantable MEMS Systems.
Keywords: Implantable MEMS systems, Nonhermetic Micro-Package , Chronic Micro-package
1. Introduction As MEMS technology continues to be developed for medical implant applications, new packaging technologies that match the small size, weight and flexibility of the MEMS devices themselves are required. The package must be biocompatible and have a small form factor while providing mechanical and electrical protection to the implanted device. In this paper, we introduce a non-hermetic micropackaging technology developed at CWRU for chronic implantable electronic systems, based on Ko’s multilayer multi-material coating theory developed from previous research work [1-5]. The micropackaging technology aimed to have ultra-low volume (less than 200um thick), pressure sensor compatible and more than 2 year’s lifetime. Customized IDE (interdigitated figure) resistance sensor and ASIC pressure telemetry device were designed for evaluation. Body temperature 40oC and accelerated 85oC saline baths were used for lifetime test, the life time projection rate of 10 was selected to convert results from 85oC to 40oC tests. Implantable animal experiments are being conducted for biocompatibility study. 2. Method of Micropackage technology The proposed multi material micropackaging process includes cleaning and multi-materials thin film layers coating process. The cleaning process is used to remove all contaminations. And the multi-layers Multi-materials coating process consists of optimized parylene-C chemical vapor deposition (CVD), as well as Hysol epoxy and PDMS roller coatings. Leakage current test down to 10-15 Amp were measured to screen off early failure samples with Keithly 4200 SCS semiconductor characterization system. SEM, AFM and high resolution microscope were used to fetch the morphology of the thin packaging layers for failure mode analysis. Bonding strength, electric noise was also measured during life time test to characterize the package quality. 3. Experimental Results a). PDMS and parylene single material multi-layer coating 24 IDE resistive sensors were packaged by PDMS multi-layer roller-casting. Accelerated life test in 85oC saline solution showed projected 3.6 years lifetime in the body and 100% yield rate. 20 IDE resistive sensors were coated with optimized parylene CVD with 100% yield and 2.2 years’ projected lifetime in body. The test is still continuing on. b). Multiple materials multi-layer coating results Two pressure telemetry device with ASIC were fabricated and packaged with parylene-C, epoxy and PDMS and evaluated in 85oC bath. Changes of pressure sensor’s sensitivity and nonlinearity were within 5% and 0.3% respectively in 71 days. The devices are still under test. The projected lifetime up to now is 2 years with 100% yield. 10 more similar devices were packaged and implanted in animal for biocompatibility study. These results verified the feasibility of our approach for chronic implant applications. 4. Discussion Parylene-C CVD processes were studied to develop a pinhole free conformal coating process. Electric noise was analyzed during the lifetime test. The noise showed 3 distinguished phases which may be used to identify the remaining lifetime of package. Packaging process and its effect on pressure sensor performance was studied. Varies physical sensors package will be developed with implant system. Further development will be carried out aimed for 2-10 years’ life time micropackage.
Wen H. Ko, Professor Emeritus
Case Western Reserve Univerasity
Cleveland , Ohio

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