In Vivo Biostability of CVD Silicon Oxide and Silicon Nitride Films
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J14.3.1
In Vivo Biostability of CVD Silicon Oxide and Silicon Nitride Films John M. Maloney, Sara A. Lipka, and Samuel P. Baldwin MicroCHIPS, Inc. 6B Preston Court Bedford, MA 01730 ABSTRACT Low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) silicon oxide and silicon nitride films were implanted subcutaneously in a rat model to study in vivo behavior of the films. Silicon chips coated with the films of interest were implanted for up to one year, and film thickness was evaluated by spectrophotometry and sectioning. Dissolution rates were estimated to be 0.33 nm/day for LPCVD silicon nitride, 2.0 nm/day for PECVD silicon nitride, and 3.5 nm/day for PECVD silicon oxide. A similar PECVD silicon oxide dissolution rate was observed on a silicon oxide / silicon nitride / silicon oxide stack that was sectioned by focused ion beam etching. These results provide a biostability reference for designing implantable microfabricated devices that feature exposed ceramic films. INTRODUCTION Microfabricated devices have been used in a variety of biomedical implant applications, including neural stimulation, sensing, and drug delivery. These devices often rely upon exposed conductive and dielectric films for operation. For example, microchips have incorporated LPCVD or PECVD ceramic films, most commonly silicon oxide and silicon nitride, to selectively passivate electrodes against the in vitro and in vivo environments [1-3]. These films have also been used as insulating layers between metal features and a silicon substrate. In some applications, silicon oxide and silicon nitride films have been combined to act as a barrier to ion transport [4] or to balance opposing residual stresses [5]. Silicon oxide and silicon nitride are known to dissolve in aqueous media [6-8]. At the hydrated surface of the film, silicon oxide dissolves to form aqueous Si(OH)4. Si-O bonds replace Si-N bonds in silicon nitride films, releasing NH3 into the surrounding medium. There have been previous reports of the dissolution rate of CVD dielectric films in saline with a focus on ranking material candidates in vitro [9]. An additional complication for implanted devices is exposure to a complex biochemical environment composed of blood or interstitial fluid along with a transient chemical profile due to wound healing [10]. The biological response to any implanted medical device is a function of the physical characteristics of the device, the properties of the fabrication materials, the location of the implant, and the skill of the surgeon. The normal tissue response to an injury follows a wellestablished series of cellular and biochemical pathways, beginning with infiltration of polymorphonuclear leukocytes into the site, followed by the appearance of macrophages and fibroblasts. In the subcutaneous space, a fibrous capsule consisting primarily of collagen eventually forms around the device. The corrosive in vivo environment promotes dissolution of ceramic films, leaving electrodes unprotected from degradation m
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