Material loss of silicon nitride thin films in a simulated ocular environment
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TECHNICAL PAPER
Material loss of silicon nitride thin films in a simulated ocular environment Christoph Schade1 • Alex Phan1 • Kevin Joslin1 • Frank E. Talke1 Received: 24 October 2020 / Accepted: 1 November 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The dissolution behavior of silicon nitride in a simulated eye environment is studied as a function of temperature and ion concentration. Thin films of silicon nitride were manufactured using plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD), respectively. The thickness of the films was measured as a function of time under various conditions. The experimental results showed that the films dissolve in saline solution over time. For the case of a thin membrane of 500 lm diameter and 200 nm thickness, typical dissolution rates are 1 nm/day for PECVD and 0.3 nm/day for LPCVD films. Dissolution of silicon nitride was found to be reduced using sputtered titanium oxide as protective overcoat.
1 Introduction Many implantable MEMS devices utilize silicon nitride as functional material or barrier coatings because of the outstanding mechanical strength and chemical properties of this material (Kaloyeros et al. 2017). Examples include blood pressure sensors, drug delivery devices, or neurophotonic probes (Elman 2009; Song 2020; Sacher 2019). Furthermore, silicon nitride has been used in a number of micro electromechanical (MEMS) devices such as tactile sensors in robotics (Abels et al. 2017), gas flow sensors (Palmer et al. 2012) or high power RF switches (Rahmann 2013). (Lee et al. 2017) used a silicon nitride membrane in an intraocular pressure sensor based on the principle of frequency shift, and (Phan et al. 2018) have used silicon nitride in an implantable intraocular pressure sensing device based on interferometry, where the sensor uses the deflection of a thin deformable silicon nitride membrane for pressure measurement of the intraocular pressure in the eye. The measurement of the intraocular pressure is important for glaucoma, an eye disease leading to irreversible blindness (Quigley and Broman 2006). Glaucoma is characterized by an increase in the eye pressure which causes retinal cell damage that destroys the optic nerve. & Frank E. Talke [email protected] 1
Implantable pressure sensors are seen as a possible way to continuously measure the eye pressure and provide physicians with a powerful tool to prescribe the best treatment for the patient (Weinreb et al. 2014). In Fig. 1a a pressure sensor as published in (Phan et al. 2018) is shown after implantation in the eye of a New Zealand White rabbit. A schematic of the sensor is depicted in Fig. 1b, revealing that the sensor is a small cavity enclosed by a silicon nitride membrane at the bottom of the cavity. Depending on the pressure in the eye, the membrane will deflect, changing the distance between the bottom of the membrane and the boron silicate glass base layer of the sensor. If a light be
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