Assessing failure in epitaxially encapsulated micro-scale sensors using micro and nano x-ray computed tomography
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Research Letter
Assessing failure in epitaxially encapsulated micro-scale sensors using micro and nano x-ray computed tomography Lizmarie Comenencia Ortiz, David B. Heinz, Ian B. Flader, Anne L. Alter, Dongsuk D. Shin, Yunhan Chen, and Thomas W. Kenny, Department of Mechanical Engineering, Stanford University, 440 Escondido Mall, Stanford, CA, 94305, USA Address all correspondence to Lizmarie Comenencia Ortiz at [email protected] (Received 31 January 2018; accepted 19 March 2018)
Abstract Millions of micro electro mechanical system sensors are fabricated each year using an ultra-clean process that allows for a vacuum-encapsulated cavity. These devices have a multi-layer structure that contains hidden layers with highly doped silicon, which makes common imaging techniques ineffective. Thus, examining device features post-fabrication, and testing, is a significant challenge. Here, we use a combination of micro- and nano-scale x-ray computed tomography to study device features and assess failure mechanisms in such devices without destroying the ultra-clean cavity. This provides a unique opportunity to examine surfaces and trace failure mechanisms to specific steps in the fabrication process.
Introduction Micro electro mechanical systems (MEMS) are micro-scale structures with mechanical and electrical components designed for applications such as high-performance inertial sensors, timing references, and others. Typical MEMS fabrication is based on materials and processes from integrated circuits (ICs) (clusters of electronic circuits fabricated on a single silicon chip). However, MEMS have different features due to their need for deep trenches, high aspect ratios, and release processes required for their dynamic components.[1] MEMS have revolutionized many industries, including the sensors industry, due to their small size, low cost, and low-power consumption. However, it is difficult to determine their sources of failure. To enable continued development of high-performance MEMS sensors for a wide variety of applications, it is critical to better understand the failure mechanisms that affect these devices.[1,2] Despite the importance of ongoing research in this area, a gap remains in studying the surface features of highly doped encapsulated MEMS sensors after fabrication and testing, without disrupting the encapsulation. Traditional means of analysis such as scanning and transmission electron microscopy are unavailable due to the opaque, hermetic encapsulation. Infrared (IR) microscopy is available but limited in resolution. X-ray microscopy is an attractive technology for through-encapsulation imaging, due to its access through multiple layers. Previous x-ray computed tomography (XCT) studies of MEMS devices have been conducted but have been largely limited to studying features above the millimeter scale and non-encapsulated devices.[3] One study uses XCT with micro-scale resolution to examine encapsulated MEMS devices post-fabrication, but it focuses on
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