Wafer-Level Vacuum Sealing by Transfer Bonding of Silicon Caps for Small Footprint and Ultra-Thin MEMS Packages

Abstract

Vacuum and hermetic packaging is a critical requirement for optimal performance of many micro-electromechanical systems (MEMS), vacuum electronics, and quantum devices. However, existing packaging solutions are either elaborate to implement or rely on bulky caps and footprint-consuming seals. Here, we address this problem by demonstrating a waferlevel vacuum packaging method featuring transfer bonding of 25-µm-thin silicon (Si) caps that are transferred from a 100-mm-diameter silicon-on-insulator (SOI) wafer to a cavity wafer to seal the cavities by gold-aluminum (Au-Al) thermocompression bonding at a low temperature of 250 • C. The resulting wafer-scale sealing yields after wafer dicing are 98% and 100% with sealing rings as narrow as 6 and 9 µm, respectively. Despite the small sealing footprint, the Si caps with 9-µm-wide sealing rings demonstrate a high mean shear strength of 127 MPa. The vacuum levels in the getter-free sealed cavities are measured by residual gas analysis to be as low as 1.3 mbar, based on which a leak rate smaller than 2.8 × 10 −14 mbarL/s is derived. We also show that the thickness of the Si caps can be reduced to 6 µm by post-transfer etching while still maintaining excellent hermeticity. The demonstrated ultra-thin packages can potentially be placed in between the solder bumps in flip-chip interfaces, thereby avoiding the need of through-capvias in conventional MEMS packages. [2018-0257] Index Terms-Vacuum, hermetic, packaging, sealing, MEMS, ultra-thin package, small footprint, transfer bonding, 3D integration, flip chip, aluminum, gold, thermo-compression bonding. I. INTRODUCTION V ACUUM packaging is a process for encapsulating a device (e.g. a MEMS or nano-electro-mechanical system (NEMS) device) in a vacuum environment and maintaining the internal vacuum level using hermetic seals. It enables functionality and long-term reliability of various MEMS devices, such as inertial sensors, pressure sensors and infrared detectors, but typically also constitutes a significant part of their cost in high-volume production [1], [2]. As a general trend,

MEMS for Photonic Integrated Circuits

“…Another source of noise, which is common in MEMS, is environmental variation in suspended geometries, usually caused by humidity or particles. Such disturbances are typically minimized by hermetic packaging [114], [115], which will likely be used for MEMS-based PICs as well. Thermomechanical noise may also cause fluctuations in small gap spacing; however, at room temperature operation, even with low mechanical quality factor, these perturbations are on the picometer-scale and do not present an issue.…”

MORPHIC: Programmable photonic circuits enabled by silicon photonic MEMS

“…In MORPHIC, we add hermetically sealed thin caps to the MEMS cavities with a wafer-level bonding and cap transfer process, using the metal bond-pads present on the silicon photonics wafers as a landing surface. 39 Fig. 5 shows a part of a wafer having MEMS cavities with caps of different sizes.…”

“…(a) Proposed method for hermetic sealing of Photonic MEMS wafers using wafer bonding and transferred thin silicon caps. (b) Demonstration of the sealing method on a dummy wafer with empty cavities 39.…”

Evaluation of Silicon Diaphragms for Hermetic Packaging of Microbolometer Arrays

“…In the case of wafer-level packaging (WLP) the dicing will occur after packaging and could potentially cause diaphragm failure if the diaphragm is too thin. Even thicker diaphragms can be considered since IR light have low optical loss within 300 µm [7]. This will, however, give a lower cavity volume being more sensitive to vacuum deterioration compared with a higher cavity volume and equal leak rate.…”

“…A thinner diaphragm is desirable for higher IR transmission and for obtaining larger cavity volume to maintain high vacuum [6]. Thin diaphragms (< 300 µm) made by optically transparent materials is able to provide the low loss transmission of IR, which is critical for the operation of MBAs [7]. Diaphragms with a thickness lower than 100 µm can also be useful for the applications of measuring the vacuum level inside hermetic packages as a larger diaphragm deflection will provide better pressure sensitivity [8,9].…”