Qualification of Metallized Optical Fiber Connections for Chip-Level MEMS Packaging

Abstract

A MEMS-based Safety and Arming (S&A) device is being developed for the next generation of Navy torpedoes. The MEMS-based S&A consists of a high aspect ratio MEMS chip fabricated by deep reactive ion etching (DRIE) of silicon on insulator substrates (SOI). The micro-machined structures, which include environmental sensors, actuators, and optical components, are susceptible to stiction related failures. A robust package is essential to transform the fragile MEMS S&A device into a rugged package capable of reliably functioning throughout the military stockpile to target sequence. To adequately protect the MEMS device from deleterious effects of the external environment, the package must be housed in a hermetic, organic-free package. This dissertation presents the design of, analyzes, and qualifies a die-level fluxless packaging concept. The die-level package consists of a metallized seal ring patterned around the perimeter of the chip, including the fiber groove, sidewalls, and base. The fiber grooves provide a fiber optic interconnect between the microstructure area and the macro-environment. A cap chip, with a matching seal ring, completes the clamshell package. Solder is deposited onto the seal ring and in the grooves at the wafer-level on the device and cap chips. A fluxless, and hence organic-free, soldering process joins and seals the fiber-chip assembly on the chip-level. The conditions that govern fluxless soldering are addressed and tailored for success in the developed design. Surface energy models are used to understand the fluxless soldering conditions and to study the geometric stability of fluid solder joints at the fiber to chip interface. Several techniques for fabrication of the chips and assembly of the packages are investigated. The effects of leak rate of the package seal on the internal package environment are discussed in detail to establish an acceptable leak rate of small volume MEMS packages. The calculations are then furthered to determine the acceptable leak path dimensions to ensure moisture does reach unacceptable levels during the package life. The presented work represents the first reported organic-free (fluxless) die-level package seal with optical fibers that cross the seal boundary.