We outline several approaches to allow individual die to be encapsulated within a silicon substrate, which we define as a cavity wafer, without causing wafer bow. This technique forms the basis for a novel integrated ultra high density (i-UHD) wafer-level packaging platform.The iUHD process begins with a standard Si wafer that is patterned and dry etched to form cavities that accept buried components. After etching, the wafer is blanket metalized. Individual commercial off-the-shelf (COTS) die are placed onto an adhesive film and precision transferred to the substrate wafer. Low coefficient of thermal expansion (CTE) encapsulant is injected into the cavity surrounding the die. Finally, the adhesive film is removed to reveal a planar surface on the reconstructed core wafer.Multilayer interconnect is fabricated on both sides of the core using standard wafer fabrication techniques.A challenge to this approach has been that curing and shrinkage of the encapsulant, as well as its CTE mismatch with silicon, creates wafer bow. In this paper we present a technique that eliminates bow by mirroring the die-side wafer cavities about the neutral bending axis.

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Draper has built zero-power MEMS wake-up sensors for DARPA’s N-ZERO Program. This program aims to enable unattended sensor arrays that last for years, limited only by battery discharge rates. For some targets, characteristic frequency signatures allow detection using narrow band passive acoustic resonators. The MEMS sensors use ambient acoustic inputs to actuate a wake-up electrical relay. Resonant sensors from 30 Hz to 180 Hz have been fabricated. The device rotates in response to an acoustic input, thereby avoiding large displacements due to gravity which would occur with a linear actuator. An adjustable acoustic cavity designed as part of the package is used to tune the resonant frequency to match a particular target. FEA Modeling was performed to achieve desired spring constants and resonant frequency. A rotary-acoustic lumped element equivalent circuit model was used to analyze the effect of the cavity and leakage resistances on the device performance. We will show finished MEMS devices and acoustic test data. [This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA).]