Michael W. Putty

Wafer-to-wafer bonding of nonplanarized MEMS surfaces using solder

The fabrication and reliability of a solder wafer-to-wafer bonding process is discussed. Using a solder reflow process allows vacuum packaging to be accomplished with unplanarized complementary metal-oxide semiconductor (CMOS) surface topography. This capability enables standard CMOS processes, and integrated microelectromechanical systems devices to be packaged at the chip-level. Alloy variations give this process the ability to bond at lower temperatures than most alternatives. Factors affecting hermeticity, shorts, Q values, shifting cavity pressure, wafer saw cleanliness and corrosion resistance will be covered.

Process Integration for active polysilicon resonant microstructures

Abstract Microsensors based on active polysilicon resonant microstructures are attractive because of their wide dynamic range, high sensitivity and frequency shift output. In this paper, we discuss processing issues for integrating electrostatically-driven and -sensed polysilicon microstructures with on-chip nMOS device. Surface-micro-machining using sacrificial spacer layers is used to obtain relased microstructures. A novel feature is the use of rapid thermal annealing (RTA) for strain relief of the ion-implanted, phosphorous-doped polysilicon. Resonance frequencies of cantilever beams indicate a lower-bound Youngs modulus of about 90 GPa and an upper-bound compressive residual strain of only 0.002%, indicating that RTA is potentially useful for strain relief.

Resonant-bridge two axis microaccelerometer

A resonant bridge two-axis microaccelerometer is disclosed comprising polysilicon resonant bridges orthogonally attached to a silicon proof mass, such that the silicon proof mass is suspended by the resonant bridges. Acceleration in the plane of the substrate causes differential axial loads on the opposing microbridges in each pair, thereby shifting their resonant frequencies. The acceleration component aligned with a pair is measured by the difference in resonant frequencies.

One-port active polysilicon resonant microstructures

Theoretical and experimental characteristics of a two-terminal, or one-port, resonant microstructure are discussed. An equivalent circuit model that is useful for design and analysis of these devices is presented. This model is verified by experimental measurements, with a worst-case error between model and experimental parameters of 30%. A process for integrating polysilicon resonant microstructures with on-chip NMOS (N-metal oxide semiconductor) circuitry is also described. A novel feature of this process is the use of rapid thermal annealing (RTA) for strain-relief of the non-implanted phosphorus-doped polysilicon. The RTA-strain-relieved polysilicon has a Youngs modulus of 0.9.10/sup 12/ dynes/cm/sup 2/ and residual strain of 0.002% as measured by resonant frequency techniques. This low value of strain indicated that RTA is a useful strain-relief technique.<<ETX>>

An electroformed CMOS integrated angular rate sensor

A CMOS integrated, surface-micromachined angular rate sensor utilizing an electroformed vibrating metal ring structure on a silicon IC has been developed. Substantial signal-conditioning circuitry is included on the IC with the vibrating structure. Tests of the sensor demonstrate that its performance is equivalent to that required for a variety of angular rate applications. Stiction of electroformed micromachines and other reliability issues will be discussed.

Enhanced pyroelectric sensitivity using ferroelectric active mode detection

Active pyroelectric detection (APD), using ferroelectrics as the sensing materials, is described and compared to traditional-passive pyroelectric modes of operation. The active approach yields a number of distinct advantages over its passive counterparts, including greater effective pyroelectric coefficient and improved signal to noise ratio. Thin film test structures formed from strontium bismuth tantalate (SrBi2Ta2O9) are used to demonstrate the APD principle.

Characterization of electroformed nickel microstructures

Electroformed nickel resonators were constructed and tested in the temperature range of - 40 to 110 degrees C. The temperature sensitivities of the resonant frequencies are - 150 ppm/degrees C, - 200 ppm/degrees C, and - 3000 ppm/degrees C for cantilever beams, ring structures, and clamped-clamped bridges, respectively. The built-in stress for the bridge was estimated to be approximately 2 X 109 dyne/cm2. No resonant frequency shift was detected after long-term (over 60 days), large amplitude vibration. This implies that the electroformed nickel is a viable material for the construction of resonant mechanical sensors.

Fabrication options and operation principle for single-crystal silicon vibratory ring gyroscope

Four different approaches for the construction of a single crystal silicon vibratory ring gyroscope were investigated. All of them require deep trench etching (DRIE)of silicon, anodic bonding of silicon structures to a glass wafer, and a dissolve silicon wafer process. In the first approach, regular single crystal silicon was used as the starting sensor structural material. Heavy boron diffused silicon (B++Si) was formed followed by anodic bonding to the glass plate. The undoped silicon was then dissolved and the device structure was fabricated by deep trench etching of the B++Si. Due to the slow boron diffusion process, this approach severely limits the attainable thickness of the device structure. In the second approach, deep trench etching was carried out first followed by boron diffusion. In order to reduce RIE lag and boron diffusion time, the larger features were subdivided into smaller ones before DRIE process. We found that RIE lag still existed which had a detrimental effect on the sensor performance. In the third approach, silicon-on-insulator (SOI) wafer was used and the sensor structures were built in the Si-epi layer by DRIE. Because of the very slow etch rate of SiO2 in the DRIE process, RIE lag can be avoided. However, the associated footing problem makes the device dimensional control difficult. In the fourth approach, a layer of epi-GeB++Si on silicon wafer was used to build the sensor structures. The fabrication process was similar to that used in the third approach. Both RIE lag and footing problems were avoided. In the operation of the vibratory ring gyroscope, it is highly desirable of having the resonant frequencies around the ring to be isotropic. In this work, theoretical and experimental studies were conducted aiming at achieving the isotropy of the resonant frequency of the single crystal silicon ring, which has an orientation dependent modulus of elasticity. We found that for odd flexural vibration modes, the resonant frequencies of the ring were isotropic, whereas, for even modes they became anisotropic.

The Formation Of Electroplating Molds By Reactive Ion Etching

Polyimide, polymethylmethacrylate (PMMA) and polyester-based polymers were investigated as potential electroplating mold materials. They showed high tensile stress (/spl equiv/10/sup 8/ dyne/cm/sup 2/) after heat treatment at temperatures /spl ges/ 200/spl deg/C. Conventional reactive ion etching techniques together with a trilevel masking scheme was used to delineate the polymer layers. The etch mask used was either spin-on glass or aluminum, and the etch gas was oxygen. With proper power density and etch gas pressure, aspect ratios >5 were obtained. Compatibility issues between RIE and the etch mask materials, as well as between RIE and the plating base materials are discussed.