Sunny Kedia

Fabrication of Integrated Vertical Mirror Surfaces and Transparent Window for Packaging MEMS Devices

A scheme for creating metal-coated vertical mirrors in silicon, along with an integrated transparent package lid for assembling, packaging, and testing microelectromechanical systems (MEMS) devices is presented. Deep reaction ion etching (DRIE) method described here reduces the loading effect and maintains a uniform etch rate resulting in highly vertical structures. A novel self-masking lithography and liftoff process was developed to ensure that the vertical mirrors undergo uniform metallization while leaving a transparent window for optical probing. Front side of a Si wafer was shallow-etched using DRIE to define an eventual optical window. This surface was then anodically bonded to a Pyrex wafer. Backside Si was then patterned to define thin channels around the optical window. These channels were vertically etched using DRIE, after which the unattached portions of the window region were removed. Negative photoresist was spun on the remaining vertical structures and the stack was exposed from the Pyrex side using Si structures as a self-mask. Subsequent metal sputtering and liftoff results in the metallized top and mirror sidewalls while leaving a clear window. These integrated mirrors and lids are then bonded to the active MEMS mirrors. Various processes and results are illustrated with an example of packaged corner cube retroreflectors (CCRs)

Fabrication processes for packaged optical MEMS devices

Processes for the fabrication of packaged optical MEMS devices are presented. A single structural layer surface micromachining process for creating MEMS actuators and sensors is discussed. A base metal layer allows electrical routing. The structural layer is made of a stack of metal, dielectric, and metal to allow electrostatic actuation of parts, stiffness, and high optical reflectivity. All three structural layers are patterned using a single mask. The lower structural metal can be additionally patterned to allow isolated areas for electrical switching applications. Toward the goal of packaged optical devices, a new scheme for creating optically-transparent package lids, which are subsequently thermo compression bonded onto the surface micromachined parts, is also introduced. The fabrication technique allows creation of extremely vertical through-wafer surfaces in silicon, with minimal surface damage to the co-bonded glass lid. An optical corner cube retroreflector (CCR) communication device is presented as one application.

Handheld Interface for Miniature Sensors

Miniaturization of laboratory sensors has been enabled by continued evolution of technology. Field portable systems are often desired, because they reduce sample handling, provide rapid feedback capability, and enhance convenience. Fieldable sensor systems should include a method for initiating the analysis, storing and displaying the results, while consuming minimal power and being compact and portable. Low cost will allow widespread usage of these systems. In this paper, we discuss a reconfigurable Personal Data Assistant (PDA) based control and data collection system for use with miniature sensors. The system is based on the Handspring visor PDA and a custom designed motherboard, which connects directly to the PDA microprocessor. The PDA provides a convenient and low cost graphical user interface, moderate processing capability, and integrated battery power. The low power motherboard provides the voltage levels, data collection, and input/output (I/O) capabilities required by many MEMS and miniature sensors. These capabilities are relayed to connectors, where an application specific daughterboard is attached. In this paper, two applications are demonstrated. First, a handheld nucleic acid sequence-based amplification (NASBA) detection sensor consisting of a heated and optical fluorescence detection system is discussed. Second, an electrostatically actuated MEMS micro mirror controller is realized.

Small form factor microsensor system using optical MEMS for passive optical digital communications (PODC)

A small form factor microsensor system with optical MEMS devices is discussed in this paper. The key components in the microsensor system include a temperature and humidity sensor for environmental monitoring, a microprocessor for signal processing, and an optical MEMS device (active corner cube retroreflector or CCR) for remote free space optical communication. A flexible circuit design and a folded packaging scheme have been utilized to minimize the overall form factor. Flat, flexible polymer batteries are incorporated to minimize the vertical profile to a few millimeters. The entire fully packaged sensor system is about 30mmx30mmx6 mm. MEMS design of the CCR, fabrication, hermetic packaging of CCR, flexible circuit design and fabrication, flip chip bonding of die form microprocessor, and a battery replacement scheme for extended operation lifetime are crucial elements for the development of a real product for the microsensor system. Optical MEMS CCR is a torsion mirror design and was fabricated using surface micromachining with Si3N4 as a structural layer. A finite element analysis (FEA) model was developed to optimize design and performance of the MEMS structures. The sensor system has a miniature mechanical switch for local actuation and an optical switch for remote actuation. The applications of such a microsensor system include both tracking, tagging, locating (TTL) and remote sensing.

MEMS mirror arrays for use in optical spectrometric detection

Optical spectrometers are used in a variety of chemical and biological analytical instruments. Typically these employ a single input slit, a spectrograph, and a CCD or photodiode array for sensing. Only a few wavelengths may be of interest to the operator in many applications, due to absorption or fluorescence occurring within these specific optical regions. In the case of fluorescence, the excitation light intensity can be orders of magnitude greater than the fluorescence signal. In lieu of a detector array, a setup where a microelectromechanical system (MEMS) fabricated mirror array directs only the wavelengths of interest to a few detectors can be advantageous over sequential-readout arrayed detector systems. The MEMS mirrors and detector combination allows the desired wavelengths to be simultaneously and rapidly measured, with specialized detectors or electronics dedicated to each band. Integration time and electronic filtering may be adjusted independently, yielding better sensitivity and dynamic range. This combination is especially relevant in the infrared region, where arrayed detectors can be noisy or expensive, and arrays of dedicated amplifiers and filters are not cost effective. This paper reports on the design, fabrication, testing and control of MEMS-fabricated one-dimensional micromirror arrays for use in visible or infrared spectrometer applications. The micromirrors are fabricated using a surface micromachining process. A multiplexing method is introduced in the design to enable positioning a large number of mirrors from a few electrical inputs, which is necessary for practical applications when integrated control circuitry cannot be created on-chip with the MEMS devices. This approach also enables separate optimization of the actuation and control sections, and significantly reduces the number of drive signals required.