W. Noell

Applications of SOI-based optical MEMS

After microelectromechanical systems (MEMS) devices have been well established, components of higher complexity are now developed. Particularly, the combination with optical components has been very successful and have led to optical MEMS. The technology of choice for us is the silicon-on-insulator (SOI) technology, which has also been successfully used by other groups. The applications presented here give an overview over what is possible with this technology. In particular, we demonstrate four completely different devices: (a) a 2 /spl times/ 2 optical cross connector (OXC)with an insertion loss of about 0.4 dB at a switching time of 500 /spl mu/s and its extension to a 4 /spl times/ 4 OXC, (b) a variable optical attenuators (VOA), which has an attenuation range of more than 50 dB (c) a Fourier transform spectrometer (FTS) with a spectral resolution of 6 nm in the visible, and (d) an accelerometer with optical readout that achieves a linear dynamic range of 40 dB over /spl plusmn/6 g. Except for the FTS, all the applications utilized optical fibers, which are held and self-aligned within the MEMS component by U-grooves and small leaf springs. All devices show high reliability and a very low power consumption.

Near-field fluorescence imaging with 32 nm resolution based on microfabricated cantilevered probes

High-resolution near-field optical imaging with microfabricated probes is demonstrated. The probes are made from solid quartz tips fabricated at the end of silicon cantilevers and covered with a 60-nm-thick aluminum film. Transmission electron micrographs indicate a continuous aluminum layer at the tip apex. A specially designed instrument combines the advantages of near-field optical and beam-deflection force microscopy. Near-field optical data of latex bead projection patterns in transmission and of single fluorophores have been obtained in constant-height imaging mode. An artifact-free optical resolution of 31.7±3.6u200anm has been deduced from full width at half maximum values of single molecule images.

A clean wafer-scale chip-release process without dicing based on vapor phase etching

A new method to release MEMS chips from a wafer without dicing is presented. It can be applied whenever SOI wafers are used that are structured from both the device and the handle side using DRIE. This method enables the release of extremely fragile structures without any mechanical impact on the chips. No more dicing residues or debris are created and deposited onto the wafer. The basic idea consists of etching deep surrounding trenches on the device and the handle layer that are displaced by about 20 /spl mu/m and thus create overlapping areas. For release, the buried silicon dioxide between the overlapping areas is etched away using hydrofluoric acid vapor phase etching.

Three-dimensional SOI-MEMS constructed by buckled bridges and vertical comb drive actuator

A new method for realizing three-dimensional structures based on the standard silicon-on-insulator microelectromechanical systems is developed using vertically buckled bridges as structural elements. The vertical displacement, profile of the bridge, and obtainable accuracy of the displacement are examined. Using the lateral dimension control of the bridge and the supporting beams, the vertical positioning is realized based on the planer photolithography. As a demonstration, a vertical comb drive actuator is prepared and its performance is examined.

Arrays of High Tilt-Angle Micromirrors for Multiobject Spectroscopy

Micromirror arrays are promising components for generating reflective slit masks in future multiobject spectrographs. The micromirrors, 100 mum times200 mum in size, are etched in bulk single crystal silicon, whereas a hidden suspension is realized by surface micromachining. The micromirrors are actuated electrostatically by electrodes located on a second chip. The use of silicon on insulator (SOI) wafers for both mirror and electrode chip ensures thermal compatibility for cryogenic operation. A system of multiple landing beams has been developed, which latches the mirror at a well-defined tilt angle when actuated. Arrays of 5times5 micromirrors have been realized. The tilt angle obtained is 20deg at a pull-in voltage of 90 V. Measurements with an optical profiler showed that the tilt angle of the actuated and locked mirror is stable with a precision of 1 arcmin over a range of 15 V. This locking system makes the tilt angle independent from process variations across the wafer and, thus, provides uniform tilt angle over the whole array. The surface quality of the mirrors in actuated state is better than 10-nm peak to valley and the local roughness is about 1-nm root mean square

Micromachined photoplastic probe for scanning near-field optical microscopy

We present a hybrid probe for scanning near-field optical microscopy (SNOM), which consists of a micromachined photoplastic tip with a metallic aperture at the apex that is attached to an optical fiber, thus combining the advantages of optical fiber probes and micromachined tips. The tip and aperture are batch fabricated and assembled to a preetched optical fiber with micrometer centering precision. Rectangular apertures of 50 nm X 130 nm have been produced without the need of any postprocessing. Topographical and optical imaging with a probe having an aperture of 300 nm demonstrate the great potential of the photoplastic probe for SNOM applications.

4X4 matrix switch based on MEMS switches and integrated waveguides

We present the development of a 4×4 matrix of optical switches based on silicon microfabricated switches and integrated waveguides. The device consists of two chips connected together by flip-chip bonding. The first chip has 16 latched silicon switches with micro-mirrors, while the second one contains the integrated waveguides and ensures the electrical connection of the 16 switches. The incorporation of integrated waveguides, patterned using a SU-8 mask and deep reactive ion etching, in our switch devices allows both the size and the packaging complexity to be reduced, and a technology for higher order matrices to be developed.

Fabrication of optical MEMS in sol-gel materials

A method for fabricating optical MEMS in sol-gel-materials is presented. The basic process involves deposition and patterning of a sacrificial spacer layer and a combined molding and photolithography step. The method described allows the fabrication of micromechanical elements incorporating micro-optical structures such as lenses (diffractive or refractive), gratings (for polarizers or resonant filters), waveguides or other micro-optical relief structures fabricated by UV-curing replication processes.

Micro-machined XY stage for fiber optics module alignment

A novel silicon micro-machined XY stage with a hybrid micro-lens for fiber optics module alignment is presented. A MEMS micro-alignment method and silicon chip design are described. Finally the micro-fabricated device performance is discussed.

Microfabrication of a combined AFM-SNOM sensor

The objective of this work is to fabricate a scanning probe sensor that combines the well-established method for atomic force microscopy, employing a micro-machined Si cantilever and integrated tip, with a probe for the optical near field. A photosensitive pn-junction is integrated into the tip for that purpose and an Al coating is applied to the tip. It comprises an aperture of 50-70 nm in diameter at the apex of the tip in order to spatially limit the interaction of the tip to the optical near field of the sample. Characterization of the tip and first results of simultaneously recorded force and photon images are presented.