Wolfgang Kronast

LPCVD against PECVD for micromechanical applications

After discussion of the basic aspects of CVD and its reaction kinetics LPCVD and PECVD will evolve as techniques commonly used at high temperature and lower temperature , respectively. Films deposited by these two techniques differ in several aspects, i.e., thickness, uniformity, purity, density, electrical properties, adhesion, step coverage, etc. Reactor designs are discussed in brief for optimization of the process parameters to yield optimized film properties. Then each of the major film materials such as polysilicon, SiN, , , SiC and some exotics such as diamond films are discussed with respect to their application in microstructures and their film properties in dependence on the deposition technique and follow-on processing, e.g., internal stresses due to imperfection in structure and composition or clamping, film density, pinhole density, and etchability. The discussion then moves to the application of LPCVD and PECVD in microstructures. A few typical examples will be presented for functional layers: films for membranes, cantilevers, etc in mono- and heterostructures, or ion sensitive films including passivation films as used in many sensors (e.g., microphones) and actuators (e.g., micromotors), especially such as fabricated by surface micromachining. Some room is also given to SiC, a new micromechanical material. A summary and weighting of the two CVD techniques is given.

Ultrastable assembly and integration technology for ground- and space-based optical systems.

Optical metrology systems crucially rely on the dimensional stability of the optical path between their individual optical components. We present in this paper a novel adhesive bonding technology for setup of quasi-monolithic systems and compare selected characteristics to the well-established state-of-the-art technique of hydroxide-catalysis bonding. It is demonstrated that within the measurement resolution of our ultraprecise custom heterodyne interferometer, both techniques achieve an equivalent passive path length and tilt stability for time scales between 0.1 mHz and 1 Hz. Furthermore, the robustness of the adhesive bonds against mechanical and thermal inputs has been tested, making this new bonding technique in particular a potential option for interferometric applications in future space missions. The integration process itself is eased by long time scales for alignment, as well as short curing times.

Micromachined Hotplate Platform for the Investigation of Ink-Jet Printed, Functionalized Metal Oxide Nanoparticles

This paper describes a novel micromachined platform serving as an interface between nanosized, gas sensitive metal oxide particles, and the macroscopic world. Through a combination of ink-jet printing and microelectromechanical systems technologies, it thus becomes possible to quickly test and characterize new nanosized metal oxide particles with respect to their gas sensitivity. Within the framework of this report, we describe the design considerations, thermal finite-element method simulations, processing, characterization, and utilization of the platform. Due to the low-power consumption, the hotplate provides an experimental platform to test nanoparticle-based metal oxide gas sensors for mobile systems.

Active focusing device based on MOEMS technology

A MEMS based device for active focus control is presented. The concept has been developed using coupled field FEM simulation. The focus length is adjusted by a reflective membrane which is electro-statically deformed. Using a special ring shaped counter electrode and an optimized weak membrane suspension, a perfect parabolic shape of the deformed membrane is obtained over a very large diameter at reasonable low driving voltages. The counter electrode is part of the chip package which simplifies the fabrication process. Using SOI-technology, the realization of stress free membranes with a diameter up to 10 mm has been proven. The device can be used in active optical applications where large numerical apertures are needed. Potential applications are e.g. confocal microscopy or scanning applications for focus control. In this paper, detailed results of the design optimization process are presented.

Porous silicon as multifunctional material in MEMS

Porous Si with pore size in the range of a few nanometers can be used as multifunctional material in different MEMS applications. In this paper, we first discuss the fabrication and characterization of porous Si. Porous Si has been used as sacrificial layer for the realization of micro-heaters, micro-hotplates and microphones. A new type of surface micromachining process for the fabrication of sub-/spl mu/m freestanding crystalline structures is presented. The use of porous Si as functional layer for different types of microsystems is discussed.

A miniaturized single-chip silicon membrane microphone with integrated field-effect transistor

A silicon microphone with integrated field-effect transistor which is fabricated on a single chip was simulated, designed and built. The oscillating single-crystal silicon membrane is structured using the electrochemical etch-stop technique with KOH solution. It contains the channel of the transistor. The gate electrode of the transistor is made of thick polysilicon provided with ventilation holes. The membrane sidelengths vary from 500 to .

Development of a tilt actuated micromirror for applications in laser interferometry

A silicon micromirror with 3x3 mm² surface area and a thickness of 100 μm has been designed and realized for the future space mission LISA (Laser Interferometer Space Antenna). The mirror is electrostatically actuated. The tilt movement of the mirror is provided by torsional load of the mirror suspension. 3D FEM simulations have been used for optimization of the layout of the mirror device. A torsion angle of ± 1.9 mrad is achieved at a driving voltage of U=200V. The demanding requirements on the laser interferometer in the mission LISA in respect to mechanical stability, noise performance and especially piston effect, (i.e. the requirement that under rotation of the mirror no significant z-movement of the reflection surface occurs) are fulfilled with a new design and fabrication concept for the micromechanical device. The piston-effect is avoided by a rotational axis of the micromirror which coincides exactly with the surface of the mirror. This is achieved by using a symmetric SOI-wafer (Silicon on Insulator) with handle and device wafer having exactly the same thickness. The mirror plane is formed by the handle wafer. The suspending beams are realized from both, the handle and the device wafer of SOI-wafer. Thus the central axis of the beams coincides with the reflecting plane. In addition, the z-displacement of the mirror under rotation due to the attracting electrostatic force is minimized by optimization of the beams and the counter electrode using FEM simulation. Fabricated devices are characterized by special interferometric optical measurements.

Development of a focusing micromirror device with an in-plane stress relief structure in silicon-on-insulator technology

Abstract. A new design concept for a dynamically focusing silicon membrane mirror with 6-mm diameter and electrostatic actuation was realized. With this concept, membrane buckling by residual compressive stress inside the membrane can be avoided, which is observed even in crystalline membranes fabricated in silicon-on-insulator (SOI) technology and leads to severe distortion of stress-sensitive devices, such as membrane-based micromirrors. To eliminate the influence of residual stress (compressive or tensile), a membrane suspension with a new stress-relief design was developed by the use of finite element (FEM) simulations. The improvement was achieved by a special tangential-beam suspension, which allows an in-plane expansion or contraction of the membrane, which reduces the stress-induced deformation and leads to substantially flat and distortion-free micromirrors (distortion<λ/10). Measurements of realized devices are in very good agreement with the prediction of the FEM simulation. A comparison of membranes with the new stress-relief suspension shows, for example, for a membrane with 6-mm diameter and 10-μm thickness, a distortion of 54 nm compared to 340 nm for a conventional rigidly clamped membrane. A focus range between 97 mm and infinity (flat position) can be used in accordance with the simulation.

Optimisation and characterisation of parabolic membrane mirrors

MOEMS-based thin silicon membrane mirrors with a useable diameter of 5mm and fast (up to 1kHz) tunable focal length (80 mm to 1m) have been realized. A ring shaped counter electrode is used to achieve a parabolic membrane deformation by electrostatic forces. A circular kerf at the outer perimeter of the membrane provides a soft suspension to the rim and thus reduces the needed driving voltage. FEM has been used for optimisation of the design, especially of the soft suspension, which is realized by a controlled thinning of the outer rim of the Si-membrane. A critical issue for demanding applications is the membrane distortion induced by material stress and the fabrication process. Membrane residual stress reduction has been obtained by using SOI-technology (c-silicon) and by optimisation of the Al deposition process (Al-coated Si-membrane). For dynamic tests of the optical mirror properties a stroboscopic interferometer has been realized. A pulsed laser diode with a pulse duration of 10μs is used as a light source which is synchronized with the modulated electrical field driving the membrane mirror. The interference pattern is recorded with a CCD and evaluated with conventional phaseshift techniques. The geometry is similar to a Mach-Zehnder interferometer. The reference path length can be varied with a piezoceramic to induce the phase shift.

Development of a focusing micromirror device with an in-plane stress relief structure in SOI technology

A new design concept for a dynamically focusing micromachined silicon membrane mirror with 6 mm diameter and electrostatic actuation was realized. To eliminate the influence of residual stress a special stress relief design of the membrane`s suspension was developed in order to achieve a distortion-free optical mirror (distortion < λ/10 (λ = 1064 nm). Even silicon membranes fabricated in SOI technology mostly suffer from buckling by residual compressive stress caused by mismatch in the coefficients of thermal expansion between silicon and the buried silicon oxide layer [1, 2].This often leads to severe distortion of stress sensitive devices such as membrane based micro mirror devices [3]. Even though a tensile pre-stress might improve the distortion in case of a non-deformed membrane, a tensile stress in the membrane increases the stiffness and thus reduces the sensitivity e.g. for capacitive sensors or for actuating devices. Different methods are reviewed for stress compensation or stress relief in membranes. We developed and fabricated a new stress relief structure which reduces the stress induced deformation of membranes and leads to substantially flat micromirrors of high optical quality. This is achieved by a special tangential beam suspension which allows an in-plane expansion or contraction of the membrane proportional to its inherent compressive or tensile stress. Optimized beam structures and the voltage dependence of the mirror’s deflection were determined by 3D FEM simulations. For membranes with a compressive pre-stress of -20MPa simulations show a decrease in bow to values < 18 nm in comparison with 700 nm for a conventional rigidly clamped membrane. A deflection of 16 μm within an aperture of 5 mm diameter is theoretically achieved by a voltage U0 = 200 V resulting in a minimal focal length of 97 mm. The fabricated devices have been characterized by the means of interferometric optical measurement. The measurement results are in good agreement with the theoretical prediction of FEM simulations.