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2002 | Buch

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Optical Microscanners and Microspectrometers using Thermal Bimorph Actuators

verfasst von: Gerhard Lammel, Sandra Schweizer, Philippe Renaud

Verlag: Springer US

Buchreihe : Microsystems

Enthalten in: Professional Book Archive

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Optical Microscanners and Microspectrometers using Thermal Bimorph Actuators shows how to design and fabricate optical microsystems using innovative technologies and and original architectures. A barcode scanner, laser projection mirror and a microspectrometer are explained in detail, starting from the system conception, discussing simulations, choice of cleanroom technologies, design, fabrication, device test, packaging all the way to the system assembly.

An advanced microscanning device capable of one- and two-dimensional scanning can be integrated in a compact barcode scanning system composed of a laser diode and adapted optics. The original design of the microscanner combines efficiently the miniaturized thermal mechanical actuator and the reflecting mirror, providing a one-dimensional scanning or an unique combination of two movements, depending on the geometry. The simplicity of the device makes it a competitive component.

The authors rethink the design of a miniaturized optical device and find a compact solution for a microspectrometer, based on a tunable filter and a single pixel detector. A porous silicon technology combines efficiently the optical filter function with a thermal mechanical actuator on chip. The methodology for design and process calibration are discussed in detail. The device is the core component of an infrared gas spectrometer.

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Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract

In this book, we explore the application of MEMS technologies in micro-optical devices that use integrated micro-actuators for scanning mirrors and optical interference plates. These devices, as part of the so-called MOEMS (Micro Opto Electro Mechanical Systems) family, are primarily intended to be used as key components for linear and two-dimensional light beam scanning as well as for tunable optical filters.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

2. Basics for a Thermally Actuated Micromirror

Abstract

The following microactuator principle will be used for various applications: a miniature scanning device for barcode reading, a microprojector and a tunable filter in a microspectrometer. The specifications are similar.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

3. Microscanner Technology

Abstract

The microscanners are based on a simple low-cost process, using standard materials and techniques. This chapter presents the fabrication technologies using standard silicon micromachining [107, 71] and process improvements to achieve acceptable yield and reliable devices for reasonable manufacturing costs.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

4. One-Dimensional Microscanner

Abstract

This chapter describes quantitatively the actuation of the optical scanner as well as of the tunable optical filter, the latter will be described more in detail in chapter 6 – 8. The fabrication technology of both devices is different, but the physics behind it is the same.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

5. Two-Dimensional Microscanner

Abstract

The two-dimensional microscanner contains an asymmetry in the beam to induce a perpendicular motion. The bimorph beam has an “L”-shape (Fig. 5.1 a). The electrical path is incorporated on one beam. The high sensitivity of the thermal bimorph actuator allows a slow (non-resonant) movement of the scanner in one direction. This movement is mainly created by the large deflection of the long part of the beam and the scanner rotates around the y-axis (Fig. 5.1 b). This movement will be called frame scanning at low frequency. The second orthogonal scanning direction is a resonance motion due to the movement of the short beam. In this mode, the scanner twists around the x-axis and this movement provides raster scanning at high frequency [162, 116].

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

6. Advanced Optical Filters of Porous Silicon

Abstract

Porous silicon is fabricated by electrochemical etching of monocrystalline silicon in hydrofluoric acid (HF). A multilayer dielectric interference filter of porous silicon is based on the effect that the index of refraction depends on the degree of porosity, which can be chosen individually for every layer by the fabrication process. The porosity, i.e. the ratio of void volume to total volume in the layer, depends on the current density during the layer formation by electrochemical etching. To fabricate an optical filter the etch current is varied over time according to the desired refractive index profile of the layer stack. With this technology, optical filters of porous silicon can be produced within minutes in a single process step that generates a modulated density (Fig. 6.1).

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

7. Micromachining Using Porous Silicon

Abstract

Porous silicon was proposed since 1994 for micromachining, but at first only for sacrificial layers [193–198]. Lehmann considered it as new base material for MEMS using the high internal surface of macropores in 1996 [228]. Tjerkstra combined porosification and electropolishing of Si to form free-standing layers of porous Si to separate concentric microchannels [229] and uses isotropic electropolishing to micromachine buried microchannels [230]. All the devices have in common that they are mechanically static.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

8. Tunable Optical Filter and IR Gas Spectroscopy

Abstract

Large, flat, flip-up optical interference filters of porous Si were fabricated with areas ranging from 250 by 750 µm² to 2400 by 4000 µm² and a typical thickness of 30 µm. Fig. 8.1 shows the first design with a ribbon actuator (compare Fig. 7.13 A). In the rest position, the plate is lifted about 45° out of plane. This angle can be increased with a longer actuator. Experiments with different thicknesses of the porous Si layer show that the plate begins to warp when it is thinner than 15 µm and is difficult to lift up when it is thicker than 40 µm, because the actuator gets also thicker and more rigid.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

9. Conclusions and Outlook

Abstract

A new type of thermally actuated micromirror capable of one- and two-dimensional scanning has been presented. This work involves a complete study of the thermal bimorph actuator, an adapted inexpensive technological process development, analytical models and simulations for the dynamic behavior of the microscanner, characterization and testing of the devices and finally their evaluation for specific applications. An important feature of the developed microscanner is its simplicity, which makes it a cost effective component. Obtaining high fabrication yields and inexpensive devices is still a great challenge for MEMS components.

Gerhard Lammel, Sandra Schweizer, Philippe Renaud

Backmatter

Titel: Optical Microscanners and Microspectrometers using Thermal Bimorph Actuators
verfasst von: Gerhard Lammel
Sandra Schweizer
Philippe Renaud
Copyright-Jahr: 2002
Verlag: Springer US
Electronic ISBN: 978-1-4757-6083-5
Print ISBN: 978-1-4419-4946-2
DOI: https://doi.org/10.1007/978-1-4757-6083-5