Microscanner

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A micro scanner ( English micro-scanner or micro-scanning mirror ) is a micro-opto-electro-mechanical system (MOEMS) from the class of micro-mirror actuators for dynamic modulation of light . Depending on the design, the modulating movement of a single mirror can take place in a translatory manner or in a rotary manner around one or two axes. In the first case, a phase-shifting effect is achieved, in the second case, the deflection of the incident light wave.

Resonant translation mirror in pantograph design with a deflection of ± 500 µm

With microscanners, the modulation is generated via a single mirror. They are therefore to be distinguished from other micromirror actuators, which require a matrix of individually addressable mirrors in their mode of operation, the surface light modulators . In contrast, a microscanner array is involved when the effect of a single array mirror already fulfills the function but several mirrors are connected in parallel in an array to increase the light yield, for example.

properties

Usual chip dimensions are 4 mm × 5 mm for mirror diameters between 1-3 mm. However, larger mirror apertures with edge dimensions of up to approx. 10 mm × 3 mm can also be produced. Scan frequencies are up to 50 kHz, depending on the type and size of the mirror. In the case of microscanners that perform a tilting movement, the light can be guided or "scanned" across a projection surface. Mechanical deflection angles can reach up to ± 30 °. With translational microscanners, a mechanical stroke of up to approx. ± 500 µm can be achieved.

Drive principles

The driving forces required to move the mirror plate can be provided by various physical operating principles. In practice, the electromagnetic , electrostatic , thermoelectric and piezoelectric operating principle are particularly relevant here .

Since the operating principles differ in their advantages and disadvantages, a suitable drive principle must be selected for the specific application. Electromagnetic drives are characterized by large actuating forces, but the high power consumption is disadvantageous for mobile devices . In addition, the required high magnetic field strengths can only be achieved using external permanent magnets , so that the miniaturization of the microscanner is limited.

Electrostatic drives offer similarly high driving forces as electromagnetic ones, but consume 2–3 orders of magnitude less power . In contrast to an electromagnetic drive, the force effect resulting between the drive structures cannot be reversed. For the realization of quasi-static components with a positive and negative direction of action, two drives that act in opposite directions are necessary. Vertical comb drives are usually used for this. When controlling or regulating electrostatic-quasi-static drives, however, the drive character, which is often strongly non-linear in parts of the deflection area, acts as a hindrance due to the principle. Many highly developed electrostatic microscanners today therefore rely on a resonant operating mode in which a mechanical eigenmode (here, the oscillation mode) is excited. The resonant operation is particularly favorable in terms of energy. However, quasi-static drives are still of great interest for beam positioning and applications in which statically actuated or linearized scanning is to be carried out.

Thermoelectric drives generate large driving forces, however, due to their principle, have some technical disadvantages. The required heating power for heating the thermal bimorph actuators is comparatively high. At the same time, components must be thermally well insulated from the environment and preheated to prevent thermal drift due to environmental influences. Another disadvantage is the short travel distances that can only be translated into usable deflections by using the leverage. These structures are particularly unsuitable for high-frequency components because of the additional natural modes that occur. The low-pass behavior of thermal actuators with fast switching cycles should also not be neglected .

Piezoelectric drives also generate small deflections compared to electromagnetic and electrostatic drives so that they share the disadvantages of electrothermal actuators in this regard. However, they are less susceptible to thermal environmental influences and can also transmit high-frequency drive signals well.

Fields of application

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  LDC module with 1D microscanner and optical position detection integrated on the rear

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  An electrostatic 2D microscanner in a DIL20 housing

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  MEMS scanner module for 3D distance measurement (LIDAR) with a single transmitter mirror (mirror dimensions approx. (9.5 × 2.5) mm²) and a synchronized microscanner array (2 × 7) as a receiver unit.

The possible uses of tilting microscanners are diverse and include projection displays, image recording z. B. for technical and medical endoscopes, barcode reading, spectroscopy, laser marking and processing of materials, object measurement / triangulation, 3D cameras, object recognition, 1D and 2D light curtains, confocal microscopy / OCT, fluorescence microscopy and laser wavelength modulation.

Translational microscanner applications include Fourier transform infrared spectrometers , confocal microscopy, and focus variation.

Manufacturing

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  With the Fraunhofer AME75 process, finished microscanners with design-specific structuring based on a bare BSOI wafer.

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  Finished with the Fraunhofer AME75 process microscanners from the VarioS modular system based on preprocessed BSOI wafers.

Microscanners are usually manufactured using surface or volume micromechanical processes. As a rule, silicon or BSOI substrates ( Bonded Silicon on Insulator ) are used here.

Advantages and disadvantages of microscanners

The advantages of microscanners over macroscopic light modulators such as B Galvanometer scanners have a very small form factor, low weight and minimal power consumption. Further advantages arise from the possibility of integrating position sensors and evaluation electronics into the component. In addition, microscanners are characterized by a high level of robustness against environmental influences. For example, the microscanners developed at Fraunhofer IPMS have a shock resistance of at least 2500 g. Provided that they are encapsulated in a dust-tight and moisture-proof manner, they are maintenance-free and can be operated at temperatures from −20 to 80 ° C.

The manufacturing-related disadvantages include the high costs for individual components and long delivery times. To address this problem are researchers at the Fraunhofer IPMS with the VarioS called MEMS kit a platform technology to minimize this problem.

Web links

• MEMS scanner . Fraunhofer Institute for Photonic Microsystems
• ARI MEMS Micromirror Demonstration Devices . Adriatic Research Institute
• Getting Started with Analog Mirrors . Texas Instruments (product page)

Individual evidence

1. ↑ VarioS microscanner kit (PDF; 231 kB). Fraunhofer Institute for Photonic Microsystems IPMS (product description).
2. ↑ ^{a b} T. Sandner, T. Grasshoff, M. Wildenhain, H. Schenk,: Synchronized micro scanner array for large aperture receiver optics of LIDAR systems . In: Proc. SPIE 7594 - MOEMS and Miniaturized Systems IX . 2010, p. 75940C , doi : 10.1117 / 12.844923 .
3. ↑ ^{a b} C. Drabe, R. James, H. Schenk, T. Sandner: MEMS Devices for Laser Camera Systems for Endoscopic Applications . In: Proc. SPIE 7594 - MOEMS and Miniaturized Systems IX . 2010, p. 759404 , doi : 10.1117 / 12.846855 .
4. ↑ T. Sandner, T. Grasshoff, H. Schenk, A. Kenda: Out-Of-Plane Translatory MEMS actuator with extraordinary large stroke for optical path length modulation . In: Proc. SPIE . 7930 - MOEMS and Miniaturized Systems X, 2011, p. 79300I , doi : 10.1117 / 12.879069 .
5. ↑ D. Jung, T. Sandner, D. Kallweit, T. Grasshoff, H. Schenk: Vertical comb drive microscanners for beam steering, linear scanning and laser projection applications . In: MOEMS and Miniaturized Systems XI . 2012, p. 82520U-1-10 .
6. ↑ Arda D. Yalcinkaya, Hakan Urey, Dean Brown, Tom Montague, Randy Sprague: Two-Axis Electromagnetic Microscanner for High Resolution Displays . In: Journal of Microelectromechanical Systems . tape 15 , no. 4 , 2006, p. 786-794 , doi : 10.1109 / YWAMS.2006.879380 .
7. ↑ Michael Scholles, Andreas Bräuer, Klaus Frommhagen, Christian Gerwig, Hubert Lakner, Harald Schenk, Markus Schwarzenberg: Ultracompact laser projection systems based on two-dimensional resonant microscanning mirrors . In: Journal of Micro / Nanolithography, MEMS and MOEMS . tape 7 , no. 2 , 2008, p. 021001 , doi : 10.1117 / 1.2911643 .
8. ^ A. Wolter, H. Schenk, E. Gaumont, H. Lakner: MEMS microscanning mirror for barcode reading: from development to production . In: Proc. SPIE . 5348 - MOEMS Display and Imaging Systems II, 2004, p. 32-39 , doi : 10.1117 / 12.530795 .
9. ^ J. Grahmann, T. Grasshoff, H. Conrad, T. Sandner, H. Schenk: Integrated piezoresistive position detection for electrostatic driven micro scanning mirrors . In: Proc. SPIE . 7930 - MOEMS and Miniaturized Systems X, 2011, p. 79300V , doi : 10.1117 / 12.874979 .

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