Microscanner

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 acting motion of a single mirror can translationally or rotationally by one and two axes done. In the first case, a phase-shifting effect in the second case, the deflection of the incident light wave is achieved.

In microscanners the modulation on a single mirror is produced. They are therefore distinguished from other micro mirror actuators that require an array of individually addressable mirrors, in their effect, the spatial light modulators. To a micro scanner array is on the other hand, if the effect of a single array mirror already fulfills the function but for example to increase the light yield several mirrors in an array are connected in parallel.

Properties

Usual chip dimensions are 4 mm × 5 mm for mirror diameters between 1-3 mm. However, with larger Spiegelaperturen Kantenabmaßen of up to about 10 mm x 3 mm may be manufactured. Scan frequencies depending on the design and mirror size between 0.1 and 50 kHz. The deflection movement takes place periodically. For micro scanners can perform a tilting movement the light out grazing on a projection or "scanned". Mechanical deflection can thereby reach up to ± 30 °. In translationally working microscanners a mechanical stroke of up to about ± 500 microns can be achieved.

Drive principles

The driving forces required to move the mirror plate can be provided by various physical principles. Relevant in practice, in particular electromagnetic, electrostatic, piezo-electric, thermoelectric, and mode of action.

Since distinguish the active principles in their advantages and disadvantages is a suitable drive principle application-specific to choose. Electromagnetic drives are by large restoring forces from but the high power consumption for mobile applications is disadvantageous. Furthermore, the required high magnetic field strengths are attainable only by the use of external permanent magnets, so that the miniaturization of the micro- scanner is limited.

Electrostatic actuators offer similarly large driving forces, such as electromagnetic, however, consume 2-3 orders of magnitude less power. Unlike an electromagnetic drive, the resulting structures between the driving force effect can not reverse the polarity. For the realization of quasi-static components with positive and negative effective direction acting actuators are therefore two oppositely necessary. For this purpose, vertical comb drives are usually used. In the control or regulation of electrostatically - quasi-static drives, however, the often highly non-linear in parts of the deflection range drive character acts inherently a hindrance. Many sophisticated electrostatic micro- scanner therefore today focus on a resonant mode of operation in which a mechanical eigenmode (in this case, the oscillation mode ) is excited. The resonant mode is energetically favored. To be used for the beam positioning and applications where static actuated or linearized scanned but are quasi-static actuators continue to be of great interest.

Thermoelectric actuators produce large driving forces, however, have inherently some technical drawbacks. Thus, the heating capacity required for the heating of the thermal bimorph actuators is relatively high. At the same components must be thermally well isolated from the environment, and are preheated by thermal drift due to environmental influences to prevent. Another disadvantage is the low adjustment paths which can be translated only about the use of leverage to usable deflections. These structures are particularly suitable for high-frequency components due to additional emerging eigenmodes. Not to be forgotten is also the low-pass behavior of thermal actuators with fast switching cycles.

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

Fields of application

An electrostatic 2D micro scanning in a DIL20 housing

MEMS scanner module for the 3D distance measurement ( LIDAR) with single transmitting mirror ( Spiegelabmaße about (9.5 x 2.5 ) mm ²) and a synchronized micro array scanner (2 × 7) as a receiving unit.

The applications of tilting working microscanners are many and include:

• Projection displays
• Image capture, for example, in industrial and medical endoscopes
• Bar code reading
• Spectroscopy
• Laser marking and processing of materials
• Property Surveying / triangulation
• 3D cameras
• Object recognition
• 1D and 2D light curtain
• Confocal microscopy / OCT
• Fluorescence microscopy
• Laser wavelength modulation

Applications of translational micro scanners include:

• Fourier - transform infrared spectrometer
• Confocal microscopy
• Focus variation

Production

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

Microscanner are usually produced by surface-or volume- micromechanical methods. These usually come silicon or BSOI substrates (English Bonded Silicon on Insulator ) are used.

Pros and Cons of microscanners

The advantages of micro- scanners over macroscopic light modulators such as galvanometer scanners B lie within a very small form factor, light weight and minimal power consumption. Further advantages arise from the ability to integrate position sensor and readout in the component. In addition, micro scanner distinguished by a high robustness against environmental influences. For example, developed at the Fraunhofer IPMS micro scanners have a shock strength of at least 2500 g Assuming an appropriate dust-and moisture- tight encapsulation, they are maintenance-free and can operate at temperatures from -20 to 80 ° C.

The production-related disadvantages include the high cost of individual components and long delivery times. To address this problem are researchers at the Fraunhofer IPMS with the VarioS MEMS kit a platform technology available for minimizing this problem.