Ramon Hahn

Silicon mirrors and micromirror arrays for spatial laser beam modulation

This contribution deals with the design, technology and experimental investigations of mirrors and micromirror arrays made of monocrystalline silicon. Electrostatically operated two-directionally deflecting mirrors and mirror arrays for continuous scanning with working frequencies between several 100 Hz and 200 kHz are presented. The modified BESOI technology used and the experimental-data-based method to improve the accuracy of model parameters for simulations and to determine the cross-coupling between array cells are new in the field of micromechanics. Furthermore, results of application-related experiments of laser projection are given.

Silicon mirror arrays fabricated by using bulk and surface micromachining

Recently scanning actuator arrays have developed using metal, e.g. aluminium, or polysilicon as mirror material. Design and technology of micro mirror arrays mad of monocrystalline silicon are discussed in this paper as well as experimental results characterizing the arrays. Micro mirror arrays with up to 1000 simultaneously movable electrostatically operated cells convenient for continuous scanning with frequencies of several hundred Hz up to some kHz will be presented. The technological approaches consist of the use of silicon wet- and dry-etching, wafer bonding and metallization. A novel modified BESOI technology with CMP, wafer bonding with buried refractory metal electrodes and sacrificial layer etching has ben developed and will be discussed. The design process is based on simple analytical calculations of the mechanical behavior, the fluid flow surrounding the movable mirror and the electrostatic field as well as numerical simulations by means of the finite element method and network analysis. Furthermore, some experimental methods to characterize the electro-mechanical behavior of micro mirror arrays are discussed. In order to evaluate theoretical models describing the behavior, the natural frequencies, the damping coefficients and the frequency transfer function are measured. The adaptation of the model parameters leads to more accurate values simulating the behavior.

A novel 24-kHz resonant scanner for high-resolution laser display

This contribution deals with design, fabrication and test of a micromachined resonant scanner usable for horizontal deflection of the laser beam in a projection display. The electrostatically driven plate is separated from the mirror in order to reduce air damping and electrostatic non linearity. The device consists of a circularly shaped mirror which is suspended by torsion beams in the center of an elastically suspended driving plate. A resonator with two rotational degrees of freedom is arranged in this way. The rotation axes of mirror and driving plate are the same. A suitable design of the properties of the two degrees of freedom resonator leads to a significant amplification of the oscillation of the mirror compared to the oscillation of the driving plate. The first resonant mode is a rotation of both plates with nearly the same magnitude at a frequency of approx. 5 kHz. The second mode with paraphase deflection at 24 kHz shows a deflection amplification by a ratio of 53 and is used for scanning operation. A supporting part made of glass carries two electrodes in the region of the driving plate and has a micro sandblasted hole beneath the mirror. Bulk micromachining KOH wet etching of the electrode gap size on the back side of the driving plate, reactive ion etching for contour shaping of the mirror, of the driving plate and of the torsion beams and anodic bonding have been used for fabrication of the mechanical structure. The mirror is evaporated by an aluminum layer. Applying a voltage of 380V results in a mechanical deflection of ± 5.5 degrees at 24 kHz at atmosphere pressure. The device shows very small dynamic warp (<100nm) of the mirror plate even though the relatively large size of 2.2 mm diameter because of the thickness of 280 µm. The measured mechanical Q-factor is 5100.

Low-temperature approaches for fabrication of high-frequency microscanners

Within this paper novel applications of low temperature silicon wafer bonding technologies for the fabrication of high frequency silicon microscanners are presented. Two technological approaches are discussed, both using low temperature bonding as a key technological step. Results of the integration of a special low temperature bonding process within the bulk technology approach are shown. Micromirror arrays fabricated with this technology are presented and show promising results for optical applications.

Synchronously working micromirrors for beam steering: design and application aspects

A new application of synchronously working micromachined silicon mirrors for a laser beam projection technique producing images on the whole circumference of a cylindrical shaped screen will be discussed within this contribution. The mechanically active mirror area of 12 square millimeters has been divided into 49 single mirrors in order to achieve both fast steering and light power distribution. Diffraction of the laser beam due to the regularly arranged mirrors and its influence to the contrast has been experimentally determined on a small sized projection system. Analytically calculated thermal effects on the mirror shape caused by light power dissipation and heat transfer correspond to the measurements by thermographic imaging and topographic metrology. Investigations concerning phase lag and amplitude difference between the mirrors lead to a design of specially shaped hinges with reduced sensitivity to the main fabrication tolerances.

Analogously working micromirror arrays

This paper deals with micromirror arrays for high frequency applications. The use of monocrystalline silicon as mechanical material results in a low hysteresis, good reproducibility and high reliability. General characteristics, advantages and limitations of large analogously working mirror arrays in comparison with single elements will be discussed with respect to the requirements of desired applications. Special aspects of the design of micromirror arrays, experimental results concerning the behavior of the electromechanical system and optical properties will be presented as well. It will be shown that cascaded elements allow beside higher quality factors or resonant band width concerning the dynamic behavior also higher quasistatic deflection angles at diminished electrode gaps and lower driving voltages.

Performance and reliability test of MEMS optical scanners

MEMS scanners are among the devices which have been investigated since the very beginning of MEMS development. The main concern of this paper is to report and discuss suitable measurement techniques and to compare and verify this methods on different scanners for examples. Analysis of scanner reaction based on image processing is covered in a first section. It is shown that an Electronic Speckle Interferometer (ESPI), a Scanning Laser Doppler Interferometer (SLDI) and a phase shift interferometer are suitable for measuring different motion characteristics. The SLDI is used for rapid measurement of FRF at a large number of locations at the scanner. A phase shift interferometer with stroboscopic illumination has been utilized for measuring the deformation of scanners operating them at resonant frequency. Measurement of static displacement and of thermal deformation is the main application of ESPI technique and shown on the example of a galvanometric scanner. A next section is dedicated to functional tests and to qualification of methods for wafer level test. The application of a tilt angle measuring set up containing a position sensitive semiconductor device and a laser diode and of a laser Doppler interferometer is analyzed. Measurement of resonant frequencies in an early production state is the topic of a third section. It provides information about geometry properties of the scanners suspension and about mechanical stress inside suspending torsion or bending beams. Moreover it enables selection of scanner chips with characteristics which do not meet the specification and the cost for packaging of this bad dies is saved.

Novel Hadamard transform spectrometer realized using a dynamically driven micromirror array as a light modulator

The paper deals with a novel setup of a Hadamard transform spectrometer (HTS) which encoding mask is realized by a micro mirror array. In contrast to other applications of an HTS the mirrors of the array are not statically switched but dynamically driven to oscillate at the same frequency. The Hadamard transform is obtained by shifting the phase shift of oscillation. The paper gives a brief introduction in the entity of the Hadamard transform technique. The driving and detection circuits are presented and first measurement results are discussed.

Application of micromirror arrays for Hadamard transform optics

The paper presents a novel kind of Hadamard transform optic. First investigations are made with a micro mirror array in a Hadamard transform spectrometer (HTS) whereby the usually used detector array is replaced by the micro mirror array. All the mirrors are imaged onto a single detector. The measurement is performed using a Hadamard matrix, i.e. while each detector reading a certain combination of mirrors given by the matrix is reflecting the light towards the detector. All the rest of them are reflecting the light beside it. The consequence is an improvement of the signal to noise ratio (SNR). The novelty of the realized spectrometer is that in contrast to other applications the mirrors are not statically switched but they are forced to oscillate at their resonant frequency. By this way a special Hadamard matrix can be used that improves the SNR best.