A Scan Head package including two 1D resonant electrostatic driven micro scanning mirrors with piezoresistive
position detection was developed. The scanning frequency of the slow and the fast axis is 100Hz and 29,05kHz,
allowing WVGA-resolution. Thereby the Scan Head design reduces vertical distortion strongly and can potentially
be assembled automatically. In addition FPGA based video processing electronic was developed to improve
the sorting of the picture information corresponding to the Lissajous figure with the objective of high picture
contrast and a homogeneous brightness.
A new two-dimensional and resonantly driven scanning micro mirror has been simulated, fabricated and characterized.
Features are a small chip size of 2900 μm x 2350 μm with a frame oscillating at frequencies in the range of 1 kHz. The
frame carries a mirror of 500 μm diameter in a gimbal mounting oscillating at frequencies in the range of 16 kHz. The
characteristic mechanical amplitudes are 21o and 28o respectively. Voltages of 60 V and less than 140 V were necessary
to accomplish this. Much higher amplitudes have been achieved on the mirror axis without breaking the torsion bars.
Initial difficulties in realizing the high amplitudes have been overcome by improving the geometry of the suspension.
The initial design is presented as well as the measurement results of the initial and improved design. The device was
used to develop a micro laser camera with high depth of focus. Pictures taken with the system are presented revealing
the excellent resolution.
This paper discusses the fracture strength study of torsion springs in MEMS microscanners, which are fabricated in
silicon-on-insulator (SOI) with deep-reactive-ion-etch (DRIE) process. High performance microscanners are of particular
interest for scanning laser projection displays. To produce high resolution images, scanners are required to rotate with
large actuation angles (>10 degrees mechanical angle) at designated resonant frequencies. While the designs are pushed
closer to material limits, it is essential to acquire knowledge of single-crystal-silicon's fracture strength. We have
designed samples for fracture strength tests, which reach failure angle (> 20 degrees) with low driving voltage (< 50
volts) under vacuum. The tests are performed with real-time optical feedback to ensure resonance operations. A voltage
ramp is applied to scanners until fractures occur; the ramp-rate and starting angle are chosen such that failures occur
within thirty minutes of operation. Torsional stresses at fracture are calculated from failure angles via an ANSYS(R)
model. In the experiment, forty samples from two spring designs with a cross-section of 14x30 um and a length of 240
um are tested. Because fracture angles scatter around a mean value, Weibull statistics is used to treat the characteristic
behaviors of the tested samples to better interpret the test results. The Weibull characteristic fracture strengths are 2.97
GPa and 2.58 GPa. With a stress limit of less than 2 GPa, we can achieve a 86% reliability SVGA microscanner design
with a 1 mm diameter, a 32 KHz resonance frequency, and a single-side mechanical scan angle of 13 degrees.
A novel translational micro mirror with a circular shape of 3 mm diameter and oscillation frequencies of 500 Hz and 1000 Hz is presented including a design study based on analytical and numerical calculations. The study takes mechanical limits like stress and shock resistivity into account as well as fabrication issues resulting in the design points presented. Considerations and results of this study including stress limits for single crystalline silicon and a FE analysis of the main oscillation mode of the resonant structure will be illustrated. Based on an SOI process with 30 μm thick and highly doped single crystalline silicon several devices were fabricated. For the characterization of the devices a Michelson interferometer set-up was used which allows determining the voltage-deflection curves as a function of the air pressure. Deflections of more than ± 50 μm for the 500 Hz device and ± 85 μm for the 1000 Hz have been achieved at a pressure of 10 Pa. The target is at ± 250 μm and ± 180 μm amplitude. In the outlook packaging requirements and approaches will be shown.
Design, fabrication and characterization of a novel out-of-plane vertical comb-drive actuator based Fourier transform microspectrometer (FTS) is presented. The spectrometer utilizes resonant mode vertical comb actuators as a variable-depth diffraction grating and a single photodetector to monitor the 0th order of the diffraction pattern. The spectrum of the source illuminating the gratings is computed by Fourier transforming the
0th order intensity as a function of the optical path difference. The vertical comb actuators have a travel range of 100 μm under atmospheric pressure with 28V excitation, which yields a theoretical spectral resolution of 0.5nm in the visible and better than 5nm in the telecom wavelengths.
We present a scanning micromirror with 5x better flatness of the mirror plate compared to our previous devices. The devices are designed for a laser scanning displays with VGA resolution. Scanning laser displays are certainly the most demanding application for scanning micromirrors. The fast axis must provide a large mirror plate that remains flat, when deflected to large angles at high frequency. The presented devices meet the specifications for VGA-resolution (640x480 pixels). Oscillation frequency is 16kHz. The mirror-plate has 1mm diameter and can be deflected by +/-10°. Dynamic deformation is below lamba/10 under these conditions. The devices are fabricated in the established SOI process of Fraunhofer IPMS Dresden. Mirror plate and springs are made of 30um of crystalline silicon. Operation is resonant with lateral out-of-plane comb-drives. In this article we present the design, simulation results and measurement results.
Light and electricity are two major sources leading technology advances into the future. Micro-opto-electro-mechanical-systems (MOEMS) devices combine these two sources in an ideal manner: they are electronically addressable devices comprising optical elements to modulate light temporally and/or spatially. Further, MOEMS devices take advantage of high integration density, high reliability, high bandwidth, and low cost fabrication for mass production. While in some cases MOEMS technology focuses on the replacement of conventional devices, the majority of developments uses the unique potential of this technology to create devices based on novel principles with extended or even new functionality for advanced applications. Products based on MOEMS technology have already entered or are only a few steps away from entering the market in various fields, e.g., in consumer, information, and communication technology, medicine, biology, and metrology. This work gives an overview of MOEMS development activities with special emphasis on devices for light beam deflection and modulation. Single micromirrors, e.g., for scanning or laser beam positioning, are also presented and discussed as micromirror arrays and membrane mirrors for image generation and phase modulation. Technology trends are derived from the current development activities and an outlook to future work is given.
Optical spectroscopy is a common tool for many applications. Micro systems most often use fixed gratings and array detectors. In the infrared wavelength range above the limit for Si-detectors (1100nm) and Ge-detectors (1700nm) respectively, this is either very expensive or almost impossible. Micro opto electro mechanical systems (MOEMS) offer very promising options. A movable grating can be realized by a silicon chip, using the technology of a well established scanner mirror chips in combination with the realization of a reflective grating either through etching of the aluminium mirror layer or even a more sophisticated technology. The patented resonant drive realizes a mechanical angle of ±7° with CMOS compatible voltages of approximately 20V. This technology leads to the realization of a set up close to a classical Czerny-Turner spectrometer using a single
detector only. The device offers the capability to be scaled down to the size of a cigarette box. The spectrometer presented here was adjusted to 900...2500nm range. The scanning grating chip has either 500, 625 or 714 lines/mm. As detector serves a fast InGaAs photodiode, read out through a 12 Bit AD converter. The sinusoidal
movement is unfolded by a signal processor (TI TMS320F2812) which also computes the spectrum. Acquired data can be shown by a display or transmitted to a host PC. System tests have been performed using infrared LEDs. Wavelengths have been 1300, 1400 or 1550nm for example.
The spectrometer is working accurately. First result of micro shaped grating structures to enhance the sensitivity are presented.
This contribution presents an optical module for projection of still images and video sequences. It consists of a laser source, miniature collimator optics, and a special MEMS device, a two-dimensional resonant micro scanning mirror. The laser beam is focused onto the micro mirror by the collimator optics. The micro mirror reflects the beam onto the desired projection area with a flare angle of up to 15 degrees for both axes. Given the resonant oscillation of the mirror, the beam follows a Lissajous figure. By choosing appropriate oscillation frequencies, it can be ensured that the laser beam hits every pixel of a pre-defined geometrical image resolution at a given frame rate. Limitations result from mechanical stability of the mirror plate that has a typical diameter of 1 mm and the CMOS-compatible fabrication process of the MEMS device. Projection of images and video sequences is achieved by modulating the laser diode. An external electronics receives data and transforms it into necessary modulation signals. Since frequency and amplitude of oscillation of the micro mirror are highly precise, no electrical feedback from the mirror to the modulation electronics has to be implemented. The system can be operated in open-loop modus. Currently, a monochrome demonstrator with VGA (640 x 480 pixels) resolution and 50 frames per second has been realized. Because of the compact size of the mirror, integration into mobile devices is fairly easy.
Micro Opto Electro Mechanical Systems (MOEMS) reach more and more importance in technical applications. They are smaller than conventional devices, less expensive when fabricated in higher numbers and offer new options concerning reliability and measuring methods. Resonant movable micro-mirrors produced as single crystalline chips with CMOS-compatible technologies provide a broad field of applications. In this paper, we will present different micro-mirrors, which are developed by the Fraunhofer IPMS in Dresden, Germany. They have different layouts and are thus suitable for several applications. Fabricated 1D-mirrors with mechanical angles of ± 16° can be used for laser deflection in bar-code-scanners, 2D-mirrors with different sizes and frequencies are suitable for imaging, displaying etc. Furthermore processes to apply diffractive structures on the micro-mirror surface were developed, showing an increased efficiency in the first diffraction order. Thus a micro-spectrometer has been built up, working in a wavelength range of 900-2500 nm. Due to the Czerny-Turner set-up, only one fast single InGaAs-photodiode is required.
This contribution presents a new scanning principle and device for 3-dimensional digital capturing and measurement of objects based on the triangulation method. The key elements are MOEMS, in particular electrostatically excited, harmonically oscillating micromechanical mirrors, which are useful means for light projection as well as for light detection. A configuration for capturing the trace of a static illumination is described, which applies a micro scanning mirror that oscillates in two axes. A synchronization method is proposed in order to apply micro scanning mirrors for both patterned illumination and light detection. For proving both techniques a test setup has been designed and assembled, and first results based on a static illumination are outlined.
Micro scanning mirrors are quite versatile MEMS devices for the deflection of a laser beam or a shaped beam from another light source. The most exciting application is certainly in laser-scanned displays. Laser television, home cinema and data projectors will display the most brilliant colors exceeding even plasma, OLED and CRT. Devices for front and rear projection will have advantages in size, weight and price. These advantages will be even more important in near-eye virtual displays like head-mounted displays or viewfinders in digital cameras and potentially in UMTS handsets.
Optical pattern generation by scanning a modulated beam over an area can be used also in a number of other applications: laser printers, direct writing of photo resist for printed circuit boards or laser marking and with higher laser power laser ablation or material processing.
Scanning a continuous laser beam over a printed pattern and analyzing the scattered reflection is the principle of barcode reading in 1D and 2D. This principle works also for identification of signatures, coins, bank notes, vehicles and other objects. With a focused white-light or RGB beam even full color imaging with high resolution is possible from an amazingly small device. The form factor is also very interesting for the application in endoscopes.
Further applications are light curtains for intrusion control and the generation of arbitrary line patterns for triangulation. Scanning a measurement beam extends point measurements to 1D or 2D scans. Automotive LIDAR (laser RADAR) or scanning confocal microscopy are just two examples. Last but not least there is the field of beam steering. E.g. for all-optical fiber switches or positioning of read-/write heads in optical storage devices. The variety of possible applications also brings a variety of specifications. This publication discusses various applications and their requirements.
MEMS (micro electro-mechanical systems) are often expected to take a development as microelectronics did in the last 35 years. Several devices are already established in mass markets like acceleration sensors, gyros, pressure sensors, ink jet heads and the DLP micromirror array. On the other hand many companies have stopped their business after the telecom bubble. Others are struggling. Many dreams based on MEMS-devices that were not at all mature and could not be manufactured in high numbers. When a commercial product is the goal, several questions must be answered already in concept phase. The specifications must clearly reflect the requirements of the application. Performance and price must be competitive to any other technology.
The relation between fabrication process and design is strong and mutual. The process must create all features of the device and the design must consider the limitations of the process. Only if the design is tolerant against all process variations reproducible performance can be achieved. And only if the design is robust in all process steps the devices can survive. Regarding the time and cost frame it is always preferable to change the layout rather than the process. This article looks at MEMS technology and identifies what has been adopted from CMOS, what is desirable to adopt and what needs new solutions. Examples are given in the fields of design, modeling layout, process, test, and packaging.
In this paper we present the analytical and experimental investigation of the air damping of micromachined scanning mirrors with out-of-plane comb drive actuation. A simple, compact model for the damping torque is derived by estimating the orders of magnitude of certain damping contributors. Viscous damping in comb finger gaps is estimated to be the dominant contributor. Because the comb fingers disengage as the scan amplitude increases, the damping coefficient is dependent on the amplitude of angular vibrations. Experimental measurements are presented for a variety of comb-finger geometries. The comb finger length, width, and the gap between comb fingers are varied, and the damping behaviour for single-axis scanning is characterised by measuring the decay rate of free oscillations. The damping is characterised by the exponential decay constant δ, found by fitting to the decaying oscillation amplitude. The predictions of the analytical model are compared to these experimental damping measurements.
New MEMS device are perpetually being proposed and concepts are approved by means of demonstrator devices. Volume production of MEMS however requires more. The fabrication process must be suitable for large numbers of wafers at acceptable cost and yield. Devices must be tested and packaged. Both are major cost factors. Reliability must
be qualified. Finally the product must compete with other (established?) solutions in cost, performance and reliability. We report on the fabrication end-test of the micro scanning mirror, a MEMS device for the resonant large-angle deflection of a laser beam at low operation voltage. The end-test involves: 1. wafer-level end-test of critical parameters on 100% of the chips, 2. full characterization of a random sample, and 3. reliability tests on representative samples. Emphasis is put on the wafer-level end-test of the mechanical properties.
In the last few years the importance of Micro Opto Electro Mechanical Systems (MOEMS) increased significantly in technical applications. This is caused by the possibility of combining micro optical elements with micromachining technology that makes it feasible to develop new systems with high volumes and low prices. In this article, we report on the realization of a NIR (near infrared) spectrometer in the range of 900 - 2000 nm using MOEMS technology. It is based on a scanning mirror chip, which mirror plate is structured with a diffractive aluminium layer on top. This offers the possibility to fabricate a spectrometer, which needs only one single InGaAs detector photo diode. In contrast to common CCD arrays, the obtained resolution is only limited by the performance of the spectrometer (entrance slit, exit slit, focus length, diffractive element). The scanning grating chip operates at a frequency of 500 Hz, at an optical scan range of ± 4°. The whole spectrometer has a size of 90 x 60 x 50 mm. For first investigations of the performance, IR LEDs (light emitting diode) with 1300, 1450 and 1550 nm wavelength have been measured.
The micro scanning mirror with lateral out-of-plane comb drives is on its way to volume fabrication. This article reviews the development and highlights the most important activities and decisions that are representative for many MEMS devices that are supposed to go the same way. Careful analysis of the product requirements, design for reliability, design for testability, design for packaging, a mature process, and automated testing preferably on wafer-level have been identified as keys to volume fabrication of MEMS.
Light and electricity are said to be the all purpose tools for the next decades. Photonic Microsystems combine this tools in an ideal manner: They are electronically addressable devices with an optical functionality allowing to modulate light temporally and/or spatially. Further, they take advantage of high integration density, high reliability, high bandwidth and low cost fabrication for serial production. While in some cases Photonic Microsystem Technology is focused on the replacement of conventional devices, the majority of developments uses the unique potential of this technology to create devices based on novel principles with extended or even new functionality for advanced applications. Products based on Photonic Microsystem Technology have already entered or are only a few steps away from entering the market in various fields e.g. in information and communication technology, medicine, biology and metrology. This paper gives an overview of the Photonic Microsystems development activities with special emphasis on devices for light deflection and light modulation. Single micro mirrors e.g. for scanning or laser beam positioning are as well presented and discussed as micro mirror arrays and membrane mirrors for image generation and phase modulation. Technology trends are derived from the current development activities and an outlook to future work is given.
One dimensional torsional micro mirrors for laser steering applications have been developed and manufactured at Fraunhofer Institute of Photonic Microsystems. Several design variations with rectangular plates are available. The device can be operated in resonant mode and quasistatic mode as well. The device is fabricated out of a BSOI wafer and a second conductive silicon wafer. The structure is assembled by conductive adhesive bonding.
Torsional springs connect the mirror plate to the mirror frame mechanically and electrically. Filled isolation trench structures separate volumes of different electrical potentials at the frame and at the deflective mirror respectively. Comb drive structures at both sides of the deflectable mirror and the part of frame located opposite increases capacitance at both mirror half sides. Applying a low level drive voltage between the combs, the mirror can be operated in resonant mode.
The second silicon wafer is placed below the deflective mirror and is electrically at ground. Applying a electrical potential of higher level to one side of the deflectable mirror, the mirror can be driven quasistatic and resonant as well. While the drive voltage is applied to one side of the mirror, the comb drive structure of the opposite side can be used for capacitance based position read out.
This paper presents a demonstrator of a low cost image projection device that has been developed at the Fraunhofer Institute of Photonic Microsystems. The image projection is not based on the common line by line raster scanning of the image. Instead, a resonant 2-dimensional micro scanning mirror is used for the deflection of a modulated laser beam. The mirror is operated at a low ratio of horizontal and vertical oscillation frequency. In particular, a ratio with a small shift from an integer value is used to enable a scan of the whole projection screen with a Lissajous pattern. The control circuit performs an excitation of both mirror axes by driving them with fixed frequency according to the response curves of the actuator. Programmable counters are used to generate the driving frequencies and to determine the actual beam position during the scanning process. That enables a very simple and low cost control circuit. A micro scanning mirror, fabricated at Fraunhofer IPMS, was used in the demonstrator set up. It is operated at oscillation frequencies of 1.4 kHz (slow axis) and 9.4 kHz (fast axis). The control circuit was realized and successfully tested with a FPGA implementation. The image resolution provided by the control circuit is 256 x 256 pixels.
Special micro scanning mirrors have been designed for the investigation of torsional stress in micro-scale hinges made of crystalline silicon. The setup with precise logging of resonant frequency and deflection amplitude of the MEMS-scanners is described. First results on fatigue and fracture strength are presented.
Fracture of torsion beams with 6.6 μm x 30 μm cross-section occurred at 2.0 GPa to 2.4 GPa. No sign of fatigue was observed in operation for 512 h at 1.4 GPa torsional stress in resonance at 2260.7 Hz oscillation frequency. Measured frequency variation was 0.06% without any trend.
Micro Optical Electro Mechanical Systems (MOEMS) gain more and more importance in technical applications. The combination of optical actuators and micromachined silicon technology arise possibilities to realize equipment in high volumes for reasonable prices, that have formerly been expensive laboratory equipment. This paper reports on the performance and applications of a spectrometer in MOEMS technology. It is based on a scanning mirror chip with a grating structure on top. Thus a spectrometer with selectable wavelength range was realized. The resolution is not limited by line width of a multisensor detector, as it is the case for state of the art low cost spectrometers. A single detector is used, cheaper than arrays and available for all wavelength ranges. The setup can be small and light, the grating withstands shocks and vibration much better than a classical spectrometer. The grating moves with a frequency of 500 Hz respectively 1000 Hz, a whole spectrum is acquired within milliseconds. The resolution is given by the grating line density and the spectrometer dimensions, as it is valid for every single detector spectrometer. Depending on the number of systems built, a price of the system can be expected significantly below those of low cost systems with fixed grating. Many possibilities arise in every application where light is analyzed spectroscopically.
Micro Opto Electro Mechanical Systems (MOEMS) gain more and more importance in technical applications. The combination of optical actuators and micromachined silicon technology arise possibilities to realize equipment in high volumes for reasonable prices, that have formerly been expensive laboratory equipment. This paper reports on the realization of a spectrometer in MOEMS technology. It is based on a scanning mirror chip with a grating structure on
top. Thus a spectrometer was realized, on which the wavelength range can be selected. The resolution is not limited by line width of a multisensor detector, as it is the case for state of the art low cost spectrometers. A single detector is used, cheaper than arrays and available for all wavelength ranges. The setup can be small and light, the grating withstands shocks and vibration much better than a classical spectrometer. The grating moves with a frequency of 500 Hz respectively 1000 Hz, a whole spectrum is acquired within milliseconds. The resolution is given by the grating line density and the spectrometer dimensions, as it is valid for every single detector spectrometer. Depending on the number of systems built, a price of the system can be expected significantly below those of low cost systems with fixed grating.
The Fraunhofer Institute of Microelectronic Circuits and Systems, Dresden, has been developing resonant Micro Scanning Mirrors for several years. They are designed for large deflection angles at low driving voltages in resonant operation. A couple of changes and optimizations in the layout of the 2-scanner that have improved performance and reliability are presented and discussed.
Different variants have been fabricated by bulk-micromachining in bonded silicon-on-insulator-substrates and have been characterized. Mirror plate and gimbal can be rectangular or elliptical, now. New comb structures of the driving electrodes allow optimization of either capacitance, damping or electromechanical stability. Complex insulation structures reduce parasitic capacitance and increase reliability and mechanical stability.
Various design variants were fabricated and characterized. Low-frequency devices with characteristic frequencies under 500 Hz reached scan ranges over 45° (±11.2 degrees mechanically) at voltages below 20V. The high-frequency devices with 8.4 kHz / 1 kHz reached 17.5° / 35° at 40 V / 30 V, respectively.
A design optimization of the electrostatically driven 1D Micro Scanning Mirror developed at the Fraunhofer Institute of Microelectronical Circuits and Systems has been carried out. The improvements are based on the use of non-Manhattan shaped structures for the mirror plate and the driving electrode combs. Several new design variants have been fabricated, characterized and are compared with devices of the previous design comprising quadratic mirror plates. The advantage of lower inertial moment favors the circular and elliptic design of the mirror plate. The capacity variation has been increased significantly by a special arrangement of the driving electrode fingers. Especially, a comb with star shaped fingers allows us to enhance the capacity variation remarkably. The experimental characterization of the devices shows that the elliptic plate with star shaped electrode combs is the variant to favor when the application requires large deflection angles for a given driving voltage and characteristic frequency. This meets the theoretical based expectation although the experimentally determined damping factor of devices with this design is significantly larger than for design variants with elliptic mirror plate and parallel electrode fingers. Devices of the novel design achieve mechanical deflection angles of up to +/- 14.0 degree(s) at a driving voltage of 11 V at low oscillation frequency. In comparison to the previous design this is an increase of 35 %.
Over the last few years, high resolution spatial light modulators (SLMs) have been developed at the IMS Dresden. These are fabricated using one of two different technological processes. In one version a flexible, highly reflecting aluminum coating of about 50nm is evaporated onto a elastic layer, while the other version has quite rigid aluminum mirrors that are suspended by flexible hinges above the substrate. Both versions are fabricated on top of a CMOS DRAM matrix, which allows the addressing of individual pixels. So far SLMs with over 2 million pixels have been produced. In order to ensure a high quality of these SLMs a map of the SLM under test is needed showing the exact position of defective pixels together with the type of defect e.g. not responding, always deflected, wrong spring constant, poorly reflecting surface. Additionally information on the local and global flatness is required. This task can only be handled by an automated test stitching together many single measurements. A test system has bene set up using a white light interferometer. This allows to measure the response of each and every SLM pixel to applied voltages.
Spatial light modulators (SLM) with addressing through a silicon backplane are gaining importance as microdisplays, for pattern generation in direct-writing systems and for several other applications. The use of CMOS technology has lead to small devices with a high grade of inspection. With the focus on direct-writing systems for photolithographic patterning we have developed tow micromechanical actuator technologies for SLMs with silicon backplane. Here we report on a third technology that employs an oil film on a mirror as micromechanical actuator. The principle of operation is explained and fabrication is described in detail. Demonstrators with 256 by 256 pixels have been fabricated with 16 micrometers and 20 micrometers pixel size. Photographs of programmed images are presented. Measurements of passive devices demonstrated a contrast ratio of 43:1 and a switching time of below 5ms.
Three different types of deformable mirror Spatial Light Modulators (SLMs) based on device concepts like Viscoelastic Control Layer (VCL), Cantilever Beam Mirror (CBM), and Moving Liquid Mirror (MLM) have been developed. All of them allow to create deformation profiles which act as phase gratings whose period is defined by the pitch of the pixel electrodes. The diffraction of the incident light is used to achieve spatial light modulation. The operation principles of the different types of SLMs are outlined in detail. All the mentioned SLMs can be manufactured on top of a high voltage CMOS circuitry. SLMs with up to 2 million pixels in analog operation mode have been realized up to now. The benefits of the different approaches with respect to fabrication aspects and respect to different applications will be addressed. For the angular deflection of light a new type of resonant microscanner mirror was developed. The device is based on a silicon micromechanical torsional actuator. The new approach for the configuration of the electrodes and the resulting driving principle allows to achieve large scanning angles (plus or minus 30 degree optically at atmospheric pressure) at low driving voltages (20 V max.) and low power consumption (less than 1 (mu) W). The operation principle of the new device enables the realization of 2D scanners as well.