2D-MEMS scanners for the deflection of Laser light in two directions are used to illuminate a measurement volume within 40° in horizontal and vertical direction. This solid angle of about 0.02 is scanned by a 658nm Laser beam with a maximum repetition rate of 350MHz digital pulses with an intensity of about 50mW. Reflected light is detected through an objective by an APD with a bandwidth of 80MHz. The phase difference between the scanned Laser light and the light reflected from an object is analyzed by sub-Nyquist sampling allowing the calculation of its distance and velocity. Presently, the achieved accuracy of the system is between 5mm and 10mm and the measurement range is about 2m. The experimental set-up of the Lidar system is presented in detail and first measurements demonstrating the capability of the system are discussed.
Electrostatic driven 2D MEMS scanners resonantly oscillate in both axes leading to Lissajous trajectories of a digitally modulated laser beam reflected from the micro mirror. A solid angle of about 0.02 is scanned by a 658nm laser beam with a maximum repetition rate of 350MHz digital pulses. Reflected light is detected by an APD with a bandwidth of 80MHz. The phase difference between the scanned laser light and the light reflected from an obstacle is analyzed by sub-Nyquist sampling. The FPGA-based electronics and software for the evaluation of distance and velocity of objects within the scanning range are presented. Furthermore, the measures to optimize the Lidar accuracy of about 1mm and the dynamic range of up to 2m are examined. First measurements demonstrating the capability of the system and the evaluation algorithms are discussed.
A higher achievable scan speed and the capability to integrate two scan axes in a very compact device are fundamental
advantages of MEMS scanning mirrors over conventional galvanometric scanners. There is a growing demand for
biaxial high speed scanning systems complementing the rapid progress of high power lasers for enabling the
development of new high throughput manufacturing processes. This paper presents concept, design, fabrication and test
of biaxial large aperture MEMS scanning mirrors (LAMM) with aperture sizes up to 20 mm for use in high-power laser
applications. To keep static and dynamic deformation of the mirror acceptably low all MEMS mirrors exhibit full
substrate thickness of 725 μm. The LAMM-scanners are being vacuum packaged on wafer-level based on a stack of 4
wafers. Scanners with aperture sizes up to 12 mm are designed as a 4-DOF-oscillator with amplitude magnification
applying electrostatic actuation for driving a motor-frame. As an example a 7-mm-scanner is presented that achieves an
optical scan angle of 32 degrees at 3.2 kHz. LAMM-scanners with apertures sizes of 20 mm are designed as passive
high-Q-resonators to be externally excited by low-cost electromagnetic or piezoelectric drives. Multi-layer dielectric
coatings with a reflectivity higher than 99.9 % have enabled to apply cw-laser power loads of more than 600 W without
damaging the MEMS mirror. Finally, a new excitation concept for resonant scanners is presented providing
advantageous shaping of intensity profiles of projected laser patterns without modulating the laser. This is of interest in
lighting applications such as automotive laser headlights.
The design and manufacturing of a piezoelectrically driven translatory MEMS actuator is presented, which features a 7
mm aperture and four thin-film PZT actuators achieving large displacements. The actuator performs piston mode
oscillation in resonance which can serve for Fourier Transform Infrared Spectroscopy (FTIR). Thereby vertical
displacements in piston mode of up to ± 800 μm at 163 Hz and 25 V driving sinusoidal voltage has been achieved under
ambient conditions. Due to the low frequencies and the low driving voltages only low power consumption is required.
The effect of residual gas friction and internal friction on the piezo-driven MEMS actuator is analyzed by measuring Qvalues
associated with the piston mode. Laser Doppler Vibrometry (LDV) was also used to detect and analyses the
parasitic effects especially tilting which superimposes the vertical movement of the mirror. The deviation from the pure
vertical piston mode was found to 1.3 μm along the x and 3 μm in the y-axis.
PZT driven resonant micromirrors offer advantages of large scan angles and decreasing power consumption due to the
benefits of resonant driving and high torque delivered by PZT actuators. Therefore they are entering into different
application fields recently, for example as laser projection or head-up displays. For many uses position sensing of the
micromirrors is necessary to set up closed loop controls. Thus, the development of integrated position sensors is aimed in
this work. Investigation and evaluation of different position sensing principles have been performed. In previous works
1D and 2D PZT driven resonant micromirrors have been presented, which feature various spring suspensions and thinfilm
PZT actuators as drivers. Due to the considerably different motion modes and resonant frequencies, which vary
from 100 Hz up to 64 kHz, various position detection methods have been investigated. This work presents primarily
fabrication and characterization results of the position sensors based on the direct piezoelectric effect, which will be
compared to the position sensors using metallic strain gauge realized by the same fabrication technology. Analyses of the
sensitivity, linearity and dynamic behavior of the sensors have been performed, by means of comparing the sensor
signals and the micromirror position signals measured by a Position-Sensitive-Device. Advantages and drawbacks of the
sensors are discussed and methods for eliminating the drawbacks are proposed.
Typical applications for resonantly driven vacuum packaged MEMS scanners including laser projection displays
require a feedback signal for closed-loop operation as well as high accuracy angle synchronization for data processing.
A well known and widely used method is based on determining the angular velocity of the oscillating
micromirror by measuring the time derivative of a capacitance. In this work we analyze a capacitive sensing approach
that uses integrated vertical comb structures to synchronize the angular motion of a torsional micromirror
oscillating in resonance. The investigated measurement method is implemented in a laser display that generates
a video projection by scanning a RBG laser beam. As the 2D-micromirror performs sinusoidal oscillations on
both perpendicular axes a continuously moving Lissajous pattern is projected. By measuring the displacement
current due to an angular deflection of the movable comb structures an appropriate feedback signal for actuation
and data synchronization is computed. In order to estimate the angular deflection and velocity a mathematical
model of the capacitive sensing system is presented. In particular, the nonlinear characteristic of the capacitance
as a function of the angle that is calculated using FEM analysis is approximated using cubic splines. Combining
this nonlinear function with a dynamic model of the micromirror oscillation and the analog electronics a mathematical
model of the capacitive measurement system is derived. To evaluate the proposed model numerical
simulations are realized using MATLAB/Simulink and are compared to experimental measurements.
2D MEMS scanners are used for e.g. Laser projection purposes or Lidar applications. Electrostatically driven resonant torsional oscillations of both axes of the scanners lead to Lissajous trajectories for Laser beams reflected from the micro mirror. Wafer level vacuum encapsulation with tilt glass capping ensures high angular amplitudes at low driving voltages additionally preventing environmental impacts. Applying Laser Doppler Vibrometry, the effect of residual gas friction, squeezed film damping and internal friction on 2D MEMS scanners is analyzed by measuring the Q-values associated with the torsional oscillations. Vibrometry is also used to analyze the oscillatory motion of the micro mirror and the gimbal of the scanners. Excited modes of the scanner structures are identified giving rise to coupling effects influencing the scanning performance of the 2D MEMS mirrors.
This paper presents designs and fabrication process of two single-axis PZT micromirrors with 1 mm diameter and 1.4 mm × 4 mm apertures, whose frequencies are 60 kHz and 17 kHz, respectively. These micromirrors achieve large optical scan angles of about 40° driven by 10 V rectangular pulses and show high Q-factors of more than 1000. The investigation on the long-term stability of a PZT driven micromirror has detected more than 100 Billion cycles. The combined results of experimental diagnostics and FEM analyses give rise to new designs iteratively leading to a larger deflection and appropriate frequencies, which are currently fabricated.
Hermetic wafer level packaging of optical MEMS scanning mirrors is essential for mass-market applications. It is the
key to enable reliable low-cost mass producible scanning solutions. Vacuum packaging of resonant MEMS scanning
mirrors widens the parameter range specifically with respect to scan angle and scan frequency. It also allows extending
the utilizable range of mirror aperture size based on the fact that the energy of the high-Q oscillator can be effectively
conserved and accumulated. But there are also some drawbacks associated with vacuum packaging. This paper discusses
the different advantageous and disadvantageous aspects of vacuum packaging of MEMS scanning mirrors with respect to
laser projection displays. Improved MEMS scanning mirror designs are being presented which focus on overcoming
previous limitations. Finally an outlook is presented on the suitability of this technology for very large aperture scanning
mirrors to be used in high power laser applications.
Low-cost automotive laser scanners for environmental perception are needed to enable the integration of advanced driver assistant systems into all automotive vehicle segments, which is a key to reduce the number of traffic accidents on roads. Within the scope of the European-funded project MiniFaros, partners from five different countries have been cooperating in developing a small-sized low-cost time-of-flight-based range sensor. An omnidirectional 360-deg laser scanning concept has been developed based on the combination of an omnidirectional lens and a biaxial large aperture MEMS mirror. The concept, design, fabrication, and first measurement results of a resonant biaxial 7-mm gimbal-less MEMS mirror that is electrostatically actuated by stacked vertical comb drives is described. Identical resonant frequencies of the two orthogonal axes are necessary to enable the required circle scanning capability. A tripod suspension was chosen, since it minimizes the frequency splitting of the two resonant axes. Low-mirror curvature is achieved by a thickness of the mirror of more than 500 μm. Hermetic wafer-level vacuum packaging of such large mirrors based on multiple wafer bonding has been developed to enable a large mechanical tilt angle of ±6.5 deg in each axis. Due to the large targeted tilt angle of ±15 deg and because of the MEMS mirror actuator having a diameter of 10 mm, a cavity depth of about 1.6 mm has been realized.
Low-cost automotive laser scanners for environment perception are needed to enable the integration of advanced driver assistant systems (ADAS) into all automotive vehicle segments, a key to reducing the number of traffic accidents on roads. An omnidirectional 360 degree laser scanning concept has been developed based on combination of an omnidirectional lens and a biaxial large aperture MEMS mirror. This omnidirectional scanning concept is the core of a small sized low-cost time-of-flight based range sensor development. This paper describes concept, design, fabrication and first measurement results of a resonant biaxial 7mm gimbal-less MEMS mirror that is electrostatically actuated by stacked vertical comb drives. Identical frequencies of the two resonant axes are necessary to enable the required circle scanning capability. A tripod suspension was chosen since it allows minimizing the frequency splitting of the two resonant axes. Low mirror curvature is achieved by a thickness of the mirror of more than 500 μm. Hermetic wafer level vacuum packaging of such large mirrors based on multiple wafer bonding has been developed to enable to achieve a large mechanical tilt angle of +/- 6.5 degrees in each axis. The 7mm-MEMS mirror demonstrates large angle circular scanning at 1.5kHz.
This paper presents design, fabrication and measurements for single-axis piezoelectric MEMS micromirrors with 1 mm2 apertures. These micromirrors, which feature thin-film PZT actuators and mechanical leverage amplification, are dedicated for laser projection and meet the requirements of high resonant frequency and large deflection angles. To identify the optimal micromirror geometries a parametric study by means of FEM simulations and analytic modeling has been performed. Characterization, related to the material qualities of PZT and the mechanical performance of the micromirrors, have verified the reliability of the process, the robustness and the performance of the fabricated prototypes. According to the measurements the fabricated micromirrors feature high Q-factor about 1570. The micromirror reaches the θopt·D product of 42.5 °·mm at 32 kHz driven by a low voltage of 7 V. Furthermore, new designs with larger apertures and deflections are currently being developed.
For many applications it is inevitable to protect MEMS devices against environmental impacts like humidity which can affect their performance. Moreover recent publications demonstrates that micro mirrors can achieve very large optical scan angles at moderate driving voltages even exceeding 100 degrees when hermetically sealed under vacuum. While discrete chips may be evacuated and sealed on single die level using small can packages like TO housings, it is obvious that for high volume production a much more economical solution for the realisation of transparent optical packages already on wafer level must be developed. However, since any laser beam crossing a transparent glass surface is partly reflected even when anti-reflective coatings are applied, the construction of a wafer level optical housing suitable for laser projection purpose requires more than the integration of simple plane glass cap. The use of inclined optical windows avoids the occurrence of intense reflections of the incident laser beam in the projected images. This paper describes a unique technology to fabricate glass packages with inclined optical windows for micro mirrors on 8 inch wafers. The new process uses a high temperature glass forming process based on subsequent wafer bonding. A borosilicate glass wafer is bonded together with two structured silicon wafers. By grinding both sides of the wafer stack, a pattern of isolated silicon structures is defined. This preprocessed glass wafer is bonded thereon on a third structured silicon wafer, wherein the silicon islands are inserted into the cavities. By setting a defined pressure level inside the cavities during the final wafer bonding, the silicon glass stack extruded and it is out of plane during a subsequent annealing process at temperatures above the softening point of the glass. Finally the silicon is selectively removed in a wet etching process. This technique allows the fabrication of 8 inch glass wafers with oblique optical surfaces with surface roughness <1 nm and an evenness of < 300 nm.
Small size, low power consumption and the capability to produce sharp images without need of an objective make
MEMS scanning laser based pico-projectors an attractive solution for embedded cell-phone projection displays. To fulfil
the high image resolution demands the MEMS scanning mirror has to show large scan angles, a large mirror aperture
size and a high scan frequency. An additional important requirement in pico-projector applications is to minimize power
consumption of the MEMS scanner to enable a long video projection time. Typically high losses in power are caused by
gas damping. For that reason Fraunhofer ISIT has established a fabrication process for 2D-MEMS mirrors that includes
vacuum encapsulation on 8-inch wafers. Quality factors as high as 145,000 require dedicated closed loop phase control
electronics to enable stable image projection even at rapidly changing laser intensities. A capacitive feedback signal is
the basis for controlling the 2D MEMS oscillation and for synchronising the laser sources. This paper reports on
fabrication of two-axis wafer level vacuum packaged scanning micromirrors and its use in a compact laser projection
display. The paper presents different approaches of overcoming the well-known reflex problem of packaged MEMS
scanning mirrors.
The use of microscanning mirrors in mobile laser projection systems demands for robust fabrication technologies. Dust,
change in humidity and temperature can only be tolerated if the fragile devices are enclosed in a hermetic package. A
novel fabrication process is presented based on two 30 micron thick epitaxially deposited silicon layers and a buried
interconnection layer. This technology allows the fabrication of stacked combdrives for electrostatic mirror actuation and
lateral feedthroughs needed for hermetic encapsulation with standard wafer bonding processes. High display resolution
requires large scan angles of the mirror plate. Therefore, a fabrication technology for structured glass wafers is presented
to provide deep cavities for large mirror plate movements. A solution for effective laser spot reflex suppression is
presented based on a static tilt of the mirror plate in relation to the glass cover wafer during eutectic bonding. By doing
so, the reflex generated at the glass surfaces is shifted out of the image area. The cavity pressure of packaged devices has
been measured showing the necessity of a getter layer in order to provide cavity pressures below 1 mbar. The
performance of a packaged device with integrated getter layer has been evaluated. A driving amplitude of only 6 V is
needed to achieve scan angles of above 50 deg. White light interferometric measurements showed excellent planarity of
the mirror plate with a radius of curvature of about 18 m.
Ulrich Hofmann, Marten Oldsen, Hans-Joachim Quenzer, Joachim Janes, Martin Heller, Manfred Weiss, Georgios Fakas, Lars Ratzmann, Eleonora Marchetti, Francesco D'Ascoli, Massimiliano Melani, Luca Bacciarelli, Emilio Volpi, Francesco Battini, Luca Mostardini, Francesco Sechi, Marco De Marinis, Bernd Wagner
Scanning laser projection using resonant actuated MEMS scanning mirrors is expected to overcome the current
limitation of small display size of mobile devices like cell phones, digital cameras and PDAs. Recent progress in the
development of compact modulated RGB laser sources enables to set up very small laser projection systems that become
attractive not only for consumer products but also for automotive applications like head-up and dash-board displays.
Within the last years continuous progress was made in increasing MEMS scanner performance. However, only little is
reported on how mass-produceability of these devices and stable functionality even under harsh environmental
conditions can be guaranteed. Automotive application requires stable MEMS scanner operation over a wide temperature
range from -40° to +85°Celsius. Therefore, hermetic packaging of electrostatically actuated MEMS scanning mirrors
becomes essential to protect the sensitive device against particle contamination and condensing moisture. This paper
reports on design, fabrication and test of a resonant actuated two-dimensional micro scanning mirror that is hermetically
sealed on wafer level. With resonant frequencies of 30kHz and 1kHz, an achievable Theta-D-product of 13mm.deg and
low dynamic deformation <20nm RMS it targets Lissajous projection with SVGA-resolution. Inevitable reflexes at the
vacuum package surface can be seperated from the projection field by permanent inclination of the micromirror.
The formation process for trench capacitor etching and its mechanisms in single-crystal silicon with a Cl2/SiCl4 reactive plasma using a multi frequency discharge etch reactor is developed. Trenches are etched using a SiO2 mask on wafers with 150 mm diameter. The influence of process gas flow, pressure, rf-power levels, and temperature is investigated revealing relevant process mechanisms. Attention is paid on the uniformity, reproducibility, and long term stability of the process. Determining the process window by varying the process parameters the changes in trench shapes are investigated giving access to the possibility of a sensitive control of the desired trench profile. The trench profiles achieved under our process conditions meet all requirements demanded by the application in advanced production lines.
Oxygen plasmas either in a reactive ion etching reactor or in a reactive ion beam etcher are used to demonstrate the capability to produce sub-half-micron features in photoresists with high aspect ratios in multi-level technique. Lower local etch rates for structures with increasing aspect ratios are evaluated. The geometry limited flux of neutrals into the structures leads to decreasing etch rates of the bottom resist with increasing aspect ratio. The role of sidewall passivation films for highly anisotropic etching is discussed. Sidewall passivation films are extremely stable with respect to further processing. Even highly reactive plasmas are not able to remove the passivating films completely. In all our experiments of re sist patterning in 02-plasmas we saw that highly anisotropic etching works only with a sidewall passivating layer.
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