Optical Modulators Based on Microfabricated Tilting Flaps Operating at low Actuation Voltages

In this thesis the concept, modelling, design, fabrication process and characterization of novel optical modulators are presented. The devices consist of flaps exhibiting a large tilt with electrostatic actuation at low voltages. The applications comprise, but are not limited to, a monochromatic reflective display, a transmissive shutter array and switchable diffraction-gratings. Nowadays the LCD-technology for reflective displays which is the most widely used technology exhibits a low contrast and reflectivity. Different technologies have been developed or commercialised, but still do not reach the quality of printed paper. It is demonstrated that the technology developed in this thesis is very competitive. If it is applied as reflective display it could reach an overall reflectivity of 45% in the white state, a contrast ratio of 50, a 100 dpi resolution, a fast switching time of 0.5ms and an actuation voltage below 40V; leading to low power consumption. When the device is employed as a transmissive shutter array a 40% fill-factor of the modulated area is obtained. The modulators consist of flaps with 250 µm width and 50 µm height. They are in vertical position at rest and in particular after fabrication. The flaps in array-configuration are suspended by a torsion beam and are electrostatically tilted to almost horizontal position. For applications as a reflective display, light gets absorbed on an underlying black-layer as the flaps are in vertical rest state, giving a black pixel. At tilted position incoming irradiation is reflected and the pixel is perceived as white. In the application as transmissive shutter array light passes through as the flaps are in rest-state and light is blocked in the actuated-state. A model of the device is presented which permits us to properly design the dimensions, knowing the scaling laws. A novel bulk microfabrication process was developed that employs micro-moulding for obtaining flaps and torsion beams made out of poly-silicon. Thin high aspect-ratio trenches are refilled by poly-silicon followed by dry etching of the surrounding material. With this process a large variety of flap shapes are fabricated, such as a wave-, lens- , comb- and grid-form. For electromechanical and optical characterization special setups and methodologies were conceived. Typically 60° tilt angles for 60V actuation voltages were measured. Depending on the geometry there was a hysteresis in the actuation trajectory and some flaps exhibited tilt angles close to 90°. The actuation voltage ranged from 35 to 85V. The resonance frequency was at 700 Hz in the rest state and at 2.3kHz in the actuated state. The poly-silicon flap surface had a reflectivity of 37% and an absorbance of 85 to 96%, depending on the wavelength. When the configuration was used as a reflective display a maximal contras ratio of 145, with typical values between 30 and 40, was measured on the active modulation area. The use of a blazed-grating shaped flap as switchable diffraction-grating was also demonstrated. Besides the concept of vertical flaps, a study is presented for the design of surface micromachined horizontal flaps actuated electrostatically to vertical position towards a transparent electrode.

Development of Tunable Optical MEMS for Advanced Laser Beam Shaping, Homogenizing and Speckle Reduction

Jonathan Masson

In this thesis, the development, design, fabrication and characterization of three innovative laser beam shaping, homogenizing and speckle reduction devices are reported. Typical beam shaping devices are static and not easily adaptable to a large variety of optical setups. Furthermore, the main issues when shaping coherent light are diffraction effects, speckle patterns, interferences and wavelength dependency. The devices presented in this work solve these issues. The main subjects discussed are beam shaping of coherent light and speckle reduction. Beam shaping and speckle reduction are important in many applications like laser machining, illumination, laser displays and photolithography. The first device presented is a dynamic diffuser and homogenizer for line generation. The 1D linear diffuser consists of a large single crystal silicon membrane of 5x5, 10x10 or 15x15 mm2 areas and having a thickness of 5 μm to 10 μm. This optical surface has a 100 % fill factor. The goal is to deform the membrane in one dimension only so that the light is also diffused in one dimension. The free standing continuous silicon membrane has a bridge type configuration. The actuation of the device is made electromagnetically at resonance by the use of the Lorentz force. Stiffening beams are fabricated below the membrane to decouple the 1D and 2D modes and to prevents 2D dynamic deformations. The device is fabricated on SOI wafer using standard microfabrication technologies. The devices are characterized using a laser doppler vibrometer (LDV) and a goniometer. The LDV measurements have shown that 1D and 2D modes can be excited separately. The diffusion angle is tunable between 0 to 22 mrad. The device is used to smooth out the interference effects created by a static 1D beam shaping optical element. The relative optical power efficiency of is 99.8 %. The device is capable to sustain high optical load of at least 140 W/cm2. It is now being tested in an industrial environment with high power CO2 laser for sealing glass capillaries. The second device presented is a 2D dynamic diffuser. It has to fulfill several characteristics, namely to diffuse light with small and tunable angle with high efficiency, reduce the speckle contrast and interference patterns, have a simple driving and packaging scheme and handle high optical power. A device with a 5x5 mm2 optical surface is designed, fabricated and characterized. The mirror is made of a thin and large deformable a-Si membrane which is supported by an array of posts. The disposition of the posts determines how the membrane is deformed in smaller sub-reflecting elements. A periodic and a pseudo random arrangements of the post arrays are designed. Electrostatic actuation is used to deform the membrane. The whole v deformable mirror is fabricated over a scanning stage. Out of plane resonant comb drive actuators are used to actuate the scanning stage. The diffusion of the light is performed by the deformation of the membrane while the homogenization of the beam profile is accomplished by the simultaneous scanning of the whole deformed membrane. Optical simulations are successfully used to predict the optical properties of the diffuser and to set the requirements for the electro-mechanical part of the device design. The fabrication process is using parylene-C as a refilling material and as a sacrificial layer to enable the fabrication of a 100 % fill factor deformable mirror. The membrane is continuous and release hole free. It is aluminum coated and showed an absolute optical power efficiency of 81 % in the actuated state. The high optical power handling of the device could not be tested. Nevertheless , the material of the device is silicon and the stage is connected with large springs to the frame for high heat conductivity. The pseudo random devices showed ability to shape an incoming beam regardless of its intensity profile into a output beam having a Gaussian profile. Besides, the diffusion angle is small and tunable between 0 to 10 mrad FWHM. The periodic devices is able to transform an incoming Gaussian beam into a super Gaussian intensity profile. Finally, the dynamic diffuser is capable of reducing the speckle contrast of a laser beam projected on a diffusing screen. The speckle contrast is reduced from 0.7 to 0.21 using the device alone and it is reduced to 0.11 when the dynamic diffuser is used in combination with a static diffuser. Tunable diffusion angle, smoothing of interferences, tunable beam shaping and speckle re- duction is realized while keeping the degree of freedom of the device to a minimum. By taking advantage of the optical design of the pseudo random diffuser, only one control electrode is needed to deform the membrane and one signal is used to scan the diffuser. The result is a very compact device, simple actuation and a straightforward packaging schemes. The last device presented is a very small footprint speckle reduction device. A simple actuation scheme using comb drive actuators and four folded springs enable large in plane displacement of a random phase plate. 180 μm of in plane displacement is achieved with a driving voltage as low as 29 V. The speckle contrast is reduced to 0.21 when the device is translating at 314 Hz. A comprehensive study of the diffuser characteristics is made to understand the surface parameters of the diffuser that influence directly the performances on the speckle reduction. The surface profile of the binary height has to be well defined to insure a phase shift of π. This very small device is a good candidate for integration in any portable device where speckle reduction is needed.

EPFL2012

Reliable MEMS Actuators for Driving Large Microsystems

Nicolas Bernard Golay

Nowadays many consumer electronic devices are using small engines (micromotors). Thanks to technological innovation, the components can be imbedded into numerous portable systems. The key factors for a nomadic integration are: reduction of power consumption, reliability, durability and manufacturing cost. From the perspective of an always denser integration, conventional fabrication techniques of motors are reaching their limits (windings, size of the permanent magnet). The technologies developed for the micro-electronics (thin film processes) offer new possibilities to create microscopic size motors. The potential for innovation in this field being very wide, many new concepts are based on this technology. Lots of research centers are trying to find new principles of actuation to increase the overall efficiency of motors. With the challenge of replacing a small "classical" motor by a micromotor based on thin film technologies, the electrostatic actuation is going to have a great potential in the future. Many problems are related to the compatibility of both technologies in terms of coexistence. This is mainly due to problems of dimensions and associated tolerances. The challenge of this research was to propose a solution for the creation of a micromotor including the following concepts: usable mechanical interface between an outer gear and the mechanical output torque of the micromotor (about 0:5 µNm), moderate energy consumption, relatively small chip size in relation to the required performances and finally ability to be easily implemented in a mechanism (module). This thesis presents the different aspects of the concept, design, simulation, fabrication and characterization of a MEMS based motor using silicon micro-technologies. The micromotor is constituted of a fixed base (stator) with an electrostatic actuator (force generator), a rotor and a protective cover (encapsulation). The principle of the electrostatic actuation is based on a comb inter-digitized configuration. A first interface (ratchet system) was developed to allow for the conversion of the back and forth movement of the linear type actuator to a circular motion of the silicon rotor. A challenge was to create an actuator able to deliver an electrostatic force of 500 µN with a applied potential of 40 V. To obtain this force, the actuator itself has an active area of over 10 mm2. The first concept has two large actuators representing 60% of the total area of the stator. In the second concept, the occupied area of the three actuators represents 40 % of the total area of the stator. Two systems were created during this work, based on the same principles of actuation and of actuator/rotor interfacing. The first system uses a crank mechanism to interface the silicon rotor and a metallic pinion. The encapsulation was achieved via a metal support on which a plastic lid was fastened. These two pieces were fabricated by conventional machining (milling) techniques. The micromotor module was made by assembling the different components. The second system was made based on a direct interface type (direct link between internal teeth of the silicon rotor and outer teeth of a metallic gear). The encapsulation concept uses only micro-technologies. The cover was made of a glass substrate on which a silicon spacer was bonded (anodic bonding process). The motor module was then achieved by assembling a silicon base (stator), a silicon rotor and a silicon/glass cover. In accordance with the specifications, the obtained results are very promising. The electrostatic force generated by the actuator is in the order of few hundreds of micronewton. The power consumption is in the order of few microwatts. Many innovative concepts have been studied and manufactured. Some of those have been patented, showing the interest of these new ideas. Experiments have demonstrated the functionality of a MEMS engine.

EPFL2010

Large Hybrid High Precision MEMS Mirrors

Dara Zaman Bayat

This thesis presents the design and microfabrication technology of a tip-tilt, 2 degree of freedom "mirror system" made out of Silicon On Insulator (SOI) wafers with two movable mirrors that have diameter of 10 mm. The system was intended for precise beam steering applications and tracking for inter-satellite telecommunication. Implementation of two mirrors allowed one mirror to have large static mechanical scan angles (±3.5°) and the other to have fast fine pointing capabilities within ±0.2° static mechanical scan angle. The first mirror was magnetically actuated and has a resonance frequency of 200 Hz and the large rotation angle. The second mirror was designed to use either electrostatic actuation or electromagnetic actuation. The resonance frequency is 1 KHz. The main objective of this thesis was the microfabrication of large mirrors (10 mm) and actuators. The study of upper limits of achievable resonant frequencies in combination with static deflection was another major aim of the project, as well as optimization of the flatness of the mirrors. The undertaken research presents a study of different non-contact actuation schemes for use in MOEMS devices, namely electrostatic and magnetic actuation schemes. Determination of the best magnetic actuation scheme for use in this thesis was carried out according to the design constraints and size limitations. It was concluded that the best magnetic actuation scheme consists of a moving magnet and a microfabricated stationary coil. This scheme created high force and the stationary microfabricated coils could easily dissipate heat through conduction into a heat sink. This configuration was of great advantage in space where heat convection does not exist. The considered electrostatic schemes included parallel plate actuators and vertical comb actuators. The vertical comb actuator was chosen to be fabricated due to its scalability that allows the microfabrication process to be easily adapted to smaller or larger devices. The microfabrication and implementation of the magnetically actuated mirrors was proven to be very straightforward resulting in high yield and uniformity of both the high and low frequency devices. The microfabrication of the electrostatic actuator was also achieved successfully. The operation performance of the electrostatically actuated mirror was not as satisfactory as the magnetically actuated mirrors. It was concluded that electrostatic actuation might be more suited to smaller devices, rather than the devices designed in the present study. The fabrication and actuation performance of the components of a "mirror system" that combines fast and large angle actuation was successfully demonstrated in this thesis. Very good mirror flatnesses were also obtained with RMS flatness of λ/10 for near infrared wavelengths. This project shows the general advantage of magnetic actuation for large MOEMS devices with large actuation ranges and proves the feasibility of their fabrication. Potential applications of these devices are robotic 3D vision, imaging LIDARs (Light Detection And Ranging), docking sensors and inter-satellite laser communications.

EPFL2011