The development of devices that are based on MEMS-on-CMOS technology becomes increasingly time-consuming since System-on-a-Chip (SoC) solutions for highly integrated and miniaturized devices are approaching smaller feature sizes. In order to reduce the development costs and shorten the time-to-market periods, the combination of commercially available CMOS processes from foundries with the subsequent processing in a dedicated MEMS facility is beneficial. This concept offers the possibility to separate the different technological requirements of conventional CMOS manufacturing and MEMS actor processing, which may follow different design rules and process specifications. As a representative of the dedicated MEMS foundries, Fraunhofer IPMS performs surface micromachining on 200 mm wafers for a variety of MEMS devices, in particular for spatial light modulators (SLM). Over the past decade, much experience was gained in development activities for customer specific applications like micro mirror arrays. In this paper, we will discuss essential requirements and upcoming challenges for the monolithic integration of surface micro-machined optical MEMS on foundry-fabricated CMOS backplanes, as conventional (i-Line) lithography is approaching patterning limits. We will present approaches of tuning the planarization of the CMOS chip surface to achieve an excellent mirror array flatness with CMOS compatible inorganic sacrificial layer techniques. Concepts like Mix&Match lithography for achieving high overlay accuracy and the litho stitching technique for the patterning of large chips will be reviewed and a brief outline of our roadmap for the implementation of DUV lithography will be presented.
We developed a novel 512 x 320 tip-tilt micro mirror array (MMA) together with the entire related technology platform, including mirror fabrication process, integrated CMOS address circuitry and external drive electronics. The MMA itself consists of 2axis-tip-tilt actuators at 48μm pixel size, allowing a continuous pure tip-tilt motion up to 3.5° in arbitrary directions, fully calibratable at standard deviations of better than 0.025°. The mirrors are realized within a 2-level architecture defined by three structural layers, two for hinge and reinforcement suspension and one for the overlying mirror. They are fabricated by surface-micromachining within a fully CMOS compatible process. MMA programming is accomplished by an underlying CMOS backplane supporting drive voltages up to 27V and frame rates up to 3.6kHz.
Large micromechanical mirror arrays (MMA) with analog pixel deflection integrated onto active CMOS address circuitry require both high-quality planar reflective optical surfaces and a stable deflection versus voltage characteristic. However, for implementing a CMOS-compatible surface-micromachining process, certain obstacles such as a restricted thermal budget and a limited selection of suitable materials must be overcome. Amorphous TiAl is presented as a new actuator material for monolithical MEMS integration onto CMOS circuitry. TiAl films may be sputter deposited at room temperature, have an x-ray amorphous structure, and a low stress gradient. The glassy structure and high melting point make TiAl less vulnerable to stress relaxation, which makes TiAl an ideal spring material. One-level actuators with TiAl or Al-TiAl-Al structural layers and two-level actuators with separate TiAl spring and Al-alloy mirror layers were fabricated and tested with respect to their drift stability. The stability of TiAl-based actuators was found to be superior in comparison to one-level Al-alloy actuators. Two-level actuators with TiAl hinges emerge as the most promising design.
In order to demonstrate and to quantitatively evaluate the wavefront correction capabilities of a spatial light modulator
(SLM) for optical imaging enhancement in Adaptive Optics (AO) a compact and flexible demonstration system and test
bed has been developed. It basically consists of a projection system, where image objects of different complexity and
spatial resolution can be implemented and imaged through Adaptive Optics onto a CCD camera. Furthermore, static and
dynamic wavefront errors of different severeness can be introduced by means of fixed and rotating phase plates. With
this system for the first time the optical performance of the Fraunhofer IPMS 240 × 200 micro mirror SLM for highresolution
wavefront control has been characterized. For an incoherent or partially coherent imaging as employed in this
case the image quality normally is assessed in terms of the Modulation Transfer Function (MTF). Therefore, a
quantitative evaluation has been carried out by measuring the system MTF including the SLM for a number of spatial
frequencies as well as for a variety of different complex aberrations without and with applied correction. Besides a
description of the system set-up the obtained results on the imaging improvement and MTF measurement are presented.
The large-scale integration of analog operable MEMS micro-mirrors onto active CMOS address circuitry requires high
quality planar reflective optical surfaces but also a stable deflection vs. voltage characteristic. However, for
implementing a CMOS compatible surface micromachining process, certain obstacles like a restricted thermal budget
and a limited selection of suitable materials must be overcome. In this paper, amorphous TiAl is presented as a new
actuator material for monolithical MEMS integration onto CMOS circuitry at room temperature. Sputter deposited TiAl
has an x-ray amorphous structure and a low stress gradient. The missing long range order and the high melting point help
to virtually eliminate stress relaxation effects, i.e. TiAl hinges behave almost perfectly elastic. In a first study, 40 &mgr;m
wide piston mirrors have been implemented onto substrates with fixed wired address electrode arrays. The actuators had
a 300 nm TiAl core sandwiched between two layers of 25 nm Al. The devices reach a maximum deflection of about 500
nm at a dc voltage of about 23V. The drift-stability of the deflection has been tested at "worst case" conditions close to
the deflection limit. During 30 min of continuous deflection near 500 nm a mechanical drift below 25nm has been
observed. TiAl offers the perspective for actuators capable of a stable analog operation, which is essential to many
applications, such as adaptive optics.
Various applications in modern optics are demanding for Spatial Light Modulators (SLM) with a true analog light processing capability, e.g. the generation of arbitrary analog phase patterns for an adaptive optical phase control. For that purpose the Fraunhofer IPMS has developed a high-resolution MEMS Micro Mirror Array (MMA) with an integrated active-matrix CMOS address circuitry. The device provides 240 x 200 piston-type mirror elements with 40 μm pixel size, where each of them can be addressed and deflected independently at an 8bit height resolution with a vertical analog deflection range of up to 400 nm suitable for a 2pi phase modulation in the visible. Full user programmability and control is provided by a newly developed comfortable driver software for Windows XP based PCs supporting both a Graphical User Interface (GUI) for stand-alone operation with pre-defined data patterns as well as an open ActiveX programming interface for a direct data feed-through within a closed-loop environment. High-speed data communication is established by an IEEE1394a FireWire interface together with an electronic driving board performing the actual MMA programming and control at a maximum frame rate of up to 500 Hz. Successful application demonstrations have been given in eye aberration correction, coupling efficiency optimization into a monomode fiber, ultra-short laser pulse modulation and diffractive beam shaping. Besides a presentation of the basic device concept the paper will give an overview of the obtained results from these applications.
Electrostatic Micro-actuators are being increasingly used for a wide variety of applications such as spatial light modulators, scanning mirrors, optical cross connects, micro-valves, and others. Usually the electrical forces operate in one direction and are balanced by a mechanical spring. The resulting deflection is then either defined by a mechanical stop, or it is only a meta-stable equilibrium position: at an additional external force or deflection it will snap to a different position, frequently again defined by a mechanical stop. This issue is well known and is often called 'pull-in'. In the often used parallel-plate capacitor actuator, the instability already begins at a deflection of only on third of the original capacitor plate separation. For safety reasons and due to the steep response-curve one can only use an even smaller fraction of the mechanically possible movement. This means, that the gap below the actuator has to be designed very much larger than the required maximum deflection. To get the pre-described force and deflection, a much higher voltage is needed than for potential smaller gap widths. The useable range of deflection for many types of micro-actuators can be extended without the penalty of large drive voltage or low shock resistivity, by employing springs with steeper-than-linear restoring force. Alternatively, the voltage needed for a given range of deflection may be reduced. This paper shows the benefits and how to design and dimension these types of springs.
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.
The MEMS Phase Former Kit developed by the Fraunhofer IPMS is a complete Spatial Light Modulator system based on a piston-type Micro Mirror Array (MMA) for the use in high-resolution, high-speed optical phase control. It has been designed for an easy system integration into an user-specific environment to offer a platform for first practical investigations to open up new applications in Adaptive Optics. The key component is a fine segmented 240 x 200 array of 40 μm piston-type mirror elements capable of 400 nm analog deflection for a 2pi phase modulation in the visible. Each mirror can be addressed and deflected independently by means of an integrated CMOS backplane address circuitry at an 8bit height resolution. Full user programmability and control is provided by a newly developed comfortable driver software for Windows XP based PCs supporting both a Graphical User Interface (GUI) for stand-alone operation with pre-defined data patterns as well as an open ActiveX programming interface for a closed-loop operation with real-time data from an external source. An IEEE1394a FireWire interface is used for high-speed data communication with an electronic driving board performing the actual MMA programming and control allowing for an overall frame rate of up to 500 Hz. Successful proof-of-concept demonstrations already have been given for eye aberration correction in ophthalmology, for error compensation of leightweight primary mirrors of future space telescopes and for ultra-short laser pulse shaping. Besides a presentation of the basic device concept and system architecture the paper will give an overview of the obtained results from these applications.
Micro Mirror Arrays (MMAs) offer the potential of a high spatial and temporal resolution technology for wavefront control applications. In this paper a new Micro-Electro-Mechanical-System (MEMS) based MMA type is investigated. As opposed to most other MMA technologies which involve flip mirrors with only two possible orientations, this system can support two different mirror designs, piston-type mirrors for a continuous phase adjustment and tilt mirrors for light discarding purposes. The MMA's wavefront correction capabilities are being investigated in a breadboard which simulates continuous distortions and step errors, such as those that could be expected from lightweight primary mirrors of space telescopes or segmented mirrors, respectively. The wavefront is corrected by the MMA, then coupled into a monomode fiber. Four different correction methods have been tested, two stochastic approaches, a closed-loop Shack-Hartmann approach and an interferometric approach. Comparison of coupling efficiency is made between these approaches and against theoretical calculations.
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.
For the correction of the human eye´s higher order aberrations in vision science we developed a new micromirror device with an monolithically integrated active CMOS address matrix providing a fine segmented array of 240 × 200 mirror elements across an active area of 9.8 × 8.0 mm2. The micromirrors possess a piston-type architecture for a pure phase shifting capability and are fabricated by means of aluminum surface-micromachining. Using a basic pixel size of 40 × 40 μm2 a mechanical stroke of at least 450 nm is obtained at address voltages below 30V, which is suitable for both active matrix addressing and a modulo 2π phase correction in the visible. Furthermore, an active CMOS address matrix similar to a DRAM was developed providing one switching transistor and one storage capacitor for each mirror cell. Those devices were fabricated within a special high voltage CMOS process providing a full analog address capability of up to 30V at an 8 bit resolution defined by the external driving board. Using interferometric surface profile and laser vibrometer measurements we will present latest experimental results of the mirrors’ electromechanical properties. For the first time those micromirror devices now also have been implemented into an ophthalmic diagnosis system for the measurement and correction of the human eye’s wave aberrations. Therefore, first results of the obtained aberration reduction as well as the impact on vision enhancement will be presented.
For an enhanced wavefront correction in Adaptive Optics especially in the case of high-order aberrations we developed a new monolithic integrated micromirror device providing a dense array of 240 x 200 piston-type mirror elements on top of an active CMOS address matrix for a closer wavefront approximation. After an analytical and numerical modeling the micromirrors were designed and fabricated by means of aluminum surface-micromachining. Using a basic pixel size of 40 x 40 micrometers 2 a mechanical stroke of at least 450 nm could be achieved at address voltages below 30V, which is suitable for both active matrix addressing and a phase correction modulo 2p in the visible. This also includes the option of an incremental increase of the actual mirror size in units of the address grid in order to allow for an extended analog deflection range. Furthermore, we designed and fabricated an active address matrix using a special high voltage CMOS process providing a full analog capability for address voltages up to 35V. Thereby, also a special light shielding as well as chemical mechanical polishing (CMP) for a high surface planarization have been incorporated. The completed devices were experimentally characterized by surface profile measurements using white light interferometry. After determining the deflection characteristic we successfully demonstrated the analog operation capability by programming different height patterns into the matrix at an 8 bit resolution provided by the external driving board.
For wavefront correction in adaptive optics mirror devices are required, which provide a pure phase shift capability with a fine partition over the optical cross section. For this purpose we investigated arrays of piston-type micromirrors. In order to predetermine the basic deformation characteristics and to estimate appropriate design parameters a simple analytical model was derived. We then designed and fabricated arrays of different mirror elements on top of passive address devices by means of surface-micromachining realizing pixel side lengths of 75, 100, 120 and 150 micrometers . Experimental investigations of the electromechanical behavior were done by surface profile measurements using white light interferometry, which reveals a good overall functionality of the mirror arrays. Furthermore, an analog deflection range of up to 1.2 micrometers at address voltages below 32V were obtained together with a load dependent height level accuracy of 80 to 100 nm.
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.
We report on a new system for submicron lithography by fast laser direct writing, where a programmable phase modulating spatial light modulator (SLM) is imaged onto the wafer using flash on the fly exposure with an excimer laser light source. This new principle for maskless optical lithography has been investigated for the first time. A SLM with 512 X 464 pixels has been developed and fabricated using a CMOS active matrix and a reflective, deformable viscoelastic layer on top. Using this light modulator for image generation a demonstrator exposure tool for 0.6 micrometers minimum feature size has been designed and set up including all the components necessary for the exposure of a complete lithographic layer from CAD layout data. The demonstrator is shown to give good quality photoresist pattern on the wafer at a throughput of roughly one 4"-wafer per hour. Based on our experimental results we propose a tool with a throughput of nine 6"-wafers per hour and conclude that the new principle of operation has the potential of high performance optical direct writing lithography.