An overview of advances in MOEMS devices and technologies for high quality imaging systems is provided. A particular focus is laid on recent technological further developments possibly opening gateways to unprecedented device and system functionality by e. g.: increase of pixel count towards higher parallel operation, decrease of the mirror pitch in large arrays towards applications like high-performance holography, novel technologies for higher operation bandwidth, increase of aperture size for scanning applications like LIDAR, integration of high reflection coatings for processing of multi Watt laser radiation for marking and engraving, and phased arrays for high speed laser beam steering.
Micromirror arrays (MMA) are spatial light modulators (SLM) used in a wide variety of applications for structured light manipulation <i>i.e.</i> structured illumination microscopy.<p> </p> In our setup, we use a combination of two micromirror arrays, which allow not only to spatially structure the light in the field of view, but also to control the direction and angle of the incident light. In order to achieve this, a first MMA is imaged in the focal plane and used as a black and white (or even greyscale) mask. With a fully illuminated objective, this image would normally be formed from the complete light cone. By imaging the second MMA onto the backfocal plane of the objective only a portion of the light cone is used to form the image. This enables avoiding the unwanted illumination of out of focus objects. The MMAs in our setup consist of an array of 256x256 micromirrors, that can each be individually and continuously tilted up to 450nm, allowing the creation of greyscale images in real time in the illumination pattern. The mirrors themselves can be tilted for times as short as 10μs up to several seconds. This gives unprecedented control over the illumination times and intensities in the sample. Furthermore, our enhanced coating technology yields a high reflectivity over a broad optical spectrum (240- 1000nm). <p> </p>Overall, the setup allows targetted illumination of subcellular regions enabling the precise, localized activation of optogenetic probes or the activation and deactivation of signaling cascades using photo-activated ion-channels.
Diffractive micromirror arrays (MMA) are a special class of optical MEMS, serving as spatial light modulators (SLM)
that control the phase of reflected light. Since the surface profile is the determining factor for an accurate phase
modulation, high-precision topographic characterization techniques are essential to reach highest optical performance.
While optical profiling techniques such as white-light interferometry are still considered to be most suitable to this task,
the practical limits of interferometric techniques start to become apparent with the current state of optical MEMS
technology. Light scatter from structured surfaces carries information about their topography, making scatter techniques
a promising alternative. Therefore, a spatially resolved scatter measurement technique, which takes advantage of the
MMA’s diffractive principle, has been implemented experimentally. Spectral measurements show very high contrast
ratios (up to 10 000 in selected samples), which are consistent with calculations from micromirror roughness parameters
obtained by white-light interferometry, and demonstrate a high sensitivity to changes in the surface topography. The
technique thus seems promising for the fast and highly sensitive characterization of diffractive MMAs.
Fraunhofer IPMS has developed a one-dimensional high-speed spatial light modulator in cooperation with Micronic
Mydata AB. This SLM is the core element of the Swedish company’s new LDI 5sp series of Laser-Direct-Imaging
systems optimized for processing of advanced substrates for semiconductor packaging. This paper reports on design,
technology, characterization and application results of the new SLM. With a resolution of 8192 pixels that can be
modulated in the MHz range and the capability to generate intensity gray-levels instantly without time multiplexing, the
SLM is applicable also in many other fields, wherever modulation of ultraviolet light needs to be combined with high
throughput and high precision.
Facing the recent developments in the area of (quasi) continuous wave lasers towards higher power the Fraunhofer IPMS
introduces a novel light modulator incorporating an innovative architecture optimized for high laser power applications
requiring a fast device. As a novelty each pixel is composed of a number of micro mirrors, aligned in a row. That
approach allows for, in principle, very long pixels with uniform surface properties. This concept in turn results in
reduction of power density at the light modulator surface and hence opens the way to high power applications allowing
power densities in the range of several ten W/cm<sup>2</sup> at the light modulator surface. Each pixel can be switched to black,
white or even arbitrary gray values with very high speed. This paper summarizes the device design, working concept,
mechanical properties for both static and dynamic operation, and surface properties. Application relevant subjects as
stability under intense laser illumination complete the discussion.
Spatial light modulators (SLM) developed at the Fraunhofer Institute for Photonic Microsystems (Fraunhofer IPMS) are
based on arrays of tiltable micro mirrors on a semiconductor chip. Development and optimization of such complex micro-
opto-electro-mechanical systems (MOEMS) require detailed knowledge of the device behaviour under application
specific operating conditions. In this context, the need for a high resolution surface topography measurement under laser
exposure (in situ) was identified, complementing ex situ characterizations where laser exposure and micro-mirror topography
measurements are carried out sequentially. For this purpose an interferometric setup using the phase-shift principle
was designed and is presented in this paper. For setup verification SLMs were irradiated at 248 nm (KrF) with energy
densities of up to 10 mJ/cm2. In general, the setup is neither limited to a specific illumination wavelength nor to micromirrors
as structures under test. Influences of different illumination parameters such as energy density, laser repetition
rate etc. on the mirror topography can be studied in detail. Results obtained so far reveal valuable feedback for further
technological optimization of mirror array devices.
The present article discusses an optical concept for the characterization of diffractive micromirror arrays (MMAs) within
an extended wavelength range from the deep ultra-violet up to near-infrared. The task derives from the development of a
novel class of MMAs that will support programmable diffractive properties between 240 nm and 800 nm. The article
illustrates aspects of the achromatic system design that comprises the reflective beam homogenization with divergence
control and coherence management for an appropriate MMA illumination as well as the transfer of phase modulating
MMA patterns into intensity profiles for contrast imaging. Contrast measurements and grey scale imaging demonstrate
the operation of the characterization system and reflect the encouraging start of technology development for
multispectral, diffractive MMAs.
A new generation of micromirror arrays (MMAs) with torsional actuators is being developed within the European
research project MEMI in order to extend the usable spectral range of diffractive MMAs from deep ultraviolet into the
visible and near infrared. The MMAs have 256 x 256 pixels reaching deflections above 350 nm at a frame rate of 1 kHz,
which enables an operation in the target wavelength range between 240 nm and 800 nm. Customized driver electronics
facilitates computer controlled operation and simple integration of the MMA into various optical setups. Tests in the
visible wavelength range demonstrate the functionality and the high application potential of first MMA test samples.
The Fraunhofer Institute for Photonic Microsystems (IPMS) develops and fabricates MOEMS micro-mirror arrays for a
variety of applications in image generation, wave-front correction and pulse shaping. In an effort to extent the
application range, mirrors are being developed that withstand higher light intensities.
The absorbed light generates heat. Being suspended on thin hinges, and isolated from the bulk by an air gap, the mirrors
heat up. Their temperature can be significantly higher than that of their substrate.
In this paper we describe an experiment carried out to verify simulations on the temperature within the mirror plates
during irradiation. We created a structure out of electrically connected mirror plates forming a four-point electrical
resistor, and calibrated the thermal coefficient of the resistor in a temperature chamber. We irradiated the resistor and
calculated the mirror temperature.
In the experiment, the temperature in the mirror plates increased by up to 180 °C. The mirrors did not show significant
damage despite the high temperatures. Also, the experiment confirms the choice of heat transport mechanisms used in
the simulations. The experiment was done on 48 μm x 48 μm mirrors suspended over a 5 μm air gap, using a 355 nm
solid-state laser (4 W, up to 500 W/cm<sup>2</sup>).
The Fraunhofer IPMS, in cooperation with Micronic Laser Systems, develops and fabricates micromirror arrays used as
spatial light modulators (SLM) for image generation in microlithography. The SLMs used consist of 2048×512
individually addressable micromirrors of 16×16μm<sup>2</sup> and can be operated in an analog mode at a frame rate of up to
2 kHz. There are continued efforts to improve the performance of the mask writers with respect to stability and CD
uniformity, which include measures to improve the SLMs used, especially with respect to the optical quality and the
Therefore, a new technology has been introduced which allows to use different materials for the mechanical suspension
and the mirror, thus optimizing them separately. The hinges are made of a thin layer of a material with very good creep
resistance, while the mirrors consist of a thick aluminium alloy with high reflectivity in DUV. Furthermore, the same
inorganic material is used for the planarization of the electrodes (by means of chemical mechanical polishing) and as
sacrificial layer for the actuator fabrication. Thus, at the end of the process, all sacrificial material, including that
between the electrodes is removed. In this way, the charging effects caused by dielectrics between the electrodes (as seen
in the previous devices) are eliminated.
The first devices using the technology described above have been fabricated and tested. The first tests in a lithography
machine show that considerable improvements in machine performance can be expected. The next steps are to stabilize
and optimize the process.
We describe charging effects on spatial light modulators
SLM. These light modulators consist of up to one million mirrors that
can be addressed individually and are operated at a frame rate of up to
2 kHz. They are used for deep ultraviolet DUV mask writing where they
have to meet very high requirements with respect to accuracy. To be
usable in a mask-writing tool, the chips have to be able to work under
DUV light and maintain their performance with high accuracy over a long
period of time. Charging effects are a problem frequently encountered
with MEMS, especially when they are operated in an analog mode. In
this work, the issue of charging effects in SLMs used for microlithography,
their causes and methods of their reduction or elimination, by
means of addressing methods as well as technological changes, is
discussed. The first method deals with the way charges can accumulate
within the actuator. It is a simple method that requires no technological
changes but cannot always be implemented. The second involves
the removal of the materials within the actuator where charges
The present article discusses steps for the realistic description of optical properties of micro-mirror arrays (MMA),
which are utilized as programmable masks for microlithography. The article focuses on global contrast as an
elementary example for the understanding of MMA's diffractive operation principle. Central point will be a
discussion of those MEMS properties that influence the global MMA contrast, and how to introduce them into
simulation. Surface corrugations of single mirrors and slit properties will be taken into account. Comparison is
made with experimental contrast data to validate the theoretical assumptions.
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.
This paper describes charging effects on spatial light modulators (SLM). These light modulators consist of up to one
million mirrors that can be addressed individually and are operated at a frame rate of up to 2 kHz. They are used for
DUV mask writing where they have to meet very high requirements with respect to accuracy.
In order to be usable in a mask-writing tool, the chips have to be able to work under DUV light and maintain their
performance with high accuracy over a long time. Charging effects are a problem frequently encountered with MEMS,
especially when they are operated in an analog mode.
In this paper, the issue of charging effects in SLMs used for microlithography, their causes and methods of their
reduction or elimination, by means of addressing methods as well as technological changes, will be discussed. The first
method deals with the way charges can accumulate within the actuator, it is a simple method that requires no
technological changes but cannot always be implemented. The second involves the removal of the materials within the
actuator where charges can accumulate.
The Fraunhofer Institute for Photonic Microsystems (IPMS) Dresden (Germany) has developed a one megapixel SLM device with 512 x 2048 individually addressable tilting micromirrors optimized for micro-lithography applications. Besides many other chip parameters SLM surface planarities strongly determine the pattern performance on the
lithography masks. This paper presents results on the so called Global Flatness (GF) of the chips which takes into account the complete active area of the large mirror matrix. A description and definition of GF is presented, followed by measurement results on GF prior to chip packaging. Different adhesives are tested for the die bonding process. Bonding on flat test substrates enables the separation of different influences on GF. Impacts of the topography of the die attach and of die bonding process parameters on GF are investigated, optimization potentials such as different dispense modes are tested and discussed. Influences of the chip layout on GF are evaluated. A transfer of Global Flatness measurements to other large area optical chips, e. g. image sensors, is outlined. In the future GF improvements are expected to gain importance due to the ever increasing requirements on CD and CD uniformity performance of mask writers.
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.
Aluminum alloy beams having the same width but different lengths were made with semiconductor fabrication methods at the IPMS. The beams are clamped on one end with posts to the underlying plane. They are illuminated with 248 nm UV radiation created by an excimer laser and the bending was investigated in dependence on energy density and repetition rate. This behaviour is important for the development and the operation of MOEMS structures when used in UV applications. Ultraviolet radiation is used for lithography as well as material processing. The light-material interaction is well investigated for high energy densities (Ed) which are used for drilling holes or ablation of materials. In this work the influence of 248 nm low energy pulses (Ed < 200 μJ/cm<sup>2</sup>) on thin beams of aluminum alloys having a thickness of several hundreds of nanometer is analyzed. The frequency of the laser radiation is varied from 1000 to 2000 Hz. The beams have different lengths and are clamped on one end. Before illumination the beams are planar, after illumination the beams show a curvature which is related to internal stresses. The amount of curvature is dependent on the geometry of the beam, the energy density and the repetition rate of the radiation pulses. Also the relaxation behaviour of the curved beam is examined, i. e. the curvature change after the end of irradiation. The results will help to predict the practicability of materials for MOEMS in UV applications (mirror structures) and to understand their behaviour.