Applications like computer generated holography or wavefront shaping call for spatial light modulators (SLMs) with millions of phase-shifting pixels of only a few micrometers size which are precisely addressable in an analogue way at frame rates of many kHz. While liquid crystal on silicon (LCoS) devices are commonly used, they can't offer very high frame rates and their polarization effect is often considered a drawback. On the other hand, MEMS devices with phaseshifting micro mirrors are not readily available but have a high potential to fulfill the requirements. It is quite challenging to reach the desired deflection range with the limited available addressing voltages. For symmetry usually at least two springs are squeezed into the tight space the small pixels offer. This paper presents an innovative way to get a good sensitivity by using a single hinge per pixel that therefore can be made especially weak. It might be astonishing but there actually are various ways to design electrostatic actuators that deliver pure piston movement with only one spring. We discuss the basic concepts of such force-balanced single spring actuators and give a variety of examples and guidelines to tune a basic pixel layout for pure piston motion. Simulations show the tilt-free response and the stability against tilting torques that might be the result of possible imperfections in manufacturing, like lithographical overlay errors or stress gradients. We also show the possibility to dynamically balance the pixel for a pure piston first eigenmode.
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.
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.
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.
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.
The Fraunhofer IPMS and Micronic Laser Systems AB have developed a technology for microlithography using spatial light modulation (SLM). This technology uses an array of micromirrors as a programmable mask, which allows parallel writing of 1 million pixels with a frame rate of up to 2 kHz. The SLM is fabricated at the IPMS using its high-voltage CMOS process. The mirrors are fabricated by surface micromachining using a polymer as sacrificial layer. The mirrors are operated in an analog mode to allow sub-pixel placement of pattern features. This paper describes the function of the SLM with an emphasis on the stability of the mirror deflection and a method to improve it which has been implemented.
The Fraunhofer IPMS and Micronic Laser Systems AB have developed a technology for the maskless DUV microlithography using spatial light modulation (SLM). This technology uses an array of micromirrors as a pro-programable mask, which allows writing up to 1 million pixels with a framerate of up to 2 kHz. The SLM is fabricated at the IPMS using its high-voltage CMOS process. The mirrors are fabricated by surface micromachining using a polymer as sacrificial layer. The mirrors are operated in an analog mode to allow sub-pixel placement of the features.
The Fraunhofer Institute for Microelectronic Circuits and Systems (FhG-IMS) has developed spatial light modulators (SLM), which are used in a pattern generator for DUV laser mask writing developed by Micronic Laser Systems. They consist of micromirror arrays and allow massive parallel writing in UV mask writers. The chip discussed here consists of 2048 × 512 individually addressable mirrors and can be run at a frame rate of 1 to 2 kHz. For this application it is necessary that the SLMs can be operated under DUV light without changing their performance. This paper discusses a failure mechanism of the SLMs when operated in DUV light and countermeasures to eliminate this effect.
The Fraunhofer IMS in Dresden is developing and fabricating spatial light modulators (SLMs) for micro lithography with DUV radiation. The accuracy of analog modulation is very important for the resulting accuracy of the generated features. On the other hand, fabrication tolerances create variations for example in spring constant, zero voltage deflection, and reflectivity. The slightly different response curves of the individual pixels therefore require an individual calibration. The parameters of these are stored in a look-up table so that the proper addressing voltage for the required optical response can be selected. As the deflection angle as well as the size of the SLM pixels are quite small, a direct measurement of the pixel response is not straightforward. An optical system similar to the one in the lithography machine has been set up, where the SLM is operating as a phase grating and the image is generated by a spatial filter. The pixel deflection can be calculated from the aerial image for isolated deflected pixels. The background pixels, that are not calibrated yet, contribute some error to this calculation. However, this error is not very large. Simulations regarding the accuracy of this measurement are discussed, and experimental results are shown.
Modern UV-lithography is searching for new highly parallel writing concepts. Spatial light modulation (SLM) offers such possibilities but special emphasis must be put on the ability of SLM devices to handle ultraviolet light (UV). We designed and fabricated micromirror arrays which fulfill these requirements. Possible applications for such UV-SLMs are direct writing systems for semiconductor and printing, and UV-stimulated biochemistry. For deep UV laser pattern generation (248 nm) e.g. we designed and fabricated a 2048x512 pixel UV-SLM with individually addressable aluminum micromirrors. They are illuminated by an excimer laser pulse and imaged onto a photomask substrate. A complete pattern is stitched together at a rate of 1 kHz. The minimum feature size is 320 nm and analog modulation of the pixels allows to realize an address grid of only 1.6 nm. The design of the array is modular so that other array sizes can be tailor made to customers needs. Design and fabrication aspects for a CMOS compatible realization of these micromirror arrays are addressed as well as their performance in lithography applications.
A new breed of pattern generators for photomasks using a new DUV spatial light modulator (SLM) technology is under development in a collaborative effort between Micronic Laser Systems AB, Taby, Sweden and the Fraunhofer Institute for Microelectronic Circuits and Systems (FhG-IMS), Dresden, Germany. Current pattern generator architectures using a limited number of scanning beams will not be able to support future production requirements with ever-increasing data complexity and resolution. The new SLM technology provides a means for high resolution and massive parallel exposure to alleviate these difficulties. There are many architectural similarities to that of a modern stepper and the technology can provide the resolution to rival that of e-beam pattern generators, yet with the productivity of laser patterning. In this paper we describe the architecture of an SLM exposure system, the SLM technology, and will consider key aspects for the intended application.
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.
Design and modeling aspects of torsional 1D and 2D Micro Scanning Mirrors are presented. During the oscillation of the mirror plate the inertial moment gives rise to a deformation of the plate. This dynamic deformation results in a defocusing of the reflected laser beam. Therefore, the scan frequency of a device with a given size of the mirror plate and deflection angle is limited. Further restrictions arise from the demanded mechanical robustness like resistivity against shock and torsional stress. This leads to a minimum eignefrequency of the device which in the case of a rectangular shaped is proportional to the width of the spring. To enable a single mode operation it is advantageous that the torsion around the springs is the lowest mode sufficiently separated from all others. A FEM-analysis has been carried out to determine the mode sequence of a 1D and a 2D Micro Scanner respectively. The analytical calculated eigenfrequencies agree well with the numerical determined. Taken into account the result of the analytical and numerical investigations 1D and 2D Micro Scanning Mirrors have been designed and fabricated. The mirror and the springs are defined in a 20 to 30 micrometers thick single crystal silicon layer. The results of the experimental investigations with respect to the shock resistivity and the long run behavior probe the suitability of the modeling.
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.