Micromirror based spatial light modulators (SLMs) developed by the Fraunhofer Institute for Photonic Microsystems are well established in microlithography applications. Serving, e.g., as reflective, programmable photomasks in deep-UV mask writers, they enable highly flexible pattern generation. During operation, the micromirror bow significantly impacts contrast and the resolvable feature size of generated patterns. In some situations, MEMS micromirrors tend to change their bow during laser irradiation. A test regime including a characterization unit for the in situ analysis of MEMS micromirror topology has been developed to measure the bow change under various irradiation conditions. Experiments in which SLMs were irradiated by a 1-kHz, 248-nm pulse laser revealed that mirror bowing can occur in both directions (concave and convex). The bowing direction is dependent upon the applied irradiation parameters such as pulse-energy density, pulse number, and the deposited energy. Sustained irradiation at energy densities exceeding a certain limit can potentially become a limiting factor for the resolvable feature sizes of the patterns generated and, therefore, for the usable SLM lifespan.
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 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
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.
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.
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 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.
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.
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 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.
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.
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.