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/cm2).
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/cm2) 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.
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.