Jan-Uwe Schmidt

Advances in SLM development for microlithography

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 16×16μm2 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 stability. 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.

Technology development for micromirror arrays with high optical fill factor and stable analogue deflection integrated on CMOS substrates

At Fraunhofer IPMS Dresden micromechanical mirror arrays are developed and fabricated using a high-voltage CMOS process for applications such as lithographic mask writers and adaptive optics. Different approaches for the fabrication of micromechanical mirror arrays with up to 1 million analogue addressable pixels in a MEMS-on-CMOS technology are discussed: sacrificial layer technologies of 1-level actuators made from a single Al-TiAl-Al structural multilayer or 2-level actuators with an additional TiAl hinge layer respectively. Also the fabrication of single crystalline Si micro-mirrors using layer-transfer bonding is discussed.

Amorphous TiAl films for micromirror arrays with stable analog deflection integrated on complementary metal oxide semiconductors

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.

One Megapixel SLM with High Optical Fill Factor and Low Creep Actuators

A one megapixel micromirror based SLM with two layer torsional mirror actuators was monolithically integrated on CMOS. The mirrors have a high optical fill factor and a very low level of mechanical creep when actuated

High-speed one-dimensional spatial light modulator for Laser Direct Imaging and other patterning applications

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.

Technology development of diffractive micromirror arrays for the deep ultraviolet to the near infrared spectral range

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.

Calibration of diffractive micromirror arrays for microscopy applications

We report on our investigation to precisely actuate diffractive micromirror arrays (MMA) with an accuracy of λ/100. The test samples consist of analog, torsional MEMS arrays with 65 536 (256x256) mirror elements. These light modulators were developed for structured illumination purposes to be applied as programmable mask for life science and semiconductor microscopy application. Main part of the work relies on the well known characterization of MEMS mirrors with profilometry to automatically measure and approximate the MMA actuation state with high resolution. Examples illustrate the potential of this strategy to control the tilt state of many thousand micromirrors within the accuracy range of the characterization tool. In a dynamic range between 0 and >250 nm the MMA deflection has been precisely adjusted for final MMA application in the deep-UV - VIS - NIR spectral range. The optical properties of calibrated MMAs are tested in a laser measurement setup. After MMA calibration an increased homogeneity and improved image contrast are demonstrated for various illumination patterns.

Multispectral characterization of diffractive micromirror arrays

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.

One-dimensional light modulator

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/cm2 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.