Analog Spatial Light Modulators Based on Micromirror Arrays

The Fraunhofer Institute for Photonic Microsystems (IPMS) has been developing and manufacturing micromirror arrays for more than 20 years. While originally focusing on applications related to microlithography and therefore mainly for light in the deep ultraviolet range, the range of applications has been expanded since, including applications in the visible and near-infrared range. This paper gives an overview of the devices and their designs, fabrication, and characterization.

Development of optical MEMS-based micromirror arrays on flexible substrate for curvilinear surfaces

This research work is targeting to demonstrate that micromirror arrays can also be fabricated successfully on flexible substrates—not only on glass substrates. The paper reports on the technological data of a successful process on a flexible substrate for curvilinear surfaces. Polyethylene naphthalate substrate was selected for its low cost, strong resistance against chemicals which are used in fabrication process and sufficient optical transmission for the visible and near infrared spectrum. Finally, electrostatic actuation of the fabricated micromirror structures is proven experimentally.

Study of Residual Stress Compensation in Continuous Membrane Micromirrors Based on Surface Micromachining Processes

Residual stress is one of the key factors that directly determines the optical quality of micro-optical devices. With the same residual stress, the larger the aperture is, the worse the optical quality is. Therefore, continuous micromirrors are more affected by residual stress than segmented micromirrors. However, due to the complexity of boundary conditions, the influence of residual stress in segmented micromirror arrays on the device performance has been widely investigated in theory and practical applications, but only a few research results about the influence of residual stress in the continuous micromirror arrays have been reported. In this work, the residual stress both in continuous and segmented micromirror arrays is analyzed and summarized, then an accurate model for continuous micromirrors is developed. Compared with the existing models, it combines two additional factors, layer plate and point supported boundary conditions. Based on the proposed model, the change of critical stress of continuous micromirrors induced by different thicknesses of residual stress compensated membrane is theoretically investigated. Finally, the compensating experiment has been carried out, and the results show that the optical quality of micromirror can be remarkably improved, almost two orders of magnitude, with the introduction of residual stress compensation.

Planar free-standing metal layer fabrication: implementing sub-structures in micromirror arrays for light steering applications

We present a method to fabricate planar metal layers to be used as micromachined mirrors. Released mirrors of pure metal involve severe stress and reveal specific challenges to obtain planar mirror structures. Introducing sub-structures generating corrugated patterns, the metal mirror layers can be mechanically stabilized and undesired mirror bending can be reduced. For our investigations we used different arrangements of line structures on our metal mirrors, such as a group of straight or curved lines oriented differently. Comparing all the implemented different designs, planar micromirrors were achieved via sub-structures with a combination of straight lines arranged orthogonally to a single line. These planar micromirrors allow steering of the incident light by reflection and adjustment of the window transmittance. The presented low-cost method is suitable for large area fabrication of micromirror arrays, but also can be customized for other applications, where planar free-standing metal layers are required.