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Static and Dynamic Optical Metrology of Micro-Mirror Thermal Deformation

C.R. Forest, P. Reynolds-Brown, O. Blum Spahn, J. Harris, E. Novak, C.C. Wong, S. Mani, F. Peter and D. Adams
Sandia National Laboratories, US

Keywords: stroboscopic interferometry, MOEMS, optical switch, thermo-mechanical behavior

Abstract:
A two-position micro-optical switch is being developed at Sandia National Laboratories. This MEMS device is shown in Figure 1 as a gear with a through-hole; the actuation mechanism is not shown. In the ''on'' position, incident radiation from a laser reflects from the gear surface to a detector as in Figure 2a. Figure 2b illustrates the ''off'' position, in which rotating the gear 180? allows the radiation to pass through the hole in the gear and a via in the underlying substrate. The design is desirable since physical gear absence results in an ''off'' switch state. Previous work on the design and performance of this device has been reported [1]. Incident infrared laser radiation at 850 nm is transmitted, absorbed, and reflected. The absorbed component can cause local heating. The resulting thermal expansion and deformation creates undesirable mechanical and optical performance, such as spurious reflections or binding between the gear and hub, which can prevent actuation. Understanding the heat transfer model for this system is crucial to mitigating these concerns. A thermo-mechanical heat transfer model in which absorbed radiation results in predictable deformation can be used to redesign the switch to prevent binding by minimizing deformation or moving deformation to an area where there is no performance impact. This model must be validated with empirical data. Therefore, we have pursued metrology of the deformed switch. This work, specifically, concerns the measurement of elastic and plastic deformation of the optical switch and its surface temperatures as a function of incident optical powers in order to create and validate a suitable heat transfer model. To measure the thermal deformations, a Wyko optical profilometer [2] utilizing a Mirau interferometer was selected for its high spatial resolution, accuracy, and repeatability. Static measurements of the switch were taken before heating and after the switch thermal response had reached steady state to determine the threshold optical power for plastic deformation and corresponding deformation amplitude (See Figure 3). These tests provided the boundary conditions for a purely elastic thermal response. Stroboscobic interferometry [3] was then utilized to capture the transient thermal heating and cooling behavior, which have time constants on the order of three milliseconds and amplitudes of hundreds of nanometers. These novel observations of repeatable, transient MEMS thermal response provide an essential contribution to modeling the system. In addition, these types of experiments can help us to characterize thin film properties better and how thin films behave under different conditions.

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