This Small Business Technology Transfer (STTR) Phase II project will develop a commercial prototype of an aberration compensated focus control device. This device, based on a MEMS technology, will allow the user to deflect a deformable membrane mirror in a controlled manner in order to select a desired focal length. The device also features active control of low-order aberrations. This technology will enable the next generation of biomedical imaging devices for microscopy applications by enabling focus control and aberration correction in a simple, compact and low-cost sensor.
The broader impacts of this research are primarily in biomedical imaging. An industry partner is interested in using the technologys aberration correction capabilities to improve skin cancer detection with their confocal microscopy product line. Microscopy and endomicroscopy researchers at the University of Arizona have stated that this technology will be a valuable asset in their research in the fight against cancer. The company will also team with a recognized leader in MEMS technology to enable enhanced imaging capabilities, primarily for imaging in the field of ophthalmology.
In 2010, Bridger Photonics, Inc. (Bridger) and Montana State University (MSU) were awarded a NSF STTR Phase II to develop a compact focus control device for biomedical applications based on MEMS deformable mirrors. In addition, Bridger and MSU were awarded a Phase IIB grant in 2012. The primary technical objectives of the Phase II and IIB effort were to optimize the manufacturing of the MEMS devices, analyze and characterize the device performance and develop a compact mirror system, complete with packaging, drive electronics and software, for benchtop use in imaging and focus control applications. We successfully completed all of the technical objectives and arrived at the end of the effort with a market-ready device. Notably, Bridger has produced a benchtop device that can be mated with any number of standard 1" optical mounts, along with user friendly drive electronics and software. Bridger has been marketing this device as a general purpose device, while exploring more advanced biomedical imaging applications. Figure 1 shows the final MEMS device and accompanying drive electronics. Bridger and MSU researchers have fabricated and characterized low-voltage electrostatically actuated, deformable membranes. The mirrors have three concentric actuation electrodes in order to control of low order rotationally symmetric aberrations. Membrane sizes ranged between 1 mm and 5 mm in diameter with an overall footprint (including bonding pads) of 1 cm x 1 cm. The overall surface quality of the mirrors is excellent; when unactuated, they are optically flat (12 nm RMS surface deviation). Membrane stroke is 10-12 μm. At the maximum stroke, this corresponds to a focal length of < 25 mm for a 2 mm mirror diameter and 130 mm for a 5 mm diameter. The required actuation voltage depends on device diameter and ranges from ~400 V for smaller diameters to as little as 150 V for larger diameters. Next generation devices can support modulation bandwidths greater than 10 kHz. Bridger and MSU overcame several technical challenges to develop a detailed manufacturing procedure for the MEMS devices with device yield reaching 90% or better. All mirrors were fabricated at the Montana Microfabrication Facility located at MSU. This facility can support manufacturing of thousands of devices per year, if necessary. Figure 2 shows individual MEMS mirrors at 1,2, and 3 mm diameters. Bridger and MSU carried out a full characterization of the mechanical properties of the MEMS mirrors including environmental and lifetime testing. Highlights includes 90 billion actuations without failure and 360,000 snapdown events before failure. Lastly, Bridger and Montana State University, along with Boston Micromachines Corporation, characterized the optical properties of the MEMS mirrors and showed that they are suitable for a wide range of imaging applications including: confocal microscopy, non-mechanical focus control or zoom cameras, and adaptive optics.
