Theses and Dissertations at Montana State University (MSU)
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Item MEMS 3-dimensional scanner with SU-8 flexures for a handheld confocal microscope(Montana State University - Bozeman, College of Engineering, 2018) Liu, Tianbo; Chairperson, Graduate Committee: David DickensheetsThe conventional method for diagnosing skin cancer is to perform a biopsy followed by pathology. However not only are biopsies invasive and likely to leave permanent scarring, they also sample the body sparsely. Fortunately, a non-invasive method of imaging called confocal laser scanning microscopy has shown great potential to replacing invasive biopsies. Confocal microscopy can use light to achieve high-resolution imaging of cells that lie underneath the surface of the skin. However, the large size of current confocal microscopes limits their application to all but the most accessible sites. In this dissertation, I address the miniaturization of confocal microscopy through the development of a new microelectromechanical systems scan mirror that can scan a focused beam in three dimensions. The scanner has a 4 mm aperture, and has the capability to replace all of the bulky beam scanners and focus mechanisms that contribute to the large size of current confocal microscopes. The fabrication of the scanner explores the use of the polymer SU-8 for its mechanical structures. The gimbal mirror has demonstrated scan angles in excess of plus or minus 3° mechanical for lateral scanning, and its deformable surface provided controllable deflection up to 10 microns for focus control. This newly developed scanner was integrated into a confocal system to test its imaging capabilities. The device demonstrated high-resolution scanning with simultaneous focus adjustment suitable for the next generation of miniaturized confocal laser scanning microscopes.Item Imaging performance of elliptical-boundary varifocal mirrors in active optical systems(Montana State University - Bozeman, College of Engineering, 2015) Lukes, Sarah Jane; Chairperson, Graduate Committee: David L. DickensheetsMicro-electro-mechanical systems deformable-membrane mirrors provide a means of focus control and attendant spherical aberration correction for miniaturized imaging systems. The technology has greatly advanced in the last decade, thereby extending their focal range capabilities. This dissertation describes a novel SU-8 2002 silicon-on-insulator wafer deformable mirror. A 4.000 mm x 5.657 mm mirror for 45° incident light rays achieves 22 micron stroke or 65 diopters, limited by snapdown. The mirrors show excellent optical quality while flat. Most have peak-to-valley difference of less than 150 nm and root-mean-square less than 25 nm. The process proves simple, only requiring a silicon-on-insulator wafer, SU-8 2002, and a metal layer. Xenon difluoride etches the silicon to release the mirrors. Greater than 90% of the devices survive fabrication and release. While current literature includes several aberration analyses on static mirrors, analyses that incorporate the dynamic nature of these mirrors do not exist. Optical designers may have a choice between deformable mirrors and other types of varifocal mirrors or lenses. Furthermore, a dynamic mirror at an incidence angle other than normal may be desired due to space limitations or for higher throughput (normal incidence requires a beam splitter). This dissertation presents an analysis based on the characteristic function of the system. It provides 2nd and 3rd order aberration coefficients in terms of dynamic focus range and base ray incidence angle. These afford an understanding of the significance of different types of aberrations. Root-mean-square and Strehl calculations provide insight into overall imaging performance for various conditions. I present general guidelines for maximum incidence angle and field of fiew that provide near diffraction-limited performance. Experimental verification of the MEMS mirrors at 5° and 45° incidence angles validates the analytical results. A Blu-ray optical pick-up imaging demonstration shows the utility of these mirrors for focus control and spherical aberration correction. Imaging results of the first demonstration of a deformable mirror for dynamic agile focus control and spherical aberration correction in a commercial table-top confocal microscope are also shown.Item A Confocal Microscope and Raman Spectroscopy Probe for Mars exploration(Montana State University - Bozeman, College of Engineering, 2002) Crowder, Dawn MichelleItem Mems 3-D scan mirror for an endoscopic confocal microscope(Montana State University - Bozeman, College of Engineering, 2005) Shao, Yuhe; Chairperson, Graduate Committee: David L. Dickensheets.Optical MEMS is at a very exciting stage and has become an enabling technology for a variety of applications in telecommunications and high-resolution display and imaging. Among many novel MEMS devices, MEMS scanners and deformable membrane mirrors are especially useful for scanned-beam imaging systems. MEMS scanners are usually used for laser beam scanning. Deformable mirrors provide an approach to modify the optical wavefront adaptively. In optical microsystems, packaging is often a critical part of the system design. Combining functionalities at a single device can reduce complexity of the system. Biaxial beam scanning and focus control are combined together in the MEMS 3-D scan mirror. The mirror surface is made of a goldcoated silicon nitride deformable membrane and works as a positive lens of variable focal length. The membrane sits on a torsional plate that can scan about two orthogonal axes. This architecture is able to move the focus of a laser beam throughout a threedimensional space with a single optical surface. The overall size of the 3-D mirror is 1.5 mm with a usable optical aperture of 0.7 mm. Both the inner and outer scanning axes achieved more than 5o zero-to-peak mechanical scan angles; the deformable membrane achieved a maximum center displacement of more than 3.7 æm, corresponding to an adjustable focal length from infinity to approximately 8 mm. For a confocal laser scanning microscope with illumination wavelength at 500 nm, they provide Nx = Ny = 488 resolvable spots for lateral resolution, and Nz = 32 depth-of-focus distances for depth resolution. To drive the 3-D mirror, the inner axis is operated near resonance (~ 650 Hz); and the outer axis quasi-statically. Operating the outer axis at 2 Hz provides a line resolution of 325 lines/frame at a refresh rate of 2 frames/second. The performance of the 3-D mirror is well matched to the intended application of endoscopic confocal microscopy, and similar devices could prove useful in a variety of optical microsystems needing beam scanning and focus control. This dissertation describes the design, fabrication, characterization, imaging experiment and target applications of the MEMS 3- D scan mirror.Item Numerical modeling of the deflection of an electrostatically actuated circular membrane mirror(Montana State University - Bozeman, College of Engineering, 2011) Moog, Eric John; Chairperson, Graduate Committee: Steven R. Shaw; David L. Dickensheets (co-chair)This thesis outlines a numerical modeling method to describe the deflection behavior and investigate control schemes for an electrostatically actuated deformable membrane mirror, with application to focus control and aberration correction in micrelectromechanical systems. The physics of the membrane are approximated using a finite difference approach with parameters obtained from measurements of a physical device. The model is validated by comparison of simulated and measured mirror position under static and dynamic conditions. This thesis provides simulation results for control schemes that would be difficult or potentially destructive if implemented using real devices. We suggest that the model may be useful for the development of future control strategies and in refining device design. Finally, a number of capacitive sensing circuits are presented as position feedback mechanisms and the capabilities and limitations of each are examined.Item Surface micro-machined SU-8 2002 deformable membrane mirrors(Montana State University - Bozeman, College of Engineering, 2011) Lukes, Sarah Jane; Chairperson, Graduate Committee: David L. DickensheetsImaging systems are continually decreasing in size, especially in applications such as microscopy and cell phone cameras. Much research is being done to increase focus control capabilities of these instruments. This paper describes a wet-etch release and a dry-etch release fabrication technique for SU-8 2002 surface micro-machined deformable mirrors for focus control and compensation of focus-induced spherical aberration. Producing a good quality SU-8 2002 membrane layer proved difficult and a detailed discussion of the recipe development is presented. A thorough review of both release processes is also included. The dry-etch release process has high yield and realizes larger mirrors with greater than a two-fold improvement in stroke, relative to the wet-etch release. This paper presents deflection vs. voltage plots, mechanical frequency response measurements, surface roughness and flatness, and intrinsic stress for the 750 micron - 4.24 mm nominal dimension circular and elliptical boundary membrane mirrors. The use of a 3 mm x 4.24 mm elliptical boundary mirror for 45° incidence focus control in microscopy is also demonstrated. The intrinsic stress of the film indicates that devices of similar size should be capable of 30 micron displacement in the future. This would allow for sufficient focus control in high NA microscopy applications, while the mirror or mechanism for such control could fit into an endoscopic sized imaging system.