Novel Actuation Mechanisms for MEMS Mirrors

Ph.DDOCTOR OF PHILOSOPH

Multi-spectral Dual Axes Confocal Endomicroscope with Vertical Cross-sectional Scanning for In-vivo Targeted Imaging of Colorectal Cancer

Pathologists review histology cut perpendicular to the tissue surface or in the vertical cross-section (XZ-plane) in order to visualize the normal or abnormal differentiation patterns. The epithelium of hollow organs, such as the colon, is the origin of many important forms of cancer. The vertical cross-section provides a comprehensive view of the epithelium which normally differentiates in the basilar to luminal direction. Real-time imaging in this orientation has not been fully explored in endomicroscopy because most instruments collect images in the horizontal cross-section (XY-plane). Imaging microstructures from the tissue surface to about half a millimeter deep can reveal early signs of disease. Furthermore, the use of molecular probes is an important, emerging direction in diagnostic imaging that improves specificity for disease detection and reveals biological function. Dysplasia is a pre-malignant condition in the colon that can progress into colorectal cancer. Peptides have demonstrated tremendous potential for in-vivo use to detect colonic dysplasia. Moreover, peptides can be labeled with NIR dyes for visualizing the full depth of the epithelium in small animals. This study aims to demonstrate large FOV multi-spectral targeted in-vivo vertical optical section with a dual axes confocal endomicroscope enabled by MEMS technology. The NIR multi-spectral fluorescence images demonstrate both histology-like morphology imaging and molecular imaging of specific peptide binding to dysplasia in the mouse colon. The specific aims of this study are: (1) to develop miniature vertical cross-sectional scan engine based on MEMS technology for imaging on XZ-plane; (2) to integrate micro-optics and develop multi-spectral dual axes confocal endomicroscope imaging system; (3) to perform in-vivo targeted vertical cross-sectional imaging with large FOV on colorectal cancer mouse model.PhDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107154/1/zqiu_1.pd

Applications of programmable MEMS micromirrors in laser systems

The use of optical microelectromechanical systems (MEMS) as enabling devices has been shown widely over the last decades, creating miniaturisation possibilities and added functionality for photonic systems. In the work presented in this thesis angular vertical offset comb-drive (AVC) actuated scanning micromirrors, and their use as intracavity active Q-switch elements in solid-state laser systems, are investigated. The AVC scanning micromirrors are created through a multi-user fabrication process, with theoretical and experimental investigations undertaken on the influence of the AVC initial conditions on the scanning micromirror dynamic resonant tilt movement behaviour. A novel actuator geometry is presented to experimentally investigate this influence, allowing a continuous variation of the initial AVC comb-offset angle through an integrated electrothermal actuator. The experimentally observed changes of the resonant movement with varying initial AVC offset are compared with an analytical model, simulating this varying resonant movement behaviour. In the second part of this work AVC scanning micromirrors are implemented as active intra-cavity Q-switch elements of a Nd:YAG solid-state laser system. The feasibility of achieving pulsed laser outputs with pulse durations limited by the laser cavity and not the MEMS Q-switch is shown, combined with a novel theoretical model for the Q-switch behaviour of the laser when using a bi-directional intra-cavity scanning micromirror. A detailed experimental investigation of the pulsed laser output behaviour for varying laser cavity geometries is presented, also discussing the influence of thin film coatings deposited on the mirror surfaces for further laser output power scaling. The MEMS Q-switch system is furthermore expanded using a micromirror array to create a novel Q-switched laser system with multiple individual controllable output beams using a common solid-state gain medium. Experimental results showing the simultaneous generation of two laser outputs are presented, with cavity limited pulse durations and excellent laser beam quality.The use of optical microelectromechanical systems (MEMS) as enabling devices has been shown widely over the last decades, creating miniaturisation possibilities and added functionality for photonic systems. In the work presented in this thesis angular vertical offset comb-drive (AVC) actuated scanning micromirrors, and their use as intracavity active Q-switch elements in solid-state laser systems, are investigated. The AVC scanning micromirrors are created through a multi-user fabrication process, with theoretical and experimental investigations undertaken on the influence of the AVC initial conditions on the scanning micromirror dynamic resonant tilt movement behaviour. A novel actuator geometry is presented to experimentally investigate this influence, allowing a continuous variation of the initial AVC comb-offset angle through an integrated electrothermal actuator. The experimentally observed changes of the resonant movement with varying initial AVC offset are compared with an analytical model, simulating this varying resonant movement behaviour. In the second part of this work AVC scanning micromirrors are implemented as active intra-cavity Q-switch elements of a Nd:YAG solid-state laser system. The feasibility of achieving pulsed laser outputs with pulse durations limited by the laser cavity and not the MEMS Q-switch is shown, combined with a novel theoretical model for the Q-switch behaviour of the laser when using a bi-directional intra-cavity scanning micromirror. A detailed experimental investigation of the pulsed laser output behaviour for varying laser cavity geometries is presented, also discussing the influence of thin film coatings deposited on the mirror surfaces for further laser output power scaling. The MEMS Q-switch system is furthermore expanded using a micromirror array to create a novel Q-switched laser system with multiple individual controllable output beams using a common solid-state gain medium. Experimental results showing the simultaneous generation of two laser outputs are presented, with cavity limited pulse durations and excellent laser beam quality

Two-dimensional microscanners with t-shaped hinges and piezoelectric actuators

For a wide range of application areas such as medical instruments, defense, communication networks, industrial equipment, and consumer electronics, microscanners have been a vibrant research topic. Among various fabrication methodologies, MEMS (microelectromechanical system) stands out for its small size and fast response characteristics. In this thesis, piezoelectric actuation mechanism is selected because of its low voltage and low current properties compared with other mechanisms, which are especially important for the target application of biomedical imaging. Although 1- and 2-dimensional microscanners with piezoelectric actuators have been studied by several other groups, this thesis introduces innovative improvements in design of the piezoelectric MEMS microscanner. A novel T-shaped hinge geometry is proposed, which is flexible in whole six directions and also free from the crosstalk issue found in the earlier designs by other groups. The piezoelectric actuator of the microscanner is comprised of five layers; a top electrode, a piezoelectric layer (lead zirconate titanate or PZT), a bottom electrode, a dielectric layer, and a mechanical support. The microscanners were analyzed using both analytical formulas and numerical simulations. Based on the analysis, the microscanners were designed and fabricated with four mask levels¯top electrodes, bottom electrodes, bonding pads, and substrate etching windows. During the silicon substrate wet etching process in KOH, ProTEK@ B3 was coated in the front to protect the devices. Polarization-voltage (P-V) measurement of deposited PZT was performed using RT66B. Actuation of the piezoelectric cantilevers were observed under a microscope by applying voltage

Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

MEMS Technology for Biomedical Imaging Applications

Biomedical imaging is the key technique and process to create informative images of the human body or other organic structures for clinical purposes or medical science. Micro-electro-mechanical systems (MEMS) technology has demonstrated enormous potential in biomedical imaging applications due to its outstanding advantages of, for instance, miniaturization, high speed, higher resolution, and convenience of batch fabrication. There are many advancements and breakthroughs developing in the academic community, and there are a few challenges raised accordingly upon the designs, structures, fabrication, integration, and applications of MEMS for all kinds of biomedical imaging. This Special Issue aims to collate and showcase research papers, short commutations, perspectives, and insightful review articles from esteemed colleagues that demonstrate: (1) original works on the topic of MEMS components or devices based on various kinds of mechanisms for biomedical imaging; and (2) new developments and potentials of applying MEMS technology of any kind in biomedical imaging. The objective of this special session is to provide insightful information regarding the technological advancements for the researchers in the community

MEMS-Based Endoscopic Optical Coherence Tomography

Early cancer detection has been playing an important role in reducing cancer mortality. Optical coherence tomography (OCT), due to its micron-scale resolution, has the ability to detect cancerous tissues at their early stages. For internal organs, endoscopic probes are needed as the penetration depth of OCT is about 1–3 mm. MEMS technology has the advantages of fast speed, small size, and low cost, and it has been widely used as the scanning engine in endoscopic OCT probes. Research results have shown great potential for OCT in endoscopic imaging by incorporating MEMS scanning mirrors. Various MEMS-OCT designs are introduced, and their imaging results are reviewed in the paper

Thin-Film PZT Scanning Micro-actuators for Vertical Cross-sectional Imaging in Endomicroscopy

The advancement of optics and the development of microelectromechanical systems (MEMS) based scanners has enabled powerful optical imaging techniques that can perform optical sectioning with high resolution and contrast, large field of view, and long working distance to be realized in endoscope-compatible form factors. Optical endomicroscopes based on these imaging techniques can be used to obtain in vivo vertical cross-sectional images of dysplastic tissues in the hollow organs before they progress to mucosal diseases such as colorectal cancer. However, existing endomicroscopic systems that use imaging modalities compatible with the use of fluorescent biomarkers are not capable of deep vertical sectioning in real time. This work proposes a unique MEMS-based scanning mechanism to be incorporated into endoscopic microscopes for real-time in vivo deep into-tissue scanning for early cancer detection. For this task, a class of novel multi-axis micro-scanners based on thin-film lead-zirconate-titanate (PZT) has been developed. Leveraging the large piezoelectric strain coefficient of PZT, the prototypes have demonstrated more than 400 μm of out-of-plane displacement with bandwidths on the order of 100-200 Hz in only a 3.2 mm-by-3.2 mm footprint, which meets the requirements for this application. The scanners have a central rectangular-shaped reflector, whose corners are supported by four symmetric PZT bending legs that generate vertical translation. This design gives the reflector a three-axis motion. The challenges to fabricate high performance piezoelectric actuators are discussed with device failure mechanisms observed during the fabrication of the 1st generation scanners. Improved fabrication steps are presented that solve the issues with the 1st generation devices and enhance the robustness of the scanners for instrument integration. Remaining non-ideal fabrication outcomes cause MEMS devices to produce unwanted motions, which can degrade imaging quality. To overcome this problem, a method to drive MEMS actuators having multiple vibration modes with close frequencies to produce a desired motion pattern with a single input is presented, and was used to generate a pure vertical motion for imaging. Two-photon based vertical cross-sectional images of mouse colon was obtained in real time for the first time using a thin-film piezoelectric microscanner. To understand the effects of fabrication non-idealities on the device behavior and produce more robust scanner performance, analytical models that describes large vertical and rotational motions including multi-axis coupling were developed. A static model that was initially developed for design optimization was calibrated, along with a transient model, using experimental data to incorporate the effects of dimensional variations and residual stress. This models can be used with future integrated sensors and feedback controllers for more precise and robust motion of the scanner. This calibration technique can be useful in developing analytic models for MEMS devices subject to fabrication uncertainty. In addition, nonlinear dynamic behavior due to large vertical stroke in the presence of fabrication non-idealities is captured by linearizing an expanded dynamic model about different static positions obtained by numerically solving the expanded nonlinear model.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137124/1/jongs_1.pd

MEMS devices for the control of trapped atomic particles

This thesis presents the design and characterisation of novel MEMS scanners, for use in systems involving trapped atomic particles. The scanners are manufactured using multiuser silicon-on-insulator MEMS fabrication processes and use resonant piezoelectric actuation based on aluminium nitride thin films to produce one dimensional scanning at high frequencies, with resonance tuning capabilities of up to 5 kHz. Frequencies of ~100kHz and higher are required to enable for example resonant addressing of trapped atomic particles. This work demonstrates how the 200 μm and 400 μm diameter scanners can produce optical deflection angles upwards of 2° at frequencies from 80 kHz to 400 kHz. It proposes an addressing scheme based on Lissajous scanning to steer laser pulses onto 2D grids at a scale compatible with experiments involving single trapped atoms. It also examines frequency tuning capabilities of the scanners using localized on-chip Joule heating and active cooling ; frequency tuning and synchronization are shown to be critical to the implementation of 2-dimensional scanning with multiple scanners. These features are then demonstrated in a prototype implementation using fluorescing samples as a mock target to evaluate the optical performance of the scanning system. Finally, the thesis describes a proof-of-concept for integration of the scanners in a trapped atoms experiment, in which rubidium atoms trapped inside a magneto-optical trap are selectively pumped into a fluorescing state using a beam steered by the MEMS scanners.This thesis presents the design and characterisation of novel MEMS scanners, for use in systems involving trapped atomic particles. The scanners are manufactured using multiuser silicon-on-insulator MEMS fabrication processes and use resonant piezoelectric actuation based on aluminium nitride thin films to produce one dimensional scanning at high frequencies, with resonance tuning capabilities of up to 5 kHz. Frequencies of ~100kHz and higher are required to enable for example resonant addressing of trapped atomic particles. This work demonstrates how the 200 μm and 400 μm diameter scanners can produce optical deflection angles upwards of 2° at frequencies from 80 kHz to 400 kHz. It proposes an addressing scheme based on Lissajous scanning to steer laser pulses onto 2D grids at a scale compatible with experiments involving single trapped atoms. It also examines frequency tuning capabilities of the scanners using localized on-chip Joule heating and active cooling ; frequency tuning and synchronization are shown to be critical to the implementation of 2-dimensional scanning with multiple scanners. These features are then demonstrated in a prototype implementation using fluorescing samples as a mock target to evaluate the optical performance of the scanning system. Finally, the thesis describes a proof-of-concept for integration of the scanners in a trapped atoms experiment, in which rubidium atoms trapped inside a magneto-optical trap are selectively pumped into a fluorescing state using a beam steered by the MEMS scanners