This article presents design and development of a novel 3D micromirror for large deflection scanning application in invivo
optical coherence tomography (OCT) bio-imaging probe. Overall mirror chip size is critical to reduce the diameter of the probe; however, mirror plate itself should not be less than 500 μm as smaller size means reducing the amount of light collected after scattering for OCT imaging. In this study, mirror chip sizes of 1 × 1 mm2 and 1.5 × 1.5 mm2 were developed with respectively 400 and 500 micrometer diameter mirror plates. The design includes electro thermal excitation mechanism in the same plane as mirror plate to achieve 3D free space scanning. Larger deflection requires longer actuators, which usually increase the overall size of the chip. To accommodate longer actuators and keep overall chip size same curved beam actuators are designed and integrated for micromirror scanning. Typical length of the actuators was 800 micrometer, which provided up to 17 degrees deflection. Deep reactive ion etching (DRIE) process
module was used extensively to etch high aspect ratio structures and keep the total mirror chip size small.
A biocompatible housing for an optical bio-probe is developed for OCT (Optical Coherence Tomography)
imaging application. Silicon micro machined 3D mirror is used to steer the optical beam in to the sample
of interest. A Grin lens fiber assembly is used to couple the light from the light source to the micro mirror.
A Silicon Optical bench (SiOB) is used to integrate the optical components and the 3D mirror. The
integrated assembly is housed in a poly carbonate housing with AR (anti reflection) coating on the inner
and outer sides of the housing wall. Micro injection molding method is used t o fabricate a miniaturized
probe housing which is transparent to 1300nm. Miniaturized housing is tested in an OCT setup and the
captured image is processed.
In this paper, we present a non-rotatory circumferential scanning optical probe integrated with a MEMS scanner for in
vivo endoscopic optical coherence tomography (OCT). OCT is an emerging optical imaging technique that allows high
resolution cross-sectional imaging of tissue microstructure. To extend its usage to endoscopic applications, a
miniaturized optical probe based on Microelectromechanical Systems (MEMS) fabrication techniques is currently
desired. A 3D electrothermally actuated micromirror realized using micromachining single crystal silicon (SCS) process
highlights its very large angular deflection, about 45 degree, with low driving voltage for safety consideration. The
micromirror is integrated with a GRIN lens into a waterproof package which is compatible with requirements for
minimally invasive endoscopic procedures. To implement circumferential scanning substantially for diagnosis on certain
pathological conditions, such as Barret's esophagus, the micromirror is mounted on 90 degree to optical axis of GRIN
lens. 4 Bimorph actuators that are connected to the mirror on one end via supporting beams and springs are selected in
this micromirror design. When actuators of the micromirror are driven by 4 channels of sinusoidal waveforms with 90
degree phase differences, beam focused by a GRIN is redirected out of the endoscope by 45 degree tilting mirror plate
and achieve circumferential scanning pattern. This novel driving method making full use of very large angular deflection
capability of our micromirror is totally different from previously developed or developing micromotor-like rotatory
MEMS device for circumferential scanning.
Design and development of a 3D scanning MEMS micromirror integrated miniaturized optical probe has been presented
in this article. The probe is designed to be less than 2 mm in diameter and has dynamic scanning modality for larger field
of view. Scanning is achieved using 3D micromirror device, which has 16º out of plane and 360º beam rotation
capability. Initial target of 45º out of plane deflection is yet to be achieved. The probe being developed currently would
have scanning capability in one quarter of 360º full rotation. The field of view would still be very large and multiple
optical biopsies would be possible for planned cancer model diagnostics. The feasibility of using scanning mirror into an
optical probe was demonstrated using scanning repeatability and OCT imaging tests. Geometrical optics and package
design using silicon optical bench have been established. Miniaturized 3D scanning micromirror have been designed and
developed with 16º out of plane deflection demonstrated. Probe package integration and optical testing are carried out.
The influence of LPCVD process parameters on stress in polysilicon films has been investigated for surface micromachined structures. The as deposited films show a large strain which can be considerably reduced by post deposition annealing. The polycrystalline film deposited at 605 degree(s)C and 250 mTorr is found to have minimum residual stress. The rapid thermal annealing (RTA) at 1100 degree(s)C for 30 sec relieves the stress completely. Further, the RTA is shown to be a superior process compared to the conventional furnace annealing for obtaining stress free films.
The C-V measurement for MOS capacitor technique has been adapted to investigate the anisotropic electrochemical etching of silicon for bulk micromachining C-V characteristics of the etching cell have been obtained during the electrochemical etching process. The behavior of the characteristics has been explained on the basis of various layers at the etching interface.
A new approach has been proposed to realize suspended microstructures such as cantilevers, diaphragms, springs, spirals, etc. over pits of controlled depth in bulk silicon micromachining process using direct wafer bonding technology. The structures have been realized in heavily boron doped silicon and dielectric layers. The electrostatic actuation voltage of the cantilever beams have been computed as a function of physical parameters and compared with experimentally measured values. LPCVD polysilicon has been investigated with a view to obtain low stress films for use in surface micromachining technology. The effect of deposition parameters on stress in the polysilicon films has been studied. Post deposition annealing is shown to have profound effect on the stress properties of the polysilicon films. Rapid thermal annealing is found to be much more effective in reducing the stress compared to conventional furnace annealing. The advantage of direct wafer bonding technology for electrostatically actuated microstructures have been presented. The application of this technology for MEMS have been discussed.
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