Current medical imaging technologies allow visualization of tissue anatomy in the human body at resolutions ranging from 100 micrometers to 1 millimeter. These technologies are generally not sensitive enough to detect early-stage tissue abnormalities associated with diseases such as cancer and atherosclerosis, which require micrometer-scale resolution. Here, optical coherence tomography was adapted to allow high-speed visualization of tissue in a living animal with a catheter-endoscope 1 millimeter in diameter. This method, referred to as "optical biopsy," was used to obtain cross-sectional images of the rabbit gastrointestinal and respiratory tracts at 10-micrometer resolution.

In Vivo Endoscopic Optical Biopsy with Optical Coherence Tomography

We describe a high-speed optical coherence domain reflectometer. Scan speeds of 40 mm/s are achieved with a dynamic range of >90 dB and a spatial resolution of 17 μm. Two applications are presented: the noninvasive measurement of anterior eye structure in a rabbit in vivo and the characterization of reflections and interelement spacing in a multielement lens.

High-speed optical coherence domain reflectometry

Laser scanners have been an integral part of MEMS research for more than three decades. During the last decade, miniaturized projection displays and various medical-imaging applications became the main driver for progress in MEMS laser scanners. Portable and truly miniaturized projectors became possible with the availability of red, green, and blue diode lasers during the past few years. Inherent traits of the laser scanning technology, such as the very large color gamut, scalability to higher resolutions within the same footprint, and capability of producing an always-in-focus image render it a very viable competitor in mobile projection. Here, we review the requirements on MEMS laser scanners for the demanding display applications, performance levels of the best scanners in the published literature, and the advantages and disadvantages of electrostatic, electromagnetic, piezoelectric, and mechanically coupled actuation principles. Resonant high-frequency scanners, low-frequency linear scanners, and 2-D scanners are included in this review.

/pdf/mems-laser-scanners-a-review-3umum14m4r.pdf

MEMS Laser Scanners: A Review

In recent years, Light Detection and Ranging (LiDAR) has been drawing extensive attention both in academia and industry because of the increasing demand for autonomous vehicles. LiDAR is believed to be the crucial sensor for autonomous driving and flying, as it can provide high-density point clouds with accurate three-dimensional information. This review presents an extensive overview of Microelectronechanical Systems (MEMS) scanning mirrors specifically for applications in LiDAR systems. MEMS mirror-based laser scanners have unrivalled advantages in terms of size, speed and cost over other types of laser scanners, making them ideal for LiDAR in a wide range of applications. A figure of merit (FoM) is defined for MEMS mirrors in LiDAR scanners in terms of aperture size, field of view (FoV) and resonant frequency. Various MEMS mirrors based on different actuation mechanisms are compared using the FoM. Finally, a preliminary assessment of off-the-shelf MEMS scanned LiDAR systems is given.

MEMS Mirrors for LiDAR: A review.

An actuator comprising a flexible substrate having a surface comprising one or more active regions, each of the active regions having disposed thereon an electrochemically active film comprising a plurality of 1T phase 2D MoS2 nanosheets which impart a first force upon the flexible substrate in response to electrochemical charging of ions in the 2D MoS2 nanosheets and impart a second force upon the flexible substrate in response to electrochemical discharging of ions in the 2D MoS2 nanosheets, wherein the first and second forces imparted to the flexible substrate each being sufficient to displace the flexible substrate. The flexible substrate may comprise a polymer substrate and the electrochemically active film may comprise a conductive layer having disposed thereon the plurality of 1T phase 2D MoS2 nanosheets.

Metallic molybdenum disulfide nanosheets-based electrochemical actuators

This paper presents the design, optimization, fabrication, and test results of an electrothermally actuated tip-tilt-piston micromirror with a large optical aperture of 1 mm. The fabrication of the device is a combination of thin-film surface micromachining and bulk silicon micromachining based on silicon-on-insulator wafers. The device has 3-DOF of actuations, including rotations around two axes in the mirror plane, and out-of-plane piston actuation. The micromirror shows an optical scan range of plusmn30deg about both x- and y-axes and displaces 480 mum in the z-axis, all at dc voltages that are less than 8 V. Dynamic testing of the micromirror shows that the thermal response time of each actuator is about 10 ms. Resonant frequencies of the piston and rotation motion are 336 and 488 Hz, respectively. The unique structural design of the device ensures that there is no lateral shift for the piston motion and no rotation-axis shift for the rotation scanning. With the large tip-tilt-piston scan ranges and low driving voltage, this type of device is very suitable for biomedical imaging and laser beam steering applications.

An Electrothermal Tip–Tilt–Piston Micromirror Based on Folded Dual S-Shaped Bimorphs

This paper reports a novel dual-chopper amplifier (DCA) and its application to monolithic complementary metal-oxide semiconductor-microelectromechanical systems accelerometers. The DCA design minimizes the power consumption and noise by chopping the sensing signals at two clocks. The first clock is a high frequency for removing the flicker noise while the second clock is a significantly lower frequency to keep the unit gain bandwidth low. A monolithic three-axis accelerometer integrated with the DCA on the same chip has been successfully fabricated using a post-CMOS micromachining process. The measured noise floors are 40 μ g/√Hz in the x - and y -axis and 130 μ g/√Hz in the z -axis, and the power consumption is about 1 mW per axis.

A Low-Power Low-Noise Dual-Chopper Amplifier for Capacitive CMOS-MEMS Accelerometers

This paper presents the design, fabrication and characterization of a piezoelectrically actuated high-fill-factor tip-tilt-piston (TTP) micromirror with small tilt and lateral shift during scanning. The piezoelectric material is a sol–gel lead zirconate titanate (PZT) thin film with a Zr/Ti ratio of 53/47. The small initial tilt and lateral-shift-free (LSF) of the mirror plate is achieved by a folded, three-segment piezoelectric unimorph actuator design. The piezoelectric unimorph actuation beams consist of Pt/Ti/PZT/Pt/Ti/SiO 2 multilayers, which are released via undercutting the substrate silicon. The fabricated piezoelectric micromirror can be actuated about two rotational axes in the mirror plane and for translational vertical scan (piston actuation). The resonant frequencies of the piston and rotation modes are 316 Hz and 582 Hz, respectively. At their respective resonant frequencies, the maximum piston magnitude at resonant driving is 32 μm, and the two-dimensional rotating scan ranges are about 5°, both measured at a 2V pp sinusoidal driving voltage.

A piezoelectric unimorph actuator based tip-tilt-piston micromirror with high fill factor and small tilt and lateral shift

This paper presents the design, fabrication and characterization of a tip–tilt–piston (TTP) piezoelectric micromirror. A unique double S-shaped unimorph piezoelectric (dSUP) actuator design is employed. The dSUP actuator design can generate large vertical displacement. With four such actuators symmetrically located on the four sides of the mirror plate, both tip and tilt as well as piston motion can be realized. Lead zirconate titanate (PZT) is used due to its high piezoelectric coefficient and it is fabricated with a sol–gel method. The Zr/Ti ratio of the PZT material is 53/47. The dSUP actuators consist of Pt/Ti/PZT/Pt/Ti/SiO2 multilayers. The thickness of the PZT layer is 0.7 μm. The 1.1 mm × 1.1 mm mirror plate is backed by a 50 μm-thick single-crystal silicon layer to ensure the flatness. Backside deep silicon etching and front-side isotropic silicon undercut are used to release the device. Experiments show that the resonant frequency of the tip–tilt scan modes is 3.5 kHz. 27 μm vertical displacement was observed under 5 V DC driving of one pair of opposite actuator and the scan angle can achieve 9.65° under sine wave resonant driving at 2 V amplitude on one pair of opposite actuator.

A tip-tilt-piston micromirror with a double S-shaped unimorph piezoelectric actuator

This paper presents the design, fabrication, packaging, and characterization of a novel high-fill-factor micromirror array (MMA) actuated by electrothermal bimorphs. In this paper, 4 × 4 MMA devices with an 88% area fill factor at normal incidence, 1.5 mm × 1.5 mm subaperture size, and single-crystal-silicon-supported mirror plates have been fabricated based on a single silicon-on-insulator wafer, without additional bonding/transfer processes. The bimorph actuators are hidden underneath the mirror plates, which are also protected by silicon walls. The MMA devices can directly be surface mounted onto driving circuit chips or printed circuit boards after fabrication. The subapertures can generate large tip-tilt-piston scanning and can individually be addressed. Static characterizations of the packaged devices show that each subaperture can achieve a piston stroke of ~310 μm and optical deflection angles greater than ±25° in both the x- and y-axes, all at 8-V dc. The preliminary laser-steering optical-phased-array capability of the obtained MMA device has also experimentally been demonstrated. This paper is based on the conference proceedings presented at the 23rd IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2010), Hong Kong.

Kemiao Jia

Papers

An Electrothermal Tip–Tilt–Piston Micromirror Based on Folded Dual S-Shaped Bimorphs

A Low-Power Low-Noise Dual-Chopper Amplifier for Capacitive CMOS-MEMS Accelerometers

A piezoelectric unimorph actuator based tip-tilt-piston micromirror with high fill factor and small tilt and lateral shift

A tip-tilt-piston micromirror with a double S-shaped unimorph piezoelectric actuator

High-Fill-Factor Micromirror Array With Hidden Bimorph Actuators and Tip–Tilt-Piston Capability