Yingshun Xu

A two axes scanning SOI MEMS micromirror for endoscopic bioimaging

A novel silicon on insulator (SOI) MEMS process has been designed and developed to realize a two axes thermally actuated single crystal silicon micromirror device, which consists of a mirror plate, four flexural springs and four thermal actuators. The mirror plate has the same thickness as a SOI device layer i.e. 4 µm. The SOI layer is selectively thinned down to 2 µm for fabricating flexural springs and thermal actuators. The thinning of the SOI layer is essential to lower (control) the flexural rigidity of the springs and the actuators and thus to achieve a higher tilt angle at low thermal power. The developed single wafer process is based on dry reactive ion etching CMOS compatible chemistries. The minimum chip size design of 1 mm × 1 mm has a 400 µm diameter mirror plate. Other chip designs include the mirror diameters in the range from 200 to 500 µm. This paper also presents a study on the mirror plate curvature, thermal actuation mechanism and the experimental results. The measured maximum angular deflection achieved was 17° at an operating applied voltage of less than 2 V, and the radius of curvature of the mirror plate was in the range from 20 to 50 mm. The micromirror was developed for a miniature catheter optical probe for optical coherence tomography in vivo imaging. A low cross-sectional size of the probe and higher resolution are essential for investigating inaccessible pathologies in vivo. This required a compact micromirror chip and yet sufficiently large mirror plate (typically ~500 µm or more), this trade-off was the key motivation for the research presented in this paper.

Design and development of a 3D scanning MEMS OCT probe using a novel SiOB package assembly

A MEMS optical coherence tomography (OCT) probe prototype was developed using a unique assembly based on silicon optical bench (SiOB) methodology. The probe is formed by integrating a three-dimensional (3D) scanning micromirror, gradient refractive index (GRIN) lens and optical fiber on SiOB substrates having prefabricated self-aligned slots. The two-axis scanning micromirror is based on electrothermal actuation with required voltage less than 2 V for mechanical deflections up to 17°. The optical probe was enclosed within a biocompatible, transparent and waterproof polycarbonate tube with a view of in vivo diagnostic applications. The diameter of the miniature probe is less than 4 mm and the length of its rigid part is about 25 mm. The probe engineering and proof of concept of the probe were demonstrated by obtaining en face and three-dimensional OCT images of an IR card used as a standard sample.

Ultra long high resolution beam by multi-zone rotationally symmetrical complex pupil filter.

An ultra long high resolution beam with extension of depth of focus (DoF) in the axial direction as well as high resolution in the transverse direction has been demonstrated by a seven-zone rotationally symmetrical complex pupil filter imposed at the aperture of a focusing lens. Both amplitude and phase of the transmitted light are modulated in different zones. The scalar diffraction theory is used to optimize the zone parameters. Simulation results show that extended DoF of the beam is increased by 16 times while the spot size at the beam waist is reduced to 0.7 times.

Two-Axis Gimbal-Less Electrothermal Micromirror for Large-Angle Circumferential Scanning

In this paper, we present the design, simulation, fabrication, and characterization of a two-axis gimbal-less micromirror optimized for large-angle circumferential scanning applications. The single-crystal silicon micromirror consists of novel folded bimorph electrothermal actuators, flexural springs, and a mirror plate coated with a high-reflective Cr/Au thin film. A modified silicon-on-insulator microelectromechanical system process has been applied on the micromirror fabrication. The single-wafer process is based on deep reactive ion etching in order to achieve high fill factor as well as small outline of the micromirror chip. A new mechanical design of bimorph actuators is developed to further increase the mechanical deflection. Both theoretical and experimental results demonstrate that the micromirror can provide a uniform mechanical deflection up to ~9deg with multichannel driving voltages of less than 2.3 V to any angle in full 360deg circumference.

Microfabricated endoscopic probe integrated MEMS micromirror for optical coherence tomography bioimaging

In this paper, we present a miniaturized endoscopic probe, consisted of MEMS micromirror, silicon optical bench (SiOB), grade index (GRIN) lens, single mode optical fiber (SMF) and transparent housing, for optical coherence tomography (OCT) bioimaging. Due to the use of the MEMS micromirror, the endoscopic OCT system is highly suitable for non-invasive imaging diagnosis of a wide variety of inner organs. The probe engineering and proof of concept were demonstrated by obtaining the two-dimensional OCT images with a cover slide and an onion used as standard samples and the axial resolution was around 10µm.

Platinum microheater integrated silicon optical bench assembly for endoscopic optical coherence tomography

A novel platinum microheater and comb isolator integrated silicon optical bench (SiOB) assembly method has been successfully demonstrated to provide electrical connection and high precision alignment for a two-axis gimbal-less micromirror. Localized heating and wetting of plastic core micro solder balls is achieved by the integrated platinum heater, and the maximum measured temperature of the platinum heater is about 250 °C. In addition, assembly procedures are significantly simplified by involving a comb isolator made by deep reactive ion etching (DRIE) in comparison with our previously reported design. DRIE is also applied to form a 45° trench on the lower silicon substrate with a nearly vertical sidewall for the micromirror. Hence, the overall dimensions of the SiOB assembly can be reduced further to meet the requirements of endoscopic optical coherence tomography (EOCT) for miniaturization.

A large rotational angle micromirror based on hypocycloidal electrothermal actuators for endoscopic imaging

The paper presents a large rotational angle micromirror base on hypocycloidal electrothermal actuators for circumferential endoscopic imaging. The micromirror consists of a double-side Cr/Au coated high reflective mirror plate (1mm by 0.8mm) laterally supported by two hypocycloidal electrothermal actuators on both sides (Fig. 1(a)). In our design, 1µm PVD Al deposited on 2µm single crystal silicon (SCS) forms a bimorph microstructure with the length of 800 µm and the width of 60µm. Four bimorph structures were staggerly connected in parallel to form a hypocycloidal electrothermal actuator. In this configuration, a metal layer was on a silicon backbone in one bimorph structure while the metal layer was deposited below the silicon backbone in adjacent bimorph structures (Fig. 1(b)). Since the radius of curvature of each bimorph structure is the same, the deflection of each structure is the same. Hence the rotational axis keeps still and there is no lateral shifting effect. Simulations via finite element analysis (FEA) show that the mechanical deflection angle of a micromirror significantly increases by using this actuator design. 141.2° was found in the design with fully double-side Al coated actuators (Fig. 2(a, b)) and 68.6° was found in the design with only frontside Al coated actuators (Fig. 2(c, d)). Micromirrors were fabricated by a post-CMOS MEMS process on 8 inches SOI wafers. An optical microscopic image and a scanning electron microscope (SEM) micrograph of a released micromirror are shown in Fig. 3(a) and (b), respectively. However, so far we have not successfully patterned Al layer below the SCS layer as part of the actuator and therefore only micromirrors equipped by frontside Al coated actuators were experimentally characterized (Fig. 4). ∼35° mechanical deflection was achieved by 2.6 V DC input voltage (Fig. 5). It has a discrepancy in comparison in comparison with the FEA simulation. −3dB cutoff frequency was found to be about 29 Hz as the large signal frequency response (Fig. 5). Current-voltage relationship of an electrothermal actuator is also shown in Fig. 5. A series of frames from a video of a switching micromirror shows various tilting angles of the micromirror under a sinusoidal drive signal with the amplitude of 2.6 V was still with absence of microstructures with backside Al coated, the concept of achieving large deflection angle by using hypocycloidal electrothermal actuators has been demonstrated. Both FEA simulation and experimental results prove the capability of the Single-axis rotational micromirror device.

Novel 3D micromirror for miniature optical bio-robe SiOB assembly

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.