Yong-chul Cho

Electrostatic 1D microscanner with vertical combs for HD resolution display

An electrostatic 1 dimensionally (1D) scanning mirror for HD resolution display is introduced. Vertical comb drive was used to tilt the micro mirror. To minimize the moment of inertia and maximize the tilting angle of the mirror having the diameter of 1.6 mm, the rib was patterned on the backside of the mirror surface and optimized. Via the finite element simulation, the dynamic deformation of 45nm was achieved within the reflecting area in operating resonant mode thanks to the optimized rib structure. The actuating part of scanner was also optimized manipulating with several design variables to get maximum tilting angle. As the fabrication result, mechanical tilting angle of ±12.0 degree was achieved with the resonant frequency of 24.75kHz and the sinusoidal driving voltage of 280Vpp. For stable resonant motion of the scanner, the feedback control algorithm was realized in the driving circuit. Rigorous reliability characterization was carried out using statistical analysis on the fabricated samples. As a result, HD-resolution image with 720 progressive horizontal lines was demonstrated.

Slow scanning electromagnetic scanner for laser display

A small sized, low power consuming, shock proven optical scanner with a capacitive comb-type rotational sensor for the application of mobile projection display is designed, fabricated, and characterized. To get a 2-D video image, the present device horizontally scans a vertical line image made through a line-type diffractive spatial optical modulator. To minimize, device size as well as power consumption, the mirror surface is placed on the opposite side of the coil actuator. To prevent thermal deformation of the mirror, the mirror is partially connected to the center point of the coil actuator. To be shock proof, mechanical stoppers are constructed in the device. The scanner is fabricated from two silicon wafers and one glass wafer using bulk micromachining technology. The packaged scanner consists of the scanner chip, a pair of magnets, yoke rim, and base plate. The fabricated package size is 9.2×10×3 mm (0.28 cc) and the mirror size is 3×1.5 mm. The scanner chip receives no damage under the shock test with an impact of 2000 G in 1 ms. In the case of a full optical scan angle of 30 deg at 120-Hz driving frequency, linearity, repeatability, and power consumption are measured at 98%, 0.013 deg, and 60 mW, respectively, which are suitable for mobile display applications.

Three-dimensional imaging using fast micromachined electro-absorptive shutter

Abstract. A 20-MHz switching high-speed light-modulating device for three-dimensional (3-D) image capturing and its system prototype are presented. For 3-D image capturing, the system utilizes a time-of-flight (TOF) principle by means of a 20-MHz high-speed micromachined electro-absorptive modulator, the so-called optical shutter. The high-speed modulation is obtained by utilizing the electro-absorption mechanism of the multilayer structure, which has an optical resonance cavity and light-absorption epilayers grown by metal organic chemical vapor deposition process. The optical shutter device is specially designed to have small resistor–capacitor–time constant to get the high-speed modulation. The optical shutter is positioned in front of a standard high-resolution complementary metal oxide semiconductor image sensor. The optical shutter modulates the incoming infrared image to acquire the depth image. The suggested novel optical shutter device enables capturing of a full high resolution-depth image, which has been limited to video graphics array (VGA) by previous depth-capturing technologies. The suggested 3-D image sensing device can have a crucial impact on 3-D–related business such as 3-D cameras, gesture recognition, user interfaces, and 3-D displays. This paper presents micro-opto-electro-mechanical systems-based optical shutter design, fabrication, characterization, 3-D camera system prototype, and image evaluation.

Gimbaled 2D Scanning Mirror with Vertical Combs for Laser Display

We designed a gimbaled 2D scanning mirror (GSM) which is driven electrostatically by vertical comb fingers. The horizontal scanning mirror with a diameter of 1 mm is rotating with respect to y-axis in the eye-shaped frame. For vertical scanning, the eye-shaped frame is rotating with respect to x-axis which is perpendicular to the horizontal scanning axis. This GSM can be actuated in two axes simultaneously resulting in the realization of 2D image in the screen with single beam raster scanning. We obtained a mechanical scanning angle of plusmn8deg(resonance) and plusmn4.2deg(non-resonance), with the resonant frequency range of 24~24.5 kHz and 800~900 Hz under the sinusoidal and triangular driving voltages for horizontal and vertical scanning, respectively

A robust design and fabrication of micromachined electro-absorptive optical modulator for 3D imaging

A time-of-flight (TOF) based three dimensional (3D) image capturing system and its enhanced optical modulating device are presented. The 3D image capturing system includes 850nm IR emitter (typically compact Laser diodes) and high speed image modulator, so called optical shutter. The optical shutter consists of multi-layered optical resonance cavity and electro-absorptive layers. The optical shutter is a solid-state controllable filter which modulates the IR image to extract the phase delay due to TOF of the emitting IR light. This presentation especially addresses robustness issues and solutions when operated under practical environments such as ambient temperature variation and existence of strong ambient light (e.g. outdoors). The wavelength of laser diode varies substantially depending on the ambient temperature, which degrades the modulation efficiency. To get a robust operation, the bandwidth of transmittance of the optical shutter is drastically improved with a novel coupled Fabry-Perot resonance cavity design to come up with the wavelength variation of the laser diode. Also, to suppress the interference of solar irradiance to IR source signal, a novel driving scheme is applied, in which IR light and optical shutter modulation duties are timely localized, i. e. ‘bursted’. Suggested novel optical shutter design and burst driving scheme enable capturing of a full HD resolution of depth image under the realistic usage environments, which so far tackle the commercialization of TOF cameras. Design, fabrication, and evaluation of the optical shutter; and, 3D capturing system prototype, image test results are presented.

Slow Scanning Electromagnetic MEMS Scanner for Laser Display

A small size, low power consuming, shock proven optical scanner with capacitive comb type rotational sensor for the application of mobile projection display was designed, fabricated, and characterized. To get a 2-dimensional video image, the present device horizontally scans a vertical line image made through a line-type diffractive spatial optical modulator. In order to minimize device size as well as power consumption, the mirror surface was placed on the opposite side of the coil actuator. To prevent thermal deformation of the mirror, the mirror was partially connected to the center point of the coil actuator. For shock proof, mechanical stoppers were constructed in the device. The scanner was fabricated from two silicon wafers and one glass wafer using a bulk micromachining technology. The packaged scanner consists of the scanner chip, a pair of magnets, yoke rim, and base plate. The fabricated package size is 9.2mmx10mmx3mm (0.28cc) and the mirror size is 3mmx1.5mm. The scanner chip has no damage under the shock test with impact of 2,000G in 1ms. In case of full optical scan angle of 30° at 120Hz driving frequency, linearity and power consumption are measured 98% and 60mW, respectively, which are suitable for mobile display applications.

Reliability Assessment of MEMS Gyroscope Sensor

Reliability of MEMS devices is receiving more attention as they are heading towards commercial production. In particular are the reliability and long-term stability of wafer level vacuum packaged MEMS gyroscope sensors subjected to cyclic mechanical stresses at high frequencies. In this study, we carried out several reliability tests such as environmental storage, fatigue, shock, and vibration, and we investigated the failure mechanisms of the anodically bonded vacuum gyroscope sensors. It was found that successful vacuum packaging could be achieved through reducing outgassing inside the cavity by deposition of titanium as well as by pre-taking process. The current gyroscope structure is found to be safe from fatigue failure for 1000 hours of operation test. The gyroscope sensor survives the drop and vibration tests without any damage, indicating robustness of the sensor. The reliability test results presented in this study demonstrate that MEMS gyroscope sensor is very close to commercialization.

MOEMS-based time-of-flight camera for 3D video capturing

We suggest a Time-of-Flight (TOF) video camera capturing real-time depth images (a.k.a depth map), which are generated from the fast-modulated IR images utilizing a novel MOEMS modulator having switching speed of 20 MHz. In general, 3 or 4 independent IR (e.g. 850nm) images are required to generate a single frame of depth image. Captured video image of a moving object frequently shows motion drag between sequentially captured IR images, which results in so called ‘motion blur’ problem even when the frame rate of depth image is fast (e.g. 30 to 60 Hz). We propose a novel ‘single shot’ TOF 3D camera architecture generating a single depth image out of synchronized captured IR images. The imaging system constitutes of 2x2 imaging lens array, MOEMS optical shutters (modulator) placed on each lens aperture and a standard CMOS image sensor. The IR light reflected from object is modulated by optical shutters on the apertures of 2x2 lens array and then transmitted images are captured on the image sensor resulting in 2x2 sub-IR images. As a result, the depth image is generated with those simultaneously captured 4 independent sub-IR images, hence the motion blur problem is canceled. The resulting performance is very useful in the applications of 3D camera to a human-machine interaction device such as user interface of TV, monitor, or hand held devices and motion capturing of human body. In addition, we show that the presented 3D camera can be modified to capture color together with depth image simultaneously on ‘single shot’ frame rate.