Won-Ick Jang

Bistable planar polysilicon microactuator with shallow arch-shaped leaf springs

Bi-stable microactuators are necessary to implement optical switch and microrelay with low power and high reliability. In this work, we analyzed the buckling and vibration characteristics of a planar microactuators with shallow arch- shaped leaf springs. To investigate elastic stability of the proposed microactuator, we derived static buckling modes. A concentrated force of 0.35 muN at the center of beam was required for the snap-through motion for the beam length of 1600 micrometer, thickness of 3 micrometer, beam width of 6.5 micrometer and initial rise of 15 micrometer considering only the first buckling mode. We also analyzed vibration characteristics of arch-shaped leaf spring. The resonant frequencies of the first modes across over the second mode and keeps constant resonant frequencies over the cross point. On the contrary, the resonant frequencies of second modes become almost constant regardless of initial rise. The planar microactuator with shallow arch-shaped leaf springs at both sides were fabricated using silicon micromachining technology. The vertical structure of the planar microactuator features simplicity and consists of p-doped polysilicon as a structural layer and LTO (Low Temperature Oxide) as a sacrificial layer. The polysilicon was annealed for the relaxation of residual stress and HF GPE (gas-phase etching) process was finally employed in order to release the microactuators. These bi- stable planar microactuators with shallow arch-shaped leaf springs showed a high stiffness against external disturbance, and would be very useful for the stable operation of micro optical switch and microrelay.

Focusing and leveling system using position-sensitive detectors for the wafer steppers

An optical focus and leveling system for ETRI KrF excimer laser stepper is developed using position sensitive detectors (PSD) and optical magnification method. This type of detection method showed focusing and leveling accuracies of about +/- 0.1 micrometers and +/- 1.0 arcsec (+/- 0.5 X 10-5 rad) respectively. Also, we confirmed experimentally the autofocus system has +/- 0.15 micrometers signal stability within the controlled temperature range of +/- 0.1 degree(s)C. In this paper, we report the design concepts of the focusing and leveling system and the characteristics of the system parameter applied to ETRI KrF excimer laser stepper.

Formation of low-stress multilayered thick polysilicon films for fabrication of microsystems

The effects of impurity doping and heat treatments on the characteristics of thick polysilicon films were studied for development of the structural materials in the MEMS. In this study, 8-15 layers of 6.5-12 micrometers thickness polysilicon films were deposited to have a symmetrical structure using low-pressure chemical vapor deposition with a novel stacking method. We have measured the physical and structural characteristics using micromachined test patterns to verify the minimal stress and stress gradient in the polysilicon layers, according to the film stacking, doping, and thermal treatment methods. The multilayer film revealed the complex orientation composed of (100), (220) and (311) grains after annealing and showe4d a higher doping concentration induced a higher compressive stress of 70 Mpa since phosphorus gave rise to a compressive stress in a polysilicon film. However, the doping method for the most uniform distribution of phosphorus induced the lowest stress gradient among all samples. A polysilicon microresonator with thickness of 6.5 micrometers were manufactured by the symmetrical stacking and optimum doping method in which the dopant concentration was lowered and annealing at 1000 degrees C. The film had a low stress of 7.6 MPa and a low stress gradient of -0.15 MPa/micrometers and revealed good slopes of sidewalls after dry etching. The fabricated test structure for a micro gyroscope showed that the driving resonant frequency and the sensitivity was measured as 9,175 Hz and 5 mV-sec/deg under the condition of a static angular velocity, respectively.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Silicon surface micromachining of a deep vacuum cavity structure and its application to a microflow sensor

Main interests for MEMS devices are to reduce thermal, dielectric and magnetic loss in active areas due to a substrate and an air medium. For this purpose, deep vacuum cavity structures with planarized stacked membranes were fabricated by the DECTOR process based on silicon surface micromachining. We discuss details of the developed process, especially the effects of a Si trench geometry, post- annealing of the poly-Si layer and HF release conditions on completion of the vacuum structure. To identify validity of the proposed microstructures, thermal microflow sensors having an n+-doped heater and two n+- /p+-doped thermopiles with poly-Si lines were implemented on the various cavity structures of 100 by 100 by 6.2 micrometers 3 using additional CMOS batch processing. The heating efficiency of the sensor on the vacuum cavity is increased by a factor of 5.8 and 1.7 compared to the structures with residual oxides and the air cavity, respectively. It is also found that the sensitivity using the downstream thermopile of 2.5 M(Omega) , 1.53 by 10-1 mV/(m/s)/mW under 10 mW input power, is about ten and three times higher than corresponding values with residual oxides and the air cavity. Therefore, the configuration employing the deep vacuum cavity structure has advantages of low power consumption and the high sensitivity. These results support versatile MEMS applications.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Design and development of a prototype excimer-laser-based stepper

This paper describes the design and development of a KrF excimer laser stepper and discusses the detailed system parameters and characterization data obtained from the performance test. We have developed a deep UV step-and-repeat system, operating at 248 nm, by retrofitting commercial modules such as a KrF excimer laser, precision wafer stage and fused silica illumination and 5X projection optics of numerical aperture 0.42. What we have developed, to the basic structure, are wafer alignment optics, reticle alignment system, autofocusing/leveling mechanisms and an environment chamber. Finally, all these subsystems were integrated under the control of microprocessor-based controllers and a computer.

Micromachined silicon actuators with low driving voltage and high accuracy for optical switches and tunable filters

With the great demand for WDM (Wavelength Division Multiplexer) optical communications, optical switches are expected to become essential components in future networks. A micromachined optical device has been developed for optical communications due to its high reliability, low power, low crosstalk, and low insertion loss. In this paper, we present two types of new lateral actuators for optical switches and tunable filters. The microactuator for an optical switch utilized triple-folded springs with higher compliance for low voltage operation, and electrostatic comb driver for large stroke with low power, respectively. For higher resolution of tunable filter, the microactuator employed a stroke reduction mechanism with meander-type springs. In order to verify the effectiveness of a proposed microactuators, we fabricated the prototypes of polysilicon microactuators for optical switch and tunable filter. The lateral microactuator consists of a polysilicon of 6.5 micrometers thickness as a structural layer and thermal oxide of 2 micrometers thickness as a sacrificial layer. The structures of silicon microactuators are patterned by RIE (Reactive Ion Etching), and finally released by using newly developed HF GPE (Gas- Phase Etching) process with virtually no stiction. We showed the theoretical and experimental driving characteristics of the fabricated microactuators and also discussed the optical properties of a designed optical switch with a focusing mirror.

Bandwidth Analysis of a Microgyroscope vibrating in two orthogonal axes on the substrate plane

A microgyroscope, which vibrates in two orthogonal axes on the substrate plane, is designed and fabricated. The shuttle mass of the vibrating gyroscope consists of two parts. The one is outer shuttle mass which vibrates in driving mode guided by four folded sprints attached to anchors. And the other is inner shuttle mass which vibrates in driving mode as the outer frame does and also can vibrate in sensing mode guided by four folded springs attached to the outer shuttle mass. Due to the directions of vibrating modes, it is possible to fabricate the gyroscope with simplified process by using polysilicon on insulator structure. Fabrication processes of the microgyroscope are composed of anisotropic silicon etching by RIE, gas-phase etching of the buried sacrificial oxide layer, metal electrode formation. An electromechanical model of the vibrating microgyroscope was modeled and bandwidth characteristics of the gyroscope were analyzed firstly. The analyzed characteristics of the gyroscope were evaluated by experiment. The gyroscope operates at DC 4V and AC 0.1V in a vacuum chamber of 100mtorr. The detection circuit consists of a discrete sense amplifier and a noise canceling circuit. Using the evaluated electromechanical mode, an operating condition for high performance of the gyroscope is obtained.