Sung-Hoon Choa

Package-level integrated antennas based on LTCC technology

We propose a novel package topology integrating multilayer miniaturized antennas. Such a functional package is suitable for the design of a system-on-chip device, or of system-on-package applications. A stacked patch antenna is designed and integrated in a package using a low temperature co-fired ceramic process. The overall size of the package is 10.3times10.3times1.3 mm3, and this package contains an 8.3times8.3times0.7 mm3 internal space for the integration of chip-scale packaged components. The package is mounted on a 20times20 mm2 ground plane to miniaturize the volume of the system. The antenna is designed to have two neighboring resonant frequencies at 5.264 and 5.355 GHz, resulting in a 140 MHz impedance bandwidth. However, the measured resonant frequencies occur at slightly higher frequencies due to manufacturing tolerances. Radiation patterns are similar to a conventional patch antenna. In addition, various parasitic effects rooted in the package size, ground size, antenna height, SMA connector, via misalignment, and the number of via holes and their locations are fully investigated

Development of Micro-Heaters with Optimized Temperature Compensation Design for Gas Sensors

One of the key components of a chemical gas sensor is a MEMS micro-heater. Micro-heaters are used in both semiconductor gas sensors and NDIR gas sensors; however they each require different heat dissipation characteristics. For the semiconductor gas sensors, a uniform temperature is required over a wide area of the heater. On the other hand, for the NDIR gas sensor, the micro-heater needs high levels of infrared radiation in order to increase sensitivity. In this study, a novel design of a poly-Si micro-heater is proposed to improve the uniformity of heat dissipation on the heating plate. Temperature uniformity of the micro-heater is achieved by compensating for the variation in power consumption around the perimeter of the heater. With the power compensated design, the uniform heating area is increased by 2.5 times and the average temperature goes up by 40 °C. Therefore, this power compensated micro-heater design is suitable for a semiconductor gas sensor. Meanwhile, the poly-Si micro-heater without compensation shows a higher level of infrared radiation under equal power consumption conditions. This indicates that the micro-heater without compensation is more suitable for a NDIR gas sensor. Furthermore, the micro-heater shows a short response time of less than 20ms, indicating a very high efficiency of pulse driving.

Application of Au-Sn eutectic bonding in hermetic radio-frequency microelectromechanical system wafer level packaging

Development of packaging is one of the critical issues toward realizing commercialization of radio-frequency-microelectromechanical system (RF-MEMS) devices. The RF-MEMS package should be designed to have small size, hermetic protection, good RF performance, and high reliability. In addition, packaging should be conducted at sufficiently low temperature. In this paper, a low-temperature hermetic wafer level packaging scheme for the RF-MEMS devices is presented. For hermetic sealing, Au-Sn eutectic bonding technology at temperatures below 300°C is used. Au-Sn multilayer metallization with a square loop of 70 µm in width is performed. The electrical feed-through is achieved by the vertical through-hole via filling with electroplated Cu. The size of the MEMS package is 1 mm × 1 mm × 700 µm. The shear strength and hermeticity of the package satisfies the requirements of MIL-STD-883F. Any organic gases or contamination are not observed inside the package. The total insertion loss for the packaging is 0.075 dB at 2 GHz. Furthermore, the robustness of the package is demonstrated by observing no performance degradation and physical damage of the package after several reliability tests.

Reliability of vacuum packaged MEMS gyroscopes

The greatest challenge for the successful commercialization of MEMS (micro-electro-mechanical system) technology is proving its reliability. Of concern 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 and investigated the failure mechanisms of the anodically bonded vacuum gyroscope sensors designed for commercial electronic products. Particularly we studied mechanical reliability issues such as fatigue, shock, and vibration. It was found that successful vacuum packaging could be achieved through the optimization of the bonding process by reducing leakage and the deposition of titanium coating for reducing out-gassing inside the cavity. The effects of the pre-baking process are also described in this study. The current design of the gyroscope structure is found to be safe from fatigue failure for the 1000 h of operation test. The MEMS gyroscope sensor survives the drop and vibration qualification tests for electronic products without any damage, indicating the robustness of the sensor. The reliability test results presented in this study demonstrate that the MEMS gyroscope sensor is very close to commercialization.

A high yield rate MEMS gyroscope with a packaged SiOG process

MEMS devices such as a vibratory gyroscope often suffer from a lower yield rate due to fabrication errors and external stress. In the decoupled vibratory gyroscope, the main factor that determines the yield rate is the frequency difference between the sensing and driving modes. The gyroscope, fabricated with a SOI (silicon-on-insulator) wafer and packaged using anodic bonding, has a large wafer bowing caused by thermal expansion mismatch as well as non-uniform surfaces of the structures caused by the notching effect. These effects result in a large distribution in the frequency difference, and thereby a lower yield rate. To improve the yield rate we propose a packaged SiOG (silicon-on-glass) technology. It uses a silicon wafer and two glass wafers to minimize the wafer bowing and a metallic membrane to avoid the notching. In the packaged SiOG gyroscope, the notching effect is eliminated and the warpage of the wafer is greatly reduced. Consequently, the frequency difference is more uniformly distributed and its variation is greatly improved. Therefore, we can achieve a more robust vibratory MEMS gyroscope with a higher yield rate.

Mechanical Flexibility of ZnSnO/Ag/ZnSnO Films Grown by Roll-to-Roll Sputtering for Flexible Organic Photovoltaics

The mechanical flexibility of ZnSnO (ZTO)/Ag/ZTO multilayer films prepared on flexible poly(ethylene terephthalate) (PET) substrates by roll-to-roll (R2R) sputtering was investigated for use in cost-efficient flexible organic solar cells (FOSCs). The change of resistance (ΔR) of the ZTO/Ag/ZTO films was measured by means of lab-made outer/inner bending, twist bending, and stretching test systems. The failure bending radii of the ZTO/Ag/ZTO film in the outer and inner bending tests were 3 and 4.5 mm, respectively. In addition, the twisting test showed that the resistance of the ZTO/Ag/ZTO multilayer began to increase at an angle of 38°. Furthermore, the stretching test showed that the strain failure of the ZTO/Ag/ZTO multilayer film was 3%. The superior flexibility of the ZTO/Ag/ZTO films is attributed to the existence of a ductile Ag layer and the mechanical stability of the amorphous ZTO film. Similar performances of the FOSCs with flexible ZTO/Ag/ZTO anodes to reference OSCs with ITO anodes indicate that the R2R sputter-grown ZTO/Ag/ZTO multilayer is promising as an indium-free flexible anode for cost-efficient FOSCs.

Magneto-Logic Device Based on a Single-Layer Magnetic Tunnel Junction

A magnetic tunnel junction (MTJ) element can compute Boolean functions and also store the output of its last operation. Therefore, the MTJ shows potential for a universal logic element to implement sequential-logic functions as well as combinatorial ones. The established magneto-logic element has been designed and fabricated based on a triple-layer MTJ. We present a novel magneto-logic structure that consists of a single-layer MTJ and a current driver, which requires less processing steps with enhanced functional flexibility and uniformity.

MECHANICAL FLEXIBILITY OF ZINC OXIDE THIN-FILM TRANSISTORS PREPARED BY TRANSFER PRINTING METHOD

In the present study, we demonstrate the performance of Zinc oxide thin film transistors (ZnO TFTs) array subjected to the strain under high bending test and the reliability of TFTs was confirmed for the bending fatigue test of 2000 cycles. Initially, ZnO TFTs were fabricated on Si substrate and subsequently transferred on flexible PET substrate using transfer printing process. It was observed that when the bending radius reached ≥ 11 mm then cracks start to initiate first at SiO2 bridges, acting as interconnecting layers among individual TFT. Whatever the strain is applied to the devices, it is almost equivalently adopted by the SiO2 bridges, as they are relatively weak compared to rest of the part. The initial cracking of destructed SiO2 bridge leads to the secondary cracks to the ITO electrodes upon further increment of bending radius. Numerical simulation suggested that the strain of SiO2 layer reached to fracture level of 0.55% which was concentrated at the edge of SiO2 bridge layer. It also suggests that the round shape of SiO2 bridge can be more fruitful to compensate the stress concentration and to prevent failure of device.

Development of a high-sensitivity strain measurement system based on a SH SAW sensor

A strain measurement system based on a shear horizontal surface acoustic wave (SH SAW) was developed. The developed system is composed of a SAW microsensor, a printed circuit board (PCB), an adhesive and a strain gauge. When a compression force is applied to the PCB by the strain gauge, the PCB is bent so that external strain energy can be evenly delivered to the microsensor without any detachment of the sensor from the board. When a stretching force is applied to the PCB under the condition that one side of the PCB is fixed and the other side is modulated, the actual length of the SAW delay line between the two interdigital transducers (IDTs) is increased. The increase in the delay line length causes a change in the time for the propagating SAW to reach the output IDT. If strain energy is applied to the piezoelectric substrate, the substrate density is changed, which then changes the propagation velocity of the SAW. Coupling-of-modes modeling was conducted prior to fabrication to determine the optimal device parameters. Depending on the strain, the frequency difference was linearly modulated. The obtained sensitivity for stretching was 17.3 kHz/% for the SH wave mode and split electrode. And the obtained sensitivity for bending was 46.1 kHz/% for the SH wave mode and split electrode. The SH wave showed about 15% higher sensitivity than the Rayleigh wave, and the dog-bone PCB showed about 8% higher sensitivity than the rectangular PCB. The obtained sensitivity was about five times higher than that of existing SAW-based strain sensors.

Dynamic absorber for actuator arm in a disk drive

Dynamic absorber is introduced to assist in controlling the amplitude and acceleration of the actuator arm in disk drive so that it can improve the shock handling capability in both operation and non-operating condition. The effect of the dynamic absorber on shock performance is examined through numerical simulation and experimental test.