Jong-seok Kim

Frequency shifts in two-level ultra-deep reactive ion etched slow-wave structures for 0.1 THz backward-wave oscillations

We present microfabricated slow-wave structures for millimeter- or terahertz-wave vacuum electronic sources. A two-level ultra-deep reactive ion etching (u-DRIE) on highly doped silicon wafers has been employed and allowed for complicated 3-dimensional structures with high aspect ratio. The measured spectra of return loss, however, show 1.2% and 6.8% upshifts in both cutoff and resonant frequencies, respectively. We found the suppression of two-level u-DRIE at the narrow channel between resonant cavities has caused the change of aspect ratios, i.e., saddle-shaped bottom surfaces, which is proved to be associated with the difference in frequency shifts as well as RF attenuation by comparison with theoretical prediction.

Microfabricated coupled-cavity backward-wave oscillator for terahertz imaging

A design study of a 0.1-THz coupled-cavity backward-wave oscillator (CCBWO) is presented using a finite element method and a particle-in-cell simulation. In experiments, the Si-based technology of 2-step deep reactive ion etching (DRIE) and eutectic bonding shows a good potential for the microfabrication of the interaction circuit. A miniaturized field emission cathode using carbon nanotubes grown by chemical vapor deposition (CVD) is also a good candidate for the electron beam generation. An integration of the cathode and the interaction circuit will be discussed. In addition, a preliminary design of a continuous-wave (CW) THz imaging system is demonstrated for the THz propagation and detection.

Quality factor measurement of micro gyroscope structure according to vacuum level and desired Q-factor range package method

A micro-machined gyro chip of gyroscope is normally packaged in specific vacuum level to get the specific quality factor(Q-factor). If the Q-factor is too high, frequency tuning and the approximate matching between driving and sensing comb structure become difficult, and if the Q-factor is too low, its sensitivity decreases. The optimum Q-factor of our gyro chip design is 4000 range. To get this range, we measured the drive mode Q-factor as vacuum level of our gyro chip and we found that the vacuum level of the desired Q-factor 4000 is in the range of 740 mTorr. Based on this data, we fabricate the wafer level package gyro chip of the desired Q-factor by controlled the basic pressure of package bonding chamber just prior to the bonding process. After wafer level package process, we measured Q-factor of whole samples. Among 804 samples, 502 packaged gyro chips are worked and the Q-factor of 67% samples is between 3500 and 4500 range.

A thermocompressive bonding method using a pure sputtered Au layer and its wafer scale package application

The authors have developed a thermocompressive bonding method using a sputtered Au layer and applied it to the coplanar waveguide (CPW) package. The bonding temperature is 350°C, the bonding pressure is 63MPa, and the duration time is limited to 30min. The bonding strength is evaluated using the Scotch tape and tweezers detaching method and the hermeticity is evaluated using helium leak detection work station. The measured hermeticity is 1.0×10−9Pam3∕s. To measure the electrical properties, the CPW line is packaged and the rf characteristics and dc resistance are measured. The insertion loss of the packaged CPW line is −0.074dB at 2GHz and the dc resistance is from 0.019to0.046Ω. This result shows that their Au compressive bonding method is very useful and good for microdevice wafer level packaging 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.

Study of sand blaster dry etched glass wafer surface for micro device package

In this paper, glass cap wafer for MEMS device package is fabricated by using sand blaster dry etcher and Its surface is studied. The surface of dry etched glass is analyzed by using SEM, and many glass particles and micro cracks are observed. If these kind of particles were dropped from glass to the surface of device, It would make critical failure to the operation of device. So, several cleaning and etching methods are induced to remove these kinds of dormant failure mode and optimized condition is found out.

Bi-polar high-voltage pulse generator using semiconductor switches

A very fast bi-polar high voltage pulse power generator using semiconductor switches is proposed. The proposed system is made up of a thyristor based-rectifier, DC link push-pull resonant inverter, a high voltage transformer, charging capacitors, a high voltage bilateral IGBT switch stacks, and a variable capacitor. The proposed system makes bi-polar high voltage sinusoidal waveform using resonance between leakage inductance of the transformer and secondary capacitor and transfers energy to output load at maximum of the secondary capacitor voltage. The proposed pulse power supply has simple structure using one switch module without DC energy storage and gives high efficiency.