Heung-Woo Park

A novel low-loss wafer-level packaging of the RF-MEMS devices

In this work, the flip-chip method was used for packaging an RF-MEMS switch on a quartz substrate with low losses. 4-inch Pyrex glass was used as a package substrate and was airblast punched with 250 /spl mu/m diameter holes. A Cr/Au seed layer was deposited on it and the vias were filled by plating with gold. After forming molds on the holes with thick photoresist, bumps were plated on holes. The package substrate was bonded with a quartz substrate using B-stage epoxy. The loss of the overall package structure was tested with a network analyzer and was within -0.05 dB. This structure can be used for wafer level packaging of both RF-MEMS and other MEMS devices.

Emission characteristics of the molybdenum-coated silicon field emitter array

Uniform and reproducible silicon tip arrays were fabricated using the reactive ion etching followed by the re-oxidation sharpening. Molybdenum was coated on the some of the silicon tip array. Current-voltage characteristics and current fluctuations were tested in the high vacuum level. Turn-on voltage of the uncoated tip was 35 V and maximum current was 1 mA, while turn-on voltage of the coated tip was 15 V and maximum current was 6 mA. While uncoated silicon tip was destroyed after 13 minutes after operation and showed rapid lowering of emission current, coated tip showed stable emission characteristics. Obtained currents were proved to be field emission currents using the Fowler-Nordheim plot studies.

Packaging of the rf MEMS switch

In this work, the flip-chip method was used for packaging of the RF-MEMS switch on the quartz substrate with low losses. The 4-inch Pyrex glass was used as a package substrate and it was punched with airblast with 250 micrometers diameter holes. The Cr/Au seed layer was deposited on it and the vias were filled with plating gold. After forming the molds on the holes with thick photoresist, the bumps were plated on holes. The package substrate was bonded with the quartz substrate with the B-stage epoxy. The loss of the overall package structure was tested with a network analyzer and was within -0.05 dB. This structure can be used for wafer level packaging of not only the RF-MEMS devices but also the MEMS devices.

Electrostatically vacuum sealed tunneling magnetic field sensors

This work reports the tunneling effects of the lateral field emitters. Tunneling effect is applicable to the VMFS(vacuum magnetic field sensors). VMFS uses the fact that the trajectory of the emitted electrons are curved by the magnetic field due to Lorentz force. Poly-silicon cantilevers were used as field emitters and anode materials. Thickness of the emitter and the anode were 2μm, respectively. PSG(phospho-silicate-glass) was used as a sacrificial layer and it was etched by HF. Cantilevers were doped with POCl3(1020cm3). 2μm-thick cantilevers were fabricated onto PSG(2μm-thick). Sublimation drying method was used at releasing step to avoid stiction. Then, the device was vacuum sealed. Device was fixed to a sodalime-glass#1 with silver paste and it was wire bonded. Glass#1 has a predefined hole and a sputtered silicon-film at backside. The front-side of the device was sealed with a sodalime-glass#2 using the glass frit. After getter insertion via the hole, backside of the glass#1 was sealed electrostatically with a sodalime-glass#3 at 10-6 torr. After sealing, getter was activated. Sealing was successful to operate the tunneling device. The packaged VMFS showed reduced emission current compared with the chamber test prior to sealing. The emission currents were changed when the magnetic field was induced. A VMFS of angular anodes were tested and its sensitivity was about 3%.

Fabrication of an uncooled infrared sensor using pyroelectric thin film

In conventional IR-sensors, there are problems of needing cooler and sensing wavelength limitation. These problems can be achieved by using un-cooling thermal IR senors. However, they raise the problems of the attack of pyroelectric thin film layer during the etching of sacrificial layer as well as the thermal isolation of the IR detection layer. In order to fabricate uncooled IR-sensor using pyroelectric film, multilayer should be prepared pyroelectric thin film and thermally isolating membrane structure of square-shaped microstructures. We used the direct bonding technique to avoid the thermal loss by silicon substrate and the attack of pyroelectric thin film by etchant of the sacrificial layer. Metallic Pt layer used as a top and a bottom electrodes were deposited by E-beam sputtering method, while pyroelectric thin films were prepared Sol-Gel techniques. Because the pyroelectric thin film with c-axial orientation raised thermal polarization without the polling, the more integrated capability could be achieved. We investigated the characterized of the pyroelectric thin films: P-E loop, dielectric constant, XRD etc.

Field emission properties of Mo-coated Si field emitter arrays and formation of molybdenum silicide

Uniform and reproducible silicon tip arrays were fabricated using the reactive ion etching followed by the re-oxidation sharpening. Molybdenum was coated on the some of the silicon tip array. Current-voltage characteristics and current fluctuations were tested in the high vacuum level. Turn-on voltage of the uncoated tip was 35 V and maximum current was 1 mA, while turn-on voltage of the coated tip was 15 V and maximum current was 6 mA. While uncoated silicon tip was destroyed after 13 minutes after operation and showed rapid lowering of emission current, coated tip showed stable emission characteristics. Obtained currents were proved to be field emission currents using the Fowler-Nordheim plot studies.