Soon-Don Choi

Tin oxide microsensor for LPG monitoring

Abstract A silicon-based SnO 2 gas sensor has been fabricated for monitoring liquified petroleum gas (LPG), commonly used as town gas. The gas sensor is made by silicon IC technology together with SnO f2 thin-film processing. The whole chip with a size of 9 mm x 9 mm consists of nine sensors (three by three array). each sensor is supported by a thin membrane of SiO 2 /Si 3 N 4 /SiO 2 layers that provides a low thermal mass and prevents heat conduction through the surrounding substrate material. Tin oxide thin film is prepared by thermal evaporation of metallic tin granules and subsequent thermal oxidation of the metallic film at 600 °C. To form the SnO 2 (Pt) thin film, a layer of Pt with a thickness of several tens of angstroms is sputtered onto the tin oxide film and heat treated at 500 °C in air for several hours in order to stabilize its electrical response. The fabricated SnO 2 (Pt) microsensors exhibit about 85 and 92% sensitivities to 5000 ppm C 3 H 8 and 5000 ppm C 4 H 10 (the main components of LPG) at 250 °C, respectively, and show a rapid response time of less than 5 s.

Carbon dioxide sensor using NASICON prepared by the sol-gel method

Abstract Solid-electrolyte sensors using NASICON for detecting CO2 gas are prepared by the sol-gel method. Starting materials are the alkoxides Si(OC2H5)4, Zr(OC2H7)4 and NaOC2H5. Because of their fine powders, the sintering temperature can be low. The phase identification is carried out by X-ray diffractometry (XRD). E.m.f. cells are constructed by fixing the NASICON disk to the end of a quartz tube with an inorganic adhesive. The devices with (Li, Ba)CO3 as sensing electrode and air as reference electrode show high sensitivity to 300–5000 ppm CO2 gas and good stability for a sintering temperature of 1000 °C.

Inertial-grade out-of-plane and in-plane differential resonant silicon accelerometers (DRXLs)

Inertial-grade vertical-type and lateral-type differential resonant accelerometers (DRXL) are designed, fabricated and tested for navigation application, using one mask process. The accelerometers consist of an out-of-plane (for z-axis) accelerometer and in-plane (for x, y-axes) accelerometers. The sensing principle of the accelerometer is based on gap-sensitive electrostatic stiffness changing effect. It says that the natural frequency of the accelerometer can be changed according to an electrostatic force on the proof mass of the accelerometer. The out-of-plane resonant accelerometer shows bias stability of 2.5 /spl mu/g, sensitivity of 70 Hz/g and bandwidth of 100 Hz at resonant frequency of 12 kHz. The in-plane resonant accelerometer shows bias stability of 5.2 /spl mu/g, sensitivity of 128 Hz/g and bandwidth of 110 Hz at resonant frequency of 23.4 kHz. The measured performance of two accelerometers are suitable for an application of inertial navigation.

A robust MEMS probe card with vertical guide for a fine pitch test

A vertically guided MEMS probe card was designed to deflect 50 ?m at a force of 1.5 g and achieve less than 50 ?m of pad pitch. Based on our experimental results, the measured average contact resistance of a device under test (DUT) was approximately 0.2 ? at 1.44 g of force and the leakage current between two tips in the distance of one pitch was about 10 pA. In addition, tip planarity was about ?6 ?m with x?y alignment errors within ?8 ?m. A reliability test showed that the average contact resistance was 0.34 ? and the probe tip wear was less than 1 ?m after the 10?000 timed touchdowns. To be capable of fine pitch probing, a cantilever beam was constructed by dry etching using a positive photoresist. After the cantilever beam was formed by silicon etching using a deep RIE etcher inside a deep-recessed trench, a vertically guided structure was created from the cantilever beam. Furthermore, to make a horizontally stopped structure, the cantilever beam was designed to have a pyramid tip with a width bigger than that of the beam itself. This kind of structure is mechanically stable when the tip is applied with an oblique force. Because the probe card can be guided vertically and horizontally, it can be neither broken nor deformed by any directional force. In that respect, this newly proposed probe card is suitable for wafer-level testing and fine pitch device testing.

SO2 sensing characteristics of Nasicon electrolytes

Abstract The SO 2 sensing characteristics of the Nasicon electrolyte cells Pt,SO 2 ,O 2 (I) I Na 2 SO 4 I Nasicon I O 2 (II) in air, Pt and Pt,SO 2 ,O 2 (I) l Na 2 SO 4 (BaSO 4 ) I Nasicon I Na 2 SiO 3 ,Pt have been studied in 5–98 ppm SO 2 concentration range at 300–550°C. The cell with the O 2 reference electrode was very unstable. However, good agreement between the measured and calculated slopes in the Nernst equation was obtained for the cell with the Na 2 SiO 3 solid reference electrode. The addition of 40 mol% BaSO 4 to the Na 2 SO 4 auxiliary electrolyte could eliminate the water vapor effect under the wet SO 2 atmosphere.

A novel MEMS silicon probe card

We have developed a novel cantilever-type probe which is capable of less than 70 of pitch and 12g of force. This probe is suitable for wafer-level burn-in testing, function testing and circuit-board O/S testing including memory and RF devices. The probe was fabricated with epitaxial polysilicon on silicon substrate. The through via hole interconnection was formed in silicon wafer by nickel electroless plating and copper electroplating. The electroless plating is easy method and can deposit film uniformly for deep trench and through hole. Especially, it can deposit film on any substrates without seed layer and allow it to be electroplated. The aspect ratio of through via hole was larger than 10:1 and the contact resistance was less than 1 ohm.

Extension of binary analytical equations to ternary ones

Abstract A series of analytical expressions of predicting and representing ternary thermodynamic properties using binary data has been derived by applying the subregular solution model and the Krupkowski equation. The geometrical methods employed for the derivation are the Muggianu-, the Kohler-, the Colinet- and the Toop- methods. The expressions have been used to predict the partial molar excess free energy in the Cd-Pb-Sn system at 760K. The experimental data of the liquid Cd-Pb-Sn solutions are found to be excellently represented by the derived expressions.

Thermodynamic Properties of Liquid Water up to 8000 Bar and Between 25 and 150°C

Abstract A simple equation of state for the condensed phase, proposed by Cho, is applied to liquid water under pressure up to 8000 bar and between 25 and 150°C. The agreement between measured and calculated volumes is satisfactory over the entire range of temperatures and pressures. The equation shows that the abnormalities in the temperature and pressure dependence of the thermal expansivity, isothermal compressibility and the heat capacity are predicted quite accurately from PVT data only. The values of thermal expansivity, isothermal compressibility, and heat capacity at constant pressure are calculated, and detailed comparisons with the experimental values are made.

A Novel MEMS LC Tank for RF Voltage Controlled Oscillator(VCO)

A linearly tunable MEMS capacitor changing the overlap area and a high-Q electroplated solenoid inductor are developed. The single crystal silicon MEMS structure is bonded with the pyrex glass substrate to make the designed capacitor. The pyrex glass is adopted as a substrate instead of the silicon to reduce the RF losses through the substrate for both a capacitor and a inductor. The measured capacitor shows the nominal capacitance of 1.4 pF, 10 % tuning range at 2 GHz and the inductor has inductance between 1 nH and 5 nH, maximum Q factor over 10 at 2 GHz. The models are also developed and proven by Serenade software of Ansoft Corporation respectively.

Study of Plating Layer Formation of Lightweight Magnesium Alloy (AZ31B)

Magnesium alloys is the lightest by structural metals, but it is not good corrosion resistant because of pit, void. Particularly, AZ31B magnesium alloy sheets that have slag, scratch by rolling process indicate some defects. The objective of this research is to perform uniform plating on AZ31B by studying etching and zincate process. Especially, zincate treatment by zinc salt and pyrophosphate is the most important in the decoration plating. Dissolution of magnesium is reduced by the formation of uniform zinc conversion layer during strick and post process, which decreases defects for plating process.