Bumkyoo Choi

Diagnostic microstructures for the measurement of intrinsic strain in thin films

Freestanding flexible microstructures fabricated from deposited thin films become mechanically unstable when internal stresses exceed critical values. A series of structures with varying geometries is used to determine the critical geometry at which buckling occurs. Observation with an optical microscope quickly reveals qualitative and quantitative information about the internal strain in the film. Strain values between +or-1.5% can be measured for a 2.0 mu m thick film using doubly-supported beams for compressive strain fields, and ring and beam structures for tensile strain fields. Parametric formulae are developed for diagnostic structure response with selected verification by finite element computations.

Improved analysis of microbeams under mechanical and electrostatic loads

Stretching effects in axially constrained doubly clamped microbeams have been analysed for mechanical and electrostatic loads. A computer code based on the shooting method, a numerical integration procedure, has been developed for calculating the induced tension, deflections and stresses. Analytic solutions for concentrated force, distributed pressure and electrostatic loading were derived and verified by numerical simulation. Typical dimensions for microbeams considered are . Various cases have been evaluated, with and without induced tension and/or residual strain. Displacements and stresses are reduced when induced tension is included and are considerably smaller with intrinsic tension.

BIOCOMPATIBILITY OF A PDMS-COATED MICRO-DEVICE： BLADDER VOLUME MONITORING SENSOR

We evaluated the biocompatibility of a dimethylpolysiloxane-coated micro-device which had been designed for monitoring real-time bladder volume in previous studies. The extract assay with dimethylpolysiloxane which had been used for coating the micro-device to measure the bladder volume was performed as an in vitro cytotoxicity test. For in vivo biocompatibility testing, the inflammatory responses around the implantation site of the micro-device in subcutaneous tissue of rat were assessed by light microscope with H&E stain and fluorescence microscope with ED1 stain and von Willebrand factor stain. The averages of cell viability in dimethylpolysiloxane group were 84.6% and 82.3% at 24 h and 72 h incubation, respectively. The qualitative evaluations with light and fluorescence microscope revealed that the inflammatory changes peaked during 2 weeks but almost disappeared at 4 weeks after implantation of devices. The quantitative evaluations for granulation layer formation and neovascularization showed that the thickness of the layer in dimethylpolysiloxane group peaked during 2 weeks but it came to be stabilized at 4 weeks as thin as at 2 weeks in control group, and the frequency of neovascularization was higher in dimethylpolysiloxane group than in control group but it was not increased with time. The dimethylpolysiloxane-coated micro-device is thought be a reliable bio-medical device.

Design of a high-density thermal inkjet using heat transfer from CVD diamond

A novel high-density thermal inkjet (TIJ) was proposed to overcome the defect due to the limitations of the nozzle density in conventional TIJs, and its feasibility was verified by a bubble visualization test. To develop the high-density TIJ, two concepts were introduced. One is an original method to use heat transfer from CVD diamond by means of placing heaters on the sidewall of the ink inlet in order to minimize the area that is occupied by the heaters. The other is the concept of placing some parts concerning the flow of ink (e.g. manifold, inlet, pressure chamber and nozzle) in line. In order to determine the dimensions of the suggested model, heat transfer analysis has been performed and shapes of the heater based on the analysis have been described. To demonstrate the feasibility of these concepts, the TIJ was designed and fabricated for the bubble visualization test. The CVD diamond on the sidewall was deposited using hot filament CVD (HFCVD). The bubble generated has a better symmetry as the power applied increases, which confirms the possibility of bubble formation by heat transfer from diamond. In addition, there was a difference in the rate of bubble growth between the top and the bottom of the CVD diamond.

Real-time bladder volume monitoring by the application of a new implantable bladder volume sensor for a small animal model

Although real‐time monitoring of bladder volume together with intravesical pressure can provide more information for understanding the functional changes of the urinary bladder, it still entails difficulties in the accurate prediction of real‐time bladder volume in urodynamic studies with small animal models. We studied a new implantable bladder volume monitoring device with eight rats. During cystometry, microelectrodes prepared by the microelectromechanical systems process were placed symmetrically on both lateral walls of the bladder, and the expanded bladder volume was calculated. Immunohistological study was done after 1 week and after 4 weeks to evaluate the biocompatibility of the microelectrode. From the point that infused saline volume into the bladder was higher than 0.6 mL, estimated bladder volume was statistically correlated with the volume of saline injected (p < 0.01). Additionally, the microelectromechanical system microelectrodes used in this study showed reliable biocompatibility. Therefore, the device can be used to evaluate changes in bladder volume in studies with small animals, and it may help to provide more information about functional changes in the bladder in laboratory studies. Furthermore, owing to its biocompatibility, the device could be chronically implanted in conscious ambulating animals, thus allowing a novel longitudinal study to be performed for a specific purpose.

Measurement and analysis of micro-scale adhesion for efficient transfer printing

The adhesion-based transfer printing process allows the devices to be fabricated through low temperature process, which is important for realizing flexible electronics with high mobility. Therefore, comprehensive understanding about adhesion between the solid object and the surface of elastomeric stamp (generally, polydimethylsiloxane is used) and the optimal strategy to control adhesion can support the effective and high yield transfer. In this paper, a novel adhesive force measurement system is developed to measure dynamic adhesive forces from the surface on the flexible stamp. The thermodynamic work of adhesion at the moment of debonding of a tip of sensor from the flexible stamp is modeled and used for developing the adhesion control strategy. The measurement results show that the work of adhesion is strongly dependent on the peel-off velocity of tip, while the indentation force has only minor effects on it. The effect of Young’s modulus of elastomeric stamp is also investigated using the model of wor...

Temperature uniformity across an x‐ray mask membrane during resist baking

We have studied temperature uniformity during the post‐exposure bake process across a 40 mm diameter of an x‐ray mask membrane. Membrane temperature was determined by measuring line size as a function of position across the membrane. A two‐dimensional finite element model (FEM) was used to analyze the results and optimize the design of a new bake chuck. The 3σ variation across the diameter of the mask was reduced to 27 nm. The FEM was also used to examine issues associated with resist baking on the ARPA‐NIST X‐ray Mask Standard and the initial results are discussed.

Mechanical analysis of pressure transducers with two-sided overload protection

Computational mechanics is used to analyse and design a new class of subminiature differential pressure transducers. Double-sided overload protection for a planar transducer is achieved with a metal stop, produced by a process involving X-ray lithography and precision electroplating. The metallic stops limit motion, suppress diaphragm stress and facilitate the option of a second signal to verify performance. Finite element stress analysis shows that the proposed device can generally sustain pressures substantially greater than those producing initial contact between the diaphragm and the overload stop. Mechanical stresses are minimized by thick stops but thermal stresses are lowest for thin stops. However finite element thermal strain computations indicate that interface stress does not increase monotonically with stop thickness.

Vibration Mode-Dependent Energy Harvesting Characteristics of Magnetoelectric Composite Cantilevers

In this letter, we simply fabricate magnetoelectric (ME) composite cantilevers through bonding 30 μm thick Metglas and 52 μm thick polyvinylidene fluoride with an epoxy, and report resonant and energy harvesting characteristics with applied DC and AC magnetic fields ranging from 0 to 3 kA/m. Generated ME voltages from a 40 mm long, 12.3 mm wide, and 112 μm thick cantilever are measured and analyzed as a function of the applied magnetic field. In our experiment, higher ME voltages are generated when both the DC and AC magnetic fields are present and the ME voltages around the second flexural bending mode are approximately five times higher than those around the first mode. The higher ME voltage with the second mode is also confirmed with finite element analysis.

Final Focusing Magnetic Lenses for Light Ion Beam Fusion Reactors

A magnetic lens has been designed which will focus 30 MeV Li+ ions emitted from a diode onto a DT filled target as required in the LIBRA-LiTE fusion reactor study. Thirty such lenses are located 2.05 m from the target. Each lens is a 5 turn solenoid magnet, 0.5 m long with a bore of 0.18 m and an average B field of 1.2 tesla. Liquid Li is used as the electrical conductor and heat transfer fluid. The front end of the magnet is fabricated from porous TZM alloy with a wetted Li surface. The lifetime of the magnet in this high radiation environment is predicted to be one calendar year.