Beomjoon Kim

High-Spatial-Resolution Surface-Temperature Mapping Using Fluorescent Thermometry**

The characterization of temperature and thermal properties is of particular importance in micro- and nanotechnology. Considering the highly increased density of structures and the increased power dissipation per unit area associated with miniaturization, good thermal design is of great importance for device reliability and performance. Locating hot spots, for example, on a microelectronic circuit, can be of great value in evaluating a design, optimizing the performance, and performing failure analysis. [1,2] Apart from the industrial applications of micro- and nanoscale thermometry, fundamental questions of the thermal behavior, for example, thermal transfer at a scale comparable to the phonon wavelength, [3] could be more effectively addressed with improved characterization tools. The common approach for mapping temperature on the microscale is based on infrared microscopy, which relies on the analysis of the thermal radiation that is emitted from any material. IR microscopy is a well-established technique and can be used with relative ease for temperature mapping on large scales. However, the technique suffers from a diffractionlimited resolution, giving it an optimal spatial resolution of around 5 mm. [2,4,5] Nanoscale scientists typically use scanning thermal microscopy (SThM) for high-resolution measurements. Since the invention of the scanning probe microscope at the beginning of the 1980s, [6] several scanning probes for thermal characterization have been developed. The thermal probes used are generally based on either thermocouple or thermistor elements. [7–11] Other approaches have proposed bimaterial cantilevers or fluorescent particles as temperaturesensing probes. [12–14] The highest spatial resolution obtained

Transfer of thin Au films to polydimethylsiloxane (PDMS) with reliable bonding using (3-mercaptopropyl)trimethoxysilane (MPTMS) as a molecular adhesive

This paper describes the transfer of thin gold films deposited on rigid silicon substrates to polydimethylsiloxane (PDMS) with reliable and strong bonding. Modification of the Au surfaces with (3-mercaptopropyl)trimethoxysilane (MPTMS) as a molecular adhesive was carried out to promote adhesion between Au and PDMS. The degree of bonding with respect to the concentration of MPTMS, treatment time and methods of deposition was investigated by a simple adhesion test using two different adhesive tapes. The effect of hydrolysis of MPTMS is discussed based on the bonding mechanism of MPTMS to the PDMS prepolymer. Also, the adsorption of MPTMS on Au deposited by different methods is discussed. The results indicate that liquid deposition of MPTMS provides the strongest adhesion between Au and PDMS among the different deposition methods and the different linker molecules. Based on these studies, the Au patterns with linewidth of less 2 μm were successfully transferred to PDMS with reliable and strong bonding in a full 3 inch wafer scale, using a dry peel-off process. (Some figures may appear in colour only in the online journal)

Optical-softlithographic technology for patterning on curved surfaces

We have established a novel concept of hybrid lithographic technology for non-planar surface patterning. Softlithography and photolithography are properly combined to transfer micro-patterns onto a curved area in an easy, low-cost way. As a first step, a film type of a photomask with micro-metal features is fabricated by the direct pattern transfer technique that has been presented in our preliminary work. Then, a flexible polymer photomask is wrapped on a curved surface to make conformal contact, and a variety of micro-features are formed on the surface via the photolithographic process. We have confirmed the validity of the technique for application in the industrial process by comparing the results transferred via the conventional photolithography with a rigid flat photomask. Subsequently, 3D polymer structures with a high aspect ratio (A/R) are fabricated on curved surfaces using this technique, followed by a discussion on several drawbacks due to the shape of the substrate. Overall, this paper has demonstrated a new method of micro-patterning, which would promise an emerging field of micro-fabrication on non-planar substrates.

Development of microfluidic device for electrical/physical characterization of single cell

A novel device with microchannels for flowing cells and twin microcantilever arrays for measuring the electrical impedance of a single cell is proposed. The fabrication process is demonstrated and the twin microcantilever arrays have been successfully fabricated. In our research, we measured the electrical impedance for normal and abnormal red blood cell over the frequency range from 1 Hz to 10 MHz. From the electrical impedance experiment of normal and abnormal red blood cell, it was examined that the electrical impedance between normal and abnormal red blood cells was significantly different in magnitude and phase shift. In this paper, we show that the normal cell can be taken apart from the abnormal cell by electrical impedance measurement. Therefore, it is expected that the applicability of this technology can be used in cellular studies such as cell sorting, counting or membrane biophysical characterization.

The vibroscanning method for the measurement of micro-hole profiles

In this paper, we present technologies for measuring inner dimensions of small holes. In the first method with a single probe system, the electrical contact between a vibrating probe and the inner surface of a hole is detected and the duty factor of the contact is measured. Through controlled scanning by a probe with a constant duty factor, data on the ups and downs of the surface profile are obtained. To characterize inside profiles of micro-holes regardless of materials, we developed a new technology with a twin-probe system. With a silicon-based micro-twin probe 3 mm in length, a micro-hole with a diameter of around 125 µm was measured and the estimated imprecision of measurement on this set-up is smaller than 0.5 µm.

A self-assembled monolayer-assisted surface microfabrication and release technique

This paper describes a method of thin film and MEMS processing which uses self-assembled monolayers as ultra-thin organic surface coating to enable a simple removal of microfabricated devices off the surface without wet chemical etching. A 1.5-nm thick self-assembled monolayer of dodecyltrichlorosilane reduces the adhesion between the SiO2 substrate surface and a 100-nm thick evaporated aluminum film. A 100-μm thick layer of photoplastic SU-8, which is spun and structured by lithography and development on top of the monolayer/aluminum sandwich layer, can be mechanically lifted off the surface with the aluminum layer. The organic monolayer provides enough stability for the microfabrication process including photoresist spinning and thermal steps. The aluminum film has a surface roughness of less than 1 nm rms as measured by AFM. Photolithographic microstructuring of the aluminum film prior to the photoplastic process allows for transparent embedded bottom-side metal electrodes. As first application example, molded nanoprobes for scanning near-field optical microscopy, has been demonstrated using this technique.

Twin-Probe Vibroscanning Method for Dimensional Measurement of Microholes

Abstract We present a new twin-probe vibroscanning method for measuring the inside profile of microholes in electrically nonconductive materials. Instead of the single cantilever probe used in the conventional vibroscanning method, a probe with two elements in electrical contact is used to detect the surface. Following a feasibility test with a macroscale prototype device, the measurement of microholes was realized using a silicon-based microprobe.

All-photoplastic microstencil with self-alignment for multiple layer shadow-mask patterning

A rapid and simple method to fabricate tiny shadow-masks and their use in multi-layer surface patterning with in situ micromechanical alignment is presented. Instead of using silicon micromachining with through-wafer etching to define the thin membrane with etched apertures, we are using photoplastic SU-8-based resist as structural material of both membrane and support rim. Two layers, 5 and 150 μm thick, are structured by lithography and finally released from the surface. The free-standing SU-8 membranes have apertures ranging from 6 to 300 μm. They are placed and mechanically fixed to the surface, which needs to be patterned. Deposition by evaporation of Cr, Au, Al or other material through the membrane apertures results in an accurate 1:1 replication of the aperture pattern. In view of multi-layer patterning, we used in situ micromechanical alignment pins or jigs and achieved an overlay precision of <2 μm in both x- and y-directions. The reusable shadow-masks allows for a low-cost surface patterning technique without the need for photolithography related process steps. It allows unconventional surfaces to be patterned in a rapid and vacuum-clean way on arbitrary surfaces.

Microfluidic electro-sonoporation: a multi-modal cell poration methodology through simultaneous application of electric field and ultrasonic wave

A microfluidic device that simultaneously applies the conditions required for microelectroporation and microsonoporation in a flow-through scheme toward high-efficiency and high-throughput molecular delivery into mammalian cells is presented. This multi-modal poration microdevice using simultaneous application of electric field and ultrasonic wave was realized by a three-dimensional (3D) microelectrode scheme where the electrodes function as both electroporation electrodes and cell flow channel so that acoustic wave can be applied perpendicular to the electric field simultaneously to cells flowing through the microfluidic channel. This 3D microelectrode configuration also allows a uniform electric field to be applied while making the device compatible with fluorescent microscopy. It is hypothesized that the simultaneous application of two different fields (electric field and acoustic wave) in perpendicular directions allows formation of transient pores along two axes of the cell membrane at reduced poration intensities, hence maximizing the delivery efficiency while minimizing cell death. The microfluidic electro-sonoporation system was characterized by delivering small molecules into mammalian cells, and showed average poration efficiency of 95.6% and cell viability of 97.3%. This proof of concept result shows that by combining electroporation and sonoporation together, significant improvement in molecule delivery efficiency could be achieved while maintaining high cell viability compared to electroporation or sonoporation alone. The microfluidic electro-sonoporation device presented here is, to the best of our knowledge, the first multi-modal cell poration device using simultaneous application of electric field and ultrasonic wave. This new multi-modal cell poration strategy and system is expected to have broad applications in delivery of small molecule therapeutics and ultimately in large molecule delivery such as gene transfection applications where high delivery efficiency and high viability are crucial.

Towards single cell heat shock response by accurate control on thermal confinement with an on-chip microwire electrode

Metal electrodes with micron scale width enable the heating of less than a dozen cells in a confluent layer at predictable temperatures up to 85 °C with an accuracy of ±2 °C. Those performances were obtained by a preliminary robust temperature calibration based on biotin-rhodamine fluorescence and by controlling the temperature map on the substrate through thermal modeling. The temperature accuracy was proved by inducing the expression of heat shock proteins (HSP) in a few NIH-3T3 cells through a confined and precise temperature rise. Our device is therefore effective to locally induce a heat shock response with almost single-cell resolution. Furthermore, we show that cells heated at a higher temperature than the one of heat shock remain alive without producing HSP. Electrode deposition being one of the most common engineering processes, the fabrication of electrode arrays with a simple control circuit is clearly within reach for parallel testing. This should enable the study of several key mechanisms such as cell heat shock, death or signaling. In nanomedicine, controlled drug release by external stimuli such as for example temperature has attracted much attention. Our device could allow fast and efficient testing of thermoactivable drug delivery systems.