Ohmyoung Kwon

Design and batch fabrication of probes for sub-100 nm scanning thermal microscopy

A batch fabrication process has been developed for making cantilever probes for scanning thermal microscopy (SThM) with spatial resolution in the sub-100 nm range. A heat transfer model was developed to optimize the thermal design of the probes. Low thermal conductivity silicon dioxide and silicon nitride were chosen for fabricating the probe tips and cantilevers, respectively, in order to minimize heat loss from the sample to the probe and to improve temperature measurement accuracy and spatial resolution. An etch process was developed for making silicon dioxide tips with tip radius as small as 20 nm. A thin film thermocouple junction was fabricated at the tip end with a junction height that could be controlled in the range of 100-600 nm. These thermal probes have been used extensively for thermal imaging of micro- and nano-electronic devices with a spatial resolution of 50 nm. This paper presents measurement results of the steady state and dynamic temperature responses of the thermal probes and examines the wear characteristics of the probes.

Infrared vision using uncooled micro-optomechanical camera

This letter presents the design, fabrication, and imaging results of an uncooled infrared (IR) camera that contains a focal plane array of bimaterial microcantilever sensors, and an optical readout technique that measures cantilever deflections in the nanometer range to directly project a visible image of the IR scene on the human eye or a visible camera. The results suggest that objects at temperatures as low as 100 °C can be imaged with the best noise-equivalent temperature difference (NEΔT) in the range of 10 K. It is estimated that further improvements that are currently being pursued can improve NEΔT to about 50 mK.

Quantitative Measurement with Scanning Thermal Microscope by Preventing the Distortion Due to the Heat Transfer through the Air

Because of its high spatial resolution, scanning thermal microscopy (SThM) has been developed quite actively and applied in such diverse areas as microelectronics, optoelectronics, polymers, and carbon nanotubes for more than a decade since the 1990s. However, despite its long history and diverse areas of application, surprisingly, no quantitative profiling method has been established yet. This is mostly due to the nonlocal nature of measurement by conventional SThM: the signal measured by SThM is induced not only from the local heat flux through the tip-sample thermal contact but also (and mostly) from the heat flux through the air gap between the sample and the SThM probe. In this study, a rigorous but simple and practical theory for quantitative SThM for local measurement is established and verified experimentally using high-performance SThM probes. The development of quantitative SThM will make possible new breakthroughs in diverse fields of nanothermal science and engineering.

Quantitative scanning thermal microscopy using double scan technique

Although scanning thermal microscope has shown the highest spatial resolution in local temperature and thermophysical property measurement, its usefulness has been severely limited due to difficulties in quantitative measurement. We propose a double scan technique that measures temperature only from the heat transfer through the tip-sample contact by the subtraction of the signal due to the heat transfer through the air. A rigorous theoretical model for this technique is derived. The effectiveness of the double scan technique in quantitative temperature measurement is demonstrated experimentally.

Scanning thermal wave microscopy (STWM)

This paper presents a technique, scanning thermal wave microscopy (STWM), which can image the phase lag and amplitude of thermal waves with sub-micrometer resolution by scanning a temperature-sensing nanoscale tip across a sample surface. Phase lag measurements during tip-sample contact showed enhancement of tip-sample heat transfer due to the presence of a liquid film. The measurement accuracy of STWM is proved by a benchmark experiment and comparison to theoretical prediction. The application of STWM for sub-surface imaging of buried structures is demonstrated by measuring the phase lag and amplitude distributions of an interconnect via sample. The measurement showed excellent agreement with a finite element analysis offering the promising prospects of three-dimensional thermal probing of micro and nanostructures. Finally, it was shown that the resolving power of thermal waves for subsurface structures improves as the wavelengths of the thermal waves become shorter at higher modulation frequencies.

Novel nanoscale thermal property imaging technique: The 2ω method. I. Principle and the 2ω signal measurement

In this and the following companion articles, the authors present the 2ω method, a novel ac mode local thermal property imaging technique with nanoscale spatial resolution. To demonstrate the use of the thermoelectric probe as an active one that can function as both a heater and a temperature sensor, the authors develop and implement the 2ω signal measurement technique, which can extract thermoelectric signals from a thermocouple junction while electrically heating it simultaneously. The principle of the 2ω signal measurement technique is explained by a steady periodic electrothermal analysis. The authors use a specially designed test pattern to experimentally verify that the 2ω signal is caused by the temperature oscillation induced by Joule heating. In addition, based on the results from an experiment using a cross-shaped pattern, the measurement accuracy of the 2ω method depends on the junction size of the thermoelectric probe. The 2ω method is implemented and compared with other methods in the followi...

Quantitative temperature measurement of an electrically heated carbon nanotube using the null-point method

Previously, we introduced the double scan technique, which enables quantitative temperature profiling with a scanning thermal microscope (SThM) without distortion arising from heat transfer through the air. However, if the tip-sample thermal conductance is disturbed due to the extremely small size of the sample, such as carbon nanotubes, or an abrupt change in the topography, then quantitative measurement becomes difficult even with the double scan technique. Here, we developed the null-point method by which one can quantitatively measure the temperature of a sample without disturbances arising from the tip-sample thermal conductance, based on the principle of the double scan technique. We first checked the effectiveness and accuracy of the null-point method using 5 μm and 400 nm wide aluminum lines. Then, we quantitatively measured the temperature of electrically heated multiwall carbon nanotubes using the null-point method. Since the null-point method has an extremely high spatial resolution of SThM and is free from disturbance due to the tip-sample thermal contact resistance, and distortion due to heat transfer through the air, the method is expected to be widely applicable for the thermal characterization of many nanomaterials and nanodevices.

The effect of electrode heat sink in organic-electronic devices

Most of organic devices showed poor thermal stability and short lifetime due to Joule heating by current injection during operation. To increase the lifetime of the devices, thermal management must be considered. We demonstrated the polymer light-emitting diodes with thermally conductive substrate and Al/Cu double cathode to enhance the thermal stability of the device. Also, we proposed the correlation between lifetime (Δt) and device heat sink (ΔT). The heat sink of all organic devices is required to enhance device durability.

Novel nanoscale thermal property imaging technique: The 2ω method. II. Demonstration and comparison

This paper presents the 2ω method, a newly developed ac mode local thermal property imaging technique with nanoscale spatial resolution. The authors batch-fabricate the thermoelectric probe, whose junction size is about 200nm, with yield higher than 95%. The shortest time constant of the thermoelectric probe is measured to be 0.72ms. They experimentally demonstrate that the 2ω method can map out the local thermal property of a sample by monitoring the amplitude of the 2ω signal from the thermocouple junction of a probe heated by an ac. By comparing with the thermal property images obtained by other methods, they also show that the 2ω method using the point-heating and point-sensing scheme is the most suitable for the nanoscale thermal property imaging.

Direct-view uncooled micro-optomechanical infrared camera

This paper presents the design, fabrication, and the first imaging results of a new uncooled infrared (IR) camera based on thermomechanical sensing and a novel optical readout technique that directly interfaces with the human eye. The system contains a focal plane array (FPA) consisting of bimaterial cantilever beams in each pixel. Absorption of the incident IR radiation by each cantilever beam raises its temperature, resulting in proportional deflection due to mismatch in thermal expansion of the two cantilever materials. A visible optical system is used to simultaneously measure the deflections of all the cantilever beams of the FPA using either Fabry-Perot interferometry or deformable diffraction gratings, and collectively project a visible image of the spatially-varying IR radiation directly on the human eye. The camera is designed to be sensitive in the spectral range of 8-14 /spl mu/m which is key to night vision. The first results suggest that objects at temperatures as low as 100/spl deg/C can be imaged with the best noise-equivalent temperature difference (NE/spl Delta/T) in the range of 10 K. It is estimated that further improvements that are currently being pursued can improve NE/spl Delta/T to about 50 mK.