Jaehun Chung

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 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.

Enabling low-noise null-point scanning thermal microscopy by the optimization of scanning thermal microscope probe through a rigorous theory of quantitative measurement

The application of conventional scanning thermal microscopy (SThM) is severely limited by three major problems: (i) distortion of the measured signal due to heat transfer through the air, (ii) the unknown and variable value of the tip-sample thermal contact resistance, and (iii) perturbation of the sample temperature due to the heat flux through the tip-sample thermal contact. Recently, we proposed null-point scanning thermal microscopy (NP SThM) as a way of overcoming these problems in principle by tracking the thermal equilibrium between the end of the SThM tip and the sample surface. However, in order to obtain high spatial resolution, which is the primary motivation for SThM, NP SThM requires an extremely sensitive SThM probe that can trace the vanishingly small heat flux through the tip-sample nano-thermal contact. Herein, we derive a relation between the spatial resolution and the design parameters of a SThM probe, optimize the thermal and electrical design, and develop a batch-fabrication process. We also quantitatively demonstrate significantly improved sensitivity, lower measurement noise, and higher spatial resolution of the fabricated SThM probes. By utilizing the exceptional performance of these fabricated probes, we show that NP SThM can be used to obtain a quantitative temperature profile with nanoscale resolution independent of the changing tip-sample thermal contact resistance and without perturbation of the sample temperature or distortion due to the heat transfer through the air.

Nanoscale range finding of subsurface structures by measuring the absolute phase lag of thermal wave

The need for a subsurface imaging technique to locate and characterize subsurface defects in multidimensional micro- and nanoengineered devices has been growing rapidly. We show that a subsurface heater can be located accurately using the phase lag of a thermal wave. We deduce that the absolute phase lag is composed of four components. Among the four components, we isolate the component directly related to the position and the structure of the periodic heat source. We demonstrate that the position of the heater can be estimated accurately from the isolated phase lag component.

Measurement of thermal contact resistance between CVD-grown graphene and SiO 2 by null point scanning thermal microscopy

For graphene-based electronic devices, it is crucial to measure and analyze the thermal contact resistance between the graphene and the insulating layer. Herein, we measure the thermal contact resistance between CVD-grown graphene and a SiO2 layer using null point scanning thermal microscopy (NP SThM), which can profile the temperature distribution quantitatively with nanoscale spatial resolution by preventing the influence of both the heat flux through the air gap and the variation of sample surface properties such as hydrophilicity. Through the comparison of the temperature jump across the interface of the electrically heated graphene and SiO2 layer with the temperature profile without the thermal contact resistance modelled with finite element method, the thermal contact resistance between the graphene and SiO2 is obtained as 10 × 10-8 ~ 45 × 10-8 m2K/W.

Non-collapsible PDMS nanochannel fabrication with tunable width and height using single master mold

A tunable polydimethylsiloxane (PDMS) nanoslit fabrication process was developed for biological sample manipulation. A microcontact printing (µCP) of laterally spreading self-assembled hexadecanethiol (HDT) layer, combined with in-situ curing of a sliding SU-8 droplet, enabled precise and independent tuning of a nanoslit-mold width and height using single µCP master mold. The SU-8 nanoslit-mold was replicated using a hard-soft composite PDMS to prevent channel collapse at low ( < 0.2) aspect ratio (height over width). The fluidic characteristics as well as dimensions of nanoslits fabricated with various conditions are analyzed using fluorescein sample and AFM images. Finally, concentration polarization-based sample preconcentration is successfully demonstrated at the nanoslit boundary where an electric double layer is overlapped.

Quantitative temperature mapping of carbon nanotube using null point method

Despite the high spatial resolution of scanning thermal microscope, its usefulness has been limited because of its lack of quantitative measurement. In this study, utilizing the principle of double scan technique, we developed the null-point method by which one can measure the temperature of a nanoscale sample quantitatively without the disturbances due to the heat transfer through the air and the variation of tip-sample conductance caused by the change of tip-sample contact area. We first checked the effectiveness and accuracy of null point method using 5 µm and 400 nm wide aluminium line whose temperature can be easily controlled and measured. Then, we measured the temperature of electrically heated multi-walled carbon nanotube (MWCNT) via null point method and the temperature profile around it using double scan technique.

Exact locating of sub-surface microelectronic structures using scanning thermal-wave microscopy

With the fast advance of ultra large scale integrated (ULSI) circuit technology, the need for sub-surface imaging technique to locate and characterize sub-surface defects in ULSI circuits has been growing. In this study we advance scanning thermal wave microscopy further so that the absolute phase lag of the thermal waves generated by an electrically heated sub-surface microelectronic structure buried in an ULSI circuit can be measured. The measurement of the absolute phase lag allowed exact locating of the vertical and horizontal position of buried microelectronic structures and evaluation of their soundness nondestructively.