Gwangseok Hwang

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.

Measuring the size dependence of thermal conductivity of suspended graphene disks using null-point scanning thermal microscopy

Using null-point scanning thermal microscopy (NP SThM), we have measured and analyzed the size dependence of the thermal conductivity of graphene. To do so, we rigorously re-derived the principal equation of NP SThM in terms of thermal property measurements so as to explain how this technique can be effectively used to quantitatively measure the local thermal resistance with nanoscale spatial resolution. This technique has already been proven to resolve the major problems of conventional SThM, and to quantitatively measure the temperature profile. Using NP SThM, we measured the variation in the thermal resistance of suspended chemical vapor deposition (CVD)-grown graphene disks with radii of 50-3680 nm from the center to the edge with respect to the size. By thoroughly analyzing the size dependence of the thermal resistance, we show that, with increasing graphene size, the ballistic resistance becomes more dominant in the thermal resistance experienced by a heat source of finite size and that the thermal conductivity experienced by such a heat source can even decrease. The results of this study reveal that the thermal conductivity of graphene detected by a heat source depends on the size of the heat source relative to that of the suspended graphene and on how the heat source and graphene are connected. As demonstrated in this study, NP SThM will be very useful for quantitative thermal characterization of not only CVD-grown graphene but also various other nanomaterials and nanodevices.

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 conductivity of Bi2Te3 nanowire using high-vacuum scanning thermal wave microscopy

With the increasing application of nanomaterials in the development of high-efficiency thermoelectric energy conversion materials and electronic devices, the measurement of the intrinsic thermal conductivity of nanomaterials in the form of nanowires and nanofilms has become very important. However, the current widely used methods for measuring thermal conductivity have difficulties in eliminating the influence of interfacial thermal resistance (ITR) during the measurement. In this study, by using high-vacuum scanning thermal wave microscopy (HV-STWM), we propose a quantitative method for measuring the thermal conductivity of nanomaterials. By measuring the local phase lag of high-frequency (>10 kHz) thermal waves passing through a nanomaterial in a high-vacuum environment, HV-STWM eliminates the measurement errors due to ITR and the distortion due to heat transfer through air. By using HV-STWM, we measure the thermal conductivity of a Bi2Te3 nanowire. Because HV-STWM is quantitatively accurate and its spe...

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.

Fabrication of scanning thermal microscope probe with ultra-thin oxide tip and demonstration of its enhanced performance

With the vigorous development of new nanodevices and nanomaterials, improvements in the quantitation and resolution of the measurement of nanoscale energy transport/conversion phenomena have become increasingly important. Although several new advanced methods for scanning thermal microscopy (SThM) have been developed to meet these needs, such methods require a drastic enhancement of SThM probe performance. In this study, by taking advantage of the characteristics of micromechanical structures where their mechanical stability is maintained even when the film that composes the structures becomes extremely thin, we develop a new design of SThM probe whose tip is made of ultra-thin SiO2 film (~100nm), fabricate the SThM probes, and demonstrate experimentally that the tip radius, thermal time constant, and thermal sensitivity of the probe are all improved. We expect the development of new high-performance SThM probes, along with the advanced measurement methods, to allow the measurement of temperature and thermal properties with higher spatial resolution and quantitative accuracy, ultimately making essential contributions to diverse areas of science and engineering related to the nanoscale energy transport/conversion phenomena.

Development of a Quantitative Nanoscale Thermal Conductivity Profiling Technique by the Removal of Influence Due to Heat Transfer Through the Air

We developed a quantitative thermal property profiling technique that measures the thermal property of the sample from the tip-sample heat transfer only using SThM. The principle of the technique is explained rigorously through a theoretical analysis of the heat transfer phenomena. The spatial resolution of this technique was demonstrated by obtaining the thermal conductivity profile of samples in which a thin silicon oxide layer is sandwiched between single crystal silicon layers. For a sample with 1.4 μm thick silicon oxide layer, its thermal conductivity was quantitatively profiled. However, for a sample with 100 nm thick silicon oxide layer, the obtained profile was not quantitative. From the experimental results the quantitative spatial resolution of this technique is estimated to be around 200 nm. In order to further improve the quantitative spatial resolution of this technique, the tip radius of the completed thermocouple SThM probe should be reduced further.Copyright

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.