Ali Assy

Heat transfer at nanoscale contacts investigated with scanning thermal microscopy

This article investigates heat transfer at nanoscale contacts through scanning thermal microscopy (SThM) under vacuum conditions. Measurements were performed using two types of resistive SThM probes operating in active mode on germanium and silicon samples. The experiments measure the heat transfer through the nanoscale point contacts formed between the probe apex, platinum-rhodium alloy, or silicon nitride depending on the probe used, and the samples. The thermal resistance at the probe apex-sample interface becomes extremely important as the contact size becomes smaller or comparable to the phonon mean free path within the materials in contact. This resistance is derived from the measurements using a nanoconstriction model. Consistent to what is expected, the interfacial thermal resistance is found to be dependent on the tip and sample. Assuming perfect interfaces, the thermal boundary resistance Rb is determined for the different contacts. Results obtained for Rb range from 10−9 m2 K W−1 up to 14 × 10−...

Analysis of heat transfer in the water meniscus at the tip-sample contact in scanning thermal microscopy

Quantitative measurements of local nanometer-scale thermal measurements are difficult to achieve because heat flux may be transferred from the heated sensor to the cold sample through various elusive mixed thermal channels. This paper addresses one of these channels, the water meniscus at the nano-contact between a heated atomic force microscopy probe and a hydrophilic sample. This heat transfer mechanism is found to depend strongly on the probe temperature. The analysis of the pull-off forces as a function of temperature indicates that the water film almost vanishes above a probe mean temperature between 120 and 150 oC. In particular, a methodology that allows for correlating the thermal conductance of the water meniscus to the capillary forces is applied. In the case of the standard scanning thermal microscopy Wollaston probe, values of this thermal conductance show that the water meniscus mechanism is not dominant in the thermal interaction between the probe and the sample, regardless of probe temperature.

Temperature-dependent capillary forces at nano-contacts for estimating the heat conduction through a water meniscus

The temperature dependence of the capillary forces at nano-sized contacts is investigated. Two different resistive scanning thermal microscopy (SThM) nanoprobes are used in this study. Measurements of the capillary forces are reported as a function of the probe temperature on hydrophilic samples of different thermal properties. These forces appear to be largely reduced for probe temperatures larger than a threshold temperature, where the value depends on the sample thermal conductance. This could pave the way to an alternative solution to reduce the stiction in nano/ micro-electromechanical (NEMS/MEMS) devices. The dimensions of the water meniscus at the probe-sample contact were then estimated. Moreover, these results help the evaluation of thermal conductance through the water meniscus. It is found, through this work, that the values of the thermal conductance through the water meniscus can represent 6% of those of the contact thermal conductance in the case of the KNT probe (from Kelvin nanotechnology). These values can be equal to 4% of those of thermal conduction in the cantilever-sample air gap in the case of a doped-silicon probe.

Heat transfer mechanisms quantified at submicron scales in scanning thermal microscopy

This work investigates the heat transfer between scanning thermal microscopy (SThM) probes and samples. It presents a detailed study of the heat transfer mechanisms that operate between the probe and the sample. Two SThM resistive probes of different sizes were used in active mode. Depending on the experimental conditions, the heat transferred to the sample through water meniscus, solid-solid contact and air is quantified. The methodology established to estimate the heat conduction through water meniscus shows that this mechanism is not dominant in the tip-sample heat exchange. Based on experimental results of measurements performed under vacuum conditions, the thermal boundary resistance at different contacts is estimated and in accordance with literature values. Through measurements performed under ambient conditions, the heat conduction through air appears to be strongly dependent on the sample thermal conductivity.

Investigation of the thermal properties of thin solid materials at different temperature levels using a set of microresistors

The measurement of thermal properties of solid materials at different temperatures above ambient is investigated using a set of microresistors. Samples consisted of suspended films with sets of long, parallel resistive wires deposited on their surfaces. One resistive wire was heated by an alternating current. Surface temperature changes in DC and AC regimes were then detected by measuring the change in electrical resistance of the other wires deposited on the surface. The length of wires was chosen so that they may be assumed isothermal and such that heat diffusion acts perpendicularly to their axes. By measuring the dependence of the surface alternating temperature oscillation on the modulation frequency f and on the separation between the heating wire and the probing wires, the thermal diffusivity of the sample was determined. Through adjustment of the alternating current amplitude in the source wire, the temperature at which the thermal diffusivity of the sample was evaluated was finely controlled. For the validation of the method, pure silicon samples were first studied. An experimental bench was set up and resistive source and probes were experimentally characterized. Results obtained from ambient temperature to 500 K for pure silicon are in accordance with reference data found in the scientific literature.

Chapter 9:Scanning Thermal Microscopy

Scanning Thermal Microscopy (SThM) allows nanoscale temperature and heat flow measurements as well as thermal characterization of materials. This chapter focuses on fundamentals and applications of SThM methods. It reviews the main Scanning Probe Microscopy techniques developed for thermal imaging with nanoscale spatial resolution and presents selected SThM applications. After reviewing the fundamentals of thermal metrology by contact, it describes the approaches currently used to calibrate SThM probes. In many cases, the link between the nominal measured signal and the investigated parameter is not yet fully understood, due to the complexity of the micro-/nanoscale interaction between the probe and the sample. Special attention is given to this interaction, which conditions the tip–sample interface temperature. Some examples of the main applications of SThM are presented. Finally, future challenges and opportunities for SThM are discussed.