Nanoscale spatial resolution probes for scanning thermal microscopy of solid state materials

Abstract

Scanning Thermal Microscopy (SThM) uses micromachined thermal sensors integrated in a force sensing cantilever with a nanoscale tip can be highly useful for exploration of thermal management of nanoscale semiconductor devices. As well as mapping of surface and subsurface properties of related materials. Whereas SThM is capable to image externally generated heat with nanoscale resolution, its ability to map and measure thermal conductivity of materials has been mainly limited to polymers or similar materials possessing low thermal conductivity in the range from 0.1 to 1 Wm -1 K -1 , with lateral resolution on the order of 1 µm.In this paper we use linked experimental and theoretical approaches to analyse thermal performance and sensitivity of the micromachined SThM probes in order to expand their applicability to a broader range of nanostructures from polymers to semiconductors and metals. We develop physical models of interlinked thermal and electrical phenomena in these probes and their interaction with the sample on the mesoscopic length scale of few tens of nm and then validate these models using experimental measurements of the real probes, which provided the basis for analysing SThM performance in exploration of nanostructures. Our study then highlights critical features of these probes, namely, the geometrical location of the thermal sensor with respect to the probe apex, thermal conductance of the probe to the support base, heat conduction to the surrounding gas, and the thermal conductivity of tip material adjacent to the apex. It is furthermore allows us to propose a novel design of the SThM probe that incorporates a multiwall carbon nanotube (CNT) or similar high thermal conductivity graphene sheet material with longitudinal dimensions on micrometre length scale positioned near the probe apex that can provide contact areas with the sample on the order of few tens of nm. The new sensor is predicted to provide greatly improved spatial resolution to thermal properties of nanostructures, as well as to expand the sensitivity of the SThM probe to materials with heat conductivity values up to 100-1000 Wm -1 K -1 .

Quantification of probe–sample interactions of a scanning thermal microscope using a nanofabricated calibration sample having programmable size

“…At the contact point (x = 0), heat transfers from the sample surface to the probe through the tip-surface contact with a thermal conductance defined as Gts. At the base of the cantilever, x = L, the temperature was assumed to be ambient TRT, as the silicon base is treated as a perfect heat sink in common with other studies [3,22,55]. Therefore, two boundary conditions at x = 0 and x = L are: Solving the equation requires knowledge of various thermal properties within the system.…”

“…The thermal resistance of the standard cantilever has been found to be a limitation in detecting materials with thermal conductivity lower than 1 W/mK or higher than several hundred [22][67] [68]. The lumped system model provides a simple way to evaluate new probe designs, which may be modified to obtain a higher sensitivity to specific sample thermal conductivities.…”

“…FEA is commonly used for modelling the temperature distribution of devices at the microscopic scale [22] [44].…”

“…When working in "active mode" (probe is both a heater and a thermometer), SThM can be used to explore the thermal properties of materials, for example nanowires [17] [18], nanotubes [19] and graphene [20][21] [22]. In order to make these measurements quantitative, it is important to understand probe thermal interactions at the nanoscale.…”

Scanning thermal microscopy with heat conductive nanowire probes

“…Modern materials science and technology is increasingly devoted to the control of matter on the nanoscale, with local thermal properties playing a major role in the diverse materials used in renewable energy generation (thermoelectrics, photovoltaics), structural composites and in optical and electronic devices [1][2][3][4][5][6]. In semiconductor processors, the inability to dissipate increasing power density leads to the failure of Moore's law due to nanoscale thermal management problems [7][8][9].…”