On the Limits of Scanning Thermal Microscopy of Ultrathin Films

Abstract

Heat transfer processes in micro- and nanoscale devices have become more and more important during the last decades. Scanning thermal microscopy (SThM) is an atomic force microscopy (AFM) based method for analyzing local thermal conductivities of layers with thicknesses in the range of several nm to µm. In this work, we investigate ultrathin films of hexagonal boron nitride (h-BN), copper iodide in zincblende structure (γ-CuI) and some test sample structures fabricated of silicon (Si) and silicon dioxide (SiO2) using SThM. Specifically, we analyze and discuss the influence of the sample topography, the touching angle between probe tip and sample, and the probe tip temperature on the acquired results. In essence, our findings indicate that SThM measurements include artefacts that are not associated with the thermal properties of the film under investigation. We discuss possible ways of influence, as well as the magnitudes involved. Furthermore, we suggest necessary measuring conditions that make qualitative SThM measurements of ultrathin films of h-BN with thicknesses at or below 23 nm possible.

Scanning Thermal Microscopy of Ultrathin Films: Numerical Studies Regarding Cantilever Displacement, Thermal Contact Areas, Heat Fluxes, and Heat Distribution

“…FEA is a versatile tool to simulate, e.g., heat transfer or mechanical problems that can be described by mathematical equations. Similar SThM measurements with h-BN have been performed in recent works [ 12 ]. We tried to create theoretical measurement setups that comply with real scenarios so that the theoretical results may be adopted by other researchers to compare them with practical measurements or to explain certain effects of SThM applied to ultrathin films.…”

“…The temperature of the topside of the cantilever is defined using the parameter temp , which varies between 50 °C and 200 °C depending on the simulations. These are appropriate cantilever temperatures for practical SThM investigations [ 12 ]. The remaining outer areas of the cantilever were defined as thermal isolating as well as the top surface of the sample, while the remaining outer areas of the sample take over the ambient temperature of exactly 293.15 K. An overview of the simulation setup and boundary conditions is given in Figure 4 .…”

“…As a result, SThM measurements create two images simultaneously, one topography and one thermal image of the same position. Such images can be found in [ 12 ]. For more details regarding the SThM measurement process, please refer to [ 22 ].…”

“…In our field of research, special focus is on the thermal characterization of ultrathin films such as h-BN with thicknesses of approximately 10 nm, which are supposed to have anisotropic thermal properties. Recent works demonstrated the possibility of such SThM measurements [ 12 ]. This work continues said research and compares it to “standard” SThM measurements of bulk Si with isotropic thermal properties.…”

“…The sample is scanned in a special pattern to obtain local thermal properties. SThM thermal images are likely to be influenced by the sample’s topography, which has been explained in literature in recent years [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. To ensure a correct interpretation of the recorded measurement results, a deep understanding of heat transfer during SThM measurements is essential to comprehend the origin and impact of those and further effects.…”