Scanning thermal probe calibration for accurate measurement of thermal conductivity of ultrathin films
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Research Letter
Scanning thermal probe calibration for accurate measurement of thermal conductivity of ultrathin films Adam A. Wilson , National Academy of Sciences, National Research Council, 500 E. 5th Street NW, Washington, DC 20001, USA; Sensors and Electron Devices Directorate, US Army Research Laboratory, 2800 Powder Mill Rd, Adelphi, MD 20783, USA Address all correspondence to Adam A. Wilson at [email protected] (Received 11 January 2019; accepted 15 March 2019)
Abstract Scanning thermal microscopy allows thermal characterization with nanoscale resolution. However, quantitative usage has been met with skepticism, because no standard exists for calibrating probe–sample thermal exchange. In this paper, three published strategies for calibrating probe–sample thermal exchange are directly compared, then used to measure bulk and thin-film thermal conductivity. It is shown that with an appropriately calibrated probe and film-on-substrate heat conduction model, thermal conductivity values of ultrathin-film (2.9– 202 nm) Al2O3 on silicon are within 20% deviation of independently measured values, while more commonly used methods yield values that may deviate by more a factor of two.
Introduction Nanoscale heat transfer is becoming an increasingly important topic for a number of industries. From miniaturization of electronics which is reaching the nanoscale,[1] to the development of heat-assisted magnetic recording,[2] to tailoring materials and devices for thermal energy harvesting,[3] the need to reliably measure heat conduction at the nanoscale is a critical component which is currently lacking.[4,5] Scanning thermal microscopy (SThM) boasts ultra-high lateral resolution (on the order of tens of nanometers or less),[1] but has generally been regarded as useful for qualitative investigations only; use as a quantitative measurement tool has been met with apprehension. This may largely be attributed to a lack of consistent, reliable methods for probe-to-sample heat transfer calibration. Recent studies have demonstrated that not all calibrations yield accurate results.[6–8] Therefore, in this paper, three methods for probe calibration which appear in the literature are compared: the implicit method, the step method, and the intersection method (each is described in section “State of the art calibration strategies”). Bulk reference samples with well-established thermal conductivity values are used to calibrate the probe via all three methods. With probe calibration obtained by each of these methods, the validity of each is explored by measuring the thermal conductivity of a series of film-on-substrate samples which had been previously characterized by time-domain thermoreflectance (TDTR).[9] In that study, thermal conductivity of atomic-layer-deposited amorphous Al2O3 thin films on silicon (with film thickness, tf, of 2.9–202.3 nm) was determined by either treating the film as a separate layer in the heat transfer model (for tf >20
nm) or by treating the film as an extra thermal resistance at the transducer–substrate
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