Thermal conductivity measurements of sub-surface buried substrates by steady-state thermoreflectance

TL;DR

This paper demonstrates that steady-state thermoreflectance (SSTR) can effectively measure the thermal conductivity of buried substrates, overcoming limitations of traditional pump-probe methods, through experimental, numerical, and sensitivity analyses.

Contribution

It introduces the use of SSTR for probing sub-surface buried substrates and provides sensitivity calculations to estimate measurement uncertainties.

Findings

SSTR can measure thermal conductivity of buried substrates inaccessible to traditional methods.

Sensitivity calculations guide the estimation of measurement uncertainties.

SSTR is suitable for thermal characterization in typical device geometries.

Abstract

Measuring the thermal conductivity of sub-surface buried substrates are of significant practical interests. However, this remains challenging with traditional pump-probe spectroscopies due to their limited thermal penetration depths (TPD). Here, we experimentally and numerically investigate the TPD of recently developed optical pump-probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates typically inaccessible by traditional pump-probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries.