Jean Spiece 1
, Charalambos Evangeli 1
, Alexander Robson 1 2
, Benjamin Robinson 1
, Francesc Alzina 3
, Oleg Kolosov 1 2 1
Physics, Lancaster University, Lancaster United Kingdom, 2
, Lancaster Materials Analysis Ltd, Lancaster United Kingdom, 3
, ICN, Barcelona Spain
Measuring thermal conductivity of nanoscale thin layers in a multilayer device is a fundamental task critical for the semiconductors (processors and memory), energy storage and nanoscale sensors. Unfortunately, major questions – getting access to a particular buried layer, and how to decouple boundary thermal resistances from the intrinsic material thermal conductivity, present significant challenge for the quantitative measurements.
Here we present a new paradigm combining a a cross-section of the material or device via Ar ion polishing followed by the quantitative measurement of the heat transport via scanning thermal microscopy – cross-sectional SThM, or x-SThM. The studied multilayer structure is first polished via “beam-exit cross-sectional polishing” (BEXP), that unlike traditional ion polishingwith beam impinging on the surface, directs polishing Ar-ion beam to the side of the sample  that exits the front surface at a glancing angle of about 50 creating a close to open angle wedge cross-sectioning the inner layers at the oblique angle. Glancing incidence produces a minimal damage to the layer and excellnt close-to atomic flat surface.
The thermal resistance R for the heat transferred through this wedge to the substrate is directly measured by the SThM  as a function of the wedge thickness d. The increase of the thermal resistance with thickness Rx=dR/dt reflects only the thermal conductivity of the sample, eliminating both the SthM tip-layer thermal resistance and layer-substrate thermal resistance, two notorious unknown parameters that render majority of SThM measurements to be merely qualitative. By comparing Rx with the same value for the known material – e.g. SiO2 wedge on Si substrate – RSiO2 we obtain true quantitative data for the thermal conductivity of the unknown material.
We applied x-SThM in vacuum and air environments to measure the thermal conductivity of SiGe alloys of gradient composition with a few 10-nm lateral resolution, for SiGe layer grown on a Si substrate with Ge composition varying from 0 to 23%. While SiGe is a highly promising material for both high speed processors demonstrating 0.8 THz transistors and future thermoelectric applications  fully compatible with Si processing, its nanoscale thermal conductivity remains a big unknown, providing a major challenge for development of advanced chips and nano-thermoelectrics.
x-SThM allows to measure and map thermal conductivity of a wide range of organic and inorganic nanoscale layers. Being suitable for almost any material, ranging from simple compounds to nanodevices, the high flexibility of the process opens wide scopes of research in nanothermal transport realm and beyond.
 Kolosov OV, Grishin I Method and apparatus for ion beam polishing. USA patent 9,082,587. 2015 July 14, 2015.
 Pumarol, M., Rosamond, M., Tovee, P., Petty, M., Zeze, D., Falko, V., & Kolosov, O. Nano letters, 12(6), 2906 (2012).
 Lee EK et al. Nano Letters. 2012;12(6):2918-23.