Session: 08-01: Micro/Nanoscale Heat Conduction

Paper Number: 133020

133020 - Size Effects on the Electron and Phonon Scattering and Thermal Conductivity of Thin Metal Films and Multilayers for Metal Interconnects: Looking Beyond Copper 

Abstract: 

The progressive reduction in the characteristic length scales of logic technology nodes in very-large-scale integration (VLSI) has led to the need to find replacements for copper being used as interconnects.  Both the thermochemical and thermomechanical stability of Cu at these length scales, along with the strong reduction in both electrical and thermal conductivity due to length scales reducing to those less than its electronic mean free path has led to underperformance, which in part can be ascribed to deleterious heating effects.  While Ru and W interconnects are currently being evaluated, a range of additional metals and metallic systems (alloys, eutectics and multilayers) are also of note due to their potential mechanical and thermal properties that are superior to Cu at the < 100 nm length scale.  In this presentation, I will discuss our recent efforts in measurements of thermal conductivity and electron-phonon scattering rates of thin metal films for interest as next-generation metal interconnects, including Ru, W, Ir, Pt, Mo, Co and Ta.  First, I will discuss the use of steady state thermoreflectance (SSTR) as a measurement platform to measure the in-plane thermal conductivity (k) of thin metal films.  This measurement of in-plane allows for direct comparison to k derived from electrical resistivity measurements and application of the Wiedemann-Franz (WF) Law.  We find that in most cases, the application of the WF law with the low temperature value of the Lorenz number does not sufficiently predict the total thermal conductivity.  To understand the mechanisms that drive the thermal transport of these metal films, we use both infrared pump-probe measurements (< ps) and infrared variable angle spectroscopic ellipsometry (IR-VASE) the measure the electron scattering rates, demonstrating the relatively thickness independent scattering processes in these films, providing strong promise in the scaling of these metals to technology node length scales.  Finally, I will talk about classes of multilayer metal/metal and metal nitride/metal carbides in which interfaces do not scatter electrons and phonons strongly enough to impact the thermal resistance, thus introducing a series of “interface transparent” metal multilayers that do not exhibit traditional size effects in their thermal conductivity.  I will conclude by commenting on new thermal metrologies that can provide truly nanoscale thermal resistance measurements and the elucidation of new mechanisms of interfacial thermal transport, including infrared variable angle spectroscopic ellipsometry (IR-VASE) and Nanoscale Thermal Microscopy (NTM), a recently developed microscope by Laser Thermal Inc.* that combined pump-probe thermoreflectance with scanning probe microscopy methods.

*Disclosure, Hopkins is a co-Founder

Presenting Author: Patrick Hopkins Professor

Presenting Author Biography: Patrick E. Hopkins is a Professor in Department of Mechanical and Aerospace Engineering at the University of Virginia, with courtesy appointments in the Department of Materials Science and Engineering and the Department of Physics. Patrick received his Ph.D. in Mechanical and Aerospace Engineering at the University of Virginia in 2008 under the mentorship of Professor Pamela Norris. After his Ph.D., Patrick was one of two researchers in the nation to receive a Truman Fellowship from Sandia National Laboratories in 2008, working under the mentorship of Dr. Leslie Phinney. In 2011, Patrick returned to the University of Virginia and joined the faculty. Patrick’s current research interest are in energy transport, charge flow, laser-chemical processes and photonic interactions with condensed matter, soft materials, liquids, vapors and their interfaces. Patrick’s group at the University of Virginia uses various optical thermometry-based experiments to measure the thermal conductivity, thermal boundary conductance, emissivity, thermal accommodation, strain propagation and sound speed, and coupled electron, phonon, and photon mechanisms in a wide array of bulk materials and nanosystems. In 2021, Patrick co-founded Laser Thermal, Inc., a company based in Charlottesville Virginia that is commercializing thermal conductivity measurement systems that provide non-contact, automated metrologies for thermal properties of thin films, coatings and bulk materials.

In the general fields of nanoscale heat transfer, laser interactions with matter, and energy transport, storage and capture, Patrick has authored or co-authored over 300 technical papers (peer reviewed) and been awarded 5 patents focused on materials, energy and laser metrology for measuring thermal properties. Patrick has been recognized for his accomplishments in these fields via AFOSR and ONR Young Investigator Awards, the ASME Bergles-Rohsenhow Young Investigator Award in Heat Transfer, and a Presidential Early Career Award for Scientists and Engineering (PECASE, awarded by DoD/ONR). Patrick is a fellow of ASME and was awarded the ASME Gustus L. Larson Memorial Award. During 2021-2022, Patrick was awarded a Humboldt Fellowship to work on laser thermometry of materials in extreme environments at the Joint Research Center in Karlsruhe, Germany.

Authors:

Patrick Hopkins Professor