ME 381R Lecture 10: Thermal Measurement Techniques for Thin Films and Nanostructured Materials Dr. Li Shi Department of Mechanical Engineering The University of Texas at Austin Austin, TX

Outline Thermal Property Measurements: --Thin films --Nanowires and Nanotubes Thermal Microscopy Reading: Ch2 in Tien et al

GMR Cu Interconnects Thin Films and Interfaces

Thin Film Thermal Conductivity Measurement I 0 sin(  t) L2b Thin Film Substrate Metal line The 3  method Cahill, Rev. Sci. Instrum. 61, 802 (1990) I ~ 1  T ~ I 2 ~ 2  R ~ T ~ 2  V~ IR ~3  V Substrate contributionFilm contribution

Data Analysis Dotted line -  T s +  T f Solid line -  T s Slope of solid line  k s  T f  k f

Thermal Conductivity of Thin Si Films (M.Asheghi,etc.,1997) Size effect on the conductivity can exceed two orders of magnitude for layers of thickness near 1  m at T<10k.

Silicon on Insulator (SOI) IBM SOI Chip Ju and Goodson, APL 74, 3005 Lines: BTE results Hot spots!

Thin Film Superlattices Increased phonon-boundary scattering  decreased k + other size effects  High thermoelectric figure of merit (ZT = S 2  T/ k) SiGe superlattice (Shakouri, UCSC)

Thermal Conductivity of Si/Ge Superlattices Period Thickness (Å) k (W/m-K) Bulk Si 0.5 Ge 0.5 Alloy Circles: Measurement by D. Cahill’s group Lines: BTE / EPRT results by G. Chen

Si x Ge 1-x /Si y Ge 1-y Superlattice Films AIM = 1.15 Superlattice Period Huxtable et al., “Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices,” Appl. Phys. Lett. 80, 1737 (2002). Alloy limit With a large AIM,  can be reduced below the alloy limit.

Anisotropic Polymer Thin Films By comparing temperature rise of the metal line for different line width, the anisotropic thermal conductivity can be deduced Ju, Kurabayashi, Goodson, Thin Solid Films 339, 160 (1999)

Nanowire Materials Sb 2 Te 3 nanowires (potentially high ZT) (X. Li et al., USTC) Ge nanowires (B. Korgel, UT Austin) ZnO nanowires (Z.L. Wang, GaTech) SnO 2 nanowires (Z.L. Wang, GaTech)

The 3  method for Nanowires Conditions: The sample needs to have a large temperature coefficient of resistance TCR= (dR/dT)/R The electrical contact has to be perfect Substrate Wire Electrode -- Lu, Yi, Zhang, Rev. Sci. Instrum. 72, 2996 (2001) Low frequency: V(3  ) ~ 1/k High frequency: V(3  ) ~ 1/C Tested for a 20  m dia. Pt wire I 0 sin(  t) V

Pt resistance thermometer Suspended SiN x membrane Long SiN x beams Q I Thermal Measurements of Nanowires Kim, Shi, Majumdar, McEuen, Phys. Rev. Lett. 87, Shi, Li, Yu, Jang, Kim, Yao, Kim, Majumdar, J. Heat Tran 125, 881

Device Fabrication Si SiO 2 SiN x Pt Photoresist (a) CVD (b) Pt lift-off (c) Lithography (d) RIE etch (e) HF etch

Sample Preparation Wet deposition Spin Chip Nanostructure suspension Pipet Direct CVD growth SnO 2 nanobelt Nanotube bundle Individual Nanotube Dielectrophoretic trapping

Thermal Conductance Measurement T 0 T 0 G -1 G b -1 T h T s Q Q h 2Q L G b -1

Collaboration: N. Mingo, NASA Ames Shi et al., Appl. Phys. Lett. 84, 2638 (2004) SnO 2 Nanobelts 64 nm 53 nm 39 nm 53 nm,  i -1 =10  -1 i, bulk 64 nm 53 nm Circles: Measurements Lines: Simulation Diffuse phonon-boundary scattering is the primary effect determining the suppressed thermal conductivities

Diameter Solid line: Theoretical prediction Si Nanowires Li et al., Appl Phys Lett 83, 2934 (2003) Phonon-boundary scattering is the primary effect determining the suppressed thermal conductivity except for the 22 nm sample, where boundary scattering alone can not account for the measurement results and confinement effects on density of states might have played an role Source Drain Gate Nanowire Channel Si Nanotransistor (Berkeley Device group) Hot Spots in Si nanotransistors!

Si/SiGe Superlattice Nanowires Si SiGe Si/Si 0.95 Ge 0.05 Li et al., Appl Phys Lett 83, 3186 (2003) Alloy limit Boundary scattering of long- wavelength further reduces the thermal conductivity below the alloy limit

Atomically-smooth surface, absence of defects: Long mean free path l & Strong SP 2 bonding: high sound velocity v  high thermal conductivity:  = Cvl/3 ~ 6000 W/m-K Carbon Nanotubes Nanotube Electronics (Avouris et al., IBM)

Thermal Conductivity of Carbon Nanotubes CVD SWCN An individual nanotube has a high k ~ W/m-K at 300 K k of a CN bundle is reduced by thermal resistance at tube-tube junctions Potential applications as heat spreading materials for electronic packaging applications CNT

Silicon Nanoelectronics Heat dissipation influences speed and reliability Device scaling is limited by power dissipation IBM Silicon-On-Insulator (SOI) Technology

Carbon Nanoelectronics TubeFET (McEuen et al., Berkeley) Nanotube Logic (Avouris et al., IBM) Current density: 10 9 A/cm 2 Ballistic charge transport - V

Infrared Thermometry 1-10  m* Laser Surface Reflectance 1  m* Raman Spectroscopy 1  m* Liquid Crystals 1  m* Near-Field Optical Thermometry < 100 nm Scanning Thermal Microscopy (SThM) < 100 nm Techniques Spatial Resolution *Diffraction limit for far-field optics Thermometry of Nanoelectronics

X-Y-Z Actuator Scanning Thermal Microscopy Sample Temperature sensor Laser Atomic Force Microscope (AFM) + Thermal Probe Cantilever Deflection Sensing Thermal X T Topographic X Z

Microfabricated Thermal Probes Pt-Cr Junction Shi, Kwon, Miner, Majumdar, J. MicroElectroMechanical Sys., 10, p. 370 (2001) 10  m Pt Line Cr Line Tip Laser Reflector SiN x Cantilever

Thermal Imaging of Nanotubes Multiwall Carbon Nanotube 1  m Topography 1  m 3 V 88  A Distance (nm) Thermal signal (  V) nm Distance (nm) Height (nm) 30 nm Distance (nm) Height (nm) 30 nm Thermal Shi, Plyosunov, Bachtold, McEuen, Majumdar, Appl. Phys. Lett., 77, p (2000) Spatial Resolution

Metallic Single Wall Nanotube TopographicThermal 1  m ABCD Low bias: Ballistic High bias: Dissipative (optical phonon emission)  T tip 2 K 0

SiGe Devices Future Challenge: Temperature Mapping of Nanotransistors SOI Devices Low thermal conductivities of SiO 2 and SiGe Interface thermal resistance Short ( nm) channel effects (ballistic transport, quantum transport) Phonon “bottleneck” (optical-acoustic phonon decay length > channel length) Few thermal measurements are available to verify simulation results