ME 381R Lecture 10: Thermal Measurement Techniques for Thin Films and Nanostructured Materials

1. 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 78712 www.me.utexas.edu/~lishi lishi@mail.utexas.edu

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

3. Thin Films and Interfaces GMR Cu Interconnects

4. Thin Film Thermal Conductivity Measurement The 3w method Cahill, Rev. Sci. Instrum. 61, 802 (1990) Metal line Thin Film L 2b V • I~ 1w • T ~ I2 ~ 2w • R ~ T ~ 2w • V~ IR ~3w I0 sin(wt) Substrate Substrate contribution Film contribution

5. Data Analysis • Dotted line - Ts+  Tf • Solid line -  Ts • Slope of solid line  ks • Tf  kf

6. 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.

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

8. Thin Film Superlattices SiGe superlattice (Shakouri, UCSC) • Increased phonon-boundary scattering • decreased k • + other size effects  Highthermoelectric figure of merit(ZT = S2sT/k)

9. Thermal Conductivity of Si/Ge Superlattices k (W/m-K) Bulk Si0.5Ge0.5 Alloy Circles: Measurement by D. Cahill’s group Lines: BTE / EPRT results by G. Chen Period Thickness (Å)

10. SixGe1-x/SiyGe1-y Superlattice Films Superlattice Period AIM = 1.15 Alloy limit With a large AIM, k can be reduced below the alloy limit. Huxtable et al., “Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices,” Appl. Phys. Lett.80, 1737 (2002).

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

12. Nanowire Materials ZnO nanowires (Z.L. Wang, GaTech) Sb2Te3 nanowires (potentially high ZT) (X. Li et al., USTC) Ge nanowires (B. Korgel, UT Austin) SnO2 nanowires (Z.L. Wang, GaTech)

13. The 3w method for Nanowires -- Lu, Yi, Zhang, Rev. Sci. Instrum. 72, 2996 (2001) • Low frequency: V(3w) ~ 1/k • High frequency: V(3w) ~ 1/C • Tested for a 20 mm dia. Pt wire V I0 sin(wt) Electrode Wire Substrate • Conditions: • The sample needs to have a large temperature coefficient of resistance TCR= (dR/dT)/R • The electrical contact has to be perfect

14. Q I Thermal Measurements of Nanowires Suspended SiNx membrane Long SiNx beams Pt resistance thermometer Kim, Shi, Majumdar, McEuen,Phys. Rev. Lett. 87, 215502 Shi, Li, Yu, Jang, Kim, Yao, Kim, Majumdar, J. Heat Tran 125, 881

15. (c) Lithography Device Fabrication Photoresist (a) CVD SiNx SiO2 (d) RIE etch Si (b) Pt lift-off Pt (e) HF etch

16. Pipet Nanostructure suspension Spin • Direct CVD growth • Dielectrophoretic trapping Sample Preparation • Wet deposition Chip SnO2 nanobelt Nanotube bundle Individual Nanotube

17. Thermal Conductance Measurement T - 1 - 1 - 1 G T G G h s b b T T 0 0 Q 2QL Q h

18. SnO2 Nanobelts 64 nm 64 nm 53 nm 39 nm Collaboration: N. Mingo, NASA Ames 53 nm 53 nm, ti-1 =10t-1i, bulk Circles: Measurements Lines: Simulation • Diffuse phonon-boundary scattering is the primary effect determining the suppressed thermal conductivities Shi et al., Appl. Phys. Lett. 84, 2638 (2004)

19. Si Nanowires Diameter Si Nanotransistor (Berkeley Device group) Gate Drain Source Solid line: Theoretical prediction Nanowire Channel Hot Spots in Si nanotransistors! Li et al., Appl Phys Lett83, 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

20. Si/SiGe Superlattice Nanowires Si/Si0.95Ge0.05 Si SiGe Boundary scattering of long- wavelength further reduces the thermal conductivity below the alloy limit Alloy limit Li et al., Appl Phys Lett83, 3186 (2003)

21. Carbon Nanotubes Nanotube Electronics (Avouris et al., IBM) • Atomically-smooth surface, absence of defects: Long mean free path l &Strong SP2 bonding: high sound velocity v •  high thermal conductivity:k = Cvl/3~ 6000 W/m-K

22. Thermal Conductivity of Carbon Nanotubes CVD SWCN CNT • An individual nanotube has a high k ~ 2000-11000 W/m-K at 300 K • k of a CN bundleis reduced by thermal resistance at tube-tube junctions • Potential applications as heat spreading materials for electronic packaging applications

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

24. Carbon Nanoelectronics TubeFET (McEuen et al., Berkeley) Nanotube Logic (Avouris et al., IBM) • Current density: 109 A/cm2 • Ballistic charge transport V -

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

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

27. 10 mm Microfabricated Thermal Probes Pt Line Tip Pt-Cr Junction Laser Reflector SiNx Cantilever Cr Line Shi, Kwon, Miner, Majumdar, J. MicroElectroMechanical Sys., 10, p. 370 (2001)

28. Thermal Imaging of Nanotubes Thermal 30 10 10 20 5 5 Height (nm) Height (nm) 30 nm 30 nm 10 0 0 0 -400 -200 0 200 400 -400 -400 -200 -200 0 0 200 200 400 400 Distance (nm) Distance (nm) Multiwall Carbon Nanotube Topography Topography 3 V m 88 A m m 1 1 m m Spatial Resolution V) m 50 nm Thermal signal ( Distance (nm) Shi, Plyosunov, Bachtold, McEuen, Majumdar, Appl. Phys. Lett., 77, p. 4295 (2000)

29. Low bias: Ballistic High bias: Dissipative (optical phonon emission) Metallic Single Wall Nanotube Topographic Thermal DTtip A B C D 2 K 0 1 mm

30. Future Challenge: Temperature Mapping of Nanotransistors SOI Devices SiGe Devices • Low thermal conductivities of SiO2 and SiGe • Interface thermal resistance • Short (10-100 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