Uncovering new thermal and mechanical behavior at the nanoscale using coherent extreme ultraviolet light by Kathleen Marie Hoogeboom-Pot B.S., Calvin College, 2008 M.S., University of Colorado Boulder, 2012 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Physics 2015 This thesis entitled: Uncovering new thermal and mechanical behavior at the nanoscale using coherent extreme ultraviolet light written by Kathleen Marie Hoogeboom-Pot has been approved for the Department of Physics Prof. Margaret M. Murnane Prof. Henry C. Kapteyn Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. Hoogeboom-Pot, Kathleen Marie (Ph.D., Physics) Uncovering new thermal and mechanical behavior at the nanoscale using coherent extreme ultraviolet light Thesis directed by Prof. Margaret M. Murnane and Prof. Henry C. Kapteyn Tremendous recent progress in nanofabrication capabilities has made high-quality single- atomic layers and nanostructures with dimensions well below 50 nm commonplace, enabling un- precedented access to materials at the nanoscale. However, tools and techniques capable of char- acterizing the properties and function of nanosystems are still quite limited, leaving much of the fundamental physics that dominates material behavior in the deep nano-regime still unknown. Further understanding gained by studying nanoscale materials is critical both to fundamental sci- ence and to continued technological development. This thesis applies coherent extreme ultraviolet (EUV) light from tabletop high harmonic generation to study nanoscale systems on their intrinsic length and time scales (nanometers and femtoseconds, and above), specifically following thermal transport and acoustic dynamics. These studies have shown where and how nanostructured mate- rial properties can be quite different from their bulk counterparts. This has in turn allowed us to develop new theoretical descriptions to guide further work. By observing heat dissipation from the smallest nanostructure heat sources measured to date (at 20 nm in lateral size), this work uncovers a previously unobserved and unpredicted nanoscale thermal transport regime where both size and spacing of heat sources play a role in determining the heat dissipation efficiency. Surprisingly, this shows that nanoscale heat sources can cool more quickly when spaced close together than when far apart. This discovery is significant to the engi- neering of thermal management in nanoscale systems and devices while also revealing new insight into the fundamental nature of thermal transport. Furthermore, we harness this new regime to demonstrate the first experimental measurement of the differential contributions of phonons with iv different mean free paths to thermal conductivity, down to mean free paths as short as 14 nm for the first time. The same technique is then applied to the study of acoustic waves in nanostructured ma- terials, where they are used to characterize mechanical properties at the nanoscale. This thesis demonstrates the application of EUV nanometrology for the complete characterization of isotropic ultrathin films down to 50 nm in thickness across a broad range of stiffnesses. By simultaneously measuring both longitudinal and transverse waves, we are able to study trends in elastic properties that are normally assumed to be constant because it is difficult to measure them. This work also extends the technique to study anisotropic materials. Finally, by observing the acoustic resonances of nanostructured ultrathin bilayers, this work is the first to apply EUV nanometrology to layers with sub-10nm thickness and to measure the mechanical properties of nanostructures down to single monolayer levels. Here it is shown that the density ratio of the ultrathin layers is not substantially altered from the bulk material counterpart, but the nanoscale elastic properties do deviate significantly and follow opposing trends for two different metallic materials. Dedication To Justin for constant love and friendship. To my family for always encouraging deep curiosity about the world. And to Him who is able to do immeasurably more than all we ask or imagine. vi Acknowledgements My time in grad school and in JILA has been a fantastic experience thanks in no small part to the many great people in the KM group that I've spent this time with and the great leadership, guidance and mentorship that Margaret and Henry provide. While I can't possibly extend thanks to all those who deserve it here, a few particular highlights must be mentioned. Through Margaret and Henry's excellent examples, I learned how important it is to develop the skill of good scientific story-telling | a lesson I will benefit from for the rest of my life. Special thanks to Tory and Craig who taught me that secret stashes of critical lab supplies must be maintained. Thanks to Dan Adams for sharing your truly impressive optics expertise and many excellent discussions which always furthered my understanding of the rigorous science behind my more intuitive ideas. Mark, your friendship and willingness to help us with experimental quirks and understanding more background have been wonderful. Most importantly, thanks to my Boulder family: Qing, you taught me everything I needed to be able to go on and delve deeply in these experiments, and you were a great friend in my first years of late nights in the lab. Travis, it has been a pleasure to watch you gaining the same kind of familiarity I learned from Qing, and you give me great confidence in turning over the ongoing experiments to you and Nico. Damiano and Nico, I think I've spent more of my life in Boulder with you than just about anyone else, and your research expertise and friendship made this all work so well. And those crazy nights when we finally felt like we were figuring it all out were unforgettable. Finally, thanks to Justin whose constant encouragement, support and sanity-keeping | and not a small number of late dinner deliveries in lab made all of this work possible. Contents Chapter 1 Introduction 1 2 Experimental background 7 2.1 Pump-probe spectroscopy . .7 2.2 Experimental setup . 12 2.2.1 Tabletop EUV from high harmonic generation . 14 2.2.2 Building a signal . 19 2.2.3 Diffraction as Fourier transform . 23 2.3 Why use EUV? . 29 2.4 Conclusion . 32 3 New regime in nanoscale thermal transport 34 3.1 Theoretical background . 35 3.2 Previous work . 40 3.3 Observing non-diffusive thermal transport . 44 3.3.1 Sample design . 45 3.3.2 Quantifying deviations from diffusive transport . 47 3.3.3 Experimental results . 53 3.4 Understanding a new thermal transport regime . 58 3.4.1 Quasi-ballistic model for isolated heat sources . 58 viii 3.4.2 Interacting heat sources, grey phonon approximation . 60 3.4.3 Interacting heat sources, full phonon distribution . 65 3.5 Alternate understanding through effective conductivity . 67 3.6 Outlook for the study of nanoscale thermal transport . 71 3.6.1 Extension to 2D heat sources . 71 3.6.2 Opportunities with dynamic imaging . 73 3.6.3 Avenues for further theoretical development . 75 3.7 Conclusion . 76 4 Probing phonon mean free path spectra 78 4.1 Developments in phonon MFP spectroscopy . 79 4.2 Experimental measurement of MFPs down to 14 nm . 82 4.3 Applying MFP spectroscopy with EUV light . 87 4.4 Conclusion . 88 5 Thin film metrology 91 5.1 Elastic properties of materials . 92 5.2 Existing metrology techniques . 93 5.3 EUV nanometrology . 97 5.3.1 Extracting acoustic velocities . 100 5.3.2 Elastic properties from acoustic velocities . 102 5.4 Anisotropic materials . 109 5.5 Future opportunities in nano-mechanical characterization with EUV . 113 5.6 Conclusion . 114 6 Nanostructure metrology 115 6.1 Nanoscale departures from bulk properties . 116 6.2 Samples for study of patterned films with sub-10nm layer thickness . 117 ix 6.3 Measuring material density with the surface acoustic resonance . 121 6.3.1 Extracting small frequency shifts with high resolution using CZT . 122 6.3.2 Interpreting frequency-shift data . 122 6.4 Characterizing elastic properties with the longitudinal acoustic resonance . 125 6.4.1 Extracting LAW periods from fast damping using MPM . 125 6.4.2 Interpreting LAW oscillation periods . 132 6.4.3 Potential mechanisms for modified nanoscale elastic parameters . 138 6.5 Conclusions and outlook . 142 7 Conclusions and Future Opportunities 143 7.1 Ongoing efforts . 145 7.2 Future opportunities . 146 Bibliography 148 Appendix A MATLAB program for frequency peak-finding using chirp z-transform 164 B Mathematica code for the Matrix Pencil Method 168 B.1 Set up special functions and modules . 168 B.2 Import data and extract signal components . 170 C MATLAB program for least-squares fitting with the acoustic transfer matrix 172 x Tables Table 3.1 Material parameters for thermal simulations . 49 6.1 Nanoscale density results from SAW under Ni/Ta bilayers . 124 6.2 Best-fit effective LAW velocities in ultrathin Ni and Ta . 137 Figures Figure 1.1 Exponential scaling of transistor feature size . .2 1.2 Hard drive data storage density . .3 2.1 Highlights in the history of time resolution . .9 2.2 Scientific photography: Marey's falling cat . 11 2.3 Experimental setup . 13 2.4 Three-step model for HHG . 16 2.5 Phase-matching for bright HHG source . 17 2.6 Timing schematic . 20 2.7 From diffraction image to signal . 21 2.8 EUV harmonics respond differently to height changes . 22 2.9 Data separates dynamics by timescale . 24 2.10 Diffraction peaks highlight specific SAW orders . 26 2.11 SAW selectivity observed .