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Abstract Govindaraju, Nirmal ABSTRACT GOVINDARAJU, NIRMAL. Thermal Conductivity Analysis of Diamond Films. (Under the direction of Prof. Zlatko Sitar (Chair) and Prof. John Prater (Co-chair).) Thermal conductivity is a fundamental material property which governs heat transport in solids. A basic material property like thermal conductivity is influenced by the structure of the material, and hence the method of synthesis and the defects present in the material. With the advent of high speed microelectronics with extremely large device densities, there is an ever increasing need to find solutions for effective heat dissipation, especially in the context of Silicon On Insulator (SOI) technology. One solution to this problem would be to use diamond as the buried dielectric. Therefore, for the effective use of diamond films, it is essential that the thermal properties, especially the thermal conductivity, of the material be characterized. A brief background of the structure, properties and synthesis of diamond is presented. This is followed by a review of some of the experimental methods used for measuring the thermal conductivity of diamond. In order to measure the thermal conductivity of CVD diamond, two different measurement approaches are used. The first is the 3ω technique, which uses thermal waves generated by a heater/thermometer to probe the underlying material and extract the thermal conductivity from the measured temperature rise. Measurements on bulk substrates of different materials, including diamond, yield results consistent with values found in literature. However, in the case of diamond thin films, the method proves to be unreliable since there is significant deviation from the assumption of one dimensional heat transport through the solid. An alternative method, known as the heated bar technique, is therefore employed to measure the thermal conductivity of the sample. The heated bar technique employs one dimensional steady state heat transfer to measure the thermal conductivity of the material. An assembly of thin film heaters and thermocouples is deposited on a free standing substrate. A known quantity of heat is input into the material and the resulting temperature profile is measured using thin film thermocouples. From the measured temperature profile and the applied heat flux, the thermal conductivity of the material is extracted. Thermal conductivity measurements are performed on diamond films as a function of morphology and thickness. It is found that there is a significant difference in the measured value of the thermal conductivity between highly oriented diamond (HOD) samples and samples having fiber texture. This is attributed to the presence of low angle grain boundaries in the HOD films, which serve to refract rather than scatter phonons which are the dominant heat carriers in the case of diamond. The effect of the nucleation layer on the lateral thermal conductivity of the material is ascertained by performing thermal conductivity measurements before and after etching of 10 µm of diamond from the nucleation side of a textured substrate. It is found that the removal of the thermally poor quality material does not significantly affect the lateral thermal conductivity of thick diamond films. THERMAL CONDUCTIVITY ANALYSIS OF DIAMOND FILMS by NIRMAL GOVINDARAJU A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy MATERIALS SCIENCE AND ENGINEERING Raleigh 2004 APPROVED BY: –––––––––––––––––––––––– –––––––––––––––––––––– –––––––––––––––––––––––– –––––––––––––––––––––– Chair of Advisory Committee Co-chair of Advisory Committee To my family ii Biography Nirmal Govindaraju obtained his Bachelor of Engineering in Metallurgy from Regional Engineering College, Durgapur, India in 1998. He then went on to obtain a Masters Degree in Materials Science and Engineering from Washington State University, Pullman, WA in 2000 under the direction of Dr. Ben Q. Li. His thesis work involved the development of a parallel computing model to study of the effect of electromagnetic stirring on the evolution of solidification microstructures in metallic castings. Since the year 2000 he has been under the tutelage of Prof. Zlatko Sitar as a member of the Laboratory for Growth and Characterization of Wide Bandgap Semiconductor Materials at North Carolina State University. His area of research as a Doctoral student has been the thermal characterization of diamond thin films. iii Acknowledgements The author would like to express his gratitude to the members of his committee, Prof. Zlatko Sitar, Prof John Prater, Prof. Jerome Cuomo and Prof. Carlton Osburn, for the time and resources expended in evaluating his research. The author in particular would like to thank Prof. Zlatko Sitar and Prof. John Prater for their support, guidance and insightful discussions throughout the course of study. The author would also like to thank all the members of Prof. Sitar’s group for their help and support. The author is grateful for the help of Prof. Raoul Schlesser with regard to the experimental setup of the 3ω method. Also, the author is thankful for the useful discussions with Dr. Ramon Collazo and for the samples provided by Dr. Scott Wolter, Dr. Fuminori Okuzumi, Dr. Aleksandar Aleksov, and Xianglin Li. iv Table of Contents List of Tables ................................................................................................................... viii List of Figures.....................................................................................................................ix 1. Introduction to the Structure, Properties and Synthesis of Diamond......................1 1.1 Introduction ...........................................................................................................1 1.2 Structure and Properties of Diamond......................................................................2 1.2.1 Diamond ........................................................................................................3 1.2.2 Graphite.........................................................................................................4 1.3 Synthesis ...............................................................................................................6 1.3.1 Synthesis at High Pressures and High Temperatures......................................7 1.3.2 Synthesis at Low Pressures and High Temperatures.......................................8 1.4 Fundamentals of Microscale Heat Transfer ..........................................................12 2 Experimental Methods for Determination of Thermal Conductivity ....................20 2.1 Introduction .........................................................................................................20 2.2 Thermal Wave Methods......................................................................................21 2.2.1 Thermal Wave Generation ..........................................................................21 2.2.2 Photothermal Radiometry ............................................................................30 2.2.3 Photoacoustic Method..................................................................................31 2.2.4 Photothermal Deflection Method (“Mirage Effect”).....................................33 2.2.5 A.C. Calorimetry..........................................................................................35 v 2.3 Pulsed Methods...................................................................................................38 2.3.1 Flash Method ..............................................................................................38 2.4 Steady State Methods...........................................................................................42 2.4.1 Insulated Plate ..................................................................................................42 2.4.2 Radiating Bar....................................................................................................44 3. Application of the 3ω Method for Measuring the Thermal Conductivity of Diamond ...................................................................................................................45 3.1 Introduction .........................................................................................................45 3.2 Theoretical Basis for the 3ω Method....................................................................46 3.3 Experimental Implementation of the 3ω Method..................................................54 3.4 Limitations of the 3ω method...............................................................................63 3.4.1 Heat Conduction Through a Multilayered Material......................................64 4 Measurement of Thermal Conductivity of Diamond Films by the Heated Bar Technique.................................................................................................................71 4.1 The Heated Bar Method.......................................................................................71 4.1.1 Implementation of the Heated Bar Method ...................................................72 4.1.2 Estimate of Convective and Radiative Heat Transfer Losses.........................76 4.2 Thermal Voltage Generation and the Equivalent Thermocouple Circuit ...............77 4.3 Rationale Behind Thermal Conductivity Measurements.......................................80 4.3.1 Structure of Diamond Thin Films .................................................................80
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