Electronics Protection - July/August 2013 - (Page 8)

Feature Diamond Heat-Spreaders: Growth Methods and Applications Dr. Richard S. Balmer, Principal Scientist Dr. Daniel J. Twitchen, Chief Technical Officer, Technologies Thomas Obeloer, Business Development Manager Dr. Geoff A. Scarsbrook, Technologies R&D Manager Adrian S. Wilson, Head of Technologies Element Six For more than 50 years, synthetic diamond manufactured using high pressure and high temperature techniques (HPHT) has been used for abrasive applications, exploiting its extreme hardness and wear resistance. Over the last 20 years, new methods of growth based on chemical vapor deposition (CVD) have been commercialized that allow for the cost effective growth of single crystal and polycrystalline diamond. These methods of synthesis have enabled the exploitation of diamond’s optical, thermal, electrochemical, chemical and electronic properties. Today diamond is used extensively in both the optical and semiconductor industries. This article focuses on the thermal benefits of diamond, introduces how diamond works as a heat spreader, presents a summary of growth methods, summarizes some of the more common applications of diamond (including how to apply it) and concludes with a view of what we can expect from diamond in the future. We begin with a summary of how and why diamond is the best thermal conductor of any solid at room temperature. How Diamond Works Diamond is a cubic crystal consisting of covalently bonded carbon atoms. Many of the extreme properties of diamond are a direct result of the stiffness of the sp³ bonds forming its rigid structure together with the low mass of the carbon atoms. In contrast to a metal where heat is transmitted via free electrons and high thermal conductivity is associated with high electrical conductivity, heat transfer in diamond is solely carried by lattice vibrations, i.e. phonons. Diamond’s extremely strong inter-atomic bonding gives a rigid lattice with high vibrational frequencies and thus a high characteristic Debye temperature of 2,220°K. As most applications are well below the Debye temperature, phonon-phonon scattering is small resulting in little impedance for the phonon-mediated heat transport. However, it is the nature of crystalline materials that any lattice imperfections act to reduce the thermal conductivity by scattering phonons. Typical imperfections in diamond include point defects such as the heavier 13°C isotope, nitrogen impurities and vacancies; extended defects such as stacking faults and dislocations; and 2D defects such as grain boundaries. As an element used purely for thermal management, natural diamond was used in some early microwave and laser diode devices [1], [2]. However, the availability, size and cost of suitable natural diamond plates limited its penetraFigure 1. Free standing CVD diamond wafer tion into that market. With the advent of microwave assisted CVD polycrystalline diamond with thermal properties similar to those of type IIa natural diamond (Fig 1) the availability problem was solved. Today, a range of thermal grades of diamond are available off the shelf from a number of sources. Since freestanding polycrystalline diamond is grown in large wafers up to 8 140 mm in diameter (Figure 1), size is no longer limited to single devices or small arrays, but can extend to arrays many centimeters across. Consequently, CVD diamond has proven practical, being built into many devices from the 1990’s onwards. As shown in Figure 2, TM200 (TM for thermal, 200 refers to conductivity >2,000 Wm−1 K−1) has a room temperature thermal conductivity of 2,200 Wm−1 K−1 exceeding that of copper by a factor of five (see Table 1). Element Six offers a range of products so that the thermal conductivity and costs Figure 2. Through-plane thermal conductivity can be tailored versus temperature measured using the laser flash to the technical method for natural type IIa [3], [4], [5] and polyrequirement and crystalline diamond grades TM200, TM180, TM150 and TM100 grown by microwave assisted CVD. budget. With a room temperature conductivity >1,000 Wm−1K−1 TM100 exceeds ceramic materials such as aluminium nitride by a factor of four to six. The thermal performance advantage for the higher grades improves further below room temperature, with the conductivity increasing dramatically at temperatures down to 100°K. Figure 2 shows that TM180 and TM200 grades exhibit a very similar trend and performance with temperature as the type IIa natural diamond. Using characterisation techniques, we have carried out a detailed analysis of the microstructure of the different grades. The conductivity of TM100 is fairly insensitive to temperature over the range studied. The size of the grains in CVD diamond increases with thickness grown and has a strong influence on conductivity. For CVD diamond with comparable grain size, the point defect density in both TM100 and TM180 was similar, however the dislocation density was three orders of magnitude higher in TM100 compared to TM180. This difference plays a major role in phonon scattering and has a significant effect on the conductivity. The measured dislocation density for TM180 and TM200 is similar; however the small difference in conductivity at lower temperatures is explained by the grain size and a five-fold lower point defect density in the TM200. As will be explored in the next section, the grain size, purity and dislocations of other growth techniques vary dramatically with an equally dramatic impact on thermal performance. Certain segments of the semiconductor market, for example power convertors and solid-state RF power amplifiers, are driving towards higher power densities, increasing the burden on local thermal management. CVD diamond, with its value proposition of combining extreme properties such as high thermal conductivity and electrical isolation, is uniquely positioned to address this need. Our measurements show that the in-plane versus through-plane conductivity for microwave assisted CVD diamond is comparable to the measurement uncertainty and less than 10 percent. Isotropic thermal properties and electrical isolation are important attributes of a heat-spreader material in many thermal applications. July/August 2013 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - July/August 2013

Electronics Protection - July/August 2013
Newer Technology Releases Next-Generation Power2U AC/USB In Wall Charging Solution
Subzero Engineering Introduces PolarXpress DCiM SaaS Monitoring Solution
Specifiers of Enclosures for Components in Outdoor Applications: Be Aware of Material Selection Issues
Formable Phase Change Materials as Latent Heat Sinks for Portable Electronic Devices
How to Protect Electronic Circuits from Power Surges
Diamond Heat-Spreaders: Growth Methods and Applications
Standards-Based Design & Electronic Packaging Solutions
Reducing Room-Level Bypass Airflow Creates Opportunities to Improve Cooling Capacity and Operating Costs
Harnessing Laboratory Wind Tunnels
New DVR Enclosure with Prepackaged Thermoelectric Cooler
New Silver Conductive Aqueous Based Sodium Silicate System for EMI/RFI Shielding
Citizen and MechaTronix LED Cooling Cooperate On New Generation CitiLED COB’s
Eaton Expands 5P UPS Product Line to Provide Efficient Protection for Any IT Environment
Flex-Block System Puts Free-Standing Enclosures on New Footing
Gore Introduces Acoustic Vent for Protecting Electronic Devices
Industry News
Calendar of Events

Electronics Protection - July/August 2013