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DIAMOND HEAT SPREADERS ———– Tips & FAQ for Diamond Heat Spreaders LEVERAGING THE PROPERTIES OF DIAMOND

Facing high heat flux from high power density hot spots in optoelectronics? Chemical Vapour Deposition (CVD) diamond is the key to reliability and success. The use of CVD grown diamond as high performance heat spreader is enabling for a wide range of disruptive electronics from GaN solid state RF X-band PAs to advanced ASICs to laser diodes. Naturally, understanding the fundamentals of science and engineering of the thermal properties of grown diamond is an essential part of any such building blocks. So too is the proper application needed to leverage the properties of diamond.

THE MATERIAL WHAT DOES A HEAT SPREADER DO? The material is 100% sp3-bonded diamond made FOR HIGH POWER RF AND OPTOELECTRONICS, CVD DIAMOND by a microwave generated plasma CVD process. The ENABLES DEVICES TO: technology is proven to yield the highest quality diamond, ––Run at higher power levels without increasing junction as defined by superior mechanical, optical and thermal operating temperature properties. Through precise process control, the growth parameters are closely set and monitored to generate ––Run at the same power level, but much cooler, thereby material with the specific required increasing lifetime and reliability by customers. We offer CVD diamond with thermal conductivities ranging from 1000 W/mK to 2000 W/mK. ––Wide optical transmission enables CVD diamond heat Standard grades are offered as 1000 W/mK, 1300/1500 spreaders to operate within an optical path, such as in W/mK, 1800 W/mK and 2000 W/mK. Naturally different laser cavities, without optical performance degradation price points are applied to different grades. Diamond wafers (substrates) are grown in diameters from 138 FOR HIGH-VOLTAGE POWER DEVICES, CVD DIAMOND DELIVERS: mm for 1000 W/mK grade to 115 mm for 2000 W/mK. ––Improved reliability and increased efficiency Material property control through direct and indirect by lowering device operating temperature thermal conductivity measurements are in place to ensure that all material produced meet the stated specifications. ––Reduced system weight and footprint TRANSFORMING THERMAL MANAGEMENT ––Reduction or elimination of auxiliary cooling systems Simply stated, diamond has the highest isotropic thermal conductivity of any known material at room temperature; heat is the single biggest cause of failure in TYPICAL PACKAGE GEOMETRY WITH CVD electronics. Statistically, reducing the operating junction DIAMOND MOUNTED IN MODULE temperature by 10°C can double a device lifetime. CVD diamond outperforms today’s common materials for Hot spot Chip or 'die' TIM1 - primary interface General heat thermal management, such as , silicon carbide and flow direction CVD Diamond TIM2 - low melting solder nitride by factors of 3 to 10 times. Heat Spreader

THERMAL CONDUCTIVITY Heat DIAFILM TM200 Sink DIAFILM TM180 DIAFILM TM150 Package DIAFILM TM130 DIAFILM TM100 Cu BeO Figure 2. AIN 0 500 1000 1500 2000

Figure 1. THERMAL CONDUCTIVITY ( W / mK)

Figure 1.

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2 OPTIMIZING DIAMOND HEAT SPREADERS

AVAILABLE THICKNESSES AND SIZES HEAT SPREADER ATTACHMENT AND METALLIZATION For the majority of applications, the full benefit of Though non-metallized heat spreaders are available diamond heat spreader could be realized using material from Element Six, we encourage our customers to in the thickness range between 0.30 mm and 0.50 mm. It procure already metallized diamond heat spreaders. should be noted that for achieving the proper spreading, For successful integration of diamond heat spreaders, the thickness of diamond should be kept to at least it is imperative that the metallization be of the highest equal, but preferably larger than, the size of the hot spot. quality and correct thickness, as any additional layer or Though this ‘rule of thumb’ has proven to work rather material will result in additional increase in thermal well for many applications, comprehensive and detailed resistance. Typical metallization stack would consist of Ti/ thermal simulation examples are available from Element Pt/Au (100/120/1000 nm). This configuration is suitable Six. For devices requiring heat dissipation over larger for a wide range of solder processes as well as wire areas such as disk lasers, VCSEL or LED arrays thicker bonding. The titanium (adhesion layer) and platinum heat spreaders are proven to be beneficial. Most grades (barrier layer) thicknesses are usually kept fixed, but the of diamond heat spreaders are available at thicknesses gold (solder layer) thickness can be made to customer of up to 1.00 mm. The 2000 W/mK grade of material is specifications. While gold thickness on the order of commercially available at up to 3.50 mm in thickness. 1500 nm is a common choice for Au-alloy soldering, it is possible to increase the gold thickness to as high as 3500 SURFACE PREPARATION nm for applications with high electrical current densities. Surfaces need to be prepared for device mounting. This As an example, a 3000 nm gold layer has been proven to includes a smooth surface and an appropriate degree work with up to 120 A of drive current without adding of flatness to ensure good thermal contact at the TIM1 significant heat loads to the assembly. Additionally, pre- (primary thermal interface). Hence the critical surface of deposited solder materials (AuSn or AuGe) are available the heat spreader is polished to a fine finish (Ra<50 nm). at different compositions (Sn–rich or Au-rich eutectics) to Flatness of the TIM1 surface is usually between 5 and meet any alloying needs for a durable solder attachment. 10 μm over the heat spreader surface. For some critical These solder layers are typically applied in thicknesses applications, such as LDAs (Laser diode arrays) a flatness between 2500 and 4000 nm resulting in thin bond lines, value of <1 μm are achievable on the supplied parts to and hence improving overall device performance in customers. As the heat spreads out through TIM1 and combination with the advanced thermal spreading. diamond to a larger surface on the opposite side, TIM2 surface finish becomes a little less critical than TIM1.

Element Six provides a fine lapped (Ra<250 nm) surface for TIM1. The fine lapped finish which meets all needs for good thermal contact to the . It also provides a visual difference for the recognition of the primary and TYPICAL APPLICATIONS secondary diamond heat spreader interface, enabling easy detection of each sides. HIGH-POWER RF DEVICES HIGH VOLTAGE POWER ––Base station RF DEVICES amplifiers ––Automotive sub systems VOLUME SUPPLY POINT ––Satellite RF uplink ––Aerospace sub systems amplifiers ––Energy distribution In response to demand for high performing CVD ––Microwave amplifiers ––DC/DC converters diamond heat spreaders, Element Six has developed a significant fabrication and manufacturing capacity in HIGH-POWER SEMICONDUCTOR EQUIPMENT the Ascot, UK and Santa Clara, CA. OPTOELECTRONICS ––Characterization ––Laser diodes and laser testing EXAMPLE OF APPLICATIONS diode arrays ––Die-attachment ––Optical planar IC ––High Power RF Devices processes ––High Power Optoelectronics modules ––High-brightness LEDs ––High Voltage Power Devices ––Semiconductor Equipment Figure 3.

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3 REDUCING THERMAL RESISTANCE

A CASE STUDY (BONDED CVD DIAMOND FOR RF PACKAGING) The following case example demonstrates the significance of replacing BeO ( oxide) with 1000 W/mK grade diamond. Not only is a serious source of toxicity eliminated from the package, but the overall thermal resistance between the junction and the base is reduced by 30%. Substituting 1800 W/mK grade diamond for BeO results in a further reduction in the overall thermal resistance.

TEMPERATURE AS FUNCTION OF HEAT SPREADER TM100, 0.299 MM HEIGHT, 180 MIL WIDE

50 BeO HEAT CuMo flange RF device SPREADER 45 Degrees C 28.147 30.072 40 31.998 33.924 35.849 35 37.775 DIAMOND 28.147 39.701 30.072 TEMPERATURE HEAT 31.998 41.627 30 33.924 35.849 43.552 SPREADER 37.775 39.701 45.478 41.627 43.552 25 45.478 0.0 0.5 1.0 1.5 2.0 2.5

DISTANCE TO BOTTOM [mm]

Figure 4.

THERMAL RESISTANCE OF RF PACKAGING 100

90 FOR MORE INFORMATION 80 Element Six Technologies US Corporation* 70 3901 Burton Drive, Santa Clara, CA 95054, USA Call: +1 408 986 2400 60 Email: [email protected] Website: www.e6.com

RELATIVE THERMAL RESISTANCE (%) 50 *Registered with the Department of State for handling ITAR sensitive BeO 1mm BeO 0.3mm TM100 0.3mm TM180 0.3mm and controlled defense projects.

Figure 5.

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