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Findings From The Lab
Thermal Management Using Diamond
by Dr Charles Willingham, Leo Paradis
Have you ever taken a magnifying glass and charred a piece of paper using sunlight? Doing that required your concentrating the sun's energy 2 or 3 times. Now, imagine the damage potential of sunlight concentrated 20,000 times onto the surface of a semiconductor. What would you do if you were faced with the problem? Maybe, you'd drop by the Materials Engineering Laboratories (part of the RAL) for a piece of diamond to help out.
Many of the advanced systems being developed throughout Raytheon require that magnitude of problem be solved to ensure reliable electronics operation. Heat is the bane of electronics' reliability, and there is no graceful degradation at that heat flux. From the solid state lasers being developed to the ship's power electronics heat at those levels is being addressed every day. As you would expect, there are numerous strategies to manage that power, and one of the best is using diamond to spread the heat out to more-manageable levels.
For high thermal conductivity, diamond is the answer.
Figure 1 compares the thermal conductivity of many of the materials used in electronic systems. Since thermal resistance is inversely proportional to thermal conductivity, you can see that a piece of diamond adjacent to a heat source can immediately reduce your problem a lot.
Figure 1 - Thermal Conductivity of Raytheon's Optical Quality
Grade Diamond Compared with Common Materials
Raytheon was one of the pioneers in manufacturing diamond use of diamond in thermal control, and other applications. Raytheon's Materials scientists developed the Microwave Assisted Chemical Vapor Deposition (MA/CVD) approach in the late '80s, and development work continues to this day. The polycrystalline diamond is grown at a relatively slow rate in reactors. Bulk deposits of diamond are comprised of small crystals that are optically transparent and exhibit very high thermal conductivity and working strength. Figure 2 illustrates how the reactors work.
Figure 2 - Diamond is grown from a plasma generated by a gaseous absorption of high power microwave energy
One of diamond's notable early applications was for thermal control of IMPATT diodes, see Figure 3. More recently, it has been developed for Infrared (IR) domes, windows and electromagnetic (EM) windows for directed energy weapons applications. Again, due to its phenomenal thermal conductivity, diamond is essentially impervious to thermal shock. Its thermal shock resistance vastly exceeds those of all other materials. . One of the latest developments has been the manufacture of an all-diamond microchannel heat sink, successfully tested during Autumn, 2002. Figure 4 shows a 1 cm by 2 cm heat sink manufactured for that program. The dark, parallel bands are coolant flow channels, 150 micrometers across.
Figure 3 - Diamond used for spreading heat generated
by IMPATT diodes. Each dark disc is 2mm across, and 0.5mm thick.
The microchannel heat sink was invented by Tuckerman, who, as a graduate student at Stanford in the early '80s, foresaw that power dissipated by Silicon-based processors would one day be so great as to prohibit conducting heat to distant heat sinks. He proposed that very narrow channels (micro-channels) for coolant be etched into the chip's rearface, and demonstrated the tremendous thermal performance to be realized by such an approach. Now, two decades later, microchannels are at the leading edge of thermal control technology. And, the best microchannel ever (as far as we know) was produced in Lexington, using Raytheon's home-grown diamond.
Figure 4- Diamond microchannel heat sink. The sink (dark area) is soldered to a supporting, diamond 'strongback'. Its performance exceeds that of all other microchannel heat sinks described in literature.
Thermal management is an important and basic property of electronic components and systems. This diamond-based thermal management technology, developed by the Advanced Materials group for more than a decade, will make its way into components and devices of the future.