The Ideal Material: Synthetic Diamond's Role in RF Thermal Management
By Adrian Wilson,
Head of Technologies Division
Suited for thermal management of RF semiconductor packaging, synthetic diamond is ideal as it combines exceptionally high thermal conductivity with electrical isolation.
RF devices are being pushed to higher and higher power levels. Radar systems are increasingly required to sense more and reach farther while weighing less. Cellular base stations need to provide greater bandwidth for the increasing number of smartphones. Satellite communications must handle more HDTV channels. As power levels go up, so must power densities, but without any drop in reliability.
Studies have shown that when it comes to the reliability of packaged chips, most failure processes follow a temperature dependent behaviour. Every 10°C of increase in junction temperature represents a 2x decrease in device lifetime. In fact, more than half of failures in today’s electronic systems are due to temperature.
This thermal challenge is at the forefront of RF package designers’ minds as they struggle to design packages to meet today’s thermal requirements. What’s more, this trend is only going to get worse. Device power densities are on a trajectory to be well above 100 W/cm2, according to the ITRS Roadmap.
Higher thermal conductivity materials are being explored to provide better heat extraction as compared to incumbent materials such as copper. Synthetic diamond is ideally suited for thermal management of semiconductor packaging, especially for today’s advanced electronic systems driving towards higher and higher power density, as it combines exceptionally high thermal conductivity with electrical isolation.
Diamond’s thermal conductivity at room temperature is an amazing five times that of copper. In addition, for mobile and aerospace applications, diamond has the advantage of low density (3.52 g/cm3) which, combined with its high thermal conductivity, enables small heat spreader dimensions for a very low weight thermal management solution. For rugged applications, the high Young’s modulus of diamond (1000 to 1100 GPa) helps increase the reliability of the entire package or module.
Widespread industry adoption of diamond in IC packaging has been slow, however some sectors are recognizing its benefits. It is being effectively integrated into packages for high power, LED and RF devices. However, the economics of diamond synthesis only really work if you can create high quality, thick diamond plates in high volume, which has only been achievable in the last 5-10 years. New materials to the semiconductor industry take anywhere from 5-15 years to be adopted. Synthetic diamond is now moving from the “early adopter” stage to the “early majority” stage of its life cycle, yet further awareness is critical to fully transition through the product life cycle model to full-scale use.
Addressing the criteria above, synthetic diamond, by way of microwave chemical vapor deposition (CVD) delivers:
High quality: High quality is relevant since the method of heat transfer within diamond is by lattice vibration, i.e. the transport of phonons. Any material impurities will hinder this lattice vibration and thus reduce the thermal conductivity. Synthetic diamond manufacturers understand this need and have patented such methods as using a microwave source that creates high energy atomic hydrogen, which strips away impurities in synthetic diamond during growth.
Thick diamond plates: Thermal conductivity is a three dimensional problem. As such, the diamond needs to be of sufficient thickness to rapidly dissipate localized semiconductor heat spots and optimally transfer the heat effectively from the semiconductor to the heat sink. Microwave assisted CVD is a scalable technology which deposits diamond over large areas and thicknesses at a cost similar to semi-insulating SiC wafers.
High volume: With an uptick in adoption, more manufacturers will look to expand capacity, as some have already done. Expanded capacity enables the industry to synthesize diamond at the appropriate scale to meet the price points required for both high power and RF device packages.
Integrating Synthetic Diamond in IC Design
Thermal conductivity alone is not the whole story. The effectiveness of CVD diamond as a heat spreader in electronic packages depends very much on how it is integrated into the module. To optimize the thermal management solution, engineers must consider carefully how the die and heat spreader will be attached, device operating requirements, the dimensions and surface conditions of the heat spreader, thermal expansions mismatch, and cost.
Die attach requirements: thermal barrier resistance of the TIM1 interface between the die and heat spreader must be minimized to optimize diamond heat spreader effectiveness. A metallic bond to the die, such as a solder joint, typically creates the least thermal barrier resistance.
Because diamond is a chemically inert material, carbide forming materials must be used to metalize diamond with sufficient adhesion. A commonly used metallization scheme of Ti/Pt/Au ensures carbide formation at the Ti/diamond interface to achieve the best result.
Device requirements: a heat spreader can be electrically conductive or insulating, and both options are possible using CVD diamond. The diamond itself is electrically insulating, but can be made conductive by means of covering the side faces with metals or laser drilling vias, with metal fillings, through the diamond.
Heat spreader characteristics: apart from choosing the grade of diamond to be used (from 1000 to 2000W/mK), the size of the heat spreader must also be determined. Heat spreaders are typically sized 50-100 μm longer than the die in each lateral dimension to ensure good solder fillets. Typical spreader thickness varies from 350 to 400 μm for a wide range of devices.
The Future for Synthetic Diamond
Given the combination of the RF semiconductor industry roadmap for increasing power densities and the increasing availability and affordability of synthetic diamond, the industry can expect to see a rapid increase in the adoption of this engineering material. Synthetic diamond will be used with a broad range of RF semiconducting materials, including GaAs and GaN.
As adoption increases, so will the desire to optimize TIM1 – the primary interface between die and diamond. No doubt new interface materials will be explored and potentially direct methods of bonding. In fact, making diamond heat spreaders easy to integrate into RF semiconductor packages and modules, through the implementation of standard package components for instance, will be a key element to the industry’s increasing adoption of this thermal management solution.
To this end, synthetic diamond manufacturers such as Element Six, in combination with its acquisition of the assets and IP of Group4 Labs, is bringing a GaN-on-diamond substrate to market. This substrate provides a highly optimized TIM1 interface and is already in use by U.S.
defense contractors for RF power amplifiers, and such early adopters indicate an increase in its use for the defense sector. Synthetic diamond technology will also reach into telecom infrastructure applications such as satellite communications and mobile base stations. For these applications, synthetic diamond enables higher power density, thus lowering system costs and increasing performance. It also allows for operation in hotter ambient environments, which leads to lowered cooling costs and increased lifetimes.
The unique combination of properties synthetic diamond possesses makes it one of the most exciting supermaterials in the world. And the list of applications for synthetic diamond is only expected to grow, making synthetic diamond manufacturers well-positioned to collaborate with industry partners to ensure future innovative applications for the material.
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