ADRIAN S. WILSON, Element Six Technologies Ltd., Berkshire, U.K.
Synthetic diamond is ideally suited for thermal management of semiconductor packaging, as it combines exceptionally high thermal conductivity with electrical isolation.
Studies have shown that when it comes to the reliability of packaged chips, most failure processes follow a temperature dependent behavior. 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 (FIGURE 1).
This thermal challenge is at the forefront of 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 at 14 nm (according to the ITRS Roadmap). When combined with the need for higher power solid state switching devices for power converters and high frequency components for military, cellular, and satellite communications, the need to manage higher power densities and the associated heat is an issue spanning all major segments of the industry.
|Figure 1: failure modes in electronic systems.|
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 lifecycle, yet further awareness is critical to fully transition through the product lifecycle model to full-scale use.
|Figure 2: CVD synthetic diamond wafer, up to 140 mm in diameter and 3 mm in thickness.|
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 (FIGURE 2) 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.
|Figure 3: Diagram of heat spreader in LED package.|
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 (FIGURE 3) 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 results.
• 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 (FIGURE 4).
The Future for synthetic diamond
The combination of the semiconductor industry roadmap for increasing power densities and the increasing availability and affordability of synthetic diamond will result in a rapid increase in the adoption of this engineering material.
Synthetic diamond will be used with a broad range of semiconducting materials, such as SiC, 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 semiconductor packages and modules, through the implementation of standard package components for instance, will be a key element to the industries 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, 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 or increasing performance, and allows operation in hotter ambient environments, thus lowering cooling costs and/or increasing lifetimes.
The unique combination of properties synthetic diamond possesses makes it one of the most exciting supermaterials in the world. In a few years, perhaps diamond will become the semiconductor material itself for applications requiring extremely high breakdown voltages. 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. •
ADRIAN S. WILSON is the Head of Technologies Division, Element Six Technologies Ltd., Berkshire, U.K.