CVD Diamond Solves Thermal Challenges
10/01/2006
BY DWAIN A. AIDALA, sp3 Diamond Technologies, Inc.
The miniaturization and increased functionality of electronic devices is causing manufacturers to seek new thermal management solutions. When compared to conventional materials, chemical vapor deposition (CVD) diamond is emerging as an effective heat spreader. Thick-film, polycrystalline CVD diamond is a material solution for first-level mounting of packaged semiconductor devices. With an isotropic thermal conductivity of 1000 to 1200W/m-K, diamond provides a mounting surface with 3 times the thermal conductivity of copper, and 5 to 6 times the thermal conductivity of more complex materials, such as BeO and AlN. The cost of thick-film CVD diamond has decreased due to its increased use in both electronic and cutting-tool applications. While the cost of CVD diamond is higher on a volume basis when compared to other heat-spreader materials, its bill-of-materials cost is low - 1-2% of the assembly bill-of-materials cost. The use of diamond heat spreaders in multiple market applications to provide device power and reliability improvements will continue to reduce its portion of total device cost.
Broad-based challenge
Many high technology applications experience constrained performance growth due to thermal issues, with increased performance restricted by reliability or stability concerns. The electronics and communications industries continue to struggle with thermal management challenges. While new technology provides the path to smaller device geometries, this same scaling path further increases the difficulty of getting heat away from the active portions of the circuits. Consider these thermal challenges in some key industries.
Illumination: The worldwide illumination market is beginning to transition from incandescent to solid-state lighting. This is driven by such factors as bulb life, form factors, and cost. This transition is well under way in the automotive market, and in the early stages for the consumer electronics display market. It is accelerating because of the high growth curve of flat panel displays, with 2006 shipments of flat-panel TVs 26 inch and larger are forecasted to grow 110% to 32 million units. The U.S. Department of Energy (DOE) has aggressive lumens-per-watt targets for white-light LED, with the high-brightness LED market forecasted to be at $10.8B by 2010. Similarly, laser diodes are vying to be the illumination device of choice for rear-projection TVs and consumer projectors.
 Figure 1. Integration of diamond heat-spreaders in laser packaging. |
Thermal management is key to both LED and laser devices as they move into these new markets. Light output drops by a factor of 4 as the junction temperature increases from 25° to 115°C in red LEDs, with green devices also significantly affected. Junction temperature also has an effect on the light color of LEDs, with green seeing the greatest shift with increase of junction temperature. One company* states that every 10°C rise in junction temperature causes a 1-nm shift in the dominant wavelength. Control of this is critical in TV applications.
Communications: High-density radar for defense, and eventually commercial, applications are driving heat densities of high-power devices. Device-level densities of 500W/mm2 are being targeted, while at the package level, 1KW/cm2 is expected. At both the device and package level, thermal management is recognized as a key challenge to achieving targeted output and reliability.
Power Semiconductor: The transportation markets’ shift to electric power (hybrid automobiles and electric trains) is requiring higher efficiencies combined with increased drive power. The power semiconductor market is focused on devices at >500V and 50A. As frequencies are increasing, power transistor modules are shrinking by as much as 50%. All of these lead to exponential increases in power densities. The continuance of both trends will be highly dependent on the continued development of cost-effective thermal solutions.
Processor: The processor market is divided into two major segments: the general-purpose processor market used for PCs, notebooks, and servers; and the graphics processor market for video gaming. Surprisingly, it is the latter that drives technological development. In 2005, over $10.5B was spent on gaming hardware and software in the US. Applications are driving the need for faster processors while manufacturing cost-driven line-width reductions are driving transistor densities into new territory. It is well known that a major general purpose processor manufacturer canceled a $4B development project because of continuing thermal hurdles. It is clear that junction temperature management solutions are needed at the package and device levels.
Diamond technology as a solution
This multitude of thermal challenges has led to an increasing interest in CVD (chemical vapor deposition) diamond. Thick film polycrystalline CVD diamond offers high thermal conductivity and easy metallization for first level mounting of solid-state optical devices and packaged semiconductor devices. Application of a titanium (Ti) layer to the diamond enables other metallization, such as platinum or gold, to be used. This enables the use of standard attachment materials such as solders or epoxies. Additionally, thick-film CVD thermal diamond can be polished and laser-cut using the same methods and equipment developed for cutting tool tips and dresser bars.
Laser and LED sub-mounts
Both laser and LED device manufacturers are using CVD diamond thermal submounts. Figure 1 illustrates the integration of diamond heat spreaders in laser packaging. Depending on the specific application and laser type, diamond may be a sufficient thermal solution. Other critical applications, such a telecommunications devices, may require the addition of an active thermo-electric cooler (TEC) to maintain a precise device junction temperature. Even in low-cost LED packages, there are opportunities for small segments of diamond to enhance color output stability.
The market driver behind high-power LED and lasers is the opportunity to replace high-power incandescent lamps. Copper has been the material of choice because of its thermal properties and low cost. However, copper is not sufficient for the multiple emitter arrays being used in these consumer applications. With a single emitter, there is no discernable difference, but for a 15-emitter array, a 33% increase in per-emitter-power was obtained.
 Figure 2. Diamond “pins” placed in a base plate made of composite material for packaged high-power components. |
CVD diamond is also making inroads into high-heat flux communications and power semiconductor devices. This is occurring with the use of metallized diamond segments as the mounting surface for an active device or as diamond pins placed in a base plate made of composite material for packaged high-power components (Figure 2).
Diamond on Silicon
A developing application of CVD diamond as a thermal management tool for both power devices and processors is the work being done with diamond-on-silicon (DoS). Using large-area deposition reactors, it is possible to produce single 300-mm diameter DoS wafers -which are state of the art for CMOS processors - in a single run, or multiple 100-mm diameter wafers - which are state of the art for GaAs or GaN power devices. Using GaN devices for illustration (Figure 3), the purpose of the diamond layer is to provide thermal spreading as part of the active device structure to move the heat away quickly from the device hot spots and broadly to the package heat spreader.
CVD diamond provides design engineers with a choice of how variables are traded - maintain junction temperature and greatly increase power out; drop junction temperature and increase device life and reliability; or some combination of both.
Summary
The high-heat flux device thermal management opportunity is continuing to grow as government- and business-focused devices’ performance is limited because of thermal issues, or pushed to replace more traditional solutions. CVD diamond, either thin- or thick-film, has a place in the choice of solutions to many of these thermal issues.
* Lumileds
DWAIN A. AIDALA, president and COO, may be contacted at sp3 Diamond Technologies, Inc. Santa Clara, CA 95050; 408/492-0630; E-mail: [email protected].