Coating at its best

Spraying graphite parts with tantalum carbide cuts the cost of producing SiC crystals and epilayers.

BY CHRISTIAN REIMANN, MATTHIAS TREMPA, KEVIN SCHUCK AND MICHAEL LANG FROM FRAUNHOFER IISB, AND TORSTEN KORNMEYER AND DIRK MUETZENICH FROM THE NIPPON KORNMEYER CARBON GROUP

The semiconductor industry is booming, with one prime example the field of SiC power electronics. Here many high-volume fabs are either under construction or expansion, to support an increase in the production of SiC devices, which are being deployed in electric vehicles.

While every boom offers a great opportunity to increase profits, it also brings its own unique challenges. Crucial to ramping global SiC chip production is the manufacture of high-quality SiC crystals, wafers and epitaxial layers. In this context, it is crucial to consider the role of semiconductor-grade graphite materials, which are used in the equipment that enables the growth of SiC crystals and the deposition of SiC epitaxial layers. Graphite, which is valued for its thermal insulation capabilities and inertness, is the preferred material for crucibles and susceptors in crystal-growing systems, and for planetary discs and satellites in epitaxial systems.

Figure 1. The TaC process technology developed at Fraunhofer IISB allows adjustment of desired coating properties, such as thickness, shown here in the range 35 µm to 110 µm.

Killer degradation…
The production of SiC crystals involves harsh process conditions. The total corrosion of a graphite crucible may occur after just a few processes, due to temperatures of over 2000 °C and extremely corrosive gas species that contain silicon. For instance, the production of SiC epitaxial layers involves: process temperatures that peak at 1600 °C; the combination of silane and propane, which are responsible for the growth of the SiC layer; and hydrogen and in some cases hydrochloric acid. The latter two can be used as carrier gases, or for very corrosive etching.

As well as simply eating up the graphite components, the harsh process conditions alter the emissivity and morphology of the component surfaces. This is a significant issue, severely impairing the reproducibility of the production processes and in turn having an impact on the resulting process stability – and thus the material quality of the SiC crystals and epitaxial layers. Another major drawback is that the harsh conditions can cause free carbon and carbon particles to be released from the corroded graphite parts, threatening the production of high-quality SiC semiconductor material.

Due to these considerable concerns, process engineers tend to only use these heavily stressed components a few times, and sometimes just once. It is a constraint that causes consumable material graphite to contribute significantly to the manufacturing costs of SiC crystals and epitaxial layers.

…and graphite shortages
It’s of little surprise that graphite is in great demand in many sectors, due to its desirable physical and mechanical properties and its capability to form a wide range of geometries. Unfortunately, today’s graphite industry cannot keep pace with the strong expansion plans of the global SiC industry, and competition between end users for the best possible supply of graphite is already underway.

Fortunately, there’s a game-changing solution that kills two birds with one stone. It’s the introduction of protective layers, which tackle both the degradation of graphite parts and the graphite delivery shortage. It’s a most welcome introduction, given the drastic increase of lead times for graphite components, driven by the booming semiconductor sector.

Today, protective coatings based on tantalum carbide (TaC) are already in use. The merits of this material include a melting temperature of over 3800 °C and a very good chemical resistance to reactive gases. Up until now, the approach to producing state-of-the-art TaC protective coatings is to use Chemical Vapour Deposition (CVD) to deposit this material on graphite components, a process that produces semiconductor-clean and gas-tight layers with a thickness of typically up to 35 mm. However, there are disadvantages of this technology, with the main drawbacks including high manufacturing costs, and once again, long delivery times. In addition, there are material issues, with cracks appearing through 100 percent dense crystalline TaC layers during repeated heating and cooling of the components. These cracks expose the underlying graphite, which strongly degrades over time and has to be replaced.