Graphite vs. Solid SiC Susceptors for Thermal Uniformity Control

For the semiconductor and epitaxy process the susceptor is a crucial element within the reactor. It carries the wafer and serves the purpose of distributing heat while working under high temperatures. The two materials typically used as susceptors are solid silicon carbide ( Cvd sic ) and Graphite crucible . They have different ways of distributing heat which influence the wafer quality and production output.

Comparing Thermal Conductivity in Graphite and Solid SiC Susceptors

It matters how the heat is spread out through a susceptor, in terms of process stability. High thermal conductivity of Graphite is strong as the heat quickly travels through the medium, this would lead to reaching target temperature faster which in turn reduces the heating and cooling time. Graphite does not, but spread the heat in equal directions. It is possible that theformation ofhot spots and variation of the wafer layer thickness is present due to a non uniform temperature inside the graphite structure.

Solid SiC and Cvd sic coating behaves differently. While it can take longer to reach target temperature than graphite, the heat is more evenly distributed over the wafer surface area, which gives more stable temperatures at center and edge of the wafer.

It was observed that when they were using a graphite susceptor the temperature across the wafer was uneven. Once they had switched over to solid High purity CVD SiC raw material the wafer temperature distribution was stable, even though the heating time was more.

So, Graphite is faster, Solid SiC is uniform.

Why CVD SiC Coated Graphite Remains Popular in AIXTRON G5/G10

But, many epitaxy systems still continue to use the CVD SiC coated graphite, specifically in high volume manufacturing. Graphite can be readily and rapidly heated; thus decreasing cycle times. SiC Coating over graphite ensures the durability against the high temperatures and corrosive gases while simultaneously ensuring a smooth wafer handling surface.

So, the benefits from both are realized; namely heat transfer from the graphite and durability and low wear from the SiC coating.

One production line, for instance, was experiencing particle problems with the (old) susceptors. They found that instead of replacing their old, coated graphite with solid SiC, they were able to achieve stable temperatures and the original production rate by replacing the old graphite part with a new, coated graphite part.

One final factor in the continuing acceptance of this material is maintenance. The maintenance of a coated graphite part ( machining and refurbishment) is simpler than maintaining a solid SiC part. When a part begins to show wear, a majority of it can simply be re-coated or machined, reducing costs on a large scale.

In essence, for the end-user, there is no single "best" material. Their decision rests on process stability, consistent maintenance schedules, and cost control. With an AIXTRON furnace and an established production line, CVD SiC coated graphite offers the solution they need.

Cost, Lifetime, and Performance Comparison for Epitaxial Parts

In making a choice, most people tend to balance cost, longevity, and process performance when considering epitaxial parts.

Graphite is the least costly and easiest part to manufacture, but if it's not protected, the high temperatures and reactive gases quickly eat the graphite away. In time it can warp or even shed tiny particles that ruin the quality of the wafer.

CVD SiC-coated graphite represents a compromise between cost and performance. The SiC layer offers excellent resistance to chemical attack and an effective reduction in surface wear.

Because many CVD coated parts can be repaired or recoated once the protective layer has worn thin, they are effectively made new again and cost less to replace. Solid SiC offers an exceptionally high level of durability at a higher cost, but will generally remain very dimensionally stable and provide a steady process over long periods of use.

It will also require longer heat-up and cool-down cycles, and will cost more to replace if broken.

In short, Graphite is least costly, least enduring; CVD is SiC-coated graphite: Compromise between cost, endurance, and performance; Solid SiC has a greatest endurance and stable performance, but more costly to purchase.

The most appropriate part depends upon production demands and budgetary constraints.

Thermal Stress Distribution in Large-Diameter SiC Epitaxy Processes

It is important to control the temperature stress as the silicon carbide wafers become larger. Temperature variations within the wafer, even by small amounts, are capable of inducing stresses in the crystal.

The heat source within the reactor comes from the susceptor, flow gases, and chamber walls. Different parts of the wafer expand more or less than other parts of the wafer if the temperature is not the same over the whole area of the wafer. If this happens, bending or defects or non uniform epitaxial growth will occur.

Wafers usually cool faster around the edges and are more uniform over the center of the wafer. This phenomenon can lead to these stresses. Engineers manage these stresses by, The geometry of the susceptor, Rotation speed, Quality of the coating, and Flow gas inside the reactor. When the coating deteriorates over time or the susceptor becomes deformed, heat is no longer distributed as desired and a rise in stress may occur. The coating and/or susceptor can then be repaired or replaced, which often rectifies these stresses.

Having a uniform temperature across the wafer surface improves wafer quality and yield.

Choosing the Right Susceptor Material for Advanced Power Devices

There are a large variety of potential susceptor materials which may be selected for a manufacturing process, dependent on application and requirements.

Graphite will heat up rapidly thus shortening the process time. Generally, the material is well suited for development or processes where a rapid thermal response is desirable, however, less durable, will wear away at a high rate and particles will be shed if there is no coating.

CVD SiC coated Graphite is among the most popular materials choice for manufacturing. It offers durability over Graphite, while retaining rapid heating time, and a cleaner process with minimal additional cost.

Solid SiC has the greatest stability of the choices listed, no coating is needed for protection. Solid SiC allows for improved wafer to wafer temperature uniformity over its competitors reducing potential variation in process parameters, making it ideal for high precision / demanding applications.

However, it is significantly more expensive, and will also generally allow for a greater process time. For one specific process, one manufacturer switched from SiC coated graphite to Solid SiC, while the uniformity was improved greatly, the heat up time and process also needed to be adapted.

So, use graphite for rapid heating requirements, use CVD SiC coated Graphite for a cost effective balance between speed and durability, and use Solid SiC for superior stability and thermal uniformity.