CVD SiC vs TaC: Which Lasts Longer in High-Temp Epitaxy?

High temperature epitaxy processes inevitably involve high temperature, gas flow and chemical attack to their components inside the reactor. Examples of protective coating material are Cvd sic and CVD TaC. There is a lot of speculation as to which of them holds up? The answer is not straightforward as it relies on the range of temperatures, the chemistry of the process and the number of cleaning cycles used. Fabs with more challenging environments can cause TaC to fail in different ways while SiC coated parts can run for months in fabs. The article discusses the behaviour of both, and the typical observation of the operator at the end of the long run. Let's take a look at this here in a simplified way so it's easier to understand.

Oxidation Resistance and Corrosion Behavior in Epitaxy Environments

The first impression people usually have on seeing CVD SiC and CVD TaC is their response to heat. SiC is noted for its great stability and smoothness. It has a good tolerance for long production runs at moderate temperatures. With appropriate cleaning and with not too hard a thermal cycle, SiC coated parts can be used for a long time for many silicon carbide epitaxy tools. This is generally the preferred option by the operators as the surface is not as dirty and thus the accumulation of unwanted particles is minimised. TaC is, however, malleable for more extreme weather. More resistant to high peak temperature than SiC and more resistant to chemical attack in some hostile gas environments. In real fab use, parts are often Ta coated for use in processes where SiC starts to degrade faster, like more aggressive etch chemistry, or higher thermal swings. One such difference is observed when it comes to thermal stress. One of the typical responses of SiC to heating and cooling is that it is not very sensitive (as other materials are), therefore there are less micro-cracks after time. The resistance to high temperature of TaC is relatively high, but if the temperature change is too fast and the change is not even, the stress will occur at the interface, which may cause coating problems. The other is surface behaviour. SiC is more chemically passive and so are deposition layers. Depending on the recipe, TaC can react differently with some gases, resulting in a speedy surface change.

Performance in AIXTRON G5/G10 and Veeco EPIK Platforms

There are many practical, and very different CVD SiC and CVD TaC, real production tools available. Many times these machines are coated and even minor differences in coating can impact the yield and particle count of the coating and maintenance scheduling. Within AIXTRON G5 and G10 reactors SiC coated parts are frequently used if the gas flow in the reactor is stable and the temperature control is good. SiC is reported to have good performance in long deposition runs as it will not release particles easily when the process is stable and the surface will be smooth. In these systems, once a regular thermal cycling situation arises, there are likely to be issues. This leads to surface wear due to small cracks that develop over time, especially near edges, and in highly stressed locations. Usually, other systems from AIXTRON have other coatings (such as TaC) in the hotter regions and/or chemistries. They have a high resistance to high temperatures and unaffected by chemical reactions in the tough recipes. Maintenance operators report the presence of rough surface areas in some situations, caused by cleaning cycles that are too aggressive and/or poorly controlled, resulting in rough surfaces on the TaC surface. This makes it more critical to do so and to check at regular PM periods. SiC has proven to be a reliable and stable material choice for high volume epitaxy applications on Veeco's EPIK platforms such as silicon and compound semiconductors epitaxy. It helps to keep the wafer quality of the product even while operating in an optimised mode. CVD TaC is suitable for higher demanding steps, especially in presence of more reactive process gases, in order to meet higher demands for durability. SiC has been shown to be stable in more controlled processes, and TaC more stable in more aggressive processes, both platforms share this fact. Typically, engineers will not be able to use one location of the chamber, but rather many different locations to balance materials.

Thermal Shock Resistance and Long-Term Stability

Thermal shock is among the more challenging stress tests for any coating that is applied in epitaxy tools. The components are heated to a very high temperature and then cooled for maintenance or process change in reactors. If this occurs quickly, it causes expansion and contraction. This will be where cracks will occur and coats will fail over time. The thermal shock resistance of CVD SiC is satisfactory in a slow and controlled temperature changing process. It has a stable structure and it expands in a more predictable manner. With mild heating and cooling ramps, a Cvd sic coating component can be inserted into the daily production process in a fab and processed through numerous cycles without any damage being observed. Issues will arise with the operator's fast ramp rates or if the system is switched on and off many times. Small stress points are possible especially on edges or sharp shapes. These small defects may not be noticed initially, but over time these defects will increase and could cause flaking of the coating as usage progresses.

Cost vs. Lifetime Trade-Off for Epitaxial Parts

There are a number of factors for choosing between CVD SiC and CVD TaC, but cost is one of them. The selection is typically based on the intervals between replacing or cleaning the parts as well as the duration of the parts in production. Generally, the cost of CVD SiC parts are less expensive. This makes it necessary to have them on lines where the product parts are being consumed in a large number. A stable service-life can be achieved using SiC, if the chemistry and thermal cycling is controlled and the epitaxy is more or less steady. It is often used for operators as the replacement cost is known, and it is stable, as long as there is no significant change in conditions. Unfortunately, this means that under extreme conditions, SiC might wear out more rapidly. Under more extreme gas chemistry or higher temperature cycling may cause the surface to become more rough sooner and may result in particle issues or replacement sooner. Initially, the price of CVD TaC components is more expensive. This brings the team to reconsider the option as a more costly one. However, in a few applications such as high temperature (above 600 °C) and/or chemically corrosive processes, TaC will perform longer in service than SiC. This service life may decrease the need to open the chamber, thus stabilizing production. The fewer the downtime, the more wafers per maintenance cycle.