Detecting Micro-Cracks in TaC Coating Before Delamination Occurs

Micro-cracks in TaC coating are easy to overlook at the beginning stage. They are usually very small and not noticeable until stressed and over a heat cycle. A small crack in these kinds of tools would be very easy to become a weak point in which the coating starts peeling or raises. Under such conditions, the performance drops and components start to break down prematurely. Delamination has started and most engineers are just becoming aware of the problem. The idea is to detect these cracks in their early stages, before they become serious problems.

Formation Mechanism of Micro-Cracks in CVD TaC Coating

Usually before any visible damage occurs, the non-sticking CVD TaC coating layer will start to develop Micro-cracks. Stress that has developed due to deposition, cooling, is the most common cause. TaC expands in response to a change in temperature in a qualitatively different way than does graphite. Tac coating is stiff/hard whereas graphite expands/contracts. When the coating is bonded to the underlying material, the coating starts to battle with the underlying material throughout the heating and cooling process. High temperature during the coating process to create a dense TaC layer. All this can appear fine at this point. The trouble begins upon the cooling of the system. The outer layer (TaC) contracts at a different rate than the graphite below. This mismatch creates high stresses in the coating. When the stress becomes too great, the cracks start to grow and may be seen at the grain boundaries or at irregular thicknesses. The condition of the surface also has a significant factor. If the coating surface is not smooth or if it has small pores, coating cannot be evenly distributed on the surface. Later, these dished areas will be the points of stress. In real production cases the wafers exhibited early cracking around the edges of the susceptors where the temperature gradients are higher, on wafers grown in AIXTRON G5/G10 systems. Afterwards, engineers discovered that these areas were subject to multiple heat stresses throughout the course of the day. The deposition rate also affects the growth of the coating, and the flow of the gas also plays a role. The rapid deposition may result in stresses from inside the layer; the uneven distribution of gas may create some weak points. Over time, these undetected defects create and develop small cracks. Once formed the micro-cracks are not resting. They are opened slightly more with each thermal cycle. They are able to weave into each other, after multiple runs and create path for delamination. Understanding the initiation of these cracks and the starting mechanism is crucial in the stability and durability of the parts which are coated with TaC during actual use.

Non-Destructive Inspection Methods for Early Detection

The micro-cracks in the coating of CVD TaC cannot be seen with the naked eye. The majority of cracks will not show up in their closed state, but only when they start to open or link up with another crack. This is why it's preferable to use a non-destructive inspection to determine early indications and avoid real damage. The most popular technique is known as ultrasonic testing. Produces sound waves and absorbs reflected sound waves. When the coating is solid the signal will not change. Wave pattern changes when there are any small cracks or weak bonding regions. After thermal cycling of a coating, engineers use this technique to find out whether there has been any damage to the coating, but the damage is not visible. Dye penetrant inspection is one of the effective techniques. A special liquid is applied to the surface and left for a short time. The liquid penetrates into the surface if there are micro-cracks which would pierce the surface. Once the surface is cleaned a developer is applied, and cracks show up as narrow lines. This approach is easy, but only applicable when cracks are present at the surface. In the lab, a scanning electron microscopy (SEM) device is used to carry out more detailed checks. Provides a very close view of the coating surface, revealing the crack patterns not visible with optical tools. In some actual production failure samples, SE images showed the initiation of a network of cracks around high-stress sites near the gas inlet regions of epitaxy systems. These cracks were not visible in the normal visual examination and were active in growth stages.

Correlation Between Micro-Cracks and TaC Coating Peeling

When a micro-crack is created, it is a small surface defect. Heat and gas are able to then flow through these small gaps during operation. Then, the crack begins to breathe each time it is heated, opening up and closing down on each thermal cycle. This cycling action slowly and gradually widens the crack. As time goes on, a number of micro-cracks can coalesce. In such instances, they produce a larger weak zone under the coating. This is the stage of the beginning of failure of adhesion. The coating is no longer well adhered to the substrate and small patches start to peel off. Many cycles will see this phenomenon in a real production environment in an epitaxy tool. In some processes in the AIXTRON G5/G10 system running at high temperature the edges of the susceptor have started peeling ahead of the rest of the susceptor. Such areas were found to be already ‘cracked' when examined later on. It was not a single crack but a series of small cracks that were fused together over time which caused the peeling. The other popular pattern is gas penetration. When cracks have a certain size, process gases will be allowed into the gap between coating and substrate. The interface strength further decreases and the peeling process accelerates. A small fault on the surface slowly develops to flaking or delamination.

Critical Thresholds for Replacement in Epitaxial Parts

It is not always simple to determine when to replace epitaxial parts that are coated with TaC. Even though they are deteriorating internally, a lot of the parts can still be easily used. This is why it's not a good thing to wait until your engineers break down and bite off the head of the jackass. The first indication is the existing crack density that is clearly visible. If networks of small isolated micro-cracks start to develop or they start crossing the coating has been put under serious stress and needs to be monitored. When cracking starts to extend over a larger surface, it is best to make plans to replace it as soon as possible, even if peeling has not begun. Edge condition is one of the important conditions. In many actual instances, damage starts at the edges of susceptors and/or high heat areas. When micro-cracks begin to lengthen and/or bend inward, it's typically an indication that the stress is too high. Surface variations are also a good indicator. The surface may look a little more irregular with the use of tools as a TaC coating is aged. This is usually caused by multiple thermal cycles, and cracking, which is below the surface. If you can see the rough surface area on a new piece, it's likely that this is at the end of the period of stable use. Performance drift is also monitored by engineers in some production lines. For instance, parts are frequently inspected when the uniformity of wafer temperatures starts to vary more than usual or when more particles are observed in the chamber, even if the condition appears to be coated parts. In a few reported cases these minor process step changes have been linked to the onset of ‘micro-cracking' in the coating. A simple rule for maintenance planning is that if cracks can be easily seen in more than one zone then the part should not be continued in manufacture, or changes in the surface should relate to changes that were observed at the time of delamination. Once in that condition, it no longer can be fixed, it should be replaced. In fact, these can best be determined early and avoided leading to unwanted shut down and destabilisation of the entire epitaxial process.

Case Study: Failure Analysis in Veeco Spare Parts

A real-life application using Veeco spare parts for epitaxy started with minor surface modifications, which were not noticed. After several thermal cycles, during routine checks, the TaC-coated susceptor was found to have a temperature on the wafer, which was not stable, and to have a slight increase in the number of particles in the chamber. The first issue that the maintenance staff could think of was gas flow or cleaning issues. Parts were not near End of Life and replacement was not a priority. Upon further investigation, more detailed surface scanning and ultrasonic testing, a pattern of micro cracks were detected around the high heat areas of the coated parts. The cracks were narrow and short, mainly at the edges where there are more differences in temperature. This case was won because the following quick development was crucial. As production proceeded a number of micro-cracks started to coalesce. After the connection was made some of the coating (TaC) began to peel away from the graphite substrate. This resulted in early stage peeling and in turn during the wafer processing, in uneven heating. Following replacement, the system was operating in a stable manner. The temperature of the wafers was reduced to minimise the temperature difference and the particle count was restored to normal. This failure in this instance was not unexpected, but rather a progressive failure that was not carefully monitored.