TaC Coating Delamination on Graphite: Causes & Prevention Guide

Graphite material with a coating of TaC has many advantages under thermal and chemical stresses, and is widely used in high temperature semiconductor and industrial processes. However, in actual production problems do occur and a common problem is delamination. The coating starts to peel or chip off of the graphite base here. May begin as a small spot or small cracks and eventually progress to a point where it interferes with function. It isn't until a few cycles of heat have been performed that this problem becomes apparent and can be predicted in some factories. Knowing why it is and how to deal with it can also prevent parts replacement unexpectedly, and downtime.

Poor Adhesion Mechanism Between TaC and High-Purity Graphite

There is weak bonding between the coating and the graphite base which is one of the primary reasons for the failure of Tac coating on graphite. The smoothness of the microstructure of graphite makes it hard for TaC to grab on during deposition. Even if the coating seems to be firmly adhered at first, the bond between the two layers may not be strong enough to withstand the repeated heat cycles. This is commonly seen in actual production in the high temperature epitaxy tools. Small flakes may be noticed after a few runs by a technician. Although it may look like normal use, if viewed under magnification the TaC is not abrading evenly, but is rippling off the graphite base. One of the most typical occurrences is in parts that have to undergo fast rate heating and cooling. The graphite expands and contracts with respect to the TaC layer and this exerts a stress over time on the weak interface. Another one is the condition of the surface just prior to coating. The dust, oil residue or machining waste may hinder proper bonding even if it is a very thin layer if it is not properly cleaned. At times, the coating does not adhere completely, resulting in essentially patches of non-bonding areas underneath. The first areas to suffer in case of thermal stress accumulation.

Surface Preparation Defects Leading to TaC Coating Peeling

Surface preparation is one of such steps which may appear easy on paper, but can determine either the long lasting or early peeling of CVD TaC coating. A microscopic film of oil, dust, moisture or from a machining process may be present on high purity graphite, which can be invisible to the naked eye. Such will not occur evenly, creating an uneven coating. Can pick up in some areas and not pick up in other areas and then patchy delamination. Mechanical roughening also plays a big role. In some processes, it is necessary to roughen the surface of graphite with grit blasting to allow it to adhere to the coating. Light blasting will not produce enough surface roughening. If too aggressive it can create weak spots in the graphite. In both cases, coating liftoff can occur early when heat stress occurs. The other is moisture. Graphite can absorb small amounts of water from the air, especially if it is stored in humid conditions. If the part is not dried or pre-baked before it is coated, then moisture may be trapped and expand under the heat, forcing against the TaC layer.

Thermal Stress & Microstructure Evolution During Deposition

If surface preparation is performed correctly, the coating process TaC can still encounter difficulties during the deposition process due to the behaviour of heat in the material. This leads to the introduction of thermal stress and microstructure changes. There are very high temperatures in the deposition process for both the graphite base and TaC layer. The problem is their reaction to the heat is different. Graphite's coefficient of expansion/contraction is higher than TaC's. Therefore, when the system heats up the graphite attempts to expand slightly, and the TaC layer opposes this expansion. The difference will create stress at the bonding location. Initially, the coating can cope with it. Repeated heating and cooling however, will begin to cause small internal cracks to develop within the TaC layer or at the TaC layer/interface. These cracks are usually not apparent unless a careful inspection is conducted. As they join over time they form bigger weak points at which the coating starts to come off. The changes in the microstructure of the coating also play a role. TaC crystallizes in small crystals during the course of deposition. These grains can form unevenly if the temperature, gas flow or deposition rate are not stable. In some places it gets compacted to become dense and tight, in others it becomes porous or loose. The weak zones serve as natural starting points for peeling, when stress accumulates. In the epitaxy production line, sometimes operators will observe that parts from the same batch behave differently in the real production line. Some maintain stability and some have early edge lifting. The underlying cause is often a slight change in the deposition conditions which produced a variation of the microstructures throughout the coating.

Process Control Strategies to Prevent Delamination

The key to preventing TaC coating delamination is to ensure that all process steps are stable. Changes in temperature, gas flow or handling can add up slowly to cause adhesion later on if it is not handled during the process. The best approach is to tightly control the temperature profile of the coating. It is common to find abrupt temperature changes as a cause of internal stress between TaC and graphite. A slow ramp up of heat allows both material to adjust, lessens strain at the interface. The same applies when cooling. Slow cooling in a uniform manner prevents the development of internal stresses which may manifest as peeling in the part. Another important factor is the stability of gas flow. In the real production lines, the fluctuation of gas distribution, even in a small scale, may impact the growth of the crystals on the surface of a TaC. In uneven flow the area that receives the more concentrated flow is coated, and the area that receives the less concentrated flow is weakened or made more porous. Pre-coating checks too are very important. Many problems are caused by small things such as cleaning residue, fingerprints or moisture on the graphite components. Oftentimes a short pre-bake is performed to remove any hidden moisture prior to the commencement of deposition. A simple one but many adhesion issues down the road can be avoided.

Inspection Standards for Early Detection in Epitaxial Parts

The most common way to detect a problem with TaC coating is to accurately inspect the parts before and after each use. Many epitaxial systems have small defects that begin long before the delamination process is complete, so the intent is to detect changes before they become obvious such as peeling. The most common approach is to examine the object visually, under good lighting. The operators tend to turn the graphite part at a slow speed and check the surface for dullness, colour changes or minor texture variations. Another level of control is achieved by looking at the microscope. Initial defects such as micro-cracks, cluster of pinholes or small lift edges can be observed when magnified. Such defects typically occur in areas of stress, including corners, edges, or areas with varying thickness of coating. If they are caught at this stage, parts can be removed before failures occur while the system is operating. Some facilities track weights as well. A coated part has a consistent base weight. The weight may vary only a little bit if the coating begins to peel off or flake off before it is seen. It is not a single check, it is used in conjunction with other checks to create a clearer picture.