Why TaC Coated Rings Are Preferred in SiC Epitaxial Reactors？

Selection of materials for the reactor internal components is a key factor for the process stability and wafer quality. Consider for instance support rings which guide the wafer during the growth process and which must withstand both high temperatures and corrosive gases. Certain materials for support rings can erode over time and even contaminate the substrate, leading to layers which are defective, incomplete or non-uniformly deposited. For this reason, most engineers resort to the use of Tac coating rings. A tantalum carbide (TaC) layer provides both the support rings high abrasive durability and high resistance to chemical attacks, as well as stabilizes the rings over extended operational periods. Such a mastery of TaC-coated rings makes it possible without major obstacles for the production of quality SiC layers for electronic devices.

Reactor ring functions and positioning in SiC epitaxy tools

The construction of reactor rings in SiC epitaxial reactors is relatively uncomplicated, but nevertheless, they have an important role. These are usually realized as circular bands which are located in the reactor chamber so as to encircle the wafers. The main purpose of the reactor rings is to assist in the retention of the wafers, and to help achieve a uniform gas flow distribution over the active surfaces of the wafers. Incorrect positioning and/or damage to the rings may lead to uneven growth and localized overheating, as well as possible deposition of undesirable contaminants on the wafer, and loss of the entire wafer batch. Most reactors are designed with rings arranged in the vertical direction within the reactor, at different levels. Each ring aligns with the wafer carrier or susceptor to ensure the wafers are adequately held and not in contact with the walls of the chamber. Proper positioning of the rings is done to ensure impeding flow of vapors such as silane, hydrogen, and carbon sources to each wafer. The gas flows evenly when the distance between the rings and wafers is correct; if the distance is too small, proper gas flow cannot occur, and if it is too large, wafers can shift resulting in the growth rate being non-uniform. In addition to supporting the wafers, the rings also serve the purpose of shielding the chamber, and they protect the vulnerable region from the corrosive and high-temperature chemicals, so that the metal or graphite parts do not corrode too quickly. Rings become subjected to high temperature and to chemical corrosion, and erosion of the ring material can occur, which can lead to the generation of particles if the material is not selected carefully. This is the main role of the coatings, especially TaC, which helps in maintaining the rings in an intact state for long periods of time, and also keeping the surfaces of the rings, so that they do not generate too much damage to the wafers. In order to keep the wafers in good condition and minimize losses in the growth process, the operators need to know the role and the placement of the rings. Correct alignment, stacking, and monitoring of the wear of the rings are simple, but they are the most effective in eliminating any interruptions to the growth process and protecting the wafers from damage.

Chemical and thermal challenges unique to SiC processes

The high temperatures, usually above 1500 °C, and the highly reactive environment, controlled by the flow of hydrogen, silane, and hydrocarbons, make the SiC epitaxy very difficult for any material inside the reactor. At high temperatures, the gases become active and can corrode the metal or graphite parts of the reactor, causing erosion, pitting, and the generation of particles. This environment is directly experienced by the reactor rings, and that is why careful attention is devoted to the selection of materials and coatings used to make the rings. In addition to being high, temperatures in a reactor chamber can be uneven. Even small hot or cold spots can influence the interaction of gases with the wafer surface. If a reactor ring expands unevenly, shifts, or is worn, it can create turbulence or dead zones in gas streams which can lead to uneven epitaxial layers, or worse, defects. Instability also can lead to cracking or warping over time in rings that can't withstand the repeated thermal cycling. The unique combination of chemistry and heat makes SiC epitaxy different from standard silicon processing, and complicates the choice of components for the reactor. Reactor components, especially rings, must be able to withstand the temperatures and chemical-induced wear, and the use of coated rings (like graphite that is coated with TaC) can mitigate those problems by offering a surface that can withstand the repeated thermal cycling and remain chemically stable. Knowing these limitations, engineers can better design the planned maintenance, to anticipate the ring inspections, and to manage risks of the issues that would negatively affect device yield or performance.

TaC coating benefits for corrosion and particle control

The TaC coating of reactor rings improves their robustness in the extreme conditions prevalent in a SiC epitaxial reactor. The best part of the coating is its ability to withstand the charged particles. With ring coatings, it is also possible to have a robust surface that can withstand repeated thermal cycling without deterioration, which is a key ingredient in the performance of the coated rings. Without this coating, there will always be erosion for graphite or metallic rings which will change shape or erode the rings by releasing tiny fragments. TaC provides a soft, chemically stable barrier that will protect the material, and minimize the exposure of harmful or contaminating materials underneath. TaC provides an added benefit in controlling Harmful materials. Even at normal temperatures, tiny, worn or destroyed ring flakes in a reactor will damage the reactive wafer. TaC helps in controlling contaminants and improving productivity. A smooth and evenly applied TaC coating minimizes flaking and the coating is designed to withstand the reactive environment and high temperatures. This will leave the chamber clean, and ensure that there is an unhindered consistent flow of gas across all the wafers to ensure the same quality for all the layers of the wafers. A reactor employs a cycle of extreme environments that will leave cracks and warping in any materials that don't have TaC. TaC rings will keep their shape and ensure that the reactor can operate for long periods of time without the need to replace the rings which will improve maintenance of clean wafers. In fact, you can expect low contaminated wafers. Production lines that have switched to TaC have defected particles and this leads to better standards in the overall quality of Wafer. This cleanliness will improve the overall growth and will make TaC coating worth the investment.

Influence on edge exclusion and wafer uniformity

Managing layer precision during SiC epitaxy is critical but also highly challenging. In this regard, the importance of the reactor rings should not be overlooked. The purpose of these rings is to control gas flow over the wafers. Edge exclusion is a phenomenon where unreacted gas flows toward the radial edges of the wafer, excluding edge areas of the wafer less than optimally, thereby increasing the number of defects generated during a run. In the case of unreacted gas, the gas does not enter the reactor chamber and is therefore not reacted and as such, the outer edge of the wafer receives unreacted gas. Because of the uniform and consistent flow of gas this elicited, TaC-coated rings are exacerbated to a lesser degree than competitor rings. Thus, there is less alteration (moat formation) of the various wafer layers, which contributes to an expansion of the edge exclusion. A larger number of SiC layers is also appreciated. A result of this is the increased zone of commercially viable defects. By preventing uncoated rings from being excessively molten, these TaC rings maintain their precision, and maintain it consistently despite the multiple furnace runs. As such, the gas flow patterns remain consistent and thus, the edge region of each wafer receives a consistently identical chemically unreacted edge zone, which improves the chemically unreacted edge zone of the wafer.