How TaC Coated Rotation Susceptors Enable Uniformity in Advanced MOCVD Production

For the advanced MOCVD manufacturing environment, to have a thin film on all wafers being uniform throughout the entire susceptor has always been the key difficulty. Another of the minor parts is rotation susceptor which can be significant if Tac coating is involved. TaC coating is able to render the susceptor more temperature resistant and more stable for the long term use. In fact for the manufacturing lines of LEDs and power devices, only temperature difference could have an influence to the yield. Once you incorporate TaC coated rotation susceptors, most engineers discover that wafer to wafer, center and edge are more reproducible. This is simple hardware change but the influence to the overall production can result in more predictable results of your day-to-day operation.

Importance of Wafer Rotation in Epitaxial Thickness and Composition Control

In MOCVD, the wafers are being held and heated at the same time as being passed through with gases. These gases are used to transport the ingredients to deposit layers onto the surface of the wafer. One of the problems associated is that, is that you cannot always assume the flow and temperature is uniform across the chamber. If the wafer is not moving some of the wafer will receive higher amount of material/ heat compared to others, resulting in non-uniform thickness or changing material properties. These effects could have great effects on devices such as LED's or power electronics. This is where rotation comes into play. As the wafer is being grown, it rotates and so at any given time, different sections of the wafer are being held within different areas of gas flow/ thermal zone. The small fluctuations at different parts of the chamber will eventually be leveled out due to rotation. Just like turning a steak on a barbecue, the food will receive more heat in some places than others resulting in a burned section and an undercooked section. In a real production situation, the rotation rate can also vary according to the materials grown. For example, layers of Gallium Nitride are used to grow LEDs, a simple change in thickness can change the color output of the chip, this can be minimized by altering the rotation rate to be able to produce the layer of a uniform thickness across the wafer, resulting in consistent brightness/ color wafer-to-wafer. The other use for the rotation is the consistency of the material constituents during deposition. Compound semiconductors must be consistent across the wafer; the characteristics of the material will change if a higher concentration of one constituent is allowed to accumulate in one section of the wafer. The rotation process can prevent this from happening by moving the wafer, the effects will have diminished to become non-existent. Users of the MOCVD process are highly advised to investigate and/or optimize their rotation settings. Start using equipment manufacturer's suggested values and make incremental changes while observing thickness maps and material quality. Altering rotation can provide benefits in terms of uniformity and reduced number of wasted wafers.

Mechanical and Thermal Demands on Rotation Susceptors

The rotation susceptor's job is difficult in an MOCVD reactor. Must maintain the steady position of the wafer, spin it at a controlled speed and remain stable under very high heat. The susceptor is a component right in the middle of most of the processes which can be carried out at temperatures above 1000°C. Meanwhile it is suffering from high gas flow rates and frequent temperature variations. This both mechanically and thermally stressed the material. Mechanically balance is a key factor. Even if the susceptor is not balanced, the vibration can be produced by a small imbalance when the susceptor rotates. This vibration can cause uneven wafer placement over time or even cause damage to the equipment. Susceptors are therefore meticulously machined and tested before use. In actual production lines, engineers may regularly check for wear, warping or perhaps very small cracks that may expand during operation. Another issue is thermal stress. The susceptor heats up rapidly and needs to distribute the heat uniformly over the wafer. When one area becomes hotter than another, it can have an impact on the growth of layers. Meanwhile, the material will expand upon heating and contract on cooling. With lots of repetitions, this repetitive expansion can cause fatigue. This could cause the susceptor material to lose shape or have surface defects if it cannot withstand it. That's where coatings such as CVD TaC come into play. A TaC coating can withstand high temperature and can protect the base material from chemical attack from reactive gases. Also it will improve heat distribution over the surface which aids in even growth. This in practice translates to a longer life and increased maintenance cycle performance stability. For operators, it is important to monitor temperature uniformity and physical condition. Avoiding sudden temperature changes, proper cleaning and regular checks can help prolong the life of the susceptor. Taking little actions like these can help to avoid bigger problems and help the process stay on track.

TaC Coating Performance under Thermal Cycling and Dynamic Stress

The susceptor is not in a steady state during normal MOCVD operation. It spins, cools and heats concurrently. This is a combination of a thermal cycle and dynamic stresses, which can erode many materials. The more runs that are done, the more cycles added, and the small changes can become significant issues over weeks or months if the coating is not strong enough.This is an environment in which a TaC coated graphite heater can cope. They are characterized by their ability to withstand repeated heating and cooling. A TaC layer doesn't break apart when the material expands, but it is intact. This helps to keep the surface of the susceptor smooth and even, which is crucial for consistent contact and heat transfer between the susceptor and the wafer.Also, there is added stress. While the susceptor is rotating it will be subjected to centrifugal forces which may gradually worsen any irregularity in surfaces or the imbalance and the coating wears over time as a result of the movement which helps in making a durable TaC coating, which will be strongly adhered to the base material and unlikely to delaminate over numerous cycles, thus in production less interruptions will be caused for re-coating/changing parts.

Contribution to Large-Area and Multi-Wafer Production Consistency

Consistency is much more difficult to achieve once MOCVD is scaled up from small wafers to production wafers. As wafers get larger, or multiple wafers in the same chamber, more variation occurs across the chamber. The flow of gas and the distribution of heat and edge effects all increase in prominence. The small difference can add up when one is producing hundreds or thousands of wafers per day.That's where TaC coated rotation susceptors come into play to help maintain stability. For large area processing, the heat must be distributed uniformly over a larger surface area. This can be supported by a good TaC coating, which will enhance the thermal balance of the susceptor. If the temperature is constant throughout the wafer, the wafer layers are more evenly developed. This helps to minimize issues such as thickness variation or composition variation within the wafer.The task is greater in multi-wafer systems. The position of each wafer within the reactor is not the same. If the outer wafers are not well controlled, they could develop differently than the inner wafers. While rotation helps to minimize this difference, it is still a surface effect. The TaC coated surface ensures a more uniform thermal response for all the wafer positions, which results in more similar conditions for the growth process of each wafer.This difference is sometimes observed by engineers in a real production environment when scaling up. This might work on one wafer, but begin to vary when scaled up to a lot. Once the coated susceptor is changed to a more stable design, the variation may decrease and yield increases. In the LED and power device industry, minute deviations can impact performance and/or lifespan.The other practical one is process repeatability. If the susceptor behaves the same from run to run, it is easier to adjust recipes and to achieve consistent yields. This saves operators from wasted time in correcting for drift and increases production stability. In the long run, this allows factories to keep their system in quality without having to constantly make changes.

Application Trends Driving Demand for TaC Coated Rotation Designs

The trend towards improved yield and tighter control continues to increase at semiconductor factories. The wafer quality is vital to the performance of devices for EVs, 5G and power electronics, and slight changes in the film thickness can have an impact. In view of this, the manufacturers of the equipment are increasing their focus on components within the MOCVD reactor that contribute to stable growth conditions. Coated rotation susceptors for heat and chemical stability over long production runs receive more attention because of the use of TaC.The direction now seems to be towards larger wafers and higher output per run. As the factories move from smaller wafers to 200mm and even 300mm wafers, controlling temperature over the whole surface area is becoming more difficult. The coated susceptor that is able to distribute heat uniformly has become even more essential. Coated designs are also becoming more common for high volume production lines due to this.Another driver is that as the SiC and GaN Devices enter the scene. For that, a very narrow range of temperatures is necessary for Epitaxial growth. Small changes on that process condition will cause defects on that layer. Currently many manufacturers have used improved susceptor coating for better condition maintain for many cycle. As reports showed by industry analysis, it demand growth is strongly related to wide bandgap semiconductor industry growth.Also there is definitely a trend of longer equipment life and minimized down time. The factories require tools which could run longer without regular change. New coating technology have been used so the susceptors could resist more time with the wear and tear process so smooth process could be guaranteed. It is also means less stop time for cleaning and changing part in real factory environment which enable factories maintain production rate.And there are demand change from automation and process monitoring too. Temperature and wear could be monitoring in real time in modern fab. Consistent susceptor coating will guarantee that recipe is stable without manual adjustment, it fit well to the trend of intelligent production line.The difference will be detected usually by engineers when scaling. The process that could be stable in small batch is possibly not stable when it scale up to full production size batch. That distance can be narrowed with improved TaC coated rotation design and that provide smoother scale up.