Why TaC Coated Susceptors Are Essential for Deep UV LED MOCVD Growth

During the MOCVD production of deep UV LEDs very tight control of the growth is required and the susceptor has a much greater impact than is often anticipated. At high temperatures and when reactive gases are used in the process, the surface on which the wafers are held should be stable and clean. Susceptors get the job done with the assistance of Tac coating susceptors. They help to maintain a level environment that results in more uniform crystal layers and fewer defects. In real factory environments, engineers occasionally see improved wafer uniformity when changing to improved coated surfaces, particularly for deep UV structures as they are more sensitive than the standard LED layers.

Process Characteristics and Material Challenges of Deep UV LED Epitaxy

Deep UV LED epitaxy is a challenging process because all things occur at the same time and under extreme conditions. The temperature is high, the gas reactivities are high, the growth range is narrow. The formation of the crystal layers is sensitive to changes in heat balance and surface conditions, even for small variations. So, engineers spend so much of their time tuning the process, rather than running it.The significant challenge is posed by aluminum-rich materials such as AlGaN. These layers are required for deep UV emission but difficult to grow in a stable manner. The surface will have defects if the temperature is not uniformly distributed. In certain production lines, one can observe uneven brightness and/or lower output of wafers, due to an off-center thermal field during growth.Contamination is an issue also. It appears that even slight reactions of the gases with chamber components will form film defects at these temperatures. Over time, this buildup appears and is witnessed as batch-to-batch variation. Many engineers report that a few first runs after maintenance seem to work fine, then slowly degrade as the chamber wears in.The function of the susceptor surface does not play a large role in this scheme of events but it does have one. Not only does it keep the wafer it has an effect on both heat diffusion and the stability of the reaction zone. When dealing with actual graphite surfaces and experimenting and looking at coated surfaces verses bare graphite surfaces, one may find a disparity in uniformity when working below wavelengths of 280nm.One of the most effective methods for enhancing the performance is by monitoring thermal mapping and wafer-to-wafer variability. Minor changes in the rotation speed, gas flow balance and surface condition checks can help minimize defects. For many of the process engineers, the long-term drift is monitored rather than just single runs as deep UV growth issues may appear over time rather than all at once.

Limitations of Conventional Susceptor Materials in High-Al AlGaN Growth

The traditional susceptor materials such as bare graphite or even common SiC-coated graphite begin to exhibit clear limitations in high-Al AlGaN growth as soon as deep UV growth is achieved. The main issue comes from stability. The surface of conventional graphite is slowly reacted with process gases at high temperatures required for layers containing aluminium. This will alter the surface condition and impact the transfer of heat to the wafer over time.If the heat transfer is not steady then the wafer temperature can change while growing. The slight change may result in non-uniform incorporation of aluminum. In practice, this manifests itself in the form of wafer bowing or wavelength variation across the face of the wafer. Engineers might be able to run the same recipe, but still get different results batch to batch, frustrating in the scale-up process.Another is the risk of contamination. Carbon based by products from graphite can enter the reaction zone in an aggressive MOCVD environment. The traces are very small and Al-rich AlGaN layers are highly sensitive. A low level of impurity can lead to a higher number of defects or reduced internal quantum efficiency. When it comes to the baseline stability, some fabs claim they have to clean chambers more often with conventional susceptor surfaces to maintain the baseline stability.Some of these limitations can be overcome using coated versions such as standard SiC, but there are also limits for coated SiC under deep UV conditions. Once used for a long time, degradation of the coating may begin to start, particularly around the edges where there is greater thermal stress. After the coating surface is uneven, the uniformity of temperature decreases once again, and the process drift is restored.In practice it has been found that it is more difficult to scale up high Al content recipes using standard susceptors. It can function in short production runs but is less stable in long production runs. That is why the material selection at the susceptor level is now a major component of the process development, rather than just a detail.tor level has become a key part of process development, not just a supporting detail.

Advantages of TaC Coatings in Ammonia-Rich and High-Temperature Conditions

In ammonia-rich MOCVD growth, the chamber environment becomes very aggressive, especially when working at high temperatures for Al-rich AlGaN layers. Ammonia becomes very reactive when heated, generating reactive species of N and H. These eventually can attack its common susceptor surfaces and gradually alter its behaviour. CVD TaC coated susceptors are more tolerant to this environment, as the surface remains stable in harsh conditions.The great benefit is chemical resistance. TaC is not easily reactant with ammonia or by-products produced during deep UV growth. This is useful in actual production to minimize surface wear and improve the uniformity of the susceptor condition in extended runs. The engineers can find that the chamber remains cleaner for a longer cycle without maintenance.Another factor of importance is thermal stability. High temperature growth, typically > 400C, is often used for deep UV LED growth because of the need for high aluminum content layers. In these cases, some coating materials start to soften or start to slowly degrade. TaC has strength to retain its structure and maintains uniform heat distribution throughout the wafer. A more uniform wafer will form when the thermal field remains constant, as there will be fewer differences between the edge and the center.It's also beneficial in terms of reducing contamination. When using older setups, carbon related or reaction related residues may gradually accumulate and impact the crystal quality. The buildup is significantly reduced in TaC coated graphite heater . An example of a production case provided by one process engineer showed that the use of a more stable coated susceptor decreased the number of chamber cleaning steps and improved the run-to-run stability particularly for wavelengths below 270 nm.A long-term repeatability is another aspect that engineers will monitor. Small changes in the condition of the susceptor in high volume production may cause drift in output power or change in wavelength. TaC coated surfaces exhibit a longer lasting performance, helping to minimize batch to batch variability.In general, the TaC coating in ammonia-rich and high-temperature MOCVD conditions can serve as a stable barrier to ensure the protection of the process environment and the stability of the growth process, thus simplifying the control of deep UV LED structures.

Influence on Epitaxial Uniformity, Yield, and Device Performance

As deep UV LED epitaxy scales to production size, minor variations in the growth environment become obvious in the quality of the wafers. Susceptor surface is directly involved in uniformity, yield and final device performance in this way.The first thing that engineers watch is epitaxial uniformity. If the heat is not distributed uniformly over the wafer, the AlGaN layers will grow at a slightly different rate from the wafer center to the edge. This results in thickness and consistency variations. This may present as a shift in the peak wavelength on the wafer map in real fab runs. The TaC coated susceptors maintain more of a stable thermal field resulting in more balanced growth rate. The outcome is a smaller spread of wavelengths and improved wafer to wafer uniformity.Another area that benefits is yield. A production process with deep UV is very sensitive to any defect, even a small one, may result in the failure of a die. Unstable growth conditions are a cause of things such as dislocations, point defects, or rough interfaces. If the susceptor surface is stable in ammonia, high-temperature environment, the fluctuation is reduced in long runs. After changing to more stable coated systems, engineers easily experience the reduced number of rejected wafers, particularly after longer production cycles.Uniformity and defect control are closely related to performance of the devices. An uniformly grown epitaxial layer will result in higher internal quantum efficiency and better uniformity of light output throughout the wafer. In practice, it translates to LEDs that not only work better, but are more predictable.In practice, it translates to LEDs that work better and are more predictable. Other production teams report that the width of the binned spread becomes narrower which makes it easier to sort downstream and reduces waste.There can also be a long term effect that is not apparent initially. With normal surfaces, the drift is small over weeks and months as the chamber ages. The more stable the coating is the lesser the drift, and the closer the baseline performance will be to the original recipe target.To sum up, a stable susceptor surface is not only foundational for growing. It stealthily influences the consistency, efficiency and reliability that the final deep UV LEDs will possess in mass production.

Key Selection Criteria for Deep UV LED Susceptor Solutions

The selection of a susceptor for deep UV LED growth is not a simple matter of selecting a part which has a long life. It has a direct impact on the long term stability of your MOCVD process. The growth of Al-rich AlGaN is very sensitive to process window, and any susceptor weakness can rapidly result in yield or performance drift.Thermal stability is one of the first things that engineers inspect. It is necessary to spread heat uniformly over the wafer, even at ultra-high temperatures, and this is the function of the susceptor. When one area heats up more quickly than another, the crystal layers are not uniformly formed. This translates to a shift in the wavelength over the extent of the wafer during actual production, making post correction difficult. A good susceptor maintains the temperature field constant from run to run.Another important consideration is chemical resistance. Deep UV growth employs ammonia rich environments, which can slowly attack weaker materials. This results in surface changes over time which impact process stability. The ability to resist these reactions and maintaining the surface condition after many cycles is provided by a strong coating.Surface purity is also more important than is generally believed. When the UV wavelengths are in the deep UV, small amounts of contamination can make a difference in device efficiency. Where trace impurities are released during long runs, some fabs are aware of a higher defect rate. A stable coating can help minimise this risk and maintain a cleaner growth environment.Long-term durability usage is also crucial. The susceptor may be used over and over again for high volume production. Early degradation of the coating can lead to a slow drift of the process, which can be hard to monitor. In critical applications, such as the aerospace industry, engineers want materials that will perform steadily even after prolonged exposure to hot gases and other reactive gases.Last but not least, there's consistency between batches. The ideal susceptor solution should have the same characteristics on each use. Quick performance variations with use makes it more difficult to keep a steady production recipe.It is generally better to select the one which will minimize surprises in a real manufacturing setting. A stable susceptor is not only a growth supporter, it also ensures a predictable process, a key factor in deep UV LED production.