Improving Epitaxial Wafer Uniformity via Gas Float Disk Design

Small changes in hardware can mean big changes in wafer quality in epitaxial wafer processes. The gas float disk is a part that is commonly forgotten. A thin layer of gas is used to control the positioning and processing of the Sic epitaxial wafer , in a controlled way. If the design is unbalanced, the wafer will slightly tilt or have unbalanced gas pressure. This leads to non-uniform wafer surface growth. The gas float disk can be re-designed to provide the wafers with more stability and levelness, to encourage the uniform growth of the layers and to achieve a better uniformity from edge to center.

Gas Flow Dynamics in Gas Floating Disk Systems

Balance is the key word with gas flow through the floating disk system. The disk sits on a thin layer of gas, and this cushion relies on the uniform distribution of this gas across the surface of the disk. The wafer is flat and stable to Epitaxial growth of silicon when the flow is smooth and stable. As the flow becomes turbulent, issues begin to appear on the wafer surface. In actual manufactures, the engineer may find that small changes in the gas inlet pressure cause changes in wafer behavior. In a reactor performing silicon epitaxy, for instance, one channel might be slightly obstructed, thus raising one side of the wafer relative to the other. The non-uniform lift may cause thickness differences between the left and right side of the wafer. At the deposition level, even a very subtle difference can manifest itself into a visible pattern. Gas speed is a factor as well. If the gas comes in too quickly in one place, it will push the wafer up to the local level. If it is too slow in any other area, the wafer could touch down slightly. Both situations cause a non-uniform separation between the wafer and the disk surface. When that gap is not consistent, then there will also be uneven heat transfer and material deposition. One effective method of dealing with this is to regularly inspect gas channels for build-up and to keep them clean. Routine flow mapping is used in many fabs to visualize the way that gas flows across the disk. They are comparing the pressure at various points and seeking imbalance at an early stage, before it impacts production. Some also make changes to the inlet design to achieve a more even distribution of flow than just a single inlet. Another useful thing is to observe the behaviour of the system over time, beyond just start up. A stable system at room temperature may not be stable during long runs at a higher temperature. Flow patterns change for small changes in gas density at high temperature. If the flow of gas remains equalised, the floating disk functions more as a stable platform. The wafer is flat and the Epitaxial layer is less prone to variations from the center to edge.

Design Optimization for Uniform Heat Transfer

The uniform heat transfer of epitaxial systems is greatly influenced by the design of the gas float disk. The disk serves not only to retain the wafer but also to facilitate the subsequent manipulation of it. It also affects the transport of heat over the surface during processing. One of the most frequently encountered problems in actual service is the unequal cool/heat areas beneath the wafer. When the gas passageways in the disk are improperly constructed, some parts of the disk receive more gas flow and others receive less gas flow. This results in little temperature gradients across the wafer. A few degrees can make all the difference in the world to the rate of deposition in that area. For example, in high volume silicon carbide epitaxy tools, it is possible to see a practical application. There are some production runs which exhibit thicker film at the edge of the wafer. Once inspected, engineers frequently discover that the flow of gas around the outside is sufficient to increase the cooling effect at the edges, but was not enough to provide adequate cooling at the center. This results in unbalanced growth. An easy way to do this is to modify the channel layout within the gas float disk. A number of small inlets may be used to provide more uniform distribution of gas. This can help to minimize the temperature gradients. Another important factor is surface flatness. In small areas it is possible that there are micron-sized warps on the disk surface, which will cause gasses to shift, and heat transfer to change. The type of material is also a factor. There are some coatings that distribute heat more even and more slowly than raw graphite, particularly in the case of long runs. Predictable thermal behaviour from batch to batch is maintained with a stable surface. The behavior of the system after repeated cycles is also observed. With time, small deposits or wear may cause the gas paths to become slightly blocked. By regular cleaning and inspection, heat transfer is maintained. These factors combined result in a more uniform thermal field for the wafer. This results in more uniform epitaxial layers and fewer differences at the edge-to-center transition.

Influence of CVD SiC Coating Surface on Gas Distribution

One of the main reasons why the surface of a CVD SiC coating is more important to the gas distribution than many realize. On a first glance, the coating appears smooth and uniform. However, under process conditions, even the smallest surface features can alter transport of the gas flow over the gas float disk and around the wafer. The flow of gas is highly sensitive to the friction at the surface in the system. A smoother SiC coating will provide even spread of gas, and a rougher surface will cause gas to be slower in some areas and faster in others. These small differences in pressure beneath the wafer will result from that uneven motion. These differences can be manifested in the form of non-uniform film thickness over time. This is apparent in actual production applications when the deposition run is long. In one epitaxy line with several wafers in a batch, for instance, engineers observed that the wafers on one side of the susceptor grew at slightly different rates. Following inspection, it was discovered that the cause of the problem was very fine scratches on the surface of the CVD SiC coating. The marks were not seen until a rapid inspection, but they did cause a disturbance in local gas flow. Another aspect to monitor is surface energy. A good SiC coating will ensure that the gas flow is more stable as it will minimise random sticking or turbulence around the surface. If there are any micro-defects or uneven grains in the surface coating, the gas will be able to flow around them and create small swirling areas. Although these are seemingly insignificant, they cause the pressure distribution under the floating disk to be uneven. Cleaning and maintenance is also important. In the course of time, residues can cover the surface of SiC. A thin coat of deposit will alter the way the gas flows over the disk. Cleaning helps to maintain a close pattern to the original design. A stable CVD SiC coating surface facilitates smooth distribution of the gas, which is then conducive to even distribution of the wafer in processing. If there is no imbalance in the gas flow the epitaxial layer will be grown with fewer regional variations, and the end wafer quality will be more uniform throughout the entire surface.

Application in AMAT Centura5200 and Advanced Epitaxy Tools

Gas float disk design becomes an important component of maintaining wafer quality stability in advanced epitaxy systems such as the AMAT Centura5200. The process windows on these platforms are very narrow and process changes due to flow or temperature will be evident very soon on the surface of the wafer. That control system is balanced by the gas float disk which is located in the center. Centura5200-type systems have a chamber that is designed to be repeatable. Even with a poorly functioning gas float disk, operators still notice the variation. A typical example is the slight edge non-uniformity of the wafer. The entire center can be well-formed, and the edges can be thicker or thinner. This is often due to the imbalance of gas pressure under the disk or small flow imbalance around the support channels. This is similar to that of advanced epitaxy tools available from other platforms. Gas needs to flow through more complicated paths in multi-wafer or high throughput systems. If a coating change is uneven on the surface of the float disk, or the surface is unevenly worn, then gas may flow from an area of less resistance to an area of greater resistance. This moves the wafer slightly, enough to affect the amount of heat exposure while the wafer is growing. The engineers who develop these systems tend to think about optimizing the disk design with conditions in the process. For instance, a recipe that has a greater number of temperature steps might require a more even SiC-coated surface of a disk to prevent local turbulence. Lower temperature processes, on the other hand, may have a bit more leeway in terms of surface texture but still require good flow channels. The other practical point is the aging of the system. Well-designed Centura5200 chambers will perform over time, however, if they stray from their original performance, they can become problematic. The flow symmetry changes slowly over time as there are small deposits on the gas float disk or changes in the condition of the coating. These changes can be detected early if they are inspected regularly before they become regular wafer variation. When operating in the field, the goal is: to maintain the wafer stability, to keep the gas movement even across the wafer and to avoid having localized stress points. With the proper operation of the gas float disk, AMAT Centura5200 and similar epitaxy tools can have excellent uniformity over long production runs with fewer surprises between batches.

Maintenance and Repair Strategies for Long-Term Stability

One of the most effective methods to ensure uniformity of wafers over time is to maintain a gas float disk in good condition. However, performance drift typically doesn't occur in one stroke. It accumulates gradually, caused by the coating wear, gas residue and slight changes of surface condition. Most of the stability problems can be managed if detected early enough and do not impact yield. Surface cleanliness is one of the first checks that engineers make. The spreading of the gas under the disk changes even if there is a thin layer of process residue. This leads to uneven lift and/or wafer tilt during epitaxy. Cleaning on a regular basis helps, but the important thing is to be consistent. Even after a few cycles, it is possible to detect changes in the flow of sensitive processes. Another key factor is coating conditions. Even though a CVD SiC coated disk may appear to be perfect, it can actually be in the process of developing a micro-wear on the disk's surface. It is these small variations that can upset the pathways of the gases. In certain types of production, the technicians use a reference surface profile to identify early signs of wear on the used disks. This can be used to determine if cleaning or repair is required. Typical repair methods are concerned with the restoration of surface smoothness and seepage of minor defects. The gas can be brought back to uniform behavior by light polishing or re-coating. In more severe cases, disk or part of the disk replacement or key portions replacement might be needed. The aim is not only to repair the visible damage, but to re-establish the even distribution of gases throughout the entire surface area. As part of regular maintenance, gas channels are inspected. Pressure imbalance can change gradually over time when the channel is partially or totally blocked. Even a small blockage in such tools can create edge-to-center variation on wafers in tools with long deposition cycles. These problems can be caught early during a simple airflow check off downtime.