Why Gas Floating Disks Become Uneven in High-Temperature Epitaxy

The floating gas disks used in high temperature epitaxy processes provide stability to the wafers and keep them supported with uniform flow of gas. In the production environment, operators may observe that over a period of time these disks lose their parallelism. Although a slight change may be negligible for the uniformity of the gas flow and stability of the wafers, a tilted disk may begin from a little wear or an accumulation on the surface of the disk, and may worsen over the time. This is what needs to be understood to avoid downtime and failures of the product. Over the years, over daily routine in the fab, slight change in temperature and flow conditions can nudge the disks out of alignment today.

Thermal Deformation Mechanisms in AMAT Centura5200 Gas Floating Disks

The gas floating disk is prone to losing its flatness in an AMAT Centura5200 system partly because of heat. During high temperature Epitaxy growth , the disk is heated non-uniformly by the reactor chamber. It receives much more radiation and plasma energy from above, and cool gas flows from below. The temperature difference leads to thermal stresses in the material. Repeated cycling of the metal or coated surface with different temperatures leads to the expansion and contraction of the disk material. Though the stress at each thermal cycle may be relatively small, when performed repeatedly for hundreds or thousands of times, the accumulated residual stress begins to develop the thermal deformation. Some areas expand and stretch more than others and lead to a slight warp in the disk. This slight tilt can disrupt the gas cushion which allows the disk to float evenly. The thermal deformation can also be induced by temperature gradients across the disk. One part of the disk could be close to the heat source or received slightly less cooling gas flow. One part then stretches more than the other. This problem may arise after maintenance or cleaning as gas flow characteristics are often altered, and it is common to observe unstable disk leveling after the above operation. Creep is also an issue. High temperature induces gradual deformation of the disk material under a sustained load. This is a slow process without the sudden breakage; however, it represents the slow "memory of stress". In some fabs, it has been reported that a slight tilt was found in the disk after months of service without apparent defects. An illustrative example can be observed from a Sic epitaxy line when wafer showed non-uniform thickness. Careful investigation of gas floating disk in the chamber revealed a concave disk. The root cause was analyzed to be from high temperature cycles, and non-uniform back gas cooling flow. Stability of the gas floating disk is primarily associated with the controlled temperature distribution, reliable gas flow, and replacing the disks before significant creep is initiated.

How CVD TaC Coating Uniformity Affects Disk Flatness and Stability?

CVD TaC coatings are usually used for gas floating disks because it's well-developed, high performance on resisting hotness and corrosion under the hard epitaxy environment. But even a tough coating material of tantalum carbide cannot avoid the critical importance of uniform coating thickness on flatness and stability during operation of a gas floating disk.When the coating thickness is perfectly uniform across the entire surface, the disk under high temperature and high pressure of gases will expand symmetrically, heat spreads evenly across the disk with less internal stress. At the same time, the gas cushion beneath the disk could be formed more uniformly, which helps the position of the wafer to be steady during growth.The problem occurs when there's a difference in coating thickness on the entire surface. One side that has a slightly thicker coating section adds extra rigidity in that region and will respond differently under high temperature, it might expand slowly or even contract relatively in comparison to the thinner area. Through continuous cyclic heating, the strain accumulation caused by those differences may generate internal stress and result in mild bending of the disk base which was perfectly flat originally.In actual production, we observe this problem as unstable float height, or the centers of the wafers start shifting slightly, even when everything else seems perfectly normal. A production engineer may find it when they observe that the center to edge thickness variation of the deposited wafer increased unexpectedly, yet, when checking the chamber, they may realize that the floating disk already has a not-very-obvious "lift pattern" which usually relates back to coating variation.I can recall one example of this problem on the SiC production line. A gas floating disk looks good visually right after coating. It then began to show mild warping after a period of high temperature operating time. We found it actually showed several micrometres of difference in thickness with analysis, the value that small was responsible for the slightly bent disk operating at 1500C.A proper control of gas flow rate, temperature profile and rotation of disk during coating are required in the processes, with measurement of the coating thickness routinely measured to prevent an early drift of process conditions, and the coating process will be re-calibrated.

AMAT Gas Float Disk Repair Standards for Precision Epitaxy Processes

Gas floating disks in epitaxy equipment for precision tools such as the AMAT Centura are essential to stabilize the wafer during high temperature growth. There is strict protocol involved when repairing worn, deformed or unevenly coated disks. Any missteps in the repair process will affect the gas cushion behavior and thus the wafer quality.The most immediate aspect evaluated after repair is the surface flatness. The repair must meet very tight flatness tolerances, as even a minor amount of warp can distort the gas flow balance. Any grinding or polishing must ensure that an equal amount of material is removed from the entire surface; uneven polishing is the cause of most early disk failure.Coating repair also is a sensitive issue. Most disks are protected with CVD TaC and similar hard coatings. Upon re-coating, it is critical to ensure that the layer thickness is uniform over the entire surface of the disk. Uneven coating thickness causes that portion to respond differently when heated and can contribute to tilt or uneven lift during operation. Fabs have witnessed disks pass visual inspection following a repair, but develop unstable float heights after a few cycles due to this defect.Although it seems trivial, cleaning prior to repair is critically important. Unremoved contaminants or reaction products on the base substrate will compromise the integrity of the new coating bond. On one SiC epitaxy line, a repaired disk was delivered back to production and after one week exhibited vibration marks on the wafer. Post-repair inspection of the disk revealed trapped residue underneath the repair coating that was slowly expanding due to thermal cycling.Post-repair, disks often are conditioned at elevated temperatures to relieve stresses incurred during repair. Operators monitor the disk during this process to detect early indicators of warping or inconsistent lifting. It is important to not just repair the damage, but to bring back the original balance of thermal properties, surface flatness, and gas flow balance to ensure the repaired disk will perform like new during production.

Relationship Between Heat Distribution and Wafer Levitation Stability

During the gas flow leveling the wafer during epitaxy, how evenly heat is distributed across the disk and chamber is the key. If heat is spread evenly, the gas film beneath it will be uniform and the wafer will remain balanced. Uneven heating will begin to upset the delicate balance.During an actual AMAT Centura-type process the center of the wafer will be much hotter than the edges. The reason for this is that the heater itself isn't producing an entirely uniform heat map across the wafer surface, and the backside cooling of the gas may not be uniform. When the wafer grows hotter in one spot, it expand more at that particular location, which ever so slightly shifts the flow pattern of gas underneath.Once the gas flow shifts, the levitation height of the wafer is altered. One side of the wafer will float a little bit higher than the other. While this may not always be visible to the naked eye, a process tool can read this as small movement or oscillation during the growth process.The easiest way to visualize this is to imagine a very thin air hockey puck but with one side of the playing surface a bit warmer; the puck would still float, but drift or tilt towards the cooler side of the surface. A similar concept applies to the growth chamber.During actual production this will show up as edge to center thickness variation on wafers, and/or slight shifts in the alignment after long runs. One fab team had experienced slightly unstable epitaxy growth after performing maintenance, and discovered through inspection that a modification had been made to the chamber heating profile; they slightly shifted the hot zone, which prevented the gas leveling the disk from performing as it should with the uneven heat.

In order to minimize instability of the levitating disk, operators will generally pay attention to three main things: the matched profiles of the heater elements, consistent backside gas flow, and any changes in the wear of the chamber O-ring and on the surface of the disk.

Optimizing Gas Floating Disk Design for Long-Term Thermal Reliability

Designing gas floating disk for use for many cycles in an epitaxy tool mostly involves thermal performance. Not only must it perform at high temperatures, it must do so after 100s to 1000s of runs, without slowly going out of flatness.Material choice is critical here. The substrate material must be capable of undergoing thermal cycling without generating a large stress profile over time. The stresses will accumulate if the material has high coefficients and undergoes rapid thermal cycles. The substrate's thermal expansion must also be well-matched to the overcoat. In a fab environment even minor thermal mismatches between materials will add up over time and the disk will warpage, although initially it may look fine.Overcoat design is equally critical. An even CVD TaC coating across the disk will help it to absorb heat evenly across its surface. If the coating has varying thickness it will have a differing thermal reaction rates across the surface, creating thermal induced stress which will warpage the disk over time. It is believed that in some production teams this may occur over long runs as the disks slowly transition from flat to a domed shape.Geometrical features are very critical as well. Any deviations in the hole pattern, edge thickness, support regions, and other features, can vary the behavior of gas flow under the disk. Non uniform flow can cause the disk to lift unevenly under process conditions, generating extra stresses during thermal cycling, which will accelerate wear.An example of an improved design working under thermal stress was seen on an SiC epitaxy line; prior to their switch to an updated disk design, they had significant leveling drift after long runs. Upon switching to a disk with optimized flow channels, the drift reduced, as the lift profile of the disk was maintained more accurately throughout operation as confirmed by long-term maintenance checks of both original and upgraded disks.Thermal reliability also depends on cooling characteristics. If the disk does not cool evenly, this can "lock in" the stress and as a result cause further degradation with cycling.Essentially long-term flatness is derived by achieving a balance between material and coating performance and the designed gas flow through the disk.