In both semiconductor device and microelectromechanical systems (MEMS) manufacturing, wafer processes are generally divided into single wafer and batch processing. As the name implies, batch processing calls for multiple wafers to be processed at the same time. This cost-effective approach is typically used for thermal oxidation, low pressure chemical vapor deposition (LPCVD) of silicon nitride, resist strips and wafer cleaning processes.
Recently, however, tightening requirements to achieve finer features is driving a need for high quality deposition and etch processes. As a result, there has been a significant shift towards single wafer processing. The main considerations of a batch vs single wafer approach are reviewed here, highlighting reasons where one method may be preferred over another.
Put simply, the per-wafer process cost is much lower when batch processing is implemented. For example, the cost of running a thermal oxidation furnace to the same process temperature with the required process gas is the same whether you’re processing one or 25 wafers. This makes the decision of using a a batch process easy. Such deposition results are consistent, high quality and not possible at single wafer level.
Most of the early front-end of line (FEOL) processing revolves around wafer cleaning and surface preparation. This requires etch solution baths for cleaning many wafers at once, and in some cases, on both sides. These processes clearly cannot be recreated at the single wafer level while also replicating the low cost that the batch method affords.
Thus, there is is a consistent thread among processes operating at batch level. While repeatable, they are easily controlled, from the wet etch clean that has a linear relationship with etch rate vs. time to film deposition, with its constant deposition rate.
Single Wafer processing
On the other hand, single wafer processing is more prevalent when higher control levels are required, or where hardware configurations limit the number of wafers that can be processed.
Sputter, chemical vapor deposition (CVD), reactive ion etch (RIE) and more recently, vapour phase etching systems are all available as single wafer systems. These systems typically perform high precision processes requiring high levels of repeatability. For example, single-wafer plasma-enhanced CVD (PECVD) systems for oxide and nitride depositions are more common, however, batch systems are available.
According to existing literature, with single wafer systems, etch control appears to be more precise for users when aspects such as etch uniformity and repeatability are involved. The vapor-phase etch process typically has a more dynamic mechanism, and requires significantly greater process control. This control becomes harder to maintain in high-volume batch processes, as the wafers begin to influence one another within the process chamber. To counter this, additional process modifications are required that reduce throughput as well as other key resulting criteria.
Cost of Ownership and Yield
Cost of ownership and yield are other important aspects to consider. Cost of ownership factors in cost of materials as part of operation, utilization, throughput and yield as part of the equation.
In an overall time comparison over a 25-wafer batch, the throughput is typically higher in batch systems than single wafer systems. There are some instances where a single wafer system may be preferred however. For example, when the facility is working with smaller batches and time can be saved in a single wafer system, or where a single wafer system is capable of processing wafers fast enough to beat the 25-wafer time of a batch tool.
However, yield is a more difficult specification to meet in a batch system. As previously mentioned, batch processing tends to be used in simple processes where a linear behavior can be used. When more advanced process requirements are present, such as in the inclusion of plasma units or complex chemical processes, batch systems struggle to meet yield requirements. This is because they are not designed to accommodate the ‘finer’ approach taken by single wafer systems. Thus, when high levels of process uniformity and device yield are required, such as in a release etch process, a single-wafer process is the better option.
In conclusion, single wafer vs batch processing is determined by a few key parameters; the cost of the process, the strictness of the required result, and the final process. In processes like wet etch, where basic control is required to provide optimal results, batch systems are low-cost, easy-to-use options. When the goal is to create highly repeatable process that achieve high final device yield, batch systems no longer offer sufficient control and single wafer processes should be considered.