Optimizing Lot Releases: A Key to Efficiency in Semiconductor Manufacturing
Imagine you arrive at the airport on time, or perhaps even early, but the airplane has a mechanical issue, causing a delay in the scheduled takeoff. Clearly, arriving on time does not get you to your destination as planned. So, would it not be better to stay at home, wait for confirmation that the plane is ready, and only then head to the airport?
Another thought would be to attempt to catch another flight to reach your destination on time. By going to the airport at the “wrong time” and simply waiting, you’re only contributing to your own work-in-progress (WIP) which in this case is waiting time. You’re also contributing to airport congestion, which could push back other flights and possibly cause other customers to miss their flights.
Such problems can be avoided by an intelligent planner or release strategy that can figure out exactly when you should leave home, given the traffic situation, the speed of cars, the parking time, and the time it takes to go through security.
This type of predictive planning is ideal for releasing lots in a semiconductor manufacturing line. Depending on the product mix, resource availability, masks, and WIP, it can be determined which lots should be released, and which ones should be held back to optimally meet the following three objectives:
- Equipment utilization
- Delivery performance
In a high-mix environment, an increase in WIP does not necessarily imply additional wait times or delivery delays because of multiple routes and the system’s balanced allocation of jobs. The following diagram shows the relationship between these three parameters and how they change with an increase in WIP. This is indicated in the diagram as the optimal region of operation where the desired objectives can be achieved.
In environments where there is a high mix of products, such as foundries, we can increase the number of lots released without increasing their waiting time by ensuring that they are balanced across different bottleneck equipment such as lithography equipment.
Given the complexity of such environments, each process has 400-600 steps and uses hundreds of types of equipment. These processes require anything from 10 minutes to 10 hours to complete, with highly sensitive setup times (such as for implanters) or batching requirements (like those for ovens). Therefore, one must intelligently plan ahead and review past activities to ensure the proper balance of lots re-entering the process, or entering the process with different priorities.
Unfortunately, sequencing engines with simplistic rules have been given too much attention in order to solve such a complex problem. Through years of R&D, we have concluded that unless a proper release strategy is deployed, sequencing would not be of much value. It is a reactive engine, not a preventive one. But more importantly, in the presence of an adequate release strategy, sequencing can be a liability in the sense that it would try to resolve issues locally without being aware of the potential issues it might be causing 50 steps down the line.
Can you imagine being at the gate, and the airline personnel trying to sequence your entry into the plane when the plane is not even at the gate or being repaired?
One other myth is the use of simulation tools to plan fabs. Simulation tools look nice and show movement. It is like a video game, and we all enjoy watching it.
However, they do not provide a strategy. They only show you where the problem might lie without telling you how to avoid it. How could they? They do not look ahead; by definition, simulation is one sequence at a time!
As in the opening example, a good release strategy is aware of the right mix of products in the fab. It also considers the workload of each piece of equipment, both currently and in the future. The strategy is constantly trying to balance what needs to go next, ensuring that the bottlenecks, as they are changing, are fully utilized. At the same time, it keeps in mind which lots need to be ready and when, to ensure on-time delivery.
In fact, our research shows that in the presence of a good release strategy, a simple ‘First In, First Out (FIFO)’ is the best sequence for the resources. In the context of our airport example, if you leave your home at the right time, as you approach your gate, without a long wait, you will show your boarding pass and get into your seat for takeoff. No need to be sequenced.
Let’s make accurate plans together!