Why a scheduling optimizer is the answer to balancing reticle moves and cycle times
Photolithography processes in semiconductor manufacturing are often considered bottlenecks due to their reliance on a critical secondary asset; reticles. Reticles play a crucial role in the photolithography process yet they are limited in number and very delicate in nature. Their scarcity makes them a difficult asset to schedule and if the reticles are broken frequently during transport, it can ramp up costs for the fab. Reducing the amount of times reticles are moved can help decrease the likelihood of them being damaged, but this sacrifices the fab’s fundamental objective of reducing cycle times.
This presents operators with a tradeoff that we have assessed by investigating three case studies where the Flexciton scheduling optimizer has been used to explore the different outcomes. The scheduler allows the user to define their own risk profile for reticle moves thus enabling the operator to quantify how the fab will perform when different KPIs are prioritised. This article summarises the findings from our analysis - if you are interested in learning about the case studies in more detail then click here to read the technical article.
Case Study 1: Focusing solely on cycle times
In each of the studies, the wafers require 48 steps across 6 different machines. For the first case study, the scheduling optimizer has been instructed to not penalise reticle moves and exclusively focus on minimising cycle times. As a result, there are a total of 7 reticle moves and total priority weighted cycle times (TWCT) equates to 16.79 hours for all 48 wafer steps.
Case Study 2: Moderate penalisation of cycle times
The smart scheduler in case study 2 has been told to avoid reticle moves that have little benefit to the cycle time, since the risk of potential damage is deemed higher than in case study 1. The outcome of this is a total of 5 reticle moves and a TWCT of 18.73 hours, a 12% increase over the first study.
Case Study 3: Only necessary reticle moves
The final case study looks at the extreme case of configuring the smart scheduler to use a very high penalty for reticle moves. In cases such as this, the operator is willing to accept sub-optimal lot-machine allocation decisions and delayed scheduling of high-priority wafers in order to keep the risk of reticle damage to an absolute minimum. This results in just 2 reticle moves for all 48 wafer steps and a TWCT of 23.20 hours, a 38% increase over the time taken in case study 1.
How does it perform in the real world?
When put into practice, two separate schedules were computed: one with low and one with high penalisation of reticle moves. The smart scheduler was able to compute the two schedules in just a few minutes each. The results obtained from the reticle machines suggested that reticle movements could be reduced by around 26% while only leading to an increase in total cycle time of around 2%.
In order to reduce the risk of damage or distortion during transport, the number of times a reticle is moved must be managed conservatively. This inadvertently clashes with the fab manager’s fundamental objective of meeting cycle time KPIs.
Utilising smart scheduling optimization allows the user to define their own risk profile and explore the tradeoff frontier in detail. This enables operators to quantify how their fab’s performance may change with a more relaxed or constrained attitude towards reticle movements.