Round world vs. square world
Nowhere is the paradigm difference between wafer manufacturing and reticle manufacturing as obvious as in the very shape of the items being manufactured. Wafers are round. Reticles are square. Indeed, this characteristic led to the colloquial description of wafer manufacturing as “round world” and reticle manufacturing as “square world.”
The round world’s pervasive view of reticle manufacturing is a bit dismissive. After all, it only takes some tens of process steps to manufacture a reticle, whereas it takes many hundreds of process steps to manufacture a semiconductor device. So how hard could it be to manufacture reticles?
The comparison of manufacturing volume furthers this rather dismissive view; wafer fabs are typically high-volume manufacturing, building massive quantities of chips that are ultimately sold on the mass market. By contrast, mask shops operate as “job shops,” building a single unique reticle for each order. However, each of those reticles must be perfect, or the wafer fab’s yield will suffer. This is where square world manufacturing is severely underestimated.
Yield vs. efficiency
Square world yield is binary; a reticle is either good or bad. Each reticle must be perfect when shipped to a wafer fab customer or it is scrapped and the process restarted until a perfect reticle is manufactured. Round world yield, on the other hand, is continuous. With hundreds of die per wafer and 25 wafers per lot, there are thousands of candidate die per lot that can be shipped to customers. It’s possible that even with several defective die on each wafer, the lot’s yield is still considered acceptable. It’s even possible that several wafers can be scrapped for the lot to have reasonably good yield.
However, while yield is important in round world, manufacturing efficiency is perhaps even more so, because wafer fab process flows commonly consist of many hundreds or even thousands of steps and cycle time is measured in months.
But the calculus is quite different in square world, where process flows are only a few tens of steps and cycle time is measured in hours or days. Here, the focus is on shipping a perfect reticle on time. That is the measure of efficiency and on time delivery is the key metric. The mask shop revolves around customers’ delivery deadlines and will go to great lengths to meet them – even to the extent of building multiple duplicate copies of a reticle with low expected yield simultaneously to improve the odds of meeting the deadline. To the mask shop’s wafer fab customers, a delayed reticle may mean that the wafer fab can’t expand production on time or that production of a new device is delayed waiting for a reticle, resulting in a potentially significant monetary impact.
Process efficiencies
High manufacturing volume enables several round world process efficiencies. Not every wafer or every lot needs to be measured or inspected to ensure product quality, and this can significantly increase overall factory output. A statistical sample plan can define the fraction of lots that need to be measured or inspected to guarantee a statistically high probability of good overall product quality.
But that’s definitely not the case in square world. Given that each reticle is unique and the manufacturing volume is relatively small, at least compared to the wafer fab, every reticle must receive both metrology and inspection to guarantee that each is perfect before shipping to the wafer fab.This requirement for 100% metrology and 100% inspection not only means higher overall cycle time, but it also alters the basic economics of the mask shop because additional metrology and inspection tools are required to minimize the impact on cycle time and attain the expected factory output.
Process control
Process control is another area where there are significant differences between round world and square world. To ensure that consistent process control is maintained in high-volume manufacturing, statistical process control (SPC) is used extensively in round world to monitor the metrology and inspection results of the fraction of wafers that pass through those steps. Traditional SPC rules can be used to determine if processes are in control or trending out of control. This provides an effective early warning system for the wafer fab to identify potential process problems and correct them before they become yield problems.
By contrast, SPC is of limited use in square world for two important reasons. The first is that the mask shop’s relatively low manufacturing volume doesn’t provide a large enough sample size to be statistically valid. Even if the manufacturing volume were high enough to exceed the threshold for a statistical sample size, the fact that each reticle is unique invalidates traditional SPC, which relies on the manufacture of identical items using identical processes.
The role of the MES
Wafer manufacturing versus reticle manufacturing. Round world versus square world. Each has its own distinct manufacturing paradigm. For both, however, an advanced Manufacturing Execution System (MES) can play a crucial role in improving operational efficiency and product quality by providing real-time data and insights into production processes.
For the mask shop, the MES facilitates seamless communication across the factory, optimizing resource allocation, minimizing delays, and ensuring on-time delivery. By facilitating the creation of detailed process flows and simplifying process flow optimizations in a structured, controlled manner, an MES also contributes to the efficient manufacture of perfect reticles.
An MES can help mask shops achieve higher throughput, improved quality, and better alignment with wafer fab customer expectations that perfect reticles will be shipped on time every time.
