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The Relationship Between Cycle Time and Capacity

In the (imaginary) no-variability case, cycle time remains constant as start rate is increased, up to the maximum system capacity. At that point, if start rate is increased further, cycle time increases linearly. This is shown in the following chart.

In a real fab, with variability, cycle time tends to increase with start rate (or throughput rate - the two measures are directly related by line yield). Exactly how much the cycle time increases will depend on the amount of variability in the system. In most fabs, once the system is loaded above approximately 85%, cycle time starts to increase rapidly. This is sometimes called the “hockey stick effect,” as illustrated below.  

Cycle time limits the effective capacity of a wafer fab. Even the low variability system cannot be run at 100% of the maximum throughput, because cycle time increases rapidly to unacceptable levels. In fact, the limiting case for systems with any variability is that as the factory loading approaches 100%, the cycle time approaches infinity. In the real world, factory planners account for this by including “catch-up capacity” in their plans. That is, they typically plan for about 15% idle time on all of their tool groups. This keeps factories out of the steep portion of the curve shown above. 

We have worked on projects in which the factory performance measure used was cycle-time-constrained capacity. This is defined as the maximum capacity at which the factory can achieve a given average cycle time, expressed as a multiple of raw process time. The multiple of raw process time is usually called an X-factor. So, the shorthand term for three times raw process time is 3X. In the chart above, the 3X cycle-time-constrained capacity (or 3X capacity) for the low variability system is approximately 80% of the maximum theoretical capacity of the system. For the highest variability factory, the 3X capacity is only about 45% of the maximum theoretical capacity. 

The maximum theoretical capacity of a factory is the start rate that drives the bottleneck to 100% loading. The only way to increase it is to add equipment, or make process changes that reduce the load on the bottleneck. However, as the example chart shows, cycle-time-constrained capacity can sometimes be dramatically improved by reducing variability in the factory. This is one of the key strategies used for low-cost cycle time reduction efforts.