Page 448 - Design for Six Sigma for Service (Six SIGMA Operational Methods)

406 Chapter Eleven

have also been developed, such as Sigma quality level and DPMO (defects per
million opportunities).
In this section, we first discuss the process capability index, and then we
discuss other Six Sigma metrics.

11.4.1 Process Performance and Process Capability
Process performance is a measure of how well a process performs. It is
measured by comparing the actual process performance level versus the
ideal process performance level. For the oxide film building process, per-
formance may be measured by the oxide film thickness, and its ideal per-
formance level would be 560 Å in thickness. For most processes, the
performance level is not constant. We call this variation the process vari-
ability. If the process performance can be measured by a real number, then
the process variability can usually be modeled by the normal distribution,
and the degree of variation can be measured by the standard deviation of
that normal distribution.

If process performance level is not a constant but a random variable, then
we can use the process mean and process standard deviation as key per-
formance measures. The mean performance can be calculated by averaging
a large number of performance measurements.

If processes follow the normal probability distribution, a high percentage of
the process performance measurements will fall between ±3s of the process
mean, where s is the standard deviation. That is, approximately 0.27 percent
of the measurements would naturally fall outside the ±3s limits and the
balance of them (approximately 99.73 percent) would be within the ±3s
limits.

Since the process limits extend from −3s to +3s, the total spread amounts
to about 6s total variation. This total spread is often used to measure the
range of process variability, also called the process spread.

For any process performance measure, there are usually some performance
specification limits. For example, if the oxide film thickness in a wafer is too
high or too low, then the wafer will not function well. Suppose it is required
that deviation from the target value of 560 Å cannot be more than 100 Å;
then the specification limits would be 560 ± 100 Å, or we say that its speci-
fication spread is (460, 660), where 460 Å is the lower specification limit
(LSL) and 660 Å is the upper specification limit (USL).