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74 August 31, 2006 2:55 Process Variations and Their Estimates
Variation

Process Variation Measurement Variation

within unit within instrument
between units between instruments
between lots between calibrations
between operators between operators

between machines
across time
between set-ups
across time
Figure 6.1 Sources of variation.

how well these services, products and processes are meeting their requirements and
expectations; in other words, how well they conform to standards. Distinguishing the
sources of process variation is paramount because the appropriate measurements are
different for each. Without this distinction, five-step problem-solving approach of the
Six Sigma methodology -- define, measure, analyze, improve, and control (DMAIC)
cannot be utilized effectively (the detailed and extended DMAIC approach can be
found in the book by Harry and Schroeder). 1
Many processes have more than one source of variation in them. In order to re-
duce variation in processes, these multiple sources must be understood. As shown
in Figure 6.1, process variation consists of within-unit variation, between-units vari-
ation, between-lots variation, between-operators variation, between-machines varia-
tion, between-set-ups variation, and, lastly, variation over time.
For example, in the semiconductor industry, a batch process may operate on several
wafers at a time. The defects found at different locations on a wafer are considered as
a within-unit variation, as there are many other locations where the wafer is defect-
free. On the other hand, one might detect the defect at one particular location of one
wafer but not of another wafer in the same process batch; such variation is catego-
rized as between-unit variation. The difference in defective rate between each lot is
considered as between-lots variation. Similarly, the disparity between the products
or services provided by different operators or machines is designated as between-
operators variation, or between-machines variation, respectively. The difference in
process yield for different set-ups can be treated as between-set-ups variation. Due to
aging, the performance of the process will deteriorate, and this variation is unavoid-
able over the lifetime of a process. In addition, the cumulative effect of many small,
essentially unavoidable causes will lead to different process outcomes; these chance
causes are an inherent part of the process.