pg_0039
5.
Process Improvement
5.3.
Choosing an experimental design
5.3.2.
How do you select and scale the process
variables.
Guidelines
to assist the
engineering
judgment
process of
selecting
process
variables
for a DOE
Process variables include both inputs and outputs - i.e., factors and responses. The
selection of these variables is best done as a team effort. The team should
Include all important factors (based on engineering judgment).
q
Be bold, but not foolish, in choosing the low and high factor levels.
q
Check the factor settings for impractical or impossible combinations - i.e.,
very low pressure and very high gas flows.
q
Include all relevant responses.
q
Avoid using only responses that combine two or more measurements of the
process. For example, if interested in selectivity (the ratio of two etch
rates), measure both rates, not just the ratio.
q
Be careful
when
choosing
the
allowable
range for
each factor
We have to choose the range of the settings for input factors, and it is wise to give
this some thought beforehand rather than just try extreme values. In some cases,
extreme values will give runs that are not feasible; in other cases, extreme ranges
might move one out of a smooth area of the response surface into some jagged
region, or close to an asymptote.
Two-level
designs
have just a
"high" and
a "low"
setting for
each factor
The most popular experimental designs are two-level designs. Why only two
levels. There are a number of good reasons why two is the most common choice
amongst engineers: one reason is that it is ideal for screening designs, simple and
economical; it also gives most of the information required to go to a multilevel
response surface experiment if one is needed.
5.3.2. How do you select and scale the process variables.
http://www.itl.nist.gov/div898/handbook/pri/section3/pri32.htm (1 of 3) [5/7/2002 4:01:45 PM]
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