Today, Statistical Process Control (SPC) is the gold standard of quality control because it helps manufacturers to maximize production of on-spec product with minimal waste and
SPC uses graphical control charts to determine
when a process should be adjusted. The following
guidelines can help silicon chip and solar cell man-
ufacturers make the best use of SPC for cleanroom
• List the criteria that will maintain a stable environ-
ment and assure product quality to focus monitor-
ing on critical process control conditions.
• Structure the testing process to focus on these criteria and avoid extraneous testing.
• When troubleshooting, think outside the box or
farther down the line.
• Use computational fluid dynamics (CFD) modeling to help identify the source of a problem.
Here are insights on how.
List the criteria that will maintain a stable environment. It’s important to narrow the focus to monitoring only those conditions that are critical for maintaining process control. Specific SPC monitoring
criteria are determined by the type of facility. The
major environmental factors that affect the quality
of a silicon chip or solar cell are airflow temperature,
humidity, air pressure, particles, light wavelength and
electromagnetic interference (EMI). Therefore, these
should be the key monitoring criteria in any facility.
Conductivity is sometimes overlooked as a monitoring requirement. During design and construction,
the facility should have been properly grounded to
control conductivity, directing energy to ground at
the proper rate without creating sparks. Nevertheless,
conductivity should be monitored to assure that the
grounding system is properly functioning.
Monitoring criteria should be based on the proj-
ect’s requirement for manual versus automated mon-
itoring based on monitoring frequency. This can be
determined by comparing the costs of manual moni-
toring with the capital and life cycle operational costs
of an automated system. If the quality specifications
of the product require hourly points of monitoring,
it makes good economic sense to use an automated
system. However, when data monitoring is less fre-
quent, the payback period on the high capital cost
of automated systems may be too long. Similarly,
the data from an initial period of monitoring may
demonstrate that the frequency can be reduced,
based on data that demonstrate consistent achieve-
ment of performance/quality standards. Monitoring
should be fine-tuned based on the data as the SPC
implementation period progresses.
Structure the testing process to focus on each of
the criteria identified above in an orderly sequence
along the critical path. Not only is it important to
understand what to test, but also how to test it. If
a fault is discovered toward the end of the process,
the product has to go through a long and expensive incubation process. This is made worse today
because a smaller wafer has more capacity than it
did in the past and therefore each batch represents a
more expensive loss.
In most cases, SSOE, a top five design firm serving the semiconductor industry, sets up a fishbone
(cause and effect) diagram or uses another statistical
tool to help owners determine the types of issues
that could be creating a problem. Troubleshooters
can check off the potential causes one by one until
they discover a demonstrable cause. When the test is
well-structured, troubleshooting is a simple matter
of using the tools and methods that process control
experts routinely use to ferret out a problem.
Brainstorming can contribute to understanding
all of the potential sources of process control problems. Some can be logically eliminated without testing until the field is narrowed, after which a series of
tests can be implemented. For example, if there have
been temperature variations, monitoring can be set
up to determine the precise variation. If the problem is particle control, some of those particles can
be captured and sent out for analysis. The chemical
composition of the particles suggests the most probable sources.
All too often, it is easy to get distracted from the
real possibilities along the critical path. For example,
as wafers and dyes get smaller and line widths get
narrower, there are some sectors in the industry that
must test for and control particle contamination
even at the molecular level.
When troubleshooting, think outside the box or
further down the line. Particles can be blown long
distances from the source because of eddy currents
and other phenomena that occur around devices.
Vibration is another common source of product
contamination, and its source is often unexpected
and sometimes distant from the process line. One
must examine all of the potential sources of these