Metrology and
Calibration
The importance of
measurement
in our daily lives can not be overemphasized. Every new technology or
science breakthrough, industrial development, or commercial success
depends on one form of measurement or another. In these modern
times, we practically measure everything we encounter: the weight of our
food, the volume of our fuel, the distance between two points,
temperature, pressure, humidity, light, current, voltage, power, speed,
energy, etc.
Needless to say, reliable
measurement is very important to the semiconductor industry, or any
industry for that matter. In fact, it is so important that there's
already a science behind it. Known as
metrology,
it was developed and systematized to
ensure that all measurements performed are meaningful and according to
international standards.
After
all, one can not control or improve something that one can not measure,
and semiconductor manufacturing is one complex game of high-precision
control and continuous improvement.
Good measurement relies on the integrity of the
measuring equipment used. Unfortunately, no matter how sophisticated a
measuring equipment is, it degrades with time due to thermal,
mechanical, electrical, and environmental effects. This
degradation is called drift,
and it is unavoidable. However, the effects of drift on the
reliability of the measurements may be offset by a process known as
calibration.
Calibration
is simply the comparison of the measuring
instrument or equipment's performance to a reference standard of known accuracy.
In addition to this determination and reporting of
deviation from nominal, it may also include correction (adjustment) to minimize
the errors. Properly calibrated equipment provides confidence that the
company's products and services meet customer specifications
all over the world. In
the semiconductor industry, all critical equipment used in manufacturing
are required to undergo periodic calibration.
Another
commonly-encountered term in relation to calibration is verification.
Verification is not the same as
calibration because it refers to the comparison of the
measurement results against a specification, usually the manufacturer's published
performance figures for the product.
Calibration Traceability
The basic concept behind calibration is
that the measuring equipment should be tested against a standard of higher
accuracy. To illustrate this, below is a typical hierarchical
relationship among the various levels of calibration/measurement
activities within a company:
National Standard..........................
Accurate to 0.002%
Calibration
Laboratory........................................0.01 %
Company "Master"
Item.....................................0.07 %
Company Production
Equipment.........................1.0 %
Produced
Product............................................10.0 %
These calibrations need to be done on a planned,
periodic basis with evidence of the comparison results being recorded
and maintained. The records must include identification of the specific
standards used (which must be within their assigned calibration
interval), as well as the methods/conditions used in the calibration
process. These records should demonstrate an unbroken chain of
comparisons that ends at the agency responsible for maintaining
and developing a
country's measurement standards (now generically known as a
national metrology institute). This demonstrable linkage to national
standards, with known accuracy, is known as 'traceability'.
<Proceed to Page 2 - Metrology/Calibration
Terminology>
See Also:
Quality Systems;
Monitors and Controls;
Document Control;
The ISO9000 Standard;
Gage Repeatability and
Reproducibility
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