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'.

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See Also:  Quality Systems;  Monitors and Controls;  Document Control;  The ISO9000 Standard;

Gage Repeatability and Reproducibility