Ed’s Threads 070914Musings by Ed Korczynski on September 14, 2007
Missing micrograms and measurement accuracy
The “one true” kilogram cannot be trusted anymore. All standards must be based on a reference, and the master reference for mass on planet earth is a platinum-iridium-alloy cylinder kept in a special vault in Sevres, southwest of Paris. The 118-year-old master cylinder now appears to have lost 50µg compared with the average of dozens of copy-masters, and the reason is a mystery. "They were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart," said Richard Davis of the International Bureau of Weights and Measures in Sevre, France. "We don't really have a good hypothesis for it."
Each copy-master, officially termed a “National Prototype,” is used as the main reference in different countries (the Figure shows the US National Prototype Kilogram, held by NIST) to calibrate measurement systems. Scientists tend to care that a kilogram is absolutely a kilogram. Engineers tend to care that they get about the same amount of something every time, relatively speaking. The difference is between “accuracy” and “precision” in measurements.
Accuracy is defined as how closely a measurement matches an actual or “true” value
, while precision is the repeatability of multiple measurements
. How we can ever really determine the true value is another question.
The real world of our experience is never “ideal.” The surface of our planet is hot enough that random kinetic energy within atoms as lattice vibration induces finite vapor-pressure so solids may alter and be altered by their environment. Thus the act of measurement may alter that which is being used to measure, which is not a macro-scale variation on Heisenberg’s Uncertainty Principle
, but an honest acceptance of the fact that macroscopic solid surfaces interact with their environments. Copies and redundancy may be used to detect any such drift of mass, and this is where we now find a problem—either the copy-masters accreted mass due to some as-yet-inconceivable phenomenon, or the master lost mass. Neither scenario is easily explained.
How might this possible loss of an absolute mass reference effect semiconductor manufacturing? Though chip fabs use technologies in common with other industries such as specialty gases and vacuum pumps, relative references are sufficient. Based on the inputs, engineers always “center processes” which then become relative standards. "Copy Exactly", as defined and developed by Intel
, fully embraces this concept; once an input is proven in manufacturing, external references may be ignored. As long as a process is very reproducible—precisely—it’s accuracy can be relative.
Absolute standards just aren’t essential for this industry to test chips before shipping them to customers either. Digital chips are designed to functions as circuits of binary units, so a slight shift in internal relative values wouldn’t matter. Even analog chips or sensors are still designed to typically allow for calibration of some sort, so for example the gain could be tweaked to allow for a drift in a basic parameter. Given the inherent variability of batch processing with the need for consistent IC functionality, the industry has learned to handle slight shifts in parameters.
So, we can all relax and not worry about our industry losing its way if “The kilogram” has lost 50 parts-per-billion (ppb) of mass. Companies such as Process Specialties Inc.
and VLSI Standards
still provide “NIST-traceable reference standards” for the industry, which are more than adequate for our needs. What more can be done?
For over two years now, NIST and other standards groups have advocated for a kilogram standard
based on something beyond a physical master, though more work is needed. One option would be to assume Avogadro’s constant (the number of atoms in a mole of matter) and then measure spacing in a “perfect crystal” to determine the number of atoms in a reference mass. Another option would be to count the number of electrons flowing through superconducting coils needed to balance a mass accelerated by gravity. “Currently, both methods are 10-100x less precise than the measurement uncertainty produced when comparing the kilogram artifact to national standards,” according to consensus from the Royal Society of London.
Supposedly, one of the leading alternatives for a 21st-century kilogram is a sphere made out of a Silicon-28 isotope crystal, though to the best of my knowledge any macro-scale crystal made up of gazillions (a technical term) of atoms on the surface of planet earth (with temperature ~298°K resulting in “random” energy) will have defects. The lattice spacing may be uniform and measurable, but vacancies and defects will still exist. These are some of the issues associated with pushing the limits of physical standards.
Humans have imagined absolute standards for thousands of years. Just like the conceived Platonic Solids
, however, absolutes don’t exist in the real world. So we can keep dreaming of perfect standards, but back in reality we’ll still be counting exceptions and measuring variations.
Labels: accuracy, kilogram, measurement, precision, standard
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070914: Missing micrograms and measurement accuracy