Calibration Accuracy – The Use of Line Scales in VIEW Micro Metrology Systems

In high-tech industries from consumer electronics to semiconductors  and medical devices, advanced manufacturing is critically reliant on precise dimensional measurements to ensure superior product quality and optimize process efficiency. As manufacturers increasingly depend on the accuracy offered by metrology solutions providers, it is imperative that the industry establishes traceability chains that link equipment calibrations to reliable material standards.

To meet this criteria, VIEW Micro Metrology engineers calibrate field-installed systems using line scales certified in a fully accredited ISO 17025 Line Scale Calibration Laboratory. 

What is a Line Scale?

Metrologists and calibration experts utilize a range of material standards, or “artifacts,” for verifying and calibrating measurement equipment, including microscopes and dimensional metrology systems. These reference objects, crafted from specific materials with geometric features, include ball bars, line grids, gauge blocks, step gauges, and other three-dimensional objects, tailored to specific measurement systems.  However, in general, metrologists recognize line scales, also known as linear scales or graphical scales, as the premier standard for calibrating optical measurement machines.

A line scale typically comprises a flat substrate, such as glass, quartz, or steel, with parallel, precisely spaced lines or graduations etched or imprinted onto the surface. Each graduation signifies a specific distance on the object.

Line scales are characterized by very low uncertainty, defined as imprecision in measurement. Every measurement is subject to some degree of uncertainty, introduced by the object itself, the measuring tool, the operator, and environmental factors such as temperature and humidity. The variance from the true measurement is expressed using standard deviation in a statistical bell curve. The lower the uncertainty in the artifact, the more accurate the resulting equipment calibrations will be.

Who Calibrates Line Scales?

Line scales and other calibration artifacts are typically verified by national laboratories operated by governments worldwide. These institutions offer scientific and technical services to interested parties. For instance, in the United States, the National Institute of Standards and Technology (NIST), part of the Department of Commerce, provides a comprehensive range of measurement and standards services. These services ensure measurement traceability, facilitate quality assurance, and support regulatory practices among manufacturers. Companies seeking to calibrate their equipment can send line scales to NIST for measurement and receive certificates of uncertainty in return.

Beyond NIST, Germany supports manufacturers through the Physikalisch-Technische Bundesanstalt (PTB), while Japan has the National Metrology Institute of Japan (NMIJ), and Sweden is served by the National Metrological Institute (NMI). An elite group of other such organizations operate globally, as countries strive to promote sound manufacturing practices within their economies.

What is a Line Scale Calibration Bench?

National laboratories typically utilize line scale benches for the calibration of line scales. These benches vary in design, with the most fundamental being a one-dimensional (single-axis) coordinate measurement machine (CMM). For instance, the National Institute of Standards and Technology (NIST) employs the NIST Line Scale Interferometer (LSI) for this purpose. As detailed on the NIST website, “This instrument consists of a scanning electro-optical line detector, a high precision one-axis motion system, and a high accuracy heterodyne interferometer for determining the displacement of the test artifact beneath the line detector.”  The entire setup is maintained within a temperature-controlled chamber, with meticulous monitoring of environmental conditions to ensure minimal uncertainty.

Why Did QVI Build a Line Scale Calibration Bench?

Historically, the excessive costs associated with these machines and their stringent environmental requirements made outsourcing line scale calibrations more economical for manufacturers. However, as the demand for enhanced measurement precision grows, private enterprises are increasingly establishing their own laboratories.

In 2020, Quality Vision International (QVI), parent company of VIEW Micro Metrology, constructed a fully accredited ISO 17025 Line Scale Calibration Laboratory in its Rochester, New York location.  QVI established the Line Scale Calibration Laboratory for three reasons: 

• ACCURACY:  With manufacturers now seeking measurements with an uncertainty of 250 nanometers (1/4 of a micron), Quality Control teams are increasingly demanding systems capable of achieving significantly lower levels of uncertainty.
• CONVENIENCE:  As a manufacturer of metrology systems committed to rigorous quality standards, QVI requires frequent calibrations of our standards. An in-house laboratory offers the advantage of conducting these calibrations on-site.
• VERTICAL INTEGRATION:  Finally, by mirroring the capabilities of a national laboratory, QVI has achieved a significant milestone in vertical integration while offering a distinctive service to our clients.

About the QVI Line Scale Calibration Bench

The QVI Line Scale Calibration Bench is located within the Calibration Laboratory at the North Street building on the QVI campus in Rochester, NY. It is a single-axis coordinate measurement machine (1D CMM), designed & built by QVI, and features a moving bridge-type carriage that glides 2.5 meters on air bearings along the top flat surface of a substantial granite beam. The Bench is equipped with a motor drive for precise positioning of the carriage and utilizes an optical video camera as the primary sensor for automatically capturing images along the line scale. The maximum scale dimensions are 2200mm (L) x 140mm (W) x 100mm (H).

During the calibration process, a technician places the line scale on the bench and allows it to acclimate to the room’s temperature and humidity conditions. Once the measurement routine begins, the carriage moves over the line scale, and a laser interferometer collects data to measure the displacement – essentially, how far the carriage has traveled down the bench. At the same time, the camera mounted on the carriage automatically captures images of the line scale and employs QVI edge detection algorithms to identify the lines.

While this process may seem straightforward, two types of errors can negatively impact measurement results: Abbé Error and environmental conditions.

What is Abbé Error?

Abbé Error refers to an angular error in the motion system—a slight deviation from perfect straight-line motion that can cause the point of interest to move in a manner not perfectly aligned with the intended path. This misalignment is exacerbated by the distance (offset) between the point of interest and the origin of the error.

To mitigate Abbé Error, the QVI Line Scale Calibration Bench incorporates a unique design feature: the open corner cube (OCC) retroreflector mounted on the carriage. This allows the carriage to straddle and pass over the line scale workpiece, ensuring that the metrology point of the laser interferometer system can coincide (to within less than a millimeter) with the metrology point of the line sensing microscope system. Consequently, the Abbé error, due to carriage pitch and yaw motion errors, is minimized to a negligible level (a few nanometers).

Environmental Errors and the Edlén Equation

All national laboratory institutions perform calibration operations within chambers that meticulously control temperature, pressure, and humidity. Advanced systems with continuous monitoring are essential to maintain these stringent environmental conditions.

The QVI Line Scale Calibration Bench is situated in a precisely regulated laboratory, equipped with devices that log temperature, pressure, and humidity (water vapor content) at specific time intervals, synchronized with the images captured by the metrology camera. Specialized software leverages these environmental conditions to adjust measurements using the Edlén equation, a formula that calculates the refractive index of air, which is a measure of how greatly light bends when passing through air. Since the wavelength of light undergoes slight changes as it travels through air, determining the refractive index is vital for achieving precise length measurements in line scales. To summarize, the QVI line scale bench employs the Edlén equation to account for the influences of temperature, pressure, and humidity, ensuring each optical measurement on the line scale is adjusted for optimal accuracy

Accuracy of the QVI Line Scale Calibration Bench

As a result, the QVI Line Scale Calibration Bench has a Calibration and Measurement Capability (CMC) uncertainty as low as 22 nm, as verified by an independent audit (Q[22, .072L] with L in mm; 2400 mm total length). This bench, offering capabilities on par with a national laboratory, is accessible to VIEW Authorized Representatives and customers for the precise measurement of optical calibration artifacts.

VIEW Micro Metrology Systems – Calibrated to Extreme Accuracy

Given the superior capability of QVI’s Line Scale Calibration Bench, particularly for line scale lengths over 314 mm, VIEW Micro Metrology dealers and field service experts use line scales calibrated on the bench.  By calibrating VIEW systems with QVI lab-approved line scales, we ensure that VIEW machines deliver unparalleled accuracy in the field.