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NPL Launch High Precision Surface Topography Areal Standard

The UK’s National Physical Laboratory (NPL) has recently launched a single-chip, cost efficient, high precision areal standard, designed to traceably calibrate areal surface topography measuring instruments in accordance with the upcoming ISO 25178-700 standard.

The new NPL areal standard enables users to:

  • Adopt good measurement practice and meet the requirements of ISO standards for determination of 3-D surface texture in a cost-effective way;
  • Calibrate common optical instruments equipped with 10×, 20× and 50× magnification lenses, and contact stylus instruments;
  • Automate calibration procedures due to the unique combination of alignment markers and a comprehensive range of calibration features on a single plate, which reduces the time and cost involved in characterising measuring instruments, handling and maintenance;
  • Validate instrument performance using two complementary electroformed areal irregular pattern artefacts, AIR-B40 and AIR-B70.

The NPL Areal Standard can be calibrated to different reference uncertainty levels depending on the user’s needs and budget. The NPL areal standard can also be used for in-line process inspection. A good example is the case where the NPL Areal Standard has been used to verify the performance of a fast, in-process confocal 3D line sensor.

The NPL Areal Standard is complemented by two separate electroformed deterministic pseudo-roughness artefacts – Type AIR (areal irregular patterns); AIR-B40 and AIR-B70 for the validation of instrument performance.

NPL Addresses Industrial Need For Standardization and Traceability

In advanced manufacturing, surface texture of the component plays an important role on its functionalization. Topography is critical to part performance. Routine dimensional inspection of surface texture as well as small features such as pits, embossed channels and printed conductors, requires calibration support to ensure traceability. This need has increased the demand for surface texture instruments for formal quality control in advanced manufacturing supported by the appropriate standardisation activities. The ISO 25178 series of standards covers areal surface texture measurements and the upcoming ISO 25178 part 700 describes a series of tests that can be carried out to calibrate the ‘metrological characteristics’ (MCs) of surface texture measuring instruments. Much like power, fuel efficiency and load-carrying specifications for a new vehicle, MCs describe the performance of a topography instrument and can be independently quantified by anyone with the right tools, without taking apart the instrument. Surface topography measurement uncertainty can be estimated from appropriate calibration of a short list of these instrument MCs, such as flatness deviation, noise, scale linearity and amplification.

The NPL areal standard is a 10 mm square silicon chip bearing calibration patterns with depths ranging from 50 nm to 2 µm (Figure 1). The silicon chip is set in a 50 mm square toughened glass plate, engraved with a unique serial number. The NPL areal standard consists of three areal cross gratings (for x and y axis calibration), four 3D resolution stars (for lateral period limit estimation at 50 nm and 200 nm), six rectangular step features (for z axis calibration), and a flat region for noise and flatness determination. Also included are a 2D resolution (spatial frequency response) target, and vertical and horizontal 8 mm linescales for navigation and coarse stage scale calibration.

Figure 1: Features on the NPL Areal Standard

The NPL Areal Standard is supplied in a clean-room compatible container with two deterministic pseudo-roughness (areal irregular) standards that may be used to validate overall instrument performance; a flash drive with calibration data and certificate is also included (Figure 2).

Figure 2. The NPL Areal Standard (left) packaged with two deterministic pseudo roughness artefacts (centre) 

NPL offers Good Practice Guides on the calibration of popular classes of areal surface texture instrument: coherence scanning and phase shifting interferometers (GPG127) and imaging confocal microscopes (GPG128).

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