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ISO 25178-700: Calibration, Adjustment and Verification of Areal Topography Measuring Instruments
Geometrical Product Specifications (GPS) — Surface Texture: Areal — Part 700
1. Calibration Framework for Areal Topography Instruments
ISO 25178-700:2022 specifies generic procedures for the calibration, adjustment, and verification of metrological characteristics common to all areal topography measuring instruments. This standard bridges the gap between the metrological characteristics defined in ISO 25178-600 and practical calibration procedures using traceable material measures. In the GPS concept, design values and tolerances on workpieces are compared with measurement results and their associated uncertainties, making instrument calibration essential for reliable geometrical product specification and verification. The default procedures ensure traceability to the meter through national metrology institutes or qualified laboratories.
The standard defines three distinct activities. Calibration determines metrological characteristic values using material measures traceable to the meter. Adjustment modifies the instrument to bring its characteristics within specification. Verification confirms that the specified requirements are met, optionally after adjustment. These three activities form a comprehensive quality assurance framework for surface topography measurements. The calibration of metrological characteristics enables verification of instrument specifications and comparison of systems from different manufacturers that may use different measurement principles. The complete evaluation of measurement uncertainty may include additional factors beyond instrument calibration, such as operator variability, changing environmental influences, and thermal and mechanical stresses on the sample.
Calibration determines metrological characteristics using material measures traceable to the meter. Adjustment modifies the instrument to meet specifications. Verification confirms specifications are met. All three are part of comprehensive quality assurance for surface topography measurements. The standard defines default methods when no other calibration procedures have been specified.
| Metrological Characteristic | Calibration Section | Default Material Measure | Assessed Parameter |
|---|---|---|---|
| Measurement noise NM | 6.5 | Smooth, flat surface | Sq of difference map divided by sqrt(2) |
| Flatness deviation zFLT | 6.6 | Optical flat (type AFL) | Sz (peak-to-valley) |
| z-amplification coefficient | 6.7 | Step height (type PGR) | Groove depth d |
| z-linearity deviation lz | 6.8 | Multi-step material measure | Max deviation from ideal |
| x,y amplification and mapping | 6.9 | Cross grid (type ACG) | Grid coordinate differences |
2. Detailed Calibration Procedures
The default method for measurement noise determination is the subtraction method: measure the same location twice under identical conditions, subtract the two topographies, and calculate the root-mean-square height Sq of the difference topography divided by the square root of 2. This approach isolates the random noise component by canceling the actual surface topography. For a stabilized noise estimation, at least three repeated measurements are recommended, with the averaging method providing improved statistical confidence. The standard carefully distinguishes between instrument noise (minimum noise under ideal conditions) and measurement noise (noise under actual working conditions including environmental contributions). The material measure for noise estimation is a smooth, flat surface such as an optical flat that does not need to be calibrated for this purpose.
Flatness deviation calibration uses an optical flat (type AFL) as the reference surface. The topography measured on the optical flat is evaluated after removing a least-squares plane, with the Sz parameter (peak-to-valley) as the assessed metric. Improved estimation is achieved by measuring at multiple lateral locations on the optical flat and averaging the results, which helps distinguish instrument flatness from imperfections in the optical flat itself. For z-axis amplification, type PGR material measures with calibrated groove depths are recommended. These are rectangular or trapezoidal groove profiles with certified depth values traceable to national standards. Measurements at multiple z-positions across the instrument range enable simultaneous calibration of both the amplification coefficient and linearity deviation, providing a comprehensive assessment of z-axis metrological performance.
Non-measured points and spurious data must be handled carefully during calibration. By default, no interpolation or filling of non-measured points is applied. Spurious data points that deviate significantly from reasonable values based on a priori knowledge should be removed from the region of interest. These procedures ensure that calibration results reflect true instrument performance rather than artifacts of data processing.
3. Advanced Characteristics and Practical Considerations
Topographic spatial resolution (WR) lacks a single default calibration method due to its multidimensional nature. The standard recommends several complementary approaches. ITF (instrument transfer function) measurement uses chirped or star-shaped groove material measures (type ASG) to characterize the instrument response as a function of spatial frequency. The lateral period limit (DLIM) is determined from the 50% transmission point of the ITF curve, representing the finest spatial period that the instrument can faithfully reproduce. For optical instruments, the standard allows the use of Rayleigh, Sparrow, or Abbe resolution criteria as lateral resolution parameters when appropriate.
Topography fidelity (TFI) serves as the comprehensive characteristic that captures all remaining measurement errors after the other six characteristics have been calibrated and accounted for. It includes slope-dependent effects, edge response characteristics, and material property influences that are not captured by the other individual characteristics. The small-scale fidelity limit (TFIL) quantifies the smallest lateral feature that can be measured with a deviation under specified amounts, typically 10%. Perpendicularity of the instrument z-axis with respect to the x-y reference surface is also addressed, as it affects the accurate measurement of vertical features. For a practical calibration strategy, engineers should characterize measurement noise first, then calibrate flatness deviation using an optical flat, determine z-amplification and linearity with certified step-height standards, calibrate x-y mapping using cross-grid artifacts, and finally assess topography fidelity for the specific application. All measurement conditions including temperature, humidity, scan speed, and filter settings should be documented.
Practical calibration strategy: (1) characterize measurement noise first, (2) calibrate flatness deviation using an optical flat, (3) determine z-amplification and linearity with certified step-height standards, (4) calibrate x-y mapping using a cross-grid artifact, (5) assess topography fidelity for the specific application. Always document temperature, humidity, scan speed, and filter settings for complete measurement traceability.
4. Frequently Asked Questions
Q: What is the default method for measuring instrument noise?
A: The subtraction method: measure the same location twice under identical conditions, subtract the two topographies, calculate the RMS height Sq of the resulting difference map, and divide by the square root of 2 to isolate the random noise component.
A: The subtraction method: measure the same location twice under identical conditions, subtract the two topographies, calculate the RMS height Sq of the resulting difference map, and divide by the square root of 2 to isolate the random noise component.
Q: What material measures are needed for z-axis calibration?
A: Type PGR material measures with calibrated groove depths traceable to the meter. Multi-step measures covering the full z-range of the instrument are preferred for comprehensive calibration of both amplification and linearity.
A: Type PGR material measures with calibrated groove depths traceable to the meter. Multi-step measures covering the full z-range of the instrument are preferred for comprehensive calibration of both amplification and linearity.
Q: How is flatness deviation calibrated?
A: Using an optical flat (type AFL). The Sz parameter (peak-to-valley) is calculated after removing a least-squares plane from the measured topography. Multi-location averaging across the flat improves estimation reliability.
A: Using an optical flat (type AFL). The Sz parameter (peak-to-valley) is calculated after removing a least-squares plane from the measured topography. Multi-location averaging across the flat improves estimation reliability.
Q: What is the difference between calibration, adjustment, and verification?
A: Calibration determines characteristic values using traceable standards. Adjustment modifies the instrument to meet specifications. Verification confirms that the specified requirements are satisfied, optionally after adjustment has been performed.
A: Calibration determines characteristic values using traceable standards. Adjustment modifies the instrument to meet specifications. Verification confirms that the specified requirements are satisfied, optionally after adjustment has been performed.
