Areal calibration set

Calibration of surface topography measuring instruments

This traceable high precision areal standard, is designed to calibrate the metrological characteristics of areal surface topography measuring instruments. The NPL areal standard enables users to:

• Adopt good measurement practice and meet the requirements of the forthcoming 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 unique combination of alignment markers and a comprehensive range of calibration features on a single plate
• Validate instrument performance using two complementary electroformed areal irregular pattern artefacts, AIR-B40 and AIR-B70.

NPL areal standard
The NPL areal standard is a 50 mm square toughened glass plate bearing a silicon wafer of 10 mm square in the middle of the plate. The silicon wafer contains geometrical patterns, designed for the calibration of areal surface topography measuring instruments, including vertical and lateral scale calibration, flatness deviation and testing the lateral resolution. The patterns on the NPL areal standard that are calibrated are listed in the table below.

Name	Description	Feature	Size
Type PGR	Groove, Rectangular (profile)	Step height	2 µm
			1 µm
			500 nm
			200 nm
			100 nm
			50 nm
Type ACG	Cross grating (areal)	Pitch	100 µm
			50 µm
			20 µm
Type AFL	Flat	Flatness	Sz <=30nm
Type ASG	Star shape grooves (areal)	Resolution	Range	Depth
			0.7 µm to 50 µm	200 nm
			0.7 µm to 25 µm	200 nm
			0.7 µm to 25 µm	50 nm
			0.2 µm to 6 µm	50 nm
Type AIR	Irregular (areal)	Sa, Sq, Sz, Ssk, Sku	40 µm autocorrelation length
			70 µm autocorrelation length

Influence parameters and metrological characteristics
The metrological characteristics of an areal surface texture instruments are shown in the table below. The NPL areal standard has been designed to directly address each of the metrological characteristics.

Metrological Characteristics	Symbol*	Main Potential Error along	Calibrated patterns that addresses characteristic
Amplification coefficient	ax , ay , az	x, y, z	Type PGR Type ACG
Linearity	lx , ly , lz	x, y, z
Perpendicularity	ΔPERxy	x, y
Flatness deviation	ZFLT	z	Type AFL
Measurement noise	NM	z
Lateral period limit	DLIM	z	Type ASG

*As per ISO DIS 25178-600:2016, Date: 20-02-2016

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