QMT Features: November 2013
Thinking outside the Bento Box
NPL develops a new set of calibration artifacts for areal surface topography measurement. By Richard Leach, Engineering Measurement, National Physical Laboratory, UK

The world is currently on the cusp of a revolution in the way surfaces are used in manufactured products. Previously, stochastic and random surfaces, or the machining marks left by the manufacturing process, were most often used to impart functionality into the surface. More recently, deterministic patterning is being used to critically control the function of a surface. This deterministic method is the way the process of evolution has led to the creation of various functional surfaces – oft-cited examples are the lotus leaf, shark skin or eye of a fly.

Examples from advanced manufacturing industry include the use of laser dimpling to reduce friction in bearings, high aspect ratio features to control the wetting characteristics of glasses, patterned topographies to enhance the adhesion of biological molecules, and so on – the list grows daily. To take advantage of the multitude of controllable functions offered by the use of structured surfaces, a measurement infrastructure is required. The latest advances made by the National Physical Laboratory (NPL) to put such a measurement infrastructure into place are described here.

The measurement and characterisation of surfaces using the profile method has been used in manufacturing industry for over a century. However, whereas the profile method may be useful for showing manufacturing process change, much more functional information about the surface can be gained from an analysis of the areal surface topography. The main instruments used to measure areal surface topography are either contact (stylus) based or non-contact (optical) based. For good reasons, stylus instruments are often considered “reference” instruments and can achieve nanometre resolution for height measurements. However, stylus instruments require physical contact with the surface being measured, are band-limited due to their finite tip geometry and can be very slow when measuring areal surface topography. Therefore, optical instruments are becoming more popular and can overcome many of the limitations of stylus instruments.

There is now a large range of commercially available optical instruments designed for surface topography measurement, for example coherence scanning interferometers (CSIs – often described as white light interferometers), confocal microscopes and focus variation microscopes. Optical instruments are also band-limited and suffer from what are termed “optical artifacts” that cause them to be considered less accurate than stylus instruments. The new calibration techniques that are reported here go some way to addressing the differences between the contact and non-contact instruments, and allow instrument users to have a high degree of confidence when making surface topography measurements.

Traceability is one of the most fundamental concepts in metrology and is a fundamental basis required by all measurements that claim to be accurate. Traceability is defined in international guidance documents as the following:
Traceability is the property of the result of a measurement whereby it can be related to stated references, usually national or international standards, through a documented unbroken chain of comparisons all having stated uncertainties.

To give an example, consider the measurement of surface profile using a stylus instrument. A basic stylus instrument measures the topography of a surface by measuring the displacement of a stylus as it traverses the surface. It is important to ensure that the displacement measurement is “correct”. To ensure correctness, the displacement measuring system must be compared, or calibrated, against a more accurate displacement measuring system. This calibration can be carried out by measuring a range of calibrated step height artifacts (known as transfer artifacts).

Assume the more accurate instrument measures the height of the step using an optical interferometer with a laser source. The frequency of this laser source is calibrated against the frequency of the iodine-stabilised laser that realises the definition of the metre and an unbroken chain of comparisons has been assured. Moving down the chain from the definition of the metre to the stylus instrument that is being calibrated, the accuracy of the measurements usually decreases.

Traceability helps to ensure that measurements are consistent and accurate. Any quality system in manufacturing will require that all measurements are traceable and that there is documented evidence of this traceability. If component parts of a product are to be made by different companies (or different parts of an organisation) it is essential that measurements are traceable so that the components can be assembled and integrated into a product.

Whilst there has been traceability for surface profile measurements for some time, traceability for areal surface topography measurements is only just coming to fruition. Figure 1 shows the steps necessary for an areal surface topography traceability infrastructure. Firstly, it was necessary to develop a primary instrument with direct traceability to the realisation of the metre. NPL has developed a stylus instrument that uses laser interferometers to determine the position of the stylus tip; the interferometers are traceable to the metre via their laser sources. The primary instrument is then used to calibrate transfer artifacts which can be used to calibrate commercial instruments.

Before discussing the transfer artifacts, it is advantageous to look at the current state of play with international specification standards for areal surface topography. In 2002, ISO technical committee 213 formed working group (WG) 16 to address standardisation of areal surface texture measurement methods. WG 16 is developing a number of draft standards encompassing definitions of terms and parameters, calibration methods, file formats and characteristics of instruments.

Several of these standards have now been published and a number are at various stages in the review and approval process (readers are encouraged to comment on the public drafts). All the areal standards are part of ISO 25178, which will consist of at least the parts shown in table 1 (correct at the time of publication), under the general title Geometrical product specification (GPS) — Surface texture: Areal.

Only the standards relating to calibration will be discussed here. Part 70 of ISO 25178 describes the artifacts that are used to calibrate areal surface topography measuring instruments and includes the profile calibration artifacts from ISO 5436 part 1 (2000), but with new names. There are four part 60X standards that have been published: part 601 (stylus instruments), part 602 (confocal chromatic probes), part 603 (phase stepping interferometers) and part 604 (CSI). At the time of writing, parts 605, 606 and 607 are drafts at the working stage. The 60X standards currently contain common terminology, metrological characteristics (see below) and a list of parameters that can influence the uncertainties when using the instrument. The standards also contain technical annexes that discuss the theory and operation of the instruments. However, as the 60X series developed, it was realised that there are a large number of sections in the various parts of the 60X series that are common to all instruments based on a microscope objective. For example, research has shown that a common set of metrological characteristics can be found that does not differ for each instrument type. Therefore, a new standard is under development (part 600), which will cover all the common aspects. Once part 600 is published, the 60X series will be withdrawn and re-issued with the common sections removed. Part 701 is concerned with the calibration of stylus instruments. Part 700, which is still under development, will cover the calibration of surface topography measuring instruments and is expected to be common across all instruments types. Once part 700 is published, part 701 will be withdrawn.

The philosophy being developed by the ISO committee to deal with the calibration of areal instruments is as follows. The calibration process for a surface topography measuring instrument should involve the determination of the characteristics of the scales of the instrument, and a determination of the instrument’s spatial frequency response, i.e. how the instrument will respond to complex surface features. Often the latter part of this process is overlooked and an instrument is considered “calibrated” if only the characteristics of the scales have been determined. With this limited calibration, it is then perfectly feasible to use the instrument to measure linear characteristics, for example, step height or lateral spacing, but not for the measurement of a complex surface, where the ability to measure slopes (and curvature) needs to be determined. At this stage, there is still some research to be carried out on how to determine the spatial frequency response of an instrument.

Therefore, the draft ISO standards being developed do not cover this aspect in detail. Once the members of the ISO committee are in consensus that methods to determine an instrument’s spatial frequency response have been developed that are robust and universally accepted, then new ISO specification standards will be developed.

An areal surface topography measuring instrument provides a three-dimensional map of a surface. The three-dimensional map is made up of a set of points measured with respect to three orthogonal length scales. The scales of an areal surface topography measuring instrument are nominally aligned to the axes of a Cartesian coordinate system. The axes are physically realised by various components that are part of the metrological loop of the instrument. Hence, the quality and the mutual position of these components partially confer the quality of the coordinate measurements. The coordinate measurements produced by areal surface topography measuring instruments are also affected by other influence factors, such as ambient temperature, mechanical noise and electrical noise. The effect of a single influence factor, or a combination of influence factors, on the quality of the areal measurements are quantified by experimentally determining the metrological characteristics of the instrument. In ISO/WD 25178 part 600, these characteristics include the noise of the instrument; the linearity, amplification and resolution of the scales; the deviation from flatness of the areal reference and the squareness of the axes.

NPL has recently developed a series of artifacts that can be used to determine the above metrological characteristics. These artifacts include step heights, lateral grids, star-shapes and an optical flat for calibration; and irregular artifacts that can be used to verify the instrument’s performance. Whilst complex machining methods have been used to manufacture the artifacts, replication techniques have then been used to ensure the cost-effectiveness of artifacts for sale. The full set of artifacts, known affectionately as the Bento Box, is now commercially available along with a set of free good practice guides that describe the calibration process in simple yet detailed terms. Figure 2 shows the full set of artifacts. The Bento Box artifacts are calibrated at NPL using traceable instrumentation and analysis software is freely available.

With the new calibration methods developed at NPL, it is possible to determine the characteristics of the instrument scales. Whilst this calibration method is a step in the right direction, the method does not allow calibration of the instrument to be used to measure a complex surface – for this the ability of the instrument to measure slopes (and curvature) on the surface must be characterised. Any surface has a finite spatial frequency bandwidth, that is, it can be represented as a series of sinusoidal profiles with given amplitudes and wavelengths that are simply added together to produce the surface. In this way, a simple surface could have a single frequency and amplitude (a sine wave) and a step height would comprise an infinite number of sine waves.

An instrument will have a finite spatial frequency response, that is to say it will transmit some spatial frequency components from the surface, it will block some components and others will be attenuated. If it is assumed that this process is linear, then the instrument simply acts as a linear filter with a specific transmission characteristic. To calibrate how the instrument responds to surfaces, the transmission characteristic needs to be determined. NPL and Loughborough University are developing a framework for measuring the spatial frequency response of areal surface topography measuring instruments by determining what is known as the transfer function of the instruments. With a knowledge of the transfer function, it is possible to correct many of the “optical artifacts” that cause discrepancies between different instrument types, therefore, allowing meaningful instrument comparisons to be made. Good progress has been made so far with interferometric instruments and progress with other types of instruments will follow in due course. Preliminary discussions are underway in the working group to incorporate transfer function techniques into the specification standards.

We have come a long way since the early days of surface topography measurement – there are now a vast, often bewildering, number of measurement and characterisation methods available. NPL, and other members of the ISO 213 community, have now taken the first steps in developing the necessary infrastructure to allow traceability and calibration of these methods. These steps will accelerate the use of complex surfaces in a range of high-tech, consumer and industrial products that will significantly improve their performance, efficiency and functionality. Nature has been taking advantage of the use of complex surface structuring for millions of years, now it is our turn..l
email: richard.leach@npl.co.uk
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