MICRO METROLOGY | ARTICLE
THINKING OUTSIDE THE BENTO BOX NPL develops a new set of calibration artefacts for areal surface topography measurement
Richard Leach | Engineering Measurement, National Physical Laboratory
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.
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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 artefacts’ 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.
he 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
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 artefacts (known as transfer artefacts). 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 artefacts
20 | commercial micro manufacturing international Vol 7 No.1
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