MICRO METROLOGY | ARTICLE
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 artefacts that can be used to determine the above metrological characteristics. These artefacts include step heights, lateral grids, star-shapes and an optical flat for calibration; and irregular artefacts that can be used to verify the instrument’s performance. Whilst complex machining methods have been used to manufacture the artefacts, replication techniques have then been used to ensure the cost-effectiveness of artefacts for sale. The full set of artefacts, 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 artefacts. The Bento Box artefacts 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.
<< Figure 2: The NPL areal calibration artefacts. >>
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 artefacts’ 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.
National Physical Laboratory
www.npl.co.uk/nanometrology
23 | commercial micro manufacturing international Vol 7 No.1
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52