Christopher W Jones

Report on the first international comparison of small force facilities: a pilot study at the micronewton level

Measurements of forces less than a micronewton are critical when examining the mechanical behaviour of materials and devices at characteristic length scales below a micrometre. As a result, specification standards for nanomechanical tests and test equipment are being proposed by international standards organizations, and an infrastructure for traceable small force calibration is developing. In this context, results are reported from the first interlaboratory comparison of micronewton-level force metrology. The basis of the comparison was the calibration of a set of five piezoresistive cantilever force sensors similar to those used for atomic force microscopes but employed here as transfer artefacts. The artefacts were circulated among four national metrology institutes with each using their own force balance to calibrate the stiffness (force change per unit displacement) and sensitivity (signal output change per unit force) of the artefacts. By considering the weighted mean of the stiffness and sensitivity values reported for a given artefact, reference values were obtained. The largest contributing uncertainty components were due to the transfer artefacts themselves, rather than from the measurements of the physical quantities of force, voltage and displacement. The results imply that it should be possible to determine cantilever stiffness using force balance techniques with an accuracy of better than 1% if necessary, but that improvements in the ability to orient the transfer artefacts, to characterize the non-linearity of their output, and to compensate for the stiffness of the associated fixtures and load frames are required if the resolution of future comparisons is to improve.

Advances in engineering nanometrology at the National Physical Laboratory

The National Physical Laboratory, UK, has been active in the field of engineering nanometrology for a number of years. A summary of progress over the last five years is presented in this paper and the following research projects discussed in detail. (1) Development of an infrastructure for the calibration of instruments for measuring areal surface topography, along with the development of areal software measurement standards. This work comprises the use of the optical transfer function and a technique for the simultaneous measurement of topography and the phase change on reflection, allowing composite materials to be measured. (2) Development of a vibrating micro-CMM probe with isotropic probing reaction and the ability to operate in a non-contact mode. (3) A review of x-ray computed tomography and its use in dimensional metrology. (4) The further development of a metrology infrastructure for atomic force microscopy and the development of an instrument for the measurement of the effect of the probe?surface interaction. (5) Traceable measurement of displacement using optical and x-ray interferometry to picometre accuracy. (6) Development of an infrastructure for low-force metrology, including the development of appropriate transfer artefacts.

Open questions in surface topography measurement: a roadmap

Control of surface topography has always been of vital importance for manufacturing and many other engineering and scientific disciplines. However, despite over one hundred years of quantitative surface topography measurement, there are still many open questions. At the top of the list of questions is ?Are we getting the right answer?? This begs the obvious question ?How would we know?? There are many other questions relating to applications, the appropriateness of a technique for a given scenario, or the relationship between a particular analysis and the function of the surface. In this first ?open questions? article we have gathered together some experts in surface topography measurement and asked them to address timely, unresolved questions about the subject. We hope that their responses will go some way to answer these questions, address areas where further research is required, and look at the future of the subject. The first section ?Spatial content characterization for precision surfaces? addresses the need to characterise the spatial content of precision surfaces. Whilst we have been manufacturing optics for centuries, there still isn?t a consensus on how to specify the surface for manufacture. The most common three methods for spatial characterisation are reviewed and compared, and the need for further work on quantifying measurement uncertainties is highlighted. The article is focussed on optical surfaces, but the ideas are more pervasive. Different communities refer to ?figure, mid-spatial frequencies, and finish? and ?form, waviness, and roughness?, but the mathematics are identical. The second section ?Light scattering methods? is focussed on light scattering techniques; an important topic with in-line metrology becoming essential in many manufacturing scenarios. The potential of scattering methods has long been recognized; in the ?smooth surface limit? functionally significant relationships can be derived from first principles for statistically stationary, random surfaces. For rougher surfaces, correlations can be found experimentally for specific manufacturing processes. Improvements in computational methods encourage us to revisit light scattering as a powerful and versatile tool to investigate surface and thin film topographies, potentially providing information on both topography and defects over large areas at high speed. Future scattering techniques will be applied for complex film systems and for sub-surface damage measurement, but more research is required to quantify and standardise such measurements. A fundamental limitation of all topography measurement systems is their finite spatial bandwidth, which limits the slopes that they can detect. The third section ?Optical measurements of surfaces containing high slope angles? discusses this limitation and potential methods to overcome it. In some cases, a rough surface can allow measurement of slopes outside the classical optics limit, but more research is needed to fully understand this process. The last section ?What are the challenges for high dynamic range surface measurement?? presents the challenge facing metrologists by the use of surfaces that need measurement systems with very high spatial and temporal bandwidths, for example, those found in roll-to-roll manufacturing. High resolution, large areas and fast measurement times are needed, and these needs are unlikely to be fulfilled by developing a single all-purpose instrument. A toolbox of techniques needs to be developed which can be applied for any specific manufacturing scenario. The functional significance of surface topography has been known for centuries. Mirrors are smooth. Sliding behaviour depends on roughness. We have been measuring surfaces for centuries, but we still face many challenges. New manufacturing paradigms suggest that we need to make rapid measurements online that relate to the functional performance of the surface. This first ?open questions? collection addresses a subset of the challenges facing the surface metrology community. There are many more challenges which we would like to address in future ?open questions? articles. We welcome your feedback and your suggestions.

Adding a dynamic aspect to amplitude-wavelength space

Amplitude–wavelength (AW) space is a well-established technique for depicting the operational performance of surface texture measuring instruments. The introduction of a third parameter to AW space, the in-plane scanning speed, is described. We present a discussion of the equations derived from operational constraints and parameter interdependencies for a scanning stylus instrument: the National Physical Laboratorys Areal Instrument. Constraints derive from instrument range, resolution, range of measurement force, probe tip geometry and the synchronization of data acquisition. The equations combine to form a polygon in amplitude–wavelength–scanning speed space that defines the operating region of the instrument.

Aperiodic interferometer for six degrees of freedom position measurement

We present a new class of interferometer system that is capable of simultaneous measurement of absolute position and rotation in all six degrees of freedom (DOF) with nanometer precision. This novel capability is due to the employment of a system of interference fringes that is not periodic. One of the key strengths offered by this new approach is that the absolute position of the system can be determined with a single measurement, rather than by counting fringes during displacement from a known location. The availability of a simultaneous measurement of all six DOF eliminates many problems associated with conventional interferometry.

Towards a Traceable Infrastructure for Low Force Measurements

Over the past ten years or so the need for the measurement of low forces ranging from newtons down to attonewtons has become increasingly important. As we begin to manufacture and manipulate structures on the micrometre to nanometre scale, the forces that are exerted in such processes must be controlled. To control such forces requires some form of measurement, either a direct measurement of the force, or a measurement of the effect the force has on the structure it is applied to. This paper is primarily concerned with the development of a traceability infrastructure for forces in the range from 1 nN to 10 μN. The lower end of this force range does not cover chemical or most biological forces (usually in the femtoto piconewton range) despite the increasing importance of accurately measuring such forces. Further work is still required to push the limits of force traceability to these levels. At the upper end of the force range considered here, more traditional methods for measuring forces can be used that are traceable to the unit of mass, i.e. the force is realised as a mass in a gravitational field. The force range discussed in this paper applies to many nano-and micrometre scale manipulation and assembly applications, including micro-grippers, handlers and force feedback devices. Further applications that fall into the force range discussed here include the force exerted on a surface by atomic force microscopes and other scanning probe instruments, forces in the area of materials property measurement using indentation technology, the forces found in micro-electromechnical systems (MEMS) and the forces exerted by artificial biological tissues, for example muscle fibres. The two main force generation mechanisms that are found in nature and engineering are the weight of the mass of an object in a gravitational field and the deflection of an element with a finite spring constant. On the microto nanometre scale the spring force is more usually used to produce or react to a force, for example an AFM cantilever.

Dimensional nanometrology at the National Physical Laboratory

The growth in nanotechnology has led to an increased requirement for traceable dimensional measurements of nanometre-sized objects and micrometre-sized objects with nanometre tolerances. To meet this challenge NPL has developed both purpose built instrumentation and added metrology to commercially available equipment. This paper describes the development and use of a selection of these instruments that include: atomic force microscopy, x-ray interferometry, a low force balance, a micro coordinate measuring machine and an areal surface texture measuring instrument.