Sesión número 2

Advances in research have made possible the traceability of force at the macro to nano level. The twentieth century has seen a rapid increase in the demands of scientific study and a jump in the sensitivity of instruments to measure very small forces. For example, in bioscience the detachment force using a bio-membrane probe is measured in the ranges 10−2pN and 100pN [58]. Also the mechanical force realization based on flux quantization in the pico-Newton range is proposed by [59], [60]. More generally, the use of soft materials (polymers and cells etc.) in research or industry situation risks distortion or damage unless probe forces are carefully controlled. The twentieth century has also seen major growth in highly miniature systems and true micro electro mechanical systems (MEMS) involving a wider range of materials and manufacturing procedures, e.g., the need to monitor and control forces on gripper of micro-robots and other micro-mechanical processes). The sur- face characterisation tools needed for such applications and for high precision macroscopic products (optics and bearings, etc) therefore often involve smaller and smaller forces in order to deliver necessary performance; consider for example, the stylus profilometer, and micro CMM, micro and nano-hardness testers, micro- and nano-tribometers as well as probe microscopy mentioned in chapter 1. It might further be noted that pharmaceutical and some other sectors often rely on weighing very small doses of powder, with a need to control to below a milligram mass (∼10µN weight).

The scientific and technical situations just discussed to cover, perhaps, forces from a large fraction of a newton to nano-newton, with likely growth in the use of even smaller ones where at least pico-newton resolution is needed. This is clearly to wide to cover by one method and a series of techniques with overlapping ranges will be required. There then arise a major challenges over how traceable force metrology can be archived for them.

The newton derives in the SI system from the kilogram, which is extremely difficult to realise even in a best standards laboratories to precision much better than (1×10−9_{). 1µN}

relates to 0.1 mg (1×10−7 kg) which on interpolation would, therefore, only be traceable to at best 1%. Fortunately, nano-science and ultra-precision technology are providing

actively on them. Generally there are at least two stages involved. First, a high-specialised traceable secondary standard is needed, which focuses directly on small forces. Following this, there needs to be adequate means of transferring information from secondary stan- dards to working devices within laboratories and factories.

This thesis takes the assumption that one of the most important ranges of low-force measurement for the next few years covers broadly from 1µN to around 100 mN. It is chosen for several reasons. First, it is highly relevant to mechanical characterisation and micro- manipulation for already established, economically important industries. Second there is good evidence that NMIs will be able to provide reasonable traceable reference instruments for this vision. There is likely to be a stable market for simpler small instruments and transfer artefacts that at low cost can either fully calibrate or at least diagnose out-of- specification conditions on user instruments. Before proceeding to consider the latter, it is helpful to review the capabilities of typical NMIs and to consider the range of sensing principles that might be applied.

2.2.1 Traceability and fundamental force concepts in metrology

Traceability

“Traceability is defined as the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty” [61], [45].

or

“Traceability may also be defined as an unbroken record of documentation (documentation traceability) or an unbroken chain of measurements and associated uncertainties (metro- logical traceability)“ [1].

Measurements are made everywhere in the world, in houses, schools, colleges, universi- ties, industries and international laboratories. It is very difficult to find the exact relation- ship between the actual and measured value if the observations vary with time, since this may perturb the reference scale. The variation in the measurement processes are controlled by repeating the observations made during precision measurements of any parameter, but

rare changes exist that were found to be identical under the same conditions. The vari- ation in the measurement process occurs, which may be caused by many factors, such as standards, work pieces, instruments, persons and procedures and environment [62]. The methods to estimate the uncertainty in measurement should be need rigorously. The ap- proach to uncertainty of fundamental concept of traceability described in [63], [64], [65] is highly encouraged and considered to be essential in the engineering experiments. As an example figure 2.1, considered here deliberately from outside our immediate context that shows a common path for a calibrated thermometer [1]. The other example is measurement of the surface profile using stylus instrument. The stylus instruments are commonly used for a measurement of topography by measuring the displacement of stylus as it traverses the surface [45].

Fundamental force concept in conventional metrology

Force is a derived unit in the SI, which means that primary standards of the force are derived from the fundamental definition of force by using three basic units kilogram (kg), mass (m), and second (s). One newton is defined as the force required to accelerate a mass of one kg at a rate of one meter-(sec)−2. 1kg is defined as the mass of primary kilogram, a platinum-iridium cylinder stored in Paris. The weight of a body is a gravitational force acting on a body and measured in kg as mass [66]. Conventionally, the force is measured by using strain gauges, load cells, resonance structured, electric balances, force transducers, piezoelectric crystal and pressure. For a known acceleration due to a gravity, the downward force generated by the earth field can be calculated. This is the basic principle that works behind the deadweight standards machines [67], [68].

The forces on the micro and nanotechnology scale are measured by using different prin- ciples as compared to measurement of macro scale forces. The uncertainty increases of the scale of masses are reduced for deadweight methods. A mass of 1kg may be measured with a standard uncertainty of 1µg (1 part in 109) [45]. Small masses (0.5mg) are calibrated at

NIST. The handling of such small masses becomes difficult and their relative uncertainties increases inversely proportional to the decrease in mass [69].

2.2.2 Low force traceability in metrology

The low force measurement is becoming essential in the national metrology institutes (NMI). In order to meet the demand of traceable micro to nano-newton force, NMIs worldwide are working together to extend the range of traceable force measurement further down to the nano-Newton level [70], [71], [72], [29], [73], [74], [47] . These NMIs have developed their own small force facilities based own designs and operating principles. Each NMI has realised diverse paths of calibration for small force facilities for primary realisation and dissemination routes. A variety of artefacts and methods have been developed by them.

Figure 2.1: A common traceability path(from [1]) .

The small force standards are still in a nascent stage. First an informal study pi- lot comparison program was organized by the NMIs, who have small force facility and standards. The comparison study program among four NMIs, KRISS, NIST, NPL and PTB [75]. The primary realisation of a small force in the pilot study program is di- vided into two types: electrical force-based and mass-based methods. Both NPL and NIST facilities realise traceable forces based on electrostatic methods through electro- static balance principle and traceability is derived from international system of units SI (i.e. meter, capacitance & voltage, while KRISS and PTB primary standards are realised based on deadweight principles by using high-precision mass comparators. The calibration procedures employed by them are not standardised yet and deviate from those used in the macro-force metrology such as standardised procedure described in ISO 376 [76].

In this pilot study comparison programm, it was decided by consensus of the participants not to adhere to the existing macro-force protocols. They agreed that comparison would be performed for the measurement of spring constant and force sensitivity for each of a set of five piezoelectric cantilevers. The pilot study comparison programm was conducted among NMIs from February 2008 to February 2010. The results of calibration capability are in agreement, suggesting that their small force facility are equivalent within their reported uncertainties. The detail of the reported uncertainties may be seen [77]. It was concluded by the authors that for future comparison a more rigorous technical protocol should be developed and adopted.