Принципы нанометрологии (1027623), страница 61
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The green arms connect the x and y axes tothe machine and also hold them orthogonal to the machine.Rather than moving orthogonally and independently of each other, as is thecase for most CMMs, the x and y axes are connected together at right anglesand move as a single unit.
This acts to increase the stiffness and accuracy ofthe machine. The extensive use of high-quality air bearings to support the xyframe and a large granite base also help to increase the stability of the system.FIGURE 9.4 Schema of the kinematic design of the Zeiss F25 CMM.273274C H A P T ER 9 : Coordinate metrologyDuring the redesign process of the original TUE machine, Zeiss changedmany of the component parts so they became serviceable, and addeda controller and software. The other main additions to the redesign wereaimed at increasing the overall stiffness of the system, and included theaddition of high-quality air bearings and a general increase in mass of allthe major components.The F25 is subject to only thirteen geometric errors and has minimalAbbe error in the horizontal mid-plane.
The measurement capacity is 100 mmby 100 mm by 100 mm. The resolution on the glass-ceramic linescales on allmeasurement axes is 7.8 nm and the quoted volumetric measurementaccuracy is 250 nm. The F25 has a tactile probe based on silicon membranetechnology (see section 9.5) with a minimum commercially available stylustip diameter of 0.125 mm.The F25 also includes a camera sensor with an objective lens that is usedto make optical 2D measurements.
The optics are optimized to exhibit a highdepth of field and low distortion. The whole system allows measurements tobe taken from the optical sensors and the tactile probe whilst using the sameprogrammed coordinate system. A second camera is used to aid observationof the probe during manual measurement and programming.9.4.1.2 A laser interferometer-based miniature CMMThe Nanomeasuring Machine (NMM) was developed by the IlmenauUniversity of Technology [28,29] and is manufactured by SIOS MesstechnikGmbH.
The device implements sample scanning over a range of 25 mm by25 mm by 5 mm with a resolution of 0.1 nm. The measurement uncertaintyis 3 nm to 5 nm and the repeatability is 1 nm to 2 nm. Figure 9.5 illustratesthe configuration of a NMM, which consists of the following maincomponents:-traceable linear and angular measurement instruments;-a 3D nanopositioning stage;-probes suitable for integration into the NMM;-control equipment.Both the metrology frame, which carries the measuring systems (interferometers), and the 3D stage are arranged on a granite base. The upperZerodur plate (not shown in Figure 9.5) of the metrological frame is constructed such that various probes can be installed and removed.
A cornermirror is moved by the 3D stage, which is built in a stacked arrangement.The separate stages consist of ball-bearing guides and voice coil drives. TheMiniature CMM probesFIGURE 9.5 Schema of the NMM.corner mirror is measured and controlled by single, double and triple beamplane mirror interferometers that are used to measure and control the sixdegrees of freedom of the 3D stage.The three laser interferometer measuring beams are reflected from theouter surfaces of the corner mirror, whereby the virtual extension of thereflected beams intersect at the point of contact between the specimen andthe sensor (see Figure 9.6).
Because the sample, as opposed to the probe, isscanned in the NMM, the Abbe principle is realised over the entiremeasuring range. Angular deviations of the guide systems are detected at thecorner mirror by means of a double and a triple beam plane mirror interferometer. The detected angular deviations are compensated by a closed-loopcontrol system. The NMM can be used with a range of probes, including bothtactile and optical probes.9.5 Miniature CMM probesMany research groups have developed miniature CMM probes and a selectfew probes are now available commercially (see [30] for a review of highaccuracy CMM probes that includes miniature probes). Whilst sometimesreferred to as ‘micro-CMMs’, most miniature CMMs usually have a standardprobe tip of diameter 0.3 mm (although tips with a diameter of 0.125 mm are275276C H A P T ER 9 : Coordinate metrologyFIGURE 9.6 Schema of the NMM measurement coordinate measuring principle.readily available).
This is far too large to measure a typical MEMS structure,for example a deep hole or steep DRIE trench. What are required are smaller,micrometre-scale probe tips that measure in 3D. This is not simply a matterof scaling the size of the probe in direct analogy with probes on conventionalCMMs. CMM probe heads that have been simply scaled down in size haveachieved measurement uncertainties of 50 nm [25].
They have beenmeticulously designed to reduce the probing force and ensure equal probingforces in each measurement axis. However, even with extensive redesign,these probes tend to have an overall mass of several grams. With stylus tipdiameters needing to be sub-millimetre these probes are quite destructive atany probing force above 1 mN [31].Addressing the problem of contact force reduction is one major area ofdevelopment in micro-scale probe design and manufacture as it is one of severalpotential sources of error that, on the scale where micro-scale probes will beoperating, is of the same order of magnitude as the desired probing accuracy.The pressure field generated at the surface when a miniature tip comes intocontact may be sufficient to cause plastic deformation [32].
Reducing thecontact force during measurement will greatly reduce the possible damagecaused and also increase the accuracy of the measurement. Monitoring ofthe tunnelling current between the probe tip and the sample being measuredhas been proposed to avoid physical contact with the surface [33].In an attempt to reduce the probing force, silicon flexures, membranes ormeshes are used to suspend the probe shaft. Using methods for chemicalMiniature CMM probesetching and vapour deposition developed by the IC industry, highly complexprobes can be made consisting of multiple layers of electrical connections,strain gauges, flexures, meshes or membranes.
As well as reducing the overallcontact force exerted on the measurement surface, using silicon to suspendthe microprobe also serves to make surface contact detection more sensitive.As the stem diameter gets smaller its compliance increases and it becomesmore difficult to sense a deflection of the probe using conventional elastichinges.
These methods have been demonstrated as viable for micro-scaleprobe production and at the Eindhoven University of Technology (TUE) theyhave produced a probe with a measurement uncertainty of 30 nm [34]. Usingthese production methods presents major design challenges and has radicallychanged what we perceive to be a CMM probe (as seen in Figure 9.7).
However,because of the prolific use of these etching methods, large-scale production ofthese probes can easily be realised, substantially reducing costs.Research at PTB has developed a highly accurate silicon micro-scaleprobe that is designed around a silicon membrane onto which a micro-stylusis attached [35] (a similar probe was also developed elsewhere [36]). Both theTUE and the PTB probes are instrumented with piezoelectric strain sensorsthat have been etched onto the silicon suspension membrane.
These willdetect when the probe makes contact with the measurement surface byproducing a voltage signal when membrane deformation occurs.FIGURE 9.7 Silicon micro-scale probe designed by [34], produced by chemicaletching and vapour deposition.277278C H A P T ER 9 : Coordinate metrologyWhen probes are refined to operate at even lower probing forces theirsilicon membranes or flexures can have the adverse effect of giving falsereadings due to inertia.
This means that the probes must be moved at veryslow speeds, which slows the measurement process. Membrane probes arealso unable to exert similar forces in all measurement axes, reducing theiraccuracy as true three-dimensional probes [37].In a further attempt to reduce the surface damage caused by probeinteractions, probes have been developed in both PTB [38] and NIST [39]that take optical measurements from illuminated glass fibres. One suchdesign is shown in Figure 9.8. The operating principle of fibre probe systemsis surprisingly simple.
The contact element of the probe is formed bya microsphere that is attached to the tip of a single optical fibre. An opticalsystem is then focused on the microsphere or the shaft of the glass fibre.Analysing the movement of either identifies contact with a measurementsurface. This results in a probe with a measurement force of the order of a fewhundred nanonewtons and a very high aspect ratio.
Using a probe witha diameter of 75 mm and a length of 50 mm NIST has been able to investigatethe nozzles of fuel injectors and the diameters of optical ferrules to anuncertainty of 70 nm [39].FIGURE 9.8 The fibre probe developed by PTB. Notice the second microsphere onthe shaft of the fibre; this gives accurate measurement of variations in sample ‘height’(z axis) [38].Miniature CMM probesEven though reducing the probing interaction force reduces the problemof plastic deformation of the sample, it does not address the problems due tosurface forces. These surface forces, including electrostatic, van der Waalsand the resulting interactions due to liquid films (see section 7.3.7), couldconceivably cause a false trigger in a low-force probe. These forces also havethe effect of producing ‘snap back’ on low-force probes when they retract fromthe measurement surface.
Once the probe tip has come into contact with themeasurement surface, regardless of possible initial attraction or false triggering, the surface forces will tend to hold the probe head on the surface, evenwhile the CMM head is retracting. The result is that the probe will ‘snapback’ from the measurement surface, a movement that could cause seriousdamage to the probe. The surface forces also cause a stick-slip phenomenonthat can seriously reduce the speed of measurement [40].Both measurement force and surface force problems have been addressedby the development of vibrating probes, whose basic concept is shown inFigure 9.9. Such probes are forced to vibrate at a specific frequency and smallamplitude by piezoelectric elements at the top of the probe shaft.
Any contactmade with a measurement surface will result in a change in the frequency atwhich the probe vibrates that is detected by piezoelectric sensors [30].Modern piezoelectric sensors can detect very small changes in vibrationamplitude. Therefore, registering a contact using this detection techniqueallows the measurement force to be greatly reduced.FIGURE 9.9 A vibrating fibre probe.
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