Принципы нанометрологии (1027506), страница 59
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One such method that evaluates the uncertainty by numericalsimulation of the measuring process is described in ISO/TS 15530 part 4 [16].To allow CMM users to easily create uncertainty statements, CMMsuppliers and other third-party companies have developed uncertaintyevaluating software, also known as virtual CMMs [17]. Even by adoptingISO 15530 part 4, there are many different approaches to the implementation of a virtual CMM [18–20].271272C H A P T ER 9 : Coordinate metrology9.4 Miniature CMMsThe advent and adoption of the CMM greatly reduced the complexity, downtime and operator skill required for measurements in a production environment. It is difficult to imagine a modern successful automobilemanufacturing plant that does not employ CMMs. The ‘CMM revolution’has yet to come to the MNT manufacturing area.
Once again many instruments are employed to measure the dimensions of MNT parts but there arenow additional problems despite their tiny size: many of the parts that needmeasuring are very complex, high-aspect-ratio structures that may be constructed from materials that are difficult to contact with a mechanical probe(for example, polymers or bio-materials). Also, there is often a need tomeasure the surface topography of steep walls found in, for example, deepreactive ion etched (DRIE) structures used for MEMS.
The only instrumentsthat are available are those which essentially ‘measure from above’ and weretraditionally used to measure surface topography. These instrumentsgenerally lack traceability for surface topography measurements (see section6.10). Therefore, it is difficult to measure with any degree of confidence thecomplex structures that are routinely encountered in MNT products.In recent years many groups have developed ‘small CMMs’, typically withranges of tens of millimetres and tens of nanometres accuracy in the x, y and zdirections. These miniature CMMs come in two forms: those that aredeveloped as stand-alone CMMs and those that are retrofitted to macro-scaleCMMs.
One of the first miniature CMMs of the latter form was the compacthigh-accuracy CMM developed at NPL [21]. This CMM used the movementscales of a conventional CMM with a retrofitted high-accuracy probe with sixdegrees of freedom metrology. This CMM had a working volume of 50 mm by50 mm by 50 mm with a volumetric accuracy of 50 nm. Retrofitted CMMs willnot be discussed in detail as they are simply a combination of conventionalCMMs (see section 9.1) and micro-CMM probes (see section 9.5).One technical challenge with probing MNT structures arises due to theinability to actually navigate around the object being measured without somelikelihood of a collision between the probe and the part being measured.Typical miniature probing systems are less robust than on larger CMMs thatincorporate collision protection.
Future research must concentrate on thesedifficult technical issues associated with micro-CMMs if they are to becomeas widely used as conventional CMMs. However, the technical barriersassociated with mechanical contact of probes at the micro-scale may forceresearchers to look into completely novel approaches such as SEM-basedphotogrammetry or x-ray computed tomography [22].Miniature CMMs9.4.1 Stand-alone miniature CMMsOnly two examples of stand-alone miniature CMMs are given here becausethey are the only two that are both currently commercially available and forwhich quite extensive information is available in the open literature. Twofurther examples are the Mitutoyo Nanocord and the IBS ISARA (based onwork by [23] but with roller as opposed to air bearing slideways). There aremany instruments that are at the research stage (see for example [24]) andsome that were developed but are not currently commercially available (seefor example [23,25,26]).9.4.1.1 A linescale-based miniature CMMThe F25 is a miniature CMM based on a design by the Eindhoven Universityof Technology (TUE) [27] and is available commercially from Carl Zeiss.
TheF25 has a unique kinematic design that helps to eliminate some of thegeometric errors inherent in conventional CMMs. The basic kinematiclayout is shown schematically in Figure 9.4. The red arms are stationary andfirmly attached to the machine. The blue arms form the x and y measurement axes and are free to move. 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]).
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