Hoffmann 2008]_CIRP JoMSaT_Electrical Probing for Dimensional Metrology

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Electrical probing for dimensional micro metrology

J. Hoffmann *, A. Weckenmann, Z. SunChair Quality Management and Manufacturing Metrology, University Erlangen-Nuremberg, Germany

1. Introduction

By far the most devices for dimensional metrology arecomposed out of an axes system and an either tactile or opticalprobing system. With the axes system the relative positionbetween the probing system and a workpiece is manipulatedand the absolute position of the probing systemis measured, whilethe probing system determines the relative position of points onthe workpiece surface with respect to a reference point in theprobing system. While appropriate axes systems for micro andeven nanometrology are commercially available [14] , there arestill many obstacles to overcomefor theconstruction of a universalprobing system capable for most topography and coordinatemeasurements on complex shaped micro-sized parts.

2. Limitations of optical and tactile probing

For 2.5 D topography measurements (only one height value canbe measured at each point of the lateral plane) very often opticalprobing systems are used due to the achievable high point rate, thetypically good resolution in beam direction and the non-destructive functional principle. Drawbacks on the other handare difculties in measuring high surface slopes andthe diffractionlimit of lateral resolution (Abbe limit) [5] . This can be illustrated at

measurements of a white light interferometer, although it

generally applies to far eld optical measurements.Fig. 1 shows a measurement of a part of a MEMS deviceperformed with a Taylor-Hobson TalySurf CCI white lightinterferometer with 50 Mirau objective. The two aluminiumcontacts are connected with a 200 nm wide carbon bridge thatcould be evidenced by conductivity measurements but not byoptical measurements due to the width being smaller than themean wavelength of the interferometer light source. The carbonbridge also causes heavy artefacts in the optical measurement of the contacts, Fig. 1 inside by ellipse. Additionally all points on thesteep anks are result of interpolation, but not of measurementand hence do not give any real information. Interpretation of thoseareas can thus be misleading.

The maximum measurable slope angle for optical methods isalways considerably below 90 8 (strict geometrical limit), a typicalvalue is around 3045 8 depending on the surface roughness, thenumerical aperture of the objective and the method of measure-ment [6] so that, in general, only small portions of true three-dimensional objects (e.g., spheres) can be measured. The necessityfor a direct linear connection between the point to be probed andthe optical sensor causes further limitations of optical systems fortrue 3D measurements (several surface points may have the samelateral position, e.g., at undercuts) of complex parts. This is one of the reasons why virtually all probing systems for 3D coordinatemeasuring machines detect the workpiece surface by touching itwith a probing element and measuring the probing force or thedisplacement of the probing element due to the probing force [7] .

CIRP Journal of Manufacturing Science and Technology 1 (2008) 5962

A R T I C L E I N F O

Article history:Available online 11 July 2008

Keywords:Scanning tunnelling microscope (STM)Coordinate Measuring Machine (CMM)Sensor

A B S T R A C T

Each dimensional measurement is based on probed points on the surface of the measured object.However, the well-established tactile and optical probing techniques face limitations when small and

delicate objectswith complex shape have to be measured. Withtactile measurements there is always thedanger of damaging the workpiece by the probing force and the measurable point rate is quite low. Withoptical probing there is a principal resolution limit and accessibility to complex surfaces is hindered bythe limitedacceptable surface slope. Also undercuts are not measurable.To overcome these limitations aprobing system based on an electrical probing interactionwith a direct currentof a few nanoampereshasbeendeveloped, tested andcompared withtraditional technologies.With this probingsystemcoordinatemeasurements of micro parts as well as nanometer resolved surface topography measurements arefeasible. By applying a wide range of probes accessibility problems can be drastically reduced comparedto tactile or optical micro probing systems.

2008 CIRP.

* Corresponding author.E-mail address: [email protected] (J. Hoffmann).

Contents lists available at ScienceDirect

CIRP Journal of Manufacturing Science and Technology

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1755-5817/$ see front matter 2008 CIRP.

doi: 10.1016/j.cirpj.2008.06.002
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Opposite to light propagation,mechanical force can be transmittedalong dened curved and articulated paths, i.e., the stylus carryingthe probing element. Also it is possible with a spherical probingelement to exert a force in any direction, whereas optical systemsusually only establish their probing interaction in one direction(mostly z ). Measurement of high slopes, undercuts and interiorfeatures in different orientations demands for such directionallyindependent probing and guided propagation of the probinginteraction Fig. 2.

Also the resolution limit does not apply to tactile probingsystems, what is used, e.g., in atomic force microscopy [1] , where ane tip witha curvature radius considerably below the wavelengthof visible light is used to scan surfaces. So the topography of thesample is transformed into a movement of the cantilever the tip isattached to. The much larger backside of the cantilever thenenables optical measurement of its movement.

Unfortunately tactile probing also shows principal disadvan-tages. The serial measurement and the necessity of moving massfor probing leads to a much lower measurable point rate by unittime and the force needed for probing might cause damages of theworkpiece and/or the probe [8,9] . Up to now there is no tactilemicro probing system that gives the possibility of using articulatedor arborescent stylii [10] , so many advantages of macroscopictactile 3D probing systems do not apply to miniaturized systems.When the size of the probing element is reduced for the benet of better accessibility to small interior features, higher spatialresolution and lower moment of inertia, several challenges directlyarise from the tactile working principle. Hertzian stress in thecontact area grows quickly with smaller tip balls [7] , raising the

danger of plastic deformation of the workpiece and largemeasurement errors [8] . For a ruby probing sphere of 0.125 mmplastic deformationof 5 nm hasbeen reportedwhen an aluminiumsurface is probed with a force of less than 1 mN [9] , which ischallenging to be controlled appropriately especially because alsodynamic forces are to be considered. As the stem diameter has tobe smaller, than the tip ball diameter, also stylus bending may getcriticaland lead to deterioratedsignal to noise ratio when small tipballs are used.

In comparison to staticcontactforce, thedynamic probing forceF = m a (m: moving probe mass, a: deceleration upon contact) canbe much higher depending on the moving probe mass and theapproach speed, what is limiting the usable approach speed inpractice [8,9] .

With ber probes [7] it is possible to probe with very low staticprobing force (e.g., 1 m N) reducing the risk of damaging theworkpiece to a minimum. On the other hand they have strictlylimited capability for 3D measurements due to the non-isotropicexibility of the glass ber stylii and the typically opticalmeasurement of the probe position.

In summary real 3D measurement of complex shapedparts is ingeneral only feasible using directionally independent probing andguided propagation of the probing interaction. Both can beachievedto a large extentwith mechanical probing using sphericalprobing elements and articulated stylii. Mechanical force is a veryuseful probing interaction for large and medium sized mechani-cally stable parts, but meets its limits when very small probingelements have to be employed or delicate structures are to beprobed.

A forceless probing interaction that gives the possibility of directionally independent probing and guided propagation of theinteraction is thus highly desirable.

3. Electrical probing

For 2.5 D topography measurement of conductive and semi-conductive workpieces the possibility of electrical probing is usedin eld emission microscopy [11] and scanning tunnellingmicroscopy [12] already and proved the excellent achievableresolution and non-destructiveness there for one dimensionalprobing.

Electrical probing interactions or the measurable informationabout it (i.e., current) can be easily guided along a dened path byusing an appropriately shaped conductor. Also it is feasible to use aspherical electrode as probing element, so the prerequisites for realthree-dimensional probing are given for the electrical interaction.

To investigate the practical usabilityof electrical probing for themeasurement of complex shaped parts an experimental probingsystem [13] was designed, set-up and integrated into a long-rangenanopositioning unit with Laser-interferometric position mea-

surement [14] . The function of the probing system is based on themeasurement of a small direct current (0100 nA) which resultsfrom a bias voltage between 2 and 2 V between probe andworkpiece when the distance between both is sufciently small.The relationship between probe travel during approach to thesurface and the resulting current can be evaluated by simulta-neously recording the probe signal and the HeNe-Laser inter-ferometer signal of the nanopositioning unit in the axis parallel toprobe movement, Fig. 3. For the depicted curves, a bias voltage of 1 V was used between a high alloy steel sample and a tungstencarbide probing sphere ( d = 0.3 mm). The probe was approachedand subsequently withdrawn four times with a speed of 20 nm/s.

The current decreases with increasing relative distance;however, at small relative distances, a drastic non-linear changing

is observed for a current higher than 14 nA. Especially the lower

Fig. 1. Abbe limit of lateral resolution shown at the measurement of an MEMSdevice.

Fig. 2. Improved 3D ability by guided propagation of the probing interaction.

J. Hoffmann et al. / CIRP Journal of Manufacturing Science and Technology 1 (2008) 596260


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current part (lower than 14 nA) can be used for dimensionalmeasurement due to the good repeatability. It is assumed thatthese phenomena are consistent with the occurrence of currentowing between narrowly separated electrodes (probe andsample), which is widely known as tunnel effect [12,15] . Bothshow an exponential behaviour and very similar electrontransmission coefcients at similar bias voltages; however, theslope of the curve of the investigated probing system is muchsmaller, than for conventional scanning tunnelling microscopes,which are using probes with a much smaller radius of curvature(e.g., 10 nm) and typically work in vacuum. Exact theoreticalcalculation of the curve is hindered by the fact that there is nocommon analytical solution for the three-dimensional barriermodel and that the one-dimensional approximation of the barrieris not valid for probes with a much larger radius of curvaturecompared to the probe-sample distance [16] . Additionally inambient atmosphere Schottky-emission may lead to a much lowereffective barrier height what can explain the large observed range,low slope of the characteristic curve [17] and the differences totheoretically predicted characteristic curves for operation in ultrahigh vacuum [18] . The decreasing slope of the curve for probeswith larger radii of curvature has also been observed by othergroups in conventionalscanningtunnelling microscopy, Fig.4 [16] ,where it is always aimed at small radii of curvature for the benetof better lateral resolution.

It is worthy to point out that the usefulness of the investigatedmeasurement system for real 3D measurements stems from thefact that it is sensitiveto the full three-dimensional structure of thesurface due to the employment of spherical probes with very largeradii (e.g., 0.15 mm) compared to conventional STM tips.

4. Experimental results

For evaluating system performance for dimensional metrologyand comparability of the results with tactile and optical measure-

ments, 2.5 D topography measurements with ranges between1 m m 1 m m and 10 mm 10 mm as well as 3D coordinatemeasurements of a micro ball bar have been performed andcompared withcommercial optical and tactile high-end metrologydevices.

4.1. 2.5 D topography measurements

For 2.5 D surface topography measurements the distancedependent current can be used for controlling the z-position of the probe during scanning along x or y axis, so that the probefollows the surface topography with constant distance. Whenusinga sharp probe,lateral resolution well below the Abbe limit foroptical measurements can be achieved, Fig. 5. The effectiveachievable resolution is considerably better than expected fromthe radiusof curvature of the used probe( r = 15 m m), soit has tobeassumed that only a small portion of the spherical tip end iseffectively in interaction with the surface to be probed. Thescanning speed has been reduced from 100 m m/s for the largestmeasurement range in Fig.5 to 1 m m/s forthe smallest one. At eachmeasuring range 1000 1000 measurement points have beencollected. Due to the robustness and virtual absence of wear anddrift of the electrical probing system also very large measuringranges up to 25 mm 25 mm are feasible with the experimentalset-up.

4.2. 3 D coordinate measurements

The ability for probing true three-dimensional objects can beinvestigated at the measurement of spheres, which show normalvectors in all directions. This is also reected in the procedure fordetermining 3D probing uncertainty of probing systems for

coordinate measuring machines according to ISO 10360. There ahemisphere of negligible form deviation is probed with 25 evenlydistributed points and the maximum deviation from the Gaussiant is evaluated. Fig. 6 shows a measurement of a polished steelsphere electrically probed with a monolithic tungsten carbideprobe with a 0.3 mm tip ball. Before the measurements no formcalibration of the tip ball was done, so the data represents acombination of probing uncertainty (not reproducible) and(reproducible) form deviations of the probing sphere and themeasured sphere.

The maximum deviation of the measured data from a Gaussiant sphere is 1.085 m m, while the mean difference per pointbetween two subsequent measurements is only 37 nm (maximumdifference: 151 nm), so the shown deviation from the Gaussian

sphere represents the form deviation of the probing sphere (form

Fig. 3. Characteristic curves of the electrical probing system when approaching to asurface and withdrawing from the surface (four cycles).

Fig. 4. Characteristiccurves of a commercialSTM with a sharp tip(a) anda blunt tip

(b) [16] .

Fig. 5. Realization of vastly differing measuring ranges on a ground steel plate.

J. Hoffmann et al. / CIRP Journal of Manufacturing Science and Technology 1 (2008) 5962 61


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deviation specied by manufacturer: 2 m m) and the measuredsphere (form deviation specied by manufacturer 135 nm) ratherthan the probing uncertainty of the electrical probing system. Thedistance between two spheres of an Invar ball bar could bemeasured with a standard deviation of 36 nm (90 measurements)and in reasonable agreement (difference 109 nm) with thecommercial micro CMM Werth VideoCheck UA 400(MPE = 0.6 m m). Main difculty for evaluation of measurement

uncertainty is the lack of electrically conductive standards whichare calibratedwith appropriate uncertainty. Further investigationsfor evaluating measurement uncertainty are in progress.

5. Investigation of invasiveness

When probing micro parts it is often critical to ensure non-invasiveness, so that the measured object is not altered by themeasurement. Invasiveness of electrical probing has been inves-tigated by repeated 2.5 D measurements of the same surface areaand changing fast and slow scan axis. As this could not reveal anytraces caused by the measurement, a cross shaped scan path on asteel sphere has been measured 300 times with adequate settings(control set point 5 nA). After that three of the four arms of the

cross have been measured once with real electric contact to the

probe, resulting in a current of about 1 m A. After that the probedsphere was measured with an Alicona Innite Focus Microscopeand a Taylor-Hobson TalySurf CCIwhite light interferometer, Fig.7 .

When measured with real electric contact (1 m A) scratches of up to 300 nm depth are left on the measured surface after only onemeasurement, while even after 300 measurements no trace at allcould be found when the current was limited to 5 nA byappropriate distance control, Fig. 7.

6. Summary

The feasibility of electrical probing for 2.5 D topographymeasurements and also true 3D coordinate measurements hasbeen demonstrated. Special advantages of electrical probing arethe enormous exibilitytowards the measuring task, the verygoodrepeatability, non-invasiveness and good robustness compared tooptical or tactile probing.A principal disadvantage is the limitationto conductive samples. Due to the fact that the probe stem only hasto conduct a very small current, but practically no mechanicalforce, further miniaturization and application of multiple articu-lated and or arborescent stylii for the access to complex interiorworkpiece features is expected to be possible.

References

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[2] Hansen, H.N.,Carneiro,K., Haitjema, H., De Chiffre, L., 2006,Dimensional Microand Nano Metrology, Annals of the CIRP, 55/2: 721744.

[3] Jaeger, G., Gruenwald, R., Manske, E., Hausotte, T., Fuessl, R., 2004, A Nano-positioning and Nanomeasuring Machine: Operation-Measured Results,Nanotechnology and Precision Engineering, 2/2004 (2): 8184.

[4] Hoffmann, J., Weckenmann, A., 2006, Coordinate Measurements with Nan-ometerResolution, in: Proceedingsof VIIthInternational Scientic Conference,Coordinate Measuring Technique (03.-05.04.2006, Bielsko-Biala, Poland),pp.113122.

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[8] van Vliet, W.P., Schellekens, P., 1996, Accuracy Limitations of Fast MechanicalProbing, Annals of the CIRP, 45/1: 483488.

[9] Meli, F., Kueng, A., 2007, AFM Investigation on Surface Damage Caused byMechanical Probing with Small Ruby Spheres, Meas Sci Technol, 18:496502.

[10] Neugebauer, M., Hilpert, U., Bartscher, M., Gerwien, M., Krystek, M., Schwehn,C., Trenk, M., Goebbels, J., 2008, Untersuchungen zur Messung von Mikrogeo-metrien mit groen taktilen KMGs und Anwendung bei einem Pru fko rper fu rMikro-CT-Messungen, Tm Technisches Messen, 75:187198.

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Fig. 7. Investigation of invasiveness with optical microscopy (left) and white lightinterferometry (right).

Fig. 6. Measurement results at a steel sphere d = 4 mm with 25 evenly distributedprobing points.

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Hoffmann 2008]_CIRP JoMSaT_Electrical Probing for Dimensional Metrology

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