ELECTROMAGNETIC CHANGER FOR AFM TIPS

B. Goj1, N. Vorbringer-Dorozhovets2, C. Wystup1, M. Hoffmann1, E. Manske2

1 Micromechanical Systems Group, IMN MacroNano®, Ilmenau University of Technology 2 Measurement Systems Group, IMN MacroNano®, Ilmenau University of Technology

Abstract — We present an automatic electromag-

netic changer which is utilized in a nanopositioning and nanomeasuring machine. The first application, an atomic force microscope, is described and first measurements are shown. The main advantage of using a changer is that the measurement chamber has not to be conditioned after an exchange of the nanoanalytic.

Keywords : electromagnetic changer, nanoposi-

tioning and nanomeasuring machine (NPMM), atomic force microscopy

I – Introduction The crucial question during the design of nanoposi-

tioning and nanomeasuring machines (NPMM) is how nanoanalytics can be integrated and exchanged without great efforts. Usually the measurement tools are manu-ally exchanged after opening the machine. Thus, cool-ing conditions and chamber vacuum are lost, and the NPMM has to be conditioned before further operations. This procedure takes a long time and goes along with additional costs. [1]

Figure 1: Nanopositioning and nanomeasuring machine

NPMM [2]

The integration of an automatic changer into the NPMM which comprises different nanoanalytics is the solution to overcome the expensive and time-consuming exchange procedures. Additionally, an automatic changer avoids large position deviations between the nanoanalytic and the NPMM probe head which occur during a manual exchange.

However, some crucial questions have to be solved during the design of an automatic changer.

How can a reproducible positioning of the nano-analytic to the specimen take place after ex-change?

How is the nanoanalytic fixed to the NPMM probe head?

In this paper the design and simulation of an elec-tromagnetic changer is described which is suitable for the integration into the NPMM (Figure 1). The whole system is described under consideration of atomic force microscopy (AFM, [3-4]) as an exemplary application. The alignment of the AFM tips on the NPMM probe head and the referencing after exchange are described in the following.

II – Design of the electromagnetic changer The designs of two electromagnetic changers are

shown in Figure 2 and 3. Both systems comprise elec-tromagnets (which carry the nanoanalytics in the so-called plugs), the specimen holder for referencing the nanoanalytic and clamps to fix the substrate. The design of the NPMM probe head (which is called plug holder in the following) comprises three magnets and three triangular moulds which define the contact with the electromagnetic conductive plugs (ball-plane connec-tion, Figure 4).

Figure 2: Electromagnetic changer for the NPMM

Figure 3: Electromagnetic changer with stationary electro-

magnets

During the design process the challenge was to find a compromise between low heat dissipation and large measurement area of the NPMM. The first electromag-netic changer satisfies the demand of a high measure-ment area employing a rotating changer wheel which is driven by a step motor (Nanotec SP1008). However, the high heat dissipation of the step motor during rotation of the wheel is the drawback of this system. Thus, a second electromagnetic changer (cf. Figure 3) was developed which copes without the step motor and comprises three stationary electromagnets. A lower measurement range and a smaller number of portable plugs are the disad-vantages of this system. Nevertheless, both systems are suitable for the NPMM and will be investigated.

Figure 4: Plug holder and plug

Figure 5: Reference mark of

the changer

The electromagnets can be employed as polarized or non-polarized systems. The polarized electromagnet has the advantage of low heat induction into the NPMM. However, the permanent magnets can influence the measurement of the nanoanalytics because of disturbing electromagnetic fields. In chapter III the simulation of the electromagnet is discussed in detail.

The reference substrate is another crucial part of the electromagnetic changer. Therewith, the nanoanalytics are referenced after an exchange, and the position on substrate can be reproducible found. The reference marks include cross lines with different dimensions for referencing an AFM [3-4] or a focus sensor [5-6] (cf. Figure 5).

AFM measurements with the reference substrate have the advantage that measurements can be continued after the AFM-tip is broken and exchanged. In chap-ter V measurements of the reference marks are present-ed and discussed.

III – Simulation of the electromagnets For the simulation of the electromagnets Ansoft

Maxwell was used. The aims of the simulation were: Miniaturization of the electromagnets in order to

increase the measurement area in the NPMM, Decrease of the heat dissipation, and Optimization of the polarized magnetic circuit

(coils and permanent magnets). The whole simulation was supplemented by a sim-

plified calculation of the heat transfer [7-8]. Measurements of the connection between the plug

and plug holder were done to determine the maximum

force which is necessary to transfer the plug from the plug holder to the electromagnet. Therefore, different magnet types with different dimensions were investigat-ed. The gap between the plug and the electromagnet was 200 microns utilizing a capton foil which functions as a damping and heat isolation layer.

Table 1: Release forces for the connection between plug and plug holder with different disk magnets

Type Material h in mm d in mm F in N Sm2Co17 S280 2 2 3,2

NdFeB N35 2 2 3,9 NdFeB N35 0,5 2 0,6 The smallest magnet (height 0,5 mm, diameter

2 mm) was considered in the simulation because the holding force can be low for the benefit of low masses of the AFM tip itself. Nevertheless, for other nanoana-lytics which have a high mass the other permanent magnets are reasonable.

The solutions of the simulation are shown in Figures 6 and 7. The surrounding fluid was air and all magneti-cally conductive materials were made of steel with a relative permeability of 800. NdFeB (N35) was utilized as material for the ring-shaped permanent magnet.

The gap-reluctance force characteristic shows that a magnetic current of approximately 80 ampere is needed to transfer the plug from the plug holder to the electro-magnet (safety factor 1,5). The electromagnet has a height of 6 mm and a diameter of 11 mm. The axially polarized permanent magnet is 1,5 mm thick and 1 mm high.

Figure 6: Design of the electromagnet: 1 plug, 2 electric sheet,

3 permanent magnet, 4 coil, 5 magnet pot

Figure 7: Gap-reluctance force characteristic of the electro-

magnet

The coil was optimized with regard to a low heat dissipation to avoid thermal drift during an exchange of the nanoanalytics. The optimized coil has a winding number of 190, a wire diameter of 15 microns and a maximum heat dissipation of 0,3 watts during the exchange.

The main advantage of the presented electromagnet is the modular design. In case of disturbing electromag-netic fields the permanent magnet can be left out and the system uses the coil for nanoanalytic transfer only. Additionally, the system allows an exchange of the plug design without high efforts. Only, the electric sheet has to be changed if the plug geometry has changed.

IV – An application for the changer: AFM tips

An AFM tip is the first nanoanalytic which was placed on a plug and integrated into the electromagnetic changer (Figure 8).

Figure 8: Plug with attached AFM tip

The utilized AFM system comprises a laser interfer-ometer and a quadrant diode. The laser interferometer measures the axial position of the AFM tip. However, for high accuracy measurements the tilt because of the bending AFM tip has to be considered. Therefore, the value of the bending angle φ is required which can be determined with the quadrant diode. [3-4]

The measurement principle of the AFM requires a defined position of the laser spot on the AFM tip in reference to the measurement standard of the NPMM so that the ABBE criterion is fulfilled [3-4]. Thus, the position of the AFM tips on different plugs must be reproducible in the Cartesian directions and the rotation φ. Otherwise, a manual alignment of the plug with AFM tip (which is placed in the NPMM probe head) is needed which is a long and expensive procedure.

Figure 9: Alignment system for the AFM tips

An alignment device was designed which positions and fixes the AFM tip on the plug. It comprises a vacuum gripper for holding the tip, an x-y-z-φ-positioner and an optical alignment system which equals the AFM measurement design [3-4]. Therewith, a reproducible alignment of the AFM tips on the plug is possible so that no manual alignment of the tips is required. Nevertheless, the positioning accuracy is about 10 µm so that reference positions are required to align the specimen to the AFM tip. Thus, a reference sub-strate was designed which is explained in the next chapter.

V – Reference marks for a NPMM The navigation and measurement of different struc-

tures over an area of 25 mm times 25 mm (measuring range of the NPMM) utilizing tactile probing systems (AFM tips [3-4]) as well as non-tactile probes (e. g. focus sensor [5-6]) require reference identification marks.

Reference marks enable the reproducible location of a measurement structure after exchanging the AFM tip. An easily traceable structure (utilizing a camera sys-tem), applicability for different nanoanalytics, high fabrication accuracy and fast measurability are the main demands on the reference marks.

A substrate with four reference marks was fabricated utilizing UV lithography (Figure 10). The reference marks comprise crosses with dimensions from 800 µm to 40 µm to enable measurements with different nano-analytics. The centre points of three crosses (I, II, III) can be utilized for locating a measurement point after exchange of the AFM-tip.

Figure 10: Reference identification marks: a) complete design, b) reference field with the structures (different cross patterns), c) AFM reference structure (determination of the cross center)

AFM measurements of the cross patterns I-III were completed in order to test the repeatability and accuracy of the centre position of the crosses. The measurements were performed using an interferometer-based metro-logical scanning probe microscope (SPM, [3-4]) which is included into the NPMM with a resolution of 0,1 nm and a temperature stability of the complete measuring setup of 20 mK. [2]

The centres of the crosses were measured with the centre of gravity method. Therefore, four scanning lines are needed for each cross. One cross line can be deter-mined with two parallel scanning lines and an evalua-tion of the edges and the midpoints. The intersection of the two perpendicular cross lines determines the centre of the cross. Table 2: Position stability of three cross centres (10 measure-

ments) Position

in mm Maximum deviation

in nm I x 0.785 313 2 4.0

y 2.374 147 9 2.1 II x 0.785 288 7 3.8

y 2.374 135 1 3.4 III x 0.784 875 6 7.6

y 2.373 862 9 9.4 The repeatability of the coordinates of the cross cen-

tres are investigated with 10 measurements of the cross patterns at different edge positions (cf. Table 2). The measurements show that the centre position of the crosses can be reproducibly found with a deviation of 10 nm. For the location of features with dimensions of 100 nm the introduced reference substrate is suitable. Specimens with smaller dimensions require more accurate reference substrates which can be generated by electron beam lithography.

VI – Conclusion and Outlook We introduced two electromagnetic changers which

comprise electromagnets for holding the nanoanalytics (e. g. AFM tips) and a reference substrate for referenc-ing the nanoanalytic after exchange. Advantages of utilizing changers are:

The avoidance of a conditioning of the meas-urement chamber (cooling, vacuum, damping) after an exchange of nanoanalytics,

The high reproducible location of the same measurement point after an exchange,

The fast exchange procedure in comparison to manual exchange.

The electromagnets are modularly designed so that different plug designs for different nanoanalytics can be integrated into the changer. The electromagnets can be employed as polarized or non-polarized systems so that disturbing electromagnetic field can be avoided.

In the future, other nanoanalytics will be integrated into the changer such as tactile and optical probing

systems which are developed at Ilmenau University of Technology [5-6, 9-11].

Figure 11: Nanopositioning and nanomeasuring machine with

integrated electromagnetic changer

Acknowledgement

The presented works were funded by the German Research Foundation (DFG) under contract SFB 622. References [1] Weckenmann, A. et al: Probing Systems In Di-

mensional Metrology. In: Annals of the 54th CIRP General Assembly, p. 657-684, February 2004

[2] Hausotte, T.: Nanopositionier- und Nanomessma-schine. PhD Thesis, Ilmenau University of Tech-nology, 2002

[3] Dorozhovets, N. et al.: Novel investigations and developments in a metrological scanning probe microscope. - In: Proceedings ICPM (Ilmenau), 2008. ISBN 978-3-938843-38-3

[4] Dorozhovets, N et al.: Development of the inter-ferometrical scanning probe microscope. - In: In-terferometry XIII: applications / SPIE Conference (San Diego, California), 2006

[5] Machleidt, T. et al.: Area-based optical 2.5D sensors of a nanopositioning and nanomeasuring machine. - In: Meas. Sci. Technol. 23. Bristol: IOP Publ., ISSN 13616501, 2012

[6] Mastylo, R.; Manske, E.; Jäger, G. Entwicklung eines Fokussensors und Integration in die Nanopo-sitionier- und Nanomessmaschine. - In: Tech-nisches Messen. München: Oldenbourg, ISSN 01718096, Bd. 71, 2004

[7] Lunze, K.: Einführung in die Elektrotechnik. Berlin: VEB Verlag Technik, vol. 9, 1979

[8] Philippow, E. S.: Basics of electrical engineering. Berlin: Technik Verlag, vol. 9, 2000

[9] Goj, B.; Hoffmann, M.: Design of a resonant triaxial nanoprobe fully integrated in a silicon sub-strate. Tönsberg (Norway):22nd MME 2011, 2011

[10] Goj, B.; Hoffmann, M.: Resonant Nanoprobe with Integrated Measurement System. Bremen: Actua-tor, 2012

[11] Balzer, F. et al. Tactile 3D microprobe system with exchangeable styli. In: Meas. Sci. Technol. 22. Bristol: IOP Publ., ISSN 13616501, (2011)