We present a new cantilever for magnetic-force microscopy (MFM), the CantiClever, which is not derived from atomic-force microscopy (AFM) probes but optimized for MFM. Our design integrates the cantilever and the magnetic tip in a single manufacturing process with the use of silicon micromachining techniques, which allows for batch fabrication of the probes. This manufacturing process enables precise control on all dimensions of the magnetic tip, resulting in a very thin magnetic element with a very high aspect ratio. Using. the CantiClever, magnetic features down to 30 nm could be observed in a CAMST reference sample

Abelmann, Leon

Heskamp, I.R.

Heskamp, Iwan

Lodder, J.C.

Siekman, Martin Herman

van den Bos, A.G.

English

We present a new cantilever for magnetic-force microscopy (MFM), the CantiClever, which is not derived from atomic-force microscopy (AFM) probes but optimized for MFM. Our design integrates the cantilever and the magnetic tip in a single manufacturing process with the use of silicon microma-chining techniques, which allows for batch fabrication of the probes. This manufacturing process enables precise control on all dimensions of the magnetic tip, resulting in a very thin magnetic element with a very high aspect ratio. Using the CantiClever, magnetic features down to 30 nm could be observed in a CAMST reference sample

University of Twente Research Information

IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002 2441The CantiClever: A Dedicated Probe for MagneticForce MicroscopyArnout van den Bos, Iwan Heskamp, Martin Siekman, Leon Abelmann, and Cock LodderAbstract—We present a new cantilever for magnetic-forcemicroscopy (MFM), the CantiClever, which is not derived fromatomic-force microscopy (AFM) probes but optimized for MFM.Our design integrates the cantilever and the magnetic tip in asingle manufacturing process with the use of silicon microma-chining techniques, which allows for batch fabrication of theprobes. This manufacturing process enables precise control on alldimensions of the magnetic tip, resulting in a very thin magneticelement with a very high aspect ratio. Using the CantiClever,magnetic features down to 30 nm could be observed in a CAMSTreference sample.Index Terms—Cantilevers, integration, magnetic force mi-croscopy (MFM), magnetic tips.I. INTRODUCTIONW ITH magnetic force microscopy (MFM) a high-resolu-tion image of the stray field of a magnetic sample canbe obtained. A typical resolution of 50 nm can be achievedwith very little sample preparation [1]. This makes MFM a veryuseful tool for the study and characterization of magnetic ma-terials for high-density magnetic recording. As the areal den-sity of magnetic recording systems increases very rapidly, thecurrent resolution of MFM needs to be improved in order to re-main useful as a measurement tool [2]. The resolution of MFMis determined by a combination of tip geometry and measure-ment noise. The thermal vibration of the cantilever currently de-termines the measurement noise levels in modern instruments;this thermal noise may be reduced by using cantilevers with lowspring constants, high resonant frequencies, and high quality ofresonance. This imposes a great challenge on the design of dy-namic mode control electronics. However, at the moment it ap-pears that the resolution of MFM is limited by the geometry ofthe magnetic tip. The dimensions as well as the shape of themagnetic tip have a big influence on the resolution. The idealtip shape would be that of an elongated bar with a flat end, asdepicted in Fig. 1 [3].An elongated shape induces a strong shape anisotropy, whichincreases the stability of the magnetic tip against remagneti-zation by the sample. A long magnetic element also behavesas a monopole when imaging small magnetic features, whichincreases the signal output. Moreover, for thick films, a longtip also improves the output at very long wavelengths. The flattip-end causes most of the magnetic charges to be confined asclose as possible to the sample. This results in an improvementManuscript received February 14, 2002; revised May 28, 2002.The authors are with the Systems and Materials for Information StorageGroup, MESA Research Institute, University of Twente, 7500 AE Enschede,The Netherlands (e-mail: a.g.vandenbos@el.utwente.nl).Digital Object Identifier 10.1109/TMAG.2002.803585.Fig. 1. Left: ideal tip shape for a MFM tip. Right: cantilever with the tip plane.in resolution as compared to point sharp tips, which suffer fromhaving a large proportion of their magnetic charges at a greaterdistance from the sample [4]. Commercially available MFM tipsconsist of an atomic-force microscopy (AFM) tip, coated witha thin layer of magnetic material, usually cobalt alloys [5]. Thepyramidal shape of these tips does not bear much resemblanceto the ideal tip shape mentioned above. A better approximationto this ideal shape is obtained with electron-beam-induced de-posited (EBID) carbon needles [6]–[8], coated on one side witha thin magnetic layer. However, the EBID carbon needle has ashape similar to that of a cone. When coated with magnetic ma-terial, this causes not all the magnetic charges to be confined tothe end of the tip, making the tip sensitive to stray fields from alarger area. Another major drawback of the EBID is their serialmanufacturing process. Therefore, making one tip takes a con-siderable amount of time and, even worse, the tip shape is not ex-actly reproducible. Another method to make tips specially suitedfor MFM measurement is used by Phillips [9], who depositsmagnetic material onto an AFM cantilever and uses focusedion-beam equipment to remove unwanted areas of the mag-netic coating. With this serial production method 8-m-longand 50–nm-wide needles could be realized. The new design pre-sented in this paper results in an MFM tip with a nearly perfectshape that is produced in a highly reproducible batch manufac-turing process.II. DESIGNA magnetic tip that is suitable for high-resolution MFMshould have lateral dimensions in the nanometer regime. Onewould like these dimensions to be controllable and variable to0018-9464/02$17.00 © 2002 IEEE2442 IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002Fig. 2. Tip plane on the cantilever for both cantilever orientations. Left:conventional approach with perpendicularly vibrating cantilever; right: newapproach with laterally vibrating cantilever.be able to customize the MFM tip for different types of measure-ments or samples. The CantiClever design accomplishes this bydefining both lateral dimensions by thin film deposition tech-niques. The magnetic tip is made by deposition of magnetic ma-terial on the side of a free-hanging, very thin layer called the tipplane. The width and thickness of the magnetic tip are definedby the thickness of the tip plane and the magnetic layer, respec-tively. The length of the tip is defined using photolithography.A schematic drawing of the structure is shown in Fig. 1.Such a structure is very difficult to realize when using theconventional fabrication technique in which the cantilevers aresituated such that the oscillation direction is perpendicular to thesurface of the substrate. Using this approach, the very thin tipplane needs to be fabricated as a free-standing layer perpendic-ular to the substrate surface. To avoid this, the free-hanging tipplane is made with a completely new approach. During fabrica-tion, the cantilevers are tilted 90compared to the conventionalcantilevers, creating a laterally oscillating cantilever with its os-cillation direction parallel to the substrate surface, as shown inFig. 2.This approach makes the fabrication of the cantilever moredifficult compared to conventional cantilevers, but at the sametime enables precise control over the cantilever resonance fre-quency which is given by [10](1)where is the eigenvalue of the system corresponding to itsfundamental frequency, having a value of 1.875,the thicknessof the cantilever, its length, the Young’s modulus of silicon(150 GPa) and the density of silicon (2330 kg/m). In con-trast to the fabrication method used for conventional cantilevers,this approach allows both the cantilever length as well as thethickness to be controlled and varied lithographically, since bothdimensions are defined parallel to the substrate surface. A singlesubstrate can carry a large number of cantilevers with differentresonance frequencies, suitable for different applications. Fur-thermore, standard deposition and etching techniques can beused to define the tip plane, as it is also oriented parallel to thesubstrate surface. The result is a reproducible manufacturingprocess that incorporates both the cantilever and the magnetictip and allows for batch fabrication of the probes.Fig. 3. Influence of the cutoff corner of the tip plane on the imaging resolution.Two factors play an important role in realizing a high-resolu-tion MFM tip with the ideal tip shape, as previously described.First, the free-standing tip plane should be very thin, as this de-fines the width of the tip, but also strong enough for contactimaging. At the same time, the tip plane material should havelow residual stress to prevent bending after release. Second, thetip plane should have a well-defined and very sharp cutoff corneras illustrated in Fig. 3 and the etching process should be direc-tional to ensure that the tip end remains flat.III. FABRICATIONThe cantilevers are fabricated from (110) silicon wafers usingKOH wet anisotropic etching. The (110) oriented silicon wafershave a and a plane perpendicular to thewafer surface.The etch rate of the family of {111} planes inKOHsolution is so low that these planes can be considered as etch stopplanes. Precise alignment of the cantilever beams to these planeswill, therefore, enable lateral cantilevers to be made with a thick-ness that only depends on the dimensions of the mask because al-most no underetching will occur. An advanced method was usedto ensure precise alignment to the crystal planes [11]. The crystalplane etch stop also ensures very smooth surfaces, suitable forinterferometric deflection detection.KOH etching of the cantilevers is done in two steps: the firststep etches the wafer from the backside, defining the width ofthe cantilevers. In a second step, the cantilevers are etched fromthe frontside of the wafer. This procedure ensures that the widthof the cantilevers can be precisely controlled. The masks usedduring KOH etching are thin silicon nitride layers, deposited ina low-stress LPCVD process. SiN layers can withstand the KOHprocess, making them a perfect material for fabrication of the tipplanes. The SiN masking layer used during KOH etching is pat-terned afterwards to form the tip planes. The sharp cutoff cornerof the tip plane is obtained by removing the rounded end of thetip plane by means of reactive ion etching using a Cr mask layer.As a final step, a thin Co coating is evaporated on the front sideof the tip plane. To reduce the amount of magnetic material de-posited on the sides of the tip plane, accurate alignment of theprobe with respect to the evaporation source is needed. A scan-ning-electron microscope (SEM) photograph of a CantiCleveris shown in Fig. 4. The free-hanging tip plane and the smoothsides of the cantilever itself can clearly be distinguished.IV. M EASUREMENTSA CantiClever with a resonance frequency of 60 kHz,a 300- m-long cantilever, and a 50-nm Co layer on theVAN DEN BOS et al.: THE CANTICLEVER: A DEDICATED PROBE FOR MAGNETIC FORCE MICROSCOPY 2443Fig. 4. SEM photograph of the CantiClever showing the cantilever and the tipplane.Fig. 5. MFM measurement on a CAMST reference sample.50-nm-thick tip plane, was mounted in a DI3100 microscopefor AFM and MFM measurements.Fig. 5 shows the result of a 11- m MFM scan on a CAMSTreference sample as used in [1]. For this scan, the CantiCleverwas operated in lift mode, detecting the frequency shift at a15-nm tip to sample distance. In this scan, magnetic featuresdown to 30 nm could be distinguished. Currently, work is beingdone on improvement of the resolution by, among others, re-ducing the thickness of the tip plane. It is expected that the tipplane thickness can be reduced below 10 nm.V. CONCLUSIONA new design of a cantilever dedicated for MFM has beendeveloped. The new design integrates the cantilever and the tipin one single batch manufacturing process with the use of sil-icon micromachining techniques. The MFM tip is defined bydeposition of a thin magnetic film on the side of a free-standingSiN layer. In this way, both thickness and width of the mag-netic tip can be defined very accurately by layer depositiontechniques. To facilitate the realization of the free-standing SiNlayer, the cantilever itself is rotated 90with respect to con-ventional manufacturing techniques. A smooth upper surface,suitable for interferometry, is obtained by anisotropic etchingof (110) Si wafers in KOH. The manufacturing process is veryreliable and a very precise control over the tip diameter is pos-sible. Magnetic features down to 30 nm could be observed ina CAMST reference sample, using a tip with a 5050–nmcross section. The authors believe that this technique will allowto produce MFM tips with the ideal tip shape and a cross sectionbelow 10 10 nm.Because the concept of the new probe is centered around theuse of a thin film as a support structure one could imagine othertypes of sensors being integrated on the tip plane. For magneticimaging, a magnetoresistive element could, for instance, be re-alized on the tip plane which would make high-resolution mag-netoresistance microscopy possible.REFERENCES[1] L. Abelmann, S. Porthun, M. Haast, C. Lodder, A. Moser, M. E. Best,P. J. A. Vanschendel, B. Stiefel, H. J. Hug, G. P. Heydon, A. Farley, S.R. Hoon, T. Pfaffelhuber, R. Proksch, and K. Babcock, “Comparing theresolution of magnetic force microscopes using the CAMST referencesamples,”J. Magn. Magn. Mat., vol. 190, no. 1–2, pp. 135–147, 1998.[2] L. Folks, M. E. Best, P. M. Rice, B. D. Terris, D. Weller, and J. N.Chapman, “Perforated tips for high-resolution in-plane magnetic forcemicroscopy,”Appl. Phys. Lett., vol. 76, no. 7, pp. 909–911, 2000.[3] S. Porthun, L. Abelmann, and C. Lodder, “Magnetic force microscopyof thin film media for high density magnetic recording,”J. Magn. Magn.Mat., vol. 182, no. 1–2, pp. 238–273, 1998.[4] S. Porthun, L. Abelmann, S. J. L. Vellekoop, J. C. Lodder, and H. J. Hug,“Optimization of lateral resolution in magnetic force microscopy,”Appl.Phys. A—Mater. Sci. Processing, vol. 66, pp. S1185–S1189, 1998.[5] T. R. Albrecht and C. F. Quate, “Atomic resolution with the atomic forcemicroscope on conductors and non conductors,”J. Vac. Sci. Technol.A—Vac. Surf. Films, vol. 6, no. 2, pp. 271–274, 1988.[6] P. B. Fischer, M. S. Wei, and S. Y. Chou, “Ultrahigh resolution magneticforce microscope tip fabricated using electron beam lithography,”J. Vac.Sci. Technol. B, vol. 11, no. 6, pp. 2570–2573, 1993.[7] G. D. Skidmore and E. D. Dahlberg, “Improved spatial resolution inmagnetic force microscopy,”Appl. Phys. Lett., vol. 71, no. 22, pp.3293–3295, 1997.[8] M. Ruhrig, S. Porthun, J. C. Lodder, S. McVitie, L. J. Heyderman, A. B.Johnston, and J. N. Chapman, “Electron beam fabrication and charac-terization of high-resolution magnetic force microscopy tips,”J. Appl.Phys., vol. 79, no. 6, pp. 2913–2919, 1996.[9] G. N. Phillips, L. Abelmann, M. Siekman, and J. C. Lodder, “High res-olution magnetic force microscopy using focussed ion beam modifiedtips,” Appl. Phys. Lett., 2000, to be published.[10] L. Meirovitch, Fundamentals of Vibrations, International ed, ser. Me-chanical Engineering. New York: McGraw-Hill, 2001.[11] M. Vangbo and Y. Bäcklund, “Precise mask alignment to the crystallo-graphic orientation of silicon wafers using wet anisotropic etching,”J.Micromech. Microeng., vol. 6, pp. 279–284, 1996.

The CantiClever: A Dedicated Probe for Magnetic Force Microscopy

Bolhuis, Thijs

NARCIS 

With the current rate of increase in areal density of magnetic recording systems, todays resolution of MFM needs to be improved in order to remain useful as a measurement tool. The resolution of MFM is amongst others determined by the geometry of the magnetic tip, which for commercial probes consists of a thin magnetic coating deposited on a pyramidal Si AFM tip. Analysis of the imaging process shows that the shape of these tips is not ideal. For higher resolution, the tip must be shaped as an elongated bar with a flat front end. In this contribution we present a completely new MFM cantilever, the CantiClever, which is not derived from traditional AFM probes but optimized for MFM. Our design incorporates the cantilever and the magnetic tip in a single manufacturing process with the use of silicon micromachining techniques, allowing for batch fabrication of the probes. We realise the ideal tip shape by deposition of the magnetic material (Co) on the side of a free hanging silicon nitride layer

Undetermined

The CantiClever. A dedicated probe for magnetic force microscopy

Comparing the resolution of magnetic force microscopes using the CAMST reference samples,”

Electron beam fabrication and characterization of high-resolution magnetic force microscopy tips,”

Fundamentals of Vibrations, International ed, ser. Mechanical Engineering.

High resolution magnetic force microscopy using focussed ion beam modified tips,”

Improved spatial resolution in magnetic force microscopy,”

Magnetic force microscopy ofthinfilm mediafor highdensitymagnetic recording,”J.Magn.Magn.

Perforated tips for high-resolution in-plane magnetic force microscopy,”

Precise mask alignment to the crystallographic orientation of silicon wafers using wet anisotropic etching,”

Quate,“Atomicresolutionwiththe atomicforce microscope on conductors and non conductors,”

Ultrahigh resolution magnetic forcemicroscopetipfabricatedusingelectronbeamlithography,”J.Vac.

http://purl.utwente.nl/publications/55657

The CantiClever: a dedicated probe for magnetic force microscopy

The CantiClever: a dedicated probe for magnetic force microscopy

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