Scanning rates of the atomic  force microscope (AFM) could be significantly \ud
increased by integrating the force sensing probe  with microelectromechanical systems (MEMS). We present a micromachining method for batch fabrication of in-plane AFM probes that consist of an ultra-sharp silicon nitride tip on a single \ud
crystal silicon cantilever. Our fabrication method is fully compatible with the silicon-on-insulator (SOI) micromachining allowing a straightforward monolithic integration of the AFM probes with high-aspect-ratio monocrystalline silicon MEMS. Scanning probes with a sharp tip having diameter of less then 10 nm are successfully realized and tested in a commercial AFM set-up demonstrating \ud
feasibility and the large innovation potential of this method

Abelmann, L.

Berenschot, J.W.

Geerlings, J.

Sarajlic, E.

Siekman, M.H.

Tas, N.R.

Scanning rates of the atomic force microscope (AFM) could be significantly increased by integrating the force sensing probe with microelectromechanical systems (MEMS). We present a micromachining method for batch fabrication of in-plane AFM probes that consist of an ultra-sharp silicon nitride tip on a single crystal silicon cantilever. Our fabrication method is fully compatible with the silicon-on-insulator (SOI) micromachining allowing a straightforward monolithic integration of the AFM probes with high-aspect-ratio monocrystalline silicon MEMS. Scanning probes with a sharp tip having diameter of less then 10 nm are successfully realized and tested in a commercial AFM set-up demonstrating feasibility and the large innovation potential of this method

Sarajlic, Edin

Berenschot, Johan W.

Siekman, Martin Herman

Tas, Niels Roelof

Abelmann, Leon

University of Twente Research Information

Back to the ProgrammeDESIGN, FABRICATION AND CHARACTERIZATION OF AN IN-PLANE AFM PROBE WITHULTRA-SHARP SILICON NITRIDE TIPE. Sarajlic 1, J. Geerlings 2, J.W. Berenschot 2, M.H. Siekman 1,2, N.R. Tas 2 and L. Abelmann 21SmartTip, Enschede, The Netherlands2MESA+ Research Institute, University of Twente, PO Box 217, 7500 AE Enschede, The NetherlandsAbstract — Scanning rates of the atomicforce microscope (AFM) could be significantlyincreased by integrating the force sensing probewith microelectromechanical systems (MEMS).We present a micromachining method for batchfabrication of in-plane AFM probes that consistof an ultra-sharp silicon nitride tip on a singlecrystal silicon cantilever. Our fabrication methodis fully compatible with the silicon-on-insulator(SOI) micromachining allowing a straightforwardmonolithic integration of the AFM probes withhigh-aspect-ratio monocrystalline silicon MEMS.Scanning probes with a sharp tip having diameterof less then 10 nm are successfully realized andtested in a commercial AFM set-up demonstratingfeasibility and the large innovation potential of thismethod.Keywords: Atomic Force Microscopy, Probes,KOH etching, Video-rate AFMI – IntroductionIn atomic force microscopy (AFM) a strong driveexists towards video-rate imaging. In addition to con-venience, the video rate AFM will enable observationof many real time dynamic processes that are currentlyimpossible to study e.g. diffusion of individual atoms,atom clusters or molecules, film growth or catalyticreactions [1].In a conventional AFM system the motion in thevertical z-direction is fastest and requires the highestfrequency components. Therefore, the mechanical res-onance frequencies of a cantilever force-sensor [2] andthe driving piezoelectric element [3] are the main speedlimiting factors in the conventional AFM systems. Inorder to obtain higher scanning rates, the external piezo-electric actuation can be substitute with a micromechan-ical actuator having an integrated force-sensing element[4]. Small dimensions and extremely small mass ofthe microactuators will improve vibration isolation andenable higher scanning rates.In this paper we present a novel micromachiningmethod for bulk fabrication of AFM probes with anultra-sharp tip. This method, which is fully compat-ible with silicon-on-insulator (SOI) micromachining,allows for easy integration of the scanning probeswith high-aspect-ratio monocrystalline silicon microac-tuators. Distinguishing characteristics of our processare as follows: (i) It allows fabrication of the AFMcantilevers with an in-plane tip (see Figure 1). In theFigure 1: Typical AFM probe with out-of-plane tip and lateralcantilever with in-plane tip.in-plane configuration, the oscillation direction of thecantilever is parallel to the wafer plane and coincideswith the preferable motion direction of different elec-trostatic microactuators (e.g. parallel plate actuators orcomb drives) allowing their straightforward integra-tion. Furthermore, the resonance frequency of the in-plane cantilever probes can be more precisely controlledbecause both length and thickness are controlled bylithography. Therefore the resonance frequency is lessdependent on the process parameters. (ii) The processresults in a monocrystalline silicon cantilever. Superiormechanical properties of single crystal silicon such aspractically no stress and low intrinsic damping [5] willlead to improved probe performance. (iii) An importantinnovative aspect of our method is the use of siliconmoulds obtained by KOH etching to form in-plane sili-con nitride tips. Silicon nitride, which is a wear resistantmaterial will result in the improved life time of theprobes. (iv) The in-plane tips are ultra-sharp allowingfor high resolution imaging because the resolution of anAFM set-up is mainly determined by the tip sharpnessof the probe.Outline of this paper is as follows. In Section II wedescribe the basic steps of the microfabrication processand present two different methods for the fabricationof in-plane tips. In Section III we give details on thedesign of the first probe prototype. The fabricationresults and experimental characterization of the probesis presented in Section IV. Conclusions are drawn inthe last section.MME2010 Workshop 24Back to the ProgrammeII – In-plane AFM probeA. Microfabrication processThe fabrication process, schematically shown in Fig-ure 2, is based on an SOI wafer with a (100) top siliconlayer.(a) DRIE and wet anisotropic KOH etch.(b) Thermal oxidation and LPCVD silicon nitride.(c) RIE outline probe and mask for opening near tip.(d) Anisotropic and isotropic DRIE steps.(e) RIE top layer silicon nitride and oxide etch.Figure 2: SOI compatible fabrication process of in-plane AFMprobes.In the top layer, a cavity is etched by combination ofdeep reactive ion etching (DRIE) and wet anisotropicetching in KOH (a). After etching of the cavity, thermaloxidation is performed followed by LPCVD depositionof silicon nitride (b). Next, the outline of the scanningprobe is defined in the silicon nitride/silicon oxidestack by reactive ion etching (RIE). Subsequently, anopening near the probe tip is lithographically defined ina photoresist layer (c) and the tip is partially releasedby combination of anisotropic and isotropic dry siliconetching in a DRIE plasma system. After the tip release,the photoresist mask is removed and the probe layout isetched by DRIE using the silicon nitride/silicon oxidestack as the etching mask. Subsequently, a back etchis preformed (d). Next, the top silicon nitride layer isremoved by a blanket RIE step. Finally, the probe isreleased by etching of silicon oxide (e). The processresults in a monocrystalline silicon scanning probe withan in-plane silicon nitride tip.B. Tip formationIn our process, the probe tip is formed by refilling asilicon mould with a silicon nitride layer. The sharpnessof the probe tip is defined by the profile of the mould.In this paper, we devised and tested two innovativemethods to obtain sharp in-plane tips.In the first method, which is shown in Figure 3, anoctahedron is used as a mould. The silicon octahedronis formed by directional plasma etching followed bywet anisotropic etching in aqueous KOH solution. Thein-plane tip is formed in one of the corners of theoctahedron, which are sharpened by thermal oxidationprior to silicon nitride deposition. This method resultsin a plane-symmetrical tip with four faces defined by<111> crystallographic planes of the silicon octahe-dron. In order to obtain a single sharp tip a perfectsymmetrical octahedron is required.(a) Cross-sectional view of a cavityobtained by DRIE and KOH etching.(b) Tip sharpening by oxidation.Figure 3: Four-face tip formation.In the second method, illustrated in Figure 4, apyramidal silicon mould is formed using an L-shapedmask opening. Silicon oxide is used as the maskinglayer. The tip formed in the top corner of the mould isbounded by two <111> planes of the silicon pyramidand the top plane of silicon oxide. As the three planesalways intersect in a single point, we expect that thisdesign will results in an ultra-sharp tip.III – DesignTo demonstrate the new fabrication method two dif-ferent types of probes were designed. AFM probes withfour-face nitride tips and probes with three-face nitridetips. The layout of silicon cantilever and holder blockare the same for both type of probes.When the tips are formed (as described in the previ-ous section), the next step is the formation of the blockMME2010 Workshop 25Back to the Programme(a) Topview L-shape mask.(b) Cross-sectional view of a cavityobtained by KOH etching.(c) Tip sharpening by oxidation.Figure 4: Three-face tip formation.and cantilever that holds the tip (see right side of Fig-ure 1). Because (100) wafers are used, the octahedron(after KOH) is aligned to the flat of the wafer. However,the tip of the octahedron must be perpendicular to thecantilever. Because oblique lines are difficult to obtain(as mask writers have a limited step resolution) thewafer must be rotated 45 degrees clockwise during thelithography steps needed to form the cantilever andholder block.Initially the cantilever width is equal to the thicknessof the wafer. To reduce the width of the cantilever, abackside etch step is applied. The depth of the backsideetch therefore determines the width of the cantilever.When a SOI wafer is used, the oxide layer serves as etchstop layer. In this design a width of 40-50 µm was aimedfor. The thickness of the cantilever is chosen equal to theminimal accurate thickness that can be obtained, whichis 3 µm in this case. Length of the cantilever is calcu-lated from the desired resonance frequency. For AFMtapping mode the resonance frequency must be between100 and 300 kHz and for contact mode between 5and 10 kHz. Table 1 summarizes the chosen cantileverdimensions and calculated approximate resonance fre-quencies. Definitions of the dimension variables areshown in Figure 5.Because the resonance frequency does not dependon the width of the cantilever, the back etch step is notcritical for the resonance frequency. The cantilever isconnected to a block with length of 3000 µm, thickness450 µm and width determined by the thickness of thewafer (380 µm in this case). This block is necessary forhandling the probe and placing it in the AFM machine.Table 1: Probe dimensions and calculated resonance frequen-cies of the cantilever.Probe 01 Probe 02 Probe 03 Probe 04CantileverLength (LC) 200 µm 300 µm 400 µm 500 µmThickness (TC) 3 µm 3 µm 3 µm 3 µmWidth (WC) 50 µm 50 µm 50 µm 50 µmResonance (f0) 108 kHz 48 kHz 27 kHz 17 kHzHolder blockLength (LB) 3000 µm 3000 µm 3000 µm 3000 µmThickness (TB) 450 µm 450 µm 450 µm 450 µmWidth (WB) 380 µm 380 µm 380 µm 380 µmFigure 5: In-plane probe with definition of variables.The holder block with cantilever is connected to thewafer by means of a breakout beam.IV – Results and DiscussionA. Four-face tip designFigure 6 shows SEM images of the fabricated in-plane AFM probe with four-face tip.Figure 6: SEM pictures of the silicon nitride in-plane AFM tip(four-face) on a mono crystalline cantilever. Tip diameter (seeinset) is 24 nm (left part of the double tip).Clearly visible is the double tip in Figure 6 whichindicates that during KOH etching a ridge was formedin the tip corner, instead of a single sharp tip. Thisinvalidates the initial assumption that the four faces ofthe octahedron (which bound the tip) always intersect inone point, regardless of the shape of the starting cavity.The role of the starting cylinder on the final cavitywas studied by KOH simulations in simulation programACES [6].MME2010 Workshop 26Back to the ProgrammeBoth graphical analysis and simulations of theoctahedron formation showed that a misalignmentbetween bottom plane and top plane of the startingcylinder causes the four planes not to intersect in onepoint. This is also the case when the bottom plane is notcompletely flat. Because DRIE etching is not perfectlyuniform, a perfect cylinder can not be obtained. Thefour faces of the octahedron will therefore alwayslead to a ridge instead of a tip. This was practicallyconfirmed during inspection of the fabricated tips in theSEM (shown in Figure 6).B. Three-face tip designBecause three non-parallel planes always intersect inone single point, this design will result in probes withsingle tips. In Figure 7 the fabrication results of thethree-face tip design is shown. Ultra-sharp single tipswere indeed observed during SEM inspection of theprobes (see Figure 7).Figure 7: SEM pictures of the nitride in-plane AFM tip (three-face) on a mono crystalline cantilever. Tip diameter (see inset)is 9 nm.On several cantilevers a reflection coating wasdeposited by evaporation. This coating consisted of a 5nm Cr layer and a 15 nm Au layer. These probes weremounted in a Nanoscope Dimension 3100 [7] to obtainAFM images. An AFM image (in tapping mode) of asilicon wafer (with native oxide) is shown in Figure 8.A three-face probe was used for this image. Featuresless than 10 nm can be observed, indicating the tipsharpness.V – ConclusionsA new batch manufacturing process for in-planeAFM probes was presented. By using a silicon mouldultra-sharp durable nitride tips can be constructed ona monocrystalline silicon cantilever. This fabricationmethod was successfully demonstrated for two differentprobe designs. The design using an octahedron mouldlead to a double tip, however, the design with apyramidal mould (partially covered with oxide) lead toa single ultra-sharp tip. These probes were successfullymounted in a commercial AFM set-up. AFM imagesFigure 8: AFM image of a silicon wafer obtained with a three-face tip probe in tapping mode at 34 kHz.with a resolution less than 10 nm were obtained. Thefabrication method is fully SOI compatible whichallows for easy integration of an (electrostatic) MEMSactuator. This enables a higher frequency in the z-direction and makes video-rate AFM feasible.VI – AcknowledgmentsThe authors would like to thank Mark Smithers fortaking the SEM images.References[1] M.J. Rost, G.J.C. van Baarle, A.J. Katan, W.M. vanSpengen, P. Schakel, W.A. van Loo, T.H. Oost-erkamp, and J.W.M. Frenken. Asian Journal ofControl, 11(2):110–129, 2009.[2] G.E. Fantner, G. Schitter, J.H. Kindt, T. Ivanov,K. Ivanova, R. Patel, N. Holten-Andersen,J. Adams, P.J. Thurner, I.W. Rangelow, and P.K.Hansma. Ultramicroscopy, 106(8-9):881–887,2006.[3] S.R. Manalis, S.C. Minne, and C.F. Quate. AppliedPhysics Letters, 68(6):871–873, 1996.[4] F. Levent Degertekin. In Lasers and Electro-OpticsSociety, 2007. LEOS 2007. The 20th Annual Meet-ing of the IEEE, pages 832 –833, 21-25 2007.[5] K.E. Petersen. Proceedings of the IEEE, 70(5):420– 457, may 1982.[6] Z. Zhu and C. Liu. Journal of Microelectromechan-ical Systems, 9(2):252–261, 2000.[7] Veeco Instruments. http://www.veeco.com/, 2010.MME2010 Workshop 27

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Design fabrication and characterization of an in-plane AFM probe with ultra-sharp silicon nitride tip

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Design fabrication and characterization of an in-plane AFM probe with ultra-sharp silicon nitride tip

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