Scanning Probe Nanoimprint Lithography

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Scanning probe nanoimprint lithography

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Please note that terms and conditions apply. IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 21 (2010) 075305 (6pp) doi:10.1088/0957-4484/21/7/075305 Scanning probe nanoimprint lithography

F Dinelli1, C Menozzi2,4, P Baschieri1, P Facci2 and P Pingue3,5

1 IPCF, Consiglio Nazionale delle Ricerche, CNR Campus, Via G. Moruzzi 1, Pisa PI 56100, Italy 2 CNR-INFM S3 National Research Center on Nanostructure and BioSystems at Surfaces, Via Campi 213/a, 41100 Modena, Italy 3 NEST, Scuola Normale Superiore and CNR-INFM, Piazza San Silvestro 12, I-56127 Pisa, Italy 4 Dipartimento di Fisica, Universit`a di Modena e Reggio Emilia, Via Campi 213/a, 41100 Modena, Italy

E-mail: [email protected]

Received 5 November 2009, in ﬁnal form 23 December 2009 Published 21 January 2010 Online at stacks.iop.org/Nano/21/075305

Abstract The present paper reports on a novel lithographic approach at the nanoscale level, which is based on scanning probe microscopy (SPM) and nanoimprint lithography (NIL). The experimental set-up consists of an atomic force microscope (AFM) operated via software specifically developed for the purpose. In particular, this software allows one to apply a predefined external load for a given lapse of time while monitoring in real-time the relative distance between the tip and the sample as well as the normal and lateral force during the embossing process. Additionally, we have employed AFM tips sculptured by means of focused ion beam in order to create indenting tools of the desired shape. Anti-sticking layers can also be used to functionalize the tips if one needs to investigate the effects of different treatments on the indentation and de-molding processes. The lithographic capabilities of this set-up are demonstrated on a polystyrene NIL-patterned sample, where imprinted features have been obtained upon using different normal load values for increasing time intervals, and on a thermoplastic polymer film, where the imprint process has been monitored in real-time.

Studies of polymeric films by means of atomic force measure normal and lateral forces. AFM can thus provide microscopy (AFM) [1] have begun almost immediately after topographical, mechanical and adhesion properties of the the technique’s inception [2]. These investigations have sample investigated [12, 13], making it an ideal tool for dealt with a wide range of issues, from the molecular lithographic applications. In the end, it can be stated that NI organization [3] to wear [4], lithography [5] and mechanical and AFM techniques can be found very useful for studying properties [6]. This has originated from the absolute novelty surface properties, used separately or in combination. represented by investigating non-conductive surfaces on the The initial aim of the work herein presented was nanoscale. In parallel, the development of the nano-indenter to develop a novel approach that might allow one to (NI) [7] has provided the scientific community with another perform scanning probe nanoimprint lithography (SP-NIL). powerful tool for material characterization. The two techniques Specifically, we intended to investigate the phenomena have different characteristics and each one can be appropriated occurring during the process of pattern formation when a stamp for addressing different material properties. On the one is placed in contact with a polymeric film as in standard hand, NI can provide a quantitative analysis of Young’s and NIL. Towards the accomplishment of this goal, we have plastic moduli, not achievable with the same accuracy with individuated AFM as the most suitable tool due to its imaging, AFM [8, 9]. For instance, Cross and co-workers have recently alignment and positioning capabilities, its high sensitivity to demonstrated the NI capabilities of studying thin films on adhesive forces and dynamics analysis [14]. We have thus rigid substrates [10, 11]. On the other hand, AFM, initially developed for imaging rather than testing surface properties, is assembled an experimental set-up that allows one to define the very sensitive to small force values and one can simultaneously indentation parameters and to monitor the cantilever deflection and torsion during the indentation process. In particular, we 5 Author to whom any correspondence should be addressed. have introduced in standard force versus distance curves the

Figure 1. (A) Schematics of the experimental set-up. The cantilever is mounted on a piezo-tube. SEM images of a tip modified by means of FIB in order to create a square-shaped punch for indenting and a small tip for imaging: (B) low magnification lateral view; (C) top view; (D) high magnification lateral view.

‘hold’ time, defined as the lapse of time during which the tip cycle typically observed for any piezoelectric material. This and the surface are in contact under a given load. We have also phenomenon reflects itself in the two branches of the force used a focused ion beam (FIB) to modify AFM tips in order to versus distance curves: they do not fully superimpose although make them suitable for both imaging and indenting [15, 16]. the x-axis values are nominally the same ones. For a correct The experimental set-up is briefly described in the data analysis, one needs to record force versus distance curves following (see schematics in figure 1(A)). The microscope on a rigid surface as reference (e.g. silicon oxide) [18]. Due head is a commercial one (SMENA, NT-MDT). In the reported to the high stiffness of the rigid surface, one can assume that configuration, the cantilever is always moved with respect to the deformation of the two bodies is negligible and that all the the sample when imaging or indenting. The cantilever/sample displacement between the cantilever base and the specimen is alignment can be adjusted in two ways: with a screw that tilts converted into cantilever deflection. Under this assumption, the plane where the AFM head stands along the direction of one can calibrate the optical lever detection system. As the cantilever long axis; or with the three legs of the AFM described above, for a complete monitoring of the whole head that can be independently shortened or lengthened. The process, an additional parameter has been implemented in our electronics (based on a DSP) and software have both been software. It allows one to set a certain thold during which the developed in our laboratory [17]. They allow one to approach tip is maintained in contact with the surface at a given Fset. the tip to the sample and stop at a given force value that we This approach is derived from force clamp experiments [19] call the ‘maximum load’ (Fmax). At this moment a feedback where the cantilever deflection and torsion are monitored while circuit is activated, keeping the normal force constant at a value keeping the load constant. thold is also very similar to the previously set (Fset). Fmax and Fset can be varied in order to so-called ‘dwell time’ that is defined in e-beam lithography reduce undesired effects due to creep of the piezo-actuator. The (EBL) or in dip pen nanolithography (DPN) [20]. During this normal load is maintained at Fset for a variable lapse of time interval, we are therefore able to monitor and record data such called in the following the ‘hold’ time (thold), during which the as the cantilever deflection and torsion as well as the vertical vertical displacement and the lateral forces are recorded. displacement of the tip with respect to the sample. The actuator of our AFM head is a tube made of As mentioned earlier, one of our goals was to mimic and piezoceramic material. The tube elongation is not linear study the NIL process. It is known that homogeneous NIL with the applied voltage and is characterized by a hysteresis on large areas can be rather difficult to perform. Besides

2 Nanotechnology 21 (2010) 075305 F Dinelli et al alignment issues, problems still unsolved regard fluid-dynamic phenomena of polymers filling the mold. Addressing this issue at the nanoscale level would be beneficial to finding viable solutions. We have thus designed tips with the desired shapes as a way to realize indenters directly on AFM cantilevers. In order to shape the tip according to our purposes, an FIB system has been employed [16]. This procedure can be in principle applied to any tip, shaping it in nearly any arbitrary fashion. In this first exemplifying experiment, we have decided to employ a very stiff cantilever in order to perform the NIL process on a polymeric sample at room temperature, where high pressure is required to produce significant embossing. In more detail, the employed cantilever had an elastic constant of ∼38 N m−1. Figures 1(B)Ð(D) show SEM images of the tip used to collect the data presented. One can see that a square punch has been fabricated at the apex of the cone and that a smaller tip has also been realized in the same region for imaging purposes. The plane where the two objects lie forms an angle of about 10◦ with respect to the cantilever long axis (figure 1(B)). This modification has been devised in order to facilitate the alignment of the scanning plane with the specimen. Once the plane is parallel to the surface, the imaging tip protrudes 250 nm off the punch (figure 1(D)). Anti- sticking treatments can be applied if one needs to study and minimize the effects of the adhesive forces on the indentation processes. In the present case, the recipe used to obtain a hydrophobic coverage is the following: 15 silanization solution (dimethyldichlorosilane in heptane by Fluka©), 15 hexane, 15 acetone, 15 hexane and rinsing in deionized water. Adjustments of the relative sampleÐcantilever tilting are initially performed in order to align the tip and the sample. The general procedure consists in imaging a calibration grating made of tips with high aspect ratio (purchased from NT-MDT) while working in tapping mode. As already described, the relative tilting can be corrected via a screw mounted on the sample stage or via the three screws present on the head. The procedure is iterated until the tip imaged via the calibration grating results in alignment as one desires, while the planarity with respect to the employed polymeric sample was obtained by placing it adjacent to the grating. Figure 2. (A) AFM image of an area of a PS-patterned film where a The sample has been fabricated by a standard thermal single indentation has been performed by means of the sculptured tip NIL process using a silicon mold and a commercial imprinter shown in figure 1. In this case the parameters were: Fset equal to (Obducat 2.5 ) with a stamp made of parallel grooves with a Fmax of 13 μNandthold of 5 . A square mark produced by the punch periodicity of 1.4 μm. The imprinted sample is made of atactic is very visible, while the hole below was produced by the ‘imaging’ polystyrene (PS, from Sigma-Aldrich). This patterned sample tip. (B) Apparent depth profile along the dotted line of the previous image. (C) Normal force (F ) versus vertical distance (z) curve on represents an ideal system to demonstrate that indentations can N PS at room temperature (thold is equal to zero): green arrows be performed where we want and under predefined conditions highlight the tip-to-punch distance, while the black circular one is the (load, speed, etc) imposed via the lithographic software. In force versus distance cycle. figure 2(A) a typical indentation obtained at Fmax equal to Fset of 13 μNandathold of 5 is shown. One can see a tip compatible with the distance measured by means of SEM square mark produced by the punch along with a mark due to (see figure 1(C)). Experiments carried out at lower Fmax values the penetration of the ‘imaging’ tip. The hole created by the do not lead to the formation of square features. In figure 2(B), imaging tip appears larger than the imaging tip itself. This is we report a typical force versus distance curve (thold equal to probably due to tip sliding that occurs in the direction of the zero) recorded during an indentation process. The hysteresis slow scan speed and takes place when the punch touches the is pronounced as Fmax is rather large. The cartoons placed polymer surface. The square mark indicates instead that the beside different curve portions help to visualize the indentation punch has successfully indented the polymer. It shows a shape sequence. The first green arrow on the right-hand side of very similar to the punch itself at a distance from the imaging the graph indicates the point of initial contact. Let us now

3 Nanotechnology 21 (2010) 075305 F Dinelli et al

Figure 3. Left panels: AFM images of three indentations made varying Fset (A) or thold (C). Above the holes produced by the ‘imaging’ tip one can see the square marks made by the punch. For each point is indicated the Fset or thold value during which the tip is left in contact with the surface (Fset of 13 μN). Right panels: graphs where the behavior of the maximum apparent indentation depth versus applied load (B) and time interval (D) are reported, both for tip (squares) as for punch (triangles). focus on the region indicated by the second green arrow. It set to zero. In our opinion, these results find an explanation in can be noticed that the curve changes slope suddenly and the fact that the applied pressure was always above the plastic becomes less steep. This is likely to indicate that, after yield value. In figure 3(B), the maximum apparent depth of the initial contact with the ‘imaging’ tip, the square punch the imaged holes (imaging tip and punch) are reported as a comes into contact with the surface. In principle, this should function of the applied load. It can be noticed that the square determine an increase of the contact area and consequently of hole depth results are deeper for increasing Fmax, indicating the total stiffness but (as discussed in more detail afterward) a higher penetration of the punch in the sample while the also a leverage effect of the punch that causes an anomalous maximum tip penetration decreases. This can be ascribed to a bending of the cantilever. Depending on the laser position leverage effect of the cantilever acting on the punch as a pivot on the cantilever, one could detect an apparent decreasing of (see the schematics in the inset of figure 3(B)). the applied load, which the feedback tries to compensate by In figure 3(C), we show the outcome of a similar further extending the z actuator. The relative distance between experiment in which the tip is placed in contact with the surface the two arrows is roughly equal to 250 nm, corresponding to at an Fset of 13 μN for three different thold values. The numbers the height difference between tip and punch (see figure 1(D)). in white indicate the duration of each indentation. Three Notice also that no adhesion is observed at the pull-off position couples of marks appear again as a result of the indentations. In in the retracting curve due to the anti-sticking treatment of figure 3(D) the maximum apparent indentation depth (δz,MAX) the tip. Nevertheless we noticed double-tip artifacts in the is reported versus thold. It is evident that time is an important imaging subsequent to the imprint process, probably due to factor: the longer thold the more defined the mark and the some deterioration of the probe after various lithographic and deeper its indentation in the polymer. In more detail, the imaging steps. leverage effect observed before is decreased by the viscous In figure 3(A), three couples of holes can be individuated. behavior of the polymer in this time and pressure range. This They have been created upon three indentations at the Fmax allows a deeper penetration of the tip itself while the maximum values indicated beside the square holes. In all cases, thold was penetration of the punch seems to reach a plateau.

4 Nanotechnology 21 (2010) 075305 F Dinelli et al ◦ an indentation process @ 85 C, with Fset equal to 600 nN and thold equal to 120 . Whereas the deflection is maintained constant (green dotted curve in figure 4(A)), the variation in the vertical displacement increases with time and tends to saturate at a value of 90 nm (see the averaged blue dotted curve in figure 4(C)). This corresponds to an expansion of the piezotube (i.e. moving the tip towards the surface) needed to maintain the cantilever deflection constant. The film thickness was estimated to be of the order of 100 nm, as measured by means of a profilometer. During the approaching part of the force versus distance graph, the tip already penetrates into the film and something happens at the dip indicated by the blue arrow. We deduce that the probe ‘feels’ the rigid substrate and, therefore, that the observed saturation exactly corresponds to the rigid surface stopping the tip penetration. It is also very important to monitor the cantilever torsion (FL). In the present case it remains nearly constant for most of the time, indicating that the lateral position does not vary during the imprint process. However, in correspondence to the arrow drawn in the graph (figure 4(B)), a sudden reduction of the average value occurs after a slight build-up of its average value and just after a small variation in the vertical displacement graph (see the blue arrow in figure 4(C)). This is likely to indicate a lateral movement of the tip probably caused also in this case by the interaction with the rigid substrate and that can be detrimental to a correct analysis of the process. Upon monitoring FL, it is thus possible to discard the curves where such an event has occurred and to have information about the polymer flow during the embossing process. A recent paper indicates a way to overcome the problem, i.e. keeping the cantilever torsion constant by moving laterally the cantilever holder when it varies [21]. This is an interesting approach that, combined with our system, would provide the opportunity to fully control the forces involved during each indentation. Finally, one can envision a profitable use of our set-up in Figure 4. Plots of (A) the normal load (FN), (B) the cantilever torsion (FL) and (C) the vertical displacement (z) as a function of the study of a thin film phase transition. The capabilities ◦ ◦ time during an indentation process on mr-I 7020 @85 C. The tip is of quantifying the tip penetration and controlling FL can be placed in contact with the surface with Fset of 600 nN. The dotted of great added value once it is combined with techniques lines indicate the average Fset, FL, and absolute z displacement, previously developed [22]. respectively, whereas the arrows highlight the critical points at which a tip movement occurs. Inset in (A): AFM image of the tip In conclusion, in this paper we have presented an indentation on the polymeric film. experimental apparatus that allows one to perform indentation on the nanoscale while monitoring the whole process. This approach exploits AFM characteristics such as precise In order to better evaluate the capabilities of our positioning via lithography software, definition of the experimental set-up in terms of real-time monitoring of the indentation parameters such as the maximum load, the speed normal and lateral forces during the imprint process, we have and the hold time. Additionally, we have employed tips employed as a specimen, a thermoplastic film heated at a sculptured into predefined shapes. We have thus demonstrated temperature (T ) above its glass transition temperature (Tg). In that with a modified tip one can simulate nanoimprint these conditions the embossing process results are possible by lithography processes. The system developed allows one to means of an indenter having an increased sensitivity to very fully follow an indentation in real-time monitoring of the low normal and lateral forces (a standard conical tip with a cantilever deflection and torsion as well as the relative distance cantilever having a lower stiffness). The specimen was realized between the tip and the surface. This opens up the possibility by spin-coating a solution of mr-I 7020 (a thermoplastic from of investigating the dynamics of viscoelastic phenomena to a Micro Resist©) onto silicon oxide substrates. In a second deeper level. ◦ moment, thermal annealing is performed above its Tg (60 C) in In the future we intend to employ our set-up in order to order to fully evaporate the solvent trapped in the film. Finally, allocate a UV waveguide under a transparent sample holder. the imprinting process was realized by the previously described Using a polymer sensitive to UV, we can irradiate the film procedure. In figure 4, we plot the signals acquired during while the tip is in contact with the sample. In this way the mark

5 Nanotechnology 21 (2010) 075305 F Dinelli et al made by the sculptured tip can be instantaneously frozen due [6] Overney R M and Meyer E 1993 MRS Bull. 18 1 to the photo-induced polymerization [23], avoiding in this way [7] Pethica J B, Hutchings R and Oliver W C 1983 Phil. Mag. A the thermal drift of the sample caused by temperature raising 48 593 as in the standard hot-embossing technique. The sample can [8] Oliver W C and Pharr G M 1992 J. Mater. Res. 7 1564 then be imaged with no deformation due to polymer relaxation. [9] Oliver W C and Pharr G M 2004 J. Mater. Res. 19 3 [10] Cross G L W, O’Connell B S, Ozer H O and Pethica J B 2007 This will allow one to investigate the indentation procedures Nano Lett. 7 357 and select the optimum tip treatment for SP-NIL applications. [11] Cross G L W, O’Connell B S and Pethica J B 2005 Appl. Phys. Finally, another possible development of this experimental Lett. 86 1902 approach will be related to the use of thermal probes having [12] Cappella B, Kaliappan S K and Sturm H 2005 Macromolecules the heater integrated onto the cantilever itself [24]6.FIB 38 1874 sculpturing of these probes could allow the study of the hot- [13] Tallal J, Gordon M, Berton K, Charley A L and Peyrade D 2006 embossing lithographic technique and polymer viscoelastic Micro Electron. Eng. 83 851 behavior at the nanoscale level. [14] Borkenzko T, Tormen M, Hock V, Liu J, Schmidt G and Molekamp L W 2001 Micro Electron. Eng. 57/58 389 [15] Calabri I, Pugno N, Rota A, Marchetto D and Valeri S 2007 Acknowledgments J. Phys.: Condens. Matter 19 395002 [16] Menozzi C, Calabri L, Facci P, Pingue P, Dinelli F and The authors would like to acknowledge Marco Cecchini from Baschieri P 2008 J. Phys: Conf. Ser. 126 012070 Laboratorio NEST, Scuola Normale Superiore in Pisa, for NIL- [17] Cappella B, Baschieri P, Frediani C, Miccoli P and imprinted PS samples. Ascoli C 1997 Nanotechnology 8 82 [18] Clifford C A and Seah M P 2005 Appl. Surf. Sci. 252 1915 [19] Samori B, Zuccheri G and Baschieri P 2005 ChemPhysChem References 6 2 [20] Salaita K, Wang Y H and Mirkin C A 2007 Nat. Nanotechnol. [1] Binnig G, Quate C F and Gerber C 1986 Phys.Rev.Lett. 2 145 56 930 [21] Shegaonkar A C and Salapaka S M 2007 Rev. Sci. Instrum. [2] Sarid D 1991 Scanning Force Microscopy (New York: Oxford 78 3706 University Press) [22] Overney R M, Buenviaje C, Luginbuhl R and Dinelli F 2000 [3] Hobbs J K, Humphris A D L and Miles M J 2001 J. Therm. Anal. Calor. 59 205 Macromolecules 34 5508 Dinelli F, Buenviaje C H and Overney R M 2000 J. Chem. [4] Leung O M and Goh M C 1992 Science 255 64 Phys. 113 2043 [5] Jung T A, Moser A, Hug H J, Brodbeck D, Hofer R, Cappella B, Kaliappan S K and Sturm H 2005 Macromolecules Hidber H R and Schwarz U D 1992 Ultramicroscopy 38 1874 42–44 1446 Sohn L L and Willet R L 1995 Appl. Phys. Lett. 67 1552 [23] Colburn M et al 1999 Proc. SPIE 3676 379 Pingue P, Lazzarino M, Beltram F, Cecconi C, Baschieri P, [24] The ‘millepede’ project: Cherubini G et al 2002 ESSCIRC Frediani C and Ascoli C 1997 J. Vac. Sci. Technol. B 2002: Proc. 28th European Solid-State Circuits Conf., 2002 15 1398 (Sept. 2002) pp 121Ð5

6 Thermal probes are commercially available, e.g. by Anasysinstruments www.anasysinstruments.com and Veeco www.veeco.com