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A route to fabricate nanocontacts by X-ray lithography for the realization of single electron transistors and highly sensitive biosensors

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A route to fabricate nanocontacts by X-ray lithography for the realization of single electron transistors and highly sensitive biosensors
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  A route to fabricate nanocontacts by X-ray lithography for the realization of single electron transistors and highly sensitive biosensors Sandro Carrara  a, ⁎ , Davide Ricci  a  , Ermanno di Zitti  a  , Enzo Di Fabrizio  b,c ,Matteo Altissimo  c , Massimo Tormen  c a   Department of Biophysics and Electronics Engineering (D.I.B.E.), University of Genoa, Italy  b Università Magna Græcia, Campus Germaneto, Viale Europa, I-88100 Catanzaro, Italy c TASC-INFM CNR, Beam Line Lilit, Elettra Synchrotron, Basovizza, Trieste, Italy Received 14 October 2005; accepted 22 March 2006Available online 21 April 2006 Abstract The fabrication of single electron transistors and/or highly sensitive biosensors is still a challenging task on account of the tight control requiredto get proper shapes and size of the electrodes. The nanosized tips and the separation of a few nanometers between electrode pairs are criticalfeatures. Conventional lithography is not suited to obtain these features because of the resolution limits, so that previous alternative approacheshave involved the use of electron beam lithography, focused ion beam lithography or scanning probe nanolithography. The novel approach presented in this letter is the exploitation of X-ray lithography in the Elettra synchrotron to fabricate arrays of nanocontacts spaced a fewnanometers, devoted to the design of a new class of nanodevices based on nanoparticles and/or single molecules, including single electrontransistors and highly sensitive biosensors. The method to fabricate such devices is illustrated and discussed. Experimental details of thefabrication process are given and preliminary results are presented through SEM and AFM images. It is worth noting that this paper presents aviable method to produce nanocontacts by using the X-ray lithography by synchrotron radiation source, that has not yet been reported together with experimental, though preliminary, data.© 2006 Elsevier B.V. All rights reserved.  Keywords:  Nanocontacts; X-ray lithography; Synchrotron radiation source 1. Letter It is widely recognized that new generations of nanoelec-tronic devices and biosensors will allow efficient addressing of single molecules inside simple devices. The rapid progress of nanomaterials and nanotechnology has allowed the utilizationof nanostructured systems like nanoparticles [1], nanotubes [2] and fullerenes [3] in device structures, and also has providedmethods to obtain electrochemical mediators in biosensor ap- plications. Nevertheless, the key problem is the addressabilityof single molecules at nanometer scale. For instance, in singleelectron devices the proper coupling of a single molecule tosource and drain electrodes would imply the possibility to inject only few electrons, while high-sensitivity biosensors [4] wouldincrease their performance. It is worth noting that in suchcontext many recent published works are related to singlemolecules but the addressability/sensing of a single molecule ina device structure has not yet exploited. On the other hand, theemerging area of molecular electronics has already showed the possibility to make conductivity measurements on single mole-cules [5], thus opening the possibility to fabricate single-molecule nanodevices. To this end, it is first necessary to findefficient ways to make nanocontacts, since the availablemethods limit the device fabrication to one device at a time[6]. Moreover, the success of such devices will depend also onthe possibility to exploit single-electron phenomena at roomtemperature. These phenomena are related to the quantizedvariation of the number of electrons in a conductive island anddepend strongly on the size of the island and on the temperature, Materials Letters 60 (2006) 3682 – 3685www.elsevier.com/locate/matlet  ⁎ Corresponding author. Present address: Biochemistry Department «Gio-vanni Moruzzi», Bologna University. Via Irnerio, 48 - 40126 Bologna (Italy).Tel.: +39 051 209 43 88; fax: +39 051 209 43 87.  E-mail address:  sandro.carrara@unibo.it  (S. Carrara).0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.03.084  under the condition that the charging (electrostatic) energy of the island would be higher than thermal fluctuations by a fewkT. The theory [7] shows that the room temperature operationrequires the use of nanostructures having a conducting islandwith size below 10 nm, placed between an electrode pair (nanocontacts) and separated from them by tunnelling barriers.Hence it follows that the development of room temperatureSingle Electron Transistors (S.E.T.) involves non trivialconstraints. Up to now, all the proposed techniques have ena- bled the fabrication of single S.E.T. prototypes involving highcosts that are unsuitable for industrial applications where parallel mass production is mandatory. Nanostructures neededfor the S.E.T. fabrication were usually fabricated by litho-graphic methods producing only a structure at a time, likeelectron beam lithography [8], Focused Ion Beam (FIB)lithography [9] or anodic oxidation by Scanning ProbeMicroscopy (SPM) [10]. On the other hand, the nanocontact fabrication can be parallelized by a multiple exposure strategyin photolithography. The progressive scaling of microelectronicstructures has relied on planar photolithography, capable toreturn many identical structures (up to 10 9 on the same chip)fabricated with few shots. High-resolution X-ray lithography(XRL) can replicate a multilevel mask by amplifying thethickness profile and can generate complex 2D structures withmultiple exposure at tilted angles, thus allowing the fabricationof structures with nanometric features [11]. In particular, theaim of this research, still running at the Lilit Beam-line installedat Elettra Synchrotron, is to exploit X-ray lithographic tools toobtain nanocontacts useful for biosensors, bioelectronic devicesand S.E.T. fabrication.The methodology relieson the fabrication of verysharp tipsinfront of straight lines in order to obtain suitable nanocontacts for addressing single molecules deposited, in a second time, by self-assembly or by Langmuir techniques. Fig. 1 sketches a pair of such contacts (source/drain in a S.E.T.) after the deposition of nanoparticle ultra-thin film on the structure. The nanoparticleshaveagoldcoreandanorganiccapping(alkanethiol)thatprevent them from spontaneous aggregation. Such nanoparticles can besynthesized [12] with different core diameters and, by usingstabilizing molecules with different length, can be organized inlattices that have different spacing, so that they can be adapted tothe resolution obtained by the parallel X-ray process.The strategy for fabricating the aforesaid structures schema-tized in Fig. 2 has been divided in two process sequences. Thefirst one ends with the formation of an array of sharp metal tips.This sequence comprises two consecutive X-ray exposures andrespective developments of two misoriented gratings on super-imposed photoresist layers followed by a metal lift-off. Thesecond step is a more conventional process consisting in thefabrication of counter-electrodes by single X-ray exposure andmetal lift-off. In this letter we describe the concepts and theactual process sequence employed for the most technicallychallenging part, i.e. that for fabricating an array of nanotips.The main objective in our investigation was to overcome theresolution limits of the employed lithographic technique. Gene-rally, the difficulty in the fabrication of structures with very finedetails is hindered by various blurring effects, such as proximityin electron beam lithography or diffraction in optical and X-raylithography. In particular, the intensity distribution of theradiation delivered to the resist through an X-ray mask does not reproduce the pattern of a sharp detail of the mask (in the tens of nm range) due to intense diffraction effects. In order tocircumvent such a problem, we have addressed the fabricationof sharp features by geometrical intersection of smoother struc-ztures. For example, the intersection of two superimposedgratings with different orientations can be obtained by a metalevaporation which is the physical realization of the orthogonal projection onto the substrate of the intersection of the openedareas of both layers. In order to obtain the sharpest possible tipswith this technique, it is crucial that the fabrication of the twosuperimposed orders of gratings does not introduce any corre-lation between their structure, in particular at the crossing points between the lines. For this reason, the choice for the resist has toconsider that the spin-coating, exposure and development steps Fig. 1. Scheme of the structure to fabricate a S.E.T. showing how the singleelectron trapping island is provided by a gold nanoparticle stabilized by thiolsdeposited in Langmuir  – Schaeffer thin films. The unit   U  is few nm.Fig. 2. Scheme of the structure with sharp tips in front of straight lines showingthe parallel strategy used in these experiments.3683 S. Carrara et al. / Materials Letters 60 (2006) 3682  –  3685  of the second resist should not affect the pattern obtained in thefirst resist. In our experiments, the resist for the first exposurewas Polymethylmethacrylate (PMMA) with a thickness of 450 nm. The resist for the second X-ray exposure was UVIII ™ with a thickness of 500 nm. UVIII is a chemically amplifiedresist, consisting of a copolymer of 4-hydroxystyrene and t- butylacrylate [13] that requires a much lower dose with respect to PMMA. Moreover, the developer of UVIII does not affect PMMA. The X-ray beam exposure dose for PMMA was of 3000 mJ/cm 2 , whereas the optimal exposure dose for UVIII wasof 160 mJ/cm 2 . Experiments using different combinations of resists, like PMMA and poly-(dimethyl-glutarimide) (PMGI),PMMA and LOR, PMMA and SAL-601 were tried. However, better results were obtained with the combination of PMMAand the UVIII. The substrates for the experiments were Siliconwafers with 300 nm thermally grown silicon oxide layer. Themask consisted of a 400 nm wide, 10  μ m straight trenches in a450 gold film obtained by electrolytic growth on a 1  μ m thick Si 3  N 4  membrane. A sample holder allows the mounting of thesystem consisting of X-ray mask and sample onto the stage of the X-ray stepper, ensuring a permanent and fixed contact  between mask and sample. 6  μ m thick aluminum spacers wereinterposed between the mask and the resist surface of the samplein order to avoid damage of the fragile silicon nitride membrane.Finally, the fabrication of metallic structures were done byevaporation of a 3 nm layer of Chromium and of a 10 nm layer of gold followed by the stripping of the PMMA and of theUVIII resists in warm acetone at 50 °C. The Scanning ElectronMicroscope (SEM) images taken after the first processing stepshow sharp triangular tips (Fig. 3), with curvature radii in therange of few tens of nanometers (Fig. 4). These structures havealso been characterized by Atomic Force Microscopy (AFM).The AFM images show compact structures with decreasingthickness and very high curvature radius of the apical part of thetips. A typical fabricated sharp tip is shown in Fig. 5: the AFMcross-sections taken at different heights in the range between 5and 20 nm indicate that the corresponding widths at half-height are in the range 40 – 70 nm. However, convolution effects withthe AFM might have supplied and over-estimation of the radius Fig. 3. The fabricated sharp tips by Scanning Electron Microscopy (SEM)showing the parallel process of fabrication.Fig. 4. A fabricated sharp tips by SEM microscopy showing the very highcurvature radius of the apical part of the tips.Fig. 5. A fabricated sharp tips by AFM microscopy showing the very highcurvature radius of the apical part of the tips.Fig. 6.  (a)  Cross-section along the longitudinal axis of the contact. The arrowshows where the lateral cross-section has been measured.  (b)  Cross-sectionalong a direction perpendicular to the longitudinal axis of the contact (taken at the position indicated by the arrow in (a). The width at half-height is 42 nm.3684  S. Carrara et al. / Materials Letters 60 (2006) 3682  –  3685  of curvature. Fig. 6 displays the cross-sectional line profilesalong the two principal axes of the contact at a height well below 10 nm, that show a suitable shape to address island sizesof S.E.T. operating at room temperature.These results, though preliminary, are encouraging to pursuethe fabrication of nanodevices based on the addressing a singlenanoparticles by exploiting synchrotron X-ray lithography. Toour knowledge, this is the first report on a strategy and on the preliminary results for the fabrication of nanocontacts by usingthe X-ray lithography. References [1] V.V. Shumyantseva, S. Carrara, V. Bavastrello, D.J. Riley, T.V. Bulko,K.G. Skryabin, A.I. Archakov, C. Nicolini, Biosensors and Bioelec-tronics 21 (2005) 217 – 222.[2] C. Chai, J. Chen, Analytical Biochemistry 325 (2004) 285 – 292.[3] I. Szyma ń ska, H. Radecka, J. Radecki, D. Kikut-Ligaj, Biosensors andBioelectronics 16 (2001) 911 – 915.[4] D.L. Graham, H.A. Ferreira, P.P. Freitas, J.M.S. Cabral, Biosensors andBioelectronics 18 (2003) 483 – 488.[5] M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin, J.M. Tour, Science 278(1997) 252 – 254.[6] S. Carrara, D.J. Riley, V. Bavastrello, E. Stura, C. Nicolini, Sensors andActuators. B, Chemical 105 (2005) 542 – 548.[7] K.K. Likharev, Proc. IEEE 87 (1999) 606 – 632.[8] Y.T. Tan, T. Kamiya, Z.A.K. Durrani, H. Ahmed, Journal of AppliedPhysics 94 (2003) 633 – 637.[9] T. Nagase, K. Gamo, T. Kubota, S. Mashiko, Thin Solid Films 499 (2006)279 – 284.[10] K. Matsumoto, Physica, B 227 (1996) 92 – 94.[11] F.Romanato,E.DiFabrizio,L.Vaccari,M.Altissimo,C.Cojoc,L.Businaro,S. Cabrini, Microelectronic Engineering 57 – 58 (2001) 101 – 107.[12] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, Journal of theChemical Society, Chemical Communications (1994) 801 – 802.[13] D.P. Mancini, J.W. Thackeray, M. McCord, Proceedings of SPIE, theInternational Society for Optical Engineering 2723 (1996) 112.3685 S. Carrara et al. / Materials Letters 60 (2006) 3682  –  3685
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