通过自上而下制备垂直硅纳米线阵列

Realization of ultra dense arrays of vertical silicon nanowires with defect free surface and perfect anisotropy using a top-down approachXiang-Lei Han a ,Guilhem Larrieu a ,b ,⇑,Pier-Francesco Fazzini b ,Emmanuel Dubois aa IEMN/UMR CNRS 8520,Avenue Poincaré,BP 60069,59652Villeneuve d’Ascq,France bLAAS-CNRS,Universitéde Toulouse,7av.du Col.Roche,31077Toulouse,Francea r t i c l e i n f o Article history:Available online 4January 2011Keywords:Electron-beam lithographyHighly dense arrays of vertical Si nanowires Oxidation of Si nanostructure Top-down approacha b s t r a c tThe routine synthesis of ultra dense nanowires arrays appears as an inescapable requirement to imple-ment future generations of nanodevices.In this study,we demonstrate the fabrication of vertical of ultra dense (4Â1010cm À2)Si NWs arrays using a top-down fabrication strategy.The developed process also feature nearly perfect anisotropy (98.5%),100%yield and an excellent surface cleanliness based a self-limiting oxidation mechanism that develops in 1D nanostructure.Ó2011Published by Elsevier B.V.1.IntroductionNanodevice based silicon nanowires (Si NWs)have been identified as potential candidates for ultimate complementary me-tal-oxide-semiconductor (CMOS)as well as more-than-Moore applications,as reported in the international technology roadmap for semiconductors (ITRS).The fabrication of ultra dense arrays of Si NWs is a requirement to implement future generations of nanodevices [1],including gate-all-around (GAA)MOSFETs [2],high sensitive biochemical sensors [3]or photovoltaic applications [4].Top-down and bottom-up approaches have their own advanta-ges and drawbacks:the bottom-up route has the potential to growth NWs with a virtually unlimited variety of materials while the top-down approach has the capability of quick integration in standard CMOS flow,with a very good reproducibility and control of the vertical NWs (position,diameter and pitch).Unlike bottom-up growth methods based on catalytic growth,the more conven-tional top-down approach is free of metallic contamination.In this study,Si NWs are defined using electron-beam lithography over a single layer of hydrogen silsesquioxane (HSQ)that acts as a robust mask to structure NWs by anisotropic plasma etching followed by a tightly controlled oxidation step.Taking advantage of the self-limited oxidation mechanism due to mechanical stress build-up in 1D nanostructure,a thin sacrificial oxide layer is grown to improve both the anisotropy and surface quality of NWs,resulting in a final tapered profile.The fabrication of Si NWs arrays with anultra high density (4Â1010cm À2),a 100%yield,excellent surface cleanliness and 98.5%etching anisotropy are demonstrated.2.Experimental procedures2.1.Realization of a hard mask by electron-beam lithography and etching vertical Si NWs arrayA single layer of negative-tone electron-beam resist,namely,hydrogen silsesquioxane (HSQ),was used as a hard mask.This choice was motivated by its excellent contrast properties upon e-beam exposure and development as well as its inorganic nature that provides an excellent etching selectivity with respect to sili-con.A solution of HSQ diluted in isobutyl ketone marketed by Dow Corning under the name of FOx-12and FOx-16was used.HSQ was spin coated on (100)Si wafers and baked at 80°C for 60s to evaporate the solvent.Electron-beam exposure was per-formed with an EBPG 5000+system from LEICA at the high energy of 100KeV.A 100pA beam current gives an extremely small spot size,estimated at 5nm.An optimum dose of 2750l C/cm 2coupled to correction factors that take into account proximity effects and pattern sizes were selected to generate HSQ nanocolumns.After exposure,the HSQ resist was developed by manual immersion in 25%tetramethylammonium hydroxide (TMAH)at 20°C for 60s,rinsed in methanol and dried using a supercritical carbon dioxide process to avoid the collapse of nanopillars caused by capillary forces that develop in conventional techniques that use gas blow [5].Using this process sequence,HSQ nanopillars arrays with very high contrast and density are obtained as shown in Fig.1(left)(diameter =27nm,pitch =24nm).0167-9317/$-see front matter Ó2011Published by Elsevier B.V.doi:10.1016/j.mee.2010.12.102⇑Corresponding author at:LAAS-CNRS,7av.du Col.Roche,31077Toulouse,France.E-mail address:rrieu@laas.fr (rrieu).Under electron beam exposure,HSQ has the remarkable prop-erty to evolve from a cage-like monomer to a network-like poly-mer that approaches the structure of silicon dioxide(SiO x),[6] improving the selectivity against plasma etching.The vertical HSQ nanopillars were transferred to the silicon bulk substrate by reactive ion etching(RIE)using a PlasmaLab100chamber from Ox-ford Instruments.A chlorine based plasma chemistry along with an optimized parameter selection(low pressure,without inductive plasma coupling)was used to obtain an anisotropy of92%without observing any traces of micro-trenching effect or‘grass effect’. More details were presented in our previous works[7].This ap-proach enabled the realization of vertical NWs arrays with a 19nm diameter and a nanowire density up4Â1010cmÀ2as given in Fig.1(right),which represents the highest density published up to now.2.2.Wet oxidation Si NWsAfter patterning,Si NWs were subjected to thermal wet oxida-tion a conventional tubular furnace(TEMPRESS)at850°C under a flow of1.5L/min of O2,2.5L/min of H2and a variable time.The SiO2layer grown was then stripped in a diluted HF solution.Based on high resolution SEM characterization,diameters at mid-height of the oxidized Si NW(d ox)and after stripping of the SiO2layer (d Si)were measured and the thickness of the grown SiO2layer (t oxide)deduced as following:t oxide¼ðd OXÀd SiÞ=2ð1Þ3.Results and discussion3.1.Self-limited oxidation phenomenon in1D Si nanostructureThermal oxidation is identified as an effective method to realize ultra-small diameter Si NWs by tapering the dimension[8].The comprehension of the different mechanisms that compete during the oxidation of a1D nanostructure,such as surface reaction and oxidant diffusivity,is a prerequisite to perfectly control the process at such dimensions.The oxidation rate of Si NWs is governed by a competition between stress build-up as the volume of oxide layer expands and stress relaxation by viscousflow.Buttner et al.[9] suggested that the stress increase is responsible for the retarded oxidation mechanism which cannot be relaxed by viscousflow of the oxide in case of dry oxidation.Under wet oxidation condition, the effect of stress relaxation by viscousflow of the oxide is more significant than in the case of dry oxidation due to the presence of hydroxyl ions[10].In Fig.2(a),the thicknesses of the oxide layer grown at850°C for different starting diameters of Si NWs are plot-ted as a function of oxidation time.The oxide thickness resulting from an one-dimensional oxidation of a planar(100)Si bulk wafer (dashed line)is given for comparison.Firstly,the shape of curve for the oxidation of Si wafer is nearly linear compared with the para-bolic profile associated to Si NWs.During the1D wet oxidation of a wafer at950°C and below,compressive stress is generated in the SiO2[11].A biaxial compressive stress at the SiO2/Si interface due to the increase of atomic volume from Si(20Å3)to SiO2 (45Å3)leads to the bending from a plane surface to a convex sur-face,resulting in a limitation in the diffusion of oxidizing agents. This phenomenon becomes even more significant in the oxidation of nanoscale cylindrical structures,such as NWs.In Fig.2(a),oxida-tion of the Si NWs is obviously retarded due to the reduced oxygen diffusion and decrease of the interface reaction rate by the com-pressive stress normal to the SiO2/Si interface.It is mentioned that this stress is strengthened in the oxidation process.Secondly,the NW oxide is grown anomalously faster than on a bulk Si wafer in the initial stage due to a supply of oxidizing species at the SiO2/ Si surface which is enhanced by the convex geometry.It this latter case,the surface of oxide shell is larger than the area at SiO2/Si interface.[12,13]Furthermore,during short oxidation on the con-vex surfaces,a tensile stress is created by the stretching of the al-ready grown oxide as it expands to a larger circumference leading to an increase of the oxidant diffusivity and solubility.As oxidation proceeds,the normal compressive stress becomes more important and retardation of oxidation is observed[13].Fig.2(b)shows the effect of the convex NWs geometry on the oxidation rate for sev-eral starting NWs diameters as function of the oxidation time.It is obvious that the self-limiting behavior is more apparent with smaller NWs diameters because of a higher surface/volume ratio which leads to a faster stress build-up.For example,considering a diameter below50nm,this dimension remains nearly constant after20min of oxidation.3.2.Improving anisotropy and smoothing surface of Si NWsOptimized RIE parameters associated to chlorine based chemis-try give anisotropy of about90%.The ideal vertical etched profile2.(a)Dependence of the oxide thickness with the oxidation time for severaldiameters obtained at850°C.The dashed line is an oxidation reference obtained on a(100)Si bulk wafer.(b)Evolution of several NW diameters function of oxidation time.(c)Dependence of the oxidation rate with oxidation time several NW diameters.(i.e.100%anisotropy)can not be reached due to the nature be-tween ionized chlorine and silicon that involves physical but also chemical reactions.In particular,lateral etching is introduced by chemical reaction and ions scattering.As previously discussed, the oxidation rate rapidly decreases and saturates to a very low rate with the oxidation duration,as shown in Fig.2(c).In other words,the rate of Si consumption by thermal oxidation in the out-er part of a large Si NW is faster than for smaller diameter NWs. Therefore,this mechanism can be used to improve the anisotropy while controlling the shrinking of diameter.Experimental results in Fig.3show Si NWs arrays before(left)and after(after)the wet oxidation step.The Si NWs diameter was reduced from42to 16nm while the anisotropy was improved from92%to98.5%.Finally,plasma-induced damage is recognized as a source of de-vice performance and reliability degradation,including UV radia-tion,electrostatic discharge and physical damage due to ionic bombardment.Ion-induced point defects and surface or interface nonstoichiometric states resulting from plasma exposure hold a major responsibility in degradation of carrier mobility,and sub-threshold metal–oxide-semiconductor characteristic[14].To cope with this problem,the vertical Si NW arrays were cured by wet oxidation and the SiO2layer grown was stripped in diluted HF. Using high-resolution TEM characterization shown in Fig.4,an atomically abrupt surface is observed after an oxidation step at 850°C/20min(right image)compared with the rough surface ob-tained after plasma etching(left image).4.ConclusionIn this study,a simple method of top-down fabrication cou-pled to a perfectly controlled oxidation step is demonstrated to realize vertical Si NWs arrays with an ultra high density (4Â1010cmÀ2),a perfect anisotropy(98.5%),100%yield and a sharply defined clean surface.The understandings of the differ-ent mechanisms that compete during the oxidation of1D nano-structure are presented in order to perfectly control the process at the nanometer scale.The mechanism of self-limited oxidation is advantageously used to effectively improve the anisotropy of vertical Si NWs.The authors thank the CNRS-CEMES laboratory (Toulouse,France)for the use of their TEM facilities. AcknowledgementsThis work was supported by the European Commission through the NANOSIL Network of Excellence(FP7-IST-216171). 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