Hang, Shuojin

Kalhor, Nima

Mizuta, Hiroshi

Moktadir, Zakaria

English

Southampton (e-Prints Soton)

Direct helium ion milling technology: towards the fabrication of extremely down-scaled graphene nanodevices   Shuojin Hang1, Zakaria Moktadir1, Nima Khalor1, Shinichi Saito1 and Hiroshi Mizuta1,2   1Nano Research Group, University of Southampton, Southampton, Hampshire, UK. E-mail: sh13g08@ecs.soton.ac.uk     2 School of Materials Science, JAIST, Nomi, Ishikawa, Japan. E-mail: mizuta@jaist.ac.jp  1.  Introduction Currently, the most established graphene device fabrication technique uses electron-beam (EB) lithog-raphy to pattern resist deposited on top of the graphene, followed  by  oxygen  plasma  etching  [1,2].  However, parameters  such  as  proximity  effect,  thickness  uni-formity  of  resist  layer,  and  manual  development  of samples after e-beam lithography have limited the res-olution  of  this  method.  Recently,  a  new  patterning technique based on direct milling of graphene using a focused beam of helium ions generated in a helium ion microscope (HIM) has emerged [3,4]. In this work, we present  our  recent  attempts  of  introducing  an  atom-ic-size  helium  ion  beam  to  pattern  graphene nanostructures  directly  (fig.1)  and  also  investigate damage caused by helium ion bombardment onto car-rier transport mechanisms in bi-layer graphene nano-ribbons (GNRs).  2.  Hybrid HIM/EB Fabrication Process          We exfoliated bilayer graphene flakes from highly oriented pyrolytic graphite onto heavily P-doped n-type Si substrate covered with 300nm SiO2. The number of layers was determined by Raman spectroscopy[5]. The coarse isolation patterns on graphene were first defined by  EBL  followed  by  15  seconds  etching  in  oxygen plasma.  EBL-patterned  source/drain  electrodes  were deposited by e-beam evaporation of 5nm Ti and 70 nm Au and a subsequent metal lift-off.  Up to this point, graphene nanoribbons (GNR) down to 80nm wide can be prepared in this way with high reproducibility (fig. 2.a). The samples were annealed at 300° C in forming gas (5%H2 and 95%N2) for 1.5h for the removal of resist residues. In the last step, we milled the fine structure by scanning a highly focused helium ion beam within a small  area  on  the  GNR  (fig.2.  b).  The  accelerating voltage is tuned to be equal to the ‘best imaging voltage (~30kV) where the tungsten source emits the ions most effectively. Standard beam adjustment (e.g. beam tilt, beam align, wobble and stigmation) were performed to optimize the resolution. A dose test can be carried out on the nearby graphene flake. Usually a dose ranging from 0.6 to 1.5        is used, depending on the layer thickness,  cleanness  and  beam  condition.  Since  the pattern  generator  in  the  HIM  only  exposes  the pre-defined  ‘unmasked’  area,  therefore  the  damage induced by the ion bombardment should be minimized.   3.  Impact of He+ induced point defects on carrier transport in bilayer GNRs        We  systematically  investigated  the  impact  of beam-induced  defects  to  graphene  by  measuring  the conductivity of a bilayer graphene filed effect transistor (BGFET) subject to precisely controlled He+ irradiation (fig.3). From the D-peak analysis for Raman spectros-copy, it was shown that point defects (PDs) are induced by  the  He+  irradiation  [6].  Fig.  4(a)  shows  the  re-sistance R = Vd/Id (Vd = 7 mV) of three BGTEFs as a function  of  the  gate  voltage  and  for  three  different channel widths: 20 nm, 50 nm and 200 nm and a similar length of 2 um. Also shown is the resistivity of the BGFETs as a function of the channel width (Fig. 4(b)). This dependence is explained by a theoretical model which takes into account the diffusive scattering at the GNR  channel  edge,  the  short-range  force  scattering caused by PDs in the channel and the screened Cou-lomb scattering due to impurities at the GNR/SiO2 in-terface. Helium ion bombardment was carried out in a 200 nm wide BGFET channel. In Fig. 5 we show the conductivity of the bombarded BGFET as a function of the  gate  voltage  for  different  ion  doses.  The  above mentioned theoretical model shows very good agree-ment with the experimental conductivity by reproduc-ing the transition from sublinear (before irradiation) to linear  (after  irradiation  of  4.16x1013  ions/cm2)  de-pendence of the conductivity on carrier concentration. From this analysis it is manifested that the dominant carrier transport mechanism changes from the diffusive edge scattering to PD scattering with an increase in He+ dose. We evaluated that the concentration of induced PDs increases up to 5.2 x 1010 cm-2. In addition we found for the BGFETs with the 200-nm-wide channel that the associated carrier mean free path (MFP) varies from 185 nm for the GNRs before irradiation, to 23 nm for those with He+ dose of 4.16x1013 ions/cm2, which corresponds  approximately  to  the  average  distance between adjacent PDs.   4. Conclusion        We demonstrated an integrated fabrication process of graphene nanostructures combining e-beam lithog-raphy and helium ion milling whereby <10nm resolu-tion can be achieved. We also examined the impact of He+ irradiation on graphene. It was found that the re-sistivity  of  the  channel  increases  with  decreasing channel width as a result of the diffusive edge scattering. Introduction of point defects using the helium ion beam changes the dominant scattering mechanism from the diffusive edge scattering to the short-range scattering due to He+ induced PDs.  Acknowledgment The authors thank Dr S. Boden and Prof. D. Bagnall for their support to helium ion beam technology and Dr D. Williams of the Hitachi Cambridge Laboratory for a partial financial support to the present work.    References [1] M. Y. Han et al, Phys. Rev. Lett, vol. 98, 1–4 (2007). [2] C. Stampfer et al, Appl. Phys Lett, 92, 012102 (2008). [3] D. C. Bell et al, Nanotechnology, 20, 455301 (2009). [4] H. I. Beam et al,  ACS nano, 3, 2674–6, (2009). [5] A. C. Ferrari et al, Phys. Rev. Lett 97, 187401 (2006) [6] S. Boden et al, Graphene 2011, Bilbao, Spain (2011)    Fig.1 a) Conceptual demonstration of HIM based fabrication process of a double quantum dot (DQD) structure. The crit-ical  pattern  in  the  center  is  carved  onto  pre-defined  EBL structure using focused He+ beam.      Fig. 2 Fabrication result on a double quantum dot (DQD) structure carved on a GNR. a) A GNR of 150nm was pre-pared by EBL lithography and RIE etching. The circled labels the area where subsequent HIM milling is performed. b) A well-defined  50nm  DQD  structure  with  25nm  wide  con-strictions were then carved by HIM directly on the GNR.   Fig. 3 The sketch of the device showing the BGFET channel, the radiating focused helium ion beam, the source, the drain and the dimensions of the channel. The highly doped silicon substrate is used as a control gate.   Fig. 4 a) Resistance of three BGFETs with widths of 20 nm, 50 nm and 200 nm and a length of 2 m as a function of the gate voltage, for Vd = 7 mV. The gate voltage is defined relative to the voltage at the neutrality point. b) Resistivity as a function of the channel width for two values of the gate voltage Vg = -2 V and Vg = -6 V. The solid lines show theo-retical resistivity.  Fig.  5  Plots  of  the  conductivity  vs  hole  concentration  for different doses as indicated. The continuous lines show cal-culated conductivity. 

Direct helium ion milling technology: towards the fabrication of extremely down-scaled graphene nanodevices

Abstract

Similar works

Full text

Available Versions

Southampton (e-Prints Soton)