Fabrication of Solid State Nanopores

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Fabrication of Solid State Nanopores

• Presented by
• Sabina Koukourinkova
• 28 March 2008

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Outline

• What are Nanopores?
• Fabrication of Solid State Nanopores
• Creating a Nanopore FIB
• Sculpting a Nanopore FIB Exposure, Broad Area
  Exposure
• Fabrication Control
• Ion Beam Sculpting Apparatus

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Nanopores

• Definition a nanoscale pore in an electrically
  insulating membrane
• - a biological protein channel in a lipid
  membrane
• - a pore in a solid-state membrane (most
  commonly Si3N4)
• Detection Principle
• - translocation event pulse
• Applications
• - DNA sequencing
• - separation of ssDNA and dsDNA in solution
• - determining the length of polymers

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Translocation of a DNA through a Biological
Channel 58mer DNA Strand Passing through a-
Hemolysin Transmembrane Pore
http//www.ks.uiuc.edu/Research/hemolysin/
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Translocation of a DNA through a Synthetic
Nanopore

• http//

www.biophysj.org/cgi/content/full/87/3/2086
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Fabrication of Solid State Nanopores (Overview)

• Si3N4 thin membranes are deposited by sputtering
  on a 4-inch Si wafer.
• Photoresist (PR) is applied to both sides of the
  wafer.
• Anisotropic RIE is used to remove the thin layer
  of Si3N4 not protected by the photoresist
• Anisotropic Wet Etching uses a chemical agent KOH
  to remove bulk material from the wafer
• Focused Ion Beam is used to drill a hole through
  the free standing membrane

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Fabrication of Solid State Nanopore
(Step-by-Step Method)

• Wafer (Substrate)
• A thin piece of superconducting material (Si
  crystal) 1 - 11.8
• The mechanical strength of the material
  determines the thickness of the wafer
• Fabrication Czochralski method (1916)

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Sputter Deposition of Si3N4 on Si Wafer

• Sputtering
• (Vacuum Evaporation/
• Deposition Technique)
• Sputtering of light elements (Ne, Ar) versus
  heavy elements (Kr, Xe)
• Sputtering of conducting (DC) versus
  non-conducting materials (radiofrequency)

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Photoresist (Masking Material) Application

• Remove any moisture from the wafer
• Clean the wafer from any contaminants
• Apply adhesion promoter HMDS (hexametyldisilazan
  e)
• Apply viscous liquid photoresist to the wafer
• Spin the wafer to produce uniform coating 0.5
  2.5µm
• 1200-4800rpm, 30-60s
• Pre-bake the photoresist (90-100C, 5-30 min) -
  removal of excess solvent
• Expose the photoresist to intense UV light
  (photolitography)
• Selectively remove material of a certain pattern
  using a photomask.
• Post-bake the photoresist (120180C)
  solidification

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Etching

• Anisotropic Reactive Ion Etching (RIE) precise
• Isotropic chemical etching
• damage to the masking material and erosion of
  the substrate
• Wet Anisotropic KOH Etching

http//www.utdallas.edu/gpp052000/Docs/TechnicsRI
E_Manual.pdf
http//en.wikipedia.org/wiki/Etching_(microfabrica
tion)
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Drilling a Hole

• Creating a hole (FIB) 100nm
• Through hole versus blind hole
• The process can take place at room temperature or
  temperature as low as -120C
• Focused ion beam (FIB)
• Ga LMIS (liquid metal ion source)
• Tungsten needle and heat
• Ionization of Ga, E 5-50KeV

http//www.fibics.com/FIBBasics.html
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Sculpting of the Nanopore

• FIB Exposure
• Ga are implanted in the surface (amorphous
  surface)
• Hole closing
• Signal from ejected particles is collected to
  form an image
• Drawback
• Hard to control the size of the hole
• Continuous sputtering of material during imaging
  of the hole
• Sculpting of the nanopore
• Hole closing large scan area and low rate of
  sputtering
• Hole opening small scan area and high sputtering
  rate

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Sculpting of the Nanopore

• Broad Area Ion Exposure Ar source beam
• Ion Beam Sculpting Apparatus close/open nanopores

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Schematic of Ion Beam Sculpting
Apparatus(Physics Dept., University of Arkansas)
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Sculpting of Nanopores Sputtering versus
Lateral Mass Transport
http//www.mcb.harvard.edu/branton/IonBeamSculptin
gNanopores.jpg
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Lateral Mass Transport

• Phenomenon which occurs when a surface of a solid
  is bombarded with heavy ion beams resulting in
  surface deformation

Surface profile of InP after 531014 ions/cm2 Se
24-MeV irradiation at 7.
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Lateral Mass Transport

• Ditch and Dike Formation (3D)
• The solid has to be deformable
• Sin? Cos? Dependence
• ? angle between the incident ion beam to the
  normal of the surface
• Sin ? the direction of the mass transfer is in
  the direction of the off-normal component of the
  momentum parallel to the surface
• Cos ? as ? increases, the incident beam
  penetrates less deep into surface, and results in
  lateral mass transport.
• Amount of Mass Displaced

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Ion Beam Sculpting Apparatus

• Lateral mass transport is induced on the surface
  of a nanopore more quickly with heavier rather
  than lighter ions
• The thickness of the lateral mass build-up
  depends on the penetration depth of the ions
• Heavier ions have less penetration depth, and so
  form thinner coating
• The flux of the ion beam is inversely
  proportional to the effectiveness of closing the
  pore.
• The size of the hole is affected by temperature
  changes
• For a constant flux and ion mass, the size of the
  hole is inversely related to the variable
  temperature.

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Closing of a Nanopore
TEM image of FIB Hole
Closing of an FIB Hole
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References

• Aleksij Aksimentiev, Jiunn B. Heng, Gregory Timp
  and Klaus Schulten.Microscopic Kinetics of DNA
  Translocation through Synthetic Nanopores
  ltwww.biophysj.org/cgi/content/full/87/3/2086gt
• Alpha-Hemolysin Self-Assembling Transmembrane
  Pore. lthttp//www.ks.uiuc.edu/Research/hemolysin/gt
  
• Bradley Ledden, Eric Krueger, Jiali Li. Study of
  Nanopore Sculpting with Noble Gas Ion Beams at
  Various Energies. 2006 APS Meeting Poster
  Session II.
• Derek M. Stein, Ciaran J. McMullan, Jiali Li.
  Feedback-controlled Ion Beam Sculpting
• Apparatus. Rev. Sci. Instrum 75, 4 (2004)
• Introduction Focused Ion Beam Systems lt
  http//www.fibics.com/FIBBasics.htmlgt
• Ion Beam Sculpting. lthttp//en.wikipedia.org/wiki/
  Ion-beam_sculptinggt
• M. Chicoin, S. Roorda, and L. Cliché. Directional
  effects during ion implantation Lateral mass
  transport and anisotropic growth. Phys Rev. B 56,
  3 (1997)
• Nanopore. lthttp//en.wikipedia.org/wiki/Nanoporegt
• Photolitography. lthttp//en.wikipedia.org/wiki/Pho
  tolithographygt
• Qun Cai, Brad Ledden, and Eric Krueger. Nanopore
  Sculpting with Noble Gas Ions. J. Appl. Phys 100,
  024914 (2006)
• Wafer (Electronics). lthttp//en.wikipedia.org/wiki
  /Wafer_28electronics29gt
• What is Sputtering? lthttp//www.tcbonding.com/sput
  tering.htmlgt