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					                  Nanofabrication




               Lithography


+ bio                          Directed Assembly


+ bio + info   Self-assembly
                             Lithography


Precise, but expensive and difficult at small sizes (< 50 nm)




    Photolithography: Widely used for microchip mass production
    Electron-Beam Lithography: High resolution, individual research devices
    Ion Beam Lithography: Special purpose (milling, direct deposition)
Resolution limit /2


                       Large object:
                          Optical ruler
                          counts  /2
                          interference
                          fringes


                        /2 limit



                       Smaller objects
                       need shorter 
                Going to Shorter Wavelength (DUV)




Can’t go farther: There is one more excimer laser line at 157 nm (the F2 laser).
However, one cannot produce good enough optics with CaF2 (or any other material
that remains transparent at such a short wavelength).
                    Trick 1 to Push beyond /2 :
                        Immersion Lithography




The higher refractive index of water reduces the wavelength (n = 1.44 at 193 nm).
                      Trick 2 to Push beyond /2 :
        Phase Shift Mask + Enhanced Resist Contrast




     Absorbing Mask                 Phase Mask                Enhanced Contrast



In contrast to the traditional absorbing masks, a phase shift mass contains regions
of transparent material with high refractive index for shifting the phase. Thereby
the oscillations originating from diffraction are converted to a damped decay.

A photoresist with a high contrast narrows the decay width. This requires very
good control of the exposure and the resist development.
                          Leapfrog to 13 nm (EUV)




  Use synchrotron radiation for testing.
  Need lab-based light source for mass production.



Need to go to mirror optics, since all materials absorb. Regular mirrors only reflect
at oblique incidence, leading to asymmetric optics that are difficult to control. Use
multilayer mirrors, where interference of multiple layers enhances the reflectivity.
13 nm is preferred, because it allows the use of silicon-based multilayer mirrors.
(Si begins to absorb below 13 nm due to the Si 2p core level at about 100 eV.)
                   EUV Interference Lithography

   Two, three, or four diffracted beams                         PMMA
 interfere to yield dense lines and spaces,
    or cubic or hexagonal arrays of dots




                                                   Cubic Array of Holes, 57 nm pitch




  By interference of the 1st orders
  one can cut the mask period in half.


Paul Nealey (Madison), Harun Solak (Switzerland)        1:1 Lines, 55 nm Pitch
                 Self-assembly


Cheap, atomically-precise at small sizes (< 5 nm),
but poor positioning at large distances (> 50 nm)
          Nanocrystals




These are surprisingly simple to make
    Synthesis of Nanocrystals in Inverse Micelles I


Surfactant:    Hydrophilic Head                  Example: Phospholipid
              + Hydrophobic Tail




        Micelle:                         Inverse Micelle:
        Heads outside, Water outside     Heads inside, Water inside




                                       A nanoscale chemical beaker
                                       with aqueous solution inside
     Synthesis of Nanocrystals in Inverse Micelles II


Recipe:
1)   Fill inverse micelles with an ionic solution of the desired material.
2)   Add a reducing agent to precipitate the neutral material.
3)   Narrow the size distribution further by additional tricks.
Nanocrystals
 with equal
 size form
  perfect
   arrays




Lin, Jaeger,
Sorensen,
Klabunde,
J. Phys. Chem
B105, 3353
(2001)
     "Perfect" Magnetic Particles: FePt (4nm)
3D array           2D array




                                                          Oleic acid
                                                          spacer ad-
                                                           justs the
                                                           distance



   Sun, Murray , Weller, Folks, Moser, Science 287, 1989 (2000)
Shape control of nanocrystals via selective surface passivation
by adsorbed molecules. Only the clean surface facets will grow.




             Manna, Scher, Alivisatos, JACS 122, 12700 (2000)
        Supported Catalysts
Rhodium nanoparticles on a TiO 2 support
Zeolites

                 O
                   Si,Al



            Tetrahedra


           Channels for
           incorporating
           catalysts or
           filtering ions
Self-assembled Nanostructures at Surfaces
    Push Nanostructures to the Atomic Limit
            Reach Atomic Precision
Si(111)7x7                Hexagonal                      fcc (diamond)
                          (eclipsed)                      (staggered)
 Most stable
silicon surface




                  > 100 atoms rearrange themselves to minimize broken bonds.
Si(111)7x7 as
2D Template

 One of the two
  7x7 triangles
is more reactive.

   Aluminum
  sticks there.




Jia et al.,
APL 80, 3186 (2002)
Stepped Si(111)7x7




1 kink in 20 000 atoms

Straight steps because
of the large 7x7 cell.

Wide kinks cost energy.




Viernow et al.,
APL 72, 948 (1998)
                     15 nm
  Stepped Si(111)7x7
    as 1D Template


The 7x7 unit cell provides a
precise 2.3 nm building block




 x-derivative of the topography
  “ illumination from the left ”
                                   Step   Step
                 Atomic Perfection by Self-Assembly
                                Works up to 10 nm

          5.731 592 8 nm




One 7x7 unit cell per terrace                Kirakosian et al., APL 79, 1608 (2001)
Sweep out Kinks
into Bunches by
Electromigration




 Yoshida et al.,
 APL 87, 032903 (2005)
         "Decoration" of Steps  1D Atomic Chains

Clean
Triple step + 7x7 facet




With Gold
1/5 monolayer


                Si chain


             Si dopant
                     One-Dimensional Growth
                          of Atom Chains




                      0.02 monolayer
                      below optimum
Chains                 Au coverage
         Clean 77
                                   Unexpected
                                   Structures :
                                Gold at the center,
                                  not the edge !
                                 Graphitic silicon
                                     ribbon !

                                   First Principles
                                    Calculations:
Graphitic Gold                   Sanchez-Portal et al.,
 Silicon chain                   PRB 65, 081401 (2002)
                                 Crain, Erwin, et al.,
                                 PRB 69, 125401 (2004)


                                  X-Ray Diffraction:
                 Si(557) - Au    Robinson et al.,
                                 PRL 88, 096104 (2002)
Free-standing Nanowires
                                     Carbon Nanowire
                                     inside a Nanotube




Zhao et al., PRL 90, 187401 (2003)
    Silicon Nanowire Growth




Works also for carbon
nanotubes with Co, Ni as
catalytic metal clusters.

                              Wu et al., Chem. Eur. J. 8, 1261 (2002)
Catalytic Nanowire Growth of Ge by Precipitation from Solution in Au




                                      Phase diagram for immiscible solids :
                                      The melting temperature of a mixture
                                      is lower than for the pure elements.
                                      (L = liquid region)

                                      Wu and Yang, JACS 123, 3165 (2001)
              ZnO Nanowires Grown by Precipitation from a Solution




SEM images of ZnO nanowire arrays grown on sapphire substrates. A top view of the well-faceted hexagonal nanowire
tips is shown in (E). (F) High-resolution TEM image of an individual ZnO nanowire showing its <0001> growth direction.
For the nanowire growth, clean (110) sapphire substrates were coated with a 10 to 35 Å thick layer of Au, with or
without using TEM grids as shadow masks.


   Peidong Yang et al., Science 292, 1897 (2001) and Int. J. of Nanoscience 1, 1 (2002)
                ZnO Nanowires for Solar Cells




Need to collect the electrons quickly in a solar cell to prevent losses.
This can be achieved by running many nanowires to the places where
electrons are created (here in CdSe dots which coat the ZnO wires).

                                     Leschkies et al., Nano Letters 7, 1793 (2007)
                                           Striped Cu/Co Nanowires Grown by
                                            Electroplating into Etched Pores
                                           (Superlattices for efficient sensors)




Ohgai, … , Ansermet, Nanotechnology 14, 978 (2003)
       Directed Assembly


    The best of both worlds


Use lithography to define a grid.
Then attach self-assembled nano-
 objects (dots, wires, diodes, … ).
Assembly of Block Copolymers on Lithographically-Defined Lines

        Unpatterned Surface                      Patterned Surface (48 nm pitch)




      • Perfect positioning over large distances
      • Perfect line width, defined by the size of a molecule

     S. O. Kim, H. H. Solak, M. P. Stoykovich, N. J. Ferrier, J. J. de Pablo, P. F. Nealey,
                                 Nature 411, 424 (2003).
                                     Transfer dot patterns
Park, Chaikin, Register, ...   from a block copolymer into a metal
        Guided Self-Assembly of Block-Copolymers:
       From a random “fingerprint” patterns to an ordered lattice




                                                 shear




Polymer in groove:         Shear via PDMS:
Thomas, Smith (MIT)        Chaikin (Princeton)           On a chemical pattern:
Naito et al. (Toshiba)                                   Kim et al. (Madison)
Patterned Magnetic Storage Media for Perfect Bits
Co-polymers as etch masks
Spiral grooves as guide for dots




       Naito et al. (Toshiba)
       IEEE Trans. Magn. 38, 1949 (2002)
Side view




            A single magnetic dot
            for storing one bit.
Magnetic force microscope    Normal microscope
dark: spin  light: spin       Dot pattern

				
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