Fabrication at the for

Fabrication at the nanoscale for nanophotonics

Ilya Sychugov,KTH Materials Physics, Kista

silicon nanocrystal by electron beam induced deposition lithography

• Devices with characteristic size d comparable with the wavelength of light λ(waveguides, modulators, amplifiers,…) can be manufactured by optical lithography

• Different fabrication methods are needed for nanostructures with d<< λ(plasmonic structures, quantum dots, nanowires,…)

Outline of basic nanofabrication methods

Precise Patterning with Sub‐Wavelength Tools‐ electron beam (lithography, deposition)‐ scanning probe (STM, AFM‐based deposition, oxidation, lithography)‐ optical near field (deposition, lithography)

Mass Fabrication‐ plasma discharge, target sputtering (Si NCs)‐ chemical synthesis (Gold Nanorods, Si NCs)‐ epitaxy and self‐assembly (InAs, InGaAs QDs)‐ deposition through a nanomask (porous alumina, nanosphere lithography)‐ pattern transfer (nanoimprint, direct stamping)‐ and many more…

Sub‐wavelength tools for precise patterning

Electron, ion beams: de‐Broglie wavelength λ = h/p (for 10 keV electrons: λ ≈ 0.01 nm)

‐ in practice it is the electromagnetic lens system, which limits spatial resolution(astigmatism, various aberrations: chromatic, spherical, coma…)difficult to make as good lenses for electrons by field as glass lenses for visible light

Tunneling electrons: resolution ≈ 0.1 nm for the scanning tunneling microscope probe

‐ atomic resolution by electrical current from the probe tip on clean surfaces

Electric field: optical near‐field at the edge of metal nanostructures, fiber tip openings

‐ not a propagating wave, but a close‐range electric field enhancement ‐ parallel patterning possible for arrays of metal nanostructures

Electron beam as a nanofabrication tool

Lithography

modifying polymer resist spincoated on the sample to create a mask for subsequent processing:‐ positive resist (creating openings)‐ negative resist (hardening the polymer)

Deposition

unwanted effect in a scanning electron microscope: deposition of contaminations by e‐beam

can be used to fabricate nanostructures of different chemical compositions

W. F. van Dorp, PhD thesis, Delft University, 2008

Electron beam induced deposition

‐ fabrication of nanostructures by decomposition of precursor gas molecules adsorbed on the surface

‐ any precursor gas can be used

‐ size of nanodeposits can be controlled at sub‐10 nm scale by e‐beam dose, gas pressure, etc.

‐ nanodeposits are amorphous or polycrystalline and contain elementsfrom surface contaminations as well

Introduce a substrate into an SEM chamber Introduce a precursor gas, W(CO)6 used hereMolecules adsorb on the sample surfaceFocus the electron beam on the area of interestDecomposition of molecules, volatile species are removedSupply of precursor molecules by surface diffusionBuilding up of the nanodeposit


sample as‐prepared pre‐heated sample (120oC, 10 min)

J. Phys. Chem. 113, 21516 (2009)

Nanodeposits by cross‐sectional TEM imaging and elemental analysis


Smallest resolution achievable

‐ by SEM: 2‐3 nm Pt dots L. van Kouwen et al. NL, 9, 2149 (2009)

‐ by TEM even smaller: 1‐2 nm W dots M. Tanaka et al. Surface and Interface Analysis, 37, 261 (2003)

Electron beam induced deposition for lithography

‐ EBID of tungsten nanodots on thin silicon‐on‐insulator wafers with subsequent etching and mask removal

‐ Crystalline Si nanocrystal and nanowire fabrication

Nanotechnology 21, 285307 (2010)

Scanning probe induced nanofabrication

tunneling atomic force optical near‐field

Types of scanning probe microscopes

Pt‐Ir tip Sharpened fiber coated with Pt‐PdSi tip

Scanning probe nanofabrication: tunneling microscope

• Chemical vapor deposition similar to EBID, but precursor molecule decomposition is induced by the tunneling current: ‐ Si nanodots from SiH2Cl2 gas with 3.4 nm FWHM‐ Fe nanowires from ferrocene gas

W. W. Pai et al., J. Vac. Sci. Tech. B 15, 785 (1997)

• Direct deposition of the tip material under high bias:‐ Ag nanodots by Ag‐coated tip‐ Au nanowires by Au‐coated tip

A. A. Tseng et al., J. Vac. Sci. Tech. B 23, 877 (2005)

Scanning probe nanofabrication: atomic force microscope

• Local oxidation of silicon and metals

• Dip‐pen lithography (AFM tip coated with a film of ink molecules reacting with surface)

K. Salaita et al. Nat. Nanotech. 2, 145 (2007)

‐molecules migrate viaa water meniscus from the tip to the surface

‐ parallel writing is possible by tip arrays(40x40 Au gold dots)

P. Avouris et al., APL 71, 285 (1997)

‐ in ambient atmosphere by water molecules‐ with conductive AFM tips‐ oxide lines < 10 nm wide can be produced

Scanning probe nanofabrication: near‐field microscope

• Chemical vapor deposition by molecule decomposition with optical near field

‐ subwavelength nanostructures of Zn, Al

M. Ohtsu et al. IEEE J. Sel. Top. Quant. Elec. 8, 839 (2002)

Optical near‐field induced lithography

• Using near‐field effects to expose resist

‐ silver mask is fabricated on a quartz substrate‐ illumination light intensity is enhanced at the edges‐ subwavelength (~ 50 nm) lines are formed in the resist

X. Luo and T. Ishihara, APL 84, 4780 (2004)

Mass fabrication methods

Gas discharge (e.g. plasma enhanced chemical vapor deposition):decomposition of precursor gas molecules in plasma and clustering of fragments‐ from silane gas (SiH4) to Si nanocrystals

L. Mangolini et al., NL 5, 655 (2005)


Co‐sputtering from solid targets:‐ deposition of films with varying chemical composition in an inert gas environment (Ar)‐ subsequent thermal treatment of films for nanostructure formation


Chemical synthesis:‐ gold nanorods by reduction of metal salts and growth from the spherical seeds in a solution

C.J. Murphy et al., J. Phys. Chem. B 109, 13857 (2005)


‐ plasmon resonance depends on the aspectratio – tunable absorbance

‐ fine structure of the peak: transverse and longitudinal modes(possible to estimate aspect ratio from absorption measurements and Gans Theory)

J. Phys. Chem. B 111, 14299 (2007)


‐ Self‐assembly (spontaneous ordering oncrystal surfaces of 1D, 2D and 3D structures)

V. Shchukin and D. Bimberg, Rev. Mod. Phys. 71, 1125 (1999)

‐ Stranski‐Krastanov growth mode for fabrication of InAs quantum dots on GaAs

‐ uniform nanocrystals (low inhomogeneous broadening) can be fabricated as measured by low‐temperature single dot spectroscopy

A. Mohan et al., Small 6, 1268 (2010)


• Deposition through a nanomask:

‐ porous alumina (Al2O3) with nanometer‐sizedpores prepared by anodic etching serves as a mask‐ gold nanodots with 40 nm average diameter can be formed by evaporation

H. Masuda and M. Satoh JJAP 35, 126 (1996)

‐ spincoating of polystyrene nanospheres of 200‐500 nm in diameter in one or two layers‐ deposition of silver through the openings createsa 2D lattice of nanodots, smallest ~45 nm wide

J. Hulteen et al., JPC B 103, 3854 (1999)


Nanoimprint lithography:‐ a mold is mechanically pressed onto a resist layer‐ after mold removing the resist is etched in oxygen plasma to create full openings

S. Chou et al., JVST B 15, 2897 (1997)

‐ array of 10 nm gold nanodots was producedby subsequent metal deposition and liftoff


Pattern transfer by stamping (nanotransfer printing):

‐ superlattices grown by epitaxy are used as astamp, where metal is deposited on selectivelyetched AlGaAs layers‐ the stamp is pressed onto en epoxy layer ontop of the silicon wafer with subsequent etchingof the GaAs oxide at the metal‐stamp interface

N. Melosh et al., Science 300, 112 (2003)

‐ metal nanowire arrays were produced, 10 nm in diameter (Pt) stretching > 100 um

‐ the pattern was then transferred to the top silicon layer of SOI wafers to form Si NWs

Some review papers on the subject

‐ B. Gates et al. “New Approaches to Nanofabrication…”, Chem. Rev. 105, 1171 (2005)‐ V. Shchukin et al. “Spontaneous Ordering of Nanostructures on Crystal Surfaces”Rev. Mod. Phys. 71, 1125 (1999)‐ K. Salaita et al. “Applications of Dip‐Pen Nanolithography”, Nat. Nanotech. 2, 145 (2007)‐ A. Tseng et al. “Nanofabrication by Scanning Probe Microscopy…”, JVST B 23, 877 (2005)‐ I. Utke et al. “ Gas‐assisted Focused Electron Beam and Ion Beam Processing and Fabrication”, JVST B 26, 1197 (2008)

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Fabrication at the for