Fabrication at the nanoscale for nanophotonics Ilya Sychugov, KTH Materials Physics, Kista silicon nanocrystal by electron beam induced deposition lithography
Outline of basic nanofabrication methods 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, ) 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 elements from surface contaminations as well Decomposition Supply Focus Introduce of Molecules of precursor the molecules, electron Building a substrate precursor adsorb molecules beam volatile up on of into on gas, the by species the W(CO) surface nanodeposit sample area SEM are 6 of chamber diffusion used surface removed interest here
Electron beam induced deposition Nanodeposits by cross sectional TEM imaging and elemental analysis sample as prepared pre heated sample (120 o C, 10 min) J. Phys. Chem. 113, 21516 (2009)
Electron beam induced deposition by SEM: 2 3 nm Pt dots L. van Kouwen et al. NL, 9, 2149 (2009) Smallest resolution achievable 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 Types of scanning probe microscopes tunneling atomic force optical near field Pt Ir tip Si tip Sharpened fiber coated with Pt Pd
Scanning probe nanofabrication: tunneling microscope Chemical vapor deposition similar to EBID, but precursor molecule decomposition is induced by the tunneling current: Si nanodots from SiH 2 Cl 2 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 in ambient atmosphere by water molecules with conductive AFM tips oxide lines < 10 nm wide can be produced Dip pen lithography (AFM tip coated with a film of ink molecules reacting with surface) molecules migrate via a water meniscus from the tip to the surface P. Avouris et al., APL 71, 285 (1997) parallel writing is possible by tip arrays (40x40 Au gold dots) K. Salaita et al. Nat. Nanotech. 2, 145 (2007)
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 (SiH 4 ) to Si nanocrystals L. Mangolini et al., NL 5, 655 (2005)
Mass fabrication methods 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
Mass fabrication methods 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)
Mass fabrication methods plasmon resonance depends on the aspect ratio 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)
Mass fabrication methods Self assembly (spontaneous ordering on crystal surfaces of 1D, 2D and 3D structures) Stranski Krastanov growth mode for fabrication of InAs quantum dots on GaAs V. Shchukin and D. Bimberg, Rev. Mod. Phys. 71, 1125 (1999) uniform nanocrystals (low inhomogeneous broadening) can be fabricated as measured by low temperature single dot spectroscopy A. Mohan et al., Small 6, 1268 (2010)
Mass fabrication methods Deposition through a nanomask: porous alumina (Al 2 O 3 ) with nanometer sized pores 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 creates a 2D lattice of nanodots, smallest ~45 nm wide J. Hulteen et al., JPC B 103, 3854 (1999)
Mass fabrication methods 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 produced by subsequent metal deposition and liftoff
Mass fabrication methods Pattern transfer by stamping (nanotransfer printing): superlattices grown by epitaxy are used as a stamp, where metal is deposited on selectively etched AlGaAslayers the stamp is pressed onto en epoxy layer on top of the silicon wafer with subsequent etching of 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)