NANO 101 Introduction to Nanotechnology Lithography NANO 101 Introduction to Nanotechnology

Lithography “Photoengraving” Photolithography Transfer pattern into reactive polymer film (“resist”) Use resist to replicate pattern into thin film or substrate Photolithography Electron beam lithography X-ray lithography Focused ion beam lithography Lithography

Photolithography “Printing” Transfer of the pattern using an optical technique Resist = photoresist; photoactive polymer, positive or negative Coat substrate with resist Mask; expose with light Develop (dissolve exposed OR unexposed areas with chemicals) Etch unprotected areas or deposit layer of metal Strip resist Photolithography

Maximum resolution (minimum size) of individual features limited by diffraction bending of light around an edge through a slit past an object (edges) direction changes wavelength, frequency stay the same Photolithography

Photolithography Maximum resolution: λ b s d http://www.nanophotonics.info/eng/about/gallery.html b s d

Three Modes of Photolithography http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html

Photolithography: Shadow Printing Contact mode: s = 0 Best resolution; near 100% accuracy Maximum resolution rarely achieved Substrate and resist film rarely completely uniform Proximity mode: Small gap between mask and substrate Need extremely flat substrates and resist films

Photolithography: Projection Printing Resolution Worse resolution than shadow printing Lens imperfections Increased diffraction type of resist material optical system (apertures) exposure wavelength (λ) Best resolution ~ λ/2

Radiation with smaller wavelengths Deep Ultra-Violet (DUV): λ < 300 nm Need to use special lasers Minimum pattern size ~ 100 nm Extreme UV (EUV): λ = 11-13 nm Minimum pattern size ~ 60 nm Strong absorption of light by lenses Low reflectance of light by mirrors http://www.lcse.umn.edu/specs/labs/images/spectrum.gif

Current State of EUV Sustained 100W average source power 1,000 wafers processed in 24 hrs http://www.extremetech.com/computing/199782-tmsc-announces-lithography-milestone-as-euv-moves-closer-to-production

Other Options: ArF laser (193 nm) is current state of art Immersion (change refractive index) Double Patterning Diffraction limit n = index of refraction n air : 1.0 n water: 1.3 http://www.extremetech.com/computing/190845-intel-forges-ahead-to-7nm-without-the-use-of-euv-lasers

Double Patterning http://willson.cm.utexas.edu/Research/Sub_Files/DoubleExposure/index.php

X-Ray Lithography X-Rays System 0.04 nm < λ < 0.4 nm Mask X-ray absorbing material pattern on a thin X-ray transparent material X-ray source Bright enough in necessary wavelength range Expensive X-ray sensitive material http://www.camd.lsu.edu/microfabrication/latech.htm

LIGA Lithography, Electroplating, Molding http://sylmand.info/features/x-ray-lithography/

Electron Beam Lithography E-beam: Finely focused beam of electrons (few nm dia.) Electrons deflected accurately and precisely to “write” pattern without mask Resolution Diffraction not an issue λ < 1 Å (0.1 nm) Scattering Forward (in resist layer) Backwards (substrate) http://nanotechweb.org/cws/article/tech/18642

Electron Beam Lithography System Electron source (gun) Electron column (forms beam) Mechanical stage Control Computer https://smif.lab.duke.edu/pict.htm

Focused Ion Beam (FIB) Lithography Ions scatter much less than electrons Sources: Liquid metal ions (Ga; Au-Si-Be alloys) Long lifetime, high stability Resolution sub-µm dimensions (~250 nm) High resist exposure sensitivity Negligible ion scattering in resist Low back scattering from substrate Extensive substrate damage Also used for etching, deposition, and doping

Focused Ion Beam (FIB) Applications Etching Physical sputtering etching Bombard areas to be etched with energetic ion beams Simple, applicable to any sample material Chemical etching Chemical reactions between substrate surface and gas molecules adsorbed onto surface Increased etching rate, little residual damage Deposition Direction deposition (low energy ions) Chemical-assisted deposition

FIB – etching/deposition Nano Factory Achieved by Focused Ion Beam Toshiaki Fujii, and Takashi Kaito, Microsc Microanal 11(Suppl 2), 2005 - See more at: http://glia.ca/meanderings-wordpress/focus#sthash.06BIxsK8.dpuf

FIB - Deposition Deposition of Pt on Al substrate to form micro-grating for measuring material deformation

Soft Lithography Alternative to photolithography Cheaper / more flexible Printing of Self-Assembled Monolayers (SAMs) Molding of liquid precursors Techniques: Microcontact printing Nanoimprint

Microcontact Printing “Stamp” made Pour liquid polymer into a mold to make a “stamp” Mold often made by photolithography “Ink” the stamp Dip into solution so SAM formed on surface of stamp Stamp the substrate Place the inked stamp on a substrate SAM transferred to substrate in specific pattern

Microcontact Printing Voskuhl, J., Wendeln, C., Versluis, F., Fritz, E.-C., Roling, O., Zope, H., Schulz, C., Rinnen, S., Arlinghaus, H. F., Ravoo, B. J. and Kros, A. (2012), Immobilization of Liposomes and Vesicles on Patterned Surfaces by a Peptide Coiled-Coil Binding Motif . Angew. Chem. Int. Ed., 51: 12616–12620. doi: 10.1002/anie.201204836

Nanoimprint Make template Coat substrate with polymer 3. Press stamp into polymer at high temperature; polymer deforms 4. Cool polymer and pull stamp away 5. Polymer can be then be etched or used as is D.R. Hines et al., Appl. Phys. Lett. 86 (16), 163101 (2005).

Nanomanipulation and Nanolithography Based on Scanning Probe Microscopy (SPM) techniques - can be used for molecular manipulation Types of Scanning Probe Microscopy (SPM) Scanning Tunneling Microscopy (STM) Electrically conducting materials Atomic Force Microscopy (AFM) Dielectric (insulating) materials

Nanomanipulation STM with Tungsten tip Placed Xenon atoms on surface UHV and low temperature Clean environment and surface Absence of thermal diffusion on surface Nanomanipulation D.M. Eigler and E.K. Schweizer, Nature 344, 524 (1990).

The World’s Smallest Movie

Nanomanipulation with AFM Perpendicular Processes: atoms lifted, then dropped Parallel Processes: atoms dragged along surface Pushing Pulling Sliding C. Baur, et al., Nanotechnology 9, 360 (1998).

SPM Nanofabrication Advantages Disadvantages Nanoscale control in three dimensions, necessary for atomic manipulation Manipulation and characterization Disadvantages Small scanning area Slow scanning Tips must be high quality and consistent Surface must be flat and smooth UHV and low temperatures SPM Nanofabrication

Dip-Pen Nanolithography (DPN) Works under ambient conditions Scan tip across substrate, atoms or molecules move from AFM tip to substrate “Water-filled capillary” Chemisorption: Made 15 nm dots, spaced ~5 nm apart to form an N (gold substrate) Dip-Pen Nanolithography (DPN) C. Mirkin, Northwestern Univ.

Polymer Pen Lithography DPN + µCP 11 million pen array http://cen.acs.org/articles/87/i12/Boron-Dreams.html Science 19 September 2008:vol. 321 no. 5896 1658-1660

Summary Photolithography Soft Lithography SPM-based techniques Reaching size limits (diffraction, etc.) Soft Lithography Relatively new Fabrication of nanostructrures and nanodevices SPM-based techniques Promise for using atoms and molecules as building blocks