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  1. 1. ETE444 :: Lecture 6 Nano Fabrication Dr. Mashiur Rahman
  2. 2. Limitations of Photolithography Current photolithography techniques used in microelectronics manufacturing use a projection printing system (known as a stepper). In this system, the image of the mask is reduced and projected, via a high numerical aperture lens system, onto a thin film of photoresist that has been spin coated onto a wafer. The resolution that the stepper is capable of is based on optical diffraction limits set in the Rayleigh equation . In the Rayleigh equation, k1 is a constant that is dependent on the photoresist, λ is the wavelength of the light source, and NA is the numerical aperture of the lens. The minimum feature size that can be achieved with this technique is approximately the wavelength of the light used, λ; although theoretically, the lower limit is λ /2. So, in order to produce micro- or nanoscaled patterns and structures, light sources with shorter wavelengths must be used. This also makes manufacturing more difficult and expensive.
  3. 3. Nano Fabrication • Electron Beam Lithography • Soft Lithography • Scanned Probe Techniques • Self-Assembly and Template Manufacturing
  4. 4. Nano Fabrication • Electron Beam Lithography • Soft Lithography • Scanned Probe Techniques • Self-Assembly and Template Manufacturing
  5. 5. Electron Beam Lithography  Introduction  Applications  Electron Beam Writing tool  Advantages  Limitations
  6. 6. Electron Beam Lithography • Very popular in research environments • Used for mask making commercially • Typically, EBL is direct write serial (slow) process • Projection EBL systems have been developed – e.g., SCALPEL(SCALPEL = Scattering with Angular Limitation Projection Electron-beam Lithography
  7. 7. Applications of Electron Beam Lithography • Research - Nanopatterning on Nanoparticles - Nanowires - Nanopillars - Gratings - Micro Ring Resonators - Nanofluidic Channels • Industrial / Commercial - Exposure Masks for Optical Lithography - Writing features
  8. 8. Examples Bragg-Fresnel lens for x-rays Paul Scherrer Institute
  9. 9. • Suspended AuPd wires made by standard e-beam lithography and etching techniques. The inset is a blowup view of one of the wires. The scale bar is 1 micron.
  10. 10. SEM images of multi-layer line-array structures made of electron-beam sensitive polymers. These structure can serve as 3D photonic crystals (upper-left image) and quasi-3D suspending slab photonic crystals (lower-right image). The structures were fabricated by e-beam lithography with single- step 100keV-exposure, and multiple- development steps.
  11. 11. Scanning electron microscopy image of a regular and homogeneous assembly of GaAs nanowires. The nanowire growth is catalyzed by a 2D array of Au dots defined by e- beam lithography.
  12. 12. Electron Beam Write • An electron gun or electron source that supplies the electrons. • An electron column that 'shapes' and focuses the electron beam. • A mechanical stage that positions the wafer under the electron beam. • A wafer handling system that automatically feeds wafers to the system and unloads them after processing. • A computer system that controls the equipment.
  13. 13. Electron energy deposition in matter • Electron trajectories in resist: An incident electron (purple) produces secondary electrons (blue). Sometimes, the incident electron may itself be backscattered as shown here and leave the surface of the resist (amber).
  14. 14. EBL resists Important parameters  Resolution (nm)  Sensitivity (C/cm^2) PMMA has extremely high resolution, and its ultimate resolution has been demonstrated to be less than 10 nm. But its major problems are its relatively poor sensitivity, poor dry etch resistance, and moderate thermal stability. Electron beam exposure breaking the polymer into fragments Recent progress in electron-beam resists for advanced mask-making by D.R.Medeiros, A.Aviram, C.R.Guarnieri, W.S.Huang, R.Kwong, C.K.Magg, A.P.Mahorowala, W.M.Moreau, K.E.Petrillo, and M.Angelopoulos
  15. 15. Advantages • High resolution – down to 5 nm • Useful design tool – direct write allows for quick pattern changes (no masks are needed)
  16. 16. Limitation • Cost (up to $6 –10 million for hardware) • Direct write has low throughput slow and expensive – E-beam lithography is not suitable for high-volume manufacturing because of its limited throughput. – The serial nature of electron beam writing makes for very slow pattern generation compared with a parallel technique like photolithography (the current standard) in which the entire surface is patterned at once. – To pattern a single wafer with an electron beam lithography system for sub-100 nm resolution, it would typically take days, compared to the few minutes it would take with a photolithography system. – Currently an optical maskless lithography tool is much faster than an electron beam tool used at the same resolution for photomask patterning.
  17. 17. Nano Fabrication • Electron Beam Lithography • Soft Lithography • Scanned Probe Techniques • Self-Assembly and Template Manufacturing
  18. 18. Soft Lithography • Introduction • Nanoimprint Lithography • Micro contact printing (μCP)
  19. 19. Introduction Soft lithography is called ‘‘soft’’ because an elastomeric stamp or mold is the part that transfers patterns to the substrate and this method uses flexible organic molecules and materials rather than the rigid inorganic materials commonly used during the fabrication of microelectronic systems. This process, developed by George Whitesides, does not depend on a resist layer to transfer a pattern onto the substrate. Soft lithography can produce micropatterns of self- assembled monolayers (SAMs) through contact printing or form microstructures in materials through imprinting (embossing) or replica molding.
  20. 20. Nanoimprint lithography (NIL) • Nanoimprint lithography (NIL) has primarily been used to emboss hard thermoplastic polymers. The micromolding and embossing of elastomers has attracted considerable interest as these materials have found important applications in softlithographic techniques such as microcontact printing (µCP). • In this technique, a monolayer of a material is printed off an elastomeric stamp [made of poly(dimethylsiloxane) (PDMS)] after forming conformal contact between stamp and substrate. Sub- micron surface relief structures can easily be introduced in PDMS by curing the polymers against a lithographically prepared master. • Feature sizes in the 10–100 nm size range. • After imprinting the polymer film, further etching can transfer the pattern into the underlying substrate. Alternatively, metal evaporation and lift-off of the polymer mask produces nanopattern metal features.
  21. 21. Advantages • Nanoimprint lithography (NIL) has the potential of high-throughput due to the parallel processing, does not require sophisticated tools, and allows nanoscale replication for data storage. • NIL is also compatible with conventional device processing techniques. The quality of the nanoimprinting process depends on a number of experimental parameters like T, viscosity in the melt, adhesion of the polymer to the mold, etc.
  22. 22. NanoimprintLithography R. Waser (ed.), Nanoelectronics and Information Technology, Chapter 9
  23. 23. NanoimprintLithography Patterns
  24. 24. Micro contact printing (μCP) • Micro contact printing (or μCP) uses the relief patterns on a PDMS stamp to form patterns of self-assembled monolayers (SAMs) of inks on the surface of a substrate through conformal contact. Micro contact printing differs from other printing methods, like inkjet printing or 3D printing, in the use of self-assembly (especially, the use of SAMs) to form micro patterns and microstructures of various materials. • The advantage of µCP is the ability to pattern surfaces chemically at the sub-micron level.
  25. 25. μCP process • An elastomeric stamp is inked with small molecules (thiols or silanes) and pressed against a clean substrate (gold or silicon wafer). Where the stamp is in contact with the surface, a monolayer of material is transferred to the substrate. A second thiol or silane is used to fill in the background to provide a chemically patterned surface.
  26. 26. ODT from the solution settles down onto the "Inking" a stamp. PDMS stamp PDMS stamp. Stamp now has ODT attached to it with pattern is placed in which acts as the ink. Ethanol and ODT solution The PDMS stamp with the ODT is placed on the gold substrate. When the stamp is removed, the ODT in Sarfus image of streptavidin contact with the gold stays stuck to the gold. Thus the deposited by soft lithography with pattern from the stamp is transferred to the gold via the PDMS stamp. ODT "ink."
  27. 27. Stamps
  28. 28. Nano Fabrication • Electron Beam Lithography • Soft Lithography • Scanned Probe Techniques • Self-Assembly and Template Manufacturing
  29. 29. Scanned Probe Techniques • SPM systems are capable of controlling the movement of an atomically sharp tip in close proximity to or in contact with a surface with subnanometer accuracy. – Scanning Probe Induced Oxidation – Dip Pen Lithography
  30. 30. STM AFM
  31. 31. Local oxidation nanolithography •In 1990 Dagata and co-workers modified a hydrogen-terminated silicon surface by the application of a bias voltage between an STM tip and the surface. •In 1993 it was demonstrated that local oxidation experiments could be performed with an atomic force microscope. Local oxidation nanolithography (LON) is sometimes called scanning probe oxidation, nano-oxidation, local anodic oxidation or generically AFM lithography.
  32. 32. • Examples of local oxidation nanopatterns. (a) Periodic array of 10 nm silicon oxide dots. The lattice spacing is 40 nm. (b) Alternating insulating (bright) and semiconducting rings. (c) First paragraph of Don Quixote .
  33. 33. Scanning Probe Induced Oxidation • Nanometer-scale local oxidation of various materials can be achieved using scanning probes operated in air and biased at a sufficiently high voltage. Tip bias of −2 to −10V is normally used with writing speeds of 0.1– 100μm/s in an ambient humidity of 20–40%. • It is believed that the water meniscus formed at the contact point serves as an electrolyte such that the biased tip anodically oxidizes a small region of the surface.
  34. 34. Scanning Probe Resist Exposure and Lithography • Electrons emitted from a biased SPM tip can be used to expose a resist the same way e-beam lithography does. Various systems have been used for this lithographic technique. These include constant current STM, noncontact AFM, and AFM with constant tip-resist force and constant current.
  35. 35. Dip Pen Lithography • Dip pen lithography is a type of scanning probe lithography. In this lithographic technique, the tip of an atomic force microscope (AFM) is used to create micro- and nanoscaled structures by depositing material onto a substrate. The AFMtip delivers the molecules to the substrate surface using a solvent meniscus that forms in ambient atmospheres. Structures with features ranging from several hundreds of nanometers to sub-50 nm can be generated using this technique
  36. 36. This image was written using Dip-Pen Nanolithography, and imaged using lateral force microscopy mode of an atomic force microscope. Courtesy the Mirkin Group, Northwestern University. From "There's Plenty of Room at the Bottom" By Professor Richard P. Feynman, December 29th, 1959.
  37. 37. Nano Fabrication • Electron Beam Lithography • Soft Lithography • Scanned Probe Techniques • Self-Assembly and Template Manufacturing
  38. 38. Self-Assembly and Template Manufacturing • Nanopatterning of self-assembled monolayers • Template growth of organic and biological structures onto nanopatterns
  39. 39. Nanopatterning of self-assembled monolayers • Self-assembly, chemical functionality and nanopatterning are concepts very akin to nanotechnology, so it is not surprising to discover various approaches to modify self-assembled monolayers or to induce a selective self assembly process by LON. • Sugimura and co-workers pioneered the protocol to generated coplanar nanostructures consisting of two different types of self-assembled monolayers (SAM).
  40. 40. Scheme of the hierarchical self- assembled approach developed by Sagiv et al. (a) SAM on Si substrate. (b) Patterned SAM by local oxidation of methyl terminated groups. (c) and (d) Different steps in the formation of a second monolayer in the patterned region. The transformation of the vinyl- terminated overlayer in amino-terminated requires the reaction of NTS groups with formamide and its further reduction with BH3.THF.
  41. 41. Template growth of organic and biological structures onto nanopatterns • Developing methods that allow the deposition of small functional molecules at pre- determined positions on a substrate is one of the exciting challenges for alternative nanolithographies. In this section we illustrate the potential of LON in this topic by describing three applications, fabrication of gold patterns and nanowires onto SAM templates, patterning of proteins (ferritin) and fabrication of conjugated molecular tracks and nanowires.
  42. 42. Template-guided self-assembly of gold nanoparticles on a organosilane bilayer template fabricated according to the scheme of (a) Template bilayer. (b) Deposition of water-soluble (Au- citrate) colloidal particles on amino-terminated template patterns. (c) Fabrication of gold electrodes and wires. (d) Patterning of a Picasso drawing. The patterning was carried out with a 800 × 800 raster-scanned points at 3.3 ms per point and by applying a tip-surface voltage of 8.5 V.
  43. 43. Thanks • Questions. • Quiz will be on the next day: Lec 5 & Lec 6