The document discusses electron beam lithography (EBL) for nano fabrication. EBL uses an electron beam to directly write nanoscale patterns on a resist-coated substrate. It allows for very high resolution down to 5 nm but has low throughput as it is a serial writing process. The key components of an EBL system include an electron gun, electron column for beam shaping and focusing, mechanical stage, wafer handling system, and control computer. EBL resists like PMMA can achieve high resolution but have limitations in sensitivity, etch resistance and thermal stability. EBL is widely used for research applications and mask making due to its high resolution, though it is too slow for high-volume manufacturing.

1. ETE444 :: Lecture 6
Nano Fabrication
Dr. Mashiur Rahman

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. Nano Fabrication
• Electron Beam Lithography
• Soft Lithography
• Scanned Probe Techniques
• Self-Assembly and Template
Manufacturing

4. Nano Fabrication
• Electron Beam Lithography
• Soft Lithography
• Scanned Probe Techniques
• Self-Assembly and Template
Manufacturing

5. Electron Beam Lithography
 Introduction
 Applications
 Electron Beam Writing tool
 Advantages
 Limitations

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. 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. Examples
Bragg-Fresnel lens for x-rays Paul Scherrer Institute

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. 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. 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. 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. 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. 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. Advantages
• High resolution
– down to 5 nm
• Useful design tool
– direct write allows for quick pattern changes (no
masks are needed)

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. Nano Fabrication
• Electron Beam Lithography
• Soft Lithography
• Scanned Probe Techniques
• Self-Assembly and Template
Manufacturing

18. Soft Lithography
• Introduction
• Nanoimprint Lithography
• Micro contact printing (μCP)

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. 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. 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.

23. NanoimprintLithography
R. Waser (ed.), Nanoelectronics and Information Technology, Chapter 9

24. NanoimprintLithography Patterns

25. 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.

26. 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.

27. 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."

28. Stamps

29. Nano Fabrication
• Electron Beam Lithography
• Soft Lithography
• Scanned Probe Techniques
• Self-Assembly and Template
Manufacturing

30. 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

31. STM AFM

32. 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.

33. • 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 .

34. 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.

35. 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.

36. 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

40. 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.

41. Nano Fabrication
• Electron Beam Lithography
• Soft Lithography
• Scanned Probe Techniques
• Self-Assembly and Template
Manufacturing

42. Self-Assembly and Template
Manufacturing
• Nanopatterning of self-assembled
monolayers
• Template growth of organic and
biological structures onto
nanopatterns

43. 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).

44. 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.

45. 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.

46. 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.

47. Thanks
• Questions.
• Quiz will be on the next day: Lec 5 & Lec 6