Nanolithography 

- Definition

Nanotechnology has given birth to many useful sub technologies, one of them is the
Nanolithography. It is a science which deals with structure at microscopic level for detecting,
writing, printing and communicating their characteristics to concerned parts. Or It refers to the
fabrication of nanometer structures and patterns in diverse dimensions. It includes different
techniques for modifying semiconductor chips at the atomic level for Integrated Fabrications
(ICs Fabrications).

Working of nanolithography::::
Nanolithography is used in the many fields of applications, it is  a branch of nanotechnology
which is used for further fabrications of nanometer materials and molecules. It is used during the
integration of edge semiconductors, integrated nanocircuitry or nanoelectromechanical systems.
Two of most important instruments used in the nanolithography processing are 

 Scanning Probe Microscope (SPM)

 Atomic Force Microscope (ATM). 

The Scanning probe microscope permits the surface viewing in advanced detail without
modifying it and both SPM and the ATM could be engaged to write, or print on a surface in
individual atom direction.

Types of Nanolithography 
Nanolithography has many a type according to the area of work in which it is used. Some of
them are listed below with brief description.

1)Optical nanolithography
Optical lithography, is a technique for patterning the various surfaces and have the capability for
producing  sub patterns upto to 100 nm with minor wave lengths. Optical nanolithography
requires the use of liquid immersion and resolution host.Most experts feel that optical nano
lithography techniques are most cost effective then traditional  methods of lithography.

2)X-ray Nanolithography 
Another type of nanolithogrphy is the x ray nanolithography which is quite different from
traditional xray lithography.It has the ability to improve and extend  optical resolution of 15 nm
by using the short wavelengths of 1 nm for the illumination. This is perfromed by printing
approach and is used for batch processing.
The X-ray interference lithography facility at the Swiss Light Source (SLS) is a unique tool to
obtain periodic nanostructures with periods as small as 25nm.The beamline provides spatially
coherent beam in the Extreme Ultraviolet (EUV) energy range. Because of this the technique is
also called Extreme-Ultraviolet Interference Lithography (EUV-IL)
Nanolithography Techniques

1) Electron Beam Direct –Write Lithography

The most famous nanolithography method is electron beam direct Write lithography(EBDW)
technique, which makes use of electron beam to draw a pattern. It is mostly used in the polymers
to obtain different patterns of polymeric structures

Electron beam write time

The minimum time to expose a given area for a given dose is given by the following formula:[2]

where T is the time to expose the object (can be divided into exposure time/step size), I is
the beam current, D is the dose and A is the area exposed.

For example, assuming an exposure area of 1 cm2, a dose of 10−3 Coulombs/cm2, and a

beam current of 10−9 Amperes, the resulting minimum write time would be 106 seconds
(about 12 days). This minimum write time does not include time for the stage to move back
and forth, as well as time for the beam to be blanked (blocked from the wafer during
deflection), as well as time for other possible beam corrections and adjustments in the
middle of writing. To cover the 700 cm2 surface area of a 300 mm silicon wafer, the
minimum write time would extend to 7*108 seconds, about 22 years. This is a factor of
about 10 million times slower than current optical lithography tools. It is clear that
throughput is a serious limitation for electron beam lithography, especially when writing
dense patterns over a large area.

E-beam lithography is not suitable for high-volume manufacturing because of its limited
throughput. The smaller field of electron beam writing makes for very slow pattern
generation compared with photolithography (the current standard) because more exposure
fields must be scanned to form the final pattern area (≤mm2 for electron beam vs.
≥40 mm2 for an optical mask projection scanner). The stage moves in between field scans.
The electron beam field is small enough that a rastering or serpentine stage motion is
needed to pattern a 26 mm X 33 mm area for example, whereas in a photolithography
scanner only a one-dimensional motion of a 26 mm X 2 mm slit field would be required.
Defects in electron-beam lithography
Despite the high resolution of electron-beam lithography, the generation of defects during
electron-beam lithography is often not considered by users. Defects may be classified into
two categories: data-related defects, and physical defects.
 Data-related defects may be classified further into two sub-
categories. Blanking or deflection errors occur when the electron beam is not deflected
properly when it is supposed to, while shaping errors occur in variable-shaped beam
systems when the wrong shape is projected onto the sample. These errors can originate
either from the electron optical control hardware or the input data that was taped out. As
might be expected, larger data files are more susceptible to data-related defects.

Physical defects are more varied, and can include sample charging (either negative or
positive), backscattering calculation errors, dose errors, fogging (long-range reflection of
backscattered electrons), outgassing, contamination, beam drift and particles. Since the
write time for electron beam lithography can easily exceed a day, "randomly occurring"
defects are more likely to occur. Here again, larger data files can present more opportunities
for defects.

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).
2) Extreme ultraviolet lithography 
Extreme ultra violet wave lithography is commonly called EUV. It is type of optical lithography
which makes use of highly active light beam such as ultra violet radiations and these are used to
produce and measure the wavelengths of different kind of materials. It is also known as NGL
method.

Top: EUV multilayer and absorber (purple) constituting mask pattern for imaging a line. Bottom: EUV
radiation (red) reflected from the mask pattern is absorbed in the resist (amber) and substrate (brown),
producing photoelectrons and secondary electrons (blue). These electrons increase the extent of chemical
reactions in the resist, beyond that defined by the original light intensity pattern. As a result, a secondary
electron pattern that is random in nature is superimposed on the optical image. The unwanted secondary
electron exposure results in loss of resolution, observable line edge roughness and line width variation.
3) Charged-particle lithography 
This technique also has the capability of producing high resolution patterns and deals with the
broad beam of ions and can also produce patterns having very high resolution. Broad beam of
ion have highly charged particles which when hit the surface designs a specific desired pattern.

1. A charged particle beam lithography apparatus comprising: a charged particle beam source for
generating a charged particle beam, a deflector for deflecting said charged particle beam, plural stencil
masks, each of said stencil mask having plural transferal apertures and a transmission aperture, each of
said plural transferal apertures being passed through by said charged particle beam so as to shape said
charged particle beam, a transfer mechanism for transferring said stencil masks, and a control part for
controlling said transfer mechanism so as to move other of at least one of said stencil masks, while said
charged particle beam is projected on a specimen through said transfer aperture provided on said one of
said stencil masks, wherein said charged particle beam starts being projected in a case that said charged
particle beam is finished being projected by said at least one stencil mask before said other of said stencil
masks is finished being moved and one of transferal apertures having a pattern to be transferred to the
next is positioned in a deflection range of said charged particle beam. 

2. A charged particle beam lithography apparatus as defined in claim 1, wherein said control part controls
said transfer mechanism so as to repeatedly project said charged particle beam on said specimen through
the transferal aperture of said at least one of stencil masks and the transferal aperture of said other of
stencil mask
Soft Lithography
Soft lithography is so called because it utilises cast moulded stamps made from
flexible materials.

The process begins with the creation of a master. The master is made by etching a
blank – normally a silicon wafer – with a negative photoresist. This gives a raised
pattern of nanometer sized features on the silicon wafer that corresponds with the
required channels in the polymer stamp.

A liquid polymer is then poured on top of the silicon wafer mould. The polymer is
commonly a resin like PDMS (poly(dimethylsiloxane)) or fluorosiliconeheat. The
polymer is heat cured and peeled off the mould.

The mould can now be used in a number of ways. These various alterations to the
process determines the sub type of soft lithography.

Mechanism ::
Soft Lithography is an umbrella term for a set of techniques that rely on printing
and molding to make microstructures and nanostructures. It was originally
developed in order to circumvent the limitations of photolithography, which has
been the basic technology used for making all microelectronic systems. The
invention of photolithography is arguably as important as that of the wheel, bronze,
or movable type in terms of its impact on society. It is, however, a technology that
is specialized for use in microelectronics.

For making other kinds of micro-systems, photolithography is not necessarily the

right technology to use. It is not only limited in the materials it can use and in the
geometries it can produce, but it is expensive and can only pattern a small area at
any given time. In addition, the size of the features one can make with
photolithography is limited by diffraction of light. As a result, to a first
approximation, photolithography is confined to extremely flat silicon substrates;
curved surfaces, for example, cannot evenly accommodate light beams moving in a
straight line. One could not, for example, fabricate electronic circuits on a plastic
sheet or a flexible display on a curved car dashboard. While Soft Lithography has
different limitations, the physics-based constraints in these techniques are
relatively minimal, especially relative to the broad range of capabilities this
technique enables.
It is important to emphasize that while Soft Lithography clearly is of value for
electronics, it is by no means limited to this field. In fact, Soft Lithography is
finding application in a range of different fields from consumer products to
industrial processes to life sciences, because the fundamental capability it enables
is critical to so many development challenges: the exquisite control over an infinite
range of structures and chemistries from the nano- to the meso-scale, and the
integration of these into useful systems and devices.

The basic principle of the first phase for any Soft Lithographic technique is
illustrated below. We start with the fabrication of a ‘Master’ using proven
techniques, such as photolithography, e-beam, or micro-machining. A Master
could also be an existing structure that doesn’t require processing like a human hair
or some woven fabric. An elastomer, such as polyurethane or a silicone, is poured
onto the Master, hardened using heat or ultraviolet light, and peeled off to yield a
‘mold’. The resulting mold is the exact structural inverse of the original Master -
down to nanometer accuracy depending on the combination of materials used and
the precision of the replication process.

Thanks to their distinct physical characteristics, such as softness, flexibility,

elasticity and minimal stickiness, these polymer molds can be used as stamps for
transferring the Master pattern to virtually any surface. While our techniques often
begin with polymer stamps and molds, we could just as easily impart structures
and chemistries onto a variety of non-plastic surfaces (such as metals, ceramics or
oxides) of practically any shape or size. Furthermore, we can pattern these diverse
materials with a broad range of materials, including silicon on glass or organic
molecules on metal.

Note that a single Master can be used tens to hundreds of times, depending on
application, to produce tens to hundreds of molds, and each mold can be used to
transfer the pattern tens to a hundred times depending on application. And each
mold can also act as a Master from which we can again accurately replicate tens to
hundreds of molds. The result is a highly scalable and economical process. It is
also important to note that molds can, in principle, be fabricated with meter-sized
dimensions and surface feature sizes ranging in size from nanometers to
millimeters.

The transfer of a pattern from a patterned stamp, created using Soft Lithography, to
a surface requires an ‘ink’. We use conventional inks to create color effects on
surfaces much like we would use a stamp to imprint a return address on an
envelope. Industries are interested in the wide range of specialized ‘inks’ that we
pattern on surfaces that modify the characteristics of the material, including its:
water-repellency, interfacial energy, electrical conductance, heat conductance,
optical properties, stiffness, strength and other physical properties. The chemical
and physical properties of the ‘ink’ may be just as important for the performance of
the integrated system or device as the design on the stamp.

Examples of Patterned Chemistry

 Organic molecules (e.g. self-assembled monolayers)

 Metals
 Charge (electrets)
 Crystals
 Liquid crystals
 Proteins and other biological molecules (e.g. DNA, antigens/antibodies)
 Cells (mammalian; bacterial)
 Solids and liquids
The micro-patterned metal spheres below illustrate the ability to transfer a pattern
onto a non-flat surface using Soft Lithography. The rainbow colors visible on the
spheres results from our fabrication of nano-scale structures on the sphere’s
surface.
We fabricated the structures enlarged on the right on a Master, which we then
replicated into a deformable plastic mold. We used this mold as a conformal phase-
mask to transfer the pattern lithographically to the curved surface of a sphere—a
surface that is practically impossible to pattern through conventional lithography.
Each structure is approximately 500 nanometers in diameter, with line widths in
the 50 to 100 nanometer range. Similar in principle to rainbows or butterfly wings,
these structures diffract white light with wavelengths of 400 to 800 nanometers
hitting the sphere’s surfaces into the spectrum of colors visible in the image above.
This type of structural color could find practical use in industry as a replacement
for die/pigment-based color or as an anti-counterfeiting measure imprinted on
packaging or the product itself.

Soft Lithography can be used to pattern or replicate structures ranging from

passive components such as electronic interconnects, optical lenses, filtration
membranes, or ultra-efficient micro-structured heat sinks, to active components
such as transistors or display pixels. Additional examples of structures that can be
fabricated include:

Examples of Fabricated Structures

 Anti-reflective structures
 Circuitry for electronics on flexible substrates
 Diffraction gratings
 Heat exchanges
 Micro-lense arrays
 Optical elements
  Filtration membranes

Types of Soft Lithography

1. Micromoulding in Capillaries (MIMIC)
The stamp can be brought into contact with a solid substrate and capillary action used
to add a polymer material to the channels. The polymer is cured and the stamp
removed leaving a pattern with features as small as 1 µm.

2. Near-Field Phase Shift Lithography

Near-Field Phase Shift Lithography uses a transparent PDMS phase mask. The mask
is placed on a photoresist and exposed to light. The relief on the mask shift the light
phase and allows features between 40 and 100 nm to be produced on the photoresist.

3. Microtransfer Moulding
A prepolymer or ceramic precursor is added to the stamp which is placed on a
substrate. After curing the stamp is removed leaving a pattern with features down to
250nm.

4. Solvent-Assisted Microcontact Moulding (SAMIM)

In Solvent-Assisted Microcontact Moulding (SAMIM) the stamp is coated with a
solvent and then placed on a polymer photoresist matched to the solvent. The solvent
causes the polymer to swell and spread into the features of the stamp. The resulting
photoresist canhave features as small as 60 nm.

5. Replica Moulding
In replica moulding the original master is not required to be a negative of the final
piece. After producing a PDMS stamp of the original master, the stamp then is used as
a secondary master and another stamp made from it. In this manner the original
master cannot be damaged or degraded as multiple copies are made.
 6. Microcontact Printing
In microcontact printing the stamp is ‘inked’ with selected chemicals, normally
alkanethiols. The stamp is then pressed onto the substrate and removed leaving a 1
molecule thick layer with features down to 300 nm.

Advantages of Soft Lithography

Soft lithography is a low cost production method that allows for the creation of three-
dimensional patterns at room temperatures and pressures. Additionally, because the
stamp is flexible the substrate material need not be perfectly flat. It can be flat curved,
spherically curved or in some instances contain surface features or roughness.

Applications of Soft Lithography

Soft lithography can be used for the production of:

         lab-on-chip systems

         biosurfaces

         biochips

         microfluidics

         microreactors

         sensors

         microelectromechanical systems (MEMS)

         microoptics
Near-Field Multiphoton Nanolithography
INTRODUCTION:
There has been great recent excitement over the development of techniques for exceeding
the diffraction limit in high-resolution optical imaging and photolithographic nanometer-scale
device fabrication. In particular, there have been two different approaches to improving optical
resolution below the limits dictated by diffraction. The first approach takes advantage of the
nanoscale aperture of a near-field scanning optical microscope (NSOM) probe[1-3], while the
second approach is based on the non-linear absorption of light by a chromophore exposed
to an intense electromagnetic field.
The NSOM approach to improved spatial resolution takes advantage of the small, nanometer-
sized aperture formed by a metal-coated optical fiber tip from which light can be either directed
to or collected from the sample. Unfortunately, due to the propagation cut-off of the waveguide
mode in fibers with such restricted apertures, only a very small fraction of the light can be
transmitted through the tip, making it difficult to use such probes in lithographic applications.
Apertureless near-field scanning optical microscopy (ANSOM) has overcome this problem by
using sharp metallic tips instead of fiber apertures to achieve nanometer scale resolution. In
ANSOM, light polarized along the sharp axis of a metallic tip induces a high concentration of
surface Plasmon’s that results in strong enhancement of the electromagnetic field in the local
vicinity of the tip (typically over a few tens of nanometers). This localized enhancement in the
light field has been widely applied in near-field imaging, single molecule excitation, and most
recently, in one-photon near-field optical lithography, where a metal-coated atomic force
microscope (AFM) tip was used to enhance the local electromagnetic field to pattern a
photoresist.
The multi-photon absorption approach to improved optical spatial resolution, in contrast,
does not require an external element (such as an AFM tip) to provide local enhancement of the
electromagnetic field, but instead relies on the non-linear optical properties of a material. When
high-intensity light shines on a material, the probability for multiphoton absorption is
proportional to the square of the field intensity, and thus is greatest at the center of a Gaussian
laser spot. By carefully choosing the laser pulse energy and duration so that only this central
region is above the intensity threshold for multiphoton absorption, a material can absorb two
photons of light over a much smaller spatial region than could be achieved using one-photon
absorption. Multiphoton absorption-based photolithography experiments utilizing this idea
recently have achieved ~ 120-nm spatial resolution using femtosecond laser pulses with a 780-
nm central wavelength.
While each of the two approaches, ANSOM and multiphoton absorption, can achieve sub-
diffraction-limited spatial resolution, a combination of these two techniques can provide even
higher spatial resolution, as demonstrated recently in near-field fluorescence and second
harmonic microscopy experiments. In spite of this potential for outstanding spatial resolution,
however, there have been no reports on using the marriage of these two techniques to further
improve the resolution of photolithography. Thus, in this Letter, the combination of apertureless
near-field enhancement and nonlinear absorption techniques is applied in photolithography to
achieve a spatial resolution as high as λ/10, nearly a factor of two smaller than the resolution
achieved in previous far-field multiphoton lithography. In our experiments, we make use of a
metallic ANSOM tip and femtosecond laser pulses to demonstrate lithographic patterning with ~
70 nm spatial resolution in a commercial negative photo resist, SU-8. Since none of the
experimental parameters—the nature of the tip, the choice of resist, and the wavelength of the
laser pulses––have been optimized for this application, we expect that even greater spatial
resolution should be possible using this multiphoton apertureless near-field lithographic
technique.

Ti: sapphire Laser AFM

790nm~120fs 1KHz Contact Mode

60nm SU-8 thin film

AFM images of multiphoton produced line structures in SU-8 exposed using the field
enhancement of the ANSOM tip with far-field intensities of 0.9 TW/cm2