C hapter 1 Nanomaterials and Nanotechnologies: an overview
6
is produced on the wafer surface. The system is then placed in an
acidic solution, which attacks the silica but not the photoresist and
the silicon. Once the silica has been removed, the photoresist can
be etched away in a different acidic solution. Though the concept
of photolithography is simple, the actual implementation is very
complex and expensive. This is because (1) nanostructures significantly smaller than 100 nm are difficult to produce due to diffraction effects, (2) masks need to be perfectly aligned with the pattern
on the wafer, (3) the density of defects needs to be carefully controlled, and (4) photolithographic tools are very costly, ranging in price
from tens to hundreds of millions of dollars.
As a response to these difficulties, electron-beam lithography and
X-ray lithography techniques have been developed as alternatives
to photolithography. In the case of electron beam lithography,
the pattern is written in a polymer film with a beam of electrons.
Since diffraction effects are largely reduced due to the wavelength
of electrons, there is no blurring of features, and thus the resolution
is greatly improved. However, the electron beam technique is very
expensive and very slow. In the case of X-ray lithography, diffraction
effects are also minimized due to the short wavelength of X-rays, but
conventional lenses are not capable of focusing X-rays and the radiation damages most of the materials used for masks and lenses.
Due to these limitations, the most recent lithography methods,
such as printing, stamping, and molding, use mechanical processes instead of photons or electrons. These methods are normally
called soft lithography methods because they involve the use of
polymers. They are discussed in Chapter 8. The process starts with
the fabrication of a mold by photolithograpy or e-beam lithography. Subsequently, a chemical precursor to polydimethylfiloxane
(PDMS) is applied over the mold and cured. As a result, a solid
PDMS stamp that matches the pattern of the mold is produced.
The stamp can then be used in several ways to make structures at
the nanoscale. For example, in the case of microcontact printing
the stamp is coated with a solution consisting of organic molecules
(thiol ink) and compressed into a thin film of gold (Au) deposited
on a silicon (Si) wafer. In this fashion the thiol ink is transferred
from the stamp to the gold surface, reproducing the original pattern
of the stamp. In the case of micromolding, the PDMS stamp sits
on a flat, hard surface and a liquid polymer is injected into any
available cavity between the stamp and the surface. Subsequently,
the polymer cures into the patterns provided by the cavities. The
advantages of using soft lithography methods are (1) no special
equipment is required, and (2) versatility in the range of materials
6
is produced on the wafer surface. The system is then placed in an
acidic solution, which attacks the silica but not the photoresist and
the silicon. Once the silica has been removed, the photoresist can
be etched away in a different acidic solution. Though the concept
of photolithography is simple, the actual implementation is very
complex and expensive. This is because (1) nanostructures significantly smaller than 100 nm are difficult to produce due to diffraction effects, (2) masks need to be perfectly aligned with the pattern
on the wafer, (3) the density of defects needs to be carefully controlled, and (4) photolithographic tools are very costly, ranging in price
from tens to hundreds of millions of dollars.
As a response to these difficulties, electron-beam lithography and
X-ray lithography techniques have been developed as alternatives
to photolithography. In the case of electron beam lithography,
the pattern is written in a polymer film with a beam of electrons.
Since diffraction effects are largely reduced due to the wavelength
of electrons, there is no blurring of features, and thus the resolution
is greatly improved. However, the electron beam technique is very
expensive and very slow. In the case of X-ray lithography, diffraction
effects are also minimized due to the short wavelength of X-rays, but
conventional lenses are not capable of focusing X-rays and the radiation damages most of the materials used for masks and lenses.
Due to these limitations, the most recent lithography methods,
such as printing, stamping, and molding, use mechanical processes instead of photons or electrons. These methods are normally
called soft lithography methods because they involve the use of
polymers. They are discussed in Chapter 8. The process starts with
the fabrication of a mold by photolithograpy or e-beam lithography. Subsequently, a chemical precursor to polydimethylfiloxane
(PDMS) is applied over the mold and cured. As a result, a solid
PDMS stamp that matches the pattern of the mold is produced.
The stamp can then be used in several ways to make structures at
the nanoscale. For example, in the case of microcontact printing
the stamp is coated with a solution consisting of organic molecules
(thiol ink) and compressed into a thin film of gold (Au) deposited
on a silicon (Si) wafer. In this fashion the thiol ink is transferred
from the stamp to the gold surface, reproducing the original pattern
of the stamp. In the case of micromolding, the PDMS stamp sits
on a flat, hard surface and a liquid polymer is injected into any
available cavity between the stamp and the surface. Subsequently,
the polymer cures into the patterns provided by the cavities. The
advantages of using soft lithography methods are (1) no special
equipment is required, and (2) versatility in the range of materials