336
NANOMACHINES AND NANODEVICES
Optical lithography is an important manufacturing tool in the semiconductor
industry. However, to fabricate semiconductor devices smaller than 100 nm, ultraviolet light of short wavelengths (1 93 nm) is required, but this will not work because
the materials are not transparent at these wavelengths. Electron-beam and X-ray
lithography, discussed in earlier chapters, can be used to make nanostructures, but
these processes are not amenable to the high rate of production that is necessary for
large-scale manufacturing. Electron-beam lithography uses a finely focused beam of
electrons, which is scanned in a specific pattern over the surface of a material. It can
produce a patterned structure on a surface having 10-nm resolution. Because it
requires the beam to hit the surface point by point in a serial manner, it cannot
produce structures at sufficiently high rates to be used in assembly-line manufacturing processes. X-ray lithography can produce patterns on surfaces having 20-nm
resolution, but its mask technology and exposure systems are complex and
expensive for practical applications.
More recently, a technique called nanoimprint lithography has been developed
that may provide a low-cost, high-production rate manufacturing technology.
Nanoimprint lithography patterns a resist by physically deforming the resist shape
with a mold having a nanostructure pattern on it, rather than by modifying the resist
surface by radiation, as in conventional lithography. A resist is a coating material that
is sufficiently soft that an impression can be made on it by a harder material.
A schematic of the process is illustrated in Fig. 13.3. A mold having a nanoscale
structured pattern on it is pressed into a thin resist coating on a substrate (Fig. 13.3a),
creating a contrast pattern in the resist. After the mold is lifted off (Fig. 13.3b), an
etching process is used to remove the remaining resist material in the compressed
regions (Fig. 13.3~). The resist is a thermoplastic polymer, which is a material that
softens on heating. It is heated during the molding process to soften the polymer
relative to the mold. The polymer is generally heated above its glass transition
temperature, thereby allowing it to flow and conform to the mold pattern. The mold
can be a metal, insulator, or semiconductor fabricated by conventional lithographic methods. Nanoimprint lithography can produce patterns on a surface
having 10-nm resolution at low cost and high rates because it does not require the
use of a sophisticated radiation beam generating patterns for the production of each
structure.
The scanning tunneling microscope (STM), described in detail in Chapter 3, uses
a narrow tip to scan across the surface of the material about a nanometer above it.
When a voltage is applied to the tip, electrons tunnel from the surface of the material
and a current can be detected. If the tip is kept at a constant distance above the
surface, then the current will vary as the tip scans the surface. The amount of
detected current depends on the electron density at the surface of the material, and
this will be higher were the atoms are located. Thus, mapping the current by
scanning the tip over the surface produces an image of the atomic or molecular
structure of the surface.
An alternate mode of operation of the STM is to keep the current constant, and
monitor the deflection of the cantilever on which the tip is held. In this mode the
recorded cantilever deflections provide a map the atomic structure of the surface.
NANOMACHINES AND NANODEVICES
Optical lithography is an important manufacturing tool in the semiconductor
industry. However, to fabricate semiconductor devices smaller than 100 nm, ultraviolet light of short wavelengths (1 93 nm) is required, but this will not work because
the materials are not transparent at these wavelengths. Electron-beam and X-ray
lithography, discussed in earlier chapters, can be used to make nanostructures, but
these processes are not amenable to the high rate of production that is necessary for
large-scale manufacturing. Electron-beam lithography uses a finely focused beam of
electrons, which is scanned in a specific pattern over the surface of a material. It can
produce a patterned structure on a surface having 10-nm resolution. Because it
requires the beam to hit the surface point by point in a serial manner, it cannot
produce structures at sufficiently high rates to be used in assembly-line manufacturing processes. X-ray lithography can produce patterns on surfaces having 20-nm
resolution, but its mask technology and exposure systems are complex and
expensive for practical applications.
More recently, a technique called nanoimprint lithography has been developed
that may provide a low-cost, high-production rate manufacturing technology.
Nanoimprint lithography patterns a resist by physically deforming the resist shape
with a mold having a nanostructure pattern on it, rather than by modifying the resist
surface by radiation, as in conventional lithography. A resist is a coating material that
is sufficiently soft that an impression can be made on it by a harder material.
A schematic of the process is illustrated in Fig. 13.3. A mold having a nanoscale
structured pattern on it is pressed into a thin resist coating on a substrate (Fig. 13.3a),
creating a contrast pattern in the resist. After the mold is lifted off (Fig. 13.3b), an
etching process is used to remove the remaining resist material in the compressed
regions (Fig. 13.3~). The resist is a thermoplastic polymer, which is a material that
softens on heating. It is heated during the molding process to soften the polymer
relative to the mold. The polymer is generally heated above its glass transition
temperature, thereby allowing it to flow and conform to the mold pattern. The mold
can be a metal, insulator, or semiconductor fabricated by conventional lithographic methods. Nanoimprint lithography can produce patterns on a surface
having 10-nm resolution at low cost and high rates because it does not require the
use of a sophisticated radiation beam generating patterns for the production of each
structure.
The scanning tunneling microscope (STM), described in detail in Chapter 3, uses
a narrow tip to scan across the surface of the material about a nanometer above it.
When a voltage is applied to the tip, electrons tunnel from the surface of the material
and a current can be detected. If the tip is kept at a constant distance above the
surface, then the current will vary as the tip scans the surface. The amount of
detected current depends on the electron density at the surface of the material, and
this will be higher were the atoms are located. Thus, mapping the current by
scanning the tip over the surface produces an image of the atomic or molecular
structure of the surface.
An alternate mode of operation of the STM is to keep the current constant, and
monitor the deflection of the cantilever on which the tip is held. In this mode the
recorded cantilever deflections provide a map the atomic structure of the surface.