Title: Nanolithography
1Nanolithography
- Lecture 15
- G.J. Mankey
- gmankey_at_mint.ua.edu
2Writing and Printing
- To go far beyond the Raleigh limit of optical
processes, other probes must be applied. - By definition a nanostructure is one with
nanometer dimensions, but in practice true
nanoscience implies a paradigm shift of the
lithography process to below 100 nm. - Sub 100-nanometer writing and printing requires
the use of x-rays, electron beams, focused ion
beams, atomic force probe tips, or a nanoimprint
mask. - Printing processes are x-ray lithography and
nanoimprint lithography. - The writing processes all involve scanning the
probe across the surface writing a single spot at
a time and are thus time consuming and expensive.
3Why Nano?
- Semiconductors and magnetic storage devices will
get smaller. - A current bit on a hard drive is 300 nm by 100
nm. - The future evolution of this technology will
require nanoscale readers, writers and possible
media. - To be marketable, these devices must be
inexpensive and reliable. - The challenge for physics is not only to make it
possible, but routine!
ref W. Eberhardt in Frontiers in Surface and
Interface Science (2002).
4Resolution Limit of e-beams
- The wavelength of an electron is
- The diffraction limited beam has a diameter d
0.6l/a, where a is the beam divergence (0.01). - A 10 kV beam has l 0.01 nm and this simple
analysis gives a 1 nm spot. - In general, the spot sizes are added in
quadrature, as shown in the diagram.
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
5Electron-Solid Interactions
- Increasing electron beam energy arbitrarily will
not result in a smaller spot in the resist. - Higher energy electrons produce more secondary
electrons which travel through the solid and
expose a larger volume of resist. - This is similar to photoelectron blur in x-ray
lithography which ultimately puts a limit on the
smallest achievable feature. - Features 10 nm are theoretically possible, but
rarely demonstrated.
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
6Two Layer Resist Processing
- The best results of e-beam have been achieved for
making a pattern of a single layer of evaporated
or sputtered material. - The two-layer process produces an undercut
structure which functions as a shadow mask for
the deposition process. - After deposition from a collimated source, the
resist is removed with a solvent.
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
7Resist Contrast
High Contrast
Low Contrast
- Resist contrast determines sensitivity to process
parameters such as exposure time and resist
thickness. - Higher contrast resist usually has a more
vertical sidewall angle. - The contrast is defined as
D1
Film Retention ()
D2
Log (Electron Dose)
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
8Dot Arrays Fabricated by EBL
- AFM images of dot arrays of different period
fabricated by e-beam lithography, followed by
thermal evaporation and lift-off. - The pitch, P, is varied systematically while the
raster rate and beam current are constant. - Under the experimental conditions, uniform arrays
are obtained when the period is greater than
45nm. The density of the dots is up to 300
Gdot/in2. - Image size is one square micrometer.
9X-Ray Lithography
- X-rays are produced by synchrotron radiation from
a bending magnet in a high energy electron
storage ring. - Resist, usually PMMA, is exposed through an x-ray
mask in proximity or in contact with the wafer. - This technique has the advantage that very high
aspect ratios can be achieved, since the x-rays
are highly penetrating.
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
10Resolution Limit of XRL
- The wavelength of an x-ray is given by l
1.2/E(keV), so one would expect higher energies
to be more desirable. - Blur, caused by the excitation of photoelectrons
and the associated shower of secondary electrons
limits the patterning ability to about 20 nm. - XRL has been applied to MEMS, where the high
aspect ratio is a distinct advantage.
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
11AFM Lithography
- The AFM tip is used to "bulldoze" through a top
layer of resist. - Submersion in developer produces an undercut in
the lower layer of resist. - The top layer then serves as a shadow mask for
deposition. - After removal of resist, the deposited
nanostructure remains. - For nanocontact printing, the initial structuring
is performed in parallel with a contact mask.
Undercut
Developing
AFM Bulldozing
Depositing
Lift Off