21-12-2012, 06:42 PM
Advanced Optical Lithography
1Advanced Optical.ppt (Size: 855.5 KB / Downloads: 156)
Resolution Limit
The basic rule of thumb is that the minimum line width achievable is basically equal to the wave length of the incident radiation.
This limit is for classical optics, there are three "tricks" which may be applied to lower this limit:
Interference lithography
Phase mask lithography
Near-field lithography
Interference Lithography
Two coherent beams of radiation from a laser interfere to make periodic fringes with a smaller separation than the optical wavelength.
By optimizing the sensitivity of the resist, features as small as 1/10 of the optical wavelength can be obtained.
Interference Lithography Setup
For large area exposures, the apparatus is complicated, since during the exposure, the system must be mechanically and optically stabilized to sub-wavelength precision.
Laser wavelengths for an Ar laser of 350 nm have been applied to produce 100 nm features such as dots and holes.
We tried this without stabilization--small vibrations completely smear out the pattern.
Achromatic Interference Lithography
The mirrors are replaced by gratings to selectively diffract the light to the exposure spot which relaxes the requirements on the coherence of the light source.
This technique has been applied to produce 13 nm wide posts of PMMA on Si using an excimer laser.
This system is under development by Michael Walsh, James M. Carter, Robert C. Fleming, Timothy A. Savas, Dr. Mark L. Schattenburg, and Professor Henry I. Smith at MIT.
Near Field lithography
An intensity mask with sub-wavelength gaps is placed in close proximity to the resist.
The evanescent wave emanating from the gap exposes the resist.
This evanescent wave occurs only in the near field at a distance much less than the wavelength of the light.
Limitation of Near Field Lithography
The evanescent field penetrates only a few tens of nanometers into the resist.
A two-layer resist process may be utilized to make this work.
The two-layer process is frequently used in e-beam and AFM lithography which will be discussed next.