IBM's experimental "hot electron lithography" uses Fowler-Nordheim tunneling as a lithography tool.........
eet.com
Excerpt (click on the link for a diagram of the process):
<<Hot-electron litho
Scientists at IBM Watson have come up with an alternative to the electron,
ion and X-ray lithography schemes that have been proposed: hot-electron
emission lithography. The scheme uses a mask manufactured by standard
MOS technology that is applied in a 1:1 projection system. It can print ICs
with an exposure time of only a few seconds, and the mask's lifetime is
300,000 exposures.
This hot-electron mask is based on a tunnel cathode formed by a SiSiO2Al
junction and has been pursued by a number of industry researchers. IBM
has produced a projection system for printing 160-nm lines on an entire
wafer in an e-beam resist. The mask is based on a silicon wafer patterned
with an oxide of two discrete thicknesses. A thin-metal layer on top of the
mask is used as the gate electrode. Biasing this electrode positively allows
electrons to tunnel from the substrate into the oxide, where they are
ballistically accelerated.
Some of the hot electrons
pass through the thin gate
electrode and are emitted
into the vacuum. Tunneling
depends exponentially on the
oxide thickness according to
the Fowler-Nordheim
equations; thus, large
emission contrast is obtained
between regions of thinner
and thicker oxide. The
researchers said the two
oxide thickness values were
chosen as a compromise
among competing goals: long mask lifetime (a small oxide field); short
exposure times (a large oxide field); high emission contrast; and mechanical
stability for mounting.
Research-staff member Markus Poppeller, in a paper submitted to Applied
Physics Letters this month, states that the theoretical resolution limit for the
experimental conditions used is 90 nm. Calculations show that the resolution
of the system can be improved by a factor of two by increasing the electron
acceleration voltage by a corresponding factor of two, or by cutting the gap
between target wafer and mask in half, according to Poppeller and his team.
"We have been working on this research for two years," he said at the
Science Day event, "and we believe that hot-electron emission masks and
1:1 HEEL provide new opportunities for sub-100 nm lithography, without
going to a fancy, novel exposure system like X-ays or direct e-beam.">> |
