Page 315 - A Practical Guide from Design Planning to Manufacturing

Semiconductor Manufacturing 285

UV Light UV Light

Etched region
Quartz
Quartz Quartz
Chrome

Photoresist Constructive Photoresist Destructive
interference interference
Without phase shift With phase shift

Light
Intensity

Figure 9-16 Phase-shift masks.

reaching the center from both sides is out of phase. Destructive inter-
ference cancels out the light from both sides, allowing the light intensity
to reach zero at the center as desired.
By making the drop off of intensity more abrupt, the thickness of the
lines patterned is more easily controlled. Small changes in the intensity
of the light or the sensitivity of the photoresist will have less effect on the
width of the line ultimately produced. Side riggers are made too narrow
to expose the photoresist themselves, but they provide light of the oppo-
site phase to help make the shadows of neighboring lines more distinct.
The processing of adding OPC and PSM features is typically done during
fracture when the layout is converted to the form to be used in making the
masks. The result is masks that look very different from the layout from
which they are derived and from the features they ultimately produce.
Although OPC and PSM have prevented the need for lithography
wavelengths to match the pace of feature size reductions, concerns
remain about the long-term future of lithography. Lithography at reso-
lutions dramatically better than in use today has been demonstrated
using electron beams, x-rays, or extreme ultraviolet (EUV), but it is
feared that any of these alternatives could be vastly more expensive than
current methods. Indeed, x-ray lithography has been proposed as the
future of lithography since the late 1970s, but new ways of incremen-
tally improving current methods have continually pushed back the need
for more radical solutions.