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This process reverses
the action of positive resist so negative images can be formed with the
same resolution and processing ease that a positive resist allows.
What’s more, image reversal allows variations of the slope of the
photoresist sidewall for higher resolution and/or lift off profiles.
For example, if the normal slope of approximately 68 degrees is altered
to 90 degrees vertical, resolution is increased 20%. And, if an angle of
112 degrees is achieved, there is a strong overhang that makes metal
deposition followed by liftoff controllable.
Why not just use a negative resist?
Positive resist offers far better resolution, and it doesn’t require
solvents for development. In contrast, the resolution limit of negative
resist is approximately 1 micron, and it requires solvents known to be
carcinogens. |
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YES image reversal
systems deliver anhydrous ammonia (NH3) into a vacuum oven. There are
two advantages of an ammonia system: 1) easier control of vapor
pressure, and 2) an absence of residual water vapor, which will react
with the NH3 to produce NH3OH, a corrosive to the system that also
creates contaminating particles.
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