Plasma ashing is an economical and efficient technique for removing photoresist from a
wafer.
A mechanical vacuum pump is used to reduce the pressure of the plasma chamber. A throttle valve
between the pump and chamber allows for control of chamber pressure. Oxygen
is injected at a controlled rate into the chamber.
Molecular oxygen (O2) does not react appreciably with photoresist at room temperature.
It must be in a higher energy state in order for it to react. Oxygen radicals, such
as O, must be created to initiate a low temperature reaction.
A radio frequency (13.56MHz) oscillator connected in a capacitively coupled arrangement
provides the energy to create the oxygen radicals (plasma). A small portion of the O2
will dissociate to form monatomic O in the plasma (along with ionized species).
The monatomic O reacts with the organic photoresist
to form volatile (gaseous at low pressure) products such as CO, CO2, and water. The reaction products are
then pumped out of the chamber.
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