RF plasma etching of a polyimide film with oxygen mixed with nitrogen trifluoride - Polyimides and Other High Temperature Polymers - page 411

Figure 6. Relationship between the CO / CO 2 ratio and NF 3 concentration in the inlet gas mixture.

CO and CO 2 concentrations during etching were analyzed using a gas chro-
matograph GC-8A (Shimadzu Co. Ltd.). The rates of formation of CO and CO 2
were 3.6 cc/min and 3.0 cc/min.
If etched PI products are only CO and CO 2 , the reaction is:

C O N H + z O → 12CO + 10CO +5H O + y NO

22 5 2 10 2 2 2 x

Considering that the PI etching rate was 5.4 mg/min, CO and CO 2 formation
rates should be 3.9 cc/min and 3.3 cc/min, respectively.
These results mean that more than 90% of carbon in the PI structure is con-
verted into CO and CO 2 with 70% O 2 / 30% NF 3 plasma etching. These results
agree with the QMS analyses in that the main peaks of the carbon compounds in
the QMS spectrum shown in Fig. 5b were only CO

+

and CO 2 + .

Figure 6 shows the relationship between the CO / CO 2 ratio and NF 3 concentra-
tion in the inlet gas mixture. The CO / CO 2 ratio showed the highest value at the
highest etching rate.

3.3. Analysis of CO and CO 2 formation mechanism
In order to investigate the etching mechanism of the PI due to NF 3 / O 2 plasma,
the reactivity of the PI with oxygen and NF 3 was evaluated by calculation of reac-
tion enthalpies. Tetracarboxylic anhydride, diamine and benzene were used for
model compounds of the PI structure. The main active species are considered to
be oxygen and fluorine radicals because the mean free path of oxygen and fluo-
rine ions is considered to be much shorter than the ion sheath at the pressure dur-
ing the etching. The reaction enthalpies between these compounds (tetracarbox-