hi all!

I want to make microfluidic channels on PFA (Polytetrafluoro ethylene) wafer
using thermal nanoimprinting. I bought some commercial PFA sheets as wafer.
However, that PFA sheets were obviously crooked. So I should use thermal
nanoimprinting (without mold) to make it flat at first. My protocal is (PFA
sheets are 7cm x 6cm, the thickness is 2mm):

Heating to: 290 degree Celsius
Pressure: 2000N
Time (at 290 degree): 1800s
Cooling to: 70 egree Celsius (cooling with oil)

However, it didn't work at all. The sheets are still crooked.

In my opinion, I believe it was caused by "internal stress". The melting
point of PFA is 302-310 degree Celsius. I don't know the glass transition
temperature of PFA exactly. But I had made microfluidic channels on PFA
wafer using thermal nanoimprinting under 270 degree, so it must below than
270 degree. If internal stress is the reason, some methods should be taken
to remove it.

So, why PFA sheets are crooked after thermal nanoimprint? Internal stress or
other reasons? If the reason is internal stress, how can I get rid of it?

Any suggestions are welcome.

Thank you in advance.