LithoTalk

[baturin]

Hello!
Seems to be the first question in the backing sections on this forum
There are a lot of heating stages during lithography process like prebake, hardbake, image reversal etc... Most of the processes require quite a few minutes at some specified temperature.
The problem arise with pyroelectric substrates, especially like lithium niobate, litthium tantalate single crystals with very high spontaneous polarization (which is of course temperature dependent). Quick heating/cooling of such substrates (cut perpendicular to polar axis) induce huge pyroelectric fields, which leads to wafer sticking to hotplate like by epoxy, electrical sparking over the polar surface and even wafer cracking.
There are techniques which can partially compensates these charges. It is also known as a rule of thumb that the heating/cooling should be limited to around 3-5 degrees per minute.
Is it possible to correct the baking recipes using such slow heating, especially when doing hardbacking (say for AZ1500 series resists)? What principles to use for the development of some starting recipes?

Thank you in advance for any comments on that issue.

[Guest]

Hello,

to my opinion, there is no reason why the heating/cooling should not be applied very slowly:

1) In case of the softbake, one has to consider the thermal decomposition of the photo active compound. Thus, slow heating/cooling to e. g. 100°C might lower the later development rate, if the time span the resist film sees >90°C is very long. Maybe it's beneficial to lower the peak temperature to 90 or 80°C if the heating/cooling takes very long.

2) The same is valid for the image reversal or post exposure bake. The longer the ramp is, the lower the maximum temperature should be. However, especially in case of the image reversal or post exposure bake, these steps are very temperature-critical, thus some experiments should be made to check the result.

3) The hardbake is uncritical - the photo activity is no longer important after development.

Generally, slow heating/cooling is beneficial for resist films and lowers the danger of cracks in the resist film induced by mechanical stress, especially for applications with thick resist films, and high baking temperatures such as the hardbake.

Does this help a little bit?

[baturin]

Thank you for your ideas.

Basically I wonder if there are some temperature threshold effects which can lead to incomplete backing when using slow heating to lower maximum temperature. If the process is of thermo-activation type like common chemical reactions, things should go fine with slower heating, just the right procedure should be developed from scratch (lower the max temperature).

[guest]

Hello
Have you tried a vacuum oven bake for the resist softbake?
this can be performed at 25°C for ~30mins and most 'thin'resists will be dry. you have to have a rehydration time after bake before exposure but it works.

[baturin]