I setup a CO2 laser system for drilling ceramic. 
But I found the tape angle of vias were too large, 400um entrance and
150um exit with 650um thickness ceramic.
The CO2 laser is Coherent Diamond 64, 150Watts.
The optical components from laser to the object are a phase retarder
module ,a beam expander(2X),and a focus lens(f=2.5") and 3 mirrors .
And there is a gas nozzle after focus len to inject air when drilling.
The substrates are made by CoorsTek and Kyocera.

I have tried lots of laser parameters that I can do, but the tape
angle was still the same.

Is there any way to modify the beam delivery to improve it?
Is there any good process that can be adapted?
What's the best laser parameters to drill ceramica?
How to reduce the heat effect area around the hole?

I hope the tape(entrance-exit)can be less than 10% of substrate
thickness.
And the hole diameter can keep around 150um.

Thanks!

I'm not sure of your system's 'tape' angle, but I will try to help
with the optics. If the Beam expander is one of the last optical
components in the system, you could try moving if back and forth in
the z-axis, or even removing it. The beam expander should double the
size of the beam, but if it is before the mirrors and the phase
retarder you could end up buring one of your optics, and optics for a
CO2 laser can be pretty expensive. Adjusting the focus lens might
raise the power density of the beam allowing you to burn through the
ceramic. The smaller the beam the faster the ceramic will 'burn' away
and the better hole profile you will have. Also make sure that the
ceramic is at the focal length of the focusing lens, there might be a
misalignment, and being too far or too close will lower the power
density.

Hello Ruger,

Thank you for your comment!
I removed the beam expander, but the tape was still the same.
The section viewing of the via looks like below.
____        _________
    \      / 
     \    /
     |    |
     |    |
     |    |
____/      \_______

The tapes shows both on the entrance and exit.
The entrance diameter are almost 2 times of the neck diameter(125um)of via.
I would like to try to minimize the entrance diameter as small as the neck.

Do you have any advice to do ?

Thank you!

Best Regards,
Oliver