How to improve shader to render UI text correctly / DOF Cam /

Hi,

I want to make text floating in the 3d space using Unity GUI. I've Canvas Screen Space - Camera and I think all is set correctly but I've problem with rendering - (shader?). Also on my cam is Depth of field (DOF) component. I tried to make custom material for this but without success. On the part, where the font is overlaping another object is rendered correctly, but if there is void behind the object it got some blur ( please see screenshot)

Am I thinking correctly towards shader? If yes, what shader you suppose to modify? Thank you...

btw... it's sprite graphics / 2D /