You shouldn't have to coat the entire wafer - usually if you place sufficient material in the center it will be OK.  If your thickness is noticeably uneven, then either your solution needs adjusting - if the solvent evaporates too quickly, this is a problem (typically want a BP around 150 C), if the viscosity is too high OR too low there can be problems.  A solution which pours like motor oil or syrup, with a spin speed of ~1000 RPM ought to give you good films of ~ 1 micron.

James Hanson pretty much said it all except two things....

1) Be sure to thoroughly clean your substrate. The solution will not "wet" a poorly cleaned substrate resulting in bad film quality.

2) When you say center to edge you must remember that the very edge will not have the same properties as the the rest of the film since the last few microns can suffer from instabilities.

Good Luck.

Thanks guys, I should clarify that I want films (or pads) of around 150 um. I have not tried adding a solvent to the pdms though. I will give that a try

Roddel makes a good point about the substrate - must be clean and sometimes helps to derivatize.  We used to do a plasma treatment to clean off any organics and to make a thin SiO2 layer, then treat with hexamethyldisilazane vapor to cap the OH groups with TMS - but that was micron films of polystyrene derivatives in a clean room.  You are making thick films (150 micron) and the PDMS may have decent adhesion to the wafer.