Hi.

The simple way to create structures is to reflow resist.  However, this will
only create concave, spherical surfaces.  The alternative is to use grey
scale resist exposure, but this tends to create stepped structures.  You
probably could not create structures thick enough anyway (up to 300 microns
high).

A more complex solution to your problem is as follows:

Create a convex lens with the required radius of curvature using the
resist reflow technique.
Duplicate this into silicon using 1:1 dry etching.
Now use this to create an imprint in photoresist on a second silicon wafer.

Duplicate the impression in the resist into the silicon (as before).

Good luck.  Martin.