The following factors may improve your etching distribution:

- wafers should be in vertical position in the solution;
- isopropyl alcohol should be added to the solution as a wetting agent (to
remove
  bubbles from the wafer surface);
- you can also use outside megasonic sourse in order to remove bubbles from
wafers;
- be sure the temperature distribution in your bath is uniform;
- just before etching remove native oxide from the wafers surface by using HF
acid.

Best regards,

Andrzej Prochaska