Al203 is difficult to etch using RIE. I used to etch Sapphire samples using a mixture of SF6 and 02 gas at relatively high power. The etch rate was very small (about a few nanometers per minute only). 

You need BCL3 gas. It is the only way to achive good etch rates and for this processes SiO2 mask better. 

Ivan is absolutely right. BCl3 is much better as a reactive gas for Al203 etching . 

some papers are here : 

http://www.sciencedirect.com/science/article/pii/S0921510704006737

http://www.transeem.org/Upload/files/TEEM/2%20JEEMT10-019(202-205).pdf

Great thanks for the help! I'll look into BCL3 and take a look at these papers.