Just as I thought. You're screwed! You have to install another layer of 5/8" plywood underlayment. It needs to be exposure 1 grade C/C/ plugged or better. Do NOT buy CDX grade, it is not for subfloors, it's for sheathing. Once it's properly installed you can then install either a 1/4" backer board or an isolation membrane such as Ditra. Then your tiles.
BTW, you can not install anything directly over a single 1/2" ply, not even carpeting.
TILE GUY - retired- TROY, MI - Method & Product suitability consulting.