Ceramic, slate and glass are popular tile materials for the back walls of outdoor showers. Stacked river rock tiles are increasingly popular, too. To install, you'll need a thinset mortar (check with your supplier to be sure the type of thinset is suited to the application and type of tile). Apply the thinset to the cement backer board, which will allow the tiles to adhere to the cement.
The drawback to ceramic, slate and glass tiles for the back wall of the outdoor shower is that you may need to have the store cut individual tiles to cleanly fit the edges of the cement backer, depending on the size of tiles and the pattern you choose. Stacked river rock tiles are often the material of choice for the back wall of the outdoor shower because the sheets can be resized by simply cutting the mesh to which the stones are attached to attain a smooth edge to the sides of the shower's back wall. River rock tile comes in various sheet sizes and may come in cases of 12-inch by 4-inch (30.4-centimeter by 10.1-centimeter) or 12-inch by 12-inch (30.4-centimeter by 30.4-centimeter), for example. As with all high-quality tile, prices vary, so you can pay way more than you would for natural stone or ceramic tile, or roughly the same, depending on your selection.