Thinning and dicing substrates using inductively coupled plasma reactive ion etching (ICP RIE). When dicing, a hard photo-resist pattern or metal mask pattern that defines scribe lines is formed on a sapphire substrate or on a semiconductor epitaxial layer, beneficially by lithographic techniques. Then, the substrate is etched along the scribe lines to form etched channels. An etching gas comprised of BCl3 and/or BCl3/Cl2 gas is used (optionally, Ar can be added). Stress lines are then produced through the substrate along the etched channels. The substrate is then diced along the stress lines. When thinning, a surface of a substrate is subjected to inductively coupled plasma reactive ion etching (ICP RIE) using BCl3 and/or BCl3/Cl2 gas, possibly with some Ar. ICP RIE is particularly useful when working sapphire and other hard substrates.