Improved method of heat-treating a glass substrate, especially where the substrate is thermally treated (such as formation of films, growth of films, and oxidation) around or above its strain point. If devices generating heat are formed on the substrate, it dissipates the heat well. An aluminum nitride film is formed on at least one surface of the substrate. This aluminum nitride film acts as a heat sink and prevents local concentration of heat produced by the devices such as TFTs formed on the glass substrate surface.