Residual linear birefringence is an important property for quality control of optical components used in optical lithographic instruments. This paper shows that it is especially critical to control the residual linear birefringence in the substrate of photomasks at a very low level. A birefringence measurement system, known as Exicor®, was used for measuring both the magnitude and angular orientation of residual linear retardance in photomask substrates. Different patterns and levels of residual linear birefringence in these samples were identified. The effect of residual linear birefringence in photomask substrates, in determining wafer imaging quality, is discussed.