The present invention generally comprises a low cost TFT and a method of manufacturing a TFT. For TFTs, soda lime glass would be an attractive alternative to non-alkali glass, but a soda lime glass substrate will permit sodium to diffuse into the active layer and degrade the performance of the TFT. Substrates comprising a polyimide, because they are flexible, would also be attractive to utilize instead of non-alkali glass substrates, but the plastic substrates permit carbon to diffuse into the active layer. By depositing a silicon oxynitride adhesion layer over the soda lime glass substrate and a silicon rich barrier layer over the adhesion layer, diffusion may be reduced and deposition may occur at high temperatures. Thus, a lower cost TFT may be produced with a soda lime glass substrate or a substrate comprising a polyimide as compared to a non-alkali glass substrate.