phase shift mask(psm mask),generates 180° phase difference via mosi layer so as to lower the diffraction effect of light and improve the mask graphics resolution ratio.

advantages of psm technology:reduce the proximity effect; utilize the coherence of light to offset part of diffraction and extension effect; change the light distribution of space in order to make more energies distributed to high frequency from low frequency, offset the disadvantages of projection in low frequency at passing and high frequency at resistance, improve the contrast of space image, change the image quality and increase the resolution ratio and focal depth.