The Mask Error Enhancement Factor

Optical lithography at the limit of resolution is a highly non-linear pattern transfer process. One consequence of this is an apparent magnification of mask errors. The paper first demonstrates early experimental evidence of this effect. Then it assesses the influence of pattern geometry, of the lithography tool setup, and of different optical enhancement techniques on the MEEF using primarily simulation data. The correspondence of MEEF as an effect of mask linewidth variations to the increased printability of mask defects is illustrated. Strategies to minimize the MEEF - like alternating phase shift masks - are presented. Because the MEEF describes conveniently the concerted action of all components on the whole lithographic pattern transfer process, it is proposed to use the MEEF as a new yardstick to characterize the degree of difficulty of a given lithographic process.