Discussion of Mask Alignment Accuracy for EUV Lithography

The shorter lithography printing light source wavelength is, the more accurate alignment is required for resist printing of high packing density integrated circuits (ICs). As to longer wavelength light source than extremely ultraviolet, view systems of alignment mark are sophisticated by using video camera system. However, it seems that it is difficult to sophisticate EUV mask-aligner alignment view system due to shorter light wavelength. Actually, old mask aligner systems such as contact aligners of Cobilt, projection aligners of Perkin Elmer, and direct steppers of GCA have alignment process visualisation for human eye by using cut off filters of UV light. However, EUV use also requires alignment system accuracy in order to increase EUV use value for high packing density on a IC chip. Since, EUV can not be visible for human naked eyes and detection of EUV is relatively difficult, the author thinks that implement of direct view alignment system by using EUV light source is difficult. However, since only use of printing of mask pattern such as printing of grating do not need accurate alignment or alignment itself, EUV lithography printing without alignment process is relatively easily applicable to grating photolithography. Since energy of EUV light is higher than that of UV, it may be necessay to treat or consider prevention against resonant turbulence of edges of photomask metal thin film patterns and photoresist small-size contact hole resonance interferences due to nuclear body resonance vibrations, by intentionally additions of heavy nuclei such as heavy impurites of metals. Through discussion on various kinds of mask aligner alignment processes, EUV lithography processes will be discussed.