In the fabrication of integrated circuits, lithography is used to generate pattern structures on the semiconductor and various materials for the construction of the desired circuit structures. A continuing demand in view of the ever increasing desire in the semiconductor industry for higher circuit density in microelectronic devices has prompted lithographic engineers to develop improved lithographic processes. Especially, a lithographic process can provide improved linewidth control. In addition to the linewidth control, as the circuit density of semiconductor devices increases, higher resolution of circuit patterns in resist films is increasingly demanded. One way of improving the resolution in resist is to migrate to shorter wavelength from 365 nm to 248 nm, then to 193 and 157 nm, and further to extremely short optical wavelengths like EUV (extreme ultraviolet), or to adopt non optical system such as E-beam. The EUV lithography with exposure wavelengths below 40 nm would allow the industry to print features beyond the diffraction limit of the current 193 nm lithography without resorting to the adoption of tricks using double or triple patterning.
In the photolithographic exposure processes, patterns formed in a mask are projected onto the resist material. The EUV masks used are reflective and generally contain a substrate with a reflective multilayer deposited on it and a patterned absorbing layer deposited on the multilayer. Krautschik et al. in a 2001 SPIE paper (Proc. of SPIE, Vol. 4343, pp. 392-401, 2001) discuss the asymmetry of Bossung curves (linewidth vs. focus plot) and the shift in focus position in EUV lithography caused by the EUV mask. The focus shift was found to be pitch dependent. The focus shift is due to a phase error caused mainly by the interference of the reflected light from the reflective substrate with the diffracted light at the absorber edge or boundary and with the partially transmitted light from the absorber edge. However, Yan in a 2002 SPIE paper (Proc. of SPIE, Vol. 4889, pp. 1106-1112, 2002) concludes that a dark field mask or a space is insensitive to such a mask edge phase error.
It is found that a regular EUV mask containing assist features larger than the EUV wavelength causes an unwanted (but hard to avoid) phase shift present on the mask, which in turn causes focus variation at different pitches for a selected dimension in the printed photoresist images. Contrary to Yan's finding, the phase error occurs on the EUV mask with trench pattern containing an assist feature (a dark field mask). This phase error is caused by the difference in size of main and assist features and gets larger with closer proximity of the different sized features. Therefore, the EUV lithography requires a new structure on the mask to address this focus variation issue.
Depositing phase shifting material above multilayer reflector of an EUV mask to correct phase error has been reported previously. For example, Yan, in U.S. Pat. No. 6,818,357, teaches the use of a selected thin layer material with the thickness of the thin layer producing a phase correction that offsets a phase error such that a common process window of the mask is maintained above a threshold level. Yan also teaches that the mask includes a multilayer reflector and portions of the multilayer reflector are etched adjacent to features of the mask. The main teaching is to correct the phase shift using boundary patterning which is not easy to implement.
In addition, Holfeld, in U.S. Pat. No. 8,142,958, teaches a method which includes the steps of determining the position of a defect area of the substrate, in which a phase shift difference of an exposure radiation is caused by the defect, and depositing a phase-shifting material above the multilayer in one portion of the substrate, in which the portion at least partially contains the defect area. The main teaching of Holfeld is to fix a defect in an EUV mask by depositing the phase shifter only to cover the area or near the area of the multilayer reflector containing the defect.