1. Field of the Invention
This invention relates to a Levenson-type phase-shift mask, in particular to a Levenson-type phase-shift mask which is capable of minimizing the dimensional deviation of transferred pattern and of inhibiting the misregistration of transferred pattern.
2. Description of the Related Art
In recent years, concomitant with the trend to increasingly refine a semiconductor circuit pattern, there is also a surging trend to further refine the photomask to be employed in the formation of semiconductor circuit pattern. Therefore, it is now desired to enhance the resolution of the photomask. Under the circumstances, there has been proposed a so-called phase-shifting technique by Levenson et al. In this phase-shifting technique, the light is projected passing through neighboring apertures of the photomask in such a manner that it creates a phase shift of 180° between the neighboring apertures, thereby enhancing the resolution of the transferred pattern.
This phase-shifting technique is based on the principle that a phase-shifting portion is provided at one of the neighboring apertures, thereby enabling the light transmitted through this phase-shifting portion to take the opposite phase (a deviation of 180°) relative to another light transmitted through another aperture. As a result, these lights transmitting through the boundary portion of the transferred pattern are mutually weakened to separate the neighboring transferred patterned portions, thus making it possible to enhance the resolution of transferred pattern.
The photomask which is designed to cause the phase inversion of transmitting light through the phase-shifting portion at one of the neighboring apertures is generally called a Levenson-type phase-shift mask.
As for the Levenson-type phase-shift mask provided with a phase-shifting portion at one of the neighboring apertures, a dugout-type phase-shift mask wherein a phase shifter as well as a dugout portion (recessed portion) which is equivalent to the phase shifter is formed in a transparent substrate is frequently employed.
FIG. 1 shows a cross-sectional view for illustrating the construction of the trench-type phase-shift mask. Referring to FIG. 1, a light-shielding film 2 is formed on the surface of a transparent substrate 1. In this light-shielding film 2, an aperture 3 exhibiting a phase shift of 0° (0-phase portion) and an aperture 4 exhibiting a phase shift of 180° (π-phase portion) are formed. A portion of the transparent substrate 1 where the aperture 4 is located is dug out in such a manner that undercut portions 5, each being over-hanged by the light-shielding film at a width of “a”, are formed. The reference symbol “b” in FIG. 1 represents a phase shift, and the reference symbol “c” is called chrome CD (CD: Critical Dimension, which is a line width when a line pattern is an isolated pattern) and represents a dimension to be created when chromium is employed as the light-shielding film. The pitch “p” is a distance between an edge of one aperture of light-shielding pattern and an edge of the neighboring aperture of light-shielding pattern.
Although the undercut portions 5 is provided in order to prevent the generation of unbalance in exposure intensity that may be produced due to the transmitted light entering the side-wall of the trench of substrate in the case of the Levenson-type phase-shift mask having a single trench structure shown in FIG. 1 (for example, JP-A 08-194303), there is also known a structure wherein a space bias “s” is added to the original dimension of the aperture for the same purpose as described above as shown in FIG. 2 (for example, JP Patent No. 3127148 and JP-A 2003-255511).
However, the aforementioned Levenson-type phase-shift mask of trench type is accompanied with the following problems.
Due to the non-uniformity in thickness of the resist film or due to the projected/recessed surface of the underlying layer, the deviation of focusing (hereinafter referred to as defocus or defocusing) on the occasion of exposure is produced. Further, since the fluctuation in contrast of the transmitted light at the 0-phase portion is caused to differ from that of the transmitted light at the π-phase portion due to the aforementioned defocus, the distribution of exposure intensity is brought into an unbalanced state as shown in the dotted line 12 of the graph of FIG. 4 illustrating the relationship between the exposure intensity and the location of phase portion. Namely, there are problems that the misregistration 14 of transferred pattern and the CD error 13 are produced. Further, the magnitude of misregistration of transferred pattern differs depending on the pitch of pattern. The solid line 11 in FIG. 4 represents a distribution of exposure intensity which is excellent in a state of balance. The reference number 15 in FIG. 4 denotes the exposure intensity at the 0-phase portion and the reference number 16 denotes the exposure intensity at the π-phase portion.
As for the measures to prevent the generation of misregistration of transferred pattern, there is known a technique wherein an original mask pattern is created based on a predetermined mask pattern data, and then, by making use of classified mask patterns, the magnitude of misregistration between each of the classified mask patterns and the original mask pattern is calculated to determine the correcting quantity of mask pattern for off-setting the magnitude of misregistration thus calculated. Then, based on the correcting quantity of mask pattern, the original mask pattern is corrected to obtain the final mask patter (JP-A 2002-357889).
However, this technique is designed to correct the size of pattern in taking measures against the misregistration of resist pattern that can be brought about due to the effects of wave-guide. Namely, this technique is designed to correct the size of pattern so as to minimize the misregistration under the condition of best focusing, so that the misregistration due to the defocusing is required to be offset. Thus, although it is possible to minimize the average value of misregistration at a desired range of focusing width, it is impossible to sufficiently correct the misregistration of mask pattern since the range thereof is unchanged.