Alternating-aperture masks ready for dense patterns

12/01/2003

Alternating-aperture phase-shifting masks (alt-PSM) rely on the fact that photoresists are insensitive to the phase of the light exposing them. Thus, spaces that differ only in phase print identically, at least in theory. Alt-PSMs have been used successfully to pattern relatively isolated narrow gates on microprocessor and ASIC chips, but a problem has limited their use for dense line-space structures. The physical mask structures that correspond to 0° and 180° spaces are somewhat different in most reticle designs, potentially making the images of one or the other type of space brighter, even if the CDs on the mask are the same. That brightness asymmetry can cause displacements of neighboring dark lines (without generally changing their CDs), thus contributing to overlay error. We have demonstrated two different ways to overcome this difficulty, enabling alt-PSMs to meet theoretical expectations for fabricating memory chips and other denser structures.

The 180° spaces correspond to trenches dry-etched into the fused silica reticle substrate, whereas the 0° space openings are at the original plate surface. Even when the widths of the openings in the opaque mask layer are identical, the brightness of the two types of spaces can differ. One reason is that the bottoms of the etched trenches may be rough, reducing their transmission. Another is that the trench walls can scatter and absorb light, reducing brightness. Even worse, the trench walls can alter the effective phase of the light passing by, introducing a phase error even when the geometrical trench depth corresponds to a 180° shift. Whenever the effective phases of the two spaces differ by something other than 180°, defocus can introduce brightness asymmetry. The resulting defocus-dependent shifts limit the range of focus and reduce one key advantage of alt-PSM technology.

Figure 1. SEMs of the a) single-trench, single-undercut and b) SCAA mask structures that produce images that fulfill theoretical expectations for alt-PSMs through focus. The CDs of 100nm dark lines and both types of 100nm spaces vary <10% through focus for the single-undercut and SCAA masks in c) and d), respectively. (Photos courtesy of Canon USA)Click here to enlarge image

Isotropic etching after dry etch can reduce the roughness of the trench bottoms and walls, increasing the transmission. Also, the wet etching widens the trenches, causing the chrome layer to extend beyond the trench wall (Fig. 1a) and thus limiting wall influence. The phase shift produced by the final structure must be 180°, requiring excellent precision for both the wet and dry etch steps, independent of trench width. It happens that the extraneous phase-shifts produced by the trench walls depend on the trench width. Eliminating that effect requires the chrome layer to extend >80nm beyond the trench wall. Undercutting the chrome layer by such a large amount can lead to adhesion problems that make the final reticle delicate.

Figure 1a shows a SEM of the single-trench, single-undercut alt-PSM structure that overcomes these problems for 248nm exposure. The 0° and 180° openings in the opaque chrome on a conventional mask plate are etched; then the 180° trenches are etched through the chrome opening, with the other spaces blocked by resist. Careful control of both dry and wet etch steps results in the structure shown with effective phase shifts of 180±2°. Since only the 180° spaces are etched, the chrome film will be undercut only on one side, thereby retaining adequate adhesion.

A different alt-PSM structure appears in Fig. 1b. This sidewall chrome alternating-aperture (SCAA) mask is made by etching the phase trenches first, but oversizing them by the average of the opaque lines on either side. A wet etch smoothes the substrate, which is then re-coated with a chrome layer that conforms to the 3D substrate topography. The openings in that chrome layer are written last, defining the 0° and 180° transparent spaces. The physical environments of the two types of spaces are identical, limiting optically induced phase errors. Since the chrome is supported everywhere by the substrate, this SCAA mask structure is even more robust.

Figures 1c and 1d display the variations of the CDs of the dark lines and both types of bright space in 100nm line/space patterns printed in 180nm of TOK DP6116 resist coated over 130nm DUV42 BARC and exposed using a Canon FPA-5000-ES3 at 245J/m². The NA was 0.73 with s = 0.3, implying k1 = 0.29. The plots were obtained using an Applied Materials Vera-SEM 3D top-down e-beam metrology tool. Both types of masks print line and space CDs that vary <10% over a 0.6µm range of focus. The slight difference in slope of the space CDs in Fig. 1c suggests that there may be a residual phase error for the single-trench and undercut mask, whereas the slight difference in CD at best focus for the SCAA mask may indicate a slight difference in space transmission. Both effects are well within typical production specifications and near the limits of the measurement procedure.

Similar plots obtained for arrays of 100nm lines with pitches between 200nm and 400nm were free of spacewidth asymmetry over a 0.6µm common range of focus. While other types of alt-PSMs can be carefully etched to eliminate that asymmetry at one pitch, the variation of effective phase with trench width may still cause it to reappear. These results show that alt-PSMs with either of these two structures can be used — along with suitable trim masks — to pattern 100nm node chips using 248nm exposure. Printing 65nm node chips will require 193nm light, and Beach et al. have already demonstrated that the SCAA mask eliminates spacewidth asymmetry down to 140nm pitch [1].

Focus-dependent spacewidth asymmetry has been the most problematic artifact observed in images projected by alt-PSMs. With two different mask structures capable of eliminating this artifact over a range of pitches, alt-PSMs are now ready for mainstream fabrication technology.

Reference

1.James V. Beach, et al., "Evaluation of SCAA Mask Technology as a Pathway to the 65 nm Node," SPIE 5040–107, 2003.

Takeaki (Joe) Ebihara received his bachelors in electrical engineering from Waseda U. in Japan in 1996. He is an application engineer with Canon USA, 3300 North First St., San Jose, CA 95134; ph 408/468-2220, fax 408/468-2219, e-mail [email protected].

Yasutaka Morikawa received his bachelors in chemical engineering from Tokushima U. in 1986. Dai Nippon Printing Co. Ltd.; e-mail [email protected].