To improve the CD controllability of isolated lines, we have developed a new method using multiple-focus exposure in alternating phase-shift lithography. In this paper, the imaging performance such as DOF, exposure latitude, mask linearity, and CD controllability is discussed through both experiments and simulation. Multiple-focus exposure experiments were performed using a KrF scanner by giving a tilt offset between the image focal plane and the wafer leveling plane. For the conventional alternating phase-shift method, the CD-focus curve showed a strong concave shape and thus the DOF was rather small. By applying multiple-focus exposure, the shape of the CD-focus curve changed from concave to flat, and therefore the DOF was much improved. We have also found that the CD controllability considering focus errors can be improved by our method.
We compared the topography effect of two types of alternating PSMs; single-trench type with side etching and dual-trench type. The side etching value and dual-trench depth were adjust to give same linewidth in 0 degree and 180 degree regions for 0.2 micrometer L/S pattern. Several test patterns having different width and length were formed on these alternating PSMs. These two PSMs were evaluated by using an x4, 0.6 NA, KrF exposure tool. For longer patterns (similar to L/S pattern), pattern size differences were very small; the mask topography effect was negligible. However, pattern size differences of shorter patterns (similar to window pattern) were large with both Alt PSMs. Therefore, optimization of the side etching value or the trench depth is required for each mask pattern.
An assist-feature mask was fabricated for 0.2 micrometers window pattern formations using a dry etching process. Although the mask's assist-features were as small as 0.68 micrometers , mask inspection was successfully carried out using the cell-shift method. In addition, defects in assist-features were repaired by use of a laser mask repair system. The lithographic performance of this assist-feature mask was compared with that of a conventional mask, using a 4x KrF excimer laser exposure tool and a 0.7 micrometers thick positive resist. The numerical aperture (NA) of the exposure tool was 0.55 and annular illumination was used. The depth of focus of the 0.2 micrometers window was improved from 0.4 to 0.6 micrometers . Moreover, it was confirmed that defects in the assist- feature have little influence on the focus latitude of the main pattern. The DOF of patterns repaired with this technique recovered to nearly the same as that of the no- defect pattern.
To improve the depth of focus of isolated windows, large assist feather technique has been proposed. This large assist method uses the assist features having almost the same size as main patterns, and the quartz substrate was vertically etched at the assist features. These large assist features were not printed on a wafer by mask topography effect; that is, the light intensity at large assist feature was decreased by the scattering effect of the vertical quartz edges. In this large assist feature masks, the phase shift angle of an assist feature has large effect on focus latitude. We chose two phase shift angles: 180 degrees for small sigma illumination and 360 degrees for annular illumination. The performances of two large assist feature masks were evaluated by using a 0.55 NA, valuable sigma, and KrF excimer laser stepper. Moreover, we applied surface insoluble layer to the assist feature method. Large assist features having the same size as main patterns were not printed on resist surface for 0.16 - 0.2 micrometer windows. Wide DOF (0.8 micrometer) of 0.16 micrometer window was obtained by using this method.
Optical proximity correction (OPC) was applied to alternating phase shift masks to improve printed resist pattern fidelity. Mask patterns were modified with jog type corrections. DRAM cell patterns were exposed by using a 0.55 NA, 0.36/0.55 (sigma) , KrF excimer laser stepper onto 0.5 micrometers thick chemically amplified negative resist. With 0.55 (sigma) , OPC was effective and printed resist pattern was very close to designed one. However, with 0.36 (sigma) , large pattern deformation was observed due to coma aberration.
To improve the depth of focus (DOF) of isolated lines, attenuated assist feature (AAF) technique has been proposed; AAFs having more than 20 % transmittance were located around an isolated line. In this mask, the transmittance & phase shift angle of AAF as well as its position & width have effects on lithographic performance. In particular, the phase shift angle has strong effect on focus latitude. The performances of two AAF masks (65 % transmittance/ 28° phase shift and 40 % transmittance/ 54° phase shift) were evaluated by using an NA=0.6, σin/σout = 0.42/0.7, i-line stepper. The focus latitude of 0.3 μm isolated line became flat around the best focus position with 28(degree) phase shift AAFs. In conclusion, we can obtain wide DOF for isolated lines by selecting optimum phase shift angle of AAF.
To improve resist pattern fidelity, partial attenuated phase-shift mask (PA PSM) was developed. On this mask, some portions of opaque regions were changed to attenuated phase- shift regions. The performances of two PA PSMs (8 percent and 13 percent transmittance) were evaluated by using an NA equals 0.6, annular illumination i-line stepper. Resist pattern shortening of longer side was alleviated to the half of a conventional mask, and corner rounding was also improved without deteriorating process margin. Moreover, the width of attenuated region did not have much effect on the resist pattern size; almost the same pattern length was obtained with any phase shifted region width (0.2 approximately 0.4micrometers ). Therefore, we have a large process margin in this mask fabrication. Moreover, KrF PA PSM (7 percent transmittance) was fabricated and evaluated. The same effect was confirmed in KrF excimer laser lithography. In conclusion, PA PSM is a very promising technique for precise pattern formation.
In attenuated phase-shift mask technique for window pattern formation, the amount of mask bias has to be optimized, not only to obtain wider focus margin but also to avoid resist film thickness loss at side-lobe position. In general, larger mask bias is necessary to preclude side- lobe printing, but depth of focus decreases with increasing mask bias. In this paper, surface insoluble layer was applied to prevent this side- lobe printing, instead of larger mask bias addition. At first, the pattern formation capability of 0.35 micrometers window was investigated with an attenuated phase-shift mask of 8% transmittance in i-line lithography (NA equals 0.6, (sigma) equals 0.3, i-line stepper; 1 micrometers thick novolac type resist). Surface insoluble layer was formed by alkaline developer (2.38 wt% tetramethylammonium hydroxide TMAH) treatment followed by water rinse before i-line exposure. After more than 1 min treatment, deep resist film dimple due to side lobe was suppressed almost perfectly, even in small mask bias (<EQ 0.05 micrometers ) condition. As a result, the depth of focus of 0.35 micrometers window increased to 2.0 micrometers (mask bias, 0.05 micrometers ) from 1.4 micrometers (mask bias, 0.1 micrometers ). Next, this technique was also applied to KrF excimer laser lithography to improve the process margin of 0.25 micrometers window formation. A 5% transmittance attenuated phase-shift mask and the combination of KrF excimer laser stepper with 0.5 NA and 0.3 (sigma) and 0.7 micrometers thick chemically amplified positive resist were used. It was found that surface insoluble layer can be formed by the same TMAH alkaline treatment in KrF chemical amplified resist. As a result, the focus margin off 0.25 micrometers window was improved to 2.0 approximately equals 2.5 micrometers (mask bias, 0.0 micrometers ) from 1.5 approximately equals 2.0 micrometers (mask bias, 0.03 micrometers ). In conclusion, we can select more suitable mask bias by means of this surface insoluble layer formation technique, indicting that wider (approximately equals 2.0 micrometers ) depth of focus of window pattern is achieved in both i-line and KrF excimer laser lithography.
Three types of half-tone phase-shift masks (shifter overcoated, substrate etched, and monolayer) have been investigated for window pattern formation from the viewpoint of lithographic performance (depth of focus, DOF and window size fidelity). A 0.35 micrometers window pattern was formed by using an NA equals 0.6, (sigma) equals 0.3, i-line stepper onto a bare-Si wafer coated with 1 micrometers thick novolac positive type resist. Mask bias value necessary for 0.35 micrometers window formation depended on shifter structure; 0.05 micrometers in the shifter overcoated, 0.075 micrometers in the substrate etched and 0.025 micrometers in the monolayer. However, the lithographic performance of every half-tone mask was almost the same; 1.6 micrometers wide DOF was obtained in 0.35 micrometers window with these three masks, in comparison with 1.0 micrometers DOF with a conventional mask. This fact indicates that topography effect only works as mask bias; vertical or tapered phase shifter edge reduces pattern size owing to the scattering exposure light. Therefore, mask bias must be adjusted for each mask structure. In conclusion, all mask structures evaluated are available for window formation.
The deviation of phase shift angle from 180° seriously deteriorates the focus latitude. In order to obtain the expected performance of phase shift mask, a Chromium(Cr)/Phase-Shifter/Quartz(Qz) structure is investigated. In this phase shift mask structure, the shifter thickness i.e., phase shift angle, can be precisely controlled, compared with a conventional Shifter/Cr/Qz structure. Spin-on-grass(SOG) is used as the phase shifter material because of its excellent thickness uniformity. Alternating phase shift mask that has the Cr/SOG/Qz structure was fabricated using Ar-laser writing method, and evaluated using a NA=0.45, 6=0.3-0.5,I-line stepper. Obtained results show that this phase shift mask structure is very promising for the subhalfmicron pattern formation.