Takayuki Uchiyama

Reduction of mask error enhancement factor (MEEF) by the optimum exposure dose self-adjusted mask

To reduce mask error enhancement factor (MEEF), we have developed the new type half-tone phase shift mask (HTPSM) in which transparent regions are surrounded by opaque rims. We evaluated the imaging performance of contact hole patterns including the MEEF and the depth of focus (DOF). Using this new method, we obtained about 2.0 MEEF and 0.7-micrometers DOF for 180-nm isolated hole, which was much better than that in the conventional mask such as binary mask or HTPSM (the MEEF more than 3). The advantage of our method was that it was possible to attain both the MEEF reduction and the DOF enhancement by the optimization of mask hole size and rim width. Furthermore, we confirmed that this new method was effective not only for improving the exposure dose latitude but also for attenuating side-peak effect.

Comparison between optical proximity effect of positive and negative tone patterns in KrF lithography

In this study, the optical proximity effect (OPE) of positive and negative tone line patterns is compared under a variety of exposure conditions. 0.25 micrometers lines with various pitch sizes were printed by a KrF stepper, and the CD variation as a function of pitch was evaluated. We found that the OPE was suppressed significantly in negative patterns under various conditions. The effect of resolution enhancement techniques on the OPE is also investigated. We found that negative patterning with the combination of off- axis illumination and attenuated phase-shift masks not only improved DOF but also gave a small OPE.

Reticle Critical Dimension Latitude for Fabrication of 0.18 µm Line Patterns

The reticle critical dimension (CD) latitude of 0.18 µm line patterns with various pitches was investigated from the standpoint of mask linearity, optical proximity effect (OPE) and depth of focus (DOF). We propose a global reticle CD latitude which is defined by the common range of allowable reticle CDs for various pitches. The global reticle CD latitude was enhanced under illumination conditions in which the OPE was suppressed, e.g., conventional illumination, and at the optimum numerical aperture (NA) which gave a wider DOF, especially for isolated lines. We found experimentally that the global reticle CD latitude (on x4 reticle, range) of 0.18 µm line patterns was 20 nm at NA=0.60 and σ=0.75 conventional illumination. Furthermore, from aerial image simulation results, the global reticle CD latitude was expected to be enhanced at the optimum NA (~0.68) by KrF, and at 0.60 NA by ArF lithography.

Standing Wave Effect of Various Illumination Methods in 0.25 μm KrF Excimer Laser Lithography

Illumination dependence of the standing wave effect was investigated by experiment and simulation in variously sized line and space (LS however, in the defocused region of ±0.6 µ m, necessary for actual device fabrication, the amplitudes of these illuminations were smaller than that of low σ illumination. Moreover, it was found that the amplitude variation was closely related to the log-slope value of exposure light intensity distribution at the nominal pattern edge.

Effect of stage synchronization error of KrF scan on 0.18-μm patterning

The stage vibration effect on imaging performance, such as DOF and CD uniformity is evaluated experimentally and compared with simulation analysis. Various kinds of 0.25 - 0.18 micrometer patterns are investigated by using KrF excimer scanner with 0.6 NA and 0.75 partial coherency and two types of chemically amplified positive resists. In the case of a standard resist for 0.25 micrometer level patterning, the CD at the best focus changed and the DOF decreased rapidly with increasing moving standard deviation (MSD) in 0.18 micrometer level pattern formation. Allowable MSD value of L&S pattern was estimated to be around 25 nm. To improve the stage synchronous error margin, the application of a high resolution resist was effective on L&S and isolated space patterns (about 40 nm), but showed little improvement for isolated line and hole patterns. Therefore, totally allowable MSD value was still about 30 nm. In particular it was found that both isolated line and hole patterns were very sensitive to stage vibration effect. Strict stage control has to be required for 0.18 micrometer patterns even if the high resolution resist is used.

DOF enhancement of isolated line patterns by newly developed assistant pattern method

To enhance depth of focus (DOF) for isolated line patterns, we have developed a new assistant pattern method. In this method, opaque additional patterns are placed beside the attenuated phase-shift main pattern. DOF enhancement effects of this and conventional assistant pattern method were evaluated by means of a KrF excimer exposure tool with variable NA (0.45, 0.50 and 0.55). Using the new method with off-axis illumination, we obtained 1.5 micrometers DOF for 0.25 micrometers isolated line pattern, much wider than that achieved in the conventional method (0.9 micrometers ). Furthermore, we confirmed that this new method was effective not only for improving the exposure dose latitude but also for reducing the optical proximity effect.

Influence of resist process on the best focus shift due to lens spherical aberration

The influence of spherical aberration on imaging performance was evaluated by resist simulation for various resist thicknesses and other resist parameters. The best focus variation in terms of pattern size in L and S was not appropriate as a lithographic criterion because it varied not only with pattern size but also with resist characteristics and thickness. General rules for the best focus of L and S based on CD-defocus characteristics are proposed. The lithographic performance represented by the CD uniformity and the best focus variation of isolated patterns were predicted approximately from the result of aerial image calculation for ideal resist performance. In conclusion, the reduction in spherical aberration that leads to improved CD accuracy is not always achieved by the decrease in best focus shift.