Reticle Critical Dimension Research Articles

Design correction in extreme ultraviolet lithography

Extreme ultraviolet (EUV) lithography is currently the most promising technology for advanced manufacturing nodes. This study aims to assess in detail the quality of a full chip optical correction for a EUV design, as well to discuss the available approaches to compensate for EUV-specific effects. Extensive data sets have been collected on the ASML EUV Alpha-Demo Tool using the latest Interuniversity Microelectronics Center baseline resist Shin-Etsu SEVR59. In total ~1300 critical dimension (CD) measurements at wafer level and 700 at mask level were used as input for model calibration and validation. The smallest feature size in the data set was 32 nm. Both one-dimensional and two-dimensional structures through CD and pitch were measured. The reticle used in this calibration exercise allowed one to modulate flare by varying tiling densities. The shadowing effect was modeled by means of a single bias correction throughout the design. Horizontal and vertical features of different types through pitch and CD were used to calibrate the shadowing correction. The model calibration yielded an root-mean square of ~1 nm, which was observed to improve by including reticle CD data. An EUV mask fully corrected for optical proximity correction, flare and shadowing was fabricated and qualified, demonstrating the effectiveness of the implemented corrections.

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.

Optically induced mask critical dimension error magnification in 248 nm lithography

One form of optical proximity effect that further complicates lithography is the unexpected response of the printed image to small perturbations [critical dimension (CD) errors] in the reticle. In this way mask CD errors are actually magnified (they are reduced by less than the reduction factor of the optics) during the optical transfer to the wafer. This effect will require even tighter specifications for mask CD control when the error magnification factor is significantly above unity. The effect is particularly pronounced for tight pitches of small features, but can also impact the printing of small isolated lines. Both resist and optical nonideal responses are involved in this mask error factor (MEF). This article discussed the optical effects that produce the MEF. This article will show where the MEF due to optical effects can be ignored and where they cannot when using 248 nm lithography with a high numerical aperture (NA) tool. We will demonstrate how the NA, partial coherence, and variations in focus can effect the mask error magnification factor. We will also show that resolution enhancement techniques can be used to reduce the mask error magnification factor. In particular, Levenson phase shift masks show particularly low mask error magnification factors for small lines. For some applications it should be possible to design the mask so that the mask error magnification factor of the smallest features is significantly below unity. This would allow loosened reticle CD specifications and/or better CD control of the lithography process.