We propose a new criterion for mask birefringence in polarized illumination. Mask birefringence is one of the
critical properties of polarized illumination, because the illumination polarization is disturbed by the birefringence of a
mask substrate. From this point of view, the allowable mask birefringence has already been analyzed. In these analyses,
only the absolute values of birefringence have been specified. As has been pointed out, the mask is a rotation retarder
for the polarized illumination. Therefore, the angle of the fast axis of mask birefringence also affects the state of
The new criterion of mask birefringence which we propose here adopts the angle of fast axis as well as the
absolute value of birefringence. This new criterion correlates well with the printed critical dimensions (CDs). To
demonstrate this, printed CDs were calculated as a function of birefringence. A lithography simulator was used to verify
the fit of the new criterion. In this simulation, experimentally measured absolute values of birefringence and the angle
of fast axis were used. The simulation showed that there was poor correlation between printed CDs and the absolute
values of birefringence. On the other hand, the new criterion exhibited a good correlation with the printed CDs. This
difference is attributed to the effect of the angle of fast axis.
For an ultra-high numerical aperture (NA), such as that exceeding 0.9, the p-polarized component of light that has passed through a region at the limit of the NA of a high-NA lithography tool, degrades contrast because of the so-called vector imaging effect, and is therefore detrimental to the formation of optical images. Polarized illumination removes the effect of the p-polarized light component and provides illumination light composed of s-polarized light. The higher the NA, the greater are the benefits of polarized illumination. Therefore, in lithography at the 45-nm node and below, polarized illumination is viewed as an indispensable technology. We explore the applicability of polarized illumination to device manufacturing processes at the 45-nm node and beyond, with a focus on the utilization of azimuthally polarized illumination, which enables one mask exposure. The data used in this research were obtained through imaging simulations and experiments using a dry lithography tool equipped with a 0.92-NA projection lens. In imaging simulations using a lithography simulator, the application of azimuthally polarized illumination improved image contrast in resists by approximately 20% for half pitch (HP) 65-nm dense patterns. As a result, device patterns showed enhanced robustness with respect to exposure dose error; extended process windows; and reduced mask error enhancement factor (MEEF), line edge roughness (LER), and line end shortening (LES). This paper examines the results of experiments conducted using imaging simulations and lithography tools on other product device like patterns (besides special patterns in which benefits can clearly be expected, including dense (L/S) patterns), and reports the results.
As the pattern feature-size of devices shrinks down to below 65 nm, it becomes more important to establish methods to control lithographic critical dimension (CD) that enable better controllability of CD after etching. This paper introduces new methods to control and optimize lithography process by precisely anticipating CD after etching. Before establishing the methods, relationships between CD after resist development and after etching were measured by using a scatterometry (iODP100 by Tokyo Electron Ltd.). As CD and side wall angle (SWA) of resist profiles can be controlled independently by adjusting exposure dose and focus offset in lithography process, it is possible to control the CD after etching by adjusting them. Moreover, since current lithography and etching process are designed based on CD budgets that have several elements such as intra-shot, intra-wafer, wafer-to-wafer and lot-to-lot CD uniformities, it is preferable to control total CD errors after etching by adjusting the elements in lithography process in anticipation of CD after etching. In addition, it is also important to control CD for various patterns, such as isolated, dense, and other patterns in various design layouts. As current optical proximity effect correction (OPC) techniques can not fully eliminate pattern density effects induced by etching, controlling CD by adjusting lithography conditions to compensate such effects will be one of the feasible solutions.