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The recent developments in scanner technology improved the ability to control intra-field overlay at high-order across the exposure field. However, this is still in several millimeter lateral resolution control ability at its best, leaving residual errors in the sub-millimeter to few millimeter regime without the ability to further suppress them to the target specification, nevertheless, not to the sub-nanometer magnitude.
In this work, we have empirically evaluated the ZEISS state-of-the-art mask tuning solution named ForTune ERC (Enhanced Registration Control). This solution is based on laser processing of the mask bulk by the ZEISS ForTune tool. It allows to suppress few nanometer overlay residuals (post the scanner best-can-do) down to deep sub-nanometer, all even at sub-millimeter sampling resolution (x1 wafer level) and low-to-high residuals modulating frequency.
For the sake of this study, we have used a dual-image mask to form one overlay signature at wafer side. Two wafers have been exposed prior to the laser-based tuning of the mask bulk; the wafers overlay error was measured and used as an initial overlay problem to begin with. A second exposure of two additional wafers was performed post the problem-solving by the ERC model and the consequent mask laser-based tuning. The pre/post wafers were then compared to examine the improvement in overlay at wafer side. CD uniformity (CDU) data has been collected as well, to confirm no degradation in CDU due to the ForTune ERC process.
The combination of this advanced method of intra-field control with high-order correction per exposure (CPE) by the scanner, provides an efficient co-optimized solution to tightly control the overlay of existing and future nodes at DUV litho.
This paper will demonstrate the complementary RegC® and TWINSCANTM solution for improving the OPO by cooptimizing the correction capabilities of the individual tools, respectively. As a consequence, the systematic intra-field fingerprints can be decreased along with the overlay (OVL) error at wafer level. Furthermore, the application could be utilized for extending some of the scanner actuators ranges by inducing a pre-determined signatures. These solutions perfectly fit into the ASML Litho InSight (LIS) product in which feedforward and feedback corrections based on YieldStar overlay and other measurements are used to improve the OPO. While the TWINSCANTM scanner corrects for global distortions (up to third order) - scanner Correctable Errors ( CE), the RegC® application can correct for the None Correctable Errors (NCE) by making the high frequency NCE into a CE with low frequency nature. The RegC® induces predictable deformation elements inside the quartz (Qz) material of the reticle, and by doing so it can induce a desired pre-defined signature into the reticle. The deformation introduced by the RegC® is optimized for the actual wafer print taking into account the scale and ortho compensation by the scanner, to correct for the systematic fingerprints and the wafer overlay. These two applications might be very powerful and could contribute to achieve a better OPO performance.
Six production masks were measured on a standard registration metrology tool and the registration errors were calculated and plotted. Specially developed algorithm along with the RegC Wizard (dedicated software) was used to compute a correction lateral strain field that would minimize the registration errors. This strain field was then implemented in the photomask bulk material using an ultra short pulse laser based system. Finally the post process registration error maps were measured and the resulting residual registration error field with and without scale and orthogonal errors removal was calculated.
In this paper we present a robust process flow in the mask shop which leads up to 32% registration 3sigma improvement, bringing some out-of-spec masks into spec, utilizing the RegC® process in the photomask periphery while leaving the exposure field optically unaffected.
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