High NA anamorphic EUV scanner has anamorphic optics with 8x demagnification in y direction and thus twice smaller exposure fields 26x16.5 mm2. In-die stitching may be required in order to create dies larger than High NA exposure field. In this work we consider stitching of vertical lines and spaces (LS) and establish methodology of stitching evaluation including detailed contour metrology at stitch, across wafer performance, process window and contrast metrics and sensitivity to single layer overlay between two stitched fields.
The cost of EUV lithography in chip production calls for imaging solutions that increase scanner throughput via dose reduction. By enhancing imaging via alternative low-n mask absorber materials, the imaging mask becomes a knob for dose reduction. Current low-n masks have an absorber thickness of 40 to 50nm. Thinning down the absorber results in image contrast losses, limiting their dose reduction potential in EUV lithography. Here we present wavefront optimization enabling thinning down low-n mask absorbers to 24 to 26nm. For single print EUV logic applications, this achieves a dose reduction of 20 to 30% compared to current low-n and Ta-based mask absorber thicknesses. In this simulation study, we first show that pole-to-pole image shifts drive the observed contrast losses at reduced low-n absorber thickness. Using wavefront optimization to overcome such image shifts, we then demonstrate that the low-n absorber thickness can be further reduced. The imaging potential of thinner low-n masks for current (0.33NA) and future (0.55NA) metal logic applications is evaluated by rigorous simulation overlapping process window (oPW) analysis. We also show that source/mask/wavefront co-optimization can enable a superior oPW depth of focus for the thinner low-n mask compared to the current generation of Ta-based and low-n masks. Finally, we propose available absorber materials with suitable optical properties for the practical implementation of thinner low-n masks and show their imaging strategy can be achieved at full illumination efficiency for single patterning metal logic applications down to a dense pitch of 20nm. In conclusion, our results prove that by employing wavefront optimization, thinner low-n masks provide similar or improved imaging at a much lower exposure dose.
The standard tantalum mask gives strong 3D electromagnetic effects, hence, its utilization in foundries to enable further downscaling to the A14 node might reach its limits even with the help of high NA EUV scanners (with 0.55 numerical aperture “NA”). The use of alternative mask absorber materials together with inverse lithography techniques (ILT), such as source mask optimization (SMO), can improve printing of metal logic layers with a target pitch of 20 and 24nm. This is possible due to thinner mask absorber thickness (around 40nm instead of 60nm for Ta-based mask) and due to different EUV optical properties of the mask material causing a different behavior of the light that is reflected by the masks. To better mitigate the light behavior during and after reflection from the mask, materials covering a good portion of the n-k graph (EUV refractive index, n, by EUV extinction coefficient, k) were chosen. The study proposes a comparison between baseline (Ta-based mask) and five new mask absorber candidates, ranging from three materials with lower refractive index and varied extinction coefficient (“low-n” with low-, mid-, and high-k), and two candidates with higher extinction coefficients (“highk” with mid- and high-n). This paper contains simulation results with the Siemens EDA Calibre tool and demonstrates theoretical proof that alternative mask materials bring significant gain when compared to the tantalum-based mask absorber. Firstly, we optimized the source and aerial image intensity threshold on a set of predefined clips (with SMO techniques). Secondly, we applied ILT techniques to correct for the full chip mask based on a horizontal layout of a metal logic layer on imec’s roadmap. We then compare the tantalum-based mask with the alternative masks using imaging criteria, such as DoF (depth of focus), NILS (Normalized Image log slope), EPE (edge placement error), pattern shifts through focus, process variation band, source telecentricity errors, and MEEF (mask error enhancement factor) on a variety of features in the metal logic clip to maximize the overall process window.
Mask stacks comprising of alternative absorber materials with various optical properties (n and k values) may allow further improvements in EUV imaging. In a strive towards dose reduction and advancement of resolution limits in EUV lithography, such masks are brought up for consideration. In this work, we evaluate a novel low-n absorber mask with a low EUV absorber reflectivity for dark field Line/Space (LS) printing and compare it to a traditional Ta-based absorber mask. For the novel low-n mask, we experimentally confirm the reflectivity vs. the Ta-based reference mask. Through simulations and experiments at 0.33 numerical aperture (NA), we evaluate the LS imaging performance in terms of best focus through pitch. At the anchor LS pitch 28nm, we report the exposure latitude and the Mask Error Enhancement Factor (MEEF) and compare these metrics to the imaging performance of a Ta-based mask. This work adds understanding to the patterning benefits and limitations of alternative absorber mask stacks in the case of Metal direct print applications.
High NA EUV lithography has become a reality. The high NA EUV scanner (EXE:5000) produces exposure fields of 26x16.5 mm2 which is twice smaller than standard fields on other scanners. For certain use cases (e.g. when a die is larger than the High NA field) stitching between two exposure fields might be required. Stitching of vertical lines across two exposure fields has already been demonstrated in several publications. In this publication, we pay attention to photomask related aspects of stitching which are multifold. We draw attention to the need for mask resolution enhancement which will enable advanced OPC at stitching. We will show stitching behavior on both Tantalum and low-n masks and demonstrate low-n absorber reflectivity suppression by means of sub-resolution gratings which is required for stitching. We explore the behavior of the exposure field black border (BB) edge and formulate recommendations for specifications on BB edge control as well as pattern placement and pattern fidelity at the black border. Finally, we conclude that the mask performance is a key enabler of High NA stitching.
The combination of High NA EUV anamorphic projection optics and unchanged mask-blank size result in a “High NA field” with a maximum size of 26x16.5 mm² at wafer level. Therefore, to create a die larger than the High NA full field, two images are stitched together. So-called in-die stitching is enabled by a combination of design, mask, OPC, process, and scanner solutions. We present an overview of our learnings about at-resolution stitching based on a representative experimental study at NA=0.33, in preparation for tomorrow’s NA=0.55. For a pitch 28nm vertical line-space, optimum conditions are confirmed experimentally to create a robust stitch. A P28 LS is measured post-stitching utilizing either a Ta absorber or a low-n absorber. For the latter, the higher reflectivity is experimentally mitigated by using sub-resolution-gratings. We also quantify the imaging impact of the transition between the absorber and the black border in the stitching region.
In this publication, we consider stitching enablement for High NA EUVL, specifically ‘zooming in’ on vertical line stitching used to create a physical connection between fields on wafer. We discuss stitching CD metrology and analysis using experimental and simulation results for pitch 36 nm dense lines. Experimental results were obtained on the NXE:3400B scanner at imec. CD uniformity across wafer and through slit are investigated as well as the impact from image to image overlap variation and the contribution of reticle CD errors and mask 3D shadowing. In the previous publications, we gave an overview of stitching challenges and various interactions in the stitching zone. In this publication, we focus on the aerial image interaction. Along a stitched vertical line, there are variations in CD creating a certain CD profile. These CD variations were modeled in a rigorous simulator but also observed experimentally. In order to characterize this behavior, we perform CD profile metrology at the stitch. We investigate the root causes of CD variability at the stitch and propose control mechanisms of stitching optimization. A key control mechanism being optical proximity correction (OPC) as well as overlay control.
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