EUV lithography has recently been implemented in high volume wafer production. Consequently, maximizing yield is gaining importance. One key component to achieve optimal yield is using a pellicle to hold particles out of the focal plane and thereby minimize the printing of defects. The Carbon Nano Tube (CNT) pellicle is a membrane consisting of a network of carbon nanotubes, and demonstrated EUV transmission up to 98%. The challenge is to balance the CNT material parameters for optimal performance in the EUV scanner: low probability for particles to pass, low impact on imaging through scattered light, high durability in the scanner environment, while maintaining high transmission. We report results of the first full-field CNT pellicle exposures on an NXE EUV scanner. We demonstrate handling of the pellicles on the scanner, without breakage, and provides a first assessment of their imaging behavior. Multiple single- and double-walled uncoated CNT pellicles with EUV transmission up to 97.7% were exposed on the NXE scanner at imec, and minimal impact on the imaging is confirmed. In these exposures, uncoated CNT pellicles were used which will not meet the specifications regarding lifetime. Therefore, current ongoing developments focus on CNT coating and durability in scanner environment. The presented demonstration proves the value of a CNT-based EUV pellicle solution.
We evaluated the printability of patterns relevant for Logic Metal at P28nm (L/S and T2T) on wafer using EUV single expose. We compare illumination sources with and without fading correction as well as Bright field / Dark field mask tonalities and NTD MOR / PTD CAR resist. In simulations, Bright field (BF) imaging gives better image quality than Dark field (DF) at small pitch/CD. It also enables smaller T2T. To avoid tone inversion (assuming dual damascene processing), BF imaging requires the use of a NTD resist. On wafer, exposure latitudes increase for a BF/NTD choice, concurrent with simulations, even after correcting out SEM shrinkage. Also, T2T CD is reduced. In terms of illumination, we compare dipole sources to fading corrected sources. As fading correction, we have both induced aberrations (Z6-corrected dipole) and monopoles. As expected, a fading correction significantly reduces best focus differences of L/S through pitch and T2T. Moreover, the Z6-corrected dipole is optimal to print small T2T with better uniformity. Finally, we observe that PTD and NTD MOR resist utilize the same aerial image differently. NTD resist can leverage pupil shapes with high exposure latitude, but low depth of focus, better than PTD resist. Fading correction via induced aberrations naturally produces such sources. In summary, the preferred option is a Z6-corrected dipole for best focus alignment and sharp T2T, together with BF imaging to allow higher L/S exposure latitudes and small T2T. Combining this choice with NTD MOR resist avoids tone inversion and leverages the illumination source optimally.
We demonstrate P24 line/space and P28 contact hole printing on wafer using a NXE:3400B EUV scanner. The goal is to enable ecosystem development towards high-NA in a Fab-like environment. We allow for pupil fill ratios down to 6% (illumination efficiency ~35%) and use fading correction by induced lens aberrations. We show that the dose sensitivity for P24 L/S can be improved by more than 30% compared to a standard (leaf-shaped dipole) pupil. For contact holes, both single expose and double L/S expose schemes print contact holes at pitch 28nm in metal oxide resist (NTD), albeit at very different dose.
A parallelism is reported between reticle lifetime experiments undertaken on TNO’s EBL2 platform and wafer printing on the ASML NXE EUV scanner installed at imec. EBL2 mimics reticle impact due to exposure of ten thousand wafers in NXE representative conditions in less than a day. In-situ X-ray Photoelectron Spectroscopy (XPS) has shown that a local high-dose EUV exposure removes surface carbon and reduces ruthenium oxide to ruthenium. These effects not only happen at the directly exposed location, but equally centimeters away. Repeating XPS after a period of reticle storage outside of the vacuum, revealed regrowth of such contamination layer and re-oxidation of ruthenium. This learning based on EBL2 explains a small but significant trend noticed in critical dimension measurement results on wafer through a batch of wafers exposed on NXE, depending on the prior storage conditions of the reticle. During first exposures following reticle entry into vacuum reticle storage effects become gradually undone. Both storage-induced mask contamination effects are shown to build-up beyond one month. Local effects of the high-dose EUV exposure remain measurable by EUV reflectometry after several weeks of storage in air.
In this work, we demonstrate the use of a scatterometry-based technique to accurately monitor the dose variations seen on an EUV scanner. By carefully setting up the exposure conditions and data analysis, we can separate scanner-driven dose effects, mask reflectivity changes and process variations into its individual components. These measurements have a very high repro and throughput allowing us to use this technique to both monitor and provide active feedback to improve overall EUV cluster stability. We have used this technique to achieve a day-to-day dose variation of +-0.5% over a 6-month period on the NXE:3400B at IMEC.
Sustaining optimum EUV litho-cluster performance requires monitoring the dimensional variance of representative patterns at high speed and sampling density. We present NXE3400 monitoring results obtained with an optical technique whose amplified sensitivity to dimensional variance increases with decreasing pitch. The scalable technique enables quantitative tracking of site-to-site pattern profile and fidelity and it is applied here on 5nm-node-relevant EUV dense features (line/space width, line/space ends in elongated contact holes). We benchmark against conventional monitoring methods to demonstrate capability improvements. We finally introduce complementary methods which enable a robust manufacturing insertion of EUV technology; such as 2-dimensional feature monitoring and stochastic defect prediction.