193nm mask inspection challenges and approaches for 7nm/5nm technology and beyond (2019)

193nm mask inspection will remain a viable solution for inspection of ArF technology masks for the 7nm/5nm technology node and beyond, even in the era of EUV lithography. In the ArF technology, pitch multiplication (SADP, SAQP, etc.) will continue to be used along with aggressive OPCs to achieve scaling. Although no major technology inflection is seen, mask capacities will continue to grow until EUV will be fully inserted into mass production. As a result, mask inspection sensitivity and defect dispositioning will remain a gating factor. Moreover, mask metrology will become a critical factor in wafer fabrication and process control. In this paper, the mask inspection challenges for 7nm/5nm and beyond are described and suggested solutions are outlined. One of the main challenges in mask pattern inspection is reducing false defects by filtering the additive white Gaussian noise (AWGN) added to the pattern image (e.g. shot-noise). Common solutions for reducing AWGN are: creating multi reference (such as ‘cell to cell’ and ‘die to many dies’) and spatial averaging (such as ‘matched filter’). However, extra sensitivity is needed at 7nm/5nm technology inspection where defect signal is very weak and close to the noise level. We propose the ‘Multi-Shot’ method as a solution for this problem. ’Multi Shot’ is based on multiple acquisitions and inspections of every location in the mask. The ‘Multi-Shot’ information is exploited through the entire detection flow, taking advantage of information that cannot be used independently such as: defect polarity (random noise does not retain polarity over multiple instances while real defects do), averaged signal and defect rank (local SNR). The added throughput impact of the ‘Multi-Shot’ approach is negligible due to pixel-size optimization. Theoretical framework predicts a ~30% sensitivity (SNR) increase by this method over current approaches, corroborated by experimental data testing. Another significant inspection challenge is the difference between defect measurement methods. The captive mask shops, the merchants and wafer FABs all are interested in the amount of edge dispositioning caused by the defects, measured in units of nm, while traditionally the inspection output is defined by pattern intensity changes due to the defect, measured in grey level units. Translation from intensity to edge dispositioning requires two conditions: The first- applying an Aerial imaging with exact exposure conditions which enables correct dispositioning assessment; and the second- estimating a gray level threshold (print threshold) to be used to convert an Aerial image to a binary printing image (as an equivalent to the resist threshold used in wafer fabrication). Defect dispositioning measurement enables nuisance filtering (by ignoring non-printing defects and defects with very small dispositioning values even if they have high intensity values). The innovation of the solution described in this paper is the integration of metrology and inspection to provide robust detection solutions. 193nm wavelength inspection will continue to be a critical factor in mask manufacturing as well as one of the strongest candidates available today for the initial EUV mask inspection approach. In this aspect we are working to implement ArF new development for future EUV mask inspections.