Multi-focal imaging for improved depth of focus; principles and application to ArFi use cases (Conference Presentation)

Over the years, lithography engineers have continued to focus on CD control, overlay and process capability to meet node requirements for yield and device performance. The use of ArFi lithography for advanced process nodes demands challenging patterning budget improvements in the sub-nm range.1 In 3D NAND devices, the height differences between the cell and periphery create issues with the ability to adequately image and maintain a useable process window in both regions. Previous work by Fukuda2 developed a multi-exposure technique at multi-focus positions to image contact holes with adequate DOF. Lalovic3 demonstrated a fixed 2 wavelength technique to improve DOF called RELAX. ASML introduced EFESE Rx, a method of tilting the stage during exposure to create multiple focus positions and finally Lalovic4 introduced a broadband laser solution to provide additional DOF. All of these techniques suffered from a number of problems that limited usability. In this work the authors will introduce a new method to increase DOF through alternating wavelength’s from an ArFi light source. This technique, called MFI (multi-focal imaging), can be tuned specifically to provide the required amount of wavelength separation for a specific DOF need. Two focal positions are created that are averaged over the exposure field. The authors will review this wavelength “dithering” approach which can be turned on and off, thus eliminating any potential scanner calibration issues. Initial simulation studies with a fixed source and mask indicated increased DOF with wavelength separation. These DOF improvements have been confirmed with on-wafer single-exposure data. The Tachyon MFI aware engine flow will be reviewed using several customer use cases that have been analyzed to demonstrate maximum DOF and ILS vs wavelength separation. The authors will also review the optimization of new pupils and OPC solutions that are unique with each wavelength separation case and maximize process capability. The presentation will close with a product availability timeline and roadmap.