EUV mask contact layer defect printability and requirement

In this paper, the printability of Extreme ultraviolet (EUV) mask contact layer defects at 90 nm contact size and above is studied via ultra-thin DUV resist and 10x EUV Microstepper. The EUV mask contact defect size requirement is determined by taking into account the wafer process critical dimension (CD) variability. In the experiment, two types of contact mask defect were studied. They are opaque defect placed at both edge and center of a contact and clear defect at edge of a contact. The programmed EW absorber defect mask was fabricated by subtractive metal patterning on a Mo/Si multilayer-coated silicon wafer substrate(1). The 10X experimental EUV lithography system with 13.4 nm exposure wavelength and 0.088 NA imaging lens was used to expose the programmed defect mask. The response of the printed resist contact area to the metal absorber mask defect area is measured under different process conditions, i.e., different exposure doses. It is found that the EUV resist contact area responds to the mask defect area linearly for small mask defects. From such a set of contact area change vs. defect area response lines, the allowable absorber mask defect requirement for the contact layer is assessed via statistical explanation of the printable mask defect size, which is tied to the wafer process specifications and the actual wafer process CD controllability. Our results showed that a clear and an opaque intrusion corner absorber mask defect as small as 70-80 nm (4x) is printable for 90 nn contacts when 10% contact area change (or 5% contact CD change) due to defect alone is allowed. The effect of an opaque defect at center of a contact is found similar to that of corner opaque defect for smaller defect. It becomes much worst than that of at edges when defect is large. Based on the statistical defect printability analysis method that we have developed, the printable mask defect size can always be re-defined without additional data collection when the process controllability or the process specification changes.