Tomohiro Tsutsui

Mask Quality Assurance in Cleaning for Haze Elimination Using Flexible Mask Specifications

We propose a new method of quality assurance for attenuated phase shifting mask (PSM) using the concept of the flexible mask specifications to extend the life of PSM [1]. The haze on PSM is a major issue for ArF lithography in semiconductor device manufacturing since it causes decline of device yield. PSM irradiated by ArF laser is periodically cleaned before haze is printed on wafer, which is a killer defect. Repetition of cleaning causes great changes of properties, i.e. phase, transmittance. Therefore, the number of times cleaning is performed has been limited by predetermined specifications based on ITRS. In this paper, relaxation of the pass/ fail criteria are studied as one solution to this limitation problem. In order to decide a suitable number of times for cleaning to be performed, we introduce the concept of flexible mask specifications, taking lithography margin into account. Firstly, we obtained mask parameters before cleaning; these parameters were, for instance, phase, transmittance and CD. Secondly, using these parameters, we simulated images of resist pattern exposed on wafer and obtained exposure latitude at desired depth of focus. Thirdly, we simulated mask parameters and exposure latitude when the mask was cleaned several times and obtained correlation between number of times cleaning is performed and exposure latitude. And finally, we estimated suitable pass/ fail criteria of mask parameters and the maximum number of times cleaning should be performed for each mask at the standard exposure latitude. In the above procedure, the maximum number of times cleaning should be performed exceeded that determined in the case of conventional specifications based on ITRS.

Leaking light through embedded shifter-type opaque ring for attenuated phase-shift mask

Critical dimension error on a wafer caused by leaking light through embedded shifter type opaque ring on an i-line attenuated phase-shift mask has been studied. We have produced the mask that includes small pinhole-array pattern as the opaque ring, and confirm that transmittance through the opaque ring depends on pinhole size in good agreement with coherent theory. Our experimental result shows that the leakage must be less than 0.125% in transmittance in order to control resist dimension error less than 0.01 micrometer on a wafer for 0.35 - 0.4 micrometer devices. We have also derived an analytical form to represent leaking light, which shows good fit to the transmittance measurements with the various pinhole size. Then we have estimated the allowable error in phase difference and transmittance of the shifter, and that in pinhole size, applying for this formula. We also discuss the process feasibility for embedded shifter type opaque ring.