Junji Fujikawa

Manufacturing of half-tone phase-shift masks I: blank

Half-tone phase shift mask (HT-PSM) blanks for i-line (365 nm) and g-line (436 nm) lithography, using chromium composites as a half-tone shifter, are brought into production. A bilayer structure of a 10 - 20 nm thick opaque, conductive chrome layer and a phase-shifting CrON layer is proposed, which can be formed by continuous deposition of the two layers and etched continuously by the process similar to that of the conventional chrome photomask. It shows low visible light transmission of less than 30% so that it can be inspected, and also shows enough conductivity to decay the excess charge during electron beam writing. HT- PSMs made of these blanks can be cleaned by sulfuric acid at 100 degree(s)C and can be used at least up to an irradiation of 1 MJ/cm2, when used for i-line exposure. The specification for the transmission is (target +/- 1)% for any point on any plate, and 0.7% range for any point on one plate, where the target ranges from 6% to 10%. The specification for the phase shift is currently (180+/- 10) degree(s).

Performance of gas-assisted FIB repair for opaque defects

Until recently, opaque defects on photomask have been removed by laser evaporation. However there are several disadvantages in this repair technique. First an operator must visually align the laser irradiation spot and pattern edge, so high repair accuracy can not be obtained. Also when the area to be repaired is large the laser sputters the surrounding surfaces and the evaporation tends to redeposit on pattern edges causing them to swell thus increasing the probability of detection by inspection equipment. Additionally the laser repair technique has proven to be very difficult if the pattern defect is any complex geometry. The repair using Focused Ion Beam (FIB) has been developed to solve some of these issues, however this method has not yet been applied to the production line because of degradation of transmission rate and the damage to the glass substrate. This paper reports the evaluation on the performance of the FIB repair using a newly developed gas assisted etching (GAE) technique and the status of using FIB with GAE in the production line. By using GAE, the degree of glass damage has been reduced by a factor of ten as compared with the FIB repairs without GAE. A transmission rate of 94% (i-line) could be obtained on a conventional mask and 92% (i-line) on the Halftone Phase Shift mask (HT-PSM). Furthermore, by applying the post-treatment after the repair, the transmission rate of the repaired area could be recovered to the same level as the normal glass area. The printing characteristics for i-line of the GAE repaired in both conventional mask and HT-PSM has been also good, showing that the GAE FIB repair is applicable in the normal photolithographic process window and that this method can achieve the similar printing result in comparison with non- repaired area.

Recent progress in repair accuracy of the focused ion-beam mask repair system

To improve the depth of focus (DOF) of isolated lines, attenuated assist feature (AAF) technique has been proposed; AAFs having more than 20% transmittance were located around an isolated line. In this mask, the transmittance and phase shift angle of AAF as well as its position and width have effects on lithographic performance. In particular, the phase shift angle has strong effect on focus latitude. The performances of two AAF masks (65% transmittance/28 degree phase shift and 40% transmittance/54 degree phase shift) were evaluated by using an NA equals 0.6, sigma- in)/(sigma) out equals 0.42/0.7, i-line stepper. The focus latitude of 0.3 micrometer isolated line became flat around the best focus position with 28 degree phase shift AAFs. In conclusion, we can obtain wide DOF for isolated lines by selecting optimum phase shift angle of AAF.

Development of bilayered TaSiOx-HTPSM: II

TaSiOx shifter has been developed for HT-PSM for ArF and F2 laser lithography. Adopting bilayered structure and embedding an etch-stop function into the transmittance control layer enable us to fabricate a TaSiOx-HT without quartz damage and to control the phase precisely. And less impact of TaSiOx shifter etching to CD was confirmed. It was confirmed this TaSiOx-HT was inspected by conventional inspection system without any problem.