Z-correction, a method for achieving ultra-high absolute pattern placement accuracy of large area photomasks

摘要

Photomasks are used in the production of LCD, OLED and other kinds of displays. For TV displays these photomasks, made of quartz glass with a Cr pattern, may have sizes up to 1.62 × 1.78 m~2 and a thickness up to 16 mm. The absolute placement accuracy, i.e. X,Y position of a pixel or line in the mask pattern needs to be better than 150 nm (3σ). The demand for higher resolution displays has led to tighter flatness requirements of the photomask, to secure that the chrome pattern is always in best focus. In contrast to small area semiconductor masks with dimensions up to 300 × 300 mm~2 and three point supports, the large area photomasks have to rest on a large stage in the mask writer. It is then unavoidable that distortions will be induced due to the fact that the glass backside or stage surface is not perfectly flat. If not corrected for, these distortions in Z direction can easily generate geometrical errors in the X,Y plane corresponding to pattern displacements of several hundred nanometers. To avoid these X,Y errors we have developed a technique called Z-correction. It is a function developed for correcting the mask pattern placement prior to the writing process in the pattern generator or in a verification measurement in the MMS15000 metrology tool. This is the first time this method is used for improving the accuracy of photo masks. It is based on height measurements of the quartz glass when it is resting on the stage during the temperature stabilizing time. Without using Z-correction it is very challenging to achieve an absolute uncertainty better than ～200 nm (3σ) over an area of 0.8 × 0.8 m~2. With Z-correction it is possible to enhance this number to ＜ 100 nm (3σ). In the MMS15000 metrology tool the performance is even better, ～50 nm (3σ) over a 0.8 × 0.8 m~2 stage area when using Z-correction in the self-calibration process.