Z-correction, a method for achieving ultraprecise self-calibration on large area coordinate measurement machines for photomasks

High-quality photomasks are a prerequisite for the production of flat panel TVs, tablets and other kinds of high-resolution displays. During the past years, the resolution demand has become more and more accelerated, and today, the high-definition standard HD, 1920 x 1080 pixels(2), is well established, and already the next-generation so-called ultra-high-definition UHD or 4K display is entering the market. Highly advanced mask writers are used to produce the photomasks needed for the production of such displays. The dimensional tolerance in X and Y on absolute pattern placement on these photomasks, with sizes of square meters, has been in the range of 200-300 nm (3 sigma), but is now on the way to be <150 nm (3 sigma). To verify these photomasks, 2D ultra-precision coordinate measurement machines are used with even tighter tolerance requirements. The metrology tool MMS15000 is today the world standard tool used for the verification of large area photomasks. This paper will present a method called Z-correction that has been developed for the purpose of improving the absolute X, Y placement accuracy of features on the photomask in the writing process. However, Z-correction is also a prerequisite for achieving X and Y uncertainty levels <90 nm (3 sigma) in the self-calibration process of the MMS15000 stage area of 1.4 x 1.5 m(2). When talking of uncertainty specifications below 200 nm (3 sigma) of such a large area, the calibration object used, here an 8-16 mmthick quartz plate of size approximately a square meter, cannot be treated as a rigid body. The reason for this is that the absolute shape of the plate will be affected by gravity and will therefore not be the same at different places on the measurement machine stage when it is used in the self-calibration process. This mechanical deformation will stretch or compress the top surface (i.e. the image side) of the plate where the pattern resides, and therefore spatially deform the mask pattern in the X- and Y-directions. Errors due to this deformation can easily be several hundred nanometers. When Z-correction is used in the writer, it is also possible to relax the flatness demand of the photomask backside, leading to reduced manufacturing costs of the plates.