A detection method of mura on a coated layer using interference light

Here, we describe a method to detect mura during the display devices manufacturing process on a uniformly coated thin photoresist layer. A mura is an irregular variation of lightness on a uniformly manufactured surface. Display devices are manufactured through photolithographic process, and every imaging process requires a uniformly coated photo resist layer. Mura detection on the layer is necessary to keep the device quality high. The mura has been visually inspected by tilting a glass under a sodium lamp in order to create the largest amount of interference. However, tilting the glass is difficult due to the growing device size currently four square-meters in the latest line. Rather than tilting the glass, we have developed an apparatus which illuminates the glass via a set of narrow bandpass filtered light of different wavelength. The apparatus observes the interference of light reflected from the surface and from the bottom of the thin layer in order to inspect the mura on the glass. The reflection intensity as a function of the layer thickness shows a sinusoidal periodic characteristic, which means that the mura detection sensitivity depends strongly on the optical path length in the layer. We have developed a method to compensate the periodic sensitivity fluctuation by employing a reflection ratio that enables us to detect the local thickness fluctuation with an accuracy of one nanometer.