Accurate and repeatable mask defect measurements for quarter-micron technology

Abstract

A highly repeatable and accurate mask defect measurement has been developed. Defects from 0.1 to 1.5 microns in diameter are measured by computing the total light absorbed or transmitted by a defect. RMS repeatability of better than 9 nanometers on 0.4 micron defects has been achieved. Reliable measurement of defect size is important for developing lithography technologies for smaller geometries, and for commercial mask production. As mask feature sizes have dropped below the wavelength of visible light, getting reliable defect size measurement has become nearly impossible. Even scanning electron microscope (SEM) and atomic force microscope (AFM) measurements have not yet proven reliable even though they provide resolutions down to a few nanometers. This technique of measuring flux absorption or transmission allows reliable measurement of defects that are several times smaller than the wavelength of light used to examine them, with repeatability of 2 - 10 nanometers, depending on the image source. Transmitted light images are acquired from KLA-3xx, Starlight, KLA-219, DRS-1, DRS-2, or other video microscopes. Then the amount of light flux absorbed (by a spot or chrome extension), or transmitted (by a hole or clear intrusion) is measured. That change in flux is converted to an area, which can then be converted to a diameter. This system is currently in use in several large mask shops. It promises to be a powerful QA and analysis tool for developing masks for .25 micron and smaller geometries. Accuracy and repeatability tests have been performed on reference defects on Dupont VeriMasks, and using PSL spheres. Repeatability is limited by vibration of the image and by pixel artifacts in the images from KLA-3xx machines. Accuracy cannot be objectively assessed because there is no 'NIST traceable' reference for defect sizes. However, chrome defect size appears to be linearly correlated to absorbed or transmitted flux, as one would expect from the physics, so defect area accuracy is expected to be similar to the repeatability, around 10 nanometers. This technique does not easily provide separate x- and y- dimensions for non-round defects smaller than the wavelength of light used in the microscope (typically 0.5 micron). Larger defects can be measured in two dimensions using conventional techniques, and Fourier transform techniques can be used to provide useful estimates of x- and y- dimensions of smaller defects.