The defect detection capability for a minute pinhole by a newly developed mask inspection system MC-3500 with DUV reflected light source is reported. The detection sensitivity of a minute pinhole less than 180 nm on a KrF phase shift mask (PSM) with transmitted light source is limited because the pinhole signal intensity is influenced by the diffraction light. The signal intensity of the pinhole both by the reflected light source and transmitted light source was calculated by an optical simulator, and the actual pinhole signal of the KrF PSM and that of the ArF PSM were measured using the MC-3500 with reflected light source. It was found that the 100 nm minute pinhole, which was not detected by the inspection with the transmitted light source, was detected by the inspection with the reflected light source. This shows the effectiveness of the reflective inspection, thus proved that the newly developed MC-3500 inspection system with reflective inspection capability has very high defect detection sensitivity for the advanced masks of 100-130 nm rule and below devices.
Binary (Chromium) and, KrF/ArF phase shift masks (PSM) were inspected by MC-3000, which uses DUV (257nm) light source, and an evaluated results of these sensitivities are shown. In the case of the chromium mask, sufficient detection sensitivity for 130nm-device inspection was obtained. For KrF and ArF phase shift masks, the detection sensitivities of the edge and the corner areas are practically equivalent to that of chromium. Though the detection sensitivity of a minute pinhole is slightly lower under the influence of the diffracted light. With an ArF phase shift mask, the contrast of absorber and a glass portion is low, and so improvement of the signal noise ratio of a sensor becomes essential for false-defect control. Additionally, the minute pinhole detection sensitivity will be higher, if a reflective inspection etc. is carried out.
We have developed a new system to measure the phase shift caused by the phase shift mask (PSM) for a deep-UV stepper. The system uses two types of 248 nm wavelength deep-UV light sources. One is an Hg-Xe lamp, and the other is a KrF excimer laser. These are interchangeable to match the illumination light source of the stepper. The phase is measured with a polarizing shearing interferometer. It has two wollaston prisms, a condenser lens, and an objective. The objective has a long working distance of more than 7 mm. This is necessary in order to measure the PSM with a pellicle. A light beam is separated into two orthogonal polarized light beams and sheared on the PSM, and then recombined on a sensor. The transmission beams through a shifter and a substrate interfere each other. Then the phase difference from the shifter is obtained from the difference between the two beams. We evaluated phase measurement repeatability and the measurable minimum pattern width. In the measurement of a halftone PSM, the repeatability was +/- 1 degree over several months, and the minimum pattern width was 2 micrometers or less.
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