The optical inspection system has been applied for mask inspection. The small but fatal detects on the mask can not be detected minutely by the optical system because of the limitations of optical resolution. We have developed the Defect Imaging System (DIS-05) using Backscattered Electron Images (BSI). DIS-05 is composed of 3 units: (1) SEM with a newly developed Backscattered Electron detector, (2) CAD computer to create CAD Image and, (3) Main computer to control the SEM and CAD computer. One of key technologies for DIS-05 is the technique of detecting BSI at a high contrast. Moreover, we herewith describe "Superimposed Image", which compares BSI with CAD one. Finally, we also report the possibility of detecting "haze on masks" using DIS-05.
For evaluation of high-end photomasks for under 65 nm design rule wafers, Holon has developed EMU-Navi, optional software for Holon EMU-series mask Critical Dimension Scanning Electron Microscope (CD-SEM), which helps automated and accurate CD measurement on high-end masks with complicated patterns after optical proximity correction (OPC) processing. As CD measurement preparation, the user makes one file indicating points to measure and the other containing template bitmaps from Electron beam (EB) writing output format data, which are to be used for pattern matching to SEM images. During measurement, EMU-Navi compares each SEM image to the corresponding template bitmap in order to have EMU move its stage accurately to the point to measure where EMU measures the CD in the SEM image. This function is especially effective in positioning complicated features in SEM images. After measurement, the user can examine whether mask patterns have been precisely processed. In this manuscript, the flow of CD measurement procedure is described.
Recently, in Critical Dimension (CD) measurement on high-end masks, Optical Proximity Correction (OPC) pattern measurement is on increase and it has become important to measure angled lines. In CD searching on a CAD layout viewer, the exact CD values can be detected for the OPC patterns because they consist of a lot of rectangles. While, the CD values for angled lines have not been detected in it. Meanwhile the mask Critical Dimension Scanning Electron Microscope (CD-SEM) can measure angled lines, but measurement accuracy cannot be verified because there is no reference standard sample available for calibration of the CD values. In this study, we made the prototype of a standard sample for CD measurement with 0 degree and 45 degree angled lines by using VLSI Standards Inc. Nano Lattice Standard. The shape is the same as 6025 mask. We measured CDs of angled lines of the above sample using Holon EMU-260 and examined the calibration
method. We are going to discuss the CD marking method on a CAD layout viewer in order to automate measurement of angled lines in near future.
Rapid changes in the feature size of photo masks have made it clear that there is an obvious limitation to the use of optical measurement tools, and mask makers now have the necessity to use CD-SEMs as a measurement tool for forefront patterns. The level of measurement precision and accuracy required for mask CD metrology has reached a point at which magnification calibration using the standard scale becomes indispensable. Since CD-SEM measurements are heavily influenced by local pattern irregularities compared to optical measurements, however, proper statistical treatment of data is necessary to estimate accurate values. In this paper, the tool repeatability, sample dispersion, line edge roughness amount and tool's long-term precision will be determined by the treatment of numerous measurement data. The general calibration of the tool is done by line pitch measurement of JQA standard, in that case the disagreement between line width measured values and user's desired values could be appeared. In this case we propose the method to change threshold value of measurement, and in this paper the evaluation of this method will be shown.
We evaluated an advanced CD-SEM as a photomask CD guarantee tool. Measurement repeatability was 2.3 nm (3 (sigma) ) for each measurement, and reproducibility (the range of average value in 5 days) is 3 nm. Contamination effect was evaluated by measuring some isolated Cr line and isolated space patterns. The contamination effect on CD measurement value was estimated as 0.02 nm/scan (reticle scale) from the result of 500 scans at the same position. The results were quite different from the results of the aerial image of around 0.3 nm/scan (reticle scale) evaluated by MSM100 ((lambda) equals 248 nm). Difference between these two evaluation results was considered to be due to the transmittance reduction of substrate. There were no degradation on measurement repeatability and no image shift from charging effect for most severe condition (0.4 micrometer Cr isolated dot, global coverage of 20%, and local coverage of 0.05% under the magnification of 88,000 X).