Critical dimension photomask metrology tool requirements for 0.25 um and future microlithography

Resolution enhancement techniques, such as optical proximity correction and phase shifting mask, have been extensively examined to extend the optical lithography capability to meet advanced lithography requirements. It is also challenges the mask technology. As the minimum feature size decreases, the effect of proximity effect, resist slope, chrome slope, chrome edge roughness, and pattern corner rounding on defining the true critical dimension is significant. Increasing the sample size of mask CD measurement is inevitable to support the wafer fabs to characterize the wafer lithography process and product performance. The need to define the pattern fidelity, such as corner rounding of the optical proximity correction patterns, is emerging. Low voltage CD SEM, confocal microscope, and Atomic Force Microscope(AFM) have been developed to improve the precision of sub-micron mask CD measurement. The AIMS system, applying microlithography simulation techniques, provides a promising method to predict the linewidth verses stepper dose, linewidth verses depth of focus, and correction between mask and wafer geometries. At Intel, we have developed a technique to magnify the image on the microscope to improve the precision and linearity of the CD metrology system. This newly developed technique which digitizes the measurement image and uses a gray level selective display improves the signal to noise ratio and enhances the image contrast and pattern edge detectability. This enables us to measure high resolution patterns, contacts, and curves of rounded features with high precision. The automatic measurement file creation software developed by Intel through downloading coordinates, automating measurements, and uploading data improves the measurement throughput in the production environment. In this paper, the current issues concerning CD measurements will be discussed. These issues include the effects of proximity effect, resist slope, chrome slope, chrome edge roughness, and pattern corner rounding on the true critical dimension. This paper will also present the user with a discussion on the current CD metrology tool capability. Most importantly, this paper will outline the requirements of future CD metrology tools, which will be vital to the accurate and descriptive measurement of photomask critical dimensions.