Contact hole characterization by SEM waveform analysis

a new algorithm for an Applied Materials CD-SEM metrology tool has been developed which gives a quantitative characterization of the quality of contact holes in photoresist. This is a non-destructive technique that allows users to assess the cross sectional profile of a contact hole from top-down measurements thus avoiding the time-consuming and expensive task of measuring cross sections. This analysis is based on the shape of the SEM (Scanning Electron Microscope) waveform that is created when a contact hole is measured. The algorithm generates a numerical grade for the contact profile, which is based on the shape of the waveform. The classification of the contact hole into different Profile Grades (either open, closed or transitional) can then be determined due to the strong correlation between the shape of the waveform and the cross-section profile of the contact hole. The Profile Grades have been found to be in excellent agreement with SEM cross-sections. When this technique is applied to contact holes across a focus-exposure matrix wafer, the algorithm gives grades which clearly delimit the domain in which the process parameters produce open, sharp-edged contacts. In many cases it was found that the CD measurement alone was insufficient to determine if a given contact hole was actually open. The combination of the CD measurements with the Profile Grades is a particularly powerful tool to determine ideal process parameters for lithography, and determination of the Profile Grade becomes essential as tool-sets are pushed toward the limits of their capability. The following paper concentrates on 250 nm contact holes but presents data on contact holes ranging in size from 200 nm to 500 nm.