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Currently, most production fabs use critical dimension (CD) measurements as their primary means for process control in printing lines, spaces and contacts. Historically, this has been adequate to control the lithography and etch processes and produce reasonable yields. However, as the industry moves from 0.25 micrometer manufacturing to 0.18 micrometer and beyond, it is becoming increasingly obvious that CD measurements alone do not provide enough information about the printed structures. As the geometry shrinks, slight changes in the shape and profile can significantly affect the electrical characteristics of the circuit while maintaining the same CD value. In this paper, we will describe a method which, in conjunction with the CD measurements, better characterizes the circuit structures and therefore provides valuable feedback about the process. This method compares stored image and linescan information of a 'golden' (correctly processed) structure to that of the structure being measured. Based on the collected data, it is possible to distinguish between different profiles and determine if a process shift has occurred, even when the measured CD remains within specification. The correlation score therefore provides an additional constraint that better defines the true process window and provides an additional flag for process problems. Without this information, the process used may not be truly optimized, or a shift may occur that is not detected in a timely manner, resulting in the loss of yield and revenue. This data collection has been implemented in production on a local interconnect lithography process. Before the correlation information was available, it was very difficult to detect the scumming within the LI trench, in which it was a time consuming and labor intensive procedure to identify problem lots. The correlation scores, collected automatically and concurrently with the CD measurement, allowed tracking through the SPC chart and the automatic flagging of problems while the lot was still in the photolithography module. The result has been faster feedback control and thus less scrap material.
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