Application of a develop end point detector as a timely and cost-effective alternative to using an SEM for CD measurement in production photolithography

This paper focuses on the use of a track-mounted, in-situ develop end point (EP) to reduce CD measurement via scanning electron microscope (SEM) in the production environment. Commonly, a few wafers are selected from a lot and examined to ensure conformance with desired CDs. Typically, the wafer's image linewidth is measured. If required, a corresponding adjustment is made to the stepper's exposure energy in order to obtain the targeted CD. While this methodology works, it does have its shortcomings such as: incomplete determination of wafer-to-wafer variations, the impossibility of detecting random 'problem' wafers, and the additional backlog CD measurement adds to the SEM station -- which generally results in production bottlenecks. The use of a track-mounted, in-situ develop end point detector facilitates rapid and accurate measurement of photoresist characteristics. Due to the direct correlation between develop end point time and CDs, a detector can be used to signal out-of- spec conditions. The nature of the data collected and analyzed by the detector allows exposure and other process variations to be revealed. In addition to providing this 'watchdog' function, some detectors can be programmed to automatically correct out-of-spec CDs by varying the development cycle time. Use of an in-situ detector, as described, will have a direct bearing on reducing the use of high-cost SEMs in the production environment as well as minimizing the number of scrapped wafers.