Critical dimension control : influencing factors and measurement

• Main Author: Binnie, Iona B.
• Published: University of Edinburgh 1991
• Subjects: 621.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.641625

Advanced Lithography continues to be the limiting factor in the drive for higher levels of microcircuit integration. The key to the successful management of a lithography process is the integration of full measurement and instrumentation functions with the process, and the adoption of effective process control strategies. The aim of this research is to improve the understanding of critical dimension (CD) control by an investigation of the sources of variations in linewidth dimensions. Having identified the key factors, it should be possible to characterize and control their influence. Experimental analysis suggests that film thickness and photoresist thickness have a profound effect on linewidth dimensions. Simulation techniques are used to establish a theory which uses standing wave patterns within film stacks to predict reflectance and exposure threshold, as well as the dimensions of the developed resist images. This theory is later corroborated by measurements on test wafers. Having established the need to monitor film thickness variations, a novel metrology technique which incorporates both film thickness and linewidth uniformity measurements is introduced. The technique is based on the optical characteristics of a 'chequerboard' test pattern, consisting of clear and opaque squares. The chequerboard effectively enhances deviations in CD by translating changes in linewidth into an area change on the chequerboard. The technique was originally based on the measurement of light transmitted through glass wafers. The implementation of the technique using reflectance from silicon wafers is described, and possible future developments of the system are discussed.