Thomas R. Glass

Supersparse overlay sampling plans: an evaluation of methods and algorithms for optimizing overlay quality control and metrology tool throughput

Traditionally, overlay metrology has ben primarily used for quality control of developed wafers. The secondary roles of overlay metrology, stepper modeling and feedback control, have become more prominent in recent years. In particular, the design of overlay sampling plans is usually strongly influenced by the requirements imposed by stepper modeling. Previous research has shown that intrafield sampling plans must be symmetric and repeating in order to support stable feedback control. These constraints impact the tradeoff between overlay tool throughput and quality control by imposing a higher than necessary cost on measuring additional exposure fields for irregular errors. In this paper we examine the concept of using super sparse overlays among plans along with innovative analysis methods and algorithms for stepper modeling. Super sparse sampling plans are defined as where the field by field intrafield sampling is irregular and asymmetric. When compared to typical sampling plans, super sparse plans allow the overlay tool to examine a 2 to 4 times the number of fields on a wafer with the same number of total measurements. In order to support super sparse sampling, it is necessary to employ innovative algorithms, which are capable of performing stepper modeling without losing result integrity. In this paper we report on the relative performance of regular versus super spares sampling plans for quality control, stepper modeling and overlay tool throughput.

Exposure effects on deep-ultraviolet resist thickness

Most deep ultraviolet (DUV) resist models available today utilize the Dill parameters to characterize resist exposure. These models assume that the thickness of the resist remains constant through exposure and post-exposure bake (PEB). The thickness is only affected by development in the models when resist is removed from the exposed or unexposed regions, depending on whether it is a positive or negative resist. It has been observed that a number of DUV resists change thickness upon exposure. This effect is expected to have an impact on the post-exposure acid profile calculated for modeling purposes. In this paper, we present data on the thickness changes for different resists and the effect of exposure to PEB delay on the change.