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In this paper we discuss the mechanism by which process variations determine the overlay accuracy of optical metrology. We start by focusing on scatterometry, and showing that the underlying physics of this mechanism involves interference effects between cavity modes that travel between the upper and lower gratings in the scatterometry target. A direct result is the behavior of accuracy as a function of wavelength, and the existence of relatively well defined spectral regimes in which the overlay accuracy and process robustness degrades (`resonant regimes'). These resonances are separated by wavelength regions in which the overlay accuracy is better and independent of wavelength (we term these `flat regions'). The combination of flat and resonant regions forms a spectral signature which is unique to each overlay alignment and carries certain universal features with respect to different types of process variations. We term this signature the `landscape', and discuss its universality. Next, we show how to characterize overlay performance with a finite set of metrics that are available on the fly, and that are derived from the angular behavior of the signal and the way it flags resonances. These metrics are used to guarantee the selection of accurate recipes and targets for the metrology tool, and for process control with the overlay tool. We end with comments on the similarity of imaging overlay to scatterometry overlay, and on the way that pupil overlay scatterometry and field overlay scatterometry differ from an accuracy perspective.

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A diffraction-based measurement of overlay requires a target composed of two cells per direction of measurement, with induced shifts of opposite signs designed into each of the cells. We present a method for a measurement which only requires a single cell per direction. This is achieved by resolving the image in the pupil plane and using the angle of incidence inlieu of the induced shift. The use of single-celled targets reduces the target size by half and enables the placement of the target in-die, as well as reducing the measurement time. This single-cell measurement requires the calibration of the target’s optical stack height, which is done on a small number of two-cell targets. This calibration also produces a stable map of the aligned layers’ height profile across the wafer.

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In semiconductor chip manufacturing, dedicated metrology targets are used for measuring overlay (OVL) between layers after the lithographic process step. Until recently, overlay targets have had typical dimensions of 20x20 microns, but have been reduced in size to allow for more critical product real estate. Reducing metrology target size, however, increases optical crosstalk and diffractions from the target’s edges which can introduce significant errors in the reported OVL values. In this work, we will evaluate the following parameters: 1. The beam spot shape on the metrology target 2. The best target design under size/wavelength constraints 3. Improved OVL extraction algorithm We will present the hardware and software optimizations for experimental measurements taken over a cascade of targets. While decreasing the target’s size, we quantified the OVL measurement performance via dynamic precision, tool-induced shift (TIS), and results through focus (Z position). We will demonstrate how these optimizations enable the measurement of targets as small as 6x6 microns without compromising throughput or measurement quality.