Reflectometry substrate modeling

To enable reflectometry to provide accurate, reproducible, and efficient measurements, several factors must be considered. Choice of the substrate material, substrate modeling, number of measurements per wafer, choice of the measurement patterns, and the setup of the pattern recognition program are all critical to the measurement process, as discussed in the following. 

In reflectometry, the light passes through the films to be measured. Beneath the transparent films, there must be an opaque substrate through which light does not pass. The substrate characteristics must be modeled correctly to calculate the thicknesses of the films above. In silicon processing, theoretically, any of the commonly used metal materials, such as the titanium nitride (TiN), aluminum (Al), and tungsten (W), can be used as substrates. However, in reality, whereas a PMD oxide can be measured on the polysilicon material used in poly interconnections, an ILD oxide can not be measured directly on TiN, because the TiN layer used is too thin to be opaque. TiN is semitransparent if its thickness is less than 1000 A. A thin... 

Reflectometry, where only the intensity of the reflected light is measured, requires a thin film adsorbed onto a reflecting substrate. The thin film has a different refractive index than both the bulk solution and the substrate so interference occurs between the rays reflected from the substrate/fllm and film/bulk solution interfaces. By model fitting the refractive index-distance behavior, it is possible to extract the volume fraction profile of the adsorbed polymer layer (15). 

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