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Due to necessities of semiconductor manufacturing some of the most critical lithographic layers may utilize only dielectric anti-reflection coatings (DARC) and not the organic anti-reflective coatings in order to minimize substrate effects on critical dimension (CD) control and in order to position the process at a best possible node. As a result of this relationship, stricter limits of control of index of refraction and extinction coefficient are generally imposed on the DARC process. While the DARC process may utilize a gas flow adjustment in order to control the optical constants, one of the biggest obstacles becomes the film thickness metrology, which is most often used via either the Bruggeman or the Harmonic Oscillator models to measure the desired optical coefficients. Unfortunately, the control of optical properties to within a few percent is generally outside of the window of specifications of even the latest generation of film thickness metrology tools. Furthermore, with each subsequent exposure node, the wavelength of interest for the optical coefficients is also near the limit of the lamp or radiation source on the film thickness metrology tool thus creating additional noise and measurement instability. An interesting situation is depicted in this paper where the metrology variation in measurement of the optical coefficients for a single stack DARC film is greater than the variation of twenty process chambers. The metrology variation was confined in major part to consist of tool-to-tool variation and of tool changes after any work on the ellipsometer. A systematic way of reducing this measurement variation is presented which allows for introduction of a floating standard tied to the combined average performance of all metrology tools without necessarily using a golden tool or a golden set of wafers. At the same time, offsets are applied to each metrology tool thus ensuring a much tighter population. Although the described situation is not ideal, with the current specifications on measurement of optical coefficients it is one of few methodologies necessary for adequate process control without the expenditure for a new toolset.
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