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Monitoring of Low Dielectric Constant Parylene Films using Spectroscopic Ellipsometry by Carlos L. Ygartua, Process Module Manager; Duncan W. Mills, Staff Software Engineer; Clive Hayzelden, Senior Technical Marketing Manager
Parylene-F is one of the most promising materials for use as an intermetal dielectric at the 0.18 µm technology node. Due to its anisotropic refractive index, however, parylene-F cannot be examined using conventional spectroscopic ellipsometric techniques. In this article, the results of developing sophisticated data collection and analysis algorithms to determine the differences between in-plane and out-of-plane refractive indices are presented. The development of new intermetal low dielectric constant materials is a critical requirement for reducing parasitic capacitance and cross-talk in the increasingly finescale fabrication of semiconductor devices. Parylene-F (AF-4) offers a low dielectric constant (<2.3), high thermal stability (>450oC) and ease of deposition1. Ideally, measurement techniques for parylene AF-4 should be rapid, non-invasive, and similar to methods already in use for dielectric film monitoring. Spectroscopic ellipsometry (SE) has already achieved considerable acceptance as a monitoring tool. To achieve widespread acceptance of AF-4 in high volume manufacturing, however, it will be necessary to have methods and equipment for production monitoring of film thickness, uniformity, and refractive index. In this article, a method is presented for simultaneously measuring thickness, in-plane, and out-of-plane refractive indices of parylene. The UV-12X0SE and ASET-F5 film thickness measurement tools use two technologies: broadband (visible plus ultra-violet) Dual Beam Spectrometry (DBS), and Spectroscopic Ellipsometry (SE). KLA-Tencor measurement tools characterize films by providing reflectivity spectra and calculating the values of film parameters — the thicknesses, t, refractive indices, n, and extinction coefficients, k — from the best fit between theoretical and measured spectra. The DBS tool subsystem obtains the measured spectrum, Rm(λ), that represents the reflected light intensity as a function of the wavelength, λ. In the case of SE, the
reflected light is elliptically polarized. It can be represented by two components: a p-component, Rp, with polarization parallel to the plane of the incident and reflected beams, and an s-component, Rs, with polarization perpendicular to that plane (figure 1). Rp and Rs are complex quantities, defined by their intensities, |Rp| and |Rs|, respectively, and their phase difference, ∆. TanΨ and cos∆ are the standard ellipsometry parameters that describe the polarization state of the reflected light. They are defined by: Rp Rp = .exp (i∆) = tan Ψ.exp(i∆) Rs Rs TanΨ is the ratio of the p- and s-component intensities, and cos∆ is the real part of the complex quantity exp(i∆). To properly analyze the uniaxial birefringent nature of these films, KLA-Tencor has developed and implemented an algorithm designed to model the effects of the ordinary and extraordinary refractive next
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nord Figure 1. Showing the s and p polarization states of the reflected light together with the ordinar y (in-plane) and extraordinar y (out-of-plane) refractive indices of the par ylene.
Autumn 1998
Yield Management Solutions
37