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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.
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DBS Reflectivity Measured – DBS
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xylylene polymerizes instantly on the wafer giving parylene AF-4 films. The vapor phase polymerization method at Novellus provides parylene AF-4 films with excellent uniformity and conformity on 200 mm wafers. The deposited parylene AF-4 films are semi-crystalline and the polymer chains are composed of -[C6H4-CF2-CF2]- structure. We have measured both as-deposited samples and samples that have received a 400ºC anneal in a nitrogen atmosphere.
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Ellipsometric analysis of several parylene films, including PPX-N, PPX-C, and PPX-D has indicated that they are uniaxial, with an extraordinary refractive index normal to the film surface, leaving two degenerate ordinary refractive indices lying in the plane of the film2. The nature of the birefringence differs between the types of parylenes; as an example, PPX-D is positive uniaxial (nextraordinary > nordinary), whereas most parylenes are negative
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Figure 2. SE and reflectivity spectra for as-deposited AF-4. Thickness = 1075 nm.
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indices (RI) upon the amplitude and phase of the reflected signal. Fortunately, uniaxial films of the parylene type contain no in-plane anisotropy, so they cause no mixing of the two independent p and s polarization states. Consequently, the “traditional” tanΨ and cos∆ parameters defined for isotropic films also apply to the present analysis. Film deposition
In a typical parylene AF-4 deposition process, the AF-4 dimer, octafluoro-[2,2]-paracyclophane, is sublimed in a vaporizer and the sublimed gaseous dimer is passed through a pyrolyzer maintained at 600ºC - 700ºC. The generated monomer, a,a,a’,a’-tetrafluoro-p-
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Wavelength (nm) Figure 3. Dispersion of the real (n) and imaginar y (k) parts of the refractive index for ordinar y and extraordinar y modes for the as-deposited sample. n ord (633 nm) = 1.5179 and n ext (633 nm) = 1.4217.
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Table 1 shows a summary of measurement results for the as-deposited and annealed samples. The in-plane (nordinary) and outof-plane (nextraordinary) refractive indices of the as-deposited material were determined to be 1.5137 and 1.4135, respectively. Following annealing, the in-plane (nordinary) and out-of-plane (nextraordinary) refractive indices of the material were determined to be 1.5879 and 1.4170, respectively.
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with an offset of ~0.1. The unified SE plus DBS measurement results for this sample match prism coupler index measurements at 633 nm (nordinary = 1.53 and nextraordinary = 1.42) and profilometer measurements (thickness ∼ 1030 nm). Figure 4 shows SE and reflectivity spectra for an annealed AF-4 sample with a thickness of 915 nm. The dispersion curves are shown in figure 5.
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Figure 4. SE and reflectivity spectra for annealed AF-4. Thickness = 915 nm.
uniaxial. Our analysis has indicated that AF-4, both as-deposited and after-annealing, is negative uniaxial, with fairly pronounced birefringence compared to other parylenes. Results
Figure 2 shows SE and DBS spectra for as-deposited parylene AF-4 on silicon, the reflectivity rapidly drops to less than 10 percent at a wavelength of 272 nm and the SE spectra oscillations are damped out in the 260-275 nm range. The variation of n and k with wavelength are shown in figure 3. A strong absorption peak at 272 nm is seen in agreement with the observed minimum in reflectivity. In the 300-750 nm range, where k=0, nordinary and nextraordinary are essentially parallel,
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Wavelength (nm) Figure 5. Dispersions of the real (n) and imaginar y (k) parts of the refractive index for ordinar y and extraordinar y modes for the annealed sample. n ord (633 nm) = 1.5879 and n ext (633 nm) = 1.4170.
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As deposited Ordinary Extraordinary Delta 1075 n/a n/a 1.5137 1.4135 0.1002 0.0349 0.0509 -0.0160 0.0157 0.0696 -0.0539
Summary
KLA-Tencor found that the refractive index (RI) dispersion for the two optical axes and the film thickness can be determined from coupling spectroscopic ellipsometry (SE) and normal incidence reflectance spectroscopy (DBS). The negative uniaxial optical anisotropy (nordinary - nextraordinary) is shown to increase from 0.1002 to 0.1709 following annealing at 400ยบC. The measurement technique is suitable
Ordinary 915 1.5879 0.0370 0.0488
Annealed Extraordinary n/a 1.4170 0.0505 0.0671
Table 1. Summar y of
Delta n/a 0.1709 -0.0135 -0.0183
for routine measurement of film thickness and refractive indices of parylene-F films deposited during integrated circuit processing. Acknowledgments
We gratefully acknowledge our collaboration with James Stimmell and Devendra Kumar of Novellus Systems, Inc., San Jose, CA. We would also like to acknowledge the contributions of the Specialty
measurement results.
Coatings Systems Division of Alpha Metals, Inc. for providing the high quality cyclic dimer AF-4 starting material used in this study. 1 M.A. Plano, D. Kumar and T.J. Cleary, The
Effect of Deposition Conditions on the Properties of Vapor-Deposited AF-4 Films. Mat. Res. Soc. Proc. Vol 476, 1997. 2 J.G. Gaynor and S. B. Desu, Optical Properties
of Polymeric Thin Films Grown by Chemical Vapor Deposition, J. Mater. Res., vol. 11, No. 1, Jan. 1996.
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