Measuring and testing – With fluid pressure – Porosity or permeability
Reexamination Certificate
2002-07-12
2003-12-16
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Porosity or permeability
C073S800000, C073S866000, C073S789000, C702S043000, C702S082000
Reexamination Certificate
active
06662631
ABSTRACT:
BACKGROUND OF THE INVENTION
As integrated circuit feature sizes continue to shrink, new low dielectric constant (low-k) materials are needed to address problems with power consumption, signal propagation delays, and cross talk between interconnects. One avenue to low-k dielectric films is introduction of nanometer scale pores to lower its effective dielectric constant. However, the pore structure strongly affects important material properties such as mechanical strength, moisture uptake, thermal expansion, and adhesion to different substrates. Therefore, characterization of the pore structure, in particular the pore size distribution and mechanical properties, is strongly needed to optimize and develop new low-k materials and processes.
Traditional methods used for the porosity characterization in bulk materials are hardly applicable to thin films because the total pore volume and surface area are too small. For this reason, advanced non-destructive methods, such as small-angle neutron and X-ray scattering (SANS and SAXS) combined with specular X-ray reflectivity (XRR) and positron annihilation spectroscopy (PALS, PAS) have recently been developed to characterize the pore size and porosity of thin porous films. Although these new techniques are based on different physico-chemical principles, few systematic studies reported so far show that the results of the measurements are in reasonable agreement.
Low stiffness properties of porous low-K films is one of key factors limiting their introduction into ULSI technology. A compromise must be reached between low dielectric constant and sufficient mechanical strength for the material to survive technological steps. There is also lack of useful and accessible techniques, which can accurately provide absolute values of the mechanical characteristics.
In PALS and PAS, films are irradiated with a focused beam of several keV positrons. Positrons form positronium (Ps)—the electron-positron bound state—that is trapped in the pores where their natural lifetime of 142 ns is reduced by annihilation during collisions with the pore walls. The reduced lifetime &tgr;(Ps) can be correlated with pore size. Ps lifetime histograms are recorded, and the lifetime distribution curves are obtained with a fitting program specified for this purpose. The distribution curves are transformed into pore size data, using pore geometries. The film porosity can be calculated by comparison of measured photon annihilation ratio of Ps atoms. In PALS, the porosity characterization needs deposition of a special barrier to compare the Ps intensity in free and capped films. PAS and PALS are efficient for the evaluation of bi-modal pores (like MSSQ). If pores are bi-modal, they give information related to their size and relative concentration. PALS and PAS are useful for characterization of pore interconnectivity and can be used for evaluation of diffusion barriers by detecting of Ps escaping from the film trough the voids in the barrier. However, because &tgr;(Ps) also depends on the wall nature, sometimes it is difficult to obtain the pore size from &tgr;(Ps): for instance, in the case of organic polymers. If all pores are open, one needs to apply a capping layer: otherwise Ps escape to vacuum and give the natural &tgr;=142 ns.
In SANS, the absolute scattered neutron intensity, I, plotted against the scattering vector q=(4&pgr;/&lgr;)sin(&thgr;/2) where &thgr; is the scattering angle from the incident beam path and &lgr; is the neutron wavelength (6 Å). The SANS intensity plotted versus q is a function of the porosity and wall density. The functional form is determined assuming a random two-phase (void+solid) structure. The film thickness and overall electron density are evaluated by the XRR measurements and are combined with the film composition data obtained by RBS and FRES so that the overall film density is determined. Since the film density is also a function of the porosity and skeleton density, these values are obtained by solving for the unknowns in the equations from SANS and XRR.
Recently, a simple X-ray scattering method for thin film evaluation was reported. The pore size is calculated by comparing the observed profile of scattering intensity and results of simulation. This approach is convenient to get general information without details because only effective pore size is calculated. If the film has bi-modal pores, the effective pore size depends on the ratio between small and large pores. For instance, if they have the same volume, the SAXS pore size is closer to the small pores due to the larger number of interfaces. The film porosity is calculated by normalization of the XRR film density to the skeleton density. This necessitates the assumption that the skeleton is identical to the dense, non-porous prototype. Sometimes such an assumption is not justified. The non-porous prototype may also be not available (for instance, in the case of CVD SiOCH films). This method is not efficient for evaluation of the pores interconnectivity, for evaluation of diffusion barriers and, generally, SAXS is not able to distinguish between pores and particles.
Nanoindentation (NI) is the most common method for obtaining stiffness of thin films. However, NI overestimates stiffness because of several possible reasons: (a) Stiffening by the substrate. For such thin films the NI tip may always feel the effect of the substrate and thus overestimate Young's Modulus (E); (b) Viscoelasticity. Polymers are known to show large viscoelastic effects which are likely to cause higher E values to be obtained; (c) Tip-film interactions. Effects such as densification or pile-up under the tip have not been quantified. The interactions of a tip with such a porous matrix are not well understood. Additionally, NI is destructive and therefore it is not applicable for in-line monitoring of low-k films.
Two different non-destructive methods have recently been successfully used for evaluation of stiffness properties of porous low-K films. Results of the stiffness measurements of MSSQ based porous low-K films by Surface Acoustic Wave Spectroscopy (SAWS) and Brillouin Light Scattering (BLS) are in good agreement one with another but E values calculated for various low-K films are ≈3 times lower than NI. The film density and porosity calculated from the same SAWS data correlate excellently with Specular X-Ray Reflectivity. These facts suggest that the E values obtained by SAWS and BLS are real and accurate.
In the SAWS method for non-destructive characterization of density and Young's Modulus of low-k films, surface acoustic wavepackets are generated thermoelastically from absorption of laser pulse energy at the layer/substrate interface. The laser pulse energy (337 nm wavelength) is focused into a thin line on the sample, and causes rapid expansion of the locally heated source, giving rise to stresses and generating surface acoustic wavepackets propagating along the sample. The wideband SAW wavepackets are detected by a piezoelectric foil with a steel-wedge transducer at different relative propagation distances (here 15 mm) on the sample. The broadband SAW wavepacket (approx. 20-100 MHz frequency range) propagates in both layer and substrate and becomes dispersed because waves of different frequency sample a different proportion of layer and substrate, with different net elastic properties, and the wave velocity is therefore frequency dependent. From a Fourier transform technique one extracts the frequency-dependent velocity dispersion curve. Assuming that thickness and Poisson's ratio are known, the density and Young's modulus of the layer are obtained from the best-fit parameters of the theoretical to the measured dispersion curve. The SAWS film density is in good agreement with XRR. Although SAWS is not able to measure the pore size, a unique feature of this method is the possibility of non-destructive evaluation of mechanical properties (Young's Modulus).
SUMMARY OF THE INVENTION
It is the aim of the invention to provide a method and apparatus
Baklanov Mikhail Rodionovich
Dultsev Fedor Nikolaevich
Maex Karen
Mogilnikov Konstantin Petrovich
Shamiryan Denis
Interuniversitair Microelektronica Centrum
McDonnell Boehnen Hulbert & Barghoff
Wiggins David
Williams Hezron
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