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Metrology must advance to support 45nm

Posted: 01 Mar 2004 ?? ?Print Version ?Bookmark and Share

Keywords:metrology? spectrometry? ellipsometric? porosimetry? porous?

Andreas Knorr is Program Manager at International Sematech

With the steady development of porous low-k films for the 45nm and 32nm nodes comes the need for film-specific metrology to support timely ramp-up and manufacturing. While many current metrology techniques, such as spectroscopic ellipsometry, are mostly transparent in their use for porous low-k materials, the measurement of several aspects of film properties remains challenging.

Two of those aspects are the average pore diameter and its size distribution in the film. These parameters are critical in predicting integratability, mastering process development and succeeding in yield learning. The criticalities in tool and material qualification and statistical process control (SPC) in mass manufacturing are open questions at this point.

While such techniques as positronium annihilation lifetime spectroscopy and small-angle neutron scattering exhibit the ability to characterize the pore structure, they are not readily available or feasible for fast-turnaround, in-line use.

Two emerging methods, small-angle X-ray spectrometry (SAXS) and ellipsometric porosimetry (EP), are competing for acceptance as commercial porosimetry solutions.

The SAXS method is based on diffuse scattering of X-rays from heterogeneous electron density distributions. Commercial solutions all use grazing incidence X-rays. But, as Wen-Li Wu of the National Institute of Standards and Technology pointed out during a recent International Sematech workshop on porosimetry, challenges remain. Foremost are data convolution due to detector geometry, signal-to-noise issues, surface and interface roughness as sources of additional scattering, and the complex theory.

The companies marketing porosimetry tools claim they have developed fixes for those problems. Roughness can be treated theoretically and therefore can be integrated into the simulation algorithm. And suppliers are promoting detectors to address the signal-to-noise and data deconvolution issues.

EP is the only commercial method competing with SAXS. EP is based on the adsorption of a liquid from a saturated-gas phase. The adsorbate volume as function of pressure is monitored by recording the change of the optical properties of the film using ellipsometry. The science of liquid adsorption into porous media has been well-established for decades. For pores larger than 2nm in diameter, the Kelvin equation is used to calculate pore size from pressure and adsorbate volume.

In tests, both methods clearly demonstrate sensitivity to changing porosity parameters. Randomized sets of samples, within which one of these parameters was systematically varied, were put in the proper sequence by each method, with some differences among tool suppliers.

This is encouraging news for the potential application of porosimetry as an in-line measurement in fabs. However, comparisons of pore size and distribution width of particular films show differences among suppliers. Since there is no generally accepted porous-film standard, these differences are unlikely to be resolved in the near future. Thus far, films with pores of a particular diameter that could be used as a standard have only been available with pore diameters larger than 5nm.

Porosimetry is a key technology for the development of porous low-k dielectrics and also proves useful in the prediction of integratability of these materials. This will drive widespread usage of porosimetry tools in laboratories during early stages of material development and integration.

But the question of whether porosimetry will ever be used in volume manufacturing as a means of SPC is still open. Since spectroscopic ellipsometry can be used to monitor the pore volume fraction, the only unique use of porosimetry in a production environment is determination of pore size. So far, no claims have been made that pore size is a critical parameter to monitor in production.

- Andreas Knorr

Program Manager

International Sematech

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