SAWS Accurately Characterizes Porous Low-k Dielectrics

Methylsilsesquioxane (MSQ) films are considered one of the most promising materials for ultralow-k dielectrics. This is partially due to the fact that relatively moderate pore volumes (~40%) can be added to the bulk MSQ material (k~2.7-3.0) and still produce films with k values in the 2.2-2.0 range. In contrast, ~70% pore volume is needed to attain similar k values in porous silica.

A crucial challenge in the integration of ultralow-k films in dual-damascene interconnects is having the mechanical strength needed to perform chemical mechanical planarization and packaging processes. The industry is attempting to identify the physical properties (density, porosity, pore size and Young's modulus) needed for process control. High-yielding multilevel interconnect performance and reliability are tied to the uniformity of these film properties.

Researchers at XPEQT (Leuven, Belgium), a spin-off business of IMEC , recently demonstrated how surface acoustic wave spectroscopy (SAWS) can be used to provide accurate values of porosity and Young's modulus of low-k films. SAWS is a fairly inexpensive technique that tests samples non-destructively and rapidly (~1 min). Upon testing a variety of porous MSQ films from various suppliers, the researchers were able to independently validate the results against specular X-ray (density), Brillouin light scattering (Young's modulus), and ellipsometric porosity measurements (pore size and distribution), showing excellent correlation.

At the same time they determined that SAWS provides a more accurate measurement of thin-film stiffness than nanoindentation (NI), the most commonly used method today. NI gives consistently higher stiffness values than SAWS, which XPEQT researchers attribute to a combination of viscoelasticity, substrate stiffening and tip/film interactions. They reported their findings in June at the IEEE International Interconnect Technology Conference (IITC).

In the SAWS technique, wideband surface acoustic wavepackets are generated thermoelastically from absorption of laser pulse energy at the layer/substrate interface. In this study, a laser pulse (0.5 mJ, 0.5 nsec, 337 nm wavelength) was focused into a thin line on the sample (10 µm × 6 mm), which expands, gives rise to stresses, and generates surface acoustic wavepackets that propagate along the sample. The broadband (20-100 MHz) wavepackets become dispersed because waves of different frequency sample a different proportion of layer and substrate with different net elastic properties. The wavepackets are then detected by a piezoelectric foil with steel-wedge transducers at different propagation distances. Fourier transform is used to give a frequency-dependent velocity dispersion curve. If the film's thickness and Poisson's ratio are known, Young's modulus (E) and film density (r) can be obtained from the best-fit parameters of the theoretical to measured dispersion curves.

The researchers tested 1-µm-thick MSQ films on silicon (001) wafers with pore concentrations of 1-30%. The SAWS density correlated extremely well with SXR, with strong relationships between porogen concentration, porosity and density. The BLS E values and SAWS E values as a function of porosity were within 20%, which is acceptable given the longitudinal mode measurement of BLS E (~8 GHz) and the SAWS value in a primarily transverse mode (~100 MHz). SAWS analysis showed the dependence of decreased stiffness with increasing porosity for several MSQ films.

The researchers determined that NI for the case of soft, thin films on a stiff substrate is more complicated than stiff-on-soft cases. NI provided significantly higher (~3×) values of stiffness. They recommend its use only for relative stiffness comparisons.

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