Spectroscopic Ellipsometry as a Potential In-Line Optical Metrology Tool For Relative Porosity Measurements of Low- K Di
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Spectroscopic Ellipsometry as a Potential In-Line Optical Metrology Tool For Relative Porosity Measurements of Low- K Dielectric Films N.V. Edwards,a) J. Vella,a) Q. Xie,a) S. Zollner,a) D. Werho,a) I. Adhihetty,a) R. Liu,a) T.E. Tiwald,b) C. Russell,c) J. Vires,d) and K.H. Junkerd); a) Motorola Process and Materials Characterization Laboratory, Mesa AZ 85202; b) J.A. Woollam Co., Inc., Lincoln, NE 68508; c) Bede Scientific Inc., Englewood, CO 80112; d) Motorola Advanced Products Research and Development Laboratory, Austin, TX 78721
ABSTRACT The optical properties of organosilicate glass (OSG) samples were investigated with spectroscopic ellipsometry. We found that samples with dramatically higher hardness had higher indices of refraction (RI) and thus higher electron densities and lower relative porosities than films with lower hardnesses. The reverse was true for films with low hardnesses. As well, these films did not have the same optical properties as porous SiO2 across the spectral range measured, which we show has significant implications for the in-line optical metrology of these materials. INTRODUCTION The International Technology Roadmap for Semiconductors predicts that the progressive downward scaling of device dimensions required for the 100nm technology node will require materials with dramatically lower static dielectric constants, achieved without sacrificing robust mechanical properties.[1] Device down-scaling requires a minimized resistance-capacitance product of the backend metal interconnect and the low-κ interlayer dielectric material, where reduced capacitance can be achieved when κ is minimized. One strategy for minimizing κ is to incorporate porosity into otherwise dense dielectrics, yet there is no definitive in-line metrology solution that that enables a controlled incorporation. This is crucial, not only to achieve increasingly smaller device dimensions but also to ensure device reliability, since increased porosity can compromise the mechanical strength of a material. As the mechanical stresses inherent to multi-level IC fabrication and packaging are likely to subject these porous materials to stress level approaching their yield strength and interfacial fracture toughness, a fast and nondestructive means of identifying structurally-compromised materials in a manufacturing environment is sorely needed. Organosilicate glasses (OSGs) are one of several candidate low-k materials. They are best described as C-doped silica, where it is assumed that the incorporation of CHx species introduces porosity into the SiO2-like matrix. Our results obtained on OSG suggest that optical techniques could provide a crucial first step toward an in-line metrology solution: mechanical hardness can be correlated with ellipsometrically-obtained electron densities. Indeed, measurements with spectroscopic ellipsometry (SE), a fast and non-destructive optical characterization technique that can be used in-line, yielded: (1) Samples with dramatically higher hardness had higher indices of refraction (RI) and thus high
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