High Sensitivity, Positive Tone, Low-k Polynorbornene Dielectrics

Permanent dielectric materials are necessary in microelectronic devices to separate electronic components.  Low dielectric constant (low-k) materials mechanically and electrically isolate on-chip, chip-to-chip, chip-to-package, or on-package interconnections.¹  Polymers are widely used for these purposes, as they offer a low stress, easily processed alternative to inorganic materials.  Additionally, permanent low-k polymeric materials can often be formulated to be directly patternable by photolithography.

     Currently, most positive tone, permanent dielectrics utilize the diazonaphthoquinone-based photochemistry, which provides relatively low sensitivity (D₁₀₀ > 300 mJ/cm²) and low contrast (< 2.5).^2,3  In order to pattern very thick films or high aspect ratio structures, materials with greatly improved lithographic properties are needed.  This can be accomplished with a photochemistry known as chemical amplification.⁴  In this case, a photo-generated acid catalyzes a deprotection reaction.  This deprotection increases the solubility of the film in aqueous base developer, providing the photo-definability.  The low loading of the photoactive compound provides high sensitivity and high contrast films.  However, cross-linking of these positive tone films after patterning is also desirable, and the presence of the strong acid makes cross-linking difficult.

     To this end, our lab has previously reported a polymer chemistry that is chemically amplified and cross-linkable.⁵  The proof of concept polymer was synthesized as a random copolymer of tert-butyl methacrylate and 2-hydroxyethyl methacrylate.  In this case, the pendent tert-butyl group is deprotected by the photogenerated acid to produce a carboxylic acid.  This deprotection makes the film soluble in aqueous base developer, whereas the unexposed film is insoluble.  Formulations of 1 and 3 parts per hundred photoacid generator were measured to have contrasts of 12.7 and 5.2, respectively, and sensitivities D₁₀₀ of 50.2 and 32.2 mJ/cm², respectively.  These films could then be cross-linked via a Fischer esterification between the pendent carboxylic acid and the pendent alcohol in the presence of acid.  The esterification cross-linking reaction is much slower than the deprotection reaction, so it did not interfere with photo-patterning.  Cross-linking was proven by changes in residual stress and solubility.

     The results of this polymer are promising, but polymethacrylates are not appropriate for many permanent, low-k dielectric applications.  A polymer with similar lithographic and cross-linking functionality is desired, which has higher glass transition and decomposition temperatures.  To achieve this, a polymer with a polynorbornene (PNB) backbone has been synthesized with similar pendent functional groups.  These polymers are patternable and cross-linkable by similar mechanisms to the polymethacrylate dielectric.

     The PNB polymer was synthesized with similar tert-butyl ester and alcohol pendent groups.  The D₁₀₀ of and contrast of the PNB were found to be 41.1 mJ/cm²and 12.4, respectively, which are comparable to the values measured for polymethacrylate.  The PNB, however, exhibits a large difference in solubility in developer.  To quantify this, dissolution characteristics of these films will be evaluated by quartz crystal microbalance analysis.  Initial mechanical and electrical property measurements will be presented.  Finally, the challenges associated with this chemistry will be addressed in an effort to formulate a robust dielectric with improved lithographic, mechanical, and electrical properties.

References

[1] Tummalla, R. R.; Swaminathan, M. Introduction to System-on-Package; McGraw-Hill: New York, 2008.

[2] Y. So, E. J. Stark, Y. Li, S. Kisting, A. Achen, K. Baranek, D. Scheck, J. Hetzner, J. J. Folkenroth, M. Töpper, and T. Baumgartner, IEEE Trans. Adv. Packag.2006, 29, 4, 741–750.

[3] X. Z. Jin and H. Ishii, “Novel positive-type photosensitive polyimide with low dielectric constant,” J. Appl. Polym. Sci.2005, 98, 15–21.

[4] Reichmanis, E.; Houlihan, F. M.; Nalamasu, O.; Neenan, T. X. Chem. Mater. 1991, 3, 394

[5] B. K. Mueller and P. A. Kohl, “Chemically amplified, positive tone, cross-linkable thick-film polymer,” J. Appl. Polym. Sci. 2013, 130, 2, 759–765.