Positive Tone, Low-k Polynorbornene Dielectric Crosslinking
Authors: Schwartz Jared , Mueller Brennen K , Kohl Paul A 
Published: February 2020

Research in microelectronics packaging has recently been driven by the reduction in size of the die the package contains. Die-die, interconnect-interconnect, and substrate-die distances are rapidly decreasing in size. As the electronic system shrinks, the need for photo-definable, permanent low-k dielectric materials becomes more important to electrically and mechanically isolate increasingly denser electrical pathways.
There are two photochemistries that make films photodefinable. Negative tone materials decrease in solubility in a developer when exposed to UV radiation. Positive tone materials increase in solubility when exposed to UV radiation. Positive tone photochemistry has a distinct advantage over negative tone because of a reduced sensitivity to particulates on a photo mask (1). Additionally, positive tone chemistries allow an aqueous develop instead of the organic solvents used in most negative tone systems that are an environmental concern.
Polynorbornene (PNB) has shown promise as a permanent dielectric material because of a relatively low dielectric constant, 2.2 (2). Synthesizing a PNB copolymer with substituted pendent groups allows cross-linking and photo-definability, although it raises the dielectric constant by introducing polarizable groups to the polymer. In this case, diazonaphthoquinone (DNQ), a well-studied positive tone photoactive compound, is used to shift the solubility of a film containing the PNB copolymer.
Previously, our group studied the effects of cure temperature on a negative tone formulation of PNB (3). A photo acid generator was used to create crosslinking via an epoxy ring-opening reaction. To quantify the cross-linking, the elastic moduli of thin films were determined. Over the cure temperatures investigated, a maximum elastic modulus of 2.8 GPa was observed at 160 ºC. The modulus then showed a steady decline to 2.4 GPa at 240 ºC. This degradation in modulus was assumed to be a result of the acidity of the film (3).
In this work, a comparison between the positive and negative tone films is made to understand the cross-linking of the more desirable positive tone PNB. During normal processing, the negative tone films exhibit a highly acidic pH prior to cure, where the positive tone films exhibit a neutral or basic pH prior to cure. To further investigate the effects of pH on the PNB film crosslinking, films with the exposed photo acid generator were swelled with base prior to curing to get a comparable pH to the positive tone material.
Elastic modulus results showed no dependence on pH for the negative tone films. Despite the stark contrast in film acidity, a peak modulus of 3.0 GPa at a 140 ºC cure temperature was observed, declining to 2.4 GPa at 220 ºC. Positive tone results showed a different dependence, however. With a neutral film, crosslinking occurred with DNQ acting as the cross-linker, giving a modulus of 4.0 GPa. When developed, DNQ-loaded films showed a decline in modulus to 2.6 GPa. This is likely due to the reactions of DNQ with aqueous base (4). An epoxy cross-linker added to the developed positive tone film helped cross-linking by inhibiting base uptake and allowing DNQ to act as a cross-linker. With the correct processing conditions and additives, the modulus and dielectric constant of the PNB film can be controlled.
References
1. B. K. Mueller, E. Elce, A. M. Grillo, and P. A. Kohl, “Positive-tone, aqueous-developable, polynorbornene dielectric: Lithographic, and dissolution properties,” J. Appl. Polym. Sci., vol. 127, no. 6, pp. 4653–4661, (2013).
2. G. Maier and D.- Garching, “Low dielectric constant polymers for microelectronics,” vol. 26, (2001).
3. M. Raeis-Zadeh, N. D. Melendez, Y.-C. Chen, and P. a. Kohl, “Aqueous-Develop, Photosensitive Polynorbornene Dielectric: Optimization of Mechanical and Electrical Properties,” J. Electron. Mater., vol. 40, no. 10, pp. 2126–2138, (2011).
4. M. Koshiba, M. Murata, M. Matsui, and Y. Harita, “Thermally Induced and Base Catalyzed Reactions of Naphthoquinone Diazides,” Adv. Resist Technol. Process. V, vol. 920, pp. 364–371, (1988).