(672d) Aqueous-Develop, Photosensitive Polynorbornene Dielectric for High Aspect Ratio All Copper Interconnects

A challenging application space exists for photosensitive, thick-film, high aspect ratio polymers in microelectronics applications. In this work, characterization has been performed on Avatrel 8000P (Promerus LLC) a new high aspect ratio, aqueous base develop, photosensitive polymer. The polymer consists of a norbornene backbone with fluorinated alcohol groups which provide solubility in an aqueous base and carboxylic acid groups which provide cross-linking sites with epoxy units. The polymer formulation is a mixture containing a multifunctional epoxy cross-linker, a photo-package and an adhesion promoter.

High aspect ratio features of 7:1 (height:width) were produced in 70 µm thick films in a single coat with straight side-wall profiles and high fidelity. The polymer films studied had a contrast of 11.6 and a low absorption coefficient. To evaluate the polymer's suitability to microelectronics applications, epoxy cross-linking reactions were studied as a function of processing condition through Fourier transform infrared spectroscopy, nano-indentation, and dielectric measurements. The fully cross-linked films had an elastic modulus of 2.9 GPa and hardness of 0.18 GPa.

In addition to its excellent photo-definition properties, its high mechanical strength and excellent thermal stability make it suitable for microelectronics packaging. In this project, Avatrel 8000P will be used as imprint material to generate high aspect ratio chip to substrate interconnects. Implementation of all copper chip to substrate interconnects could provide high conductivity electrical connections, resistance to electromigration while avoiding formation of brittle intermetallics. However, to achieve mechanical compliance these structures need to have high aspect ratios. High aspect ratio hollow structures are challenging to fabricate with traditional photolithography because the transport of the developer in the core is slow compared with the transport of developer around the perimeter of the feature. As a result hollow structures require longer development times than solid features of comparable aspect ratios and at high aspect ratios delamination of the hollow core structure occurs before the core can be fully developed.

Development of an imprint process to fabricate high aspect ratio polymer molds for copper electroplating interconnects could enable improved IC electrical performance and reliability. A shallow photo-imprint stamp has been developed to physically displace material in the polymer core to reduce the development times necessary for these structures. Work has been conducted to clear a hollow core polymer structure twice the height of the imprint stamp. Work will be continued to fabricate photo-imprint stamps that can achieve the same aspect ratio hollow features as solid features. Subsequently, electroplating these high aspect ratio Avatrel 8000P molds will be able to generate high aspect ratio copper interconnects that can are mechanically compliant.