High Sensitivity, Positive Tone, Low-k Polynorbornene Permanent Dielectric for Electronics Packaging
Authors: Mueller Brennen K , Schwartz Jared , Sutlief Alexandra , Kohl Paul A 
Published: February 2020

Permanent dielectric materials are necessary in microelectronic devices to separate electronic components. Low dielectric constant (low-k) materials are especially important because they electrically isolate on-chip, chip-to-chip, chip-to-package, or on-package interconnections. 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). 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 well-known photochemistry known as chemical amplification. In this case, a photo-generated acid catalyzed a deprotection reaction that leads to the solubility switch of the film. 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 developed a functional chemistry for chemically amplified photo-patterning and subsequent cross-linking. Of particular interest is the low dielectric constant, ca. 2.2 to 2.8 depending on composition, without added porogens. This was achieved on a polynorbornene backbone with pendent tert-butyl ester and fluoroalcohol moieties. Films of this polymer are cast with small loadings of a photoacid generator (PAG). These films are originally insoluble in aqueous base. Exposure of the PAG to ultraviolet light produces a strong acid that catalyzes the deprotection of the tert-butyl ester. This deprotection results in a carboxylic acid, increasing the solubility of the film in aqueous base. A film with 3 parts per hundred PAG was measured to have a D100 of 8.09 mJ/cm2, and contrast of ≥ 14.2, significant improvements over existing positive tone dielectric values. After photolithography, the unexposed regions of the dielectric are thermally cured for 2 hours at 250°C. During this step, the PAG thermally forms the acid catalyst and deprotects the tert-butyl ester. The resulting carboxylic acid undergoes a Fischer esterification reaction with the alcohol functionality to form a covalent cross-linked network.

It was found that the monomer ratio of the tert-butyl ester and the fluoroalcohol had a significant effect on the mechanical and electrical properties. Increasing the fluoroalcohol content decreased the dielectric constant, likely because of the high carbon-fluorine bond content. However, increasing the fluoroalcohol decreased modulus and hardness. This is likely due to the fact that the carboxylic acid will readily cross-link with another carboxylic acid to form an anhydride, confirmed by infrared spectroscopy. A polymer with 60 mol% fluoroalcohol was measured to have a dielectric constant of 2.78, a modulus of 2.95 GPa, and a hardness of 243 MPa. These values are appropriate for use as a permanent dielectric material.

In order to further improve electrical and mechanical properties, a third monomer was incorporated into the polynorbornene backbone. This third monomer has an alkyl pendent group which does not participate in photolithography or cross-linking. This monomer addition does not come without challenges, namely in solubility. The alkyl monomer lacks an acidic proton and lowers solubility in aqueous base. The physical properties of this dielectric will be presented. Further, the challenges associated with this chemistry will be addressed in an effort to formulate a robust dielectric with improved lithographic, mechanical, and electrical properties.