Novomer polypropylene carbonate (PPC) is a revolutionary low temperature sacrificial material which enables innovative solutions for the most pressing challenges faced by the microelectronics and MEMS industries. Used in conjunction with conventional processes, PPC applications include microstructure formation, wafer level MEMS packaging and temporary wafer bonding. Novomer PPC decomposes into volatile gases which permeate through solid overcoats and leave virtually no residue, thus avoiding additional solvent cleanup steps. PPC dissolves in a number of commonly available solvents and can be applied to a substrate using standard spin coating processing. It can be overcoated with traditional electronic chemicals such as photoresists, inorganic glasses, organic polymers and metals.
Microstructure Formation
Novomer polycarbonates are a valuable temporary placeholder for forming encapsulated air-channels. These gaps can serve a number of functions, including as low-k dielectric air spaces between interconnects and as enclosed channels for microfluidic devices and micro–reactors. Generally, fabrication involves three steps:
- Coat the wafer with Novomer polycarbonate and pattern.
- Overcoat Novomer material with desired permanent layer.
- Heat to decompose Novomer sacrificial material, leaving open structures.
Selectively removing patterned PPC to form microfeatures can be achieved using two methods. Standard PPC can be patterned using a hard mask overcoat and standard lithography techniques. Direct patterning is also possible by the addition of a photoacid generator, making the material photodefinable.
Temporary Wafer Bonding
Novomer materials offer simple, cost effective wafer thinning & handling solutions for 3-dimensional integrated circuit (3D-IC) applications. Novomer PPC can be used as an adhesive glue for wafer-to-wafer, die-to-wafer, or wafer-to-substrate bonding. Post processing, de-bonding is easily achieved by thermal decomposition of the polypropylene carbonate material with no residual ash.
Wafer Level MEMS Packaging
Novomer sacrificial materials provide a low cost packaging technique for chip-level encapsulation. Used to create a temporary span between the active part of a MEMS device and a solid overcoat, Novomer PPC forms a cavity when thermally decomposed. This package system has a number of advantages including low temperature processing suitable for sensitive MEMS devices, a reduced number of processing steps, and lower cost.
All photographs are courtesy of Professor Paul Kohl at Georgia Institute of Technology
a) HARPSS polysilicon ring gyroscope (~200um wide). b) After dispensing PPC. c) After encapsulation with a dielectric polymer and thermal decomposition. d, e, and f) After breaking the cap, no residue is present.
70 micrometer wide air-channel fabricated using PPC encapsulated in a dielectric polymer.