Leuven, Belgium-based research center IMEC has teamed up with the University of Ghent to develop a new process flow for ultra-thin bendable chip packages with as little as 50-µm thickness. Thanks to its flexibility, the technology enables the embedding of packaged chips in bendable boards. These chips can be used for a wide variety of applications, including smart textiles and flexible displays.
The process has been demonstrated with silicon chips thinned down to 20 to 30 µm. The addition of polyimide layers and metal bring the total thickness of the package to just 50 µm, making the whole package bendable. The ultra-thin chip package offers a contact fan out with more relaxed pitches.
The process starts with a 20-µm thick polyimide layer base substrate spin-coated on a rigid glass carrier. For the fixation and the placement of the chips on the polyimide layer, bicyclobutane less than 5 µm thick is used as adhesive. Bicyclobutane is resistant to the high curing temperature of the top polyimide since its solvents evaporate during a pre-curing phase.
Void-free bonds can be obtained by placing the chips properly, either in vacuum or with a dispensed bicyclobutane. Current research focuses on the optimization of the chip placement on dispensed (pre-cured) bicyclobutane and on preventing voids by controlling the dispensed quantity. This way, no vacuum environment will be required.
After the cure of the bicyclobutane at 350°C, the chip is fixed on the polyimide layer. A covering polyimide layer is spin-coated on the fixed die with a thickness of 20 µm. Contact openings to the bumps of the chips are laser drilled. By using a shaped laser beam, via diameters with a top diameter down to 20 µm can be realized.
A top metal layer of 1-µm titanium-tungsten/copper is sputtered and photolithographically patterned, metallizing the contacts to the chip and providing a fan out to the contacts of the chips. The complete, bendable package is then released from the carrier.
University of Ghent