Scalable Batch Transfer of Individual Silicon Dice for Ultra-Flexible Polyimide-Based Bioelectronic Devices.

Abstract

Demands on flexible neural interfaces in terms of functionality, spatial resolution and longevity have increased in the past years. These requirements can be met by sophisticated integrated circuits developed in CMOS (complementary metal oxide semiconductor) technology. Embedding such fabricated dice into flexible polymeric substrates greatly enhances the adaption to the mechanical environment in the body. With the process developed here, 100 % of individual dice (n = 34, 390 x 390 μm2) could be transferred simultaneously into polyimide (PI) substrates with simple and exact positioning (0.2° rotational and 5 μm translational error). Levelled layer build-up and standard microfabrication technologies could be used for CMOS-post-processing in order to manufacture metal interconnections between contact pads of 100 μm thin dice and PI insulation as selectively patterned device substrate. The process allows for individual positioning according to desired shape of the final chip-in-foil-system and for upscaling the number of dice to be transferred. Furthermore, final distribution and embedding of dice on the flexible substrate is independent from their distribution on the CMOS fabrication wafer the and does not require additional adhesion promoters. During fabrication the transfer method is insensitive to high temperatures (450 °C in this study) and hence enables a wide range of post-processes. Shear strength between dice and PI substrate was characterized by shear tests and results (58.1 ± 13.7 MPa) are in the range achieved with the adhesive benzocyclobutene (BCB).