Abstract
Abstract The demands on flexible implants for recording of neural signals and electrical stimulating have increased in recent years with regard to their functionality, miniaturization, and spatial resolution. These requirements can be met best by embedding powerful complementary metal oxide semiconductor (CMOS) microchips into thin biocompatible polymer substrates. So-called chip-in-foil systems thus combine mechanical properties of a polymer substrate and performance of CMOS technology. The development of a process for direct transfer of multiple CMOS microchips (edge length <400 μm) simultaneously into thin polyimide (PI) substrates is subject of this study. It allows the use of standard microelectromechanical systems (MEMS) processes for further levelled superficial layer build-up. This is achieved with the help of a silicon carrier wafer equipped with cavities for precise chip placement and a sacrificial layer to facilitate release of the chip-in-foil systems. In a post-processing step all silicon chips are thinned down to 100 μm. With this process a transfer yield of 100 % (n = 34) was achieved for the silicon chips on a die level. Chip rotational error on substrates was determined to be as low as 0.21° ± 0.10°. Die adhesion was examined by shear tests, resulting in shear strength of 58.1 MPa ± 13.7 MPa, which dropped to 15.2 MPa ± 10.5 MPa after accelerated ageing in 60 °C phosphate buffered saline solution (PBS) for 16 days (equivalent to 78 days at 37 °C). This study demonstrated a reliable microchip transfer process with low positioning error into flexible PI substrates with post-processing thinning of the dies. The use of a carrier silicon wafer allowed precise electrical interconnect fabrication with standard MEMS processing techniques and without handling of thin and fragile chips. These results are a prerequisite to meet needs of reliability and structural biocompatibility in implantable flexible bioelectronic devices.
Highlights
Neural electrodes are frequently used for recording and stimulation in the central and peripheral nervous system to help physically impaired patients in their everyday life [1, 2]
This study demonstrated a reliable microchip transfer process with low positioning error into flexible PI substrates with post-processing thinning of the dies
Prior to back side grinding down the microchips and carrier wafer to the desired thickness of 100 μm, E3215 KL tape was added to the front side for added stability and protection (Figure 1(iv))
Summary
Neural electrodes are frequently used for recording and stimulation in the central and peripheral nervous system to help physically impaired patients in their everyday life [1, 2] Improving functions of such implantable devices often results in complexity increase, as for higher spatial resolution and selectivity e.g. high-density electrode arrays are required [3]. Complex integrated circuit systems (ICs) can be realized in thin complementary metal oxide semiconductor (CMOS) microchips and transferred into flexible thin-film polymer substrates with integrated metal interconnects [4, 5]. The combination of both allows for CMOS performance with conformability of polymer-based substrates [4]. Chip-in-foil systems pose a viable solution, but additional challenges in embedding multiple microchips and their alignment arise
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