ISSCC 2013: Silicon and organic stacks for prosthesis

Through silicon vias (TSVs) represent one of the weapons in making 3DIC a reality. Although much of the attention is on mainstream applications such as lower-power memory stacks for mobile phones, TSVs are already in mass production in more specialized fields, such as image sensors. Now they are being applied to medical applications, as demonstrated by a Taiwan-based project to build a denser neural sensor for prosthesis control.

Researchers from the National Chiao Tung University, China Medical University in Taichung and ASE Group described at the International Solid State Circuits Conference (ISSCC) in San Francisco on Monday (18 February) how they used TSVs to fabricate a double-side integrated microsystem for neural sensing. The TSVs were used to provide the shortest possible connection between the microprobe array used for neural sensing and the processing electronics. The 350µm-thick design allows for further stacking using TSVs.

The CMOS circuits were made at UMC on a 0.18µm process on 200mm wafers. The TSVs were made using a via-last approach to allow the ICs to be made on a fully standard CMOS line, using solid copper TSVs applied to the front side of the wafer. A redistribution layer was applied to the front to form connections to the package. A deep etching process was used on the backside of the wafer to form the microprobe array followed by two thin-film MEMS processes to form the sensor layer. After the wafer was diced, the chips were fully encapsulated and then selectively etched to expose the platinum-tipped probe elements.

Workers at the University of Tokyo turned to a thin stack of organic materials to create a sensing sheet intended to help with prosthetic hand control and overcome the problems of conventional electrodes, which tend to be too rigid for long-term use.

However, organic electronics remains at an early stage of development and the team had to deal with a process that forces the use of very large area circuits, particularly for amplifiers, as well as mismatch problems. The process only currently allows for p-channel devices today as well, which limits the gain of amplifiers.

To work around the shortcomings of organic transistors, the researchers opted for a distributed shared amplifier architecture and a post-manufacture “select and connect” strategy to identify well-matched transistors and halve power consumption compared with a conventional parallel-transistor architecture.

The electronics were organized into two stacked sheets, one used for electrodes and the other for the amplifier array. Because of the size of the amplifiers, each one is shared between four electrodes and then addressed using a memory-like scheme of word and bit lines.