Design of a high density CMOS array for the bidirectional coupling with electrogenic cells

In this work, a high density CMOS chip for the bidirectional coupling with electrogenic cells was designed, fabricated, post-processed and successfully tested using a newly developed measurement and control system. In addition, a setup for the single cell stimulation with multi electrode arrays was developed and successfully tested with cortical rat neurons. The CMOS chip consisted of a 64 x 64 pixel array with on-chip amplifier and output buffer circuits. A calibration mechanism minimised the output signal mismatch between the different pixels that either worked in the stimulation or in the recording mode. The chip provided an overall signal gain of 80 mS which was linear for the expected input signal range of +-5 mV. The pixels had a pitch 12.5 um and the entire array could be read out with a frame rate of 3 kHz. The design and simulation of the chip was performed at the transistor level for a 0.5 um CMOS process with three metal layers using standard industry tools. In every pixel a floating-gate field-effect transistor served as sensor. It was connected with the electrolyte through a capacitor. The bottom electrode of latter was formed in the third metal layer with a 4.1 x 4.1 um opening in the final silicon nitride passivation above it. The dielectric was deposited in a post-process. Several deposition methods and material systems were tested for the post-process with a 50 nm atomic layer deposited Al2O3/HfO2 multi-layer system having the best properties. The electrochemical IV curve showed a leakage of around 35 nA/cm^2 which did not increase significantly until 6 V and the dielectric constant of 11.5 was sufficient for first experiments. After the deposition the chips were bonded and encapsulated inside a silicon glue package. This was necessary to adapt the chip to an operation in a liquid environment. A versatile measurement and control system was developed for the chip. Its central control unit was formed by a high speed sequencer combined with an advanced multitasking-capable microcontroller. The measurement parameters were sent to the controllers using a software bundle and then the system performed the required experiment autonomously. A detailed analysis of the various test structures, the sensor array as well as the amplifier and buffer circuits was performed. Pixels in the sensor array were selected, calibrated and read out successfully. Also, the measured data agreed well with the circuit simulations performed during the design process. Finally, it was shown that the Al2O3/HfO2 multi-layer was biocompatible and that neural cells grew nicely on the structured surface of the chip