Verlag des Forschungszentrums Jülich
JUEL-4264
Schindler, Mathias
Design of a High Density CMOS Array for the Bidirectional Coupling with Electrogenic Cells
122 S., 2008
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 ±5mV. The pixels had a pitch 12.5 μm 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 μm
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 μm 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/cm2 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 multitaskingcapable
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 multilayer
was biocompatible and that neural cells grew nicely on the structured surface of the
chip.
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