Analysis of the passivation layer by testing and modeling a cell impedance micro-sensor

This paper describes the fabrication and experimental results of a microchip for cell impedance spectroscopy. A model with parameters extracted from the measurement is used to investigate the influence of the passivation layer on the impedance measurement sensitivity. The 2 mm square chip fabricated with a simple two-mask process contains a sensor with a 1 mm square sensing window with 20 sensing micropatterned electrodes. The chip successfully monitored the growth of MDA-MB-231 human breast cancer cells and recorded the impedance from 1 kHz to 1 MHz and demonstrated a peak frequency of about 100 kHz in the impedance spectrum. Thorough analysis of the result revealed that the passivation layer, which is typically used to reduce the sensing window size, results in a drop in sensitivity of 13%. This observation is consistent with the simulation results based on parameters extracted from measurements. Thus, the passivation layer needs to be provided special consideration in future designs of the sensor as it can modify the response of the sensor, in order to alleviate its detrimental effect.