Surface Modification for High Photocurrent and pH Sensitivity in a Silicon-Based Light-Addressable Potentiometric Sensor

Abstract

A device structure made from the Si substrate etched from the smooth surface by KOH wet etching is investigated for use as a light-addressable potentiometric sensor (LAPS). Stacked Si3N4/SiO2 layers were used as the sensing membrane on the top side, and a hard mask layer generated by KOH etching comprises the bottom side of the LAPS. A single-side polished P-type Si wafer with a thickness of $350~\mu \text{m}$ was reduced to $175~\mu \text{m}$ by KOH wet etching of the polished side. A high photocurrent was observed in an LAPS sample with increased surface roughness while under backside illumination. Additionally, the increased roughness of the sensing membrane on the top side generated by the unpolished Si surface in the same sample produced higher pH sensitivity than conventional polished Si surfaces. The pH sensitivity and linearity were 45.4 mV/pH and 99.7%, respectively, for pH values ranging from 2 to 12. A 4.4-fold higher photocurrent was achieved using the thin-Si LAPS with a thickness of $175~\mu \text{m}$ measured at a light-modulated frequency of 1 kHz compared with that using a Si substrate with a thickness of $550~\mu \text{m}$ . An acceptable photocurrent was obtained with a modulated illumination frequency up to 20 kHz, which is beneficial for 2-D images acquired by high-speed scanning. This simple device structure consisting of a thin-Si LAPS increases the photocurrent and pH sensitivity. The thickness of the LAPS Si substrate could be reduced in the future to optimize the process stability and sensing performance.