Miniaturized pH‐Sensitive Field‐Effect Capacitors with Ultrathin Ta₂O₅Films Prepared by Atomic Layer Deposition

Abstract

Miniaturized electrolyte–insulator–semiconductor capacitors (EISCAPs) with ultrathin gate insulators have been studied in terms of their pH‐sensitive sensor characteristics: three different EISCAP systems consisting of Al–p‐Si–Ta2O5(5 nm), Al–p‐Si–Si3N4(1 or 2 nm)–Ta2O5 (5 nm), and Al–p‐Si–SiO2(3.6 nm)–Ta2O5(5 nm) layer structures are characterized in buffer solution with different pH values by means of capacitance–voltage and constant capacitance method. The SiO2 and Si3N4 gate insulators are deposited by rapid thermal oxidation and rapid thermal nitridation, respectively, whereas the Ta2O5 film is prepared by atomic layer deposition. All EISCAP systems have a clear pH response, favoring the stacked gate insulators SiO2–Ta2O5 when considering the overall sensor characteristics, while the Si3N4(1 nm)–Ta2O5 stack delivers the largest accumulation capacitance (due to the lower equivalent oxide thickness) and a higher steepness in the slope of the capacitance–voltage curve among the studied stacked gate insulator systems.

Multiplexing System for Automated Characterization of a Capacitive Field‐Effect Sensor Array

“…The pH sensitivity of each EISCAP, determined from the respective calibration curve, is given in Table 1. The average pH sensitivity of 16 Ta 2 O 5 -gate EISCAP sensors was 56.4 AE 1.5 mV pH À1 , which is in good agreement with the pH sensitivity of Ta 2 O 5 layers prepared by atomic layer deposition, [1,47] electron-beam evaporation [48] and thermal oxidation of sputtered Ta films. [49,50]…”

“…The determined average pH sensitivity of 56.4 AE 1.5 mV pH À1 is in good agreement with literature data for the pH sensitivity of Ta 2 O 5 layers. [1,[47][48][49][50] For the detection of diverse analytes, the mitigation of interfering influences, such as temperature, drift, or pH variations in the measurement buffer, is desirable. This can be achieved by incorporating a differential measurement setup.…”

“…The determined average pH sensitivity of 56.4 ± 1.5 mV pH −1 is in good agreement with literature data for the pH sensitivity of Ta 2 O 5 layers. [ 1,47–50 ]…”

“…The Ta 2 O 5 -gate EISCAPs were used for multiplexed pH measurements, because Ta 2 O 5 belongs to the best pHsensitive materials, exhibiting a nearly-Nernstian pH sensitivity, small drift and a low hysteresis. [1,25,27,49,60] The fabrication process steps of EISCAPs were described previously (see e.g., [7,17] ). Briefly, the Ta 2 O 5 layer (60 nm) was prepared by electron-beam evaporation of a 30 nm tantalum layer and subsequent oxidation under oxygen atmosphere at 520 °C for 60 min.…”

“…Similar to other types of field-effect biochemical sensors, EISCAPs are very sensitive to any kind of potential (or charge) changes induced via biochemical reactions at or nearby the gate-insulator/electrolyte interface. Due to the simple structure and easy fabrication, the EISCAP platform has been widely applied for designing a large variety of biochemical sensors capable of concentration measurements of ions [1][2][3] and metabolites, [4][5][6][7][8][9] as well as for the label-free detection of charged biomolecules, [10][11][12][13][14][15] virus particles, [16][17][18] carbon nanotubes, [19] and nanoparticle/molecule hybrids [20][21][22] by their intrinsic charge. Moreover, EISCAPs have been applied for the creation of biomolecular logic gates.…”