Fabrication and characterization of lon-sensitive field-effect transistors using silicon-on-insulator technology

Ion-sensitive field effect transistors (ISFETs) were reported for the first time by Bergveld in the 1970s. During the last decade, ISFETs experienced a revival on the nanometer scale as the downscaling and the simultaneous detection of multiple targets make SiNW-ISFETs a promising candidate for a cheap and multifunctional sensor.
In this dissertation the fabrication and characterization of silicon nanowire ISFETs (SiNW-ISFETs) is presented.
In chapter I, the transistor physics and the working principle of an ISFET are briefly introduced. The sensing principle of ISFETs is based on the adsorption of charged particles on the sensor surface, which lead to a change in the surface potential and thereby to a change of the transistor current.
In chapter II, we present the process flow of a SiNW-ISFET fabricated from a silicon-on-insulator (SOI) wafer with aluminum oxide or hafnium oxide as gate dielectrics. Both oxide types were grown by atomic layer deposition (ALD).
In chapter III, the characterization of fabricated SiNW-ISFETs is described, which was performed in liquid environment and in air, with respect to sensing and electrical properties, as well as noise. The fabricated SiNW-ISFETs show a nearly ideal and linear pH- response of 59.5 mV/pH at 300 K with aluminum oxide or hafnium oxide. Furthermore, a systematic study of the effect of the nanowire width on the pH-response is presented with ISFETs having SiNW widths ranging from 100 nm to 1 um. No influence of the nanowire width on the pH response was observed. A size dependence on the pH-response is not expected as long as the oxide-/electrolyte interface of the nanowire surface provides a large surface buffer capacity for protons. Apart from the sensing properties also the electrical properties are exceptionally good and reproducible. A negligible hysteresis in the transfer curves and leakage currents less than 2 nA are the result of a reliable fabrication process. Hole mobilities and dielectric constants of the gate oxides are in agreement with reported values. Furthermore, we analyzed 1/f noise in SiNW-ISFETs which were operated under different gating conditions, in order to determine the noise source. To do so, the measured source-drain current noise was converted into a gate referred voltage noise and also compared with different noise models. A constant value of the gate referred voltage noise within a wide range of parameters suggests that the noise is dominantly generated by the gate. This result was further confirmed by additional measurements of the gate referred voltage noise performed with SiNW-ISFETs having two different gate oxides but otherwise similar device parameters. The measured noise data could be described by the trap state noise model which suggests, that the source of the 1/f noise is due to trap states, residing in the gate oxide. Additionally, we determined from the noise data of a 1 um wide SiNW a sensor accuracy of 0.017 per cent of an ideal pH-response of 59.5 mV/pH. The sensor accuracy was found to be inversely proportional to the nanowire width for a constant nanowire length.
Chapter IV comprises investigations with SiNW-ISFETs having a sensor surface chemically modified with functional groups. We demonstrated that the surface functionalization enables the differential and selective detection of potassium and sodium ions and the integration of a stable reference electrode.
The results are summarized in chapter V with the conclusion that the developed sensor platform might become a future analytical sensor.