Implementing Silicon Nanoribbon Field-Effect Transistors as Arrays for Multiple Ion Detection

Ionic gradients play a crucial role in the physiology of the human body, ranging from metabolism in cells to muscle contractions or brain activities. To monitor these ions, inexpensive, label-free chemical sensing devices are needed. Field-effect transistors (FETs) based on silicon (Si) nanowires or nanoribbons (NRs) have a great potential as future biochemical sensors as they allow for the integration in microscopic devices at low production costs. Integrating NRs in dense arrays on a single chip expands the field of applications to implantable electrodes or multifunctional chemical sensing platforms. Ideally, such a platform is capable of detecting numerous species in a complex analyte. Here, we demonstrate the basis for simultaneous sodium and fluoride ion detection with a single sensor chip consisting of arrays of gold-coated SiNR FETs. A microfluidic system with individual channels allows modifying the NR surfaces with self-assembled monolayers of two types of ion receptors sensitive to sodium and fluoride ions. The functionalization procedure results in a differential setup having active fluoride-and sodium-sensitive NRs together with bare gold control NRs on the same chip. Comparing functionalized NRs with control NRs allows the compensation of non-specific contributions from changes in the background electrolyte concentration and reveals the response to the targeted species

Competing surface reactions limiting the performance of ion-sensitive field-effect transistors

© 2015 Elsevier B.V. All rights reserved.Ion-sensitive field-effect transistors based on silicon nanowires are promising candidates for the detection of chemical and biochemical species. These devices have been established as pH sensors thanks to the large number of surface hydroxyl groups at the gate dielectrics which makes them intrinsically sensitive to protons. To specifically detect species other than protons, the sensor surface needs to be modified. However, the remaining hydroxyl groups after functionalization may still limit the sensor response to the targeted species. Here, we describe the influence of competing reactions on the measured response using a general site-binding model. We investigate the key features of the model with a real sensing example based on gold-coated nanoribbons functionalized with a self-assembled monolayer of calcium-sensitive molecules. We identify the residual pH response as the key parameter limiting the sensor response. The competing effect of pH or any other relevant reaction at the sensor surface has therefore to be included to quantitatively understand the sensor response and prevent misleading interpretations

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

A Verilog-A model for silicon nanowire biosensors : from theory to verification

Silicon nanowires offer great potential as highly sensitive biosensors. Since the signals they produce are quite weak and noisy, the use of integrated circuits is preferable to read out and digitize these signals as quickly as possible following the sensing event to take full advantage of the properties of the nanowires. In order to design optimized and tailored circuits, simulations involving the sensor itself in the design phase are needed. We propose here a Verilog-A model for silicon nanowire-based biosensors. The model can easily be applied using commercially available electronic design automation (EDA) tools that are commonly used for integrated circuit design and simulations. The model is quite general and comprehensive; it can be used to simulate different types of sensing events, while still being quite simple and undemanding in terms of computational power. The model is described in detail and verified with measurements from two different nanowire sensors featuring aluminum-oxide and hafnium-oxide coatings. Good agreement has been achieved in all cases, with errors never exceeding 21%

Label-Free FimH Protein Interaction Analysis Using Silicon Nanoribbon BioFETs

The detection of biomarkers at very low concentration and low cost is increasingly important for clinical diagnosis. Moreover, monitoring affinities for receptor-antagonist interactions by time-resolved measurements is crucial for drug discovery and development. Biosensors based on ion-sensitive field-effect transistors (BioFETs) are promising candidates for being integrated into CMOS structures and cost-effective production. The detection of DNA and proteins with silicon nanowires has been successfully demonstrated using high affinity systems such as the biotin-streptavidin interaction. Here, we show the time-resolved label-free detection of the interaction of the bacterial FimH lectin with an immobilized mannose ligand on gold-coated silicon nanoribbon BioFETs. By comparing our results with a commercial state of the art surface plasmon resonance system, additional surface effects become visible when using this charge based detection method. Furthermore, we demonstrate the effect of sensor area on signal-to-noise ratio and estimate the theoretical limit of detection

Silver-epoxy microwave filters and thermalizers for millikelvin experiments

Peer reviewe

Competing surface reactions limiting the performance of ion-sensitive field-effect transistors

Ion-sensitive field-effect transistors based on silicon nanowires are promising candidates for the detection of chemical and biochemical species. These devices have been established as pH sensors thanks to the large number of surface hydroxyl groups at the gate dielectrics which makes them intrinsically sensitive to protons. To specifically detect species other than protons, the sensor surface needs to be modified. However, the remaining hydroxyl groups after functionalization may still limit the sensor response to the targeted species. Here, we describe the influence of competing reactions on the measured response using a general site-binding model. We investigate the key features of the model with a real sensing example based on gold-coated nanoribbons functionalized with a self-assembled monolayer of calcium-sensitive molecules. We identify the residual pH response as the key parameter limiting the sensor response. The competing effect of pH or any other relevant reaction at the sensor surface has therefore to be included to quantitatively understand the sensor response and prevent misleading interpretations

NON-UNIFORM POROUS BEARING OF CHANGING THICKNESS AND OF FINITE LENGTH WITH LUBRICATION THROUGH PORES OF SHELL

There the mathematical simulator for prediction of operation of non-uniform porous bearing of changing thickness in the radial and axial lubrication has been obtained. The construction of cermet shell which has the risen load-carrying capacity has been developed. The bearing has been brought in the Donetsk Pilot Works of Precision Equipment. The bearings to be offered may be used in the friction units of machines in loads up to 7,5 MPa over 2 - 8 m/s range of speeds, the mathematical simulators may be used in designsAvailable from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio

Investigation of the dominant 1/f noise source in silicon nanowire sensors

We analyzed 1/f noise in silicon nanowire ion-sensitive field-effect transistors (SiNW-ISFETs) having different wire widths ranging from 100 nm to 1 pin and operated under different gating conditions in order to determine the noise source and the sensor accuracy. We find that the gate-referred voltage noise S-VG (power spectral density) is constant over a large range of SiNWs resistances tuned by a DC gate voltage. The measurements of S-VG for SiNWs with two different gate-oxide thicknesses, but otherwise similar device parameters, are only compatible with the so-called trap state noise model in which the source of 1/f noise is due to trap states residing in the gate oxide (most likely in the interface between the semiconductor and the oxide). S-VG is found to be inversely proportional to the wire width for constant wire length. From the noise data we determine a sensor accuracy of 0.017% of a full Nernstian shift of 60 mV/pH for a SiNW wire with a width of 1 pm. No influence of the ions in the buffer solution was found. (C) 2013 Elsevier B.V. All rights reserved