Poornika G. Fernandes

Control and stability of self-assembled monolayers under biosensing conditions

The ability to stabilize and control the attachment of cells on the surfaces of a variety of inorganic materials is important for the development of biomedical devices and sensors. An important intermediate step is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with proteins. The stability of such SAMs is critical to withstanding subsequent processing and measurement conditions (e.g. long exposure to a buffer solution) to avoid artifacts resulting from such deterioration during electrical measurements. This work highlights the importance of surface cleaning and SAM chain length by comparing two commonly used short alkyl chains, aminopropyltriethoxysilane (APTES) or 3-(trimethoxysilyl)propyl aldehyde (C4-ald) molecules, with their long-chain counterparts, amino-undecilenyltriethoxysilane (AUTES) and 11-(triethoxysilyl)undecanal (C11-ald). Using IR spectroscopy, spectroscopic ellipsometry, and electrical measurements, a cleaning method is developed, based on a short room temperature (RT) SC-1 treatment, to remove photoresist without degrading device performance as is the case with currently used oxygen plasma methods. The spectroscopic and electrical measurements also show that short-chain SAMs, typically used for pH- or bio-sensing, do not have the stability suitable for biosensor environments. In contrast, long-chain SAMs display much higher stability and can be reproducibly grafted. These findings are the basis for a reliable preparation and robust operation of biosensors.

One-step selective chemistry for silicon-on-insulator sensor geometries.

A one-step functionalization process has been developed for oxide-free channels of field effect transistor structures, enabling a self-selective grafting of receptor molecules on the device active area, while protecting the nonactive part from nonspecific attachment of target molecules. Characterization of the self-organized chemical process is performed on both Si(100) and SiO(2) surfaces by infrared and X-ray photoelectron spectroscopy, atomic force microscopy, and electrical measurements. This selective functionalization leads to structures with better chemical stability, reproducibility, and reliability than current SiO(2)-based devices using silane molecules.

Comparison of Methods to Bias Fully Depleted SOI-Based MOSFET Nanoribbon pH Sensors

The potential and electric field boundary conditions for the Gouy-Chapman model of the electrolyte diffuse layer are used to properly couple the potentials in the silicon-on-insulator-based metal-oxide-semiconductor field-effect transistor to the electrolyte. This analysis is possible because the active silicon channel is fully depleted. Both the subthreshold and linear regimes are simulated. An operation with electrolyte floating and bias applied to the substrate is compared with the other methods of biasing the sensor.

Effect of mobile ions on ultrathin silicon-on-insulator-based sensors

The presence of mobile Na+ and K+ ions in biological solutions often lead to instabilities in metal-oxide-semiconductor devices and is therefore an important consideration in developing sensor technologies. Permanent hysteresis is observed on silicon-on-insulator field-effect-transistors based sensors after exposure to Na+-based buffer solutions but not after exposure to K+-based solutions. This behavior is attributed to the difference in mobilities of the ions in silicon dioxide. Mobile charge measurements confirm that ions can be transferred from the solution into the oxide. Self-assembled monolayers are shown to provide protection against ion diffusion, preventing permanent hysteresis of the sensors after exposure to solutions.

Lithographically Defined Si Nanowire Field Effect Transistors for Biochemical Sensing

A process integration of e-beam lithography, plasma etching, and Si processing have been developed to pattern Si nanowires on crystalline Si on insulator wafers. Si nanowires of 12-50 nm linewidth, 30-70 nm height, and 10 mum length have been made. Using these Si nanowires as conducting channels, field effect transistors using the back Si substrate as gate have been fabricated. Good I-V characteristics have been obtained. With the back-gate configuration, the surface of Si nanowires can be functionalized for biochemical sensing applications.

Noise effects in field-effect transistor biological sensor detection circuits

Affinity-based biological sensor field-effect transistors (BioFETs) exhibit a large amount of noise in their drain current under constant bias. In this work, we use SPICE to simulate the effect of sensor noise on a differential pair amplifier detection circuit. This is accomplished by the generation of a realistic noise signal with 1/f power spectrum which is applied to the back gate and reference electrode of a nanoribbon BioFET sensor. The resulting output signal from the transient simulation is time-averaged to obtain the noise rms amplitude. The ability to distinguish the sensor signal from the noisy environment is examined for various amounts of noise power and integration time. Corresponding minimum theoretical direct detection limits for of Biotin-Streptavidin attachment and DNA hybridization are also provided, along with discussion of possible methods to reduce the noise effects.

Impact of back-gate biasing on ultra-thin silicon-on-insulator-based nanoribbon sensors

In this study, we demonstrated that electrolytes in contact with SOI-based back-gated sensors are complexely coupled to applied back gate biases. Because to this, the liquid voltage modulates the top channel and controls the operating point of the device. This dual gating behavior has strong implications on the performance of both nanoribbon and nanowire sensors.

Effects of fluid media on ultra-thin SOI based pH sensors

For over 30 years, field effect transistors (FETs) have been used as ion sensors [1]. Recently, functionalized silicon nanowires have been used to detect a variety of species including proteins and single viruses [2]. In addition to the specific design of the device, careful consideration must be given to the properties of the liquid media and its interaction and coupling effects with the FET. This is paramount to the resolution of the “true” sensing signal from that of other factors, and is important not only for pH sensors, but also for bio-sensors. Some of these effects, such as screening due to salt concentrations, changes in sensitivity of devices to dielectric constants [3] and signal to noise ratios [4] have been considered. In this work, we explore other important factors including ionic response to applied voltages, salt contamination of oxides, bias dependence of signal drift and hysteresis in the presence of fluid.

2009 International Semiconductor Device Research Symposium, ISDRS '09

For over 30 years, field effect transistors (FETs) have been used as ion sensors [1]. Recently, functionalized silicon nanowires have been used to detect a variety of species including proteins and single viruses [2]. In addition to the specific design of the device, careful consideration must be given to the properties of the liquid media and its interaction and coupling effects with the FET. This is paramount to the resolution of the “true” sensing signal from that of other factors, and is important not only for pH sensors, but also for bio-sensors. Some of these effects, such as screening due to salt concentrations, changes in sensitivity of devices to dielectric constants [3] and signal to noise ratios [4] have been considered. In this work, we explore other important factors including ionic response to applied voltages, salt contamination of oxides, bias dependence of signal drift and hysteresis in the presence of fluid.