O. V. Naumova

SOI nanowires as sensors for charge detection

The properties of silicon-on-insulator nanowires (SOI NWs) fabricated by means of electron lithography and gas etching of SOI in XeF2 or SF6:CFCl3 have been investigated. The method used to fabricate the nanowires was found to require no additional anneal to be given to the final structure for defect removal after nanostructuring. The sensitivity of SOI NWs to negative protein BSA molecules in the pH 7.4 buffer solution was shown to be as high as 1 femtomoles. The gate characteristics of SOI NWs were used to determine the charge density of particles adsorbed on the NW surface. A charge density of 4.6 × 1011 cm−2 was estimated for a 1 femtomole protein concentration. The combined use of open-channel structures with top gates was employed for determining the charge state of structure surfaces after different chemical treatments. Chemical treatments giving rise to a density of the negative charges on the surface of NWs ranging in the interval (7–23) × 1011 cm−2 were examined. Treatments in methanol (after removal of the native oxide) were found to provide stabilization of the SOI surface over a 3-h interval after the treatments.

SOI nanowire for the high-sensitive detection of HBsAg and α-fetoprotein

Silicon-on-isolator-nanowires (SOI-NWs) were used for the label-free, real-time biospecific detection of the hepatitis B marker HBsAg and cancer marker α-fetoprotein (AFP). Specific protein-protein recognition was carried out using individual NWs that were functionalized with antibodies. To solve the problem of non-specific binding of target protein molecules to the sensor element the use of a reference NW with immobilized antibodies against non-target proteins was proposed. Using individual SOI-NW surface functionalization allowed the fabrication of a NW array, containing working NWs and reference NWs within one chip. It was shown that this approach allows us to reach a detection limit up to 10(-14) and 10(-15) M for HBsAg and AFP, respectively. Our investigations also allowed us to reveal the influence of the charged state of the target protein molecules and antibodies in solutions with various pH values on the target protein detection limit. A high sensitivity NW-detector is of interest for the creation of diagnosticums for hepatitis B and for the early stages of cancer diseases.

Properties of silicon-on-insulator structures and devices

The physical grounds for making SOI structures by the DeleCut (ion irradiated deleted oxide cut) method are considered. This method is a modification of the commonly known Smart Cut® technique and aims at eliminating the disadvantages of the basic method [1]. The proposed method makes it possible to considerably lower the annealing temperature and the content of radiation defects in SOI structures. It allows the thickness of a split-off Si layer and a transition layer between the SOI layer and a buried oxide to be reduced. The method also reduces the nonuniformity in the thickness of the SOI layer and the insulator to several nanometers. By using DeleCut, new SOI structures were formed on wafers with diameters as large as 150 mm; the structures included dislocation-free SOI layers of 0.003–1.7 µm in thickness and a buried thermal SiO2 oxide (0.05–0.5 µm). These structures have good electrical characteristics, which is supported by fabricating the submicrometer (0.2–0.5 µm) SOI-based CMOS transistors and test integrated circuits.

Silicon nanowire transistors for electron biosensors

A method of nanostructuring of silicon-on-insulator (SOI) layers on the basis of gas etching in XeF2 or SF6:CFCl3 is developed for the purpose of obtaining SOI nanowire structures. SOI nanowire transistors (SOI NWTs) with free channels, used as sensors in electron detectors, are fabricated and tested. The results of experiments show that the method used to fabricate nanowires requires no high-temperature operations for elimination of defects after nanostructuring of SOI layers. The sensitivity of SOI NWTs to test molecules of bovine serum albumin is 10−15 mole/liter, which is one of the best results for nanowire biosensors.

SOI-nanowire biosensor for detection of D-NFATc1 protein

The nanowire (NW) detection is one of the fast-acting and high-sensitive methods, which can recognize potentially relevant protein molecules. A NW-biosensor based on the silicon-on-insulator (SOI)-structures has been used for biospecific label-free real time detection of the NFATc1 (D-NFATc1) oncomarker. For this purpose, SOI-nanowires (NWs) were modified with aptamers against NFATc1 used as molecular probes. It was shown that using this biosensor it is possible to reach sensitivity of 10−15 M. This sensitivity was comparable to that of the NW-biosensor with immobilized antibodies used as macromolecular probes. The results demonstrate that approaches used in this study are promising for development of sensor elements for high-sensitive diagnostics of diseases.

SOI nanowire transistor for detection of D-NFATc1 molecules

Nanowire (NW) detection is one of the fast and highly sensitive methods. An NW biosensor based on silicon-on-insulator (SOI) structures are used in the reported study for real-time label-free biospecific detection of the NFATc1 (D-NFATc1) cancer marker. For this purpose, the SOI NWs are functionalized with NFATc1 aptamers used as macromolecular probes. It is demonstrated that such a biosensor can ensure a detection limits up to 10−15 M, which is comparable with the sensitivity ensured by an NW biosensor with immobilized antibodies used as macromolecular probes. The results of this study demonstrate that such approaches to the development of sensor elements for highly sensitive diagnostics of diseases are really promising.

Charge state of SOI nanowire sensors

The initial charge state of SOI nanowire sensors after their long storage in air ambient has been identified by measuring the drain-gate characteristics of SOI nanowire transistors. In the examined structures, the density of the surface charge was found to be ranging within the interval 2×1012–4×1012 cm−2. Two surface treatments were given to the samples to clean the surface of SOI nanowire sensors from contaminations; among these treatments an optimal one was chosen enabling stabilization of the charge state of the sensor surface. The value of the surface charge in sensors immersed in buffer solutions with different pH values was measured.

The charge accumulation in an insulator and the states at interfaces of silicon-on-insulator structures as a result of irradiation with electrons and gamma-ray photons

The accumulation of charge in an insulator and the states at interfaces in silicon-on-insulator structures irradiated with 2.5-MeV electrons and 662-keV gamma-ray photons were studied. It was found that an additional positive charge appears in the buried insulator of the structures as a result of irradiation. The concentration of hole traps generated by radiation in the oxide is higher at the boundary with substrate than at the bonding interface between a split-off silicon layer and oxide. It is shown that the presence of even a weak built-in field in the structures (F≳5×103 V/cm) gives rise to efficient separation of charge carriers. There is no generation of additional states at the Si/SiO2 interfaces in the silicon-on-insulator structures for both irradiation types, although this generation is observed in the initial thermal oxide.

Ultra-thin SOI Layer Nanostructuring and Nanowire Transistor Formation for FemtoMole Electronic Biosensors

In this work nanostructuring of SOI layers with tenth nanometer thickness was made at last stage of electronic biochip producing using fluorine gas-plasma content etching. Electrical characterization verities that used fabrication approach produces high-quality devices operating up to temperatures close to liquid helium. The sensitivity of SOI NWs to negative ions Cl- in aqua solution (pH 6) was shown to be as high as 10 femtoMoles. The sensitivity of SOI NWs to negative protein BSA molecules in pH 7.4 buffer solution was shown to be as high as 1 femtoMoles.

Modification of silicon-on-insulator structures under nano-scale device fabrication

Based on the characterization of the ultrathin silicon-on-insulator (SOI) test structures, the effects of charge at the interface between the top silicon layer and the buried oxide (bonded interface) are studied during different technology steps (thinning of the top silicon layer and plasma etching). SOI structures were fabricated by wafer bonding and hydrogen slicing technology. An increase in the charge with the thinning of the top silicon layer and introduction of the mobile charge or the slow interface states after the plasma etching were found for the SOI structures. The conductance oscillations have been observed at room temperature in the ultrathin silicon layers due to formation of tunnel barriers for one type of carriers caused by charge fluctuation.