Anne-Claire Salaün

Step-gate polysilicon nanowires field effect transistor compatible with CMOS technology for label-free DNA biosensor

Currently, detection of DNA hybridization using fluorescence-based detection technique requires expensive optical systems and complex bioinformatics tools. Hence, the development of new low cost devices that enable direct and highly sensitive detection stimulates a lot of research efforts. Particularly, devices based on silicon nanowires are emerging as ultrasensitive electrical sensors for the direct detection of biological species thanks to their high surface to volume ratio. In this study, we propose innovative devices using step-gate polycrystalline silicon nanowire FET (poly-Si NW FETs), achieved with simple and low cost fabrication process, and used as ultrasensitive electronic sensor for DNA hybridization. The poly-SiNWs are synthesized using the sidewall spacer formation technique. The detailed fabrication procedure for a step-gate NWFET sensor is described in this paper. No-complementary and complementary DNA sequences were clearly discriminated and detection limit to 1 fM range is observed. This first result using this nano-device is promising for the development of low cost and ultrasensitive polysilicon nanowires based DNA sensors compatible with the CMOS technology.

Fabrication of polycrystalline silicon nanowires using conventional UV lithography

Silicon nanowires are processed by using the sidewall spacer formation technique. This technique uses craftily a drawback of anisotropic etching to go beyond optical limits with conventional UV lithography for precision patterns. The final width of the spacer is controlled by the steepness of the etching side and by the uniformity of the wall recovering layer. In our process, a polysilicon layer is deposited by low pressure chemical vapour deposition technique on SiO2 wall network patterned by conventional UV lithography technique. Accurate control of the etching rate of the polysilicon leads to the formation of nanometric size sidewall spacers with a curvature radius below 100nm. Networks of such parallel polysilicon nanowires were electrically tested in function of temperature (530K 300K) with thermal activation EA ~ 0.3 eV

Polycrystalline silicon thin films for MEMS applications

Abstract Thanks to its interesting mechanical and electrical properties, silicon represents the first candidate as a structural material in the Micro Electro Mechanical Systems field. Doped polycrystalline silicon films are generally used, particularly when electrostatically movable mechanical structures are needed. Here, we investigate the effects of the doping type as well as the post-deposition thermal treatments on the mechanical behaviour of in situ doped polycrystalline silicon films deposited by low pressure chemical vapour deposition from a mixture of silane SiH4 and phosphine or diborane. Stress measurements, performed using micro-Raman spectroscopy, are related to the behaviour of micro fixed–fixed beams as determined from optical and scanning electron microscopy observations. The films, regardless of their doping type, are found tensely stressed when the amorphous deposited films are solid phase crystallised at 600 °C. The tensile stress is reduced becoming compressive when the crystallization temperature is increased. An optimum tensile stress value, corresponding to the maximum of the beam free length, is determined. Finally, air-gap thin film transistors (TFTs) using these doped fixed–fixed beams are realised. Electrical parameters of these TFTs (field effect mobility, threshold voltage, and subthreshold slope) may be considered as good. Particularly the low value of the threshold voltage, 2.5 V, is very interesting for handling devices where the power consumption saving is crucial.

Fabrication and electrical characterization of silicon nanowires based resistors

Silicon nanowires (SiNWs) are synthesized via the Vapor-Liquid-Solid (VLS) mechanism using gold (Au) as metal catalyst and silane (SiH4) as precursor gas. Au nanoparticles are employed as liquid droplets catalysis during the SiNWs growth performed in a hot wall LPCVD reactor at 480°C and 40 Pa. SiNWs local synthesis at micron scale is demonstrated using classical optical photolithography process. SiNWs grow with high density anchored at the dedicated catalyst islands. This resulting network is used to interconnect two heavily doped polysilicon interdigitated electrodes leading to the formation of electrical resistors in a coplanar structure. Current-voltage (I-V) characteristics highlight a symmetric shape. The temperature dependence of the electrical resistance is activated, with activation energy of 0.47 eV at temperatures greater than 300K.

Transferrin Electronic Detector for Iron Disease Diagnostics

Field effect transistor (FET) structure with suspended bridge shows the ability to detect proteins with very high sensitivity. This sensor is tested for the transferrin, a specific blood plasma protein, relevant in iron disease diagnostic. This system is based on field effect transistor with a suspended bridge used as a gate electrode. The sensitive layer is made of silicon nitride as in the ISFET (ion sensitive field effect transistor) technology. The surface micro-technology ensures a small height suspended-bridge (0.5 mum) composed of poly-silicon insulated with silicon nitride. To improve sensitivity and to limit biofouling, proteins are selected by antibodies covalently bound to organic layers. The specific charge of the fixed transferrins translates the transistor transfer characteristics. For low concentration, resulting shift is important and confers a good sensitivity to our electronic transducer.

Highly sensitive suspended-gate ion sensitive transistor for the detection of pH

This paper presents a new device for the pH detection. It is based on a suspended polysilicon gate field effect transistor (SGFET). The sensitive layer is made of silicon nitride as for ISFET technology. The suspended bridge, used as gate electrode, is formed with doped polysilicon covered with silicon nitride layers for electrical insulation. The layers are deposited by Low Pressure Chemical Vapor Deposition (LPCVD). Surface micro-technology allows to obtain a small height (0.5μm) suspended-bridge. In this case, the solution penetrates under the gate. The high field effect in the gap between the gate and the channel is enough to change the charges distribution. Very high pH sensitivity, greater than 200 mV/pH, is found with this new structure and it is much higher than the usual Nernstian sensitivity of ISFETs. The device concept, electrical characteristics, and the effect of the thickness of the gap between the bridge and the sensitive layer on the pH sensitivity are discussed in this study.

Low Concentrated DNA Detection by SGFET

In this work a device based on oligonucleotide hybridization charged detection is presented. This biosensor is a SGFET (suspend gate field effect transistor), it is based on a MOSFET (metal oxide semiconductor field effect transistor) structure whose gate is suspended. The insulating layer (empty gap) is chemically modified by immobilized molecular receptors that enable miniaturized label free DNA detection. This method allows direct, time-resolved and in situ detection of specific nucleic acid binding. By the use of SGFET, the immobilization of DNA sequences brings negatives charges due to their phosphate groups under the gate and induce large shift of transfer characteristic. One of the applications of this device can be clinical testing.

Direct Electrical Detection of Biological Species

Suspended-Gate FETs, namely SGFET, with sub-micron gap, is shown to be able to detect biological species with very high sensitivity in a large range of concentration. Examples of detection of DNA (through the DelF508 mutation of the cystic fibrosis gene that is widely disseminating in Europe and North America, and one mutation of BRCA1 gene that is the main indication of the possibility for a woman to have breast cancer) and proteins (through the transferrin that is the only carrier of iron in blood) are presented. This very sensitive electrical detection without any labelling is shown to deliver directly readable signal.

Suspended-gate thin film transistor as highly sensitive humidity sensor

The paper deals with a very high sensitive integrated humidity sensor compatible with CMOS technology. This sensor is a polysilicon Suspended Gate Thin Film Transistor (SGTFT), fabricated using a low temperature surface micromachining process. Microtechnology technics using sacrificial layer are used to fabricate polysilicon bridge which acts as the transistor gate. Transistors are characterized at various humidity rates and transfer characteristics show highly sensitive dependence with humidity. The small air-gap (0.5 μm) between the gate and the channel explains the amplifying effect of the sensitivity: threshold voltage shift is more than 17V when the humidity ratio varies from 20 to 70%.

High performance polysilicon air-gap thin film transistor on low temperature substrates

Three-dimensional polysilicon microstructures, as cantilevers or bridges, give new opportunities to increase the application field of polycrystalline silicon. Polysilicon devices with new functions can be imagined. Air-gap thin film transistors can be built from polysilicon bridges for example. They use a highly doped polysilicon bridge as gate; the gate insulator is the air or any other ambience. Then, these devices can be used as ambience sensors including chemical or biological sensors. They are very interesting as they are deposited on any substrate. Moreover, their main interest is their full compatibility with the microelectronics technology on glass performed at temperature lower than 600°C and developed first for the flat panel displays addressing. Fully integrated sensing systems are then imaginable. Here we present a low temperature process (< 600 °C) to built air-gap-polysilicon-thin-film-transistor with high mechanical and electrical performance. The transistor-active-layer is made of Low Pressure Chemical Vapor Deposition (LPCVD) undoped polysilicon while the suspended gate is made of LPCVD highly doped polysilicon. Deposition and annealing conditions of the doped polysilicon has first optimized to obtain high mechanical performance for the polysilicon micro-bridges. Hence, an optimal tensile stress to obtain longer bridges has been found. The mechanical stress has been determined from micro-Raman analysis. Then, air-gap transistors with 500 nm thick air-gap as gate insulator have been designed and fabricated. The transistor channel is 7-μm long and 7-μm wide, with a 15-μm long and 10-μm wide bridge. Their subthreshold slope, threshold voltage, and field effect mobility are 0.22 V/dec, 2.4 V and 130 cm2/V.sec respectively. These parameters are comparable to that of the best polysilicon thin film transistors with thin silicon dioxide (100 nm) as gate insulator. Then, these transistors are interesting as electronic devices firstly. Moreover they can be involved in ambience sensors. Indeed, they are checked as oxygen sensor in a preliminary experiment. Transistor characteristics are measured in air ambience, then in oxygen and finally in air. The electrical parameters have shown a considerable dependence on the ambience with an excellent recovering after removing oxygen.