F. Le Bihan

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.

N-TYPE POLYCRYSTALLINE SILICON FILMS OBTAINED BY CRYSTALLIZATION OF IN SITU PHOSPHORUS-DOPED AMORPHOUS SILICON FILMS DEPOSITED AT LOW PRESSURE

The low‐pressure chemical‐vapor deposition of phosphorus‐doped silicon film on glass at 550 °C was investigated as a function of silane pressure (1–100 Pa) and phosphine/silane mole ratio ranging between 4×10−6 and 4×10−4. At this low temperature the film is homogeneous in thickness and the silicon is amorphous except for low pressure (1 Pa). Phosphorus concentration varies linearly with mole ratio in amorphous deposited films. The resistivity of films annealed at 600 °C decreases while the incorporation of phosphorus (mole ratio) increases, and varies with phosphorus concentration from 101 to 10−3 Ω cm. For the same phosphorus content, the resistivity is lower if the silicon film is amorphous deposited and subsequently crystallized, than if the film is polycrystalline deposited. Carrier concentration and mobility are measured using the Hall method. Doping efficiency and electrical properties are discussed.

pH sensing from frequency response of SGFET

This paper presents suspended gate field effect transistors (SGFET) used as highly sensitive pH sensors. The devices are there characterized in dynamic mode. The gain, measured versus frequency, is studied for different pH values and shows a resonance frequency depending on the pH value. These results, obtained with different geometries of SGFET, give an opportunity to develop new microsystems, CMOS compatible, highly sensitive to pH.

Electrical detection of very low content of transferrin in view of iron metabolism characterization

Suspended-gate FETs, namely SGFET, with sub-micron gap are used to detect electrically transferrin concentration directly without any labelling. The fabrication of the device and the process to functionnalize it, so that it will be able to detect transferrin, are given. The feasibility of the detection is demonstrated and a range of detectable concentrations is determined. Concentration, as low as 100 ng/mL, is measured. The maximum concentration in the linear regime, 5 mug/mL, is under the range of the standard clinical techniques. The fabrication of the device is compatible with usual microelectronic tools. Its functionalization can be easily implemented in any chemical environment. Present results open the way to simultaneous detection of many proteins giving in real time indication on complicated biologic functions as liver metabolism.

Development of suspended gate field effect transistors array-based microsystem for pH measurements

This work presents the specifications of a microsystem which allows pH electrical measurements of solution especially for water monitoring applications. The architecture of this microsystem consists in SGFET (Suspended gate Field Effect Transistor) array-based, control and processing modules. The array is composed of 3 × 3 identical SGFET sensors. Each SGFET can be addressed independently to increase the accuracy and the reliability of the global measurements.

Magnetic sensors with polysilicon TFTs

This paper deals with magnetic position sensors compatible with large area electronics using polycrystalline silicon deposited by a low-pressure chemical vapour deposition (LPCVD) technique. The principle of this large area position sensor is a matrix of thin film field effect transistors (TFT) with two additional Hall probes. The performances of the TFT based cells are linked to the crystalline quality of the active polysilicon layer, which depends on the deposition conditions and on the technological process. Layers are made from two precursor gases, silane or disilane and two processes. We have compared the sensitivity (absolute or relative) of devices and measured their power consumption. Sensors made from disilane have a sensitivity of 18 mV/T, and the ones made with a monolayer process a sensitivity of 28 m V/T. We propose a simple model, which describes the bias dependency of the sensitivity. The offset voltage is also studied in order to determine the role of geometry and of the layer morphology.

Electrical quantification of low concentrations of specific protein in complex biological solutions

A sensitive immunosensor based on charge detection is developed for proteins. Its aim is to select and quantify low protein concentrations in a complex environment. This highly sensitive sensor is a SGFET (Suspended Gate Field Effect Transistor) whose gate is suspended at 500 nm high above the silicon channel and embedded in silicon nitride. This electrical insulation and the general design of the sensor allow to graft biomolecules and to experiment in liquids and particularly in complex biological solutions. To prove its efficiency, a blood plasma protein is used as an example : the transferrin. Specific antibodies are covalently immobilized under the gate by a new highly reproducible chemical functionalization. SGFET is duplicated in a 3x3 matrix that permits to produce statistical results and to increase reliability. Moreover, our device can detect and quantify a specific protein, here transferrin with 10 to 100 more sensitivity than current clinical tests in a complex solution.