Enakshi Bhattacharya

Physical Model for the Resistivity and Temperature Coefficient of Resistivity in Heavily Doped Polysilicon

One of the key benefits of using polysilicon as the material for resistors and piezoresistors is that the temperature coefficient of resistivity (TCR) can be tailored to be negative, zero, or positive by adjusting the doping concentration. This paper focuses on optimization of the boron doping of low-pressure chemical vapor deposited polysilicon resistors for obtaining near-zero TCR and development of a physical model that explains quantitatively all the results obtained in the optimization experiments encompassing the doping concentration ranges that show negative, near-zero, and positive TCR values in the polysilicon resistors. The proposed model considers single-crystal silicon grain in equilibrium with amorphous silicon grain boundary. The grain boundary carrier concentration is calculated considering exponential band tails in the density of states for amorphous silicon in the grain boundaries. Comparison of the results from the model shows excellent agreement with the measured values of resistivity as well as TCR for heavily doped polysilicon. It is shown that the trap density for holes in the grain boundary increases as the square root of the doping concentration, which is consistent with the defect compensation model of doping in the amorphous silicon grain boundaries

Estimation of Stiction Force From Electrical and Optical Measurements on Cantilever Beams

In this paper, we estimate the stiction force from electrical (current-voltage) measurements on surface micromachined polysilicon cantilever beams. A bias voltage was applied between the beam and the substrate. At the pull-in voltage, the beam collapsed to the substrate and the current rose rapidly from zero. Similarly, at the pull-out voltage during bias sweep back, the current dropped rapidly to zero when the contact between the beam and the substrate was broken. An analytic model for the stiction force was developed in terms of the pull-in and pull-out voltages and was used to estimate a stiction force of about 70 nN from the measured electrical characteristics. This method of characterization is suitable for use in packaged devices. An analytic model was developed to estimate stiction force from optical surface-profile measurements of the curvature of long collapsed cantilever beams in a cantilever-beam array, in the absence of any electrostatic actuation. The force per unit length of about 14 nN/m thus obtained was used to compare the effects of surface roughness on stiction.

Comparison of a potentiometric and a micromechanical triglyceride biosensor

Sensitive biosensors for detection of triglyceride concentration are important. In this paper we report on two types of silicon based triglyceride sensors: an electrolyte-insulator-semiconductor capacitor (EISCAP) which is a potentiometric device and a polysilicon microcantilever. The detection principle for both sensors is based on the enzymatic hydrolysis of triglyceride though the sensing mechanisms are different: electronic for the EISCAP and mechanical for the microcantilever. The characteristics and performances of the two sensors are critically compared. The EISCAP sensor necessitates the presence of a buffer for stable measurements which limits the sensitivity of the sensor at low concentrations of the bioanalyte to 1mM. The cantilever sensor works without a buffer which improves the lower level of sensitivity to 10 microm. Both sensors are found to give reproducible and reliable results.

MEMS Composite Porous Silicon/Polysilicon Cantilever Sensor for Enhanced Triglycerides Biosensing

A novel composite porous silicon/polysilicon microcantilever for biosensing applications with enhanced sensitivity is reported. It is fabricated by surface micromachining of polysilicon cantilevers followed by the formation of the surface porous layer after release by Reaction Induced Vapor Phase Stain Etch. The microcantilevers with porous surface layer are characterized by their morphology that exhibits a dual macro and nanostructure for very effective immobilization of biomolecules. The current work focuses on the fabrication of composite porous silicon/polysilicon microcantilevers, characterization of their morphology and demonstration of improved immobilization of enzymes resulting in enhanced sensing of triglycerides.

Enhancement of the sensitivity of pressure sensors with a composite Si/porous silicon membrane

Porous silicon (PS) has many interesting and unique properties that make it a viable material in the field of MEMS. In this paper, we investigate the application of PS in improving the sensitivity of bulk micromachined piezoresistive pressure sensors. The property of a low Youngs modulus of PS and its dependence on porosity have been exploited to obtain a higher sensitivity compared to pressure sensors with single crystalline silicon membranes. Simulation was carried out to represent the Si/PS composite membrane by two layers with Youngs modulus corresponding to silicon and PS. The behavior of this membrane was studied for various values of porosity and thickness of the PS layer. Composite Si/PS membranes were fabricated by converting a part of the silicon membrane thickness into PS by electrochemical etching in an HF-based electrolyte. Polysilicon piezoresistors were formed on the membrane in the form of a Wheatstone bridge for the measurement of sensitivity. When compared to membranes of silicon, the sensitivity of the composite Si/PS membrane is found to be higher showing improvement with an increase in the porosity and thickness of the PS layer.

Parameter extraction from simple electrical measurements on surface micromachined cantilevers

In surface micromachined structures, many parameters like geometry and Youngs modulus depend on the process steps and need to be measured for accurate prediction of their functionality. This work discusses simple electrical measurement techniques on surface micromachined cantilever beams to determine Youngs modulus, the gap between the beam and the substrate, and the thickness of a deposited aluminum layer on the beam. Cantilevers are ubiquitous in most microelectromechanical system (MEMS) sensors and actuators, and hence are ideal test structures. Pull-in, and a novel resonance frequency measurement based on the pull-in technique, are done on oxide anchored doped polysilicon beams at the wafer level, and some of the device and material properties are extracted from these measurements. The extracted values are compared with those determined from established methods like vibrometry and surface profiler measurements, and show good agreement. Since the measurements are all electrical, they can be part of standardized testing and are also suitable for packaged devices.

Miniaturised silicon biosensors for the detection of triglyceride in blood serum

This paper reports on the design and fabrication of electrolyte insulator semiconductor capacitor (EISCAP) devices to detect triglycerides in the form of microreactors fabricated by bulk micromachining of silicon. We have developed a complete triglyceride biochip wherein the enzyme for hydrolysis and the counter electrode for signal transduction are integrated with a miniaturised EISCAP sensor. A compact readout system that measures the triglyceride concentration in blood serum under test has been developed and is implemented in a Programmable System on Chip (PSoC®). The miniaturised EISCAP devices are tested using blood serum samples to estimate the triglyceride concentration within the clinical range of 50 to 150 mg dL−1 and the time taken by the readout system to calibrate the sensor and to measure the triglyceride is less than 5 minutes.

Studies on varying n-alkanethiol chain lengths on a gold coated surface and their effect on antibody–antigen binding efficiency

Self-assembled monolayer (SAM) of n-alkanethiols of different chain lengths (n = 2, 3, 6, 11, 16) on a gold surface are used to immobilize antibodies which in turn bind to a antigen. The antibody and antigen used in this study have similar molecular weights i.e. ∼150 kDa. The antibody [1.5 μg cm−2] immobilized varied with the surface packing density of the SAM of carboxylic acid-terminated n-alkanethiols of different lengths. In comparison, the efficiency of antibody immobilization was lowest on the loosely packed SAM of n-alkanethiols (n ≤ 3) and the highest on the densely packed SAM of n-alkanethiols (n ≥ 11). However, increased immobilization of antibodies with increasing chain length of the n-alkanethiols [n > 11], did not result in a corresponding increase in antigen binding. An attempt to explain this phenomenon based on packing density and an orientation of the captured antibody is presented.

Knudsen force based MEMS structures

Knudsen forces are gas molecular forces which originate from the differential temperatures in rarefied gases. We report measurements of these forces at normal ambience on test structures made by surface micromachining of polysilicon. Using these results, a surface micromachined Knudsen vacuum sensor has been simulated, fabricated and characterized. The vacuum sensor has an area of 1 mm2. The fabricated device has a sensitivity of 40 fF Pa−1 in the pressure range of 0.1–10 Pa. The measured data is analysed and the magnitude of the Knudsens force is extracted. The paper also suggests ways to enhance the range and improve the sensitivity of such sensors.

A Miniaturized pH Sensor With an Embedded Counter Electrode and a Readout Circuit

Electrolyte insulator semiconductor capacitors (EISCAPs) show a shift in the measured capacitance-voltage (C-V) characteristics with changes in the pH of the electrolyte and has the potential to be used as biosensors. The choice of an electrode to the EISCAP is important for reliable measurements. Here, we discuss a silicon-based EISCAP sensor bonded to a glass wafer with an embedded electrode. Three noble metal electrodes (Pt, Au, Ag) are studied for the ease of integration and performance and it is found that the chloridized Ag electrodes exhibit the highest pH sensitivity and the lowest electrode potential drift with time. A readout system that measures the pH of the electrolyte under test is developed and implemented in a programmable system on chip. Calibration of the EISCAP to account for sensor process variations is also incorporated. The pH measurement data on the miniaturized EISCAPs is presented.