Giorgio Ferrari

Dielectric-constant measurement of thin insulating films at low-frequency by nanoscale capacitance microscopy

We demonstrate a method for quantitatively probing the local low-frequency dielectric constant of thin insulating films by nanoscale capacitance microscopy. The calibrated capacitance-distance curves are measured on the dielectric film and analyzed by using a tip-sample capacitance model here proposed. Applied to SiO2 films as small as 1×1μm2 area and 20–30nm thickness, the method gives a dielectric constant on the submicron scale in agreement with the value determined on the large scale. The observed precision is set by the capacitance noise level of the instrument and the tip radius.

Spectrum analyzer with noise reduction by cross-correlation technique on two channels

A spectrum analyzer with a sensitivity better than few pV/Hz in voltage noise measurements and better than 1 fA/Hz in current noise measurements is presented. It has two distinct and independent input amplifiers in parallel, connected to the same device under test (DUT) and is based on the suppression of their uncorrelated noises. The instrument is modular with different front-end amplifiers conceived to optimize the measurement of low impedance or high impedance DUTs. The instrument can cover 8 decades of frequency span, from 10 mHz to 1 MHz. The improvement of sensitivity with respect to a traditional system and the simplicity in the connection and biasing of the DUT makes it perfectly suited to measure ultralow noise levels in semiconductor devices, like trapping noise, shot noise associated with tunneling in fractional quantum Hall systems, 1/f and channel noise in metal–oxide–semiconductor field effect transistors operated below threshold.

Transimpedance Amplifier for High Sensitivity Current Measurements on Nanodevices

The paper presents a very high sensitivity transimpedance amplifier in standard CMOS 0.35 mum technology suited for sensing current signals from molecular and nanodevices systems. The circuit, based on an integrator followed by a differentiator configuration, features i) a low-noise time-continuous feedback loop to cope with possible standing currents from the device under test as high as few tens of nA without limiting the signal dynamic range; ii) active current-reducers to implement very high value equivalent resistances of hundreds of GOmega with high linearity irrespective to the current direction and characterized by a shot noise current level (2qI) which is, for low standing current, few orders of magnitude smaller than a physical resistor of equal value and iii) nested-Miller compensation networks to ensure strong stability over a bandwidth of few MHz. Thanks to the ability to draw large standing currents, the circuit is suitable for a use in biological systems where physiological medium is co-present. The measured input equivalent noise of 4 fA/radic(Hz) at about 100 kHz, recorded when the input dc current is lower than 10 pA, allows the chip to be used, among others, in impedance spectroscopy measurements at the nanoscale with a capability of detecting capacitance variations in sub-attofarad range to cope with the challenges of single-chip instrumentation.

Quantitative Nanoscale Dielectric Microscopy of Single-Layer Supported Biomembranes

We present the experimental demonstration of low-frequency dielectric constant imaging of single-layer supported biomembranes at the nanoscale. The dielectric constant image has been quantitatively reconstructed by combining the thickness and local capacitance obtained using a scanning force microscope equipped with a sub-attofarad low-frequency capacitance detector. This work opens new possibilities for studying bioelectric phenomena and the dielectric properties of biological membranes at the nanoscale.

Spectroscopic performance of the DePMOS detector/amplifier device with respect to different filtering techniques and operating conditions

A DePMOS structure provides detection and amplification jointly, and it is free of interconnection stray capacitance. To fully exploit the intrinsic low noise of the device an electrical model has been developed. The most relevant parameters have been measured in order to choose an adequate readout electronics. DePMOS can operate in continuous mode, i.e., without applying any clear pulse during the signal processing, and can be read out by a time-continuous shaper amplifier. An unequalled noise of 2.2 electrons rms at room temperature has been measured. In this mode DePMOS can be used, for example, as the readout device for silicon drift detectors. Anyway , DePMOS was developed to be the basic element of an active pixel sensor suitable to cope with the requirements of the XEUS Wide Field Imager. In a matrix arrangement, each pixel must be read out by a time-variant filter. A multichannel integrated shaping amplifier, based on multicorrelated double sampling, has been designed. Spectroscopic resolution obtained filtering the pixel matrix with this readout chip is in agreement with measurements in continuous mode and matches the predictions of the model presented. It has also been experimentally proved that the clear procedure does not introduce additional noise contribution, even in the very low noise range achieved. This qualifies DePMOS as a reset-noise-free device

High sensitivity noise measurement with a correlation spectrum analyzer

A spectrum analyzer with enhanced sensitivity has been built and used in noise measurements. It is based on the processing of the input signal by two independent channels in parallel and takes advantage of the incoherent property of the noise in each of the two input stages. The instrument has demonstrated an improvement in sensitivity of at least 50 dB with respect to a traditional system, and therefore can measure low input signals down to the hundred pV//spl radic/Hz range.

Wide bandwidth transimpedance amplifier for extremely high sensitivity continuous measurements

This article presents a wide bandwidth transimpedance amplifier based on the series of an integrator and a differentiator stage, having an additional feedback loop to discharge the standing current from the device under test (DUT) to ensure an unlimited measuring time opportunity when compared to switched discharge configurations while maintaining a large signal amplification over the full bandwidth. The amplifier shows a flat response from 0.6 Hz to 1.4 MHz, the capability to operate with leakage currents from the DUT as high as tens of nanoamperes, and rail-to-rail dynamic range for sinusoidal current signals independent of the DUT leakage current. Also available is a monitor output of the stationary current to track experimental slow drifts. The circuit is ideal for noise spectral and impedance measurements of nanodevices and biomolecules when in the presence of a physiological medium and in all cases where high sensitivity current measurements are requested such as in scanning probe microscopy systems.

Tracking of conduction phenomena and degradation in organic light emitting diodes by current noise measurements

Noise current analysis, both in time and frequency, is proposed as a means to sense variations of the microscopic conduction in organic light emitting diodes and to track their time evolution. The sensitivity of the technique would allow to correlate the carriers conduction properties with the corresponding changes in the microscopic morphology of the organic layers as obtained with structural or spectroscopic investigations. The method is shown to be very effective also in sensing the initial state and the growth of organic diodes catastrophic degradation in large advance to current monitoring or other techniques.

CMOS fully compatible microwave detector based on MOSFET operating in resistive regime

A microwave detector featuring full compatibility with standard CMOS process is presented. It is based on the channel resistance nonlinearity of a MOSFET operating in ohmic regime. The detecting sensitivity is shown to be tuned to below mW power by properly setting the bias voltage of the gate and of the drain of the transistor. Experiments with 180-nm gate length transistor have confirmed detecting operation up to 34GHz. The absence of additional technological steps required for the detector fabrication with respect to a standard CMOS process opens the realm of RF monitoring in products at virtually no cost.

Attofarad resolution potentiostat for electrochemical measurements on nanoscale biomolecular interfacial systems.

We present an instrument that enables electrochemical measurements (cyclic voltammetry, impedance tracking, and impedance spectroscopy) on submicrometric samples. The system features a frequency range from dc to 1 MHz and a current resolution of 10 fA for a measurement time of 1 s, giving a sensitivity of few attofarads in terms of measurable capacitance with an applied voltage of only 100 mV. These performances are obtained using a low-noise wide-bandwidth integrator/differentiator stage to sense the input current and a modular approach to minimize the effect of input stray capacitances. A digitally implemented lock-in filter optimally extracts the impedance of the sample, providing time tracking and spectroscopy operating modes. This computer-based and flexible instrument is well suited for characterizing and tracking the electrical properties of biomolecules kept in the physiological solution down to the nanoscale.