A. Nascetti

Counting and integrating readout for direct conversion X-ray imaging concept, realization and first prototype measurements

A novel signal processing concept for X-ray imaging with directly converting pixelated semiconductor sensors is presented. The novelty of this approach compared to existing concepts is the combination of charge integration and single photon counting in every single pixel. Simultaneous operation of both signal processing chains extends the dynamic range beyond the limits of the individual schemes and allows determination of the mean photon energy. Medical applications such as X-ray computed tomography can benefit from this additional spectral information through improved contrast and the ability to determine the hardening of the tube spectrum due to attenuation by the scanned object. A prototype chip in 0.35-micrometer technology was successfully tested. The pixel electronics are designed using a low-noise differential current mode logic and provide configurable feedback modes, leakage current compensation and various test circuits. This paper will discuss measurement results of the prototype structures and give details on the circuit design

Hydrogenated amorphous silicon ultraviolet sensor for deoxyribonucleic acid analysis

In this letter, we show the results achieved using a hydrogenated amorphous silicon photosensor for a “label-free” deoxyribonucleic acid (DNA) analysis based on the measurements of the absorbance in the ultraviolet range. The optimization of the sensor structure allowed us to bring the detection limit for a 30-mer DNA sample down to 1nM×cm, limited by the experimental setup. Taking advantage of the hypochromic effect, we also demonstrated the detection of single- and double-stranded DNA molecules in a melting experiment. From the noise characterization of our setup, we estimated the minimum DNA absorbance required to detect the occurrence of a hybridization/separation process to be below 10−3.

Analysis of lead oxide (PbO) layers for direct conversion X-ray detection

Lead oxide (PbO) is a candidate direct conversion material for medical X-ray applications. We produced various samples and detectors with thick PbO layers. X-ray performance data such as dark current, charge generation yield and temporal behavior were evaluated on small samples. The influence of the metal contacts was studied in detail. We also covered large a-Si thin-film transistor (TFT)-plates with PbO. Imaging results from a large detector with an active area of 18 cm /spl times/ 20 cm are presented. The detector has 960 /spl times/ 1080 pixels with a pixel pitch of 184 /spl mu/m. The modulation transfer function at the Nyquist frequency of 2.72 linepairs/mm is 50%. Finally, a full size X-ray image is presented.

PbO as direct conversion X-ray detector material

A flat X-ray detector with lead oxide (PbO) as direct conversion material has been developed. The material lead oxide, which has a very high X-ray absorption, was analysed in detail including Raman spectroscopy and electron microscopy. X-ray performance data such as dark current, charge yield and temporal behaviour were evaluated on small functional samples. A process to cover a-Si TFT-plates with PbO has been developed. We present imaging results from a large detector with an active area of 18 × 20 cm2. The detector has 1080 × 960 pixels with a pixel pitch of 184 μm. The linearity of detector response was verified. The NPS was determined with a total dark noise as low as 1800 electrons/pixel. The MTF was measured with two different methods: first with the analysis of a square wave phantom and second with a narrow slit. The MTF at the Nyquist frequency of 2.72 lp/mm was 50 %. We calculated first DQE values of our prototype detector plates. Full size images of anatomic and technical phantoms are shown.

On-chip detection of multiple serum antibodies against epitopes of celiac disease by an array of amorphous silicon sensors

In this paper, we present the preliminary results of an ELISA-on-chip device, intended as a technological demonstrator of a novel analytical system suitable for the diagnosis and follow-up of celiac disease. The idea of the work is to combine an array of amorphous silicon photosensors with a pattern of a poly(2-hydroxyethyl methacrylate) polymer brush film, which acts as anchor for the immobilization of gliadin peptides containing the celiac disease epitopes. Recognition relies on a sandwich immunoassay between antibodies against the peptides and secondary antibodies marked with horseradish peroxidase to obtain a chemiluminescent signal. Detection is based on the measurement of photocurrent induced in the array of amorphous silicon photosensors by the chemiluminescent signal. An ad-hoc procedure has been developed in order to enable the fabrication of the photodiode array and the polymer brush pattern on the two sides of the same glass substrate ensuring the compatibility of the different technological steps. The sensitivity and the selectivity of the chip for multiplex immunoassays were demonstrated using two gliadin peptides (VEA and DEC). In particular, we found that the average amount of the bound HRP revealed by our analytical protocol is 3.5(±0.3) × 10−6 pg μm−2 and 0.85(±0.3) × 10−6 pg μm−2 for specific and non-specific interactions, respectively.

Detailed Study of Amorphous Silicon Ultraviolet Sensor With Chromium Silicide Window Layer

In this paper, we present a detailed investigation of an amorphous silicon sensor for the detection of ultraviolet (UV) radiation. The device is an n-i-p stacked structure with a grid-patterned top metal contact through which the incident radiation reaches the active layers. The performances of the sensor have been enhanced by using a very thin chromium silicide (CrSi) film formed on top of the p-doped layer. In particular, this film enhances the surface conductivity, reducing the effect of the self-forward bias that occurs in the device due to the high resistivity of the p-doped layer. As a result, the sensitivity and the linearity of the response increase, reaching a responsivity above 60 mAAV at 254.3 nm. Furthermore, the CrSi layer leads to a stable device because it hides the effect of the p-doped layer resistivity variation under UV radiation. The comparison between two sets of devices with different grid geometries, one with and one without the CrSi film, demonstrates the effectiveness of the alloy film.

Microfluidic Chip With Integrated a-Si:H Photodiodes for Chemiluminescence-Based Bioassays

On-chip optical detection of chemiluminescent reactions is presented. The device is based on the integration of thin film hydrogenated amorphous silicon photosensors on a functionalized glass substrate ensuring both a good optical coupling and an optimal separation between biological or chemical reagents and the sensing elements. The sensor has been characterized and optimized using the chemiluminescent system composed by the enzyme horseradish peroxidase (HRP) and luminol/peroxide/enhancer cocktail. The detectability of HRP is at the attomole level with a sensitivity of 1.46 fA/fg. Experiments, involving the detection of immobilized bio-specific probes on the functionalized surface have been performed both in bulk and microfluidics regime, proving the ability of the system to effectively detect chemiluminescent reactions and their kinetics. In particular, results achieved using conventional polydimethylsiloxane microfluidics for samples and reagents handling confirmed the good detection capabilities of the proposed system.

Monitoring of Temperature Distribution in a Thin Film Heater by an Array of a-Si:H Temperature Sensors

In this paper, we propose the use of an array of amorphous silicon (a-Si:H) p-i-n diodes to monitor the spatial temperature distribution over a thin film heater used for thermal treatments in lab-on-chip systems. The effects of heater geometry and operating conditions on the spatial temperature distribution have been preliminarily investigated by using COMSOL Multiphysics, coupling the electrostatic problem with the thermal problem via the Joule effect. Depending on the analyzed system, nonuniform temperature profiles can be induced over the heater surface revealing the need for a temperature point-monitoring. An example of whole device, constituted by a serpentine shaped TiW/Al/TiW thin film heater and five a-Si:H diodes deposited between the resistor meanders, has been fabricated on a microscope glass slide and characterized. Voltage-temperature characteristics of the a-Si:H sensors, measured at constant forward current, show a sensitivity around . The spatial temperature distribution along the heater has been derived measuring the voltage across each a-Si:H diode. A good agreement between modeled and measured data is obtained, demonstrating the suitability of the a-Si:H array as temperature distribution sensors in lab-on-chip application.

Amorphous Silicon Sensors for Single and Multicolor Detection of Biomolecules

In this paper, we report on a system for single and multicolor detection of biomolecules based on amorphous silicon photosensors. The system promises to be compact, portable, and low cost. It allows the quantitative detection without using optics for focusing both the excitation and the emitted radiation. The revealed biomolecules can be chemi- or naturally luminescent or can be labeled with fluorochromes. Here, we focus on the detection of DNA molecules labeled with a single or with two fluorochromes by using a p-i-n and a p-i-n-i-p amorphous silicon stacked structure, respectively. The device design has been optimized in order to maximize the signal-to-noise ratio and to match the sensor spectral response with the emission spectra of the fluorochromes. This optimization process has been carried out by means of a numerical device simulator, which takes into account the optical and electrical properties of the amorphous silicon. Detection limit in the order of a few nmol/l have been achieved for both the single and the two-color photosensors. Comparison with commercial measurement equipment shows the suitability of our system for practical applications.

Amorphous Silicon Photosensors for Detection of Ochratoxin a in Wine

The presence of ochratoxin A (OTA) in different food commodities deserves great attention because of its toxic and carcinogenic effects on humans and animals. In this paper, we report on the detection of OTA both in standard solutions and in contaminated samples of red wine by using amorphous silicon photosensors. The method relies on the excitation by ultraviolet radiation of the toxin molecules and on the absorption of the toxin reemitted light by the photosensor. The device is a p-i-n stacked structure, whose electro-optical characteristics have been optimized in order to maximize the photosensor responsivity and the limit of detection. For standard solutions, we found minimum detected OTA amount to be equal to 0.1 ng, while for contaminated red wine samples the technique coupled with very simplified and rapid extraction procedures has allowed the detection of OTA at the 1-ppb level.