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Chiara Guazzoni |
Chiara Guazzoni was born in Milano, Italy in 1972. She received the Laurea Degree in Physics cum laude in March 1996 from Universita’ degli Studi, Milano, Italy where during 1996 she attended the Master Course in Nuclear Physics. At the end of 1996 she won the competition to enter the PhD program in Electronic Engineering at Politecnico di Milano, Italy. Since 1994 she has collaborated with the research group of Prof. Gatti in Politecnico di Milano for the development of new semiconductor detectors for X-rays and for the design of nuclear electronics. She carries out her experimental and theoretical research activity in the laboratories of Politecnico di Milano and of Universita’ degli Studi di Milano. She collaborates with the Brookhaven National Laboratory and with the Halbleiterlabor of Max Planck Institut (Munich, Germany) where she has worked for short periods. The main research fields on which she works are:
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Chiara Guazzoni |
She is co-inventor of a new semiconductor detector for position sensitive X-ray spectroscopy called Controlled-Drift Detector for which an Italian patent extended to Europe and to the United States has been obtained. During the Laurea degree she cooperated in the characterization of a new Silicon Drift Detector. Unlike conventional Silicon Drift Detectors, the lateral (that is, in the direction orthogonal to the drift) broadening of the signal charge generated by the incident radiation is prevented by means of deep p implants. This improves the rate capabilities and allows sub-micron resolution to be achieved in the position measurement.
Her main PhD research activity is devoted to the complete development of a new kind of semiconductor detector for high energy resolution X-ray spectroscopy retaining the position information of the incident X-rays. The new detector, named Controlled-Drift Detector, is almost a hybrid between two well known detectors; i) Fully Depleted Charge Coupled Devices (CCDs) and ii) Silicon Drift Detectors (SDDs). It combines the positive features of each detector without their drawbacks. It retains the readout speed of the SDD without the requirement for the independent knowledge of the time of the X-ray conversion and the pixel structure typical of the CCDs with much faster transport of the signal charge. Moreover it retains the very low capacitance of the charge collecting anode that allows the precise charge and hence energy measurement typical of both CCDs and SDDs. The Controlled-Drift Detector is operated in integrate-readout mode thanks to the possibility of externally switchable longitudinal channel stops. When longitudinal channel stops are switched on, the integration phase takes place and the detector accumulates charges in discrete pixels. When the longitudinal channel stops are switched off, the detector acts as a Silicon Drift Detector with limited lateral diffusion. Charge accumulated during the integration phase is allowed to drift towards the anode at a velocity typical of a SDD. The drift time, that is, the time between the removal of the channel stops and the arrival of electrons at the anode, defines the position of the pixel from which the charge was released.
Three different schemes for switching the longitudinal channel stops were developed. From the very beginning she participated in the new detector project. She analysed and designed the different implementations producing the layouts for the different detectors. The designed detectors were produced at the MPI Halbleiterlabor in Munich, Germany. She took care of the development of the experimental apparatus for the detectors characterization. During the last two years she characterised two of the prototypes, solving unforeseen problems and difficulties. Both prototypes are properly functioning. The measured readout times are less than 3 microseconds for a 1cm long detector, well below the time required to readout a CCD for spectroscopic applications.
During the last year she also participated in the design of front-end electronics for high count rate X-ray spectroscopy detectors to be integrated on the detector chip. On-chip JFETs have already been produced and tested in a source follower configuration. Efforts are being devoted to change to a charge amplifier configuration that intrinsically features very high gain stability. The main problem is to find a reset mechanism for the accumulated charge that is efficient and stable and, last but not least, suitable for the integration on the detector chip. Two solutions have been found that correspond to these requirements. They are suitable for continuous reset mode operation and can be integrated with no need for additional steps in the detector production. The solution is to use an active device (pnp bipolar transistor or p-MOSFET) for the reset path. Comparison of the performance achievable with the two devices shows that the better solution, in terms of added noise and of linearity of the response, would be a bipolar transistor for the values of leakage currents typical for semiconductor detectors for high resolution spectroscopy. However, the available technological process and the layout compatibility have driven towards a p-MOSFET operate in the sub threshold mode. She designed the p-MOS transistor embedded in the front-end JFET taking care of both the simulation and the design of the structure. Many problems had to be solved due to the reduced space for the integration of the MOSFET and to keep the JFET properly operating even with the introduction of the reset device. At present the layouts have been delivered to the Halbleiterlabor of the Max Planck Institut in Munich, Germany, where these detectors will be produced. Moreover, she designed the transresistance amplifier in BiCMOS technology to be coupled to the on-detector chip structure. Many problems arise in the amplifier design to cope with the particular characteristics of the front-end transistor whose transconductance is lower than that of commercially available JFETs. The requirements in terms of noise and bandwidth are very demanding to cope with the design criteria of this electronics. At present the transresistance amplifier is under test.
She participated in the development of a new method for the simulation of semiconductor detectors, based on the solution of the Poisson equation. The aim of the proposed method is to take into account the effects of an accumulation layer of mobile electrons in close proximity of the Silicon-oxide interface without solving the continuity equations for both electrons and holes. The method is based on a physical model of the interface that allows correct approximations of the boundary condition in the interface region. The model assumes that the interface region is divided in two regions, an equipotential region that corresponds to the region where the electron accumulate and a fully depleted region. The extension and the potential of the electron layer are calculated with the desired precision by an iterative procedure. This method has been implemented in a 3D Poisson solver previously developed in the group of Prof. Gatti. The main advantage of the proposed method is that it properly simulates semiconductor detectors that operate in conditions of full depletion by solving only the Poisson equation. She directly validated the proposed method by comparing the results achievable with the proposed simulator with a conventional two dimensional drift-diffusion simulator. This simulator has been very useful in the study of the potential profile within the volume of a semiconductor detector in different biasing conditions at low CPU time.
She analysed the non-destructive readout, a noise reduction technique. This technique is usually applied to high resolution CCDs for visible light applications. However, the optimum weighting function for this kind of readout was never studied. She applied an innovative method for optimum filter synthesis in the presence of arbitrary noise densities (previously developed by Prof. Gatti’s group) to the non-destructive readout technique. The non-destructive technique is particularly suitable for the reduction of the 1/f noise contribution that often imposes the lower limit in the achievable resolution. The method has been fully developed, implemented on a personal computer and applied to study the effects of multiple readouts on the different noise contributions. She also studied the effects of thermally generated leakage current on the performance achievable with this readout technique. Moreover, she designed a new device to embody the multiple readout in X-ray detectors based on a high resistivity substrate, like the fully depleted pn-Charge Coupled Device, the Controlled-Drift Detector, the Silicon Drift Detector. At present the device is under test.
Chiara Guazzoni can be reached at Dipartimento di Elettronica e Informazione - Politecnico di Milano - P.za Leonardo da Vinci, 32 - 20133 Milano - Italy; Phone: ++39 02 2399 6147; Fax: ++39 02 2367604; E-mail: Chiara.Guazzoni@mi.infn.it; WWW: http://www.elet.polimi.it/Users/DEI/Sections/Electronic/Chiara.Guazzoni/