Gian Franco Dalla Betta
Doctoral programme – Materials, Mechatronics and Systems Engineering
Department of Industrial Engineering
The research activity of Gian-Franco Dalla Betta is concerned with the design, simulation, fabrication and experimental characterization of silicon integrated devices and circuits, with emphasis on radiation sensors over a broad range of radiation types and energies.
A. RADIATION DETECTORS
This is the main research activity. It started in 1994 with the PhD activity and it has continued for more than 20 years, involving many different detector types and applications, as briefly summarized in the following.
1. PIN diode and drift detectors for X-ray spectroscopy and imaging
This activity initiated the development of radiation detection processes on high-resistivity silicon at ITC-IRST of Trento in 1994. As such, it was crucial in establishing a production capability for radiation detectors that is unique in Italy. Achievements included: (i) the development of a state-of-the-art, low leakage process for the fabrication of PIN diode detectors, making use of extrinsic gettering techniques; (ii) the design, modeling, and experimental characterization of PIN diode detectors with good electro-optical and spectroscopic properties; (iii) the study of the accuracy of generation lifetime measurements in high-resistivity silicon using gate-controlled diodes and the design and characterization of optimized test structures; (iv) the development of PIN diode arrays coupled to CsI scintillators to be used in an X-ray imaging system for the on-line inspection of materials and structures; (v) the development of a fabrication process for drift detectors, and the fabrication of the first prototypes of these detectors ever made in Italy.
2. Strip and pixel detectors for tracking of charged particles
This activity, strongly fostered by INFN, allowed to fully accomplish the goal to establish at ITC-IRST, within a relatively short period of time, a production capability of medium volumes of advanced detectors for tracking of charged particles in High Energy Physics and space experiments. Achievements included: (i) the development of the critical process steps (e.g., integrated coupling capacitors, bias resistors, etc.) for microstrip detectors; (ii) the design, fabrication and electrical characterization of several prototype batches of double-sided microstrip detectors with excellent electrical properties and very low defect density; (iii) the development of an analytical model for the interstrip resistance on the ohmic-side of double-sided microstrip detectors; (iv) the development of an automatic test method for the electrical characterization of double-sided microstrip detectors and of a computerized analysis tool for process defect identification; (v) the development of a state-of-the-art process for double-sided pixel sensors of the n-on-n type, oriented to the ATLAS Pixel Detector.
Results were so good that ITC-IRST was included among the few silicon foundries at world level entitled to bid for the tenders relevant for the production of radiation detectors for CERN experiments, and it soon obtained important achievements: the production of 600 double-sided microstrip detectors for the silicon tracker of the AMS2 experiment (installed on board of the International Space Station), and 800 double-sided microstrip detectors for the silicon tracker of the ALICE experiment (one of the main experiments at CERN-LHC). Both productions were completed in due time and with very good results, both in terms of detector quality and of fabrication yield.
More recently, double-sided striplet detectors and pixel detectors were designed, fabricated on thin silicon substrates (200 mm), and fully characterized with good results. These detectors represented the baseline option for the Layer 0 of the foreseen SuperB Silicon Vertex Tracker, but the project was cancelled at the end of 2012 and these developments had to stop.
3. Radiation damage effects and radiation tolerant detectors
Theoretical and experimental studies have been carried out to investigate radiation damage effects on radiation detectors, and to develop new technological/design solutions for radiation-hard detectors to be used at future experiments at the High-Luminosity LHC Upgrades.
Achievements included: (i) the TCAD analysis and experimental characterization of irradiated FOXFET transistors to be used as bias structures in microstrip detectors; (ii) the TCAD analysis of the edge-generated leakage current in single-sided microstrip detectors irradiated beyond the type-inversion fluence; (iii) the development of special fabrication technologies by means of substrate engineering (e.g., oxygen enrichment of Float Zone substrates, pre-irradiation of Float Zone substrates by high fluences of fast neutrons, use of oxygen-rich substrates such as epitaxial, Czochralsky or Magnetic Czochralsky wafers); (iv) the design, fabrication and characterization of radiation-hard detectors featuring the above mentioned substrate engineering options; (v) the design, fabrication and characterization of thin radiation detectors obtained by local thinning of the substrate by TMAH etching; (vi) the measurement of the impact ionization coefficient of electrons in highly irradiated silicon; (vii) the development of a comprehensive model of bulk and surface radiation damage in highly irradiated silicon sensors.
For their importance and impact on the recent research activity, 3D and active edge detectors are described separately.
4. Multiple guard-ring edge terminations for radiation detectors
The edge termination is one of the most important problems in the design of radiation detectors. Dedicated efforts were devoted to this purpose with very good results. Achievements included: (i) the design, TCAD optimization and characterization of termination structures with multiple guard rings enabling the high voltage (>1000V) behavior of silicon radiation detectors both before irradiation and after heavy particle irradiation; (ii) the introduction of a new termination principle (namely the “all-p-type” multiple guard ring termination) aimed at improving the long term stability of radiation detectors subject to varying environmental conditions while using a simplified fabrication technology; (iii) the design, TCAD simulation and experimental characterization of several variants of the “all-p-type” termination (e.g., by using different combinations of inward and outward field plates) under different environmental conditions and irradiation scenarios.
5. Radiation detectors with integrated electronics
The integration of the radiation detector and at least part of the front-end electronics on the same high-resistivity substrate can significantly improve the noise performance, owing to the reduction of the stray capacitances associated with the connections from the charge collecting electrode and the preamplifier. For this reason, and also as a demonstration of the advanced processing capabilities achievable at ITC-IRST, an important R&D effort was started in the mid 90’s and continued for about 10 years, leading to interesting results.
Achievements included: (i) the development of a fabrication process (the so called JSD technology) for detector-compatible n-channel JFETs; (ii) the design, fabrication and characterization of a test chip, demonstrating the feasibility of JFETs with good electrical properties while maintaining a very low detector leakage current; (iii) the design and characterization of PIN diode detectors with integrated JFETs, showing good performance; (iv) the modification of the JSD technology to allow for other active and passive devices to be fabricated (e.g., MOS transistors, BJT transistors, poly-Si resistors, coupling capacitors); (v) the design and fabrication of JFET-based charge sensitive amplifiers monolithically integrated on the detectors substrate and their extensive characterization, including radiation damage effects; (vi) the TCAD study and explanation of some anomalies in the JFET characteristics, causing excess noise, and the optimization of the fabrication process by implementing a high-energy p-well implant to fix these issues; (vii) the successful demonstration of the low noise features of modified JFET transistors and of JFET-based detectors and integrated amplifiers; (viii) the design, fabrication and characterization of microstrip detectors with embedded, JFET-based source-follower amplifier; (ix) the design, fabrication and characterization of arrays of active pixels featuring JFET and MOSFET transistors embedded within the charge collection element.
6. Radiation detectors based on bipolar junction transistor
Initially obtained as a side-product of the JSD process, radiation detectors based on bipolar junction transistors (BJT) were extensively characterized and optimized, finally leading to their use in a commercial product. Achievements included: (i) the TCAD analysis and experimental characterization of BJT-based radiation sensors featuring different design and technological options; (ii) the design, implementation and experimental validation of different bias methods for BJT detectors; (iii) the design, implementation and characterization of an original detector concept featuring monolithic arrays of BJT detectors with shared collector (substrate) and emitters shorted by a metal grid; (iv) the development of an alpha particle detector system based on BJT detectors and suited for Radon concentration measurements.
7. Radiation detectors for medical imaging
The detector technologies developed for High Energy Physics experiments were adapted to fabricate other types of detectors aimed medical imaging applications. Achievements included: (i) the optimization of the technological processes for very thick substrates (up to 1mm), as required for increased detection efficiency for X-rays; (ii) the design, fabrication and experimental characterization of several batches of pixel detectors for digital mammography, compatible with the read-out chips of the MEDIPIX family; (iii) the design, fabrication and experimental characterization of arrays of PIN diodes coupled to scintillators for g-ray detection, to be used in a functional imaging instrument for scintigraphy.
8. Silicon photomultipliers
Silicon photomultipliers (SiPM) are the hottest topic in photodetection and candidate themselves to replace photomultiplier tubes in most applications. Achievements included: (i) the development of the first prototypes of silicon photomultipliers at ITC-IRST by contributing to the design, the definition of the fabrication process, and the experimental characterization, in particular of the timing properties; (ii) the development of a compact model for the circuit simulation of SiPM; (iii) the development of a SiPM-based detection system for Positron Emission Tomography; (iv) the feasibility study of a novel SiPM-based detection system for Positron Time-of-Flight measurements, to be used for defect analysis in material science.
9. Radiation detectors with three-dimensional electrodes (3D detectors)
This research activity has been the most important one for Gian-Franco Dalla Betta in the past few years. His contribution was fundamental in bringing 3D detectors from their infancy to their maturity, allowing for their first application in a High Energy Physics experiment at CERN-LHC, and paving the way for their use in other experiments. Moreover, other activities later started based on 3D detector technology, as reported in the following.
Achievements included: (i) the introduction of modified 3D detector concepts, alternative to the original one proposed by S. Parker (e.g., single-type column 3D detectors; double-sided double-type column 3D detectors with partially-through and full-through electrodes), and the development of the related technologies; (ii) the design, fabrication and experimental characterization of several batches of 3D detectors in different configurations (e.g., pad, strip, pixels) and featuring different design and technological options; (iii) the study and TCAD analysis of non idealities in non irradiated and irradiated 3D detectors; (iv) the first observation of charge multiplication effects in irradiated 3D detectors; (v) the development and industrialization of 3D pixel detectors for the ATLAS Insertable B-Layer; (vi) the optimization of 3D detector design and technology for higher breakdown voltage; (vii) the introduction of original design and technological solutions for a new generation of downscaled (smaller and thinner) 3D pixel detectors for future applications at the High-Luminosity LHC.
10. Active-edge and slim-edge terminations for radiation detectors
Based on the experience with 3D technology, active-edge and slim-edge detectors have been developed allowing for the minimization of the dead-area at the edge and for the realization of large area seamlessly tiled detector matrices. Achievements included: (i) the development of fabrication technologies and design solutions for planar active-edge detectors; (ii) the design, TCAD simulation, fabrication, and experimental characterization of planar active-edge diode and strip detectors; (iii) the introduction of original slim-edge concepts in double-sided 3D detectors, also suitable for planar detectors, and their implementation in the ATLAS IBL design; (iv) the design, TCAD simulation, fabrication, and experimental characterization of different slim-edge structures in double-sided 3D detectors, with minimum dead area at the edge; (v) the contribution to the validation of the scribe-cleave-passivate (SCP) post-processing technique for slim edge in 3D detectors; (vi) the development of high-efficiency, active edge planar pixel sensors for experiments at next generation Free Electron Lasers (FELs).
11. Hybrid neutron detectors
Also based on the experience with 3D technology, hybrid detectors of neutrons have been developed. Achievements included: (i) the design of novel 3D sensors for neutron detection, also compatible with pixelated read-out chips for neutron imaging; (ii) the fabrication and experimental characterization of prototype hybrid detectors based on 3D sensors and different converter materials for the detection of both thermal and fast neutrons; (iii) the optimization of the 3D sensors by GEANT4 simulations and the design of a new batch with enhanced efficiency; (iv) the fabrication and experimental characterization of 3D sensors from the second batch, with emphasis on the feasibility neutron/gamma discrimination by means of proper choice of the bias voltage.
12. Low Gain Avalanche Detectors
Low Gain Avalanche Detectors (LGADs) are radiation sensors with intrinsic signal amplification based on the avalanche effect. LGADs are a recent, hot topic in the field of radiation detectors, since they promise to open several new application opportunities, in particular owing to the possibility to provide an excellent timing resolution (~10s of ps) besides the usual high spatial resolution. Achievements included: (i) the contribution to the optimization of design parameters of LGADs; (ii) the proposal of a new, double-sided LGAD concept to allow for segmented detectors with uniform gain; (iii) the definition of a fabrication process and the design of a first batch of double-sided LGADs; (iv) the fabrication and experimental characterization of different LGAD structures from the first batch; (v) the definition of a fabrication process and the design of a second batch of thin, single-sided LGADs.
13. Monolithic Active Pixel Sensors
Within a large collaboration project funded by INFN, Monolithic Active Pixel Sensors (MAPS) have been developed for particle tracking in high-energy physics experiments (in particular for the Layer 0 of SuperB project). The specific role of Gian-Franco Dalla Betta has been the design and numerical simulation of CMOS radiation sensors embedded in the MAPS pixels. Achievements included the design and experimental characterization of several chips of the so-called APSEL family in 0.13 um CMOS technology, featuring pixels with full signal processing capabilities and on-chip data sparsification, also suited to vertical integration. The experimental characterization in the laboratory and at beam tests confirmed the high tracking efficiency of these devices.
B. CMOS PHOTODETECTORS AND IMAGE SENSORS
This activity includes both a device-related part aimed at the realization of non-standard photodetectors in commercial CMOS technologies and a circuit/system part devoted to the realization of pixel arrays for different imaging applications.
Achievements included: (i) the design, modeling and experimental characterization of photon mixing devices based on different structures (e.g., MSM, interdigitated photodiodes, pinned photodiodes) suitable for the implementation of the Indirect Time-of-Flight technique at the pixel level for distance measurements; (ii) the development of versatile models for the simulation of optical sensors at the circuit level; (iii) the design, modeling and experimental characterization of avalanche based photodetectors, operated either in the linear mode (APD) or in the Geiger mode (SPAD) in different sub-micron CMOS technology nodes; (iv) the porting of a Current-Assisted Photonic Demodulator (CAPD) to a 0.18um CMOS process; (v) the design and characterization of smart pixels based on the previously mentioned sensors and their implementation in functional 3D image sensors with good performance; (vi) the design, modeling and characterization of CMOS SPAD-based pixel arrays with time-resolved readout channels, which enable single photon imaging with sub-ns resolution, for time resolved fluorescence measurements; (vii) the design and implementation of image sensors featuring novel active pixel concepts enabling outstanding performance in terms of dynamic range (higher than 130 dB); (viii) the design and characterization of a prototype hybrid camera based on a CMOS chip coupled to an array of organic photodiodes, aimed at the development of an image sensor with chemically-tunable spectral response also extending to the infrared; (ix) the design of dual-tier SPAD sensor arrays for coincidence charged particle detection.
C. DEVICES AND CIRCUITS FOR SENSING APPLICATIONS
Achievements included: (i) the design, fabrication and experimental characterization of high performance optical sensors (photodiodes, phototransistors, photoASICs) for position encoders to be used in industrial control applications; (ii) the development and successful demonstration of technological modules (p-channel JFET, filterless color sensor based on multiple overlapped junctions, LED structures based on MOS diodes biased in the Fowler-Nordheim regime) fully compatible with the CMOS process available at ITC-IRST, and aimed at enhancing the design flexibility; (iii) the design and implementation of novel blue-sensitive optical transducers (finger shaped photodiodes and BJT phototransistors) for gas sensing (electro-optical noses), showing good response down to 300 ppm concentration of ethanol; (iv) the design and implementation of CMOS circuits (main building blocks and full interfaces) for optical sensors and chemical sensors; (v) the development of silicon nanowires featuring palladium gate contacts for hydrogen detection; (vi) the TCAD and experimental validation of an active method to improve the reliability of RF-MEMS switches.
D. CMOS ANALOG CELLULAR NEURAL NETWORKS
This activity was started at the University of Bologna for the MSc thesis, and then was reprised for some time while at ITC-IRST. Achievements included: (i) the design and simulation of novel CMOS implementations of analog programmable Cellular Neural Networks (CNNs), considering small area occupation and low power consumption as main constraints; (ii) the design, fabrication, and test of an original CMOS test structure for the mismatch characterization of MOS transistors, based on a circuit topology typical of neural circuits; (iii) the design and test of prototype CNN circuits showing good electrical figures and correct performance in some basic image processing applications (noise removal, hole filling, edge detection, shadow detection, etc).