Elisa Vianello

Slim edges in double-sided silicon 3D detectors

Minimization of the insensitive edge area is one of the key requirements for silicon radiation detectors to be used in future silicon trackers. In 3D detectors this goal can be achieved with the active edge, at the expense of a high fabrication process complexity. In the framework of the ATLAS 3D sensor collaboration, we produced modified 3D silicon sensors with a double-sided technology. While this approach is not suitable to obtain active edges, because it does not use a support wafer, it allows for a new type of edge termination, the slim edge. In this paper we report on the development of the slim edge, from numerical simulations to design and testing, proving that it works effectively without increasing the fabrication complexity of silicon 3D detectors, and that it could be further optimized to reduce the insensitive edge region to less than 100 μm.

Development of Double-Sided Full-Passing-Column 3D Sensors at FBK

We report on the main design and technological characteristics related to the latest 3D sensor process developments at Fondazione Bruno Kessler (FBK, Trento, Italy). With respect to the previous version of this technology, which involved columnar electrodes of both doping types etched from both wafer sides and stopping at a short distance from the opposite surface, passing-through columns are now available. This feature ensures better performance, but also a higher reproducibility, which is of concern in medium volume productions. In particular, this R&D project was aimed at establishing a suitable technology for the production of 3D pixel sensors to be installed into the ATLAS Insertable B-Layer. An additional benefit is the feasibility of slim edges, which consist of a multiple ohmic column termination with an overall size as low as 100 μm. Eight batches with two different wafer layouts have been fabricated using this approach, and including several design options, among them the ATLAS 3D sensor prototypes compatible with the new read-out chip FE-I4.

Development of active and slim edge terminations for 3D and planar detectors

We report novel solutions for the edge termination in silicon detectors. In the framework of a project aimed at the optimization of 3D detectors with active edge, we have developed both active edges using a single sided process with support wafer, and slim edges using a double sided process without support wafer. TCAD simulations and experimental tests have been carried out to validate and compare the proposed approaches. While active edges can provide a better sensitivity up to a few microns from the physical edge, slim edges can simplify the fabrication technology while limiting the dead area at the edge to about 50 µm. The main design and technological issues are reported in this paper, along with selected results from TCAD simulations and electro-optical tests performed on these devices.

Optimization of double-side 3D detector technology for first productions at FBK

We report on the optimization of the technology aimed at the production of Double-Sided, Double-Column 3D detectors with full passing columns (3D-DTTC) at FBK (Trento, Italy). This R&D project is aimed at establishing a suitable technology for the production of 3D pixel sensors to be installed into the ATLAS IBL. We describe the main process modifications adopted on more recent 3D batches, to overcome the limitations affecting the first 3D batch, as arisen from its electrical characterization.

Development of modified 3D detectors at FBK

We report on the main design and technological issues related to a modified 3D-DTTC (Double-side, Double-Type-Column) detector process at FBK (Trento, Italy). With respect to the previous versions of this technology, which involved columnar electrodes of both doping types etched from both wafer sides and stopping at a short distance from the opposite surface, passing-through columns are now available. This is expected to enhance the performance but most of all to make it more reproducible, having in mind medium volume productions. An additional benefit is the feasibility of slim edges, which consist of a multiple ohmic column termination with an overall size in the order of 200 μm. Two batches of detectors have been fabricated at FBK using this modified 3D-DDTC approach, and with two different wafer layouts including several design options, among them the ATLAS 3D sensor prototypes compatible with the new read-out chip FE-I4. Selected results from the characterization of test structures from the first processed wafer are reported.

Simulations and electrical characterization of Double-side Double Type Column 3D detectors

We report here the results of the electrical characterization of 3D-DDTC diode samples in virgin conditions and after exposition to reactors neutrons with a fluence of 1015 neq · cm−2. CV and IV measurements before irradiation have been employed to calibrate a commercial TCAD simulation tool, which has been then used to reproduce the effects of radiation damage in the detector. Simulation results are compared to post irradiation measurements. A systematic study of the effects of the positive oxide charge on the breakdown behaviour reveals a non-monotonic trend which causes the VBR to remain almost unchanged compared to measured pre-irradiation values.

3D silicon sensors - Large area production, QA and development for the CERN ATLAS experiment pixel sensor upgrade

3D silicon sensors, where electrodes penetrate fully or partially through the silicon substrate, have been successfully fabricated in different processing facilities in Europe and the USA. They key to 3D fabrication is the use of plasma micromachining to etch narrow deep vertical openings which allow dopants to be diffused in and form the electrodes of the p-i-n junctions. Similar openings can be used at the sensors edge to reduce the perimeters dead area to be as narrow as 4 μm. Since 2009, four fabrication facilities of the 3D ATLAS R&D Collaboration started a joint effort aimed at one common design and compatible processing strategy for the production of 3D sensors for the LHC Upgrade and in particular for the ATLAS pixel Insertable B-Layer (IBL). In this project where the installation is aimed for 2013, a new layer will be inserted as close as 3.4 cm from the proton beams inside the existing pixel layers of the ATLAS experiment. The detector proximity to the interaction point will therefore require new radiation hard technologies for both sensors and front-end electronics. The latter, called FE-I4 is processed at IBM and is the biggest front end of its kind, with a surface area of about 4 cm2. This paper will discuss some design aspects, and the different approaches taken by the facilities. Results from both the qualification runs and the current production runs for the IBL are also reported.