HOW ?

The diode structure of present silicon pixel sensors strongly penalises the enormous potential of silicon-based time measurement. The MONOLITH project introduces a novel silicon-sensor structure devised to overcome the intrinsic limits of present sensors. This goal is achieved by the Picosecond Avalanche Detector (PicoAD), a novel sensor exploiting a fully depleted multi-PN junction (European Patent 18181123.3, 2 July 2018). The remarkable performance of the PicoAD, combined with the simplified assembly process and reduced production cost offered by the monolithic implementation in standard CMOS processes, represent the required breakthrough.

In addition to the  multi-junction PicoAD sensor, the cornerstones of the project are the low-noise very-fast SiGe HBT frontend and the TDC with robust synchronisation method (patent EP 18 207 008.6, 19 November 2018) already produced in preliminary versions.

 

The MONOLITH project encompasses four work packages, as shown by the figure on the right.

 

We are presently developing our circuits and sensors using the IHP SG13G2 130nm process. Large-volume foundry processes will also be investigated.

European industrial partners have been contacted for commercial exploitation of the detector.

 

 

RESULTS so far:

1. A proof-of-concept PicoAD prototype was produced by IHP.  Wafers with epitaxial layers of 5µm (absorption region) and 10µm (drift region) were produced, with four different implant doses in between for the gain layer. Lab tests with 55Fe source showed that the detector works as expected, and provides a gain of 23 at HV=125V for the highest implant dose, see https://iopscience.iop.org/article/10.1088/1748-0221/17/10/P10032; the corresponding gain for minimum-ionizing particles is estimated to be 60-70.
2. In testbeam measurements the proof-of-concept prototype proved full efficiency and and average time resolution of 17 ps, with 13 ps at the center of the pixel and 25 ps in the inter-pixel region https://iopscience.iop.org/article/10.1088/1748-0221/17/10/P10040.
3. A second prototype was produced recently with several improvements in the electronics. Produced on a 50µm epilayer of resistivity of 350 Ωcm, WITHOUT INTERNAL GAIN LAYER, we obtained the following results:
• in a the testbeam:   we obtained full efficiency and average time resolution of 20 ps, https://iopscience.iop.org/article/10.1088/1748-0221/18/03/P03047, which is a striking result for a PN-junction sensor without a gain layer
• using  a 90Sr radioactive source:  we proved that SiGe HBTs are radiation tolerant for proton fluence of 10^16 1 MeV n_eq/cm^2, with a timing performance of 40 ps, https://iopscience.iop.org/article/10.1088/1748-0221/19/01/P01014
• using a femtosecond laser, we measured time resolutions down to 3 ps  https://arxiv.org/pdf/2401.01229.pdf

We are looking forward to early 2024 when a  sensor will be produced based on these foundry masks and PicoAD wafers.

 

Participating researchers:  Giuseppe Iacobucci, Lorenzo Paolozzi, Thanushan Kugathasan, Roberto Cardella, Stefano Zambito, Didier Ferrére, Yannick Favre, Stéphane Débieux, Matteo Milanesio, Théo Moretti, Antonio Picardi, Luca Iodice, Carlo Alberto Fenoglio, Andrea Pizarro-Medina, Ivan Semandyaev

 

Former participants: Magdalena Muenker, Pierpaolo Valerio, Fulvio Martinelli