Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - UFSD (Ultra-Fast Silicon Detectors: Enabling Discoveries)

Teaser

The measurement of trajectories of charged particles is ubiquitous in many areas of science, ranging from space science to the determination of the mass of molecules, from beam monitoring in medical treatments to research on fundamental particle interactions in nuclear and...

Summary

The measurement of trajectories of charged particles is ubiquitous in many areas of science, ranging from space science to the determination of the mass of molecules, from beam monitoring in medical treatments to research on fundamental particle interactions in nuclear and particle physics.

State-of-the-art tracking devices work similarly to our pocket camera: they take a snapshot of a given volume and show what is contained in the volume in the instant the picture is taken. For example, a camera can take a landscape picture with the sky, hills, trees, maybe a lake or the seaside. Consider now what happens if many birds fly crisscross through the sky while we take a picture: the snapshot will show the birds trajectories crossing each other, but it will not tell us if the birds flew together, or one after the other.

The same fact happens in charge particle tracking: if a lot of particles go through the tracking volume during the snapshot time, the reconstruction of what happens become very difficult, at time even impossible, and the results are not very precise.
Pocket cameras have now a way to avoid this problem: they can make movies.

The UFSD project proposes to transform present tracking devices by adding the “movie capability”, dividing a single snapshot into a sequence of frames separated by a very small time difference. In so doing, particles overlapping in space will be distinguished by their different times of passage, obtaining a series of much crispier pictures.

Once space-time tracking will be an established technology, it will be use in the next generation of many high-tech instruments. Currently, even at this very early stage of the project, there are already groups exploring usage of 4D tracking for beam monitoring in cancer treatments with hadron beams, next generation of mass spectroscopy instruments, and high-energy physics experiments.

Work performed

1) Production of thin Ultra-fast silicon detectors (UFSD)
The key technological development that has been proposed and produced in the first 18 months of the project is the production of thin low gain avalanche diodes (LGAD), that we called Ultra-Fast Silicon Detectors (UFSD). UFSD are a new concept in silicon detector design, merging the best characteristics of standard silicon sensors with the main feature of Avalanche Photo Diodes (APD). The overarching idea is to design silicon detectors with signals that are large enough to assure excellent timing performances, but to keep the gain as low as possible. The complicate technological step that needs to be mustered is the capability of keeping the gain value low, and be able to segment it.
Charge multiplication in silicon sensors happens when the charge carriers are in electric fields of the order of E ∼ 300 kV/cm. Under this condition the electrons (and to less extent the holes) acquire sufficient kinetic energy that are able to generate additional e/h pairs. A field value of 300 kV/cm can be obtained by implanting an appropriate charge density that locally generates very high fields (ND ∼ 1016/cm3). The gain has an exponential dependence on the electric field N(l) = Noeα(E)l, where α(E) is a strong function of the electric field and l is the path length inside the high field region. The additional doping layer present at the n − p junction in the UFSD design, Figure 1, generates the high field necessary to achieve charge multiplication.
Charge multiplication in silicon sensors happens when the charge carriers are in electric fields of the order of E ∼ 300 kV/cm. Under this condition the electrons (and to less extent the holes) acquire sufficient kinetic energy that are able to generate additional e/h pairs. A field value of 300 kV/cm can be obtained by implanting an appropriate charge density that locally generates very high fields (ND ∼ 1016/cm3). The gain has an exponential dependence on the electric field N(l) = Noeα(E)l, where α(E) is a strong function of the electric field and l is the path length.
Following our design, the Centro Nacional de Microelectrónica (CNM) Barcelona, the first production of thin UFSD (50 μm) by CNM was presented in 2016. The Fondazione Bruno Kessler (FBK) has also designed and produced UFSD sensors, up to now only 300-micron thick; first FBK production of thin UFSD is expected in early 2017. At the 2016 IEEE conference it was announced that Hamamatsu photonics has produced successfully thin UFSD (50- and 80-micron thick).

2) Development of the simulation program Weightfield2.
We have developed a full simulation program, Weightfield2 (WF2) with the specific aim of assessing the timing capability of silicon sensors with internal gain. The program (see Figure_2) has been validated by comparing its predictions for minimum ionizing particles and alpha particles with both measured signals and TCAD simulations, finding excellent agreement in both cases. Figure 3 shows on the left the comparison WF2-TCAD for the predicted current produced by a MIP in a 300-micron silicon sensors with gain = 14 while on the right the comparison between WF2 and the impulse measured at a beam test with 120 GeV/c pions, using as read-out a charge sensitive amplifier. The left side therefore shows the good agreement in the simulation of the mechanisms involved in the current signal, while the right side shows how the program also correctly simulates the electronic response.

3) Design and production of a custom VLSI chip to read-out UFSD sensors.
We designed of a full custom amplifier-comparator readout chip for silicon detectors with internal gain designed for precise timing applications. The ASIC has been developed in UMC 110 nm CMOS technology and is aimed to achieve a time resolution of ∼ 30 ps. Each channel is independent and the signal processing chain, is composed by: (i) trans-impedance amplifier, (ii) single threshold discriminator, (i

Final results

The first 18 months of the projects have been really productive, and we have already transformed the track-tracking established paradigm.

We have successfully completed the design and production of a new type of silicon sensor, the so-called Ultra-Fast Silicon Detector – UFSD. We have also developed a new family of very low noise amplifiers, designed both as custom designed VLSI chip and discrete components boards, specifically tailored for the read-out of the UFSD sensors.

Using the combination of UFSD sensors and these custom-made low noise amplifiers we have measured the time of passage of charge particle with accuracy 3 times better (35 picosecond instead of more than 100 ps) than what has ever done before in silicon tracking.

There is strong interest from industry in our project, with now three silicon foundries producing prototype sensors, including the largest silicon detector company in the word, Hamamatsu photonics. Hamamatsu sees a clear market for time-tracking devices, and it is investing its own money in pursuing our ideas.

Several experiment in high-energy physics are discussing the use of UFSD sensors, and one, the CMS experiment at the CERN laboratory in Geneva, has committed to the purchase of more than 2 ml euros of such detectors.

The project is therefore already having a significant impact on the advancement of the production capabilities of European silicon foundries in the high-end market and it is focusing new resources toward new technologies

Website & more info

More info: http://personalpages.to.infn.it/.