N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 Future silicon trackers: 4D tracking, very high fluences, very small pixels Nicolò Cartiglia INFN - Italy 1
Outline N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 A brief history of silicon trackers • Requests for the next generation of silicon trackers • 4D tracking: • - what is it - is it possible? Sensors for extreme fluences • 2
A brief history N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 The beginning of the Silicon detector era is set in the period 1978-1982 , and the NA11/NA32 experiments are credited to be the first one to have used a silicon tracker Shortly after, successful tests of silicon strip detectors with VLSI readouts were carried out in 1985. During the 1990s, CDF and the LEP experiments were instrumented with Silicon trackers, with the electronics at the edges. Here at DESY, we even manufactured a curved silicon detector, to be placed near the proton beam. The ZEUS experiment was also instrumented with the silicon vertex detectors 3
Evolution up to LHC N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 This incredible evolution was made possible by the development of the “silicon” industry and by the collaboration of our community with several silicon foundries 4
The LHC and HL-LHC era N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 Incredible development of manufacturing capability Very good understanding of the silicon properties under irradiation: modelling of silicon detectors and the effect of irradiation is well modelled. Taken from Doris Eckstein Similar development in read-out capability HL-LHC : the CMS-ATLAS upgrades are very large, however, they are in spirit similar to the present LHC detectors. Higher radiation levels, more channels and much more performing electronics. One novel request: need to measure the time of each track , to bundle correctly the tracks of each vertex. 5
What’s next N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 There are many futures in Silicon trackers: some are clever redesigns of existing systems, some requires much higher radiation tolerance, some extremely good position resolution. One of the most challenging design: the Future Circular Collider tracker Tracker requirements: position : 7.5 - 9.5 μm time resolution = 5 ps Radiation levels : up to ~1E17 n/cm2 Note: there are many R&D directions in Silicon detectors. This presentation is not a review but it is about a possible future. 6
Tracking particles in space and time at FCC First question : Can we design a single detector that can concurrently measure (a) time with ~ 10 ps precision (b) position with N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 ~ 10 micron precision This is an extraordinary challenge in sensor design and ASICs Second question : can we make silicon detectors able to work at fluences about 1E16 – 1E17 n/cm 2 ? A lot has been understood regarding the design of radiation hard silicon detectors, with a key contribution from Hamburg, however, currently we don’t know how to do design a sensor for extreme fluences, F = 1E16 – 1E17 n/cm2 7
First question: ~ 10 micron and 10 ps precision Silicon sensors were never considered accurate timing devices However, in the last 10 years there has been a very intense R&D N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 At present, silicon sensors are the ONLY detector able to provide excellent timing capability (~ 30 ps) , good radiation hardness (fluence ~ 1E15 n/cm2), good pixelation (10um – 1 mm), and large area coverage (many m 2 ) Important: Sensors provide the current signals, read-out chips use them Timing is the to combination of these two parts, that succeed and fail together 8
The effect of timing information The inclusion of track-timing in the event information has the capability of changing radically how we design experiments. Timing can be available at different levels of the event reconstruction, in increasing order of complexity: 1) Timing in the event reconstruction è Timing layers (time, position) N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 this is the easiest implementation, a layer ONLY for timing • 2) Timing at each point along the track è 4D tracking (time, position) tracking-timing • Timing 3) Timing at each point along the track at high rate è 5D tracking (time, position, and rate) Very high rate represents an additional step in complication, very different read-out • chip and data output organization 9
Signal formation in silicon: induced current The charge carriers motion induces Induced variable charge on the read-out charge ++++ ++++++ electrode. N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 The signal ends when the charges are collected Signal shape is determined by Ramo’s Theorem: i ∝ qvE w Drift velocity Weighting field 10
The sensors’ role: provide good signals The goal of a sensor designer is to minimize the differences in the sensor’s output , providing well defined, uniform current signals to the electronics. N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 The prerequisite for this goal is the capability of simulating the physics of the particle-sensor interaction. Chip designers need to test their solutions on a realistic sets of current signals that reproduce the full variability of the sensor’s output. Good sensor simulation is necessary to achieve excellent time resolution 11
Simulator Weightfield2 Available at: http://personalpages.to.infn.it/~cartigli/Weightfield2/Main.html It requires Root build from source, it is for Linux and Mac. It will not replace TCAD, but it helps in understanding the sensors response N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 12
Weightfield2 and friends Weightfield2: - It is completely open source - It is fast - It generates the signal from several sources (MIP, alpha, lasers..) - Runs in batch mode writing output files N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 - It loads/save configurations - It has basics electronics simulation It crashes occasionally Other simulators: KDrtSim, https://indico.desy.de/indico/event/12934/session/3/contribution/26/material/slides/ TRACS https://indico.desy.de/indico/event/12934/session/3/contribution/29/material/slides/ 13
The art of weighing field Calculating the correct weighting field for a variety of situations is a very difficult task. Most of the time we rely on simulator to do it. Please note a series of papers that are approaching this problem analytically: N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 W. Riegler, “ An application of extensions of the Ramo-Shockley theorem to signals in silicon sensors ” Nucl.Instrum.Meth. A940 (2019) 453-461 arXiv:1812.07570 Academic training at CERN: https://indico.cern.ch/event/843083/ Joern Schwandt, Robert Klanner, On the weighting field of irradiated silicon detectors , https://arxiv.org/abs/1905.08533 14
Silicon time-tagging detector (a simplified view) N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 Time is set when the signal crosses the comparator threshold The timing capabilities are determined by the characteristics of the signal at the output of the pre-Amplifier and by the TDC binning. Strong interplay between sensor and electronics 15
Time resolution # %&'() # = + ∆'&/'01"'&/ # + ∆(213) # + 456 # ! " *+/*" Subleading, Sensor design ignored here Usual “Jitter” term Amplitude variation: N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 Here enters everything that is “Noise” and variation in the total charge the steepness of the signal Shape distortion : non homogeneous energy deposition total current electron current hole current total current electron current Need large dV/dt hole current 16
# Sensor geometry: how to minimize its contribution to ! " Signal shape is determined by Ramo’s Theorem: i ∝ qvE w N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 Drift velocity Weighting field The key to good timing is the uniformity of signals: Drift velocity and Weighting field need to be as uniform as possible Basic rule: parallel plate geometry 17
Larger dV/dt from thick detectors? (Simplified model for pad detectors) Thick detectors have higher number of charges: + - d + - Q tot ~ 75 q*d N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 + - However, the weighting field is 1/d, so each charge + - D contributes to the initial current as: + - i ∝ qv 1 + - + - d The initial current for a silicon detector does not depend on how thick (d) the sensor is: i = Nq k v = (75 dq ) k − 6 A v = 75 kqv ~1 − 2*10 d d Number of e/h = 75/micron velocity è Initial current = constant Weighting field 18
Summary “thin vs thick” detectors (Simplified model for pad detectors) Thin detector + - d + - i(t) N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019 + - S + - Thick detector D + - + - + - dV dt ~ S ~ const t r Thick detectors have longer signals, not higher signals We need to add do something about this problem… 19
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