1. Tobias Bus, Benjamin Linnik, Michael Deveaux, Ali Yazgili
ADvanced
MOnolithic
Sensors for
News on radiation hardness
studies for the CBM MVD
Tobias Bus, Benjamin Linnik,
Michael Deveaux, Ali Yazgili
Institut für Kernphysik, Goethe Universität Frankfurt am Main
Ladies and gentleman, my name is Tobias Bus and ill give you a short talk about radiation hardness of MAPS for the cbm mvd.

2. Leakage currents in MAPS
+3.3V
Reset
+3.3V
Output
SiO2
SiO2
SiO2
N++
N++ P+ N+
Ileak
50µm P-
15µm
Lets have a look at the sensor. The active volume is in the middle. Above is the higher doped P-Well, which has N implantations, that create depleted areas at the pn-junctions. The diode here is collecting charge from the active volume. By charge I mean signal electrons as well as leakage current. So – what do we know about leakage current? P+
Leakage current:
Adds shot noise
May clear the signal from the pixel (more later)
Tobias Bus, DPG 2018, Bochum

3. Radiation tolerance: 5 ∗10 14 𝑛 𝑒𝑞 𝑐𝑚²
PIPPER-2: „Fully depleted“ MAPS with AC- coupling
Last year we tested some monolithic active pixel sensors, that were manufactured in a tower jazz process. An improvement of this process is the AC coupling which allows us to apply a higher voltage to the collecting diode, which increases the depleted volume of the sensor. The advantage of fully depleted sensors is a higher radiation tolerance, but on the other hand they struggle with effects of higher leakage currents, which could be a thing for the PIPPER-2 sensor as we can see on this plot: The sensor works fine, if it is heavily cooled. With higher teamperatures only a fraction of the original charge is collected. This could be a cause of charge clearing induced by high leakage currents.
Signal amplitude highly relies on temperature.
Charge clearing due to high leakage current?
Tobias Bus, DPG 2018, Bochum

4. Leakage current – What do we know?
Fully depleted PIN-diodes:
Which volume?
Δ𝐼=𝛼⋅ Φ 𝑒𝑞 ⋅𝑉
𝛼≈4⋅ 10 −17 𝐴/𝑐𝑚
G. Lindström, NIM-A (2004): 30-43
Leakage current:
scales with volume and radiation dose.
does not depend on doping.
What do we know?
Earlier studies have shown, that for fully depleted sensors the leakage currents scales linearally with the radiation dose and the volume and does not depend on doping.
Is it the same for the PIPPER-2 sensor and if yes, which volume is meant in the formula? Is it the active – or just the depleted volume? Does the fully depleted sensor has a higher leakage current than the not fully depleted one? So it has to be checked which volume causes the leakage current.
Known to hold for depleted sensors. MAPS are not (always) depleted.
Open issue: Valid for MAPS?
Measure leakage current
Tobias Bus, DPG 2018, Bochum

5. Open issue: How to measure depleted volume
Seed
(1 Pixel)
Ndepl
X-rays
Depleted
Summed
(up to second
next neighbor)
Nepi
Depletedvolume:
𝑉 𝑑𝑒𝑝𝑙 ≈ 𝑁 𝑑𝑒𝑝𝑙 𝑁 𝑒𝑝𝑖 ⋅ 𝑉 𝑝𝑖𝑥𝑒𝑙
Known from design
Therefore studies with older Mimosa19 sensors were made, because they have 3T pixels, which allow us to directly measure the leakage current and they are not fully depleted. This comes in handy for us because just a fraction of the photons convert in the depleted volume and deposite its charge there. The charge of all other hits is shared by several adjacent diodes. To determine the just mentioned fraction one can devide the number of entries in this peak where all signal electrons are gathered by the entries of all hits
Hierf+ür wurden Studien an veralteten Mi-19 Sensoren durchgeführt. Diese Sensoren eigenen sich besonder für eine solche Studie, da sie 3T Pixel enthalten und dadruch der Leckstorm direkt gemessen werden kann. Außerdem sind die Sensoren NICHT voll depletiert, sodass sich das depletierte Volumen vom aktiven Volumen deutlich unterscheidet. Daher kann man prüfen, welches der beiden Columina letzzendlich den Leckstrom ausmacht.
Tobias Bus, DPG 2018, Bochum

6. Leakage current in MAPS?
Literature
𝛼≈4⋅ 10 −17 𝐴/𝑐𝑚
Observed
𝛼≈3.4⋅ 10 −17 𝐴/𝑐𝑚
MIMOSA-19
Non-depleted sensor
Preliminary
Mismatch compatible with room temperature annealing.
Ali Yazgili, bachelor thesis, in progress
Depleting also increases the leakage current
Depleted volume was used.
Epitaxial layer delivers mostly no leakage current.
Leakage current seems to scale with depleted volume.
Tobias Bus, DPG 2018, Bochum

7. Leakage current in SB pixels
Visible signal Qs
Qphy
Signal charge [a.u.]
Lost
Visible signal Qs
Qphy
High leakage current
Lost
Requirementsfor
good (>90%) signal:
𝜏 𝑚𝑖𝑛 >10 𝑡 𝑖𝑛𝑡
=50µs
=> 𝐼 𝐿𝑚𝑎𝑥 ≈500 fA
t [readout cycles]
Collected charge
Leakage current
𝑄 𝑆 = 𝑄 𝑃ℎ𝑦 ⋅ exp − 𝑡 𝜏
with 𝜏≈ 𝑘 𝐵 𝑇 𝐶 𝑒 ⋅ 1 𝐼 𝑙𝑒𝑎𝑘 with 𝐶≈10 fF
Clearing of the pixel is accelerated by radiation induced leakage current.
To obtain good signal leakage current must remain limited

8. Relevant for depleted sensors?
Estimate highest acceptable leakage current, e.g. for MAPS with 5µs integration time
Requiredforgood>90% signal:
𝜏 𝑚𝑖𝑛 >10 𝑡 𝑖𝑛𝑡 =50µs
=> 𝐼 𝐿𝑚𝑎𝑥 ≈500 fA± a factor
More depletion
(full matrix)
Comparison with data:
Ileak of heavily irradiated sensor exceeds limit.
Cooling helps.
500 fA/pixel
CBM-MVD
PIPPER-II, IPHC Strasbourg
Unsere Messungen haben gezeigt, dass diese Bedingung nur erfüllt ist, wenn wir die stark depletierten sensoren (zeige auf den den >20 V Bereich), wenn extrem stark gekühlt wird. In diesem Plot nur bis -15 Grad. Extreme Unterschiede im Leckstrom in Abhängigkeit von der Bestrahlung uind Temperatur.
Austoben. Min 2 Min.
Conclusion: Depletion…
Improves charge collection in heavily irradiated sensors.
Increases leakage current of heavily irridiated sensors.
=> Fast readout and/or cooling is needed to exploit advantages.
Tobias Bus, DPG 2018, Bochum