1. Double Sided Silicon Strip Detectors
Effect of the interstrip gap on the efficiency and response of
Double Sided Silicon Strip Detectors
Domenico Torresi
Dipartimento di Fisica e Astronomia - Università di Catania
INFN – Laboratori Nazionali del Sud
In this talk I will describe a study of the response of DSSSD.
Nucleus Nucleus 2015 – Catania, June 20th-26th, 2015

2. DSSSD characteristcs MICRON semiconductor
Double Sided Silicon Strip Detector design W1.
Both electrodes are segmented in strips.
thickness: μm
full depletion: V
metallization: 3000 Å
Si dead layer: 0.5 μm
50x50 mm2 active area
16 strip front side (p)
16 strip back side (n)
strip width 3mm
strip length 50 mm
interstrip gap is a SiO2 layer
100 mm wide
1 mm thick
The detectors under test are the DSSSD acronym for Double Side Silicon Strip Detector. They are widely used in nuclear physics. Infact thanks to their very high granularity and large area they are useful to extract very precise angular distributions or to study reactions were the detection of two or more particles is required to fully characterize the final state and, more in general, when a small resolution in position is required.
The detectors we have characterized are DSSSD manufactured by Micron semiconductor. Both electrodes are segmented in 16 strips, vertical for the front side and horizontal for the back side.
Some of the main characteristic are listed here. They are 1000 micron thick, I would liketo stress that the segmantation of the electrodes is obtained by a Silicon Oxide gap 100 micron wide and 1 micron thick.

3. Aim of the experiment
It is known from the literature that the segmentation of the electrodes is responsible for:
charge sharing between neighboring strips.
opposite polarity signals in coincidence with normal polarity signals.
Yorkston, NIM A262 (1987) 353;
Blumenfeld NIM A421 (1999) 471.
It was observed that not all events are detected with full energy but some events (up to 30% of the total) are detected with lower energy.
Aims:
Efficiency for full energy detection.
Measurements of the effective interstrip width.
Does it depend on the incident energy, ion species and applied bias?
What is the origin of these phenomena.
Searching for a selection procedure.
Why we studied the response of these detector that are used since long time (more o less two decades). It is known in literature that in this type of segmented detector are present phenomena such us charge sharing between neighboring strips and the presence of anomalous polarity signals when a particle enter the detector through an interstrip region.
This means that not all the particle entering the detector are detected with the full energy. The number of events detected with smaller energy is not negligible and can reach up to the 30% of the total.
Thus this study came from the necessity to know precisely the efficiency for the full energy detection of this detectors.
In literature (to my knwoledge) there are no investigation of interstrip effects as a function of the energy of the incoming particle.
Thus the aim of the following experiment is to study the efficiency for the full energy reconstruction, its dependence on the energy on the ion species and on the applied bias.

4. DSSSD Experiment at LNS
Tandem Beams:
7Li E= MeV
16O E= MeV
Beam:
very low intensity (<1000 pps).
defocused ( Ø ≈ 2 cm)
directly on the detector placed at 0°.
The experiment was performed LNS in Catania. Tandem beams of 7Li and 16O in these energy ranges were used.
Three detectors, two of them 1000 thick and the third one 500 micron thick. They were placed in a rotating plate allowing to place each of them at zero degree. The beam, that was sent directly on the detector had very low intensity, less than 1000 pps, and was defocused, the diameter on the detector was about 2 cm.
Both positive and negative signals were acquired at the same time in order to observe anomalous polarity signals.
2 detectors micron
Bias:
1 Full Depletion Voltage
1.4 Full Depletion Voltage
Both positive and negative signals are acquired at the same time.
D. Torresi et al., NIMA 713 (2013) 11

5. Front and back strip spectra.
back strip spectrum
Lithium beam at 14.6 MeV.
front
full energy peak.
pedestal (0 energy).
negative signals.
front strip spectrum
Here are shown two typical energy spectra for a back and a front strip.
The peak on the right is the full energy peak, The peak on the left is the pedestal, that is zero energy.
In the front strips are present negative energy events that correspondes to opposite polarity signals while there is no negative signals in the backstrip.
It is important also to note the presence of a large number of events having energies lower than the full energy. These are respectively front and back interstrip events. That are shown in the next slide.
The ratio of the interstrip events and total number of events as a function of the incident energy is shown in this plot. The number of this events go from 20% to 30%
Yorkston, NIM A262 (1987) 353;
Blumenfeld NIM A421 (1999) 471.

6. Event Selection. EFront = EBack EFront = EBack
Independently on the origin of these phenomena a procedure is required to select the full energy events.
By using the event selection
EFront = EBack_i+EBack_i±1
back interstrip event are recovered
EFront = EBack
EFront = EBack
Our aim was to find a selection procedure for the events.
A common way to select data is to compare the energy detected from back and front side.
Here is shown the back strip spectra before selection in red and after the selction (in blu). This is a good selection since almost all te lower energy events are removed.
But if you look at this plot where efficiency vs incident energy is shown you see a dependence of the efficiency on the energy. In green is shown the effficiecy with the selction front equal to back for 1FDV, in red is shown the same efficency for 1.4 FDV in red. So The efficiency depends on incident energy and bias.
But if you recover the back interstrip comparing the front side with the energy with the sum of the energy of two adjacent back strips the efficiency encrease and the dependence on the energy and bias is removed.
With the event selection E(Front) = E(Back)
we get an efficiency which is dependent on energy and bias!
By selecting event with E(Front) = E(Back_i)+E(Back i±1)
the efficiency is increased and the energy and bias dependence is removed

7. Microprobe scattering chamber
DSSSD Experiment at Ruđer Boskovic Institute (Zagreb)
By using a microbeam it is possible to obtain precise information on the position of the impinging particle with a precision of few μm.
Proton microbeam with energies from 0.8 MeV to 6 MeV
corresponding range in Si mm mm
Bias: 200 V (1FDV) and 280 V (1.4 FDV)
Microprobe scattering chamber
Before continuing with the results of the experiment at LNS, I would like to show some very preliminary results from the experiment performed in Zagreb.
In this experiment we use a proton microbeam, the microbeam properly focused is sent into a scattering chamber equipped with a system which allows a precise positioning of the sample in (XYZ), in which is placed the detector. Inside the chamber a camera is placed for visual monitoring of the detector. This camera is connected to a microscope that allows to center the region of the detector we are interested in, in a very simple way.
Magnification of the interstrip region of the detector seen with the control camera.
L. Grassi et al., NIMA 767 (2014) 99

8. DSSSD Experiment at Ruđer Boskovic Institute (Zagreb)
A microbeam was used to scan de interstrip region of DSSSDs. The position of each incoming particle is known with a precision of few microns. x y
FRONT
BACK y
Front
Back
A sampling box of 400x400 micron is irradiated, inside the box the position of the incoming particle are known with a precision of few microns.
In this case the sampling box is centered on a back interstrip as shown in the picture.
In this plot it is shown the energy detected (not calibrated) as a function of the y position. For the front side the energy detected is the full energy in each point of the sampling box.
When the beam is on one back strip the full charge is collected, as the beam moves over the edge there is a region where the charge is shared between the two neighboring strips. As the beam moves on the next strip the charge is entirely collected by that strip.
The effective interstrip region here is about 200 micron while the geometrical one is 100 micron.
strip 7 strip 8
front side measure the full energy.
in the back side there is a narrow region (200 μm) where the charge is shared between the two neighboring strips.
geometrical interstrip region 100 μm !

9. DSSSD front interstrip events.
By using a micro-beam is possible to study the correlation between the energy measured by two adjacent strips and the impact position of the incoming particle.
Definition of the effective interstrip width is simply and direct!
Energy signal amplitude as a function of the position in the interstrip region
BACK
FRONT x

10. Behavior of front and back interstrip with the Bias at different energies: The tick detector.
Increasing the bias the effective interstrip width and the amplitude of the opposite polarity signals decreases.
front
1000 μm
Effective interstrip width for three different energies as a function of the bias voltage
back

11. Behavior of front and back interstrip with the Bias at different energies: The thin detector
The Front Effective interstrip width shows a non-monotone trend a s a function of the polarization voltage and a region where the EIW is minimum.
front
75 μm
The back EIW decreases with increasing bias voltages, but the overall values are smaller than those measured for the thick detector.
back

12. back front Thin detector in punch through
2 MeV protons on 75 μm thick DSSSD
back
For particles punching through the thin detector
Opposite polarity signals appear in the back side
Pulse hight defect appear for the front interstrip events
front

13. A simplified simulation based on the Shokkley-Ramo-Gunn framework
In a system at the electrostatic equilibrium the instantaneous current ii induced at the i-th electrode by the motion of a point charge q is:
𝒊=−𝒒 𝒗 ∙ 𝑬 𝒘,𝒊
Where 𝑬 𝒘,𝒊 is the weighting field associated with i-th read-out electrode.
The time integration of eq 1 gives the total charge induced by q at the i-th read-out electrode
𝑸 𝒊 =𝒒∙ 𝝋 𝒘,𝒊 𝒙 𝒇 − 𝝋 𝒘,𝒊 𝒙 𝒊
Qi is proportional to the difference of the weighting potential at the initial and final position.

14. Origin of the opposite polarity pulsesIf
All holes are collected by Fi
All electrons are collected by Bi
Both electrons and holes induce positive polarity signals on front strip Fi
Full energy amplitude is detected in Fi
Weighting potential maps associated with the read-out electrode for the thin detector (75 μm).
Anyway
Signals induced by electrons in Fj is positive
Signals induced by holes in Fj is negative
Signal in Fj is bipolar
The time integration of the induced signals on Fj gives total charge zero
Inverted polarity pulses can occur
If electrons are trapped or recombined along their path.
Electric field lines are bend at the Si SiO2 interface by a build up of an effective surface charge density Yorkstone et al. NIMA 262, (1985) 353

15. Origin of the opposite polarity pulses
Assuming a build up of an effective surface charge density at the Si SiO2 interface the electric field is distorted.
Simulation performed by means of the finite element solver Comsol multiphysics 4.3

16. Results of the theoretical calculations: front interstrip
The weighting potential and electric field assuming a positive charge density at the front Si-SiO2 interface of
𝝈=𝒆∙𝟓∙ 𝟏𝟎 𝟗 𝒄𝒎 −𝟐
1.0 B/D
0.5 B/D
1.5 B/D
The simulation reproduces qualitatively rather well the observed data.
Simulated energy signals for keV protons as a function of position along the front interstrip.

17. Results of the theoretical calculations: back interstrip
Simulated (left) and measured (right) energy signals for keV protons as a function of position along the front interstrip for the thick detector

18. Summary and Conclusions
A systematic study of the effect of the segmentation on the response of DSSSDs for different thicknesses (75 and 1000 μm) for different incident ions energies for different detector bias voltages was performed.
The results show that both front and back Effective Interstrip Widths can be larger than the nominal width and depend on the energy of the ions and on the bias voltage.
Determine a simple procedure for the selection of event detected with the full energy which is independent on the energy and bias.
The experimental observations were compared with the results of simplifyed simulations based on Shockley-Ramo-Gunn framework. Satisfactory qualitative reproduction of all the observed inter-strip effects was obtained assuming the buildup of positive charge at the SiO2 interface in the front inter-strip and of negative charge at the SiO2 interface in the back inter-strip.
For those experiments aiming at an accurate measurements of quantity dependent on the efficiency for full energy detection a complete characterization of the detector should be required

19. Collaboration
D. Torresi, L. Grassi, J. Forneris, L. Acosta, A. Di Pietro, P. Figuera, M. Fisichella, V. Grilj, M. Jaksic, M. Lattuada, T. Mijatovic, M. Milin, M. Musumarra, M.G. Pellegriti, L. Prepolec, V. Scuderi, N. Skukan, N. Soic, D. Stanko, E. Strano, V. Tokic ,M. Uroic, M. Zadro
INFN - LNS
Università degli studi di Catania
University of Zagreb
Ruder Boskovic institute
Università di Messina
Università di Torino
This work is a result of a collaboration between: lns university of Catania Messina and Zagreb.

20. EXTRA SLIDE

22. Schematic Layout
of the DSSSD

23. Event Selection.
Without using back information it is not possible to select properly the full energy data.

24. Energy vs position (II) – front interstripx y
BACK
FRONT x
Front
Back δE
Here are shown the plots when the sampling box is centered on a front interstrip region, and we are looking at the energy detected as a function of the x direction.
For the front strip we still see a charge sharing but negative signals appear, moreover the interstrip front affects the charge collection in the back strip. There is an energy defect measured in the back for those events that cross a front interstrip region not observed in the previous case.
A charge sharing effect is also present in the front interstrip region but negative signals appear.
Particles hitting the front interstrip generate lower energy signals in the back strip.

25. Theoretical model and interpratetion
Interstrip front
Weighting potential maps associated with the read-out electrode for the thin detector (75 μm).
The electric field lines originating from Bs diverge from the median interstrip coordinate x0
At the front size the weighting filed decreases from the value 1 in Fi to 0 in Fj
Interstrip back
Simulation performed by means of the finite element solver Comsol multiphysics 4.3.

26. Behavior of front and back interstrip with the Bias at different energies: The tick detector.x
Increasing the bias the effective interstrip width and the amplitude of the opposite polarity signals decreases.
1000 μm
Effective interstrip width for three different energies as a function of the bias voltage