1. 4D Sensors: Unifying the Space and Time Domain with Ultra-Fast Silicon Detectors
Hartmut Sadrozinski, Scott Ely, Vitaliy Fadeyev, Zachary Galloway, Jeffrey Ngo, Colin Parker, Brett Petersen, Abe Seiden SCIPP, Univ. of California Santa Cruz Nicolo Cartiglia, Amadeo Staiano INFN Torino Mara Bruzzi, Riccardo Mori, Monica Scaringella, Anna Vinattieri Universita de Firenze

2. “4D” 2 questions need to be addressed for UFSD:
Ultra-Fast Silicon Detectors (UFSD) incorporate the time-domain into the excellent position resolution of semiconductor sensors
they provide in the same detector and readout chain
ultra-fast timing resolution [10’s of ps]
precision location information [10’s of mm]
A crucial element for UFSD is the charge multiplication in silicon sensors investigated by RD50, which permits the use of very thin detectors without loss of signal-to-noise.
2 questions need to be addressed for UFSD:
can they work: signal, capacitance, collection time vs. thickness
will they work: required gain and E-field, fast readout
We hope that we will answer the questions within an RD50 Common Project (Giulio Pellegrini)

3. Motivation for UFSD
Up to now, semiconductor sensors have supplied precision data only for the 3 space dimensions (diodes, strips, pixels, even “3D”), while the time dimension has had limited accuracy (e.g. to match the beam structure in the accelerator).
We believe that being able to resolve the time dimension with ps accuracy would open up completely new applications not limited to HEP
Proposal: Combined-function pixel detector will collect electrons from thin n-on-p pixel sensors read out with short shaping time electronics
Charge multiplication with moderate gain g ~10 increases the collected signal
Need very fast pixel readout

4. Time of Flight for Particle Identification in Space.
The Alpha Magnetic Spectrometer (AMS) detector, operating in the International Space Station since 2011, performs precision measurements of cosmic ray composition and flux.
The momentum of the particles is measured with high-resolution silicon sensors inside a magnetic field of about 1 m length.
A time resolution
of 10 picoseconds,
the “Holy Grail” of
Cosmic Ray Physics:
the distinction between
anti-carbon ions and
anti-protons can be achieved
up to a momentum of 200 GeV/c.

5. Future: 4-D Ultra-Fast Si Detectors ?
Protons of 200 MeV have a range of ~ 30 cm in plastic scintillator. The straggling limits the WEPL resolution.
Replace calorimeter/range counter by TOF:
Light-weight, combine tracking with WEPL determination
Range straggling limit for 200 MeV p
Hartmut F.-W. Sadrozinski: UFSD, Tredi 2013

6. Positron Emission Tomography PET
Study accumulation of radioactive tracers in specific organs.
The tracer has radioactive positron decay, and the positron annihilates within a short
Distance with emission of 511 keV γ pair, which are observed in coincidence.
Perfect Picture:
Resolution and S/N Effects:
Resolution of detector (pitch)
Positron range
A-collinearity
Parallax (depth)
T: true event
S: Compton Scatter
R: Random Coincidence
A. Del Guerra, RESMDD12
Hartmut F.-W. Sadrozinski, UFSD, Tredi 2013

7. Reduce Accidentals & Improve Image: TOF-PET
Localization uncertainty:
Dd = c x Dt /2
When Dt = 200 ps
➔ Dd = 3 cm
As you know, in Time-Of-Flight Positron Emission Tomography, TOF information provides better localization of the annihilation event and improves the signal-to-noise ratio of the reconstructed image.
For example, when timing resolution is 300ps, position resolution is 4.5cm along the line of response.
In fact, timing resolution depends on the scintillation light yield, detector performance and read out noise.
Multi-Pixel Photon Counter, MPPC is a silicon photomultiplier and expected for use in PET-MR.
Timing resolution required for the MPPC is less than 200ps.
@ VCI
K. Yamamoto 2012 IEEE NSS-MIC
Hartmut F.-W. Sadrozinski: UFSD, Tredi 2013

8. TOF – PET SNR Improvement
The improved source localization due to timing
leads to an improvement in signal-to-noise
and an increase in Noise Equivalent Count NEC
For a given acquisition time and dose to the patient, TOF can provide better image quality and improved lesion detection. OR
with TOF the scan time and dose can be reduced while keeping the same image quality ( better clinical workflow and added comfort for the patient).
M. Conti, Eur. J. Nucl. Med. MoI Imaging (2011) 38:
Hartmut F.-W. Sadrozinski: UFSD, Tredi 2013

9. UFSD Pixel / Strips Collected Charge
Signal = thickness*EPM (EPM = 73 e-/mm
Collection time = thickness/vsat (vsat = 80 mm/ns)
Per 1 cm
Realistic
gain & cap
Good time
resolution
For thickness > 5 um, Capacitance to the backplane Cb << Cint
For thickness = 2 um, Cb ~ ½ of Cint, and we might need bipolar (SiGe)?

10. Impact Ionization Charge multiplication in path length ℓ :
A. Macchiolo,16th RD50 Workshop Barcelona, Spain, May 2010
Charge multiplication in path length ℓ :
At the breakdown field in Si of 270kV/cm:
ae ≈ 1 pair/um
ah ≈ 0.1 pair/um
→ In the linear mode (gain <10), consider electrons only
Raise maximum and minimum E-field
as close to breakdown field as possible

11. Non-uniform E-Field across a pixel/strip
Example of electric field:
Ф = 1.6·1015 n/cm2, U = 900 V
Non-uniform Field across the implant results in charge collection difference
Gregor Kramberger, 19th RD50 Workshop, CERN, Nov 2011
A. Macchiolo, 16th RD50 Workshop Barcelona, Spain, May 2010
Even if non-uniformity of field across the implant is only 30%, a large fraction of the center of the implant does not exhibit charge multiplication !

12. Epi, short drift on planar diode g = 6.5
Using red laser and a’s probes E-field and gain close to the junction, where it counts. Diode gives uniform field.
Charge multiplication in 3D sensors:
M. Koehler et al.
J. Lange et al., Nucl.Instrum.Meth A622, 49-58, 2010.
Need uniform field: 3D or diode-like

13. What about fast readout:
CERN fixed-target experiment (NA62) needs very fast pixel sensors: Gigatracker (GTK)
Prototype CFD system (INFN Torino) has ~ 100 ps resolution, predicted to be 30 ps in next iteration.
Optimized for 200mm sensors and hole collection (?), could it be re-designed for electron collection from 2 – 10mm sensors?

14. Firenze ps Laser Data: 50 um p-on-n 6kW-cm
1.2 ps pulse width & 10 ns period: pulse distortions
740 nm wavelength for red light, penetrate ~6 um.
Oscilloscope: 500 MHz bandwidth
2 BNC into 50 Ohm, 500 Ohm in scope
Charge collection of electrons moving away from laser spot
Terminal velocity ~ 100um/ns, i.e. expected collection time ~500ps for high fields

15. Base line shift due to AC coupling?

16. Timing Properties of ps Laser signal
Pulse is convolution of electronic shaping and charge collection, Fit pulses to extract shaping time trise & 10%-90% RT FWHM is considered best measure of the convolution
FWHM becomes constant at about 150 V bias
Rise times RT and trise are ~ independent of bias: Bandwidth of system

17. Collection of ps Laser signal
Above 120V bias, the field > 25kV/cm, i.e. large enough to saturate the drift velocity, i.e. constant collection time.
Below 120V bias, the unsaturated drift velocity increases the charge collection time, causing the pulses to widen.

18. Study of ’s from Thorium 230 50 um thick P-type epi diodes
A’s from
Challenge for manufacturers:
For charge multiplication,
need breakdown voltage >1000V!!

19. NIST  Range
Thorium 230: 4.7 MeV

20. Pulses from Th 230 alpha’s Do NOT expect: Charge multiplication Expect
collection time of ~ 500ps for over-depleted bias (drift velocity is saturated)
large rise time dispersion for low bias voltage

21. Bias dependence of pulse (mean values vs bias voltage)

22. Expected Performance from 50um epi Diode

23. Thin Sensor Simulations Preliminary Results
Colin Parker
UCSC / UNITN
1/18/13

24. Finite Element Mesh

25. Electric Field Width = 2um Depth = 2um

26. Electric Field Width = 20um Depth = 2um

27. Electric Field Width = 10um Depth = 5um

28. Breakdown Voltage vs. Implant Dimension
2um deep * width (um)
Vbd 2
190 10
250 20
340 39
>500
5um deep * width (um)
Vbd 2
350 10
450 20
>500 39

29. Doping Profiles & Biasing
Assume doping distributions with maximum 2 doping distributions, uniform in depth,“pad fields”
Explore two basic conditions:
Emax = 270 kV/cm, adjust bias -> maximum gain
Common Bias for pairs of distributions: all 100 W-cm vs. 100 W-cm with p+ implants of 5, 10, 20 W-cm (4um deep in 20 um, 2 um 5 um)
Thickness 20, 5 um
Resistivity 10, 50, 100, 1000 Ohm-cm p +
substr.
p- epi n

30. Emax = 270 kV/cm, (Bias adjusted), 5 mm
[3um 100 W-cm + 2um of p+ implants (5, 10, 15 W-cm)] vs. [5um 100W-cm]

31. Emax = 270 kV/cm, (Bias adjusted), 5 mm
[3um 100 W-cm + 2um of p+ implants (5, 10, 15 W-cm)] vs. [5um 100WW-cm]
Shower development in Space
Shower development in Time

32. Emax = 270 kV/cm, (Bias adjusted), 5 mm
[3um 100 W-cm + 2um of p+ implants (5, 10, 15 W-cm)] vs. [5um 100W-cm]
Gain vs. Resistivity
Collected Charge vs. Resistivity

33. Emax = 270 kV/cm, (Bias adjusted), 20 mm
[16um 100 W-cm + 4um of p+ implants (5, 10, 15 W-cm)] vs. [20um 10, 50, 100, 1000 W-cm]

34. Emax = 270 kV/cm, (Bias adjusted), 20 mm
[16um 100 W-cm + 4um of p+ implants (5, 10, 15 W-cm)] vs. [20um 10, 50, 100, 1000 W-cm]
Shower development in Space
Shower development in Time

35. Emax = 270 kV/cm, (Bias adjusted), 20 mm
[16um 100 W-cm + 4um of p+ implants (5, 10, 15 W-cm)] vs. [20um 10, 50, 100, 1000 W-cm]
Gain vs. Resistivity
Collected Charge vs. Resistivity

36. Common Bias of 100 W-cm Bulk and Implant, 5 mm
[3um 100 W-cm + 2um of p+ implants (5, 10, 15 W-cm)] vs. [5um 100W-cm]

37. Common Bias of 100 W-cm Bulk and Implant, 5 mm
[3um 100 W-cm + 2um of p+ implants (5, 10, 15 W-cm)] vs. [5um 100W-cm]

38. Common Bias of 100 W-cm Bulk and Implant, 20 mm
[16um 100 W-cm + 4um of p+ implants (5, 10, 15 W-cm)] vs. [20um 100W-cm]

39. Common Bias of 100 W-cm Bulk and Implant, 20 mm
[16um 100 W-cm + 4um of p+ implants (5, 10, 15 W-cm)] vs. [20um 100W-cm]

40. Collected Charge in 5 & 20 um
[16um 100 W-cm + 4um of p+ implants (5, 10, 15 W-cm)] vs. [20um 100W-cm]
[3um 100 W-cm + 2um of p+ implants (5, 10, 15 W-cm)] vs. [5um 10, 100, 1000W-cm]