Universita de Firenze 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 4D Sensors: Unifying the Space and Time Domain with Ultra-Fast Silicon Detectors

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  m] 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) 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. “4D”

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

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. Time of Flight for Particle Identification in Space. 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 Range straggling limit for 200 MeV p Future: 4-D Ultra-Fast Si Detectors ? Hartmut F.-W. Sadrozinski: UFSD, Tredi 2013 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

6 A. Del Guerra, RESMDD12 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. Resolution of detector (pitch) Positron range A-collinearity Parallax (depth) T: true event S: Compton Scatter R: Random Coincidence Resolution and S/N Effects: Hartmut F.-W. Sadrozinski, UFSD, Tredi 2013 Perfect Picture:

Reduce Accidentals & Improve Image: TOF-PET t1 t2 Localization uncertainty:  d = c x  t /2 When  t = 200 ps ➔  d = 3 cm PET VCI K. Yamamoto 2012 IEEE NSS-MIC Hartmut F.-W. Sadrozinski: UFSD, Tredi

8 1.For a given acquisition time and dose to the patient, TOF can provide better image quality and improved lesion detection. OR 2.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). TOF – PET SNR Improvement M. Conti, Eur. J. Nucl. Med. MoI Imaging (2011) 38: The improved source localization due to timing leads to an improvement in signal-to-noise and an increase in Noise Equivalent Count NEC Hartmut F.-W. Sadrozinski: UFSD, Tredi 2013

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

Impact Ionization A. Macchiolo,16th RD50 Workshop Barcelona, Spain, May 2010 Charge multiplication in path length ℓ : At the breakdown field in Si of 270kV/cm:  e ≈ 1 pair/um  h ≈ 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

Non-uniform E-Field across a pixel/strip Non-uniform Field across the implant results in charge collection difference Example of electric field: Ф = 1.6·10 15 n/cm 2, U = 900 V 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 ! Gregor Kramberger, 19th RD50 Workshop, CERN, Nov 2011A. Macchiolo, 16th RD50 Workshop Barcelona, Spain, May 2010

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

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 200  m sensors and hole collection (?), could it be re- designed for electron collection from 2 – 10  m sensors?

Firenze ps Laser Data: 50 um p-on-n 6k  -cm Laser  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

Base line shift due to AC coupling?

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 Timing Properties 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. Collection of ps Laser signal

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

NIST  Range Thorium 230: 4.7 MeV

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

Bias dependence of pulse (mean values vs bias voltage)

Expected Performance from 50um epi Diode

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

Finite Element Mesh

Electric Field Width = 2um Depth = 2um

Electric Field Width = 20um Depth = 2um

Electric Field Width = 10um Depth = 5um

2um deep * width (um)Vbd >500 5um deep * width (um)Vbd >500 39>500 Breakdown Voltage vs. Implant Dimension

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  -cm vs. 100  -cm with p+ implants of 5, 10, 20  -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

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

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

Emax = 270 kV/cm, (Bias adjusted), 5  m [3um 100  -cm + 2um of p+ implants (5, 10, 15  -cm)] vs. [5um 100  -cm] Gain vs. ResistivityCollected Charge vs. Resistivity

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

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

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

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

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

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

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

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