june 1-5, santa fe n. m. vertex 2015 1 ultra-fast silicon detectors june 3, 2015 abe seiden uc santa...

June 1-5, Santa Fe N. M. Vertex 2015 1

Ultra-fast Silicon Detectors

June 3, 2015

Abe Seiden UC Santa Cruz


Outline

• General Idea for Ultra-Fast Silicon Detectors.• Measurements made.• Plans for the future.Goal is a detector that combines excellent timing and position measurement.

Acknowledgments:My thanks to RD50 and particularly the groups at: UC Santa Cruz Torino Barcelona and CNM Trento and FBK Ljubljana


Ultra-Fast Silicon Detectors: Detectors with Gain and Large Electron Drift Velocity

Goal: Gain field ~ 300 kV/cm.Bulk field ~ 20 kV/cm, gives a saturated electron drift velocity ~ 107cm/sec. Want to have gain for electrons but not holes, leads to gain ~ 10.


Signal Amplitude – Measured with Laser

Reference, ordinary sensor

Gain ~ 10

Plot of signal for several sensors (~ 8mm pads, 285 m thickness, from CNM) shows the modest sensitivity of the gain to the detector voltage.


Measuring the Time

The timing capabilities are determined by the characteristics of the signal at the output of the pre-Amplifier and by the TDC binning:

Total2 = Time-Walk

2 + Jitter 2 + TDC

2

Single Comparator: Time is set when the signal crosses the comparator threshold.

The time-walk term is due to amplitude variations, the jitter term is due to noise.To do better likely will want to use a constant fraction discriminator (to reduce time-walk) or to sample the signal in time-bins and fit the start time (reduces time-walk and averages jitter).


Time Resolution and Slew Rate

To minimize the time resolution, we need to maximize the slew-rate, dV/dt of the signal, for a given noise value. This minimizes the error in extrapolating back to the start time of the signal. Need both large and fast signals. We choose the slew-rate as a good figure of merit to use. It is enhanced by gain and its impact is shown in later slides showing simulations. Slew rate is determined by intrinsic detector signal characteristics and the amplifier rise time (tr).Other important factor is the noise for which the ENC2 has termswhich depend on the amplifier details, and detector characteristics: Current Noise2 ~ detector leakage current x tr

Voltage Noise2 ~ Cdet2/tr ~ Cdet for tr ~ Cdet

To minimize the voltage noise we need to keep the detector capacitance small, which also minimizes shot noise from the leakage current (small collection area) and rise time (gives large slew rate).


Detector Thickness and Signal Shapes

Typical signal characteristics versus detector thickness for pad detector with saturated drift velocities.

Conventional detector: Rise time similar for thick and thin, same slew rate.

Detector with a gain of 10: Rise time (and slew-rate) are different. Rise time ~ electron collection time, is proportional to detector thickness.


Slew-rate as a Function of Sensor Thickness

Weightfield2

simulation

N. Cartiglia, F.

Cenna

2014 IEEE NSS-

MIC

50 micron:

~ 3x improvement

with gain = 10

Significant improvements in time resolution expected for thin detectors.


Signal Simulations: n-on-p pad detector

Components making up signal (50 m thick detector): primary e- signal collected in about 0.5 nsec, gain signal peaks at about 0.5 nsec.


Have performed two beam tests of the pad detectors. The first at Frascati using electrons, with timing for two pad detectors compared (constant fraction discriminators were used to reduce time-walk). Second at CERN using a high energy pion beam. Start time provided by SiPM trigger. Occcasional events that have 2 or 3 particles in a beam spill provide events with large pulses.

For CERN beam test the Detector Capacitance = 11 pF, gain about 10. Scanning the detector with a laser indicates that the gain is constant over the pad face to about 3%.

Some Results from Recent Beam Tests


Typical pulse.50 psec bins.

20 bins smoothing.

Both overlapped.

Simulation, no electronic noise included.

Pulse Shapes in Pion Beam Test


Time Resolution

MIP

Fixed threshold = 10 mV, approximately 15% of MIP.

Mean is average time referenced to trigger, sigma is the variation around mean.

Data indicate importance of having a pulse height correction. Sigma for MIP is about 170 psec.

12


Weightfield2 simulation shows that changing to smaller sensors increase the slew rate and significantly improves the time resolution.

Effect of Detector Capacitance

Blue: Measurements from electron beam test or laser. Red: Simulation.


Latest Detectors from CNM

Gain


Comparison to Conventional PIN Diode


Next Step: Program to Make Detailed Measurements for Pad Detectors of Varying Dimensions

(For details of fabrication from CNM see talk of Pellegrini at Trento Workshop, Feb. 2015).

Array of pads of size varying from 300x300 m to 1x1cm. Thickness is 300 m. Next run: same pad sizes, thickness 200 m, due shortly. For Summer/Fall production of 100 m and 50 m arrays. Plan is to do detailed measurements of performance, especially establish correlation of timing accuracy with sensor thickness and sensor capacitance


AC Coupling to Achieve Segmentation (Ideal Configuration)

gain layer

AC coupling

n+ electrode

Detector Detector Detector Detector Detector Detector

AC coupling

The signal is briefly frozen on

the resistive sheet, and is AC

coupled to the electronics.


FBK Fab: Next Fall, Winter.

DC Single diodes:

0.5 - 5 mm2 single diodes,

reading either the n- or p-

side. DC read-out.

p-side

n-side

p-side

n-side

p-side

n-side

ACp-side

n-side

ACp-side

n-side

AC

AC Single diodes:

AC with different metal pads.

Fixed oxide thickness (250

nm). Two different sheet

resistance values: 0.5-2 kOhm.

(For details see: G.-F. Dalla Betta, et al., NIM A (2015)).


Layout - Multipads

6x6 mm2, 1 mm pixels (36 channels). DC or AC p-side

readout.

6 m

m

6 mm n-side

p-side

n-side

p-sideAC

10x10 mm2, several pixels of

different width (~ 20 channels).

DC readout on the p-side.

10 m

m10 mm


Other Applications: Forward Calorimeter

Calorimeter with layers providing excellent timing to pin down vertices by timing differences. Optimized design will depend on whether used as pre-shower or inside shower.An example: Possible unit cell made of 4 pads, each 3mm x 3mm, 4 pads readout by one chip at center.

3 mm


Protons of 200 MeV have a range of ~ 30 cm in plastic scintillator. The straggling limits the WEPL (water equivalent path length) resolution.

Replace calorimeter/range counter by TOF:Light-weight, combine tracking with WEPL determination

Future: 4-D UFSD Detector in pCT


Radiation Hardness Question

Detectors tested so far have the gain-generating implant made with boron. Radiation tests indicate an ~20%, drop in gain after a fluence of 1014 n/cm2 equivalent dose. This can be compensated for to a degree by raising the voltage but it would be much better to have a more radiation hard device since the problem will worsen at larger fluences. Plan is to try replacing the boron with gallium. The larger atomic mass is expected to lead to more radiation hardness according to the experts. Will be testing this in the future.


Conclusions

Detectors fabricated with gain layer show that stably operating detectors with gain ~ 10 can be made with good gain uniformity. These are capable of high rate operation. Simulation program (Weightfield2) allows prediction of performance based on detector and electronics parameters

In the coming year expect to perform extensive tests to establish dependence of timing accuracy on detector capacitance and thickness and to develop a pixel version of the detector to allow much better position measurement than with small pads, while maintaining the timing accuracy.

june 1-5, santa fe n. m. vertex 2015 1 ultra-fast silicon detectors june 3, 2015 abe seiden uc santa...

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