The Charm of Small Pixels ULITIMA 2018 Ronald Lipton - Fermilab - PowerPoint PPT Presentation

The Charm of Small Pixels ULITIMA 2018 Ronald Lipton - Fermilab Jason Thieman - Purdue University

Introduction There has been increasing interest in fast timing and “intelligent” detector systems. I would like to present some ideas for alternate designs of such systems based on small pixel size detectors. This talk is really about an exploration of ideas rather than a finished product. We focus on capabilities enabled by new technologies that provide small pixels with low capacitance and sophisticated processing • 3D integration of sensors and electronics • Monolithic active devices � 2


 
 
 
 
 Some Basics - time resolution • The rule of thumb for the time resolution of a system dominated by jitter is: 
 Jitter Front end noise Time resolution ( ) 2 + t d 2 4 ktA ⎛ ⎞ ∂ V ⎛ ⎞ Noise 2 = C L 2 t a C L σ t ~ σ noise σ n ⎜ ⎟ ~ t r ⎜ ⎟ σ t ~ ⎝ ⎠ ∂ t ⎝ ⎠ Signal g m t a Signal g m t a slew rate (dV/dt) is related to the inverse amplifier rise time, C L is the load capacitance t d and t a are the detector and amplifier rise times and g m is the input transistor transductance - related to input current, and A is a characteristic of the amplifier. • Fast timing -> large S/N, fast amp, small load capacitance • There are tradeoffs available � 3

Small Pixels In HEP, a current focus is in improving time 
 resolution by increasing the S/N by using 
 low gain (10-20) avalanche diodes of 
 ~1x1 mm to increase signal. • Large pixels - load capacitance of ~4 pf • Goal is time resolution of ~30ps These parts are sensitive to radiation damage due to the moderate doping of the gain layer An alternative is to increase S/N by lowering noise in a low capacitance finely pixelated sensor • These are now possible based on 3D integration of sensors and electronics as well as CMOS monolithic active pixels • Multi-tier 3D processing of small pixels can also enable sophisticated extraction of information in thicker sensors 
 � 4

3D Integration Fermilab has been involved the development of 3D sensor/ASIC integration for almost a decade and have demonstrated (with industrial partners): • Hybrid bonding technology Wafer-wafer bond Chip-wafer bond • Oxide bonding with imbedded metal 3 through silicon vias (TSV) 34 µ 2 • Bond pitch of 4 microns 1 • First 3-tier electronics-sensor stack • Small pixels with ADC, TDC (24 microns) 500" • Small TSV capacitance (~7 ff) 450" Unbonded" 400" Bump"bonded" The noise in hybrid bonded VIPIC 3D • 350" Fusion"Bonded" 300" assembly is almost a factor of two lower Counts' 250" than the equivalent conventionally bump 
 200" 150" bonded parts due to lower C load 100" 50" 0" 25" 30" 35" 40" 45" 50" 55" 60" 65" � 5 noise'(electrons)'


 
 
 
 
 TSV Test Structure Pixel Capacitance A detector with low capacitance can provide excellent time resolution: 
 σ t , pixel ∼ C pixel ≥ 10 2 × 1 1 × 20 ∼ 5 σ t , LGAD C LGAD Gain LGAD Before this improvement is realized other effects will dominate including charge deposition variations. Power TCAD Simulation considerations will limit front-end current 3 E-14 which will reduce transistor 200 Micron Thick Detector Capacitance (farads) 2 .5E-14 transductance 
 Farads/micron 2 E-14 1 σ t ∼ α g m ∼ I d , ( α ~ 1) 1 .5E-14 g m 1 E-14 However with “spare” margin we can 5 E-15 y = 2E-18x 2 + 1E-16x + 1E-15 become more adventurous 0 0 20 40 60 80 100 120 � 6 Pixel Pitch (microns)

3D 9 pixel model Methodology We explore simple systems with various pixel sizes, detector thickness and pulse shapes • Build a (Silvaco) TCAD (2D or 3D) detector model • Inject a Q tot =4 fc pulse • Extract the capacitance and pulse shapes at the electrodes • Inject the resulting pulse into a SPICE model of a generic 65nm charge sensitive amplifier including Pulses from “x-ray” at~100µ noise 200µ thick detector • Analyze the characteristics of the 
 resulting output pulses • Monoenergetic - no time walk or 
 ionization fluctuations in this study This allows fast turn around studies of 
 various configurations 3ns � 7

Example - MIP in a 50 micron Detector σ ~25 micron pitch, 50 microns thick 200 V, sensor potential distribution Pulse on central pixel 2ns Amplifier output with noise, 20ff load Timing histogram h h Entries Entries 26 26 Mean Mean 102.3 102.3 12 Std Dev Std Dev 0.01593 0.01593 c c 2 2 / ndf / ndf 0.6285 / 1 0.6285 / 1 ± ± Constant Constant 12.34 12.34 3.24 3.24 ± ± Mean Mean 102.3 102.3 0.0 0.0 ± ± Sigma Sigma 0.01657 0.01657 0.00311 0.00311 10 σ ~16ps 8 Threshold 6 4 2 � 8 0 102 102.1 102.2 102.3 102.4 102.5 102.6 102.7

Signal Development • Signal induced by moving charges ! E w × ! depends on work done by circuit. i = − q v The charge induced on an electrode ! d ! depends on the coupling between ∫ ∫ Q s = idt = q E w x the moving charge and the electrode Q 1 → 2 = q ( V w 2 − V w 1 ) (Ramo’s theorem) • In a multi-electrode system the induced current on an electrode depends on the velocity of the charge and the value of the effective “weighting” field • Weighting field is calculated with 1 V on measuring elected, 0 V on others • There are fast transient induced currents on neighbor electrodes that integrate to zero - can we use them? � 9

Example - X-Rays Suppose an application requires fast timing on high energy x-rays • Usually we would like thin detectors for fast timing, but thin detectors imply low efficiency - can we used induced currents? 200 µ detector, charge at 185 microns, n-on-p • • Initial current spike is ~identical for all channels, central pixel rise is late - due to the weighting field Central pixel Initial Current Spike Edge neighbor Corner neighbor � 10

Pulse Shapes - 200 micron detector Χ -ray Central Electrode z=10 z=10 z=100 z=190 2ns 2ns z=190 z=100 Central = n n+1 n+2 n+3 2ns 2ns � 11

Time resolution of a thick detector • We use the TCAD/SPICE simulation chain to model an x-ray in the thicker detector Central pixel 
 peak at 1V, 12 ns Electrode 3 – Deposit at Z=25 Electrode 1 – small pulse, but fast rise Deposit at Z=185 – note scales are not equal Edge pixel peak at 25 mV, 2 ns � 12 100ns 110ns

With Noise at 185/200 micron depth • Apply a constant threshold of E1~730 mV, E4~850 mV • Tabulate time at threshold crossing including noise E1 Edge pixel E1 σ ~ 30ps Timing histogram h h 10 25 25 Entries Entries Mean Mean 104.5 104.5 0.0204 0.0204 Std Dev Std Dev χ χ 2 2 / ndf / ndf 0.4421 / 2 0.4421 / 2 ± ± 9.179 9.179 2.457 2.457 Constant Constant Central pixel 104.5 104.5 ± ± 0.0 0.0 Mean Mean ± ± Sigma Sigma 0.02177 0.02177 0.00445 0.00445 8 σ ~ 22ps E4 6 4 2 � 13 0 104 104.2 104.4 104.6 104.8 105

Comments The 20-30 ps resolution will be degraded in a real system However: • All pixels with spacing small compared to depth will have similar signals ~ 16 pixels for a 25x200 micron sensor x 4 in (uncorrelated) time resolution • The central pixel will see a large signal within a few ns of the leading edge - initial thresholds can be set low and signals latched if a central pixel fires at a higher threshold • The pattern of pixels will also provide depth information • Multiple thresholds or more sophisticated processing can give a time walk correction if needed • These results are for n-on-p with maximum field at the top. n- on-n sensors have a maximum field at the bottom. The field profiles can be adjusted to suit the application by varying the applied bias � 14

CMS “P t module” Example - Pattern Recognition Collider based experiments have to 
 deal with increasingly complex events • HL LHC with ~200 interactions per 
 crossing • The CMS experiment is addressing this with stacked sensor arrays to distinguish low from moderate momentum tracks • Can we do this in a single sensor? • Muon collider experiments with huge decay backgrounds • Muon collider studies use timing - fall x 100 short • Backgrounds are from various absorber surfaces/angles • We can use the pattern of electrode signals to distinguish between signal and background tracks signatures To get a feeling for this we use the same electrode geometry in a ~300 micron thick sensor. � 15

Charge Motion Visualization 15 degree track, n on n, maximum field at bottom. electrons holes .1 ns .5 ns 1.0 ns 1.5 ns 2 ns 2.5 ns � 16

MIPs at various angles 0 Deg 5 Deg 10ns 10ns 10 Deg 15 Deg � 17 10ns 10ns