1. LePIX: first results from a novel monolithic pixel sensor
Serena Mattiazzo
University & INFN of Padova
M. Caselle, Y. Ikemoto, K. Kloukinas, C. Mansuy, A. Marchioro, W. Snoeys
CERN CH-1211, Geneve 23, Switzerland
D. Biselloa, L. Demariab, P. Giubilatoa, S. Mattiazzoa, D. Pantanoa, A. Potenzab, A.Rivettib, L. Silvestrina
a: University and INFN section of Padova, Italy
b: INFN Section of Torino, Italy
P. Chalmet, H. Mugnier, J. Rousset
MIND-Micro Technologies-Bât Archamps
M. Battaglia
UC Santa Cruz
12th Pisa Meeting on Advanced Detectors

2. 12th Pisa Meeting on Advanced Detectors
Outline
LePIX concept
Pixel design layout
Results:
Test in laboratory
Test beam with high momentum particles
Radiation damage tests
Low power architecture
Conclusions
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

3. Standard hybrid pixels and MAPS
In hybrid pixels detectors and electronics are fabricated on different substrates.
The choice of the detector material can be optimized, depending on the application (Si, GaAs, etc). For HEP: high resistivity silicon
Bump-bonding: expensive, high material budget, parasitic capacitance
Charge collection is by drift, good radiation hardness.
Pixel size: 50m  50m to 50m  400m.
Complex front-end electronics, high read-out speed.
Typical power density: 250 mW/cm2.
Monolithic Active Pixel Sensors (MAPS): detector and electronics on the same substrate.
Only commercial CMOS technologies.
Pixel size: 20m  20m or lower.
Limited amount of charge collected in the thin (10-20 m) epitaxial layer; more sensitive to bulk damage (diffusion mechanism).
Lower read-out speed (often rolling shutter).
Power density for fast options: 100mW/cm2.
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

4. LePIX: a monolithic detector in advanced CMOS
Electronics
Collection electrode
High energy particle
Sensitive layer
Develop monolithic pixel detectors integrating readout and detecting elements by porting CMOS to moderate resistivity substrates
Motivation: very interesting for tracker and pixels !
Good radiation hardness (charge collection by drift).
Monolithic integration
Low capacitance for low power consumption (low material budget)
Easier overall integration
High production rate (20 m2 per day…) and cost per unit area less than traditional detectors
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

5. Low Capacitance for Low Power
Let’s take the transistor noise at 40 MHz BW for 1 A
(1A/100100 m pixel  10 mW/cm2) + - n+ p V
Collection depth
300 mm
30 mm
3 mm
Noise is only very weakly dependent on the current (NI-m, m<1/2):
For the same , the power consumption ~ (Q/C)-2..4
more Q/C allows very significant power reduction for same S/N
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

6. Circuitry in the collection electrode
Only few transistors in the collection electrode for a low C
P-substrate
N-well collection electrode
P-well
NMOS in P-well
PMOS in N-well
Figure not to scale!
Would like small collection electrode for minimum capacitance (C); in-pixel circuitry placed in the N-well collection electrode prevents loss of signal charge, but higher input capacitance & risk of coupling circuit signals into input
A uniform depletion layer for uniform response: larger pixels more difficult
Effective charge resetting scheme robust over a large range of leakage currents
Pattern density rules in very deep submicron technologies very restrictive.
Insulation of the low-voltage transistors from the high voltage substrate.
Sensor needs to be designed in close contact with the foundry
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

7. The submission
Feedback from foundry: with a reverse bias of up to 100 V on moderate resistivity substrate we can collect signal charge by drift over several tens of micron
Low-K dielectrics in the metal stack beyond 130nm
Non standard: ESD protection, special layers, mask generation, guard rings.
7 chips submitted: 4 test matrices (analog and digital readout and 2 collection electrode sizes), large diode for radiation tolerance, breakdown test structure, transistor test
Small thin oxide PMOS transistor
Larger thin oxide PMOS transistor
Thick oxide PMOS transistor
NMOS transistor
3232 pixels
50m pitch
8 sectors
Active
reset
Continuous reset
Diode
Breakdown test
Transistor test
Serena Mattiazzo

8. Design: pixel circuit architecture
Switches for storage
Switches for readout
To readout
Bias circuit
Reset
Store reference
Reset
Store signal
Readout
Integration time
nwell collection diode
Pmos input device.
Only one PMOS transistor in the pixel (or maybe very few…)
No clock is distributed in the pixel; use analog power to send the signal to the periphery
Each pixel is permanently connected to its front-end electronics located at the periphery of the matrix (90 nm CMOS).
The analog signal is sampled twice, once after reset and once after the integration time. The difference between the two values is the signal collected in that time interval (Correlated Double Sampling, CDS).
Storing is done in parallel for all pixels in the matrix, while readout is sequential (integration time independent of the readout speed). Working on fast architecture
Prototype with preamplifier/shaper and discriminator currently under test in Torino
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

9. First results from submission
Actually two submissions made: very same design but different substrates: standard and high resistivity one. In the first submission there was a short due to mask generation issue: the guard ring received p+ implant creating a short
Corrections were implemented on the «on hold» high resistivity substrate lot on 6 different masks; new design submitted one year ago
Received prototypes on high resisitivity silicon on Nov 10th (started testing Nov 11th)
-200 V on large diodes
-100 V on matrices in the lab
Differences above not understood
No hard breakdown, above limit gradual increase with time (thermal runaway ?)
Values reached encouraging
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors 9
Large diode
Matrices

10. Depletion depth (diode)
Resistivity:
400-cm   31013cm-3
45 m for Vsub = -40V: very good for Q/C
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

11. 12th Pisa Meeting on Advanced Detectors
Matrix: tests with 55Fe
55Fe
55Mn
K = 6.4keV
K = 5.9keV  1640e-
  140eV (40e-)
Fe-55 spectrum at room temperature
The two peaks at 5.90keV and 6.45keV can be distinguished
Sigma 140eV or 40e- RMS (10s of integration time, Vsub = -20V)
Illustrates S/N potential of the device
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

12. Matrix: test beam @ PSI, 300 MeV - (PiM1 beamline) Layer 3 Layer 2
LePIX
Power supply filters
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12th Pisa Meeting on Advanced Detectors

13. Signal and multiplicity
@ PSI, 300 MeV - (PiM1 beamline)
Cluster Vsub = -60V
Cluster Vsub = -60V
Sector 0
Small thin oxide PMOS transistor
Larger thin oxide PMOS transistor
Thick oxide PMOS transistor
NMOS transistor
Sector 1 13
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

14. Drift vs diffusion @ PSI, 300 MeV - (PiM1 beamline)
Plane 1
Plane 4
Plane 2
Plane 3
Proton, 0 Volts bias
Diffusion: at zero bias, incident protons generate on average clusters of more than 30 5050m pixels.
Drift: for significant reverse bias (60V) cluster reduced to a few pixels only
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

15. Matrix: efficiency @ PSI, 300 MeV - (PiM1 beamline) 40 Volts bias
Plane 2
Plane 3
Plane 1
Plane 4
Track in telescope of 4 planes
Depletion is not uniform: signal charge is collected only from the pixel center, not from the pixel corners.
This leads to a 70 % efficiency for MIPs
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

16. 12th Pisa Meeting on Advanced Detectors
Laser scan
Scan of one row of pixels with backside laser illumination
(IR 1060nm, 6ns) y
The signal shape (sum of the signal in two adjacent pixels) confirms that signal charge is collected only from the pixel center, not from the pixel corners.
Simulations and focused ion beam repair (FIB) are in progress to fully understand and fix this issue
The collected charge increases with reverse bias x
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

17. 12th Pisa Meeting on Advanced Detectors
Depletion evolution
For the moment simulation can reproduce the shape, but not voltages at which pixel enters and leaves the “bubble shaped” depletion regime
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

18. Radiation damage: Total Dose
See poster by Alberto Potenza
10 Mrad X-ray irradiation (4.8 krad/min – irradiation biased – anneal not)
pre-rad (red), post-rad (green), after one week at room T (yellow)
Pulse train is serial analog output from small pixel matrix
Some sectors more affected than others immediately after irradiation,
reasonable recovery after one week.
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

19. Radiation damage: Bulk Damage
Neutron irradiated samples :
Fe-55 spectra
(Irradiation by Ljubljana with Alice help (P. Riedler))
After 1014neq/cm2
@ -24°C
After 1014neq/cm2
@ -16°C
See poster by Alberto Potenza
Threshold current (Ithr) above which pixel capacitance becomes prohibitive
Below this threshold current the pixel operates normally with leakage dominated noise increasing with T and dose
Capacitance increases by about 10% between 21013 and 1014 (below Ithr)
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

20. Large cluster if charge is collected by diffusion
Monolithic pixels offer:
Summary
Small pixels
Low material budget
Low capacitance
Low power consumption
Monolithic pixels for
LOW MATERIAL BUDGET
Depleted devices for LARGE SIGNAL
and SMALL CLUSTERS
Small Pixels
Low Power consumption in the pixels
To fully exploit the low power consumption offered by the monolithic approach, also the architecture must be low power: few transistor per pixels, no clock across the matrix, etc…
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

21. Low power architecture: Orthopix (by P. Giubilato)
Smart self-aware pixels (current trend in HEP): they determine whether they contain important data, need brain power
Stupid pixels (Orthopix approach) do not care, just transfer info to the periphery, no brain to feed!
Add more projections with respect to traditional sparsification XY (which fails in case of multiple hits, but it reduces data from N2 to 2N in a static way)
The projections defined by Orthopix are implemented within the matrix itself using metal connections and the compressed data are made available at the chip periphery without requiring further processing
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

22. 12th Pisa Meeting on Advanced Detectors
Conclusions
LePIX tries to integrate a commercial deep submicron CMOS electronics into a moderate resistivity substrate to create a monolithic pixel
Low capacitance  Low Power (target 20mW/cm2)  Low mass
Collection by drift  Radiation tolerant
Tests on the first prototype on high resistivity substrate
Promising resistivity  High Q/C
Good 55Fe spectrum  Low C and low noise
Beam test  depletion not uniform (trying to investigate and fix)
Radiation damage: TID OK, Bulk: more work & understanding
Work on the digital matrix to be started in Torino
Work on high speed architecture
Low Power Architecture: OrthoPIX
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

23. 12th Pisa Meeting on Advanced Detectors
Thank you
for your attention
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

24. 12th Pisa Meeting on Advanced Detectors
Backup slides
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

25. Orthopix (by P. Giubilato): smaller and simpler pixels
Smart self-aware pixels (current trend in HEP): they determine whether they contain important data, need brain power
Stupid pixels (orthopix approach) do not care, just transfer info to the periphery, no brain to feed!
Sparsification and data transfer to the periphery done by the same mechanism
Traditional sparsification (XY) fails in case of multiple hits, but it reduces data from N2 to 2N in a static way
Minimum metal pitch P in 90 nm CMOS is 0.28 μm for most routing metals. For a pixels column of 1 cm height and one metal layer used for routing this yields a minimum pixel size of about 53 um. This minimum pixel size decreases with the square root of the number of routing layers but increases with the size of the matrix
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

26. Orthopix: N maps static sparsification
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

27. Orthopix: reconstruction purity
NN matrix
Indicates 2 Ghits/cm2 feasible with small pixels
Simple pixel & immediate sparsification => low power
Hits are not lost, remain available for tracking
Fixed maximum occupancy depending on pixel count
Would like small clusters
Assuming uniform hit distribution, needs to be verified vs real physics events
Technology independent
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

28. First submission: mask generation issue
Actually two submissions made: very same design but different substrates: standard and high resistivity one.
In the first submission there was a short due to mask generation issue: the guard ring received p+ implant creating a short (which transforms into a ~80 ohm resistor due to series resistance)
Discovered on standard substrate, exists on all structures (4 matrices, test diode and breakdown structure).
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

29. First submission: circuitry operational
Digital circuitry was operational: a lot of measurements and tests have been carried out on the “flawed” standard resistivity lot structures.
High resistivity substrate lot has been kept on hold
4 zones of 8 columns with different input transistor clearly visible
Difference between active and diode reset
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

30. First submission: breakdown
The breakdown test structure contains a 23 pixel matrix surrounded by a ring of pixels and guard (see picture)
The central pixels can be reverse-biased alone, maintaining the guard at the same potential as the substrate
Breakdown < -30V on standard substrate, close to expected for a planar junction
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

31. 55Fe charge collection and field region
Point corresponding to chip without repair
Separated electrodes – using Focused Ion Beam (FIB) – so that bias could be optimized separately for different zones in the field area
Rate increases – keeping all other conditions the same – reaching a plateau around -6 V for the poly at ~ % rate compared to unrepaired chip
In the beam test efficiency was 70%, further verifications in progress but rate increase observed here coherent with full efficiency recovery…
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

32. Noise vs integration time
T = 22°C
Vsub = -50V
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12th Pisa Meeting on Advanced Detectors

33. 12th Pisa Meeting on Advanced Detectors
Spherical junction R1 R2
Pixel pitch: 50m
Planar junction
Spherical junction
For a planar junction:
Cubic root!
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

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Spherical junction
Resistivity: 400-cm  31013cm-3
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

35. 12th Pisa Meeting on Advanced Detectors
Recalling the S/N relation…
…and assuming a constant S/N…
Weak… Strong inversion
…segmentation affects power consumption
For constant signal-to-noise, analog power decreases with segmentation (will eventually saturate at high segmentation)!
Higher segmentation is very good!
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

36. 12th Pisa Meeting on Advanced Detectors
Landau peaks
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12th Pisa Meeting on Advanced Detectors

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Vsub = -20V
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

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Vsub = -60V
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

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Leakage current
nA for Vsub = -30V (on the diode)
3-4 pA/pxl
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

40. 12th Pisa Meeting on Advanced Detectors
Pixel capacitance
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors

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Poly and Metal effects
Serena Mattiazzo
12th Pisa Meeting on Advanced Detectors