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Development of an Active Pixel Sensor Vertex Detector. H. Matis, F. Bieser, G. Rai, F. Retiere, S. Wurzel, H. Wieman, E. Yamamato, LBNL S. Kleinfelder, K. Singh, UCI H. Bichel, U. Washington. STAR Needs a Thin Vertex Detector to Measure Charm at RHIC. High precision - ~4 µm resolution - PowerPoint PPT Presentation
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Development of an Active Pixel Sensor Vertex Detector
H. Matis, F. Bieser, G. Rai, F. Retiere, S. Wurzel, H. Wieman, E. Yamamato, LBNL
S. Kleinfelder, K. Singh, UCI
H. Bichel, U. Washington
H. Matis ([email protected]) 2Pixel2002
STAR Needs a Thin Vertex Detector to Measure Charm at RHIC • High precision - ~4
µm resolution• Low mass - 1 GeV/c
particles - need low multiple scattering
• Medium radiation environment - 50 krad/y - @ 40x RHIC luminosity
40 µm 80 µm
160 µm 320 µm
640 µm
H. Matis ([email protected]) 3Pixel2002
Active Pixel Sensor (APS) – Attractive Technology• Has same advantages of CCDs
– Small pixels– Can thin wafers
• Plus– Standard CMOS process– More radiation resistant– Low power– Put extra circuits on chip
• Minus– New Technology– Lots to learn
H. Matis ([email protected]) 4Pixel2002
Epitaxial Sensor Medium
• High-resistivity epitaxial silicon used as a sensor
• Higher doped P bulk reflects and confines electrons
• Slower, more lateral diffusion and recombination
• 100% fill factor achieved
n+ diffusiondepletion region
p- epi.
p+ bulk
Particle track Electrostatic potential
H. Matis ([email protected]) 5Pixel2002
CMOS APS with Epitaxial Sensor
n+ diffusion
p epi.
p+ bulk
Particle track
Electrostatic potential
p well
n well
nMOS
(A) (B)
(A) (B)
H. Matis ([email protected]) 6Pixel2002
Three Example CMOS Pixel Circuits
• Passive Pixel Sensor (PPS, left)• Active Pixel Sensor (APS, middle)• APS with sample and hold / shutter (right)
APS pixel
ResetAccessPower
Output
PPS pixel
Access
Output
APS pixel with shutter
ResetAccessPower
Output
Shutter
H. Matis ([email protected]) 7Pixel2002
“EPI-1” Prototype Epi / APS Imager
• 0.25 µm CMOS
• 128 x 128 array
• 4 pixel variants
• 20 x 20 µm pixels
• 8-10 µm Epi
• Fabbed at TSMC
H. Matis ([email protected]) 8Pixel2002
4 Configurations
• 4 variants:
– Small pickup
– 4x small pickups
– Small pickup +
direct injection
– Large pickup +
Direct injection
H. Matis ([email protected]) 9Pixel2002
APS Pixel QuadrantsAPS3 (upper left)
ResetRead
PowerBias
1x diff> well Output
APS1 (lower left)ResetRead
PowerBias
APS2 (lower right)
1x 4x
Output
TX-S/H
APS4 (upper right)
Output4x diff> well
Output
H. Matis ([email protected]) 10Pixel2002
Sr90 Electron Source
• Quadrant of 64 x 64 pixels with (left) and without (right) Sr90 source applied.
H. Matis ([email protected]) 11Pixel2002
1.5 GeV electron source (ALS)
• Quadrant with (left) and without (right) electron source applied.
H. Matis ([email protected]) 12Pixel2002
Energy spectrum of 1.5 GeV electrons
• Circles are measured points, dotted line shows calculated result for 8 µm epitaxial layer.
H. Matis ([email protected]) 13Pixel2002
Version II - 16 different configurations
• Row 1 - Pixels with one to four distributed diodes.
• Increase in charge collected within one pixel– Less charge diffusion to neighboring pixels– But lower gain due to increased capacitance
H. Matis ([email protected]) 15Pixel2002
Speed Matters• Output of ADC• Currently reading a pixel with
500 kHz clock - limited by external ADC
• Easily could read at 1 MHz• Need 250 ms to read out
1000 x 1000 chip with 4 channels at this speed
• Working to improve speed for next generation
1 µs/division
H. Matis ([email protected]) 16Pixel2002
Total Collected Charge (Fe-55)
600
800
1000
1200
1400
1600
1800
1 10 100# of summed pixels (log scale)
Mean charge collected, electrons
32 ms Frame Period
2 ms Frame Period
H. Matis ([email protected]) 17Pixel2002
Signal to Noise (Fe-55)
0
5
10
15
20
25
30
35
40
45
1 10 100Number of summed pixels (log scale)
Signal to noise ratio
32 ms Frame Period
2 ms Frame Period
H. Matis ([email protected]) 18Pixel2002
Diode Topology vs.. Collected Charge
• Normalized charge plots. • More diodes yields greater percentage of charge collected
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1 10 100
Number of summed pixels
Ratio of collected to peak charge
One diodeTwo diodesThree diodesFour diodes
H. Matis ([email protected]) 19Pixel2002
Diode Topology vs. Signal to Noise
• More diodes reduces S/N except for the single pixel case (no summation of neighboring pixels)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1 10 100
Number of summed pixels
Normalized signal to noise ratio
One diodeTwo diodesThree diodesFour diodes
H. Matis ([email protected]) 20Pixel2002
Other Configurations - Rows 2-4
Row 2 - Same as Row 1 except larger output transistor
Row 3 - Centered pixel1 small pickup2 medium well pickup3 large well pickup4 large diffusion
Row 4 - Sample and Hold 1 small well pickup2 medium well pickup3 large diffusion4 large diffusion
H. Matis ([email protected]) 21Pixel2002
S/N All SectorsRow 2: S/N vs Pixels Summed
0
5
10
15
20
25
1 10 100
Pixels Summed
S/N
Row 2 Col 1
Row 2 Col 2
Row 2 Col 3
Row 2 Col 4
Row 3: S/N vs Pixels Summed
0
5
10
15
20
25
1 10 100
Pixels Summed
S/N
Row 3 Col 1
Row 3 Col 2
Row 3 Col 3
Row 3 Col 4
Row 4: S/N vs Pixels Summed
0
5
10
15
20
25
1 10 100
Pixels Summed
S/N
Row 4 Col 1
Row 4 Col 2
Row 4 Col 3
Row 4 Col 4
Row 1: S/N vs Pixels Summed
0
5
10
15
20
25
1 10 100
Pixels Summed
S/N
Row 1 Col 1
Row 1 Col 2
Row 1 Col 3
Row 1 Col 4
Row 1 Row 2
Row 3 Row 4
H. Matis ([email protected]) 22Pixel2002
Charge Diffusion
• Increasing number of diode collection points increases collection with lower signal
• Sample and Hold collects charge in few pixels but much lower signal
1 100
100
200
300
Pixels Summed
Quad 1-1
Quad 1-4
Quad 4-1
Quad 4-2
Quad 4-4Row 1 - 1 diode
Row 1 - 4 diode
Row 4 -Sample and Hold
H. Matis ([email protected]) 23Pixel2002
Radiation Hardness
• CCDs show radiation effects ~ krad
• 3 year RHIC design luminosity - 3.5 krad or 1 x 1011 55 MeV p’s/cm
• 3 year RHIC II at 40x design - 140 krad
• Expose unpowered chips to 55 MeV p’s at 88” cyclotron
H. Matis ([email protected]) 24Pixel2002
Pre Radiation
Post Radiation
€
10
1012 protons at 55 MeVEquivalent to 3 years at RHIC at 40x currentluminosity
H. Matis ([email protected]) 25Pixel2002
Mean Leakage Current Per Pixel vs 55MeV protons
0
100
200
300
400
500
600
1.00E+11 1.00E+12 1.00E+13
protons
electrons / frame period
Mean Leakage Current Per Pixel vs 55 MeV protons
0
100
200
300
400
500
600
1.00E+11 1.00E+12 1.00E+13
protons/cm2
electrons / frame period
1.5 1011 protons, 55 MeVEquivalent to 0.5 y @ 40x currentLuminosity of RHIC (RHIC II)
3 y @ RHIC II
1.5 y @ RHIC II
9 y @ RHIC II
30 y @ RHIC II
Mrad
H. Matis ([email protected]) 26Pixel2002
> Mrad exposure
H. Matis ([email protected]) 27Pixel2002
Signal Loss to Radiation
• Signal does decrease with radiation dose
• Noise increases • Small change in
radiation region of our interest
• Significant Mrad effects
Fe55 Peak vs. Radiation
0
50
100
150
200
250
300
0 2E+12 4E+12 6E+12 8E+12 1E+13
Intensity - 55 MeV protons/cm2
ADC Counts
Noise vs. Radiation
0
5
10
15
20
25
0 2E+12 4E+12 6E+12 8E+12 1E+13
Intensity - 55 MeV protons/cm2
ADC Counts
H. Matis ([email protected]) 28Pixel2002
Thinned Silicon• CCD detector thin to epi
layer (with backing)• Testing 50 µm and 100
µm wafers• 50 µm wafer can be
stretched to >1 kg (limit is our stain gauge)
• Build mechanical support easy to replace modules - beam accident
Silicon
H. Matis ([email protected]) 29Pixel2002
Mechanical Configuration
H. Matis ([email protected]) 30Pixel2002
Summary and Conclusions• A CMOS active pixel sensor array using an epitaxial silicon sensor has
been designed and tested.
– Two 128 by 128 pixel arrays were fabricated
– Both used a standard digital 0.25 micron CMOS technology
– Both used 8-10 micron epitaxial silicon sensors
• Variety of pixel topologies and circuits were tested.
• Optimum performance in sparse-event environment was obtained by simplest, highest gain pixel circuits.
• Tested with 1.5 GeV electrons and Fe-55 X-rays
• Obtained 13 electrons RMS noise and an SNR for single Fe-55 X-rays (5.9 keV) of greater than 30.
• Standard digital CMOS APS can resolve individual gamma rays and minimum-ionizing charged particles.
• CMOS technology appropriate to radiation environment of RHIC.
H. Matis ([email protected]) 31Pixel2002
Future
• Must fully understand noise sources – improve signal to noise. Reduce charge diffusion
• New faster chip in 0.5 µm process ready soon. Larger epi layer
• Increase speed of chip– 1000 x 1000 array with four parallel channels - 50
ns readout 12.5 ms cycle time• Mechanical Prototyping. Fixture ready in a week.• Great promise of APS technology at RHIC
The End