Development of an Active Pixel Sensor Vertex Detector

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


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


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


CMOS APS with Epitaxial Sensor

n+ diffusion

p epi.

p+ bulk

Particle track

Electrostatic potential

p well

n well

nMOS

(A) (B)

(A) (B)


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


“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


4 Configurations

• 4 variants:

– Small pickup

– 4x small pickups

– Small pickup +

direct injection

– Large pickup +

Direct injection


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


Sr90 Electron Source

• Quadrant of 64 x 64 pixels with (left) and without (right) Sr90 source applied.


1.5 GeV electron source (ALS)

• Quadrant with (left) and without (right) electron source applied.


Energy spectrum of 1.5 GeV electrons

• Circles are measured points, dotted line shows calculated result for 8 µm epitaxial layer.


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


Sample Fe55 Spectra

1638 electrons


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


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


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


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


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


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


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


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


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


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


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


Pre Radiation

Post Radiation

€

10

1012 protons at 55 MeVEquivalent to 3 years at RHIC at 40x currentluminosity


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


> Mrad exposure


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


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


Mechanical Configuration


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.


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

Development of an Active Pixel Sensor Vertex Detector

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