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November 3-8, 2002 D. Bortoletto - Vertex 2002 1

Silicon Sensors for CMS Silicon Sensors for CMS

Daniela BortolettoPurdue University

Grad students: Kim Giolo, Amit Roy, Seunghee Son

Engineering Physicist: Gino Bolla

• OUTLINE– Design consideration for Pixel sensors for the LHC: p-on-n

versus n-on-n and p-stops versus p-sprays – Summary results from CMS Forward Pixel (FpiX) first

prototype submission Sintef 1999 (received 2000)– Design improvements and results from Sintef 2001

submission (received 2002)– Irradiation studies up to 1015 neq/cm2

– Barrel sensor design (Tilman Rohe)– Conclusions


FPIX COLLABORATIONFPIX COLLABORATIONPSI (Horisberger) ETHU. ZurichU. BaselIHEP WienRWTH Aachen

US CMSUC Davis NorthwesternFermilab PurdueJohns Hopkins Rutgers Mississippi

BARREL

2 Layers, 17(27) Mpixels

FORWARD DISKS:4 disks, 12 Mpixels1.5<<2.5


Design ConsiderationsDesign Considerations• The LHC detectors will be

hybrid pixels– Readout chip is very

complex (500 K transistors)

– Sensor are simpler (50k diodes)

• Irradiation changes silicon– Type inversion of the bulk

material n p– Increase of effective doping and

full depletion voltage– Complex annealing and anti-

annealing behavior– Undepleted bulk becomes high

resistive– Increase trapping of signal

charge


Radiation HardnessRadiation Hardness• The CMS pixel design has

been optimized for a dose of 61014 neq/cm2

• Fluence is dominated by ’s. Oxygenation is expected to be useful

• Crucial to limit the periods without cooling because of anti-annealing

Rose collaboration


Design ConsiderationsDesign Considerations

p-on-n

n-on-n


Design considerations Design considerations • p-on-n option

– require sensors to be depleted for operation:

• High voltage after irradiation

• Complex guard ring design• Difficult module

construction • Possible damage to the

chip because of high V and small distance between chip and the sensor

• Protection of unconnected pixels may be necessary

• To reduce trapping small gap between pixels

Tilman Rohe pixel 2002


Design considerations Design considerations • n-on-n option:

– Allows operation of undepleted sensors after type inversion

– Requires double sided processing

• More expensive• Lower yield • Testing with bias grid

(Atlas), resistive network (CMS)

– N-side pixel isolation• P-stops (CMS)• P-spray (Atlas)

– Design optimized for irradiation

• Guard rings • Unbonded pixel protection

Tilman Rohe pixel 2002


Guard ring Design Guard ring Design • Guard rings must satisfy two requirements:

– Limit the lateral extension of the depletion region– Prevent breakdown at the device edge

• These goals can be achieved by:– Gentle potential drop towards

the edge– Increasing gaps from inner to

outer region– Field plates to reduce the field


Eleven guard ring design (Sintef 1999)

After irradiation11 Guard Ring Diodes

1.E-10

1.E-09

1.E-08

1.E-07

0 500 1000

Reverse Bias Voltage (V)

Cu

rren

t (A

)

Diode 3Diode 4 Diode 11Diode 14Diode 20

Guard Ring PerformanceGuard Ring Performance

Diode

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0 100 200 300 400 500 600 700 800

Reverse Bias (V)Le

akag

e C

urre

nt (A

)

S22P47

S22P29

S24P29

S4P47

= 61014 neq/cm2

Before irradiation

No breakdown up to 800 V even after irradiation to = 61014 neq/cm2 Guard ring design frozen.


N-side isolation N-side isolation • P-stops

– Standard processing for most vendors– Additional mask– Alignment and design rules can lead to

large gaps• P-sprays

– No extra mask– Lower cost – No alignment– Better performance after irradiation

• Moderated p-sprays– No additional mask– Good performance before and after irradiation


N-side isolation N-side isolation • Charge trapping in Oxyde layer

P-stops P-sprays

N.I.M. A 377 (1996) 412

0.2

0.2 3.0

3.0


N-side isolation N-side isolation • Sintef 1999 submission focused

on double open p-stop ring (CMS Tracker-TDR baseline)

• We tested 8 p-stop options. Best designs have open p-stop rings (A, F and G)

• Opening between p-stops provides resistive network

F: Single open ring G: Double open ring 2A : Double open ring


P-stop performance P-stop performance • Performance was measured before and after

irradiation

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0 100 200 300 400 500Reverse Bias (V)

Lea

kag

e C

urr

en t

(A) Pixel Current

Guard Ring Current

Before irradiation After irradiation

• IV measurements at -10 C after irradiation show:

• Vbias< 300 V: Normal operation

• 300V< Vbias<550 linear increase of the leakage current from the pixel area (soft breakdown)

• Vbias>550V breakdown

0

5

10

15

20

25

30

0 200 400 600 800 1000

Breakdown Voltage Distribution - All Pixels

Range

1% 1% 1%

4%

6%

8%8%

13%

26%

31%

Design G

T=-10 0C


P-stop performance P-stop performance

• TDR Sensor was connected to prototype chip at PSI.

• “Soft breakdown” current is draw by few pixels that become noisy at around 300 V

• Noisy pixels are uncorrelated to missing bond connections


P-stop performance P-stop performance • Design with one open ring

(F):– Allows for smaller gaps– Shows improved

performance after irradiation

– No hard breakdown up to 800 V

– Lower slope of leakage current increase after “soft breakdown”

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0 100 200 300 400 500 600 700 800

Bias Voltage (V)

Leak

age

Curr

ent (

A)

S24P4F S22P4F

S4P4Foxy S21P4F

S4P4Foxy

= 11014 neq/cm2

= 61014 neq/cm2

Design F

= 11014 neq/cm2 = 61014neq/cm2

A(TDR) at 300V ~5.0nA/pixel >10nA/pixel G at 300V ~1.9nA/pixel ~5.0nA/pixelF at 300V ~0.5nA/pixel ~4.0nA/pixel

T=-10 0C


Sintef 2001 submissionSintef 2001 submission • Wafer Layout:

– 125x125 Finalize single pixel design (PSI-30 36 40 pixels Honeywell chip)

– 150x150 to match existing DMIL PSI-43 full size 52 53 pixels chip)

– 150x100 to match IBM 0.25m compatible layout

• 15 wafers Instrument 5 blades

• Bulk: (1,0,0) Resistivity=1-2 Kcm, thickness 275 m, several oxygenated wafers


Single pixel design Single pixel design P-stopP-stop • Sintef 2001 (received in Summer

2002) submission focuses on single open p-stop. Small modifications: – improve yield (F design baseline).– Reduce inter-pixel regions to improve

charge collection efficiency (FM design).– Field plates to improve breakdown

FM

Field

Plate

Average Breakdown voltage increases by 200 V


• July 2002: Irradiated 85 structures (single ROC silicon sensors + diodes) at IUCF with 200 MeV protons.– 15 pixels sensors and 10 diodes @ = 1x1014 p/cm2

– 24 pixels sensors + 8 diodes @ = 6x1014 p/cm2

– 20 pixel sensors + 8 diodes @ = 1x1015 p/cm2

• We measured the properties of the chips at room T and -10 0C– Half of the structures have been kept at -7.5 0C at

all time but for a few hours– Half of the structures were annealed for 4 minutes

at 80 0C following the procedure established by the Rose collaboration.

Irradiation at IUCF


0.E+00

2.E-02

4.E-02

6.E-02

0 50 100 150 200 250 300

Reverse Bias [V]

1/C

^2 [p

F(̂-

2)]

10 KHz (W7D43)

Vdeplete = 20 V

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

0 200 400 600 800 1000

Reverse Bias (V)

Lea

kag

e C

urr

ent

(A) W16P2-P+ W16P2-GR

W16P5-P+ W16P5-GRW16P7-P+ W16P7-GRW7P1-P+ W7P1-GRW7P7-P+ W7P7-GRW7D43-P+ W7D43-GR

SENSOR Current

Guard Ring Current

Single pixel design Single pixel design P-stopP-stop • Measurements at T=-10 0C

Dose:11014np/cm2

• Depletion voltage:20V• Some pixel sensors show

increased guard ring current at around 600 V


1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

0 200 400 600 800 1000

Reverse Bias (V)

Lea

kag

e C

urr

ent

(A) W16P2-P+ W16P2-GR

W16P5-P+ W16P5-GRW16P7-P+ W16P7-GRW7P1-P+ W7P1-GRW7P7-P+ W7P7-GRW7D43-P+ W7D43-GR

SENSOR Current

Guard Ring Current

Single pixel design Single pixel design P-stopP-stop

Dose:11014np/cm2

• Several sensors showed “breakdown” before irradiation but not after irradiation.

• The guard current was higher than expected before irradiation

Before Irradiation

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0 100 200 300 400 500 600 700 800 900 1000

Reverse Bias [V]

Lea

kag

e C

urr

ent

[A]

W16P2 -P+ W16P2 - GRW16P5 - P+ W16P5 - GRW16P7 - P+ W16P7 - GRW7P1 - P+ W7P1 - GRW7P7 - P+ W7P7 - GRW7D42 - P+ W7D43 - GR


Single pixel design Single pixel design P-stopP-stop • Sintef 2001

Dose 6e14 at -10C

1.E-07

1.E-06

1.E-05

1.E-04

0 200 400 600 800 1000

Reverse Bias (V)

Leakag

e C

urr

en

t (A

)

W14P7-P+ W14P7-GRW4P1-P+ W4P1-GRW16P3-P+ W16P3-GR

W16P6-P+ W16P6-GRW14D45 - P+ W14D45 - GR

SENSOR Current

Guard Ring Current

CV for Dose 6e14 at -10C

0.E+00

1.E-02

2.E-02

3.E-02

4.E-02

5.E-02

0 100 200 300 400

Reverse Bias [V]

1/C

^2

[pF

^(-

2)] 10 KHz (W14D45)

Vdeplete = 220 V

Dose: 61014np/cm2

• Depletion voltage:220V• Some pixel sensors

show increased guard ring current at around 700 V


CV for Dose 1e15 at -10C

0.E+00

1.E-02

2.E-02

0 100 200 300 400 500

Reverse Bias [V]

1/C

^2 [p

F^(

-2)]

100 Hz (W14D43)

Single pixel design Single pixel design P-stopP-stop • Sintef 2001 Dose: 11015np/cm2

Dose 1e15 at -10 C

1.E-07

1.E-06

1.E-05

1.E-04

0 200 400 600 800 1000

Reverse Bias (V)

Lea

kag

e C

ure

nt

(A)

W14P5-P+ W14P5-GR

W4P2-P+ W4P2-GR

W7P5-P+ W7P5-GR

W14D43-P+ W14D43-GRGuard Ring Current

SENSOR Current

• Depletion voltage >500V

• Some pixel sensors show increased guard ring current at around 700 V


• Calculate single pixel current increase due to radiation using:

I = V =410-17 A/cm3 (Rose Collaboration)• We determine the expected current for = 1x1014 p/cm2,

=6x1014 p/cm2 and = 1x1015 p/cm2.

– Expectations at -10 0C for a single pixel I= 0.85 10-9,5.0910-9, 8.4910-9 A

– Measurements at -10 0C,

– @Vbias=300 V I: = 0.6210-9, 3.5910-9, 5.7510-9 A

– @Vbias=500 V I: = 0.6510-9, 3.8210-9, 6.10 10-9 A

– @Vbias=1000V I: = 0.7810-9, 5.0810-9, 7.39 10-9 A

Increase in leakage current

21

21

2

2

1

2leak

1leak

TT

T-T

2k

Eexp

T

T

)(TI

)(TI


• Performance of p-spray and open p-stop appears to be similar:

Increase in leakage current

W14D43 - Dose 1e15 - -10C

0.E+00

1.E+00

2.E+00

3.E+00

4.E+00

5.E+00

6.E+00

7.E+00

0 200 400 600 800 1000Reverse Bias (V)

Leak

age

Cu

rre

nt

(uA

)

P+

Guard Ring

P-spray –18 0C P-stops –10 0C


PSI sensors developmentPSI sensors development • PSI has made a submission with CIS, Erfurt,

Germany.• One wafer contains:

– one full size barrel sensor with 150 m 150 m pixels (one open p-stop ring)

– one full size barrel sensor with the "1/4 micron“ pitch of 100 m 150 m (p-spray design).

– 27 sensors with pitch 125 m 125 m to fit the old Honeywell PSI30 chip.


PSI sensors developmentPSI sensors development • Technology options aim to

suppress soft breakdown

– moderated p-spray (similar to ATLAS design).

– "open p-stop" but with p-stop dose starting from 1014 cm-2 down to 31012cm-2.

• Several design options were tried:

– p-spray with different gap width 15, 20, 30 m

– Standard p-stop

– p-stop rotated by 900 between pixels.

– crosses.


PSI sensors developmentPSI sensors development

• PSI has received 10 wafers from CIS + 10 “dummies” (full size sensors are damaged).

• Five wafers were measured. Good yield on the small sensors (only 2 of 68 were bad Vbreak<Vdep+50V).

• Irradiation and beam test planned


ConclusionsConclusions• Probe station measurements indicates

that the new p-stop design is robust up to fluence of 11015 neq /cm2

• We are currently evaluating the PSI43 chip

• 4 sensors wafers have been tested and they will be sent to bump bonding companies in November

• Beam tests and/or source data will be used to understand noise, and charged collection efficiency of the current design.