Test of Pixel Sensors for the CMS experiment

Amitava Roy

Purdue University

Pixel Sensors

• The CMS experiment at the LHC will have a silicon pixel detector as its innermost tracking device.

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2 barrels, 17(27) Mpixels

4 forward disks, 12 Mpixels

How does a Silicon detector works?

+-

- +

P

N+ - - - - - - -

+ + + + + + +

Hole and free electron free region

Particle

Newly generated electron and hole pair

•A detector has to be fully depleted!•What is depletion? Giving enough reverse bias voltage so that no free carriers are available. No more a diode now. It’s a resistor!• Why? In fully depleted condition, if a particle goes through the detector it will make electron and hole pair. They are attracted to the opposite terminals and get

collected - we get a signal!

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How does CMS pixel looks?

N

N+

•It’s N+ on N! Not a diode!

•And it has 11 Guard rings (what’s that?)

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P+

It’s going to be a Diode later!

•In LHC it will be a harsh radiation environment. In 6 years of running, fluence will be 6x1014 /cm2 at r=7cm•This radiation make the N to P! Nuclear interaction with the incoming particle, Si => 25Mg. A considerable concentration of is 25Mg is achieved. •Magnesium is a donor. That will make the N to P!

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What does the Guard ring guard?

N

•To protect from outer environment detector is covered with SiO2.•SiO2 traps electrons at the surface that causes a high voltage drop in a small area.•High electric field - silicon breaks down.•To maintain a uniform voltage drop guard rings are used!

V

V

Breakdown prone area

SiO2

P+

N+

Trapped electrons

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Neutron irradiation

Proton irradiation

What voltage we need?

•So we have to operate at 400 V

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No full depletion!

•We are going to operate in 300V - partial depletion.•We will be collecting the electrons still!

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Wafers

•We received sensors from two vendors. •3 wafers from CSEM 300m.2 wafers from Sintef 275 m.

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Single and double sided

measurement

N

A

A

•Measurement can be done from P+ side only!

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P+

N+

Guard Ring

Leakage Current - Diodes

1.E-10

1.E-09

1.E-08

1.E-07

0 200 400 600 800 1000

Reverse Bias Voltage (V)

Cu

rren

t (A

)Diode 3

Diode 4

Diode 11

Diode 14

Diode 20

•Guard ring design good enoughto have Vbreakdown > 1000V!

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Pixels

•Vdepletion~ 155 V•Some pixels has metalon top, some doesn’t.•There are 8 different p-stop designs.•Vbreakdown between 200-800 volts.

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Metalized

Non-metalized

Pixels

A,B,C - not metalizedD,E,F,G,H - metalized

•Metalized pixels have higherbreakdown voltage.

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Pixels

•Among 8 different type of p-stops design, design A, F and G have higher breakdown voltage.

Design A

Design F

Design G

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Best Pixels

Leakage Current - Sensors

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

0 200 400 600 800 1000

Reverse Bias Voltage (V)

Cu

rren

t (A

)

Design A

Design A

Design A

Design F

Design G

•Design A, F and G with metal on top have the best performances. Vbreakdown ~ 800V

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Interpixel resistance

I1

I2

V

•We took a pixel wellinside the array so that it is isolated from the n-ring•R=V/(I2-I1)

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P+N N+

Interpixel resistance

-4.50E+01

-4.00E+01

-3.50E+01

-3.00E+01

-2.50E+01

-2.00E+01

-1.50E+01

-1.00E+01

-5.00E+00

0.00E+00

5.00E+00

-200-150-100-500

Reverse Bias (V)

V a

t pix

el (V

)

V at pxl

Inter Pixel Resistance

0.00E+00

2.00E+06

4.00E+06

6.00E+06

8.00E+06

1.00E+07

1.20E+07

1.40E+07

1.60E+07

1.80E+07

2.00E+07

-200-195-190-185-180-175-170

Bias Voltage

Resis

tan

ce

Resistance MRD 12

•V at the center pixel showsthat p-stop start kicking in after 150 volts. A sign that the whole pixel array is getting biased.

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Conclusions

•Single sided probing is possible.•Guard Ring design is good for operational voltage > 1000V•We optimized the p-stop design. Design A,F and G breaks down around 800V, far above 300V, the operating voltage in LHC.•P-stop design gives high resistance to isolate the pixels after depletion voltage.

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