Post on 21-Jan-2016
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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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