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Test of Pixel Sensors for the CMS experiment

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. 2 barrels, 17(27) Mpixels. 4 forward disks, 12 Mpixels.

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Test of Pixel Sensors for the CMS experiment

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  1. Test of Pixel Sensors for the CMS experiment Amitava Roy Purdue University

  2. Pixel Sensors • The CMS experiment at the LHC will have a silicon pixel detector as its innermost tracking device. 2 barrels, 17(27) Mpixels 4 forward disks, 12 Mpixels NP 2000 A. Roy

  3. How does a Silicon detector works? • 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! P + + + + + + + N+ - - - - - - - Hole and free electron free region Newly generated electron and hole pair Particle - + - + NP 2000 A. Roy ROC

  4. How does CMS pixel looks? N N+ P+ • It’s N+ on N! Not a diode! • And it has 11 Guard rings (what’s that?) NP 2000 A. Roy

  5. 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! NP 2000 A. Roy

  6. What does the Guard ring guard? SiO2 V • 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! N P+ N+ Breakdown prone area Trapped electrons V NP 2000 A. Roy

  7. What voltage we need? Neutron irradiation Proton irradiation • So we have to operate at 400 V NP 2000 A. Roy

  8. No full depletion! • We are going to operate in 300V - partial depletion. • We will be collecting the electrons still! NP 2000 A. Roy

  9. Wafers • We received sensors from two vendors. • 3 wafers from CSEM • 300 mm. • 2 wafers from Sintef 275 mm. NP 2000 A. Roy

  10. Single and double sided measurement • Measurement can be • done from P+ side • only! N P+ N+ A A NP 2000 A. Roy

  11. Guard Ring • Guard ring design good enough • to have Vbreakdown > 1000V! NP 2000 A. Roy

  12. Pixels • Vdepletion~ 155 V • Some pixels has metal • on top, some doesn’t. • There are 8 different • p-stop designs. • Vbreakdown between • 200-800 volts. Metalized Non-metalized NP 2000 A. Roy

  13. Pixels A,B,C - not metalized D,E,F,G,H - metalized • Metalized pixels have higher • breakdown voltage. NP 2000 A. Roy

  14. Pixels Design G Design A • Among 8 different type of p-stops design, design A, F and G • have higher breakdown voltage. Design F NP 2000 A. Roy

  15. Best Pixels • Design A, F and G with metal • on top have the best • performances. Vbreakdown ~ 800V NP 2000 A. Roy

  16. Interpixel resistance I1 • We took a pixel well • inside the array so that • it is isolated from the • n-ring • R=V/(I2-I1) N N+ P+ I2 V NP 2000 A. Roy

  17. Interpixel resistance • V at the center pixel shows • that p-stop start kicking in • after 150 volts. A sign that the • whole pixel array is getting • biased. NP 2000 A. Roy

  18. 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. NP 2000 A. Roy

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