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. NP 2000 A. Roy 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! ROC NP 2000 A. Roy

How does CMS pixel looks? N N+N+ It’s N + on N! Not a diode! And it has 11 Guard rings (what’s that?) NP 2000 A. Roy P+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 6x10 14 /cm 2 at r=7cm This radiation make the N to P! Nuclear interaction with the incoming particle, Si => 25 Mg. A considerable concentration of is 25 Mg is achieved. Magnesium is a donor. That will make the N to P! NP 2000 A. Roy

What does the Guard ring guard? N To protect from outer environment detector is covered with SiO 2. SiO 2 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 SiO 2 P+P+ N+N+ Trapped electrons NP 2000 A. Roy

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

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

Wafers We received sensors from two vendors. 3 wafers from CSEM 300 m m.  2 wafers from Sintef 275 m m. NP 2000 A. Roy

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

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

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

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

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

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

Interpixel resistance I1I1 I2I2 V We took a pixel well inside the array so that it is isolated from the n-ring R=V/(I 2 -I 1 ) NP 2000 A. Roy P+P+ N N+N+

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

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