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Silicon Sensors for CMS

Silicon Sensors for CMS. 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)

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Silicon Sensors for CMS

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  1. Silicon Sensors for CMS • 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 Daniela Bortoletto Purdue University Grad students: Kim Giolo, Amit Roy, Seunghee Son Engineering Physicist: Gino Bolla D. Bortoletto - Vertex 2002

  2. FPIX COLLABORATION BARREL 2 Layers, 17(27) Mpixels PSI (Horisberger) ETH U. Zurich U. Basel IHEP Wien RWTH Aachen US CMS UC Davis Northwestern Fermilab Purdue Johns Hopkins Rutgers Mississippi FORWARD DISKS: 4 disks, 12 Mpixels 1.5<<2.5 D. Bortoletto - Vertex 2002

  3. Design 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 D. Bortoletto - Vertex 2002

  4. Radiation Hardness • The CMS pixel design has been optimized for a dose of 61014 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 D. Bortoletto - Vertex 2002

  5. Design Considerations p-on-n n-on-n D. Bortoletto - Vertex 2002

  6. 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 D. Bortoletto - Vertex 2002

  7. 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 D. Bortoletto - Vertex 2002

  8. 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 D. Bortoletto - Vertex 2002

  9. Guard Ring Performance • Eleven guard ring design (Sintef 1999) • After irradiation • Before irradiation  = 61014 neq/cm2 • No breakdown up to 800 V even after irradiation to  = 61014 neq/cm2  Guard ring design frozen. D. Bortoletto - Vertex 2002

  10. 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 D. Bortoletto - Vertex 2002

  11. N-side isolation • Charge trapping in Oxyde layer 3.0 0.2 0.2 3.0 P-stops P-sprays N.I.M. A 377 (1996) 412 D. Bortoletto - Vertex 2002

  12. 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 A : Double open ring F: Single open ring G: Double open ring 2 D. Bortoletto - Vertex 2002

  13. P-stop performance • Performance was measured before and after irradiation T=-10 0C Design G After irradiation Before 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 D. Bortoletto - Vertex 2002

  14. 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 D. Bortoletto - Vertex 2002

  15. 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”  = 61014 neq/cm2  = 11014 neq/cm2 Design F T=-10 0C  = 11014 neq/cm2= 61014neq/cm2 A(TDR) at 300V ~5.0nA/pixel >10nA/pixel G at 300V ~1.9nA/pixel ~5.0nA/pixel F at 300V ~0.5nA/pixel ~4.0nA/pixel D. Bortoletto - Vertex 2002

  16. Sintef 2001 submission • Wafer Layout: • 125x125Finalize single pixel design (PSI-30 36 40 pixels Honeywell chip) • 150x150to match existing DMIL PSI-43 full size 52 53 pixels chip) • 150x100 to match IBM 0.25m compatible layout • 15 wafers Instrument 5 blades • Bulk: (1,0,0) Resistivity=1-2 Kcm, thickness 275 m, several oxygenated wafers D. Bortoletto - Vertex 2002

  17. Single pixel design P-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 Average Breakdown voltage increases by 200 V Field Plate D. Bortoletto - Vertex 2002

  18. Irradiation at IUCF • 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. D. Bortoletto - Vertex 2002

  19. Single pixel design P-stop • Measurements at T=-10 0C Dose:11014np/cm2 • Depletion voltage:20V • Some pixel sensors show increased guard ring current at around 600 V D. Bortoletto - Vertex 2002

  20. Single pixel design P-stop Dose:11014np/cm2 • Several sensors showed “breakdown” before irradiation but not after irradiation. • The guard current was higher than expected before irradiation D. Bortoletto - Vertex 2002

  21. Single pixel design P-stop • Sintef 2001 Dose: 61014np/cm2 • Depletion voltage:220V • Some pixel sensors show increased guard ring current at around 700 V D. Bortoletto - Vertex 2002

  22. Single pixel design P-stop • Sintef 2001 Dose: 11015np/cm2 • Depletion voltage >500V • Some pixel sensors show increased guard ring current at around 700 V D. Bortoletto - Vertex 2002

  23. Increase in leakage current • Calculate single pixel current increase due to radiation using: I =  V  =410-17 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  10-9,5.0910-9,8.4910-9 A • Measurements at -10 0C, • @Vbias=300 V  I: = 0.6210-9,3.5910-9, 5.7510-9 A • @Vbias=500 V  I: = 0.6510-9,3.8210-9,6.10 10-9 A • @Vbias=1000V  I: = 0.7810-9,5.0810-9,7.39 10-9 A D. Bortoletto - Vertex 2002

  24. Increase in leakage current • Performance of p-spray and open p-stop appears to be similar: P-spray –18 0C P-stops –10 0C D. Bortoletto - Vertex 2002

  25. PSI 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. D. Bortoletto - Vertex 2002

  26. PSI 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 31012cm-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. D. Bortoletto - Vertex 2002

  27. PSI 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 D. Bortoletto - Vertex 2002

  28. Conclusions • Probe station measurements indicates that the new p-stop design is robust up to fluence of 11015 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. D. Bortoletto - Vertex 2002

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