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Design Considerations for Silicon Pixel Sensors in the LHC: p-on-n vs. n-on-n Technologies

This document outlines the design considerations for pixel sensors used in the Large Hadron Collider (LHC), comparing p-on-n and n-on-n architectures, alongside the implications of p-stops and p-sprays. It summarizes key results from the CMS Forward Pixel (FpiX) prototypes submitted in 1999 and 2001, detailing advancements in sensor design and performance evaluations under high radiation conditions. The work addresses the challenges of sensor construction, radiation hardness, guard ring performance, and overall sensor optimization for robust operation in high-fluence environments.

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Design Considerations for Silicon Pixel Sensors in the LHC: p-on-n vs. n-on-n Technologies

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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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