November 3-8, 2002D. Bortoletto - Vertex Silicon Sensors for CMS Daniela Bortoletto Purdue University Grad students: Kim Giolo, Amit Roy, Seunghee Son Engineering Physicist: Gino Bolla 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 n eq /cm 2 –Barrel sensor design (Tilman Rohe) –Conclusions

November 3-8, 2002D. Bortoletto - Vertex FPIX COLLABORATION PSI (Horisberger) ETH U. Zurich U. Basel IHEP Wien RWTH Aachen US CMS UC Davis Northwestern Fermilab Purdue Johns Hopkins Rutgers Mississippi BARREL 2 Layers, 17(27) Mpixels FORWARD DISKS: 4 disks, 12 Mpixels 1.5<  <2.5

November 3-8, 2002D. Bortoletto - Vertex 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

November 3-8, 2002D. Bortoletto - Vertex Radiation Hardness The CMS pixel design has been optimized for a dose of 6  n eq /cm 2 Fluence is dominated by  ’s. Oxygenation is expected to be useful Crucial to limit the periods without cooling because of anti-annealing Rose collaboration

November 3-8, 2002D. Bortoletto - Vertex Design Considerations p-on-n n-on-n

November 3-8, 2002D. Bortoletto - Vertex 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

November 3-8, 2002D. Bortoletto - Vertex 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

November 3-8, 2002D. Bortoletto - Vertex 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

November 3-8, 2002D. Bortoletto - Vertex Eleven guard ring design (Sintef 1999) After irradiation Guard Ring Performance  = 6  n eq /cm 2 Before irradiation No breakdown up to 800 V even after irradiation to  = 6  n eq /cm 2  Guard ring design frozen.

November 3-8, 2002D. Bortoletto - Vertex 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

November 3-8, 2002D. Bortoletto - Vertex N-side isolation Charge trapping in Oxyde layer P-stops P-sprays N.I.M. A 377 (1996)

November 3-8, 2002D. Bortoletto - Vertex 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 F: Single open ringG: Double open ring 2 A : Double open ring

November 3-8, 2002D. Bortoletto - Vertex P-stop performance Performance was measured before and after irradiation Before irradiation After irradiation IV measurements at -10 C after irradiation show: V bias < 300 V: Normal operation 300V< V bias <550 linear increase of the leakage current from the pixel area (soft breakdown) V bias >550V breakdown Design G T=-10 0 C

November 3-8, 2002D. Bortoletto - Vertex 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

November 3-8, 2002D. Bortoletto - Vertex 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”  = 1  n eq /cm 2  = 6  n eq /cm 2 Design F  = 1  n eq /cm 2  = 6  n eq /cm 2 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 T=-10 0 C

November 3-8, 2002D. Bortoletto - Vertex Sintef 2001 submission Wafer Layout: –125x125 Finalize single pixel design (PSI  40 pixels Honeywell chip) –150x150 to 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

November 3-8, 2002D. Bortoletto - Vertex 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 Field Plate Average Breakdown voltage increases by 200 V

November 3-8, 2002D. Bortoletto - Vertex July 2002: Irradiated 85 structures (single ROC silicon sensors + diodes) at IUCF with 200 MeV protons. –15 pixels sensors and 10  = 1x10 14 p/cm 2 –24 pixels sensors + 8  = 6x10 14 p/cm 2 –20 pixel sensors + 8  = 1x10 15 p/cm 2 We measured the properties of the chips at room T and C –Half of the structures have been kept at C at all time but for a few hours –Half of the structures were annealed for 4 minutes at 80 0 C following the procedure established by the Rose collaboration. Irradiation at IUCF

November 3-8, 2002D. Bortoletto - Vertex Single pixel design P-stop Measurements at T=-10 0 C Dose:1  n p /cm 2 Depletion voltage:20V Some pixel sensors show increased guard ring current at around 600 V

November 3-8, 2002D. Bortoletto - Vertex Single pixel design P-stop Dose:1  n p /cm 2 Several sensors showed “breakdown” before irradiation but not after irradiation. The guard current was higher than expected before irradiation

November 3-8, 2002D. Bortoletto - Vertex Single pixel design P-stop Sintef 2001 Dose: 6  n p /cm 2 Depletion voltage:220V Some pixel sensors show increased guard ring current at around 700 V

November 3-8, 2002D. Bortoletto - Vertex Single pixel design P-stop Sintef 2001 Dose: 1  n p /cm 2 Depletion voltage >500V Some pixel sensors show increased guard ring current at around 700 V

November 3-8, 2002D. Bortoletto - Vertex Calculate single pixel current increase due to radiation using:  I =  V   =4  A/cm 3 (Rose Collaboration) We determine the expected current for  = 1x10 14 p/cm 2,  =6x10 14 p/cm 2 and  = 1x10 15 p/cm 2. –Expectations at C for a single pixel  I= 0.85  10 -9,5.09  10 -9, 8.49  A –Measurements at C, bias =300 V   I: = 0.62  10 -9, 3.59  10 -9, 5.75  A bias =500 V   I: = 0.65  10 -9, 3.82  10 -9, 6.10  A bias =1000V   I: = 0.78  10 -9, 5.08  10 -9, 7.39  A Increase in leakage current

November 3-8, 2002D. Bortoletto - Vertex Performance of p-spray and open p-stop appears to be similar: Increase in leakage current P-spray –18 0 C P-stops –10 0 C

November 3-8, 2002D. Bortoletto - Vertex 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.

November 3-8, 2002D. Bortoletto - Vertex 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 cm -2 down to 3  cm -2. Several design options were tried: –p-spray with different gap width 15, 20, 30  m –Standard p-stop –p-stop rotated by 90 0 between pixels. –crosses.

November 3-8, 2002D. Bortoletto - Vertex 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 V break <V dep +50V). Irradiation and beam test planned

November 3-8, 2002D. Bortoletto - Vertex Conclusions Probe station measurements indicates that the new p-stop design is robust up to fluence of 1  n eq /cm 2 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.