Metal overhang: an important ingredient against “micro-discharge” L.Demaria – INFN Torino 21/04/2004 Sensor meeting Introduction and R&D results Simulation results Use of overhang on TK modules ST simulation results Conclusions

High Stability: critical fields around strips The breakdown at the strips happens because a very high electric field is present at the edge of the p + implant. The magnitude of the field depends on the geometry: It is expected to be higher for lower values of the w/p ratio. The breakdown field for silicon is about 30 V/mm. In practice breakdown can occur at lower fields if the high electric field triggers a breakdown in a few imperfect strips and impair, at least locally, proper operation of the detector. - LD - NIM A447 (2000) - CERN group (MM,LD,EM et al.) CMS-Note 2000/011 NIM A485 2002

Improving stability with newer design Improvement of the breakdown performances of the detectors can be obtained if the region with the higher electric field is moved away from the p + implant and into the oxide where the breakdown field is approximately 600 V/mm, which is an order of magnitude higher than for the silicon.  This can be achieved by the use of metal strips which are wider than the implant. METAL OVERHANG

Al readout strips referred to GROUND TOB1-TIB3 Interesting results were found for irradiated detector comparing standard strip geometries with those with 8-mm metal overhang Al readout strips referred to GROUND TIB1 - LD - NIM A447 (2000) - CERN group (MM,LD,EM et al.) CMS-Note 2000/011 NIM A485 2002

Simulation studies Without overhang Pitch=240 mm w/p=0.1 w/p=0.3 Here are evident the effects of w/p and of pitch D.Passeri, G.M.Bilei et al. – IEEE TNS 48 (2000)

Overhang effect w=72 mm, p=240 mm, Vbias=250V. Highest electrical field is at p+ implant corner in the n-bulk silicon Highest electrical field is moved to the oxide layer close to the end of metal overhang E-field in the n-bulk silicon is lower E-field in the silicon bulk in further decreased w=72 mm, p=240 mm, Vbias=250V.

Vbias=500V, p=240 mm w/p=0.1 w/p=0.3 ov=0 ov=4 ov=10

V bias = 500V, p=60 mm w/p=0.1 w/p=0.3 ov=0 ov=4 ov=10

Irradiation effects

E-max for unirradiated sensors and different pitches and widths Vbreak E-max for irradiated sensors Vbreak D.Passeri, G.M.Bilei et al. – IEEE TNS 48 (2000)

MOS effect of metal overhang The overhanging contact acts there as a p-channel MOS gate, which, under given bias conditions, suppresses the accumulation layer and tends to deplete the underlying region. This indicates a deep change in the electric field profile at the edge of the p+ implant, depending on the overhang extension. D.Passeri, G.M.Bilei et al. – IEEE TNS 48 (2000)

<100> material <111> material Q~5-10 times than for <100> - LD - NIM A447 (2000) - CERN group (MM,LD,EM et al.) CMS-Note 2000/011 NIM A485 2002

TK Modules Unfortunatelly the TK modules have the Al referred to ~0.8V The overhang effect makes the silicon sensor to be more stable at high voltage bias since it reduces the E-max close to the p+ implant. All the results seen so far, both real measurements and silumation studies, if the AL lines are referred to GROUND i.e. if the voltage of the p+ implant is positive w.r.t. that of the Al. This is specially important for the MOS effect that otherwise does not happen or even worse instead of working in depletion mode it works in accumulation mode, making the E-max to increase Unfortunatelly the TK modules have the Al referred to ~0.8V (input stage of APV) and p+ basically at GROUND.

ST simulations A B C D Vbias Qoxide Val Case + 500 Volts Pitch = 180 mm Vbias Qoxide Val Case + 500 Volts 1.0·1010 e-/cm2 GND A + 10 Volts B 1.5·1011 e- /cm2 C D Good <100> (low Vfb) material has Q~ 1.0·1010 e-/cm2

A: Val=GND E-max(Sil)~8 V/mm Oxide Al W= 40 mm P =180 mm W/P=0.22 Qox=1.0·1010 e-/cm2 Oxide Al W= 40 mm P =180 mm W/P=0.22 Over=6 mm

B: Val=10V E-max(Sil)~12 V/mm Al Oxide W= 40 mm P =180 mm W/P=0.22 Qox=1.0·1010 e-/cm2 Al Oxide W= 40 mm P =180 mm W/P=0.22 Over=6 mm

C: Val=GND E-max(Si)~16 V/mm Oxide Al W= 40 mm P =180 mm W/P=0.22 Qox=1.5·1011 e-/cm2 Oxide Al W= 40 mm P =180 mm W/P=0.22 Over=6 mm

C: Val=GND E-max(Si)~20 V/mm Oxide Al W= 40 mm P =180 mm W/P=0.22 Qox=1.5·1011 e-/cm2 Oxide Al W= 40 mm P =180 mm W/P=0.22 Over=6 mm

Comparison of E-field Vbreak Big dependence on Qox (factor ~2) D : Val=10V, Q= 1.5·1011 e-/cm2 C : Val=GND, Q= 1.5·1011 e-/cm2 B : Val=GND, Q= 1.0·1010 e-/cm2 A: Val=GND, Q= 1.0·1010 e-/cm2 Big dependence on Qox (factor ~2) Val > Vp+ is worse (factor>30%)

Conclusions (1) E-field is very high close to the p+ implant corner (=E-max) E-max increases with pitch and decrease with width of p+ implant MORE critical for TOB1-2 than for TIB At high bias voltage E-max can become close to E-break (30V/ mm) so to cause strip, small area instabilities with I- leakage increase Metal overhang decrease E-max  makes silicon sensor more stable at high bias voltage, provided Al at lower bias than p+ implant Unfortunatelly TK-modules have Val~0.8V and Vp+~0V (!) Oxide charge (~high Vfb) increase E-max close to implant if no metal overhang is used or if it has a positive voltage w.r.t p+ implant (this is even worse since MOS effect might increase local charge)

Conclusions (2) Is microdischarge in TOB modules due to Vbreak in p+ strips ? If yes, then: the process behind is well understood and should have no dependence on time sensor testing made with strip floating is not ideal… can the overhang be better used by increasing Vp+ (to 2.5V) ? This would improve module stability at high voltage Vfb now is lower in ST sensors. This should decrease ‘microdischarge’ appearance smaller pitches should be more ‘microdischarge’ resistant