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Pixel Sensors

Pixel Sensors. Sally Seidel University of New Mexico NSS’99 Short Course on Pixels 24 October 1999. The usual pixel environment is one of High luminosity the good news: sensitivity to rare events. the bad news: in many cases, high rad damage . Close proximity to the particle source

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Pixel Sensors

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  1. Pixel Sensors Sally Seidel University of New Mexico NSS’99 Short Course on Pixels 24 October 1999

  2. The usual pixel environment is one of • High luminosity • the good news:sensitivityto rare events. • the bad news:in many cases, high rad damage. • Close proximity to the particle source • the goodnews:precision tracking; on-line triggering can examine tracks while curvature is small, simplifying algorithms and speeding decisions • the bad news:rad damage 1/r1.7; jets of particles are most compact near the production point, so a close-in tracker needs high granularity. NIM A 383 (1996) 155

  3. The desire for fine granularity makes silicon detectors a natural choice for tracking, but very small silicon structures means • good news: low capacitance, low noise, good S/N, and low occupancy per channel which reduces event buffering requirements, and • bad news: high density circuit---harder to read out, cool, handle mechanically, and the rad damage increases capacitance and creates charge traps.

  4. The focus of R&D on pixel sensors has consequently been on: (1) Engineering for robustness of rad-damaged sensors (2) Maximizing the rad hardness of future designs (3) Minimizing capacitance and maximizing signal (4) Exploring new design concepts

  5. (1) Engineering for robustness of rad-damaged detectors: i. Minimizing their depletion voltage and leakage current ii. Guaranteeing implant isolation iii. Developing termination/guard structures to suppress currents and breakdown at the edges iv. Anticipating type inversion v. Suppressing “anti-annealing”

  6. (2) Maximizing their radiation hardness i. Understanding the correlation between microscopic damage and macroscopic failure mechanisms, then doing “defect engineering” ii. Utilizing substrates with large band gaps: diamond, GaAs, and SiC in addition to Si iii. Correlating rad hardness to ingot production and processing options (3) Minimizing capacitance and maximizing signal (4) Exploring new paths i.Monolithic ii. “3D”

  7. Before discussing techniques for maximizing rad hardness, recall the nature of radiation damage in silicon… • Rad damage is caused by the passage of particles through the detector. • The main source of charged particles is collisions at the interaction point of the experiment, so their fluence   1/r2. • The main source of neutrons is backsplash from the calorimeter, so their  depends on shielding and design. • Microscopically, 2 kinds of effects occur: • (1) Bulk damage • (2) Surface damage • Consider these separately...

  8. Radiation Effect #1: Bulk damage • Through-going particles cause dislocations in the Si lattice which disrupt the band structure. • The displaced atom (the Primary Knock-on Atom (PKA)) becomes a silicon interstitial (Sii) and leaves a vacancy. Vacancy + Sii A Frenkel Pair • In Si, 25 eV needed to displace the PKA. • The hypothesis that bulk damage depends exclusively on non-ionizing energy loss (the “NIEL scaling hypothesis”) has been demonstrated. • Model: the recoiling PKA strikes neighboring lattice atoms, so typically damage sites occur in clusters; subsequent evolution of those clusters is thought to produce certain macro effects (see below). NIM A 426 (1999) 1

  9. Fluences are generally expressed in terms of the equivalent damage done by 1 MeV neutrons. • Pions cause the worst damage to silicon in HEP experiments through -resonance production in the -nucleus collision.

  10. The displacement damage functions for several particle species, calculated from reaction cross sections, energy distribution of the recoils, and partition between ionizing and non-ionizing energy loss of the recoils, summed over all reaction channels for the initial particle and its energy: A. Vasilescu, DESY-PROC-1998-02

  11. Radiation Effect #2: Surface damage • Bulk Si naturally develops a layer of SiO2. Ionizing radiation generates bound charge in that layer and at the interface between the Si and SiO2. • The macro effects ofbulk damage (see below) are more lethal to detectors than are surface effects so surface damage is studied less; e.g., surface damage currents have been observed but are less important than bulk currents. • The interface states become completely filled and saturate @ 100 kRad. • (No saturation of bulk effects has been observed up to  = (a few)  1015/cm2.) IEEE Trans. Nucl. Sci. 45, 3 (1998) 295

  12. Rad Damage Effects in Semiconductors (1) Leakage current increase (2) Change in effective dopant concentration (3) Signal loss (4) Increased resistivity of undepleted bulk

  13. (1) Leakage current increase: • Empirically, • J() =  + Jintrinsic, • where: • J = leakage current density, •  = fluence, and •  = the “current-related damage constant.” • J is due to the development of generation-recombination centers in the band gap. It causes • stochastic noise in the amplifier, • heat which can lead to thermal runaway.

  14. The J is a solvable problem if the thermal environment can be controlled, because: (a) the current can be suppressed after the damage has occurred: NIM A 265 (1998) 105

  15. (b) the rate of damage that continues to develop after the irradiation is over (“reverse annealing”) also is temperature-dependent: where t = time t0= reference time (end of irradiation) T = temperature 1= characteristic time associated with annealing , , and  are annealing functions defined below.

  16. The formula for fits well to data from a variety of processes and irradiation levels: NIM A 426 (1999) 87

  17. A much worse problem is: (2) The change with fluence in the effective dopant concentration, Neff ,of the substrate. Empirically: where: NC = Neff0 - Nc0 (1 - e-c) - gc :“Stable damage”: no time constant. Na= “short-term beneficial annealing”: Insignificant after 2 days @ room temperature. NY = -gY (1 - e-t/):“reverse annealing” or “anti-annealing.” Its value begins at 0 for t = 0 and grows to saturate @ gY as t  . E. Fretwurst et al., Proc. 3rd Int. (Hiroshima) Symp. on Semiconductor Tracking Det.

  18. The effect of annealing upon Neff, as a function of time after irradiation: M. Moll, Ph.D. thesis, U. Hamburg

  19. The annealing constants: Change in the effective dopant concentration versus fluence: NIM A 426 (1999) 1; DESY F35D-97-08

  20. Neff has 2 consequences: 1) Unlimited growth of depletion voltages 2) Eventual conversion of n-type substrates to p-type (“type inversion”)

  21. A few words aboutannealing, the observed change in Neffand Ileakage with time after the irradiation process has stopped... • It occurs in bothp- and n-type substrates • There is not universal agreement on • i.) Whether the Vdep and Ileakage are due to the same micro-process, or • ii.) Exactly what micro-processes(es) is/are responsible. The majority opinion is that it is due to deep acceptor creation and possibly donor removal; however some investigators subscribe to donor compensation by deep acceptors only. This is important because it influences which remedies are tried. NIM A 418 (1998) 138

  22. Some comments from the literature on annealing: • “The compensation model could not explain the observed exponential decrease.” • “Compensation [is correct] because the inversion fluence was found proportional to the initial resistivity in a wide resistivity range.” • “Reverse annealing [probably comes from] a rearrangement of interstitial-related defects…[but] clusters formed by hadronic damage have so far evaded direct observation.” • “Anti-annealing is due to the release of metastable acceptors.” • “There is no clear understanding of the underlying physics.” NIM A 409 (1998) 180; NIM A 426 (1999) 114; IEEE Trans. Nucl. Sci. 45, 3 (1998) 295

  23. Some investigators have attempted to correlate annealing with specific traps they observe. • Annealing behavior is independent of material and inversion status (this contradicts earlier studies which were done without consistent use of guard rings): NIM A 409 (1998) 194; NIM A 426 (1999) 87

  24. Other consequences of rad damage: (3) Creation of defects that act as trap sites: leads to up to 15% signal loss @ the highest LHC fluences for typical LHC collection times. P. Allport, Proc. VERTEX'97

  25. (4) Decrease of the undepleted bulk’s electrical conductivity with fluence. NIM A 342 (1994) 105

  26. Consequences of rad damage in Si detectors Bulk: Unlimited growth of depletion voltages: w = wafer thickness  = permittivity  = carrier mobility  = resistivity q = charge Neff = effective dopant concentration

  27. Vdep   means detector may eventually have to be operated partially depleted if Vdep > Vbreakdown. • In the depleted region:rapid signal collection on the junction side. • n-side signal (e-’s) collected in 8 ns. • p-side signal (holes) collected in 21 ns due to 2.6x lower mobility. IEEE Trans. Nucl. Sci. 45, 3 (1998) 295

  28. ohmic side signal diffused, w/ long collection time. ATLAS SCT p+-n (strip detector) research found that after inversion, the high resistivity of the undepleted bulk along the cut edge suppressed current there and thus suppressed otherwise expected breakdown. NIM A 409 (1998) 184

  29. Bulk:Neff  , so Vdep   -and- Ileakage  .  Power (required and dissipated)  2

  30. Surface:the layer of fixed charge is positive; an “accumulation layer”of e-’s is permanently attracted to it from the bulk. The acc layer can compromise the isolation of the implants on the n-side unless special isolation features are included. NIM A 418 (1998) 120

  31. Surface: Inter-implant capacitance increases with ---caused by an effective widening of the implants due to the acc layer: With acc layer present, the E field lines in the bulk can terminate on the acc layer rather than just on the implants. Cinterstrip = 20 - 50% for p-side strips after  = (few)  1013/cm2: SCIPP 93/16 (1993)

  32. Surface:Microdischarge---the reversible increase in noise in individual channels @ low bias; noise increases sharply, and neighboring channels become affected, as bias increases. Thought to be due to a tunneling or avalanche breakdown caused by high fields (along the junction implant edge inside the Si bulk or due to oxide charge trapped in the SiO2 or at the Si-SiO2 interface.) NIM A 383 (1996) 116

  33. Bulk:Type inversion i) Movement of the main junction: Inversion is not a lethal problem if the design anticipates it. When n p, the junction simply moves from the front (p) side to the back (n) side. Design features to address this are mentioned below. NIM A 311 (1992) 98

  34. ii) Development of a second junction: After inversion a second junction (associated with a ~15 m thick n-type region) begins to appear on the p-side. This has been observed and reproduced in simulation. Model: “If more than 1 defect type is present (e.g., a dominant acceptor level and an additional donor level), trapped charge is not distributed uniformly along the device. Holes…are more efficiently trapped close to the p+ junction-side: such a region is therefore less inverted than the deeper bulk,…within a certain range of fluences, a depletion layer can simultaneously originate from doping discontinuities at both ends of the detector…” NIM A 426 (1999) 99; NIM A 426 (1999) 131

  35. “If e--hole pairs are injected in the low field (p+) side, electrons drift across the first depleted regiondriven by the field,inducing charges on device extremities; from this process takes rise a field…in theelectrically neutralbulk,able tore-inject holes and electrons, respectively, in the first and the second depleted region.” NIM A 426 (1999) 135

  36. Techniques for increasing the radiation robustness of proven sensor designs: 1) decrease the operating temperature 2) defeat the acc layer 3) use guard rings 4) select the right crystal orientation [5) consider p-bulk] 6) optimize for low capacitance

  37. (1) Decrease the operating temperature: • Recall Neff = NC + Na + NY. • When  reaches 1014 /cm2, V(NC) 200 V, and V(Na + NY)  600V. • But Na and NY can be completely suppressed at low temperature. (Model: reduced thermal diffusion of the defects slows the crystal evolution.) • So 200V  Vdep(T)  800V. Need temperature high enough to activate beneficial annealing but low enough to suppress anti-annealing. NIM A 342 (1994) 96

  38. -10°C < T < -5°C appropriate for LHC lifetimes + fluences. • A radical temperature effect---“The Lazarus Effect”: • A 300 m Si detector irrad to 1.2  1015/cm2, then biased to 250V, showsno signal @ 195K, recovers a 13000 e- signal with usual fast (< 5ns) characteristics when cooled to 77K. NIM A 381 (1996) 338; NIM A 413 (1998) 475

  39. No further change when cooled to 4.2K. • Device is stored at room temperature and only operated cold (so this is different from the effect that suppresses annealing). • The model: @ cryo temperatures, carrier mobility increases; the reduced thermal energy in the lattice causes the detrapping rate to be reduced; so a large fraction of deep levels are filled and deactivated.

  40. (2) Defeat the accumulation layer: • As  increases, bound (positive) surface charge increases, attracting e- to the interface. This can short the n-implants. • 2approaches: p-stops and p-spray • i) p-stops • implanted p+ channels between neighboring n-channels • borrowed by pixels from strip sensors; used, e.g., by CMS • principal benefit of this choice: extensive experience of the vendors NIM A 277 (1989) 147

  41. Design options: “common” (and “ordinary”), “atoll,” or “combined”: IEEE Trans. Nucl. Sci. 45, 3 (1998) 303

  42. p-stop design issues: • a) Charge collection efficiency (CCE): • CCE diminished between atoll implants due to lack of a clear potential ridge. • Atoll and commondo not completely divide the acc layer. Charge falling between atolls can be coupled away by the conductive acc layer. IEEE Trans. Nucl. Sci. 45, 3 (1998) 303

  43. Conclusion about p-stop effect on charge collection: • Best CCE: ordinaryp-stop • Second best: combined.

  44. b) Capacitance (noise) depends on the p-stop design • Combined p-stop has lowest cap. • for high Vbias, Catoll approaches Ccombined. • interstripcap sensitive to implant separation but not width. • Studies with strip detectors conclude that “combined” is best compromise. IEEE Trans. Nucl. Sci. 45, 3 (1998) 303

  45. ii) p-spray: • Before any other processing is done, a shallow p-type (B) layer is implanted across full wafer w/o mask. Dopant concentration is matchedto the (well-known) surface charge saturation value, 3  1012/cm2. Layer is over-compensatedby n-implants wherever those are needed. NIM A 377 (1996) 412

  46. As damage occurs, oxide layer grows, attracts acc layer, which compensates the dopant acceptors, so spray layer becomes more intrinsic and E field decreases with . NIM A 377 (1996) 412

  47. Benefits to p-spray: • no mask needed: lowers cost and allows closer placement of n-structures w/o alignment requirements. • Vbreakdown increases with .

  48. (3) Control the electric field at the edges: guard rings • The issues: • i) as depletion region develops from junction, it expands toward cut edge which is conductive due to dangling bonds. Guard ring needed to drop V in a controlled way from interior of detector face to edge: guarantee V = 0 across edge. • ii) as irrad proceeds, oxide charges develop at the Si-SiO2 interface in the absence of implantation. The presence of the oxide attracts the acc layer which deforms the depletion region and generates high field points. Oxide layer is unstable and sensitive to changes in the environment. • So the guard ring needs to stabilize the oxide and control the potential drop.

  49. Guard ring design components: a) metal linesatop the oxide [this achieves (ii) but not (i)]. b) one or more ring-shaped p-n junctions that surround the diode array but are not contacted or biased directly.

  50. Example---Several p+-implanted diffusions with non-overlapping gates in n-bulk:) The rings are a serial connection of p-channel enhancement MOSFETs, where the gate only covers 1/2 of the distance between the drain and source of the detector. The gates are connected to the sources, not the drains. NIM A 326 (1993) 27

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