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INSTITUT FÜR EXPERIMENTELLE KERNPHYSIK

HPK Workshop , Jan. 2013 – Tracker Upgrade Sensor Simulation studies of isolation techniques for n-in-p silicon sensors on behalf of the Simulation Working Group. INSTITUT FÜR EXPERIMENTELLE KERNPHYSIK. p-stop and p-spray strip isolation.

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INSTITUT FÜR EXPERIMENTELLE KERNPHYSIK

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  1. HPK Workshop , Jan. 2013 – Tracker Upgrade Sensor Simulation studies of isolation techniques for n-in-p silicon sensors on behalf of the Simulation Working Group INSTITUT FÜR EXPERIMENTELLE KERNPHYSIK

  2. p-stop and p-spray strip isolation • Needed to isolate adjacent n-strips due to fixed and trap charges in • Si/SiO2 interface  create an inversion layer on the n-side • Impact on sensor performance !?: CCE, Ccoup, Cint, max E-Field, breakdown Voltage… • Weconsider 2 p-stopconfigurations: Atoll und Common •  impact on Signal-to-Noise (see Manfred Valentan, HEPHY) Fig.: detail of Layout GDS file for masking shop showing the two p-stop techniques; left: p-stop atoll, and right: p-stop common HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  3. P-spray N+ strips Fig.: Electric Field strenght V/cm-1 • P-spray isolation technique • Uniform layer, covering whole wafer; no additional mask • High electric fields at n+ implant edges, as the p-spray layer is in direct contact to the n+ strips! • Simulation studies consider electrical fields depending on doping concentration, fixed oxide charge at the bulk/oxide interface, interstrip capacitance… HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  4. Fig.: eDensity on sensor front side; slice 100nm underneath the bulk/oxide interface Fig.: eDensity on sensor front side • P-stop isolation technique • P-stop structure cuts the accumulation layer • Electric fields depend on placement, width, doping conc., structure… • Additional photolitography mask is needed HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  5. Validation of Simulation, University of Dehli, Silvaco Silvaco TCAD Sim. Paper Silvaco TCAD Sim. Paper HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors • Comparison with Review article: “Device Simulations of Isolation Techniques for Silicon Microstrip Detectors Made on p-Type Substrates” by Claudio Piemonte, IEEE TRANS. NUCL. SCI., VOL. 53, NO. 3, JUNE 2006. • General Trends were same (although we don’t have exact simulation parameters as used by Claudio). Validation with some other papers are also carried out.

  6. 12 Configurations of Multi-SSD with P-stop in HPK • Experimental results of Cint taken from Lyon Tracker Upgrade Database (thanks to Robert) http://ikcms02.fzk.de/probe/lyon2/login.php • n+-p--p+ configuration chosen for these results. • For each design, two separate results for non-irradiated sensors measured by FIRENZE probe station and one result obtained by simulation have been compared Strip length = 3.0490 cm HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  7. Simulation of Cint for Multi-SSD with Double P-stops Simulated Structure – zoomed region Double P-stops 4µm wide separated by 6µm • Instead of two adjacent half- P+neighbouring strips, we have considered five strips in which Cint is evaluated by sending AC signal to central electrode and measuring it w.r.t. two adjacent strips (which are shorted). HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  8. Simulation Parameters – Same for all 12 configurations 1. Substrate Doping Conc. (NB) = 3.4x1012 cm-32. Surface Charge Density  (QF) = ? 3. Temp = 21o C corresponding to 294 K4. p+ implant of 30 micron from the back side5. n+ strip junction depth of 1.5 mm6. Strip length for normalization = 3.0490 cm7. Total device depth is 320 mm.8. Frequency = 1MHz 9. P-stop Junction width = 4 mm, P-stop separation = 6 mm10. P-stop junction depth = ? 11. P-stop peak doping conc. (Npst) = ? Other than Width and Pitch, nothing else is changed in 12 Configurations. MO = 6.5 mm on either side is considered on each strip as per MSSD design. HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  9. Simulation of Cint for Multi-SSD with P-stops, Silvaco T-CAD • We performed “sensitivity studies” for peak P-stop Doping Density (Npst) , P-stop Junction Depth (XJ),Surface charge density (Qf) for structure no-1 • P-stop peak doping density (Npst) and P-stop junction depth do not affect Cint in a significant manner ensuring desired isolation. Structure # 1 P-stop Xj = 0.5,1,1.5µm Structure # 1 NPST = 5e16, 1e17, 2e17, 1e18 cm-3 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  10. Simulation of Cint for Multi-SSD for Structure #1, Silvaco T-CAD • QF effect on Cint is quite significant! • Comparison with measurement shows QF = 5x10cm-2 matches closely with measurement HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  11. MSSD: Measurement vs. Simulation, Silvaco T-CAD • For FZ320P, Double P-stop (each 4µm wide separated by 6µm), Optimized Parameters: • peak doping density = 5x1016cm-3 • P-stop doping depth = 1µm • QF= 5x1010 cm-2 • Observations: • Initial drop in Cint is not reproduced by Simulations • Cint plateau values are within 20% of the experimental values HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  12. For FZ 320Y: Cint vs. Bias Voltage, Silvaco T-CAD • Simulation vs. Measurement for FZ320Y • All 12 N+-P--P+ configurations (non-irradiated) • For FZ320Y, Peak P-spray doping density = • 4x1016cm-3, P-spray doping depth = 0.2µm • Only FIRENZE Probe station measurements are used for comparison HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors January 22nd, 2013

  13. Simulations of irradiated MSSDs with p-stop, HIP, • Synopsys T-CAD 5-strip structure for 7-80 and 5-120 regions Two-level radiation damage model [1] 1 MeV neutron equivalent doses for the simulations (from CMS irr. campaign) p-stop configurations (preliminary) Total p-stop width = constant [1] M. Petasecca et al. NIM A 563 (2006) 192-195. HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors January 22nd, 2013

  14. Interstrip capacitances of measured and simulated nonirradiated 200P detectors @ +20 °C , Synopsys • Simulation dp = 1.5 μm • Qf = 5e10 cm-2 • Simulation dp = 1.5 μm • Qf = 5e10 cm-2 region 5 region 7 Initial dip not produced by simulation TEST ID: 15016 TEST ID: 15010 • By lowering the value of Qf from 5e10 cm-2 to 2.7e10 cm-2 the initial dip of measured curve is produced by simulation. At higher voltages simulated curve is within ~2 % of measurement. • At higher value of non-irradiated inversion layer oxide charge Qf = 4e11 cm-2 simulation does not match the measurement and fails at ~600 V due to high E at p-stop edges. • For the irradiation simulations, p-stop depth was varied by dp = 1.0 and 1.5 μm • Irradiated inversion layer oxide charge Qf = 1e12 cm-2 region 7 • Simulation dp = 1.5 μm • Qf = 2.7e10 cm-2 TEST ID: 15016 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors January 22nd, 2013

  15. Irradiated 200P 7-80 region for varyingNp and p-stop count/width @ -20 °C, Synopsys T-CAD • Radiation damage removes electrons • from the inversion layer → isolation • reached at lower Vd? • Widest wphas the lowest CintandVd • where minimumCint is reached • for Np=3e16 Cint p-stop thickness = 1.0 μm Cback f = 1.0 MHz f = 1.0 kHz • Are the strips short-circuited for Np = 3e16 • until Vd has removed electrons from the inversion • layer? • e- density between two strips and p-stops for • Φ = 7e14 cm-2 and wp= 2μm. Isolation fails for • smallerNp @ low Vd? Qf = 1e12 cm-2 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors January 22nd, 2013

  16. Irradiated 200P 7-80 region for varyingNp and p-stop count/width @ +20 °C, Synopsys T-CAD p-stop thickness = 1.0 μm Cback Cint • At higher T electrons are removed • from inversion layer more quickly • → min.Cint reached at lower Vd • 350 V (-20 °C) → 290 V (+20 °C) • e- density between two strips and p-stops for • Φ = 7e14 cm-2 and wp= 2μm HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors January 22nd, 2013

  17. Irradiated 200P 7-80 region for varyingNp and • p-stop count/width @ -/+20 °C, Synopsys T-CAD p-stop thickness = 1.5 μm • e- density between two strips and • p-stops for Φ = 7e14 cm-2 and wp= 2μm Cint @ -20 °C n @ -20 °C • No step in leakage current • observed for both Np Ileak @ -20 °C Cint @ +20 °C • No high values of Cint observed • after ~30 V n @ +20 °C Ileak @ +20 °C • Low e- density even at • low voltage for both Np Superior configuration to dp = 1.0 μm HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors January 22nd, 2013

  18. p=90um, w=20um, w/p=0.22 • With increasing doping conc of p+ dopants (boron), max. electric fields increase too! • Si breakdown field about 3x105 V/cm !!!! • max. E-fields raise exponentially with increasing doping conc. !!! • Hence, in order to achieve a sufficient strip isolation and simultanously low electric fields at the strips, the doping conc. of p-spray layer must be calculated carefully ! P-spray: Electric fields on p+ doping concentration, nonirradiated, Synopsys T-CAD HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  19. Cint is almost! independent of p+ doping conc. (until 1e16cm-3) for a given oxide charge but then increases with increasing doping conc. ->higher acceptor concentration N_p determines a narrower lateral depletion region and therefore a tighter coupling!? Ok, but why not observed for concentrations up to 8e15cm^-3 ? • For a given doping conc Cint decreases slightly with increasing oxide charge -> are e and h removed from surface region? -> the p-spray layer gets partialy depleted and less conductive… • P-spray isolation and interstrip capacitance Cint, Synopsys T-CAD HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  20. Fig.: Electric field strength N+ strip P-stop atoll • Max. electric field strengths saturate with increasing doping conc. !!! • Placement of p-stop affects more significantly the e-fields than p-stop doping conc.!!! • Calculation of p-stop doping conc. much more easier compared to p-spray technique… Electric field strength with P-spray!! P-stop: electric field on p+ doping concentration, Synopsys T-CAD HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  21. max. electric field strength on p-stop distance, Synopsys T-CAD • Regarding E-field strenght, the p-stops should be implanted in the center of adjacent strips • No constraint through process technology (but lateral diffusion) • Experimental studies also prefer p-stop distances of at least 15um for pitch 90um. • SNR, CCE. ( M.Valentan,HEPHY) 0 10 20 30 40 50 P-stop distance [um] P-stop P-stop Fig.: strip sensor (two half electrodes) with p-stop atoll config HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  22. dist 0.2 width 8 um dist 0.2 width 4 um dist 0.8 width 8 um dist 0.8 width 4 um Edge n+ strip Edge p-stop E-Field [Vcm^-1] Potential defines electric field and max electric field strength at the edges of n+ and p+ implants Like on the slide before, wider p-stop dist- ance and lower p-stop width ensures high- voltage operation Optimization of p-stop/p-spray dose profile to ensure good isolation and satisfying the breakdown performance potential is higher with wider p-stop electric field at p-stop edges very high Potential [V] E-field and electrostatic potential, Synopsys T-CAD HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  23. P-stop conc. 5e16cm-3 Oxide charge. 1e11cm-2 It seems that the accumulation layer „extends“ the n+ strips -> strip geometry „changes“ and significantly affects Cint… P-stop and interstrip capacitance, Synopsys T-CAD Gap between strip and p-stop pattern doesn´t affect Cint but is significantly influenced by surface damage?! Just interface charge, we also have to consider oxide charge growth models… Experimental studies necessary to confirm simulation outcomes HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

  24. Summary • We are able to reproduce HPK MSSD measurements like depletion voltages, currents (see Ranjeet´s and Robert´s talk) and interstrip capacitances within 20 % of experimental values. • Parameters taken for correct simulation of sensors agree with values from spreading resistance measurements (example: p+, n+ doping concentrations, see Wolfgang´s talk on on processing!?) • Comparison of p-stop and p-spray techniques with the goal to determine the perfect pattern or concentrations etc. is difficult without knowledge of real process parameters as some variables we assume from calculations. • Nevertheless, simulations indicate the p-stop technique to be more convenient compared to p-spray. • P-spray doping conc. is difficult to calculate and electric fields raise exponentially with conc.! But interstrip capacitance is less affected compared to p-stop technique. • P-stop doping conc. can be chosen higher than necessary because of no significant effect on electric field strenghts. Cint is strongly affected by surface damage! • Combination of both has still to be studied. • Simulations exclude some p-stop patterns or distances in agreement with first exp. studies • Radiation damage of surface is used but models for oxide charge growth or interface traps have to be considered. • Next step could be the implemantation of trap models in order to investigate bulk defects and their impact on p+ isolation. • See Robert´s talk HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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