Simulation studies of isolation techniques for n-in-p silicon sensors 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 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… We consider 2 p-stop configurations: 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

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… P-spray N+ strips Fig.: Electric Field strenght V/cm-1 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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 Fig.: eDensity on sensor front side Fig.: eDensity on sensor front side; slice 100nm underneath the bulk/oxide interface HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

Validation of Simulation, University of Dehli, Silvaco 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. Paper Silvaco TCAD Sim. Silvaco TCAD Sim. Paper HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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

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

Simulation Parameters – Same for all 12 configurations 1. Substrate Doping Conc. (NB) = 3.4x1012 cm-3 2. Surface Charge Density  (QF) = ? 3. Temp = 21o C corresponding to 294 K 4. p+ implant of 30 micron from the back side 5. n+ strip junction depth of 1.5 mm 6. Strip length for normalization = 3.0490 cm 7. Total device depth is 320 mm. 8. Frequency = 1MHz 9. P-stop Junction width = 4 mm, P-stop separation = 6 mm 10. 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

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

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

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

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 January 22nd, 2013 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

Synopsys T-CAD Simulations of irradiated MSSDs with p-stop, HIP, 5-strip structure for 7-80 and 5-120 regions Two-level radiation damage model [1] Parameter Value n+ / p+ doping [cm-3] 5e18 Neff (320P) [cm-3] 3.4e12 Neff (200P) [cm-3] 3e12 Neff (120P) [cm-3] 1.5e12 implant thickness [µm] 1.5 backplane diffusion depth (320P) [µm] 33 backplane diffusion depth (200P) [µm] 115 backplane diffusion depth (FTH200P) [µm] 10 backplane diffusion depth (120P) [µm] 215 Np [cm-3] 3e16, 4e16, 5e16 p-stop thickness [µm] 0.5, 1.0, 1.5 SiO2 thickness [µm] 0.5 T [K] 253, 273, 293 area factor 30490 F (Cint) 1.0 MHz F (Cback) 1.0 kHz Qf (irr.) [cm-2] 1e12 [1] Qf (unirr.) [cm-2] 1e10, 5e10, 4e11 Level [eV] σn [cm-2] σp Introduction rate [cm-1] EC - 0.42 2e-15 2e-14 1.613 EC - 0.46 5e-15 5e-14 0.9 1 MeV neutron equivalent doses for the simulations (from CMS irr. campaign) Device thickness [μm] Proton 1 MeV neqv fluence [cm-2] 1 MeV n fluence 1 MeV neqv summed fluence ≥ 200 3.0e14 4.0e14 7.0e14 1.0e15 5.0e14 1.5e15 6.0e14 2.1e15 ≤ 200 3.0e15 3.7e15 < 200 1.3e16 1.4e16 p-stop configurations (preliminary) p-stop count p-stop width [µm] spacing 2 10 4 6 1 14 Total p-stop width = constant [1] M. Petasecca et al. NIM A 563 (2006) 192-195. January 22nd, 2013 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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 January 22nd, 2013 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

Irradiated 200P 7-80 region for varying Np and p-stop count/width @ -20 °C, Synopsys T-CAD Radiation damage removes electrons from the inversion layer → isolation reached at lower Vd? Widest wp has the lowest Cint and Vd where minimum Cint is reached for Np = 3e16 p-stop thickness = 1.0 μm Cint 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 smaller Np @ low Vd? Qf = 1e12 cm-2 January 22nd, 2013 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

Irradiated 200P 7-80 region for varying Np 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 January 22nd, 2013 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

Irradiated 200P 7-80 region for varying Np 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 January 22nd, 2013 HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

P-spray: Electric fields on p+ doping concentration, nonirradiated, Synopsys T-CAD 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 ! HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

P-spray isolation and interstrip capacitance Cint, Synopsys T-CAD 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… HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

P-stop: electric field on p+ doping concentration, Synopsys T-CAD 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!! HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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

E-field and electrostatic potential, Synopsys T-CAD 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] HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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 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… HPK Workshop CERN Studies of isolation techniques for n-in-p silicon sensors

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