Simulating Radiation-Induced Shifts in MOSFET Threshold Voltage
Contents
1. E E Cc D v 4 fo 5 a ec H _ S a a o D N S Position X um Figure 2 Increase in ionized donor trap concentration after ir radiation leakage current is affected by it Results As the paired n MOSFET device is irradiated a charge builds up within the oxide layer in the trench This is illustrated in Figure 2 which shows the ionized donor trap concentration along a cut plane 0 1 um below the top of the device The charge that has built up within the oxide layer opens a channel around the trench A leakage current flows in this channel from the vdd electrode to the ground electrode Figure 3 shows the current density vectors in the device af ter it has received a radiation dose of 4 M rad As continued exposure to radiation builds up charge in the trench oxide the leakage channel widens and the leakage current increases Figure 4 shows how the calculated leak age current from the vdd electrode increases as a function of the radiation dose under conditions of constant bias At any given radiation dose level the magnitude of the leakage current around the trench depends on the bias put on the nearby electrodes Figure 5 shows how the cal culated leakage current depends on the vdd bias and the radiation dose Notice that as the dose is increased from 1 M rad to 2 M rad the dependence of the leakage E gt fe a o a Materials Silicon OxyNitride2. ing the way to required tests to reduce uncertainties The damage associated with the collision between ener getic particles and atoms within the crystal lattice is de fects that can trap electrons and holes The density of defects is represented by the equation below N a E Density Fluence EQ 1 where e N is the defcts cm F e a is the damage factor which represents the number of defect states created per unit energy deposited in the material e E is the Non lonizing Energy Loss in MeV cm g par ticle e Density is the mass density of the material in g cm e Fluence is the particle fluence in particles cm where the particle species can be alpha electron ion neu tron photon proton or user defined The Displacement Damage capability within the Radiation Effects Module of Victory Device supports combinations of particle species with separate fluence for each type October November December 2014 Victory Process cell mode Export Victory conformal Data from radex12 0 str 68 o s Net Doping cm3 1 20 1 a 25 di Xz Materials Silicon 5102 Polysilicon Aluminum Figure 1 4T CMOS image sensor Radiation Example radex12 demonstrates a displace ment damage of a 4T CMOS Image Sensor structure A Victory Process file creates the 4T CMOS Image Sensor structure which is depicted in Figure 1 This radiation example sweeps the fluence of 1 8Mev Pro tons from 1
3. separated by a trench that are bombarded by x rays The structure and doping of these MOSFET s are shown in Figure 1 In Victory Device the radiation models only apply to semiconductors Consequently to model radiation ef fects in an insulator you must tell Victory Device to re gard the material as a semiconductor To do this set the SEMICONDUCTOR flag on an appropriate MATERIAL statement You may also need to define a limited num ber of semiconductor properties for the material For this simulation we set the following Semiconductor properties for oxynitride material material oxynitride semiconductor material material oxynitride nc300 2e19 nv300 2e19 eg300 9 affinity 0 9 permittivity 3 9 mun 1 mup le 3 m vthp 1 Semiconductor properties for oxide nMOS nMOS Structure Materials Polysilicon _ sio2 OxyNitride _ Silicon Figure 1 nMOS nMOS Structure used to Simulate Radiation Induced Leakage October November December 2014 material material oxide semiconductor material material oxide nc300 2e19 A nv300 2e19 eg300 9 affinity 0 9 mun 1 mup le 3 m vthp 1 We also specify the following traps within the oxide and oxynitride materials and on the interfaces between oxynitride and silicon Conditions for radiation induced oxide charging intoxidecharging rlmaterial oxynitride r2material silicon jmodel p nt p 1lel4 A sigmat p 1 5e 13 sigman p le 30 sigmaph p 1 5e 13 jmodel n nt n 1
4. SiO2 Polysilicon Si3N4 Conductor Electrodes Position X um Figure 3 Leakage path around trench The Simulation Standard Vdd Current A Vvdd 50 mv Vanode 1 V 1e 06 2e 06 3e 06 Total Radiation Dose rad Figure 4 Increase in leakage current with radiation dose current on the vdd voltage the slope of the curves in the figure increases as well Conclusion We have shown how ionizing radiation can create a leakage path around a trench that separates two MOSFET s The leakage path is produced because radiation causes charging of the trench oxide and the charged oxide then acts as the gate of a parasitic MOSFET that forms around the trench be tween the vdd and ground electrodes of the simulated device The leakage current increases the power consump tion of the device and may facilitate latchup A simulator such as Victory Device which can model radiation effects can help to identify potential problems like this in electronic devices that will be exposed to ionizing radiation References 1 Simulating Radiation Induced Shifts in MOSFET Thresh old Voltage Simulation Standard July August Septem ber 2014 2 Victory Device User s Manual Chapter 4 Radiation Ef fects Module REM Silvaco 2014 S i w O ks D gt 0 4 0 6 Vdd Voltage V Figure 5 Dependence of leakage current on total ionizing dose and Vdd bias October November December 20
5. of pairs that undergo geminate recombination and V N fis the probability that a phonon created by a gemi nate recombination event will interact with a filled trap to empty it Simulation To demonstrate the effects of insulator charging due to exposure to high energy radiation we shall use Victory Device to simulate an n MOSFET that is bombarded by gt 2 gt p 2 uw o a log Net Doping 19 3 18 8 18 3 17 9 Materials 17 4 SiO2 Silicon Conductor Electrodes Position X um Figure 3 n MOSFET structure used in simulation of radiation ef fects October November December 2014 x rays The structure and doping of the MOSFET are typi cal as shown in Figure 3 In Victory Device the radiation models only apply to semiconductors Consequently to model radiation ef fects in an insulator you must tell Victory Device to re gard the material as a semiconductor To do this set the SEMICONDUCTOR flag on an appropriate MATERIAL statement You may also need to define a limited num ber of semiconductor properties for the material For this simulation we set the following Semiconductor properties for oxide MATERIAL MATERIAL oxide NC300 2e19 NV300 2e19 EG300 9 SEMICONDUCTOR MATERIAL MATERIAL oxide MUN 1 MUP 1e 3 MATERIAL MATERIAL oxide M VTHP 1 We also specify the following traps in the insulator body and on its interface with the semiconductor These will become charged as the device is irrad
6. 14 Displacement Damage Two fundamental damage mechanisms take place when devices are exposed to particle fluences ionization and lattice displacement or just displacement damage Ion ization has previously been addressed in other simula tion standard articles Neutrons protons alpha particles heavy ions and very high energy photons cause lattice displacement or just displacement damage Particle bombardment can change the arrangement of the atoms in the crystal lattice creating lasting damage and increase the number of recombination defect centers depleting the minority carriers and degrading the analog proper ties of the affected semiconductor junctions High dose rates of particles particles area s can cause partial an nealing healing of the damaged lattice leading to a lower degree of damage than with the same doses deliv ered in low intensity over a longer time period The displacement damage effects can vary depending on parameters such as type of particle radiation total dose and radiation flux combination of types of radiation and device operating frequency operating voltage actual state of the de vice during the instant of irradiation and intrinsic and extrin sic shielding These issues makes thorough testing difficult time consuming and requiring a significant number of test samples Silvaco s displacement damage capability with Vic tory Device can assist in reducing the cost of testing by point
7. Simulation Standard Engineered Excellence A Journal for Process and Device Engineers Simulating Radiation Induced Shifts in MOSFET Threshold Voltage Introduction Irradiation by energetic particles can degrade semicon ductor device performance The particles involved can be electrons positrons neutrons protons alpha particles heavy ions or high energy photons As they pass through a device these particles interact with the lattice Energy deposited through these interactions may damage the lattice directly by displacing its atoms or may result in the creation of electron hole pairs A sudden excess of electron hole pairs may trigger a latchup possibly dam aging the device through overcurrent Holes generated within an insulator may become trapped there leading to a gradual accumulation of charge that worsens perfor mance and eventually causes the device to fail Consid eration and modeling of these effects is important when designing semiconductor devices that will be exposed to high energy radiation The device simulator Victory Device has models to ac count for the following radiation effects 1 Local electron hole pair generation along particle tracks caused by individual particle strikes single event upsets 2 Generation and recombination of electron hole pairs caused by the ongoing radiation of a device 3 Insulator charging caused by the trapping of radia tion generated electrons and holes withi
8. ce and towards the The Simulation Standard Page 4 Concentration of Trapped Holes at Oxide Channel Interface Response to Irradiation under Forward and Reverse Bias Forward Bias With Forward Bias Removed Reverse Bias __With Reverse Bias Removed a E U a 104 v o Uv uv v po uv a pe E 5e 05 le 06 1 5e 06 Total Radiation Dose rad Figure 5 Response of trapped interface holes to gate bias and irradiation gate Consequently little or no trapping takes place at the interface When the reverse bias is removed approxi mately half the holes generated in the oxide start diffus ing towards the interface and some of them are trapped there With irradiation continuing at zero gate bias both the upper and the lower curves in Figure 5 appear to be headed towards the same level Indeed under conditions of constant irradiation and constant bias the distribution of trapped holes eventually approaches an equilibrium value that depends only on the final bias Conclusion We have shown one way in which the operation and per formance of semiconductor devices can be altered or de graded by exposure to high energy radiation However these effects can be mitigated if they are taken into con sideration during the design of a semiconductor device A simulator such as Victory Device which can model radiation effects can be a useful tool in the design of radiation har
9. cesses The capture process for insulator traps is the same as for traps in ordinary semiconductors but the emission pro cess appears to be different According to the model of Kimpton and Kerr the primary energy source stimulat ing the detrapping process is the geminate recombination Victory Device Default Yield Fraction 1666686 1e 06 Electric Field Magnitude VY cn Figure 2 Geminate yield functions for SiO October November December 2014 of electron hole pairs during irradiation As illustrated in Figure 1 a geminate recombination event emits a quantum of energy that may prompt the emission of a hole from a donor like trap or the emission of an electron from an acceptor like trap Meanwhile the trap energies are assumed to be far from the band edges so the ordi nary thermally stimulated emission processes are negli gible According to these assumptions donor like insulator traps emit holes at the rate Op 3 C U YV N f where e G is the generation factor with units of generated pairs cm rad e is the dose rate in rad s e Yis the geminate yield Vis the emission interaction volume for a trap e Nis the density of traps and e fis the probability that a trap is filled The expression for the emission of electrons from accep tor like traps is similar Breaking down the rate equation G 6 is the rate of elec tron hole pair creation per unit volume 1 Y is the frac tion
10. defects to specific locations or material using the local ization parameter within the defect statement as described in section 6 4 of the Victory Device manual To sweep the Fluence value one can use a feature de scribed in Appendix B of the VWF Interactive Tools Manual as shown below solve init solve previous Deplete the Image Sensor of Electrons log outfile radex12_0 log solve Vcgate 3 3 ramptime 1e 6 dt 1e 8 tstop 1e 6 solve tstop 2e 6 dt 1e 8 Dark Recovery Time solve tstop 1 dt 1e 7 go internal load infile radex12_1 in sweep parameter FLUENCE type list data 1e10 1e11 1e12 with the radex12_1 in file being go victorydevice set FLUENCE 1e8 mesh infile radex12_0 str radiation proton energy 1 8 fluence FLUENCE material dam proton 1e3 dam niel 3 1 defects fluence model sigtae 1 e 17 sigtah 1 e 15 sigtde 1 e 15 sigtdh 1 e 17 models consrh cvt fermi The Simulation Standard Page 8 VICTORY Data from multiple files y cis conc 7 Y zZ 3 4 v lt radex12_0 log radex12_1x1010 log lt radex12_1x10 1 Jog radex12_1x10 2 log Figure 2 4T CMOS Image Sensor degrading electron concen tration from base through 1e10 1e11 and 1e12 proton fluences output band param con band val band probe n conc x 4 25 y 3 25 z 1 name cis_conc probe potential x 1 y 4 5 z 0 002 name fd_poten tial probe potential x 4 25 y 3 25 z 1 tial name cis_poten method pam gmres norm scalin
11. dened electronics References 1 R J Milanowski et al TCAD Assisted Analysis of Back Channel Leakage in Irradiated Mesa SOI nMOSFETs IEEE Transactions on Nuclear Science Vol 45 No 6 1998 2593 2599 2 C M Dozier et al An Evaluation of Low Energy X Ray and Cobalt 60 Irradiations of MOS Transistors IEEE Transactions on Nuclear Science Vol 34 No 6 1987 1535 1539 3 D Kimpton and J Kerr A Simple Trap Detrap Model for Accurate Prediction of Radiation Induced Threshold Voltage Shifts in Radiation Tolerant Oxides for all Static or Time Variant Oxide Fields Solid State Electronics Vol 37 No 1 1994 153 158 October November December 2014 Radiation Induced Current Leakage Between Two n MOSFET s Introduction The Simulation Standard article Simulating Radiation Induced Shifts in MOSFET Threshold Voltage gives a brief overview of the ways that ionizing radiation can affect semiconductor devices and considers insula tor charging in particular In the Victory Device User s Manual there is a more extensive discussion of radiation effects Here we look at how insulator charging due to ionizing radiation can induce a leakage current between two MOSFET s separated by a trench Simulation To demonstrate how exposure to high energy radiation can lead to a breakdown of the isolation between separate devices we shall use Victory Device to simulate a pair of n MOSFET s
12. e8 to 1e12 and plots the electron concentration as the fluence is swept through its values The fluence value is initially set to 1e8 using the set con struct set FLUENCE 1e8 The Victory Process generated structure is imported mesh infile radex12_0 str The radiation statement is declared and parameterized with protons of 1 8 MeV at a fluence of 1e8 from the set statement radiation proton energy 1 8 fluence FLUENCE Using the material statement the damaging particles are defined to be protons with a NIEL value of 3 1MeV cm g material damage proton 1e3 damage niel 3 1 The displacement damage defects model is declared by setting the fluence model flag on the defects statement The density of defects is calculated using EQ 1 above This density of defects applies individually to both ac ceptor like and donor like defects states The energy of these defect states are assumed to be uniformly distrib The Simulation Standard uted across the band gap If one wanted to describe only defects of a single type one must explicitly set NUMA or NUMD parameter to zero The tail state parameters SIG TAE SIGTAH SIGTDE and SIGTDH are used to specify the cross sections defects fluence model sigtae 1 e 17 sigtah 1 e 15 sigtde 1 e 15 sigtdh 1 e 17 Victory Device allows one to specify combinations of par ticle species but each requires a separate radiation state ment for each different species and supports the bounding of
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14. g local solve init solve previous Deplete the Image Sensor of Electrons log outfile radex12_ FLUENCE log solve Vcgate 3 3 ramptime 1e 6 dt 1e 8 tstop 1e 6 solve tstop 2e 6 dt 1e 8 Dark Recovery Time solve tstop 1 dt 1e 7 The simulation of increasing fluence from a base to 1e10 1e11 and final to 1e12 is shown by Figure 2 The Figure shows that at a fluence of 1e12 that well collection no lon ger exists and the Image Sensor is no longer functioning properly October November December 2014 Mixed Mode Radiation Event Detector A reversed biased PIN diode acts as a radiation event detector sensing the occurrence of an ionizing radiation pulse which is connected to a threshold detector the re sistor Ra y that drives the base input of a comple mentary Q1 and Q2 pulse amplifier The output of the amplifier is supplied to the base of Q4 that drives a pulse circuit Q5 and Q6 with a time constant set by RT x CT The Radiation Event Detector circuit is shown in Figure 1 The PIN diode structure is generated using Victory Process Since the PIN diode is cylindrically symmetric one can use 2D Victory Process to generate the PIN diode structure as shown in Figure 2 A and B This structure once generated can now be called from a Data from pin_3D str mixed mode simulation as shown below ADIODE PWR15V cathode PIN NEG anode GUARD _ RING pgate infile pin 3D str cylindrical The generated user defined ioniz
15. generated become active in the device Some pairs recombine so soon after they are gen erated that neither carrier has a chance to be transported This is called geminate recombination It is accounted for by multiplying the basic generation rate by a yield function Based on the work of Dozier et al and others the yield function used by Victory Device takes the form A Y where El Y IEI E El is the magnitude of the electric field Y is the zero field yield factor E is a critical field value and A moderates the growth rate of the function Here A is non negative The Simulation Standard Hole Electron Pair Generation and Subsequent Geminate Page 2 Conduction Band Electric Field Energy Quantum Oxide Valence Band Hole Trap Energy Quantum from Geminate Recombination Process Releases Recombination Trapped Hole The parameters of the yield function depend on both the type of the radiation and the material being irradiated Typical curves for SiO are shown in Figure 2 If an electric field is present electrons and holes from pairs that survive the geminate recombination process drift apart under the influence of the field Insulator charging occurs when some of these carriers become trapped by impurities or other crystal defects present within the insulator ma terial The magnitude of this charging is determined by a balance between carrier capture and emission pro
16. he second is the electric field that arises from the applied biases which tends to separate the carriers This simple description is sufficient provided that that distinct pairs of electrons and holes are far enough apart that the sepa rate pairs do not interact A sparse distribution like this is denoted by the adjective geminate Continued on page 2 INSIDE Radiation Induced Current Leakage Between Toon MOSFET Displacement Damage Mixed Mode Radiation Event Detector SILVACO Photon e Oxide e N Y Y N y Hole Trap Untrapped Hole Trapped Hole Figure 1 Physical processes leading to insulator charging In accounting for the generation of electron hole pairs the generation rate due to high energy radiation can be expressed as the product of the irradiation dose rate and a generation factor that is specific to the irradiated material This generation factor is basically just the material density divided by the energy it takes to create an electron hole pair in that substance Departures from this basic rate can be accounted for by applying a numerical enhancement fac tor Taking the density of SiO as 2 2 g cm and the forma tion energy of an electron hole pair as 18 eV we calculate a basic generation factor for SiO of 7 6x10 pairs cm rad Multiplying this by a dose rate expressed in rad s gives us a generation rate in units of pairs cm s However not all electron hole pairs that are
17. iated Conditions for radiation induced oxide charging INTOXIDECHARGING RIMATERIAL oxide R2MATERIAL silicon JMODEL P NT P 3e12 SIGMAT P 1 5e 13 SIGMAN P le 30 SIGMAPH P 1 5e 13 JMODEL N NT N 1e4 SIGMAT N 1e 30 SIGMAP N le 30 SIGMAPH N 1e 30 MFP PHONON 0 013 OXIDECHARGING MATERIAL oxide JMODEL P NT P 4e18 SIGMAT P 1 5e 13 SIGMAN P le 30 SIGMAPH P 1 5e 13 JMODEL N NT N 1e10 SIGMAT N 1e 30 SIGMAP N 1e 30 SIGMAPH N 1e 30 The source of radiation will be x rays at a dose rate of 1 rad s Radiation environment RADIATION DOSERATE 1 XRay The simulation in our example consists of two parts In the first part we begin by irradiating the device while it is under a forward gate bias of 5 V up to an exposure of 1 M rad In the second part we remove the gate bias and continue the irradiation up to a total exposure of 2 M rad At the start of the simulation after the first part and at the end we shall sweep the gate bias to determine the threshold voltage The Simulation Standard IdVg Threshold Shift due to Irradiation Before irradiation After Irradiation under Forward Bias After Irradation with Bias Removed Drain Current A Gate Voltage V Figure 4 Shift in IDVG threshold voltage due to insulator charging Results Irradiation under forward bias induces a shift in the threshold voltage of the IV curve as seen in Figure 4 This is due to the trapping of holes in the o
18. ing pulse is done using the combination of statements below RAD PWL 0 0 0 9e 06 0 1 0e 06 1 0 2 0e 06 1 0 2 1e 06 0 0 The RAD statements generates a piece wise linear pulse at 0 9us with a rise time of 0 1us to a value of 1 with a width of lus and then a fall time of 0 1us starting at 2us Figure 2 B PIN diode structure showing anode and guard band pgate contact to a value of 0 at 2 lus This RAD pulse generations PIN DOCE 5 D ADIODE PIN NEG a a ia Lu wi Lu wi lt RThreshold 100 gt CT XRT Time Constant for NED Pulse Figure1 Radiation Event Detector Circuit October November December 2014 Page 9 The Simulation Standard electron hole pairs using the radiation statement as de scribed in chapter 4 of the Victory Device manual VIPIN_NEG V radiation g0 4 0e13 doserate 1 0e8 The result of the ionizing pulse is shown in Figure 3 Mixed Mode simulation of this Radiation Event Detector allows the user to explore threshold sensitivity by chang ing the value of RThreshold from 10 ohm to 10K ohms and the response time of the circuit by changing C3 and R6 values as well as associated device models for the transistors Q1 and Q2 Figure 3 Radiation Event Detector Circuit signaling that a lon izing Pulse of 1E8 for 1us occurred The Simulation Standard Page 10 October November December 2014 USA Headquarters Silvaco Inc 4701 Patrick Henry Drive Bld
19. le4 sigmat n le 30 sigmap n le 30 Sigmaph n le 30 mfp phonon 0 013 oxidecharging material oxynitride jmodel p nt p 2e18 sigmat p 1 5e 13 A Sigman p le 30 sigmaph p 1 5e 13 jmodel n nt n 1el0 sigmat n le 30 A Sigmap n le 30 sigmaph n le 30 oxidecharging material oxide vmodel p nt p 2e18 sigmat p 1 5e 17 Sigman p le 34 sigmaph p 1 5e 13 vmodel n nt n lel0 sigmat n 2e 32 Sigmap n 2e 32 sigmaph n 1le 30 The source of radiation will be x rays at a dose rate of 1 rad s Radiation environment radiation doserate 1 Xray In the paired n MOSFET structure shown in Figure 1 there is a polysilicon layer in the trench This layer is pres ent for stress relief but may act as a floating electrode For the purpose of illustrating radiation induced leakage around the trench our simulation assumes that this poly silicon layer labeled anode in the figure has floated to a bias of 1 V although this represents something of a worst case scenario With a bias of 50 mV on the electrode labeled vdd we use Victory Device to simulate the de vice performance as it is irradiated up to a dose of 4 M rad in order to examine the effect of dose on the leakage current Also at dose levels of 0 1 2 3 and 4 M rad we sweep the vdd bias between 0 V and 1 V to see how the The Simulation Standard Charged Donor Hole Traps Cutline at y 0 1 microns Silicon OxyNitride SiO2 Polysilicon Si3N4 Ga O et
20. n insulator materials 4 Lattice dislocation defects caused by the accumulated flux of radiation through a device In this article we illustrate some effects that radiation may have on the electrical characteristics of a device We shall consider an n MOSFET that is exposed to x rays consequently experiencing radiation induced generation and recombination that leads to charging of the oxide re gion below the gate Volume 24 Number 4 October November December 2014 Insulator Charging In insulator materials because very few carriers are pres ent trap states usually do not become charged although quantum mechanical tunneling may cause some charg ing near interfaces Irradiation by energetic particles however can generate electron hole pairs within the body of an insulator Under the influence of an electric field the electrons and holes from these pairs can sepa rate and become trapped leading to a gradual accumula tion of charge both within the insulator and on its sur face The processes involved are illustrated in Figure 1 An energetic particle such as an x ray photon first enters the body of an insulator There it may be scattered by the crystal lattice and in the process generate an electron hole pair Once an electron hole pair has been created it becomes subject to two opposing forces The first force arises from the Coulomb attraction between the electron and hole which tends to cause the pair to recombine T
21. xide With a positive bias on the gate electrode holes generated with in the oxide are pushed away from the gate so most of the trapping takes place near the oxide silicon interface When the gate bias is removed only half of the radiation generated holes will migrate towards the oxide silicon interface while the other half migrate towards the gate Consequently irradiation with the bias removed releases some of holes that were trapped near the interface even tually reducing the threshold shift by about half In Victory Device we can set a PROBE near the oxide silicon interface to investigate how the areal concentra tion of trapped holes changes during the course of this simulation Probe conditions near the oxide silicon interface PROBE MATERIAL oxide INT DONOR TRAPS X 0 Y le 6 NAME trapped int holes The results are shown in Figure 5 With irradiation under a forward gate bias the concentration of holes trapped at the interface gradually increases Eventually it should saturate but the saturation level is not reached during the course of this simulation After the bias is removed the rate at which holes impinge on the traps is reduced by roughly half while the hole emission rate remains nearly the same leading to a reduction in the concentration of trapped holes Figure 5 also shows a curve for a reversed gate bias Under a reversed bias holes generated in the oxide are pulled away from the oxide silicon interfa
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