Silicon Oxides - University of Texas at Austin

Silicon Oxides - University of Texas at Austin

Silicon Oxides: SiO2 Uses: diffusion masks surface passivation gate insulator (MOSFET) isolation, insulation Formation: grown / native thermal: highest quality anodization deposited: C V D, evaporate, sputter vitreous silica: material is a GLASS under normal circumstances can also find crystal quartz in nature m.p. 1732 C; glass is unstable below 1710 C BUT devitrification rate (i.e. crystallization) below 1000 C negligible Dean P. Neikirk 1999, last update January 31, 2020 1 bridging oxygen non-bridging oxygen silicon network modifier network former hydroxyl group Dept. of ECE, Univ. of Texas at Austin Growth of SiO2 from Si in dry (<< 20 ppm H2O) oxygen Si + O2 SiO2 once an oxide is formed, how does this chemical reaction

continue? does the oxygen go in or the silicon go out? density / formula differences SiO2 = 2.25 gm/cm3 , GMW = 60 Si = 2.3 gm/cm3 , GMW = 28 oxide d thick consumes a layer 0.44d thick of Si original silicon surface d SiO2 0.44d bare silicon in air is always covered with about 15-20 of oxide, upper limit of ~ 40 it is possible to prepare a hydrogen terminated Si surface to retard this native oxide formation Dean P. Neikirk 1999, last update January 31, 2020 2 Dept. of ECE, Univ. of Texas at Austin Wet oxidation of Si overall reaction is Si + 2 H2O SiO2 + H2 proposed process H2O + Si-O-Si Si-OH + Si-OH diffusion of hydroxyl complex to SiO2 -Si interface Si - OH Si - O - Si + Si - Si Si - O H + H2 Si - O - Si this results in a more open oxide, with lower density, weaker structure, than dry oxide wet 2 . 15 gm / cm3

Dean P. Neikirk 1999, last update January 31, 2020 3 Dept. of ECE, Univ. of Texas at Austin Oxide growth kinetics basic model is the Grove and Deal Model supply of oxidizer is limited by diffusion through oxide to growth interface N Ficks First Law: flux j D oxidizer concentration x moving growth interface N0 N1 gas oxide silicon x simplest approximation: N N0 N1 x x Dean P. Neikirk 1999, last update January 31, 2020 4 Dept. of ECE, Univ. of Texas at Austin concentration Oxidizer concentration gradient and flux N0 gas oxide x N1 moving

growth interface silicon N N N j D oxidizer D 0 1 x x N0 is limited by the solid solubility limit of the oxidizer in the oxide! N0O2 ~ 5 x 1016 cm-3 @ 1000 C N0H2O ~ 3 x 1019 cm-3 @ 1000 C flux of oxidizer j at SiO2 / Si interface consumed to form new oxide j' kN1 k is the chemical reaction rate constant in steady state, flux in must equal flux consumed j' steady state j N0 N1 kN1 D x Dean P. Neikirk 1999, last update January 31, 2020 5 j DN0 x Dk solve for N1, sub back into flux eq Dept. of ECE, Univ. of Texas at Austin Relation between flux and interface position flux: #oxidizer molecules crossing interface per unit area

per unit time # cm-2 sec-1 rate of change of interface position: dx / dt (interface velocity) cm sec-1 n: # of oxidizer molecules per unit volume of oxide: # cm-3 SiO2 NA 2 for H2O 2 for H2O 22 3 n 2.25 10 cm GMWSiO2 1 for O2 1 for O2 then relation is just dx dt j DN0 n n x Dk now integrate with appropriate initial condition Dean P. Neikirk 1999, last update January 31, 2020 6 Dept. of ECE, Univ. of Texas at Austin Grove and Deal relation setting 2D/k = A function of whats diffusing, what its diffusing in, and what it reacts with 2DN0/n = B function of whats diffusing and what its diffusing in initial condition x (t = 0) = xi integration gives

x t A t 1 2 1 2 A 4B LiveMath where represents an offset time to account for any oxide present at t = 0 Dean P. Neikirk 1999, last update January 31, 2020 xi 2 7 Axi B Dept. of ECE, Univ. of Texas at Austin Limiting behavior of Grove & Deal oxidation model short times x t t A t 1 2 2 A 4B 1 2 A

x t x t A 2 1 t 1 A 2 4B 4B A t 1 B t 1 12 2 2 A A 4B thickness is linearly increasing with time characteristic of a reaction rate limited process B/A is the linear rate constant N k B 2 D N 0 2 D 0 n A n k linear rate constant depends on reaction rate between oxidizer and silicon (k) AND solid solubility of oxidizer in oxide (N0) temperature dependence mainly from reaction rate

Dean P. Neikirk 1999, last update January 31, 2020 8 Dept. of ECE, Univ. of Texas at Austin Rate constants and Arrhenius plots 0.000 8 EA = 0.5eV 0.000 6 thermally activated process i.e., process must thermally overcome an energy barrier y y o e EA kT 0.000 4 y EA = 1eV 0.000 2 400 LiveMath 600 800 1x10+003 1.2x10+003 1.4x10+003 temperature plot log(y) vs 1/T EA = 0.5eV if process has the simple thermally-activated behavior you will get a straight line! 0

log[y] 0 EA = 1eV 0.001 0.001 5 0.002 0.002 5 0.003 1/[temperature] Dean P. Neikirk 1999, last update January 31, 2020 9 Dept. of ECE, Univ. of Texas at Austin Rate constant behavior: linear rate constant A 2 1 B x t t A t 1 A 2 4B t B 2DN0 2D N0 k A n k n linear rate constant depends on reaction rate between oxidizer and silicon (k) is orientation

dependent temperature dependence mainly from energy required to break Si-Si bond 1200C 1000 C 10 solid solubility of oxidizer in oxide (N0) 10 800 C H20 (640 Torr) EA = 2.05 eV 1 (111) Si 10-1 (100) Si (111) Si B A 111 1.68 B A 100 10-2 (100) Si 10-3 dry O2 EA = 2.0 eV adapted from Ghandhi 10-4 0.6 AND Dean P. Neikirk 1999, last update January 31, 2020 A2 4 B Linear rate constant B/A (m/hour) x t 0.7

0.8 0.9 1.0 1.1 Temperature 1000/T (K-1) plane available bonds (100) 6.8 x1014cm-2 (111) 11.7 x1014cm-2 ratio 1.7 (from Sze, 2nd ed., p. 110) Dept. of ECE, Univ. of Texas at Austin Limiting behavior of Grove & Deal oxidation model long times x t A t 1 2 2 A 4B t 1 x t A 2 1 t

1 A 2 4B A2 4 B x t A t 2 2 A 4B Bt dependence is parabolic: (thickness)2 time characteristic of a diffusion limited process B is the parabolic rate constant B 2DN0 n parabolic rate constant depends on diffusivity of oxidizer in oxide (D) AND solid solubility of oxidizer in oxide (N0) temperature dependence mainly from diffusivity Dean P. Neikirk 1999, last update January 31, 2020 11 Dept. of ECE, Univ. of Texas at Austin parabolic rate constant depends on diffusivity of oxidizer in oxide (D) and solid solubility of oxidizer in oxide (N0) Solid solubility @ 1000C H2O 3 x 1019cm-3 O2

5 x 1016cm-3 is NOT orientation dependent IS oxidizer dependent temperature dependence mainly from diffusivity of oxidizer in oxide Dean P. Neikirk 1999, last update January 31, 2020 12 600C 700C 800C 900C 1000C 2DN0 n 1200C B Parabolic rate constant B ( m2/ hour) x t B t 1100C Rate constant behavior: parabolic rate constant 1 H 2 0 (76 0 T orr ) 10 -1 E A = 0 .7 1 eV 10 -2 dr y O 2 10 -3 E A = 1 .2 4 eV 10 -4 0. 7 0. 8 0. 9 1. 0 1. 1 1. 2

Tempera tur e 1000 /T ( K -1 ) adapted from Ghandhi Dept. of ECE, Univ. of Texas at Austin Effect of Si doping on oxidation kinetics k = Cox / CSi ~ 3 dopants accumulate in oxide little effect on linear rate constant B/A ( = Nok / n) can increase parabolic rate constant B ( = 2DNo / n ) concentration boron k > 1, slow SiO2 diffuser oxide silicon really only significant for Nboron > ~1020 cm-3 phosphorus k = Cox / CSi ~ 0.1 dopants pile-up at silicon surface little effect on parabolic rate constant B increases linear rate constant B/A again, really only significant for Nphosphorus > ~1020 cm-3 Dean P. Neikirk 1999, last update January 31, 2020 13 concentration k < 1, slow SiO2 diffuser oxide silicon Dept. of ECE, Univ. of Texas at Austin Oxidation thicknesses

640 Torr partial pressure is typical (vapor pressure over liquid water @ 95C) dry oxidation 101 100 (100) silicon steam (100) silicon dry Oxide thickness (microns) Oxide thickness (microns) wet oxidation 1100C 1200C 10-1 1000C 900C 10-2 10-1 100 10-1 1100C 1050C C 00 0 1 C 0 5 9 C 0 0 9 10-2

101 10-1 Time (hours) Dean P. Neikirk 1999, last update January 31, 2020 100 1150C C 0 0 8 100 101 Time (hours) 14 Dept. of ECE, Univ. of Texas at Austin Pressure Effects on Oxidation grow thick oxides at reduced time / temperature product use elevated pressures to increase concentration of oxidizer in oxide for steam, both B and B/A ~ linear with pressure rule of thumb: constant growth rate, if for each increase of 1 atm pressure, temperature is reduced ~ 30 C. pressures up to 25 atm have been used (commercial systems: HiPOx, FOX) Dean P. Neikirk 1999, last update January 31, 2020 Pyrogenic steam 900 C Oxide Thickness (microns)

20 atm 10 atm 5 atm 1 1 atm (111) (100) 0.1 adapted from Sze, 2nd, p. 122 0.2 1 10 Time (hour) 15 Dept. of ECE, Univ. of Texas at Austin Oxidation isolation how do you insulate (isolate) one device from another? what about using an oxide? try growing it first since itll be long, high temp now etch clear regions to build devices in oxide devices silicon oxidation , etch it doesnt do any good!! Dean P. Neikirk 1999, last update January 31, 2020 16 Dept. of ECE, Univ. of Texas at Austin Masking of oxidation and isolation techniques would like to form thick oxide between device regions with minimum step heights mask oxidation using material with low water diffusivity / solubility: Local Oxidation of Silicon (LOCOS) process Si3N4 (silicon nitride)

induces high stress, must place pad layer below to prevent dislocations for oxide pad layer, tpad ~ 0.25 tnitride do have lateral diffusion at mask edge, produces birds beak lateral encroachment oxide thickness can reduce using more complex pads SiO2 / poly / nitride (~15nm/50nm/150nm) helps nitride mask pad SiO2 silicon birds beak before oxidation Dean P. Neikirk 1999, last update January 31, 2020 after oxidation 17 Dept. of ECE, Univ. of Texas at Austin Use of SiO2 as a diffusion mask Oxide Mask Thickness (microns) mask against boron, phosphorus, arsenic diffusion most impurities transported to slice surface as an oxide P2O5, B2O3 reacts with oxide to form mixed phosphosilcate (or borosilicate) glass reaction continues until full thickness of masking oxide is converted doping of underlying Si commences 101 adapted from Nicollian and Brews, p. 732 phosphorus 100 1200 C 1100 C 1000 C 900 C simple parabolic relationship: 1200 C 10-1 boron 10-2

10-3 phosphorus: 1100 C 1000 C xm2 900 C tm boron: xm2 101 102 Time (minutes) tm 10 3 Dean P. Neikirk 1999, last update January 31, 2020 18 7 1.7 10 e 4.9 10 5 e 1.46eV cm2 k T sec 2.80eV 2 kT cm sec Dept. of ECE, Univ. of Texas at Austin Other problems in oxidation: MOS threshold stability

for an MIS structure if ionized impurities are in the insulator, what happens? if the ionized impurities are close to the metal, they are screened, and the silicon surface remains unchanged Metal - threshold voltage of MOS device is critically dependent on the location and amount of mobile ionic contamination in SiO2 gate insulator stability can be adversely affected Dean P. Neikirk 1999, last update January 31, 2020 image charge Silicon Oxide + + + + + + + + + - image charge + fixed surface states mobile ionized impurities in oxide (sodium), Qm 19

Dept. of ECE, Univ. of Texas at Austin Bias-Temperature-Stress (BTS) technique apply field of approximately 1MV/cm between capacitor metallization and substrate heat sample to ~200o C to increase diffusion rate of Na+ E 1MV/cmMV/cm T 200CC Metal Oxide + + + + + + + + + + allows evaluation of contamination levels Silicon - - mobile ionized impurities in oxide (sodium), Qm Dean P. Neikirk 1999, last update January 31, 2020 20 Dept. of ECE, Univ. of Texas at Austin BTS impurity drift apply field of approximately 1MV/cm across gate insulator heat sample to ~200o C to increase diffusion rate of Na+ Metal

image charge external - electric field + - most of the impurities have drifted to near the silicon surface screening now due to electrons at silicon surface can cause surface inversion!! process is reversible! Dean P. Neikirk 1999, last update January 31, 2020 Oxide - + Silicon + + + + + + + + - mobile ionized impurities in oxide (sodium), Qm 21 image charge fixed surface states Dept. of ECE, Univ. of Texas at Austin Cox after BTS Qm / Cox capacitance C-V measurements p-type substrate before BTS CT

voltage (metal wrt substrate) measure C-V curves before and after BTS Qm Cox x Vt Qm / toxAcapq this occurs even at room temperature! threshold voltage shift Vt can be MANY volts! Dean P. Neikirk 1999, last update January 31, 2020 22 Dept. of ECE, Univ. of Texas at Austin Other problems in oxidation: oxidation induced stacking faults typically ~95% of all stacking faults are OSFs essentially an extra (111) plane density can vary from ~0 to 107 / cm2 highly dependent on process (high temperature) history size (length) at surface can be many microns growth is related to presence of excess unoxidized Si at Si-SiO2 interface heterogeneous coalescence of excess Si on nucleation centers produce OSFs any process that produces an excess of Si vacancies will inhibit OSF formation/growth Dean P. Neikirk 1999, last update January 31, 2020 23 Dept. of ECE, Univ. of Texas at Austin Chlorine Gettering in SiO2 effects of chlorine during oxidation reduction of OSFs increased dielectric breakdown strength improved minority carrier lifetimes improved MOS threshold stability when oxide is grown in presence of chlorine the Cl- getters ionic contaminants such as Na+ Cl in gas stream reacts with Na diffusing from the furnace walls Cl- is incorporated into the grown oxide near the silicon interface (~20nm from Si) and can capture mobile sodium preventing threshhold instabilities efficiency of chlorine gettering can be evaluated by alternately drifting the Na+ back and forth from metal to silicon side of the test capacitor

only effective for oxidation temperature above ~1100 C Dean P. Neikirk 1999, last update January 31, 2020 24 Dept. of ECE, Univ. of Texas at Austin Restrictions on the use of Cl Gettering chlorine injection techniques mix HCl in O2 gas stream must be VERY dry or severe corrosion problems can occur produces large quantities of H2O in furnace: 4HCl + O2 2Cl2 + 2H2O mix trichloroethylene (TCE) / trichloroethane (TCA) in O2 gas stream produces less water: C2HCl3 + 2O2 HCl +Cl2 + 2CO2 for TCE must have large excess of O2 present to prevent carbon deposits under low temperatures and low oxygen conditions can form phosgene COCl2 only effective if oxidation temperature 1100 C large Cl concentrations, very high temperatures, or very long oxidation times can cause rough oxides very high Cl can cause blisters and separation of SiO2 from Si Dean P. Neikirk 1999, last update January 31, 2020 25 Dept. of ECE, Univ. of Texas at Austin Minimum feature (microns) 10 1000 1K 4K 16K 64K 1 256K

100 1M 4M 16M 64M 256M 1G 16G 10 0.1 1970 1975 1980 1985 1990 1995 2000 2005 year (dram: intro or production) Oxide thickness (Angstroms) Gate Oxide thickness trends MOS oxide thickness scales with gate length circa 1998 thickness was ~ 4nm (64M and 256M DRAM technology) Dean P. Neikirk 1999, last update January 31, 2020 26 Dept. of ECE, Univ. of Texas at Austin Oxidation thicknesses: Grove & Deal calculations wet oxidation 640 Torr partial pressure fits very well to measurement 101 (100) silicon steam 1MV/cm0 1MV/cm000C Oxide thickness (microns) oxide thickness (microns) (1MV/cm00) Si

1MV/cm 1MV/cm1MV/cm00C 0.1MV/cm 0.01MV/cm 0.01MV/cm 900C 100 10-1 1150C 1050C C 0 0 10 C 0 95 C 0 90 10-2 0.1MV/cm 1MV/cm 1MV/cm0 wet oxidation time (hours) 10-1 1MV/cm00 C 0 0 8 100 101 Time (hours) calculation Dean P. Neikirk 1999, last update January 31, 2020 1100C measurement 27 Dept. of ECE, Univ. of Texas at Austin

Problems with the simple Grove/Deal model of oxidation Arrhenius plots of linear and parabolic rate constants are not straight at low temperatures (T < 900o C). dry oxidation growth curves do not extrapolate back to zero oxide thickness at zero time A (m) (hr) xi (nm) 1200 0.05 0.720 14.4 0 0 Wet 1100 0.11 0.51 4.64 0 0 (640 Torr H2O) 1000 0.226 0.287 1.27 0 0 920

0.5 0.203 0.406 0 0 1200 0.04 0.045 1.12 0.027 20 Dry 1100 0.09 0.027 0.30 0.076 19 (760 Torr O2) 1000 0.165 0.0117 0.071 0.37 23 920 0.235 0.0049

0.02 1.4 26 700 ? ? ~2.6 x10-3 81 B B/A 2 ( m /hr) ( m/hr) 1MV/cm Dry Oxidation Thickness (m) T (C) (1MV/cm00) Si 1MV/cm1MV/cm00C 1MV/cm000C 0.1MV/cm 900C 0.01MV/cm 0.1MV/cm 1MV/cm time (hours) 1MV/cm0 must use non-physical boundary condition of either an offset time or xi200 in order to fit data Dean P. Neikirk 1999, last update January 31, 2020 28 Dept. of ECE, Univ. of Texas at Austin Possible models to explain rapid initial growth micropores and intrinsic stress in low temperature thin oxides micropores:

~10 about 100 apart can visualize as small irregularities that mask oxidation, adjacent regions grow up around "holes" diffusion of O2 down pores can fit rapid initial growth stage and curvature of Arrhenius plots Dean P. Neikirk 1999, last update January 31, 2020 29 Dept. of ECE, Univ. of Texas at Austin Field-Enhanced Diffusion experiments with external applied fields imply O2 is charged during oxidation: ~D O2 O2- + h+ because of mobility differences have ambipolar diffusion effects: range of effect is approximately the Debye length D D 1/NN gas + + + - O + - 2 + + hole + + + + + oxide silicon ~150-200 for O2 in SiO2 ~5 for H2O in SiO2 Dean P. Neikirk 1999, last update January 31, 2020 30 Dept. of ECE, Univ. of Texas at Austin Thin oxide growth

thin oxides can be grown controllably use reduced pressure usually dry O2 oxidation need pressure (1 atms = 3x1019) near solid solubility limit (5x1016 @ 1000C) ~ 10-3 atms (0.25 - 2 Torr used) use low temperature!!! intrinsic stress in low temperaturegrown oxides: (100) silicon, dry oxygen 1000 C 950 C Oxide thickness (nm) low temp oxides tend to have higher density not thermal expansion stress at high temp viscosity of SiO2 low enough to allow plastic flow at low temp viscosity is too high 102 900 C 850 C 800 C 101 adapted from Sze, 2nd ed, p. 1MV/cm1MV/cm6. 100 Dean P. Neikirk 1999, last update January 31, 2020 10-1 31 100 101 102 Oxidation time (minutes) 103 Dept. of ECE, Univ. of Texas at Austin ITRS roadmap (2000) requirements (http://public.itrs.net/Files/2001ITRS/Home.htm ) year min gate

length (nm) equivalent gate oxide thickness (nm) 130 2002 85-90 1.5-1.9 90 2005 65 1.0-1.5 60 2008 45 0.8-1.2 40 2011 32 0.6-0.8 technology node (nm) 2005 projections require lgate ~ 65nm, effective oxide thickness ~ 1-1.5nm EOT : EOT t physical r r SiO2 problem: excessive leakage current and boron penetration for oxide thicknesses < 1.5nm alternative high k dielectrics Dean P. Neikirk 1999, last update January 31, 2020 32

Dept. of ECE, Univ. of Texas at Austin Diffusion Mechanisms probability of movement 4 o e E kT , o ~ 1013 - 1014 sec-1 substitutional impurity vacancy interstitial diffusers Emove ~ 0.6 - 1.2 eV T = 300K: ~ 1 jump per minute T = 1300K: ~ 109 jumps per sec substitutional diffusers Emove ~ 3 - 4 eV T = 300K: ~ 1 jump per 1030 - 1040 years! T = 1300K: ~ few jumps per sec Dean P. Neikirk 1999, last update January 31, 2020 interstitial impurity 33 interstitialcy mechanism Dept. of ECE, Univ. of Texas at Austin

Recently Viewed Presentations

  • Diapositiva 1

    Diapositiva 1

    Preparación del ensilaje. Cortado, picado, compactado (60% de humedad) y cubrimiento. Es un proceso que se divide luego en 4 etapas.
  • Organic Chemistry

    Organic Chemistry

    Some Properties of Alkanes Alkanes with 1-4 carbon atoms are methane, ethane, propane, and butane. gases at room temperature. used as heating fuels. Some Properties of Alkanes Alkanes with 5-8 carbon atoms are liquids at room temperature. pentane, hexane, heptane,...
  • The Constitutional Scenarios - Kvasaheim

    The Constitutional Scenarios - Kvasaheim

    The Constitution The Constitutional Scenarios Directions You will be broken into several groups to determine if your scenario is constitutional Your argument of constitutionality will include the proper citations from the original Constitution Safe Harbors To make our harbors safer...
  • The War Correspondent - WordPress.com

    The War Correspondent - WordPress.com

    The title 'Out of the Bag' suggests something being revealed or something being born - links to how the child thinks that the babies come out of the doctor's bag (stork story) STRUCTURE Enjambment - "With his large pink index...
  • Black Economy, Underestimation of Unemployment and Budget ...

    Black Economy, Underestimation of Unemployment and Budget ...

    Learning Pundits on Budget analysis REPP class 9
  • The insider Automation: threat  you didnt know Cyber

    The insider Automation: threat you didnt know Cyber

    Thank you for coming. My name is John. And this talk is about: ... My Name is John. I am a DevOps Engineer with Independent Security Evaluators. We are a security consultancy and research firm based out of Baltimore Maryland.
  • Allowed vs. Aloud

    Allowed vs. Aloud

    Allowed vs. Aloud. Allowed - past tense verb - to let do or happen. I am not allowed to talk on the phone after 10 p.m.. Aloud - adverb - with use of voice or in a loud tone. Please...
  • Safety of Decreased Heparin Flush Concentration During Peripheral

    Safety of Decreased Heparin Flush Concentration During Peripheral

    Times New Roman Century Gothic Arial Ribbons design template Microsoft Office Excel Chart Safety of Decreased Heparin Flush Concentration During Peripheral Blood Stem Cell Collection Rose Batiste RN, MSN, AOCN Mary Brush RN, MS, AOCN Cathy Del Sarto RN Ursula...