Text Book: Silicon VLSI Technology Fundamentals, Practice ...zyang/Teaching/20182019... · Silicon VLSI Technology Fundamentals, Practice and Modeling Authors: J. D. Plummer, M. D.
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Oxidation - Chapter 6
Text Book:Silicon VLSI Technology
Fundamentals, Practice and Modelingg
Authors: J. D. Plummer, M. D. Deal, and P. B. Griffina d G
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Structure of Silica GlassSh t d A h h dShort range order maintained
Amorphous material Non-bridging oxygen in
fused Silica (not present in crystalline SiO2)
hydrogen
Si can be replaced by deposits. B,P,As or Sb = network formers.
Network modifier ----> Qm
• SiO is amorphous even though it grows on a crystalline substrate (Figure 3 15)• SiO2 is amorphous even though it grows on a crystalline substrate (Figure 3-15)– Based on SiO4 tetrahedra shown above. – Bridging oxygen atoms share to form crystal like quartz– Time to form appropriate rotational forms for full crystallization not
available; therefore forms rarely observed in IC
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
available; therefore, forms rarely observed in IC– Lattice doesn’t match Si, but there is a short range order
Oxidation - Chapter 6
Si/SiO2 Stresses• Compressive stress due to constrained growth region
– Grow upward– Stress as large as 5 x 109 dyne cm-2
• At high temperature, viscous flow may reduce stress
• There is a large difference in the thermal expansion coefficients– Stress as large as 1-2 x 109 dyne cm-2
• Wafer curvature can be produced from unbalanced stress between the top and bottom of a wafer– Selective etching of one surface will produce curvature
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Selective etching of one surface will produce curvature
8
Oxidation - Chapter 6
Oxide Layer: Intel 90 nm Process
• SiO2 is amorphous even though it grows on a crystalline substrateon a crystalline substrate
– Lattice doesn’t match Si– There is a short range order
I t l SiO f 3 5 t i l• Intel SiO2 of approx. 3-5 atomic layers• http://leitl.org/docs/intel/90nm_press_briefing-technical.pdf
• Deal and Grove (1965) showed that SiO2 growth follows a linear parabolic law.
Wh d l i d t thi id id i i d bi t– Where model inadequate: thin oxides, oxides grown in mixed ambient, oxides grown on 2D and 3D Si surfaces, oxides grown on heavily doped substrates
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
• Deal defined nomenclature in 1980 for electrical charge defects
• Processing to reduce charges:– High temperature inert anneals in Ar or N2 toward the end of process flow.– Moderate temperature anneal (400 ºC) in H2 or forming gas (N2/H2)
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
• Oxidation systems are conceptually very simple.– Dry or wet, 600 – 1200 ºC
I ti t d ti l fQuartz Carrier
Resistance Heating
• In practice today, vertical furnaces, RTO systems and fast ramp furnaces all find use.
H2O2 Gate OxidesLOCOSor STI DRAM Dielectrics
• Thermal oxidation canpotentially be used in manyplaces in chip fabrication.In practice, deposited SiO2In practice, deposited SiO2layers are increasingly beingused (lower Dt).
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Color chart (xox > 50nm) –> not destructive interference will affect the reflected light –> color correlated with thickness of a dielectric layer (10-20 nm accuracy)
For monochromatic light minima and maxima in the reflected beam
( )
L321atmaxima
cos2 01
=
⋅⋅⋅=
mm
xn βλ
allows to determine xox (fringes, spectrometers with sweeping wavelength λ for fixed φ we can find extrema). [Good for a few tens of nm]
Ellipsometry uses polarized light and detect the change in polarization
L
L
,25,
23,
21atminima
,3,2,1atmaxima
=
=
m
m
( )⎞⎛ i φ
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Electrical Measurements• Direct electrical measurement of device type
parameters• Oxides are typically used as the dielectric layer in a
capacitorDoped silicon conductor SiO dielectric doped polysilicon or– Doped silicon conductor, SiO2 dielectric, doped polysilicon or metal conductor layers similar to an MOS transistor.
• Typically Capacitance-Voltage or C-V measurements are taken– A DC bias with an AC voltage source to measure changes inA DC bias with an AC voltage source to measure changes in
impedance with frequency– Widely used for MOS devices, gate oxide parameters, and
carrier lifetimes
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
inversionQD follows QG @ High freq C ~ f(COX, CD )
Deep Depletion
DC Gate VoltageVTH
• LF curve - inversion layer carriers can be created and recombine at AC signal frequency so Cinv is just Cox.
• HF curve (100 kHz to 1 MHz) - inversion layer carriers cannot be generated fast enough to follow the AC signal so Cinv is Cox + CD.
• Deep depletion - “DC” voltage is applied fast enough that inversion layer carriers cannot follow it so C must expand to balance the charge on the gatecannot follow it, so CD must expand to balance the charge on the gate.
• C-V measurements can be used to extract quantitative values for:– tox - oxide thickness– N - the substrate doping profile
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
F1: Transport of Oxygen to oxide surfaceGas phase diffusion through stagnant boundary layerE ilib i t ti i lid (b d ti l )Equilibrium concentration in a solid (based on partial pressures)
F2: Diffusion of oxidant through the oxide F3: Reaction with the Silicon surface
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Oxidation rate for (100) silicon in dry O Oxidation rate for (100) silicon in wet O
• Wet O2 rate is significantly higher than dry O2. The oxidant solubility is higher. • Dry O2 used for thin oxides and controlled depths, wet O2 for thicker films.
Oxidation rate for (100) silicon in dry O2. Oxidation rate for (100) silicon in wet O2.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Time for dry oxidation would be unrealistically long
Oxidation - Chapter 6
Thin Oxide Growth Models• A major problem with the Deal Grove model was recognized when it was
first proposed - it does not correctly model thin O2 growth kinetics.• Experimentally O2 oxides grow much faster for ≈ 20 nm than Deal Grove
predicts.
• MANY models have been suggested in the literature.
1. Reisman et. al. Model
xO = a t + t i( )b or xO = a t + x i⎛
⎝⎜
⎞
⎠⎟
1b
⎛ ⎜ ⎜
⎞ ⎟ ⎟
b
O i( ) o O a⎝
⎜ ⎠ ⎟
⎝ ⎜ ⎜
⎠ ⎟ ⎟
• Power law “fits the data” for all oxide thicknesses.• a and b are experimentally extracted parameters• a and b are experimentally extracted parameters.
• Physically - interface reaction controlled, volume expansion and viscous flow of SiO2 control growth.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
• Second parallel reaction added - “fits the data” ” for all oxide thicknesses.• Three parameters (one of the A values is 0).
• Second process may be outdiffusion of OV and reaction at the gas/SiO2Second process may be outdiffusion of OV and reaction at the gas/SiO2interface.
3. Massoud et. al. Modeldx B x⎛ ⎞
dxO
dt= B
2xO + A+ Cexp −
xO
L⎛
⎝ ⎜
⎞
⎠ ⎟
• Second term added to Deal Grove model - higher dx/dt during initial growth.L ≈ 7 nm second term disappears for thicker oxides• L ≈ 7 nm, second term disappears for thicker oxides.
• Easy to implement along with the DG model, ∴ used in process simulators.• Data agrees with the Reisman, Han and Massoud models. (800˚C dry O2
model comparison below.)
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
2D SiO2 Growth KineticsDifference in volume -> problems when expansion is restricted (SiO2 confined)restricted (SiO2 confined)
Experiments by Kao et al.:• Retardation at sharp corners (2X for 500 nm SiO2)• Retardation larger @ low T (no effect @ 1200 °C)Retardation larger @ low T (no effect @ 1200 C)• Interior (concave) corners oxidize slower than
exterior (convex) but both slower than flat Si
Reasons
Poly-Si for contrast
Reasons• Crystal orientation• Diffusion of oxidant through amorphous SiO2 is the
same -> no dependence on directionS ( l diff ) SiO d l• Stress (volume difference): SiO2 under large compressive stress -> affect both oxidant transport and reaction at the Si surface
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
1/r µmVR, VT and VS are reaction volumes andare fitting parameters.
(Kao et.al)
Oxidation - Chapter 6
Simulating StressI dditi th fl ti f th SiO d t b d ib d b t• In addition, the flow properties of the SiO2 need to be described by a stress dependent viscosity
η(stress ) = η(T) σ SVC / 2kT
sinh σ SVC / 2kT( )where is the shear stress in the oxide and VC is again a fitting parameter. σ S
Parameter Value SiOSi3N4
-0.2
-0.4
Parameter ValueVR 0.0125 nm3 VD 0.0065 nm3
VS, VT 0 VC 0.3 nm3 @ 850˚C
Silicon
SiO 2
0
0.2
0 4Mic
rons
VC 0.3 nm @ 850 C0.72 nm3 @ 1050˚C
η(T) - SiO2 3.13 x 1010 exp(2.19 eV/kT) poise η(T) - Si3N4 4.77 x 1010 exp(1.12 eV/kT) poise
0.4
0.6
0.8
M
• These models have been implemented in modern process simulators and allow them to predict shapes and stress levels for VLSI structures (above
Microns0 0.4 0.8-0.4-0.8
Microns0 0.4 0.8-0.4-0.8
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin
Summary of Key Ideas• Thermal oxidation has been a key element of silicon technology since its
inception.
• Thermally, chemically, mechanically and electrically stable SiO2 layers on silicon distinguish silicon from other possible semiconductors.
• The basic growth kinetics of SiO2 on silicon are controlled by oxidant diffusion and Si/SiO2 interface chemical reaction.
• This simple Deal-Grove model has been extended to include 2D effects highThis simple Deal Grove model has been extended to include 2D effects, high dopant concentrations, mixed ambients and thin oxides.
• Oxidation can also have long range effects on dopant diffusion (OED or ORD) hi h d l d th h i t d f t i t tiORD) which are modeled through point defect interactions.
• Process simulators today include all these physical effects (and more) and are quite powerful in predicting oxidation geometry and properties.
SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin