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PECVD and HDPCVD Basics
12015
Outline
Introduction to plasma enhanced deposition
General equipment configuration
PECVD film properties
Films of interestSiO2, SiNx, a-Si:H
HDP CVD
Backup slidesGeneral operational guidance
General process parameter trends (temperature, pressure, frequency, flow)
2
Good source of information:http://www.timedomaincvd.com/CVD_Fundamentals/Fundamentals_of_CVD.html
3
What is PECVD?
PlasmaEnhancedChemical VaporDeposition
Goal: Deposition of thin films using plasma assistance to avoid undesirable temperatures
4
Technology for PECVD Applications
Passivation/Encapsulation/Insulation
Capacitor dielectricMasking
General Case of Chemical Vapor Deposition
Precursors (reagents) are gasesReactions occur - solid (film) is formed as by-productHighly simplified examples:
SiH4 → Si + 2 H2
SiCl2H2 + 2 N2O → SiO2 + 2 N2 + HCl3 SiH4 + 4 NH3 → 3 Si3N4 + 12 H2
Energy input is needed for chemical reactions to proceed
5
Thermally driven(low pressure CVD)
(~600-900°C)
AActivation Energy EA
BReactants
Products
Furnace
Heat
Reactions occur at lower temperaturePlasma input EA - EA*
Attractive for temperature sensitive substrates (e.g. III-V materials, polymers, some silicon devices)
Lowers activation energy (→ lower temperature required), EA* < EA
Potential increase to deposition rate
Affects on film characteristics (e.g. stress, density through energetic ions)
PECVD – Plasma EnhancementPlasma supplied energy (excited species)
6
A*
A
EA
EA*
Reactants
Products
Temperature can be reduced to ~150 to 400°C
PECVD – Plasma Enhancement
Polysilicon growth Without plasma enhancementHigh activation energy, EA
Strong dependence on temperature(e.g. ~0 Å/min vs. ~130 Å/min at ~500°C)
Kinetic regime vs. diffusion regime athigher temperatures
Plasma enhancedLow activation energy, EA
Relatively weak dependence on temperature
Source: Hajjar et al, J. Electronic Mat., 15, 279 (1986)
48 kcal / mol
7 kcal / mol
7
8
PECVD – Film Formation
Radicals are formed by electron impactSiH4 + e- → SiH3 + H + e-
SiH3 + e- → SiH2 + H + e-
NH3 + e- → NH2 + H + e-
Radicals adsorb on substrate surface
Reactions occur on surfaceThermally driven, may be some plasma interaction
Film forms and by-products generated
Plasma added energy
9
PECVD: Qualitative Model of Film Formation
e-
precursors
e-
electron impact(precursor formation)
surface adsorption → film formationsurface reactions →Substrate
Inlet(reagents,
carrier gases)
Exhaustby-products,
excess reagents, carrier gases
Transportreactants to
substrate Desorptionof byproducts
Transportreactants to
growth regionTransport
byproducts to main gas stream
PECVD – Silane Based Films
Important films in semiconductor industry
Silicon dioxide, SiO2 SiH4 + N2O
Silicon nitride, SiNx SiH4 + NH3 or + SiH4 + N2
Silicon oxynitride, SiON SiH4 + NH3 +N2O
Amorphous silicon, a-Si:H SiH4 + He or Ar
Silicon carbide, SiC SiH4 + CH4
10
Plus carrier gas (He, N2)
when dilute SiH4is used for
safety reasons
Outline
Introduction to plasma enhanced deposition
General equipment configuration
PECVD film properties
Films of interestSiO2, SiNx, SiON , a-Si:H
HDP CVD
Backup slidesGeneral operational guidance
General process parameter trends (temperature, pressure, frequency, flow)
11
Good source of information:http://www.timedomaincvd.com/CVD_Fundamentals/Fundamentals_of_CVD.html
PECVD – Equipment Configuration
Geometry (similar to plasma enhanced configuration)Vacuum system configured for high gas flow – uniform pumping
Relatively high pressure regime (100’s mTorr → few Torr)Viscous flow regime, not diffusionHigh speed pumping (500-2000 sccm)
Electrode SpacingConfined plasma – need to be close (smaller gap higher rate)Uniformity requires optimization (e.g. parallel)Must consider handling limitations
Top electrodePowered (single or multiple frequencies)Serves as gas inlet
High gas flows – litres/min ( note: mass balance/dilute silane)Showerhead gas inlet for uniform gas distribution (viscous flow regime)
Lower electrodeHeated to drive thermal reactionsSingle or multi-wafer loading capabilities
12
Pump
Gas Inlet
RF + Match
PECVD – Basic Reactor Configuration
13
Distributed pumping
Gas Inlet
RF + Match
Powered electrode, showerhead
Variations: • Heated walls (lower particulates, lower deposition
rate)• Multi-wafer (batch)• Adjustable electrode spacing (affects deposition
rate and uniformity)
Lower electrodeHeated & Grounded
Blower Pump
Backing Pump
SubstratePlasma
MFC MFC
ThrottleValve
PressureGauge
High speed, moderate vacuum, oil free
Increases power delivery efficiency, protects generator
Pressure control feedback
14
PECVD – Distributed pumping
PECVD is in viscous flow regime (>>~50mTorr)
Distributed pumping needed to avoid nonuniform gas flow and film
deposition (~500 – 2000 sccm)
Peripheral pumping for uniform gas flow
15
Productivity and Film Quality Design Aspects
Isothermal DesignHigher quality films
Improved particle performance and uniformity
Increased mean time between cleans
Increased etch back (plasma clean) rates (2kW rf supply optional)
In situ thickness monitor
Outline
Introduction to plasma enhanced deposition
General equipment configuration
PECVD film properties
Films of interestSiO2, SiNx, a-Si:H
HDP CVD
Backup slidesGeneral operational guidance
General process parameter trends (temperature, pressure, frequency, flow)
16
Good source of information:http://www.timedomaincvd.com/CVD_Fundamentals/Fundamentals_of_CVD.html
PECVDProcess and Film Evaluation Criteria
Electrical properties (breakdown)Mechanical properties (film stress)AdhesionPinholes Conformal (step coverage)Induced damageFill capability (without voids)Uniformity thickness, refractive indexParticulatesComposition (H conc. refractive index)Wet etch rate (BHF solution)Density
Deposition rateFast for high throughputSlow for control of thin films
“Robustness”Reproducible(within wafer, within batch, run-to-run)Wide process windowReliability
MaintenanceLong intervals between cleans (MTBC)Short, efficient clean cycles
Handling (single, batch)Endpoint
17
18
PECVD: Basic Film Properties
Most films formed are amorphousNo crystalline structure (not even micro-crystallineexcept for some Si)
Films formed are not perfectly stoichiometricOften significant H incorporationSilicon dioxide is not “SiO2” – but SiOx: H Silicon Nitride is not “Si3N4 ” – but SiNx: HAmorphous Si is really a-Si:H
Film properties depend onPlasma conditions (pressure, flow, power, reactor geometry)Substrate temperatureSubstrate material (surface, thickness) More on this later…
19
PECVD – “Edge Effect” Typical Deposition Within Wafer Uniformity
-1
0
1
2
3
4
5
6
7
8
9
0 5 10 15 20
Distance From Edge (mm)
Film
Thi
ckne
ss C
hang
e (%
)
Flow disturbances at wafer edge
20
PECVD – Controlling Edge EffectsRecessed Wafer “Pocket”
Gap and delta important for uniformity(some gap needed to allow for reliable wafer transfer)
Carrier (pallet) example(7x 3”)
delta
Wafer ThicknessGapRecess Depth
(Not to scale)Electrode or Carrier
Gap
21
PECVD – Uniformity Dependence on “Gap”Example
SiO2 .025" wafer in .050" recess
4040
4080
4120
4160
4200
4240
4280
4320
4360
4400
4440
4480
4520
0 5 10 15 20Distance from edge (mm)
Film
Thi
ckne
ss (Å
)
Gap = 0mmGap = 0.75mmGap = 1.5mmGap = 2.25mmGap = 3mmGap = 5mm
~1%
SiO2 on silicon wafer
optimum gap(constant delta)
Optimum gap and delta can be process specific
PECVD Process Parameters: Frequency -Plasma Effects
Low frequency (50-100 kHz)Ions traverse sheath before electric field reversesElectrons still preferentially diffuse out of plasma contributing to negative biasWider ion energy distribution at lower frequencies
High frequency (>1MHz)Electrons respond to plasma and less likely to diffuse to walls causing less bias and more reagent dissociationLess bias corresponds to lower ion acceleration voltageIons cannot traverse sheath before field reverses - thus lower ion energy
22
Increased stress not always bad –Sometimes used as a stress
compensation layer to achieve a desired stress for a film stack
PECVD Process Parameters: Frequency Film Effects
more compressivelower H concentrationhigher density
General statement since dependent on geometry,
pressure, power, etc.
23
0
2
4
6
8
10
12
0 100 200 300 400 500
RF Power (W)
BO
E R
atio
Ts = 350 ºC
Data NormalizedTo Thermal Oxide
More LF power → increased ion bombardment and densification
lower refractive indexlower wet etching ratePotential damage to sensitive devices
BOE Rate ~1/2 with low frequency
PECVD Step Coverage of Deposited Films
24
Uniform coverage resulting from rapid surface migration (e.g. high temp)
Nonconformal step coverage for long mean free path and no surface migration
(e.g. PVD)
Nonconformal step coverage for short mean free path and no surface migration
(e.g. PECVD)
Example: SiO2 (AR=4.2:1)
Example: SiNx (AR=1.5:1)
Example of “keyhole” formation
As aspect ratio increases keyhole more readily formed
25
Step Coverage (Aspect Ratio Dependence)
26
SiO2 Step coverage SiNx Step coverage
10 µm x 10 µm trench
0
20
40
60
80
100
0.0 1.0 2.0 3.0 4.0 5.0Aspect Ratio
Cov
erag
e (%
)
Sidewall (half way)
Bottom Surface
SiNx coverageAR 0.25:1 - 4.2:1
0
20
40
60
80
100
0.0 1.0 2.0 3.0 4.0 5.0Aspect Ratio
Cov
erag
e (%
)
Sidewall (half way)
Bottom Surface
SiO2 coverageAR 0.25:1 - 4.2:1
More surface mobility for
SiNx
Breakdown Voltage – Process Dependent and Metrology Dependent
Strong Process Dependence:
2 systems, 3 different conditions
~85V ~120V ~160V
27
Note: Breakdown must be defined: current, slope,
device size (edge effects), etc.
Example for SiNx
28
Refractive Index ControlGas Ratios & Temperature
1.92
1.96
2.00
2.04
2.08
2.12
0.25 0.5 0.75 1 1.25 1.5NH3 (ratio to SiH4)
Ref
ract
ive
inde
x
140C120C100C200C250C300C350C
Example for SiNx
29
Hydrogen Content Strong Temperature Effect
05001000150020002500300035004000
350oC
05001000150020002500300035004000
05001000150020002500300035004000
250oC
120oC
Si-HSNx ExampleFTIR Spectra
N-H
Be careful with interpretation (large error bars)
Si-N
Basic Thin Film PropertiesStress
Mechanical Integrity & StabilityToo compressive → film will buckle or blisterToo tensile → film will crackHandling/clamping issues (bowing)Photolithography (depth of focus)Pattern distortion upon etchingTotal stress proportional to total film thickness
Electrical & Optical PerformanceSiO2 Optical waveguides, 10 - 20 µm
birefringence & mode distortion
Stress not always bad – compensation layers
tensile
zerofilm
substrate
compressivefilm
expands
filmcontracts
Origin: Chemical, Ion Bombardment, MicrostructureFunction of atoms per volume and forces affecting bond lengths
What is Low Stress?
Need determines acceptable stress. Example: MEMS membranes
31
tensile
Thick substrate Thin substrate
Thin filmThick film
Bowing depends on substrate rigidity and film thickness
Film thickness and intrinsic stress
compressive too low tensile
General SiNx Stress Guidelines
Nitride Thickness Suspended StructureStress Target
<0.05 μm <350 MPa
0.05 to 0.1μm <300 MPa
0.1 to 0.5 μm <250 MPa
0.5 to 1 μm <200 MPa
1 to 1.5 μm <150 MPa
1.5 to 2 μm <100 MPa
2 to 3 μm <50 MPa
32
Source: after Rogue Valley Microdevices, www.roguevalleymicro.com
PECVD: Basic Film PropertiesStress
Total stress is function of film thicknessThicker films need lower inherent stress
Electrical & Optical PerformanceOptical waveguides – SiO2 Film thickness: 10 - 20 µm
Stress-induced bow
Birefringence & mode distortion
Packaging – Film thickness: 10 - 40 µm
33
f
s
f
s
s
tt
Yr
2
2 13 Bow,
0
1
2
3
4
0 10 20 30 40
Film Thickness, tf (µm)
Waf
er B
ow,
(mm
)
100
200
400
600
300
500
Stress(MPa)
Si Wafer
Stoney’s equation
Bow ~2x typical wafer
SiNx Stress Modification Techniques
Stress Control Parameter Benefits Limitations
Temperature Provides ability to reduce stress @ low temperature
Film quality is reduced for most applications
Power/Pressure Provides flexible stress control Narrow process window
Chemical Composition(Refractive Index Shift)
Provides good stress control for applications not index
sensitive
Not recommended for index sensitive applications or
stoichiometric films
Mixed Frequency on Powered Electrode
Can provide some stress control for oxide films
Concerns for damage sensitive applications
Helium Dilution Excellent control for critical thin films
Slower deposition rates. Requires He.
CONFIDENTIAL 34
35
PECVD SiNxUsing Temperature to Adjust Stress
Deposition Temperature (°C)
0
100
200
300
400
500
600
50 100 150 200 250 300 350
Tens
ile S
tres
s (M
Pa)
SiH4 /NH3 / N2 Chemistry
36
PECVD SiNxUsing Pressure and Power to Control Stress
N2 onlyFilm compressive @ low pressure
Lower dep rate
-250
-200
-150
-100
-50
0
50
100
150
300 400 500 600 700 800Pressure (mTorr)
Stre
ss (M
Pa)
Compressive
Tensile
75 W
100 W
50 W
N2 1000 sccm
120 W
37
PECVD Silicon Oxynitride (SiOxNy:H)Useful Stress and Index Control
Extremely useful film when index or stress control is essentiale.g. anti-reflection coatings (ARC)
Plasma chemistry: SiH4 +NH3 + N2O with He or N2 carrier gasFilm is SiOxNy:HFilm properties can be varied
High N2O/NH3 ratio – oxide likeLow N2O/NH3 ratio – nitride likeRefractive index 1.5 1.9Stress compressive tensile
Stress ControlAdjustment of Film Stoichiometry – SiOyNx
-300
-200
-100
0
100
200
0 2 4 6 8 10N2O / NH3 Ratio
Stre
ss (M
Pa)
Tensile
Compressive
-300
-200
-100
0
100
200
1.45 1.50 1.55 1.60 1.65Refractive Index
Stre
ss (M
Pa)
Compressive
Tensile
Silicon oxynitride, SiOyNx formed from (SiH4, NH3, N2O, N2)Zero stress achieved by simple adjustment of N2O/NH3 ratio
39
PECVD SiOxNy:HRefractive Index Control
1.4
1.6
1.8
2.0
2.2
2.4
2.6
0 2 4 6 8 10
Ref
ract
ive
Inde
x, n
N2O / SiH4 Ratio
Refractive Index can be adjusted for antireflection performance
= 633 nm
Nitrogen comes form N2O (avoiding NH3 for resist processing)
Note: Index increase is in Si rich regime
Without NH3 process
nitride-like
oxide-like
With NH3 process
Refractive Index can be adjusted for antireflection performance
40
PECVD SiNxUsing Low Frequency to Adjust Stress
Add LF (< 1 MHz) to RF power
Stress control achievable
High energy ion bombardment results in film compression
Possibility of plasma-induced damage
Additional hardware needed
41
PECVD SiNxStress Control with He Dilution
-400
-300
-200
-100
100
200
300
400
Stre
ss (M
Pa)
10 20 30 40 50 60 70 80 90 100
20 Watts
50 Watts
100 WattsCompressive
Tensile
0
% N2 / (N2 + He)
Low, adjustable stress
0
← Increasing He
42
PECVD SiNxStress Control with He Dilution
Independent comparison between low dual frequency and helium dilution process
Conclusions:Low frequency approach induces “irreversible damage” to III-V device layersNo detectable damage with helium dilution processNo additional hardware required
W. S. Tan, P. A. Houston, G. Hill, R. J. Airey, and P. J. ParbrookJ. Electron. Mat. 33, 400 (2004).
Damage Effects: Dual frequency vs. Helium Dilution (SiNx example, GaN HEMT device)
43
Helium dilution
With dual frequency
0
-20
-40
-60
-80
-100 0 100 200 300 400 500RF Power (W)
Stre
ss (M
Pa)
Ts = 350 ºC
PECVD SiO2 (example):Stress vs. RF Power
Compressive filmsLower stress with at high deposition power (less dense→ lower stress)
44
-400
-300
-200
-100
-0
0 20 40 60 80LF Power (%)
(LFPower/(LFPower+ HFPower))
Stre
ss (M
Pa)
More LF power → increased ion bombardment and densification →
more stress
PECVD – Process Parameter Effect Summary
45
Pressure↑
Gas Flow↑
Power↑
ElectrodeSpacing ↑
Temperature↑
Frequency↑
Residence Time ↑
Deposition Rate ↑ ↑ ↑ ↑ ↑↓― ― ↓Uniformity ↓ ↑↓ ↓ ↑↓ ↑↓ ― ―Damage ― ― ↑ ↑ ― ↑ ―Step coverage ↑ ― ― ― ↑ ― ↓Density ― ― ↓ ― ↑ ↑ ―Stress ↓ ↑ ↑↓ ― ↑↓ ↑ ―Refractive index ― ↑ ↑ ― ― ↓ ―Wet etch rate ― ― ↓ ― ↓ ↓ ―
Outline
Introduction to plasma enhanced deposition
General equipment configuration
PECVD film properties
Films of interestSiO2, SiNx, a-Si:H
HDP CVD
Backup slidesGeneral operational guidance
General process parameter trends (temperature, pressure, frequency, flow)
46
Basic process: Silane + Oxygen source (O2, N2O, NO, CO2)Low N-O bond energy (1.7 eV) makes N2O preferred (vs. CO2)
SiH4 + 4 N2O SiO2 + 2 H2 + 4 N2
SiH4 + O2 SiO2 + 2 H2 (reactive at RT, particles)
But O2 spontaneously reacts with SiH4 (best to keep separated!)
Silane concentration affects dep rate (~ 500A/min to 5000A/min)
SiH4/N2O ratio affects refractive index and stress
Three Regimes:Excess Oxygen+ SiH4 SiO2:(OH) + nH2OBalanced Oxygen + SiH4 SiO2 + 2H2
Deficient Oxygen + SiH4 SiO2:H + nH2
PECVD: Silicon Oxide Chemistry
, plasma
, plasma
47
48
Parameter Values Comments
Deposition Rate ~400-4000 Å/min Function of SiH4 concentration, temp, power
Hydrogen Content 2-9 % Present mainly as Si-H
Resistivity 1013-1017 ohm-cm Decreases with increasing Si/O ratio
Breakdown Field 2 - 6 x 106 V/cm Si rich has lower Ebreak
Refractive Index 1.45-1.50 Increases with increasing Si/O ratio
Film Stress Compressive
0 – 300 MPa
Function of temp, gas composition, power, pressure
BOE Etch Rate ~2-6x Thermal oxide Function of power, temp, comp, etc.
Temperature 200-400 ºC
Silicon Dioxide (SiO2 )Typical Properties
PECVD: Silicon Nitride (SiNx)
Advantages Good insulator/dielectricGood passivation film for III/VWide range of deposition rates
Stable processes at low powersControllable refractive index
SH4/NH3 ratio, temperatureExcellent step coverageContains significant hydrogenControllable stress
He dilution, low frequency, pressure, compositionTensile to ~0 to Compressive
Concern over H and N incorporationTypically 15-30% hydrogen bonded to Si or N
49
At high temperature (LPCVD regime)H content decreases
SiH4 + N2 or NH3 → Si
N
N
N
N + H2↑
PECVD SiNxSiH4 + Nitrogen source (NH3, N2)
At low temperature (PECVD regime)H included at N and Si sites
Low N-H bond energy make NH3 preferred for parallel place PECVD
400 700500 600300SiH4 + N2 or NH3 → Si
N
N
N
N + H2↑
H
H
Atomic %
50
PECVD SiNx:Typical Properties
Parameter Values Comments
Deposition Rate 100-500 Å/min Function of Si-H concentration, temp, power
Hydrogen Content 15-30% Present as Si-H and N-HResistivity 105-1019 ohm-cm Decreases with increasing Si/N ratio
Breakdown Fields 1-6 x 106 V/cm Si rich has lower Ebreak
Refractive Index 1.90-2.10 Increases with increasing Si/N ratio
Film Stress compressive to tensile Function of temp, gas composition, power, pressure, frequency
BOE Etch Rate <0.7 Å/sec Decreases with increasing Si/N ratio. H content increases BOE.
Temperature 250-350C Lower wet etch rate with increased temperature
Optical Gap 3-4 eV 5 eV for stoichiometric Si3N4
51
PECVD: Amorphous Si (a-Si:H)
Plasma chemistry: SiH4 with carrier gas e.g. He, H2100% SiH4 is best
Film Formation:
SemiconductorHighly photoconductiveDoped n-type or p-type (addition of PH3 or B2H6)Incorporation of H is critical to achieve semiconductor properties
H helps reduces concentration of electronic defects (dangling bonds)and allow sensitive doping. (removes mid gap states and better minority carrier lifetimes)
Typical deposition rates: 100 - 1000 Å/min
52
SiH4 → SiH4* → SiH3 + H→ SiH3 + H2
→ SiH2 + 2H
e-
52
PECVD Undoped a-Si:HExample
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
2.0 2.5 3.0 3.5
1000/T (K-1)
Con
duct
ivity
(W-1
cm-1
)
(Dep Rate = 380 Å/min)
EA = 0.88 eV (from fitted slope)σo = 14,500 Ω-1 cm-1
σRT = 1.1 x 10-11 Ω-1 cm-1 (high resistivity)Photoconductivity (~ AM1) = 4.6x10-5 Ω-1cm-1
53
Evidence of intrinsic materialMid-band Fermi level
Low conductivity material
PECVD: a-Si:HGas Phase Doping
Toxic gases – requires loadlocks, sensorsGas-phase doping with PH3 and B2H6can change a-Si resistivity by <1010
Undoped a-Si: 1010 Ω.cmn+ a-Si: 100 Ω.cmp+ a-Si: 1000 Ω.cm
Note: B2H6 thermally unstable and degrades in the gas cylinder and also contaminates the process chamber.(CH3)3B is a better alternative. It has a similar doping efficiency to B2H6.
Example of Doping Curve (after Spear & LeComber)
PECVD a-Si:HGas Doping Examples
10-5
10-4
10-3
10-2
10-1
2.0 2.5 3.0 3.51000/T (K-1)
Con
duct
ivity
(W-1
cm-1
)
EA = 0.36 eVσo = 88 Ω-1 cm-1
σRT = 5.1 x 10-5 Ω-1 cm-1
(10,000 ppm B2H6 /SiH4 , 1000 Å/min)
100
10-3
10-2
10-1
2.0 2.5 3.0 3.51000/T (K-1)
Con
duct
ivity
(W-1
cm-1
)
EA = 0.21 eVσo = 18 Ω-1 cm-1
σRT = 3.9 x 10-3 Ω-1 cm-1
(10,000 ppm PH3 /SiH4 , 1000 Å/min)
n+ a-Si:Hp+ a-Si:H
55
Evidence of material dopingEA indicates Fermi level (p or n) type
Highly conductive materials
56
Parameter Values Comments
Deposition Rate ~100-1000 Å/min Function of SiH4 concentration, power, dilution
Hydrogen Content ~ 10 % Present as Si-H
Resistivity ~ 1010 ohm-cm undoped
~ 102 ohm-cm n-type (1% PH3/SiH4)
~ 103 ohm-cm p-type (1% B2H6/SiH4)
Film Stress Low Compressive
Refractive Index ~ 3.6
Optical Gap ~ 1.8 eV 1.1 eV, crystalline Si
Temperature ~ 250 ºC
Amorphous Si (a-Si:H)Typical Properties
Sources for a-Si:H information• Hydrogenated Amorphous Silicon: R. A. Street, (1991) Cambridge University Press • Electronic Transport in Hydrogenated Amorphous Semiconductors (1989) Springer • Thin-Film Silicon Solar Cells: Arvind Shah (ed.) (2010) EPFL Press
Outline
Introduction to plasma enhanced deposition
General equipment configuration
PECVD film properties
Films of interestSiO2, SiNx, a-Si:H
HDP CVD
Backup slidesGeneral operational guidance
General process parameter trends (temperature, pressure, frequency, flow)
57
Why HDPCVD?
Trend to lower temperature processing e.g. <150°C)Better quality dielectric films than PECVD at lower temperatures
Independent control of the ion flux and ion energyMinimize ion damageEfficient gap fill capability
High dissociation efficiencyNH3 free SiNx process is possible
58
Furnace
Oxidation(~1100°C)
Furnace
LPCVD~650°C
HDPCVD<150°C
Gas
Gas
PECVD~200-350°C
HDPCVD ReactorInductively Coupled Plasma SourceICP Source High ion density
13.56 MHz biasControllable ion energyDeposition plus simultaneous ion bombardment
Wafer clamped and He cooledAllows low temperature deposition
Secondary gas inlet (gas ring) for SiH4 introduction
Allows use of SiH4 and O2without hazard
59
60
HDPCVD SiO2Comparison with PECVD and Thermal
100
1000
10000
100000
0 100 200 300
Deposition Temperature (°C)
BH
F E
tch
Rat
e (Å
/min
)
PECVD
HDP CVD
THERMAL OXIDE
HDPCVD SiNx:Wet Etch HDPCVD vs. PECVD
61
Deposition Temperature (C)
BH
F E
tch
Rat
e (Å
/min
)
62
HDPCVD SiO2 (90ºC) FTIR Spectrum to Determine H Content
0
0.2
0.4
0.6
0.8
1
1.2
05001000150020002500300035004000
Wavenumber (cm-1)
Abs
orba
nce
(arb
. uni
ts)
Si-OStretch Mode
Si-OBend Mode
Weak Si-OHStretch Mode
Low H content
Principal Vibration Modes
*Careful with interpretation
63
HDPCVD SiNxFTIR Spectra to Determine H Content
0
0.2
0.4
0.6
0.8
1
05001000150020002500300035004000
Wavenumber (cm-1)
Abs
orba
nce
(arb
. uni
ts)
Si-NStretch Mode
Si-HStretch ModeN-H
Stretch Mode
Principal Vibration Modes
05001000150020002500300035004000
PECVD SiNx120oC
Low H content90°C
*Careful with interpretation
SiO2 HDPCVDImproved Gap Filling
64
1 µm gap completely filledAR ~ 2.5:1
Gap completely filledand surface nearly planarized
Isolated trench fill Multiple trench fill
65
HDPCVD SiO2Deposition Rate Versus RF Bias Power
1000
1200
1400
1600
1800
2000
0 100 200 300 400
RF Chuck Power (W)
Dep
ositi
on R
ate
(Å/m
in)
10 mTorr, 50 sccm SiH4
O2 / SiH4 Ratio = 1.3400 W ICP, 100 °C
Film sputter rate increases with power
RF Bias Power (W)
Dep
ositi
on R
ate
(A/m
in)
HDPCVD SiO2: Example Process Capability and Typical Process Parameters
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Deposition Temperature <180oC
Plasma Chemistry SiH4, O2, Ar (O2:SiH4 ~1.2)
Chamber pressure 2 mT to 20 mT
ICP power 400 – 1000 W
Bias power ~5 – 200 W
Deposition Rate 1000 - 2000 Å/min
Refractive Index (controlled w/ SiH4/O2) 1.46 – 1.50
Film Stress (controlled w/ RF bias) ~ -300 MPa (compressive)
BOE ~0.2x PECVD at 200oC
HDPCVD SiNx: Example Process Capability and Typical Process Parameters
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Tricky!Film stress typically too compressive (film delaminates)
Deposition Temperature <180oC
Plasma Chemistry SiH4, N2, Ar
Chamber pressure 2 mT to 20 mT
ICP power 400 – 1500 W
Bias power ~5 – 200 W
Deposition Rate 500 - 1000 Å/min
Refractive Index (controlled w/ SiH4/O2) 1.95 – 2.05
Film Stress (controlled w/ RF bias) ~ -300 MPa (compressive)
Definitions: Amorphous, Microcrystalline, Polycrystalline
Crystalline: very long range order
Poly-crystalline: composed of crystallites (often referred to as grains) Micro-crystalline: containing small crystals (typically microscopic)Nano-crystalline: containing crystals on the order of nanometers
Amorphous: Non-crystalline, without long-range order
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A-Si:H nano and micro-Si:H poly-Si:H
1 A 10A 100A 1kA 10kA 100kA
Short range order
Extra Material
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Outline
Introduction to plasma enhanced deposition
General equipment configuration
PECVD film properties
Films of interestSiO2, SiNx, a-Si:H
HDP CVD
Backup slidesGeneral operational guidance
General process parameter trends (temperature, pressure, frequency, flow)
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Chamber Cleaning
Films deposit on all plasma contacted surfacesPowdery deposit on non-plasma contacted surfacesHigher wall temperatures improves adhesion
Deposition on chamber surfaces must be removedOtherwise eventually flaking/particle formation
Cleaning conditions in conflict with deposition conditions Very different plasma conditions are optimum
SF6 gas, low pressure, high RF power levels
Endpoint technology (OES) can help minimize cleaning “downtime” and improve productivity
F2Too dangerous and toxic
SF6 ~1000 to 2000 Å/minInexpensiveRelatively low toxicityGood cleaning rate
NF3 ~2500-4000 Å/minRelatively expensiveToxicFast cleaning rate
CF4 ~500-1000 Å/minInexpensiveOkay cleaning rate
Chamber CleaningGas Choices
Dependent onGeometryPressurePowerTemperatureFrequency
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PECVD – General Guidance
Thermal equilibrium –Allow a thermal “soak” at ~1 Torr (without silane) for 1-2 min prior to runTime depends on process temperature, single wafer or carrier, open load or loadlock
AdhesionSi – quick HF dipMetals – short (~15s) Ar plasma (with low frequency if available)
Wet cleansAvoid scrubbing showerhead (particles can clog)
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Troubleshooting Particles
Problem Cause Potential Solutions
Poor Process (too high a deposition rate)
Process Adjustment (lower power, reduce reagent concentrations)
Infrequent Cleaning Clean More Frequently (determine cleaning rate)
Oxygen Leak Vacuum Integrity (leak test)
Low Wall Adhesion Chamber Design (hot walls, rough walls, eliminate corners, nitrogen curtains)
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Problem Cause Potential Solutions
Change in pumping speed Monitor pumping efficiency Maintain pumping port with clean
cycles
Change in reagent flow (new gas bottle?) Keep MFCs calibrated
Change in electrode temperature Periodic checks
TroubleshootingProcess Drift
Changes in deposition rates and/or film quality
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PECVD Parameter Effects:Gas Flow
Total Gas Flow vs. Partial Gas Flow of ReactantsTotal gas flow includes carrier or dilution gasPartial gas flow is only “active” species
Deposition Rate: typically increases with gas flowUniformity: gas dynamics (flow directions) have effectDamage: little effectStep Coverage: little effect Index: increases slightly (as SiH4 flow increases)Density: minimal effectStress: slight decrease (as SiH4 flow increases)
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PECVD Parameter Effects:Power
Deposition Rate: Higher power increases concentration of reactive species. Increases deposition rate until reagent limited.
Uniformity: Tends to worsen slightly
Damage: slight increase with increased self bias.
Step Coverage: little effect
Index: typically increases
Density: may decrease at high power
Stress: strong effect
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PECVD Parameter Effects:Temperature
Probably the most important parameter.
Higher temperatures produce higher quality films
Deposition Rate: weak influences with temperature
SiO2 SiNx
Uniformity: Better temperature uniformity better uniformity
Damage: Minimal effect
Step Coverage: Improves slight with elevated temperature
Density: Increases with temperature
Stress: Increases slightly but overall a weak effect
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PECVD Process Parameters:Residence Time
Reactive species in steady state can be increased with shorter residence times
Short Residence TimeLess time for gas phase nucleationMore reagent available for film forming thus faster deposition
Long Residence TimeMore time for gas phase nucleationLess reagent available for film forming
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PECVD Parameter Effects:Pressure
Total Pressure vs. Partial Pressure of ReactantsTotal pressure includes carrier or dilution gasPartial pressure is only “active” species
Deposition Rate: Increases with pressureUniformity: Lower partial pressure improves uniformityDamage: Minimal effectStep Coverage: Higher pressure slightly improves coverageIndex: Minimal effectDensity: Minimal effectStress: Typically low effect
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