PECVD and HDPCVD Basics - Amazon S3€¦ · Introduction to plasma enhanced deposition General...

PECVD and HDPCVD Basics

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)

Good source of information:http://www.timedomaincvd.com/CVD_Fundamentals/Fundamentals_of_CVD.html

What is PECVD?

PlasmaEnhancedChemical VaporDeposition

Goal: Deposition of thin films using plasma assistance to avoid undesirable temperatures

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

Thermally driven(low pressure CVD)

(~600-900°C)

AActivation Energy EA

BReactants

Products

Furnace

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)

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

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

PECVD: Qualitative Model of Film Formation

precursors

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

Plus carrier gas (He, N2)

when dilute SiH4is used for

safety reasons

Outline

Films of interestSiO2, SiNx, SiON , a-Si:H

HDP CVD

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

Gas Inlet

RF + Match

PECVD – Basic Reactor Configuration

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

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

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

HDP CVD

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

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…

PECVD – “Edge Effect” Typical Deposition Within Wafer Uniformity

0 5 10 15 20

Distance From Edge (mm)

Flow disturbances at wafer edge

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”)

Wafer ThicknessGapRecess Depth

(Not to scale)Electrode or Carrier

PECVD – Uniformity Dependence on “Gap”Example

SiO2 .025" wafer in .050" recess

0 5 10 15 20Distance from edge (mm)

ss (Å

Gap = 0mmGap = 0.75mmGap = 1.5mmGap = 2.25mmGap = 3mmGap = 5mm

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

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.

0 100 200 300 400 500

RF Power (W)

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

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

Step Coverage (Aspect Ratio Dependence)

SiO2 Step coverage SiNx Step coverage

10 µm x 10 µm trench

0.0 1.0 2.0 3.0 4.0 5.0Aspect Ratio

Sidewall (half way)

Bottom Surface

SiNx coverageAR 0.25:1 - 4.2:1

0.0 1.0 2.0 3.0 4.0 5.0Aspect Ratio

Sidewall (half way)

Bottom Surface

SiO2 coverageAR 0.25:1 - 4.2:1

More surface mobility for

Breakdown Voltage – Process Dependent and Metrology Dependent

Strong Process Dependence:

2 systems, 3 different conditions

~85V ~120V ~160V

Note: Breakdown must be defined: current, slope,

device size (edge effects), etc.

Example for SiNx

Refractive Index ControlGas Ratios & Temperature

0.25 0.5 0.75 1 1.25 1.5NH3 (ratio to SiH4)

140C120C100C200C250C300C350C

Example for SiNx

Hydrogen Content Strong Temperature Effect

05001000150020002500300035004000

Si-HSNx ExampleFTIR Spectra

Be careful with interpretation (large error bars)

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

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

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

2 13 Bow,

0 10 20 30 40

Film Thickness, tf (µm)

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

PECVD SiNxUsing Temperature to Adjust Stress

Deposition Temperature (°C)

50 100 150 200 250 300 350

SiH4 /NH3 / N2 Chemistry

PECVD SiNxUsing Pressure and Power to Control Stress

N2 onlyFilm compressive @ low pressure

Lower dep rate

300 400 500 600 700 800Pressure (mTorr)

Compressive

Tensile

N2 1000 sccm

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

0 2 4 6 8 10N2O / NH3 Ratio

Tensile

Compressive

1.45 1.50 1.55 1.60 1.65Refractive Index

Compressive

Tensile

Silicon oxynitride, SiOyNx formed from (SiH4, NH3, N2O, N2)Zero stress achieved by simple adjustment of N2O/NH3 ratio

PECVD SiOxNy:HRefractive Index Control

0 2 4 6 8 10

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

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

PECVD SiNxStress Control with He Dilution

10 20 30 40 50 60 70 80 90 100

20 Watts

50 Watts

100 WattsCompressive

Tensile

% N2 / (N2 + He)

Low, adjustable stress

← Increasing He

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)

Helium dilution

With dual frequency

-100 0 100 200 300 400 500RF Power (W)

Ts = 350 ºC

PECVD SiO2 (example):Stress vs. RF Power

Compressive filmsLower stress with at high deposition power (less dense→ lower stress)

0 20 40 60 80LF Power (%)

(LFPower/(LFPower+ HFPower))

More LF power → increased ion bombardment and densification →

more stress

PECVD – Process Parameter Effect Summary

Pressure↑

Gas Flow↑

Power↑

ElectrodeSpacing ↑

Temperature↑

Frequency↑

Residence Time ↑

Deposition Rate ↑ ↑ ↑ ↑ ↑↓― ― ↓Uniformity ↓ ↑↓ ↓ ↑↓ ↑↓ ― ―Damage ― ― ↑ ↑ ― ↑ ―Step coverage ↑ ― ― ― ↑ ― ↓Density ― ― ↓ ― ↑ ↑ ―Stress ↓ ↑ ↑↓ ― ↑↓ ↑ ―Refractive index ― ↑ ↑ ― ― ↓ ―Wet etch rate ― ― ↓ ― ↓ ↓ ―

Outline

HDP CVD

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

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

At high temperature (LPCVD regime)H content decreases

SiH4 + N2 or NH3 → Si

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 + H2↑

Atomic %

PECVD SiNx:Typical Properties

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

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

SiH4 → SiH4* → SiH3 + H→ SiH3 + H2

→ SiH2 + 2H

PECVD Undoped a-Si:HExample

2.0 2.5 3.0 3.5

1000/T (K-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

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

2.0 2.5 3.0 3.51000/T (K-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)

2.0 2.5 3.0 3.51000/T (K-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

Evidence of material dopingEA indicates Fermi level (p or n) type

Highly conductive materials

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

HDP CVD

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

Furnace

Oxidation(~1100°C)

Furnace

LPCVD~650°C

HDPCVD<150°C

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

HDPCVD SiO2Comparison with PECVD and Thermal

100000

0 100 200 300

Deposition Temperature (°C)

HDP CVD

THERMAL OXIDE

HDPCVD SiNx:Wet Etch HDPCVD vs. PECVD

Deposition Temperature (C)

HDPCVD SiO2 (90ºC) FTIR Spectrum to Determine H Content

05001000150020002500300035004000

Wavenumber (cm-1)

Si-OStretch Mode

Si-OBend Mode

Weak Si-OHStretch Mode

Low H content

Principal Vibration Modes

*Careful with interpretation

HDPCVD SiNxFTIR Spectra to Determine H Content

05001000150020002500300035004000

Wavenumber (cm-1)

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

1 µm gap completely filledAR ~ 2.5:1

Gap completely filledand surface nearly planarized

Isolated trench fill Multiple trench fill

HDPCVD SiO2Deposition Rate Versus RF Bias Power

0 100 200 300 400

RF Chuck Power (W)

10 mTorr, 50 sccm SiH4

O2 / SiH4 Ratio = 1.3400 W ICP, 100 °C

Film sputter rate increases with power

RF Bias Power (W)

HDPCVD SiO2: Example Process Capability and Typical Process Parameters

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

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

A-Si:H nano and micro-Si:H poly-Si:H

1 A 10A 100A 1kA 10kA 100kA

Short range order

Extra Material

Outline

HDP CVD

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

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)

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)

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

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)

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

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

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

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

PECVD and HDPCVD Basics - Amazon S3€¦ · Introduction to plasma enhanced deposition General...

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