THIN FILM DEPOSITION – Chapter 9 Introduction Many films, made of many different materials are...

THIN FILM DEPOSITION – Chapter 9Introduction

P

P WellN Well

PNP+ P+ N+ N+

• Many films, made of many different materials are deposited during a standard CMOS process.• Requirements or desirable traits for deposition:1. Desired composition, low contaminates, good electrical and mechanical properties.2. Uniform thickness across wafer, and wafer-to-wafer.3. Good step coverage (“conformal coverage”).4. Good filling of spaces.5. Planarized films .

SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin

© 2000 by Prentice HallUpper Saddle River NJ

€

Aspect ratio=AR =hw

• Note the aspect ratios and the need for new materials.• Note also the number of metal layers requiring more deposition steps.

Year of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018

Technology Node (half pitch) 250 nm 180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm

MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm

Min Metal 1 Pitch (nm) 214 152 108 76 54 42

Wiring Levels - Logic 10 11 12 12 14 14

Metal 1 Aspect Ratio (Cu) 1.7 1.7 1.8 1.9 2.0 2.0

Contact Aspect Ratio (DRAM) 15 16 >20 >20 >20 >20

STI Trench Aspect Ratio 4.8 5.9 7.9 10.3 14 16.4

Metal Resistivity (µohm-cm) 3.3, 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2

Interlevel Dielectric Constant 3.9 3.7 3.7 <2.7 <2.4 <2.1 <1.9 <1.7 <1.7



Metal line

a

Contact hole

b

wh

wh

Thin Film Deposition

• Quality – composition, defect density, mechanical and electrical properties

• Uniformity – affect performance (mechanical , electrical)

Thinning leads to R

Voids: Trap chemicals lead to cracks (dielectric) large contact resistance and sheet resistance (metallization)

AR (aspect ratio) = h/w with feature size in ICs.

Examples

Poor step coverage with increasing AR

Thinning causes metal resistance to increase, generates heat and lead to failure

Chemical Vapor Deposition

Flat on the susceptor

Cold wall reactor

Methods of Deposition:

Chemical Vapor Deposition (CVD):APCD, LPCVD, HDPCVD Physical Vapor Deposition (PVD: evaporation, sputtering)

Atmospheric Pressure : APCVD

Cold wall reactors (walls not heated - only the susceptor)

Low pressure: LPCVD – batch processing.

Hot wall reactor

Atmospheric Pressure Chemical Vapor Deposition

Transport by forced convection

By diffusion through boundary layer

Diffusion through the B. L

Desorption of by products

Transport of byproducts by forced convection @ the surface (4):

decomposition, reaction, surface migration attachment etc.(3) May be desorption which depends on a sticking coefficient (4)

Growth rate for Si deposition N=5•1022cm-3

Mole fraction of the incorporating species in the gas phase.

Partial pressure

Total concentration in the gas phase

CT = 1 * 1019 cm-35 * 1022

V = 0.14 m/min

PG @ 1 torr

Ptotal = 1 atm = 760 torr

adsorption

transport

reaction

Steady state

Steps in deposition

As in Deal-Grove model for oxidation

Growth Kinetics

Determined by the Smaller of ks or hGTwo limiting cases

1) Surface reaction ks << hG

Control: fast transport

slow reaction

YkN

Cv S

T=

2) Mass transfer or gas phase diffusion hG << ks

YhN

Cv G

T=

Fast reaction and slow transport.

Temperature uniformity more important than the gas flow wafers vertically poly-Si

Put wafers flat to ensure flow uniformity @ the Si surface.

EpitaxyAPCVD

SiO2

(111) Si shows slower v – fewer attachment sites than in (100) Si

Ea 1.6 eV for all Si sources H desorption from the Si surface.

With H2 as a gas carrier

Light mass

heavy

Limited by transport

Both are linear with time (t)

SiH4 the fastest growth

kS limited deposition is VERY temp sensitive.hG limited deposition is VERY geometry (boundary layer) sensitive

Boundary Layer – Diffusion to the Surface

Gas moves with the constant velocity U.

Boundary layer (caused by friction ) increases along the susceptor, mass transfer coefficient hG decreases, gas depletion caused by consumption of the reacting species (concentrations decrease)Growth rate decreases along the chamber

• Use tilted susceptor

• Use T gradient 5-25°C

• Gas injectors along the tube

• Use moving belt

Deposition of alloys DIFFICULT – various reactions, kinetics (species, precursors)

Use PVD rather than CVD

B.L.

viscosity

gas density

Doping in CVD for EPITAXY

Intentional and Unintentional

Si source + Dopant (AsH3, PH3, or B2H6)

Autodoping: 1 - 4

outdiffusion

autodopingThe growth is

faster than the diffusion

Dtvt >>

The dopant sources at the surface go through:

• dissociation of hydride gas

• lattice site incorporation

• burying of dopants by other atoms in the film

Simulation very inaccurate : chamber design etc.

In deposition , the doping,

€

C ∝ Pi for low growth rates

C ∝Pi

v for high growth rates

800-1000°C

outdiffusion

autodoping

T&time of CVD

Calculate all distributions (=contributions) to get C(x,t)

Low Pressure Chemical Vapor Deposition

Operate at the surface reaction limited regime lower deposition temperatures

Larger diffusion @ lower P s

GG

Dh

δ=

YN

c

hk

hkv T

Gs

Gs

+=

hG increases ~ 100 X for 760 torr 1 torr

Surface reaction controls the growth

LPCVD reactors use

• Vertical wafer stacking

• P = 0.25 – 2.0 torr

• T = 300 – 900 °C ( + 1 °C) temperature gradient increase 5 – 25% to compensate reactants depletion distributed feeding.

• Less autodoping (at lower P)

• Fewer particulates.

Possible disadvantages:

• Deposition rates may be too low, Film quality decreases

• Shadowing (less gas-phase collisions) due to directional diffusion to the surface deterioration of the step coverage and filling.

transport less important

low pressure

DG1/Ptotal

Plasma Enhanced Chemical Vapor Deposition(PECVD) Used when :

• Low T required (dielectrics on Al, metals) but CVD at decreased T gives increased porosity, poor step coverage.

• Good quality films – energy supplied by plasma increases film density, composition, step coverage for metal decreases but WATCH for damage and by product incorporation.Outgassing , peeling ,

cracking stress.

P 50 mtorr - 5 torr

Plasma: ionized excited molecules, neutrals, fragments, ex. free radicals very reactive reactions @ the Si surface enhanced increase deposition rates

• High Density Plasma CVD dense layers ( SiO2) at low T (150 °C) and low P ( 1- 10 m torr); T increases to 400°C by bombardment

• Separate RF (gives substrate biasing bombardment) from plasma generation (Electron Cynclotron resonance ECR and Inductively coupled Plasma ICP)

• Controlled bombardment (angular -> sputtering) preferential sputtering of sloped surface improved planarization and filling

200- 350 °C

13.56 MHz

Ions, electrons, neutrals = bombardment

High Density Plasma (HDP) CVD

gas inlet

plasma

magnetic coil

RFbias supply

(13.56 MHz)

wafer

gas outlet,pump

Microwavesupply

(2.45 GHz)

• Remote high density plasma with independent RF substrate bias.• Allows simultaneous deposition and sputtering for better planarization and void-free films (later). • Mostly used for SiO2 deposition in backend processes.



Physical Vapor Deposition (PVD) – no chemical reactions (except for reactive sputtering)

Evaporation

Advantages:

• Little damage

• Pure layers (high vacuum)

Disadvantages:

• Not for low vapor pressure metals

•No in-situ cleaning

• Poor step coverage

Very low pressure (P < 10 –5 torr) - long mean free path.

• purer – no filaments, only surface of the source melted

• X-rays generated trapped charges in the gate oxides anneal it !

Evaporation

k2cos

rN

vopRev θ

Ω= ki2

coscosrN

vopRev θθ

π=

Ideal cosine emission

Wafer holders to increase uniformity of deposition

Projected area

Knudsen –cell like behavior

The largest flux for the perpendicular direction

Affected by a crucible (melt,) l

r and cos θk v

Practical cases

Use spherical holders & rotate them in a planetary configuration

Deposition rates:

nonuniform deposition

lower because of cosθi emission

source here is || to the wafer

Emitted fluxes from crucibles

Evaporation

Partial Pressure (Pe) of the source (target)

e

2

1

s2

evap PT

mA1083.5R ⎟

⎠

⎞⎜⎝

⎛×= −

Needed for reasonable v 0.1 - 1m/min

No alloys – partial pressure differences

Use separate sources and e-beam

incident

reactedc F

FS =

Step Coverage Poor :

• Long mean free path (arrival angle not wide = small scattering) and low T (low energy of ad-atoms)

• Sticking coefficient high (@ T) no desorption and readsorption poor step coverage

• Heating can increase Sc but may change film properties (composition, structure)Rarely used in IC fabrication

Sticking coefficient

1-10 mtorr

Deposition

Sputter Deposition

Higher pressures 1 –100 mtorr ( < 10-5 torr in evaporation) -> contaminations!

Use ultra clean gasses and ultra clean targets

Alloys (TiW, TiN etc)

• good step coverage

• controlled properties

DC Sputtering (for metal)

Conductive

Al, W, Ti

Ar inert gas at low pressure.

No free radicals formed by Ar (ex. O, H ,F as was for PECVD)

Major Technique in Microelectronics for:

DC Sputtering

Low concentrations of electrons -> few collisions -> large voltage here0.1 –10

mm

Ar ions (+) strike the target and sputter ions electrons do not have high energy yet dark glow anode sheath.

e-

I+

e- collide with Ar atoms excitation=glow

(Crookes dark space)

positive potential (form next to each surface = anode)

I+ and e- strike the surface:µe larger (smaller mass than for ions) more electrons than ionsE field due to charge imbalance Vp (10 V develops) to decrease e- accumulation

+ potential

secondary electrons!They sustain the plasma

ions

The source of material to be deposited

Conductive !

low e--conc.

Very small sputtering at the anode can be used for bias sputtering and ionized sputtering

Ions get neutralized by e- and diffuse to the wafer surface

Sputtering

10-20 eV

secondary electrons

electrons sustain the plasma - ionization of the gas Bombardment by

I+ , e- charges and neutrals

adsorption

migration

desorption (small)

Cathode

DC Sputtering

heating

can be used in hot sputterreflow

Reactive SputterDeposition = add gas:. N2 Ti N. O2 Ti O2

Can be incorporated in the film

CONDUCTOR

WAFERS

SPUTTERING YIELD

Effect of mass (gas) & Energy TiW

steady state sputtering

Effect of mass (target)

implantation

Target

Target atom

gas

much smaller differences in sputtering yields then in partial pressures of components (target) in evaporation

+

Neutral

RF Sputter DepositionDielect

ric

Instead of DC: 13.6 MHz RF coupled capacitively to plasma

several 100V

wafers

DC sputter cannot be used for dielectrics

secondary e-

plasma extinguished (VZ ) in 1-10µs

More on the walls charge built-up

potential VP

potential

@ the target ( area)

-

= NON-CONDUCTINGOscillating (with RF) e- ionization yield

pressure

+ magnet e- trajectory Magnetron Sputter Deposition have better ionization yields

deposition rates (10-100X)better film quality (Ar needed)use in DC & RF ( heating of the target since I+ )

large A1 area

A2

A2A1

e- charge on electrode (e- are fast so they keep up with RF)

e- d

tenths of volts

faster, smaller

can be used

V1/V2=(A2/A1)m

m=1-2

For A1=A2 Ions would bombard the target and the deposited layer

BIAS SPUTTERING - Small (-) Bias @ the Wafer Chuck50 – 300V on wafers 700 – 2000V on target

For :

• precleaning = sputter etch

• better planarity step coverage

• properties (stress, compos.)

Not used much particulates from flaking

Use High Density Plasma CVD instead or

Collimated Sputtering and Ionized Sputtering

small Wider arrival

in

extended sources = not point sources evap;in point sources

Collimate the beam by using holes to direct the ions to the wafers : that

Ionized Sputter Deposition or HDP Sputtering

DC target bias

RF substrate bias

Inductivelycoupled RF

antenna

Al →

Al+ + e-

Al target

• In some systems the depositing atoms themselves are ionized. An RF coil around the plasma induces collisions in the plasma creating the ions (50-85% ionized).

• This provides a narrow distribution of arrival angles which may be useful when filling or coating the bottom of deep contact hole.

a) b)

Highly directional fluxBetter solution than a collimator (holes)



VARIOUS DEPOSITION TECHNIQUES

3326AsH,PH,BH

give better step coverage than sputtering

0.5 – 2 torr 80 – 200 torr

Barrier

plugs

400 – 7000C

VLSI !

• mass transfer regime

• T oxide evaporation

• Cl- cleaning (HCL etc)

& better doping control

• Selectivity with Cl, poor for SiHCL

porous BPSG 4-8% B, P)

reflow for planarization, steam, N2 ! Al – TIBA or

DMAH Al-H C - better Cu or Al-Cu

sputtered on CVD Al diffusion @ 400 0C

= salicide

Low T: -Si instead of poly_Si

Conditions: density of SiO2, step coverage, contaminations (C, H, N) all that determines T, pressure and gas used.

TEOS

LPCVD (conformal, 650-800°C)

Polysilicon@ low T amorphous Si

columnar structure

As & P deposition rate of poly – Si use doping after poly deposition

B Vpoly - Si

As, P segregate @ the grain boundaries ( B does

not ! )

625 °C

Models and Simulation

Ar +

Direct flux,neutrals

Al

Resputtered flux

Desorbed(emitted)

flux

Redeposited fluxes

Al

Al

Surface diffusion flux

AlAl+ Direct flux, ions

€

Fneti =Fdirect(neutrals)

i + Fdirect(ions)i + Fredep

i + Fdiff .ini

−Femittedi −Fsputtered

i −Fdiff .outi

• Within the past decade, a number of simulation tools have been developed for topography simulation. • Generalized picture of fluxes involved in deposition. (No gas phase boundary layer is included, so this picture doesn't fully model APCVD.)• Essentially the same picture will be used for etching simulation (in Chapter 10).

(14)

• To simulate these processes, we need mathematical descriptions of the various fluxes. • Modeling specific systems involves figuring out which of these fluxes needs to be included.



• Finally, ions striking the surface can sometime enhance the deposition rate (by supplying the energy to drive chemical reactions for example), so that

€

Fion−inducedi =K i ∗Fions

i (22)

LPCVD Deposition Systems

€

Fdirect(neutrals)i Yes

Fdirect( ions)i No

Fdiff (net)i =Fdiff ( in )

i −Fdiff (out)i No

Femittedi Yes

Fredep(emitted)i Yes

Fsputteredi No

Fredep(sputtered)i No

Fion−inducedi No

Furnace - with resistance heaters

Standup wafers


Desorbed(emitted)

flux

Redeposited fluxes • In these systems there are no ions involved and hence no sputtering. Surface diffusion also is usually not important.



PECVD Deposition SystemsRF power input

Electrode

WafersPlasma

Gas outlet, pump

Heater

Gas inlet( SiH4, O2)


Desorbed(emitted)

flux

Redeposited fluxes

+ Direct flux, ions

€


Fdirect( ions)i No



Femittedi Yes


Fsputteredi No

Fredep(sputtered)i No

Fion−inducedi Yes

• In these systems an ion flux can enhance the deposition rate by changing the surface reactions. Sputtering is usually not significant because the ion energy is low, nor is direct deposition of ions significant.

€

rate=ScKdFd( ) + K iFi( )

N• Thus (25)

where Kd and KI are relative rate constants for the neutral and ion-enhanced components respectively.



Ionized PVD Deposition Systems

+Direct flux,

neutrals

Resputtered flux Desorbed(emitted)

flux

Redeposited fluxes

Direct flux, ions+

DC target bias

RF substrate bias

Al → Al+ + -e

Al target

€


Fdirect( ions)i Yes



Femittedi Yes


Fsputteredi Yes

Fredep(sputtered)i Yes

Fion−inducedi Yes

• These systems are complex to model because both ions and neutrals play a role.• They are often used for metal deposition so that Ar+ ions in addition to Al+ or Ti+ ions may be present.• Thus almost all the possible terms are included

€

rate=ScFd( ) + Fi − K spYFi( ) + K rdFrd( )

Nwhere Fd includes the direct and redeposited (emitted) neutral fluxes, Fi includes the direct and ion-induced fluxes associated with the ions, and Frd models redeposition due to sputtering.



High Density Plasma CVD Deposition Systems

gas inlet

plasma

magnetic coil

RFbias supply(13.56 MHz)

wafer

gas outlet,pump

Microwavesupply

(2.45 GHz)

Resputtered flux

Redeposited fluxes

Direct flux, ions+

+

• Very similar to IPVD (except neutral direct flux not as important):

€

rate =KiFi( ) − KspYFi( ) + KrdFrd( )

N(29)



Models in SPEEDIE

LPCVD:

PECVD:

Standard PVD:

High T PVD:

Ionized PVD:

HDP CVD:

€

rate= ScFddensity

€

Fd = Fdirect(neutrals)i + Fredep(emit)

i( )

€

rate=ScKdFd( ) + K iFi( )

density

€

Fi =Fions( )

€

rate= ScFddensity

€

rate=ScFd + Ds

kTγsΩ∂

2K

∂s2

density

€

rate=ScFd( ) + Fi − K spYFi( ) + K rdFrd( )

density

€

rate=ScK iFi( )− K spYFi( ) + K rdFrd( )

densitySILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin


Calibration of Models - Example: SPEEDIE

Overhang test structure allows calibration of different components

o Neutral Precursor • Ions θ SiOx•

••o(1)

(2)(4)

(3) θ θ

(1) CVD component(2) - Ion induced deposition(3) - Sputtering with angle dependent sputter yield(4) Redeposition(5) Backscattered deposition

(5)

- Poly Si overhangSiO2

•

θ



DIRECT DEPOSITION

SURFACE DIFFUSION

DEPOSITION PRECURSORS

POLY-Si OVERHANG

SILICON SUBSTRATE

OXIDE

16 µm

1-4 µm

RE-EMISSION

OVERHANG TEST STRUCTURE

INDIRECT DEPOSITION

BY OBSERVING DEPOSITION PROFILES IN THE CAVITY CONCLUSIONS CAN BE DRAWN ABOUT THE DEPOSITION MECHANISMS

* TAPERING OF THICKNESS ON TOP SURFACE

* INFLUENCE OF CAVITY HIGHT ON DEPOSITION ON THE UNDERSIDE

J.P. McVittie, J.C. Rey, L.Y. Cheng, and K.C. Saraswat, "LPCVD Profile Simulation Using a Re-Emmission Model", IEDM Tech. Digest, 917-919 (1990).

L-Y. Cheng, J. P. McVittie and K. C. Saraswat, " Role of Sticking Coefficient on the Deposition Profiles of CVD Oxide, "Appl. Phys. Lett., 58(19), 2147-2149 (1991).



PECVD LPCVD

J.P. McVittie, “Test Structure and Modeling Studies of Deposition and Etch Mechanisms”, Talk TC1-WeM6, AVS mtg in Orlando, Florida, 1993



Parameter Values for Specific Systemsn

(exponent in cosinearrival angledistribution)

SC

(stickingcoefficient)

Sputter deposition-standard ~ 1 - 4 ~1-ionized orcollimated

8 - 80 ~1

Evaporation 3 - 80 ~1

LPCVD silicon dioxide- silane 1 0.2 - 0.4-TEOS 1 0.05 - 0.1

LPCVD tungsten 1 0.01 or lessLPCVD polysilicon 1 0.001 or less

• PVD systems - more vertical arrival angle distribution (low pressure line of sight or field driven ions). n > 1 typically.• CVD systems provide isotropic arrival angle distributions (higher pressure, gas phase collisions, mostly neutral molecules). n ≈ 1 typically.• PVD systems usually provide Sc of 1. Little surface chemistry involved. Atoms arrive and stick.• CVD systems involve surface chemistry and Sc <<1. Molecules often reemit and redeposit elsewhere before reacting.CVD systems provide more conformal deposition.



Topography Simulation (Using SPEEDIE)‘

microns-2.0 -1.0 0.0 1.0 2.0

-0.5

0.0

0.5

1.0

1.5

2.0

microns

a)‘

microns-2.0 -1.0 0.0 1.0 2.0

-0.5

0.0

0.5

1.0

1.5

2.0

microns

c)

• SPEEDIE simulations for LPCVD deposition of SiO2 with Sc = 1 (which is more typical of PVD than LPCVD) and varying values of n, the arrival angle distribution factor: (a) n=1; (c) n=10.• Worse step coverage results as n increases (the arrival angle distribution narrows).• Even for n = 1, conformal coverage is not achieved.

‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

a)

‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

a)

‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

b)

• SPEEDIE simulations for LPCVD deposition of SiO2 in a narrow trench with the same isotropic arrival angle distribution (n=1) but different values of Sc: (a) Sc = 1; (b) Sc = 0.1; and (c) Sc = 0.01.• Reducing Sc is much more effective than changing n if conformal deposition is desired.



Simulations

90˚ 85˚ 80˚

• Results of SPEEDIE LPCVD simulations with the sidewall angle changed. Sc = 0.2 and n = 1. Note the improved trench filling.

‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

a)

‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

b)

• SPEEDIE simulations comparing LPCVD and HDPCVD depositions. (a) LPCVD deposition of SiO2 over rectangular line. Sc = 0.1 and n = 1. (b) HDPCVD deposition, with directed ionic flux and angle-dependent sputtering, over rectangular line showing much more planar topography.• CMP might still be required in the HDPCVD case to fully planarize the surface.

Simulations - CVD



‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

c)

‘

-1.00 1.000.0-0.5

0.0

0.5

1.0

1.5

2.0

microns

microns

d)

• SPEEDIE simulations comparing LPCVD and HDPCVD depositions. (c) LPCVD deposition in trench, showing void formation. Sc = 0.2 and n = 1. (d) HDPCVD deposition in trench, showing much better filling.• HDPCVD has a strong directed ion component and any overhangs that form are sputtered away.

0.5 micron

oxide

metal

0.5 micron

oxidemetal

• Actual SEM images of HDP oxide deposition.

Simulations - CVD



Summary of Key Ideas

• Thin film deposition is a key technology in modern IC fabrication.• Topography coverage issues and filling issues are very important, especially as geometries continue to decrease. • CVD and PVD are the two principal deposition techniques.• CVD systems generally operate at elevated temperatures and depend on chemical reactions.• In general either mass transport of reactants to the surface or surface reactions can limit the deposition rate in CVD systems. • In low pressure CVD systems, mass transport is usually not rate limiting.• However even in low pressure systems, shadowing by surface topography can be important. • In PVD systems arrival angle distribution is very important in determining surface coverage. Shadowing can be very important.• A wide variety of systems are used in manufacturing for depositing specific thin films. • Advanced simulation tools are becoming available, which are very useful in predicting topographic issues.• Generally these simulators are based on physical models of mass transport and surface reactions and utilize parameters like arrival angle and sticking coefficients from direct and indirect fluxes to model local deposition rates.



THIN FILM DEPOSITION – Chapter 9 Introduction Many films, made of many different materials are...

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Transcript of THIN FILM DEPOSITION – Chapter 9 Introduction Many films, made of many different materials are...