Thin Film Disposition

8/12/2019 Thin Film Disposition

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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 N ode (half pi tch) 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 Meta l 1 Pitch (nm) 214 152 108 76 54 42

Wiring Levels - Logic 10 11 12 12 14 14

Me tal 1 Aspect Ratio (Cu) 1.7 1.7 1.8 1.9 2.0 2.0

Contact As pect Ratio (DRAM) 15 16 >20 >20 >20 >20

STI Trench As pect Ratio 4.8 5.9 7.9 10.3 14 16.4

Me tal Resistivi ty (ohm-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


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Thin Film Deposition

Quality composition, defect density, mechanical and electrical properties

Uniformity affect performance (mechanical , electrical)

Thinningleads 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.


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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 wallreactor


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Atmospheric Pressure Chemical Vapor Deposition

Transport by forcedconvection

By diffusionthrough boundarylayer

Diffusion through the B. L

Desorption of by products

Transport of byproducts byforced convection

@ the surface (4): decomposition,

reaction, surface migration attachment etc.

(3) May be desorption which depends ona sticking coefficient (4)

Growth rate for Si deposition N=510 22cm -3

Mole fraction of the incorporatingspecies in the gas phase.

Partial pressure

Total concentration in the gas phase

CT = 1 * 10 19 cm -3 5 * 10 22

V = 0.14 m/min

PG @ 1 torrPtotal = 1 atm = 760 torr

adsorption

transport

reaction

Steady state

Steps in deposition

As in Deal-Grovemodel for oxidation


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Growth Kinetics

Determined by the Smaller of k s or h GTwo limiting cases

1) Surface reaction k s


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Boundary Layer Diffusion to the Surface

Gas moves with the constant velocity U.

Boundary layer (caused by friction ) increases alongthe susceptor, mass transfer coefficient h G decreases,gas depletion caused by consumption of the reactingspecies (concentrations decrease)

Growth rate decreases along the chamber

Use tilted susceptor

Use T gradient 5-25C

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


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Doping in CVD for EPITAXY

Intentional and Unintentional

Si source + Dopant(AsH 3, PH 3, orB2H6)

Autodoping: 1 - 4

outdiffusion

autodopingThe growth is faster thanthe diffusion

Dt vt

The dopant sources at thesurface go through:

dissociation of hydride gas

lattice site incorporation

burying of dopants by otheratoms in the film

Simulation very inaccurate :chamber design etc.

In deposition , the doping,

C P i for low growth rates

C P iv

for high growth rat

800-1000C

outdiffusion

autodoping

T&time of CVD

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


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Low Pressure Chemical Vapor Deposition

Operate at the surface reaction limited regime lower deposition temperatures

Larger diffusion @ lower P s

GG

Dh

Y N c

hk hk

v T G s

G s

hG increases ~ 100 X for760 torr 1 torr

Surface reaction controlsthe growth

LPCVD reactors use

Vertical wafer stacking

P = 0.25 2.0 torr T = 300 900 C ( + 1 C) temperaturegradient increase 5 25% to compensatereactants 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 directionaldiffusion to the surface deterioration of the step coverage andfilling.

transport less important

low pressure

DG 1/P total


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Plasma Enhanced Chemical Vapor Deposition(PECVD)

Used when :

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

Good quality films energy supplied by plasma increases film density,composition, step coverage for metaldecreases but WATCH for damage and by

product incorporation.

Outgassing , peeling , cracking

stress.

P 50 mtorr - 5 torr

Plasma: ionizedexcited molecules,neutrals, fragments,ex. free radicals very reactivereactions @ the Sisurface enhanced increase depositionrates

High Density Plasma CVD dense layers ( SiO 2) at low T (150 C) and low P ( 1- 10 m torr); T increases to 400C by bombardment

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

Controlled bombardment (angular -> sputtering) preferential sputtering of sloped surface improved planarization andfilling

200- 350 C

13.56 MHz

Ions, electrons,neutrals =

bombardment


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High Density Plasma (HDP) CVD

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 SiO 2 deposition in backend processes.

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

2000 by Prentice HallUpper Saddle River NJ


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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 thesource melted

X-rays generated trapped charges in thegate oxides anneal it !


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Evaporation

k 2 cosr N vop Re

v

k i2 coscosr N vop Re

v

Ideal cosine emission

Wafer holders to increaseuniformity of deposition

Projectedarea

Knudsen cell like behavior

The largest fluxfor the

perpendiculardirection

Affected by a crucible(melt,)

l rand cos k

v

Practical cases

Use sphericalholders & rotatethem in a

planetaryconfiguration

Deposition rates:

nonuniform deposition

lower because ofcos i emission

source here is ||to the wafer

Emitted fluxes from crucibles


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Evaporation

Partial Pressure (P e) of the source(target)

e

21

s2

evap P T m

A1083.5 R

Needed forreasonable v 0.1 - 1 m/min

No alloys partial pressure differences

Use separate sources and e-beam

incident

reacted c

F

F S

Step Coverage Poor :

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

Sticking coefficient high (@ T) no desorption andreadsorption poor step coverage

Heating can increase S c but may change film properties (composition, structure)

Rarely used in IC fabrication

Sticking coefficient

1-10 mtorr

Deposition


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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 wasfor PECVD)

Major Technique in Microelectronics for:


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DC Sputtering

Low concentrations of electrons ->few collisions -> large voltage here

0.1 10 mm

Ar ions (+) strike the target and sputterions electrons do not have highenergy yet dark glow anodesheath.

e-

I+

e- collide with Ar atomsexcitation=glow

(Crookes dark space)

positive potential (form next to eachsurface = anode)

I+ and e - strike the surface:

e larger (smaller mass than for ions)more electrons than ionsE field due to charge imbalance V p (10 Vdevelops) 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 biassputtering and ionized sputtering

Ions get neutralized by e - anddiffuse to the wafer surface


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Sputtering

10-20 eV

secondary electrons

electronssustain the

plasma -

ionization ofthe gas

Bombardment by I + ,e- charges and neutrals

adsorption

migration

desorption (small)

Cathode DC Sputtering

heating

can beused inhot sputter

reflow

Reactive SputterDeposition = add gas:. N2 Ti N. O2 Ti O 2

Can beincorporated

in the film

CONDUCTOR

WAFERS


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SPUTTERING YIELD

Effect of mass (gas) & EnergyTiWsteady statesputtering

Effect ofmass (target)

implantation

Target

Target atom

gas

muchsmallerdifferencesin sputteringyields thenin partial

pressures ofcomponents(target) inevaporation

+

Neutral


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RF Sputter Deposition Dielectric

Instead of DC: 13.6 MHz RFcoupled capacitively to

plasma

several 100V wafers

DC sputter cannot be used for dielectrics

secondary e - plasma extinguished (V Z ) in 1-10s

More on the walls charge built-up

potential V P

potential

@ the target ( area)

-

= NON-CONDUCTINGOscillating (with RF) e -

ionization yield pressure

+ magnet e - trajectoryMagnetron Sputter Depositionhave better ionization yields

deposition rates (10-100X)

better film quality (Ar needed)

use in DC & RF ( heating of thetarget since I + )

large A1 area

A2

A2 A1

e - charge onelectrode (e - are fast sothey keep upwith RF)

e - tenths of volts

faster, smaller

can be used

V1/V2=(A 2/A1)m m=1-2

For A 1=A 2 Ions would bombard the target and thedeposited layer


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

For :

precleaning= sputter etch

better planaritystep coverage

properties(stress, compos.)

Not used much particulates from flaking

Use High Density Plasma CVDinstead or

Collimated Sputtering and Ionized Sputtering

smallWider arrival

in

extendedsources = not

point sourcesevap;

in point sources

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


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Ionized Sputter Deposition or HDP Sputtering

In some systems the depositingatoms themselves are ionized.An RF coil around the plasmainduces collisions in the plasmacreating the ions (50-85% ionized).

This provides a narrowdistribution of arrival angleswhich may be useful when fillingor coating the bottom of deepcontact hole.

a) b)

Highlydirectional fluxBetter solution

than a collimator(holes)



VARIOUS DEPOSITION TECHNIQUES


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VARIOUS DEPOSITION TECHNIQUES

3 3 2 6AsH ,PH , B H

give better step coverage

than sputtering

0.5 2 torr80 200 torr

Barrier

plugs

400 700 0C

VLSI!

mass transfer regime

T oxide evaporation

Cl- cleaning (HCL etc)

& better doping control Selectivity with Cl, poorfor SiHCL

porousBPSG 4-8% B, P)

reflow for planarization, steam, N

2

! 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 SiO 2,step coverage, contaminations(C, H, N) all that determinesT, pressure and gas used.

TEOS

LPCVD (conformal, 650-800C)


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Polysilicon@ low T amorphous Si

columnarstructure

As & P deposition rate of poly Si use dopingafter poly deposition

B V poly - Si

As , P segregate @ the grain boundaries

( B does not ! )

625 C


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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

Fsputteredi

Fdiff .outi

Within the past decade, a number ofsimulation tools have been developed

for topography simulation. Generalized picture of fluxesinvolved in deposition. (No gas

phase boundary layer is included, sothis picture doesn't fully modelAPCVD.)

Essentially the same picture will beused for etching simulation (inChapter 10).

(14)

To simulate these processes, we need mathematical descriptions of the variousfluxes.

Modeling specific systems involves figuring out which of these fluxes needsto be included.




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Finally, ions striking the surface can sometime enhance the deposition rate (bysupplying 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)

iF

diff ( in )

iF

diff (out )

i No

Femittedi

Yes

Fredep(emitted)i

Yes

Fsputteredi

No

Fredep(sputtered )i

No

F ion inducedi

No

Furnace - with resistance heaters

Standup wafe rs

Directflux,neutrals

De sorbed(emitted)

flux

Redepos ited fluxes In these systems there are no ions involved andhence no sputtering. Surface diffusion also isusually not important.




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PECVD Deposition SystemsRF power input

Electrode

WafersPlasma

Gas outlet, pump

Heater

Gas inlet( SiH 4, O 2)

Direct flux,neutrals

Desorbed(e mitted)

flux

Redeposited fluxes

+ Direct flux, ions

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 Yes

In these systems an ion flux can enhance the depositionrate by changing the surface reactions. Sputtering isusually not significant because the ion energy is low,

nor is direct deposition of ions significant.

rateS c K d Fd K i Fi

N Thus (25)

where K d and K I are relative rate constants for theneutral and ion-enhanced components respectively.




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Ionized PVD Deposition Systems

+Direct flux,

neutrals

Resputtered fluxDesorbed(e mitted)flux

Redeposited fluxes

Direct flux, ions+

DC targe t bias

RF substrate bias

Al Al + + e-

Al target

Fdirect(neutrals)i

Yes

Fdirect(ions )i

Yes

Fdiff (net)i

Fdiff ( in )i

Fdiff (out)i

No

Femitted

i Yes

Fredep(emitted )i

Yes

Fsputteredi

Yes

Fredep(sputtered )i

Yes

Fion inducedi

Yes

These systems are complex to model because bothions 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 S c Fd Fi K sp YF i K rd Frd

Nwhere F d includes the direct and redeposited (emitted)neutral fluxes, F i includes the direct and ion-inducedfluxes associated with the ions, and F rd models

redeposition due to sputtering.SILICON VLSI TECHNOLOGYFundamentals, Practice and ModelingBy Plummer, Deal & Griffin



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High Density Plasma CVD Deposition Systems

gas inlet

plasma

magneti c coil

RFbias supply

(13.56 MHz )

wafer

gas outlet,pump

Microwavesupply

(2.45 GHz )

Resputtered flu

Redeposited fluxes

Direct flux, i ons

+

+

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

rate K iF i K sp YF i K rd Frd N

(29)




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Models in SPEEDIE

LPCVD:

PECVD:

Standard PVD:

High T PVD:

Ionized PVD:

HDP CVD:

rate S c Fd

density

Fd Fdirect(neutrals)i F redep(emit)

i

rateS c K d Fd K i Fi

density F i Fions

rate S c Fd

density

rate

S c Fd D skT s

u2 K

s2

density

rate

Sc

Fd

Fi K

spYF

i K

rdF

rd density

rate

S c K i Fi K sp YF i K rd Frd density




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OVERHANG TEST STRUCTURE


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DIRECT DEPOSITION

SURFACE DIFFUSION

DEPOSITION PRECURSORS

POLY-Si OVERHANG

SILICON SUBSTRATE

OXIDE

16 m

1-4 m

RE-EMISSION

OVERHANG TEST STRUCTURE

INDIRECTDEPOSITION

BY OBSERVING DEPOSITION PROFILES IN THE CAVITY CONCLUSIONS CAN BEDRAWN 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).




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PECVD LPCVD

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




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Parameter Values for Specific Systemsn

(exponent in cosinearrival angledistribution)

SC(sticking

coefficient)

Sputter deposition-standard ~ 1 - 4 ~1-ionized orcollima ted

8 - 80 ~1

Evaporation 3 - 80 ~1

LPCVD silicon dioxide- silane 1 0.2 - 0.4

-TEOS 1 0.05 - 0.1LPCVD tungsten 1 0.01 or lessLPCVD polysilicon 1 0.001 or less

PVD systems - more vertical arrival angle distribution (low pressure line of sightor 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 S c of 1. Little surface chemistry involved. Atomsarrive and stick.

CVD systems involve surface chemistry and S c


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Topography Simulation (Using SPEEDIE)

SPEEDIE simulations for LPCVD deposition of SiO 2 with S c = 1 (which is more typical of PVD than LPCVD)and varying values of n, the arrival angle distributionfactor: (a) n=1; (c) n=10.

Worse step coverage results as n increases (the arrivalangle 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

SPEEDIE simulations for LPCVDdeposition of SiO 2 in a narrowtrench with the same isotropicarrival angle distribution (n=1) butdifferent values of S

c: (a) S

c = 1;

(b) S c = 0.1; and (c) S c = 0.01. Reducing S c is much more effective

than changing n if conformaldeposition is desired.




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Simulations

Si l i CVD


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90 85 80

Results of SPEEDIE LPCVD simulations with the sidewall angle changed. S c = 0.2 and n = 1.

Note the improved trench filling.

SPEEDIE simulations comparing LPCVD andHDPCVD depositions. (a) LPCVD depositionof SiO 2 over rectangular line. S c = 0.1 and n = 1.(b) HDPCVD deposition, with directed ionicflux and angle-dependent sputtering, overrectangular line showing much more planartopography.

CMP might still be required in the HDPCVD

case to fully planarize the surface.

Simulations - CVD



Si l i CVD


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SPEEDIE simulations comparing LPCVDand HDPCVD depositions. (c) LPCVDdeposition in trench, showing void formation.S

c = 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 aresputtered away.

Actual SEM images of HDPoxide deposition.

Simulations - CVD



S f K Id


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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 beimportant.

In PVD systems arrival angle distribution is very important in determining surfacecoverage. Shadowing can be very important.

A wide variety of systems are used in manufacturing for depositing specificthin 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 andsurface reactions and utilize parameters like arrival angle and sticking coefficientsfrom direct and indirect fluxes to model local deposition rates.

SILICON VLSI TECHNOLOGYF d l P i d M d li 2000 b P i H ll

Thin Film Disposition

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