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Fundamental Beam Studies of Radical Enhanced Atomic Layer Deposition

(RE-ALD)SFR Workshop & Review

April 17, 2002

Frank Greer, David Fraser, John Coburn, David GravesBerkeley, CA

2002 GOALS: 1). to investigate and model the adsorption of TiCl4on industrially relevant surfaces. 2). to investigate fundamental surface reactions mechanisms during N/H radical exposures.

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RE-ALD for TiN Diffusion Barriers• Future generation microelectronic devices require highly conformal, ultra-

thin TiN Diffusion Barriers • Atomic layer deposition (ALD) and Radical Enhanced ALD

– Uniform deposition over large area– Uniform deposition for arbitrary topography– Precise growth rate control

• Many different approaches and films reported1,2,3,4

– TiCl4 + NH3à TiN + HCl – TiCl4 + N + H à TiN + HCl– TaCl5 + H à Ta + HCl– Good resistivity reported– With Plasmas/Radicals, low impurity content at low substrate temperatures

• Can achieve Cl <0.3%, Resistivity ~1000 µΩ.cm• High Cl% may lead to corrosion of metallic films

1. Satta et al., MRS 2000 Spring Mtg. (D6.5)2. Ritala et al., JES 147, 2000

3. Greer et al., J.Vac.Sci.Tech. B. submitted4. Kim et al., MRS 2002 Spring Mtg. (B8.5)

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Adsorption Issues in RE-ALD of TiN• RE-ALD1 uses a volatile metallic precursor and a radical source to deposit a film

• TiCl4/H/N Reactants introduced separately to achieve self-limiting growth

– Purpose is to grow film 1 monolayer at a time

• Film thickness controlled by repeating TiCl4/H/N cycles

• Nucleation regime observed by many authors

• Growth Rate typically less than one monolayer/cycle even after nucleation regime

• Most studies done at higher temperatures > 200oC due kinetic constraints

1A. Sherman U.S. Patent 1999.

TiCl4Plasma (N/H)Ar

Time

Rea

ct. C

onc.

Film Thickness

Cycle Number

NucleationRegime ?

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Surface Reaction Products

Atom Source

Schematic of the Beam Apparatus in Cross-section (Top View)Quadrupole Mass

Spectrometer (QMS)

Main Chamber

Analysis SectionRotatable Carousel

+Experimental Diagnostics1. QCM

- Measures mass change of film

2. QMS- Measures products

formed on film- Characterize beams

Si/Cu/W/Porous SiO2(low-k)-coatedQuartz Crystal

Microbalance (QCM)

Ion Source

Tuning forkchopper

X

TiCl4

Precursor Doser

X = N or D

Load Lock

In-vacuoAuger

Analysis

+

Atom Source

NH3

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TiCl4 Precursor Adsorption

• Two distinct ads. regimes (Initial rapid followed by significantly slower)• Desorption of precursor à deposition rate/cycle lower at higher temperatures• Adsorption appears more uniform at higher temperatures • Adsorption likely submonolayer in all cases due to steric hinderance

*TiClx

TiCl4

Adsorption

*

TiCl4

0 2000 4000 6000 8000 10000

0.00E+000

2.00E+014

4.00E+014

6.00E+014

8.00E+014

1.00E+015

# of

TiC

l 2 Mol

ecul

es A

dsor

bed/

cm2

TiCl4 Exposure (1015 molecules/cm2)

Temp300K Temp350K Temp410K Temp445K

Submonolayer Adsorption

TiClx

Si QCM

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BET Predictions for Multilayer Adsorption• BET Isotherm is an equilibrium

calculation• Assumes surface interactions of

multilayers similar to liquid phase • Adsorption inconsistent with

multilayer adsorption (equilibrium)– P/Pvap ~ 4x10-3 à # Layers = 1

• Shows all adsorption is chemisorption of initial monolayeron surface, but is affected by temperature

*TiClx

*

TiCl4

Number of TiCl4 Layers Vs. Vapor Pressure Ratio

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1Delivery Pressure/Vapor Pressure

Nu

mb

er o

f A

dso

rbed

Lay

ers

TiClx

# of Layers =1

1-(P/Pvap)2

*TiClx

*TiClx

Keq~1/Pvap

TiCl4

*TiClx*

TiClx

TiCl4

OperatingPoint

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Adsorption Model

*TiClx

*

TiCl4

TiClx

Desorption Diffusion

•Initial fraction of monolayer adsorbs due to line of sight collisions

• Subsequent adsorption proceeds through weakly bound intermediate species due to steric hinderance of TiCl2

•Increase in substrate temperature increases desorption more rapidly than diffusion1,2

•Two Site Model Fit to Data

• R = s1ΓTiCl4θ1+ s2ΓTiCl4θ2

–Sticking probabilities

–Active site (*) densities

• s2 = f(T)1Widdra, W. et al. Phys. Rev. Let. 74(11), 19952Kota, G.P.; Coburn, J.W.; Graves, D.B., J. Appl. Phys. 85(1), 1999.

0 4000 8000 12000 16000 20000

0.00E+000

2.00E+014

4.00E+014

6.00E+014

8.00E+014

1.00E+015

# of

TiC

l 2Mol

ecul

es A

dsor

bed/

cm2

TiCl4 Exposure (1015Molecules/cm

2)

Raw Adsorption Data (410K)Rapid Adsorption SitesSlow Adsorption SitesSum of Models

Unhindered sites rapidly filled by TiCl4

Hindered sites slowly filled (thermally activated)

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Adsorption Model

*TiClx

*

TiCl4

TiClx

Desorption

Diffusion

Activation Energy Plot

9

9.2

9.4

9.69.8

1010.2

0.002 0.0025 0.003 0.00351/T

ln(1

/s2

)

• Rates of diffusion and desorption both increase with T

• Model fit gives apparent activation energy (Eapp = -0.08 eV)

•Consistent with other surface processes proceeding via weakly bound surface intermediates

• Activation energy negative because desorption increases faster

Slope = -0.08 eV

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0 100 200 300 400 500 600 700 800 900 1000-1.00E+014

0.00E+000

1.00E+014

2.00E+014

3.00E+014

4.00E+014

5.00E+014

6.00E+014

7.00E+014

8.00E+014

TiC

l 2 Mol

ecul

es A

dsor

bed

(#/c

m2 )

TiCl4 Exposure (1015 Molecules/cm2)

Silicon TiN W Cu

Adsorption on Different Surfaces

0

2E+15

4E+15

6E+15

8E+15

1E+16

1.2E+16

1.4E+16

0 1000 2000 3000 4000 5000Exposure (1015 TiCl4 Molecules/cm2s)

TiC

l 2m

olec

ules

/cm

2 Porous SilicaSample

Native Oxide

• Two regimes of adsorption not unique to Si surface

• Same qualitative behavior observed for TiN, W, Cu, and SiO2

• Porous material behaves much differently due to larger surface area

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00.5

11.5

22.5

33.5

44.5

0 10 20 30 40

Dechlorination ResultsD

Cl

QCM

Dechlorination

• Residual Cl% can be controlled

through TiCl4 and D dosages

• Increasing relative D exposure time reduces Cl content to detection limit of XPS! (<0.3%)

• Comparable thermal ALD process

yields 1.5% Cl at 400oC1

•H recombination very important– γDH ~ 0.04 >> γDCl (3x10-4)–Much of D from plasma lost in ineffective channels

•N recombination difficult to measure in this system

– γNN ~ O(0.1)2

1 Satta, A. et al. MRS 2000 Spring Meeting (D6.5)

Residual Chlorine vs. Relative D Exposure

Cl C

onte

nt (%

)

Relative H or D Atom/Cl Exposure

Results for 30-100oC

D atoms (D2)H atoms (NH3)

2H. Singh, D.B. Graves, JAP 38 (6) 2000.

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Nitrogenation Results• Excess nitrogen may be adsorbed by Si substrate1

• Incubation time for deposition is about 10 cycles• ex-situ XPS measurement after 20 cycles shows N/Ti ~ 3

# TiN Layers Deposited/Cycle

-1.6E+15

-8E+14

0

8E+14

1.6E+15

2.4E+15

0 50 100

Cycle Number#T

iN/c

m^2

Comparison of Layer 1 and Layer 12

0

1E+15

2E+15

3E+15

4E+15

5E+15

6E+15

7E+15

0 200 400 600 800Nitrogen Exposure (L)

Nit

rog

en U

pta

ke (#

N/c

m2 ) Nitrogenation of Layer 1

Nitrogenation of Layer 12

1H. Niimi, and G. Lucovsky, G., JVST B 17 (6) 1999.

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Conclusions/2002 and 2003 Goals• RE-ALD processing questions

– Precursor (TiCl4)

• Adsorption is monolayer-like over the range of surface T investigated

• Desorption of weakly bound TiCl4 precursor important in initial ML formation

– Radicals (D,N)

• D radicals can reduce Cl content to < 0.3%

• N radicals likely can nitrogenate the substrate during first few cycles

• NH3 plasma results consistent with separate N and H fluxes

• Goals– Explore adsorption kinetics of other RE-ALD precursors

– Construct high throughput RE-ALD system or partner with industrial liasons to deposit thick films for characterization (TEM/XRD/etc.)

– Investigate integration issues (adhesion, etc.)