4th Annual SFR Workshop & Review, April. 17, 2002

8:30 – 9:00 Research and Educational Objectives / Spanos

9:00 – 9:45 CMP / Doyle, Dornfeld, Talbot, Spanos

9:45 9:45 –– 10:30 10:30 Plasma & Diffusion / Plasma & Diffusion / Graves, Lieberman, Cheung, HallerGraves, Lieberman, Cheung, Haller

10:30 – 10:45 break10:45 – 12:00 Poster Session / Education, CMP, Plasma, Diffusion

12:00 – 1:00 lunch, / New Microlab Planning Update

1:00 – 1:45 Lithography / Spanos, Neureuther, Bokor

1:45 – 2:30 Sensors & Controls /Aydil, Poolla, Smith, Dunn, Cheung, Spanos

2:30 – 2:45 Break2:45 – 4:30 Poster Session / all subjects

3:30 – 4:30 Steering Committee Meeting in room 606 Soda4:30 – 5:30 Feedback Session

4th Annual SFR Workshop & Review, April. 17, 2002

4/17/2002 SFR Workshop - Plasma & Diffusion

2

Diffusion and Plasma

UC-SMART Major Program Award

E. Haller, N. Cheung, D. Graves, and M. Lieberman,

University of CaliforniaFourth Annual Workshop & Review

4/17/2002


3

Fermi Level DependentDiffusion in Silicon

SMART/SFR Workshop and ReviewApril 17, 2002

Hughes Silvestri, Ian Sharp, Hartmut Bracht and Eugene E. Haller

University of California at Berkeley andLawrence Berkeley National Laboratory

Berkeley, CA 94720


4

Collaborators

• Hughes H. Silvestri, Graduate Student Research Assistant, MSE, UC Berkeley

• Ian D. Sharp, Undergraduate Student Research Assistant, Chem. Eng. & MSE, UC Berkeley

• Dr. Hartmut A. Bracht, Dept. of Materials Physics, University of Münster, Germany

• Dr. Steven Burden, ISONICS Corp., Golden, CO


5

Motivation• Downscaling of devices is, in part, limited by diffusion of dopants

(see: Paul Packan, Intel Corp., MRS San Francisco, April 19, 2001, Symposium X3, “Materials issues related to dopant diffusion and activation to form shallow, low resistance profiles will be addressed.”).

• Advanced modeling and control of diffusion requires an improved basic understanding of diffusion processes.

• Quantitative knowledge of the properties of silicon interstitials, one of the major active native defect assisting diffusion, is extremely limited. What are its charge states? Where do the energy levels lie in the silicon bandgap? What are the formation and migration energies? What are the individual contributions of the native defects to self-diffusion?


6

Approach

• Use undoped and doped Si isotope multilayer structures to study self- and dopant diffusion.

• Determine simultaneously the diffusivities of dopants and silicon as a function of temperature and Fermi level position.

• Major issue: Introduce dopants without causing transient phenomena (i.e., avoid “interstitial wind” caused by oxidation, implantation or clustering of excess dopant concentrations.)

• Use a low-temperature MBE-grown amorphous silicon cap layer, implanted at low energies as dopant diffusion source.

• Boron acceptor and Arsenic donor diffusion have been studied (see poster); Phosphorus diffusion is under investigation.


7

Experimental results: Boron as-implanted/Si isotope multilayer structure

1.0E+17

1.0E+18

1.0E+19

1.0E+20

1.0E+21

1.0E+22

1.0E+23

0 200 400 600 800 1000 1200 1400 1600 1800

depth (nm)

con

cen

trat

ion

(cm

-3)

B 30Si 28Si

28Si enriched layers

Natural Si layers

Artifact caused by SIMS etch pits

Amorphous Si layer

(260 nm)

Natural Si cap layer (200 nm)


8

Simultaneous diffusion of B and SiDiffusion anneal → SIMS → analysis: Bi

0 ⇔ Bs- + I 0,+ and

Bi0 + V 0,+ ⇔ Bs

-

1017

1018

1019

1020

1021

1022

0 600 1200 1800

30Si

B

depth (nm)

conc

entr

atio

n (c

m-3

)

T=950°C, t=15h+57 min

0.01

0.1

1

10

0 600 1200 1800

V +

V 0 I +

I 0

depth (nm)C

X /

Ceq x

B diffusion and doping favorsSi diffusion via I 0 and I + and suppresses Si diffusion via V


9

Temperature dependence of B and Si diffusion

10-20

10-18

10-16

10-14

10-12

10-10

0.6 0.7 0.8 0.9

1400 1200 1000 800

Bracht et al.(1998)

DSi(p)

DSi(ni)

DB

Antoniadis et al. (1978)

103/T (K-1)

D (

cm2 s-1

)

T (oC)

• B diffusion coefficients are consistent with literature data

• Si diffusion is enhanced under p-type doping and mainly controlled by I 0 and I +


10

Contribution of vacancies to Si self-diffusion?

1017

1018

1019

1020

1021

1022

0 500 1000 1500

30Si

As

depth (nm)

conc

entr

atio

n (c

m-3

)

as-implanted Si isotope structure

→ Diffusion of As in Si is known to be mediated both by I and V:

As implantation150 keV, 1016 cm-2

natSi (amorphous)

120 nm

natSi

28Si

1.3 µm

250 nm


11

Simultaneous diffusion of As and Si

1016

1018

1020

1022

0 500 1000 1500

V 2- V -

V 0

30Si

As

depth (nm)

conc

entra

tion

(cm

-3)

As diffusion: T=950°C, t=122h+14 min

Diffusion model: (AsV)0 ⇔ Ass+ + V - and (AsI)0 ⇔ Ass

+ + I 0

• As diffusion mediated by V - and I 0

• yields V - and I 0 contribution to DSi


12

Conclusion

• Boron diffusion via the kick-out mechanism Bi0 ⇔ Bs

- + I 0,+ ;interstitialcy mechanism1 ( [BI ]0 ⇔ Bs

- + I 0,+ ) not very likely; fBI ≤ 0.3

• Arsenic diffusion via vacancy and interstitialcy mechanism[AsV]0 ⇔ Ass

+ + V - and [AsI]0 ⇔ Ass+ + I 0 ; no V 0 and

V 2-

• I +, I 0, and V - contributions to Si diffusion add up accurately to DSi


13

Summary

• Boron diffusion in Si isotope structure is mediated by I+ and I0

– kick-out mechanism more likely than interstitialcy mechanism– native defect-controlled mode of B-diffusion– I+ and I0 contributions to Si self-diffusion– Q(I+) = 4.74 eV ; Q(I0) = 4.56 eV ; Q(B) = 3.56 eV

• Arsenic diffusion in Si isotope structure is proposed to be mediated by V - and I0

– interstitialcy and vacancy mechanisms both contribute– dopant defect-controlled mode of As diffusion– V - and I0 contributions to Si self-diffusion– Q(V -) = 5.18 eV ; Q(I0) = 4.32 eV ; Q(As) = 4.16 eV


14

Outlook and Future Work• As diffusion model is not unique concerning the defects V - and I0, i.e.,

fitting is also possible with I - and V0. Consequences: – only neutral vacancies? (in contradiction to our current understanding) – three charge states for self-interstitials? (further evidence for I - required)

• P diffusion studies in Si isotope structure are very important– P is considered to diffuse via the vacancy and the kick-out mechanism; native

defect-controlled mode of P diffusion– expect to distinguish between V - and I0 contributions to Si self-diffusion

• Upon completion of our studies, we will have comprehensive information about: detailed mechanisms of dopant diffusion (B, As, P, Sb); charge states of native point defects; individual contributions to Si self-diffusion

• Milestone 2002: Fermi level dependent diffusion studies with phosphorus and arsenic

• Milestone 2003: Dopant and self-diffusion in isotopically controlled SiGe structures (relaxed and strained)


15

Plasma Enhanced Bonding

SFR WorkshopApr 17, 2002

Yonah Cho and Nathan CheungBerkeley, CA

2002 GOAL: Enable wafer/die scale direct bonding of surfaces at low temperature (<200oC)


16

Problem Statement

..

Systems Integration

Enhanced Adhesion between low k /metal

Metal

polymerMetal

SOI Technology

Encapsulation of power sources

Plasma Enhanced Bonding Applications


17

State-of-the-ArtNo universal model on plasma assisted bonding mechanisms-Chemical force ?- Electrostatic force ?- Surface smoothing ?- Nano-plasticity of interfacial layer ?

Hydrophobic

Hydrophilic

3000

2500

2000

1500

1000

500

0100 200300400500 600700800 900

Surf

ace

Ene

rgy

(mJ/

m2 )

Annealing Temperature (oC)

OO22 plasmaplasmaTong et al.

APL 64, 625 (94)

UCBData


18

Our Approach

Bonding for a self-powered chip

Cross-section view

Power unit LED unit

Top view

a

b

a

b

Process flow:1. Si etch in KOH for wells2. Metal deposition for anode (-) & metal lift-off for patterning3. Metal bonding of LED ~200 oC4. Battery insertion5. Meta deposition for cathode (+) & patterning6. RT bonding of SiO2/Si7. Mechanical separation for top membrane

Si-undoped

SiO2


19

Low temperature encapsulation demonstration

SiO2, Si3N4 masks

Mask patterning

Si etch in KOHMask etch (SF6/He)

Piranha cleaning O2 plasma exposure

Bonding

InsertionPolymer film


20

Results/Justification for Approach

Demonstrated Low Temp Encapsulation: 25 oC- 150 oC

Infrared Transmission Image of Bonded Interface

3 cm

1.5 cm

1 cm 0.5 cm

Parafilm

Transparency

Cavity


21

New Approaches for low temperature Bonding

•Surface smoothing improves gap filling ability•Getters processing contaminants to form benign compounds at Si/BOX interface

F/Cl plasma or beam surface treatment

•Reduced surface Young’s modulus improves gap filling ability

Beam/Plasma implantation to modify nano-plasticity of surface layer

•Generation of silanol groups to increase surface bonding energy•H incorporation passivates oxide charge at Si/BOX interface

Water plasma treatment of Si-based surfaces


22

Justification for Approach Gap Closing Criterion for nm surface roughness

1x10 -8 1x10 -7 1x10 -6 1x10 -5 1x10-4 1x10-310 -9

10 -8

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

10 -1

100

101

Op

en

ing

Ra

diu

s (c

m)

Height (cm)

R >2d

R < 2dGap opening

γ =10mJ/m2

R

2h d

γ =100mJ/m2

Gap closing

Wafer thickness = 0.5 mm


23

Justification for Approach Molecular Dynamics: Ar+ impacts on Si Amorphize Top Layer

(D. Graves ,UCB)


24

Plans

–– 20022002•• Hermetic testing of encapsulated battery with plasma Hermetic testing of encapsulated battery with plasma

bonding (with B. Dunn)bonding (with B. Dunn)•• Plasma and Ion Beam experiments to verify bonding Plasma and Ion Beam experiments to verify bonding

nanonano--plasticity andplasticity and silanolsilanol group mechanisms (with group mechanisms (with D. Graves)D. Graves)

–– 20032003• Bipolar Plasma Substrate Bias for enhanced

adhesion with polymer surfaces


25

SFR Workshop and ReviewApril 17, 2001Berkeley, CA

F. Greer, M. Nierode, M. Radtke, D. Fraser, J. Coburn and D. Graves Berkeley, CA

Plasma-Assisted Processes for Small Feature Reproducibility


26

Fundamental Beam Studies of Radical Enhanced Atomic Layer Deposition (RE-ALD)

• Barrier layers for interconnect must be very thin and highly conformal

• Atomic layer deposition (ALD) has been proposed for deposition of barrier films

• Our studies focus on the use of radicals for part of the atomic layer deposition process

• This offers the promise of greatly reduced deposition temperature

• Use of radicals for ALD must be characterized and understood


27

Progress and Research Activitiesü Conducted series of experiments demonstrating radical-enhanced

ALD and obtained fundamental kinetic parameters

ü Continued plasma testbed and model development for plasma processing of new materials

Since November

ü Studied adsorption kinetics of TiCl4 as a function of Tsurface and surface type

ü Demonstrated sub-monolayer precursor adsorption

ü Developed parameterized model of adsorption kinetics

ü Created FEMLAB-based model of non-isothermal, compressible neutral transport for plasma tool analysis

ü Installed new FTIR spectrometer on plasma experimental testbed


28

Milestones

• September 30th, 2001 ü Demonstrate TiN barrier layers deposited by novel ALD technology: REALCVD. Conduct preliminary film deposition studies. Test self-limiting film formation

• September 30th, 2002 üStudy effects of surface preparation, surface type and surface temperature on REALCVD film properties. Measurekey radical-surface kinetics. Project completion; student graduation.

• September 30th, 2003 Plasma processing of novel materials: experiments and modeling. Plasma and ion beam bonding experiments/models; (with N. Cheung)


29

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 ?


30

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


31

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)


32

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


33

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.


34

Conclusions: RE-ALD• RE-ALD processing questions

– Precursor (TiCl4)

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

• Desorption of weakly bound TiCl4 precursor important in precursor adsorption

– 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


35

Future Plans

• Plasma processing of novel materials: experiments and plasma modeling.

•High k, low k, photoresist and metal gate materials

•Etch products, etch product interactions with surfaces

• Plasma and ion beam experiments to verify bonding nano-plasticity and silanol group mechanisms (with N. Cheung)

• Bipolar Plasma Substrate Bias for enhanced adhesion with polymer surfaces (with N. Cheung)


36

Plasma Sources for Small Feature Reproducibility

SFR WorkshopApril 17, 2002Berkeley, CA

M. A. Lieberman, A. J. Lichtenberg, A. M. MarakhtanovUC Berkeley, CA

P. Chabert (Ecole Polytechnique, France)M. Tuszewski (Los Alamos National Laboratory)


37

Milestones

• September 30, 2002- Characterize plasma instability using V-I-phase probe. - Model for reduced electron temperature and density.

• September 30, 2003- Develop and test instabilities control. Reduce electron

temperature and density in discharges.


38

Instabilities in Processing Plasmas

1 10 1000

200

400

600

800

1000

CAPACITIVE

INSTABILITY

INDUCTIVE

Abs

orbe

d P

ower

(W

)

Gas Pressure (mTorr)

1 10 1000

200

400

600

800

1000

INSTABILITY

CAPACITIVE

INDUCTIVE

Abs

orbe

d P

ower

(W

)Gas Pressure (mTorr)

Instability range in SF6 plasma Instability range in Ar/SF6 (1:1) plasma

Plasma instabilities have been observed in low pressure inductivedischarges with electronegative gas feedstocks (O2, SF6, etc)


39

Experiment (TCP)

QMS

PMT

∅ 30 cm

20 cm

GAS FEED

PUMP

Z-Scan V-I Probe

13.56 MHzPOWERSUPPLY

C1

C2

Planar Probe

Langmuir ProbeINDUCTIVE COIL


40

Ar/SF6 Plasma Parameters vs. Instability

0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

8

9

10

Te

Ele

ctro

n T

empe

ratu

re T

e (e

V)

Time (ms)

0.0 0.5 1.0 1.5 2.01E7

1E8

1E9

1E10

ne

Ele

ctro

n de

nsity

ne (

cm-3)

Ar/SF6 (1:1) 5 mTorr, 350 W

Inductive mode (high density)

Capacitive mode (low density)

Instabilities occur in the transitionfrom capacitive (E) to inductive (H)mode of the discharge


41

Plasma Instabilities vs. Electron Power Absorption

Electron power absorptionper unit volume

+

++

= 20

202

nnnn

nnn

fRIPe

e

ce

ccindrfabs

capacitive inductive0 1

0

1

P ele

ctro

n/ V

olum

e

ne

capacitive inductive

Instability

Does reduction of capacitive coupling eliminate plasma instability?


42

Effect of Capacitive Coupling on Instability

Electrostatic (Faraday) Shield with Variable Area

Variable Faraday Shield§ up to 20 flat copper wedges§ shielded area: 0…100 %§ grounded

18°

TCP Coil


43

020

40

60

80 0100

200300

4000

4

8

12

16

Fre

quen

cy o

f Lig

ht, k

Hz

Power, W

Area covered

by shield , %

0 10 20 30 40 50 60 700

50

100150

200

250

300350

400STABLE INDUCTIVE

STABLE CAPACITIVE

UNSTABLE

Pow

er, W

Area covered by shield, %

Effect of Capacitive Coupling on Instability

§ Capacitive coupling and stable capacitive region reduced by Faraday shield§ Plasma instability continuously reduced by increase of the shielded area from

25 to 65 %§ Plasma relaxation oscillation frequencies decrease with shielded area increase § No instability observed for shielded area >65%

Ar/SF6 (1:1) 5 mTorr


44

Spatial Variations During Instability

0 1 2 3 4 50.0

0.4

0.8

1.2

1.6

20 mm 95 mm 170 mm

Pos

itive

ion

dens

ity (

a.u.

)

Time (ms)

170 mm

3 2 1

95 mm

20 mm

Probe @ -50 V

Ar/SF6 (1:1) 40 mTorr, 200 W

§ Spatial variations seen at higher pressures§ Instability decays more rapidly as pressure increases


45

Instabilities in Pure Inductive Discharges

0.000 0.001 0.002 0.003 0.004 0.005-3

-2

-1

0

1

2

3

Time (s)

Flo

atin

g pr

obe

pote

ntia

l (V

)

Ar/SF6 (1:1) 40 mTorr, 660 W

Instabilities have been observed in pure inductive discharges in Ar/SF6for pressures >20 mTorr (these fluctuations are small)


46

Future Work• Examine spatial variations during instability• Continue to study types of instabilities

a) Inductive to capacitive transition (kHz)b) Propagating inductive (kHz)c) Low frequency (Hz)d) Low pressure (< 3 mTorr)

• Continue to study capacitive coupling effect on instability• Continue to investigate the chemical composition of the discharge to

make improvements to the instability global model• Investigate instability effect on substrate

Publications1. M.A. Lieberman, A.J. Lichtenberg, and A.M. Marakhtanov, Instabilities in Low-pressure Inductive

Discharges with Attaching Gases, Appl. Phys. Lett. 75, 3617 (1999)2. P. Chabert, A.J. Lichtenberg, M.A. Lieberman, and A.M. Marakhtanov, Instabilities in Low-pressure

Electronegative Inductive Discharges, Plasma Sources Sci. Technol. 10, 478-489 (2001)

4th Annual SFR Workshop & Review, April. 17, 2002

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Transcript of 4th Annual SFR Workshop & Review, April. 17, 2002