Introduction to Plasma and Surface...

Introduction to Plasma and Surface Diagnostics

Vincent M. Donnelly

fUniversity of Houston

GEC Pre‐Conference TutorialfPrinceton NJ, Sept. 28 2013

Support from the Department of Energy Office of Fusion Energy Science contract DE SC0001939

1

Support from the Department of Energy, Office of Fusion Energy Science, contract DE-SC0001939

OUTLINE

• Very brief introduction to Langmuir Probes

• Selected topics on Optical Emission Spectroscopy (OES)

• Brief review of OES with actinometry

• Issues concerning plasma‐surface interactionsssues co ce g p as a su ace te act o s

• Requirements for plasma‐surface probes

R i f l t d i it th d• Review of selected in‐situ methods

• XPS for in‐vacuum sample analysis

• Specific example of applications of the above

2

THE FIRST “LANGMUIR PROBE” EXPERIMENT ‐ 1752 B j i F kli Phil d l hiBenjamin Franklin – Philadelphia

Is the key Kid, touch Is the key at Vp or Vf?

that key and see what

happens.

3

ERA OF MODERN PLASMA PHYSICSIrving Langmuir General Electric Co 1924Irving Langmuir – General Electric Co. 1924

Paper on what became known as the Langmuir probe technique:I. Langmuir and H. Mott‐Smith, Gen. Elec. Rev. 27, 449,538,616,762,810 (1924).

4

Optical Emission Spectroscopy and

ActinometryActinometry

5

THE SECOND PLASMA OPTICAL EMISSIONSPECTROSCOPY EXPERIMENT‐ 1666SPECTROSCOPY EXPERIMENT 1666

Isaac Newton – CambridgeUniversityUniversity

The first plasma optical emission experimentwas less than a total success:

6

Optical Emission Spectroscopy

O ti l i i t (OES) f th t id l d di ti• Optical emission spectroscopy (OES), one of the most widely used diagnostictechnique in plasma processing, was first used in plasma etching byHarshbarger, et al. in 1977 to study a CF4/O2 plasma during Si etching in parallelplate plasma reactor.p p

• They identified F, O, Si and CO emissions and showed that F and Si emissionexhibited a maximum as a function of O2 addition to CF4.

• Optical emission in etching plasmas is a mostly from electron-impact excitation.

• Most atomic and diatomic species can be monitored by OES.

• Some triatomic molecules (e.g. CF2, SiCl2, NH2 and CO2+) can too.

• Emission from larger molecules is either lacking or broad and featureless• Emission from larger molecules is either lacking, or broad and featureless.

• Because of the complexity of the excitation mechanism, OES is usuallyqualitative, which is OK for endpoint detection, but makes it difficult (but notimpossible) to derive relative and absolute species number densities.

7

“A Study of the Optical Emission S udy o e Op c ss ofrom an rf Plasma during Semiconductor Etching”, Applied Optics, 31, 201 (1977). W. R. HARSHBARAR R A PORTER T AHARSHBARAR, R. A. PORTER, T. A. MILLER, and P. NORTON Bell Telephone Laboratories Inc. Allentown, PA and Murray Hill, NJ

CF4/O2 plasma

8

Typical optical emission spectra of a chlorine plasma during fast etching of Si and slow etching of SiO2

• Spectra are dominated by emission from Cl.

• With Si present and the substrate stage RF‐bias strong Si SiCl SiCl2 andWith Si present and the substrate stage RF bias, strong Si, SiCl, SiCl2, and SiCl3 (and/or SiCl3+) emissions are also observed.

• Emission from Cl2 is also apparent in the spectrum recorded during slow h f

16

18

20Cl2 plasma, Si etching

SiClCl

SiCl3SiCl2

etching of SiO2.

8

10

12

14

TEN

SIT

Y

2

4

6

8Cl2 plasma, SiO2 etching

Cl2Cl2

Si Si

INT

200 300 400 500 600 700 800 9000

WAVELENGTH (nm)9

Actinometry

• Assume excited state (k) of species X is populated solely by e-impact from its ground state (i).

Th it b l t d t t b d it ( ) i l t d t th i t it

dvvfvvbQnaI )()()(4 3

• Then its absolute ground state number density (nX) is related to the intensity (IX,i,j,k) of emission at wavelength j,k accompanying the transition Xk Xj by

dvvfvvbQnaI eXkjAkAXkjAXkjiX )()()(40

,,,,,,,,

a(,j,k) = spectrometer sensitivity at A,j,k( ) ti t l t d f X X

)( 11 PkQ

X,i,k(v) = cross section at electron speed v for e + Xi Xk + efe(v) is the electron speed distribution function (4v2fe(v)dv is thenumber of electrons with speeds between v and v+dv)

is the quantum yield for emission by X where and)( PkQ qk

kkj XXkjA iib

,,,

is the quantum yield for emission by Xk where andkq are the radiative lifetime and quenching rate constant for Ak by allspecies at total pressure P

is the branching ratio for the transition Xk Xj

10

Actinometry (cont.)

Th l d di ib i d h i li• The electron speed distribution and the proportionality constant aredifficult to determine.

• Consequently rare gas actinometry is often used to convert emissionConsequently rare gas actinometry is often used to convert emissionintensities into quantitative, relative number densities.

• Technique was first applied in plasmas by Coburn and Chen.

• Add nA amount of a rare gas, A, with an excited state Ak at energy closeto that of Xk.

• Assuming that rare gas emissions are caused solely by e-impactexcitation of the ground state, then it emission intensity is

dvvfvvbQnaI eAkjAkAAkjAAkjiA i)()()(4

0

3,,,,,,,,

11

Actinometry (cont.)

• If A(v) X(v) at any v, then nX can be simply expressed as

)/( ,,,,,,, kjiAkjiXAAXX IInan

where aX,A is a proportionality constant.

• Relative densities of atoms (F, Cl, H, O) and small moleculesRelative densities of atoms (F, Cl, H, O) and small molecules(Cl2, CF, CF2, BCl) have been determined by this method.

• In a few cases, absolute number densities have also beend th h l lib ti th dmeasured through several calibration methods.

• Lets look at an example.

12

Transformer‐Coupled Plasma (TCP) Reactor + OES

TCPAntennaMatching

network

Quartz window

Feed Gas: Cl2 + 5%He/Ne/Ar/Kr/Xe

Pump

Oxidized Si wafer, electrically floating stage, quartz and stainless steel walls

OpticalEmission

Spectrometer

#13

Oxidized Si wafer, electrically floating stage, quartz and stainless steel walls(TRG‐OES)

Electron‐Impact Excitation Emission Processes

eVEesCleCl thgg 4.8)4( 122

ÅhClCl 3060)()4( 11

Cl2 emission:

ÅhClsCl ugg 3060,)()4( 12

12

eVEeJPpCleCl h 5910)2/34( 02

Cl emission:

ÅhJPClJPCl

eVEeJPpClCleCl

eVEeJPpCleCl

th

th

7924)2/14()2/34(

0.13)2/3,4(

59.10)2/3,4(

402

022

eVEpXeeSXe 949)2()1( 0

ÅhJPsClJPpCl 7924,)2/1,4()2/3,4( 402

Xe emission:

ÅhXX

eVEpXeesXe

eVEpXeeSXe

th

th

8280)1()2(

5.1)2()1(

94.9)2()1(

55,3

5

#14

ÅhsXeepXe 8280,)1()2( 55

OES Emission Intensities: 1mTorr Cl2/5% rare gases plasma

1 Xe 8280 ÅCl 7924 Å

0.1

Cl 7924 Å Cl2 3060 Å

nits

)

0.01

y (a

rb. u

n

1E-3

Inte

nsity

1E-4

#15

10 100 1000

Power (W)

CORRECTION FACTOR FOR EFFECT OF Xe METASTABLES ON ACTINOMETRYPROPORTIONALITY CONSTANT a

1.6

PROPORTIONALITY CONSTANT aX,A

1.4

1.5 1 mTorr 2 mTorr 5 mTorr 10 mTorr

1.3

20 mTorr

,Te,l

,p)

1.1

1.2

a'(n

e,

1.0

#16

10 100 1000

Power (W)

Absolute Cl2/Xe and Cl/Xe (at high power)

Number Density Ratios

100

dashed line:Cl calculated from Cl2 and mass balance

t l tiCl

Cl21 mTorr

wer

)

extrapolationto zero poweryields absolutecalibration ofCl2/Xe (a

t hig

h po

w

10

t d f di i ti2/nX

e, nC

l/nX

e (

10 100 1000

uncorrected for dissociativeexcitation from Cl2n C

l 2

Power (W)

#17

Cl and Cl2 Number Densities in a Cl2 Plasma

0.1

1 nCl

nCl21 mTorr

10

• nCl > nCl2 at high power andall pressures

hi h d

0 1

1

10

2 mTorr

-3)

• nCl >> nCl2 at high power andlow pressure

• The nCl = nCl2 power0.1

1

10

5 mTorr

n Cl2 (1

013cm

- Cl Cl2 pincreases with increasingpressure

1

10

10 mTorr

n Cl,

n

20 mTorr10

100

#18

20 mTorr10 100 1000

1

Power (W)

Actinometry for [O] measurement

• O (844.6nm) to Ar (750.4 nm)emission intensity ratio used tod t i O t d it [1]

1E-9

1E-8 a) O(1S) + e -> O(3p3P) + e (dir.)

determine O atom density [1].

1E-12

1E-11

1E-10

O(1D) + e -> O(3p3P) + e (dir. + cas.)

O + e -> O(3p3P) + e (dir.+cas.)3

k (c

m3 s-1

)

• Use e-impact excitation cross

K: rate coefficient I: emission intensity

b: branching ratio : spectral response

1000

10000

1E-13

b)O(1S) + e -> O(3p3P) + e (dir.)

O + e -> O(3p3P) + e (dir.)

Ar + e -> Ar(2p1) + eO2 + e -> O + O(3p3P) + e

psections for O and Ar to compute KO, KAr. 10

100O(1D) + e -> O(3p3P) + e (dir. + cas.)

O*)

/k(A

r)

• Use direct + cascade: optically thick.

U di t ti ll thi 1 2 3 4 5 6 7 80.01

0.1

1 O + e -> O(3p3P) + e (dir.+cas.) O + e -> O(3p3P) + e (dir.)

O2 + e -> O + O(3p3P) + e

k(O

• Use direct: optically thin.

[1] N. C. M. Fuller, et al. PSST 9, 116 (2000)

1 2 3 4 5 6 7 8

Te(eV)

Plasma‐Surface Interactions

20

REQUIREMENTS FOR PLASMA‐SURFACE DIAGNOSTIC METHOD

• The methods should provide as many as possible of the following:

‐ elemental analysis of the near‐surface layery y‐ chemical identification (bonding, stoichiometry, etc.)‐ adsorbate coverages (relative and absolute)‐ sticking coefficientsrecombination probabilities‐ recombination probabilities

‐ reaction probabilities‐ branching ratios‐ activation energies‐ pre‐exponential factors‐ reaction orders

• The method should also be:• The method should also be:

‐ non‐perturbing‐ not perturbed by the plasma‐ capable of providing time‐dependent measurements‐ easy to interpret

Some of the Techniques Applied to Plasma Surface Interactions

• Ellipsometry (single wavelength and spectroscopic)

• Infrared spectroscopy

• x‐ray photoelectron spectroscopy (XPS) with vacuum sample transfer

• Laser desorptionLaser desorption

• Spinning wall

22

Ellipsometry

• Yields n and k, the real and imaginary parts of the complex index of refraction of the film, respectively, but no chemical information.

• Spectroscopic ellipsometry (SE) in the ultraviolet visible region combined with• Spectroscopic ellipsometry (SE) in the ultraviolet‐visible region, combined withmodeling, can be used to distinguish crystalline and amorphous material in the film as well as sense the presence of voids.

• In situ SE has been used to probe surfaces and thin films during sputter‐etching of Si,54 etching of Si in a chlorine plasma,55 plasma oxidation of Si,56 and PE‐CVD ofamorphous silicon.57

23

Refs from: “Critical review: Plasma-surface reactions and the spinning wall method”, V. M. Donnelly, J.Guha, and L. Stafford, J. Vac. Sci. Technol. A, 29, 010801-1 (2011).

Example of SE Probing of Si Etching in a Chlorine Plasma

“Cl2 Plasma Etching of Si(100): Damaged Surface Layer Studied by In Situ Spectroscopic Ellipsometry”, N L di V M D ll J T C L d F P Kl J V S i T h l A 15 604 (1997)

24

N. Layadi, V. M. Donnelly, J. T. C. Lee, and F. P. Klemens, J. Vac. Sci. Technol. A. 15, 604 (1997).

Spectroscopic Ellipsometry as a Diagnostic Method


‐ elemental analysis of the near‐surface layer: NOh i l id tifi ti (b di t i hi t t ) NO‐ chemical identification (bonding, stoichiometry, etc.): NO

‐ adsorbate coverages (relative and absolute): YES* ‐ sticking coefficients: NO ‐ recombination probabilities: NO p‐ reaction probabilities: NO ‐ branching ratios: NO ‐ activation energies: NO pre exponential factors: NO‐ pre‐exponential factors: NO

‐ reaction orders: NO

• The method should also be:

‐ non‐perturbing: YES‐ not perturbed by the plasma: YES ‐ capable of providing time‐dependent measurements YES‐ capable of providing time‐dependent measurements YES ‐ easy to interpret : NO

Infrared Absorption (IR) Spectroscopy

• Measures absorption of infrared light as a function of wavelength due to excitation of vibrations and rotations of species in the beam path.

• Scanning IR spectroscopy with a grating spectrometer takes too long hence the• Scanning IR spectroscopy with a grating spectrometer takes too long, hence the time‐dependent intensity of “white‐light” IR as a function interferometer plate separation is measured and Fourier transformed from the time to frequency domain.

• This FT‐IR signal is still very weak for a thin (i.e. ~monolayer) film, hence a multipass arrangement is needed.

• Total internal reflection (TIR) FTIR the preferred method is limited to IR‐• Total internal reflection (TIR) FTIR, the preferred method is limited to IR‐transparent substrates (e.g. GaAs)

• Ullal et al. used TIR‐FTIR to characterize the buildup of SiO and SiOCl groups on GaAs during etching of Si in a chlorine ICP as well as to monitor its removal in a SF6 plasma,17, 50, 51 to detect SiHx during plasma‐assisted deposition of hydrogenated amorphous Si in SiH4 plasmas.52

26


TIR‐FTIR Setup on ICP

Aydil and co‐workers 17, 50, 51, 52

27


TIR‐FTIR as a Diagnostic Method


‐ elemental analysis of the near‐surface layer: YES (but limited)h i l id tifi ti (b di t i hi t t ) YES (b t li it d)‐ chemical identification (bonding, stoichiometry, etc.): YES (but limited)

‐ adsorbate coverages (relative and absolute): YES ‐ sticking coefficients: NO ‐ recombination probabilities: NO p‐ reaction probabilities: NO ‐ branching ratios: NO ‐ activation energies: NO pre exponential factors: NO‐ pre‐exponential factors: NO



‐ non‐perturbing: YES‐ not perturbed by the plasma: YES ‐ capable of providing time‐dependent measurements YES ( but long times)‐ capable of providing time‐dependent measurements YES ( but long times)‐ easy to interpret : MAYBE

Laser-desorption, laser-induced fluorescence and plasma induced emission

• In-situ technique for probing surfaces during etching.

1) Si etching in a Cl2 plasma has a chlorinated (SiClx) surface layer.2 x

2) The surface is irradiated by a 15 ns long, XeCl (308 nm) excimer laser pulse that begins to heat the surface.

3) Later in the pulse, the surface temperature continues to rise and Si products (e.g. SiCl and SiCl2) desorb. The latter portion of the laser pulse excites SiCllaser-induced fluorescence very close to the surface.

Laser‐induced heatingSiCl desorptionPlasma1) 3)

4) Later, and far from the surface, e-impact excites emission from products.

excimer laser pulse

SiCl desorptionSiCl excitation (by laser,close to the substrate)

SiCl excitation by plasma(l d f f h

SiClxlayerSi

1)

2)

3)

4)

29

excimer laser pulse (later and far from thesubstrate)

) 4)

Laser-desorption, laser-induced fluorescence (LD-LIF) spectra of SiCl during p , ( ) p gCl2 plasma etching of Si(100)

Bottom trace: LD-LIF spectrum with no plasma, due to laser-induced etching. Upper trace: with plasma

8

r=3/2 21 v' = 0SiCl (B2+ -> X2r)

UN

ITS

)Upper trace: with plasma.

4

6r 3/2r=1/2

1 2 v" = 0

plasma on

TY (A

RB

. U

2

4

no plasma

F IN

TEN

SIT

2800 2850 2900 2950 3000 30500S

iCl L

IF

30

WAVELENGTH (Å)

Laser-desorption, plasma-induced emission (LD-PIE) spectrum, along with the plasma induced emission (PIE) spectrum without laser irradiation, during

20Si

Si

etching of Si(100) in a Cl2 plasma.

15 LD-PIE

SiCl (B' 2 -> X2r)

. UN

ITS)

10

ClSITY

(AR

B.

5

PIE

Cl2

INTE

NS

2500 2600 2700 2800 29000

WAVELENGTH (Å)

31

• NOTE: The absence of a Cl2 band in the LD-PIE spectrum means that Cl2 is not present on the surface.

Laser-desorption, laser-induced fluorescence (LD-LIF) measurements of SiCl(proportional to Cl coverage) during Cl plasma etching of Si(100)

2.5 1.25

LD-LIF MEASUREMENT OF Cl COVERAGE DURING AND AFTER ETCHING OF Si(100) IN A Cl2 PLASMA

Cl2 plasma etching (steady-state)

(proportional to Cl coverage) during Cl2 plasma etching of Si(100)

1.5

2.0

0.75

1.00

(1015

cm-2)

plasma offplasma on desorption:

2B

. UN

ITS

)

0.5

1.0

0.25

0.50

l CO

VE

RA

GE

(plasma on

Cl2/laser-etching( t d t t )

after Cl2

p after Cl2 plasma

Cl /laser-etchingNTE

NS

ITY

(AR

B

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0 0.00

Cl (steady-state)

100 laser pulses

Cl2/laser etching (steady-state)

IN

TIME (min)cl2_066d

• Cl coverage doubles when the plasma is on (Cl and Cl2 chlorinate surface with plasma on but only Cl2 does with plasma off)• Maybe the roughly doubling of the ion assisted yield with Cl compared to Cl is due to

32

• Maybe the roughly doubling of the ion‐assisted yield with Cl, compared to Cl2, is due to Cl chlorination roughly doubling the Cl‐content of the layer.• The chlorinated layer does not desorb in the absence of a plasma or laser irradiation

SiCl LD-LIF signal (i.e. Cl coverage) vs. laser repetition rate

• At low pressure the laser removes the layer at a rate that is too fast for the• At low pressure, the laser removes the layer at a rate that is too fast for theplasma to keep up with.• The lines are a simple Langmuir-Hinshelwood model of the chlorination of thesurface from all sources of Cl as a function of laser repetition rate. Suggest Cl

1 2

1.4

1.2

1.4

S)

sticking coefficient of ~0.1.

0 8

1.0

1.2

0.8

1.0

1.2

P=2.1 mTorr Cl2

E (X

1015

)

Y (A

RB

. UN

ITS

0 4

0.6

0.8

0.4

0.6

0.8

CO

VE

RA

GE

F IN

TEN

SIT

Y

0.0

0.2

0.4

0.0

0.2

Cl

P=0.6 mTorr Cl2

SiC

l LD

-LIF

33

0 10 20 30 40 50 60 70 800.0

LASER REPETITION RATE (Hz)

Th th d h ld id ibl f th f ll i

LD‐LIF/LD‐PIE as a Diagnostic Method


‐ elemental analysis of the near‐surface layer: YES (but limited, especially LIF)‐ chemical identification (bonding, stoichiometry, etc.): MAYBE QUALITATIVE ( g y )‐ adsorbate coverages (relative and absolute): YES (but limited) ‐ sticking coefficients: ‐ recombination probabilities: reaction probabilities:‐ reaction probabilities:

‐ branching ratios: YES (but requires special conditions andother diagnostics)

‐ activation energies: ‐ pre‐exponential factors: ‐ reaction orders:

• The method should also be:• The method should also be:

‐ non‐perturbing: YES (up until the instant when it may melt the surface)‐ not perturbed by the plasma: YES (up until the instant when it may melt

the surface) ‐ capable of providing time‐dependent measurements: YES‐ easy to interpret : MAYBE

X‐ray photoelectron spectroscopy with vacuum sample transfer

• Transfer sample to XPS analysis chamber after plasma‐surface exposure.

35

XPS with Vacuum Transfer as a Diagnostic Method


‐ elemental analysis of the near‐surface layer: YES (only H not detectable)h i l id tifi ti (b di t i hi t t ) YES‐ chemical identification (bonding, stoichiometry, etc.): YES

‐ adsorbate coverages (relative and absolute): YES ‐ sticking coefficients: NO ‐ recombination probabilities: NO p‐ reaction probabilities: NO ‐ branching ratios: NO ‐ activation energies: NO pre exponential factors: NO‐ pre‐exponential factors: NO



‐ non‐perturbing: YES‐ not perturbed by the plasma: YES ‐ capable of providing time‐dependent measurements NO (at t )‐ capable of providing time‐dependent measurements NO (at t )‐ easy to interpret : YES

Low resolution spectra after Si etching in chlorine ICP

• Si and Cl 2s and 2p core level featuresp

• Plasmon losses associated with both the Si and Cl indicate that the surface layer contains the equivalent of a couple of monolayers of chlorine.

25000 Si(2p)99.4 eV

Si(2s)150.6 eV

Cl(2p)199.2 eV

Cl(2s)269 9 eV

15000

20000 = 20°269.9 eV

30°

rb. u

nits

)

10000

15000

60°

45°

nten

sity

(ar

0

500085°

75°I

37

300 250 200 150 1000

siclreg3.opjG0223004

Figure 2Bogart et. alBinding Energy (eV)

High resolution spectra after Si etching in chlorine ICPSi(2p3/2) core level spectrum with 2p1/2 removed

• Strong peak (off scale h ) i f d l i

30

10 0 sccm Cl

here) is from underlying Si(100) substrate.

• SiCl, SiCl2 and SiCl320

SiCl

SiClSi

10.0 sccm Cl2 = 30°

nts), 2 3

binding energy shifts in excellent agreement with published values. 10

SiCl O ( )

SiCl3

SiCl2

Si·

ensi

ty (c

ou

• A small feature at low binding energy believed to be Si with a dangling bond

0

SiCl3O (none)Inte

and 3 Si-Si bonds. 97 98 99 100 101 102 103 104 105Binding Energy (eV)

38

ANGLE-RESOLVED X-RAY PHOTOELECTRON SPECTROSCOPY (XPS)

Some length scales: - x-ray penetration: 17,000 A into Si

•

• Angle-resolved measurements provide a depth resolutionof ~6Å

- electron escape depth (): 22 A for Si

of ~6Å

electron energyanalyzer x-raysx-rays

e's e

more bulk sensitive

d = sin d

ore

surfa

ceen

sitiv

e

d

39

more bulk sensitive

mo

se

Model of XPS Signal Dependence on Silicon Chl id S iChloride Species

• Model • Model dependence of XPS signals100

Si(bulk)

0.4 sccm Cl2

• Invert signal dependence

10SiClSiCl

Si(bulk)

nten

sity

dependence to derive depth profile for each 1

Si·

SiCl O

SiCl3

SiCl2In

component0 10 20 30 40 50 60 70 80 90

Figure 8

SiCl3O

peakfitter05.opjAG0619003 (°)g

Bogart et. al

Depth Profiles: Silicon Chloride Speciesp p

• SiClx ~ 16 Å

0.25

SiCl 0 8

1.0

• SiCl>SiCl2>SiCl3>SiCl3O depth and concentration0 15

0.20SiCl

atio

n

0.4

0.6

0.8

Si

ve C

once

ntra

tion

concentration

• Si∙deep into layer0.10

0.15

SiCl2

Con

cent

ra

0 5 10 15 20 25 30 35 40 45 50

0.0

0.2 SiClxRel

ativ

• Some Si near the surface?

0.05

10.0 sccm 5.0 sccm 2.5 sccm1 0 sccmSi·

SiCl3

Rel

ativ

e Depth (Å)

-5 0 5 10 15 20 25 30 35 40 45 50

0.00

1.0 sccm 0.5 sccmSiCl3O

Depth (Å)

Kinetic Molecular Dynamics Simulations by Graves and Co-workers

• Si(100) surface• Bombardment with 50 eV Ar+ created a disordered but dense damaged layer• Bombardment with 50 eV Cl+ created a disordered, roughened layer with sub-

f id S h l i bi di i f Cl Cl2 d i

a) Ar + bombardment b) Cl + bombardment

surface voids. Such a layer contains more binding sites for Cl, Cl2 adsorption

Thi k iThickness in reasonable agreement

with i t

10 Å

experiment

42

Cl Coverage vs. Ion Energy

3.5

4.0Barker Cl2/Ar+ beams

2 5

3.0

(1015

cm-2)

Coburn Cl2/Ar+ beams

2.0

2.5

Cov

erag

e (

ICP Data

1 0

1.5

Coburn Cl2 beam

Cl C

0 5 10 15 20 25 301.0

Eion1/2(eV1/2)

• Cl coverage increases with increasing ion energy but saturates at

43

Cl coverage increases with increasing ion energy, but saturates at high Ar+ energy.

HBr/Cl2-containing plasmas

0.35

0.40

0.45

0.50 DURING ETCHING

Cl + Br

rmal

ized

)

• Si takes up Br and Cl in proportion to the HBr:Cl2 feed gas ratio, confirmed later by Sawin and co workers

0 10

0.15

0.20

0.25

0.30 Cl

Br

F S

IGN

AL

(norco-workers.

• Neither study was capable of detecting H, so falloff in total

0 45

0.50 AFTER ETCHING

0.00

0.05

0.10

LD-L

Ihalogen coverage with HBraddition could be due to adsorbed H blocking sites for Cl or Br adsorption It could also be from

0.25

0.30

0.35

0.40

0.45

Cl + Br

Clrela

tive

to S

i)

adsorption. It could also be from steric hindrance.

• Etching rate fall-off with HBrdditi t Cl h b tt ib t d

0.05

0.10

0.15

0.20

0.25

Br

XP

S S

IGN

AL

(raddition to Cl2 has been attributed to:

- reduced adsorbed halogencoverage, and/or

44

0 10 20 30 40 50 60 70 80 90 100

0.00

X

%HBr in FEED GAS (balance is Cl2)

co e age, a d/o- lower ion flux in HBr plasmas

Spinning Wall Method

• A portion of the plasma wall is rotated so that it is exposed to the plasma and then to an analysis method.

• The method currently has been demonstrated with cylindrical substates and is• The method currently has been demonstrated with cylindrical substates and is primarily for studying plasma‐wall interactions and not flat substrates such as silicon wafers.

45

WHY ARE PLASMA-WALL INTERACTIONS IMPORTANT?

2O2Cl Cl + SiCl SiCl 2O

O2

2Cl

Cl2SiCl SiCl

+ SiCle

Cl + SiCl SiCl2

SiCl2 SiCl4

• Neutral reactions on surfaces usually dominate those in the gas phase, especiallyat low pressure.

• Recombination of feed gases

• Product transport to walls

46

• Product formation

WHY ARE PLASMA-WALL INTERACTIONS IMPORTANT?

• Species number densities are determined by the balance betweenformation and loss reactions.

• Species are formed by association reactions of smaller radicals on thewall of the plasma chamber.

• Loss processes include wall reactions such as recombinationLoss processes include wall reactions, such as recombination.

• At the low pressures (~1-100 mTorr) of many plasma etching processes,three-body (M) gas-phase association reactions A + B + M AB + M are

lvery slow.

• On the other hand, diffusion becomes faster at low pressure, andsurfaces provide an efficient “third body” for association reactions.su aces p o de a e c e t t d body o assoc at o eact o s

• Therefore, many important reactions in plasma etching occur on the wallsof the chamber, or on the wafer.

47

INDUCTIVELY‐COUPLED PLASMA AND SPINNING WALL

Feed gases

differentialdifferentialpumping

mass spectrometer

tuning forkchopper

electro‐magnet

Langmuirprobe

differentialpumping

differentialOES i 1 C

anodized Al reactor

pumping pumpingOES point 2: EDGE

OES point 1: Center

“SPINNING WALL” Method for Studying Plasma-SurfaceInteractions

topumpp p

plasma

to differentially-pumped mass spectrometer, or 10-3P

Auger Electron spectrometer

high-

10-6P pres.=P

spinning cylinder surface exposed

high-speed motor

pto plasma

VIEW FROM PLASMA CHAMBER

SIDE VIEW FROM FIRST DIFFERENTIALLY PUMPED CHAMBER

10 m

Anodized Al

Studies of Si Etching and Product Deposition on Walls

Plasma density: 1 x 1011 cm-3

Feed gases

• Plasma density: ~1 x 1011 cm-3

• Erosion of quartz discharge tube leads to trace O in plasma• Si wafer etched in Cl plasma: g

ICP coil

Quartz di h

• Si wafer etched in Cl2 plasma:2000 A/min with rf bias (110 Vdc)30 A/min with rf bias

coil

S i i ll

discharge tube

chopperSpinning wall

Si Wafer

MassSpectrometerSpectrometer(Hiden EQS)

Mass Spectra of Products Desorbing from the Spinning Wall in a Cl2 ICP with and without a Si Substrate

Mass specPlasma chamber

2

Spinning wall(19000 rpm)

3.0

3.5

SiCl 0.30

0.35

Si2OCl3

1.5

2.0

2.5

Si OClSi2Cl2 SiCl4

SiCl3SiCl2

Cl

arb.

uni

ts)

Bias on

0.15

0.20

0.25

Si2OCl4Si3O2Cl5 Si Cl

Si3O2Cl7Si3OCl7

Si3Cl7Si3O3Cl5Si3Cl6

Si2OCl5Si2Cl5

(arb

. uni

ts)

Si2Cl4Bias on

0 0

0.5

1.0

5 Si2OCl Si2Cl3Cl2

Sign

al (a

ICP on0.00

0.05

0.103 2 5

Si3OCl5Si4Cl8

Sign

al

ICP on

60 70 80 90 100 110 120 130 140 150 160 170

0.0

m/e175 200 225 250 275 300 325 350 375 400

m/eBias = –108 VDC, Cl2 pressure = 2.5 mTorr, ICP power = 400 W, MS electron energy = 70 eV

Mass Spectrometer Signals as a Function of Time After Extinguishing Just Bias, or ICP and Bias

a) SiClICP, bias on b) SiClICP, bias on 1.2 c) SiClICP, bias on

g g

1.0

1.5

2.0

2.5

ICP off, bias off

ICP off, bias off

l (ar

b. u

nits

)

a) SiCl

ICP on, bias off

0.10

0.15

0.20

0.25

ICP off,Bias off

(arb

. uni

ts)

b) SiCl2ICP on,Bias off

ICP off, Bias off

,

0.6

0.8

1.0

ICP off,Bias off

l (ar

b. u

nits

)

c) SiCl3,

ICP on,Bias off

ICP off,Bias off

0 2 4 6 8 10 120.0

0.5

1.0 bias off

Sign

al

Time (min)0 2 4 6 8 10

0.00

0.05

Sign

al

Time (min) 0 2 4 6 8 10 12 14 16 180.0

0.2

0.4

Sign

al

Time (min)

0.2

0.3

0.4

ICP on,Bias off

ICP, bias on e) Si2OCl3

rb. u

nits

)

0.3

0.4

0.5

rb. u

nits

)

d) SiCl4ICP, bias on

ICP off,

ICP on,Bias off

ICP off,Bias off 0.15

0.20

0.25

0.30

ICP on,Bias off

ICP, bias on f) Si2OCl5

rb. u

nits

)

0 2 4 6 8 100.0

0.1

ICP off, Bias off

Bias off

Sign

al (a

Time (min)0 2 4 6 8 10 12

0.0

0.1

0.2

Sign

al (a

r ICP off, Bias off

Bias off

0 2 4 6 8 100.00

0.05

0.10ICP off, Bias offSi

gnal

(ar

Time(min)Time (min) Time (min) Time (min)

18 1 5

2.0

Bias on,nits

)

I(SiCl)/I(Xe) desorbed SiCl (m/e = 63)

c) SiClOptical Emission of Cl2 ICP with andwithout Si substrate bias

12141618

0.2

0.3 Cl2

SiCl

nsity

(a.u

.)

SiSini

ts) 0.5

1.0

1.5

Bias off,ICP on

ICP on

Sign

al (a

rb. u

n

468

10

2800 2900 3000 3100 3200 3300

0.0

0.1

SiCl

Emis

sion

Inte

n

Wavelength (A)SiClSiCligna

l (ar

b. u SiCl

Bias on Bias off

2 4 6 8 10 120.0

Time (min)

0.15

0.20

Bias on,

I(SiCl2)/I(Xe) desorbed SiCl2 (m/e = 98)

ts)

b) SiCl2

3000 3500 4000024 Wavelength (A)SiCl3

SiCl2Si

Wavelength(A)0.05

0.10

,ICP on

Bias off,Sign

al (a

rb. u

nit

2.5 mTorr400 WWavelength (A)

2 4 6 8 10 120.00

ICP on

Time (min)

1.6

2.0

Bi

I(SiCl3/I(Xe) desorbed SiCl3 (m/e = 133)

ts)

a) SiCl3

3 0

3.5

ICP on

Bias off,ICP on

ty (1

013 c

m-3)

Bias on400 W-108 VDC

0.4

0.8

1.2

Bias onICP on

Sign

al (a

rb. u

nit

Bias off,ICP2 0

2.5

3.0

umbe

r Den

sit

determined by OESactinometry with Xe

2 4 6 8 10 120.0

0.4

Time (min)

ICP on

0 2 4 6 8 10 12

2.0

Cl N

u

Time (min)

Three Surfaces were Prepared and Reactions Studied:

1. Lightly oxidized: Cl2 ICP, bias on Si substrate, 10 min; thenCl2 ICP, no bias on Si substrate, 60 min.

2. Oxidized: Cl2/5%O2 ICP, bias on Si substrate, 10 min; thenCl2/5%O2 ICP no bias on Si substrate 60 min.Cl2/5%O2 ICP, no bias on Si substrate, 60 min.

3. O2 plasma‐treated: oxidized surface in (2); then O2 ICP, 60 min;then Cl2/5%O2 ICP, no bias on Si substrate, 30 min.

20

25 O2 plasma-treatedSiO Cl

5

10

15oxidized

I/dE)

Ag lightly oxidized

AES e’s

AESe‐beam

plasma

-10

-5

0E(d g lightly-oxidized

high‐speed motor

0 500 1000 1500-15

Kinetic Energy (eV)

Cl2 MS and Pressure Rise Signals as a Function of Wall Rotation frequency

4

6

rr) a) 100% Cl2 (lightly-oxidized)

Wall Rotation frequency

60

2

4

(10-8

To

2

4

6

b) Cl2 / 5% O2(oxidized)

re R

ise

0

4

6

Pres

su

c) Cl2 / 5% O2

0

2

302520155 10

Cl 2

(O2 plasma-treated)

0 302520155 10rpm (103)

0

Extracting Cl L-H Recombination Probabilities 10

Cl2 plasma

Cl atoms

O l t t d fm-2

s-1 )

(L-H) Cl2

1

O2 plasma-treated surface

x 10

14 c

m

Mass Spec.P2

0.1

re

cf

(

oxidized surface

• As desorption rate rotation rate, mass spec. and P2 values their avarage values.

0 5 10 15

Reaction time, tr = 1/2f (ms)

fD 6

• Therefore as f (i.e. t 0) it is as though the sample were continuously exposed to a Cl flux of 1/3 that in the plasma, Cl.

Cl

fCl • Therefore LH recombination probability,

Cl Langmuir‐Hinshelwood Recombination Coefficients and O‐atom Near‐Surface Concentrations

65

(%

)0 07

0.08

ent, C

l

505560

trat

ion

(

0 05

0.06

0.07

oeffi

cie

354045

co

ncen

t

0.03

0.04

0.05

na

tion

co

253035

urfa

ce c

Stainless 0.01

0.02

0.03

com

bin

20

O2 plasma-treated

oxidized

O s

u

AnodizedAluminum

Stainless Steel

lightlyoxidized

0.00

Cl r

ec

treated Aluminumoxidized

Why does Cl Increase with Increasing O Coverage?

• O‐depleted conditions: every O bound to 2 Si.

Cl

Si

O

Si+ Cl

Si

O

Si• O‐rich conditions: occasionally O cannot find a second Si to bond to.

Si

Cl

Si

O

SiSi

O

Si

O

Si

+ Cl

Si

O

SiSi

O

Si

O

Si

OOO+ Cl

ClOOO

+ Cl2(g)Si SiSiSi Si Si SiSiSi Si

25

Cl2 MASS SPECTROMETER SIGNALS: PLASMA ON OR OFF

20

25

600 W

5 mTorr Cl2

units

)J. Guha, V. M. Donnelly, Y‐K. Pu, J. Appl. Phys. 103, 013306 (2008);

10

15

. Sig

nal (

arb.

5

10 100 W

l 2 Mas

s S

pec.

0 5 10 15 20 25 30 350

0 WC

Rotation Frequency (103rpm)Rotation Frequency (10 rpm)

• Plasma‐ON signals are a result of desorption of Cl2 formed by recombination of Cl on the spinning wall surface.

• Plasma‐OFF signal is a result of desorption of physisorbed Cl2.

Cl Atom LH Recombination Probabilities on Anodized Al as a Function of Cl‐to‐Cl2 Number Density Ratio

0.1

1.25mT 5mT 10mT 20mT

t ( C

l)co

effic

ient

m

bina

tion

c

0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8

0.01

Rec

om

• Cl scales with Cl‐to‐Cl2 flux ratio.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

nCl/nCl2

• Suggests Cl2 may block sites for Cl adsorption and recombination.• See J. Guha, V. M. Donnelly, Y‐K. Pu, J. Appl. Phys. 103, 013306 (2008); L. Stafford, R. Khare, J. Guha, V. M. Donnelly, J‐S. Poirier and J. Margot, J. Phys. D, Appl Phys. 42, 055206 (2009).

Proposed Site Blocking Mechanism for Cl Heterogeneous Recombination in Cl2 PlasmasRecombination in Cl2 Plasmas

High Cl2/Cl density

Low Cl2/Cl density

h h d h ld d bl f h f ll

Spinning Wall Method


‐ elemental analysis of the near‐surface layer: YES (only H not detectable)‐ chemical identification (bonding, stoichiometry, etc.): SOME (with Auger) ( g, y, ) ( g )‐ adsorbate coverages (relative and absolute): YES ‐ sticking coefficients: NO ‐ recombination probabilities: YES

ti b biliti YES (b t i th di i ti )‐ reaction probabilities: YES (but requires other diagnistics) ‐ branching ratios: YES ‐ activation energies: SHOULD BE (not done yet) ‐ pre‐exponential factors: MAYBE ‐ reaction orders: MAYBE


‐ non‐perturbing: YES‐ not perturbed by the plasma: YES ‐ capable of providing time‐dependent measurements SORE OF (at t > 0.5

ms after plasma exposure)‐ easy to interpret : NO

For more details see:

“Interactions of chlorine plasmas with silicon chloride-coated reactor walls during and after silicon etching” Rohit Khare Ashutosh Srivastava and Vincent M Donnelly J Vac Sci Technol A 30etching , Rohit Khare, Ashutosh Srivastava, and Vincent M. Donnelly, J. Vac. Sci. Technol. A, 30, 051306-1 (2012).

Cl atom recombination on Silicon oxy-chloride layers deposited on chamber walls in chlorine-oxygen plasmas”, Rohit Khare, Ashutosh Srivastava, and Vincent M. Donnelly, J. Vac. Sci. Technol. A 30 051306 1 (2012)A, 30, 051306-1 (2012).

“Critical review: Plasma‐surface reactions and the spinning wall method”, V. M. Donnelly, J. Guha and L. Stafford, J. Vac. Sci. Technol. A, 29, 010801‐1 (2011).

Introduction to Plasma and Surface...

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