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Plasma Diagnostics Monday Afternoon Tutorial for UC-DISCOVERY

Major Program Award on Feature Level Compensation and Control

Eray S. AydilChemical Engineering Department

University of California Santa Barbara

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Central Problem in Plasma Etching

ion flux, J+

radical fluxes, i

ion energy, E

Internal plasma parameters

pressure, gas flow rateand composition,rf power, rf-bias power,wafer temperature

Externally controlledvariables

etch rate, anisotropyselectivity, uniformity,reproducibility

Figures of merit(process outcome)

• To understand how externally controlled variables affect the process outcome through the internal plasma parameters.

• Plasma diagnostics are experimental methods based on various electrical and spectroscopic techniques that allow the measurement of internal plasma parameters.

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Ion and etchant fluxes impinging on the wafer surface determines the etch rates and profile

evolution in plasma etching processes.+ +

Passivating oxide layer

ER = f (J+, E, i, T)

Example: SF6/O2 etching of Si

ER = f (J+, E, F, O, T)

Would like to measure or

estimate J+, E, F, O

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Langmuir Probes

I

VI=Iionsat

I=Iesat

Vf

Vp

21

)( probepionsat VVaI

ii M

eeAna

2

2

e

probepesate kT

VVeII exp

www.staldertechnologies.comwww.hidenanalytical.com

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On Wafer Ion Flux Probe Measurements

Measurement Probe

V

R

Bias Voltage

rf filter

Heavily Doped Conducting Si wafer

Probe mounted on 8” heavily doped Si wafer.

Probe biased at ~ -70 V with respect to the Si wafer

Ion current determined by measuring the voltage drop across a known resistance.

Both reference and measurement probe are isolated from ground using a floating power supply.

Plasma sees the same surfaces during etching of a wafer.

Probe and reference are etched but measurement is not affected.

SiKapton

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Ion flux measurements in SF6/O2 plasmas

0 20 40 60 80 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Ion

Curr

ent D

ensi

ty (m

A/cm

2 )

Pressure (mTorr)

I-V probe ion flux probe

Effect of TCP power and pressure50% O2, 80 sccm total flow

0

1

2

3

4

0 200 400 600 800 1000

TCP power, W

Ion

curr

ent,

mA/

cm2

5 mTorr15 mTorr

25 mTorr

Measurements were done in plasmas containing SF6, O2, HBr, Cl2, NF3 and probe worked well for extended periods of time.

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Laser Induced Fluorescence

Measuring radical concentrations in a plasma

Line of Sight Appearance

Ionization Mass Spectrometry

Optical Emission Spectroscopy with

Actinometry

UV Absorption

IR Absorption

Time,Cost,Footprint

Accuracy

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Monochromator & PMT

Optical Emission Spectroscopy

Imaging Spectrographs with

CCDs

Photodiode and narrow pass filter

Cost

Resolution, nm

Integrated spectrographs and data acquisition

$ 25 K

$ 10 K

$ 3 K

$ 0.5 K

10 1 0.1

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Optical Emission Spectra

0

1

2

3

4

5

300 400 500 600 700 800Wavelength (nm)

Opt

ical

em

issi

on (a

.u.)

Ar

F

O40 SF6/ 40 O2/ 5 Ar

735 740 745 750 755 760 765

Cl749.2 nm

Ar 751.5 nm

Ar750.4 nm

Cl2/Ar plasma

Ar plasma

Em

issi

on I

nten

sity

W avelength (nm)

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Optical Emission

*k*u

*u

k

XX

eXeXem

e

ground state

u

h

e

0

2 d)(f)(m

k exe

ex

eXex*Xem

em

eXex*X

*XemeXex*X

nnk)(Snk)(SIk

nnkn

nknnkdt

dn

0

Emission intensity depends on nx, ne and Te

Emission intensity is not a measure of X concentration

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Optical Emission Actinometry

eArAr,exArAr

eXX,exXX

nnk)(SInnk)(SI

Ar

X

Ar

X

nn

II

Ar,exAr

X,exX

k)(Sk)(S

J. Coburn and M. Chen, J. Appl. Phys. 51, 3134(1980).

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Actinometry Requirements Excitation to the emitting states of X and actinometer

(e.g., Ar) must have similar magnitude cross sections and thresholds.

ex, X ex, Ar then is a weak f(Te). ex, X ex, Ar then is f(Te) which must be determined. Emitting state must only be populated by electron impact

excitation of the ground state.

eCleCl

eClCleCl

eCleCl

*m

*2

*

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Example: Use of OES and ion flux measurements in SF6/O2 etching of Si

0

1

2

m

10 mT 25 mT 40 mT 75 mT

800W TCP/-20 V rf-bias/40 SF6/40 O2/150 sec

• Etch rate has a maximum at some intermediate pressure (~25 mTorr).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-20 V

-40 V

Etch

Rat

e (

m/m

in)

0 20 40 60 800.0

0.5

1.0

1.5

Pressure (mTorr)Io

n Fl

ux (1

016 c

m-2s-1

)0

1

2

3

4

5

6

F Density (a.u.)

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Example: Absolute measurements of Cl and Cl2 concentrations in Cl2 plasma

A number of emission lines for Cl2, Cl and Ar studied for suitability (Donnelly, et al.)

305 nm Cl2 emission (Eth = 8.4 or 9.2 eV)

822 nm Cl emission (Eth = 10.5 eV)

750.4 nm Ar emission (Eth = 13.5 eV)

The Ar emitting state has unusually low for excitation from Arm but threshold does not match the Cl2 or Cl thresholds

must be corrected for Te dependence

Donnelly, J. Vac. Sci. Technol. A 14, 1076 (1996)Malyshev, Donnelly, Kornblit, and Ciampa, J. Appl. Phys. 84, 137 (1998)Ullal, Singh, Daugherty, Vahedi and Aydil J. Vac. Sci. Technol. A 20, 1195 (2002).

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From emission intensities to absolute concentrations

0 200 400 600 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

nCl2

from actinometry n

Cl from actinometry

nCl

from mass balance

Cl o

r Cl 2 C

once

ntra

tion

( x10

14 #

/cm

-3)

TCP Power (W)

P = 10 mTorr, no wafer, Q = 100 sccm Cl2

In the limit power 0; dissociation 0 nCl2 ng = Pg/kBTg

Repeat zero-power extrapolation at different pressures to determine Cl2 (Te)

Determine Cl concentration by mass balance or

Use nCl at single point to determine Cl,Ar

0powerArCl

ArgAr,Cl II

nn

2

2

PowerHighCl

Ar

Ar

CleCl I

Inn)(T

α

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Studying the Effect of Walls on the Cl2 Dissociation

Using OES and Actinometry

0 100 200 300 400 500 600 700 8000

25

50

75

100

TCP Power(W)

% o

f und

isso

ciat

ed C

l 2

5 mTorr 10 mTorr

0 100 200 300 400 500 600 700 8000

25

50

75

100

5 mTorr 10 mTorr

% o

f und

isso

ciat

ed C

l 2

TCP Power (W)

SiO2 covered walls: low Cl sticking probability ~0.03

Alumina reactor walls: high Cl sticking probability ~1

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Mass Spectrometry

http://www.mcb.mcgill.ca/~hallett/GEP/PLecture1/MassSpe_files/image011.gif

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Line of Sight Threshold Ionization Mass Spectrometry

• Threshold ionization can be used for detecting ALL radicals in a plasma

• Density of radicals is obtained at the substrate plane

O + e O+ + 2e : 13.6 eV (E1)

O2 + e O+ + O + 2e : 19.0 eV (E2)

Principle of TIMS

• Since E1 > E2, an electron energy scan can differentiate the two products

• E2-E1 is typically equal to the bond energy of the bond that is broken during dissociative ionization

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Dissociation on the ionizer filament also produces radicals which must be distinguished from the radicals in the beam extracted from the plasma

e-

Filament(Thoria Coated Iridium)

Beam

1800 K

Ionization cage(3-5 V DC bias)

To Bessel Box

(thermal dissociationinto radical species)

Emission Control

Id

Ie

Ie

If

A

Molecules are thermally dissociated on the filament and ionized resulting in a spurious background signal.

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Heated Electrode

Turbo Pump200 l/s

4 mm

Beam Chopper

HidenQMS

to Turbo Pump 60 l/s

to Turbo Pump 900 l/s

4 mm

Plasma

1st stage

2nd stage

3rd stage

183

mm

Radical/ion beam

1 mm Sampling Orifice25-200 mTorr

~10-5 Torr

~10-7 Torr~10-9 Torr

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Beam-to-Background Ratio

• Pure O2: Beam-to-background ratio 3.2 at 25 mTorr and 2.0 at 200 mTorr.

• For radicals, the beam-to-background ratio will depend on the sticking probability of the radical.

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O atom detection in O2 plasmaO + e O+ + 2e : 13.6 eV

O2 + e O+ + O + 2e : 19.0 eVm/e = 16

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O in the beam = Signal w/chopper open – Signal w/chopper closed

O + e O+ + 2e : 13.6 eV

O2 + e O+ + O + 2e : 19.0 eV

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Quantifying the Mass Spectrometer signal

ionizere nIS

where,

S : QMS signal in c/s : product of m/e-ratio dependent factors

Ie : electron current of the ionizer

: cross-section of the ionization process

nionizer : number density of neutrals in the ionizer (nbeam+ nbackground)

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Calibration• CH4 (m/e =16) is used for calibration.

• QMS signal for CH4 is measured for a known pressure of the gas in the plasma

chamber under plasma-off condition

• CH4 calibration must be done right after the O concentration measurements to

avoid the effect of drifts in the SEM sensitivity

4

4 4

4 4

CHS

SO nn

OO

CHCH

CHCH

OO

Singh, Coburn, and Graves, JVST A 17, 2447 (1999).Singh, Coburn, and Graves, JVST A 18, 299 (2000).Agarwal, Quax, van de Sanden, Maroudas and Aydil, JVST A 22, in press (2004).Agarwal, Hoex, van de Sanden, Maroudas and Aydil, Appl. Phys. Lett, in press (2003).

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Absolute O atom concentrations in O2/Ar discharge

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Example: N2* (metastable A3u+ state) and N

concentrations in N2 plasmaN2

* + e N2+ + 2e : 9.4 eV (??)

N2 + e N2+ + 2e : 15.6 eV

•In plasma assisted MBE of GaN, N2* may be preferred over N as the nitrogen precursor.

•Can N2* be detected and absolute concentrations of N and N2* measured?

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Probable Franck-Condon Transitions

0.40

0.8 1.2 2.0r (Å)

2.8 3.6

E (e

V)

2

4

6

8

10

12

14

16

18

20

22

24

26

B u+2

A u2

X g+2

X g+1

A u+3

B g3

E ion

= 1

4.53

eV

0

5

10

15

20

0

05

510

152010

15

0

50

0

5

1015

1015

20

20

25

9.4

eVa

b

cTransition ‘a’: ~11 eV

Transition ‘b’: ~12 eV

Transition ‘c’: ~14 eV

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Absolute N2* and N Concentrations

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Summary Simultaneous with the emergence of plasma processing as an enabling

technology, a variety of plasma diagnostic methods have been developed over the last two decades to measure internal plasma properties.

Ion current probe

OES and actinometry

Line of sight threshold ionization mass spectrometry

Ease of implementation range from methods that take ~days-week to “Ph.D. lifetime.”

To save time and money the first ask “What do we want to measure and how accurately do we want to measure it?”

Measurement of radical concentrations over the wafer and ion flux impinging on its surface help in process development and improve fundamental understanding of etching and deposition processes.

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Acknowledgements

Sumit Agarwal (now @ University of Massachusetts)

Jun Belen (UCSB)

Dr. Sergi Gomez (UCSB)

Bram Hoex (now @ Eindhoven Univ. of Technology)

Guido Quax (now @ Eindhoven Univ. of Technology)

Saurabh Ullal (now Lam Research Corporation)