spectroscopic ellipsometry for in situ applications...spectroscopic ellipsometry is a useful...

© 2019 J.A. Woollam jawoollam.com 1

Greg PribilApplications Engineer

Spectroscopic Ellipsometry for In Situ Applications

Harvard University, NNCI ALD/MOCVD/MBE Symposium – October 2019


Outline

Spectroscopic Ellipsometry (SE) Introduction

In Situ SE Integration and Common Considerations

In Situ SE Examples

© 2019 J.A. Woollam www.jawoollam.com 3

Founded in 1987 by Prof. John A. Woollam based on

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© 2019 J.A. Woollam www.jawoollam.com 4

What is Polarization? Describes shape of Electric Field relative to direction of travel.

Linear: arbitrary amplitudes, in-phase Circular: equal amplitudes, 90° phase difference Elliptical: arbitrary amplitudes, arbitrary phases

X

Y

Z

wave1

wave2


Spectroscopic Ellipsometry (SE)

SE measures the change of

polarization as light interacts

with a thin film coated surface

Coherent interference occurs when

light recombines after traveling

different paths through thin film

t

2~n

1~n

0~n


What Can SE Determine?


In Situ Ellipsometry Real-time monitoring and control

Complete sample history

– Before, during and after process

Direct measurement of surfaces & interfaces

Atomic Layer

Deposition

J.N. Hilfiker “In situ spectroscopic ellipsometry

(SE) for characterization of thin film growth”,

chapter in In situ characterization of thin film

growth, G. Koster and G. Rijnders, eds.,

Woodhead Publishing: Cambridge, UK (2011)


In Situ Spectroscopic Ellipsometry Source and Receiver attach to standard window flanges on

process chamber for non-invasive, non-contact measurements.

CCD detection: Possible to measure

>1000 wavelengths from UV to NIR –

collected multiple times each second.

High Sensitivity to Thin Film properties:

Thicknesses from <1nm to 10+ microns.

Measurement is ratio, so accurate

with fluctuating intensity due to

window coating, moving sample,

etc.


In Situ Spectroscopic Ellipsometry Data

Spectral Datastudy sample

before, during, &

after film growth

Dynamic

SE Datacontains

continuous

Spectral Data

Time in Minutes

Me

asu

red

Yin

deg

rees

200 1000

0

15

30

45

60

75

90

90

00 8

Wavelength in nm

45

75

60

30

15

Yin

deg

rees


CompleteEASE User Interface In situ Tab: Analysis Tab: Measurement Tab:

“Custom” tabs (e.g., Heat Cell)

Optical Models: Gen-Osc

B-Spline

Cauchy …


Application: Thickness and Growth Rate

Most common in situ SE application:

Real-Time film thickness.

Film thickness and growth rate shown

for sputter deposition of SiO2 thin film.

9.9 10.2 10.5 10.8 11.1 11.4 11.7

Time in Minutes

400

500

600

Layer

thic

kness in

Å

Target thickness

Shutter close

time

SE data

points

Thickness control algorithm for

precise process control.

Controlled growth shown for a 583Å

GaAs thin film.


Advantages of Spectroscopic Ellipsometers

Die

lectr

ic f

unction

Die

lectr

ic f

unction

Langereis et al., J. Phys. D: Appl. Phys. 42, 073001 (2009)

Wide spectral range

– Near IR range for conductive film

– UV range for semiconductors and

dielectrics

No gaps in spectrum

– Interference enhancement to study thin

metal films

Software with extensive modeling

options for most-general samples

– General Oscillator models

• Drude for conductivity

• Tauc-Lorentz for amorphous materials

• Parameterized semiconductor models

• ....

– B-spline parameterizations


Chamber Integration Requires optical access to the Sample

during measurement.

SE measurements can be continuous,

periodic, or triggered.

SE measurement angle defined from

sample normal to ellipsometer viewport.

– Source Angle = Receiver Angle

– Typical “default” port angle ~ 70°,

with most chambers designed for port

angle between 60° to 75°.

Source Unit

Source Stage

Receiver Unit

Receiver Stage

Viewport

(Window)Viewport

(Window)

Angle


Examples: In Situ/In-Line SE Implementations

Moving metal s

heet

Ellipsometer housed in single unit to protect from harsh industrial environment.

Double reflection

Conveyor belt

Atomic Layer DepositionMetal Roll-to-RollDoor mounted


Common In Situ Considerations

Nominal Angle of Incidence

–Determined during „System Check‟ calibration

Window Effects

Substrate Temperature

Substrate Wobble


Viewport Window Effects (Strain) SE polarization measurements are not affected

by transparent coatings on chamber windows.

– Beam transmits through window at/near normal incidence.

– Absorbing coatings can decrease signal-to-noise or

eventually limit spectral range if coatings become opaque.

Calibration is required for window birefringence,

which changes the polarization.

– Patented 2nd order window correction applied.

Calibration Data, Channel #10 (3342 Å)

Polarizer Azimuth in degrees

60 80 100 120 140 160 180

Fourier

Coeffic

ients

-0.6

-0.3

-0.0

0.3

0.6

0.9

1.2

AlphaBetaResidualFit

No Window

Correction

Calibration Data, Channel #10 (3342 Å)

Polarizer Azimuth in degrees

60 80 100 120 140 160 180

Fourier

Coeffic

ients

-0.6

-0.3

-0.0

0.3

0.6

0.9

1.2

AlphaBetaResidualFit

Patented 2nd Order

Window Correction

RCE Data Fits, Native Oxide on Si, with Windows

Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

in d

egre

es

100

120

140

160

180

Model Fit Exp. Data with offset correction applied'raw' Exp. Data

Mount windows / viewports

evenly to minimize induced

window mounting strain


Substrate Temperature

Variable Temperature Silicon Optical Constants

Substrate commonly not at room temperature.

Optical constants are temperature dependent.

Temperature and composition dependent material files

available or can be created within the software.


Beam Wobble During Substrate Rotation Substrate rotation may cause

beam “wobble”.

– Minimize rotator wobble if possible.

– Use large beam diameter to always

cover receiver (detector) aperture.

– Synchronize data acquisition period

with substrate rotation period.

Or Trigger measurements for specific

position.

Ellipsometer

beam

rotation

Beam

Trajectoryaxis of

rotationq

Beam trajectory due

to Substrate rotation

Detector

aperture

Ellipsometer

beam


Synchronization with Rotation Period Beam wobble from rotating substrates can

produce beat pattern in dynamic data when

acquisition period does not match substrate

rotation period.

Right figures show dynamic data initially out

of sync with substrate rotation period.

Data acquisition time changed after 2 minutes

to match substrate rotation period.

Experimental Data

Time in Minutes

60 62 64 66 68 70

Delta

in d

egre

es

99.0

99.5

100.0

100.5

101.0

101.5

102.0

Exp 2.1024eV


Growth Example: ALD WO3

Data courtesy

of Veeco/CNT


Fast Process Development & Optimization

0 50 100 150 200 250 3000

4

8

12

16

20

F

ilm t

hic

kn

ess (

nm

)

Number of cycles

15 s

60 s

15 s

30 s

0 20 40 60 80 100 120 1400.00

0.02

0.04

0.06

0.08

0.10

Gro

wth

ra

te (

nm

/cycle

)

Plasma exposure time (s)

single deposition run

separate depositions

Monitor film thickness while changing

precursor/reactant dosing time

Calculate growth per cycle from slope

to establish saturation curves

Ex situ measurements not strictly necessary as

single deposition run yields saturation curves, etc.

Slide courtesy of Eindhoven University of Technology


Initial Growth: Substrate Dependence, Nucleation Delay


0 50 100 150 2000

2

4

6

8

10

TaN on HF-last Si

TiN on SiO2 (1000

oC)

Film

th

ickn

ess (

nm

)

Number of cycles

0 50 100 150 200 250

0

2

4

6

8

10

12 Plasma ALD Pt

Thermal ALD Pt

Th

ickn

ess (

nm

)

Number of cycles

TiN from TiCl4 nucleates poorly on surfaces

with low —OH densities,

TaN from Ta[N(CH3)2]5 nucleates easily

Very long nucleation delay of Pt by

thermal ALD on oxide surfaces,

plasma-assisted ALD nucleates easily

With in situ SE nucleation delay of ALD processes can be investigated

and film thickness can be controlled exactly


Distinction of Phase Compositions


1 2 3 4 5 60

10

20

30

Die

lectr

ic f

un

ctio

n

2

Photon energy (eV)

Different TaNx phases deposited by varying ALD conditions

Clear distinction TaNx phases from dielectric function:

• Conductive TaN shows Drude absorption by free electrons

• Semiconductive Ta3N5 shows Tauc bandgap behavior

TaN0.45 (390 µ cm)

TaN0.49 (1.0×103 µ cm)

TaN1 (1.1×104 µ cm)

Ta3N4.7

Ta3N5

Langereis et al., J. Appl. Phys. 102, 083517 (2007)


Evolving Film Properties: ALD TiN


Summary

Spectroscopic Ellipsometry is a useful technique for

monitoring in situ dynamic processes.

– Determine thickness, growth/etch rate, optical constants…

Determine other material properties such as

composition, resistivity, and crystallinity from

optical property changes.

Spectroscopic ellipsometers have been integrated

on a variety of in situ and in-line systems.

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spectroscopic ellipsometry for in situ applications...spectroscopic ellipsometry is a useful...

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