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Model meets measurement thechallenges of optical spectrum simu-lation facing real thin film deposition

W.TheissW.Theiss Hard- and Software, Aachen, Germany

AbstractOptical spectrum simulation is a powerful technique

providin valua!le information for the control of thin film

deposition devices. The required inredients of this

method are descri!ed. Successful optical models have

ad"usta!le parameters which reflect the possi!le variations

of the deposition hardware. The values of these parameters

can !e determined analy#in spectra recorded !y an

appropriate networ$ of spectrometers. This way the

current state of the coatin deposition is $nown and - if

necessary - corrections leadin to ideal production

conditions can !e made.

%eywords& optical spectrum, simulation, thin film,production control

1. Introduction

Optical spectroscopy is a powerful method to characteri#e

thin films. Applied durin or directly after the deposition,

it supplies information a!out the thic$ness and the

comple' refractive inde' of the deposited material. The

refractive inde' (or more eneral& the optical constants)

reflect, in an indirect way, the composition of the film.

Hence quantities li$e electrical conductivity or o'yen

content can !e deduced from a quic$, non-destructive

optical inspection.

The !est technique to analy#e optical thin film spectra is

to ad"ust the parameters of a suita!le optical model in

such a way that compara!le simulated and measured

spectra match. This parameter fittin method provides the

fle'i!ility required in cases of floatin production

conditions (due to, for e'ample, taret erosion in

sputterin devices, as pressure variations, temperature

drifts). *ependin on the wanted information, optical

models can !e made very simple deliverin only a few $ey

parameters li$e thic$nesses, or rather sophisticated,

providin information a!out su!tle details of the

produced coatins.

Section + summari#es required features of a spectrum

simulation software to !e used with comple' deposition

devices (li$e, for e'ample, production lines for

architectural lass coatins). Then, in the main section ,

the systematic development of optical models is discussed.

The article closes with some remar$s in section a!out

the setup of an optical inspection networ$ providin the

information required for deposition control.

2. Optical spectrum simulation

Optical inspection of thin film deposition requires thecom!ination of spectroscopic hardware and data analysis

software. To !e equipped for cases of comple' coatin

equipment for multilayer systems the applied spectrum

simulation proram should have the followin features&

fle'i!le optical constant models

several layer stac$s, partially connected

simultaneous computation of several spectra

n the followin sections some details concernin these

points are discussed.

2.1 Optical constant models

Althouh a lot of effort is usually put into the deposition

hardware to produce thin films with sta!le properties, the

optical constants in the various layers of a coatin cannot

!e considered to !e the same all the time. This e'cludes

the use of fi'ed ta!les of literature data for optical

constants. nstead, one must employ fle'i!le models that

can follow driftin material properties. The !est choice is

to wor$ with physical models !ased on parameters which

have a (more or less) intuitive meanin.

/owerful optical constant models to !e applied inspectroscopic production control must !e availa!le from

the 01 (wavelenth 2 33 nm) to the 45 (up to

67833 nm). This spectral rane can !e covered !y fast

array spectrometers. n some cases - if 9T5 (9ourier

Transform nfra-5ed) spectrometers are used - also the

mid-infrared ( +333 nm : : +3333 nm) is of interest.

Typical material e'citations are electronic inter!and

transitions and the acceleration of free chare carriers in

conductive materials. Ta!le 7 ives an overview on

appropriate models for the various types of e'citations ;

the optical constants of a iven material are then o!tained

!y the superposition of several suscepti!ility terms.


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availa!le in the software. n most cases it is sufficient to

use the simple Drueman mi'in formula ?E@.

2.2 Layer stacks

Once the optical constant models are set for all relevant

materials, one has to define the sequence of these

materials in the coatin.

n order to ma$e use of measured spectra of incomplete

coatins, ta$en in !etween two deposition steps, thesoftware should !e a!le to handle several layer stac$s

within one optical model. t must !e possi!le to connect

layer thic$nesses of different stac$s in order to use

information o!tained for one stac$ in another.

>iht propaation in thin films must !e computed with

coherent superposition of partially reflected and

transmitted waves. 9or thic$ su!strates li$e lass panes

incoherent superposition (no interference patterns visi!le

in the spectrum) should !e selected.

2.3 Spectra

9or each spectrometer in the optical inspection networ$there must !e its counterpart in the model, i.e. a simulated

reflectance, transmittance or ellipsometry spectrum.

*etails li$e anle of incidence and polari#ation of the

incident radiation should match. This applies to the

normali#ation as well& f the hardware records relative

spectra, the software must do that as well.

0sually there is a unique assinment of layer stac$s and

spectra& 9or each spectrum there is a correspondin layer

stac$. However, in the case of products with a

microstructure on a lenth scale much smaller than the

optical measurement spot, the spectrometers see a mi'ture

of several layer stac$s. n order to analy#e such spectra thesimulation proram must !e a!le to averae the spectra of

several layer stac$s, ta$in into account their individual

area fractions.

2.4 rediction o! coatin" per!ormance

t can !e advantaeous to !e a!le to compute technical

data li$e color coordinates ?8@, emissivities ?F@, 0 ?F@and

?@values for the currently produced coatin. This way a

direct prediction of the final products performance is

possi!le which simplifies decisions concernin the

modification of deposition parameters.

3 #e$elopin" optical models

Good optical models used for production control can save

a lare amount of money ; hence their development has

to !e done as careful as possi!le. The most efficient way to

wor$ is to use the same optical model for the desin of

new coatins and their production control. This ensures

that $nowlede a!out production pro!lems and

phenomena (li$e mi'in of ad"acent layers) can !e ta$en

into account in the desin phase already. 4ew desins, on

the other hand, can !e turned into methods for

production control most easily.

3.1 Sin"le layers

The !asis of any success are relia!le optical constants of

the deposited materials. 9or dielectric materials (o'ides,

nitrides) it is recommended to deposit thin and thic$

sinle layers on typical su!strates. The spectra of the thic$

layers should e'hi!it at least one interference ma'imum or

minimum . This ensures a quite safe determination of the

optical constants. =omparin measured spectra of the thin

layer versions with simulated spectra o!tained from asimple thic$ness fit (Ifro#en optical constant model) one

can easily chec$ if the o!tained optical constants wor$ for

!oth thin and thic$ layers. f not, a systematic study of the

variation of the optical constants with thic$ness miht !e

useful.

The test layers should !e produced with the final

deposition equipment, even if it hurts to run an e'pensive

production line for several sets of sinle layers.

nformation o!tained directly from the factory may turn

out to !e very valua!le, in particular if you allow a

feed!ac$ of the production model to the one used for

coatin desin.

Whenever possi!le, one should use parameters which are

reflectin the controls of the deposition equipment. f, for

e'ample, the reactive sputterin of an o'ide is reulated !y

the settin of a lam!da-pro!e and the value of the

electrical power, there should !e two correspondin

parameters in the model. f the applied optical constant

model requires more parameters, it is useful to investiate

how the model parameters depend on the machine

settins. 9i. 7 shows how the ap enery of an O> model

?+@ varies with the value of a deposition parameter. The

quite simple relation can easily !e e'pressed !y

polynomial interpolation. f it is possi!le to e'press alloptical model parameters as simple functions of the

machine parameters, these relations can !e implemented

in the optical model itself& The machine parameters are

then used as Imaster parameters in the model (here&

lam!da-pro!e settin, electrical power), and the optical

constants of the produced material are computed usin

Islave parameters li$e ap enery or oscillator strenths.

9i. 7. J'ample for a simple relation !etween a deposition

parameter (lam!da-pro!e settin) and an optical

parameter (ap enery of a reactively sputtered o'ide).

,.,

,.-

,.B

,.E

,.8

3 7 + , - B E

%achine settin" &m'(

)apener"y&e'

(


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n luc$y cases a sinle parameter li$e a lam!da-pro!e

settin determines the optical constants of the material,

and an independent second parameter li$e the electrical

power is responsi!le for the achieved layer thic$ness. 9i. +

shows an e'ample.

9i. +. Optical constants of a reactively sputtered o'ide&

The upper curves show the real part of the comple'refractive inde' n, the lower curves the imainary part $.

The solid lines ive the optical constants for E m1, the

dashed ones for 3 m1.

n real systems the correlation of deposition and model

parameters may not !e perfect.


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9i. & Schematic s$etch of a satisfyin silver layer model&

The rouh interfaces of the Ireal system on the left are

appro'imated !y thin effective-medium layers, and the

reduction of electron mo!ility due to diffuse scatterin at

the rouh !oundaries is ta$en into account !y an increase

of the dampin constant in the *rude model.

3.3 Application e*ample

After careful development of a coatin model, ta$in into

account realistic sinle layer results and the interaction

effects of neih!oured layers in the stac$, it is possi!le to

achieve a ood areement of model and measured coatin

properties even for stac$s with many layers. 9i. shows

the comparison of measured and simulated reflectance

and transmittance spectra of a commercial solar control

coatin. The development of this hih performance

product has !een supported !y model computations,

which helped sinificantly to reduce the amount of

required e'pensive production of test samples in the plant.

9i. & Simulated (dashed) and measured (solid) spectra of

a commercial solar control coatin (SJeary et al., .Appl. /hys. F+, 8 (78) -3.

?@ =.=. %im et al., /hys. 5ev. D B (+3) (7+) 778-778E8.

?@ /.*rude, Ann. /hys. (733) E.

?B@ *

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