Referenced Spectroscopic Ellipsometry

Niklas Reineking

nre@accurion.com info@accurion.com Accurion GmbH Stresemannstraße 30 D-37079 Göttingen, Germany www.accurion.com

Introduction Ellipsometry is a very sensitive optical method which has

been used for about a hundred years to derive information

about surfaces. It makes use of the fact that the polarization

state of light may change when the light beam is reflected at

a surface. If the surface is covered by a thin film (or a stack

of films), the combined optical system of film & substrate in-

fluences the change of polarization. Thus it is possible to de-

duce information about the film properties, especially the film

thickness.

Here we present a special type of ellipsometer which com-

pares the sample to a reference being arranged within the

beam path.

Due to the orientation of the reference, none of the optical

components need to be moved or modulated during meas-

urement, and the full high resolution spectrum can be ob-

tained in a single-shot measurement.

This way one hundred spectra per second are acquired. The

synchronized x-y-stage enables acquisition of high-

resolution, large-field film-thickness-maps of the sample

within a few minutes.

▲ Fig. 1 The linear polarized white light is reflected from the reference at an an-gle of incidence of 60°. The reference is rotated by 90° referring to the beam axis. Thus the p- and s-polarized components of light interchange in relation to the sample. If the sample and the reference are identical, the outgoing light is linear polarized.

Setup For illumination a supercontinuum-laser is used. The colli-

mated, linear polarized white light is reflected at an angle of

incidence (AOI) of 60° from the reference-sample which is

rotated by 90° around the main beam axis (see Fig. 1). After-

wards the light is reflected by the sample at the same AOI.

The light passes through an analyzer and is detected by a

spectrometer.

In case of homogeneous, isotropic samples, their behavior

can be described by the Jones-Matrix

,

where rs and rp are determined by the Fresnel-equations.

The light gets reflected at a reference which is rotated by 90°

with respect to the beam axis. This can be described by a ro-

tation-matrix:

For a reflection coefficient of ri,sam=ri+δri with ri=ri,ref and i=s,p

the total reflection matrix becomes:

Contact

If the reference is equal to the sample, the second term in

(eq. 3) is zero and the identity matrix remains. So the light

flux can be extinguished with crossed polarizer and analyzer.

This shows that the rotated reference compensates the po-

larization effects of the sample – it acts as an ideal compen-

sator for all wavelengths which is a substantial ad-

vantage over conventional compensator designs.

If the sample differs from the reference, the second term in

(eq. 3) is non-zero and the outgoing light is elliptically polar-

ized. The spectrum of the light passing through the analyzer

contains information about the difference between sample

and reference, e.g. a variation in film thickness.

Outlook A hyperspectral imaging concept will enable a spectral-line-

scan-imaging at very high measurement speeds with a lat-

eral resolution down to a few micrometers.

Dedicated versions capable of in-line and in-situ measure-

ments are projected as well as extended wavelength range

up to SWIR.

Measurements With each single measurement a full 2048-channel spectrum

is generated. Due to the special configuration every wave-

length contains ellipsometric information about the sample.

At the default configuration 100 Spectra per second are col-

lected. In combination with a motorized and synchronized x-y

-stage, high speed mapping is possible. The lateral resolu-

tion at AOI=60° is around 50 x 100µm using a micro-spot op-

tion.

Being an ellipsometric method, the measured data needs to

be fitted to an optical model to obtain optical parameters like

the complex refractive index and/or the film thickness. To

deal with the high data rate a look up table fitting was imple-

mented. Prior to the measurement a look up table is calculat-

ed. The measured data can then be fitted in real time and in

high resolution.

Figure 3a shows the film thickness map of SiO2-coated 4’’-

silicon wafer. In Fig. 3b the corresponding look up table is

shown, which shows the spectral variation caused by a film

thickness variation.

Due to the spectral ellipsometric information it is possible to

achieve a multi layer fit. Figure 2a+b show the film thickness

fit results for a layer stack of graphene on SiO2 on silicon.

▲ Fig. 2a+b The maps show the film thickness fit results for a graphene coated sample. The monolayer of graphene was applied on 300 nm SiO2 on a Si100 substrate. Due to the spectral information of the measurement it was possible to fit the film thick-ness of the graphene and the film thickness of the SiO2 simultaneously. The results show, that the 10x10mm sample is homogenously coated with graphene (fit result 0.4nm). The SiO2 shows film thickness variations of a few nanometers, which seems to be caused by polishing.

◄ Fig. 3a This map shows the film-thickness-variations of a SiO2-coated 4’’-wafer. The central film-thickness is around 400nm, but in-creases to up to 500nm in the border region. The measured field of 25x35mm was mapped within 5:40min generating 8800 spectra.

▼ Fig. 3b This multiplot shows the spectral variation due to the vari-ation in film-thickness of the SiO2.

► Fig. 4a The picture shows native silicon samples. The upper sample is still equipped with a piece of physisorbing plastic foil which should be removed without any contaminations. An equivalent piece has been already removed from the lower sample with no visible residues.

▼ Fig. 4b The intensity-map clearly shows the shape of the stripe - obviously some invisi-ble residues remained. Problems in deposition processes may occur due to such contamina-tions.

Fig. 4a/b show contaminations on bare silicon due to the us-

age of a physisorbing plastic foil. Those foils should not

leave any residues and none are actually visible under a

standard microscope. The ellipsometric measurement re-

veals clear contaminations which may cause serious prob-

lems in following deposition processes.