Raman
Spectroscopy

Resolving micron-sized layers
in multilayer films with Raman
microscopy by cross-section analysis
and confocal depth profiling

Application
NotePolymers
RA89

Lama KHALIL, Thibault BRULÉ, Ludivine FROMENTOUX, and Massimiliano ROCCHIAHORIBA FRANCE SAS, 14 Boulevard Thomas Gobert - Passage Jobin Yvon, CS45002 - 91120 Palaiseau, France

Abstract: Multilayer polymer films, composed of different materials, are used in a variety of industrial applications. The
analysis of these multilayers is important to support production, identify defects and perform failure analysis, understanddelamination problems due to adhesion issue (lack or non-uniform distribution of the adhesive) and for reverse engineering
investigation. Here, we use Raman microscopy for cross-section analysis and confocal depth profiling to investigate thechemical composition of two multilayer films. It is also shown how Layers, the new LabSpec6 software app from HORIBA for
cross-section and confocal analysis, can efficiently provide such information, allowing a detailed identification of the chemicalcomposition and spatial distribution of each component within a multilayer structure.
Keywords: Raman microscopy, Cross-section analysis, Confocal depth profiling, Polymer multilayers, Layers.

Introduction
Multilayer polymer films, produced using co-extrusion andlamination processes, exhibit characteristics and properties
not achievable by their individual constituent monolayers.
These composite materials are widely used in several
industrial applications, including consumer product andfood packaging, as they are strong and flexible thus able topreserve their contents. Each layer of the complex structure
provides a specific functionality to the full film, acting asoxygen, moisture, dust, microbes, or light barrier. Typically,
conventional polymers like ethylene vinyl alcohol (EVOH),
ethylene vinyl acetate (EVA), polypropylene (PP), polyethylene(PE), polyethylene terephthalate (PET) and polyamide (PA) are
employed to produce multilayer structures.
Since the design of such structures has become more
complex and the manufacturing specifications have becometighter, there is an increasing need of new hardware andsoftware solutions that can easily and straightforwardly
characterize these multilayer films. In this respect, the solution
should provide both chemical and thickness information andmoreover it should consider that nowadays the new multilayer
structure has an overall thickness that is decreasing (few tensof microns) and that the number of layers is increasing (10 to15 layers) with respect to the past.

Confocal Raman microscopy is the perfect technique toprobe multilayer polymer film and the combination of Raman
microscopy together with our customized polymer vice
(Figure 1) and the “Layers” software app of LabSpec6 is theperfect solution. Raman microscopy combines the advantagesof Raman spectroscopy allowing chemical identification of
the constituents with those of optical imaging (i.e., opticalmicroscopy) showing a high spatial resolution image of thefilms and of the different layers. By using Raman microscopy,
the layer identification (chemical nature and thickness)of multilayer polymer film can be made by cross-section
analysis or by confocal depth profiling. The advantage ofconfocal measurement is that it requires no or minimal sample
preparation for samples that cannot be cross-sectioned andallows defect analysis without the need to bring the defect tothe surface by sectioning the sample, which can lead to lossof defects. Cross-section Raman mapping, on the other hand,offers the best lateral resolution ensuring the analysis of layers
down to sub-micron thickness.
In this application note, two different samples, namely amultilayer polymer film and a screen protection film, areanalyzed using the cross-section and confocal modes,respectively. It is also shown how Layers, the new LabSpec6

software application for Raman cross-section and confocal
depth analyses, allows an automatic identification of thechemistry and thickness of each constituent layer within themultilayer film.

Instrument and methods
Two distinct samples were selected for the analysis: amultilayer polymer film and a screen protection film. The
multilayer polymer film was analyzed in cross-section modeusing the easy-to-use kit developed by HORIBA. This includes
a “polymer vice” to hold the multilayer films in cross-section
or to allow an easy cut of them, a stage mount to securely
hold the vice on the manual and/or automatic stage of theHORIBA Raman microscopes, single-use sharp blades tocut the multilayer films, and a screwdriver to lock the polymer
vice into the stage holder for reliable and accurate Raman
measurements (see Figure 1).
Due to its hardness, the screen protection film was notcross-sectioned; it was analyzed in confocal depth profilingmode after immobilization on a glass slide. For both samples,the micro-Raman measurements were performed using theconfocal HORIBA LabRAM SoleilTM (Figure 2), offering fullautomation. A 532 nm laser excitation was focused down toa micrometric spot on both sample surfaces through a 100xobjective (NA = 0.9; WD = 1 mm). The selected spectrometer
grating allowed collection of a full spectral window from
30 cm-1 to 3600 cm-1 in one shot. The laser power was set
to 4.8 mW and 6.2 mW at the surface of the multilayer film
and screen protection film, respectively. The LabRAM SoleilTM
instrument was directly controlled via the LabSpec6 software.

The Layers app, integrated as part ofthe LabSpec6 software, was used toanalyse the Raman maps of the twosamples and to determine the chemicalstructure, by means of a customised
HORIBA polymer library, and thicknessof each of the layers. Layers app is also
exploiting other modules to display andanalysis the data such as the Multivariate Analysis Plus (MVAPlusTM) application which is embedded in it.

Figure 2: LabRAM Soleil Raman microscope.

Figure 1: Multilayer kit made of a multilayer sample holder, sharp single-use blades, and a screwdriver.

Results

A

Score (a.u.)B

100

Polyethylene
Polypropylene
Polyamide copolymer
EVOH

500-1001020X (µm)

C
Polyethylene

Intensity (counts)

The chemical identification of the different
components was carried out using the HORIBA
polymer library integrated in the KnowItAll®
software, after applying minimal preprocessing
step (i.e., baseline correction) to the raw spectral
data. Figure 3a shows the optical image of themultilayer polymer film on which the Raman map
is superimposed (each color identifies a different
polymer). Four compounds were found by meansof the Multivariate Analysis Plus (MVA PlusTM)
module: linear low density polyethylene (cyan),polypropylene (orange), polyamide copolymer(blue), and poly(ethylene vinyl alcohol) (EVOH;purple); see their representative Raman spectra in
Figure 3c. Note that the presence and thickness of
EVOH is of great importance in food packaging as
it acts as an oxygen barrier; its barrier properties
change according to the thickness. In Figure 3a, we
present the distribution of these compounds acrossthe mapped region. From this distribution it ispossible to automatically determine the thickness ofeach component, which is then displayed in a table,
Figure 3d. Some comments can also be added tothe table (see Figure 3d), which can be very useful todescribe the layer functionality for non-expert users
and/or to detail any features and characteristics ofthe specific layer. Here, the thickness of the thinnest
layer (EVOH layer) is 2.40 µm, directly determinedfrom Figure 3d. Layers app hence combines bothchemical and morphological information to provide
a complete picture of each individual layer in a
multilayered sample.

Polypropylene
Polyamide copolymer

EVOH
Raman shift (cm -¹)

DLayer #From (µm)

To (µm) Thickness(µm)

ID

1-19.30-14.96

4.34

Polyamide copolymer

2-14.96-11.37

3.59

Polyethylene

3-11.37

-8.07

3.30

Polypropylene

4-8.07

-5.08

3.00

Polyethylene

5-5.08

-0.58

4.49

Polyamide copolymer

6-0.58

1.81

2.40

EVOH

71.81

7.21

5.39

Polyamide copolymer

87.21

24.43

17.22

Polyethylene

ColorLoading

Spectrum

Comment

Oxygen barrier

Figure 3: A) Raman map of the multilayer polymer film superimposed on the optical image. Cyan, orange, blue and purple colours
correspond to linear low density polyethylene, polypropylene, polyamide copolymer, and poly(ethylene vinyl alcohol) (EVOH), respectively.
B) Score profile showing the distribution of the different components. C) Raman reference spectra of the different components. D) Table
generated by the Layers application, displaying the thickness, chemical ID, loading (calculated spectra from the multivariate analysis) andaverage spectrum for each layer.

As mentioned, the screen protection film was analyzed
in confocal depth profiling mode without any sample
preparation; for this type of analysis the size of the pinhole
is an important parameter in the instrument configuration, as
it spatially filters the volume to be analysed allowing perfectcontrol of the confocal resolution. The highly flexible HORIBALabRAM SoleilTM Raman microscope allows the pinhole
size to be tuned from 500 µm to 1 µm directly throughthe LabSpec6 software, ensuring a fine adjustment of theconfocal resolution; here, the confocal resolution was set to≈1 µm. Figure 4a shows the depth profile of the protection
film consisting of multiple layers of bis(2-ethylhexyl) adipate(purple), polyethylene terephthalate (PET; red), silicone oil andPET (orange), and a coating layer (green). The corresponding
Raman spectra of these pure and mixed chemical compounds
within the protection film are presented in Figure 4b. Figure
4c shows the table obtained from the Layers application,
displaying the thickness of each individual layer. As shownin Figure 4, a detailed information on the structure of theprotection multi-layered film was obtained using the depth
profiling capabilities of LabRAM SoleilTM.

Conclusion

AB

CLayer #From (µm)

To (µm)Thickness(µm)

ID

1-75.00-19.5055.50

Coating

2-19.50

18.50

38.00

PET3

18.50

37.50

19.00

Silicone oil + PET

437.50

74.50

37.00

PET574.50

160.0085.50

Bis(2-ethylhexyl) a...

ColorLoading

Spectrum

CommentFigure 4: A) Depth profile acquired from the protection film, showing the thickness of the constitutive layers. B) Raman reference spectra ofthe layers, identified as bis(2-ethylhexyl) adipate (purple), polyethylene terephthalate (PET; red), silicone oil and PET (orange), and a coating
layer (green). C) Table obtained from Layers, revealing the thickness, chemical ID, loading (calculated spectra from the multivariate analysis)and average spectrum of each individual layer.

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To conclude, we have analyzed two samples using Raman
microscopy in cross-section and confocal depth profilingmode. Raman microscopy was not only able to probe thechemical composition of the constituent layers, but also to
determine the distribution and thickness of each component.
It was also shown that the HORIBA polymer kit in combination
with the Layers application and the high sensitivity and spatial
resolution capabilities of the LabRAM SoleilTM can provide afull solution for analysing any multilayer structure.