w.elsevier.com/locate/tsf

Thin Solid Films 495 (

Growth of Ag films on polyethylene terephthalate (PET) deposited

by electron beam

I. Prosy*evas a, J. Puixo a,b,*, S. Tamulevi*ius a,b, A. Juraitis a, M. Andrulevi*ius a, B Cy”iute b

a Institute of Physical Electronics, Kaunas University of Technology, Savanoriu 271, LT-50131, Kaunas, Lithuaniab Department of Physics, Kaunas University of Technology, Studentu 50, LT- 51368, Kaunas, Lithuania

Available online 30 September 2005

Abstract

The optical and structural properties of silver (semi-continuous and continuous) layers on optical polymer (PET) deposited by electron beam

evaporation were investigated at different growth stages. The experimental results have shown that thermal annealing of discontinuous silver

layers at temperatures close to the polymer glassing temperatures brings to formation of nanostructured interface.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Polyethylene terephthalate; Electron beam evaporation; Nanostructures; Glassing temperature; Ag films

1. Introduction

Metallization of polymers is essential for realizing their

application in elements of microelectronics, photonics devices,

biosensors and medical implants [1–16]. The main reason of

interesting in polymer metallization is the low-cost and high-

performance polymer electronic devices, a very low absorption

in the visible wavelength range [14]. Outstanding properties of

polymer webs compared to glass substrates are their lightweight

and their flexibility, thus they are used especially on curved

surfaces and in portable electronic devices [4,5,12]. The

adhesion between metals and polymers is normally very poor.

The cohesive energy of not too reactive metals is typically two

orders of magnitude higher than that of polymers. Furthermore,

the interaction between moderately reactive metals and poly-

mers is generally very weak in comparison to the strong metal–

metal-binding forces. As a consequence, these metals are

expected to exhibit a strong aggregation tendency, and their

solubility in polymers should be extremely low under equilib-

rium conditions. The polymer surfaces are usually non-polar or

have very low wettability. For the same reasons metals of low

reactivity do not wet untreated polymer surfaces. They form

0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2005.08.278

* Corresponding author. Department of Physics, Kaunas University of

Technology, Studentu 50, LT- 51368, Kaunas, Lithuania. Tel.: +37037

313432; fax: +37037 314432.

E-mail address: puiso@ktu.lt (J. Puixo).

3D-clusters during vacuum deposition, which finally coalesce,

and form a continuous film at high metal coverage [1–10].

Usually metals are deposited on top of polymer films by thermal

evaporation under a high vacuum condition. The metal atoms

can readily diffuse in to the top surface region of the polymer

film and form a diffuse interface of several nanometers with a

depth distribution of metal atoms [9]. Sometimes metal atoms

react with the polymer and produce nanocomposites [3,9].

The use of a polyethylene terephthalate (PET) web as an

optical polymer substrate for the deposition of silver layer

stacks has various application advantages: high melting

temperature, good mechanical properties and recyclablity [3].

Optical properties of Ag polymer system can be affected by

film thickness and deposition parameters for the Ag film [13].

The quality of the interfaces between the silver layer and PET,

when the deposition temperature is close to PET glassing

temperature is a key factor in the manufacturing of induced

transmittance filters and mirrors.

This paper presents the research on Ag-PET system

produced by electron beam evaporation concentrating on the

mechanism of interface formation at temperatures close to the

PET glassing temperature.

2. Experimental

Substrates PET (polyethylene terephthalate, 25 Am) were

metallized with silver Ag (Tmelting=961.93-C) (purity 99,99%,

2006) 118 – 123

ww

40 50 60 70 80 90 100400

600

800

1000

1200

PET

PETAgc(111)

2ΘIn

tens

ity

(a.u

.)

20 nm on PET as-deposited

20 nm on PET after annealing at 1400C in air PET

a

40 50 60 70 80 90 1000

500

1000

1500

2000

2500

3000

3500

4000

4500

Ag2Oh (003)

Agc(311)

Agc(200)

Agc(222)

PET

Agc(111)

Inte

nsit

y (a

.u.)

100 nm on PET

100 nm on PET annealing at 1400C in air

PET

c

40 50 60 70 80 90 100400

600

800

1000

1200

Agc(311)

Agc(200)

PET

PET

Agc(111)

2Θ

2Θ

Inte

nsit

y (a

.u.)

50 nm on PET as-deposited

50 nm on PET after annealing at 1400C in air PET

b

Fig. 1. XRD pattern of Ag films on PET substrate of different thickness as

deposited and under annealing for 30 min at 140 -C. a) 20 nm, b) 50 nm, c)

100 nm.

I. Prosycevas et al. / Thin Solid Films 495 (2006) 118–123 119

Sigma Aldrich, UK) of different thickness: 20 nm, 50 nm, 100

nm by electron beam evaporation (Tsubstrate=80 -C, residualgas pressure 10�4 Pa, deposition rate v=1–2 nm/s). Before

coating substrates were cleaned using oxygen plasma proces-

sing (RF=13.56 MHz, P=0.3 W/cm2, t=5–60 s) [17]. The

contact angle of water on polymer substrate was measured by

self-made contact angle analyzer [17]. The film thickness

during deposition was monitored with a quartz balance

technique. The temperature of the substrates during deposition

was controlled with a precision T0.5 -C. After thin film

deposition part of Ag-PET structures were thermally annealed

at 140 -C for 30 min. The X-ray diffraction measurements for

the as-deposited as well as-annealed films were recorded at

room temperature in a DRON-3.0 diffractometer using Ni-

filtered CuKa radiation operating at 30 kV and 30 mA in a step

scan mode with a step size of 0.02-, for 2h and registration

time of 20s per step. XRD analysis of Ag films on PET was

performed to define values of interplanar distance d111, lattice

constant as well as grain size and microstrain. Winfit!1.2

software was used in these calculations [18]. Atomic force

microscope NANOTOP-206 operating in a contact mode

(cantilever force constant 0.9 N/m) was used to investigate

morphology of the surface. Image processing and analysis of

scanning probe microscopy data was performed using a

Windows-based program ‘‘Surface View version 5.0’’. Statis-

tical evaluation of the surface morphology was performed for

the scanning area 13 Am�13 Am. The optical transmittance

and reflection of the films were measured by the spectrometer

UV-VIS Specord in the 300–800 nm wavelength ranges. The

composition and chemical state of the elements in the silver

film surface were studied by X-ray Photoelectron Spectroscopy

(XPS) on KRATOS ANALYTICAL XSAM800. The energy

scale of the spectrometer was calibrated using Au 4f7/2 and Cu

2p3/2 Ag 3d5/2 peaks, [19]. The source was operating in

aluminum anode mode: Al Ka with photon energy of 1486.6

eV. The hemispherical analyzer with pass energy of 20 eV (0.1

eV energy increment) and fixed analyzer transmission (FAT)

mode were used. Carbon, oxygen and silver relative atomic

concentrations were calculated from appropriate peak area with

respect to the sensitivity factors, using original KRATOS

software. The ‘‘XPSPEAK41’’ software was employed for the

peak fitting procedure. The Shirley background with Lorenz to

Gauss rate 40:60 and asymmetric line shape (asymmetry

parameters TS: 0.4; TL: 40) for metallic silver were used.

Lorenz to Gauss rate 0 :100 and symmetric line shape for other

peaks were used. All fitting procedures are made using the

Gaussian–Lorentzian sum function. The results of surface

composition of thin silver films deposited onto PET were

compared with the ‘‘bulk’’ silver surface analysis results.

Surface cleaning with Ar+ions (3 keV, 18 AA/cm2) was used

to remove adsorbed impurities. The argon-ion cleaning was

applied to all samples.

3. Results and discussion

After 60 s exposure oxygen plasma contact angle for PET

substrate was reduced from 72- to 10- respectively, illustrating

the improvement of wettability of polymers that is a result of

the increase of its surface polar groups [11]. Oxygen plasma

like chemical etching, corona discharge plasma treatment,

irradiation by gamma ray, ultraviolet light and electrons beams

appears to be efficient treatment way of polymers. Such kind of

Table 1

The crystallite size and microstrain of Ag films on PET (as deposited and annealed at 140 -C for 30 min)

Ag cubic (111) 20 nm of Ag on PET 50 nm of Ag on PET 100 nm of Ag on PET

As-deposited Annealed As-deposited Annealed As-deposited Annealed

Crystallite size, nm 15.6 8.4 14.1 14.5 17.7 18.5

Mictrostrain, r.u. 3.54I10�3 6.64I10�3 3.92I10�3 3.84I10�3 3.14I10�3 3.00I10�3

I. Prosycevas et al. / Thin Solid Films 495 (2006) 118–123120

treatment produces relative strong adhesion of metallic film to

polymer that was demonstrated for aluminum or copper thin

films with polymers [11]. In addition [11], as the describe

observation is the same as in the case of PET ( PET: 70 -to 10-,Polysiloxan 70 - to 10 -).

3.1. XRD analysis results

XRD pattern of 20, 50 and 100 nm Ag on PET as deposited

and annealed are presented in Fig. 1. XRD peaks of Ag films

on PET substrates corresponding to (111), (200) and (311) and

(222) planes were observed. The position of these lines in XRD

patterns is similar to those observed for bulk silver [20–23].

The relative intensity of (111) peak is significantly higher than

that for Ag powder diffraction, suggesting a preferred

orientation of some Ag grains starting at very early growth

stages. According to our results the silver layers on PET of 20

nm thicknesses have strong (111) textures. This fact was also

observed for the earliest stages of the Ag (111) film on the flat

and defect-free substrate (mica) substrates [23]. Intensity of

these peaks increased with the thickness of films. The

crystallite size and microstrain of the Ag film calculated

according to [17] are presented in Table 1.

The crystallite size for the Ag films (Table 1) increased from

15.6 to 17.7 nm with increment of Ag film thickness.

According to the X-ray diffraction studies of Ag films on

PET [5] at low thickness (up to 10 nm) the crystallite size is

equal to the film thickness. Above a thickness of 10 nm the

Y, um

a

c

b

d

Z, nm

1.0

0.5

7.50

0.5

1.0

X, um

Y,

1.0

Y,

1.0

Y, um

Z, nm

1.0

0.5

170

0.5

1.0

X, um

Fig. 2. AFM images of Ag film on PET thickness: a) 20 nm, c) 50 nm, (a

crystallite size is still proportional to the film thickness, but the

crystallite growth is slower than the increment in the film

thickness [7]. In our case after annealing Ag films on PET at

140 -C for 30 min we didn’t observe drastical increase of the

average in-plane crystallite size (like it was observed in the

case of Ag films on crystalline silicon substrates after annealing

at <150 -C temperatures) [20].

Thermal annealing in our case resulted in an increase of

the grain size by 3–5% for the film of 50 and 100 nm

thickness, and decrease from 15.6 to 8.4 nm for the thickness

20 nm. The behavior of 50 and 100 nm Ag films on PET

wasn’t dependent on the dynamics of motion of PET

macromolecules taking place at temperature close to glassing

point. These films behave like films on a solid substrate

where the crystallite size increases and microstrain decreases

with annealing. Completely different behavior was observed

for films of 20 nm Ag thickness where the microstrain

increase up to 47% was found. At the early stage of growth

(up to 20 nm Ag films on PET) behavior of this film is

similar to Ag films on sapphire. At this thickness Ag films

are discontinuous over the sapphire surface [8] and film with

a thickness of less than 15 nm prepared by evaporation onto a

glass or quartz substrates consists of isolated metal islands

[22]. Exposition in air at ambient temperature and atmosphere

of such discontinuous films and thin film of 100 nm resulted

in formation of few tens of nanometer porous oxide [22]. In

our case hexagonal (Ag2O) was detected for the Ag-PET film

annealed at 140 -C.

um

Z, nm

0.5

220

0.5

1.0

X, um

um

Z, nm

0.5

450

0.5

1.1

X, um

s deposited), b) 20nm and d) 50 nm (annealed at 140 -C for 30 min).

Table 2

Morphology and grain size of Ag films on PET (as deposited and annealed at 140 -C for 30 min)

20 nm of Ag on PET 50 nm of Ag on PET 100 nm of Ag on PET

As-deposited Annealed As-deposited Annealed As-deposited Annealed

A (max. height), nm 14.9 22.3 22.0 62.3 18.1 41.8

Ra (average roughness), nm 1.34 1.98 2.22 7.57 1.45 2.66

Rq (rms), nm 1.72 2.57 2.84 9.46 1.90 3.77

Rsk (skewness), nm �0.10 0.31 �0.19 0.09 0.07 0.90

Rku (kurtosis), nm 3.51 3.74 3.25 3.03 4.17 7.02

I. Prosycevas et al. / Thin Solid Films 495 (2006) 118–123 121

3.2. AFM analysis results

Fig. 2 and Table 2 present the AFM images and statistical

analysis of morphology of evaporated and annealed Ag films.

The difference in microstucture and the particle shape of

discontinuous and semi-continuous Ag samples are clearly

seen. The grain height increased from 14.9 to 22.0 nm with

the increase of thickness of Ag film. (For comparison islands

of 20 Ag films on sapphire have typically vertical and lateral

dimensions in the 100–300 nm ranges 10–60 nm in

diameter [24]). 20 nm Ag films on PET are discontinuous

like it was for 20 nm Ag films on sapphire [8] and silicon

[24].

3.3. Optical properties

Fig. 3 shows the optical-density spectra of the Ag film on

PET in the wavelength range 300–800 nm. According to the

optical measurements, reflectivity of the film for 100 nm was

close to 10% and optical density was measured for the films

of lower thickness only. The reflectivity of 50 nm Ag film on

PET was found to increase with increasing the wavelength

from 500 to 800 nm. In these spectra, the peak around 400

nm [13] that is attributed to the surface plasmon transverse

resonance wasn’t observed. For the low thickness film (i.e. 20

nm) the absorption peak due to the surface plasmon of

longitudinal resonance of electric field vector along the length

axis [13] was found at around 750 nm wavelengths. The

400 500 600 700 800

0.2

0.4

0.6

0.8

1.0

1.2

1.4

surface plasmon

a)b)c)d)

λ, nm

Opt

ical

den

sity

(a.

u)

Fig. 3. Optical density of Ag films on PET versus wavelength (before and after

annealing) a) 20 nm, c) 50 nm, (as deposited), b) 20nm and d) 50 nm (annealed

at 140 -C for 30 min).

thermal annealing induced red shift of this peak to the higher

wavelength. According to Gan’s theory [25], the longitudinal

plasmon position shift to longer wavelength is attributed the

increase of aspect ratio (length/diameter) of the metal

nanorods formed in polymer matrix. After annealing the

intensity of this peak decreased, that could be explained as

variation of silver particles density.

3.4. Surface composition

The results of surface chemical analysis of Ag films on PET

are presented in Table 3 and Fig. 4. There are two peaks (Fig.

4) located at the same binding energy of 386.0 and 374.0 eV,

which correspond to a doublet Ag3d5/2 and Ag3d3/2 [26–28].

The Ag 3d position was stable, during thermal annealing and

surface cleaning. The presence of small Ag–O peak after

thermal annealing in air suggests that the film within the XPS

sampling depth is not fully oxidized.

100 nm silver films exhibit the same behavior during

processing of the samples— after annealing (or ageing) process

relative amount of carbon increases while amount of oxygen is

almost changeless. The relative amount of carbon after short

Ar+ion sputtering decreases sharply. On the other hand, different

picture appears for 20 nm silver film sample. The relative

amount of oxygen drops sharply after sputtering process, while

amount of carbon shows almost no changes. The peak with the

binding energy off 531.5 eV is seen in spectra for all samples. To

compare this value with X.–Y. Gao [28] results one can suggest

that this peak originates from the mixture of atomic oxygen and

hydroxyl groups. It can be found that the surface concentration

of O is very high, which could be attributed to the chemosorption

process. The significant C1s peak, which indicates to high

surface concentration of C, usually results from the adventitious

Table 3

Surface atomic concentration of Ag films on PET (d=20 and 100 nm,

Tanneal=140 -C)

Ag3d5/2 (%) O1s (%) C1s (%)

Ag 20 nm 25 26 50

Ag 20 nm on PET after annealing

at 140 -C12 33 54

Ag 20 nm on PET after annealing

at 140 -C after Ar+sputtering

44 7 49

Ag100 nm 29 22 49

Ag 100 nm on PET after annealing

at 140 -C

21 23 56

Ag 100 nm on PET after annealing

at 140 -C after Ar+sputtering

56 17 27

10000

12000

14000

16000

18000

20000

22000

280282284286288290292294

Binding Energy, eV

Inte

nsity

, a.u

.O=C-O

C-C, C-H

Fig. 5. XPS C1s spectra for 20 nm Ag film on PET annealed at 140 -C: dashed

line with crosses – acquired data.

I. Prosycevas et al. / Thin Solid Films 495 (2006) 118–123122

contaminant from the atmosphere and from the PET substrate.

On the basis of the binding energy data [27,28] fitting was for

done Ag 3d5/2. In this we suggested the presence of silver oxide

and surface plasmons. It is difficult to determine the oxidation

states of Ag because the peaks are rather close i.e. 368–368.2 for

Ag- [25,27] and 367.4 for AgO [27,29] and 367.8 for Ag2O

[27,28]. In [22] it was also observed the surface plasmon loss

peak (+3.78 eV) of silver around the interface between the silver

and dielectric layer. Fitted Ag 3d spectra (Fig. 4) for annealed at

140 -C 20 nm silver film on PET show high intensity peak at

368.2 eV and low intensity peak at 367.7 eV. Therefore we

assigned high intensity peak to metallic silver and low intensity

peak to Ag – O bounds. Relative peak area for metallic silver

and Ag – O bounds in this case is 75.9% and 24.1% respectively.

Relative peak area of metallic silver and Ag – O bounds for 100

nm oxidized silver peak is 86% and 12.5%. The Ar+ ion

sputtering of the sample surface removes oxides completely

from all samples.

AnalyzingtheC1speak(Fig.4)ofAgonPETonecanconclude

that it contains three well-separated peaks corresponding to

carbon atoms bound to carbon and hydrogen in benzene ring

(284.6 eV) and to ester carbon atoms (288.75 eV). Methylene

carbons singly bound to oxygen peak (286.3 eV) is observed in

O1s (533.8 eV), respectively. It is known [29] that in C 1s

spectra of pure PET surface the rate between C–C, C–H and

O=C–O relative peak area is 80% and 20% respectively. For

100 nm of Ag on PET C–C, C–H and O=C–O relative peak

area is 97% and 3%, for 20 nm it 89% and 10%, respectively.

The C1s peaks fitting results of all samples show the

unchangeable behavior of C1s bound for 20 nm silver films

on PET (even after annealing and sputtering) (Fig. 5). The

relative area of O=C–O bound is more higher then twice

compared to other samples (50, 100 nm). The appearance of

15000

35000

55000

75000

95000

115000

135000

364366368370372374376378Binding Energy, eV

Inte

nsity

, a.u

Ag 3d5/2

Ag 3d3/2

Ag - O

Ag

sat.

Fig. 4. Fitted XPS Ag 3d spectra for 20 nm Ag film on PET annealed at 140

-C,: crosses— acquired data, dashed line —fitted data, thin line— metallic

silver, thick line— silver bounded to oxygen, ‘‘sat.’’— satellite signal due to

nonmonochromatized Al Ka radiation.

intense O=C–O peak is clear evidence of discontinuous of thin

metal film [30].

4. Concluding remarks

Ag thin film structure, morphology and optical and surface

analysis results suggest that Ag film deposited on PET at

elevated temperatures are (111) textured, and depending on the

thickness are discontinuous (20 nm) or continuous (50–100 nm)

films. The preferred orientation of some Ag grains starts at very

early growth stages (i.e 20 nm). Formation of Ag nanorods in

PET matrix was indicated by surface plasmon longitudinal

resonance peak in UV-VIS spectra and typical peak on the XPS

spectra. Thermal annealing of these discontinuous films at

temperatures close to the glassing temperatures brings to the red

shift of plasmon resonance wavelength. Formation of discon-

tinuous structure was observed for 50 nm of Ag film on the PET

substrate, i.e. the reflectivity of this film increased with

increasing the wavelength from 500 to 800 nm. The continuous

Ag films (100 nm) presented close to 100% reflectivity. Some

kind of similarity between ‘‘bulk’’ silver and 100 nm Ag film on

PET samples surface composition can be noticed. The silver

oxidation product as hexagonal silver (I) oxide (Ag2O) was

detected on 100 nm silver layer surface after 140oC annealing.

Acknowledgements

The Lithuanian Science and Study Foundation has sup-

ported this work and is greatly acknowledged.

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