moisture barrier films deposited on pet by icpecvd of sinx

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Moisture Barrier Films Deposited on PET byICPECVD of SiNx

Rainer Wolf,* Klaus Wandel, Christine Boeffel

In this work, the inductively-coupled PECVD (ICPECVD) technology has been used to depositmoisture barrier coatings in the range of 25–1 200 nm on a 100 mm thick polyethyleneterephthalate (PET) and polyethylene naphthalate (PEN) substrate. The water vapour trans-mission rate (WVTR) was determined at room temperature using the Calcium test method. Adecrease in the metallic calcium layer thickness leads to an increase in transmission coeffi-cient, related to the amount of absorbed water as a function of time. At a coating thickness ofaround 25 nm, the WVTR is reduced to less than 0.1 g �m�2 �d�1. With increasing SiN thickness,the transmission rate decreases to an excellent value of 2�10�3 g �m�2 �d�1.

Introduction

The use of polymer-based substrates enables the fabrica-

tion of flexible electro-optical devices like OLED on such

substrates. This polymer material has the limitation that

oxygen and moisture rapidly diffuse through the polymer

and subsequently degrade the electro-optical devices.

Inorganic barrier layers like SiNx and SiOx obtained by

PECVD can decrease the permeation through substrates by

several orders of magnitude to an asymptotic minimum

value attributed to microscopic defects in the coating.[1]

Schaepkens et al reported on SiNx coatings deposited

either by expanding thermal plasma (ETP) or PECVD on

0.175 mm thick polycarbonate film substrates using silane

and ammonia.[2] WVTR below 0.15 g �m�2 �d�1 (100%

relative humidity and 25 8C) has been measured for

200 nm ETP-SiNx and for 15–30 nm PECVD-SiNx in contrast

to 11 g �m�2 �d�1 for the bare polycarbonate film. The

needed lower thickness of PECVD-SiNx is attributed to the

higher material density. The use of a multilayer structure

R. WolfOUT e.V., Kopenicker Str. 325b, 12555 Berlin, GermanyE-mail: [email protected]. WandelSENTECH Instruments GmbH, Carl-Scheele Str. 16, 12489 Berlin,GermanyC. BoeffelFraunhofer Institute for Reliability and Microintegration, Kantstr.55, 14513 Teltow, Germany

Plasma Process. Polym. 2007, 4, S185–S189

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

consisting of organic-like and inorganic-like layers im-

proves the barrier performance by more than one order of

magnitude.

It is well known that PECVD-SiNx deposited at tem-

peratures below 200 8C shows high BHF etch rates, high

hydrogen content and low density. One way to improve

these properties is the use of high-density plasma sources.

In ref.,[3] we reported on low-temperature (80–130 8C)high-quality silicon nitride using a SiH4/NH3/Ar chemistry

and ICPECVD technology.[3] The lowest etch rates are found

for nearly stoichiometric SiNx with a refractive index close

to 2 and were only two- and four-fold, respectively, higher

than those for LPCVD SiN (780 8C).The moisture barrier properties of such ICPECVD-SiNx

layers are therefore of great interest and will be reported

here.

Experimental Part

The SiNx films were deposited in the high-density plasma chemi-

cal vapour deposition (HDPCVD; ICPECVD) system SI 500 D

(SENTECH Instruments GmbH). This system is based on the planar

ICP source PTSA 200 (Planar Triple Spiral Antenna). The antenna

resides on a 15 mm thick quartz window of 320 mm diameter

attached at the top of the reactor chamber. The wafers are

mechanically clamped on the lower electrode. Helium backside

cooling allows the deposition of SiNx films at temperatures as low

as 75 8C. The gases NH3 and Ar were fed directly into the plasma

source through peripheral openings. The SiH4 (5% in He) entered

DOI: 10.1002/ppap.200730608 S185

R. Wolf, K. Wandel, C. Boeffel

S186

the chamber through a gas ring above the substrate. Typical

process parameters are: substrate temperature 70–120 8C, pres-sure 3–5 Pa, ICP-Power 500–750 W, SiH4 flow 145–210 sccm, NH3

flow 7–20 sccm, Ar flow 140 sccm.

The SiNx films were deposited on 400 Si wafers and on polymer

foil substrates such as polyethylene terephthalate (PET, 125 mm)

and polyethylene naphthalate (PEN, 125 mm). For the WVTR

measurements, the polymer substrates were cut into 50� 50mm2

squares, cleaned with isopropanol and degassed. For the deposi-

tion a carrier was used.

The SiNx filmswere characterized by spectroscopic ellipsometry

(SE 850 spectroscopic ellipsometer by SENTECH Instruments). The

refractive index nb at l¼ 632.8 nm can be varied between 1.9 and

2.2 depending on the deposition process. The average stress in the

SiNx filmswas determined from the opticalmeasured curvature of

the silicon wafers using the Stoney formula.

For the WVTRmeasurements the calciummirror test was used.

ASTM E 96 and the so called Mocon test allow for the relatively

comfortable determination of the water vapour permeation on

adhesive foils. But this test is limited to 5� 10�3 g �m�2 �d�1

for H2O. The calcium (Ca) mirror test is better suited for such

analyses.[4] Here, a 100 nm thick Ca-layer is vacuum-deposited

onto SiNx on polymer foils. The whole substrate (Ca side) is then

sealed by a cover glass. The measurement of penetrating

moisture and oxygen is determined indirectly via the transmis-

sion change of the Ca-layer. According to the following chemical

equations:

Plasma

� 2007

Caþ 2H2O ! CaðOHÞ2 þ H2

2Caþ O2 ! 2CaO(1)

the opaque Ca-layer is changed to transparent calcium hydroxide

or calcium oxide, respectively, meaning that also the penetrating

oxygen is recorded by this measuring method. For the measure-

ment, the Ca-layer is divided into various sectors.

The optical transmission of the layer in each

single sector is determined via a laser and a

photodiode. Via the averaged transmission, a

water equivalent and therefore the WVTR can be

determined. At a storage temperature of 25 8Cand ambient humidity (30–50%), the calcium

degradation over time was monitored.

Figure 1. Film stress of the SiNx layers on Si as a function of the reactor pressure fortwo series. For each parameter set, the NH3 gas flow rate is corrected to adjust therefractive index close to 2.

Results and Discussion

At first, the deposition of the polymer

substrates requires a deposition temperature

below 100 8C. Secondly, the material density

of a thin film to a certain extent should be

related to the barrier performance. Therefore,

SiNx films with the lowest etch rates should

be promising candidates for the WVTR tests.

Thirdly, the stress in the silicon nitride layers

should be as small as possible.

The effects of source power, pressure,

and gas flow rates on refractive index,

deposition and etch rates were discussed by

Process. Polym. 2007, 4, S185–S189

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Wolf et al.[3] The lowest etch rates (BOE, 8 nm �min�1 were

achieved for films with a refractive index close to nb� 2.0

with deposition rates above 40 nm �min�1 at lower gas

flow rates of reactants and at a higher source power.

Generally, the refractive index nb decreases with increa-

sing NH3 gas flow rate, so that for a given deposition

parameter set it is easy to adjust nb.

Figure 1 shows the effect of pressure on the film stress

for two parameter sets.

The compressive film stress increases with increasing

source power and decreasing pressure.

It is well known that ion bombardment causes com-

pressive film stress. A higher source power causes a higher

ion density, and the mean free path increases with

decreasing pressure – both leading to higher compressive

stress. So the way to get zero stress is to choose a higher

pressure or a lower power. The lower etch rates of SiNx

films of series (B) favour their application as barrier films

on polymers.

To control the growth on polymer substrate, 400 nm

thick SiNx films of series (B) were deposited on PET

(Figure 2).

The SiNx film showing the highest compressive stress on

Si causes the highest bowing of the SiN coated PET foil. On

the other hand, the parameter set giving the lowest stress

on Si results in the lowest bow for PET. This means that the

growth on Si and on PET is nearly the same.

Detailed measurements were carried out for various

SiNx layer thickness to determine the WVTR using the

Calciummethod.What is revealing aswell is the process of

the reduction of the Ca-layer thickness. The decrease in the

DOI: 10.1002/ppap.200730608

Moisture Barrier Films Deposited on PET by ICPECVD of SiNx

Figure 2. 400 nm thick SiNx films of series (B) deposited on PET. The bow demonstrates the stress of the SiNx layers.

Ca layer thickness and the concomitant increase in the

relative transmission of the Ca layer over the storage time

at 25 8C and 30–50% relative humidity (RH) are shown in

Figure 3 and 4, respectively.

At a storage temperature of 25 8C and ambient humidity

(30–50%), aWVTR of 0.61 g �m�2 �d�1 has been determined

for bare 125 mm PET and a WVTR of 0.14 g �m�2 �d�1 for

bare 125 mm PEN substrate.

For SiNx deposited polymer foils, a significantly lower

WVTR can be observed (Figure 4). The WVTR determined for

a 100 nm SiNx film on PET lies at 0.029 g �m�2 �d�1, more

than one order of magnitude less than for the bare foils.

WVTR measured for PET and PEN samples of varying

SiNx thickness are shown in Figure 5.

From Figure 5, it can be seen that with the ICPECVD-SiNx

layers an improvement in WTRV values over two orders of

magnitude over bare PET/PEN is realized. The best WVTR

lies near 2� 10�3 g �m�2 �d�1. This behaviour is an

indication that SiNx films deposited by high density

plasma are superior barriers towater vapour transmission.

Figure 3. Ca thickness decreases and relative transmission increasestime for the bare PET foils at 25 8C and 30–50% RH.



The behaviour of WVTR for coated polymer foils is

described in the literature.[1,2,5]

Schaepkens et al reported on SiNx coatings deposited by

either ETP or PECVD on 0.175mm thick polycarbonate film

substrates using silane and ammonia.[2] Water vapour

transmissions rates (WVTR) below 0.15 g �m�2 � d�1 (100%

RH. 25 8C) have beenmeasured for 200 nmETP-SiNx and for

15–30 nm PECVD-SiNx. The WVTR value of the uncoated

substrate lies near 11 g �m�2 �d�1.

Erlat et al reported on AlOxNy coatings deposited by

reactive magnetron sputtering on 50 mm thick PET. WVTR

measurements were performed by MOCON test at 30 8C and

100% relative humidity.[5] For>60 nm AlOxNy they reported

0.11 g �m�2 � d�1 and for bare substrate 3.89 g �m�2 �d�1.

The higher WVTR reported for coated and uncoated

polymer foils are caused by different material thicknesses

and humidity values and are measured by different

methods.

To compare the measurement methods, let us examine

the WVTR of uncoated PET. The permeability of a polymer

over the storage

foil does not alter with varying thickness.

For a rough estimation, the measured

WVTR� 3.89 g �m�2 �d�1 in ref.[5] can be

corrected by thickness and by RH value,

(50 mm/125 mm)�0.4 and agrees very well

with our measured WVTR� 0.6 g �m�2 �d�1. This means that both methods, the

MOCON test and the Calcium test, give

comparable values in this range.

Even if we take into account the

difference in thickness of polymer foils,

the ICPECVD-SiNx films are excellent

barriers and promising candidates for

layer stacks.

It is interesting that the bare PEN subs-

trate obviously shows the lower WVTR

than PET. However, the values of coated

foils are lower for PET with film thickness

up to 100 nm. These differences level out

for thicker films. This indicates a more

perfect surface of the PET foil and a need

of thicker films to cover a rough surface.

www.plasma-polymers.org S187

R. Wolf, K. Wandel, C. Boeffel

Figure 5. WVTR as a function of SiNx coating thickness on PET or PEN (WVTRPET: 0.60 g �m�2 � d�1, WVTRPEN: 0.13 g �m�2 �d�1).

Figure 4. Ca thickness decreases and relative transmission increases over the storage time for the 100 nm SiNx/PET at 25 8C and 30–50% RH.

S188

Conclusion

High-quality, low-temperature and low-stress silicon

nitride was deposited by ICPECVD on Si, PET and PEN

foils. The stress values are adjustable maintaining other

layer properties like the refractive index. TheWVTR of SiNx

coated PET and PEN substrates were measured using the

Calcium method. The superior properties of these nitride



films result in low values for WVTR as 0.002 g �m�2 �d�1

and make these films suitable for applications such as

single film barrier or as barrier layer in a layer stack.

Acknowledgements: We gratefully acknowledge the FederalMinistry of Economics and Technology (BMWi, Germany, contractno. 1179/03) for supporting parts of this work.

DOI: 10.1002/ppap.200730608

Moisture Barrier Films Deposited on PET by ICPECVD of SiNx

Received: September 9, 2006; Accepted: November 16, 2006; DOI:10.1002/ppap.200730608

Keywords: calcium method; high-density plasma; inductivelycoupled; moisture barrier; PET; silicon nitride

[1] A. S. da Silva Sobrinho, G. Czeremuszkin, M. Latreche,G. Dennler, M. R. Wertheimer, Surf. Coat. Technol. 1999,116–119, 1204.



[2] M. Schaepkens, T. Kim, A. G. Erlat, M. Yan, K. Flanagan,C. Heller, P. McConnelee, GE Global Research, Report number2003GRC334, Febr. 2004.

[3] R. Wolf, K. Wandel, B. Gruska, Surf. Coat. Technol. 2001,142–144, 786.

[4] G. Nisato, P. C. P. Bouten, P. J. Slikkerveer, W. Bennett, G. Graff,N. Rutherford, L. Wiese, ‘‘Evaluating High Performance Diffu-sion Barriers: The Calcium Test’’ Proceeding Asia Display/IDW’01, 2001, pp 1435–1438.

[5] A. G. Erlat, B. M. Henry, J. J. Ingram, D. B. Mountain,A. McGuigan, R. P. Howson, C. R. M. Grovenor, G. A. D. Briggs,Y. Tsukahara, Thin Solid Films 2001, 388, 78–86.

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moisture barrier films deposited on pet by icpecvd of sinx

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