Thin Solid Films 532 (2013) 84–88

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Thin Solid Films

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All hot wire chemical vapor deposition low substrate temperature transparent thinfilm moisture barrier

D.A. Spee ⁎, M.R. Schipper, C.H.M. van der Werf 1, J.K. Rath, R.E.I. SchroppNanophotonics — Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus, Building 5, 5656 AE Einhoven, The Netherlands

⁎ Corresponding author.E-mail address: d.a.spee@uu.nl (D.A. Spee).

1 Presently at: Energy Research Centre of the NethCampus 5, 5656 AE Eindhoven, The Netherlands.

0040-6090/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2012.11.146

a b s t r a c t

a r t i c l e i n f o

Available online 22 December 2012

Keywords:Hot wire chemical vapor depositionThin film encapsulationFlexible electronicsMoisture barrier

We deposited a silicon nitride/polymer hybrid multilayer moisture barrier for flexible electronics in a hot wirechemical vapor deposition process, entirely below 100 °C. We were able to reach a water vapor transmissionrate (WVTR) as low as 5×10−6 g/m2/day at a temperature of 60 °C and a relative humidity of 90% for a simplethree-layer structure consisting of two low-temperature silicon nitride (SiNx) layers and a polymer layer inbetween. This WVTR is low enough for organic and polymer devices. In a second experiment it is investigatedhow the yield of our samples increases with the number of SiNx layers, while keeping the total SiNx thicknessconstant. Cross sectional scanning electron microscopy images of degraded samples reveal a high structural ro-bustness of our multilayers.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Sensitive electronic devices can easily be damaged by oxygen andwater vapor permeating into their active layers. This is an importantissue for devices made on flexible plastic substrates, which, contraryto glass, have a high permeability to oxygen and water. This meansthat permeation barrier layers on flexible substrates are requiredfor many devices, including organic light emitting diodes, flexiblesolar cells, and rollable displays [1–3]. One way to create such a bar-rier, is to deposit an inorganic/organicmultilayer in which the organ-ic layers decouple defects in consecutive inorganic layers. In this waychances of permeation paths like pinholes or cracks propagatingthrough the entire multilayer are decreased. A combination of SiNx

and polymer is very suitable to create such a multilayer [4]. We areusing poly(glycidyl methacrylate) (PGMA) as the polymer. This is awidely available and inexpensive material. SiNx is deposited by hotwire chemical vapor deposition (HWCVD) and PGMA by initiatedchemical vapor deposition (iCVD), a variant of HWCVDwhere an ini-tiator is dissociated into two radicals at a hot filament and starts thepolymerization process [5]. HWCVD and iCVD use radical formationat heated wires, which provide a linear source of radicals. Whenthe substrate is moved perpendicular to the wires the depositionwill be homogeneous in both dimensions [6]. This makes it easy to

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implement in a roll-to-roll or inline process. Since in the HWCVDprocess no high energy ions are present, layers can be deposited ondelicate substrates. To be able to deposit SiNx on PGMA layers, twomain requirements need to be fulfilled: (i) the PGMA should bevery stable against the flux of chemical species during SiNx deposi-tion [7] and (ii) the SiNx must be deposited at low enough substratetemperatures to be compatible with common plastic substrates andalso PGMA. Here, besides discussing a low temperature hybrid mois-ture barrier, we will investigate how the yield of our samples in-creases by adding extra layers and address the robustness of themultilayer.

2. Experimental details

2.1. Layer deposition

The SiNx layers were deposited in a multichamber depositionsystem using pure silane (SiH4) and ammonia (NH3) as source gas-ses. A dedicated low temperature hot wire chamber was used inwhich the wire-substrate distance could be as large as 20 cm. Sinceradiative heating is reduced drastically in this manner, process tem-peratures never exceeded 100 °C [8]. Thermindex TC8020 tempera-ture indication stickers were attached on the front side of thesubstrate to be able to accurately determine themaximum temperaturereached during deposition. The PGMA layers were deposited in a home-made iCVD reactor [9] designed after a reactor at Massachusetts Insti-tute of Technology labs [10]. The layers were deposited at a depositionrate of 0.8 nm/s. As a monomer GMA (97%, Aldrich) was used and asan initiator tert butyl peroxide (98%, Aldrich). All samples were depos-ited on glass.

Fig. 1. The development in time of 5×5 mm2 Ca squares coveredwith: 100 nm low temperature SiNx (top), 100 nm state-of-the-art high-temperature SiNx (middle), and a low temperaturemultilayer (bottom).

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2.2. Characterization

The permeation measurements were performed using the Ca meth-od [11]. In this method squares of metallic calcium on glass are coveredby the barrier layer. Upon oxidation the calcium becomes transparent.In this way the permeatedwater can accurately be determined and pin-holes can bemade visible. By keeping the samples in a climate chamberat a temperature of 60 °C at a relative humidity (RH) of 90%, the mea-surement was accelerated [2]. The Scanning Electron Microscopy

Fig. 2. Graph of the permeation through the multilayer. Fitting a linear function, a WVTR of5×10−6 g/m2/day was found.

(SEM) images were made using a FEI (Nova Nanolab 600 Dualbeam)combined Focussed Ion Beam (FIB) and SEM system [12]. Images weremade in back scattered electron mode using an operating voltage of2.0 kV. The cross sectional SEM images were made under an angle of52°, which means the heights in the images should be multiplied by1/Sin(52°)=1.3.

Fig. 3. SEM image of amultilayer, consisting of four SiNx layers and three PGMA layers, onglass. The cross sectionwasmade by the FIBmethod. The samplewas covered byplatinumto avoid electrical charging.

Fig. 4. The development in time of arrays of 5×5 mm2 Ca squares coveredwith: 4 layers of 50 nmSiNx (top), 3 layers of 67 nm SiNx (middle), and two 100 nm thick SiNx (bottom), with200 nm PGMA interlayers.

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3. Results

3.1. Low temperature double layer

First we developed highly transparent, compact HWCVD SiNx at asubstrate temperature below 100 °C, compatible with our PGMA layersdeposited by iCVD. This facilitated the deposition of transparent SiNx/PGMAmultilayers [13]. Second we tested the permeability of three dif-ferent samples: a 100 nm thick low temperature SiNx layer, a 100 nmthick state-of-the-art SiNx layer [14] and a multilayer consisting oftwo 100 nm low-temperature SiNx layers sandwiching a PGMA layer.For the multilayer we chose a very thin interlayer of only 200 nmPGMA. On the one hand, the decoupling function of the layer will prob-ably decrease with decreasing thickness. On the other hand, for the bar-rier function of amultilayer to be as high as possible, thinner interlayersare beneficial, since they create a more tortuous permeation path [1].We chose a thin polymer layer, since Bakker et al. [10] demonstratedthat PGMA layers made by iCVD are excellent planarizing layers even

Fig. 5. The yield of our samples, as a function of number of SiNx layerswithin themoisturebarrier. For all types 36 samples were studied.

at thicknesses around 200 nm. The optical transmission of Ca squarescovered by the three samples after various incremental exposure dura-tions is shown in Fig. 1. Comparing the Ca degradation of the two singlelayer samples, the behavior is as expected. Higher porosity and a higherdensity of pinholes are expected in low temperature samples comparedto the high temperature ones, since the surface mobility of precursorsduring deposition goes down with decreasing substrate temperature.However, a configuration of two of these layers with a PGMA interlayershows an entirely different behavior. No significant degradation of theCa layer is visible, even after 190 days in the accelerated measurement.In Fig. 2, the permeation graph of this sample is shown. The initial jumpin the permeated water graph was attributed to residual moisture dif-fusing out of the as-deposited polymer layer. Therefore the data of thefirst day was not included in the data used for fitting. When a linearfit is applied, a water vapor transmission rate (WVTR) of 5×10−6 g/m2/day is found, compared to 3×10−5 g/m2/day for a single layer ofstate-of-the-art SiNx. For the individual low temperature layer wewere not able to determine the WVTR because it deteriorates withintwo days by permeation through the pinholes. The excellent barrierperformance of our multilayer can be explained by the fact that our or-ganic layer is kept relatively thin: 200 nm is probably significantlysmaller than the average distance between defects in the inorganiclayers, even if the number of pinholes in the low temperature SiNx ishigh. In this way an extremely tortuous path is created for the permeat-ing moisture [1]. Also the good barrier performance of the double layerbeing partly caused by the filling up of defects in the SiNx layer by PGMA[15] cannot be ruled out. Since the WVTR of 5×10−6 g/m2/day wasfound at 60 °C and 90% RH, an excellent performance at room tempera-ture can be expected [2]. The graph shown in Fig. 2 flattens over time,indicating that the extremely high barrier function is most probably aconsequence of a low equilibrium permeation rather than a high lagtime of thewater vapor [16]. Our experiment shows themultilayers' ro-bustnesswith respect to defects originating from the deposition and thesubstrate. It was however difficult to avoid contamination withdefect-creating dust particles, since the present facilities in our labora-tory are such that the samples had to be moved outside vacuum be-tween different reactors to deposit the multilayer. This affected the

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yield of our samples. In practical production it is straightforward toavoid such issues by transporting the samples between subsequent re-actors in vacuum.

3.2. Increasing yield by increasing the number of layers

To investigate the effect of an increasing number of SiNx layers onthe yield of our samples, i.e. their robustness with respect to environ-mental dust, a second experiment was done, in which three differentconfigurations were tested. Here yield is defined as the fraction of thesamples which does not show any form of deterioration, measuredover 36 samples. We deposited multilayers consisting of two, threeand four SiNx layers, keeping the total thickness of all SiNx layers con-stant at 200 nm: two layers of 100 nm, three layers of 67 nm and fourlayers of 50 nm. In between these layers were always 200 nm thickPGMA interlayers.

A SEM image of a cross section of themultilayer containing four SiNx

layers is shown in Fig. 3. The cross section was made by FIB. The imageshows that the SiNx layers are around 50 nm, as expected. The PGMAlayers however, are around 150 nm, while 200 nm was intended. Ifmore layers are deposited on top, the surface becomes less planar.Knowing that as deposited PGMA layers exhibit a rms roughness lowerthan 5 nm (measured over an area of 10×10 μm) [7], this indicatesthat there is still some slight deformation of the PGMA layers during de-position of the SiNx. It is difficult to see the abruptness of the interfacesin the SEM image, since the electrons have some penetration depth andthe cross section is seen under an angle.

In Fig. 4 the typical development of Ca squares covered by our differ-ent barriers are shown after incremental exposures. Considering thesquares that were not affected by pinholes after 30 days, we see thattheWVTR goes down slightly with an increasing number of SiNx layers:8.3×10−5 g/m2/day for two layers, 3.6×10−5 g/m2/day for threelayers and 2.8×10−5 g/m2/day for four layers, even though the totalamount of SiNx is constant.

The most distinct effect in the second experiment however, is an in-crease in the yield with an increasing number of layers. In Fig. 5 it isshown that the yield increases from 0.06, using two layers, to 0.37using four layers, as measured over 36 samples. Probably there aretwo factors that contribute to this increase: the probability that pinholesin all consecutive layers are laterally in the same spot or less than rough-ly the interlayer thickness apart is decreasing rapidly with the numberof layers. Secondly, the chance of dust particles being entirely covered

Fig. 6. SEM images of: (a) a dust particle causing a pinhole in the multilayer, (b) the rectanarea and (d) an enlarged view of the multilayer being detached from the substrate due to t

increases, since the thickness of the total multilayer increases due tothe increased accumulated PGMA thickness.

3.3. Robustness of the multilayer

In the SEM images of Fig. 6, a dust particle causing a pinhole in a bar-rier coating consisting of four SiNx and three PGMA layers is shown. Theparticle was in the center of a typical pinhole, i.e. a pinhole throughwhich a constant amount of moisture is permeating, causing a whitespot growing linearly in time [10]. In Fig. 6 (a) it can be seen that thepinhole is caused by a dust particle, which is around 10 μm in diameter,cutting through the entiremultilayer. Directly around the pinhole, largebubble-like structures have formed, indicating that themultilayer is de-tached from the substrate. To create a cross section of the white spotarea, a rectangular piece of material was removed by FIB to a depth of3 μm, as is shown in Fig. 6 (b). The cross section and an enlarged detailof it are shown in Fig. 6 (c) and (d). The delamination of the multilayerfrom the substrate is clearly a result of the deterioration of the coveredcalcium layer bymoisture. In this way it is revealed that the adhesion ofthe different organic and inorganic layers and the robustness of themultilayer are high: its structure is retained even when the multilayerstands alone.

4. Conclusions

In a process entirely below 100 °C we deposited a multilayer byHWCVD, consisting of only two SiNx layers with a PGMA layer in be-tween [17]. Using iCVD to deposit PGMA we were able to create aninterlayer with a thickness of only 200 nm, which is very effectivelydecoupling defects in consecutive SiNx layers. Although the low-temperature SiNx itself exhibited a high porosity, this three-layer struc-ture is pinhole free and shows a water vapor transmission rate (WVTR)as low as 5×10−6 g/m2/day at 60 °C and a relative humidity of 90%.This value is low enough for any possible electronic device. Due to thelow radiation and consequently low deposition temperature and theabsence of high energy particles (ions) in the process, these layers canbe deposited directly onto organic of polymeric active devices.

In a second experiment itwas shown that the robustness of ourmul-tilayerswith respect to defect-creating dust particles increaseswhen in-creasing the number of SiNx interlayers while keeping the totalthickness of SiNx the same. In practical production however, the issueof environmental dust can be avoided by transporting samples in

gular area removed by FIB to create a cross section, (c) the cross section of the pinholehe calcium that has reacted with water.

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vacuum. Cross sectional SEM images of a degraded sample reveal thatthe structural robustness of ourmultilayer is high, as is the adhesion be-tween the organic and inorganic layers.

Acknowledgments

We thankDr. P. van deWeijer of Philips Research Laboratories for theCa corrosion tests andD.A.M. deWinter for the FIB-SEM imaging andweacknowledge the financial support for this research from the “StichtingTechnische Wetenschappen” (project Thin Film Nanomanufacturing10019).

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