Surface & Coatings Technology 231 (2013) 496–500

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Surface & Coatings Technology

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Effect of magnetron sputtered silicon nitride on the water-vapor-permeation-rate ofpolyimide thin film

I-Hsiang Tseng a, Chi-Jung Chang b, Chin-Wen Chang a, Horng-Hwa Lu c, Mei-Hui Tsai a,⁎a Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, No.57, Sec.2, Zhongshan Rd., Taipin Dist., Taichung 41170, Taiwanb Department of Chemical Engineering, Feng-Chia University, No.100, Wenhwa Rd., Seatwen Dist., Taichung 40724, Taiwanc Department of Mechanical Engineering, National Chin-Yi University of Technology, No.57, Sec.2, Zhongshan Rd., Taipin Dist., Taichung 41170, Taiwan

⁎ Corresponding author. Tel.: +886 4 23924505; fax:E-mail address: tsaimh@ncut.edu.tw (M.-H. Tsai).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2012.07.038

a b s t r a c t

a r t i c l e i n f o

Available online 20 July 2012

Keywords:PolyimideWater-vapor-transmission-rateSilicon nitrideBendingRF magnetron sputtering

A series of PIs were first derived from 4,4′-oxydianiline (ODA) and various dianhydrides, including pyromelliticdianhydride (PMDA), 4,4′-oxydiphthalic dianhydride (ODPA), and 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (IDPA), to produce PI matrix with various degrees of rigidity. In order to reduce thewater-vapor-transmission-rate (WVTR) of PI, a moisture barrier was deposited on each PI substrate by an RFmagnetron sputtering system and theWVTRwas investigated as a function of rigidity of PImatrix. The depositedthin film (SiNxOy) with the thickness of 100 nmon PI is an excellent moisture barrier that theWVTR of all PI thinfilms is significantly reduced. The relatively rigid PImatrix leads to a denser packing of barrier layer, and thus ex-hibits lower WVTR than other two PI systems. The WVTR of ODA–PMDA/SiNxOy declines to 5 g/m2-day com-pared with 123 g/m2-day for parent PI film. On the other hand, the moisture barrier on a flexible PI matrix(ODA–IDPA) shows a higher reliability on the WVTR after bending treatment to 15,000 cycles. In additionto the moisture barrier feature, the SiNxOy-deposited PI thin films maintain favorable thermal stability andmechanical strength for practical applications.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polyimides (PIs) have been considered as potential candidates tosubstitute glass substrates in microelectronic applications due to theirexcellent thermal and mechanical characteristics [1–3]. However, highwater absorption or permeation as well as high oxygen permeation ofPI limit the performance or service life of those electronics using PIas a substrate [1–4]. Sykes [4] investigated the gas and water vaportransmission rates (GTR and WVTR) of a series of PI films derivedfrom dianhydrides PMDA or BTDA and various diamines. The PI filmswith relatively rigid structure exhibit low WVTR. The WVTR is 341and 130 g/m2-day, respectively, for an ODA–PMDA film and an ODA–BTDA film (same thickness: 25 μm) at 37.7 °C and 100%RH. The wateror gas permeability of polymers can be reduced by atomic layer de-position (ALD) or chemical vapor deposition (CVD) of inorganic mate-rials, such as Al2O3, SiO2 and SiNx, on polymer substrates [3,5–8]. PI(Kapton) films prepared by Burek et al. [9] exhibit enhanced moistureresistance after coating with monolayer organic film by Langmuir–Blodgett technology. The WVTR through Kapton film (thickness: 75 μm)is 168 g/m2-day and drops to as low as 28 g/m2-day for LB-coated ones.Si3N4 has been considered as a superior moisture resistance materialsfor semiconductor application [7]. Wuu et al. [1] deposited silicon nitrideand parylene thin films on flexible PI substrate by plasma-enhanced

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CVD method. After deposited with 100 nm SiNx on PI, the WVTR ofPI-SiNx films reduces to 0.06 g/m2-day compared with 20 g/m2-dayfor pure PI film. With the presence of parylene layer between SiNx andPI, theWVTR further decreases to 0.01 g/m2-day. However, the high de-position temperature is required, up to 200 °C, to reduce the defectquantity. In previous study [10], we found the working pressure of RFsputtering significantly affects the roughness and packing density ofSi3N4 barrier film on PI/Al2O3 hybrid film aswell as theWVTR of PI com-posites. A denser structure of Si3N4 on PI substrate leads to a lowerWVTR.

In this study, a series of aromatic PI films were synthesized fromODA and three different dianhydrides, PMDA, ODPA, and IDPA to pro-duce rigid, semi-rigid and flexible PI matrix. Those PI films were thendeposited with a moisture barrier by RF sputtering. The effects of PIbackbone rigidity together with the properties of deposited layer onWVTR of PI films were investigated. Moreover, the reliability of WVTRof PI film after bending tests was evaluated in this work.

2. Experimental

2.1. Materials

Dianhydrides, including pyromellitic dianhydride (PMDA), 4,4′-oxydiphthalic dianhydride (ODPA), and 4,4′-(4,4′-isopropylidene-diphenoxy) bis(phthalic anhydride) (IDPA), from Taimide Tech. Inc.were purified by recrystallization from acetic anhydride and then dried

497I.-H. Tseng et al. / Surface & Coatings Technology 231 (2013) 496–500

in a vacuum oven at 125 °C for 24 h. 4,4′-diaminodiphenyl ether (ODA)purchased from TCI was vacuum-dried for 12 h at 105 °C prior to use. N,N-dimethyl acetyamide (DMAc) was dried over molecular sieves beforeusage. A silicon nitride disk (purity: 99.99%) from Summit Techwas usedas the target for sputtering.

2.2. Preparation of PI

Diamine ODA was first dissolved in DMAc at room temperature(R.T.) with nitrogen purging. Equal molar amount of dianhydride(PMDA, ODPA or IDPA) was then added into the stirred ODA solutionby four portions. The homogeneous solution was stirred at R.T. for 6 hunder N2 to obtain poly(amic acid) (PAA) solutions with a solid con-tent of 18 wt.%. The reaction scheme is shown in Fig. 1. The PAA solu-tion was then casted on a glass substrate by a doctor blade. The castedfilms were cured in an air-circulating oven at 110, 170, 230 and300 °C, each for 1 h. The peeled freestanding PI film has a thicknessranging from 19 to 22 μm.

2.3. Deposition of barrier film on PI

The SiNxOy film was deposited on PI hybrid films by a RF magne-tron sputtering system under the conditions shown in Fig. 1. Thedeposited PI films was denoted PI/SiNxOy or ODA–PMDA/SiNxOy,ODA–ODPA/SiNxOy and ODA–IDPA/SiNxOy, respectively, to indicatethe monomers used in PI synthesis.

2.4. Measurements

The water vapor transmission rate (WVTR) of samples with thesize of 10 cm2 was measured using a Permatran-w3/61 model systemat atmospheric pressure, 40 °C and 100% relative humidity. The thick-ness of the deposited films was measured with an alpha-step profiler

Fig. 1. The synthesis route of P

(Kosaka ET 4000a). The morphology of thin films was conductedwith a scanning electron microscope (SEM, JEOL JSM-6700F). Thecrystalline structure of deposited film was identified by X-ray diffrac-tion (XRD, Shimadzu XRD-6000) equipped with a CuKα radiation.The bonding structure of deposited film on PI was examined byXPS spectra using an X-ray photoelectron spectroscopy (UlvacPHI-5000). The dynamic mechanical analysis (DMA, TA 2980) wasperformed at a frequency of 1 Hz and a heating rate of 3 °C/min.Thermogravimetric analysis (TGA, TA Q500) was performed with aheating rate of 20 °C/min under nitrogen. The in-plane coefficient ofthermal expansion (CTE) of samples was determined from TMA mea-surements (TA Q400) at a heating rate of 10 °C/min.

3. Results and discussion

3.1. Characteristics of deposited film on PI

Fig. 2 shows the XRD patterns of deposited films on PI composites.There is no significant difference between three PI composites. How-ever, the positions of the diffraction peaks at 2θ=37.6°, 43.7°, 64.6°and 77.7° are not consistent with standard Si3N4, Si2N2O or SiC dif-fraction data reported in literature [7,11–15]. XPS analysis alsoshows only little nitrogen but abundant oxygen present in the depos-ited films. We believed the crystalline structure is SiNxOy. Byselecting the most intense diffraction peak at 43.7°, the crystallinesize of deposited silicon-related compound could be estimated bythe Debye–Scherrer equation [13]. The calculated average crystallinesize (d) of this Si-based compound is around 12 nm, which is not af-fected by the PI structure.

The XPS depth profiling was performed to study the chemicalcomposition of deposited film by using an Ar+ ion gun at a sputteringrate of 11.2 nm/min. The narrow scan XPS spectra of Si2p and O1s ofdeposited PI films are illustrated in Fig. 3. Fig. 3a shows the Si2p

I and PI/SiNxOy thin films.

15 20 25 30 35 40 45 50 55 60 65 70 75 80

Inte

nsit

y (a

.u.)

(c)

(b)

(a)

2θ

Fig. 2. XRD patterns of (a) ODA–PMDA/SiNxOy, (b) ODA–ODPA/SiNxOy and (c) ODA–IDPA/SiNxOy thin films.

110 108 106 104 102 100 98 96

(103.5 eV)O-Si-O

Inte

nsit

y (a

.u.)

Inte

nsit

y (a

.u.)

Binding energy (eV)

Binding energy (eV)538 536 534 532 530 528 526

Si-O-Si(532.4 eV)

(a)

(b)

Fig. 3. XPS (a) Si2p and (b) O1s spectra and peak deconvolution results of ODA–IDPA/SiNxOy thin film.

1000 800 600 400 200 0

Inte

nsit

y (a

.u.)

Si2pSi2s

N1s

O1sODA-IDPA/SiNxOy

Binding energy (eV)

Fig. 4. Full scan XPS spectrum of ODA–IDPA/SiNxOy thin films.

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photoelectron emission peaks after 1 min Ar+ sputtering (i.e. aboutthe depth of 10 nm below the surface) on PI (ODA–IDPA). Aftercurve fitting, the Si2p peak demonstrates the dominated Si–O signalwith the binding energy of SiO2 at 103.5 eV [10,11,16]. Moreover,the deconvolution of O1s XPS spectrum (Fig. 3b) of this sampleshows a single Si–O peak at 531.8 eV [11,16,17], which is in agree-ment with the presence of SiO2 on the deposited film. Similar resultswere observed for other deposited PI composites. Surprisingly, the in-tensity of collected N1s spectra under depth profiling, from top sur-face to the middle of the deposition layer, are all very weak. Fig. 4 isthe XPS full scan spectrum of ODA–IDPA/SiNxOy thin film. Thecomposition of N from obtained survey spectra is only 0.1 mol%, com-pared with 30 mol% for Si on three PI films. The relative compositionof O to Si for ODA–IDPA/SiNxOy sample is around 2.2 and is not func-tion of the Ar+ sputtering period within 5 min. The composition ofSi2p and O1s photoelectrons of deposited layers from top surfaceto the depth of 50 nm (half of the deposition thickness) is almostidentical. The O/Si ratio for other PI composites is 2.38 and 2.24, re-spectively, for ODA–PMDA and ODA–ODPA composites. It has beenobserved that the oxidation usually occurs on the uppermost surfacelayer of Si3N4 [11]. The surface oxygen composition significantly in-creases with the period of storage, especially in the environmentwith high humidity. In this work, the large amount of oxygenpresenting in the deposited film from top surface to half the depthsuggests the severe oxidation of Si3N4 target occurred during the stor-age. The further confirmation is still in progress.

The morphology of deposited layer on three PI films is shown inFig. 5. The size of deposited SiNxOy particles shown in SEM imagesis close to the crystalline size calculated from XRD results. The effectof PI structure on the average size of deposited particles is negligible.In contrast, a relatively denser packing of SiNxOy particles are re-vealed in ODA–PMDA film (Fig. 5(a)). The low packing density andpoor homogeneity of particle size are revealed on the surface ofODA–ODPA and ODA–IDPA films.

3.2. Mechanical and thermal properties of pure PI and PI/SiNxOy thinfilms

TGA, TMA and DMA analyses were utilized to reveal the mechan-ical and thermal properties of the pure PI and PI/SiNxOy films.The DMA storage modulus for both pure PI and deposited PI filmssynthesized from ODA–PMDA are higher than from ODA–ODPA orODA–IDPA (Fig. 6). These results contribute to the more rigiddianhydride unit of PMDA compared to ODPA or IDPA, resulting in

Fig. 5. Morphology of deposited-film on PI composites: (a) ODA–PMDA–SiNxOy, (b) ODA–ODPA–SiNxOy and (c) ODA–IDPA–SiNxOy.

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compact packing with smaller free volume in bulk PI films. Conse-quently, the ODA–PMDA films exhibit more rigid for their mechanicalproperties (enhanced storage modulus) than other PI films. In addi-tion, there is no significant difference for storage modulus betweenpure PI and PI/SiNxOy films, meaning that the deposition did not af-fect the mechanical properties of the resulting hybrid PI films.

The tanδ curves of the PI films are shown in Fig. 7. The glass tran-sition temperature (Tg) is determined from the tanδ peak. The PI fromIDPA has a lowest Tg of 225 °C. On the other hand, the PI from PMDAshows highest Tg of 419 °C. These results can be explained by chainflexibility. The IDPA have alkyl groups and oxygen linkage (ethergroups), allowing more degrees of conformational freedom so that

60 100 200 300 400

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Stor

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Pa)

0

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Stor

age

mod

ulus

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Pa)

Temperature (ºC)

60 100 200 300 400Temperature (ºC)

ODA-PMDA

ODA-ODPA

ODA-IDPA

ODA-PMDA/SiNxOy

ODA-ODPA/SiNxOy

ODA-IDPA/SiNxOy

(a)

(b)

Fig. 6. Storage modulus of (a) pure PI and (b) SiNxOy-deposited PI thin films.

its polymer chains can archive relaxation state at lower temperature.The PMDA lacks of flexible structures (CH3 or –O–) so that highertemperature is needed to move the resulting PI chains, leading tohighest Tg. Similarly, the damping intensity of the relaxation peak(tanδ) decreased significantly by using the PMDA as the dianhydrideunit compared to ODPA and IDPA, indicating the formation of morerigid polymer network. The changes of tan δ curve of PI film aftersputtering are not significant.

The dimension stability is one of the important designing param-eters for the application of polymer multilayer substrate in microelec-tronic field. Hence, the CTE of the deposited PI films were evaluatedand their results were summarized in Table 1. The ODA–PMDA/

100 200 300 400

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0.5

1.0

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2.0

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ta

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100 200 300 400Temperature (ºC)

ODA-PMDA ODA-ODPA ODA-IDPA

ODA-PMDA/SiNxOy ODA-ODPA/SiNxOy ODA-IDPA/SiNxOy

(a)

(b)

Fig. 7. The tan delta curves of (a) pure PI and (b) SiNxOy-deposited PI thin films.

Table 1Thermal properties and water-vapor-transmission-rate (WVTR) of pure PI and PI/SiNxOythin films.

Sample Tga (°C) CTEb (ppm/K) Tdc (°C) WVTRd (g/m2-day)

ODA–PMDA 419 –e 562 123ODA–ODPA 275 –e 557 133ODA–IDPA 225 –e 524 178ODA–PMDA/SiNxOy 404 26 566 5ODA–ODPA/SiNxOy 270 54 557 6ODA–IDPA/SiNxOy 219 92 521 25

a Determined from the peak temperature of tan delta curve.b Determined from the TMA results in the temperature range of 100 to 200 °C.c Determined from the TGA curves at the temperature of 5 wt.% weight loss.d Before bending test.e Not determined in this work.

0 5000 10000 15000

0

20

40

60

80

100

120

ODA-PMDA/SiNxOy

ODA-ODPA/SiNxOy

ODA-IDPA/SiNxOy

Bending cycles

WV

TR

(g/

m2 -d

ay)

Fig. 8. Effects of bending cycle and PI matrix on the WVTR of PI/SiNxOy thin films.

500 I.-H. Tseng et al. / Surface & Coatings Technology 231 (2013) 496–500

SiNxOy film have a CTE of 26.4 ppm/°C, in the temperature range of100 to 200 °C, which is lower than that of ODA–ODPA/SiNxOy andODA–IDPA/SiNxOy films. This could be easily understood by consider-ing that the ODA–PMDA/SiNxOy has higher Tg than ODA–ODPA/SiNxOy and ODA–IDPA/SiNxOy. The SiNxOy was more dense accumu-lation in deposition process, thus may show a lower CTE.

3.3. WVTR of pure PI and PI/SiNxOy thin films

The WVTRs of pure PI films as well as sputtered PI films are listedin Table 1. The WVTR of as-prepared ODA–PMDA is the lowest one,123 g/m2-day, compared with 133 and 178 g/m2-day, respectively,for ODA–ODPA and ODA–IDPA PI films. A more rigid PI structure indi-cates more compact polymer chains and consequently exhibits small-er free volume. Hence, the PI film derived from relatively more rigiddianhydride unit PMDA exhibits lowest WVTR. With the depositionof SiNxOy barrier films on pure PI films, the WVTR drops dramaticallyto 5, 6, and 25 g/m2-day for ODA–PMDA, ODA–ODPA, and ODA–IDPAfilms, respectively. With the same deposition thickness, significantdecrease in WVTR (nearly 96%) occurs on ODA–PMDA/SiNxOy. Recallthe SEM images shown in Fig. 3, a denser packing of SiNxOy particlesis observed on rigid ODA–PMDA substrate thus the barrier film effi-ciently decreases the WVTR.

In addition, the effect of bending cycles on the WVTR wasperformed on deposited PI films. As shown in Fig. 8, the WVTR ofPI/SiNxOy increases with the increasing bending cycles. Notably, theincrease in WVTR with bending cycles for ODA–IDPA/SiNxOy is notas significant as other PI systems with relatively rigid backbones. Amoisture barrier on flexible PI matrix exhibits a higher reliability onWVTR after bending test. Wuu et al. [1] have mentioned that the in-ternal stress control is critical in flexible substrates deposited with in-organic layer. The PI substrate with relatively flexible structure maylead to less cracks formation on the deposited inorganic layer and con-sequently has lower increase in WVTR value after bending treatment.The increase in WVTR for ODA–IDPA/SiNxOy within 5000 times ofbending treatment is negligible and is less than 40 g/m2-day after10,000 bending cycles.

4. Conclusions

The moisture barrier layer was successfully deposited on the sur-face of synthesized PI films. The WVTR of PI/SiNxOy composite withmore rigid PI structure (i.e. ODA–PMDA) is lower than that of otherPI films. The WVTR significantly decreases from 123 g/m2-day forODA–PMDA to only 5 g/m2-day after sputtering process. The bendingtreatment may lead to the formation of cracks in deposited films thatthe WVTR increases with bending cycles. Notably, the increase inWVTRwith bending cycles is smaller for PI substrate withmore flexiblePI structure. The deposited moisture barrier on a flexible PI matrix(ODA–IDPA) shows a higher reliability on the WVTR after bendingtreatment.

Acknowledgment

The authors would like to acknowledge the financial support fromtheMinistry of Economic Affairs, Taiwan, through the project on flexiblepolymericmaterials for electronic application (99-EC-17-A-07-S1-120).

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