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Atomic Layer Deposition Process for Barrier Applications of Flexible Packaging

Petri Johansson, Kimmo Lahtinen and Jurkka Kuusipalo

Tampere University of Technology, Paper Converting and Packaging Technology (TUT/PCT)

Tommi Kriinen, Philipp Maydannik and David Cameron

Lappeenranta University of Technology, Advanced Surface Technology Research Laboratory (LUT/ASTRaL)

ABSTRACT

Atomic layer deposition (ALD) is a process where thin films of material are deposited in a controlled manner one

atomic layer at a time. The main advantages of ALD are the extreme degree of conformality and uniformity which

can be obtained regardless of the orientation or shape of the substrate; there are no pinholes in the film. The abilityto do roll-to-roll coating would open huge possibilities for the technique, because barrier layers are required to avoid

the diffusion of water or oxygen through polymer or paper webs for example in the food packaging industry.

According to barrier test results, the aluminum oxide coating provided by the ALD batch process improved the

oxygen and water vapor barrier as a function of coating thickness by a large amount. As an oxygen barrier, a thin 10

nm coating is already able to obtain ten times lower transmission levels than the uncoated structure. As a watervapor barrier, the thickness of the polymer layer becomes practically meaningless once the thickness of the ALD

layer increases above 40 nm. With over 100 nm thickness, the water vapor transmission rates of the structures were

reduced below the level of 1 g/m2/24h (23C, 50% RH). These results encourage the continuation of investigations

towards roll-to-roll solutions of ALD processing.

INTRODUCTION

The purpose of atomic layer deposition (ALD) process is to produce a thin, gas-tight and stable coating from

gaseous precursors. ALD is based on chemical reactions on the surface of a substrate. Precursors are fed to thereaction chamber, including the substrate, one at the time such that the substrate is exposed to only one precursor ata time. The molecules are attached to the substrate as a monolayer via covalent bonding. After the reaction is

finished and the surface is saturated with attached precursor molecules, the excess gas and reaction products are

removed from the chamber by flushing the space with an inert gas such as nitrogen. A typical ALD cycle consists of

four stages. The cycle starts by introducing the first precursor into the chamber which is flushed in the second stage.

Then, another precursor is fed in and flushed, completing the four stages. After the complete reaction cycle, one

atomic layer of desired coating is chemically bonded to the substrate surface. Figure 1 shows the principles of theALD reaction cycle. /1,4/

The actual ALD coating consists of several reaction cycles. One reaction cycle is able to achieve about 0.1 nm layer

thickness depending on the coating material and process parameters. The thickness of a typical ALD coating is

determined by the number of cycles and varies roughly from 1 to 100 nm. Temperature in the chamber can varyfrom room temperature to several hundreds of degrees Celsius. The role of temperature is to provide activation

energy for the ALD process (thermal ALD). The reaction can also be activated using plasma (plasma assisted ALD).

Plasma-assisted ALD provides lower cycle periods and process temperatures for the system than thermal ALD. /1-

3/


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Fig. 1. Principles of ALD reaction cycle /4/.

It is possible to use ALD to deposit a range of different materials. These include e.g. oxides, nitrides and carbides

/1/. In addition, it is possible to produce nanolaminates using different materials in atom-thick layers one after

another. ALD is particularly applicable to substrates with different shapes, even 3D, because thickness variations ofALD coatings are very low since the coating thickness is controlled by a self-limiting reaction at the surface.

Because of the chemical bonding the adhesion with the substrate is commonly excellent. The oxide precursors havebeen reported to adhere to most kinds of polymers even if they lack typical chemical functional groups such as

hydroxyl (-OH) species. /3,5/

Batch atomic layer deposition process

ALD processing techniques can be divided in two categories regarding substrate handling: batch and continuous

processes. The batch process is a traditional technique in which the substrate to be coated remains stationary and the

precursors are pulsed in turn onto the surface of the substrate. The actual coating process occurs in a modularchamber which is filled with substrates In a chamber the substrates can be located close to each other because the


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In the literature, significant barrier properties have been reported for Al2O3layers deposited on polymer surfaces by

the ALD method /6-8/. According to Groner et al. /6/, WVTR values as low as 1*10-3g/m2/24h have been achieved

for very thin (10-25 nm) Al2O3films. The purpose of this study was to investigate the applicability of such ALD

layers for flexible packaging. Batch ALD equipment was used to provide Al2O3coatings on LDPE, PP, PET and

PLA extrusion-coated papers. The WVTRs and O2TRs of the produced materials were tested in order tocharacterize the barrier properties obtained. The results of this study are employed as trend-setters for the futurestudies concerning continuous ALD process.

MATERIALS AND METHODS

The materials to be tested in the study were paper/polymer/Al2O3structures in which the polymer layer wasdeposited by extrusion coating and the aluminum oxide in a batch ALD process. The extrusion coating trials were

performed at the pilot line of TUT/PCT. Four extrusion coating polymers were chosen for the study: LDPE

(CA7230, Borealis), PP (WF420HMS, Borealis), PET (Lighter C98, Equipolymers) and PLA (test grade). The aimwas to select polymers with significant scatter in heat resistance, transport and surface properties. The PLA coating

brought also the aspect of biodegradable polymer into the study. Table 1 shows selected thermal and physicalproperties of the used polymers.

Table 1. Selected thermal and physical properties of the polymers used in the study /9-11/.TmC

TgC

WVTR (38C, 90% RH)

g/m2/24h

O2TR (23C, 0% RH)

cm3/m

2/24h

LDPE 109 -110 17 9400

PP 163 -18 10 4000

PET 247 78 69 150

PLA 150 - 165 55-65 290 660

*Barrier values measured for 25 g/m2extrusion-coated paper

The LDPE-coated paper samples were produced with three different coating weights: 18, 27 and 36 g/m2. With the

other polymers, only one coating weight was performed:25 g/m2. A one-side mineral-coated paper, Lumiflex 90

from Stora Enso, was used as a substrate. The non-pigmented side of the paper was extrusion-coated. Beforehand,

the substrate was pre-treated with corona discharge equipment (2 kW) in order to obtain good adhesion with the

polymer.

The batch ALD equipment shown in Figure 2 (Beneq TFS 500 operated by LUT/ASTRaL) was used to producealuminum oxide (Al2O3) coatings on polymer surfaces. The thicknesses of the layers were 10-200 nm measured by

spectroscopic ellipsometry from Silicon (100) reference samples placed in the same deposition batch.

Trimethylaluminum (TMA) and ozone (O3) were used as precursors for the coatings. All the coatings were produced

in thermal ALD processes having different reactor temperatures (65, 100 or 150C). One deposition cycle consistedof a 250 ms TMA pulse, 6 s purge, 3 s ozone pulse and 6 s purge. The deposition started with 40 s ozone pulse and

90 s purge.

The water vapor transmission rate (WVTR) measurements of the study were made according to SCAN P22:68 (cup

method) In the method a sample is placed against an aluminum dish containing calcium chloride The sample is


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The oxygen transmission rate (O2TR) measurements of the study were performed with Mocon Ox-Tran Model 2/21

according to a standard ASTM D 3985. In the performed test series, the coated side of the sample was sealed against

a test cell using vacuum grease, thus the paper side was facing oxygen during measurements. The active test area

was 50 cm2. 10% oxygen was used as a test gas. The O2TR measurements were made at 23C, 0% RH conditions.

Both WVTR and O2TR results tabulated in the diagrams of this paper are mean values of two parallelmeasurements.

RESULTS AND DISCUSSION

Al2O3layers deposited on LDPE- and PP-coated paper

In a preliminary test, thin Al2O3layers were deposited on the surface of LDPE-coated papers. A low reactor

temperature (65C) was used for the depositions because of the low melting point of LDPE. 120, 350 and 600 cycles

provided approximately 10, 25 and 40 nm Al2O3coatings, respectively. With three LDPE coating weights (18, 27and 36 g/m2) and three ALD layer thicknesses, nine different structures were obtained from the depositions. Figures

4 and 5 show the WVTR (38C, 90% RH) and O2TR (23C, 0% RH) results measured for the structures. The resultsare compared to the reference values measured for the corresponding structures without an Al2O3layer.

0

4

8

12

16

20

24

28

LDPE 18 g/m2 LDPE 27 g/m2 LDPE 36 g/m2

WVTR(g/m2/24h)

Paper / LDPE / Al2O3 (T = 65 C)

Refer.

10 nm

25 nm

40 nm

Fig. 4. WVTR (38C, 90% RH) results for paper/LDPE/Al2O3structures having 10-40 nm Al2O3layer.


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0

2000

4000

6000

8000

10000

12000

14000

LDPE 18 g/m2 LDPE 27 g/m2 LDPE 36 g/m2

O2TR

(cm3/m2/24h)

Paper / LDPE / Al2O3 (T = 65 C)

Refer.

10 nm

25 nm

40 nm

Fig. 5. O2TR (23C, 0% RH) results for paper/LDPE/Al2O3structures having 10-40 nm Al2O3layer.

According to the results, a thin 10 nm ALD coating is already able to obtain considerably improved oxygen barrier

properties for the structure. Concerning water vapor barrier, the decrease of the transmission level is not as dramatic.

Polyethylene, being a good moisture barrier itself, provides already an efficient moisture block which is only

moderately improved by the ALD coating. Nonetheless, the WVTR levels are still considerably decreased and thebarrier effect of the polymer becomes almost meaningless with 40 nm Al2O3layer.

In order to obtain further barrier improvements, thicker A12O3coatings were produced in the study. By performing

more ALD cycles, coatings with thicknesses between 100 and 200 nm were deposited on LDPE- and PP-coatedpapers. This time, the WVTRs were measured in two atmospheric conditions (23C, 50% RH and 38C, 90% RH) in

order to characterize the influence of climatic conditions on the results. The O2TRs were again measured at 23C,

0% RH. The results of the WVTR and O2TR tests are shown in Figures 6 and 7. The thicknesses of the LDPE and

PP layers in the structures were approximately 25 g/m2.


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LDPE LDPE + 100 nm PP PP + 130 nm

23C, 50% R H 4,55 0,39 4,24 0,29

38C, 90% R H 16,74 2,56 10,48 1,94

0

1

2

3

4

56

7

8

9

10

11

12

13

14

15

16

17

WVTR(g/m2/24h)

Paper / Polymer / Al2O3 (T = 65 C)

Fig. 6. WVTR (23C, 50% RH and 38C, 90% RH) results for paper/polymer/Al2O3structures having 100 nmAl2O3layer.

LDPE LDPE + 100 nm PP PP + 130 nm

23 C, 0 % RH 9400 590 4000 230

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

O

2TR(cm3/m2/24h)

Paper / Polymer / Al2O3 (T = 65 C)


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Influence of reactor temperature on the barrier performance

In general, the thermal ALD process produces more controlled ALD layers when using higher reactor temperature

/1/. The influence of reactor temperature on the moisture barrier properties was tested by using two temperatures, 65and 100C, for the Al2O3deposition. PP-coated paper was used as a substrate in the test due to the PPs suitablemelting temperature. The results obtained from the WVTR test are shown in Figure 8.

reference130 nm / 1200

cycles, 65 C,

130 nm / 1660

cycles, 65 C,

80 nm / 1200

cycles, 100 C,

90 nm / 1660

cycles, 100 C,

23 C, 50 % RH 4,24 0,29 0,22 0,59 0,48

38 C, 90 % RH 10,48 1,94 2,69 3,54 4,02

0

1

2

3

4

5

6

7

8

9

10

11

WVTR(g/m2/24h)

Paper / PP / Al2O3

Fig. 8. Influence of reactor temperature and ALD layer thickness on WVTR of PP-coated paper.

According to the results, the use of higher reactor temperature leads to a thinner Al2O3coating and, thus, higherWVTR value. In other words, the higher deposition speed achieved with low reactor temperature (65C) produces

better barrier properties for the structure with an equal number of ALD cycles. This was also observed from the

O2TR results. The higher film growth rate at lower deposition temperature is a well known behavior for this type of

ALD processes /4,12,13/. The phenomenon can be explained through incomplete chemical reactions during the ALD

cycles leading to a higher concentration of compounds formed by hydrocarbons in the film. In result, the lower

reactor temperature used seems to have an advantage over the higher ones regarding the barrier properties of the

obtained structure, even if a perfectly controlled ALD process is not achieved in the reactor.

Al2O3layers deposited on PET- and PLA-coated paper

In addition to LDPE and PP, the ALD coatings were deposited on PET- and PLA-coated paper substrates. The ALDcoatings were again made at 65C temperature except for the two test points with paper/PET/Al2O3structure that

were made at 150C. Figure 9 shows the WVTR results obtained for the paper/PET/Al2O3structures made at 65 and

150C temperatures. According to the results, the increased reactor temperature increased also the WVTR of the

t i l th ith PP Th f th 65C t t t b id d ti l t


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reference

120 nm /

1200 cycles

65 C

190 nm /

1660 cycles

65 C

80 nm / 1200

cycles 150 C

90 nm / 1660

cycles 150 C

23 C, 50 % RH 19 0,1 0,6 6 5,1

38 C, 90 % RH 69 0,8 5,6 21 17

0

10

20

30

40

50

60

70

WV

TR(g/m2/24h)

Paper / PET (27 g/m2) / Al2O3

Fig. 9. WVTR (23C, 50% RH and 38C, 90% RH) results for paper/PET/Al 2O3structures.

reference65 C, 1200

cycles

65 C, 1660

cycles

150 C, 1200

cycles

150 C, 1660

cycles

23 C, 0 % RH 150 2 0,9 2000 2000

0

20

40

60

80

100

120

140

160

180

200

O2TR(cm3/m2/

24h)

Paper / PET (27 g/m2) / Al2O3

Fig. 10. O2TR (23C, 0% RH) results for paper/PET/Al2O3structures.

Figure 10 shows the O2TR results for the paper/PET/Al2O3structures. Remarkable oxygen barrier improvements

were found for the structures when using 65C reactor temperature. The O 2TR values found were significantly lowerthan the ones achieved with the other polymers. However, with the 150C ALD process the oxygen barrier of the


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reference 65 nm / 600 cycles110 nm / 1200

cycles

23 C, 50 % RH 72 3,9 4,1

38 C, 90 % RH 290 55 53

0

50

100

150

200

250

300

WVTR(g/m2/24h)

Paper / PLA (25 g/m2) / Al2

O3

(T = 65 C)

Fig. 11. WVTR (23C, 50% RH and 38C, 90% RH) results for paper/PLA/Al 2O3structures.

reference65 nm / 600

cycles

110 nm / 1200

cycles

200 nm / 1660

cycles23 C, 0 % RH 660 94 53 12

0

100

200

300

400

500

600

700

O2TR(cm3/m2/2

4h)

Paper / PLA (25 g/m2) / Al2O3 (T = 65 C)

Fig. 12. O2TR (23C, 0% RH) results for paper/PLA/Al2O3structures.

CONCLUSIONS


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LDPE ref PP ref PET ref PLA ref LDPE + ALD PP + ALD PET + ALD PLA + ALD

23 C, 50 % RH 4,6 4,2 19 72 0,4 0,3 0,1 4,1

38 C, 90 % RH 17 10 69 290 2,6 1,9 0,8 53

0

10

20

30

40

50

60

70

80

WVTR(g/m

2/24h)

Paper / Polymer / Al2O3 (T = 65 C) [Ref. and 1200 cycles]

Fig. 13. Comparison between the WVTR results.

1000

1500

2000

2500

3000

3500

4000

4500

5000

O2TR(cm3/

m2/24h)

Paper / Polymer / Al2O3 (T=65 C) [Ref. and 1200 cycles]


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ACKNOWLEDGEMENTS

The financial supports of the Finnish Funding Agency of Technology and Innovation (TEKES), Stora Enso Oyj,UPM-Kymmene Oyj and Savcor Face Group Oy are gratefully acknowledged.

REFERENCE

1. Ritala M and Leskel M. Atomic Layer Deposition. In: Nalwa (ed) Handbook of thin film material, Vol. 1.

Deposition and processing of thin films. Stanford Scientific Corporation. USA, 2002.

2. Sneck S. High Capacity Atomic Layer Deposition for Industrial Coating Applications. SVC 50th

AnnualTechnical Conference Proceedings. April 28- May 3, 2007. Louisville, KY.

3. Groner M, Fabreguette F, Elam J and George S. Low-Temperature Al2O3Atomic Layer Deposition. Chemistryof Materials 16, 4 (2004).

4. Puurunen R. Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water

process. Journal of Applied Physics 97, 12 (2005).

5. Ferguson J, Weimer A and George S. Atomic Layer Deposition of Al2O3on Polyethylene Particles. Chemistry

of Materials 16, 26 (2004).

6. Groner M, George S, McLean R and Carcia P. Gas diffusion barriers on polymers using Al2O3atomic layer

deposition. Applied Physics Letters 88, 051907 (2006).

7. Carcia P, McLean R, Reilly M, Groner M and George S. Ca test of Al2O3gas diffusion barriers grown by atomic

layer deposition on polymers. Applied Physics Letters 89, 031915 (2006).

8. Langereis E, Creatore M, Heil S, van de Sanden M and Kessels W. Plasma-assisted atomic layer deposition of

Al2O3moisture permeation barriers on polymers. Applied Physics Letters 89, 081915 (2006).

9. Borealis. Polyolefins for Extrusion Coating, 2ndEd. Austria, 2008.

10. Equipolymers. Lighter C98. Product Data Sheet. July 2004.

11. Callister Jr, W. Chapter 16 in: Materials Science and Engineering, An Introduction, 5thEd. John Wiley & Sons,

Inc. New York, NY, 2000.

12. Matero R, Rahtu A, Ritala M, Leskel M and Sajavaara T. Effect of water dose on the atomic layer depositionrate of oxide thin films. Thin Solid Films 368, 1 (2000).

13. Elliot S, Scarel C, Wiemer C, Fanciulli M and Pavia G. Ozone-Based Atomic Layer Deposition of Alumina

from TMA: Growth, Morphology, and Reaction Mechanisms. Chemistry of Materials 18, 16 (2006).


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Atomic Layer Deposition Processfor Barrier Applications of

Flexible Packaging

Presented by:Petri JohanssonTampere University of Technology


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IntroductionIntroduction

The purpose of atomic layer deposition (ALD)process is to produce a thin, tight and stablecoating from gaseous precursors.

The main advantage of ALD is the conformalityand uniformity which can be obtained

regardless of the orientation or shape of thesubstrate. I.e., there are no pinholes in thefilm.


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In ALD process, thin films of material aredeposited one atomic layer at a time.

.


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exhaust

pump

precursor A precursor B

movement gas leakageweb

purge purge

L

One possible solution of continuous ALD

Roll-to-roll ALD would open huge possibilities. Itwould allow all the flexibility and surface control

capabilities of ALD to a large scale production.


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The ALD coating consists of several reactioncycles. One reaction cycle is able to achieveabout 0.1 nm layer depending on the coatingmaterial and process parameters. Thickness of

a typical ALD layer can vary from 1 to over 100nm.

The role of temperature in the ALD process isto provide activation energy. The reactortemperature can vary from room temperature

to several hundreds of degrees Celsius.


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MaterialsMaterials and methodsand methods

The materials tested in the study werepaper/polymer/Al

2

O3

structures.

Polymer layer via extrusion coating

Al2O3 layer in a batch ALD process

4 extrusion coating polymers were used in the

study: LDPE, PP PET and PLA.


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Trimethylaluminum (TMA) and ozone (O3) wereused as precursors for the Al

2

O3

-coatings.

All the coatings were produced in thermal ALDprocesses having different reactor

temperatures of 65, 100 or 150 C. One deposition cycle consisted of a 250 ms

TMA pulse, 6 s purge, 3 s ozone pulse and 6 spurge. The deposition started with 40 s ozonepulse and 90 s purge.


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The barrier properties against water vapor andoxygen were measured for the obtainedmaterials.

The water vapor transmission rate (WVTR)

measurements were made according to SCANP22:68 (cup method).

2 atmospheric conditions (normal and tropical)were used in the WVTR -measurements:

23 C, 50 % RH and 38 C, 90 % RH.


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The oxygen transmission rate (O2TR)measurements were made according to astandard ASTM D 3985.

The active test area was 50 cm2 ,10 % oxygen

was used as a test gas and the measurementswere made at 23 C, 0 % RH conditions.

Each WVTR and O2TR result shown in thediagrams is a mean value of two replicates.


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ResultsResults and discussionand discussion

Al2O3 layers deposited on LDPE-coated paper


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Al2O3 layers deposited on LDPE-coated paper


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Al2O3 layers deposited on LDPE- and PP-coated paper


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The influence of reactor temperature


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Al2O3 layers deposited on PET-coated paper


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Al2O3 layers deposited on PET-coated paper


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Al2O3 layers deposited on PLA-coated paper


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Al2O3 layers deposited on PLA-coated paper


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ConclusionsConclusions

Comparison between the WVTR results


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ConclusionsConclusions

Comparison between the O2TR results


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AcknowledgementsAcknowledgements

We wish to thank:

TEKES, Stora Enso, UPM-Kymmene and Savcorfor financial support.


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Presented by:

Petri JohanssonResearcher

Tampere University of

[email protected]

biobarrier harlin al2o3johansson

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