XPS studies on aluminum ions modified polyimide with the PIII techniqueZhao Jun Han, Beng Kang Tay, Peter C.T. Ha, Jia Yin Sze, and Daniel H.C. Chua Citation: Journal of Applied Physics 101, 053301 (2007); doi: 10.1063/1.2709578 View online: http://dx.doi.org/10.1063/1.2709578 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/101/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Micro/nanofabrication of poly(L-lactic acid) using focused ion beam direct etching Appl. Phys. Lett. 103, 163105 (2013); 10.1063/1.4825277 Effect of ion beam irradiation and rubbing on the directional behavior and alignment mechanism of liquid crystalson polyimide surfaces J. Appl. Phys. 105, 014507 (2009); 10.1063/1.3033380 Passivation layer on polyimide deposited by combined plasma immersion ion implantation and deposition andcathodic vacuum arc technique J. Vac. Sci. Technol. A 25, 411 (2007); 10.1116/1.2712196 Characteristics and anticoagulation behavior of polyethylene terephthalate modified by C 2 H 2 plasmaimmersion ion implantation-deposition J. Vac. Sci. Technol. A 22, 170 (2004); 10.1116/1.1633569 Ion-beam-induced surface damages on tris-(8-hydroxyquinoline) aluminum Appl. Phys. Lett. 75, 1619 (1999); 10.1063/1.124773

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XPS studies on aluminum ions modified polyimide with the PIII techniqueZhao Jun Han,a� Beng Kang Tay, and Peter C.T. HaSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798

Jia Yin SzeData Storage Institute, Singapore 117608

Daniel H.C. ChuaSchool of Material Science and Engineering, National University of Singapore, Singapore 117576

�Received 18 July 2006; accepted 9 January 2007; published online 2 March 2007�

Polyimide samples modified by aluminum �Al� ions produced by filtered cathodic vacuum arc�FCVA� with plasma immersion ion implantation �PIII� technique, under ambient argon and oxygengases �flow rate Ar:O2=2:1� were investigated by x-ray photoelectron spectroscopy �XPS�. Theworking pressure was about 8�10−4 Torr and the plasma density was estimated to be 109 ions/cm3.The applied bias voltages were varied from 5 to 12.5 kV but with fixed frequency at 900 Hz andduty time of 15 �s. For 1 min process time, C 1s and O 1s spectra for modified samples clearlyindicated that the carbonyl group �C�O� was largely destroyed by incident Al ions while Al 2pspectra suggested Al atoms remain inside polyimide matrices in the form of C-O-Al complexes. Fora 5 min process time, when the ion fluence became large, both C 1s and O 1s spectra suggested astructure of “aluminum oxide-mixed layer-polyimide” and Al 2p spectra confirmed that most Alatoms were bonded to oxygen atoms on the top surface. These XPS results revealed the chemicalbonds between implanted and deposited Al ions and polyimide matrix by using the PIII technique.The structural information can also be suggested. Furthermore in this paper, some discussions withthe theoretical �the stopping and range of ions in matter �SRIM�� simulation were also mentioned inorder to explore the effectiveness of Al ions irradiation on polyimide. �DOI: 10.1063/1.2709578�

I. INTRODUCTION

Plasma modification is a versatile technique in changingpolymers surface properties without affecting their counter-part bulk properties. Various surface properties such as sur-face morphology, surface electrical conductivity, opticaltransmittance, reflectance and refractive index, and surfacechemical activeness can be improved to large extent byplasma modification.1 However, conventional plasma gener-ated by rf power, microwave, etc., is “cold plasma” and canonly modify a very shallow polymer surface layer. Thebeam-line ion implantation technique, which accelerates ionsto higher energy, was considered to overcome this problem.2

For example, 1 MeV proton implanted poly�methyl meth-acrylate� �PMMA� at low fluence �5�1013/cm2� showed in-creased refractive index depth up to 26 �m, which may besuitable for using as waveguide materials.3 However, the dis-advantages of using beam-line ion source are the “line-of-sight” restriction and limited ion density. To further improvethis, the plasma immersion ion implantation �PIII� technique,by applying a high negative bias voltage �1–50 keV� to thesubstrate is considered.4 Ions inside the plasma, either gas-eous or metallic, will be accelerated to the substrate from alldirections. The main advantages for using the PIII techniqueare the non-“line-of-sight” restriction and uniform retainedion density in the substrate.

Polyimide is a high performance polymer and has appli-

cations in electronics, such as electronic packaging, and inaerospace applications by utilizing its thermal stability �up to400 °C�.5 However, it can have improved applications inthose areas if the surface is functionalized.6 Studies onplasma modified polyimide were extensively performed inthe past years, mainly by using gaseous plasmas and ionbeams but rarely metallic plasmas. X-ray photoelectronanalysis is one of the main techniques used to investigatesurface chemical changes in situ and/or after themodification.7 In our experiment, an XPS analysis was per-formed on Al ions modified polyimide surface by using com-bined filtered cathodic vacuum arc �FCVA� and the plasmaimmersion ion implantation �PIII� technique.8 The modifica-tion depth could be as deep as 100 nm. The motivation fordoing such investigation is to see in what ways high ener-getic metal ions react with polyimide as they are inherentlydifferent from volatile gaseous ions.

II. EXPERIMENT

Polyimide film �brand name Kapton® HN� were fromGoodfellow. Its structure formula is shown in Fig. 1. Thefilm thickness is 0.125 mm and the density is 1.42 g/cm3.Pieces with dimension 1�1 cm were cut for the experiment.The Al plasma was generated by the filtered cathodicvacuum arc �FCVA� technique at 60 A dc power supply. Thedescription of the FCVA technique can be found elsewhere.9

Plasma density was estimated at 109 /cm3, a typical value forour cathodic vacuum arc system. Gases of argon and oxygenwere used and the flow rate ratio of Ar:O2 is 2:1. Here, Arwas used in order to sustain the aluminum arcing while O2

a�Author to whom correspondence should be addressed. Electronic mail:hanz0002@ntu.edu.sg

JOURNAL OF APPLIED PHYSICS 101, 053301 ��

0021-8979/2007/101�5�/053301/7/$23.00 ©101, 053301-1

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was used as a reactive gas to Al atoms and perhaps polyim-ide as well. The base pressure was 1�10−5 Torr while theworking pressure is 1�10−3 Torr.

Two batches of experiments were done according totheir different process time. The first batch of modificationused 1 min process time. The bias voltages were varied at5 , 7.5, 10, and 12.5 kV with the frequency fixed at 900 Hzand the duty time at 15 �s. The second batch of modificationhad a 5 min process time. Other experimental conditionswere the same as the first batch. When considering the heat-ing effect, it was noted that for 1 min processing, the tem-perature of polyimide during the experiment was roughly atroom temperature, far below the glass transition temperatureof polyimide ��400 °C�. For a 5 min processing, the tem-perature is much higher. However, it is still estimated to belower than the glass transition temperature.

The XPS instrument for measurement used a Mg K�x-ray source at 1253.6 eV. The wide scan took steps of 0.875eV for measurement and the high-resolution scan took stepsof 0.05 eV. For dissolving the peaks a program written byKwok, from the Chinese University of Hong Kong, wasused.10 In this program, plotting of background baseline usesthe “Shirley-linear” method and peak shapes use Gaussian-Lorentzian at different mixing ratios. Moreover, Gaussian-Lorentzian product function and Newton’s nonlinear least-square method is used to minimize the standard derivations.All resolved peaks are mainly used as qualitative identifica-tion with a few quantitative calculations completed when it isnecessary for supporting suggestions made in the article.

III. RESULTS AND DISCUSSION

The structure formula of polyimide is shown in Fig. 1.The carbon bonds inside the structure were grouped intothree categories according to their estimated binding ener-gies. According to Atanasoska et al.,11 labels for bond I–IIIare indicating the carbon-hydrogen bonds �C-H� dangling inthe benzene rings, the ether bonds �C-O-C�, C-N-C, and car-bon bonds in imides group, and the carbonyl bonds �C�O�pendent to the benzene rings, respectively.

A. 1 min modification

The wide scan spectra from a binding energy of 0–1200eV are shown in Fig. 2. Compositions of C, N, and O arefound in pristine polyimide �see Fig. 2�a��� and extra Al isfound in modified polyimide �see Fig. 2�b��. The intensity of

N is greatly reduced while C and O keep roughly unchangedafter the modification. The high-resolution spectra of C 1sare shown in Fig. 3. Figure 3�a� is the spectrum of pristinepolyimide. There are three prominent peaks resolved by theXPS analysis program, C1, C2, and C3. The positions fortheir binding energies are about 287, 288, and 291 eV, re-spectively. Based on published data from other researchers,12

peak C1 is assigned to bond I while peak C2, whose bindingenergy is about 1 eV larger, represents bond II. It is reason-able for such assignment because oxygen atoms have moreelectronegativity than hydrogen atoms when bonding to car-bon atoms, making the electrons from carbon inner core lev-els have higher binding energy. The carbon-hydrogen bondsin the imide group are special cases that the binding energyfor them is higher because of the electron deficiency causedby strong electron withdrawal by imide carbonyl bonds.11

The third peak C3 at 291 eV represents bond III, the carbonylbond. It is noted here that the binding energy for bond I is

FIG. 1. The structural formula of polyimide, bonds I–III are indicating the carbon-hydrogen bonds �C-H� in the benzene rings, the ether bonds �C-O-C andC-N-C� and carbon bonds in the imide group, and the carbonyl bonds �C�O� pendent to the benzene rings, respectively.

FIG. 2. The wide scan spectra of �a� pristine and �b� 1 min Al plasmamodified polyimide with the PIII technique at a bias voltage of 10 kV.

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not calibrated to the usual value of 285 eV and all the fol-lowing discussions on peak shift will be based on the mea-sured value of bond I here at 287 eV.

Figures 3�b�–3�e� are C 1s spectra for modified polyim-ide at bias voltages of 5, 7.5, 10, and 12.5 kV, respectively.Comparing them with pristine polyimide �Fig. 3�a�� after thepeak resolving, some obvious differences can be obtained.First, the intensity and area of peak C3 are largely reduced.That implies the carbonyl bonds were largely destroyed byAl ions. According to others,11,13 Al atoms will preferablyreact with the pendent oxygen atoms and the carbonyl bondswere broken to form the carbon-oxygen-metal �C-O-Al�complexes. A simple explanation from a thermodynamicpoint of view was given in Ref. 12 on why the complexeswere preferred. This reaction between Al atoms and polyim-ide could happen at a very low energy, e.g., Al atoms gener-ated by evaporation. Second, the intensity for C2 is reducedwhile the intensity of C1 remains more or less the same forall the modified polyimide samples. In other words, the ratioof intensity of C2−C1 is reduced. This reveals that some of

bond II were broken after the modification. If we recall thewide scan spectra in Fig. 2, we find that the intensity of N islargely reduced while the intensity of O remains unchanged.A possible reaction during this process is the chain scissionthat energetic Al ions collide with the polyimide backboneand break the chains. N atoms will diffuse out and suffer lossin intensity from the spectra but oxygen will not because theambient gases contain oxygen plus some C-O bond in thenewly formed C-O-Al complexes. This could be reasonableas Al ions at this energy range ��50 keV� has dominantnuclear stopping power over electronic stopping power whenimplanted into the polyimide. The chain scission effect couldbe serious.

For Al ions generated by the cathodic vacuum arc, theaverage charge density defined as the average ionized chargewhich an Al ion possesses, is about 1.7. If we also considerthe charging effect due to the nonconductivity of polyimidesurface,14 the final implantation energy for Al ions is ap-proximately 1.5 times the applied bias voltages, i.e., approxi-mately 8, 12, 16, and 20 keV. The plot of nuclear stopping

FIG. 3. The XPS spectra of C 1s for �a� pristine polyimide and �b�–�f� 1 min Al plasma modified polyimide with the PIII technique at a bias voltages of 5,7.5, 10, and 12.5 kV, respectively.

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power versus the implantation energy from a the stoppingand range of ions in matter �SRIM�15 simulation is shown inFig. 4�a� while the ratio of area C2 to area C1 is shown inFig. 4�b�. We can see that they are correlated by an inverseproportionality. Our XPS results agrees that when nuclearstopping power decreases, chain scission becomes less.

The O 1s spectra are shown in Fig. 5. They have a rela-tively simple peak resolution compared to the C 1s spectra.As shown in Fig. 5�a�, the pristine O 1s spectrum has tworesolved peaks, O1 and O2, which represent the electron corelevel binding energy of the oxygen atoms in a double andsingle bond, respectively. O1 �double bond� is at a bindingenergy of approximately 534 eV and O2 �single bond� isapproximately 535 eV. Figures 5�b�–5�e� are O 1s spectra forthe modified polyimide under different bias voltages of 5,7.5, 10, 12.5 kV, respectively. All of them have only onepeak remaining, the O2 peak, at approximately 535 eV. Thisagrees with the conclusion drawn from the C 1s spectra thatthe pendent oxygen atoms were destroyed by Al atoms, leav-ing only the oxygen atoms in single bonds. These singlebonds may arise from the remaining ether bonds as well asnewly formed C-O-Al complexes.

The Al 2p spectrum for 1 min 5 kV modified polyimideis shown in Fig. 6. The spectra for other bias voltages showquite a similar shape as that, and only 5 kV spectrum is usedhere as a representative. There is no Al detected for the pris-tine polyimide and only one peak for modified samples. Thisagrees that Al atoms are contributed to the formation ofC-O-Al complexes. The binding energy for Al 2p electrons is

about 77.5 eV for all bias voltages. It is worth mentioningthat the binding energy for Al 2p in the complexes will bedifferent from that in Al atoms only bonded to O in theoxide. This sole peak implies that, besides the C-O-Al com-plexes, no Al atoms are either bonding to ambient oxygenatoms or remaining as its atomic form.

B. 5 min modification

The spectra of 5 min modification time were estimatedto be quite different from the 1 min case. This is because alonger process time with the PIII technique does not onlyindicate a higher dose in implantation, but it also indicatesdeposition on the top surface. The deposition happens duringthe pulse-off period in the PIII operation. Besides that, an-other important phenomenon is the thermal spike.16 Thermalspike is a model that is advocated extensively by researchersin order to explain a variety of experimental results in ionirradiated substrates, such as mixing, compositional change,structural change, and track formation.17 Even when the bulktemperature is less than the glass transition temperature ofthe polyimide during modification, thermal spike could hap-pen in the track of single energetic ions. This enhances iondiffusion and mixing with polyimide matrices. By applyingthis concept, it is suggested here that a mixed layer of Alatoms and polyimide will be present on top of the polyimide.Inside this mixed layer, smaller molecular weight polyimidechains, carbon tracks, as well as incident ions will be present.

Furthermore, because the ambient oxygen exists, alumi-num oxide instead of metallic aluminum may be easilyformed on top of the mixed layer. Thus a structure of“polyimide-mixed layer-aluminum oxide” cross sectionviewed from bottom to top will be shown as a schematic inFig. 7. The mixed layer is also believed to contribute to thelargely enhanced interfacial adhesion.18

The wide scan of 5 min modification process time isshown in Fig. 8. Comparing it with the pristine one, theintensity is reduced. This is due to the coverage of aluminumoxide on the top surface so that the detections of O and Cbecome weaker. The C 1s spectra are shown in Fig. 9. Thedisappearance of the bond I peak �i.e., peak C1 in Fig. 3� at287 eV implied that most of the C-H bonds were lost in theXPS detectable range. Some theoretic works already showedthat the hydrogen atoms dangling to the benzene rings willbe replaced by metal ions and form hydrogen gas in elevatedtemperature.19 That is confirmed here. The C-H bonds dan-gling to the underneath unaffected polymer chains are unde-tectable because the XPS sampling depth is smaller than that.By a simple calculation, when the incident Al ions have anenergy of 20 keV, the projection depth for polyimide calcu-lated from SRIM is 43 nm. If we consider the thickness ofaluminum oxide, the depth of “aluminum � mixed layer”will be even larger.

The binding energies of peaks C2 and C3 are notchanged, at 288 and 291 eV, respectively. C2 peaks havecomparable intensity and area as in the pristine sample andC3 peaks have larger intensity and area. This may indicatethat for a 5 min modification, the ambient oxygen will oxi-dize polyimide by forming C-O or C�O bonds.

FIG. 4. The plots of �a� nuclear stopping and �b� ratio of areas C2 to area C1

vs ion energy from SRIM. The lines are linear fitting to the plot.

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O 1s spectra on Fig. 10 show two peaks for all the modi-fied polyimide samples. The O1 peak at binding energy of535 eV represents the O atoms in the ether bond and isconsistent with the peak in pristine samples as shown in Fig.5�a�. The O2 peak at a binding energy of 537 eV representsthe O atoms bonded to Al atoms in the aluminum oxide. The

FIG. 5. The XPS spectra of O 1s for �a� pristine polyimide and �b�–�f� 1 min Al plasma modified polyimide with the PIII technique at a bias voltages of 5,7.5, 10, and 12.5 kV, respectively.

FIG. 6. The Al 2p spectrum for 1 min Al plasma modified polyimide withthe PIII technique at a bias voltage of 5 kV. The sole peak indicates theformation of C-O-Al complexes.

FIG. 7. The proposed cross-section schematic of 5 min Al ion modifiedpolyimide by the PIII technique that has the structure of “polyimide-mixedlayer-aluminum oxide.”

FIG. 8. The wide scan spectrum of 5 min, 5 kV Al ions modified polyimide.

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peak for O atoms in the carbonyl bonds are not resolvedeither due to the inability of this XPS analysis program ordiscernible composition percentage.

The Al 2p spectra confirmed the above suggestions fromC 1s spectra and O 1s spectra. As shown in Fig. 11, there aretwo peaks resolved for Al atoms. The very intensive Al2 peakrepresents the Al atoms bonded to oxygen atoms in the alu-minum oxide while the weak peak Al1 indicates the Al atomsin the polymer matrix. The area under the peaks may not be

useful for a quantitative calculation for Al atoms percentageas the sampling depth is different for the two Al states.

IV. CONCLUSIONS

This work studied the XPS spectra for Al ions modifiedpolyimide with the PIII technique at different process times

FIG. 9. The C 1s spectra for 5 min Al plasma modified polyimide with the PIII technique at bias voltages of �a� 5, �b� 10, and �c� 12.5 kV, respectively.

FIG. 10. The O 1s spectra of 5 min Al ions modified polyimide with the PIII technique at bias voltages of �a� 5, �b� 10, and �c� 12.5 kV, respectively.

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and bias voltages. It was suggested that for 1 min processing,the carbonyl bonds pendent in pristine polyimide structurewere largely destroyed by energetic Al ions and the etherbonds were also reduced according to the nuclear stoppingpower dissipated. The increasing ratio of area C2 to area C1,which implies an increased concentration of ether bonds,agrees that the inversely proportional relationship withgradually reduced nuclear stopping power for the Al ionsenergy range of 8–20 keV from the SRIM simulation. Inaddition, Al atoms were suggested to react with polyimide toform C-O-Al complexes. For 5 min processing, a structuralof “aluminum oxide-mixed layer-polyimide” was proposed.Furthermore, from the XPS results, Al atoms in the mixedlayer are preferred to react with pendent hydrogen atoms inpolyimide matrices and hydrogen gas is released eventually,leaving no carbon-hydrogen bonds detected.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financialsupport from the Ministry of Education �MOE�, Singaporeand the Agency for Science, Technology, and Research�ASTAR�, Singapore.

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FIG. 11. The Al 2p spectra for 5 min Al ion modified polyimide with thePIII technique at a bias voltage of 5 kV. All the other spectra for other biasvoltages modification show a similar shape.

053301-7 Han et al. J. Appl. Phys. 101, 053301 ��

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