Characterization of ion-beam modified polyimide layers

Surface and Coatings Technology 139 Ž2001. 257᎐264

Characterization of ion-beam modified polyimide layers K. Sahre a,U , K.-J. Eichhorn a , F. Simona , D. Pleul a , A. Janke a , G. Gerlach b b

a Institute of Polymer Research Dresden, Hohe Strasse 6, Dresden 01069, Germany Dresden Uni¨ ersity of Technology, Institute for Solid-State Electronics, Mommsenstr. 13, Dresden 01062, Germany

Received 20 October 2000; accepted in revised form 19 January 2001

Abstract Thin chemically modified polyimide films are widely used as functional layers for new microelectronic sensors. Modification of the chemistry of these polymers can lead to different mechanical, optical and electrical properties. Ion implantation is a preferred method to modify polyimide structures. In this work the ion-induced changes of chemical structures of three polyimides were analyzed by attenuated total reflection. Fourier transform infrared spectroscopy ŽATR-FTIR.; X-ray photoelectron spectroscopy ŽXPS.; Raman spectroscopy; and spectroscopic ellipsometry and atomic force microscopy ŽAFM.. The results indicate that during the implantation process the imide structures were partly destroyed. Carbon-rich, graphite-similar and amorphous structures were formed in the surface-near area of the polyimide layers. The changes in molecular structures especially depend on the dose of implanted boron ions. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyimide; Ion implantation; ATR-FTIR; XPS; Raman spectroscopy; Spectroscopic ellipsometry; AFM

1. Introduction Polymer films show a lot of advantages for use in sensors as functional layers with excellent mechanical and dielectric properties as well as improved chemical resistance. Recently, we studied the microphysical mechanisms of water adsorption in polyimide layers w1x, which are frequently used in microsystem technology, especially the swelling behavior in atmospheres with varying humidity w2x. As the controlled humidity-induced changes in the polyimide films are quite large and typically reversible, they have been exploited to design humidity sensors, namely bimorphic piezoresistive humidity sensors w3x. To improve the performance and reliability of the polyimide films for sensor device

U

Corresponding author. Tel.: q49-351-4658-228; fax: q49-3514658-284. E-mail address: [email protected] ŽK. Sahre.

applications a further modification of the chemical and physical structures of the polymer films is necessary w4x. The ion implantation is described as a very powerful method for modification of polyimide layers. The goal is to reach definite changes of macroscopic properties of polymer surfaces through ion-beam induced chemical modifications on a molecular level w5x. The chemical processes that happen during and after the ion implantation are not explained sufficiently. It is shown that cross-linking unsaturated bond formation and chain scission including gas evolution take place during boron ion bombardment of polyimide films w6᎐11x. Highly condensed aromatic ring systems or complex ‘graphitelike’ carbon structures can be found in the upper surface regions. Nevertheless, all surface changes strongly depend on implantation parameters Ženergy and dose. as well as the polymer type. Up to now, not the sensoric properties, but the improvement of surface hardness, electrical conductivity, chemical and wear resistance are in the focus of interest w12x. The first

0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 0 1 3 - 1

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K. Sahre et al. r Surface and Coatings Technology 139 (2001) 257᎐264

results on multiplying sensitivity and tailoring transfer functions of bimorphic humidity sensors by boron-ion bombardment of our polyimide layers were published recently w13x. In this paper detailed studies in the characterization of boron ion-induced molecular modifications of our aromatic polyimide layers using methods of vibrational spectroscopy ŽATR-FTIR, Raman spectroscopy., XPS, spectroscopic ellipsometry and AFM are presented.

2. Experimental 2.1. Sample preparation Different derivatives of polyamic acid were dissolved in N-methyl-2-pyrrolidone and spin-coated on silicon wafers. Prior to coating the thermal oxide layer on the silicon wafers were modified with a coupling agent. After deposition the solvent was removed at 90⬚C for 30 min. The thickness of remaining precursor films was determined at 1.5᎐3.5 ␮m. The precursor systems were baked at 400⬚C for 60 min to convert them into the polyimide structures shown in Table 1. 2.2. Ion implantation To modify the polyimide surface, boron ions were implanted at energies from 50 to 180 keV and doses from 10 13 to 10 16 Bqrcm2 . The boron ions are generated from BF3 molecules, filtered and definitely deposited inside the implanter. 2.3. Characterization of the chemical structures of ¨ irgin and ion-implanted polyimides ATR-FTIR spectroscopic measurements were performed to monitor the ion-beam induced chemical changes in the surfaces using a Bruker spectrometer IFS 66vrs with a ‘Golden Gate’ ATR unit ŽGe crystal, penetration depth 166᎐1106 nm at 4000᎐600 cmy1 ..

The spectra were recorded with a resolution of 4 cmy1 and 200 scans were co-added. A MCT detector was applied. IR transmission measurements were also done with an IFS 66. XPS was carried out using a ESCA-LAB 220i spectrometer ŽVacuum Generators, UK. equipped with a non-monochromatized Mg K ␣ X-ray source. The kinetic energy of photoelectrons was determined using a hemispherical analyzer with a constant pass energy of 80 eV for survey spectra and 25 eV for high-resolution spectra. All spectra were referenced to the hydrocarbon reference peak C1s at the binding energy BE s 285 eV w14x and recorded using take-off angles ⌰ of 0, 45 and 60⬚. Here, the take-off angle is defined as the angle between the sample’s surface normal and the electronoptical axis of the spectrometer. The maximum information depth of XPS is not more than 10 nm. Therefore, this method is very sensitive for the description of elemental compositions and the bonding states in surface areas of very thin polymer layers. Quantitative elemental compositions were determined from peak areas after using Wagner’s sensitivity factors w15x and the spectrometer transmission function. The high-resolved spectra were decomposed by means of the VG ECLIPSE routines. The free parameters of component peaks were their binding energy, height, full width at half maximum and the Gaussian᎐Lorentzian ratio. Raman spectroscopy Žlaser wavelength 785 nm. was carried out by a HoloProbe 785 ŽKaiser Optical Systems, Inc.. with a sapphire probe. The spectra were recorded with a resolution of 4 cmy1 , 20 scans were co-added. The ellipsometric measurements were performed using a Variable Angle Multiwavelength Ellipsometer M44, J.A. Woollam Co., Inc. The ellipsometric angles ␺ and ⌬ were measured in a spectral range from 428 to 763 nm at four angles of incidence Ž60, 65, 70 and 75⬚.. The spectra depend on optical constants of the substrates as well as layers, e.g. indices of refraction n, extinction coefficient k and layer thickness d. These unknown parameters were determined by applying an iterative regression fitting algorithm. For this purpose the software offers different adjusting models of generated to measured data. In our case, models based on anisotropic layers and multi-layer systems were applied to evaluate the optical constants and the layer thickness of the modified polyimide films. AFM experiments were carried out in tapping mode under ambient conditions with a Nanoscope IIIa BioScope, Digital Instruments. Pointprobe silicon cantilevers for force modulation mode from Nanosensors Žresonance frequency 75 kHz, force constant 2.8 Nrm. were used for both height and phase imaging. Accord-

K. Sahre et al. r Surface and Coatings Technology 139 (2001) 257᎐264

ing to Magonov w16x the scan conditions were chosen to reveal stiffness contrast in the phase image: free amplitude A 0 ) 100 nm; and set point ratio rsp approximately 0.5. Under these conditions, soft areas in the phase images are dark and hard areas are bright. Image modification Žbinary image. and analysis gave the relation between hard and soft areas. Height images for roughness measurements were recorded in soft tapping mode Ž rsp approx. 0.9.. The roughness average Sa of the surface was determined: Sa s

1 Lx Ly

Ly

Lx

H0 H0

< f Ž x, y . < d xd y

with f x , y the surface relative to the center plane, L x and L y , dimensions of the surface.

3. Results and discussion 3.1. Structural changes ATR-FTIR spectroscopy analyzes only the surfacenear region of the layers. Therefore, the structural changes can be excellently detected due to boron ion bombardment. So the ion-beam induced changes of the chemical structure of polyimides are particularly well proven at doses higher than 10 15 Bqrcm2 . Fig. 1 demonstrates an example of ATR spectra of virgin and polyimide PI 2808 modified at 180 keV and 10 13 ᎐10 16 Bqrcm2 . The spectra show changes of the characteristic absorption bands of imide and aromatic groups which are typical for the modified polyimide region. The intensi-

259

ties of the bands at 1776, 1720 Ž ␯ 1 stretching vibrations CsO., 1376 Ž ␯ CNC. and 1500 cmy1 Ž ␯ CC aromatic . are reduced because the imide and defined aromatic structures were partly destroyed. Simultaneously a broad band appears in the region of 1550᎐1650 cmy1 based on the forming of graphite-like and carbon-rich amorphous structures in the stronger modified surface-near areas. New B᎐H, C⬅C or C⬅N bonds can be detected at 2331 and 2208 cmy1 w5,8,17x only at a dose of 10 16 Bqrcm2 . The ATR spectra of PI 2566 show the formation of CFsCF and CFsCF2 bands at 1733 and 1795᎐1780 cmy1 after ion implantation. Polarized IR spectroscopic transmission experiments at normal incidence of the infrared beam were carried out to study the molecular alignment of polyimide chains in plane. Measuring with parallel and perpendicular polarizers settings at different positions of the layered substrate showed no dichroic behavior. That means that the polymer chains in plane are not oriented in a preferred direction. This result is important for the modeling of the ellipsometric data. XPS investigations confirm the results of ATR spectroscopy. Generally, with increasing in dose the imide structures in the surface area are destroyed to a large extent as it is also known from the literature w5,17,18x. Thus, the XPS spectra of modified polyimides compared with XPS spectrum of the virgin polyimide show decreasing intensities of the C1s peak and N1s peak of the imide group at binding energies BE s 289 eV and BE s 400 eV. Figs. 2 and 3 demonstrate the features of PI 2540 virgin and modified at 100 keV and 10 16 Bqrcm2 . The survey photoelectron spectra and the high-resolved C1s spectra are shown. The spectra were normalized to equal areas of C1s peaks. The compo-

Fig. 1. ATR spectra of virgin and modified PI 2808.

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K. Sahre et al. r Surface and Coatings Technology 139 (2001) 257᎐264

Fig. 4. Structural units of the component peaks.

Fig. 2. Survey XPS spectra of polyimide films PI 2540 virgin Ža. and modified at 100 keV and 10 16 Bqrcm2 Žb..

nent peak B represents hydrocarbon surface contamination. Component peak F appears from polymer species decomposed by the ion bombardment. The structure units of the other component peaks are given in Fig. 4. The ratio of the element concentrations wNx:wCx was determined from the high-resolution spectra using Wagner’s sensitivity factors and the spectrometer transmission function Žsee Fig. 5.. The partly decomposition of imide groups relating to increasing dose is

Fig. 3. High-resolved C1s spectra of polyimide films PI 2540 virgin Ža. and modified at 100 keV and 10 16 Bqrcm2 Žb..

proved. Moreover, the results of measurements using the three take-off angles show a stronger destruction of imide groups directly at the surface. The Raman spectroscopic measurements indicate the development of non-ordered amorphous carbon-like structures during ion implantation at a high energy at 180 keV and a dose) 10 15 Bqrcm2 . In Fig. 6, the Raman spectra of virgin and modified PI 2540 are shown. The Raman spectrum of virgin PI 2540 is completely overlaid by the fluorescence spectrum. The Raman spectrum of modified polyimide corresponds to the Raman spectrum of graphite structure, which does not show fluorescence effects w19x. The background is also influenced by the fluorescence spectrum caused to polyimide structure. However, a broad asymmetric band is created in the region from 1650 to 1100 cmy1 as the result of carbonization and formation of graphitic structure islands. These results agree with studies in literature w5,12,18,20᎐22x. 3.2. Optical properties Both the film thickness and the refractive index of virgin and modified polyimide layers were determined using spectroscopic ellipsometry. To fit the ellipsometric data we used an optical multi-layer model, which represents the physical reality of the samples satisfactorily.

Fig. 5. Ratio of wNx:wCx in relation to dose of ion implantation.

K. Sahre et al. r Surface and Coatings Technology 139 (2001) 257᎐264

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Fig. 6. Raman spectra of virgin and modified PI 2540 at 180 keV, 10 15 Bqrcm2 .

The UV-VIS spectra of the investigated polyimides show that they do not have notable absorption bands in the VIS region w23x. That means the parameter of the extinction coefficient can be fixed as unity Ž k s 0. for the polyimide layers in the optical model. It is well known that spin-coated layers of stiff aromatic polyimides are optically anisotropic w5x. Therefore, different indices of refraction parallel ‘in plane’ Ž n ip . and perpendicular ‘out of plane’ Ž n oop . to the lateral plane of the substrate must be considered. Kasten et al. w24x have determined both values for a polyimide layer having the same chemical structure and preparation as the PI 2611 investigated here. Using variable angle null ellipsometry at a laser wavelength of 632.8 nm they found an anisotropy ⌬ n s n ip y n oop s 0.241 for the PI layer with a thickness of ds 1357 nm. We applied an anisotropic multi-layer Cauchy model for characterization of our virgin polyimides onto silicon substrates. Our corresponding model based on an assembly of silicon substrate Ž0.rSiO 2 layerŽ1.runmodified anisotropic PIŽ4. layer which is split into two so-called ‘dummy layers’ Žwith thickness ds 0 nm. to model its refractive index in plane n ip Ž2. and out of plane n oop Ž3., which meets the features of the Woollam software package. In Fig. 7a,b the Cauchy model of virgin PI 2611 is shown as an example. With this, the thickness of these virgin polyimide layers was determined as 1853 " 10 nm and the values of the refractive indices are n ip s 1.92 and n oop s 1.69 at 500 nm Ž ⌬ n f 0.23., which is in excellent agreement with the data reported by Kasten et al. w24x. The results also correspond to the values which we have determined by interferometry Ž ⌬ n f 0.23.. Table 2 shows the refractive indices and the anisotropy of the three investigated PI at 500 nm. For the characterization of modified polyimide lay-

ers, the multi-layer Cauchy model of Fig. 7a was extended assuming that in a top layer a change of anisotropic molecular structures into isotropic ones is caused by the ion bombardment Žsee Fig. 8.. The most unmodified layer material should be kept in the anisotropic state. Therefore, the corresponding parameters of virgin polyimide are used for this biaxial bottom layer Ž4.. The thickness of the modified isotropic top layer amounts to 367 " 7 nm Ž n isotrop s 1.94 at 500 nm. and that of the non-influenced biaxial bottom layer to 1531 " 10 nm. Generally, the refractive indices n isotrop Ž500 nm. of the top layer for all three polyimides amount to 1.9᎐2.1 at doses ) 10 15 Bqrcm2 Ž50, 130 and 180 keV., caused by the formation of carbon islands in the modified polyimides. The ellipsometric measurements of all polyimide films deliver a range of thickness from 200 to 400 nm for the modified layer. That means the changes in optical constants, which are necessary for ellipsometric detection of the modified region as a new separate layer take place only in a small part at the surface with drastic changes in chemical structure. So the thickness values of the modified top layers do not correspond to the penetration depth of implanted Bq in polyimide films Žapprox. 800 nm. determined by the Variable Energy Positron Beam w25x, but to the range of ion distribution after the implantation calculated by TRIM code Ž200᎐400 nm. w26x. 3.3. Surface properties Atomic force microscopy was used to study morphological aspects of the polyimide surfaces w27x. The

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262

4. Conclusions The modification of the chemical structure of thin polyimide layers caused by boron ion implantation was analyzed using different surface-sensitive techniques: ATR-FTIR; Raman; and XPS spectroscopy. The ATR spectra of modified layer regions show new bands, which are characteristically of additional aromatic, olefinic, carbonyl and hydroxyl groups formed during bombardment. The changes in molecular structures are especially dependent on the dose of implanted boron ions. The XPS measurements confirm the decomposition of the imide structure. Moreover, the Raman spectra of modified polyimides Ž180 keV ) 10 15

Fig. 7. Optical properties of virgin polyimide PI 2611. Ža. Cauchy model with calculated values for the thickness of SiO 2 Ž1. and anisotropic PI layer Ž4. and Žb. Ellipsometric angles ␺ and ⌬.

AFM᎐images show the change in the surface morphology during modification. The roughness Sa ŽFig. 9. increases by more than 25%, from 0.40 nm of the virgin PI 2808 to 0.51 nm of PI 2808 modified at 180 keV, 10 16 Bqrcm2 . The ratio of hardrsoft areas increases from 72% of virgin PI 2808 to 80% of modified PI 2808 ŽFig. 10.. These results indicate the formation of crosslinked and carbon-rich structures in the surface area of polyimides during the implantation process. Table 2 Refractive indices n ip , n oop and anisotropy ⌬ n at 500 nm Polyimide

nip

noop

⌬n

PI 2611 PI 2540, 2808 PI 2566

1.92 1.8 1.64

1.69 1.71 1.63

0.23" 0.01 0.09" 0.01 0.01" 0.001

Fig. 8. Optical properties of polyimide PI 2611 modified at 180 eV, 10 15 Bqrcm2 . Ža. Cauchy model with calculated values for the thickness of the layers Žnative SiO 2 . and Žb. Ellipsometric angles ␺ and ⌬.

K. Sahre et al. r Surface and Coatings Technology 139 (2001) 257᎐264

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Fig. 9. AFM images Ž500 = 500 nm. of topography of virgin Ža. and modified PI 2808 at 180 keV, 10 16 Bqrcm2 Žb..

Fig. 10. Distribution of hard Žwhite. and soft areas of virgin Ža. and modified PI 2808 at 180 keV, 10 16 Bqrcm2 Žb..

Bqrcm2 . are equivalent to the spectrum of graphite. A broad asymmetric band is formed in the spectral region of different CC bonds. The spectroscopic ellipsometry was used to determine the thickness and corresponding optical constants of the virgin and ion bombarded polymer layers. The ellipsometric fit procedures give thickness data from 200 to 400 nm of the isotropic top layers, which are formed during bombardment. The refractive indices of the top layers amount to 1.9᎐2.1 at high doses and energies caused by carbon islands. The hardrsoft ratio and roughness of the surface determined by AFM show an increase of hard areas and roughness in relationship to the increase of dose of implantation. The combination of the analytical methods employed in these studies has shown good correlation. The results indicate that the typical imide structures were partly destroyed and carbon-rich, graphite-similar and amorphous structures were formed in the surface area of modified polymer layers. These results are very important to the studies of mechanical, electrical and swelling properties of ion-implanted polymers in connection with the development of the new desired sensors.

Acknowledgements The authors thank Mr. Baumann of AktivSensor GmbH ŽStahnsdorf. and Dr. Richter of Forschungszentrum Rossendorf e. V. for realization of the ion implantation processes and gratefully acknowledge financial support of this work from the DFG ŽDeutsche Forschungsgemeinschaft .. References w1x R. Buchhold, A. Nakladal, G. Gerlach, K. Sahre, M. Muller, ¨ K.-J. Eichhorn, M. Herold, G. Gauglitz, J. Electrochem. Soc. 145 Ž11. Ž1998. 4012᎐4018. w2x R. Buchhold, A. Nakladal, G. Gerlach, M. Herold, G. Gauglitz, K. Sahre, K.-J. Eichhorn, Thin Solid Films 350 Ž1-2. Ž1999. 178᎐185. w3x R. Buchhold, A. Nakladal, U. Buttner, G. Gerlach, Meas. Sci. ¨ Technol. 9 Ž1998. 354᎐359. w4x M. Guenther, K. Sahre, G. Suchaneck, G. Gerlach, K.-J. Eichhorn, accepted to publication in Surf. Coat. Techn. w5x E.H. Lee, in: K. Malay, K.L. Ghosh ŽEds.., polymides, Mittal Marcel Dekker, New York, 1996, pp. 471᎐502. w6x W.L. Brown, Radiat. Eff. 98 Ž1986. 115᎐137. w7x M.B. Lewis, E.H. Lee, Nucl. Instr. Meth. Phys. Res. B 69 Ž1992. 341᎐348.

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