- Email: [email protected]
Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization
Thin Solid Films 476 (2005) 101 – 107 www.elsevier.com/locate/tsf
Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization J.E. Klemberg-Sapiehaa,*, L. Martinua, N.L.S. Yamasakib,1, C.W. Lantmanc,2 a
Department of Engineering Physics, Ecole Polytechnique, P.O. Box 6079, Station Centre Ville, Montreal, Qc, Canada H3C 3A7 b Optical Coating Laboratory Inc., JDS Uniphase, Santa Rosa, CA 95407, USA c Optical Coating Laboratory Inc., JDS Uniphase, Santa Rosa, CA 95407, USA Received 4 June 2004; received in revised form 17 September 2004; accepted 17 September 2004 Available online 10 November 2004
Abstract Adhesion of plasma-deposited optical and protective coatings, such as amorphous hydrogenated silicon nitride, SiN1.3, on polymethylmethacrylate (PMMA) substrates has been found to be limited by a cohesive failure inside the PMMA bulk. Using direct exposure to a low pressure plasma in helium or to vacuum ultraviolet (VUV) radiation generated from plasma, the adhesion of SiN1.3 at high humidity and elevated temperature has been substantially increased. Using a multitechnique analytical approach, the enhanced adhesion was attributed to the initial etching of the weak boundary layer followed by formation of a crosslinked, graded, mechanically stabilized layer in the interfacial region (interphase), which possesses a physical thickness of 50 to 100 nm and a microhardness of about 2 GPa. D 2004 Elsevier B.V. All rights reserved. Keywords: Adhesion; Polymers; X-ray photoelectron spectroscopy (XPS); Ultraviolet surface treatment
1. Introduction Polymethyl-methacrylate (PMMA) is frequently used as a substrate material for precision optics components due to its high transparency in the visible region, low refractive index, and hence reduced optical loss [1]. For different applications, such as ophthalmics, displays, medical instruments and others, PMMA is coated with antireflective films or interference filters [2]. However, the use of PMMA is often limited by insufficient adhesion of coatings, usually related to a cohesive failure in the PMMA bulk close to the film/PMMA interface. This problem is particularly pronounced for PMMA prepared by polymerization in the reactive injection molding process when no conditioning agents and additives are employed. This gives rise to a non-
* Corresponding author. Tel.: +1 514 340 5747; fax: +1 514 340 3218. E-mail address: [email protected] (J.E. Klemberg-Sapieha). 1 Presently with Younger Optics, Torrance 90503, CA, USA. 2 Presently with SRI International, Menlo Park, CA 94025, USA. 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.09.050
uniform distribution of density in the polymer bulk, and it often leads to a soft surface. Adhesion to polymeric substrates can be enhanced by different methods [3]; this includes wet-chemical or mechanical treatment, exposure to flames, ultraviolet radiation, corona discharges, ion beams and deposition of intermediate adhesion layers. Among these, low pressure plasma modification appears very powerful since it can address all adhesion mechanisms related to plastics [4–7]. In fact, there are four principal effects of plasma on polymeric surfaces. Each of them is always present to a certain degree, but one may dominate, depending on the type of polymer, gas and operating conditions. They are the following: (i) surface cleaning, (ii) ablation (or etching), (iii) crosslinking, and (iv) surface chemical functionalization. In most cases, the effects (i), (ii) and (iv) appear to substantially contribute to enhanced adhesion by removing the weak boundary layer, and by forming strong covalent bonds at the film–substrate interface. Formation of film– N–C and film–O–C covalent chemical bonds has been found to correlate well with enhanced adhesion of
102
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
dielectric (e.g. silicon nitride or oxide) or metal (e.g. Ag, Cu) films on polycarbonate [8, 9], polyethylene terephthalate, polypropylene [10, 11], polyethylene [12], polytetrafluoroethylene such as Du Pont’s Teflon PTFE [13], and others. In addition, the role of interface roughness [14], as well as interface stabilization by crosslinking [4,6,8,15–17] has been recognized. These observations gave rise to a general conclusion that adhesion improvement of films to plastic substrates using plasma processes can be attributed to the presence of a physically thick interfacial region, binterphaseQ. This interphase is structured, and it consists of a crosslinked layer followed by a transition layer formed by covalent bonds between the two adherent materials [4,18], and possibly voids [17]. The role of vacuum ultraviolet (VUV, kb200) radiation has been particularly investigated [19,20]. In fact, energetic VUV photons generated by plasma can either be favoured deliberately by using, for example, hydrogen (strong Lyman-a line at 121 nm) or helium, or accidentally, due to the presence of hydrogen originating from dissociated water molecules or fragmented polymer surfaces [21, 22]. In the present work, we study the adhesion of amorphous hydrogenated silicon nitride (SiN1.3) films prepared by plasma enhanced chemical vapor deposition [23], considered as an optical and protective coating on PMMA and other polymers. Even using the knowledge described above, adhesion enhancement to PMMA proved to be difficult, since PMMA is readily susceptible to radiation damage. In addition, using X-ray photoelectron spectroscopy (XPS), we found that a cohesive delamination occurred inside the PMMA substrate. Therefore, the main goal of this work was to stabilize the mechanical properties of the interphase between SiN1.3 and PMMA. At the same time, we systematically studied the effect of different treatments on adhesion, and used a multitechnique approach to determine the thickness, the microstructure and the properties of the interface region.
2. Experimental methodology 2.1. Surface pretreatment The substrates used in this study were optical grade rectangular PMMA plates (2.5 mm thick, 7515 mm2 area) prepared by an injection molding process and supplied by Netra, a subsidiary of the Optical Coating Laboratory, Santa Rosa, CA. Prior to coating, the substrate surface was pretreated using two types of approaches. 2.1.1. Direct exposure to microwave (MW, 2.45 GHz) plasma In this case the substrates were mounted on a substrate holder facing a fused silica window, through which microwave (MW) power was applied from a microwave
applicator (for more detail, see Ref. [8]). The treatments were performed using different working gases (Ar, NH3, N2, O2, He) at a pressure of 27 Pa (200 mTorr), a flow rate of 30 sccm, and a MW power of 150 W. These conditions proved most suitable in our earlier studies aimed at adhesion enhancement to other plastics [8,13,17]. 2.1.2. Exposure to VUV radiation The VUV radiation was generated from MW plasma inside a quartz tube with a diameter of 13 mm and a length of 400 mm. In this experiment the sample compartment was at a pressure of 10 6 Torr, and it was separated from the active plasma zone by a MgF2 window with a cut-off wavelength of k=112 nm (for more detail, see Refs. [19,20]). The plasma in the MW cavity was excited in H2 or He using a power of 100 W, a pressure of 200 mTorr, and a flow rate of 100 sccm. H2 plasma was specially selected because of its strong emission in the VUV region (between 112 and 160 nm) due to the Lyman and Werner series below 160 nm, and the Lyman-a line at 121.5 nm [19]. 2.2. Film deposition and adhesion testing Following surface pretreatment, 500 nm thick SiN1.3 films were deposited in a dual–mode microwave/radiofrequency (MW/RF) plasma. The substrate holder was capacitively coupled to a radiofrequency (RF, 13.56 MHz) power supply, while the MW power was simultaneously applied through a fused silica window using a slow wave applicator [7]. The SiN1.3 films were prepared from a SiH4/ NH3 (8/24 sccm) gas mixture using a pressure of 120 mTorr, a MW power of 100 W, an RF power of 30 W, and a substrate bias of V B = 150 V. When direct plasma pretreatments were used (see Section 2.1.1), the depositions were performed in situ without breaking the vacuum. Following VUV pretreatments (approach described in Section 2.1.2), the samples were exposed to the laboratory air before deposition. After film fabrication, the samples were inspected for their adhesion using an adhesive tape peel test (M 610 Scotch tape) and a microscratch testing instrument (MST, CSEM, Neuchaˆtel, Switzerland). In the MST, a hemispherical Rockwell C diamond stylus was moved along the film surface with a progressively increasing load. The level of adhesion was semi-quantitatively determined in terms of a critical load, L c, when the film started to delaminate from the substrate [16]. The adhesion tests were repeated for samples submitted to accelerated aging consisting of a high relative humidity (RH=97%) at an elevated temperature (50 8C) for a period of 24 h. 2.3. Characterization of the treated PMMA surface In order to elucidate the mechanism of adhesion, different complementary techniques were used for the characterization of the surface and of the near-surface regions of the
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
plasma-treated PMMA. These were: (i) Angle-resolved XPS, VG-ESCALAB Mk-II instrument, using non-monochromated MgKa radiation with an energy resolution of about 0.7 eV. In order to avoid degradation of PMMA the radiation power during analysis was kept below 200 W. For further analysis, the background signal was subtracted using the Shirley method [24], and the high-resolution spectra were fitted with 40% Laurenzian/60% Gaussian distributions similarly to our earlier work [8,17]. (ii) Atomic force microscopy (AFM, Topometrix Discover III); (iii) Variable Angle Spectroscopic Ellipsometry (VASE, J.A. Woollam, in the 260–1700 nm wavelength range); and (iv) Surface solubility test for chemical stability, using acetone. Following the MST, the scratch track was observed in a JSM 820 (JEOL) scanning electron microscope (SEM), in order to evaluate the mode of delamination.
3. Results and discussion 3.1. Evaluation of adhesion In contrast to our previous work on the deposition of optical coatings on PC substrates [8], the SiN1.3 films deposited onto PMMA did not pass the Scotch-tape peel test. Therefore, in the initial part of this study we attempted to enhance adhesion by pre-treatment of the PMMA surface in a MW plasma excited in different gases such as Ar, NH3, N2 and O2, using different conditions and exposure times, similarly to our previous experiments [8, 17]. In addition, we also investigated other treatment approaches aimed at surface stabilization; this included (i) heating at 60 8C or 90 8C, (ii) surface activation by inert gas plasma, followed by exposure to acrylate vapours, and finally, (iii) by deposition
Fig. 1. C (1s) XPS spectra of PMMA identified on both surfaces of a delaminated SiN1.3/PMMA sample: identical surface indicate a cohesive failure in the PMMA.
103
Fig. 2. Effect of treatment time on the critical load, L c, for SiN1.3/PMMA for different pre-treatment conditions (full paints). Effect of exposure to 97% RH at 50 8C for 24 h, indicated by empty points.
of intermediate adhesive layers such as hydrogenated amorphous silicon (a-Si:H) or carbon (a-C:H). In summary, all the above experiments resulted in unsatisfactory coating adhesion since they did not pass the peel test. The main reason for this observation was the fact that a cohesive failure occurred in the PMMA bulk, as confirmed by XPS analyses (see Fig. 1). On both sides of the delaminated surfaces, one could clearly distinguish the chemical features corresponding to PMMA, documented by an appearance of the C1, C2 and C3 peaks due to CUC and CUH; CUO; and CMO chemical bonds, respectively, according to the PMMA chemical structure. From the Si and N atomic concentrations at different take-off angles we deduced that the failure occurred close to the interface for an untreated sample (probably less than
Fig. 3. C (1s) XPS spectra of untreated PMMA and PMMA treated by direct He plasma and VUV radiation generated by H2 plasma.
104
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
Fig. 4. Atomic concentration of oxygen on PMMA surface as a function of time for different treatment conditions.
2 nm deep), while it occurred deeper following plasma treatment. For example, we estimate that the locus of failure was about 4 and 6 nm from the interface following
30 and 120 s treatments in Ar plasma, respectively. This means that with plasma exposure the interface stabilizes due to crosslinking, as described in our earlier studies [8,17] and documented by an increased surface hardness [25], but the cohesive failure occurs progressively deeper inside PMMA. Therefore, our subsequent study focused on the removal of the weak boundary layer and on the stabilization of the interface region at a greater depth in the PMMA, by using processes characterized by intense VUV radiation. Substantial adhesion improvement of SiN 1.3 was obtained only when PMMA was exposed to direct MW plasma in He, which is a rich source of VUV photons (process (A), using the conditions of Section 2.1.1), and to VUV radiation generated from H2 plasma behind the MgF2 window (process (B), using the conditions of Section 2.1.2) for an extended period of time. This is illustrated in Fig. 2 by a substantial increase of the critical load L c used as a measure of adhesion. It was observed that a longer exposure time prior to deposition is required for VUV irradiation process (B), while the effect of direct He plasma treatment
Fig. 5. AFM images of differently treated PMMA.
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
105
h at 97% RH, but the samples still successfully passed the adhesion tape peel test. For comparison, no improvement of adhesion has been observed following exposure to the radiation generated by He plasma behind the MgF2 window. The main reason is that the strong line at ~53 nm, from He plasma, was eliminated by the MgF2 window in the experiments of Section 2.1.2. On the contrary, strong emission due to transitions in molecular H2 (120–170 nm) and the strong Lyman a line contribute to surface crosslinking [19]. 3.2. Analysis of the interphase Adhesion improvement of SiN1.3 on PMMA was successful only when the polymer surface had been exposed for an extended period of time to an environment containing strong VUV radiation. During such treatment, the weak boundary layer was possibly removed by surface ablation induced by plasma and by photons [22], while forming a relatively thick crosslinked layer at the interface. This is in agreement with a model of the structured interfacial region discussed in more detail in recent literature [2,4,6–8,12]. The crosslinked layer exhibits a thickness of many tens of nanometers, and a hardness of about 2 GPa, 10 times more than that of an untreated polymer [25]. In the following, we further characterize the crosslinked stabilized layer. Following surface treatment, we did not observe any discoloration of the PMMA surface. Crosslinking of PMMA was detected by XPS analysis, which exhibited a substantial loss of O, pronounced broadening of the C (1s) peak, and decrease of its components (Fig. 3). The effect of VUV radiation from H2 plasma was found more efficient than He plasma. The oxygen concentration decreased with increasing treatment time from 25 at.% for untreated PMMA to 13.5 at.% after 35 min exposure to direct He plasma, and to 7 at.% after 90 min VUV radiation generated by H2 plasma
Fig. 6. SEM micrographs of the scratch-track region for differently treated PMMA.
process (A) is faster, and it achieves maximum effectiveness in about 25 min treatment time (Fig. 2). Exposure to high humidity was also evaluated, and the results are represented by the open points in Fig. 2. Typically, L c values decreased by about 20–30% after 24
Fig. 7. Surface appearance after the solubility test in acetone for untreated (1) and He plasma treated PMMA (2) for 5 min, (3) for 10 min and (4) for 15 min.
106
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
4. Conclusions
Fig. 8. Effect of treatment on the depth profile of the refractive index at 550 nm near the PMMA surface: Full line—direct exposure to microwave plasma in He; dotted line—exposure to VUV radiation generated by H2 plasma behind the MgF2 window.
behind the MgF2 window (Fig. 4). The effect of VUV with He plasma behind the MgF2 window was much less pronounced due to the elimination of the strong He line at 53 nm (21 at.% of O after 60 min treatment). In all measurements, the values obtained at different take-off angles did not exhibit any substantial changes. This suggests that the modified surface region is thicker than the maximum sampling depth (~8 nm) probed by XPS, in agreement with the optical analysis discussed below. AFM images, together with the scan profiles, indicated a decrease in surface roughness following both treatments processes (A) and (B) (see Fig. 5). SEM observations of untreated PMMA show microcracks at the bottom of the scratch-track, suggesting a cohesive failure in the polymer (see Fig. 6). After surface treatment, deep cracks are much less abundant, and removal of the scratched hardened layer appears. Presence of a crosslinked skin has also been confirmed by the surface solubility test in acetone (Fig. 7). After exposures to direct He plasma and VUV radiation from H2 plasma, the chemical attack was substantially reduced, similarly to our earlier observations for polycarbonate [17]. VASE measurements on treated PMMA surfaces were used to estimate the thickness of the modified surface layer (Fig. 8). This was accomplished by fitting the experimental ellipsometric angles W(k) and D(k) with a simple optical model consisting of a thin layer with linearly graded refractive index (n) values on top of the buntreatedQ PMMA bulk. These measurements indicated an increase of n by about 4% (from 1.50 to about 1.57 at 550 nm). The thickness of this graded layer varied between 50 and 96 nm, depending on the treatment time. This is in good agreement with the values obtained for other polymers such as polycarbonate [17], and with the penetration depth of VUV photons [26]. Thickness values of the crosslinked near-surface layer are higher than the electron escape depth in XPS, and thy are comparable with the skin detachment observed by SEM.
In the present work, we investigated the characteristics of PMMA surfaces treated by low pressure plasma under conditions leading to optimized adhesion of plasma-deposited films, namely silicon nitride. We found that relatively long exposures (10–30 min) to both direct He plasma or H2 plasma-generated VUV radiation lead to the removal of the surface weak boundary layer, and to the formation of a crosslinked surface layer, about 50–100 nm thick, which mechanically stabilizes the interface. The presence of this layer was determined by complementary techniques, namely XPS, SEM, and VASE, and by the chemical solubility test. Contribution of a crosslinked layer to the structured interfacial region, as a consequence of plasma treatment, is in agreement with the phenomenological model of adhesion enhancement described in the literature [4,6,16]. Mechanical stabilization of the interface by exposure to plasma or to VUV radiation appears very attractive for enhanced device performance in various applications such as optical coatings on organic substrates.
Acknowledgments The authors wish to thank Drs. A. Holl7nder, and G. Czeremuszkin, and Prof. M.R. Wertheimer for stimulating discussions, Dr. A. Fozza for his assistance with VUV treatment, and Dr. D. Poitras and Mr. A. Bergeron for the assistance with optical measurements. This work was supported by a contract with Optical Coating Laboratory Inc./JDS Uniphase.
References [1] J.M.E. Harper, Handbook of Plastics, Elastomers and Composites, McGraw-Hill, New York, NY, 1992. [2] L. Martinu, J.E. Klemberg-Sapieha, Optical Interference Filters, in: N. Kaiser, H. Pulker (Eds.), Springer Verlag, Berlin, 2003, p. 359. [3] A. Pizzi, K.L. Mittal (Eds.), Adhesion Promotion Techniques, Marcel Dekker, New York, 1999. [4] E.M. Liston, L. Martinu, M.R. Wertheimer, J. Adhes. Sci. Technol. 7 (1993) 1091. [5] F.D. Egitto, L.J. Matienzo, IBM J. Res. Develop. 38 (1994) 433. [6] M.R. Wertheimer, L. Martinu, E.M. Liston, Handbook of Thin Film Process Technology, in: S.I. Shah, D.A. Glocker (Eds.), IOP Publishing, Bristol, 1996, p. 33. [7] M.R. Wertheimer, L. Martinu, J.E. Klemberg–Sapieha, G. Czeremuszkin, in: K.L. Mittal, A. Pizzi (Eds.), Chapter 5 in Adhesion Promotion Techniques, Marcel Dekker, New York, 1999, p. 139. [8] J.E. Klemberg-Sapieha, D. Poitras, L. Martinu, N.L.S. Yamasaki, C.W. Lantman, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 985. [9] S. Vallon, B. Dre´villon, F. Poncin-Epaillard, J.E. Klemberg-Sapieha, L. Martinu, J. Vac. Sci. Technol., A, Vac. Surf. Films 14 (1996) 3194. [10] S. Vallon, A. Hofrichter, B. Dre´villon, J.E. Klemberg-Sapieha, L. Martinu, F. Poncin-Epaillard, Thin Solid Films 290/291 (1996) 68. [11] S. Vallon, R. Brenot, A. Hofrichter, B. Dre´villon, A. Gheorghiu, C. Se´ne´maud, J.E. Klemberg-Sapieha, L. Martinu, F. Poncin-Epaillard, J. Adhes. Sci. Technol., A 10 (1996) 1313.
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107 [12] L. Gerenser, J. Vac. Sci. Technol., A, Vac. Surf. Films 6 (1988) 2887. [13] J.E. Klemberg–Sapieha, M.K. Shi, L. Martinu, M.R. Wertheimer, Proc. of the Society of Plastic Engineers, Ann. Tech. Conf. ’95, Boston, MA, May 1995, p. 286. [14] J.E. Klemberg–Sapieha, in: R. d’Agostino, F. Fracassi, P. Favia (Eds.), Chapter 4 in Plasma Treatments and Deposition of Polymers, Kluwer Academic Publishers, Dordrecht, 1997, p. 233. [15] R.H. Hansen, H. Schonhorn, J. Polym. Sci., B, Polym. Lett. 4 (1966) 203. [16] L. Martinu, in: R. d’Agostino, F. Fracassi, P. Favia (Eds.), Chapter 4 in Plasma Treatments and Deposition of Polymers, Kluwer Academic Publishers, Dordrecht, 1997, p. 247. [17] A. Bergeron, J.E. Klemberg-Sapieha, L. Martinu, J. Vac. Sci. Technol., A, Vac. Surf. Films 16 (1998) 285. [18] A.S. da Silva Sobrinho, N. Schqhler, J.E. Klemberg-Sapieha, M.R. Wertheimer, M. Andrews, S.C. Gujrathi, J. Vac. Sci. Technol., A, Vac. Surf. Films 16 (1998) 2021.
107
[19] A.C. Fozza, J. Roch, J.E. Klemberg-Sapieha, A. Kruse, A. Holl7nder, M.R. Wertheimer, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 131 (1997) 205. [20] A.C. Fozza, J.E. Klemberg-Sapieha, M.R. Wertheimer, Plasmas Polym. 4 (1999) 183. [21] M.R. Wertheimer, A.C. Fozza, A. Holl7nder, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 151 (1999) 65. [22] J. Hong, F. Truica-Marasescu, L. Martinu, M.R. Wertheimer, Plasma Polym. 7 (2000) 245. [23] L. Martinu, D. Poitras, J. Vac. Sci. Technol., A, Vac. Surf. Films 18 (2000) 2619. [24] D.A. Shirley, Phys. Rev. 5 (1972) 4709. [25] S. Dahl, D. Rats, L. Martinu, J. von Stebut, J.E. Klemberg-Sapieha, Thin Solid Films 355/356 (1999) 290. [26] A. Holl7nder, R. Wilken, J. Behnisch, Surf. Coat. Technol. 116–119 (1999) 788.
Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization J.E. Klemberg-Sapiehaa,*, L. Martinua, N.L.S. Yamasakib,1, C.W. Lantmanc,2 a
Department of Engineering Physics, Ecole Polytechnique, P.O. Box 6079, Station Centre Ville, Montreal, Qc, Canada H3C 3A7 b Optical Coating Laboratory Inc., JDS Uniphase, Santa Rosa, CA 95407, USA c Optical Coating Laboratory Inc., JDS Uniphase, Santa Rosa, CA 95407, USA Received 4 June 2004; received in revised form 17 September 2004; accepted 17 September 2004 Available online 10 November 2004
Abstract Adhesion of plasma-deposited optical and protective coatings, such as amorphous hydrogenated silicon nitride, SiN1.3, on polymethylmethacrylate (PMMA) substrates has been found to be limited by a cohesive failure inside the PMMA bulk. Using direct exposure to a low pressure plasma in helium or to vacuum ultraviolet (VUV) radiation generated from plasma, the adhesion of SiN1.3 at high humidity and elevated temperature has been substantially increased. Using a multitechnique analytical approach, the enhanced adhesion was attributed to the initial etching of the weak boundary layer followed by formation of a crosslinked, graded, mechanically stabilized layer in the interfacial region (interphase), which possesses a physical thickness of 50 to 100 nm and a microhardness of about 2 GPa. D 2004 Elsevier B.V. All rights reserved. Keywords: Adhesion; Polymers; X-ray photoelectron spectroscopy (XPS); Ultraviolet surface treatment
1. Introduction Polymethyl-methacrylate (PMMA) is frequently used as a substrate material for precision optics components due to its high transparency in the visible region, low refractive index, and hence reduced optical loss [1]. For different applications, such as ophthalmics, displays, medical instruments and others, PMMA is coated with antireflective films or interference filters [2]. However, the use of PMMA is often limited by insufficient adhesion of coatings, usually related to a cohesive failure in the PMMA bulk close to the film/PMMA interface. This problem is particularly pronounced for PMMA prepared by polymerization in the reactive injection molding process when no conditioning agents and additives are employed. This gives rise to a non-
* Corresponding author. Tel.: +1 514 340 5747; fax: +1 514 340 3218. E-mail address: [email protected] (J.E. Klemberg-Sapieha). 1 Presently with Younger Optics, Torrance 90503, CA, USA. 2 Presently with SRI International, Menlo Park, CA 94025, USA. 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.09.050
uniform distribution of density in the polymer bulk, and it often leads to a soft surface. Adhesion to polymeric substrates can be enhanced by different methods [3]; this includes wet-chemical or mechanical treatment, exposure to flames, ultraviolet radiation, corona discharges, ion beams and deposition of intermediate adhesion layers. Among these, low pressure plasma modification appears very powerful since it can address all adhesion mechanisms related to plastics [4–7]. In fact, there are four principal effects of plasma on polymeric surfaces. Each of them is always present to a certain degree, but one may dominate, depending on the type of polymer, gas and operating conditions. They are the following: (i) surface cleaning, (ii) ablation (or etching), (iii) crosslinking, and (iv) surface chemical functionalization. In most cases, the effects (i), (ii) and (iv) appear to substantially contribute to enhanced adhesion by removing the weak boundary layer, and by forming strong covalent bonds at the film–substrate interface. Formation of film– N–C and film–O–C covalent chemical bonds has been found to correlate well with enhanced adhesion of
102
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
dielectric (e.g. silicon nitride or oxide) or metal (e.g. Ag, Cu) films on polycarbonate [8, 9], polyethylene terephthalate, polypropylene [10, 11], polyethylene [12], polytetrafluoroethylene such as Du Pont’s Teflon PTFE [13], and others. In addition, the role of interface roughness [14], as well as interface stabilization by crosslinking [4,6,8,15–17] has been recognized. These observations gave rise to a general conclusion that adhesion improvement of films to plastic substrates using plasma processes can be attributed to the presence of a physically thick interfacial region, binterphaseQ. This interphase is structured, and it consists of a crosslinked layer followed by a transition layer formed by covalent bonds between the two adherent materials [4,18], and possibly voids [17]. The role of vacuum ultraviolet (VUV, kb200) radiation has been particularly investigated [19,20]. In fact, energetic VUV photons generated by plasma can either be favoured deliberately by using, for example, hydrogen (strong Lyman-a line at 121 nm) or helium, or accidentally, due to the presence of hydrogen originating from dissociated water molecules or fragmented polymer surfaces [21, 22]. In the present work, we study the adhesion of amorphous hydrogenated silicon nitride (SiN1.3) films prepared by plasma enhanced chemical vapor deposition [23], considered as an optical and protective coating on PMMA and other polymers. Even using the knowledge described above, adhesion enhancement to PMMA proved to be difficult, since PMMA is readily susceptible to radiation damage. In addition, using X-ray photoelectron spectroscopy (XPS), we found that a cohesive delamination occurred inside the PMMA substrate. Therefore, the main goal of this work was to stabilize the mechanical properties of the interphase between SiN1.3 and PMMA. At the same time, we systematically studied the effect of different treatments on adhesion, and used a multitechnique approach to determine the thickness, the microstructure and the properties of the interface region.
2. Experimental methodology 2.1. Surface pretreatment The substrates used in this study were optical grade rectangular PMMA plates (2.5 mm thick, 7515 mm2 area) prepared by an injection molding process and supplied by Netra, a subsidiary of the Optical Coating Laboratory, Santa Rosa, CA. Prior to coating, the substrate surface was pretreated using two types of approaches. 2.1.1. Direct exposure to microwave (MW, 2.45 GHz) plasma In this case the substrates were mounted on a substrate holder facing a fused silica window, through which microwave (MW) power was applied from a microwave
applicator (for more detail, see Ref. [8]). The treatments were performed using different working gases (Ar, NH3, N2, O2, He) at a pressure of 27 Pa (200 mTorr), a flow rate of 30 sccm, and a MW power of 150 W. These conditions proved most suitable in our earlier studies aimed at adhesion enhancement to other plastics [8,13,17]. 2.1.2. Exposure to VUV radiation The VUV radiation was generated from MW plasma inside a quartz tube with a diameter of 13 mm and a length of 400 mm. In this experiment the sample compartment was at a pressure of 10 6 Torr, and it was separated from the active plasma zone by a MgF2 window with a cut-off wavelength of k=112 nm (for more detail, see Refs. [19,20]). The plasma in the MW cavity was excited in H2 or He using a power of 100 W, a pressure of 200 mTorr, and a flow rate of 100 sccm. H2 plasma was specially selected because of its strong emission in the VUV region (between 112 and 160 nm) due to the Lyman and Werner series below 160 nm, and the Lyman-a line at 121.5 nm [19]. 2.2. Film deposition and adhesion testing Following surface pretreatment, 500 nm thick SiN1.3 films were deposited in a dual–mode microwave/radiofrequency (MW/RF) plasma. The substrate holder was capacitively coupled to a radiofrequency (RF, 13.56 MHz) power supply, while the MW power was simultaneously applied through a fused silica window using a slow wave applicator [7]. The SiN1.3 films were prepared from a SiH4/ NH3 (8/24 sccm) gas mixture using a pressure of 120 mTorr, a MW power of 100 W, an RF power of 30 W, and a substrate bias of V B = 150 V. When direct plasma pretreatments were used (see Section 2.1.1), the depositions were performed in situ without breaking the vacuum. Following VUV pretreatments (approach described in Section 2.1.2), the samples were exposed to the laboratory air before deposition. After film fabrication, the samples were inspected for their adhesion using an adhesive tape peel test (M 610 Scotch tape) and a microscratch testing instrument (MST, CSEM, Neuchaˆtel, Switzerland). In the MST, a hemispherical Rockwell C diamond stylus was moved along the film surface with a progressively increasing load. The level of adhesion was semi-quantitatively determined in terms of a critical load, L c, when the film started to delaminate from the substrate [16]. The adhesion tests were repeated for samples submitted to accelerated aging consisting of a high relative humidity (RH=97%) at an elevated temperature (50 8C) for a period of 24 h. 2.3. Characterization of the treated PMMA surface In order to elucidate the mechanism of adhesion, different complementary techniques were used for the characterization of the surface and of the near-surface regions of the
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
plasma-treated PMMA. These were: (i) Angle-resolved XPS, VG-ESCALAB Mk-II instrument, using non-monochromated MgKa radiation with an energy resolution of about 0.7 eV. In order to avoid degradation of PMMA the radiation power during analysis was kept below 200 W. For further analysis, the background signal was subtracted using the Shirley method [24], and the high-resolution spectra were fitted with 40% Laurenzian/60% Gaussian distributions similarly to our earlier work [8,17]. (ii) Atomic force microscopy (AFM, Topometrix Discover III); (iii) Variable Angle Spectroscopic Ellipsometry (VASE, J.A. Woollam, in the 260–1700 nm wavelength range); and (iv) Surface solubility test for chemical stability, using acetone. Following the MST, the scratch track was observed in a JSM 820 (JEOL) scanning electron microscope (SEM), in order to evaluate the mode of delamination.
3. Results and discussion 3.1. Evaluation of adhesion In contrast to our previous work on the deposition of optical coatings on PC substrates [8], the SiN1.3 films deposited onto PMMA did not pass the Scotch-tape peel test. Therefore, in the initial part of this study we attempted to enhance adhesion by pre-treatment of the PMMA surface in a MW plasma excited in different gases such as Ar, NH3, N2 and O2, using different conditions and exposure times, similarly to our previous experiments [8, 17]. In addition, we also investigated other treatment approaches aimed at surface stabilization; this included (i) heating at 60 8C or 90 8C, (ii) surface activation by inert gas plasma, followed by exposure to acrylate vapours, and finally, (iii) by deposition
Fig. 1. C (1s) XPS spectra of PMMA identified on both surfaces of a delaminated SiN1.3/PMMA sample: identical surface indicate a cohesive failure in the PMMA.
103
Fig. 2. Effect of treatment time on the critical load, L c, for SiN1.3/PMMA for different pre-treatment conditions (full paints). Effect of exposure to 97% RH at 50 8C for 24 h, indicated by empty points.
of intermediate adhesive layers such as hydrogenated amorphous silicon (a-Si:H) or carbon (a-C:H). In summary, all the above experiments resulted in unsatisfactory coating adhesion since they did not pass the peel test. The main reason for this observation was the fact that a cohesive failure occurred in the PMMA bulk, as confirmed by XPS analyses (see Fig. 1). On both sides of the delaminated surfaces, one could clearly distinguish the chemical features corresponding to PMMA, documented by an appearance of the C1, C2 and C3 peaks due to CUC and CUH; CUO; and CMO chemical bonds, respectively, according to the PMMA chemical structure. From the Si and N atomic concentrations at different take-off angles we deduced that the failure occurred close to the interface for an untreated sample (probably less than
Fig. 3. C (1s) XPS spectra of untreated PMMA and PMMA treated by direct He plasma and VUV radiation generated by H2 plasma.
104
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
Fig. 4. Atomic concentration of oxygen on PMMA surface as a function of time for different treatment conditions.
2 nm deep), while it occurred deeper following plasma treatment. For example, we estimate that the locus of failure was about 4 and 6 nm from the interface following
30 and 120 s treatments in Ar plasma, respectively. This means that with plasma exposure the interface stabilizes due to crosslinking, as described in our earlier studies [8,17] and documented by an increased surface hardness [25], but the cohesive failure occurs progressively deeper inside PMMA. Therefore, our subsequent study focused on the removal of the weak boundary layer and on the stabilization of the interface region at a greater depth in the PMMA, by using processes characterized by intense VUV radiation. Substantial adhesion improvement of SiN 1.3 was obtained only when PMMA was exposed to direct MW plasma in He, which is a rich source of VUV photons (process (A), using the conditions of Section 2.1.1), and to VUV radiation generated from H2 plasma behind the MgF2 window (process (B), using the conditions of Section 2.1.2) for an extended period of time. This is illustrated in Fig. 2 by a substantial increase of the critical load L c used as a measure of adhesion. It was observed that a longer exposure time prior to deposition is required for VUV irradiation process (B), while the effect of direct He plasma treatment
Fig. 5. AFM images of differently treated PMMA.
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
105
h at 97% RH, but the samples still successfully passed the adhesion tape peel test. For comparison, no improvement of adhesion has been observed following exposure to the radiation generated by He plasma behind the MgF2 window. The main reason is that the strong line at ~53 nm, from He plasma, was eliminated by the MgF2 window in the experiments of Section 2.1.2. On the contrary, strong emission due to transitions in molecular H2 (120–170 nm) and the strong Lyman a line contribute to surface crosslinking [19]. 3.2. Analysis of the interphase Adhesion improvement of SiN1.3 on PMMA was successful only when the polymer surface had been exposed for an extended period of time to an environment containing strong VUV radiation. During such treatment, the weak boundary layer was possibly removed by surface ablation induced by plasma and by photons [22], while forming a relatively thick crosslinked layer at the interface. This is in agreement with a model of the structured interfacial region discussed in more detail in recent literature [2,4,6–8,12]. The crosslinked layer exhibits a thickness of many tens of nanometers, and a hardness of about 2 GPa, 10 times more than that of an untreated polymer [25]. In the following, we further characterize the crosslinked stabilized layer. Following surface treatment, we did not observe any discoloration of the PMMA surface. Crosslinking of PMMA was detected by XPS analysis, which exhibited a substantial loss of O, pronounced broadening of the C (1s) peak, and decrease of its components (Fig. 3). The effect of VUV radiation from H2 plasma was found more efficient than He plasma. The oxygen concentration decreased with increasing treatment time from 25 at.% for untreated PMMA to 13.5 at.% after 35 min exposure to direct He plasma, and to 7 at.% after 90 min VUV radiation generated by H2 plasma
Fig. 6. SEM micrographs of the scratch-track region for differently treated PMMA.
process (A) is faster, and it achieves maximum effectiveness in about 25 min treatment time (Fig. 2). Exposure to high humidity was also evaluated, and the results are represented by the open points in Fig. 2. Typically, L c values decreased by about 20–30% after 24
Fig. 7. Surface appearance after the solubility test in acetone for untreated (1) and He plasma treated PMMA (2) for 5 min, (3) for 10 min and (4) for 15 min.
106
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107
4. Conclusions
Fig. 8. Effect of treatment on the depth profile of the refractive index at 550 nm near the PMMA surface: Full line—direct exposure to microwave plasma in He; dotted line—exposure to VUV radiation generated by H2 plasma behind the MgF2 window.
behind the MgF2 window (Fig. 4). The effect of VUV with He plasma behind the MgF2 window was much less pronounced due to the elimination of the strong He line at 53 nm (21 at.% of O after 60 min treatment). In all measurements, the values obtained at different take-off angles did not exhibit any substantial changes. This suggests that the modified surface region is thicker than the maximum sampling depth (~8 nm) probed by XPS, in agreement with the optical analysis discussed below. AFM images, together with the scan profiles, indicated a decrease in surface roughness following both treatments processes (A) and (B) (see Fig. 5). SEM observations of untreated PMMA show microcracks at the bottom of the scratch-track, suggesting a cohesive failure in the polymer (see Fig. 6). After surface treatment, deep cracks are much less abundant, and removal of the scratched hardened layer appears. Presence of a crosslinked skin has also been confirmed by the surface solubility test in acetone (Fig. 7). After exposures to direct He plasma and VUV radiation from H2 plasma, the chemical attack was substantially reduced, similarly to our earlier observations for polycarbonate [17]. VASE measurements on treated PMMA surfaces were used to estimate the thickness of the modified surface layer (Fig. 8). This was accomplished by fitting the experimental ellipsometric angles W(k) and D(k) with a simple optical model consisting of a thin layer with linearly graded refractive index (n) values on top of the buntreatedQ PMMA bulk. These measurements indicated an increase of n by about 4% (from 1.50 to about 1.57 at 550 nm). The thickness of this graded layer varied between 50 and 96 nm, depending on the treatment time. This is in good agreement with the values obtained for other polymers such as polycarbonate [17], and with the penetration depth of VUV photons [26]. Thickness values of the crosslinked near-surface layer are higher than the electron escape depth in XPS, and thy are comparable with the skin detachment observed by SEM.
In the present work, we investigated the characteristics of PMMA surfaces treated by low pressure plasma under conditions leading to optimized adhesion of plasma-deposited films, namely silicon nitride. We found that relatively long exposures (10–30 min) to both direct He plasma or H2 plasma-generated VUV radiation lead to the removal of the surface weak boundary layer, and to the formation of a crosslinked surface layer, about 50–100 nm thick, which mechanically stabilizes the interface. The presence of this layer was determined by complementary techniques, namely XPS, SEM, and VASE, and by the chemical solubility test. Contribution of a crosslinked layer to the structured interfacial region, as a consequence of plasma treatment, is in agreement with the phenomenological model of adhesion enhancement described in the literature [4,6,16]. Mechanical stabilization of the interface by exposure to plasma or to VUV radiation appears very attractive for enhanced device performance in various applications such as optical coatings on organic substrates.
Acknowledgments The authors wish to thank Drs. A. Holl7nder, and G. Czeremuszkin, and Prof. M.R. Wertheimer for stimulating discussions, Dr. A. Fozza for his assistance with VUV treatment, and Dr. D. Poitras and Mr. A. Bergeron for the assistance with optical measurements. This work was supported by a contract with Optical Coating Laboratory Inc./JDS Uniphase.
References [1] J.M.E. Harper, Handbook of Plastics, Elastomers and Composites, McGraw-Hill, New York, NY, 1992. [2] L. Martinu, J.E. Klemberg-Sapieha, Optical Interference Filters, in: N. Kaiser, H. Pulker (Eds.), Springer Verlag, Berlin, 2003, p. 359. [3] A. Pizzi, K.L. Mittal (Eds.), Adhesion Promotion Techniques, Marcel Dekker, New York, 1999. [4] E.M. Liston, L. Martinu, M.R. Wertheimer, J. Adhes. Sci. Technol. 7 (1993) 1091. [5] F.D. Egitto, L.J. Matienzo, IBM J. Res. Develop. 38 (1994) 433. [6] M.R. Wertheimer, L. Martinu, E.M. Liston, Handbook of Thin Film Process Technology, in: S.I. Shah, D.A. Glocker (Eds.), IOP Publishing, Bristol, 1996, p. 33. [7] M.R. Wertheimer, L. Martinu, J.E. Klemberg–Sapieha, G. Czeremuszkin, in: K.L. Mittal, A. Pizzi (Eds.), Chapter 5 in Adhesion Promotion Techniques, Marcel Dekker, New York, 1999, p. 139. [8] J.E. Klemberg-Sapieha, D. Poitras, L. Martinu, N.L.S. Yamasaki, C.W. Lantman, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 985. [9] S. Vallon, B. Dre´villon, F. Poncin-Epaillard, J.E. Klemberg-Sapieha, L. Martinu, J. Vac. Sci. Technol., A, Vac. Surf. Films 14 (1996) 3194. [10] S. Vallon, A. Hofrichter, B. Dre´villon, J.E. Klemberg-Sapieha, L. Martinu, F. Poncin-Epaillard, Thin Solid Films 290/291 (1996) 68. [11] S. Vallon, R. Brenot, A. Hofrichter, B. Dre´villon, A. Gheorghiu, C. Se´ne´maud, J.E. Klemberg-Sapieha, L. Martinu, F. Poncin-Epaillard, J. Adhes. Sci. Technol., A 10 (1996) 1313.
J.E. Klemberg-Sapieha et al. / Thin Solid Films 476 (2005) 101–107 [12] L. Gerenser, J. Vac. Sci. Technol., A, Vac. Surf. Films 6 (1988) 2887. [13] J.E. Klemberg–Sapieha, M.K. Shi, L. Martinu, M.R. Wertheimer, Proc. of the Society of Plastic Engineers, Ann. Tech. Conf. ’95, Boston, MA, May 1995, p. 286. [14] J.E. Klemberg–Sapieha, in: R. d’Agostino, F. Fracassi, P. Favia (Eds.), Chapter 4 in Plasma Treatments and Deposition of Polymers, Kluwer Academic Publishers, Dordrecht, 1997, p. 233. [15] R.H. Hansen, H. Schonhorn, J. Polym. Sci., B, Polym. Lett. 4 (1966) 203. [16] L. Martinu, in: R. d’Agostino, F. Fracassi, P. Favia (Eds.), Chapter 4 in Plasma Treatments and Deposition of Polymers, Kluwer Academic Publishers, Dordrecht, 1997, p. 247. [17] A. Bergeron, J.E. Klemberg-Sapieha, L. Martinu, J. Vac. Sci. Technol., A, Vac. Surf. Films 16 (1998) 285. [18] A.S. da Silva Sobrinho, N. Schqhler, J.E. Klemberg-Sapieha, M.R. Wertheimer, M. Andrews, S.C. Gujrathi, J. Vac. Sci. Technol., A, Vac. Surf. Films 16 (1998) 2021.
107
[19] A.C. Fozza, J. Roch, J.E. Klemberg-Sapieha, A. Kruse, A. Holl7nder, M.R. Wertheimer, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 131 (1997) 205. [20] A.C. Fozza, J.E. Klemberg-Sapieha, M.R. Wertheimer, Plasmas Polym. 4 (1999) 183. [21] M.R. Wertheimer, A.C. Fozza, A. Holl7nder, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 151 (1999) 65. [22] J. Hong, F. Truica-Marasescu, L. Martinu, M.R. Wertheimer, Plasma Polym. 7 (2000) 245. [23] L. Martinu, D. Poitras, J. Vac. Sci. Technol., A, Vac. Surf. Films 18 (2000) 2619. [24] D.A. Shirley, Phys. Rev. 5 (1972) 4709. [25] S. Dahl, D. Rats, L. Martinu, J. von Stebut, J.E. Klemberg-Sapieha, Thin Solid Films 355/356 (1999) 290. [26] A. Holl7nder, R. Wilken, J. Behnisch, Surf. Coat. Technol. 116–119 (1999) 788.