Preparation of periodic surface structures on doped poly(methyl metacrylate) films by irradiation with KrF excimer laser
© Kalachyova et al.; licensee Springer. 2014
Received: 12 August 2014
Accepted: 19 October 2014
Published: 28 October 2014
In this work, we describe laser modification of poly(methyl methacrylate) films doped with Fast Red ITR, followed by dopant exclusion from the bulk polymer. By this procedure, the polymer can be modified under extremely mild conditions. Creation of surface ordered structure was observed already after application of 15 pulses and 12 mJ cm−2 fluence. Formation of grating begins in the hottest places and tends to form concentric semi-circles around them. The mechanism of surface ordered structure formation is attributed to polymer ablation, which is more pronounced in the place of higher light intensity. The smoothness of the underlying substrate plays a key role in the quality of surface ordered structure. Most regular grating structures were obtained on polymer films deposited on atomically ‘flat’ Si substrates. After laser patterning, the dopant was removed from the polymer by soaking the film in methanol.
Illumination of polymers by polarized UV laser beam under specific conditions can induce formation of surface ordered structures [1–6]. Properties and mechanism of grating formation have been extensively studied after their first observation . Now, it is apparent that periodical structures develop on polymeric material surface as a result of the interference between attenuated and diffracted beams . Creation of surface ordered structures occurs only if a threshold value of laser energy is exceeded. Periodicity and amplitude of surface structures depend on applied laser wavelength, polymer's properties, and angle of incidence of the laser beam [9–16]. Grating creation may open new possibilities in a wide range of applications, including microfabrication, surface modifications, and medical applications fields [17–20].
Sufficient amount of absorbed laser energy is necessary to induce a polymer mass redistribution or ablation and creation of periodical structures at polymer surface. Polymers which are not absorbing at the wavelength of laser irradiation may be sensitized by doping with suitable compounds [21, 22]. Polymer doping may lower the threshold of ablation and therefore the costs of machining. Broad spectrum of periodic structures may be prepared by properly choosing the polymer and dopant from many possible combinations and by adjusting experimental conditions [23–26].
Poly(methyl methacrylate) (PMMA) is a common material with excellent optical and electrical properties. PMMA weakly absorbs at 248 nm, and the first excimer modification of this polymer, carried out by Kawamura et al. in 1982, was performed on PMMA doped with benzoin . Later, significant attention was paid onto ablation of both pristine PMMA [28–32] and PMMA doped with different compounds [33–37]. Wochnowski et al.  and Scully et al.  investigated the modification mechanism of PMMA induced by excimer laser irradiation at different wavelengths. The threshold value of the laser energy for the creation of periodical structure on pristine PMMA surface was found to be 400 mJ cm−2 (at 248 nm). Srinivasan et al.  reported the possibility of decreasing this threshold to 90 mJ cm−2 by addition of 2 wt.% of acridine. However, compared to other common polymers (e.g., polystyrene, polyethylene, polyethylene terephthalate), the threshold energy needed for grating formation on PMMA is significantly higher.
Despite a lot of works regarding the excimer modification of PMMA [33–40], several key issues remain unclear: (i) coalescence of grating features, which occurs under higher dose of illumination and leads to abnormal dependence of structure parameters on experimental conditions; (ii) influence of the underlying substrate; and (iii) the possible removal of the dopant after PMMA modification.
Based on the available information, we firstly introduced the method of ripple structure preparation on pristine PMMA at extremely ‘mild’ conditions (for common polymers). Decreasing the threshold value was carried out through the addition of a dopant, followed by laser illumination and dopant removal. We describe the effect of KrF laser irradiation of the doped PMMA in dependence on the substrate's roughness. We also investigated the dynamic of the surface structure formation and prepared a detailed report about the dependence of structure parameters on the condition of both laser treatment and properties of pristine polymer films.
In this work, a new method of grating formation on PMMA films deposited on a carrier substrate at extremely mild conditions is described. The threshold value of the laser energy is lowered by PMMA doping with 2-methoxy-5-(diethylaminosulfonyl)aniline, better known as Fast Red ITR (FR). The surface ordered structures are created by irradiation with KrF laser light, and the dopant is removed from the PMMA bulk after irradiation. The dynamic of the grating formation is described, and the dependence of laser irradiation effects on the smoothness of the carrier substrate is investigated.
The Fast Red-doped PMMA films were prepared by separately dissolving the PMMA (Mw ~ 1,500 K) and the Fast Red ITR in 1,2-dichloroethane. Then, 7.0 wt.% PMMA and 2.8 wt.% FR solutions were mixed and spin-coated onto freshly cleaned silicon wafers (crystallographic orientation (100), resistance 0.002 Ω cm, refractive index n = 3.50) or glass substrates (supplied by Glassbel Ltd., Prague, Czech Republic). The prepared samples were dried under ambient conditions for 24 h. The thicknesses of the polymer films were measured by profilometry.
The samples were irradiated with KrF excimer laser pulses (40 ns, λ = 248 nm) at a repetition rate of 10 Hz (Lambda Physik COMPexPro 50, Coherent Inc., Göttingen, Germany). The laser beam was polarized linearly with a cube of UV-grade fused silica with an active polarization layer. According to the preliminary experiments, it was decided to apply several levels of the irradiation energy, relating to different numbers of pulses and fluencies. The samples were irradiated by 1 to 350 laser pulses with laser fluencies from 7 to 13.5 mJ cm−2. The samples were mounted onto a translation stage, and different angles (from 0° to 67°) between the sample surface normal and the laser beam were chosen.
The surface morphology of PMMA and the grating structures was examined with an AFM technique using a VEECO CP II device (‘tapping’ mode, probe RTESPA-CP, spring constant 50 N·m−1, Veeco Instruments Inc., Plainview, NY, USA). The mass loss of the samples was measured using a Mettler-Toledo UMX2 microbalance (Mettler-Toledo, Inc., Greifensee, Switzerland). UV/vis spectra were measured using a UV/vis spectrometer Lambda 25 (PerkinElmer, Inc., Waltham, MA, USA).
Results and discussion
PMMA is known to have weak absorption peak at 248 nm [41, 42]. Therefore, the modification of PMMA can be performed by application of ‘high’ laser fluence which can be reduced by adding a dopant with high extinction coefficient. In our case, we used FR as the dopant (chemical structure of FR is presented in Figure 1). FR is well compatible with PMMA. The prepared films are homogeneous, and no dopant aggregation on the polymer surface or in the bulk is observed. The FR compound exhibits strong absorption at 248 nm and shifts optical properties of PMMA into the desired range.
Dependences of weight loss and the gratings amplitude on the angle of incidence of laser beam
Laser irradiation angle (°)
∆ m (μg)
114 ± 7.2
97 ± 8.6
43 ± 5.5
16 ± 6.5
The grating formation by laser light diffraction on the surface of a PMMA-FR film under extremely mild conditions was described. The grating pattern was observed already after irradiation with 15 pulses of light from KrF laser and 12 mJ cm−2 fluence. Creation of surface ordered structures begins at hot places and tends to form concentric semi-circles around. When the number of hot places is high and they are located closely to each other, created structures overlap during further irradiation stages and form highly regular grating pattern. The mechanism of surface ordered structure formation was attributed to ablation, which is more pronounced in the places of higher light intensity. The smoothness of substrate and its high refractive index play a key role in the quality of the prepared gratings. When atomically ‘flat’ Si substrates were used, nearly ideal grating structures are obtained. In some experiments, FR dopant was removed after laser patterning from bulk polymer by soaking the film in methanol. After dopant removal, the grating structures on dopant-free PMMA were obtained. After dopant extraction, the quality of surface ordered structures was partially degraded, but it still remains optically active. It can be assumed that the proposed method of grating creation could find applications in the preparation of optical components, including Bragg grating, wavelength splitter, or active substrates for surface enhancement of luminescence or Raman signals .
This work was supported by GACR under the project P108/12/G108.
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