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Preparation of periodic surface structures on doped poly(methyl metacrylate) films by irradiation with KrF excimer laser.

Kalachyova Y, Lyutakov O, Slepicka P, Elashnikov R, Svorcik V - Nanoscale Res Lett (2014)

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Solid State Engineering, Institute of Chemical Technology, Prague 166 28, Czech Republic.

ABSTRACT
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.

No MeSH data available.


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Dependence of grating amplitude on the concentration of FR. Laser treatment with 50 pulses and 12 mJ cm−2. Insets are AFM images.
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Figure 4: Dependence of grating amplitude on the concentration of FR. Laser treatment with 50 pulses and 12 mJ cm−2. Insets are AFM images.

Mentions: The quality of the surface ordered structures prepared by laser beam interference depends strongly on the regularity of interference pattern. In our case, the interference occurs between the incident laser beam and the beam reflected from the underlying substrate. Th properties of incident beam are affected by laser source and optical path (beam collimator, polarizer, presence of impurities, and so on). Reflection from the substrate may introduce additional deviation from ideally collimated and polarized laser beam. This can occur because of different inhomogeneities on the substrate surface, which can scatter the laser beam or change the plane of laser beam polarization. To examine the effect of the underlying substrate, we compare gratings prepared under the same experimental conditions on PMMA-FR films deposited onto microscopic glass and extremely ‘flat’ monocrystalline silicon. Silicon oxide surface layer, typical for Si substrate, was removed by HF etching immediately before polymer deposition. The results of this experiment are shown in Figure 3. It is seen that the quality of grating created on the PMMA-FR/Si substrate is much higher. In case of PMMA-FR/glass, the surface structures consist of several domains, with variable mutual orientation. Grating features are often disrupted or the coalescence with each other is observed. This picture is typical for common case of grating preparation by excimer laser ablation. In the case of PMMA-FR/Si sample, the prepared grating also exhibits some deviation from the ‘ideal’ case, but these deviations are negligible compared with those in the previous case. It appears that the homogeneity of the carrier substrate is critical for obtaining close-to-ideal surface pattern by laser light diffraction. Difference in the refraction indexes of glass and polymer or silicon and polymer must also be taken into account. Glass and polymer have similar refractive indices (1.51 and 1.49, respectively) [43,44]. On the other hand, the difference between polymer and silicon is sufficiently greater (1.49 for polymer and more than 3.5 for Si) [44]. According to Fresnel rules, the amount of reflected energy on polymer/silicon surface is sufficiently higher than that on the polymer/glass surface. Strong reflection can better conserve the laser beam shape, coherence, and light phase shift. Additionally, substrate/polymer reflection will affect the amplitude of interference pattern on polymer surface. So, the difference in grating regularity can be attributed to both parameters - smoothness of substrate's surface and substrate's reflectivity. All further prepared samples were deposited onto silicon substrates.In the next step, we performed a systematic investigation of the parameters of the prepared gratings and their quality which depend on the experimental conditions. At first, we investigated the dependence of the grating amplitude on the amount of added FR which is shown in Figure 4 together with AFM images illustrating the pattern quality. As can be expected, the increase of FR concentration leads to higher energy absorption and amplitude increase. However, not all structures were as regular as that shown in Figure 3B. A too high amount of FR leads to structure disruption and coalescence of grating features (see Figure 4 insets). A too low concentration leads to the structure creation only at several ‘hot’ places randomly distributed on polymer surface. The origin of these hot places can be attributed to inhomogeneities of the polymer film, random deviation in FR concentration, or variations in space distribution of laser beam energy. The range of optimal FR concentration, in which prepared gratings were ‘more’ regular, is depicted in the graph by a shaded area. Some qualitative conclusions about surface structure formation can be made, namely creation of regular grating begins at hot places and tends to form concentric semi-circles with the center at the initial hot place. When the number of hot places is high, they are located closely to each other and created structures overlap during further stages of formation. As a result, a grating pattern is formed. When the number of hot places is small, structure creation occurs only at separate local points. Increasing the laser fluence leads to rather curved geometry of the prepared surface structures, as depicted in Figure 4 insets.


Preparation of periodic surface structures on doped poly(methyl metacrylate) films by irradiation with KrF excimer laser.

Kalachyova Y, Lyutakov O, Slepicka P, Elashnikov R, Svorcik V - Nanoscale Res Lett (2014)

Dependence of grating amplitude on the concentration of FR. Laser treatment with 50 pulses and 12 mJ cm−2. Insets are AFM images.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4215891&req=5

Figure 4: Dependence of grating amplitude on the concentration of FR. Laser treatment with 50 pulses and 12 mJ cm−2. Insets are AFM images.
Mentions: The quality of the surface ordered structures prepared by laser beam interference depends strongly on the regularity of interference pattern. In our case, the interference occurs between the incident laser beam and the beam reflected from the underlying substrate. Th properties of incident beam are affected by laser source and optical path (beam collimator, polarizer, presence of impurities, and so on). Reflection from the substrate may introduce additional deviation from ideally collimated and polarized laser beam. This can occur because of different inhomogeneities on the substrate surface, which can scatter the laser beam or change the plane of laser beam polarization. To examine the effect of the underlying substrate, we compare gratings prepared under the same experimental conditions on PMMA-FR films deposited onto microscopic glass and extremely ‘flat’ monocrystalline silicon. Silicon oxide surface layer, typical for Si substrate, was removed by HF etching immediately before polymer deposition. The results of this experiment are shown in Figure 3. It is seen that the quality of grating created on the PMMA-FR/Si substrate is much higher. In case of PMMA-FR/glass, the surface structures consist of several domains, with variable mutual orientation. Grating features are often disrupted or the coalescence with each other is observed. This picture is typical for common case of grating preparation by excimer laser ablation. In the case of PMMA-FR/Si sample, the prepared grating also exhibits some deviation from the ‘ideal’ case, but these deviations are negligible compared with those in the previous case. It appears that the homogeneity of the carrier substrate is critical for obtaining close-to-ideal surface pattern by laser light diffraction. Difference in the refraction indexes of glass and polymer or silicon and polymer must also be taken into account. Glass and polymer have similar refractive indices (1.51 and 1.49, respectively) [43,44]. On the other hand, the difference between polymer and silicon is sufficiently greater (1.49 for polymer and more than 3.5 for Si) [44]. According to Fresnel rules, the amount of reflected energy on polymer/silicon surface is sufficiently higher than that on the polymer/glass surface. Strong reflection can better conserve the laser beam shape, coherence, and light phase shift. Additionally, substrate/polymer reflection will affect the amplitude of interference pattern on polymer surface. So, the difference in grating regularity can be attributed to both parameters - smoothness of substrate's surface and substrate's reflectivity. All further prepared samples were deposited onto silicon substrates.In the next step, we performed a systematic investigation of the parameters of the prepared gratings and their quality which depend on the experimental conditions. At first, we investigated the dependence of the grating amplitude on the amount of added FR which is shown in Figure 4 together with AFM images illustrating the pattern quality. As can be expected, the increase of FR concentration leads to higher energy absorption and amplitude increase. However, not all structures were as regular as that shown in Figure 3B. A too high amount of FR leads to structure disruption and coalescence of grating features (see Figure 4 insets). A too low concentration leads to the structure creation only at several ‘hot’ places randomly distributed on polymer surface. The origin of these hot places can be attributed to inhomogeneities of the polymer film, random deviation in FR concentration, or variations in space distribution of laser beam energy. The range of optimal FR concentration, in which prepared gratings were ‘more’ regular, is depicted in the graph by a shaded area. Some qualitative conclusions about surface structure formation can be made, namely creation of regular grating begins at hot places and tends to form concentric semi-circles with the center at the initial hot place. When the number of hot places is high, they are located closely to each other and created structures overlap during further stages of formation. As a result, a grating pattern is formed. When the number of hot places is small, structure creation occurs only at separate local points. Increasing the laser fluence leads to rather curved geometry of the prepared surface structures, as depicted in Figure 4 insets.

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Solid State Engineering, Institute of Chemical Technology, Prague 166 28, Czech Republic.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus