Chapter Synthesis and characterization of novel polymer used as fluorescent label 138 5.1 Introduction In the integrated circuit (IC) fabrication industry, the demand for pushing to finer features appears to be intense and it is necessary to develop new technologies. Several lithographic pathways to reduce feature size are under continuous development1. Except for the control of critical dimension pattern of sub-100 nm, pattern-placement metrology also plays a critical role in the nanofabrication. The patterning of mask requires strict fidelity and placement accuracy. The accuracy of the pattern placement is widely recognized as a major hurdle in developing sub-100 nm technologies. A successful lithographic technology must control mask distortions, alignment and other sources of overlay errors to less than 20% of the critical dimensions2-4. Such precision is not achieved with traditional e-beam lithography due to the difficulty in monitoring the electron beam on the substrate directly. As the feature size decreases, the contribution of the electron-beam patterning step towards the errors in the maskmaking process become larger due to many reasons, which include the stage accuracy, substrate heating and long term drifts. All such cumulative effects become substantial in the final output5. Spatial-phase-locked electron-beam lithography (SPLEBL) is being developed to significantly improve the pattern-placement accuracy. Currently, SPLEBL is the only known solution for achieving a sub-20 nm pattern-placement accuracy4-13. A critical component in the SPLEBL technique is the fiducial grid, which is patterned on a substrate and provides a direct reference for the position of ebeam. As the e-beam scans the substrate writing patterns, a weak interaction between the substrate and the grid generates a periodic signal, which gives information about the e-beam position on the substrate. Accordingly, the e-beam’s position can be locked on to the spatial phase of the fiducial grid in a feed back scheme.7 Good improvement of the pattern-placement precision is reported using scintillating global- 139 fiducial grid and through the membrane signal monitoring method 2, 3, 10 - 13. However, the observed signal-to-noise ratio is poor. One of the schemes proposed to avoid the problems in focusing e-beam, is to lay down Smith’s fiducial grids as a feedback system. To avoid the poor signal-to-noise ratio, the grids are formed by adding fluorescent labels14-16 to the resist film and an optical signal with high contrast is generated using an electron-beam activation. A successful fluorescent label should be highly sensitive to e-beam and significant fluorescent contrast would be generated between exposed and unexposed area. UVactive materials obtained considerable interest in the last two decades especially in the area of photoresist materials2. The light-induced changes of photo-physical properties of polymeric materials generate a good contrast between exposed and unexposed areas and may be suitable for generating a fiducial grid signal. Among the various photoactive systems, anthracene derivatives attracted significant interest in the past. Paul et al. 17-19 incorporated anthracene groups onto a polymer backbone and used the polymer as potential photoimageble materials. The arrangement and the high concentration of the chromophores on the polymer backbone gave good excimer fluorescence intensity upon excitation. Other aromatic chromophores immobilized polymers are also developed as potential fluorescence labels20, 21. Here we report the synthesis and characterization of novel chromophoreincorporated polymers to enhance the chromophore density in the resist medium without losing the processability. In this approach, we incorporated two chromophores per each monomeric unit on a methacrylate polymer backbone and used them as e-beam active fluorescent labels for e-beam writing applications. This simple concept also allows us to prevent the crystallization or microphase separation 140 of chromophores inside the polymer film. The polymers are also tested as potential fluorescent label dispersed in polymethyl methacrylate (PMMA) matrix. 5.2 Experimental section 5.2.1 Materials and reagents All reagents were obtained from commercial companies such as Sigma Aldrich, Fluka and used without further purification unless noted otherwise. Tetrahydrofuran (THF) was dried over metallic sodium and distilled under nitrogen. N,N-dimethylformamide (DMF) was dried with calcined Å molecular sieves (Aldrich). Flash column chromatography was performed using silica gel (60-120 mesh, Aldrich). 5.2.2 Instrumentation Fourier transform infrared (FT-IR) spectra were obtained using a Perkin-Elmer 1616 FT-IR spectrometer as KBr discs. 1H NMR, 13C NMR spectra were recorded on a Bruker ACF 300 MHz spectrometer. Thermal properties of the polymers were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) on a TA-SDT2960 and a TA-DSC 2920 at a heating rate of 10 °C min-1 under nitrogen environment. Gel permeation chromatography (GPC) were conducted with a Waters 2696 separation module equipped with a Water 410 differential refractometer HPLC system and Waters Styragel HR 4E columns using THF as eluent and polystyrene as standard. UV and fluorescence spectra were performed on a HP 8452A spectrophotometer and SHIMDZU RF 500 spectrofluorophotometer. The contrast of optical signal was realized on a Zeiss laser scanning confocal microscope. The excitation laser with a wavelength of 488 nm was used. The optical band-pass filter before the photomultiplier tube on the microscope was from 510 nm to 565 nm. 141 5.2.2 Synthesis Poly(1, 3-bis(1-naphthyloxy)-2-propylmethacrylate) (3a), poly(1, 3-bis(4- biphenyloxy)-2-propylmethacrylate) (3b), and poly(1, 3-bis(9-phenanthryloxy)-2propylmethacrylate) (3c) were synthesized starting from the commercially available 1,3- dibromo-2-hydroxypropane, as shown in Scheme 5.1. Scheme 5.1. Synthetic scheme for the monomers and polymers. General procedure for the synthesis of 1, 3-bisaryloxy-2-propanol (1) To a 250 ml three-neck round bottom flask fitted with a reflux condenser, addition funnel and a nitrogen inlet, 100 ml DMF, 50 mmol of the appropriate phenol and potassium carbonate (10.4 g, 100 mmol) were added. The mixture was stirred at 80 °C for hr under nitrogen atmosphere. A solution of 1,3-dibromo-2-propanol (2.4 ml, 142 23.4 mmol) in 25 ml DMF was added dropwise using a dropping funnel. The reaction mixture was stirred at 80 °C for 12 hr and filtered. The volatile components were removed under reduced pressure and excessive phenol was extracted with 2M sodium hydroxide solution followed by water (3 × 100 ml). The resulting crude product was purified using column chromatography on a silica gel column with hexane and dichloromethane (10:1) as the eluent. 1, 3-bis(1-naphthyloxy)-2-propanol (1a): Yield: 3.2g (41 %). 1H NMR (300 MHz, CDCl3, δ ppm) 8.3-7.8, (m, ArH, H), 7.5-6.9 (m, ArH, 10 H), 4.7 (m, J = 5.2Hz, CCH(O)-, H), 4.5 (d, J = 5.5 Hz, O-CH2-, H), 2.7 (s, -OH, H). 13 CNMR (75.4 MHz, CDCl3, δ ppm) 154.2, 137.2, 134.7, 127.7, 126.7, 126.0, 125.7, 125.5, 121.0, 105.3 (ArC), 69.4 (O-CH-), 69.2 (-O-CH2-). Anal. calcd for C23H20O3: C, 80.23 %; H, 5.81 %. Found: C, 80.61 %; H, 5.82 %. 1, 3-bis(4-biphenyloxy)-2-propanol (1b): Yield 5.6 g (61 %). H NMR (300MHz, CDCl3, δ ppm) 7.29-7.57 (m, ArH, 14 H). 7.03 (d, ArH, H), 4.45 (m, C-CH(O)-C, H), 4.23 (m, O-CH2-R, H), 2.58 (s, -OH, H). 13 CNMR (75.4 MHz, CDCl3, δ ppm) 157, 140, 134.4, 130.8, 128.7, 128.2, 126.7, 114.8 (ArC), 75.4 (O-CH), 68 (-O-CH2-). Anal. calcd for C27H24O3: C, 81.79 %; H, 6.10 %. Found: C, 81.53 %; H, 6.18 %. 1,3-bis(9-phenanthryloxy)-2-propanol (1c): Yield: 3.6 g (35 %). H NMR (300 MHz, DMSO-d6, δ ppm) 8.6-7.1 (m, ArH, 18 H), 5.7 (d, J = 5.14 Hz, -OH, H), 4.6 (m, R-CH(O)-R,1 H), 4.5 (d, J = 4.8Hz, O-CH2-R, H). 13 C NMR (75.4 MHz, DMSO-d6, δ ppm) 157, 137.5, 135.7, 132.6, 132.4, 132.2, 130.9, 130.8, 129.6, 128.8, 127.8, 127.7, 127.4, 108.1 (ArC), 74.5 (O-CH-), 72.6 (-O-CH2-). Anal. calcd for C31H24O3: C, 83.78 %; H, 5.41 %. Found: C, 83.65 %; H, 5.61 %. 143 General procedure for the synthesis of 1, 3-bisaryloxy-2-propyl methacrylate (2) Triethylamine (3 ml, 22 mmol) and the appropriate 1, 3-bisaryloxy-2-propanol (1) (10 mmol) were dissolved in 50 ml dry THF placed in a 100 ml round-bottom flask. This solution was cooled to °C, and methacryloyl chloride (2 ml, 20 mmol) dissolved in ml THF was added. The reaction mixture was warmed to room temperature, stirred for h, filtered and the volatile components were removed under reduced pressure. The resulting crude product was dissolved in 25 ml dichloromethane, and washed with sodium bicarbonate solution (50 ml), followed by water (3 × 50 ml). The organic layer was dried over anhydrous magnesium sulfate, filtered and the excess solvent was removed under reduced pressure to yield the monomer (2) 1, 3-bis(1-naphthyloxy)-2-propylmethacrylate (2a). Yield: 3.2 g (83 %). H NMR (300MHz, CDCl3, δ ppm) 8.3 - 6.9 (m, ArH, 14 H), 6.2 (s, CH2=C, H), 5.6 (s, CH2=C, H), 5.9 (m, -CH(O)-R, H), 4.6 (m, O-CH2-C, H,), 2.0 (s, -CH3, H). 13 C NMR (75.4 MHz, CDCl3, δ ppm) 166.5 (C=O), 146, 137, 135.5, 134.4, 127.7, 126.4, 126.2, 126.0, 125.6, 124.9, 122.0, 117.7 (ArC, C=C), 70.2 (O-CH-), 66 (OCH2-), 18.2 (-CH3). Anal. calcd for C27H24O4: C, 78.64 %; H, 5.83 %. Found: C, 78.96 %; H, 5.97 %. 1, 3-bis(4-biphenyloxy)–2-propylmethacrylate (2b). Yield: 3.0 g, (64 %). H NMR (300 MHz, CDCl3, δ ppm) 7.30 - 7.58 (m, ArH, 14 H), 7.03 (d, ArH, H), 6.19 (s, CH2=C-, 1H), 5.62 (m, -CH(O)-R, C=CH, H), 4.38 (d, O-CH2-, H), 1.99 (s, -CH3, H). 13 C NMR (75.4 MHz, CDCl3, δ ppm) 166.5 (C=O), 146, 140.6, 135.8, 134.4, 128.7, 128.1, 127.7, 126.7, 126.0, 117.7 (ArC, C=C), 70.7 (O-CH-), 66.2 (O-CH2-), 18.2 (-CH3). Anal Calcd for C31H28O4: C, 80.17 %; H, 6.03 %. Found: C, 80.17 %; H, 6.26 %. 144 1, 3-bis(9-phenanthryloxy)–2-propylmethacrylate (2c). Yield: 1.7 g (65 %). H NMR (300 MHz, CDCl3, δ ppm) 8.3 – 7.1 (m, ArH, 18 H), 6.3 (s, C=CH2, H), 5.6 (s, C=CH2, H), 6.0 (m, -CH(O)-R, H), 4.7 (m, J = 4.8 Hz, O-CH2-, H,), 2.0 (s, CH3, H). 13 C NMR (75.4 MHz, CDCl3, δ ppm) 166.6 (C=O), 151, 137, 132, 131, 127,126.9, 126.4, 126.2, 126.1, 124.2, 124, 122.2, 121.1, 102.9 (ArC, C=C), 70.5 (OCH-), 66.5 (O-CH2-), 17.9 (-CH3). Anal. calcd for C35H28O4: C, 82.03%; H, 5.47%. Found: C, 82.45 %; H, 5.71 %. General procedure for the preparation of poly (1, 3-bis(aryloxy)-2-propyl methacrylate) (3) The appropriate monomer (2 mmol) and 2, 2’-azobisisobutyronitrile (AIBN) (0.02 g, 0.01 mmol, 0.5 mol %) were dissolved in 20 ml dry THF. The solution was thoroughly degassed and flushed with nitrogen for 30 minutes, heated to 70 - 80 °C and stirred for 18 hr under nitrogen atmosphere. The polymer was isolated by precipitation from methanol. Poly(1,3-bis(1-naphthyloxy)-2-propylmethacrylate) (3a). Yield: 0.79 g (powder, 91 %). H NMR (300 MHz, CDCl3, δ ppm) 8.353-7.08 (b, ArH, 14 H), 6.06 (b, -CH(O)R, H), 4.17 (b, -CH2-O, H), 2.01 (b, -CH2-, H), 1.26 (b, -CH3, H). FT-IR (KBr, cm-1): 3054 (ArH stretching), 2931 (-CH2- stretching), 1731 (ester C=O stretching), 1580, 1509, 1460 (Ar, C=C stretching), 1268, 1156, 1020 (C-O-C stretching). Poly(1, 3-bis(4-biphenyloxy)-2-propylmethacrylate) (3b). Yield 0.72 g (powder, 79 %). 1H NMR (300 MHz, CDCl3, δ ppm) 7.6 – 6.5 (b, ArH, 18 H), 4.4 – 3.6 (b, OCH2, H), 2.3 - 1.7 (b, -CH2-, 2H), 1.4 - 0.9 (b, -CH3, H). FT-IR (KBr, cm-1): 3029 (ArH stretching), 2930 (-CH2- stretching), 1729 (ester C=O stretching), 1602,1527,1486 (Ar, C=C stretching), 1240, 1174, 1047(C-O-C stretching). 145 Poly(1, 3-bis(9-phenanthryloxy)-2-propylmethacrylate) (3c). Yield: 0.8 g (powder, 75%). H NMR (300 MHz, CDCl3, δ ppm) 8.1 - 7.0 (b, ArH, 18 H), 6.36 (b, -CH(O)-, H), 3.95 (b, -CH2-O, H), 1.87 (b, -CH2-, H), 1.0 (b, -CH3, H). FT-IR (KBr, cm-1): 3067 (ArH stretching), 2920 (-CH2- stretching), 1724 (ester C=O stretching), 1626, 1602, 1527 (Ar, C=C stretching), 1268, 1150, 1020 (C-O-C stretching). 5.2.4 Molecular weight measurement The molecular weights of polymer 3a, 3b and 3c were determined using GPC with solutions of polymers in THF (table5.1). Table 5.1. Number average (Mn), weight average (Mw) molecular weight and polydispersity (PD) of polymers Polymer Mn Mw PD 3a 0.60 × 104 1.71 × 104 2.9 3b 0.81 × 104 2.26 × 104 2.7 3c 0.51 × 104 1.16 × 104 2.2 5.2.3 Techniques and methods for re-structuring by high-energy electrons The re-structuring process was carried out by irradiating the polymer film with a beam of high-energy electrons using a predesigned pattern. The irradiation of electron-beam is similar to an electron-beam direct writing step for the fabrication of a mask in the IC industry. The beam writer is a JEOL JBX-5DII using LaB6 as its filament. The accelerating voltage for the electrons was 25 kV. 0.5 µm lines with an interline spacing of µm were made using an e-beam with a dosage of 300 µC/cm2. The polymer film was deposited on a silicon wafer via spin-coating a homogenous mixture of polymers (25 wt % of the PMMA solid), PMMA (5 wt %) in chlorobenzene, 2, 2’-p-phenylenebis-(5-phenyloxazole) (POPOP) (5 wt % of the 146 solid). Generally the POPOP was added as a wavelength shifter to reduce the absorption of the scintillated photon by the polymer host, PMMA. 5.3 Results and Discussion Methacrylate monomers carrying two chromophores were synthesized in moderate to high yield using a simple synthetic strategy as shown in Scheme 5. 1. Highly soluble polymers with high chromophore density along the polymer backbone were obtained through free radical polymerization and fully characterized using spectroscopic techniques. 5.3.1 Thermal properties The thermal stability of the novel polymers in nitrogen was evaluated using thermogravimetric analysis (TGA). All polymers showed a weight loss at 350-355 °C at a heating rate of 10 °C /min above which the polymer start to degrade completely 3b 3c 100 Weight percent (%) 80 3a 60 40 20 100 200 300 400 500 600 o Temperature ( C) Figure 5.1. TGA traces of polymer 3a, 3b and 3c measured in nitrogen atmosphere 147 The thermal properties of all polymers were also analyzed by differential scanning calroimetry (DSC) at a heating rate of 10 °C /min (Figure 5.2). A glass transition (Tg) was observed at about 80 °C for polymer 3a, but no melting point was observed. The DSC plots of polymer 3b, and 3c showed corresponding Tg at 85 °C and 112 °C, respectively. However, polymer 3c showed a higher Tg than the other two polymers, which might be due to the presence of rigid and bulky pendant groups. o Heat Flow (mW) Tg 112 C 3c o Tg 85 C 3b o Tg 80 C 3a 50 100 150 200 o Temperature ( C) Figure 5.2. DSC traces of polymer 3a, 3b and 3c measured in a nitrogen atmosphere. 5.3.2 Optical properties The spectroscopic properties of polymers 3a, 3b and 3c were measured in chloroform solution. The absorption and emission spectra of the polymers are shown in Fig.5.3 and the values as given in Table 5.2. The UV spectrum of polymer 3a showed absorption maximum at 256 nm with a weak absorption at 308 nm. When excited at 256 nm, polymer 3a showed two strong emission peaks at 362nm and 379nm. Polymer 3b exhibits the absorption maximum at 266 nm. When excited at 266 nm, polymer 3b displayed emission peaks at 333nm. The spectrum of polymer 3c shows 148 maximum absorption at 258 nm with a shoulder around 298 nm. When excited at 258 nm, it displayed two strong emission peaks at 340 and 357nm. All polymers showed fluorescence active around 330-390 nm. These can be ascribed to the attached chromophore groups. absorption emission Intensity (a.u.) 3c 3b 3a 300 400 500 wavelength (nm) Figure 5.3. UV-vis absorption and Fluorescence spectra of polymers measured in chloroform at room temperature Table 5.2 The detailed data of UV-vis absorption and Fluorescence spectra polymer 3a, 3b and 3c Polymer λ abso (nm) λ emiss (nm) 3a 256, 308 362, 379 3b 266 333 3c 258, 298 340, 357 149 Polymers incorporated with biphenyl (3a), naphthyl (3b) to phenanthryl (3c) chromophore group, a red shift on the fluorescence spectrum were observed. Other photophysical properties of the polymers are currently under investigation. 5.3.3 The fiducial-grid signal The enhancement of pattern placement accuracy in a SPLEBL scheme depends upon the fidelity of the grid, the grid’s period and the quality of the periodic signal from the grid. However, the contrast of the fiducial grid patterns is low due to the inherent nature of interferometric lithography, which utilizes interference of two UV standing waves (wavelength of 351 nm). In order to maintain the requirement of maskless approach, the fiducial grid is patterned by e-beam, which generates high image contrast at the exposure level. The signal from the grid is in the form of laserstimulating fluorescence and can be detected by a photomultiplier tube. The preliminary experiment on the effect of electron excitation was carried out using the polymers film under a conventional JEOL SEM. The results are shown in Figure 5.4. Poly(methyl methacrylate)(PMMA) and its derivatives have been used as e-beam resists11-12. Generally, the host polymer itself is not a good fluorescent active matrix and hence the presence of an UV-active additive is necessary to pattern fiducial grid. The final mixture of PMMA resist constitutes three components: (i) the host polymer, (ii) the added chromophore and (iii) the wavelength shifter. Owing to the poor solubility, the chromophores or fluorophores such as anthracene tend to crystallize and precipitate out of the resist film and show a heterogeneous distribution. Here we use the chromophore-immobilized polymers to prevent the heterogeneous distribution of the chromophores and to obtain a high contrast for the pattern. From fluorescence spectra of the pure polymers in solution as well as in thin film, little fluorescence emission was observed between 515-565 nm (Table 5.2). This is 150 important to avoid the emission from the background. After irradiation by e-beam, a bright green fluorescent line appeared at the exposed area (Figure 5.4). It is believed that the interaction of e-beam with the chromophore on the film is primarily responsible for such emission. Paul et al. described the mechanism of chemical changes in anthracene-containing materials after UV exposure 17, 22. Hence we believe that the high-energy e-beam interacts with the substrate, and causes the depolymerization of PMMA. However, the chromophors incorporated on each repeat units remain on the substrate and produce a strong emission signal. The exact mechanism of this emission is still under investigation. Our results indicate that both polymers (3a and 3b) are active under e-beam irradiation and may be useful as a fluorescence label in the e-beam writing. A well-designed pattern was generated onto the polymer film with different chromophoric groups by using an electron-beam direct write method, which is a standard step in mask fabrication. Figure 5.5 shows two typical fluorescence images of the resist surface after patterned with an e-beam. A set of green 0.5 µm lines can be resolved clearly with a spacing of µm using an irradiation dosage of 300 µC/cm2. The contrast between exposed and unexposed area is sufficient to give the image of the pattern. However, polymer 3c appeared weak e-beam reactive matrix under the experimental conditions used here. A dosage of µC/cm2 did not print any lines and there was no image contrast between exposed and unexposed area. It is generally believed that the host polymer is highly absorptive and hence a wavelength shifter, POPOP, is added to increase the scintillation efficiency. 151 a b Figure 5.4. The fluorescence image of (a) pure 3a and (b) pure 3b after excitation by electron beam of 13 kV voltage (the magnification of the picture is only 100 times). a b Figure 5.5. The fluorescence image of a set of 0.5 µm lines, patterned by 25 kV ebeam excitation and dosage of 300 µC/cm2, of the fluorescent PMMA resist mixture. The mixture contains < 0.4 wt % POPOP and 25wt % various chromophore (a) 3a (b) 3b 5.3.4 Sensitivity of the chromophore The effect of the concentration of chromophore-immobilized polymers on the image contrast was investigated. The results are shown in Figure 5.6. Anthracene is dissolved in PMMA matrix to set a benchmark for reviewing the performance of the 152 synthesized polymers 3. The results showed that with the increase of the chromophore concentration, more irradiation dosage is necessary so that the contrast between exposed and unexposed area is sufficient to give a clear fluorescent image of the patterns. Compared with anthracene, the sensitivity of chromophore-containing polymers is lower, and high irradiation dosage was required to generate strong fluorescence. However, anthracene was precipitated and crystallized in the PMMA matrix and created defects in the patterns in our experiments. Exposure dose (C/cm) 400 Anthrance 3b 3a 200 20 40 60 80 100 Loading of chromphore (wt%) Figure 5.6. The effect of the concentration of chromophoric polymers on sensitivity of the photo resists The good solubility and lack of crystallization of the chromophore-immobilized polymers 3a and 3b in the resist film allowed them to be potential candidates as fluorescent labels in e-beam writing. Even though, the reported polymers need high dosage for patterning, the images obtained were intense with high contrast. Since there are many chromophores available, we believe that the performance can be 153 optimized via immobilization of other chromophores on the polymer backbone in the near future. 5.4 Conclusion A series of chromophore-immobilized polymethacrylate were synthesized via radical polymerization and tested them as potential fluorescent labels for e-beam writing. The chromophore groups such as naphthyl, biphenyl and phenathryl groups were incorporated on the polymethacrylate backbone and demonstrated the potential of using these polymers for increasing the pattern placement accuracy of an e-beam writer. The fluorescent label can be pre-irradiated to form the fiducial grids, which would be used for getting the feedback of the electron beam position on the patterning area. This scheme is believed to be capable of improving the placement accuracy over the conventional registration marks that are outside of the patterning area. References 1. Cerrina, F. J. Phys. D: Appl. 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European Polymer Journal 2001, 37, 411-416. 21. Itoh, Y.; Inoue, M.; European Polymer Journal 2000, 36, 2605-2610. 22. Kawashima, Y.; Hashimoto, T.; Nakano, H. and Hira, K. Theor. Chem. Acc. 1999, 102, 49-64. 156 [...]... presence of rigid and bulky pendant groups o Heat Flow (mW) Tg 112 C 3c o Tg 85 C 3b o Tg 80 C 3a 50 100 150 200 o Temperature ( C) Figure 5. 2 DSC traces of polymer 3a, 3b and 3c measured in a nitrogen atmosphere 5. 3.2 Optical properties The spectroscopic properties of polymers 3a, 3b and 3c were measured in chloroform solution The absorption and emission spectra of the polymers are shown in Fig .5. 3 and. .. magnification of the picture is only 100 times) a b Figure 5. 5 The fluorescence image of a set of 0 .5 µm lines, patterned by 25 kV ebeam excitation and dosage of 300 µC/cm2, of the fluorescent PMMA resist mixture The mixture contains < 0.4 wt % POPOP and 25wt % various chromophore (a) 3a (b) 3b 5. 3.4 Sensitivity of the chromophore The effect of the concentration of chromophore-immobilized polymers on... excited at 258 nm, it displayed two strong emission peaks at 340 and 357 nm All polymers showed fluorescence active around 330-390 nm These can be ascribed to the attached chromophore groups absorption emission Intensity (a.u.) 3c 3b 3a 300 400 50 0 wavelength (nm) Figure 5. 3 UV-vis absorption and Fluorescence spectra of polymers measured in chloroform at room temperature Table 5. 2 The detailed data of UV-vis... heterogeneous distribution Here we use the chromophore-immobilized polymers to prevent the heterogeneous distribution of the chromophores and to obtain a high contrast for the pattern From fluorescence spectra of the pure polymers in solution as well as in thin film, little fluorescence emission was observed between 51 5 -56 5 nm (Table 5. 2) This is 150 important to avoid the emission from the background After... enhancement of pattern placement accuracy in a SPLEBL scheme depends upon the fidelity of the grid, the grid’s period and the quality of the periodic signal from the grid However, the contrast of the fiducial grid patterns is low due to the inherent nature of interferometric lithography, which utilizes interference of two UV standing waves (wavelength of 351 nm) In order to maintain the requirement of maskless... electron-beam direct write method, which is a standard step in mask fabrication Figure 5. 5 shows two typical fluorescence images of the resist surface after patterned with an e-beam A set of green 0 .5 µm lines can be resolved clearly with a spacing of 2 µm using an irradiation dosage of 300 µC/cm2 The contrast between exposed and unexposed area is sufficient to give the image of the pattern However, polymer 3c... used here A dosage of 5 µC/cm2 did not print any lines and there was no image contrast between exposed and unexposed area It is generally believed that the host polymer is highly absorptive and hence a wavelength shifter, POPOP, is added to increase the scintillation efficiency 151 a b Figure 5. 4 The fluorescence image of (a) pure 3a and (b) pure 3b after excitation by electron beam of 13 kV voltage... UV-vis absorption and Fluorescence spectra polymer 3a, 3b and 3c Polymer λ abso (nm) λ emiss (nm) 3a 256 , 308 362, 379 3b 266 333 3c 258 , 298 340, 357 149 Polymers incorporated with biphenyl (3a), naphthyl (3b) to phenanthryl (3c) chromophore group, a red shift on the fluorescence spectrum were observed Other photophysical properties of the polymers are currently under investigation 5. 3.3 The fiducial-grid... of chromophore-containing polymers 3 is lower, and high irradiation dosage was required to generate strong fluorescence However, anthracene was precipitated and crystallized in the PMMA matrix and created defects in the patterns in our experiments Exposure dose (C/cm) 400 Anthrance 3b 3a 200 0 0 20 40 60 80 100 Loading of chromphore (wt%) Figure 5. 6 The effect of the concentration of chromophoric polymers. .. and Mullen, K Acta Polymer 1994, 45, 2 352 43 19 Paul, S.; Halle, Olaf ; Mullen, K Thin Solid Films 1996, 288, 150 - 154 155 20 Itoh, Y.; Inoue, M.; Kusano, S and Goshima, T European Polymer Journal 2001, 37, 411-416 21 Itoh, Y.; Inoue, M.; European Polymer Journal 2000, 36, 26 05- 2610 22 Kawashima, Y.; Hashimoto, T.; Nakano, H and Hira, K Theor Chem Acc 1999, 102, 49-64 156 . J = 5. 2Hz, C- CH(O)-, 1 H), 4 .5 (d, J = 5. 5 Hz, O-CH 2 -, 4 H), 2.7 (s, -OH, 1 H). 13 CNMR ( 75. 4 MHz, CDCl 3 , δ ppm) 154 .2, 137.2, 134.7, 127.7, 126.7, 126.0, 1 25. 7, 1 25. 5, 121.0, 1 05. 3. 100 times). a b Figure 5. 5. The fluorescence image of a set of 0 .5 µm lines, patterned by 25 kV e- beam excitation and dosage of 300 µC/cm 2 , of the fluorescent PMMA resist mixture Figure 5. 6. The effect of the concentration of chromophoric polymers on sensitivity of the photo resists The good solubility and lack of crystallization of the chromophore-immobilized polymers