Synthesis and characterization of novel jacketed polymers and investigation of their self assembly and application 5

Chapter 5 Synthesis and characterization of novel polymer used as fluorescent label... 17-19 incorporated anthracene groups onto a polymer backbone and used the polymer as potential pho

Chapter 5

Synthesis and characterization of novel polymer used

as fluorescent label

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 mask-making 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 e-beam 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-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 UV-active 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 chromophore-incorporated 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

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 4 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

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)-2-propylmethacrylate) (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 1 hr under nitrogen atmosphere A solution of 1,3-dibromo-2-propanol (2.4 ml,

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, 4 H), 7.5-6.9 (m, ArH, 10 H), 4.7 (m, J = 5.2Hz, C-CH(O)-, 1 H), 4.5 (d, J = 5.5 Hz, O-CH2-, 4 H), 2.7 (s, -OH, 1 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 %) 1 H NMR (300MHz, CDCl3, δ ppm) 7.29-7.57 (m, ArH, 14 H) 7.03 (d, ArH, 4 H), 4.45 (m, C-CH(O)-C, 1 H), 4.23 (m, O-CH2-R, 4 H), 2.58 (s, -OH, 1 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 %) 1 H NMR (300 MHz, DMSO-d6, δ ppm) 8.6-7.1 (m, ArH, 18 H), 5.7 (d, J = 5.14 Hz, -OH, 1 H), 4.6 (m, R-CH(O)-R,1 H), 4.5 (d, J = 4.8Hz, O-CH2-R, 4 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 %

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 0 °C, and methacryloyl chloride (2 ml, 20 mmol) dissolved in

4 ml THF was added The reaction mixture was warmed to room temperature, stirred for 4 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 %) 1 H NMR (300MHz, CDCl3, δ ppm) 8.3 - 6.9 (m, ArH, 14 H), 6.2 (s, CH2=C, 1 H), 5.6 (s,

CH2=C, 1 H), 5.9 (m, -CH(O)-R, 1 H), 4.6 (m, O-CH2-C, 4 H,), 2.0 (s, -CH3, 3 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 CH-), 66

(O-CH2-), 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 %) 1 H NMR (300 MHz, CDCl3, δ ppm) 7.30 - 7.58 (m, ArH, 14 H), 7.03 (d, ArH, 4 H), 6.19 (s,

CH2=C-, 1H), 5.62 (m, -CH(O)-R, C=CH, 2 H), 4.38 (d, O-CH2-, 4 H), 1.99 (s, -CH3,

3 H) 13C 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 %

1, 3-bis(9-phenanthryloxy)–2-propylmethacrylate (2c) Yield: 1.7 g (65 %) 1 H NMR (300 MHz, CDCl3, δ ppm) 8.3 – 7.1 (m, ArH, 18 H), 6.3 (s, C=CH2, 1 H), 5.6 (s, C=CH2, 1 H), 6.0 (m, -CH(O)-R, 1 H), 4.7 (m, J = 4.8 Hz, O-CH2, 4 H,), 2.0 (s,

-CH3, 3 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 (O-CH-), 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 (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 3 was isolated by

precipitation from methanol

Poly(1,3-bis(1-naphthyloxy)-2-propylmethacrylate) (3a) Yield: 0.79 g (powder, 91

%) 1 H NMR (300 MHz, CDCl3, δ ppm) 8.353-7.08 (b, ArH, 14 H), 6.06 (b,

-CH(O)-R, 1 H), 4.17 (b, -CH2-O, 4 H), 2.01 (b, -CH2-, 2 H), 1.26 (b, -CH3, 3 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,

O-CH2,5 H), 2.3 - 1.7 (b, -CH2-, 2H), 1.4 - 0.9 (b, -CH3, 3 H) FT-IR (KBr, cm-1): 3029 (ArH stretching), 2930 (-CH2- stretching), 1729 (ester C=O stretching),

Poly(1, 3-bis(9-phenanthryloxy)-2-propylmethacrylate) (3c) Yield: 0.8 g (powder,

75%) 1 H NMR (300 MHz, CDCl3, δ ppm) 8.1 - 7.0 (b, ArH, 18 H), 6.36 (b, -CH(O)-,

2 H), 3.95 (b, -CH2-O, 4 H), 1.87 (b, -CH2-, 2 H), 1.0 (b, -CH3, 3 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

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 2 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 3 (25 wt % of the PMMA solid), PMMA (5 wt %) in

chlorobenzene, 2, 2’-p-phenylenebis-(5-phenyloxazole) (POPOP) (5 wt % of the

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

0 20 40 60 80

100

3c 3b

3a

Temperature ( o

C)

Figure 5.1 TGA traces of polymer 3a, 3b and 3c measured in nitrogen atmosphere

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

3c

3b

3a

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