3.1 Materials 3.1.1 Chemicals
(1) Initiator(3-chloropropyl)(triethoxy)silane
CAS No. 5089-70-3, purity=95%, product by TCI
Si
MeO OMe
OMe
Cl
(2) Catalyst
(a) Copper (I) bromide
CuBr(I), CAS No. 7787-70-4, purity = 98%, product by Sterm Chemicals
(b) N,N,N’,N”,N”-pentamethyldiethylenetriamine
PMDETA, CAS No. 3030-47-5, purity = 99+%, product by ACROS
N NH
NH
(3) Monomer Styrene
St, CAS No. 100-42-5, purity = 99%, product by SHOWA
(4) Polymer Polystyrene
PS, CAS No.9003-53-6, standard, MW:90,000 g/mole, product by Aldrich
*
*
n
3.1.2 Matrix
methyltrimethoxysilane
MTMS, CAS No. 1185-55-3, purity = 97%, product by ACROS
Si
O O
O
3.1.3 Solvent
(1) TolueneToluene, CAS No.108-88-3, product by TEDIA
(2) Tetrahydrofuran
THF, CAS No. 109-99-9, purity =99.9, product by ECHO
O
(3) Ethanol
EtOH, CAS No.64-17-5, purity =95, product by ECHO
OH
(4) n-Hexane
Hex, CAS No.110-545-3, purity >95 (HPLC), product by Aldrich
3.1.4 Acid
Hexafluorophosphoric acid
HPF6, CAS No. 16940-81-1, purity = 60wt%, product by ACROS
H
+P
-F F
F F F
F
3.2 Preparation
3.2.1 Purification of styrene (St) and CuBr(I)
CuBr was sublimated before using. CuBr (20~30g) were added into a 250 mL flask with equipped magnetic stirring bar, and then added 1L acetic acid. The flask was wrapped by aluminum foil overnight. Acetic acid would be removed by methanol, and then removed methanol at 80 °C oven. The result was CuBr(I). Styrene was sublimated before using. Styrene was distilled to remove the inhibitor.
3.2.2 Synthesis of α-siloxane-polystyrene (PS-siloxane)
In this experiment, CuBr(I) (0.28g, 0.001 mole) was added to a 250mL flask, equipped with a magnetic stirring bar, and then the flask was degassed. Initiator SiCl (0.4g, 0.002 mole) was dissolved in 100 mL Toluene three freeze–pump–thaw cycles.
Styrene (5.6g, 0.054 mole) was added in a 100 mL flask and degassed with three freeze–pump–thaw cycles. PMDETA (0.328g, 0.0002 mole) was transferred into the 250mL flask which carried CuBr(I) through a syringe, and CuBr(I) would dissolve in PMEDTA. PMEDTA was chosen as ligand to chelate CuBr(I), and activated CuBr(I).
Toluene and initiator were transferred into the flask through a syringe. After 1 min, styrene was transferred into the flask through a syringe and stirred at 80 °C for 12 h under nitrogen atmosphere. The polymerization would be stopped by adding THF and Cu2+ was removed by an alumina column. The polymer (polystyrene-siloxane, PS-Silocane) was obtained by precipitating the solution into alcohol and dried in the vacuum overnight. Yield: 1.3g (58.6%).
In this study, the ratio of styrene and initiator would be adjusted to change PS molecular weight. Table 3.1 shows the recipe of ATRP reactions for various PS molecular weight .
Table 3. 1 The recipe of ATRP reaction for various PS molecular weight.
styrene [I] CuBr PMDETA Toluene Time
(1) 5.6 0.4 0.28 0.328 3ml 12 h
(2) 5.6 1.2 0.28 0.328 3ml 12 h
(3) 5.6 2.4 0.28 0.328 3ml 12 h
3.2.3 Synthesis of MSQ from MTMS using sol-gel
MTMS (3.3g, 0.024 mole) was added into an alumina pan, DI water (0.285g, 0.018 mole) was added to hydrolysis MTMS, and added HPF6 (0.0375g, 0.0003 mole) to dehydrate MTMS and start the sol-gel reaction. The pan was put into the oven at 80
°C for 7.5 mins to yield the final product, MSQ.
3.2.4 Grafting PS onto MSQ through siloxane-PS
Siloxane-PS was first dissolved in 2 mL THF in a 100 ml flask. Upon the completion of the sol-gel reaction, MSQ (1g) was immediately added into the flask.
The reaction proceeded at 50 °C under oil bath for 14 h. The product was MSQ-g-PS, which was stored in freezer prior to usage.
3.2.5 Preparation of porous low-k film
MSQ-g-PS was dissolved in THF to form a 20 wt% solution. Before spin coating, the solution was initially filtered through a 0.45 μm PTFE filter (Millipore Inc.) in another bottle. The MSQ-g-PS solution was spin coating onto a silicon wafer to obtain the desired film thickness. The MSQ-g-PS film was then cured on a hot plate preheated at 200 °C for 30 mins, and at 400°C for 60 mins.
study to compare the PS sizes to that in pure THF solvent. Typically, THF was first added to dissolve MSQ-g-PS. Then ethanol or hexane was added to the MSQ-g-PS solution in order to examine coils size of PS long chain. Meanwhile, the thin film preparation method was the same as MSQ-g-PS porous low k film.
3.3 Experimental techniques
3.3.1 Nuclear Magnetic Resonance Spectroscopy (NMR)
1H-NMR was employed to study the chemical structure of PS-Siloxane using Varian Unity-300 NMR. Chemical compound was dissolved in CDCl3. The relative sensitivity was 1.00 ppm, and the sample was scanned for 32 times.
29Si-NMR was employed to study the chemical structure of using Bruker DSX-400WB NMR. Time was 0.0246260 sec, Repetition was 4096 times. Sample was solid.
3.3.2 Gel Permeation Chromatography (GPC)
GPC was employed to calculate the molecular weight of PS-Siloxane using GPC Water 1515, and sample was dissolved in THF. The ratio of sample/THF was 2mg/1mL. Flow was 1 mL/min at 45 °C.
3.3.3 Different Scanning Calorimetry (DSC)
DSC was employed to study the glass transition temperature of PS-Siloxane, which was produced by Perkin-Elmer. Sample weight must higher than 2 mg. The heating rate is 10 °C/min from 0 °C to 250 °C in nitrogen.
3.3.4 Thermal Gravimetric Analyses (TGA)
TGA was employed to study the decomposition temperature of PS-Siloxane and
MSQ-g-PS using TA Q500. Sample weight must higher than 5 mg. The heating rate is 10 °C/min from 0 °C to 900 °C in nitrogen.
3.3.5 Fourier-transform infrared spectroscopy (FTIR)
Fourier-transform infrared spectroscopy (FTIR) was employed to study chemical structures of PS-Siloxane, MSQ, and MSQ-g-PS using MAGNA-IR Technology Protage 460 (Nicolet Inc.) from 400 to 4000 cm-1. A transmission mode is typically used for porous low-k films onto a silicon wafer, which is infrared transparent. The total number of scans was 32 and the resolution was 4 cm-1.
3.3.6 Scanning Electron Microscope (SEM)
A dual beam FIB/SEM system was used to examine the pore size of porous low-k films. Voltage was kept at 5 kV, while current was 98 pA.
3.3.7 X-ray reflectivity (XRR)
XRR was employed to characterize the porosity of porous low-k films using Beamline 13A1 in NSRRC, Taiwan. The scanning angle ranged from 0° to 2°, while the resolution was at 0.002°.