Studies on MCM-41/PDMS based hybrid polybenzoxazine nanocomposites for interlayer low k dielectrics

Abstract

With a view to replace the conventional insulating silica material, a thermally stable mesoporous silica/siloxane based hybrid polybenzoxazine nanocomposite was developed as an interlayer low k dielectric material. The precursors such as benzoxazine terminated polydimethylsiloxane (PDMS-Bz) and benzoxazine terminated mesoporous MCM-41 silica (BTMS) materials were synthesized and are copolymerized by a thermal curing process to obtain polybenzoxazine nanocomposites. The cured polybenzoxazine nanocomposites were characterized using different analytical techniques such as FTIR, NMR, XRD, SEM, HRTEM, DSC and TGA. The dielectric behavior of different weight percentages of BTMS reinforced PDMS polybenzoxazine (BTMS/PDMS-PBz) nanocomposites were described based on the concepts of polarization and porosity of the material. Among the different weight percentages of BTMS reinforced composites 7 wt% BTMS/PDMS-PBz possesses the lowest value of dielectric constant (2.06) with enhanced thermal stability.

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Introduction

The performance of microelectronics devices is usually limited by increase in crosstalk, propagation delay and power dissipation.¹ Consequently the microelectronics industries are continuously trying to find new technological solutions to reduce such shortcomings. In view of this, the conventional insulator (SiO₂, k = 3.9–4.3) used in ultra large scale integrated circuits was gradually replaced by low-k dielectric materials.² Thus, the semiconductor technologies are forced to move from Al/SiO₂ to Cu/low-k interconnect technology with a view to improve the device performance by reducing resistance–capacitance (R–C) time delay.³ Besides the silica materials, less polar organics and porous structured organic–inorganic hybrid polymer nanocomposites might be a good candidate for low-k interlayer dielectric materials.⁴ In recent years, the polymers were hybridized with porous silica materials such as POSS, SBA-15, MCM-41, etc., for the reduction of dielectric constant to the possible extent and also for retaining the thermo-mechanical behaviors.^5–8 Among the silica materials, MCM-41 is one of the most abundant mesoporous silica materials, which is widely used to develop porous structured polymer composites.⁸ The pores present in the hybrid nanocomposites were filled by air or vacuum which helps to reduce the extent of polarization and inturn contributes to the reduction of value of dielectric constant. In addition, moisture absorption property of dielectric material also one of the important factors in the reduction of dielectric constant, because of the presence of moisture inherently enhances their orientation polarization throughout the matrix.^9,10 Thus the hydrophobic character of the polymer composites was improved by the introduction of less polar functional groups, siloxane and silica based compounds.^11–13 Different types of polymeric materials namely polyimides, fluoropolymers, polyaryl ethers, polybenzoxazines, polysilsesquioxanes, etc., were used as potential low-k dielectrics for microelectronics devices.^14–16

It is well known that the polyalkylsilsesquioxanes (PASSQ) are high performance materials which possess relatively low-k values (2.6–3.2), minimal moisture uptake, flexibility, high thermal stability and outstanding radiation resistance as well as good toughness contributed by methyl siloxane (–Si–O) chains.¹⁷ Moreover, polybenzoxazine is an excellent thermosetting polymer which has good thermal and mechanical properties, low moisture absorption, low shrinkage and excellent electrical properties.^18–20 The polydimethylsiloxane (PDMS) is a versatile starting platform for the synthesis of benzoxazine monomers. Hence, the development of PDMS based polybenzoxazine hybrids are warranted to utilize them for low-k dielectrics applications.^12,15 In addition, the porous structured MCM-41 has been considered as a suitable reinforcement and it can be used to develop porous structured polymer composites to reduce the value of dielectric constant.

Hence, in the present work, BTMS/PDMS-polybenzoxazine nanocomposites have been developed as a high performance interlayer low-k dielectric material. To study the changes of dielectric behavior and thermal properties of nanocomposites, varying weight percentages of benzoxazine terminated mesoporous MCM-41 (BTMS) have been reinforced into the PDMS based polybenzoxazine through copolymerization. The developed composite materials were characterized by different analytical techniques and the data resulted are discussed and reported.

Experimental

Materials

Analytical grades of paraformaldehyde, aniline, chloroform and toluene were purchased from SRL, India. 2-Allyl phenol, hydride terminated polydimethylsiloxane (PDMS, molecular weight ∼ 540) and platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene [Pt(dvs)] catalyst were purchased from Sigma-Aldrich and were used as received without further purification. Mesoporous MCM-41 was prepared as per the previous report.⁸

Synthesis of ortho-allyl phenol benzoxazine (OAP-Bz) (Scheme 1a)

OAP-Bz was synthesized and confirmed as per our earlier report¹⁸ is as follows; to a stirred solution of aniline (10 ml, 0.110 mol) in chloroform, formaldehyde (6.6 g, 0.22 mol) was added at 0 °C and stirred for 30 minutes at the same temperature. Subsequently, 14.2 ml of ortho-allyl phenol (OAP) (0.110 mol) was added to the reaction mixture and stirred for overnight at 70 °C. After the completion of reaction (monitored by TLC), the reaction mixture was extracted with ethyl acetate and washed with 2 N NaOH, water, brine and concentrated the extracted organic portion to yield (96%) the product.

Scheme 1 Synthesis of OAP-Bz and PDMS-Bz.

Synthesis of PDMS benzoxazine monomer (PDMS-Bz) (Scheme 1b)

The synthesis of benzoxazine terminated PDMS monomer is as follows; to a stirred and cooled solution of H-PDMS (10 g, 0.022 mol) in toluene, Pt(dvs) catalyst (6 drops) was added under nitrogen atmosphere and stirred for 30 minutes at 30 °C. After that the solution of OAP-Bz (11.17 g, 0.044 mol) in toluene was added to the reaction mixture and stirred for overnight at 100 °C. After the completion of reaction the temperature was reduced to 30 °C and activated charcoal was added to the reaction mixture and then the charcoal was filtered out through Celite bed, the filtrate was concentrated under reduced pressure to yield (94%) the product.

Synthesis of triethoxysilane terminated benzoxazine (TES-Bz) (Scheme 2a)

3-Aminopropyltriethoxysilane (0.02 mol) was added to a suspended solution of paraformaldehyde (0.04 mol), calcium hydride (CaH₂) and chloroform (30 ml) at 65 °C under inert atmosphere. Subsequently, the phenol (0.02 mol) was added to the reaction mixture and stirred for 3 h at the same temperature. After the completion of reaction, the solid residues were filtered out and the filtrate was concentrated to obtain the product (91% yield).

Scheme 2 Synthesis of TES-Bz and BTMS.

Preparation of benzoxazine terminated MCM-41 (BTMS) (Scheme 2b)

BTMS was obtained by refluxing 1 g of MCM-41 and 3 g of TES-Bz in ethanol/water (9 : 1) for 24 h. After that the product was cooled and filtered and the obtained solid was washed with ethanol and dried overnight at 70 °C.

Preparation of neat polybenzoxazine (PBz) matrix

A solution of PDMS-Bz in tetrahydrofuran (THF) was poured to a respective glass mold and the solvent was evaporated overnight at 60 °C and then cured stepwise at 120, 140, 160, 180, 200, 220 and 240 °C for 1 h each to obtain a film.

Preparation of BTMS/PDMS polybenzoxazine (PBz) composites (Scheme 3)

The varying weight percentages of BTMS (1, 3, 5, 7, and 10 wt%) were added separately to a solution of PDMS-Bz (2 g) in 10 ml THF and stirred for 30 min at 30 °C. The solutions were poured into a respective glass mold and heated at 60 °C for overnight and then cured stepwise at 120, 140, 160, 180, 200, 220 and 240 °C for 1 h each.

Scheme 3 Preparation of BTMS/PDMS-PBz nanocomposites.

Characterization

A small amount of sample was grind with 100 mg of potassium bromide (KBr) and the resulting fine powder was made as a disk using handmade pellet maker. Subsequently, the Fourier-transform infrared (FTIR) spectra of the disks were carried out using a Bruker Tensor 27 FT-IR spectrophotometer. ¹H and ¹³C NMR spectra of benzoxazine monomers were recorded on a Brucker-300 NMR spectrometer. X-ray powder diffraction patterns were recorded on a Siemens D500 diffractometer using a monochromated CuKα radiation in the angular range from 1 to 10° (2θ); the 2θ scanning speed was 0.6° min⁻¹. Thermogravimetric and differential scanning calorimetric analyses of polybenzoxazine nanocomposite films were carried out with a Exstar 6300 at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The surface overview of the nanocomposites was identified from VEGA3 TESCAN scanning electron microscope (SEM). Samples required for the high resolution transmission electron microscopy (HRTEM) analysis were prepared by dispersions in ethanol and sonicated for 15 minutes. After that, the dispersed solution was dropped over the mesh of 200 copper nets. HRTEM images were captured using TECNAI G₂ S-Twin transmission electron microscope, with an acceleration voltage of 250 kV. The polybenzoxazine thin films were slashed with 11 mm diameter and measured the thickness of a film. Subsequently, the slashed film was placed between the electrodes and capacitance of the sample was measured by applying an external electric field using Broad band Dielectric Spectrometer (BDS), NOVOCONTROL Technologies GmbH & Co. (model Concept 80) at 30 °C and then the dielectric constant and dielectric loss were determined from the measured values of capacitance, diameter and thickness of the samples.

Results and discussion

Scheme 1 shows the molecular structure of OAP-Bz and PDMS-Bz which were confirmed by ¹H, ¹³C NMR and FTIR spectral data. Fig. 1 shows the ¹H and ¹³C NMR spectra of PDMS-Bz, the presence of respective proton and carbon peaks as described in the part of ESI† confirm the successful formation of PDMS-Bz. The formation of PDMS-Bz was further supported by FTIR spectra as shown in Fig. 2. The bands appeared at 1081 cm⁻¹ and 1023 cm⁻¹ represents the presence of Si–O–Si bond and the bands related to N–C–O and C–O–C of benzoxazine ring can be seen at 943 cm⁻¹ and 1220 cm⁻¹, respectively.¹⁸ Moreover, the ESI† exhibits the ¹H NMR spectral data of OAP-Bz which confirms the formation of OAP-Bz. The formation of TES-Bz was confirmed by ¹H, ¹³C (Fig. S2, ESI†) and ²⁹Si (Fig. S3, ESI†) NMR spectra. The ²⁹Si NMR spectrum shows the peak at −51.82 represents the triethoxysilane. Fig. S1 (ESI†) shows the FTIR spectra of OAP-Bz and TES-Bz, the appearance of bands at 941 cm⁻¹ and 1242 cm⁻¹ represents N–C–O and Ar–O–C which confirm the oxazine ring formation. Subsequently, the molecular structure of benzoxazine terminated MCM-41 (BTMS) was confirmed by FTIR spectra (Fig. 2), the bands associated to Si–O–Si, N–C–O and C–O–C are appeared at 1081 cm⁻¹, 941 cm⁻¹ and 1232 cm⁻¹, respectively. Similarly, the hybrid BTMS/PDMS-PBz nanocomposites were confirmed by FTIR spectra and are presented in Fig. 3. The disappearance of bands at 943 cm⁻¹ (N–C–O), 1220 cm⁻¹ (C–O–C) and 1468 cm⁻¹ (tri-substituted benzene ring) and the band appeared at 1422 cm⁻¹ (tetra-substituted benzene ring) indicates the occurrence of ring opening and addition polymerization of benzoxazine and the band at 1093 cm⁻¹ represents the presence of Si–O–Si bond. In addition, the formation of neat PDMS-PBz and hybrid BTMS/PDMS-PBz composites were examined by DSC analysis. Fig. 4a shows an exothermic peaks maximum at 218 °C and 225 °C for PDMS-Bz and BTMS respectively infer the presence of exothermic polymerization reaction. The DSC profile of PDMS-PBz and 7 wt% BTMS/PDMS-PBz shows (Fig. 4b) the absence of an exothermic peak (at 218 °C/225 °C) confirm the successful formation of polybenzoxazine and hybrid polybenzoxazine nanocomposites.¹⁹

Fig. 1 ¹H and ¹³C NMR spectra of PDMS-Bz.

Fig. 2 FTIR spectra of PDMS-Bz and BTMS.

Fig. 3 FTIR spectra of neat PDMS-PBz and BTMS/PDMS-PBz nanocomposites.

Fig. 4 DSC profile of PDMS-Bz, BTMS/PDMS-Bz (a) and PDMS-PBz, BTMS/PDMS-PBz nanocomposites (b).

Fig. 5 shows the XRD pattern of synthesized MCM-41 and BTMS. XRD pattern of MCM-41 and BTMS clearly shows three peaks of (100), (110) and (200) planes are attributed to the hexagonal symmetry of mesoporous structure.²¹ However, the BTMS shows the preserved peak planes of MCM-41 peak with less intensity inferred that the surface modification of MCM-41 with TES-Bz does not affect the framework integrity of the ordered hexagonal structure of mesoporous MCM-41.

Fig. 5 XRD pattern of MCM-41 and BTMS.

The microstructure of neat polybenzoxazine and hybrid polybenzoxazine nanocomposites were investigated by scanning electron microscope (SEM) images and are presented in Fig. 6. The SEM image of neat PDMS-PBz shows (Fig. 6a) smooth surfaces, whereas, that of 7 wt% BTMS/PDMS-PBz exhibit (Fig. 6b) light colored spots with homogeneous distributions due to the presence of mesoporous silica nanoparticles.²² Besides, the 10 wt% BTMS/PDMS-PBz shows (Fig. 6c) an agglomeration of silica nanoparticles on the surface of the film. In addition, the homogeneous distribution of MCM-41 in the polybenzoxazine matrix significantly increased the porosity of that material by introducing pores, but the reverse trend was observed for higher weight percentages of BTMS reinforced polybenzoxazine matrix which might be due to the agglomeration of silica nanoparticles. The internal microstructure of a material can be seen from transmission electron microscope (TEM) images. Fig. 7 shows the HRTEM images of 7 wt% of BTMS reinforced PDMS-PBz which clearly indicates the homogeneous distribution of silica nanoparticles and porous nature of the hybrid polybenzoxazine nanocomposites. However, increasing the porosity of a material could effectively reduce the density of that material which could reduce the polarization as well as the dielectric constant of a material.

Fig. 6 SEM images of PDMS-PBz (a), 7% BTMS/PDMS-PBz (b) and 10% BTMS/PDMS-PBz (c).

Fig. 7 HRTEM images of 7% BTMS/PDMS-PBz (a and b).

To investigate the thermal stability of cured neat PDMS-PBz and BTMS/PDMS-PBz composites, thermogravimetric analysis (TGA) was carried out under nitrogen atmosphere. Fig. 8 shows the weight loss of cured polybenzoxazine with respect to temperature, the absence of any weight loss below 150 °C indicate that the complete removal of water or solvent from the polybenzoxazine films.⁵ The weight loss occurred at above 350 °C, 5% weight loss of neat PDMS-PBz matrix and BTMS/PDMS-PBz composites are presented in Table 1. The major weight loss exhibited in the temperature ranges between 400 °C and 600 °C and gradually losses their weight to yield residual char at 900 °C. Fig. 8 clearly shows the single step degradation of polybenzoxazine systems indicate the well coordination between the incorporated silica and PDMS-PBz system. Moreover the incorporation of mesoporous silica into the polybenzoxazine system significantly enhances their thermal stability. The residual char yield increases as the increasing concentration of mesoporous silica inferred that the thermal stability of the resulting nanocomposites. The improved thermal stability of composite films suggest that the existence of strong chemical interaction between the polybenzoxazine and mesoporous silica. Also, higher char yield of the composite films inferred that the excellent flame retardant efficiency. On the basis of the experimental data, it can be ascertained that the thermal stability of a material has been improved by the introduction of inorganic material such as silica. Besides, higher thermal stability of 7% BTMS reinforced PDMS-PBz nanocomposites could possesses enhanced longevity.

Fig. 8 TGA curve of PDMS-PBz and BTMS/PDMS-PBz nanocomposites.

Table 1 Weight loss, char yield (Y_c), dielectric constant and dielectric loss of neat PDMS-PBz and BTMS/PDMS-PBz nanocomposites

Sample	T5 (°C)	Yc (%) at 900 °C	Dielectric constant (ε′) at 1 MHz	Dielectric loss (ε′′) at 1 MHz
PDMS-PBz	399.4	53.4	3.12 ± 0.01	0.0917 ± 0.001
1% BTMS/PDMS-PBz	387.5	54.9	2.98 ± 0.01	0.0689 ± 0.001
3% BTMS/PDMS-PBz	399.4	55.1	2.64 ± 0.01	0.0568 ± 0.001
5% BTMS/PDMS-PBz	398.4	57.6	2.37 ± 0.01	0.0355 ± 0.001
7% BTMS/PDMS-PBz	411.3	58.2	2.06 ± 0.01	0.0173 ± 0.001
10% BTMS/PDMS-PBz	394.7	63.5	2.41 ± 0.01	0.0401 ± 0.001
50% BTMS/PDMS-PBz	—	—	2.58 ± 0.01	0.0611 ± 0.001

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The hydrophilicity of materials is a disadvantage for interlayer low k dielectrics, due to the absorption of water significantly affects the dielectric behavior of a material by increasing the orientation polarization throughout the matrix.^9,10 Water contact angle analysis infers the affinity between the films and water. Fig. 9 and Table 2 show the contact angle value of water and di-iodomethane (DIM) for polybenzoxazine materials, PDMS-PBz possesses hydrophobic in nature. Despite, the moisture resistance of 7% BTMS/PDMS-PBz hybrid nanocomposites is higher than that of neat PDMS-PBz. The water and DIM contact angle becomes larger when the BTMS weight percentage increases. The water and DIM contact angles of the resultant composites increase from 95.8 to 106.4° and 59.7 to 68.6° respectively, when the contents of BTMS increase from 0 to 7 wt%. In addition, the water and diiodomethane contact angle of 10% BTMS/PDMS-PBz are 104.5° and 67.2°, respectively, which are less than that of 7% BTMS/PDMS-PBz. This might be due to the agglomeration of silica nanoparticles which makes uneven surfaces and that was identified from the SEM images. Further, the surface free energy (SFE) of a sample (Table 2) was determined (Neumann's method)²³ from water and DIM contact angle values. The 7% BTMS/PDMS-PBz possesses a relatively low surface energy (23.6 mJ m⁻²) and it tends to lead to surface enrichment, which results in the enhanced hydrophobicity of the BTMS/PDMS-PBz hybrid nanocomposites.

Fig. 9 Water and DIM contact angle of PDMS-PBz (a & f), 3% BTMS/PDMS-PBz (b & g), 5% BTMS/PDMS-PBz (c & h), 7% BTMS/PDMS-PBz (d & i) and 10% BTMS/PDMS-PBz (e & j) nanocomposites.

Table 2 Water and DIM contact angle and surface free energy values of PDMS-PBz and BTMS/PDMS-PBz nanocomposites

Sample	Contact angle (θ)		Surface free energy (mJ m^−2)
	Water	Diiodomethane
PDMS-PBz	95.8	59.7	28.6
3% BTMS/PDMS-PBz	99.5	62.3	26.9
5% BTMS/PDMS-PBz	102.6	64.7	25.5
7% BTMS/PDMS-PBz	106.4	68.6	23.6
10% BTMS/PDMS-PBz	104.5	67.2	24.45

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The dielectric constant value of conventional silicon dioxide (SiO₂, k = 3.9) is higher than the requirement of ITRS (international technology roadmap for semiconductors, <k = 2.1).²⁴ Thus the reduction of dielectric constant of materials is warranted without affecting their physical properties. Accordingly, mesoporous silica/siloxane based material has been developed with low dielectric constant (2.04) and with high thermal stability. Fig. 10 displays the values of dielectric constant of PDMS-PBz and BTMS/PDMS-PBz composites as the function of frequency. It describes the effect of reinforcement (BTMS) in BTMS/PDMS-PBz composites in terms of dielectric constant. The neat PDMS-PBz inherently exhibit the lower value of dielectric constant which might be due to the less polar Si–CH₃ groups were attached to the siloxane unit and also the hydrophobic nature of PDMS.¹² From Fig. 10 it can be ascertained that the value of dielectric constant of composites decreases as the increasing concentration of BTMS upto 7 wt% of BTMS (k = 2.06) and subsequently the reverse trend was observed at above the particular concentration of BTMS. Besides, the 50% BTMS/PDMS-PBz possesses slightly higher value of dielectric constant than that of 10% BTMS/PDMS-PBz which might be due to the agglomeration of silica nanoparticles. The SEM images of 10 wt% BTMS/PDMS-PBz evidently support that the agglomeration of silica nanoparticles, which exhibit slightly higher value of dielectric constant (2.41) than that of 7 wt% BTMS/PDMS-PBz. In addition, it is submitted that there is a possibility of benzoxazine functionalization both inside and outside the MCM-41. However, the functionalization is more predominant on outer surface of the MCM-41, due to the occluded template which slows the diffusion of the grafting reactant into the pores. However, a small part of the reactants diffuse into the structure and functionalize the inner pore surface to a little extent. Thus the less amount of benzoxazine could be grafted into the inner pore surface of the MCM-41 and the pore size of the benzoxazine functionalized MCM-41 (BTMS) would reduced slightly than that of MCM-41.²⁵ However, the residual pores could contribute to the reduction in the value of dielectric constant. From that it can be described as the introduction of mesoporous silica into PDMS-PBz effectively increases the pores in the resulting nanocomposites which in turn contribute to the reduction of dielectric constant value by reducing the polarization of a material. In addition, the TEM images illustrate the existence of pores in the hybrid BTMS/PDMS-PBz nanocomposites and these pores were filled by vacuum/air which has a dielectric constant of about 1.¹⁴ Thus, the pores exist in the hybrid nanocomposites mainly attributed to the reduction of value of dielectric constant to an appreciable extent. The effect of loss of electrical energy causing the relaxation of dipole is named as dielectric loss. Due to the dielectric loss, the integrated circuitry device consumes more electrical power. Since, the reduction of dielectric loss also significantly reduces the power consumption. Fig. 11 shows the dielectric loss of neat PDMS-PBz matrix and BTMS/PDMS-PBz nanocomposites with respect to frequency. The incorporation of BTMS into the PDMS-PBz composites considerably reduces the value of dielectric loss and the values are listed in Table 1. 7 wt% of BTMS incorporated PDMS-PBz shows the lowest value of dielectric loss of 0.017 which could contribute to the reduction of power consumption of a dielectric material.

Fig. 10 Frequency dependence of dielectric constants of PDMS-PBz and BTMS/PDMS-PBz nanocomposites.

Fig. 11 Frequency dependence of dielectric losses of PDMS-PBz and BTMS/PDMS-PBz nanocomposites.

Conclusion

A class of hybrid BTMS/PDMS polybenzoxazine nanocomposite has been developed based on MCM-41/PDMS with a view to reduce the value of dielectric constant with enhanced thermal stability. As expected, the reinforcement BTMS significantly contributes to the reduction of value of dielectric constant of the resulting nanocomposites through the reduction of polarization of the matrix by the introduction of pores. The pores exist in the hybrid nanocomposites were confirmed from the TEM images. The experimental values of dielectric constant inferred that the increasing concentration of BTMS considerably reduces the value of dielectric constant up to 7 wt% of BTMS (2.06) and the reverse trend was observed beyond 7 wt% of BTMS. In addition to low k values, the higher degradation temperature and higher char yield of the resulting nanocomposites indicate that these materials possess an excellent thermal stability and flame retardant properties.

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Footnote

† Electronic supplementary information (ESI) available: Data's of FTIR, ¹H, ¹³C and ²⁹Si NMR spectra of benzoxazine monomer are provided. See DOI: 10.1039/c5ra02842f

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