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Structural Analysis and Dielectric Property of Novel Conjugated Polycyanurates.

Structural characterizations reveal that the plasma polymerization of three cyanate ester monomers proceeded mainly via the opening of [pi]-bonds of the cyanate ester functional groups and transforming them to a large [pi]-conjugated structure, which is noticeably different from the conventional thermal or catalytic cyclo-trimerization reaction of cyanate ester monomers. The dielectric properties of the resulting plasma thin films were evaluated and results show that the dielectric constant of the three polycyanurate thin films decreased with increasing frequency, while in contrast, the dielectric loss factor increased with the increasing frequency. POLYM. ENG. SCI., 54:812-817, 2014. [c] 2013 Society of Plastics Engineers

INTRODUCTION

The advances in microelectronics technology and semiconductor lithography have resulted in a large scale integration of circuit board functions with microelectronic components. Advanced circuit boards are now multilayer boards with more stringent demands lor high-speed transmission, lovi heal losses, higher circuit density, minimal dimensions, and reduced power consumption (1). For this reason, many technological advances have been made in recent years to meel the ever increasing requirements tor the electrical performance of on-chip wiring (2), one of them is the exploration of new dielectric materials as a substitution of [SiO.sub.2] lo be ihe dielectric between layers and lines (3-5). Among the broad variety of accessible alternatives, polymers have been some of the most widely studied materials due to their outstanding and promising mechanical and physicochemical properties. Advantage can be taken of the unique combination of excellent properties of polymers such as chemical inertness, good mechanical properties, thermal stability, electrical properties, safely, weather resistance, formability. and easy shaping. Thus, one of the exciting and promising developments in material science today is the design and synthesis of novel low-dielectric-constant polymer materials, which are found to have potential applications in the field of ultra large-scale integration, capacitors, and other electronic circuits as insulating and/or dielectric materials (6), A number of studies have been conducted in this area. The candidate materials include polynitriles (7, 8). polyimides (9, 10). benzocyelobutene resins (11. 12). poly (binaphlhylene ether) (13, 14). polyquinolines (15), poly-norbomenes (16), polylvinylidenc fluoride) (17), and organic-inorganic hybrid polymers (18, 19).

Cyanate ester resins have an attractive range of dielectric properties when considered againsi other candidate materials. The lower polarity associated with the cyanate ester resin's cyanurate linkage offers a way to maintain these properties associated wilh a ihertnosct while depressing the dielectric constant lo the 2.5-3.0 range. With the current demand for the higher performance resulting from substitution of a lower dielectric resin, cyanate esters offer a good cost-performance ratio compared with other engineering plastics (20). Il is well known lhal ihe conventional thermal or catalytic polymerization of cyanate ester monomers was performed through polycv-clolrimerizalion lo form polycyanurate containing tria/ine ring structure. The catalysts are usually Lewis acids and transition metal complexes or amines. Generally they are cured wilh a transition metal calalyst or chelate catalyst in the presence of a hydrogen donor. Although the synthesis technology based on cyanate ester has taken large strides, the very basic information on its polymerization mechanism remain disputed (21).

Until now, researchers have proposed several possible polymerization mechanisms. The thennal cure reaction of bisphenol A dicyanate studied by Gupta and Macosko (22) using a mono functional model compound, namely 2-(4-cyanalophenyl)-2-phenyl propane suggested trimeri-/ation to be the major reaction. Shimp J 231 reported that in the presence of metal ions, the cyanate groups are believed to be coordinated through the metal ion to allow ring closure through a step growth or ionic path. Grenier-Loustalot et al. investigated the molten state reactivity of three different cyanate ester monomers and showed trimerizalion as the major phenomenon, but not the exclusive reaction (24). The same team (25. 26) also investigated the mechanism of polymerization of mono and dicyanates by high perfonnance liquid ehromatogra-phy (HPLC) and spectroscopic methods. Using HPLC/ UV. secondary products like phenolic derivatives of the cyclic trimer and pentamer and carbamates could be detected. The authors proposed formation of dimer. which could inhibit the polymerization and delay gelation. Mass spectral evidence has been reported by Fang and Houlihan for the existence of hydraled cyanate dimer. believed to be the actual calalyst for the uncata-lyzed polymerization of cyanates (27). This hydraled dimer mediated mechanism has gained more suppon w ith evidence for ils presence being presented by researchers. Simon and Gillham (28) proposed a clotri-merization reaction and kinetic model of dicyanate ester, which has considered all possible reaction paths and intennediales. Brownhill and coworkers (29) reported the [TiCI.sub.4]-catalyzed polymerization of cyanate esters and proposed a four-step reaction process model. The results show that the transition metal coordinates with the nitrogen, enhancing the electrophilidty of the nitrite group for addition of another cyanale group on lo it. George ami coworker (30) reported the phoiocaialyzcd polymerization of cyanale ester in the presence of iricarbom I cyclopcntadienyl manganese, li was found that the reaction was first order with respect to concentration of both cyanate and catalyst. The proposed mechanism involves a photosubsiitution of the carbonyl group in ihe catalyst by the cyanate group lo form a new complex, which produces an active catalyst on thermal aelivation. The activation energy decreased with irradiation time.

In the present article, the technique of plasma polymerization was used in the preparation of poly cyan urates with the aim lo develop a novel conjugated polymer thin film. Apart from ihe preparation of polycyanurate dielectric thin films with a uniform and defect-free surface, another objective of this work is io investigate whether and how the cyanate ester monomers undergo plasma polymerization.

EXPERIMENTAL DETAILS

Materials

The monomers. 4-methoxyphenol cyanate ester (MPCE), 4-cumylphenol cyanate esier (CPCE). and 4-phenylphcnol cyanate ester (PPCE), were obtained from the Aldrich Chemical Company. The substrates used were heavily doped, p-type. (100) silicon wafers, quartz glass and freshly pressed infrared-grade potassium bromide (KBr). The argon used in the plasma system was highly-purified grade.

Plasma Deposition of Cyanate Ester Monomers

Plasma polymerization of MPCE. CPCE. and CPCE was carried out using a radiofrcquency capacitive coupled glow discharge system. Our plasma reaction room is made up of two components, name I) the main chamber and the auxiliary chamber. Before the reaction actually look place, the monomer is placed inside an ampoule with eight identical holes arranged neatly around it, and then the whole set was placed into the auxiliary chamber. The auxiliary chamber was then evacuated and purged with high purity argon for three limes. The substrates were centered on ihe bottom electrode of the main chamber, after evacuation and purging wilh high purity argon three times, a cleaning/etching operation in argon plasma was performed subsequently al a discharge power of SOW for about 2 min. After this process, we adjust the vacuum pressure in both the main and auxiliary chambers, and then transmit the monomer from the auxiliary chamber to the main chamber. Adjustments were done to the RF system in the main chamber to achieve the preset powers, and a glow discharge was igniied for fixed time laps. After the plasma was extinguished. Ihe chamber was evacuated for 3 min before ihe high purity argon was induced until the pressure of reactor exceeded 267 Pa. After the pressurizing process, the reactor was brought back to atmospheric pressure with air. The plasma polymerization conditions and physical properties of these plasma polycyanurate thin films arc shown in Table 1.

TABLE 1. Plasma polymerization conditions and
Dim thickness.
                              Discharge    Film
Plasma              Pressure    power    thickness
Polymers  Monomers    (Pa)       (W)       (nm)
PPMPCE      MPCE    6.0-7.3       3         402
PPPPCE      PPCE    6.7-K.O       10          297
PPCPCE      CPCE    6.7-8.0       10          472


Characterization

An S-4800 scanning electron microscope (SEM) was used to examine the surface structure of the polymer thin films obtained. The electron spin resonance (ESRI spectra were measured on a Bruker ER200D-SRC electron spin resonance spectrometer. The FTIR spectra were measured on a Perkin-Elmcr System 2(KK) FTIR spectrometer. An HP 4284 A semiconductor parameter analyzer available from Hewlett-Packard Company was used to measure the capacitance and dielectric loss of the plasma polycyanurate thin films in the frequency range I(X) kHz tol MHz. The dielectric constant ([[epsilon].sub.r]) was calculated from the known values of eapacilance(C), film thickness (d), the area of aluminum dor. (A), and the permittivity of free space ([[epsilon].sub.0]) using ihe following relation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

RESULTS AND DISCUSSION

Morphology Charm terization

The morphologies of PPMPCE. PPPPCE. and PPCPCE ihin films deposited on silicon wafers were observed using SEM. The SEM micrographs in Fig. 1 show that the plasma polycyanurate thin films were all smooth, homogeneous, and pinhole-free. suitable for the measurement of dielectric properties. Funhennore. the obtained polycyanurate Ihin films show a strong adhesion to the substrale. which may be ascribed to the fomiation of highly reactive free radicals on the substrate surface by electron bombardment and radiation of various wavelengths produced during the plasma polymerization. Such highly reactive free radicals can lead lo bond formation between the substrale and growing plasma polycyanurate chains. In order to prove the above inference, the ESR measurements were carried out for the PPMPCE and PPPPCE using a Bruker ER200D-SRC Electron Spin Resonance Spectrometer. As shown in Fig. 2, it is clear that both plasma polycyanurate samples exhibit a strong ESR signal, indicating that there is a high concentration of radicals in the plasma polycyanurate samples.

Structural Characterization ami Reaction Mechanism

Figure 3 illustrates the FTIR spectra of ihe three cyanate ester monomers. Ii is clear from Fig. 3 that two or three characteristic absorption bands of the cyanate esier functional group (O-C[equivalent to]N) were observed between 2235 and 2280 [cm.sub.-1) in the infrared spectrum of monomer MPCE, PPCE, and CPCE. The plasma polymerization reaction of cyanale ester monomers can be followed by-monitoring the corresponding changes in ihe absorbance bands of the O-C[equivalent to]N functional group. The plasma-polymerized cyanate ester films were deposited on KBr pellets directly in the process of plasma polymerization, and Ihe FT IR spectra were recorded. Figure 4 shows the typical FTIR spectra of ihe plasma polycyanurate ihin films prepared from monomer MPCE. PPCE. and CPCE. respectively. The corresponding FTIR hand assignments of three plasma polymers are summarized in Table 2.

TABLE 2. FTIR bacid assignments of the
plasma polycyanuraies.
Plasma    FTIR hands
polymers  ([cm.sup.-1))  Assignments
PPMPCE    1677             Conjugated C=N
                           stretching
                           vibration
          1607, 1504,      Benzene ring
          1459             backbone
                           stretching
                           vibrations
          3074, 3121       Aromatic
                           C[sbond-H
                           stretching
                           vibration
          1206, 1028, X32  p-Substituted
                           benzene ring
          2838. 2948       [CH.sub.3]
                           stretching
                           vibration
PPPPCE    1674             Conjugated C=N
                           stretching
                           vibration
          1655, 1544,      Benzene ring
          1459             backbone
                           stretching
                           vibrations
          3014, 3038       Aromatic C-H
                           stretching
                           vibration
          1260, 1055, 802  p-Substituted
                           benzene ring
          747, 694         Mono-substituted
                           benzene ring
          2832, 2965       [CH.sub.3]
                           stretching
                           vibration
PPCPCE    1660             Conjugated C=N
                           stretching
                           vibration
          1608, 1505,      Benzene ring
          1445             backbone
                           stretching
                           vibrations
          3023, 3059       Aromatic C-H
                           stretching
                           vibration
          1273, 1069, 835  p-Substituted
                           benzene ring
          765, 703         Mono-suhstituted
                           benzene ring
          2878, 2963       [CH.sub.3]
                           stretching
                           vibration


It can be seen lhal the intensity of absorption bands corresponding io ihe O-C[equivalent to]N functional group has diminished significantly in three plasma polycyanurate thin films. Al the same lime, there appeared a broader and considerably stronger absorption band at 1677 [cm.sub.-1) to PPMPCE, 1674 [cm.sub.-1) for PPPPCE, and 1660 [cm.sup.-1) for PPCPCE, respectively, attributed lo the conjugated C=N stretching vibration, which indicated that extensively conjugated C=N double bonds were fonned during Ihe plasma polymerization of three cyanale ester monomers. Since the characteristic absorption bands at 1360 and 1570 [cm.sup.-1) corresponding to ihe triazine ring structure (31 1 were nol observed in three plasma polycyanurate thin films, we can derive thai the plasma polymerization of MPCE, PPCE. and CPCE is quite different from the conventional thennal or catalytic polymerization reaction of cyanale ester monomers, in which the cyanate ester monomer undergoes thennal or catalytic cyelolrime-rizaiion lo fonn polycyanurate containing triazine ring structure. Furthermore, ihe corresponding characteristic absorption of the monomers resulting from aromatic ring are evident for the plasma polycyanurates, although broadening effects characteristic to the plasma polycyanurates were observed. Take the PPCPCE thin film as an example, the absorption bands al 3023 and 3059 [cm.sup.-1) were assigned to the aromatic C- H stretching vibration. The absorption bands al 1273 and 1069 [cm.sup.-1) are due to the in-plane bending vibration of the benzene ring C- H bonds, and those al 835. 765. and 703 [cm.sub.-1) are due in their oul-of-planc bending vibration. The absorption bands at 1608, 1505, and 1445[cm.sup.-1) are atlribuied to the benzene ring backbone stretching vibrations. Moreover, il is also clear from Fig. 4 that the characteristic absorptions al 2878 and 2963 [cm.sup.-1) for the [CH.sub.3] stretching vibration of the monomer CPCE have also been preserved to a large extent in ihe PPCPCE thin film, indicating greater retention of ihe starting monomer structure including aromatic rings and methyl groups in the PPCPCE deposited film. Similar results were also observed in the PPMPCE and PPPPCE ihin films (see Fig. 4). Thus, the FTIR results suggest lhal the plasma polymerization of monomer MPCE. PPCE. and CPCE proceeded mainly via the opening of [pi]-bonds of the cyanale ester functional groups which are further formed into a large [pi]-conjugaied system, and nol through ihe conventional method ol forming polycyanurate through polycyelotrimerizaiion. The different reaction mechanisms of cyanale ester monomers for plasma polymerization and ihermal or catalytic cyclo-irimerization are shown in Fig. 5.

Dielectric Properties

The dielectric properties of three plasma polycyanurale thin films were measured by the semiconductor parameter analyzer in the frequency range 1(X) kHz tol MHz. The variation of dielectric constant with frequency is shown in Fig. 6. It can be seen thai in the whole frequency range there is an increase in dielectric constant with a decrease in frequency. The observed frequency dependence of the dielectric constant might be attributed to the combination of iwo factors. First, ii has been reported (32) thai there are some charges present in dielectric materials which can migrate some distance through the dielectric as an electric field is applied. When such carriers are impeded in their motion, il results in space charges and macroscopic field distortions. As the charge carriers migrate under ihe influence of an electric field, il is possible lhal they gel blocked ? the electrode dielectric interface, which leads to the inlerfacial polarization (33J. Such distortion causes an increase in capacitance of the plasma polycyanurale thin films at low frequencies (34). Second, the plasma deposited polycyanurale ihin films have a high concenlralion of radicals, and the rotation of radicals in polycyanurale thin films may result in an increase in the polarization in the low frequency region (321. And thus, the corresponding dielectric constant of three plasma polycyanurate ihin films will increase wilh a decrease in frequency. Figure 7 shows ihe variation of the dielectric loss factor (tan [delta]) as a function of frequency for three polycyanurate Ihin films. It can be seen that the dielectric loss factor increases with increasing the frequency. There is no relaxation effect observed in the variation of tan [delta] in this frequency range.

CONCLUSIONS

Plasma polymerization of three cyanale esler monomers were investigated from the viewpoint of chemical structure and composition, and the iransformation of the cyanale ester functional group (O-C[equivalent to]U)during the plasma polymerization was discussed. The structure and morphology of Ihe obtained plasma polycyanurate ihin films were analyzed by FTIR and SEM. We can derive the following conclusions from the results obtained in this study.

The plasma polymerization of three cyanale esler monomers proceeded mainly via the opening of [pi]-bonds of the cyanate ester functional groups which are further being formed into a large [pi]-conjugated system, this unique process is noticeably different from the conventional thennal or catalytic cycloirimerizaiion of cyanale esler monomers.

The operation of plasma polymerization under mild plasma polymerization condilions such as lower discharge power is favorable for ihe preparation of plasma thin films with a high reteniion ol" the aromatic ring structure of the starting monomer.

The plasma-prepared polycyanurale ihin films are smooth, homogeneous, and pinhole-free. suitable for the measurement of dielectric properties. The dielectric measurement shows thai the [[epsilon].sub.r] value of the three polycyanurate thin films decreased with increasing the frequency, and the dielectric loss factor was found to increase with increasing the frequency.

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Correspondent c to:Xmng-Yan Zhao: e-mail: /Jiaoxyofx[R]l26.eom Contract grant sponsor: Natural Science Foundation of Hebei Province; contract grant number: 20I220S026; contract grant sponsor: Key Project of Education Department of Hebei Province. P. R. China: eontrac! grant number: ZD20I0KIX. DOl IO.I002/pen.2J62X Published online in Wiley Online Library twileyonlinelihrary.com). [c]2013 Society of Plastics Engineers

Xiong-Yan Zhao, (1), (2) Ming-Zhu Wang, (3) Jiao-Qing Ji, (1) Tian-Heng Wang, (1) Fei Yang, (1) Juan-Min Du (1)

(1) College of Material Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, People's Republic of China

(2) Hebei Key Laboratory of Material Near-Net Forming Technology, Hebei University of Science and Technology, Shijiazhuang 050018, People's Republic of China

(3) Hebei Province Analysis and Testing Research Centre, Hebei University of Science and Technology, Shijiazhuang 050018, People's Republic of China
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Author:Zhao, Xiong-Yan; Wang, Ming-Zhu; Ji, Jiao-Qing; Wang, Tian-Heng; Yang, Fei; Du, Juan-Min
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Apr 1, 2014
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