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Preparation, characterization and dielectric properties of 4,4-diphenylmethane diisocyanate (MDI) based cross-linked polyimide films
European Polymer Journal 42 (2006) 1370–1377
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Preparation, characterization and dielectric properties of 4,4-diphenylmethane diisocyanate (MDI) based cross-linked polyimide films Hu¨seyin Deligo¨z a
a,*
¨ zgu¨mu¨s a, Saffettin Yildirim , Tuncer Yalcinyuva a, Saadet O
b
Istanbul University, Engineering Faculty, Chemical Engineering Department, Chemical Technologies Group, 34320 Avcılar, Istanbul, Turkey b Istanbul University, Faculty of Science, Physics Department, 34459 Vezneciler, Istanbul, Turkey Received 10 August 2005; received in revised form 25 November 2005; accepted 6 December 2005 Available online 25 January 2006
Abstract Four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared by the reaction of different types of conventional poly(amic acid) intermediates with MDI as a cross-linking agent. Subsequently, they were thermally imidized in order to obtain corresponding cross-linked polyimide structure. The results of FTIR-ATR showed that MDI can effectively react with carboxylic acid groups of PAA to form cross-linked polyimide films. TGA, FTIR-ATR and SEM analyses were carried out for characterization of cross-linked polyimide (CPI) films. Moreover, the electrical properties such as dielectric breakdown strength, dielectric constant, I–V characteristics and loss factor of MDI based cross-linked polyimides have been checked. In addition, some physical properties such as water uptake, adhesion, hardness and solubility properties of the films were investigated. The results showed that all CPI films have good insulating properties such as high dielectric breakdown voltage, low loss factor (tan d), leakage density and excellent physical properties. 2006 Elsevier Ltd. All rights reserved. Keywords: Cross-linking; Dielectric properties; Polyimide; Thin film
1. Introduction Polyimides (PI) have attracted great attention in the last a few decades due to their high thermal stability, good solvent resistance, excellent dielectric properties, low dielectric constant, high dielectric * Corresponding author. Tel.: +90 212 591 2479/212 473 7070x17758; fax: +90 212 473 7180. E-mail address: [email protected] (H. Deligo¨z).
strength [1–3]. They are widely used in membrane industry as gas separation membranes [2–10] and in microelectronic industry as an interlayer dielectric (ILD) material due to a novel combination of these outstanding properties [1,11,12]. Cross-linked polyimide (CPI) films were especially prepared for improving gas separation and transport properties [2–10]. Gao et al. have reported thermally cross-linkable PIs containing the 1,2-diphenylcyclopropane unit in the backbone [3]. Lu Shao et al. has
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H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
reported that polyimide was modified by a linear aliphatic diamine (ethylene diamine) to enhance transport properties of the films and it was reported that CO2/CH4 separation is especially suitable for this membrane [4]. Thermal treatment and chemical cross-linking methods are used to enhance the physical properties of polyimide membranes against plasticization and impurity hydrocarbon attack [13,14]. The major drawback of heat treatment at an elevated temperature is to deteriorate the subtle structures of asymmetric membrane and their gas permeation properties. As a result, cross-linking method performed at a low temperature is necessary for the modification of polyimide films [15]. PIs are generally chosen for high temperature applications and insulators for electronic parts [1,11,12] due to their good thermal stability, high planarization, low dielectric constant, leakage current density, high dielectric breakdown strength and good processibility. There are several methods and reports for lowering dielectric constants of PIs [16–19]. As an alternative approach, Yamazaki et al. [20] has offered that CPI thin films may be interesting because the gel structure is expected to make porous polyimides which can be used as low dielectric material. While there are lots of studies on the gas separation properties of CPI films, only few pioneering studies have been published on general and electrical properties of CPI which cross-linked at higher temperatures [12,21,22]. Dunson has studied synthesis and characterization of cross-linkable groups containing polyimide oligomers as possible candidates for thin film interlayer dielectrics in electronics packaging [12]. In the other study, electrical properties of cross-linked poly(ether-imide)(PEI) were studied and reported by Xu et al. [21]. It was found that the prepared samples have moderate dielectric constant and dielectric breakdown strength. On the other hand, some reports were published on room temperature preparation of polyimide and PAA gels [23,24], although no electrical data were given in these studies. To the best of our knowledge, no study has been reported on the electrical properties of the CPI films which was cross-linked at low temperatures. In this study, four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared. Firstly, previously prepared four different types of conventional PAA intermediates were reacted with MDI as a crosslinking agent. Subsequently, they were thermally
1371
imidized in order to obtain corresponding crosslinked polyimide structures. The effects of cross-linking modification on the thermal stability, solubility and physical properties (hardness and adhesion) of the CPI films were studied. Furthermore, preliminary studies on the electrical properties such as dielectric constant and dielectric breakdown strength and I–V characteristics of MDI based CPI films were investigated and presented comparatively. Using this method, we recently reported Ac and Dc electrical properties of a cross-linked and a conventional PI films in comparison [25]. 2. Experimental 2.1. Materials Diphenylmethane diisocyanate (MDI), diaminodiphenyl ether (DDE, >99 purity), diaminodiphenyl sulfone (DDS, >99 purity), pyromellitic dianhydride (PMDA, >97 purity) and benzophenonetetracarboxylic dianhydride (BTDA, >97 purity) were supplied from Merck Company, Germany and used as received without further purification. N-methyl pyrolidone (NMP) was purified by distillation under reduced pressure and NMP was stored over molec˚. ular sieve 5 A 2.2. Measurements (instruments) Thermal analyses were carried out with a Linseis thermal analyzer with a heating rate of 5 C/min in air. The sample weights were nearly 10 mg in all experiments. Infrared spectra of the samples were recorded by Perkin Elmer Spectrum 2000 model with ATR (Attenuated total reflectance) unit in the range of 400–4000 cm1. Dielectric constant and loss factor of the CPI films were measured by the HP 4192 A LCR meter Impedance Analyzer at various frequencies and room temperature. Prior to coating, glass substrates were cleaned with isopropyl alcohol, acetone and water to obtain very clean surface. After coating two surfaces of the films with gold, the contacts were done by using indium oxide. The dielectric constant was calculated from the measured capacitance data as follows: e¼
Cd ; e0 A
ð1Þ
where C is the capacitance, e is the dielectric constant, e0 is the permittivity of the free space (8.85 · 1012 MKS unit), d and A are the film
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Reaction conditions were given in Table 1. Chemical structure of the PAA is depicted in Scheme 1.
Table 1 Formulations for CPAA gel preparations Film
Diamine (g)
Dianhydride (g)
[NH2]/ Solvent Gelation [COO] (%) time
CPI-1 CPI-2 CPI-3 CPI-4
DDE (2.40) DDS (1.49) DDE (2.62) DDS (1.49)
PMDA (2.62) PMDA (1.29) BTDA (3.89) BTDA (1.93)
1 1 1 1
90.2 90.3 88.8 90.0
3 days 10 min 5 days 20 min
thickness and electrode area, respectively. Dielectric strengths of the films were measured with Electrotechn. Laboratorium D-7015 ‘‘Insulation Breakdown Tester’’ UH 270, with resolutions of 50 V for 2.5 kV and 100 V for 5 kV. Current–voltage (I–V) characteristics of the films were performed by Keithley Instruments, 410 A picoampermeter. Thicknesses of the films were measured with Mitutoyo Model, 0–25 lm with 0.001 mm (1 lm) resolution. Water uptake properties of the CPI films were investigated. For this purpose, the films were dried at 80 C for 10 h in a vacuum oven and weighted. Subsequently, they were kept in a box with 55% relative humidity (RH) for 1 week and re-weighted. The water uptake values of the films were calculated from the weight changes of each film. SEM analyses were carried out by using Jeol JXA 840 A model apparatus. The solubility tests were carried out by immersing the films into various solvents for one week. The hardness of the prepared films were determined by Ko¨nig Pandulum [DIN 53 157] and given in Ko¨nig sec. The adhesion test was determined as reported in ASTM 359-76 standard. The thicknesses of the self-standing films CPI-1, CPI-2, CPI-3 and CPI-4 were measured by means of a stylus profilometer and found to be 8, 10, 8 and 7 lm, respectively. All donations are shown in Table 1. 2.3. Reactions 2.3.1. Preparation of poly(amic acid) (PAA) Into a 100 mL three necked, round bottomed flask equipped with a thermometer, nitrogen gas inlet and outlet and a magnetic stirrer, 0.012 mol of diamine monomer and NMP were placed to obtain a 10% (wt/wt) solution. The dianhydride monomer (0.012 mol) was added in small quantities over a period of 10 min at 30 C. The reaction took place in 3 h at this temperature under N2 atmosphere. Then, the final mixture was kept in a freezer in order to prevent side reactions until further use.
2.3.2. Preparation of cross-linked poly(amid acid) (CPAA) All CPAAs were prepared by dropwise addition of the isocyanate solution in NMP into the PAA solution dissolved in NMP and the mixture was kept under nitrogen atmosphere at 0 C. All reactions were carried out without a catalyst. The reaction took place in 5 min. Cross-linking has occurred in different times after the reaction has stopped. The gelation times were given in Table 1 and determined as the time of the magnetic stirring bar becoming immobile in the PI solution due to the dramatic increase in the viscosity. As an example, chemical structure of CPAA-1 derived from PMDA-DDE is shown in Scheme 1(a). 2.3.3. Preparation of cross-linked polyimide (CPI) films from CPAA solutions Before cross-linking of PAAs, solutions were immediately applied to a cleaned glass substrate by a doctor blade, then the prepared films were cured at 100 C for 1 h, 200 C for 1 h and 300 C for 1 h in order to obtain corresponding crosslinked polyimide films. Then the films were removed from the glass plate by immersing into hot water. The chemical structure of CPI-1 is depicted in Scheme 1(b). 3. Results and discussion In this study, the cross-linked polyimide films have been prepared in three stages. At the first stage, the reaction of a diamino compound with a dianhyride compound has been carried out in order to obtain PAA intermediate. At the second stage, PAA was reacted with equivalent amount (to the carboxylic acid groups of PAA) of MDI as a cross-linker to prepare PAA gels. At the third stage, reaction solution was urgently applied on a well-cleaned glass substrate. Finally cross-linked polyimide films have been prepared by thermal imidization of corresponding PAA gels. As one can see from Table 1, the gelation times vary between 10 min and 5 days. This may be explained by the reactivity and nature of prepared PAAs. In particular, DDS-derived PAAs were cross-linked faster than DDE-derived one. In all experiments, amounts of isocyanate and carboxylic acid groups were taken equivalent.
H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
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O
O
O
O O
H 2N
+
NH 2
O
O
Diaminodiphenylether (DDE)
Pyromellitic dianhydride (PMDA)
30 ºC and 3h.
O
O O
HN
OH NH
OH O
O
Poly(amic acid)-PAA 1 R=
CH 2 0 ºC and 5 min.
Diphenylmethane diisocyanate (MDI) O
O O
HN
NH
OH
O NH
n
O
R NH
O
OH
O
HN
HN O
O
O
1 (a)
n
Thermal imidization at 100 ºC 1h, 200 ºC 1h, 300ºC 1h
O
O
N
N NH
O
O
n
R NH
O
N
N O
O
O
n
1 (b) Scheme 1. (a) Chemical structure of CPAA-1 and (b) chemical structure of CPI-1.
On the other hand, in this study, it was aimed to observe the effect of the pore formation that may
occur during the cross-linking reaction on the electrical properties of the polyimide thin films. The
3.2. Thermal properties of CPI films prepared from PAA
1369
1775
817
1776
1645
1360
1722
CPI-2Film (PMDA+DDS+MDI)
1717
1370
1777
3353
718
819
CPI-3 Film (BTDA+DDE+MDI)
825
CPI-4 Film
826
1370
719
1716 1645
(BTDA+DDS+MDI)
4000
3000
2000
1500
1000
650
Fig. 1. FTIR spectra of MDI based CPI films.
Table 2 Thermal properties of MDI based CPI films Film
T10% (C)a
T50% (C)b
Char yieldc
CPI-1 CPI-2 CPI-3 CPI-4
420 425 365 385
498 520 500 505
0.9 7.2 6.4 10.6
a b c
The thermal properties of the CPI films were performed by TGA under air atmosphere with a heating rate of 5 C/min. It was observed that the thermal decomposition of cross-linked polyimide films starts in the range of 300–400 C. All thermal and oxidative degradation patterns exhibit the same two-step degradation behavior. Firstly, amide groups in the polymer backbone start to decompose and this is followed by the decomposition of the imide structure. As compared to PMDA-DDE derived conventional polyimide film (commercially known as Kapton), thermal stability of the CPIs have exhibited a slight decline. As seen from Table 2 and Fig. 2, CPI-1 (PMDA-DDE-MDI) has slightly better thermal and oxidative stability, among the prepared films.
3353
Transmittance
3.1. FTIR spectra of cross-linked polyimide films When 4,4-diphenylmethane diisocyanate (MDI) is added as cross-linking agent, a peak attributed to the presence of isocyanate groups has been observed in the range of 2250–2270 cm1. After the cross-linking reaction and thermal imidization of prepared poly(amic acid), it was not observed an absorption band at 2250–2270 cm1 corresponding to the isocyanate group. Therefore, the result confirmed that isocyanate compound has entirely reacted with carboxylic groups of PAA. Furthermore, new peaks at 1777 and 1716 cm1 (C–O symmetric stretching), 1370 cm1 (C–N stretching), 826 and 719 cm1(C–O asymmetric stretching) corresponding imide structures were observed. On the other hand, the peaks belong to amide structure that are formed by the cross-linking reaction were detected as a strong peak at 3353 cm1 (N–H primary and secondary amide stretching) and an increase in the peak intensity at 1645 cm1 (C@O amide I) and 1586 cm1 was observed. The FTIR spectra of CPI films are depicted in Fig. 1.
1720
722
CPI-1 Film (PMDA+DDE+MDI)
1774
3348
pores that may form on the surface of the film can improve the insulating properties such as dielectic constant and dielectric breakdown voltage. However, from the SEM figures, it was observed no pore formation on the surface of the films. Consequently, we believe that this result may be related with the thermal imidization procedure of the films.
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3352
1374
The temperature where 10% weight loss has occurred. The temperature where 50% weight loss has occurred. Weight of residue polymer at 750 C.
3.3. Scanning electron microscope (SEM) SEM analyses of the CPI films were carried out in order to characterize surface properties. According to the SEM results, formed films are very dense, smooth and have no pores, cracking or delamination. The figures were not included since all SEM figures were completely black. 3.4. Electrical properties of the cross-linked PI films 3.4.1. Dielectric constants of the cross-linked PI films Dielectric constants (e1) of the CPI films were calculated from the measurements of the capacity at
H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
100
Weight %
80 60 CPI-1 (PMDA+DDE+MDI) CPI-2 (PMDA+DDS+MDI) CPI-3 (BTDA+DDE+MDI) CPI-4 (BTDA+DDS+MDI)
40 20 0 0
100
200
300
400
500
600
700
800
Temperature ºC Fig. 2. TGA curves of MDI based CPI films.
Table 3 Dielectric properties of MDI based CPI films Film
Dielectric constant
CPI-1 CPI-2 CPI-3 CPI-4
10 kHz
100 kHz
1 MHz
10 MHz
4.4 8.9 6.3 2.4
4.3 8.7 6.2 2.3
4.1 8.4 6.1 2.2
4.2 9.2 6.2 2.2
Dielectric breakdown strength (kV/mm)
Water uptake (%)
117.8 80.6 80.2 80.4
1.53 1.19 1.57 1.18
various frequencies (1 kHz to 10 MHz) at room temperature. The dielectric constant values versus frequencies for CPI films are shown in Table 3 and Fig. 3. One can see from Fig. 3 that e1 decreases with increasing frequency when temperature is stayed constant. Previous studies indicated that
12
Dielectric Constant
10 8
CPI-1 (PMDA+DDE+MDI) CPI-2 (PMDA+DDS+MDI) CPI-4 (BTDA+DDS+MDI)
6 4 2 0 0
5000
10000
15000
Frequency (KHz) Fig. 3. Dielectric constant (e) versus frequency plot of MDI based CPI films at room temperature.
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dielectric constants of PIs decrease gradually with increasing frequency [25,26]. This behavior can be attributed to the frequency dependence of the polarization mechanism. From the results, CPI-4 has the lowest dielectric constant value (2.2 at 1 MHz). This value is lower than the dielectric constant of the conventional polyimide film. We explained this situation by cross-linking caused a network structure, which restricts orientation and relaxation of dipoles, in our recent publication [25]. Considering dielectric constant, dielectric breakdown strength and water uptake values of CPI-4, it can be said that this sample satisfies all required physical properties for being a good ILD in microelectronic industry [11]. On the other hand, we expected the same results for all CPI films, but surprisingly CPI-2 and CPI-3 have shown higher dielectric constants (8.4 and 6.1 at 1 MHz, respectively). The typical value for the dielectric loss (tan d) of polyimides is nearly 0.002 [27,28]. Low values of tan d are indicative of minimal conversion of electrical energy to heat and, thus, signify minimal overall power loss in the dielectric material. It is advantageous to have low values for both tan d and dielectric constant because electrical signals will lose less of their intensity in the dielectric medium. Dielectric loss (tan d) values of all prepared MDI based CPI films are in the range of 0.01–0.015. 3.4.2. Dielectric breakdown strength of the CPI films Polyimides are typically chosen to fit in these high temperature applications as coating material and insulator for electronic parts due to their high dielectric strength and low dielectric constant [1,2]. The dielectric strength is an important parameter for selecting an appropriate electrical insulation material to avoid short circuiting, especially at high temperature operations. The dielectric strength of a material measures its ability to withstand high voltages without breakdown or the passage of considerable amounts of current. It is determined as the minimum value in the applied voltage (Vb) at which breakdown occurs. In particular, dielectric strength depends on temperature, specimen thickness and humidity [12,29]. Dielectric breakdown strengths of the MDI based CPI films are in the range of 80–118 kV/mm and related data are given in Table 3. From the results, it can be said that the value of dielectric breakdown strengths of all CPIs are quite appropriate for many
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microelectronic applications. For comparison, the electrical porcelains and sol–gel ceramics typically have a dielectric strength of 25 and 24–22 kV/mm, respectively. 3.4.3. I–V characteristics of MDI based cross-linked PI films I–V characteristics of MDI based cross-linked PI films were measured at different voltage values and depicted in Fig. 4. Below 50 V, currents are in the range of 48–230 pA. It means that all of the CPI films have very low leakage current density and they can be used as a capacitor in the electronic applications. While the current values of CPI-4 are 20.5 and 50 pA at 25 and 50 V, respectively, CPI-2 has 84 pA and 300 pA current values at the same voltages, respectively. This difference in the current values can be attributed to the film thickness and pore volume in the structure. 3.5. Solubility, water uptake and some physical properties of the films Solubility properties of the films were investigated by immersing into various solvents such as
NMP, THF, MeCl2 (methylene chloride) and toluene for one week. All the cross-linked films were not dissolved in any of employed solvents as expected. Therefore, this result shows that crosslinked films prepared by our method have excellent solvent resistance due to the formation of crosslinked structure. Water absorption behavior of the polymers heavily influences their dielectric constants and limits their applications in the electric-microelectronic industry. Furthermore, water absorption can increase the conductivity of the dielectric materials and may cause corrosion of metal conductors, which can potentially lead to device failure. For this reason, it is important to prepare polyimides with very low water absorption. Prepared CPI films have water uptakes in the range of 1.2–1.5%. Water uptake data are given in Table 3. For comparison, Kapton film has water uptake values in the range of 1.3–3.5% [11]. Adhesion of the PI films is known to be excellent. Our results indicate that all CPI films exhibit 80–100% adhesions to the glass substrate. The data are given in Table 4. The determined hardness values are very high and close to glass hardness. 4. Conclusion
300
CPI-2 (PMD A+DDS +MDI) CPI-1 (PMD A+DDE +MDI) CPI-4 (BTDA+DDS +MDI)
Current (picoamper)
250 200 150 100 50 0 0
10
20
30
40
50
Voltage (Volt) Fig. 4. I–V graph of MDI based CPI films at room temperature.
In this work, cross-linked polyimide films have been prepared by the reaction of 4.4-diphenylmethane diisocyanate (MDI) as a cross-linking agent with carboxylic acid groups of the prepared conventional poly(amic acid) (PAA). Gelation times were determined and they were in the range of 10 min to 5 days. We believe that this depends on the reactivity of the structure of PAA. FTIR-ATR results showed that MDI can effectively react with carboxylic acid groups of PAA in order to perform crosslinked polyimide films. CPI-1 (PMDA-DDE-MDI) has slightly better thermal and oxidative stability, among the prepared films. Dielectric constant values of all CPIs decrease with increasing frequency when
Table 4 Physical properties of MDI based CPI films Film
Diamine
Dianhydride
Physical properties Hardness (Ko¨nig sec.)
Adhesion %
Appearance
CPI-1 CPI-2 CPI-3 CPI-4
DDE DDS DDE DDS
PMDA PMDA BTDA BTDA
223 227 217 217
90 80 100 80
Homogenous, Homogenous, Homogenous, Homogenous,
transparent, transparent, transparent, transparent,
brown brown brown brown
H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
temperature is stayed constant. CPI-4 has the lowest dielectric constant (2.2 at 1 MHz). Even under 50 V, currents are in the range of 48–230 pA. These values indicate that the prepared samples have low leakage current density and can be used as a capacitor in electronic industry. From the SEM figures, no pore formation was observed on the surface of the film. This result can be attributed to thermal imidization procedure of the films. All CPI films have exhibited excellent solvent resistance due to the formation of cross-linking structure between the units. On the other hand, the adhesion and hardness values of CPI films were found to be outstanding. The films were light colored, transparent, flexible and smooth. Diamine compounds can be also used for crosslinking reaction. The effect of various diisocyanate compounds or cross-linking agents and thermal imidization procedures on the electrical properties of the polyimide films is currently under investigation. Acknowledgements We gratefully thank to Mr. Ismet Cakar, General Director of POLIYA company, for kindly financial support. We also wish to express our thanks to Siemens Company for helping in Dielectric Breakdown Strength measurements. References [1] Abadie MJM, Sillion B. Polyimides and other high temperature polymers. Amsterdam: Elsevier; 1991. [2] Ghosh MK, Mittal KL. Polyimides, fundamentals and applications. New York: Mercel-Dekker. Inc; 1996. [3] Gao CP, Paventi M, Hay AS. J Polym Sci Part A: Polym Chem 1996;34:413–20. [4] Shao L, Chung T-S, Goh S-H, Pramoda KP. J Membr Sci 2005;256:46–56.
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[5] Ayala D, Lozano AE, Abajo JD, Garcia Perez C, Dela Campa JG, Peinemann KV. J Membr Sci 2003;215:61. [6] Chang TS, Kafchinski RE. J Membr Sci 1992;75:181. [7] Shao L, Chung T-S, Goh S-H. J Membr Sci 2004;238: 153–65. [8] Staudt-Buckel C. Soft Mater 2003;1:277–93. [9] Tin PS, Chung TS, Liu Y, Wang R, Liu SL, Promada KP. J Membr Sci 2003;225:77–90. [10] Sullivan DM, Bruening ML. J Membr Sci 2005;248:161–70. [11] Maier G. Prog Polym Sci 2001;26:3–65. [12] Dunson DL. PhD thesis, Virginia Polytechnic University, Virginia, USA, 2000. [13] Bersama JN, Klijustra SD, Balster JH, Van der vegt NFA, Koops GH, Wesseling M. J Membr Sci 2004;238:93. [14] Wind JD, Staudt-Bickel C, Paul DR, Koros WJ. Macromolecules 2003;36:1882. [15] Liu Y, Wang R, Chung T-S. J Membr Sci 2001;189:231–9. [16] Fay CC, St. Clair AK. J Appl Polym Sci 1998;69:2383–93. [17] Chern YT. Macromolecules 1998;31:5837–44. [18] Chern YT, Shiue H-C. Macromolecules 1997;30:4646–51. [19] Park S-J, Cho K-S, Kim S-H. J Colloid Interface Sci 2004; 272:384. [20] Yamazaki O, Yamashita T, Horie K. React Funct Polym 2000;43:173–81. [21] Xu K, Economy J. Macromolecules 2004;37(11):4146–55. [22] Eiju I, Hideyuki I, Masatoshi M. JP2004285084, 2004. [23] He J, Yamashita T, Horie K. Macromol Chem Phys 1998; 199:949–54. [24] Microyannidis JA. J Appl Polym Sci 1994;53:1357–63. ¨ zgu¨mu¨ßs S, Yıldırım S, Deger D, [25] Deligo¨z H, Yalc¸ınyuva T, O Ulutasß K. Polymer 2005;46:3720–9. [26] Muruganand S, Naryandass SaK, Mangalaraj D, Vijayan TM. Polym Int 2001;50:1089–94. [27] Feger C, Franke H. Polyimides in high performance electronics packaging and optoelectronic applications. In: Feger Ghosh MK, Franke Mittal KL, editors. Polyimides: fundamentals and applications. New York: Marcel Dekker, Inc; 1996. [28] Tummala RR, Rymaszewski EJ, Klopfenstein AG, editors. Microelectronics packaging handbook: semiconductor packaging, Pt. II, 2nd. ed. Chapman & Hall, Inc.; 1997. [29] Magaraphan R, Lilayuthalert W, Sirivat A, Schwank JW. Compos Sci Technol 2001;61:1253.
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Preparation, characterization and dielectric properties of 4,4-diphenylmethane diisocyanate (MDI) based cross-linked polyimide films Hu¨seyin Deligo¨z a
a,*
¨ zgu¨mu¨s a, Saffettin Yildirim , Tuncer Yalcinyuva a, Saadet O
b
Istanbul University, Engineering Faculty, Chemical Engineering Department, Chemical Technologies Group, 34320 Avcılar, Istanbul, Turkey b Istanbul University, Faculty of Science, Physics Department, 34459 Vezneciler, Istanbul, Turkey Received 10 August 2005; received in revised form 25 November 2005; accepted 6 December 2005 Available online 25 January 2006
Abstract Four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared by the reaction of different types of conventional poly(amic acid) intermediates with MDI as a cross-linking agent. Subsequently, they were thermally imidized in order to obtain corresponding cross-linked polyimide structure. The results of FTIR-ATR showed that MDI can effectively react with carboxylic acid groups of PAA to form cross-linked polyimide films. TGA, FTIR-ATR and SEM analyses were carried out for characterization of cross-linked polyimide (CPI) films. Moreover, the electrical properties such as dielectric breakdown strength, dielectric constant, I–V characteristics and loss factor of MDI based cross-linked polyimides have been checked. In addition, some physical properties such as water uptake, adhesion, hardness and solubility properties of the films were investigated. The results showed that all CPI films have good insulating properties such as high dielectric breakdown voltage, low loss factor (tan d), leakage density and excellent physical properties. 2006 Elsevier Ltd. All rights reserved. Keywords: Cross-linking; Dielectric properties; Polyimide; Thin film
1. Introduction Polyimides (PI) have attracted great attention in the last a few decades due to their high thermal stability, good solvent resistance, excellent dielectric properties, low dielectric constant, high dielectric * Corresponding author. Tel.: +90 212 591 2479/212 473 7070x17758; fax: +90 212 473 7180. E-mail address: [email protected] (H. Deligo¨z).
strength [1–3]. They are widely used in membrane industry as gas separation membranes [2–10] and in microelectronic industry as an interlayer dielectric (ILD) material due to a novel combination of these outstanding properties [1,11,12]. Cross-linked polyimide (CPI) films were especially prepared for improving gas separation and transport properties [2–10]. Gao et al. have reported thermally cross-linkable PIs containing the 1,2-diphenylcyclopropane unit in the backbone [3]. Lu Shao et al. has
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.12.005
H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
reported that polyimide was modified by a linear aliphatic diamine (ethylene diamine) to enhance transport properties of the films and it was reported that CO2/CH4 separation is especially suitable for this membrane [4]. Thermal treatment and chemical cross-linking methods are used to enhance the physical properties of polyimide membranes against plasticization and impurity hydrocarbon attack [13,14]. The major drawback of heat treatment at an elevated temperature is to deteriorate the subtle structures of asymmetric membrane and their gas permeation properties. As a result, cross-linking method performed at a low temperature is necessary for the modification of polyimide films [15]. PIs are generally chosen for high temperature applications and insulators for electronic parts [1,11,12] due to their good thermal stability, high planarization, low dielectric constant, leakage current density, high dielectric breakdown strength and good processibility. There are several methods and reports for lowering dielectric constants of PIs [16–19]. As an alternative approach, Yamazaki et al. [20] has offered that CPI thin films may be interesting because the gel structure is expected to make porous polyimides which can be used as low dielectric material. While there are lots of studies on the gas separation properties of CPI films, only few pioneering studies have been published on general and electrical properties of CPI which cross-linked at higher temperatures [12,21,22]. Dunson has studied synthesis and characterization of cross-linkable groups containing polyimide oligomers as possible candidates for thin film interlayer dielectrics in electronics packaging [12]. In the other study, electrical properties of cross-linked poly(ether-imide)(PEI) were studied and reported by Xu et al. [21]. It was found that the prepared samples have moderate dielectric constant and dielectric breakdown strength. On the other hand, some reports were published on room temperature preparation of polyimide and PAA gels [23,24], although no electrical data were given in these studies. To the best of our knowledge, no study has been reported on the electrical properties of the CPI films which was cross-linked at low temperatures. In this study, four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared. Firstly, previously prepared four different types of conventional PAA intermediates were reacted with MDI as a crosslinking agent. Subsequently, they were thermally
1371
imidized in order to obtain corresponding crosslinked polyimide structures. The effects of cross-linking modification on the thermal stability, solubility and physical properties (hardness and adhesion) of the CPI films were studied. Furthermore, preliminary studies on the electrical properties such as dielectric constant and dielectric breakdown strength and I–V characteristics of MDI based CPI films were investigated and presented comparatively. Using this method, we recently reported Ac and Dc electrical properties of a cross-linked and a conventional PI films in comparison [25]. 2. Experimental 2.1. Materials Diphenylmethane diisocyanate (MDI), diaminodiphenyl ether (DDE, >99 purity), diaminodiphenyl sulfone (DDS, >99 purity), pyromellitic dianhydride (PMDA, >97 purity) and benzophenonetetracarboxylic dianhydride (BTDA, >97 purity) were supplied from Merck Company, Germany and used as received without further purification. N-methyl pyrolidone (NMP) was purified by distillation under reduced pressure and NMP was stored over molec˚. ular sieve 5 A 2.2. Measurements (instruments) Thermal analyses were carried out with a Linseis thermal analyzer with a heating rate of 5 C/min in air. The sample weights were nearly 10 mg in all experiments. Infrared spectra of the samples were recorded by Perkin Elmer Spectrum 2000 model with ATR (Attenuated total reflectance) unit in the range of 400–4000 cm1. Dielectric constant and loss factor of the CPI films were measured by the HP 4192 A LCR meter Impedance Analyzer at various frequencies and room temperature. Prior to coating, glass substrates were cleaned with isopropyl alcohol, acetone and water to obtain very clean surface. After coating two surfaces of the films with gold, the contacts were done by using indium oxide. The dielectric constant was calculated from the measured capacitance data as follows: e¼
Cd ; e0 A
ð1Þ
where C is the capacitance, e is the dielectric constant, e0 is the permittivity of the free space (8.85 · 1012 MKS unit), d and A are the film
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Reaction conditions were given in Table 1. Chemical structure of the PAA is depicted in Scheme 1.
Table 1 Formulations for CPAA gel preparations Film
Diamine (g)
Dianhydride (g)
[NH2]/ Solvent Gelation [COO] (%) time
CPI-1 CPI-2 CPI-3 CPI-4
DDE (2.40) DDS (1.49) DDE (2.62) DDS (1.49)
PMDA (2.62) PMDA (1.29) BTDA (3.89) BTDA (1.93)
1 1 1 1
90.2 90.3 88.8 90.0
3 days 10 min 5 days 20 min
thickness and electrode area, respectively. Dielectric strengths of the films were measured with Electrotechn. Laboratorium D-7015 ‘‘Insulation Breakdown Tester’’ UH 270, with resolutions of 50 V for 2.5 kV and 100 V for 5 kV. Current–voltage (I–V) characteristics of the films were performed by Keithley Instruments, 410 A picoampermeter. Thicknesses of the films were measured with Mitutoyo Model, 0–25 lm with 0.001 mm (1 lm) resolution. Water uptake properties of the CPI films were investigated. For this purpose, the films were dried at 80 C for 10 h in a vacuum oven and weighted. Subsequently, they were kept in a box with 55% relative humidity (RH) for 1 week and re-weighted. The water uptake values of the films were calculated from the weight changes of each film. SEM analyses were carried out by using Jeol JXA 840 A model apparatus. The solubility tests were carried out by immersing the films into various solvents for one week. The hardness of the prepared films were determined by Ko¨nig Pandulum [DIN 53 157] and given in Ko¨nig sec. The adhesion test was determined as reported in ASTM 359-76 standard. The thicknesses of the self-standing films CPI-1, CPI-2, CPI-3 and CPI-4 were measured by means of a stylus profilometer and found to be 8, 10, 8 and 7 lm, respectively. All donations are shown in Table 1. 2.3. Reactions 2.3.1. Preparation of poly(amic acid) (PAA) Into a 100 mL three necked, round bottomed flask equipped with a thermometer, nitrogen gas inlet and outlet and a magnetic stirrer, 0.012 mol of diamine monomer and NMP were placed to obtain a 10% (wt/wt) solution. The dianhydride monomer (0.012 mol) was added in small quantities over a period of 10 min at 30 C. The reaction took place in 3 h at this temperature under N2 atmosphere. Then, the final mixture was kept in a freezer in order to prevent side reactions until further use.
2.3.2. Preparation of cross-linked poly(amid acid) (CPAA) All CPAAs were prepared by dropwise addition of the isocyanate solution in NMP into the PAA solution dissolved in NMP and the mixture was kept under nitrogen atmosphere at 0 C. All reactions were carried out without a catalyst. The reaction took place in 5 min. Cross-linking has occurred in different times after the reaction has stopped. The gelation times were given in Table 1 and determined as the time of the magnetic stirring bar becoming immobile in the PI solution due to the dramatic increase in the viscosity. As an example, chemical structure of CPAA-1 derived from PMDA-DDE is shown in Scheme 1(a). 2.3.3. Preparation of cross-linked polyimide (CPI) films from CPAA solutions Before cross-linking of PAAs, solutions were immediately applied to a cleaned glass substrate by a doctor blade, then the prepared films were cured at 100 C for 1 h, 200 C for 1 h and 300 C for 1 h in order to obtain corresponding crosslinked polyimide films. Then the films were removed from the glass plate by immersing into hot water. The chemical structure of CPI-1 is depicted in Scheme 1(b). 3. Results and discussion In this study, the cross-linked polyimide films have been prepared in three stages. At the first stage, the reaction of a diamino compound with a dianhyride compound has been carried out in order to obtain PAA intermediate. At the second stage, PAA was reacted with equivalent amount (to the carboxylic acid groups of PAA) of MDI as a cross-linker to prepare PAA gels. At the third stage, reaction solution was urgently applied on a well-cleaned glass substrate. Finally cross-linked polyimide films have been prepared by thermal imidization of corresponding PAA gels. As one can see from Table 1, the gelation times vary between 10 min and 5 days. This may be explained by the reactivity and nature of prepared PAAs. In particular, DDS-derived PAAs were cross-linked faster than DDE-derived one. In all experiments, amounts of isocyanate and carboxylic acid groups were taken equivalent.
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O
O
O
O O
H 2N
+
NH 2
O
O
Diaminodiphenylether (DDE)
Pyromellitic dianhydride (PMDA)
30 ºC and 3h.
O
O O
HN
OH NH
OH O
O
Poly(amic acid)-PAA 1 R=
CH 2 0 ºC and 5 min.
Diphenylmethane diisocyanate (MDI) O
O O
HN
NH
OH
O NH
n
O
R NH
O
OH
O
HN
HN O
O
O
1 (a)
n
Thermal imidization at 100 ºC 1h, 200 ºC 1h, 300ºC 1h
O
O
N
N NH
O
O
n
R NH
O
N
N O
O
O
n
1 (b) Scheme 1. (a) Chemical structure of CPAA-1 and (b) chemical structure of CPI-1.
On the other hand, in this study, it was aimed to observe the effect of the pore formation that may
occur during the cross-linking reaction on the electrical properties of the polyimide thin films. The
3.2. Thermal properties of CPI films prepared from PAA
1369
1775
817
1776
1645
1360
1722
CPI-2Film (PMDA+DDS+MDI)
1717
1370
1777
3353
718
819
CPI-3 Film (BTDA+DDE+MDI)
825
CPI-4 Film
826
1370
719
1716 1645
(BTDA+DDS+MDI)
4000
3000
2000
1500
1000
650
Fig. 1. FTIR spectra of MDI based CPI films.
Table 2 Thermal properties of MDI based CPI films Film
T10% (C)a
T50% (C)b
Char yieldc
CPI-1 CPI-2 CPI-3 CPI-4
420 425 365 385
498 520 500 505
0.9 7.2 6.4 10.6
a b c
The thermal properties of the CPI films were performed by TGA under air atmosphere with a heating rate of 5 C/min. It was observed that the thermal decomposition of cross-linked polyimide films starts in the range of 300–400 C. All thermal and oxidative degradation patterns exhibit the same two-step degradation behavior. Firstly, amide groups in the polymer backbone start to decompose and this is followed by the decomposition of the imide structure. As compared to PMDA-DDE derived conventional polyimide film (commercially known as Kapton), thermal stability of the CPIs have exhibited a slight decline. As seen from Table 2 and Fig. 2, CPI-1 (PMDA-DDE-MDI) has slightly better thermal and oxidative stability, among the prepared films.
3353
Transmittance
3.1. FTIR spectra of cross-linked polyimide films When 4,4-diphenylmethane diisocyanate (MDI) is added as cross-linking agent, a peak attributed to the presence of isocyanate groups has been observed in the range of 2250–2270 cm1. After the cross-linking reaction and thermal imidization of prepared poly(amic acid), it was not observed an absorption band at 2250–2270 cm1 corresponding to the isocyanate group. Therefore, the result confirmed that isocyanate compound has entirely reacted with carboxylic groups of PAA. Furthermore, new peaks at 1777 and 1716 cm1 (C–O symmetric stretching), 1370 cm1 (C–N stretching), 826 and 719 cm1(C–O asymmetric stretching) corresponding imide structures were observed. On the other hand, the peaks belong to amide structure that are formed by the cross-linking reaction were detected as a strong peak at 3353 cm1 (N–H primary and secondary amide stretching) and an increase in the peak intensity at 1645 cm1 (C@O amide I) and 1586 cm1 was observed. The FTIR spectra of CPI films are depicted in Fig. 1.
1720
722
CPI-1 Film (PMDA+DDE+MDI)
1774
3348
pores that may form on the surface of the film can improve the insulating properties such as dielectic constant and dielectric breakdown voltage. However, from the SEM figures, it was observed no pore formation on the surface of the films. Consequently, we believe that this result may be related with the thermal imidization procedure of the films.
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3352
1374
The temperature where 10% weight loss has occurred. The temperature where 50% weight loss has occurred. Weight of residue polymer at 750 C.
3.3. Scanning electron microscope (SEM) SEM analyses of the CPI films were carried out in order to characterize surface properties. According to the SEM results, formed films are very dense, smooth and have no pores, cracking or delamination. The figures were not included since all SEM figures were completely black. 3.4. Electrical properties of the cross-linked PI films 3.4.1. Dielectric constants of the cross-linked PI films Dielectric constants (e1) of the CPI films were calculated from the measurements of the capacity at
H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
100
Weight %
80 60 CPI-1 (PMDA+DDE+MDI) CPI-2 (PMDA+DDS+MDI) CPI-3 (BTDA+DDE+MDI) CPI-4 (BTDA+DDS+MDI)
40 20 0 0
100
200
300
400
500
600
700
800
Temperature ºC Fig. 2. TGA curves of MDI based CPI films.
Table 3 Dielectric properties of MDI based CPI films Film
Dielectric constant
CPI-1 CPI-2 CPI-3 CPI-4
10 kHz
100 kHz
1 MHz
10 MHz
4.4 8.9 6.3 2.4
4.3 8.7 6.2 2.3
4.1 8.4 6.1 2.2
4.2 9.2 6.2 2.2
Dielectric breakdown strength (kV/mm)
Water uptake (%)
117.8 80.6 80.2 80.4
1.53 1.19 1.57 1.18
various frequencies (1 kHz to 10 MHz) at room temperature. The dielectric constant values versus frequencies for CPI films are shown in Table 3 and Fig. 3. One can see from Fig. 3 that e1 decreases with increasing frequency when temperature is stayed constant. Previous studies indicated that
12
Dielectric Constant
10 8
CPI-1 (PMDA+DDE+MDI) CPI-2 (PMDA+DDS+MDI) CPI-4 (BTDA+DDS+MDI)
6 4 2 0 0
5000
10000
15000
Frequency (KHz) Fig. 3. Dielectric constant (e) versus frequency plot of MDI based CPI films at room temperature.
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dielectric constants of PIs decrease gradually with increasing frequency [25,26]. This behavior can be attributed to the frequency dependence of the polarization mechanism. From the results, CPI-4 has the lowest dielectric constant value (2.2 at 1 MHz). This value is lower than the dielectric constant of the conventional polyimide film. We explained this situation by cross-linking caused a network structure, which restricts orientation and relaxation of dipoles, in our recent publication [25]. Considering dielectric constant, dielectric breakdown strength and water uptake values of CPI-4, it can be said that this sample satisfies all required physical properties for being a good ILD in microelectronic industry [11]. On the other hand, we expected the same results for all CPI films, but surprisingly CPI-2 and CPI-3 have shown higher dielectric constants (8.4 and 6.1 at 1 MHz, respectively). The typical value for the dielectric loss (tan d) of polyimides is nearly 0.002 [27,28]. Low values of tan d are indicative of minimal conversion of electrical energy to heat and, thus, signify minimal overall power loss in the dielectric material. It is advantageous to have low values for both tan d and dielectric constant because electrical signals will lose less of their intensity in the dielectric medium. Dielectric loss (tan d) values of all prepared MDI based CPI films are in the range of 0.01–0.015. 3.4.2. Dielectric breakdown strength of the CPI films Polyimides are typically chosen to fit in these high temperature applications as coating material and insulator for electronic parts due to their high dielectric strength and low dielectric constant [1,2]. The dielectric strength is an important parameter for selecting an appropriate electrical insulation material to avoid short circuiting, especially at high temperature operations. The dielectric strength of a material measures its ability to withstand high voltages without breakdown or the passage of considerable amounts of current. It is determined as the minimum value in the applied voltage (Vb) at which breakdown occurs. In particular, dielectric strength depends on temperature, specimen thickness and humidity [12,29]. Dielectric breakdown strengths of the MDI based CPI films are in the range of 80–118 kV/mm and related data are given in Table 3. From the results, it can be said that the value of dielectric breakdown strengths of all CPIs are quite appropriate for many
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microelectronic applications. For comparison, the electrical porcelains and sol–gel ceramics typically have a dielectric strength of 25 and 24–22 kV/mm, respectively. 3.4.3. I–V characteristics of MDI based cross-linked PI films I–V characteristics of MDI based cross-linked PI films were measured at different voltage values and depicted in Fig. 4. Below 50 V, currents are in the range of 48–230 pA. It means that all of the CPI films have very low leakage current density and they can be used as a capacitor in the electronic applications. While the current values of CPI-4 are 20.5 and 50 pA at 25 and 50 V, respectively, CPI-2 has 84 pA and 300 pA current values at the same voltages, respectively. This difference in the current values can be attributed to the film thickness and pore volume in the structure. 3.5. Solubility, water uptake and some physical properties of the films Solubility properties of the films were investigated by immersing into various solvents such as
NMP, THF, MeCl2 (methylene chloride) and toluene for one week. All the cross-linked films were not dissolved in any of employed solvents as expected. Therefore, this result shows that crosslinked films prepared by our method have excellent solvent resistance due to the formation of crosslinked structure. Water absorption behavior of the polymers heavily influences their dielectric constants and limits their applications in the electric-microelectronic industry. Furthermore, water absorption can increase the conductivity of the dielectric materials and may cause corrosion of metal conductors, which can potentially lead to device failure. For this reason, it is important to prepare polyimides with very low water absorption. Prepared CPI films have water uptakes in the range of 1.2–1.5%. Water uptake data are given in Table 3. For comparison, Kapton film has water uptake values in the range of 1.3–3.5% [11]. Adhesion of the PI films is known to be excellent. Our results indicate that all CPI films exhibit 80–100% adhesions to the glass substrate. The data are given in Table 4. The determined hardness values are very high and close to glass hardness. 4. Conclusion
300
CPI-2 (PMD A+DDS +MDI) CPI-1 (PMD A+DDE +MDI) CPI-4 (BTDA+DDS +MDI)
Current (picoamper)
250 200 150 100 50 0 0
10
20
30
40
50
Voltage (Volt) Fig. 4. I–V graph of MDI based CPI films at room temperature.
In this work, cross-linked polyimide films have been prepared by the reaction of 4.4-diphenylmethane diisocyanate (MDI) as a cross-linking agent with carboxylic acid groups of the prepared conventional poly(amic acid) (PAA). Gelation times were determined and they were in the range of 10 min to 5 days. We believe that this depends on the reactivity of the structure of PAA. FTIR-ATR results showed that MDI can effectively react with carboxylic acid groups of PAA in order to perform crosslinked polyimide films. CPI-1 (PMDA-DDE-MDI) has slightly better thermal and oxidative stability, among the prepared films. Dielectric constant values of all CPIs decrease with increasing frequency when
Table 4 Physical properties of MDI based CPI films Film
Diamine
Dianhydride
Physical properties Hardness (Ko¨nig sec.)
Adhesion %
Appearance
CPI-1 CPI-2 CPI-3 CPI-4
DDE DDS DDE DDS
PMDA PMDA BTDA BTDA
223 227 217 217
90 80 100 80
Homogenous, Homogenous, Homogenous, Homogenous,
transparent, transparent, transparent, transparent,
brown brown brown brown
H. Deligo¨z et al. / European Polymer Journal 42 (2006) 1370–1377
temperature is stayed constant. CPI-4 has the lowest dielectric constant (2.2 at 1 MHz). Even under 50 V, currents are in the range of 48–230 pA. These values indicate that the prepared samples have low leakage current density and can be used as a capacitor in electronic industry. From the SEM figures, no pore formation was observed on the surface of the film. This result can be attributed to thermal imidization procedure of the films. All CPI films have exhibited excellent solvent resistance due to the formation of cross-linking structure between the units. On the other hand, the adhesion and hardness values of CPI films were found to be outstanding. The films were light colored, transparent, flexible and smooth. Diamine compounds can be also used for crosslinking reaction. The effect of various diisocyanate compounds or cross-linking agents and thermal imidization procedures on the electrical properties of the polyimide films is currently under investigation. Acknowledgements We gratefully thank to Mr. Ismet Cakar, General Director of POLIYA company, for kindly financial support. We also wish to express our thanks to Siemens Company for helping in Dielectric Breakdown Strength measurements. References [1] Abadie MJM, Sillion B. Polyimides and other high temperature polymers. Amsterdam: Elsevier; 1991. [2] Ghosh MK, Mittal KL. Polyimides, fundamentals and applications. New York: Mercel-Dekker. Inc; 1996. [3] Gao CP, Paventi M, Hay AS. J Polym Sci Part A: Polym Chem 1996;34:413–20. [4] Shao L, Chung T-S, Goh S-H, Pramoda KP. J Membr Sci 2005;256:46–56.
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