Effect of film formation process on residual stress of poly(p-phenylene biphenyltetracarboximide) in thin films

Soluble poly(p-phenylene biphenyltetracarboxamine acid) (BPDA-PDA PAA) precursor, which was synthesized from biphenyltetracarboxylic dianhydride and p-phenylene diamine in N-methyl-2-pyrrolidone (NMP), was spin-cast on silicon substrates, followed by softbake at various conditions over 80–185°C. Softbaked films were converted in nitrogen atmosphere to be the polyimide films of ca. 10 μm thickness through various imidizations over 120–400°C. Residual stress, which is generated at the polymer/substrate interface by volume shrinkage, polymer chain ordering, thermal history, and differences between properties of the polymer film and the substrate, was measured in situ during softbake and subsequent imidization processes. Polymer films imidized were further characterized in the aspect of polymer chain orientation by prism coupling and X-ray diffraction. Residual stress in the polyimide film was very sensitive to all the film formation process parameters, such as softbake temperature and time, imidization temperature, imidization step, heating rate, and film thickness, but insensitive to the cooling process. Softbaked precursor films revealed 9–42 MPa at room temperature, depending on the softbake temperature and time. That is, residual stress in the precursor film was affected by the amount of residual solvent and by partial imidization possibly occurring during softbake above the onset of imidization temperature, ca. 130°C. A lower amount of residual solvent caused higher stress in the precursor film, whereas a higher degree of imidization led to lower stress. Partially imidized precursor films were converted to polyimide films revealing relatively high stresses. After imidization, polyimide films exhibited a wide range of residual stress, 4–43 MPa at room temperature, depending on the histories of softbake and imidization. Relatively high stresses were observed in the polyimide films which were prepared from softbaked films partially imidized and by rapid imidization process with a high heating rate. The residual stress in films is an in-plane characteristic so that it is sensitive to the degree of in-plane chain orientation in addition to the thermal history term. Low stress films exhibited higher degree of in-plane chain orientation. Thus, residual stress in the film would be controlled by the alignment of polyimide chains via the film formation process with varying process parameters. Conclusively, in order to minimize residual stress and to maximize in-plane chain orientation, precursor films should be softbaked for 30 min-2 h below the onset imidization temperature, ca. 130°C, and subsequently imidized over the range of 300–400°C for 1–4 h by a two-step or multi-step process with a heating rate of ? 5.0 K min⁻¹, including a step to cover the boiling point, 202°C, of NMP. In addition, the final thickness of the imidized films should be <20 μm. © 1997 Elsevier Science Ltd.     

A chemically amplified positive-working photosensitive polyimide based on a blend of poly(amic acid ethoxymethyl ester) and poly(amic acid)

We prepared a novel chemically amplified photosensitive polyimide based on a blend of poly(amic acid ethoxymethyl ester) (PAAE) and poly(amic acid); this blend produces polyimide (PI) films with improved mechanical properties after imidization with photoacid generator (PAG). PAAE and poly(amic acid) were end-capped with 5-norbornene-2,3-dicarboxylic dianhydride and 2,3-dimethyl maleic anhydride, respectively, to lower their molecular weights without compromising the properties of the resulting PI films. As a result of the blending of these PI precursors, the mechanical properties of the PI films were found to be less affected by the strong acid generated from the PAG than PI films fabricated by imidization of PAAE alone. The relatively high solubility of the blended PI precursor film in basic aqueous solutions was found to be effectively controlled by the use of a high-temperature post-exposure bake process to partially imidize the end-capped PAA. It was found that a 10-μm-thick film of the PSPI precursor system containing 13 wt% PAGs exhibits a sensitivity (D⁰) of 700 mJ/cm² when developed with 2.38 wt% aqueous tetramethyl ammonium hydroxide solution at room temperature. A fine positive pattern was fabricated in a 12 μm thick film with 1000 mJ/cm² of i-line exposure. The resultant PI film was also found to exhibit excellent mechanical and thermal properties, which are critical to its practical use as a stress buffer layer in semiconductor packaging.     

The effects of varied imidization conditions on rubbed polyimide film surface morphology

The thermal histories of polyimide films were varied to study the effect of the extent of imidization on the response of films to rubbing. Films rubbed prior to imidization formed tears that extended to the substrate; tears in these films formed at smaller film thicknesses than in films imidized prior to rubbing. Aligned nanoscopic islands were also seen. The alignment of these islands disappeared upon imidization. Films imidized at a low temperature showed less alignment of islands than conventionally prepared films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1192–1197, 2004     

Photosensitive poly(amic acid)/organoclay nanocomposites

A project was carried out aimed at reducing the coefficient of thermal expansion (CTE) of photosensitive polyimide formulations (photoresists) through the incorporation of small amounts of an organoclay. The organoclay was formed by a cation exchange reaction between a NA⁺-montmorillonite clay and an ammonium salt of dodecylamine. Two polyimide precursors, a poly(amic ester) (PAE) and a poly(amic acid) (PAA), were used in this study. The PAE was prepared by direct polymerization of 2,2′-bis-(3-amino-4-hydroxyphenyl)hexafluoropropane and bis(n-butyl)ester of pyromellitic acid in the presence of phenylphosphonic dichloride as an activator. The polymer had an inherent viscosity of 0.23 dL/g. The PAA copolymer was prepared by polymerization of pyromellitic dianhydride, oxydiphthalic anhydride and oxydianiline. The polymer had an inherent viscosity of 1.00 dL/g. Two photosensitive resin/clay formulations were prepared from these two PI precursors using 2,3,4-tris(1-oxo-2-diazonaphthoquinone-5-sulfonyloxy)-benzophenone as the photosensitizer and 3 wt% organoclay. The films obtained from the PAA formulation were transparent and tough, while the films prepared from the PAE formulation were opaque and brittle. Both X-ray diffraction and transmission electron microscope analyses showed that, although the organoclay was not dispersed well in the PAE matrix, it was dispersed in the PAA matrix on a nanometer scale. The clay particles remained well dispersed after the PAA film was thermally imidized. The CTE of the polyimide film obtained was 23% lower than that of a similar film that did not contain the organoclay. The temperature at which the polyimide underwent a 5% weight loss when subjected to TGA in nitrogen was also increased by 13%. The photosensitive PAA/clay nanocomposite showed a sensitivity of 301 mJ/cm² and a contrast of 1.66 when a 0.2 wt% tetramethylammonium hydroxide developer was used. A line/space pattern with a resolution of 10 μm was obtained from this formulation.     

An ellipsometric study of polymer film curing: 2,6-Bis(3-aminophenoxy) benzonitrile/4,4′oxidiphthalic anhydride poly(amic acid)

The curing of 2,6-bis(3-aminophenoxy)benzonitrile/4,4′oxidiphthalic anhydride ((β-CN) APB/ODPA) has been investigated using spectroscopic ellipsometry on films with various degrees of imidization. Results indicate that much of the film imidization is accomplished at 200 °C and above. Three absorption peaks have been observed (4.1, 5, and 6 eV) which correspond to intra- and inter-molecular optical transitions. A comparison of the film optical constants for the pristine poly(amic acid) and the fully cured polyimide shows film densification upon imidization. A curing timeline has been obtained using in situ real-time spectroscopic ellipsometry, and ellipsometry is shown to serve as a general technique for studying organic film curing.     

Double phase separation in preparing polyimide/silica hybrid films by sol-gel method

In this paper, the phase separation process of the polyimide/silica hybrid films made from polyamic acid (PAA) and precursor (TEOS-A) hydrolyzed tetraethoxysilane under acidic condition in N-methyl-2-pyrrolidone (NMP) through sol-gel method was investigated by the scanning electron microscope (SEM). A double phase separation was discovered for the preparation of the hybrid films. With evaporation of the solvent NMP at lower than 100 °C, the component miscibility of TEOS-A and PAA decreases so that the first phase separation took place and a larger particle phase of TEOS-A precursor with size around 2.0 μm was formed. The second phase separation from the matrix phase appeared, as PAA was imidized at elevated temperature, which destroyed the interaction between carboxyl group of PAA and hydroxyl group of TEOS-A, and a nanoscale SiO₂ particle phase formed. The formation mechanism of the double phase separation was explained by the “capture-release” model. According to the model, the second phase separation can be controlled by synthesizing amic acid-imide copolymer with different contents of carboxyl group.     

Effect of thermal history during drying and curing process on the chain orientation of rod-shaped polyimide

Chain orientation in polyimide (PI) film is influenced by the thermal history during drying and curing process. The amount of residual solvent and the degree of imidization, among other factors, play a major role in determining the chain orientation during the process. In the present study, poly(amic acid), the precursor of PI, coated on the glass substrate was imidized to PI through different drying and curing protocols. On the way of complete imidization, the residual solvent concentration and the degree of imidization were characterized using confocal Raman spectroscopy. The poly(amic acid) began to imidize quickly while retaining more solvent in the film as the initial drying temperature increased. The degree of in-plane chain orientation in fully imidized PI film made by different process protocols was compared using polarized Raman spectroscopy. The fully imidized PI showed the lowest degree of in-plane chain orientation when it was processed by the protocol with the highest drying temperature. The difference in the degree of in-plane chain orientation among different PI films significantly influenced the in-plane thermal expansion coefficient, while no significant change in crystallinity or glass transition temperature was observed.     

聚酰胺酸溶液酰亚胺化程度控制及其对PI薄膜性能影响

乙酐/吡啶混合溶剂加入均苯四羧酸二酐(PMDA)–4,4’–二氨基二苯醚(ODA)型聚酰胺酸溶液中,发生化学酰亚胺化,得到酰胺酸-酰亚胺共聚物(PA–I)。通过旋转黏度计和傅立叶变换红外光谱的系统研究,当酰亚胺化程度达到26%左右时,溶液发生相分离。选择合适的溶液浓度、乙酐含量、反应时间,能够对PA–I均相溶液的酰亚胺化程度实施控制。用不同酰亚胺化程度的PA–I溶液进行热酰亚胺化,获得聚酰亚胺薄膜。研究结果表明,酰亚胺化程度较高的PA–I溶液,所制聚酰亚胺薄膜呈现出较高有序程度的聚集态结构,较高的拉伸强度和拉伸弹性模量。     

A solid-state NMR study of aromatic polyimides based on 4,4′-diaminotriphenylmethane

The effect of different synthesis routes on the chemical and molecular order of polyimides based on 4,4′-diaminotripenylmethane (DA-TPM) and various aromatic dianhydrides (PI-TPM) was studied by solid-state carbon-13 nuclear magnetic resonance (¹³C-NMR). Polyimides were prepared by three different methods including a two-step procedure with either thermal or chemical imidization of precursor poly(amic acid)s (PAA) and one-step high-temperature polycondensation in phenolic solvents. Model compounds were also obtained and used in the assignment of the NMR signals. The NMR spectra for PI-TPMs obtained by one-step high-temperature polycondensation and—to a lesser extent—by thermal imidization of PAA, show sharper lines than those observed in the spectra of polymers prepared from PAA via chemical imidization. These differences are due mainly to the lower degree of ordering of the latter polyimides. WAXD patterns of polyimide films also indicated a less-ordered structure of the polymers resulting from the chemical imidization of PAA. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1053–1064, 1998     

Studies on syntheses and properties of novel CeO2/polyimide nanocomposite films from Ce(Phen)3 complex

A series of cerium dioxide (CeO₂)/polyimide (PI) nanocomposites were successfully prepared from Ce(Phen)₃ and polyamic acid (PAA) via the solution direct-dispersing method, followed by a step thermal imidization process. TGA and XPS studies showed that the cerium complex decomposed to form CeO₂ during the thermal imidization process at 300 °C. SEM observation showed that the formed CeO₂ as nanoparticles was well dispersed in polyimide matrix with a size of about 50-100 nm for samples with different contents of CeO₂. Thermal analysis indicated that the introduction of CeO₂ decreased the thermal stability of nanocomposite films due to the decomposition of Ce(Phen)₃ in the imidization process, while the glass transition temperature (T_g) increased obviously, especially nanocomposite films with high loading of CeO₂ exhibited a trend of disappearance of T_g. DMTA and static tensile measurements showed that the storage modulus of nanocomposite films increased, while the elongation at break decreased with increasing CeO₂ content.     

Effects of reactive end-capper on mechanical properties of chemical amplified photosensitive polyimide

Photosensitive polyimide (PSPI) was synthesized and characterized to replace the conventional polyimide buffer layer because direct patterning with PSPI could reduce the processing procedure to the half. Since PSPI should be dissolved in alkaline aqueous solution and have good mechanical properties after imidization, low molecular weight of PSPI was synthesized with reactive end-capper, which could extend the chain length of PSPI during imidization. Therefore norbornene end-capped PSPI precursor was synthesized with various 5-norbornene-2,3-dicarboxylic anhydride (NDA) content.Although molecular weight of PSPI decrease with increasing NDA content, the elongation at break and the glass transition temperature (T_g) of PSPI films imidized at 300 °C increased with increasing NDA content. On the other hand, elongation at break of PSPI films imidized at 350 °C decreased but T_g of those increased with increasing NDA content. Above T_g, thermal expansion coefficient decreased dramatically by introducing NDA end-capper. From mechanical and thermal properties of PSPI, it appears that low molecular weight of PSPI can be chain-extended and crosslinked during imidization.     

Characteristics of polyimide/barium titanate composite films

In this study, the characteristics of the polyimide/BaTiO₃ composite films with various amounts of BaTiO₃ were evaluated. Modifier 1-methoxy-2-propyl acetate was added during composite preparation to disperse the BaTiO₃ particles in polyimide matrix. Conversion of polyamic acid (PAA) to polyimide was not completed for the composite film with a high BaTiO₃ loading (90 wt%). Dielectric constant of the film increases from 3.53 to 46.50, at the sweep frequency of 10 kHz, as the BaTiO₃ content increases from 0 to 90 wt% (0–67.5 vol.%), which is mainly due to the relatively high dielectric constant of BaTiO₃ particles in the polyimide matrix. The dielectric losses at 10 kHz is ranging from 0.005 to 0.015, which is due to the switching of the domain wall. Water absorption decreases considerably with increasing BaTiO₃ content. With 10 wt% (2.5 vol.%) BaTiO₃ addition, the water absorption of the composite film reduces 45% from that of pure polyimide. Also, high loading of BaTiO₃ is not beneficial to reduce the water absorption of the composite film.     

Preparation and Characterization of Aminopropylsilatrane Endcapped Polyimide Films

γ-Aminopropylsilatrane (APS)/γ-aminopropyltriethoxysilane (APTES) end capped polyimide films were prepared by thermal imidization method. Polyamic acid (PAA) was prepared by the reaction of 4,4′-oxydianiline (ODA) with 4,4′-oxydipthalicdianhydride (ODPA) using dimethylacetamide (DMAc) as solvent. The end group of prepared PAA was capped by different percentage of APS/APTES. The polyimide films were characterized by different advanced instrumental techniques for chemical/physical properties. APS end capped PI films show better thermal and mechanical properties and air permeability than APTES end capped polyimide films.     

Processable conductive blends of polyaniline/polyimide

By using camphorsulfonic acid (CSA) to protonate polyaniline (PANI), the counterion enabled the PANI–CSA complex processable as a solution phase. So camphorsulfonic acid (CSA)-doped polyaniline/polyimide (PANI/PI) blend films were prepared by the solvent casting method using N-methylpyrrolidinone (NMP) as a cosolvent followed by thermal imidization. The conductivity of the PANI–CSA/PAA (50 wt % PANI content) is greater than that of the pure PANI sample at room temperature. As the thermal imidization proceeded, molecular order of polymer chain structure was improved in the resulting PANI–CSA/PI film due to the annealing effect of PANI chain, and this PANI–CSA/PI film showed higher conductivity than PANI–CSA and PANI–CSA/PAA film. PANI–CSA/PI blend films had a good thermal stability of conductivity at high temperature. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1863–1870, 1998     

A novel cross-linked polyimide film: synthesis and dielectric properties

A novel cross-linked polyimide (CPI) has been prepared by imidization of cross-linked poly(amic acid) (CPAA). In this work, the Ac conductivity and dielectric properties of this polyimide are presented comparitively with those of conventional polyimide (PI), in the 0.2-100 kHz frequency range and 300-463 K temperature interval. Although the frequency and temperature dependencies of dielectric constant of both conventional and cross-linked polyimides show the same behaviour, the dielectric constant of CPI takes lower values. The Ac conduction studies suggest that electron hopping is responsible for conduction of the PI and CPI films. The activation energy calculated in 296-353 K temperature interval and the ß-relaxation was also observed for CPI.     

Carbonization and graphitization of polyimide films: Effect of size of leaving group at imidization

Polyamide acid was prepared from pyromellitic dianhydride and p-phenylenediamine and was then reacted with NaH and various kinds of alkyl halides to transform into alkyl esters. The cast films were imidized as fixed on glass substrate to give polyimide films and were then carbonized by heating to 900°C. The electrical conductivity of the carbonized films decreased with the increase of the size of the leaving group at the imidization step. The carbonized films were further heated to 2800°C for graphitization. Their degrees of graphitization and orientation of the graphite crystallite as a function of weight loss at imidization were studied by X-ray diffraction measurement at room temperature and magnetoresistance measurement at liquid nitrogen temperature. Both measurements clearly indicate that the graphitized films prepared from polyamide acid alkyl ester have high degrees of graphitization. It was also made clear that the orientation of the graphitized films increased with the increase of the size of the leaving group at the imidization step. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci John Wiley & Sons, Inc.J Appl Polym Sci 68: 1613–1620, 1998     

Thermal imidization of poly(amic acid) precursors on surface-modified poly(tetrafluoroethylene) films via graft copolymerization with glycidyl methacrylate

A novel method for preparing composites of polyimides (PI) laminated to poly(tetrafluoroethylene) (PTFE) films is reported. PI/PTFE composites were developed through thermal imidization of poly(amic acid) (PAA) precursors on surface-modified PTFE films. Surface modification of PTFE films was carried out via Ar plasma pretreatment of the films, followed by UV-induced graft copolymerization with glycidyl methacrylate (GMA). The surface composition and topography of the graft copolymerized PTFE films and the delaminated PI and PTFE surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The adhesion strengths of the PI (imidized PAA) on the GMA graft copolymerized PTFE films were evaluated as a function of various thermal imidization schedules. The adhesion reliability of the PI/PTFE composites was tested by a series of hydrothermal cycles. The development of strong Tpeel adhesion strengths of about 8 N/cm with excellent reliability for the PI/PTFE composites was attributable to the synergistic effect of coupling the curing of the epoxide functional groups of the grafted GMA chains with the imidization process of the PAA and the fact that the GMA chains were covalently tethered onto the PTFE surface. The PI/PTFE composites delaminated via cohesive failure inside the PTFE substrates. The delaminated PI film with a covalently adhered 'rough' PTFE surface layer exhibited a water contact angle as high as 140°.     

The chemistry involved in the loading of silver(I) into poly(amic acid) via ion exchange: A metal-ion-induced crosslinking behavior

Metal-ion-induced crosslinking of poly(amic acid) (PAA) was observed in the incorporation of silver ions into PAA through ion exchange. Studies on the interaction of silver ions with 3,3′,4,4′-benzophenonetetracarboxylic acid/4,4′-oxidianiline (BTDA/ODA)-based PAA suggest that the ion exchange reactions between poly(amic acid) and silver ions are not so simple as what we generally believed. It involves not only the formation of silver carboxylate but also the generation of diversified silver chemical entities arising from the strong chemical bonding of metal ions with the functional groups, such as carbonyl groups and amide groups, in the polymer chain, which are suggested to be responsible for the crosslinking behavior. Moreover, silver ions loaded into the film are readily self-reduced and provides us a convenient route to disperse very small metal nanoparticles into the polymeric matrix. Meanwhile, strong accelerating effect of silver ions was observed on the hydrolysis of PAA molecules and the characterization results indicate that about 14-16 wt% precursors were dissolved during the ion exchange in the aqueous silver ion solutions. Fortunately, it is found that the metal-ion-induced crosslinking structure formed in the silver-doped film has helped to prevent the damaging effect of silver ions and the essential structural features of PAA were retained in the remaining polymer matrix.     

Synthesis and properties of fluorene‐based polyimide adhesives

Five kinds of fluorene‐based polyimides (PIs) based on 4,4′‐oxydiphthalicanhydride (ODPA), 9,9′‐bis(4‐aminophenyl)fluorene (BAFL), and 3,4′‐diaminodiphenyl ether (3,4′‐ODA) were synthesized through two‐step method. The partially or fully imidized PI films were cast from poly(amic acid) (PAA) solution and were imidized by far‐infrared radiation at various temperatures. The degree of imidization was characterized by FT‐IR and TGA. The fully imidized PI films were characterized by DMTA, TGA, and tensile tests. The partially imidized PI films were adhered to stainless steel plates for preparing the single lap joints. Lap shear strength (LSS) at room temperature was measured to compare the adhesive strength of single lap joint. Fractured surfaces were analyzed using scanning electron microscopy (SEM). The effects of fluorene content on thermal, tensile, and adhesion properties of PIs were elaborately studied. The results showed that PI films exhibited high glass transition temperature (T_g), good thermalplasticity, and thermal stability. The LSS of PIs increased abruptly with the incorporation of fluorene groups. The LSS of PI‐50/50 was the highest, which was 22.3 MPa. The LSS of PI‐50/50 was also measured at high temperature to investigate the thermal resistance of fluorene‐based PI adhesive. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers.     

Polyimide blend alignment layers for control of liquid crystal pretilt angle through baking

Polyimide films were used for liquid crystal (LC) alignment layers to control LC pretilt angles over the full range (8°-90°). The pretilt angles could be controlled using polyimide films prepared from polyamic acid for vertical LC alignment and using polyimide blend films prepared from two types of polyamic acids, one for vertical LC alignment and the other for planar LC alignment, by changing the baking times ranging from 40 to 180 min at 230 °C. The polyimide blend film could control the pretilt angle better than the polyimide prepared from just one polymer component. The LC alignment behavior was well correlated with the wettability of the polyimide films due to the fragmentation of the long alkyl side group on the polyimide surfaces by the baking process.