Cured Polyimide Films Research Articles

Positive-tone chemically amplified photosensitive polyimides with controllable taper angles

Current methods for tuning the taper angle of photosensitive polyimide (PSPI) often require additional equipment or materials, and has limited achievable angle tuning range while maintaining resolution. To overcome these limitations, a positive tone chemically amplified photosensitive polyimide (p-CA-PSPI) was prepared from tert-butylcarbonate-protected polyimide (t-Boc-PI), photoacid generator (PAG), other additives, and solvent. By optimizing the protection ratios of t-Boc-PI resins, the p-CA-PSPI achieved a high film retention rate and wide processing window. The photo-patterning conditions were able to manipulate the sidewall profile and taper angle within the range of 11–53°, maintaining a resolution of 3 μm for patterns with taper angles higher than 28° at 1.5 μm film thickness, by controlling the acid diffusion. The thermally cured polyimide films demonstrated excellent mechanical properties, including a tensile strength of 95.7 MPa, tensile modulus of 2.9 GPa, and elongation at breakage of 24.2%, high thermal stability with a glass transition temperature of 343 °C, decomposition temperature of 409 °C, and good chemical resistance. This novel taper angle tuning method has the potential to optimize the internal stress of semiconductor devices and advance the development of display devices, MEMs, and multifunctional electronics that require a single package containing multiple or unique taper profiles.

(Invited) Fan-out and Cu- Compatible Dielectric Processing of Low Temperature Polyimides for Next-Generation 3D Advanced IC Back End Applications

Fan-out (FO) wafer and panel level processing in advanced backend applications requires the curing of RDL (Redistribution Layers) or interconnect dielectrics at lower temperatures in order to manage stress and relevant warpage for multi-level or 3D processing. In addition, the need to transition to all-Cu interconnect has resulted in a temperature limit of 230°C for large numbers of end products. Most of these products employ the advanced packaging capabilities of heterogeneous and 3D integration, 3D stacking and system-in-package (SIP) technology, whereby a variety of chiplets and other components are stacked, delivering high compute density and high performance at low cost. Multi-layer RDLs are used to connect these dissimilar chips on organic, glass, silicon or fan-out substrates. Consequently, process technology for polyimide, photo imageable dielectric (PID) and photo sensitive polyimide (PSPI) must now enable a faster cure at lower temperatures to achieve lower stress, while also delivering better film properties.In this talk, the physical, mechanical, thermal, and electrical properties of different types of polyimide materials will be discussed as a function of different process parameters, under atmospheric and vacuum process conditions. The talk references a study in which a variety of low temperature RDL polyimides such as HD-7110, Asahi BL-301 and Fuji-LTC9320 were processed at a temperature of less than 230°C, and then compared to conventional polyimide HD-4100. Vacuum cure even at lower temperatures resulted in higher thermal stability, lower outgassing, and better thermal properties. In electrical characterization, vacuum cure showed better dielectric strength as well as a lower dissipation factor. These results are consistent with the assumption that cure under vacuum reduces the amount of volatile and volatilizable material remaining in the film after the cure process.The use of sub-atmospheric curing or annealing technology at low temperature improved both the quality and the performance of the cured polyimide films for RDL applications. These results are relevant to the multi-level metallization process of Fan Out Wafer Level Packaging (FOWLP) and Fan Out Panel Level Packaging (FOPLP), and therefore to 3D stack integration.

A comprehensive study of pyrazine-contained and low-temperature curable polyimide

One challenge in tailoring polyimide (PI) for microelectronics is to obtain excellent properties of films when cured at low temperature. To meet this challenge, this study put forwards an investigation of pyrazine moieties incorporated polyimide, which is comprehensively investigated on curing temperature dependence with thermal, mechanical, and dielectric properties. Self-catalytic effect stemming from pyrazine moiety has a significant improvement on imidization degree of PI films from 10.1% to 13.3% under 200 °C as revealed with ATR-FTIR spectrum. The resulting low temperature cured polyimide films with higher imidization degree exhibit excellent comprehensively properties. It is demonstrated that PI-1 shows high tensile strength (136 MPa), high Young's modulus (3.4 GPa) and low CTE value (4.7 ppm/°C) when cured at 200 °C. Besides, the temperature for 5% weight loss as high as 556 °C and glass transition temperature (Tg) as 367 °C of the PI film turn out to be superior thermal stability. Moreover, molecular dynamics simulation was carried out to get a theoretical perspective on the effect of intrinsic factors on a microscopic level associated with the molecular structure of the polyimides to gain a deeper view on the effect of imidizaiton degree on the Tg, chain mobility and conformational rigidity. This research gives a comprehensive study of pyrazine constructed polyimide for low-temperature cured polyimide and paves a way for its application in the microelectronics.

Highly soluble phenylethynyl terminated oligoimides derived from 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane, 4,4′-oxydianiline and mixed thioetherdiphthalic anhydride isomers

Novel highly soluble phenylethynyl-endcapped oligoimides derived from mixed thioetherdiphthalic anhydride isomers (m-TDPA) and 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI), 4,4′-oxydianiline (4,4’-ODA) with 4-phenylethynylphthalic anhydride (PEPA) as a reactive endcapping agent were synthesized. The calculated molecular weights of all the oligoimides were 2500 g·mol−1.The effect of the mole ratio of DAPI/4,4’-ODA on solubility and melt viscosity of oligoimides as well as the thermal and mechanical properties of cured polyimide films was investigated. Experimental results indicated that DAPI greatly improved the solubility (>30 wt.%) of the oligoimides in low boiling points solvents when the content of DAPI was more than 10 mol% per total diamine. All the oligoimides exhibited very good processability with minimum melt viscosity lower than 60 Pa·s at about 330 °C. Tough and brown films were obtained after thermally cured at 370 °C for 1 h and dynamic mechanical analysis (DMA) showed that glass transition temperatures of the cured films increased up to 340 °C with the increasing content of DAPI. However, the thermal stability and mechanical properties decreased with the increase of DAPI content. The temperature of 5% weight loss was higher than 470 °C in both air and N2 atmosphere obtained by thermogravimetric analysis (TGA). And the tensile strengths of the cured films were about 60 MPa.

Synthesis and characterization of phenylethynyl-terminated polyimide oligomers derived from 2,3,3′,4′-diphenyl ether tetracarboxylic acid dianhydride and 3,4′-oxydianiline

With the goal of improving processability of imide oligomers and achieving high toughness of thermosetting polyimides, a series of 4-phenylethynylphthalic anhydride (PEPA)-terminated imide oligomers prepared by the reaction of 2,3,3′,4′-diphenyl ether tetracarboxylic acid dianhydride (a-ODPA) and 3,4′-oxydianiline (3,4′-ODA) with different molecular weights (degree of polymerization: n = 1–9) were formed. The resultant oligomers with different molecular weights were characterized for their chemical architecture, cure behavior, thermal properties, solubility in organic solvents and rheological characteristics. Besides, the thermal properties and tensile test of cured polyimide films were also evaluated. The imide oligomer (degree of polymerization: n = 1) has some somewhat crystalline phase, and imide oligomers (degree of polymerization: n = 2–9) showed excellent solubility (40 wt%) in N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) at room temperature. Furthermore, the rheological properties of imide oligomers showed very low melt viscosity and wider processing window. The cured films exhibited good thermal properties with the glass transition temperatures of 282–373 °C and 5 wt% thermal decomposition temperatures higher than 551 °C in nitrogen atmosphere. The elongation at break of the prepared films was found to be high (almost > 9.3%).

The effect of film thickness on the depth‐wise chain orientation of rod‐shaped polyimide

ABSTRACTControlling the chain orientation of polyimide is important because it affects the physical and electrical properties of the film. When a polyimide film is thick, the chain orientation has an inhomogeneous distribution along the thickness direction. In this study, poly(amic acids), the precursor of polyimide, with different coating thicknesses are dried, and the distribution of chain orientation in the thickness direction is investigated by measuring the residual solvent content with Raman spectroscopy. The effect of film thickness on the imidization rate is also studied by measuring the depth‐wise degree of imidization at the curing step. With the final cured polyimide film, the depth‐wise chain orientation is quantified by introducing the Fraser distribution function using polarized Raman spectroscopy. The thicker film has a lower degree of in‐plane orientation of polyimide chains, particularly near the substrate. This distribution of polyimide chain orientation in the thickness direction is similar to that of poly(amic acid) after drying. Fast imidization with higher solvent content for thick polyimide retards the formation of a well‐ordered structure with a high degree of in‐plane orientation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 848–857

Synthesis and characterization of ternary copolyimides derived from aromatic dianhydride and aromatic diisocyanates

A series of polyimide (PI) copolymers were synthesized via a one-step polycondensation procedure from benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride, toluene diisocyanate (TDI), and 4,4′-methylene bis(phenyl isocyanate) (MDI) with various mole ratios of TDI and MDI ranging from 70:30 to 100:0. The inherent viscosities of the resulting PIs were in the range of 0.51–0.80 dL g−1. The prepared PIs showed good solubility in common organic as well as in less polar solvents. All copolymers afforded tough and flexible films that had good thermal stability with glass transition temperature of 308–325°C and the temperatures of 5% weight loss in air and nitrogen being 420 and 460°C, respectively. The resultant PI films with thickness about 15 μm exhibited high transparency (>88%) in visible region (400–700 nm). Also, the cured PI films possessed good mechanical properties with tensile strength of 60.52–69.31 MPa and tensile modulus of 0.98–1.30 GPa.

Dielectric properties of on-chip-cured polyimide films

Spin-coated polyimide thin films were locally cured using microelectromechanical system microhotplates as an alternative approach to the conventional wafer-level curing process. The on-chip cured polyimide films were characterized Fourier transform infrared spectroscopy. The polyimide was found to be fully cured at a temperature over 350 °C for 1 h. The dielectric properties of 350 °C-cured polyimide films were measured over broad frequency, temperature, and humidity ranges. As is expected for most polymer materials, the dielectric constant decreased with increasing frequency due to dielectric relaxation, which may be due to the fact that multiple relaxation times (i.e., several types of polarization), rather than a single one are involved. The dissipation factor was very low and nearly flat for excitation frequencies less than 1 MHz. Both the dielectric constant and the dissipation factor increased with increasing temperature from 10 °C to 90 °C, and relative humidity (RH) from 30%RH to 95%RH, with a steeper slope at lower frequencies. These results indicate that on-chip-cured polyimide films have weak low frequency dispersion due to the aging effect at high temperature and humidity.

Fully aromatic polyimide gate insulators with low temperature processability for pentacene organic thin-film transistors

This paper demonstrates the effects of the imidization ratio of polyimide gate insulators on the performance of organic thin-film transistors (OTFTs). We report the synthetic results of polyimide films imidized at a temperature of 200 °C along with an easily removed organic base catalyst (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU), and their application in gate insulators of organic thin-film transistors. The degree of imidization increased to almost 100% after a thermal treatment at 200 °C for 40 min in the presence of DBU. The performance of the pentacene OTFT dramatically improved by using low temperature cured polyimide film as the gate insulator.

Synthesis and characterization of novel polyimides with 1‐octadecyl side chains for liquid crystal alignment layers

AbstractA series of poly(amic acid)s have been synthesized from cyclobutane‐1,2,3,4‐tetracarboxylic dianhydride (CBDA), a functional diamine, 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide and 4,4‐diaminodiphenylmethane (DDM) to give liquid crystal alignment layers with high pretilt angle after a rubbing process for vertically aligned liquid crystal display devices. Inherent viscosities of the poly(amic acid)s measured were in the range 0.2–1.56 dl/g, which decreased with an increase of content of 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide. The cured polyimide films had good transparency, which was above 95% at 400 nm. Surface energies of all the polyimide films prepared from 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide were below 40.7 dyn/cm before the rubbing process, which resulted in very high pretilt angles above 89°. By a rubbing process, the surface energy of the polyimide prepared from diamine mixture with 10 mol% of 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide and 90% of DDM increased from 40.7 to 42.7 dyn/cm. Copyright © 2007 John Wiley & Sons, Ltd.

Effect of Thermal Curing on Molecular Orientation in Polyimide Films Having Rodlike Molecular Skeleton Formed on Substrates

Films of a single polyimide having a rodlike molecular skeleton of uniform thickness were prepared on silicon and quartz substrates under several different heating rates during the thermal curing cycle. The molecular orientation in the cured polyimide film was evaluated by polarizing infrared and visible light investigation. The molecular orientation in the film was very sensitive to the heating rate during the curing cycle at the interface with the substrate, but less sensitive to that at the surface. The rodlike molecules in the film which were slowly heated were highly oriented and nearly parallel to the film plane, whereas those which were quickly heated were randomly oriented at the interface. It was shown that the increase in thermal stress in the polyimide film with increasing heating rate reflected the degree of decrease of the in-plane orientation.

Study of local stress, morphology, and liquid-crystal alignment on buffed polyimide surfaces

The magnitude of local stress applied to a polymer surface in the buffing process, which is a critical method to uniformly align liquid crystals for displays, has been evaluated from the polarizing optical microscopic investigation of nematic liquid crystals in contact with polyimide surfaces which were suitably buffed in order to visualize the individual tracks of the buffing rayon fibers. The estimated magnitude of local stress is most likely to be far greater than the yield stress of the polymer film, thereby causing permanent molecular orientation along the buffing direction. The surfaces of poly(amic acid) and cured polyimide films, subjected to different buffing and cure profiles, have been examined by atomic force microscopy. The ability of these buffed polymer surfaces to align nematic and chiral smectic C (ferroelectric) liquid crystals were also investigated. These studies show that the presence of microgrooves on buffed polymer surfaces is not necessary for alignment of liquid crystals. The contribution of polymer molecules from the buffing fibers, deposited and oriented on the polyimide surfaces during buffing, can also be ruled out. Rather, it is concluded that the liquid-crystal alignment is mainly caused by the anisotropic interactions between liquid-crystal molecules and the polymer segments oriented through plastic deformation, with little contribution from frictional heating. For the alignment of smectics, both the degree of order and mechanical properties of polyimide films are found to be important factors.

Effect of Thermal Curing on Thermomechanical Properties of Polyimide Films Having Rodlike Molecular Skeleton Formed on a Silicon Substrate

Stresses in polyimide films coated on silicon wafers were measured in situ during the curing and cooling cycles. The films had a single rodlike molecular skeleton. Stress measurements were performed on films of uniform thickness under several different heating rates during the curing cycle. After the stress measurements, the cured polyimide films were examined by thermal mechanical analysis. The stress after the curing cycle increased with increasing heating rate during the curing cycle. Similarly, the thermal coefficient of expansion of the cured polyimide film increased with increasing heating rate. This phenomenom was thought to be caused by the decrease in the degree of in-plane orientation of the polyimide molecular chain with increased heating rate. The in-plane orientation was presumed to be influenced by the evaporation rate of the residual solvent.

Mechanisms Of Liquid Crystal Alignment On Buffed Polyimide Surfaces

AbstractThe surfaces of poly(amic acid) and cured polyimide films, subjected to different buffing and cure profiles, have been examined by atomic force microscopy (AFM). The ability of these buffed polymer surfaces to align nematic and chiral smectic C liquid crystals were also investigated. These studies show that the presence of microgrooves on buffed polymer surfaces are not necessary for alignment of liquid crystals. Rather, it is concluded that the liquid crystal alignment is mainly caused by the anisotropic intermolecular interactions between liquid crystal molecules and the polymer chains oriented by buffing. For the alignment of smectics, both the degree of order and mechanical properties of polyimide films are found to be important factors.

Non‐destructive characterization of polymide/copper and polyimide/gold interphases using surface‐enhanced Raman scattering and reflection–absorption infrared spectroscopy

AbstractThe molecular structure of interfaces formed by curing the polyamic acid of pyromellitic dianhydride (PMDA) and oxydianiline (ODA) against copper and gold substrates was determined using surface‐enhanced Raman scattering (SERS) and reflection‐absorption infrared spectroscopy (RAIR). It was found that acid groups of the polyamic acid reacted with metal ions on the substrates to form carboxylate salts when the polyamic acid was spin‐coated onto metal substrates. Formation of carboxylates on the copper surface extended several hundred angstroms into the polyamic acid films, while only trace levels of carboxylate were formed on the gold surface. Formation of carboxylate salts suppressed the curing of polyamic acid on copper substrates. However, the degree of curing of the polyamic acid was high on gold surfaces. These results were compared to our previous work concerned with the molecular structure of interfaces between PMDA/ODA polyimides and silver substrates. The extent of carboxylate formation on silver was intermediate between those on copper and gold. Results obtained from SERS and RAIR also indicated that cured polyimide films above the partially cured layers were oriented edge‐on, with the PMDA moieties perpendicular to the metal surface and the ODA moieties mostly parallel to the surface.

Ultraviolet absorption and photoconduction spectra of polyimide films fabricated at various curing temperatures

The absorption and photoconduction spectra in the ultraviolet region for PMDA-ODA polyimide films fabricated under various curing temperatures have been measured. There are four absorption peaks located at 3.65, 4.35, 5.65, and 6.40 eV in the fully cured polyimide films. The peak at 6.40 eV is practically independent of the curing temperature, but the other three peaks increase with increasing curing temperature. These four peaks are due to intra- and intermolecular optical transitions. In the photoconduction quantum efficiency spectra, there are also four peaks in the fully cured polyimide films but they are located at 3.55, 4.05, 4.90, and 5.80 eV. Analysis of the photoconduction quantum efficiency spectra yields values of 4.17 and 4.37 eV as the potential barrier heights for photoemission from Au and Ag electrodes to polyimide, respectively. The quantum efficiency increases with increasing applied electric field, following closely the Onsager theory with a thermalization separation 30 Å in the uv region. The primary yield increases with increasing curing temperature. All experimental results are briefly discussed.

Effect of precursor history on residual stress and relaxation behaviour of high temperature polyimides

Using a wafer bending technique, residual stress and its relaxation were investigated for four different kinds of high temperature polyimides: rodlike PMDA-PDA, semi-rigid BPDA-PDA, semi-flexible PMDA-ODA, and flexible BTDA-ODA. Residual stress was measured in situ on silicon wafers during thermal imidization of the polyimide precursors and subsequent cooling of the resulting polyimides as a function of temperature over the range of 25–400°C. The stress of the cured films at room temperature was significantly relaxed by the moisture uptake of the films rather than their creep behaviour. Both residual stress and moisture-induced stress relaxation in the cured polyimide films were strongly dependent upon the molecular nature (that is, molecular chain rigidity, degree of molecular orientation and packing), as well as the precursor origin. In addition, the diffusion coefficients of water in the cured polyimide films were estimated by best-fitting stress relaxation against time profiles.

Coating thickness effect on the orientation and thermal expansion coefficient of polyimide films

From X-ray diffraction experiments, it is observed that on-substrate cured polyimide films are so oriented that the imide chains are preferentially aligned in the plane direction of the films. For films with rigid-rod polyimide, film orientation is especially prominent and decreases significantly as the film thickness increases. The effect of coating thickness on the orientation and ordering of polyimide films is most pronounced in pure pyromellitic dianhydride- p-phenylenediamine (PMDA-PDA) films, slightly less in films with 50% pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) and 50% PMDA-PDA, and relatively insignificant in those with 100% PMDA-ODA. According to the C=O and C-N stretching absorption bands from Fourier-transform infra-red-attenuated total reflectance spectroscopic experiments, imide molecules located near the centre of the film exhibit poorer structure ordering than those near the surface. This reveals why the average film orientation decreases and the corresponding thermal expansion coefficient increases with increase of film thickness.

Effect of plasma treatment on adhesion of low thermal expansion polyimide films

AbstractTo fabricate a multilayer wiring using a low thermal expansion polyimide film, the adhesion of polyimide film on a thermally cured polyimide film must be secured. When the surface was plasma‐treated by O2‐CF4 = 1:1 gas, the adhesion was 30 times stronger (peel strength: at least 500 g/cm) than in the untreated case (peel strength: 15 g/cm). The film on the plasma‐treated surface maintained strong adhesion for long periods of time in a high‐temperature and high‐humidity environment. The cause of improved adhesion was investigated by evaluating the plasma‐treated polyimide surface by XPS and it was found that functional group, which seems to be COF, was formed.

Surface modifications of Kapton and cured polyimide films by ArF excimer laser: applications to imagewise wetting and metallization

Polymer surfaces of certain kinds, particularly polyimide, become highly hydrophilic upon irradiation by deep UV or UV laser in air. Unlike UV lamp light, laser exposure in air is extremely efficient in making polymer surfaces hydrophilic and wettable with aqueous solutions or similar liquids. The change in chemical nature of the polymer surfaces upon laser and deep UV irradiation is studied by contact-angle measurements in both advancing and receding modes. The results are compared with those of PPQ, PPES, polystyrene and PMMA. Under UV irradiation in air, polymer surfaces become highly hydrophilic, resulting in a decrease of the contact angles in both modes. Laser exposure in air makes the surfaces highly hydrophilic and wettable by both photo-induced reactions and debris/roughness formation. Water spray reduces the hysteresis by removing the debris, but the polymers still keep certain roughness and hydrophilicity on the remaining surfaces. Laser irradiation makes imagewise wetting possible with polyimide films at a low fluence. Direct imagewise metallization is also observed with polyimides and PPQ films. The laser-exposed areas cannot have electrically conducting metal films deposited on them while on unexposed surfaces uniform metal film deposition took place. Although the mechanism is still not clear, oxygen RIE is found to be possible in the exposed, non-conducting part, while galvanic plating was carried out on unexposed, conducting surfaces with a Au-Pd metallization.