Photosensitive Polyimide Research Articles

Novel Structure and Fabrication Process for Integrated RF Microelectromechanical-System Technology

This paper describes a novel structure and fabrication process for the integration of several types of RF microelectromechanical-system (MEMS) device, such as switches and varactors having different structures. It also describes an encapsulation technique suitable for the integrated devices to protect movable parts during packaging. An adaptable multilayer structure and its fabrication process, which includes planarization with photosensitive polyimide, are proposed for integration. A capsule structure fabricated using spin-coating film transfer and hot-pressing technology is also proposed for protection. Several types of RF MEMS device were simultaneously fabricated on the same substrate using these techniques. The results confirm that these techniques will pave the way for the development of single-chip RF transceivers with integrated RF MEMS devices.

A high‐performance positive‐working photosensitive polyimide: Effects of reactive end groups on the physical properties of the films

AbstractTo investigate the effects of reactive end‐cappers on the performance of PI precursors and the resulting polyimide (PI) films, poly(amic acid)s (PAAs) as a base polymer of the positive‐working photosensitive polyimides (PSPIs) were synthesized via ring‐opening polymerization of 4,4′‐oxydiphthalic anhydride and 4,4′‐oxydianiline with four different reactive end‐cappers [maleic anhydride, citraconic anhydride, 2,3‐dimethylmaleic anhydride (DMA), and 5‐norbornene‐2,3‐dicarboxylic anhydride (NDA)]. During imidization of these end‐capped PAAs to form the corresponding PI films, chain extension and crosslinking reactions of the respective end groups occurred, resulting in an improvement in the mechanical and thermal properties despite the low molecular weight of the precursors. However, the UV transmittance at ˜365 nm, an important property of PSPIs for thick‐film applications, such as stress buffer layers, was strongly influenced by the type of end‐capper used. These behaviors were understood in terms of the optimized geometries and the simulated UV–vis spectra of modeled end groups determined from density functional theory calculations. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:2180–2188, 2006

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 ( D 0) of 700 mJ/cm 2 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/cm 2 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.

Surface protection of oxide circuit by polyimide thin film

Photosensitive polyimide was selected as a protection layer for the YBa2Cu3O7−x circuits and stability of ramp-edge-junction characteristics against atmospheric moisture was investigated. Both coating and patterning of the protection layer could simultaneously be formed on the ramp-edge-junctions by using the polyimide film. Characteristic change of the ramp-edge-junctions that were coated with a polyimide film were less than 10% after thermal cycling more than ten times between room temperature and liquid He temperature. On the other hand, characteristic change was much more than 20% for the bare junction as narrow as 2μm.

Positive-type photosensitive polyimide based on a photobase generator containing oxime-urethane groups as a photosensitive compound

The chemical structure of a semi-aromatic polyimide-I, which was prepared by the chemical imidization of cyclopentanetetracarboxylic dianhydride and 2,2-bis(4-aminophenyl)hexafluoropropane, was characterized by13C-NMR spectroscopy. The chemically imidized polyimide-I was used for the preparation of a photosensitive polyimide (PSPI)|through the addition of benzophenone and benzophenone oxime hexamethylene diurethane (BOHD), a photobase generator containing oxime-urethane groups. The polyimide-I film containing benzophenone and BOHD was not soluble in 2.38 wt% tetrabutylammonium hydroxide solution in H2O. However, it became soluble following irradiation with 310 nm UV light. A positive tone image with a resolution of 5 μm was obtained with this PSPI, having sensitivity (Dc)|of 1.2 J/cm2 and contrast (γp)|of 1.08. Thus, a polyimide, which is not intrinsically photosensitive, can become photosensitive through the addition of a photobase generator containing oxime-urethane groups as a photosensitive compound.

Liquid crystal photoalignment by soluble photosensitive polyimide with methylene cinnamate side units

The photoalignment techniques and materials for liquid crystal display (LCD) have attracted considerable interests, and been studied extensively as an alternative method to the rubbing technique. The alignment of liquid crystals (LCs) is possible by using a rubbing-free process, photo-irradiation of linearly polarized UV light (LPUVL) on the photoreactive thin film surface. The photoinduced chemical anisotropy of the alignment surface is transferred to LC layers to cause homogeneous alignment of LC molecules. Photoalignment is also attractive because its photo-patterning capability can give it an extra potential value for the manufacture of next generation LCDs, such as large area, multidomain, vertically aligned, and/or in-plane switching (IPS) mode displays. Wide range of photoreactive materials have been investigated so far, which are mainly based on various photochemical reactions such as cis-trans conformational transition, photodimerization, and photo-degradation. During the last decade, there have been significant advances improving the performance of photoalignment materials by designing the molecular structure, and the alignment performance of the best photoalignment materials approaches that of the rubbed polyimide. However, as an organic thin film to be used in the LCD industry, more improvements in physical properties are required. These include solution processibility, thermal and photochemical stability, adhesion, and optical transparency in addition to a high photosensitivity and a superior aligning performance. Polymers containing photodimerizable chromophores, such as cinnamate, coumarin, and chalcone units, at the side groups or in the polymer main chain, have been investigated more extensively due to their advantageous properties than other photochemical systems. Unlike azobenzene-type cistrans isomerizable polymers, these are photochemically irreversible and chemically stable after UV irradiation. Also, these polymers are relatively more sensitive to UV light, requiring a much lower exposure dose to achieve a saturated alignment than the photodegradation-type polymers. Poly(vinyl cinnamate) derivatives are among the most studied photoalignment materials based on photodimerization. LPUVL irradiation on their film provides homogeneous alignment of LCs with significantly high photosensitivity. However, the main problem hindering the commercial application of these polymers is their poor thermal stability due to their high chain flexibility. The deterioration of aligning performance was observed when the LC cells were annealed at high temperature. Recently, there have been many attempts to improve the thermal stability by using various polymer backbones, such as polyesters, poly(arylene ether), and polyimides, with crosslinkable chromophores in the structure. Polyimide is the most attractive one owing to its verified properties as an alignment material in the LC industry. It has stable alignment ability, good adhesive strength, high moisture resistance, and dimensional stability. The LC alignment on a soluble PI film with cinnamate side units was reported to be thermally stable up to 200 C, as described by Ree et al. On the other hand, the photosensitivity of these polymers was usually compromised by their rigid main chain. A high exposure dose of 0.5 J/cm was required for the saturated alignment of LCs in these systems. Therefore, it is necessary to explore new photoalignment materials having higher photosensitivity and superior thermal stability. In this communication, we report a soluble fluorinated polyimide bearing methylene cinnamate side groups, PIMC (Figure 1), as a photoalignment material of excellent thermal

A novel negative photoinitiator-free photosensitive polyimide

A novel negative photoinitiator-free photosensitive polyimide (PFPS–PI) was synthesized through introducing the photosensitive 4,4-bis[(4-amino)thiophenyl] benzophenone (BATPB) into backbone chain and methyl acrylate group into side-chain of the polyimide, respectively. Photosensitive properties study revealed its good photolithographic properties, with a resolution about 5 μm and a sensitivity of 150 mJ/cm 2.

A novel positive‐type photosensitive polyimide based on soluble block copolyimide showing low dielectric constant with a low‐temperature curing process

AbstractA soluble block copolyimide (Bco‐PI) was prepared by direct one‐pot polycondensation of 2,2‐bis‐(3‐amino‐4‐hydroxyphenyl)hexafluoropropane, 2,2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, and cyclohexane‐tetracarboxylic dianhydride in the presence of γ‐valerolactone and a pyridine catalyst system using γ‐butyrolactone as the solvent. The thermal transmission temperature (Tg) of Bco‐PI was 282°C. By having the curing process occur at 250°C, which was below the Tg, colorless and transparent films could be obtained. The film showed excellent optical characteristics. Such properties could not be attained by the conventional high‐temperature thermal imidization process of poly(amic acid). The hydroxy groups in the polyimide backbone gave the Bco‐PI the potential to become alkaline. To the Bco‐PI was added 15 wt % ester of 2,3,4‐trihydroxybenzophenone with 1,2‐naphthoquinone‐(2) diazide‐5‐sulfonic acid (NT200) as the photoreactive compound. The system worked as a positive‐type photosensitive polyimide (PSPI). The sensitivity and contrast of the PSPI system were 220 mJ/cm2 and 1.27, respectively, when exposed to UV light, followed by development with a 5% tetramethylammonium hydroxide (TMAH) aqueous solution at room temperature. After curing at 250°C for 1 h, the average refractive index of Bco‐PI with and without NT200 was 1.5543 and 1.5563, and the optically estimated dielectric constant of the polyimides was 2.66 and 2.67, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4240–4246, 2006

SYNTHESIS AND CHARACTERIZATION OF SOLUBLE PHOTOSENSITIVE POLYIMIDES CONTAINING PHOSPHINE OXIDE MOIETIES

The soluble photosensitive polyimides were synthesized by the copolymerization of 3,3′-diaminochalcone (mDAC),bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine oxide (mDA6FPPO) and 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA).The polymers were characterized by FT-IR spectrum, and the properties of polymers were also discussed.The results show that the polyimides have excellent solubility in conventional organic solvents such as NMP,DMF,THF and acetone.The T_ gs of polyimides are ranged in 256℃～277℃.The polymers are stable up to over 400℃ and the residual weights at 800℃ are between 46%～50%.The resulting polyimides have good adhesive properties,and they were found to increase with increasing the mDA6FPPO content.The photosensitivity of the polymers was investigated by observations of UV-Vis spectral changes during UV irradiation.

Direct patterning of poly(amic acid) and low‐temperature imidization using a photo‐base generator

AbstractA negative‐type photosensitive polyimide (PSPI) based on poly(amic acid) (PAA) and a photo‐base generator (PBG) has been developed. PAA‐2, the polyimide precursor, was prepared from oxydianiline (ODA) and 4,4′‐biphthalic dianhydride (BPDA) in N,N‐dimethylacetamide (DMAc), and used directly for lithographic evaluations. Since a photo‐generated base, dimethyl piperidine (DMP), was effective for the imidization of PAA‐2 to the corresponding PI‐2 at temperatures up to 200°C, the dissolution behavior of this system was studied in relation to PBG content, bake‐temperature dependence, and composition of the aqueous alkaline developer. The PSPI using 85 wt% PAA‐2 and 15 wt% synthesized PBG showed a sensitivity of 220 mJ/cm2 and contrast of 11.7 when exposed to 365‐nm light (i‐line) and developed with an aqueous alkaline developer (aqueous tetramethylammonium hydroxide and iso‐propanol, TMAHaq/iPrOH). A clear negative image with 8‐µm features was produced by contact‐printing and converted into the PI‐2 pattern upon heating at 200°C, confirming the results obtained by Scanning electro microscopy (SEM) imaging and infrared spectroscopy. Copyright © 2006 John Wiley & Sons, Ltd.

Synthesis and characterization of a photosensitive polyimide precursor and its photocuring behavior for lithography applications

Photosensitive polyimide (PSPI) is of great interest, especially in microelectronic industry and potentially in the photonics and MEMS applications. Here we report on our studies for optimization of synthesis conditions for a PSPI which can be used in lithography. The direct polymerization method using phenyl phosphonic dichloride (PPD) as an activator was used to synthesize poly(amic ester) (PAE) from pyromellitic dianhydride (PMDA), 4,4′-oxydianiline (ODA) and 2-hydroxyethylmethacrylate (HEMA). Main parameters of the synthesis procedure were investigated by differential scanning calorimetry (DSC) analyses. From that, synthesis conditions were determined. Characteristics of the synthesized PAE and the imidized film were studied by the means of DSC, thermalgravimetric analyses (TGA) and Fourier transform infrared spectroscopy (FTIR). The glass transition temperature and the thermal stability of the imidized film were determined. Additionally, formulations of photo-PAE and the Irgacure 369 as photoinitiator were prepared and photocuring was tested by lithography. The results show that the images can be transferred to the PAE film by selective exposure.

Positive-Working Photosensitive Alkaline-Developable Polyimide Precursor Based on Semi-Alicyclic Poly(amide acid), Vinyl Ether Crosslinker, and a Photoacid Generator

A positive-working photosensitive polyimide based on semi-alicyclic poly(amide acid) (PAA), 1,1,1-tris(4-[2-(vinyloxy)ethoxy]phenyl)ethane (TVPE) as a crosslinking dissolution inhibitor and a photoacid generator has been developed. PAA was prepared from pyromellitic dianhydride and alicyclic diamine in N,N-dimethylacetamide (DMAc), which has excellent transparency above 365 nm and good solubility for 1 % sodium carbonate aqueous solution (aq.). The resist from the polymerization solution was formulated with photosensitive additives without isolation of PAA. A 23 μm thick film of the PSPI precursor system containing 25 wt% TVPE, 5 wt% PAG exhibited a sensitivity of 160 mJ/cm2 and contrast of 1.3 when it was exposed to broadband UV light followed by development in a 1 % sodium carbonate aq. at room temperature. A positive image featuring 40 μm line and space patterns was observed by the contact mode.

Mechanical Properties of Photosensitive Polyimide Film by Nanoindentation and Microtensile Test

A free-standing photosensitive polyimide film with thickness of 10µm is fabricated with the different curing temperatures using a micro fabrication process. The microtensile specimens of a strip type are made to facilitate a tensile testing. The Young’s modulus and yield strength of photosensitive polyimide film are measured with various strain rates from 10-4 /s to 10-3 /s by using a microtensile test. Also, the hardness and Young’s modulus of polyimide films are obtained from nanoindentation test. Finally, the mechanical properties measured from microtensile test are compared with those from nanoindentation test.

Preparation of a chemically amplified photosensitive polyimide based on norbornene‐end‐capped poly(amic acid ethoxymethylester)

AbstractA positive‐working chemically amplified photosensitive polyimide (PSPI) developable with basic aqueous solutions was obtained from poly(amic acid ethoxymethylester) (PAAE) as a polyimide precursor and diphenyliodonium 5‐hydroxynaphthalene‐1‐sulfonate (DINS) as a photoacid generator. The norbornene‐end‐capped PAAE based on 4,4′‐oxydiphthalic anhydride and 4,4′‐oxydianiline exhibited high transparency at 365 nm. The protection ratio of the ethoxymethyl groups was optimized to maximize the difference between the dissolution rates of the exposed and unexposed areas. The acid generated from DINS in the UV‐exposed region effectively deprotected the ethoxymethyl groups of PAAE by a chemical amplification mechanism. A 10‐μm‐thick film of the PSPI precursor system containing 16 wt % DINS exhibited a sensitivity (Do) of 1100 mJ cm−2 when developed with a 2.38 wt % aqueous tetramethyl ammonium hydroxide solution at room temperature. A fine, positive, 5‐μm line‐and‐space pattern was fabricated in a 15‐μm‐thick film with 1500 mJ cm−2 of UV exposure. This resolution is excellent in comparison with those previously reported for chemically amplified PSPIs, and such a film can thus be used as a buffer coating in semiconductor packaging. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5520–5528, 2005

Novel positive‐type photosensitive polyimide with low dielectric constant

AbstractA novel positive‐type photosensitive polyimide (PSPI) with a low dielectric constant was developed. The PSPI system was composed of soluble block PI (Bco‐PI) with a hydroxy group and diazonaphthoquinone as a photoreactive compound. The base Bco‐PI was prepared by a direct one‐pot polycondensation of 2,2‐bis(3‐amino‐4‐hydroxy‐phenylhexafluoropropane), 2,2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, and cyclohexanetetracarboxylic dianhydride in the presence of a γ‐valerolactone and pyridine catalyst system using N‐methyl‐2‐pyrrolidone as the solvent. The film of Bco‐PI was colorless and transparent, both important factors for a PSPI. Photosensitive soluble block PI (Bco‐PI), containing 20 wt % ester of 2,3,4‐trihydroxybenzophenone with 1,2‐naphthoquinone‐(2)‐diazide‐5‐sulfonic acid (NT200), showed a sensitivity of 350 mJ/cm2 and a contrast of 1.20 when it was exposed to UV light, followed by development with 5% tetramethylammonium hydroxide aqueous solution at room temperature. The estimated optical dielectric constants of the PIs with and without NT200 were 2.68 and 2.75, respectively. These values were significantly lower than those of conventional aromatic PIs, such as Kapton film (50EN). © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 15–21, 2005

Development of Photosensitive Porous Polyimide with Supercritical Carbondioxide (scCO<sub>2</sub>) Extraction

Development of Photosensitive Porous Polyimide with Supercritical Carbondioxide (scCO<sub>2</sub>) Extraction

Piezoactuator-integrated monolithic microstage with six degrees of freedom

In comparison with miniature XY-stages in term of driving methods: electrostatic, electromagnetic and piezoelectric actuators, the piezoelectric actuators have high area efficiency but no machinability by conventional microfabrication techniques. In this paper we have proposed the novel fabrication method of a double-layered piezo-stack actuator made from a monolithic PZT plate based on a planar fabrication method, and integrated the actuators into an XYZ-stage with six degrees of freedom, i.e., X, Y, Z, θ x , θ y and θ z directions. The stacked piezoactuators were formed on both sides of the PZT plate. The double stacked-piezoactuators, which are arranged around the stage, can stretch in plane or bend to Z-direction. The fabrication method includes dicing, electroplating, and laser machining. After making grooves by dicing, metal electroplating was performed and buried Ni into the grooves, which was done on both sides of the PZT plate. After polishing the surface, metal electrodes were formed on an insulating photosensitive polyimide layer. Finally, the complete structure was defined by laser machining. The test results show that the X-direction displacement of the actuation arms was about 2 μm when 40 V was applied to both of double-layered piezo-stacked actuators. The Z-direction displacement of the actuation beam was about 2 μm when 40 V was only applied to one stacked actuator. These results show the possibility of the microstage with six degrees of freedom.

Photosensitive polyimide/silica hybrid optical materials: Synthesis, properties, and patterning

In conventional ionic salt photosensitive polyimides, large volume shrinkage during imidization would be occurred due to eliminating pendant photosensitive moieties, such as 2-methyl acrylic acid 2-dimethylamino-ethyl ester (MDAE). In this study, the volume shrinkage of photosensitive poly(4,4′-(hexafluoroisopropylidenediphthalic anhydride)- co-oxydianiline) (6FDA-ODA)/MDAE was largely reduced by photocrosslinking MDAE with a coupling agent and the silica domain in the hybrid materials. The used coupling agents were 3-methacryloxypropyl trimethoxysilane (MPTMS) or (4-vinylphenethyl)trimethoxysilane (VPTMS). The coupling agent and the silica domain are designed primarily for reducing the volume shrinkage and enhancing the thermal properties, respectively. The retention of MDAE in the prepared hybrid films is supported by X-ray photoelectron spectroscopy (XPS) and thickness variation during curing process. The silica domain in the hybrid materials from TEM analysis was in the range of 10–50 nm, which was formed by the coupling agent and tetramethoxysilane. The silica domain significantly enhanced the thermal properties of the prepared hybrid films in comparison with parent fluorinated polyimide, including the glass transition temperature and coefficient of thermal expansion. The prepared hybrid materials also exhibited reduced refractive index and optical loss by increasing the silica. The SEM diagram suggested the prepared photosensitive hybrid materials could obtain lithographical patterns with a good resolution. These results indicate that the newly prepared photosensitive polyimide/silica hybrid materials may have potential applications for optical devices.

New Fabrication Process for Superconducting Quantum Interference Devices With Nb/AlAlOx/Nb Junctions by Using Photosensitive Polyimide Insulation Layers

We propose a new fabrication process for Superconducting Quantum Interference Devices (SQUIDs) with Nb/Al/AlOx/Nb junctions by using photosensitive polyimide insulation layers. The photosensitive polyimide, which is synthesized by multi-block copolymerization, can be formed into a thin layer with spin-coating, and patterned with conventional photolithography. Compared to forming a SiO/sub 2/ insulation layer with a sputtering and etching process, the fabrication is simplified and the coverage over the edge of the patterns can be improved due to planarization. SQUIDs with a flux transformer having two insulation layers were successfully fabricated with this process and were characterized. The critical current of the flux transformer was more than 20 mA, and a flux noise of less than 1/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/Hz was obtained in the white region. With an axial gradiometer in combination with this SQUID, a field resolution of 1.5 fT//spl radic/Hz was achieved.

X-Ray Detection Using Superconducting Tunnel Junctions With Polyimide Insulation Layer

We have fabricated Nb-based superconducting tunnel junctions (STJs) with Al trapping layers using the photosensitive polyimide insulation layer for X-ray detector. It is possible to simplify the fabrication process of the STJs, because the photosensitive polyimide is used as the insulation layer instead of conventional inorganic insulation films without an etching process in a vacuum chamber. The polyimide insulation layer expects to be stable for thermal cycling between 0.3 K and room temperature. The preliminary experiments of 5.9 keV radiation shows the energy resolution of 86 eV.