Free-standing Polyimide Films Research Articles

Enhancing the Toughness of Free-Standing Polyimide Films for Advanced Electronics Applications: A Study on the Impact of Film-Forming Processes.

High-quality and free-standing polyimide (PI) film with desirable mechanical properties and uniformity is in high demand due to its widespread applications in highly precise flexible and chip-integrated sensors. In this study, a free-standing PI film with high toughness was successfully prepared using a diamine monomer with ether linkages. The prepared PI films exhibited significantly superior mechanical properties compared to PI films of the same molecular structure, which can be attributed to the systematic exploration of the film-forming process. The exploration of the film-forming process includes the curing procedures, film-forming substrates, and annealing treatments. Additionally, the thickness uniformity and surface homogeneity of free-standing films were crucial for toughness. Increasing the crystallinity of the PI films by eliminating residual stress also contributed to their high strength. The results demonstrate that by adjusting the above-mentioned factors, the prepared PI films possess excellent mechanical properties, with tensile strength and elongation at break of 194.71 MPa and 130.13%, respectively.

New Approach for Fabricating of Deep Submicron Free-Standing Polyimide Thin Film

A new approach for manufacturing ultrathin free-standing polyimide film was proposed. This approach involves casting polyamic acid solution to a Al-doped zinc oxide film coated substrate sputtered at room temperature and to form polyamic acid film. The polyamic acid film was released from substrate in 0.5% diluted hydrochloric acid. Then polyamic acid was reattached to glass substrate and was transformed into polyimide film by thermal imidization with gradient heat treatment process. After that, thickness of the1000 nm polyimide film was reduced by KrF excimer laser ablation with 100 pulses at 45 mJ/cm2 and 30 pulses at 75 mJ/cm2, and 200 nm free-standing polyimide film was obtained. By this method, free-standing polyimide films of 1000 nm and 200 nm thickness were fabricated without and with excimer laser ablation reduction, respectively.

Photothermal Effect Induced Negative Photoconductivity and High Responsivity in Flexible Black Phosphorus Transistors.

This paper reports negative photoconductivity mechanism in flexible black phosphorus (BP) transistors built on freestanding polyimide film. Near-infrared laser (λ = 830 nm) excitation leads to significantly suppressed device on-state current with a very high responsivity of up to 53 A/W. The underlying mechanism of the negative photoconductivity is attributed to the strong photothermal effect induced by the low thermal conductivity of the polyimide substrate used. The heat generated by the infrared light illumination results in enhanced phonon scattering, reduced carrier mobility, and consequently negative photocurrent. Such a phenomenon was not observed in similar BP devices built on SiO2/Si substrates whose thermal conductivity is much higher. The above photothermal mechanism is also supported by temperature-dependent electrical characterization and device simulation. Such a flexible BP infrared photodetector with ultrahigh responsivity may find potential applications in future wearable and biointegrated imaging systems.

Ultrathin flexible dual band terahertz absorber

We propose an ultrathin and flexible dual band absorber operated at terahertz frequencies based on metamaterial. The metamaterial structure consists of periodical split ring resonators with two asymmetric gaps and a metallic ground plane, separated by a thin-flexible dielectric spacer. Particularly, the dielectric spacer is a free-standing polyimide film with thickness of 25µm, resulting in highly flexible for our absorber and making it promising for non-planar applications such as micro-bolometers and stealth aircraft. Experimental results show that the absorber has two resonant absorption frequencies (0.41THz and 0.75THz) with absorption rates 92.2% and 97.4%, respectively. The resonances at the absorption frequencies come from normal dipole resonance and high-order dipole resonance which is inaccessible in the symmetrical structure. Multiple reflection interference theory is used to analyze the mechanism of the absorber and the results are in good agreement with simulated and experimental results. Furthermore, the absorption properties are studied under various spacer thicknesses. This kind of metamaterial absorber is insensitive to polarization, has high absorption rates (over 90%) with wide incident angles range from 0° to 45° and the absorption rates are also above 90% when wrapping it to a curved surface.

Effects of mechanical stresses, thickness and atmosphere on aging of polyimide thin films at high temperature

This work deals with the isothermal aging mechanisms and their impact on the behaviour of polyimide thin films, usually used as dielectric material in high temperature electronic applications. The objectives were to achieve a better knowledge of the atmosphere and mechanical stress effects on the degradation mechanisms of deposited thin films during high temperature isothermal aging. Hence, BPDA/PDA aromatic polyimide (PI) thin films deposited on two different substrates (stainless steel and silicon) were aged slightly below their glass transition temperature (0.8 Tg) in air and in nitrogen atmospheres. During the aging, the dielectric strength, morphological characteristics (thickness, surface profile) and chemical variation were monitored for different initial film thicknesses (<10 μm). Results show that films aged under nitrogen and those deposited on silicon substrates aged in air do not show any remarkable degradation during the aging period. In contrast, the films deposited on stainless steel substrates present significant degradation during aging in air and the material lifetime is thickness dependent. In the latter conditions, the measured mean thickness losses for long aging periods are about 0.5 nm/h for all initial thicknesses, indicating that the degradation is not related to the bulk variation of the material but to a surface degradation mechanism. In addition, the breakdown field presents stabilization after an initial aging period, leading us to conclude that the bulk of the material is not affected by the aging. The coupled results indicate that the BPDA/PDA thermal degradation mechanism at temperature below its glass transition one is due to the attack on the outer film layer, related to the presence of oxygen in the atmosphere and strongly dependent on thermo-mechanical substrate properties. Hence, studying of the thermo-oxidative stability of freestanding polyimide films can lead to misestimated reliability.

Aliphatic Polyimides Derived from Polyalicyclic Monomers

Tetracarboxylic dianhydrides bearing a polyalicyclic structure, bicyclo[2.2.1]-heptane-2,3,5,6-tetracarboxylic 2,3:5,6-dianhydrides (BHDA), bicyclo[2.2.2]-octane-2,3,5,6-tetracarboxylic 2,3:5,6-dianhydrides (BODA), (4arH,8acH)decahydro-1t,4t:5c,8c-dimethanonaphthalene-2,3,6,7-tetracarboxylic 2,3:6,7-dianhydrides (DNDA), and bicyclo[2.2.1]heptane-2-exo,3-exo,5-exo-tricarboxyl-5-endo-acetic 2,3:5,5-dianhydride (BSDA) were synthesized and polycondensated with aromatic diamines or a polyalicyclic diamine, bis(aminomethyl)bicyclo[2.2.1]heptane (BBH), in DMAc, NMP, DMI or HMPA at room temperature or 60–105°C under nitrogen. The polymerization solutions gave the flexible and free-standing polyimide films after being cast on glass plates and then cured. The films were soluble in aprotic polar solvents such as DMAc and NMP at room temperature. Among the films, some dissolved in CHCl3 or 1,4-dioxane. The polyimides showed good thermal stability and the 5% weight loss temperatures in N2 were over 450°C. The polyimide films exhibited Tg values in the range of 280–400°C. The cutoff wavelengths of the non-aromatic polyimides such as PI(DNDAdx-BBH) and PI(BHDAdx-BBH) were shorter than 235 nm, and around 290–330 nm for the semi-aromatic polyimides. The transparencies in the visible region (400–780 nm) were over 85%. The flexible polyimide films possessed an average refractive index range of 1.522–1.617, and the optical anisotropies were 0.000–0.017. The dielectric constant ε values that were estimated from the refractive indices according to the modified Maxwell’s equation were 2.55–2.87.

Synthesis of Fully Alicyclic Polyimides from 2,5(6)-Bis(aminomethyl)bicyclo[2.2.1]heptane.

Bicyclo[2.2.1]heptane-2-exo, 3-exo, 5-exo-tricarboxyl-5-endo-acetic dianhydride (4a, BSDA) and its 5-endo-carboxyl-5-exo-acetic isomer (4b) with an alicyclic and spiro structure were prepared from a Diels-Alder adduct of itaconic anhydride with cyclopentadiene via three steps. Fully alicyclic polyimides were synthesized by polycondensations of an alicyclic diamine, 2, 5(6)-bis(aminomethyl)bicyclo[2.2.1]heptane (BBH) with four kinds of cycloaliphatic dianhydrides including BSDA in HMPA. Although inherent viscosities of the poly(amic acid) were somewhat low, the polymer solutions gave the free-standing polyimide films after curing. The films were soluble in organic polar solvents such as DMAc and NMP.

Colorless Full-alicyclic Polyimides with Low Dielectric Constant.

The fully alicyclic polyimides were prepared by a 2-step polycondensation of 2, 5(6)-bis(aminomethyl)bicyclo[2.2.1]heptane with cycloaliphatic dianhydrides such as bicyclo[2.2.1]heptane-2, 3, 5, 6-tetracarboxylic dianhydride. These polyimides possessed a 5% weight-loss temperature range of 422-459°C in N2, and 411- 416°C in air. The polyimides had the Tg's over 200°C, especially those of polyalicyclic polyimides were over 250°C. The free-standing polyimide films exhibited cutoffs at a wavelength shorter than 235nm, and are colorless. The averaged refractive indices were 1.52-1.54 and the optical anisotropies, birefringencies, were nearly zero. The dielectric constants that were evaluated from the refractive index were 2.55-2.73.

Infrared refractive index and extinction coefficient of polyimide films

We have measured the transmittance of several polyimide (C22H10N2O4) films at wave numbers from 6000 to 500 cm−1 (wavelengths from 1.67 to 20 μm) using a Fourier-transform infrared (FT-IR) spectrometer. The free-standing polyimide films are made by spin coating and thermal curing processes. The thickness of the films ranges from 0.1 to 4 μm. In the nonabsorbing region from 6000 to 4000 cm−1, the minimum transmittance caused by interference is used to obtain the refractive index for film thicknesses greater than 1 μm. The film thicknesses are determined by fitting the spectral transmittance using the refractive index. Molecular absorption strongly reduces the transmittance at wave numbers from 2000 to 500 cm−1. The optical constants, i.e., the refractive index and the extinction coefficient, are determined from the measured transmittance for several films of different thickness using a least-squares method. A Lorentzian oscillator model is also developed, which in general agrees well with the measured transmittance at wave numbers from 6000 to 500 cm−1. This study will facilitate the application of polyimide films in the fabrication of infrared filters and other optoelectronic applications. The methods presented in this paper can be used to determine the optical constants of other types of thin-film materials.

Atomic force microscopy images of ion-implanted 6FDA-pMDA polyimide films

The surface structure and morphology of ion-beam irradiated 6FDA-pMDA films were investigated using atomic force microscopy (AFM). A beam of 140 keV N + ions with a low current density was used in this work. Three irradiation fluences (2 x 10 14 /cm 2 , 1 x 10 15 /cm 2 , and 5 x 10 15 /cm 2 ) were chosen to represent three different regimes of ion-beam influence on material properties based upon previous diffusion and gas permeation results of implanted polyimide films. Detailed roughness and bearing analyses of the AFM images indicate that freestanding polyimide films have deep surface valleys which can extend to a depth of several micrometers. Ion-beam irradiation, even at a small dose, alters the microstructure of the surface layer and forms a modified layer which eliminates the initial deep valleys. The AFM analysis shows that small fluence irradiation induced microvoids in the surface layer of the polymer, and high fluence irradiation resulted in a large number of small-size microvoids in the surface. All of these results agree well with the ion-beam irradiation effects on iodine diffusion and gas permeation properties of the polyimides. A ripple topographical structure with a wavelength of 25 μm and an amplitude of 2 nm was also observed for irradiated samples.

Laser release process to obtain freestanding multilayer metal-polyimide circuits

Some applications in microelectronics call for freestanding polyimide films with fine metallic wiring patterns that are thinner than commercially available copper-clad polyimide sheets, which are typically greater than 25 µm in thickness. This work describes a laser-assisted technique to fabricate freestanding multilayer thin-film wiring with polyimide dielectric insulating layers that are less than 10 µm thick. A release layer consisting of a thin polymeric film is first deposited on an optically transparent carrier and the multilayer thin-film structure is then fabricated on this substrate, with the polymeric release layer sandwiched between the transparent carrier and the multilayer structure. Excimer laser light passes through the transparent carrier and ablates the polymeric layer at the transparent carrier/polymer interface, resulting in separation of the sacrificial carrier from the multilayer structure. The optimal release process is carried out using a 308-nm XeCl excimer laser operati ng at a fluence of about 100 mJ/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>

Characterization of 6FDA-APBP polyimide films through impulsive stimulated thermal scattering

The elastic and thermal properties of 4,4′-(hexafluoroisopropylidene)-bis (phthalic anhydride)-4,4′-bis (4-aminophenoxy) biphenyl (6FDA-APBP) free-standing polyimide films (2–10 μm) were investigated using impulsive stimulated thermal scattering (ISTS), a non-contact optical method for thin film characterization. Shear and longitudinal velocities, Young's modulus, Poisson's ratio and the in-plane thermal diffusivity are reported.

Optical constants of gold in the soft-x-ray region from reflection and transmission measurements

The optical constants of gold deposited on 300-nm-thick freestanding polyimide films have been measured in the energy range of 40-1350 eV. The optical constants of the polyimide films were known from a previous investigation. By performing both transmission measurements with Kramers-Kronig analysis and multiangle reflection measurements with analysis by fitting to a Fresnel multilayer model including roughness, one could carry out a detailed error analysis. In the energy range above 500 eV systematic deviations between the results obtained by both methods were found. For gold films the discrepancies can be attributed to deviations from Beckmann-type behavior at small grazing-incidence angles.