Thin Polyimide Layers Research Articles

High Performance Polymer Thin Film Transistors Array Printed on a Flexible Polycarbonate Substrate

ABSTRACTHigh performance poly (3-hexylthiophene) (P3HT) thin film transistors (TFTs) array was fabricated on a polycarbonate substrate by micro-contact printing method. A thin polyimide layer (40 nm) was applied before silicon oxide deposition to improve the electrical properties of the TFT device. Also, the effects of O2 plasma treatment on the field effect mobility and output current behaviors of the devices were investigated. By plasma treatment, the surface roughness of gate dielectric was improved which accounts for the increased field effect mobility and the hole Schottky barrier height in electrode/semiconductor interface was lowered resulting in large drain current in the device. Based on the experiments, we fabricated P3HT TFTs array with 0.025 cm2/V·s in saturation field effect mobility and on/off current ratio of 103 ∼ 104 on a polycarbonate substrate.

Accelerated plasticization of thin-film composite membranes used in gas separation

Permeation experiments with He, N2, O2 and CO2, have been carried out with double layer composite membranes consisting of a silicone rubber support layer and a thin polyimide layer determining the permeation properties. The estimated thickness of the polyimide layer in the composite membranes was between 1.5 and 4 microns. This paper describes the phenomenon of accelerated plasticization of such thin polyimide layers used in gas separation membranes. The pressure-normalized fluxes were determined at 5 bars for N2, O2 and in the range of 1?8 bars in the case of He and CO2. Helium permeation decreased with increasing feed pressure and no hysteresis behavior was found for successive increasing and decreasing feed pressure steps. For CO2, the pressure-normalized flux did not follow the typical behavior of glassy polymers but increased continuously with increasing feed pressure. Also, CO2 permeation showed a clear hysteresis effects resulting in an increasing permeability with time and an elevated magnitude in successive decreasing feed pressure steps. Hence, stronger plasticization effects in the composite membranes must be concluded in comparison with thick single film membranes. The plasticization effects are significantly pronounced as the polyimide concentration in the solution from which the membranes are cast become more dilute. The latter is interpreted as accelerated plasticization with decreasing film thickness.

Static charge reducible liquid crystal alignment layer based on PAAc doped PMDA-ODA/polyaniline blend

We have studied liquid crystal alignment on the thin layer of polyimide derived from pyromellitic dianhydride and oxydianiline (PMDA-ODA) and polyaniline (PANi) which can be used as a static charge reducible alignment layer. The static charge arising from rubbing was reduced by introduction of poly(acrylic acid) (PAAc) into PMDA-ODA/PANi blend due to the increase of doping level and the high dielectric constant of PAAc.

Azimuthal Anchoring Energy and Pretilt angle of Photo-aligned Nematic Liquid Crystals on a Polyimide and Poly(vinyl cinnamate) Blend Alignment Layer

The azimuthal anchoring energy and pretilt angle of a nematic liquid crystal (E7) which was aligned on a thin layer of polyimide (PI) and poly(vinyl cinnamate) (PVCi) blend by the irradiation of polarized UV on the surface, were studied. Within a PVCi content of 10–100 wt%, the azimuthal anchoring energy remains unchanged with the decrease of the PVCi content in the blend, which is attributed to the preferential occupation of the surface by cinnamate chromophore. The blend alignment layer revealed a significantly enhanced thermal stability of azimuthal anchoring in comparison with the pure PVCi based alignment layer on account of the stiffening of the layer by the incorporated PI, which caused low thermal relaxation of the structural anisotropy developed from PVCi in the blend. The addition of PI into PVCi was also found to be effective for improving the thermal stability of the pretilt angle.

Measurement of Interfacial Fracture Toughness Under Combined Mechanical and Thermal Stresses

The interfacial fracture resistance of a polyimide passivation/underfill interface was measured using an asymmetric double cantilever beam technique. A thin layer of polyimide was coated on a standard epoxy as one of the beams and an underfill was flowed over the polyimide film and cured to form the second beam. The relative contributions of the thermal and mechanical stress were systematically varied by varying the thickness ratio of the two beams. The results indicate that the effect of thermal residual stress must be taken into account in the calculation of critical strain energy release rate to obtain the interfacial fracture energy.

Design studies on piezoresistive humidity sensors

The present paper summarizes a detailed design study of a novel piezoresistive humidity sensor, the operation of which relies on the humidity-induced swelling of thin polyimide layers. The purpose of this study was to gain a better understanding of the general metrological behavior of the sensors, with a systematic optimization of the device structure in mind. The design study is based on newly developed analytical models for the offset and full-scale output parameters of the sensor.

Thin polyimide layers—preparation and properties

Two types of vacuum deposited polyimide layers differing in their mode of preparation—deposition of the monomer vapours on rotary and linearly moving substrates—have been investigated. The monomers were PMDA and ODA evaporated from two Knudsen sources on to soda lime glass substrates. The deposits were transformed to polyimide by heating in air. It was established that there was a strong influence of the dynamic state of the substrate during layers formation on the cross-section of the samples. The growth of deposits on rotary substrates were characterized by a higher microhardness and an increased (25%) stability during reactive ion etching in an oxygen atmosphere. It was experimentally shown that the microhardness depended on the conditions of thermal treating for imidization of the deposited layers. The properties of the two types of layers are discussed on the basis of the principle differences in condensation of the monomers.

GaAs versus quartz FGC lines for MMIC applications

The performance of finite-ground coplanar (FGC) lines on GaAs and quartz for high-frequency monolithic-microwave integrated-circuit (MMIC) applications is experimentally investigated in this paper. The FGC lines on GaAs are covered with a thin layer of polyimide for passivation purposes. Permittivity and attenuation characteristics for these lines up to 118 GHz are presented and compared with the corresponding characteristics of FGC lines on a quartz substrate, which are commonly used for millimeter-wave applications. The impact of different characteristic impedance values in attenuation properties for both GaAs and quartz is also addressed. Results indicate that the loss on the lines does not depend on the substrate material, but rather on line geometry. All the lines tested show low-loss and low-dispersion characteristics.

Highly integrated three-dimensional MMIC technology applied to novel masterslice GaAs- and Si-MMICs

A novel three-dimensional (3-D) masterslice monolithic microwave integrated circuit (MMIC) is presented that significantly reduces turnaround time and cost for multifunction MMIC production. This MMIC incorporates an artificial ground metal for effective selection of master array elements on the wafer surface, resulting in various MMIC implementations on a master-arrayed footprint in association with thin polyimide and metal layers over it. Additionally, the 3-D miniature circuit components of less than 0.4 mm/sup 2/ in size provide a very high integration level. To clearly show the advantages, a 20-GHz-band receiver MMIC was implemented on a master array with 6/spl times/3 array units including a total of 36 MESFETs in a 1.78/spl times/1.78 mm area. Details of the miniature circuit components and the design, closely related to the fabrication process, are also presented. The receiver MMIC exhibited a 19-dB conversion gain with an associated 6.5-dB noise figure from 17 to 24 GHz and an integration level four times higher than conventional planar MMICs. This technology promises about a 90% cost reduction for MMIC because it can be similarly applied to large-scale Si wafers with the aid of an artificial ground.

Humidity-dependent mechanical properties of polyimide films and their use for IC-compatible humidity sensors

Thin polyimide layers have been investigated to determine their volume expansion as a result of humidity absorption and desorption, respectively. Furthermore, we have used the experiments in order to clarify the physical and chemical processes of polyimide films in a wet environment as well as their humido-mechanical properties. It can be shown that the humidity-induced material extension is fairly linear in a wide range of humidity. The observed material behaviour strongly depends on the conditions of the polyimide fabrication process. Analogous to the well-known temperature extension coefficient, a humidity extension value of about 60 to 80 ppm/%RH is observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bimetal elements influenced by temperature.

Humidity-Induced Volume-Expansion of Polyimide Films

Thin polyimide layers were investigated to determine their volume expansion as a result of humidity absorption and desorption, respectively. Furthermore, we used the experiments in order to clarify the physical and chemical processes of polyimide films in wet environment as well as their humido-mechanical properties. It could be shown that the humidity-induced material extension is fairly linear in a wide range of humidity. The observed material behaviour strongly depends on the conditions of the polyimide fabrication process. Analogous to the well-known temperature extension coefficient, a humidity extension value of about 60 to 90 ppm/%RH was observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bimetal elements influenced by temperature.

Optical properties induced by the formation of carbon nanoparticles in irradiated polyimide

Abstract Ultra thin polyimide layers have been prepared by the Langmuir Blodgett method and bombarded with a beam of 170 keV Ne+. Optical absorption and specular reflectance studies have been performed in the spectral domain 2.5–6.5 eV. The energy loss function is deduced and shows the development of a new absorption band at 3 eV, which accounts for the shift of the absorption edge towards the visible in thick films. An effective medium calculation shows that this resonance can be attributed to graphitic nanoparticles dispersed in the polymeric matrix. The development of these conducting domains accounts for the transition to a conducting regime at high irradiation fluence.

Development of mustrip gas chambers with multi-chip technology

We have developed the mustrip gas chamber (MSGC) by using multi-chip technology which supports high-density assembly of bare LSI chips on a silicon wafer. Our MSGC was operated steadily with ∼ 103 gain more than one week. An energy resolution of 15% for 5.9 keV X-ray of a 55Fe source was obtained. With a very thin polyimide substrate of 16 μm thickness, two interesting phenomena were observed; one was a strong dependence of gains on backplane potential, and the other was little time variation of gains. We also found that a guard mask of a thin polyimide layer on the cathode edge reduced incidental electrical discharges between anode and cathode strips.

Effects of aging on polyimide: A study of bulk and interface chemistry

AbstractWe are investigating the bulk and interfacial chemistry of polyimide (PI) exposed to heat and relative humidity (RH) stress (85°C/85% RH) over long periods to understand the long‐term reliability aspects of PI for microelectronic applications. The PI surface chemistry and the PI–Si interface chemistry was followed with electron spectroscopy for chemical analysis (ESCA). For exposure times greater than 550 h, the PI surface exhibits significant changes in emission structure and atomic concentration. The PI–Si interface indicates little change in the PI chemistry when compared to control samples. The water‐contact angle at the PI surface shows no statistically significant change as a function of exposure time. Bulk and surface chemistry were followed with Fourier transform infrared spectroscopy (FTIR) in a grazing incidence reflection configuration. There were measurable differences in FTIR spectra between aged and control samples only for the case of very thin PI layers. These results suggest that for exposure times of less than 1100 h, chemical changes in PI occur primarily at the PI–air interface for samples 1 μm thick and greater. These changes represent a possible rearrangement of the imide structure and may be a form of deimidization of the polymer, but the final state is not polyamic acid. The PI surface chemistry changes observed with ESCA are reversible upon reheating to the cure temperature.

Large anisotropy in optical properties of thin polyimide films of poly( p-phenylene biphenyltetracarboximide)

Thin films ranging from 400 nm to 4 μm thickness of poly( p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by the optical waveguide spectroscopy. These polyimide films, most prominent for potential applications as the interlevel dielectrics in multilevel interconnect technologies owing to their low coefficients of thermal expansion and excellent thermal/mechanical properties, are found to exhibit an extraordinarily large anisotropy in the refractive index, with the measured in-plane refractive index n∥≂1.852 and the out-of-plane value n⊥≂1.612 at 632.8 nm wavelength, nearly independent of the film thickness. This large optical anisotropy indicates a very strong preference of polymer chains to orient along the film surface, and suggests a considerably larger (by ca. 27%) dielectric constant in the film plane than that along the film thickness. Moreover, there is some evidence for the existence of a very thin polyimide layer of slightly lower density and higher anisotropy adjacent to the substrate.