Spin-coated Polyimide Research Articles

Development of Localized Plasma Etching System for Failure Analyses in Semiconductor Devices: (3)Etching-Monitoring Using Quadrupole Mass Spectrometry

Quadrupole mass spectrometry (QMS) has been applied to monitor the etching processes in a localized plasma etching system. An inward plasma was employed for etching in which the etching gas was discharged in the narrow gap between the etched sample and the entrance of an evacuating capillary tube. As the etching products are immediately evacuated through the capillary, a QMS system equipped at the capillary exit is able to analyze the products without any loss in concentration via diffusion into the chamber. Two kinds of samples, thermally grown SiO2 on Si and spin-coated polyimide film on Si, were etched, and the chemical species in the evacuated etching gas were analyzed with QMS, which enables monitoring of the composition of the surface being etched. Samples of thermal SiO2 were etched with CF4 plasma. The peak height of the SiF3+ signal during the SiO2 etching was lower than that observed during etching of the silicon substrate, leading to endpoint detection. The endpoint detection of the polyimide film etching was conducted using two etching gases: pure O2 and pure CF4. When O2 was used, the endpoint was detected by the decrease of the mass peak attributed to CO. When CF4 was employed, the plasma was able to etch both the polyimide film and Si substrate. Then the endpoint was detected by the increase of the mass peak of SiF3+ produced by the etching of the Si substrate.

Efficient Direct Electron Transfer with Enzyme on a Nanostructured Carbon Film Fabricated with a Maskless Top-Down UV/Ozone Process

We have developed a new carbon film electrode material with thornlike surface nanostructures to realize efficient direct electron transfer (DET) with enzymes, which is very important for various enzyme biosensors and for anodes or cathodes used in biofuel cells. The nanostructures were fabricated using UV/ozone treatment without a mask, and the obtained nanostructures were typically 2-3.5 nm high as confirmed by atomic force microscopy measurements. X-ray photoelectron spectroscopy and transmission electron microscopy revealed that these nanostructures could be formed by employing significantly different etching rates depending on nanometer-order differences in the local sp(3) content of the nanocarbon film, which we fabricated with the electron cyclotron resonance sputtering method. These structures could not be realized using other carbon films such as boron-doped diamond, glassy carbon, pyrolyzed polymers based on spin-coated polyimide or vacuum-deposited phthalocyanine films, or diamond-like carbon films because those carbon films have relatively homogeneous structures or micrometer-order crystalline structures. With physically adsorbed bilirubin oxidase on the nanostructured carbon surface, the DET catalytic current amplification was 30 times greater than that obtained with the original carbon film with a flat surface. This efficient DET of an enzyme could not be achieved by changing the hydrophilicity of the flat carbon surface, suggesting that DET was accelerated by the formation of nanostructures with a hydrophilic surface. Efficient DET was also observed using cytochrome c.

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.

Ultra-thin crystalline silicon films produced by plasma assisted epitaxial growth on silicon wafers and their transfer to foreign substrates

We have developed a new process to produce ultra-thin crystalline silicon films with thicknesses in the range of 0.1 − 1 μ m on flexible substrates. A crystalline silicon wafer was cleaned by SiF4 plasma exposure and without breaking vacuum, an epitaxial film was grown from SiF4 , H2 and Ar gas mixtures at low substrate temperature (T sub ≈ 200 °C) in a standard RF PECVD reactor. We found that H2 dilution is a key parameter for the growth of high quality epitaxial films and modification of the structural composition of the interface with the c-Si wafer, allowing one to switch from a smooth interface at low hydrogen flow rates to a fragile one, composed of hydrogen-rich micro-cavities, at high hydrogen flow rates. This feature can be advantageously used to separate the epitaxial film from the crystalline Si wafer. As a example demonstration, we show that by depositing a metal film followed by a spin-coated polyimide layer and applying a moderate thermal treatment to the stack, the fragile interface breaks down and allows one to obtain an ultrathin crystalline wafer on the flexible polyimide support.

Fabrication of Flexible TFTs on a Heat-Resistant Transparent Polymer using the Standard a-Si TFT Process above 300ºC

A method for fabricating thin-film transistors (TFTs) on flexible substrates using a spin-coated heat-resistant transparent polyimide analog polymer, polybenzoxazole (PBO), was developed. Since this material is heat-resistant up to 350ºC, it can withstand the conventional above-300ºC a-Si TFT process. With XeCl excimer laser exposure, the PBO layer with a-Si TFTs could be easily separated from the glass substrate without any damage to the TFTs.

Low-Temperature Annealing Combined with Laser Crystallization for Polycrystalline Silicon TFTs on Polymeric Substrate

The formation of polycrystalline layers on flexible plastic substrates, through plasma enhanced chemical vapor deposition and excimer laser annealing, is investigated. Combining low-temperature annealing with laser dehydrogenation/crystallization produces good-quality polycrystalline silicon with a reduced shot density. By using optimal crystallization conditions it is possible to achieve a superlateral growth crystallization regime, with a grain size up to , and void-free material, as confirmed by the presented structural analysis. The beneficial effect of the low-temperature thermal annealing has been related to the removal of nonbound hydrogen, as supported by the elastic recoil detection analysis and IR analysis of the samples. To validate the process, we fabricated non-self-aligned polysilicon thin-film transistors (TFTs) directly on spin-coated polyimide substrates, with a maximum processing temperature of and with a relatively low shot density ( shots/point). The TFTs presented good electrical characteristics with an on/off ratio , a field-effect mobility up to , and a threshold voltage of . These results confirmed that the developed crystallization process is suitable to fabricate polysilicon TFTs on polymeric substrates, allowing an increased process throughput.

A novel fabrication process of MEMS devices on polyimide flexible substrates

Micro-electro-mechanic-system (MEMS) devices on flexible substrate are important for non-planar and non-rigid surface applications. In this paper, a novel and cost-effective fabrication process for an 8 × 8 MEMS temperature sensor array with a lateral dimension of 2.5 mm × 5.5 mm on a polyimide flexible substrate is developed. A 40 μm thick polyimide substrate is formed on a rigid silicon wafer using as a mechanical carrier throughout the fabrication by four successive spin coating liquid polyimide. The arrayed temperature sensing elements made of 1200 Å sputtered platinum thin film on polyimide substrate show excellent linearity with a temperature coefficient of resistance of 0.0028/°C. The purposed sensor obtains a high sensitivity of 0.781 Ω/°C at 8 mA at constant drive current. Because of the low heat capacity and excellent thermal isolation, the temperature sensing element shows excellent high sensitivity and a fast thermal response. The finished devices are flexible enough to be folded and twisted achieving any desired shape and form. Employing spin-coated liquid polyimide substrate instead of solid polyimide sheet minimizes the thermal cycling as well as improves the production yield. This fabrication technique first introduces the spin-coated PDMS (Polydimethylsiloxane) interlayer between the silicon carrier and the polyimide substrate and makes the polyimide-based devices separate much easier and greatly simplifies the fabrication process with a high production yield. A non-successive two-stage cure procedure for the polyimide precursor is developed to meet low-temperature requirement of the PDMS interlayer. The fabrication procedure developed in this research is compatible with conventional MEMS technology through an optimized integration process. The novel flexible MEMS technology can benefit the development of other new flexible polyimide-based devices.

Ni–Cr thin film resistor fabrication for GaAs monolithic microwave integrated circuits

Different Ni–Cr alloys were sputter-deposited on silicon nitride-coated GaAs substrates and covered with a spin-coated polyimide layer to develop thin film metal resistors for GaAs monolithic microwave integrated circuits (MMICs). The contact to the resistors was made through vias in the polyimide layer by sputter-deposited Ti/Au interconnect metal. The variation of contact resistance, sheet resistance ( R S) and temperature coefficient of resistance (TCR) of the Ni–Cr resistors with fabrication process parameters such as polyimide curing thermal cycles and surface treatment given to the wafer prior to interconnect metal deposition has been studied. The Ni–Cr thin film resistors exhibited lower R S and higher TCR compared to the as-deposited Ni–Cr film that was not subjected to thermal cycles involved in the MMIC fabrication process. The change in resistivity and TCR values of Ni–Cr films during the MMIC fabrication process was found to be dependent on the Ni–Cr alloy composition.

Micro-scale metallization of high aspect-ratio Cu and Au lines on flexible polyimide substrate by electroplating using SU-8 photoresist mask

In order to fabricate flexible microelectronic devices, fabrication of metallization lines and metal electrodes on the flexible substrate is essential. Cu lines are often used as interconnect lines in electronic devices and Au as microelectrodes in organic transistors and bioelectronics devices due to its good electrochemical stability and biocompatibility. For minimizing the size of device, the realization of metallization lines and microelectrodes with the scale of a few micrometers on the flexible substrate is very important. In this work, micro-scale metallization lines of Cu and Au were fabricated on the flexible polyimide (PI) substrate by electroplating using the patterned mask of a negative-tone SU-8 photoresist. Surface of PI substrate was treated by O 2 inductively coupled plasma for improvement of the adhesion strength between Cr layer and PI and in situ sputter-deposition of 100-nm thick Cu seed layers on the sputter-deposited 50-nm thick Cr adhesion layer was followed. Electroplating of high aspect-ratio Cu and Au lines using a sulfuric acid and a noncyanide solution with the patterned SU-8 mask, respectively, removal of SU-8, and selective wet etch of Cr adhesion and Cu seed layers were carried out. Micro-scale Au electrode lines were successfully fabricated on the PI substrate. Micro-scale gap-filled Cu lines with spin-coated polyimide on the PI substrate with the thickness of 6–12 μm and the aspect ratio of 1–3 were successfully fabricated.

Microbolometers on a Flexible Substrate for Infrared Detection

Uncooled semiconducting YBaCuO infrared microbolometers have been fabricated on a flexible polyimide substrate formed by spin-coating a silicon wafer with a release layer. The wafer was used as a carrier for the flexible substrate during fabrication. The finished microbolometers on the flexible substrate showed a temperature coefficient of resistance (TCR) TCR =(1/R)(dR/dT) of -3.03% K/sup -1/, at room temperature, which is comparable to the TCR values observed for semiconducting YBaCuO microbolometers fabricated directly on Si. In order to provide protection and better mechanical integrity, some of the devices were encapsulated. The microbolometers attained a responsivity and detectivity as high as 3.5/spl times/10/sup 3/ V/W and 1/spl times/10/sup 7/ cm/spl middot/Hz/sup 1/2//W, respectively, at 2.88 /spl mu/A of current bias. The responsivity and detectivity of the encapsulated microbolometers, on the other hand, were 1.6/spl times/10/sup 3/ V/W and 4.9/spl times/10/sup 6/ cm/spl middot/Hz/sup 1/2//W, respectively at 1 /spl mu/A of current bias. Spin-coated liquid polyimide solved two major problems previously encountered with the solid polyimide sheets when used as a flexible substrate. First, flatness of the flexible substrate was maintained with no air bubbles. Second, the thermal expansion of the flexible substrate during the fabrication process due to thermal cycling was minimal. All measurements reported in this paper, were taken prior to releasing the flexible substrate from the Si wafer containing the finished microbolometers.

Organic thin-film transistors with ODPA–ODA polyimide as a gate insulator through vapor deposition polymerization

A new process for polymeric gate insulator in field-effect transistors was proposed. Fourier transform infrared absorption spectra were measured in order to identify ODPA–ODA polyimide. Its breakdown field and electrical conductivity were measured. Organic thin-film transistors with a stacked–inverted top-contact structure was fabricated to demonstrate that thermally evaporated polyimide films could be used as a gate insulator. As a result, the transistor performance with evaporated polyimide was similar with spin-coated polyimide. It seems that the mass-productive in-situ solution-free processes for organic thin-film transistors are possible by using the proposed method without breaking vacuum.

Single-electron transport in metal/polyimide:C 60/metal junction

Metal/organic film/metal (MIM) junctions were fabricated using C 60 dispersed in spin-coated polyimide (PI) ultra-thin film. The current–voltage ( I– V) characteristics of these new types of junctions exhibit a Coulomb staircase behavior at a temperature of 15.5 K, a behavior not observed with C 60-free MIM junctions. It is obvious that the Coulomb staircase behavior observed was induced by C 60 acting as a central electrode of the single-electron tunneling junction. We then analyzed the I– V characteristics of this MIM junction, taking into account both the Coulomb charging energy and the discrete energy states of the molecular central electrode. Comparing the experimental ( V/ I)d I/d V– V characteristics with the calculated ones, a possible energy structure for C 60 in the junction was estimated.

Nonlinear Phenomena in the Mass Loss of Polyimide Films under Hyperthermal Atomic Oxygen Beam Exposure

Erosion phenomenon of polyimide film under the hyperthermal atomic oxygen beam exposure, which is a simulated low Earth orbit space environment, has been investigated. The polyimide film was spin-coated on a sensor crystal of a quartz crystal microbalance, and the mass of the film was measured under the atomic oxygen beam exposure. The spin-coated polyimide film which was exposed to a 4.7 eV atomic oxygen beam showed a mass gain at the beginning of the reaction and then steady-state mass loss followed. The experimental results of the mass change was analysed by the computational model, and the results showed that the carbon abstraction rate at the oxygen-adsorbed sites was two orders higher than that at the unoxidized polyimide surface. The computational results suggested that a large fraction of the carbon abstraction reaction occurred in the oxygen-adsorbed site through a Langmuir–Hinshelwood reaction mechanism.

Studies of dielectric characteristics and surface energies of spin‐coated polyimide films

AbstractA two‐step method, that is, polyamic acid formation with subsequent curing, was used to synthesize six kinds of polyimides. Dielectric constants and surface energies were investigated to determine the nature of the fluorinated and nonfluorinated polyimides. The dielectric constant decreased from 3.3 (at 100 kHz) for PMDA/ODA to 2.6 (at 100 kHz) for 6FDA/4,4′‐6F when the fluorine content increased from 0 to 30.7 wt %. Simultaneously, the water contact angle increased from 65° for PMDA/ODA to 78° for 6FDA/4,4′‐6F. Experimental results indicated that fluorinated polyimides contained a lower dielectric constant with improved water resistance. The surface energy values obtained from experiments agreed well with Holmes' correlation between surface energy and dielectric constant. The surface energies and dielectric constants were significantly affected by the polymer backbone structures, especially by the fluorination effect. Therefore, by choosing the appropriate monomers, polyimides of low dielectric constants with hydrophobic surfaces could be obtained. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1642–1652, 2001

Impact of molecular orientation on thermal conduction in spin-coated polyimide films

Polymer films are serving as passive regions in fast logic circuits and as active regions in organic optoelectronic devices, such as light-emitting diodes. Recent data illustrated the strong anisotropy in the thermal conductivity of polyimide films of thickness near 1 μm, with the in-plane value larger by a factor of approximately six. This manuscript extends previous theoretical work on heat conduction in stretched bulk polymers to model the conductivity anisotropy in spin-coated polymer films. Predictions are based on the standard deviation of the angle of molecular orientation with respect to the film in-plane direction, which can be investigated using birefringence data, and the expected conductivity anisotropy in a material with perfectly aligned chains. The modeling and previous data indicate that the anisotropy factor could increase to a value larger than 10 for polyimide films much thinner than 1 μm.

Measurement of the thermal conductivity anisotropy in polyimide films

Polymer films are playing an important role in the development of micromachined sensors and actuators, fast logic circuits, and organic optoelectronic devices. The thermal properties of polyimide films govern the temporal response of many micromachined thermomechanical actuators, such as ciliary arrays. This work develops three experimental techniques for measuring both the in-plane and the out-of-plane thermal conductivities of spin-coated polyimide films with thicknesses between 0.5 and 2.5 /spl mu/m, which are common in MEMS. Two of the techniques use transient electrical heating and thermometry in micromachined structures to isolate the in-plane and out-of-plane components. These techniques establish confidence in a third, simpler technique, which measures both components independently and uses IC-compatible processing. The data illustrate the anisotropy in the thermal conductivity of the polyimide films investigated here, with the in-plane conductivity larger by a factor between four and eight depending on film thickness and temperature. The anisotropy diminishes the time constants of thermal actuators made from polyimide films.

低フラックス原子状酸素ビームとポリイミド薄膜との表面反応 ２ 紫外線の同時照射効果

The synergistic effect of atomic oxygen and ultraviolet rays on the spin-coated polyimide film is investigated. The X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy suggest that the formation of carbonyl group at the film surface occurs during atomic oxygen exposures. The mass loss of the polyimide film and the enhancement carbon dioxide formation are observed during simultaneous irradiation of the low flux atomic oxygen and ultraviolet rays. These experimental results are explained by the photo-assisted chemical reaction of carbonyl groups formed by atomicoxygen attack. The fact that the synergistic effect of atomic oxygen and ultraviolet rays is obvious on the polyimide film, which is known as the material showing no synergistic effect in low Earth orbit, may reveal the importance of relative intensities of atomic oxygen and ultraviolet rays on the ground-based synergistic testing of the materials.

Effect of molecular orientation on the dielectric properties of spin‐coated polyimide films

AbstractThe effect of molecular orientation on the dielectric properties of spin‐coated polymide films has been studied in Situ for pyromellitic dianhydride with 4,4′‐oxydianiline (PMDA/ODA) and biphenyldianhydride with p‐phenylendiamine (BPDA/PPD). The degree of molecular orientation is characterized by the optical anisotropy between the in‐plane and the through‐plane, refractive indices. The through‐plane dielectric properties are measured by fabricating parallel‐plate capacitors directly onto the silicon substrate. Both the birefringence and the dielectric constant of PMDA/ODA polyimide vary with film thickness; however, these properties are independent of film thickness for BPDA/PPD films. To confirm that the measured dielectric constant obtained from the parallel‐plate structures is free from a significant edge effect, finite element modeling of the electrstatic potential within the dielectric edge effect, finite element modeling of the electrostatic potential within the dielectric is performed. Models for both isotropic and anisotropic dielectric properties indicated that the fringing effects are indeed negligible for the film thicknesses examined. Thus, the changes observed in the measured dielectric constant are attributed to the variations in the molecular orientation. The optical anisotropy observed for the polyimides suggests a corresponding dielectric anisotropy in the films. An estimation using the Maxwell relation indicates that the dielectric anisotropy at 105 Hz is 0.31 for PMDA/ODA and 0.85 for BPDA/PPD. © John Wiley & Sons, Inc.

Transmission of polyimide during pulsed ultraviolet laser irradiation

Experiments that demonstrate quantitatively the importance of laser absorption dynamics for ultraviolet laser ablation of organic materials are presented. Laser pulse transmission measurements have been performed on 0.1 μm spin-coated polyimide films at three ultraviolet wavelengths (193 nm, 248 nm, and 355 nm) over the fluence range 10−3 −10 J/cm2. Target transmission is observed to increase with increasing fluence by a factor of ∼5 at 193 nm, and a factor of ∼10 at 248 nm. In contrast, transmission decreases by approximately one half during 355 nm target irradiation. These results are analyzed theoretically with a two-level model of chromophore absorption. This theory is also applied to reported pulsed UV-laser polyimide ablation data. It is shown that an accurate description of the fluence-dependent film absorption leads to a prediction of the etch depth versus pulse fluence relationship in good agreement with experimental data.

Laser Induced Periodic Structures on Polymer Surfaces

Abstract Subhalf micron (0.2 nm) space and amplitude linear periodic structures and an array of dot images are obtained with the Nd:YAG laser irradiation on Kapton polyimide films, poly(ethylene terephthalate) films, spin-coated polyimide films and others. Different from the excimer laser irradiation, which requires a polarizer, our solid state Nd:YAG laser source provides polarized beams without a polarizer with advantage over excimer laser irradiation. AFM and SEM studies have been carried out. XPS studies of laser exposed areas indicate no significant chemical reactions took place on exposed areas.