Layers Of Polyimide Films Research Articles

Distributed salinity sensor based on optical frequency domain reflectometry with polyimide coated single mode fiber

A distributed optical fiber sensor for salinity sensing is proposed and analyzed. The sensor uses a single mode fiber coated with polyimide and is based on optical frequency domain reflectometry. By applying the polyimide solution to the surface of the fiber, a layer of polyimide film is formed by heating. As the polyimide coating is very sensitive to changes in the salinity of the external solution, the polyimide film will expand or shrink as the external concentration changes. This converts the value of salt concentration into the frequency shift in the spectrum through cross-correlation. To characterize the sensor, four polyimide-coated sensors with different average coating thicknesses of 126 µm, 170 µm, 192 µm and 249 µm were fabricated. The sensitivities of -21.71 GHz/(mol/L), -28.55 GHz/(mol/L), -29.97 GHz/(mol/L) and -42.78 GHz/(mol/L) were achieved when the salinity was measured from 0 mol/L to 3.09 mol/L, respectively. The sensitivity and response time of polyimide fibers with different diameters were measured. The minimum salinity measurement uncertainty of the sensor is 0.05 mol/L. The sensor has high sensitivity and has promising applications in observing ocean parameters and ion chemical sensing.

Anomalous Mobility Improvement in Ultra-Low-Temperature Polycrystalline-Silicon Thin-Film Transistors on Flexible Substrate after Laser Lift-Off Process

This work investigates an improvement in anomalous mobility in high-performance ultra-low-temperature polycrystalline-silicon (ULTPS) thin-film transistors (TFTs) on flexible substrate after laser lift-off process. The experiment results reveal that the improvement was observed in p-type TFTs but not observed in n-type ones. It was contributed to the boron atom has more kinetic energy than phosphorous one during laser lift-off process. Consequently, the effect of slight boron movement is to reduce the effective p-TFT channel length.Electronic devices fabricated on flexible substrates have drawn a lot of attention to recent researches in these years. Transistors on flexible substrates were the hearts of flexible technologies. Many literatures reported the implementing methods including carbon-nano-tubes transistors, amorphous silicon TFTs, amorphous IGZO TFTs, and etc.. Some of them suffered from either high temperature process or poor transistor characteristics. And the most important, some of them suffered from single polarity transistors, such only n-type transistors. A very attractive technology for flexible electronics was using ULTPS TFTs fabricated direct on flexible substrate on carrier glass [1]. It could not only realize large-size manufacturing process but also is compatible with current CMOS LTPS TFTs circuits. Since the ULTPS TFTs on flexible substrates needs to be lift-off from carrier glass, the electrical characteristics between before and after lift-off process should be studied. However, there were lacks of study of anomalous mobility improvement in p-channel ULTPS TFTs after laser lift-off process. In this experiment, the complementary ULTPS TFTs, i.e. n- and p- channel devices, were both fabricated on flexible material coated on carrier glass then lift-off by laser irradiation. The brief fabrication process was as followings. First, the carrier glass was coated by a layer of polyimide film. After curing, the complementary TFTs were fabricated under ultra-low temperature process by fine tune the recipes. Finally, to implement flexible electronic applications, the ULTPS TFTs were lift-off from the carrier glass by virtue of excimer laser irradiation. The laser irradiation energy densities were varied to investigate the lift-off process effects on the electrical performance of ULTPS TFTs, as shown in Fig1(a)-(c). One sample, named high-energy laser lift-off (HELLO), was irradiated at 320 mJ/cm2. The other sample, named low-energy laser lift off (LELLO), was irradiated at 300 mJ/cm2. And the device dimension was W/L=20um/4.5um for both n- and p- channel TFTs. As fabricated, the HELLO TFTs have been demonstrated the field-effect electron and hole mobility of 82 and 85 cm2/V-s, respectively, while the corresponding characteristics were 78 and 86 cm2/V-s for the LELLO TFTs. Both of them had the similar performance on carrier glass. After laser lift-off process, it was observed that the electron mobility were 81 and 77 cm2/V-s for HELLO and LELLO devices, accordingly. The hole mobility for LELLO TFTs was 88 cm2/V-s. In the contrast, that for HELLO TFTs was improved to 92 cm2/V-s. It could be seen that the electron mobility was not improved for both HELLO and LELLO devices. And no improvement in hole mobility was observed in LELLO devices, but on the contrary an anomalous hole mobility improvement was found in HELLO devices. In the meanwhile, no significant changes were observed in the other electrical parameters, such as threshold voltage and subthreshold swing[2]. The anomalous field-effect hole mobility improvement may be resulted from the reduction of interface trap density state and the shrinkage of channel. Since the subthreshold swing reflected the interface trap density state between gate insulator and channel and no changes were found, that interpreted that the interesting improvement was not caused by the reduction of interface trap density. On the other hand, since only the HELLO p-channel exhibited the interesting anomalous improvement, it is believed that the boron atom is lighter than phosphorous and diffused during laser lift-off process. Therefore, the boron diffusion occurred only when the laser lift-off energy density. Consequently, the source and drain dopants, namely boron, were slight diffusing into the channel to reduce effective channel length. In conclusion, we investigated the anomalous improvement in p-channel ULTPS TFTs after the high-energy laser-lift-off process. The hole mobility was improved from 85cm2/V-s to 92cm2/V-s. It is believed that these anomalous phenomena may be contributed to boron diffusion caused reduction of channel length. The ULTSP TFTs are promising to future flexible electronic circuits. Reference [1] ISDRS 2009, December 9-11, 2009, College Park, MD, USA [2] IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 11, NOVEMBER 2010 Figure 1

Design of the New SC Bus-Line System for the EAST

A new external superconducting (SC) bus-line system consisting of 13 pairs of bus-lines has been designed for the upgrade project of the experimental advanced SC tokamak. The old system integrated into two vacuum ducts will be extended approx. 23 m with new cable in conduit conductors (CICCs) connecting by twin-box lap joints. The leakage currents of the CICCs electrically insulated with four layers of polyimide films and pre-impregnation fiberglass tapes are <10 μA under a 10 kV electrical potential at 77 K, 0.1–100 Pa in helium atmosphere. Each bus-line pair is bound together by 50 % overwrapped with three layers of Kevlar polyester tapes to eliminate its adverse effects of electromagnetic forces with a maximum of 2.44 kN/m. The bus-lines designed with at least four bends are flexible to absorb the thermal shrinkage which is approx. 90 mm in total. The maximum shift of the CICCs is 26.1 mm when cooled down. The total thermal loads to 5 K components in bus-line group 1 and 2 are <0.3 W/m as required.

Transferable and flexible thin film devices for engineering applications

Thin film devices can be of significance for manufacturing, energy conversion systems, solid state electronics, wireless applications, etc. However, these thin film sensors/devices are normally fabricated on rigid silicon substrates, thus neither flexible nor transferrable for engineering applications. This paper reports an innovative approach to transfer polyimide (PI) embedded thin film devices, which were fabricated on glass, to thin metal foils. Thin film thermocouples (TFTCs) were fabricated on a thin PI film, which was spin coated and cured on a glass substrate. Another layer of PI film was then spin coated again on TFTC/PI and cured to obtain the embedded TFTCs. Assisted by oxygen plasma surface coarsening of the PI film on the glass substrate, the PI embedded TFTC was successfully transferred from the glass substrate to a flexible copper foil. To demonstrate the functionality of the flexible embedded thin film sensors, they were transferred to the sonotrode tip of an ultrasonic metal welding machine for in situ process monitoring. The dynamic temperatures near the sonotrode tip were effectively measured under various ultrasonic vibration amplitudes. This technique of transferring polymer embedded electronic devices onto metal foils yield great potentials for numerous engineering applications.

Effect of thermal expansion coefficient on the stress distribution in solar panel

Residual thermal stress, which affects structural safety, would be produced in the solar panel under the temperature field because of the multilayer structure of the solar panel. The stress distribution and the effect of thermal expansion coefficient on the stress of the solar panel under the temperature field using analytic method were studied. The analytical results showed that the maximum of structural tensile/compression stress and shear stress present in the layer of polyimide film and silicone rubber which bonded carbon fiber composite and polyimide film, respectively. The maximal stress value in the structure was decreased remarkably by choosing to use polyimide film with lower thermal expansion coefficient. The one dimensional model used in this work would exert an important influence on the design and structural optimization of the solar panel.

Passive microelectrode arrays for recording of neural signals: A simplified fabrication process

A new process for the fabrication of microelectrode arrays is developed that is fully compatible with standard integrated circuit fabrication schedules. The process involves two innovations: the use of initially thin silicon substrates (50 μm), and the use of sputtered SiO2 as a plasma etching mask. The finished electrode array consists of a very thin silicon substrate (&amp;lt;20 μm) that provides a mechanically stable carrier for the electrodes. The electrode metal conductors are sandwiched between two layers of polyimide films that adhere firmly to the substrate. Small electrode interface and contact windows are made with lithographically limited minimum feature sizes. Dry plasma etching is employed both to open the windows and to thin the substrate. For the former, we have determined the appropriate etching conditions and thicknesses necessary for SiO2 to be a viable mask in this step. For the latter step, the electrode profile is first etched to the final desired substrate thickness from the front (with the same oxide as the mask), and the wafer is then turned over and fully etched until the electrodes separate. Using initially thin wafers simplifies considerably this final etch.