Polyimide substrate textured by copper-seeding technique for enhanced light absorption in flexible black silicon

we studied the director configuration in thin liquid crystalline layers of 5 CB using a null ellipsometry technique. Liquid crystal (LC) material was spin- coated on polyimide (PI) substrates. The influence of different treatment of PI substrates on he director configuration was investigated. The PI substrates without additional treatment provide homeotropic LC alignment. The UV irradiated substrates provide the formation of biaxial (fan-like) LC structure that is symmetric with respect to the film normal. The rubbed PI substrates provide the splayed optic axis distribution with the average tilt angle of 25° with respect to the film normal.

The fabrication of a flexible electronic system which consists of a flexible polyimide (PI) substrate and a rigid silicon thin chip is demonstrated. The 50 μm thick rigid silicon chips are bonded to the PI substrate using a die attach film (20 μm) and electrically interconnected to an adjacent metal pad which is deposited on the PI substrate and silicon chips. A 3D step covered interconnection consisted of silver nanoparticles and polymer are achieved by using an electrohydrodynamic system. The mechanical and electrical characteristic was investigated by performing a Kelvin resistance measurement on the jetted lines while the substrate was subjected to bending modes of varying diameters.

The aim of this research is to develop epoxy based adhesive for screen printing on aluminum (Al) and polyimide (PI) substrates in flexible printed circuit (FPC) manufacturing. Two types of aromatic epoxy resins are evaluated in this work including diglycidyl ether of bisphenol-A (DGEBA) and epoxy phenolic novolacs (EPN). The two epoxy resins will also be blended with aliphatic epoxy (PPGDE) for adhesive property enhancement purpose. These epoxy systems can be cured at 170oC for 15 min using metaphenylene diamine (m-PDA) as a curing agent. The compositions of the aromatics epoxies were varied in a range of 50 to 90wt%. From the results, it was found that peel strength of EPN/PPGDE blends was higher than peel strength obtained from DGEBA/PPGDE systems. The maximum peel strength of EPN/PPGDE blends was measured to be 0.561 N/mm at 50/50 mass ratio. These blends were also found to show outstanding adhesive bonding with the aluminum substrate. In addition, it was also found that when the EPN/PPGDE blend at 50/50 mass ratio was modified with 5 phr of CTBN rubber, the resulting adhesive exhibited a good bonding with polyimide substrate providing maximum peel strength of about 0.706 N/mm. The above results also suggest difference in bonding mechanisms of the two adhesive layers and the Al or PI substrates. Consequently, two adhesive layers of epoxy systems with the first layer of EPN/PPGDE blend was applied on Al substrate whereas the CTBN modified EPN/PPGDE blend, a second adhesive layer, which provides a good adhesion on PI film was casted on top of the EPN/PPGDE layer. The obtained peel strength of this two-layered epoxy blends was found to be as high as 0.813 N/mm meeting the industrial requirement. These two-layered epoxy adhesive systems are a good candidate for adhesive bonding of Al and PI substrates through screening printing process.

Chemistry and physics of solid surfaces VI: Edited by R. Vanselow and R. Howe. Springer-Verlag, New York, 1986. $83.00

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Device fabrication was carried out on both rigid silicon and flexible polyimide substrate, and I-V performance for both devices was measured. The optimized LLO process for the whole GaN LED film transfer would be applicable in flexible LED applications without compromising electrical properties.

1. Introduction The resistive random access memory (ReRAM), which is a nonvolatile memory device, on flexible plastic substrate have been widely investigated for future wearable device. The flexible ReRAM based on metal-insulator-metal (MIM) structure is generally fabricated at relatively low temperature (~250°C) from the viewpoint of heat resistance of plastics. Numerous active layers, including Al2O3-TiO2, Al2O3-HfO2, and Al2O3-Ta2O5 bilayers have been studied in a aim of improving resistive swithing reliability. In our previous paper, we found that Pt/Al2O3/TiO2/Pt capacitor fabricated on Si substrate by atomic layer deposition (ALD) and a post-deposition annealing (PDA) process at 200°C, exhibited bipolar resistive swithing charactreistics with a high resistive state (HRS) and a low resisitive state (LRS) in a small switching voltage of ±0.8V.In this paper, we fabricated Pt/Al2O3/TiO2/Pt capacitors on polyimide (PI) substrate by ALD and PDA process at 200°C and examined reliabilities of resistive swithing properties under tensile stress. 2. Experiment MIM capacitors with Al2O3/TiO2 active layer were prepared as follows. First, a 200-nm-thick SiO2 layer as gas barrier was deposited on PI substrate. Pt bottom electrode (BE-Pt) was deposited on PI/SiO2 layer by DC sputtering. A 5-nm-thick Al2O3 layer was deposited on BE-Pt by ALD at 200°C using Al(CH3)3 precursor and H2O gas and subsquently PDA was carried out at 200°C in N2. Next, a 60-nm-thick TiO2 layer was deposited on Al2O3 layer by ALD at same temperature, using Ti[N(CH3)2]4 precursor and H2O gas and PDA was performed at 200°C in O2. Finally, Pt top electrode (TE-Pt) was fabricated on the TiO2 layer by DC sputtering. The area of the TE-Pt was approximately 1.5 ´ 10-4 cm-2. 3. Results and discussion Each of Al2O3 and TiO2 layer of the flexible Pt/Al2O3/TiO2/Pt capacitor on PI had an amorphous structure because no spotty electron diffraction pattern was observed by TEM and EDS measurement. There were no severe chemical reactions in any of the layers.Fig 1 shows the J-V characterestics of the flexible Pt/Al2O3/TiO2/Pt capacitor. The first forming process was performed to occur the irreversible breakdown of the Al2O3 layer when applied voltage was increased to + 7V. The second forming process was the creation of a TiO2-x layer due to the oxygen vacancies (Vo) near the TE-Pt and TiO2 layer when voltage was applied -3V. The set process (switching from the HRS to the LRS) caused at an applied voltage of about +1V and the reset process (switching from the LRS to the HRS) occurred at about -1V. These set and reset processes appeared due to the Vo transfer into TiO2 layer when voltage applied in positive and negative directions. Stable bipolar switching properies between the HRS and the LRS was obtained until the number of swithing cycles of 20. We also observed resistive swithing characteristics of about 2 orders of magnitude between the HRS and the LRS under tensile stress condition. 4. Summary We conclude that flexible Pt/Al2O3/TiO2/Pt capacitors on PI exhibited stable bipolar switching characteristics of the HRS and LRS after two forming steps of breaking Al2O3 layer and creating a TiO2-x layer due to the Vo. Furthermore, a similar resistive switching property was obtained regardless of tensile stress. Acknowledgement This work was partially supported by JSPS KAKENHI Grant Number of JP20H02189. The authors thank all staff members of the Nanofabrication group of the National Institute for Materials Science (NIMS) for their support. Figure 1

Corrosion testing of anisotropic conductive adhesive interconnections on FR4, liquid crystal polymer and polyimide substrates

Electrical and optical properties of SnO2:Sb films prepared on polyimide substrate by r.f. bias sputtering

Flexible transparent conductive films (TCFs) are used in a variety of optoelectronic devices. However, their use is limited due to poor thermostability. We report hybrid TCFs incorporation in both aluminum-doped zinc oxide (AZO) and silver nanowires (AgNWs). The layered AZO/AgNWs/AZO structure was deposited onto a transparent polyimide (PI) substrate and displayed excellent thermostability. When heated to 250 °C for 1 h, the change in resistivity (Rc) was less than 10% (Rc of pure AgNW film > 500) while retaining good photoelectric properties (Rsh = 8.6 Ohm/sq and T = 74.4%). Layering the AgNW network between AZO films decreased the surface roughness (Rrms < 8 nm) and enhances the mechanical flexibility of the hybrid films. The combination of these characteristics makes the hybrid film an excellent candidate for substrates of novel flexible optoelectronic devices which require high-temperature processing.

The aim of this work is to study the effect of Na on low temperature growth of CIGS solar cells on polyimide (PI) substrates. The Na is introduced via evaporation of a NaF precursor layer prior to the absorber deposition. As reported previously, the devices at nominal Tsub,max = 500 °C are characterized by a higher Voc when the Na content increases. However, lowering process temperature to nominally 450 °C together with high Na concentration produces a pronounced band gap grading through the absorber layer, as detected by energy dispersive X‐ray spectroscopy (EDX). The absorber layers and the devices were studied by grazing incidence X‐ray diffraction, external quantum efficiency and drive level capacitance profiling. A limit Tsub,max is identified as detrimental on the PV performance when high Na concentration is added, mainly due to a reduced Voc. 12.2% of efficiency (active area) is achieved reducing the Na concentration for the lower Tsub,max. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Counterpoise-assisted annealing effects on enhanced photoluminescence and electrical properties of sol–gel-derived ZnO thin films grown on polyimide substrates

Solution-based processes are emerging in the fabrication of flexible electronics owing to their cost-effectiveness, low-temperature processing capabilities, and vacuum-free operations. Currently, printing techniques, as the most widely used solution-based processes, suffer from low resolution and poor pattern fidelity. Although surface energy-directed assembly (SEDA) process enables unparalleledresolution and fidelity, multilayer fabrication and applicationon flexible substrates are rarely attempted. Here, a SEDA process is used for fabricating metal oxide thin film transistors (TFTs) on ultrathin and flexible polyimide (PI) substrates with a thickness of ≈35µm. Comprehensive procedures to render PI substrates hydrophobic as well as to homogenize the hydrophobicity of the PI substrates and as-assembled layers are developed. All-solution-processed flexible TFTs are constructed by assembling indium oxide semiconducting channels, indium tin oxide source/drain electrodes, aluminum oxide gate dielectric layers, and indium tin oxide gate electrodes ordinally. The metal oxide TFTs exhibit excellent electrical properties with an average mobility of 22.01 cm2V-1 s-1 and stable operation under mechanical strain. Flexible inverters with a high voltage gain of 78 and a high dynamic operation frequency of up to 1kHz are also constructed, representing new opportunities of the SEDA process for the fabrication of next-generation flexible electronics.

Micro-scale twistable organic field effect transistors and complementary inverters fabricated by orthogonal photolithography on flexible polyimide substrate

Phase transition behavior and electrical properties of C/Sb2Te3 multilayer films on flexible polyimide substrates

The use of evaporated chromium as an intermediate layer to provide adhesion between polyimide (PI) insulator materials and other metals has numerous applications throughout first-level microelectronics packaging. In this work, PI substrates are considered to determine microprocess environments during electron beam evaporated deposition of Cr, with possible ramifications for the integrity of the Cr/PI interfaces. Auger electron spectroscopy (AES) analysis-in-depth was used to evaluate electron beam evaporated chromium thin films (1000 Å) both as-deposited and after a simulated assembly process in which the Cr/PI structures were heated to 150 °C. The AES results indicated that the effects of water outgassing from the PI substrate during deposition were the dominant influence on oxidation of chromium as opposed to subsequent thermal processing. It was found that the degree of water outgassing, and thus the extent of incorporation of O into growing Cr films, was determined by both the cumulative thickness of the PI underlayers and the thermal processing of these PI structures prior to deposition, all other things remaining the same. The greatest amount of oxygen [∼20% Atomic Concentration (AC)] incorporated throughout the layer of Cr was for a substrate which contained multiple levels of PI having a total thickness of 21 μm which was patterned with interconnecting metallurgy. A ∼4X decrease in both the number of wiring levels and total PI thickness resulted in a reduction of oxygen in the bulk Cr to just detectable levels. The final cure temperature of the PI for these structures was 350 °C. An increase in the final cure temperature to 400 °C prior to deposition of Cr was also found to reduce the amount of O; for the same thickness of PI, the level of oxygen in the Cr decreased from 20% to ∼15% AC. The effects of a postdeposition thermal anneal in nitrogen ambient on Cr oxidation was assessed for all types of structures; there was no measurable increase in chromium oxidation found for any of the samples after they were annealed at 150 °C.