Thick Polyimide Foil Research Articles

Shadow masks as an alternative method to lithography for the structuring of thin film layers on LTCC substrates

The combination of low temperature cofired ceramic multilayer technology with thin film deposition methods enables new functional principles by expanding the available portfolio of materials. Low-stress integration of micromechanical chips can be achieved, for example, by using reactive multilayers as local heat sources, which can be deposited directly onto the ceramics. Shadow masks are ideal for structuring these multilayers. The technology has the advantage that the layers are patterned without the use of etching chemicals and no residues of masking layers or photoresist remain on the non-coated areas. Flexible polyimide used to cover the non-coated areas can adapt to the surface unevenness of the ceramic. The polymer has excellent temperature stability and is compatible with vacuum coating processes. It is available in various thickness gradations and can be easily structured by laser cutting. The accuracy of the mask fabrication by means of laser cut is studied in this work. Structures with a line width of 30 μm can be precisely cut into 75 μm thick polyimide foils. Mask and chip are mechanically aligned, thus a positioning accuracy of 70 μm and better when using the outer edge of mask and chip for alignment is achievable. Major influences of the laser process on the precision of the mask and the resulting transfer fidelity to the ceramic surface are discussed. The method is suitable for reliably reproducing layers with structure sizes from 30 μm with a pitch of 150 μm.

Wireless System for In Situ Monitoring of Moisture Ingress in PV Modules

Recently, a new in situ measurement technique has been developed using miniature digital relative humidity (RI!) sensors. The measurement technique proved to be a valuable tool not only for in situ monitoring of water concentration in the photovoltaic (PV) modules exposed to accelerated test conditions in climatic chambers but also for long-term outdoor monitoring under field conditions. In this paper, we present a completely new in situ setup, which is based on a wireless radio-frequency identification (RFID) technology. In our design, up to seven digital humidity sensors can be soldered on a 130 μm thick polyimide foil that also integrates RFID antenna and all required readout electronics. Their ultra-small size and wireless design allows placing them anywhere in the PV module either in front or at the back side of the solar cells. All measurement data can be read by a dedicated RFID reader which also provides the power needed during the measurement and communication cycle. The systems were applied in mini-modules with one full-size crystalline silicon solar cell and exposed to the elevated temperature and humidity conditions (40 °C / 85 %RI! and 50 °C / 70 %RI!). The acquired moisture ingress data agree well with the two-dimensional simulations using the realistic dimensions and diffusion coefficient of ethylene-vinyl acetate.

The stopping power and energy straggling of the energetic C and O ions in polyimide

The stopping power and energy straggling of 12Cn+ and 16On+ heavy ions in the energy range 5.3–8.0MeV in 8μm thick polyimide (PI) foil were measured by means of an indirect transmission method using a half-covered a PIPS detector. Ions scattered from thin gold layer, under the scattering angle 150° were detected and the spectrum of ions penetrating the PI foil and without foil was recorded. The values of the experimentally determined stopping powers were compared to the calculated data by SRIM-2013 and MSTAR codes. Measured data were in good agreement with data calculated by SRIM-2013, especially for C ions was observed better agreement than for O ions. The energy straggling was determined and compared to those calculated by using Bohr’s, Bethe–Livingston and Yang models. The measured energy straggling values in the PI foil was corrected for foil roughness and thickness inhomogeneity determined from AFM. Bethe–Livingston predicting formula has been modified to make it appropriate for thicker targets. The energy straggling determined in our experiment was obtained higher than Bohr’s predicted value; the predictions by Yang are in good agreement with our experiment. Bethe–Livingston formulation of the energy straggling shows better agreement with the experimental data after the modified formula implementation which assumes that the thick target was consisted to be composed of n-number of thin layers. Influence of the charge-exchange phenomena to the energy straggling of C and O ions in PI was discussed.

Light Weight and Flexible High‐Performance Diagnostic Platform

A flexible diagnostic platform is realized and its performance is demonstrated for early detection of avian influenza virus (AIV) subtype H1N1 DNA sequences. The key component of the platform is high-performance biosensors based on high output currents and low power dissipation Si nanowire field effect transistors (SiNW-FETs) fabricated on flexible 100 μm thick polyimide foils. The devices on a polymeric support are about ten times lighter compared to their rigid counterparts on Si wafers and can be prepared on large areas. While the latter potentially allows reducing the fabrication costs per device, the former makes them cost efficient for high-volume delivery to medical institutions in, e.g., developing countries. The flexible devices withstand bending down to a 7.5 mm radius and do not degrade in performance even after 1000 consecutive bending cycles. In addition to these remarkable mechanical properties, on the analytic side, the diagnostic platform allows fast detection of specific DNA sequences of AIV subtype H1N1 with a limit of detection of 40 × 10(-12) m within 30 min suggesting its suitability for early stage disease diagnosis.

Excitation functions of longer lived radionuclides formed by deuteron irradiation of germanium

The cross sections of nuclear reactions induced by deuteron particles on natural germanium were investigated by using the stacked target technique, activation method and standard high resolution gamma spectrometry. Targets were natural germanium, vacuum evaporated onto 25micron thick polyimide (kapton) foils with nominal thickness of about 2μm. The stack was composed of 26 kapton–Ge–Ge–kapton sandwich targets foils and 24 aluminum and 25 titanium foils with nominal thickness of 50 and 11μm respectively. The Al foils served as monitors and at the same time as energy degraders in the high energy part of the stack. Ti foils were included in the stack to monitor the beam parameters in the low energy region. The irradiation was done with 50MeV deuteron particles with a beam current of about 50nA for about 1h. Activation cross sections were determined for production of the 70,71,72,73,74,76As, 69,75,77Ge and 66,67,73Ga radionuclides. The deduced experimental cross sections were compared to the results of theoretical calculations taken from the TENDL-2013 data library based on the TALYS computer code. A comparison was made with available experimental data measured earlier. Thick target yields were deduced from the experimental cross sections and were compared with the data published before. The possible production routes of arsenic radioisotopes are discussed.

Optimization of Ti/TiN/Mo back contact properties for Cu(In,Ga)Se 2 solar cells on polyimide foils

Molybdenum is conventionally used as electrical back contact for Cu(In,Ga)Se 2 (CIGS) solar cells. In this work, a multifunctional stack of Ti/TiN/Mo is introduced as back contact for flexible CIGS solar cells. The multilayer back contact was deposited on 25 μm thick polyimide foil by means of DC reactive sputtering. To optimize electrical conductivity and film stress of the alternative back contact sputter parameters such as total gas pressure, sputtering power, substrate temperature and RF substrate bias have been varied. XRD measurements and quantitative analysis of foil curvature revealed that the film stress is significantly influenced by the argon gas pressure and sputtering power. The electrical conductivity was improved by applying higher sputtering power or RF substrate bias. Analysis of the film microstructure with SEM shows that applied substrate bias influences the density of the sputtered film. The solar cells processed on Ti/TiN/Mo as well as on a conventional Mo bilayer back contact have been compared using standard current density to voltage (J–V) measurements and external quantum efficiency measurements. Conversion efficiencies of 13.4% for the alternative and 14.9% for the conventional design have been obtained.

Ultraflexible amorphous silicon transistors made with a resilient insulator

The conventional, brittle, silicon nitride barrier layer and gate insulator in amorphous silicon thin-film transistors (a-Si:H TFTs) on 50 μm thick polyimide foil was replaced by a resilient, homogeneous, hybrid of silicon dioxide and silicone polymer. The transistor structures can be bent down to 0.5 mm radius (5% strain) in tension and down to 1 mm radius (2.5% strain) in compression. This pronounced flexibility shifts the criterion for reversible bending away from a-Si:H TFT backplanes and toward the materials for substrate and encapsulation. It qualifies a-Si:H TFTs for pull-out display screens in handheld devices.

Laser-driven proton acceleration from a near-critical density target

The duration-controlled amplified spontaneous emission with intensity of 1013W/cm2 is used to convert a 7.5 μm thick polyimide foil into a near-critical plasma, in which the p-polarized, 45 fs, 1019 W/cm2 laser pulse generates 3.8 MeV protons, emitted at some angle between the target normal and the laser propagation direction of 45°. The mechanism which explains the proton generation from the near-critical plasma cloud is discussed using the two-dimensional (2D) particle-in-cell (PIC) simulation.

Temperature and humidity effects on the stability of on-plastic a-Si:H thin film transistors with various conduction channel layer thicknesses

ABSTRACTStability is an important issue for the application of TFTs. In this paper, we present the effects of humidity and temperature on the stability of inverted-staggered back-channel-cut a-Si:H TFTs with various conduction channel layer thickness. We evaluated the stability of on-plastic TFTs of different conduction layer thicknesses made at a process temperature of 150°C on 51-μm thick Kapton polyimide foil substrates.. With conduction channel layer thickness of 50nm, humidity reversibly varies the characteristics of TFTs, but TFTs of conduction layer thickness greater than 100 nm is pretty immune to the humidity change. The temperature dependent stability and characteristics of TFTs were analyzed from 20°C to 60°C. Rising temperature from 20°C to 56°C, the threshold voltage (Vt) drops about 2 volts; on-off current ratio decreases by one order of magnitude mainly due to thermally excited carriers in the off-region.

Stability of Amorphous Silicon Thin Film Transistors under Prolonged High Compressive Strain

AbstractWe studied the effect of prolonged mechanical strain on the electrical characteristics of thin-film transistors of hydrogenated amorphous silicon made at a process temperature of 150°C on 51-μm thick Kapton polyimide foil substrates. Effects are observed only at very high compressive strain of 1.8%. Tensile strain up to fracture at 0.3% to 0.5% does not show any effect, nor does compressive strain substantially less than 1.8%. The TFTs were stressed for times up to 23 days by bending around a tube with axis perpendicular to the channel length, and were evaluated in the flattened state. The changes observed are small. The threshold voltage is increased, the “on” current and the field effect mobility remain essentially constant, and the subthreshold slope, “off” current and gate leakage current drop somewhat. Overall, the observed changes are small. We conclude that mechanical strain caused by roll-to-roll processing and permanent shaping will have negligible effects on TFT performance.

43.3: A Rugged Conformable Backplane Fabricated with an a‐Si:H TFT Array on a Polyimide Substrate

AbstractFlexible active matrix TFT array backplanes were fabricated on 51 μm thick polyimide foil (Kapton®E) substrates. Expected applications include low cost flexible lightweight displays using electrophoretic, OLED, and PDLC displays. TFT performance of the Kapton backplane is nearly identically to the same structure fabricated on a glass substrate. Measurement of the transfer and output characteristics while the sample was bent at an inward radius of curvature of 2.5 cm indicate no loss in TFT performance.

150°C Amorphous Silicon Thin-Film Transistor Technology for Polyimide Substrates

We have developed a 150°C technology for amorphous silicon thin-film transistors (a-Si:H TFTs) on polyimide substrates deposited by plasma enhanced chemical vapor deposition. The silicon nitride gate dielectric and the a-Si:H channel material were tailored to provide the least leakage current and midgap defect density, respectively. In addition, we conducted experiments on the TFT structure and fabrication with the aim of obtaining high electron mobility. TFTs with back-channel etch and channel-passivated structures were fabricated on glass or 51 μm thick polyimide foil. The a-Si:H TFTs have an on/off current ratio of and an electron mobility of ∼0.7 cm2/V s. © 2001 The Electrochemical Society. All rights reserved.

Degradation of polyimide by 100 keV He +, Ne +, Ar + and Kr + ions

50 μm thick polyimide (Kapton, PI) foils were irradiated with 100 keV He +, Ne +, Ar + and Kr + ions to fluences from 1×10 12 to 1×10 16 cm −2, and the structural and compositional changes in the polymer surface layer, degraded by the ion bombardment, were studied by RBS and ERDA techniques. The sheet resistance at room temperature was also measured. Significant hydrogen and oxygen depletion is observed for the ion fluences above 1×10 14 cm −2 in the PI surface layer, the thickness of which is an increasing function of the ion projected range. Measured H and O depth profiles are structureless with the concentration monotonically increasing from the sample surface to the sample bulk where it achieves the value typical for pristine PI. The sheet resistance remains unchanged up to the fluence of 10 14 cm −2 and then it declines rapidly by several orders of magnitude, the decline being more pronounced for heavier ions.

DC-gate-bias stressing of a-Si:H TFTs fabricated at 150°C on polyimide foil

We investigated the electrical stability of a-Si:H TFTs with mobilities of /spl sim/0.7 cm/sup 2//Vs fabricated on 51 /spl mu/m thick polyimide foil at 150/spl deg/C. Positive gate voltage V/sub g/ ranging from 20 to 80 V was used in the bias stress experiments conducted at room temperature. The bias stressing caused an increase in threshold voltage and subthreshold slope, and minor decrease in mobility. Annealing in forming gas substantially improved the stability of the TFTs. The threshold voltage shift exhibited a power law time dependence with the exponent /spl gamma/ depending on the gate bias V/sub g/. For V/sub g/=20 V, /spl gamma/=0.45, while for V/sub g/=80 V, /spl gamma/=0.27. The threshold voltage shift also exhibited a power law dependence on V/sub g/ with the exponent /spl beta/ depending slightly on stress duration. /spl beta/=2.1 for t=100 sec and 1.7 for t=5000 s. These values fall into the range experimentally observed for a-Si:H TFTs fabricated at the standard temperatures of 250-350/spl deg/C.

Electrical Stability of a-Si:H TFTs Fabricated at 150°C

ABSTRACTWe investigated the electrical stability of amorphous silicon thin-film transistors (a-Si:H TFTs) fabricated on 51 μm thick polyimide foil at 150°C. Gate dc-voltage bias stressing caused an increase in threshold voltage and subthreshold slope, and a minor decrease in mobility. Annealing in H2+N2 substantially improved the stability of the TFTs. In annealed TFTs the threshold voltage shift exhibited a power law dependence on time with the exponent γ depending on gate bias Vg. For Vg = 20 V, γ = 0.45, while for Vg = 80 V, γ = 0.27. The threshold voltage shift also exhibited a power law dependence on Vg with the exponent β depending slightly on stress duration. β = 2.1 for t = 100 sec and 1.7 for t = 5000 sec. These values fall into the range experimentally observed for a-Si:H TFTs fabricated at the standard temperatures of 250-350°C.

Development and operation of laser machined microwell detectors

Arrays of 100 μm diameter cylindrical wells were laser micromachined on a 200 micrometer Cartesian grid, producing MicroWell Detectors (MWD). The substrate was 125 μm thick polyimide foil, more than twice as thick as a typical GEM or WELL detector. An advantage of the laser micromachining process is that the wells are produced with nearly vertical sidewalls, in contrast to the sloping sidewalls characteristic of conventional chemical etching processes. With the steeper sidewall, active elements may be more closely packed than is possible with wet etching techniques. Thicker substrates can be patterned, increasing the length of the charge multiplication region and reducing the internal capacitance per unit element. A series of prototypes have been produced and tested in a counting gas composed of 85% argon and 15% carbon dioxide, with a maximum measured gas gain of approximately 12 000.

Amorphous silicon thin-film transistors on compliant polyimide foil substrates

Much of the mechanical strain in semiconductor devices can be relieved when they are made on compliant substrates. We demonstrate this strain relief with amorphous silicon thin-film transistors made on 25-/spl mu/m thick polyimide foil, which can be bent to radii of curvature R down to 0.5 mm without substantial change in electrical characteristics.

Observation of latent heavy-ion tracks in polyimide by means of transmission electron microscopy

The structure of latent tracks produced by high-energy heavy ions in an organic material has been revealed for the first time by transmission electron microscopy. The experiments involved 9.3 MeV/A 197Au and 11.1 MeV/A 208Pb ion beams and ∼50-nm thick polyimide foils (ion stopping power ∼110 MeV mg −1 cm 2). Two-week old tracks are open channels, indicating the importance of violent energy equilibration events. Four-month old tracks engulf small amounts of material. Hence, damage sites exhibit chemical activity on a timescale of months, with possible absorption of material from the atmosphere. The change of material density at the track boundary is sharp. The track width distribution is broad. This may have resulted from the inhomogeneity of the polymer density on a molecular scale. The mean track diameter is 8.1±1.7 nm. There is no evidence for any severe structural alteration outside the track core.

Rugged a-Si:H TFTs on Plastic Substrates

AbstractMuch of the mechanical strain in semiconductor devices can be relieved when they are made on compliant substrates. We demonstrate this strain relief with amorphous silicon thin-film transistors (a-Si:H TFTs) made on 25-μm thick polyimide foil, which can be bent to radii of curvature R down to 0.5 mm without substantial change in electrical characteristics. At R= 0.5 mm the channel area of the TFTs is strained by ~ 1%. The reduction in bending radius, from R ≠ 2 mm on steel foil of the same thickness, agrees with the theoretical prediction that changing from a stiff to a compliant substrate can reduce the bending strain in the device plane by a factor of up to 5.

Young's modulus and tensile strength of CuNi(Mn) thin films on polyimide foils by tensile testing

Force-strain curves were measured for 1.0 μm and 1.5 μm thick Cu0.57Ni0.42Mn0.01 films on 8 μm thick polyimide foils by tensile testing. By separating the force working on the polyimide foil from that working on the metal-polyimide compound, stress-strain curves for the CuNi(Mn) films were obtained. Young's modulus and tensile strength were determined for as-deposited and annealed [350 °C, 1 h, N2/H2(5 vol%) atmosphere] films by this method. Crack propagation starts at the end of the elastic region at 0.2 to 0.7% strain, depending on the film thickness and the thermal treatment. The cracking behavior is described by a steady-state approximation.