Pulse plated silver metallization on porosified LTCC substrates for high frequency applications

Advanced high frequency systems such as needed in modern radar applications, require high conductive metallizations as well as substrates with areas of variable permittivity. This paper presents the combination of the selective porosification technology of low temperature co-fired ceramics (LTCC) and electro pulse plated silver microstrip lines. By means of selective plating methods, line widths of 20 μm can be manufactured featuring low resistivity values down to 2.33 μΩ cm, without detectable pore penetration. The substrate permittivity is measured facilitating a combined method of ring resonator detuning and 3D field simulations resulting in a reduction of 6.5% with a shift from approx. 7.52 to 7.03 at 66 GHz due to the porosification. As often outlined in literature, the major challenge in using silver as a conductor lies in its high tendency of agglomeration and microstructural transformation especially in oxygen containing atmosphere even at low temperatures. Therefore, the effect of different temperature loads up to 500 °C on the dc film resistivity is measured using the van der Pauw technique and is compared to scanning electron microscope analyses.     

A printed OTFT-backplane for AMOLED display

An AMOLED panel driven by an OTFT-backplane is an attractive display because OTFTs and OLEDs use organic materials with unique characteristics such as low temperature and solution processing ability, and thus are able to implement the key features of future displays. In this study we applied some printing technologies to fabricate an OTFT-backplane for AMOLEDs. Screen printing combined with photolithography with Ag ink was used for the gate electrodes and scan bus lines and contact pads. Ag metal lines with a width of 20 μm and thickness of 60 nm and resistivity of 3.0 × 10^?5 Ω cm were achieved. Inkjet printing was applied to deposit TIPS-pentacene as an organic semiconductor. The OTFT-backplane using the Ag gate electrodes and TIPS-pentacene exhibited uniform performance over 17,500 pixels on a 7 in. panel. The mobility was 0.31 ± 0.05 cm²/V s with a deviation of 17%. The AMOLED panel successfully demonstrated its ability to display patterns.     

Photosintering and electrical performance of CuO nanoparticle inks

A copper oxide (CuO) nanoparticle ink was inkjet printed and photosintered in order to optimize electrical performance as a function of pattern dimension. For a given photosintering condition, electrical conductance varied strongly with line widths, ranging from 100 to 300 μm, illustrating the implications of printing and sintering complex circuit designs with varying feature sizes. By tuning the time delay between printing and sintering, exposure wavelength, radiant energy, pulse width and the distance between the light-source and substrate, photosintering conditions were optimized so that variations in sheet resistance for different line widths were minimized. Using optimized photosintering conditions, a sheet resistance value as low as 150 mΩ/□ (resistivity of 9 μΩ cm) and current carrying capacity of 280 mA for a 300 μm wide trace was achieved.     

RF measurements to pinpoint defects in inkjet-printed,thermally and mechanically stressed coplanar waveguides

In this work 10-GHz-band RF measurement and microscopy characterizations were performed on thermally and mechanically long-term-stressed coplanar waveguides (CPW) to observe electrical and mechanical degradation in 1-mm-thick PPO/PPE polymer substrates with inkjet-printed Ag conductors. The structure contained two different CPW geometries in a total of 18 samples with 250/270 μm line widths/gaps and 670/180 μm line widths/gaps. A reliability test was carried out with three sets. In set #1 three 250 μm and three 670 μm lines were stored in room temperature conditions and used as a reference. In set #2 six samples were thermally cycled (TC) for 10,000 cycles, and in set #3 six samples were thermally cycled and bent with 6 mm and 8 mm bending diameters.Thermal stressing was done by cycling the samples in a thermal cycling test chamber operating at 0/100 °C with 15-minutes rise, fall, and dwell times, resulting in a one-hour cycle. The samples were analyzed during cycling breaks using a vector network analyzer (VNA). In addition to optical microscopy, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) imaging were used to mechanically characterize the structures.The results showed that the line width of 670 μm had better signal performance and better long-term reliability than the line width of 250 μm. In this study, the average limit for proper RF operation was 2500 thermal cycles with both line geometries. The wide CPW lines provided more stable characteristics than the narrow CPW lines for the whole 10,000-cycle duration of the test, combined with repeated bending with a maximum bending radius of 6 mm. A phenomenon of nanoparticle silver protruding from cracks in the print of the bent samples was observed, as well as fracturing of the silver print in the CPW lines.     

Low temperature fabrication of Ni–P metallic patterns on ITO substrates utilizing inkjet printing

A low temperature process to fabricate high resolution metallic lines on indium tin oxide (ITO) substrates using inkjet printing and subsequent electroless plating is described in this study. In this method, a thermo-sensitive (styrene-co-NIPAAm)/Pd (St-co-NIPAAm/Pd) nanoparticle-based ink was printed onto ITO substrates to create patterned catalytic sites, where nickel is subsequently deposited by electroless plating to form metal lines with desired width and conductivity. The inkjet printing variables such as droplet spacing and printing voltage, as well as the Ni electroless deposition variables such as deposition time and temperature were systematically investigated to obtain the optimum parameters. The adhesion of the deposited Ni–P coating to the ITO substrate was evaluated by a scotch tape test method. Optical microscope observation shows that a continuous pattern was formed with a printing voltage of 37 V and a droplet spacing of 50 μm. The irritating coffee ring effect was significantly suppressed by raising the substrate temperature to 50 °C and increasing the electroless plating temperature to 75 °C. High-resolution conductive metal lines can be easily and successfully fabricated using our method, which shows good potential for preparing the metallic lines as front or back electrodes in solar cells.     

Gravure printed organic rectifying diodes operating at high frequencies

Organic rectifier diodes operating at 10 MHz made using roll-to-roll compatible mass printing processes to define patterns and deposit inks are reported. The diodes consist of a layer of poly(triarylamine) sandwiched between layers of silver and copper. No high resolution prepatterning of any surfaces was performed, thus the entire process could be carried out on large-scale roll-to-roll production lines. The organic diode based rectifier circuit generates a DC output voltage of approximately 2.7 V at 10 MHz, using an input signal with zero-to-peak voltage amplitude of 10 V. The result demonstrates the possibility of printed organic diodes for RFID applications.     

Invited paper: Short pulse generation in mid-IR fiber lasers

Mode-locked fiber lasers emitting short pulses of light at wavelengths of 2 μm and longer are reviewed. Rare-earth doped silica and fluoride fiber lasers operating in the mode-locked regime in the mid-IR (2–5 μm) have attracted attention due to their usefulness to spectroscopy, nonlinear optics, laser surgery, remote sensing and ranging to name a few. While silica fiber lasers are fundamentally limited to emission wavelengths below 2.2 μm, fluoride fiber lasers can reach to nearly 4 μm. The relative infancy of fluoride fibers as compared to silica fibers means the field has work to do to translate the mode-locking techniques to systems beyond 2 μm. However, with the recent demonstration of a stable, mode-locked 3 μm fiber laser, the possibility of achieving high performance 3 μm class mode-locked fiber lasers looks promising.     

Low temperature fabricated conductive lines on flexible substrate by inkjet printing

The optimal conditions of inkjet-printed nano-silver suspension and silver nitrate solution for fabricating continuous narrow conductive lines on a polyimide substrate are investigated by varying the driving pulse and droplet overlap. The dimensionless Weber number and Reynolds number are used to evaluate the droplet size after impact. It was found that the presence of a suspension of nanoparticles increases droplet diameter. With appropriate droplet overlap and driving pulse conditions, continuous lines of AgNO₃ with 24.3 μm in width and nano-silver suspension with 33 μm in width were fabricated. In addition, the effects of driving pulse voltage and droplet coverage on the bulging of as-printed conductive lines are also examined.     

Measurements of n-InGaAs with MBE layers: Relevance of negative magnetoresistance in the investigated samples

Measurements of n_H were performed. n_H values showed a distinct increase at temperatures below ~90 K (1.1 μm n-InGaAs samples) and a decrease at temperatures below ~30 K (7 μm n-InGaAs samples), depending on the doping level. These trends might be related to the magnetoresistance (MR) of the n-InGaAs samples. The MR behavior of the n-InGaAs samples with respect to magnetic field and temperature was apparently dependent on the doping level. Two n-InGaAs samples, one of which had a thin InGaAs epilayer (1.1 μm) and the other with a thicker (7 μm) epilayer, showed interesting behavior at low temperature. Their behavior at magnetic fields of approximately −15000 to +15,000 G were determined. The resistivity ((ρ_G – ρ₀)/ρ₀) of the 1.1 μm sample was negative at temperatures lower than 30 K.     

Direct imprint of Al foil for metallization of high-aspect ratio Al lines in nano/micro patterned SiO2/Si

Patterning techniques of Al micro/nano-structures become more and more critical as optical components and microelectronic devices continue to be scaled down. In this work, we fabricated gap-filled Al lines in SiO₂/Si masters by using the direct thermal imprint of molten Al. As a result, gap-filled Al lines with width ranging from 0.25 to 20 μm and depth ranging from 6 to 127 μm could be achieved without any further processing step such as CVD and PVD. The process studied here has shown the possibility to extend trench filling capability to 0.25 μm structures with 24:1 aspect ratio, which are difficult to be obtained by other conventional Al metallization methods.     

Drivability improvement in Schottky barrier source/drain MOSFETs with strained-Si channel by Schottky barrier height reduction

Due to an extra barrier between source and channel, the drivability of Schottky barrier source/drain MOSFETs (SBMOSFETs) is smaller than that of conventional transistors. To reach the drivability comparable to the conventional MOSFET, the Schottky barrier height (SBH) should be lower than a critical value. It is expected that SBH can be effectively reduced by a bi-axially strain on Si. In this letter, p-channel MOSFETs with PtSi Schottky barrier source/drain, HfAlO gate dielectric, HfN/TaN metal gate and strained-Si channel are demonstrated for the first time using a simplified low temperature process. Devices with the channel length of 4 μm have the drain current of 9.5 μA/μm and the transconductance of 14 μS/μm at V_gs − V_th = V_ds = −1 V. Compared to the cubic Si counterpart, the drain current and the transconductance are improved up to 2.7 and 3.1 times respectively. The improvement is believed to arising from the reduced barrier height of the PtSi/strained-Si contact and the enhanced hole mobility in the strained-Si channel.     

Sub-micron imaging on high-topography wafers using spray coating and projection lithography

We propose a method to image inside deep trenches (50 μm) using spray-coated resist and the ASML PAS 5500/100 system with the new functionality multi-step imaging. Multi-step imaging allows extending the focus offset range of the PAS 5500/100 system from ±30 μm to ±200 μm. Isolated trenches and contact holes were both imaged inside the deep trenches and on the surface of the wafer to study the versatility of the new functionality. A resolution of 700 nm in 3 μm thick photoresist, at the bottom of 50 μm deep, 200 μm wide trench, was obtained with this process. Finally, multi-focus exposure that consists in exposing the same image several times at various focus offsets was performed in order to image thick photoresist on high topographic substrates.     

Biaxially aligned YBCO film tapes fabricated by all pulsed laser deposition

Biaxially aligned yttria-stabilized zirconia (YSZ) films on Ni-based alloy substrates were realized with high deposition rate of 0.5 μm min⁻¹ by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing effect in the ISD process. To raise I_c values, YBCO thickness was increased up to 5 μm. Thick YBCO films with high J_c values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high I_c value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved.     

Fabrication and advanced electrical and stability characterization of laser-shaped thick-film and LTCC microresistors for high temperature applications

Thick-film and LTCC (Low Temperature Co-fired Ceramics) technologies are well-established and relatively low-cost fabrication method of passives. This paper presents systematic studies of fabrication and a wide spectrum of geometrical and electrical properties of thick-film and LTCC microresistors with dimensions down to 30 × 200 μm². The geometrical parameters (average length, width and thickness, relations between designed and real dimensions, distribution of planar dimensions) are correlated with basic electrical properties of resistors (sheet resistance, temperature dependence of resistance), long-term stability as well as durability of microresistors to short electrical pulses in the temperature range from 25 °C to 400 °C.     

High-resolution spatial control of the threshold voltage of organic transistors by microcontact printing of alkyl and fluoroalkylphosphonic acid self-assembled monolayers

A dense array of 500 organic TFTs with two different threshold voltages arranged in a checkerboard pattern has been fabricated. The threshold voltages were defined by preparing self-assembled monolayers (SAMs) of either an alkyl or a fluoroalkylphosphonic acid on the gate-oxide surface of each TFT, using a combination of microcontact printing from an elastomeric stamp and dipping into a solution. The threshold voltages are −1.01 ± 0.15 V for the TFTs with the fluoroalkyl SAM and −1.28 ± 0.23 V for the TFTs with the alkyl SAM. ToF-SIMS analysis shows that the two SAMs can be patterned with a pitch of 10 μm and without significant cross-contamination. Cross-sectional TEM and NEXAFS characterization of the SAMs indicate that the properties of the SAMs prepared by microcontact printing and dipping are essentially identical.     

Ultra-thin flexible screen printed rechargeable polymer battery for wearable electronic applications

This research has demonstrated how an ultra-thin rechargeable battery technology has been fabricated using screen printing technology. The screen printing process enabled the sequential deposition of current collector, electrode and separator/electrolyte materials onto a polyethylene terephthalate (PET) substrate in order to form both flexible and rechargeable electrodes for a battery application. The anode and cathode fabricated were based on the conducting poly (3,4-ethylenedioxythiophen): poly (styrene sulfonate) (PEDOT: PSS) and polyethyleneimine (PEI) which were combined to form the electrodes. The difference in the oxidation level between the two electrodes produced an open circuit voltage of 0.60 V and displayed a practical specific capacity of 5.5 mAh g⁻¹. The battery developed had an active surface area of 400 mm² and a device thickness of 440 μm. The chemistry developed during this study displayed long-term cycling potential and proves the stability of the cells for continued usage. This technology has direct uses in future personal wearable electronic devices.     

Fabrication of a conductive nanoscale electrode for functional devices using nanoimprint lithography with printable metallic nanoink

This paper proposes two metal patterning processes. In each process, nanoimprint lithography (NIL) is used with commercialized particle-based silver nanoink which has appropriate properties for NIL. One process is a direct NIL process with a polydimethylsiloxane (PDMS) stamp; the other is a combined NIL and lift-off process. The direct NIL process is executed by using a xylene-absorbed PDMS stamp to decrease the curing time and minimize the residuals. A flexible PDMS stamp can also be wrapped around a quartz cylinder and used as a roll stamp to enlarge the patterned area. The direct NIL process successfully produced silver line patterns in the range of 200–300 nm, and the combined NIL and lift-off process successfully produced silver line patterns in the range of 15–60 nm.     

High-resolution direct-writing of metallic electrodes on flexible substrates for high performance organic field effect transistors

We report on organic field-effect transistors (OFETs) with sub-micrometer channels fabricated on plastic substrates with fully direct-written electrical contacts. In order to pattern source and drain electrodes with high resolution and reliability, we adopted a combination of two digital, direct writing techniques: ink-jet printing and femtosecond laser ablation. First silver lines are deposited by inkjet printing and sintered at low temperature and then sub-micrometer channels are produced by highly selective femtosecond laser ablation, strongly improving the lateral patterning resolution achievable with inkjet printing only. These direct-written electrodes are adopted in top gate OFETs, based on high-mobility holes and electrons transporting semiconductors, with field-effect mobilities up to 0.2 cm²/V s. Arrays of tens of devices have been fabricated with high process yield and good uniformity, demonstrating the robustness of the proposed direct-writing approach for the patterning of downscaled electrodes for high performance OFETs, compatibly with cost-effective manufacturing of large-area circuits.     

Flatly broadened mid-infrared supercontinuum generation in a cascade of thulium-doped silica fiber amplifiers

Single-mode, flatly broadened supercontinuum (SC) generated in a thulium two-stage fiber amplifier spanning nearly the mid-infrared band is reported. The output average power and 10 dB bandwidth of the obtained SC are over 2.3 W and 570 nm (from 1.95 μm to 2.52 μm), respectively. For wavelengths beyond 2.4 μm the output power was 495 mW constituting almost 21% of the total output power. Applying Tm-doped single-mode silica fibers as nonlinear and amplification media it was possible to extend the long wavelength cut-off to 2.7 μm.     

Degradation of all-inkjet-printed organic thin-film transistors with TIPS-pentacene under processes applied in textile manufacturing

Printed electronics represent an alternative solution for the manufacturing of low-temperature and large area flexible electronics. The use of inkjet printing is showing major advantages when compared to other established printing technologies such as gravure, screen or offset printing, allowing the reduction of manufacturing costs due to its efficient material usage and the direct-writing approach without requirement of any masks. However, several technological restrictions for printed electronics can hinder its application potential, e.g. the device stability under atmospheric or even more stringent conditions. Here, we study the influence of specific mechanical, chemical, and temperature treatments usually appearing in manufacturing processes for textiles on the electrical performance of all-inkjet-printed organic thin-film transistors (OTFTs). Therefore, OTFTs where manufactured with silver electrodes, a UV curable dielectric, and 6,13-bis(triisopropylsilylethynyl) pentance (TIPS-pentacene) as the active semiconductor layer. All the layers were deposited using inkjet printing. After electrical characterization of the printed OTFTs, a simple encapsulation method was applied followed by the degradation study allowing a comparison of the electrical performance of treated and not treated OTFTs. Industrial calendering, dyeing, washing and stentering were selected as typical textile processes and treatment methods for the printed OTFTs. It is shown that the all-inkjet-printed OTFTs fabricated in this work are functional after their submission to the textiles processes but with degradation in the electrical performance, exhibiting higher degradation in the OTFTs with shorter channel lengths (L = 10 μm).