An Approach to Simultaneously Test Multiple Devices for High-Throughput Production of Thin-Film Electronics

New generation of thin-film transistors (TFTs), where the active material is amorphous oxide, conjugated polymer, or small molecules, have the advantage of flexibility, high form factor, and large scale manufacturability through low cost processing techniques, e.g., roll-to-roll printing, screen printing. During high-throughput production using these techniques, the probability of defects being present increases with the speed of manufacturing and area of devices. Therefore a high-throughput and low cost testing technique is absolute essential to maintain high quality of final product. We report a Simultaneous Multiple Device Testing (SMuDT) approach which is up to 10 times faster and cost effective than conventional testing methods. The SMuDT approach was validated using circuit simulation and demonstrated by testing large scale indium gallium zinc oxide (IGZO) TFTs. A method to ‘bin’ the tested devices using Figure of Merit was established.The authors acknowledge the support of this project provided by the EPSRC and Innovate UK through the AUTOFLEX Project (grant no. EP/L505201/1) and CIMLAE Project (EP/K03099X/1). AK and AJF would like to thank PragmatIC Printing Ltd. for wafer samples. Additional data related to this publication which is not of a commercially sensitive nature is available at the DSpace@Cambridge data repository (www.repository.cam.ac.uk).This is the final version of the article. It first appeared from IEEE via http://dx.doi.org/10.1109/JDT.2015.246229

Instability mechanisms in amorphous oxide semiconductors leading to a threshold voltage shift in thin film transistors

Amorphous indium gallium zinc oxide (a-IGZO) has been successfully employed commercially as the channel layer in thin film transistors (TFTs) for active-matrix flat panel displays. However, these TFTs are known to suffer from a threshold voltage shift upon application of a gate bias. The threshold voltage shift is reversible through annealing. A similar phenomenon is observed in other TFTs with an amorphous oxide semiconductor channel. The migration of oxygen vacancies is proposed as being the microscopic mechanism causing this effect as it can lead to a change in the equilibrium distribution of defect states in the band gap of the semiconductor. This would manifest itself as a reversible threshold voltage shift in the TFT transfer characteristics, as observed experimentally.The support of this work by the Engineering and Physical Sciences Research Council (EPSRC) through project EP/M013650/1 is acknowledged

Analysis of amorphous indium-gallium-zinc-oxide thin-film transistor contact metal using Pilling-Bedworth theory and a variable capacitance diode model

It is widely reported that threshold voltage and on-state current of amorphous indium-gallium-zinc-oxide bottom-gate thin-film transistors are strongly influenced by the choice of source/drain contact metal. Electrical characterisation of thin-film transistors indicates that the electrical properties depend on the type and thickness of the metal(s) used. Electron transport mechanisms and possibilities for control of the defect state density are discussed. Pilling-Bedworth theory for metal oxidation explains the interaction between contact metal and amorphous indium-gallium-zinc-oxide, which leads to significant trap formation. Charge trapping within these states leads to variable capacitance diode-like behavior and is shown to explain the thin-film transistor operation.This work was supported by the Engineering and Physical Sciences Research CouncilThis is the author's accepted manuscript of the publication: http://dx.doi.org/10.1063/1.480199

Novel Tunnel-Contact-Controlled IGZO Thin-Film Transistors with High Tolerance to Geometrical Variability.

Thin insulating layers are used to modulate a depletion region at the source of a thin-film transistor. Bottom contact, staggered-electrode indium gallium zinc oxide transistors with a 3 nm Al2 O3 layer between the semiconductor and Ni source/drain contacts, show behaviors typical of source-gated transistors (SGTs): low saturation voltage (VD_SAT ≈ 3 V), change in VD_SAT with a gate voltage of only 0.12 V V-1 , and flat saturated output characteristics (small dependence of drain current on drain voltage). The transistors show high tolerance to geometry: the saturated current changes only 0.15× for 2-50 µm channels and 2× for 9-45 µm source-gate overlaps. A higher than expected (5×) increase in drain current for a 30 K change in temperature, similar to Schottky-contact SGTs, underlines a more complex device operation than previously theorized. Optimization for increasing intrinsic gain and reducing temperature effects is discussed. These devices complete the portfolio of contact-controlled transistors, comprising devices with Schottky contacts, bulk barrier, or heterojunctions, and now, tunneling insulating layers. The findings should also apply to nanowire transistors, leading to new low-power, robust design approaches as large-scale fabrication techniques with sub-nanometer control mature

Nanostructured plasmonic metapixels

State-of-the-art pixels for high-resolution microdisplays utilize refective surfaces on top of electrical backplanes. Each pixel is a single fxed color and will usually only modulate the amplitude of light. With the rise of nanophotonics, a pixel’s relatively large surface area (~10μm$^2$), is in efect underutilized. Considering the unique optical phenomena associated with plasmonic nanostructures, the scope for use in refective pixel technology for increased functionality is vast. Yet in general, low refectance due to plasmonic losses, and sub-optimal design schemes, have limited the real-world application. Here we demonstrate the $\textit{plasmonic metapixel}$; which permits high refection capability whilst providing vivid, polarization switchable, wide color gamut fltering. Ultra-thin nanostructured metal-insulatormetal geometries result in the excitation of hybridized absorption modes across the visible spectrum. These modes include surface plasmons and quasi-guided modes, and by tailoring the absorption modes to exist either side of target wavelengths, we achieve pixels with polarization dependent multicolor refection on mirror-like surfaces. Because the target wavelength is not part of a plasmonic process, subtractive color fltering and mirror-like refection occurs. We demonstrate wide color-range pixels, RGB pixel designs, and in-plane Gaussian profle pixels that have the potential to enable new functionality beyond that of a conventional ‘square’ pixel.EPSRC Integrated Photonics & Electronic Systems Centre for Doctoral Training (Grant number: EP/L015455/1); Cambridge Commonwealth, European & International Trust