Adhesion lithography for large-area patterning of asymmetric nanogap electrodes

As the resolution of devices in the electronics industry has hit the nanoscale, device fabrication costs have rapidly increased. Whilst commercial technologies such as electron-beam lithography are able to define nanoscale features, they are costly and unsuitable for large area electronics. Research is now focusing on fabrication techniques that can pattern features on the nanoscale on flexible substrates, over large areas without incurring these high costs, such as adhesion lithography (a-Lith). A-Lith is a large-scale fabrication technique for producing planar asymmetric nanogap electrodes [1]. Devices have been created with gap width:length aspect ratios \u3e100000. The technique can be carried out in air and at ambient temperature making it ideal for the field of plastic electronics [2]. The a-Lith technique relies on a self-assembled monolayer (SAM) molecule selectively coating a prepatterned metal (M1) which then changes the adhesion forces. A second metal (M2) is then deposited over the top and can be specifically patterned when peeled using an adhesive due to its reduced adhesion on M1 relative to elsewhere. M2 only remains in the areas where there is no M1 (in the areas where it directly contacts the substrate). Where M2 fractures at the edge of M1, a nanogap (≈10 nm) is formed between the two metals [1]. A-Lith has shown improved device performance across many areas of device electronics as the ability to pattern electrodes side-by-side largely eliminates parasitic capacitances. Such electrodes have been utilized in device applications including high responsivity photodiodes [3], nano organic light emitting diodes [4], memristors [2] and high speed diodes [5]. This fabrication technique was previously only successfully carried out with Al, Au and Ti as M1, and Al and Au as M2, with the Al and Au (with an Al adhesion layer) thermally evaporated. In this work, a-Lith has been successful executed with a variety of materials sputtered including Cu, Ni, Ti, Mo, Cr and Al as M1. M2 is shown to be successful with Al, Ni, Cu and Cr. This has allowed for further devices applications to be explored including devices utilizing 2D materials. References [1] D. J. Beesley et al., “Sub-15-nm patterning of asymmetric metal electrodes and devices by adhesion lithography.” Nat. Commun., vol. 5, (2014), p. 3933. [2] J. Semple et al., “Large-area plastic nanogap electronics enabled by adhesion lithography,” npj Flex. Electron., vol. 18, (2018). [3] G. Wyatt-Moon, et al., “Deep Ultraviolet Copper(I) Thiocyanate (CuSCN) Photodetectors Based on Coplanar Nanogap Electrodes Fabricated via Adhesion Lithography,” ACS Appl. Mater. Interfaces, vol. 9, (2017), p. 41965. [4] G. Wyatt-Moon, et al., “Flexible nanogap polymer light-emitting diodes fabricated via adhesion lithography (a-Lith),” J. Phys. Mater, vol. 1, (2018). [5] J. Semple et al., “Radio Frequency Coplanar ZnO Schottky Nanodiodes Processed from Solution on Plastic Substrates,” Small, vol. 12, (2016), p. 1993

Adhesion Lithography Peel Tool Design

This document describes the adhesion lithography peel tool, designed for the automation of the final step of the adhesion lithography process

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

Invited; P-channel metal oxide thin film transistors for flexible CMOS logic: Challenges and opportunities

The ‘unique selling point’ of thin film transistors (TFTs) compared with MOSFETs is that the former do not require the substrate to be a semiconducting material. It is for this reason that TFTs are required for active matrix display backplanes. However, the development of the ‘Internet of Things’ (IoT) presents a new opportunity for TFTs as it becomes possible to build complex logic or memory circuits on flexible substrates that can be more easily incorporated into products such as clothing or packaging without the form factor restrictions that rigid semiconducting substrates impose. There have been recent reports of the successful fabrication of basic microprocessors comprising TFTs on plastic substrates instead of MOSFETs [1]. Please click Download on the upper right corner to see the full abstract

A novel split mode TFBAR device for quantitative measurements of prostate specific antigen in a small sample of whole blood.

Easy monitoring of prostate specific antigen (PSA) directly from blood samples would present a significant improvement as compared to conventional diagnostic methods. In this work, a split mode thin film bulk acoustic resonator (TFBAR) device was employed for the first time for label-free measurements of PSA concentrations in the whole blood and without sample pre-treatment. The surface of the sensor was covalently modified with anti-PSA antibodies and demonstrated a very high sensitivity of 101 kHz mL ng-1 and low limit of detection (LOD) of 0.34 ng mL-1 in model spiked solutions. It has previously been widely believed that significant pre-processing of blood samples would be required for TFBAR biosensors. Importantly, this work demonstrates that this is not the case, and TFBAR technology provides a cost-effective means for point-of-care (POC) diagnostics and monitoring of PSA in hospitals and in doctors' offices. Additionally, the accuracy of the developed biosensor, with respect to a commercial auto analyser (Beckman Coulter Access), was evaluated to analyse clinical samples, giving well-matched results between the two methods, thus showing a practical application in quantitative monitoring of PSA levels in the whole blood with very good signal recovery

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

Abstract: We report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10−4 A (a photo-to-dark current ratio of ~ 107) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10−7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC

Inkjet printed circuits with two-dimensional semiconductor inks for high-performance electronics

Air-stable semiconducting inks suitable for complementary logic are key to create low-power printed integrated circuits (ICs). High-performance printable electronic inks with two-dimensional materials have the potential to enable the next generation of high performance, low-cost printed digital electronics. Here we demonstrate air-stable, low voltage (< 5 V) operation of inkjet-printed n-type molybdenum disulfide (MoS2) and p-type indacenodithiophene-co-benzothiadiazole (IDT-BT) field-effect transistors (FETs), estimating a switching time of {\tau} ~ 3.3 {\mu}s for the MoS2 FETs. We achieve this by engineering high-quality MoS2 and air-stable IDT-BT inks suitable for inkjet-printing complementary pairs of n-type MoS2 and p-type IDT-BT FETs. We then integrate MoS2 and IDT-BT FETs to realise inkjet-printed complementary logic inverters with a voltage gain |Av| ~ 4 when in resistive load configuration and |Av| ~ 1.36 in complementary configuration. These results represent a key enabling step towards ubiquitous long-term stable, low-cost printed digital ICs

Photoconductive laser spectroscopy as a method to enhance defect spectral signatures in amorphous oxide semiconductor thin- film transistors

Defects in semiconductor thin-films often leave optical spectral signatures that can be used for their identification. In this letter, we report on spectrally resolved photoconductivity measurements of amorphous oxide semiconductor thin-film transistors. In contrast to previously reported photoconductive spectroscopy measurements recorded using spectrally filtered broadband light sources, we used a wavelength tunable picosecond laser to illuminate the thin-film. We extracted the absorption coefficient as a function of wavelength from the photocurrent measurement and showed that it followed the typical characteristic behaviour previously reported for amorphous oxide semiconductor thin-films. However, in addition, we observed several sharp spectral peaks in the photoconductivity spectrum which can be associated with sub-bandgap defects. These enhanced peaks are not normally visible in previously reported photoconductivity spectra. Furthermore, we show that we can control the sensitivity of our measurement by changing the applied gate bias voltage when the thin-films were fabricated into transistors. The enhancement achieved by using the wavelength tunable laser makes this a particularly sensitive characterisation tool and can additionally be used to discriminate between defects which have been incorporated after device fabrication