SVILUPPO DI DISPOSITIVI E CIRCUITI ORGANICI REALIZZATI SU SUBSTRATI PLASTICI FLESSIBILI MEDIANTE TECNICHE DI STAMPA

Abstract

Nowadays electronics has new requirements, it is expected to be cost-effective, environmentally friendly, disposable, lightweight, unbreakable and even biocompatible. Printed organic electronics appears to be an appropriate solution to these issues, due to low-cost fabrication and to the possibility to obtain large area multifunctional devices on flexible substrates. Althoughsignificant progresses, the potential of organic materials is still not fulfilled due to the lack in high performance optimized ink formulations. Moreover, it is required the improvement of the existing printing techniques in order to meet the microelectronics constraints. Among the printing techniques, the gravure printing is promising since it combines high throughput with suitable film thicknesses, homogeneity, resolution and registration accuracy. Accordingly, this technique has been chosen for this thesis work to print thin films of organic materials. The printability of an ink is connected to its fluid dynamic behaviour during each step of the printing process, which is related to both the material and the printing system characteristics. Hence, as detailed hereafter, during this thesis the current issues in obtaining high performance printed devices have been faced up working on both the gravure printing system and the ink formulations. At first, the gravure printing system has been modified and improved (Fig. 1) In particular, it has been developed a glass cliché that allowed to overcome several limits of standard clichés. The procedure to define the cells on the glass cliché has been optimized, achieving the following technical features: - etching control down to 0.5 μm; - lateral etch compensation; - homogeneous etch over the cliché area; - resolutions down to 2 μm; - improvement of the cell top edge profile; - cell bottom surface roughness down to 10 nm; - control of surface wettability by optimized functionalization. Moreover, combining the glass cliché with the use of proper plastic blades, the metallic particle contamination and the excessive wear issues have been worked out. Secondly, high performance electronic ink formulations have been assessed with this optimized gravure system. Continuous and homogeneous films have been obtained, with suitable resolution and registration, without degradation of the material electrical properties. In particular, organic dielectric and organic conductive ink formulations have been studied. It has been evaluated the influence of the forces acting on the fluid during the printing process through the analysis of several significant fluid dynamic parameters. It has been defined a range of parameters suitable to obtain acceptable printing outcomes with the optimized printing system. The rheological study has allowed to identify the presence of intrinsic instabilities in the ink formulations, such as thixotropic loops, which make the inks hardly manageable. The elastic behaviour of the inks has been studied through the variations of the G’ and G’’ moduli, resulting in significant differences in the ink transferring on the substrate as a function of the cell geometry. Formulations with low yield point have shown an enhancement in the levelling step resulting in an improvement of the final film homogeneity and ensuring the achievement of a good interface between the semiconductor and the dielectric. It has been pointed out the possibility to use a Newtonian-like ink to achieve a highly uniform film over large areas, while it has been taken advantage of a pseudoplastic behaviour for highly scaled features. Furthermore, the gravure printing process of two polymeric organic semiconductors, a p-type and an n-type, has been optimized, by changing printing parameters, in order to get continuity and homogeneity of the layer. As a result of this work, several of the current issues of the gravure printing technique have been overcome and two inks based on organic materials, a conductive and a dielectric one, have been refined. Besides, the inkjet printing technique has been applied to electrode realization and resistors definition. This technique has been chosen as it is an effective method for rapid prototyping and it allows to obtain high resolution patterns. Different procedures have been optimized for the inkjet printing of silver nanoparticles based ink, depending on the application requirements. The following results has been achieved: - arrays of printed areas down to 200x100μm, for high resolution requirements; - realization of top contacts with low temperature process, down to 80°C, without damaging the dielectric layer, as confirmed by the electrical measurements; - low resistivity source and drain (S/D) contacts optimized for two different layer thicknesses, with reduced coffee ring effect and roughness. The printing procedure to obtain small area resistors on flexible substrates has been optimized. The resistors have been realized using a carbon ink, achieving resistances from 0.2 MΩ up to 17 MΩ in an area of 1 mm2 (Fig. 2). The devices have demonstrated to be suitable for future applications in mechanical sensors or as load for inverters. Then, exploiting the optimized printing processes and materials, p-type and n-type Organic Thin Film Transistors (OTFTs) have been realized and electrically characterized, testing their stability under bias, light and bending stresses (Fig. 3). Fully printed devices channel length, L, and width, W, ranged from 30 to 400 μm and from 100 to 900 μm, respectively. The p-type OTFTs with gold S/D contacts patterned by laser lithography have shown a remarkable field effect mobility, µLIN, up to 0.22 cm2V -1 s -1 , however they suffered from a strong influence of the contact resistance and an imperfect dielectric layer.. They have shown instabilities under bias stress both in the ON- and OFF-regions, with a complete, but slow, recovery in the ON region after bias removing. On the contrary, the instabilities induced by light exposure have been recovered quite fast, in particular in the OFF-region. Furthermore, the lower IOFF measured after dark recovery has increased significantly the ION/IOFF of these devices, up to 103÷104 . The fully printed p-type OTFTs with silver inkjetted S/D contacts have shown very low leakage current Igs < 2 pA at Vg = -20 V and small parasitic contact resistance. Mobility µLIN up to 0.24 cm2V - 1 s -1 and ION/IOFF ≈ 103 have been measured. The shifts of the transfer characteristics induced by bias stress, both in the ON- and OFF-region, have been completely recovered in time. The light effects on the subthreshold region of the transfer characteristics have been recovered after time intervals comparable to the stress time, while the OFF-region recover was slow. Substrate bending influences the device transfer characteristics inducing shifts up to ΔVg ≈ 1.6 V for a 2% of strain. This instability has been completely recovered removing the mechanical stress. The sufficient reliability and reproducibility of these devices have suggested the possibility of implementing integrated circuits. The fully printed p-type OTFTs with organic S/D contacts have provided a good charge injection in the semiconductor without the use of self-assembled monolayer functionalizations, resulting in improved contact characteristics. These devices have shown a µLIN up to 0.06 cm2V -1s-1 and low leakage current (Igs < 20 pA at Vg = -20 V). The subthreshold slope has been measured to be down to 842 mV/dec, while the ION/IOFF ≈ 105. Considering the current state of the art, these devices are the best performing fully printed p-channel OTFTs with organic S/D realized with roll to roll compatible techniques and are well comparable with the best ones obtained with inkjet printing techniques. They have shown not yet satisfying stability under bias and light stress, suggesting disposable devices as suitable applications. The fully printed n-type OTFTs realized have shown very low Igs < 10 pA at Vg = 50 V, subthreshold slope ≈ 4600 mV/dec, ION/IOFF ≈ 106 . These devices have not shown reliable characteristics and the main issue has been identified in the optimization of the semiconductor/dielectric interface. At last, inverters with resistive or active load configuration have been realized and their static and dynamic behaviours have been characterized. As transistors, the fully printed OTFTs with silver inkjetted S/D contacts have been used. The devices with external resistive load have shown interesting features. They have been tested in dynamic until 1 kHz, with supply voltage down to 10 V, still showing a suitable logic swing and stable output waveforms. Future perspectives include the integration of a printed resistance on the same substrate, as the results previously obtained with the inkjet printed resistors are compatible with the implementation of this application. The fully printed active load inverters have shown good performance, with regenerative property and gain around 2.2, while applying a supply voltage of -25 V. The fall time has resulted to be the limiting factor for scaling up the operating frequency of the device (Fig. 4). Further improvements would be achieved reducing the parasitic capacitances of the OTFTs, optimizing the layout, and defining the most performing coupling of different W/L OTFTs as drive and load transistors.