Making flexible electronics

Manufacture of electric circuits on polymer substrates is broadly referred to as flexible electronics and has gained significant interest as a pathway to low-cost or large-area electronics.^1,2 Although conventional vacuum deposition and photolithographic patterning methods are well developed for inorganic microelectronics, they are not appropriate for this application. Flexible polymer substrates are chemically incompatible with resists, etchants, and developers used in conventional integrated circuit (IC) processing. In practice, the usual IC fabrication processes involve multiple steps and high processing temperatures and produce toxic waste, all of which add to their cost. Furthermore, the increasing size of electronic devices such as displays poses great difficulty in adapting standard microfabrication techniques, including lithographic patterning.

The Laser Thermal Lab at the University of California, Berkeley, in collaboration with the UC Berkeley College of Chemistry and the Swiss Federal Institute of Technology, Zurich, has demonstrated a series of approaches to realize flexible electronics by combining three different techniques to achieve effective deposition of high-resolution metal patterns on heat-sensitive, low-cost, and lightweight plastic substrates at low temperature and under ambient atmospheric pressure. The new method does not use any expensive, toxic, or time-consuming lithographic methods.^1–4

The first technique exploits the lowered melting temperature of nanoparticles to reduce the processing temperature. Nanomaterials exhibit several interesting mechanical, chemical, optical, and electrical properties that cannot be observed in their bulk counterparts due to their large surface-to-volume ratio, substantial surface energy, and spatial confinement.^4,5 For example, bulk gold starts to melt above 1063*C. However, the melting temperature of the material drops to around 150*C when the size of the nanoparticle shrinks below 2nm due to the thermodynamic size effect. By using metal nanoparticles, we can handle and treat metal components at low plastic-compatible processing temperature using an inkjet-based solution process instead of a vacuum deposition method.^1,2

A second technique uses a focused laser beam to induce highly localized and efficient nanoparticle melting. Nanoparticle sintering could be achieved by applying an energy source such as a furnace or a laser beam. Local laser heating is preferable for two reasons: it produces a smaller heat-affected zone, and the laser deposits energy more efficiently because it takes advantage of the strong absorption peak generated by surface plasmon oscillation modes around the 520nm wavelength.^1–4,6-8 Both provide benefits when using polymer substrates with low transition/ melting temperatures. Laser-sintered gold lines showed much greater uniformity and higher resolution (down to 1–2μm) than the ones deposited by an inkjet process and sintered by a heater (~100μm).²

The third technique employs inkjet printing to apply a nanoparticle suspension to the substrate. Inkjet printing can deliver a vast range of functional materials with minimum waste and is easily integrated with computer-aided design (CAD) systems. The resolution of the conventional inkjet process is limited, but combined with laser processing can be enhanced to a level comparable with lithography.^1–3 In addition, this step can be replaced by very high (submicron) resolution direct nanoparticle nanoimprinting.⁹

We recently demonstrated fabrication of the basic building blocks for complex flexible electronics, organic field effect transistors (OFETs).^1–3 The process consists of first depositing metal nanoparticles using inkjet printing, subsequent laser processing of the nanoparticles, and then using inkjet printing to deposit a semiconducting polymer. The gold nanoparticles (1-3nm diameter) encapsulated by a hexanethiol self-assembled monolayer in organic solvent were synthesized using a two-phase reduction method.² The semiconducting polymer used in this research is a new, modified polythiophene containing electron-withdrawing alkyl carboxylate substituents. It exhibits high charge mobility as well as air stability, ^2,3 which is one of the key characteristics necessary for producing cost-effective electronics.

Figure 1 shows the two laser-based nanoparticle-processing approaches used to create high-resolution OFETs. The OFET arrays on a polymer substrate exhibit similar overall performance to the conventional photolithographically processed OFET with a SiO₂ dielectric layer (see Figure 2).

Figure 1. Laser-processed organic field effect transistors (OFETs). (a) Process steps for OFET fabrication by laser ablation and sintering of metal nanoparticle ink. (b) An OFET made using only nanoparticle selective laser sintering.² (c) An OFET produced by a combination of nanoparticle laser ablation and sintering. This novel multilayer processing scheme¹ allows selective laser ablation of an unsintered nanoparticle ink line to bisect the inkjet-printed nanoparticle line into two electrodes separated by a narrow channel. The two vertical lines are source (S) and drain (D) electrodes, and the underlying horizontal line represents a gate (G). Purple circles are inkjet-printed semiconducting polymer.

Figure 2. (a) OFET arrays on a polymer substrate fabricated by selective laser sintering.² (b) Comparison of normalized output characteristics shows that an OFET made by nanoparticle laser ablation and sintering (open symbols) on a SiO₂/P+ wafer has properties similar to an OFET prepared by conventional photolithography (solid symbols).

Based on the successful demonstration of the OFET array on a polymer substrate, the Laser Thermal Lab at UC Berkeley will pursue full active-matrix organic LED (OLED) display fabrication. Our research results, however, are of general utility beyond just displays. The methods can be applied to manufacturing low- cost, high-resolution, lightweight, flexible, and portable electronics without using any conventional vacuum or photolithography processes. In addition to flexible displays, we intend to expand the current research to other applications such as biological sensors and photovoltaics on polymer substrates.