Brian Mattis

Printed electronics for low-cost electronic systems: Technology status and application development

In recent years, printing has received substantial interest as a technique for realizing low cost, large area electronic systems. Printing allows the use of purely additive processing, thus lowering process complexity and material usage. Coupled with the use of low-cost substrates such as plastic, metal foils, etc., it is expected that printed electronics will enable the realization of a wide range of easily deployable electronic systems, including displays, sensors, and RFID tags. We review our work on the development of technologies and applications for printed electronics. By combining synthetically derived inorganic nanoparticles and organic materials, we have realized a range of printable electronic ldquoinksrdquo, and used these to demonstrate printed passive components, multilayer interconnection, diodes, transistors, memories, batteries, and various types of gas and biosensors. By exploiting the ability of printing to cheaply allow for the integration of diverse functionalities and materials onto the same substrate, therefore, it is possible to realize printed systems that exploit the advantages of printing while working around the disadvantages of the same.

A novel transparent air-stable printable n-type semiconductor technology using ZnO nanoparticles

We report on a novel, air-stable, printable, transparent, NMOS semiconductor technology using soluble ZnO nanoparticles. We demonstrate solution-processed transistors with mobility > 0.1 cm/sup 2//V/spl middot/s, which is the highest solution-processed NMOS mobility reported to date. The air-stability and transparency make this device an ideal candidate for low-cost printed displays and CMOS circuitry.

Inkjetted Organic Transistors using a Novel Pentacene Precursor

Pentacene is one of the most promising organic materials for organic transistor fabrication, since it offers higher mobility, better on-off ratio, improved environmental stability, and better reliability than most other organic semiconductors. However, its severe insolubility renders it useless for the solution-based fabrication of electronic devices. Solution-based processing is the key to enabling ultra-low-cost circuit fabrication, since it eliminates the need for lithography, subtractive processing, and vacuum-based film deposition. Using a recently developed soluble pentacene precursor, we demonstrate the first inkjet-printed pentacene transistor fabricated to date. This is achieved using a substrate-gated transistor structure in conjunction with an inkjetprinted pentacene precursor active layer. After deposition, the precursor is converted to pentacene via heating, through the decomposition of the Diels-Alder product. As the anneal temperature increases above 120°C, performance increases dramatically. The process is therefore compatible with numerous low-temperature plastics. As the anneal time is increased to several minutes, performance likewise increases through increased precursor decomposition. However, exposure to excess temperatures or times tends to degrade performance. This is caused by morphological and chemical changes in the pentacene film. Optimization of the anneal process alone has resulted in the demonstration of transistors with an on-off ratio of >10 5 and field-effect mobility of >0.01cm 2 /V-s, attesting to the great promise of this material.

Effect of thermal cycling on performance of Poly(3-hexylthiophene) Transistors

We present the results of studies on the electrical and physical modifications to Poly(3hexylthiophene), upon thermal annealing. Thermally-induced performance modifications and thermal stability of polythiophene thin film transistors are explored. We observe substantial mobility improvements in devices annealed at low temperatures (<80°C), as well as increases in on/off ratios by two orders of magnitude at moderate anneal temperatures (~120°C). We document changes in conductivity, mobility, on current, and on/off ratio with anneal temperature and total thermal budget. In conjunction with material analysis, we develop qualitative models for the mechanisms involved in the annealing/ degradation processes. Hence, this study provides a comprehensive analysis of the effect of thermal cycling of polythiophene TFTs on various device performance metrics, and identifies the relevant thermal limits and failure mechanisms.

Printed organic transistors for low-cost RFID applications

Printed electronics is attractive as a pathway towards the realization of ultra-low-cost RFID tags for replacement of conventional optical barcodes. While this application has received tremendous attention in recent years, it also represents one of the most challenging applications for organic transistors, based on both the performance requirements and the process complexity and cost implications. Here, we report on our progress in developing materials and processes for the realization of printed transistors for low-cost RFID applications. Using inkjet printing of novel conductors, dielectrics, and organic semiconductors, we have realized printed transistors with mobilities >0.1cm2/V-s, which is approaching the requirements of certain RFID applications. We review the performance of these devices, and discuss optimization strategies for achieving the ultimate performance goals requisite for realizing printed RFID.

Stacked low-power field-programmable antifuse memories for RFID on plastic

We demonstrate a stacked antifuse-based field-programmable memory utilizing organic materials on plastic to enable applications such as the encoding of low-cost RFID tags. We demonstrate 2 stacked levels with 100 bits/level, delivering excellent read margins and disturb immunity. These devices show high-speed programming capability (25ns) and ultra-low programming energy requirements of <1nW/bit. This technology is therefore extremely promising for plastic electronics applications

Nanoscale device isolation of organic transistors via electron-beam lithography

We investigated the use of electron-beam lithography on pentacene and poly(3-hexylthiophene) field-effect transistors to achieve device isolation and enable the realization of nanoscale organic circuits. We determined the doses and exposure linewidths needed to suppress carrier transport, enabling direct patterning at the nanoscale. The precision limits were also studied through an analysis of proximity scattering effects. With an optimized pattern and exposure, we reduced off-currents by almost four orders of magnitude and gate leakage by three orders of magnitude on backgated devices. Our electron-beam isolation methods also increased the on/off ratio and drastically improved the subthreshold swing, thus attesting to the viability of this technique for patterning of organic circuits at the nanoscale.

A field-programmable antifuse memory for RFID on plastic

We demonstrate an integrated field-programmable nonvolatile memory technology on plastic, thus realizing a low-cost memory technology for systems on plastic such as RFID tags and sensors that require post-fabrication encoding with unique ID numbers. The crossbar memory array is integrated with steering diodes for every memory element, thus ensuring excellent addressability and scalability to large array sizes. Pentacene-aluminum schottky diodes were combined with a polyvinylphenol (PVP) dielectric to create field programmable nonvolatile memory crossbar arrays on a flexible PEN substrate. Our devices show high programmed/unprogrammed current margins (up to 10,000) at read voltages of 6V. Programming voltages are >20V, providing excellent read-write margin.