Published online on April 10, 2012 by Jennifer Ernst
Printed electronics memory: challenges of logic and integration
Printed electronics have long been touted as disruptive technology. With printed electronic memory now commercially available, the next phase for building the ecosystem will be adding logic to enable new applications. But how is that done? What technical challenges does it entail? What are the types of applications that it will enable?
Currently, non-volatile, rewritable printed memory is commercially available in the form of stickers that can be attached to cards, toys, and other flat or smoothly curved surfaces. They are among a new generation of products that take advantage of printing’s ability to not only coat a surface, but to create patterns on it. While lower in their performance than electronics developed in conventional silicon, printed electronic devices have advantages in thin and flexible form factors, disposability, distributed and environmentally friendly manufacturing, and cost. These advantages become most prominent when comparing systems in which multiple printed components are working together, but until recently, a key piece has been missing to drive the industry forward — the logic to enable integration.
The addition of printed logic makes it possible to connect memory with other printed components, such as sensors and displays. This integration opens the doors for fully printed sensor tags, disposable price labels, wireless ID tags, and other smart objects, at a fraction of the cost of conventional silicon-based electronics. Printed rewritable memory is also expected to be a key enabler for mass-scale adoption of near field communications (NFC).
Given that printed electronics have been in research for several decades, and printing processes themselves have been around for centuries, the obvious questions are: Why has it taken so long for commercial products to emerge, and what is different today?
Fundamentally, the design of any printed electronic device, whether memory, logic, or integrated systems, starts with the materials used to create it. Printed electronics are made with special functional inks that, like their counterparts in traditional electronics, must be optimized for both their function in the device and the technologies used to deposit the materials. How large the circuits are, what functionality and precision can be achieved, how much power is required to drive a device — everything is dependent, at the first order, on the performance of the materials themselves.
In recent years, chemical advances have yielded materials that are “good enough” for commercial application, in some cases approaching or matching amorphous silicon performance. Advances from companies like Polyera, BASF, and Merck in developing n-type materials opened the door to more power-efficient circuitry through the use of complementary materials, in the same way circuits are designed in industry-standard CMOS. Finally, companies have focused on compatibility between materials that are used to create different layers — their own and those of others companies. What used to be the black magic of the research lab is becoming the product suite of major materials companies.
Think of this as vertical compatibility. Each layer — from silver electrodes to memory film, from dielectrics to semiconductors — must be compatible with the layers below and above it in multiple ways. For example, the solvents used in one ink cannot dissolve the layer below when that ink is printed. Adhesion in the wet state of the ink must be optimized to create the right shapes and patterns, so that the ink neither spreads too much nor pulls away from the layer below. Adhesion in the dry state must be optimized so that when the finished device is subjected to quality testing, layers do not separate and peel away. Much work has to be done empirically on these factors, as few design rules have been established to date centric to the printing of multi-layer electronic devices.
A second dimension of compatibility can be thought of as horizontal compatibility — from component to component. It has been shown not merely that the individual components of memory and logic can be constructed by printing, but that the resulting devices have compatible performance specifications. For example, organic circuits are intrinsically noisier than their silicon counterparts, with more variability from transistor to transistor. Despite such variability, though, the integrated memory-plus-logic device can be written reliably, in sequence.
The integrated memory-plus logic device, or addressable memory, is now a building block for adding additional components. For example, a printed temperature sensor will be able to write data to the memory, which can then be read through an external reader, a display or an RF circuit. As each type of component is shown to work with other pieces of the system, the number of applications that can be addressed by printed electronic devices increases exponentially. The integration of components into systems significantly decreases the cost-per-function. So while the selling price of an individual printed memory today is slightly less than the lowest cost silicon options, the cost of an integrated sensor-plus-digital-readout device will be 1/10th to 1/100th the cost of current silicon systems.
Once devices are designed, how they will be manufactured, or in this case, printed, must be determined. Device feature size and ink compatibility were major considerations for designing logic devices that can be printed in a high-speed roll-to-roll process.
Some of the most important recent advances in printed electronics haven’t been on the technical side, but on the business side. In the past, printed components have been demonstrated at universities and research labs, and presented to the market in isolation. Product developers were left to figure out how, or if, they could use such components in combination with traditional silicon or with other printed devices. In the past two years, though, several companies have demonstrated the value of working together to deliver complete solutions. These types of business partnerships will be central to establishing the total supply chain for printed electronic products.
Printed addressable memory is as an important building block for fully printed systems that can address a myriad of markets. Where memory provides the ability to store data on any object, to retrieve when and where it’s needed, printed logic is the glue that will enable other devices to generate, store and read that data. Literally any object can be “smart.”