Printed Logic and Memory
December 1,2011, Printed Electronics USA 2011, Santa Clara, CA—Janos Veres from PARC detailed the developments in printed logic and memory devices. Their approach is to create technologies and systems in building-block form.
PARC has extensive experience in printed sensors, logic, memory, and active matrix backplanes. They continue to push capabilities and limits in all areas of the technology for internal developments in micro-systems, MEMS, lasers, and LEDs. So far, they have work in amorphous and poly-silicon as well as in organics, oxides , and nanowires.
Their focus is on flexible circuits applied through printing processes like inkjet and gravure. Organic electronics, and hybrid printed and IC systems are candidates for the many application development areas. The delivery platforms for printed electronics include inkjet, laser, gravure, screen, and lithography. To ensure their results, they have extensive metrology and automate testing facilities. They take advantage of the many different capabilities, through the circuit design and simulation, and tie the designs to careful device and molecular modeling.
An example of their work include active matrix TFT arrays for display backplanes. The system-level design process is simplified by a library of printed circuit building blocks. These blocks include ring oscillators, shift registers, amplifiers, and memory. Much of the circuitry is designed with printed complementary TFTs that have closely matched P and N carrier mobility of about 0.1 cm2.Vs at 20 V.
Interconnects and contacts are printed with silver-based inks. The complementary devices offer a self-compensation mechanism that partially cancels the shifts in operating points for the devices. So far, the main source for variation is due to the semiconductor morphology. The printing variations at the resolutions they are using are secondary effects.
Systems are not just logic and sensors, so they are working on a number of memory types and configurations. The active matrix memory uses a multiplexed row and column architecture in a thin film organic logic process. Memories are built up as arrays of cells, but are limited to fairly small sizes. Another memory type is a memristor which changes value based upon the integral of the applied voltage over time.
Sensors are available for pressure, sound, light, and x-rays. All of these can be applied with inkjet printing. Temperature sensors can be screen printed, and one application is a multi-parameter printed sensor tape. This tape can monitor the environment to prevent traumatic brain injury. It measures pressure, acceleration, sound, and light. The various signals are acquired, processed, and stored in the tape. The non-volatile memory can hold the data for one week. Data is downloaded to an external computer for incident and trend analysis.
Their temperature sensors can support a 0.1° C resolution in the body temperature range for medical devices. In other configurations, they can be used for package tracking up to 70° C. The materials are screen printable and stable with good flex characteristics that avoids de-lamination and cracking.
To characterize and monitor their devices, they have developed a number of circuits. Circuits designs have to be resilient and robust to operate over the wide range of device parameters, inverters can measure up to 9x variation in mobility. Other circuits are building blocks. Currently, most of the circuits are small scale functions like inverters, NAND, NOR, transmission gates, shift registers, amplifiers, and ring oscillators.