Ferroelectronic ram memory family for critical data storage

Abstract

Ferroelectric materials show a spontaneous electrical polarization that can be reversed in sense by an applied external electric field. It should, therefore, be feasible to build a ferroelectric memory device that can store information in digital form. Early attempts to build such a memory have failed for various reasons, the major one being a lack of a well defined and stable coercive field, which resulted in the eventual loss of data due to half-select pulses applied to unselected cells in the crosspoint array architecture. Fatigue or wear out was also a problem in that the amount of available signal depended upon the number of polarization reversals. We have been able to overcome these problems by a combination of design innovations and process/materials breakthroughs. We have chosen PZT (lead zirconate titanate) as the basic ferroelectric material and integrated it into a standard 1.5μ CMOS process. PZT has a wide temperature range (+ 350°C Curie Temperature), low coercive voltage, high specific polarization charge (10-20 μC/cm2), good retention, and good endurance. The lack of a well defined coercive field was overcome with a DRAM like circuit architecture, which provides for transistor switches in series with each ferroelectric element preventing disturb pulses from affecting the unselected cells. A family of 1 K, 2 K, 4 K, 8 K, and 16 K-bit Ferroelectronic RAM memories has been developed for critical data storage applications. These FRAM® products are built by Ramtron using an integrated ferroelectric/semiconductor process initially demonstrated at the University of Colorado Microelectronic Research Lab and currently being fabricated at ITT Semiconductors in West Germany. The FRAM is the world's first integrated “nonvolatile static RAM.“