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Ferroelectric random access memory (FeRAM) is an attractive candidate technology for embedded nonvolatile memory, especially in applications where low power and high program speed are important. Market introduction of high-density FeRAM is, however, lagging behind standard complementary metal-oxide semiconductor (CMOS) because of the difficult integration technology. This paper discusses the major integration issues for high-density FeRAM, based on SrBi2Ta2O9 (strontium bismuth tantalate or SBT), in relation to the fabrication of our stacked cell structure. We have worked in the previous years on the development of SBT-FeRAM integration technology, based on a so-called pseudo-three-dimensional (3D) cell, with a capacitor that can be scaled from quasi two-dimensional towards a true three-dimensional capacitor where the sidewalls will importantly contribute to the signal. In the first phase of our integration development, we integrated our FeRAM cell in a 0.35μm CMOS technology. In a second phase, then, possibility of scaling of our cell is demonstrated in 0.18μm technology. The excellent electrical and reliability properties of the small integrated ferroelectric capacitors prove the feasibility of the technology, while the verification of the potential 3D effect confirms the basic scaling potential of our concept beyond that of the single-mask capacitor. The paper outlines the different material and technological challenges, and working solutions are demonstrated. While some issues are specific to our own cell, many are applicable to different stacked FeRAM cell concepts, or will become more general concerns when more developments are moving into 3D structures.

The difficult scaling of ferroelectric random access memories with the complementary metal-oxide semiconductor technology roadmap requires integration of three-dimensional (3D) ferroelectric capacitors (FeCAP’s). In this work the unusual electrical behavior of 3D FeCAP sidewalls was studied by comparing the electrical properties of two-dimensional and 3D integrated FeCAP structures. We evidenced composition variations of the SrBi2Ta2O9 (SBT) film in the sidewalls with marked bismuth segregation during metal-organic chemical-vapor deposition (MOCVD) of the SBT film. The segregation was reduced after decreasing the deposition temperature from 440°C, whereby the Bi-rich phase in the sidewalls does not contribute to polarization, down to 405°C, whereby sidewall SBT contributes to polarization. After further optimization of the MOCVD conditions at 405°C, the segregation is minimized and the ferroelectric contribution of the sidewall SBT is almost the same as the contribution of the planar SBT. As a result, 3D FeCAP’s integrated up to metal interconnection exhibit a remnant polarization Pr∼7.5μC∕cm2.

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