An Overview of FeRAM Technology for High Density Applications
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An Overview of FeRAM Technology for High Density Applications
Nicolas Nagel, Thomas Mikolajick, Igor Kasko, Walter Hartner, Manfred Moert, Cay-Uwe Pinnow, Christine Dehm and Carlos Mazure Infineon Technologies, Memory Product Division Otto-Hahn Ring. 6, 81739 Munich, Germany
ABSTRACT Ferroelectric random access memories (FeRAMs) are new types of memories especially suitable for mobile applications due to their unique properties such as nonvolatility, small DRAM - like cell size, fast read and write as well as low voltage / low power behavior. Although standard CMOS processes can be used for frontend and backend / metallization processes, FeRAM technology development has to overcome major challenges due to new materials used for capacitor formation. In this paper, advantages and disadvantages of different ferroelectric materials and major development issues for high density applications are discussed. Results of a 0.5µm ferroelectric process using SrBi2Ta2O9 (SBT) as ferroelectric layer, Pt as electrode material, and 2-layer tungsten / aluminum metallization are discussed.
INTRODUCTION In recent years ferroelectric memories (FeRAMs) have attracted considerable attention as a possible next generation nonvolatile memory technology [1]. This is due to the fact that FeRAMs promise to combine DRAM benefits like small cell size, low voltage and fast access time with a nonvolatile data storage (see table I). Up to now only low density products have appeared in the market. For high density applications some major technological issues remain to be solved. An overview of these problems is given in the following sections.
Table I. Comparison of FeRAM with DRAM, Flash and SRAM
Read cycles Write cycles
FeRAMa
DRAM Flash
SRAM
1012 (1015)
1015
1015
1015
Access time
1012 (1015) 1015 106 1015 100ns (20ns) 40-70ns 40-70ns 6-70ns
Write time
100ns (20ns) ns
µs
Ns > 4x
Relative cell size 2x-5x (1x)
1x
1x
Data retention
None
10 years None
a
10 years
The numbers in parentheses refer to the prognosticated values for further generations.
CC1.1.1
Table II. Comparison of PZT and SBT as ferroelectric layer in FeRAMs Coercive Voltage
Read/Write Cycles
Leakage Current Density (A/cm2)
Switched Polarization (µC/cm2)
Processing Temperature (°C)
PZT 2V @ 50nm
1012
10-7-10-6 @ 4V and 200nm
30-60
550-700
SBT 2V @ 200nm
> 1014
10-9-10-8 @ 6V and 200nm
15-25
650-800
Either Pb(Zr,Ti)O3 (PZT) or SrBi2Ta2O9 (SBT) is used in current applications. SBT offers the benefits of low fatigue and low voltage operation while PZT offers seemingly easier integration scheme due to its lower processing temperature and higher polarization values (see table II for details). Ferroelectric memories can be operated in a 2T / 2C (2 transistor / 2 capacitor) or in a 1T / 1C (1 transistor / 1 capacitor) mode. The 2T / 2C mode offers the benefit of one reference capacitor for each cell and therefore an increased reliability margin. However, for high density applications a 1T / 1C approach is essential. The simplest way to integrate a f
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