730
24 Transistors
(a)
(b)
Fig. 24.41 a Cross-section TEM image of a cell from a Ramtron 4 Mb FeRAM. The information is stored in the
electric polarization of a polycrystalline Pb(Ti x Zr 1−x )O 3 (PZT) island, contacted on the bottom and top with platinum
and iridium oxide, respectively. Adapted from [2074]. b Cross-section TEM of the magnetic tunneling junction from a
Freescale 4.2 Mb MRAM, located between the M4 and M5 interconnect layers. The magnetization of the free layer can
be switched, that of the fixed layer remains constant. Adapted from [2074]
consumer grade. Recently also triple level cells (TLC), storing 3 bit (8 states) are commercialized,
however the increased storage density comes at high cost of reliability [2093].
The ultimate limit, explored currently, is to use a single electron charge to cause such an effect in
the single-electron transistor (SET).
Future Concepts
Memory concepts beyond the storage of free charges include information storage via
• the static polarization in a ferroelectric material (either crystalline or polymer) (FeRAM [2094],
Fig. 24.41a) which can be switched by an electric field.
• the phase change between amorphous and polycrystalline phases in a chalcogenide layer (typically
GeSb [2095] or Ge 2 Sb 2 Te 5 , GST [2096, 2097] with an α ↔ c transition, Fig. 24.42) upon local
heating (similar to a rewritable DVD) and the related change is resistivity (PCM, phase change
memory).
• the storage of magnetization direction (MRAM [2098, 2099]) and subsequent resistance change of a
magneto-tunneling junction (MTJ) whose resistance depends on the relative magnetization (parallel
or perpendicular) of two magnetic layers separated by a thin tunneling insulator (Fig. 24.41b). The
largest TMR (tunnel-magnetoresistance) effect has been achieved with MgO as insulator [2099].
The magnetization of the bottom magnetic layer of the MTJ is fixed. The magnetization directions
±45
◦ are written into the free layer with the magnetic fields of two perpendicular high current wires
in two subsequent back-end interconnect layers sandwiching the MTJ.
• resistance change based on solid electrolytes (PMC, programmable metallization cell memory). The
lowering of the resistance is attained by the reduction of ions in a fairly high resistivity electrolyte
(e.g. from the system Cu,Ag–Ge–Se,S,O [2100, 2101] or oxides [2102]) to form a conducting bridge
between the electrodes. The resistance is returned to the high value via the application of a reverse
bias that results in the breaking of the conducting pathway.
• resistance change in transition metal oxides such as perovskites, e.g. SrTiO 3 :Cr [2103, 2104] or
NiO:Ti (RRAM). Electrical pulses of opposite polarity switch the resistance reversibly between a
high- and a low-resistance state. Oxygen-vacancy drift modulates the valence of the mixed-valence
transition-metal ion (e.g. Ti
3+ –Ti
4+ ) and thus the conducting state [2105].
24 Transistors
(a)
(b)
Fig. 24.41 a Cross-section TEM image of a cell from a Ramtron 4 Mb FeRAM. The information is stored in the
electric polarization of a polycrystalline Pb(Ti x Zr 1−x )O 3 (PZT) island, contacted on the bottom and top with platinum
and iridium oxide, respectively. Adapted from [2074]. b Cross-section TEM of the magnetic tunneling junction from a
Freescale 4.2 Mb MRAM, located between the M4 and M5 interconnect layers. The magnetization of the free layer can
be switched, that of the fixed layer remains constant. Adapted from [2074]
consumer grade. Recently also triple level cells (TLC), storing 3 bit (8 states) are commercialized,
however the increased storage density comes at high cost of reliability [2093].
The ultimate limit, explored currently, is to use a single electron charge to cause such an effect in
the single-electron transistor (SET).
Future Concepts
Memory concepts beyond the storage of free charges include information storage via
• the static polarization in a ferroelectric material (either crystalline or polymer) (FeRAM [2094],
Fig. 24.41a) which can be switched by an electric field.
• the phase change between amorphous and polycrystalline phases in a chalcogenide layer (typically
GeSb [2095] or Ge 2 Sb 2 Te 5 , GST [2096, 2097] with an α ↔ c transition, Fig. 24.42) upon local
heating (similar to a rewritable DVD) and the related change is resistivity (PCM, phase change
memory).
• the storage of magnetization direction (MRAM [2098, 2099]) and subsequent resistance change of a
magneto-tunneling junction (MTJ) whose resistance depends on the relative magnetization (parallel
or perpendicular) of two magnetic layers separated by a thin tunneling insulator (Fig. 24.41b). The
largest TMR (tunnel-magnetoresistance) effect has been achieved with MgO as insulator [2099].
The magnetization of the bottom magnetic layer of the MTJ is fixed. The magnetization directions
±45
◦ are written into the free layer with the magnetic fields of two perpendicular high current wires
in two subsequent back-end interconnect layers sandwiching the MTJ.
• resistance change based on solid electrolytes (PMC, programmable metallization cell memory). The
lowering of the resistance is attained by the reduction of ions in a fairly high resistivity electrolyte
(e.g. from the system Cu,Ag–Ge–Se,S,O [2100, 2101] or oxides [2102]) to form a conducting bridge
between the electrodes. The resistance is returned to the high value via the application of a reverse
bias that results in the breaking of the conducting pathway.
• resistance change in transition metal oxides such as perovskites, e.g. SrTiO 3 :Cr [2103, 2104] or
NiO:Ti (RRAM). Electrical pulses of opposite polarity switch the resistance reversibly between a
high- and a low-resistance state. Oxygen-vacancy drift modulates the valence of the mixed-valence
transition-metal ion (e.g. Ti
3+ –Ti
4+ ) and thus the conducting state [2105].