Integration of Redox-Based Resistive Switching Memory Devices
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Beschreibung
The steadily growing market for consumer electronics and the rapid proliferation
of mobile devices such as tablet computers, MP3 players and smart phones make
high demands for the nonvolatile memory. Present FLASH memory technology approaches
to the end due to physical scalability limits. Therefore, an alternative
technology must be developed. For memory technology, not only the storage density
and cost are important factors but the power consumption and the writing/reading
speed must also be taken in account. Redox-based resistive memory (ReRAM) offers
a potential alternative to the FLASH technology and presently is in the focus of research
activities. The operating principle of the ReRAM is based on the non-volatile
reversible change in resistance by electrical stimuli in a simple metal-insulator-metal
(MIM) device architecture. This simple structure enables the integration of ReRAM
in passive crossbar arrays, in which each crosspoint consumes only 4F2 (F- feature
size) device area. This leads to an ultra-high storage density at reduced cost.
Research on the ReRAM memory elements requires a technology platform that ensures
a cost-effective fabrication of the crossbar devices with nanometer feature size.
In this thesis, the fabrication processes have been developed based on the nanoimprint
lithography, which facilitates both the high resolution (<50nm) and the high
throughput at low cost. The stamp for the UV-nanoimprinting is developed with
plasma etching and electron-beam lithography. This process facilitates the fabrication
of the ReRAM devices sizes ranging from 40x40 nm2 to 100x100 nm2. The
fabricated nano-crosspoint ReRAM of different switching layer thickness and different
device areas are electrically characterized. In order to toggle the resistance state
in the ReRAM device, an electroforming step is generally required. In this work, a
systematic analysis of the electroforming process is carried out on TiO2 and WO3-
based ReRAM cells and the respective switching characteristics are investigated. The
switching mechanism is explained by the filamentary conduction model. The forming
voltage decreases with decreasing oxide layer thickness whereas it increases for the
smaller device size. Due to overshoot phenomena during the electroforming process,
these devices show a significant increased switching current, lower non-linearity, and
lower endurance. The ReRAM device performance is improved by integration in the
backend of a 65nm CMOS process. In the integrated 1T-1R stack, the electroforming
is performed by controlling the current flow with the gate electrode. By employing
this approach, the switching current in the ReRAM devices is reduced to 1 A. In
order to lower the sneak path current in the passive crossbar arrays, a high degree of
nonlinearity is required. This nonlinearity parameter has been investigated with 100
ns transient pulses in the nano-crossbar devices and in the 1T-1R structures. This
parameter depends on the switching current and switching material properties. The
lower switching current in the TiO2 ReRAM leads to the higher nonlinearity.
Furthermore, the ReRAM nanodevices inherently exhibit open clamp voltage in the
switching characteristics. This phenomenon is explained by the electromotive force
(EMF). The amplitude of the generated EMF voltage depends on the nature of the
switching materials and can be several hundred mV. This degrades the conducting
filament and thereby limits the ON state retention properties of the ReRAM devices.
Additionally, the non-zero crossing of the I-V characteristics, caused by the
EMF voltage demands the refinement of the memristor theory.
Autor*in
Florian Lentz