Resistive random access memory (ReRAM) is a non-volatile, advanced memory technology. It is a top choice for replacing existing non-volatile memory technologies, for example flash. According to researchers, ReRAM is capable of achieving a switching time of less than 10 ns, and is more energy efficient and faster than traditional non-volatile memories.
This technology can be scaled down to smaller than 30 nm, which makes it more compatible with future semiconductor processing nodes. Furthermore, ReRAM can be integrated into three-dimensional device structures and this increases its density and versatility significantly.
The basic structure of a ReRAM device is made up of a thin semiconductor or insulator layer between two metal layers. This structure is generally known as a metal-insulator-metal or MIM device. Different types of insulator material have been investigated by scientists, who have discovered that specific oxides work well for ReRAM devices. These include TiO2, NiO, TaO2, SiOx and ZnO.
Modern electronic systems are reliant on devices that work in two different states – “off” and “on”. The basis of binary code is formed by this on-off functionality, and is comprised of “1”s (on) and “0”s (off). Binary code is the language of electronics. ReRAM devices also work in two distinct off and on states. The insulating layer functions as a variable resistor which provides the on-off functionality.
A low resistance pathway is formed when the ReRAM device is turned on. This allows electrical current to pass easily between the metal electrodes. When the device is turned off, the flow of electrical current is hindered because the insulator has a large resistance.
Scientists have suggested that the “turn on” mechanism is caused by the formation of metallic filament, enabling the electrical current to flow easily.
The Protochips Fusion heating and electrical biasing system is ideally suited to analyzing electric devices using in situ electron microscopy. Voltage and current measurements on devices can be easily obtained using the custom Fusion electrical biasing software.
Users can simultaneously image a sample and compare the electrical measurements with SEM and TEM observations. These include structural changes, diffraction, chemical changes in electron energy loss (EELS) and energy dispersive x-ray (EDS) spectra.
In the following experiments, researchers from National Chiao Tung University in Hsinchu, Taiwan developed a thin sample from a ReRAM device. This was made up of a ZnO layer between two Pt electrodes. The ZnO layer had a thickness of 100 nm and was deposited through RF magnetron sputtering.
In order to create a sample suitable to TEM, a focused ion beam (FIB) system was used to cut out a small section and thin it to approximately 50 nm. FIB induced metal deposition was used to make electrical connections from the metal leads on an E-chip to the device.
When the device had been inserted into the TEM, electrical biasing tools, integrated into the system, were used to apply a voltage to the device. At the same time they measured the current in situ. Simultaneously, scientists used JEOL 2100F TEM that operates in bright-field mode to observe device behavior in real-time. The ZnO structure changes were also examined using dark field imaging, diffraction, EELS and EDS.
The researchers imaged the formation of Zn filament in several areas of the device and behaviors observed corresponded well with earlier reports. The team was also able to elucidate the switching mechanism more clearly and ultimately proposed a model to describe the behavior. This was possible because they were able to directly observe the behavior of the Zn filament in real time and control the electrical stimuli.
By combining information from high-resolution images, EELS spectra and diffraction data to support their proposed model, the scientists were able to describe the physical and chemical behavior of filament formation. The formation of Zn filament is the result of a redox process, where oxygen atoms migrate, leaving oxygen depleted regions of Zn and ZnO1-x.
Previous reports support this observation and have demonstrated that oxygen species are more mobile than Zn in an electric field.
The formation of filament and the redox process is visually described in the schematic above. Filament formation often starts with a conical shape and then changes to a dendritic shape (Figure 2). This is due to the electric field enhancement at the tip of the cone, which causes the filament to branch out. It is possible to reset the device to its original state by applying an appropriate voltage. Therefore, the process can be observed multiple times.
As feature sizes of electronic devices become increasingly smaller, the TEM becomes a more useful and more powerful tool for investigating the behavior and operation of these devices. This is because it can resolve features down to the anatomic scale. The in situ heating and electrical biasing capabilities, combined with the resolving power of the Fusion system, make new and existing TEMs a more valuable tool for analysis.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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