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Fundamentals of resistive memory devices

Posted: 24 Jul 2014 ?? ?Print Version ?Bookmark and Share

Keywords:non-volatile memory? NVM? NAND flash memory? MOSFET transistors? Resistive random-access-memory?

For many portable electronics, the non-volatile memory (NVM) technology of choice has long been the floating-gate NAND flash memory. Flash cells are implemented on the foundation of MOSFET transistors, so they have standard source, gate, drain, and bulk/substrate connections. Fowler-Nordheim current tunnelling through gate oxide and Hot Carrier Injection represent the two standard methods for storing and removing charge from the floating gate. These methods are degradation mechanisms of standard (non-NVM) MOSFET transistors, which are also responsible for the finite number of write/erase cycles (endurance) of flash memory.

Manufacturers of consumer products that incorporate memory devices are increasingly concerned that floating-gate flash memory won't be able to continue providing higher storage capacities at the ever lower cost-per-bit requirements that drive the NVM market. Electrical characterisation is key to making the transition from materials research to a commercial product. Resistive random-access-memory (ReRAM or simply RRAM) offers the potential to be an alternative to floating-gate flash technology and it's now inching closer to commercial production. In fact, one manufacturer, Crossbar Inc., has already developed a high-density, high-speed, filament-based ReRAM memory structure.

A typical ReRAM cell has a switching material with different resistance characteristics sandwiched by two metallic electrodes. The switching effect of ReRAM is based on the motion of ions under the influence of an electric field or heat and also the switching material's ability to store the state of the ion distribution. This in turn causes a measurable change in the device resistance.

ReRAM is faster and requires lower voltage than traditional flash memory. It is bit-alterable, making it suitable for use in both embedded and solid-state drive (SSD) applications. The ReRAM cell structure offers high area efficiency and scalability. It also offers the potential for 3D integration. ReRAM requires lower programming currents than phase-change memory (PC RAM) or magneto-resistive memory (MRAM), with comparable retention and endurance.

Table: Summary of the important test parameters for characterizing ReRAM devices.

NVM characterisation basics
Electrical characterisation of floating-gate flash memory was traditionally performed using DC instruments, such as source measure unit (SMU) instruments, after pulse generators had programmed and/or erased the memory cell. This required some type of switch to apply the DC or pulse signal alternately to the test device. Occasionally, oscilloscopes were used to verify pulse fidelity (pulse width, overshoot, pulse voltage level, rise time, and fall time) at the device under test (DUT). Measuring the pulse is important because the flash memory state is quite sensitive to the pulse voltage level.

However, the use of oscilloscopes was relatively rare, even in research, because the required setup for oscilloscope measurements differed from that for the pulse-source/DC-measure approach. Even when scopes were used for flash characterisation, the complexity of measuring the transient current meant that voltage was the only measurement taken while pulsing. The transition to smaller geometries and multi-bit cells has increased the need for more precise pulse source and measurement for floating-gate flash development.

In addition to accurate pulse levels, characterizing the newest NVM technologies requires the ability to produce complex, easily adjustable waveforms, not just a single standard square pulse. For example, testing ReRAM devices often requires pulse sweep up/sweep down profiles while simultaneously measuring the current. This memory technology requires the ability to output multi-pulse waveforms consisting of arbitrary segments, together with multiple measurements within each waveform. Endurance testing requires the ability to output complex arbitrary waveforms quickly without the need for additional setup time or overhead.

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