Avalanche Technology’s Space Grade Ava-MRAM is built with industry leading data retention in mind. Ava-MRAM stores its information within a Perpendicular Magnetic Tunnel Junction (pMTJ) whose underlying physics grant its extraordinary data retention time. For space missions of any type, ensuring your data stays intact should not be constrained by the memory hardware. While many technologies claim to be “Space Grade,” true Space Grade requires that 5 objective criteria be met simultaneously. Space Grade technology must:
- Survive radiation without disruption
- Retain data for the life of a mission
- Endure unlimited writes
- Commit data instantly
- Prove real space heritage
This blog covers the second pillar of Space Grade, RETAIN, describing the ability to accurately retain data beyond the requirements of any given mission.
Memory Solutions Available Today
There are many considerations made when choosing the right memory for any application. Sustained exposure in extreme environments, such as space, poses additional difficulties for traditional memory devices. Consider the following memory types: FRAM, RRAM, and Toggle MRAM. All of these memories position themselves suitable for extreme industrial environments and space. Ferroelectric RAM, or FRAM, is a non-volatile memory technology that uses ferroelectric materials for durable storage with high endurance for read/write cycles. It offers fast write speeds, low power consumption, and strong resilience to radiation and extreme environments. Resistive RAM, or RRAM, is another non-volatile memory, built using a metal-insulator-metal structure, that promises similar features like low power operation and radiation resistance. In the future, it may offer high-density integrations, fast switching, and has a potential for scalable and low cost production. Toggle MRAM is a non-volatile technology that uses magnetic tunnel junctions with field-switched toggle-mode switching to store data via electron spin states. It provides SRAM-like speed, high endurance, and high reliability.
However, while these different memory technologies theoretically provide benefits for space applications, they have not been manufactured into real-world products at high densities. For FRAM, RRAM, and Toggle MRAM, their maximum densities available today are 16 Megabit, 12 Megabit, and 16 Megabit maximum. When comparing these memories to Avalanche’s Space Grade Ava-MRAM, the density differences are clear: Avalanche today offers up to an 8 Gigabit discrete product. The other memory technologies aren’t Space Grade because of their low densities, and will be excluded from this discussion.
Data Retention and Endurance: Measuring
JEDEC (formerly Joint Electron Device Engineering Council) is a global semiconductor industry standards organization. It develops and maintains technical standards for memory, storage, and other microelectronic components to ensure interoperability, reliability, and consistent testing methods across manufacturers. Accelerated testing is one such procedure outlined by JDEC. Because of the distinct physical structures of each memory technology, memory cell breakdown occurs in unique ways. Specifically, the failure mode of each memory cell type results from different stress vectors applied before or during a high-temperature bake.
Failure Modes
NAND and NOR flash memory function on a charge-trap based memory cell. A floating gate holding a charge is read as a “logical 0” and a floating gate not holding a charge is a “logical 1”. The thin oxide layer surrounding the floating gate breaks down over time and with the stress of write cycles, also called program cycles, on a cell. This damage to the oxide layer allows electrons to escape from the floating gate. The memory cell is dead when it functionally gets locked into a “logical 1” state. The number of times a cell is write-cycled dramatically affects the total lifespan of flash memory.
STT-MRAM cells operate on a different physical mechanism and consequently have a different failure mode. The base structure of the STT-MRAM cell is the Perpendicular Magnetic Tunnel Junction (pMTJ). This structure contains one tunnel barrier layer fixed between two ferromagnetic layers. The free layer can have its magnetic direction changed externally through thermally activated magnetic switching. This causes either high or low resistance within the tunnel which indicates a “logical 1” or “logical 0”, respectively. The mechanism of compromise for the pMTJ is Time-Dependent Dielectric Breakdown (TDDB). When the barrier oxide layer between the fixed and free layer breaks down, conductive paths can build through it and cause a low resistance path through the cell. Functionally, it locks the pMTJ into a “logical 0” state.
NAND and NOR vs STT-MRAM Data Retention Testing
JEDEC data retention validation uses the data gathered from accelerated testing and the use of an Arrhenius relationship to extrapolate lifespan at normal operating temperatures. When accelerated testing, a device is raised to an elevated temperature and held there for a determined period of time. Both for flash memory and for STT-MRAM devices, accelerated testing is used for data retention validation. The difference being that the activation energy term in Arrhenius equation does not apply to MRAM devices since NOR/NAND and MRAM have differences between their structure and failure modes.
Because STT-MRAM cells operate through thermally activated magnetic switching, reliable data storage depends on a device’s high temperature tolerance over long periods of time. The testing procedure begins with loading the device with a known data set. Then, it is subjected to accelerated testing. Bit error rate increase determines device’s reliability in data retention.
NAND and NOR flash devices require different testing considerations compared to STT-MRAM. This is because the total number of write cycles sustained on the memory cell affect data retention. In testing, dedicated memory blocks are write-cycled between 0% and 100% of their maximum cycle specification. A known data pattern is then loaded onto the devices followed by a high temperature bake. A pass is given if the devices tested meet the data sheet specification and if it has accurately retained the same data pattern.
Data Retention Across Memory Types
Initially, SRAM and DRAM may appear as attractive choices for space missions. They have a proven history as reliable terrestrial memory devices. But for applications in long term data storage, their volatility is their main weakness. Both aeronautic and astronautic applications are strictly constrained by their power generation and energy storage capabilities. So, for SRAM and DRAM, even though they work well in a terrestrial environment, they do not appeal to this use case.
NAND flash provides the highest density of modern memory devices. Unfortunately, as NAND flash density increases, long-term data retention reliability is demonstrably compromised. Even within the common NAND technologies available, that is SLC, MLC, and QLC, only the lowest density SLC provides the best data retention figures. The higher density cells are more sensitive to charge loss as their data is stored within narrower voltage thresholds. Also as discussed above, the amount of memory write-cycling done to a device severely affects its data retention. For SLC NAND, data retention is about 10 years at room temperature, or 25°C. There exists an additional downside to using NAND as long term non-volatile storage. For data integrity purposes, periodic scrubbing is required. The device needs to be occasionally powered up so the memory controller can perform read-scrub-write operations to maintain data integrity.
NOR flash has a more robust memory cell composition. Although much less dense than NAND flash, it provides notable gains to data retention. Like NAND, memory cell write-cycling severely impacts total data retention time as cells become worse at holding onto charge. Therefore, only uncycled memory figures will be considered in this comparison. The data retention period for uncycled NOR flash is about 20 years at 70°C. It is common practice for NOR flash subjected to extreme environments to have periodic data scrubbing like NAND flash. Meaning, periodic powering of the device is required to achieve these data retention figures. If one were to accept these same downsides, it is a major improvement in total data retention time compared to NAND flash.
Other formulations of NOR flash have been developed to increase its durability and radiation resistance. SONOS NOR flash boasts high data retention figures over traditional NOR flash.
Avalanche Technology’s Space Grade Ava-MRAM provides exceptional data retention at industry-leading densities. Its proprietary pMTJ formulation and integrated reliability engineering outperform other STT-MRAM solutions, enabling memory devices that withstand 125°C for 10 years. Extrapolated to room temperature (25°C), that translates to a remarkable 7 million years of data retention, reliably exceeding the lifespan of any mission. The pMTJ’s stable structure provides true non-volatility, meaning the device requires no power for lifelong data integrity. Additionally, its SRAM-like operation eliminates the need for flash management overhead.
Achieving Space Grade
For mission designers operating in the unforgiving conditions of space, data retention cannot be an afterthought or a managed compromise. The RETAIN pillar of Avalanche’s Space Grade standard reflects a straightforward principle: memory hardware should never be the limiting factor in a mission’s lifespan. The structure of Avalanche Technology’s proprietary formulation of pMTJ provides extraordinary data retention. It is a physics-defined advantage over other memory technologies available on the market today with superior data density.
Ultimately, there is no one requirement of Space Grade that stands above the rest. Avalanche reaffirms its leadership as the only provider of true Space Grade memory, delivering solutions that meet all five non-negotiable criteria required for space missions: Survive, Retain, Endure, Commit, and Prove. Space Grade technology must survive radiation without disruption, retain data for the life of a mission, endure unlimited writes, commit data instantly, and prove real space heritage.
In space systems, a technology that survives radiation but loses data, wears out over time, delays data commitment, or lacks flight heritage is not Space Grade. Partial compliance leads to failure. Avalanche Space Grade Ava-MRAM uniquely delivers on all 5 criteria simultaneously, through radiation immunity, permanent data retention, unlimited endurance, deterministic nanosecond writes, and proven flight qualification, without tradeoffs.
