Effect of Radiation on Solid State Drive ( SSD)
Effect of Radiation on SSD for Space Applications.
As the need for lower-cost, higher-density mass storage increases, Aerospace developers and Space engineers are looking towards non-volatile memories such as NAND flash and solid state drives as alternatives for mass storage in space applications.Because of the harsh environment and radiation effects, there are many factors to consider when looking for the best storage solution and to ensure future unmanned deep-space mission success.
Why the need for Radiation Hardened SSD ?
SSD’s are used for storing mission critical data,- – Radiation caused SSD functional failure can result into catastrophic event.
• No testing is performed by SSD Manufacturers to understand the impact of radiations on functionality of SSD drives.
• Study will provide valuable data for SSD drives under consideration for Enterprise usage.
• Test Results will help better understand various type of failure modes in SSD under radiation environment.
Types of Memories
The types of digital storage devices can be divided into two very broad categories: • Volatile memory describes devices that must remain powered on for storage to persist. Examples include the DRAM and SRAM used in computers that only remember information while a computer is turned on.
• Nonvolatile memory devices can retain stored data without application of power. Examples include magnetic tape, PROMs and EEPROMs, hard drives, solid-state drives, and flash drives. Flash Memory—Solid-State Drives
• NAND logic – Can be written and read in blocks of bytes – Has shorter erase and write times – Needs less die area per cell (therefore lower cost per MB) – Has longer endurance (more write/read cycles) – NAND architecture is inherently more susceptible to total ionizing dose damage than the NOR architecture
• Solid-state drives – Combine a flash memory (usually NAND) with a controller as an interface to the host computer – Controller can include functions such as error correction, encryption, rapid erase, etc. – Greater density, lower power, and lower cost than SRAM/DRAM solid state recorders
Typical Space Requirements Many storage technologies are not practical for space because of the particular requirements for satellites and space probes:
• Low mass
• Small volume
• Low power consumption
• Must be able to function in zero gravity
• High reliability (repairs not possible)
• Low error rate
• Long life: 5 – 15 years with no maintenance
• Operating range of -55°C to +125°C
• Radiation-hard: requirements vary considerably by application
– Total dose
– Dose rate upset
– Dose rate survivability
• Tape and Magnetic Disk
• Flash memory—solid-state drives – NOR – NAND – Solid-state drives (SSD)
• SRAM – Battery-backed SRAM – Nonvolatile SRAM
Sources of radiation
Electronic memory performance is highly affected by cosmic rays. While we have the ozone layer to shield us from radiation while we are on earth, the picture changes once our computers are above the earth’s ozone layer. The explosive energy from the sun and stars can generate tremendous waves to wipe out memories in our space vehicles. The sun produces electromagnetic radiation of particles in the form of x-rays and gamma rays. Cosmic rays from other stars produce ionizing radiation primarily in the form of protons, nuclei, electrons and gamma radiation.
What does radiation do to the Non-Voltile memory ?
There are basically 3 types of radiation disruption effects in space flight. The most serious is Displacement Damage Dose ( DDD) effect. In this case, the radiation is sufficiently strong to cause permanent damage to the semiconductor memory cells. The next is Total Ionizing Dose (TID) effect. They slowly ionize the chip changing balance of energy. It makes some switches harder to turn on and the others harder to turn off. This makes the memory operation highly un-predictable. The 3rd type is “SEE”, Single Event Effect ( SEE). “SEE” happen when the collision of a radiation particle with a chip produces enough energy to cause a “soft error” in the memory system.
Focus in Single Event Effect
Cause of Soft errors
a. Ion creates electron hole pairs in the silicon
b. Charges drift and collect at nodes, producing a prompt current
c. Charges diffuse toward the nodes, producing lower current
If the current is large enough, storage nodes such as SRAMs,DRAM,NAND FLASH can switch states
Single Event Effect ( SEE or SEU) in memory is recoverable once the radiation level returns to normal. The soft error cells can be re-written and errors might never happen again until the next high dose of radiation comes through. How to make Storage and Flash emory work in space ? There are two ways to make Storage and Flash memory work in space.
One is to invent better semiconductor process to make the memory cell less susceptible to direct radiation. Researchers found that performance under radiation can be improved by repositioning various component junctions or by using Silicon-On-Saphire (SOS) or Silicon-On-Insulator technologies (SOI). Increasing the separation between circuit elements reduces the likelihood that the energy of a radiation strike in one part of the chip will affect enough of the circuit to be a problem. All these methods require redesign of the memory chips and calls for new process technologies that is not widely used for memory chips. That presents a risk in production cost and in scalability when memory technology changes.
The second way is to use off-the-self memory parts. NASA Engineers have found that every memory chip exhibits slightly different characteristic under radiation environment. Even the same batch of chips can react quite differently among each other. Some chip works better in the energy environment while others fail miserably. Therefore, the most economical way is to test-and-select. NASA decided to use Off-the-self parts even in the International Space Station. This is obviously for cost reason and to allow upgrade path as non-volatile memory technology advances during their course.
How is radiation test performed?
Since more tests can yield radiation hardened memory chips, the focus is now turned to “how to test them for radiation hardening?” Any low cost SSD tester can fulfill the task. The device under test (DUT) is placed in a chamber under radiation and particle bombardment. The tester head should have a four inch “riser” to elevate the DUT from the tester. This is to allow the beam to focus onto the SSD and not expose the SSD tester to the beam. To protect the test operator, controls the tester at a safe location outside of the radiation chamber.
Radiation is usually applied to the DUT for a very long period of time to induce SEU, Single Event Upset. Once a cell is upset, it usually stays as an error until the memory content is renew or rewritten. Therefore, the test result has no direct relationship with test time. This factor allows the very low cost SSD Tester to come into the play. The most important thing is, therefore, the test algorithm. Error Capture for Analysis and Record is Needed The SSD tester has the capability to map the locations of the radiation sensitive cells. It has to record the address and bit columns of the upset cells. The feature is called “Error Capture”. Since error recording is a slow process, the engineer might not want to finish testing the entire SSD when the error count exceeds a certain percentage. That is why a function call “STOP” is added to stop the test once the error percentage is over a preset threshold. Based on the SEU rate and their rate of recovery, engineers will be able to determine which SSDdrive qualifies to be used in space application.
System application on Radiation hardened SSD
Since SEU in memory will definitely occur in space flights, there are computer architectural strategies to keep the systems in operation. One way is to add a storage array of SSD hoping that the radiation upset on one SSD is different than the other.The computer system can thus operate normally in the abnormal environment. Conclusion Radiations in space can cause memory and storage failure on in flight computers. Radiation hardening test can select memories and storage device that are least susceptible to radiation conditions. Radiation SSD and Flash memory testers do not have to be expensive. They can be a simple modified version of a standard low cost memory tester. Smart system architecture would allow selected memories to be used safely in space systems.
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