NAND Flash in Space: JPL’s Strauss reports advanced Flash devices with finer geometries better for space-borne applications

Yesterday, I blogged about a presentation on embedded SSDs given at the Flash Memory Summit by Viking Modular Solutions during a panel on embedded Flash. Today, I want to discuss the subsequent talk on the same panel, a presentation by Karl F Strauss of NASA’s Jet Propulsion Lab (JPL). Strauss discussed the use of Flash memory for data storage in spacecraft. You might think that shrinking device geometries make newer, more advanced Flash parts increasingly unsuitable for the radiation environments in space--at least I did based on old stuff I learned about semiconductors and radiation back in the 1980s--but the opposite is actually true and Strauss has the data to back up this claim (no pun intended).

First, Strauss discussed spacecraft as embedded systems. Qualitatively, the same project goals taunt spacecraft designers as much as other designers of embedded systems: power, mass, volume, and environment. Power is a problem for cell phone, laptop, and tablet designers who must worry about talk or operating time, standby time, and recharge time. Embedded designers trade off battery capacity and size, overall product size, and weight for power consumption. Power is also a problem for spacecraft designers. Spacecraft power comes from one or more of three sources. Two of those sources are familiar to more earthbound applications: lithium-ion batteries and solar cells. One power source, the plutonium-powered GPHS-RTG (general-purpose heat source, radioactive thermal generator) is unique to spacecraft and early model, time-traveling DeLoreans.

Although they have constantly improving charge/discharge cycle specifications and can last a long time, rechargeable lithium-ion batteries have clear, finite storage capacity and must be recharged when drained or the mission ends. Solar cells can recharge lithium-ion batteries (as on the Mars Rovers) but they only work when the sun shines and they cannot provide sufficient power out beyond the orbit of Mars. In fact, the Mars Rovers don’t really get enough solar power to operate during the Martian winter when the sun is low on the horizon so the rovers must position themselves advantageously, hunker down, and nearly hibernate during the winter.

For each spacecraft and mission, there’s a specification for the power source’s weight and volume and those specifications determine how much power is available to the rest of the system. Plutonium is the answer to continuous power for long-lived missions in space. GPHS-RTGs essentially put out full power--about 300W--for a dozen years and then power slowly tapers off. That’s why they’re used for deep-space missions. Strauss noted that the GPHS-RTGs on the two Voyager spacecraft--which have now entered the termination shock region between the solar system and interstellar space--are still operating at about 3% of their initial power rating, 33 years after launch, and the Voyagers are still phoning home.

The mass of a system is perhaps even more critical for spacecraft than for cell phones and other earthbound embedded applications because it’s expensive to send mass into earth orbit and even more expensive to kick the spacecraft’s mass further out into space. For example, said Strauss, a Minotaur rocket--derived from the Minuteman and Peacekeeper intercontinental ballistic missile weapons delivery systems--can place 1000 kilograms of payload into polar Earth orbit for $13,000/kilogram. (A GPHS-RTG masses 57 kilograms.) So spacecraft designers are always trading mass off against mission specs. They can reduce weight by reducing the number of science experiments (resulting in fewer instruments) or by reducing the mission duration (the space-borne equivalent of talk time). Naturally, they prefer to do neither and they devote a lot of time and effort to doing clever things with less mass. They must perform the same tricks with respect to volume as well, both because volume and mass are closely interrelated and because there’s only so much room under that payload shroud capping the top of the launch vehicle.

Finally, there’s environment. Those who have not designed spacecraft (including me) might think that both temperature and radiation play a role. Not really so, says Strauss. Spacecraft designers can keep electronic components in relatively benign, even balmy thermal environments that humans would not find uncomfortable. They do this with a combination of good thermal insulation and heaters (including electrical heaters, lumps of heat-giving plutonium, and self-heating of the electronic circuitry) to maintain even temperatures for the spacecraft’s electronics systems. The only place that they cannot maintain such benign temperatures, at least not for long, is on Venus where the surface temperature is a uniform 480C--considerably above the melting point of lead (or solder). A spacecraft on Venus would need to use active cooling and, with currently available power sources, would only be able to do so for a limited time before the systems overheat and fail.

That leaves radiation, which finally brings us back to advanced Flash memory for space borne applications. Storage systems based on NAND Flash memory can provide high storage capacity with low mass, volume, and power requirements. Here on earth, we call such storage systems SSDs and SSDs would be ideal for data storage in space (low mass, low volume, low power requirements) except for their susceptibility to radiation. Three types of radiation can damage semiconductors. Gamma radiation causes transistor threshold voltages to shift and can be mitigated with controlled semiconductor processing and special doping profiles. Ion strikes can flip bits (SEFI or single-event functional interrupts) and can actually cause a semiconductor device to go into latchup followed by catastrophic failure from current-induced thermal overload (known technically as a “bad thing”). Neutron radiation can disturb the semiconductor lattice, upset device parameters, and cause faulty operation.

It turns out that NAND Flash devices are most susceptible to ion strikes and that they have been growing less and less susceptible to such strikes as device features shrink. For a Flash cell, radiation susceptibility is merely a matter of mass--the smaller the amount of oxide insulation in the Flash memory cell, the less the ability of an ion or photon to become trapped at a defect site and induce leakage. Because radiation tolerance is inversely proportional to memory-cell volume, Flash memory’s radiation tolerance has been steeply increasing over the last few years to the point where only the on-chip charge pump is really vulnerable to ion strikes. Consequently, Flash memory is becoming a very viable candidate for data storage on spacecraft because it is so attractive with respect to the other critical characteristics. As a result, the projected storage capacity on spacecraft, which has been essentially flat at 1-2 Gbits from 1977 to the present time, is now expected to climb rapidly into the Tbit region. In fact, said Strauss, spacecraft storage capacity will track Moore’s Law now and into the future, riding on the projected increases in Flash capacity.

The detailed presentation is available here to Flash Memory Summit attendees: http://www.flashmemorysummit.com/