1 Radiation Challenges

Different semiconductor technologies have different inherent radiation tolerances (see TI’s Radiation Handbook for Electronics for more details). At the same time, two products using the same process technology or node might have totally different radiation responses due to how the product is designed and which modules in the process are used. As a result, customers need to spend time and resources in order to evaluate the reliability and radiation performance of these devices. To enable shorter development times, Texas Instruments’ Space EP products provide extensive radiation characterization to meet the requirements of LEO missions.

In addition, there are many generalities floating around about radiation tolerance that are not true for all cases. A 65-nm process is likely to be SEL immune, but only for the 1.1-V circuits. If a product uses higher voltage circuits, it is more likely to have SEL. Having an epi or SOI substrate does not necessarily mean that a CMOS product is SEL immune. For most CMOS products, the use of an epi substrate has no impact on SEL susceptibility and SOI only assures SEL immunity if the field oxide (STI) reaches all the way through the active layer down to the buried oxide.

A supplier such as TI has the knowledge of the process used on its products. Using this information, TI can choose products that have a high probability of being radiation tolerant and often uses a process or design change to meet the radiation goal. After choosing a part, TI then verifies the choice with heavy ion, neutron displacement damage (NDD), and total ionizing dose (TID) testing.

TI’s Space EP flow also follows a single production flow and provides radiation lot acceptance testing (RLAT) in order to reduce the risk with lot-to-lot variation. Most wafer fabs do not have monitors or controls in place for radiation tolerance. Modern wafer fabs maintain very tight controls to ensure consistent electrical performance, but the parameters that are controlled are not the same ones that impact radiation tolerance. For instance, the stoichiometry and thickness of passivisation layers have little impact on electrical performance but can be huge variables in radiation tolerance. In an extreme case, there was a product where one lot passed 100 krad(Si) and a lot processed in the same wafer fab a month later only passed 10 krad(Si). That is why radiation lot acceptance testing (RLAT) is so important.

Customers can design with Texas Instruments’ Space EP products to help bring new space systems faster to market and ensure these systems meet the radiation requirements for LEO missions. Each device is radiation tested up front and has TID, SEE and often NDD characterization provided in separate radiation reports available in the product folder. For enhanced radiation reliability, the Space EP products use only one production flow and each lot gets RLAT, eliminating the risks of lot-to-lot variations.