| Summary: | Multiple cell upsets (MCUs) were first observed in static random access memory (SRAM) in the 1980s. As microelectronics technology scaled, the number of cells affected by an ion strike increased. In order to assess test data, the MCU patterns first need to be associated to mechanisms. This research provides that link by re-evaluating the underlying soft error mechanisms for the SRAM cell and array. Modeling, simulation, and experimental approaches were developed to determine the MCU characteristics and to associate them with mechanisms.
The study of SRAM single event upset led to the identification of the well-collapse source-injection (WCSI) mechanism. This mechanism was encountered when charge collection in the well/substrate p-n junction exceeded the amount that can be supplied by the well and/or substrate contacts. When this occurs, the additional photocurrent is supplied by source/body p-n junction diodes in forward-bias. The forward-bias condition injects minority carriers near the MOSFET drains to induce drain current. The WCSI is shown to have a large range of effect, collapsing an entire well and forward-biasing all of the source/body p-n junctions.
Technology computer aided design (TCAD) modeling of the WCSI mechanism showed that SRAM cell upset was dependent upon two factors: (1) the forward-bias current and (2) the relative resistances between the source/body p-n junctions and the well-collapse region. These factors enabled the MOSFETs to be replaced with source diodes having an equivalent resistive path. That breakthrough allowed many more devices to be simulated in one TCAD model and enabled the MCU properties of the WCSI mechanism to be understood.
A 65-nm SRAM provided experimental data sets containing MCUs; these were analyzed for the WCSI mechanism. The first data set verified TCAD simulations for ion strikes at high incident angles as well as provided many examples of MCU patterns. The second set of data showed that the observed SEU cross section depended upon the SRAM power supply voltage. Overall, the WCSI mechanism increased the number of upsets found in the experimental data.
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