**1. Introduction**

The dimensions of integrated circuit devices decreased in each successive technology gener‐ ation. The goal of this scaling is, on the one hand, to improve the performance of integrated circuits and, on the other hand, to integrate a greater number of devices per unit area. Static random access memories (SRAMs) are not an exception to this evolution, the dimensions of the transistors forming memory cells decreased roughly following Moore's Law. Consequently,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the area occupied by each cell decreased from generation to generation [1]. Current techno‐ logical processes used to manufacture complementary metal‐oxide‐semiconductor (CMOS) SRAM memories are in the nanometer region, since the nominal characteristic dimensions of the transistors forming each cell are of the order of tens of nanometers.

The supply voltage of SRAMs has also been reduced. However, this decrease did not follow the predictions of the International Technology Roadmap for Semiconductors (ITRS), in fact, it was more moderate. This is mainly due to the limitation imposed on the transistors threshold voltage scaling to avoid an excessive increase of leakage current [2]. To meet performance demands of current electronic systems, large capacity integrated SRAMs are usually needed, and in fact, FPGA‐SRAMs are not an exception. This requirement results in a large proportion of area dedicated to SRAM memory. Forecasts indicate that in the coming years this figure may reach 90% [3]. Of course, integrating large memories has an adverse impact on circuit area, which in turn results into higher costs. For this reason, designers try to integrate the largest possible number of SRAM cells per unit area. This leads to cells designs with small sizes to squeeze the full potential of technology. SRAMs are usually designed with transistors close to the minimum possible size, and arranged with the highest possible density. In addition, to reduce power consumption, voltages are kept as low as possible. Although, as mentioned before, the expected voltage reduction have not been fully implemented in real technology.

As a result of the decrease in device dimensions and of the reduction of supply voltage in successive technology generation, designing SRAM faces two major challenges: the first one is related to the stability of the cells and second one has to do with their susceptibility to radiation‐induced transient events. This chapter focuses on the second challenge, the CMOS SRAM radiation problem. However, SRAM stability issues are also discussed.

SRAMs are one of the most sensitive to radiation parts of a circuit. They are especially sensitive to those effects caused by a single energetic particle. These effects are the so‐called single event upsets (SEUs). They are considered soft errors (SE) because they trigger an error without permanently damaging the circuit. This chapter focuses on six‐transistor (6T) CMOS SRAM SEUs and on a technique to mitigate its effects, which is easily implementable in current FPGA design workflows. The architecture of 6T RAMS cell is described in Section 2.

Regarding the process that generate SEUs, the interaction of an energetic particle creates electron‐hole pairs, so that part of this deposited electric charge can be collected by a sensitive node affecting its voltage. If this node is the node of an SRAM and the perturbation is high enough, it can flip the cell state altering data stored in it, and thus generating an error. These errors are not necessarily destructive. In particular, in an SRAM, a particle is capable of modifying data stored in one or more memory cells without damaging them. This means that cells can be rewritten and operate normally. Nevertheless, cell data has been corrupted, and if the cell is read before a new write occurs, a read error will be produced.

The problem of radiation effects in integrated circuits is not new. It has been studied and taken into account for decades by designers in areas such as the aerospace industry and, since the mid‐1990s, also by the aeronautics manufacturers [4]. This is due to the high flow of energetic particles that devices operating in these high‐altitude environments are exposed to. The atmosphere shields part of the energetic particles that come from outside the Earth, so that, the higher the altitude, the higher the particle flux. To mitigate these effects, radiation shields, redundant components, techniques of error detection and correction and radiation tolerant elements are used. The implementation of these measures ranges from technological aspects of architecture to system level. Many of these measures increase costs and negatively impact circuit performance. There exist many well‐known techniques to mitigate SEU effects, such as triple modular redundancy (TMR), which can be suitable for certain applications. However, most of them involve high penalties in terms of cost, power, or performance, which can be affordable for the space industry but could be non‐acceptable for other FPGA fields of application.

the area occupied by each cell decreased from generation to generation [1]. Current techno‐ logical processes used to manufacture complementary metal‐oxide‐semiconductor (CMOS) SRAM memories are in the nanometer region, since the nominal characteristic dimensions of

The supply voltage of SRAMs has also been reduced. However, this decrease did not follow the predictions of the International Technology Roadmap for Semiconductors (ITRS), in fact, it was more moderate. This is mainly due to the limitation imposed on the transistors threshold voltage scaling to avoid an excessive increase of leakage current [2]. To meet performance demands of current electronic systems, large capacity integrated SRAMs are usually needed, and in fact, FPGA‐SRAMs are not an exception. This requirement results in a large proportion of area dedicated to SRAM memory. Forecasts indicate that in the coming years this figure may reach 90% [3]. Of course, integrating large memories has an adverse impact on circuit area, which in turn results into higher costs. For this reason, designers try to integrate the largest possible number of SRAM cells per unit area. This leads to cells designs with small sizes to squeeze the full potential of technology. SRAMs are usually designed with transistors close to the minimum possible size, and arranged with the highest possible density. In addition, to reduce power consumption, voltages are kept as low as possible. Although, as mentioned before, the expected voltage reduction have not been fully implemented in real technology.

As a result of the decrease in device dimensions and of the reduction of supply voltage in successive technology generation, designing SRAM faces two major challenges: the first one is related to the stability of the cells and second one has to do with their susceptibility to radiation‐induced transient events. This chapter focuses on the second challenge, the CMOS

SRAMs are one of the most sensitive to radiation parts of a circuit. They are especially sensitive to those effects caused by a single energetic particle. These effects are the so‐called single event upsets (SEUs). They are considered soft errors (SE) because they trigger an error without permanently damaging the circuit. This chapter focuses on six‐transistor (6T) CMOS SRAM SEUs and on a technique to mitigate its effects, which is easily implementable in current FPGA

Regarding the process that generate SEUs, the interaction of an energetic particle creates electron‐hole pairs, so that part of this deposited electric charge can be collected by a sensitive node affecting its voltage. If this node is the node of an SRAM and the perturbation is high enough, it can flip the cell state altering data stored in it, and thus generating an error. These errors are not necessarily destructive. In particular, in an SRAM, a particle is capable of modifying data stored in one or more memory cells without damaging them. This means that cells can be rewritten and operate normally. Nevertheless, cell data has been corrupted, and if

The problem of radiation effects in integrated circuits is not new. It has been studied and taken into account for decades by designers in areas such as the aerospace industry and, since the mid‐1990s, also by the aeronautics manufacturers [4]. This is due to the high flow of energetic particles that devices operating in these high‐altitude environments are exposed to. The

SRAM radiation problem. However, SRAM stability issues are also discussed.

design workflows. The architecture of 6T RAMS cell is described in Section 2.

the cell is read before a new write occurs, a read error will be produced.

the transistors forming each cell are of the order of tens of nanometers.

198 Field - Programmable Gate Array

In addition, due to technology scaling, SEUs are becoming a major reliability concern for electronic devices in general and SRAMs in particular, not only in harsh radiation environ‐ ments but also at ground level, where radiation fluxes are low. In the case of SRAMs, this is due to the fact that the number of errors per time unit in SRAM memories due to radiation‐ induced transient events has increased with technology scaling [3, 5]. This fact has two main causes. The first cause has to do with both reducing the dimensions of the transistors forming the cells and with decreasing the supply voltage. Both factors contribute to reduce the amount of electrical charge used by a cell to store one bit of information. Thus, it is easier that the charge induced by the interaction of a particle upsets the cell content. The second cause includes three related factors: the increase in the number of cells integrating SRAMs, the higher density of cells, and the amount of chip area occupied by SRAM cells. All of them contribute to increase the probability that an energetic particle interacts with a sensitive area of a memory causing a transient event that leads to cell data corruption. In a FPGA, this can be a serious problem, since SRAM‐based FPGAs rely on SRAMs to store configuration bits. An SEU affecting one of those bits can produce an unpredictable behavior or even a complete system failure.

To conclude, SEU effects are not a new problem and the space industry has developed specialized techniques to deal with them for decades. However, FPGAs are used in a broad range of applications, and in many of them circuits are not subject to high radiation fluxes. Nevertheless, due to technology scaling, they are becoming sensitive to radiation either from the environment or from the circuit materials. For this reason, it is necessary to implement some radiation hardening techniques, especially if the circuit is operated in critical systems. Traditional aerospace techniques are not suitable for most SRAM‐based FPGA applications, since they involve high costs or significant performance degradation, which cannot be assumed. One of the most paradigmatic examples is commercial electronics or any other FPGA application field where FPGAs are attractive due to its fast time to market, flexibility, and reprogrammability, which reduce costs while keeping good performance. Thus, the aim of this chapter is to present a technique that fills this gap and can be used as a suitable technique to improve radiation reliability in a broad range of FPGA‐SRAMs applications. More specifically, the technique works at the cell design level, and its goal is to enable the design of intrinsically more robust cells. In addition, the technique is also attractive because it is compatible with current memory compilers, since it does not change SRAMs cell architecture.
