Radiation-Hardened 14T SRAM Bitcell With Speed and Power Optimized for Space Application
In this paper, a novel radiation-hardened 14-transistor SRAM bitcell with speed and power optimized [radiation-hardened with speed and power optimized (RSP)-14T] for space application is proposed. By circuit- and layout-level optimization design in a 65-nm CMOS technology, the 3-D TCAD mixed-mode simulation results show that the novel structure is provided with increased resilience to single-event upset as well as single-event–multiple-node upsets due to the charge sharing among OFF-transistors. Moreover, the HSPICE simulation results show that the write speed and power consumption of the proposed RSP-14T are improved by ∼65% and ∼50%, respectively, compared with those of the radiation hardened design (RHD)-12T memory cell.
SINGLE-event upset (SEU) is a soft-error and nondestructive form of single-event effects (SEEs). In the radiation environment, when the heavy ion is incident on the semiconductor material, the particles will be ionized. These excess charges will be collected by the sensitive nodes of the device. As a result, a voltage perturbation will appear at those nodes. For SRAM bitcell, when the amplitude of the voltage perturbation is strong enough and exceeds the logic threshold level of the inverter, the data stored might be turned over, as shown in Fig. 1; that is, an SEU is caused. With the continuous scaling of CMOS technology, the minimum spacing between the transistors is decreased.
Figure 1: SEU induced by an ion strike in an SRAM memory
As a result, multiple transistors are susceptible to the charge deposited from a single particle strike compared to older processes where only one transistor was affected. The charge sharing results in single-event–multiple-node upsets (SEMNUs), which is becoming the main effect of energetic particle strikes in emerging nanometer CMOS technology. In addition, supply voltage reduction further increases the susceptibility of circuits to radiation. Thus, the development of radiation-hardened technologies in digital circuits is extremely urgent. Due to the larger sensitive volume per bit and lower node capacitance than the dynamic counterpart, SRAM is more prone to soft errors. Therefore, the soft error rate (SER)  in SRAM is increased with the technology scaled in the nanometer regime. In order to reduce the SER, numerous alternatives have been proposed to the standard 6T SRAM cell. The main reinforcement method is through constructing special topology of transistor connections inside cells to achieve circuit-level protection. The soft error robust Quatro-10T SRAM cell, offering differential read operation with large noise margin was proposed. However, it can only recover from “1” to “0”; thus, it cannot immune SEU completely. Due to the feedback of the dual node, the dual interlocked storage cell (DICE) can fully immune against single-event transient (SET) occurring on any of its single nodes. However, the very minimum ability of SEMNUs immunity and radiation hardness performance of it has yet to be improved. In Schmitt trigger based (STB)-13T memory cell with fully SEU immune was proposed. However, the limited promotion of SEMNUs immune ability of it is achieved at the expense of writing speed, power consumption, and layout area compared with DICE. Based on the STB13T, two novel hardened memory cells with more reliability, radiation hardened design (RHD)-11T and RHD-13T, were proposed in . Unfortunately, the writing speed, as well as write margin, of them is deteriorated. In low power and highly reliable radiation-hardened application, the RH memory (RHM)-12T was proposed; however, the authors used nMOS as pull-up devices causing worse read noise margins. Recently, the RHD-12T memory cell with favorable radiation hardness performance, as shown in Fig. 2(a), was proposed. Besides the toleration for an SEU on any of its internal single nodes, it can also provide the SEMNUs immune to some extent. Unfortunately, the slow write speed as well as large power consumption limits the application of it. In addition to the reinforcement at the circuit level, specific layout techniques as an alternative method for improving the radiation tolerance have also been proposed. As presented in a new layout technique named layout design through error-aware transistor positioning (LEAP) was applied to DICE, resulting in a new sequential element, LEAP-DICE. The TCAD simulations show that it is effective for increasing the linear energy transfer (LET) upset threshold. In order to investigate the charge sharing, a Monte Carlo simulation platform named tool suite for radiation reliability assessment (TIARA). By analysis of the TIARA simulations results, the layout optimization of the most vulnerable transistor pairs will be targeted.
In this paper, the radiation-hardened with speed and power optimized (RSP)-14T bitcell is proposed. Compared with RHD-12T, its radiation hardness has been improved by the reinforcement of redundant nodes with two extra pMOS transistors. Furthermore, due to the supply of the branch where the redundant nodes located are controlled by the extra PMOSs, during the write operation, the feedback mechanism will be interrupted easily. Thus, the write speed and power consumption have been improved effectively. Generally, SPICE simulations by using the double-exponential current source model are applied for evaluating the radiation tolerance of the circuit, which is time saving. However, the model relies on calibration parameters that are not physical. The charge sharing between transistors will be neglected; it may overestimate the SEU immune ability of other SRAM cells. Thus, in order to consider the charge sharing between transistors as well as reducing the CPU burden, TCAD mixed-mode simulation as a good qualitative approach to valuate SEU immune is adopted in this paper. Combined with the layout-level design, the simulation results show that the proposed circuit has better SEU immunity.
- More Soft Error Rate
- More Power consumption
- More immunity and radiation hardness
- Read and Write Operation
The schematic of the proposed RSP-14T is shown in Fig. 2(b). Here, the transistors N4 and N5, controlled by a word line (WL), are access transistors, which control the connection between the bit lines (BL and BLB) and the storage nodes (Q and QB). The nodes S1 and S0 are redundant nodes of Q and QB. If the stored bit is “1,” the logic values at nodes Q, QB, S1, and S0 are “1,” “0,” “1,” and “0,” respectively.
Figure 2 : (a) RHD-12T bit cell (b) Proposed RSP-14T bit cell
The functional analysis of the proposed RSP-14T is sequentially presented: 1) write; 2) read; and 3) hold operation. In write operation, we assume that Q = “1” and QB = “0” and the bitlines BL and BLB are set to “0” and “1,” respectively. When the WL is activated, the value stored in Q and QB will be changed to “0” and “1,” respectively. After that, once WL is discharged to “0,” the new state of the memory cell is stored. For a read operation, the BL and BLB are precharged to “1.” When the selected WL is enabled, the transistors N4 and N5 are turned on, BLB will be discharged through transistors N4 and N1. As a result, the differential voltage of the BLs will be generated and amplified by the sense amplifier. During the hold operation, WL is deactivated and the storage nodes are isolated from the BLs; thus, they maintain the initial state. In this paper, the transistors P0 and P7 are used to control the connection or cutoff between the power supply and transistors P1/P5, which is beneficial to improve the write speed and power consumption compared with RHD-12T.
- Error Tolerance Analysis
Regardless of the charge sharing between the transistors in the actual layout, the analyses of SEU recovery behavior at circuit level are given. Assuming that Q = “1,” QB = “0,” S1 = “1,” and S0 = “0,” respectively, the analyses of the nodes (Q, QB, S1, and S0) are demonstrated as follows.
Case 1 (Positive Transient Pulse at Node S0):When the drain of P1 is hit by a particle, it will collect positive charge and increase the voltage at node S0 (i.e., S0 will be changed from “0” to “1”). As a result, P6 and P5 will be turned off. However, it cannot further affect the OFF/ON-states of other transistors, and the storage status of Q and S1 nodes will remain unchanged. Therefore, the transient fault at S0 cannot propagate inside the cell. Finally, the nodal logic level will be recovered after the radiation events.
Case 2 (Positive Transient Pulse at Node QB):When the drain of P2 is hit by a particle, it will collect positive charge and increase the voltage at node QB (i.e., QB will be changed from “0” to “1”). As a result, N2 and N0 will be turned on. Correspondingly, Q and S1 will be changed from “1” to “0,” P0 and P1 will be turned on, and N3 will be turned off, and then S0 will be changed from “0” to “1.” Finally, the storage state of the cell will be turned over. (It has been made difficult to change in the layout-level design as presented in Section III.) Due to the transistors being stacked and topology optimized, the parasitic bipolar amplification effect of P2 (the source of P3 is connected with VDD, whereas the source of P2 with weak connection “1”) is mitigated. As a result, the quantity of charge collected by the drain of P2 is reduced, which improves the SEU tolerance of node QB.
Case 3 (Negative Transient Pulse at Node S1):When the drain of N0 is hit by a particle, it will collect negative charge and S1 will be discharged from “1” to “0,” and P3 and P1 will be turned on. However, due to the blocking effect of transistors P2 and P0, the fault at S1 cannot further propagate in the cell. Therefore, QB and S0 will remain in their original status. Due to the low status at QB and S0, P7 and P5 are always at open state. Hence, the current provided by P7 and P5 will charge S1 continuously. This positive feedback will accelerate the recovery process of S1. Finally, the nodal storage status will be recovered after the radiation events.
Case 4 (Negative Transient Pulse at Node Q):When the drain of N2 is hit by a particle, negative charge will be collected and Q will be discharged from “1” to “0,” and then N1 and N3 will be turned off. This is very similar to case 1, thus, the storage status of Q will finally recover after the radiation events.
For time efficiency, in order to prove that the above-mentioned analyses are correct and locate the most vulnerable node of the proposed RSP-14T, the transient injections at S0, QB, S1, and Q nodes are simulated, as shown in Fig. 3, by the double-exponential current source. On this basis, further analysis of SEU is given by TCAD in Section III. Here, the double-exponential current is expressed as
I(t) = I0(e−t/τα − e−t/τβ ) (1)
I0 = Q/(τα − τβ) (2)
where I0 is the peak of current source, Q is the amount of deposited charge, τ α is the collection time constant of the junction, and τ β is the time constant for initially establishing the ion track. In this paper, I0 is set as ∼174 μA, while τ α and τ β are set as 200 and 50 ps, respectively.
- Less Soft Error Rate
- Less Power consumption
- Less immunity and radiation hardness
 P. E. Dodd and L. W. Massengill, “Basic mechanisms and modeling of single-event upset in digital microelectronics,” IEEE Trans. Nucl. Sci., vol. 50, no. 3, pp. 583–602, Jun. 2003.