Novel MTJ-Based Sensing Inverter Variation Tolerant Nonvolatile Flip-Flop in the Near-Threshold Voltage Region

Abstract

In recent years, the low power design of Internet of Things devices has become increasingly important. The most basic method to implement a low power design is to reduce static power consumption. In this regard, a spin-transfer-torque magnetic-tunnel-junction-based nonvolatile flip-flop (NVFF) is a promising candidate for storing the computing data while the power is off. However, as the technology node scales down, the process variation increases, leading to a restoration failure in previous NVFFs, especially in the near-threshold voltage (NTV) region. For better energy efficiency when operating the NVFF in the NTV region, this paper presents a novel NVFF that guarantees a target restore yield of 4σ and a target read disturbance margin of 6σ in the NTV region (0.6 V supply voltage), along with auto-zeroing and dynamic reference voltage (DRV) techniques, using only a single capacitor to improve the NVFF's restore yield. The Monte Carlo HSPICE simulation results using industry-compatible 28-nm model parameters show that the proposed NVFF satisfies both the target restore yield and the read disturbance margin, saving 44% restoration time compared with the current state-of-the-art NVFF, which does not satisfy the target restore yield despite having offset cancellation characteristic. INDEX TERMS Auto-zeroing, double sensing margin, dynamic reference voltage (DRV), magnetic tunnel junction (MTJ), near-threshold voltage (NTV), nonvolatile flip-flop (NVFF), sensing inverter variation tolerant (SIVT), sensing margin (SM).

Analysis of State-of-the-Art Spin-Transfer-Torque Nonvolatile Flip-Flops Considering Restore Yield in the Near/Sub-Threshold Voltage Region

“…To date, several separated latch and sensing circuit (SLS) structure based NVFFs have been proposed [10][11][12][13] to independently optimize the circuit and flip-flop core. Even though they satisfy the target restore yield of 4σ (it was set to 4σ to guarantee a 96.88% (=Φ(4σ) 1000 ) yield when 1000 NVFFs are assumed, where Φ() is the cumulative distribution function of the standard normal distribution [11][12][13]) in super-threshold voltage region, it is hard for them to satisfy target restore yield of 4σ in the near/sub-threshold voltage region no matter how much the size and time increase.…”

“…To date, several separated latch and sensing circuit (SLS) structure based NVFFs have been proposed [10][11][12][13] to independently optimize the circuit and flip-flop core. Even though they satisfy the target restore yield of 4σ (it was set to 4σ to guarantee a 96.88% (=Φ(4σ) 1000 ) yield when 1000 NVFFs are assumed, where Φ() is the cumulative distribution function of the standard normal distribution [11][12][13]) in super-threshold voltage region, it is hard for them to satisfy target restore yield of 4σ in the near/sub-threshold voltage region no matter how much the size and time increase. Furthermore, although the NVFF operating in the near-threshold voltage region [11] was proposed, it is difficult to satisfy the target restore yield of 4σ if the target read disturbance margin of 6σ (it was set to guarantee 99% yield when 10,000 access per a single cell is assumed by considering the stochastic nature of MTJ [13]) is considered.…”

“…Even though they satisfy the target restore yield of 4σ (it was set to 4σ to guarantee a 96.88% (=Φ(4σ) 1000 ) yield when 1000 NVFFs are assumed, where Φ() is the cumulative distribution function of the standard normal distribution [11][12][13]) in super-threshold voltage region, it is hard for them to satisfy target restore yield of 4σ in the near/sub-threshold voltage region no matter how much the size and time increase. Furthermore, although the NVFF operating in the near-threshold voltage region [11] was proposed, it is difficult to satisfy the target restore yield of 4σ if the target read disturbance margin of 6σ (it was set to guarantee 99% yield when 10,000 access per a single cell is assumed by considering the stochastic nature of MTJ [13]) is considered. Thus, most of the state-of-the-art STT-MTJ-based NVFFs cannot satisfy the target restore yield and target read disturbance margin simultaneously at all corners in the near/sub-threshold voltage region.…”

“…In this paper, among the various recently proposed state-of-the-art NVFFs employing different emerging nonvolatile elements, such as field-induced magnetization reversal MTJ [14,15], STT-MTJ [10][11][12][13][16][17][18][19][20][21][22][23], complementary polarizer MTJ [24], spin-orbital-torque MTJ [25,26], memristor [27][28][29][30], ferroelectric capacitor (FeCAP) [31][32][33][34], and ferroelectric field-effect transistor (FeFET) [35,36], four state-of-the-art STT-MTJ-based NVFFs with the SLS structure were simulated and described why the Among various nonvolatile elements, the spin-transfer-torque magnetic tunnel junction (STT-MTJ) is regarded as a strong candidate because of its various advantages, such as high endurance, scalability, and easy integration with CMOS technology [5,6]. The STT-MTJ can save the computing data by using the two different resistance values such as low resistance (R L ) and high resistance (R H ).…”

“…In this paper, among the various recently proposed state-of-the-art NVFFs employing different emerging nonvolatile elements, such as field-induced magnetization reversal MTJ [14,15], STT-MTJ [10][11][12][13][16][17][18][19][20][21][22][23], complementary polarizer MTJ [24], spin-orbital-torque MTJ [25,26], memristor [27][28][29][30], ferroelectric capacitor (FeCAP) [31][32][33][34], and ferroelectric field-effect transistor (FeFET) [35,36], four state-of-the-art STT-MTJ-based NVFFs with the SLS structure were simulated and described why the NVFFs do not satisfy the target restore yield according to V DD . The analysis of the silicon-proven NVFFs with the merged latch and sensing circuit (MLS) structure can be found in the reference texts (see [13]). The rest of this paper is as follows.…”