Semiconductor Static Random Access Memory Research Articles

Conductivity Mapping in Graphene through Scattering-Type Scanning Near-Field Optical Microscopy in the Mid-Infrared and Terahertz Spectral Region with 25nm Spatial Resolution

Terahertz (THz) imaging and spectroscopy has emerged as a powerful tool for analysis of biological specimens or for studying and controlling low-energy excitations in solid-state systems [1]. One major limitation of THz spectroscopy is, however, the spatial resolution that is limited by diffraction to about 100-1000 µm, making it impossible to extract intrinsic, local material characteristics of nanoscale Graphene or other 2D-material structures or devices. Scattering-type scanning near-field optical microscopy (s-SNOM) bypasses the diffraction limit, enabling optical measurements with extreme sub-wavelength spatial resolution of below 20 nm [2]. THz time-domain s-SNOM spectroscopy of a semiconductor static random access memory (SRAM) sample enables quantitative analysis of free carrier concentration and scattering rates of nanoscale device structures at unprecedented surface sensitivity. Similarly, THz s-SNOM measurements at single frequencies enabled highlighting highly conductive nanostructures in transistor devices [3]. Using a similar experimental concept to generate a local photo-current by THz radiation allows for studying nanoscale conductivity in a biased graphene device [4] or to characterize carrier scattering in a graphene sheet that has been encapsulated in hexagonal boron nitride (h-BN) and placed on a split metallic film [5]. Using an SRAM sample as a method to calibrate the amplitude contrast of the s-SNOM THz measurement with known free carrier concentrations the concept is extended to graphene. Single to few layer graphene on h-BN is imaged with a broadband THz pulse and reveals changing amplitude contrast independently of layer number (Fig. (a), (b), zoom in (d)). In a corresponding s-SNOM measurement carried out at 10 µm an inverted contrast can be observed (Fig. (c)). This contrast inversion is a direct consequence of the scattering frequency dependent free carrier response in graphene [6]. Extending the already established concepts of mid-IR s-SNOM imaging and spectroscopy of 2D materials [7] to the THz frequency range a plethora of fundamental and applied new insights into graphene and other 2D device structures can be expected from THz near-field nanoscopy measurements. [1] M. Tonouchi, Nat. Photonics 2007, 1, 97; [2] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 2004, 362, 787; [3] A. J. Huber et al. Nano Lett. 2008, 8, 3766; [4] P. Alonso-Gonzalez, et al., Nat. Nanotechnol. 2016, 12, 31; [5] B. Lundeberg, et al., Science 2017, 357, 187; [6] J. Horng et al., Phys. Rev. B 2011, 83, 165113; [7] D. N. Basov et al., Science 2016, 354, 1992. Figure 1

Mechanism of multiple bit upsets induced by localized latch-up effect in 90 nm complementary metal semiconductor static random-access memory

By using the pulsed laser single effect facility, the single event upset and latch-up phenomenon are studied, and the bitmap of 90 nm complementary metal oxide semiconductor (CMOS) static random-access memory (SRAM) is mapped. It is shown that many multiple bit upsets occur and pulsed supply current of 20 mA amplitude is monitored. Based on the technology computer aided design (TCAD), it is found that the localized latch-up in CMOS SRAM is the main reason for the single event multiple bit upsets. Finally, by analyzing the results of the pulsed laser experiment and TCAD, it is found that the P/N well potential collapse is the key physical mechanism responsible for the spreading of the single event latch-up effect in 90 nm CMOS SRAM.

The supply voltage scaled dependency of the recovery of single event upset in advanced complementary metal—oxide—semiconductor static random-access memory cells

Using computer-aided design three-dimensional simulation technology, the supply voltage scaled dependency of the recovery of single event upset and charge collection in static random-access memory cells are investigated. It reveals that the recovery linear energy transfer threshold decreases with the supply voltage reducing, which is quite attractive for dynamic voltage scaling and subthreshold circuit radiation-hardened design. Additionally, the effect of supply voltage on charge collection is also investigated. It is concluded that the supply voltage mainly affects the bipolar gain of the parasitical bipolar junction transistor (BJT) and the existence of the source plays an important role in supply voltage variation.

Recovery of single event upset in advanced complementary metal—oxide semiconductor static random access memory cells

Using computer-aided design three-dimensional (3D) simulation technology, the recovery mechanism of single event upset and the effects of spacing and hit angle on the recovery are studied. It is found that the multi-node charge collection plays a key role in recovery and shielding the charge sharing by adding guard rings. It cannot exhibit the recovery effect. It is also indicated that the upset linear energy transfer (LET) threshold is kept constant while the recovery LET threshold increases as the spacing increases. Additionally, the effect of incident angle on recovery is analysed and it is shown that a larger angle can bring about a stronger charge sharing effect, thus strengthening the recovery ability.

Tunnelling-based SRAM

This paper describes the use of low-current-density tunnelling devices to obtain dense low-power (sub-100 nW/bit) compound semiconductor static random access memory (SRAM); this cell power is over two orders of magnitude lower than the best previously demonstrated in III-V materials. This same circuit topology promises orders of magnitude reduction in static power dissipation for silicon memories, assuming a suitable Si-based negative differential resistance device, at sub-pA current levels, is developed. The concept of low-standby-power tunnelling-based SRAM (TSRAM) is presented in the context of a scaling theory for all memories and a comparison is given across technologies. Experimental results for a 50 nW TSRAM gain cell using ultralow current density (~1 A cm-2) resonant tunnelling diodes and low-leakage heterostructure field-effect transistors are discussed. We describe a one-transistor TSRAM cell, verify its operation, and discuss merits of TSRAM relative to other memory types. Silicon TSRAM standby power is projected to be lower than that of conventional-style silicon SRAM or DRAM.

Magnetic tunneling applied to memory (invited)

Random access magnetoresistive memories have been designed using anisotropic magnetoresistive (AMR) material and more recently giant magnetoresistive (GMR) material. The thin films in these memories have low sheet resistivities (about 10 Ω/sq), resulting in cell resistances of 10 to 100 Ω at competitive areal densities. High sense currents of a mA or more are required to get signals on the order of a few mV. Spin dependent tunneling (SDT) devices are intrinsically high impedance, with typical equivalent resistance values of 104–109 Ω for a square micron area. SDT cells have the potential for signals on the order of 10 mV with lower sense currents, and hence, faster access times than GMR memory. A GMR pseudospin valve memory concept is presented for comparison with SDT memory. Three different design approaches are discussed for SDT memory: (1) high-density memory arrays similar to those in AMR and GMR memories, (2) a transistor per cell approach similar to semiconductor dynamic random access memory, and (3) embedded SDT devices in a flip–flop cell similar to semiconductor static random access memory. The conclusions are: (1) SDT memory is potentially higher speed than GMR memory, (2) SDT memory has no area advantage compared with dense GMR memory, and (3) risks with SDT memory include (a) processing ultrathin dielectric layers uniformly and reliably that are compatible with integrated circuits and (b) attaining sufficiently low impedance levels to get a satisfactory signal-to-noise ratio in a small area cell.