# Advances in Emerging Memory Technologies: From Data Storage to Artificial Intelligence
## 3 Overview of Emerging Memory Technologies
### Emerging Memory Technologies
#### 3.1.4 Ferroelectric Memory  
Ferroelectric memories (FeRAM) are based on the polarization of ferroelectric materials (Figure 5). In an FeRAM, thin ferroelectric films are sandwiched between two metallic electrodes and used as capacitors. When an electric field is applied across the stack, the states are aligned with the electric field leading to a low-energy state. Analogically, the high-energy state is obtained when the polarization alignment is antiparallel to the electric field. Moving from the high-energy state to the low-energy state or vice versa produces  
Figure 4: TEM cross-sections of perpendicular STT-MRAM integrated in a four kilobyte array in the BEOL of a 130 nm CMOS process, from [82]. energy as a charge and is normally called a switch charge (Qs). Therefore, two Q(V) curves are obtained as a function of the polarization of the electric field applied. An access transistor is used to sense the state of the ferroelectric film, while the typical polarization depends on the electric field. Ferroelectrics are theoretically an ideal solution for low write power nonvolatile memories. However, the complexity of ferroelectric perovskites has hindered the scaling of such devices to competitive feature sizes. The discovery of ferroelectricity in hafnium oxide solved this issue, and led to renewed interest by the scientific community in this concept for various applications, due to its CMOS compatibility [92; 93; 94]. Three types of ferroelectric-based memories are:  
* Ferroelectric FET (FeFET): the ferroelectric material is embedded in the gate stack of a transistor. The nonvolatile polarization of the material acts as a remnant control gate and leads to a threshold voltage shift of the characteristics. This concept offers ultra-low power but is a three terminal device and can thus hardly be envisaged for high-density applications. Moreover, the effect vanishes for thin layers, making the concept hardly scalable. Finally, the degradation of the interface layer between the ferroelectric and the semiconductor channel limits endurance, in particular, due to trapped charge that affects the conduction of the FET below the ferroelectric [92]. For all of these reasons FeFET are targeting Flash or EEPROM rather than DRAM replacement. In particular, it is now seen as an alternative to Flash for ultra-low power applications [91], due to its 10 fJ/bit consumption and five-nanosecond programming speed. FeFET based eNVM solutions were integrated into leading edge technologies: GlobalFoundries FeFET technology was embedded into the 28 nm gate first HKMG low power CMOS platform, showing 68 distribution, reasonable endurance, and stable data retention [95]. GF also demonstrated a 22 nm node on FDSOI CMOS technology [96]. * Capacitor based ferroelectric RAM (FeRAM): in this case, the cell resembles a DRAM with the capacitor dielectric replaced by the ferroelectric. Recent reports verified anti-ferroelectric properties for pure ZrO\({}_{2}\) dielectrics used in DRAM stacks. By employing electrodes with different work function values, a built-in bias is introduced within the anti-ferroelectric stack, thus creating two stable nonvolatile states [97]. It demonstrated 10\({}^{10}\) endurance and 10 ns speed combined with 100 \({}^{\circ}\)C retention, making this concept very promising for a dense (6 F\({}^{2}\)) and new class of nonvolatile DRAM. In FeRAM, reading is destructive as it is performed by switching the ferroelectric into a specific direction and measuring the contrast between a switching and a nonswitching event. Thus, programming is required after each reading operation. In terms of FeRAM reliability challenges, trapped charges at the ferroelectric-electrode interface have to be controlled to improve retention (requiring careful interface engineering), while dielectric breakdown induced by high coercive film has to be prevented to insure high endurance [92].