The Ox-RRAM NVM project for 0.13 µm BiCMOS IHP Technology
The aim of the NVM project in the Materials Research department at IHP is to increase the functionality of 0.13µm IHP BiCMOS by integrating innovative NVM cell concepts. The following criteria were considered of utmost importance to select an appropriate memory concept for IHP BiCMOS technologies:
• cost-effectiveness
Back-end-of-line integration is preferred to save valuable wafer area
• fast read / write times
Fast read / write times are required for embedded NVM cells in BiCMOS technology
• low power
Portable electronics require NVM concepts with low power consumption
• high reliability
Retention & endurance must be guaranteed for future BiCMOS technology qualification.
• innovative memory concept
R&D work at IHP-Microelectronics must focus on upcoming, innovative cell concepts
Based on these criteria, the decision was made for the so-called “Ox-RRAM” NVM approach.
Fundamental Materials Research on Resistive Switching Phenomena in Dielectrics
The resistive RAM (RRAM) concept is based on a RAM architecture in which the non-volatile memory phenomenon is realized by reversibly changing the resistance of a thin film layer material (chalcogenides, amorphous semiconductors, dielectrics) sandwiched between two metal electrodes by applying external voltage pulses. Among these materials systems, the Ox-RRAM NVM approach is a special case in which the resistive switching of oxide thin film structures (Nb2O5 [6], NiO [7], TiO2 [8], Perovskites [9, 10, 11] etc.) in metal - insulator – metal (MIM) cells is exploited. Fig. 6 shows an ideal I-V curve of such Ox-RRAM - MIM structures [12]:
Blue branch: Cell is switched from off-state (high-resistance) to on-state (low-resistance) by applying a certain positive voltage and limiting the current.
Green branch: Reducing the voltage, the cell in the on-state follows an ohmic behaviour.
Red branch: Ohmic state is conserved also under negative voltage; under a too high voltage, a high current destroys the ohmic state and switches the cell back in the off-state.
Black branch: Under a high negative voltage and current control, the cell can be set in the on-state also under negative voltage polarity.
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Figure 6: Resistive switching phenomenon of RRAM cells |
It is interesting to note that, by applying a voltage in the “read” window, a non-destructive read-out of the cell is possible. Furthermore, high data retention and endurance were demonstrated for various dielectrics, even under high-temperature operation. A problem is however the fact that power consumption for Ox-RRAM cells is in general still too high.
Due to the fact that the microscopic mechanism of the resistive switching behaviour of the thin film materials in the RRAM MIM structures is still controversially discussed in the literature, worldwide R&D work focusses to solve this issue. Certainly, a detailed understanding of the resistive switching phenomena in oxides is required to tailor the materials of interest in such a way that the power consumption can be reduced for future device applications.
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Figure 7: Examples of resistive switching mechanisms in RRAM cells |
Fig.7 shows typical microscopic mechanisms discussed in the literature to explain the resistive switching behaviour in thin film materials. Both models part from an insulating material which defines the “off-state” of the cell. The left side depicts the so called “nanofilament model” in which the “on-state” is produced by the formation of metallic nanofilaments which short circuit the metal electrodes [13]. The right image is a sketch of the so called “defect band model” in which the conducting “on-state” of the insulator is produced by partial filling of a defect band structure [14].