Projection PCM rethinks the design of PCM device structures and demonstrates the ability to isolate from the read process the effect of undesirable changes in parameters of the active region of a MLC-PCM in its amorphous state
The Multi-Level Cell / Phase Change Memory (MLC-PCM) team at IBM in Zurich have just released the details of a its latest work which offers in a single structure the potential to eliminate a number of MLC-PCM performance problems and represents yet another significant waypoint on its roadmap towards the possible future commercial success of MLC-PCM.
Under the descriptive term “Projection” and embracing many different types of PCM device structures, both vertical and lateral, Projection PCM adds a new acronym, P-PCM, to the non-volatile memory (NVM) lexicon. This piece work began with a complete rethink of the design of PCM device structures to see if a means could be found of decoupling the physical mechanism of resistance storage from the information retrieval process. The achievement of “projection” is its demonstrated ability to isolate from the read process the effect of undesirable changes in parameters of the active region of a MLC-PCM in its amorphous state, such as drift, resistance and threshold voltage.
What is projection? Figure 1(a) and (b) illustrates the device structure and associated current flow during the read operation for a device in the (a) SET and (b) RESET states. In this example the structure is a conventional “pore” vertical planar structure except the difference is the inner surface of the pore is lined with a cylindrical film called the projection segment. The resistance of the projection film is carefully chosen so that it has only a marginal influence on the write operation but a significant influence on the read operation.
Figure 1: (a), (b) The planar structure of the P-PCM with equivalent circuit, the projection element (brown), amorphous material (red), (c) the P-PCM I-V characteristics.
Starting with the active material in the crystallized SET state, figure 1(a) illustrates the current flow though the structure and the lumped equivalent resistance components. During read the current will flow directly though the low resistance crystallized material, with very little current flowing through the projection film. The same current localization will occur during RESET.
When the central region of the chalcogenide memory material is RESET to its amorphous state, the region shaded red in Figure 1(b), the resistance of the amorphous material is higher than that of the projection material, which means that almost all of the read current will flow or be projected into the latter. The information that is stored as the length of the amorphous material is “projected” into the stable projection film. This means quite large variations in parameters like drift, resistance and threshold voltage changes can occur in the amorphous material without any significant effect of the read current.
During writing to the low resistance state and prior to threshold switching, the observed resistance will be dominated by the resistance of the projection element, as illustrated in Figure 1(c), also benefiting from the non-linearity of the I-V characteristics of the amorphous material. The degree to which heat conducted from the projection element will lower the threshold. Voltage is a variable that must be considered in the design of the structure and selection of material.
While lateral or “gap” PCM structures may or may not have a role to play in any PCM arrays of the future, the IBM team viewed that as the type of structure as the best one to use in order to understand, explore, quantify and evaluate the effects of projection. While most of today's focus is on the digital application for PCM, this work also exposed a possible future role for P-PCM as an analog memory component.
An example of one of a family of lateral structures evaluated is illustrated in figure 2(a), a simple elongated resistor structure with the amorphous film deposited directly on the 2.8nm thick projection film of titanium nitride (TiN); all deposited on and enclosed in a dielectric oxide film. To demonstrate the versatility of the technique, two different compositions of phase change material were used. The active films were either a 15nm thick AgInSbTe (AIST) or a 30nm thick GeTe, the latter deposited in its crystalline phase. Those particular compositions also offered levels of resistivity compatible when TiN is used as the projection material.
Figure 2: (a) Illustration of the new lateral P-PCM structure, with (b), (c) and (d) SEMs of the variety of different structures evaluated.
Figure 2 (b), (c) & (d) are SEMs of three examples of the structures evaluated, from the simplest almost point contact tapered electrode, to the elongated resistor and finally the stepped structure.
The more complex winged or stepped structures allow the controlled development of localized hotspots which can be expanded along the center line of the device structure, with sufficient control to provide analogue-like storage with a high level of precision.