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The Advantages of Phase Change Memory

Phase change memory (PCM) is a type of advanced non-volatile memory where the information is encoded in the phase (i.e. the atomic arrangement) of a material. Phase change materials are usually based on chalcogenides (they contain elements in group 16 on the periodic table, typically those below oxygen). Their atomic structure can be reversibly changed between crystalline and amorphous states by using an external electrical field or optical pulse to heat and cool the material over a specific time scale. This rapid amorphous to crystalline transition leads to change in electrical resistance up to several hundred orders of magnitude, and these low- and high-resistance states define the digital 1s and 0s of stored data. The amorphous-crystalline phase change can be triggered repeatedly with minimal electrical power in a nanosecond, so novel information storage memory built from chalcogenide materials is anticipated to out-perform some other advanced memory technologies with lower power consumption, faster switching, and higher endurance for storage class memory applications.

The performance of PCM cells are largely influenced by the elemental compositions of chalcogenide alloys. Different alloys will have distinctly different physical and electrical properties in terms of the crystallization speed, thermal stability, switching power and resistance contrast. For example, some Ge-rich chalcogenide films tend to show superior amorphous-crystalline phase changes with higher thermal stability and better data retention while Te-rich chalcogenides demonstrate highly non-linear, volatile Ovonic Threshold Switching (OTS). The former is suitable for use as PCM element and the latter is considered as a compelling candidate for memory interface devices called selectors. When chalcogenide-based PCM and OTS elements are integrated together in vertically stacked sandwich architecture, the density of memories cell can be significantly increased as a result of reduction of cell size to 4F2. Non-volatile memories (NVM) stacked in 3D cross-point and 3D vertical architecture made from chalcogenides are envisioned to be the most promising solutions to overcome the memory scaling limits for high density memory applications.

Integration techniques for chalcogenide materials in PCM memories

Physical vapor deposition (PVD) has been the primary technique employed to integrate the chalcogenide materials for applications in stacked PCM and OTS cells. In this method, solid elemental or compound chalcogenide targets of interest are vaporized and precipitated onto planar memory structures to form the sandwiched PCM/OTS stacks. The main advantage of PVD films is that the composition can be precisely targeted simply by changing the mixture of elements used in the target. In fact, using Intermolecular’s high-throughput PVD experimental platform, a wide range of chalcogenide compositions have been quickly screened for target memory applications (check out our webinar on this topic).
However, the drawback of PVD is the poor step coverage and film non-uniformity, which is problematic for 3D architectures and for nodes where the size of the memory cell becomes very small. The stacked memory elements in 3D using PVD use multiple masking steps, and therefore very costly sequential processing must be employed.

By contrast, in atomic-layer deposition (ALD), the chalcogenide films are deposited chemically instead of physically. Each element is contained in a chemical precursor and is introduced to the target substrate sequentially between inert carrier gas purges. The ligand exchange reaction of precursors leads to formation of chalcogenides films of high purity, and because each precursor is given time to saturate the surface, even high-aspect ratio structures can be covered. The advantage of ALD is its highly conformal and uniform film deposition on complicated nanoscale structures, making it the most suitable candidate for realizing 3D vertical memory architectures. ALD is much more complex than PVD because composition tuning relies on stacking materials in an appropriate layering sequence. Further, the precursor chemistries must be carefully selected to ensure saturation of the surface and efficient chemical reactions to produce the desired films.

Intermolecular progress in ALD of chalcogenide films

Intermolecular’s high-volume manufacturing compatible 300 mm ALD reactor has been successfully used for rapid screening of chalcogenide precursors for PCM applications. In-situ ellipsometry has enabled dynamic monitoring of the precursor nucleation and film growth, which accelerates precursor screening and development of new ALD processes by looking into the mechanics of the ALD process at each precursor step. Furthermore, by experimenting with various stacking sequences of binary ALD processes, Intermolecular has successfully demonstrated a wide range of nano-laminated chalcogenide film compositions, spanning the PCM and OTS application spaces. In particular, we have demonstrated binary GeTe, GeSe, and Sb2Te3, and ternary Ge-Se-Te and Ge-Sb-Te that show excellent conformality and great potential for integration into 3D memories. For more information, check out our Editor’s pick article in JVST A.