New Series – NV Memory Selectors: Forming the Known Unknowns

Photo of Ron Neale, Renowned Phase-Change Memory ExpertThe Memory Guy is pleased to announce a new series contributed by guest blogger and PCM expert Ron Neale.

The series will appear in five sections detailed below:

Part 1: The series starts by analyzing some very detailed research work on selectors, which highlighted the subject of Forming. While the measurements were detailed, the question as to cause was left unanswered; it became the “known unknowns” which is the title for this series.  This unknown has possible reliability and performance implications for future 3D stacked memory arrays of all types.

Part 2:  Explains the mechanism of Forming in the operation of selectors and memory in a memory array and raises a series of important questions, which if answered would provide all the necessary understanding of when and why Forming occurs giving its potential implications on performance and reliability.

Part 3: In trying to provide answers to the many Forming questions raised in Part 2, it is key to have a clear and unambiguous explanation of threshold switching.  The available research literature has very many options from which to choose, some of which are briefly reviewed in this post. Our answer is to consolidate these options  into a single all-purpose mixed thermal-electronic model for threshold switching which accommodates most of the options and provides a direct link to localised Forming

Part 4: Offers some physical explanations for Forming with speculative models which focus on percolation paths based on localised permanent changes in structure, or result from filamentary composition changes or chemical reactions.

Part 5:  Some observations suggest a few experiments which might help uncover the mysteries of Forming.  With scaling these experiments would enable us to identify whether Forming and threshold switching is based on filamentary, contact, or bulk effects.  The objective is to forever remove the known unknown of Forming as an ill-defined final fabrication step.


This post is launch of a five-part series on 2-terminal chalcogenide-based memory selectors and Forming authored by Ron Neale and published in The Memory Guy blog in February-November 2019.  The links below will take you to each part of the series:

  1. Selector Forming, What is Known and what is Unknown
  2. The “Hows” and “Whys” of Selector Endurance
  3. Narrowing the Many Explanations for Forming
  4. Physical Explanations for Forming
  5. Where to Go from Here?

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