PhD Oral Examination - SangBum Kim, Tuesday, March 16, 2010: 10:00AM

Special University PhD Oral Examination

 

Scalability and Reliability Study of Phase-Change Memory

 

SangBum Kim

Advisor: Professor H.-S. Philip Wong

 

Department of Electrical Engineering, Stanford University

 

Tuesday, March 16, 2010, 10:00 – 11:00 AM

Paul Allen Building Auditorium (CISX-101)

(Refreshments served at 9:45 AM)

 

Abstract

 Phase-change memory (PCM) is one of the most mature emerging memory technologies. Superior scalability and reliability are important features that phase-change memory (PCM) technology should demonstrate to expand its usage in various memory applications. In the first part of the talk, scalability study of PCM will be presented. The first principle for PCM scaling rule is studied using analytical analysis. As a PCM cell scales down, the interfacial effect becomes more prominent in material properties and device characteristics due to larger surface area to volume ratio. Various structures have been fabricated to measure these interfacial effects and study its impact on the operation of PCM. In search for a high current-density selection-device with low processing temperature which is needed for integration of PCM in a 3-dimensional 4F² cross-point array, a Ge nanowire diode has been successfully integrated with a PCM memory cell.

 

 In the second part of the talk, reliability related study is presented. Interaction of thermal program-disturb mechanism and temperature dependence of PCM characteristics poses unique challenges for reliability of PCM devices. To study temperature dependence of PCM characteristics, the micro-thermal stage (MTS) with a fast on-chip heater has been integrated with a PCM cell. The MTS enables experimental study on temperature dependence of crystallization time, drift speed, and threshold switching in microsecond time scale. Thermal disturbance and its impact on PCM operation are measured with the MTS and the relevant model is developed. Based on measurement and modeling results, a new scheme is proposed to improve stability of PCM with a short time annealing pulse.