Electrothermal Model-based Analysis for Novel Phase-Change

Memory Structure with Decoupled Program and Read Paths

Date : 2021. 03. 31

Inhyuk Choi

Neuromorphic Materials and Devices Laboratory (NMDL)

Department of Materials and Science and Engineering, Seoul National University

1

Outline 2

Introduction• Phase Change Memory (PCM)• Issues on conventional PCM structure

Novel PCM structure• Concept• Electrothermal model: Joule heating • Electrothermal model: Joule heating + Thermoelectric effects• Possibility of cyclic endurance enhancement

Summary

Outline 3

Introduction• Phase Change Memory (PCM)• Issues on conventional PCM structure

Novel PCM structure• Concept• Electrothermal model: Joule heating • Electrothermal model: Joule heating + Thermoelectric effects• Possibility of cyclic endurance enhancement

Summary

Introduction 4

Storage Class Memory Neuromorphic Computing

A. Lotnyk et al., Nanoscale Adv., (2019)

Q. Z. Wan et al., Adv. Mater. Technol. Rev., (2019)

Introduction 5

Non-Volatile

Memory

Key properties Endurance

Power consumption

Write & Read Speed

PRAM RRAM MRAM

S. Bhatti et al., Materials Today, (2017)

G. W. Burr et al., Adv. Physics X, (2017)

Motivation 6

Issues in conventional PCM structures: 1) 𝐈𝐈𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 𝐯𝐯𝐯𝐯 𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 Trade-off

Reduction of RESET power by increasing 𝑹𝑹𝒕𝒕𝒕𝒕 Contact / Cell size reduction increases both 𝑅𝑅𝑡𝑡𝑡 & 𝑅𝑅𝑒𝑒𝑒𝑒 Material doping increases both 𝜌𝜌𝑡𝑡𝑡 & 𝜌𝜌𝑒𝑒𝑒𝑒 (by W-F Law)

However, for fast readout (< 𝟓𝟓𝟓𝟓𝟓𝟓𝟓𝟓),

𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺 < ~𝟏𝟏𝟓𝟓𝟓𝟓𝟏𝟏𝛀𝛀 is required!Boniardi, M. et al., IEDM, (2014)

Lacaita, A. L. et al., Microelectron. Eng., (2013)

7Motivation

• Stuck SET: Gradual elemental segregation

• Stuck RESET: Void formation near the contact

Burr, G. W. et al., J. Vac. Sci. Technol., (2010) Yang, T-Y. et al., Appl. Phys. Lett., (2009)

High electric fields in phase change material is one of the main driving forces.

Due to integrated program and read paths

Issues in conventional PCM structures: 2) Cyclic Endurance Failure

Outline 8

Introduction• Phase Change Memory (PCM)• Issues on conventional PCM structure

Novel PCM structure• Concept• Electrothermal model: Joule heating • Electrothermal model: Joule heating + Thermoelectric effects• Possibility of cyclic endurance enhancement

Summary

Novel PCM structure 9

Separated program and read paths.

Breaks out of the 𝑰𝑰𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺 vs 𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺 trade-off.

Enhancement of the cyclic endurance.: Significant reduction of the electromigration.

Thermoelectric effects can be maximized.: Reduction of the power consumption.

Concept

Model-based Analysis 10

Geometry Equations

𝛻𝛻 � 𝐽𝐽 = −𝛻𝛻 � 𝜎𝜎 𝛻𝛻𝛻𝛻 = 0

𝑑𝑑𝐶𝐶𝑃𝑃𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 − 𝛻𝛻 � (𝜅𝜅𝛻𝛻𝑑𝑑) =

𝐽𝐽 � 𝐽𝐽𝜎𝜎

2D Axisymmetric analysis

The heater needs to be stable at high temperatures (e.g. Refractory metals)

Comparison of 𝑰𝑰𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺 𝒗𝒗𝟓𝟓 𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺 curve with reducing ‘r’ of heating materials

Electrothermal model: Joule Heating effects

𝐽𝐽: Current density𝜎𝜎: Electrical Cond.𝛻𝛻: Electric potential𝑑𝑑: Mass density𝐶𝐶𝑝𝑝: Heat capacity𝜅𝜅: Thermal Cond.

Results 11

(𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅: when 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅/𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 ≈ 103)

Consistent 𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺 while reducing 𝑰𝑰𝑹𝑹𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺.: Cell scaling no longer affects read speed.

Energy efficiency should be improved.: Thermoelectric effects for additional heat.

Thermoelectric effects- Seebeck + Peltier + Thomson effect.- Conversion between heat and electricity.

Comparison of 𝐈𝐈𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 𝐯𝐯𝐯𝐯 𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 Trend

12Model-based Analysis Electrothermal model: Joule heating + Thermoelectric effects

Geometry Equations

𝛻𝛻 � 𝐽𝐽 = −𝛻𝛻 � 𝜎𝜎 𝛻𝛻𝛻𝛻 + 𝑆𝑆𝛻𝛻𝑑𝑑 = 0

𝑑𝑑𝐶𝐶𝑃𝑃𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

− 𝛻𝛻 � 𝜅𝜅𝛻𝛻𝑑𝑑 =𝐽𝐽 � 𝐽𝐽𝜎𝜎

− 𝑑𝑑𝐽𝐽 � ∆𝑆𝑆 − 𝑑𝑑𝐽𝐽 �𝑑𝑑𝑆𝑆𝑑𝑑𝑑𝑑

𝛻𝛻𝑑𝑑

Assumption: Two heater materials

𝐽𝐽: Current density𝜎𝜎: Electrical Cond.

𝐶𝐶𝑝𝑝: Heat capacity𝜅𝜅: Thermal Cond.

𝛻𝛻: Electric potential𝑑𝑑: Mass density

Results 13

R-I curve change according to the ‘S’ value

< Seebeck effect only > < Peltier effect only >

14

Temperature distribution

Joule Heating(JH)

JH + PH(∆𝑺𝑺 = 𝟐𝟐𝟓𝟓𝟓𝟓)

JH + PC(∆𝑺𝑺 = −𝟐𝟐𝟓𝟓𝟓𝟓)

(Under positive polarity)

𝑺𝑺𝑨𝑨 > 𝟓𝟓, 𝑺𝑺𝑩𝑩 < 𝟓𝟓 Heaters become resistive by

the Seebeck effect.

∆𝑺𝑺 = 𝑺𝑺𝑨𝑨 − 𝑺𝑺𝑩𝑩 > 𝟓𝟓 ∆𝑆𝑆 > 0: Peltier Heating(PH). ∆𝑆𝑆 < 0: Peltier Cooling(PC).

Reduction of 𝐈𝐈𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑

(Under the same voltage)

Results

𝐑𝐑𝐦𝐦𝐦𝐦𝐦𝐦 = 𝟐𝟐𝟓𝟓𝟐𝟐𝟓𝟓𝟐𝟐𝐑𝐑𝐦𝐦𝐦𝐦𝐦𝐦 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟓𝟓𝟐𝟐 𝐑𝐑𝐦𝐦𝐦𝐦𝐦𝐦 = 𝟏𝟏𝟏𝟏𝟐𝟐𝟓𝟓𝟐𝟐

15ResultsComparison of 𝐈𝐈𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 𝐯𝐯𝐯𝐯 𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 Trend, Power consumption

101 102 10310-1

100

101

102

R(I)

/R(I=

200µ

A)

IRESET [µA]

Confined cell Confined cell (+TE) Novel cell Novel cell(+TE) Ref. 1 Ref. 2

r = 4nm, 𝑺𝑺𝟓𝟓= 100 → 500

Discussions 16

Possibility of cyclic endurance enhancement

Summary 17

Based on the electrothermal model, the novel PCM structure with the separated program and read paths no longer has a 𝐈𝐈𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 𝐯𝐯𝐯𝐯 𝐑𝐑𝐑𝐑𝐑𝐑𝐑𝐑 trade-off of the existing PCM structures.

Energy efficiency can be greatly improved by maximizing the thermoelectric effects in the novel PCM structure.

Cyclic endurance can be enhanced in the novel PCM structure since the electromigration in the phase change material is expected to be significantly reduced during the program.