• 沒有找到結果。

氮氧化鉭薄膜於電阻式記憶體的製作與轉態特性之研究

N/A
N/A
Protected

Academic year: 2021

Share "氮氧化鉭薄膜於電阻式記憶體的製作與轉態特性之研究"

Copied!
121
0
0

加載中.... (立即查看全文)

全文

(1)

國 立 交 通 大 學

電子工程學系電子研究所碩士班

碩士論文

氮氧化鉭薄膜於電阻式記憶體的製作與轉態特性之研

Study of Resistance Switching Characteristics and

Fabrication in TaON Thin Film for Resistive Random

Access Memory

研究生: 邱奕介

指導教授: 施敏 院士

張鼎張 博士

(2)

氮氧化鉭薄膜於電阻式記憶體的製作與轉態特性之研究

Study of Resistance Switching Behavior in TaON and Fabrication Thin

Film for Resistive Random Access Memory (RRAM)

研 究 生:邱奕介 Student:Yi-Chieh Chiu

指導教授:施敏 院士 Advisor:Prof. S. M. Sze

張鼎張 博士 Prof. Ting-Chang Chang

國 立 交 通 大 學

電子工程學系 電子研究所

碩 士 論 文

A Thesis

Submitted to Department of Electronics Engineering and Institute of Electronics

College of Electrical Engineering and Computer Science National Chiao Tung University

in partial Fulfillment of the Requirements for the Degree of

Master in

Electronics Engineering June 2011

Hsinchu, Taiwan, Republic of China

(3)

I

氮氧化鉭薄膜於電阻式記憶體的製作與

轉態特性之研究

研究生:邱奕介 指導教授:施敏 院士

張鼎張 博士

國立交通大學電子工程學系電子研究所碩士班

摘要

由於近幾年來,非揮發性記憶體的研究與發展備受關注,又因為傳統浮閘快 閃記憶體無論在垂直或水平微縮上受到挑戰,所以發展新穎式的非揮發性記憶體 是重要的趨勢。其中又以電阻式非揮發性記憶體元件具有低耗損能量、低操作電 壓、高記憶密度、且具快的操作速度快以及高耐久度等優點,最重要的是結構簡 單(金屬/介電質/金屬) ,使其成為最具有取代快閃記憶體的新穎式非揮發性記 憶體元件。 在本篇論文中,主要是研究探討氮化鈦/氮氧化鉭/白金結構與氮化鈦/氮氧 化鉭/銅結構的電阻式記憶體的轉態特性探討和電流傳導物理機制。其實驗內容 主要可以分為四大部分,第一部份為白金上電極結構元件的二極性基本電性與轉 態機制研究,且強調正負形成電壓的機制差異。第二部份為銅上電極結構元件的 二極性基本電性與轉態機制研究。第三部份為利用變溫、定電流等量測方式來證 明兩元件的轉態機制與差異,探討在薄膜內轉態的情形以及上電極對轉態有甚麼 影響。第四部份為兩元件的單極性基本電性與轉態機制研究,並與雙極性轉態機 制做比較。

(4)

II

Study of Resistance Switching Characteristics and

Fabrication in TaON Thin Film for Resistive Random

Access Memory

Student: Yi-Chieh Chiu Advisor: Prof. S. M. Sze

Prof. Ting-Chang Chang

Department of Electronics Engineering and Institute of Electronics

National Chiao Tung University

Abstract

Recently, there are intensive researches and development of various non-volatile memories gradually. Because the traditional floating gate flash memory in terms of vertical or horizontal scaling is being challenged. One of the most important advanced memories is the resistance random access memory (RRAM). The device has the advantages of low power consumption, low operating voltage, high density, fast speed and high endurance and retention. The most important attribute of the device is that it has a simple structure (metal / dielectric / metal).Thus, we may eventually replace Flash memory with RRAM for non-volatile memory application.

This thesis is the studying the resistive switching properties and the physical mechanism of current conduction of structure of titanium nitride / tantalum oxynitride / platinum and the structure of titanium nitride / tantalum oxynitride / copper . The

(5)

III

content can be divided into four parts. In part one, we study the bipolar resistive switching characteristic of structure TiN/TaON/Pt, and propose a model which oxygen ions migration makes the device’s resistance switched and focus on positive and negative Forming explanation. In part two, we change top electrode to copper and study the bipolar resistive switching characteristic of structure TiN/TaON/Cu. We also propose a model which copper ions redox and electron-migration makes the device’s resistance switched. In part three, we use many experiments like temperature measurement, constant current stress measurement to prove two devices’ mechanisms. With different methods we study the resistive switching phenomena in film and what effect will happen in different top electrode. In final part, there is unipolar characteristic in two devices and we explain resistive switching mechanism of two devices as well.

(6)

IV

Acknowledgement

完成整個碩士的畢業論文,是我人生中非常寶貴的經驗與歷程,使我深深的體會 到要呈現優秀的實驗數據是一件不容易的事,需要擁有精良的實驗製程機台和精 密的量測儀器,至此更學會許多製程及量測的技術。首先,我要感謝的是我的指 導教授施敏和張鼎張博士,在他深入淺出的教導之下,使我受益良多,獲益匪淺, 很快就接受那些新觀念,他也指引我實驗的方向,使我能完成此畢業論文,接著, 要感謝薄膜電晶體平面顯示及半導體積體電路實驗室的各位學長姐們,其中,我 要特別感陳仕承學長的指導,不管是儀器的操作與使用、理論概念的釐清抑或是 數據的討論,都給了我莫大的幫助,感謝黃正杰學長幫我切FIB與看TEM。感謝富 彥、美娜、書瑋、原瑞、柏鈞、敏甄、佳盛、志豪、宛芳、德智、詠恩、冠張、 禹鈞、侑廷、聖堯、學志、文宏、侑廷、耿維、君寶、天宇、慶恩等學長姐於研 究過程中給予建議及指正,感謝一起走過這段日子的同學們:柔妙、菀琳、儀憲、 凱弘、雅琪、承瑋、國孝、偉立、岳恆、祐松、冠任、志誠等,感謝你們的陪伴 與鼓勵 ,還有感謝眾多學弟妹們:昌蓓、哲丘、君昱、明諺、峻豪、天健等總能不吝的 給我意見並教導我許多不足的地方,陪我一起把實驗給做出來。最後,我要感謝 我最親愛的家人與女友,在精神上所給予我的支持跟鼓勵,感謝你們多年來的栽 培,讓我可以全力以赴、無後顧之憂,順利完成此畢業論文,在此獻上內心最深 的謝意。 邱奕介 謹識 交通大學 2011 年

(7)

V

Contents

Abstract (Chinese) ………..……….I Abstract (English) ………..……….II Acknowledgement………..………IV Contents………...……….V Table Lists………...IX Figure Captions………...X Chapter 1 Introduction………1 Chapter 2 Literature 2-1 Introduction of Memory………..3 2-2 Advanced Non-volatile Memories

2-2.1 FeRAM (Ferroelectric Random Access Memory)……..………..4 2-2.2 MRAM (Magnetic Random Access Memory)………..4 2-2.3 PCRAM (Phase Change Random Access Memory)……….5 2-2.4 RRAM (Resistance Random Access Memory)……….5

2-3 The Materials of RRAM

2.3.1 Perovskite………..………6 2.3.2 Transition metal oxides……….8 2.3.3 Organic materials……….……….8

2.4 The Resistive Switching Mechanism of RRAM

2.4.1 Filament

2.4.1.1 Joule heating effect……….…………....9 2.4.1.2 Redox processes by cation migration……….……..10 2.4.1.3 Redox processes by anion migration…………...……….10

(8)

VI

2.4.2 Charge-trap in small domain……….………..11

2.4.3 Modified Schottky barrier model………..………..11

2.5 The Mechanism of Current Conduction 2.5.1 Ohmic conduction………...……12

2.5.2 Schottky emission………...……13

2.5.3 Poole-Frenkel emission………...13

2.5.4 Space charge limited current………...………14

Chapter 3 Experiment 3-1 Process Flow 3-1.1 Pt/TaON/TiN………...………27

3-1.2 Pt/Cu/TaON/TiN………...…………..27

3-2 Material Analysis of Pt or Cu/ TaON/TiN Structure 3-2.1 Material analysis of N&K………...………28

3-2.2 Material analysis of Mid-IR………28

3-2.3 Material analysis of XPS………...….28

3-2.4 Material analysis of TEM………..……….29

3-3 Electrical Measurement……….29

Chapter 4 Results and Discussion 4-1 The Resistive Switching Features of Different Top Electrode……36

4-2 Bipolar Resistive Switching Feature of Pt / TaON/ TiN Structure 4-2.1 Current-Voltage characteristics………...……36

4-2.2 Reliability 4-2.2.1 Endurance………...……….38

(9)

VII

4-2.2.2 Retention………..39 4-2.2.3 Multi-level………39

4-3 The Bipolar Resistive Switching Mechanism Discussion of the Structure Pt/ TaON/ TiN

4-3.1 Forming of bipolar………..40 4-3.2 I-V feature of bipolar………..42 4-3.3 The mechanism discussion the first Reset of positive and

negative Forming………...……….44

4-4 Bipolar Resistive Switching Feature of Pt/Cu/TaON/TiN Structure

4-4.1 Current-Voltage characteristics………..…….46 4-4.2 Reliability

4-4.2.1 Endurance…………...……….47 4-4.2.2 Retention………..………48

4-5 The Bipolar Resistive Switching Mechanism Discussion of the Structure Pt/ Cu/ TaON/ TiN

4-5.1 Forming of bipolar………..………49 4-5.2 I-V feature of bipolar………..………50

4-6 The Current Fitting of Structure Pt / TaON/ TiN and Pt/Cu/

TaON/ TiN

4-6.1 Current-Voltage curve fitting of structure Pt/TaON/TiN…....52 4-6.2 Current-Voltage curve fitting of structure Pt/Cu/TaON/TiN...53

(10)

VIII

Pt/Cu/ TaON/ TiN

4-7.1 Conduction mechanism of Structure Pt / TaON/ TiN……...54

4-7.2 Conduction mechanism of Structure Pt /Cu/ TaON/ TiN...55

4-8 Temperature Method Explains Copper Conduction Filament in TaON………56

4-9 Sampling Method to Prove the Devices of Top Electrode Cu’s Resistive Switching Mechanism………57

4-10 Another Resistive Switching Characteristic of Pt/ TaON/ TiN Structure

4-10.1 Current-Voltage characteristics………….………..59 4-10.2 DC endurance……….60 4-10.3 Retention……….60 4-10.4 The discussion of resistive switching mechanism

4-10.4.1 Forming of unipolar and the first Reset……...…..61 4-10.4.2 I-V feature of unipolar………...………62

4-11Another Resistive Switching Characteristic of Pt/Cu/ TaON/ TiN Structure

4-11.1 Current-Voltage characteristics………..….63 4-11.2 The discussion of resistive switching mechanism………..64

Chapter 5 Conclusions………...………..100 References……….………101

(11)

IX

Table Lists

Chapter 2

Table 2-1Comparison different memory technology [1]……….….16 Table 2-2 Basic current conduction [25]……….………….16

Chapter 3

Table 3-1 The recipe of film TaON……….……….30 Table 3-2 The recipe of film Pt………30 Table 3-3The recipe of film Cu………....31

(12)

X

Figure Captions

Chapter 2

Fig. 2-1 The P-E hysteresis of ferroelectric material [2][3]……….….17 Fig. 2-2 The MRAM structure operating schematic diagram [5]………..…17

Fig. 2-3 The PCRAM structure diagram and operating schematic

diagram [2]……….…..18

Fig. 2-4 The RRAM structure diagram [6]………18

Fig. 2-5 (a) Nonpolar (unipolar) switching (b) Bipolar switching[7]………..……19

Fig. 2-6 Perovskite structure [8]………..………..20 Fig. 2-7 Nonvolatile resistance v.s. electrical pulse number for a PCMO

thin film sample [14]………..20 Fig. 2-8 Switching curves of ZnO device: (a) with Pt as cathode, and

(b) with TiN as cathode.[15]……….….21 Fig. 2-9 The device of the resistance switching is bi-stable symmetric

[16]………...………22

Fig. 2-10 the structure of the device and the chemical structure of the

organic material [17]………..22 Fig. 2-11 the formation and rupture of conductive filaments(1)forming,

(2) reset and (3) set process [18]………..………23 Fig. 2-12 the temperature distribution of Joel heating effect [19]………23 Fig. 2-13 the sketch map of Redox processes by cation migration [21]…………...24 Fig. 2-14 Schematic cross-section structure of Pt/TiO2/TiN devices [22]…………24 Fig. 2-15 the mechanism of redox processes by anion migration [22]…………...25 Fig. 2-16 Schematic view of the model with three kinds of domains [24]………...25

(13)

XI

Fig. 2-17 Schottky barrier model and Schematic view of the model [18]……….26

Chapter 3

Fig. 3-1 (a)Pt/ TaON/TiN device’s schematic diagraph(b) The image of cross section for Pt/ TaON/ TiN(TEM)………...….31 Fig. 3-2 (a)Pt/Cu/TaON/TiN device’s schematic diagraph(b) The image of cross

section for Pt/ Cu/TaON/ TiN(TEM)………..………..32

Fig. 3-3 electrical measurement schematic diagraph………32 Fig. 3-4 the material analysis of N&K(a) the fitting of reflective wave

(b) n&k……….………33 Fig. 3-5 the material analysis of Mid-IR………..………….34 Fig. 3-6 the material analysis of XPS (a) Ta 4f7/2 spectra (b) O 1s

spectra………...…………...35

Chapter 4

Fig. 4-1 Pt/TaON(20nm)/TiN current-voltage curve bias on TiN(a)

positive bias Forming (b) negative bias Forming………...….65 Fig. 4-2 Pt/TaON(20nm)/TiN current-voltage characteristic operation

curve………...…..65 Fig. 4-3 Pt/TaON(20nm)/TiN current-voltage operation curve DC

sweep 100 times………...……66 Fig. 4-4 Pt/TaON(20nm)/TiN device DC endurance @0.2V………66 Fig. 4-5 Pt/TaON(20nm)/TiN AC endurance ―set‖ pulse and ―reset‖

pulse operation……….67 Fig. 4-6 Pt/TaON(20nm)/TiN device AC endurance @0.2V………67 Fig. 4-7 Pt/TaON(20nm)/TiN device at temperature 850C retention

(14)

XII

Fig. 4-8 Pt/TaON(20nm)/TiN device multi-level change current

compliance (a) I-Vcurve (b) on/off ratio @0.2V……….………68 Fig. 4-9 Pt/TaON(20nm)/TiN device multi-level change voltage stop (a)

I-Vcurve (b) on/off ratio @0.2V(c) Vset distribution of different

Vstop….………...69

Fig. 4-10 Pt/TaON(20nm)/TiN device +Forming and –Forming overlap

I-V curve………..…….70 Fig. 4-11 Pt/TaON(20nm)/TiN device positive Forming schematic

diagram (a) Initial (b) Forming process……….………..70 Fig. 4-12 Pt/TaON(20nm)/TiN device negative Forming schematic

diagram (a) Initial (b) Forming process……….………..71 Fig. 4-13 Pt/TaON(20nm)/TiN device DC endurance overlap @0.2V after

operating positive Forming and negative Forming………..…71 Fig. 4-14 Pt/TaON(20nm)/TiN device operation (a)I-V curve (b)―reset‖

process schematic diagram………..…….72 Fig. 4-15 Pt/TaON(20nm)/TiN device operation (a)I-V curve (b) ―set‖

process schematic diagram………..……….72 Fig. 4-16 Pt/TaON(20nm)/TiN device operation schematic diagram

(a)Before Forming (b) After -1.9V ―reset‖ (c) After -1.6V ―reset‖

(d) After -1.1V ―reset……….……….73 Fig. 4-17 Pt/TaON(20nm)/TiN device +Forming and –Forming first

―reset‖ overlap I-V curve………..…….74 Fig. 4-18 Pt/TaON(20nm)/TiN device band diagram (a)no bias (b)

+Forming (c) –Forming………..74 Fig. 4-19 the schematic diagram of oxygen anion moving in the first

(15)

XIII

Fig. 4-20 Pt/TaON(20nm)/TiN device schematic diagram (a)first ―reset‖

after -Forming (b) first ―reset‖ after +Forming…………...……….75 Fig. 4-21 Pt/Cu/TaON(20nm)/TiN current-voltage curve bias on TiN

(a) positive bias Forming (b) negative bias Forming………..……76 Fig. 4-22 Pt/Cu/TaON(20nm)/TiN current-voltage characteristic operation

Curve………..………76 Fig. 4-23 Pt/Cu/TaON(20nm)/TiN current-voltage operation curve DC

sweep 100 times………...77 Fig. 4-24 Pt/Cu/TaON(20nm)/TiN device DC endurance @0.2V………...…77 Fig. 4-25 Pt/Cu/TaON(20nm)/TiN AC endurance ―set‖ pulse and ―reset‖

pulse operation………...……..78 Fig. 4-26 Pt/Cu/TaON(20nm)/TiN device AC endurance @0.2V………..………….78 Fig. 4-27 Pt/Cu/TaON(20nm)/TiN device room temperature retention

@0.2V………..………..79 Fig. 4-28 Pt/Cu/TaON(20nm)/TiN device +Forming and –Forming

overlap I-V curve……….……….79 Fig. 4-29 Pt/Cu/TaON(20nm)/TiN device schematic diagram

(a)+Forming process (b)(c)(d) – Forming process………..……….80

Fig. 4-30 Pt/Cu/TaON(20nm)/TiN device DC sweep 100 times cycle Vset

distribution after +Forming or – Forming………80 Fig. 4-31 Pt/Cu/TaON(20nm)/TiN device schematic diagram operation

process after Forming (a) -Forming (b)+ Forming………..………….81 Fig.4-32 Pt/Cu/TaON(20nm)/TiN device operation (a)I-V curve

(b)―reset‖ process schematic diagram………...……….81 Fig. 4-33 Pt/Cu/TaON(20nm)/TiN device operation (a)I-V curve (b) ―set‖

(16)

XIV

Fig. 4-34 Pt/TaON(20nm)/TiN device operation process current fitting………….…82 Fig. 4-35 Pt/Cu/TaON(20nm)/TiN device operation process current

Fitting……….83 Fig. 4-36 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―set‖ process [Ohmic]………..……83 Fig. 4-37 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―set‖ process [Schottky]………...…84 Fig. 4-38 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―set‖ process [Poole-Frenkel]………...………84 Fig. 4-39 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―set‖ process [SCLC]………...…85 Fig. 4-40 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―reset‖ process [Ohmic]………...…85 Fig. 4-41 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―reset‖ process [Schottky]………..……..86 Fig. 4-42 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―reset‖ process [Poole-Frenke]……….………86 Fig. 4-43 Pt/TaON(20nm)/TiN device energy band diagram for the high

resistance of ―reset‖ process [SCLC]………...….87 Fig.4-44 Pt/Cu/TaON(20nm)/TiN device no bias energy band diagram…..…………87 Fig. 4-45 Pt/Cu/TaON(20nm)/TiN device electrons conduction schematic

diagram of ―set‖ process………...………..88 Fig. 4-46 Pt/Cu/TaON(20nm)/TiN device energy band diagram for the

high resistance of ―set‖ process [Ohmic]………..………88 Fig. 4-47 Pt/Cu/TaON(20nm)/TiN device energy band diagram for the

(17)

XV

Fig. 4-48 Pt/Cu/TaON(20nm)/TiN device electrons conduction schematic

diagram of ―reset‖ process……….………..89 Fig. 4-49 Pt/Cu/TaON(20nm)/TiN device energy band diagram for the

high resistance of ―reset‖ process [Ohmic]……...………90 Fig. 4-50 Pt/Cu/TaON(20nm)/TiN device energy band diagram for the

high resistance of ―reset‖ process [Schottky]………..…..90 Fig. 4-51 Temperature measurement at high resistance state @0.2V

Pt/TaON(20nm)/TiN (b) Pt/Cu/TaON(20nm)/TiN………...……91 Fig. 4-52 Temperature measurement at low resistance state @0.2V

(a) Pt/TaON(20nm)/TiN (b) Pt/Cu/TaON(20nm)/TiN……….………….91 Fig. 4-53 use CCS(constant current sampling) to switch device’s

resistance at temperature 300K(a)Positive CCS (b) before and

after CCS I-V curve……….……….92 Fig. 4-54 use CCS(constant current sampling) to switch device’s

resistance at temperature 300K(a)Negative CCS (b) before and

after CCS I-V curve………..………92 Fig. 4-55 use CCS(constant current sampling) to switch device’s

resistance at temperature77K(a)Negative CCS (b) before and

after CCS I-V curve………..………93 Fig. 4-56 Pt/TaON(20nm)/TiN unipolar current-voltage characteristic

operation curve……….93 Fig. 4-57 Pt/TaON(20nm)/TiN unipolar current-voltage ―set‖ process

double 0~ -3 and -3~0……….……….94 Fig. 4-58 Pt/TaON(20nm)/TiN unipolar current-voltage operation

curve DC sweep 100 times after +Forming(a) ―set‖ (b) ―reset‖……..……94 Fig. 4-59 Pt/TaON(20nm)/TiN unipolar current-voltage operation curve

(18)

XVI

DC sweep 100 times after -Forming(a) ―set‖ (b) ―reset‖……….……95 Fig. 4-60 Pt/TaON(20nm)/TiN device unipolar operation DC endurance

@ 0.2V(a) after +Forming (b) after – Forming……….….95 Fig. 4-61 Pt/TaON(20nm)/TiN device unipolar operation retention at

850C@ 0.2V……….96

Fig. 4-62 Pt/TaON(20nm)/TiN device Forming process schematic

diagram (a) positive bias (b) negative bias………...………96 Fig. 4-63 Pt/ TaON(20nm)/TiN device operation (a)I-V curve (b) ―set‖

process schematic diagram………...………97 Fig. 4-64 the schematic diagram of oxygen anions moving during ―reset‖

Process………..……97 Fig. 4-65 Pt/ TaON(20nm)/TiN device operation (a)I-V curve (b) ―reset‖

process schematic diagram………..…….98 Fig.4-66 Pt/Cu/TaON(20nm)/TiN unipolar current-voltage characteristic

operation curve………..……….98 Fig. 4-67 Pt/ Cu/TaON(20nm)/TiN device operation process schematic

(19)

1

Chapter 1

Introduction

In the advanced rapidly age, more and more technology products are invented to improve people’s lives. The people’s demands for consumer electronics are surged. No matter what types of consumer electronics are. Such as, mobile phone, smart phone, camera, GPS, and so on, any product is created to let human convenient. Those consumer electronics all must have memory which can save data permanently and can’t let data lose. However, in present, in order to carry conveniently, the products are designed smaller and smaller. Thus, the memories also become smaller and smaller and doesn’t affect their capacity. Even the capacity of memory is increased. Due to the memory Flash encountering above problems, making new evaluation of memory device are significant subjects in the near future. The ideal memory devices own advantages, such as simple structure , low energy consumption ,higher storage density , higher operation speed, better endurance and retention.

Recently, the development of traditional memory devices have encountered some problems .Therefore, we will have to further research new structure or new material. The recent studies indicate that RRAM devices have the benefits of non-volatile, low energy consumption, high retention and endurance, fast programming. Most important thing of all is that the RRAM has a simple structure (metal-insulator-metal) which lets device fabrication easy. In spite of the wonderful characteristics of RRAM, there is not a complete and clear system to explain the resistive switching mechanism of RRAM.

(20)

2

mechanism for different top electrode. We also use some equipment to certify resistive switching mechanism. In the device fabrication, we choose the material Pt, the insulator of TaON , TiN to make our device .Because those materials have been used in the traditional CMOS process. For example, TaN is used to resist copper diffusion, TiN is used to be anti-reflection layers or adhere layers, Pt is inertia metal. In addition, we substitute our device top electrode Cu for Pt.

(21)

3

Chapter 2

Literature

2-1 Introduction of Memory

We know that the memory can be distinguished simply into two types: volatile and non-volatile. The volatile memory needs power to maintain their memory state. When the power is turned off, the charges will lose and cause data to disappear. However, the memory has advantages of fast access speed. The volatile memories include two types: SRAM (Static Random Access Memory) and DRAM (Dynamic Random Access Memory).

Comparing with the volatile memories the non-volatile memory can’t lose their data when the power is turn off. In the other way, we can conserve our data for a long time without giving any energy. When supplying the power, the information can be programmed, erased, or read. The Flash memory the typical non-volatile memory is the most widely used now. However, we encounter critical problems recently. Due to increasing density of storage, we have to scale down our devices. When the devices are scaled down continuously, the tunnel oxide layer will go thinner. The consequence which the charges can leak and lead to loss all stored information will happen. The other problem is that the programming and erasing are slow.

Recently many non-volatile memories are developed. The memory can be divided into four types: (1) FeRAM (Ferroelectric Random Access Memory) (2) MRAM (Magnetic Random Access Memory) (3) PCRAM (Phase Change Random Access Memory) (4) RRAM (Resistance Random Access Memory). The comparisons

(22)

4

of memory characteristic of non-volatile memory are listed in Table2-1. [1]

2-2 Advanced Non-volatile Memories

2-2.1 FeRAM (Ferroelectric Random Access Memory)

The ferroelectric feature is the some substance having P-E hysteresis effect. When giving the external voltage to them, the interior ions movement cause polarization phenomenon Figure 2-1. [2][3]Besides, the produced electric dipole doesn’t completely disappear as removing the voltage. This phenomenon is called remnant polarization. The direction of remnant polarization is different as applied different direction of electric field. From the direction of remnant polarization [4] we can distinguish the signals of ―1‖ and ―0‖. The FeRAM has many advantages, such as low power operation, high density, and low voltage operation, but the FeRAM also face the limits of size.

2-2.2 MRAM (Magnetic Random Access Memory)

The access mechanism of MRAM is that using top and bottom two conductive metal layers, and the middle layer is tunneling magnetic resistive (TMR) or giant Magnetoresistance (GMR) cell. The top is the bit line and the bottom is the word line like Figure 2-2. [5] .When giving a pulse to bit line, the magnetic field is induced by the current. In that time, the current will affect the direction of polarization to shift in free layer. If giving a pulse current to word line, as well, the polarization direction of free layer will be changed completely by induced magnetic. Therefore, two polarization of ferroelectric layer will be arranged forward (In the moment low magnetic resistive is ordered ―0‖) or be arranged reverse (In the moment high magnetic resistive is ordered ―1‖). The data which are stored will not lose for a long

(23)

5

time just like hard disk. It has characteristic of low power consumption, fast response time, and high density of storage. The MRAM is a complex and spin-electric device. Thus, if we want to achieve ultra high density of storage and a good performance, we need to solve some critical problems, such as vortex, the leakage current effect of access, power consumption and thermal stability. Therefore, we need to improve further those disadvantages.

2-2.3 PCRAM (Phase Change Random Access Memory)

The major material of the PCRAM is Chalcogenide. After adsorbing thermal energy, Chalcogenide will become two reversible type: amorphous and poly-crystal. As the current pass through the small area for a long time to maintain high temperature, the atoms can arrange into poly-crystal .This poly-crystal has low resistance. [2] Therefore, we define the different resistance of amorphous and poly-crystal states. The amorphous state is ―0‖and the poly-crystal state is ―1‖ like

Figure 2-3[2]. In the contrast, when the current only pass for a short time, the atoms

can’t arrange into uniform structures. So it becomes amorphous and the resistance of amorphous is high. There are some advantages in PCRAM. For example, the fast programming speed, less power consumption, high reliability, and process compatibility. But the heat produced will affect the around devices. Maybe solve this problem first.

2-2.4 RRAM (Resistance Random Access Memory)

The RRAM is made of a transistor and a resistor like Figure 2-4 [6]. The resistor has a structure MIM (metal-insulator-metal).The top and bottom metals are the electrodes. And then, middle thin insulator film is a switching layer. The basic mechanism of access is that when giving the appropriate voltage to top or bottom

(24)

6

electrode, the resistor’s resistance will change. We can decide that higher resistance state is ―0‖ and lower resistance state is ―1‖.The main switching characteristic could be divided into two types: unipolar and bipolar. Unipolar Figure 2-5(a) [7] is that switching between high resistance state and low resistance state is controlled by the applied voltage magnitude rather than polarity. On the contrary, bipolar Figure 2-5(b)

[7] is that switching was dependent with voltage polarity and magnitude. The main

materials of insulator film can be divided into three types: (1) Perovskite (2) organic materials (3) transition metal oxide. There are many advantages in RRAM .Such as, low cost, simple structure, low operating voltage, fast access speed, high retention and endurance, high density of storage. Therefore, thanks to so many advantages, the greatest potential method is that we substitute RRAM for the Flash memory.

2-3 The Materials of RRAM

There are various materials which can be observed the resistive switching, such as perovskite, transition metal oxides, and organic materials.

2.3.1 Perovskite

One extensively studied material group is perovskite. The perovskite structure is ABO3.The ―A‖ is the cation with the longer radius and is localized at the eight corner

of the crystal structure. The ―B‖ is the cation with the shorter radius and occupies the body-centered of crystal. The ―O‖ is the oxygen ion and is situated at the face-centered of crystal like Figure 2-6 [8]. In the electric and magnetic characteristic Perovskite is special. By doping characteristics of Perovskite are affected.Thus,

recently many materials derived from perovskite are Cr-doped SrZrO3 [9], BSZT[10],

(25)

7

In 2000, A. Beck et al. [9] said that the SrZrO3 film doping with Cr can enhance the device. The bottom electrode is SrRuO3 or Pt. The top electrode is Pt or Au. Applying positive voltage is SET, and applying negative voltage is RESET.SET is that making the state of device from high resistive state to low resistive state. And then, RESET is that making the state of device from low resistive state to high resistive state. The resistance switching depends on polarity of applied voltage.

BSZT is(Ba,Sr)(Zr,Ti)O3.The material has already been studied as high-k

dielectric for a long time [10].Someone uses BSZT as RRAM with doping V, Cr and so on. Due to the more compositions and the more complex chemical environment, the control of the composition proportion is not easy as binary oxides.

PCMO (PrCaMnO3) has the famous Perovskite structure and has electric and magnetic characteristics, like magnetoresistance [11] and colossal electroresistance .

[12]

In 2000, Sawa et al.[13] discovered electric hysteresis curve in

Ti/PCMO/SRO device and think the resistance switching at the interface between Ti and PCMO. Liu et al. [14] fortunately changed the resistance of Ag/PCMO/Pt structure by electric pulse without magnetic field under the room temperature. In the

Figure 2-7, the resistance depends on electric field polar direction and the on/off ratio

is about three orders. They also thought that ferromagnetic clusters would be arranged directionally to form filament path for carrier passing through the conductive clusters. On the other hand, reverse bias could also rearrange ferromagnetic clusters to higher resistive state. However, this oxide contains more than three elements, so the crystal structure, process technology and stoichiometry are difficult under the control. For the current semiconductor process technologies the material is difficult to connect with. In addition, the board needs to have a good crystalline substrate before producing high

(26)

8

quality film, and efficient etching process has not yet been made.Thus, the type of material has its limitation in volatile resistive RAM development in the future.

2.3.2 Transition metal oxides

Currently using a variety of transition metal oxide thin films in resistive memory.In the ternary transition metal oxides BaTiO3, and SrTiO3 and their doping elements have received attention. And then, in binary transition metal oxides TiO2, Ta2O5, ZnO, ZrO2, HfO2, Nb2O5, NiO, MoO3,, CuxO with WO3 and so on have received attention in the current. These oxides in the physical and chemical have their unique characteristics, and their characteristics perhaps are related with metal electrode with a barrier and work function.

In 2008, B. Gao [15] had also observed resistance switching of the ZnO thin films deposited on Pt/Ti/SiO2/Si. The resistance switching of the device is bipolar

Figure 2-8 and cathode is Pt (a) or TiN (b). They proposed a oxide-Based RRAM

Switching Mechanism in Reset process about ion-transport-recombination.

In 2004, S. Seo et al. [16] had discovered resistance switching of the NiO thin films also deposited on Pt/Ti/SiO2/Si. The resistance switching of the device is bi-stable symmetric Figure 2-9. They thought that the reason of the resistive switching is the contribution of nickel vacancies.

2.3.3 Organic materials

In 2002,L. P. Ma et al. [17] had observed an organic bi-stable device and the structure is made of organic/metal/organic as Figure 2-10. When giving a voltage over a critical value, the resistance switching from high resistance state to low resistance state is SET. The high resistance state restored by giving a reverse voltage is RESET, and the high resistance state was maintained without voltage. The structure

(27)

9

of organic/metal/organic has a non-volatile feature.

2.4 The Resistive Switching Mechanism of RRAM

In the near future, the phenomenon of the resistance switching has been found out continuously in a variety of materials. Besides, many people have proposed some models in order to explain the mechanism of resistive switching. The switching mechanism can be distinguished into two types: bulk control and interface control. Bulk control is that the switching region is occurred in the insulator layer. The resistive switching model like filament, charge-trap in small domains and conduction paths are classified in Bulk control. Interface control’s switching region is occurred in the layer which is between the metal layer and insulator layer.

We will discuss these mechanisms respectively.

2.4.1 Filament

The fundamental theory is that the formation and rupture of the conductive filaments in the insulator, like Figure 2-11 [18]. The Figure shows the mechanism of filamentary type RRAM. Applying the voltage to make the filament formed in insulator layer is Forming process, and the device maintain in on state. After that, rupturing the filaments to make the device changed to off state is Reset process. Then, the filaments formed again to on state is Set process. The Joule heatingeffect Figure

2-12 [19], redox processes induced by cation migration, redox processes induced by

anion migration or anodization near the interface between the metal electrode and the insulator layer are considered to form and rupture the filaments.

2.4.1.1 Joule heating effect

(28)

10

Y. Sato et al. [20] had found out the temperature of filaments with NiO2 thin film. The temperature in each Reset process was the same from thermal conductive equation .We can calculate the temperature of conductive path. They defined that the reset process is a thermal chemical reaction with electrical current heating.

2.4.1.2 Redox processes by cation migration

The principle is that when giving the positive voltage on top electrode, metal atoms will be oxidized to metal ions. The metal ions are driven by the positive electricity. Due to positive voltage on the top electrode, the metal ions will be moved to bottom electrode by the downwards electric field. As soon as the metal ions arrive at the bottom electrode, it will be reduced to the metal atoms. When the above process happens continuously, the metal atoms will be deposited gradually until the filament formed makes the top electrode connected with the bottom electrode. This phenomenon is on state. On the contrary, applying the positive voltage on bottom electrode is the process of rupturing the filaments. The above process will make the metal atoms in the filament oxidized to metal ions and moved to top electrode by the upwards electric field. And then the phenomenon is off state. The device is just like

Figure 2-13. [21]

2.4.1.3 Redox processes by anion migration

In 2006, M. Fujimoto et al. [22] had observed resistance switching behavior of the Pt/TiO2/TiN/Pt device like Figure 2-14. The device Pt/TiO2/TiN/Pt [23] has the bipolar resistive switching characteristic. When giving a negative pulse, donor concentration could be increased. The upwards electric field was produced and made oxygen ions depart from bonding and flow to bottom electrode. In that time, the oxygen vacancies were left. The above process makes device switched to on state. On

(29)

11

the other hand, when giving a positive pulse to the top electrode, the oxygen ions were filled up the oxygen vacancies. When the oxygen vacancies disappeared in the film, the device were switched to off state .These processes was shown in Figure

2-15[22]. They thought that giving a pulse to the electrode was enough for the donor

concentration abrupt increasing or decreasing in the layer.

2.4.2 Charge-trap in small domain

In 2004, M. J. Rozenberg et al.[24] create a model to explain multilevel resistive switching phenomenon. In this mechanism the insulator is at medium and top and bottom is two electrodes .Besides, contained metallic domains which is the same as charge traps in the real system like dopants , vacancies, metallic clusters, nano-domains, and so on. The three kinds of domains is in the Figure 2-16 [24].The top and bottom domains are smaller than medium. When giving a negative pulse to fill up the bottom domains and then to let the top domains in vain. Due to low probability of carriers transferred into the filled bottom domains, carriers were transferred out of the emptied top domains to the electrode. Thus, the system was at high resistance state. On the contrary, they gave a positive pulse. Due to carriers transferred from the bottom electrode to the empty bottom easily, carriers were transferred from the fully filled top domains to the top electrode. Therefore, the system came back to low resistance state.

2.4.3 Modified Schottky barrier model

In the metal/insulator/metal structure the work function is not the same in these materials that constructs the different contact situation between them. The contact condition can be distinguished two kinds: Ohmic contact and Schottky contact. The mechanism of Schottky barrier model is introduced by A. Sawa et al. [18]. The

(30)

12

structure of device is Ti/PCMO/SRO. Ti and PCMO become the Schottky contact. Due to the catching oxygen ability, Ti will catch many oxygen ions at the interface and leave many oxygen vacancies in the interface. Oxygen vacancies are positive charge and make the energy band bended. If the vacancies are too much, the Fermi level will never changed by given bias. The barrier height will not be changed. When giving the reverse bias, the electrons will be accumulated at the interface. On the contrary, the electron will be depleted likeFigure 2-17[18]. They considered the more

or less of charges can alter the height and width of barrier and make the resistance switched high or low.

2.5 The Mechanism of Current Conduction

Even though the current flowing though the ideal insulator has to be zero, the current can pass through the insulator at higher temperature or higher electric field actually. Thus, the device has leakage current. In the RRAM, when we operate the device, there are much current conduction. Such as Ohmic conduction, Schottky emission, Poole-Frenkel emission, space charge limit current, and so on. Fitting current in current-voltage curve can let us understand what current conduct in operation process in insulator. There are four current conductions in our equipments, so we introduce the current conduction mechanism in advance.

2.5.1 Ohmic conduction

Generally, applying to the condition of low electric field region in which thermally-generated carriers are dominant in conduction. When giving the external electric field, the free electron flowing can generate the current.

(31)

13 exp ac c E J qN E kT

    

which,

N

c= effective density of state

(

)

ac C F

E

E

E

= electron activation energy

2.5.2 Schottky emission

When the electrons jump over interface barrier between the metal and insulator or metal and semiconductor by the heat, the phenomenon is called Schottky emission. And then, in electric field the electron flowing can generate the current. The Schottky emission is related strongly with temperature. Lowing the barrier height will happen when the image charges caused by electrons. Thus, due to lowing barrier, making the electrons can hop through the barrier easily. The barrier height is affected by the materials’ work-function .The interface trap, defect, carrier density, and process condition can also make the barrier change. If the barrier height is higher, the less electrons can jump over the barrier with the heat.The formula of Schottky emission is

** 2 4 exp q B qEi i J A T kT            or 2

exp

B

q

J

T

a V

kT

which, A = effective Richardson constant **

0

i r

 

,

r= dynamic dielectric constant

B

= barrier height

(32)

14

Poole-Frenkel emission is similar to Schottky emission mechanism. Poole-Frenkel emission is that the electrons overcome the barrier caused by the defects in the insulator by the heat. And then, under the electric field the electron flowing generates the current. The Poole-Frenkel emission is also related strongly with temperature, while electric field plays a more important role in Poole-Frenkel emission than in Schottky emission, which implies that field effect has greater impact on defect-related behavior. The barrier height is determined by the deep of defect. And then, the barrier lowering in the Poole-Frenkel emission is twice as large as that in Schottky emission, because of the captured charges being fixed not like image charges in the metal. The deeper defects make less electrons hop through the barrier. The formula of Poole-Frenkel emission is

exp B i i i q qE J E kT            or

exp

2

B

q

J

V

a V

kT

which,

i

 

r 0,

r = dynamic dielectric constant

B

= barrier height

2.5.4 Space charge limited current

The electrons restricted to inject the insulator by the electrons existing in insulator that is Space charge limited current. The block phenomenon happens. The formula of space charge limited current is

2 3 9 8 i V J d    or JV2 which,

= mobility

(33)

15

The Table 2-2 [25] is the every current conduction .That shows the relationship between the current and electric field, the current and voltage, or other factors.

(34)

16

過程 表示式 電壓和溫度關係性

Table 2-1Comparison different memory technology [1]

(35)

17

Figure 2-1 The P-E hysteresis of ferroelectric material [2][3]

(36)

18

Figure 2-3

The PCRAM structure diagram and

operating schematic

diagram [2]

(37)

19

(38)

20

Figure 2-6 Perovskite structure [8]

Figure 2-7 Nonvolatile resistance v.s. electrical pulse number for a

PCMO thin film sample [14]

(39)

21

Figure 2-8 Switching curves of ZnO device: (a) with Pt as cathode, and

(b) with TiN as cathode.[15]

(40)

22

Figure 2-9 The device of the resistance switching is bi-stable symmetric [16]

Figure 2-10 The structure of the device and the chemical structure of the

organic material [17]

(41)

23

Figure 2-11 the formation and rupture of conductive filaments, (1) forming,

(2) reset and (3) set process [18]

(42)

24

Figure 2-13 The sketch map of Redox processes by cation migration [21]

(43)

25

Figure 2-15 The mechanism of redox processes by anion migration [22]

(44)

26

(45)

27

Chapter 3

Experiment

We specify the process flow, material analysis ,and electrical measurement of every structure in this chapter.

3-1 Process Flow

There are two devices in the experiment. The one has insulator TaON and the top electrode Pt , and the other has insulator TaON and the top electrode Cu. The same thickness of insulator TaON in two devices is 20nm and the bottom electrode is the same material TiN. The insulator TaON is deposited by radio-frequency magnetron sputtering and the top electrodes Pt and Cu are deposited by DC magnetron sputtering. Finally, the produced structures of two devices are Pt/TaON/TiN and Pt/Cu/TaON/TiN.

3-1.1 Pt/TaON/TiN

First, getting the defined via size devices are that the TiN bottom electrode has defined within photo-resist protecting the exposed bottom electrode and without photo-resist on via hole. The insulator TaON is deposited by radio-frequency magnetron sputtering on bottom electrode TiN and the recipe is Table 3-1. After depositing TaON, the Pt top electrode is deposited by DC magnetron sputtering on insulator like Table 3-2. Lifting off the photo-resist on bottom electrode is final step.

Figure 3-1(a) is the schematic diagraph of device Pt/TaON/TiN .

3-1.2 Pt/Cu/TaON/TiN

(46)

28

defined within photo-resist protecting the exposed bottom electrode and without photo-resist on via hole. The insulator TaON is deposited with the insulator of device Pt/TaON/TiN simultaneously by radio-frequency magnetron sputtering on bottom electrode TiN at the same time with and the recipe is Table 3-1. After depositing TaON, the Cu top electrode is deposited by DC magnetron sputtering on insulator like

Table 3-3. And then, the Pt which protects Cu from oxidized is deposited by DC

magnetron sputtering on insulator like Table 3-2, but deposition time/thickness is 80 sec/ 10nm. Lifting off the photo-resist on bottom electrode is final step. The Figure

3-2(a) is the schematic diagraph of device Pt/Cu/TaON/TiN .

3-2 Material Analysis of Pt or Cu/ TaON

/

TiN Structure

In order to understand the structure of Pt / TaON/TiN, there are some methods of the material analysis to be used to analysis the device.

3-2.1 Material analysis of N&K

The Figure 3-4 shows thickness of film TaON. The analysis is that energy gap is 2.5eV [26] and thickness is about 20nm.

3-2.2 Material analysis of Mid-IR

The Figure 3-5 shows bonding of film TaON. The N -Ta-O, Ta -O, and Ta-N is the main bonding [27][28].

3-2.3 Material analysis of XPS

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique and a power tool. Using the method can measure the elemental composition, chemical state and electronic state of the elements existing within a material. XPS

(47)

29

spectra are obtained by using X-rays to shoot the thin films and simultaneously detecting the kinetic energy and number of electrons escaping from the material.

The X-ray photoelectron spectroscopy (XPS) pictures Figure3-6(a) and

Figure3-6(b) show the composition of the insulator of TaON. According to XPS,

we can observe that the peaks existing in the Ta 4f7/2 spectra Figure3-6(a) are Ta metal Ta 4f7/2 (21.8eV) , Ta metal Ta 4f5/2 (23.7eV), TaON Ta 4f7/2 (25.8eV), and TaON Ta 4f5/2 (27.7eV). On the other hand, the peak exists in the O 1s spectra

Figure3-6(b) is TaON (530.8eV)[26].

3-2.3 Material analysis of TEM

The Figure 3-1(b) shows the image of cross section for Pt /TaON/ TiN structure. The thickness of TaON which matches with N&K is about 20nm. The Figure 3-2(b) shows the image of cross section for Pt /Cu/TaON/ TiN structure. Its TaON thickness 20nm also matches with N&K.

3-3 Electrical Measurement

In the experiment, using the Agilent-4156 and Agilent B1500 to do the electrical measurement is the main method. Applying bias on the bottom electrode TiN and top electrode Cu or Pt grounded like Figure 3-3.

(48)

30

The recipe of film TaON

Target 4‖ TaN

Substrate temperature room

Working pressure 8m torr

Gas flow Ar 30 sccm O2 30 sccm

power RF 100 W

Deposition time/thickness 2000 sec/20nm

The recipe of film Pt

Target 4‖ Pt

Substrate temperature room

Working pressure 8m torr

Gas flow Ar 30 sccm

power DC 100 W

Deposition time/thickness 500 sec/80nm

Table 3-1

(49)

31

The recipe of film Cu

Target 4‖ Cu

Substrate temperature room

Working pressure 4m torr

Gas flow Ar 30 sccm

power DC 200 W

Deposition time/thickness 200 sec/100nm

Table 3-3

Figure 3-1 (a)Pt/ TaON/TiN device’s schematic diagraph

(b)

The image of cross section for Pt/ TaON/ TiN(TEM)

TaON

TiN

Pt

(a)

(50)

32

Figure 3-2 (a)Pt/Cu/TaON/TiN device’s schematic diagraph

(b)

The image of cross section for Pt/ Cu/TaON/ TiN(TEM)

Figure 3-3 electrical measurement schematic diagraph

TaON

Cu

TiN

(a)

(51)

33

Wavelength(nm)

200 400 600 800 1000

R

eflec

tan

ce(

%

)

0 20 40 60 80 100

R

exp

R

cal

Wavelength(nm)

200 400 600 800 1000

n&

k

0.0 0.5 1.0 1.5 2.0 2.5 3.0

k

n

Figure 3-4 The material analysis of N&K(a) the fitting of

reflective wave (b) n&k

(a)

(52)

34

(53)

35

(b)

(a)

Figure 3-6 The material analysis of XPS (a) Ta 4f7/2 spectra

(b) O 1s spectra

(54)

36

Chapter 4

Results and Discussion

4-1 The Resistive Switching Features of Different Top

Electrode

In the section, first, we discuss primarily that the different two top electrodes (Cu,Pt) affect the resistive switching characteristics. So we fixed the thickness of the insulator, TaON, 20nm and the bottom electrode TiN. We discuss the mechanism of resistive switching by the two devices. Because we know that Cu’s redox is easier than Pt’s, our inference is that the one device (Cu top electrode) has the resistive switching by Cu cations redox and migration in the film and the other (Pt top electrode) has the resistive switching by oxygen anions migration (oxygen vacancy remains) in the film.

4-2 Bipolar Resistive Switching Fature of Pt / TaON

/ TiN

Structure

4-2.1 Current-Voltage characteristics

We introduce our electrical measurement at first. For our structure Pt/TaON/TiN, it is at high resistance state original before it is processed Forming. ―Forming‖ is the initial applied large voltage process which can produces large electric field in the insulator and makes the insulator TaON produce impact ionization. We calls this process soft break down .When soft break down happens, device’s resistance will become low. Therefore, the large current will pass through the TaON. But soft break

(55)

37

down which can recover from low resistance to high resistance differs from hard break down. Like Figure 4-1(a), we choose one device that bottom electrode TiN is applied positive DC sweep from 0V to 10V with voltage step increasing 10mV and top electrode is common. We can observe TaON film break down when the voltage value is 6V which is Forming voltage. Due to avoiding the insulator TaON causing the permanent destruction, we have to set the current compliance (Icomp) about

5mA.Then, we choose another device that bottom electrode TiN is applied negative DC sweep from 0V to -10V with voltage step increasing 10mV and top electrode is common and also set current compliance like Figure 4-1(b). We observe that the negative Forming is successful as well. After Forming, we can operate the devices with bipolar method.

Figure 4-2 is the operating devices current-voltage characteristics. We give the

bias to bottom electrode and top electrode is common. The figure shows when the positive bias sweeps from 0V to 2V, the current suddenly increases to current compliance which we set is the same as Formig . In that time, the resistance state of the device switches from high to low which is ―set‖ and the voltage which lets current increase suddenly is ―set voltage‖ .When bias sweeps from 2V back to 0V, the resistance state keep in low. The negative bias sweeps from 0V to -1.6V, we can find about 0.6V afterward that the current doesn’t increase along with voltage increasing, but the current decreases instead. In the end, the resistance state of the device switching from low to high is ―reset‖ process and the voltage which begins to make the current decreased along with voltage increasing is ―reset voltage‖. The positive bias Forming has the same operation as the negative bias Forming.

4-2.2 Reliability

(56)

38

data loss or change. Thus, we have to operate our devices and understand our devices’ fundamental characteristics. We will measure devices’ endurance, retention, and multi-level. We hope that our devices can be operated more times, memorized data for a long time, and have a large capacity.

4-2.2.1 Endurance

Endurance is how many times device can be operated. We wouldn’t program and erase wrong. In the memories, endurance is very important. So we also apply DC bias and AC pulse to device’s (Pt/TaON/TiN +Forming) bottom electrode and top electrode is common.

In DC endurance test, we apply DC sweep and read the current values of 0.2 V, and then calculate the resistance. The Figure 4-3 is the current-voltage curve of DC sweep 100cycles. The Figure 4-4 shows the resistance of the 100 times DC sweep cycles at 0.2V. We can observe the on state resistance ( low resistance) / off state resistance (high resistance) have about 2 orders and is stable for 100 times. In order to measure our device whether can endure more operation times or not, we use AC pulse.

AC Pulse is faster operation than DC sweep, so device is operated more times per second. In AC endurance test, first, we must measure condition what pulse can ―set‖ or ―reset‖ the device. Giving a positive bias pulse to device which is at the high resistance state so as to make switch to low resistance state .In order to make sure the device’s state, we apply a small voltage sweep from 0V to 0.2V to make sure the device ―set‖ successfully. Afterward we give a negative bias pulse to the same device and apply a small voltage sweep from 0V to 0.2V to make sure the device ―reset‖ successfully. In addition, avoiding wasting time, in one time applying 1000 times continuous positive bias pulses and negative bias pulses. This process is like operating

(57)

39

the device ‖set‖ and ―reset‖ 1000 times .Repeating to apply 1000 times continuous positive bias pulses and negative bias pulses one hundred times that means we will operate the device’s ―set‖ and ―reset‖ 100,000 times.

The Figure 4-5 is asignal we set. ―Set‖ pulse width is 10us, and voltage is 2.2V. ―Reset‖ pulse width is 18us, and voltage is -2V. The Figure 4-6 is AC endurance at room temperature. We observe the on/off ratio about 1.5 orders.

4-2.2.2 Retention

Another important feature of memories is retention. Retention is how long can the memory memorize and no datum losing. Of course we hope that memorial time is longer and longer. At retention test we maintain device (Pt/TaON/TiN +Forming ) at high or low resistance state . We measure the device’s resistance value by 0V~0.2V sweeping at the time passing about 1s, 30s, 100s, 300s, 1000s, 3000s, 5000s, 7000s, 9000s, 10000s at 850C.In that way ,we can check out device’s resistance state changed or not after long time passes. In Figure 4-7, we see the device maintain the on/off ratio about 2.5 ordersafter 10,000 seconds.

4-2.2.3 Multi-level

Multi-level is the important feature of memory. Due to smaller and smaller memory technology, we hope that the memory can be stored more and more data in a finite space. If one device can save one over data, we don’t need many devices to save data. Thus, we can save space and miniature memories.

We can operate two methods to get multi-level. The one is changing the current compliance and the other is changing voltage stop. The two methods make on/off ratio order under control in order to increase the density of storage. The method which is changing current compliance to operate multi-level is in Figure 4-8(a). We control the current compliance ( Icomp ) at 1 mA, 5 mA, and 10mA. From Figure 4-8(b)

(58)

40

20cycs per current compliance, we observe the on/off ratio orders changed from 1 to 2. Although the high resistance state value doesn’t be changed, the low resistance state value is decreased along with current compliance increasing. The other method which is changing voltage stop to operate multi-level is in Figure 4-9(a). We control the voltage stop ( Vstop )at -1.1V, -1.6V, and -1.9V. From Figure 4-9(b) 20cycs per voltage

stop, we get the one result instead which is the high resistance state value increased along with voltage stop increasing and the low resistance state value is not changed. The other result in Figure 4-9(c) 20cycs per voltage stop is that the ―set‖ voltage ( Vset ) distribution is increased by voltage stop increasing.

4-3 The Bipolar Resistive Switching Mechanism Discussion

of the Structure Pt/ TaON/ TiN

After we are familiar with our device characteristic, we can construct some models to explain these phenomena. Due to some unusual phenomena, published resistive switching mechanism didn’t explain. So we propose some suit mechanism explanation for our data.

4-3.1 Forming of bipolar

The RRAM is the structure of MIM (metal /insulator/ metal). There is not any RRAM characteristic before Forming process. Thus, we have to apply Forming bias which is the first step to break down the insulator. So that electrons can be conducted easily in the insulator via the conductive path which is constructed by the breaking down. However, general break down is not revivable. The breaking down is called Soft break down which can be recovered and is the key phenomenon in our device. In the other word , when our device is operated Formig, device’s state is switched from high resistance state to low resistance state, and then device can be operated ―reset‖

(59)

41

from low resistance state to high resistance state.

In the Figure 4-10, we set current compliance about 5mA and apply DC sweep to bottom electrode and top is common. In positive Forming process, we give DC sweep from 0V to 10V. Beginning the current keeps in very small value about 10-13A~10-12A. However, the current elevates abruptly from 10-12A to current compliance (5mA) at the Forming voltage about 6V. The phenomenon we can specify with schematic diagram. The Figure 4-11(a) is device initial state. In the Figure

4-11(b) when we apply a large positive bias on bottom electrode, there is upward

electric field produced in the insulator. The electric field has large strength will make the weakest place of insulator soft break down. Once the soft break down happens, the tantalum-oxygen bonds and the nitrogen-oxygen bonds will be broken. Then, oxygen will be ionized and is charged negative charge. Since the ionized oxygens are anions, they will drift along with reverse direction of electric field and be stored in the TiN electrode which is the nice oxygen ions storage tank (from Gibb’s free energy TiO 672.4kj/mol). Therefore, there are many oxygen vacancies left in the insulator. Those oxygen vacancies are just like a conductive filament which electrons hopping via. In that time, the large current pass through insulator, so we know that the device’s resistance state is switched from high to low.

In Figure 4-10, we set current compliance about 5mA and apply DC sweep to bottom electrode and top is common. In negative Forming process, we give DC sweep from 0V to -10V. We also find out the negative bias Forming is successful and forming voltage value is the almost same as positive bias Forming. The phenomenon we explain in Figure 4-12(a) and (b). Due to electric field direction downwards, the only special thing is that oxygen anions drift towards top electrode .The Pt electrode isn’t the nice oxide storage tank (Gibb free energy PtO is 391.6 kJ/mol ), and oxygen anions will run away by Pt’s grains. However, the soft break down needs adequate

(60)

42

oxygen anions to be recovered. This moment the insulator’s oxygen anions are not enough to rebound the insulator. Thus, observing Figure 4-13 100 times DC sweep

endurancecan let us know that the positive bias Forming operation is more stable than the negative bias Forming operation. Due to the oxygen anions running away, insufficient oxygen anions fill the oxygen vacancies in negative bias Forming. The condition make the high and low resistance state value smaller in negative bias Forming process than in positive bias Forming process .

4-3. 2 I-V Feature of bipolar

We know that the resistance of the device will change from high to low by Forming process. But we can’t conclude the device which is workable. The most important thing of all we hope Forming process is soft break down rather than hard break down. Therefore, We must make it’s resistance from low to high by ―reset‖ and then from high to low by ―set‖ many times.

In the Figure 4-14(a), we successfully let the device ―reset‖ (resistance state from low to high) ,maintain resistance state in high state. In the ―reset‖, we apply the DC sweep from 0V to -1.6V. The -1.6V is voltage stop which we can set to change the high resistance state of our device like multi-level. Now we focus on about -0.6V. We find that the current value become smaller and smaller from about -0.6V to voltage stop -1.6V. That is the unusual condition. Thus, we think the condition is the resistive switching from low resistance to high resistance. In order to explain the condition, we construct the model. The resistive switching is related with ionized oxygen moving. The electrons hopping by oxygen vacancies forming conductive filament causes insulator conductible. After the positive bias Forming process, the conductive filament appears. In that time, the oxygen ions( negative charges ) move towards TiN electrode by the electric field (upwards) reverse direction. In the ―reset‖ process , applying

數據

Figure 2-7 Nonvolatile resistance v.s. electrical pulse number for a    PCMO thin film sample [14]
Figure 2-8 Switching curves of ZnO device: (a) with Pt as cathode, and  (b) with TiN as cathode.[15]
Figure 2-9 The device of the resistance switching is bi-stable symmetric [16]
Figure 2-12 The temperature distribution of Joel heating effect [19]
+7

參考文獻

相關文件

• The approximate and introduces a false positive if a negative example makes either CC( X ) or CC(Y) return false but makes the approximate and return true. • The approximate

Rather than requiring a physical press of the reset button before an upload, the Arduino Uno is designed in a way that allows it to be reset by software running on a

For importation of cinema film, no matter whether the film is positive or negative, an approval issued by Bureau of Audiovisual and Music Industry Development, Ministry of Culture

Laughing (a positive outlook) can beat negative emotions during hard times1. Laughing (a positive outlook) can beat negative emotions during

Laughing (a positive outlook) can beat negative emotions during hard times2. Laughing (a positive outlook) can beat negative emotions during

Hope theory: A member of the positive psychology family. Lopez (Eds.), Handbook of positive

Define instead the imaginary.. potential, magnetic field, lattice…) Dirac-BdG Hamiltonian:. with small, and matrix

• The particles of the atom are the negative electron, the positive proton, and the uncharged neutron.. • Protons and neutrons make up the tiny dense nucleus