High k dielectric MIM capacitors for silicon RF and analog applications

500oC respectively………...28 Figure 2.5 Spectral dependence of refractive indexes of HfO2 films deposited at a various substrate temperatures oxygen pressure: 50 mTorr and b various deposi

High-κ Dielectric MIM Capacitors

for Silicon RF and Analog Applications

HU HANG (M Sc., Jilin University)

A thesis submitted in partial fulfillment of the requirements for

the degree of Doctor of Philosophy

Electrical and Computer Engineering Department

National University of Singapore

Singapore

December, 2003

ABSTRACT

Metal-insulator-metal (MIM) capacitors in silicon integrated circuits have attracted great attention due to their high conductive electrodes and low parasitic capacitance The conventional MIM capacitors using SiO2 and Si3N4 usually provide low capacitance density, which is far from the requirement predicted by ITRS roadmap Therefore, to adopt high-κ materials is an unavoidable choice to improve the overall electrical performance by using physically thicker dielectric films

In this thesis, a thorough research has been done for high-κ MIM capacitors using HfO2 based dielectrics for the first time Various fabrication methods such as pulsed-laser deposition, sputtering, and atomic-layer-deposition have been employed

to prepare high-κ dielectrics, and different dielectric structures like laminate, stack, sandwich, etc, have also been explored as well

Extensive electrical characterization was conducted to evaluate HfO2 based high-κ MIM capacitors DC properties in terms of leakage, voltage coefficients, reliability etc, have been analyzed which are strongly correlated to the preparation methods and material properties In addition, well behaved RF characteristics of these dielectrics have been demonstrated showing the almost invariable dielectric constants

of HfO2 based dielectrics in RF regime As a result, all the experimental results justify the suitability of HfO2 based dielectrics for MIM capacitors application

Mechanisms with regard to the electronic conduction in high-κ dielectrics, voltage coefficients of capacitance (VCCs) dependency, oxide degradation etc., have been discussed and clarified A good understanding of process-structure-property

correlation is thus been achieved for high-κ dielectrics fabrication in back-end of line process, and the information obtained in this thesis is paramount for the operation of MIM capacitor devices

Finally, a free carrier injection model has been employed to understand VCCs’

mechanism of MIM capacitors The results reveal that, the thickness (t) dependence of

quadratic VCCs is an intrinsic problem due to electrical field enhancement in the scaled dielectric film, which exhibits a relation of (n~2) Besides, the frequency

dependence of VCCs, and the stress modified VCCs could also been well interpreted using this model

n

t−

∝α

TABLE OF CONTENTS

Page No

CHPATER 1

INTRODUCTION OF HIGH-Κ MIM TECHNOLOGY

1.1 Capacitors in Si technology……… ……… 1

1.2 Review of the literature……… 4

1.2.1 Motivation of metal-insulator-metal (MIM) technology……….4

1.2.2 Current status of MIM technology……… 5

1.2.3 High-κ dielectrics for MIM capacitors application……….7

1.2.4 Challenges and unsolved problems……… 12

1.3 Contribution of this thesis……….12

1.4 Thesis outline……….……….13

References……….15

CHAPTER 2 HFO 2 MIM CAPACITORS BY PULSED-LASER DEPOSITION (PLD) 2.1 Introduction……… ……….22

2.2 Experiments……… 24

2.3 Results and discussion………25

2.3.1 Physical characterization of PLD processed HfO2……… 25

2.3.2 Electrical characterization of HfO2 MIM capacitor……….35

2.4 Limitations of PLD for thin film fabrication……….43

2.5 Conclusion……….……46

References……….48

CHAPTER 3 CHARACTERIZATION OF HFO 2 MIM CAPACITORS FOR RF APPLICATION 3.1 Introduction……… 53

3.2 Experiments……… 54

3.2.1 RF MIM capacitor fabrication………54

3.2.2 S-parameters for RF characterization………57

3.3 Results and discussion………58

3.3.1 RF characterization………58

3.3.2 DC and low frequency measurements…… ………63

3.4 Conclusion……….70

References……….72

CHAPTER 4 HFALO X MIM CAPACITORS BY ATOMIC-LAYER-DEPOSITION (ALD) 4.1 Introduction……… 76

4.1.1 ALD method for thin films fabrication………76

4.1.2 Characteristics of ALD processed HfO2 and Al2O3……… 77

4.2 Experiments……… 80

4.3 Electrical characterization of HfO2-Al2O3 laminated MIM capacitors………81

4.3.1 RF characteristics of laminated MIM capacitors….………82

4.3.2 Leakage and breakdown characteristics of laminated MIM capacitors………84

4.3.3 VCCs dependence and reliability of laminated MIM capacitors………93

4.4 Effects of dielectric structures on the electrical properties……….100

4.5 Conclusion……… 105

Reference……….106

CHPATER 5 UNDERSTANDING VOLTAGE COEFFICIENTS OF HIGH-Κ MIM CAPACITORS 5.1 Introduction……….112

5.2 Theory……… 113

5.3 Results and discussion……….115

5.3.1 Thickness dependence of VCCs for HfO2 MIM capacitor……… 115

5.3.2 Frequency dependence of VCCs……… 123

5.3.3 Electrical stress modified VCCs……… 125

5.3.4 Prediction of VCCs……… 126

5.4 Conclusion……… …129

References……….130

CHAPTER 6 Summary and future works………134

6.1 Summary……… 134

6.2 Future works………135

LIST OF FIGURES

Figure 1.1 Dielectric constant κ versus band gap for oxides……… ….8 Figure 2.1 Experimental configuration of pulsed-laser deposition system in this

work……… 23 Figure 2.2 XRD patterns of HfO2 thin films deposited on Si(100) substrates at

various substrate temperatures……… 26 Figure 2.3 Deposition rates of HfO2 thin films deposited on Si substrates at various

substrate temperatures……… 27 Figure 2.4 Three dimensional AFM images of HfO2 thin films deposited on Si

substrates at various substrate temperatures of (a) 25, (b) 200, (c) 300, and (d) 500oC respectively……… 28 Figure 2.5 Spectral dependence of refractive indexes of HfO2 films deposited at (a)

various substrate temperatures (oxygen pressure: 50 mTorr) and (b) various deposition pressures (all deposited at room temperature)……32 Figure 2.6 Spectral dependence of extinction coefficients of HfO2 films deposited

at (a) various substrate temperatures (oxygen pressure: 50 mTorr) and (b) various deposition pressures (all deposited at room temperature) 34 Figure 2.7 TEM photos of 56 nm HfO2 MIM capacitor fabricated at 200oC…….36 Figure 2.8 Current-voltage characteristic of HfO2 MIM capacitors prepared at 200,

300, and 400oC respectively……… 37 Figure 2.9 Capacitance versus frequency at zero bias for HfO2 MIM capacitors

prepared at 200, 300, and 400oC respectively……… 38

Figure 2.10 Normalized capacitance of HfO2 MIM capacitors prepared at (a) 200, (b)

300, and (c) 400oC as a function of voltage applied at a frequency of 1 kHz, 10 kHz, 100 kHz, and 1 MHz respectively………39 Figure 2.11 Normalized capacitance of HfO2 MIM capacitor prepared at 200oC as a

function of temperature……… ……… 42 Figure 2.12 SEM top views of HfO2 film surfaces prepared with the laser fluence of

(a) 4.0 and (b) 7.0 J/cm2 respectively (fabricated at room temperature)……… 44 Figure 3.1 Major fabrication steps and schematic top views of RF HfO2 MIM

capacitor and open dummy structure……….56 Figure 3.2 The definition of S-parameters for a two-port network……….57 Figure 3.3 The equivalent circuit model for capacitor simulation at RF regime…59 Figure 3.4 The measured and simulated S-parameters for (a) HfO-1 and (b) HfO-2

(Simulation and parameter extractions were done by ICCAP.)………60 Figure 3.5 High frequency response of PVD HfO2 MIM capacitors from 50 MHz

to 20 GHz for HfO-1 and HfO-2……… 62 Figure 3.6 The frequency dependence of capacitance density for PVD HfO2 MIM

capacitors HfO-1 and HfO-2………62 Figure 3.7 Stress induced leakage currents (SILCs) characteristics of (a) HfO-1

and (b) HfO-2 under the constant voltage stress at 1.5 V……….64 Figure 3.8 Stress time dependence of (a) the quadratic voltage coefficients and (b)

the linear voltage coefficients for HfO-1 under the constant voltage stress at 1.5 V………66

Figure 3.9 Stress time dependence of (a) the quadratic voltage coefficients and (b)

the linear voltage coefficients for HfO-2 under the constant voltage stress at 1.5 V………68 Figure 3.10 The equivalent circuit for HfO2 MIM capacitors after stress The added

branch stands for the generated trapped states in MIM capacitor after stress……… 69 Figure 4.1 The growth rates dependence on deposition cycles for ALD processed

(a) HfO2 and (b) Al2O3………79 Figure 4.2 TEM cross section of 13 nm HfO2-Al2O3 laminated dielectric……….81 Figure 4.3 Measured and simulated S-parameters for (a) 13 nm, (b) 31 nm and (c)

43 nm laminated MIM capacitors………83 Figure 4.4 The capacitance density dependence on frequency for laminate

capacitors with three thicknesses, the inset shows high frequency response of laminate MIM capacitors from 50 MHz to 20 GHz…… 84 Figure 4.5 J-V characteristics of 13, 31 and 43 nm laminated capacitors measured

at 125oC.……… ……….85 Figure 4.6 J-V characteristics of 13 nm laminated MIM capacitor as a function of

temperature……… ……….85 Figure 4.7 Conduction mechanisms for the 13 nm laminated MIM capacitor: (a)

Poole-Frenkel mechanism occurring at high electric field, exhibiting a shift to lower electric field with increasing the temperature, (b) Schottky emission fitting at low electric field……… ………….87 Figure 4.8 The characteristics of leakage current versus stress time under 4V stress

for the 13 nm laminated MIM capacitor Square and round symbols

represent the 1st stress and the 2nd stress after an interruption of 10 hours, respectively……… ……….90 Figure 4.9 I-V measurements showing the hysteresis loop of 13 nm laminated

MIM capacitor……… ………90 Figure 4.10 (a) The typical breakdown characteristics of 13 nm laminate under

different constant voltage stress; (b) the cumulative probability dependence on breakdown voltage for the laminated MIM capacitors with different thicknesses… ………92 Figure 4.11 (a) The voltage-dependent normalized capacitance (∆C/C0) at 1 MHz

for 13, 31 and 43 nm laminated capacitors, fitted by a second order polynomial equation; and (b) the corresponding plot of ∆C/C0 versus electric field (E)……… ……… 94 Figure 4.12 Frequency dependences of α for 13, 31 and 43 nm laminated capacitors,

showing a linear fitting in log-log scale……… ……….95 Figure 4.13 Thickness dependence of quadratic VCC (α) for laminated MIM

capacitors.……… 95 Figure 4.14 Temperature dependences of α and β at 100 kHz for 13, 31 and 43 nm

laminated capacitors……… … 96 Figure 4.15 The dependence of α/α0 on stress time at 10 kHz, 100 kHz and 1 MHz

The inset shows stress time dependence of β/β0 at the same frequencies

α0 and β0 represent the data before voltage stress (β0 is of negative sign.), α and β denote the data after different time stress……….97 Figure 4.16 (a) Cumulative TDDB curves under various constant voltages stress for

13 nm laminated MIM capacitor measured at room temperature, (b)

lifetime projection of 13 nm laminated MIM capacitor, using 50% failure time as the criteria……… …… …… ………99 Figure 4.17 Illustrations of five different HfO2-Al2O3 material structures for

electrical characteristics comparison……… ……….101 Figure 4.18 Typical J-V characteristics for MIM capacitors with different dielectric

structures at 125oC The inset shows the corresponding breakdown characteristics obtained at the same temperature (Device area: 10-4

Figure 4.19 TEM photos of (a) 13 nm HfO2-Al2O3 laminate, (b) 10 nm HfO2 and

(3) 30 nm HfO2 films, illustrating the amorphous structure of laminate film and improved crystallinity with the increase of HfO2

thicknesses……… 102 Figure 4.20 Evolution of C0 at zero DC bias with stress time, illustrating the highest

stability for the laminated capacitor compared to other dielectric structures……….……….104 Figure 5.1 Schottky plot of 30 nm HfO2 MIM capacitor The inset shows the

typical J-V curve……… 116 Figure 5.2 Measured and simulated normalized capacitance as a function of

voltage n 0 and µ are extracted by fitting the measured data.…… …117 Figure 5.3 Carrier concentration pre-factor (n0) dependence on thickness…… 118 Figure 5.4 Simulated normalized capacitance as a function of voltage for different

thickness of 20, 30, 40, 50, and 60 nm………… ……… 119 Figure 5.5 The simulated VCCs as a function of thickness with and without taking

account of the change of pre-factor (n0) with thickness……….…….120

Figure 5.6 Linear voltage coefficients versus the capacitance density for HfO2

based high-κ dielectrics at 100 kHz……… ……… 121 Figure 5.7 Normalized capacitance versus DC bias measured at 100 kHz, showing

good symmetrical CV characteristics (small linear coefficients β) for

sandwich and laminate structures………122

Figure 5.8 The measured VCCs for 30 nm HfO2 MIM capacitor together with the

extracted carrier mobility at frequencies of 10k, 100k, 500k, 1 MHz……….…124 Figure 5.9 Simulated normalized capacitance as a function of voltage for 30 nm

HfO2 MIM capacitors at frequencies of 10k, 100k, 500k, and 1MHz……… 124 Figure 5.10 (a) The simulated VCCs of HfO2 MIM capacitors as a function of

thickness with different carrier concentration pre-factor (n0), and (b) the simulated VCCs as a function of thickness with different carrier mobility in dielectric film……… ……….127

LIST OF TABLES

Table 1.1 Mixed-signal capacitor technology requirements ― Short-term… ….2 Table 1.2 Mixed-signal capacitor technology requirements ― Long-term …….2 Table 1.3 Integration of MIM capacitors into Al BEOL ― Current status… …6 Table 1.4 Integration of MIM capacitors into Cu BEOL ― Current status… 6 Table 2.1 The ratios of SIMS intensities O/Hf for HfO2 thin films prepared at

various substrate temperatures (oxygen pressure: 50 mTorr)…… …30 Table 2.2 The ratios of SIMS intensities O/Hf for HfO2 thin films prepared at

various oxygen pressures (All samples are prepared at room temperature 25oC.)……… 30 Table 2.3 Voltage linearity coefficients as a function of frequency for HfO2 MIM

capacitors prepared at 200, 300, and 400oC respectively……… 41 Table 4.1 ALD process conditions for the deposition of HfO2 and Al2O3…… 77 Table 4.2 Variations of VCCs and leakage current under different condition

(frequency: 100 kHz; stress voltage: 4V; area: 1×10-4 cm-2)…………98 Table 4.3 Comparison of various high capacitance density MIM capacitors using

high-κ dielectrics (year 2002-2003)………100 Table 5.1 Different structural HfO2-Al2O3 high-κ MIM capacitors prepared by

ALD method……… ……… …122

ACKNOWLEDGMENTS

I would like to take this opportunity to express my gratitude to all the people

who make it possible to complete this thesis work

First and foremost, I would like to give my great thanks to my principle

supervisor, Dr Zhu Chunxiang, who provides me with an interesting project, constant

direction, valuable advice, and most of all, for providing me with opportunity He has

my tremendous appreciation and respect

I am deeply indebted to my co-supervisors, Dr Lu Yongfeng currently in

University of Nebraska Lincoln and Dr Subhash Chander Rustagi from Institute of

Microelectronics, Singapore, for being a constant source of help and advice; I truly

appreciate the time, support and encouragement they have given me during the course

of my PhD study

I owe most thanks to Prof Li Ming-Fu, A/P Cho Byung Jin, A/P Yoo Won

Jong, Prof Albert Chin from National Chiao Tung University, Taiwan, and Prof

Lee-Dim Kwong from UT at Austin, for their always available helping hands, many great

conversations My special thanks go to Dr Ding Shi-Jin, I feel privileged to have had

the opportunity to work with him during my time in the PhD program, lots of

collaboration work and fruitful discussions contribute to my thesis development

I would like to thank my peers with Silicon Nano Device Laboratory in

alphabetical sequence: Chen Jingde, Chen Jinghao, Chen Xiaoyu, Joo Moon Sig, Kim

Sun Jung, Loh Wei Yip, Park Chang Seo, Poon Chyiu Hyia, Debora, Ren Chi, Tan

Kian Ming, Wang Yingqian, Wu Nan, Yang Tian, Yeo Chia Ching, Yu Xiongfei, and

Zhang Qingchun I have benefited the collaboration work with them, and their

friendship makes my stay in NUS more enjoyable

Last, and certainly the most, I would like to thank my parents for their love and

support I can never forget their inspiration and encouragement during my education

years, their constant love and support made the long hours and frustrations bearable

Chapter 1 Introduction of High-κ MIM Technology

Basic passive devices including capacitor, inductor, and resistor are indispensable elements in Si integrated circuits (ICs) Intuitively, passive devices can only consume or store energy, where active device like metal-oxide-semiconductor field effect transistor (MOSFET) can also provide amplification A precise definition

of passive elements was given by Desoer et al [1] Given a one-port with port voltage )

where )ε(t0 is the energy stored by the one-port at time t Similarly, with the aid of 0

the scattering matrix usually used for high frequency measurement, one could also deduce that the definition of passivity implies

1

0≤ S ij ≤ (1.2) [2] Compared to active devices such as MOSFET in the ultra large scale integrated circuit (ULSI) technology, passive devices played a relatively minor role However, the recent advances in wired and wireless communication trigger demands for high quality passive devices for radio frequency (RF) and mixed signal applications, and

therefore spawned a revival interest in passive devices A good introduction of passive component technology could be found elsewhere given by R K Ulrich [3]

Among the basic passive devices, capacitor is one of the essential elements, which may find its wide applications in RF circuits for oscillators and phase-shift networks, in various configurations of analog ICs such as the converters and filters, and decoupling capacitance in microprocessor units (MPUs), and so on

Table 1.1 Mixed-signal capacitor technology requirements ― Short-term [4]

Year of Production 2001 2002 2003 2004 2005 2006 2007

Q (1/KQ 2 •/µm 2 •GHz) 200 300 300 300 450 450 450 Voltage linearity

(ppm/V 2 ) 100 100 100 100 100 100 100

Analog capacitor

3 σ Matching (%•µm 2 ) 4.5 3 3 3 2.5 2.5 2.5 Density (fF/µm 2 ) 7 7.5 8 9 10 11 12

Q (1/KQ 2 •/µm 2 •GHz) 22 25 25 29 30 30 30

RF bypass

capacitor Voltage linearity

(ppm/V) 1000 1000 1000 1000 1000 1000 1000 Table 1.2 Mixed-signal capacitor technology requirements ― Long-term [4]

Q (1/KQ 2 •/µm 2 •GHz) 700 1000 1500 Voltage linearity

Manufacturable Solutions Exist, and Are Being Optimized

Manufacturable Solutions are Known

Manufacturable Solutions are Not Known

Based on the international technology roadmap for semiconductors (ITRS roadmap) [4], the main requirements and specifications for capacitors are summarized

in Table 1.1 and Table 1.2, where aggressive projections have been extent to year 2016 with ever increasing performance requirements

According to Table 1.1 and 1.2, capacitors are categorized into analog and RF bypass capacitors by ITRS roadmap, and it is believed that the requirements of analog capacitor are more difficult to be achieved compared to RF bypass capacitor Here, we may detail generally the above technique specifications as follows:

Q factor is a measure for parasitic effects including the distributed resistance

and inductance, which could be computed by Q=imag(Z Cap)/real(Z Cap)[5]

4 Voltage coefficients of capacitance (VCCs)

VCC can be approximated by C(V) = C0 (αV2+βV+1) [6], where C0 is the capacitance at zero volt and α, β are the quadratic and linear voltage coefficients of the capacitance respectively

5 Temperature coefficients of capacitance (TCCs)

TCC can be usually defined as: ppm C

dT

dC T

The capacitors’ fabrication needs to be compatible to existing ULSI backend technology Thus, high quality dielectric must be formed at a very low temperature of ~400oC limited by backend process

1.2.1 Motivation of metal-insulator-metal (MIM) technology

Traditionally, metal-insulator-silicon (MIS) [8, 9] structure has been used in Si ICs However, this structure was replaced by polysilicon-oxide-polysilicon (double-poly) capacitor since the electrical performance of double-poly structure was superior

in terms of small VCCs and stray capacitance [10], where the capacitors’ precision is paramount for those of applications such as A/D converter Accordingly, double-poly structure was established as a mature analog component In addition, the improved capacitor structures like metal-ploy have also been reported [11, 12]

Though the polysilicon structure could be tailored in many ways to yield good electrical properties making it suitable for many analog applications, it suffered from limited RF capability in multi-GHz range [13] The limitations in the quality factor are primarily due to the large resistive loss from the electrodes, and the parasitic capacitance because of the proximity to the lossy silicon substrate [14] Therefore, metal-insulator-metal (MIM) structures have been proposed as the next generation capacitor structure due to their high conductive electrodes and low parasitic capacitances In addition, the intrinsic depletion free MIM structures would provide better voltage linearity property [15]

Except the applications in Si ULSI circuits, MIM capacitor is also a key element in GaAs based monolithic microwave integrated circuits (MMICs) [16, 17] In addition, the above mentioned problems are also anticipated by dynamic random access memories (DRAM) that use MIS structure as the charge storage cell Therefore, advanced DRAM cell with MIM structures have also been studied [18, 19, 20] It is possible to implement MIM for DRAM application beyond the 90 nm node in 2004, according to ITRS roadmap [4] In this work, our focus is on high-κ MIM capacitors integrated into BEOL process for Si RF and analog applications, which is much different with the requirements of MIM capacitors in MMICs and DRAM cells in terms of materials, structures, process flow, and other aspects

1.2.2 Current status of MIM technology

As an emerging technology, MIM capacitors draw great attentions among semiconductor industry companies in the very recent years Based on the literature survey, we summarize the reported MIM capacitors technology from several major semiconductor companies, which are presented in Table 1.3 and Table 1.4 for Al and

Cu BEOL integration respectively As can be seen, SiO2 and Si3N4 are usually chosen

as the dielectric materials for MIM capacitors fabrication in the current technology node In comparison, Si3N4 has a higher dielectric constant (κ) of 7 compared to SiO2

(~3.9), which usually provides relatively higher capacitance density than SiO2 MIM capacitors In addition, Si3N4 could also be used to serve as a good Cu diffusion barrier [21], therefore eliminating Cu barrier metal stacks usually required in Cu BEOL process [21, 22] However, the frequency dependence of capacitance and voltage linearity for Si3N4 capacitors may degrade the capacitors’ accuracy, which was

proposed to be originated from bulk traps in nitride films [23] Low temperature

deposited Si3N4 was reported to show higher relaxation recovery voltage than oxide

[24] When compared to LPCVD SiO2, the breakdown field strength of Si3N4 is lower,

and both its voltage and temperature coefficients are usually higher Therefore,

schemes such as nitrous oxide plasma treatment [25], silicon oxynitride [26], SiO2

-Si3N4 stacks [27] have also been explored to combine the merits of SiO2 and Si3N4

Though SiO2 and Si3N4 MIM capacitors with excellent electrical performance have

been successfully demonstrated in Al and Cu BEOL process; the capacitance density is

still low, usually ≤ 2 fF/µm2

Table 1.3 Integration of MIM capacitors into Al BEOL ― Current status

Conexant system [15] IBM [28] Toshiba [29]

Dielectric PECVD Nitride (30~60 nm) (SiOSingle/multi layers

2 /Si 3 N 4 , 50-125 nm) (Ta(Si 32NO45, 50 nm), 50 nm) Bottom electrode Ti/TiN/AlCu/Ti/TiN TiN/AlCu/TiN WSi 2

C (fF/µm 2 ) 30-60 nm thick film 1.0-1.9 for 0.44-1.40 4.36 1.01

Good leakage property obtained after 300 o C furnace annealing Table 1.4 Integration of MIM capacitors into Cu BEOL ― Current status

Lucent [21] IBM [22] Motorola [30] TSMC [31]

diffusion barrier

Leakage sensitivity to T ox

Q 2GHz = 30-200

at 3-0.1 pf

Q=100 (2.4 GHz) and 40 (5.3 GHz)

at 1.1 pf

Furthermore, it was noted that these of SiO2 and Si3N4 works are focused on the integration of MIM capacitors, and the process related issues have thus been well addressed Planar structures were usually implemented for MIM capacitors integrated

in BEOL process, and positioning the capacitors beneath the final metal level could further minimize the loss to the substrate At or below 0.18 µm technology, Cu metallization is used instead of Al metallization due to copper’s low resistivity and feasibility of thick and fine pattern through damascene process [4] However the introduction of Cu interconnects will create unique challenges for fabricating high reliability MIM capacitors, such as surface roughness of Cu on the reliability of MIM capacitors [21], the proper choice of Cu barrier metal stack [22], and the compatibility

of capacitor dielectric with inter-level dielectric [22], etc

1.2.3 High-κ dielectrics for MIM capacitors application

As described above, SiO2 and Si3N4 are dielectrics that are commonly used in conventional MIM capacitors [6-31] Although these SiO2 and Si3N4 MIM capacitors could provide excellent electrical properties, their capacitance densities are limited due

to their low dielectric constants (κ~3.9 for SiO2, κ~7 for Si3N4) This is far from the requirement on capacitance density projected by the 2002 ITRS roadmap [4]

Further reduction in dielectric thicknesses of SiO2 and Si3N4 can increase the capacitance density, but it may offset leakage current, breakdown voltage, and voltage linearity property [22, 29] For instance, it was reported that a 30-nm-thick SiO2 MIM capacitor has a voltage linearity of ~ 20 ppm/V2 [27] From the 1/t2 (t: thickness) dependence [32], the voltage linearity of 14-nm-thick SiO2 MIM capacitor is supposed

to reach an upper limit of 100 ppm/V2 according to ITRS roadmap [4] However, the capacitance density of 2.5 fF/µm2 is low

Figure 1.1: Dielectric constant κ versus band gap for oxides [34]

Therefore, the adoption of high-κ materials is imperative to meet the requirements of MIM capacitors in Si RF and analog IC applications This is because

of the fact that using physical thicker high-κ dielectric films may potentially improve the overall electrical performance In the search to find suitable high-κ dielectrics, Figure 1.1 presents a compilation of a few potential high-κ dielectric candidates indicating the relationship of dielectric constant versus band gap This provides a simple criterion of selecting suitable high-κ materials as the dielectrics for MIM capacitors It is important to note that the general band gap reduction with the increase

of κ value for dielectrics is a limitation that must be considered when selecting a

suitable high-κ material for MIM capacitor application [33, 34] The decrease in band gap is usually coupled with the reduction of breakdown voltage for the dielectric materials [35]

Among various high-κ candidates for MIM capacitors application, Ta2O5,

Al2O3, and HfO2 high-κ dielectrics are of great interests among researchers due to their relatively good materials properties andindustry’s familiarity

Ta2O5 based high-κ dielectrics have drawn a great attention, which may be inspired by memory capacitor applications and the resultant semiconductor manufacturing tool infrastructure [18, 36] T Yoshitomi et al demonstrated pure

Ta2O5 MIM capacitors by reactive sputtering [29] With an O2 annealing at 300oC, the

Ta2O5 MIM capacitor exhibit superior electrical performance when compared to its

Si3N4 counterpart at the same equivalent oxide thickness (EOT) in terms of leakage property T Ishikawa et al integrated Ta2O5 MIM capacitors into Cu BEOL process, and insertion of thin Al2O3 layer between Ta2O5. The bottom electrode was designed to improve the interface quality and the resulting electrical performance [37] Y L Tu et

al achieved a very good voltage linearity (25 ppm/V2 and 13 ppm/V) for 4 fF/µm2

Ta2O5 MIM capacitor when compared to TaOxNy, HfO2, Al2O3 and Ta2O5/Al2O3

stacks MIM capacitors in their work [38] However, the electrical properties of those

Ta2O5 MIM capacitors have been reported to be strongly dependent on the fabrication methods and the following thermal treatments [36, 38] Therefore, a good understanding of process-structure-property correlation is of great importance before selecting Ta2O5 thin films for MIM capacitors application

Compared with other high-κ candidates, Al2O3 has a moderate dielectric constant of ~9, making it to be only a short term solution for industry’ need However, the low oxygen diffusivity of Al2O3 [34, 39] may improve the interface quality by

reducing the chemical reaction with metal electrode A large band gap of 8.9 eV is also beneficial for the improvement of leakage and breakdown characteristics For MIM capacitors application, Al2O3 based high-κ materials including pure Al2O3 [39], Ti doped Al2O3 [39], and Ta doped Al2O3 [40, 41] have been investigated using an evaporation/oxidation method, and a high capacitance density of 17 fF/µm2 has been achieved for AlTaOx MIM capacitor [41] In particular, the RF performance of high-κ MIM capacitors have been studied for Al2O3 based dielectrics up to 20 GHz A mathematical method was recently proposed for the computation of VCCs in RF regime for Al2O3 based dielectrics [40] However, the low thermal budget in the fabrication of those Al2O3 based high-κ materials is probably responsible for their marginal electrical performance

HfO2 has the advantages of high dielectric constant (~25), high heat of formation (271 Kcal/mol), and large band gap (5.68 eV), etc [34] Most important of all, HfO2 based high-κ materials are well established as the next generation gate dielectric in MOSFETs [42] and DRAM [20] HfO2 MIM capacitor was first reported using a pulsed-laser deposition (PLD) method [43] Following that, other fabrication techniques including PVD [44], atomic-layer deposition (ALD) [45] have also been demonstrated These techniques are more favourable for the mass production compared to PLD method In addition, materials engineering of HfO2 dielectric such

as Tb doping [44], Al alloying [45], and novel structures of HfO2-Al2O3 laminate [46] and stacks [47] have been further explored to improve the leakage and voltage linearity properties of HfO2 MIM capacitors In summary, compared to the reported Ta2O5 and

Al2O3 MIM capacitors, HfO2 based high-κ MIM capacitors exhibited nearly the best overall electrical properties, indicating that they are very promising for the next generation MIM capacitors application

It was also noticed that some novel oxide systems have been explored for MIM capacitors application, such as Zr-Sn-Ti oxide with a high dielectric constant of 62 [32], Nb stabilized Ta2O5 [48], etc However, the final justification of these oxide systems for MIM capacitors application needs more study, especially on their capacitance characteristics

Finally, it is worthwhile to point out that there are very few systematic reports

on high-κ MIM capacitors’ reliability and mechanism study The possible reason is that high-κ MIM capacitor is an immerging technology This is in sharp contrast with the extensive and thorough investigation of high-κ materials for gate oxides applications The reliability assessments for high-κ MIM capacitors could be found for HfO2 based high-κ dielectrics in [46, 47] and for Ta2O5 in [49] Among them, HfO2-

Al2O3 laminate and pure Ta2O5 MIM capacitors both show promising reliability characteristics for 10-year lifetime under optimized process conditions [46, 49]

The conduction mechanism in dielectric insulating films is another subject with extensive theoretical and experimental investigation A good understanding of electronic transport phenomena is paramount for the operation and leakage improvement of practical devices To be specific, Poole-Frenkel effect [44, 46, 50], Shottky emission [46, 50], trap assisted tunnelling [50] are found to be prevalent in the leakage components in thick high-κ films used in MIM capacitors Other major issues related to MIM capacitors, such as VCCs dependency [27, 28, 32, 37-41, 43-46], the dielectric relaxation [24, 51, 52], etc have also been frequently observed These phenomena are worthwhile for further investigations considering the poor knowledge thus far

1.2.4 Challenges and unsolved problems

Though the implementation of MIM structure and high-κ dielectrics appears to

be straightforward, putting high-κ MIM capacitors into practical use may be problematic until a clear understanding and thorough research have been done The main challenges and problems for high-κ materials to be used in MIM capacitors are as follows:

1 Suitable high-κ systems for MIM capacitors need to be identified

2 The effects of fabrication techniques, structures, thermal treatment on high-κ MIM capacitors need to be studied

3 Knowledge on electrical characteristics of high-κ MIM capacitors such as DC characteristics, RF performance, reliability, etc must be obtained

4 The mechanisms with regard to high-κ MIM capacitors, such as conduction, voltage linearity dependency (parameters including thickness, frequency, and electrical stress), dielectric relaxation, etc must be understood

1.3 Contribution of this thesis

The followings are the research contribution of this project:

1 A thorough research has been done for HfO2 based high-κ dielectrics using various fabrication methods for the first time The electrical performance has been demonstrated to be superior to SiO2 and Si3N4 counterparts in many aspects

2 The RF characteristics of HfO2 based MIM capacitors have been reported for the first time The good capacitance-frequency dependency indicate the usefulness of HfO2 based dielectrics for Si RF and mixed signal applications

3 We firstly observed that the voltage linearity of high-κ HfO2 based MIM capacitors could be modified after the electrical stress, and a possible explanation was given with the aid of a phenomenological circuit model

4 For the first time, a free carrier injection model has been employed to understand voltage coefficients of capacitance (VCCs) of MIM capacitors The

thickness (t) dependence of quadratic VCCs, which exhibits a relation of n

t−

∝

α (n~2), is an intrinsic property due to electrical field enhancement in the

scaled dielectric film

1 4 Thesis outline

In Chapter 2, HfO2 high-κ dielectrics prepared by PLD have been employed for MIM capacitors application with systematic material characterization, and excellent electrical properties had been demonstrated as well

In Chapter 3, RF characteristics of HfO2 MIM capacitors were investigated In addition, thickness dependence of stress induced leakage currents and the evolution of voltage linearity with stress time for HfO2 MIM capacitors had been discussed with the help of proposed circuit model

In Chapter 4, high performance HfO2-Al2O3 laminate MIM capacitors have been developed using ALD method The conduction mechanism, voltage linearity dependency, reliability issue have been investigated for these laminate films In addition, the laminate structure has also been compared with other material structures such as stack, sandwich, etc

In Chapter 5, the free carrier injection model was employed successfully to explain VCCs dependence on dielectric thickness, frequency, and electrical stress for high-κ MIM capacitors A unified understanding of VCC is achieved for the first time

Finally, Chapter 6 concludes with suggestions for future work

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Over the past few years, PLD is increasingly being used to prepare a wide variety of materials in thin film forms [1, 2] Its low start-up cost, laser energy source independent of the deposition system, the stoichiometric removal of constituent species from the target during ablation, and the relatively small number of control parameters appeal more and more attention More importantly, the PLD technique is well known for the quality of the layers grown at relatively lower substrate temperatures than other thin film deposition methods [1, 2] This is desirable for MIM capacitors integrated into back-end of line (BEOL) process where a low thermal budget (~400oC) is needed

Figure 2.1: Experimental configuration of pulsed-laser deposition system in this work

For successful integration of MIM capacitors, extremely reliable and high quality HfO2 thin films are needed Several thin film growth techniques such as atomic-layer-deposition (ALD) [3, 4], evaporation with ion-assisted deposition (IAD) [5], sol-gel [6], sputtering [7, 8], in-situ rapid thermal CVD (RTCVD) [9, 10], and metallorganic chemical vapor deposition (MOCVD) [11, 12] have been employed to fabricate good quality HfO2 thin films The properties of HfO2 thin films have been reported to be strongly dependent on the fabrication method Therefore, an understanding of process-structure-property correlation is of great importance to exploit HfO2 for thin film devices application

In this chapter, HfO2 thin films were fabricated using PLD at various substrate temperatures and oxygen pressures, in order to study physical properties dependence

on process parameters In addition, excellent electrical performance of HfO2 MIM

capacitor had been demonstrated for the first time [13, 14], suggesting the great potential of HfO2 for MIM capacitors application

2.2 Experiments

The deposition was accomplished in a stainless-steel vacuum chamber of the PLD system that was evacuated by a turbo-molecular pump to 1×10-5 Torr, the oxygen pressure was measured by a convectron vacuum gauge (1 mTorr-1000 Torr) and a single gauge TM (< 1 mTorr) respectively, and it was controlled and adjusted by a gate valve The purity of Hf target is 99.5%; it was placed on the target holder that rotated constantly by an external motor to prevent surface craters The target was irradiated by

a focused KrF excimer laser beam (λ = 248 nm, τ = 30 ns) at an incidence angle of 45o

with a repetition rate of 5-10 Hz The laser fluence was set between 3.0 and 7.0 J/cm2

In this experiment, for physical properties characterization, silicon substrates were used for HfO2 thin films deposition at various substrate temperatures ranging from room temperature (25oC) to 750oC and at various oxygen pressures varying from

10 to 200 mTorr For electrical and dielectrical measurement, MIM capacitors with

HfO2 high-κ dielectric films were fabricated on a layer of 500 nm SiO2 deposited on silicon substrate The SiO2 is for isolation purpose A layer of Ta film (~1000 Å) was

then deposited ex situ by sputtering on the SiO2 layer as the bottom electrode Following that, high-κ HfO2 dielectric films were deposited in an oxygen ambient of

50 mTorr at 200, 300, and 400oC It is noted that the deposition temperature was kept

at or below 400oC for MIM capacitor fabrication in order to meet the thermal budget requirement of BEOL process Finally, Al was deposited by evaporation and patterned

as top electrode by wet etching with the device area being 2.54×104 µm2

The crystal structures of HfO2 thin films deposited on Si substrates were investigated by X-ray diffraction (Phillips PW 1729 X-Ray diffractometer, Cu Kα radiation, λ = 0.5418 nm) The surface morphology was obtained using an Au-to-Probe CP atomic force microscope (AFM) in the contact mode The film thicknesses were measured by an Alpha-step 500 surface profiler (Tencor Instruments) and further confirmed by cross-sectional transmission electron microscopy (TEM) analyses Scanning electron microscopy (SEM) was also used to inspect particulates generation

on the surfaces of HfO2 films Time of Flight Secondary Ion Mass Spectroscopy IV (TOF-SIMS IV) was employed to characterize the change of the relative stoichiometry

of the HfO2 films prepared at various deposition conditions The optical constants, i.e refractive index and extinction coefficient, of HfO2 thin films were evaluated by a variable angle spectroscopic ellipsometer (VASE) For electrical measurements, the leakage current was measured using an HP4155A parameter analyzer, and the capacitance was characterized using an HP4284A precision LCR meter at frequencies varied from 500 Hz to 1 MHz

2.3 Results and discussion

HfO2 is a material forming several polymorphs, although pure HfO2 tends to appear in the monoclinic phase at room temperature and atmospheric pressure, orthorhombic and tetragonal phases could be formed at high pressures and/or high temperatures [15, 16], which is generally thought to be metastable if present at room temperature and atmospheric pressure Figure 2.2 shows the XRD patterns of HfO2