Metal-Dielectric Interfaces in Gigascale Electronics: Thermal and Electrical Stability

154 Pages·2012·5.556 MB·English

by  Ming He, Toh-Ming Lu (auth.)

Springer Series in Materials Science
Volume 157
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classes of materials.
Ming He Toh-Ming Lu
•
Metal-Dielectric Interfaces
in Gigascale Electronics
Thermal and Electrical Stability
123
MingHe Dr. Toh-MingLu
Department of Physics Center forIntegrated Electronics
AppliedPhysics andAstronomy, Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute 110,8th Street, CII6013
110,8th Street Troy,NY12180-3590
Troy,NY12180-3590 USA
USA e-mail: [email protected]
e-mail: [email protected]
ISSN 0933-033X
ISBN 978-1-4614-1811-5 e-ISBN978-1-4614-1812-2
DOI 10.1007/978-1-4614-1812-2
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Preface
Microelectronic devices contain multiple layers of dissimilar materials, including
metals, dielectrics, and semiconductors. When integrating these heterogeneous
materials to create a device, the boundary that separates each pair of materials is
called their interface. Examples are dielectric-semiconductor interfaces, metal-
semiconductor interfaces, andmetal-dielectric interfaces.As thedevice structures
in gigascale electronics continue to shrink, their functionality is not only affected
by the film quality of these dissimilar materials, but also is more and more
determined by the stability of their interfaces. Interface stability has become an
increasingly important issue that has a profound impact on the reliability of
electronic devices during operation. For instance, the dielectric-semiconductor
interface,suchastheSiO –Siinterface,wasintenselystudiedfordecadesinorder
2
to understand its critical role on the operation of the metal-oxide-semiconductor
field-effecttransistor(MOSFET);meanwhilemetal-semiconductorinterfaceshave
beenwidelyresearchedforyearssincetheyarethefoundationfortheoperationof
rectifyingdiodes,suchasSchottkydiodes.Inthismonograph,wefocusonmetal-
dielectric interfaces, whose stability is key to understanding the reliability of
multilevel interconnect structures in gigascale integrated circuits (IC), and more
recently, of memristor devices.
Inmultilevelinterconnectstructures,metalsarerequiredtobeveryconductive,
while dielectrics have to be highly resistive. Butthese requirements have become
more and more challenging as the width of the dielectric layer between the metal
lines gets very thin (tens of nm). Any disturbances at the metal–dielectric
boundary can significantly modify their electrical properties. The typical distur-
bance we discuss in this monograph is the penetration of metal species into
dielectricfilms.Suchmetalcontaminationisbelievedtobethemaincauseofearly
dielectric breakdown. There are two ways that metal can migrate into dielectrics:
neutral metal atom diffusion and metal ion drift. The latter is exacerbated by the
fact that the effective electric field across the dielectric film between metal lines
has reached MV/cm range during device operation.
Research on metal-dielectric interfaces is receiving increasing attention, par-
ticularly after the introduction of Cu technology one and a half decades ago and
v
vi Preface
the recent use of low dielectric constant (low-k) interlayer dielectric materials.
There are many challenging fundamental questions that need to be answered, and
some are very controversial and still under debate. Are conventional refractory
metal barriers stable on low-k materials? What is the origin of metal-ion genera-
tionatthemetal-dielectricinterface?Whydosomemetalsmigratemainlyinionic
statesinsidedielectricswhileothersmigrateonlyinatomicform?Whatroledoes
moisture play in the instability of metal-dielectric interfaces? How is metal con-
tamination affecting the lifetime of the dielectric? All these unknowns have
become central research topics for the development offuture IC technology. This
monograph is an attempt to provide a unifying picture of the diverse but some-
times contradictory findings that are reported in the literature. This unifying pic-
ture is built upon fundamental physical and chemical principles and can provide
readers with a clear account of the origins of the observed phenomena involving
metal-dielectric interfaces.
The monograph consists of nine chapters. In Chap. 1, we introduce different
types of materials of interest and their interfaces. We also introduce the types of
instabilities that can occur at metal–dielectric interfaces, including thermally
induced atomic diffusion and electric field-enhanced ion drift. In Chap. 2, we
formulate the fundamental laws that govern thermal diffusion and field-enhanced
ion drift. We discuss chemical interactions and the tendency toward oxide for-
mation at the metal-dielectric interface, which is believed to be an important
source of metal ion generation. The main test structures used to study interface
stability, including metal–dielectric–metal (MIM) and metal–dielectric–semicon-
ductor (MIS), are reviewed in Chap. 3. Electrical characterizations include I–V
(current vs. voltage), C–V (capacitance vs. voltage), bias-temperature stress
(BTS), and triangular voltage sweep (TVS). Elemental measurements include
secondaryionmassspectroscopy(SIMS),Rutherfordbackscatteringspectroscopy
(RBS), and energy dispersive X-ray spectroscopy (EDX). Al–dielectric interfaces
are discussed in Chap. 4. This chapter explains the origin of the stable Al–SiO
2
interface, which has served the IC industry for many decades. This chapter also
discusses many Al/low-k interfaces, which, to the surprise of many researchers,
fail badly under an electrical field. The stability of Cu–dielectric interfaces is
presentedindetailinChap.5.TheoriginofCuiongenerationanditsrelationship
with interface Cu oxide formation are described. The impact of moisture and
oxygen on interface stability is discussed. It is shown that, in the absence of
oxidationagents,Cuiongenerationanddriftcanbeavoided.Wedemonstratethe
severityofthermaldiffusionofCuinlow-kdielectricsatlowtemperatures,evenin
theabsenceofanexternalelectricfield.InChap.6,weshowthatwhilerefractory
metal-dielectric interfacescouldhave excellent thermal stability, they may notbe
stable under an electric field, depending on the chemical nature of the dielectric
surfaces. For most low-k dielectrics, contrary to common belief, refractory metal
ions can be generated and can drift into the dielectrics under an external electric
field due to the formation of metal sub-oxides at the refractory metal/low-
k dielectric interfaces. In Chap. 7, advanced concepts of self-forming barrier
strategies are explored. Processing challenges to the implementation of self-
Preface vii
formingbarrierstrategiesarereviewed.InChap.8,themathematicalformulations
of the kinetics of metal ion drift in dielectrics are presented. We also discuss the
methods of extracting ion diffusivity by experiments. The relevancy of metal
migration inside dielectrics and the popular time-dependent dielectric breakdown
(TDDB) reliability tests are discussed in Chap. 9. We also explore the possible
future research directions in this chapter.
Although this monograph focuses on the metal-dielectric interfaces in the
interconnect structure, the fundamental understanding gained here can be applied
to the study of metal gate/high-k dielectric interfaces for future MOSFET appli-
cations.Also,theknowledgecouldbeusefulformemristordeviceresearch,whose
operationrelieson‘‘unstable’’metal-dielectricinterfaces.Overall,thismonograph
can be used as a reference for university researchers and industrial scientists
working in the areas of electronic materials processing, semiconductor manufac-
turing, memory chips, and IC design.
TMLwouldliketothankhismentor,ProfessorS.P.Murarka,forhisinspiration
and collaboration over the years. We thank Semiconductor Research Corporation
(SRC) for its generous support of our research which has led to this monograph.
We would also like to thank our colleagues Drs. P.-I.Wang, A. Mallikarjunan,
Y. Ou, J. Juneja, J. Senkevich, C. Jezewski, H. Li, G.-R. Yang, P. Leung,
T.Spooner,G.Spencer,H.Bakhru,J.Plawsky,andW.Gill,whohavecollaborated
withusonmuchoftheresearchthatisincludedinthismonograph.Thesupportof
theRPIcleanroomstaffforfabricationandtestingisgreatlyappreciated.Finally,
wethankB.J.Lewis,G.-C.Wang,andS.Nasonfortheireditingandproofreading
ofthismonograph.
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Metal-Dielectric Interfaces in IC Chips . . . . . . . . . . . . . . . . . . 1
1.2 Materials Choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Thermal and Electrical Stability . . . . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Metal–Dielectric Diffusion Processes: Fundamentals . . . . . . . . . . . 11
2.1 Thermal Diffusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Field-Enhanced Ion Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Thermodynamics and Chemical Interactions. . . . . . . . . . . . . . . 15
2.3.1 Pt–Dielectric Interface. . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 Al–Dielectric Interface. . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.3 Ta–Dielectric Interface . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.4 Cu–Dielectric Interface . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Experimental Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1 Test Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.1 Metal–Insulator–Metal. . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.2 Metal–Insulator–Semiconductor . . . . . . . . . . . . . . . . . . 25
3.2 Electrical Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.1 Bias-Temperature Stress. . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.2 Triangular Voltage Sweep . . . . . . . . . . . . . . . . . . . . . . 35
3.2.3 Leakage Current–Voltage. . . . . . . . . . . . . . . . . . . . . . . 36
3.2.4 Time-Dependent Dielectric Breakdown . . . . . . . . . . . . . 38
3.3 Elemental Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.1 Rutherford Backscattering Spectrometry . . . . . . . . . . . . 39
3.3.2 Energy-Dispersive X-Ray Spectroscopy. . . . . . . . . . . . . 40
3.3.3 X-Ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . 41
ix
x Contents
3.3.4 Secondary Ion Mass Spectrometry . . . . . . . . . . . . . . . . 41
3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4 Al-Dielectric Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.1 Al-SiO Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2
4.2 Al/Low-k Dielectric Interfaces . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3 Chemical Identification of Al-Ion Drift . . . . . . . . . . . . . . . . . . 50
4.4 SiO as a Dielectric Barrier Against Al-Ion Drift . . . . . . . . . . . 52
2
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5 Cu–Dielectric Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.1 Stability of Cu-SiO in an Oxygen-Free Environment . . . . . . . . 58
2
5.2 Instability of Cu-SiO in an Oxygen-Containing
2
Environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2.1 Cu–SiO Under Thermal Stress . . . . . . . . . . . . . . . . . . 59
2
5.2.2 Cu–SiO Under BTS. . . . . . . . . . . . . . . . . . . . . . . . . . 61
2
5.2.3 Leakage Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3 Origin of Cu Ions in SiO . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
2
5.4 Cu-Ion Diffusivity Inside SiO . . . . . . . . . . . . . . . . . . . . . . . . 64
2
5.5 Cu Ions in Porous Low-k Dielectrics. . . . . . . . . . . . . . . . . . . . 65
5.6 Pre-Cleaning of Cu/Low-k Dielectric. . . . . . . . . . . . . . . . . . . . 67
5.7 Cu Atoms in Porous Low-k Dielectric . . . . . . . . . . . . . . . . . . . 69
5.8 Dielectrics Containing No Oxygen. . . . . . . . . . . . . . . . . . . . . . 71
5.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6 Barrier Metal–Dielectric Interfaces. . . . . . . . . . . . . . . . . . . . . . . . 75
6.1 Barrier Metals on SiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
2
6.2 Barrier Metals on Low-k Dielectrics . . . . . . . . . . . . . . . . . . . . 77
6.2.1 C–V Detection of Barrier Metal Ions
in Low-k Dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.2.2 I–V Detection of Barrier Metal Ions
in Low-k Dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6.2.3 SIMS Detection of Barrier Metal Ions
in Low-k Dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.2.4 TEM and EDX Detection of Barrier-Metal Ions
in Low-k Dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Contents xi
7 Self-Forming Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.1 General Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
7.2 Cu(Al) Self-Forming Barrier. . . . . . . . . . . . . . . . . . . . . . . . . . 95
7.3 Cu(Mg) Self-Forming Barrier . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.4 Cu(Mn) Self-Forming Barrier . . . . . . . . . . . . . . . . . . . . . . . . . 101
7.5 Refractory Metal Self-Forming Barrier Alloys . . . . . . . . . . . . . 104
7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
8 Kinetics of Ion Drift. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
8.1 Ion Distribution Simulations. . . . . . . . . . . . . . . . . . . . . . . . . . 110
8.2 Leakage Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
8.3 C–V Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
9 Time-Dependent Dielectric Breakdown (TDDB)
and Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
9.1 Time-Dependent Dielectric Breakdown . . . . . . . . . . . . . . . . . . 127
9.1.1 Intrinsic Breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . 128
9.1.2 Ion-Induced Breakdown. . . . . . . . . . . . . . . . . . . . . . . . 131
9.1.3 Additional Concerns in TDDB Modeling. . . . . . . . . . . . 135
9.2 Dielectric Pore-Sealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
9.3 Resistance-Switching Memory . . . . . . . . . . . . . . . . . . . . . . . . 141
9.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147