Klaus Wetzig studied physics at the University of Technology in Dresden, receiving his licence in 1963, his doctorate in 1967 and his habilitation in 1973. In 1975 he moved to the Academy of Sciences, and since 1992 he is Professor of Materials Analysis at the University of Technology in Dresden and Director at the Leibniz Institute for Solid State and Materials Research Dresden.
His research interests include materials analysis and microstroctures, especially electron microscopy of functional materials, characterization of thin films for electronics, and nanostructural features in general.
Claus Michael Schneider studied physics at the Institute of Technology Aachen receiving his diploma in 1985. He obtained his phD in 1990 at the Free University of Berlin and his habilitation in 1996 at Martin-Luther-University Halle. In 1998 he moved to the Leibniz-Institute of Solid State and Materials Research Dresden, heading the department of thin film systems andnanostructures.
His research interests include solid state physics, thin film systems and surface magnetism as well as the physics of nanostructures.
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About the Author
Klaus Wetzig is Director of the Leibniz Institute of Solid State and Materials Research, Dresden. His research interests include materials analysis and microstructures, especially electron microscopy of functional materials, characterization of thin films for electronics, and nanostructural features in general.
Claus Michael Schneider is Director at the Institute of Solid State Research of the Research Center Jülich (IFF-IEE). His research interests include solid state physics, thin film systems and surface magnetism as well as the physics of nanostructures.
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Metal Based Thin Films for Electronics
John Wiley & Sons
Copyright © 2003
Klaus Wetzig, Claus M. Schneider
All right reserved.
Electronic devices have found widespread use in our everyday lives. The applications cover
many areas such as consumer electronics, information technology, engineering, automotive
application, transportation, medical diagnostics and treatments, etc. The construction of these
devices and their building blocks involves elaborate fabrication processes which are based on
a thorough understanding of materials science and solid state physics. The device functionality
may involve conventional microelectronic, acoustoelectronic, optoelectronic, or future
spinelectronic elements, or a combination of these (Figure 1.1). The functionality is achieved
by a carefully engineered and complex combination of metallic, semiconducting, and insulating
layers. These layers are often micro- and nanostructured by sophisticated lithography
techniques in order to achieve the desired properties. Sometimes, as in the case of microprocessors,
the structuring involves several levels. The individual feature sizes created by the
structuring processes may be as small as 100 nm and are expected to become even smaller in
the future in leading edge applications.
[Figure 1.1 Omitted]
The fabrication of these electronic devices requiresa very good control of the materials
properties. This concerns not only the physical material parameters, but also the film structure
and morphology. The latter are largely determined by the details of the deposition process
and postgrowth processing procedures. In addition, the interfaces between different materials
and material classes are also becoming of crucial importance. In this situation, a wide
variety of analysis tools must be used to ensure a reliable process control and - if necessary - a
precise failure analysis. These tools include not only different real space (microscopy) and
reciprocal space (diffraction) techniques, but also spectroscopic techniques, electrical transport
measurements, stress and strain analyses, migration investigations, etc.
Novel device technologies are often closely linked to the use of new materials or material
classes. One recent example is the replacement of the conventional Al interconnects in microprocessors
by Cu ones. This step not only involves new fabrication procedures, such as
the "damascene" technique, but also requires new barrier layers to avoid the mixing of Cu
and Si. Another example is the emerging technology of magneto- or spinelectronics. In its
present state it employs complex magnetic units composed of metal or metal/insulator layer
stacks. In addition to the electrical properties, the layers must also provide a distinct magnetic
functionality. Since all of the classical ferromagnets Fe, Co, Ni and many antiferromagnets
used in magnetoelectronics are metals, this adds another and very exciting facet to
the application of metal-based films in electronics.
From the above considerations follows quite clearly that metal-based thin films play a
central role in the different steps of the fabrication and for the specific functionality of electronic
devices. The most evident use concerns conducting lines and interconnects. Less obvious
is their employment as barrier layers against interdiffusion and segregation. Also very
important are metallization layers, for example, in acoustoelectronic devices. In chemically
complex systems, the physical properties can be conveniently changed by the chemical composition.
This is particularly true for the conductivity and is exploited in silicides for thermoelectric
applications. Metal-based films are also very important for X-ray optical techniques
used to fabricate (X-ray lithography) and analyze (X-ray diffraction and spectroscopy)
electronic device structures.
Since metal-based films have such a widespread use in the different areas of microelectronics,
knowledge of the respective properties and phenomena is distributed over various
fields of physics and materials science. As a consequence, one usually has to consult many
different sources in order to get the desired piece of information or a broader overview of a
specific issue. Considering the importance of metal-based films in the field of electronics it is
thus justified to describe and discuss these systems, the associated effects and phenomena,
and their applications in one place.
Organization, Aim and Content of This Book
The main purpose of this book is two-fold. On the one hand, it is meant to serve as a compendium
for metal-based thin film systems and their usage in electronics technology. As
such, it addresses both the scientist and the research engineer. On the other hand, the book
also includes a more tutorial part which is intended to bridge the gap between fundamental
phenomena and their technological applications. It may therefore also serve as a textbook for
advanced students in solid state physics, materials science, and electronics engineering.
The book is organized into several chapters covering the range from principal aspects and
phenomena over contemporary challenges in materials science to actual device concepts and
applications. We thereby mainly concentrate on the relevant fields of interconnects, acoustoelectronics,
thermoelectrics, magnetoelectronics, and X-ray optics.
In Chapter 2 we review the various fundamental aspects of metal and metal-based films
with respect to the individual fields and applications addressed in this book. This chapter is
mainly intended to convey background information for the advanced student in a more tutorial
form. It forms a basis for the discussion of the future challenges and the device-related
topics in the subsequent chapters. The first section is devoted to a key aspect in microelectronics,
namely the means to transfer and distribute information and power in a microelectronic
device, for example, in a microprocessor. This is achieved by means of metallic interconnects
which are usually arranged in very complicated and delicate three-dimensional
networks. The contribution discusses both Al and Cu-based technologies for interconnects
and highlights the specific implications and problems associated with each technology. A
somewhat less familiar, though not less eminent area of microelectronics is acoustoelectronics.
Acoustoelectronic devices are based on the exploitation of phenomena involving the
generation, transport, and filtering of surface acoustic waves. Their functionality is largely
determined by the interaction between a piezoelectric substrate and a metallic film serving as
an electrode. Surface acoustic wave devices play a strategic role in telecommunication and
other high frequency applications. A rather novel facet of microelectronics is called magnetoelectronics
or "spintronics" which is the topic of the third section of Chapter 2. Spintronics
is still an emerging technology which is based on the transport of spins and charges, rather
than just charges. It thus combines magnetic functionalities and materials with established
microelectronics concepts. Current spintronics applications concern read heads in hard disk
drives, magnetocouplers, or nonvolatile magnetic random access memories (MRAM). In the
long run, reprogrammable magnetic logic circuits or active magnetoelectronic devices, such
as a spin transistor, may be expected. The section reviews the fundamental aspects of spindependent
transport and magnetic coupling phenomena in thin films and layer stacks. It also
discusses the basic thin film arrangements and their specific properties with respect to a
particular device functionality. Thermoelectricity exploits the conversion of thermal energy
into electrical energy and vice versa for power generators, cooling devices, and temperature
or radiation sensors. The particular relevance of thermoelectrical systems for microelectronics
arises - among other reasons - from the increasing need for efficient thermal and power
management of chip devices. The implementation of Peltier elements in the chip architecture
can provide on-chip cooling facilities. The recovery of excess heat and its conversion into
electrical energy may help to reduce the overall power consumption and represents a step
towards future self-sufficient systems. The different material systems and thermoelectric
concepts which are currently under consideration are treated in the fourth section. Particular
emphasis is put on the role of the various materials properties with respect to the thermoelectrical
efficiency parameters and figure of merit. The final section of the chapter deals with
the role of metallic layers and multilayer systems for X-ray optics. The connection of X-ray
optics to microelectronics comes from the progress in optical lithography techniques which
aim at feature sizes well below 100 nm. Because of the smaller wavelengths, the novel lithography
approaches can no longer be based on transmission optics, but have to use reflective
optics instead. Metal thin film systems are therefore needed to realize the appropriate
optical elements (mirrors, gratings, etc.). The section discusses the fundamental aspects of X-ray
optics with respect to thin film systems based on reflection and diffraction.
Chapter 3 is devoted to the deposition techniques used to prepare thin film systems and to
the main analytical approaches employed to study their behavior. The analysis involves microscopy,
spectroscopy, or diffraction techniques and gives access to different properties,
such as the film morphology, chemical composition, crystallographic and electronic structure.
Deposition techniques for thin metallic films exist in a wide variety and are described in
Chapter 3.1. Today vacuum based physical and also chemical deposition techniques play the
dominant role in the preparation of thin metallic films, but non-vacuum based deposition
methods such as electroplating or the modified CVD technique ALD (atomic layer deposition)
are also of growing interest and will therefore be discussed in this book. Both transmission
electron microscopy (TEM) and electron diffraction are strong techniques for studying
micro- and nanostructures in metal based thin films. Furthermore, with enhancement of an
analytical TEM by spectroscopic attachments for such as energy dispersive X-ray spectroscopy
(EDXS) and electron energy-loss spectroscopy (EELS) it is also possible to receive
chemical information (element distribution and chemical binding) in the nanometer range of
thin films. A powerful method for the immediate study of electrical thin film properties is in
situ scanning electron microscopy (SEM). In situ SEM methods allow the investigation of
potential contrast, ferroelectric domains, electron beam induced current (EBIC) and processes
of electro- or acoustomigration respectively. X-ray scattering techniques are discussed
as a widely-used tool for structural information on thin films. Both the possibilities and limitations
of angle diffraction, reflectometry, soft X-rays and magnetic scattering are discussed.
Spectroscopic techniques allow the element distribution and depth profile analysis of thin
films. They can be carried out by electrons, X-rays or ions and are frequently used in connection
with imaging techniques such as scanning or transmission electron microscopy. In contrast
to bulk materials, thin films on substrates are usually under mechanical stress. Thus,
stress measurement methods play an important role in the characterization of thin films for
electronics. Different techniques such as the substrate curvature and the [sin.sup.2] [PSI] method are
discussed under application aspects.
As one of the core parts of this book Chapter 4 addresses current challenges in the investigation
and application of metal-based thin films. These include the aspects of thermal stability,
acousto- and electromigration, barrier and nucleation layers, functional electric and
magnetic layers and mulilayers for X-ray optical purposes. These topics represent the forefront
of the current research in materials science and solid state physics. Because of the continuing
downscaling in the architecture of integrated circuits electromigration is a life- limiting
process in metallization layers. The damage analysis is discussed both for Al and Cu
interconnects. The introduction of copper as the conducting material for interconnects requires
effective diffusion barriers since copper readily diffuses into silicon oxide and silicon.
The optimization of barriers and new barrier/ seed concepts are therefore the focus of attention.
Migration effects are also observed in surface acoustic waves (SAW) structures, as a
result of diminishing structure dimensions (< 1 µm) and increasing electrical input power
values (> 1 W) which cause very high power density levels and therefore high stress loading
of metallization. Thus, new metallization concepts have to be discussed in detail. Spintronic
applications of functional magnetic layers, such as for sensors and MRAMs, may be realized
by thin film systems which may be grouped into multilayers, spin valves and tunnel junctions.
These systems excel in a precisely defined functionality which is strongly influenced
by temperature. Therefore the thermal stability plays a dominant role in both the manufacture
and operation of functional magnetic layers, as will be demonstrated for magnetoresistive
layer stacks. A further group of thin film based components with growing importance are
multilayers for X-ray optical purposes, e.g. as reflectors for X-rays. Finally, the last part of
Chapter 4 is focused on functional electric layers with well-defined electronic and electrical
transport properties. Such thin film materials are used as resistance layers, thermoelectric
sensors and generator devices. The optimization of the electrical and thermoelectric film
parameters will be discussed in depth.
The application of metal-based thin film systems in electronic and microelectronics-related
devices is the focus of Chapter 5. The diversity of the devices treated in this chapter
highlights the widespread application areas of metal film systems. The first section deals
with interconnect technologies for memory and logic products. Of particular interest are the
combination of Cu interconnects and low-k dielectrics. The subsequent section on surface
acoustic wave devices gives examples of high frequency filters, resonators and delay lines.
The device concepts range from relatively simple transversal bandpass filters to programmable
phase shift keying filters. The magnetic and magnetoelectronic sensor devices are mainly
related to automotive applications and thus emphasize one of the growing future markets for
microelectronics products. There, magnetic sensors are employed to measure positions, angles,
rotational speeds, or torques with the aim of improving fuel economy, vehicle and passenger
safety, and driving comfort in the present and new generations of automobiles. Reducing
the energy consumption and improving the energy efficiency is also the driving force
in the development of thermoelectric sensors and transducers.
Excerpted from Metal Based Thin Films for Electronics
Copyright © 2003 by Klaus Wetzig, Claus M. Schneider.
Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents1. Introduction
2. Thin Film Systems: Basic Aspects
- Interconnects for Microelectronics
- Metallization Structures in Acoustoelectronics
- Silicide Layers for Electronics
- Complex Layered Systems for Magnetoelectronics
- Multilayer and Single-Surface Reflectors for X-Ray Optics
3. Thin Film Preparation and Characterization Techniques
- Thin Film Preparation Methods
- Electron Microscopy and Diffraction
- X-Ray Scattering Techniques
- Spectroscopic Techniques
- Stress Measurement Techniques
4. Challenges for Thin Film Systems Characterization and Optimization
- Electromigration in Metallization Layers
- Barrier and Nucleation Layers for Interconnects
- Acoustomigration in Surface Acoustic Waves Structures
- Thermal Stability of Magnetoresistive Layer Stacks
- Functional Magnetic Layers for Sensors and MRAMs
- Multilayers for X-Ray Optical Purposes
- Functional Electric Layers
- Devices Related Aspects for Si Based Electronics
- SAW High Frequency Filters, Resonators and Delay Lines
- Sensor Devices
- X-Ray Optical Systems
- Thermoelectric Sensors and Transducers
What People are Saying About This
"...ist das Buch sowohl in seiner Praxisorientierung als auch in seiner didaktischen Gestaltung auf den Praktiker zugeschnitten. Es kann jedem Werkstofftechniker, Physiker oder Prozessingenieur, der auf dem immer noch aufregenden Gebiet der Mikroelektronik tätig ist oder tätig werden will, wärmstens empfohlen werden."
Winfried Blau, Europäische Forschungsgesellschaft Dünne Schichten e.V., Dresden
"Es kann jedem Werkstofftechniker, Physiker oder Prozessingenieur, der auf dem immer noch aufregenden Gebiet der Mikroelektronik tätig ist oder tätig werden will, wärmstens empfohlen werden."
Winfried Blau, Europäische Forschungsgesellschaft Dünne Schichten e.V., Dresden Vakuum in Forschung und Praxis 2/04