Table Of ContentContributors to This Volume
FRED J. CADIEU NEELKANTH G. DHERE
MICHAEL S. MENDOLIA K.V. REDDY
PAUL H. WOJCIECHOWSKI
Physics of Thin Films
Advances in Research and Development
THIN FILMS FOR EMERGING
APPLICATIONS
Edited by
Maurice H. Francombe
Department of Physics
The University of Pittsburgh
Pittsburgh, Pennsylvania
John L. Vossen
John Vossen Associates
Technical and Scientific Consulting
Bridgewater, New Jersey
VOLUME 16
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Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
FRED J. CADIEU (146), Queens College of the City University of New York,
Department of Physics, Flushing, New York 11367
NEELKANTH G. DHERE (1), Florida Solar Energy Center, 300 State Road 401, Cape
Canaveral, Florida 32920
K.V. REDDY (234), Department of Physics, Indian Institute of Technology, Madras
600 036, India
MICHAEL s. MENDOLIA (272), Department of Material Science and Engineering,
University of Pennsylvania, Philadelphia, Pennsylvania 19104
PAUL H. WOJCIECHOWSKI (272), Department of Mechanical Engineering, Rochester
Institute of Technology, Rochester, New York 14623
ix
Preface
Volume 16 of Physics of Thin Films emphasizes two main technical themes. The
first is essentially an extension of the topical thrust on "Thin Films for Advanced
Electronic Devices," developed in Volume 15 of this series. The second deals
primarily with the physical and mechanical behavior of films and the influence of
these in relation to various applications. The first of the four articles in this
volume, by Neelkanth G. Dhere, discusses high-transition-temperature (Γ)
superconducting films. Since their discovery in 1986, both world-wide research
activity and published literature on high-Γ oxide films have exploded at a
phenomenal rate. In his treatment, Dr. Dhere presents an effective survey of the
already vast literature on this subject, discusses the numerous techniques under
development for the growth of these perovskite-related complex oxides, and
describes their key properties and applications. In particular, factors affecting the
epitaxial structure, critical current capability, and microwave conductivity in Y-
Ba-Cu-0 and Bi-Sr-Ca-Cu-O-based film compositions are evaluated in relation
to their use at 77 K. An overview of potential applications in a variety of
microwave devices, wide-band optical detectors, SQUID-type high-sensitivity
magnetometers, etc., is included.
The second article, by Fred J. Cadieu, discusses the synthesis, properties, and
applications of permanent thin films. The film compositions considered are
primarily of the rare-earth-transition metal type (RE-TM), discussed by Krusor
and Connell in Volume 15 of this series, for use in magneto-optic recording.
However, the objective here is to synthesize very high magnetic energy density
layers, mainly for incorporation into monolithic integrated circuits. Professor
Cadieu's treatment deals in particular with the challenging problems of develop-
ing highly anisotropic crystalline structures in sputtered films based on the
Sm(TM), Nd-Fe-Ti, and similar systems, with energy products approaching
values of 20 MGOe and with geometries suitable for use in microwave magneto-
static wave {cf. Adam et al. in Volume 15), magneto-optic, magneto-electron
beam, and other devices.
xi
xii PREFACE
Semiconductor integrated circuit failures due to electromigration in the metal-
lic conductors has long been a source of concern. In the very small geometry
devices now being designed, and the even smaller geometry devices that will be
needed in the near future, the failures due to this phenomenon are so severe that
a complete rethinking of metallization materials and processing techniques is now
underway in the integrated circuit industry. Traditional metallization materials
and techniques have just about exhausted their usefulness. In Volume 7 of this
series, d'Heurle and Rosenberg presented a comprehensive discussion of the
mechanisms involved in the generation of defects such as voids and whiskers by
diffusion under high current density conditions. In the third article of this volume,
on lateral diffusion and electromigration in metallic thin films, Κ. V. Reddy
revisits and updates this topic. Based upon somewhat more sophisticated and
quantitative experimental techniques, such as the radioactive tracer method, he
explains self-diffusion and electromigration behavior for a wide range of metallic
films that are of present and potential importance in integrated circuits.
The fourth article, by Paul H. Wojciechowski and Michael S. Mendolia,
reviews the causes and interpretation of fracture and cracking phenomena in thin
films adhering to high-elongation substrates. These phenomena are crucial to
many applications of thin films on flexible polymeric substrates (webs) such as
permeation barrier coatings of Al and oxides in packaging of pharmaceuticals,
food, solar control films on polymers for attachment to windows, antireflection
coatings deposited on polymers for attachment to cathode ray tubes and other
display devices, etc. The phenomena are closely related to the failure effects
occurring in widely-used composite materials made up of bonded fibers or sheets.
The origins and strain-dependence of multiple fracture in the low-elongation
component are explained on the basis of yield and load transfer (from film to
substrate) effects, and the modulating role of interfacial adhesion is analyzed.
M. H. Francombe
J. L. Vossen
High-Γ Superconducting Thin Films
NEELKANTH G. DHERE
Florida Solar Energy Center
Cape Canaveral, Florida
I. Introduction 2
A. Characterization of High-Γ Superconducting Thin Films 5
II. Sputtering and Ion-Beam Deposition 14
A. YBa2Cu307t and Other Rare-Earth-Based Thin Films 17
1. In Situ or Low-Temperature Process 18
2. Buffer Layers 26
3. Degradation 28
B. Bi-Sr-Ca-Cu-0 Thin Films 28
1. In Situ or Low-Temperature Process 29
2. 110 Κ Phase Formation 32
C. Tl-Ba-Ca-Cu-O Thin Films 34
III. Vacuum Evaporation 39
A. YBa2Cu307r and Other Rare-Earth-Based Thin Films 41
1. Fluoride* Route 42
2. In Situ and Low-Temperature Process 51
B. Bi-Sr-Ca-Cu-O Thin Films 52
C. Tl-Ba-Ca-Cu-O Thin Films 54
IV. Laser Ablation 55
A. YBa2Cu307r and Other Rare-Earth-Based Thin Films 59
1. In Situ and Low-Temperature Processing 61
B. Bi-Sr-Ca-Cu-0 Thin Films 78
C. Tl-Ba-Ca-Cu-O Thin Films 80
V. Metalorganic Chemical Vapor Deposition (MOCVD) 81
A. YBa2Cu307x Thin Films 85
B. Bi-Sr-Ca-Cu-O Thin Films 92
C. Tl-Ba-Ca-Cu-O Thin Films 94
VI. Metalorganic Deposition (MOD) and Other Processes 95
A. YBa2Cu307x Thin Films 96
B. Bi-Sr-Ca-Cu-O Thin Films 98
C. Tl-Ba-Ca-Cu-O Thin Films 100
VII. Applications 101
A. Microwave Components 101
1. Resonators and Oscillators 101
2. Band-Pass and Band-Reject Filters 105
3. Delay Lines 106
4. Antennas 108
B. Passive Interconnections in Microelectronic Systems 108
C. Optical Sensors and Radiation Detectors 109
D. Josephson Junction 110
E. SQUIDs 114
F. High-Speed Computer Switches and Transistors 123
References 123
Copyright © 1992 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-533016-2
2 Ν. G. DHERE
I. Introduction
The new era of high critical-transition-temperature (Γ) superconductors
began in 1986 with the discovery of superconductivity above 30 Κ in La (Ba,
2x
Sr)Cu0 perovskite oxide by Bednorz and Muller (7). Wu et al (2) discov-
x 4
ered superconducting transition temperature with zero resistance (T ) at 93 Κ
co
in YBa Cu 0 . Following the observation of superconductivity in Bi-Sr-Cu-
2 3 7 x
O by Raveau et al, Maeda and co-workers (3) announced superconductivity at
110 Κ in Bi-Sr-Ca-Cu-0 (with a probable composition Bi SrCaCuO ).
2 2 2 3 10
Sheng and Hermann (4) presented a Tl-Ba-Ca-Cu-O system (Tl BaCaCu
2 2 2 3
O ) with superconductivity at 123 K. A Y Ba Cu 0 (or YBa Cu 0 ) phase
10 2 4 8 1c6 j 2 4 8 x
has been isolated by Char et al. (5) and Mandlich et al (6). Other rare-earth
materials have been substituted for yttrium. All show transition temperatures
in the same range. Amador et al (7) have isolated a Bi SrCaCuO_ phase
2 2 2 gx
with Γ of 55 K. In the superconductors described above, the charge carriers
are holes or vacancies in the valence band. An electronic-charge carrier super-
conductor, Nd Ce^Cu0 , with Γ of 20 Κ was first reported by Tokura et al
2 x 4
(8). There have also been some unconfirmed reports of superconductivity at
much higher temperatures.
The high-Γ superconducting oxides belong to the perovskite, CaTi0 , crys-
3
tal structure. Y von and François (Ρ) have reviewed the work on the structure
analysis of high-Γ superconducting oxides carried out by x-ray diffraction
(XRD), using single crystals, and powder neutron diffraction, emphasizing
precision and completeness. The principal feature of high-Γ superconducting
compounds is the formation of infinite layers of Cu0 stoichiometry consist-
2
ing of square or pseudo-square Cu0 planes interconnected by shared-corner
4
oxide ions. The Cu0 planes do not have a strictly tetragonal symmetry. There
2
exists a small but easily detectable orthorhombic distortion. Increasing the
number of Cu-0 planes from one to three has resulted in Τ values of <23 K,
90 K, and 110 Κ in Bi-based materials and <80 K, 105 K,'and 125 Κ in Tl-
based materials. Wasa et al (10) grew Bi-based thin films with controlled
number of Cu-0 planes and found that Γ diminished when the number of Cu-
O planes was increased to four or more. In Bi-based superconductors, the
nearest-neighbor Bi-Bi separation along the b axis is not constant, but shows a
sinusoidal modulation between 6.6 Â and 5.2 Â, with an approximate period-
icity of five unit cells (77). This probably leads to the formation of a 5x
superstructure on the 6-axis, which is often observed in Bi-based compounds.
YBaXu.O^ and YBa,Cu„O phases are also referred to as 123 and 124
e
*
2 3 1-x 2 4 8-JC
phases, respectively. Similarly, Bi-Sr-Ca-Cu-0 and Tl-Ba-Ca-Cu-O are re-
ferred to as 2212, 2223, etc., phases, based on the proportion of the cations.
HIGH-rc SUPERCONDUCTING THIN FILMS 3
Both La-Sr-Cu-0 and Y-Ba-Cu-0 systems are very sensitive to oxygen
composition. Bulk YBa Cu 0 superconducting compounds are prepared by
2 3 7 x
firing mixtures of oxides of Y and Cu and Ba carbonates at 850-900°C in
flowing oxygen followed by a slow cooling in the range 400-500°C to help in
the incorporation of oxygen. Fueki et al (12,13) have annealed La-based and Y-
based materials at temperatures in the range 350-l,100°C. In both materials,
oxygen content decreases at high temperatures and at low oxygen pressures.
Additionally, in La Sr CuO , the oxygen content was found to be controlled
Q 920 08 4
only by content of Sr at low temperatures (<800°C) and high oxygen pressure (1
atm). In YBaCu0_, the metal ratio is fixed and the oxygen content is con-
2 37x
trolled only by the temperature and oxygen pressure. The oxygen content is the
lowest in samples annealed at high temperatures (>1,000°C) and low oxygen
pressures. At the normal synthesis temperatures, the tetragonal phase
YBa^Cu.CX is more stable than the orthorhombic YBa Cu,0_ phase with x—> 0.
0
2 3 6 2 3 l-x ~
Increasing the oxygen pressure during the synthesis does not result in the
formation of the orthorhombic YBa Cu 0 phase. In fact, the higher oxygen
2 3 7
pressure can impede the formation of the tetragonal YBa Cu 0 phase. It is
2 3 6
preferable to synthesize the tetragonal YBa Cu 0 phase in bulk or thin film
2 3 6
form, maintaining an appropriately low pressure of oxygen. The oxygen content
can then be increased by annealing in appropriate ambients at temperatures
below 500°C. Specht et al. (14) have carried out high-resolution x-ray diffrac-
tion measurements of the lattice constants of YBa Cu 0 ^ during heating and
2 37
cooling through the orthorhombic-tetragonal transition in different oxygen
partial pressures and have found that the transition is sharp, continuous, and
reversible in temperature and pressure. Above the transition temperature of
~676°C, in 1 atm 0 , the superconducting orthorhombic YBa Cu 0 structure
2 2 3 7x
loses a significant amount of the oxygen and transforms into a non-supercon-
ducting tetragonal phase. The orthorhombic-tetragonal transition temperature
drops with a decrease in oxygen partial pressure, becoming ~521°C at 2% 0 in
2
He. On being cooled slowly in oxygen, the structure picks up oxygen again at
temperatures of 400-500°C and retransforms to the orthorhombic phase, ab-
sorbing the quantity necessary for the transformation. The average valence of
Cu increases with increasing oxygen content. With decreasing oxygen content
in the formula unit, the cell volume of the orthorhombic YBa Cu,0^ phase
0
' 2 3 l-x *
increases; the structure becomes unstable and gradually transforms to a tetrago-
nal phase. The oaxis parameter of the superconducting orthorhombic
YBa Cu 0 phase, which has a value of 11.679 Â at 20°C in 1 atm 0 ,
2 3 679 2
increases rapidly around an oxygen content of 6.5. The tetragonal phase
YBa Cu 0 is non-superconducting and has a oaxis value of 11.817 Â at 20°C,
2 3 6
in 1 atm 0 . A direct correlation has been shown to exist between the increase in
2
the oxygen content and the decrease in the oaxis parameter (75). With an
4 Ν. G. DHERE
increase in the proportion of the orthorhombic phase, there is a corresponding
increase in Γ in Y- and other rare-earth-based superconductors.
Morris et al. (16) have shown that the critical temperature, Γ, of Bi-based
superconductors can be shifted reversibly over a range of >15 Κ by changing
the oxygen concentration. For example, Γ is lowered by heat treatment at
600°C in high oxygen pressures of ~100 atm, while the highest Γ can be
achieved by heat treatment at 600°C at P < 10 Torr. These results show that
Q2
the Bi Sr CaCu 0 phase has a T range of 85-95 K. Hence, it is now
2 2 28 χ co
referred to as the 90 Κ phase, rather than the 85 Κ phase as was done earlier.
Bi-based superconductors may be able to provide the advantage of supercon-
ducting critical temperatures comparable to those from Tl-based materials with-
out the necessity of processing a toxic material such as thallium. However, as
compared to Tl BaCaCuO , the preparation of 110 Κ Bi SrCa Cu O has
2 2 2 3 10 2 2 2 3 10
been found to be more difficult. Chavira et al. (17) have shown that partial
substitution with Pb facilitates the growth of 110 Κ phase. There were uncon-
firmed reports of T at 132 Κ and 153 Κ by partial substitution of Bi with Pb
co
and Sb (18). Dou et al. (19) have found that Pb addition helps in stabilization of
110 Κ phase, and that while Sb addition depresses Γ for heat treatment in pure
oxygen, it does not seem to aid in the formation of higher-Γ superconducting
phases. Shi et al. (20) have observed cooperative nucleation and growth of
Bi Sr CaCu 0 and CaCu from amorphous matrix of Bi-Sr-Ca-Cu-0
2 2 28 χ 3 3
glasses. The growth of 110 Κ phase at the interface between the crystallized
phases is controlled by the diffusion of Ca and Cu atoms and hence is favored
by initial high Ca and Cu content and longer anneals at 870°C. They also
believe that Pb addition advances the diffusion process. Endo et al. (21) have
found that annealing in low oxygen partial pressures of 70 Torr can enhance the
formation of the 110 Κ 2223 phase in bulk Bi-based materials.
Aselage et al. (22) have found that the vapor pressure established by pure,
condensed thallium oxide exceeded the stability limits of the Tl-based super-
conducting phases, and hence these phases do not coexist in equilibrium with
T10 vapors. Special precautions are essential in processing and disposing of
2
the Tl-based materials to avoid risks from their toxicity (23).
There is considerable interest in thin films of high-Γ superconducting mate-
rials because of the possibility of fabricating cryoelectronic devices. Of the
many techniques for the deposition of thin films, vacuum evaporation, sputter-
ing, metalorganic chemical vapor deposition (MOCVD), molecular beam epi-
taxy (MBE), and ion-assisted deposition are used widely in laboratories and
industries, and have been treated extensively in textbooks (24-27). The other
techniques are laser ablation, metalorganic deposition (MOD), spray pyrolysis,
electrodeposition, plasma spray, screen printing, dip coating, solution growth,
etc. The definitions of thin and thick films are not based entirely on their
