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Note that the absolute values of the GB diffusivity P
Ni
in NiAl are appre-
ciably smaller than those in Ni
3
Al and pure Ni at the same temperatures.
4.6.5 Fe
3
Al
Fe GB diffusion in the Fe
3
Al compound was investigated in the
extended temperature interval 733 to 1065 K.
[110]
The temperature depend-
ence of P
Fe
is presented in Fig. 4.23. The compositional dependence of
Fe GB diffusivity has not been studied. Tokei et al. found
[110]
that the
A2 B2 transition does not remarkably affect the GB diffusion, although
a negative deviation of the P
Fe
values from the otherwise linear Arrhenius
dependence is indicated below the B2 D0
3
transition temperature (see
Fig. 4.23). These deviations are not caused by a hypothetical transition
from the B to the C GB diffusion regime, since the B regime conditions
were realized even at the lowest temperature.
[110]
The smaller diffusivities
of GBs in the D0
3
phase field suggest a higher degree of order in the GB
region of the corresponding phase in comparison with the B2 structure.
Atomistic calculations of defect formation and migration enthalpies
would give more insight into this problem.
It was also proposed
[110]
that the GB segregation of Al in Fe
3
Al can
modify the order-disorder transition temperature in GBs as compared to
the bulk transition temperature, especially for the A2 B2 transition.
4.7 Summary
Self-diffusion of the transition metal component in Ni
3
Al, Ti
3
Al, and
TiAl mainly occurs by nearest neighbor jumps via vacancies in the transi-
tion metal sublattice (sublattice diffusion mechanism). This mechanism
(that is, next–nearest neighbor jumps of Ni atoms) does not dominate in
NiAl. The triple-defect mechanism produces the main contribution to dif-
fusion in the near-stoichiometric NiAl alloys. Under specific conditions
(high temperatures in TiAl, T 1450 K, or large Ni content in NiAl,
c
Ni
55 at.%), the antistructure bridge mechanism also becomes impor-
tant in these intermetallics.
The transition metal sublattice diffusion mechanism superimposed by
the ASB mechanism governs Al diffusion in Ni
3
Al, Ti
3
Al, and TiAl. Ga,
which dissolves exclusively on the Al sublattice in Ni and Ti aluminides,
is a suitable solute to simulate Al diffusion in these compounds because it
diffuses by the same mechanisms as Al. Al diffusion in near-stoichiometric
NiAl alloys mainly occurs by the triple-defect mechanism, and the ASB
mechanism dominates Al diffusion in Al-rich compositions.
232 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS
Chapter-04.qxd 11/29/04 6:36 PM Page 232
DIFFUSION IN INTERMETALLIC COMPOUNDS, HERZIG, DIVINSKI 233
91011121314
T
-1
[10
-4
K
-1
]
10
-25
10
-24
10
-23
10
-22
10
-21
10
-20
10
-19
P [m
3
s
-1
]
8001100
900
1000
T [K]
A2
D0
3
B2
Figure 4.23 Arrhenius plot of Fe grain boundary diffusion in Fe
3
Al.
[110]
The tem-
perature intervals of the A2, B2, and D0
3
phases are indicated.The solid line rep-
resents an Arrhenius fit of the data measured in the A2 and B2 phase regions.
Chapter-04.qxd 11/29/04 6:36 PM Page 233
Bulk self-diffusion in the antistructure-defect-type Ni and Ti alu-
minides (Ni
3
Al, Ti
3
Al, and TiAl) reveals only a marginal compositional
dependence, whereas a pronounced compositional dependence of D
Ni
is
observed in the triple-defect-type NiAl phase. This behavior is related to
the fact that the sublattice diffusion mechanism operates mainly in Ni
3
Al,
Ti
3
Al, and TiAl.
Self-diffusion of the transition metal component in the closed-packed
Ni and Ti aluminides, if measured at the same reduced temperature,
decreases systematically when changing from pure metal to A
3
B and to
AB compound.
Grain boundary self-diffusion in closed-packed A
3
B compounds
(Ni
3
Al and Ti
3
Al) reveals a distinct compositional dependence, despite a
marginal compositional dependence of the bulk self-diffusivity in these
compounds. The deviation from the stoichiometric composition in these
alloys produces different effects on the transition metal GB diffusion in
Ni
3
Al and Ti
3
Al: An increase of P
Ni
and a decrease of Q
Ni
gb
in Ni
3
Al, and
the opposite in Ti
3
Al (a decrease of the P
Ti
values and an increase of Q
Ti
gb
in the Al-rich compositions).
Acknowledgments
The present investigation was supported by funding from the
Deutsche Forschungsgemeinschaft (Projects He89825-1 and 25-3).
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5 Diffusion Barriers in Semiconductor
Devices/Circuits
Shyam P. Murarka
Rensselaer Polytechnic Institute
Troy, New York
5.1 Introduction
When two different materials are brought in contact with each other, there
is a tendency for them to mix. This mixing, a naturally driven process that
leads to a lowering of the free energy of the system comprising these materi-
als, occurs by a transfer of matter from one part of the system to another as a
result of the random atomic or molecular motion. This is diffusion, which is
measured in terms of a quantity called diffusivity or diffusion coefficient D.
D is defined, by Fick’s first law of diffusion in one dimension, as:
J D [∂c(x, t) ∂x]. (1)
Here c is the concentration of the diffusing species; it is a function of the
position x in the sample and the diffusion time t. D is the proportionality
constant relating the flux (J) of the diffusing species to the concentration
gradient (∂c∂x). The magnitude of D determines the extent of mixing and
is dependent on the materials type (its melting point, structure, and the
defect type and content), the temperature, and the surroundings of the
materials in contact with each other. Such diffusion and mixing lead to
changes in the properties of the materials, sometimes to a better material
and other times to an undesirable material behavior.
Most solid-state transformations are the result of interdiffusion. In
microelectronic devices and circuits, semiconductors, metals, and insula-
tors coexist with each other (generally in a multilayer scheme), leading to
an ongoing concern about the intermixing and resultant loss of the desired
functional behavior. There are several known examples of failures associated
with this solid-state diffusion or mixing. Interaction of Al with underlying
Si in the contact windows of the devices has led to considerable current
leakages and contact failures. Diffusion of Cu into SiO
2
is known to cause
increased leakage through the oxide and thus to cause an insulator failure.
[1]
In these and similar circumstances, we would like to inhibit the diffusion
Ch_05.qxd 11/29/04 6:19 PM Page 239
and intermixing, and thus obtain the expected performance and reliability
of the materials involved. In the simplest way, we can find and interpose
a layer of a third material between the two interacting materials to achieve
a successful inhibition of the interaction. The material used for this diffu-
sion barrier must have certain characteristics and must be compatible in
properties with the materials it joins. For example, an electrically con-
ducting barrier is required between two metals; it is also preferred
between an insulator and a metal. On the other hand, an insulating barrier
is needed between two insulating materials.
The desired characteristics of an ideal diffusion barrier material, X,
between two materials, Aand B, have been listed by Nicolet:
[2]
1. The material transport rate of A and B across X should be
negligible.
2. The material transport of X into Aand B should be negligible.
3. X should be thermodynamically stable against A and B.
4. X should adhere strongly to A and B.
5. The specific contact resistance of A to X and of X to B
should be small.
6. X should be laterally uniform in thickness and structure.
7. X should be resistant to mechanical and thermal stress.
8. X should be a good electrical and thermal conductor.
With the advent of subquarter mm dimensions, high aspect ratio vias
and trenches, and Cu as the interconnection metal in the silicon integrated
circuits, a few new requirements have emerged:
9. X should be as thin as needed (for example, the projected
need for the years 2005 through 2007 for the high-resistivity
barriers, such as TaN, is a barrier less than 5 nm thick, and
preferably 2 to 3 nm thick) or should have electrical con-
ductivity similar to that of Cu (for example, alloys of Cu,
if they work).
10. Deposition of X should be carried out in such a way that it
coats the via/trench walls and bottoms conformally.
11.X can be an insulator, but it should not be thicker than a
few monolayers, not contribute significantly to the capac-
itance of the interlayer dielectric (ILD), and be anisotrop-
ically dry-etchable by reactive ions from the bottoms of
the vias and trenches.
It is obvious that the defined requirements for the diffusion barrier char-
acteristics are changing with the application, although Nicolet’s criteria 1
240 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS
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DIFFUSION BARRIERS IN SEMICONDUCTOR DEVICES, MURARKA 241
through 4, 6, and 7 hold for all uses of the diffusion barriers. Criteria 5 and 8
are specific to applications in a metallic system where metallic behavior
is essential. Criteria 9, 10, and even 11 are specific to advanced subquar-
ter mm design rule microelectronic applications. With the advent of nano-
electronics and nanostructured materials, these requirements are expected
to change, and we may be looking for very unconventional ways of
inhibiting or eliminating the interactions caused by the diffusion/interdif-
fusion. This chapter focuses on the diffusion barriers for application in
microelectronics, and especially between semiconductor and insulator or
metal, insulator, and metal, and between different metal systems used. It
is important to note that with use of new materials in these systems of
semiconductor devices/circuits, we invariably find a need for a suitable
adhesion promoter between two materials, thus making Nicolet’s criterion
4 essential. Our goal, therefore, is to consider thin films that will function
as adhesion promoters and diffusion barriers (APDBs) to optimize mate-
rial systems with respect to the layer thicknesses along with the process
costs. The ultimate objective of any research focused on APDBs is to
identify the ideal APDB for a given specific application, in our case for
use in a multilevel metallization scheme for advanced microelectronics.
Like everything else in microelectronics, the requirements of the so-called
ideal APDB films are changing with time. Thickness is reducing; so is the
temperature at which the films should be reliable. The introduction of
newer materials is sought to achieve better performance and reliability of
the device/circuit. However, an ideal APDB becomes feasible in a real
situation only if it can be fabricated without incurring disproportionate
expenses, and if the process of fabrication is reproducible and reliable.
This chapter briefly reviews the past history of diffusion barriers, fol-
lowed by a short discussion of the available theories of diffusion, relation
to structure of materials, manipulation of the structures, and related con-
siderations. The future requirements of the specific applications, and var-
ious approaches taken to satisfy these requirements, both conventional
and unconventional, are discussed. Finally, specific examples of new
approaches are reviewed.
5.2 Diffusion Barriers from the 1960s
Through the 1990s
Historically, the interaction between metals and the surrounding envi-
ronment has been mitigated by alloying with selected impurities, which
reduced the diffusion coefficient D and the tendency to react. A few clas-
sic examples are the making of steel, stainless steel, and copper with a
small amount of boron. Yet in microelectronic applications, thin films of
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