U.S. patent number RE32,625 [Application Number 06/898,032] was granted by the patent office on 1988-03-15 for dynamic testing of electrical conductors.
This patent grant is currently assigned to Syracuse University. Invention is credited to Robert W. Pasco, James A. Schwarz.
United States Patent |
RE32,625 |
Schwarz , et al. |
March 15, 1988 |
**Please see images for:
( Certificate of Correction ) ** |
Dynamic testing of electrical conductors
Abstract
A technique is described that permits direct and accurate
evaluation of a thin film conductor's reliability which requires
only a few hours to carry out. The technique involves a temperature
ramp procedure which dynamically exposes a conductor operating
under constant current stress to a linear (in time) rise in
temperature. Changes in resistivity of the conductor provides
kinetic data that is directly related to both the electromigration
process and the reliability of the device.
Inventors: |
Schwarz; James A.
(Fayetteville, NY), Pasco; Robert W. (Wappingers Falls,
NY) |
Assignee: |
Syracuse University (Syracuse,
NY)
|
Family
ID: |
27038005 |
Appl.
No.: |
06/898,032 |
Filed: |
August 18, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
455852 |
Jan 5, 1983 |
04483629 |
Nov 20, 1984 |
|
|
Current U.S.
Class: |
374/57; 324/703;
324/750.05 |
Current CPC
Class: |
G01R
27/14 (20130101); G01R 31/2849 (20130101); G01R
31/2805 (20130101); G01R 31/2642 (20130101) |
Current International
Class: |
G01R
27/14 (20060101); G01R 31/26 (20060101); G01R
31/28 (20060101); G01N 027/14 (); G01R
027/02 () |
Field of
Search: |
;374/45,46,44,57
;324/300,73PC ;219/504 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Measurement of Thermal Diffusivity Using a Pyroelectric Detector",
C. E. Yeack et al., J. Appl. Phys. 53 6, Jun. 1982, pp. 3947-3949.
.
"The Effect of Hydrogen Ambient on Electromigration in Al and Al-Cu
Thin Films", George Sardo, Syracuse University, 1979. .
"Circuit Board Testing", H. E. Meier IBM Tech. Disclosure Bulletin,
vol. 23 No. 9, Feb. 1981, 324-73 PC..
|
Primary Examiner: Yasich; Daniel M.
Attorney, Agent or Firm: Bruns and Wall
Claims
We claim:
1. A method for determining the kinetic parameters of activation
energy and pre-exponential factor which characterizes
electromigration failure in a thin film conductor that is subject
to high current densities at elevated temperatures, said method
.[.includes.]. .Iadd.including .Iaddend.the steps of
placing a thin film conductor in a .[.predetermined atmosphere.].
.Iadd.testing device.Iaddend.,
electrically stressing the thin film conductor, while in said
.[.atmosphere.]. .Iadd.testing device.Iaddend., by applying a
uniform current thereto,
heating the stressed conductor to increase the conductor
temperature at a linear rate with respect to time,
measuring changes in resistance of the current stressed conductor
as it is being heated at a linear rate,
reducing the measured changes in resistance by those changes due to
the temperature dependent components of resistance to determine the
changes in resistance produced by the electromigration failure
process,
determining the activation energy and pre-exponential factor by
relating the measured changes in resistance with respect to time
produced by the electromigration failure process to the following
zeroth order rate expression:
where: R.sub.o is the initial resistance of the thin film
conductor, dR/dt is the variation in the conductor resistance
produced by the electromigration process, Q is the activation
energy for the process, A is the pre-exponential for the process, k
is Boltzmann's constant and T is absolute temperature.
2. The method of claim 1 that further includes the step of
annealing the .[.unstressed.]. conductor prior to heating the said
conductor at a linear rate.
3. The method of claim 1 including the further step of determining
the activation energy for the electromigration process by
plotting:
where: .DELTA.R.sub.em is the change in resistance of the conductor
produced by the electromigration process.
4. The method of claim 3 wherein the activation energy of the
process is determined from the plot according to the
relationship
where: s is the slope of the plotted line.
5. The method of claim 3 that includes the further step of
determining the pre-exponential factor from the relationship:
where: .beta. is the heating rate, and I is the intercept of the
plotted line.
6. The method of claim 5 that includes the further step of
determining the life expectancy of the conductor by the
relationship: ##EQU7## where: t is time in seconds.
7. Apparatus for testing a thin film conductor to determine the
kinetic parameters of activation energy and pre-exponential factor
which lead to electromigration failure in the conductor that
includes
.[.a hermetically sealed chamber having.]. means to support a thin
film conductor .[.in said chamber.].,
.[.means to maintain a controlled atmosphere within the
chamber.].
heating means to increase .[.the temperature of the atmosphere
within the chamber.]. at a linear rate .[.whereby.]. the
temperature of the conductor .[.is also heated at a linear
rate.].,
electrical means for maintaining a constant current flow through
the conductor as .[.is.]. .Iadd.the letter .Iaddend.is being heated
.Iadd.by said heating means .Iaddend.at said linear rate,
first means to measure the changes in resistance of the conductor
as the temperature thereof is changed .Iadd.by said heating means
.Iaddend.at a linear rate over a preselected given period of time,
and
said means to measure the change in temperature of the
.[.atmosphere within the chamber.]. .Iadd.thin film conductor
.Iaddend.whereby the increase changes in the conductor resistance
due to the electromigration failure process can be determined by
relating said changes to an activation energy and pre-exponential
factor, wherein a record of the changes in resistance over said
given time period .[.time.]. is obtained.
8. The apparatus of claim 7 which further includes a recording
means operatively connected to said first and second means for
providing the record of the changes in resistance and temperature
over the given .Iadd.time .Iaddend.period .[.of time..].
9. The apparatus of claim 7 wherein .[.the.]. .Iadd.said means to
support the thin film conductor includes a hermetically sealed
chamber, and .Iaddend.means to control the chamber atmosphere
.[.further.]. .Iadd.which .Iaddend.includes valve means for
regulating pressure of the atmosphere within the chamber.
10. The apparatus of claim 7 wherein said heating means includes an
electrical heater .[.that is wrapped about the outside of the
chamber.]..
Description
BACKGROUND OF THE INVENTION
This invention relates generally to the mass transport of atoms in
a conductor and, in particular, to a process for evaluating the
reliability of a thin film interconnector of the type typically
used in micro-electronic devices.
As pointed out in U.S. Pat. No. 3,474,530 to Ainslie et al, thin
film conductors as typically employed in microcircuits are
subjected to a process of electromigration which can under certain
conditions lead to early circuit failure. This type of failure
generally involves the movement of atoms in the direction of
current flow from a first donor region into a second acceptor
region. As noted by d'Heurle and Ho in a publication entitled "Thin
Films--Interdiffusion and Reactions" published by
Wiley-interscience, New York 243-303 (1978), electromigration
failure occurs in two separate stages. During the first stage of
failure, herein referred to as the electromigration damage (EMD)
stage, atoms move out of the donor region under relatively well
defined conditions leaving behind voids in the material. The
transported atoms are deposited in the acceptor region thereby
creating hillocks. The second stage of electromigration failure,
which is herein referred to as the catastrophic failure process or
(CFP) stage, is characterized by complex temperature and current
density variations that lead to a rapid and complete failure of the
device.
It is important to note that the two stages of electromigration
failure occur in sequence with (EMD) being first in time. The
damage that takes place in these early stages of the process
proceed under well defined conditions of temperature, temperature
distribution, and current density. These conditions remain
relatively constant during (EMD) and to a great extent controls the
failure process over most of the conductor's life. The second, more
dramatic, stage of the failure process, while still an
electromigration event, is not characterized by the initial
conditions of temperature and current density previously
experienced by the conductor but rather by local current densities
and temperatures that develop in the now highly stressed donor
region. Microscopic defects produced in the conductor by complex
temperature and current density variations increase flux
divergences in the previously damaged regions thus bringing on
rapid, catastrophic and total failure. Although the second stage of
failure is a consequence of the first, it nevertheless occurs with
rapid kinetics and under less well-defined conditions than those
experienced during the earlier stages.
It is important to note that (EMD) occurs over a major portion of
the conductor life while (CFP) takes place during a relatively
short period at the end of this life span. Accordingly, the
physical changes in the conductor which controls the overall
failure process typically requires an extremely long period of time
to produce catastrophic failure. Heretofore, the kinetics of an
electromigration process have been determined by life test
experiments known as Mean Time to Failure (MTF) tests. In this type
of testing a specimen is generally electrically stressed under
isothermal conditions. The kinetics of electromigration are then
determined as weighted averages which are taken over a relatively
long period of time beginning with the initial stressing of the
specimen and ending with failure. As can be seen, these weighted
averages are characteristic of both (EMD) and (CFP). The results
obtained are therefore of a generally questionable nature in light
of the fact that (CFP) depends upon the initial state of the
specimen and the specific damage produced during the first stage
(EMD) of the process. Testing of many samples is needed to
determine (MTF). These tests clearly show that the life span of the
specimens can vary by as much as a factor of four. Furthermore,
using the test results to determine kinetic parameters relating to
the process is also questionable because, as noted, local
temperatures and current densities change dramatically during the
latter stages of failure and, as a consequence, the time-to-failure
values are sometimes more likely to reflect variations in (CFP)
rather than (EMD).
Electrical resistance or resistivity measurements have also been
used to study the kinetics of the electromigration process.
However, like (MTF) measurements, these isothermal resistivity
measurements require testing of many samples over long periods of
time to determine the kinetic parameters of electromigration. A
more thorough treatment of this type of testing is given by Hummel
et al, in the Journal of Physics and Chemistry of Solids, Pergamon
Press, 1967, Vol. 37, at pp. 73-80, (printed in Great Britain).
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to improve the
method and apparatus for evaluating the reliability of a conductor
and in particular, a thin film interconnect as typically used in a
micro-electronic device.
It is a further object of the present invention to shorten the time
required to evaluate the reliability of a conductor.
A still further object of the present invention is to provide for
dynamic evaluation of a thin film conductor so that the kinetic
parameters controlling failure can be accurately and quickly
determined.
Yet another object of the present invention is to accurately
predict the life span of a conductor.
Another object of the present invention is to provide a method of
characterizing electromigration in a conductor using a temperature
ramp technique.
Still another object of the present invention is to solve
reliability problems related to large surface-to-volume ratio
submicron sized circuit components and the resulting increased
material transport rates afforded by high diffusivity paths and
short diffusion distances.
While a further object of the present invention is to accurately
determine both the activation energy (Q) and pre-exponent (A)
values associated with an electromigration process within a
relatively short period of time by use of a single experiment.
These and other objects of the present invention are attained by
placing a conductive specimen within a controlled ambient and
applying a constant current to the specimen while simultaneously
therewith heating the specimen at a constant rate. Changes in the
specimens resistivity are noted and the activation energy and
pre-exponent for the process determined from the observed data.
Using a zeroth order relationship for the activated process, the
life expectancy of the conductor can also be determined.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention reference is
had to the following detailed description of the invention which is
to be read in conjunction with the associated drawings,
wherein:
FIG. 1 is a schematic presentation of an apparatus embodying the
teachings of the present invention;
FIG. 2 is an enlarged view of a typical test specimen utilized in
the apparatus of FIG. 1;
FIG. 3 is a characteristic plot for determining activation energy
from data obtained through use of apparatus shown in FIG. 1 wherein
an aluminum specimen is stressed at a constant current while being
heated at a linear rate;
FIG. 4 illustrates characteristic plots similar to the one shown in
FIG. 3 for two identical aluminum specimens that have been tested
in different ambients while being stressed at the same current and
heated at the same rate; and
FIG. 5 also illustrates characteristic plots similar to the one
shown in FIG. 3 for two specimens that are formed of different
materials that have been stressed in the same ambient at the same
current heated at the same rate.
DESCRIPTION OF THE INVENTION
The current trend in the electronics industry toward very large
scale integration of circuits emphasizes the importance of research
concerning the reliability problems associated with submicron sized
devices and, in particular, to studying the effects of high
material transport rates produced in high diffusity paths having
short diffusion distances. One such transport phenomena that leads
to shortened conductor life and ultimate circuit failure results
from the current induced transport of atoms in the conductor
material from a donor region to an acceptor region. This phenomenon
is referred to as electromigration and is well known in the
semiconductor industry. The kinetic parameters of this type of
failure process are the activation energy (Q) and pre-exponential
factor (A) for the process.
These parameters are known to be sensitive to both the conductor
material and the environment in which it operates. See G. M. Sardo,
Masters Thesis, Solid State Science and Technology, Dept. of
Chemical Engineering and Materials Science, Syracuse University
(1981).
As previously noted, electromigration failure in integrated circuit
conductors occurs in two sequential stages. Electromigration damage
(EMD) occurs first in time and controls the failure process for a
major part of the conductor life. It is therefore fundamentally
more important to define or characterize the earlier stages of
(EMD) not only because it controls the process but also because it
proceeds under clear and well defined conditions of temperature,
temperature distribution and current density. It is possible
through the measurement of the kinetics of electromigration during
the early stages of (EMD) to gain an understanding of the mechanism
of the solid state transport phenomena that is associated with the
latter failure. As will be explained in greater detail below, this
can be accomplished by resistivity measurements.
The kinetics of electromigration have heretofore been determined
through Mean Time to Failure (MTF) experiments and are described
according to the relationship:
where:
A is the pre-exponential factor for the process;
J is the current density applied to the conductor;
n is an exponent that is generally in the range of
Q is the activation energy for the process;
k is Boltzmann's constant; and
T is the process temperature.
The discussion presented by Ho et al in Electro- and
Thermo-Transport in Metals and Alloys, TMS-AIME, New York (1977)
provides a summary of the application of this equation.
As reported by Hummel, DeHoff and Geier in the Journal of Physics
and Chemistry of Solids, Pergamon Press, vol. 37, pp. 73-80 (1976)
resistance measurements have been used to study electromigration
kinetics. These studies show that the kinetics of the process are
well defined only during the early stages of electromigration
during which a 5% to 10% relative resistance change takes place. As
will be explained in greater detail below, an expression for an
activated process similar to the (MTF) expression can be written
for resistance measurements. In analogy to equation (1) the
following expression for small changes in resistance of a conductor
can be written: ##EQU1## where: m is current density exponent
determined from resistance change measurements in the range of
R.sub.o is the initial resistance of the conductor at room
temperature; and
dR/dt is the variation in the conductor resistance under (EMD)
conditions.
Equation (2) reflects the observed linear time dependence of
changes in resistivity as measured during the early stages of
electromigration. Deviations from the observed linear behavior at
higher resistivity changes and longer time periods cannot be
described by a simple kinetic process of integer order. These
deviations are likely produced by localized temperature and current
density changes due to electromigration damage and signal the onset
of catastrophic failure.
Although these and other techniques have been used to study
electromigration, none of these techniques are able to supply a
dynamic description of the process. It is well known from studies
of diffusion in bulk materials that order of magnitude errors can
be made by extrapolating high temperature results to lower
temperature when changes in the process mechanisms occur. In the
present method, resistivity measurements of a conductor are taken
through a given temperature range over a predetermined period of
time so that the kinetics of (EMD) can be studied dynamically. This
method permits systematic investigation of lower temperature
process that are normally ignored by isothermal tests. By current
stressing a test specimen, such as a thin film conductor, as it is
being heated at a uniform linear rate, variations in the
electromigration kinetics at the higher temperature can be
determined in a relatively short time. These values can then be
extrapolated to lower temperatures and the reliability of the
device determined.
THEORY
The present method employs a linear temperature ramp to determine
the kinetic parameter of an electromigration process. The kinetic
parameters, as herein used, refers to the activation energy (Q) and
the pre-exponent (A) for the process. As previously noted, in a
constant temperature or isothermal electromigration test, the
resistance of the conductor is a function of time only so that
R=R(t). In the instant process, the process temperature (T) is
increased linearly with respect to time so that:
where:
T.sub.o is the initial conductor temperature;
.beta. is the heating rate; and
t is time.
As can be seen, the conductors resistance now becomes a function of
both time and temperature. Assuming Matthiessen's Rule is obeyed
(see J. Bass, Advances in Physics, Vol. 21, pg. 431, 1972), it is
possible to separate the total resistance of the conductor into two
independent additive components:
where:
R.sub.T is the temperature component;
R.sub.em is the electromigration component.
The temperature component is well represented by a linear
relationship over the range of interest and therefore:
where .alpha. is the temperature coefficient of resistivity. From
equations (3) and (5):
so that:
The subscript zero in the above noted equations refers to the
initial test conditions at time zero. Stated more concisely, the
electromigration component of the relative resistance change is the
total relative resistance change minus the linear baseline which
consists of the temperature component. The assumption of
Matthiessen's Rule that the electromigration component of
resistance is independent of the temperature component is implicit
in all resistance techniques and is used herein to separate
resistance changes due to temperature change from changes due to
electromigration.
By subtracting the linear baseline from the total resistance change
which can be measured as raw data, the remaining electromigration
component of resistivity is related to temperature and time by the
zeroth order kinetics for the activated process according to
equation (2): ##EQU2## Noting that dt=dT/.beta., and R.sub.em at
time zero is equal to zero: ##EQU3## The integral in equation (9)
appears often in temperature ramp experiments and can be
approximated as: ##EQU4## This relationship is valid so long was
Q/2kT>>1. The second term of equation (10) can be dropped
since it is relatively small in comparison to the first term.
Combining equations (9) and (10) provides:
and rearranging:
and taking the logarithm of both sides of equation (12):
A characteristic plot of the left hand term in (13) is shown in
FIG. 3 and provides a slope (S) and an intercept (I) so that:
where:
so that:
Checking the accuracy of the above approach using calculated data
for reasonable values of Q and A, it was determined that the slot
shown in FIG. 3 gives accurate determinations of Q values to within
2% or 3%. The pre-exponent values (A), however, were found to be
less accurate with errors being in the 10% to 20% range.
Accordingly, the (Q) values are first determined using measured
resistance values and the characteristic plot as shown in FIG. 3
and the determined (Q) value for the process is then employed to
numerically integrate the right hand side of equation (9) to
determine the (A) value for each data point. The A values are then
averaged to determine the best pre-exponential value for the
process.
EXPERIMENTAL
Turning now to FIGS. 1 and 2, there is shown the apparatus for
carrying out the present invention. The apparatus includes an
electric furnace 10 that is capable of being accurately controlled
so that the temperature of the ambient within the furnace can be
increased at a desired linear rate. The furnace includes a test
chamber 11 that is wrapped by electrical heating coils 12 and an
insulating blanket 13. The heating coils are connected to a
suitable temperature control unit 15 and the coil temperature
monitored via a thermocouple 17 so as to hold the coils at a
desired .Iadd.temperature .Iaddend.level.
A test specimen 20 is mounted in the chamber 11 upon a support 21
which, in practice, can be a pinned electronic package of the type
used in industry to package semiconductive devices and the like.
The test specimen is shown in greater detail in FIG. 2. For the
purposes of explanation the instant test specimen is a thin film
conductor formed from an aluminum film that has been magnetron
sputtered onto a 0.8 .mu.m silicondioxide on a silicon wafer. The
device further includes two parallelly aligned conductive side
members 22 and 23 that are electrically joined by a strip 25 that
forms the specimen under test. The side members include a pair of
enlarged upper pads 26 and 27 that are connected in series with a
power supply 28 (FIG. 1) so that a current is caused to flow
through the strip. A second smaller pair of lower pads 30 and 31
are connected to a voltmeter 32 which is situated in recorder 29
(FIG. 1) and used to determine the resistivity of the strip.
A flanged connector 33 is secured in the top wall 34 of the chamber
through which the chamber communicates with a source of gas (not
shown), the previously noted power supply 28 and the chart recorder
29. The power supply is adapted to apply a constant current to the
test specimen throughout the test period. A temperature sensor 37
is positioned within the chamber in close proximity to the specimen
and provides a constant stream of ambient temperature information
to the recorder. The term ambient, as herein used, shall mean the
atmosphere maintained within the chamber which is typically at some
pressure that is equal or greater than the atmospheric pressure
surrounding the chamber. Related voltage data is also brought out
of the chamber through the flanged connector to the recorder. This
information is recorded on a strip chart along with the temperature
information.
Prior to testing, the specimen is annealed by heating it within the
chamber for about four hours in a helium or inert atmosphere. This
prevents the specimen from becoming annealed during the test period
and thus prevents erroneous measurements from being generated.
The ambient within the chamber is controlled by means of the gas
inlet 40 and a gas purge line 41 passing out of the bottom wall of
the chamber. Once the specimen has been mounted, the chamber is
sealed and an inert or reactive gas is fed into the chamber to
totally replace the air atmosphere.
Raw experimental data are in the form V.sub.s /V.sub.o =R/R.sub.o
as a function of temperature. The low temperature portion of the
resistivity curve is quite linear and represents resistance changes
produced by changes in temperature only. Low temperature data is
therefore used to determine the temperature coefficient of
resistivity. In accordance with equation (7), subtraction yields
the electromigration component as a function of temperature. The
actual .[.stripe.]. .Iadd.strip .Iaddend.temperature, however, is
needed in order to determine the kinetics of the process.
The .[.stripe.]. .Iadd.strip .Iaddend.temperature is higher than
the ambient temperature because of the current stressing. The
resistance of the .[.stripe.]. .Iadd.strip .Iaddend.as a function
of current density is measured over a given temperature range and
the average .[.stripe.]. .Iadd.strip .Iaddend.temperature rise
above the ambient rise for various current densities is found. This
added rise in .[.stripe.]. .Iadd.strip .Iaddend.temperature is
constant over the observed range. Using this data measured ambient
temperatures are corrected to obtain average .[.stripe.].
.Iadd.strip .Iaddend.temperatures and the data recorded in terms
thereof.
FIG. 3 shows the results of a test conducted in the apparatus
described above using a conductive test stripe of aluminum having a
thickness 0.8 .mu.m, a width of 6.35 .mu.m and a length of 380
.mu.m. The specimen was current stressed at 3.times.10.sup.6
amp/cm.sup.2 and the heating rate was held at a constant rate of 5
K..degree./min. From previously conducted MTF experiments conducted
upon similar aluminum conductors, it was indicated that the
activation energy should be about 0.43 eV. As can be seen, the test
results showed the value of (Q) to be 0.48 eV which compares quite
favorably with the previously indicated results. Similar favorable
results were also found for pre-exponential values by integrating
the right hand side of equation (9).
The invention will now be explained in greater detail with
reference to the following examples:
Example 1
Employing the apparatus illustrated in FIGS. 1 and 2, a pair of
identically constructed aluminum test specimens were mounted in the
furnace. The first specimen, specimen 1, was exposed to a helium
atmosphere during testing while the second specimen, specimen 2,
was exposed to a hydrogen atmosphere. Each specimen was stressed at
3.times.10.sup.6 amps/cm.sup.2 while being heated to a constant
linear rate of 1.degree. K./min from room temperature to about
600.degree. K.
Tests described about were conducted on both specimens in different
ambients and characteristic plots, similar to that shown in FIG. 3,
for the activated process prepared. These plots are illustrated in
FIG. 4. The pre-exponential (A) and activation (Q) values were
determined using this data. The values for specimen contained in
the helium ambient were:
while the values for the specimen contained in the hydrogen ambient
were:
From the integrated form of equation (2) above, the following
relationship can be written: ##EQU5## Rearranging equation (17)
provides: ##EQU6## Experience has shown that the .DELTA.R.sub.em
/R.sub.o factor representing failure has a typical value of between
0.10 and 0.50. An arbitrary value of 0.20 was selected which would
be representative based on previously observed resistivity changes.
Using the known values in equation (18) and a Boltzmann's constant
of 8.617.times.10.sup.-5 eV/K..degree. the .DELTA.t value for each
specimen was calculated for room temperature at two different
current densities. The results are tabulated below:
______________________________________ SPEC NO ATMO J = 3 .times.
10.sup.6 A/cm.sup.2 J = 5 .times. 10.sup.6 A/cm.sup.2
______________________________________ #1 He 76.6 days 7.6 years*
#2 H.sub.2 17.5 years 629 years* FOR: .DELTA.R.sub.cm = 0.20 Ro T =
298.degree. K. ______________________________________ *Assuming A a
J.sup.2 (m = 2)
As can be seen from the table above, the specimen contained in the
hydrogen atmosphere had a life expectancy that was greater than the
specimen contained in the helium atmosphere by a factor of about 83
for specimens stressed at 3.times.10.sup.6 amps/cm.sup.2.
EXAMPLE 2
Two new specimens were constructed as noted above from two
different materials. One specimen, specimen 3, was constructed of
aluminum while the other specimen, specimen 4, was constructed of
aluminum containing about 2% copper. Again using the above
described apparatus and procedures of the present invention, each
specimen was tested in a hydrogen atmosphere while being stresses
at 3.times.10.sup.6 A/cm.sup.2. The temperature of each specimen
was increased at a linear rate of 1.degree. K./min. The
characteristic plots of the two activated processes are illustrated
in FIG. 5. As explained in reference to Example 1, the data was
used to find the life expectancy of each specimen and the results
are tabulated below:
______________________________________ SPEC NO MATL J = 3 .times.
10.sup.6 A/cm.sup.2 J = 5 .times. 10.sup.6 A/cm.sup.2
______________________________________ #3 Al 17.5 years 629 years*
#4 Al--CU 2.0 years 73.2 years* FOR: .DELTA.R.sub.cm = 0.20 Ro T =
298.degree. K. ______________________________________ *Assuming A a
J.sup.2 (m = 2)
From the results of Example 2 it can be seen that the life
expectancy and/or reliability of a pure aluminum conductor is
considerably greater than that of one alloyed with copper when
operating at room temperature in a hydrogen environment.
These results clearly show that detailed information concerning a
given conductor can be obtained in one experiment using the method
and apparatus of the present invention. While this invention has
been described with reference to the structure disclosed herein, it
is not necessarily confined to the details as set forth in the
application and the invention is intended to cover any
modifications or changes that might come within the scope of the
following claims.
* * * * *