techniques to measure the intrinsic tensile strength and intrinsic
toughness of interfaces have been developed. Notable among these
is a laser spallation experiment for interface strength measurement.
The latter is a synthesis of two inventions, (1) nanosecond rise-time
stress pulses with tailless profiles, and (2) a wide-angle interferometer
for non-specular surfaces. Both these inventions are contained in
United States Patent No
5,438,402: System and Method for Measuring The Interface
Tensile Strength of Planar Interfaces.
Using these techniques, and high resolution transmission electron
microscopy, the atomic-level structural and chemical basis for such
measurements have been uncovered in several engineering and model
Interface strength degradation as
a function of relaxed interfacial structure has been established
in polyimide/Si systems. The relaxation occurs on account of moisture
segregation when exposed to humidity/time/temperature variables.
These strength degradation charts now allow reliable prediction
of electronic devices when used in conjunction with established
coupled mass, heat, and mechanical stress analyses simulations.
This, plus the extension of the laser spallation technique for in-situ
adhesion measurement in geometrically heterogeneous electronic devices
and packages have led to technology transfer to Intel Corporation
Discovery of Glass-Modified Stress Waves with Rarefaction
Observations of laser-generated
stress wave profiles with rarefaction shocks (almost discontinuous
or zero post-peak decay times) in glasses have been uncovered. Figure
1 shows a series of stress pulse profiles with increasing laser
fluence, measured in a soda lime glass. At low stress amplitudes,
the profiles are similar to that in Si and other materials, having
a finite (1-2 ns) rise-time and gradual post-peak decay (14-18 ns).
However, as the stress pulse amplitude exceeds a certain threshold,
the rise-time of the stress wave gets longer but the post-peak stress
profile starts to decay rather quickly. Ultimately a profile is
attained with the post-peak stress dropping instantaneously (the
drop time is within the resolution limit of our instrumentation),
much like a “rarefaction shock.” The modification in
the stress wave profile due to glass compared with that due to Si
can be appreciated by viewing Fig 3a.
Figure 1. Varying stress pulses in soda lime glass
Figure 2 shows the pulse profiles
measured in Pyrex, soda lime, quartz, and borosilicate glasses.
The magnitude of the above effect was found to vary from glass to
glass, but all showed the formation of the rarefaction shock.
Figure 2. Pulse profiles in various substrates
The rarefaction shock stems from
an initially increased compressibility of glass under increasing
pressures. The decrease in the wave speed is evident in the increase
in the rise-time of the stress wave with its amplitude (Fig. 1).
Thus, the formation of rarefaction shock can be thought to occur
due to overcrowding of the faster moving post-peak wave profile
into the significantly decelerating pulse front. The initial ramp-like
behavior has been attributed to the nonlinear elastic response in
The technological importance of
these pulses in measuring the tensile strength of very thin film
interfaces has been demonstrated using the laser spallation experiment.
Because of the rarefaction shock, glass-modified waves allow generation
of substantially higher interfacial tensile stress amplitudes compared
with those with finite post-peak decay profiles. This is demonstrated
in Figure 3. Figure 3a shows the failure inside the Si in a Cu(1400nm)/TiN(70nm)/Si
system when no glass was used. Figure 3b shows a high magnification
view of failure caused by a wave generated inside the glass and
transferred to the backside of the Si substrate. The arrow shows
the failure to be at the Cu/TiN interface with no fracture inside
the Si. An interface tensile strength value of 2.62 GPa was calculated.
This is a rather high value, which was not possible to attain using
the basic spallation setup for the film thicknesses tested. This
is demonstrated in Figures 3 (c) and (d). Figure 3 (c) shows the
measured free surface velocity profiles in the borosilicate glass
and Si substrates corresponding to the failures shown in Fig. 3
(a) and (b). The interface tensile stress history corresponding
to each profile is shown in Fig. 3 (d). The dramatic increase in
the interface tensile stress due to the glass-modified wave is evident.