Ryan McFadden
Scanning Acoustic Microscopy
Research on Material Bonding using the Scanning Acoustic Microscope
Abstract
Through the course of the fall semester, I conducted research in conjunction with Jeong
Nyeon Kim using the Scanning Acoustic Microscope (SAM). The SAM was a way of nondestructively evaluating multi-layered, structured, thin-filmed samples. We utilize the SAM in an
attempt to effectively measure and average out the surface acoustic waves of the varying samples
we used. The surface acoustic wave is essential because it provides a correlation between the
bonding strength of materials and the speed of the surface wave. The SAM measures this wave
by the use of a V(z) curve. The V(z) curve displays the speed at which the surface acoustic wave
traveled across the scanning width (approximately 2 millimeters) as a function of depth. The
V(z) curve simulation program simulates a transducer’s output when an ultrasound wave is
transmitted into the sample. Using the V(z) curve graph created from scanning the specimen,
comparisons were made between these results and the simulated results using the MATLAB
program. For the multi-layered samples, Silicon (Si) (100) was used as a substrate. Titanium (Ti)
(thickness: 10 nm) and Platinum (Pt) (thickness: 100 nm) were deposited, respectively.
Introduction
A multilayered thin film structure is extensively used in today’s time because of its
specific properties. The structure has enhanced resistance to varying types of damage, as well as
has implications in reducing frictional and electrical resistance. These samples can also,
depending on their specific properties, contain special magnetic or dielectric properties. The
SAM is especially effective in quantitatively measuring and giving solid data relating to the
specimen’s material properties. The SAM non-destructively does this by scanning images of the
surface and subsurface of the sample. The images it produces are on the micrometer scale, and
the microscope requires no special treatment to the sample to produce such high resolution
images. The strength of the bond between materials can be measured because the SAM
propagates its acoustic waves across the scanning area of the sample. Propagation of acoustic
waves are directly related to the elastic properties of the specimen. The specimen’s layered
structure is shown below:
Theory
To broaden the use of the V(z) curve in this study, a program was created in MATLAB to
simulate the output voltage of the transducer. This output voltage would realistically cause
emission of ultrasound on the multilayered sample, and the program was designed to simulate
this. The software applies Atalar’s angular spectrum approach1 with a Fourier spatial
transformation2 to find the acoustic field as well as approximate the V(z) curve equation. The
V(z) curve in this experiment is representative of the output voltage of the transducer versus the
depth as the lens scans the surface/subsurface of the material. The curve usually appears to be
periodic, and this behavior can be attributed to, for example, the interference between specularly
reflected and leaky surface waves. These waves depend on the materials and their elastic
properties contained in the multi-layered specimen. The mathematical expression for the V(z)
curve is transformed into a Fourier series as shown:
V(z) = F{C-1u2(kz)P2(kz)kzR(kz)}
(1)
where C is represented as an integral:
(2)
F{} denotes the Fourier transform. In Eqn. (1), kz is the z-component of the wave number k in
the coupling medium. Also in Eqn. (1) u and P specify the acoustic signal and pupil function
respectively. Both of these are determined from the transducer parameters and lens geometry.
Continuing, by setting and defining for the experiment the Reflectance function, R, for the
material, the V(z) curve can be simulated and calculated for comparison with the results from the
SAM.
Experimental Procedure
To complete the procedure of taking images and scanning the samples, a number of steps
needed to be completed in a specific order to allow for precise accuracy in measurements. First, I
had to set up the SAM so that its settings, frequency, lens, et cetera were set to our preferences.
The SAM as a wide frequency range, but for this experiment we examined on a frequency of 400
MHz. This frequency box needed to be inserted into the SAM in conjunction with the
corresponding lens before the SAM was to be turned on. Next, the sample needed to be cleaned
of all blemishes and spots using Q-tips. This obviously allows for less irregular/erratic results.
The sample was then placed on the stage, and, using a dropper, the spot that I wanted to scan was
immersed in distilled water. The purpose of distilled water is so that there are no bubbles or other
obstacles to deflect/divert the beam from the lens. The water further acts as impedance matching
agent in order to deliver acoustic energy to the scanning area. Next, the lens had to be lined up
exactly with the spot from which I wanted to analyze. Using the controls on the SAM, I would
move the stage in the x, y, and z directions so that the lens would appear to be directly above the
indicated spot on the sample.
On the specimen, we had marked off seven distinct areas, as shown in the image below:
where the spots were numbered from left to right (1-7). The lens of the microscope had to be
lined up right in the center of the four dots.
Once I felt the sample was lined up with the lens, I would use the forward and reverse
buttons to take an image of what the SAM was seeing. An example of such an image is show
below:
The dots provided a water-mark of sorts; it allowed me to know I was in the right area to take a
V(z) curve measurement. Once I had an image shown on the microscope monitor comparable to
the image above, I would scan again and stop the microscope in the center of the set of dots. By
doing this, I would stop the scan right where I wanted to measure the velocity of the surface
acoustic wave. After stopping the scan, I would use the knob on the side of the SAM to find the
focal point of the material (the focal point optimized the V(z) curve results). Then I would
defocus the microscope to -250 micro-meters, and adjust the brightness of the Sam depending on
the signal I was getting from the focal point. Finally, I would take a V(z) curve measurement
(five per spot). This entire procedure was repeated for each of the seven spots on the specimen.
Results
After completing the five scans per spot, I had the data required to analyze and make
conclusions. The calculated data is shown below (The red font specifies the fastest velocity and
the green is for the lowest velocity for each individual point):
This data is collected and calculated by the use of the MATLAB programmed referenced earlier.
The program allows me to compare the data collected from the microscope and compare it with
the simulation from the program itself. The program takes the graph inputted from the
microscope, and calculates the average surface acoustic wave using a Fourier approximation. An
example of a simulation graph compared to a SAM V(z) curve graph is shown below:
The average V(z) curve obtained at point 1 on the sample produced an average surface acoustic
wave (vsaw) of approximately 4948.85 m/s. This is fantastic compared to the experimental
simulation on MATLAB, which produced a vsaw of about 4940.97 m/s. This allowed us to
validate the simulation’s accuracy to less than .15 percent difference.
Conclusion
From all of this collected data, it appeared that as I worked down the sample from spots 1
through 7 that the average surface acoustic wave linearly decreased. The simulation in MATLAB
continued to follow the experimental results to great accuracy. Because of the proximity in the
vsaw, a conclusion can be made that the simulation in MATLAB is very effective in predicting the
material behavior in multi-layered specimens.
References
[1] A. Atalar, "An angular-spectrum approach to contrast in reflection acoustic microscopy,"
Appl. Phys. Lett. 49, 1530-1539 (1978).
[2] Goodman, J. W., [Introduction to Fourier Optics], McGraw-Hill, New York, 48 (1968).