New Optical Three Dimensional Structure
Measurement Method of Cone Shape Micro Bumps
Used for 3D LSI Chip Stacking
Masahiro Aoyagi, Naoya Watanabe, Motohiro Suzuki,
Katsuya Kikuchi, Shunsuke Nemoto
Nano-electronics Research Institute (NeRI),
National Institute of Advanced Industrial Science and
Technology (AIST)
Tsukuba, Ibaraki, Japan
e-mail: [email protected]
Abstract—3D LSI chip stacking technology has been
developed in AIST using cone shape micro bumps fabricated by
nanoparticle deposition method. The cone shape bumps are
suitable for a thermocompression bump joint process with low
temperature and low load force, where the bumps are easy to
collapse with loading due to the pointed structure. High yield
micro bump joints can be obtained. The three dimensional
measurement of the cone shape bumps can be done using laser
scanning microscope or scanning electron (ion) beam microscope
(SEM, SIM). It takes much long time to get the precise three
dimensional measurement data. It is not suitable for a mass
production inspection test. The highly efficient measurement
technique is required for this purpose. We propose a new optical
three dimensional structure measurement technique using optical
microscopes with CCD cameras or line sensors.
In this
technique, the dimensional measurement of bumps is done by
image processing with three images captured by three
microscopes, where one microscope is for a top view and the
other two are for slanting views from two directionals which
counter.
Keywords—3D LSI, chip stacking, cone shape bump, three
dimensional structure measurement
I. INTRODUCTION
Application technology of nanoparticle deposition method
has been developed in AIST. We can fabricate a nanoparticle
condensed film, where the jet stream of helium gas containing
metal nanoparticles is sprayed on a sample. 3D LSI chip
stacking technology has been also developed in AIST using
cone shape micro bumps fabricated by nanoparticle deposition
method [1-2]. The cone shape bumps are suitable for a
thermocompression bonding process with low temperature and
low load force, where the bumps are easy to collapse with
loading due to the pointed structure. High yield and high
reliable micro bump joints can be obtained.
The three dimensional measurement of the cone shape
bumps can be done using scanning laser microscope or
scanning electron (ion) beam microscope (SEM, SIM). It takes
much long time to get the precise three dimensional
measurement data. For this reason, it is difficult to measure a
large number (1000-10000) of bumps used for 3D LSI chip
Noriaki Arima, Misaki Ishizuka, Koji Suzuki,
Toshio Shiomi
SoftWorks Co. Ltd.
Hamamatsu, Shizuoka, Japan
e-mail: [email protected]
stacking technology. Moreover, it is not suitable for a mass
production inspection test. The highly efficient measurement
technique is required for this purpose.
In this study, we propose a new three dimensional structure
measurement technique for the cone shape bumps using three
sets of the optical microscopes with CCD cameras (for mass
production: line sensors). By this measurement technique, the
dimensional measurement of bumps is efficiently performed by
observing the measured sample with three microscopes to get
the top view and two slanting views, where the two
microscopes for slanting views are arranged so that it may face
each other. Using three pictures acquired by three microscopes,
image processing is carried out in a PC system in order to
obtain the detailed 3D structure data of a bump. For the
inspection test in a mass production line, high-speed picture
acquisition and image processing can be realized by using line
sensors and an FPGA accelerating board.
In this paper, the design and the prototype fabrication of a
three-dimensional structure optical measurement system, and
the experimental measurement results are reported in detail.
The experiment was carried out using the prototype system
with test samples including cone shape bumps with 10 μm
diameter.
II. PROTOTYPE DESIGN AND FABRICATION OF A THREEDIMENSIONAL STRUCTURE OPTICAL MEASUREMENT SYSTEM
FOR CONE SHAPE BUMPS
A. The hardware component of a three-dimensional structure
optical measurement system
Inspection test systems of electronic devices and
components for a mass production line have been developed
and provided as a commercial product by SoftWorks Co. Ltd.
The company has a technical experience for 22 years about
optical sensing technology. Recently, they have developed a
stud bump inspection test system, where the system can
measure the complex shape of a stud bump with 70 μm height
and diameter [3]. A stud bump consists of disk shape
basement and tiny cone shape tip. The inspection system has
been developed under the combination of the high-speed high-
resolution image acquisition and high-speed image processing,
specially using optical line sensors with non-shadow plane
lighting. They achieved the high speed defect detection for
40000 bumps on a 4 inch wafer for 3 minutes.
set to 5 million-pixel CCD type, and magnification was set to
20 times. The slanting view optical system and the top view
optical system have been arranged with some separation in the
depth direction as shown in Fig. 1.
In this study, three-dimensional structure optical
measurement system for cone shape micro bumps was
developed, based on the concept of the stud bump inspection
system, where three sets of the high resolution optical
microscopes equipped with CCD devices were used.
Using blue LED lighting, a clear bump shadow image is
obtained for slanting view observation. Under the combination
of the blue LED coaxial lighting and blue LED ring lighting, a
clear bump outline image is obtained for top view observation.
The block diagram and appearance photograph of a cone
shape bump three-dimensional structure optical measurement
system are shown in Fig. 1 and Fig. 2. The outline of system
specification is explained as follows.
Z-axis
From the three cameras, three image data were collected
and processed through the image-processing boards in a PC
system.
An XY stage with 300 mm x 300 mm of a stroke area has
been used on the baseplate of the optical system, where the
wafer size from 4 inch to 8 inch can be handled. Moreover, the
porous vacuum chucking plate was used to keep a surface
flatness of the measured sample. It is very important to reduce
the error factor in the bump height measurement. The XY
stage was controlled through the control driver using the
controller board in a PC system. Moreover, in the top view
observation optical system, Z-axis drive mechanism was
equipped, where autofocus was made possible. The autofocus
was carried out by X-axis movement in the slanting view
observation optical system.
B. The measurement software section of a three-dimensional
structure optical measurement system
X-axis
Fig. 1
The block diagram of the three-dimensional structure optical
measurement system corresponding to a cone micro bump
The measurement software of the three-dimensional
structure optical measurement system corresponding to cone
shape bumps was designed to perform high speed image
processing to 3 CCD camera images acquired through three
microscopes. In order to realize an efficient three-dimensional
structure measurement, the height and the shape anomaly (2D
diameter, 2D shape anomaly, 3D base width, 3D peak shift, 3D
shape distortion, 3D side slope angle, etc.) of the cone shape
bump can be evaluated as numerical values.
The measurement algorithm was developed to measure the
height, the diameter, and the shape of a cone shape bump.
Several image processing functions (binarization, labeling,
edge detection, etc.) are included.
The measured bump
features are explained in details as follows.
Major axis, minor axis:
An
elliptical
approximation
is
calculated from an outline. The major
axis and the minor axis are obtained.
These values are well reproducible and
suitable to evaluate the distortion of the
bump shape.
径
短
Two Leica Z16 microscopes were used for 3D slanting
view observation from a direction of 45 degrees, where two
microscopes have been arranged so that it may face each other,
two cameras were set to 5 million-pixel CCD type, and the
maximum magnification was about 15 times. For a 2D top
view observation, the Nikon engineering microscope and one
Nikon object lens have been used in the center, a camera was
長
径
Fig.2 The appearance photograph of the three-dimensional structure optical
measurement system corresponding to a cone micro bump
[2D measurement]
Diameter of X, diameter of Y:
The maximum diameters of the X-axis
and the Y-axis of the stage are obtained.
In the diameter measurement of a cone
shape bump, the measured results using a
measuring microscope can be compared.
Shape anomaly:
In the shape anomaly case with a dust
adhering to the bump surface, the
maximum gap between the outline of an
elliptical approximation and the actual
outline is evaluated.
[3D measurement]
Base width:
Width of the bump basement is obtained.
It is the same value as a diameter of Yaxis in 2D measurement.
III. BUMP MEASUREMENT EVALUATION OF THE THREEDIMENSIONAL STRUCTURE OPTICAL MEASUREMENT SYSTEM
FOR CONE SHAPE BUMPS
A. Measurement evaluation using a cone shape bump
evaluation sample
Measurement evaluation of the three-dimensional structure
optical measurement system was carried out using the
evaluation sample by which the conical golden micro bump
was formed with the nanoparticle deposition method on the
silicon substrate. Specifically, the linear array of 20 cone
shape Au micro bumps with 10 μm diameter and 11μm height
was used.
Tilt anomaly of the side slope:
The tilt angle of the side slope is
evaluated in order to find out some
distortion of the bump shape.
Shift of the peak position from the ideal
center:
The shift of the peak position from the
ideal center is evaluated from two images
of both slanting views.
The maximum difference from the side
slope with dust:
The maximum difference between the
straight line of the side slope and the
actual outline is evaluated in order to find
out the size of the dust adhering to the
bump surface. This value shows only
distortion of Y-axis.
Fig. 3 Total 2D and 3D measurement GUI screen
White spot size of the dust adhering to
the bump surface:
In the shape anomaly case with a dust
adhering to the bump surface, the dust
looks a white spot. The size of the white
spot is evaluated. This value shows only
distortion of X-axis.
Outline height:
Bump height is evaluated from the
actual outline of the bump. If the tip
shape is flat, it shows as it is. Even in the
case of the ideal pointed tip, the tip image
may be rounded due to the slightly low
resolution of the microscope.
Side slope height:
Bump height is evaluated from the
intersection point of the two side slope
straight lines. Even if the tip image may be
rounded, it shows the estimated value.
About this value, the reproducibility is very
high because the intersection point is
calculated from the large number of pixel data.
Fig. 4 2D measuring result on GUI screen
The total 2D and 3D measurement GUI screen of the threedimensional
structure
optical
measurement
system
corresponding to a cone shape bump measurement are shown
in Fig. 3. Three pictures obtained from three microscopes are
displayed on the left. 2D and 3D measurement results and
measurement control parameters are displayed on the right. 2D
measurement result on the GUI screen is expanded and shown
in Fig.4. 3D measurement result on the GUI screen is
expanded and shown in Fig. 5, where 3DR means right slanting
image, 3DL means left slanting image.
The measurement result of a cone shape bump with a tiny
dust adhering to a bump slope is shown in Fig. 6. The outline
of the bump is properly extracted without any distortion
affected from the dust. The enlarged picture of the tiny dust
adhering to the bump slope is also shown in Fig. 6. The
position of the dust is properly recognized and indicated with a
cross mark.
B. Measurement quality evaluation of a three-dimensional
structure optical measurement system
In order to obtain a clear cone shape bump image with high
contrast and no reflection using the three-dimensional structure
optical measurement system, the contrast and the reflection
affected from the layout of lighting and microscopes was
investigated. Fig. 7 shows the slanting observation image of
the cone shape bumps with 10 μm diameter in the conditions of
the optimal layout [4-5]. Clear bump picture was obtained by
precisely adjusting the relative positions between lighting and
microscopes.
Fig. 7 Observed slanting pictures of cone shape bumps with 10 μm diameter
(optimal condition)
Bump
Height
(μm)
Bump Number
Fig. 8 Measured result of bump height for 20 cone shape bumps (blue-green
line: a scanning laser microscope, purple and green lines: three-dimensional
structure optical measurement system, blue line: SIM observation)
Fig. 5 3D measuring result on GUI screen (3DR: right slanting image, 3DL:
left slanting image)
Fig. 6 Measurement result of a cone shape bump with a tiny dust adhering to a
bump slope (Magnified view: cross mark shows a position of the dust)
The measurement performance of the three-dimensional
structure optical measurement system was investigated in
details.
A confocal short wavelength laser microscope
(Keyence VK-X200) was used in order to perform comparative
evaluation of the bump height measurement. In Fig. 8, the
measured result of bump height for 20 cone shape bumps with
10 μm diameter is shown. Blue green line shows the height
measurement result using the scanning laser microscope.
Purple and green lines show the height measurement result as
an outline height using the three-dimensional structure optical
measurement system.
Blue line shows the height
measurement result using SIM observation.
The measured value of the bump height using the threedimensional structure optical measurement system and the
scanning laser microscope were well matched each other, and
the average difference was less than 0.1 μm. For the bump of
No.6, the bump height became a higher value because a dust
adhered to the tip during the experiment.
Variation of bump height measurement was evaluated as a
measurement quality evaluation using a three-dimensional
structure optical measurement system. The experimental data
for 10 times measurement using 20 cone shape bumps with 10
μm diameter was statistically analyzed.
The standard
deviations were obtained to be 0.27 μm for outline height and
0.21 μm for side slope height, resulting less than 0.3 μm for
both cases. The good reproducibility of the bump height
measurement was confirmed.
In order to perform a strict comparison of a bump height
measurement, scanning ion microscope (SIM) observation was
performed as a higher-precision method. To observe precisely
the cross-sectional shape of the bump, 20 cone shape bump
array was etched by FIB processing in a direction of the bump
side. SIM observation was carried out to get a cross-sectional
view of a bump for each bump (shown in blue line of Fig. 8).
The cross-sectional photographs of SIM observation for 20
cone shape micro bumps are shown in Fig. 9. From the SIM
photographs, it turns out that the actual bump tip shape was
slightly rounded. As shown in Fig. 8, the height measurement
value by SIM observation was larger than that of outline height
by 0.5μm. This difference can be used as a correction value for
calibration.
No.1
No.2
No.3
No.17
Fig. 9
bumps
No.18
No.19
No.20
Cross-sectional photographs of SIM observation for 20 cone shape
IV. CONCLUSION
The cone shape bumps are suitable for three-dimensional
LSI chip stacking technology because high yield and high
reliable micro bump joints can be obtained. The new threedimensional structure measurement technique corresponding to
a cone shape bump was successfully developed. Three
dimensional structure measurement of a cone shape bump can
be realized by image processing from three pictures captured
through three optical microscopes. The prototype system was
designed and fabricated. The bump height measurement with a
standard deviation of less than 0.3 μm was confirmed using 20
cone shape micro bumps with 10 μm diameter. The correction
value of bump height measurement for calibration was found
out from comparison with SIM observation. This measurement
technique can be extended to a mass-production inspection by
improving the speed of bump image acquisition and
processing.
No.4
ACKNOWLEDGMENT
This research and development are carried out as a research
and development project supported by Ministry of Economy,
Trade and Industry (METI). Authors are deeply thankful to
Mr. Yoshihiro Gomi (Mikuni Kogyo Co. Ltd.) for his
cooperation of the nanoparticle deposition process.
No.5
No.6
No.7
No.8
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