588
Y. Li and D. Goyal
where “E” stands for Young’s modulus, “μ” is the Poisson’s Coefficient. Thus
Acoustic Impedance is defined by density, Young’s modulus, Poisson’s Coefficient
[22, 23] and is characteristic of materials.
From Table 18.3 [23], showing Acoustic Impedance of common materials in
microelectronic packages, the Acoustic Impedance of air is about 0.389 × 10
−3
[Kg/m
2 s] × 10
6 , very small comparing with other materials listed in the table. By
Eqs. (18.3) and (18.4) with Z 2 ≈ 0, the Reflection coefficient “R” and Transmission
coefficient “T” from any medium to air are equal to −1, and 0, respectively, Indicating almost 100% reflection. Therefore SAM is very sensitive to air gap defects in
microelectronic packages, like delamination, blisters, cracks, and voids.
The SAM technique illustrated in Fig. 18.11 is also called Pulse-Echo method
[24–26], which provides information associated with both intensity and time of flight
of the ultrasonic beam. The same ultrasonic transducer that transmits the acoustic
pulse is also used for receiving echoes occurring at boundaries of different acoustic
impedance values. As illustrated in the scan location 2 of Fig. 18.11, the echo reflected
by a UF void shown in blue has a unique time of flight compared with the die back
side, thus contains the z dimensional information of the defect, and could be obtained
by either C-scan SAM (CSAM) with Tomographic Acoustic Micro Imaging (TAMI)
technique [27, 28] or B-scan SAM [29]. CSAM provides a plane view of the location
and size of features of the test specimen. By setting a sequence of multiple gates of
data acquisition adjusted to resolve the interfaces of interest, TAMI CSAM provides
images from multiple depths within the sample simultaneously. B-scan SAM is
virtual cross sectioning of the CSAM plane, and could present a cross sectional
view of defects [29]. As illustrated in Fig. 18.12, the UF void area show up as “white
contrast” in gate 7 to gate 9 of the TAMI CSAM images by employing a CSAM
recipe with 12 gates regulated to analyze all the interfaces from the die backside to
the substrate of the flip chip package shown in Fig. 18.11.
To image subtle defects in advanced 3D microelectronic packages, both lateral and
axial resolution of the CSAM scan need to be improved. The resolution of CSAM
scans is usually determined by CSAM transducers. As shown in Fig. 18.13, the
characteristic features of CSAM transducers include frequency, shape of the cavity,
focal length, spot size in water, and depth of field. Lateral or X-Y dimensional
resolution of CSAM images is related to transducer beam spot size, the smaller the
better. While axial or Z dimensional resolution exposing the depth of delamination
gaps, is related to transducer frequency, the higher the better [29, 30]. However
higher frequency acoustic waves have bigger attenuation in the medium, which may
limit the non-destructive applications of high frequency transducers, especially for
samples with thick Si. CSAM images of the same units performed with transducers
having different frequency, spot size, focal length, and depth of field can be compared
to optimize the CSAM recipe for a specific product. It is found that axial resolution
sometimes is more critical for defect detection [30].
As indicated in Fig. 18.14a, b, the same sample is imaged by CSAM with transducers having different frequencies [30]. Figure 18.14a shows the image taken by a
lower frequency transducer of 110 MHz, no obvious defect is observed. The image
obtained with a higher frequency transducer of 230 MHz is shown in Fig. 18.14b.
Y. Li and D. Goyal
where “E” stands for Young’s modulus, “μ” is the Poisson’s Coefficient. Thus
Acoustic Impedance is defined by density, Young’s modulus, Poisson’s Coefficient
[22, 23] and is characteristic of materials.
From Table 18.3 [23], showing Acoustic Impedance of common materials in
microelectronic packages, the Acoustic Impedance of air is about 0.389 × 10
−3
[Kg/m
2 s] × 10
6 , very small comparing with other materials listed in the table. By
Eqs. (18.3) and (18.4) with Z 2 ≈ 0, the Reflection coefficient “R” and Transmission
coefficient “T” from any medium to air are equal to −1, and 0, respectively, Indicating almost 100% reflection. Therefore SAM is very sensitive to air gap defects in
microelectronic packages, like delamination, blisters, cracks, and voids.
The SAM technique illustrated in Fig. 18.11 is also called Pulse-Echo method
[24–26], which provides information associated with both intensity and time of flight
of the ultrasonic beam. The same ultrasonic transducer that transmits the acoustic
pulse is also used for receiving echoes occurring at boundaries of different acoustic
impedance values. As illustrated in the scan location 2 of Fig. 18.11, the echo reflected
by a UF void shown in blue has a unique time of flight compared with the die back
side, thus contains the z dimensional information of the defect, and could be obtained
by either C-scan SAM (CSAM) with Tomographic Acoustic Micro Imaging (TAMI)
technique [27, 28] or B-scan SAM [29]. CSAM provides a plane view of the location
and size of features of the test specimen. By setting a sequence of multiple gates of
data acquisition adjusted to resolve the interfaces of interest, TAMI CSAM provides
images from multiple depths within the sample simultaneously. B-scan SAM is
virtual cross sectioning of the CSAM plane, and could present a cross sectional
view of defects [29]. As illustrated in Fig. 18.12, the UF void area show up as “white
contrast” in gate 7 to gate 9 of the TAMI CSAM images by employing a CSAM
recipe with 12 gates regulated to analyze all the interfaces from the die backside to
the substrate of the flip chip package shown in Fig. 18.11.
To image subtle defects in advanced 3D microelectronic packages, both lateral and
axial resolution of the CSAM scan need to be improved. The resolution of CSAM
scans is usually determined by CSAM transducers. As shown in Fig. 18.13, the
characteristic features of CSAM transducers include frequency, shape of the cavity,
focal length, spot size in water, and depth of field. Lateral or X-Y dimensional
resolution of CSAM images is related to transducer beam spot size, the smaller the
better. While axial or Z dimensional resolution exposing the depth of delamination
gaps, is related to transducer frequency, the higher the better [29, 30]. However
higher frequency acoustic waves have bigger attenuation in the medium, which may
limit the non-destructive applications of high frequency transducers, especially for
samples with thick Si. CSAM images of the same units performed with transducers
having different frequency, spot size, focal length, and depth of field can be compared
to optimize the CSAM recipe for a specific product. It is found that axial resolution
sometimes is more critical for defect detection [30].
As indicated in Fig. 18.14a, b, the same sample is imaged by CSAM with transducers having different frequencies [30]. Figure 18.14a shows the image taken by a
lower frequency transducer of 110 MHz, no obvious defect is observed. The image
obtained with a higher frequency transducer of 230 MHz is shown in Fig. 18.14b.