SONIX Wafer Capability Brochure

Scanning Acoustic Microscopy For
Metrology of 3D Interconnect Bonded Wafers
Jim McKeon, Ph.D. - Sonix, Director of Technology
Sriram Gopalan, Ph.D. - Sonix, Technology Engineer
8700 Morrissette Drive
8700 Morrissette Drive
Springfield, VA 22152
Springfield, VA 22152
tel: : 703-440
440-02220222
tel: : 703-440
440-02220222
fax: 703-440
440-9512
fax: 703-440
440-9512
e-mail: info@sonix
sonix.com
e-mail: info@sonix
sonix.com
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Ultrasonic Inspection of 3D Architectures
• 3D architectures present challenges to
conventional processing and inspection tools
• Ultrasonic inspection is inherently a 3D technology
• Ultrasound provides defect detection, metrology,
and monitoring for process control
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Ultrasonic Inspection
Ultrasound
•• A transducer transducer produces produces a high high
frequency frequency sound sound wave wave which which
interacts interacts with with the the sample. sample.
•1 •1– 300 300 MHz MHz
•• High High frequency frequency sound sound waves waves can can
not not propagate propagate through through air. air.
Pulse Echo Inspection
•A •A change change in in acoustic acoustic impedance impedance (Z) (Z)
results results in in some some sound sound reflected reflected and and
some some transmitted
transmitted
•Air •Air has has Z = 0
Transducer
•• Couplant- Couplant-A material material used used to to carry carry
the the high high frequency frequency sound sound waves. waves.
•Water •Water is is the the most most common common
couplant couplant
Si Wafer 1
Glass wafer
Si Wafer 2
H 2 O
Couplant
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SAM Capabilities
Bonded wafer
Si Wafer 1
Si Wafer 2
MEMS pressure sensor
Si Wafer
Glass Wafer
Alignment of bonded wafer pair
•Voiding in wafer bonding
•Bond delaminations for hermetic
seal applications (MEMS)
•Metrology for wafer pair
alignment post bonding
Si Wafer 1
Si Wafer 2
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Example Images
MEMS Inspection
Bonded Wafer Voiding
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A-Scan
Initial Pulse
Front surface
Interface of interest
Back surface
Transducer
Si Wafer 1
Si Wafer 2
The raw ultrasonic data. It is the received RF signal from a single point (x,y).
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C-Scan
Data from a specified depth over the entire scan area. (Horizontal cross-section).
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Determining Void Size Distribution:
Cluster Analysis
•Cluster Analysis determines void size distribution based on user-defined amplitude
and size criteria
•Void count and percentage of void area are indicated
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Process Validation
Original process
Modified process
11.664% defect area
1.086% defect area
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Metrology
Via measurement - spatial
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Metrology
Via measurement - depth
Cu velocity = 4.66 mm/µs
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Metrology
Trench depth measurement
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Metrology
Wafer Thickness Measurement
Si velocity = 8.6 mm/µs
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Metrology
Wafer Thickness Measurement
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Defect detection
• Air Gap thickness
•Confirmed detection of 10 nm air gaps with UHF
and greater frequencies
•Confirmed detection of 50 nm air gaps with
25MHz and greater
•Smaller Smaller air gap thickness may also be detectable
• Spatial Detection
•Defects Defects with diameter of few microns detectable
•Resolution of closely spaced defects depends on
the sample type and the transducer used
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Closely spaced defects
•When When the pitch of the defects/ features approach
the ultrasonic beam width, the resolution of the
defects/features becomes challenging
•The The variables affecting the beam width include:
•the bonded wafer materials
•thickness of the layers
•Transducer selected
•To To evaluate the effect of these variables for small
pitch defects a calibration wafer has been
developed
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Calibration Wafer
• Anodic bond
• Defects etched
into oxide
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Schematic of the device
Silicon (Die 1)
Air gap
Bonding
layer
• The thickness of the Silicon (Die 1) was
varied (200um, 400um, and 700um)
• The feature/pitch size varied from 1-
500um
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SAM image of the device
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Silicon thickness : 200um
Transducer used: UHF 2mm
7um pitch resolved
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Die thickness : 400um
Transducer used: 200MHz
4mm
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Die thickness : 400um
Transducer used: 200MHz
8mm
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Discussion
• Best pitch resolution was obtained with the thinnest
silicon layer
• Transducer frequency was the same in each case, but
the focal length had to vary due to silicon thickness
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Alignment of wafers in 3D
metrology
• Wafer alignment is typically checked
pre-bonding using built-in in optical or
other techniques.
• Post bond inspection is of interest to
ensure that the wafers are still properly
aligned before adding extra cost into
the product.
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Alignment of wafers in 3D
metrology
• Technologies such as Infrared (IR) and X-ray
technology have been used to inspect post
bond wafer alignment, but they require
additional handling and process steps
• Scanning Acoustic Microscopy (SAM) is
widely used to evaluate the integrity of
wafer bonding by detecting voids/
delamination at the bonding interface
• Wafer Alignment could be checked in the
same step reducing process steps and cost
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Alignment of wafers in 3D
metrology
∆y
∆x
∆x
∆y
•Typically a cross / box
overlay structure is used for
alignment purposes
•Post bonding of wafer, the
variation in ∆ X and ∆ y
values will determine how
well the wafers have been
aligned
∆y
∆x
∆y
• The actual X and Y values
used can vary
∆x
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Alignment of wafers in 3D
metrology
• In 3D Interconnect wafers, the
alignment features are typically
etched and filled with Copper on both
sides
• Leaving the etched features unfilled
will benefit acoustic detection
• This is due to the difference in acoustic
impedance between air and copper
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Reflection vs. Transmission: Intensity Coefficients
Greater Greater the the difference difference in in acoustic acoustic impedance impedance
at at a boundary, boundary, greater greater the the reflection. reflection.
Silicon Z 1
Copper Z 2
Incident Sound
Reflected Sound
Z 3
Air
Transmitted Sound
Z 1
= ρ C where:
ρ=2.33 gram/cm 3
C= 8.6 mm/µs
Z 1
= 2.00 g/µs cm 2
Z 2
= ρ C where:
ρ =8.6 gram/cm 3
C= 5.01 mm/µs
Z 2
= 4.31 g/µs cm 2
Z 3
= ρ C where:
ρ =0 gram/cm 3
C= 0.344 mm/µs
Z 3
= 0 g/µs cm 2
Silicon - Copper boundary
Ri
Ri
Ri
=
=
=
( Z
2
− Z
1
)
( Z + Z )
( 4 . 31
( 4 . 31
. 13
2
−
+
1
2
2
2 . 0 )
2 . 0 )
2
2
Ri
Ri
Ri
Silicon – Air boundary
=
airgap
airgap
( Z
2
− Z
1
)
( Z + Z )
2
1
( 0 −
=
( 0 +
= 1
2
2
2 . 0 )
2 . 0 )
2
2
100% refelcted
versus 13%
reflected
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Alignment of wafers in 3D
metrology
• As shown in the case of calibration
wafer, SAM can resolve line pairs up to
7um using 200 MHz 2mm transducer for
Si wafer thickness of 200um
• This indicates the viability of using SAM
for checking the alignment of wafers
post bonding to within a 7um shift
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Future Work
• Using 300 MHz transducers finer features
should be resolved allowing or finer
alignment measurements
• This will be tested with the calibration wafers
• 3D Interconnect wafers with unfilled
alignment features are being fabricated to
test the viablity of SAM to measure post
bonding alignment
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