11TM
Scanning Acoustic Microscopy For Metrology of 3D Interconnect Bonded Wafers
Jim McKeon, Ph.D. - Sonix™, Director of TechnologySriram Gopalan, Ph.D. - Sonix™, Technology Engineer
8700 8700 MorrissetteMorrissette DriveDrive
Springfield, VA 22152Springfield, VA 22152
teltel: 703: 703--440440--02220222
fax: 703fax: 703--440440--95129512
ee--mail: info@mail: [email protected]
8700 8700 MorrissetteMorrissette DriveDrive
Springfield, VA 22152Springfield, VA 22152
teltel: 703: 703--440440--02220222
fax: 703fax: 703--440440--95129512
ee--mail: info@mail: [email protected]
22TM
Ultrasonic Inspection of 3D ArchitecturesUltrasonic Inspection of 3D Architectures
•• 3D architectures present challenges to 3D architectures present challenges to conventional processing and inspection toolsconventional processing and inspection tools
•• Ultrasonic inspection is inherently a 3D technologyUltrasonic inspection is inherently a 3D technology
•• Ultrasound provides defect detection, metrology, Ultrasound provides defect detection, metrology, and monitoring for process controland monitoring for process control
33TM
Ultrasonic InspectionUltrasonic Inspection
Ultrasound
• A transducer produces a high frequency sound wave which interacts with the sample.
•1 – 300 MHz
• High frequency sound waves can not propagate through air.
• Couplant- A material used to carry the high frequency sound waves.
•Water is the most common couplant
Ultrasound
• A transducer produces a high frequency sound wave which interacts with the sample.
•1 – 300 MHz
• High frequency sound waves can not propagate through air.
• Couplant- A material used to carry the high frequency sound waves.
•Water is the most common couplant
Pulse Echo Inspection•A change in acoustic impedance (Z) results in some sound reflected and some transmitted
•Air has Z = 0
Pulse Echo Inspection•A change in acoustic impedance (Z) results in some sound reflected and some transmitted
•Air has Z = 0
Transducer
Si Wafer 1
Si Wafer 2
Glass wafer
H2O
Couplant
44TM
SAM CapabilitiesSAM Capabilities
Bonded wafer
Si Wafer 1
Si Wafer 2
MEMS pressure sensor •Voiding in wafer bonding
•Bond delaminations for hermetic seal applications (MEMS)
•Metrology for wafer pair alignment post bonding
Glass Wafer
Si Wafer
Alignment of bonded wafer pair
Si Wafer 1
Si Wafer 2
55TM
Example ImagesExample Images
MEMS Inspection Bonded Wafer Voiding
66TM
AA--ScanScanInitial Pulse
Front surface
Interface of interest
Back surface
Si Wafer 1
Si Wafer 2
Transducer
The raw ultrasonic data. It is the received RF signal from a single point (x,y).
77TM
CC--ScanScan
Data from a specified depth over the entire scan area. (Horizontal cross-section).
88TM
Determining Void Size Distribution: Determining Void Size Distribution: Cluster AnalysisCluster Analysis
•Cluster Analysis determines void size distribution based on user-defined amplitude and size criteria•Void count and percentage of void area are indicated
99TM
Process ValidationProcess ValidationOriginal process Modified process
1.086% defect area11.664% defect area
1010TM
MetrologyMetrology
Via measurement - spatial
1111TM
MetrologyMetrology
Via measurement - depth
Cu velocity = 4.66 mm/µs
1212TM
MetrologyMetrology
Trench depth measurement
1313TM
MetrologyMetrology
Wafer Thickness Measurement
Si velocity = 8.6 mm/µs
1414TM
MetrologyMetrology
Wafer Thickness Measurement
1515TM
Defect detection Defect detection
•• Air Gap thicknessAir Gap thickness
••Confirmed detection of 10 nm air gaps with UHF Confirmed detection of 10 nm air gaps with UHF and greater frequenciesand greater frequencies
••Confirmed detection of 50 nm air gaps with Confirmed detection of 50 nm air gaps with 25MHz and greater25MHz and greater
••Smaller air gap thickness may also be detectableSmaller air gap thickness may also be detectable
•• Spatial Detection Spatial Detection
••Defects with diameter of few microns detectableDefects with diameter of few microns detectable
••Resolution of closely spaced defects depends on Resolution of closely spaced defects depends on the sample type and the transducer usedthe sample type and the transducer used
1616TM
Closely spaced defects Closely spaced defects ••When the pitch of the defects/ features approach When the pitch of the defects/ features approach the ultrasonic beam width, the resolution of the the ultrasonic beam width, the resolution of the defects/features becomes challengingdefects/features becomes challenging
••The variables affecting the beam width include: The variables affecting the beam width include: ••the bonded wafer materialsthe bonded wafer materials
••thickness of the layersthickness of the layers
••Transducer selectedTransducer selected
••To evaluate the effect of these variables for small To evaluate the effect of these variables for small pitch defects a calibration wafer has been pitch defects a calibration wafer has been developeddeveloped
1717TM
Calibration WaferCalibration Wafer
•• Anodic bondAnodic bond•• Defects etched Defects etched
into oxideinto oxide
1818TM
Schematic of the deviceSchematic of the deviceSilicon (Die 1)
• The thickness of the Silicon (Die 1) was varied (200um, 400um, and 700um)
• The feature/pitch size varied from 1-500um
Air gapBonding layer
1919TM
SAM image of the deviceSAM image of the device
2020TM
Silicon thickness : 200umSilicon thickness : 200umTransducer used: UHF 2mmTransducer used: UHF 2mm
7um pitch resolved
2121TM
Die thickness : 400umDie thickness : 400umTransducer used: 200MHz Transducer used: 200MHz
4mm4mm
2222TM
Die thickness : 400umDie thickness : 400umTransducer used: 200MHz Transducer used: 200MHz
8mm8mm
2323TM
DiscussionDiscussion
•• Best pitch resolution was obtained with the thinnest Best pitch resolution was obtained with the thinnest silicon layersilicon layer
•• Transducer frequency was the same in each case, but Transducer frequency was the same in each case, but the focal length had to vary due to silicon thicknessthe focal length had to vary due to silicon thickness
2424TM
Alignment of wafers in 3D Alignment of wafers in 3D metrologymetrology•• Wafer alignment is typically checked Wafer alignment is typically checked
prepre--bonding using builtbonding using built--in optical or in optical or other techniques. other techniques.
•• Post bond inspection is of interest to Post bond inspection is of interest to ensure that the wafers are still properly ensure that the wafers are still properly aligned before adding extra cost into aligned before adding extra cost into the product. the product.
2525TM
Alignment of wafers in 3D Alignment of wafers in 3D metrologymetrology•• Technologies such as Infrared (IR) and Technologies such as Infrared (IR) and XX--ray ray
technologytechnology have been used to inspect post have been used to inspect post bond wafer alignment, but they require bond wafer alignment, but they require additional handling and process stepsadditional handling and process steps
•• Scanning Acoustic Microscopy (SAM) is Scanning Acoustic Microscopy (SAM) is widely used to evaluate the integrity of widely used to evaluate the integrity of wafer bonding by detecting voids/ wafer bonding by detecting voids/ delamination at the bonding interfacedelamination at the bonding interface
•• Wafer Alignment could be checked in the Wafer Alignment could be checked in the same step reducing process steps and costsame step reducing process steps and cost
2626TM
Alignment of wafers in 3D Alignment of wafers in 3D metrologymetrology
∆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
• The actual X and Y values used can vary
∆x
∆y
∆x ∆x
∆y ∆y
2727TM
Alignment of wafers in 3D Alignment of wafers in 3D metrologymetrology•• In 3D Interconnect wafers, the In 3D Interconnect wafers, the
alignment features are typically alignment features are typically etched and filled with Copper on both etched and filled with Copper on both sidessides
•• Leaving the etched features unfilled Leaving the etched features unfilled will benefit acoustic detectionwill benefit acoustic detection
•• This is due to the difference in acoustic This is due to the difference in acoustic impedance between air and copperimpedance between air and copper
2828TM
Reflection vs. Transmission: Intensity CoefficientsReflection vs. Transmission: Intensity CoefficientsIncident Sound
Transmitted Sound
Silicon Z1
Copper Z2
Reflected SoundGreater the difference in acoustic impedance at a boundary, greater the reflection.
Greater the difference in acoustic impedance at a boundary, greater the reflection.
Z3
Air
Z1 = ρ C where:ρ=2.33 gram/cm3
C= 8.6 mm/µsZ1 = 2.00 g/µs cm2
Z2 = ρ C where:ρ =8.6 gram/cm3
C= 5.01 mm/µsZ2 = 4.31 g/µs cm2
Z3 = ρ C where:ρ =0 gram/cm3
C= 0.344 mm/µsZ3 = 0 g/µs cm2
Silicon - Copper boundary Silicon – Air boundary100% refelcted versus 13% reflected
( )( )
1)0.20()0.20(
2
2
212
212
=+−
=
+−
=
airgap
airgap
Ri
Ri
ZZZZRi( )
( )
13.)0.231.4()0.231.4(
2
2
212
212
=+−
=
+−
=
Ri
Ri
ZZZZRi
2929TM
Alignment of wafers in 3D Alignment of wafers in 3D metrologymetrology•• As shown in the case of calibration As shown in the case of calibration
wafer, SAM can resolve line pairs up to wafer, SAM can resolve line pairs up to 7um using 200 MHz 2mm transducer for 7um using 200 MHz 2mm transducer for Si wafer thickness of 200umSi wafer thickness of 200um
•• This indicates the viability of using SAM This indicates the viability of using SAM for checking the alignment of wafers for checking the alignment of wafers post bonding to within a 7um shiftpost bonding to within a 7um shift
3030TM
Future WorkFuture Work
•• Using 300 MHz transducers finer features Using 300 MHz transducers finer features should be resolved allowing or finer should be resolved allowing or finer alignment measurementsalignment measurements
•• This will be tested with the calibration wafersThis will be tested with the calibration wafers•• 3D Interconnect wafers with unfilled 3D Interconnect wafers with unfilled
alignment features are being fabricated to alignment features are being fabricated to test the test the viablity viablity of SAM to measure post of SAM to measure post bonding alignment bonding alignment
3131TM