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: info@sonixsonix.com.com

8700 8700 MorrissetteMorrissette DriveDrive

Springfield, VA 22152Springfield, VA 22152

teltel: 703: 703--440440--02220222

fax: 703fax: 703--440440--95129512

ee--mail: info@mail: info@sonixsonix.com.com

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