Scanning Acoustic Microscopy For Metrology of 3D Interconnect Bonded Wafers

Transcription

1 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 Springfield, VA tel: : tel: : fax: fax: info@sonix sonix.com info@sonix sonix.com 1

2 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 2

3 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 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 3

4 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 4

5 Example Images MEMS Inspection Bonded Wafer Voiding 5

6 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). 6

7 C-Scan Data from a specified depth over the entire scan area. (Horizontal cross-section). 7

8 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 8

9 Process Validation Original process Modified process % defect area 1.086% defect area 9

10 Metrology Via measurement - spatial 10

11 Metrology Via measurement - depth Cu velocity = 4.66 mm/µs 11

12 Metrology Trench depth measurement 12

13 Metrology Wafer Thickness Measurement Si velocity = 8.6 mm/µs 13

14 Metrology Wafer Thickness Measurement 14

15 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 15

16 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 16

17 Calibration Wafer Anodic bond Defects etched into oxide 17

18 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 18

19 SAM image of the device 19

20 Silicon thickness : 200um Transducer used: UHF 2mm 7um pitch resolved 20

21 Die thickness : 400um Transducer used: 200MHz 4mm 21

22 Die thickness : 400um Transducer used: 200MHz 8mm 22

23 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 23

24 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. 24

25 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 25

26 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 26

27 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 27

28 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= mm/µs Z 3 = 0 g/µs cm 2 Silicon - Copper boundary Ri Ri Ri = = = ( Z 2 Z 1 ) ( Z + Z ) ( ( ) 2. 0 ) 2 2 Ri Ri Ri Silicon Air boundary = airgap airgap ( Z 2 Z 1 ) ( Z + Z ) 2 1 ( 0 = ( 0 + = ) 2. 0 ) % refelcted versus 13% reflected 28

29 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 29

30 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 30

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