Analytical Systems
Application Note
Scanning Acoustic Microscopy
2 The World of Scanning Acoustic Microscopy
SAM Operation Principle
Scanning Acoustic Microscopy (SAM) utilizes ultrasound to non-destructively examine surfaces and internal structures of a solid sample. The resulting acoustic image is used to analyze features such as cracks, delaminations, and voids.
The Scanning Acoustic Microscope operates with the pulse reflection method. The special acoustic objective-centerpiece of the microscope, produces, transmits and receives short sound pulses of high penetration rate. The acoustic lens converts high frequency electromagnetic oscillations into a mechanical wave which are propagated as a plane parallel wave field inside the lens. The cavity focuses the sound field on the sample through the coupling medium (water, IPO, inert liquid). The acoustic lens receives the sound pulses reflected from the sample. The transducer transforms the sound pulses into electromagnetic pulses which are displayed as pixels with defined gray values. To produce an image the acoustic objective scans the sample line by line. A transducer with good focusing properties on axis can be used for both transmitting and receiving the signal. The image is formed by scanning the transducer mechanically over the sample.
Transducer
Crack
UltrasoundPath in
ReturnedEcho
Interface
Particles,inclusions,precipitations
Voids,bubbles,holes
Specimen
Delamination, adhesion
artefact
Grain boundary structures,
texturesAco
ustic
Imaging and Analysis
Reception of sound pulses from the sampleTransmission of the sound pulses into the sample
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Frequency ranges with the attainable imaging resolution and corresponding applications
10 MHz 30 MHz 100 MHz 2000 MHz1000 MHz
0.15 mm 50 µm 15 µm > 1 µm 0.2 µm
Quality Control
Grains
Material AnalysisSemiconductorSemiconductor Material Research
Wafer FibersPackagesWelding Structure
Low transducer frequencies have a higher penetration depth but less image resolution while higher frequencies have improved image resolution but less penetration depth. This factor must be taken into utmost consideration during analysis depending on the goal of the investigation. The most suitable frequency must be used to attain the desired resolution at a particular area of interest.
Attainable imaging resolution depending on the working frequency of the transducer
PVA TePla Analytical Systems acoustic microscopes provide the largest frequency range for non-destructive ultrasonic testing. For routine use 2000 MHz is the highest applicable frequency, which offers a resolution of about 0,3 µm
Specimen: Bone. By courtesy of Dr. Raum, University of Halle, Germany
25 MHz ( l ≈ 60 µm) 50 MHz ( l ≈ 30 µm) 100 MHz ( l ≈ 15 µm)
Osteocyte
200 MHz ( l ≈ 7,5 µm) 400 MHz ( l ≈ 3,7 µm) 900 MHz ( l ≈ 1,6 µm)
Haversian channel
Lamellae
Application Note Scanning Acoustic Microscopy4
High resolution transducer up to 2000 MHz
The frequency of 2000 MHz is the worldwide highest practical frequency supplied by PVA TePla Analytical Systems GmbH for acoustic imaging and analysis, which offers resolution values down to 0,3 µm. PVA TePla Analytical Systems GmbH provides transducers from 800 - 2000 MHz for different high resolution applications. There are many situations where for thin coating layers the measurement of the elastic properties of the layer structure could be important to understand the properties of coating processes. Even more desirable in some applications is knowledge of stresses in the layer. It is now possible to perform measurements by quantitative acoustic microscopy that will yield this kind of information. Accurate elastic measurements by acoustic microscopy were pioneered using cylindrical lenses which brought the acoustic beam to a focus along a line. This means that Rayleigh waves were excited in the surface of the material in one azimuthal direction in the surface only, so that even if the sample was anisotropic only one Rayleigh velocity would be involved. Line-focus-beam lenses give particularly well defined oscillations in the variation of signal with defocus - the so-called V(z) curve - and by suitable Fourier analysis surface wave velocities can be determined from the period of the oscillations with an accuracy which can exceed 0.1 %. The attenuation of surface waves can also be measured.
5-200 MHz transducer f
100-400 MHz acoustic objectives f
800-2000 MHz ultra high frequency acoustic objectives f
Investigation methods for bonded areas
Technology Highest lateral resolution
Highest Z resolution Investigation time (wafer)
Inspection area
IR >250 µm 250 µm 3 min whole wafer
X-ray Topography >10 µm >2 µm several hours whole wafer
SAM >5 µm >5 nm 2-8 min whole wafer
Magic Mirror >1 mm 0.25 mm 3 min whole wafer
Interfacing etching >20 µm >20 µm several hours whole wafer
TEM > 1 nm correspond to foil thickness (>50 nm) several weeks 1x1 mm2
Semiconductor wafer bonding, science and technology, Q.Y. Tong, U. Goesele, the electrochemical science series
Comparison of bonded wafer analysis
Optical SAM
IR X-ray CT
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6 Application Note Scanning Acoustic Microscopy
Specimen
Scanning modes
Scanning acoustic imaging basically consists of pulse echo or transmission (or through scan) analysis.
Echo mode (or pulse echo) – A single transducer emits and receives pulse of ultrasonic energy. This allows the examination on a particular depth of the sample. Flawed areas appear as bright areas while well-bonded regions appear as dark.
Transmission mode (or through scan) – Two transducers are involved. One transducer emits and another receives the pulse of ultrasonic energy after it passed through a tested sample. This type of analysis offers an overview inspection of the sample. Typically, this is used for a “Go / No Go” testing. Since the ultrasound waves cannot pass through air (delamination, crack or void), the obstruction of ultrasound signal is observed as black areas in a through scan image.
In SAM, a sample can be non-destructively analyzed using various scanning modes depending on the particular area of interest and objective of the test. Surface, volume, or depth investigations can be performed.
The available scanning modes are discussed using the same tested specimen (in this case, an integrated circuit or IC).
Reference optical Image of the tested Specimen Schematic cross-section diagramm
A scan
The A scan is a real-time waveform display of acoustic signals based on the reflected ultrasound echoes from an analyzed specimen. It shows the quantitative time-of-flight information or time dependent component, which corresponds to depth, of the reflected ultrasonic wave. Electronic gate positions and other parameters are set with reference to the A-scan.
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C scan
The C scan provides a two dimensional acoustic image of the reflected ultrasonic echoes from a focused X-Y plane or specific planar depth of a tested specimen. Delaminated areas, as indicated by phase inversion in the corresponding A scan, can be displayed automatically in red for ease of visual identification.
X
Z
Y
B scan
The B scan generates a cross-sectional image of a tested specimen in the X-Z plane. This provides information of the various interfaces along the depth. B scan images are usually based on the C scan.
X
Z
Y
C scan image showing the B scan location
Application Note Scanning Acoustic Microscopy8
D scan
D scan generates a diagonal section of a tested specimen. It is a combination of B scan and C scan.
X scan
X scan provides simultaneously scan of equally spaced slices through the depth of a tested sample. It is usually used to acquire an overview of the layers of an unknown sample.
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G scan
G scan produces multiple C scan images at a single scan. Simultaneous scans with varying parameters or settings can be achieved.
P scan
P scan creates cascading cross-sections along the X-Z plane of a tested specimen. It is similar to segmentally recording multiple B scans.
X
Z
Y
Application Note Scanning Acoustic Microscopy10
Transducer 1
Transducer 2
S scan
This is a simultaneous C scan and Through scan imaging. An overview of the internal structure of a tested specimen is easily obtained.
X
Z
Y
+
Z scan
The Z scan automatically acquires the acoustic volume information of a tested sample for any further virtual scanning. It enables offline reconstruction of A, B, C, D, P, and X scans. This is typically used if the availability of a specimen for testing is limited.
Z scan of an other IC sample
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T scan
The T scan or tray scan enables automatic evaluation of a series of equispaced and identical specimen in succession. A specimen tray or holder is typically needed.
G-Tray scan
The G Tray scan is a combination of T scan and G scan. A series of equispaced and identical specimen is tested in succession with several set of parameters, which results to multiple acoustic images, per scan.
Application Note Scanning Acoustic Microscopy12
The SAM Product Line
Product Frequency range Application
AM 300 5 to 250 MHz Industrial and research
SAM 300 SAM 300 TWIN SAM 300 QUAD
5 to 400 MHzDedicated research, quality control and process control
SAM 400 SAM 400 TWIN SAM 400 QUAD
5 to 400 MHz High throughput applications
Product Frequency range Application
SAM 1000 SAM 2000
100 to 1000 MHz 100 to 2000 MHz
High-end research and industrial
SAM 1000 inverted SAM 2000 inverted
100 to 1000 MHz 100 to 2000 MHz
High-end research and industrial
The Premium SAM Product Line
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Product Frequency range Application
SAM 300 AUTO WAFER SINGLE / TWIN / QUAD
5 to 400 MHz
"Inline" production control
The SAM Wafer Automation Product Line
Product Application
AUTO INGOT Volume inspection of monocrystalline ingots
The Product Line for Quality Control of Ingots
14 SAM Applications, Semiconductor packages
Optical Image Acoustic Image
Dual inline package Misalignment of the die is clearly visible. Transducer: 30 MHz
Transistor outline package
Acoustic image shows severe solder die-attach delamination. Remaining well-bonded areas between the lead frame and die backside (represented as the green regions within the die) were localized at the center of the die. Transducer: 50 MHz
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Optical Image Acoustic Image
Shrink small outline package Mold to leadframe delamination was automatically displayed in red. Measurements showed 4.65% delaminated area. Transducer: 30 MHz
Shrink small outline package
Shrink small outline package
Transducer: 30 MHz
Acoustic 3D image clearly shows the mold-leadframe and mold-die delamination as represented by the blue areas. Transducer: 30 MHz
16 SAM Applications, bonded wafers
Polymer bonded wafers. Acoustic images were taken using 175 MHz transducer. Pinholes and disbonds or delamination at the interface of the 2 polymer bonded wafers are displayed as white areas.
Acoustic image was taken using 150 MHz transducer. Cracks can easily be identified in the acoustic image as distinct black lines.
Si-Si molecular bonding
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Resolution test, taken at 175 MHz/8mm in 800µm depth bonded interface with Hilbert filter
Wafer bond delaminations. Image taken at 110 MHz
18 SAM Applications, bonded wafers
Crack detection in bonding area
Si-Si molecular bonded interface: Rayleigh wave mode
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Other examples of defects in bonding interfaces
Acoustic image taken at 175 MHz shows delamination caused by impurities or contaminants in the wafer interface.
Top surface
Bottom surface
Acoustic images taken at 175 MHz show delamination within the wafer.
20 SAM Applications, Solar wafer / cell
Optical Image Acoustic Image
Solar cell Small internal crack was easily detected in the acoustic image wherein cracks are distinctively shown as clear black lines. Such a crack was not evident in the optical image even at higher magnification. Transducer: 100 MHz
Solar wafer
Acoustic image shows the polycrystalline structure of the solar cell. Transducer: 100 MHz
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Acoustic image of solar wafer cracks.
Acoustic image of Solar Cell Busbar delamination (Transducer: 100 MHz)
22 SAM Applications, Ingot inspection (Pinholes)
Acoustic images of a Si ingot: Pinholes inside the Si - ingot, 3-D plots
23SAM Applications, Material and Biology Sciece
Corrosion in 2-component polyurethane varnish and primer sealed stainless steel. Corrosion spreads from the induced stratch. Transducer: 100 MHz
A 1 GHz-transducer with a focal length of 80 µm was used for the cell investigations. The cell images were recorded at different defocus positions along the V(z) curve.
1000
PVA TePla Analytical Systems global presence
Headquarter of PVA TePla AG in Wettenberg, Germany Company Building in Aalen, Germany
PVA TePla Analytical Systems GmbH Deutschordenstrasse 38 D-73463 Westhausen, Germany
Phone +49 (0) 7363 9544 0 Fax +49 (0) 7363 9544 113 E-Mail [email protected] Internet www.pva-analyticalsystems.com
AN e V 1 2011