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

3

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

5

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.

7

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.

9

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

11

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

13

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

15

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

17

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

19

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

21

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 saminfo@pvatepla.com Internet www.pva-analyticalsystems.com

AN e V 1 2011