SAM Functional principle (Scanning Acoustic Microscope)
An Acoustic Microscope (Ultrasonic Microscope) is based on the same working principle as the well known medical ultrasonic examinations: A probe is moved (within a coupling fluid)across the area of interest. An image comes up showing the interior without violating the "object" under investigation.
The heart of the Acoustic Microscope is the probe (=Transducer; loudspeaker and microphone in one piece). It converts the electrical signal into the acoustic signal. The sound waves are focussed and transmitted to the sample by the coupling fluid (normally water). The ultrasonic waves interact with the sample; one part is reflected back to the transducer, the other part is transmitted.
Basically there are 2 methods of ultrasonic imaging: In the majority of cases the "Pulse-Echo"-Mode is used. Amplitude, phase and time of flight of the reflected soundwave are analysied to create the pixel-by-pixel image information. This mode operates with one transducer.
The couterpart of this mode is the "Transmission Mode" (=Throgh Scan Mode). In this process a second transducer underneath the sample receives the transmitted part of the soundwaves. This transmitted signal is the base for the acoustiv throgh scan image.
At both methodes (Puls-Echo- and Transmission- Mode) the inspected sample is scanned pixel by pixel and line by line. The movement of the transducer is realised by a x-y-mechanic, the so called Scanner.
The time of a sample inspection is depending on the size of the sample and the chosen resolution and can last a few seconds till several minutes.
The features of the transducer (frequence and focal length)must be aligned with the application.
For this reason there are a huge number of transducers; comparable to the multiplicity of objects used for high quality cameras.
To choose the right Transducer some things have to be regarded: Transducers with high frequence and short focal length produce a high resolution in comparison with the penetration depth.
The choice of the correct Transducer for the respective application is very important. An agreement between favoured resolution and penetration depth have to be found.
How are contrasts created in an acoustic image?
Ultra sonic waves need instead of light a propagation medium. The higher the mediums density the better sonic waves can expand. This can also be recognized in everyday life: in air (comparatevely "light" medium) ultrasonic waves can expand slow and not so far. For example: loud music in a building can be heard only 2-3 rooms further. Dense materials carried sound much better. For example: metal pipes carry sound excellent; the heaters at the end of a pipe attend to "loud speakers".
Sonic acoustic microscopy make use of the physical circumstances. After leaving the transducer the ultrasonic waves are transported by the couple medium (generaly water) to the sample. Inside the sample it strikes a lot of different materials and fringe areas that affect the amplitude, the phase and the elapsed time of the ultrasonic signal. The evaluation unit of the sonic acoustic microscope recognized these partly lowest changes and creates an image out of this inforamtions.
In summary: ultrasonic waves react very sensitive to changes and inhomogeneity of the medium it moves in. Primarily differences in density and elasticity are mentioned.
Patented based FCT Transducer by KSI
Due to the Transducers new, optimized design turbulences (blisters) and wave formation in the coupling medium ars reduced. This is necessary, because the maximum scan speed of the new scanning acoustic microscope v-400 rises from 1 m/s up to 2.0 m/s.
SAM Scanmodes
With scanning acoustic microscopes the whole volume of a sample can be penetrated and inspected without any change or damage. To get all information needed and a detailed understanding of the inspected sample, different scanmodes are used. The following pictures and examples explain the scanmodes and their specific advantages.
A - Scan
"A" means Amplitude-Scan. The A-Scan shows the reflected ultrasonic echo over the time. The information is contained in the way the acoustic wave is reflected from the specimen. Time of flight ("TOF"), Amplitude and Phase of the reflected echo are the base for the acoustic image. Only certain parts of the echo signal are image-relevant. They can be selected by setting gates within the A-Scan histogram. Position and length of these gates define depth and thickness of the investigated layers.
B - Scan
The scanner is moving once in x-direction to get a cross section image in x-direction. The position of the B-Scan can be defined by software.
C - Scan
The C-Scan records an image of layer parallel to the surface. Depth and thickness of the layer can be adjusted as required. The scanner is moved in a meander pattern across the sample in acquire to collect the image date pixel by pixel, line by line.
D - Scan
D-Scan (Diagonal-Scan)is a combination of the B-Scan and C-Scan functions. A meander scan is carried out and the position of the gate is altered at the same time. The resulting image represents a diagonal section through the sample.
G - Scan
The G-Scan is able to record up to 32 C-scan images at the same time. Different settings can be stored for automatic evaluation.
P - Scan
The P-Scan records several B-Scans automatically. Number and resolution lead back to the prevoius taken C-Scan.
TT - Scan
TT-Scan ("Transmission"- or "Through"-Scan) is an optional Scan-mode. Additional hardware - a second Transducer - is necessary to perform a Through-Scan. The transducer above the sample emits an ultrasonic signal that is detected by a second transducer placed underneath the sample. This image mode provides lower resolution but a fast and easy evaluation of delaminations.
Tray - Scan
This scan is a special mode for the analysis of IC-samples that are placed on a JEDEC-tray. Dimensions, size and number of the ICs have can be edited and saved. After that all devices are scanned one after the other automatically.
W - Scan
W-Scan stands for Wafer-Scan and is a combination of a G-Scan and a Tray-Scan adapted to wafers. This application comes with a customer-specific sample stage, automatic image analysing software and special scanning software.
S - Scan
S-Scan stands for simultaneous- scan. During one scan a C-Scan and a TT-Scan is carried out to reduce scantime.
G - Tray - Scan
G-Tray-Scan is a scanmode for the analysis of a number of samples that are fixed to JEDEC-trays in a defined distance (see also "Tray-Scan"). With the G-Tray-Scan up to 32 images of each sample can be generated at the same time.
X - Scan
The X-Scan gives you a fast overview over the inner structure of the sample. With one click several images are created - beginning at the surface going down more and more layer by layer.
Z - Scan
Z-Scan - Automated volume acquisition which enables an offline reconstruction of B-, C-, D-, P-,X-, A-, 3D-Scans and time of flight images with free selectable gate windows.
Scientific Applications
V(z)-Curve-Rayleigh wave lens
V(z)-Curves are produced with the help of special transducers that create not only longitudinal-waves but also Rayleigh-waves. V(z)-Curves contain all the material parameters determinable with the help of the acoustic imaging. To produce a V(z)-Curve the focal point is software-controlled displaced from the surface into the sample, the intensity of the reflected ultrasonic signals is measured (V=voltage) and plotted against the distance the acoustic lens is lowered (z in µm).
The result is a frequency dependent curve of the material or layer combination, characteristic for the sample investigated: V(z)-Curves allow a completely new, non-destructive type of material characterisation in the micron range as they are the key to a wide range of quantitative measurements. V(z)-Curves are influenced by varieties in film thickness and reveal defect structures under the surface.
