Using the interaction between waves and the interface is a non-destructive approach. But what kind of waves should be used? Electromagnetic waves in the visible range depend on wave attenuation in the material, infrared waves also depend on the thickness and X-ray waves have a too short a wave length to detect interface defects. In this article, the use of acoustic waves is proposed for non-destructive examination of molecular assembly interfaces.
Acoustic wave propagation is very sensitive to variations in interface characteristics depending on whether the waves are reflected or transmitted. To improve the sensitivity and resolution of this technique, small wave lengths have been used with a scanning acoustic microscope (S.A.M.) with a band width from 1MHz to 400 MHz. After a short description of the principle of the method, results are given for different types of components. Different applications of acoustic microscopy are proposed for non-destructive examination of interfaces and defect detection in materials.
SCANNING ACOUSTIC MICROSCOPY (S.A.M.) (1)
In this technique a piezoelectric transducer emitting and receiving ultrasonic energy pulses scans across the surface of a component immersed in a water bath. The ultrasonic sound beam is focused using lenses (hence the term “microscopy”) at different levels within the component in order to image a particular layer. The resulting image is referred to as a C-Scan. Scanning acoustic microscopes have two distincts operating modes from which images can be generated.
The reflective mode or pulse echo mode is a single-sided technique using one transducer to send a pulse out and receive the return echo from the component being tested. Amplitude, phase change and depth information are extracted from the echo for imaging the part non-destructively.
Through-transmission is a dual-sided technique which uses two transducers, one to send the ultrasonic pulse and the other transducer to receive it. Typically only amplitude, or the amount of sound transmitted, is measured in order to create an image of the component. Although it is similar to X-ray examination in concept, the capability of this test is very different.
Using sound to penetrate materials and to characterise the interface is not commonplace (2). The following equation illustrates this with: where Zi = ρivi with ρi density and Vi wave velocity.
This equation is always used in the acoustic field when discussing the ability of sound to travel into the device and return to the receiver. R is the reflected pressure at zero angle of incidence, Z1 is the acoustic impedance of the material the sound is leaving and Z2 is the acoustic impedance of the material the sound is entering. In most applications relating to the microelectronics industry, the user is looking at the bond integrity between different materials. In case of heterogeneities at the interface of the molecular assembly the difference between Z1 and Z2 will change and the heterogeneity will thus be detected.
Acoustic tomography can be performed (3). The depth resolution depends on wave length.
CHARACTERISTICS OF THE EQUIPMENT
Scanning area : Adaptable
Linear Encoder resolution : 1μm
Step of scanning : 1μm mini
Band width : up to 400 MhZ
Analogic / Digital conversion rate : 1 GhZ (1ns step)
Reference of equipment : SONIX S.A.M. – UHR 2000
EXAMPLES OF RESULTS
3.1. Specimens diameters : 50 mm
Total thickness : 20 mm
Material : optical glace
Optical examination : No defect have been detected
Similar results are obtained with SiC materials in which porosities or inclusions resulting from the sintering technique have been detected. In such material, it is not possible to make an optical examination of the interfaces.
Different information can be used for image representation: the reflection amplitude, Fig.4a, or the travel time from the surface, Fig.4b, which can be used to distinguish between different kinds of defects in relation to their position at the interface.
3.2. Silicon / Silicon interface
Specimen diameter : 200 mm
Total thickness : 2 mm
Materials : Si
Results are given in Fig.5: Small defects of 15 μm width at the interface have been detected. Regardless of their size, these defects cannot be detected by infrared techniques.
S.A.M. APPLICATION FOR MOLECULAR ASSEMBLY
➢ Detection of variations in acoustic reflection at the interface
➢ Detection and selection of different kinds of defects
➢ Defects in sintered materials such as SiC
➢ Interface parallelism and thickness
➢ Multi-interface acoustic tomography
➢ Thickness measurement
➢ Kinetics of interface deterioration under mechanical or thermal stress
A non-destructive technique, acoustic microscopy, has been used for the examination of molecular assembly interfaces.
Defects have been detected in opaque materials such as SiC. Small defects of 15 μm width can be detected in Si/Si assemblies that other techniques cannot detect.
Scanning Acoustic microscopy (S.A.M.) can be used for process qualification, quality control, correlation between mechanical results and defects. The kinetics of defect propagation at the interface could be archived