What Is an Acoustic Microscope?
An acoustic microscope is a microscope that uses ultrasonic waves to observe the state of a sample by detecting the strength and position of the reflected waves from the sample.
Although its resolution is lower than that of an electron microscope, which is often used in industrial fields, its main feature is that it can measure and observe minute areas with higher resolution than an optical microscope.
Uses of Acoustic Microscopes
Acoustic microscopes are often used in the industrial field to inspect products. An acoustic microscope is characterized by its ability to observe not only the surface of a sample but also its interior and bottom surface by changing the depth of incidence.
Specifically, the microscope is used for internal inspections of electronic components and other general materials, checking the adhesiveness of adhesives, and detecting cracks, voids, and delamination on bonded surfaces. It is also useful for internal inspections of electronic components, etc., internal inspections of general materials, and checking the adhesiveness of adhesives. Both are used to detect cracks, voids, delamination, etc., on bonded surfaces.
Ultrasonic waves propagate while attenuating in areas where materials are continuous, but cannot propagate in non-continuous areas such as gaps, and most of the waves are reflected at boundary surfaces. Therefore, reflected waves reflected from delaminated areas and voids can be detected.
Principle of Acoustic Microscopes
An acoustic microscope utilizes the property that when ultrasonic waves are propagated between different materials at adjacent points, some of them are reflected and some are transmitted. When the reflected wave is detected and observed, the microscope is of the reflection type, and when the transmitted wave is detected and observed, the microscope is of the transmission type.
1. Transmission Type
A transmission type acoustic microscope consists of two acoustic lenses placed so that they face each other with the specimen in between. One acoustic lens has a piezoelectric element that receives ultrasonic waves, and the other acoustic lens has a piezoelectric element that receives ultrasonic waves.
During measurement, ultrasonic waves are injected through one acoustic lens, and the ultrasonic waves transmitted through the sample placed at the focal point are received by the piezoelectric element on the other acoustic lens.
2. Reflective Type
In a reflective acoustic microscope, the acoustic lens and piezoelectric element are placed only on one side. The piezoelectric element receives the incident ultrasonic wave and the reflected wave from the material. Both transmission and reflection ultrasonic microscopes can measure and observe the surface and internal conditions of a sample by analyzing the intensity and phase of the received ultrasonic waves.
Reflective acoustic microscopes are more commonly used commercially than transmissive microscopes. The main reason for this is that the reflective type does not require the sample to be sandwiched between acoustic lenses, so there is no limit to the thickness of the sample. Another major advantage of the reflective type acoustic microscope is that it is simpler to operate and easier to handle, since there is no need to place the acoustic lens in confocal, and image formation and measurement of propagation velocity are also possible.
Other Information on Acoustic Microscopes
1. Advantages of Reflective Acoustic Microscopes
The pulse reflection method, a measurement method using a reflective acoustic microscope, is a strong method for detecting exfoliation and voids inside the sample. In this method, a water-immersed sample is irradiated with ultrasonic waves of pulsed waveforms, and information about the interior of the object is obtained from the internally reflected pulses.
The intensity of this pulse reflection depends on the difference in acoustic impedance of the two materials that serve as boundary surfaces. Acoustic impedance is a quantity defined as the product of the density of the material and the speed of sound, and is almost 100% reflective of the air layer. This makes this pulse reflection method a strong method for detecting delamination and voids in the sample.
2. Cell Observation Using an Acoustic Microscope
Acoustic microscopes that can visualize the inside of cells using high frequencies such as 100 MHz to 200 MHz have been developed. The use of high frequencies during measurement has no adverse effect on cells in water, and such acoustic microscopes enable observation of cells in a living state.
Therefore, changes in intracellular structural proteins can be confirmed without dye staining, as in the past. In addition to the established technology for propagating high-frequency ultrasound through a thin fiber, the tip of the fiber is processed on a concave surface to converge the ultrasound waves, thereby increasing the azimuthal resolution to the cellular level.
Acoustic microscope produces images based on the density and viscoelasticity of intracellular substances, making it extremely suitable for observing the cytoskeleton, which controls cell proliferation and other processes. Taking advantage of this property, a technique has been developed to identify cancer cells from a cancer cell population. Using this technology, fluorescent proteins can be expressed so that normal cells can be identified and how anticancer drugs have acted on cancer cells can be confirmed.