JPH023454B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for measuring changes in the sound speed of surface acoustic waves due to thin film deposition on a fixed surface, surface treatment, surface defects, etc. This change in the sound speed of surface acoustic waves can be detected by leaky surface acoustic waves (Leaky Surface Acoustic Waves) produced by ultrasonic lenses used in ultrasonic microscopes.
A measurement method that utilizes the excitation detection effect of waves has recently been reported. This method moves the ultrasonic lens perpendicular to the sample surface,
The reflected wave amplitude V is measured as a function of the distance z between the sample surface and the lens focal point, and the sound speed υ _R of the surface acoustic wave is determined from the periodic minimum interval Δz appearing on the obtained V(z) curve. , which is attracting attention as a new non-destructive testing method because it enables the measurement of elastic properties of minute regions on solid surfaces. However, to measure the sound velocity υ _R , it is necessary to mechanically move the lens in a direction perpendicular to the sample surface using a micrometer, etc., and it takes at least several seconds to measure one point on the sample. In addition to this, mechanical accuracy is also an issue.
Therefore, there is a problem in terms of speed when measuring the sound velocity υ _R at a large number of points on a sample and examining its distribution, or when performing real-time measurement of a surface condition that changes from time to time. In order to overcome such problems, the present invention has the following features:
By using an ultrasonic lens that excites short-duration pulsed ultrasonic waves and eliminating the need for mechanical movement by simply processing electrical signals, the sound speed υ _R of leaky surface acoustic waves can be reduced very quickly. Moreover, it is designed to be able to measure with high precision. In other words, the sound velocity measurement method of the present invention radiates pulsed ultrasonic waves through an ultrasonic lens so as to converge on a focal point located within the sample, and the components that are specularly reflected back at the sample surface and the leakage surface elasticity of the sample surface are The method is characterized in that the speed of sound of the surface acoustic wave for the sample is determined based on the return time difference between the component that returns after being transmitted as a wave and the component that returns after being internally reflected by an ultrasonic lens. The method of the present invention will be explained in more detail below with reference to the drawings. In Fig. 1, reference numeral 1 denotes an ultrasonic lens similar to that used in ultrasonic microscopes, and a cylindrical part 1a made of single crystal alumina or fused silica has a concave spherical recess 1b on the tip surface. The longitudinal wave excited by the provided piezoelectric transducer 2 is refracted at a concave spherical surface facing the sample 3, so that it is converged to a focal point O. The transducer connected to the piezoelectric transducer 2 excites short-duration pulsed ultrasonic waves and receives reflected waves through the piezoelectric transducer 2. A time interval measuring device is connected to measure the time interval of pulsed reflected waves. Note that when measuring the sound speed of a surface acoustic wave at a specific location on the sample 3, it is not necessary to move the ultrasonic lens 1 in the axial direction, etc., but it is necessary to measure the sound speed υ _R at many points on the sample. In order to examine the distribution of the sample, it is desirable to provide a means for two-dimensional scanning along the sample surface. In a measuring device having such a configuration,
When pulsed ultrasonic waves are emitted from the transducer through the ultrasonic lens 1, they are separated into the following components. First, it passes through the center of the ultrasonic lens 1 from point E, passes through a liquid such as water used as coupler 4, and returns through specular reflection at point F on the surface of sample 3, thereby tracing the path of EFE-→. There is a specularly reflected wave component (hereinafter referred to as an S wave) that is traced. A component that becomes a leaky surface acoustic wave (hereinafter referred to as an L wave) passes through a portion away from the center of the ultrasonic lens 1, is refracted at point A of the concave spherical surface, passes through the coupler 4, and is refracted at the surface of the sample 3. point B is reached, where parallel to the sample surface,
Moreover, it is converted into a surface acoustic wave that propagates only near the surface of the sample, and when it reaches point C from point B through point F,
It is emitted into the coupler 4 again, and from the point D to the lens 1.
and return to transducer 2. In the present invention, the sound speed υ _R at which this L wave becomes a surface acoustic wave at BFC-→ and travels on the sample surface is determined.
This sound velocity υ _R contains information that has changed due to defects near the sample surface or surface treatment. There is also an internally reflected wave (hereinafter referred to as I wave) which is a component that is internally reflected at point E of the ultrasonic lens 1 and returns. FIG. 2 shows the time relationship in which the above-mentioned I wave, S wave, and L wave return to the piezoelectric transducer 2. The time difference between the I wave and the S wave reaching the piezoelectric transducer 2 is Δt _O , the same time difference between S wave and L wave is Δt _R , these are as follows: Δt _O =2(f−z)/υ _O …… (1) Δt _R =2z/υ _O (1−√1− _O ² _R ² )... is given by (2). However, f is the lens focal length, and υ _O is the sound speed in the liquid used as the coupler 4. Here, the distance z between the sample surface and the lens focus is
It changes depending on the unevenness of the sample surface, so if the unevenness of the sample is unknown, the sound speed υ _R cannot be determined from the time difference Δt _R using equation (2) above. Therefore, by eliminating z using equations (1) and (2), υ _R = [Δt _R /υ _O (f−υ _O Δt _O /2) − Δt _R ² /4 (f−υ _O Δt _O /2) ² 〕 ^-1/2 ...(3) That is, the sound speed υ _R can be determined only by time difference measurement, regardless of the unevenness of the sample surface. In the measurement method of the present invention, the time interval measurement does not involve mechanical movement of an ultrasonic lens, etc., and can be performed only by processing electrical signals, so it is possible to measure the sound speed υ _R in an extremely short time and with high precision. can be measured. Next, examples will be shown. In the apparatus shown in the first figure, a 30 μm thick PVDF film was used as the broadband piezoelectric transducer, and fused silica was used as the lens material. The focal length f is 4 mm, water is used as the coupler, and the sound speed is 1486 m/s. The time intervals between the S wave, I wave, and L wave were measured based on the rising time of each pulse. Measurement is SKD11 steel, soda glass, Si (111)
Table 1 shows a comparison of the measured values of the sound velocity υ _R with the literature values for the Si (100) surface, the MgO (100) surface, and the Saffire (0001) surface.

[Table] As can be seen from Table 1 above, the sound speed value is 2940m/
We were able to perform measurements within a 4% error range in the range from 5,660 m/s to 5,660 m/s.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an apparatus for implementing the method of the present invention, and FIG. 2 is a diagram showing the temporal relationship of each reflected wave. 1... Ultrasonic lens, 3... Sample.

Claims (1)

[Claims] 1 Emit pulsed ultrasonic waves through an ultrasonic lens so as to converge on a focal point located within the sample,
Based on the return time difference between the component that returns after specular reflection on the sample surface, the component that returns after propagating through the sample surface as a leaky surface acoustic wave, and the component that returns after being internally reflected by the ultrasonic lens, the surface of the sample is determined. A method for measuring the sound speed of surface acoustic waves, which is characterized by determining the sound speed of elastic waves.