JPH0658222B2 - Ultrasonic microscope lens - Google Patents

Description

TECHNICAL FIELD The present invention relates to an ultrasonic microscope lens suitable for measuring the thickness of a film formed on a substrate.

2. Description of the Related Art An ultrasonic microscope developed in recent years applies high-frequency electric pulses to a piezoelectric body to generate ultrasonic waves, converges the ultrasonic waves with an acoustic lens, and irradiates the surface of the sample with the acoustic lens or the XY sample. By scanning, the intensity or phase of the reflected ultrasonic wave from the sample surface or the transmitted ultrasonic wave transmitted through the sample at that time is received by the acoustic lens and the piezoelectric body and recorded in synchronization with the XY scanning. The elastic information immediately below the surface of the sample is obtained as an image.

Here, the role of the acoustic lens is to focus the ultrasonic beam generated by the piezoelectric body on a minute region on the sample surface to improve the resolution of the ultrasonic microscope image. As an acoustic lens used for this purpose, there is typically one in which a piezoelectric body is adhered to one end surface of a cylinder such as sapphire or quartz glass and a concave spherical surface is provided on the other end surface. The ultrasonic wave generated by the piezoelectric body travels in the cylinder as a plane wave, and when it reaches a concave spherical surface, it is refracted there and converges on a minute area on the sample surface. In addition, a concave spherical surface is provided on one end surface of a similar cylinder, and a piezoelectric body is further adhered along the surface to form a concave spherical piezoelectric body, which is used to directly generate a convergent spherical wave. Microscope lenses have also been announced. All of these aims at converging the ultrasonic beam to a minute area on the sample surface to obtain high resolution. However, in the so-called V (Z) curve method, which is one of the measuring methods using an ultrasonic microscope. Is also used. This is because the distance between the sample surface and the acoustic lens is changed while the sample or the acoustic lens is fixed at a fixed position on the sample surface without XY scanning, and the intensity change of the reflected ultrasonic wave from the sample surface at that time is changed. When recording, it is observed that the intensity changes periodically with a period peculiar to the material of the sample, so it is possible to measure the elastic characteristics of the sample surface and immediately below the surface from the recording period obtained by the same method. is there. This periodic change in the reflection intensity is caused by the interference between the vertically incident component and the so-called Rayleigh angle incident component on the sample surface of the converged ultrasonic wave generated from the concave spherical lens. It depends on the speed of sound of nearby Rayleigh waves.

As described above, most acoustic lenses for ultrasonic microscopes that have been conventionally used are characterized in that spherical focused ultrasonic waves are generated and focused on the sample surface.

On the other hand, as what is known as an ultrasonic microscope lens for generating ultrasonic waves other than spherically focused ultrasonic waves, Japanese Patent Laid-Open No. Sho 5 (1999)
The ultrasonic microscope lens described in JP-A 8-166258 can be used. This is done by setting a conical recess at the tip of a cylinder made of single crystal alumina or fused silica, and setting the conical surface so that the refracted convergent beam is focused on the line segment on the center line of the cone. On the other hand, the ultrasonic microscope lens is characterized in that a piezoelectric thin film is provided on the base end face facing the front end face of the cylinder. The objective is to achieve high resolution by converging the ultrasonic beam small in the conventional concave spherical lens, but on the other hand, the depth of focus is shallow and when observing below the sample surface, it gradually increases in the depth direction. The focus position must be changed, and non-destructive inspection of industrial products is not efficient in detecting the presence or absence of defects, so the depth of focus is increased and the focus is moved in the depth direction even if the resolution is reduced. Without trying to detect defects on the surface of the sample and near the surface. As this purpose shows, in this lens, it is necessary to receive only the irradiated ultrasonic beam that is incident on the inside of the sample from the surface of the sample and reflected by internal defects, so the component reflected by the surface is not directly received. It is indispensable to provide a sound absorber. As described above, the conventional ultrasonic microscope lens is configured for the purpose of observing the intensity or phase of the reflected ultrasonic wave from the ultrasonic wave by converging the ultrasonic wave on the spot or line segment on the sample surface or inside. Is.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The inventors of the present invention have found that when a film formed on a solid substrate is irradiated with ultrasonic waves, the ultrasonic waves irradiated at the incident angle peculiar to the material of the substrate and the film are materials. It was found that the reflectance has a minimum value at a frequency peculiar to the film thickness, and a method for measuring the film thickness by utilizing this phenomenon was invented, and an application has already been filed. The measuring method according to this application is a method of measuring the film thickness of a film formed by plating, coating, clad, coating, etc. formed in layers on a solid substrate by ultrasonic waves, in which a liquid for ultrasonic wave propagation to a substrate is used. The incident angle of the incident ultrasonic wave is a value peculiar to the substance of the substrate, liquid and film, and the ratio of the wavelength (in) of the ultrasonic wave to the film thickness (d) is a value peculiar to the substance of the substrate, liquid and film ( In the case of (H), the wavelength (on) of the ultrasonic wave at which the intensity of the reflected wave is reduced is measured by utilizing the fact that the reflectance of the ultrasonic wave is reduced, and the following formula (A) d = on · H …… ( This is a film thickness measurement method characterized in that the film thickness (d) is obtained by A), but the ultrasonic beam required at that time is incident on the sample at an incident angle peculiar to the system defined by the substrate and film. Must. Further, what is to be measured is the frequency dependence of the reflectance on the sample surface, and therefore, only the ultrasonic waves reflected on the surface of the ultrasonic waves applied to the sample surface must be received. In other words, in this film thickness measuring method, it is necessary to irradiate the surface of the sample with an ultrasonic wave of a predetermined incident angle and receive the reflected wave from the surface, which is the purpose of the conventional ultrasonic microscope lens shown above. Can not compete.

Means for Solving the Problems In order to solve the above problems, the present invention provides a system determined by the material of the substrate and the film when measuring the thickness of the film on the substrate based on the above measurement principle. The present invention provides an ultrasonic microscope lens capable of irradiating an ultrasonic wave beam having an incident angle peculiar to the sample surface and receiving an ultrasonic wave reflected from the surface.

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the present invention will be described in detail with reference to the drawings. First
The figure is an explanatory view of an ultrasonic microscope lens according to the present invention and an example of use thereof, and FIG. 2 is an explanatory view showing behavior of ultrasonic waves incident on a substrate on which a film is formed. As can be understood from FIG. 2, the ultrasonic beam 14 incident from the liquid 11 has an incident angle of
It is incident on the film surface at 16 and is divided into a transmitted wave and an incident wave, and the ultrasonic reflected beam 15 is irradiated at the same angle 16 ′ as the incident angle 16.
At this time, as described above, the reflectance when the product of the frequency of the ultrasonic wave and the film thickness has a specific value determined by the material, in the ultrasonic wave incident at the specific incident angle determined by the material of the film 12 and the substrate 13. May be extremely small. This phenomenon has been discovered by the present inventors, and the ultrasonic wave incident on a sample (plating film thickness is 0 μm, 2 μm, 3 μm, 4 μm) in which gold is plated on the surface of 42% nickel alloy is shown in FIG. The degree of change in reflectance with respect to the incident angle of is shown for each gold plating film thickness. As can be seen from the drawing, in the case of a sample in which a 42% nickel alloy substrate is coated with a gold plating film, the reflectance is minimal at a film thickness of 3 μm when the incident angle is 32 °. However, the frequency is 100MHz in this example.
Is.

Therefore, as an ultrasonic microscope lens that can be used in this film thickness measuring method, the sample surface can be irradiated with an ultrasonic beam having an incident angle peculiar to the system determined by the substrate of the sample to be measured and the constituent material of the film. It is necessary to have the property of receiving the ultrasonic waves reflected by. Therefore, in the present invention, as shown in FIG. 1, a film formed on the solid substrate 36.
When measuring the film thickness of 37, the incident angle 33 of the ultrasonic wave 38 incident on the solid substrate 36 from the liquid 35 for ultrasonic wave propagation is a value specific to the substance of the substrate 36, the liquid 35 and the film 37, and the wavelength of the ultrasonic wave. (On)
When the ratio of the film thickness (d) to the substrate (36), the liquid (35) and the film (37) is specific to the substance (H), the intensity of the reflected wave is reduced by utilizing the fact that the reflectance of ultrasonic waves is reduced. Ultrasonic wavelength (on)
Is measured, and an ultrasonic microscope lens used when the film thickness is obtained by the following formula (A) d = on · H ... (A) is used. A piezoelectric film (32, The conical recess 39 is provided at each end, and the apex angle 34 of the conical recess 39 is the ultrasonic wave incident on the solid substrate 36 from the liquid 35.
By setting 38 so as to be incident on the substrate 36, the liquid 35, and the substance of the film 37 with the above-mentioned value peculiar to the substance, it is possible to impart the characteristics described above.

Specifically, in FIG. 1, when the incident angle of the ultrasonic wave 38 on the sample surface is θ, the sound speed of the ultrasonic wave propagation material 31 is Co, and the sound speed of the liquid 35 is Cf, the angle 34 of the conical recess 39 is As φ Therefore, the ultrasonic wave propagation material 31 is fused quartz, liquid.
35 for water, solid substrate 36 for 42% nickel alloy, film 37 for gold,
If the incident angle θ is set to 32 ° as described above, the recess 39
The angle 34 should be 48.5 °. When the film thickness of the measurement sample is measured by the above-described measuring system and measuring method, the incident angle is 32 when (frequency) × (film thickness) = 300 m / s.
Since the reflectance becomes minimal at °, the frequency dependence of the emission intensity is measured by using an ultrasonic microscope lens capable of generating ultrasonic waves with an incident angle of 32 °, and the reflectance becomes minimal. If the frequency is obtained, the film thickness can be measured by (film thickness) = 300 / (frequency).

On the other hand, as a lens similar to the ultrasonic microscope lens having such a configuration, as described above, JP-A-58-1662.
There is an ultrasonic microscope lens disclosed in Japanese Patent Laid-Open No. 58-58, but this lens is for making a beam with a deep depth of focus enter the inside of a sample and receiving the reflected wave thereof. Therefore, the reflected wave from the sample surface is received. It is configured to install the ultrasonic absorber at the apex of the conical recess so as not to be specified, and is not configured to specify the incident angle of the ultrasonic wave. It is completely different from such an acoustic microscope.

EFFECTS OF THE INVENTION Since the present invention is configured as described above, when the combination of the substrate of the object to be measured and the material of the film is determined, and further the ultrasonic wave propagating liquid is determined, the ultrasonic wave is emitted at a unique incident angle of the ultrasonic wave. A measurement in which the ratio of the intensity of the reflected wave to the incident wave, that is, the frequency at which the reflectance becomes a minimum is measured with a frequency measuring instrument to obtain the frequency, and then the film thickness of the film of the object to be measured is calculated according to the formula A. When applied to the method, the film thickness of the film formed on the substrate and the two-dimensional distribution thereof can be measured accurately in a short time.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an ultrasonic microscope lens according to the present invention and its use, FIG. 2 is an explanatory diagram showing behavior of ultrasonic waves incident on a substrate on which a film is formed, and FIG. 3 is incident on a sample. It is explanatory drawing which shows the degree of the reflectance with respect to the incident angle of the ultrasonic wave. 31 ... Ultrasonic wave propagating material, 32 ... Piezoelectric film 33 ... Incident angle, 34 ... Apex angle, 35 ... Liquid 36 ... Solid substrate, 37 ... Film, 38 ... Ultrasonic wave 39 ... Recess

Claims (1)

[Claims] 1. When measuring the film thickness of a film formed on a solid substrate, the incident angle of ultrasonic waves incident on the solid substrate from a liquid for ultrasonic wave propagation is a value specific to the substance of the substrate, liquid and film, Moreover, when the ratio of the wavelength (in) of the ultrasonic wave to the film thickness (d) is a value (H) peculiar to the substrate, the liquid and the material of the film, the reflected wave of the ultrasonic wave decreases In the ultrasonic microscope lens used when measuring the wavelength (in) of the ultrasonic wave at which the intensity of is decreased and determining the film thickness by the following formula (A) d = in.H ... (A), Has a piezoelectric film on one end and a conical recess on the other end, and the apex angle of the conical recess is such that the ultrasonic waves incident on the solid substrate from the liquid impinge on the substance of the substrate, liquid and film. An ultrasonic microscope lens, characterized in that it is set so as to be incident with the above-mentioned unique value.