JPH0526655A - Film thickness measuring method and device - Google Patents

Abstract

PURPOSE:To measure the film thickness of a sample and improve resolution by measuring the reflected waves from the sample installed on the inside of the focal point of a focal probe. CONSTITUTION:Ultrasonic waves excited on the lower face of the piezoelectric element 6A of a focal probe 6 when the pulse voltage from a high-frequency pulse oscillator 1 is applied are focused by the concave face of the radiation face 6C of the acoustic lens 6B of the probe 6. The ultrasonic waves are radiated to a sample 16 to be inspected via a liquid sound field medium 15, a stage 17 is moved in X, Y, Z-directions by a computer 9 controlled by a computer 4, and the reflected waves from the sample 16 are collected by the probe 6 and converted into an electric signal, the signal is amplified by a receiver 2 and A/D-converted 3 then stored in the memory of the computer 4, and the analyzed and processed result of the signal is displayed on a monitor 5. The sample 16 is installed on the inside of the focal point of the probe 6, the frequency characteristic of the reflected waves from the sample 16 is determined, and the film thickness can be obtained from it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film thickness measuring method and apparatus using a focus type probe.

[0002]

2. Description of the Related Art In film thickness measurement using ultrasonic waves, a flat type probe 10 radiates plane waves having different frequencies to a sample 11 as shown in FIG. 4, and the film thickness is measured from the intensity of a reflected wave at each frequency. To measure. FIG. 5 shows an example of film thickness measurement of the thin film 13 formed on the substrate 12. The plane wave ultrasonic waves reach the thin film 13 through a medium such as water, but the reflection intensity R at this time
Is

[0003]

[Equation 1] Z = A / B Where A and B are

[0004]

[Equation 2] Becomes Z ₁₂ and Z ₂₃ in [Equation 2] are as follows.

[0005]

Z ₁₂ = Z ₁ / Z ₂ Z ₂₃ = Z ₂ / Z ₃ where Z ₁ is the acoustic impedance in the medium 14, Z ₂ is the acoustic impedance in the thin film 13, and Z ₃ is the substrate 12. The acoustic impedance. Furthermore, k in [Equation 2]
₂ is the wave number in the thin film 13.

When Z ₂ <Z ₃ between the acoustic impedances Z ₂ and Z ₃ of the thin film 13 and the substrate 12, the thickness d of the thin film 13 is
It is known that the reflectance R becomes minimum at 1/4, 3/4, 5/4, ... This is used in the film thickness measurement. FIG. 6 shows a relationship diagram between the reflectance R and d / λ under the condition of Z ₂ <Z ₃ . This relationship diagram is an example in which the substrate 12 is sapphire and the thin film 13 is quartz glass. The sound velocity of the sapphire is 11000 m / s, the density is 4.0 g / cm ³ , the sound velocity of the quartz glass is 5900 m / s,
The density was 2.2 g / cm ³ . In FIG. 6, (d / λ) =
It can be seen that the minimum value appears at two points of 0.25 and 0.75.

In order to measure the film thickness from FIG. 6, the frequency of ultrasonic waves is set to a small value, and the reflectance R is calculated for each frequency while gradually increasing the frequency. Then, find the minimum value that appears first. The minimum value that appears first is (d / λ) = 0.25. On the other hand, the sound velocity v, the ultrasonic frequency f _i, and the wavelength λ are

[0008]

Since the relationship of λ = v / f _i , the film thickness d is

[0009]

## EQU5 ## It can be obtained by the mathematical expression d = 0.25 (v / f _i ).

[0010]

In the prior art, a flat probe must be used for plane wave radiation. As shown in FIG. 4, the flat probe is a plane whose emission surfaces are parallel to each other, and the area where ultrasonic waves are emitted is determined by the diameter D of the probe. And the measurement result is an average value of the entire radiated area. As a result, the resolution of the measurement position decreases, and when measuring the film thickness at positions close to each other on the sample, measured values of the same film thickness are obtained, and it is not possible to measure the actual film thickness variation. It was

It is an object of the present invention to provide a film thickness measuring method and apparatus which enable film thickness measurement using a focus type probe instead of a flat type probe.

[0012]

The measuring method of the present invention uses a focus type probe as a probe, and reflects from the sample when the sample is installed inside the focus of the focus type probe. The frequency characteristic of the reflected wave is obtained, and the film thickness of the thin film is obtained from this frequency characteristic (claim 1).

Further, the measuring device of the present invention comprises a variable frequency embossing pulse source, and a focus type probe which emits an ultrasonic wave toward a sample placed inside the focal point in accordance with a pulse having a variable frequency. Of the reflected waves from the sample, refer to the relationship between the pulse frequency and the reflected signal (reflectance) of the direct wave and the means for extracting only the directly reflected wave using the time difference from the surface acoustic wave. And a means for calculating the thickness of the thin film from the pulse frequency and the speed of sound, which is the minimum value.

Further, in the measuring apparatus of the present invention, the pulse width which gives the pulse frequency is the distance Δ from the focus to the inner installation position.
The value is smaller than the time difference in Z (claim 3).

[0015]

According to the present invention, the film thickness can be measured by placing the sample inside the focus of the focus type probe and measuring the reflected wave (Claim 1).

Further, according to the present invention, only the direct reflection wave is detected while changing the frequency of the pulse signal, and the film thickness can be measured from the minimum value in the same way as the plane wave (claims 2 and 3).

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an embodiment of the film thickness measuring device of the present invention. It consists of the following components. High frequency pulse oscillator 1 ... An oscillator capable of generating a variable frequency pulse. Here, the variable frequency means that the pulse width is variable. Receiver 2 ... Receives and amplifies the reflected wave signal (high frequency pulse voltage) returned from the probe 6. AD converter 3 ... AD-converts the reflected wave signal received by the receiver 2. Computer 4 ... Data of the reflected wave waveform obtained by the AD converter 3 is stored in a buffer (not shown) and analysis processing is performed. Monitor 5 ... Displays the analysis / processing result on the computer 4 and presents it to the observer. The probe 6 is composed of a piezoelectric element 6A and an acoustic lens 6B,
The emission surface 6C of the acoustic lens 6B has a concave shape. The shape of this concave surface determines the focus of ultrasonic waves. In the one-point focus type, the concave surface portion 6C has a conical shape, and in the line focus type, the entire probe 6 is rectangular and the concave surface portion 6C has a cylindrical shape from the paper surface to the back surface. In this probe 6, ultrasonic waves excited by applying a high frequency pulse voltage are reflected by the lens 6B.
The light is focused by the concave surface portion 6C of and is discharged.

Medium 15: A medium for transmitting ultrasonic waves between the probe 6 and the probe 6 without attenuating the medium, and is made of, for example, water. Sample 16 ... It has a film thickness on its surface. Stage 17 ... A mechanical system that holds the sample 16 and can move in the X, Y, and Z directions. Computer 9 ... Installed for the purpose of controlling the stage 17. The computer 4 may also serve as the computer. With the above configuration, since the probe 6 is a focus type, how to obtain a plane wave becomes a problem. The ultrasonic waves emitted from the probe 6 are a direct wave (vertical incident wave) that travels straight from the right-angled (normal) direction to the sample directly below, and an oblique wave that enters at a critical angle and acts as a surface acoustic wave. Yes, only direct waves are considered to have the character of plane waves. Therefore, the present embodiment is characterized in that only the direct wave is detected as the reflected wave. The direct wave and the oblique wave (surface wave) may interfere with each other as a reflected wave. In this embodiment, the direct reflected wave is detected under the condition that there is no interference.

First, the operation of FIG. 1 will be briefly described. When a pulse voltage generated by the high frequency pulse oscillator 1 is applied to the piezoelectric element of the probe 6, ultrasonic waves are excited on the lower surface of the piezoelectric element. The excited ultrasonic waves are focused by the concave surface provided on the other end surface of the probe 6. The focused ultrasonic waves are applied to the object 16 to be inspected via the liquid sound field medium 15. At that time, the stage 17 moves in the X, Y, and Z directions according to a signal from the computer 9. Then, the reflected wave of the inspection object 16 is collected by the probe 6, converted into an electric signal, and amplified by the receiver 2. The amplified electric signal is converted into a digital value by the AD converter 3, stored in the memory of the computer 4, and the result of analyzing and processing the signal is displayed on the monitor 5.

In this embodiment, the sample 17 is placed in front of the focus position (defocus position) and the pulse frequency is changed to detect the nth-order reflected wave in the multiple reflected waves. The relationship between the reflectance R due to the nth-order reflected wave and the frequency is obtained while changing the frequency from a low value to a high value. When the sample 17 is present in front of the focus position, the direct arrival wave and the surface acoustic wave from the oblique wave have different arrival times to the vibrator 6A, and the higher the order of the multiple reflected waves, the higher the order. ,
This time difference becomes large. Therefore, a reflected wave with a high order is detected, the position where the minimum value is obtained is detected from the relationship between the reflectance and the frequency, and the thickness d of the thin film of the sample 17 is calculated using [Equation 4] and [Equation 5]. . Further details will be described.

(1) Relationship between defocus and multiple reflected waves. When the inspection object 16 is located at the focal position of the probe 6, the positional relationship of the reflected waves on the time axis is as shown in FIG. 2, lens echo (1) → inspection object reflected echo → lens echo (2). Becomes When the distance between the probe 6 and the object 16 to be inspected is reduced from this state, that is, defocusing is performed, the positional relationship of the reflected waves becomes as shown in FIG. The reflected ultrasonic waves are reflected again on the spherical surface of the lens, travel toward the object 16 to be inspected, and multiple reflected waves (1) → (2) → (3) appearing on the surface of the object 16 to be inspected. When the probe 6 is defocused, the ultrasonic waves that contribute to the output from the piezoelectric element 6A are a direct incident wave (a right angle incident wave) near the center axis due to the lens effect, and a surface acoustic wave water 15 propagating on the surface. There are two types of re-radiation wave (oblique wave) into the inside, but by shortening the pulse width Δt of the launch pulse,
These two types of waves on the surface of the piezoelectric element are temporally separated from each other and can be prevented from interfering with each other. That is, since the two types of waves have different propagation distances and propagation velocities, the time required to reach the piezoelectric element surface is different and they can be separated on the time axis.

(2) Separation of multiple reflected waves. The time difference between the multiple reflected waves can be calculated. This is shown below. The definitions used in the trial calculation formula are as follows. Sound velocity of lens material = Sound velocity of C _L medium = Sound velocity of C _W surface wave = C _R Radius of lens spherical surface = R Focal length F = R / {1- (C _W / C _L )} Further, in FIG. The positional relationship between the probe 6 and the sample 18 is shown. In FIG. 7, assuming that the ultrasonic wave on the central axis propagates in the lens 6B as a distance La and the ultrasonic wave transmitted as a surface wave propagates in the lens 6B as a distance Lb, La and Lb are as follows.

[0023]

[Equation 6] On the other hand, if the sample is moved closer to the lens side by ΔZ from the focus position,

[0024]

[Equation 7] Becomes Therefore, the time t ₁ until the direct wave (vertical incident wave) at the center is multiply reflected n times on the sample surface and returns to the piezoelectric element again
Is

[0025]

[Equation 8] Becomes Also, the sample surface propagates as a surface wave, and A → B
→ C → D → C → B → A → B ... The time t ₂ required for multiple reflections n times and returning to the piezoelectric element again is

[0026]

[Equation 9] Becomes

As an example, the radius R of the spherical surface is R = 2 mm, the sound velocity of the lens material is C _L = 11000 m / s, and the sound velocity of water is C _W = 1500.
m / s, sound velocity of surface wave C _R = 3000 m / s, axial length L ₁ =
FIG. 8 shows an example of calculating the time difference with respect to the defocus amount ΔZ in the case of 11 mm. The numbers in the figure are the orders of multiple reflection and are shown up to the third order. From the figure, it can be seen that the propagation time difference increases for the same defocus amount as the order increases. Also, focusing on the line of order 1, when ΔZ = 1 mm, the time difference is 0.234 μs. Therefore, the pulse width of the launch pulse is set to 0.234.
If μs or less, the interference of two waves does not occur when ΔZ = 1 mm.

(3) Means for separation. In this embodiment, first, the sample 18 is defocused (distance Δz) so as to approach the lens from the focal position. The larger the defocus amount ΔZ is as shown in FIG. 8, the better.
ΔZ and the pulse width must be selected so that the pulse width of the oscillation pulse (launching pulse) of the pulse oscillator 1 is smaller than the time (t ₁ −t ₂ ) of the multiple echo at the position of the defocus amount ΔZ. . Further, while changing the frequency f of the oscillation pulse from the small value to the large value at the position of ΔZ, only the reflected signal as the direct wave is selected and the reflectance is obtained. Here, the echo order of the reflected signal is determined in advance. Thus, the minimum value of the reflectance that appears first is found, and the film thickness is measured according to [Equation 4] and [Equation 5]. The above procedure for measuring the film thickness is performed by the operator's instruction, the instruction of the computer 4, and the processing.

[0029]

The film thickness can be measured from the reflection echo of only the direct wave using the focus type probe. Further, since it is a focus type, the resolution of the measurement position in film thickness measurement can be improved as compared with the flat type.

[Brief description of drawings]

FIG. 1 is an embodiment of a film thickness measuring device of the present invention.

FIG. 2 is a diagram showing how a lens echo and a reflection echo appear.

FIG. 3 is a diagram showing how a lens echo and multiple reflection echoes appear.

FIG. 4 is a diagram showing an example of film thickness measurement using a conventional flat probe.

FIG. 5 is a diagram showing impedance in each layer for film thickness measurement.

FIG. 6 is a diagram showing a relationship between d / λ and reflectance.

FIG. 7 is a diagram illustrating a time difference in multiple reflection echo according to the present invention.

FIG. 8 is a diagram showing a specific example of a defocus amount ΔZ and a time difference in the embodiment of the present invention.

[Explanation of symbols]

1 High frequency pulse oscillator 4 computers 6 Focus type probe 16 samples 17 stages

Claims (3)

[Claims] 1. A focus type probe as a probe in a film thickness measuring method in which ultrasonic waves are radiated to a sample having a thin film on its surface and the thickness of the thin film is obtained from the frequency characteristics of the ultrasonic waves reflected from the sample. Is used to obtain the frequency characteristic of the reflected wave reflected from the sample in the state where the sample is installed inside the focus of the focus type probe, and the film thickness of the thin film is obtained from this frequency characteristic. Thickness measurement method. 2. A film thickness measuring apparatus for irradiating a sample having a thin film on its surface with ultrasonic waves, and determining the thickness of the thin film from the frequency characteristics of the ultrasonic waves reflected from the sample, wherein a variable frequency punching pulse source and The time difference between the focus probe that emits ultrasonic waves toward the sample placed inside the focal point according to the pulse with variable frequency and only the direct reflected wave among the reflected waves from the sample and the surface acoustic wave. The pulse frequency that is the minimum value of the reflected signal is found by referring to the relationship between the pulse frequency and the reflected signal (reflectance) of the direct wave, and the thin film thickness is calculated from this pulse frequency and sound velocity. And a film thickness measuring device comprising: 3. The film thickness measuring apparatus according to claim 2, wherein the pulse width that gives the pulse frequency is a value smaller than the time difference at the distance ΔZ from the focus to the inner installation position.