JP3271994B2 - Dimension measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel dimension measuring method for analyzing the dimensions of the inside of an optically opaque object.

[0002]

2. Description of the Related Art For example, when considering the manufacture of a magnetic head for performing magnetic recording on a floppy disk, a hard disk, a video tape, an audio tape, and the like, at present, inspection of defective products depends on visual observation under a microscope. is there. Also,
The measurement of the finished dimension of the magnetic head (for example, the depth of the gap) is performed by relying on an indirect method such as irradiating light from the side and observing it under a microscope. However, in such an indirect measurement, an internal defect is not known and an error is large, so that accurate measurement and evaluation are difficult.
Therefore, there is a need to develop a device capable of directly observing the internal structure in the state of a chip.

[0003] An ultrasonic microscope is known as a measuring device capable of measuring internal dimensions and having sufficient accuracy. There are two types of measurement techniques in this ultrasonic microscope, one using a longitudinal reflected wave as used for flaw detection and one using a surface acoustic wave excited on a sample. The former is effective for relatively thick ones (millimeter order or more), and the latter is effective for extremely thin ones of several μm or less.

[0004]

However, for example, the depth (so-called depth) of the gap of the magnetic head as described above is about tens of μm, and the measurement using an ultrasonic microscope is required regardless of any of the above-mentioned measurement techniques. Not suitable for Also,
In the measurement using the above ultrasonic microscope, it is necessary to use water (coupler) or the like at the time of measurement, and there is a possibility that the sample may be soiled. Accordingly, the present invention has been proposed in view of such conventional circumstances, and provides a dimension measuring method capable of measuring an internal dimension on the order of about several tens to several hundreds of micrometers in a non-destructive manner. With the goal. A further object of the present invention is to provide a dimension measurement method which can perform measurement in the atmosphere and does not require an environment such as immersion in water.

[0005]

Means for Solving the Problems The present inventors have made intensive studies on achieving the above object and completed the present invention by focusing on the photoacoustic effect. That is, the present invention
A photoacoustic signal generated by the photoacoustic effect is measured in the vicinity of the invisible end face of the object to be measured, and the dimension of the invisible end face depth of the object to be measured is obtained nondestructively.

The acoustic signal due to the photoacoustic effect can be measured by various devices. The photoacoustic devices applicable to the present invention are listed below. 1. A continuous wave laser beam that is periodically modulated is condensed on a sample placed in a photoacoustic cell via a micro optical system such as a microscope objective lens, and heat waves and thermoelasticity generated by absorption of the irradiated light are collected. A wave is detected by a sensor such as a microphone or a piezoelectric element, and the signal is directly captured by an A / D converter, or a synchronous detection is performed by a lock-in amplifier, and then a data bus such as GP / IB or an A / D converter is used. A photoacoustic apparatus characterized in that the photoacoustic apparatus is loaded into a signal processing device.

[0007] 2. The modulation of the laser light of the above-mentioned device 1 is replaced with a modulation in a random time series, and a time correlation between a photoacoustic signal generated by a correlator or the like and a random excitation light pulse sequence is taken, thereby obtaining a signal under the sample surface. A photoacoustic device for obtaining information on an internal structure.

[0008] 3. Pulse lasers are used in place of the lasers and modulators of the above devices 1 and 2, and a high-speed responsive piezoelectric element or surface displacement meter is used as the sensor, and the detected photoacoustic signal is combined with a method such as Fourier transform. A photoacoustic apparatus characterized in that information on an internal structure below a sample surface is obtained by the above method.

4. The photoacoustic apparatus according to the first aspect, wherein information is processed based on an amplitude image of a photoacoustic signal generated from the sample.

[0010] 5. The photoacoustic apparatus according to the first aspect, wherein information is processed based on a phase image of a photoacoustic signal generated from the sample.

6. In the above apparatus, when the position where the sample is irradiated with the laser beam is (x, y), the amplitude image of the photoacoustic signal is A (x, y), and the phase image is φ (x, y), the expression H (X, y) = A (x, y) · [1 + cos (φ (x,
y))] or H (x, y) = A (x, y) ² · [1 + cos (φ
(X, y))] A photoacoustic device based on processing information by a “hologram” defined by:

7. The photoacoustic apparatus according to any one of the above items 1 to 6, characterized in that the function is provided by controlling the position of the laser beam relative to the sample and scanning the sample.

8. The photoacoustic apparatus according to any one of the above items 1 to 6, wherein the function is provided with high accuracy by controlling and scanning the sample by a mechanical method such as a pulse stage.

9. In each of the above devices 1 to 8, more information about the sample is obtained by separating the optical properties and the thermal / acoustic properties of the sample by using the wavelength change of the laser scanning the sample. A photoacoustic device characterized by obtaining:

10. The photoacoustic apparatus according to any one of the above items 1 to 9, wherein noise is reduced by using an optical or electrical difference method.

[0016]

The photoacoustic effect is, as shown in FIG. 1, the generation of heat due to the modulated light absorption of a substance (measurement object 1). ,
This is a phenomenon in which an acoustic signal is generated by repeating contraction. For example, an acoustic signal obtained via the microphone 2 or the like
It contains information on the light absorption properties in the light irradiation area 1a and the thermal properties in the thermal diffusion length area 1b determined by the following equation (1).

(Equation 1) Therefore, the photoacoustic measurement is used as a method for detecting a defect inside an opaque material that is not optically observable.

On the other hand, the present invention quantitatively obtains the dimensions of the end surface of the object to be measured by utilizing the edge effect near the invisible end surface. That is, when photoacoustic measurement is performed near the edge of the device under test, a phenomenon called “edge effect” is observed. For example, a phenomenon in which the amplitude intensity of the photoacoustic signal increases exponentially from the inside toward the edge. Here, paying attention to the edge effect in the measured object having a thickness of about the thermal diffusion length, the photoacoustic signal changes in proportion to the dimension of the end face of the measured object. Therefore,
By measuring the photoacoustic signal, the dimension of the invisible end face depth of the measured object is obtained.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments to which the present invention is applied will be described in detail based on experimental results relating to phase measurement as an example. First, the photoacoustic apparatus used in this embodiment is:
As shown in FIG. 2, it is composed of an optical system, a scanning system, a sample storage unit, and a signal processing system. The optical system includes an Ar gas laser device 3 as a light source, a light chopper 4 as a modulator, a plano-convex lens 5 for condensing, and an optical microscope 6. Here, the plano-convex lens 5 is mounted at a position separated by a focal length from a mirror described later so that the light beam to be scanned becomes parallel. The objective lens of the optical microscope 6 was used for an ultra-long working distance so as not to come into contact with the photoacoustic cell. Note that a modulated laser can be used instead of the Ar gas laser device 3 and the light chopper 4.

As a scanning system, two mirrors 7,
8, a scanner control driver 9 for controlling the mirrors 7 and 8, and a personal computer 10 (including a D / A conversion board 11) for control are provided. On the other hand, a closed cell 13 is used for storing the sample 12 for measurement by the microphone method, and a microphone 14 and a preamplifier 15 are stored in the cell 13 in addition to a space for storing the sample 12. There is a space for

The signal processing system includes a microphone 14 as a detector, a preamplifier 15 for signal amplification, and a lock-in amplifier 16, and an output from the lock-in amplifier 16 is transmitted through an A / D converter 17 to the personal computer. It is directly taken into the computer 10. Here, the microphone 14 is used as the detector, but detection using a sensor such as a piezoelectric element may be considered. Further, the output from the lock-in amplifier 16 is directly taken in through the A / D converter 17, but various variations such as passing through a GP-IB bus are conceivable.

In the photoacoustic apparatus having the above-described structure, it is expected that electric noise and vibration noise are generated. The former includes a device that is picked up externally by a cord between the microphone 14 and the amplifier 15 and a device that uses the elements of the microphone 14 and the preamplifier 15 as a generation source. Therefore, a battery is used as a power source for the microphone 14 and the preamplifier 15, and the sealed cell 13 is grounded to the outside so that the outside is not affected as much as possible. In the latter, various vibrations transmitted through the floor are
Is probably due to picking up. Therefore, the entire apparatus is mounted on a vibration isolating table to remove external vibrations as much as possible.

On the other hand, as shown in FIG. 3, software in the above-described photoacoustic apparatus includes a part 18 for controlling a scanning system.
And a part 19 for processing the measurement data and displaying an image. The parameters for processing the scanning system include a scanning width and a scanning speed, and some values can be selected for each. Then, the set parameters are stored in the database 20 at the time of measurement. X-
The scanner control signal processed in the Y mirror control processing step 21 is transmitted from the personal computer 10 to the D / D
The data is sent to the scanner control driver 9 via the A conversion board 11, and the X-axis scanning 22 and the Y
Two-channel signals are used for axis scanning 3.

An electric signal emitted from the microphone 14 passes through a preamplifier 15 and a lock-in amplifier 16 and
Sent to the / D converter 17 and the personal computer 1
Input to 0. The input data captured during measurement is
The image is processed to be displayed as a pseudo-color on a display as a two-dimensional image, and is sequentially transferred to the database 20 via a data input step 24, a data imaging step 25, and a data output step 26.

The above is the configuration of the hardware and software of the photoacoustic apparatus used in the present embodiment. Using this photoacoustic apparatus, actual measurement was performed on the sample 12 shown in FIG. Hereinafter, the measurement results will be described in detail. The measured sample is ferrite having a tapered portion 12a as shown in FIG. The sample 12 is housed in a closed cell 13 and tightened with a bolt to make the inside of the sample closed.
The optical image was set at a position near the optical focus. Next, the surface of the sample 12 was irradiated with an appropriate power by oscillating the Ar gas laser 3, and the laser was again focused.

Then, a photoacoustic measurement of the tapered portion of the sample 12 (indicated by an arrow A in the figure) was performed. As a result, FIG.
As shown in FIG. 7, a change in phase due to the edge effect of the end face 12b of the sample 12 was observed. That is, as shown in FIG. 5, the phase value gradually decreases as approaching the end face 12b, and shows a local minimum value near the end face 12b.
In 2b, the edge rises sharply and an edge is observed.

Therefore, when the same measurement was repeated for samples having different dimensions of the end face 12b, the difference between the maximum value and the minimum value in phase change (variation B) and the dimension (depth) of the end face 12b were as shown in FIG. As shown in FIG. Note that FIG.
The relationship between the change amount B and the depth when the frequency is set to 00 Hz and 800 Hz is displayed. Therefore, it was found that the size of the end face 12b can be obtained based on the correlation by obtaining the amount of change B, which can be applied to, for example, depth measurement of a magnetic head.

[0027]

As is apparent from the above description, in the present invention, the dimensions of the invisible end face of the object to be measured are determined by using the photoacoustic effect, and the
It is possible to accurately measure internal dimensions on the order of m to several hundreds of μm. Further, in the present invention, the measurement can be performed in the atmosphere, and the problem of contamination caused by immersion in water can be avoided. Therefore, for example, it is possible to quantitatively measure the internal dimensions of the components constituting the magnetic circuit of the magnetic head, and it is also possible to non-destructively evaluate and inspect the internal quality. Sex is big.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating the principle of the photoacoustic effect.

FIG. 2 is a schematic diagram illustrating a hardware configuration of a photoacoustic apparatus used in the embodiment.

FIG. 3 is a schematic diagram illustrating a software configuration of a photoacoustic apparatus used in the example.

FIG. 4 is a schematic perspective view showing the shape of a sample actually measured.

FIG. 5 is a characteristic diagram showing a change in phase due to an edge effect.

FIG. 6 is a characteristic diagram showing a correlation between a phase change amount and an end face dimension (depth).

Continuation of front page (72) Inventor Tsutomu Hoshimiya 1-13-1 Chuo, Tagajo City, Miyagi Prefecture Tohoku Gakuin University (56) References JP-A-61-256213 (JP, A) JP-A-61-90073 (JP, A) JP-A-57-153209 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01B 17/00-17/08 G01B 11/00-11/30 102

Claims (1)

(57) [Claims] 1. In the vicinity of an invisible end face of an object to be measured,
A dimension measuring method comprising: measuring a photoacoustic signal generated by a photoacoustic effect; and non-destructively determining a dimension of an invisible end face depth of the measured object.