JPS637601B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for optically measuring a minute gap between two objects to be measured.

FIG. 1 shows a diagram of the principle of a measurement method using optical interferometry, and a case will be described in which, for example, a minute gap between a disk and a head of a magnetic disk device is measured using this principle.

Reference numeral 1 denotes a light source section that generates monochromatic light having a wavelength λ from light including wavelengths in a wide band.The monochromatic light from this light source section 1 is bent downward in the figure by a beam splitter 2. 3 is a transparent disk that is one of the objects to be measured;
Reference numeral 4 designates an opaque head which is the other object to be measured, facing the one disk 3. This head 4
A minute gap h is formed between the disc 3 and the disc 3 when the disc 3 rotates.

In the above configuration, a monochromatic light beam generated from the light source section 1 passes through the disk 3 and enters the head 4. At this time, the beam is partially reflected by the front surface 3a and back surface 3b of the disk 3, and is also reflected by the surface 4a of the head 4. In this way, the beam reflected by the disk 3 and the head 4 passes through the beam splitter 2 and enters the photoelectric conversion element 5. Here, if we focus on the interference light generated by interference between the beam reflected from the front surface 4a of the head 4 and the back surface 3b of the disk 3, the brightness of this interference light will vary depending on the size of the gap h between the disk 3 and the head 4. changes as shown in FIG.

That is, when the gap h=0, the brightness of the interference light is also the darkest, and when the gap h=λ/4, the brightness of the interference light is the brightest. Therefore, by converting the amount of change in brightness of this interference light into a voltage signal using the photoelectric conversion element 5 and measuring it, the gap h can be determined.

However, this measurement method has the following problems.

(a) The brightness of the interference light is related not only to the gap h but also to the brightness of the reflected light. Therefore, the brightness of the interference light varies depending on variations in the amount of incident light and the reflectance of the disk 3 and head 4, resulting in a decrease in measurement accuracy.

(b) The gap h consists of an average value component h ₀ of the gap and a dynamic component h _t that changes over time. Here, if h ₀ ≒ 1/8λ, it is possible to measure within the range of h _t ≒±1/8λ, but for example, if the disk 3
When the rotational speed of h ₀ changes, the measurable range of h _t changes accordingly.

(c) The gap measurement range is limited to 0 to 2/4.

In view of the above points, the present invention provides an optical measurement method for a microgap that accurately measures the average amount of a microgap formed between two objects to be measured and the amount of the gap that changes over time. The purpose is to

An embodiment of the measuring method of the present invention will be described below with reference to the principle diagram shown in FIG.

In FIG. 3, the same reference numerals as in FIG. 1 indicate the same parts.

In FIG. 3, 6 is a mirror, 7 is a photoelectric conversion unit that converts an interference fringe image generated from the reflected light from the disk 3 and head 4 into an electrical signal, and 8 is an interference fringe imaged on the photoelectric conversion unit 7. An observation device that observes
Reference numeral 9 denotes a measurement circuit that outputs a signal A corresponding to a minute gap from the interference fringe image converted into an electric signal by the photoelectric conversion section 7. That is, the average position of the interference fringes is detected based on the output signal from the photoelectric conversion unit 7, and the amount by which the position of the interference fringes changes dynamically around that position is detected, and voltages corresponding to each are output. Since the gap is zero when the disk 3 is at rest, there is only a dark area over the entire surface of the surface 4a of the object to be measured. When the disk 3 rotates, a small gap h is formed between the disk 3 and the head 4, but since 4a generally forms a small angle θ with respect to 3b, the interference fringes are not of uniform brightness but are bright. The result is a striped pattern that includes dark and dark areas.

As the gap changes, the position where the dark part of the interference fringes occurs changes. (Hereinafter, the bright areas can be considered in the same way as the dark areas, so the explanation will be omitted.)
At this time, the measurement circuit 9 calculates the average value of the gaps.
Outputs an electrical signal corresponding to h ₀ and the dynamic component h _t .
In order to make the dark part of the interference fringes appear stationary on the detector surface of the photoelectric conversion section 7, the output signal of the measurement circuit 9 is applied to a deflection drive circuit (described later) of the photoelectric conversion section 7, thereby forming a small gap. An electrical signal proportional to the change in can be obtained with high precision. By measuring this electrical signal, changes in the minute gap can be measured with high precision.

The configuration of the photoelectric conversion section 7 is shown in FIG. Fourth
In the figure, the photoelectric conversion unit 7 includes an optical lens 10 for adjusting magnification, a detector 11, and a video preamplifier 1.
2. Deflection circuit 13, XY deflection drive circuit 14, TV
It is composed of a signal output circuit 15 and the like. Detector 11
further includes a photocathode 16, an aperture 17, and an electrode 18.
It consists of

The operation of the photoelectric conversion unit 7 is to convert the interference fringe image formed on the photoelectric conversion surface 16 via the optical lens 10 into an electronic image, and then use the XY deflection drive circuit 14 from the outside.
The converted electron image of the interference fringes is deflected by the deflection voltage applied to the electrode 1, and the electrons passing through the aperture 17 are transferred to the electrode 1.
Detected at 8. In this way, the interference fringe image is arbitrarily scanned, and the video preamplifier 12 outputs an electrical output proportional to the brightness. On the other hand, the TV signal output circuit 15 generates and outputs a scanning signal (not shown because it is a general NT4C signal) for scanning the entire surface of the interference fringe image. The entire interference fringe image is observed on a TV monitor 8.

In FIG. 5, a indicates the floating state of the head 4,
b shows the interference fringes imaged on the light receiving surface of the photoelectric conversion unit 7, and c shows the output signal of the video preamplifier 12, respectively.

It is assumed that the surface of the head 4 is linear, and in the following, the scanning direction is only full-width scanning in the X direction, and the Y direction is fixed by applying a constant voltage _VY . Next, we will explain how to find the minimum gap h _nio from the position where the dark part of the interference fringes occurs. The relationship between the dark part of the interference fringes and the gap can be expressed by the following equation.

h _i = 1/2λ・i (i=0, 1, 2,...) ...(1) (i...order) Here, the order i can be determined by lowering the rotation speed and checking the 0th order interference fringes. You can decide. When two or more first-order and second-order interference fringes can be observed, and assuming that the angle of inclination of 4 with respect to the disk 3 is θ, the following equation is obtained from FIGS. 5b and 5c.

tanθ=1/2λ/x ₂ −x ₁ = h ₂ −h ₁ /x ₂ −x ₁ …(2) h ₂ = h _nio + tanθ・x ₂ …(3) h ₁ = h _nio + tanθ・x ₁ ( From equations 2) and (3), the minimum gap h _nio is h _nio = h ₁ x ₂ − _{h 2} x ₁ / x ₂ − x ₁ (4). Here, x ₁ and x ₂ indicate the positions of dark areas of interference fringes generated on the light receiving surface of the photoelectric conversion section 7 and on the surface of the head 4. h ₁ and h ₂ indicate the actual gap amount.

As shown in FIG. 5c, a brightness signal is obtained from the photoelectric conversion section 7 by scanning the interference fringes in the X direction.
The positions x ₁ , x ₂ of the interference fringes on the head 4 correspond to scanning voltage levels V ₁ , V ₂ . Signals that are directly measured include V ₁ , V ₂ and Δv (corresponding to Δx and Δh). Therefore, from equation (4), the minimum gap can be found as h _nio =h ₁ ·V ₂ −h ₂ ·V ₁ /V ₂ −V ₁ (5).

In the present invention, when the gap changes between ±Δh, the position of the dark part of the interference fringe changes on the light receiving surface.
That is, a change of ±Δv is detected in terms of scanning voltage.

This ±Δv is detected as a deviation signal in which the position of the dark area changes. By feeding back this deviation signal so that it is added to the deflection signal of the photoelectric converter 7 in a minor manner using the circuit configurations shown in FIGS. 3 and 6, the deflection signal is statically moved in response to the gap that changes from a stationary state. The values x ₁ and x ₂ can be tracked one by one. This shows that the order of the interference fringes in FIG. 5b can be uniquely determined by checking the φth order. If i=1, (5)
The formula is as follows.

h _nio = 1/2λ・V ₂ ×λ・V ₁ /V ₂ −V ₁ …(6) Note that V ₁ and V ₂ are the peak positions of the dark part of the interference fringe, and this determination is based on the brightness in Figure 5 c. It can be determined by detecting the minimum value of the signal level by configuring a comparison circuit or the like. This detection method will be omitted.

From the above, along with the fluctuation component h _t , h ₀ (average value of the gap)
It turns out that it is easy to determine. The interference fringes may be scanned at a sufficiently high scanning speed for the dynamic component h _t , and the interference fringes may be scanned at a low speed for the static component h ₀ . That is, when the rotational speed is changed, low-speed scanning is performed until a steady state is reached, and then high-speed scanning is performed and feedback is performed.

Next, the 1:1 correspondence between Δh and Δv will be explained.

When the disk 3 and the head 4 are in a floating state as shown in Fig. 5, focusing on the interference fringes generated in the gap h ₂ (when i = 1), we will consider the case where the gap changes by ±Δh with respect to h ₂ think. Figure 5b in proportion to Δh,
In c, ±Δx and ±Δv correspond. Since the attitude of the head 4 is considered to be constant in the floating state, (3)
The inclination angle θ of the disk 3 in the equation is constant. Therefore, the following equation can be obtained from the static interference fringe generation state shown in equation (3).

tanθ=Δh/Δx=K·Δh/Δv (7) Δx is equivalent to Δv, and K is a constant of proportionality. K can be defined by the following equation.

Δv=V ₂ −V ₁ /X ₂ −X ₁ ×Δx=K・Δx …(8) If we calculate Δv from equation (7), we get Δh=KtanθΔv=V ₂ −V ₁ /X ₂ −X ₁ tanθ・Δv It turns out that it can be determined as

The operating principle of the method for detecting Δv corresponding to Δh will be described below.

A method of detecting a voltage component Δv corresponding to a fluctuation component of the gap from an interference fringe image formed on the photoelectric conversion unit 7 will be described using the measurement circuit 9 shown in FIG.

First, to show the configuration, an amplifier 16 that amplifies the brightness signal, a multiplier circuit 17 that multiplies the scanning signal and the brightness signal, an integration circuit 18 that integrates the multiplied signal, and in order to control the photoelectric conversion section, first scan in advance. A scan position setting circuit 19 that sets the start point for starting the scan, a scan function oscillator 20, the voltage level and scan speed of the scan signal, and the scan function (triangular wave, sine wave, etc.)
It consists of a scanning signal setting circuit 21 for setting V xi , an adding circuit 22 for adding a preset V _xi , a scanning signal, and the output from the integrating circuit 18 , a recording section 23 , and the like.

The amplifier 16 receives and amplifies the brightness signal (video output) from the video preamplifier 12 of the photoelectric conversion section 7 and outputs it to the multiplication circuit 17 . A brightness signal is generated when a scanning signal is used to scan a dark area of the interference fringes within a range that does not exceed the adjacent bright area, and the multiplier circuit 17 multiplies this brightness signal by the scanning signal. The integrating circuit 18 integrates this output and outputs the average value of the multiplied signal. This value corresponds to a deviation signal in which the peak position of the dark part of the interference fringe deviates from the scanning center set in advance by the scanning position setting circuit 19. A method for obtaining this deviation signal will be described further later.

On the other hand, the scanning signal oscillator 20 outputs a scanning signal corresponding to the scanning function, scanning speed, and scanning width set in advance by the scanning signal setting circuit 21.
By adding V _xi , the scanning signal, and the integration circuit output in the adding circuit 22 and applying the result to the deflection drive circuit 14,
The measurement circuit 9 and the photoelectric conversion unit 7 constitute a feedback system of the miner, which constantly provides feedback on the amount of deviation from the point at which scanning was started in advance. Therefore, the output from the integrating circuit 18 represents the fluctuation component of the gap, which also provides a signal for reversing the change in the interference fringes. For example, the amount of change when the interference fringes gradually move is also included as a value by which Δv gradually changes. By increasing the scanning speed, the frequency at which fluctuation components can be measured also increases. The scanning width, scanning function, and scanning speed are arbitrarily selected depending on the state of occurrence of interference fringes and the state of occurrence of fluctuation components. Further, if two or more interference fringes are generated, it is possible to shift to adjacent interference fringes. (V _xi is changed.) By inputting the output of the integrating circuit 18 to the deflection drive circuit 14 for the miner's feedback system, the interference fringes are electrically fixed on the photoelectric conversion section. The output signal required for fixing at this time becomes a deviation signal corresponding to the gap.

Next, a method for obtaining Δv will be described. FIG. 7 shows the operating principle when a triangular wave is used as the scanning function. First, by scanning the interference fringe image of FIG. 7a over its entire width, a brightness signal as shown in FIG. 7b is output from the amplifier 16 in FIG. 6.
On the other hand, by setting the partial scanning signal as shown in FIG. 7c, a video output (brightness signal) corresponding to scanning a part of the interference fringes shown in FIG. 7a is output from the amplifier 16. be done. This means that the scanning position setting circuit 19 sets voltages V _a , V b , and _{V b corresponding} to the positions of the interference fringe images X a , X _b , X _c , and
If we scan back and forth with a triangular wave of amplitude ΔV and period T around V _c and V _d , the seventh
It is shown that F(V _a ), F(V _b ), F(V _c ), and F(V _d ) in FIG. d are obtained as the output of the amplifier 16, that is, the brightness signal. Now, let us consider the case where the brightness signals scanned back and forth at the scan centers V _b , V _c , and V _d are multiplied by the scan voltage. FIG. 8 shows its operation.

FIG. 8a shows the relationship between the position of interference fringes in the photoelectric converter 7 and the brightness of interference light, and V _b , V _c , and V _d show center voltages of partial scanning. In addition, in Figure 8b, A is the brightness signal (solid line), scanning voltage (dotted line), and brightness obtained when scanning around point b of the brightness signal of the interference light shown in Figure 8a. The signal x scanning voltage (dashed line) is shown. Similarly, B shows the waveform when scanning is performed centering on point c, and C shows the waveform when scanning is performed centering on point d.

When a partial scan of the interference fringe image is performed at V _b (centered on point b), the brightness signal becomes the solid lines in FIG. 8b and a. When this is multiplied by the scanning voltage, the value becomes smaller in the positive scanning period around point b, and becomes larger as a negative value in the negative scanning period. Therefore, integrating the multiplication output yields a negative voltage. Next, when scanning is performed at V _c (centered at point c), the brightness of the interference light becomes almost symmetrical about point c, so a signal with the same brightness is obtained every 1/2 cycle.

Therefore, the multiplier circuit 17 outputs a signal that is symmetrical in positive and negative directions. When this is input to the integrating circuit 18,
The integral output becomes 0. Further, when scanning is performed at V _d (centered at point d), a brightness signal is obtained at a higher output during scanning on the positive side. Therefore, the output of the product of the brightness signal and the scanning voltage becomes a positive voltage, contrary to the case centered at point b. When the output of this multiplier circuit 17 is input to the next integrating circuit 18, the output becomes a positive voltage. That is, a negative output is obtained at the center of point b, approximately 0 at point c, and a positive output at point d.

Therefore, the output of the multiplication circuit 17 is input to the integration circuit 18 and averaged, and this is added to the output from the scanning position setting circuit for V _b , V _c , and V _d with the sign reversed in the addition circuit 22 . If you do this, the scanning center will automatically move to the darkest part of the interference light.

Therefore, the output of the integrating circuit 18 indicates the fluctuation component of the minute gap, and by measuring the output of the integrating circuit 18, the dynamic component of the minute gap can be detected.

The above explanation was given using a triangular wave as an example, but the same applies to a sine wave as well.

The adding circuit 22 inverts the sign and adds the following:
It works to keep the interference fringe image of interest fixed at a certain point at all times.

This also shows that a dark part of the interference fringes occurs when starting from a stationary state, and if scanning is started centering on this point, it is possible to trace the first-order interference fringes until they disappear from the photoelectric conversion unit 7.

If there are two or more interference fringes, it is possible to skip to the adjacent interference fringes and measure x ₂ corresponding to h ₂ from this output voltage. Even in the case where there is only about one interference fringe, by constantly tracking it, the occurrence status of the interference fringe can be known, and changes in the gap h can be grasped.

Next, another embodiment of the present invention will be described with reference to FIG. In FIG. 9, the same reference numerals as in FIGS. 1 and 6 indicate the same parts.

24 is a white light source; 25 is a wavelength selection unit that selects an arbitrary wavelength λ from the white light source to produce a monochromatic light beam; 26
is a circuit that outputs an operation signal for selecting a wavelength based on the deviation signal from the measurement circuit 9.

Next, FIG. 10 shows a specific example of the wavelength selection section 25.

In FIG. 10, 27 is the first lens, 28
is the pinhole, 29 is the second lens, 30 is the third lens
31 is a continuous transmission wavelength variable interference filter, 32 is a linear motor that linearly moves the interference filter 31, and 33 is a position detector. The structure of the variable transmission wavelength interference filter 31 is shown in FIG. This is an arrangement of interference filters so that the transmitted wavelengths are continuously different.The light beam is narrowed down by a lens 29, and the interference filter 3 is placed at the focal point.
1, the light input to the lens 30 becomes monochromatic light with a wavelength λ ₃ , for example. The light from the lens 30 becomes a parallel beam of light and generates interference fringes like the light source described above. By driving the interference filter 31 with the linear motor 32, light of any wavelength can be obtained. When using light with a constant wavelength,
The position where the dark part of the interference fringe occurs moves as the average value of the gap changes, but according to the above embodiment, the deviation corresponding to the component of the amount of movement of the interference fringe that moves slowly over time The signal is transferred to the circuit 26 in FIG.
If the feedback is performed, the dark part of the interference fringes can be kept within the field of view of the photoelectric converter 7, regardless of changes in the average value of the gap. If dynamic changes in the gap are measured in this state using the method shown in the example above,
Measurements can be made continuously, always focusing on the same dark part of the interference fringes. At this time, the relationship between the dynamic change in the gap and the amount of movement of the dark part of the interference pattern changes depending on the wavelength used, so it is necessary to calibrate it in advance.

In FIG. 10, an interference filter whose transmission wavelength changes is arranged and operated linearly, but it is clear that the same effect can be produced even if the emission wavelength of the light source is changed. In this case, a dye laser or the like is used as a light source whose wavelength can be varied over a relatively wide band.

According to the optical measurement method of microgap of the present invention,
With respect to the interference fringes created by a normal optical system, it is possible to accurately capture the components that change the interference fringe image, so it is possible to accurately measure the average gap amount of micro gaps and the gap amount that changes over time. can be measured.

[Brief explanation of the drawing]

Fig. 1 is a principle diagram for explaining the conventional optical measurement method for micro-gaps, Fig. 2 is a diagram for explaining the relationship between the gap and the brightness of interference light, and Fig. 3 is a diagram for explaining the micro-gap according to the present invention. FIG. 4 is a diagram to explain the photoelectric conversion unit in the method of the present invention, FIG. 5 is a diagram to explain the correspondence between the gap and the interference fringe image, FIG. 6 is a diagram for explaining the measuring circuit in the method of the present invention, FIGS. 7 and 8 are diagrams for explaining the operating principle of the method of the present invention, and FIG. 9 is another embodiment of the method of the present invention. FIG. 10 is a diagram for explaining the wavelength selection section in another embodiment of the method of the present invention,
FIG. 11 is a diagram for explaining a transmission wavelength variable interference filter. 3... Disk, 4... Head, 7... Photoelectric conversion section,
9...Measuring circuit.

Claims (1)

[Claims] 1. Interference fringes are generated in the small gap between the objects to be measured using optical interferometry, and the interference fringes are scanned by a photoelectric converter, and the small gap is detected optically from the output signal corresponding to the brightness of the dark or bright part of the interference fringe. In this method, the interference fringes are scanned with a scanning width determined by the spacing of the interference fringes, and a measurement circuit generates a voltage signal corresponding to the amount of change in the interference fringes from the output signal from the photoelectric conversion unit and the scanning signal. The voltage signal is fed back to the deflection circuit of the photoelectric conversion unit to track the interference fringe so that the dark or bright part of the interference fringe appears to remain stationary, and the change in the minute gap is determined from the amount of feedback at this time. An optical measurement method for a minute gap, characterized in that the amount is measured.