JPH07229828A - Method and apparatus for measuring birefringence of optical disk substrate - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide a method and an apparatus capable of measuring the in-plane and vertical birefringence of an optical disk substrate accurately, quickly and easily, and also applicable in-line. [Structure] A parallel light beam is obliquely incident on a substrate of an optical disc having a transparent resin substrate on which at least a recording layer is provided from a surface opposite to the recording layer, and a transmitted light or a reflected light is generated. A method for measuring the birefringence of a substrate by measuring a phase difference, characterized in that the birefringence of the optical disk substrate is measured with respect to at least two directions of an incident surface including an incident light beam at right angles. Measurement method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring birefringence of an optical disk substrate, and more particularly to an in-line and vertical birefringence measuring method.

[0002]

2. Description of the Related Art More than 10 years have passed since the introduction of a hole-type recording medium as a recordable optical disk.
During this period, a magneto-optical recording medium capable of recording and erasing, a phase change recording medium capable of one-beam overwriting, etc. have been put into practical use.

A semiconductor laser is used as a recording / reproducing light source except in the very beginning, and the laser wavelength used is around 830 nm at the initial stage and recently around 780 nm. Since the spot diameter of the focused light beam can be made small if the wavelength is short, it is desired to shorten the wavelength. At present, however, the wavelength of a reliable and practical semiconductor laser is up to 780 nm. Such an optical recording medium has a recording layer, a protective layer and the like formed on a transparent resin substrate from the viewpoint of cost and mass productivity.

At present, a polycarbonate resin or the like is mainly used as a substrate. In a resin substrate, particularly a polycarbonate resin substrate, the optical anisotropy of the substrate, that is, birefringence becomes a problem. Especially in magneto-optical recording media,
To detect a small Kerr rotation angle of about 0.5 degrees,
The effect of birefringence is large.

However, the in-plane birefringence is 20 × 10 ⁻⁶ by optimizing the molecular weight of the resin and improving the molding technique.
It is suppressed to less than less than the practical level.
On the other hand, the vertical birefringence is particularly large in the polycarbonate resin substrate and reaches 500 × 10 ⁻⁶ , but with the development of the actuating optical head, its influence has been reduced to a level where there is no practical problem. However, optical discs are required to have higher densities, and semiconductor lasers having a wavelength of about 680 nm have been put into practical use, and it is expected that inexpensive and high-output optical discs will be provided in the near future.

Further, the technology for obtaining a wavelength of about 500 nm by combining a high-power semiconductor laser of about 800 to 1000 m and a non-linear element has advanced, and the head combining the laser and the non-linear optical element is becoming smaller. Furthermore, there have been reports that semiconductor lasers with a wavelength of about 500 nm have been successfully developed at the laboratory level. In this way, high-density optical discs using short-wavelength semiconductor lasers are being put into mass production in the near future, starting with wavelengths around 680 nm. At this time, 780
At about nm, there is concern that the problem of optical anisotropy of the resin substrate, which was once considered to be solved, will become a serious problem again.

That is, there are the following two problems as to the optical anisotropy (birefringence) of the resin substrate (I. Prikryl, Applied Optics, 31 (1992), p1853, Toda et al., Optical Memory Symposium). Proceedings (1986), p1
9, Yoshizawa et al., Optical Memory Symposium Proceedings (198
6), p33). 1) A phase difference that occurs when a light beam passes through a substrate. In a medium such as a magneto-optical medium which records and reproduces information by utilizing the deflection of light and the rotation of its direction, ellipticization occurs along with the rotation of linear deflection in a specific direction. This causes an increase in common mode noise.

The phase difference is determined by the incident direction of light rays, where Δn is the birefringence of the substrate, d is the substrate thickness, and λ is the wavelength.

[0009]

Since it is determined by Δn · d / λ, the phase difference substantially increases as the wavelength used for recording / reproducing becomes shorter. Therefore, shortening the wavelength, especially 70
In the magneto-optical medium used at less than 0 nm, the problem of retardation due to the birefringence of the substrate becomes serious.

2) The problem of astigmatism due to birefringence. Refraction occurs when a focused light beam impinges on a substrate obliquely rather than perpendicularly to the substrate. However, in a substrate having optical anisotropy, the refractive index differs depending on the direction of the incident ray (Yoshizawa, Optics, 15 (1986), p414). For this reason,
Originally, astigmatism occurs in the beam to be focused on the surface of the recording layer side of the substrate having a diameter of about 1 μm.

When astigmatism occurs, the recording and reproducing characteristics vary due to the difference in the optical heads where the focal plane is adjusted. Further, when the beam is an elliptical beam having a long axis in the track crossing direction, crosstalk from adjacent tracks becomes a problem. In a high-density optical disc using a short-wavelength light source, the track pitch also becomes narrower,
The problem of crosstalk becomes even more severe.

The only solution to this problem is to reduce the birefringence in the vertical direction. However, with a commonly used polycarbonate substrate, it is about 500 × 10 ⁻⁶ , and at least about 300 × 10 ^−6. Due to the existence of the birefringence of, it is almost impossible to eliminate astigmatism as long as a linearly polarized beam is used. In view of the problems described above,
It is important to control the birefringence of the substrate in the manufacturing process. For that purpose, it is necessary to measure the birefringence of the substrate accurately and promptly and reflect it in the manufacturing conditions.

Conventionally, as the in-plane and vertical birefringence measuring method of a transparent resin substrate of an optical disk, a phase difference measuring method using obliquely incident light has been used. 2 and 3 schematically show the positional relationship of the incident light beam with respect to the disk in the conventional oblique incidence measurement method, in which A is a light emitting portion for measuring parallel beam light and B is a light receiving portion.

FIG. 2 shows the measurement by the transmission method, and FIG. 3 shows the measurement by the reflection method. In order to determine the size of each axis of the index ellipsoid at one point O in the substrate, that is, the birefringence, in the usual transmission method, the incident angle θ of the incident light beam and the azimuth angle φ of the incident surface are calculated. The phase difference caused by passing through the substrate was measured by changing the points, but it was necessary to change at least two points (Yoshizawa, Kogaku, 15 (1986), p414.
-421, Toda et al., Optical Memory Symposium Proceedings, p1
9).

In the transmission method, the in-plane birefringence can be directly obtained by injecting the measurement light beam perpendicularly to the substrate and measuring the phase difference. For the birefringence in the direction perpendicular to the substrate, the light beam cannot be incident from the side of the substrate.Therefore, an obliquely incident light beam is used to measure the phase difference affected by both the vertical birefringence and the in-plane birefringence. , The contribution of the in-plane birefringence is obtained by correcting the contribution of the in-plane birefringence value obtained by the vertical incidence.

However, in this case, it is necessary to tilt the substrate to change the incident angle, which is complicated to apply to in-line measurement. On the other hand, it is known that the birefringence of the resin changes depending on the stress applied to the resin, but the stress applied to the substrate also changes depending on the optical disc manufacturing process after the substrate is molded.

For example, it may be changed by the internal stress of the recording layer, the dielectric layer used for protecting the recording layer, the hard coat layer used for protecting the substrate, and the like. Therefore, since the birefringence may change in these film forming steps and the like, although it is secondary, it is desirable to measure it again in the final step of optical disc manufacturing. Generally, since the recording layer of an optical disk is reflective, it is necessary to make a measurement light beam incident from the surface of the substrate opposite to the recording layer and perform measurement by the reflection method.

As one of the methods, a method of measuring the phase difference of reflected light using an ellipsometer has been proposed (A. Skumanich, Proceedings of Magneto-Optical Reco.
rding International Symposium '92, pp237-240). Generally, in the measurement by the reflection method, it is impossible to measure the normal incidence / reflected light on the substrate surface because the light emitting portion and the light receiving portion cannot be placed on the same line.

Therefore, it is impossible to directly measure the in-plane birefringence, and the in-plane and vertical birefringences are obtained by curve fitting with several angles of oblique incidence. This method does not cause any problems in principle, but in order to change the incident angle, it is necessary to reset both the angles of the light emitting part and the light receiving part, so the device is generally complicated and the measurement time becomes long. Therefore, it is not suitable for in-line measurement in the manufacturing process.

Further, in this method, the correct birefringence value cannot be obtained unless the direction of the principal axis and the direction of the incident surface containing the incident beam are matched. Further, in both the transmission method and the reflection method, when the optical main axis is tilted from the direction perpendicular to the substrate, both the incident angle and the incident surface are changed and accurate measurement values must be obtained at multiple points. Can't get

In addition, for complicated in-line measurement,
Furthermore, it is not applicable even to the sampling inspection.

[0022]

As described above, there is a demand for a method capable of measuring the in-plane and vertical birefringence of an optical disk substrate accurately, quickly and simply and applicable in-line.

[0023]

According to the present invention, a parallel light beam is obliquely incident on a substrate of an optical disc having at least a recording layer provided on a transparent resin substrate from a surface opposite to the recording layer. A method for measuring the birefringence of a substrate by measuring the phase difference generated in the transmitted light or the reflected light, wherein the birefringence is measured by at least two orthogonal directions of the incident surface including the incident light beam. Is a method for measuring birefringence of an optical disk substrate.

As a simpler method of implementing the present invention, if the principal axis direction of the in-plane birefringence of the substrate is known and the direction is substantially stable, the above two orthogonal directions can be used as an optical axis in the plane of the substrate. By setting the direction of the axis, necessary and sufficient information regarding in-plane and vertical birefringence can be obtained.

Furthermore, according to the method of the present invention, as means for automatically measuring the in-plane and vertical birefringence distribution at each position in the substrate plane, a linear moving mechanism that is movable in the two orthogonal directions described above, A rotary stage that is installed on a moving mechanism and can rotate around each point on the linear axis and horizontally installs a disk on it, and a phase difference is measured by injecting a light beam obliquely onto the disk surface to be measured. A birefringence measuring apparatus for an optical disc is proposed, which is composed of a light emitting section and a light receiving section.

The present invention will be described in more detail below. In the present invention, it is sufficient for the incident angle θ to be one point of oblique incidence that is appropriately inclined, and it is desirable to set it to about 30 to 70 degrees, for example. Below 30 degrees, the contribution of vertical birefringence is small,
If it is larger than 0 degree, the contribution of the in-plane birefringence is small, and it is not preferable because an error becomes large if both phase differences are obtained at the same time.

In the present invention, the azimuth angle φ of the incident surface is changed by moving only the substrate in the horizontal plane while keeping the incident angle fixed.
Change. Since it is not necessary to change the inclination of the light emitting part, the light receiving part, and the disk only by relatively shifting the position of the disk, it is possible to simplify the measurement jig and the procedure.
Therefore, it is also cheap. Also, the measurement time can be shortened.

In the present invention, as described above, the azimuth angle φ of the incident surface is changed by at least four points, the phase difference is measured, and fitting is performed with the theoretically obtained phase dependence φ dependence curve. Determine in-plane and vertical birefringence. Furthermore, it is also possible to find the direction of the optical principal axis. In FIG. 4, the azimuth of the main axis is taken in the (x ', y', z ') axis direction,
The positional relationship when the direction perpendicular to the substrate surface is the z axis and the in-plane of the substrate is the x and y axes is shown.

The Euler angles of the azimuths (x ', y', z ') of the principal axes with respect to the coordinate axes (x, y, z) are (α, β, γ).
And the refractive index corresponding to each axis of (x ', y', z ') is (nx, ny, nz). Here, α is an angle formed by the main axis z ′ axis and the z axis, and β is a plane P formed by the z axis and the z ′ axis.
The angle between 1 (the shaded area) and the y-axis, γ is the plane P1 and y'oz '
It is the angle between the faces.

(Α, β, γ), (nx, ny, nz) and the incident angle θ, the refractive indices n ′ and n ″ and the refraction angles θ ′ and θ ″ for the two propagation directions allowed in the substrate. The azimuth angle φ and the phase difference R by the transmission method when the substrate thickness d is expressed by the relational expression given by the following expression.

[0031]

## EQU00002 ## R = d.multidot. (N'cos.theta .'- n "cos.theta.") Where sin.theta. = N'sin.theta. '= N "sin.theta." The theoretical values of the plural .phi. And .theta. Curve fitting is performed using the measured values at the measurement points, and the six parameters (α, β, γ) and (nx, ny, nz) that determine the index ellipsoid are obtained.

However, in the injection-molded resin substrate which is widely used in practice, due to its symmetry, the main axis in the substrate plane is approximately in the radius and circumferential directions, which are nx and ny.
Corresponding to. Further, there is a principal axis in a direction substantially vertical to the substrate surface, which corresponds to nz. Therefore, γ may be regarded as 0 out of the 6 parameters for defining the index ellipsoid.

Birefringence δL = nx-nz, δV = ny-
Since nz is several orders of magnitude smaller than nz itself,
The phase difference is actually determined by ΔL and ΔV. There is no problem even if nz is approximately the average value of the respective refractive indices. For example, for a polycarbonate resin often used as an optical disk substrate, δL and δV may be obtained with nz set to 1.58.
That is, there are actually four unknown parameters.

The azimuth angle φ dependence of the phase difference at oblique incidence is
5 to 8 show how the four parameters (δL, δV, α, β) change. 5 shows the vertical birefringence dependence of the phase difference, FIG. 6 shows the birefringence (δL, δV) dependence of the phase difference, FIG. 7 shows the principal axis azimuth (α, β) dependence of the phase difference, and FIG. 8 shows the phase difference. Main axis direction (α, β)
It is a figure which shows a dependency.

The incident angle is 30 degrees for each map a and 60 for the map b.
Nz = 1.58, in-plane birefringence δL is 0 to 20 × 10 ⁻⁶ , and vertical birefringence δV is 0.
Although 600 × 10 ⁻⁶ , α is in the range of 0 to 10 degrees, and β is in the range of 0 to 360 degrees, the φ dependence of the phase difference has symmetry with respect to the in-plane principal axis direction. Therefore, if the phase difference is measured at at least four points that are orthogonal to φ, then φ
It can be seen that the characteristic of the dependency curve can be extracted and accurate curve fitting can be performed without changing θ.

In the present invention, the measurement of the phase difference itself is carried out by an ordinary method, that is, the linearly polarized light or the circularly polarized beam is used as the incident light, and the elliptical polarization due to the phase difference caused by the passage of the substrate is detected.

For the phase difference between the principal axes of the ellipse, a known method such as a rotation analyzer method or a method using a retardation plate may be applied ("
Crystal optics ", edited by the Japan Society of Applied Physics, Optical Round-table Conference.) In the reflection method, the measurement can be performed in exactly the same manner except that the optical path length is doubled and that a phase difference of about 180 degrees is added due to reflection on the recording layer surface. In the present invention, the incident angle θ is fixed, but there is an advantage that the index ellipsoid can be easily determined including the principal axis direction thereof regardless of the transmission method / reflection method.

Further, when the directions (α, β) of the main axes are almost fixed, this method can be simplified. That is, in an optical disk substrate manufactured by normal injection molding, the nx and ny axes are oriented in the radial direction and the circumferential direction, respectively, due to the symmetry of the resin flow, and the deviation is 5 degrees at most. Also,
The nz axis is perpendicular to the substrate, and only a 1-2 degree shift occurs.

In this case, the influence of an accurate deviation from the main axis is extremely small and can be ignored. Then, the incident azimuth is in the radial direction (φ = 0 or only 180 degrees) and in the circumferential direction (φ =
It suffices to measure the phase difference in only two points in two directions (one of 90 or 270 degrees). The relationship between the phase difference R and δL and δV is expressed by the following equation (1). In the following, δL = n _x −n
_y, and _{δV = n 0 -n z, n} 0 = the _{(n x + n y) /} 2 is defined. or the .DELTA.V is thus defined, δV = n _y -n _z or δ
Whether V = n _x −n _z is arbitrary, but normally n _x ≈n _y
It doesn't make a big difference.

[0040]

Equation 3] _{R r = d × {√ (} n y 2 -sin 2 θ) + n x / n z × √ (n z 2 -sin 2 θ)} ·· (1) d: thickness of the substrate

[0041]

Equation 4] Rφ = d × {√ (n x 2 -sin 2 θ) + n y / n z × √ (n z 2 -sin 2 θ)} ·· (2) d: thickness of the substrate

Furthermore, since δL / nx and δV / nx << 1, the above equations (1) and (2) are changed to δL / nx and δV.
Expanding about / nx

[0043]

## EQU5 ## R _r = d × {-(n ₀ ² -sin ² θ) δL / 2-sin ² θ · δV} / {n ₀ √ (n ₀ ² -sin ² θ)} (3 )

[0044]

## EQU6 ## Rφ = d × {+ (n ₀ ² -sin ² θ) δL / 2−sin ² θ · δV} / {n ₀ √ (n ₀ ² −sin ² θ)} (4)

Therefore, by adding and subtracting each of the expressions (3) and (4),

[0046]

## EQU7 ## δV = {(R _r + Rφ) n ₀ √ (n ₀ ² −sin ² θ)} / (2d sin ² θ) (5)

[0047]

## EQU8 ## δL = {(Rφ-R _r ) n ₀ √ (n ₀ ² −sin ² θ)} / {d × (2n ₀ ² −sin ² θ)} (6)

To obtain That is, by measuring two values of Rr and Rφ at a specific incident angle θ, δL and δV can be easily obtained from the equations (5) and (6). According to the method of the present invention, the measurement is performed at two points, and the values of both in-plane and vertical birefringence can be obtained by simple calculation. Since the measurement time can be greatly reduced, in-line measurement during the process becomes possible.

By using this method, the in-plane distributions of δL and δV can be easily obtained. That is, in the conventional method, it was necessary to change the incident angle at each point to obtain the incident angle dependence of the phase difference, but in the present method, the incident angle is fixed and the disk may be moved in the horizontal plane. Only.

FIG. 1 shows a conceptual diagram of the in-plane distribution measuring device of the present invention by the reflection method. In FIG. 1, a stage 2 on which a disc 1 is placed is rotatably provided on linear movement mechanisms 3 and 4 which are movable in two directions orthogonal to each other, and a mechanism which is rotatable around each point on the linear movement mechanisms 3 and 4. Have five.

A normal ellipsometer may be used to measure the phase difference itself. The angles of the light emitting portion A and the light receiving portion B do not have to be variable. With a normal injection molded disc,
Since the refractive index distribution in the circumferential direction is small, measuring only the radial direction distribution is sufficient for process control. The movement in these two directions may be achieved by simply sliding the disc on a wide horizontal stage without the mechanism shown in FIG.

[0052]

EXAMPLES The present invention will be described in more detail below with reference to examples. Example 1 A commercially available 5.25-inch size magneto-optical disk was taken out from the cartridge, and was put in two directions (φ and radial direction).
= 0, 180 degrees and 90, 270 degrees)
The phase difference was measured by the reflection method.

The material of the substrate is polycarbonate. Only the radial distribution was measured. The in-plane birefringence δL and the vertical birefringence δV were obtained based on the above equations (5) and (6). In this case, θ = 60 degrees, nx = 1.58, d =
1.2mm,

[0054]

## EQU9 ## δV = 1.160 × 10 ³ × (Rr + Rφ) δL = 4.101 × 10 ² × (Rφ-Rr), and δL and δV can be obtained only by multiplying the coefficient, and curve fitting is unnecessary. A commercially available ellipsometer (Gartner, wavelength 633 nm) was used to measure the phase difference.

One measurement is about 20 seconds, and the measurement time at a specific radius is within 1 minute. Table 1 shows the calculation results of birefringence. There is no significant difference in the measured values of the phase difference between two points in the radial direction, 90 and 180 degrees, and two points in the circumferential direction, 9 and 90 degrees.
There is no difference in phase difference at 0 and 270 degrees. Therefore,
The main axis may be oriented in a radius, a circumference, and a direction perpendicular to the substrate. That is, (α, β, γ) = (0, 0, 0).

In this case, it is sufficient to measure only two points of φ = 0 and 90 degrees, which can be further simplified. Examples 2 and 3 The transparent substrate (polycarbonate) after molding was measured by a transmission method. Incident angle θ = fixed at 30 degrees, azimuth angle φ 0, 9
There were four points of 0, 180, and 270 degrees.

Table 2 shows the measured values of the phase difference. In Example 2, there is no difference in the measured values of the phase difference between the two points in the radial direction and the two points in the circumferential direction, and the main axis is not tilted. On the other hand, Example 3
Then, we can see that this symmetry is lost and the principal axis is tilted. Table 2 also shows the results of obtaining the principal axis direction and the birefringence value.

[0058]

[Table 1]

[0059]

[Table 2]

[0060]

According to the present invention, the in-plane and vertical birefringence of an optical disk substrate can be measured accurately, quickly and simply, and it can be applied in-line.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an in-plane distribution measuring device of the present invention by a reflection method.

FIG. 2 is a diagram schematically showing a positional relationship of an incident light beam with respect to a disk in a conventional oblique incidence measurement method.

FIG. 3 is a diagram schematically showing a positional relationship of an incident light beam with respect to a disk in a conventional oblique incidence measurement method.

FIG. 4 shows the orientation of the main axis in the (x ′, y ′, z ′) axis direction, the direction perpendicular to the substrate surface is the z axis, and the in-plane of the substrate is x, y.
Diagram showing the positional relationship when the axis is taken

FIG. 5 shows that the azimuth φ dependence of the phase difference at oblique incidence is 4
Diagram of the vertical birefringence dependence of the phase difference, showing how it changes with each parameter (δL, δV, α, β)

FIG. 6 shows that the azimuth angle φ dependence of the phase difference at oblique incidence is 4
Birefringence of phase difference (δL, δV), which shows how it changes with each parameter (δL, δV, α, β)
Dependency diagram

FIG. 7 shows that the azimuth φ dependence of the phase difference at oblique incidence is 4
Principal axis azimuth (α, β) of the phase difference, showing how it changes depending on the number of parameters (δL, δV, α, β)
Dependency diagram

FIG. 8 shows that the azimuth angle φ dependence of the phase difference at oblique incidence is 4
Principal axis azimuth (α, β) of the phase difference, showing how it changes depending on the number of parameters (δL, δV, α, β)
Dependency diagram

[Explanation of symbols]

 1 Disc 2 Stage 3 Linear Moving Mechanism 4 Linear Moving Mechanism 5 Rotating Mechanism A Parallel Beam Light Emitting Section for Measurement B Light Receiving Section

Claims (3)

[Claims] 1. A parallel light beam is obliquely incident on a substrate of an optical disc having a transparent resin substrate on which at least a recording layer is provided from a surface opposite to the recording layer, and the transmitted light or reflected light is obtained. A method of measuring the phase difference produced to measure the birefringence of a substrate, wherein the birefringence of the optical disk substrate is measured with respect to at least two directions of an incident surface including an incident light beam which are orthogonal to each other. Refraction measurement method. 2. The measuring method according to claim 1, wherein the two orthogonal directions coincide with the direction of the in-plane principal axis of the substrate. 3. A linear moving mechanism that is movable in two orthogonal directions, and a rotary stage that is installed on the linear moving mechanism, is rotatable around each point on the linear axis, and horizontally installs a disk on it. And a birefringence measuring device for an optical disk substrate, which comprises a light emitting part and a light receiving part for obliquely injecting a light beam onto the surface of the disk to be measured to measure the phase difference.