JP3011036B2 - Measurement method of birefringence of optical disk substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art More than 10 years have passed since the emergence of a perforated recording medium as a recordable optical disk.
Meanwhile, magneto-optical recording media capable of recording and erasing, phase change recording media capable of overwriting by one beam, and the like have also been put to practical use.

[0003] Except for the very beginning, a semiconductor laser is used as a recording / reproducing light source, and the wavelength of the laser used is initially around 830 nm, and recently around 780 nm. Since the spot diameter of the focused light beam can be reduced 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 is obtained by forming a recording layer, a protective layer, and the like 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 the case of 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,
In order to detect a small Kerr rotation angle of about 0.5 degree,
The influence of birefringence is great.

However, by optimizing the molecular weight of the resin and improving the molding technique, the in-plane birefringence becomes 20 × 10 ^−6.
, Which is no problem in practical use.
On the other hand, the vertical birefringence is particularly large in the case of a polycarbonate resin substrate and reaches as high as 500 × 10 ⁻⁶ , but the influence has been reduced to a level that does not cause any practical problems due to the development of the working optical head. However, there is a demand for higher density optical discs, and semiconductor lasers having a wavelength of about 680 nm have been put into practical use, and it is expected that cheap 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 with a nonlinear element has been advanced, and the head combining a laser and a nonlinear optical element has been reduced in size. Furthermore, there have been reports that semiconductor lasers having a wavelength of about 500 nm have been successfully developed at the laboratory level. As described above, high-density optical discs using a semiconductor laser with a shorter wavelength are being mass-produced in the near future, beginning with a wavelength of about 680 nm. At this time, 780
At about nm, there is a concern that the problem of optical anisotropy of the resin substrate once considered to be solved will become a serious problem again.

That is, the following two problems are associated with 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., Proceedings of Optical Memory Symposium (198
6), p33). 1) A phase difference generated when a light beam passes through a substrate. In a medium such as a magneto-optical medium that records and reproduces information by utilizing the deflection of light and the rotation of its azimuth, the rotation of linear deflection in a specific direction causes an ovalization, which lowers the carrier level and reduces the working head. This leads to an increase in common mode noise.

The phase difference is determined by Δn, the substrate thickness d, and the wavelength λ, which are determined by the incident direction of the light beam.

[0009]

Since it is determined by か ら n · d / λ, the phase difference substantially increases as the wavelength used for recording / reproducing becomes shorter. Therefore, a shorter wavelength, especially 70
In a magneto-optical medium used below 0 nm, the problem of phase difference due to birefringence of the substrate becomes serious.

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

When astigmatism occurs, the recording / reproducing characteristics vary due to differences in the optical head where the focal plane is adjusted. Further, when the beam is an elliptical beam having a major axis in the cross-track direction, crosstalk from an adjacent track becomes a problem. In a high-density optical disk using a short-wavelength light source, the track pitch 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, in the case of a commonly used polycarbonate substrate, it is about 500 × 10 ⁻⁶ , and at least about 300 × 10 ^−6. It is almost impossible to eliminate astigmatism as long as a linearly polarized beam is used. In view of the problems mentioned above,
It is important to control the birefringence of the substrate in the manufacturing process. For this purpose, it is necessary to first and accurately measure the birefringence of the substrate and reflect it in the manufacturing conditions.

Conventionally, as a method for measuring birefringence in a plane and in a vertical direction of a transparent resin substrate of an optical disk, a phase difference measurement 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, wherein A is a light emitting portion of the measuring parallel light beam, 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 refractive index ellipsoid at one point O in the substrate, that is, the birefringence, in an ordinary transmission method, what is the incident angle θ of the incident light beam and the azimuth φ of the incident surface? The phase difference caused in the transmitted light by passing through the substrate is measured by changing the point, but it was necessary to change at least θ by two points (Yoshizawa, Optics, 15 (1986), p414).
-421, Toda et al., Proceedings of Optical Memory Symposium, p1
9).

In the transmission method, the in-plane birefringence can be directly obtained by measuring the phase difference by vertically irradiating the measurement light beam to the substrate. Since birefringence in the direction perpendicular to the substrate does not allow the light beam to be incident from the side of the substrate, use a diagonally incident light beam to measure the phase difference affected by both vertical and in-plane birefringence. , By correcting the contribution of in-plane birefringence using the in-plane birefringence value obtained by the above-mentioned normal incidence.

However, in this case, it is necessary to change the angle of incidence by tilting the substrate, which is complicated to apply to in-line measurement. On the other hand, it is known that the birefringence of the resin changes due to the stress applied to the resin, but the stress applied to the substrate also changes depending on the optical disk manufacturing process after molding the substrate.

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

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

Therefore, it is impossible to directly measure the in-plane birefringence, and the in-plane and vertical birefringence are determined by curve fitting with oblique incidence from several angles. Although this method has no problem in principle, changing the incident angle requires resetting both the angle of the light-emitting unit and the light-receiving unit, which generally complicates the apparatus and increases the measurement time. Therefore, it is not suitable for in-line measurement in the manufacturing process.

Furthermore, in this method, a correct birefringence value cannot be obtained unless the direction of the main axis and the direction of the incident surface including the incident beam are made to coincide. Furthermore, in both the transmission method and the reflection method, when the optical principal axis is inclined from a direction perpendicular to the substrate, accurate measurement values are obtained unless both the incident angle and the incident surface are changed and measured at a plurality of points. Cannot be obtained.

In addition, for complicated and in-line measurement,
Furthermore, it cannot be applied even to sampling inspection.

[0022]

As described above, there is a need for a method that can accurately, quickly and easily measure the in-plane and vertical birefringence of an optical disk substrate and can be applied in-line.

According to the present invention, a parallel light beam is obliquely incident on a substrate of an optical disk having at least a recording layer provided on a transparent resin substrate from a surface opposite to the recording layer and transmitted or reflected by the parallel light beam. A method of measuring the birefringence of a substrate by measuring a phase difference generated in light, and measuring a certain point.
With respect to a point, while keeping the incident angle θ of the incident light with respect to the substrate surface constant, the phase difference is measured for at least two directions perpendicular to the direction of the incident surface including the incident light beam.
The birefringence of the optical disc substrate is measured by measuring the birefringence of the one point from each of the obtained phase differences .

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

Further, as means for automatically measuring the distribution of in-plane and vertical birefringence at each position in the substrate plane by the method of the present invention, a linear movement mechanism capable of horizontally moving in the two orthogonal directions, A rotary stage, which is installed on a linear movement mechanism and is rotatable about each point on the linear axis and horizontally installs a disk thereon, and makes a light beam obliquely incident on the surface of the disk to be measured to reduce the phase difference. Measurement using a birefringence measuring device composed of a light emitting part and a light receiving part to be measured
The present invention proposes a method for measuring the birefringence of the optical disk substrate described above .

Hereinafter, the present invention will be described in more detail. In the present invention, one angle of incidence, which is appropriately inclined, is sufficient for the incident angle θ, and it is desirable that the angle of incidence is, for example, about 30 to 70 degrees. Below 30 degrees, the contribution of vertical birefringence is small,
If the angle is larger than 0 degree, the contribution due to the in-plane birefringence is small, and it is not preferable to simultaneously obtain both phase differences because an error increases.

In the present invention, only the substrate is moved in the horizontal plane while the incident angle is fixed, so that the azimuth angle φ of the incident surface is
To change. Since it is not necessary to change the inclination of the light emitting unit, the light receiving unit and the disk only by relatively shifting the position of the disk, the measuring jig and the procedure can be simplified,
Therefore, it becomes cheap. In addition, the measurement time can be shortened.

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

The Euler angles of the azimuth (x ', y', z ') of the main axis with respect to the coordinate axes (x, y, z) are (α, β, γ)
And the refractive indices corresponding to the (x ', y', z ') axes are respectively (nx, ny, nz). Here, α is the angle between the principal axes z ′ and z, and β is the plane P between the z and z ′.
1 (hatched portion) and the angle formed by the y axis, γ is the plane P1 and y'oz '
This is the angle between the faces.

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

[0031]

R = d · (n′cosθ′−n “cosθ”) where sinθ = n′sinθ ′ = n ”sinθ” The theoretical values of the plurality of φ and θ dependencies of the phase difference and the values of the plurality of measurements Curve fitting is performed using the measured values at the measurement points, and six parameters (α, β, γ) and (nx, ny, nz) for determining the refractive index ellipsoid are obtained.

However, in a resin substrate formed by injection molding, which is widely used in practical use, the principal axis in the substrate surface is substantially in the radial and circumferential directions due to its symmetry, which is nx and ny.
Corresponding to Further, there is a main axis substantially perpendicular to the substrate surface, and this corresponds to nz. Therefore, among the six parameters for defining the refractive index ellipsoid, γ may be regarded as 0.

Also, 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 each refractive index. For example, in 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
FIG. 5 to FIG. 8 show how changes are made depending on the four parameters (δL, δV, α, β). 5 shows the dependency of the phase difference on the vertical birefringence, FIG. 6 shows the dependency of the phase difference on the birefringence (δL, δV), FIG. 7 shows the dependency of the phase difference on the principal axis directions (α, β), and FIG. Main axis direction (α, β)
It is a figure which shows dependency.

The incident angle was 30 degrees for each plot a and 60 for the plot b.
Nz = 1.58, in-plane birefringence δL is 0 to 20 × 10 ⁻⁶ , and vertical birefringence δV is 0 to 20 × 10 ⁻⁶ .
Although 600 × 10 ⁻⁶ , α are in the range of 0 to 10 degrees, and β are in the range of 0 to 360 degrees, the φ dependence of the phase difference shows symmetry in the in-plane principal axis direction. Therefore, if the phase difference is measured at least at four points orthogonal to φ, φ
It can be seen that the characteristics 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 performed by a normal method, that is, by using a linearly or circularly polarized beam as incident light and detecting elliptical deflection due to the phase difference caused by passing through the substrate.

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

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

In this case, the influence of an accurate deviation from the main spindle is extremely small and can be ignored. Then, the incident azimuth is changed in the radial direction (only one of φ = 0 or 180 degrees) and the circumferential direction (φ =
The phase difference may be measured in only two points in two directions (only one of 90 or 270 degrees). The relationship between the phase difference R and δL, δV is represented by the following equation (1). In the following [delta] L = 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
V = n _x -n or a _z may have any properties, but generally n _x ≒ n _y
There is no 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

Further, since δL / nx and δV / nx << 1, the above equations (1) and (2) are replaced by δL / nx and δV
Expanding on / nx

[0043]

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]

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 adjusting each of the equations (3) and (4),

[0046]

Equation 7] _{δV = {(R r + Rφ} ) n 0 √ (n 0 2 -sin 2 θ)} / (2dsin 2 θ) ··· (5)

[0047]

ΔL = {(Rφ−R _r ) n ₀ √ (n ₀ ² −sin ² θ)} / {d × (2n ₀ ² −sin ² θ)} (6)

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

Using this method, the distribution of ΔL and ΔV in the disk plane can be easily obtained. That is, in the conventional method, it was necessary to change the incident angle at each point to determine the incident angle dependency of the phase difference, but in the present method, the incident angle is fixed and the disk may be moved in a horizontal plane. Only.

FIG. 1 shows a conceptual diagram of the in-plane distribution measuring apparatus of the present invention by the reflection method. In FIG. 1, a stage 2 on which a disk 1 is placed is rotatably provided on linear moving mechanisms 3 and 4 which can move in two orthogonal directions, and a mechanism capable of rotating about each point on the linear moving mechanisms 3 and 4. Five.

The measurement of the phase difference itself may be performed by using an ordinary ellipsometer. The angles of the light emitting unit A and the light receiving unit B do not need to be variable. With a normal injection molded disc,
Since the refractive index distribution in the circumferential direction is small, measuring only the radial distribution is sufficient for process control. The movement in these two directions may be achieved by manually shifting the disk on a wide horizontal stage without the mechanism shown in FIG.

[0052]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples. Example 1 A commercially available magneto-optical disk having a size of 5.25 inches was taken out of a cartridge, and was taken out of a cartridge in two directions (φ and φ).
= 0, 180 degrees and 90, 270 degrees) at an incident angle of 60
The phase difference was measured by the reflection method in degrees.

The material of the substrate is polycarbonate. Only the radial distribution was measured. The in-plane birefringence δL and the vertical birefringence δV were determined based on the above formulas (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 simply by multiplying the coefficients, so that curve fitting is unnecessary. The phase difference was measured using a commercially available ellipsometer (Gartner, wavelength 633 nm).

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 the birefringence. There is no significant difference in the measured value of the phase difference at two points in the radial direction, 90 and 180 degrees, and two points in the circumferential direction, 9 and 180 degrees.
There is no difference in phase difference between 0 and 270 degrees. Therefore,
The main axis may be oriented at a radius, circumference, and 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 molded transparent substrate (polycarbonate) was measured by a transmission method. Incident angle θ = 30 degrees fixed, azimuth φ 0, 9
Four points of 0, 180, and 270 degrees were set.

Table 2 shows the measured values of the phase difference. In the second embodiment, there is no difference between the measured values of the phase difference at two points in the radial direction and two points in the circumferential direction, and the main axis is not inclined. On the other hand, Example 3
Then the symmetry is lost and the main axis is tilted. Table 2 also shows the results of determining 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 accurately, quickly and easily measured, and the method can be applied in-line.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an in-plane distribution measuring apparatus of the present invention using 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 as the z axis, and the x, y in the substrate surface.
Diagram showing the positional relationship when taken on the axis

FIG. 5 shows that the azimuth angle φ dependence of the phase difference at oblique incidence is 4
Of the dependence of the phase difference on the vertical birefringence, showing how it varies with the number of parameters (δL, δV, α, β)

FIG. 6 shows that the azimuth angle φ dependence of the phase difference at oblique incidence is 4
Phase difference birefringence (δL, δV), showing how it varies with the number of parameters (δL, δV, α, β)
Dependency diagram

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Disc 2 Stage 3 Linear movement mechanism 4 Linear movement mechanism 5 Rotation mechanism A Light emitting part of parallel beam light for measurement B Light receiving part

Continuation of the front page (56) References JP-A-63-241452 (JP, A) JP-A-63-262530 (JP, A) JP-A-63-186130 (JP, A) JP-A-3-205536 (JP) JP-A-3-221842 (JP, A) JP-A-3-218440 (JP, A) JP-A-5-34274 (JP, A) JP-A-62-203046 (JP, A) 3-74348 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 21/00-21/61 G01N 21/84-21/90 G11B 7/26

Claims (3)

(57) [Claims] A parallel light beam is incident on a substrate of an optical disk having at least a recording layer provided on a transparent resin substrate from a surface opposite to the recording layer in an oblique direction with respect to the substrate. A method for measuring the birefringence of a substrate by measuring a generated phase difference, wherein a certain measurement point is measured.
With the incident angle θ of the incident light with respect to the substrate surface kept constant, the phase difference was measured for at least two directions orthogonal to the direction of the incident surface including the incident light beam .
A method for measuring the birefringence of an optical disc substrate, wherein the birefringence of the one point is measured from each phase difference . 2. The method according to claim 1, wherein the two orthogonal directions coincide with a direction of an in-plane principal axis of the substrate. 3. A linear moving mechanism capable of horizontally moving in two orthogonal directions, and a rotation installed on the linear moving mechanism, rotatable about each point on the linear axis and horizontally mounting a disk thereon. Item 1. Measurement using a birefringence measuring device composed of a stage, a light emitting unit and a light receiving unit for measuring a phase difference by irradiating a light beam obliquely to the disk surface to be measured.
The described measurement method .