JPH087883B2 - Optical disk master exposure method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical disc master exposure method.

2. Description of the Related Art Generally, in forming a stamper master for an optical disc, a groove is formed by exposing a photoresist film to a laser beam.

As the shape of the guide groove / pre-formatted substrate for an optical disc, a so-called land recording type as shown in FIG. 2 has recently attracted attention in terms of high C / N ratio.
In FIG. 2, 1 is a guide groove and 2 is a pre-format. Here, assuming that the wavelength of the laser light used is λ, theoretically, the depth of the guide groove 1 is λ / 8, and the pre-format 2
The optimum depth is λ / 4. For example, when the wavelength of the semiconductor laser is λ = 728 nm and the refractive index of the substrate 3 is 1.5, the guide groove 1 has an optimum value of λ / 8 = 728/8 × 1.5.

However, considering the case of actually forming the grooves 1 and 2 as shown in FIG. 2, two focused lasers are formed on the photoresist film 5 formed on the glass substrate 4 to have a film thickness of λ / 4. It exposes by irradiating with light. At this time, one laser beam for forming the groove of the pre-format 2 is the photoresist film 5
Is a strong laser beam that develops to the bottom, that is, the surface of the glass substrate 4. The one laser beam for the other guide groove 1 is a weak laser beam that is developed only halfway through the photoresist film 5 (that is, only the depth λ / 8 is developed). FIG. 3 shows the sectional shapes of the grooves 1 and 2 obtained by such exposure and development.

According to the sectional shape of FIG. 3, the ideal shape as shown in FIG. 2, that is, the guide groove 1 and the pre-format 2 is formed.
It can be seen that the groove shape is rectangular up to the bottom surface, but it is deformed. That is, the guide groove 1 is a groove having a Gaussian distribution shape, and the pre-format 2 is a groove in which the front end side of the Gaussian distribution shape is stopped by the glass substrate 4. This is because the laser light used for exposure is directly focused on the photoresist film 5, which is the exposure surface, in the form of Gaussian intensity.

More specifically, the intensity distribution (Gaussian distribution) of the laser light focused by the guide groove 1 and the pre-format 2
A general method for changing the NA is to change the NA (numerical aperture) of the condenser lens 6. Here, as shown in FIG. 4, the diameter W of the beam waist of the laser light condensed by the condenser lens 6 with NA = sin θ can be represented by W = k · λ / NA. Therefore, when a condensing lens with a large NA is used, the beam waist diameter W becomes small, and the laser light intensity distribution at this beam waist becomes steeper than that with a small NA, as shown in FIG. Although the steepness changes in this way, the distribution characteristic itself remains a Gaussian distribution.

When the cross-sectional shape shown in FIG. 3 is examined again, the groove of the deeper pre-format 2 is exposed and developed up to the surface of the glass substrate 4, so that the groove bottom surface becomes flat to some extent. However, the shallower guide groove 1 has a shape corresponding to the Gaussian distribution as a whole, and has a rounded bottom surface, so that the groove width becomes smaller than the ideal rectangular shape shown in FIG. is there. As a result, the tracking error signal, which plays a role of the guide groove 1, becomes unstable as compared with the ideal one shown in FIG.

The present invention has been made in view of such a point, and at the time of exposing a photoresist film, even a groove having a depth shallower than the film thickness of the photoresist film has a shape closer to a rectangular flat shape up to its bottom. It is an object of the present invention to provide an optical disc master exposure method that can be formed as a groove.

Structure In order to achieve the above object, the present invention is characterized in that a plurality of laser beams whose centers are offset from each other are superposed on each other and the photoresist film is exposed to form one groove.

An embodiment of the present invention will be described below with reference to FIG. First, a plurality of, for example, three laser lights each having an intensity distribution according to a Gaussian distribution are used. These laser lights are set with their centers (optical axes) shifted by 0.5 μm, and their intensity distributions are set to overlap each other. As described above, as a method of shifting these laser beams by 0.5 μm, for example, a method using a diffraction grating, a method of shifting the incident angle to the objective lens with a prism, or the like can be adopted. Also, the beam waist diameter of these laser beams W = 1 / e ² = 0.6μ
m. Further, in the present embodiment, the laser intensities of these laser lights are not all the same, and only the laser light arranged in the center has a central intensity higher than that of the other laser lights.
It is set to 0.8 times.

By overlapping such three laser beams, one groove, for example, a guide groove or a pre-format groove is formed. That is, these 3
The intensity distribution shape obtained by superimposing the laser lights of the book has a distribution close to a rectangular shape as shown by the synthetic laser light of the solid line.
Since the photoresist film is exposed by the synthetic laser beam having this intensity distribution, for example, even in the case of a guide groove which should be a groove having a depth half the film thickness of the photoresist film, it is shown in FIG. It is possible to form a guide groove that is rectangular and flat to the bottom close to the ideal shape as in the case of. This is the same even in the case of a groove for developing up to the glass substrate (that is, a groove having a depth corresponding to the film thickness of the photoresist film like a pre-format). That is, it is possible to obtain synthetic laser beams having various intensity distributions by appropriately setting the positions of a plurality of laser beams and the respective intensity ratios thereof as shown by.

By the way, the case of a single laser beam having a Gaussian distribution in which the intensity distribution of the combined laser beam and the intensity distribution of the central portion according to the present embodiment are the same is shown by the chain line. When the single laser light and the synthetic laser light are compared, the difference in the characteristics of the intensity distributions of both is clearly understood.

Effect As described above, according to the present invention, a plurality of laser beams whose centers are offset from each other are overlapped and the photoresist film is exposed to form one groove. Therefore, the number and position of these laser beams are set. , Synthetic laser beams having various intensity distributions can be obtained by simply setting the respective intensity ratios. Therefore, even if the guide groove has a depth less than the film thickness of the photoresist film in land recording, It is possible to form a groove close to a rectangular flat shape and obtain a sufficient tracking error signal.

[Brief description of drawings]

FIG. 1 is a distribution characteristic diagram showing an embodiment of the present invention, FIG. 2 is a perspective view showing a land recording type ideal groove shape, FIG. 3 is a sectional view showing a conventional example, and FIG. 4 is a lens NA change. FIG. 5 is a side view showing the method, and FIG. 5 is a laser light intensity distribution characteristic diagram. ...... Laser light, ...... Synthetic laser light

Claims (1)

[Claims] 1. An optical disk master disc exposure method, wherein a plurality of laser beams whose centers are offset from each other are overlapped and the photoresist film is exposed to form one groove.