JPH01194145A - Focusing stabilizing method for exposing master disk of optical disk - Google Patents

Abstract

PURPOSE:To stably execute a focusing by making a wavelength plate rotatable around an axis parallel to an optical axis by a rotating means and feedback- controlling it so that the total light receiving quantity of a light receiving element for focusing may be constant. CONSTITUTION:A wavelength plate 12 is arranged between a laser beam source 10 and a reflecting prism 14 and the wavelength plate 12 is provided rotatably around the axis parallel to the optical axis. When the wavelength plate 12 is rotated around the axis parallel to the optical axis, the quantity of light transmitting through a polarizing beam splitter 16 changes the total light receiving quantity of a light receiving element 24 for focusing changes. When a feedback is applied to the rotating means and the rotation of the wavelength plate 12 is controlled so that the total light receiving quantity of the light receiving element 24 for focusing may be always constant, the inclination of an S-curve in a focusing control can be kept always constant regardless of the fluctuation of reflectance. Thus, the focusing can be stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a focusing stabilization method in optical disc master exposure.

(Prior Art) Optical discs are recording media for recording and reproducing information using light as a medium, and are well known as laser visual discs, compact discs, and the like.

The stamper for such an optical disc is manufactured by exposing an optical disc master, which is formed by providing a photoresist layer on a glass substrate.

A groove called a preformat is formed in advance on the substrate of an optical disk. The preformat can be roughly divided into an ID groove and a tracking guide groove. The group recording method and the land recording method are conventionally known as recording methods for optical discs, but the land recording method has been widely adopted in recent years because of its high C/N ratio.

In the land recording method, the tracking guide grooves formed on the stamper are formed in the photoresist layer PR on the glass substrate GL, as shown by the symbol TG in FIG. The depth is halfway through the photoresist layer PR, and precise focusing is required to make the depth of the groove uniform.

By the way, when light such as a He--Ne laser is incident on a photoresist for focusing, the reflectance of the photoresist changes as shown in FIG. 3 depending on the thickness of the photoresist.

Various methods are known for focusing, such as the astigmatism method and the knife method, but in any case, the focusing light is focused in a straight line near the focal point depending on the distance between the objective lens and the optical disc master. A focus error signal that changes over time is obtained from a focusing light receiving element, and focusing is performed in accordance with this focus error signal.

When exposing a preformat such as a tracking guide groove on an optical disk master, a laser beam for exposure and a laser beam for focusing, which is separate from this, are usually used. In FIG. 4, a straight line 4-1 represents the relationship between the focus error signal generated by the focusing laser beam and the distance between the objective lens and the optical disc master.
This is a straight line portion of a so-called S-curve, and when the focus error is 0, the focusing laser beam is clearly focused on the optical disc master by the objective lens.

Generally, focusing lasers (e.g. He-Ne
It is difficult to match the focus position of the exposure laser (e.g. Ar laser) with the focus position of the exposure laser (e.g. Ar laser) due to chromatic aberration of the objective lens, adjustment error of the optical system, etc., and there is usually a deviation in the focus position of both lasers. . Therefore, depending on the deviation of the focusing position, for example, an offset is given by 00 minutes in FIG. I try to focus on the optical disc master. However, if there is non-uniformity in the thickness of the photoresist on the optical disc master H, the amount of light received at the edge of the focusing light receiving element varies. If, for example, the amount of light received at the edge decreases due to this fluctuation, the straight line portion of the S-shaped curve becomes, for example, a straight line 4-2 in FIG.
Due to the offset of 100 min, the focusing position of the exposure laser beam is shifted by the amount AB, resulting in an out-of-focus state.

(Purpose) The present invention has been made in view of the above-mentioned circumstances, and its purpose is to improve the focusing of an exposure laser beam even if the thickness of the photoresist layer is uneven in an optical disc master. An object of the present invention is to provide a method for stabilizing focusing during exposure of an optical disc master, which can stably perform the following.

(composition) The present invention will be explained below.

The present invention is a method for stabilizing focusing during exposure of an optical disc master formed by providing a photoresist layer on a glass substrate, and its features are as follows.

That is, a wavelength plate is placed in the optical path for focus servo,
This wavelength plate is made rotatable around an axis parallel to the optical axis by a rotating means, and the rotating means is feedback-controlled so that the amount of light received at the edge of the focusing light-receiving element is constant.

As the wavelength plate, a 174-wave plate or a 172-wave plate is suitable.

(Example) Hereinafter, a specific example will be described with reference to the drawings.

FIG. 1 shows only the essential parts of an embodiment of the present invention in an explanatory diagram. That is, FIG. 1 shows an optical system for focus servo in an optical disk master exposure apparatus.As the exposure optical system (not shown), a conventionally known optical system can be used as appropriate.

In FIG. 1, numeral 10 is a laser light source for focusing, numeral 14 is a reflecting prism, numeral 16 is a deflection beam splitter, numeral 18 is a 174 wavelength plate, numeral 20 is an objective lens, and numeral 22 is a cylindrical lens. ,
Reference numeral 24 indicates a focusing light receiving element. These constitute an optical system for focusing servo using the well-known astigmatism method, and in focusing, a predetermined offset is applied to the focus error signal obtained from the focusing light receiving element 24, so that the focus error signal is always constant. This is performed by servo-driving the objective lens 20 in the optical axis direction so as to achieve this.

Now, the present invention is implemented as follows in this embodiment.

That is, a wavelength plate 12 is disposed between the laser light source 10 and the reflecting prism 14, and the wavelength plate 12 can be rotated around an axis parallel to the optical axis by a rotating means 13.

As the wavelength plate 12, a 172-wave plate is used in this embodiment. When the wave plate 12 is rotated around an axis parallel to the optical axis (in this example, it coincides with the center ray of the laser beam), the polarization state of the beam transmitted through the wave plate 12 changes. The amount of light transmitted through the polarizing beam splitter 16 changes.

Therefore, as a result, the amount of light of the focusing laser beam incident on the optical disc master 30 changes, so even if the reflectance of the photoresistor PR on the master is constant, the total amount of light received by the focusing light receiving element 24 changes.

Therefore, if the rotation of the wavelength plate 12 is controlled by applying feedback to the rotation means so that the total amount of light received by the focusing light receiving element 24 is always constant, the photoresist P
The slope of the above-mentioned S-shaped curve in focusing control can always be kept constant regardless of variations in reflectance due to non-uniformity of the layer thickness of R, thereby making it possible to stabilize focusing. Of course, variations in reflectance due to non-uniformity in the thickness of the photoresist can be absorbed not only within the same master but also between different masters.

Note that as the rotation means 13, a known rotation control servo mechanism such as a stepping motor can be used as appropriate.

(Effects) As described above, according to the present invention, it is possible to provide a focusing stabilization method in optical disc master exposure.

Since this method is constructed as described above, even if the thickness of the photoresist on the optical disk master is somewhat uneven, it is possible to record tracking guide grooves and the like in the land recording method with extremely high accuracy.

Furthermore, since it is no longer necessary to strictly control the thickness of the photoresist, it becomes easier to produce an optical disk master, and it becomes possible to reduce the master disk cost.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram that schematically shows only the parts necessary for explaining one embodiment of the present invention, and FIGS. 2 to 4 are diagrams for explaining the problems to be solved by the present invention. 10. Laser light source for focusing, 12.
,. Wave plate, 16. , , polarizing beam splitter 118
,. 174 wavelength plate, 20. , , objective lens, 22. ,
, cylindrical lens, 24. ,, Light receiving element for focusing, Fig. 4 (zL area 2 Fig. T/:

Claims (1)

[Claims] A method for stabilizing focusing during exposure of an optical disk master comprising a photoresist layer provided on a glass substrate, the method comprising: disposing a wavelength plate in an optical path for focus servo; A method for stabilizing focusing, characterized in that a plate is rotatable around an axis parallel to an optical axis by a rotating means, and the rotating means is feedback-controlled so that the total amount of light received by a focusing light-receiving element is constant.