JPH04363649A - Defect inspection method of optical information recording medium - Google Patents

Abstract

PURPOSE:To detect an optical defect and a coating defect by taking out only a low frequency component of a focus error signal at an arbitrary point on an optical information recording medium to make it a reference signal and comparing the reference signal with the low frequency component of the focus error signal at each position. CONSTITUTION:An optical system is controlled so that a laser beam is either focused or brought in a state near a focus and only the low frequency component of a focus error signal at this point of time is taken out to make it a reference signal. A recording medium is scanned to detect only the low frequency component of the focus error signal, which is compared with the reference signal to perform the detection of a defect. Since the focus error signal indicates a defocus quantity caused by the thickness, the inclination, the change of a refraction factor and the like of the optical information recording medium, a flaw, thickness unevenness, the inclination, the optical defect and coating nonuniformity of a board can be detected with the use of the focus error signal. The high frequency component of the focus error signal is cut off so as not to detect the face deflection of the board and the like in the range without any problem is use and the judgement of the defect is performed with the use of only the low frequency component.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a defect inspection method for optical information recording media.

0002

[Background Art and Problems to be Solved by the Invention] Conventional methods for inspecting optical information recording media for defects include a method in which a CCD image sensor detects changes in transmitted light and reflected light from a tungsten halogen lamp or a mercury lamp; Alternatively, there is a method of detecting changes in the amount of transmitted light, reflected light, diffracted light, and scattered light by a laser beam using a photodetector.

Inspection methods that use irradiation with a tungsten halogen lamp or a mercury lamp and use a CCD image sensor have the advantage of short overall scanning time, but the defect detection sensitivity is not very high. On the other hand, the method using laser light irradiation has the advantage of higher sensitivity than the above methods. This defect inspection method using laser light is as follows:
There are spiral scanning methods, scanning methods using polygon mirrors, etc., but spiral scanning is the most common in terms of cost. In this spiral scanning of laser light, the beam diameter of the laser light is usually on the order of several tens of micrometers due to the scanning tact. This beam diameter of several tens of micrometers is sufficient to detect relatively large defects, but for example, defects with low contrast between light and dark, or defects that are minute compared to the beam diameter, can be detected using transmitted light, reflected light, etc. When using changes in the amount of diffracted light or scattered light, it cannot be said that there is necessarily sufficient detection sensitivity.

[0004] For such defects with low contrast between light and dark or defects that are minute compared to the beam diameter and become defects during actual recording and reproduction, it is possible to lower the threshold of the defect signal, but in this state In this case, the margin between the minute defect signal level and the noise level (signal level without a defect) becomes small, and there is a risk of erroneous detection. Furthermore, although it is possible to increase the defect detection sensitivity by narrowing down the beam diameter of the laser beam, this method is subject to certain limitations due to the inspection time.

Furthermore, when inspecting defects such as birefringence abnormality of a substrate, coating unevenness of a hard coat layer, coating unevenness of a recording film such as a dye, etc., conventional defect inspection methods cannot be used. It was almost impossible to detect those defects.

The present invention has been made in view of the actual state of the prior art, and can detect defects that were previously undetectable, such as optical defects and coating unevenness defects (defects with uneven film thickness distribution). The purpose is to provide an inspection method that can detect this.

[0007]

[Means and effects for solving the problem] The above problem is to provide a method for inspecting defects of an optical information recording medium by scanning a laser beam, by providing a means for extracting only the low frequency component of a focus error signal, and by detecting a certain arbitrary position. The optical system is controlled so that the laser beam is focused or nearly focused on the optical information recording medium, and the low frequency component of the focus error signal at that point is determined in advance by the above means and used as a reference signal. , a defect in an optical information recording medium, characterized in that the defect is detected by comparing the low frequency component of the focus error signal at each position obtained by the means with a low frequency component reference signal of the focus error signal. The problem is solved by the inspection method.

[0008] Furthermore, the above-mentioned problem is solved in a method of inspecting optical information recording media for defects by scanning a laser beam, in which the beam diameter of the laser beam is set to more than twice the track pitch, and only the low frequency component of the focus error signal is detected. The focus servo system is controlled so that the laser beam is focused or nearly focused on the optical information recording medium at a certain arbitrary position, and the low frequency component of the focus error signal at that point is A reference signal is obtained in advance by the means, and then the focus servo system is turned off and the optical system is fixed, and the low frequency component of the focus error signal at each position obtained by the means and the low frequency component of the focus error signal are The problem is solved by a defect inspection method for an optical information recording medium, which is characterized in that defects are detected by comparing frequency component reference signals.

The present invention will be explained below with reference to the drawings. As explained earlier, conventional defect inspection methods usually use beams with a diameter of several tens of microns due to the relationship between defect inspection time and defect detection sensitivity, and detect changes in the amount of light such as transmitted, reflected, and diffracted light. Detected. When performing defect inspection using this method, the detection sensitivity is still equal to or greater than the beam diameter. In contrast, the present invention narrows down the beam diameter to about 1 micron (in the case of a track pitch of about 1.6 microns) or the size of several tracks, and then performs defect inspection using the low frequency component of the focus error signal. It is.

In the method according to claim 1 of the present invention, first, focus is brought into focus at an arbitrary position, for example, a defect inspection start position, and the optical system is controlled to be in a focused state or a state close to being in focus. If it is not in focus or close to focus at the first arbitrary position, focus at another arbitrary position, capture the low frequency component of the focus error signal at that position, and scan. Just move it to the starting position. Then, the optical information recording medium is scanned, only the low frequency component of the focus error signal is detected, and defects are detected by comparing it with the focus error low frequency component reference signal. Defects can be detected using the focus error signal because the focus error signal is based on the thickness of the optical information recording medium,
This is because it indicates the amount of defocus due to changes in tilt, refractive index, etc. (see FIG. 1). Therefore, by using this focus error signal, it is possible to detect scratches, thickness unevenness, tilt, optical defects of the substrate, and even coating unevenness of the layers constituting the optical information recording medium. However, there is a risk that surface deflection of the substrate, etc., which is not a problem in normal use, may be detected. Therefore, in the present invention, the high frequency component of the focus error signal is cut,
In other words, components of the focus error signal that are difficult to detect as defects are excluded, and defects are determined only based on the low frequency components of the focus error signal (see FIG. 2). A circuit that detects only the low frequency component of the focus error signal can be constructed using known circuit technology.

As described above, the present invention makes it possible to detect minute defects that could not be detected using conventional defect inspection methods. Of course, large defects such as foreign objects and pinholes, or defects that can be detected using conventional methods, can also be detected by detecting the low frequency component of the focus error signal.

Furthermore, in the present invention, in order to improve the correspondence with the bit error rate, a detection system for the amount of reflected and transmitted light may be added as necessary. However, in this case, means for compensating for changes in light amount due to pre-pits etc. must be provided in the detection system.

Furthermore, in the present invention, in order to inspect the optical information recording medium for defects at high speed, a vibration scanning method as shown in FIGS. 3 and 4 or a skip scanning method as shown in FIG. 5 can be used.

For example, if the diameter of the laser beam in conventional laser scanning is 30 microns, vibration scanning in the method of the present invention can be performed in a range of 30 microns to obtain a sensitivity higher than that of the conventional method. Also, if the frequency of vibration is increased (l1
), further improvement in sensitivity can be expected. Furthermore, although it is related to scanning time, sensitivity can be increased by narrowing the range of vibration. In other words, with the same inspection time as the conventional method, defect detection sensitivity greater than that of the conventional method can be obtained.

Even when using the skip scanning method, if the skip interval l2 is set to 30 microns as shown in FIG. 5, or if the skip is narrowed to a certain extent, defect detection sensitivity higher than that of the conventional method can be obtained with the same inspection time as the conventional method. It will be done. The threshold level for defect detection may be set to a level higher than the variation range of the focus error signal depending on the land-groove depth and the pre-pit groove depth.

However, when using the skip scanning method, the astigmatism method or the knife edge method can be used as a focus error detection method. On the other hand, when using the vibration scanning method or a beam diameter that spans several tracks, it is preferable to use the knife edge method in order to avoid being affected by guide grooves and pre-pits.

[0017] As described above, by using the above defect inspection method, defects that affect normal recording/playback,
In other words, defects corresponding to the bit error rate can be detected at high speed.

[0018]

[Example] Next, an example of the present invention will be described. Inkjet printing is performed on the inner circumference outside the recording area of the polycarbonate substrate, and a hard coat layer (thickness: 3 μm) is formed using a spin coating method to cover the printed characters. created a defect. This defect was detected using a conventional laser scanning inspection device (transmitted light amount detection type) and a device to which the focus error skip inspection method of the present invention was applied. The results are shown in Table 1. In the table, A/B represents the number of detected defects (A) relative to the total number of defects (B).

[0019]

[Table 1]

[0020]

Effects of the Invention: By using the defect inspection method for optical information recording media of the present invention, defects that were difficult to detect using conventional methods can be detected, and defects can be detected that corresponds to bit error rates better than conventional methods. can be performed at high speed and with high sensitivity.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between a focus error signal and a defocus amount.

FIG. 2 is an explanatory diagram of the method of the present invention.

FIG. 3 is an explanatory diagram of a vibration scanning method.

FIG. 4 is an explanatory diagram of a vibration scanning method.

FIG. 5 is an explanatory diagram of a skip scanning method.

Claims (5)

[Claims] 1. In a method for inspecting defects in an optical information recording medium by scanning a laser beam, means is provided for extracting only the low frequency component of a focus error signal, and the laser beam is focused on the optical information recording medium at a certain arbitrary position. The optical system is controlled to be in focus or close to focus, and the low frequency component of the focus error signal at that point is determined in advance by the means described above and used as a reference signal. 1. A defect inspection method for an optical information recording medium, characterized in that defects are detected by comparing a low frequency component of a focus error signal with a low frequency component reference signal of the focus error signal. 2. A method of inspecting an optical information recording medium for defects by scanning a laser beam, the beam diameter of the laser beam being set to twice or more the track pitch, and means for extracting only the low frequency component of the focus error signal. and controlling a focus servo system so that the laser beam is focused or nearly focused on the optical information recording medium at a certain arbitrary position,
The low frequency component of the focus error signal at that point is determined in advance by the above means and used as a reference signal, and then the focus servo system is turned off and the optical system is fixed, and the focus error at each position obtained by the above means is determined. 1. A defect inspection method for an optical information recording medium, characterized in that a defect is detected by comparing a low frequency component of the signal with a low frequency component reference signal of the focus error signal. 3. The defect inspection method for an optical information recording medium according to claim 1, wherein the laser beam is skip-scanned in a direction perpendicular to the defect inspection scanning direction on the optical information recording medium. 4. The defect inspection method for an optical information recording medium according to claim 1, wherein the laser beam is vibrated in a direction perpendicular to the defect inspection scanning direction on the optical information recording medium. 5. A detection system for detecting reflected light of a laser beam by an optical information recording medium, a detection system for detecting transmitted light of a laser beam, or both are used. The defect inspection method for an optical information recording medium according to any one of the above.