1. Field of the Invention
The present invention relates to a surface defect tester and, particularly, the present invention relates to a surface defect tester for testing the flatness of a surface of a faceplate such as a magnetic disk or a glass substrate thereof, which can precisely detect the size of a recessed or protruded defect in the surface and, further, can precisely detect the size and the depth or height of such defect.
2. Description of the Prior Art
A magnetic hard disk used as a recording medium of a computer system is tested for defect and size thereof in a surface of the disk in a substrate state or a complete magnetic disk state in which a magnetic film is painted on the substrate.
The size of the recent magnetic disk is 3.3 inches or smaller and the recording density thereof is substantially increased with employment of a GMR (Giant Magneto-Resistance) head. In such magnetic disk, a glass substrate, which has thermal expansion coefficient smaller than that of the conventional aluminum substrate and is as thin as in a range from 0.6 mm to 0.8 mm, is used.
In the conventional surface defect tester, the detection of defect is usually performed by helically scanning a disk with a laser beam. However, in order to precisely detect the size of a recessed defect (such as dish type defect, pit type defectscratch type defect, or dimple defect, etc.,) or protruded defect (such as bump type defect, particle type defect, or stain type defect, etc.,), it is necessary to precisely set a illuminating angle of laser beam in a light illuminating system, a light receiving angle in a light receiving system and a voltage applied to each APD (Avalanche Photo-Diode). Further, in the conventional surface defect tester, it is necessary to optimally set parameters related to sensitivity of detection, such as gain of each of an amplifiers included in a signal processing circuit of the surface defect tester, threshold value for removing noise, and laser output of a laser light source, etc., thereof through a control panel of the surface defect tester. Incidentally, the sensitivity regulation of the surface defect tester is performed by using a practical disk having a sample defect having known size, such as dish type defect, pit type defect, scratch type defect or protruded defect having known height, as a sample disk for sensitivity calibration.
U.S. Pat. No. 5,875,027 assigned to the assignee of this application discloses such sample disk for sensitivity calibration.
On the other hand, it is required recently to improve the preciseness of defect configuration measurement and the preciseness of defect classification. However, it is impossible to precisely perform the classification of defects by the above mentioned prior art.
JP 2001-1744115A (JPA-H11-358769) assigned to the assignee of this application discloses a technique with which the above problem can be solved.
The technique disclosed in JP 2001-174415A (JPA-H11-358769) is featured by that a sensor arrangement including a plurality of APD elements is used as the light receiver and a zigzagged stripe pattern corresponding to the APD elements is provided in front of the sensor arrangement. The recessed defect and the protruded defect are detected on the basis of a difference in amount of received light between adjacent APD elements of the sensor arrangement.
However, this technique requires the zigzagged stripe pattern and a number of detection circuits are necessary to detect the difference in light amount between adjacent sensor elements.
U.S. patent application Ser. No. 9,907,713 assigned to the assignee of this application relates to a technique suitable to solve such problem. Although it is possible to detect a recessed or protruded defect without using a zigzagged stripe pattern according to this technique, a detection method thereof is different from that of the present invention.
An object of the present invention is to provide a surface defect tester capable of precisely detecting the size of recessed or protruded defect in a flat surface of a faceplate.
In order to achieve the above object, a surface defect tester according to the present invention is featured by comprising a light illuminating system for relatively scanning the surface of the faceplate by irradiating the surface with a light beam having width in a direction perpendicular to a main scanning direction, a light receiver having n light receiving elements arranged along a line perpendicular to a main scan direction, where n is an integer not smaller than 2, an optical system for focusing an image of a scanning position on the faceplate on a light receiving region defined by the n light receiving elements such that an amount of light received by the light receiving region becomes a peak at a center of the light receiving region in the arranging direction thereof and is gradually reduced substantially symmetrically toward both ends thereof in the same direction, at least two of the n light receiving elements being arranged in symmetrical positions in the light receiving region with the center of the light receiving region as a reference, and a defect detector for detecting the defect by using a difference in level between light receiving signals from the symmetrically arranged two light receiving elements as a defect detection signal, wherein the image focused on the light receiving region is moved in the arranging direction when the light receiver receives light reflected by the recessed or protruded defect by the scanning in the main scan direction.
In an embodiment, two groups each including a plurality of the light receiving elements are arranged symmetrically about the center portion of the arrangement of the light receiving elements as the reference. In this case, n is not smaller than 4. It should be noted that the principle of the present invention is to arrange at least two light receiving element one of which is arranged symmetrically to the other light receiving element. When a plurality of light receiving elements are arranged symmetrically to a plurality of other light receiving elements, total level values of light signals obtained from the plurality of light receiving elements on both sides are used.
In the present invention, the image of the scanning position on the faceplate is focused on the light receiving region defined by the n light receiving elements such that an amount of light received by the light receiving region becomes a peak at a center of the light receiving region in the arranging direction thereof and is gradually reduced substantially symmetrically toward both ends thereof in the same direction as mentioned above. Therefore, the levels of the light receiving signal from the light receiving elements arranged substantially symmetrically about the center of the light receiving region defined by the light receiving elements become substantially equal and there is no substantial difference therebetween if there is no defect in the surface of the faceplate. However, if there is a recessed or protruded defect in the scan position on the surface of the faceplate, the focused image is shifted from the center of the light receiving region in the arranging direction of the light receiving elements toward one of the ends of the light receiving region by light reflected from a slanted side face portion of the defect depending upon the type of the defect, that is, recessed type or protruded type. Therefore, the levels of the light receiving signal from the light receiving elements arranged substantially symmetrically about the center of the light receiving region become substantial and the defect is detected when the difference becomes a predetermined value or larger. Incidentally, the predetermined value is used to exclude noise, etc.
In the case where n is 2, one light receiving element is provided on one side of the light receiving region with respect to the center of the region and the other light receiving element is provided on the other side symmetrically about the center.
In the case where a plurality of light receiving elements are provided on each side of the light receiving region, a difference between a total amount of light receiving signals from the light receiving elements on one side (when the arranging direction of the light receiving elements is vertical, for example, the light receiving elements on an upper or lower side in the vertical direction and, when the arranging direction is horizontal, the light receiving elements on one side in the horizontal direction) and a total amount of light receiving signals from the light receiving elements on the other side is employed. If there is no defect in the surface of the faceplate, the difference becomes substantially zero or a value close thereto.
On the other hand, when there is a recessed or protruded defect, the focused image is shifted from the center of the light receiving region in the arranging direction of the light receiving elements toward one of the ends of the light receiving region by light reflected from a slanted side face portion of the defect depending upon the type of the defect, as mentioned above. Therefore, when the difference between the total amount of light receiving signals from the light receiving elements on one side and the total amount of light receiving signals from the light receiving elements on the other side becomes a positive or negative value larger than a predetermined value. Accordingly, it is possible to use the difference as a defect detection signal indicative of the recessed or protruded defect.
It is usual that the defect detection signal obtained from the difference between the light amounts received by the light receiving elements arranged symmetrically about the center of the light receiving region defined thereby has a positive peak and a negative peak regardless of the kind of defect, because either the recessed defect or the protruded defect has a pair of slanted side faces in the scan directions R and xcex8. Therefore, it is possible to detect the size of the defect on the basis of a distance between the two peaks.
The distance between two peaks is obtained by detecting scan positions (coordinates positions) at which the positive and negative peaks of the defect detection signal occur and an area of the defect can be easily calculated from a relation between the distance and the scan positions.
Further, according to the present invention, the continuity of a certain defect to another defect can be easily determined from the coordinates positions of the defects and the distances between peaks. By this determination, it is easily possible to calculate an area of a somewhat deformed defect among the recessed and protruded defects. Further, according to the present invention, it is possible to precisely detect the depth of recessed defect or the height of protruded defect by employing averages of absolute values of the positive and negative peaks.
As a result, it is possible, according to the present invention, to precisely detect the size of the recessed or protruded defect in the surface of the faceplate and to precisely detect the size and depth of the recessed defect and the size and height of the protruded defect. Therefore, it is possible to realize a surface defect tester capable of classifying the defects easily.