Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2001-07-19
2003-01-21
Pitts, Harold I. (Department: 2876)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06509966
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system for defecting a surface defect and a surface defect tester and, particularly, the present invention relates to an optical system for use in a detection of surface defect of a flat plate such as a magnetic disk or a glass substrate thereof to detect the size of irregularity of a surface of the flat plate with high precision and the surface defect tester using the same optical system.
2. Description of the Prior Art
A magnetic hard disk used as a recording medium of a computer system is tested for surface defect and size of the surface defect in a substrate state or in a complete magnetic disk state in which a magnetic film is painted thereon.
The size of the recent magnetic disk is 3.3 inches or smaller and the recording density thereof is substantially increased by the employment of a giant magneto-resistance (GMR) 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.
FIG.
5
(
a
) shows a main construction of a conventional surface defect tester
10
for magnetic disk.
The surface defect tester
10
shown in FIG.
5
(
a
) is constructed with a rotary mechanism
2
, a detection optical system
3
and a surface defect detection/processor
4
. A disk
1
to be tested is mounted on a spindle
21
of the rotary mechanism
2
and rotated by a motor (M)
22
. On the other hand, the detection optical system
3
is constructed with a light illuminating system
31
including a laser light source
311
and a condenser lens
312
and an optical light receiving system
32
including a condenser lens
321
and a light receiver
322
. A laser beam L
T
produced by the laser light source
311
is condensed by the condenser lens
312
to a laser spot Sp on a surface of the disk
1
.
When the disk
1
is moved horizontally while being rotated, the laser spot Sp moves in a radial direction R of the disk
1
, so that a surface of the disk
1
is scanned spirally. In this case, in order to make a total scan time of the disk
1
as short as possible, an area of the laser spot Sp is made ellipsoidal having a length &phgr;
1
in a minor axis direction and a length &phgr;
2
in a major axis direction as shown in FIG.
5
(
b
) and the major axis is set perpendicular to the scan direction to increase a scan width of the laser spot. The laser spot Sp is scattered by a defect F of the surface of the disk. A reflected light SR is condensed by the condenser lens
321
of the optical light receiving system
32
and the condensed light is received by the light receiver
322
including the optoelectric converter element such as, for example, an avalanche photodiode (APD) or a photo multiplier tube (PMT). An output signal of the light receiver
322
is inputted to a signal processing circuit
41
of the surface defect detection/processing unit
4
. The defect F is detected by a defect detection signal output from the signal processing circuit
41
. The size of the defect F is detected or calculated according to a level of the detection signal outputted from the light receiver
322
. The signal processing circuit
41
for detecting the defect and classifying or calculating the size of detected defects by the so-called sampling includes an amplifier for amplifying the output signal of the light receiver
322
, a sampling circuit for sampling peak values of the amplified output signal, which corresponds to a defect and is larger than noise component of the output signal with a pulse supplied from a rotary encoder
23
to detect the peak values of the sampled output signal, an A/D converter for digitizing the sampled peak value and a position data producing circuit responsive to the pulse signal from the rotary encoder
23
for producing a position data on the disk, etc.
The size data of the respective defects and the position data of the detects on the disk are converted into digital data by the signal processing circuit
41
and inputted to the data processor
44
composed of an MPU
42
and a memory
43
, etc. The number of defects of each size are counted by the data processor
44
and the size data and the count value of the defects, etc., are outputted to a printer (PR)
45
together with the position data of the defects on the disk
1
. In this case, these data may be printed out as a map on the disk. Alternatively, the defect size is displayed on a display (CRT)
46
, etc., together with the position thereof on the disk and the count value of the defects is displayed separately.
Incidentally, the rotary encoder
23
is provided in the vicinity of a rotary shaft of the motor
22
or in engagement therewith, detects an amount of rotation of the disk with reference to a reference position provided on the disk and sends a pulse signal corresponding to the amount of rotation of the disk to the signal processing circuit
41
.
In order to clearly detect the size of recessed defect and protruded defect, the surface defect tester
10
optimally sets factors related to the detection sensitivity, such as illumination angle &thgr;
T
of the laser beam L
T
of the optical illuminating system
31
, light receiving angle &thgr;
R
of the light receiving system
32
, voltage V applied to the light receiver (APD)
322
or gain of the amplifier provided in the signal processing circuit
41
, threshold voltage E for removing noise and laser output of the laser light source
311
, etc., through the control panel
47
. Incidentally, the detection sensitivity is regulated by using, as a sample disk, a practical disk having recessed defects such as dish-like defect, pit-like defect or scratch: defect having known size or a practical disk having protruded defects having a specific height.
JP H10-325713A assigned to the assignee of this application and belonging to the same technical field as that of the above mentioned prior art discloses a technique titled “Surface Defect Test Method and Surface Defect Tester”, in which a defect test is performed by using a sensitivity calibration disk having a plurality of radially extending dummy defect rows each including a plurality of protruded or recessed defects whose sizes are changed in steps: and the detection sensitivity is regulated correspondingly to the dummy defect row including defects whose sizes are increased or decreased in steps, by displaying the dummy defect row as a result of the test.
However, in such conventional defect detection system for disk in which a recessed defect of a disk or a protruded defect including extraordinary substance, etc., on the disk is detected by comparing a level of reflected or scattered light detected by a light receiver with a reference level, the size of the defect is detected as a level of received light, so that the detected defect size becomes inaccurate. Particularly, for the recessed defect or the protruded defect, depth or height of the defect influences the level of received light, so that the detection accuracy of defect size, which is an area extended in a flat plane, is low.
The sensitivity calibration disk used in this tester corresponds to U.S. Pat. No. 5,975,027, the content of which is shown in FIGS.
5
(
a
) and
5
(
b
).
Recently, it is required 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.
In order to solve this problem, JP H11-358769A assigned to the assignee of the present application discloses a technique in which a sensor arrangement including a plurality of APD elements is used as the light receiver
322
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, thi
Hitachi Electronics Engineering Co. Ltd.
Mattingly Stanger & Malur, P.C.
Pitts Harold I.
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