Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
1999-06-21
2003-05-20
Stafira, Michael P. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
Reexamination Certificate
active
06566674
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to a method and apparatus for inspecting substrates used during the manufacture of magnetic disks.
Magnetic disks are typically manufactured by the following process:
1. An aluminum alloy substrate is electroless plated with NiP.
2. The plated substrate is polished.
3. The polished substrate is then textured, either mechanically or using a laser.
4 An underlayer (e.g. Cr or NiP), a magnetic alloy (typically a Co alloy) and a protective overcoat (typically carbon, hydrogenated carbon, or zirconia) are then sputtered, in that order, onto the substrate.
5. A lubricant is then applied to the protective overcoat.
The layers formed on magnetic disks (e.g. the underlayer, magnetic layer and overcoat) are extremely thin, e.g. on the scale of several tens of nanometers. It is very important that there be no or few large defects in the substrate prior to sputtering.
It is known in the art to use laser scanning systems to inspect magnetic disk substrates prior to sputtering. Examples of such systems include the PMT Pit Detector, the Diskan
6000
, Diskan
9000
and Diskan
9001
systems manufactured by QC Optics of Burlington, Mass. Other prior art systems are discussed in U.S. Pat. Nos. 4,794,264; 4,794,265; and 5,389,794, each assigned to QC Optics.
FIG. 1
schematically illustrates a QC Optics Diskan
9001
apparatus
10
for detecting defects in a substrate, such as a substrate
12
. Referring to
FIG. 1
, apparatus
10
comprises HeNe lasers
14
a
,
14
b
for generating laser beams
16
a
,
16
b
respectively. Laser beam
16
a
is used to scan across and inspect one side of substrate
12
, while laser beam
16
b
is used to scan across and inspect the other side of substrate
12
. (Substrate
12
is typically rotated by a motor during this inspection, and laser beams
16
a
,
16
b
typically scan in the radial direction of the substrate.)
Laser beam
16
a
passes through a polarizer
18
a
, ¼ waveplate
20
a
, and a shutter
22
a
, reflects off a mirror
23
a
, passes through a lens
24
a
, a beam splitter
25
a
, and a lens
26
a
and reflects off of mirror
28
a
. Mirror
28
a
deflects laser beam
16
a
downward to substrate
12
. Substrate
12
reflects laser beam
16
a
upwardly and back to mirror
28
a
, through lens
26
a
and back to beam splitter
25
a
. Beam splitter
25
a
deflects laser beam
16
b
to a photomultiplier tube
30
a
. Of importance, if laser beam
16
a
strikes a defect in substrate
12
(either a pit or a bump), that defect will reflect laser beam
16
a
at an angle. The fact that laser beam
16
a
is reflected at an angle is detected by photomultiplier tube
30
a
. In this way, apparatus
10
can use laser beam
16
a
to determine whether there are pits or bumps in substrate
12
.
The manner in which a defect deflects a laser beam can best be understood by comparing
FIGS. 2A and 2B
. In
FIG. 2A
, laser beam
16
a
strikes a portion of substrate
12
where defect
32
deflects laser beam
16
a
at an angle &thgr;. In contrast, in
FIG. 2B
, laser beam
16
b
strikes a portion of substrate
12
where there are no defects. Thus, in
FIG. 2B
, laser beam
16
a
reflects straight back, and not at an angle. As mentioned above, photomultiplier tube
30
a
detects whether or not laser beam
16
a
is reflected at an angle by a defect on substrate
12
.
Referring back to
FIG. 1
, portions of laser beam
16
a
are also reflected past mirror
28
a
, pass through spacial filter
34
a
and lens
36
a
, and strike photomultiplier tube
38
a
. (Spacial filter
34
a
filters out light scattering caused by the texture pattern that is formed on substrate
12
.) Of importance, photomultiplier tube
38
a
determines whether light is scattered by defects or contamination on substrate
12
at a wide angle.
The optical path for laser beam
16
b
is similar to the optical path of laser beam
16
a
, and will not be described in detail, except to note that it includes two mirrors
28
b
′ and
28
b
″ instead of single mirror
28
a.
FIG. 3
is a block diagram of the circuitry coupled to photomultiplier tubes
30
a
,
30
b
,
38
a
and
38
b
. As can be seen, each of photomultiplier tubes
30
a
,
30
b
,
38
a
and
38
b
is coupled to four comparators
42
a
-
42
d
,
44
a
-
44
d
,
46
a
-
46
d
and
48
a
-
48
d
, respectively. Each of comparators
42
a
-
42
d
compares the output signal OS
30
a
of photomultiplier tube
30
a
with an associated reference voltage RV
42
a
-RV
42
d
, and provides a binary output signal BOS
42
a
-BOS
42
d
in response thereto. Binary output signals BOS
42
a
-BOS
42
d
are stored in associated latches
52
a
-
52
d
, the contents of which are loaded into a memory which can then be accessed by a central processing unit CPU (not shown). Comparators
44
-
48
similarly compare the output signals from photomultiplier tubes
30
b
,
38
a
and
38
b
to reference voltage signals RV, and generate binary output signals BOS in response thereto. These binary output signals are stored in latches
54
-
58
, the contents of which can be accessed by central processing unit CPU to determine the size and character of a defect detected by the apparatus.
While apparatus
10
can detect some defects, it would be desirable to provide improved means for detecting such defects with greater sensitivity and accuracy.
SUMMARY
A method for inspecting a substrate in accordance with our invention comprises the step of providing a laser beam that strikes and reflects off the substrate and then strikes a bi-cell photodetector. In one embodiment, the photodetector is a photodiode. The cells of the photodetector are coupled to circuitry that generates a signal equal to (L−R), where L is the strength of the signal provided by one cell of the photodetector, and R is the strength of the signal provided by the other cell of the photodetector. The signal L−R corresponds to the difference between the amount of light striking one cell of the photodetector and the amount of light striking the other cell, which in turn depends on the extent to which the laser beam is deflected by a defect. A signal equal to L+R is also developed. Signal L+R is used to “normalize” signal L−R. In other words, signal L+R is used to compensate for sources of common mode noise, e.g. fluctuations in the intensity of the laser, variations in substrate reflectivity, etc. From these two signals, a signal proportional or equal to (L−R)/(L+R) is developed. Signal (L−R)/(L+R) is compared to a set of threshold circuits to determine the size of the defect detected.
In one embodiment, the bi-cell photodetector contains two photodiodes that are biased with a bias voltage so that the photodiodes exhibit reduced capacitance. Because of this, the circuit employing the bi-cell photodetector exhibits enhanced bandwidth, thereby improving the speed at which the substrate can be inspected.
We have found that one embodiment of apparatus in accordance with our invention is more sensitive to defects than the apparatus of FIG.
3
. For example, the apparatus of
FIG. 3
was capable of detecting defects having a wall slope of about 0.05° or greater. One embodiment of our invention can detect defects having a wall slope less than 0.02°, and in one embodiment, defects having a wall slope as low as 0.005°. (A defect wall slope of 0.005° typically represents the lower limit of presently feasible substrate manufacturing processes. If one could manufacture a flatter substrate, we believe the apparatus of the present invention could detect defects having wall slopes as low as 0.003°.)
REFERENCES:
patent: 3679307 (1972-07-01), Zoot et al.
patent: 3999865 (1976-12-01), Milam et al.
patent: 4065786 (1977-12-01), Stewart
patent: 4092068 (1978-05-01), Lucas et al.
patent: 4355904 (1982-10-01), Balasubramanian
patent: 4395122 (1983-07-01), Southgate et al.
patent: 4402607 (1983-09-01), McVay et al.
patent: 4412743 (1983-11-01), Eberly
patent: 4544241 (1985-10-01), LaBudde et al.
patent: 4600996 (1986-07-01), Kawauchi
pate
Hsieh Yung-Chieh
O'Dell Thomas A.
Treves David
Komag, Inc.
Lee Andrew H.
Stafira Michael P.
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