Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2000-04-14
2003-04-15
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559460, C250S559480, C250S559490
Reexamination Certificate
active
06548821
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. In these systems, a laser beam is reflected off of a substrate, and sensors such as photomultiplier tubes detect the reflected laser beam to determine whether defects are present on the substrate. Other systems use sensors other than photomultipliers to detect the reflected laser beam.
Examples of laser scanning 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 a top side
12
a
of substrate
12
, while laser beam
16
b
is used to scan across and inspect a bottom side
12
b
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. Co-pending patent application Ser. No. 09/337,709 discloses an improved circuit using a bi-cell photodetector for receiving a reflected laser beam and generating an output signal indicative of the presence of defects on a substrate in response thereto. In particular, the circuitry described in the '709 application generates an output signal indicative of the slope of the side of a defect wall. The '709 is more sensitive to the presence of defects than the
FIG. 3
circuitry. However, it would be desirable to further improve the ability to detect defects.
SUMMARY
A method for detecting defects in a substrate comprises:
a) reflecting radiant energy off of a substrate (e.g. in the form of a laser beam);
b) generating a first signal indicative of the slope of the portion of the substrate surface reflecting said radiant energy; and
c) generating a second signal indicative of the height of the portion of the substrate surface in response to the first signal. In one embodiment, the second signal is the integral of the first signal.
If there is a defect at the portion of the substrate surface, the first signal indicates the slope of the defect, and the second signal indicates the height of the defect. Defective substrates are typically thrown out or reworked.
In accordance with another aspect of the invention, apparatus comprises means for detecting a laser beam reflected off the surface of a substrate. A first circuit within the apparatus generates a first signal indicative of the slope of the portion of the substrate where the laser strikes the substrate. A second circuit within the apparatus generates a signal indicative of the height of the portion of the substrate. In one embodiment, the second circuit functions as an integrator for integrating the first signal.
The first circuit is capable of detecting a first set of defects (i.e. defects having walls having a steepness exceeding a certain value), whereas the second circuit is capable of detecting a second set of defects (i.e. defects exceeding a certain height). By providing apparatus comprising both the above-mentioned first and second circuits, different types of defects can be detected, thereby enhancing the ability to screen out defective substrates early in the manufacturing process.
In one embodiment of the invention, the substrate is rotated during testing. During rotation, the substrate can vibrate. In accordance with one novel feature of th
O'Dell Thomas A.
Treves David
Hobden Pamela R.
Kim Robert H.
Komag, Inc.
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