Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-10-08
2003-09-30
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S495000
Reexamination Certificate
active
06628402
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a system for detecting a phase interference in an interferometer for detecting defects on a surface of a magnetic disc and so on, with use of phase interference of a light, as well as to a light detector therefor.
2. Description of Prior Art
In recent years, a standard specification is that a hard disc device is installed into a personal computer as an information storage medium thereof, and that the hard disc device has a storage capacity in an order of several G bytes. Also, in particular for a personal computer such as a note-book computer, it is desired that the hard disc device has a high density in the memory capacity but with a small volume thereof, so as to be installed therein.
For increasing the memory density of the hard disc device, it is necessary to make a magnetic head float or fly from the surface of a magnetic disc thereof at a very small amount of distance, such as from 20 nm to 50 nm. In the inspection of defects on the surface of the magnetic disc which is used in such the hard disc device, the defect inspection must be performed at high accuracy or precision corresponding to that floating or flying amount.
Conventionally, the inspection of defects on the surface of the magnetic disc is performed by an apparatus, known as a glide tester. This glide tester, while rotating the magnetic disc and flying the magnetic head at a predetermined flying amount, detects the number of collisions of the magnetic head against abnormal projections on the disc surface, thereby deciding a glide level (height of the projection on the magnetic disc surface) of that magnetic disc upon the basis of the number of the collisions.
However, when the flying amount comes to be very small in the value thereof, i.e., from 20 nm to 50 nm, the number of collision against projections also rises up, and also the magnetic head is frequently damaged by contacting or collision on the disc surface. Therefore, there is a problem that a large amount of time is needed for exchanging and positioning of the magnetic head.
Then, the height of the abnormal projections can be tested optically. An apparatus for such the test is known as “an optical glide tester”.
FIG. 5
shows the structure of the conventional optical glide tester. Basically, the optical glide tester is constructed with an interferometer. A laser device
31
emits a laser beam b
1
of wavelength 532 nm there from. The laser beam b
1
is modulated by a modulation means
32
. The modulation means
32
comprises beam splitters
33
and
34
, acousto-optic modulators (AOM)
35
and
36
, and mirrors
37
and
38
. The laser beam b
1
is divided or separated into a reflection laser beam b
4
and a penetration laser beam b
2
by the beam splitter
33
. The penetration laser beam b
2
is incident upon the AOM
35
, in which it is modulated with frequency fm, and is emitted as a laser beam b
3
(=b
2
+fm). On the other hand, the reflection laser beam b
4
is reflected by the mirror
37
again, to be incident upon the AOM
36
, in which it is modulated with frequency fn, and is emitted as a laser beam b
5
(=b
2
+fn). The laser beam b
3
emitted from the AOM
35
is reflected by the mirror
38
so as to be incident upon the beam splitter
34
. On the other hand, the laser beam b
5
emitted from the AOM
36
is also incident upon the beam splitter
34
. The laser beam b
3
and the laser beam b
5
, both being incident upon the beam splitter
34
, are combined or mixed so as to be incident upon a divider means
39
.
The divider means
39
divides the combined laser beam b
3
+b
5
into two laser beams b
31
+b
51
and b
32
+b
52
, which are separated by a predetermined distance but are same in a direction of propagation and in a light path, and then they are emitted into a polarization beam splitter
3
A. The polarization beam splitter
3
A reflects portions of the two laser beams b
31
+b
51
and b
32
+b
52
(i.e., straight polarization lights b
31
and b
32
in a predetermined direction) to irradiate them upon a reference surface
3
B, while it penetrates the remaining laser beams b
51
and b
52
so as to irradiate them on points A and B on a measurement surface
3
C, respectively. Further, between the reference surface
3
B and the polarization beam splitter
3
A and also between the measurement surface
3
C and the polarization beam splitter
3
A, there are respectively provided ¼ wavelength plates
3
D and
3
E, both converting a linear polarization light into a circular polarization light. The laser beams b
31
and b
32
reflected upon the reference surface
3
B penetrate through the polarization beam splitter
3
A to be incident upon light detectors
3
F and
3
G respectively, since each of them is converted from the linear polarization light into the circular polarization light. The laser beams b
51
and b
52
which are reflected upon the measurement surface
3
C, since they are also converted from the linear polarization lights into the circular polarization lights, are reflected upon the polarization beam splitter
3
A to be incident upon light detectors
3
F and
3
G, respectively.
The light detectors
3
F and
3
G receive the combined laser beams b
6
and b
7
of the laser beams b
31
and b
32
which are reflected upon the reference surface
3
B and the laser beam b
51
and b
52
which are reflected upon the measurement surface
3
C, and output electric signals to a phase difference measurement circuit
3
H depending thereupon. The phase difference measurement circuit
3
H measures the height of the projection on the measurement surface
3
C upon the basis of the electric signals from the light detectors
3
F and
3
G.
FIGS. 6A-6B
and
FIGS. 7A-7C
explain a principle of how the height of the projection is detected. In particular,
FIGS. 6A-6B
show a case where no projection nor step is detected on the measurement surface, while
FIGS. 7A-7C
show a case where the projection or the step (including both pits and bumps or asperities) of height &Dgr;&dgr; is detected thereon. In
FIGS. 6A-6B
and
7
A-
7
C, only the polarization beam splitter
3
A, the reference surface
3
B, the measurement surface
3
C, and the light detectors
3
F and
3
G are indicated therein. Upon incidence of the laser beams b
6
and b
7
, interference output signals are outputted from the light detectors
3
F and
3
G, as indicated at the right-hand sides of the
FIGS. 6A-6B
and
7
A-
7
C, respectively. From the laser device
31
is outputted a laser beam of wavelength 532 nm, and it is modulated by the modulator means
32
, so that a frequency difference about 10 MHz can be obtained, i.e., a modulation frequency fm=150 MHz and a modulation frequency fn=140 MHz are obtained. According to this, each the light detectors
3
F and
3
G outputs an interference output signal. Since one cycle of this interference output signal corresponds to approximately one half (½) of the wavelength of the laser beam, 266 nm, it is possible to measure the heights at the points A and B on the measurement surface, i.e., the height when it is the projection, or the distance between the step levels when it is the step (including both pits and bumps or asperities), by measuring the phase difference between the interference signals which are outputted from those light detectors
3
F and
3
G.
In the case of
FIGS. 6A-6B
, since there is no projection nor step on the measurement surface
3
C, the lights reflecting upon at the points A and B on the measuring surface
3
C are incident upon the light detectors
3
F and
3
G, after passing through the same length of the optical path. Accordingly, the phases of the interference output signals of the light detectors
3
F and
3
G are same to each other. On a while, in the case of
FIGS. 7A-7C
, since there lies a step (including both pits and bumps or asperities) upon the measuring surface, as shown in the figure, a phase difference &Dgr;corresponding to the distance betwe
Ishimori Hideo
Yamaba Tuneo
Hitachi Electronics Engineering Co. Ltd.
Lee Andrew H.
LandOfFree
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