Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-02-09
2003-11-04
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06643026
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 2000-050783 filed on Feb. 28, 2000, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system used in an oblique incidence interferometer which makes it possible to measure the planarity of a rough surface, in particular, in a noncontact manner; and an apparatus using the same.
2. Description of the Prior Art
Conventionally, various interferometer apparatus for measuring the planarity of surfaces of processed products have been known. Among them, oblique incidence interferometer apparatus have been known as an apparatus which can measure the planarity of test surfaces having large irregularities.
The oblique incidence interferometer apparatus have been used for measuring the planarity of rough surfaces and the like in a noncontact manner, since their measuring sensitivity can be set lower by making a coherent light beam bundle obliquely incident on a test surface. Letting &lgr; be the wavelength of light used for measurement, and &thgr; be the angle of incidence with respect to the test surface, the amount of irregularities of the test surface, i.e., the measuring sensitivity &Dgr; is represented by the following expression:
&Dgr;h=&lgr;/
(2 cos &thgr;)
Namely, as the incident angle &thgr; becomes greater, so that the degree of oblique incidence increases, the fringe interval becomes longer, so that the measuring sensitivity lowers, whereby it becomes possible to measure surfaces having a low surface accuracy.
FIG. 14
shows a first configurational example of conventional oblique incidence interferometer apparatus, using a planar reference plate as a reference standard. In this oblique incidence interferometer apparatus, a reference plane
116
a
of a plane parallel plate
116
and a test surface
2
a
of a sample
2
are disposed so as to oppose each other. Coherent light emitted from a laser light source
111
is turned into parallel light by a collimator lens
114
and obliquely irradiates the reference plane
116
a.
Interference fringes corresponding to the optical path difference based on the distance between the reference plane
116
a
and test surface
2
a
are projected onto a screen
118
, so as to be viewed by an observer
119
. In this example, as shown in
FIG. 14
, reference light and measurement light are separated from each other at the reference plane
116
a
and then are combined together at this plane.
FIG. 15
is a second configurational example of conventional oblique incidence interferometer apparatus, which is an example known as Abramson type using a right isosceles triangle prism as a reference standard. In FIG.
15
and its subsequent conventional examples, members similar to those of the oblique incidence interferometer apparatus shown in
FIG. 14
are referred to with numerals whose lower two digits are the same as those of their equivalents in FIG.
14
. In this apparatus, coherent parallel light is made incident on a right isosceles triangle prism
216
from an entrance surface
216
b.
As in the above-mentioned first example, reference light and measurement light are separated from each other at a reference plane
216
a
and then are combined together at this plane. This apparatus is configured such that interference fringes projected on a screen
218
are captured by a TV camera
219
so as to be viewed.
FIG. 16
shows a third configurational example of conventional oblique incidence interferometer apparatus, which is an example known as Birch type, using diffraction gratings.
In this oblique incidence interferometer apparatus, coherent parallel light is made incident on a diffraction grating
317
a,
so that its wavefront is divided into two directions. One of thus obtained light beam bundles is made obliquely incident on a test surface
2
a,
and the resulting reflected light is used as measurement light, whereas the other light beam bundle is used as reference light. The measurement light and reference light are made incident on a diffraction grating
317
b,
so as to combine their wavefronts together. Interference fringes generated by the optical interference between the measurement light and reference light emitted from the diffraction grating
317
b
in the same direction are projected onto a hologram screen
318
, and are captured by a TV camera
319
so as to be viewed. In
FIG. 16
, the zero-order diffracted light subjected to wavefront division at the diffraction grating
317
a
is used as the reference light, whereas the first-order diffracted light is used as the measurement light. The first-order diffracted light of reference light and the zero-order light of measurement light are combined together at the diffraction grating
317
b
in a later stage, so as to interfere with each other.
FIG. 17
is a fourth configurational example of conventional oblique incidence interferometer apparatus, which is an example employing a Mach-Zehnder type interferometer as an oblique incidence interferometer.
In this oblique incidence interferometer apparatus, coherent parallel light is divided into two directions by a half mirror
417
a.
One of thus obtained light beam bundles is used as measurement light so as to be made obliquely incident on a test surface
2
a
by way of a mirror
415
a,
whereas the other light beam bundle is used as reference light. The measurement light reflected by the test surface
2
a
and the reference light reflected by the mirror
415
b
are combined together by a half mirror
417
b.
Interference fringes generated by optical interference between the measurement light and reference light are projected onto a screen
418
, so as to be viewed directly or by use of a TV camera and the like.
Though optical systems for oblique incidence interferometers having various configurations and apparatus using the same have conventionally been proposed as mentioned above, the optical systems and apparatus have their own problems.
Functions required for the optical systems for oblique incidence interferometers and apparatus include easiness in viewing the interference fringes formed. One of causes obstructing the viewing is interference noise.
For example, in the configuration of the first conventional example, interference noise is likely to occur due to the reflected light at a surface (which may be an entrance or exit surface) of the planar plate other than the reference plane. This interference noise can be reduced to a certain extent if the surface is provided with an antireflection coating. Since the angle of incidence is large, however, a coating having a low reflectivity is hard to provide.
The apparatus of the second conventional example can prevent the interference noise from occurring due to such surface reflection. Nevertheless, a problem of interference noise caused by multi-reflected light between the test surface and reference plane remains even in such a configuration. For eliminating this problem, the multi-reflected light between the test surface and reference plane and the light internally reflected by the reference plane should be prevented from interfering with each other and reaching the screen surface. In order to prevent this from happening with a relative arrangement of optical members, if will be effective if the reference plane is set to a size at least twice that of the test surface, for example. The conventional configurational example using the right isosceles triangle prism
216
is problematic in that the prism
216
itself becomes very large, heavy, and expensive in order to enlarge the reference plane
216
a
as such.
There is also a problem that the luminous flux internally reflected by the reference plane
216
a
of prism
216
is emitted into the same direction as the interfering light, whereby the noise caused by this internally reflected light is superimposed on the screen
218
.
Since the third conventional example utilizes a diffraction grating, there is a fear of unnecessary orders of diffracted light bec
Kanda Hideo
Kobayashi Fumio
Watanabe Fumio
Snider Ronald R.
Snider & Associates
Turner Samuel A.
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