Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-08-24
2002-12-31
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S497000
Reexamination Certificate
active
06501553
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a surface profile measuring method and apparatus for measuring rugged surface profiles. More particularly, the invention relates to a technique of measuring surfaces in a non-contact mode by using white light.
(2) Description of the Related Art
Conventional apparatus of this type include a well-known surface profile measuring apparatus which employs a method of measuring, by means of white light interference, rugged profiles of precision products such as semiconductor wafers and glass substrates for liquid crystal displays.
As shown in
FIG. 1
, a conventional surface profile measuring apparatus comprises an interferometer, in which white light from a white light source
90
is directed through a first lens
91
to a half mirror
92
. The white light reflected by the half mirror
92
is condensed by a second lens
93
, and passes through a beam splitter
95
and irradiates an object surface
96
to be measured.
The beam splitter
95
of the interferometer divides the white light into a part that irradiates the object surface
96
, and a part that irradiates a reference surface
94
. The white light irradiating the reference surface
94
is reflected by a reflector
94
a
on the reference surface
94
, and reaches the beam splitter
95
again. The white light having passed through the beam splitter
95
is reflected by the object surface
96
, and reaches the beam splitter
95
again. The beam splitter
95
brings together, to the same path again, the white light reflected by the reference surface
94
and the white light reflected by the object surface
96
. At this time, an interference phenomenon occurs, which corresponds to a difference between a distance L
1
from the reference surface
94
to the beam splitter
95
and a distance L
2
from the beam splitter
95
to the object surface
96
. The white light with which the interference has occurred travels through the half mirror
92
and into a CCD camera
98
.
The CCD camera
98
picks up an image of the object surface
96
along with the white light with which the interference has occurred. This construction includes a varying device, not shown, for vertically shifting a unit having the beam splitter
95
. Such shifting of the beam splitter
95
varies the difference between distance L
1
and distance L
2
, thereby increasing or decreasing the intensity of the white light incident on the CCD camera
98
. When, for example, attention is directed to a particular location on the surface
96
within a region covered by the CCD camera
98
, the beam splitter
95
is moved to vary the difference from distance L
2
<distance L
1
to distance L
2
>distance L
1
. By measuring the intensity of white light having interfered (hereinafter referred to simply as “interference light”) in the particular location, a waveform as shown in
FIG. 2A
, in theory, is obtained. A height in the particular location on the object surface may be derived from a peak position in the waveform of intensity variations of the interference light. The rugged profile of the surface is measured by determining heights in a plurality of locations in a similar way.
It is to be noted that actual data obtained by measuring the intensity of interference light are discrete as shown in FIG.
2
B. It is necessary to derive a peak position in the waveform of intensity variations of the interference light from such data.
A method and apparatus for determining a peak position in a waveform are disclosed in U.S. Pat. No. 5,133,601, for example. In the method and apparatus disclosed therein, each discrete data as shown in
FIG. 2B
is squared to obtain data as shown in FIG.
2
C. Subsequently, these data are smoothed into a waveform as shown in
FIG. 2D. A
height in a particular location is determined from a peak position of the smoothed waveform.
However, such prior method and apparatus have the following drawbacks.
In U.S. Pat. No. 5,133,601 noted above, a theoretical waveform as shown in
FIG. 2A
must be reproduced since a waveform with a peak position occurring at the height in a particular location is based on data obtained through actual measurements. That is, in order to measure, with sufficient precision, a height in a particular location on an object surface being measured, intensity values of the interference light in the particular location must be sampled so finely as to enable reproduction of a theoretical waveform.
As a result, an extended sampling time is required to acquire numerous intensity values, and hence a disadvantage of consuming a long time in measuring a surface profile. Further, a huge amount of data is acquired by the sampling, which requires an increased storage capacity for storing such data. This results in an increased cost of manufacturing the apparatus, and an extended computation time for processing the large amount of data, thereby further extending the time consumed in measuring the surface profile.
SUMMARY OF THE INVENTION
This invention has been made having regard to the state of the art noted above, and its object is to provide a surface profile measuring method and apparatus for measuring rugged profiles of object surfaces at a relatively high speed and with high precision by determining heights in particular locations with increased precision from a relatively small amount of data.
The above object is fulfilled, according to this invention, by a surface profile measuring method for measuring rugged profiles of object surfaces, wherein interference fringes are produced by varying a relative distance between an object surface and a reference surface while irradiating the two surfaces with white light from a white light source, variations in intensity value of interference light occurring at this time are measured for a plurality of particular locations on the object surface, and heights in the particular locations are derived from a group of interference light intensity values acquired from each particular location, the method comprising:
a first step of limiting a frequency band of the white light from the white light source to a particular frequency band;
a second step of varying the relative distance between the object surface and the reference surface irradiated with the white light in the particular frequency band;
a third step of acquiring a group of interference light intensity values which are successively collected from each of the particular locations on the object surface at sampling intervals corresponding to a bandwidth of the particular frequency band, the intensity values corresponding to variations in the interference fringes occurring with variations in the relative distance between the object surface and the reference surface;
a fourth step of estimating characteristic functions based on amplitude components of a theoretical waveform of intensity value variations derived from the group of interference light intensity values; and
a fifth step of determining a height in each of the particular locations based on a peak position of the characteristic functions estimated.
The frequency band of the white light from the white light source is limited to a particular frequency band, and the white light in the particular frequency band irradiates the object surface and the reference surface. Interference fringes are produced according to an optical path difference between the white light reflected by the object surface and the white light reflected by the reference surface.
At this time, the relative distance between the object surface and the reference surface is varied to vary the optical path difference, and hence the interference fringes. Interference light intensity values resulting from the variations in the interference fringes are successively collected from a particular location on the object surface at sampling intervals corresponding to a bandwidth of the particular frequency band. As a result, a minimum group of interference light intensity values based on the white light in the particular frequency band is acquired.
An ideal w
Hirabayashi Akira
Kitagawa Katsuichi
Ogawa Hidemitsu
Connolly Patrick
Toray Engineering Co., Ltd.
Turner Samuel A.
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