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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for measuring extrasurface shape
and intrasurface shape by scanning with a laser beam.
2. Description of the Related Art
It has been increasingly urged to measure the extrasurface shape which is a
change in the direction of height of a precisely machined sample
maintaining a precision of 1 nm, as well as to measure the intrasurface
shape such as the size and shape of a pattern or configuration of areas
formed by different materials, respectively, formed on the surface,
maintaining a precision of 10 nm. A method which utilizes optical
interference has been employed for measuring extrasurface shapes such as
surface coarseness and the like, and an optical heterodyne interference
method is effective in highly precise measuring of a very small change in
height. This method consists of forming beat signals of a differential
frequency by interference between two laser beams of different
frequencies, and detecting a change in the phase of beat signals
maintaining a resolution of about 1/500 wavelength to measure a change of
the surface in the height direction. This change in the phase corresponds
to a difference in the length of optical path between the two beams. Among
the optical heterodyne interference methods, a differential heterodyne
interference method invented by the present inventors has been described
in Japanese Patent Publication No. 44243/1991 entitled "Device for
Measuring Surface Shape by the Optical Heterodyne Interference Method",
according to which two beams of different frequencies are generated by
driving an acousto-optical element with electric signals of two frequency
components in order to detect a change in phase between the two beams.
For measuring the intrasurface shapes, there has been widely employed a
method according to which the surface is scanned with a laser beam which
is focused into a tiny spot, and a change in the intensity of light
reflected from the sample is detected. When the surface is constituted by
a plurality of members having different reflection factors, the intensity
of the reflected light undergoes a change depending upon the distribution
of reflection factors. A change in the intensity of the reflected light
caused by the change in the reflection factor is calculated to detect an
edge which is a boundary at which the reflection factor changes, and the
size and shape of the pattern are measured from a change in the edge
position. In particular, a laser scanning-type confocal microscope has
been used in a variety of fields since it is capable of obtaining inner
surface resolution greater than that of ordinary microscopes. This method
consists of detecting (confocal detecting) the light reflected from the
sample through a pinhole, and the scattering light that becomes noise is
cut off in order to increase the inner surface resolution. Moreover, the
confocal microscope is capable of measuring a change in the height in the
direction of the focal point since it detects the intensity of light
reflected from the position of focal point of the spot beam at which it
falls on the sample. In this case, the sample is moved by a pulse stage or
the like in the direction of the optical axis, and the data related to the
intensity of the reflected light detected at each of the positions are
processed in order to measure the intrasurface and extrasurface
three-dimensional shapes.
However, though the above-mentioned laser scanning-type confocal microscope
is capable of measuring the intrasurface and extrasurface shapes, the
sample must be moved by a mechanical stage when the extrasurface shape is
to be measured. Moreover, since the focal depth of the spot light falling
on the sample is relatively shape is about 0.1 .mu.m. With the laser
scanning-type great, the resolution for measuring the extrasurface shape
is about 0.1 .mu.m. With the laser scanning-type cofocol microscope,
therefore, it is not allowed to measure a change in the extrasurface shape
which is as small as about 10 nm. The optical heterodyne interference
makes it possible to measure a change in the extrasurface shape of about
several nm but does not make it possible to measure the intrasurface
shape.
That is, the phase detection based on the above-mentioned conventional
optical heterodyne interference method makes it possible to detect a
change in the direction of height on the extrasurface but does not make it
possible to detect a change in the intrasurface shape. This is because,
the phase date include only the data related to the lengths of optical
paths in the direction of height. With the method of detecting a change in
the intensity of the reflected light, on the other hand, a change in the
intrasurface shape can be detected but a change in the extrasurface shape
cannot be detected. This is because, when a change in the direction of
height of the surface is within the focal depth of the irradiated light,
the intensity of the reflected light remains constant with respect to a
change in the direction of height. Therefore, it is not possible to
simultaneously measure by using one measuring device the extrasurface
shape and the intrasurface shape of a sample that has a shape changing on
the extrasurface and on the intrasurface. That is, separate measuring
devices have been used depending on the purpose of measurement. It is
therefore an object of the present invention to realize a measuring device
of a novel constitution which is capable of simultaneously measuring the
intrasurface shape and the extrasurface shape maintaining high precision
by solving the aforementioned problems.
SUMMARY OF THE INVENTION
In order to achieve the above-mentioned object, the present invention
provides an optical device for measuring surface shape which basically has
the technical constitution described below.
That is, according to a first aspect of the present invention, there is
provided an optical device for measuring surface shape wherein a laser
beam emitted from a source of laser beam is permitted to be incident upon
an acousto-optical element which generates at least two beams of different
frequencies, the two beams are permitted to fall on and scan the surface
of a sample using a suitable optical system, at least part of the light
reflected by the surface of the sample is split by a splitter and is
detected by a suitable light-receiving means to form a reflected light
beat signal, a change in phase between the reflected light beat signal and
a predetermined reference beat signal is detected by a phase comparator
means, the extrasurface shape sample in the direction of height of the
surface thereof is calculated by an extrasurface shape calculation means
and, at the same time, the reflected light that has passed through said
splitter is reflected by another splitter, the intensity in the intensity
distribution of the reflected light is detected by another light-receiving
means to form a reflected light intensity signal, the intrasurface shape
of said sample is calculated by an intrasurface shape calculation means
based upon a change in the intensity of said reflected light intensity
signal, and said extrasurface shape and said intrasurface shape are
measured by the same device.
According to a second aspect of the present invention, there is provided an
optical device for measuring surface shape wherein a laser beam emitted
from a source of laser beam is permitted to be incident upon an
acousto-optical element which generates at least two beams of different
frequencies of which the distribution of resultant intensities is
variable, at least part of the intensity of the two beams is reflected by
a beam splitter and is detected by a first light-receiving means to form a
reference beat signal of an AC component, at least the two beams that have
passed through said beam splitter are focused into tiny spots through an
objective lens and are permitted to fall on and scan the surface of a
sample of which the extrasurface shape and the intrasurface shape are to
be measured, part of the light reflected by said sample is reflected by
said beam splitter and is detected by a second light-receiving means to
form a reflected light intensity signal of a DC component and a reflected
light beat signal of an AC component, a change in phase between said
reflected light beat signal and said reference beat signal is detected by
a phase comparator means, the extrasurface shape of said sample in the
direction of height is measured by an extrasurface shape calculation
means, the intrasurface shape of said sample is calculated by an
intrasurface shape calculation means based upon a change in the intensity
of DC components of said reflected light intensity signal, and said
extrasurface shape and said intrasurface shape are simultaneously
measured.
In the device for measuring surface shape of a sample of the present
invention which employs the aforementioned constitution, the
acousto-optical element generates two beams of different frequencies that
travel in different directions when it is driven with electric signals of
two frequency components fa.+-.fm. The frequency fa controls the angle of
diffraction of the two beams, and the frequency fm controls the angle of
separation of the two beams. The two beams are scanned by the
acousto-optical element, and are focused into tiny spots which are then
permitted to fall on a sample, and the reflected light is detected. The
reflected light signal of the two interfering beams has a DC component on
which an AC component is superposed. These components are separately
detected. A change in the direction of height on the extrasurface is
detected relying upon an AC signal and a change in the intrasurface shape
is detected from a change in the intensity of a DC signal. Thus, by using
the optical heterodyne interferometer constitution, the reflected light
detected by the same light-receiving unit is separated and is detected,
and two operations, i.e., optical heterodyne detection and detection of
the reflected light intensity are realized using one optical device.
Concretely speaking, the angle subtended by the two beams is controlled by
a frequency fm; i.e., the separation into two beams increases with an
increase in fm, and the state of substantially one beam is established
when fm is small. In order to measure the extrasurface shape, therefore,
the state of the two beams is established, in order to detect a change in
phase between the AC beat signals detected by the first and second light
receivers by utilizing the heterodyne interference. In order to measure
the intrasurface shape, either two beams or one beam is used, and a DC
reflected light intensity signal is detected using a third light receiver.
The intensity is detected at a predetermined point in the beam scanning,
which is a cofocol detection of the reflected light through a pinhole or
the like, and features a high intrasurface resolution of measurement.
Thus, the optical heterodyne interference function and the confocal
microscope function are realized using a single optical device.
That is, the present invention has a principle in that when the
extrasurface shape is to be measured utilizing the optical heterodyne
interference, a change in the length of the optical path between the two
beams is detected from a change in phase of the AC signals. In detecting
the intensity of the reflected light, there are detected the distribution
of resultant intensities of two beams and the intensity of the reflected
light that corresponds to the distribution of reflection factors on the
surface of the sample. A change in the intensity of the reflected light is
calculated to detect the edge at which the reflection factor changes. The
shape and size are measured from a change in the edge position and from
the scanning distance of the two beams that have scanned across the edges.
In an embodiment of the present invention as will be described later,
furthermore, the reflected light signal is detected by all of the first,
second and third light receivers when the two beams are used for measuring
the intrasurface shape. In this case, it becomes possible to
simultaneously measure both the extrasurface shape and the intrasurface
shape. Here, since the light receivers have been provided in the same
optical device, both the extrasurface shape and the intrasurface shape can
be measured by one optical device. In the present invention, as explained
above, the extrasurface shape denotes a height or a depth of an irregular
portion or concaved portion formed on a surface of body, to be detected,
and the intrasurface shape denotes a characteristic configurations formed
on the surface of the body. In this case, provision is made of a variety
of polarizing elements and their polarizing axes are adjusted to set the
intensity of the reflected light incident on the three light receivers to
be an optimum value. When the intrasurface shape and the extrasurface
shape are to be separately measured, furthermore, the shape of beam
emitted from the acousto-optical element is set depending upon the kind of
measurement, the intensity of the reflected light incident on the light
receivers is maximized and the intensity of the reflected light incident
on the light receivers not used for the measurement is minimized, in order
to increase the efficiency for detecting the reflected light as well as to
increase the S/N ratio of the reflected light.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram for explaining the constitution and operation of
an embodiment in a first aspect of the present invention;
FIG. 2 is a diagram of a sample that is to be measured;
FIGS. 3(A) to 3(C) are diagrams of waveforms of detected signals of when
the extrasurface shape is measured by detecting the phase of the reflected
light and, at the same time, when the intrasurface shape is measured by
detecting the intensity while effecting the scanning with two beams;
FIGS. 4(A) and 4 (B) are a diagram of a waveform of a reflected light
intensity signal that is detected during the intrasurface shape
measurement while effecting the scanning with one beam and a diagram of a
signal waveform of when the differential processing is effected;
FIG. 5 is a diagram illustrating the constitution of an optical system
according to an embodiment of the present invention;
FIG. 6 is a block diagram of a system for explaining the constitution and
operation of an embodiment in a second aspect of the present invention;
FIG. 7 is a diagram illustrating the constitution of a scanning optical
system which effects the two-beam scanning according to the second aspect
of the present invention;
FIGS. 8(A) to 8(C) are diagrams illustrating examples of measurements
according to the second aspect of the present invention, wherein FIG. 8(A)
illustrates the constitution of a sample that is to be measured, FIG. 8(B)
illustrates the detection of a phase, and FIG. 8(C) illustrates a pattern
of the intensity of the reflected light;
FIG. 9 iS a diagram illustrating an object to be measured of the present
invention;
FIG. 10 is a block diagram illustrating the constitution of a measuring
device according to the second aspect of the present invention;
FIG. 11(A) is a diagram illustrating the state where a sample to be
measured is scanned with an optical beam according to the present
invention;
FIG. 11(B) is a diagram illustrating the distribution of intensities of the
scanning beam; and
FIGS. 12(A) and 12 (B) are graphs illustrating the results of measurement,
wherein FIG. 12(A) is a graph showing a change in the intensity of the
reflected light, and
FIG. 12(B) is a graph showing a change in the phase.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An optical device for measuring surface shape according to the present
invention will now be described in detail with reference to the drawings.
FIG. 1 is a block diagram illustrating the structure of an embodiment for
realizing the fundamental constitution of the device for measuring surface
shape according to a first aspect of the present invention, and in which
reference numeral 10 denotes a source of laser beam which may be, for
example, an He--Ne laser, a semiconductor laser or the like and emits a
laser beam 100 having linear polarization. Reference numeral 11 denotes a
first beam splitter having the constitution of a polarized beam splitter
which relies upon the polarization. With the axis of linear polarization
of the laser beam 100 being adjusted, the laser beam is permitted by
almost 100% to pass through the first beam splitter. Reference numeral 12
denotes an acousto-optical element (hereinafter abbreviated as AO) which
is driven by an acousto-optical element driver (hereinafter abbreviated as
AO driver) which receives signals from a first signal source 112 that
generates a frequency fm and from a second signal source 114 that
generates a frequency fa. Here, the frequency fm controls the shape of a
beam emitted from the AO 12 and the frequency fa controls the scanning of
the beam. When the frequency fm of the signals generated by the first
signal source 112 is low, the AO 12 emits diffracted light of
substantially one beam and when the frequency fa is high, the AO 12 emits
diffracted light which is separated into two beams that have frequencies
different from each other.
The two beams or one beam emitted from the AO 12 is suitably converted for
its axis of polarization through a 1/2 wavelength board, adjusted for its
intensity, and is permitted to be incident upon a second beam splitter 13.
The second beam splitter 13 has the constitution of a polarized beam
splitter which relies upon the polarization. The second beam splitter 13
reflects part of the intensity of the incident beam in a first direction
(X-direction) and the reflected light is detected by a first light
receiver 14. The beam that has passed through the second beam splitter 13
passes through a 1/4 wavelength board 135, focused into a tiny spot
through an objective lens 15, and is permitted to fall on and scan a
sample 16 of which the extrasurface and intrasurface shapes are to be
measured. The beam reflected by the sample 16 travels in the reverse
direction along the optical path, and part of the intensity thereof is
reflected by the second beam splitter 13 in a second direction
(Y-direction) and is detected by a second light receiver 17. In this case,
the axis of polarization is adjusted by the 1/4 wavelength board 135,
such that part of the intensity of the beam is reflected.
The reflected light signals detected by the first Light receiver 14 and the
second light receiver 17 are beat signals having a frequency 2 fm which is
a difference in the frequency between the two beams. The first light
receiver 14 generates a reference light beat signal 145 and the second
light receiver 17 generates a reflected light beat signal 175. Reference
numeral 18 denotes a phase comparator which detects a phase difference
between the reference light beat signal 145 and the reflected light beat
signal 175. The detection is based upon the optical heterodyne
interference. The phase of the reference light beat signal 145 remains
constant whereas the phase of the reflected light beat signal 175 changes
depending upon a difference in the optical path between the two beams
falling on the sample 16. Therefore, a change in the phase of the
reflected light beat signal 175 is detected by detecting the phase
difference by the phase comparator 18. Reference numeral 19 denotes an
extrasurface shape calculation unit which calculates the phase data from
the phase comparator 18 to measure the extrasurface shape of the sample
16.
The beam having part of the intensity that has passed through the second
beam splitter 13 travels in the reverse direction passing through the 1/2
wavelength board 125 and the AO 12, and is almost all reflected by the
first beam splitter 11. Reference numeral 20 denotes a pinhole, and 21
denotes a third light receiver. Here, the pinhole 20 is directly adhered
onto the surface of the third light receiver 21. The light receiver of
this constitution detects the intensity of the reflected light over part
of the range inclusive of the central portion in the intensity
distribution of the reflected light. Therefore, the confocal detection is
carried out. Moreover, this position of receiving light is a predetermined
point in the beam scanning. Therefore, no matter which position on the
surface of the sample 16 the probe light is scanning, the reflected light
is detected at a predetermined position by the third light receiver 21
through the pinhole 20. Reference numeral 22 denotes an intrasurface shape
calculation unit which calculates a change in the reflected light
intensity data of DC component detected by the third light receiver 21,
and detects edge positions of the sample 16 to measure the intrasurface
shape.
When the intrasurface shape and the extrasurface shape of the sample 16 are
to be simultaneously measured, two beams of different frequencies are
emitted from the AO 12, and axes of polarization of the polarizing
elements are so adjusted that the reflected light can be detected by all
of the three light receivers 14, 17 and 21. When the extrasurface shape
only is to be measured, the axes of polarization of the 1/2 wavelength
board 125 and of the 1/4 wavelength board 135 are adjusted, so that the
reflected light is incident only upon the first light receiver 14 and the
second light receiver 17. When the intrasurface shape only is to be
measured, on the other hand, a maximum reflected light is permitted to be
incident upon the third light receiver 21. It is also allowable to adjust
the polarization and separation by providing polarizing elements such as
polarizing boards other than the polarizing elements that are diagrammed.
That is, when the height is to be measured on the surface of the sample as
described above by using the optical device for measuring surface shape of
the present invention, the scanning is effected with two beams which are
then separated by the second beam splitter 13 to form reference beat
signals. One beam is permitted to fall on the surface of the sample 16,
and the reflected light thereof is received again by the second beam
splitter 13 and is separated to form a reflected light beat signal. A
change in the phase is then detected between the reflected light beat
signal and the reference beat signal to thereby measure the height.
In the above optical heterodyne system, it is desired that the axes of
polarization are so adjusted that the reflected light will not be incident
upon the first beam splitter.
When the intrasurface size of the sample is to be measured by using the
optical device for measuring surface shape of the present invention, the
scanning is effected with one beam which falls on the surface of the
sample 16, and the reflected light thereof is received by the first beam
splitter 11 and is separated. A change in the intensity of the reflected
light is analyzed by the third light receiver 21 to measure the size.
In this operation, the second beam splitter 13 is not necessary and should,
hence, be omitted from the optical path of the reflected beam.
For this purpose, the second beam splitter 13 is connected to a suitable
drive mechanism. When the extrasurface shape is to be measured, the drive
mechanism is actuated in order to move the second beam splitter 13 to a
position off the optical path of the reflected beam.
When the second beam splitter 13 is to be moved to a position off the
optical path of the reflected beam, it is desired to replace it by a
transparent member such as the one composed of a glass or the like having
the same shape and the same refractive index as those of the second beam
splitter 13.
The aforementioned constitution will now be concretely described. In
separately measuring the extrasurface shape and the intrasurface shape of
the sample, when the intrasurface shape is to be measured, a single beam
having a single frequency is emitted from the acousto-optical element 12.
When this single beam reflected by the surface of the sample is to be
received by the third light receiver 21, the second beam splitter 13 is
moved off the optical light of the reflected light.
In measuring the intrasurface shape according to the present invention,
furthermore, what is important is that the light reflected by the surface
of the sample 16 is separated by the first beam splitter 11 and is
received by the third light receiver 21 and, in this case, the first beam
splitter 11 is deposed between the source of laser beam 10 and the
acousto-optical element 12. According to this constitution, the reflected
light necessarily passed through the confocal point, enabling the
measurement to be correctly carried out.
According to the aforementioned embodiment of the present invention, it is
further allowable to provide a 1/2 wavelength board 145 between the source
10 of laser beam and the first beam splitter 11.
In the above-mentioned embodiment, the reference beat signals are formed by
partly separating the two beams emitted from the acousto-optical element
12 through the second beam splitter 13. However, the present invention is
not necessarily limited to the above constitution, and the reference beat
signals may be formed by any means.
For instance, the reference beat signals may be directly generated from a
signal source that drives the acousto-optical element. More concretely,
there may be used, as the reference beat signals, AC signals having a
frequency 2 fm which is equal to twice as great as the difference of
frequency between the first signal fa and the second signal fm that are
fed to the acousto-optical element to drive it.
According to the above-mentioned embodiment of the present invention, the
intrasurface shape can be measured to an accuracy of the order of microns
when the scanning is effected with two beams and to an accuracy of the
order of submicrons when the scanning is effected with one beam.
As a further constitution, the present invention provides an optical device
for measuring surface shape wherein a laser beam emitted from a source of
laser beam is permitted to pass through a first beam splitter and to be
incident upon an acousto-optical element which generates at least two
beams of different frequencies to effect the scanning, the beams are
separated into two directions through a second beam splitter, a first beam
that travels in a first direction being reflected by the second beam
splitter is detected by a first light receiver which forms a reference
beat signal, the beam that travels passing through the second beam
splitter is focused into a tiny spot through an objective lens and is
caused to fall on and scan the surface of a sample of which the shape is
to be measured, part of the light reflected by the sample is reflected by
the second beam splitter in a second direction different from the first
direction and is detected by a second light receiver which forms a
reflected light beat signal, a change in the phase between the reflected
light beat signal and the reference beat signal is detected by a phase
comparator, the extrasurface shape of the sample in the height direction
is calculated by an extrasurface shape calculation means, the reflected
light that has passed through the second beam splitter is reflected by the
first beam splitter, part of the reflected light having a predetermined
distribution of reflected light intensities is detected by a third light
receiver which forms a reflected light intensity signal, the intrasurface
shape of the sample is calculated by an intrasurface shape calculation
means based on a change in the intensity of the reflected light intensity
signal, and the extrasurface shape and the intrasurface shape are
simultaneously measured. The invention further provides an optical device
for measuring surface shape wherein a laser beam emitted from a source of
laser beam is permitted to pass through a beam splitter and to be incident
upon an acousto-optical element which generates at least two beams of
different frequencies to effect the scanning, the beams effecting the
scanning are focused into tiny spots through an optical system inclusive
of an objective lens and are caused to fall on and scan the surface of a
sample of which the shape is to be measured, at least part of the light
reflected by the sample is detected by a suitable light receiver which
forms a reflected light beat signal, a change in the phase between said
reflected light beat signal and a reference beat signal obtained from a
signal source that drives the acousto-optical element is detected by a
phase comparator, the extrasurface shape of the sample in the direction of
height is calculated by an extrasurface shape calculation means, the
reflected light is reflected by the beam splitter, part of the reflected
light having a predetermined distribution of reflected light intensities
is detected by a light receiver different from the light receiver to form
a reflected light intensity signal, the intrasurface shape of the sample
is calculated by an intrasurface shape calculation means based on a change
in the intensity of the reflected light intensity signal, and the
extrasurface shape and the intrasurface shape are simultaneously measured.
The procedure for simultaneously measuring the extrasurface shape and the
intrasurface shape according to the aforementioned first aspect of the
present invention will now be described below with reference to FIGS. 2 to
4.
FIG. 2 illustrates a shape of a sample 16 which is to be measured for its
extrasurface shape and intrasurface shape simultaneously. The sample 16
consists of a substrate portion 25 having a reflection factor Rs and a
dimensional portion 26 having a reflection factor Rm. There exist
protruding steps h of about 0.1 .mu.m between the dimensional portion 26
and the substrate portion 25. The surface of the sample is scanned with
two beams 27 and 28 which are focused into tiny spots. The distances
between the peaks of the two beams 27 and 28 are controlled by an AC
signal of a frequency fm generated from the first signal source 112. The
distances between the peaks are set to be nearly equal to the diameters of
the individual beams. Moreover, the frequency fa generated from the second
signal source 114 is changed to effect the scanning with the two beams 27
and 28.
FIG. 3 (a), 3(b) and 3(c) shows examples of signal waveforms that are
detected. The waveform 31 shown in FIG. 3(A) is that of a change in phase
of the reflected light detected by the second light receiver 17. According
to this constitution, the optical heterodyne interference is of the
differential type which detects a difference in the optical path between
the two beams 27 and 28. Therefore, the detected phase represents the
differentiation of the surface of the sample 16. When the source of laser
beam is an He--Ne laser, one degree of phase represents a difference of
0.88 nm in the optical path. In a portion where steps exist at the edges
between the substrate portion 25 and the dimensional portion 26,
therefore, the phase varies depending upon the steps. A change in phase on
the surfaces of the substrate portion 25 and the dimensional portion 26
represents the surface coarseness on the surfaces thereof. Here, a
positive sign of the phase represents that the surface between the two
beams 27 and 28 is protruding and a negative sign of the phase represents
that the surface is recessed. The thus obtained phase data are then
integrated by the extrasurface shape calculation unit 19 to measure the
surface topography.
The waveform 32 shown in FIG. 3(B) represents a reflected light intensity
pattern signal that is detected by the third light receiver 21 when the
scanning is effected with two beams 27 and 28. The surface reflection
factors of the members hold a relationship Rm>Rs. The waveform 32 includes
a change in the intensity at the central portions 322 and 324 of
intensity. The waveform 33 shown in FIG. 3(C) is a differential intensity
waveform of the reflected light intensity pattern signal 32. Detection is
made of the position of a peak 332 between the two rising peaks 330 and
331 and the position of a peak 337 between the two breaking peaks 335 and
336. The peak positions 332 and 337 represent the state where the edge
positions of the substrate portion 25 and of the dimensional portion 26
are irradiated with a central portion of the intensity distribution of the
two beams. The intrasurface shape calculation unit 22 detects the edge
positions. The size is measured from the scanning amount of the two beams
between the edge positions, and the shape is measured from a change in the
edge positions.
When the extrasurface shape and the intrasurface shape are to be separately
measured, there may be employed the scanning with two beams as explained
with reference to FIG. 3. In particular, the intrasurface shape can be
measured utilizing the scanning with one beam. FIG. 4 shows an example of
detecting an edge when the scanning is effected with one beam. The sample
is the same as the one shown in FIG. 2. The waveform 41 of FIG. 4(A)
represents a reflected light intensity signal. The rising part 410 and
breaking part 415 of the waveform monotonously increases and monotonously
decreases. The waveform 42 of FIG. 4(B) is a differential intensity signal
waveform which is obtained by subjecting the waveform 41 to the
differentiation processing. Two peak intensity positions 420 and 425 are
detected. The peak intensity positions represent the state where the edges
are irradiated with beams of peak intensities at positions where the rate
of change in the reflected light intensity is maximum.
FIG. 5 illustrates another concrete constitution of the optical system in
the device for measuring surface shape according to the present invention.
A laser beam emitted from the source 10 of laser beam passes through the
first beam splitter 11, and is converted into a sheet beam which spreads
over a plane in parallel with the paper through a combination of a
cylindrical lens 50 and a convex lens 51, and is incident on the AO 12
which generates a probe light that corresponds to the frequency fm as
mentioned earlier. The probe light coming out of the AO 12 is converted
again into a circular beam through the 1/2 wavelength board 125, a convex
lens 52 and a cylindrical lens 53. The probe light traveling as a
diverging light is split into two directions by the second beam splitter
13. The reflected light having part of the intensity is focused through a
convex lens 54 and is detected by the first light receiver 14. The passing
light is collimated through a convex lens 55, passes through the 1/4
wavelength board 135, focused by the objective lens 15, and is caused to
fall on and scan the surface of the sample 16. The light reflected by the
sample 16 is partly reflected by the second beam splitter and is detected
by the light receiver 17. The reflected light that has passed through the
second beam splitter 13 is reflected by the first beam splitter 11,
focused through a convex lens 56, and is detected by the third light
receiver 21 provided with a pinhole. The optical system of the
above-mentioned constitution makes it possible to simultaneously measure
the intrasurface shape and the extrasurface shape.
As described above, the optical device of the present invention which is
provided with the optical heterodyne interference function and the
confocal scanning microscope function, makes it possible to simultaneously
measure the extrasurface shape maintaining a precision of 1 nm and the
intrasurface shape maintaining a precision of 10 nm. The two beams emitted
from the acousto-optical element can be freely controlled for their
intensity distribution. Therefore, the conditions of the two beams can be
set depending on the purpose of measurement to carry out a wide range of
measurement. The reflected light intensity signal needs be simply
processed. Therefore, the measurement can be taken in real time using a
simply constituted operation unit. Moreover, the two beams falling on the
sample travel through nearly the same optical paths and are hardly
affected by the disturbance, making it possible to take stable measurement
that is suited for in-line measurement in the production line.
Next, the concrete constitution of the device for measuring surface shape
according to the second aspect of the present invention will be described
with reference to the drawings. That is, the fundamental constitution of
the second aspect of the present invention is as described above, and FIG.
6 is a block diagram illustrating the structure of an embodiment thereof.
FIG. 6 is a block diagram illustrating the constitution of the present
invention wherein reference numeral 201 denotes a source of laser beam
which is constituted by, for example, a He--Ne laser, a semiconductor
laser or the like, and emits a laser beam 200 having linear polarization,
and 211 denotes an acousto-optical element (hereinafter abbreviated as AO)
which is driven by an acousto-optical element driver 210 that receives
signals from a first signal source 232 that generates AC signals of a
frequency fa and from a second signal source 234 that generates AC signals
of a frequency fm. The acousto-optical driver 210 forms a drive signal
having two frequency components fa.+-.fm and drives the AO 212 to generate
two beams 220 and 222 that have different frequencies and that travel in
different directions. The frequency fm controls the angle of separation
between the two beams 220 and 222, and the frequency fa controls the
angles of diffraction of the two beams. The two beams 220 and 222 emitted
from the AO 212 are split into two directions by a beam splitter 231.
The two beams having an intensity of about 20% reflected by the beam
splitter 231 are detected by a first light receiver 213 which forms a
reference light beat signal 230 of AC component. The two beams that have
passed through the beam splitter 231 are focused through an objective 214
and are permitted to fall on and scan the surface of a sample 215 whose
extrasurface shape an | | |
