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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an interferometric method for dimensional
measurements and defect detection. More particularly, the invention is a
variable phase contrast interferometric method permitting dimensional
measurements without complex computer calculation and also defect
detection.
2. Description of the Related Art
In practically all fields of technology, the processing tolerances observed
during the manufacture of parts have been tightened considerably. This
trend has been most evident in the manufacture of integrated semiconductor
circuits in the submicron range and for the manufacture of magnetic and
optical storage disks, since the extreme miniaturization and increased
packaging densities necessitate extremely plane and flawless surfaces. To
meet these tightened processing tolerances, extremely accurate, high
speed, and automatic measuring techniques are required for the control of
materials, for the monitoring of production processes, and for final
testing.
Past methods used for dimensional measurements include ellipsometric and
stylus techniques. Ellipsometric methods use polarized radiation incident
on the surface to be measured at a large angle, the ellipticity of which
is measured after reflection at the surface measured. The necessary
complicated mathematical calculations are so time consuming that it is not
possible to examine a great number of measuring points on one sample for
manufacturing control for final test during production. Stylus measurement
techniques involve the running of a sensitive electromechanical stylus
across a surface to be measured. However, the mechanical contact
inherently required by this technique makes it impractical for use on the
delicate surfaces of miniaturized semiconductor circuits or data recording
disks.
A variety of interferometric techniques are also known for making
dimensional measurements. In general, these techniques involve the
observation of the interference pattern created by light beams reflecting
off the surface being observed Some interferometric techniques include the
use of light beams of at least two different wavelengths. Examples of
multiple wavelength interferometric methods are shown in U.S. Pat. Nos.
4,652,744 and 4,552,457. Such multiple wavelength techniques suffer from
the disadvantage that their measurements depend on the amount of light
reflected from the observed surface, which is determined in part by the
focal plane of each of the incident light beams thereon The flatness of
the observed surface thus limits the applicability of such techniques for
use in making dimensional measurements, such as step heights.
Amplitude shearing interferometry may also be used for making dimensional
measurements. This technique employs two coherent light beams generated
from the same source. To measure the step height on the surface of a
specimen, for example, the two light beams are made to reflect from the
specimen surface on different sides of the step. The step causes a
difference in the path length of the two light beams. By observing the
shift in the interference pattern of the two light beams caused by the
step, the amount of the shift can be geometrically translated into the
step height. Amplitude shearing techniques suffer from two disadvantages.
First, such measurements can only be made where the step height does not
exceed half the wavelength of the light used. This is because the amount
of shift in the interference pattern for such step heights prevents the
unequivocal location of the interference maxima and minima. Another
disadvantage of amplitude shearing interferometry is that the measurement
of small step heights is limited by one's ability to resolve the shift in
the interference pattern. The resolution of the interference pattern shift
is limited to approximately one-tenth of the wavelength of the light used
because multiple light waves will actually be detected by the system photo
detector.
Phase shearing interferometry is a technique with improved interference
resolution compared with that of amplitude shearing interferometry. Phase
shearing interferometry has been disclosed in U.S. Pat. No. 4,298,283 and
a related publication. Makosch, G., Solf, B., "Surface profiling by
electro-optical phase measurements", SPIE Vol. 316 High Resolution Soft
X-Ray Optics (1981), pgs. 40-53. The technique uses a polarized light beam
which is passed through an electro-optical phase modulator and resolved
into two orthogonally polarized beams using beam splitting optics
consisting of a Wollaston prism and a focusing lens, the two laser beams
are focused on the object surface as colinear beams. The two beams reflect
from the object surface on opposite sides of the step height therein. The
reflected beams are combined by the Wollaston prism and are brought to
interference passing through a polarizer preceding the photodetector. The
step height may be calculated mathematically from the phase shift imparted
between the two beams. The phase shift is itself calculated from the
intensity of the beams measured by the photodetector. The light intensity
measured at the photodetector varies sinusoidally with the voltage applied
to the phase modulator. By measuring the detected intensity at three
different voltages, the phase shift and hence the step height can be
calculated without variations caused by reflectivity of the object surface
This technique is an improvement over amplitude shearing interferometry in
that resolution to approximately 1/300 of the wavelength of light used can
be achieved. U.S. Pat. No. 4,358,201 and European Patent Application No.
0226658 also show phase shearing interferometers with some modification,
such as the use of a Foucalt prism in place of the Wollaston prism.
Although phase shearing interferometry allows for improved resolution, the
technique suffers from the disadvantage that three intensity measurements
at different voltages, and the calculations associated therewith, result
in slow processing times. The time required for measurements is such that
the tool is not suitable for use in the manufacturing environment. The
MP-2000 non-contact surface profiler tool, marketed by Photographic
Sciences, Corp., uses a technique similar to phase shearing interferometry
for measuring surface roughness in the manufacturing environment. The tool
is similar to a phase shearing interferometer, except that no
electro-optical phase modulator is used. Instead, a differential detector
is used after the beam is deflected by a polarized beam splitter. However,
because this tool only makes a single intensity measurement, variations in
surface flatness and reflectivity at each point of reflection will affect
the intensity detected. For this reason, the beams incident upon the
object surface are brought extremely close together, thereby making the
tool impractical for use in dimensional measurements such as for step
height. Information on the MP-2000 tool was acquired from a 1987 product
brochure, which lists the following address for contact to receive
additional information: Photographic Sciences Corp., 770 Basket Road, P.O.
Box 338, Webster, N.Y. 14580-0338.
Other interferometers are known in which the voltage applied to a light
beam phase modulator is selected so as to maximize the resolution
achievable by the system. For example, U.S. Pat. Nos. 4,286,878 and
4,280,766 disclose fiber optic interferometric gyrometers. These
gyrometers sense rotation by measuring the difference in time it takes for
light or other electro-magnetic waves to pass in opposite directions
through a common path loop whose rotation is to be measured. If the two
beams are only slightly out of phase, the light intensity at the detector
will not vary significantly for small changes in phase difference between
the two beams because the light intensity versus phase difference curve
will be at a location of substantially zero slope. To resolve this
problem, the references disclose the use of a phase difference imparted to
the two light beams reaching the detector, such that the beams are 90
degrees out of phase at a zero rotation rate of the fiber optic loop. This
assures that the system operates at a point on the light intensity versus
phase difference curve wherein any slight change in phase difference
between the beams of the pair results in a substantial change in the
intensity of light falling on the detector (i.e. at or near the point of
inflection of the curve). However, neither these or any of the
aforementioned references, provide for both dimensional measurements and
defect detection in the manufacturing environment.
SUMMARY OF THE INVENTION
In view of the foregoing, it is the principal object of this invention to
improve methods for making dimensional measurements in the manufacturing
environment.
Another object of this invention is a method for making dimensional
measurements and the detection of defects in the manufacturing
environment.
These and other objects of this invention are accomplished using phase
shearing interferometry, and the observation that the intensity
measurement at the photo detector is a sine wave type function of the
phase difference between two orthogonally polarized beams. At or near the
extrema in the intensity versus phase difference curve, the intensity does
not change much with increasing or decreasing phase difference. The
interferometer is therefore said to be operating in the "phase
insensitive" mode, whereby the intensity measured by the photodetector is
a function of the reflectivity of the surface being observed. Reflectivity
of the surface is in turn a function of the presence of defects. At or
near the points of inflection in the intensity versus phase difference
curve, the intensity changes rapidly with increasing or decreasing phase
difference. The interferometer is therefore said to be operating in the
"phase sensitive" mode, thereby making dimensional measurements possible.
Thus, by applying the appropriate voltage to the phase modulator, the
interferometer may be made phase sensitive or phase insensitive. Since
only a single measurement is taken in either mode, no complex computer
calculations are required. Because the reflectivity information in the
reflected beams can be minimized, the incident beams are not required to
be extremely close together, making the technique suitable for dimensional
measurements such as step height. In addition, because the voltage applied
to the phase modulator can be switched at rapid electronic rates, both
dimensional measurements and defect detection can be made on the fly using
the same pull in the manufacturing environment.
The foregoing and other objects, features, and advantages of the invention
will be apparent from the following more particular description of the
preferred embodiment of the invention, as illustrated in the accompanying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a schematic diagram of an apparatus suitable for carrying out
the invention.
FIG. 2 shows a plot of the intensity detected by the apparatus of FIG. 1 as
the voltage applied to the phase modulator is cycled from -V to +V volts.
FIG. 3 shows plots of the intensity detected by the apparatus of FIG. 1
when the incident beams are scanned across the same series of parallel
grooves in a disk.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the principle optical arrangement for carrying out the
invention and its operation will now be described.
Laser 1 generates a linearly polarized beam 2 which is suitably rotated by
.lambda./2-plate 3 and polarized again by polarizer 4. Phase modulator 5
resolves the beam into two orthogonally polarized beams. The individual
beams are polarized in the X and Y directions. One of the beams is phase
shifted by the phase modulator which operates electrooptically or
piezoelectrically. The amount of phase shift between the two beams is
dependent on the voltage applied to the modulator.
The X and Y beams next pass through beam splitter 6 where they reach
Wollaston prism 7. The Wollaston prism splits the X and Y beams apart as
shown in the drawing. The angle of separation between the two beams varies
according to the thickness of prism 7 and the slope of the interface of
the components which comprise prism 7. The thus separated beams are then
focused on the surface of object 9 by objective lens 8. The drawing shows
a step height in object 9, which is located between the separated beams.
However, the drawing could show a sloped object surface under the beams,
or a defect existing on the surface of the object, such as particulate
matter. The reflected beams are combined again upon passing through
objective lens 8 and Wollaston prism 7. Upon partial reflection by beam
splitter 6, the beams pass through polarizer 10 where they reach
photodetector 11. In addition, the beams pass through a phase plate
located on either side of polarizer 10, not shown in FIG. 1. Photodetector
11 senses the intensity of the light incident thereon.
The total amount of light reaching detector 11 is described by equation 1,
wherein J.sub.1 and J.sub.2 are the intensities of the particular
reflected beams, .zeta. is the inherent phase shift introduced by the
interferometer, and .zeta..sub.m is the phase shift resulting from the
difference in optical path length of the two beams.
##EQU1##
The intensity of each reflected beam is a function of its incident
intensity and the surface reflectivity of the object. The phase difference
.zeta..sub.m is a function of the surface topography, such as step heights
or slopes, of the object between the points of incidence of the two beams.
The phase difference .zeta. is a function of the optical components in the
interferometer and the voltage applied to phase modulator 5. The optical
components which affect .zeta. include Wollaston prism 7 and the direction
of polarizer 10. Hence, the light intensity measured at the photodetector
varies sinusoidally with the linear variance of the voltage applied to
modulator 5. The sinusoidal curve has a characteristic phase shift
.zeta..sub.m. .zeta..sub.m is defined in equation 2, in which .lambda. is
the wavelength of the laser and h is the optical path difference of the
two reflected beams.
##EQU2##
Thus, a geometrical step or slope in the surface of the object may be
calculated by using the intensities detected at photodetector 11 and
equations 1 and 2. By measuring the intensities at photodetector 11 at
three different modulator voltages and applying complex mathematical
analysis, the variance in intensities J.sub.1 and J.sub.2 caused by the
difference in reflectivity across the surface of the object can be
accounted for. For a further description of such analysis, one is referred
to the article by Makosch and Solf cited earlier, hereby incorporated by
reference.
The high efficiency dimensional measurement test of this invention operates
to eliminate the need for intensity measurements at three different
applied modulator voltages and the calculations associated therewith.
Instead, a single measurement is made at the modulator voltage which
maximizes sensitivity to the beam phase difference and minimizes
sensitivity to the object surface reflectivity.
FIG. 2 shows the sinusoidal variance of the intensity detected by
photodetector 11 as the voltage applied to phase modulator 5 is cycled
between an arbitrary -v and +v volts. The response of the intensity
half-way along the curve represents the situation when no voltage is
applied to modulator 5. Plots 21 through 25 were obtained by
systematically rotating the phase plate in front of the detector, thereby
introducing a phase shift between the two orthogonal beams. Thus, it can
be seen that the point on the sinusoidal curve that the interferometer is
operating at is a function of the rotation of phase plate 10. The extrema
of the curves shown, having slopes approaching zero, indicate points on
the curve in which the intensity detected is relatively insensitive to the
voltage applied to modulator 5. The points of inflection of the curves
shown, having slopes of relatively large magnitude, are relatively
sensitive points of intensity according to the variance of the voltage
applied to modulator 5. As shown in the figure, it is clearly seen that at
no applied modulator voltage curve 23 is in the phase insensitive region.
Moreover plots 21 and 25 represent a higher degree of phase sensitivity
than plots 22 and 24. It is also significant that plots 21 and 22 show
phase sensitivity in the opposite direction to that shown in plots 24 and
25. It is therefore shown that by regulating the voltage applied to
modulator 5 the sensitivity of the intensity detected at photodetector 11
to the surface reflectance of object 9 can be minimized or maximized
Dimensional measurements are taken with the sensitivity to reflectivity
minimized and the sensitivity to phase difference maximized.
FIG. 3 shows plots of the intensity detected when the beams are scanned
across the same set of grooves in a disk. Plots 31 through 35 were
generated at different applied modulator voltages. It can be seen that the
intensity detected depends on the degree of phase sensitivity selected via
the applied modulator voltage. It should, thus, be obvious to conclude
that the intensity can be made phase sensitive or phase insensitive by
selecting the appropriate interferometer conditions.
By operating the interferometer in the phase insensitive mode, defects on
the surface of object 9 may be detected. In this mode, the phase of the
reflected beams is not a function of the depth or step height; the
intensity of the beams is a function of the reflectivity of the surface of
the object. Thus, a scratch or particulate matter on the surface of the
object which causes a change in surface reflectivity, will show up as a
change in the intensity detected by photodetector 11.
The technique embodied by this invention can be used to detect both
dimensional measurements and defects on the fly in the manufacturing
environment. That is, by altering the head voltage of modulator 5
electronically to switch modes, as opposed to rotating phase plate 10,
which is a manual adjustment, the change between phase sensitive and phase
insensitive modes can be done at rapid electronic speeds. To adjust the
voltage applied to modulator 5, a regulated power supply is attached to
modulator 5, which is programmable. A wafer or disk may be scanned by the
interferometer while such is alternating back and forth between phase
insensitive and phase sensitive modes. This permits characterization of
both dimensional characteristics and defect detection across the wafer,
disk, or any other type of object. When a defect is detected its size can
be determined by continually scanning and observing when the intensity
change that has been detected ceases.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood by
those skilled in the art that various changes in form and details may be
made therein without departing from the spirit and scope of the invention.
For example, energy beams other than light beams may be used. Also, the
beams need not be orthogonally polarized, but merely of relatively
different directions of polarization.
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