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DESCRIPTION
1. Technical Field
The subject invention relates to a new and improved method and apparatus
for detecting thermal waves generated in a sample. More particularly, a
noncontact measurement technique is disclosed wherein the periodic
temperature induced change in reflectivity of a sample surface is
monitored by testing the intensity variations of an electromagnetic probe
beam that is reflected off the sample surface. The intensity variations of
the probe beam are used to detect thermal waves in a sample.
2. Background of the Invention
There is presently a significant amount of research being conducted in the
field of thermal wave microscopy. In thermal wave microscopy, a periodic
heat source is focused on the surface of a sample. The heat source is
typically supplied by either an intensity modulated laser beam or a stream
of particles, such as an electron beam. When the sample absorbs the
incident energy at or near the sample surface, a periodic surface heating
results which, in turn, generates thermal waves that propagate from the
irradiated spot. These thermal waves have the same frequency as the beam
modulation frequency. The wavelength of the thermal waves is determined
both by the frequency of the beam and by the thermal parameters of the
sample.
In a thermal wave microscope, thermal features beneath the sample surface
are detected and imaged by sensing the thermal waves that scatter and
reflect from these features. The thermal waves are highly damped such that
they travel only one or two wavelengths before becoming too weak to
detect. Nevertheless, a variety of methods have been developed capable of
sensing and measuring the thermal waves generated in the sample.
One method of detection includes the sensing of acoustic waves which are
generated by the thermal waves. More particularly, acoustic waves are
generated because the thermal waves induce stress-strain oscillations in
the heated region of the sample. These elastic waves are true propagating
waves and can be detected with conventional ultrasonic transducers. This
technique disclosed in U.S. Pat. No. 4,255,971, issued Mar. 17, 1981,
assigned to the same assignee as the subject invention, and which is
incorporated herein by reference.
As can be appreciated, the above described system, utilizing a
piezoelectric crystal, is a "contact" technique requiring the attachment
of the transducer to the sample. The latter requirement is time-consuming
and potentially contaminating and is not suitable for production
situations encountered in the semiconductor industry. Accordingly, there
has been significant work carried out in developing noncontact detection
techniques. One such noncontact detection technique is described in
copending applications, Ser. No. 401,511, filed July 26, 1982 and now U.S.
Pat. No. 4,521,118, issued June 4, 1985, and Ser. No. 481,275, filed Apr.
1, 1983, incorporated by reference.
The latter applications describe a method and apparatus for detecting
thermal waves by monitoring the local angular changes occurring at the
surface of the sample. More specifically, when thermal waves are generated
in a localized region of the sample, the surface of the sample undergoes
periodic angular changes within the periodically heated area because of
local thermoelastic effects. These angular changes occur at a frequency
equal to the frequency of the modulated heating beam. To monitor these
changes, a beam of energy, such as a laser beam, is focused on the surface
of the sample in a manner such that is reflected. Because of the local
angular changes occurring at the surface of the sample, the reflected beam
will experience angular displacements in a periodic fashion. By measuring
the angular displacements, information about the thermal wave activity in
the sample can be determined. The latter technique has proved to be a
highly sensitive process for detecting thermal waves.
The subject invention, in contrast, is directed towards an independent and
totally different method of detecting thermal waves. The technique
disclosed herein may be used as an independent basis for the detection of
thermal waves. In addition, when used in combination with any of the
earlier described techniques, new and surprising additional information
may be obtained about the characteristics of a sample. The advantages of
using two different thermal wave detection techniques to gain additional
information about a sample is described in detail in copending
application, Ser. No. 612,077, filed May 21, 1984, assigned to the same
assignee as the subject invention and incorporated herein by reference.
These advantages are discussed briefly below.
In the above described techniques, such as monitoring the deflection of a
probe beam or through detection of acoustic waves through a transducer,
the output signals generated are primarily a function of the integral of
the temperature distribution through the sample. In contrast, in the
subject system, which is based on measurements of reflectivity, the output
signals are primarily a function of surface temperature. The availability
of two independent measurements of thermal wave signals permits the
evaluation of both thickness and compositional variables in a sample. The
latter concepts are set forth in detail, and are the subject of the
copending application cited above, which is incorporated herein by
reference. It should be understood, however, that the subject invention
not only provides a new detection technique, but in addition, when
combined with other measurement techniques, defines a completely new and
powerful analytical tool with capabilities not found in the prior art.
Accordingly, it is an object of the subject invention to provide a new and
improved apparatus and method for detecting thermal waves.
It is another object of the subject invention to provide a new and improved
apparatus and method which detects thermal waves based on changes in
reflectivity of the sample.
It is still a further object of the subject invention to provide a new and
improved method and apparatus for detecting thermal waves which is based
on surface temperature variations.
SUMMARY OF THE INVENTION
In accordance with these and many other objects, the subject invention
provides for a new and improved method and apparatus for detecting thermal
waves. The method and apparatus is based on the principle that the changes
in optical reflectivity of a sample, occurring as it is periodically
heated, will vary, depending on the thermal characteristics of the sample.
It has been known that optical reflectivity is dependent, to some extent,
on temperature. This dependence is defined by the following equation:
R.sub.T =R.sub.o +(.delta.R/.delta.T)(.DELTA.T) (1)
In this equation, R.sub.o represents the reflectivity at a set temperature
and the second term in the equation gives the change of reflectivity
resulting from the change in surface temperature. The term
(.delta.R/.delta.T) is the temperature coefficient of reflectivity which
represents the rate of change in reflectivity with respect to the change
in temperature. The term .DELTA.T is the changing temperature at the
sample surface.
The first term R.sub.o is at least four orders of magnitude greater than
the second term for temperature changes, .DELTA.T of less than
100.degree.. Furthermore, the noise level associated with R.sub.o as
measured with a photodetector, is on the order of .sqroot.R.sub.o. The
latter value is still 100 times greater than the second term of the
equation which makes measurement of the second term quite difficult. In
absolute terms, the value of the ratio
(.delta.R/.delta.T)(.DELTA.T)/R.sub.o is on the order of 10.sup.-4 to
10.sup.-5 and, therefore, has not been used as a measurement parameter.
In accordance with the subject invention, this difficulty is overcome by
modulating the heating source. Periodic changes in reflectivity which are
occurring at the frequency of the modulation beam are then monitored. This
information is processed by passing the signal through narrow bandwidth
filters. The result is that only the periodic reflectivity signal
.DELTA.R.sub.T, as a result of the periodic temperature variations
.DELTA.T, is measured, rather than absolute reflectivity R.sub.T.
The periodic reflectivity signal .DELTA.R.sub.T is defined by the following
equation:
.DELTA.R.sub.T =(.delta.R/.delta.T)(.DELTA.T) (2)
As seen from the above equation, the periodic reflectivity signal
.DELTA.R.sub.T is dependent on the temperature coefficient of reflectivity
(.delta.R/.delta.T) times the periodic surface temperature (.DELTA.T). The
periodic reflectivity signal .DELTA.R.sub.T thus provides a measure of the
periodic surface temperature .DELTA.T. The periodic surface temperature,
in turn, provides information about thermal wave propagation and
interaction in the material. Thus, with suitable mathematical equations,
one can determine the thermal wave activity based on the measured changes
in reflectivity. Calculation of the thermal waves is carried out by
normalizing the signals against a known reference sample, as discussed in
greater detail below.
Based on the foregoing principles, a method and apparatus is disclosed for
detecting the presence of thermal waves. As set forth above, thermal waves
are created by generating a periodic localized heating at a spot on the
surface of a sample. In accordance with the subject invention, the
apparatus for detecting the thermal waves includes a radiation probe beam
which is directed on a portion of the periodically heated area on the
sample surface in a manner such that the radiation probe beam reflects off
that surface. A means is provided for measuring the intensity variations
of the reflected radiation probe beam resulting from the periodic heating.
Finally, a means is provided for processing the measured intensity
variations from the reflected radiation probe to detect the presence of
thermal waves.
Further objects and advantages of the subject invention will become
apparent from the following detailed description taken in conjunction with
the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a composite block and schematic diagram of the apparatus for
carrying out the detection of thermal waves in accordance with the subject
invention.
FIG. 2 is a graphical representation comparing the available signal
strength in both reflectivity deflection-type thermal wave detection
systems, measured as a function of the distance on the sample surface from
the heating source.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIG. 1, there is illustrated the apparatus 20 for carrying out
the method of the subject invention. A sample 22 is shown resting on a
platform 24. Platform 24 is capable of movement in two orthogonal
directions in a manner such that the sample can be rastered with respect
to the heating and probe beams of the subject invention. Controllable
states are well known in the art and also disclosed in U.S. Pat. No.
4,255,971, cited above.
As illustrated in FIG. 1, a means is shown for generating thermal waves.
This means is defined by laser 30 which is intensity modulated by
modulator 32. In the preferred embodiment, beam 34 is focused on the
surface of the sample by a microscopic objective 38. Beam 34 is intended
to create a periodic surface heating at the sample surface. This periodic
heating is the source of thermal waves that propagate outwardly from the
center of the beam. The thermal waves interact with thermal boundaries and
barriers in a manner that is mathematically equivalent to scattering and
reflection of conventional propagating waves. Any features on or beneath
the surface of the sample that have thermal characteristics different from
their surroundings will reflect and scatter thermal waves and thus become
visible to these thermal waves.
The intensity modulated heating source could be supplied by electromagnetic
radiation at various wavelengths, including X-rays, gamma rays, infrared,
ultraviolet, visible light, microwaves or radio frequencies. The intensity
modulated source can also be generated through thermal excitations arising
from the interaction of the sample with an intensity modulated stream of
particles, such as a beam of electrons, protons, neutrons, ions or
molecules. However, because of the ease of directing and focusing a laser
beam, it is believed that the illustrated embodiment is preferable.
The intensity modulated beam 34 is passed through dichroic mirror 36 prior
to passing through the microscopic objective 38. In the preferred
embodiment, the heating beam is an argon ion laser and the dichroic mirror
is transparent to argon ion radiation. As will be discussed below, the
dichroic mirror functions to reflect the measuring laser beam, which is
preferably generated by a helium-neon laser.
In accordance with the subject invention, a new and improved method and
apparatus is provided for detecting the thermal waves which are being
generated in the sample. The detection system includes a light probe for
emitting a beam 52 which is directed on the surface of the sample that has
been periodically heated by the modulated energy beam 34. In the
illustrated embodiment, the light probe beam 52 is generated by
helium-neon laser 50. Various other sources of electromagnetic radiation
may be used for the probe beam as long as the beam reflectivity is
affected by the temperature changes on the sample surface in a manner
which can be measured.
Probe beam 52, emanating from the helium-neon laser 50, is then passed
through a polarizing splitter 54. The polarizing splitter is oriented in a
manner such as to let the coherent light emanating from laser 50 to pass
freely therethrough. The splitter will, however, deflect all light whose
phase has been rotated through 90.degree. relative to beam 52. The reason
for this arrangement will become apparent below.
Light probe beam 52 is then passed through a 1/4.lambda.-waveplate 55.
Waveplate 55 functions to rotate the phase of the probe beam by
45.degree.. As can be appreciated, on the return path of the beam, the
waveplate will rotate the phase of the beam another 45.degree. so that
when it reaches splitter 54 the phase of the beam will have been rotated a
total of 90.degree. from the incoming orientation. By this arrangement,
the splitter 54 will deflect the retro-flected light beam up to detector
56, as discussed in more detail below.
After the probe beam 52 initially passes through waveplate 55, it is
reflected downwardly by dichroic mirror 36. As pointed out above, the
dichroic mirror is transparent to argon ion light but will reflect the
light rays in the helium-neon frequencies. In the preferred embodiment,
the heating beam and the probe beam are aligned in such a manner that they
are directed in a coincident manner down through lens 38 and focused at
the same spot on the surface of the sample. By focusing the probe beam and
the heating beam at the same spot, the maximum signal output can be
achieved.
It is to be understood that the reflectivity signals of interest exist at
any areas on the surface of the sample which has been periodically heated
by the beam 34. Therefore, the probe beam does not have to be directly
coincident with the heating beam 34 to detect the signals of interest.
Accordingly, a microscope objective is not necessary for focusing either
the heating beam 34 or the probe beam 52. Rather, it is only necessary to
direct the probe beam within at least a portion of the area periodically
heated by beam 34. A discussion and equations for calculating the size of
the periodically heated area are set forth in U.S. Pat. No. 4,521,118,
cited above. Briefly, the diameter of the heated area, which extends
radially away from the center of the heating beam, is a function of the
modulation frequency and the diameter of the heating beam and of the
thermal parameters of the sample.
Because the signals to be measured are so small, on the order of 10.sup.-5
of the DC level of the probe beam, every effort should be made to maximize
the output for detection. Accordingly, it is desirable to direct the probe
beam essentially coincident with the heating beam. Direction of the probe
beam can be accomplished by movement of mirror 36. The alignment of the
probe beam with the heating beam should be contrasted with the measuring
technique described in U.S. Pat. No. 4,521,118, cited above, wherein the
probe beam is preferably directed off center from the heating beam but
within the periodically heated area. As set forth in detail in the latter
specification, the probe beam is intended to measure angular changes in
the surface of the sample. However, the surface of the sample at the
center of the heating beam undergoes only vertical movements. The angular
surface changes occur in areas on the surface spaced from the center of
the heating beam.
A graphical representation of the signal strength available for detection
by these two techniques, as a function of the distance on the sample
surface from the heating source, is illustrated in FIG. 2. In that Figure,
the horizontal axis indicates the distance away from that central heating
point C on the surface of the sample. The vertical axis is a measure of
available signal strength. Curve 70 represents signals available in the
deflection detection technique, while curve 72 illustrates signal strength
available with the subject reflectivity detection technique. As will be
seen from curve 70, the maximum output signals measurable in the
deflection technique are at a minimum adjacent the center of the heating
beam. The signals increase at positions located radially outwardly from
the center and then taper off towards the border of the periodically
heated area. The actual dimensions of the periodically heated area can be
calculated by the equations set forth in the prior application. In
contrast, curve 72 indicates that the reflectivity output signal is
maximized when the probe beam is centered on the heating beam. From FIG. 2
it should be apparent that in the subject technique, maximum signal output
can be achieved by focusing the probe beam to be coincident with the
heating laser beam.
As the probe beam is reflected off the surface of the sample, it interacts
with the electrons and thus with the lattice structure of the sample at
its surface. The lattice structure of the sample will undergo periodic
changes as the temperature of the sample changes periodically. The probe
beam essentially "sees" the changes of this lattice structure and the
level of the intensity of the beam changes along with the changing thermal
conditions of the sample surface.
The probe beam is then reflected back up to the dichroic mirror where it
is, in turn, reflected back along the incoming path and through the
1/4.lambda.-waveplate 55. As discussed above, waveplate 55 rotates the
phase of the probe beam by another 45.degree. such that when the beam
reaches splitter 54, its phase has been rotated 90.degree. with respect to
the original beam. Accordingly, this splitter will deflect the
retro-reflected probe beam upwardly towards detector 56.
Since intensity variations of a radiation beam are to be detected, a
standard photodetector may be employed as a sensing mechanism. The
intensity variations which are measured are then supplied as an output
signal to a processor for deriving the data on the thermal waves based on
the changing surface temperature conditions as indicated by the changing
output signal.
The operation of processor 58 is dependent on the type of testing
configuration which is utilized. In all cases, the processor is designed
to evaluate the intensity changes of the incoming probe beam which are the
result of the periodic reflectivity changes caused by the periodic heating
on the sample. These periodic intensity changes are filtered to produce a
signal which may be evaluated.
The derivation of thermal wave signals from the periodic reflectivity
signal is carried out by normalizing either the phase or magnitude of the
measured signal. These normalized values are then compared to normalized
values taken from a known reference sample. Calculations of this general
type are discussed in "Thermal Wave Depth Profiling: Theory" by Jon Opsal
and Allan Rosencwaig, Journal of Applied Physics, June, 1982. The
calculations set forth in the latter article are based on detection
techniques which measure an output signal that is a function of the
integral of the temperature beneath the sample surface. As discussed
above, the subject detection system measures an output signal which is
primarily a function of the surface temperature and therefore the
calculations must be modified accordingly.
Once the thermal wave information is derived, an analysis can be performed
to provide significant information about a sample. Various types of
thermal wave analysis are set forth in copending application, Ser. No.
389,623, filed June 18, 1982, and now U.S. Pat. No. 4,513,384, issued
April 23, 1985 incorporated herein by reference. For example, an
evaluation can be made of the thickness of thin film layers. In addition
depth profiling varying thermal parameters is possible.
Referring again to FIG. 1, a controllable stage 24 is provided to simplify
the movement of the sample with respect to the heating and probe beams. By
this arrangement, a two-dimensional thermal wave image may be readily
generated. In the alternative, in a manufacturing setting, point testing
may be utilized to evaluate if a particular fabrication step has been
successful. Thermal wave analysis is particularly suited for the
evaluation of integrated circuits.
In summary, there has been disclosed a new and improved method and
apparatus for detecting thermal waves which have been generated in a
sample by a periodic localized heating at a spot on the sample surface.
The subject invention includes a radiation probe beam which is directed
onto a portion of the area which has been periodically heated, in a manner
such that it reflects off the surface of the sample. A means is provided
for measuring the intensity variations of the reflected radiation probe
beam resulting from the periodic heating. A processing means is provided
for analyzing the intensity variations of the reflected radiation probe
beam to detect the presence of thermal waves.
While the subject invention has been described with reference to a
preferred embodiment, various other changes and modifications could be
made therein, by one skilled in the art, without varying from the scope
and spirit of the subject invention as defined by the appended claims.
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