Apparatus for evaluating metalized layers on semiconductors

An apparatus for characterizing multilayer samples is disclosed. An intensity modulated pump beam is focused onto the sample surface to periodically excite the sample. A probe beam is focused onto the sample surface within the periodically excited area. The power of the reflected probe beam is measured by a photodetector. The output of the photodetector is filtered and processed to derive the modulated optical reflectivity of the sample. Measurements are taken at a plurality of pump beam modulation frequencies. In addition, measurements are taken as the lateral separation between the pump and probe beam spots on the sample surface is varied. The measurements at multiple modulation frequencies and at different lateral beam spot spacings are used to help characterize complex multilayer samples. In the preferred embodiment, a spectrometer is also included to provide additional data for characterizing the sample.

TECHNICAL FIELD 
The subject invention relates to a method and apparatus particularly suited 
for the nondestructive characterization of opaque layer structures on 
semiconductor samples. 
BACKGROUND OF THE INVENTION 
There is a great need in the semiconductor industry for metrology equipment 
which can provide high resolution, nondestructive evaluation of product 
wafers as they pass through various fabrication stages. In recent years, a 
number of products have been developed for the nondestructive evaluation 
of semiconductor samples. One such product has been successfully marketed 
by the assignee herein under the trademark Therma-Probe. This device 
incorporates technology described in the following U.S. Pat. Nos. 
4,634,290; 4,636,088, 4,854,710 and 5,074,669. The latter patents are 
incorporated herein by reference. 
In the basic device described in the patents, an intensity modulated pump 
laser beam is focused on the sample surface for periodically exciting the 
sample. In the case of a semiconductor, thermal and plasma waves are 
generated in the sample which spread out from the pump beam spot. These 
waves reflect and scatter off various features and interact with various 
regions within the sample in a way which alters the flow of heat and/or 
plasma from the pump beam spot. 
The presence of the thermal and plasma waves has a direct effect on the 
reflectivity at the surface of the sample. Features and regions below the 
sample surface which alter the passage of the thermal and plasma waves 
will therefore alter the optical reflective patterns at the surface of the 
sample. By monitoring the changes in reflectivity of the sample at the 
surface, information about characteristics below the surface can be 
investigated. 
In the basic device, a second laser is provided for generating a probe beam 
of radiation. This probe beam is focused colinearly with the pump beam and 
reflects off the sample. A photodetector is provided for monitoring the 
power of reflected probe beam. The photodetector generates an output 
signal which is proportional to the reflected power of the probe beam and 
is therefore indicative of the varying optical reflectivity of the sample 
surface. 
The output signal from the photodetector is filtered to isolate the changes 
which are synchronous with the pump beam modulation frequency. In the 
preferred embodiment, a lock-in detector is used to monitor the magnitude 
and phase of the periodic reflectivity signal. This output signal is 
conventionally referred to as the modulated optical reflectivity (MOR) of 
the sample. 
This system has the advantage that it is a non-contact, nondestructive 
technique which can be used on product wafers during processing. Using 
lasers for the pump and probe beams allows for very tight focusing, in the 
micron range, to permit measurements with high spatial resolution, a 
critical requirement for semiconductor inspection. The prior system has 
been used extensively in the past to monitor levels of ion doping in 
samples since the modulated optical reflectivity is dependent on ion 
dopant levels in the sample. This dependence is relatively linear for the 
low to mid-dose regimes (10.sup.11 to 10.sup.14 ions/cm.sup.2). At higher 
dopant concentrations, the MOR signal tends to become non-monotonic and 
further information is needed to fully analyze the sample. 
One approach for dealing with the problem of monitoring samples with high 
dopant concentrations is to measure the DC reflectivity of both the pump 
and probe beams in addition to the modulated optical reflectivity signal 
carried on the probe beam. Using the DC reflectivity data at two 
wavelengths, some ambiguities in the measurement can often be resolved. 
The details of this approach are described in U.S. Pat. No. 5,074,669, 
cited above. 
Semiconductor fabrication technology is increasing in complexity at a rapid 
pace. Various multilayer structures are being developed which makes 
testing more difficult. In addition, manufacturers are seeking to increase 
yields by fabricating chips on larger diameter wafers. As the diameter of 
the semiconductor wafers increases, the cost and value of each wafer 
increases. When using large, valuable and expensive wafers, it is no 
longer economically viable for manufacturers to rely on any forms of 
destructive testing methodologies. Therefore, there is a great need to 
provide equipment which can characterize complex structures with many 
unknowns or variables in a nondestructive manner. 
Inspection problems also arise where metalized layers are deposited on 
semiconductors. If a typical metal layers is more than 100 angstroms 
thick, it will generally be opaque to more commonly used optical 
wavelengths. Therefore, equipment designed to monitor relatively 
transparent oxide layers cannot be effectively used to inspect metalized 
layers. Therefore, some new methodologies are required in order to inspect 
semiconductors with metalized layers. These layers can be formed from 
materials, such as aluminum, titanium, titanium nitride (TiN) and tungsten 
silicide (WSi). 
SUMMARY OF THE INVENTION 
In order to obtain sufficient information to characterize more complex 
samples, a system has been developed which substantially increases the 
amount of data that can be collected. The system of the subject invention 
includes an intensity modulated pump laser beam which is focused onto the 
sample in a manner to periodically excite the sample. A probe laser beam 
is focused onto the sample within the periodically heated area. A 
photodetector is provided for monitoring the reflected power of the probe 
beam and generating an output signal responsive thereto. The output signal 
is filtered and processed to provide a measure of the modulated optical 
reflectivity of the sample. 
In accordance with the subject invention, the device further includes a 
steering means for adjusting the relative position of pump and probe beam 
spots on the sample surface. In the preferred embodiment, the steering 
means is used to move the beam spots from an overlapping, aligned 
position, to a separation of up to about 10 microns. Measurements can be 
taken as the separation of the beam spots is gradually changed or at 
discrete separation intervals. 
This approach is particularly useful for monitoring the deposition of 
opaque, thin metal films. More specifically, the measurements taken at 
different spatial distances between the pump and probe beam spots can be 
used to help more accurately characterize the thermal diffusivity of the 
layer. This information can then be used by the processor to more 
accurately characterize the sample. 
It should be noted that the concept of taking a measurement with a probe 
beam displaced from a pump beam can be found in the prior art. For 
example, in U.S. Pat. Nos. 4,521,118 and 4,522,510, both assigned to 
assignee herein and incorporated by reference, deformations at the sample 
surface, induced by periodic heating, are measured using a probe beam 
displaced from the pump beam. In the latter patents, periodic angular 
deviations of the probe beam are monitored. However, the latter patents do 
not teach or suggest that it would be desirable to take multiple 
measurements as the displacement between the two beams spots are varied. 
Obtaining measurements from a probe beam displaced from a pump beam is also 
disclosed in U.S. Pat. No. 5,228,776, assigned to assignee herein and 
incorporated by reference. In this patent, an effort is made to align the 
pump and probe beams at the opposed ends of elongated conductive features 
within the sample. Further, the focal planes of the two beams are 
displaced vertically. In the principal embodiment of the U.S. Pat. No. 
5,228,776 patent, the lateral spacing between the beams is selected and 
then fixed. There is no teaching in the U.S. Pat. No. 5,228,776 patent 
that it would be desirable to take multiple measurements as the 
displacement between the two beams is varied. 
In the preferred embodiment of the subject invention, further information 
can be obtained by varying the modulation frequency of the pump beam. 
While it has been known that obtaining information as a function of 
modulation frequency is useful, the subject invention expands upon the 
past teachings by increasing the modulation range. In particular, in the 
prior art, the modulation range was typically in the 100 KHz to 1 MHz 
range. Some experiments utilized modulation frequency as high as 10 MHz. 
In the subject device, it has been found useful to take measurements with 
modulation frequencies up to 100 MHz range. At these high frequencies, the 
thermal wavelengths are very short, enabling information to be obtained 
for thin metal layers on a sample, on the order of 100 angstroms. 
In the preferred embodiment of the subject invention, further information 
can be obtained by varying the spot sizes of either the pump or probe 
beams. Varying the spot size of the pump beam will vary the propagation 
characteristics of the thermal waves. Varying the spot size of the probe 
beam will vary the sensitivity of the system with respect to the depth of 
detection. By taking measurements at different spot sizes, some depth 
profiling information can be recorded and used to characterize the sample. 
In the preferred embodiment of the subject invention, still further 
information can be derived by obtaining independent reflectivity 
measurements at a plurality of wavelengths. More specifically, the subject 
apparatus can further include a polychromatic light source generating a 
second probe beam which is directed to the sample surface. The reflected 
beam is captured by a detector which is capable of measuring power as a 
function of wavelength. These added measurements can also be used to help 
better resolve ambiguities in the analysis and improve the 
characterization of the sample. 
It is also possible to add additional measurement modules which measure 
either reflectivity or ellipsometric parameters as a function of angle of 
incidence. Further, the system can also be used to monitor the periodic 
angular deviations of the probe beam to derive additional information. 
Further objects of the subject invention will become apparent from the 
following detailed description, taken in conjunction with the drawings, in 
which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning to FIG. 1 there is shown a schematic diagram of the apparatus 10 of 
the subject invention. The apparatus 10 is intended to permit testing of a 
semiconductor sample 12 and in particular can be used to derive 
information about layers on the sample. Sample 12 is shown resting on a 
controllable stage 14. Stage 14 is capable of precise movements and 
functions to scan the sample with respect to the laser beams. 
In the preferred embodiment, the apparatus 10 includes a pump laser 20 for 
exciting the sample and a probe laser 22 for monitoring the sample. Gas, 
solid state or semiconductor lasers can be used. As described in the 
assignees earlier patents, other means for exciting the sample can include 
different sources of electromagnetic radiation or particle beams such as 
from an electron gun. 
In the preferred embodiment, semiconductor lasers are selected for both the 
pump and probe lasers due to their reliability and long life. In the 
illustrated embodiment, pump laser 20 generates a near infrared output 
beam 24 at 790 nm while probe laser 22 generates a visible output beam 26 
at 670 nm. Suitable semiconductor lasers for this application include the 
Mitsubishi ML6414R (operated at 30 mW output) for the pump laser and a 
Toshiba Model 9211 (5 mW output) for the probe laser. The outputs of the 
two lasers are linearly polarized. The beams are combined with a dichroic 
mirror 28. It is also possible to use two lasers with similar wavelengths 
and rely on polarization discrimination for beam combining and splitting. 
Pump laser 20 is connected to a power supply 30 which is under the control 
of a processor 32. The output beam of laser 20 is intensity modulated 
through the output of power supply 30. The modulation frequency has a 
range running from 100 KHz to 100 MHz. In the preferred embodiment, the 
modulation frequency can be set up to 125 MHz. As described in the above 
cited patents, if an ion laser is used to generate the pump beam, the 
intensity modulation can be achieved by a separate acousto-optic 
modulator. 
Prior to reaching the beam combining mirror 26, the probe beam 24 passes 
through a tracker 40 and a shutter 42. Tracker 40 is used to control the 
lateral position of beam 24 with respect to the probe beam as discussed 
more fully hereinbelow. The shutter 42 is used to block the pump beam when 
other measurements which do not require periodic excitation are being 
taken. 
The beam 26 from probe laser 22 is turned by mirror 46 and passed through a 
collimator 48 which is used to match the focal plane of the probe laser 
spot with the pump laser spot. A shutter 50 is also located in the path of 
the probe beam 26 to block the probe beam when measurements that do not 
use the probe are being taken, so that no stray probe beam light hits the 
detector. 
After the beams 24 and 26 are combined, they are redirected through 
polarizing beam splitter 54 to another beam splitter 56. In between, the 
beams pass through a quarter-wave plate 58 for rotating the polarization 
of the beams by 45 degrees. The beams are directed down to the sample 12 
through a microscope objective 60. Objective 60 has a high n.a., on the 
order of 0.9, and is capable of focusing the beam to a spot size on the 
order of a few microns and preferably close to one micron in diameter. The 
spacing between the objective and the sample is controlled by an autofocus 
system described in greater detail hereinbelow. 
The returning reflected beams 24 and 26 pass through the quarter-wave plate 
58 a second time, resulting in another 45 degree polarization rotation. 
This second rotation allows the beams to pass through the beam splitter 
54. A filter 64 is provided to remove the pump beam light 24 allowing the 
probe beam light to fall on the photodetector 70. Detector 70 provides an 
output signal which is proportional to the power of the reflected probe 
beam 26. Detector 70 is arranged to be underfilled so that its output can 
be insensitive to any changes in beam diameter or position. In the 
preferred embodiment, detector 70 is a quad cell generating four separate 
outputs. When used to measure reflected beam power, the output of all four 
quadrants are summed. When the subject apparatus is operated to measure 
beam deflection, the output of one adjacent pair of quadrants is summed 
and subtracted from the sum of the remaining pair of quadrants. This later 
measurement will be discussed below. 
The output of the photodetector 70 is passed through a low pass filter 71 
before reaching processor 32. One function of filter 71 is to pass a 
signal to the processor 32 proportional to the DC power of the reflected 
probe. A portion of filter 71 also functions to isolate the changes in 
power of the reflected probe beam which are synchronous with the pump beam 
modulation frequency. In the preferred embodiment, the filter 71 includes 
a lock-in detector 72 for monitoring the magnitude and phase of the 
periodic reflectivity signal. Because the modulation frequency of pump 
laser can be so high, it is preferable to provide an initial down-mixing 
stage for reducing the frequency of detection. 
A schematic diagram of the frequency generation and detection stage is 
illustrated in FIG. 2. As shown therein, a frequency synthesizer 73 is 
provided for generating the various pump beam modulation frequencies. 
Synthesizer 73 is under the control of processor 32 and can generate an 
output from 100 KHz to at least 125 KHz. This output is delivered as a 
signal to the power supply 30 of laser 20. 
Synthesizer 73 also generates an electronic heterodyne signal for delivery 
to the lock-in amplifier 72. The heterodyne signal will be close to, but 
different from the signal sent to the pump laser. For example, the 
heterodyne signal can be 10 KHz higher than the signal sent to the pump 
laser. 
The heterodyne signal from the synthesizer is combined with the output from 
the signal detector 70 in a mixer 74. The output of the mixer will include 
signal components at both the sum and difference of the two input signals. 
The difference signal will be at the relatively low frequency of 10 KHz. 
All the signals are passed through a low pass filter 75 to eliminate the 
high frequency components from the synthesizer and the detector. The low 
frequency signal is then demodulated by demodulator 76. The outputs of 
demodulator 76 are the "in-phase" and "quadrature" signals typical of a 
lock-in amplifier. The in-phase and quadrature signals can be used by 
processor 32 to calculate the magnitude and the phase of the modulated 
optical reflectivity signal. 
In initial experiments, a model SR844 lock-in detector from Stanford 
Research Systems was utilized. This device utilizes a combination of 
analog and digital techniques to permit operation over a wide frequency 
range. In this device, an internal frequency synthesizer is used to modify 
the incoming reference signal from the synthesizer to generate two 
reference signals which differ in phase by 90 degrees. These reference 
signals are mixed with the incoming signals from the detector 70. After 
filtering, the low frequency signals are digitized by two 16-bit 
analog-to-digital converters. The digital low frequency signals are 
supplied to a DSP chip for analysis. 
As an alternative to using an electronic heterodyne down-mixing system, it 
is also possible to reduce the frequency of detection using an optical 
heterodyne approach. Such an optical approach is disclosed in U.S. Pat. 
No. 5,408,327, incorporated herein by reference. In the latter system, 
both of the laser beams are modulated but at slightly different 
frequencies. Both beams generate thermal and plasma waves at their 
respective modulation frequencies. The beam from one laser picks up an 
intensity modulation upon reflection due to the modulated optical 
reflectivity induced in the sample by the other beam. The MOR signal 
picked up upon reflection "mixes" with the inherent modulation of the 
beam, creating additional modulations in the beam at both the sum and 
difference frequency. This process is analogous to electrical 
heterodyning. The difference or "beat" frequency is much lower than either 
of the initial beam modulation frequencies and can therefore be detected 
by a low frequency lock-in amplifier. 
It should be noted that in the latter arrangement, both beams carry the 
desired MOR signal and either one or both of the reflected beams can be 
measured. Therefore, in the latter system, the term probe beam could be 
applied to either laser beam. It should also be noted that when some 
samples are exited with two modulation frequencies, nonlinear effects in 
the sample may result in periodic excitation at the sum and difference 
frequencies in the sample. This effect is usually relatively small. 
To insure proper repeatability of the measurements, the signals must be 
normalized in the processor. As noted above, the DC reflectivity of the 
probe beam is derived from detector 70. In addition, the DC output powers 
of the pump and probe lasers are monitored by incident power detectors 84 
and 86 respectively. A wedge 88 functions to pick off about one percent of 
the incident beam power and redirects it to an edge filter 90 for 
separating the two beams. The outputs of the detector 84 and 86 are passed 
through the low pass portion of filter 71 and into the processor 32. 
The signals are further normalized by taking a measurement of the power of 
the pump beam 24 after it has been reflected. This measurement is used to 
determine the amount of pump energy which has been absorbed in the sample. 
The pump beam reflected power is measured by detector 94. Wedge 88 
functions to pick off about one percent of the returning beams which are 
redirected to edge filter 96. The DC signal for both the incident pump and 
probe beam powers as well as the reflected beam powers are used to correct 
for laser intensity fluctuations and absorption and reflection variations 
in the samples. In addition, the signals can be used to help calculate 
sample parameters. 
An autofocus mechanism is used to maintain the spacing between the 
objective 60 and the sample 12 to be equal to the focal length of the 
objective. This distance can be maintained to less than one hundredth of a 
micron. 
The autofocus mechanism includes a servo motor 108 for varying the vertical 
position of the objective 60. The servo is driven by an analog detection 
loop which determines if the objective 60 is properly focusing the probe 
beam. As seen in FIG. 1, a small portion of the reflected probe beam light 
picked off by the wedge 88 is redirected by filter 96 into the main 
elements of the autofocus detection loop. The probe beam is focused by a 
lens 98 through a chopper wheel 104 located in the focal plane of the lens 
98. The light passing the chopper wheel 104 is imaged on a split-cell 
photodetector 106. 
If the objective 60 is out of focus, there will be a phase difference in 
the light striking the two sides of the split cell detector 106 which is 
measured by a phase detector 109. The phase difference is used as an input 
to an amplifier 110 which in turn drives the servo 108. This approach to 
autofocusing is known as the automated Foucault knife edge focus test. 
This system can also be used to intentionally defocus the beams, thereby 
varying the spot sizes of the pump and probe beams. This adjustment can be 
used to gain further information as discussed below. 
In the preferred embodiment, a polychromatic, or white light source 120 is 
provided to illuminate the sample to permit viewing on a TV monitor (not 
shown). The beam 122 from the white light source 120 is passed through a 
shutter 124. When measurements with the lasers 20 and 22 are being taken, 
the shutter is closed so no stray white light gets into the detection 
system. 
A portion of the white light returning from the sample is redirected by a 
beam splitter 130 to a CCD camera 132 for generating an image for the TV 
monitor. The beam will pass through a CCD lens 134 and a filter 136. 
Filter 136 functions to reduce the amount of laser light reaching the 
camera. In the preferred embodiment, a portion of the light is redirected 
by a beam splitter 140 into an array spectrograph 142 more fully described 
herein below. 
Having described the basic elements of the subject invention, its operation 
will now be discussed. 
In use, a sample 12 is placed on the stage 14. Measurements at various 
points on the sample can be made by rastering the stage to the desired 
location. During initial set up of a fabrication procedure, a relatively 
high number of test points may be desired. Once the fabrication procedure 
is up and running, only spot checks of the wafer are typically made. This 
will often entail measuring from five to twenty-five spots on a wafer. 
At each measurement point, the pump and probe laser beams 24 and 26 are 
activated. In a typical measurement, the beams will be initially 
colinearly aligned. Exact overlap will typically provide the strongest 
modulated optical reflectivity signal. Measurements can then be taken at a 
plurality of modulation frequencies of the pump beam. The range of 
modulation frequencies and the number of frequencies is selected by the 
operator. As noted above, in the preferred embodiment, the range of pump 
beam modulation frequencies covers from 100 KHz to 125 MHz. The output 
from the photodetector 70 is passed through the filter 71 (including 
lock-in 72) to the processor which records the magnitude and/or phase of 
the modulated optical reflectivity signal for each of the modulation 
frequencies selected. 
In accordance with the subject invention, once the desired measurements are 
completed with the beam spots in an overlapping, aligned position, the 
processor signals the tracker 40 to adjust the position of the pump beam 
24 so that pump and probe beam spots on the sample surface are laterally 
displaced. In one possible scenario, the beams are initially displaced a 
distance of one micron. Once in this position, a series of measurements 
are taken at different modulation frequencies in the manner described 
above. Once again, the processor will record the modulated optical 
reflectivity signal at each of these modulation frequencies. 
In accordance with the subject invention, once the second set of 
measurements are complete, the processor will again command the tracker 40 
to further separate the pump and probe beam spots to a distance of two 
microns. Measurements will then be taken at this two micron spacing and at 
subsequent spacings, each time increasing the spacing by one micron. It is 
envisioned that for complex samples, measurement might be taken at 
successive increments of 0.5 microns. Alternatively, measurements can be 
taken as the separation of the two beam spots is continuously increased. 
The span of spacings between beam spots can range from overlapping to 
about 10 microns. 
It is envisioned that the user will be able to determine what sort of 
scanning algorithm is best suited to the particular test situation. 
Variables such as separation distance and number of measurements at each 
separation can be entered by the user through a software interface. 
Once all of the measurements at various spacings and modulation frequencies 
have been taken and stored, the processor will attempt to characterize the 
sample. Various types of modeling algorithms can be used depending on the 
complexity of the sample. Optimization routines which use iterative 
processes such as least square fitting routines are typically employed. 
One example of this type of optimization routine used for thermal wave 
analysis is described in "Thermal Wave Measurements and Monitoring of 
TASi.sub.x Film Properties," Smith, et. al., Journal of Vacuum Science and 
Technology B, Vol. 2, No. 4, p. 710, 1984. An optimization routine used 
for more general optical analysis can be found in "Multiparameter 
Measurements of Thin Films Using Beam-Profile Reflectivity," Fanton, et. 
al., Journal of Applied Physics, Vol. 73, No. 11, p.7035, 1993. 
It has been found that measurements taken at different beam spot spacings 
are particularly useful in characterizing thermal diffusivity of a sample. 
Identification of this parameter can be used to determine the structure of 
the sample. In addition, measurements at very high modulation frequencies 
are useful for very thin films, since the wavelength of the thermal and 
plasma waves is very short. 
In the preferred embodiment of the subject invention, further information 
can be obtained by varying the spot sizes of either of both of the pump 
and probe beams. Varying the spot sizes of the beams will vary the 
propagation and detection characteristics of the thermal waves. In 
particular, when using a highly focused pump beam spot size, the thermal 
waves will tend to propagate in all directions (3 dimensionally) away from 
the area of focus. In contrast, as the pump beam spot size increases, the 
thermal waves will tend to propagate in a more 1-dimensional manner, 
perpendicular to the area of focus. 
With respect to the detection, a very large probe beam spot tends to see 
mostly the 1-dimensional component of the thermal wave. In contrast, a 
highly focused probe beam, being more localized, is sensitive to the 
3-dimensional character of the thermal waves. This variation in the 
generation and detection characteristics of thermal waves is characterized 
by the square root of the sum of the squares of the pump and probe beam 
diameters. When this value is small compared to the thermal diffusion 
length, the measurement is 3-dimensional in character and when the value 
is large compared to the thermal diffusion length, the measurement is 
1-dimensional in character. By taking measurements at different pump 
and/or probe beam spot sizes, some depth profiling information can be 
recorded and used to characterize the sample. 
As noted above, the spot size of both the beams can be controlled by the 
autofocus system. In order to increase the size of the beam spots, the 
processor can add an offset to the focusing algorithm which would defocus 
the beams. The beam spots can be made of different sizes by adjusting the 
collimator 48. In the preferred embodiment, measurements are taken and 
recorded at various beam spot sizes ranging from one micron to ten 
microns. This additional information can be used to characterize the 
sample. 
In the preferred embodiment of the subject invention, still further 
measurements can be taken to reduce the ambiguities of analysis. More 
specifically, in addition to the modulated optical reflectivity 
measurements, it is also desirable to monitor the periodic angular 
deflections of the probe beam due to deformations in the surface of the 
sample induced by the periodic heating. This type of measurement is 
described in detail in U.S. Pat. Nos. 4,521,118 and 4,522,510, cited 
above. As described in those patents, because of the thermal expansion 
properties of samples, the periodic heating by the pump beam will create a 
time varying "bump" in the sample surface. If the pump and probe beam are 
spaced apart, the probe beam will undergo periodic angular deviations at 
the frequency of the modulated heating. These angular deviations can be 
measured by a split cell detector. The output of the split cell is sent to 
the filter and the processor. The processor functions to calculate the 
magnitude and phase of the deflection signal that is synchronous with the 
pump beam modulation frequency. 
As noted earlier, detector 70 is preferably a quad cell detector, or a 
detector with four quadrants each generating separate output signals. 
Deflections of the probe beam in both the X and Y axis due to angular 
deviations of the probe beam can be measured with such a detector. More 
specifically and as shown in FIG. 3, to determine the deflection of the 
probe beam in the X axis, the outputs of quadrants 200 and 202 are summed 
and subtracted from the sum of quadrants 204 and 206. Alternatively, to 
determine the deflection of the probe beam in the Y axis, the outputs of 
quadrants 200 and 204 are summed and subtracted from the sum of quadrants 
202 and 206. It should be noted that these type of measurements function 
to cancel out any changes in the reflected power of the beam. As noted 
above, in operation, the output of all four detector quadrants are fed to 
the filter 71 and processor 32. If the output of the four quadrants is 
summed, the MOR signal is obtained. If the quadrant halves are summed and 
subtracted, the probe beam deflection signal can be obtained. 
In use, the deflection signal can be recorded for each of the beam spot 
positions and modulation frequencies. In practice, there will usually be 
little or no deflection signal when the two beam spots are colinearly 
aligned. However, as soon as the centers of the beam spots are separated, 
a signal can be often be detected, even if the beams are in a partially 
overlapping configuration. 
In the preferred embodiment, the subject apparatus further includes a 
spectrometer for providing additional data. As noted above, a white light 
source 120 is necessary for illuminating the sample for tracking on a TV 
monitor. This same light source can be used to provide spectral 
reflectivity data. 
As seen in FIG. 1, a beam splitter can be used to redirect a portion of the 
reflected white light to a spectrometer 142. The spectrometer can be of 
any type commonly known and used in the prior art. FIG. 4 illustrates one 
form of a spectrometer. As seen therein, the white light beam 122 strikes 
a curved grating 242 which functions to angularly spread the beam as a 
function of wavelength. A photodetector 244 is provided for measuring the 
beam. Detector 244 is typically a photodiode array with different 
wavelengths or colors falling on each element 246 in the array. The 
outputs of the diode array are sent to the processor for determining the 
reflectivity of the sample as a function of wavelength. This information 
can be used by the processor during the modeling steps to help further 
characterize the sample. 
It is also possible to provide a mechanism for measuring sample 
reflectivity as a function of the angle of incidence of the beam. To 
achieved this goal, a portion of the reflected probe beam light 26 can be 
picked off by wedge 300. This light is sent to a beam profile 
reflectometer module 302 as more clearly shown in FIG. 5. When taking 
these measurements, the pump beam is preferably turned off so that the 
probe beam will not be modulated. 
Module 302 is of the type described in U.S. Pat. No. 4,999,014 assigned to 
the assignee herein and incorporated by reference. As described therein, 
if a probe beam is focused onto a sample with a strong lens, various rays 
within the beam will strike the sample surface at a range of angles of 
incidence. If the beam is properly imaged with a relay lens 306, the 
various rays can be mapped onto a linear photodiode array 308. The higher 
angles of incidence rays will fall closer to the opposed ends of the 
array. The output from each element 309 in the diode array will correspond 
to different angles of incidence. Preferably, two orthogonally disposed 
arrays 308a and 308b are provided to generate angle of incidence 
information in two axes. A beam splitter 310 is used to separate the probe 
beam into two parts so both axes can be detected simultaneously. The 
output of the arrays will supplied to the processor 32 for storage. The 
data can be used to further characterize the sample. 
In still a further preferred embodiment, ellipsometric information can be 
derived from the sample using the reflected probe beam. As in the previous 
example, the pump beam should be turned off for these measurements. As 
seen in FIG. 1, a portion of the reflected probe beam can be picked off by 
a beam splitter 400 and redirected to a beam profile ellipsometer 402 as 
shown in FIG. 6. A suitable beam profile ellipsometer is described in U.S. 
Pat. No. 5,181,080, assigned to the assignee herein and incorporated by 
reference. 
As described above, the rays in the reflected probe beam correspond to 
different angles of incidence. By monitoring the change in polarization 
state of the beams (from the original linear polarization, to elliptical 
polarization upon reflection), ellipsometric information, such as .psi. 
and .DELTA., can be determined. 
To determine this information, the beam is first passed through a 
quarter-wave plate 404 for retarding the phase of one of the polarization 
states of the beam by 90 degrees. The beam is then passed through a 
polarizer 408 which functions to cause the two polarization states of the 
probe beam to interfere with each other. The light is then passed through 
an imaging lens 410 an onto a quadrant detector 412. Each quadrant, 414, 
416, 418, and 420 generate separate output signals proportional to the 
power of the probe beam striking that quadrant. These signals represents 
an integration of the intensities of all the rays having different angles 
of incidence with respect to the sample surface. As described in U.S. Pat. 
No. 5,181,080, ellipsometric information can be obtained if the signals 
from opposing quadrants 414 and 418 are summed and subtracted from the sum 
of the signals from opposing quadrants 412 and 420. The output of detector 
412 is sent to the processor 32 for storage and use in characterizing the 
sample. 
As can be seen, the subject device can be used to provide a large amount of 
measurement data in order to better resolve the characteristics of the 
sample. Such complete measurements are often necessary in order to 
determine the composition of a multilayer structure. 
While the subject invention has been described with reference to a 
preferred embodiment, various 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.