Process for fabricating a device using an ellipsometric technique

An ellipsometric method for process control in the context of device fabrication is disclosed. An ellipsometric signal is used to provide information about the device during the fabrication process. The information is used to better control the process. An ellipsometric signal of a particular wavelength is selected. The signal is selected based on the composition and thickness of the films on the substrate through which the ellipsometric signal will pass before it is reflected from the substrate. Once the appropriate wavelength is determined, the ellipsometric signal is used to monitor the thickness of the films on the substrate over time, to assist in controlling the deposition and removal of films on the substrate, and to perform other process control functions in the context of device fabrication. The ellipsometric method is used to control the deposition and removal of films that underlie patterned masks with aspect ratios of 0.3 or more, that overlie topography on a substrate surface, or that both underlie a mask and overlie topography.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention is directed to a process for fabricating integrated circuit 
devices and in particular utilizing a measuring technique in conjunction 
with such processes. 
2. Art Background 
The move towards smaller design rules for the fabrication of integrated 
circuits is motivated by the desire to place a greater number of devices 
on a chip. For advanced device structures, both the film thickness of 
layers used to form these structures and the structure width must decrease 
as the number of devices on a chip increase. The presence of these smaller 
features, which are smaller in width and in thickness, has increased the 
complexity of fabricating integrated circuits. It is especially difficult 
to control the etching process (for example) of an integrated circuit when 
rapidly removing multiple layers of different materials from over a thin 
layer which must not be substantially affected. Typically, such processes 
require complex changes in processing conditions that must be made 
quickly. 
An ellipsometer is frequently used to determine the characteristics of 
blanket films in the context of device fabrication. A beam of light is 
directed onto the surface of the film. The ellipsometer measures the light 
reflected from the film. From the reflected light the ellipsometer 
determines the angles DELTA (.DELTA.) and PSI (.PSI.), which are defined 
as the change in phase of the light and the arc tangent of the factor by 
which the amplitude ratio of the incident and reflected light in the 
direction parallel to the plane of incident light changes, respectively. 
These quantities are then used to determine the optical characteristics 
such as the index of refraction, the film thickness, and the extinction 
coefficient of unpatterned, uniform films. 
The use of an ellipsometer to derive .DELTA. and .PSI. coordinates of a 
polarized light beam reflected from a work piece to monitor the thickness 
of a film deposited on a substrate is described in U.S. Pat. No. 5,131,752 
to Yu et al. Yu et al. first calculate the endpoint, which is the point 
when a film of the desired thickness has been deposited, from the .DELTA. 
and .PSI. values of a film of the desired thickness. Yu et al. calculate 
end point values of .DELTA. and .PSI. from the known angle of incidence, 
the wavelength of the light source, the desired final film thickness, and 
the optical constants of the substrate and film at the process 
temperature. Therefore, the Yu et al. method is limited to controlling a 
process in which the precise endpoint is known. 
In the context of device fabrication using masks or over topography, the 
precise endpoint of any process is difficult to determine due to the 
attendant irregularity of the surface. Masks as used herein are structures 
which are used to introduce a pattern onto or into a film or films 
overlying a substrate. Topography as used herein are surface 
irregularities or structures underlying films on a substrate. The optical 
path of an incident beam of light from an ellipsometer is affected by the 
presence of a mask or topography on a wafer, because the composition 
and/or configuration of the surface is irregular. Wafer, as used herein, 
is a substrate with films thereon. These irregularities cause the light 
reflected from the surface to be different than light reflected from 
uniform, blanket films. The mask or topography also affects the 
planarization state of light and therefore creates interference which 
adversely affects the quality of the signal that is reflected from the 
surface. 
Henck, Steven A., et al., "In situ spectral ellipsometry for real-time 
thickness measurement: Etching multilyer stacks," J. Vac. Sci. Technol. 
A., 11(4): 1179 (July/Aug. 1993) propose using an ellipsometer to monitor 
the film thickness in a large unpatterned region at the center of the 
wafer during plasma etching. The ellipsometer is first calibrated using 
known techniques. Then the ellipsometric parameters, .DELTA. and .PSI., 
are determined from the thickness and the dielectric function of the film 
material. As the etch proceeds, the ellipsometer continues to transmit a 
beam of light toward the film and to measure the properties of the 
reflected light to obtain a useful signal. The changes in the reflected 
light indicate the decreasing thickness of the top layer of the film and 
the increasing proximity of the interface between the top layer and the 
layer underlying the top layer. Thus, during etching, an ellipsometer is 
used to determine how much of the top layer has been removed, which 
enables the etching to be terminated with a known film thickness 
remaining. 
The technique described by Henck et al. requires an unpatterned 
topography-free area (or test pad) on the wafer in which to make the 
required measurements. An unpatterned, topography-free area on a device is 
undesirable, because it sacrifices that portion of the device real estate. 
Also, since an unpatterned, topography-free test pad requires different 
masking, etching, and deposition steps than patterned 
topography-containing areas, it can also complicate the lithographic 
process. Therefore, manufacturing costs will increase if a significant 
portion of the wafer has to be sacrificed to provide an area on which to 
perform the technique described by Henck et al. 
Haverlag, M., et al., "In situ ellipsometry and reflectometry during 
etching of patterned surfaces: Experiments and simulations," J. Vac. Sci. 
Technol. B, 10(6):2412 (Nov./Dec. 1992) describe the use of ellipsometry 
at a single wavelength (632 nm) for the end point detection of a plasma 
etching process. Haverlag et al. conclude that such a technique cannot be 
used over typical patterned wafers because the end point was not detected 
when the incident ellipsometric light beam was aimed onto the surface in a 
direction that was perpendicular to the direction of lines formed by a 
mask over the film. Haverlag et al. observed that reflections of the light 
on the mask sidewalls inhibit the light from reaching the ellipsometric 
detector and concluded that an ellipsometric technique for plasma etch end 
point detection could only be used on patterned wafers with low, e.g., 
less than 0.3, aspect ratios. 
However, in many applications for fabricating real devices, the aspect 
ratios of masks or topography are typically greater than 1.0. Also, as 
design rules decrease, more severe topography and higher aspect ratios are 
expected. Therefore, an ellipsometric technique for process control that 
can be used in processes for fabricating small design rule devices with 
topography over essentially all of the entire surface of the wafer is 
desired. 
SUMMARY OF THE INVENTION 
The process of the present invention uses an ellipsometric technique for 
process control during device fabrication. Specifically, the process is 
used to control those aspects of device fabrication that add materials to 
or remove materials from the substrate surface. In one example, the 
process is used to detect interfaces between two layers of a film on a 
substrate. Interface, as used herein, is the boundary between two layers 
of film, or between a layer of film and the substrate underlying the film. 
The process of the present invention provides information which is then 
used to control the device fabrication process either by changing the 
conditions under which a material is being deposited on or removed from a 
substrate, or by stopping the process altogether. The process provides 
this information by directing an ellipsometric light beam at the surface 
of the wafer being processed and monitoring the signal that is reflected. 
The light signal so monitored for process control must be of a particular 
wavelength or range of wavelengths in order to provide the information 
necessary for process control when the wafer surface is irregular. Such 
surface irregularity occurs when a mask is deposited over the surface of 
the wafer being processed (a "patterned surface" herein) or there is an 
irregular surface ("topography" herein) underlying the film being 
processed. The appropriate wavelength is selected such that the absorption 
length is significantly smaller, e.g., a factor of 10, than the optical 
path through the film that is being ellipsometrically monitored. (The 
absorption length is defined as the depth in the material at which the 
incident electric field is decreased by 1/e relative to the incident field 
amplitude on the material.) 
A way in which to select the initial wavelength is by performing the 
following calculation: 
##EQU1## 
wherein d is the initial nominal thickness of the layer being probed, 
n.sub..lambda. is the refractive index of the layer, k.sub..lambda. is the 
extinction coefficient of the film material at the wavelength .lambda. and 
.phi. is the angle of the incident beam relative to the normal from the 
surface of the wafer. The optical path is reduced by about a factor of 10 
in the above calculation. However, appropriate wavelengths are obtained if 
the optical path is reduced by a factor of about 5 to about 20. Once the 
wavelength is selected, light of that wavelength is detected by the 
ellipsometer. The spectral refractive indices n.sub..lambda. and 
extinction coefficients k.sub..lambda. for various materials are obtained 
from sources for these coefficients such as Palik, D. E., et al., 
"Handbook of Optical Constants of Solids", Vol. 1 and 2 (1985). 
Another means by which the appropriate wavelength is determined is by 
scanning a number of different wavelengths in a signal reflected from the 
substrate surface during a process in which the film thickness is being 
increased or reduced. The values of .DELTA. and .PSI. over time, referred 
to as "ellipsometric traces", are observed. Those traces that exhibit an 
abrupt change in the slope of the values of .DELTA. and/or .PSI. over time 
during the process are examined to determine if the abrupt change in slope 
indicates a transition in the process from one material to another. A 
trace that indicates the point during the process when the surface 
transitions from one layer of material to another is a useful tool for 
process control. Such an indication permits the process to be stopped at 
the appropriate time, or it permits process conditions to be changed in 
real time. The wavelength at which one such trace is obtained is used as 
the signal wavelength when performing the process on subsequent substrates 
with substantially identical films thereon. 
Since ellipsometric traces are substantially identical for substantially 
identical processes on substantially identical wafers, these traces are 
used as tools for process control.

DETAILED DESCRIPTION 
Illustrated in FIG. 1 is a simplified diagram of an ellipsometer as it is 
configured for use in the process of the present invention. It is 
contemplated that the process for controlling the removal of a film will 
be used in many different processes for thin film growth or etching of 
devices such as integrated circuits and other semiconductor and optical 
devices. However, the process of the present invention has been initially 
implemented in connection with the plasma etching of films upon a 
substrate and will be described in detail accordingly. 
As shown in FIG. 1, a beam of incident light 10 is generated by the 
excitation head 20 of the ellipsometer 30. The light is generated from a 
light source (not shown) and transported to the head 20 via an optical 
fiber 25. The beam of light 10 is directed to strike a wafer 40. The 
excitation head 20 is positioned so that the angle of incidence 50 of the 
light beam 10 on the wafer 40 is about 70.degree. from normal. Other 
angles of incidence are contemplated as useful depending upon the wafer. 
Angles of about 0.degree. to about 90.degree. are contemplated. 
The incident light 10 is reflected from the wafer 40. The light signal so 
reflected is detected by the detection head 60 of the ellipsometer 30. The 
detection head 60 transmits the optical signal to a monochromator via a an 
optical fiber (not pictured). The monochromator is used to select the 
desired wavelength of light, which is then transmitted to a detector. The 
detector converts the optical signal to an electrical signal and then 
transmits that signal to a signal processor, which determines the .DELTA. 
and .PSI. values for the reflected light. 
The wafer 40 is placed in a chamber 70 in which the desired processing 
takes place. In FIG. 1, the chamber 70 depicted is suitable for plasma 
etching a film from the surface of the substrate. The chamber 70 is 
standard, except it is equipped with viewports 80 through which the 
ellipsometric signal enters and exits the chamber 70. 
The process of the present invention, unlike previous processes, utilizes 
an ellipsometer for controlling the deposition and removal of films on 
substrates when there are patterned features with aspect ratios that are 
greater than 0.3 overlying the films, topography underlying the films or 
both patterned features over the film and topography under the film. The 
ellipsometer reports the values of .DELTA. and .PSI. over time during a 
processing step in a device fabrication process. Since .DELTA. and .PSI. 
are optical parameters of light reflected from the substrate, they are 
influenced by the optical path which the light must travel, i.e., through 
the layers on the substrate. As the thickness of these layers change, so 
do the values of .DELTA. and .PSI.. The values of .DELTA. and .PSI. also 
change after a layer has been removed entirely from a portion of the 
wafer. An abrupt change in the slope of the trace of .DELTA. and .PSI. 
over time during an etching step in a device fabrication process will 
indicate a change in the wafer surface, such as an interface between two 
materials. 
However, if there is interference with the light reflected from the film, 
the changes in .DELTA. and .PSI. caused by an interface or other 
significant change in surface composition or thickness are obscured. Such 
interference results from thickness variations, such as those between 
masked and unmasked zones on the portion of the surface intersected by the 
incident beam of light from the ellipsometer. If sufficiently obscured, a 
trace of .DELTA. and .PSI. over time will not clearly indicate when an 
interface has been reached during the process. Such traces are not useful 
for the control of process parameters such as adjusting the process 
conditions, terminating the etching process, or changing the composition 
of the etchant when an interface between two layers has been reached. 
The process of the present invention achieves the desired degree of control 
by selecting a particular wavelength of light that provides a trace which 
is used for process control in the above described manner. Once the 
appropriate wavelength is selected, the ellipsometer is used to obtain 
traces of values of .DELTA. and .PSI. over time during the process. If the 
appropriate wavelength of light has been selected, the trace will indicate 
when during the process an interface between two films is reached even 
though there is a mask overlying the film and/or topography underlying the 
film. 
The ellipsometric traces are referred to as signature traces, because they 
are substantially identical for a particular film etched under particular 
conditions. For example, if five wafers as illustrated in FIG. 2 were 
etched under substantially identical conditions, the traces obtained 
during the etch would be substantially identical as well. Thus, once a 
signature trace has been obtained for a particular wafer and process, the 
trace is used to control the processing of similar wafers in the same 
process. The traces also provide an indication that an interface is about 
to be reached during the process. This advance warning allows the process 
to be controlled with even greater accuracy. 
In order to practice the process of the present invention, the wavelength 
of light used to generate the trace must be of a certain value. That 
wavelength is determined by considering a number of factors. Among these 
factors are: film thickness; the refractive index of the film material; 
the extinction coefficient of the film material at the relevant wavelength 
and the angle of incidence of the beam of light on the wafer surface. 
If .DELTA. and .PSI. are to be measured, the reflected light must be 
received by the detection head of the ellipsometer. Therefore, a 
wavelength must be selected such that the optical path through the layers 
is greater than the absorption length of the light in those layers. The 
choice of wavelength is therefore dependent on the composition and 
thickness of the film or films being investigated. 
For example, a wavelength is selected by using the following formula: 
##EQU2## 
wherein n.sub..lambda. is the refractive index of the film being probed, 
k.sub..lambda. is the extinction coefficient of the film material at the 
wavelength .lambda., and .phi. is the incident angle of the ellipsometric 
beam. The spectral refractive indices n.sub..lambda. and extinction 
coefficients k.sub..lambda. for the film materials are either obtained 
from reference data files or are determined experimentally using the 
ellipsometer to measure the spectrum of the blanket film. These values are 
also obtained in G. E. Jellison, Jr., "Optical Function of Silicon 
Determined by Two-channel Polarization Modulation Ellipsometry", Optical 
Materials, 1:46-47 (1992) and Palik, D. E., et al., "Handbook of Optical 
Constants of Solids", Vol. 1 and 2 (1985). If the film being monitored by 
the ellipsometer is a multilayer film, n.sub..lambda. and k.sub..lambda. 
are selected for the top layer material. 
This formula provides a wavelength of light for which the absorption length 
through the particular film is much less than the optical path through the 
film. It is advantageous if the absorption length is a factor of about 5 
to about 20 less than the optical path. It is particularly advantageous if 
the absorption length is at least a factor of 10 less than the optical 
path. For example, if the film on the substrate is a 2000 .ANG.-thick (d) 
film of polysilicon, the following calculations are made to determine the 
appropriate wavelength: 
##EQU3## 
The above calculations illustrate that, for a 2000 .ANG.-thick film of 
polysilicon, a wavelength that would provide a useful trace is 375 nm or 
428 nm, because at both wavelengths, the absorption length is much less 
than the optical path. The 632 nm wavelength is not useful because the 
absorption length is longer than the optical path. It is contemplated 
that, for multilayer films, the wavelength at which the trace is generated 
will be varied during the process to accommodate each of the individual 
films being deposited or removed. 
It is also possible to select the wavelength by obtaining traces of a film 
during a process at a number of different wavelengths. By observing the 
traces obtained, the trace that indicates the interface with the desired 
degree of clarity is selected. The wavelength used to obtain that trace is 
then used to monitor the processing of subsequent films. The wavelength of 
the light used to generate the trace is varied from process to process and 
from film composition to film composition because the wavelength selected 
depends upon the characteristics of the film and the process. Therefore, 
it is advantageous if the ellipsometer used in the process is equipped 
with a broad spectral light source and a mechanism on the detector for 
selecting the desired wavelength of light to generate the desired trace. 
EXAMPLE 1 
Use of the Process of the Present Invention to Control a Plasma Etch 
The ellipsometer used to determine the ellipsometric parameters (.DELTA., 
.PSI.) was a UV-visible, phase modulated, spectroscopic ellipsometer made 
by the ISA division of Jobin-Yvon of Longjumeau, France. One skilled in 
the art will appreciate that other ellipsometers are equally suited to 
practice the process of the present invention. The ellipsometer was 
equipped with a white light source from a Xenon arc lamp. The light was 
polarized and passed through a phase modulator operating at 50 KHz. The 
excitation head of the ellipsometer was positioned such that the 
elliptically polarized light will hit a wafer at an angle of about 
70.degree. from normal. A detection head was mounted at -70.degree. from 
normal. The wavelength of interest was observed by using a monochromator 
to exclude light of other wavelengths. 
The ellipsometer was used to monitor the etching of films on several 
different wafers. One such film, depicted in FIG. 2, was a 1000 .ANG. 
thick layer of titanium nitride 91 over a 2000 .ANG. thick layer of 
polysilicon 92 over a 70 .ANG. thick layer of an oxide of silicon 93. 
Another film, depicted in FIG. 3, was the same film as depicted in FIG. 2 
but with a 2000 .ANG. mask 94 of submicron features deposited thereover. A 
third film, as depicted in FIG. 4 is the film of FIG. 3, but deposited 
over a wafer with topography (not pictured) thereunder. 
The wafers were placed in an etching tool that was manufactured by Lucas 
Labs of Sunnyvale, Calif. The tool was configured for single wafer 
processing of 125 mm diameter wafers. The wafer was clamped to a chuck. 
The temperature of the wafer was controlled using backside cooling with a 
helium purge by controlling the temperature of the chuck at about 
0.degree. C. 
A low pressure, high density helicon plasma source, also made by Lucas 
Labs, was used to generate the plasma, which consisted of ions, electrons 
and reactive neutrals. The wafer was placed in the reaction chamber into 
which a gas was introduced. First, the native oxide was removed by 
igniting a plasma with 100 sccm of HBr and 20 sccm of Cl.sub.2 at an 
rf-bias of 50 W for 10 seconds. The helicon source power was 2500 W and 
the reactor pressure was 2 mTorr. 
The flow of the Cl.sub.2 was shut off and the layer of titanium nitride was 
removed using the same conditions. Ten seconds after the titanium nitride 
was removed, the etchant recipe was changed by adding 20 sccm of a 20 
percent/80 percent mixture of O.sub.2 and helium to the 100 sccm of HBr to 
etch the polysilicon layer. 
As the polysilicon layer was etched, the plasma recipe was again changed by 
reducing the rf-bias from 50 W to 25 W to avoid etching the thin oxide 
layer beneath the polysilicon. This change was made about 10 seconds after 
the O.sub.2 /helium was introduced. 
Ellipsometric traces were obtained while etching several wafers with the 
films described in FIGS. 2-4 deposited thereon. The ellipsometric traces 
were obtained using a monochromator to select various wavelengths to 
observe the effect of wavelength on the ellipsometric trace obtained. 
A trace was obtained for a blanket film as illustrated in FIG. 2. This 
trace is illustrated in FIG. 5. The wavelength used to generate this trace 
was 632 nm. This wavelength is the wavelength used by single wavelength 
ellipsometers with a helium neon light source. The trace that was obtained 
indicates at what points in the process the first layer of titanium 
nitride and the second layer of polysilicon were removed. These points 
were indicated by abrupt changes in the slope of .DELTA.80 and .PSI.90 
over time during the process. The point in the process when the titanium 
nitride was removed is indicated by point 100 and the point in the process 
when the polysilicon was removed is illustrated by point 120. Such traces 
are useful for process control during device fabrication because, by 
observing the trace, one can change the conditions to which the film is 
subjected. This is a great advantage in device fabrication, since many 
different materials are deposited on substrates and these materials react 
differently under different conditions. For example, some materials are 
very resistant to a particular etchant, while other materials may be 
removed rapidly by the same etchant. Therefore, the ability to either 
change conditions at a particular point in the process, such as when one 
layer has been removed, or to stop the process at a particular point, is 
extremely useful in the context of device fabrication. 
FIG. 6 is another ellipsometric trace obtained while etching a film under 
the conditions described above. The film is again illustrated in FIG. 2. 
The wavelength of interest was selected by the process of the present 
invention. Again, the wavelength was obtained from the reflected light 
using a monochromator. The wavelength was 375 nm. Again, the traces 
clearly indicated the points during the process when the first layer of 
titanium nitride 136 and the second layer of polysilicon 138 were removed. 
Again, these changes were indicated by an abrupt change in the slope of 
.DELTA.132 and .PSI.134 over time during the process. 
FIGS. 7-9 are ellipsometric traces obtained while etching films described 
in FIG. 3 under the above-described conditions. In FIG. 7, the trace was 
obtained by first calculating the wavelength of interest to generate the 
trace using the method described above which was again 375 nm. The film 
had a silicon oxide mask thereover that was typical of masks used for 
integrated circuit fabrication. 
The trace that was obtained clearly indicated the points during the process 
when the titanium nitride 146 and the polysilicon 148 were removed from 
the wafer. These points were again illustrated by the abrupt change in the 
slope of .DELTA.142 and .PSI.144 over time during the process. 
FIG. 8 illustrates the effect that a change in the wavelength used to 
generate the trace will have on the trace that is obtained. The trace was 
obtained while etching a film as illustrated in FIG. 3 under the 
conditions described above. The conditions were identical to the 
conditions under which the trace depicted in FIG. 7 was obtained. The only 
difference was that the wavelength used to generate the trace was 428 nm. 
FIG. 8 clearly indicates the points during the process when the titanium 
nitride 156 and the polysilicon 158 were removed. These points were again 
illustrated by abrupt changes in the slope of .DELTA.152 and .PSI.154 over 
time during the process. FIGS. 7 and 8 illustrate that a range of 
appropriate wavelengths are provided by the above formula 1. This formula 
provides a mechanism to select a wavelength in the appropriate range. This 
wavelength is then increased or decreased, depending upon the requirements 
of a particular process. Once the appropriate wavelength is approximated, 
one skilled in the art will appreciate how much the wavelength will be 
increased or decreased in order to obtain specific processing objectives. 
FIG. 9 illustrates that, when processing films with a mask thereover, not 
every wavelength of light will provide a useful trace. The trace in FIG. 9 
was obtained while etching a film as illustrated in FIG. 3 under the 
conditions described above. The mask 94 had an aspect ratio of 0.4 (2000 
.ANG. high/5000 .ANG. wide). The wavelength of light used to generate the 
trace was 632 nm, a wavelength that is typically selected for performing 
ellipsometry. The trace obtained does not indicate at what points during 
the etch that the titanium nitride and polysilicon were removed because 
there is no point during the etch when the slope of .DELTA.162 and 
.PSI.164 changed abruptly over time. 
The trace illustrated in FIG. 5 was also obtained at this wavelength. 
However, the trace in FIG. 5 does indicate when the interfaces were 
encountered during the process. This is due to the fact that the trace in 
FIG. 5 was obtained when etching a blanket film. Blanket films do not 
generate the same type of ellipsometric signals, nor do they provide the 
same degree of interference with these signals as do high aspect ratio 
masks and topography. Therefore, although a useful trace is obtained at a 
particular wavelength for a blanket film, that wavelength will not 
necessarily provide a useful trace for fabrication process control for 
patterned wafers. FIG. 9, therefore illustrates that the appropriate 
wavelength must be selected to obtain an ellipsometric trace that is 
useful for controlling a lithographic process when a mask is deposited on 
the wafer. 
A trace was obtained by etching a film as illustrated in FIG. 4 under the 
conditions described above. The wavelength of light used to generate the 
trace was 375 nm and was calculated as described above. The trace that was 
obtained is illustrated in FIG. 10. The trace clearly illustrates the 
points in the process when the titanium nitride 176 and the polysilicon 
178 are removed from the wafer. These points are again indicated by an 
abrupt change in the slope of .DELTA.172 and .PSI.174 over time. 
The process of the present invention is useful even if there is a mask or 
topography over the entire wafer surface. As mentioned previously, prior 
art ellipsometric processes required a large area with no mask and no 
topography in order to practice the ellipsometric technique. As 
illustrated by FIGS. 11 and 12, useful traces are obtained even if the 
mask is a series of submicron lines and spaces over the entire substrate 
surface. The lines in the mask used to obtain the trace depicted in FIG. 
11 had an aspect ratio of about 1. The trace in FIG. 11 was obtained by 
etching a film as depicted generally in FIG. 3, wherein the mask was a 
series of submicron lines and spaces. The incident beam was in a direction 
that was parallel to the direction of the lines and spaces on the mask. 
The wavelength used to generate the trace was 375 nm and was calculated as 
described above. The trace obtained indicates the points during the etch 
when the titanium nitride 186 and the polysilicon 188 layers were removed 
from the wafer. However, this indication was provided only by the change 
in the rate at which the value of .PSI.184 changed during the process. 
Therefore, the change in one of either optical parameter, .DELTA. or 
.PSI., will provide the information necessary for device fabrication 
process control. 
FIG. 12 was obtained under the identical conditions used to obtain the 
trace illustrated in FIG. 11, except that the direction of the incident 
beam of light was perpendicular to the direction of the lines and spaces 
on the mask overlying the film. FIG. 12 illustrates that the trace 
obtained under these conditions does indicate the points during the etch 
when the titanium nitride 196 and the polysilicon 198 were removed. In 
this trace, the point in the process at which the titanium nitride was 
removed was indicated by the peak value of .DELTA.192 and the point at 
which the polysilicon was removed was indicated by the abrupt change in 
the slope of .PSI.194. 
As mentioned previously, once a signature trace is obtained for a 
particular film under particular conditions, that trace is used to control 
the processing of substantially identical films under substantially 
identical conditions. If the appropriate wavelength is selected, the point 
in the process between films will be indicated by a marked change in the 
value or slope of at least one of the ellipsometric angles .DELTA. and 
.PSI.. The particular type of change will vary widely from trace to trace, 
however. 
It is contemplated that the process of the present invention will be used 
to observe and control numerous aspects of device fabrication that require 
deposition or removal of materials over irregular surfaces. For example, 
the process is used to control the deposition of films onto substrates in 
addition to controlling the removal of those films as described above. 
After the appropriate wavelength is selected, the process is used to 
control deposition much in the manner of the processes described in U.S. 
Pat. No. 5,091,320 to Aspnes et al. and U.S. Pat. No. 5,131,752 to Yu et 
al., both of which are incorporated by reference. It is contemplated that 
the film will be grown under the dry deposition conditions described in 
Aspnes et al. 
For example, once a signature trace is obtained, the deposition of a 
multilayer film over topography or other surface irregularity is monitored 
and controlled by observing the trace being obtained in real time during 
the deposition process. The trace indicates the thickness of the first 
layer because the slope of .DELTA. and/or .PSI. is relatively constant 
over a period of time. When that period of time has elapsed, the desired 
thickness has been deposited. 
The process conditions are then changed to begin depositing the second 
layer of film. The point in time when deposition of the second film begins 
is characterized by an abrupt change in the slope of .DELTA. and/or .PSI.. 
This indicates the point during the process when the deposition begins for 
the second layer of film. By indicating the starting point, the deposition 
of the second layer is controlled, because one can identify the point in 
time during the process when deposition of this layer commenced. From this 
information one can determine when the desired amount of material has been 
deposited by simply observing the length of time that elapses from the 
point during the process when deposition began. This observation cannot be 
made using a trace such as the one illustrated in FIG. 9, because no 
interface is indicated by the trace.