Optics: measuring and testing – Dimension – Thickness
Reexamination Certificate
2000-12-12
2003-12-02
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Dimension
Thickness
C356S632000
Reexamination Certificate
active
06657737
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a film thickness measuring apparatus for measuring a film thickness. More particularly, the present invention relates to a film thickness measuring method and a film thickness measuring apparatus in which light is projected onto a film to be measured, and reflected light from the film is received to measure the thickness of the film based on the intensity of the reflected light. It should be noted that the term “film thickness measurement” as used in this specification includes not only the measurement of a film thickness but also the detection of a substrate condition, for example, as to whether or not a metal film is present on a substrate, or the observation of a substrate condition.
There has heretofore been a film thickness measuring apparatus in which light is applied to a film under measurement, and reflected light from the upper and lower interface surfaces of the film is received. The thickness of the film is measured by making use of the phenomenon that light reflected from the upper and lower interface surfaces of the film interfere with each other, and the reflected light intensity changes according to the thickness of the film.
FIGS. 1 and 2
are flowcharts showing the process for calculating a film thickness based on the intensity of reflected light received in a conventional film thickness measuring apparatus of the type described above. To calculate a film thickness, as shown in
FIG. 1
, a spectral reflectance ratio S(&lgr;) is determined (see curve a in
FIG. 3
) from the spectral reflection intensity at a measuring point (a spot where the film under measurement is present) (step ST
1
). In addition, a spectral reflectance ratio R(&lgr;) is determined (see curve b in
FIG. 3
) from the spectral reflection intensity at a spot where the film under measurement is not present (step ST
2
). The spectral reflectance ratio S(&lgr;) is divided by the spectral reflectance ratio R(&lgr;) to obtain a spectral reflectance ratio (=measured profile) Rmeas(&lgr;)=S(&lgr;)/R(&lgr;) of the film (see curve c in
FIG. 3
) (step ST
3
). It should be noted that curves a and b in
FIG. 3
show a case where the wavelength &lgr; spectrum of reflected light is continuous when a halogen lamp is used as an incident light source, for example.
To determine a film thickness value D, a variable d is used to represent the film thickness, and d is changed in a range (from d
1
to d
2
) where the proper film thickness value is expected to be present. First, d is initialized (d=d
1
) (step ST
4
), and an evaluation value Ed is determined from the square-sum of differences between the theoretical value Rcalc(&lgr;) and the measured value Rmeas(&lgr;) of the spectral reflectance ratio at the relevant film thickness d to obtain an evaluation function E(d) (step ST
5
). A minimum unit (d of measurement is added to the film thickness d (d=d+&Dgr;d) (step ST
6
). Subsequently, it is determined whether or not d≦d
2
(step ST
7
). If d≦d
2
, the process returns to step ST
5
to repeat the processing. If d≦d
2
does not hold, the film thickness d that gives a minimum value of the evaluation function E(d) is determined to be a measured film thickness value D (step ST
8
).
FIG. 2
is a flowchart showing the processing for determining an evaluation value Ed from the square-sum of differences between the theoretical value Rcalc(&lgr;) and the measured value Rmeas(&lgr;) of the spectral reflectance ratio in the measuring wavelength range (from &lgr;
1
to &lgr;
2
) at the relevant film thickness d at the above-described step ST
5
to obtain an evaluation function E(d). First, initialization is executed (&lgr;=&lgr;
1
, Ed=0) to change the wavelength &lgr; within the measuring wavelength range of from &lgr;
1
to &lgr;
2
(step ST
11
).
Next, an evaluation value Ed is determined by the following calculation (step ST
12
). The square-sum of differences between the theoretical value Rcalc(&lgr;) and the measured value Rmeas(&lgr;) at the relevant film thickness is determined.
E
&lgr;
=(
R
meas(&lgr;)−
R
calc(&lgr;))
2
Ed=Ed+E
&lgr;
When the absorption coefficient is assumed to be zero, the theoretical value Rcalc(&lgr;) can be calculated from the following equation:
R
calc(&lgr;)=
r
1
2
+r
2
2
+2
×r
1
×
r
2
×cos &dgr;
where r
1
=(1−n
1
)/(1+n
1
); r
2
=(1−nb)/(1+nb); &dgr;=4&pgr;n
1
d/&lgr;; n
1
is the refractive index of the film; nb is the refractive index of the substrate; d is the film thickness; and &lgr; is the measuring wavelength range (from &lgr;
1
to &lgr;
2
).
Next, the resolution &Dgr;&lgr; in the direction of the measuring wavelength range is added (&lgr;=&lgr;+&Dgr;&lgr;) (step ST
13
). Subsequently, it is determined whether or not &lgr;≦&lgr;
2
(step ST
14
). If, &lgr;≦&lgr;
2
the process returns to step ST
12
to repeat the processing. If &lgr;≦&lgr;
2
does not hold, the evaluation value Ed is substituted into the evaluation function E(d) (E(d)=Ed) (step ST
15
). Then, the process proceeds to step ST
6
in FIG.
1
.
Thus, the conventional process for calculating a film thickness is as follows. To obtain a spectral reflectance ratio (=measured profile) of a film under measurement, the spectral reflection intensity at a measuring point is measured, and the spectral reflection intensity for calibration (at a spot where the film under measurement is not present) is measured. Then, the former spectral reflection intensity is divided by the latter spectral reflection intensity to obtain a spectral reflectance ratio of the film. The measured profile thus obtained is compared with a theoretically calculated spectral reflectance ratio based on an assumed film thickness, and an assumed film thickness that gives a minimum difference between the measured profile and the theoretically calculated spectral reflectance ratio is determined to be a measured film thickness.
For this type of conventional film thickness measuring apparatus, mechanical and optical schemes have been devised with an emphasis on how a measured profile is obtained accurately with a good S/N ratio by optical or other techniques meeting a demand for high accuracy. Accordingly, the spectral reflection intensity obtained from the film to be measured provides a sufficiently high intensity to obtain the desired result. Therefore, the film thickness can be measured satisfactorily by an algorithm in which the measured profile and the theoretically calculated spectral reflectance ratio are compared directly to each other as stated above.
However, in a thin-film processing apparatus such as a chemical/mechanical polishing apparatus (CMP) for chemically and mechanically polishing substrates, e.g. semiconductor wafers, there has recently been an increasing demand for measurement inside the thin-film processing apparatus and measurement during processing. In this case, a film thickness measuring apparatus must be installed without interfering with polishing or other processing for which the processing apparatus is designed. In addition, because the measurement of film thickness is an accessory function, the film thickness measuring apparatus is required to be simplified in structure with a view to minimizing costs. In other words, in the measurement of a film thickness carried out inside the thin-film processing apparatus or during processing, it is difficult to detect a sufficiently high spectral reflection intensity to obtain the desired result, which has heretofore been possible to attain without any problem.
FIGS. 3
to
6
are diagrams showing measured data concerning a SiO
2
film with a thickness of about 460 nanometers (nm) provided on a silicon (Si) substrate.
FIGS. 3 and 4
are diagrams showing measured data in a case where the spectral reflection intensity is sufficiently high to obtain a film thickness value. Curves a, b and c in
FIG. 3
are the spectral reflectance ratio
Kimba Toshifumi
Nakai Shunsuke
Ebara Corporation
Pham Hoa Q.
Wenderoth , Lind & Ponack, L.L.P.
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