X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
2001-11-07
2004-08-03
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
Reexamination Certificate
active
06771735
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the area of nondestructive film thickness measurement. In particular, the present invention relates to a method and apparatus for improving the resolution and accuracy of x-ray reflectometry measurements of thin films.
2. Discussion of Related Art
Conventional thin film thickness measurement systems often use a technique known as x-ray reflectometry (XRR), which measures the interference patterns created by reflection of x-rays off a thin film.
FIG. 1
a
shows a conventional x-ray reflectometry system
100
, as described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997 to Koppel. X-ray reflectometry system
100
comprises a microfocus x-ray tube
110
, an x-ray reflector
120
, a detector
130
, and a stage
140
. A test sample
142
having a thin film layer
141
is held in place by stage
140
for the measurement process.
To measure the thickness of thin film layer
141
, microfocus x-ray tube
110
directs a source x-ray beam
150
at x-ray reflector
120
. Source x-ray beam
150
actually comprises a bundle of diverging x-rays, including x-rays
151
,
152
, and
153
. X-ray reflector
120
reflects and focuses the diverging x-rays of x-ray beam
150
into a converging x-ray beam
160
. Converging x-ray beam
160
includes x-rays
161
,
162
, and
163
, which correspond to x-rays
151
,
152
, and
153
, respectively. Typically, x-ray reflector
120
is a monochromator that ensures that only x-rays of a particular wavelength are included in converging x-ray beam
160
.
Converging x-ray beam
160
is then reflected by thin film layer
141
as an output x-ray beam
170
onto detector
130
. A detail view of this reflection is shown in
FIG. 1
b
, with reflected x-rays
171
,
172
, and
173
corresponding to incident x-rays
161
,
162
, and
163
, respectively. The x-rays undergo specular reflection, forcing angles A
1
, A
2
, and A
3
, of x-rays
161
,
162
, and
163
, respectively, to be equal to angles A
11
, A
22
, and A
33
of x-rays
171
,
172
, and
173
, respectively.
As shown in
FIG. 1
c
, the reflected x-rays are actually formed by reflections at both thin film surface
141
a
and thin film/substrate interface
142
a
. Using x-ray
162
as an example, the incident x-ray splits into a primary ray
172
a
and a secondary ray
172
b
at thin film layer
141
. Primary ray
172
a
is reflected by thin film surface
141
a
at an angle A
22
. Secondary ray
172
b
is transmitted through thin film layer
141
and is reflected at thin film/substrate interface
142
a
, eventually exiting thin film surface
141
a
at angle A
22
.
Because both rays
172
a
and
172
b
exit thin film surface
141
a
at angle A
22
, the intensity of x-ray
172
is determined by the amount of constructive or destructive interference between the two rays. The two rays will be in phase if the difference between the optical path length of primary ray
172
a
and the optical path length of secondary ray
172
b
is equal to an integer multiple of the wavelength of x-ray
162
. (Note that the optical path length of ray
172
b
includes the distance secondary ray
172
b
travels within thin film layer
141
multiplied by the index of refraction of thin film layer
141
.) If rays
172
a
and
172
b
are in phase, the maximum intensity for x-ray
172
is achieved. However, if this optical path length difference is not an integer multiple of the wavelength of x-ray
162
, then the two rays will be out of phase, thereby reducing the intensity of x-ray
172
.
Note that the actual optical path length of secondary ray
172
b
within thin film layer
141
is controlled by the incident angle of x-ray
162
. Therefore, the intensity of x-ray
172
is ultimately determined by incident angle A
2
. By simultaneously focusing a beam of x-rays spanning a range of incident angles at the thin film layer, a reflected beam of x-rays having varying intensities can be generated. Those varying intensities can be measured by sensor
130
, as indicated in
FIG. 1
b
. For example, reflected x-rays
171
,
172
, and
173
are shown impinging on a detector plane
130
a
of detector
130
at points
181
,
182
, and
183
, respectively. Points
181
,
182
, and
183
typically comprise sensor pixels capable of measuring incident x-ray intensity. The known pixel positions allow detector
130
to correlate the intensities at points
181
,
182
, and
183
with incident angles A
1
, A
2
, and A
3
, respectively. By performing a similar correlation for all the pixels on detector surface
130
a
, a reflectivity curve can be derived for thin film layer
140
. An example reflectivity curve is shown in FIG.
2
. By measuring the fringes in the reflectivity curve, the thickness of thin film layer
140
can be determined, as described in U.S. Pat. No. 5,619,548.
However, accuracy of conventional x-ray reflectometry systems can be severely limited by problems associated with x-ray scattering and spreading at the thin film surface. For example,
FIG. 3
shows a detail view of x-ray reflectometry system
100
, with incident x-rays
164
and
165
being reflected by thin film layer
141
. X-ray
164
has an incident angle A
4
and is reflected at an angle A
44
as x-ray
174
. In accordance with the law of specular reflection, angle A
4
is equal to angle A
44
. X-ray
165
has an incident angle A
5
, and theoretically would be reflected at an angle A
55
as x-ray
175
r
, where angle A
55
is equal to angle A
5
. Because angle A
4
is different from angle A
5
, x-rays
174
and
175
r
would ideally impinge on detector surface
130
a
at points
184
and
185
, respectively. However, scattering caused by imperfections in the surface of thin film layer
141
can result in a portion or all of incident x-ray
165
splitting off as x-ray
175
s
. X-ray
175
s
leaves the surface of thin film layer
141
at an angle A
5
s
(which is not equal to incident angle A
5
). If angle A
5
s
happens to be equal to angle A
44
, both x-rays
175
s
and
174
will impinge on detector surface
130
a
at point
184
, thereby corrupting the intensity measurements at both points
184
and
185
. Scattering is most likely to occur for x-rays having incident angles near the “critical angle” where total external reflection takes place.
The accuracy of conventional x-ray reflectometry systems is further degraded by problems associated with x-ray beam spreading. For example,
FIG. 4
a
depicts the interface between x-ray beam
160
and thin film layer
141
where an illuminated spot B is formed on thin film surface
141
a
. Compared to a cross section A at the most tightly focused portion of x-ray beam
160
, illuminated spot B is significantly elongated in the beam direction.
FIG. 4
b
shows cross section A of x-ray beam
160
overlaid onto illuminated spot B. Conventional microfocus x-ray tubes produce a circular x-ray beam, as indicated in
FIG. 4
b
. Accordingly, the height H
1
and width W
1
of cross section A are the same (i.e., unitary aspect ratio). In contrast, illuminated spot B is significantly distorted as it spreads across thin film surface
141
a
, and so has a length L
2
and a width W
2
at its largest dimensions. In a direction perpendicular to the beam direction and parallel to thin film surface
141
a
(sometimes referred to as the “neutral axis”), width W
2
of illuminated spot B is increased slightly from width W
1
of beam cross section A. However, along the beam direction, height H
1
of beam cross section A is translated into a significantly greater length L
2
of illuminated spot B. This disparity in x-ray beam height and illuminated spot length increases as the incident angle of the incoming x-ray beam decreases, and so is particularly problematic for the grazing-angle x-ray beams required in x-ray reflectometry. For example, at an incident angle of 0.5 degree, the length of the illuminated spot is roughly 100 times greater than the diameter of the x-ray beam.
Because of this lengthening of the illuminated spot, the resolution of conventional x-ray r
Janik Gary
Moore Jeffrey
Bever Hoffman & Harms LLP
Church Craig E.
Harms Jeanette S.
KLA-Tencor Technologies Corporation
Kubodera John M.
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