Ellipsometer using radial symmetry

Details Ellipsometer using radial symmetry Ellipsometer using radial symmetry

2000-10-18

2004-02-17

Stafira, Michael P. (Department: 2877)

Optics: measuring and testing

By polarized light examination

Of surface reflection

C356S364000

Reexamination Certificate

active

06693711

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to ellipsometry. More particularly, the present invention pertains to ellipsometric methods and apparatus using radial symmetry.
BACKGROUND OF THE INVENTION
Ellipsometry is an optical technique that uses polarized light to probe the properties of a sample. The most common application of ellipsometry is the analysis of very thin films. Through the analysis of the state of polarization of the light that interacts with the sample, ellipsometry can yield information about such films. For example, depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, morphology, or chemical composition.
Generally, optical ellipsometry can be defined as the measurement of the state of polarized light waves. An ellipsometer measures the changes in the polarization state of light when it interacts with a sample. The most common ellipsometer configuration is a reflection ellipsometer, although transmission ellipsometers are sometime used. If linearly polarized light of a knownorientationis reflected or transmitted at oblique incidence from a sample surface resultant light becomes elliptically polarized. The shape and orientation of the ellipse depend on the angle of incidence, the direction of the polarization of the incident light, the wavelength of the incident light, and the Fresnel properties of the surface. The polarization of the light is measured for use in determining characteristics of the sample. For example, in one conventional null ellipsometer, the polarization of the reflected light can be measured with a quarter-wave plate followed by an analyzer. The orientation of the quarter-wave plate and the analyzer are varied until no light passes though the analyzer, i.e., a null is attained. From these orientations and the direction of polarization of the incident light, a description of the state of polarization of the light reflected from the surface can be calculated and sample properties deduced.
Two characteristics of ellipsometry make its use particularly attractive. First, it is a nondestructive technique, such that it is suitable for in situ observation. Second, the technique is extremely sensitive. For example, it can measure small changes of a film down to sub-monolayer of atoms or molecules. For these reasons, ellipsometry has been used in physics, chemistry, materials science, biology, metallurgical engineering, biomedical engineering, etc.
As mentioned above, one important application of ellipsometry is to study thin films, e.g., in the fabrication of integrated circuits. In the context of ellipsometry, a thin film is one that ranges from essentially zero thickness to several thousand Angstroms, although this range can be extended in many cases. The sensitivity of an ellipsometer is such that a change in film thickness of a few Angstroms can usually be detected. From the measurement of changes in the polarization state of light when it is reflected from a sample, an ellipsometer can measure the refractive index and the thickness of thin films, e.g., semi-transparent thin films. The ellipsometer relies on the fact that the reflection at a material interface changes the polarization of the incident light according to the index of refraction of the interface materials. In addition, the polarization and overall phase of the incident light is changed depending on the refractive index of the film material as well as its thickness.
Generally, for example, a conventional reflection ellipsometer apparatus, such as shown in
FIG. 1
, includes a polarizer arm
12
and an analyzer arm
14
. The polarizer arm
12
includes a light source
15
such as a laser (commonly a 632.8 nm helium
eon laser or a 650-850 nm semiconductor diode laser) and a polarizer
16
which provides a state of polarization for the incident light
18
. The polarization of the incident light may vary from linearly polarized light to elliptically polarized light to circularly polarized light. The incident light
18
is reflected off the sample
10
or layer of interest and then analyzed with the analyzer arm
14
of the ellipsometer apparatus. The polarizer arm
12
of the ellipsometer apparatus produces the polarized light
18
and orients the incident light
18
at an angle
13
with respect to a sample plane
11
of the sample
10
to be analyzed, e.g., at some angle such as 20 degrees with respect to the sample plane
11
or 70 degrees with respect to the sample normal.
The reflected light
20
is examined by components of the analyzer arm
14
, e.g., components that are also oriented at the same fixed angle with respect to the sample plane
11
of the sample
10
. For example, the analyzer arm
14
may include a quarter wave plate
22
, an analyzer
24
(e.g., a polarizer generally crossed with the polarizer
16
of the polarizer arm
12
), and a detector
26
. To measure the polarization of the reflected light
20
, the operator may change the angle of one or more of the polarizer
16
, analyzer
24
, or quarter wave plate
22
until a minimal signal is detected. For example, the minimum signal is detected if the light
20
reflected by the sample
10
is linearly polarized, while the analyzer
24
is set so that only light with a polarization which is perpendicular to the incoming polarization is allowed to pass. The angle of the analyzer
24
is therefore related to the direction of polarization of the reflected light
20
if the minimum condition is satisfied. The instrument is “tuned” to this null (e.g., generally automatically under computer control), and the positions of the polarizer
16
, the analyzer
24
, and the incident angle
13
of the light relative to the sample plane
11
of the sample
10
are used to calculate the fundamental quantities of ellipsometry: the so called Psi, delta (&PSgr;, &Dgr;) pair given by:
r
p
r
s
=
tan
⁢
 
⁢
Ψ
⁢
(
ⅇ
j
⁢
 
⁢
Δ
)
where r
p
and r
s
are the complex Fresnel reflection coefficients for the transverse magnetic and transverse electrical waves of the polarized light, respectively. Form the ellipsometry pair (&PSgr;, &Dgr;), the film thickness (t) and index of refraction (n) can be determined. It will be recognized that various ways of analyzing the reflected light may be possible. For example, one alternative is to vary the angle of the quarter wave plate and analyzer to collect polarization information.
Advances in microelectronics fabrication are rapidly surpassing current capabilities in metrology. In order to enable future generations of microelectronics, advanced specific metrology capabilities must be developed. Key among these metrology capabilities is the ability to measure the properties of complex layers of extremely thin films over sub-micron lateral dimensions.
Currently available ellipsometric techniques that measure material properties generally measure them over a large area. In other words, polarization measurements have been traditionally used to determine the thickness and refractive index of homogeneous films over a relatively large area. However, generally, determining the thickness and refractive index of homogeneous films over a relatively large area is inadequate for exceedingly small featured structures. For example, since the polarization state is effected significantly by diffraction from sub-micron features, the shape of such sub-micron features, e.g., critical dimensions of lateral or traverse structures such as gate dielectrics for transistor structures, is difficult to measure using current ellipsometric techniques that determine thickness and refractive index over relatively large areas. For example, the smallest spot that a conventional ellipsometer can measure is generally determined by the beam size, usually on the order of hundreds of microns. This essentially limits the application of conventional ellipsometers to samples with large and uniform interface characteristics.
SUMMARY OF THE INVENTION
The present invention exploits the polarization properties of hi

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