Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2001-10-23
2004-12-21
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06833914
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the simulation of the reflectometry response from grating profiles and more particularly to an efficient method for accurately simulating the integrated reflectometry response from two-dimensional grating structures using a few points.
BACKGROUND ART
Spectroscopic reflectometry and ellipsometry have been a mainstay for thin film metrology for many years. Recently, spectroscopic reflectometry and ellipsometry have been applied to characterizing patterned structures in integrated circuit (IC) processing by directing a beam of light on the patterned structures at a certain angle of incidence and measuring the spectra of the reflected light. However, due to the difficulty of rigorous simulation responses from patterned structures, empirical methods, such as neural networks (NN) or principle component analysis (PCA), are used to build up the relation between the patterned structure parameters (e.g., width of a structure (CD), grating height, sidewall angle, and other profile parameters) and reflected spectra.
FIG. 1
illustrates a typical reflectometry configuration for characterizing patterned structures located on a wafer. A broadband light beam
105
travels through an optical system
110
characterized by a numerical aperture. A lens
115
focuses the broadband light beam
105
into a spot characterized by a spot size onto a patterned structure located on wafer
120
(note that throughout this specification a lens is referred to more generically as an aperture). The light reflected off of the patterned structure located on wafer
120
is then collected by lens
115
and transmitted to a spectrometer
125
through optical system
110
. Using other metrology tools, such as a scanning electron microscope (SEM), an atomic force microscope (AFM), etc., the parameters of the patterned structure can be measured off-line. Next, neural networks or principal component analysis can be used to build up (train) a non-linear relation between the reflected spectra and the parameters of the patterned structure. However, the relation between the reflected spectra and the parameters of the patterned structure is only valid when the values of the parameters are within the range of parameters used for training. Furthermore, long turnaround times imposed by off-line metrology also result in lower yields, slower learning curves, and higher costs for new processes and products.
A multi-point rigorous simulation method can be used to avoid doing experiments to build up empirical relations between the reflected spectra and the parameters of patterned structures. Ideally, the numerical aperture should be very small (e.g. less than 0.01) so that the reflectometry response can be simulated using normal incidence. However, if the lens
115
is too small, then the light throughput is low, thus weakening the intensity of the light beam
105
incident on a two-dimensional grating structure. The weaker the intensity of the light beam
105
, the longer it takes to collect enough simulation data to achieve stability.
Starting from Maxwell's equations, the response of light reflected from patterned structures can be simulated rigorously using numerical methods. However, in an actual reflectometry system, to have an acceptable spot size (less than 100 &mgr;m) and throughput (less than 1 second), the numerical aperture (NA) is fairly large (about 0.05 or larger). Thus, the reflectance is actually the integrated response from multiple beams of light reflected off the patterned structures, where the incidence angle of each light beam is close to zero degrees.
FIGS. 2
a
and
2
b
show an example of the multi-point rigorous simulation method.
FIG. 2
a
shows light passing through numerous (e.g., 20 to 30) points
205
across an aperture
210
creating numerous light beams
215
. The aperture
210
focuses each light beam
215
onto the two-dimensional grating structure
220
at an angle close to zero degrees. Simulating the numerous light beams
215
focused onto the two-dimensional grating structure
220
yields the response distribution
225
for light beams
215
as shown in
FIG. 2
b.
The integrated response for this wavelength very closely approximates the actual reflectance and can be obtained by adding up each of the responses shown in
FIG. 2
b.
However, the simulation takes a long time.
A single point rigorous simulation method can also be used to avoid doing experiments to build up empirical relations between the reflected spectra and the parameters of the patterned structures.
FIGS. 3
a
and
3
b
show an example of the single point rigorous simulation method.
FIG. 3
a
shows a simulation of light passing through a single point O
305
located at the center of an aperture
310
creating a single light beam
315
. The light beam
315
is incident on the two-dimensional grating structure
320
at an angle of zero degrees.
FIG. 3
b
shows the simulated reflectance response
325
of the light passing through the single point O
305
. Although, the reflectance response
325
can be obtained very quickly, it is not very accurate since it only represents the reflectance response
325
from light passing through the single point O
305
. The reflectance response
325
does not take into account the reflectance response from light passing through other points across the aperture
310
, and thus cannot represent the overall reflectance response of the aperture
310
.
FIG. 4
is a simulation which compares the accuracy of the response from light reflected off of a two-dimensional grating structure using the multi-point rigorous simulation method and the single point simulation method. As shown in
FIG. 4
, the simulated response
410
obtained using the single point rigorous simulation method differs greatly from the response
405
obtained using the multi-point rigorous simulation method. The multi-point rigorous simulation method closely approximates the actual reflectance response. Thus, there is a desire for a method of simulating the reflectance response of two-dimensional grating structures that is both accurate and not time consuming.
SUMMARY OF INVENTION
The method in accordance with embodiments of the present invention relates to a method for efficient simulation of reflectometry response from two-dimensional grating structures.
In one embodiment, the light intensity distribution across the aperture is uniform. A first and a second point are determined within an aperture located in an optical system. Next, the reflectance response of light incident at the first point and the second point are simulated. The approximated integrated reflectance response of the aperture is then determined based on the reflectance response at the first point and the second point and determined characteristics of the optical system.
In another embodiment, the light intensity distribution across the aperture is not uniform. A first and a second point are determined within an aperture located in an optical system. Next, the reflectance response of light incident at the first point and the second point are simulated. The approximated integrated reflectance response of the aperture is then determined based on the reflectance response at the first point and the second point and determined characteristics of the optical system.
REFERENCES:
patent: 6292265 (2001-09-01), Finarov et al.
patent: 6614540 (2003-09-01), Stirton
Bao Junwei
Jakatdar Nickhil
Niu Xinhui
Morrison & Foerster / LLP
Rosenberger Richard A.
Timbre Technologies, Inc.
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