X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2002-02-04
2003-01-14
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
Reexamination Certificate
active
06507634
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of measurement of materials used in the fabrication of semiconductor devices. Specifically, the present invention pertains to using transmission characteristics of thin-film layers disposed on a substrate to determine the thickness of the thin-film layer.
BACKGROUND
Semiconductor wafers typically include thin-films formed on semiconductor substrates. There is a need to be able to measure and analyze characteristics of these films. Previous systems have provided a way to analyze the thickness and density of thin-films disposed on semiconductor substrates using X-ray reflectometry (XRR). For example, U.S. Pat. No. 5,619,548 and PCT Publication W01/71325 A2 (referred to herein as the '325 application) discuss different aspects of XRR systems and are hereby incorporated by reference.
XRR systems of the prior art make use of the fact that x-rays reflected off of a thin-film disposed on a substrate are detected as having different characteristics depending on the x-ray's angle of reflection relative to the surface of the structure.
FIG. 1
shows a view of a prior art XRR system for simultaneous measurements of the reflectivity over a range of angles. As shown in
FIG. 1
a source
100
generates an x-ray beam
101
that is incident upon an x-ray reflector
102
, which is typically a monochromator. X-rays are then focused upon the sample being evaluated
106
which is positioned on a supporting stage
104
. X-rays incident upon the sample are reflected and then detected with a position-sensitive detector
108
(such as a photodiode array).
Reflected x-rays
110
are captured in the top half of the detector
108
, while the incident beam
112
can be measured by lowering the stage and reading the bottom half of the detector. By properly normalizing the two profiles (as described in the '325 application) one can determine the reflectivity as a function of angle. Signals are generated by the detector
108
, and the information contained in these signals is then used by the processor system
114
to analyze the reflectivity characteristics of the sample
106
. The processor system
114
can then generate a display
116
to convey information about the sample
106
to user.
FIG. 2
shows a typical plot of angle-resolved XRR data, in a graph form which could be generated by the processor system
114
. This type of graph depicts the efficiency with which monochromatic x rays are reflected from a sample, and this type of information can characterize the reflectivity of a thin-film disposed on a substrate. Specifically,
FIG. 2
shows a graph for reflectivity of x-rays incident on a 358 Å cobalt thin film, on a substrate taken at 6.4 keV. The reflectivity signal shows a fringe pattern having peaks
206
, and these peaks correlate to different reflection angles. It will be readily appreciated by one skilled in the art that as the thickness of the film increases the difference in the reflection angle between the peaks will decrease. For thin-films of sufficient thickness, prior systems may not be able to accurately resolve the fringe pattern, and as a result it may be difficult or impossible to determine the thickness of the thin-film. One approach for dealing with this problem is to modify the resolution of the system, but in general there is a limit to how much the resolution of the system can be increased, and further increasing the resolution of the system results in an increase in the amount of time it takes to make a measurement. (Aspects of one approach to varying the resolution of the system are disclosed in co-pending commonly assigned patent application Ser. No. 10/053,373 entitled X-RAY REFLECTANCE MEASUREMENT SYSTEM WITH ADJUSTABLE RESOLUTION, filed Oct. 24, 2001, which is incorporated herein by reference.)
One example of a semiconductor wafer structure where prior art XRR techniques are often unable to accurately determine the characteristics of a thin-film, is where a thin-film of porous SiO
2
is formed on a second film, or material, which is composed of a material which is denser than SiO
2
. Using previous systems and methods it was often difficult, or impossible to accurately determine the thickness of the porous SiO
2
material, because in many applications the SiO
2
layer is thick enough that it produces a very narrow fringe pattern which is beyond the resolution of the system. What is needed is a system and method for accurately determining the thickness of a thin film layer where the thin film layer is such that it produces an fringe pattern that is cannot be accurately resolved using standard XRR systems.
SUMMARY
Prior XRR systems utilize fringe patterns in reflectivity data to determine the thickness of a thin-film layer. In general terms, the fringe pattern is caused by the interference of x-rays reflected at the several density interfaces present in a thin-film structure, such as for a thin-film layer on a substrate. Changes in the thickness of the thin-film layer will result in changes in fringe pattern.
In contrast with prior methods which focus on using the reflectivity information to determine a fringe pattern and then use this information to determine the thickness of the thin-film, the present method and system use reflectivity information to determine transmission characteristics of the thin-film layer. The transmission characteristics are then used to determine the thickness of the thin-film. A system and method which evaluates the transmission characteristics of the thin-film, as disclosed herein, can be used to determine the thickness of the thin-film structures which could not be determined using many prior systems which utilized fringe pattern analysis.
REFERENCES:
patent: 5042951 (1991-08-01), Gold et al.
patent: 5042952 (1991-08-01), Opsal et al.
patent: 5412473 (1995-05-01), Rosencwaig et al.
patent: 5619548 (1997-04-01), Koppel
patent: 5949847 (1999-09-01), Terada et al.
patent: 6434217 (2002-08-01), Pickelsimer et al.
patent: WO 92/08104 (1992-05-01), None
patent: WO 00/57127 (2000-09-01), None
patent: WO 01/71325 (2001-09-01), None
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K.N. Stoev et al., “Review on grazing incidence X-ray spectrometry and reflectometry,”Spectrochimica Acta Part B, vol. 54, 1999, pp. 41-82.
N. Wainfan et al., “Density Measurements of Some Thin Copper Films,”Journal of Applied Physics, vol. 30, No. 10, Oct. 1959, pp. 1604-1609.
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Wen-Li Wu et al., “Study of ultra-thin hydrogen silsesquioxane films using X-ray reflectivity,”Thin Solid Films, vol. 312, 1998, pp. 73-77.
E.K. Lin et al., “Structure and Property Characterization of Porous Low-k Dielectric Constant Thin Films using X-ray Reflectivity and Small Angle Neutron Scattering,”Mat. Res. Soc. Symp. Proc., vol. 612, 2000[Materials Research Society], pp. D4.1.1-D4.1.8.
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Johnson William
Koppel Louis N.
Church Craig E.
Stallman & Pollock LLP
Therma-Wave, Inc.
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