X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2002-03-07
2004-11-09
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S089000, C250S310000
Reexamination Certificate
active
06816570
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the area of thin film analysis. In particular, the present invention relates to a method and apparatus for combining multiple thin film analysis capabilities into a single instrument.
2. Discussion of Related Art
As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Typically, the manufacturing process for modern semiconductor devices includes the formation of a number of layers or “thin films”, such as oxide, nitride, and metal layers. To ensure proper performance of the finished semiconductor devices, the thickness and composition of each film formed during the manufacturing process must be tightly controlled. In the realm of thin film analysis, three basic techniques have evolved to measure film thickness and composition.
Grazing-incidence X-ray Reflectometry
Grazing-incidence x-ray reflectometry (GXR), which is sometimes referred to as x-ray reflectometry (XRR), measures the interference patterns created by reflection of x-rays off a thin film.
FIG. 1
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
, and a detector
130
. X-ray reflectometry system
100
is configured to analyze a test sample
140
that includes a thin film layer
142
formed on a substrate
141
.
Microfocus x-ray tube
110
directs a source x-ray beam
150
at x-ray reflector
120
. Source x-ray beam
150
typically comprises a bundle of diverging x-rays that can have a variety of different wavelengths. X-ray reflector
120
reflects and focuses the diverging x-rays of x-ray beam
150
into a converging x-ray beam
160
. Typically, x-ray reflector
120
is a singly- or doubly-curved monochromatizing crystal that ensures that only x-rays of a particular wavelength are included in converging x-ray beam
160
, which is directed at thin film layer
142
.
Converging x-ray beam
160
is then reflected by thin film layer
141
as an output x-ray beam
170
onto detector
130
. X-ray beam
170
forms an interference pattern on the surface of detector
130
due to constructive and destructive interference of x-ray reflections at the top and bottom surfaces of thin film layer
142
. Detector
130
is a position-sensitive detector that measures the varying intensity of this interference pattern. The resulting reflectivity curve of intensity versus position can then be used to calculate the thickness of thin film layer
142
, as described in U.S. Pat. No. 5,619,548.
GXR is best suited for measuring thickness and electron density for films in the range of 10A-2000A thick. It is well matched to the barrier/seed film stacks used in BEOL (back end of line) copper interconnects. However, GXR cannot measure thicker ECP (electrochemical plated) copper films having thicknesses greater than 1 um. Furthermore, GXR is not very good at measuring the composition of thin films—for example the composition of a barrier film such as TaN or TiSiN.
Electron Microprobe Analysis
To analyze the composition of a thin film layer, a technique known as electron microprobe (EMP) analysis is often used. EMP analysis involves the use of an electron beam (e-beam) to generate characteristic x-rays from a thin film layer.
FIG. 2
shows a conventional EMP system
200
comprising an e-beam generator
210
and an x-ray detector
230
. EMP system
200
is configured to analyze a test sample
240
that includes a thin film layer
242
formed on a substrate
241
.
To perform an EMP analysis, e-beam generator
210
directs an e-beam
250
at thin film layer
242
. The high-energy electrons in e-beam
250
cause characteristic x-rays
290
to be emitted by thin film layer
242
. The properties of characteristic x-rays
290
are then measured by x-ray detector
230
to determine the composition of thin film layer
242
.
Generally, x-ray detector
230
comprises either an energy-dispersive x-ray spectrometer (EDX or EDS) or a wavelength-dispersive x-ray spectrometer (WDX or WDS). In an EDX detector, the energies of the characteristic x-rays are used to determine the composition of the thin film.
FIG. 4
a
shows a conventional EDX detector
230
a
that includes a detector crystal
231
and a pulse analyzer
232
. Each of characteristic x-rays
290
incident on detector crystal
231
deposits an amount of charge proportional to the energy of that particular x-ray. These charge pulses are then measured by pulse analyzer
232
. Because different elements generate x-rays having different energies, the charge pulse magnitudes read by pulse analyzer
232
can be used to determine the intensity of the characteristic x-rays, which in turn can be used to determine thin film composition and thickness.
While an EDX detector provides a relatively simple means for determining the composition of a thin film layer, x-rays having closely spaced wavelengths (i.e., energies) can be difficult to distinguish. For example, an ECP copper film may be formed over a tantalum nitride barrier film. The characteristic copper x-rays (Cu-K, indicating x-rays resulting from the ionization of the K shells of the copper atoms) and the characteristic tantalum x-rays (Ta-L, indicating x-rays resulting from the ionization of the L shells of the tantalum atoms) are only separated by 100 eV, and therefore cannot be resolved by an EDX detector, which typically has a resolution limit of greater than 150 eV. Furthermore, an EDX detector cannot detect low energy x-rays, such as those emitted by the nitrogen (N-K x-rays; i.e., x-rays resulting from the ionization of the K shells of the nitrogen atoms) in a barrier film.
In contrast, WDX detectors have a much lower resolution limit of roughly 10-20 eV, and can therefore provide much more accurate measurements than an EDX detector. The low resolution limit of a WDX detector would enable Cu-K and Ta-L x-rays to be distinguished, and also enables the detection of low-energy N-K x-rays. In a WDX detector, x-rays having specific wavelengths are detected to improve the resolution of the measurement process.
FIG. 4
b
shows a conventional WDX detector
230
b
that includes an x-ray reflector
238
and a proportional counter
239
. Incoming characteristic x-rays
290
are incident on x-ray reflector
238
. X-ray reflector
238
is a monochromator, and disperses the incoming characteristic x-rays
290
according to Bragg's Law. X-ray reflector
238
is configured such that only those characteristic x-rays
290
having a specific wavelength are directed onto proportional counter
239
. The specific wavelength is selected to be the characteristic wavelength of x-rays emitted by a particular element. Therefore, the output of proportional counter
239
can then be correlated to the concentration of the particular element in the thin film layer. Often, multiple WDX detectors are used simultaneously, with each of the multiple WDX detectors being configured to respond to a different element.
Whether an EDX or WDX detector is used, EMP analysis can be performed relatively quickly due to the intense characteristic x-rays produced by the thin film in response to the e-beam. Also, by varying the energy of the e-beam, an EMP system can “depth profile” a stack of thin film layers, allowing composition measurements to be taken at various positions thoughout the film stack. However, as film thickness in the test sample increases, the electrons in the e-beam must be raised to higher and higher energies to properly penetrate the film. For example, to penetrate 1-2 um thick ECP (electro-chemical plated) copper films, electrons with at least 50 keV energy must be used. Such high-energy electrons are difficult to produce and can damage the test sample. In addition, higher power e-beam generators increase the cost of an EMP system while decreasing overall system reliability. This is in addition to the in
Janik Gary R.
Moore Jeffrey
Bever Hoffman & Harms LLP
Church Craig E.
Kiknadze Irakli
KLA-Tencor Corporation
Kubodera John M.
LandOfFree
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