X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
2002-02-19
2004-01-20
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S084000
Reexamination Certificate
active
06680996
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
BACKGROUND OF THE INVENTION
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Such measurements are particularly useful in evaluating layers deposited on semiconductor wafer substrates in the course of integrated circuit manufacture.
X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a detector mounted on a goniometer. More recently, fast X-ray reflectometers have been developed using position-sensitive detectors, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
For example, U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
U.S. Pat. No. 5,740,226, to Komiya et al., describes a method for analyzing X-ray reflectometric data to determine film thickness. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
In order to obtain accurate measurements of film thickness, it is necessary to precisely calibrate the angular scale of the reflection. Such a calibration requires, inter alia, exact control of the zero angle of reflection, so that the angle of the reflected beam relative to the surface can be determined accurately. (In the context of the present patent application and in the claims, the term “zero angle” refers to the orientation of a tangent to the reflecting surface at the point of incidence of the radiation.) To make reflectometric measurements with optimal accuracy, the zero angle at the measurement point should be known to within 0.005°.
Although semiconductor wafers appear to be flat, in practice wafers typically deform slightly when held by a vacuum chuck during production or inspection. The deformation is due both to the vacuum force exerted by the chuck and to the weight of the wafer itself. Furthermore, the chuck may have imperfections, such as a slight bend in its axis, that cause deviations in the zero angle of the wafer as it rotates. As a result, inclination of the surface at different sample points on the surface of a wafer may vary by as much as 0.1-0.2°. Therefore, to perform accurate reflectometric measurements at a well-defined measurement point, it becomes necessary to recalibrate the zero angle at each point that is tested on the wafer surface.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved methods and systems for reflectometry.
It is a further object of some aspects of the present invention to provide methods and devices that enable rapid, accurate determination of the zero angle of a surface under reflectometric inspection.
In preferred embodiments of the present invention, the zero angle of a surface under inspection is calibrated by measuring reflections of X-ray beams from the surface at two different, known wavelengths, &lgr;
1
and &lgr;
2
. The beams are aligned so as to impinge upon the surface at the same point and along substantially the same direction. Each of the beams generates a reflectometric fringe pattern, which allows the critical angle for total external reflection from the surface to be observed at each of the two wavelengths. Even when the precise zero angle of the surface is not known, the difference between the critical angles at the two different wavelengths can be measured with high precision.
In accordance with known physical principles, the critical angle at X-ray wavelengths is equal to a constant, k, which depends on the density of the reflecting surface layer, multiplied by the wavelength itself. The precise measurement of the difference in the critical angles at the two different measurement wavelengths can thus be used to accurately calculate k with respect to the surface under inspection. Once k is known, the zero angle of the surface at the measurement point is calibrated simply by subtracting k&lgr; from the observed critical angle at either of the known wavelengths. The pattern of reflected fringes at either or both of &lgr;
1
and &lgr;
2
can then be analyzed to accurately determine local surface properties including thickness, density and roughness of think film layers on the surface.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for testing a surface, including:
finding respective first and second critical angles for total external reflection of radiation from an area of the surface at first and second wavelengths; and
comparing the first and second critical angles to determine an orientation of a tangent to the surface in the area.
Preferably, comparing the first and second critical angles includes taking an angular difference between the first and second critical angles, and calculating, based on the angular difference, a property of the surface for use in determining the orientation of the tangent. Most preferably, calculating the property includes finding a constant k such that the angular difference between the first and second critical angles is substantially equal to |k(&lgr;
2
-&lgr;
1
)|, wherein &lgr;
1
and &lgr;
2
are the first and second wavelengths, respectively, and setting k&lgr;
1
, equal to the first critical angle so as to find the orientation of the tangent.
Preferably, finding the first and second critical angles includes irradiating the surface with first and second beams of the radiation at the first and second wavelengths, respectively, wherein the first and second beams both impinge on the surface in the area along substantially the same direction. Further preferably, finding the first and second critical angles includes detecting the radiation reflected from the surface using a common detector for the first and second beams. Most preferably, detecting the radiation includes detecting the radiation at the second wavelength while preventing the first beam from impinging on the surface. Alternatively, since the first and second beams have respective first and second photon e
Gvirtzman Amos
Mazor Isaac
Yokhin Boris
Church Craig E.
Jordan Valley Applied Radiation Ltd.
Kiknadze Irakli
Weingarten Schurgin, Gagnebin & Lebovici LLP
LandOfFree
Dual-wavelength X-ray reflectometry does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dual-wavelength X-ray reflectometry, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dual-wavelength X-ray reflectometry will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3250251