X-ray array detector

Details X-ray array detector X-ray array detector

1999-09-29

2002-05-14

Dunn, Drew (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Diffraction, reflection, or scattering analysis

C378S050000, C250S208100, C250S370090, C250S370100

Reexamination Certificate

active

06389102

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. Conventional 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 position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD). A method for performing the analysis to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference.
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.
Various types of position-sensitive X-ray detectors are known in the art of reflectometry. Solid-state arrays typically comprise multiple detector elements, which are read out by a CCD or other scanning mechanism. Each element accumulates photoelectric charge over a period of time before being read out and therefore cannot resolve the energy or number of incident X-ray photons. XRR using such arrays simply records the total integrated radiation flux that is incident on each element. Energy discrimination can be achieved only if an additional monochromator is used between the sample and the detector array, but this configuration results in signal throughput that is too low for practical applications.
Proportional counters are a type of gas-based, position-sensitive, X-ray detectors that do provide some energy resolution, typically about 20% (1200 eV for a 6 keV line). Such counters, however, are capable of processing only one photon at a time, leading to very slow analysis speed. Their energy resolution is inadequate for many applications.
Another common method of X-ray reflectometric measurement is described, for example, in an article by Chihab et al., entitled “New Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,” in
Journal of Applied Crystallography
22 (1989), p. 460, which is incorporated herein by reference. A narrow beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface in order to cut off the primary X-ray beam, so that only reflected X-rays reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector.
X-ray reflectometry has been combined with measurements of X-ray fluorescence (XRF) to provide additional information on the composition of thin film layers. For example, an article by Lengeler, entitled “X-ray Reflection, a New Tool for Investigating Layered Structures and Interfaces,” in
Advances in X
-
ray Analysis
35 (1992), p. 127, which is incorporated herein by reference, describes a system for measurement of grazing-incidence X-ray reflection, in which X-ray fluorescence is also measured. A sample is irradiated by an X-ray source at grazing incidence. One X-ray detector captures X-rays reflected (likewise at grazing incidence) from the surface of the sample, while another detector above the sample captures X-ray fluorescence emitted by the sample due to excitation by the X-ray source. Analysis of the fluorescence emitted when the sample is excited at an angle below the critical angle for total external reflection of the incident X-rays, as described in this article, is known in the art as total reflection X-ray fluorescence (TXRF) analysis.
A related technique is described in an article by Leenaers et al., entitled “Applications of Glancing Incidence X-ray Analysis,” in
X
-
ray Spectrometry
26 (1997), p. 115, which is incorporated herein by reference. The authors describe a method of glancing incidence X-ray analysis (GIXA), combining X-ray reflectivity and angle-dependent X-ray fluorescence measurements to obtain a structural and chemical analysis of a sample.
An alternative method for determining the thickness and composition of thin film layers is described in an article by Wiener et al., entitled “Characterization of Titanium Nitride Layers by Grazing-Emission X-ray Fluorescence Spectrometry,” in
Applied Surface Science
125 (1998), p. 129, which is incorporated herein by reference. This article describes a technique whereby a sample is irradiated by an X-ray source at normal or near-normal incidence, and fluorescent X-ray photons emitted by the sample are collected at a grazing angle, close to the surface. The spectrum of the collected photons is analyzed by a technique of wavelength dispersion, as is known in the art, and the distribution of photons by emission angle is determined, as well. The resultant data provide information about the thickness and composition of thin film layers on the sample.
Energy dispersion techniques can also be used to analyze the spectral distribution of reflected photons, as described, for example, in a paper by Windover et al., entitled “Thin Film Density Determination by Multiple Radiation Energy Dispersive X-ray Reflectivity,” presented at the 47th Annual Denver X-ray Conference (August 1998), which is incorporated herein by reference.
X-ray detector arrays with a dedicated processing circuit for each detector have been developed for use in imaging systems based on synchrotron radiation. Such arrays are described by Arfelli et al., in articles entitled “New Developments in the Field of Silicon Detectors for Digital Radiography,” in
Nuclear Instruments and Methods in Physics Research A
377 (1996), p. 508, and “Design and Evaluation of AC-Coupled FOXFET-Biased, ‘Edge-on’ Silicon Strip Detectors for X-ray Imaging,” in
Nuclear Instruments and Methods in Physics Research A
385 (1997), p. 311, which are incorporated herein by reference. The detectors in the array are read by a VLSI CMOS circuit for multichannel counting, including a preamplifier, shaper, buffer, discriminator and counter for each channel. The detector array chip is connected to the VLSI inputs by wire bonding, although the authors state that a future redesign may make it possible to mount the front-end circuits directly on the detector chip itself.
SUMMARY OF THE INVENTION
It is an object of the present invention to provi

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