U.S. patent application number 09/409046 was filed with the patent office on 2002-01-03 for x-ray array detector.
Invention is credited to DOVRAT, AMI, GVIRTZMAN, AMOS, MAZOR, ISAAC, YOKHIN, BORIS.
Application Number | 20020001365 09/409046 |
Document ID | / |
Family ID | 23618838 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001365 |
Kind Code |
A1 |
MAZOR, ISAAC ; et
al. |
January 3, 2002 |
X-RAY ARRAY DETECTOR
Abstract
Apparatus for X-ray analysis of a sample includes an X-ray
source, which irradiates the sample, and an X-ray detector device,
which receives X-rays from the sample responsive to the
irradiation. The device includes an array of radiation-sensitive
detectors, which generate electrical signals responsive to
radiation photons incident thereon. Processing circuitry of the
device includes a plurality of signal processing channels, each
coupled to process the signals from a respective one of the
detectors so as to generate an output dependent upon a rate of
incidence of the photons on the respective detector and upon a
distribution of the energy of the incident photons.
Inventors: |
MAZOR, ISAAC; (HAIFA,
IL) ; GVIRTZMAN, AMOS; (MOSHAV ZIPPORI, IL) ;
YOKHIN, BORIS; (NAZARETH ILLIT, IL) ; DOVRAT,
AMI; (HAIFA, IL) |
Correspondence
Address: |
CHRISTIE PARKER & HALE LLP
P O BOX 7068
PASADENA
CA
911097068
|
Family ID: |
23618838 |
Appl. No.: |
09/409046 |
Filed: |
September 29, 1999 |
Current U.S.
Class: |
378/89 ;
250/208.1; 250/370.09; 250/370.1; 378/50 |
Current CPC
Class: |
G01T 1/247 20130101;
G01N 23/20 20130101 |
Class at
Publication: |
378/89 ; 378/50;
250/370.09; 250/370.1; 250/208.1 |
International
Class: |
G21K 001/06; G01N
023/20 |
Claims
1. Apparatus for X-ray analysis of a sample, comprising: an X-ray
source, which irradiates the sample; and an X-ray detector device,
which receives X-rays from the sample responsive to the
irradiation, the device comprising: an array of radiation-sensitive
detectors, which generate electrical signals responsive to
radiation photons incident thereon; and processing circuitry
comprising a plurality of signal processing channels, each coupled
to process the signals from a respective one of the detectors so as
to generate an output dependent upon a rate of incidence of the
photons on the respective detector and upon a distribution of the
energy of the incident photons.
2. Apparatus according to claim 1, wherein the array of detectors
comprises an array of radiation-sensitive diodes.
3. Apparatus according to claim 2, wherein the diodes comprise
silicon diode detectors.
4. Apparatus according to claim 1, wherein each of the plurality of
signal processing channels comprises an integrated circuit disposed
on a common substrate with the respective detector.
5. Apparatus according to claim 4, wherein the common substrate
comprises a semiconductor chip including integrated circuits
belonging to a multiplicity of the signal processing channels.
6. Apparatus according to claim 1, wherein the signal processing
channels process the signals in accordance with adjustable
processing parameters.
7. Apparatus according to claim 6, wherein the processing
parameters are adjusted independently for different ones of the
channels responsive to different incidence rates of the photons at
the respective detectors.
8. Apparatus according to claim 1, wherein the signal processing
channels comprise discriminators, which reject signals
corresponding to photons outside a predetermined energy range.
9. Apparatus according to claim 8, wherein the processing circuitry
comprises a threshold control circuit, which adjusts the
predetermined energy range of the discriminators.
10. Apparatus according to claim 1, wherein the signal processing
channels comprise counters, which count the number of photons
incident on the respective detectors responsive to the energy of
the photons, and wherein the processing circuitry comprises a bus
common to a multiplicity of the channels, which receives and
outputs respective photon counts from the channels in turn.
11. Apparatus according to claim 1, wherein the X-ray detector
device receives X-rays reflected from the sample.
12. Apparatus according to claim 1, wherein the X-ray detector
device receives fluorescent X-rays emitted by the sample.
13. Apparatus according to claim 1, wherein the X-ray source
comprises a monochromator, such that the sample is irradiated with
substantially monochromatic X-rays at a predetermined energy.
14. Apparatus according to claim 13, wherein the signal processing
channels comprise discriminators, which are adjusted to reject
signals corresponding to photons outside an energy range including
the predetermined energy of the monochromatic X-rays.
15. A method for X-ray analysis of a sample, comprising:
irradiating the sample with X-rays; receiving X-rays from the
sample, responsive to the irradiation, at an array of detectors in
respective, predetermined locations, which detectors generate
electrical signals responsive to X-ray photons incident thereon;
and processing the signals from the array of detectors in
respective processing channels, so as to generate an output
indicative of a rate of arrival of the photons incident at the
respective locations and dependent upon a distribution of the
energy of the incident photons.
16. A method according to claim 15, wherein processing the signals
comprises providing a plurality of channels each comprising an
integrated circuit disposed on a common substrate with the
respective detector for processing signals generated by the
detector.
17. A method according to claim 15, wherein processing the signals
comprises processing signals in accordance with processing
parameters, which are independently adjustable for different ones
of the channels.
18. A method according to claim 17, wherein processing the signals
comprises adjusting the processing parameters in the channels
responsive to an incidence rate of the photons on the
detectors.
19. A method according to claim 15, wherein processing the signals
comprises discriminating signal levels so as to reject signals
corresponding to photons outside a predetermined energy range.
20. A method according to claim 19, wherein processing the signals
comprises counting the number of photons incident at each of the
locations within the predetermined energy range.
21. A method according to claim 20, wherein irradiating the sample
comprises irradiating the sample with substantially monochromatic
X-rays at a selected energy, and wherein discriminating the signal
levels comprises rejecting signals corresponding to photons outside
an energy range including the selected energy of the monochromatic
X-rays.
22. A method according to claim 15, wherein receiving the X-rays
comprises receiving X-rays reflected from the sample.
23. A method according to claim 15, wherein receiving the X-rays
comprises receiving fluorescent X-rays emitted by the sample.
24. Radiation detection apparatus, comprising: an array of
radiation-sensitive detectors, which generate electrical signals
responsive to radiation photons incident thereon; and processing
circuitry comprising: a plurality of signal processing channels,
each channel coupled to process the signals from a respective one
of the detectors and comprising a counter, which counts the number
of photons incident on the respective detector; and a bus common to
a multiplicity of the channels, which receives and outputs
respective photon counts from the channels in turn.
25. Apparatus according to claim 24, wherein the signal processing
channels comprise discriminators, which reject signals
corresponding to photons outside a predetermined energy range, so
that the counters count only photons within the predetermined
energy range.
26. Apparatus according to claim 25, wherein the processing
circuitry comprises a threshold control circuit, which adjusts the
predetermined energy range of the discriminators.
27. Apparatus according to claim 26, wherein the array of detectors
comprises an array of radiation-sensitive diodes.
28. Apparatus according to claim 28, wherein the diodes comprise
silicon diode detectors.
29. Apparatus according to claim 26, wherein each of the plurality
of signal processing channels comprises an integrated circuit
disposed on a common substrate with the respective detector.
30. Apparatus according to claim 39, wherein the common substrate
comprises a semiconductor chip including integrated circuits
belonging to a multiplicity of the signal processing channels.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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
[0013] It is an object of the present invention to provide improved
methods and apparatus for position-sensitive X-ray detection.
[0014] It is a further object of some aspects of the present
invention to provide improved methods and apparatus for
energy-resolved X-ray analysis of a sample, and particularly for
X-ray reflectometric analysis.
[0015] In preferred embodiments of the preferred embodiment, X-ray
detection apparatus comprises an array of X-ray sensitive
detectors, coupled to respective signal processing channels.
Preferably, the detectors comprise photodiodes, as are known in the
art, which are disposed in a linear or matrix (two-dimensional)
configuration. The processing channels comprise integrated
circuits, which are formed or mounted on a common substrate
together with the respective detectors, so that each channel is
coupled to its respective detector as an integral unit. Most
preferably, all of these units are formed together on a single
integrated circuit chip, but alternatively, the apparatus may be
made up of a number of separate components, mounted on a hybrid,
chip carrier or other printed circuit.
[0016] When an X-ray photon strikes one of the detectors, an
electrical pulse is generated, having an amplitude indicative of
the energy of the incident photon. The pulse is processed by the
respective channel in order to determine the energy of the photon,
as is known generally in the art of energy-dispersive X-ray signal
processing. Each of the channels generates an output dependent on
the rate of incidence of X-ray photons on the respective detector
and the distribution of the energy of the incident photons. The
sensitivity of the channels is automatically or manually
controlled, typically based on adjustment of the time constant and
gain of a pulse-shaping filter in each channel. Optionally, the
sensitivity in each channel is controlled separately so as to
increase the sensitivity of channels in which there is a relatively
low rate of incident photons, while the sensitivity of channels
having high incidence rates is reduced in order to allow high pulse
throughput.
[0017] The array with parallel processing of the individual channel
signals allows position-sensitive, energy-dependent X-ray photon
counting to be performed with extremely high efficiency, energy
resolution and dynamic range. These qualities cannot be achieved in
detector arrays known in the art of X-ray reflectometry, in which
multiple detectors share a common pulse processing channel, and
only the total or average flux can be measured.
[0018] In some preferred embodiments of the present invention, the
processing channels comprise energy level discriminators, which
eliminate pulses due to photons of energy outside a predetermined
range. The discriminators of all of the channels are preferably
adjustable, either individually or all together, so that only
photons within the predetermined range are counted.
[0019] In one of these preferred embodiments, the array is used to
detect X-ray reflectivity from a sample, which is irradiated by an
X-ray beam at a given, substantially monochromatic energy level.
The discriminators are set to accept only pulses due to reflected
photons, and to reject energy-shifted photons due to scattering and
fluorescent processes. The use of the array thus enables accurate
reflectance measurements to be made with high dynamic range and
high throughput, while obviating the need for filtering or
monochromatization of the beam reflected from the sample.
[0020] There is therefore provided, in accordance with a preferred
embodiment of the present invention, apparatus for X-ray analysis
of a sample, including:
[0021] an X-ray source, which irradiates the sample; and
[0022] an X-ray detector device, which receives X-rays from the
sample responsive to the irradiation, the device including:
[0023] an array of radiation-sensitive detectors, which generate
electrical signals responsive to radiation photons incident
thereon; and
[0024] processing circuitry including a plurality of signal
processing channels, each coupled to process the signals from a
respective one of the detectors so as to generate an output
dependent upon a rate of incidence of the photons on the respective
detector and upon a distribution of the energy of the incident
photons.
[0025] Preferably, the array of detectors includes an array of
radiation-sensitive diodes, most preferably silicon diode
detectors.
[0026] Further preferably, each of the plurality of signal
processing channels includes an integrated circuit disposed on a
common substrate with the respective detector. Most preferably, the
common substrate includes a semiconductor chip including integrated
circuits belonging to a multiplicity of the signal processing
channels.
[0027] In a preferred embodiment, the signal processing channels
process the signals in accordance with adjustable processing
parameters, which are optionally individually adjusted responsive
to different incidence rates of the photons at the respective
detectors.
[0028] Preferably, the signal processing channels include
discriminators, which reject signals corresponding to photons
outside a predetermined energy range, wherein the processing
circuitry includes a threshold control circuit, which adjusts the
predetermined energy range of the discriminators.
[0029] Preferably, the signal processing channels include counters,
which count the number of photons incident on the respective
detectors responsive to the energy of the photons, and the
processing circuitry includes a bus common to a multiplicity of the
channels, which receives and outputs respective photon counts from
the channels in turn.
[0030] In a preferred embodiment, the X-ray detector device
receives X-rays reflected from the sample or, alternatively or
additionally, fluorescent X-rays emitted by the sample. Preferably,
the X-ray source includes a monochromator, such that the sample is
irradiated with substantially monochromatic X-rays at a
predetermined energy. Most preferably, the signal processing
channels include discriminators, which are adjusted to reject
signals corresponding to photons outside an energy range including
the predetermined energy of the monochromatic X-rays.
[0031] There is also provided, in accordance with a preferred
embodiment of the present invention, a method for X-ray analysis of
a sample, including:
[0032] irradiating the sample with X-rays;
[0033] receiving X-rays from the sample, responsive to the
irradiation, at an array of detectors in respective, predetermined
locations, which detectors generate electrical signals responsive
to X-ray photons incident thereon; and
[0034] processing the signals from the array of detectors in
respective processing channels, so as to generate an output
indicative of a rate of arrival of the photons incident at the
respective locations and dependent upon a distribution of the
energy of the incident photons.
[0035] There is additionally provided, in accordance with a
preferred embodiment of the present invention, radiation detection
apparatus including:
[0036] an array of radiation-sensitive detectors, which generate
electrical signals responsive to radiation photons incident
thereon; and
[0037] processing circuitry including:
[0038] a plurality of signal processing channels, each channel
coupled to process the signals from a respective one of the
detectors and including a counter, which counts the number of
photons incident on the respective detector; and
[0039] a bus common to a multiplicity of the channels, which
receives and outputs respective photon counts from the channels in
turn.
[0040] The present invention will be more fully understood from the
following detailed description of the preferred embodiments
thereof, taken together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a schematic illustration of a system for X-ray
reflectometry, in accordance with a preferred embodiment of the
present invention;
[0042] FIG. 2 is a schematic block diagram illustrating X-ray
detection apparatus used in the system of FIG. 1, in accordance
with a preferred embodiment of the present invention; and
[0043] FIG. 3 is a schematic block diagram illustrating a signal
processing channel in the apparatus of FIG. 2, in accordance with a
preferred embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0044] FIG. 1 is a schematic illustration of a system 20 for X-ray
reflectometry of a sample 22, in accordance with a preferred
embodiment of the present invention. An X-ray source 24, typically
an X-ray tube, irradiates a small area 28 on sample 22 via a
focusing monochromator 26. Most preferably, monochromator 26
comprises a Kirkpatrick-Baez type device, available from Osmic
Inc., of Troy, Mich., or an X-ray Doubly-bent Focusing Crystal
Optic, manufactured by XOS (X-ray optical Systems), Inc., of
Albany, N.Y. Such monochromators are described in greater detail in
a patent application entitled "X-ray Microanalysis of Thin Films,"
filed on even date, which is assigned to the assignee of the
present patent application and whose disclosure is incorporated
herein by reference. Alternatively, any other suitable
monochromator may be used, such as those described in the
above-mentioned U.S. Pat. No. 5,619,548 and 5,923,720, as may the
knife-edge arrangement described in the above-mentioned article by
Chihab et al. A typical irradiation energy for reflectometric
measurements in system 20 is about 5.4 keV.
[0045] X-rays reflected by sample 22 are collected by an array 30
of detectors 32. The detectors are coupled to processing circuitry
34, comprising a plurality of processing channels 36, each of which
receives signals from a corresponding detector 32. Although for the
sake of simplicity of illustration, only a single row of detectors
32 is shown in FIG. 1, with a relatively small number of detectors,
in preferred embodiments of the present invention, array 30
generally includes a greater number of elements, arranged in either
a linear or a matrix (two-dimensional) array, with a corresponding
array of processing channels 36, as described further hereinbelow.
Output signals from channels 36, preferably in digital form, are
transferred to a processing and analysis block 38, typically
comprising a general-purpose computer, suitably programmed, which
is coupled to a display 40 and/or other output device.
[0046] Block 38 analyzes the outputs of channels 36, preferably so
as to determine a distribution 42 of the flux of photons reflected
from sample 22 as a function of angle at a given energy or over a
range of energies. As described further hereinbelow,
energy-dispersive processing by channels 36 obviates the need for
an additional monochromator between sample 22 and detector array
30, since energy-selectivity is provided in the signal processing.
When sample 22 has one or more thin surface layers, such as thin
films, at area 28, distribution 42 typically exhibits a periodic
structure due to interference effects among reflected X-ray waves
from interfaces between the layers. Characteristics of the periodic
structure are preferably analyzed by block 38 in order to determine
the thickness, density and surface quality of one or more of the
surface layers, using methods of analysis described, for example,
in the above-mentioned U.S. Pat. No. 5,619,548 and 5,740,226, or as
is otherwise known in the art.
[0047] Although in the preferred embodiment shown in FIG. 1, system
20, including array 30 and accompanying circuitry 34, is described
with reference to X-ray reflectometry, it will be appreciated that
the system may similarly be used, mutatis mutandis, in other fields
of X-ray analysis. Possible fields of application include X-ray
fluorescence (XRF) analysis, including particularly grazing
emission XRF, as well as other XRF techniques known in the art, as
described in the Background of the Invention. Furthermore, the
principles of system 20 may be implemented in position-sensitive
detection systems for other energy ranges, such as for detection of
gamma rays and other nuclear radiation.
[0048] FIG. 2 is a block diagram that schematically illustrates
detector array 30 and processing circuitry 34, in accordance with a
preferred embodiment of the present invention. Detectors 32
preferably comprise silicon PIN diodes, having a depletion
thickness of at least 20 .mu.m. Such detectors have the advantages
of being low in cost and integrable with circuitry 34 on a common
silicon substrate. Alternatively, any other suitable type of
detectors known in the art may be used, for example, CdZnTe
detectors, which are preferably wire-bonded to one or more silicon
chips comprising the corresponding processing channels 36.
Optionally, array 30 and circuitry 34 are cooled, preferably by a
thermoelectric cooler, to improve their signal/noise performance.
Details of channels 36 are described hereinbelow with reference to
FIG. 3.
[0049] Array 30 most preferably comprises 512 detectors 32 disposed
along a linear axis of the array, having an axial dimension of
approximately 30 .mu.m and a transverse dimension of 6-12 mm. Such
dimensions give the array an active area of about 15.times.6 mm up
to about 15.times.12 mm. The narrow axial spacing of the detectors
enhances the angular resolution that can be achieved in
measurements using array 30, while the broad transverse dimension
is useful in maximizing the sensitivity of detection, thus
increasing the XRR measurement throughput of system 20. It will be
understood, however, that these dimensions and numbers of detectors
are cited here by way of example, and detectors of any suitable
type, dimension and number can be used.
[0050] In place of the linear array shown in FIG. 2, detectors 32
may alternatively be disposed in a two-dimensional matrix array.
Such an array has the advantage of providing two-dimensional
angular resolution if desired. If two-dimensional resolution is not
needed, signal outputs may be summed over the pixels in each of the
rows of the array. The relatively small pixel size in this
configuration has at least two potential benefits: (1) saturation
at angles with high X-ray flux is avoided; and (2) the capacitance
of the detectors is reduced, which may lead to a reduction in the
overall detection noise.
[0051] Further alternatively, a mask may be placed over linear
array 30 to limit the active area of detectors 32 that is exposed
to X-rays. For example, if fine angular resolution is desired in
the transverse direction, as well as in the axial direction, the
active areas of detectors 32 may be masked so as to reduce the
transverse dimensions of the areas exposed to the X-rays. The mask
may be moved transversely and signals captured at multiple
locations if desired, to capture X-rays at different transverse
angular positions. Alternatively, a mask made up of a row of narrow
slits, each slit corresponding to one of detectors 32, may be
translated axially over the array to enhance the detection
resolution in the axial direction. Further alternatively, if there
is a substantial variation in the X-ray flux incident on array 30
as a function of angle in the axial direction (as commonly occurs
in XRR measurement), the mask may have a graduated transverse
dimension, so that detectors 32 in the high-flux region have a
smaller active area exposed to the X-rays than those in the
low-flux region. This configuration reduces the likelihood of
saturation in the high-flux region and effectively increases the
dynamic range of the array.
[0052] FIG. 3 is a block diagram that schematically illustrates one
of processing channels 36, in accordance with a preferred
embodiment of the present invention. Signals output by
corresponding detector 32 are first amplified by a charge-sensitive
preamplifier 70, typically a low-noise FET amplifier. A
pulse-shaping filter 72 smooths and shapes the signals output by
preamplifier 70, so as to generate a pulse having an amplitude
indicative of the energy of the incident photon. Preferably, a gain
and shaping control circuit 73 (not shown in FIG. 2 for the sake of
simplicity of illustration) provides appropriate control inputs to
preamplifier 70 and filter 72.
[0053] Preferably, the degree of smoothing applied by filter 72 is
adjusted based on the pulse rate encountered the detectors, i.e.,
responsive to the flux of X-ray photons incident on array 30. The
adjustment is used to increase the sensitivity of channels in which
there is a relatively low rate of incident photons, while the
sensitivity of channels having high incidence rates is reduced in
order to allow high pulse counting throughput. Typically, the
sensitivity is set so that channel 36 can accommodate at least
1.5.times.10.sup.5 pulses/sec, as determined by the pulse shaping
time of the channel. Optionally, the sensitivity of each channel or
of a group of channels is individually adjustable. Appropriate
choices of components and design parameters for channel 36 will be
clear to those skilled in the art, based on the use of similar
components and designs in conventional energy-dispersive processing
systems.
[0054] A level discriminator 74 is preferably applied to the output
of pulse shaper 72 in order to select a range of energies to be
passed to a n-bit counter circuit 76. Preferably, each of counter
circuits 76 is capable of integrating up to 10.sup.8 photon counts,
dependent on the width of a bus 60 through which the counts are
read out and on the integration time between successive readouts.
The range of discriminator 74 is selected by an energy threshold
control 52, so that only photons in the selected energy range are
chosen. Preferably, a common energy range is chosen for all of
channels 36, with an energy passband no more than about 0.3 keV
wide. In addition to rejecting photons outside the chosen passband,
the upper limit set on discriminator 74 also eliminates spurious
signals due to pulse pile-up, i.e., high-amplitude signals
generated when two photons arrive at almost the same time.
[0055] The energy discrimination afforded by array 30 and circuitry
34 is particularly useful in determining the angular distribution
of X-rays reflected from sample 22. It allows the reflected X-ray
photons (which have the same, substantially monochromatic energy as
the incident photons from source 24) to be distinguished from
photons whose wavelength is shifted due to fluorescent emission and
scattering processes. There is no need for an additional
monochromator between sample 22 and detector array 30. This energy
discrimination capability can likewise be used in distinguishing
particular X-ray fluorescence lines or scattering transitions.
[0056] Alternatively, different energies are chosen for level
discriminators 74 in different channels 36. Further alternatively
or additionally, the energy thresholds are swept over a number of
different energy levels of interest. Moreover, although channel 36
is shown in FIG. 3 as including only a single discriminator 74 and
counter 76, in alternative embodiments of the present invention,
the channels may include multiple, parallel counters, each with its
own level discriminator. In such embodiments, the parallel counters
count the number of X-ray photons incident on the corresponding
detector 32 at a number of different energy levels
simultaneously.
[0057] Returning now to FIG. 2, it is observed that certain
functions are performed collectively for the entire array 30 of
detectors 32 and corresponding processing channels 36. A
high-voltage bias circuit 50 provides a bias voltage common to all
of the detectors. Threshold control circuitry 52 preferably sets
the energy level discrimination range for all of the channels
(although as noted hereinabove, it is also possible to set
different ranges for different channels). N-bit count outputs of
counters 76 are output to common bus 60, for sequential transfer to
processing and analysis block 38, under the control of a bus
controller 54. The bus controller reads out the counts from each of
channels 36 in turn, in accordance with signals provided by a chip
reset and control circuit 56 and with address selection by a
counter address bus circuit 58. The bus addressing may read
channels 36 sequentially or by random access. The design of such
circuits will be clear to those skilled in the art. Optionally,
circuit 58 may be programmed and controlled so as to provide a
relatively longer integration time to channels in which the photon
flux is relatively low.
[0058] Each detector 32 and the corresponding channel 36 make up a
channel unit 48, which is preferably integrated on a single
substrate. Most preferably, all of units 48, i.e., all of the
detectors in array 30 and the processing channels in circuitry 34,
are produced together on a single, custom integrated circuit chip
62 on a silicon substrate. Control circuits 52, 54, 56 and 58 are
preferably included on chip 62, as well.
[0059] Other modes of integration are also possible, however. For
example, each channel unit 48 may comprise a separate integrated
circuit on a silicon substrate, or alternatively may comprise a
hybrid circuit, with several integrated circuits on a ceramic or
chip carrier substrate. Alternatively, a number of units 48
together may be contained in a single custom integrated circuit or
hybrid. These integrated channel units 48 are then combined in a
hybrid or multi-layer sandwich arrangement, or alternatively on a
printed circuit board, to make up the entire array 30 together with
circuitry 34. Those skilled in the art will be able to devise other
means for integrating the multiple channels of array 30 and
circuitry 34, all of which means are considered to be within the
scope of the present invention.
[0060] It will thus be appreciated that the preferred embodiments
described above are cited by way of example, and the full scope of
the invention is limited only by the claims.
* * * * *