Title: Imaging X-ray detector based on direct conversion
Abstract: An X-ray detector (401, 501, 601) has a detecting element that comprises a semiconductor heterostructure where an undoped Germanium layer (402, 502) is enclosed between two oppositely doped Gallium Arsenide layers (403, 404, 503, 505).
Patent Number: 6,933,503 Issued on 08/23/2005 to
| Inventors:
|
Sipilä; Heikki Johannes (Espoo, FI);
Bourgoin; Jacques (Avon, FR)
|
| Assignee:
|
Oxford Instruments Analytical Oy (Espoo, FI)
|
| Appl. No.:
|
458910 |
| Filed:
|
June 11, 2003 |
Foreign Application Priority Data
| Current U.S. Class: |
250/370.09; 250/370.11 |
| Intern'l Class: |
G01T 001/24 |
| Field of Search: |
250/37009,370.11,370.14,367,363.02,385.11
257/290,200,201,12,627
|
References Cited [Referenced By]
U.S. Patent Documents
| 5001536 | Mar., 1991 | Fukuzawa et al.
| |
| 5512756 | Apr., 1996 | Bayer et al.
| |
| 5596200 | Jan., 1997 | Sharma et al.
| |
| 5981986 | Nov., 1999 | Tsuchiya.
| |
| 6248990 | Jun., 2001 | Pyyhtiā et al.
| |
| 6403965 | Jun., 2002 | Ikeda et al.
| |
| 2005/0040445 | Feb., 2005 | Mouli.
| |
Primary Examiner: Gabor; Otilia
Attorney, Agent or Firm: Wood, Phillips, Katz, Clark & Mortimer
Claims
1. An X-ray detector comprising, as a detecting element, a semiconductor heterostructure
where an undoped Germanium layer is enclosed between two oppositely doped Gallium
Arsenide layers.
2. An X-ray detector according to claim 1, wherein the thickness of the Germanium
layer is between 200 micrometers and 2 millimeters, and the thickness of each of
the Gallium Arsenide layers is between 1 and 5 micrometers.
3. An X-ray detector according to claim 1, comprising a multitude of pixels for
detecting a spatial distribution of X-rays incident on the X-ray detector.
4. An X-ray detector according to claim 3, wherein:
the pixels comprise discrete pieces of the Gallium Arsenide layer on one side
of the Germanium layer, with physical separation between adjacent pixels, and
the Gallium Arsenide layer on that side of the Germanium layer that does not
comprise pixels is even.
5. An X-ray detector according to claim 3, wherein each of the Gallium Arsenide
layers comprises a number of stripes, the stripes of one Gallium Arsenide layer
having a general orientation that is different than a general orientation of the
stripes of the other Gallium Arsenide layer, so that intersection regions between
stripes of different Gallium Arsenide layers constitute the pixels.
6. An X-ray detector according to claim 1, being arranged to be used in room temperature.
7. An X-ray imaging arrangement, comprising:
a solid-state semiconductor detector for detecting X-rays and for converting
a detected intensity of X-rays into a digital value, and
within the solid-state semiconductor detector a semiconductor heterostructure
where an undoped Germanium layer is enclosed between two oppositely doped Gallium
Arsenide layers.
8. An X-ray imaging arrangement according to claim 7, wherein:
the X-ray imaging arrangement comprises a controllable X-ray source for controllably
illuminating an object under study with X-rays, and
the solid-state semiconductor detector is arranged to detect X-rays that have
passed through the object under study.
9. An X-ray imaging arrangement according to claim 7, wherein the solid-state
semiconductor detector is arranged to detect X-rays that come from distant, originally
unspecified sources.
10. An X-ray imaging arrangement according to claim 7, wherein:
the solid-state semiconductor detector comprises a multitude of pixels for detecting
a spatial distribution of X-rays incident on the solid-state semiconductor detector
the solid-state semiconductor detector is arranged to convert a detected intensity
of X-rays pixel-wise into a multitude of digital values and
the X-ray imaging arrangement is arranged to store such a multitude of digital
values in the form of a digital image.
11. An X-ray imaging arrangement according to claim 7, additionally comprising
a cooling arrangement for cooling the solid-state semiconductor detector in order
to reduce interference caused by thermally excited electrons in the Germanium layer.
12. An X-ray imaging arrangement according to claim 11, wherein the solid-state
semiconductor detector is arranged to convert also a detected energy of an incident
X-ray photon into a digital value, and the X-ray imaging arrangement is arranged
to produce a spectroscopic representation of a number of detected X-ray photons.
13. An X-ray imaging arrangement according to claim 7, comprising a signal processing
circuit bonded onto one of said Gallium Arsenide layers.
14. An X-ray imaging arrangement according to claim 7, comprising a signal processing
circuit processed directly onto one of said Gallium Arsenide layers.
15. A method for producing an X-ray detector, comprising the steps of:
producing a first doped Gallium Arsenide layer on a first side of an undoped
Germanium plate, and
producing a second doped Gallium Arsenide layer on a second side of the undoped
Germanium plate, which second doped Gallium Arsenide layer is differently doped
than the first doped Gallium Arsenide layer.
16. A method according to claim 15, comprising the steps of:
epitaxially growing a first Gallium Arsenide layer on a first side of an undoped
Germanium plate,
implanting said first Gallium Arsenide layer with a dopant of a first type, said
first type being either donor or acceptor,
epitaxially growing a second Gallium Arsenide layer on a second side of said
undoped Germanium plate, and
implanting said second Gallium Arsenide layer with a dopant of a second type,
which is opposite to the type of the dopant implanted into said first Gallium Arsenide
layer.
17. A method according to claim 16, comprising the steps of:
depositing a photoresist layer on one of the Gallium Arsenide layers,
exposing said photoresist through a mask that constitutes an array of pixels,
and developing the exposed photoresist so that photoresist remains on an array
of pixels on said one of the Gallium Arsenide layers, and
etching away Gallium Arsenide from between said pixels so that physical separations
appear between adjacent pixels.
18. A method according to claim 15, comprising the step of bonding a signal processing
circuit onto one of said Gallium Arsenide layers.
19. A method according to claim 15, comprising the step of processing a signal
processing circuit directly onto one of said Gallium Arsenide layers.
Description
TECHNICAL FIELD
The invention concerns generally the technology of producing digital images of
objects illuminated with X-rays. Especially the invention concerns the composition,
structure and manufacturing method of a detector that is used for producing a digital
image of the illuminated object.
BACKGROUND OF THE INVENTION
FIG. 1 illustrates schematically a conventional way of producing an X-ray image
in electronic form. An X-ray tube 101 emits X-rays that propagate through
an object under study (such as a part of a human body) 102 and enter an
image intensifier 103. Optical equipment 104 convey the output signal
of the image intensifier 103 onto a CCD array 105, from which the
two-dimensional image is read and stored into a memory 106 and/or shown
on a display 107. The drawbacks of the conventional approach arise from
the large number of different devices and components that take part in converting
the spatial distribution of differently attenuated x-rays into an actual image.
Taking the signal through a variety of devices and components reduces the efficiency
of collecting information from the object under study and introduces noise.
FIG. 2 illustrates a more recently adopted approach where detected X-rays are
converted into an image through fewer intermediate phases. The whole arrangement
of image intensifier, optics and CCD is now replaced with a single digital detector
201 the output signal of which is ready to be taken into a memory 106
and/or a display 107. The structure and operation of the digital detector
201 are thus such that the spatial distribution of X-ray intensity that
hits the detector 201 can be more or less directly converted into an array
of digital pixel values. Truly digital imaging of this kind can make the process
of producing X-ray images in electronic form remarkably simpler and more efficient
compared to the conventional approach of FIG. 1.
There remains the question of how should the digital detector 201 be
constructed. At the priority date of this description the most well-known example
of digital X-ray imaging detectors is the CsI/Si (Cesium Iodide/Silicon) detector
of GE Medical Systems Corporation. FIG. 3 is a simplified cross-section of a CsI/Si
detector panel 301. A CsI scintillator layer 302 is arranged to meet
the incoming X-ray photons. Absorption of a photon into the CsI layer 302
causes light to be emitted locally into an amorphous Si panel 303 that is
located immediately next to the CsI layer 302 and constitutes a two-dimensional
photodiode/transistor array. It absorbs the light and converts it into electronic
charge. Each photodiode in the amorphous Si panel 303 represents a pixel
in the image to be generated. An arrangement of low-noise readout electronics 304
behind the amorphous Si panel 303 is employed to collect the accumulated
charge from each photodiode and convert the individual quantities of collected
charge into corresponding digital value. The two-dimensional constellation of digital
values represents an image that can be stored into memory and/or shown on a display.
A critical factor of a detector of the type shown in FIG. 3 is the scattering
of
light between the CsI scintillator layer 302 and the amorphous Si panel
303. The objective of producing sharp images calls for keeping each burst
of scintillation photons confined into an as small spatial sector as possible.
GE Medical Systems Corporation announces having developed a "needle-like" structure
for the CsI layer that should prevent scattering to a large extent. However, producing
such a structure with good yield and highly homogenous gain throughout the detecting
surface may be problematic. Homogeneity of the detector response is of crucial
importance for example in medical imaging applications where far-reaching decisions
are made on the basis of what fundamentally is a detected spatial distribution
of received X-ray intensity. Another disadvantage of the structure of FIG. 3 are
that only approximately one half of the visible light photons can be collected.
Amorphous Si is not known to be a very good photodetector; it is used in this structure
mainly because it allows building a detector with a relatively large area. Siemens
Corporation has a corresponding product on the market with the trade name TRIXELL.
Other known techniques for obtaining digital X-ray images include slot scanning,
where a linear detector is mechanically moved across the illumination beam; using
tiled CCD arrays coupled to a scintillator plate via fiber optics; computed radiography
where electrons are trapped on photostimulated plates that are then exposed to
generate image data; and direct conversion. The last-mentioned has traditionally
meant that two-dimensional Selenium detector panels are used for receiving the
X-ray photons, which get absorbed and give rise to local accumulation of charges
in the bulk of the Se substrate. Readout electronics are then employed to collect
the accumulated charge and to convert the collected charge values into a two-dimensional
image. The drawbacks of the Se-based direct conversion detector arrangements have
been associated with questionable reliability as well as a relatively low DQE (Detective
Quantum Efficiency) values, which cause degradation to image quality and preclude
the use of Se-based direct conversion detectors in advanced applications of X-ray
diagnostics and therapy.
SUMMARY OF THE INVENTION
It is an objective of the present invention to present an X-ray detector that
is suitable for direct digital imaging, has a high efficiency in terms of utilizing
the information carried by X-ray photons incident on the detector, and is suitable
for mass production in respect of production yield, manufacturing cost and reliability
in use. Further objectives of the invention are that the X-ray detector should
have good absorption properties for X-rays and potential for good energy resolution,
as well as small leakage current through the detector.
The objectives of the invention are met by using a high-purity Germanium layer
as a central part of the detector and doped Gallium Arsenide layers on each side
of said Germanium layer.
An X-ray detector according to the invention is characterized by the features
that are recited in the characterizing part of the appended independent patent
claim directed to an X-ray detector.
The invention also applies to an X-ray imaging arrangement that is characterized
by the features that are recited in the characterizing part of the appended independent
patent claim directed to an X-ray imaging arrangement.
Additionally the invention applies to a method for producing an X-ray
detector. The method is characterized by the features that are recited in the characterizing
part of the appended independent patent claim directed to a method.
Direct conversion detectors invariably involve the conversion of the energy
of an incoming photon into a certain spatial distribution of electrical charge.
Controlling and detecting the space charge region with reasonable accuracy requires
the use of a high-quality semiconductor material, the properties of which are homogenous
enough so that detection probability and accuracy is independent of the location
at which the photon happened to hit the detector.
Germanium (or Ge for short) has been widely regarded as an unsuitable material
for direct conversion detector applications at least in room temperature, because
the excitation energy of electrons in Ge is small, only 0.65 eV, and consequently
the leakage current through the material is prohibitively high. Otherwise Ge would
have many advantageous properties. It is relatively straighforward to produce Ge
wafers of extremely high purity and of desired thickness. The absorption cross-section
of Ge is large enough for X-ray photons of the energy range used in typical imaging
applications. For example absorption in Si is far too low, and consequently Si
can be used for detecting only very low energy radiation. Ge is readily available
at reasonable cost and it can advantageously provide very good energy resolution.
For example Cadmium Telluride (CdTe) is very costly and suitable only for detectors
with very small dimensions, with little or no potential for the application of
microelectronic technology thereon.
According to the present invention, a high-purity Ge layer is complemented
with doped Gallium Arsenide (GaAs) layers on each side. A negatively doped GaAs
layer is located on one side of the Ge layer and a positively doped GaAs layer
comes on the other side of the Ge layer. The resulting structure resembles a GaAs
diode in electric operation, which means for example a relatively low leakage current
even in room temperature. One of the GaAs layers is patterned in a suitable way
to give rise to pixels. The Ge layer acts as a photoelectric absorber of the X-ray
photons: each X-ray photon that gets absorbed in the Ge gives rise to a local cloud
of electric charge, which is collected into the pixels and read with suitable integrated
readout electronics. The circuit that takes care of the reading of charge from
the pixels can be bonded onto one surface of the detector, or it may even be processed
directly onto one of the doped GaAs layers.
The Ge layer is typically very thick compared to the GaAs layers: the thickness
of the Ge layer is usually in the range between 0.2 and 2 mm, whereas the thickness
of the GaAs layers is only in micrometer range, like between 1 and 5 micrometers,
although the exact thickness value of the GaAs layers is not that important. The
Ge layer should be thick enough to absorb a large majority of incident X-ray photons.
On the other hand it is not advantageous to make the Ge layer any thicker than
what is required for good absorption, because the thicker the Ge layer is, the
larger is the minimum reasonable pixel size and the more vulnerable the structure
is to interfering effects caused by impurities in the Ge material. In general the
resistivity of the pure Ge material is high, and effects caused by residual impurities
are negligible.
The low excitation energy of Ge gives rise to large statistical fluctuations
in the number of free charge carriers in room temperature. This in turn means that
a detector according to the invention is hardly suitable for spectroscopic analysis
of the incoming X-rays unless the detector is heavily cooled. The resulting lack
of information about quantum energies has no significance in most medical imaging
applications, where essentially monochromatic X-rays are used for illumination
anyway and all useful information resides in the spatial intensity distribution
of X-rays that have propagated through the object under study. Additionally it
is relatively straightforward to built a cooling arrangement to cool down the detector
into a temperature range where thermal excitation starts to loose significance,
in which case the detector according to the invention can also be used in spectroscopical applications.
The advantageousness of combining just GaAs with Ge comes from the near sameness
of certain lattice constants of the two materials. Close lattice constants mean
that producing a nice and regularly grown epitaxial layer of one material on top
of the other in the manufacturing process is easy. Additionally recombination at
the material interface can be made negligible. The high purity of the Ge layer
means that the detection response of the detector can be made extremely homogenous
throughout the detector area.
BRIEF DESCRIPTION OF DRAWINGS
The novel features which are considered as characteristic of the invention are
set forth in particular in the appended claims. The invention itself, however,
both as to its construction and its method of operation, together with additional
objects and advantages thereof, will be best understood from the following description
of specific embodiments when read in connection with the accompanying drawings.
FIG. 1 illustrates a conventional X-ray imaging arrangement,
FIG. 2 illustrates a more advanced known X-ray imaging arrangement,
FIG. 3 is a cross section of a detector used in the arrangement of FIG. 2,
FIG. 4 is a cross section of a detector according to an embodiment of the invention,
FIG. 5 illustrates bonding a readout circuit onto a detector according to an
embodiment of the invention,
FIG. 6 illustrates integrating a readout circuit with a detector according to
an embodiment of the invention,
FIGS. 7
a and 7
b illustrate a method according to an embodiment
of the invention,
FIG. 8 illustrates an X-ray imaging arrangement according to an embodiment of
the invention,
FIG. 9 illustrates an X-ray imaging arrangement according to another embodiment
of the invention,
FIG. 10 illustrates an alternative pixelizing approach and
FIG. 11 illustrates an advantageous way of implementing readout electronics.
DETAILED DESCRIPTION OF THE INVENTION
The exemplary embodiments of the invention presented in this patent application
are not to be interpreted to pose limitations to the applicability of the appended
claims. The verb "to comprise" is used in this patent application as an open limitation
that does not exclude the existence of also unrecited features. The features recited
in depending claims are mutually freely combinable unless otherwise explicitly stated.
FIG. 4 is a schematic cross section through a stack of semiconductor layers
that together constitute the radiation-sensitive parts of an X-ray detector
401.
Basically the detector's semiconductor stack consists of a pure Ge layer
402
sandwiched between two oppositely doped GaAs layers
403 and
404.
"Oppositely doped" means that one of the GaAs layers
403 and
404
is an n-type semiconductor layer and comprises a surplus of negative charge carriers
(electrons), while the other is a p-type semiconductor layer and comprises a surplus
of positive charge carriers (holes). One of a pair of electrodes
405 and
406 is coupled to each of the doped GaAs layers
403 and
404.
The Ge layer
402 is "pure" in the sense that its purity is as high as is
commercially reasonably achievable.
Electrically the structure shown in FIG. 4 operates much like an ordinary
GaAs pn-diode, so that it can for example be biased either in the forward or in
the reverse direction. In order to be useful as an X-ray detector it is biased
in the reverse direction by applying a suitable reverse bias voltage between the
electrodes
405 and
406.
The GaAs layers
403 and
404 are thin enough so that they do not
absorb significant amounts of X-rays incident on the detector, whereas the thickness
of the Ge layer
402 is typically 200 micrometers or more. To be more exact,
the thickness of the Ge layer should be selected depending on the energy of the
X-ray photons that are to be detected. Typical photon energies are in the order
of a few keVs, tens of keVs or even some hundreds of keVs: for example mammographic
imaging applications frequently use energies of 17-25 keV while dental imaging
applications use 50-60 keV and thorax imaging could use 100 keV or more. A sufficient
thickness of the Ge layer is such that it serves to absorb a major portion (more
than 90%) of the incident X-ray photons of interest. It is not recommendable to
make the Ge layer any thicker than is necessary for achieving the absorption objective,
because the thicker the Ge layer is the more its electrical characteristics tend
to be dominated by residual impurities left in the Ge lattice. There is no practical
upper limit for the thickness of the Ge layer, but the thicker it is, the larger
must the pixels be if pixelization is required; more aspects of pixelization are
described later. A sufficient thickness for the Ge layer is in many cases less
than 2 mm, but even thicknesses of several centimeters like in certain neutron
detectors can be realized.
When an X-ray photon hits the detector, it causes a photoelectric effect in
a Ge atom, producing a photoelectron that in turn excites a number of outer electrons
from other atoms from their valence bands into the conduction band. Each excited
electron leaves behind a positive hole. The resulting cloud of free charge is concentrated
within a relatively small spatial area, in the order of some micrometers. The reverse-direction
bias voltage across the detector drives the free charge carriers towards the GaAs
layers. By suitable detection techniques it is possible to detect both the amount
of produced free charge as well as the (two-dimensional) location at which it was
produced. The amount of free charge produced by a single absorbed X-ray photon
is proportional to the incident energy of the photon, and the location where the
free charge appeared reveals the position where the photon hit the Ge layer. If
the detector is at room temperature, the statistical fluctuation of the number
of thermally excited free electrons is so large that it tends to mask the exact
relationship between the amount of photoelectrically induced charge and the energy
of the incident photon, but even in room temperature the spatial information is
preserved to a reasonably large extent. Energy resolution may be possible in a
coarse scale: intrinsic concentration of carriers is in the order of 2·10
13
1/cm, which in room temperature and an exemplary pixel volume of 100×100×100
microns means 2·10
13 electrons and a statistical fluctuation of
4500 electrons, which is equivalent to one incident photon of 20 keV. The interfering
effect of thermal excitation can be reduced by cooling the detector.
FIG. 5 is a cross section through a detector
501 according to an embodiment
of the invention. The body of the detector is a high-purity Ge layer
502.
On one planar surface of the Ge layer
502, which here is the bottom surface,
there has been produced an n-type GaAs layer
503, which is further covered
by an ohmic contact layer
504. The other planar surface of the Ge layer
502, which here is the top surface, carries a p-type GaAs layer that has
been patterned to form a continuous array of distinct p-type GaAs pixels
505.
The dimensions D of each pixel in the planar direction of the plate-like detector
are typically in the order of 50-100 micrometers. An arrangement of ohmic contacts
506 has been produced on top of the pixels
505 in order to facilitate
pixelwise reading of accumulated free charge in the detector
501.
Also very much larger pixels are possible. The limiting case is a non-imaging
detector where a single "pixel" covers the whole detector area. Pixels in the scale
of several millimeters or even centimeters can be used for example in simple "pinhole
camera" applications where X-ray radiation is allowed to enter through a small
aperture so that it hits a detector and produces a rough image of what kind of
radiation sources are located ahead in the shooting direction.
At one location of the detector
501 there has been placed an integrated
circuit
507 the task of which is to implement the readout functions. Bonding
wires
508 couple the integrated circuit
507 to all necessary ohmic
contacts on the surfaces of the detector
501. The integrated circuit
507
is typically an ASIC (Application Specific Integrated Circuit) that has been specifically
designed for this purpose.
FIG. 6 illustrates a detector
601 that is a variation of the detector
501 shown in FIG.
5. The only difference is in the implementation
of the readout circuitry. Instead of bonding a separately manufactured ASIC onto
the detector plate, in FIG. 6 a readout circuit
602 has been processed directly
into the GaAs semiconductor substrate constituted by one of the GaAs layers in
the detector plate. Using one of the GaAs layers in the detector plate as the substrate
for an integrated circuit requires the GaAs material to be relatively heavily doped,
which criterion is met since it is typical also to the GaAs-Ge-GaAs semiconductor
heterostructure used according to the invention that the dopant concentration in
the GaAs layers is high.
Following either one of the principles of FIGS. 5 and 6 (or even both simultaneously)
it is possible to place two or more separate circuit units onto the detector plate,
if necessary.
FIGS. 7
a and
7b together show an exemplary step-by-step
method for manufacturing a detector plate according to an embodiment of the invention.
The method starts at step
701 by obtaining a high-purity Ge wafer of a desired
thickness. In case the planar surfaces of the Ge wafer need polishing or other
preparation before starting the process of depositing the GaAs layers, such preparative
measures can be conceptually included in step
701. In the method of FIGS.
7
a and
7b we assume that the pixelized GaAs layer is the p-type
layer and that it is produced first, so step
702 involves epitaxial growth
of GaAs on that surface of the Ge wafer that is going to be pixelized. At step
703 acceptor ions are implanted into the epitaxially grown GaAs layer to
make it appear as a p-type semiconductor. Typical acceptor implanting may involve
something like 3×10
13 Mg
+ ions per square centimeter
implanted at an energy of 33 keV. Thermal annealing at 850° C. for the duration
of 20 s follows the implanting to allow restoring crystalline defects caused by
the implanting ion beam.
Ion implanting is not the only known way of producing doped semiconductor layers.
It is only mentioned here as en example of how the desired result can be achieved.
For the purposes of present invention the importance of step
703 (and step
723 below) is in the result achieved therethrough: when completed, the GaAs
layer must be suitably doped.
At step
704 a photoresist of the thickness of about 1 micrometer is deposited
on the ion-implanted GaAs layer. The photoresist must be selected so that it allows
chemical etching. At step
705 the photoresist is exposed through a suitable
mask and developed so that photoresist remains on pixels and all other places where
GaAs will be needed, but essentially does not remain on the pixel separator lines.
The pixel separators are etched out chemically over a depth of approximately 1-2
micrometers at step
706 by using for example a solution of NH
4OH:H
2O
2:H
2O
in ratios 1:1:50 and a 20 minutes exposure time at room temperature. At step
707
the residual photoresist that remains on the pixels and other preserved GaAs areas
is removed.
Step
708 involves plasma deposition of a thin (about 40 nanometers) insulator
substance like Si
3N
4 over the entire surface at an elevated
temperature in the order of 300° C. At step
709 another photoresist
layer of about 1 micrometer is deposited, the photoresist now having to sustain
plasma etching. At step
710 the photoresist is exposed and developed, this
time to open over the pixels and other locations where the insulator layer must
be removed. Selective removing of the insulator over the exposed areas through
plasma etching follows at step
711. Exemplary process parameters for the
plasma etching step are 50 W, 0.04 Torr, 50 cm
-;3s
-;1. Plasma
etching shoud stop when the GaAs surface is reached.
At step
712 ohmic contacts are deposited on the exposed GaAs surfaces
of
the pixels by thermal evaporation. A typical composition of an ohmic contact deposited
through thermal evaporation is 10 nanometers of Pt, followed by 30 nanometers of
Ti, another 10 nanometers of Pt and finally 300 nanometers of Au. Removing the
remaining photoresist at step
713 finalizes the manufacturing of the pixelized
p-type GaAs surface.
Preparing the other planar surface of the detector plate, which in this
example is the unpatterned n-type GaAs side, is more straightforward. If the remaining
free surface of the Ge wafer needs polishing for example to exactly determine the
thickness of the Ge layer, it is accomplished at step
721. Step
722
involves epitaxial growth of GaAs on the surface. At step
723 the GaAs layer
is implanted with donor ions and annealed. At step
724 an even back ohmic
contact is deposited, consisting for example of 15 nanometers of Ni, followed by
19.5 nanometers of Ge, 39 nanometers of Au, 50 nanometers of Ti and finally 200
nm of Au. Final thermal annealing at 400° C. for the duration of 40 s under
a H
2 flux, with the detector plate lying on the p-type side, finalizes
the manufacturing process at step
725.
FIG. 8 illustrates an X-ray imaging arrangement according to an embodiment of
the invention. An X-ray tube
801 is used as a source for X-rays in the range
of some tens of keVs. A detector arrangement
802 is arranged to receive
the X-rays that propagated through an object under study. The detector arrangement
802 comprises the pixelized GaAs-Ge-GaAs detector plate as well as readout,
amplification and A/D converter circuitry either bonded to it or processed directly
onto one of the GaAs layers. The detector arrangement
802 is coupled to
a central processing unit
803 for delivering the A/D-converted measurement
results thereto. It would also be possible to place the amplifying and A/D-conversion
circuitry at least partly into the central processing unit
803, but placing
it as near as possible to the location of the actual detection helps to eliminate
noise from the measurement results.
The central processing unit
803 is coupled to a memory
804 for
storing digital images received from the detector arrangement as well as for reading
previously stored digital images from the memory. The central processing unit has
also a user interface that comprises a display
805 for displaying digital
images and a keyboard
806 for receiving key commands from a human user.
In order to control the process of X-ray imaging the central processing unit
803
is coupled to a high voltage source
807 that generates the voltage(s) required
in the X-ray tube
801 and to an operating voltage source
808 that
provides operating voltages to the detector arrangement
802.
The purpose of use of the X-ray imaging arrangement dictates among other things
the physical size of the pixelized area in the detector arrangement
802.
Generally the size of the pixelized area is directly proportional to the size of
the object to be studied. Monolithic detectors are in many cases the most advantageous
in terms of image quality, because with a monolithic detector it is easy to obtain
a highly homogenous response over the whole imaging area. Using a 6-inch or even
an 8-inch Ge wafer as a starting point for building a detector would make is possible
to use a monolithic direct conversion detector in a mammographic X-ray imaging
application. If even larger imaging areas are required, it is possible to use an
even larger monolithic wafer (disc diameters of 300 or even 450 mm have been suggested)
or to tile several monolithic detector arrangements side by side. Careful calibrating
is needed to ensure homogeneity of imaging response if several different detector
arrangements are used. Having readout, amplification and A/D-conversion circuitry
integrated on the same substrate with the actual detector (either by bonding or
by directly processing) allows automatic compensation: it is possible to measure
the response over a detector and to program the integrated electronics so that
they automatically compensate for any possibly detected unhomogeneity in imaging response.
In an ultimately simplified case the detector arrangement
802 would not
need to comprise pixels at all, if at least one of the following conditions is met:
- only the intensity of radiation that passed through the object under
study is of importance, not its spatial distribution,
- the size of the detector arrangement 802 is small compared to
the size of the details in the object under study, so that one image obtained at
a correct location is enough to reveal the information of interest,
- the imaging arrangement comprises means for moving at least one of the
X-ray tube 801 and the detector arrangement 802 in respect of the
object under study, so that a more extensive image can be obtained by scanning.
FIG. 9 illustrates another imaging arrangement that is additionally meant to
be used for X-ray spectroscopy, which means that the arrangement should be able
to detect not only the locations of received X-ray photons on the detector plate
but also their energies. The detector arrangement
901 could be similar to
that used in the imaging arrangement of FIG. 8, but most probably it is advantageous
to use different readout, amplification and A/D-conversion circuitry that is optimized
for spectroscopy. In order to reduce the interfering effect of thermally excited
electrons in the Ge layer the detector arrangement
901 is within the influence
of a cooling arrangement
902, which may include e.g. thermoelectric (Peltier)
cooling and/or cooling through the use of liquified gas. The central processing
unit
903 is arranged to receive both spatial and spectroscopic information
from the detector arrangement
901 and to store it into a memory
904.
We assume that the imaging arrangement of FIG. 9 is built for remote-controlled
operation, which means that a telemetric transceiver
905 replaces any local
user interfaces. An operating voltage source
906 operates under the control
of the central processing unit
903 to provide operating voltages to the
detector arrangement
901. A temperature sensor
907 is provided within
the cooling arrangement
902 for providing the central processing unit with
information about the temperature of the detector arrangement
901.
FIG. 10 illustrates an alternative approach to the task of providing an X-ray
detector according to the invention with spatial resolution capability. The high-purity
Ge layer
1001 is only shown schematically with dotted lines. One planar
surface of the Ge layer
1001 comprises mutually parallel p-doped GaAs strips,
of which strips
1011 and
1012 as well as a number of strips between
them are shown. These strips have a certain longitudinal direction. The other planar
planar surface of the Ge layer
1001 comprises mutually parallel n-doped
GaAs strips, of which strips
1021 and
1022 as well as a number of
strips between them are shown. These strips have also a certain longitudinal direction,
which however is different than that of the strips
1011 to
1012 on
the first surface. The thickness of all layers is heavily exaggerated in FIG. 10
in order to enhance graphical clarity.
FIG. 11 shows how the detector structure of FIG. 10 can be used in an X-ray
imaging arrangement. The GaAs strips in the "double-striped" detector arrangement
1101 are connected to readout electronics so that the strips on one side
of the Ge layer are connected to one bias and readout circuit
1102 and the
strips on the other side of the Ge layer are connected to another bias and readout
circuit
1103 (naturally all strips could as well be connected to a common
bias and readout circuit, or to common bias circuit and to common readout circuit,
as long as knowledge about which signal came from which strip is maintained). Each
of the bias and readout circuits
1102 and
1103 detects the hit of
an X-ray photon as a transient potential swing in one of the GaAs strips connected
thereto. The corresponding readout signal is taken to signal processing electronics
where a correlation between signals from both sides of the detector arrangement
is detected. The fact that the orientation of the GaAs strips is different on different
sides of the detector means that a simultaneous signal from a certain pair of strips
can only be the result of an X-ray photon hitting the detector at the intersection
of those strips.
An arrangement of differently oriented straight GaAs lines on different sides
of the Ge layer is not the only possible geometry that can be used in the double-sided
pixelizing approach. From the general field of imaging detectors also other geometries
are known. It is straightforward as such to apply some known pixelizing geometry
to the basic idea of having oppositely doped GaAs layers on the opposite sides
of a Ge layer.
Previously the advantages of bonding a readout chip onto a detector plate
or even integrating the readout circuitry directly onto the semiconductor material
of a detector plate were discussed. On the other hand we must remember that not
placing bias and readout circuits into direct contact with the detector means that
the heat dissipated in the electronic circuits does not warm up the detector. For
example in the arrangement of FIG. 11 it may well be most advantageous to thermally
isolate the bias and readout circuits
1102 and
1103 from the detector
arrangement
1101. All embodiments of the invention can be easily implemented
so that the readout electronics are thermally isolated from the detector materials.
*
