WO2000057205A1

WO2000057205A1 - X-ray imaging device and method for making same

Info

Publication number: WO2000057205A1
Application number: PCT/FR2000/000685
Authority: WO
Inventors: Loïck Verger; Olivier Peyret; Marc Arques; Michel Wolny
Original assignee: Commissariat A L'energie Atomique
Priority date: 1999-03-23
Filing date: 2000-03-20
Publication date: 2000-09-28
Also published as: EP1166150A1; FR2791469B1; FR2791469A1

Abstract

The invention concerns an X-ray imaging device comprising at least a detecting matrix made of semiconductor material, including pixels (11), to convert the incident X-ray photons into electrical charges, and a silicon-based panel (10) for reading the electrical charges, comprising a plurality of electronic devices, each electronic device being incorporated at a pixel (11), wherein each detecting matrix is provided by a semiconductor material layer vapour-phase deposited on the panel for reading electrical charges. The invention also concerns a method for making such an imaging device.

Description

X-RAY IMAGING DEVICE AND METHOD FOR PRODUCING SUCH A DEVICE

DESCRIPTION

Technical area

The present invention relates to an X-ray imaging device, for example of large dimension, capable of operating in radiography mode or in radioscopy mode, and the method for producing such a device.

The invention applies in particular to medical imaging.

State of the art

In the field of radiological imaging, there are two types of applications which differ from each other by their acquisition principle. In the radiography application, a single image is acquired while in the radioscopy application, it is a series of images which is acquired at the video rate of twenty five images per second.

In the radiography systems currently on the market, the taking of the image is analog while in the radioscopy systems, it is digital.

The advantage of obtaining a digital image is such (possibility of image processing, data archiving ...) that, in the case of radiography, several solutions are proposed to transform the detected analog signal into digital signal.

In Disposit _/ radiographic yew, the detecting means of X-rays comprise X-ray sensitive films and emitting light, which is read by reinforcing screens (for example in BaFBr or in BaFCl).

A first embodiment making it possible to obtain digital information consists in coupling these films to a video camera, itself coupled to an image intensifier. The digital image thus obtained is instantaneous but of mediocre quality (poor spatial resolution, low conversion efficiency, noise, etc.). A second embodiment consists in replacing the film provided with reinforcing screens by a luminescent screen with photostimulable memory. This screen keeps in memory the energy stored during exposure to X-rays. The information contained in this energy is read offline after the screen has been subjected to scanning by a laser beam. This embodiment has the following drawbacks: the radiological device is bulky, the digital image is not obtained instantly and the information processing time is long (from 40 to 60 seconds).

A third embodiment consists in using a detector comprising a selenium-based photoconductor implementing the principle of xeradiography: the charge initially created on the surface of selenium by the Corona effect depends on the quantity of X photons detected. The variations in the charge are read by microprobes by capacitive effect. After exposure to X-rays and reading of the charge created, the selenium layer must be recharged. The radiological device implementing this embodiment is bulky and the reading of the information is slow, around fifteen seconds, excluding its use in radioscopy mode. In the devices used in radioscopy, the digital detection means comprise an Radiological Image Intensifier (IIR), also called a brightness amplifier. This detector allows real-time imaging, has excellent sensitivity but has an image field limited by the maximum size of the vacuum tubes (40 cm), a modest spatial resolution, image distortions and a large footprint. . In recent years, new two-dimensional direct reading digital detectors have appeared, their use being however limited to the radiography mode only. These new detectors have the particularity of being able to be produced in large dimensions (for example 40 × 40 cm ² ).

There have appeared, on the one hand detectors comprising luminescent screens associated with an optically coupled camera (CCD), requiring an optical reduction for large fields and on the other hand, detectors with flat panels based on amorphous silicon, such as described in the document referenced [1] at the end of the description.

The technology for producing flat panels based on amorphous silicon is based on that of liquid crystal displays. A panel is a charge reading matrix, made of amorphous silicon (a- Si: H), comprising pixels. The reading of the panel is carried out with a system of switches (transistors) with a command by the lines and a reading by the columns. The entire column is read during scanning and the electronic processing of the load is carried out on remote electronics. This reading process generates significant noise (2,000 to 5,000 electrons). There are two embodiments of a detector using such a reading panel:

The most common embodiment consists in covering each pixel of the reading panel with a photodiode and in contacting the photodiodes with a scintillator, for example in Csl: T1. Photodiodes convert light radiation into electrical charges read by the amorphous silicon-based panel. This type of device presents a performance problem linked to the indirect detection of photons: the detected signal is of low amplitude. Furthermore, the use of Csl does not make it possible to obtain good absorption of photons by Csl and measurements having good spatial resolution: a compromise has to be made. In addition, a phenomenon of luminescence occurring after the X-ray has stopped in the scintillator prevents the operation of this device in fluoroscopy mode. Finally, devices of this type have a low filling rate (from 50 to 70%).

A second embodiment consists in depositing a layer of amorphous selenium on the reading panel, this layer of amorphous selenium directly converting X-rays into electrical charges. Selenium imposes certain constraints linked to the fact that it is a light element. This characteristic requires it to be deposited in a thick layer in order to be able to stop the photons, to the detriment of the collection efficiency of the charge carriers. And this requires the application of a large potential difference (of the order of magnitude of 10 V / μm) to polarize the detector, which is penalizing for use in the medical field. In conclusion, to date, no device is able to operate in radiography mode and in fluoroscopy mode.

The object of the invention is to produce a digital imaging device comprising a two-dimensional digital detector, capable of operating both in radiography mode and in radioscopy mode, having good detection efficiency and being able to be produced in large dimensions.

Statement of the invention

The present invention relates to an X-ray imaging device comprising at least one detector matrix of semiconductor material comprising pixels, for converting incident X photons into electrical charges and a panel for reading electrical charges, based on silicon, comprising a plurality of electronic devices, each electronic device being integrated at a pixel level, characterized in that each detector matrix is produced by a layer of semiconductor material deposited in the vapor phase on the panel for reading electrical charges.

The invention therefore relates to a fully integrated semiconductor device used in radiological imaging making it possible to produce digital images of large areas (for example from 20 × 20 cm ² to 40 × 40 cm ² ). This device has the advantage of being a low noise structure, with advanced electronics allowing it to operate in mixed radiography / radioscopy mode with high manufacturing yields for moderate manufacturing costs.

The invention also relates to a method for producing an X-ray imaging device. comprising at least one detector matrix of semiconductor material comprising pixels, for converting the incident X photons into electrical charges and a silicon-based electrical charge reading panel comprising a plurality of electronic devices, each electronic device being integrated into the level of a pixel, characterized in that each detector matrix is obtained by depositing on the reading panel of electrical charges a semiconductor in the vapor phase.

Advantageously, the evaporation properties of this semiconductor allow deposition at low temperature.

Advantageously, the semiconductor material used to make the matrix of detector pixels is CdTe, Hgl ₂ or Pbl.

Advantageously, electronic devices made by a 1.25 μ technological sector are used. Advantageously, electronic devices produced by a technological network of 0.1 μm are used.

The process of the invention is compatible with the monocrystalline silicon technology used today in microelectronics, which has the following advantages:

• Take advantage of developments in standard micro-electronic systems, which have seen the diameter of silicon ingots increase over the years (from 10 cm in 1980 to 35 cm in 2000) in order to limit the costs of the fully integrated detector.

• Eliminate the coupling or connection steps between the two elements since a detector layer based on semiconductor is deposited directly on the reading circuit based on monocrystalline silicon including advanced electronics (preamplifier, amplifier, filters ...).

• Present the crystalline quality of the detector material by use.

Brief description of the figures

FIG. 1 illustrates the X-ray imaging device of the invention and its production method. FIGS. 2A to 2E illustrate the method for producing a radiological imaging device according to the invention.

Detailed description of embodiments The present invention relates to an X-ray imaging device which comprises at least one matrix made of semiconductor material, for converting the incident X photons into electrical charges and comprising pixels 11, each matrix being arranged on a panel 10 for reading electrical charges based on monocrystalline silicon, comprising a plurality of electronic devices, each electronic device being integrated at the level of each pixel 11 of said matrix. The charge reading panel, for example from conventional 0 J μm to 1.25 μm channels of microelectronics (diameter: a few ten centimeters) is used as a substrate on which the matrix of detector material based on semiconductor, which converts incident X photons into electrical charges.

The matrix of semiconductor material is for example deposited by the CSVT method from a source 12 containing the semiconductor material 13, in an enclosure 14 under a controlled atmosphere of an inert gas

As described in the document referenced [2], the development of thin layers by the CSVT (“Close-Spaced Vapor Transport”) method has the main characteristics of being easy to implement, inexpensive, and usable for growth. large areas.

In the invention, the source 12 comprising the semiconductor material which may be solid or in the form of powder is heated to a temperature T1 of the order of 600 ° C. The semiconductor material used can be for example CdTe, Pbl ₂ , or Hgl ₂ . This source 12 is separated from the substrate 10 by a short distance which varies from 1 to 10 mm. The temperature of the substrate is regulated at a temperature T2 lower than that of the source. It varies from 200 ° C to 600 ° C depending on the nature of the semiconductor used and the quality of the layer required. The temperature gradient which is created allows material transport between the source 12 and the substrate 10. The physical properties of the semiconductors such as CdTe, Pbl ₂ or Hgl ₂ , associated with the use of a CSVT method make it possible to spare the substrate by imposing only a temperature (200 to 450 ° C) compatible with the temperature resistance of silicon in electronic devices.

To deposit such a semiconductor layer at low temperature, various conditions must be met. We must, in fact:

- heat the source to its sublimation temperature,

- deposit on a material so that the deposited material can reorganize under layer form (the material can be preheated),

- optimize the distance between the source and the substrate so as to obtain a diffusion of the vapors between the source and the supported and not dispersed substrate,

- obtain a sufficiently high deposition rate, that is to say greater than a few μm / h, in order to produce, in a deposition time compatible with industrialization of the detector, a layer of a few hundred microns in order to effectively stop photons,

- maintain the substrate, which includes the reading circuit, at a temperature such that the circuit is not deteriorated (that is to say a temperature below 450 ° C for monocrystalline silicon, below 250 ° C for amorphous silicon).

To choose a usable semiconductor material, it is necessary to take into account all its physical properties, and to make a compromise. For a given material, we have the following data: - the higher its atomic number z, the higher its absorption,

- the higher its density, the more it absorbs X-rays for the same thickness (we aim for an absorption between 70% and 90%), - the higher its resistivity, the lower the noise of the detector,

- the lower the energy of the electron-hole pairs, the more electrical information the interaction of the material with X-rays, - the lifetime must be greater than the extraction time, which is the time necessary for the electrons and the holes to exit,

- the higher the mobility, which is a function of the atomic number and the density, the faster the flux increases,

- the higher the quality criterion μτ, which is the product of mobility over lifetime, the better the detection for a given applied electric field.

- with equivalent physical characteristics, the material requiring the application of the weakest possible electric field must be chosen.

Table 1, given at the end of the description, is a comparative table of different possible detector materials, E (V / cm) being the electric field conventionally applied to the material considered

The invention therefore combines a semiconductor-based detector material whose deposition method makes it possible to produce large areas (a few dm ² ) with a reading circuit developed on a full slice of monocrystalline silicon (diameter 10 to 30 cm ) incorporating advanced electronics dedicated to X-ray detection (amplification, filters and processing) which can be integrated into a pixel, for example from 100 to 200 μm.

A fully integrated large area X-ray imaging device is thus obtained, the performance of which in terms of signal / noise ratio is considerably increased.

In this imaging device, an electronic device is arranged as close as possible to each detector pixel. As a result, the link capacities are reduced to an extreme and this results in a significant reduction in reading noise compared to devices of the prior art.

In addition, the use of electronic devices produced from monocrystalline silicon ensures the production of an amplifier of the detected signal with excellent quality.

Finally, the combination of a detector with a low connection capacity with an electronic device comprising a good quality amplifier, gives the imaging device of the invention a negligible reading noise, lower than the noise of the photon, thus giving access images at low doses like that obtained in fluoroscopy mode.

Thus, the imaging device of the invention is able to operate both in radiography mode and in fluoroscopy mode.

Each electronic device, which is dedicated to the detection and treatment of the charge deposited in the semiconductor material, is a device which can integrate several functionalities of the detection of X-rays. By way of example, the device of the invention includes advanced electronics, as described in the document referenced [3], which can be integrated into a pixel, for example of 150 μm x 150 μm. Each electronic device can include a reading circuit and an integration circuit (which stores a quantity of electrons, which will be transformed into analog voltage which will then be digitized) and / or a counting circuit. Before this basic block, it is possible to add means to avoid saturating the reading means, for example with the continuous dark current flowing in the detector.

The invention also relates to the method for producing such an imaging device. As described above, this process therefore consists in transferring, by vapor phase, a semiconductor whose evaporation properties allow deposition at low temperature on a substrate compatible with its temperature resistance, substrate which, in the case of the present invention, is the reading circuit based on monocrystalline silicon incorporating advanced electronics.

We will now consider successively two embodiments of the imaging device of the invention.

In a first embodiment, a 30 cm silicon substrate is used and an electronics produced by a technological network of 0.1 μm is used. FIG. 2A illustrates a slice of monocrystalline silicon 20 (diameter 30 cm), the monocrystalline silicon part with integrated electronics being referenced 21. In this figure are also represented: - the piloting and control pads 22;

- 23 pixels from 100 to 200 μm including dedicated electronics.

Figure 2B illustrates the cutout 25 of 20 cm x

20 cm of a slice of monocrystalline silicon with integrated electronics used as a substrate during the deposition of a semiconductor layer by the method

CSVT.

Figures 2C and 2D illustrate the semiconductor layer 24 deposited by the CSVT method for example to form an element 25 of 20 cm x 20 cm.

FIG. 2E illustrates the splicing of four elements, for example of 20 cm X 20 cm 25 to obtain a digital detector of / large area dedicated to radiology, that is a surface (40 cm x 40 cm) according to the example chosen. Such an embodiment has the following advantages:

- obtaining a large field by assembling several detectors; - use of highly advanced electronic functions;

- realization of electronic devices by standard micro-electronics technologies.

In a second embodiment, a 15 cm silicon substrate is considered and an electronics made with a 1.25 μ technological path is used. Electronics produced with this type of technology is more than enough to integrate electronics dedicated to radiology in a 100 μm pixel. Its interest lies in its immediate availability with reduced production costs. For applications in radioscopy, four detectors of 10 cm x 10 cm can be combined in order to obtain a detection surface of 20 cm x 20 cm, sufficient surface for a medical application.

Table 1

REFERENCES

[1] "Amorphous Semiconductors Usher In Digital X-Ray Imaging" by John Ro lands and Safa Kasap (Physics Today, November 1997, pages 24 to 30)

[2] "Growth Of Semiconductors By The Close-Spaced

Vapor Transport Technique: A Review "by G.

Perrier, R. Philippe and J.P. Dodelet (J. Mater.

Res. 3 (5), September / October 1988, pages 1031 to 1042)

[3] "Readout For a 64x64 Pixel Matrix With 15-Bit

Single Photon Counting ”by M. Campbell, E.H.M.

Heijne, G. Meddeler, E. Pernigotti and W. Snoeys

(Nuclear Science Symposium, Albuquerque, November 12, 1997)

Claims

1. X-ray imaging device comprising at least one detector matrix of semiconductor material comprising pixels (11), for converting the incident X photons into electrical charges and a panel (10) for reading electrical charges, based of silicon comprising a plurality of electronic devices, each electronic device being integrated at the level of a pixel (11), characterized in that each detector matrix is produced by a layer of semiconductor material deposited in vapor phase on the reading panel electric charges.

2. Method for producing an X-ray imaging device comprising at least one detector matrix of semiconductor material comprising pixels (11), for converting the incident X photons into electrical charges and a reading panel (10) of electrical charges, based on silicon comprising a plurality of electronic devices, each electronic device being integrated at the level of a pixel (11), characterized in that each detector matrix is obtained by deposition on the panel for reading electrical charges d 'a vapor phase semiconductor (13).

3. Method according to claim 2, wherein the evaporation properties of this semiconductor allow deposition at low temperature.

4. The method of claim 2, wherein the semiconductor material used to make the matrix of detector pixels is CdTe, Hgl ₂ , or Pbl ₂ .

5. Method according to claim 2, in which electronic devices made by a technological sector of 1.25 μm are used.

6. Method according to claim 2, in which electronic devices produced by a technological channel of 0.1 μ are used.