WO2002067271A2

WO2002067271A2 - Imaging systems and particle detectors using silicon enriched by heavier elements

Info

Publication number: WO2002067271A2
Application number: PCT/IL2002/000111
Authority: WO
Inventors: Arie Ruzin
Original assignee: Ramot University Authority For Applied Research & Industrial Development Ltd.
Priority date: 2001-02-16
Filing date: 2002-02-13
Publication date: 2002-08-29
Also published as: WO2002067271A3; AU2002232089A1

Abstract

An imaging and particle detection system using silicon enriched by heavier elements. An element which has been found to be highly effective in these applications is germanium. The silicon material enriched by germanium has an improved absorption coefficient, which is essential for effective particle detection in imaging applications. At low energies, silicon alone absorbs well. At high energies, silicon enriched by germanium has proven effective. Typical applications include medical imaging systems, x-ray imaging systems, non-destructive testing systems, environmental monitoring systems, nuclear imaging systems, fluorescent chemical analysis systems, and x-ray astronomy systems.

Description

IMAGING SYSTEMS AND PARTICLE DETECTORS USING SILICON ENRICHED

BY HEAVIER ELEMENTS

FIELD OF THE INVENTION

The present invention generally relates to the realization of imaging systems and particle detectors using silicon enriched by heavier elements, and more particularly, to enrichment by germanium.

BACKGROUND OF THE INVENTION

The goal of diagnostic radiographic imaging, is to provide an image with the highest possible resolution and contrast, while using the minimum necessary irradiation dose. It is particularly important to minimize the dose and to optimize the photons energy in medical imaging, since the incident photons are damaging and may cause cancer in the living cells. In medical imaging the useful diagnostic energy range of the photons is from about 15 to 150 keV. Higher energies are known, however, in specific applications such as positron emission tomography (PET). In "X-ray imaging," the image is formed by the non-absorbed photons. At energies below 15 keV nearly all the photons are absorbed even in the soft tissue, therefore photons in this range will not contribute to the image, yet they may generate some cell damage. At energies above 150 keV the absoφtion in the human body is too small, thus the radiological contrast is greatly diminished. The radiological detector must be capable of absorbing nearly the entire radiation incident upon it, and convert the absorbed energy into an electrical signal with sufficient gain, such that the intrinsic noise of the detector is not greater than the signal generated by the high-energy photons.

Thus, high absoφtion efficiency is normally required in medical imaging systems, as well as other high flux applications, such as non-destructive testing of various products. High absorption efficiency is also required in spectroscopy detectors, which detect a single photon interaction and characterize its energy. Such applications include environmental monitoring, nuclear cameras, X-ray fluorescence for chemical analysis and X-ray astronomy where the energy source may be a gamma burst in space. The absorption efficiency is strongly dependent on the atomic number of the absorbing media. In the low energy range the absoφtion coefficient is nearly proportional to Z⁵, where Z is the atomic number, namely the number of electrons or protons in the atom. Such strong dependence of the absorption coefficient on the atomic number implies that even a low concentration of "heavier" atoms can contribute to a substantial increase in the absoφtion coefficient.

When photons enter an absorbing medium, they are absorbed according to Beers Law:

I = Ioe^,

Where Io is the original flux at depth 0, 1 is the remaining flux at depth x and , the absoφtion coefficient of the media, is roughly proportional to Z⁵ in the low to intermediate energy range

(when the photoelectric effect is the dominant absoφtion mechanism). Therefore, it would be desirable to provide improved photon detection and imaging systems, using semiconductor materials having a higher atomic number, and therefore having higher absoφtion efficiency

One traditional detector circuit is based on a reverse-biased diode, which may be fabricated on silicon semiconductor material. When the device is bombarded by photons of x-rays or gamma rays, pairs of free electrons and holes are generated through the various energy transfer mechanisms, i.e., mainly through the photoelectric effect and the Compton scattering. The free charge drifts in the electric field of the depletion region, which is created by the reverse bias voltage. While drifting, the charge induces electrical current in the external circuit. Ideally, if no significant signal trapping occurs, the integral of the induced current is proportional to the transferred energy. The diode structures provide excellent frequency response, and they are available with very low dark currents, and veiy high sensitivities. Silicon p-i-n photodiodes are particularly sensitive.

Silicon has an atomic number of 14, and an atomic weight of 28.09. At low energies, low atomic number materials, such as silicon, absorb very well. But at higher energies the silicon material is substantially transparent, i.e., there is virtually no absorption. Using even relatively small amounts of impurities of the heavier elements (preferably in the same valence group (IV) as silicon), gives substantial improvement in the absoφtion efficiency. For a few years silicon germanium (SiGe) has been grown in epitaxial layers, but only now the first successful attempts were performed to grow bulk silicon germanium. The bulk SiGe is not yet of high resistivity, and is only grown by Cz method with 1 to 2.5 inch diameter. The epitaxial SiGe is known for its use in the base region of fast bipolar transistors. Today, for high-energy resolution, pure germanium detectors are often used. They are expensive and require liquid nitrogen cooling to achieve the temperature of 77K, and therefore require heavy, bulky systems. The leakage current for germanium detectors at room temperature is high, which is why super-cooling is necessary. In addition, the Ge detector fabrication process does not benefit from the advantages of the highly developed silicon process (originally developed for VLSI technology), mainly characterized by the availability of local oxide growth. If these Ge detectors could be replaced by SiGe room temperature, or near room temperature, detectors, it would be a breakthrough.

An additional benefit from the introduction of heavier elements into the silicon crystal lattice could be an improved radiation hardness of the devices. The reason being that large elements in the lattice create local stress, and often act as gettering centers for mobile defects in the form of vacancies and interstitial atoms.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention to provide an improved photon detection system for flux imaging systems, as well as for spectroscopy systems.

It is a further object of the present invention to provide detectors of various internal structures, such as p-i-n diodes, Schottky diodes, photoconductors, etc., for imaging and spectro copy systems that incorporate the capabilities of silicon, to conveniently grow a variety of oxides, and a wide range of other available processes.

It is yet a further object of the present invention to provide detectors of various internal structures, such as p-i-n diodes, Schottky diodes, photoconductors, etc., for imaging and spectroscopy systems that incorporate the capabilities of silicon, i.e., to be able detect at room temperature.

It is still a further object of the present invention to provide detectors of various internal structures, such as p-i-n diodes, Schottky diodes, photoconductors, etc., spectroscopy detectors that incorporate bulk or otherwise grown, high resistivity SiGe in proportions for high absorption and low leakage, thereby utilizing the advantages of germanium, while overcoming the main disadvantage of germanium, i.e., the cooling requirement. It is still another object of the present invention to provide detectors of various internal structures that incoφorate besides germanium, other heavy atoms with silicon.

In accordance with a preferred embodiment of the present invention, there is provided an imaging and particle detection system using silicon enriched by heavier elements. An element that has been found to be highly effective in these applications is germanium. The silicon material, enriched by germanium, has an improved absoφtion coefficient, which is essential for effective particle detection or imaging applications. At low energies, silicon alone absorbs well. At high energies, silicon alone does not absorb well, however, silicon enriched by germanium does absorb well.

For 40 keV photons, for instance, the absoφtion coefficient of silicon is 1.45 cm^"1, yet for Sio Geo i it is 4.83 cm^"1, and for Sio₈Geo₂ it is 8.13 cm^"1. Namely an increase of factor 3.3 to 5.6 can be achieved in the absoφtion coefficient by adding 10 to 20 percent of Ge. SiGe detectors show a significant advantage in the absoφtion coefficient over purely silicon detectors up to the energy of ~ 200 keV, i.e., 2.E+05 eV, where Compton scattering becomes dominant, and the dependence of the absoφtion coefficient on the atomic number is less dramatic. That increase in the absoφtion efficiency may make these detectors usable in numerous medical applications.

Two additional advantages are expected by the use of SiGe instead of silicon. The mobility of the free carriers in the epitaxial SiGe was found to be much higher than in silicon, which should yield better charge collection and shorter time response. The germanium sites may act as getters for radiation induced defects in SiGe detectors, thereby improving their radiation hardness. Thus, the addition of germanium to silicon may be expected to prolong the "lifetime" of the detectors when used in hostile environments, such as space, nuclear reactors, and especially in high-energy physics experiments (mainly in particle colliders where large numbers of damaging panicles are present).

Alternatively, even heavier elements than Ge may be added to silicon, resulting in even higher absoφtion efficiencies, elements such as zirconium, titanium, tin, etc.

The following terms are defined in reference to detector performance terminology: Bias Voltage

The voltage applied to the detector circuit, normally DC volts; sometimes called optimum bias for values, which give optimum signal-to-noise ratios, and maximum bias for values which produce the maximum signal voltage output, or above which breakdown may occur. In the rectifying configurations, i.e. p-i-n, Schottky, etc., it is common to apply reverse bias in order to reduce the leakage current, reduce the capacitance, increase the active volume, improve the charge collection and to fasten the response.

Compton Scattering

Some scattered radiation possesses, which may be described as an elastic collision between an incident photon and an electron of the media materials. When the electron is valent, or from an inner orbit, it is freed after the collision, and it gains kinetic energy. The outgoing secondary photon has less energy and different direction than the incident one.

Dark Current

The measured current in a detector circuit when operated with no signal (no incident radiation on the detector element).

Dark Resistance

Relevant for photoconductor structure, the dark resistance is the ratio of the DC voltage across the detector to the DC current through it, when no radiation is incident on the detector.

Equivalent-Noise- Charge (ENC)

The amount of required signal electron-hole pairs in the detector element to yield a signal-to-noise ratio of one. It indicates the minimum detectable charge by the system, and therefore the smaller the ENC value, the better the performance.

Photovoltaic detector

A photon detector with a p-n or p-i-n junction, which converts radiant power directly into electrical current; also called a photodiode.

Other features and advantages of the invention will become apparent from the following drawings and description. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, in which like numerals designate corresponding elements or sections throughout, and in which:

Fig. 1 is a graph of calculated total absoφtion efficiencies, in accordance with the principles illustrating the significance of the present invention;

Fig. 2 is a schematic illustration of a transducer for the detection of high-energy photons, and for their direct translation into an electrical signal, for use with the imaging detection system of the present invention;

Fig. 3 is a schematic illustration of the details of a spectroscopy system, constructed and operated in accordance with the principles of the present invention; and

Fig. 4 is a schematic illustration of the details of the front-end electronics for imaging system operated for detection of x-rays in flux mode, including the diode transducer of fig. 2, constructed and operated in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Silicon germanium (SiGe), having a high resistivity, is a new material, which has not yet been extensively studied. Therefore, in TABLE ϊ, approximate values are given based on calculations rather than on experiments.

Fig. 1 is a graph of calculated total absoφtion efficiencies 100, in accordance with the principles illustrating the significance of the present invention. The calculated total absoφtion coefficients are plotted for Si, Sio Geo ι, SioχGe₀₂, and Ge.

It can be easily seen that the absoφtion coefficient, plotted as the Y-coordinate 105, increases substantially, except in the narrow energy range of 2 to 10 keV 110, where even pure Ge does not show significant advantage. For 40 keV 120 photons, for instance, the absorption coefficient of silicon is 1.45 cm^"1, yet for Si₀9Ge_{0 1} it is 4.83 cm^"1, and for Si₀gGe₀₂ it is 8.13 cm^"1. Thus, an increase by a factor of 3.3 to 5.6 can be achieved in the absoφtion coefficient, which is highly advantageous, by adding 10 to 20 percent of Ge.

Two additional advantages are expected by the use of SiGe instead of pure silicon. The mobility of the free carriers in the epitaxial layers of SiGe is found to be much higher than in silicon, which should yield better charge collection and shorter time response if the effect is reproduced in the bulk material. The germanium sites may act as getters for radiation induced defects in SiGe detectors, and thereby improve their radiation hardness.

TABLE I

By using Sio9Geo ι the band gap is only reduced to about 1.074 eV. The energy gap is the energy between the top of the valence band and the bottom of the conduction band that a charge carrier must obtain before it can transfer a charge. The band gap determines the temperature dependence of the electrical conductivity of a semiconductor. Germanium, with a band gap of 0.66 eV, has a high intrinsic carrier concentration, corresponding to such a narrow band gap. The leakage currents, and the resulting noise near room temperature in germanium detectors, are too great.

An x-ray direct transducer operates by converting high-energy photons (such as x-rays) directly into electric charge, which in turn drifts in the electric field (or diffuses in the diffusion chambers) and is collected by the electrodes. In the case of detector arrays, one of the electrodes is usually in the form of an array of pixels, so that during the drifting of charge, an induced current appears in the external circuit associated with a specific pixel. The resulting current may be integrated on an external capacitor, as it is done in charge sensitive preamplifiers in the spectroscopy application. The current may be amplified and converted into a voltage signal by a trans-impedance amplifier, as is often done in a flux operation. The term "direct conversion" is used.

By contrast, the concept of "non-direct conversion" is used in the case of scintillating detectors, where the high-energy photons are first translated into low energy photons (visible or near-visible), which are then translated into an electric signal by silicon diodes (operated as 'quantum detectors' with band to band excitation). The primary advantages of direct conversion of the high-energy photons into electrical signal are lower noise, higher average ionization energy and lower image blur. X-rays produce free charge directly in the absorbing SiGe transducer, and the charge can be directed by an electric field to a collecting electrode or surface. Because the x-ray signal charge is guided by an electric field, the detector can be made thick to achieve high quantum efficiency (QE).

MTF at that frequency is very low (1%). The MTF is a measure of how well spatial frequencies are transferred in an imaging system. X-rays are quanta, and apart from detector limitations, the limiting resolution is set by the number of quanta that are present per unit area at the detector. It is useful to note, that over most of the diagnostic energy range, 1 Roentgen incident on a 50% QE detector corresponds to - 1 x-ray photon absorbed per pixel.

Fig. 2 is a schematic illustration of a transducer diode 200 for the direct detection of high-energy photons for use with the imaging detection system of the present invention. The free charges are generated when high-energy photons 202 are absorbed in the active volume of the SiGe transducer. They are then separated and collected by the electric field onto the electrodes of the transducer element or pixel. Since the charge, represented by an electron-hole pair 205, is guided by the electric field, represented by arrows 208, to the metal electrodes: positive (p) 210 and negative (n) 220, the only blurring is due to the charge diffusion during the collection time. Tlie circuit consists of a p⁺/n junction (or a Schottky contact) on one side 210 or 220, and an ohmic contact on the other side. Thus, under reverse bias conditions, a large depletion layer develops in the semi-intrinsic SiGe, which constitutes the active detector volume.

If the resistivity of the so called "semi-intrinsic" SiGe is too low, then the operating bias voltage required to achieve the needed depletion layer width, comprising the active volume of material, is very high, in addition electric field 208 at such high bias voltages is also high, and may cause high leakage currents, thereby leading to high electronic noise, and even to breakdown of the diode. Recently, some SiGe has been grown with resistivities of 50 to 100 ohmxcm, which is somewhat higher than the resistivity of the common epitaxial iayers. However, for the fabrication of full size devices, semiconductor material with a resistivity of several [kΩxcm] is required, which has not yet been produced. The commercial applications of the present invention provide the impetus to produce such material, thereby furnishing a large active volume with a reasonable bias. The material used should also have a reasonable lifetime-mobility product to insure high collection efficiency.

SiGe detectors could revolutionize the fields of high-resolution spectroscopy as well as imaging. Today for high-energy resolution pure germanium detectors have to be used. They are expensive, and require liquid nitrogen cooling. If such detectors could be replaced by SiGe room temperature, or near room temperature, solid-state detectors, it would be a breakthrough. In the field of imaging the potential is enormous. According to the calculations, SiGe detectors show a significant advantage in the absorption coefficient over silicon detectors up to the energy of - 200 keV, i.e., 2.E+05 eV, where Compton scattering becomes dominant. That increase may make these detectors usable in numerous medical applications. It should be mentioned that enriching the silicon lattice with yet heavier elements might revolutionize all fields of medical imaging. If high absoφtion efficiency is achieved up to the photon energy of 140 keV, the detectors will be suitable for most existing applications, including CT.

Fig. 3 is a schematic illustration of the details of a spectroscopy system 300. In spectroscopy mode the system detects a single incident photon 305 and characterizes its energy or more precisely, it characterizes tlie energy fraction that was absorbed in the active volume of tlie detector, for example, the SiGe material. The example shown in fig. 3 is a particular case of Charge Sensitive Preamplifier (CSP) with AC coupling and resistive feedback. The induced current is integrated on tlie C_mt capacitor 350, yielding, at tlie output 355 of tlie operational amplifier 340, a voltage step proportional to the charge. The feedback resistor, R_f 370, slowly discharges the integration capacitor 350 to prevent saturation after repeating events. Other configurations are also possible. The high-energy photons 305, for example, are shown entering diode transducer. The high bias voltage 312 is supplied to the detector via bias resistor, R^ 310, which prevents the signal current from flowing to the power supply, instead of to the integrating capacitor 350. Tlie induced current flows through tlie coupling capacitor, C 325, and integrates on C_ιnt350, which should be small in order to produce a large voltage step for a small charge. Output is fed to a shaping amplifier 385, which processes the signal for optimum signal to noise ratio.

The equivalent electronic circuit 330 at the bottom of the figure represents the detector for puφoses of small signal analyses. The pulse source 334 represents the induced current 332 present during the charge drift. Equivalent capacitance 336, C_D, represents the total device capacitance and equivalent resistance 338, R_D, represents the parasitic parallel resistance.

Fig. 4 is a schematic illustration of the details of the front-end electronics for imaging system 400, operated for detection of x-rays in flux mode, including the diode transducer of fig. 2, constructed and operated in accordance with tlie principles of tlie present invention. An x-ray photon flux 405, for example, is shown entering diode transducer 425. The amplifier 440 shown in this mode of operation is a DC coupled trans-impedance configuration. The feedback capacitor 470, C_Bw, i« this case, is only reducing tlie bandwidth of tlie amplifier. Since tlie flux is made of single photons, and we are only interested in the average energy deposited per unit of time, it is sensible to filter the fluctuations produced by the particular interactions. The feedback resistor 450 is actually the converting element tlirough which the signal current flows.

The equivalent electronic circuit at the bottom of the figure represents the detector for puφoses of small signal analyses. The pulse source 334 represents the induced current present during irradiation and charge drift. Equivalent capacitance 336, Co, represents the total device capacitance and equivalent resistance 338, R_D, represents the parasitic parallel resistance.

Having described the invention with regard to certain specific embodiments, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in tlie ait, and it is intended to cover such modifications as fall within the scope of the appended claims.

Claims

I claim:

1. A system for particle detection, comprising a high resistivity silicon material used as a particle detector combined with enrichment material comprising elements having a higher atomic number than silicon.

2. The system of claim 1, wherein said enrichment material is germanium.

3. The system of claim 2, wherein said combined material is Si₀9Geo ι

4. The system of claim 2, wherein said combined material is Sio.sGeo₂.

5. The system of claim 1, wherein said enrichment material is any element.

6. The system of claim 1, wherein said system is an imaging system.

7. The system of claim 6, wherein said system is a medical imaging system.

8. The system of claim 6, wherein said system is an x-ray imaging system.

9. Tl e system of claim 1 , wherein said system is used for non-destructive testing.

10. The system of claim 1 , wherein said particles are gamma rays.

1 1. Tlie system of claim 8, wherein said system is used for environmental monitoring.

12. The system of claim 8, wherein said system is used for nuclear cameras.

13. The system of claim 8, wherein said system is used for x-ray fluorescence for chemical analysis.

14. The system of claim 8, wherein said system is used for x-ray astronomy.

15. The system of claim 1 , wherein said combined material is bulk grown.

16. The system of claim 1 , wherein said system operates substantially at room temperature.

17. A system for particle detection, comprising a high resistivity silicon material used as a particle detector combined with germanium.

18. A method for particle detection, comprising: providing a high resistivity silicon material used as a particle detector; combining said silicon material with enriclmient material comprising elements having a substantially higher atomic number than silicon.

19. The method of claim 18, wherein said enrichment material is germanium.

20. Tlie method of claim 18, wherein said enrichment material is any element.