WO2014045203A1

WO2014045203A1 - Alpha particles detector

Info

Publication number: WO2014045203A1
Application number: PCT/IB2013/058626
Authority: WO
Inventors: Giovanni Verzellesi; Gian-Franco Dalla Betta; Andrea Bosi; Luigi Rovati; Davide SAGUATTI; Giovanni BATIGNANI; Stefano BETTARINI; Marcello GIORGI; Luciano BOSISIO; Nicola Zorzi; Maurizio Boscardin; Claudio Piemonte; Gabriele GIACOMINI; Luca BIDINELLI
Original assignee: Rsens S.R.L.; Istituto Nazionale Di Fisica Nucleare (I.N.F.N.); Università Di Pisa; Università Degli Studi Di Trieste; Fondazione Bruno Kessler
Priority date: 2012-09-18
Filing date: 2013-09-18
Publication date: 2014-03-27
Also published as: ITMI20121547A1

Abstract

The invention relates to an alpha particles detector comprising a semiconductor substrate (20), a matrix (100) of pixels (11) formed in the semiconductor substrate (20) and a plurality of bipolar transistors (110) having base regions (2) and emitter regions (3) formed in every pixel (11), the semiconductor substrate (20) being configured as a common collector for all the transistors (110).

Description

"ALPHA PARTICLES DETECTOR"

DESCRIPTION

Field of application

The present invention relates to an alpha particles detector.

In particular, the present invention relates to a semiconductor detector for the detection of alpha particles.

More particularly, the present invention relates to a semiconductor detector for the detection of alpha particles designed to measure the Radon gas concentration in the environment.

The description that follows is provided with reference to this field of application in order to simplify its disclosure.

Prior art

Techniques for measuring the concentration of radioisotopes of Radon (222Rn) by means of ionization chambers, scintillation cells, nuclear track detectors, activated carbon detectors and electrets are in general known. In particular, techniques using semiconductor detectors as described here are also known.

One basic technique in semiconductor detectors uses a high-resistivity silicon detector to detect the alpha particles emitted as a result of Radon decay and/or the decay products thereof.

The electron-hole pairs generated by alpha particles in silicon are separated by the electric field generated by polarizing a p-n junction in a reverse direction and give rise to a current pulse at the terminals of the device.

This current is typically integrated within a charge preamplifier, which outputs a voltage proportional to the input charge for subsequent processing.

The information drawn from the measuring system can comprise solely a count of the alpha particles detected per unit of time, or also the energy of the alpha particles.

In this latter case it is possible to discriminate between the different decays involved (tied to Radon and the daughters thereof), something that can be exploited to obtain a more accurate measurement of the Radon concentration in the environment.

The silicon detectors typically used comprise p-n junction diodes and surface barrier diodes.

Alternatively, another technique used in semiconductor detectors provides for an npn bipolar transistor built on high-resistivity silicon, which outputs a current which is already amplified, relative to that induced by the alpha particle, by an amount approximately equal to the current gain of the transistor itself.

This internal amplification makes it possible to reduce the complexity of the electronic reader placed downstream of the detector.

This occurs at the cost of a greater capacitance associated with the terminal which collects the generated charge and a consequent reduction in the response speed as well as greater electronic noise, along with a decrease in the energy resolution of the detector.

There is an additional fundamental limitation of the bipolar transistor detector in the form proposed up to now, which compromises its applicability in systems for continuous measurement of the ambient Radon concentration.

The latter must be characterized by high sensitivities in terms of count rate (number of alpha particles detected per unit of time) per unit of Radon concentration (measured in Bq/m³), in order to provide real-time indications as to the presence of Radon levels close to the limits beyond which they pose a health hazard. This need imposes the use of detectors with a large sensitive area (on the order of a cm²).

In the case of the bipolar transistor detectors, the sensitive area corresponds to the base area and a value on the order of a cm² means an excessively high collector node capacitance.

Exploiting a large sensitive area thus entails significant functional problems. This can reduce the amplitude of the current pulses associated with the detection of alpha particles to the extent that it may fall below the threshold of discrimination of the system (fixed in relation to the amplitude of the disturbances present under the conditions of use).

This mechanism makes it extremely difficult to produce a bipolar transistor detector, in the form proposed up to now, which has the required sensitivity for continuous measurement of the ambient Radon gas concentration.

The object of the present invention is to provide an alpha particles detector capable of solving the above-described problems.

A particular object is to provide an alpha particles detector capable of functioning effectively in real time.

A further object is to provide an alpha particles detector with simplified electronics and with low power consumption in the detector.

Summary of the invention

These and other objects are achieved by an alpha particles detector according to what is disclosed in the appended claims.

The alpha particles detector as disclosed provides the following technical effects:

- it makes it possible to obtain appropriate sensitivities for continuous measurement of the ambient Radon gas concentration.

In particular, it makes it possible to obtain high sensitivity values in terms of count rate (number of alpha particles detected per unit of time) per unit of Radon concentration (measured in Bq/m³) during the measurement of the ambient Radon gas concentration and, in particular, sensitivity values that are appropriate for a continuous monitoring of Radon gas up to concentration levels comparable with the limits beyond which they pose a health hazard to humans.

In fact, the detector enables count rates greater than 10 CPH/(kBq/m³) (where CPH = 'counts per hour'), where the number of counts in the absence of Radon is negligible («0.5 CPH). - it enables the use of simplified, low-cost electronic reader with low power consumption in the detector;

it makes it possible to obtain a low power consumption, with a consequent possible use within a battery-operated measuring system having a sufficiently long-lasting charge for typical environmental monitoring applications;

it makes it possible to obtain an output signal that is already amplified relative to the charge released by the alpha particles within the silicon substrate, with the advantage of significantly relaxing the required amplification and low noise specifications for the electronic reader.

Amplification factors above 100 are obtained from well-established measurements.

The aforesaid technical effects and other technical effects of the invention will be apparent in greater detail from the description that follows of an example embodiment, given by way of illustration and not by way of limitation with reference to the appended drawings.

Brief description of the drawings

Figure 1 is a schematic view from above of an alpha particles detector according to the invention.

Figure 2 is a sectional view of the alpha particles detector according to the invention.

Figure 3 schematically shows a detail of the detector of figures 1 and 2.

Detailed description

The invention relates to an alpha particles detector comprising a semiconductor substrate 20, a matrix 100 of pixels 11 formed in the semiconductor substrate 20 and a plurality of bipolar transistors 1 0 having base regions 2 and emitter regions 3 formed in every pixel 11 , the semiconductor substrate 20 being configured as a common collector for all of the transistors.

In other words, the invention relates to a matrix of npn bipolar transistors made of high-resistivity silicon and having a common collector region and separate base and emitter regions (in the pixels), and characterized by geometries of the overall structure and of the individual pixels which are rendered optimal for the detection of alpha particles (He2⁺) emitted during radioactive decay of the Radon gas present in the environment.

With particular reference to figure 2, the alpha particles detector comprises a semiconductor substrate 20.

Preferably, the semiconductor substrate 20 has a first conductivity type, which is type n.

Preferably, the semiconductor substrate 20 comprises silicon.

Preferably, the semiconductor substrate 20 comprises high-resistivity silicon, i.e. low-doped.

The alpha particles detector further comprises a matrix 100 of pixels 11 formed in the semiconductor substrate 20.

The detector of the invention further comprises a plurality of bipolar transistors 110.

According to the invention, every bipolar transistor 110 comprises a first base region 2 formed in a pixel 11.

Preferably, the first base region 2 has a second conductivity type, which is type p.

According to the invention, every bipolar transistor 110 comprises an emitter region 3 formed in a corresponding pixel 11.

Preferably, the emitter region 3 has the first conductivity type, type n.

An emitter contact 3 is associated with the emitter region 4.

Preferably, said contact 4 has the first conductivity type, type n.

Preferably, said contact 4 is heavily doped.

According to the invention, every bipolar transistor 110 further comprises a collector region 20 consisting of the semiconductor substrate 20.

A collector contact 7 is associated with the semiconductor substrate 20.

Preferably, said contact 7 has the first conductivity type, type n.

Preferably, said contact 7 is heavily doped.

The semiconductor layer 20 is interposed between the matrix 100 and the collector contact 7.

Advantageously, therefore, according to the invention, the semiconductor substrate 20 is configured as a common collector for all of the bipolar transistors 110 of all the pixels 11.

The detector according to the invention further comprises an interconnection grid 8 (fig. 1) configured to connect together the emitter contacts 4 of every transistor 110.

By means of this grid, the emitter contact is made common to all the pixels 11 of the matrix 100.

The technical effect achieved is a distribution of the emitter voltage to all the pixels 11 of the matrix 100.

Advantageously, the implementation of an optimized emitter interconnection grid 8 makes it possible to reduce the contribution of the same to the total capacitance of the device.

In other words, as shown in figure 1 , the grid 8 connects pairs of rows of pixels 11 in such a way that not all the adjacent pixels are directly connected.

The technical effect, as said, is a low total capacitance deriving from the use of reduced grid tracks.

The low capacitance determines briefer current pulses with larger amplitude, facilitating the electronic processing system set up at the detector output.

According to the invention, the detector is configured to function between a first operating condition I, in which it is not struck by alpha particles, and a second operating condition II, in which it is struck by alpha particles.

To enable the detector to function, a positive voltage is applied to the collector contact 7 relative to that of the emitter contact 4.

The current output by the detector is given by the sum of the emitter currents of the pixels 11 making up the matrix 11.

In the first operating condition I, i.e. under quiescent conditions, the output current of the detector is negligible, as the base currents of all the individual pixels 11 are null and the bipolar transistors 110 are thus all 'off'. In operating condition II, when an alpha particle strikes the detector, it generates electron-hole pairs within the semiconductor substrate 20.

The holes are mainly collected by the base region 2 of the pixel 1 which has been struck by the alpha particle and determine therein an increase in the base-emitter voltage.

Advantageously, this gives rise to an output current pulse consisting of a rapid increase in the emitter current of the pixel struck by the alpha particle, followed by a slower return toward the quiescent condition.

The ratio between the overall charge input by the emitter 7 of the pixel 11 struck by an alpha particle and that generated by the latter and collected in the first base region 2 of the same pixel 11 approximately coincides with the common emitter current gain of the bipolar transistor 110 making up the individual pixels and as such is significantly higher than 1.

Advantageously, all the other pixels 11 not struck by an alpha particle remain in an operating region close to the quiescent state.

Consequently, their contribution to the output current of the device is negligible compared to that of the pixel struck by the alpha particle.

In the second operating condition II, if the alpha particle strikes the surface of the detector in a region falling between two or more pixels 11 of the matrix 100, the charge generated can be distributed over two or more pixels. What was said above with reference to the operation of the individual pixel struck by an alpha particle qualitatively applies for the two or more pixels involved in the collection of the charge generated by the alpha particle.

The detector as disclosed according to the invention makes it possible, as its main technical effect, to obtain values of sensitivity to the ambient Radon concentration that cannot be obtained with a single bipolar transistor detector.

This result is made possible mainly by the fact that, where AM is the overall area of the matrix 100 of bipolar transistors 110 and AP is the area of the individual pixel 11 , AM and AP can be dimensioned so as to simultaneously obtain: (i) a sufficiently large overall sensitive area of the matrix of bipolar transistors AM and (ii) a sufficiently low charge collection capacitance proportional to AP, and thus output current pulses of sufficiently large amplitude.

In other words, the area AM and area AP are dimensioned in such a way that the area AP is 2-3 orders of magnitude smaller than AM, and the area AM is of dimensions on the order of a cm2, in particular 1 cm².

A second technical effect achieved is that of obtaining an output signal that is already amplified relative to the charge released by the alpha particle inside the silicon substrate 20, with the advantage of significantly relaxing the required amplification specifications for the electronic reader. This result is made possible by the fact that every individual pixel 11 of the matrix 100 operates, throughout the duration of the current pulse output by the detector, as an individual bipolar transistor 110 in a direct active region. Advantageously, the current gain is significantly greater than 1. A third technical effect achieved is that use can be made of an electronic reader based on commercial integrated circuits, thus potentially low cost. This result is a further consequence of the previous technical effect.

The detector of the invention further comprises a blocking loop 5 (figs. 2 and 3) formed around the emitter region 3.

The blocking loop 5 has the second conductivity type, type p.

The blocking loop 5 is configured to ensure electrical isolation between the emitter region 3 and the collector region 20 in a surface region of the pixel 11.

According to the invention, the blocking loop 5 is heavily doped with the second type of doping.

The detector of the invention further comprises a second base region 6 (fig. 3) configured to reduce the surface leakage current of the individual pixels 11.

As is clearly shown in figure 3, the second base region 6 is contained within the first base region 2.

The second base region 6 had the second conductivity type, type p.

The second base region 6 is heavily doped.

Preferably, the detector further comprises a dielectric layer 10 (fig. 2) formed above the pixels 11.

The dielectric layer 10 is a passivation layer of the detector.

Preferably, the detector further comprises an optical shield 9 (fig. 2) applied above the dielectric layer 10.

Preferably, the optical shield 9 is made with a metal film.

Advantageously, according to the invention the implementation of the blocking loop 5, of the heavily doped base region 6, and of the optical shield 9, as disclosed, contribute to the technical effect of obtaining values of sensitivity to the ambient Radon concentration that cannot be obtained with a single bipolar transistor detector. This is achieved since these components serve to reduce the output current under quiescent conditions, thus making it easier to discriminate the current pulses due to the alpha particles.

Advantageously, according to the invention the first base region 2 and, optionally, also the second base region 6, i.e. the base regions having the same p-type conductivity, are configured so as to be floating.

In other words, these regions are not contacted electrically, so that the transistors remain in an off status when no alpha particle strikes the detector surface.

The output current under these conditions is equal to the leakage current (common emitter) of the bipolar transistor.

The technical effect achieved is that of obtaining low power consumption under quiescent conditions, with the consequent possible use of the matrix of bipolar transistors within a battery-operated measuring system having a sufficiently long-lasting charge for the typical applications.

This can further contribute to obtaining the aforesaid main technical effect, since it allows the output current to be reduced in the blocking loop 5 as well under quiescent conditions.

Claims

1. An alpha particles detector comprising:

a semiconductor substrate (20) of a first conductivity type; a matrix (100) of pixels (11) formed in said semiconductor substrate (20);

a plurality of bipolar transistors (110), wherein every bipolar transistor (110) comprises:

o a first base region (2) of a second conductivity type formed in every pixel (11),

o an emitter region (3) of said first conductivity type formed in every pixel ( ),

o a collector region (20) consisting of said semiconductor substrate (20);

o a second base region (6) of said second conductivity type;

wherein said semiconductor substrate (20) is configured as a common collector for all of the bipolar transistors (1 0) of said pixels (11);

• wherein said first base region (2) and said second base region (6) are configured so as to be floating;

• wherein the detector is configured to function between a first operating condition (I), in which it is not struck by alpha particles, and a second operating condition (II), in which it is struck by alpha particles, said alpha particle generating electron-hole pairs within the semiconductor substrate (20), when it strikes the detector,

o wherein the holes are mainly collected by the base region (2) of the pixel (1 ) which has been struck by the alpha particle and determine therein an increase in the base-emitter voltage, thereby giving rise to an output current pulse consisting of a rapid increase in the emitter current of the pixel struck by the alpha particle, followed by a slower return toward a quiescent condition,

o wherein all the other pixels (11) not struck by the alpha particle remain in an operating region close to the quiescent state, contributing to the output current of the device in a negligible manner compared to that of the pixel struck by the alpha particle.

2. The detector according to claim 1 , wherein said matrix (100) of bipolar transistors (1 0) has an overall area (AM) and said pixel (1 ) has an individual area (AP), where said overall area (AM) and said individual area (AP) are dimensioned so as to simultaneously obtain a sufficiently large overall sensitive area (AM) of the matrix (100) of bipolar transistors (110) and a sufficiently low charge collection capacitance proportional to said individual area (AP), this determining current pulses of sufficiently large amplitude output by the detector.

3. The detector according to claim 1 or 2, wherein each individual pixel (11) of the matrix (100) is configured to operate, throughout the duration of the current pulse output by the detector, as an individual bipolar transistor (110) in a direct active region.

4. The detector according to any of the preceding claims, further comprising

o a heavily doped emitter contact (4) of said first conductivity type associated with said emitter region (3);

o a heavily doped collector contact (7) of said first conductivity type associated with said semiconductor substrate (20) so that said semiconductor layer (20) is interposed between said matrix (100) and said collector contact (7), wherein said collector contact (7) is common to all of the pixels (11) of said matrix (100);

5. The detector according to any of the preceding claims, wherein said semiconductor substrate (20) comprises silicon.

6. The detector according to claim 5, wherein said semiconductor substrate (20) comprises high-resistivity silicon.

7. The detector according to any of the preceding claims, further comprising an interconnection grid (8) configured to connect said emitter contact (4) of every transistor (110) so as to distribute an emitter voltage to all of said pixels (11).

8. The detector according to any of the preceding claims, further comprising a blocking loop (5) of said second conductivity type formed around said emitter region (3), wherein said blocking loop (5) is configured to ensure electrical isolation between said emitter region (3) and said collector region (20) in a surface region of said pixels (11)

9. The detector according to claim 8, wherein said blocking loop (5) is heavily doped with the second type of doping.

10. The detector according to any of the preceding claims, wherein said second base region (6) of said second conductivity type is heavily doped and configured to reduce the surface leakage current of the individual pixels (11).

11. The detector according to any of the preceding claims wherein said first conductivity type is type n and said second conductivity type is type p.

12. The detector according to any of claims 2 to 11 , wherein said individual area (AP) of pixels (11) is from two to three orders of magnitude smaller than said overall area (AM) of said matrix (100).