WO2019159032A1

WO2019159032A1 - Semiconductor device for the detection of ionizing radiation provided with micrometric optical windows

Info

Publication number: WO2019159032A1
Application number: PCT/IB2019/050870
Authority: WO
Inventors: Nicolò CARTIGLIA
Original assignee: Instituto Nazionale di Fisica Nucleare INFN
Current assignee: Instituto Nazionale di Fisica Nucleare INFN
Priority date: 2018-02-19
Filing date: 2019-02-04
Publication date: 2019-08-22
Anticipated expiration: 2020-08-19
Also published as: IT201800002821A1; EP3756034A1

Abstract

A semiconductor detector device for the detection of ionizing radiation comprises, in combination: a substrate (1) of semiconductor material capable of creating electrical charges in response to the passage of ionizing radiation therethrough; at least a first electrode (2) capable of collecting the electrical charges created in the substrate (1), which said electrode (2) is arranged on a first surface of the substrate (1), a second electrode (1) is arranged on a second surface of the substrate (1) opposed to the first one; a meta l layer (3) for providing electrical contact with a reading electronics (4) placed in contact with said first electrode (2). According to the invention, said meta l layer (3) comprises at least one slit (8), having at least one dimension equal to or less than 20 μm.

Description

SEMICONDUCTOR DEVICE FOR THE DETECTION OF IONIZING RADIATION PROVIDED WITH MICROMETRIC OPTICAL WINDOWS

Technical field of the invention

A semiconductor device for the detection of ionizing radiation with optical windows, more precisely the invention relates to a device which can be qualified by the use of a laser signal.

Background art

The present invention generally relates to semiconductor devices for the detection of ionizing radiation.

Devices of this type substantial ly operate by producing an electrical signal upon the passage of ionizing radiation therethrough . In particular, during normal operation, this signal is generated by creating free charges by ionization, electrons and holes, which generate an electric current under the influence of an external electric field.

This signal is then read by an electrode conveniently connected to a reading electronics.

A problem encountered in the use of such devices is the need for qualification thereof prior to their use for the detection of ionizing radiation.

In practice, the use of ionizing radiation for the qualification of the devices encounters various difficulties directly related to the use of radioactive sources, such as for example:

radioactive sources are not always available, the energy of the emitted radiation is too low, the direction of the radiation is not known as it is emitted by the radioactive source in a 77 di recti ons ,

occupational safety regulations limit the use of radioactive sources in rooms not sped fi cal ly equipped.

For these reasons, in most cases, laser signals are used for the qualification, which “emulate” the creation of charges in the semi conductor by ionizing radiation.

The limit of the prior art is that the reading electrodes of the si licon aluminum-coated detectors do not al low the laser signal to penetrate because the metal is notoriously opaque to the radiation of the wave! engths involved.

4s a partial resol uti on of the problem, the prior art provides si licon detectors only partial ly coated with aluminum. On the other hand, the partial coating with aluminum alters the operation of the detector, thus making its behavior unpredictabl e once exposed to the particle beam.

The only information that can be obtained from the qualification concerns only the detector fraction not affected by the aluminum coating.

A further drawback of the devices of the prior art lies in the role played by the optical system used for the qua! i fi cation of the devices: the deposition of charge by the laser strongly depends on the focusing of the optical system which can vary from day to day with the same components and is therefore not known a priori .

In view of the foregoing, the need is felt for a semi conductor device for the detection of ionizing radiation which overcomes the drawbacks of the prior art and allows the qualification of the device by means of a laser signal .

Summary of the invention

The invention consists of a semiconductor device for the detection of ionizing radiation adapted to be qualified by using a laser signal in order to know a priori the response of the device to the passage of ionizing radiation therethrough .

The invention also consists in the method of using such a device in order to qualify it by using a laser signal .

Finally, the method for manufacturi ng the device described above forms part of the invention .

Brief description of the drawings

Figure 1 shows the cross-section of a semi conductor device according to the present invention ;

Figure 2 shows the above device during normal operation, i.e., during exposure to ionizing radiation (2a) and during exposure to a laser signal (2b);

Figure 3 shows a semiconductor device which forms part of the prior art;

Figures 4a, 4b and 4c show the experimental data reported by a test performed on a semiconductor device of the known type (4a) from a spatial (4b) and signal (4c) amplitude point of view.

Detailed description of the invention

Semiconductor devices for the detection of ionizing radiation are constructed by using a substrate of semiconductor material typically characterized by a p o n doping on which a thin layer of doping of a different type from the first one is implanted ( type n or type p) .

By way of a non-limiting example, the device shown in figure 1 is described. Such a device consists of a substrate 1 of semiconductor material character! zed by a p doping ( for example Silicon p ) and a layer 2 of semiconductor material with n doping ( for example silicon n ) placed on a first face of said substrate 1 to form a first electrode.

A second electrode 5 is provided at a second surface of the substrate 1, opposed to the first surface.

The first electrode 2 is typically segmented or made up of several semiconductor material regions characterized by the same type of doping, in the exemplified case n doping.

Said doping regions which form the first electrode 2 may extend along a main direction so as to form strips, giving rise to so-called microstri p detectors , or may be arranged in a matrix forming the so-called pixel detectors .

In technical jargon, the strips or pixels thus formed are referred to as the “reading electrode” . The reading electrodes are generally coated, at least in part, by a layer 3 of metal ( for example aluminum) having the function of ensuring a good electrical contact between the reading electrode and the electronics 4 required to read the electric signal generated .

For the purposes of the following explanations , it is worth noting that the creation of charges in the substrate 1 when this is crossed by ionizing radiation takes place in a substanti al ly cyl i ndrical volume, the radius of which is approxi ately 1 pm extending along the impact direction in a time of the order of the picosecond . A graphical depiction of this phenomenon is shown in figure 2a .

To obtain information on the functioning of the device even before its instal lation and/or exposure to ionizing radiation, it is useful and appropriate to perform tests for the qualification of the semiconductor devices by using a laser signal 6. A graphical depiction of this event is shown in figure 2b.

In order to know a priori the response of the device which wi l l be exposed to ionizing radiation, using a laser signal, it is essential to fol low some precautions which wi l l be better described hereinafter . In general, the optical system employed, as wel l as the features of the device, must be such as to better emulate the exposure to the ionizing radiation.

From a practical point of view and with the devices forming part of the prior art, this means using a pi colaser or a laser with a time pulse of about 10-20 picoseconds , focused as much as possible ( section diameter of approximately 10 20 pm) and with a wavelength al lowing the penetration by the whole thickness of the substrate (general ly but not exclusively equal to 1064 nm) . In the absence of optical focusing systems, there is a laser beam in which the diameter of the cross section is of the order of hundreds of microns.

According to a peculiar feature of the present invention , the semiconductor device according to the invention instead allows the qualification of the device by using a laser signal without the need to resort to optical focusing systems and without affecting the validity of the information obtained relating to the response of the device in case of exposure to ionizing radiation.

According to the i hventi on , the device comprises the following elements:

i. a substrate 1 of semiconductor material (for example of doped silicon p or n) for creating electrical charges in response to the passage of ionizing radiation therethrough ;

77. a first electrode 2, placed on a first surface of the substrate, doped in an opposite manner to the substrate 1;

777. a second electrode 5 arranged on a second surface of the substrate 1, doped in the same manner as the substrate 1;

iv. a metal layer 3, placed in contact with one of the electrodes, with a reading electronics 4 and characterized in that said metal layer 3 comprises at least one slit 8 having at least one dimension equal to or less than 20 pm.

The slit 8 may have any geometrical shape provided that, having identified two main axes, it does not exceed the length of 20 pm in the direction of at least one of these.

It is not essential that the slits 8 respect a minimum size, rather this is limited by the accuracy tha t the manufacturing process can achieve.

Such a minimum size of the sl its 8 should in any case ensure tha t the laser signa l 6 can reach the first electrode 2, and therefore cannot be less than 1/4 of the wave length of the laser signa l 6 used.

In a preferred embodiment of the i nventi on , the slit or slits 8 has/have a minimum size of 1 pm or more, which is an optima l choice for the amp litude of the s igna 1 it genera tes .

In a preferred embodiment of the i nventi on , the slit or slits has/have at least one dimension equa l to or less than 10 pm.

It is a lso convenient that the slits 8 have an area equa l to or less than 200 pm², preferab ly less than 100 pttf .

Advantageously, the choice of slits 8 having such features a l lows verifying the operation of the detection device without this being disturbed by the lack in some areas of the meta l layer 3, since the electric fie lds which determine its operation do not undergo varia tions.

A further advantage of the present invention lies in that it is comp lete ly free, during the qua lification step, from the focusing properties of the laser signa l 6, since the determining e lement for charge deposition is precise ly the size of the slits 8 and therefore it is reproducib le.

In a preferred embodiment of the i nventi on , the slits 8 are positioned at particularly significant points or where undesired effects may occur with respect to normal operation.

By way of example only, differences in the signal as a function of the impact position or crossing of the ionizing radiation with respect to the sensitive surface of the device, typical signal variations of the junctions between the pixels of the reading electrode, and distortion of the signal at the edge of a sensor may be mentioned.

In light of the foregoing , it is advisable to provide slits 8 in the central region of the device and/or at the furthest point from the reading system, as well as in the corners or in any case along the periphery of the main surface of the device.

The semiconductor materials 1 and 2 may be selected from: silicon, diamond, germanium, silicon carbide.

The metals 3 may be selected from: aluminum, titanium, gold, silver, and platinum.

The semiconductor device according to the present invention is advantageously achievable by the techniques known to those skilled in the art.

For example, the device may be made from a semiconductor substrate 1 with a first electrode 2 capable of collecting the electrical charges created in the substrate 1, which electrode is placed on a first face of the substrate itself.

The metal layer 3, which covers said first electrode 2, is appli cable by a process including the following main steps:

a. providing a substrate 1 of semiconductor material capable of creating electrical charges in response to the passage of ionizing radiation therethrough , equipped with a t least a first electrode 2 for co l lecting the e lectrica l charges created in the substrate 1, where sa id first electrode 2 is arranged on a first surface of the substrate 1, and a second e lectrode 5 is arranged on a second surface of the substrate 1 opposed to sa id first surface;

b. app lying a layer of photoresistive materia l over the who le surface on which there is the first electrode 2 or the second e lectrode 5;

c. removing, by means of a photo lithographic process, the geometry of the slits 8 to be made on the first e lectrode 2 or on the second e lectrode 5; d. depositing a meta l layer over the who le surface on which the photoresistive paste was deposited: the meta l is deposited on the photoresistive paste where it has not been removed, or at the openings described in the previous step “c.”, on the first e lectrode 2 or on the second electrode 5;

e. removing the photoresistive paste referred to in the previous step “b.” by washing.

For a more accurate description, reference may a lso be made, for examp le, to the description of the opera ting steps of production reported in https : //en . wikipedia. org/wi ki /Semi conductor_devi ce_ f abri cation.

It is hardly necessary to note that the optica l paste should be p laced where the meta l layer is not desired. For this reason, the optica l paste should be deposited to form a figure having at least one dimension equa l to or less than 20 pm.

In view of the foregoing, a semiconductor device according to the present invention may be advantageously used in a qua lification procedure which, by using a laser signa l, is able to acquire information on the measurabl e signa l during its exposure to ionizing radiation.

The qua lification of the device through the laser signa l is performed by simul ati ng the effect of ionizing radiation with the laser, and by studying the shape of the signa l read by the reading electronics 4. Among the important parameters in this qua lification are: (i) the length of the signa l, (ii) the speed at which the signa l reaches its maximum va lue, and (Hi) the duration of the signa l. The wavelength of the laser may range from 400 nm (below which the signa l does not penetrate into the materia l) up to about 2- 3000 nm, beyond which the laser does not generate enough ionization. For example, using a laser with a wavelength of 1060 nm, at a power of ~1 microwatt, a signa l simi lar to that of an ionizing particle is genera ted.

By way of example on ly, the results of measurements made on a si l icon device of the known type, shown in figure 3 with a surface of 1. 3 x 1. 3 mm² and characterized by the presence of a meta l layer provided with at least one circular shaped opening 8 with a diameter of 0. 8 mm, are reported.

Further detai ls can be found in the scientific publ ication NIMA-D-17 -01002 entitled: “Studies of uniformity of 50 urn low-gain ava lanche detectors at the Fermi! ab test beam” submitted to Nuclear Inst and Methods in Physics Research, A.

Figure 4b shows the amplitude of the signal generated by a particle striking the square silicon detector with the circular optical window. The detectors in question were previously i r radiated to perform a radiation resistance study.

/4s can be seen in the two figures 4b and 4c, the amplitude of the signal generated at the aperture 8 is smaller than the signal measured at the metal layer.

Claims

1. A semiconductor detector device for the detection of ionizing radiation comprising, in combination

a substrate of semiconductor material ( 1 ) able of creating electrical charges in response to the passage of ionizing radiation through it,

at least a first electrode (2) able of collecting the electrical charges created in the substrate (1) , which electrode (2) is arranged on a first surface of the substrate,

a second electrode (5) arranged on a second surface of the substrate ( 1 ) opposed to the first,

a metal layer (3) to provide electrical contact with a reading electronics (4) placed in contact with said first electrode (2),

characterized in that said metal layer (3) comprises at least one slit (8), having at least one dimension equal to or less than 20 pm.

2. Device according to the previous claim, characterized in that said at least one slit has at least one dimension equal to or less than 10 pm.

3. Device according to any one of the preceding claims, characterized in that said at least one slit (8) has at least one dimension equal to or greater than 1 pm.

4. Device according to any one of the preceding claims, wherein said at least one slit has an area equal to or less than 200 pm², preferably less than 100 pttf .

5. Device according to any one of the preceding claims, wherein said at least one slit (8) has an area equal to or less than 50 mhi².

6. Device according to any one of the preceding claims, wherein the material of the semiconductor substrate (1) and of the second electrode (5) are of the same type of doped silicon n or p, whereas the material of the first electrode (2) is silicon doped in opposite way to the substrate (1) , hence p or n.

7. Device according to any one of claims 1 to 5, wherein the semiconductor substrate material ( 1 ) is diamond and the electrodes (2, 3) are metallic.

8. Device according to any one of the preceding claims, wherein the metal layer (3) is made of aluminum.

9. Method for the qualification of a semiconductor detector device according to the preceding claims, by using a laser signal (6) without the need to resort to optical focusing systems and without affecting the validity of the information obtained relative to the response of the device in case of exposure to ionizing radiation, characterized in that it comprises the following main steps:

A. exposing said device to a beam of photons having a wavelength ranging from 400 nm, below which the signal does not penetrate into the material, up to about 2-3000 nm, beyond which the laser does not generate enough ionization;

B. detecting the electrical signal supplied by the reading electronics (4).

10. Method for manufacturing a semiconductor detector device according to any one of the claims 1 to 8, compri si rig the following main steps:

a. providing a substrate (1) of semiconductor material able of creating electrical charges in response to the passage of ionizing radiation through it, equipped with at least a first electrode (2) for collecting the electrical charges created in the substrate (1) , wherein said first electrode (2) is arranged on a first surface of the substrate (1) , and a second electrode (5) is arranged on a second surface of the substrate (1) opposed to said first surface; b. applying a layer of photoresistive material or paste over the whole surface on which there is the first electrode (2) or the second electrode (5);

c. removing by photolithography the geometry of the slits (8) to be made on the first electrode (2) or on the second electrode (5);

d. depositing a metal layer (3) over the whole surface on which the photoresistive paste was deposited: the metal being deposited on the photoresistive paste where it has not been removed, or at the slits (8) on the first electrode (2) or on the second electrode (5);

e. removing the photoresistive paste by washing.