PROVIDED WITH CAGE-LIKE READING ELECTRODES

Technical field of the invention

The invention relates to semiconductor devices for detecting ionizing radiation based on a resistive readout system with metal electrodes such as to prevent signal propagation to reading electrodes spaced apart from the point of impact.

Prior art

The present invention generally relates to semiconductor devices for detecting ionizing radiation, Figure 1, based on the resistive readout principle. The detectors at issue are characterized by a resistive electrode (3) and can be alternating current (AC) coupled, Figure la, or direct current (DC) coupled, Figure lb, to the readout pads (1), i.e., they may have (AC) or not (DC) an insulating layer (2) between the resistive electrode (B) and the readout pads (1).

The devices of this type substantially work by producing an electrical signal as the ionizing radiation passes therethrough. In particular, during normal operation, such a signal is generated by creating free charges, electrons, and holes by ionization, which generate an electric current under the bias of an external electric field. Such a signal is then read by readout electronics appropriately connected to an electrode. The device at issue typically has a resistive electrode (3) on which the signal generated by an incident particle is formed. The signal propagates to the resistive electrode (B) and is read by the readout pads (1). An example of how the signal is measured from multiple pads is shown in Figure 2; the particle struck the detector at the point of impact indicated by the circle. The four pads close to the point of impact saw a signal, appropriately read by the readout electronics.

A problem encountered in the use of such devices is the propagation of the initial signal to a multiplicity of reading electrodes, Figure 3a. Such a phenomenon has the following disadvantages, inter al i a :

• the initial signal generated by a single event is fractioned into a multiplicity of signals of lower intensity, sometimes too small to be visible. Therefore, some of the signal is lost. This problem is shown in Figure 3a, in which the pads identified by reference numeral (1) see a small portion of the initial signal, losing accuracy.

• By dividing between many readout pads, the signal propagates over a large surface of the detector .

This causes the likelihood of a superimposition of two or more initially spaced apart signals propagating until they are superimposed. This problem is shown in Figure 3b: the pads identified by reference numeral (2) see signals from two particles, damaging the capability of the detector to reconstruct signals from the two particles in an accurate manner.

• The number of reading electrodes concerned by the event of a single incident particle changes depending on the point of impact: in some points, two pads are involved; in others, three, four, or up to 6 pads are involved.

The limitation of the prior art is that the reading electrodes of the detectors at issue are characterized by a not well-defined signal propagation over a multiplicity of reading electrodes with all the disadvantages described above.

In light of the above, the need is felt for a semiconductor device for detecting ionizing radiation based on the resistive readout principle, which overcomes the drawbacks of the prior art and limits the signal propagation to a small number of el ect rodes .

Summary of the invention

The invention is a semiconductor device for detecting radiation based on the resistive readout design, Figures la, lb, in which the reading electrodes are shaped so that the signal propagates over a limited number of electrodes.

An example of this design is shown in Figures 4a and 4b: the signal propagates until it reaches all sides of the metal electrodes designed to prevent the signal from propagating to the furthest electrodes, in an external position with respect to those directly facing the point of impact. Figures 4a and 4b show two examples of electrodes according to the present invention, also referred to as cage-like electrodes: Figure 4a shows a geometry for dividing the signal between three readout pads, and Figure 4b for dividing it between four readout pads. The electrode branches are characterized by 3 dimensions: width (1), length (L) , and distance (d) between different electrode branches.

Brief description of the drawings

Figures la and lb show the cross-section of a semiconductor device of the AC-resistive readout type (Figure la) and DC-resistive readout type (Figure lb), respectively.

Figure 2 shows the division of a signal between 4 readout pads.

Figures Ba and 3b show two examples (a) of signal propagation between six read pads, (b) of the superimposition of two signals, one involving 4 readout pads and one involving 6 readout pads. Figures 4a and 4b show two preferred embodiments of the electrodes according to the present invention.

Figure 5 shows four further preferred embodiments of the electrodes according to the present invention.

Figure 6 shows a further embodiment of the electrodes according to the present invention.

Detailed description of the invention For the purposes of the present invention, "pad" means the usually metallic portion of the detector adapted to read the signal generated by the interaction of the ionizing radiation with the detector or detection device; "event" means the interaction of the ionizing radiation with the detection device.

As known to those skilled in the art, said events generate a charge signal detected by the metal electrodes of the device. The present invention addresses the optimal geometry of said metal electrodes, or pads, so that the signal generated by the single event is collected by a predetermined number of reading electrodes.

The semiconductor devices for detecting ionizing radiation are generally constructed by using a substrate of semiconductor material typically characterized by p-type or n-type doping on which a thin doping layer of a different type than the former (n-type or p-type) is implanted.

The device shown in Figure la is described for illustrative and non-limiting purposes only. Such a device comprises a substrate 4 of semiconductor material characterized by p-type doping (e.g., p silicon) and a layer S of semiconductor material with n-type doping (e.g., n silicon) placed on a first face of said substrate 5 to form a first electrode. A second electrode 5 is provided at a second surface of the substrate 5, opposite to the first surface. The first electrode S in the type of resistive detectors is typically continuous. An insulating material 2, typically silicon oxide or silicon nitride, can be placed above the electrode 3. In this case, metal electrodes 1 are placed above this layer.

In the absence of the insulating layer 2, the pads are placed directly in contact with layer B, Figure lb. These metal pads are referred to as reading electrodes in technical jargon. The reading electrodes generally consist of a layer 1 of metal, e.g., aluminum, serving the function of ensuring good electrical contact between the reading electrode and the electronics needed for reading the generated electrical signal. The sensor design may include a further layer 6, doped with the same dopant as the substrate 4, but with a higher level of doping, i.e., with higher concentrations of dopant.

According to the present invention, the reading electrodes 1 have such a shape as to limit the signal propagation. These electrodes can have any type of geometry which achieves the signal propagation limitation to electrodes further away from the point of radiation-detector interaction. Two examples of geometry are shown in Figure 4a, with a three-readout pad geometry, and in Figure 4b with four readout pads .

In a preferred embodiment of the invention, each metal electrode 1 is cross-shaped, i.e., comprising four branches intersecting at the central point of the electrode. The electrode array of said geometry is shown in Figure 4b. In practice, the generated signal is collected by four electrodes placed at the vertices of a square. The formed regions, i.e., surface portions, have a substant ally square area and are delimited by four pairs of branches orthogonal to each other and belonging to four different metal electrodes 1.

In a second preferred embodiment of the invention, each metal electrode 1 is star-shaped, i.e., comprising six branches intersecting at the central point of each electrode.

The electrode array of said geometry is shown in Figure 4a. The active regions formed have a substantially triangular area and are delimited by three pairs of branches adjacent to each other and belonging to three different metal detectors 1. In other words, the signal is contained between three electrodes placed at the vertices of a triangle.

Further preferred embodiments are shown in Figure 5, in which, by way of example, (a) the signal is contained between the branches of two electrodes placed at opposite vertices of a rectangle. The branches of each electrode form a cross with two arms orthogonal to each other, i.e., four branches orthogonal two by two. Two electrodes 1 are placed therebetween to delineate a surface portion embedded between a pair of arms of each electrode, respectively.

In the embodiment shown in Figure 5(b), the signal is contained by the electrodes only along one of the Cartesian directions. Each electrode has the shape of a rectangle obtained by joining together several branches. The electrodes are mutually aligned to define rows of electrodes where the branches of one electrode end when the branches of the next electrode begin. In the embodiment shown in Figure 5 (c) , the signal is contained between two electrodes. The branches of each electrode form half of a rectangle so that the branches of two opposite electrodes completely delimit a surface. Electrode pairs are positioned in sequence, so that the entire surface is divided into a sequence of electrode pai rs .

In the embodiment shown in Figure 5(d), the signal is contained within a single electrode. Four different embodiments of the branches forming the single electrode are suggested in the drawing. These embodiments differ in the shape of the metal branches, which are respectively: i. Grid-shaped with dense mesh, i i . Star-shaped , iii. Grid-shaped with coarse mesh, iv. Shaped as 4 separate squares.

Figure 6 shows a preferred embodiment of the invention in which the branches of the metal electrodes are mutually connected, thus forming a grid with readout points at the intersection points of the grid.

Further geometric configurations achieving the goal of forming active regions delimited by the branches of a predetermined number of electrodes fall within the scope of protection of the present patent.

In light of the description provided so far, the semiconductor device for detecting ionizing radiation according to the present invention comprises the following main elements:

- a substrate 4 of semiconductor material characterized by a p-type or n-type doping and having a first and a second main face opposite to each other;

- - a layer B of semiconductor material with n- type or p-type doping, different from the doping type of the substrate 4, placed on the first face of said substrate 4 to form a first elect rode;

- a second electrode 5 at the second face of the substrate 4;

- an optional insulating material layer 2 placed on top of the semiconductor layer 3;

- a multiplicity of metal electrodes 1 positioned over said insulating material layer 2 or, if absent, directly on the semiconductor layer 3, characterized in that each metal electrode consists of one or more branches and in that said electrodes 1 are positioned with respect to one another to form surface regions delimited by the branches of a defined number of electrodes. The sem conductor material layer 3 generally has a resistivity between 100 Ohm and 10 kOhm.

Considering all alternative embodiments falling within the scope of protection of the present patent, we believed that the defined number of electrodes to delimit an active surface region is from 1 to 12. Preferred embodiments, as shown in Figures 4a and 4b, are character zed by 3 and 4 electrodes, respect vely, adapted to delimit the surface concerned by the generation of the single event signal .

Generally, the width (1) of each branch of a metal electrode 1 is between 5 and 30 micrometers, the length (L) is less than one millimeter, which the positioning of the various electrodes is such that the distance (d) between the branches of one electrode and the adjacent one is less than 50 micrometers .

In the described embodiments, the branches of one electrode are spaced apart from the branches of the adjacent electrodes by a distance of less than 50 micrometers so as to form surface areas well delimited by the combination of the branches of the adjacent electrodes.

Indeed, longer distances would not ensure the confinement of the signal generated by an interaction event in a delimited portion of the detector.

Longer distances would allow the signal generated by an interaction event to propagate beyond the surface region substantially delimited by the predetermined number of electrodes 1, and therefore the desired effect of the present invention would not be ensured.

In a variant of the invention, the metal electrodes 1 are placed at mutual distances of even less than 10 micrometers, if not actually in contact two by two or in contact with one another. The effect of having electrodes 1 in mutual contact is the formation of a sort of metal grid capable of containing and collecting the signal generated by an interaction event within a predetermined surface delimited by a fixed number of electrodes 1. It is apparent that the device according to the present invention achieves advantages in terms of ability to reconstruct the signal generated by a single event and sensitivity of detecting the interaction of the device with the ionizing radiation.