A semiconductor particle-detector and methods for the manufacture thereof

Abstract: A particle-detector is formed on a die of semiconductor material (20) comprising: first and second layers (22, 23) with a first type of conductivity (N), a third layer (21) with a second type of conductivity (P), interposed between the first and second layers (22, 23), first and second means (25, 31; 26, 32) for electrical connection with the first and second layers (22, 23), respectively, disposed on the opposite surfaces thereof to those of the junctions with the third layer (21) and third means (27, 24) for electrical connection with the third layer (21). To permit large-scale industrial manufacture, the third means (27, 24) for electrical connection with the third layer (21) comprise a region (24) with the second type of conductivity (P) which extends from the front face of the die as far as the third layer (21) and means (27) for surface electrical contact with this region.

Description: The present inyention relates to devices for detecting atomic or nuclear particles and, more particularly, to a semiconductor particle-detector of the type defined in the preamble to the first of the claims which follow the present description, as well as to methods for the manufacture thereof. During the study of nuclear reactions brought about in the laboratory, it is necessary to detect and identify heavy nuclear particles, that is, ions of atoms with high atomic numbers, or nuclear fission fragments or, in any case, nuclear-reaction products. Similar requirements generally arise during the analysis of radiation phenomena with the emission of particles, as in cosmic-ray analysis. In order to identify a particle, it is necessary to measure various quantities associated therewith, such as the kinetic energy, the momentum (vectorial), and the atomic mass and number. A known method of measuring mass and energy provides for the use of a thick semiconductor detector (typically 300-400 µm thick) and a thin semiconductor detector (typically from a few µm to a few tens of µm thick) which are disposed on the path of the particle. The particle interacts firstly with the thin detector, losing only some of its energy (ΔE), and then with the thick detector, to which it yields all of its residual energy (E - ΔE), where E represents the total energy of the particle at the moment of impact with the thin detector. Since the loss of energy ΔE through the thin detector is more or less directly proportional to the mass and inversely proportional to the total energy E, a measurement of ΔE and E - ΔE enables the mass of the particle to be found. When the particles to be detected are particularly heavy or have relatively low energy, the ΔE detectors have to be very thin to take account of the low penetration capacity and are therefore very fragile and very expensive. Moreover, if the detector also has to be able to give information on the spatial distribution of the particles, that is, to be a position detector, its sensitive area must be quite extensive. Naturally, this need conflicts with the need to have very small thicknesses, and the surface areas of position detectors which can be produced by the usual techniques for reducing thickness are thus quite limited; in practice they do not exceed one square centimetre. In order to overcome this limitation, a composite ΔE and E detector, which is shown schematically in Figure 1 of the appended drawings and can be produced on a single piece of semiconductor, has been proposed, see Nuclear Instruments and Methods in Physics Research (1987) p. 365-377. It is produced by implanting boron ions with high energy (10.8 MeV) in a semiconductor wafer doped with N-type impurities. This produces a highly doped P-type layer about 0.6 µm thick, indicated 11, buried at a depth of about 12 µm from the implantation surface, that is, a P+ layer interposed between two N layers, indicated 12 and 13 in the drawing. The wafer is then cut into square dice of the desired dimensions, such as that indicated 10 in the drawing, and the individual dice are subjected to a treatment for the deposition and diffusion of boron at high temperature to form a P+ type region, indicated 14, on their side surfaces in contact with the edges of the buried P+ layer 11. Metal electrodes, indicated 15, 16 and 17, respectively, are formed on the front and rear surfaces as well as on the side surfaces in order to establish electrical contact with the two layers and with the buried layer. Before the electrodes 15 and 16 are formed on the front and on the rear of the die, the semiconductor is enriched with N-type impurities, as shown in the drawing by the N+ regions indicated 8 and 9. Two diodes having a common anode constituted by the buried layer are thus produced on the same die. The upper, thin diode constitutes the ΔE detector and the thick, lower diode constitutes the E detector. In operation, two voltages, V1 and V2, respectively, relative to a common terminal represented by the earth symbol in the drawing, are applied between the upper and lower electrodes 15 and 16 and the side electrode 17 in operation, their signs being such as to bias the two diodes in opposite directions. Two depletion regions are thus formed and constitute the active parts of the two detectors. A particle to be detected, represented by an arrow with a broken line, strikes the front surface of the composite detector and, passing through the two depletion regions causes the formation of electron-vacancy pairs which move towards the electrodes forming pulsed currents. These currents are collected and amplified by suitable circuits, generally indicated 18 and 19, and are then measured and displayed by means of a suitable electronic device, indicted 7, to give an indication of the quantities ΔE and E - ΔE, and hence of the masses of the incident particles. With the known detector described above, the ΔE detector portion can be made very thin without problems of fragility, since it is integral with the thicker E-detector portion. However, this detector is not suitable for large-scale manufacture because a considerable proportion of the necessary steps (the formation of the lateral P+ region, the formation of the electrodes) has to be carried out individually for each die after the wafer has been cut. Moreover, it cannot operate as a position detector. An object of the present invention is to propose a semiconductor particle-detector having a structure which is suitable for large-scale manufacture. Another object is to propose a semiconductor particle-detector which can operate as a position detector. Yet another object is to propose methods of manufacturing the novel particle-detector. These objects are achieved by the detector defined in claim 1 and by the methods defined in independent claims 6 and 8. Specific embodiments are disclosed in the dependent claims. The invention and the advantages derived therefrom will be understood better from the following detailed description of two embodiments thereof which are given purely by way of non-limiting example with reference to the appended drawings, in which: Figure 1, which has already been described, shows, in section, a particle-detector according to the prior art and some circuits which, together with the detector, constitute a particle-detection device, and Figures 2 and 3 show, in section, two particle-detectors according to two embodiments of the invention. As can be seen in Figure 2, the detector according to the invention is formed in a single die of semiconductor material, for example, monocrystalline silicon, generally indicated 20, which comprises three superimposed layers: two N-type layers, an upper layer 22 and a lower layer 23, and a P-type layer 21 which is highly doped and is thus marked P+ and is buried, that is, interposed between the two N-type layers. Unlike the known structure shown in Figure 1, the lower layer 23 extends to the front face of the die laterally of the surface of the upper layer 22, so as to surround the upper layer 22 and the P+ layer 21. For electrical contact with the P+ layer 21, there is a region 24 which is also a highly doped P-type region extending from the upper or front surface of the die as far as the P+ layer 21 and, on the surface, has a contact element in the form of a metal strip 27, for example, of aluminium. Seen in plan, the P+ contact region 24 preferably has the shape of a square frame. However, it could also be constituted simply by one or two strips or even by a plurality of isolated regions. Similarly, the metal contact may also be shaped differently. The upper layer 22 and the lower layer 23 are contacted electrically by means of two N-type surface regions which are highly doped and hence are marked N+, indicated 31 and 32, respectively, and by means of two metal layers 25 and 26, respectively. The latter, together with the metal strip 27, constitute the terminal electrodes of the detector and serve to connect the detector to biasing voltage sources and to amplifier circuits, processors and indicators similar to those already described briefly in connection with the known detector. The upper electrode is preferably formed as a very thin layer to reduce as much as possible its screening effect with respect to the incident particles. Alternatively, instead of extending over the entire surface region 31, it may extend solely along a peripheral region thereof, like a frame. The upper surface of the die is covered by a layer of insulating material, typically silicon dioxide, indicated 33, in which holes are formed for the contacts of the electrodes 25 and 27. It should be noted that the depletion region of the lower diode, that is, that formed by the buried P+ layer 21 and by the N layer 23, extends deeply into the N layer 23 and also laterally to the P+ regions 24. The structure is preferably of a size such that the portion of the N layer 23 between the P+ region 24 and the edge of the die 20 is wider than the maximum extent of the depletion region. The structure described above can be manufactured in large numbers from the same silicon wafer and does not require manufacturing steps after the wafer has been cut into dice. This permits large-scale industrial manufacture. Two methods of producing the detector structure according to the invention will now be described. The first method provides for the following principal steps, carried out on an N-type monocrystalline silicon wafer: a. the formation of a silicon dioxide (SiO 2 ) layer on the entire front face of the wafer by growing at high temperature in an oxidizing atmosphere, b. the opening, in the SiO 2 layer, of windows with dimensions corresponding to those desired for the P+ contact regions 24, by the usual techniques of masking and selective removal of material, c. the implantation of boron ions at a high dose (10 14 -10 15 a/cm 2 ) and with low energy (60-80 KeV) on the front face of the wafer, d. high-temperature diffusion in an oxidizing atmosphere to form P+ regions 24 and to form an SiO 2 layer on the uncovered areas of the front face of the wafer, e. the opening, in the SiO 2 layer of windows with dimensions corresponding to those desired for the buried P+ layer 21, f. the implantation of boron ions at a high dose (10 14 -10 15 a/cm 2 ) and with high energy (0.5-3MeV), so as to form the P+ layer 21 at a depth of 1-3 µm from the front face and with a thickness of 0.5-1 µm, g. the formation of a new SiO 2 layer on the uncovered front areas, h. the opening of windows with dimensions corresponding to those desired to form an enriched N+ surface region, i. the implantation of arsenic ions at a high dose (about 10 15 a/cm 2 ) and with low energy (40-60KeV) on the front face of the wafer, l. the implantation of phosphorus ions at a high dose (10 15 -10 16 a/cm 2 ) and with low energy (80 KeV) on the rear of the wafer, m. the formation of a new SiO 2 layer on the uncovered front areas, n. the opening of windows with dimensions corresponding to those desired to form the contacts for the electrodes 25 and 27, o. the deposition of a layer of aluminium on the front face of the wafer, p. the selective removal of aluminium from the layer to define the electrodes 25 and 27, q. the deposition of a layer of aluminium on the rear of the wafer, r. the division of the wafer into dice. The second method provides for the following principal steps, carried out on an N-type monocrystalline silicon wafer: initial oxidation as in step a. of the method described above, the opening of windows as in step b, the implantation of boron ions at a high dose (10 14 -10 15 a/cm 2 ) and with low energy (40-80KeV), high-temperature heat treatment (900-1100°C) to allow the boron to diffuse to a depth of about 1 µm, the removal of the SiO 2 formed during the previous step, the formation of an N-type monocrystalline layer on the front face of the wafer by epitaxial growth at high temperature to produce the N layer 22, the formation of an SiO 2 layer on the front face of the wafer (the surface of the epitaxial layer), the formation of P+ contact regions 24 as in steps b. to d. of the method described above, the formation of enriched regions and the formation of the electrodes on the front face and on the rear of the wafer as in steps h. to q. of the method described above, the division of the wafer into dice. The embodiment of Figure 3, in which the reference numerals correspond to the same parts as in Figure 2, represents a position detector. The only difference in comparison with the structure of Figure 2 is that, instead of a single contact on the front face of the die, four parallel contact strips, indicated 25a, 25b, 25c and 25d are provided and, instead of a single enriched N+ region, a number of regions, indicated 31a, 31b, 31c and 31d, equal to the number of contact strips, is provided. In operation, each electrode is disconnected from the others so as to provide a datum on the pulsed current produced solely in a strip of the underlying depletion region and, therefore, on the relative positions of the incident particles. According to another embodiment of the invention which permits even more precise determination of the impact positions of the particles, a plurality of contact elements of small dimensions as independent cathode electrodes is provided on the front face of the die. It is clear from the foregoing that the detector according to the invention can be produced without problems with very extensive active surfaces and, as already stated, by large-scale industrial production methods. Although only two embodiments of the invention have been described and illustrated, clearly many variations and modifications are possible within the scope of the invention as defined by the appended claims. For example, instead of being formed by a NP+N series of layers, the structures described could be constituted by a PN+P series of layers and the manufacturing methods could be modified correspondingly with the use of techniques wholly similar to those described and dopants of the opposite types to those indicated.

Claims: A particle-detector formed on a die (20) of semiconductor material having a front face, a rear face and a side wall and comprising: a first layer (22) of a first conductivity type having a surface on the front face of the die, a second layer (23) of the first conductivity type, thicker than the first layer (22), having a surface on the rear face of the die, a third layer (21) of a second conductivity type interposed between the first and second layers, first (25, 31) and second (26, 32) means for electrical connection with the first layer (22) and the second layer (23), respectively, disposed on the surface thereof, and third means (27, 24) for electrical connection with the third layer (21) comprising a region (24) of the second conductivity type which extends form the front face of the die as far as the third layer (21) and means (27) for surface electrical contact with this region (24), characterised in that the second layer (23) extends to the front face of the die laterally of said surface of the first layer (22), so as to surround the first (22) and the third (21) layers and the means (27) for surface electrical contact with the region (24) lies on the front face of the die. A particle-detector according to Claim 1, in which the region (24) of the second conductivity type is in contact with edge regions of the third layer (21) and extends laterally as far as a distance from the side wall of the die (20). A particle-detector according to Claim 1 or Claim 2, in which the first (25, 31) means for electrical connection with the first layer (22) comprise a plurality of strips (25a-d) of electrically-conductive material insulated from one another. A particle-detector according to Claim 1 or Claim 2, in which the first means for electrical connection with the first layer (22) comprise a plurality of contact elements insulated from one another. A device comprising the detector according to any one of the preceding claims, also comprising means (18, 19, 7, V1, V2) connected to the first, second and third electrical connection means, for biasing in opposite directions the two junctions formed by the first layer (22) and by the second layer (23) with the third layer (21), for detecting currents through the junctions, for obtaining and processing signals associated with the currents detected, and for displaying the results of the processing of the signals. A method of manufacturing a particle-detector according to any one of Claims 1-4 on a die of semiconductor material of a first conductivity type , having a front face and a rear face, comprising the following steps: forming the third layer (21) of the second conductivity type interposed between the first and second layers (22, 23) of the first conductivity type having surfaces on the front face and on the rear face of the die, respectively, by implanting doping material through the front face of the die forming the first and second electrical connection means (25, 31; 26, 32) on the surfaces of the first and second layers (22, 23), and forming the third means (27,24) for electrical connection with the third layer (21), characterised in that the step of forming the third means for electrical connection with the third layer (21) comprises forming the region (24) of the second conductivity type extending from the front face of the die as far as the third layer (21), and forming the means (27) for surface electrical contact with this region. A method according to Claim 6, in which the step of forming the region (24) of the second conductivity type takes place by implanting and diffusing doping material before the implanting step to form the third layer (21). A method of manufacturing a particle-detector according to any one of Claims 1-4 on a die of semiconductor material (20) starting with a substrate of a first conductivity type, characterised by the following steps: forming the third layer (21) of a second conductivity type by surface doping of the substrate, thereby forming a first junction between the third layer (21) and the substrate, forming the first layer (22) of the first conductivity type on the third layer (21) of the second conductivity type by epitaxial growth on the substrate, thereby forming a second junction between the third layer (21) of the second conductivity type and the first layer (22), forming the second and first electrical connection means (26, 32; 25, 31) on the surfaces of the substrate and the first layer (22) facing said first and second junctions, respectively, and forming the third means (27, 24) for electrical connection with the third layer (21) of the second conductivity type by forming the region (24) of the second conductivity type extending from the surface of the first layer (22) facing said second junction as far as the third layer (21) of the second conductivity type and by forming the means (27) for surface electrical contact with said region (24) of the second conductivity type.

Inventors: FALLICA PIERO GIORGIO, FALLICA, PIERO GIORGIO
Applicants: CO RI M ME BR CONSORZIO PER LA, CO.RI.M.ME.<BR>CONSORZIO PER LA RICERCA SULLA<BR>MICROELETTRONICA NEL MEZZOGIORNO
Application year: 1995
Main classification: H01L31
IPC codes: H01L31, G01T1