PHOTODETECTION DEVICE WHICH HAS AN INTER-DIODE ARRAY AND IS OVERDOPED BY METAL DIFFUSION AND MANUFACTURING METHOD

Abstract

A photodetection device and a method for manufacturing the device, the device including a substrate and an array of diodes, the substrate including an absorption layer including a first type of doping, and each diode including, in the absorption layer, a collection region including a second type of doping opposite to the first type. The device further includes, under the surface of the substrate, a conductive mesh including at least one conductive channel inserted between the collection regions of two adjacent diodes, the at least one conductive channel including the first type of doping and a higher doping density than the absorption layer. The doping density of the at least one conductive channel is the result of a diffusion of metal in the absorption layer from a metal mesh provided on the surface of the substrate.

Claims

1-12. (canceled)

13. A photodetection device comprising: a substrate which comprises a front face for receiving an electromagnetic radiation, a rear face opposite and substantially parallel to the front face, and an absorption layer including a first doping type; an array of diodes each including in the absorption layer a collection region being flush with the rear face of the substrate and including a second doping type opposite to the first type; a conduction meshing buried in the substrate and being flush with the rear face of the substrate, the conduction meshing comprising at least one conduction channel sandwiched between the collection regions of two adjacent diodes, the at least one conduction channel including the first doping type and a higher doping density than the absorption layer; and a metallic meshing present on the rear face of the substrate and which covers the conducting meshing, wherein the doping density of the at least one conduction channel results from a metal diffusion in the absorption layer from the metallic meshing.

14. The device according to claim 13, further comprising a peripheral substrate contact arranged on at least one side of the array of diodes.

15. The device according to claim 14, wherein the metallic meshing is in electric contact with the peripheral substrate contact.

16. The device according to claim 13, wherein the conduction meshing includes at least one conduction channel extending between two adjacent lines of diodes.

17. The device according to claim 16, wherein the conduction meshing includes a plurality of conduction channels arranged such that there is a conduction channel sandwiched between the collection regions of each of the adjacent diodes of the array of diodes.

18. The device according to claim 13, further comprising a passivation layer that covers the substrate except for contact regions of an electrically conductive bump with a collection region of a diode.

19. The device according to claim 13, wherein the absorption layer is a layer of CdHgTe.

20. The device according to claim 19, wherein the absorption layer includes an intrinsic doping by mercury vacancies or an extrinsic doping by arsenic incorporation.

21. The device according to claim 13, wherein the absorption layer includes a gradual bandgap increase from the rear face of the substrate in the thickness of the absorption layer.

22. The device according to claim 13, wherein the absorption layer includes an abrupt bandgap decrease which appears at a given depth in thickness of the absorption layer from the rear face of the substrate, and which delimits a first region having a lower bandgap in proximity of the rear face of the substrate in which there is the conduction meshing, and a second region having a lower bandgap away from the rear face of the substrate.

23. A method for manufacturing a photodetection device including a substrate and an array of diodes, the substrate including a front face for receiving an electromagnetic radiation, a rear face opposite to and substantially parallel to the front face and an absorption layer having a first doping type, each diode including in the absorption layer a collection region being flush with the rear face of the substrate and having a second doping type opposite to the first type, the method comprising: forming, buried in the substrate and being flush with the rear face of the substrate, a conduction meshing which comprises at least one conduction channel sandwiched between the collection regions of two adjacent diodes, the at least one conduction channel having the first doping type and a higher doping density than in the absorption layer; forming a metallic meshing comprising at least one metallic row on the rear face of the substrate, and wherein the forming the conduction meshing is made by metal diffusion from the metallic meshing.

24. The method according to claim 23, further comprising: after forming the metallic meshing, depositing a passivation layer onto the substrate and the metallic meshing, and wherein the forming the conduction meshing is made during an annealing for stabilizing the interface between the substrate and the passivation layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Further aspects, purposes, advantages and characteristics of the invention will better appear upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made in reference to the appended drawings in which, in addition to FIGS. 1a, 1b and 2 already described:

[0026] FIGS. 3a and 3b are respectively a top view and a cross-section view along a row of diodes of a matrix of diodes in accordance with the invention;

[0027] FIGS. 4a and 4b illustrate alternative embodiments of the invention with a bandgap variation in the thickness of the absorption layer;

[0028] FIGS. 5a-5f illustrate a possible embodiment of a method for manufacturing a photodetection device in accordance with the invention.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0029] In reference to FIGS. 3a and 3b, the invention is concerned with a photodetection device including a substrate and an array of diodes. The substrate comprises a front face for receiving an electromagnetic radiation and a rear face opposite to the front face and substantially parallel to the same. The substrate also comprises an absorption layer 1 having a first doping type, each diode includes in the absorption layer 1 a collection region 2 which is flush with the rear face of the substrate and has a second doping type opposite to the first type.

[0030] The absorption layer 1 is for example a layer of CdHgTe. Such a layer can in particular be formed by epitaxy on a substrate of CdZnTe. Its cadmium composition can be between 20 and 40%. Its thickness is for example 8 m.

[0031] The doping of the absorption layer 1 can be of the p-type. It can be an intrinsic doping by mercury vacancies or an extrinsic doping, for example by arsenic incorporation. The doping density of the absorption layer is typically between 10.sup.15 and 10.sup.17 at/cm.sup.3.

[0032] Doping of the collection region 2 is thereby of the n-type. This doping can be achieved by ion implantation, for example boron implantation.

[0033] Of course, the invention is applicable to the case of a n-type doping of the absorption layer 1, for example by indium incorporation, and a p-type doping of the collection region 2, obtained for example by arsenic implantation.

[0034] The device according to the invention further comprises, under the surface of the substrate at which the collection regions are made (by surface of the substrate, it is meant the rear face of the substrate opposite to the front face receiving the electromagnetic radiation), a conduction meshing comprising at least one conduction channel 7 sandwiched between the collection regions of two adjacent diodes. The at least one conduction channel 7 has the first doping type and a higher doping density than the absorption layer. The doping density of the at least one conduction channel is typically between 10.sup.16 and 10.sup.18 at/cm.sup.3.

[0035] The collection meshing is buried in the substrate and is flush with the rear face of the substrate. Different topologies of conduction meshing can be implemented within the scope of the invention. Preferably, this conduction meshing has at least one conduction channel which extends between two adjacent lines of diodes. In other words, this canal extends between two adjacent rows or two adjacent columns of diodes.

[0036] The conduction meshing has preferably a plurality of conduction channels which extend each between two adjacent lines (rows or columns) of diodes. In particular, a plurality of conduction channels arranged such that there is a conduction channel sandwiched between the collection regions of each of the adjacent diodes of the array of diodes can be provided. In other words, and as is represented in FIG. 3a in the case of a matrix array of diodes, each diode is separated from its adjacent diodes by four conduction channels which interleave each other.

[0037] Within the scope of the invention, the doping density of the at least one conduction channel 7 results from a metal diffusion in the absorption layer. The metal is for example gold or copper when the aim is to make a p-type doping of the conduction meshing 7. The metal is for example indium when the aim is to make an n-type doping of the conduction meshing 7.

[0038] The diffused metal comes to be placed in a substitution site in the crystal lattice of the semi-conductor by creating donor or acceptor states resulting in overdoping. Metal diffusion is preferably made by heat treatment.

[0039] In addition to stabilize the interface between the semi-conductor and the passivation layer in the same way as in patent application EP 2 806 457 A2, forming the highly doped conduction meshing by metal diffusion enables the formation of faults to be reduced with respect to making the highly doped region by ion implantation as proposed in application EP 2 806 457 A2. This reduction in the fault formation results in an improvement of the performance of diodes with a reduction in the dark current and in the number of diodes with noise fault.

[0040] Metal diffusion is made from a metallic meshing 8 directly present on the surface of the substrate, for example a gold, copper or indium meshing. This metallic meshing 8 thus directly and wholly covers the conduction meshing 7. It is thereby distributed on the surface of the substrate according to the same pattern as the overdoped conduction meshing.

[0041] The metallic meshing 8 forms an ohmic contact with the substrate in which the current is directly conducted. Thus, the presence of this metallic meshing advantageously brings about a strong reduction in the serial access resistance, and thus of the collective depolarization and RC effects.

[0042] The device according to the invention further includes a peripheral substrate contact arranged on at least one side of the array of diodes. This contact is not represented in FIGS. 3a and 3b but is similar to what is previously discussed in connection with FIGS. 1a and 1b. Such a peripheral substrate contact enables an electrical connection to be made between the substrate and the read circuit on the periphery of the array of diodes thus releasing space between the diodes, and advantageously enabling an array of diodes to be made with a small pitch between the diodes.

[0043] The conduction meshing 7 is preferably in electric contact with the peripheral substrate contact, in particular through the metallic meshing 8.

[0044] The device according to the invention further includes a passivation layer 5 which covers the substrate except for contact regions of an electrically conductive bump contact 3 with a collection region 2 of a diode, and optionally a peripheral contact region of an electrically conductive stud with the substrate.

[0045] In alternative embodiments represented in FIGS. 4a and 4b, the absorption layer has a bandgap (forbidden energy gap) variation Eg from the surface of the substrate in the thickness of the absorption layer. With an absorption layer of CdHgTe for example, this bandgap variation can be achieved by varying the cadmium content.

[0046] In FIG. 4a, the absorption layer 1 has a gradual bandgap increase from the surface of the substrate in the thickness of the absorption layer. This alternative enables diffusion of minority carriers between the diodes to be limited because of an increase in the probability of collecting charges in the pixel where they have been photo-generated. Therefore, it enables the spread of the response on the neighbor pixels to be decreased. This alternative is advantageously used in the case of a detection array with a small pitch.

[0047] In FIG. 4b, the absorption layer has an abrupt bandgap decrease which appears at a given depth in the thickness of the absorption layer from the surface of the substrate. The absorption layer thereby consists of two regions 1a, 1b with a different energy bandgap value, that is a region 1a with a high bandgap immediately under the passivation layer and a low-bandgap region 1b. The extend of the high-bandgap region 1a is such that there is the conduction meshing 7 in this region 1a. The collection regions 2 preferably in turn project from the region 1a, or extent at least in the region 1a up to be adjacent with the bandgap change interface between the regions 1a and 1b. The presence of the high-bandgap region 1a enables possibly detrimental effects related to faults induced by the formation of this region immediately under the surface of the substrate to be limited.

[0048] The invention is not limited to the photodetection device previously described, but is also applicable to a method for manufacturing a photodetection device including a substrate and an array of diodes, the substrate comprising an absorption layer 1 having a first doping type and each diode including in the absorption layer 1 a collection region 2 which has a second doping type opposite to the first type. The method comprises a step of forming a conduction meshing 7 buried in the substrate and being flush with the rear face of the substrate. The conduction meshing 7 comprises at least one conduction channel sandwiched between the collection regions of two adjacent diodes, the at least one conduction channel having the first doping type and a higher doping density than in the absorption layer. Within the scope of the invention, the formation of a metallic meshing is conducted on the rear face of the substrate and said step of forming the conduction meshing is made by metal diffusion from the metallic meshing at the surface of the substrate.

[0049] FIGS. 5a-5f illustrate an exemplary embodiment of such a method which starts with a step of providing a substrate carrying the absorption layer 1, for example of CdHgTe. After the surface of the substrate is prepared, as represented in FIG. 5a, a deposition of the metallic meshing 8 on the surface of the substrate is conducted. The metallic meshing comprises for example several rows having a width of 1 m and a thickness of 500 nm. FIG. 5b illustrates the deposition of a passivation layer 5 onto the substrate and the metallic meshing. Then, an annealing operation for stabilizing the interface between the substrate and the passivation layer is conducted. This annealing operation enables metal diffusion of the metallic meshing towards the substrate, resulting as represented in FIG. 5c, in the formation of the conduction meshing 7 in the substrate directly underneath the metallic meshing 8 at the surface of the substrate. When the passivation layer is deposited before metal diffusion, it acts as a barrier to Hg evaporation from the absorption layer and a simple annealing operation under vacuum can be used. The passivation layer can however be deposited thereafter, wherein an annealing operation under Hg saturation vapour pressure can thereby be implemented.

[0050] In reference to FIG. 5d, the formation of the collection regions 2 are then conducted by means of an ion implantation through a suitable mask. The diameter of each collection region is for example between 5 and 20 m, and these regions are spaced apart by a pixel pitch for example between 5 and 30 m.

[0051] As represented in FIG. 5e, the opening of the passivation layer 5 is then conducted at each pixel, as well as in the array periphery for the subsequent formation of the substrate contact.

[0052] As represented in FIG. 5f, the deposition of a metal layer and the etching thereof at each pixel and at the periphery are then conducted to form the diode contacts 3 and the substrate contact. The thickness of these contacts is typically lower than 1 m.

[0053] The invention is advantageously applicable to infrared imaging, and in particular full size imaging where it enables the depolarization effect to be reduced in case of high flux, to active imaging where it enables the depolarization effect and slowing-down caused by the collective RC effect to be reduced, as well as to very low pitch imaging where it enables the interface between the diodes to be stabilized and the modulation transfer function of the photodetector to be increased.