Matrix detection device incorporating a metal mesh in a detection layer, and manufacturing method

Abstract

A matrix-array detecting device including a stack comprising a matrix array of detecting-element pixels, and an active matrix array comprising a network of rows and columns for controlling the pixels and produced on the surface of a substrate, wherein the detecting-element pixels comprise: a common top electrode; a detecting layer; and discrete bottom electrodes; the device comprising a metallic mesh that is connected to the top electrode; that includes pads comprising at least one metal portion, the pads being incorporated into the detecting layer; and that is positioned in correspondence with the network of controlling rows and columns. A process for fabricating the matrix-array detecting device is also provided.

Claims

1. A matrix-array detecting device including a stack comprising a matrix array of detecting-element pixels, and an active matrix array comprising a network of rows and columns for controlling the pixels and produced on the surface of a substrate, wherein: the detecting-element pixels comprise: a common top electrode; a detecting layer; and discrete bottom electrodes; said device comprising a metallic mesh: that is connected to said top electrode; that includes pads comprising at least one metal portion, said pads being incorporated into said detecting layer; and that is positioned in correspondence with said network of controlling rows and columns, wherein at least one of said pads comprises a metal top portion and a bottom portion comprising a dielectric, wherein the metal top portion is in contact with the common top electrode.

2. The matrix-array detecting device as claimed in claim 1, wherein the detecting layer is a common detecting layer.

3. The matrix-array detecting device as claimed in claim 1, wherein the detecting pixels are photodiodes.

4. The matrix-array detecting device as claimed in claim 1, wherein the metallic mesh is separated from the network of controlling rows and columns by a dielectric layer of permittivity lower than about 2.5.

5. The matrix-array detecting device as claimed in claim 1, wherein the bottom portion of said pads includes a positive photoresist.

6. The matrix-array detecting device as claimed in claim 1, wherein detecting-element pixels comprise one or more organic materials.

7. The matrix-array detecting device as claimed in claim 6, wherein the organic material is a blend of p-type polymer and n-type polymer.

8. The matrix-array detecting device as claimed in claim 1, wherein the network of rows and columns for controlling said pixels is connected to a network of controlling transistors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other advantages will become apparent on reading the following nonlimiting description that is given with reference to the appended figures, in which:

(2) FIGS. 1a and 1b illustrate an exemplary imager according to the prior art;

(3) FIG. 2 illustrates an exemplary matrix-array detecting device according to the invention;

(4) FIGS. 3a to 3l illustrate the various steps of a first exemplary process for producing a device of the invention; and

(5) FIGS. 4a to 4c illustrate the various steps of a second exemplary process for producing a device of the invention.

DETAILED DESCRIPTION

(6) A schematic of a matrix-array detecting device of the present invention is illustrated in FIG. 2. It comprises, as is conventional, on the surface of a substrate 10, a matrix array of transistors (not shown) that are connected to rows/columns 20 for controlling the detecting pixels. It will be recalled that the controlling rows/columns allow the transistor of each pixel to be activated and that each transistor allows one detecting pixel to be controlled/read.

(7) The detecting pixels comprise:

(8) discrete bottom electrodes 40;

(9) a detecting layer 50;

(10) a top electrode that is common to said pixels 60.

(11) A dielectric layer 30 insulates the conductive rows and columns 20 from the bottom electrodes 40.

(12) According to the present invention, a mesh, which is what is called a metallic mesh, comprising pads 80 that are at least partially made of metal, is provided. The pads 80 of the metallic mesh are positioned in correspondence with, i.e. facing, the network 20 of controlling rows/columns and insulated therefrom, and separate the elements of the detecting layer 50 while making contact with said layer 50. This configuration is particularly advantageous in the case of matrix array devices including photodetecting elements, in which devices it is sought to optimize and therefore decrease the size of zones opaque to the radiation that it is sought to detect.

Exemplary Process for Fabricating a Matrix Array Photodetecting Device According to the Invention

(13) This process essentially details the production of the photodetector portion, the matrix arrays of transistors, the controlling rows/columns and the bottom electrodes of the photodetection portion having already been prefabricated on the surface of a substrate.

(14) FIGS. 3a to 3l show the various steps of the fabricating process of the invention.

First Step

(15) As shown in FIG. 3a, the controlling rows/columns 20 and the bottom electrodes 40 of the photodetecting elements are produced on a substrate 10, the insulation between transistors and photodetectors being achieved by a dielectric insulating layer 30. The rows/columns are generally produced by vacuum deposition of a conductive layer (aluminum, molybdenum, etc.) followed by a step of photolithography and etching. The insulating layer is generally of SiN (silicon nitride) and deposited by a vacuum deposition technique. Lastly, the bottom electrodes are generally of transparent conductive oxides such as ITO (indium tin oxide) and deposited by PVD (physical vapor deposition) followed by a photolithography step.

Second Step

(16) As shown in FIG. 3b, the following are deposited in succession on the preformed base: a positive photoresist 71, for example S1818 from Shipley. This type of resist generally consists of 3 components: a polymer resin matrix (Novolak), a photoactive compound (diazo-compound, generally diazonaphtoquinone) and a PGMEA (propylene glycol monomethyl ether acetate) solvent. The thickness of this layer may vary from a few nanometers to several microns, and ideally is about 1 to 2 m. The deposition may be carried out by spin coating (at 2000 rpm (revolutions/minute) for 30 seconds for a final thickness of 1.2 m) or using any other resist deposition technique, followed by a post-deposition anneal at 115 C. for 5 minutes under N.sub.2; a low-resistivity metal layer 72 of resistivity lower than 100/, and better still than 10/, and ideally lower than 1/, its resistance easily being measurable by the 4-point probe method or the TLM (transmission line model) method. It may typically be a gold layer. The thickness of this layer may vary from a few nanometers to a few microns depending on the desired sheet resistance but it must, above all, be sufficiently transparent to UV/visible light to let pass at least 10%, better still 50% and ideally 98% of the incident light flux. This layer is for example obtained by vacuum evaporation at 210.sup.6 mbar of a 10 nm layer of gold at a rate of 2 /s; a second positive photoresist 73, for example S1818 from Shipley. The thickness of this layer may vary from a few nanometers to several microns, and ideally is about 1 to 2 m. The deposition may be carried out by spin coating (at 2000 rpm for 30 seconds for a final thickness of 1.2 m) or using any other resist deposition technique, followed by a post-deposition anneal at 115 C. for 5 minutes under N.sub.2.

Third Step

(17) As shown in FIG. 3c, an exposure is performed from the back side of the carrier with an MA750 exposing system. The exposure time may vary from a few tens of seconds to several minutes depending on the transparency of the metal layer 72. For a layer of gold of about 10 nm thickness, exposure for 300 seconds with a dose of 4.3 mW/cm.sup.2 is sufficient. This allows, in a single step, the two resist layers 71 and 73 to be exposed and the resist portions that were exposed to the light rays to be made soluble in a suitable solvent.

Fourth Step

(18) As shown in FIG. 3d, the top resist layer is then developed in a commercial developer based on tetramethyl ammonium hydroxide (about 2% in water) for 1 to 5 mn. Thus an initial portion 83 made of resist of the pads of the mesh is produced on the surface of the metal layer 72.

Fifth Step

(19) As shown in FIG. 3e, the gold layer is then etched chemically with a Kl/I.sub.2 solution for 1 to 5 mn then rinsed in deionized water and dried under N.sub.2. Thus the metal portions 82 of the pads, surmounted with the portions 83 made of resist of the pads and allowing the mesh to be defined in correspondence with, i.e. facing, the addressing rows/columns 20, are obtained.

Sixth Step

(20) The top portions 83 made of resist of the pads and the resist 71 located level with (facing) the bottom electrodes 40 are then removed, so as to also uncover the latter, by reactive ion etching (RIE) in an oxygen plasma (flow rate of 150 sccm) with 2% SF.sub.6 at 10 mtorr and a power of 120 W for 10 minutes. It is then possible to deposit a layer 90 of negative resist, such as the resist SU8 from MicroChem, on the preformed carrier by spin coating, then annealing at 115 C. for 5 minutes. This negative resist 90 may then be exposed from the back side as also shown in FIG. 3f.

Seventh Step

(21) As shown in FIG. 3g, the result of the exposure of the negative photoresist is developed, allowing said negative resist to be removed facing the pads 82/81.

Eighth Step

(22) As shown in FIG. 3h, an operation for growing the metal layer is then carried out. To do this, the substrate is submerged in an electrolytic solution in order to make the metal layer grow only above the addressing rows/columns. Different complexes with their advantages and drawbacks exist, the most commonplace being aurocyanure [Au(CN).sub.2].sup.. To this complex are added conductive salts (((NH.sub.4).sub.2HPO.sub.4, K.sub.2HPO.sub.4, sodium potassium tartrate, etc.), and buffer salts to stabilize the pH. The carrier is connected to the negative terminal (anode) of a DC voltage generator and the second terminal (cathode) is connected to a platinum (or graphite) electrode. With an electrical current of 20 mA applied for 120 seconds and a deposition area of about 0.8 cm.sup.2, the thickness of gold deposited on the carrier may be about 1 m, which thickness is largely sufficient to obtain a top electrode with an excellent conductivity.

Ninth Step

(23) As shown in FIG. 3i, all of the negative-resist patterns remaining on the wafer are then chemically removed by submersion in acetone for 5 to 10 minutes, followed by a rinse in acetone and drying under a flow of N.sub.2, leaving the pads 82/81 of the metallic mesh visible.

(24) Once this step has finished, production of the actual photodetector stack may begin.

Tenth Step

(25) As shown in FIG. 3j, a photosensitive layer 50 composed of a blend of n-type and p-type polymers is then deposited. It may be deposited using techniques such as spin coating or screen printing. The proportion and concentration of the various constituents, and the thickness, the solvent or the combination of solvents used may vary depending on the deposition techniques used, the targeted final performance, the solubility of the various constituents, etc. For example, depositing by spin coating a 1 to 2 blend in xylene (with a concentration of 15 mg/mL of p-type polymer) delivers a thickness of about 250 nm on a glass substrate covered with ITO, for a spin speed of 800 rpm. In the case shown, the thickness of the layer 50 is larger than the height of the pads of the metallic mesh.

Eleventh Step

(26) As shown in FIG. 3k, an operation for laser ablating this photosensitive layer 50 is then carried out only level with the addressing rows and columns, in order to uncover the metal layer produced in the preceding steps. The laser ablation may for example be carried out with an Excimer laser: for an active layer of 270 nm thickness, ablation is observed for two laser pulses with a fluence of 440 mJ.

(27) It will be noted that depending on the thickness of the metal layer and in particular if it is too thin, it is possible that the latter will also be etched during the laser ablation. The electrical contact between the top electrode and this metal layer is nonetheless not broken but only made via the flanks of the metal layer.

Twelfth Step

(28) As shown in FIG. 3l, the top electrode 60 is deposited. To do this, as is conventional, the deposited material may be PEDOT/PSS deposited on the wafer scale by suitable deposition techniques such as screen printing, spin coating or even slot-die coating.

(29) This top electrode makes contact with the metallic mesh formed beforehand and thus its sheet resistance is spectacularly decreased.

Second Exemplary Process for Manufacturing a Matrix-Array Photodetecting Device According to the Invention

(30) It is possible to remove the step of electrolytic growth if the conductivity of the metal layer is sufficient and in particular in the case of a thick metal layer. The main advantage of this variant is to decrease the number of production steps since a negative resist, a second photolithography step and an electrodeposition step are not necessary.

(31) This process comprises first steps similar to those of the first exemplary process and illustrated in FIG. 3a.

(32) In the present example, a thick metal layer allowing a very good conductivity to be obtained is then deposited as illustrated in FIG. 4a. The preformed carrier is exposed from the front side through a mask M and from the back side (without a mask).

(33) As in the first exemplary process, the top portions 83 made of resist are developed, leaving the thick metal layer locally uncovered as illustrated in FIG. 4b.

(34) Next, the thick metal layer is etched and then the top resist layer is etched using methods identical to those described above so as to form the pads of the mesh comprising a metal top portion 82 above a resist portion 81, as shown in FIG. 4c.

(35) The subsequent steps of the process may then be identical to those of the first exemplary process of the invention.

(36) According to one variant of the invention, it is also advantageously possible to then de-wet the photosensitive layer 50 on the metal pads 82, allowing the step of laser ablation of the photosensitive layer 50 level with the addressing rows and columns to be removed. To do this, it is possible to submerge the carrier for 10 minutes in a 1% solution of 1H,1H,2H,2H-perfluorodecanethiol in water.

(37) Generally, it will thus be clear that the fabricating process of the present invention allows a mesh of rows/columns that has the aim of decreasing the parasitic capacitances between the top electrode of an active matrix array of photodetectors and the rows/columns for controlling the transistors of this matrix array to be produced. It also allows, without loss of active area, the sheet resistance of the top electrode to be notably decreased using a mesh of metallic rows/columns connected to the top electrode.