Microstructured substrate

Abstract

A microstructured substrate includes a plurality of at least one elementary microstructure. An electrical storage device, and more particularly an all-solid-state battery, can include the microstructured substrate.

Claims

1. A microstructured substrate comprising: a plurality of at least one elementary microstructure, wherein said at least one elementary microstructure, on the one hand, has an elongated shape having a longitudinal dimension (d.sub.L) and lower and upper opposite longitudinal ends, the lower end being connected to a surface of a substrate and, on the other hand, includes an open cavity at its upper end, said open cavity having a longitudinal dimension (d.sub.in) and extending longitudinally inside said at least one elementary microstructure, said at least one elementary microstructure has an external face that delimits the exterior outline of said at least one elementary microstructure and an internal face that delimits said cavity, said microstructured substrate having a conformal layer of alumina deposited directly on the surface of the substrate located at the exterior outline of said at least one microstructure, on the external face of said at least one elementary microstructure and on the internal face of said cavity, wherein the longitudinal dimension (d.sub.in) of the open cavity is substantially equal to half the longitudinal dimension (d.sub.L) of said at least one elementary microstructure.

2. The microstructured substrate according to claim 1, wherein said at least one elementary microstructure has a circular, elliptical, rectangular, square or triangular transverse cross section.

3. The microstructured substrate according to claim 1, wherein said at least one elementary microstructure has a longitudinal dimension (d.sub.L) between 5 and 200 m and a transverse dimension (OD) between 2 and 10 m.

4. The microstructured substrate according claim 1, wherein said at least one elementary microstructures of the substrate are arranged periodically over said substrate.

5. The microstructured substrate according to claim 4, wherein said at least one elementary microstructures of the substrate have a spatial period (SP) between 3 and 10 m.

6. The microstructured substrate according to claim 1, wherein said at least one elementary microstructure has an aspect ratio (r.sub.asp) higher than or equal to 10.

7. The microstructured substrate according to claim 1, wherein said substrate is made from a material chosen from silicon, silicon dioxide, gallium arsenide, silicon nitride and indium phosphide.

8. A process for obtaining a microstructured substrate according to claim 1 by microstructuring a substrate having a planar surface, said process comprising the steps of: a) coating a photoresist layer onto the planar surface of said substrate, b) producing by photolithography a repetition of at least one elementary pattern in the photoresist layer in order that the surface of the substrate presents zones exempt from photoresist, said at least one elementary pattern having an elongated shape having a longitudinal dimension (d.sub.L) and a lower and an upper opposite longitudinal ends, the lower end being connected to the surface of the substrate and, on the other hand, includes an open cavity having a longitudinal dimension (d.sub.in) at its upper end, the longitudinal dimension (d.sub.in) of said open cavity being substantially equal to half the longitudinal dimension (d.sub.L) of said at least one elementary pattern, said at least one elementary pattern has an external face that delimits the exterior outline of said at least one elementary pattern and an internal face that delimits said cavity, c) etching the zones of the surface of the substrate exempt from photoresist, d) passivating the surface of the substrate, e) repeating said etching and passivating steps c) and d), and f) depositing a conformal layer of alumina directly on the surface of the surface of the substrate located at the exterior outline of said at least one elementary pattern, on the external face of said at least one elementary pattern and on the internal face of said cavity.

9. The process according to claim 8, wherein said at least one elementary pattern obtained in the photolithography step b) has an annular shape presenting transverse dimensions comprised between 2 to 10 m.

10. The process according to claim 8, wherein the etching and passivating steps c) and d) are carried out by way of an ionized gas.

11. The microstructured substrate as defined in claim 1, wherein said microstructured substrate is configured for application within a device for storing electrical energy.

12. A device for storing electrical energy having a microstructured substrate according to claim 1, said device comprising: a substrate, a negative electrode and a positive electrode, one of which is placed on the substrate, and an electrolyte placed between the negative electrode and the positive electrode.

13. The device according to claim 12, wherein said device further comprises a first current collector placed on the substrate and a second current collector, the negative electrode and the positive electrode being placed between the first and second current collectors.

14. The device according to claim 13, wherein the substrate is made from a material chosen from silicon, silicon dioxide, gallium arsenide, silicon nitride and indium phosphide, wherein the first and second current collectors are each made from solid materials chosen from aluminium, copper, platinum and titanium nitride.

15. The device for storing electrical energy according to claim 12, wherein the negative electrode, the positive electrode and the electrolyte are each formed as a layer having a thickness between 15 nm and 200 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood, and other aims, details, features and advantages thereof will become more clearly apparent, from the following description of particular embodiments and exemplary embodiments of the invention, which description is given merely by way of nonlimiting illustration and with reference to the appended drawings, in which:

(2) FIG. 1a shows a very schematic longitudinal cross-sectional view of a microstructured substrate according to the invention;

(3) FIG. 1b is a top view of the microstructured substrate in FIG. 1a;

(4) FIG. 1c is an enlarged top view of two types of elementary microstructure placed on the microstructured substrate in FIG. 1a;

(5) FIG. 1d is a cross-sectional photograph of one portion of a microstructured substrate according to the invention;

(6) FIGS. 2a, 2b, 3a and 3b are photographs of patterns obtained in the photolithography step according to the invention;

(7) FIGS. 4a and 4b are photographs at various magnifications of a microstructured substrate according to the invention;

(8) FIG. 5a shows a schematic perspective top and partial cutaway view of a device for storing electrical energy according to the invention;

(9) FIG. 5b is an enlarged schematic transverse cross-sectional view of the constituent elements of said device for storing electrical energy in FIG. 5a; and

(10) FIGS. 6a to 6d show graphs illustrating the performance of a battery according to the invention.

DETAILED DESCRIPTION

(11) FIG. 1a shows a very schematic longitudinal cross-sectional view of a microstructured substrate 1 according to the invention including a plurality of elementary microstructures 3.

(12) The substrate is, in the embodiment shown, a silicon wafer that has a substantially planar surface and a substantially constant thickness.

(13) Said elementary microstructure 3 has an elongate shape and two opposite longitudinal ends, namely a lower end 3a and an upper end 3b.

(14) The elementary microstructure 3 is connected to the substrate 1 by way of its lower end 3a and extends away from the surface of said substrate 1. The upper end 3b of the elementary microstructure 3 is free. In addition, the longitudinal axis of said elementary microstructure 3 is substantially orthogonal to the main plane of extension of said substrate 1.

(15) Moreover, the elementary microstructure 3 includes an open cavity 5 at the upper end 3b of said microstructure 3.

(16) It will be noted that the elementary microstructure 3 has an external face 3c that delimits the exterior outline of the microstructure 3 and an internal face 3d that delimits said cavity 5.

(17) More particularly, the open cavity 5 extends longitudinally inside said elementary microstructure 3. The cavity 5 is characterized by a longitudinal dimension d.sub.in comprised between 5 and 50 m. The elementary microstructure 3 is characterized by a distance between the lower and upper end of the elementary microstructure that corresponds to the longitudinal dimension d.sub.L of the elementary microstructure 3. The longitudinal dimension d.sub.L of the elementary microstructure is comprised between 10 and 200 m.

(18) It will be noted that the longitudinal dimension d.sub.in of the cavity 5 is substantially equal to half the longitudinal dimension d.sub.L of the elementary microstructure 3.

(19) The elementary microstructure 3 is also characterized by a transverse dimension OD that corresponds to the maximum spatial extension of the elementary microstructure 3 in a plane orthogonal to the longitudinal axis of said microstructure 3. The transverse dimension OD of the elementary microstructure is comprised between 2 and 10 m.

(20) It will be noted that said elementary microstructures 3 are arranged, on the one hand, substantially parallel to one another and, on the other hand, so as to make relative to the main plane of extension of the substrate 1 an angle that is substantially right.

(21) The expression an angle that is substantially right is understood to mean an angle comprised between about 88 and 92.

(22) Said microstructures 3 are spaced apart from one another by a distance FP comprised between about 0.5 and 10 m and preferably comprised between 1 and 2 m.

(23) It will also be noted that the elementary microstructure may for example have a circular, elliptical, rectangular, square or triangular transverse cross section.

(24) As illustrated in FIG. 1b, said elementary microstructures 3 are arranged periodically over the substrate 1. The spatial period SP of the elementary microstructures 3 is for example comprised between 3 and 10 m.

(25) The spatial period SP is also equal to the sum of the transverse dimension OD of a microstructure 3 and the space (corresponding to the distance FP) separating two consecutively placed microstructures 3.

(26) FIG. 1c for its part is a schematic top view of elementary microstructures 13a, 13b having square and circular transverse cross sections, respectively.

(27) Thus, each of the elementary microstructures 3, 13a, 13b is also characterized by a transverse dimension ID corresponding to the maximum transverse spatial extension ID of the cavity 5 of the elementary microstructure 3.

(28) The elementary microstructure may thus be defined by an aspect ratio r.sub.asp that is the ratio of the longitudinal dimension d.sub.L to the distance FP separating two consecutive elementary microstructures 3. The aspect ratio of an elementary microstructure 3, 13a, 13b is for example higher than or equal to 10 and is preferably comprised between 10 and 100.

(29) FIG. 1d is a cross-sectional photograph of a microstructured substrate 1 according to the invention that includes two elementary microstructures 3. FIG. 1d is a photograph taken by a scanning electron microscope sold under the trade name Zeiss Ultra 55.

(30) The elementary microstructures 3 in FIG. 1d have the following characteristic sizes: a longitudinal dimension d.sub.1 of 10 m; a transverse dimension OD of 3 m; a longitudinal dimension d.sub.in of the cavity 5 of 5 m; a transverse dimension ID of the cavity 5 of 1 m; and a spatial period SP of 4 m (it will therefore be noted that the distance FP separating two consecutively placed microstructures is 1 m).

(31) The appended FIGS. 4a and 4b are photographs of observations at various magnifications of a microstructured substrate according to the invention. The photographs in FIGS. 4a and 4b were taken with a scanning electron microscope sold under the trade name Zeiss Ultra 55.

(32) Elementary microstructures of square cross section arranged periodically over a silicon substrate may be seen.

(33) The characteristic sizes of the microstructured substrate of FIGS. 4a and 4b are the following: a longitudinal dimension d.sub.L of 30 m; a transverse dimension OD of 2 m; a longitudinal dimension d.sub.in of the cavity 5 of 15 m; a transverse dimension ID of the cavity 5 of 1.2 m; and a spatial period SP of 4 m (it will therefore be noted that the distance FP separating two consecutively placed microstructures is 2 m).

(34) It will be noted that the microstructured substrate and its microstructures may also be produced in a material chosen from silicon dioxide, gallium arsenide, silicon nitride and indium phosphide.

(35) The microstructured substrate and the plurality of elementary microstructures form a single object (i.e. the microstructured substrate and the plurality of elementary microstructures are formed integrally from the same material).

(36) Nevertheless, in one variant embodiment, the substrate and the microstructures may be produced from different materials.

(37) It will in addition be noted that in another variant embodiment, the elementary microstructure is produced from a material chosen from silicon dioxide, gallium arsenide, indium phosphide and one of their mixtures.

(38) The microstructured substrate 1 according to the invention is obtained by an obtaining process that is described below.

(39) The process for obtaining a microstructured substrate according to the invention includes the following steps: a) A step of cleaning a silicon substrate or wafer using methods known per se and that therefore will not be detailed further here. b) A step of coating a layer of photoresist of uniform thickness onto the plane surface of the silicon wafer. More particularly, the photoresist layer is deposited by spin coating. By way of example, it is especially possible to use the photoresist sold under the name SPR 220 by Rohm and Haas or the photoresist sold under the name AZ9260 by Microchemicals GmbH. c) A step of producing by photolithography a repetition of at least one elementary pattern in the photoresist layer in order that the surface of the substrate presents zones exempt from photoresist. The step of producing by photolithography a plurality of at least one elementary pattern is a method known per se and will therefore not be detailed further here.

(40) The appended FIGS. 2a to 3b are photographs obtained by a Zeiss Ultra 55 scanning electron microscope and in which a plurality of elementary patterns produced in photoresist may be seen.

(41) The elementary patterns in FIGS. 2a and 2b are substantially the shape of a tube with a circular cross section.

(42) The elementary patterns in FIGS. 3a and 3b are substantially the shape of a tube of square cross section. d) A step of etching the zones of the substrate exempt from photoresist. The etching step is carried out by a Bosch-type deep reactive ion etching (DRIE) technique using an ionized gas such as sulphur hexafluoride (or SF.sub.6). The silicon substrate is etched at a rate of 3 m/m and the duration of the etching step is about 3 seconds. e) A step of passivating the surface of the substrate (the passivating is a selective process that occurs preferentially on the zones of the substrate exempt from photoresist). The passivating step is carried out by a Bosch-type deep reactive ion etching (DRIE) technique using an ionized gas such as octafluorobutene (C.sub.4F.sub.8). The duration of the passivating step is about 2 seconds. f) A step of repeating said etching and passivating steps in order to obtain said microstructured substrate.

(43) It will be noted that the etching and passivating steps are carried out using a plasma etching tool from SPTS.

(44) The repetition of the etching and passivating steps allows elementary microstructures having a high aspect ratio, for example higher than or equal to 10 and preferably comprised between 10 and 100, to be obtained. In addition, the elementary microstructures thus obtained extend away from and orthogonally to the main plane of extension of the substrate. g) A step of removing the photoresist carried out by a plasma surface treatment technique by way of a machine sold under the name Ashing System GIGAbatch 310 M by PVA TEPLA.

(45) It will be noted that this manufacturing technique produces elementary microstructures having irregularities such as scallops on their surface (more particularly visible in FIGS. 1d and 4b).

(46) In a variant embodiment (not shown), the step g) of removing the photoresist is followed by a step of depositing alumina (or Al.sub.2O.sub.3) on the surface of the obtained elementary microstructures. The step of depositing alumina is carried out by an atomic layer deposition (or ALD) process and allows a conformal layer of alumina to be deposited on the interior and exterior of the elementary microstructures.

(47) This alumina layer at least partially fills the scallops produced by the process for obtaining the microstructured substrate according to the invention in order to facilitate the subsequent deposition of other materials on the surface of the microstructured substrate and to electrically insulate the microstructured substrate according to the invention.

(48) The last subject of the present invention is a device for storing electrical energy, for example an electrical battery, and more particularly an all-solid-state electrochemical battery 41, that includes a microstructured substrate 43 according to the invention.

(49) FIG. 5a is a schematic perspective top and partial cutaway view of an electrochemical battery 41 according to the invention.

(50) Said battery 41 includes: a first current collector 45 placed on the microstructured substrate 43; a second current collector 47; a negative electrode 49 and a positive electrode 51, which electrodes are placed between the first and second current collectors 45, 47; and an electrolyte 53 placed between the negative electrode 49 and the positive electrode 51.

(51) The above elements 45 to 53 take the form of a stack of thin layers produced from solid materials.

(52) The current collectors are for example produced from solid materials such as aluminium, copper, platinum and titanium nitride.

(53) In addition, a protective layer 55 for example of silicon nitride or of silicon oxide is deposited on the surface of the stack of thin layers 45 to 53 in order to protect the latter from oxidation.

(54) As illustrated in FIG. 5b, the microstructured substrate according to the invention allows said thin layers 45 to 53 to be deposited on the entirety of the surface of the substrate 43. The microstructured substrate 43 according to the invention is able to accommodate a larger quantity of electrode materials while exhibiting good mechanical qualities. Thus, the battery 41 according to the invention has a better performance than prior-art batteries.

(55) It will be noted that the thin layers 45 to 53 are produced using at least one of the following methods: atomic layer deposition (or ALD), molecular beam epitaxy (MBE), chemical vapour deposition (CVD), physical vapour deposition (PVD), electrodeposition, etc.

(56) By way of example, FIGS. 6a to 6d illustrate characteristic properties of electrochemical half-cells including a microstructured substrate according to the invention and/or an electrochemical half-cell including a planar substrate.

(57) The electrochemical half-cells each include a thin layer of 30 nm of platinum forming a current collector and a thin layer of titanium dioxide (in anatase crystalline form) forming a negative electrode that has a thickness comprised between 38 nm and 150 nm.

(58) The electrochemical half-cells were tested with a liquid electrolyte that was a mixture of lithium bis(trifluoro-methanesulfonyl)imide (or LiTFSI) of 1M concentration, of ethylene carbonate and diethylene carbonate (in identical proportions by volume). In addition, these half-cells were each tested with a lithium electrode, the lithium electrode playing the role of counter electrode and of reference electrode in the electrochemical cell thus formed (redox couple Li/Li.sup.+).

(59) The various electrochemical half-cells S0 to S3 tested included substrates having the properties presented in table 1 below:

(60) TABLE-US-00001 TABLE 1 OD (m) ID (m) SP (m) d.sub.L (m) d.sub.in (m) S0 0 0 0 0 0 S1 3 0 4 9.1 0 S2 2.02 1.25 4.04 27.6 13.8 S3 2 1.5 4 45 22.5

(61) The electrochemical half-cells S0 and S1 respectively include a planar substrate and a microstructured substrate in which the elementary microstructures are pillars.

(62) The electrochemical half-cells S2 and S3 include, for their part, a microstructured substrate according to the invention.

(63) It will therefore be noted that the microstructured substrate according to the invention allows devices for storing electrical energy to be manufactured that comprise a larger amount of active materials in each of the constituent elements (for example: electrolyte, electrodes, etc.) of a device for storing electrical energy.

(64) The results of the characteristic quantities measured for the electrochemical half-cells are therefore illustrated in FIGS. 6a to 6d.

(65) FIG. 6a shows the magnitude of the current in mA (or milliamps) delivered by the electrochemical half-cell S0 including a planar substrate as a function of the voltage in V (or volts) applied across the terminals of the electrochemical cell formed.

(66) FIG. 6b shows the magnitude of the current in mA delivered by the electrochemical cells S0, S1 and S2 as a function of the voltage in V (or volts) applied across the terminals of said formed electrochemical cells.

(67) It may be seen that the maximum magnitude of the current delivered by the cell including the microstructured substrate according to the invention S2 is: at least 10 times higher, at identical voltage, than the magnitude of the current delivered by the cell S0 including a planar substrate; and at least 4 times higher, at identical voltage, than the magnitude of the current delivered by the cell S1 including a microstructured substrate.

(68) FIG. 6c shows the capacity per unit area in Ah/cm.sup.2 (or microamp-hours per centimeter squared) during a charge and during a discharge of the electrochemical cell S0 including a planar substrate, as a function of a voltage in V applied across the terminals of said cell.

(69) FIG. 6d shows the capacity per unit area in Ah/cm.sup.2 during a charge and during a discharge of the electrochemical cell S3 including a microstructured substrate according to the invention, as a function of a voltage in V applied across the terminals of said cell.

(70) FIG. 6d also includes the results illustrated in FIG. 6c for the electrochemical cell including a planar substrate.

(71) It may be seen that the maximum values of the capacity per unit area of the cell including a microstructured substrate according to the invention, whether this is during the charge or discharge of the electrochemical cell, are at least 20 times higher than the maximum values of the capacity per unit area of the electrochemical cell including a planar substrate.

(72) It will be noted that the results illustrated in FIGS. 6a-b were obtained with a voltage scanning speed of 0.15 mV/s.

(73) FIGS. 6a to 6d therefore clearly illustrate the fact that the device for storing electrical energy according to the invention allows a better performance to be obtained than prior-art devices for storing electrical energy.