WO2010043991A1

WO2010043991A1 - 3d electrochemical device

Info

Publication number: WO2010043991A1
Application number: PCT/IB2009/054253
Authority: WO
Inventors: Nynke Verhaegh; Johan Hendrik Klootwijk; Willem Frederik Adrianus Besling; Rogier Adrianus Henrica Niessen; Yann Pierre Roger Lamy; Freddy Roozeboom; Petrus Henricus Laurentius Notten
Original assignee: Nxp B.V.; Koninklijke Philips Electronics N.V.
Priority date: 2008-10-14
Filing date: 2009-09-29
Publication date: 2010-04-22

Abstract

The present invention relates to a modified rechargeable Li- ion solid- state battery design that is preferably integrated in 3D silicon. The use of photolithography in the manufacture of solid state batteries, such as lithium cells is known, as is the use of various etching techniques such as chemical, ion beam, plasma and sputter etching in the manufacture of such batteries. In particular the use of PVD, CVD and plasma CVD deposition processes and sputter etching to reduce vertical direction stresses during expansion and contraction during charge and discharge is disclosed. Lithium all- so lid- state batteries are based upon the reversible exchange of lithium ions between two electrodes (anode and cathode). These electrodes are separated by a solid-state electrolyte, that allows for lithium ion diffusion-migration and that prevents electron transport.

Description

3D ELECTROCHEMICAL DEVICE

FIELD OF THE INVENTION

The present invention relates to a modified electrochemical device such as a rechargeable Li-ion solid-state battery design, that is preferably integrated in 3D silicon.

BACKGROUND OF THE INVENTION

The use of photolithography in the manufacture of solid state batteries, such as lithium cells is known, as is the use of various etching techniques such as chemical, ion beam, plasma and sputter etching, in the manufacture of such batteries. In particular the use of PVD, CVD and plasma CVD deposition processes and sputter etching to reduce vertical direction stresses during expansion and contraction during charge and discharge is disclosed. Lithium all- so lid- state batteries are based upon the reversible exchange of lithium ions, between two electrodes (anode and cathode). These electrodes are separated by a solid-state electrolyte, that allows for lithium ion diffusion-migration and that prevents electron transport.

A SEM cross section of an example of a planar battery stack is given in Fig. 2.

The energy and power density of a solid-state-battery is e.g. determined by the amount of lithium ions that can migrate between the anode and cathode. The storage capacity can e.g. be increased by:

Increasing the anode and cathode layer thickness (during discharge of the battery the anode is the Lithium ions source and the cathode is the Lithium ions receptor; during charge of the battery it is vice versa). A typical all-solid-state battery stack is in the order of 2 μm (0.1 μm barrier layer; 0.1 μm anode; 0.5μm solid state electrolyte, 1.0 μm cathode, 0.1 μm current collector).

Increasing the surface area of the battery by growing the devices in 3 dimensional (3D) substrates.

Experience has been obtained in increasing the capacitance of capacitors by surface enhancement. Several 3D structures (pores, tripods, honeycombs etc.) have been explored. In contrast to a capacitor a battery consists of active layers: during operation the cathode and anode layers undergo volume expansion and contraction due to exchange of Lithium ions. This causes mechanical stress in a battery stack. Therefore teachings on capacitors are typically not applicable to electrochemical devices, such as solid-state batteries. The volume expansion can result in an increase of 400 volume % in case of intercalation anode materials such as Silicon. 15 Li + 4 Si ^ LiI₅S₁₄ (eq. 1)

The present inventors that for electrochemical devices, such as batteries, the requirements for 3D structures are more stringent because of this volume expansion and contraction of the active layers during operation. In a planar device this volume expansion/contraction occurs only in one dimension.

WO2008/011061 Al discloses a method for producing a thin film lithium battery, comprising applying a cathode current collector, a cathode material, an anode current collector, and an electrolyte layer separating the cathode material from the anode current collector to a substrate, wherein at least one of the layers contains lithiated compounds that is patterned at least in part by a photolithography operation comprising removal of a photoresist material from the layer containing lithiated compounds by a process including a wet chemical treatment. Additionally, a method and apparatus for making lithium batteries by providing a first sheet that includes a substrate having a cathode material, an anode material, and a LiPON barrier/electrolyte layer separating the cathode material from the anode material; and removing a subset of first material to separate a plurality of cells from the first sheet.

In some embodiments, the method further includes depositing second material on the sheet to cover the plurality of cells; and removing a subset of second material to separate a plurality of cells from the first sheet.

WO2004/093223 Al discloses a solid-state battery including at least one thin film layer, and method for making same.

The etch techniques proposed in this application are e.g. wet etch techniques. They offer the advantages and disadvantages of standard wet etch techniques.

WO2005/041324 Al discloses aperture mask deposition techniques using aperture mask patterns formed in one or more elongated webs of flexible film. The techniques involve sequentially depositing material through mask patterns formed in the film to define layers, or portions of layers, of the thin film battery. A deposition substrate can also be formed from an elongated web, and the deposition substrate web can be fed through a series of deposition stations.

WO03/022461 Al patterned thin film electrochemical devices such as batteries on fibrous or ribbon-like substrates, as well as the design and manufacture of the same, for utilization in electrochemical cells, electronic devices, optical devices, synthetic multi-functional materials, and superconducting materials, as well as fiber reinforced composite material applications.

US2002/117469 Al discloses a method of manufacturing an electrode for a secondary battery by depositing a thin film composed of active material on a current collector in which a surface-treated layer such as an antirust-treated layer is formed, including the steps of: removing at least part of the surface-treated layer by etching the surface of the current collector with an ion beam or plasma in order to improve the diffusion of the current collector material into the active material thin film; and depositing the thin film on the surface of the current collector subjected to the etching step.

US2003/138554 Al discloses a method for manufacturing an electrode for a lithium secondary battery includes a step of forming an oxide film other than a natural oxide film on a current collector by oxidizing the surface of the current collector, and a step of forming an active material layer on the oxide film by a method to provide a material for the active material layer by emitting in the vapor phase, such as PVD (physical vapor deposition) including sputtering, vapor evaporation, and the like, and CVD (chemical vapor deposition) including plasma CVD.

The sputter etch technique disclosed in the above application is directed to cleaning a surface. Thus, the above solid-state batteries contain active layers (anode and cathode) that exchange (for example) lithium ions during operation (charge/discharge). These active layers undergo volume expansion/contraction due to this lithium ion exchange. The inventors note that this induces stress in the battery stack. The above disclosures relate amongst others to deposition of layers by

PVD and LPCVD, forming batteries in 3D structures being etched, and realization of batteries on slopes for step coverage.

The above disclosures, however, relate to structures having a reduced reliability, e.g. in terms of lifetime, power delivery, breakdown of battery structure, etc., despite expectations to the contrary and despite optional advanced process control during manufacturing.

As such, there still is a need for improved electrochemical devices, such as batteries. The present invention is aimed at providing such improved structures, without jeopardizing other relevant characteristics.

SUMMARY OF THE INVENTION

The present invention relates to a method of manufacturing an electrochemical device, comprising the steps of providing a substrate with a 3D element, smoothening the edges of the element, and providing a solid state electrolyte, said electrochemical device, and devices comprising said electrochemical device.

The present invention thereby provides improved electrochemical devices, such as batteries, having an improved reliability, e.g. in terms of lifetime, power delivery, breakdown of battery structure, etc.

The present invention provides 3D structures, such as trenches. It has been found that in that case the volume expansion/contraction is almost limited to one dimension as well.

Fig. 1 shows a sketch of trenches in which the battery layers are stacked.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect the invention relates to a method of manufacturing an electrochemical device, comprising the steps of: - providing a substrate with a 3D element, smoothening the edges of the 3D element, and providing a solid-state electrolyte.

Preferably a full battery stack is provided. Further, the solid-state electrolyte is preferably deposited by PVD. The present invention preferably relates to silicon, silicon germanium and comparable substrates.

After thorough research it has been found that the prior art electrochemical devices suffer from the sharp etches formed therein, and specifically from spikes formed at the sharp edges. These spikes are amongst others caused by the processing techniques of the prior art. It has been found that the spikes cause high electromagnetic fields, which fields cause a reliability problem as described above. By providing smoothened edges the disadvantages of the prior art are overcome, whereas, on the other hand, virtually no negative effect on surface area is observed. These smoothened edges are not provided by one of the above-mentioned disclosures. Basically the prior art falls short in recognition of the problems, as well as in providing necessary measures to overcome the problems.

Furthermore, the stress built up caused by the above described volume expansion/contraction is found to be most pronounced at sharp edges of 3D structures into which 3D all electrochemical devices such as solid-state batteries are stacked.

Thus it has been found that for batteries the 3D structures should be as smooth as possible in order to prevent mechanical breakdown at sharp edges during operation. That is because it has been found that active layers undergo volume expansion and contraction during operation. The sharp edges of 3D structures should therefore be smoothened.

Preferably corners if present are smoothened and/or tapered, such that e.g. sharp corners (corners at edges have and angle of less then 90°) are rounded into a more or less smooth surface with a curvature having an approximate radius of 1 - 10 μm with edges of 10-100 μm, or e.g. a tapering of 1 - 10 μm with edges of 10-100 μm with two edges being at a specific angle, e.g. about 90°, or a tapering of a trench or pore of about 5-25 °, such as 10-15 °, or rounding by e.g. a bird's beak with a curvature having a radius of 1 - 10 μm. Capping according to the invention provides similar results.

Also the present invention provides a better step coverage in optional subsequent deposition steps. This is specifically relevant for PVD of layers. A better step coverage leads to an improved reliability in terms of more rigid structures, battery performance and lifetime.

Thus the present invention provides a solution for the above problems. The smoothening of the edges may be performed directly after forming/providing a substrate, or after depositing one or more layers on said substrate, however, before depositing a solid-state electrolyte.

The substrate may form an active part of the electrochemical device, or it may be a passive part thereof, such as a charge carrier. Furthermore, the substrate may comprise one or more further layers, such as a barrier layer, such as silicon nitride or silicon oxide, a conducting layer, such as a Pt layer or Cu layer, and one or more current collector layers, or layers functioning as such.

In a preferred embodiment of the method according to the invention, the smoothening step is performed by a wet etch, preferably an aqueous etch, such as a KOH etch, or by an isotropic dry etch, such as by SF₆, or by capping of the 3D structure, such as by fluidic based layers, or by etching the 3D structure with LOCOS bird's beak, or by ion beam etch, such as by argon, or combinations thereof. Thus, several methods are described to smoothen edges such as KOH etch; (argon) ion beam etch; isotropic etch (SF₆); capping of 3D structures with fluidic based layers; etching of 3D structures with LOCOS bird's beak; as well as products obtained by said methods.

In a second aspect the present invention relates to an electrochemical device, comprising an anode, a cathode, an electrolyte, wherein the substrate is a 3D structure with rounded edges.

The substrate may be a standard substrate, such as silicon, whereon an anode (layer) is deposited, or the substrate itself may function as anode. Preferably the substrate is also the anode.

Useful substrates include flexible and rigid polymeric substrates, glass, silica, alumina, ceramic, metal foils, fibers, fabric, paper, woven or non-woven materials, silicon or other semiconductors, and batteries. Semiconductor substrates include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures known in the art. The substrate may also comprise a separately provided thin film battery, to produce a stacked, multi-cell thin film battery. The substrate may also comprise a circuit or circuit element, such as a thin film transistor to produce an electronic device with an integral power source.

In such an embodiment preferably also at least one current collector is present. Useful materials for the cathode and anode current collectors include main group metals (including noble metals), metal alloys, metalloids, and carbon black. Preferred current collector materials include Cu, Ag, Pd, Pt and Au.

Useful materials for the anode deposited layers include lithium metal (for lithium thin film batteries) or a lithium intercalation material (for lithium ion batteries), gold, tin, tin/lead alloys or the "lithium-free" materials, whereby lithium metal is electroplated from the electrolyte in situ at the metal anode current collector upon the initial charge cycle of the battery. Other useful anode materials include SiTON, a silicon-tin oxynitride, such as SiSno.gONi.9, which may be deposited by RF magnetron sputtering of SnOz-SiO₂ in N₂, SnN_x (0 < x < 1.33), which may be deposited by reactive sputtering of Sn in an Ar + N₂ mixture, Sn₃N₄, Zn₃N₂, and InN_x (0 < x < 1), which may be deposited by reactive sputtering of In in an Ar + N₂ mixture.

Useful materials for the cathode deposited layers include crystalline TiS₂, Li containing materials such as LiMn₂O₂, LiCθo.₂Nio.s0₂, LiV₃Os, LiV₂Os, LiV₃Oi₃, LiMnO₂, crystalline LiMnO₄, crystalline LiCoO₂, crystalline and amorphous V₂O₅, and nanocrystalline Li_xMn₂ __yθ4. Preferred cathode materials include lithium transition metal oxides.

Useful electrolyte materials include lithium phosphorus oxynitride, known as LiPON, which may be deposited by RF magnetron sputtering of Li₃PO₄ in N₂. A second useful electrolyte is the NASICON-type solid electrolytes of the formula Lii_+xM'M"(P0₄)₃, where M' and M" are transition or non-transition metals with an average oxidation state lower than +4.

In a preferred embodiment of the present invention the electrochemical device is selected from the group consisting of integrated battery, and power source, preferably an integrated battery, and/or wherein the electrolyte is an inorganic solid- state electrolyte.

In a third aspect the present invention relates to a device, such as long- lifetime autonomous applications such as lighting control unit, a presence and motion detection device, a building (energy) control unit, an autonomous light source, green house sensor platform, wireless add-on sensors, medical implantable device, OLED devices, presence detection, implantables, smart cards and hearing aids, comprising an electrochemical device according to the invention.

The present invention is further elucidated by the following Figures and examples, which are not intended to limit the scope of the invention. The person skilled in the art will understand that various embodiments disclosed in the application may be combined.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 Sketch of trenches with battery layers Fig. 2 SEM cross section of typical battery stack

Fig. 3 KOH etched cavities with subsequent KOH dip (2 minutes) in order to smoothen the sharp top edge

Fig. 4 shows a straight 3D structure after 1 h ion beam etch: top edges are smoothened

Fig. 5 Capping of poly vinyl pyrrolidone polymer dissolved in an organic deposited by dipping and cured at 200 ⁰C under vacuum.

Fig. 6 shows a schematic drawing of formation of LOCOS.

Fig. 7 shows a schematic cross section of a LOCOS area.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 Sketch of trenches with battery layers. Therein a first (100) and second (170) current collector are shown. Further a substrate (110), such as a Si substrate, SiGe substrate, a barrier layer (120) covering the 3D structure, a conductor (130) provided on the barrier layer, such as silicon or doped silicon, a solid state electrolyte (140) and a cathode (150), such as LiCoO₂, are shown.

Fig. 2 SEM cross-section of typical battery stack. Therein an anode layer (17), an electrolyte (15), a cathode (13), a current collector (10), and a substrate (14) are shown. Also the lithiation and delithiation of anode and cathode is indicated. Further, on the left side, the electron current is shown.

Fig. 3a-d shows KOH etched cavities with subsequent KOH dip (2 minutes) in order to smoothen the sharp top edge.

A global smoothening action is performed. It is noted that corner softening follows Si crystallographic plans. A straight plane is shown, having rough aspect. The structure is tapered with a combination KOH softening. As a result a slope of 10-15 degrees, having a length range of about 1 - 10 μm. A slope of 10 to 15 degrees (with respect to horizontal) is formed having a length range: 1 - 10 μm.

Fig. 4a-e shows a straight 3D structure after 1 h Ar ion beam etch: top edges are smoothened. Local smoothening action is performed only on the top part of pores/trenches. A moderate softening is obtained, but with a very accurate shape. Straight plan. The obtained slope: 10 to 15 degrees and the length range: 0.5 to 3 μm.

Also an isotropic silicon etch may be preformed in high SF₆ flows. Normally silicon can be continuously etched in a SF₆IO₂ flow. The higher the SF₆ ratio, the more isotropic the silicon etching will be. This gives also a smoothening of edges.

Temporarily capping of an array of pores, trenches and the like, with fluidic based layers (sol-gel, polymers such as PMMA, poly imide poly amide, poly vinyl pyrrolidone). The natural tendency of these fluidic-based layers gives the layers a depth profile around the 3D structure (see Fig. 5). When such a layer is used as mask, the final etching profile is a projection of the fluidic-based capping layer.

Fig. 5a-c shows capping of polyvinyl pyrrolidone polymer dissolved in an organic deposited by dipping and cured at 200 ⁰C under vacuum.

A further embodiment is profiled etching of a trench edge after growing of LOCOS bird's beak. LOCOS stands for Local oxidation of silicon. The concept is explained in Fig. 6.

Fig. 6a-e shows that a buffer oxide (660) is required since the silicon nitride (680) hard mask can not be grown on silicon (610) as such, (a) The areas that should not be locally oxidized are protected by silicon nitride, (b) during oxidation oxygen diffuses through the silicon oxide and oxidizes the silicon under the silicon nitride, (c) The silicon nitride hard mask is pressed upwards, giving the so-called bird's beak. When a LOCOS bird's beak (650) is grown around the 3D structures into which a battery stack will be deposited, the silicon edge of the trench is structured. After removal of the LOCOS bird's beak the silicon edge is smoothened. Two options are shown, one having micro pores (670), and one having tapered via's (690). This is further schematically shown in Fig. 7. Fig. 7a shows a trench with thin thermal oxide layer (720) and a SiN capping layer (700) both on planar site and in trench. Further the substrate (710), typically Si, SiN barrier layer (730) and oxide layer (720) are shown.

Fig. 7b shows a silicon substrate underneath SiN capping layer is oxidized (LOCOS formation). Fig. 7c shows a silicon substrate after removal of SiN and SiO₂.

Fig. 7d shows a SEM cross-section with a Bird's beak - LOCOS approach. In this case, the corner softening is due to the thermal Si growth. The dimension of the top corner depends directly on the SiO₂ thickness and the bird's beak dimension. A curved soft corner is obtained. The curvature radius, the shape as well as the thickness of the bird beak can be fined tuned with the process options, offering further advantages and possibilities.

The obtained slope is: 5 to 45 degrees, having a curved shape. The length range is : -100 nm to 5 μm, having a depth: 1 to 5 μm.

Claims

CLAIMS:

1. Method of manufacturing an electrochemical device, comprising the steps of:

- providing a substrate (110) with a 3D element, - smoothening the edges of the element, and

- providing a solid-state electrolyte (140).

2. Method according to claim 1, wherein the smoothening step is performed by a wet etch, preferably an aqueous etch, such as a KOH etch, or by an isotropic dry etch, such as by SF₆, or by capping of the 3D structure, such as by fluidic based layers, or by etching the 3D structure with LOCOS bird's beak, or by ion beam etch, such as by argon, or combinations thereof.

3. Electrochemical device, comprising an anode, a cathode (150), and an electrolyte (140), wherein the substrate(l 10) is a 3D structure with rounded edges.

4. Electrochemical device according to claim 3, wherein the electrochemical device is selected from the group consisting of integrated battery, and power source, preferably an integrated battery, and/or wherein the electrolyte is an inorganic solid-state electrolyte.

5. Electrochemical device according to any of claims 3-4, wherein the device further comprises one or more high aspect ratio structures, selected from trenches, pillar, holes and combinations thereof.

6. Device, such as long-lifetime autonomous applications such as lighting control unit, a presence and motion detection device, a building (energy) control unit, an autonomous light source, green house sensor platform, wireless add-on sensors, medical implantable device, OLED devices, presence detection, implantables in general, smart cards and hearing aids, comprising an electrochemical device according to any of claims 3-5.