Supercapacitor with separator

Abstract

Supercapacitor (10) comprising the following: a first substrate (12) and a second substrate (12); a first electrode (18) supported by the first substrate; a second electrode (18) which is supported by the second substrate; wherein the first substrate is connected to the second substrate to form an inner chamber (16); a first separator (14) between the first electrode and the second electrode inside the inner chamber, wherein the first separator has several micro-fabricated holes (22); Electrolyte (20) with ions inside the inner chamber and in contact with the first and second electrode, wherein the micro-manufactured holes of the first separator are dimensioned to allow the passage of the ions through the micro-manufactured holes.

Description

FIELD OF INVENTION

[0001] The invention relates generally to supercapacitors, and more particularly to stacked supercapacitors GENERAL STATE OF THE ART

[0002] While the size of portable electronic devices continues to shrink, their power requirements often do not decrease comparably. For example, a next-generation MEMS accelerometer may have a volume 10 percent smaller and still require only 5 percent less power than the previous generation MEMS accelerometer. In this case, more of the MEMS die can be used for energy storage. Unfortunately, this trend can limit the miniaturization and applicability of such electronic devices.

[0003] Technology has responded to this problem by developing chip-level supercapacitors (also known as "microsupercapacitors"), which have much larger capacitances than conventional capacitors. In particular, compared to conventional capacitors and batteries, supercapacitors generally have higher power densities, shorter charge and discharge times, longer life cycles, and faster switching capabilities.

[0004] Supercapacitors sometimes have two closely spaced electrodes that can short-circuit if contact is made. While current technology places an insulating separator between the electrodes to prevent this problem, many such separators cause other issues.

[0005] US 2001 / 0 020 319 A1 describes a method for producing a double-layer capacitor comprising the following steps: impregnating a plurality of carbon preforms with a metal; forming a plurality of current collector foils, each of the plurality of current collector foils having a tab portion and a wing portion; forming a plurality of electrodes by positioning one of the plurality of carbon preforms against corresponding wing portions of each of the plurality of current collector foils, each of the plurality of electrodes comprising one of the plurality of current collector foils and one of the plurality of carbon preforms; stacking each of the plurality of electrodes such that tab portions of adjacent ones of the plurality of current collector foils are offset, thereby forming an electrode stack;Inserting respective porous separator sections between each of the plurality of electrodes, the porous separator sections acting as electrical insulators between adjacent ones of the plurality of electrodes and preventing electrical shorting to each other; applying a modest constant pressure against the electrode stack; saturating the electrode stack with an electrolytic solution; and maintaining the electrode stack immersed in the electrolytic solution. BRIEF DESCRIPTION OF DIFFERENT EMBODIMENTS

[0006] The subject matter of the invention is defined in claims 1 to 14. A method for manufacturing a supercapacitor provides a first substrate having a first base, forms a first electrode on the first substrate, and forms a separator such that the electrode is located between the first base and the first separator. The method also micromachines holes through the separator, forms a chamber, and adds electrolyte containing ions to the chamber. The electrolyte is in contact with the first electrode within the chamber. Furthermore, the holes are sized to allow the transfer of electrolyte ions through the holes.

[0007] Although the holes allow ion transfer, the separator may be formed from a material that is substantially impermeable to the ions of the electrolyte. For example, each hole in a group of holes of the separator may have a largest dimension (e.g., corner-to-corner or diameter) of between about 2 nanometers and 5 micrometers. As another example, the separator may include nitride or parylene, and the first base may include low-resistivity silicon (e.g., doped silicon). The method may also form a current collector on the first substrate. Such a current collector may be positioned between the first base and the first electrode and in electrical contact with the first electrode.

[0008] Some embodiments also provide a second substrate supporting a second electrode and wafer bonding the first substrate to the second substrate. Insulating the material at the bonding area between the two substrates may be provided to prevent electrical short circuits between the two substrates after bonding. As a result of this bonding, 1) the separator electrically prevents electrical contact between the first electrode and the second electrode, and 2) the bonded first and second wafers form the aforementioned chamber. Preferably, a microfabrication process forms the first electrode and the separator.

[0009] According to another embodiment, a supercapacitor has a first substrate and a second substrate, a first electrode supported by the first substrate, and a second electrode supported by the second substrate. The first substrate is wafer-bonded to the second substrate to form a chamber. The supercapacitor also has a first separator between the first electrode and the second electrode within the chamber, and electrolyte containing ions within the chamber. The separator has a plurality of microfabricated holes sized to allow the ions of the electrolyte to pass through the holes. Furthermore, the electrolyte is in contact with the first and second electrodes.

[0010] Another method for fabricating a supercapacitor forms 1) a first electrode on a first substrate, 2) a separator on the first substrate, 3) holes through the separator, and 4) a second electrode on a second substrate. The method also wafer bonds the first substrate to the second substrate to form a chamber, such that the separator is located between the first electrode and the second electrode. Furthermore, the method adds an electrolyte containing ions to the chamber, such that the electrolyte is in contact with the first electrode and the second electrode. The holes through the separator are sized to allow the electrolyte ions to pass through the holes. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following "Descriptions of Illustrative Embodiments" discussed with reference to the drawings summarized immediately thereafter. Fig. 1 schematically shows a supercapacitor that may be configured and formed according to illustrative embodiments of the invention. Fig. Figure 2 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 along line 2-2. Fig. Figure 3 shows a process for forming the supercapacitor of Fig. 1 according to illustrative embodiments of the invention. Fig. Figure 4 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 300 of the process of Fig. 3. Fig. Figure 5 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 302 of the process of Fig. 3. Fig. Figure 6 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 304 of the process of Fig. 3. Fig. Figure 7 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 306 of the process of Fig. 3. Fig. Figure 8 shows a schematic cross-sectional view of the supercapacitor of Fig. 1, while the process of Fig. 3 approaches step 308. Fig. Figure 9 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 308 of the process of Fig. 3. Fig. Figure 10 shows a schematic cross-sectional view of the supercapacitor of Fig. 1, while the process approaches step 310 of the process of Fig. 3 approaches. Fig. Figure 11 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 310 of the process of Fig. 3. Fig. Figure 12 shows a schematic cross-sectional view of the supercapacitor of Fig. 1 at step 312 of the process of Fig. 3. Fig. Figure 13 shows a schematic cross-sectional view of the supercapacitor of Fig. 1, while the process approaches step 314 of the process of Fig. 3 approaches. Fig. Figure 14 shows a schematic cross-sectional view of a supercapacitor of Fig. 1 at steps 314, 316 and 318 of the process of Fig. 3, but with two separators. Fig. 15 schematically shows another embodiment of the invention. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0012] Illustrative embodiments use microfabrication processes to form robust and reliable supercapacitors. To this end, such embodiments form supercapacitors with two out-of-plane electrodes that are electrically and physically separated within an electrolyte-filled chamber by a rigid and robust separator. While the separator may be made of a material that does not allow the electrolyte ions to transfer therethrough (e.g., nitride), it has multiple microfabricated holes with geometries to permit ion transfer. Accordingly, ions can freely pass through the separator holes, allowing relatively free ion movement within the chamber. Details of illustrative embodiments are discussed below.

[0013] Fig. 1 schematically shows a perspective view of a microsupercapacitor (hereinafter "supercapacitor 10") configured according to illustrative embodiments of the invention. Fig. Figure 2 schematically shows a cross-sectional view of the supercapacitor 10 along the line 2-2 of Fig. 1. Specifically, the supercapacitor 10 is a unitary chip-level device comprising a pair of multilayer substrates 12 bonded together to form an internal chamber 16. Among other things, the internal chamber 16 contains a pair of stacked electrodes 18 (i.e., out-of-plane electrodes 18 relative to the substrate 12) and electrolyte material 20 (also referred to as "electrolyte 20"), which together form a capacitance. In other words, the electrodes 18 and the electrolyte material 20 cooperate to possess the capacitance to store a prescribed electrical charge.

[0014] To prevent electrical contact between the electrodes 18, the chamber 16 also includes a unitary separator 14 that physically separates the two electrodes 18. Among other functions, the separator 14 substantially prevents the electrodes 18 from making electrical contact. As discussed below, the separator 14 is preferably formed from a material commonly used in microfabrication, such as nitride, parylene, or an oxide. As such, the separator material is generally impervious to the ions of the electrolyte material 20. Such a design, if solid, would thus be ineffective.

[0015] The inventors have recognized that a robust separator material is more desirable than the prior art separator material (e.g., polymers), but unlike prior art separators, such materials are generally impermeable to ions. This limitation makes a more desirable material impractical. Accordingly, the inventors realized that the separator 14 should have some means to allow ion transfer and, consequently, holes 22 formed through the separator 14. The holes 22 are preferably sized to have an internal dimension / geometry that allows transfer of the ions within the electrolyte material 20. For example, the holes 22 may be 1-5 micrometers wide or even as small as 2 nanometers. One of ordinary skill in the art may choose the size of the holes 22 based on the electrolyte material 20 used.Ions within the electrolyte material 20 can thus freely pass through the holes 22 in the separator 14 to optimize the storage capacity of the supercapacitor 10.

[0016] The electrodes 18 may be formed from conventional materials known in the art—preferably from a porous solid material. For example, as discussed in more detail below, the electrodes 18 may be formed from graphene, which is known to be a porous material with intertwined inner and outer surfaces. Thus, virtually any surface of the electrode 18 exposed to the electrolyte 20 can therefore be considered part of the surface area of ​​the capacitor plates, which is represented by the well-known equation: C=ε∗(A / D) where C is the capacitance, ε is a constant, A is the area and D is the distance.

[0017] Indeed, one skilled in the art may use other materials to form the electrode 18, such as activated carbon, carbon aerogel, or carbon nanotubes, to name a few. Accordingly, the discussion of graphene is merely exemplary and is not intended to limit various other embodiments of the invention.

[0018] Similarly, the electrolyte 20 may be formed from any of a wide variety of other suitable materials. For example, the electrolyte 20 may be formed from an aqueous salt such as sodium chloride, or a gel such as a polyvinyl alcohol polymer impregnated with a salt. Some embodiments may utilize an ionic liquid in which ions are in a liquid state at room temperature. Although not necessarily aqueous, such electrolytes 20 are known to be extremely conductive due to the relatively free movement of their ions. The inventors believe that such an electrolyte 20 could produce a supercapacitor 10 with a relatively high energy storage capacity because, as one skilled in the art will appreciate, the energy storage of the capacitor is a function of the square of the voltage.

[0019] As noted, the electrolyte 20 is preferably generally integrated with both the inner and outer surfaces of the electrodes 18. Among other things, the inner surfaces may be formed by intricate internal channels and pores within the electrodes 18. The external surfaces may simply be those surfaces visible from the electrode exteriors. The electrolyte 20 and the aforementioned electrode surfaces are thus considered to form an interface that stores energy.

[0020] Depending on the electrode material, electrons can flow somewhat freely within the electrodes 18. For example, electrons can flow within graphene. However, the electrolyte 20 acts as an insulator and thus does not conduct electrons from the electrodes 18. Similarly, ions in the electrolyte 20 can migrate somewhat freely to the interfaces with the electrodes 18. Like electrons in the electrodes 18, ions in the electrolyte 20 do not migrate through this interface.

[0021] When ions within the electrolyte 20 are subjected to an electric field, they migrate in such a way that they align with the electric field. This causes electrons and holes in the electrodes 18 to migrate in a corresponding manner, effectively storing charge. For example, in a prescribed electric field, positive ions in the electrolyte 20 may migrate to a first electrode surface (e.g., on the top electrode 18), and the negative ions in the electrolyte 20 may migrate to a second electrode surface (e.g., on the bottom electrode 18). In this case, the positive ions near the first electrode surface attract electrons (in the electrode 18) to that interface, while the negative ions near the second electrode surface attract holes (in the electrode 18) for that interface.The distance of the ions from the interface plus the distance of the electrons or holes from the same interface (on the opposite side of the interface) represents the distance “d” of equation 1 above.

[0022] Although graphene is useful as an electrode material, it does not possess optimal conductivity properties. Accordingly, illustrative embodiments also form a current collector 26 on or as part of the substrate 12 to provide external access to the electrodes 18. Among other materials, the current collector 26 may be formed from a highly conductive metal such as gold or a highly doped semiconductor such as polysilicon. Those skilled in the art may select other materials for this purpose.

[0023] The supercapacitor 10 has a number of additional features that are less relevant to the discussion but are worth mentioning. For example, it has positive and negative contacts (not shown, but each partially formed by the base layers of the substrate 12) and a plug 24 for hermetically sealing the electrolyte 20 in the chamber 16. In addition, the supercapacitor 10 may also have an outer package, which is not shown in the figures, or it may be considered to form a wafer-level package, as in Fig. 1 and Fig. 2 shown.

[0024] Fig. 3 shows a process of manufacturing the supercapacitor 10 of Fig. 1 and Fig. 2 according to the illustrative embodiments of the invention—using microfabrication processes. It should be noted that this process is significantly simplified compared to a lengthy process that would normally be used to form the supercapacitor 10. Accordingly, the process for forming the supercapacitor 10 includes many steps, such as testing steps or additional passivation steps, that one of ordinary skill in the art would likely utilize. Furthermore, some of the steps may be performed in a different order than shown or at the same time. One of ordinary skill in the art may therefore modify the process as appropriate. Furthermore, many of the materials and structures mentioned (e.g., silicon nitride) are only one or one of a wide variety of different materials and structures that may be used.One skilled in the art may select appropriate materials and structures depending on the application and other constraints. Accordingly, the discussion of specific materials and structures is not intended to limit all embodiments.

[0025] It should be noted that the process of Fig. 3 is a bulk micromachining process that simultaneously forms multiple supercapacitors 10 on the same wafer / base. Although much less efficient, one skilled in the art can apply these principles to a process that forms only one supercapacitor 10.

[0026] The process begins at step 300, which patterns the device layer of a silicon-on-insulator wafer (also known as an "SOI wafer 28"). In particular, as will be appreciated by those skilled in the art, an SOI wafer 28 is a three-layer wafer structure having an oxide layer 30 (e.g., silicon dioxide) between two silicon wafers 32A and 32B. Due to the perspective of the drawings, the upper silicon wafer 32A is often referred to as the "device wafer" 32A, while the lower silicon wafer 32B is often referred to as the "handle wafer 32B." Accordingly, patterned as shown in Fig. As shown in Figure 4, the process patterns the device layer of SOI wafer 28 by forming a conventional etch mask and etching through the mask's openings. Among other things, the process may perform a dry etch using silicon difluoride (XeF2). This patterned region within the device layer will ultimately form the inner chamber 16 containing the electrodes 18 and the electrolyte 20.

[0027] Next, step 302 isolates the device wafer 32A. As in Fig. 5, a portion of this insulator contacts the oxide layer 30 of the SOI wafer 28. For example, this step may coat the wafer with silicon nitride 34 (Si3N4), which defines an electrode window, together with the photoresist layer 36 to be used in the subsequent step 304. In particular, as shown in Fig. 6, step 304 deposits a thick photoresist layer 36 over the wafer, and then patterns the photoresist layer 36 to remove a prescribed portion of the silicon nitride 34 and a portion of the oxide layer 30 of the SOI wafer 28. Accordingly, this step forms a recess for forming the current collector 26 and the electrode 18.

[0028] To this end, step 306 first deposits metal to form a current collector layer, which will form the current collector 26. After the deposited metal has cooled, the process adds graphene, which forms the electrode 18 supported on the substrate 12. In illustrative embodiments, the graphene is in the form of multiple stacked graphene monolayers. This step concludes with the removal of the thick photoresist layer 36 deposited in step 304. Fig. Figure 7 schematically shows a cross-sectional view of the wafer / structure (also known as a "preliminary device") at the process stage. The dashed lines indicate where the photoresist layer 36 was positioned before it was removed.

[0029] After forming the electrode 18, the process proceeds to step 308, which 1) forms the separator 14 and 2) microfabricates holes 22 through the separator 14. In particular, as shown in Fig. As shown in Figure 8, the process forms a sacrificial layer 38 within the subchamber formed by the previous steps. The sacrificial layer 38, which may be made of any conventional material, such as silicon dioxide, completely covers the electrode 18. Next, this step may employ a chemical mechanical planarization process, a common wafer grinding process, to expose the uppermost portions of the silicon nitride 34, as shown in Figure 8. Fig. 8 shown.

[0030] This step then etches a portion of the sacrificial layer 38 (in this case, the silicon dioxide) using a conventional etchant such as hydrofluoric acid. This portion of step 308 still maintains sufficient sacrificial material to continue encapsulating the electrode 18. The remaining sacrificial material 38 thus provides a support for receiving the separate material, which in this embodiment is at least partially composed of silicon nitride. As noted above, this separate material (e.g., silicon nitride in this case) is substantially impervious to ions within the electrolyte 20, which is to be added in a later step. Finally, step 308 microfabricates a plurality of holes 22 through the silicon nitride to Fig. 9. (This figure also has a thinned substrate 12 for illustrative purposes only. A thinner substrate 12 is not essential in this step.) Accordingly, as noted above, the separator holes 22 are configured to have a size / dimension and geometry that allows ions of the added electrolyte 20 to pass through the separator 14. In the absence of the holes 22 or any other means, ions could not pass through the separator 14.

[0031] As mentioned, this process is preferably performed substantially simultaneously with several other similar structures. Alternatively, this process can be performed serially. Indeed, some of these structures do not have a separator 14. In this case, steps 300 to 308 form several structures (some with separators 14 and some without separators 14) that are subsequently used in the remaining steps of the process.

[0032] The process then goes to step 310, which performs two of the Fig. 9 shown wafers are bonded together to form the structure of Fig. 11. In this embodiment, only one of the structures to be bonded has the separator 14, while the other does not have the separator 14. To do this, the process first covers the silicon nitride separator 14 (or the wafer / structure with a separator 14) with additional silicon dioxide and then uses chemical mechanical planarization processes to remove the upper portion of the silicon nitride, thus exposing the silicon of the device wafer 32A. This part of step 310 is Fig. 10 is graphically detailed. A similar process is used to prepare the structures without separator 14. After step 310 prepares both wafers, it then uses conventional wafer bonding techniques to bond the two wafers together. This wafer bond forms the inner chamber 16.

[0033] At this point in the process, much of the sacrificial material must be removed to make room for the electrolyte 20 to be added to the inner chamber 16. To do this, the process proceeds to step 312, which forms an insulated opening 40 to the chamber 16. In particular, etch, as in Fig. As shown in Figure 12, microfabrication processes an opening 40 through the lower wafer / structure (which is now thinner) and subsequent layers further inward until the opening 40 reaches the sacrificial oxide in the inner chamber 16. To protect the oxide layer 30, the process then deposits silicon nitride on the sidewalls of the opening 40, thereby forming the aforementioned isolated opening 40. Fig. Figure 12 shows the structure at this point in the process.

[0034] The isolated opening 40 forms a channel for removing the sacrificial oxide within the inner chamber 16 and then adding the electrolyte 20 in its place. Accordingly, step 314 first removes the sacrificial oxide within the chamber 16 by conventional means such as vapor and liquid acid etching (e.g., using hydrofluoric acid) to remove the Fig. 13. Next, step 314 adds the electrolyte 20 to the inner chamber 16.

[0035] This inner chamber 16, filled with a liquid electrolyte 20, is then given time to harden, depending on the type of electrolyte. Positive or negative pressure can be applied to better integrate the electrolyte 20 with the electrodes 18. Step 316 then seals the opening 40 with an appropriate material to hermetically seal the inner chamber 16. The plug 24 is only one example of such a seal. Fig. Figure 2 shows the wafer / structure at this stage of the process. Embodiments that do not use a solid or hardened electrolyte 20 (i.e., a liquid electrolyte 20) preferably use an appropriate sealing material that prevents long-term liquid from escaping from the inner chamber 16. The process ends at step 318, which forms contacts (not shown) on the two substrates 12, separates the wafer(s), and then encapsulates the dies as required by the application.

[0036] As mentioned above, step 310 may join two identical wafers / structures that both have separators 14. In this case, the final device may be more like the die / structure of Fig. 14, which features a dual-separator design. Such a design should improve reliability and safety.

[0037] Such embodiments may also be used for a supercapacitor 10 with side-by-side / coplanar electrodes 18, such as that in Fig. 15. In this case, a single substrate 12 may support electrodes 18 within separate but connected internal chambers 16 formed by a cap wafer 41 (e.g., a silicon wafer). Specifically, the chambers (e.g., formed of nitride) have separators 14 with preformed holes 22 that communicate with through-holes 42 extending through the substrate 12 and the cap wafer 41. Fig. Figure 15 also shows through-holes 42 across cross-section AA. Various embodiments have an electrolyte 20 extending through the separator holes 22 and through-holes 42.

[0038] Accordingly, illustrative embodiments form robust and reliable supercapacitors 10 using a separator 14 formed from strong materials not previously used in supercapacitor technology (to the inventors' knowledge). Such materials are now available because the affirmatively formed holes 22 through the separator 14 allow ions to traverse the entire area of ​​the inner chamber containing the electrolyte 20.

Claims

[1] Supercapacitor (10) having: a first substrate (12) and a second substrate (12); a first electrode (18) supported by the first substrate; a second electrode (18) supported by the second substrate; wherein the first substrate is connected to the second substrate to form an inner chamber (16); a first separator (14) between the first electrode and the second electrode within the inner chamber, the first separator having a plurality of microfabricated holes (22); Electrolyte (20) with ions within the inner chamber and in contact with the first and second electrodes, wherein the micro-machined holes of the first separator are sized to allow the ions to pass through the micro-machined holes. [2] The supercapacitor (10) of claim 1, wherein each of the first and second electrodes (18) comprises a porous solid material. [3] The supercapacitor (10) of claim 2, wherein each of the first and second electrodes (18) comprises a plurality of graphene monolayers. [4] Supercapacitor (10) according to one of claims 1 to 3, further comprising a second separator (14) between the first separator (14) and the second electrode (18). [5] Supercapacitor (10) according to one of claims 1 to 4, wherein the electrolyte (20) comprises a hardened gel electrolyte. [6] Supercapacitor (10) according to one of claims 1 to 5, wherein the first separator (14) comprises nitride or parylene. [7] Supercapacitor (10) according to one of claims 1 to 5, wherein the first separator (14) is formed from a material that is substantially impermeable to the ions of the electrolyte (20). [8] Supercapacitor (10) having: a first substrate (12) and a second substrate (12) which together form at least a portion of a die; a first electrode (18) and a second electrode (18) spaced apart from each other and disposed between the first substrate and the second substrate, the first electrode forming an inner chamber (16); a separator (14) between the first electrode and the second electrode within the inner chamber, the separator having a plurality of micro-machined holes (22); Electrolyte (20) with ions within the inner chamber and in contact with the first and second electrodes, wherein the micro-machined holes of the separator are sized to allow the ions to pass through the micro-machined holes. [9] The supercapacitor (10) of claim 8, wherein each of the first and second electrodes comprises a porous solid material. [10] The supercapacitor (10) of claim 9, wherein each of the first and second electrodes comprises a plurality of graphene monolayers. [11] Supercapacitor (10) according to one of claims 8 to 10, wherein the separator comprises a first separator and a second separator between the first electrode and the second electrode. [12] Supercapacitor according to one of claims 8 to 11, wherein the electrolyte (20) comprises a hardened gel electrolyte. [13] Supercapacitor (10) according to one of claims 8 to 12, wherein the separator (14) comprises nitride or parylene. [14] Supercapacitor (10) according to one of claims 8 to 12, wherein the separator (14) is formed from a material that is substantially impermeable to the ions in the electrolyte.

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