KR102465400B1 - Light emitting device and method of fabricating the same - Google Patents

Abstract

The embodiment relates to a light emitting device including a phosphor layer having a uniform thickness and a method for manufacturing the same, and the light emitting device according to an embodiment of the present invention includes a light emitting structure including a first semiconductor layer, an active layer and a second semiconductor layer; a phosphor layer covering the first semiconductor layer exposed on the first surface of the light emitting structure; a first electrode surrounding side surfaces of the first semiconductor layer and the phosphor layer and electrically connected to the first semiconductor layer; and a second electrode electrically connected to the second semiconductor layer exposed on the second surface opposite to the first surface of the light emitting structure.

Description

Light emitting device and manufacturing method thereof

An embodiment of the present invention relates to a light emitting device.

A light emitting diode (LED) is one of light emitting devices that emits light when an electric current is applied thereto. Light-emitting diodes can emit high-efficiency light with a low voltage, and thus have an excellent energy-saving effect. Recently, the luminance problem of light emitting diodes has been greatly improved, and it has been applied to various devices such as a backlight unit of a liquid crystal display device, an electric signboard, a display device, and a home appliance.

1 is a cross-sectional view of a general light emitting device.

As shown in FIG. 1 , the light emitting device includes a light emitting structure 11 including a first semiconductor layer 11a , an active layer 11b and a second semiconductor layer 11c provided on a substrate 10 , and the light emitting structure 11 . ) and a first electrode 12a and a second electrode 12b connected to each other. When the light emitting device further includes the phosphor layer 13 , when the light generated from the active layer 11b is emitted to the outside, the color of the light emitted from the light emitting device may be determined by the phosphor layer 13 .

However, as the size of the light emitting device becomes smaller, the area over which the phosphor is applied becomes narrower, making it difficult to uniformly apply the formed body. Even when the phosphor is uniformly applied, the phosphor layer 13 with a uniform thickness flows to the edge of the light emitting device before the phosphor is cured. ) is difficult to form.

An embodiment of the present invention provides a light emitting device including a phosphor layer formed with a uniform thickness and a method of manufacturing the same.

The light emitting device of the embodiment includes a light emitting structure including a first semiconductor layer, an active layer, and a second semiconductor layer; a phosphor layer covering the first semiconductor layer exposed on the first surface of the light emitting structure; a first electrode surrounding side surfaces of the first semiconductor layer and the phosphor layer and electrically connected to the first semiconductor layer; and a second electrode electrically connected to the second semiconductor layer exposed on the second surface opposite to the first surface of the light emitting structure.

In this case, the phosphor layer is formed in a groove having a bottom surface of the first semiconductor layer and a side surface of the first electrode.

The manufacturing method of the light emitting device of the embodiment includes forming a light emitting structure including a first semiconductor layer, an active layer and a second semiconductor layer on a substrate; selectively removing the light emitting structure to expose the first semiconductor layer; forming a first electrode electrically connected to the first semiconductor layer and surrounding a side surface of the first semiconductor layer; forming a second electrode electrically connected to the second semiconductor layer; separating the substrate from the light emitting structure; further removing the first semiconductor layer exposed by the separation of the substrate to form a groove having a bottom surface of the first semiconductor layer and a side surface of the first electrode; and forming a phosphor layer in the groove.

According to the embodiment, the light emitting device of the embodiment of the present invention has the following effects.

First, after forming a groove by removing a portion of the light emitting structure, a phosphor layer may be formed in the groove. Accordingly, it is possible to implement a phosphor layer having a uniform thickness by preventing the phosphor layer from peeling off, and it is possible to reduce color deviation according to the directional angle of the light emitting device.

Second, since the phosphor layer is formed in the groove of the light emitting structure, the thickness of the light emitting device can be reduced to realize a light emitting device having a thin thickness.

1 is a cross-sectional view of a general light emitting device.
2A is a cross-sectional view of a light emitting device according to an embodiment of the present invention.
Fig. 2B is a bottom plan view of Fig. 2A;
3A to 3I are cross-sectional views illustrating a method of manufacturing a light emitting device according to an embodiment of the present invention.
4 is a cross-sectional view of a printed circuit board attached to a second electrode.

Since the present invention may have various changes and may have various embodiments, specific embodiments will be illustrated and described in the drawings. However, this is not intended to limit the embodiment of the present invention to a specific embodiment, and it should be understood to include all changes, equivalents, or substitutes included in the spirit and scope of the embodiment.

Terms including an ordinal number, such as first, second, etc., may be used to describe various elements, but the elements are not limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the second component may be referred to as the first component, and similarly, the first component may also be referred to as the second component. and/or includes a combination of a plurality of related listed items or any of a plurality of related listed items.

When a component is referred to as being “connected” or “connected” to another component, it may be directly connected or connected to the other component, but it is understood that other components may exist in between. it should be On the other hand, when it is said that a certain element is "directly connected" or "directly connected" to another element, it should be understood that no other element is present in the middle.

The terms used in the present application are only used to describe specific embodiments, and are not intended to limit the embodiments of the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as "comprise" or "have" are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It is to be understood that it does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

In the description of the embodiment, in the case where one element is described as being formed on "on or under" of another element, on (above) or below (on) or under) includes both elements in which two elements are in direct contact with each other or one or more other elements are disposed between the two elements indirectly. In addition, when expressed as "up (up) or down (on or under)", it may include the meaning of not only an upward direction but also a downward direction based on one element.

Hereinafter, the embodiment will be described in detail with reference to the accompanying drawings, but the same or corresponding components are given the same reference numerals regardless of reference numerals, and overlapping descriptions thereof will be omitted.

Hereinafter, the light emitting device of the embodiment will be described in detail with reference to the accompanying drawings.

2A is a cross-sectional view of a light emitting device according to an embodiment of the present invention, and FIG. 2B is a bottom plan view of FIG. 2A.

As shown in FIG. 2A , the light emitting device according to the embodiment of the present invention includes a light emitting structure 110 including a first semiconductor layer 110a, an active layer 110b and a second semiconductor layer 110c, and a first surface of the light emitting structure 110 . The phosphor layer 140 covering the first semiconductor layer 110a exposed in and a second electrode 120a electrically connected to the second semiconductor layer 110c exposed on the second surface opposite to the first surface of the light emitting structure 110 .

Specifically, the light emitting structure 110 includes a first semiconductor layer 110a, an active layer 110b, and a second semiconductor layer 110c sequentially formed, and the first semiconductor layer 110a includes a first electrode 120a. is electrically connected to, and the second semiconductor layer 110c is electrically connected to the second electrode 120b.

The first semiconductor layer 110a may be implemented with a group III-V or group II-VI compound semiconductor, and may be doped with a first dopant. The first semiconductor layer 110a is a semiconductor material having a composition formula of In _x Al _y Ga _{1 -x- y} N (0≤x≤1, 0≤y≤1, 0≤x+y≤1) or AlInN, AlGaAs , GaP, GaAs, GaAsP, may be formed of a material selected from AlGaInP. When the first dopant is a p-type dopant such as Mg, Zn, Ca, Sr, or Ba, the first semiconductor layer 110a may be a p-type semiconductor layer.

The active layer 110b is provided between the first semiconductor layer 110a and the second semiconductor layer 110c. The active layer 110b is a layer in which electrons (or holes) injected through the first semiconductor layer 110a and holes (or electrons) injected through the second semiconductor layer 110c meet. The active layer 110b as described above may transition to a low energy level as electrons and holes recombine, and may generate light having a corresponding wavelength.

The active layer 110b may have any one of a single well structure, a multiwell structure, a single quantum well structure, a multi quantum well (MQW) structure, a quantum dot structure, or a quantum wire structure, and the active layer 110b The structure of is not limited thereto. When the active layer 110b has a well structure, the well layer/barrier layer of the active layer 110b is InGaN/GaN, InGaN/InGaN, GaN/AlGaN, InAlGaN/GaN, GaAs (InGaAs)/AlGaAs, GaP (InGaP). It may be formed in any one or more pair structure of /AlGaP, but is not limited thereto. The well layer may be formed of a material having a band gap smaller than that of the barrier layer.

The second semiconductor layer 110c may be implemented with a group III-V or group II-VI compound semiconductor, and may be doped with a second dopant. The second semiconductor layer 110c is a semiconductor material having a composition formula of Al _x In _y Ga _(1-xy) N (0≤x≤1, 0≤y≤1, 0≤x+y≤1), InAlGaN, AlGaAs , GaP, GaAs, GaAsP, may be formed of any one or more of AlGaInP, but is not limited thereto. When the second dopant is an n-type dopant such as Si, Ge, Sn, Se, Te, or the like, the second semiconductor layer 110c may be an n-type semiconductor layer.

The light emitting structure 110 as described above may be formed of at least any one of np, pn, npn, and pnp junction structures. It may be of various structures including. In addition, doping concentrations of impurities in the first semiconductor layer 110a and the second semiconductor layer 110c may be uniformly or non-uniformly formed. That is, the doped structure of the light emitting structure 110 may be formed in various ways, but is not limited thereto.

The phosphor layer 140 may be formed in a direction in which light generated from the light emitting structure 110 is emitted. Accordingly, when the light generated from the light emitting structure 110 is emitted to the outside, the color of the light emitted from the light emitting device may be determined by the phosphor layer 140 . For example, in order for the light emitting device of the present invention to realize white light, the light emitting structure 110 may emit blue light, and the phosphor layer 140 may include a yellow phosphor.

On the other hand, as the size of the light emitting device is reduced, the area to which the phosphor is applied becomes narrow, and it is difficult to obtain the phosphor layer 140 having a uniform thickness. Furthermore, when the light emitting device is manufactured in micro size and each light emitting device is formed for each pixel of the display device, the size of the light emitting device is very small, and the phosphor layer 140 is partially removed or the thickness is not uniform. A color deviation according to the directional angle of the light emitting device of the device may increase. In this case, a problem occurs in that the color characteristics of the display device including the light emitting element are deteriorated.

Accordingly, in the present invention, the first electrode 120a electrically connected to the first semiconductor layer 110a has a structure that not only the light emitting structure 110 but also the side surface of the phosphor layer 140 is covered, so that the phosphor layer 140 is It has a structure in which an edge is surrounded by the first electrode 120a. That is, the phosphor layer 140 has a structure in which the bottom surface is the light emitting structure 110 and the side surface is filled in the groove 110h made of the first electrode 120a, so that the phosphor layer 140 having a uniform thickness can be formed. have. In this case, when the thickness of the phosphor layer 140 is too thin, the phosphor layer 140 cannot output blue light generated from the light emitting structure 110 as white light. Accordingly, the thickness of the phosphor layer 140 is preferably 0.3 μm or more.

That is, as shown in FIG. 2B , since the phosphor layer 140 is formed in the groove 110h surrounded by the first electrode 120a, it is possible to prevent the phosphor from flowing to the edge of the light emitting device before the phosphor is cured. In addition, since the phosphor layer 140 is uniformly filled in the groove 110h, the phosphor layer 140 having a uniform thickness can be implemented. Accordingly, it is possible to reduce the color deviation according to the directional angle of the light emitting element, thereby preventing the problem of deterioration of color characteristics of the display device including the light emitting element.

Moreover, in a general light emitting device, the thickness of the light emitting device increases according to the thickness of the phosphor layer 140 , but in the present invention, since the phosphor is filled in the groove 110h formed by removing the first semiconductor layer 110a, the thickness is thin. A light emitting device can be implemented.

Again, referring to FIG. 2A , the first electrode 120a and the second electrode 120b are formed of a transparent conductive oxide (TCO) or an opaque metal, or a mixture of a transparent conductive oxide film and an opaque metal, or It may be formed in a plurality of layers. At this time. Transparent conductive oxide films include Indium Tin Oxide (ITO), Indium Zinc Oxide (IZO), Aluminum Zinc Oxide (AZO), Aluminum Gallium Zinc Oxide (AGZO), Indium Zinc Tin Oxide (IZTO), Indium Aluminum Zinc Oxide (IAZO), IGZO It may be selected from (Indium Gallium Zinc Oxide), IGTO (Indium Gallium Tin Oxide), ATO (Antimony Tin Oxide), GZO (Gallium Zinc Oxide), IZON (IZO Nitride), ZnO, IrOx, RuOx, NiO, and the like. In addition, the opaque metal may be selected from Ag, Ni, Al, Rh, Pd, Ir, Ru, Mg, Zn, Pt, Au, Hf, and the like.

In particular, the first electrode 120a is preferably formed of a material having an etch rate lower than that of the first semiconductor layer 110a with respect to the etching gas for removing the first semiconductor layer 110a. To this end, the first electrode 120a may be made of gold (Au) or an alloy including gold (Au). In this case, the etching gas may be Cl ₂ or BCl ₃ or a mixed gas of Cl ₂ and BCl ₃ .

Hereinafter, a method of manufacturing a light emitting device according to an embodiment of the present invention will be described in detail.

3A to 3I are cross-sectional views illustrating a method of manufacturing a light emitting device according to an embodiment of the present invention.

As shown in FIG. 3A , the light emitting structure 110 is formed on the substrate 100 . The substrate 100 may be formed of a material selected from sapphire (Al ₂ O ₃ ), SiC, GaAs, GaN, ZnO, Si, GaP, InP and Ge, but is not limited thereto. The light emitting structure 110 may be formed by sequentially growing the first semiconductor layer 110a, the active layer 110b, and the second semiconductor layer 110c on the substrate 100 .

Then, as shown in FIG. 3B , the first semiconductor layer 110a, the active layer 110b, and the second semiconductor layer 110c are partially removed to expose the first semiconductor layer 110a. Then, as shown in FIG. 3C , isolation etching is performed to separate the adjacent light emitting structures 110 . In this case, the substrate 100 between the adjacent light emitting structures 110 is exposed through isolation etching.

And, as shown in FIG. 3D , the first electrode 120a electrically connected to the exposed first semiconductor layer 110a by partially removing the first semiconductor layer 110a, the active layer 110b, and the second semiconductor layer 110c. ) to form At this time, the first electrode 120a is connected to the exposed first semiconductor layer 110a and extends to the substrate 100 exposed between the adjacent light emitting structures 110 so as to cover the side surface of the first semiconductor layer 110a. It is an extended structure. That is, the first electrode 120a is formed to surround the edge of the light emitting structure 110 .

The first electrode 120a may be formed of a transparent conductive oxide (TCO) or an opaque metal, or may be formed of one or a plurality of layers in which a transparent conductive oxide layer and an opaque metal are mixed. In particular, the first electrode 120a is preferably made of a material that is not damaged by an etching gas during a process of removing the first semiconductor layer 110a, which will be described later, among the transparent conductive oxide film and the opaque metal described above.

For example, when the first semiconductor layer 110a is removed by a dry etching method using an etching gas such as Cl ₂ , BCl ₃ , etc., the first electrode 120a is higher than the first semiconductor layer 110a. It may be formed of a material having a low etching rate with respect to the etching gas. To this end, the first electrode 120a may be made of gold (Au) or an alloy including gold (Au).

Next, as shown in FIG. 3E , an insulating layer 130 is formed on the entire surface of the light emitting structure 110 and the substrate 100 to expose a portion of the second semiconductor layer 110c. The insulating film 130 may be formed by selecting at least one from the group consisting of SiO ₂ , Si _x O _y , Si ₃ N ₄ , Si _x N _y , SiO _x N _y , Al ₂ O ₃ , TiO ₂ , AlN, and the like. and is not limited thereto. The insulating film 130 is for insulating the second electrode 120b and the first electrode 120a. As shown, the insulating film 130 is formed to completely surround the first electrode 120a, or the first electrode ( It may be formed to cover only a part of 120a).

Then, as shown in FIG. 3F , the second electrode 120b is formed on the second semiconductor layer 110c exposed by the insulating layer 130 . The second electrode 120b is electrically connected to the second semiconductor layer 110c.

Although not shown, a reflective layer and an ohmic layer may be further formed between the second semiconductor layer 110c and the second electrode 120b. The reflective layer is for reflecting light generated from the active layer 110b in the direction of the first semiconductor layer 110a. The reflective layer as described above is formed of a material having a high reflectance such as Ag, Ni, Al, Rh, Pd, Ir, Ru, Mg, Zn, Pt, Au and Hf, or the high reflectivity material and IZO, IZTO, IAZO, A transparent conductive material such as IGZO, IGTO, AZO, and ATO may be mixed and formed.

And, the ohmic layer is for ohmic contact with the second semiconductor layer 110c. In particular, when the ohmic layer is formed of a low-resistance metal, current may easily diffuse from the second electrode 120b to the second semiconductor layer 110c through the ohmic layer. The ohmic layer as described above includes indium tin oxide (ITO), indium zinc oxide (IZO), indium zinc tin oxide (IZTO), indium aluminum zinc oxide (IAZO), indium gallium zinc oxide (IGZO), and indium gallium tin oxide (IGTO). ), AZO (aluminum zinc oxide), ATO (antimony tin oxide), GZO (gallium zinc oxide), IZON (IZO Nitride), AGZO (Al-Ga ZnO), IGZO (In-Ga ZnO), ZnO, IrOx, RuOx , NiO, RuOx/ITO, Ni/IrOx/Au, or Ni/IrOx/Au/ITO, Ag, Ni, Cr, Ti, Al, Rh, Pd, Ir, Sn, In, Ru, Mg, Zn, Pt, It may be formed including at least one of Au and Hf, but is not limited to these materials.

Then, as shown in FIG. 3G , the substrate 100 is removed from the light emitting structure 110 . The substrate 100 may be separated from the light emitting structure 110 by a method such as a laser lift off (LLO) using a laser.

Next, as shown in FIG. 3H , the first semiconductor layer 110a exposed by removing the substrate 100 is also partially removed. This is to form a groove 110h for filling a phosphor layer to be described later in the light emitting structure 110 .

That is, the groove 110h has a depth corresponding to the thickness of the removed first semiconductor layer 110a, the bottom surface of the groove 110h is the first semiconductor layer 110a, and the side surface of the groove 110h emits light. The first electrode 120a surrounds the side surface of the structure 110 . In this case, the depth of the groove is the same as the thickness of the phosphor layer to be filled in the groove 110h, and the depth of the groove 110h is such that the phosphor layer has a sufficient thickness to output the light generated from the light emitting structure 110 as white light. It is preferable that it is 0.3 micrometer or more. Also, since the depth of the groove 110h corresponds to the removed thickness of the first semiconductor layer 110a, the depth of the groove 110h may be adjusted by adjusting the degree of removal of the first semiconductor layer 110a.

Next, as shown in FIG. 3I , the phosphor layer 140 may be formed by filling the groove 110h with a phosphor and curing the phosphor. In this case, since an accurate amount of phosphor can be filled in the groove 110h, manufacturing cost can be reduced.

That is, in the light emitting device of the present invention as described above, after the substrate 100 is removed and the exposed first semiconductor layer 110a is further removed to form the groove 110h, the phosphor layer 140 is formed in the groove 110h. to form Accordingly, the phosphor layer 140 is filled in the light emitting structure 110 to prevent the phosphor layer 140 from peeling off. Accordingly, the phosphor layer 140 having a uniform thickness may be implemented.

In addition, in a general light emitting device, the thickness of the light emitting device increases as much as the thickness of the phosphor layer 140 , but in the present invention, since the phosphor is filled in the region where the first semiconductor layer 110a is removed, a light emitting device having a thin thickness can be implemented. have.

Furthermore, when the light emitting device is formed in a micro size to form the light emitting device for each pixel of the display device, the size of the light emitting device is very small, so that the phosphor layer 140 is partially removed or the thickness is not uniform. However, in the present invention, since the phosphor layer 140 is formed in the groove 110h, a phosphor layer 140 having a uniform thickness is implemented to reduce color deviation according to the orientation angle of the light emitting device, so that the light emitting device is included. It is possible to prevent a problem in which the color characteristics of the display device are deteriorated.

Meanwhile, when the light emitting device is formed for each pixel of the display device as described above, the data line of the display device is electrically connected to the first electrode 120a and the second electrode 120b is connected to the common electrode of the display device. can The connection between the light emitting element and the display device driver is not limited thereto.

In particular, the second electrode 120b may be electrically connected to the printed circuit board.

4 is a cross-sectional view of a printed circuit board attached to a second electrode.

4 , the printed circuit board 200 may be attached to be electrically connected to the second electrode 120b. In this case, the printed circuit board 200 may be connected to the second electrode 120b before the board 100 is removed. In this case, the plurality of light emitting structures 110 formed on the substrate 100 may all be attached to one printed circuit board 200 . Also, the printed circuit board 200 may be individually attached to each light emitting structure 110 after the substrate 100 is removed.

The printed circuit board 200 may be connected to the second electrode 120b through the adhesive layer 210, and for electrical connection between the printed circuit board 200 and the second electrode 120b, the adhesive layer 210 is a conductive ball (Ball) and the like may include a conductive material. For example, the adhesive layer 210 may have a structure in which conductive balls are dispersed in a resin.

In particular, as shown, when the adhesive layer 210 is formed to completely surround the light emitting structure 110 , in order to insulate the first electrode 120a from the printed circuit board 200 , the insulating layer 130 is formed on the first It may be formed to completely surround the electrode 120a.

As described above, the light emitting device of the present invention is formed to be very small in a micro size, and may be disposed in each pixel defined by the intersection of the data line and the gate line of the display device. In this case, the light emitting device is not a backlight unit disposed on the rear surface of the liquid crystal cell or organic light emitting cell of the display device, but within the pixel, the first electrode 120a is connected to the data line, and the second electrode 120b is connected to the common electrode. connected, and can be directly driven by the driving unit of the display device. In this case, the white light emitted from the light emitting device may implement various colors of light through a color filter.

The embodiments of the present invention described above are not limited to the above-described embodiments and the accompanying drawings, and it is a technical field to which the embodiments of the present invention pertain that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the embodiments. It will be clear to those with prior knowledge in

100: substrate 110: light emitting structure
110a: first semiconductor layer 110b: active layer
110c: second semiconductor layer 110h: groove
120a: first electrode 120b: second electrode
130: insulating film 140: phosphor layer
200: printed circuit board 210: adhesive layer

Claims (15)

a light emitting structure including a first semiconductor layer, an active layer, and a second semiconductor layer;
a phosphor layer covering the first semiconductor layer exposed on the first surface of the light emitting structure;
a first electrode surrounding side surfaces of the first semiconductor layer and the phosphor layer and electrically connected to the first semiconductor layer; and
and a second electrode electrically connected to the second semiconductor layer exposed on the second surface opposite to the first surface of the light emitting structure,
The phosphor layer has a bottom surface formed of the first semiconductor layer and a side surface formed in a groove formed of the first electrode,
The first electrode is in direct contact with the side surface of the first semiconductor layer and the side surface of the phosphor layer,
the first electrode protrudes outward from all sides of the phosphor layer;
A portion of the first electrode protruding from one side of the phosphor layer protrudes more than the remaining portions of the first electrode protruding from other sides of the phosphor layer,
A lower surface of the phosphor layer and a lower surface of the first electrode are disposed on the same plane.
The method of claim 1,
The first electrode is a light emitting device including gold (Au).
The method of claim 1,
Further comprising an insulating film insulating the first electrode and the second electrode,
The insulating layer is formed along an edge of the light emitting structure to surround an upper surface of the first electrode.
The method of claim 1,
Further comprising a printed circuit board electrically connected to the second electrode,
The printed circuit board is connected to the second electrode through an adhesive layer, and the adhesive layer has a structure in which conductive balls are dispersed in resin.
forming a light emitting structure including a first semiconductor layer, an active layer, and a second semiconductor layer on a substrate;
selectively removing the light emitting structure to expose the first semiconductor layer;
forming a first electrode electrically connected to the first semiconductor layer and surrounding a side surface of the first semiconductor layer;
forming a second electrode electrically connected to the second semiconductor layer;
separating the substrate from the light emitting structure;
further removing the first semiconductor layer exposed by the separation of the substrate to form a groove having a bottom surface of the first semiconductor layer and a side surface of the first electrode; and
forming a phosphor layer in the groove;
The phosphor layer is in direct contact with a side surface of the groove formed by the first electrode and a bottom surface of the groove formed by the first semiconductor layer,
the first electrode protrudes outward from all sides of the phosphor layer;
A portion of the first electrode protruding from one side of the phosphor layer protrudes more than the remaining portions of the first electrode protruding from other sides of the phosphor layer,
A method of manufacturing a light emitting device in which a lower surface of the phosphor layer and a lower surface of the first electrode are disposed on the same plane.
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