Diamond-based sunken electrode photoconductive switch and preparation method thereof

Abstract

The invention relates to a diamond-based sunken electrode photoconductive switch and a preparation method thereof. The preparation method comprises the following steps: growing a diamond etching mask layer on the surface of an insulating diamond substrate; performing photoetching on the diamond etching mask layer to define an electrode etching area of the insulating diamond substrate, and defining an alignment mark area at the same time; removing the diamond etching mask layer in the electrode etching region and the alignment mark region; etching the insulating diamond substrate in the electrode etching area and the alignment mark area; carrying out organic cleaning and RCA cleaning on the etched insulating diamond substrate; performing oxygen plasma treatment on the etched and cleaned insulating diamond substrate to form a surface oxygen terminal; defining an electrode sputtering region through the alignment mark by using a photoetching overlay process; and manufacturing an ohmic contact electrode sinking into the electrode groove in the electrode sputtering area. According to the method, the manufacturing cost and difficulty of the diamond photoconductive switch are reduced, and the voltage withstanding characteristic and the conduction characteristic of the device are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor optoelectronic devices, and in particular relates to a diamond-based sunken electrode photoconductive switch and a preparation method thereof. Background Art

[0002] With the rapid development of high voltage, pulse power and wide bandgap semiconductor technology, the demand for photoconductive switches with excellent high voltage, high temperature and radiation resistance is growing. As a key component that can control the conductive state through optical signals, photoconductive switches can withstand high voltage and high current in a short time and are widely used in high-performance fields such as pulse power systems, ultrafast lasers, radars and communications.

[0003] Diamond is known as the "ultimate semiconductor" and has a large bandgap (5.47 eV), a high breakdown electric field (10 MV / cm), a low dielectric constant (5.7), a high thermal conductivity (>20 W / cm·K), and a high carrier mobility (μ e =2200cm 2 / V·s,μ h =1600cm 2 / V·s) and other characteristics, and is very suitable for working under high voltage, high power and high frequency conditions. Photoconductive semiconductor switch (PCSS) is triggered by light pulses and controls the conductivity of semiconductor materials to control the on and off of the switch. It is often used in high-energy pulse generation, high current density and radio frequency microwave applications.

[0004] However, the breakdown voltage of existing diamond photoconductive switches is far lower than its theoretical value, and there is still huge room for improvement in the withstand voltage performance of the device. Traditional photoconductive switches are prone to breakdown failure under large electric fields. Specifically, the accumulation of charges around the electrode causes the electric field of the device to be concentrated at the edge of the electrode, resulting in excessive peak electric field and current density around the electrode, which shortens the life of the device and reduces the overall reliability. Secondly, due to the small carrier transport cross-sectional area caused by the electrode design, the conduction characteristics of the diamond planar electrode photoconductive switch are often poor, mainly manifested in high on-resistance and low conduction efficiency. Finally, the existing diamond photoconductive switch preparation scheme usually involves steps such as ion implantation, epitaxial growth and complex electrode preparation, resulting in high device manufacturing costs, complex manufacturing processes, high processing difficulty and low economic benefits, which have become factors restricting the popularization and application of high-performance diamond photoconductive switches. Therefore, how to develop a diamond photoconductive switch technology solution with a simple preparation process and the ability to effectively reduce the peak electric field, current density and on-resistance of the device under high voltage conditions has become a key technical problem that needs to be solved urgently. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a diamond-based sunken electrode photoconductive switch and a preparation method thereof. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0006] The embodiment of the present invention provides a method for preparing a diamond-based sunken electrode photoconductive switch, comprising the steps of:

[0007] growing a diamond etching mask layer on the cleaned surface of the insulating diamond substrate;

[0008] Photolithographically defining an electrode etching region of the insulating diamond substrate on the diamond etching mask layer, and simultaneously defining an alignment mark region;

[0009] Removing the diamond etching mask layer in the electrode etching area and the alignment mark area to form a diamond etching mask;

[0010] Etching the insulating diamond substrate in the electrode etching area and the alignment mark area through the diamond etching mask to form an electrode groove and an alignment mark;

[0011] Performing organic cleaning and RCA cleaning on the etched insulating diamond substrate, so that the etched insulating diamond substrate has an electronic grade clean surface;

[0012] The cleaned insulating diamond substrate is subjected to oxygen plasma treatment to form surface oxygen terminations;

[0013] Using a photolithography overlay process, the electrode sputtering region is aligned with the electrode etching region through the alignment mark, and the electrode sputtering region is defined on the surface of the insulating diamond substrate, wherein the center of the electrode sputtering region coincides with the center of the electrode etching region;

[0014] An ohmic contact metal is fabricated in the electrode sputtering region to form an ohmic contact electrode sunk into the electrode groove.

[0015] In one embodiment of the present invention, etching the insulating diamond substrate of the electrode etching area and the alignment mark area through the diamond etching mask to form the electrode groove and the alignment mark includes:

[0016] An O-based dry etching process is adopted to etch the insulating diamond substrate in the electrode etching area and the alignment mark area through the diamond etching mask to form an electrode groove and an alignment mark.

[0017] In one embodiment of the present invention, the conditions of the O-based dry etching process include: the reaction gas includes O2, the O2 flow rate is 30-80sccm, the reaction chamber pressure is 7mTorr, the upper electrode power is 400-800W, the lower electrode power is 60-200W, and the etching depth is 0.2-3μm.

[0018] In one embodiment of the present invention, after etching the insulating diamond substrate of the electrode etching area and the alignment mark area through the diamond etching mask to form the electrode groove and the alignment mark, the step further includes:

[0019] The diamond etching mask remaining on the surface of the insulating diamond substrate is removed.

[0020] In one embodiment of the present invention, the insulating diamond substrate after etching is subjected to organic cleaning and RCA cleaning so that the insulating diamond after etching has an electronic grade clean surface, including:

[0021] The etched insulating diamond substrate is placed in acetone and isopropanol solutions for ultrasonic cleaning in turn;

[0022] The etched insulating diamond substrate is placed in a mixed solution of H2O2 and concentrated H2SO4 for cleaning;

[0023] The etched insulating diamond substrate is placed in a mixed solution of NH4OH, H2O2 and H2O for water bath heating and cleaning, and the mixed solution of NH4OH, H2O2 and H2O on the surface of the sample is rinsed off with ultrapure water;

[0024] The etched insulating diamond substrate is placed in a mixed solution of HCl, H2O2 and H2O for water bath heating and cleaning, and the mixed solution of HCl, H2O2 and H2O on the surface of the sample is rinsed off with ultrapure water;

[0025] The etched insulating diamond substrate is placed in a mixed solution of BOE and ultrapure water for cleaning, and the mixed solution of BOE and ultrapure water on the surface of the sample is rinsed off with ultrapure water;

[0026] The sample was blown dry using nitrogen gas to obtain an insulating diamond substrate with an electronic grade clean surface.

[0027] In one embodiment of the present invention, in the mixed solution of H2O2 and concentrated H2SO4, H2O2 has a mass fraction of 30%, and the volume ratio of H2O2 to concentrated H2SO4 is 1:2 to 1:4;

[0028] In the mixed solution of NH4OH, H2O2 and H2O, the volume ratio of NH4OH, H2O2 and H2O is 1:1:5;

[0029] In the mixed solution of HCl, H2O2 and H2O, the volume ratio of HCl, H2O2 and H2O is 1:1:6;

[0030] In the mixed solution of BOE and ultrapure water, the volume ratio of BOE to ultrapure water is 2:3, and the BOE solution includes 49% HF solution and 40% NH4F solution in a volume ratio of 1:6.

[0031] In one embodiment of the present invention, the material of the insulating diamond substrate comprises single crystal or polycrystalline diamond, and the dark state resistance is ≥10 13 Ω;

[0032] The material of the diamond etching mask layer includes a dielectric material or a metal material, the dielectric material includes any one of silicon nitride and silicon dioxide, the metal material includes any one of aluminum, titanium and gold, and the thickness of the diamond etching mask layer is 200 to 400 nm;

[0033] The shape of the alignment mark includes a cross figure;

[0034] The area of ​​the electrode sputtering region is larger than the area of ​​the electrode etching region;

[0035] The thickness of the ohmic contact electrode is greater than the depth of the electrode groove;

[0036] The material of the ohmic contact electrode includes Ti / Pt / Au stacked metal, Ti / Au stacked metal, Ti / Al stacked metal, Ti / Pt / Au / Ti stacked metal, Ti / Au / Ti / Au / Ti stacked metal or Ti / Pt / Au / Ti / Pt / Au / Ti stacked metal, wherein the thickness of the contact layer metal Ti in each stacked metal is 5-50nm.

[0037] In one embodiment of the present invention, the electrode groove and the ohmic contact electrode both have a rectangular shape or a rounded rectangular shape.

[0038] Another embodiment of the present invention provides a diamond-based sunken electrode photoconductive switch, which is manufactured by the manufacturing method described in the above embodiment, and includes: an insulating diamond substrate, a first ohmic contact electrode, a second ohmic contact electrode, a first alignment mark and a second alignment mark, wherein:

[0039] The insulating diamond substrate is provided with a first electrode groove and a second electrode groove at intervals;

[0040] The first ohmic contact electrode and the second ohmic contact electrode are respectively disposed in the first electrode groove and the second electrode groove;

[0041] The first alignment mark and the second alignment mark are disposed on the insulating diamond substrate and are located at opposite corners of the first ohmic contact electrode and the second ohmic contact electrode formation regions.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. The preparation method of the present invention includes processes such as electrode etching area definition, insulating diamond substrate etching, RCA cleaning of insulating diamond substrate, electrode sputtering area definition and ohmic contact electrode preparation. The preparation process is simple and easy, the structural design is concise, and complex electrode shapes and complex processes such as material epitaxy and ion implantation are avoided, which reduces the manufacturing cost and difficulty of the device, improves the device preparation efficiency and economic benefits, can realize large-scale manufacturing, is easier to mass produce and promote, and meets the large-scale high-voltage and high-frequency application requirements;

[0044] 2. The present invention sinks the ohmic contact electrode into the electrode groove, thereby sinking the ohmic contact electrode into the insulating diamond substrate to form a sunken electrode structure. First, the depth distribution of the current density is more uniform, rather than concentrated in the surface area of ​​the device, which significantly improves the uniformity of the electric field, avoids local electric field concentration, reduces the peak electric field strength, and can effectively reduce the electric field concentration at the edge of the electrode, improve the voltage resistance of the device, and thus extend the service life of the device; second, the sunken electrode structure can reduce the on-resistance by increasing the cross-sectional area of ​​the conductive channel, improve the conduction efficiency and current carrying capacity of the device, and thus improve the conduction characteristics of the switch; third, the sunken electrode structure optimizes the current path, makes the current density more uniform, reduces local overheating, and reduces power loss, thereby improving the working efficiency and reliability of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 A schematic flow chart of a method for preparing a diamond-based sunken electrode photoconductive switch provided by an embodiment of the present invention;

[0046] Figure 2a-2b A front view and a top view of a diamond-based sunken electrode photoconductive switch provided by an embodiment of the present invention;

[0047] Figure 3 A schematic structural diagram of another diamond-based sunken electrode photoconductive switch provided by an embodiment of the present invention;

[0048] Figure 4 A peak electric field simulation result diagram provided by an embodiment of the present invention;

[0049] Figure 5 A diagram of current density simulation results provided by an embodiment of the present invention;

[0050] Figure 6a-6jA schematic diagram of a preparation process of a method for preparing a diamond-based sunken electrode photoconductive switch provided by an embodiment of the present invention;

[0051] Figure 7 A characterization diagram of the diamond etching morphology provided by an embodiment of the present invention;

[0052] Figure 8 A comparison diagram of the photoelectric response curves of the sunken electrode device provided in an embodiment of the present invention and the traditional planar electrode device. DETAILED DESCRIPTION

[0053] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0054] Embodiment 1

[0055] See also Figure 1 , Figure 1 A schematic flow chart of a method for preparing a diamond-based sunken electrode photoconductive switch provided in an embodiment of the present invention. The preparation method comprises the following steps:

[0056] S1. Growing a diamond etching mask layer on the cleaned surface of the insulating diamond substrate.

[0057] Specifically, the insulating diamond substrate is firstly ultrasonically cleaned with acetone and isopropanol. The material of the insulating diamond substrate includes single crystal or polycrystalline diamond, and the dark state resistance is ≥10 13 Ω.

[0058] Then, a diamond etching mask layer is grown on the surface of the cleaned insulating diamond substrate. The diamond etching mask layer adopts an etching mask with a high etching selectivity ratio, and its material includes a dielectric material or a metal material, the dielectric material includes any one of silicon nitride and silicon dioxide, and the metal material includes any one of aluminum, titanium, and gold, with a thickness of 200 to 400 nm. Preferably, a plasma enhanced chemical vapor deposition (PECVD) process is used to grow a silicon nitride film with a thickness of 200 to 400 nm as the diamond etching mask layer.

[0059] S2. Photolithographically define the electrode etching area of ​​the insulating diamond substrate on the diamond etching mask layer, and simultaneously define the alignment mark area.

[0060] Specifically, glue is coated and spun on the diamond etching mask layer, and then the sample after glue coating and spun is placed in a photolithography machine for exposure, and the exposed sample is placed in a developer to remove the photoresist in the electrode etching area and the alignment mark area, thereby forming the electrode etching area and the alignment mark area.

[0061] Specifically, the shape of the electrode etching area can be a rectangle or a rounded rectangle, and the rounded corners can be circular or elliptical. For example, the four corners of the rectangular area are all formed into 1 / 4 arc chamfers. The shape of the alignment mark area can be a cross pattern. The shapes of the electrode etching area and the alignment mark are formed by a mask.

[0062] Exemplarily, the number of the electrode etching regions and the number of the alignment marks are both two, and the two electrode etching regions are arranged at intervals. The two alignment marks are arranged at diagonal positions of the two electrode etching regions.

[0063] S3, removing the diamond etching mask layer in the electrode etching area and the alignment mark area to form a diamond etching mask.

[0064] Specifically, when the diamond etching mask layer is made of silicon nitride, an F-based dry etching process is used to remove the silicon nitride film in the electrode etching area and the alignment mark area, and the etching is terminated to the surface of the diamond substrate to form a diamond etching mask.

[0065] S4, etching the insulating diamond substrate in the electrode etching area and the alignment mark area through the diamond etching mask to form electrode grooves and alignment marks.

[0066] Specifically, an O-based dry etching process is used to etch the insulating diamond substrate of the electrode etching area and the alignment mark area through a diamond etching mask to form an electrode groove and an alignment mark. The conditions of the O-based dry etching process include: the reaction gas includes O2, the flow rate of O2 is 30-80sccm, one or more of Ar and CF4 can be added as auxiliary gas, the flow rate of the auxiliary gas is 0-15sccm, the reaction chamber pressure is 7mTorr, the upper electrode power is 400-800W, the lower electrode power is 60-200W, the etching depth is 0.2-3μm, and the etching time is determined by the etching depth. Preferably, the reaction gas is O2, the O2 flow rate is 40sccm, the reaction chamber pressure is 7mTorr, the upper electrode power is 500W, and the lower electrode power is 80W.

[0067] It can be understood that the shape of the electrode groove can be a rectangle or a rectangle with rounded corners. The shape of the alignment mark can be a cross pattern.

[0068] Exemplarily, the number of the electrode grooves and the alignment marks are both 2, and the two electrode grooves are arranged at intervals. The two alignment marks are arranged at diagonal positions of the two electrode grooves.

[0069] S5. Performing organic cleaning and RCA cleaning on the etched insulating diamond substrate, so that the etched insulating diamond substrate has an electronic grade clean surface.

[0070] Specifically, the etched insulating diamond substrate is sequentially placed in acetone and isopropanol solutions for ultrasonic cleaning; the etched insulating diamond substrate is placed in a mixed solution of H2O2 and concentrated H2SO4 for cleaning; the etched insulating diamond substrate is placed in a mixed solution of NH4OH, H2O2 and H2O for water bath heating cleaning, and the mixed solution of NH4OH, H2O2 and H2O on the surface of the sample is rinsed off with ultrapure water; the etched insulating diamond substrate is placed in a mixed solution of HCl, H2O2 and H2O for water bath heating cleaning, and the mixed solution of HCl, H2O2 and H2O on the surface of the sample is rinsed off with ultrapure water; the etched insulating diamond substrate is placed in a mixed solution of BOE and ultrapure water for cleaning, and the mixed solution of BOE and ultrapure water on the surface of the sample is rinsed off with ultrapure water; the sample is blown dry with nitrogen to obtain an insulating diamond substrate with an electronic grade clean surface.

[0071] Among them, in the mixed solution of H2O2 and concentrated H2SO4, H2O2 is used with a mass fraction of 30%, and the volume ratio of H2O2 to concentrated H2SO4 is 1:2 to 1:4, preferably 1:3. In the mixed solution of NH4OH, H2O2 and H2O, the volume ratio of NH4OH, H2O2 and H2O is 1:1:5. In the mixed solution of HCl, H2O2 and H2O, the volume ratio of HCl, H2O2 and H2O is 1:1:6. In the mixed solution of BOE and ultrapure water, the volume ratio of BOE and ultrapure water is 2:3, and the BOE solution includes 49% HF solution and 40% NH4F solution with a volume ratio of 1:6.

[0072] This embodiment uses a cleaning solution that is gentle on the surface of the diamond substrate, and some cleaning steps are combined with ultrasonic cleaning technology to avoid surface damage and ensure the physical properties and functions of the diamond substrate. Through the combination of various solutions and ratios in the organic cleaning and RCA cleaning processes, an electronic-grade clean substrate surface is obtained to ensure its bonding quality with the metal. In addition, the diamond substrate cleaning process is compatible with the silicon-based cleaning process, which is conducive to promotion and application on existing silicon-based production lines and reduces equipment modification costs.

[0073] S6. Performing oxygen plasma treatment on the cleaned insulating diamond substrate to form surface oxygen terminations.

[0074] Specifically, after electronic-grade cleaning of the insulating diamond substrate, oxygen plasma is used to treat the diamond substrate to form diamond surface oxygen terminations, reduce device leakage, and further improve withstand voltage performance. Exemplarily, the conditions for oxygen plasma treatment are: oxygen flow rate of 50 sccm, power setting of 80 W, and treatment for 30 seconds after confirmation of ignition.

[0075] S7. Use a photolithography overlay process to align the electrode sputtering region with the electrode etching region through alignment marks, and define the electrode sputtering region on the surface of the insulating diamond substrate, wherein the center of the electrode sputtering region coincides with the center of the electrode etching region.

[0076] Specifically, to facilitate the subsequent production of the ohmic electrode, this step uses a double-layer glue process. First, a stripping glue is coated on the insulating diamond substrate and spun. Then, the stripping glue is coated and spun again to form a photoresist. After that, the sample that has been coated and spun is placed in a photolithography machine for exposure. During the exposure process, alignment marks are used to ensure that the electrode sputtering area and the electrode etching area are aligned. Then, the exposed sample is placed in a developer to remove the photoresist in the electrode sputtering area. Finally, the sample is placed in an oxygen plasma stripper for primer treatment to ensure that there is no residual photoresist in the electrode sputtering area.

[0077] Specifically, the alignment mark should ensure that the electrode sputtering area can include the electrode etching area, that is, the electrode sputtering area is larger than the electrode etching area, so as to ensure the gap filling effect of the sputtered metal. Exemplarily, the geometric dimensions of the electrode etching area are 4mm×8mm, and the geometric dimensions of the electrode sputtering area are 4.1mm×8.1mm, and the centers of the two are located at the same position.

[0078] Specifically, the two alignment marks are used to achieve photolithography overlay, that is, the pattern is present on the mask for both photolithography, and is transferred to the insulating diamond substrate along with the chamfered rectangular pattern. During the second photolithography, it is necessary to ensure that the mark on the mask is aligned with the pattern of the first photolithography on the substrate to achieve precise overlay, and the ultimate goal is to ensure that the electrode sputtering area contains the electrode etching area.

[0079] S8. Making an ohmic contact metal in the electrode sputtering area to form an ohmic contact electrode sunk into the electrode groove.

[0080] Specifically, a magnetron sputtering or evaporation process is used to deposit an ohmic contact metal in the electrode sputtering area, and the sample after metal deposition is stripped to remove the ohmic contact metal, photoresist and stripping glue outside the electrode sputtering area. Finally, the device is cleaned to form an ohmic contact electrode sunk into the electrode groove.

[0081] Specifically, the thickness of the ohmic contact electrode is greater than the depth of the electrode groove, ensuring the interface contact between the electrode and the insulating diamond substrate, and the electrode and the external circuit have good conductive contact. The shape of the ohmic contact electrode can be a rectangle or a rounded rectangle.

[0082] Materials with excellent conductivity and stability are selected as electrode materials to improve the reliability and conductivity of the electrode in high-voltage applications. Exemplarily, the materials of the ohmic contact electrode include Ti / Pt / Au laminated metal, Ti / Au laminated metal, Ti / Al laminated metal, Ti / Pt / Au / Ti laminated metal, Ti / Au / Ti / Au / Ti laminated metal or Ti / Pt / Au / Ti / Pt / Au / Ti laminated metal, wherein the thickness of the contact layer metal Ti in each laminated metal is 5-50nm.

[0083] The electrode requires the bottom metal to be Ti, and the outermost layer needs to be a metal with good conductivity. The metal layer combination is preferably Ti / Pt / Au. Ti is used as the contact layer metal, and the potential barrier formed by contact with the diamond surface is low, and it can form a good ohmic contact with the diamond substrate; Pt is used as a buffer layer metal to prevent Ti and the outermost layer Au from melting at high temperature; the outermost and thickest Au is used as a conductive layer metal to prevent oxidation of the lower metal and connect to the external circuit as a conductive layer. Therefore, the material of the ohmic contact electrode is preferably Ti / Pt / Au laminated metal, and the thickness of Ti / Pt / Au is 10 / 50 / 500nm.

[0084] In a specific embodiment, a step may be further included between steps S4 and S5: removing the residual diamond etching mask on the surface of the insulating diamond substrate.

[0085] Specifically, after forming the electrode grooves and alignment marks, if silicon nitride is selected as the diamond etching mask material, the residual silicon nitride can be retained to serve as a passivation layer to further enhance the device's voltage resistance, or the residual silicon nitride mask can be removed, that is, after forming the electrode grooves and alignment marks, the diamond etching mask is first removed, and then the insulating diamond substrate is organically cleaned and RCA cleaned.

[0086] The preparation method of this embodiment includes processes such as electrode etching area definition, insulating diamond substrate etching, RCA cleaning of the insulating diamond substrate, electrode sputtering area definition and ohmic contact electrode preparation. The preparation process is simple and easy, the structural design is concise, and complex electrode shapes and complex processes such as material epitaxy and ion implantation are avoided. The manufacturing cost and difficulty of the device are reduced, the device preparation efficiency and economic benefits are improved, and large-scale manufacturing can be achieved. It is easier to mass produce and promote, and meets the needs of large-scale high-voltage and high-frequency equipment.

[0087] In this embodiment, the ohmic contact electrode is sunk into the electrode groove, so that the ohmic contact electrode is sunk into the insulating diamond substrate to form a sunken electrode structure. First, the depth distribution of the current density is more uniform, rather than concentrated in the surface area of ​​the device, which significantly improves the uniformity of the electric field, avoids local electric field concentration, and reduces the peak electric field strength. It can effectively reduce the electric field concentration at the electrode, improve the voltage resistance of the device, and thus extend the service life of the device; second, the sunken electrode structure can reduce the on-resistance by increasing the cross-sectional area of ​​the conductive channel. From the perspective of electrical transport, the sunken electrode provides more carrier drift channels, improves the current carrying capacity of the device, and improves the conduction efficiency and current carrying capacity of the device, thereby improving the conduction characteristics of the switch; third, the sunken electrode structure optimizes the current path, makes the current density more uniform, reduces local overheating, and reduces power loss, thereby improving the working efficiency and reliability of the device.

[0088] Embodiment 2

[0089] On the basis of the first embodiment, this embodiment provides a diamond-based sunken electrode photoconductive switch, which is prepared by the preparation method of the first embodiment.

[0090] See also Figure 2a-2b , Figure 2a-2b A front view and a top view of a diamond-based sunken electrode photoconductive switch provided by an embodiment of the present invention, Figure 2a For the main view, Figure 2b The photoconductive switch comprises: an insulating diamond substrate 1, a first ohmic contact electrode 4, a second ohmic contact electrode 5, a first alignment mark 6 and a second alignment mark 7, wherein the insulating diamond substrate 1 is provided with a first electrode groove 2 and a second electrode groove 3 at intervals; the first ohmic contact electrode 4 and the second ohmic contact electrode 5 are respectively provided in the first electrode groove 2 and the second electrode groove 3; the first alignment mark 6 and the second alignment mark 7 are provided on the insulating diamond substrate 1 and are located at the diagonal positions of the areas where the first ohmic contact electrode 4 and the second ohmic contact electrode 5 are formed.

[0091] Specifically, the material of the insulating diamond substrate 1 includes single crystal or polycrystalline diamond, and its dark state resistance is ≥10 13 The depth of the first electrode groove 2 and the second electrode groove 3 is 0.2-3 μm.

[0092] Specifically, the height of the first ohmic contact electrode 4 is greater than the depth of the first electrode groove 2, and the height of the second ohmic contact electrode 5 is greater than the depth of the second electrode groove 3, so as to ensure the interface contact between the electrode and the insulating diamond substrate 1, and the good conductive contact between the electrode and the external circuit. In addition, the width of the raised portion of the first ohmic contact electrode 4 is greater than the width of the first electrode groove 2, and the width of the raised portion of the second ohmic contact electrode 5 is greater than the width of the second electrode groove 3, that is, the rectangular pattern of the ohmic contact electrodes 4 and 5 is slightly larger than the rectangular electrode pattern of the electrode grooves 2 and 3, so as to ensure that the ohmic contact electrodes can contain the electrode grooves.

[0093] Specifically, the electrode grooves 2 and 3 and the ohmic contact electrodes 4 and 5 are all rectangular shapes with rounded corners, and the electric field can be uniformed by rounding the corners of the electrodes.

[0094] Specifically, the first alignment mark 6 and the second alignment mark 7 are two groups of cross patterns, which are distributed at the diagonals of the ohmic contact electrodes 4 and 5 .

[0095] See also Figure 3 , Figure 3 A schematic diagram of the structure of another diamond-based sunken electrode photoconductive switch provided in an embodiment of the present invention. The diamond-based sunken electrode photoconductive switch comprises an insulating diamond substrate 1, a first ohmic contact electrode 4, a second ohmic contact electrode 5, a first alignment mark 6, a second alignment mark 7 and a diamond etching mask 8. Among them, the insulating diamond substrate 1 is provided with a first electrode groove 2 and a second electrode groove 3 at intervals; the first ohmic contact electrode 4 and the second ohmic contact electrode 5 are respectively arranged in the first electrode groove 2 and the second electrode groove 3; the first alignment mark 6 and the second alignment mark 7 are arranged on the insulating diamond substrate 1 and are located at the diagonal positions of the first ohmic contact electrode 4 and the second ohmic contact electrode 5 forming area; the diamond etching mask 8 serves as a passivation layer, covering the surface of the insulating diamond substrate 1 other than the first ohmic contact electrode 4 and the second ohmic contact electrode 5, and the first ohmic contact electrode 4 and the second ohmic contact electrode 5 extend to the side of the diamond etching mask 8.

[0096] This embodiment designs a diamond-based sunken electrode photoconductive switch. Compared with planar electrode devices, the device electric field distribution is optimized, the sidewall electrode conductive path is increased, and the electrical transport characteristics are improved, thereby achieving the effect of reducing the peak electric field strength and on-resistance of the device, solving the problems of electric field concentration and poor conduction characteristics caused by the limitations of traditional planar electrode design, and realizing a diamond photoconductive switch with high withstand voltage and low on-resistance.

[0097] Embodiment 3

[0098] Based on the first embodiment, this embodiment further illustrates the method for preparing a diamond-based sunken electrode photoconductive switch through the following examples.

[0099] Example 1

[0100] This example uses Figure 2a-2b As an example, the structure uses an insulating polycrystalline diamond substrate 1 with a size of 1 cm square and a thickness of 0.6 mm. The substrate surface has a high-quality first electrode groove 2 and a second electrode groove 3 with a depth of 400 nm. The first ohmic contact electrode 4 and the second ohmic contact electrode 5 are filled in the first electrode groove 2 and the second electrode groove 3 respectively. The geometric dimensions of the first ohmic contact electrode 4 and the second ohmic contact electrode 5 are 4 mm (width) × 8 mm (height) and are symmetrical on both sides. The distance between the two is 1 mm. The composition of the electrode is a Ti / Pt / Au metal stack with a thickness of 10 / 50 / 500 nm. The four corners are chamfered with arcs to homogenize the electric field. Cross pattern alignment marks 6 and 7 are provided at the diagonals of the diamond substrate 1 for realizing photolithography alignment.

[0101] Before device preparation, Silvaco TCAD software was used to analyze the influence of the sunken electrode on the peak electric field and current density distribution of the device. The DC voltage of the single-point test was set to 3 kV, the contact between the electrode and the substrate was ohmic contact, and the wavelength of the excitation light source was 532 nm.

[0102] See also Figure 4 , Figure 4 This is a peak electric field simulation result diagram provided by an embodiment of the present invention. Figure 4 As shown in (a) in the figure, two tangent lines are placed when analyzing the electric field distribution, located on the upper and lower surfaces of the electrode (the contact surface between the electrode and the air and the diamond substrate), corresponding to the position where the switch surface and the interior are prone to breakdown; the simulation gives the effect of etching depth on the peak electric field strength of the device with electrodes on the same surface. The peak electric field-etching depth variation curves of the device in the dark state and the on-state are shown in Figure 4 As shown in (b) and (c), Figure 4 The points with an etching depth of 0 in (b) and (c) represent traditional planar devices without etching. Compared with traditional planar devices, the peak electric field strength on the upper and lower surfaces of the electrode of the sunken electrode device has decreased significantly, and as the electrode sinking depth increases, the peak electric field strength has a trend of continuing to decrease.

[0103] On the other hand, the sinking of the electrode area makes the electron current density more uniform in depth rather than concentrated in the device surface area. Figure 5 , Figure 5 The current density simulation result diagram provided by the embodiment of the present invention is as follows: Figure 5The photogenerated carriers are mainly concentrated between the two electrodes, while the charge accumulation effect in the area around the electrodes leads to a local high current density. Compared with the planar electrode device, the internal electron current of the sinking electrode device is still concentrated in the inner area of ​​the anode, but its value is significantly reduced, indicating that the structure effectively alleviates the excessive current density caused by charge accumulation around the electrode. From the perspective of electrical transport, the sinking electrode provides more carrier drift channels, improves the current carrying capacity of the device, and thus reduces the on-resistance of the device.

[0104] Based on the above influence of the sunken electrode structure on device performance, please refer to Figure 6a-6j As shown, Figure 6a-6j A schematic diagram of a preparation process of a method for preparing a diamond-based sunken electrode photoconductive switch provided in an embodiment of the present invention, the preparation method comprising the steps of:

[0105] S1, growing a diamond etching mask layer 8 on the surface of the cleaned insulating diamond substrate 1. Specifically comprising:

[0106] S11, performing ultrasonic cleaning on the surface of the insulating diamond substrate 1, such as Figure 6a shown.

[0107] First, the sample was ultrasonically cleaned in an acetone solution for 3 minutes to remove particles and organic contaminants on the substrate surface. Then, the sample was ultrasonically cleaned in an isopropanol solution for 3 minutes to remove residual acetone and a small amount of organic contamination. Finally, the sample was rinsed with ultrapure water (DIW) and dried with nitrogen.

[0108] S12, growing silicon nitride (Si3N4) on the insulating diamond substrate 1 as a diamond etching mask layer 8, such as Figure 6b shown.

[0109] Specifically, a Si3N4 mask layer 8 with a thickness of 200 nm is grown on an insulating diamond substrate 1 by a plasma enhanced chemical vapor deposition (PECVD) process, and the growth process conditions are: NH3 and SiH4 are used as reaction gases, the substrate temperature is 250°C, the reaction chamber pressure is 600mTorr, and the RF power is 22W.

[0110] S2, photolithographically define the electrode etching area of ​​the insulating diamond substrate on the diamond etching mask layer 8, such as Figure 6c As shown, an alignment mark area is defined at the same time.

[0111] Specifically, first, bake the sample on a hot plate at 200°C for 5 minutes; then, coat and spin the Si3N4 mask layer 8 on the diamond substrate 1, with a spinning speed of 3000 rpm and a photoresist 9 thickness of 800nm, and bake the sample on a hot plate at 90°C for 1 minute, and then put the sample that has been coated and spun into a photolithography machine for exposure. Then, bake the sample on a hot plate at 110°C for 1 minute; then, put the exposed sample into a developer to remove the photoresist in the electrode etching area, and rinse it with ultrapure water and blow it dry with nitrogen. Finally, bake the sample on a hot plate at 100°C for 1 minute. In the process of exposing and developing the photoresist, the electrode etching area and the alignment mark area are formed at the same time.

[0112] S3, removing the diamond etching mask layer in the electrode etching area and the alignment mark area to form a diamond etching mask, such as Figure 6d shown.

[0113] Specifically, the Si3N4 mask layer 8 on the diamond substrate 1 was etched by dry etching process under the conditions of reaction gas of 15sccm CF4 and 15sccm O2, reaction chamber pressure of 3Pa, upper electrode power of 60W, and lower electrode power of 15W, and the etching depth was 200nm. Then the sample was ultrasonically cleaned in acetone solution and isopropanol solution for 3min in turn to remove the residual photoresist 9 on the surface of the sample. Finally, the sample was rinsed with ultrapure water (DIW) and dried with nitrogen.

[0114] S4, etching the insulating diamond substrate in the electrode etching area and the alignment mark area through the diamond etching mask to form the first electrode groove 2 and the second electrode groove 3, the first alignment mark 6 and the second alignment mark 7, as shown in FIG. Figure 6e shown.

[0115] Specifically, the electrode etching area and the alignment mark area on the diamond substrate 1 are etched with a dry etching process under the conditions of a reaction gas of 40 sccm of O2, a reaction chamber pressure of 7 mTorr, an upper electrode power of 500 W, a lower electrode power of 80 W, and a tray bottom temperature of -5°C. The etching depth is 400 nm, forming a pair of high-quality first electrode grooves 2 and second electrode grooves 3, and simultaneously forming a cross-shaped first alignment mark 6 and a second alignment mark 7.

[0116] The improvement of the electrical performance of the photoconductive switch by the sunken electrode is determined by the high-quality diamond electrode groove. Therefore, after the electrode groove is formed, the atomic force microscope (AFM) is used to characterize the etching selectivity, diamond etching sidewall and morphology. Due to the limited scanning area of ​​the instrument, the first alignment mark 6 and the second alignment mark 7 are taken as examples. The characterization results are shown in Figure 7 , Figure 7This is a characterization diagram of the diamond etching morphology provided by the embodiment of the present invention. Figure 7 It can be seen that the etching selectivity ratio of diamond to silicon nitride is greater than 20:1, the diamond etching sidewalls are relatively steep, and the root mean square roughness of the etched diamond surface is about 1 nm, indicating that high-quality grooves with steep sidewalls and smooth surfaces can be obtained through this etching process.

[0117] Next, the sample was immersed in a solution of BOE solution (49% HF:40% NH4F=1:6, volume ratio): ultrapure water=2:3 for 2 hours to ensure that there was no Si3N4 mask residue on the diamond substrate 1, thereby removing the residual diamond etching mask 8 on the surface of the insulating diamond substrate 1. Figure 6f shown.

[0118] Finally, the samples were rinsed with ultrapure water (DIW) and blown dry with nitrogen.

[0119] S5, performing organic cleaning and RCA cleaning on the etched insulating diamond substrate, so that the etched insulating diamond substrate has an electronic grade clean surface. Specifically comprising the steps of:

[0120] S51. Place the sample in an acetone solution and perform ultrasonic cleaning for 5 minutes to remove organic pollutants such as particles and grease on the surface of the substrate.

[0121] S52. Place the sample in an isopropanol solution and perform ultrasonic cleaning for 5 minutes to further remove organic pollutants and residual acetone on the surface; then rinse in ultrapure water for 3 minutes to remove the organic solution.

[0122] S53. Place the sample in a mixed solution of 30% H2O2: concentrated H2SO4 = 1:3 = 100ml:300ml and wash for 5 minutes to dissolve and remove residual organic matter on the surface of the substrate.

[0123] S54. Place the sample in SC-1 solution (NH4OH:H2O2:H2O=1:1:5=60ml:60ml:300ml), wash in an 80°C water bath for 10 minutes to remove surface organic matter and metal ions; then rinse with ultrapure water (DIW) for 3 minutes to remove the SC-1 solution.

[0124] S55. Place the sample in SC-2 solution (HCl:H2O2:H2O=1:1:6=50ml:50ml:300ml), wash in an 80°C water bath for 10 min to remove surface alkali metal cations and other cations; then rinse with ultrapure water (DIW) for 3 min to remove the SC-2 solution.

[0125] S56. Place the sample in a solution of BOE:DIW=2:3=160ml:240ml for cleaning for 15 seconds, and then rinse the substrate surface with ultrapure water (DIW) for 3 minutes.

[0126] S57, using nitrogen to blow dry the diamond substrate 1 to obtain an electronic grade clean surface.

[0127] S6. The cleaned insulating diamond substrate is subjected to oxygen plasma treatment to form surface oxygen terminations to reduce device leakage, such as Figure 6g shown.

[0128] Specifically, the sample is placed in an oxygen plasma stripping machine, the oxygen flow rate is set to 50 sccm, the power is set to 80 W, and the process is performed for 30 seconds to form oxygen terminations on the surface of the diamond substrate 1 .

[0129] S7, using a photolithography overlay process, aligning the electrode sputtering region with the electrode etching region through alignment marks, and defining the electrode sputtering region on the surface of the insulating diamond substrate, wherein the center of the electrode sputtering region coincides with the center of the electrode etching region, such as Figure 6h shown.

[0130] First, the overlay uses a double-layer glue process, and the sample is placed on a hot plate at 200°C and baked for 5 minutes before coating. The stripping glue 10 is coated on the diamond substrate 1 and spun, and the spinning speed is 2000 rpm. The thickness of the photoresist 10 is 0.35μm, and the sample is placed on a hot plate at 200°C and baked for 5 minutes. Glue coating and spinning are performed again on the stripping glue 10, and the spinning speed is 5000 rpm to form a photoresist 11. The thickness of the photoresist 11 is about 2.8μm. The width of the photoresist 11 in the formed double-layer glue is slightly larger than the width of the stripping glue 10, and the sample is placed on a hot plate at 100°C and baked for 2 minutes. The sample that has been coated and spun is placed in a photolithography machine for exposure; this step uses cross alignment marks 6 and 7 to ensure that the electrode sputtering area is aligned with the electrode etching area in step S3. Subsequently, the sample after exposure is placed in a developer to remove the photoresist in the electrode etching area, and it is rinsed with ultrapure water and blown dry with nitrogen. Finally, the sample was placed in an oxygen plasma stripper for primer film treatment, with the oxygen flow rate set to 100 sccm, the power to 200 W, and the treatment time to 5 min to remove the residual photoresist in the electrode sputtering area.

[0131] S8. Using a magnetron sputtering process, an ohmic contact metal is manufactured in the electrode sputtering region to form a first ohmic contact electrode 4 and a second ohmic contact electrode 5 sunk into the electrode groove.

[0132] First, the sample was placed in a magnetron sputtering machine until the vacuum degree of the reaction chamber reached 2×10 -6Torr, a stacked metal layer consisting of three metal layers of Ti, Pt and Au is sputtered on the surface of the sample from bottom to top, and the thickness of the Ti / Pt / Au metal layer is 10 / 50 / 500nm. Figure 6i Then, the sample after metal sputtering is stripped to remove the photoresist and metal outside the electrode sputtering area, that is, the stripping glue 10, the photoresist 11 and the metal 12 above it, to form the first ohmic contact electrode 4 and the second ohmic contact electrode 5, as shown in FIG. Figure 6j Next, the sinking device with the electrode prepared is cleaned, rinsed with ultrapure water (DIW) and dried with nitrogen to complete the device manufacturing. The prepared device structure is shown in Figure 2a and 2b shown.

[0133] The photoelectric response test of the sunken electrode device of this embodiment and the traditional planar electrode device is carried out. The test results are as follows: Figure 8 As shown, Figure 8 A comparison diagram of the photoelectric response curves of the sunken electrode device provided in an embodiment of the present invention and the traditional planar electrode device. Figure 8 In the same test conditions (laser energy of 1mJ / pulse, power supply voltage of 200V), the output signal photoelectric response curve of the high-speed oscilloscope load resistor was plotted. The results show that while ensuring ultra-fast response speed (the half-height width of the response curve is about 0.3ns), the sinking electrode device produces a higher peak photocurrent and can effectively improve the conduction characteristics of the device (the on-resistance drops by about 30%).

[0134] Example 2

[0135] The difference from Example 1 is that the polycrystalline diamond substrate 1 of Example 1 is replaced with single crystal diamond, and the remaining steps are the same. A diamond-based sunken electrode photoconductive switch is prepared by preparing high-quality electrode grooves, and the voltage resistance of the prepared sunken device is significantly improved.

[0136] Example 3

[0137] The difference from Example 1 is that after forming the first electrode groove 2 and the second electrode groove 3, the first alignment mark 6 and the second alignment mark 7 in step S4, the diamond etching mask 8 is retained to serve as a dielectric passivation layer to further enhance the device's withstand voltage performance. Figure 3 shown.

[0138] This embodiment is directed to a process for preparing a photoconductive switch on a diamond substrate, including a systematic combination of steps such as electronic-grade cleaning of the diamond substrate, oxygen plasma surface treatment, diamond high-quality groove etching, and electrode pattern overlay process, thereby realizing a diamond-based photoconductive switch with high withstand voltage, fast response, and low on-resistance.

[0139] The present embodiment provides a simple, efficient and easy-to-promote method for preparing a diamond-based sunken electrode photoconductive switch: it includes a systematic combination of processes such as electronic-grade cleaning of the diamond substrate, reconstruction of the surface oxygen terminal, high-quality groove etching of the diamond substrate and preparation of the ohmic metal layer. The preparation method avoids complex processes such as ion implantation and epitaxial growth, and can prepare a diamond photoconductive switch with high withstand voltage and low on-resistance without the need to prepare complex electrode patterns, thereby improving the device preparation efficiency and economic benefits, making it easier to mass produce and promote, and expanding the high-voltage and high-frequency application scenarios of the diamond photoconductive switch.

[0140] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A method for preparing a diamond-based sunken electrode photoconductive switch, characterized in that: Includes steps: growing a diamond etching mask layer on the cleaned surface of the insulating diamond substrate; Photolithographically defining an electrode etching region of the insulating diamond substrate on the diamond etching mask layer, and simultaneously defining an alignment mark region; Removing the diamond etching mask layer in the electrode etching area and the alignment mark area to form a diamond etching mask; Etching the insulating diamond substrate in the electrode etching area and the alignment mark area through the diamond etching mask to form an electrode groove and an alignment mark; Performing organic cleaning and RCA cleaning on the etched insulating diamond substrate, so that the etched insulating diamond substrate has an electronic grade clean surface; The cleaned insulating diamond substrate is subjected to oxygen plasma treatment to form surface oxygen terminations; Using a photolithography overlay process, the electrode sputtering region is aligned with the electrode etching region through the alignment mark, and the electrode sputtering region is defined on the surface of the insulating diamond substrate, wherein the center of the electrode sputtering region coincides with the center of the electrode etching region; An ohmic contact metal is fabricated in the electrode sputtering region to form an ohmic contact electrode sunk into the electrode groove.

2. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 1, characterized in that: Etching the insulating diamond substrate of the electrode etching area and the alignment mark area through the diamond etching mask to form an electrode groove and an alignment mark, comprising: An O-based dry etching process is adopted to etch the insulating diamond substrate in the electrode etching area and the alignment mark area through the diamond etching mask to form an electrode groove and an alignment mark.

3. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 2, characterized in that: The conditions of the O-based dry etching process include: the reaction gas includes O2, the O2 flow rate is 30-80sccm, the reaction chamber pressure is 7mTorr, the upper electrode power is 400-800W, the lower electrode power is 60-200W, and the etching depth is 0.2-3μm.

4. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 1, characterized in that: After etching the insulating diamond substrate of the electrode etching area and the alignment mark area through the diamond etching mask to form the electrode groove and the alignment mark, the step further includes: The diamond etching mask remaining on the surface of the insulating diamond substrate is removed.

5. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 1, characterized in that: The insulating diamond substrate after etching is subjected to organic cleaning and RCA cleaning, so that the insulating diamond after etching has an electronic grade clean surface, including: The etched insulating diamond substrate is placed in acetone and isopropanol solutions for ultrasonic cleaning in turn; The etched insulating diamond substrate is placed in a mixed solution of H2O2 and concentrated H2SO4 for cleaning; The etched insulating diamond substrate is placed in a mixed solution of NH4OH, H2O2 and H2O for water bath heating and cleaning, and the mixed solution of NH4OH, H2O2 and H2O on the surface of the sample is rinsed off with ultrapure water; The etched insulating diamond substrate is placed in a mixed solution of HCl, H2O2 and H2O for water bath heating and cleaning, and the mixed solution of HCl, H2O2 and H2O on the surface of the sample is rinsed off with ultrapure water; The etched insulating diamond substrate is placed in a mixed solution of BOE and ultrapure water for cleaning, and the mixed solution of BOE and ultrapure water on the surface of the sample is rinsed off with ultrapure water; The sample was blown dry using nitrogen gas to obtain an insulating diamond substrate with an electronic grade clean surface.

6. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 5, characterized in that: In the mixed solution of H2O2 and concentrated H2SO4, the H2O2 has a mass fraction of 30%, and the volume ratio of H2O2 to concentrated H2SO4 is 1:2 to 1:4; In the mixed solution of NH4OH, H2O2 and H2O, the volume ratio of NH4OH, H2O2 and H2O is 1:1:5; In the mixed solution of HCl, H2O2 and H2O, the volume ratio of HCl, H2O2 and H2O is 1:1:6; In the mixed solution of BOE and ultrapure water, the volume ratio of BOE to ultrapure water is 2:3, and the BOE solution includes 49% HF solution and 40% NH4F solution in a volume ratio of 1:

6.

7. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 1, characterized in that: The insulating diamond substrate comprises a material of single crystal or polycrystalline diamond, and the dark state resistance is ≥10 13 Ω; The material of the diamond etching mask layer includes a dielectric material or a metal material, the dielectric material includes any one of silicon nitride and silicon dioxide, the metal material includes any one of aluminum, titanium and gold, and the thickness of the diamond etching mask layer is 200 to 400 nm; The shape of the alignment mark includes a cross figure; The area of ​​the electrode sputtering region is larger than the area of ​​the electrode etching region; The thickness of the ohmic contact electrode is greater than the depth of the electrode groove; The material of the ohmic contact electrode includes Ti / Pt / Au stacked metal, Ti / Au stacked metal, Ti / Al stacked metal, Ti / Pt / Au / Ti stacked metal, Ti / Au / Ti / Au / Ti stacked metal or Ti / Pt / Au / Ti / Pt / Au / Ti stacked metal, wherein the thickness of the contact layer metal Ti in each stacked metal is 5-50nm.

8. The method for preparing a diamond-based sunken electrode photoconductive switch according to claim 1, characterized in that: The electrode groove and the ohmic contact electrode both have a rectangular shape or a rounded rectangular shape.

9. A diamond-based sunken electrode photoconductive switch, characterized in that: The method according to any one of claims 1 to 8 comprises: an insulating diamond substrate, a first ohmic contact electrode, a second ohmic contact electrode, a first alignment mark and a second alignment mark, wherein: The insulating diamond substrate is provided with a first electrode groove and a second electrode groove at intervals; The first ohmic contact electrode and the second ohmic contact electrode are respectively disposed in the first electrode groove and the second electrode groove; The first alignment mark and the second alignment mark are disposed on the insulating diamond substrate and are located at opposite corners of the first ohmic contact electrode and the second ohmic contact electrode formation regions.

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