Vertical 4H-SiC photoconductive switch

Abstract

The invention discloses a vertical 4H-SiC photoconductive switch, which comprises a SiC substrate, a front electrode, a passivation layer and a back electrode, and is characterized in that the front electrode is located on the upper surface of the SiC substrate, and the passivation layer is located on the upper surface of the SiC substrate and is arranged at the edge of the front electrode; the back electrode is located on the lower surface of the SiC substrate; wherein the front electrode comprises an annular electrode and a mesh electrode, and the mesh electrode is arranged in the inner circle of the annular electrode and connected with the annular electrode; a plurality of concentric terminal protection rings are also arranged in the SiC substrate below the outer side of the front electrode, and the circle centers of the longitudinal projections of the plurality of terminal protection rings coincide with the circle centers of the longitudinal projections of the annular electrodes. According to the structural design, the electric field concentration effect near the electrode caused by non-uniform distribution of the electric field on the surface of the photoconductive switch is relieved, premature breakdown of the device is avoided, and the voltage endurance capability of the photoconductive switch is doubled; and meanwhile, the uniformity of current distribution in the photoconductive switch is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and in particular relates to a vertical 4H-SiC photoconductive switch. Background Art

[0002] In recent years, power electronic switches, as an important component of power electronic technology, have been widely used in smart grids, consumer electronic devices, radars, aviation systems and other fields. Common switching devices include photoconductive switches, spark gaps, backlit thyristors, rectifiers, thyristors and field effect transistors. Among them, photoconductive switches have been widely used in high-pulse and high-power systems due to their compact structure, small switching jitter, fast switching rate, small inductance and high frequency. As a third-generation semiconductor material, SiC crystal has the advantages of large bandgap, high thermal conductivity and high breakdown field strength. Among them, the comprehensive performance of 4H-SiC is more outstanding among several common configurations of SiC, and it is the most promising substrate material in the field of photoconductive switches so far.

[0003] In the design of 4H-SiC photoconductive switches, the optimization of the electrode structure is crucial, as it directly affects the electrical, thermal characteristics and reliability of the switch. The electrode structures of conventional 4H-SiC photoconductive switches are mainly divided into two categories: planar and vertical. Figure 1 As shown in the figure, the left side is a planar photoconductive switch and the right side is a vertical photoconductive switch. Since SiC has a shallow absorption of the trigger laser, the conductive channel of the planar photoconductive switch is distributed on the surface of one side of the electrode, which is prone to surface breakdown. The vertical photoconductive switch can solve this problem well and give full play to the advantage of SiC's strong critical breakdown field.

[0004] However, the existing vertical 4H-SiC photoconductive switch design still has the following problems:

[0005] First, the peak electric field is concentrated at the edge of the electrode. The electric field concentration effect near the electrode under high-voltage bias causes electrical breakdown to occur prematurely, which greatly limits the voltage resistance of the photoconductive switch. Second, a sudden peak electric field appears near the electrode at the moment of switching, causing dielectric breakdown at the ohmic contact interface of the device and cracks on the electrode surface, affecting the high-temperature stability of the ohmic contact and increasing the contact resistance. Third, the current concentration phenomenon near the electrode causes a large amount of Joule heat, generating thermal stress, leading to thermal breakdown, and thus degrading the device. Summary of the invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides a vertical 4H-SiC photoconductive switch. The technical problem to be solved by the present invention is achieved by the following technical solutions:

[0007] In a first aspect, the present invention provides a vertical 4H-SiC photoconductive switch, comprising a SiC substrate, a front electrode, a passivation layer and a back electrode, wherein the front electrode is located on the upper surface of the SiC substrate, the passivation layer is located on the upper surface of the SiC substrate and is arranged at the edge of the front electrode; and the back electrode is located on the lower surface of the SiC substrate;

[0008] The front electrode includes a ring electrode and a mesh electrode, wherein the mesh electrode is arranged in the inner circle of the ring electrode and connected to the ring electrode;

[0009] A plurality of concentric terminal protection rings are also provided in the SiC substrate below the outer side of the front electrode, and the centers of the longitudinal projections of the plurality of terminal protection rings coincide with the center of the longitudinal projection of the annular electrode.

[0010] In one embodiment of the present invention, the inner diameter of the annular electrode is 9600 μm, and the outer diameter is 10600 μm.

[0011] In one embodiment of the present invention, the mesh electrode is formed by a plurality of metal wires crisscrossing each other; the width of the metal wires is 30 μm, and the density of the mesh electrode is 4%-5% of the inner circle area of ​​the annular electrode.

[0012] In one embodiment of the present invention, the widths of the terminal protection rings are all 3 μm, and the innermost terminal protection ring is 1 μm away from the outer circle of the annular electrode; the spacing between two adjacent terminal protection rings is the same, or is distributed in segmented arithmetic from the inside to the outside.

[0013] In one embodiment of the present invention, the number of terminal protection rings is 120 or more, and the terminal protection rings are evenly distributed with a ring spacing of 1 μm.

[0014] In one embodiment of the present invention, the number of terminal protection rings is 80, and the distance between two adjacent protection rings is:

[0015]

[0016] Wherein, Sn represents the distance between the nth terminal protection ring and the n-1th terminal protection ring.

[0017] In one embodiment of the present invention, the front electrode adopts a multi-layer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Au;

[0018] The back electrode adopts a multi-layer electrode structure in the vertical direction, which is Ni / Ti / Pt / Ag / Pt / Au.

[0019] In one embodiment of the present invention, a transparent conductive film is further provided above the front electrode, and the material of the transparent conductive film is an oxide semiconductor film or graphene.

[0020] In one embodiment of the present invention, the thickness of the SiC substrate is 500 nm, the thickness of the passivation layer is 1000 nm, and the thickness of the transparent conductive film is 362 nm.

[0021] Beneficial effects of the present invention:

[0022] The vertical 4H-SiC photoconductive switch provided by the present invention comprises a SiC substrate, a front electrode, a passivation layer and a back electrode, wherein the front electrode is located on the upper surface of the SiC substrate, the passivation layer is located on the upper surface of the SiC substrate and is arranged at the edge of the front electrode; the back electrode is located on the lower surface of the SiC substrate; wherein the front electrode comprises an annular electrode and a mesh electrode, the mesh electrode is arranged in the inner circle of the annular electrode and connected to the annular electrode; a plurality of concentric terminal guard rings are also arranged in the SiC substrate below the outer side of the front electrode, and the center of the longitudinal projection of the plurality of terminal guard rings coincides with the center of the longitudinal projection of the annular electrode. On the one hand, the structural design enables each guard ring to divide the voltage well through the design of several terminal guard rings, thereby increasing the junction curvature radius, which largely alleviates the electric field concentration effect near the electrode caused by the uneven electric field distribution on the surface of the photoconductive switch, avoids premature breakdown of the device, and multiplies the voltage resistance of the photoconductive switch; on the other hand, the front electrode structure combining the ring electrode and the mesh electrode is introduced to increase the electrode area, alleviate the repetition frequency reliability problem caused by the excessive current density near the electrode, and greatly improve the uniformity of the current distribution inside the photoconductive switch while taking into account the transient output characteristics;

[0023] 2. The vertical 4H-SiC photoconductive switch provided by the present invention adopts a Ni / Ti / Pt / Au system electrode structure, which not only optimizes the ohmic contact quality, improves the thermal stability and contact reliability of the ohmic contact, but also reduces the specific contact resistance, improves the switching speed and power conversion performance;

[0024] 3. The vertical 4H-SiC photoconductive switch provided by the present invention is designed with a high-efficiency optical path structure with positive electrode transmittance enhancement and negative electrode reflection enhancement. The thickness of the front transparent conductive film is specially designed according to the refractive index and resistivity, which greatly reduces the reflectivity of the laser on the surface, meets the requirements of transmittance enhancement and conductivity, further improves the utilization efficiency of the laser, and has an absolute advantage in achieving low-energy triggering.

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure of the planar electrode and the vertical electrode of the conventional 4H-SiC photoconductive switch;

[0027] Figure 2This is a front view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present invention;

[0028] Figure 3 is a top view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present invention;

[0029] Figure 4 Schematic diagram of the structure and dimensions of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present invention;

[0030] Description of reference numerals:

[0031] 1-SiC substrate, 2-front electrode, 21-ring electrode, 22-mesh electrode, 3-passivation layer, 4-back electrode, 5-terminal guard ring. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] Research shows that the design of the electrode structure mainly needs to consider the following three issues: First, the uneven distribution of the surface electric field under high-voltage bias leads to the electric field concentration effect, which causes electrical breakdown in advance and affects the voltage resistance of the device; second, the peak electric field that suddenly changes when switching causes dielectric breakdown at the ohmic contact interface, causing cracks on the electrode surface; third, the current concentration phenomenon causes a large amount of Joule heat, generating thermal stress and leading to thermal breakdown. At present, the electrode structures with excellent performance under the vertical structure are based on transparent electrodes, and the laser is directly irradiated from the electrode. The electrode shape is a ring electrode. This structure optimizes the internal light intensity distribution of the switch, but sacrifices more electrode contact area, while the mesh electrode can take both aspects into account and has better conduction performance. However, the existence of the mesh electrode also leads to the loss of the illuminated area.

[0034] Based on this, the present invention proposes a new vertical 4H-SiC photoconductive switch structure.

[0035] Please see the joint Figure 2 and Figure 3 , Figure 2 is a front view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present invention, Figure 3: is a top view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present invention. The vertical 4H-SiC photoconductive switch provided by the present invention mainly comprises a SiC substrate 1, a front electrode 2, a passivation layer 3 and a back electrode 4, wherein the front electrode 2 is located on the upper surface of the SiC substrate 1, the passivation layer 3 is located on the upper surface of the SiC substrate 1 and is arranged at the edge of the front electrode 2; the back electrode 4 is located on the lower surface of the SiC substrate 1;

[0036] The front electrode 2 includes a ring electrode 21 and a mesh electrode 22. The mesh electrode 22 is arranged in the inner circle of the ring electrode 21 and connected to the ring electrode 21.

[0037] A plurality of concentric terminal guard rings 5 ​​are further provided in the SiC substrate 1 below the outer side of the front electrode 2 , and the center of the longitudinal projection of the plurality of terminal guard rings 5 ​​coincides with the center of the longitudinal projection of the annular electrode 21 .

[0038] On the one hand, this structural design enables each guard ring to divide the voltage well through the design of several terminal guard rings, thereby increasing the junction curvature radius. This largely alleviates the electric field concentration effect near the electrode caused by the uneven electric field distribution on the surface of the photoconductive switch, avoids premature breakdown of the device, and exponentially improves the voltage resistance of the photoconductive switch. On the other hand, the front electrode structure combining annular electrodes and mesh electrodes is introduced to increase the electrode area, alleviate the repetition rate reliability problem caused by excessive current density near the electrode, and greatly improves the uniformity of current distribution inside the photoconductive switch while taking into account the transient output characteristics.

[0039] For further information, see Figure 4 , Figure 4 It is a schematic diagram of the structural dimensions of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present invention. The lateral dimension of the vertical 4H-SiC photoconductive switch designed in this embodiment is 12000 μm, and the longitudinal dimension is 2.35 μm. Among them, the thickness of the SiC substrate 1 is 500 nm, the thickness of the passivation layer 3 is 1000 nm, the thickness of the front electrode 2, i.e., the anode, is 450 nm, and the thickness of the back electrode, i.e., the cathode, is 850 nm.

[0040] Optionally, in this embodiment, the inner diameter of the annular electrode 21 is 9600 μm, and the outer diameter is 10600 μm.

[0041] Optionally, as an implementation method, the mesh electrode 22 may be formed by a plurality of metal wires crisscrossing each other, such as Figure 3 As shown; wherein the width of the metal wire is 30μm, and the density of the mesh electrode 22 is 4%-5% of the inner circle area of ​​the annular electrode 21.

[0042] The present invention increases the electrode area by reasonably designing the mesh electrode density, which alleviates the repetition reliability problem caused by excessive current density near the electrode. On the other hand, the existence of the mesh electrode also leads to the loss of illumination area. By changing the metal line width of the mesh electrode 22, the influence of the electrode density on the transient output characteristics of the current and the temperature distribution on the electrode surface is analyzed, and a scheme of setting the metal width to 30μm and the area to 5% of the inner circle is proposed. This scheme greatly improves the uniformity of the current distribution inside the photoconductive switch while taking into account the transient output characteristics.

[0043] Furthermore, for a plurality of terminal guard rings 5, this embodiment can implement a plurality of N-type ion implantation in the SiC substrate 1 below the outer side of the front electrode 2. + -N-type terminal guard ring. It can be understood that a high concentration of N ions is implanted into the SiC substrate 1 below the front electrode 2 to make an ohmic contact. + The doping concentration of the -N guard ring terminal is exactly the same as that of the main junction below the electrode.

[0044] Optionally, as an implementation method, in this embodiment, the width of several terminal protection rings 5 ​​is designed to be 3μm, and the distance between the innermost terminal protection ring and the outer circle of the annular electrode 21 is 1μm; the spacing between two adjacent terminal protection rings is the same, or is distributed in segmented arithmetic from the inside to the outside.

[0045] For example, the number of terminal protection rings 5 ​​may be specifically set to 120 or more, and may be evenly distributed with a ring spacing of 1 μm.

[0046] For another example, the number of terminal protection rings 5 ​​may be specifically set to 80, and the spacing between two adjacent protection rings is distributed in a piecewise arithmetic progression from the inside to the outside, specifically distributed according to the following formula:

[0047]

[0048] Wherein, Sn represents the distance between the nth terminal protection ring and the n-1th terminal protection ring.

[0049] After the vertical 4H-SiC photoconductive switch designed in this embodiment introduces the N+-N terminal guard ring, as the external voltage of the device increases, the space charge region expands so that the depletion region between the first ring and the main junction is connected together, increasing the curvature radius of the curved junction, thereby improving the voltage resistance of the device. Similarly, the same result will occur between the guard rings until the last ring.

[0050] In addition, the terminal solution designs the distance between the guard ring and the main junction to be 1μm, the width of the guard ring to be 3μm, the number of rings to be 80, and the guard rings to be distributed in a segmented arithmetic distribution, which greatly improves the voltage resistance while taking into account the process complexity and device size.

[0051] Furthermore, for the front electrode 2 and the back electrode 4, the mainstream metals for preparing SiC ohmic contacts are Ni, Ti, Al, TiC, TiN, TiW and other metals and their metal combinations. The specific contact resistance of n-type 4H-SiC ohmic contacts can be as low as 1×10 -6 Ω·cm 2 .

[0052] In order to improve the quality of ohmic contact and alleviate the breakdown of interface dielectrics, the front electrode 2 of this embodiment adopts a multilayer electrode structure in the vertical direction, which is Ni / Ti / Pt / Au, with thicknesses of 200 / 50 / 100 / 100, respectively, in nm. The back electrode 4 adopts a multilayer electrode structure in the vertical direction, which is Ni / Ti / Pt / Ag / Pt / Au, with thicknesses of 200 / 50 / 100 / 300 / 100 / 100, respectively, in nm.

[0053] This embodiment adopts a Ni / Ti / Pt / Au system electrode structure, which not only optimizes the ohmic contact quality and improves the thermal stability and contact reliability of the ohmic contact; it also reduces the specific contact resistance and improves the switching speed and power conversion performance.

[0054] In addition, considering that air breakdown is easy to occur at the electrode edge, this embodiment adds a silicon dioxide passivation layer at the electrode edge for insulation protection, and applies inorganic doped, fiber reinforced new nano silver composite solder paste to the device triple point during packaging. The thickness of the passivation layer can generally be set to 1000nm.

[0055] Furthermore, in this embodiment, a transparent conductive film (not shown in the figure) is provided above the front electrode 2 , and the material of the transparent conductive film is an oxide semiconductor film or graphene.

[0056] Optionally, the transparent conductive film may be made of oxide semiconductor films such as AZO (Al-doped ZnO material), ITO (In2O3 (SnO2), tin-doped indium oxide), and SGO (Ga2O3 (SnO2), tin-doped gallium oxide).

[0057] Taking AZO conductive film as an example, research shows that the transmittance of AZO film is greater than 95% near the wavelength of 532nm, and it has the function of anti-reflection and anti-reflection, which can allow more light waves to enter the substrate; and when the thickness of AZO film is greater than 300nm, its resistivity is as low as 5.6×10-6Ω·cm, and its conductivity is close to that of metal materials, which can meet the electrode's demand for conductivity. Therefore, the present invention is based on the incident laser trigger wavelength of 532nm, and magnetron sputtering 362nm AZO film on the electrode, which maximizes the anti-reflection function of the film and more effectively utilizes the laser energy.

[0058] In summary, the vertical 4H-SiC photoconductive switch designed in the present invention has the following beneficial effects:

[0059] 1. The present invention designs the distribution of terminal guard rings so that each guard ring can divide the voltage well, thereby increasing the junction curvature radius. This largely alleviates the electric field concentration effect near the electrode caused by the uneven electric field distribution on the surface of the photoconductive switch, avoids premature breakdown of the device, and multiplies the voltage resistance of the photoconductive switch.

[0060] 2. The present invention designs a high-efficiency optical path structure with positive electrode increased transmittance and negative electrode increased reflection. The thickness of the front transparent conductive film is specially designed according to the refractive index and resistivity, which greatly reduces the reflectivity of the laser on the surface, meets the requirements of increased transmittance and conductivity, further improves the utilization efficiency of the laser, and has an absolute advantage in achieving low-energy triggering.

[0061] 3. The present invention reduces the specific contact resistance by optimizing the ohmic contact quality, thereby improving the switching speed and power conversion performance, and making the device have good high-frequency reliability, high-temperature reliability and anti-ablation performance.

[0062] 4. The present invention greatly improves the uniformity of the internal current distribution of the photoconductive switch while taking into account the transient output characteristics through the reasonable density of the mesh electrodes and the line width of the metal wires between the mesh electrodes.

[0063] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0064] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0065] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0066] 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 vertical 4H-SiC photoconductive switch, characterized in that: The invention comprises a SiC substrate (1), a front electrode (2), a passivation layer (3) and a back electrode (4), wherein the front electrode (2) is located on the upper surface of the SiC substrate (1), the passivation layer (3) is located on the upper surface of the SiC substrate (1) and is arranged at the edge of the front electrode (2); and the back electrode (4) is located on the lower surface of the SiC substrate (1); The front electrode (2) comprises an annular electrode (21) and a mesh electrode (22), wherein the mesh electrode (22) is arranged in the inner circle of the annular electrode (21) and connected to the annular electrode (21); A plurality of concentric terminal protection rings (5) are also provided in the SiC substrate (1) below the outer side of the front electrode (2), and the center of the longitudinal projection of the plurality of terminal protection rings (5) coincides with the center of the longitudinal projection of the annular electrode (21).

2. A vertical 4H-SiC photoconductive switch according to claim 1, characterized in that: The inner diameter of the annular electrode (21) is 9600 μm, and the outer diameter is 10600 μm.

3. A vertical 4H-SiC photoconductive switch according to claim 1, characterized in that: The mesh electrode (22) is formed by a plurality of metal wires that are crisscrossed; the width of the metal wires is 30 μm, and the density of the mesh electrode (22) is 4%-5% of the inner circle area of ​​the annular electrode (21).

4. A vertical 4H-SiC photoconductive switch according to claim 1, characterized in that: The widths of the terminal protection rings (5) are all 3 μm, and the distance between the innermost terminal protection ring and the outer circle of the annular electrode (21) is 1 μm; the spacing between two adjacent terminal protection rings is the same, or is distributed in segmented equal distances from the inside to the outside.

5. A vertical 4H-SiC photoconductive switch according to claim 4, characterized in that: The number of the terminal protection rings (5) is 120 or more, and they are evenly distributed with a ring spacing of 1 μm.

6. A vertical 4H-SiC photoconductive switch according to claim 4, characterized in that: The number of the terminal protection rings (5) is 80, and the distance between two adjacent protection rings is: Wherein, Sn represents the distance between the nth terminal protection ring and the n-1th terminal protection ring.

7. The vertical 4H-SiC photoconductive switch according to claim 1, characterized in that: The material of the passivation layer (3) is silicon dioxide.

8. The vertical 4H-SiC photoconductive switch according to claim 1, characterized in that: The front electrode (2) adopts a multi-layer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Au respectively; The back electrode (4) adopts a multi-layer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Ag / Pt / Au.

9. The vertical 4H-SiC photoconductive switch according to claim 1, characterized in that: A transparent conductive film is also provided above the front electrode (2), and the material of the transparent conductive film is an oxide semiconductor film or graphene.

10. The vertical 4H-SiC photoconductive switch according to claim 9, characterized in that: The thickness of the SiC substrate (1) is 500 nm, the thickness of the passivation layer (3) is 1000 nm; and the thickness of the transparent conductive film is 362 nm.

Images