Silicon carbide surface modified ohmic contact structure, silicon carbide semiconductor device and preparation method thereof

Abstract

The invention discloses a silicon carbide surface modified ohmic contact structure, a silicon carbide semiconductor device and a preparation method thereof, and belongs to the technical field of silicon carbide semiconductors. The ohmic contact structure comprises a modified silicon carbide bottom layer and a deposited metal surface layer which are directly connected, and carbon vacancies exist in the modified silicon carbide bottom layer. The preparation method comprises the following steps: depositing a nickel film on silicon carbide, and heating the surface of the silicon carbide to generate a nickel-silicon compound layer; removing the nickel-silicon compound layer and the carbon cluster to obtain modified silicon carbide; and depositing conductive metal on the modified silicon carbide. According to the method, first ohmic contact is formed through high-temperature annealing, then the nickel-silicon compound is completely corroded, corroded silicon carbide is removed, and a new clean surface is obtained. Carbon clusters are formed on the surface in the high-temperature annealing process and can be removed through an oxidation method. And then second ohmic contact is formed on the surface of the silicon carbide through low-temperature annealing, so that the gate oxide layer and the ohmic contact have good performance.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide semiconductors, and in particular to a silicon carbide surface modified ohmic contact structure, a silicon carbide semiconductor device and a preparation method thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not be necessarily regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art.

[0003] Semiconductor materials and devices have been developing towards high efficiency, high density and integration. Since silicon carbide (SiC) materials have the characteristics of wide band gap, large breakdown electric field, high electron mobility and high thermal conductivity, they are more suitable for power devices in the fields of high voltage, high power and high temperature. At present, silicon carbide-based metal semiconductor field effect transistors (MOSFET, MOS for short) have been applied in electric vehicles, charging equipment, aircraft and other fields.

[0004] Ohmic contact can reduce the contact resistance of the interface, so that the device carries the smallest possible voltage drop when it is turned on, and improve the device's conduction characteristics. The method of forming an ohmic contact at the contact interface between silicon carbide and the metal electrode is one of the key processes for preparing silicon carbide power devices, which affects the maximum current density, operating temperature, and stability of the device when it is working. Currently, there are two main processes for forming ohmic contacts: rapid thermal annealing and laser annealing. Among them, laser annealing is currently only used in the back metal process of diode devices. The rapid thermal annealing process is a method of heating silicon carbide and a metal electrode as a whole to react to form an ohmic contact. It is widely used in various device processes. When metal nickel is used to prepare an ohmic contact, its cross-sectional structure is as follows: Figure 6 As shown in (a), the reaction between metallic nickel and the silicon carbide substrate generates a nickel-silicon composite layer and carbon clusters, which affects the surface adhesion and flatness.

[0005] The biggest advantage of the rapid thermal annealing (high temperature annealing) process lies in the uniformity of temperature and the stability of the process. In the preparation process of MOSFET, the rapid thermal annealing process is carried out after the preparation process of the gate oxide layer. This is done to prevent the metal oxidation of the metal electrode, the degradation of the ohmic contact performance and the cross contamination of the chamber caused by the prepared ohmic contact during the preparation process of the gate oxide layer. However, the existing process also causes the gate oxide layer to experience temperature rise parameters that do not match it in the rapid thermal annealing process, resulting in the degradation of the gate oxide layer, the degradation of the contact interface quality between the gate oxide layer and silicon carbide, and the reduction of carrier mobility, which leads to problems such as increased device on-resistance and threshold voltage drift. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention aims to provide a silicon carbide surface modified ohmic contact structure, a silicon carbide semiconductor device and a preparation method thereof, firstly preparing Ni on the silicon carbide surface by high temperature annealing 2 Si phase, and then the generated Ni 2 The Si phase and the surface carbon clusters are removed to obtain modified silicon carbide, and metal is deposited on the surface of the modified silicon carbide to form an ohmic contact, thereby eliminating the metal-silicon complex in the modified ohmic contact structure on the silicon carbide surface; when preparing a silicon carbide semiconductor device, the modified silicon carbide is first prepared, then the gate oxide layer is prepared, and finally the ohmic contact is prepared, thereby obtaining a silicon carbide semiconductor device with better performance.

[0007] In order to achieve the above object, the present invention is implemented through the following technical solutions: In a first aspect, a silicon carbide surface modified ohmic contact structure includes a directly connected modified silicon carbide bottom layer and a deposited metal surface layer, wherein carbon vacancies exist inside the modified silicon carbide bottom layer; and the metal of the deposited metal surface layer is one of titanium (Ti), nickel (Ni) and aluminum (Al).

[0008] In a second aspect, the method for preparing the above-mentioned silicon carbide surface modified ohmic contact structure comprises the following steps: S1. Depositing a nickel film on silicon carbide, heating by a set method, generating a nickel-silicon composite layer on the surface of silicon carbide, and generating carbon clusters on the surface and inside of silicon carbide; S2, removing the nickel-silicon composite layer by corrosion, and removing the carbon clusters on the surface of silicon carbide by oxidation to obtain modified silicon carbide; S3. Depositing a conductive metal on the modified silicon carbide to form a silicon carbide surface modified ohmic contact structure in which a deposited metal surface layer and a modified silicon carbide bottom layer are directly connected.

[0009] Optionally, in S1, the silicon carbide is n-type silicon carbide or p-type silicon carbide.

[0010] Optionally, in S1, before depositing the nickel film, the silicon carbide is treated using an RCA standard cleaning method.

[0011] Optionally, in S1, the nickel film is deposited by magnetron sputtering or electron beam evaporation, and the film thickness is 100-150 nm.

[0012] Optionally, in S1, the heating method is high temperature annealing or high energy laser annealing; the high temperature annealing method is: RTP annealing at 950-1650°C for 10-15 min; the high energy laser annealing method is: 248-532nm laser at 1.5-2.5J / cm 2 power density annealing.

[0013] Optionally, in S2, the etching method is: using HF solution or HF and HNO 3 The mixed solution is used for wet etching.

[0014] Optionally, in S2, a 10-49% by volume HF solution is used during the corrosion process, or a 5-49% by volume HF solution and a 30-90% by volume HNO solution are used. 3 The corrosive solution is prepared with the solution and water in a volume ratio of 1: (1~1.5): (2~3).

[0015] Optionally, in S2, ultrasonic cleaning is performed after corrosion, and the process is repeated multiple times.

[0016] Optionally, in S2, the oxidation method includes: heating to 500-900°C in an oxygen atmosphere and keeping the temperature for more than 1 hour, or using a 100-150W oxygen plasma cleaning method for surface treatment.

[0017] Optionally, in S3, the conductive metal is one of titanium (Ti), nickel (Ni) and aluminum (Al).

[0018] Optionally, in S3, the conductive metal is prepared by magnetron sputtering, thermal evaporation or electron beam evaporation, with a thickness of 100-150 nm.

[0019] In a third aspect, a silicon carbide semiconductor device comprises the above-mentioned silicon carbide surface modified ohmic contact structure.

[0020] Optionally, the silicon carbide semiconductor device includes one or more of MOSFET, JFET and IGBT.

[0021] In a fourth aspect, the method for preparing the above-mentioned silicon carbide semiconductor device comprises the steps of: S1. Depositing a nickel film on a silicon carbide epitaxial wafer, heating by a set method, generating a nickel-silicon composite layer on the surface of the silicon carbide, and generating carbon clusters on the surface and inside of the silicon carbide; S2, removing the nickel-silicon composite layer by corrosion, and removing the carbon clusters on the surface of silicon carbide by oxidation to obtain modified silicon carbide; S3, preparing a gate oxide layer connected to the epitaxial layer on the modified silicon carbide surface; S4. Depositing a conductive metal on the modified silicon carbide to form an electrode, thereby obtaining a silicon carbide semiconductor device having an ohmic contact structure between the electrode and the silicon carbide substrate through a silicon carbide surface modification.

[0022] Optionally, in S1, the method for preparing the epitaxial layer includes: chemical vapor deposition.

[0023] Optionally, in S1, an ion implantation method is used to prepare a locally doped epitaxial layer.

[0024] Optionally, in S3, the method for preparing the gate oxide layer includes: preparing the gate oxide layer by dry oxidation, wet oxidation or chemical vapor deposition, and preparing polysilicon by low-pressure chemical vapor deposition on the surface of the obtained silicon oxide to form a gate electrode.

[0025] The beneficial effects of the present invention are as follows: In a method for preparing an ohmic contact and a gate oxide layer of silicon carbide provided by the present invention, high temperature annealing is first used to form a first ohmic contact between silicon carbide and nickel, and then the nickel-silicon composite forming the ohmic contact is completely corroded and removed, leaving the modified silicon carbide. The corroded silicon carbide obtains a new clean surface, which can improve the preparation effect of the gate oxide layer and the second ohmic contact. During the high temperature annealing process, some carbon atoms are also precipitated in the silicon carbide, and carbon clusters are formed on the surface while leaving carbon vacancies in the silicon carbide, increasing the carrier concentration to achieve modification, and the carbon clusters can be removed by oxidation. After that, metal is deposited on the surface of the silicon carbide to form an ohmic contact, and low temperature rapid annealing or laser annealing is used to improve the adhesion of the second ohmic contact between the electrode metal and the silicon carbide, so that the gate oxide layer and the ohmic contact can have good performance, and the performance of the silicon carbide is also improved after modification, so that the overall performance of the MOSFET is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0027] In order to show the positions of various parts, the distances or sizes between them are exaggerated in the figure, and the schematic diagram is for illustration only.

[0028] Figure 1 This is a flow chart of device preparation in Example 2.

[0029] Figure 2 It is a schematic diagram of the structure of the first layer of metal deposition annealing and surface modification after ion implantation in Example 2.

[0030] Figure 3 It is a schematic diagram of the structure of the MOSFET device prepared in Example 2.

[0031] Figure 4 This is a diagram showing the difference in ohmic contact results before and after surface treatment in Example 2.

[0032] Figure 5 It is a comparison diagram of the gate oxide layer of the traditional method in Example 2 and the gate oxide CV of the present invention.

[0033] Figure 61 is a comparison diagram of the metal / SiC interface of the present invention and the traditional interface in Example 1, wherein (a) is the ohmic contact cross-sectional structure in the background technology, and (b) is the silicon carbide surface modified ohmic contact cross-sectional structure in Example 1.

[0034] Figure 7 This is the test result of RCA and surface electron affinity after modification.

[0035] Figure 8 1 is an XPS test result diagram of Example 1, wherein (a) is an XPS test result diagram before the first ohmic contact, and (b) is an XPS test result diagram of the silicon carbide surface after modification before the second ohmic contact is prepared.

[0036] Among them, 1. Silicon carbide substrate; 2. Ion implantation of P-type doped area; 3. Ion implantation of N-type doped area; 4. First deposition of metal Ni layer; 5. Gate oxide layer; 6. Polysilicon gate; 7. Secondary deposition of metal Al. DETAILED DESCRIPTION

[0037] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0038] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0039] A silicon carbide surface modified ohmic contact structure comprises a directly connected modified silicon carbide bottom layer and a deposited metal surface layer, wherein carbon vacancies exist inside the modified silicon carbide bottom layer; and the metal of the deposited metal surface layer is one of titanium (Ti), nickel (Ni) and aluminum (Al).

[0040] The method for preparing the above-mentioned silicon carbide surface modified ohmic contact structure comprises the following steps: S1. Depositing a nickel film on silicon carbide, heating by a set method, generating a nickel-silicon composite layer on the surface of silicon carbide, and generating carbon clusters on the surface and inside of silicon carbide; S2, removing the generated nickel-silicon composite by corrosion, and removing the carbon clusters on the surface of silicon carbide by oxidation to obtain modified silicon carbide; S3. Depositing a conductive metal on the modified silicon carbide to form a silicon carbide surface modified ohmic contact structure in which a deposited metal surface layer and a modified silicon carbide bottom layer are directly connected.

[0041] In the above method, the first ohmic contact is formed in S1, and the silicon carbide obtains a modified internal structure during the process. The reaction products are removed in S2 to form a new modified surface. Ohmic contact is formed under low temperature conditions in S3 to complete the effective connection between silicon carbide and the electrode metal. In this process, Ni fully reacts with Si in SiC to form a nickel-silicon complex, while carbon atoms precipitate to the surface to form carbon clusters and leave carbon vacancies in a certain area below the surface of silicon carbide, thereby increasing the carrier concentration and modifying the surface of silicon carbide.

[0042] Optionally, in S1, n-type silicon carbide or p-type silicon carbide is used as the substrate.

[0043] Optionally, in S1, before depositing the nickel film, the silicon carbide is treated with the RCA standard cleaning method. The reagents used in the RCA standard cleaning method include: an APM cleaning solution for removing particles attached to the surface of silicon carbide, an HPM cleaning agent for removing metal residues on the surface of silicon carbide, an SPM cleaning agent for removing organic residues and other metal residues on the surface, and an HF cleaning agent for removing the surface oxide layer without damaging the SiC surface.

[0044] Optionally, in S1, the deposition method of the nickel film includes magnetron sputtering or electron beam evaporation, and the film thickness is 100~150nm, which serves as the nickel source for forming the first ohmic contact.

[0045] Optionally, in S1, the heating method is high temperature annealing or high energy laser annealing; the high temperature annealing method is: RTP annealing at 950-1650°C for 10-15 min; the high energy laser annealing method is: 248-532nm laser at 1.5-2.5J / cm 2 During the annealing process, various elements diffuse into each other at the interface, causing the nickel film on the surface to form Ni with the silicon carbide underneath. 2 Si phase, while the remaining C in silicon carbide diffuses to the surface, forming carbon clusters at the interface or surface, and carbon precipitation occurs inside silicon carbide to increase the number of carbon vacancies inside silicon carbide, thereby modifying silicon carbide, thereby increasing carrier mobility and improving device performance.

[0046] Optionally, in S2, the etching method is: using HF solution or HF and HNO 3 The Ni that has been produced is completely removed during the etching process. 2 Si phase, re-exposing the surface of silicon carbide, and the cleanliness of the re-exposed surface is further improved, thus laying the foundation for the subsequent preparation of the gate oxide layer or the second ohmic contact; the high concentration HF solution itself can react with Ni and Si to remove the product, and the mixed solution is HNO 3Oxidation generates oxides, and HF reacts with the oxides to prevent the formation of a dense passivation layer on the surface and increase the removal rate.

[0047] Optionally, a 10-49% by volume HF solution or a 5-49% by volume HF solution or a 30-90% by volume HNO solution is used during the etching process. 3 The etching solution prepared with the solution and water in a volume ratio of 1: (1~1.5): (2~3) can obtain a better etching effect. Optionally, in S2, ultrasonic cleaning is performed after etching, and the etching-cleaning cycle is repeated multiple times to ensure that there is no metal residue on the surface of silicon carbide to prevent contamination and impact on other steps in the device preparation process.

[0048] Optionally, in S2, the oxidation method includes: heating to 500-900°C in an oxygen atmosphere and keeping warm for more than 1 hour, or using a 100-150W oxygen plasma cleaning method for surface treatment; this process is to remove carbon residues to prevent the defect density in the gate oxide from increasing during subsequent gate oxide preparation; the carbon clusters existing on the re-exposed silicon carbide surface will affect the adhesion of the subsequent metal and the interface quality of the gate oxide layer, and therefore also need to be removed. The conventional oxidation process has a slow heating process and a long reaction time. Although it can also achieve the desired effect, oxygen plasma cleaning as a common method in the process can accelerate this process; the appearance of the by-product silicon oxide will affect the direct contact between the secondary deposited metal and silicon carbide, and HF solution needs to be used for cleaning in the process. The gate oxide patterning process in the commercial device process can remove it.

[0049] Optionally, in S3, the conductive metal is one of titanium (Ti), nickel (Ni) and aluminum (Al). As a metal source for forming a second ohmic contact, the conductive metal needs to have a work function close to the electron affinity of silicon carbide to increase the probability of electron tunneling. Experimental verification shows that common metals Ni, Ti, and Al can directly form ohmic contacts after deposition; deposition methods include magnetron sputtering, thermal evaporation, or electron beam evaporation.

[0050] Optionally, in S3, the thickness of the conductive metal is 100-150 nm, and a certain thickness of the conductive metal needs to be retained in addition to the phase required for forming the ohmic contact. Since the interface is clean, the ohmic contact effect is better than the first time.

[0051] Optionally, in S3, the low temperature annealing method includes: low temperature annealing at 450-550°C for 3-5 min, or laser annealing. Low temperature annealing can increase the adhesion between metal and silicon carbide, and will not degrade the gate oxide layer; however, the actual heating temperature of low temperature annealing or laser annealing cannot exceed 700°C, otherwise the metal and silicon carbide will re-react, or the gate oxide layer will degrade, and the electron transport mechanism will change.

[0052] A silicon carbide semiconductor device comprises the above-mentioned silicon carbide surface modified ohmic contact structure.

[0053] Optionally, the silicon carbide semiconductor device includes one or more of MOSFET, JFET and IGBT.

[0054] The method for preparing the silicon carbide semiconductor device comprises: S1, preparing an epitaxial layer on a silicon carbide surface; S2, depositing a nickel film on silicon carbide, heating by a set method, generating a nickel-silicon composite layer on the surface of silicon carbide, and generating carbon clusters on the surface and inside of silicon carbide; S3, removing the nickel-silicon composite layer by corrosion, and removing the carbon clusters on the surface of silicon carbide by oxidation to obtain modified silicon carbide; S4, preparing a gate oxide layer connected to the epitaxial layer on the modified silicon carbide surface; S5. Depositing a conductive metal on the modified silicon carbide to form an electrode, thereby obtaining a silicon carbide semiconductor device having an ohmic contact structure between the electrode and the silicon carbide substrate through a silicon carbide surface modification.

[0055] The step of depositing the conductive metal is performed after the gate oxide layer is prepared, so as to meet the process requirement of no metal in the gate oxide layer preparation process to ensure spatial purity.

[0056] Optionally, in S1, after the epitaxial layer is prepared, local doping is performed using an ion implantation method to obtain the epitaxial layer.

[0057] Optionally, in S1, the method for preparing the epitaxial layer includes: chemical vapor deposition. Optionally, in S4, the method for preparing the gate oxide layer includes: preparing the gate oxide layer by dry oxidation, wet oxidation or chemical vapor deposition, and preparing polysilicon by low-pressure chemical vapor deposition on the surface of the obtained silicon oxide to form a gate electrode.

[0058] Optionally, in S4, the gate oxide layer is prepared by a method selected from the group consisting of thermal oxidation and chemical vapor deposition.

[0059] Optionally, in S5, the conductive metal is deposited by one of magnetron sputtering, thermal evaporation or electron beam evaporation.

[0060] Example 1 A silicon carbide surface modified ohmic contact structure of N-type silicon carbide, such as Figure 6 As shown in (b), it includes a directly connected modified silicon carbide bottom layer and a deposited metal surface layer, wherein carbon vacancies exist inside the modified silicon carbide bottom layer; the deposited metal surface layer is metal aluminum; and Figure 6 The difference from (a) is that there is no metal-silicon composite between the modified silicon carbide bottom layer and the deposited metal surface layer.

[0061] The substrate is an N-type 4H-SiC substrate with a thickness of 350 μm, and the substrate surface has two epitaxial layers, the inner epitaxial layer is a 10 μm low-doped layer, and the outer epitaxial layer is a 1 μm N-type high-doped layer; the preparation method of the silicon carbide surface modified ohmic contact structure and MOSFET includes the following steps.

[0062] S1. The substrate was treated by RCA standard cleaning method, including APM, HPM, SPM and HF solution. Then, a nickel metal film with a thickness of 100 nm was deposited on the surface and back of the substrate by magnetron sputtering. The substrate was treated by rapid thermal annealing process and kept at 1050°C for 15 minutes to allow Ni and SiC to fully react to form the first ohmic contact, forming Figure 2 In the substrate structure shown, a first deposited metal Ni layer 4 is provided on both sides of a silicon carbide substrate 1, an ion-implanted P-type doping region 2 is provided on the upper surface of the silicon carbide substrate 1, and an ion-implanted N-type doping region 3; 4 is provided inside the ion-implanted P-type doping region 2.

[0063] S2, then HF solution (volume fraction 49%), HNO 3 The solution (volume fraction of 65%) and water were prepared into an etching solution. The substrate with nickel film deposited was placed in the etching solution for 3 minutes, and then ultrasonically cleaned for 5 minutes. The etching-cleaning process was repeated 3 times to remove the generated Ni 2 The Si phase is completely removed, and a substrate with carbon clusters on the surface is obtained.

[0064] The substrate with carbon clusters on the surface obtained in S1 was treated with an oxygen plasma cleaner at a power of 100 W for 5 min to completely remove the carbon clusters on the surface of the substrate, and then an HF solution with a volume fraction of 1% was used to remove the oxide layer generated during the oxygen plasma treatment.

[0065] S3. Metal Al is deposited on the surface and back of the 4H-SiC substrate by magnetron sputtering to form an aluminum film with a thickness of 100 nm. During the deposition of the metal Al, Al forms a second ohmic contact with silicon carbide.

[0066] The C element XPS test of silicon carbide was performed before the first ohmic contact was formed in step S1. The results are as follows Figure 8 In Figure (a), the CC peak binding energy is shown as the dotted line a in the figure; the modified silicon carbide formed in step S2 is subjected to C element XPS test, and the results are shown in Figure 8In Figure (b), the CC peak binding energy is shown as the dotted line b in the figure, indicating that during the SiC surface modification process (the modification process includes: removing the ohmic contact structure after the first ohmic contact is prepared on the SiC surface), the CC peak binding energy shifts toward the low energy direction, indicating that the electron concentration in the sample increases, that is, the carbon vacancies increase the carrier concentration.

[0067] Example 2 A MOSFET device, comprising the silicon carbide surface modified ohmic contact structure in Example 1, prepared by the following method: Figure 1 As shown, the following steps are included.

[0068] S1. Ion implantation is performed on the substrate with the epitaxial layer to form P-type and N-type doped regions respectively, and annealing is performed at 1650°C for activation; the substrate is treated by RCA standard cleaning method, including APM, HPM, SPM and HF solution, and then a nickel metal film with a thickness of 100nm is deposited on the surface and back of the substrate by magnetron sputtering. The substrate is treated by rapid thermal annealing process and kept at 1050°C for 15 minutes to allow Ni and SiC to fully react to form the first ohmic contact, forming as shown in the figure. Figure 2 The substrate structure is shown and IV test is performed.

[0069] S2, then HF solution (volume fraction 49%), HNO 3 The solution (volume fraction of 65%) and water were prepared into an etching solution. The substrate with nickel film deposited was placed in the etching solution for 3 minutes, and then ultrasonically cleaned for 5 minutes. The etching-cleaning process was repeated 3 times to remove the generated Ni 2 The Si phase is completely removed, and a substrate with carbon clusters on the surface is obtained.

[0070] The substrate with carbon clusters on the surface obtained in S1 was treated with an oxygen plasma cleaner at a power of 100 W for 5 min to completely remove the carbon clusters on the surface of the substrate, and then an HF solution with a volume fraction of 1% was used to remove the oxide layer generated during the oxygen plasma treatment.

[0071] Then, a gate oxide layer of 30 nm is prepared on the surface of the obtained substrate, including dry oxidation, wet oxidation or chemical vapor deposition.

[0072] On the obtained silicon oxide surface, polysilicon 1 μm is prepared by low-pressure chemical vapor deposition to form a gate electrode.

[0073] S3, using magnetron sputtering method to deposit metal Al on the surface and back of 4H-SiC substrate to form an aluminum film with a thickness of 100nm. During the deposition of metal aluminum, Al forms a second ohmic contact with silicon carbide. After patterning, Figure 3As shown, at this time, the outer side of the silicon carbide substrate 1 is respectively distributed with secondary deposited metal Al7, the upper surface of the silicon carbide substrate 1 is provided with an ion-implanted P-type doping region 2, the ion-implanted P-type doping region 2 is provided with an ion-implanted N-type doping region 3, the gate oxide layer 5 is located in the middle of the spaced ion-implanted N-type doping region 3, and the outer surface of the gate oxide layer 5 is a polysilicon gate 6.

[0074] The IV test method of this embodiment includes a test system composed of a probe station and a source meter. The IV test is performed after the S1 step and after the S3 step using a four-probe method. The results are as follows: Figure 4 Compare Figure 4 The two test lines inside the test tube show that the ohmic contact effect of the secondary deposited metal is slightly better than the first one, and the current increases slightly, indicating that the ohmic contact performance of the secondary deposited metal after surface pretreatment can meet the needs of use.

[0075] Example 3 This embodiment and embodiment 1 use the same substrate, and the difference in the preparation method is: In S1, a 150 nm nickel film was deposited on the substrate surface by electron beam evaporation, and then a 355 nm laser was used at 2.5 J / cm 2 Annealing forms an ohmic contact between the conductive metal and the N-type silicon carbide contact interface. Laser annealing can also cause the nickel film on the surface to produce Ni-SiC with the silicon carbide underneath. 2 Si phase, while the remaining C in silicon carbide diffuses to the surface, forming carbon clusters at the interface or surface, and generating carbon vacancies below the surface of silicon carbide, thereby modifying silicon carbide. In S2, a 10% volume fraction HF solution was used for wet etching for 10 minutes to completely remove the generated Ni 2 Four phases.

[0076] The oxidation method is to heat to 900°C in an oxygen atmosphere and keep it warm for 0.5h to remove carbon clusters on the surface of silicon carbide.

[0077] The gate oxide layer is prepared by chemical vapor deposition.

[0078] In S3, a thermal evaporation method is used to deposit a 150nm thick metal layer on the surface of the processed substrate using metal Al as a raw material. The second ohmic contact can be formed during the deposition process.

[0079] The sample was then subjected to 1.2 J / cm 2 During the laser annealing treatment, the actual heating temperature does not exceed 500°C, which will not cause the metal and silicon carbide to re-react, nor will it degrade the gate oxide layer. It will increase the adhesion between the metal and silicon carbide and improve device performance.

[0080] IV tests were performed on the device with the first ohmic contact obtained before wet etching in S1, the device with the second ohmic contact obtained by deposition in S3, and the device after laser annealing. The trend of the results obtained was the same as that in Example 2, indicating that laser annealing and RTP can have similar effects, and the matching conditions in the experiment can be used for equivalent treatment.

[0081] Comparative Example 1 A MOSFET device, the preparation method of which is different from that of Example 2 in that: instead of using the method of depositing a nickel film and then removing it by etching, and then oxidizing to eliminate carbon clusters on the surface of the substrate in S1 and S2, the substrate is treated by an RCA standard cleaning method, a gate oxide layer is prepared on the surface of the substrate, and then an electrode is directly formed on the surface of the substrate using metal Ni by a magnetron sputtering method, and then an ohmic contact is formed by high temperature annealing. The obtained silicon carbide surface modified ohmic contact structure is as follows Figure 6 As shown in (a), it contains a metal-silicon composite layer.

[0082] The gate oxide layer performance of the MOSFET devices prepared in Comparative Example 1 and Example 2 was tested respectively. The results are as follows: Figure 5 As shown, it is found that the flat band voltage in the CV test results of the gate oxide layer prepared in Example 2 moves toward the forward direction close to the theoretical value. This is because the process of forming the second ohmic contact does not involve high temperature annealing, so high performance can be maintained.

[0083] The affinity of the silicon carbide surface modification obtained in the MOSFET obtained in Example 2 and the RCA cleaning of the silicon carbide in Comparative Example 1 were detected respectively, and the detection method was: ultraviolet photoelectron spectroscopy detection. The results are as follows Figure 7 As shown: The work function of surface modified silicon carbide has been significantly improved.

[0084] After the substrate is treated only by the RCA standard cleaning method, a gate oxide layer is prepared on the substrate surface, and then Al is deposited on the substrate surface by magnetron sputtering. As a result, no ohmic contact can be formed, and the IV curve surface shows Schottky characteristics. It is impossible to simultaneously obtain a gate oxide layer with good performance and an ohmic contact, so it cannot be used in devices.

[0085] The device of this comparative example exhibits Schottky characteristics in the performance test, indicating that it is impossible to simultaneously obtain a device with good performance and an ohmic contact.

[0086] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A silicon carbide surface modified ohmic contact structure, comprising a directly connected modified silicon carbide bottom layer and a deposited metal surface layer, wherein carbon vacancies exist inside the modified silicon carbide bottom layer; and the metal of the deposited metal surface layer is one of titanium, nickel and aluminum.

2. A method for preparing the silicon carbide surface modified ohmic contact structure according to claim 1, characterized in that: The following steps are involved: S1. Depositing a nickel film on silicon carbide, heating by a set method, generating a nickel-silicon composite layer on the surface of silicon carbide, and generating carbon clusters on the surface and inside of silicon carbide; S2, removing the nickel-silicon composite layer by corrosion, and removing the carbon clusters on the surface of silicon carbide by oxidation to obtain modified silicon carbide; S3. Depositing a conductive metal on the modified silicon carbide to form a silicon carbide surface modified ohmic contact structure in which a deposited metal surface layer and a modified silicon carbide bottom layer are directly connected.

3. The method for preparing the silicon carbide surface modified ohmic contact structure according to claim 2, characterized in that: In S1, the silicon carbide is n-type silicon carbide or p-type silicon carbide; before depositing the nickel film, the silicon carbide is treated by the RCA standard cleaning method; the nickel film is deposited by magnetron sputtering or electron beam evaporation, and the film thickness is 100-150 nm.

4. The method for preparing the silicon carbide surface modified ohmic contact structure according to claim 2, characterized in that: In S1, the heating method is high temperature annealing or high energy laser annealing; the high temperature annealing method is: RTP annealing at 950-1650°C for 10-15 minutes; the high energy laser annealing method is: 248-532nm laser at 1.5-2.5J / cm 2 power density annealing.

5. The method for preparing the silicon carbide surface modified ohmic contact structure according to claim 2, characterized in that: In S2, the etching method is: wet etching using HF solution or a mixture of HF and HNO3; during the etching process, an HF solution with a volume fraction of 10-49% is used, or an etching solution prepared by using an HF solution with a volume fraction of 5-49%, an HNO3 solution with a volume fraction of 30-90%, and water in a volume ratio of 1: (1-1.5): (2-3) is used.

6. The method for preparing the silicon carbide surface modified ohmic contact structure according to claim 2, characterized in that: In S2, the oxidation method includes: heating to 500-900°C in an oxygen atmosphere and keeping the temperature for more than 1 hour, or using a 100-150W oxygen plasma cleaning method for surface treatment.

7. The method for preparing the silicon carbide surface modified ohmic contact structure according to claim 2, characterized in that: In S3, the conductive metal is one of titanium, nickel and aluminum; the conductive metal is prepared by magnetron sputtering, thermal evaporation or electron beam evaporation, and has a thickness of 100~150nm.

8. A silicon carbide semiconductor device, characterized in that: It comprises the silicon carbide surface modified ohmic contact structure as claimed in claim 1.

9. A method for preparing a silicon carbide semiconductor device according to claim 8, characterized in that: Includes steps: S1. Depositing a nickel film on silicon carbide, heating by a set method, generating a nickel-silicon composite layer on the surface of silicon carbide, and generating carbon clusters on the surface and inside of silicon carbide; S2, removing the nickel-silicon composite layer by corrosion, and removing the carbon clusters on the surface of silicon carbide by oxidation to obtain modified silicon carbide; S3, preparing an epitaxial layer on the modified silicon carbide surface; S4, preparing a gate oxide layer connected to the epitaxial layer on the modified silicon carbide surface; S5. Depositing a conductive metal on the modified silicon carbide to form an electrode, thereby obtaining a silicon carbide semiconductor device having an ohmic contact structure between the electrode and the silicon carbide substrate through a silicon carbide surface modification.

10. The method for preparing a silicon carbide semiconductor device according to claim 9, characterized in that: The method for preparing the gate oxide layer comprises: preparing the gate oxide layer by dry oxidation, wet oxidation or chemical vapor deposition, and preparing polysilicon on the surface of the obtained silicon oxide by low-pressure chemical vapor deposition to form a gate electrode.

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