A SiC-based Schottky device with low on-state voltage drop and a method for preparing the same

Abstract

The invention discloses a SiC-based Schottky device with low on-state voltage drop and a preparation method thereof, which comprises, from bottom to top, an ohmic contact electrode, an n++-type SiC substrate substrate, an n+-type buffer layer, a drift layer and a Schottky contact electrode; the drift layer comprises, from bottom to top, an nx first drift layer and an n-second drift layer, the doping concentration of the nx first drift layer is greater than the doping concentration of the n-second drift layer, and a p-type shielding layer is arranged in the n-second drift layer; wherein the doping concentration of the nx first drift layer changes from low to high doping from bottom to top, and then changes from high to low doping. The invention reduces the on-state voltage drop and power consumption of the SiC Schottky device, and at the same time improves the long-term stability of the device.

Description

Technical Field

[0001] The present invention relates to a SiC Schottky device and a preparation method thereof, and in particular to a SiC-based Schottky diode structure with low on-state voltage drop and a preparation method thereof. Background Art

[0002] The third generation semiconductor silicon carbide (SiC) material has better physical and electrical properties than traditional Si materials. For example, SiC has the characteristics of wide bandgap, high thermal conductivity, high breakdown field strength, high saturated electron drift rate, etc., and also has excellent physical and chemical stability, strong radiation resistance and mechanical strength. Therefore, electronic devices based on wide bandgap SiC materials can be used in power electronics fields such as high temperature, high power, high frequency, and high radiation.

[0003] SiC Schottky diode devices are already mature in the commercialization process, especially the junction Schottky devices with injected p-type shielding layer are the mainstream. The junction barrier Schottky diode integrates the PN junction in the Schottky structure, which can effectively ensure the excellent forward conduction characteristics of the low Schottky barrier. At the same time, when reverse blocking, it can limit the maximum electric field to the PN junction area, thereby reducing the electric field at the surface Schottky contact, making the reverse leakage current greatly reduced compared to the pure Schottky type. Nevertheless, on the one hand, the on-state voltage drop (V F ) is affected by the total series resistance, which includes the JFET resistance R JFET , drift region resistance R drift And the substrate resistance R SUB . There are still many contradictions in the compromise design between the blocking voltage and forward conduction characteristics of SiC Schottky devices, such as the optimization design of the doping and thickness of the epitaxial layer, which has a key impact on it. Higher drift region doping and smaller drift layer thickness are not conducive to the improvement of reverse blocking capability, and vice versa. It is not conducive to the improvement of forward conduction characteristics. On the other hand, there are still many defects on the surface of the SiC substrate. Although the quality of the subsequent epitaxial layer is improved by growing a buffer layer, there is still a strong driving effect of electrical stress after the device is powered on, which makes the defect evolution and thus affects the overall reliability of the device. Taking these factors into consideration, it is necessary to find a new technology to make SiC Schottky devices have a low on-state voltage drop, while reducing the defect expansion in sensitive areas and improving the long-term stability of the device. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a structural design scheme and preparation method of a SiC Schottky device, mainly used to reduce the on-state voltage drop and power consumption of the SiC Schottky device, while improving the long-term stability of the device, in view of the design contradiction between the on-state voltage drop and the blocking voltage of the current SiC Schottky device, and the problem of material defects expanding under electrical stress.

[0005] The technical solution of the present invention is to determine the total series resistance of the SiC Schottky device on-state voltage drop, such as the JFET resistance R JFET , drift region resistance R drift And the substrate resistance R SUB Taking all aspects into consideration, a SiC Schottky device with a variable doping epitaxial structure is proposed. The high-doped drift layer far away from the main junction area is used to reduce the series resistance of the device, and the low-doped layer in the main junction area is used to improve the protection of the p-type shielding layer and reduce the reverse leakage current of the device. At the same time, the low-doped epitaxial structure design close to the buffer layer is used to improve the reliability of the SiC Schottky device.

[0006] In order to achieve the above purpose, the technical solution of the present invention is:

[0007] A SiC-based Schottky device with low on-state voltage drop comprises, from bottom to top, an ohmic contact electrode, an n++-type SiC substrate, an n+-type buffer layer, a drift layer and a Schottky contact electrode; the drift layer comprises, from bottom to top, an nx first drift layer and an n-second drift layer, the doping concentration of the nx first drift layer is greater than the doping concentration of the n-second drift layer, and a p-type shielding layer is arranged in the n-second drift layer; wherein the doping concentration of the nx first drift layer changes from low to high doping first, and then changes from high to low doping, from bottom to top.

[0008] Optionally, the thickness of the nx first drift layer accounts for 50-80% of the thickness of the entire drift layer.

[0009] Optionally, the p-type shielding layer is disposed on the upper portion of the n-second drift layer, and a distance between a bottom of the p-type shielding layer and a bottom of the n-second drift layer is 50% to 90% of a thickness of the n-second drift layer.

[0010] Optionally, the nx first drift layer includes at least three stacked doping layers, and the doping concentration of the middle doping layer is greater than the doping concentration of the doping layers on the upper and lower sides.

[0011] Optionally, the thickness of the middle doping layer accounts for 40% to 60% of the thickness of the nx first drift layer.

[0012] Optionally, the nx first drift layer is gradually doped from bottom to top, and the doping concentration is first from low to high and then from high to low.

[0013] Optionally, the doping concentration of the nx first drift layer is no more than 1×10 17 cm -3 , minimum not less than 1×10 15 cm -3 The doping concentration of the n-second drift layer is in the range of 1×1014 ~5×10 15 cm -3 The doping concentration range of the n+ type buffer layer is 1×10 18 ~1×10 19 cm -3 .

[0014] A method for preparing the above-mentioned SiC-based Schottky device with low on-state voltage drop comprises the following steps:

[0015] 1) growing an n+ type buffer layer on an n++ type SiC substrate;

[0016] 2) growing a drift layer, first growing an nx first drift layer with a higher doping concentration on the n+ type buffer layer, and then growing an n- second drift layer with a lower doping concentration on the nx first drift layer;

[0017] 3) fabricating a p-type shielding layer in the n-second drift layer by a local doping process;

[0018] 4) Fabricating an ohmic contact electrode on the back side of the n++ type SiC substrate;

[0019] 5) Fabricating a Schottky contact electrode on the n-second drift layer.

[0020] Optionally, in the step 2), the drift layer is grown by chemical vapor deposition process, the growth temperature is 1500-1700° C., the doping source is NH 3, and the doping concentration is controlled by controlling the doping source flow rate.

[0021] Optionally, during the growth of the nx first drift layer, the flow rate of the doping source changes in stages or gradually.

[0022] The beneficial effects of the present invention are:

[0023] (1) Based on the partially highly doped drift layer, the series resistance of the SiC Schottky device is greatly reduced;

[0024] (2) By adopting a variable doping epitaxial structure, the low-doping layer in the main junction region can still improve the protection effect of the p-type shielding layer, reduce the reverse leakage current of the device, and improve the blocking capability of the SiC Schottky device.

[0025] (3) By adopting a variable doping epitaxial structure, the thickness of the high-doping layer away from the main junction area can be appropriately reduced, further reducing the device conduction voltage drop compared with traditional devices;

[0026] (4) By adopting a variable doping epitaxial structure, the doping layer close to the buffer layer can reduce the electric field and collision ionization rate at the interface between the buffer layer and the epitaxial layer, thereby reducing the probability of defect expansion.

[0027] (5) The SiC Schottky device has a higher Baliga figure of merit and a larger high-frequency switching figure of merit compared to a traditional Schottky diode. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A preparation flow chart of a SiC Schottky device with low on-state voltage drop provided by the present disclosure;

[0029] Figure 2 is a structural diagram of a SiC epitaxial material substrate involved in an embodiment;

[0030] Figure 3 is a structural diagram of a first nx drift layer 31 according to an embodiment;

[0031] Figure 4 is a structural diagram of an n-second drift layer 30 according to an embodiment;

[0032] Figure 5 is a structural diagram of a p-type shielding layer 40 according to an embodiment;

[0033] Figure 6 is a structural diagram of an ohmic contact electrode 50 according to an embodiment;

[0034] Figure 7 is a structural diagram of a Schottky contact electrode 60 according to an embodiment;

[0035] Figure 8 is a structural diagram of a first nx drift layer 31 according to an embodiment;

[0036] Fig. 9 is a structural diagram of a SiC Schottky device with low on-state voltage drop involved in Example 2;

[0037] Fig.10 is a structural diagram of the nx first drift layer 31 involved in the second embodiment;

[0038] Fig.11 is a structural diagram of a SiC Schottky device with low on-state voltage drop involved in Example 2;

[0039] Fig.12 This is a comparison diagram of the SiC Schottky device with low on-state voltage drop according to an embodiment of the present invention and a traditional device. DETAILED DESCRIPTION

[0040] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. The various drawings of the present invention are only for illustration to make it easier to understand the present invention, and the specific proportions can be adjusted according to design requirements. The upper and lower relationships of the relative elements and the definitions of the front / back in the figures described in the text should be understood by those skilled in the art to refer to the relative positions of the components, so they can all be flipped to present the same components, which should all fall within the scope disclosed in this specification.

[0041] In the present invention, a SiC Schottky device structure and a preparation method having both low on-state voltage drop and high reliability are provided. The device utilizes a variable doping epitaxial structure to greatly reduce the drift layer resistance of the SiC Schottky device, ensuring that the device has a low on-state voltage drop characteristic when forward-conducting. During reverse blocking, the self-consistent shielding effect of the adjacent p-type shielding layer effectively protects the Schottky contact, so that the electric field at the Schottky contact of the device is greatly reduced, and avalanche occurs at the PN junction of the device body region. At the same time, the electrical stress of the epitaxial layer region close to the buffer layer side is reduced, thereby improving the reliability of the device. The prepared SiC Schottky device has a lower forward on-state voltage drop and a lower reverse leakage current, and the conduction, blocking characteristics and stability of the device are improved.

[0042] Example 1

[0043] Figure 1 The present invention is a flow chart of a method for preparing a SiC Schottky device with a low on-state voltage drop, comprising the following steps:

[0044] S1. Cleaning the SiC material substrate.

[0045] S2. Growth buffer layer.

[0046] S3. Growth of drift layer.

[0047] S4. Fabricate a p-type shielding layer.

[0048] S5. Make ohmic contact.

[0049] S6. Making Schottky contacts.

[0050] in,

[0051] Step S1: Cleaning the SiC material substrate. The SiC material substrate comprises an n++ type SiC substrate substrate 10, and the substrate sample is subjected to standard cleaning, specifically:

[0052] a. Ultrasonic clean the surface with acetone and ethanol three times in sequence, and then rinse with deionized water.

[0053] b. Boil the organic ultrasonically treated SiC epitaxial material substrate in concentrated sulfuric acid and hydrogen peroxide solution for at least 10 minutes.

[0054] c. Boil the SiC epitaxial material substrate that has been boiled in concentrated sulfuric acid in liquid No. 1 and liquid No. 2 for 15 minutes, then rinse with deionized water and blow dry with nitrogen for later use. Liquid No. 1 is a mixture of ammonia water, hydrogen peroxide and deionized water, with a volume ratio of ammonia water: hydrogen peroxide: deionized water = 1:2:5, and liquid No. 2 is a mixture of hydrochloric acid, hydrogen peroxide and deionized water, with a volume ratio of hydrochloric acid: hydrogen peroxide: deionized water = 1:2:5.

[0055] d. Soak the rinsed SiC epitaxial material substrate in diluted hydrofluoric acid (hydrogen fluoride: deionized water = 1:3 by volume) for 1 min to remove the surface oxide, and then wash it with deionized water and dry it.

[0056] Step S2: growing a buffer layer. Figure 2 As shown, the n+ type buffer layer 20 is made of homogeneous materials, specifically comprising:

[0057] like Figure 2 As shown, it is obtained by physical or chemical vapor deposition, specifically: using chemical vapor deposition or physical vapor deposition to grow a homogeneous material n+ type buffer layer 20 onto the n++ type SiC substrate substrate 10 of the cleaned SiC material substrate, the growth source is SiH4 and C2H4, or other Si source and C source gases, the thickness of the n+ type buffer layer 20 is 0.5-5μm, the growth temperature of the n+ type buffer layer 20 is 1500-1700℃, and the doping source is NH3. The doping concentration range of the n+ type buffer layer 20 is 1×10 18 ~1×10 19 cm -3 .

[0058] Step S3: growing a drift layer. Figure 3 and Figure 4 As shown, the drift layer comprises an nx first drift layer 31 and an n- second drift layer 30, and the nx first drift layer 31 and the n- second drift layer 30 are made of homogeneous materials, specifically comprising:

[0059] a. Figure 3 As shown, it is obtained by physical or chemical vapor deposition, specifically: a homogeneous material nx first drift layer 31 is grown on the n+ type buffer layer 20 by chemical vapor deposition or physical vapor deposition, the growth source is SiH4 and C2H4, or other Si source and C source gases, the doping source is NH3, and the growth temperature is 1500-1700° C. The thickness of the nx first drift layer 31 accounts for 50-80% of the thickness of the entire drift layer. For example, the thickness of the nx first drift layer 31 is 5-8 μm.

[0060] The nx first drift layer 31 has the characteristics of high doping in the middle part and low doping on both sides from top to bottom, such as Figure 8As shown, as an example, the nx first drift layer 31 includes three layers of nx1, nx2 and nx3 from bottom to top, and the doping concentration of the nx2 layer is greater than the doping concentration of the nx1 layer and the nx3 layer. For example, the doping concentration of the nx2 layer is 1×10 16 cm -3 ~1×10 17 cm -3 , with a thickness of 3-4 μm; the doping concentration of the nx1 layer is 1×10 15 cm -3 ~1×10 16 cm -3 , with a thickness of 1-2 μm; the doping concentration of the nx3 layer is 1×10 15 cm -3 ~1×10 16 cm -3 , thickness is 1-2μm.

[0061] b. Figure 4 As shown, it is obtained by physical or chemical vapor deposition, specifically: using chemical vapor deposition or physical vapor deposition to grow the homogeneous material n-second drift layer 30 onto the nx first drift layer 31, the growth source is SiH4 and C2H4, or other Si source and C source gases, the doping source is NH3, and the growth temperature is 1500-1700°C. The thickness of the n-second drift layer 30 accounts for 20-50% of the thickness of the entire drift layer. The doping concentration of the n-second drift layer 30 is 1×10 14 ~5×10 15 cm -3 .

[0062] Step S4: making a p-type shielding layer 40. Figure 5 As shown, the p-type shielding layer 40 is periodically arranged within the n-second drift layer 30, and the bottom of the p-type shielding layer 40 does not exceed the bottom of the n-second drift layer 30, that is, it maintains a certain distance from the top of the nx first drift layer 31, specifically including:

[0063] A mask layer is deposited on the n-second drift layer 300 by chemical vapor deposition or physical vapor deposition. The mask layer may be SiO2 or Si3N4 or polysilicon or a metal material. A photoresist A is used to perform photolithography and patterning to form an injection mask layer. A p-type shielding layer 40 is formed in the n-second drift layer 30 by a doping method such as ion implantation. The p-type shielding layer 40 is periodically arranged in the n-second drift layer 30. The bottom of the p-type shielding layer 40 does not exceed the bottom of the n-second drift layer 30, that is, it maintains a certain distance from the top of the nx first drift layer 31. The doping concentration of the p-type shielding layer 40 is 5×10 17 cm -3 ~1×1019 cm -3 The distance between the bottom of the p-type shield layer 40 and the bottom of the n-second drift layer 30 is 50% to 90% of the thickness of the n-second drift layer 30 .

[0064] Step S5: Make ohmic contacts, such as Figure 6 As shown, specifically including:

[0065] a. The front side of the SiC substrate is coated with photoresist for protection, and the oxide layer on the back side of the n++ type SiC substrate 10 is removed with diluted HF. A metal layer is deposited on the back side using a thin film deposition method such as electron beam evaporation or sputtering. The metal layer can be AlTi, Ni, TiW, AlTi or a combination thereof to form an electrode contact, and then the front side photoresist is removed.

[0066] b: Annealing the electrode contact in a temperature range of 900° C. to 1100° C. under nitrogen or argon conditions to form an ohmic contact electrode 50 .

[0067] Step S6: Make Schottky contacts, such as Figure 7 As shown, specifically including:

[0068] a. The back of the SiC substrate is coated with photoresist for protection, and a metal layer is deposited on the front side by a thin film deposition method such as electron beam evaporation or sputtering. The metal layer can be a metal such as Ti, TiAl, etc., to form a Schottky contact. The metal Schottky contact is located on the upper surface of the n-second drift layer 30 and the p-type shielding layer 40.

[0069] b. Annealing the Schottky contact in a temperature range of 400 to 600° C. under nitrogen or argon conditions to eventually form a Schottky contact electrode 60 .

[0070] c. Photolithography patterning to form the front pad metal layer.

[0071] Complete the preparation of a SiC Schottky device with low on-state voltage drop and high reliability, such as Figure 7 and 9 shown.

[0072] Example 2

[0073] Figure 1 The present invention is a flow chart of a method for preparing a SiC Schottky device with a low on-state voltage drop, comprising the following steps:

[0074] S1. Cleaning the SiC material substrate.

[0075] S2. Growth buffer layer.

[0076] S3. Growth of drift layer.

[0077] S4. Fabricate a p-type shielding layer.

[0078] S5. Make ohmic contact.

[0079] S6. Making Schottky contacts.

[0080] in,

[0081] Steps S1-S2, and steps S4-S6 are the same as those in Example 1, except that:

[0082] Step S3: growing a drift layer. Fig.10 and Fig.11 As shown, the drift layer comprises an nx first drift layer 31 and an n- second drift layer 30, and the nx first drift layer 31 and the n- second drift layer 30 are made of homogeneous materials, specifically comprising:

[0083] a. Fig.10 As shown, it is obtained by physical or chemical vapor deposition, specifically: a homogeneous material nx first drift layer 31 is grown on the n+ type buffer layer 20 by chemical vapor deposition or physical vapor deposition, the growth source is SiH4 and C2H4, or other Si source and C source gases, the doping source is NH3, and the growth temperature is 1500-1700°C. The nx first drift layer 31 forms a gradient doping structure from bottom to top. Specifically, the doping concentration of the nx first drift layer 31 gradually increases from low to high from bottom to top, and then gradually decreases from high to low. For example, the high doping concentration in the middle part of the nx first drift layer 31 is 1×10 16 cm -3 ~1×10 17 cm -3 The thickness accounts for about 50% of the total thickness of the nx first drift layer 31. The low-doping concentration of the upper and lower sides of the nx first drift layer 31 is 1×10 15 cm -3 ~1×10 16 cm -3 , respectively, the thickness accounts for about 25% of the total thickness of the nx first drift layer 31. The thickness of the nx first drift layer 31 accounts for 50-80% of the thickness of the entire drift layer. For example, the thickness of the nx first drift layer 31 is 5-8 μm.

[0084] b. Fig.11 As shown, it is obtained by physical or chemical vapor deposition, specifically: a homogeneous material n-second drift layer 30 is grown on the nx first drift layer 31 by chemical vapor deposition or physical vapor deposition, the growth source is SiH4 and C2H4, or other Si source and C source gases, the doping source is NH3, and the growth temperature is 1500-1700° C. The thickness of the n-second drift layer 30 accounts for 20-50% of the thickness of the entire drift layer.

[0085] Finally, after steps S4-S6, a SiC Schottky device with low on-state voltage drop and high reliability is prepared. Fig.11 shown.

[0086] Comparative Example 1

[0087] The difference between Comparative Example 1 and Example 1 is that the drift layer adopts a uniform doping concentration of 5×10 15 cm -3 , thickness is 10μm.

[0088] refer to Fig.12 It can be seen that when the nx first drift layer structure of Example 1 is adopted, that is, the middle part 1×10 16 cm -3 and 5×10 on both sides 15 cm -3 The doping structure is changed, and the on-current is significantly improved. At the same time, during reverse blocking, a higher possibility of collision ionization occurs at the main junction PN, while the possibility of collision ionization on the side close to the n+ type buffer layer 20 is significantly reduced, thereby reducing the possibility of material defect expansion on one side of the buffer layer and improving the long-term stability of the device.

[0089] The above embodiments are only used to further illustrate a SiC-based Schottky device with low on-state voltage drop and a preparation method thereof of the present invention, but the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention fall within the protection scope of the technical solution of the present invention.

Claims

1. A SiC-based Schottky device with low on-state voltage drop, characterized in that: The invention comprises an ohmic contact electrode, an n++ type SiC substrate, an n+ type SiC buffer layer, a SiC drift layer and a Schottky contact electrode from bottom to top; the drift layer comprises an nx first drift layer and an n- second drift layer from bottom to top, the doping concentration of the nx first drift layer is greater than the doping concentration of the n- second drift layer, and a p-type shielding layer is arranged inside the n- second drift layer; the doping concentration of the nx first drift layer changes from low to high doping from bottom to top, and then changes from high to low doping; the doping concentration of the nx first drift layer does not exceed 1×10 17 cm -3 , minimum not less than 1×10 15 cm -3 The doping concentration of the n-second drift layer is in the range of 1×10 14 ~5×10 15 cm -3 The doping concentration range of the n+ type buffer layer is 1×10 18 ~1×10 19 cm -3 .

2. The SiC-based Schottky device with low on-state voltage drop according to claim 1, characterized in that: The thickness of the nx first drift layer accounts for 50-80% of the thickness of the entire drift layer.

3. The SiC-based Schottky device with low on-state voltage drop according to claim 1, characterized in that: The p-type shielding layer is disposed on the upper portion of the n-second drift layer, and a distance between a bottom of the p-type shielding layer and a bottom of the n-second drift layer is 50-90% of a thickness of the n-second drift layer.

4. The SiC-based Schottky device with low on-state voltage drop according to claim 1, characterized in that: The nx first drift layer includes at least three stacked doping layers, and the doping concentration of the middle doping layer is greater than the doping concentration of the doping layers on the upper and lower sides.

5. The SiC-based Schottky device with low on-state voltage drop according to claim 4, characterized in that: The thickness of the middle doping layer accounts for 40% to 60% of the thickness of the nx first drift layer.

6. The SiC-based Schottky device with low on-state voltage drop according to claim 1, characterized in that: The nx first drift layer is gradually doped from bottom to top, and the doping concentration is first from low to high and then from high to low.

7. A method for preparing a SiC-based Schottky device with low on-state voltage drop according to any one of claims 1 to 6, characterized in that: The following steps are involved: 1) Growing a homogeneous n+ type buffer layer on an n++ type SiC substrate; 2) growing a homogeneous material drift layer, first growing an nx first drift layer with a higher doping concentration on the n+ type buffer layer, and then growing an n- second drift layer with a lower doping concentration on the nx first drift layer; growing the drift layer by chemical vapor deposition process, the growth temperature is 1500-1700 ° C, the growth source is Si source and C source gas, the doping source is NH3, and the doping concentration is controlled by controlling the doping source flow rate; 3) Producing a p-type shielding layer in the n-second drift layer by a local doping process; 4) Fabricating an ohmic contact electrode on the back side of the n++ type SiC substrate; 5) Fabricate a Schottky contact electrode on the n-second drift layer.

8. The preparation method according to claim 7, characterized in that: During the growth of the nx first drift layer, the flow rate of the doping source changes in stages or gradually.

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