A Laser Annealed Silicon Carbide Ohmic Contact, Device and Its Preparation Method

Abstract

The present invention belongs to the field of semiconductor devices and relates to a laser annealed ohmic contact for silicon carbide, a device and a preparation method thereof. The N-type 4H-SiC substrate is subjected to RCA standard cleaning. There are two epitaxial layers provided on the N-type 4H-SiC substrate, namely a buffer layer and an N-type doped layer. The ion implantation method is used to fabricate a P-type doped region and an N-type doped region in the 4H-SiC epitaxial layer, and rapid thermal annealing is carried out to obtain an epitaxial wafer. A metal layer is deposited above the P-type and N-type regions of the epitaxial wafer to form a source electrode, and high-energy laser annealing is carried out on the source electrode region to form an ohmic contact. Through the cooperation of the Al / Ti / Ni or Ni / Ti metal system and the laser annealing process, the present invention enables the metal and silicon carbide to react to form a composite while forming silicon at the interface under a high energy density, effectively improving the ohmic contact effect, and can simultaneously achieve N-type and P-type ohmic contacts.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor devices, and relates to a laser annealing silicon carbide ohmic contact, a device and a preparation method thereof. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the overall understanding of the present invention, and is not necessarily regarded as an admission or any form of implication that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] In recent years, with the rapid expansion of the electric vehicle market and the growing demand for efficient and compact power electronic converters, the demand for SiC MOSFETs (metal-oxide-semiconductor field-effect transistors) has been continuously increasing. At the same time, the technology of SiC MOSFETs has also been continuously advancing, including the optimization of device structures (such as planar and planar trench structures), the improvement of gate oxide reliability, the acceleration of switching speed, and the reduction of switching losses. Although SiC MOSFETs have many advantages, their high cost and complex manufacturing process are still the key factors hindering their large-scale application. Therefore, how to reduce costs and increase production has become the focus of current research. Among them, as a new annealing process, laser annealing is superior to the existing rapid thermal annealing process in terms of production efficiency, and thus has always been a research hotspot. At present, the application of the laser annealing process in silicon carbide is in the back metal ohmic contact process of Schottky diodes, but only for N-type silicon carbide. In the MOSFET process, in addition to N-type doped silicon carbide in the source metal region, there is also a P-type doped region. In order to simultaneously achieve the N-type and P-type ohmic contact processes in the conventional rapid thermal annealing process, a compromise temperature is taken in the annealing process to achieve both at the same time, but this results in a decrease in the ohmic contact effect of both. Summary of the Invention

[0004] To solve the above problems, the present invention provides a laser annealing silicon carbide ohmic contact, a device and a preparation method thereof. Through the cooperation of the Al / Ti / Ni (aluminum / titanium / nickel) or Ni / Ti (nickel / titanium) metal system and the laser annealing process, at a high energy density, while the metal reacts with silicon carbide to form a complex, silicon is formed at the interface, effectively improving the ohmic contact effect, and simultaneously achieving N-type and P-type ohmic contacts. Compared with the RTP (rapid thermal annealing process), on the basis of ensuring the N-type specific contact resistivity, the specific contact resistivity of the P-type ohmic contact is reduced by two orders of magnitude.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In the first aspect of the present invention, a preparation method of a laser annealing silicon carbide ohmic contact is provided, including:

[0007] The N-type 4H-SiC (silicon carbide) substrate is cleaned according to the RCA standard. There are two epitaxial layers on the N-type 4H-SiC substrate, namely a buffer layer and an N-type doped layer;

[0008] The P-type doped region and the N-type doped region are fabricated in the 4H-SiC epitaxial layer by ion implantation, and then rapid thermal annealing is carried out to obtain an epitaxial wafer;

[0009] A metal layer is deposited above the P-type and N-type regions of the epitaxial wafer to form source electrodes, and high-energy laser annealing is performed on the source electrode regions to form ohmic contacts.

[0010] The thickness of the doped layer affects the performance of the SiC MOSFET. Therefore, in the present invention, the thickness of the doped layer is studied. Preferably, the thickness of the N-type doped layer is 10 μm - 25 μm.

[0011] When ion implantation is carried out in the SiC epitaxial layer, nitrogen (N) or phosphorus (P) is usually used for the N-type region, which are donor elements that are easily low-resistanced, while aluminum (Al) is usually used as an acceptor element for the P-type region. Therefore, preferably, the ions implanted by the ion implantation method are nitrogen ions and aluminum ions respectively.

[0012] The rapid thermal annealing process can repair the lattice and activate the doped ions. Therefore, in the present invention, the temperature of the rapid thermal annealing process is studied. Preferably, the temperature of the rapid thermal annealing is 1600 °C - 1650 °C.

[0013] The selection of the contact material can improve the ohmic contact performance. Therefore, through systematic research and experimental exploration in the present invention, it is found that: during laser annealing, Al / Ti / Ni or Ni / Ti reacts with silicon carbide to form a complex while forming Si (silicon), significantly improving the ohmic contact effect. Therefore, preferably, the metal layer is an Al / Ti / Ni composite metal layer or a Ni / Ti composite metal layer.

[0014] It is found through research that: when high-energy laser annealing is performed on the source electrode region, if the instantaneous temperature of the laser energy reaches above 2000 °C, the metal reacts with silicon carbide to form a complex (wherein, Ni reacts with Si to form a nickel-silicon complex, and Ti and Al react with carbon to form a copolymer), and at the same time, Si is formed at the interface, effectively reducing the contact resistance. Therefore, preferably, the conditions for the laser annealing are that the wavelength of the laser is 248 nm - 532 nm, and the energy density is 2.5 J / cm 2 -2.75 J / cm 2 .

[0015] In the second aspect of the present invention, an ohmic contact formed by the above method is provided.

[0016] The third aspect of the present invention provides a method for preparing a device, comprising:

[0017] Performing RCA standard cleaning on an N-type 4H-SiC substrate, where two epitaxial layers, namely a buffer layer and an N-type doped layer, are provided on the N-type 4H-SiC substrate;

[0018] Fabricating P-type and N-type doped regions in the 4H-SiC epitaxial layer by ion implantation and performing rapid thermal annealing to obtain an epitaxial wafer;

[0019] Preparing a gate oxide layer region above the epitaxial wafer and patterning it;

[0020] Depositing polysilicon or metal in the gate region as a gate electrode;

[0021] Depositing a metal layer above the P-type and N-type regions of the epitaxial wafer to form a source electrode, and performing laser annealing on the source electrode region to form an ohmic contact;

[0022] Depositing metal Ni on the back of the epitaxial wafer to form a drain electrode, and performing laser annealing on the drain electrode region to form an ohmic contact, thus obtaining the device.

[0023] Preferably, the conditions for the laser annealing are that the wavelength of the laser is 248 nm - 532 nm and the energy density is 1.8 J / cm 2 -2.75 J / cm 2 .

[0024] The fourth aspect of the present invention provides a device prepared by the above method.

[0025] Advantages of the present invention

[0026] (1) Through the cooperation of the Al / Ti / Ni or Ni / Ti metal system and the laser annealing process, the present invention effectively improves the ohmic contact effect by enabling the reaction between the metal and silicon carbide to form a complex and forming silicon at the interface under a high energy density, and can simultaneously achieve N-type and P-type ohmic contacts.

[0027] (2) The preparation method of the present invention is simple, the device has a small resistance, strong practicability, and is easy to promote. Brief description of the drawings

[0028] The specification drawings forming 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 an improper limitation of the present invention.

[0029] Figure 1 It is a schematic structural diagram after epitaxy and ion implantation of the present invention;

[0030] Figure 2 It is a schematic diagram after the preparation of the gate oxide layer and electrodes of the present invention;

[0031] Figure 3 This is a complete schematic diagram of the device structure of the present invention;

[0032] Figure 4 This is a schematic diagram for the preparation of the laser annealing P / N ohmic contact of the present invention;

[0033] Figure 5 This is the XRD (X-ray diffraction) test result of the annealed sample in Example 1 of the present invention;

[0034] Figure 6 This is the test result of the source N-type ohmic contact in Example 1 of the present invention;

[0035] Figure 7 The test result of the source P-type ohmic contact in Example 1 of the present invention;

[0036] Figure 8 This is the device preparation flow chart of the present invention;

[0037] Among them, 1 is the source N-type ion implantation region, 2 is the source P-type ion implantation region, 3 is the N-type epitaxial wafer, 4 is the gate oxide layer, 5 is the polysilicon gate, 6 is the source metal, 7 is the drain metal, 8 is the laser, and 9 is the SiC MOSFET. Detailed implementation manners

[0038] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0039] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are interpretations rather than limitations of the present invention.

[0040] Example 1

[0041] This embodiment is directed to the ohmic contact formation method when the source metal layer is a three-layer structure of Al / Ti / Ni / SiC. The process is as Figure 8 shown and includes:

[0042] Step 1: Use an N-type 4H-SiC substrate with a thickness of 350 μm, which has two epitaxial layers. The first layer is a 1-μm buffer layer with a doping concentration of 1×10 18 cm -3 , and the second layer is a 10-μm N-type doped layer with a doping concentration of 5×10 15 cm -3 . Perform the RCA standard cleaning process;

[0043] Step 2: Nitrogen ions and aluminum ions are implanted respectively by ion implantation to form an N-type region and a P-type region. The doping concentration of N-type ion implantation is 1×10 18 cm -3 , and the doping concentration of P-type ion implantation is 1×10 19 cm -3 to obtain a sufficiently low specific contact resistance; the structures after epitaxy and ion implantation are as Figure 1 shown.

[0044] Step 3: A rapid thermal annealing process is used to hold at 1600 °C for 15 min to repair the lattice and activate the doped ions;

[0045] Step 4: A gate oxide layer is prepared by dry oxidation and nitric oxide passivation;

[0046] Step 5: A polysilicon gate is prepared by low-pressure chemical vapor deposition; the structures after the gate oxide layer and the electrode are prepared are as Figure 2 shown,

[0047] Step 6: Ni 50 nm, Ti 50 nm, and Al 50 nm are sequentially deposited on the source electrode by magnetron sputtering and patterned by photolithography and etching;

[0048] Step 7: The back surface of SiC is thinned to reduce the on-resistance, and Ni 100 nm is deposited on the back drain by magnetron sputtering;

[0049] Step 8: As Figure 4 shown, a 248 nm laser is used. After shaping into a flat-top beam, the source electrode is laser-annealed at an energy density of 2.75 J / cm 2 , and the drain electrode is laser-annealed at 1.8 J / cm 2 . Ohmic contacts are formed on both the source and drain electrodes, and the device structure is as Figure 3 shown.

[0050] The XRD test results of the annealed sample are as Figure 5 shown, indicating the formation of Si at the interface.

[0051] The electrical properties of the sample are tested using a circular transmission line model (CTLM), and the specific contact resistance is calculated. The test results of the specific contact resistance are as Figure 6 and Figure 7 . The specific contact resistivity of the N-type and P-type ohmic contacts formed under this condition is 9.25×10 -6 Ω·cm 2 to 8.28×10 -7 Ω·cm 2, compared with the RTP process, on the basis of ensuring the specific contact resistivity of the N-type, the specific contact resistivity of the P-type ohmic contact is reduced by two orders of magnitude.

[0052] Example 2

[0053] This example is about the formation method of ohmic contact when the source metal layer is a two-layer structure of Ti / Ni / SiC.

[0054] Step 1: Use an N-type 4H-SiC substrate with a thickness of 350 μm, which has two epitaxial layers on it. The first layer is a 1-μm buffer layer with a doping concentration of 1×10 18 cm -3 , and the second layer is a 25-μm N-type doped layer with a doping concentration of 5×10 15 cm -3 . Perform the RCA standard cleaning process;

[0055] Step 2: Use the ion implantation method to implant nitrogen ions and aluminum ions respectively to form an N-type region and a P-type region. The doping concentration of N-type ion implantation is 1×10 18 cm -3 , and the doping concentration of P-type ion implantation is 1×10 19 cm -3 ;

[0056] Step 3: Use the rapid thermal annealing process to hold at 1650 °C for 15 min to repair the lattice and activate the doping ions;

[0057] Step 4: Use the methods of dry oxidation and nitric oxide passivation to prepare the gate oxide layer;

[0058] Step 5: Use the low-pressure chemical vapor deposition method to prepare the polysilicon gate;

[0059] Step 6: Use the magnetron sputtering method to deposit Ni 50 nm and Ti 50 nm on the source electrode in sequence, and pattern it by photolithography and etching methods;

[0060] Step 7: Use the thinning process to thin the substrate from the back to 120 μm to reduce the on-resistance, and use the magnetron sputtering method to deposit Ni 100 nm on the back drain;

[0061] Step 8: Use a 532-nm laser. After shaping it into a flat-top light beam, perform laser annealing on the source electrode at an energy density of 2.5 J / cm 2 , and perform laser annealing on the drain electrode at 2.0 J / cm 2 . Ohmic contacts are formed on both the source and drain electrodes.

[0062] The electrical properties of the sample were tested using the circular transmission line model (CTLM), and the specific contact resistance was calculated. The test results show that: the specific contact resistivity of the N-type and P-type ohmic contacts formed under this condition is 2×10 -5 Ω·cm 2 to 5×10 -6 Ω·cm 2 . Compared with the RTP process, on the basis of ensuring the N-type specific contact resistivity, the specific contact resistivity of the P-type ohmic contact is reduced by one order of magnitude.

[0063] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a laser annealed silicon carbide ohmic contact, characterized in that: include: Performing RCA standard cleaning on an N-type 4H-SiC substrate, wherein two epitaxial layers are arranged on the N-type 4H-SiC substrate, namely a buffer layer and an N-type doping layer; Using ion implantation to form a P-type doping region and an N-type doping region in a 4H-SiC epitaxial layer, and performing rapid thermal annealing to obtain an epitaxial wafer; Depositing a metal layer on the P-type and N-type regions of the epitaxial wafer to form a source electrode, and performing high-energy laser annealing on the source electrode region to form an ohmic contact; The metal layer is an Al / Ti / Ni composite metal layer or a Ni / Ti composite metal layer; The conditions of the high energy laser annealing are: the laser wavelength is 248nm-532nm, the energy density is 2.5J / cm2-2.75J / cm 2 , the instantaneous temperature of laser energy reaches over 2000℃; The thickness of the Al / Ti / Ni composite metal layer is Ni 50nm, Ti 50nm, Al 50nm; The thickness of the Ni / Ti composite metal layer is 50nm for Ni and 50nm for Ti.

2. The method for preparing a laser annealed silicon carbide ohmic contact according to claim 1, characterized in that: The thickness of the N-type doping layer is 10 μm-25 μm.

3. The method for preparing a laser annealed silicon carbide ohmic contact according to claim 1, characterized in that: The ions implanted by the ion implantation method are nitrogen ions and aluminum ions respectively.

4. The method for preparing a laser annealed silicon carbide ohmic contact according to claim 1, characterized in that: The temperature of the rapid thermal annealing is 1600°C-1650°C.

5. An ohmic contact formed by the method according to any one of claims 1 to 4.

6. A method for preparing a device, characterized in that: include: Performing RCA standard cleaning on an N-type 4H-SiC substrate, wherein two epitaxial layers are arranged on the N-type 4H-SiC substrate, namely a buffer layer and an N-type doping layer; Using ion implantation to form a P-type doping region and an N-type doping region in a 4H-SiC epitaxial layer, and performing rapid thermal annealing to obtain an epitaxial wafer; Preparing and patterning a gate oxide layer region above the epitaxial wafer; Depositing polysilicon or metal in the gate region for use as a gate electrode; Depositing a metal layer on the P-type and N-type regions of the epitaxial wafer to form a source electrode, and performing high-energy laser annealing on the source electrode region to form an ohmic contact; Metal Ni is deposited on the back of the epitaxial wafer to form a drain electrode, and the drain electrode region is laser annealed to form an ohmic contact. The metal layer is an Al / Ti / Ni composite metal layer or a Ni / Ti composite metal layer; The conditions for high-energy laser annealing are a laser wavelength of 248nm-532nm and an energy density of 2.5J / cm 2 -2.75J / cm 2 , the instantaneous temperature of laser energy reaches over 2000℃; The thickness of the Al / Ti / Ni composite metal layer is Ni 50nm, Ti 50nm, Al 50nm; The thickness of the Ni / Ti composite metal layer is 50nm for Ni and 50nm for Ti.

7. A device prepared by the method of claim 6.

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