Method for removing silicon nitride after LOCOS process

Abstract

The application discloses a method for removing silicon nitride after LOCOS process, which comprises the following steps: before the formal gate oxide growth, depositing a silicon nitride film on the surface and forming a LOCOS window, growing an oxidation layer with a certain thickness in the middle of the poly gate two side channels by using a local oxidation process, and making the local oxidation thickening part located at the non-channel position, then removing the silicon nitride film, and finally performing the next process. The product obtained by using the method has a significant decrease in Qgd, and finally makes Tdoff decrease by about half, so that the product has an advantage when working at high frequency, and the switching loss is obviously reduced. The VDMOS product produced by the method is used in a smart phone charger, and compared with the conventional products, the surface temperature of the charger during the charging process is about 15 degrees lower than that of the conventional charger, the safety and energy consumption are ensured, the product is suitable for industrial production, and the customer satisfaction is improved.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor technology, specifically to MOSFET and IGBT products in the field of high-frequency design. It relates to a method for removing silicon nitride in VDMOS after employing the LOCOS (Local Oxidation of Silicon) process. Background Technology

[0002] VDMOS (vertical double-diffused metal oxide semiconductor) is a vertically conductive double-diffused power device with characteristics such as high input impedance, low drive power, fast switching speed, and good thermal stability. It also has a negative temperature coefficient and does not suffer from the secondary breakdown characteristic of bipolar diodes. These advantages have led to the increasingly widespread application of VDMOS devices in extreme and complex environments such as aerospace and nuclear engineering. However, this inevitably exposes them to the effects of strong radiation environments such as space radiation and nuclear radiation, causing changes in the device's electrical parameters, resulting in varying degrees of performance degradation, decreased reliability, and even complete device failure.

[0003] See Figure 2 In a normal VDMOS cell, the gate portion, channel portion, and JFET region are all in a flat state.

[0004] However, the LOCOS process requires a silicon nitride film. After localized thick oxygen growth in LOCOS, the silicon nitride film's function is complete. The most critical step is removing the silicon nitride film; improper handling can lead to severe phosphorus contamination and ultimately product failure. There are generally two processes for removing silicon nitride films: dry and wet. Dry removal uses plasma gas to etch away the silicon nitride film, but this method easily leaves residual silicon nitride particles, which can cause white spots after subsequent oxidation, making it less effective. Therefore, most processes utilize wet removal of the silicon nitride film. The conventional wet method uses hot phosphoric acid to remove the silicon nitride film. Wet processes can thoroughly remove the silicon nitride film without leaving residue, but prolonged hot phosphoric acid treatment can cause phosphorus contamination on the product surface. Summary of the Invention

[0005] To address the aforementioned issues, this invention provides a method for removing silicon nitride after the LOCOS process. The purpose of this invention is to effectively remove the silicon nitride film after the LOCOS process to prevent the negative effects of phosphorus contamination. The advantages of the product obtained using the method of this invention are more pronounced, and its reliability is guaranteed by HTRB, ensuring it passes the standard 168-hour test.

[0006] The main process of this invention is consistent with the currently mature VDMOS process, with the addition of the LOCOS process.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for removing silicon nitride after a LOCOS process, the method comprising: depositing a silicon nitride film on the surface and forming a LOCOS window before formal gate oxide growth; growing an oxide layer of a certain thickness in the middle of the channel on both sides of the polysilicon gate using a local oxidation process, such that the locally oxidized thickened part is located in the non-channel area; removing the silicon nitride film; and finally proceeding to the next process.

[0009] This invention is based on the existing VDMOS method and adds a LOCOS process. After local thick oxide growth in LOCOS, this invention first uses a wet process to remove the silicon nitride film, then removes the residual phosphoric acid solution on the surface and removes the oxide layer contaminated by phosphoric acid, and then regenerates silicon oxide. Finally, the regenerated silicon oxide is removed to obtain a satisfactory product.

[0010] As a preferred embodiment of the present invention, the thickness of the silicon nitride film is 1000-1500 angstroms and the width of the LOCOS window is 1-2 μm.

[0011] As a preferred embodiment of the present invention, the thickness of the locally oxidized thickened portion is 3000 to 6000 angstroms.

[0012] As a preferred embodiment of the present invention, the specific steps of the method are as follows:

[0013] 1) Thick oxygen growth forms a partial pressure ring master junction;

[0014] 2) Open the active region, inject JFET and anneal;

[0015] 3) Silicon nitride film deposition to form LOCOS windows;

[0016] 4) LOCOS thick oxygen growth;

[0017] 5) Wet removal of silicon nitride, a process for removing phosphorus contaminants;

[0018] 6) Normal gate oxide growth and polysilicon gate formation.

[0019] As a preferred embodiment of the present invention, step 5) specifically comprises:

[0020] A) Use hydrofluoric acid to clean the surface of the silicon nitride film on the product to remove the oxide layer;

[0021] B) Heat phosphoric acid to boil the product to completely remove silicon nitride;

[0022] C) At room temperature, treat the product obtained in step B) with phosphoric acid and wash with a continuous overflow of ultrapure water to remove the phosphoric acid solution;

[0023] D) Treat the product obtained in step C) with mixed acid to remove the oxide layer contaminated with phosphoric acid;

[0024] E) Clean the product obtained in step D) sequentially using standard solution No. 3, solution No. 1, and solution No. 2. After cleaning, enter the oxidation furnace to regenerate silicon oxide at a depth of 400-600 angstroms, and then remove the generated silicon oxide.

[0025] In a preferred embodiment of the present invention, in step A), the mass fraction of hydrofluoric acid is 5%; in step B), phosphoric acid is heated to 150°C and the cooking time is 140-160 min.

[0026] As a preferred embodiment of the present invention, in step C), the phosphoric acid treatment time of the product at room temperature is 8 to 12 minutes, and the ultrapure water washing time is ≥30 minutes.

[0027] In a preferred embodiment of the present invention, in step D), the mixed acid is 5% hydrofluoric acid and ammonium fluoride by mass, and the treatment time is 80-100s.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The product obtained using the method of this invention exhibits a significant reduction in Qgd, ultimately resulting in a reduction of Tdoff by approximately half. This gives the product an advantage during high-frequency operation, particularly with a significant reduction in switching losses. The VDMOS product manufactured using this method, when used in smartphone chargers, has shown, through comparisons with domestic and international customers, that compared to similar conventional products, the surface temperature of the corresponding charger is approximately 15 degrees Celsius lower during charging. This ensures both safety and energy efficiency, making it suitable for industrial production and enhancing customer satisfaction. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the locally thickened gate oxide cell of the present invention.

[0031] Figure 2 This is a schematic diagram of a normal VDMOS cell.

[0032] Figure 3 This is a schematic diagram of LOCOS growth under thick oxygen.

[0033] Figure 4 This is a schematic diagram of a wet process for removing phosphorus contaminants from silicon nitride. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Unless otherwise specified, the processes described in this invention are all existing processes.

[0036] Unless otherwise specified, all raw materials used in this invention can be purchased from the market.

[0037] This invention is based on the existing VDMOS method and adds a LOCOS process. After local thick oxide growth in LOCOS, this invention first uses a wet process to remove the silicon nitride film, then removes the residual phosphoric acid solution on the surface and removes the oxide layer contaminated by phosphoric acid, and then regenerates silicon oxide. Finally, the regenerated silicon oxide is removed to obtain a satisfactory product.

[0038] See Figure 1 The locally thickened gate oxide cell obtained by the method of this invention grows an oxide layer of sufficient thickness in the middle of the channel on both sides of the polysilicon gate using a local oxidation process before the formal gate oxide growth. Generally, it is about 3000 to 5000 angstroms. In this way, the Cgd capacitance can be significantly reduced. At the same time, since the locally thickened oxide layer is not on the channel, it will not have a negative impact on Vth. The locally thickened gate oxide technology will not affect the current working principle of VDMOS.

[0039] Example 1

[0040] The method for removing silicon nitride after the LOCOS process provided in this embodiment includes the following specific steps:

[0041] 1) Thick oxygen growth, partial pressure ring main segment formation.

[0042] 2) The active region is opened, and JFET injection and annealing are formed.

[0043] 3) Silicon nitride film deposition, with a thickness of about 1400 angstroms, LOCOS window opened, and the window width designed to be between 1 and 2 μm.

[0044] 4) LOCOS growth under heavy oxygen conditions, see [reference needed] Figure 3The principle of LOCOS thick oxide growth: A silicon nitride film is retained outside the window area. During oxide layer growth, because the silicon nitride film is very dense, it will prevent oxygen from entering the underlying silicon area. Therefore, in areas where silicon nitride film is present, oxide layer cannot grow, while in areas where silicon nitride is not present, oxide layer can grow normally, forming local oxide layer growth. The thickness of local LOCOS thick oxide growth is about 5000 to 6000 angstroms.

[0045] 5) For wet removal of silicon nitride and subsequent phosphorus contamination removal processes, please refer to... Figure 4 :

[0046] A: First, use 5% HF acid for a 30-second pretreatment to remove the oxide layer on the silicon nitride surface, preventing oxide residue from causing unclean removal of the silicon nitride film in the next step.

[0047] B: Heat the H3PO4 acid to 150 degrees Celsius, then boil the silicon nitride film for about 150 minutes to completely remove the silicon nitride.

[0048] C: Treat the product with H3PO4 acid at room temperature for 10 minutes, then rinse with continuously overflowing ultrapure water for more than 30 minutes to ensure that no H3PO4 liquid remains on the surface.

[0049] D: Then treat the surface of the product with a 5% HF + NH4F mixed acid for about 90 seconds to remove the oxide layer contaminated by phosphoric acid. After this step, the local thick oxide layer will be reduced by about 1000 to 1500 angstroms, and the final thick oxide layer of LOCOS will be about 4000 angstroms.

[0050] E: The product is cleaned using standard solution No. 3, solution No. 1, and solution No. 2. Then, it is placed in an oxidation furnace to regenerate 500 angstroms of silicon oxide. The purpose is to convert the contaminated silicon on the surface into silicon oxide. Then, the generated silicon oxide is removed. This step is then complete.

[0051] 6) Normal gate oxide growth and polysilicon gate formation.

[0052] 7) Conventional P-well, N+ and P+ formation in the source region.

[0053] 8) LTO oxide layer, BPSG film deposition, pore formation, front metal layer formation.

[0054] 9) Thinning is performed to form a metal layer on the back, resulting in the final product.

[0055] Comparative Example 1: The product obtained using the normal process.

[0056] Example 2

[0057] The product obtained in Example 1 and the product obtained by normal process were tested. After using the method of the present invention, the Cgd capacitance decreased significantly, which in turn caused the dynamic switching parameter data Tdoff to decrease significantly. The results are shown in Table 1.

[0058] Table 1. Test Results

[0059] Example 1 4.4nc 133ns Comparative Example 1 1.9nc 60ns

[0060] As can be seen from Table 1, after adopting the method of the present invention, Qgd decreases significantly, and Tdoff is reduced by about half, thus giving the product an advantage when operating at high frequency, especially with a significant reduction in switching losses.

[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any form or substance. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for removing silicon nitride after a LOCOS process, characterized in that, The method is as follows: Before the formal gate oxide growth, a silicon nitride film is deposited on the surface to form a LOCOS window. A certain thickness of oxide layer is grown in the middle of the channel on both sides of the polysilicon gate using a local oxidation process, and the thickened part of the local oxidation is located in the non-channel area. Then the silicon nitride film is removed, and finally the next process is carried out. The specific steps of the method are as follows: 1) Thick oxygen growth forms a partial pressure ring master junction; 2) Open the active region, inject JFET and anneal; 3) Silicon nitride film deposition to form LOCOS windows; 4) LOCOS thick oxygen growth; 5) Wet removal of silicon nitride, a process for removing phosphorus contaminants; 6) Normal gate oxide growth and polysilicon gate formation; Step 5) specifically involves: A) Use hydrofluoric acid to clean the surface of the silicon nitride film on the product to remove the oxide layer; B) Heat phosphoric acid to boil the product to completely remove silicon nitride; C) At room temperature, treat the product obtained in step B) with phosphoric acid and wash with a continuous overflow of ultrapure water to remove the phosphoric acid solution; D) Treat the product obtained in step C) with a mixed acid to remove the oxidized layer contaminated with phosphoric acid; E) Clean the product obtained in step D) sequentially using standard solution No. 3, solution No. 1, and solution No.

2. After cleaning, enter the oxidation furnace to regenerate silicon oxide at a depth of 400-600 angstroms, and then remove the generated silicon oxide.

2. The method for removing silicon nitride after the LOCOS process according to claim 1, characterized in that, The thickness of the silicon nitride film is 1000~1500 angstroms, and the width of the LOCOS window is 1~2 μm.

3. The method for removing silicon nitride after the LOCOS process according to claim 1, characterized in that, The thickness of the locally oxidized thickened areas is 3000~6000 angstroms.

4. The method for removing silicon nitride after the LOCOS process according to claim 1, characterized in that, In step A), the mass fraction of hydrofluoric acid is 5%; in step B), phosphoric acid is heated to 150°C and the cooking time is 140~160 min.

5. The method for removing silicon nitride after the LOCOS process according to claim 1, characterized in that, In step C), the phosphoric acid treatment time at room temperature is 8-12 min, and the ultrapure water washing time is ≥30 min.

6. The method for removing silicon nitride after the LOCOS process according to claim 1, characterized in that, In step D), the mixed acid is 5% hydrofluoric acid and ammonium fluoride by mass, and the treatment time is 80~100s.

Images