Plasma etching method

Abstract

The present invention provides a plasma etching method. Inductively coupled radio frequency excitation of a mixed gas, such as fluoromethane, oxygen, a sulfur-containing oxygen gas, and hydrogen bromide, forms a plasma. After the plasma is formed, the method, under the action of pulsed bias radio frequency, etches a high-aspect-ratio hole formed by the pre-plasma etching. The method is suitable for etching a silicon nitride layer at the bottom of a high-aspect-ratio hole, which comprises a stack of dielectric and conductive layers. The method has a high selectivity for mask layers and silicon dioxide layers, and can generate a protective deposition layer on the top and sides of the mask, reducing plasma consumption of the mask. Simultaneously, the method inhibits the decomposition of the silicon dioxide layer while maintaining a high silicon nitride etching rate. This method effectively suppresses the risk of increased leakage current due to large dielectric layer loss, ensuring the safe operation of the resulting electronic device. By using pulsed bias radio frequency, the method eliminates the accumulation of small molecule polymers caused by charged particles at the bottom of the hole, maintaining a high and stable silicon nitride etching rate.

Description

Technical Field

[0001] The present invention relates to the technical field of plasma etching, and in particular to a plasma etching method with high selectivity for holes with high aspect ratio. Background Art

[0002] As the size of semiconductor devices continues to shrink and the integration density continues to increase, the application of high aspect ratio (HAR) connection hole etching in memory devices is gradually increasing. Among them, silicon nitride (Si3N4) is a commonly used material in memory devices and is usually used as a separator between multiple layers of conductive / dielectric alternating materials. In the etching process of high aspect ratio connection holes in memory devices, the etching of the bottom Si3N4 layer becomes increasingly important. The main reasons are as follows: 1) Due to subsequent process requirements, the Si3N4 layer should be completely etched away and the loss of the dielectric layer (usually silicon oxide) below it should be as small as possible to avoid an increase in leakage current, that is, the Si3N4 / dielectric layer has a high selectivity; 2) For high HAR holes with a top mask of photoresist or amorphous carbon, the Si3N4 / mask is also required to have a high selectivity to avoid the risk of dielectric breakdown due to the large lateral loss of the dielectric layer material under the mask. How to simultaneously improve the selectivity of Si3N4 / dielectric layer and Si3N4 / mask layer is of great significance for etching Si3N4 at the bottom of high HAR holes.

[0003] Existing technologies usually use fluoromethane (CH3F) and oxygen (O2) to etch silicon nitride, wherein fluoromethane reacts with silicon nitride to generate volatile gas after being excited into plasma, and oxygen prevents the active groups of fluoromethane plasma from generating polymer deposition and reduces the etching of silicon nitride, thereby achieving the purpose of etching. When plasma etching is performed on holes with high aspect ratios, the etching rate generally tends to decrease or drop to zero as the depth increases. When using the mixed gas of fluoromethane (CH3F) and oxygen (O2) in the prior art to etch holes with high aspect ratios, the etching rate generally decreases or drops to zero as the depth increases. Figure 2 When etching the Si3N4 layer at the bottom of the high-aspect-ratio hole shown in the figure, the etching stops, and the top mask layer suffers significant vertical and lateral losses (the mask layer contains a large amount of carbon, and the more oxygen it contains, the greater the loss). The lateral losses, in particular, can easily expose the underlying dielectric layer to plasma, increasing the risk of dielectric breakdown in the electronic device. Furthermore, at the bottom of the deep hole, the silicon nitride layer is covered by a silicon dioxide layer (SiO2). To ensure complete etching of the silicon nitride, sufficient overetching (typically 20-50%) is required. At this point, if the silicon nitride / silicon dioxide selectivity is low, that is, only a small amount of etching of the silicon dioxide layer results in significant silicon dioxide loss, increased leakage current, and the resulting semiconductor device also faces the risk of dielectric breakdown. Therefore, finding an effective etching method that simultaneously achieves high selectivity between the Si3N4 at the bottom of the high-aspect-ratio hole and the underlying silicon oxide layer and the top mask layer is of great practical significance. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a plasma etching method, providing an etching object, wherein the etching object is a hole with an aspect ratio greater than 10 and less than 30, the hole bottom comprising a silicon dioxide (SiO2) layer and a silicon nitride (Si3N4) layer covering the first silicon dioxide layer, and the hole wall comprising a mask layer, a dielectric layer and / or a conductive layer; the method comprises the following steps:

[0005] a) introducing reaction gases including fluoromethane (CH3F), oxygen (O2), sulfur-containing oxygen gas and hydrogen bromide (HBr) into a reaction chamber;

[0006] b) applying high frequency radio frequency to excite the gas into plasma;

[0007] c) applying bias radio frequency to etch the silicon nitride layer.

[0008] Optionally, the diameter of the pore is 100-500 nanometers.

[0009] Optionally, the conductive layer is made of polysilicon or nitride material, and the dielectric layer is made of silicon oxide material.

[0010] Optionally, the conductive layer is silicon nitride or silicon oxynitride.

[0011] Optionally, the hole wall is formed by alternating stacking of conductive layers and dielectric layers, and the thickness of the conductive layer or dielectric layer is 20-40 nanometers.

[0012] Optionally, the hole wall further includes a second silicon dioxide layer located below the mask layer.

[0013] Optionally, the mask layer material is photoresist (PR) or amorphous carbon (ACL).

[0014] Optionally, the sulfur-containing oxygen gas is hydroxyl sulfide (COS) or sulfur dioxide (SO2).

[0015] Optionally, the reaction gas also includes an inert gas.

[0016] Optionally, the inert gas is argon (Ar) or helium (He).

[0017] Optionally, the gas pressure in step a) is 10-80 mT, the flow rate of the fluoromethane is in the range of 100-200 sccm, and the flow rate of the inert gas is in the range of 0-230 sccm;

[0018] The flow ratio of oxygen to fluoromethane is (0.66-1.5):1;

[0019] The flow ratio of hydrogen bromide to oxygen is (0.45-0.85):1;

[0020] The flow ratio of sulfur-containing oxygen gas to oxygen is 0.3-1:1.

[0021] Optionally, the bias radio frequency is a pulsed radio frequency.

[0022] Optionally, the high-frequency radio frequency is 10-20 MHz, the power is 100-1500 W; the bias radio frequency is 100-800 KHz, the power is 50-800 W, and the duty cycle is 10%-90%.

[0023] Optionally, step c) is carried out at a temperature ranging from -10°C to 60°C.

[0024] Optionally, the thickness of the first silicon dioxide layer is 20-40 nanometers, and the thickness of the silicon nitride layer is 100-200 nanometers.

[0025] The advantages of the present invention are: the present invention provides a plasma etching method, which is suitable for holes with high aspect ratios composed of stacks of silicon nitride and silicon dioxide. While ensuring that the bottom silicon nitride layer has a high selectivity to the silicon dioxide layer below it, the selectivity of the silicon nitride / mask layer is significantly improved, and a protective deposition layer is generated on the top and sides of the mask, which reduces the consumption of the mask by the plasma and significantly improves the retention of the mask (especially the lateral retention). By using pulsed bias radio frequency, the deposition of polymers caused by the accumulation of charged particles at the bottom of the hole can be eliminated, and a high and stable silicon nitride etching rate can be maintained. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 A schematic structural diagram of an inductively coupled plasma processing device for implementing the present method is shown;

[0028] Figure 2 A schematic diagram of a high aspect ratio hole structure processed by the present method is shown;

[0029] Figure 3 A schematic diagram showing the structure after processing a high aspect ratio hole using the prior art;

[0030] Figure 4 A schematic diagram showing the structure after processing a high aspect ratio hole using the etching method of the present invention;

[0031] Figure 5A A line graph showing the effect of adding HBr gas on the etching rate;

[0032] Figure 5B A line graph showing the effect of adding COS gas on the etching rate;

[0033] Figure 5C A line graph showing the effect of adding COS gas on the etching rate under the condition of adding 100 sccm HBr gas;

[0034] Figure 5D A line graph showing the effect of adding HBr gas on the selectivity;

[0035] Figure 5E A line graph showing the effect of adding COS gas on the selectivity;

[0036] Figure 5F The line graph shows the effect of adding COS gas on the selectivity under the condition of adding 100 sccm HBr gas. DETAILED DESCRIPTION

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0038] like Figure 1FIG. 1 illustrates an inductively coupled plasma processing apparatus for implementing the etching method of the present invention. The apparatus in this embodiment primarily includes a reaction chamber. A susceptor 101 is disposed within the reaction chamber. An electrostatic chuck 102 is positioned above susceptor 101 to support a substrate 103. An edge ring assembly 20 is positioned around susceptor 101 to adjust the plasma coupling strength. A confinement ring 108 is positioned around edge ring assembly 20 to confine the plasma above substrate 103. A grounding ring 109 is positioned below confinement ring 108 to form a radio frequency (RF) loop. An insulating window 130 is positioned above the reaction chamber, and an inductive coil 140 is positioned above the insulating window. A high-frequency RF power supply 145 applies an RF signal to inductive coil 140, generating an alternating magnetic field that induces an alternating electric field within the reaction chamber, thereby dissociating the reactant gas entering the reaction chamber into the plasma. Reactive gas can be injected into the reaction chamber through the sidewalls, or a gas injection port can be provided in the insulating window to allow process gas to enter. The bias RF power is applied to the lower electrode assembly through a bias RF matching to control the energy distribution of the plasma. Before plasma etching, high-pressure gas is first introduced to clean the top and side walls of the reaction chamber, and then low-pressure gas is introduced to clean the base.

[0039] like Figure 2 The figure shows a high aspect ratio hole structure on a target substrate processed by the present method. Before using the present method, a first silicon dioxide layer 5, a silicon nitride layer 4, and an intermediate layer 3 on the silicon nitride layer 4 are formed by deposition on the target substrate. The intermediate layer 3 is formed by repeated stacking of a conductive layer and a dielectric layer, wherein the conductive layer can be a polycrystalline silicon material, which can be n-doped polycrystalline silicon, p-doped polycrystalline silicon, silicon nitride or silicon oxynitride according to the requirements of different semiconductor devices; the dielectric layer can be silicon oxide according to the requirements of different semiconductor devices; in other embodiments, the last layer of the intermediate layer 3 is a dielectric layer, such as silicon dioxide, and the thickness of the conductive layer and the dielectric layer of the intermediate layer 3 is between 20-40 nanometers, the thickness of the silicon nitride layer 4 at the bottom of the deep hole is between 100-200 nanometers, and the thickness of the first silicon dioxide layer 5 is between 20-40 nanometers. In some embodiments, a second silicon dioxide layer 2 is formed on the intermediate layer 3, which can provide a certain degree of protection for the intermediate layer 3 during the etching process. A mask layer 1 is formed on the second silicon dioxide layer 2, which can be made of amorphous carbon (ACL) or photoresist (PR). In this embodiment, ACL is selected as the mask layer 1 because PR and ACL have similar compositions and can achieve the same technical effect. According to the shape of the mask layer 1, in the preceding step, a mask layer 1 is formed by plasma etching. Figure 2 The deep holes shown have an aspect ratio greater than 10 and less than 30, and a diameter between 100-500 nm.

[0040] In the prior art, silicon nitride (Si3N4) is etched by fluoromethane (CH3F) and oxygen (O2), and the specific reaction is as follows:

[0041] CH3F+e→CH*+F*

[0042] Si3N4→Si*+N*

[0043] CH*+N*→HCN↑

[0044] O*+N*→NO↑+NO2↑

[0045] Si*+F*→SiF4↑

[0046] Among them, HCN, NO, NO2 and SiF4 are all volatile gases, and silicon nitride is removed based on this principle.

[0047] Fluoromethane will also etch silicon dioxide. The specific reaction is as follows:

[0048] SiO2→Si*+O*

[0049] Si*+F*→SiF4↑

[0050] The oxygen in the reaction gas inhibits the decomposition of silicon dioxide, thereby increasing the silicon nitride / silicon dioxide selectivity and preventing the first silicon dioxide layer 5 from being over-etched, thereby causing the risk of increased leakage current.

[0051] However, since the mask layer 1 contains a large amount of carbon (C), the oxygen atoms after plasma activation react with the carbon to generate volatile gases, such as CO and CO2. As a result, during the complete etching of the silicon nitride, the top and sidewalls of the mask layer 1 may be over-etched, thereby increasing the risk of damage to the intermediate layer 3. Even if a second silicon dioxide layer 2 is added, it may be over-consumed, increasing the risk of dielectric breakdown of the device. Figure 3 As shown, using the existing method, for holes with high aspect ratios, the silicon nitride at the bottom is difficult to etch. During the etching of the silicon nitride, the mask layer 1 will be over-consumed. When the side of the mask layer 1 is consumed, the second silicon dioxide layer 2 will be exposed to the plasma, especially the shoulder of the second silicon dioxide layer 2 will be etched, which increases the risk of leakage current and causes the thickness to fail to meet the device requirements.

[0052] The etching method of the present invention uses fluoromethane and oxygen as reaction gases, and adds hydroxyl sulfide (COS) and hydrogen bromide (HBr) gases. In other embodiments, sulfur dioxide can also be used instead of hydroxyl sulfide gas. Figures 5A-5CThe figure shows the effect of adding HBr and COS gases on the etching rate when the amount of fluoromethane and oxygen is constant. SN represents the etching line of silicon nitride, ACL represents the etching line of the mask, and OX represents the etching line of silicon dioxide. The horizontal axis represents the flow rate in sccm. The left vertical axis corresponds to the SN and ACL etching rate lines, and the right vertical axis corresponds to the OX etching rate line. Figure 5A As shown in Figure 2, with the increase of HBr, the etching rate of the mask first decreases significantly, then maintains a stable state, and finally increases slightly. The etching rate of silicon nitride first increases and then decreases, while the etching rate of silicon dioxide first decreases and then increases. Figure 5B As shown in Figure 2, with the increase of COS, the etching rate of the mask first decreases significantly and then maintains a stable state. The etching rate of silicon nitride decreases slowly, while the etching rate of silicon dioxide first increases and then decreases. Figure 5A and 5B ,exist Figure 5C In the figure, after maintaining the HBr gas at 100 sccm, COS gas is added. It can be seen that the etching rate of the mask is first stable, then significantly decreases, and finally slowly decreases. The etching rate of silicon nitride decreases slowly, while the etching rate of silicon dioxide first decreases, then increases, and finally slowly decreases. Therefore, according to the changing trends of the three curves, the content of HBr and COS can be selected to adjust the etching rate according to actual needs. Figures 5D-5F The effect of adding HBr and COS gases on the selectivity when the amount of fluoromethane and oxygen is constant is shown in the figure, where SN / OX is the selectivity of silicon nitride / silicon dioxide, SN / ACL is the selectivity of silicon nitride / mask layer, the horizontal axis represents the flow rate in sccm, the left vertical axis corresponds to the selectivity of silicon nitride / silicon dioxide, and the right vertical axis corresponds to the selectivity of silicon nitride / mask layer. Figure 5D As shown in FIG, with the increase of HBr, the selectivity of silicon nitride / mask layer first increases and then decreases, while the selectivity of silicon nitride / silicon dioxide first increases and then decreases and finally stabilizes; Figure 5E As shown in Figure 2, the selectivity of silicon nitride / mask layer first increases, then decreases, and finally stabilizes, while the selectivity of silicon nitride / silicon dioxide first decreases and then slowly increases. Figure 5D and 5E ,exist Figure 5F In the figure, after maintaining the HBr gas at 100 sccm, COS gas is added. It can be seen that the selectivity of silicon nitride / mask layer first decreases, then increases, and finally decreases, while the selectivity of silicon nitride / silicon dioxide increases, then decreases, and finally stabilizes. Therefore, according to the changing trends of the three curves, the appropriate ratio can be selected to achieve the purpose of simultaneously improving the selectivity of silicon nitride / silicon dioxide and the selectivity of silicon nitride / mask layer.

[0053] The fact that COS and HBr can improve the selectivity of silicon nitride / mask layer can be explained by the following principle:

[0054] HBr→H*+Br*

[0055] Si*+Br*+O*→SiBrO2

[0056] SiBrO2 will cover the top of the mask layer to prevent the oxygen in the plasma from etching the top of the mask layer.

[0057] COS+N*→C=S+C=N+O*

[0058] The C═S and C═N groups cover the sidewalls of the mask layer to prevent oxygen in the plasma from etching the sidewalls of the mask layer.

[0059] That is to say, the selectivity ratio of silicon nitride / mask layer is improved. Figures 5D-5F It can be seen that compared with the effect of increasing the gas flow rate of COS or HBr alone on the selectivity, increasing the COS and HBr gases simultaneously can further improve the selectivity of silicon nitride to silicon dioxide. Therefore, the present invention achieves a higher silicon nitride to mask layer selectivity and silicon nitride to mask layer selectivity by adjusting the ratio of oxygen, hydrogen bromide and hydroxyl sulfide, especially significantly improving the silicon nitride to mask layer selectivity, which can achieve the following: Figure 4 As shown in the etching effect, after the etching is completed, the mask layer 1 can achieve small loss in both height and width, and the second silicon dioxide layer 2 and the intermediate layer 3 are properly protected. At the same time, the silicon nitride layer 4 is completely etched, and the first silicon dioxide layer 5 is only slightly over-etched.

[0060] In some embodiments, the high-frequency radio frequency is 10-20 MHz, the power is 100-1500 W; the bias radio frequency is 100-800 KHz, the power is 50-800 W, and the duty cycle is 10%-90%. In order to release the charged particles in the deep hole and prevent excessive polymer accumulation from significantly slowing down the etching rate, it is necessary to apply pulse bias pulse radio frequency, so that in one cycle, during the non-working period of radio frequency, it is more conducive to the discharge of polymer from the deep hole. Using low-power high-frequency radio frequency to excite plasma can increase the mean free path of plasma, making it easier for plasma to reach the bottom of the hole; relatively high-power bias radio frequency is beneficial to enhance the bombardment energy and directionality of positive ions in the plasma, making it easier for plasma to reach the bottom of the hole and maintain a high etching rate.

[0061] In some embodiments, the gas composition of the present invention further includes an inert gas as a carrier gas to make the mixed gas more uniform and reduce the plasma excitation barrier. Specifically, helium, argon, etc. can be selected.

[0062] In some embodiments, the etching method of the present invention is performed at -10°C to 60°C.

[0063] Comparative Example 1

[0064] Under the conditions of 300W high-frequency RF power, 13.56MHz frequency, 150W bias RF power, 400KHz frequency, pulse mode, 40% duty cycle, and 40mT gas pressure, 180sccm CH3F gas, 120sccm O2 gas, and 200sccm He gas were selected. HBr and COS were not added. After the plasma was excited, the blank wafer and the patterned wafer were etched. The results are shown in Table 1:

[0065] Table 1 Etching results of comparative example 1

[0066]

[0067]

[0068] Comparative Example 2

[0069] Under the conditions of high-frequency RF with a power of 300W and a frequency of 13.56MHz, bias RF with a power of 150W and a frequency of 400KHz, pulse mode, a duty cycle of 40%, and a gas pressure of 40mT, 180sccm CH3F gas, 120sccm O2, 200sccmHe, and 36sccmCOS were selected to ignite the plasma and then etch the blank wafer and the patterned wafer. The results are shown in Table 2.

[0070] Table 2 Etching results of comparative example 2

[0071]

[0072] Comparative Example 3

[0073] Under the conditions of high-frequency RF with a power of 300W and a frequency of 13.56MHz, bias RF with a power of 150W and a frequency of 400KHz, pulse mode, a duty cycle of 40%, and a gas pressure of 40mT, 180sccm CH3F gas, 120sccm O2, 200sccmHe, and 50sccmHBr were selected to ignite the plasma and then etch the blank wafer and the patterned wafer. The results are shown in Table 3.

[0074] Table 3 Etching results of Comparative Example 3

[0075]

[0076] Example 1

[0077] Under the conditions of high-frequency RF with a power of 300W and a frequency of 13.56MHz, bias RF with a power of 150W and a frequency of 400KHz, pulse mode, a duty cycle of 40%, and a gas pressure of 40mT, 180sccm CH3F gas, 120sccm O2, 200sccmHe, 36sccmCOS, and 50sccmHBr were selected to ignite the plasma and then etch the blank wafer and the patterned wafer. The results are shown in Table 4.

[0078] Table 4 Etching results of Example 1

[0079]

[0080] Example 2

[0081] Under the conditions of high-frequency RF with a power of 300W and a frequency of 13.56MHz, bias RF with a power of 150W and a frequency of 400KHz, pulse mode, a duty cycle of 40%, and a gas pressure of 40mT, 180sccm CH3F gas, 120sccm O2, 200sccmHe, 48sccmCOS, and 100sccmHBr were selected to ignite the plasma and then etch the blank wafer and the patterned wafer. The results are shown in Table 5.

[0082] Table 5 Etching results of Example 2

[0083]

[0084] Through the above five sets of etching results, it can be concluded that the etching rate and selectivity results obtained by the etching simulation on the blank wafer are consistent with the actual pattern wafer etching results. The selectivity results of the blank wafer can be used as a representation of the selectivity of the etching of the actual pattern wafer using the reaction conditions. From the above embodiments, it can be seen that compared with Comparative Examples 1-3, the present invention adds the etching method of HBr and COS gases at the same time, significantly improving the selectivity of silicon nitride / mask layer and the selectivity of silicon nitride / mask layer. In addition, when the loss of the ACL mask layer is actually measured, the loss is also significantly reduced. A relatively high silicon nitride etching rate is maintained. The flow rate of HBr and COS gases can be adjusted according to the process requirements and the time connection between different etching processes to control the etching rate of silicon nitride while ensuring a high selectivity.

[0085] The selectivity data obtained by adjusting the gas pressure, high-frequency RF power and bias RF power are shown in Table 6, where the bias RF adopts a pulse mode with a duty cycle of 40%.

[0086] Table 6 Selectivity results under different conditions

[0087] Gas pressure mT High frequency RF power W Bias RF power W <![CDATA[Si3N4 / SiO2]]> <![CDATA[Si3N4 / ACL]]> 40 300 150 17.15 1.37 20 300 150 14.72 1.19 40 400 150 16.12 1.26 40 300 250 10.56 1.28 60 300 150 19.22 1.42 40 200 150 18.53 1.44

[0088] It can be seen from Table 6 that with the changes in pressure, high-frequency RF power and bias RF power, both selectivities are maintained at relatively high values, that is, the method conditions used in the etching method of the present invention have a relatively wide selection range.

[0089] The etching method of the present invention aims to retain the most of the mask layer and the silicon dioxide layer after completely etching the silicon nitride, that is, to have a high silicon nitride / mask layer selectivity and a high silicon nitride / mask layer selectivity.

[0090] The present invention provides a plasma etching method suitable for high-aspect-ratio holes composed of a stack of silicon nitride and silicon dioxide. While ensuring a high selectivity of the bottom silicon nitride layer to the underlying silicon dioxide layer, the selectivity of the silicon nitride / mask layer is significantly improved. A protective deposition layer is generated on the top and sides of the mask, reducing the consumption of the mask by the plasma and significantly increasing the mask retention (especially the lateral retention). By using pulsed bias radio frequency, polymer deposition caused by the accumulation of charged particles at the bottom of the hole can be eliminated, maintaining a high and stable silicon nitride etching rate.

[0091] The etching method disclosed in the present invention only takes an inductively coupled plasma processing device as an example in the embodiment, but does not limit the equipment used in the method of the present invention. It can also be applied to other plasma processing devices and will not be described in detail here.

[0092] Although the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description is not intended to limit the present invention. After reading the above description, various modifications and substitutions of the present invention will become apparent to those skilled in the art. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A plasma etching method, characterized in that: An etching object is provided, wherein the etching object is a hole with an aspect ratio greater than 10 and less than 30, wherein the hole bottom comprises a first silicon dioxide (SiO2) layer and a silicon nitride (Si3N4) layer covering the first silicon dioxide layer, and the hole wall comprises a mask layer, a dielectric layer and / or a conductive layer; the method comprises the following steps: a) introducing reaction gases including fluoromethane (CH3F), oxygen (O2), sulfur-containing oxygen gas and hydrogen bromide (HBr) into a reaction chamber; b) applying high frequency radio frequency to excite the gas into plasma; c) applying bias radio frequency to etch the silicon nitride layer; The flow ratio of oxygen to fluoromethane is (0.66-1.5):1; The flow ratio of hydrogen bromide to oxygen is (0.45-0.85):1; The flow ratio of sulfur-containing oxygen gas to oxygen is (0.3-1):1; While ensuring that the silicon nitride layer has a high selectivity to the first silicon dioxide layer, the selectivity of the silicon nitride layer to the mask layer is improved.

2. The etching method according to claim 1, wherein: The diameter of the pores is 100-500 nanometers.

3. The etching method according to claim 1, wherein: The conductive layer is made of polysilicon or nitride material, and the dielectric layer is made of silicon oxide material.

4. The etching method according to claim 3, wherein: The conductive layer is silicon nitride or silicon oxynitride.

5. The etching method according to claim 1, wherein: The hole wall is formed by alternating stacking of conductive layers and dielectric layers, and the thickness of the conductive layer or dielectric layer is 20-40 nanometers.

6. The etching method according to claim 1, wherein: The hole wall further includes a second silicon dioxide layer located below the mask layer.

7. The etching method according to claim 1, wherein: The mask layer material is photoresist (PR) or amorphous carbon (ACL).

8. The etching method according to claim 1, wherein: The sulfur-containing oxygen gas is hydroxyl sulfide (COS) or sulfur dioxide (SO2).

9. The etching method according to claim 8, characterized in that , the reaction gas also includes an inert gas.

10. The etching method according to claim 9, wherein: The inert gas is argon (Ar) or helium (He).

11. The etching method according to claim 10, wherein: The gas pressure in step a) is 10-80 mT, the flow rate of the fluoromethane is in the range of 100-200 sccm, and the flow rate of the inert gas is in the range of 0-230 sccm.

12. The etching method according to claim 1, wherein: The bias radio frequency is a pulsed radio frequency.

13. The etching method according to claim 12, wherein: The high-frequency radio frequency is 10-20 MHz, the power is 100-1500 W; the bias radio frequency is 100-800 KHz, the power is 50-800 W, and the duty cycle is 10%-90%.

14. The etching method according to claim 1, wherein: The step c) is carried out at a temperature ranging from -10°C to 60°C.

15. The etching method according to claim 1, wherein: The thickness of the first silicon dioxide layer is 20-40 nanometers, and the thickness of the silicon nitride layer is 100-200 nanometers.

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