A method of forming an air gap

Abstract

The application provides a method for forming air gap, comprising: providing a substrate, the substrate is provided with word line structures, the side and top of the word line structures are sequentially provided with oxide layer and redundant silicon nitride layer; removing the oxide layer and redundant silicon nitride layer on the top of the word line structures, and retaining the oxide layer and redundant silicon nitride layer on the side wall of the word line structures; removing part of the redundant silicon nitride layer; removing the remaining redundant silicon nitride layer to form air gap between adjacent word line structures; and forming self-aligned metal silicide on the word line structures. The height of the self-aligned metal silicide is much higher than that in the traditional process, thus the resistance of the self-aligned metal silicide is reduced, and the height of the air gap between the word line structures is also much higher than that in the traditional process, thus the capacitance between the word line structures is reduced, and the performance of the whole air gap is greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor, in particular to a method for forming air gap. BACKGROUND

[0002] NAND flash is an important flash memory device, because the structure has very high cell density, can achieve high storage density, and its write and erase speed is very fast, so it is widely used in various types of memory cards, and is gradually replacing the mechanical hard disk solid state disk.

[0003] With the shrinking of device size, the word line spacing size of the NAND device block area is also decreasing, which will cause serious inter-cell coupling interference problem of the floating gate type memory, thereby affecting the size of the cell threshold voltage, the programming and reading speed of the memory array. In order to solve this problem, the process technology of air gap isolation technology is introduced into the production of NAND flash, by introducing the material with the lowest dielectric constant-air between the floating gate and the floating gate, to improve the capacitive coupling effect between the word line floating gate of the device.

[0004] In the process of forming air gap in NAND flash, by using dry etching process (dry recess) and wet etching process (WET recess), in the dry etching process, when removing the residue, by controlling the selectivity ratio of polysilicon, oxide layer and silicon nitride, it is obtained, because the selectivity ratio of polysilicon (poly), oxide layer (OX) and silicon nitride (SIN) is low, in the formation of the word line side wall exposed to part of the height, causing great loss of polysilicon, in the wet etching process, without the protection of the oxide layer, also causing great loss of polysilicon, thereby reducing the height of the entire air gap. SUMMARY

[0005] The purpose of the present application is to provide a method for forming air gap, to solve the problem of great loss of word line in the process of forming air gap, and the height of the entire air gap is reduced.

[0006] To solve the above technical problems, the present application provides a method for forming air gap, comprising:

[0007] providing a substrate, the substrate is formed with the word line structure, the side and top of the word line structure are sequentially formed with an oxide layer and a redundant silicon nitride layer;

[0008] remove the oxide layer and the redundant silicon nitride layer on the top of the word line structure, and retain the oxide layer and the redundant silicon nitride layer on the side wall of the word line structure;

[0009] remove part of the redundant silicon nitride layer;

[0010] Remove the remaining redundant silicon nitride layers to form air gaps between adjacent word line structures; and

[0011] A self-aligned metal silicide is formed on the word line structure.

[0012] Optionally, the oxide layer and redundant silicon nitride layer at the top of the word line structure can be removed using a dry etching process.

[0013] Optionally, a wet etching process can be used to remove some redundant silicon nitride layers.

[0014] Optionally, the height of the removed redundant silicon nitride layer is 350 angstroms to 450 angstroms.

[0015] Optionally, a first pre-cleaning process is performed before removing the remaining redundant silicon nitride layer to remove the oxide layer remaining on top of the word line structure.

[0016] Optionally, the remaining redundant silicon nitride layer can be removed using a wet etching process.

[0017] Optionally, a second pre-cleaning process is performed before the formation of the self-aligned metal silicide to remove a portion of the oxide layer, with the thickness of the removed oxide layer being 5-10 angstroms.

[0018] Optionally, the second pre-cleaning process employs gas pre-cleaning.

[0019] Optionally, the step of forming a self-aligned metal silicide includes:

[0020] A metal layer is formed on the word line structure;

[0021] An annealing process is performed to form self-aligned metal silicides.

[0022] Optionally, the material of the self-aligned metal silicide is NiSi.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] In the method for forming an air gap provided by this invention, by removing only the oxide layer and redundant silicon nitride layer at the top of the word line structure while retaining the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure, loss to the word line structure is avoided. Since the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure are retained, the oxide layer protects the sidewalls of the word line structure during the processes of removing part of the redundant silicon nitride layer and removing the remaining redundant silicon nitride layer, increasing the selectivity ratio of the redundant silicon nitride layer to the word line structure. This completely avoids loss to the word line structure during the air gap formation process. The word line structure is much taller than that in conventional processes; that is, the height of the self-aligned metal silicide in this invention is much higher than that in conventional processes. Therefore, the resistance of the self-aligned metal silicide is reduced. Correspondingly, the height of the air gap between the word line structures is also much higher than that in conventional processes. Therefore, the capacitance between the word line structures is reduced, greatly improving the performance of the entire air gap. Attached Figure Description

[0025] Figure 1 This is a flowchart of the method for forming an air gap according to an embodiment of the present invention;

[0026] Figures 2 to 7 This is a schematic diagram of the structure corresponding to the method for forming an air gap according to an embodiment of the present invention;

[0027] In the picture,

[0028] 11 - Word line structure; 12 - Oxide layer; 13 - Redundant silicon nitride layer; 14 - ONO layer; 15 - Self-aligned metal silicide; 16 - Air gap. Detailed Implementation

[0029] The method for forming an air gap according to the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer from the following description and claims. It should be noted that the drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the present invention.

[0030] The inventors discovered that during the formation of the air gap, including dry etching and wet etching processes, the dry etching process, while removing the oxide layer and redundant silicon nitride layer at the top of the word line structure, also requires forming the sidewalls of the exposed word line structure. This means removing a portion of the oxide layer and redundant silicon nitride. Since the word line structure is made of polycrystalline silicon, the selectivity ratio of silicon oxide, silicon nitride, and polycrystalline silicon is low. Removing part of the oxide layer and redundant silicon nitride simultaneously removes a portion of the word line structure, causing significant damage. Furthermore, in the wet etching process, the etching solution is phosphoric acid. While removing the redundant silicon nitride layer, the exposed part of the word line structure reacts with the phosphoric acid. This means that the exposed sidewalls of the word line structure, without the protection of an oxide layer, react with the polycrystalline silicon, causing further damage to the word line structure. This reduces the height of the word line structure and the overall height of the air gap, failing to reduce the capacitance of the semiconductor device.

[0031] Therefore, the core idea of ​​this invention is to provide a method for forming an air gap. By removing only the oxide layer and redundant silicon nitride layer at the top of the word line structure while retaining the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure, loss to the word line structure is avoided. Since the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure are retained, the oxide layer protects the sidewalls of the word line structure during the processes of removing part of the redundant silicon nitride layer and removing the remaining redundant silicon nitride layer, thereby increasing the selectivity ratio of the redundant silicon nitride layer and the word line structure. This completely avoids loss to the word line structure during the air gap formation process, greatly improving the performance of the entire air gap. At the same time, the word line structure is much taller than the word line structure in traditional processes, reducing the device resistance.

[0032] For details, please refer to Figure 1 This is a flowchart illustrating the method for forming an air gap according to an embodiment of the present invention. Figure 1 As shown, this embodiment provides a method for forming an air gap, including:

[0033] Step S10: Provide a substrate on which the word line structure is formed, wherein the side and top of the word line structure are respectively an oxide layer and a redundant silicon nitride layer;

[0034] Step S20: Remove the oxide layer and redundant silicon nitride layer at the top of the word line structure, while retaining the oxide layer and redundant silicon nitride layer on the sidewall of the word line structure.

[0035] Step S30, to remove part of the redundant silicon nitride layer;

[0036] Step S40: Remove the remaining redundant silicon nitride layers to form air gaps between adjacent word line structures.

[0037] Step S50: A self-aligned metal silicide is formed on the word line structure.

[0038] Figures 2 to 7 This is a structural schematic diagram corresponding to the air gap formation method of this invention embodiment; the following is in conjunction with the appendix. Figures 2 to 7 The method for forming an air gap provided in the embodiments of the present invention will be described in detail.

[0039] In step S10, a substrate (not shown in the figure) is provided. The substrate can be a silicon substrate, a silicon-germanium substrate, a silicon carbide substrate, a silicon-on-insulator (SOI) substrate, a germanium-on-insulator (GOI) substrate, a glass substrate, or other III-V compound substrates. This embodiment does not limit the material or structure of the substrate. Furthermore, a device structure (not shown in the figure) can be formed on the substrate. This device structure can be a device structure formed during semiconductor front-end processes, such as a MOS transistor.

[0040] like Figure 2 As shown, in step S10, a plurality of discrete word line structures 11 are provided. The word line structures 11 are located on the substrate, and their sides are sequentially formed by an oxide layer 12 and a redundant silicon nitride layer 13. Each word line structure 11 includes a first part of the word line structure located above and a second part of the word line structure located below. The first part of the word line structure and the second part of the word line structure are isolated by an ONO layer 14. In this embodiment, the specific formation process includes: forming a first polysilicon material layer on the substrate, the first polysilicon material layer being formed by a chemical vapor deposition process; forming an ONO layer 14 on the first polysilicon material layer, the ONO layer 14 covering the first polysilicon material layer; the ONO layer 14 including a first oxide layer, a first silicon nitride layer, and a second oxide layer; the ONO layer 14 being formed, for example, by a chemical vapor deposition process; and the thickness of the ONO layer 14 being, for example, 120 angstroms-140 angstroms. A second polysilicon material layer is formed on the ONO layer 14, for example by chemical vapor deposition. The first polysilicon material layer, the ONO layer, and the second polysilicon material layer are etched to form discrete word line structures 11. An oxide layer 12 is formed on the sidewall of the word line structure, for example by chemical vapor deposition, and the thickness of the oxide layer 12 is for example 40-60 angstroms. Finally, redundant silicon nitride layers 13 are filled to fill the adjacent discrete word line structures 11.

[0041] like Figure 3As shown, in step S20, the oxide layer and redundant silicon nitride layer on the top of the word line structure 11 are removed, while the oxide layer 12 and redundant silicon nitride layer 13 on the sidewalls of the retained word lines are removed. The removal of the oxide layer and redundant silicon nitride layer on the top of the word line structure 11 can be performed using an etching process, such as a dry etching process. In the process of removing the oxide layer and redundant silicon nitride layer on the top of the word line structure 11, oxide layers and redundant silicon nitride layers of the same height are removed.

[0042] like Figure 4 As shown, in step S30, a wet etching process is used to remove a portion of the redundant silicon nitride layer 13. This wet etching process is the first wet etching process, in which the oxide layer 12 on the sidewall of the word line structure 11 is retained. The etching liquid in the first wet etching process is, for example, phosphoric acid, which does not react with the oxide layer 12. In the first wet etching process, the height of the removed redundant silicon nitride layer is 350 angstroms to 450 angstroms.

[0043] like Figure 5 As shown, between steps S40 and S30, a first pre-cleaning process is included to remove the residual oxide layer 12 on the top of the word line structure 11. Simultaneously, the oxide layer 12 on the sidewalls of the word line structure 11 is also removed. The residual oxide layer 12 on the top of the word line structure 11 is the oxide layer 12 that was not completely removed during the removal of the oxide layer on the top of the word line structure 11 and the redundant silicon nitride layer. The solution in the first pre-cleaning process is, for example, HF, the process time of the pre-cleaning process is, for example, 5-10 seconds, and the thickness of the removed oxide layer 12 is, for example, 5-10 angstroms.

[0044] like Figure 6 As shown, in step S40, a wet etching process is used to remove the remaining redundant silicon nitride layer 13 to form an air gap 16 between adjacent word line structures; an air gap is also formed between the first part of the word line structure and the second part of the word line structure. Removing the remaining redundant silicon nitride layer 13 is a second wet etching process, and the etching liquid in the second wet etching process is, for example, phosphoric acid. When removing the oxide layer and redundant silicon nitride layer at the top of the word line structure, the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure are retained, without causing damage to the word line structure. In the processes of removing part of the redundant silicon nitride layer and removing the remaining redundant silicon nitride layer, due to the protection of the oxide layer on the sidewalls of the word line structure, no damage is caused to the word line structure. Therefore, the height of the word line structure in this embodiment is higher than that in conventional processes. That is, the height of the air gap between adjacent word line structures is also higher than that of conventional air gaps. Therefore, the height of the air gap is increased, the capacitance between the word line structures is reduced, and the capacitance of the device is further reduced.

[0045] Before the self-alignment process, a second pre-cleaning process is required to ensure that the substrate surface is free of impurities or oxide layers, thus avoiding any impact on the quality of the formed metal layer and metal silicide layer. In this embodiment, the second pre-cleaning is, for example, a dry cleaning process using gas cleaning, specifically ionized HF. The second pre-cleaning process can utilize PECVD to generate plasma. In this embodiment, the substrate is introduced into the pre-cleaning chamber, and a cleaning gas is introduced into the chamber, exciting the gas into plasma to clean the substrate surface. The second pre-cleaning process removes a portion of the oxide layer, with a thickness of 5-10 angstroms. This embodiment does not limit the specific method of pre-cleaning, as long as it effectively cleans the substrate to prevent impurities or oxide layers from forming on the self-aligned silicide layer.

[0046] like Figure 7 As shown, in step S50, a self-alignment process is performed to form a self-aligned silicide 15 on the word line structure 11. The self-aligned silicide process includes:

[0047] Step S51: A metal layer is formed on the word line structure 11;

[0048] In step S51, a metal layer is formed using a deposition process. The metal compound gas used in this deposition process has selective deposition characteristics. The metal compound gas can only adsorb onto the surface of conductors and semiconductors (such as metals and silicon) and decompose to form a metal layer, but cannot adsorb onto the surface of the insulating dielectric layer. Therefore, the metal layer only covers the top and exposed sidewalls of the word line structure 11, and cannot form a metal layer on the oxide layer 12. The metal compound gas includes one or more of tungsten (W), cobalt (Co), nickel (Ni), or titanium (Ti). In this embodiment, the metal layer is, for example, Ni. However, this embodiment does not limit the specific type of metal included in the metal compound gas; any metal that can selectively adsorb onto the surface of conductors and semiconductors (such as metals and silicon) but not onto the surface of the insulating dielectric layer is acceptable, and it may include metals with this characteristic developed according to technological advancements.

[0049] In this embodiment, the deposition process is chemical vapor deposition (CVD) or atomic layer deposition (ALD) to precisely control the thickness of the metal layer.

[0050] In this embodiment, the deposition process requires a certain amount of time, such as between 1 and 10 minutes, and is performed at a certain temperature. During the deposition process, the metal layer naturally formed on the semiconductor surface reacts with the semiconductor to generate a transition metal silicide layer. The transition metal silicide layer is a transition phase that forms a self-aligned metal silicide. If the metal compound gas includes nickel (Ni) and the substrate is a silicon region, the transition metal silicide layer is a nickel-rich phase silicide such as Ni₂Si, thus eliminating the need for a dedicated annealing process to generate the transition metal silicide layer.

[0051] Step S52: Perform an annealing process to form a self-aligned metal silicide.

[0052] In step S52, the material of the self-aligned silicide is, for example, NiSi. In this embodiment, an annealing process is performed to form a low-resistance metal silicide layer, which includes a low-resistance self-aligned metal silicide layer 15 formed on the surface of the word line structure 11. Since only the residue at the top of the word line structure 11 is removed in the first etching process, without damaging the word line structure 11, and since the sidewalls are protected by an oxide layer, there is no damage to the word line structure 11 in the processes of removing part of the redundant silicon nitride layer and removing the remaining redundant silicon nitride layer. Therefore, compared with the prior art, the word line structure 11 in this embodiment is much higher than the word line structure in conventional processes. That is, the height of the self-aligned metal silicide in this embodiment is much higher than the height of the self-aligned metal silicide in conventional processes. Therefore, the resistance of the self-aligned metal silicide is reduced, further reducing the resistance of the device. The height of the self-aligned metal silicide is, for example, 400 angstroms-600 angstroms.

[0053] In this embodiment, the annealing temperature is, for example, between 400°C and 800°C. The specific annealing temperature can be determined based on the migration ability of the metal contained in the metal layer and the thickness of the metal layer.

[0054] In summary, in the air gap formation method provided by this invention, by removing only the oxide layer and redundant silicon nitride layer at the top of the word line structure while retaining the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure, loss to the word line structure is avoided. Since the oxide layer and redundant silicon nitride layer on the sidewalls of the word line structure are retained, the oxide layer protects the sidewalls of the word line structure during the processes of removing part of the redundant silicon nitride layer and removing the remaining redundant silicon nitride layer, increasing the selectivity ratio of the redundant silicon nitride layer to the word line structure. This completely avoids loss to the word line structure during the air gap formation process. The word line structure is much taller than that in conventional processes; that is, the height of the self-aligned metal silicide in this embodiment is much higher than that in conventional processes. Therefore, the resistance of the self-aligned metal silicide is reduced. Correspondingly, the height of the air gap between the word line structures is also much higher than that in conventional processes. Therefore, the capacitance between the word line structures is reduced, greatly improving the performance of the entire air gap.

[0055] The above description is merely a description of preferred embodiments of the present invention and is not intended to limit the scope of the present invention in any way. Any changes or modifications made by those skilled in the art based on the above disclosure shall fall within the protection scope of the claims.

Claims

1. A method for forming an air gap, characterized in that, include: A substrate is provided on which a word line structure is formed, wherein an oxide layer and a redundant silicon nitride layer are sequentially formed on the sidewalls and top of the word line structure. Remove the oxide layer and redundant silicon nitride layer at the top of the word line structure, and retain the oxide layer and redundant silicon nitride layer on the sidewall of the word line structure. Remove some redundant silicon nitride layers; A first pre-cleaning process is performed to remove the residual oxide layer on the top of the word line structure; Remove the remaining redundant silicon nitride layers to form air gaps between adjacent word line structures; as well as, A self-aligned metal silicide is formed on the word line structure.

2. The method for forming an air gap as described in claim 1, characterized in that, The oxide layer and redundant silicon nitride layer at the top of the word line structure are removed using a dry etching process.

3. The method for forming an air gap as described in claim 1, characterized in that, The redundant silicon nitride layer was removed using a wet etching process.

4. The method for forming an air gap as described in claim 1, characterized in that, The height of the removed redundant silicon nitride layer is 350 angstroms to 450 angstroms.

5. The method for forming an air gap as described in claim 1, characterized in that, The remaining redundant silicon nitride layer was removed using a wet etching process.

6. The method for forming an air gap as described in claim 1, characterized in that, Before the formation of self-aligned metal silicides, a second pre-cleaning process is performed to remove part of the oxide layer, with the thickness of the removed oxide layer being 5-10 angstroms.

7. The method for forming an air gap as described in claim 6, characterized in that, The second pre-cleaning process uses gas pre-cleaning.

8. The method for forming an air gap as described in claim 1, characterized in that, The steps for forming self-aligned metal silicides include: A metal layer is formed on the word line structure; An annealing process is performed to form self-aligned metal silicides.

9. The method for forming an air gap as described in claim 8, characterized in that, The material of the self-aligned metal silicide is NiSi.

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