Method for selective wet etching of silicon nitride film

Abstract

The invention discloses a method for selectively wet-etching a silicon nitride film. The method comprises the following steps: preparing a Si3N4 wafer, arranging a film formed by Si3N4 on the Si3N4 wafer 24, preparing and coating a photoresist material on the Si3N4 film, selectively wet-etching the Si3N4 wafer with the thin Si3N4 film 20 at a low temperature, cleaning the etched wafer, and removing a photoresist mask from the cleaned wafer. According to the invention, the photoresist is used as the mask, the window with the required pattern shape is firstly photoetched on the photoetched film, and then the Si3N4 thin film is etched through wet etching, so that the etching quality is higher, the etching precision is higher, the etching rate is improved at low temperature, and the problems of insufficient mask stripping and etching are solved at the same time.

Description

[Technical field]

[0001] The invention relates to the technical field of silicon nitride film etching, in particular to the technical field of wet etching of silicon nitride in a buffered hydrofluoric acid (BHF) / ammonium fluoride (NH4F) mixed solution. [Background technology]

[0002] Currently known silicon nitride (Si3N4) film etching methods include plasma etching and wet etching. Wet etching usually uses an acidic solution, such as hydrochloric acid (HF), buffered hydrochloric acid (BHF) or phosphoric acid (H3PO4) as an etchant. The wet etching of Si3N4 usually uses a photoresist material as a mask and is immersed in HF or buffered hydrochloric acid solution, usually at room temperature. The process is relatively simple and has low cost. However, there is inconvenience when using a photoresist mask to selectively etch the Si3N4 film, because the photoresist mask is easy to peel off after etching for 2 hours, and there is a problem of insufficient etching.

[0003] The problem of stripping and insufficient etching is mainly caused by using photoresist as a mask layer to etch Si3N4 for too long, which leads to the instability of the photoresist. The long etching process will cause the etchant to diffuse into the photoresist layer, causing the photoresist to swell, thereby exposing the area not protected by the mask. This may cause corrosion of the Si3N4 layer and even the silicon substrate.

[0004] On the other hand, plasma etching using photoresist as a mask provides better etching results. However, the process is complex, costly and requires high process precision.

[0005] U.S. Patent No. US8741168B2 discloses a method for etching a silicon nitride film, including the steps of providing an etch-resistant material; and etching the silicon nitride film. The step of providing the etch-resistant material includes providing the etch-resistant material to a processing surface, which includes a silicon nitride film surface and a non-etching film surface, and the material of the non-etching film is different from that of the silicon nitride film. The etching step includes etching the silicon nitride film using an etchant in a state where the etch-resistant material is relatively more densely formed on the non-etching film surface than on the silicon nitride film surface. Although an etch-resistant material is mentioned, the invention uses a self-assembled monolayer (SAM) as a mask instead of a photoresist mask, which is another mask material for creating wafer patterns.

[0006] European Patent Application Publication No. 0590876A2 discloses an integrated circuit manufacturing etching method, which etches a layer containing silicon nitride or silicon material, and the material is placed on silicon oxide, and is etched in a wet bath containing phosphoric acid, chlorofluoric acid and nitric acid. The invention aims to improve the selectivity of wet etching by combining three etchants (phosphoric acid, chlorofluoric acid and nitric acid) and replenishing chlorofluoric acid and nitric acid within a certain period of time. Although the invention can improve the selectivity of wet etching, no photoresist mask layer is used.

[0007] In view of the above, a low-cost, time-saving method for selective wet etching of Si3N4 thin films is needed, while overcoming the above-mentioned problems of mask stripping and insufficient etching. [Summary of the invention]

[0008] The purpose of the present invention is to solve the problems in the prior art and to propose a method for selective wet etching of silicon nitride film, using photoresist as a mask to increase the etching rate at low temperature and overcome the above-mentioned problems of mask stripping and insufficient etching.

[0009] To achieve the above object, the present invention provides a method for selectively wet etching a silicon nitride film, comprising the following steps:

[0010] Step 1, providing a Si3N4 wafer, on which a thin film composed of Si3N4 is provided;

[0011] Step 2, using an organic solvent to remove impurities on the Si3N4 wafer;

[0012] Step 3, treating the Si3N4 wafer with a buffered hydrofluoric acid / ammonium fluoride solution;

[0013] Step 4, dehydrating the Si3N4 wafer at a temperature ranging from 120° C. to 150° C.;

[0014] Step 5, coating a photoresist material on the dehydrated Si3N4 wafer as a mask for the thin film, then heating the wafer with the mask to 90°C-95°C, and photolithography the heated mask through a photolithography process and exposing it to ultraviolet light;

[0015] Step 6, heating the exposed wafer with the mask to 110° C.-120° C.;

[0016] Step 7, placing the wafer in a buffered hydrofluoric acid or ammonium fluoride solution at a temperature ranging from 40° C. to 80° C. to etch the thin film at a portion not covered by the mask;

[0017] Step eight, cleaning and removing the remaining photoresist material on the etched wafer.

[0018] Preferably, the organic solvent in step 2 comprises any one of acetone, deionized water and isopropanol, or any combination thereof.

[0019] Preferably, in step seven, the exposed Si3N4 wafer is placed in a buffered hydrofluoric acid or ammonium fluoride solution at a temperature range of 80° C. to etch the exposed Si3N4 film with the mask.

[0020] Preferably, the etching time is 1 minute.

[0021] The beneficial effects of the method for selective wet etching of silicon nitride film of the present invention are as follows: the present invention uses photoresist as a mask to first photoetch a window of a desired pattern shape on the photoresist film, and then etches the Si3N4 film by wet etching, so that the etching quality is higher, the precision is higher, the etching rate is increased at low temperature, and at the same time the above-mentioned problems of mask stripping and insufficient etching are overcome.

[0022] One advantage of the present invention is that after coating the photoresist mask on the Si3N4 film, the Si3N4 wafer is heated to between 110°C and 120°C for 5 to 10 minutes, thereby improving the adhesion of the photoresist mask layer on the Si3N4 wafer.

[0023] Another advantage of the present invention is that the temperature of the buffered hydrofluoric acid / ammonium fluoride solution is maintained between 40°C and 80°C, which increases the etching rate by etching the Si3N4 film in a short time, thereby preventing the photoresist mask layer from peeling off the Si3N4 film and enabling the etchant to diffuse through the Si3N4 film and the substrate.

[0024] The features and advantages of the present invention will be described in detail through embodiments in conjunction with the accompanying drawings.

Brief Description of the Drawings

[0025] Figure 1 It is a schematic diagram of step 1 of the present invention.

[0026] Figure 2 It is a schematic diagram of coating a photoresist material on a Si3N4 wafer in step five of the present invention.

[0027] Figure 3 This is a schematic diagram of performing photolithography on the heated mask through a photolithography process in step five of the present invention.

[0028] Figure 4 It is a schematic diagram of the photolithography process of the present invention when processing the mask.

[0029] Figure 5 This is a schematic diagram of the mask after photolithography.

[0030] Figure 6 This is a schematic diagram of etching the thin film in step seven.

[0031] Figure 7 This is a schematic diagram of the chip after etching in step seven.

[0032] Figure 8 Schematic diagram of the wafer after cleaning and removing the remaining photoresist material in step eight.

[0033] Fig. 9 Schematic diagram of a chip according to an embodiment of the present invention, wherein a is a schematic diagram of the chip during the photolithography process; b is a chip before removing the photoresist material mask; and c is a chip after removing the photoresist material mask.

[0034] Fig.10 1 is a graph showing the relationship between the temperature of the buffered hydrochlorofluoroacid solution and the etching rate according to data from an embodiment of the present invention.

[0035] In the figure: 20 - thin film, 22 - photoresist material, 24 - Si3N4 wafer, 26 - buffered hydrofluoric acid / ammonium fluoride solution, 30 - standard photolithography process, 34 - container, 36 - bracket. [Specific implementation method]

[0036] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below through the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the scope of the present invention. In addition, in the following description, the description of known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0037] In the description of the present invention, it should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it may be directly on the other element or indirectly on the other element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or indirectly connected to the other element.

[0038] In the description of the present invention, it should be noted that the terms "center", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, or the positions or positional relationships in which the invention product is usually placed when in use, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third" and the like are only used to distinguish descriptions, and cannot be understood as indicating or implying relative importance. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "multiple" means two or more, unless otherwise clearly and specifically defined. "Several" means one or more, unless otherwise clearly and specifically defined.

[0039] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0040] See also Figure 1-Figure 4 The present invention relates to a method for wet etching a silicon nitride Si3N4 film 20 on a Si3N4 wafer 24 through a window on a photoresist material 22 at low temperature using a buffered hydrofluoric acid / ammonium fluoride solution 26, the specific steps of which are as follows:

[0041] I. Prepare a Si3N4 wafer 24 on which a thin film 20 composed of Si3N4 is provided, such as Figure 1 II. Prepare and apply a photoresist material 22 onto the Si3N4 film 20, such as Figure 2 As shown; III. Selective wet etching of a Si3N4 wafer having a thin Si3N4 film 20 at low temperature, such as Figure 2-5 iv. Clean the etched wafer, as shown in Figure 6 As shown; v. removing the photoresist mask from the cleaned wafer, such as Figure 7 shown.

[0042] Among them, step I involves the wafer preparation process, the Si3N4 wafer 24 will first be cleaned in an organic solvent, such as acetone, isopropyl alcohol (IPA) and deionized water (DI). Then, the Si3N4 wafer 24 will be treated with buffered hydrofluoric acid / ammonium fluoride for a short time, and then the wafer will be placed on a hot plate at a temperature of 120°C to 150°C and baked for 30 minutes to remove moisture from the wafer.

[0043] Reference Figure 2-Figure 4 , involving coating a photoresist material 22 on a Si3N4 wafer 24, and then exposing the photoresist material 22 through a standard photolithography process to determine the pattern on the Si3N4 wafer 24 and improve the adhesion of the photoresist 22 material. Apply a fresh photoresist material 22 (such as fresh AZ 4620) as a mask on the Si3N4 film 20 of the Si3N4 wafer 24, and then accelerate the spin coating on the wafer spin coater at a speed of 2000-3000 rpm to ensure uniform coating of the photoresist. The coated Si3N4 wafer is then heated by pre-baking on a hot plate for 2 to 3 minutes, with the temperature set at 90°C to 95°C. Then, the coated Si3N4 wafer is exposed to ultraviolet light through a standard mask alignment photolithography device, with an exposure time of 80 to 90 seconds. The exposed Si3N4 wafer is then developed in a standard AZ4620 developer, with a dilution ratio of 1:3 (AZ400:DI). The developed Si3N4 wafer is then baked on a hot plate at a temperature of 110°C to 120°C for 5 minutes.

[0044] refer to Figure 5 , Fig. 9 , involving etching a Si3N4 film 20 with a mask on which a corresponding pattern has been etched by the aforementioned photolithography process. The wafer after the photolithography process is immersed in a hot, fresh buffered hydrofluoric acid / ammonium fluoride solution 26, maintained at a temperature between 40°C and 80°C, and the specific temperature is adjusted according to the required etching rate.

[0045] Specifically, the etching process of the Si3N4 film 20 includes the following steps: i. preparing a buffered hydrofluoric acid / ammonium fluoride solution 26 with a concentration ratio of 10:1; ii. pouring 150 ml of the buffered hydrofluoric acid / ammonium fluoride solution 26 into a container 34; iii. preparing a bracket 36 for fixing the Si3N4 wafer 24; iv. setting, controlling and maintaining the temperature of the buffered hydrofluoric acid / ammonium fluoride solution 26 between 40°C and 80°C; v. maintaining the atmospheric pressure at 1atm during the entire etching process; vi. fixing the Si3N4 wafer 24 on the bracket and immersing it in the buffered hydrofluoric acid / ammonium fluoride solution 26 for the required etching time, thereby etching the Si3N4 film 20 exposed by the mask on the Si3N4 wafer 24.

[0046] Step Iv includes a typical cleaning of the etched Si3N4 wafer, wherein the etched Si3N4 wafer is rinsed with deionized water (DI) to remove the buffered hydrofluoric acid solution / ammonium fluoride on the etched Si3N4 wafer. Then, the etched Si3N4 wafer is blown dry using nitrogen.

[0047] v. The step includes removing the residual photoresist 22 on the Si3N4 wafer after etching using an acetone solution, IPA or deionized water (DI), and then dry-blowing the Si3N4 wafer after etching using nitrogen gas.

[0048] The present invention will be described in further detail below through several non-limiting experimental examples.

[0049] Embodiment 1:

[0050] In this example, a total of five Si3N4 wafer 24 samples were prepared, and the Si3N4 film 20 of each sample was wet etched for 1 minute using a buffered hydrofluoric acid / ammonium fluoride solution at etching temperatures of 40°C, 50°C, 60°C, 70°C and 80°C, respectively.

[0051] The Si3N4 wafers 24 were prepared by cleaning each Si3N4 wafer 24 using acetone, isopropyl alcohol (IPA) and deionized water (DI). Next, the Si3N4 wafers 24 were treated with a buffered hydrofluoric acid / ammonium fluoride solution for a short time and then baked on a hot plate at 120°C to 150°C for 30 minutes to remove moisture from the wafers.

[0052] Fresh photoresist 22 (AZ 4620) was coated on the Si3N4 film 20 of the Si3N4 wafer 24 as a mask, and the spin coating was accelerated at a speed of 2000 rpm on a wafer spinner to make the surface thickness of the mask layer uniform. Figure 2 The Si3N4 wafer 24 shown is pre-baked on a hot plate at 90°C to 95°C for 2 to 3 minutes. Then, using a MaskAligner lithography device, the mask of the Si3N4 wafer 24 is exposed to standard photolithography process 30 ultraviolet light in the desired configuration for 80 to 90 seconds. The exposed photoresist is developed using a standard AZ 4620 developer (dilution ratio of 1:3, AZ400:DI). Then, Figure 5 The exposed masked Si3N4 wafer 24 is shown heated on a hot plate for 5 minutes at a temperature ranging from 110°C to 120°C.

[0053] Next, refer to Figure 6, prepare 150 ml of hydrogen fluoride solution 42 with a concentration ratio of 10:1 and pour it into a Teflon beaker 50. Fix the Si3N4 wafer 24 exposed to the mask on a Teflon holder 44 and contact it with the buffered hydrogen fluoride / ammonium fluoride solution 42 at 40°C for 3 minutes to etch the Si3N4 film 20 not covered by the mask.

[0054] refer to Figure 7 After etching for 3 minutes, the etched Si3N4 wafer 24 is rinsed with deionized water (DI) to remove the buffered hydrofluoric acid / ammonium fluoride solution 42. Then, the cleaned etched Si3N4 wafer 24 is blown dry with nitrogen. Finally, the remaining photoresist 22 on the cleaned etched Si3N4 wafer 24 is removed using acetone solution, IPA or DI, and then dry-blown with nitrogen.

[0055] Then, the experiment was repeated 4 times at etching temperatures of 50°C, 60°C, 70°C and 80°C, respectively, and each etching time was 1 minute.

[0056] Embodiment 2:

[0057] In this example, five Si3N4 wafer 24 samples are also prepared, and the Si3N4 film 20 of each sample is wet etched for 1 minute using a buffered hydrofluoric acid / ammonium fluoride solution at etching temperatures of 40° C., 50° C., 60° C., 70° C., and 80° C. The difference between this example and the first example is that the etching time of each time in this example is kept at 3 minutes.

[0058] Embodiment three:

[0059] In this example, five Si3N4 wafer 24 samples are also prepared, and the Si3N4 film 20 of each sample is wet-etched for 1 minute using a buffered hydrofluoric acid / ammonium fluoride solution at etching temperatures of 40° C., 50° C., 60° C., 70° C., and 80° C. The difference between this example and the first example is that the etching time of each time in this example is kept at 5 minutes.

[0060] The above three embodiments all use a 700 μm silicon nitride Si3N4 wafer 24 as a substrate, a 200 nm thick Si3N4 film 20, and a photoresist 22 as a mask. Then, it is etched in a buffered hydrofluoric acid / ammonium fluoride solution 42, which serves as an etchant.

[0061] A total of 15 Si3N4 wafer samples 20 were used for the experiment, and the experimental results are given in the following Table 1. The samples were etched with a buffered hydrofluoric acid / ammonium fluoride solution 42 at different temperatures (40°C, 50°C, 60°C, 70°C, 80°C) and different etching times (1 minute, 3 minutes, and 5 minutes).

[0062]

[0063] According to the results in the above table, at temperatures of 40°C, 50°C, 60°C, 70°C and 80°C, the etching rates of silicon nitride (Si3N4) film in buffered hydrofluoric acid / ammonium fluoride solution are 2.13, 4.58, 8.27, 19.02 and 40.94 nm / min respectively.

[0064] In Example 1, a silicon nitride (Si3N4) film was wet etched for 1 minute using a buffered hydrofluoric acid / ammonium fluoride solution 42 at 40°C, 50°C, 60°C, 70°C and 80°C. The results showed that the etching rates were 2.2, 4.6, 5.7, 16.2 and 43.4 nm / min, respectively, which were related to the temperature of the buffered hydrofluoric acid / ammonium fluoride solution 42 used.

[0065] In Example 2, the silicon nitride (Si3N4) film was wet etched for 3 minutes at 40°C, 50°C, 60°C, 70°C and 80°C using a buffered hydrofluoric acid / ammonium fluoride solution 42. The results showed that the etching rates were 2.2, 4.6, 8.7, 17.9 and 39.8 nm / min, respectively, which were related to the temperature of the buffered hydrofluoric acid / ammonium fluoride solution 42 used.

[0066] In Example 3, the silicon nitride (Si3N4) film was wet etched for 3 minutes at 40°C, 50°C, 60°C, 70°C and 80°C using a buffered hydrofluoric acid / ammonium fluoride solution 42. The results showed that the etching rates were 2.0, 4.6, 10.4, 23.0 and 39.6 nm / min, respectively, which were related to the temperature of the buffered hydrofluoric acid / ammonium fluoride solution 42 used.

[0067] Fig.10 The results of three groups of experiments are shown, corresponding to Table 1, and it is concluded that as the temperature of the etchant (buffered hydrofluoric acid / ammonium fluoride solution 42) increases, the etching rate of the silicon nitride (Si3N4) film 20 increases. When wet etching is performed for 1 minute at 80°C, the best etching rate is shown, which is 43.4nm / min.

[0068] The results of the three embodiments above further lead to the conclusion that the wet etching method of the present invention provides significant results compared to conventional room temperature wet etching methods, which may take up to 4 hours. Moreover, using photoresist as a mask creates a fast etching window.

[0069] It should be noted that, although the above embodiments have been described in this article, the scope of patent protection of the present invention is not limited thereby. Therefore, based on the innovative concept of the present invention, changes and modifications made to the embodiments described herein, or equivalent structures or equivalent process changes made using the contents of the present specification and drawings, directly or indirectly applying the above technical solutions to other related technical fields are all included in the scope of protection of the patent of the present invention.

Claims

1. A method for selective wet etching of a silicon nitride film, characterized in that: The following steps are involved: Step 1, providing a Si3N4 wafer, on which a thin film composed of Si3N4 is provided; Step 2, using an organic solvent to remove impurities on the Si3N4 wafer; Step 3, treating the Si3N4 wafer with a buffered hydrofluoric acid / ammonium fluoride solution; Step 4, dehydrating the Si3N4 wafer at a temperature ranging from 120° C. to 150° C.; Step 5, coating a photoresist material on the dehydrated Si3N4 wafer as a mask for the thin film, then heating the wafer with the mask to 90°C-95°C, and photolithography the heated mask through a photolithography process and exposing it to ultraviolet light; Step 6, heating the exposed wafer with the mask to 110° C.-120° C.; Step 7, placing the wafer in a buffered hydrofluoric acid or ammonium fluoride solution at a temperature ranging from 40° C. to 80° C. to etch the thin film at a portion not covered by the mask; Step eight, cleaning and removing the remaining photoresist material on the etched wafer.

2. A method for selective wet etching of a silicon nitride film according to claim 1, characterized in that: The organic solvent in step 2 includes any one of acetone, deionized water and isopropanol, or any combination of these.

3. A method for selective wet etching of a silicon nitride film according to claim 1, characterized in that: In the step seven, the exposed Si3N4 wafer is placed in a buffered hydrofluoric acid or ammonium fluoride solution at a temperature range of 80° C. to etch the exposed Si3N4 film with the mask.

4. A method for selective wet etching of a silicon nitride film as claimed in claim 3, characterized in that: The etching time is 1 minute.

Images