JP2006332359A - Method of etching substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of etching a film to be etched which is formed on a substrate wherein the roughness of substrate surface is restrained, selectivity is excellent, and cost is low. <P>SOLUTION: In a process tub 201 into which a mist 30 of a fluoric acid water solution serving as an etchant is introduced, an SOI substrate 10 is placed on which a silicon oxide film 13 and a silicon nitride film 20 serving as a film to be etched are formed, and the SOI substrate 10 is exposed to the mist 30 of the fluoric acid water solution, whereby after the silicon nitride film 20 is reformed to a water-soluble material 21 composed of ammonium fluoride salt of silicon, a substrate 10 is cleaned with water 41 to remove the water-soluble material 21, resulting in etching the silicon nitride film 20. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for etching a film to be etched formed on a substrate, and more particularly to a method for selectively etching a desired film to be etched.

  FIG. 7 is a schematic cross-sectional view showing a conventional method for manufacturing a capacitive sensor as a structure using this type of substrate.

  In this substrate, a pair of electrode parts composed of a movable electrode (movable part) 16 and a fixed electrode (fixed part) 17 which are released from each other on the substrate 10 is formed, and the movable electrode 16 is formed by applying a mechanical quantity such as acceleration. This is a capacitive sensor that is displaced.

  Here, as shown in FIG. 7A, the substrate 10 is bonded by bonding two silicon substrates, the first silicon substrate 11 and the second silicon substrate 12 with a buried oxide film 13 interposed therebetween. An SOI (silicon-on-insulator) substrate 10 is shown.

  Next, as shown in FIG. 7B, the groove 15 for defining the movable electrode 16 and the fixed electrode 17 is etched from one side of the substrate 10, that is, the second silicon substrate 12 side. Form with. This groove 15 is a groove that reaches the buried oxide film 13 from the surface of the second silicon substrate 12.

  Next, as shown in FIG. 7C, the cavity 14 is formed by performing silicon etching from the other surface side of the substrate 10, that is, the first silicon substrate 11 side. After this cavity etching, release etching is further performed to remove the buried oxide film 13 below the electrode portions 16 and 17 in the cavity 14.

  As a result, the movable electrode 16 that is the movable portion is separated from the first silicon substrate 11 that is the support portion via the groove 15 and the cavity 14 and is in a released state, that is, a released state.

  Here, the etching of the cavity 14 is conventionally performed as shown in FIG. FIG. 8 is a schematic sectional view showing a method for etching the cavity 14 in the substrate 10 shown in FIG.

  As shown in FIG. 8A, a mask 20 made of a silicon nitride film or the like is formed on the other surface of the substrate 10, that is, on the surface of the first silicon substrate 11 where the cavity 14 is not formed.

  Next, as shown in FIG. 8B, anisotropic etching using an alkaline solution such as KOH (potassium hydroxide) or TMAH (tetramethyl ammonium hydroxide) as an etchant is performed, A cavity 14 is formed in the first silicon substrate 11 on the other surface side of the substrate 10.

  In the state so far, the mask 20 is the film to be etched, and the state of FIG. 8B shows the state of the substrate 10 on which the film to be etched 20 is formed. Thereafter, as shown in FIG. 8C, the mask 20 is removed by dry etching using hydrofluoric acid (HF) or the like. Then, as described above, release etching of the buried oxide film 13 is further performed.

  By the way, the removal of the mask 20 in the cavity etching shown in FIG. 8 is conventionally performed by dry etching. Although this dry etching is suitable for microfabrication, it involves a physical impact, so the mask is removed. The surface of the substrate 10 after the removal of 20 becomes rough (see FIG. 8C). Also, dry etching requires a vacuum facility such as a vacuum chamber, and is expensive.

  Here, it is conceivable to remove the mask 20 in the cavity etching by wet etching using hydrofluoric acid instead of dry etching. This wet etching is a cheaper method than dry etching.

  However, since this wet etching cannot selectively etch the silicon nitride film of the silicon nitride film constituting the mask 20 and the silicon oxide film constituting the buried oxide film 13, the removal of the mask 20 is not possible. It is difficult to apply to. In addition, in wet etching, an etching solution does not easily enter a fine portion, and fine processing is difficult as compared with dry etching.

  The present invention has been made in view of the above-described problems, and provides an etching method for a film to be etched formed on a substrate, which provides an inexpensive etching method that suppresses surface roughness of the substrate and has excellent selectivity. With the goal.

  As a result of intensive studies in order to achieve the above object, attention was focused on performing mist etching. In this mist etching, an aqueous solution of an etching agent such as hydrofluoric acid (HF) is introduced as droplets (mist) into a treatment tank. The mechanism of this mist etching is estimated as follows.

  FIG. 9 is a diagram showing an estimated etching mechanism of mist etching. Here, an example of sacrificial layer etching for etching the buried oxide film 13 under the second silicon substrate 12 in the SOI substrate is shown.

  An aqueous solution in which HF as an etching agent is dissolved in water is prepared and sprayed from a nozzle to generate a mist 30 of the aqueous solution, and the mist 30 is introduced into the treatment tank. The mist 30 has a diameter on the order of several μm, for example, and enters a fine portion such as the groove 15 of the silicon substrate 12. Therefore, mist etching is excellent in microfabrication like dry etching.

  Further, as shown in FIG. 9, in the mist 30, HF in the mist 30 exists as a vapor 31 at the interface, and this vapor 31 forms the buried oxide film 13 as a film to be etched. The buried oxide film 13 is etched by reacting with the constituent SiO 2 (illustrated as an etching reaction portion in the figure).

  Thus, mist etching is excellent in microfabrication and has an etching mechanism that does not involve physical impact like dry etching, so that surface roughness of the substrate after etching can be suppressed.

  In addition, mist etching does not require vacuum equipment such as dry etching, and only sprays an aqueous solution of an etching agent, so that the cost is comparable to that of wet etching. Furthermore, there is an advantage that the amount of the etching agent used is significantly less than that of wet etching.

  Further examination of this mist etching has revealed that the water content of the mist 30 can be controlled by controlling the temperature by heating or the like, or by controlling the mist diameter by spraying.

  Further, it is known that a film to be etched formed on a substrate has etching selectivity by changing the amount of moisture in the etching atmosphere depending on the material. That is, depending on the material of the film to be etched, it is known that it reacts with the etching agent or does not react depending on the amount of moisture. In addition, depending on the combination of the film to be etched and the etching agent, it is known that a water-soluble substance is generated by the reaction between the two.

  For this reason, in mist etching, it was considered that different types of films to be etched can be selectively etched by controlling the amount of moisture in the mist. The present invention has been created based on such examination results.

  That is, in the invention according to claim 1, the substrate (10) on which the film to be etched (20) is formed is placed in the treatment tank (201) into which the aqueous solution of the etching agent is introduced in a mist state, The film (20) is exposed to the aqueous solution in the mist state to modify the film to be etched (20) into a water-soluble substance (21), and then the substrate (10) is washed with water to thereby form the film to be etched ( 20) is etched.

  According to the present invention, since mist etching is employed, it is easy to control the moisture content of the mist (30). By controlling this moisture content, only a desired film (20) to be etched is selectively selected. It can be modified with a water-soluble substance (21) by reacting with an etching agent.

  And since the water-soluble substance (21) of the film to be etched (20) is removed from the substrate (10) by washing with water, the selective etching of the film to be etched (20) is appropriately performed. Made.

  In addition, by employing mist etching, as described above, it is excellent in microfabrication as in the case of conventional dry etching, and can be made cheaper, and also suppresses surface roughness of the substrate (10) after etching as much as possible. be able to.

  Therefore, according to the present invention, it is an etching method for the etching target film (20) formed on the substrate (10), and it is possible to provide an inexpensive etching method that suppresses the roughness of the substrate surface and has excellent selectivity. it can.

  According to a second aspect of the present invention, in the substrate etching method according to the first aspect, the mist aqueous solution is mixed with a gas and introduced into the treatment tank (201).

  According to this, the moisture content of the mist can be controlled by controlling the temperature of the gas by heating the gas or the like.

  Furthermore, as in the invention described in claim 3, in the substrate etching method according to claim 2, the silicon nitride film (20) and the silicon oxide film (13) are formed on the substrate (10). The etching agent is hydrofluoric acid, the gas is heated nitrogen, and the silicon nitride film (20) can be selectively etched as the film to be etched.

  According to the inventor's study, the silicon nitride film (20) is modified into a water-soluble substance by hydrofluoric acid as an etchant regardless of the amount of moisture in the mist, and is etched by subsequent water washing. On the other hand, since the presence of moisture is indispensable for the reaction with hydrofluoric acid, the silicon oxide film (13) does not become a water-soluble material when the amount of moisture in the mist is small, and the water-soluble material when the amount of moisture in the mist is large. It was found that etching is possible.

  Therefore, if hydrofluoric acid is used as an etching agent and heated nitrogen is used as a gas as in the present invention, the amount of water in the mist made of hydrofluoric acid aqueous solution can be reduced as much as possible, and the silicon nitride film (20 Since the silicon oxide film (13) is hardly etched, the silicon nitride film (20) can be selectively etched as a film to be etched.

  According to a fourth aspect of the present invention, in the substrate etching method according to the third aspect, the silicon nitride film (20) is formed by a low temperature plasma CVD method.

  According to this, the modification of the silicon nitride film (20) to a water-soluble substance can be performed more easily, which is preferable.

  In addition, the code | symbol in the bracket | parenthesis of each said means is an example which shows a corresponding relationship with the specific means as described in embodiment mentioned later.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, parts that are the same or equivalent to each other are given the same reference numerals in the drawings for the sake of simplicity.

  In this embodiment, a capacitive acceleration sensor will be described as an example of a product manufactured by an etching method for a film to be etched formed on a substrate. However, application examples of the etching method of the present invention are limited to this. It is not a thing.

[Configuration of capacitive acceleration sensor, etc.]
FIG. 1 is a schematic cross-sectional view of a capacitive acceleration sensor 100 according to an embodiment of the present invention. First, the outline of the sensor configuration will be described.

  The sensor 100 includes an SOI (silicon-on-insulator) substrate 10 as a substrate in the present invention. The SOI substrate 10 is formed by laminating a first silicon substrate 11 and a second silicon substrate 12 via a buried oxide film 13 made of a silicon oxide film (SiO 2).

  In the central portion of the SOI substrate 10, a cavity 14 is formed by removing the buried oxide film 13 and the first silicon substrate 11. The second silicon substrate 12 corresponding to the cavity 14 is formed with a groove 15 penetrating from the upper surface to the cavity 14 in the thickness direction.

  By this groove 15, the second silicon substrate 12 is partitioned into a movable electrode 16 as a movable part that is displaced by application of acceleration, and a fixed electrode 17 as a fixed part.

  In the example shown in FIG. 1, the movable electrodes 16 and the fixed electrodes 17 are alternately arranged and are adjacent to each other through the grooves 15. The movable electrode 16 is released from the first silicon substrate 11 that is the support portion of the SOI substrate 10 via the cavity 14, and is movable with respect to the first silicon substrate 11.

  Although not shown, specifically, the fixed electrode 17 is supported and fixed to the first silicon substrate 11 via the buried oxide film 13, and the movable electrode 16 is a beam or the like with respect to the first silicon substrate 11. It is movable because it is elastically supported via, for example.

  These movable and fixed electrodes 16, 17 can be, for example, a well-known comb-like beam structure. In this case, for example, with the application of acceleration along the left-right direction in FIG. 1, the movable electrode 16 is displaced along the same direction, whereby the movable electrode 16 and the fixed electrode 17 that are opposed to each other with the groove 15 therebetween. The distance between changes.

  For example, the applied acceleration can be obtained by detecting a change in capacitance between the electrodes 16 and 17 based on this change in distance. Thus, in the present embodiment, the capacitive acceleration sensor 100 is configured.

[Manufacturing method]
Next, the manufacturing method of the acceleration sensor 100 of this embodiment is demonstrated with reference to FIG. 2, FIG. 2 and 3, some of the electrode portions 16 and 17 are arranged from FIG. 1, but are substantially the same as the sensor 100 of FIG. 1.

  2A to 2D and FIGS. 3A to 3D are schematic cross-sectional views illustrating a method for manufacturing the capacitive acceleration sensor 100 according to the present embodiment. In particular, FIG. 3 is a schematic cross-sectional view showing a substrate etching method according to the present embodiment. Through this manufacturing method, the sensor 100 shown in FIG. 1 is finally manufactured.

  First, as shown in FIGS. 2A and 2B, the SOI substrate 10 is prepared, and a movable electrode (movable part) is formed on the substrate 10 from the upper surface thereof, that is, the upper surface side of the second silicon substrate 12. ) The groove 15 for defining 16 is formed by etching (trench groove forming step). Here, the groove 15 is a groove that reaches the buried oxide film 13 from the upper surface of the second silicon substrate 12.

  This trench groove forming step can be performed by a normal photo etching process. Specifically, in order to form the electrode portions 16 and 17, dry etching or the like using a pattern formed by a photoresist is performed.

This dry etching can be performed using an etching gas such as SF ₆ gas. The portions left by this etching are formed as electrode portions 16 and 17. Thereafter, the photoresist is removed by a usual method. In this example using the SOI substrate 10, the etching is stopped by the buried oxide film 13, so that the controllability is excellent.

  Next, as shown in FIGS. 2C and 2D, the cavity 14 is formed by further performing silicon etching from the lower surface of the SOI substrate 10, that is, the lower surface side of the first silicon substrate 11 (cavity forming step). ).

  Specifically, as shown in FIG. 2C, a mask 20 made of a silicon nitride film, which is a film to be etched, is formed on the lower surface of the first silicon substrate 11 where the cavity 14 is not formed. In this example, the mask 20 is a silicon nitride film 20 formed by a CVD method or the like. The silicon nitride film 20 is preferably formed by low temperature plasma CVD.

  Thereafter, as shown in FIG. 2D, a predetermined region is removed from the lower surface side of the SOI substrate 10 by anisotropic wet etching using an alkaline solution or the like to remove the first silicon substrate. A cavity 14 is formed in 11.

  Here, as the alkali solution, KOH (potassium hydroxide), TMAH (tetramethyl ammonium hydroxide, tetramethylammonium hydroxide), or the like is used. Also in the case of this cavity etching, in this example using the SOI substrate 10, the controllability is excellent by stopping the etching with the buried oxide film 13.

  Before the cavity etching is performed, the upper surface side of the SOI substrate 10 is covered with a resist or the like other than the mask 20, and the resist is removed after the cavity etching is completed, for example.

  In the state shown in FIG. 2D, the mask 20 is a film to be etched, and the substrate 10 on which the film to be etched 20 is formed is shown. More specifically, in this example, the substrate 10 is an SOI substrate 10 on which a silicon nitride film 20 as a mask and a silicon oxide film 13 as a buried oxide film are formed.

  Next, in this manufacturing method, as shown in FIG. 3, the mask 20 made of a silicon nitride film as an etching target film is selectively etched and removed using mist etching.

  Here, the mist etching apparatus 200 used for this mist etching is demonstrated with reference to FIG. FIG. 4 is a diagram showing a schematic configuration of the mist etching apparatus 200.

  The apparatus 200 includes a processing tank 201 that accommodates the substrate 10 having the film to be etched 20 and in which etching is performed. The processing tank 201 forms a space partitioned from the outside inside, but does not have to function as a vacuum vessel like a chamber of a dry etching apparatus.

  A holding member 202 is provided inside the processing bath 201, and the SOI substrate 10 is held and fixed by the holding member 202. A mist generating chamber 203 is provided outside the processing tank 201.

  In the lower part of the mist generating chamber 203, an aqueous solution 32 of this etching agent is stored. In this example, hydrofluoric acid (HF) is used as a chemical which is an etching agent. Hereinafter, the hydrofluoric acid aqueous solution 32 is referred to as an etching solution 32.

  A mist generator 204 is provided in the mist generation chamber 203. The mist generator 204 is for making the etching solution 32 into a mist state and introducing the mist 30 into the processing tank 201. For example, the mist generator 204 sucks up the etching solution 32 and sprays the etching solution 32 using a spraying nitrogen (N 2) gas to generate the mist 30.

  In addition, a mist introduction rod 205 that communicates the mist generation chamber 203 and the inside of the processing tank 201 is provided. In the upper part of the mist generating chamber 203, the mist 30 sprayed from the mist generator 204 enters the mist introducing rod 205 in a state of being mixed with the above-mentioned atomizing nitrogen that is a gas, and the mist introducing rod 205 The liquid is discharged from the tip of the liquid into the treatment tank 201.

  Here, a mist heating device 206 for heating the mist 30 is provided in the middle of the mist introduction rod 205. In this example, the mist heating device 206 heats the mist 30 by introducing the nitrogen gas heated through the heater into the mist introduction rod 205.

  Note that the flow rate of the heated nitrogen supplied from the mist heating device 206 is always constant, and the supply amount of the mist 30 introduced into the treatment tank 201 is determined by the amount of nitrogen for spraying by the mist generator 204. Flow rate. That is, if the flow rate of the atomizing nitrogen is increased, the mist supply amount is also increased.

  In the mist etching apparatus 200, the mist 30 generated from the mist generator 204 is heated by the mist heating apparatus 206 and introduced into the processing tank 201 while controlling the mist supply amount by the flow rate of the nitrogen for spraying. It is like that. And the to-be-etched film | membrane 20 in the board | substrate 10 in the processing tank 201 is processed by the introduce | transduced mist 30. FIG.

  Using the mist etching apparatus 200, the etching target film 20 is etched. First, as shown in FIG. 3A, the SOI substrate 10 on which the mask 20 and the cavity 14 are formed is installed in the processing tank 201, and an etching solution mist 30 is introduced into the processing tank 201.

  For example, the temperature of the heated nitrogen introduced from the mist heating device 206 into the mist introduction rod 205 is about several tens of degrees Celsius, and the ratio of mist 30 and nitrogen released from the tip of the mist introduction rod 205 is about 1: 2. The mist 30 is a micro mist (fine particle) having a diameter of about 1 to 5 μm.

  Mist 30 mixed with heated nitrogen of about several tens of degrees Celsius (for example, 40 ° C.) has a significantly reduced amount of water in mist 30. In the SOI substrate 10 exposed in such a mist 30, the silicon nitride film 20 as the mask 20 that is a film to be etched is modified to a water-soluble substance 21, but the silicon oxide film 13 as a buried oxide film is formed. Is not modified (etching target film modification step).

  This is because in the reaction with hydrofluoric acid (HF), the silicon nitride film 20 proceeds regardless of the presence of water, whereas the silicon oxide film 13 requires the presence of water. The reason why the silicon nitride film 20 and the silicon oxide film 13 are not modified into the water-soluble substance 21 and the details of the mechanism will be described later.

  As described above, according to the etching method of this example, only the silicon nitride film 20 that is the film to be etched can be substantially modified into the water-soluble substance 21 by exposing it to the mist 30 having a small amount of water. .

  Subsequently, the SOI substrate 10 is taken out from the processing tank 201 and, as shown in FIG. 3 (c), the substrate 10 is washed with water by a method such as putting in the water tank 40 containing water 41 (water washing). Process).

  Thereby, the water-soluble substance 21 is etched and removed from the substrate 10 as shown in FIG. That is, the present etching method includes the etching target film modification step and the water washing step, and according to such a method, the silicon nitride film 20 that is the mask 20 is composed of the silicon oxide film 13 and the silicon nitride film 20. Can be selectively etched as a film to be etched.

  Thereafter, the buried oxide film 13 under the electrode portions 16 and 17 exposed through the cavity 14 is removed by dry etching. Thereby, the movable electrode 16 is separated and released from the first silicon substrate 11 (movable part releasing step).

  In this way, by etching the buried oxide film 13 and separating the movable electrode 16 from the first silicon substrate 11, the movable electrode 16 can be moved, that is, can be displaced by applied acceleration and change its capacitance. Thus, the capacitive acceleration sensor 100 described above is completed.

[About the expression of etching selectivity]
The reason why the silicon nitride film 13 is not modified, the mechanism thereof, and the like will be described by exposing the silicon nitride film 20 to the water-soluble substance 21 by exposure to the mist 30 having a small amount of moisture.

As described above, depending on the material of the film to be etched, depending on the amount of moisture, it may or may not become a water-soluble substance by reacting with the etching agent. The etching reaction with hydrofluoric acid (HF) for the silicon oxide film (SiO ₂ ) and the silicon nitride film (SixNyHz) is represented by the following chemical formulas.

(Reaction of silicon oxide film)
SiO ₂ + 2HF ₂ ⁻ + 2H ⁺ + H ₂ O ⇒ SiF ₄ + 3H ₂ O
(Reaction of silicon nitride film)
_{SixNyHz + aHF ⇒ bH 2 SiF 6} + c (NH 4) 2 SiF 6
From the above reaction equation, the etching reaction of the silicon oxide film with hydrofluoric acid does not start unless moisture (H ₂ O) is present, but the etching reaction of the silicon nitride film with hydrofluoric acid does not exist even if moisture exists. I think it will start.

  That is, the silicon nitride film 20 is modified into a water-soluble substance 21 by hydrofluoric acid and etched by subsequent water washing regardless of the amount of moisture in the mist 30. Is indispensable for the reaction with hydrofluoric acid. Therefore, when the water content of the mist 30 is small, it does not become a water-soluble material.

Further, silicon fluoride ammonium salt ((NH ₄ ) ₂ SiF ₆ ), which is a product of the etching reaction of the silicon nitride film, is a water-soluble substance. As a result of the inventor's EDX analysis and ion chromatographic analysis, the water-soluble substance 21 modified by exposing the silicon nitride film to the mist 30 in the etching shown in FIG. It was confirmed to be a salt.

  In this example, as described above, the substrate 10 has the silicon nitride film 20 and the silicon oxide film 13 formed thereon. However, hydrofluoric acid is used as the etchant, and the mist 30 of the hydrofluoric acid etching solution is used. Further, heated nitrogen is used as a gas mixed with the mist 30.

  Accordingly, in the film to be etched modification step, the water content of the mist 30 made of the hydrofluoric acid aqueous solution can be made as small as possible, and only the silicon nitride film 20 can be substantially modified into a water-soluble substance.

  Therefore, the water washing step hardly etches the silicon oxide film 13 that is not water-soluble, and the water-soluble substance 21 that is a modified product of the silicon nitride film 20 can be selectively removed and etched.

  A specific etching state of each of the films 13 and 20 will be described with reference to FIGS. The inventor investigated the etching rates of the films 13 and 20 when the mist 30 was heated and when the mist 30 was not heated in order to change the amount of water in the mist 30.

  FIG. 5 is a diagram showing the relationship between the amount of mist introduced into the silicon oxide film 13 and the etching rate, and FIG. 6 is a diagram showing the relationship between the amount of mist introduced into the silicon nitride film 20 and the etching rate.

  In FIG. 5 and FIG. 6, “mist room temperature” is introduced into the processing tank 201 by supplying room temperature nitrogen from the heating device 206 without heating by the mist heating device 206 in the mist etching device 200. The case where the mist 30 to be made is room temperature is shown.

  “With mist heating” means that the temperature of the heated nitrogen introduced from the mist heating device 206 to the mist introduction rod 205 is 40 ° C. to heat the mist 30 to the treatment tank 201 as in the etching example described above. The case where it introduces is shown.

The “mist generation N ₂ flow rate” is a flow rate of atomizing nitrogen by the mist generator 204, and its unit is liter / minute (L / min). The amount of mist 30 introduced into the processing tank 201 is substantially proportional to this “mist generation N ₂ flow rate”.

  Further, the unit of the etching rate of the silicon oxide film in FIG. 5 is nm / min, which indicates the thickness of the silicon oxide film 13 removed by mist etching per minute.

  Also, the unit of the etching rate of the silicon nitride film in FIG. 6 is nm / min. This is obtained by subtracting the thickness of the silicon nitride film 20 thinned after the water washing step from the thickness of the silicon nitride film 20 before etching, and substantially per minute. This corresponds to the thickness of the water-soluble substance 21 generated by mist etching.

As shown in FIG. 5, in the case of the silicon oxide film 13, the etching rate increases at the “mist room temperature (white circle plot)” as the “mist generation N ₂ flow rate”, that is, the mist introduction amount increases. In the case of “With mist heating (white square plot)”, the etching rate is almost zero even if the mist introduction amount is increased.

On the other hand, as shown in FIG. 6, in the case of the silicon nitride film 20, “mist” is almost the same in both “mist room temperature (white circle plot)” and “with mist heating (white square plot)”. The etching rate increases as the “generated N ₂ flow rate” increases.

  That is, in the case of the silicon oxide film 13, the etching can be suppressed if the moisture of the mist 30 is controlled and reduced by heating the mist 30, but in the case of the silicon nitride film 20, even hydrofluoric acid as an etching agent exists. Then, etching can be performed regardless of the moisture content of the mist 30. This is in accordance with the etching reaction equation described above.

  In the example of etching shown in FIG. 3, since the case of “with mist heating” in FIGS. 5 and 6 is applied to both the silicon oxide film 13 and the silicon nitride film 20, the silicon oxide film 13 is etched. However, the silicon nitride film 20 can be selectively etched.

  If the above phenomenon is used, selective etching is unnecessary, and when both the silicon oxide film and the silicon nitride film may be etched, by introducing a mist 30 having a large amount of water at room temperature, Both films can be etched.

[Effects]
By the way, according to the present embodiment, the etching target film 20 such as a silicon nitride film is formed in the processing bath 201 into which the etching solution 32 that is an aqueous solution of an etching agent such as hydrofluoric acid is introduced in the state of the mist 30. After the substrate 10 is installed and the etching target film 20 is exposed to the mist etching solution 32, that is, the mist 30, the etching target film 20 is modified to a water-soluble substance 21, and then the substrate 10 is washed with water 41. There is provided a substrate etching method characterized by etching the film to be etched 20 by removing the water-soluble substance 21.

  According to this etching method, since the mist etching is adopted, the moisture amount of the mist 30 can be easily controlled by controlling the temperature by heating or the like and controlling the mist diameter by spraying.

  Depending on the material of the film to be etched, depending on the amount of the moisture, it may or may not become a water-soluble substance by reacting with the etching agent. Only 20 can be selectively reacted with the etching agent to be modified into the water-soluble substance 21.

  Then, since the water-soluble substance 21 in the film to be etched 20 is removed from the substrate 10 by washing with water 41, the film to be etched is selectively etched.

  In the above example, in this etching method, the mist 30 of the etching solution is mixed with a gas such as nitrogen and introduced into the processing bath 201. According to this, the moisture content of the mist 30 can be controlled by controlling the temperature of the gas by heating the gas to be mixed.

  In particular, in the above example, in this etching method, it is assumed that the substrate 10 is formed with the silicon oxide film 13 and the silicon nitride film 20 to be etched, the etching agent is hydrofluoric acid, and the gas is heated nitrogen. Yes.

  As described above, the silicon nitride film 20 is modified into the water-soluble substance 21 by hydrofluoric acid regardless of the amount of water in the mist 30 and etched by subsequent water washing. If the amount of water is small, the reaction with hydrofluoric acid does not proceed and the water does not become a water-soluble substance.

  Therefore, if hydrofluoric acid is used as the etching agent and heated nitrogen is used as the gas, the amount of water in the mist 30 made of the hydrofluoric acid aqueous solution can be made as small as possible, and substantially only the silicon nitride film 20 reacts with hydrofluoric acid. Therefore, the silicon nitride film 20 can be selectively etched as a film to be etched.

  In addition, as described above, the etching method of this embodiment can be made excellent in microfabrication as in the case of the conventional dry etching by using mist etching. It is suitable for processing a microstructure such as 100.

  Furthermore, as shown in FIG. 9 above, this etching method is an etching mechanism that does not involve a physical impact as in dry etching, so that surface roughness of the SOI substrate 10 after etching can be suppressed as much as possible. it can.

  Further, in this embodiment, by employing mist etching, vacuum equipment such as dry etching is not required, and the amount of etching agent used can be significantly reduced compared to wet etching, so that a cheaper etching method can be achieved. There is also a merit.

  As described above, according to the present embodiment, it is possible to provide an etching method for the etching target film 20 formed on the substrate 10 and an inexpensive etching method that suppresses the roughness of the substrate surface and has excellent selectivity. .

  As described above, in the etching method of the present embodiment, the silicon nitride film 20 is preferably formed by a low temperature plasma CVD method.

  According to the inventor's experimental study, when the silicon nitride film 20 was formed by low temperature plasma CVD (for example, a film formation temperature of about 350 ° C.) and high temperature thermal CVD (for example, a film formation temperature of about 800 ° C.), The silicon nitride film 20 formed by low-temperature plasma CVD is more likely to produce the water-soluble substance 21 and has a higher etching rate.

  Although details are unknown about this, it is considered that the amount of hydrogen (H) in the formed silicon nitride film is different. Specifically, since the amount of hydrogen in the film is higher in the low temperature plasma CVD, it is considered that the silicon nitride film 20 is easily modified to the water-soluble substance 21.

(Other embodiments)
In the above embodiment, after the film to be etched is modified in the processing tank 201, the substrate 10 is taken out of the processing tank 201 and washed in the water tank 40. This water washing is performed by the mist etching apparatus 200. You may perform in the processing tank 201 of. For example, if a nozzle for supplying water is provided in the processing tank 201, the substrate 10 can be washed in the processing tank 201 while being held by the holding member 202 after the reforming step.

  Further, the gas introduced into the processing bath 201 after being mixed with the mist 30 of the etching solution may be an inert gas such as helium or argon in addition to the above nitrogen.

  The film to be etched may be other than the above silicon nitride film as long as it becomes a water-soluble substance when exposed to the mist of the etching solution. Further, as an example of selective etching, a combination of a silicon nitride film and a silicon oxide film has been described, but other combinations may be used.

  Further, the substrate is not limited to the above-described SOI substrate, and may be a semiconductor substrate such as a silicon substrate other than that, or a substrate made of ceramic, metal, resin, or the like. Further, the substrate may not be a simple plate, but may be an arbitrary shape such as a block.

  The present invention is not limited to application to the capacitive acceleration sensor described above, and can be applied to any structure having a step of etching a film to be etched formed on a substrate. For example, the present invention can be widely applied to various sensor devices that perform cavity processing similar to the acceleration sensor, such as an angular velocity sensor and a pressure sensor, and various semiconductor devices.

  In short, in the present invention, a substrate on which an etching target film is formed is placed in a processing tank into which an aqueous solution of an etching agent is introduced in a mist state, and the etching target film is exposed to a mist state aqueous solution to make the etching target film water-soluble. The main part is that the film to be etched is etched by washing the substrate with water after the modification to the active substance, and other parts can be appropriately changed.

It is a schematic sectional drawing of the capacitive acceleration sensor which concerns on embodiment of this invention. It is process drawing which shows the manufacturing method of the capacitive acceleration sensor which concerns on the said embodiment. FIG. 3 is a process diagram illustrating a manufacturing method subsequent to FIG. 2. It is a figure which shows schematic structure of a mist etching apparatus. It is a figure which shows the relationship between the amount of mist introduction | transduction in a silicon oxide film, and an etching rate. It is a figure which shows the relationship between the amount of mist introduction | transduction in a silicon nitride film, and an etching rate. It is a schematic sectional drawing which shows the manufacturing method of the capacitive sensor as a conventional board | substrate. It is a schematic sectional drawing which shows the etching method of the cavity in the board | substrate shown by FIG. It is a figure which shows the presumed etching mechanism of mist etching.

Explanation of symbols

10 ... SOI substrate as substrate, 13 ... Silicon oxide film as buried oxide film,
20 ... Silicon nitride film which is a film to be etched and is a mask, 21 ... Water-soluble substance,
201 ... treatment tank.

Claims (4)

A substrate (10) on which an etching target film (20) is formed is placed in a treatment tank (201) into which an aqueous solution of an etching agent is introduced in a mist state, and the etching target film (20) is placed in the mist state aqueous solution. After the film to be etched (20) is modified to a water-soluble substance (21) by exposing to water,
A method for etching a substrate, comprising: etching the film to be etched (20) by washing the substrate (10) with water. The method for etching a substrate according to claim 1, wherein the aqueous solution in the mist state is mixed with a gas and introduced into the treatment tank (201). The substrate (10) has a silicon nitride film (20) and a silicon oxide film (13) formed thereon,
The etchant is hydrofluoric acid,
The gas is heated nitrogen;
The method for etching a substrate according to claim 2, wherein the silicon nitride film (20) is selectively etched as the film to be etched. The method for etching a substrate according to claim 3, wherein the silicon nitride film (20) is formed by a low temperature plasma CVD method.

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