Film formation method

Abstract

To provide a tungsten film deposition method that can minimize rise in resistivity as much as possible, when depositing a tungsten film on a surface of an aluminum nitride film.SOLUTION: A deposition method according to the present invention includes the step of preparing a target substrate Sw with a thin film primarily composed of aluminum nitride formed on its surface, and depositing a tungsten film of a predetermined film thickness on a target substrate surface in a vacuum atmosphere. This method further includes a preliminary step for etching the surface of the aluminum nitride film at a predetermined etching rate to flatten its surface.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a film forming method including a step of forming a tungsten film with a predetermined film thickness on the surface of a substrate to be processed, on which a thin film mainly composed of aluminum nitride is formed on the surface. In the law Related to.

Background Art

[0002] For example, communication devices are provided with elastic wave devices that function as filters such as SAW devices (surface acoustic wave devices) and BAW devices (bulk acoustic wave devices) according to their frequency bands in order to remove noise contained in electrical signals. For example, a BAW device has a bottom electrode film, a piezoelectric film, and a top electrode film that are sequentially laminated, and as the piezoelectric film, a thin film mainly composed of an aluminum nitride film (AlN film or ScAlN film) is usually used (see, for example, Patent Document 1).

[0003] On the other hand, as the top electrode film, a single layer film of chromium, aluminum, titanium, copper, molybdenum, tungsten, tantalum (Ta), etc. or a laminated film thereof is used. Among them, tungsten as a high melting point metal has a large electromechanical coupling coefficient (k) and can maintain piezoelectric characteristics even in a relatively high temperature range, so it has attracted attention as this type of electrode film. For forming such a tungsten electrode film, a sputtering method using a tungsten target is generally used in consideration of productivity and the like.

[0004] Here, when a tungsten film as a top electrode film is formed on an aluminum nitride film as a piezoelectric film by a sputtering method, it has been found that the specific resistance value increases compared to the case where, for example, a silicon oxide film is formed on the surface of a silicon wafer and a tungsten film is formed on the surface of this silicon oxide film. Such an increase in the specific resistance value becomes a factor that hinders the miniaturization development of the device, so it is necessary to suppress the increase in the specific resistance value as much as possible.

[0005] Therefore, the inventors of this invention conducted extensive research and came to the following conclusion: When a tungsten film is deposited on the surface of an aluminum nitride film, the deposited tungsten film grows along the surface of the aluminum nitride, which has a needle-like surface structure, resulting in finer grain size. When the tungsten film was observed in cross-section, fine columnar structures were observed along the irregularities of the aluminum nitride surface. This reduction in the size of the tungsten film per unit volume is equivalent to an increase in grain boundaries, and an increase in grain boundaries hinders electron movement, leading to a deterioration of the electrical properties of the tungsten film. It was concluded that this reduction in the size of the tungsten film is due to the presence of fine irregularities on the surface of the aluminum nitride film (i.e., the surface on which the tungsten film is deposited). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-178187 [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention is based on the above findings and provides a method for depositing a tungsten film that can suppress the increase in resistivity when a tungsten film is deposited on the surface of an aluminum nitride film as much as possible. Law The objective is to provide it. [Means for solving the problem]

[0008] To solve the above problems, the present invention provides a film deposition method that includes a step of forming a tungsten film of a predetermined thickness on the surface of a substrate to be processed, in a vacuum atmosphere, wherein the substrate to be processed has a thin film mainly composed of aluminum nitride formed on its surface, and the method further includes a pre-step of etching the surface of the aluminum nitride film at a predetermined etching rate to flatten its surface.

[0009] According to the present invention, when dry etching is performed on an aluminum nitride film having fine irregularities on its surface with a predetermined etching rate and etching amount, the convex portions are preferentially etched due to electric field concentration, and the surface of the aluminum nitride film is flattened. When a tungsten film is deposited on the surface of the flattened aluminum nitride film by sputtering using a tungsten target, the tungsten film forms a thicker needle-like structure compared to one without the preceding process, thereby suppressing the increase in resistivity as much as possible. In this case, it is preferable to set the etching amount of the aluminum nitride film surface so that the etching amount when dry etching a thermal silicon oxide film under equivalent etching conditions is in the range of 283 nm to 1089 nm. Furthermore, it is preferable to set the etching conditions for the aluminum nitride film so that the etching rate when dry etching a thermal silicon oxide film under equivalent etching conditions is in the range of 10 nm / min to 100 nm / min. This makes it possible to set the arithmetic mean height (Sa) of the surface of the aluminum nitride film to the range of 0.4 nm to 0.8 nm.

[0010] Furthermore, even if etching is performed under etching conditions where the etching rate for the thermal silicon oxide film is slower than 10 nm / min, the aluminum nitride will not be etched. On the other hand, if etching is performed under etching conditions where the etching rate for the thermal silicon oxide film is faster than 100 nm / min, the surface of the aluminum nitride film will actually become rough due to etching. In addition, the pressure inside the vacuum chamber during dry etching with argon gas is set in the range of 0.1 Pa to 5.0 Pa. If the pressure is lower than 0.1 Pa, the increase in Vdc voltage during sputtering increases ion impact on the substrate being processed, which will actually roughen the surface of the aluminum nitride film. On the other hand, if the pressure is higher than 5.0 Pa, the etching rate will become too low.

[0011] Furthermore, in order to solve the above problems, the present invention provides an elastic wave device having a piezoelectric film and an electrode film on one side of a substrate, characterized in that the piezoelectric film is a thin film mainly composed of aluminum nitride, and its surface is planarized by dry etching with a rare gas plasma under etching conditions in which the etching rate when a thermal silicon oxide film is dry etched is in the range of 10 nm / min to 100 nm / min, and the upper electrode is composed of a tungsten film. In this case, it is preferable that the arithmetic mean height (Sa) of the surface of the aluminum nitride film is 1.0 nm or less. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic cross-sectional view of the elastic wave device of this embodiment, which is fabricated using the film deposition method of this embodiment. [Figure 2] A graph showing the change in resistivity when a tungsten film is deposited by sputtering with a bias voltage applied. [Figure 3] A schematic cross-sectional view of a dry etching apparatus capable of performing the pre-processing steps of this embodiment. [Figure 4] A graph showing experimental results illustrating the effects of the present invention. [Figure 5] These are surface observation images of a tungsten film, with (a) showing the film without dry etching and (b) showing the film with dry etching. [Modes for carrying out the invention]

[0013] Hereinafter, with reference to the drawings, the elastic wave device will be defined as a BAW device, and embodiments of the tungsten film deposition method and elastic wave device according to the present invention will be described.

[0014] Referring to Figure 1, 1 is a BAW device that utilizes bulk acoustic waves (BAW) propagating within a material. The BAW device 1 comprises a substrate 2 made of a semiconductor material such as silicon, and a bottom electrode film 3, a piezoelectric film 4, and a top electrode film 5 are sequentially laminated on one side of the substrate 2. As the bottom electrode film 3, a single layer film of chromium, aluminum, titanium, copper, molybdenum, tungsten, tantalum (Ta), or a laminate of these can be used. Then, for example, the film is deposited with a thickness in the range of 50 nm to 500 nm using a sputtering apparatus that uses a target appropriate for the thin film. Since known methods can be used for depositing the bottom electrode film 3 using such a sputtering apparatus, a detailed explanation, including the structure of the sputtering apparatus and the sputtering conditions, is omitted here.

[0015] For the piezoelectric film 4, a thin film mainly composed of aluminum nitride (AlN film or ScAlN film) is used. The aluminum nitride film is deposited using a reactive sputtering method with a thickness in the range of 300 nm to 1000 nm, using a sputtering apparatus that uses an aluminum target and nitrogen gas as the reaction gas. For the top electrode film 5, a tungsten film is used. The tungsten film is deposited using a sputtering apparatus that uses a tungsten target, with a thickness in the range of 50 nm to 200 nm. The deposition methods used for the piezoelectric film 4 and the top electrode film 5 are also known methods, as described above, so a detailed explanation, including the structure of the sputtering apparatus and the sputtering conditions, is omitted here.

[0016] Here, when the aluminum nitride film thickness was set to 40 nm and 1000 nm, and a tungsten film (top electrode film 5) was deposited on the surface of the aluminum nitride with a thickness of approximately 70 nm, as shown in Figure 2, the resistivity increased compared to, for example, when a tungsten film was deposited on the surface of a silicon oxide film formed on a silicon wafer by sputtering under the same conditions as above. It was found that the increase in resistivity was more pronounced when the piezoelectric film 4 was thinner. For reference, when bias power was applied to the silicon wafer at 0, 35 W, 60 W, and 80 W (13.56 MHz) when depositing a tungsten film on the surface of a silicon oxide film, no significant change in resistivity was observed. In Figure 2, △ represents the case when the piezoelectric film 4 thickness is 40 nm, □ represents the case when the piezoelectric film 4 thickness is 1000 nm, and ◇ represents the case when a silicon oxide film is deposited on the surface of a silicon wafer. Furthermore, the conditions for depositing the tungsten film were set as follows: a power input to the tungsten target of 0.2 kW, a bias power of 60 W, a pressure in the vacuum chamber during film deposition of 0.2 Pa (argon gas flow rate of 10 sccm), a target-substrate distance of 55 mm, and a substrate temperature of 400°C during film deposition. In order to suppress such an increase in resistivity as much as possible, the film deposition method of this embodiment includes a step (preceding step) in which the surface of the aluminum nitride film, which is the piezoelectric film 4, is dry-etched at a predetermined etching rate using the following dry etching apparatus 6 to flatten the surface.

[0017] Referring to Figure 3, the dry etching apparatus 6 is an ICP (inductively coupled plasma) type dry etching apparatus, and the dry etching apparatus 6 comprises a cylindrical vacuum chamber 61 having an upper opening 61a. The upper opening 61a of the vacuum chamber 61 is closed in an airtight manner via an O-ring by a dielectric window 62 made of a quartz plate. Above the dielectric window 62, multiple (two in this embodiment) loop-shaped antenna coils 63 are provided, and the output from the high-frequency power supply E1 is connected to the antenna coils 63. In addition, a so-called star electrode 64 is arranged between the dielectric window 62 and the antenna coils 63, although it is not shown in detail.

[0018] A stage 65 is provided in the vacuum chamber 61 positioned directly below the dielectric window 62 via an insulator 65a, and a processed substrate Sw on which a bottom electrode film 3 and a piezoelectric film 4 are formed on the surface of the substrate 2 can be held. An output from a high-frequency power supply E2 is connected to the stage 2, and a bias potential can be applied to the substrate Sw. An exhaust pipe 66 leading to a vacuum pump (not shown) is connected to the vacuum chamber 1, and the inside of the vacuum chamber 61 can be evacuated to a predetermined pressure. A gas introduction pipe 67 leading to each gas source is also connected to the vacuum chamber 61 via a flow control valve (e.g., a mass flow controller, not shown), and an etching gas composed of a rare gas can be introduced into the vacuum chamber 1 at a predetermined flow rate.

[0019] When dry-etching the piezoelectric film 4 of the processed substrate Sw using the processed substrate Sw on which the piezoelectric film 4 is formed by a dry etching apparatus 6, the processed substrate Sw is placed on the stage 65, and the inside of the vacuum chamber 1 is evacuated by the vacuum exhaust of the vacuum pump. When the inside of the vacuum chamber 1 reaches a predetermined pressure (e.g., 10 -5 Pa), an etching gas is introduced through the gas introduction pipe 67, high-frequency power is applied from the high-frequency power supply E1 to the antenna coil 63, and high-frequency power is applied from the high-frequency power supply E2 to the stage 65. As the etching gas, argon gas is used, but other rare gases such as neon, xenon, and krypton can be used. The flow rate of the etching gas introduced into the vacuum chamber 61 is 10 sccm to 100 sccm (the pressure inside the vacuum chamber 61 evacuated at a constant exhaust speed is maintained in the range of 0.1 Pa to 5.0 Pa). Also, the high-frequency power applied from the high-frequency power supply E1 to the antenna coil 63 is set to a frequency of 12.5 MHz to 13.56 MHz and a power of 400 W to 800 W. On the other hand, the high-frequency power applied from the high-frequency power supply E2 to the stage 65 is set to a frequency of 12.5 MHz to 13.56 MHz and a power of 50 W to 400 W.

[0020] Under the above etching conditions, the etching rate for the piezoelectric film 4 is in the range of 10 nm / min to 100 nm / min in terms of the etching rate of a thermally oxidized silicon film when dry-etched under the same etching conditions, that is, in terms of the etching rate conversion of the thermally oxidized silicon film. On the other hand, the etching amount for the piezoelectric film 4 is set by appropriately adjusting the etching time, for example, so that the etching amount of the thermally oxidized silicon film under the same etching conditions is in the range of 283 nm to 1089 nm. When the etching rate in terms of the thermally oxidized silicon film is slower than 10 nm / min, the piezoelectric film 4 cannot be effectively etched, while when it is faster than 100 nm / min, the surface of the piezoelectric film 4 becomes rougher due to dry etching. As a result, the average height (Sa) of the surface of the aluminum nitride film is in the range of 0.4 to 0.8 nm. Also, the pressure inside the vacuum chamber 61 during dry etching with argon gas introduced is set in the range of 0.1 Pa to 0.5 Pa. When the pressure is lower than 0.1 Pa, the ion impact on the piezoelectric film 4 increases due to the increase in the Vdc voltage, and the surface of the piezoelectric film 4 becomes rougher due to dry etching, while when the pressure is higher than 5.0 Pa, the etching rate becomes extremely slow.

[0021] According to the above, when dry etching is performed on the piezoelectric film 4 having fine irregularities on its surface at a predetermined range of etching rate and etching amount, the convex portions are preferentially etched due to electric field concentration, and the surface of the piezoelectric film 4 is planarized (that is, the surface of the piezoelectric film 4 can be made to have a predetermined arithmetic mean height suitable for forming a thick needle-shaped tungsten film (top electrode film 5)). When a tungsten film is formed on the surface of the thus planarized aluminum nitride film by sputtering using a tungsten target, the tungsten film has a thick needle-shaped structure compared to the case without the previous process, so that an increase in the specific resistance value is suppressed as much as possible. As a result, even when using a tungsten film having advantages such as a large electromechanical coupling coefficient (k) as the top electrode film 5 of the BAW device 1, an increase in the specific resistance value is suppressed as much as possible, and an elastic wave device with a low specific resistance value can be manufactured.

[0022] Next, the following experiment was conducted to demonstrate the effects of the present invention. In this experiment, the substrate to be treated, Sw, was prepared by depositing a molybdenum film with a thickness of 350 nm as a bottom electrode film 3 on the surface of a silicon substrate 2, and then depositing an aluminum nitride film with a thickness of 825 nm on the surface of the molybdenum film by reactive sputtering. As etching conditions, argon gas was used as the etching gas, and the high-frequency power supplied from high-frequency power supply E1 was set to a frequency of 13.6 MHz and 400 W, and the high-frequency power supplied from high-frequency power supply E2 was set to a frequency of 12.5 MHz and 100 W. Furthermore, the flow rate of argon gas was set so that the pressure in the vacuum chamber 61 was maintained at 0.5 Pa during dry etching. The etching rate of aluminum nitride under these etching conditions was 4.63 nm / min. In this case, when a thermal silicon oxide film was dry etched under equivalent etching conditions, the etching rate was 21.2 nm / min. Then, dry etching was performed on the aluminum nitride film at room temperature, with dry etching times set to 517 seconds, 800 seconds, 1541 seconds, and 3082 seconds, respectively. In other words, the etching amounts of aluminum nitride were 40 nm, 61.7 nm, 119 nm, and 238 nm, which, when converted to etching amounts of thermal silicon oxide film, were 183 nm, 283 nm, 545 nm, and 1089 nm. Subsequently, a tungsten film with a thickness of 70 nm was deposited on the surface of the aluminum nitride film using a known sputtering method.

[0023] Figure 4 is a graph showing the change in the average resistivity of the tungsten film with respect to dry etching time. According to this, when the aluminum nitride film was not dry-etched at all, the resistivity of the tungsten film was 14.5 μΩcm. In contrast, when dry etching was performed, the resistivity decreased as the dry etching time increased, and it was confirmed that when the aluminum nitride film was dry-etched for approximately 3000 seconds, the resistivity of the tungsten film decreased to approximately 10 μΩcm. Furthermore, when the surface state of the tungsten film after deposition was observed using an atomic force microscope (AFM), it was found that the aluminum nitride film that was not dry-etched at all had a film quality similar to a single crystal, with a needle-like structure and many internal spaces (see Figure 5(a)). In contrast, the tungsten film deposited after dry etching the aluminum nitride film for approximately 3000 seconds was found to have a thicker needle-like structure compared to the un-dry-etched film, due to the flattening of the surface of the aluminum nitride film.

[0024] While embodiments of the present invention have been described above, various modifications are possible as long as they do not deviate from the technical concept of the present invention. In the above embodiments, a dry etching method was described as an example of a pre-processing step for the film deposition process, but the invention is not limited to this, as long as a pre-processing step can be performed to planarize the surface of the piezoelectric film 4 so that the arithmetic mean height (Sa) of the surface of the aluminum nitride film is in the range of 0.4 nm to 0.8 nm. As the etching solution, wet etching using KOH or CMP can also be used. In the case of wet etching, the etching rate is set in the range of 10 nm / min to 100 nm / min. [Explanation of Symbols]

[0025] Sw...Substrate to be processed, 1...BAW device (Acoustic wave device), 2...Silicon substrate, 3...Bottom electrode film, 4...Piezoelectric film (aluminum nitride film), 5...Top electrode film (tungsten film).

Claims

1. A film deposition method comprising the step of forming a tungsten film of a predetermined thickness on the surface of a substrate to be treated, in a vacuum atmosphere, wherein the substrate to be treated has a thin film mainly composed of aluminum nitride formed on its surface, The process further includes a pre-step in which the surface of an aluminum nitride film is etched at a predetermined etching rate to flatten the surface, The aforementioned step is a film formation method characterized in that the amount of etching on the surface of the aluminum nitride film is set such that the amount of etching when a thermal silicon oxide film is dry etched under equivalent etching conditions is in the range of 545 nm to 1089 nm.

2. The method for forming a film according to claim 1, characterized in that the preceding step is dry etching by plasma of a rare gas in a vacuum atmosphere.

3. The aforementioned prior step is dry etching by plasma of a rare gas in a vacuum atmosphere, and the etching conditions for the aluminum nitride film are set such that the etching rate when a thermal silicon oxide film is dry etched under equivalent etching conditions is in the range of 10 nm / min to 100 nm / min, characterized in that the film formation method according to claim 1 or 2.

Images