JP2024000432A - Deposition method and elastic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: 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

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention provides a film forming method including the step of forming a tungsten film to a predetermined thickness on the surface of the substrate to be processed in a vacuum atmosphere, using a substrate on which a thin film mainly composed of aluminum nitride is formed. The present invention also relates to an acoustic wave device manufactured by implementing this film-forming method.

For example, communication equipment includes acoustic wave devices that function as filters, such as SAW devices (surface acoustic wave devices) and BAW devices (bulk acoustic wave devices), depending on the frequency band, in order to remove noise contained in electrical signals. Be prepared. For example, a BAW device has a bottom electrode film, a piezoelectric film, and a top electrode film that are sequentially laminated, and the piezoelectric film is usually a thin film mainly composed of an aluminum nitride film (such as an AlN film or a ScAlN film). (For example, see Patent Document 1).

On the other hand, as the top electrode film, a single layer film or a laminated film of chromium, aluminum, titanium, copper, molybdenum, tungsten, tantalum (Ta), etc., is used. Among them, tungsten, which is a high melting point metal, is This type of electrode film is attracting attention because it has a large mechanical coupling coefficient (k) and can maintain piezoelectric properties even in a relatively high temperature range. To form such a tungsten electrode film, a sputtering method using a tungsten target is generally used in consideration of productivity and the like.

Here, if a tungsten film is formed as a top electrode film on an aluminum nitride film as a piezoelectric film by a sputtering method, 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. It was found that the specific resistance value increased compared to the case where a film was formed. Such an increase in resistivity value becomes a factor that inhibits the development of miniaturization of devices, so it is necessary to suppress the increase in resistivity value as much as possible.

Therefore, the inventors of the present application have conducted extensive research and have come to the following knowledge. That is, when a tungsten film is formed on the surface of an aluminum nitride film, the formed tungsten film grows along the aluminum nitride surface having an acicular surface structure, and the crystal grain size becomes fine. When the tungsten film was observed in cross section, fine columnar structures were observed along the unevenness of the aluminum nitride surface. The narrowing of the tungsten film per unit volume is synonymous with the increase in the number of grain boundaries, and the increase in the number of grain boundaries impedes the movement of electrons and causes deterioration of the electrical properties of the tungsten film. It was considered that such narrow columnarization of the tungsten film was caused by minute irregularities on the surface of the aluminum nitride film (ie, the surface on which the tungsten film was formed).

Japanese Patent Application Publication No. 2020-178187

The present invention has been made based on the above findings, and provides a method for forming a tungsten film that can suppress as much as possible an increase in resistivity when a tungsten film is formed on the surface of an aluminum nitride film. The object of the present invention is to provide an acoustic wave device with a low specific resistance value.

In order to solve the above problems, the present invention uses a substrate to be processed on which a thin film mainly composed of aluminum nitride is formed, and forms a tungsten film with a predetermined thickness on the surface of the substrate in a vacuum atmosphere. The film forming method including the step of forming a film is characterized in that it further includes a pre-step of etching the surface of the aluminum nitride film at a predetermined etching rate to planarize the surface.

According to the present invention, when dry etching is performed on an aluminum nitride film having minute irregularities on the surface at a predetermined etching rate and etching amount, the convex portions are preferentially etched due to electric field concentration, and the aluminum nitride film is surface is flattened. When a tungsten film is formed on the surface of the planarized aluminum nitride film by a sputtering method using a tungsten target, the tungsten film becomes a thicker needle-like structure compared to one without the previous process. This suppresses the increase in specific resistance value as much as possible. In this case, it is preferable to set the etching amount on the surface of the aluminum nitride film so that the etching amount when dry etching the thermally oxidized silicon film under the same etching conditions is in the range of 283 nm to 1089 nm. Further, it is preferable that the etching conditions for the aluminum nitride film are set so that the etching rate when dry etching the thermally oxidized silicon film under the same etching conditions is in the range of 10 nm/min to 100 nm/min. This allows the arithmetic mean height (Sa) of the surface of the aluminum nitride film to be in the range of 0.4 nm to 0.8 nm.

Note that even if etching is performed under etching conditions such that the etching rate for the thermally oxidized silicon film is slower than 10 nm/min, aluminum nitride is not etched. On the other hand, if etching is performed under etching conditions such that the etching rate for the thermally oxidized silicon film is faster than 100 nm/min, the surface of the aluminum nitride film is rather roughened by the etching. Further, the pressure inside the vacuum chamber during dry etching with argon gas introduced 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 bombardment against the substrate to be processed, and rather the surface of the aluminum nitride film becomes rough. On the other hand, when the pressure is higher than 5.0 Pa, the etching rate becomes too low.

Further, in order to solve the above problems, the acoustic wave device of the present invention has a substrate having a piezoelectric film and an electrode film on one side, the piezoelectric film is a thin film mainly composed of aluminum nitride, and the piezoelectric film is a thin film mainly composed of aluminum nitride, and The surface of the silicon film is flattened by dry etching with rare gas plasma under etching conditions such that the etching rate is in the range of 10 nm/min to 100 nm/min. It is characterized by being 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.

FIG. 2 is a schematic cross-sectional view of an acoustic wave device of this embodiment manufactured using the film forming method of this embodiment. 5 is a graph showing a change in specific resistance value when a tungsten film is formed by a sputtering method while applying a bias voltage. FIG. 1 is a schematic cross-sectional view of a dry etching apparatus that can perform the pre-process of this embodiment. 1 is a graph of experimental results showing the effects of the present invention. These are surface observation images of a tungsten film, in which (a) is without dry etching and (b) is with dry etching.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for forming a tungsten film and an embodiment of an acoustic wave device according to an embodiment of the present invention will be described with reference to the drawings, assuming that the acoustic wave device is a BAW device.

Referring to FIG. 1, 1 is a BAW device that uses bulk acoustic waves (BAW) propagating inside a substance, and the BAW device 1 includes a substrate 2 made of a semiconductor material such as silicon. On one surface of the substrate 2, a bottom electrode film 3, a piezoelectric film 4, and a top electrode film 5 are sequentially laminated. As the bottom electrode film 3, a single layer film of chromium, aluminum, titanium, copper, molybdenum, tungsten, tantalum (Ta), etc. or a laminated film of these is used. Then, for example, a film is formed with a thickness in the range of 50 nm to 500 nm using a sputtering apparatus that uses a target depending on the thin film. Since a known method can be used to form the bottom electrode film 3 using such a sputtering device, detailed explanations including the structure of the sputtering device and the conditions during sputtering will be omitted here.

As the piezoelectric film 4, a thin film (AlN film or ScAlN film) whose main component is an aluminum nitride film is used. The aluminum nitride film is formed to a film thickness in the range of 300 nm to 1000 nm by reactive sputtering using a sputtering apparatus that uses an aluminum target and nitrogen gas as a reactive gas. Further, as the top electrode film 5, a tungsten film is used. The tungsten film is formed to a thickness in the range of 50 nm to 200 nm using a sputtering apparatus using a tungsten target. As for the film-forming methods used for the piezoelectric film 4 and the top electrode film 5, well-known methods can be used, as described above, so detailed explanations, including the structure of the sputtering device and conditions during sputtering, will be omitted here. .

Here, when the film thicknesses of aluminum nitride are set to 40 nm and 1000 nm, and a tungsten film (top electrode film 5) is formed to a film thickness of about 70 nm on the surface of aluminum nitride, as shown in FIG. Compared to the case where a tungsten film is formed on the surface of the formed silicon oxide film by the sputtering method under the same conditions as above, the specific resistance value increases, and at this time, when the piezoelectric film 4 is thinner, the It was found that this caused a more significant increase in the specific resistance value. For reference, when forming a tungsten film on the surface of a silicon oxide film, when bias power was applied to the silicon wafer at each power of 0, 35 W, 60 W, and 80 W (13.56 MHz), there was a noticeable change in the resistivity value. was not seen. In addition, in FIG. 2, △ indicates the case where the film thickness of the piezoelectric film 4 is 40 nm, □ indicates the case where the film thickness of the piezoelectric film 4 is 1000 nm, and ◇ indicates the case where the silicon oxide film is formed on the surface of the silicon wafer. It is. The conditions for forming the tungsten film were as follows: power input to the tungsten target was 0.2 kW, bias power was 60 W, pressure in the vacuum chamber during film formation was 0.2 Pa (argon gas flow rate 10 sccm), and the target The distance between the substrates was set to 55 mm, and the substrate temperature during film formation was set to 400°C. In order to suppress such an increase in the resistivity value as much as possible, in the film forming method of this embodiment, the surface of the aluminum nitride film as the piezoelectric film 4 is etched in a predetermined manner using the following dry etching apparatus 6. We decided to provide a process of flattening the surface by performing dry etching at a high rate (pre-process).

Referring to FIG. 3, dry etching apparatus 6 is an ICP (inductively coupled plasma) type dry etching apparatus, and includes a cylindrical vacuum chamber 61 having an upper opening 61a. The upper opening 61a of the vacuum chamber 61 is hermetically closed by a dielectric window 62 made of a quartz plate via an O-ring. A plurality of stages (two stages in this embodiment) of loop-shaped antenna coils 63 are provided above the dielectric window 62, and the output from the high frequency power source E1 is connected to the antenna coil 63. Further, although not shown in detail, a so-called star electrode 64 is arranged between the dielectric window 62 and the antenna coil 63.

A stage 65 is provided in the vacuum chamber 61 directly below the dielectric window 62 via an insulator 65a, and a substrate to be processed Sw has a bottom electrode film 3 and a piezoelectric film 4 formed on the surface of the substrate 2. can be retained. An output from a high frequency power source 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. The vacuum chamber 61 is also connected to a gas introduction pipe 67 that leads to each gas source via a flow control valve (for example, a mass flow controller) not shown, and supplies an etching gas composed of a rare gas to the vacuum chamber 1 at a predetermined flow rate. It can be introduced within.

When dry etching the piezoelectric film 4 of the substrate Sw to be processed using the dry etching apparatus 6 using the substrate Sw to be processed having the piezoelectric film 4 formed thereon, the substrate Sw to be processed is placed on the stage 65 and the substrate to be processed Sw is turned on by the vacuum pump. The inside of the vacuum chamber 1 is evacuated by vacuum evacuation. When the inside of the vacuum chamber 1 reaches a predetermined pressure (for example, 10 ⁻⁵ Pa), etching gas is introduced through the gas introduction pipe 67, and high frequency power is supplied from the high frequency power source E1 to the antenna coil 63, and at the same time, the high frequency power source E2 is supplied with high frequency power to the antenna coil 63. High frequency power is supplied to the stage 65 from the stage 65. Argon gas is used as the etching gas, but other rare gases such as neon, xenon, and krypton can also 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, which is evacuated at a constant pumping speed, is maintained in the range of 0.1 Pa to 5.0 Pa). Furthermore, the frequency of the high frequency power input from the high frequency power source E1 to the antenna coil 63 is set to 12.5 MHz to 13.56 MHz, and the power is set to 400 W to 800 W. On the other hand, the high frequency power input from the high frequency power source E2 to the stage 65 is set to have a frequency of 12.5 MHz to 13.56 MHz and a power of 50 W to 400 W.

With the above etching conditions, the etching rate for the piezoelectric film 4 is 10 nm/min to 100 nm/min in terms of the etching rate when dry etching the thermally oxidized silicon film under the same etching conditions, that is, the etching rate of the thermally oxidized silicon film. The range is . On the other hand, the etching amount for the piezoelectric film 4 is set, for example, by appropriately adjusting the etching time so that the etching amount when dry etching the thermally oxidized silicon film under the same etching conditions is in the range of 283 nm to 1089 nm. Ru. If the etching rate of the thermally oxidized silicon film is slower than 10 nm/min, the piezoelectric film 4 cannot be etched effectively, while if the etching rate is faster than 100 nm/min, the surface of the piezoelectric film 4 will become rough 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. Further, 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 bombardment on the piezoelectric film 4 increases due to the increase in Vdc voltage, and the surface of the piezoelectric film 4 becomes rough due to dry etching, whereas when the pressure is higher than 5.0 Pa, , the etching rate becomes extremely slow.

According to the above, when dry etching is performed on the piezoelectric film 4 having minute irregularities on the surface at a predetermined 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 is flattened (that is, the surface of the piezoelectric film 4 can have a predetermined arithmetic mean height suitable for forming a tungsten film (top electrode film 5) having a thick needle-like structure). When a tungsten film is formed on the surface of the planarized aluminum nitride film by a sputtering method using a tungsten target, the tungsten film becomes a thicker needle-like structure compared to one without the previous process. This suppresses the increase in specific resistance value as much as possible. As a result, even if a tungsten film, which has advantages such as a large electromechanical coupling coefficient (k), is used 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 the specific resistance value It is possible to fabricate an acoustic wave device with low

Next, the following experiment was conducted to demonstrate the effects of the present invention. In this experiment, a molybdenum film with a thickness of 350 nm was formed as a bottom electrode film 3 on the surface of a silicon substrate 2, and an aluminum nitride film with a thickness of 825 nm was formed on the surface of the molybdenum film by reactive sputtering. It was assumed that a film was formed. As etching conditions, the etching gas was argon gas, the high frequency power input from the high frequency power source E1 was set to a frequency of 13.6 MHz and 400 W, and the high frequency power input from the high frequency power source E2 was set to a frequency of 12.5 MHz and a power of 100 W. Furthermore, the flow rate of argon gas was set so that the pressure inside 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 the thermally oxidized silicon film was dry etched under the same etching conditions, the etching rate was 21.2 nm/min. Then, dry etching was performed on the aluminum nitride film at room temperature by setting dry etching times to 517 seconds, 800 seconds, 1541 seconds, and 3082 seconds, respectively. That is, the etching amount of aluminum nitride was 40 nm, 61.7 nm, 119 nm, and 238 nm, and when converted into the etching amount of the thermal oxide silicon film, it was 183 nm, 283 nm, 545 nm, and 1089 nm. Thereafter, a tungsten film with a thickness of 70 nm was formed on the surface of the aluminum nitride film by a known sputtering method.

FIG. 4 is a graph showing changes in the average specific resistance value of the tungsten film with respect to dry etching time. According to this, when no dry etching was performed on the aluminum nitride film, the specific resistance value of the tungsten film was 14.5 μΩcm. On the other hand, when dry etching is performed, as the dry etching time becomes longer, the specific resistance value decreases. It was confirmed that the specific resistance value decreased to about 10 μΩcm. Furthermore, when observing the surface condition of the tungsten film after it has been formed using an atomic force microscope (AFM), it is found that aluminum nitride films that have not been dry-etched at all have a single-crystal-like film quality, with an acicular structure. It can be seen that there is a lot of space inside (see Figure 5(a)). On the other hand, the tungsten film formed after dry etching the aluminum nitride film for about 3000 seconds flattens the surface of the aluminum nitride film, making it as if no dry etching had been performed at all. It was confirmed that it had a thick needle-like structure.

Although the embodiments of the present invention have been described above, various modifications can be made without departing from the scope of the technical idea of the present invention. In the above embodiment, the dry etching method is used as a pre-process of the film forming process. The method is not limited to this, as long as it can perform a pre-process to planarize the surface of the piezoelectric film 4, and wet etching using KOH or the like as an etching solution, CMP, etc. can also be used. In the case of wet etching, the etching rate is set in a range of 10 nm/min to 100 nm/min.

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 (5)

In a film formation method including a step of forming a tungsten film to a predetermined thickness on the surface of the substrate to be processed in a vacuum atmosphere, using a substrate on which a thin film mainly composed of aluminum nitride is formed,
A film forming method further comprising a pre-step of etching the surface of the aluminum nitride film at a predetermined etching rate to planarize the surface. The pre-process is dry etching using rare gas plasma in a vacuum atmosphere, and the nitriding process is performed so that the etching amount when dry etching the thermally oxidized silicon film under the same etching conditions is in the range of 283 nm to 1089 nm. 2. The film forming method according to claim 1, wherein the amount of etching on the surface of the aluminum film is set. The pre-process is dry etching using rare gas plasma in a vacuum atmosphere, and the etching rate is in the range of 10 nm/min to 100 nm/min when dry etching the thermally oxidized silicon film under the same etching conditions. 3. The film-forming method according to claim 1, wherein etching conditions for the aluminum nitride film are set in this manner. In an acoustic wave device having a piezoelectric film and an electrode film on one side of the substrate,
The piezoelectric film is a thin film mainly composed of aluminum nitride, and dry etching is performed using rare gas plasma under etching conditions such that the etching rate when dry etching the thermally oxidized silicon film is in the range of 10 nm/min to 100 nm/min. An acoustic wave device characterized in that the top electrode is made of a tungsten film. The acoustic wave device according to claim 4, wherein the arithmetic mean height (Sa) of the surface of the aluminum nitride film is 1.0 nm or less.

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