Thin film deposition method and acoustic wave device

Abstract

To provide a tungsten film deposition method that can minimize rise in resistivity as much as possible, when depositing a tungsten film on an LN substrate or an LT substrate.SOLUTION: This method includes a step for depositing a tungsten film of a predetermined film thickness on an LN substrate 2 in a vacuum atmosphere. Preceding this, there is a preliminary step for etching the LN substrate surface at a predetermined etching rate. The preliminary step involves dry etching with rare gas plasma in a vacuum atmosphere. Preferably, etching conditions for a lithium niobate layer or a lithium tantalate layer are set to ensure that, when a thermally oxidized silicon film is dry-etched under same etching conditions, the etching rate falls within a range of 10 nm / min to 100 nm / min.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a film forming method including a step of using a substrate including a lithium niobate layer or a lithium tantalate layer as a substrate to be processed and forming a tungsten film with a predetermined film thickness on the surface of the lithium niobate layer or the lithium tantalate layer in a vacuum atmosphere, and an elastic wave device manufactured by implementing the film forming method.

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 SAW device has an amorphous silicon oxide film and a lithium niobate film as a piezoelectric layer on one surface of a quartz substrate, and an electrode film is formed in a comb shape on the surface of the lithium niobate film (layer) (see, for example, Patent Document 1). For example, aluminum is used as the electrode film.

[0003] Here, in recent years, there has been a demand to slow down the surface wave propagation speed in order to create a Love wave type that does not attenuate in propagation, and using a metal with a relatively high specific gravity has been considered as one of the methods. Among them, tungsten has attracted attention as this type of electrode film because it has a large electromechanical coupling coefficient (k), a low thermal expansion coefficient, and good heat conduction efficiency. 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, it was found that when a tungsten film is deposited on the surface of a (single crystal) substrate containing a lithium niobate layer as a piezoelectric layer (hereinafter referred to as "LN substrate") or a (single crystal) substrate containing a lithium tantalate layer (hereinafter referred to as "LT substrate") by sputtering, the resistivity increases by more than 1.3 times compared to, for example, when a tungsten film is deposited on the surface of a silicon wafer. Since this increase in resistivity leads to a decrease in the Q value and deterioration of filter performance, it is necessary to suppress the increase in resistivity as much as possible.

[0005] Therefore, the inventors of this invention conducted extensive research and came to the following conclusion: When diffraction evaluation was performed on a deposited tungsten film at different evaluation positions, multiple different diffraction results were observed. This is presumed to be because, during the fabrication of LN and LT substrates, the surface is usually polished, but this polishing process leaves behind fine, repeating polishing marks on the surface. When a tungsten film is deposited with these polishing marks remaining, multiple crystals grow in different directions from the crystal grains that serve as nuclei for film growth in the initial stages of deposition, resulting in the formation of many crystal interfaces in the deposited tungsten film. It is thought that the presence of many crystal interfaces leads to poor electron flow, causing an increase in the resistivity. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-149724 [Overview of the project] [Problems that the invention aims to solve]

[0007] This invention is based on the above findings and aims to provide a method for depositing a tungsten film on an LN substrate or LT substrate that can suppress the increase in resistivity as much as possible, and an elastic wave device with low resistivity. [Means for solving the problem]

[0008] To solve the above problems, the present invention provides a film deposition method that includes the step of depositing a tungsten film of a predetermined thickness on the surface of a lithium niobate layer or lithium tantalate layer in a vacuum atmosphere, using a substrate containing a lithium niobate layer or lithium tantalate layer as the substrate to be processed, further comprising a pre-step of etching the surface of the lithium niobate layer or lithium tantalate layer at a predetermined etching rate. In this case, the pre-step is dry etching by a rare gas plasma in a vacuum atmosphere, and it is preferable that the etching conditions for the lithium niobate layer or lithium tantalate layer 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.

[0009] According to the present invention, when LN substrates and LT substrates are manufactured, if linear polishing marks with repeated fine irregularities remain on their surface, it has been confirmed that by performing dry etching at a predetermined etching rate, the polishing marks can be eliminated as much as possible, although there is no significant change in the arithmetic mean height (Sa). Furthermore, when a tungsten film is deposited on the surface of such an LN substrate or LT substrate, it is possible to deposit a tungsten film with large crystal grains, aligned growth orientation, and few grain boundaries. As a result, the flow of electrons improves, and the resistivity can be reduced.

[0010] In this invention, if the etching rate of the thermal silicon oxide film is slower than 10 nm / min, polishing marks on the surface of the lithium niobate layer or lithium tantalate layer cannot be removed. On the other hand, if the etching rate is faster than 100 nm / min, the surface of the lithium niobate layer or lithium tantalate layer becomes rough due to etching. Furthermore, 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 etching increases ion bombardment on the surface of the lithium niobate layer or lithium tantalate layer, causing the surface to become rough. Conversely, if the pressure is higher than 5.0 Pa, the etching rate becomes extremely slow.

[0011] Furthermore, in order to solve the above problems, the present invention relates to an elastic wave device having a lithium niobate layer or a lithium tantalate layer, on which an electrode film is formed, wherein the lithium niobate layer or lithium tantalate layer is subjected to dry etching with a rare gas plasma under etching conditions such that 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 electrode is composed of a tungsten film. [Brief explanation of the drawing]

[0012] [Figure 1] (a) and (b) are schematic plan and cross-sectional views of the elastic wave device of this embodiment, which is manufactured using the film deposition method of this embodiment. [Figure 2] A schematic cross-sectional view of a dry etching apparatus capable of performing the pre-processing steps of this embodiment. [Figure 3] A graph showing the change in resistivity of a tungsten film with respect to dry etching time. [Figure 4] Graph showing the change in arithmetic mean height (Sa) of the LN substrate and silicon substrate surface with respect to dry etching time. [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 SAW 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 SAW device for extracting only electrical signals of a characteristic frequency using surface acoustic waves. The SAW device 1 comprises an LN substrate (lithium niobate single crystal substrate) 2 as a piezoelectric layer. A pair of comb-shaped electrode layers 3a and 3b are formed on one side of the LN substrate 2, with one electrode layer 3a connected to a high-frequency power supply 4 and the other electrode layer 3b connected to a detection circuit 5. A known LN substrate 2, high-frequency power supply 4, and detection circuit 5 can also be used, so a detailed explanation is omitted here. A tungsten film is used as the electrode layers 3a and 3b, and the tungsten film is deposited using a sputtering apparatus with a tungsten target to a film thickness in the range of 10 nm to 400 nm. A known sputtering apparatus is used, so a detailed explanation, including the structure of the sputtering apparatus and the sputtering conditions, is omitted here.

[0015] Here, a tungsten film was deposited on a commercially available LN substrate 2 and a silicon substrate to a thickness of approximately 70 nm by sputtering, and the resistivity (μΩcm) was measured. The resistivity of the LN substrate was approximately 15 μΩcm, confirming that the resistivity was approximately 1.3 times higher than that of the silicon substrate. The conditions for depositing the tungsten film were set as follows: the power input to the tungsten target was 0.2 kW, the bias power was 60 W, the pressure in the vacuum chamber during deposition was 0.2 Pa (argon gas flow rate 10 sccm), the distance between the target and the substrate was 35 mm, and the temperature of the stage 65 on which the substrate to be processed Sw (described later) was placed was set to 400°C during deposition. Therefore, in the deposition method of this embodiment, a step was added in which the surface of the LN substrate 2 was dry-etched at a predetermined etching rate using the following dry etching apparatus 6 prior to depositing the tungsten film on the LN substrate 2 (pre-step).

[0016] Referring to Figure 2, 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.

[0017] A stage 65 is provided in the vacuum chamber 61, positioned directly below the dielectric window 62, via an insulator 65a, and can hold the substrate Sw to be processed. The output from the high-frequency power supply E2 is connected to the stage 65, 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. Gas introduction pipes 67 leading to each gas source via flow control valves (e.g., mass flow controllers) (not shown) are also connected to the vacuum chamber 61, and etching gas composed of noble gases can be introduced into the vacuum chamber 1 at a predetermined flow rate.

[0018] When dry etching the surface of LN substrate 2 using dry etching apparatus 6, with LN substrate 2 as the substrate to be processed Sw, the substrate to be processed Sw is placed on stage 65, and the vacuum chamber 1 is evacuated by vacuum pump. The vacuum chamber 1 is evacuated to a predetermined pressure (for example, 10°C). -5 When the pressure reaches Pa, etching gas is introduced via the gas introduction tube 67, and high-frequency power is supplied to the antenna coil 63 from the high-frequency power supply E1, and high-frequency power is supplied to the stage 65 from the high-frequency power supply E2. Argon gas is used as the etching gas, but other noble gases such as neon, xenon, and krypton can be used. The flow rate of 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). The high-frequency power supplied to the antenna coil 63 from the high-frequency power supply E1 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 supplied from the high-frequency power supply E2 to stage 65 is set to a frequency of 12.5 MHz to 13.56 MHz and a power of 50 W to 400 W, and is adjusted 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.

[0019] In this embodiment, when the etching rate conversion of the thermal oxide silicon film is slower than 10 nm / min, the polishing marks on the surface of the LN substrate 2 cannot be removed. On the other hand, when it becomes faster than 100 nm / min, rather, the surface of the LN substrate 2 becomes rough due to etching. Further, the pressure in the vacuum chamber during dry etching with the introduction of argon gas is set in the range of 0.1 Pa to 5.0 Pa. When the pressure is lower than 0.1 Pa, the ion impact on the surface of the LN substrate 2 increases due to the increase in the Vdc voltage during etching, and the surface becomes rough. On the other hand, when the pressure becomes higher than 5.0 Pa, the etching rate becomes extremely slow. Furthermore, the etching time is appropriately set within the range where the linear marks on the surface of the LN substrate 2 can be disappeared as much as possible, preferably in the range of 60 sec to 1200 sec, more preferably set to a time in the range of 300 sec to 1200 sec. However, it was confirmed that when the etching time becomes longer than necessary, the specific resistance value that has once decreased tends to increase again.

[0020] According to the above, when manufacturing the LN substrate 2, even if fine uneven polishing marks remain on its surface, by performing dry etching at an etching rate within a predetermined range, although there is no significant change in the arithmetic surface height (Sa), it was confirmed that the polishing marks can be disappeared as much as possible. Then, when a tungsten film is formed on the surface of such an LN substrate 2, a tungsten film with large crystal grains, aligned growth orientations, and few grain boundaries can be formed. By reducing the crystal interfaces of the formed tungsten film, the flow of electrons is improved and the specific resistance value can be decreased. As a result, even when using a tungsten film having advantages such as a large electromechanical coupling coefficient (k) as the electrode film 5 of the SAW device 1, the increase in the specific resistance value can be suppressed as much as possible, and an elastic wave device 1 with a low specific resistance value can be manufactured.

[0021] Next, the following experiment was conducted to demonstrate the effects of the present invention. In this experiment, a commercially available LN substrate 2 and a silicon substrate were used, and the surfaces of the LN substrate 2 and the silicon substrate were dry-etched using the dry etching apparatus 6 described above, after which a tungsten film was deposited. For the etching conditions, argon gas was used as the etching gas. The high-frequency power supplied from the high-frequency power supply E1 was set to a frequency of 13.6 MHz and 400 W, and the high-frequency power supplied from the 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 for the silicon thermal oxide film under these etching conditions was 21.1 nm / min. Dry etching was then performed at room temperature with dry etching times set to 60 sec, 300 sec, and 1200 sec, respectively. After that, a tungsten film with a thickness of 70 nm was deposited by a known sputtering method.

[0022] FIG. 3 is a graph showing the change in the average value of the resistivity of the tungsten film with respect to the dry etching time. In FIG. 3, -〇- represents a silicon substrate, and -□- represents the LN substrate 2. According to this, in the case of the silicon substrate, as the dry etching time increases, the resistivity of the tungsten film increases. On the other hand, in the case of the LN substrate 2, the resistivity of the tungsten film decreases only by performing dry etching, and when set to 1200 seconds, it was confirmed that the resistivity decreased to about 10 μΩcm. Further, when measuring the arithmetic mean height (Sa) of the surface of the LN substrate 2 with respect to the etching time, as shown in FIG. 4, in the silicon substrate, as the etching time increases, the arithmetic mean height (Sa) increases, while in the LN substrate 2, it was confirmed that there is almost no change. In FIG. 4, -〇- represents a silicon substrate, and -□- represents the LN substrate 2. Furthermore, after forming a tungsten film on the LN substrate 2 and confirming its surface state with an analytical image, when no dry etching was performed at all, a large number of linear marks could be confirmed (see FIG. 5(a)). Moreover, when diffraction evaluation was performed by changing the evaluation position with respect to the formed tungsten film, a plurality of different diffraction results were observed. On the other hand, in the case where dry etching was performed for 1200 seconds, the linear marks were significantly disappeared (see FIG. 5(b)), and even when diffraction evaluation was performed by changing the evaluation position, almost the same diffraction result was obtained.

[0023] As described above, the embodiments of the present invention have been described. However, various modifications are possible as long as they do not depart from the scope of the technical idea of the present invention. In the above embodiment, the dry etching method was described as an example for the pre-process of the film forming process. However, as long as the polishing marks can be eliminated as much as possible, it is not limited to this, and wet etching, CMP, etc. can also be used. Also, in the above embodiment, the LN substrate 2 was described as an example. However, it is not limited to this, and the present invention can also be applied when forming a tungsten film on an LT substrate that has linear polishing marks repeating fine irregularities on its surface.

Description of Reference Numerals

[0024] Sw...Substrate to be processed, 1...SAW device (Acoustic wave device), 2...LN substrate, 3...Electrode film (Tungsten film).

Claims

1. A film deposition method comprising the step of depositing a tungsten film of a predetermined thickness on the surface of a lithium niobate layer or lithium tantalate layer in a vacuum atmosphere, using a substrate containing a lithium niobate layer or a lithium tantalate layer as the substrate to be processed, A film formation method further comprising a pre-step of etching the surface of a lithium niobate layer or a lithium tantalate layer at a predetermined etching rate, characterized in that the pre-step eliminates the repeated linear polishing marks that cause fine irregularities on the surface of the lithium niobate layer or lithium tantalate layer.

2. A film formation method comprising the step of forming a tungsten film of a predetermined thickness on the surface of a lithium niobate layer or a lithium tantalate layer in a vacuum atmosphere, wherein the substrate to be processed is a substrate containing a lithium niobate layer or a lithium tantalate layer, The process further includes a pre-step of etching the surface of a lithium niobate layer or a lithium tantalate layer at a predetermined etching rate, The aforementioned prior step is dry etching by plasma of a rare gas in a vacuum atmosphere, and the etching conditions for the lithium niobate layer or lithium tantalate layer 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, and the dry etching time is set in the range of 60 sec to 1200 sec.

3. In an elastic wave device having a lithium niobate layer or a lithium tantalate layer, with an electrode film formed on its surface, The lithium niobate layer or lithium tantalate layer has no linear polishing marks on its surface that repeat fine irregularities. An elastic wave device characterized by having electrodes composed of tungsten films.

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