Piezoelectric film, MEMS sensor, and method for manufacturing piezoelectric film

Abstract

Provided is a piezoelectric film having scandium aluminum nitride, carbon, and oxygen, the carbon concentration being 2.5 at% or less, and the oxygen concentration being 0.35 at% or less. The present invention also provides an MEMS sensor provided with the piezoelectric film, and a method for manufacturing the piezoelectric film.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202180051424.9 (Invention Title: MEMS Sensor and Manufacturing Method Thereof), filed on February 21, 2023.

[0002] Cross-referencing of related applications

[0003] This application is based on Japanese Patent Application No. 2020-146975, filed on September 1, 2020, the contents of which are incorporated herein by reference. Technical Field

[0004] This disclosure relates to a MEMS (Micro Electro Mechanical Systems) sensor having a piezoelectric film made of scandium aluminum nitride (hereinafter also referred to as ScAlN) and a method for manufacturing the same. Background Technology

[0005] Previously, ultrasonic sensors having a piezoelectric film made of ScAlN have been proposed as MEMS sensors (see, for example, Patent Document 1). Specifically, in this MEMS sensor, the piezoelectric properties can be improved by specifying the carbon concentration of the piezoelectric film to be 2.5 at% (atomic %) or less.

[0006] Furthermore, in this MEMS sensor, the piezoelectric film is formed by sputtering a target. In this case, in the aforementioned MEMS sensor, by using a target composed of ScAl (scandium aluminum) with a carbon concentration of less than 5 at%, the carbon concentration in the piezoelectric film is reduced to less than 2.5 at%.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2014-236051 Summary of the Invention

[0010] However, the inventors further studied the aforementioned MEMS sensor and found that the piezoelectric properties of the piezoelectric film also decreased when the oxygen concentration of the piezoelectric film was high.

[0011] The purpose of this disclosure is to provide a MEMS sensor capable of suppressing the degradation of piezoelectric properties and a method for manufacturing the same.

[0012] According to one aspect of this disclosure, a MEMS sensor includes a substrate on which a diaphragm portion is formed, and a piezoelectric film disposed on the diaphragm portion. The piezoelectric film is composed of ScAlN nitride, with a carbon concentration of 2.5 at% or less and an oxygen concentration of 0.35 at% or less.

[0013] Based on the above configuration, in the piezoelectric film 20, the carbon concentration is specified to be 2.5 at% or less, and the oxygen concentration is specified to be 0.35 at% or less. Therefore, it is possible to suppress the decrease in the piezoelectric strain constant and suppress the decrease in piezoelectric properties.

[0014] Furthermore, according to another aspect of this disclosure, in the above-mentioned MEMS sensor manufacturing method, the steps of placing a substrate and a target material in a cavity and forming a piezoelectric film by sputtering are performed. Before the placement step, the substrate is subjected to heat treatment.

[0015] Based on the above configuration, when the piezoelectric film is formed, the increase in water vapor pressure within the chamber and the increase in oxygen concentration of the piezoelectric film can be suppressed. Therefore, it is possible to manufacture a MEMS sensor that suppresses the degradation of piezoelectric properties.

[0016] Furthermore, the reference symbols in parentheses attached to each constituent element are examples of indicating the correspondence between that constituent element and the specific constituent elements described in the embodiments described later. Attached Figure Description

[0017] Figure 1 This is a cross-sectional view of the ultrasonic sensor in the first embodiment.

[0018] Figure 2 This is a graph showing the relationship between the carbon concentration of the piezoelectric film and the piezoelectric strain constant.

[0019] Figure 3 This is a graph showing the relationship between the oxygen concentration of the piezoelectric film and the piezoelectric strain constant.

[0020] Figure 4 This is a graph showing the relationship between the carbon concentration, oxygen concentration, and piezoelectric strain constant of a piezoelectric film.

[0021] Figure 5 This is a schematic diagram showing the state during the formation of a piezoelectric film.

[0022] Figure 6 This is a graph showing the experimental results relating the oxygen concentration in the piezoelectric film, the oxygen concentration in the target material, the water vapor pressure in the chamber, and the piezoelectric strain constant.

[0023] Figure 7 It means Figure 6 A diagram showing the relationship between the water vapor pressure in the chamber, the oxygen concentration in the piezoelectric film, and the oxygen concentration in the target material.

[0024] Figure 8 It means Figure 6 A diagram showing the relationship between oxygen concentration, piezoelectric strain constant, and water vapor pressure in the target material.

[0025] Figure 9A This is a diagram showing the pressure distribution within the chamber during sputtering without heating the substrate.

[0026] Figure 9B This is a diagram showing the pressure distribution within the chamber when the substrate is placed in the chamber for sputtering after being heated. Detailed Implementation

[0027] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Furthermore, in the following embodiments, parts that are the same or equivalent to each other will be labeled with the same symbol for explanation.

[0028] (First Embodiment)

[0029] The first embodiment will be described with reference to the accompanying drawings. Furthermore, in this embodiment, an ultrasonic sensor, as a MEMS sensor, will be used as an example for description. In addition, the ultrasonic sensor of this embodiment, for example, is mounted around the bumper of a vehicle and is suitable for constituting an object detection device to detect objects around the vehicle.

[0030] The ultrasonic sensor in this embodiment is as follows: Figure 1 As shown, it is constructed by forming a diaphragm portion 11 on a substrate 10 such as silicon and forming a piezoelectric film 20 on the diaphragm portion 11.

[0031] Furthermore, the diaphragm portion 11 is not particularly limited, but in this embodiment, its planar shape is defined as circular. The piezoelectric film 20 is made of ScAlN and is smaller than the planar shape of the diaphragm portion 11. The specific structure of the piezoelectric film 20 will be described later.

[0032] Furthermore, a pad portion 30 is formed on the substrate 10, electrically connected to the piezoelectric film 20, etc., via a wiring pattern (not shown). Moreover, Figure 1 The relationship between the substrate 10 and the piezoelectric film 20 is briefly shown, but in reality, an insulating film or the like can also be formed on the substrate 10.

[0033] The above describes the basic structure of the ultrasonic sensor in this embodiment. Next, the structure of the piezoelectric film 20 in this embodiment will be explained in detail.

[0034] The piezoelectric film 20, as described above, is constructed using ScAlN. In this case, if the piezoelectric film 20 is specified as Sc... x Al 1-x For N (0 < x < 1), the higher the concentration of Sc (i.e., the higher the concentration of Sc), the more effective it is to improve sensitivity by increasing the piezoelectric strain constant. For example, Non-Patent Literature 1, etc., reports on improving sensitivity by increasing the concentration of Sc... x Al 1-xN (0 < x < 1) is defined as 0.3 ≤ x, and the piezoelectric strain constant d33 increases sharply. Therefore, if the piezoelectric film 20 is Sc x Al 1-x For N (0 < x < 1), it is preferably specified as 0.3 ≤ x (Non-patent literature 1: Keiichi Umeda, H. Kawai, A. Honda, M. Akiyama, T. Kato, T. Fukura. “Piezoelectric properties of ScAlN thin films for piezo-MEMS devices” 2013 IEEE 26th International Conference on Micro Electro Mechanical Systems, March 7, 2013).

[0035] Furthermore, regarding this piezoelectric film 20, the inventors conducted a thorough study on the relationship between the carbon concentration of the piezoelectric film 20 and the piezoelectric strain constant d33, and obtained... Figure 2 The results are shown. Furthermore, the inventors have conducted a thorough study on the relationship between the oxygen concentration of the piezoelectric film 20 and the piezoelectric strain constant d33, and obtained... Figure 3 The results are shown. Furthermore, Figure 2 and Figure 3 The Sc constituting the piezoelectric film 20 is shown x Al 1-x N(0<x<1) is defined as the result when 0.3≤x.

[0036] Specifically, such as Figure 2 As shown, it was confirmed that the piezoelectric strain constant d33 decreases slowly if the carbon concentration is greater than 0.7 at%, and decreases sharply if it is greater than 2.5 at%. Furthermore, as... Figure 3 As shown, it was confirmed that the piezoelectric strain constant d33 decreases slowly if the oxygen concentration is greater than 0.1 at%, and decreases sharply if it is greater than 0.35 at%.

[0037] Therefore, in this embodiment, the piezoelectric film 20 has a carbon concentration of 2.5 at% or less and an oxygen concentration of 0.35 at% or less. In this case, it is more preferable that the piezoelectric film 20 has a carbon concentration of 0.7 at% or less and an oxygen concentration of 0.1 at% or less. Furthermore, the at% of the carbon concentration here refers to the number of carbon atoms in the piezoelectric film 20 that is 100 at% of the total number of Sc atoms, Al atoms, and N atoms. Similarly, the at% of the oxygen concentration refers to the number of oxygen atoms in the piezoelectric film 20 that is 100 at% of the total number of Sc atoms, Al atoms, and N atoms.

[0038] Furthermore, regarding the aforementioned piezoelectric film 20, the inventors have conducted a thorough study on the interrelationship between the carbon concentration, oxygen concentration, and piezoelectric strain constant d33 of the piezoelectric film 20, and obtained... Figure 4 The results are shown. Figure 4 The values ​​in the table represent the piezoelectric strain constant d33 [pC / N]. Additionally... Figure 4 In the diagram, double circles represent piezoelectric strain constant d33 of 19 [pC / N] or higher, while single circles represent piezoelectric strain constant d33 of 17 [pC / N] or higher but lower than 19 [pC / N]. Furthermore, Figure 4 The Sc constituting the piezoelectric film 20 is shown x Al 1-x In N(0<x<1), the result is defined as when 0.3≤x.

[0039] Furthermore, by Figure 2 and Figure 3 It is known that 17 [pC / N] of the piezoelectric strain constant d33 is the value at which the piezoelectric strain constant d33 begins to decrease sharply, from... Figure 2 and Figure 3 It is known that 19 [pC / N] of the piezoelectric strain constant d33 is the value at which the piezoelectric strain constant d33 begins to slowly decrease. Furthermore, Figure 4 Region A in the diagram represents the area where the carbon concentration is below 2.5 at% and the oxygen concentration is below 0.35 at%. Furthermore, Figure 4 Region B in the diagram represents the region where the carbon concentration is below 0.7 at% and the oxygen concentration is below 0.1 at%.

[0040] like Figure 4As shown, the piezoelectric strain constant d33 is also 17 [pC / N] or more in region C, where the carbon concentration is 2.5 at% or less and the oxygen concentration is 0.1 at% or less. Furthermore, the piezoelectric strain constant d33 is also 17 [pC / N] or more in region D, where the carbon concentration is 1.0 at% or less and the oxygen concentration is 0.2 at% or less. Additionally, the piezoelectric strain constant d33 is also 17 [pC / N] or more in region E, where the carbon concentration is 0.3 at% or less and the oxygen concentration is 0.35 at% or less. From the above, it is clear that in this embodiment, it is preferable to specify the carbon and oxygen concentrations within the above ranges. By specifying the carbon and oxygen concentrations within the above ranges, the decrease in the piezoelectric strain constant d33 can be sufficiently suppressed.

[0041] Next, the manufacturing method of the piezoelectric film 20 in the ultrasonic sensor described above will be explained.

[0042] In this embodiment, when forming the piezoelectric film 20, such as Figure 5 As shown, a substrate 10 and a target 50 are arranged opposite each other within the chamber 40, and both the substrate 10 and the target 50 are connected to a high-frequency power supply 60. Furthermore, the target 50 is a ScAl alloy, and an alloy with an elemental composition ratio of approximately 0.45:0.55 of Sc and Al can be used. Such a target 50 can be formed, for example, by melting, low-oxygen sintering under conditions of reduced ambient oxygen concentration, or normal sintering with an ambient oxygen concentration similar to that of the atmosphere.

[0043] In addition, during the formation of the piezoelectric film 20, sputtering is performed to attach atoms from the target material 50 to the substrate 10, thereby forming the piezoelectric film 20. During sputtering, for example, the sputtering pressure is set to 0.16 Pa, the nitrogen concentration is set to 43 vol%, and the target electric power density is set to 10 W / cm². 2 The substrate temperature was set to 300°C, and the sputtering time was set to 200 minutes. Furthermore, during sputtering, the pressure inside the chamber 40 was reduced to 5 × 10⁻⁶. -5 Below Pa, 99.999% vol% argon and 99.999% vol% nitrogen are introduced into chamber 40.

[0044] Furthermore, during sputtering, a high-frequency voltage is applied to the high-frequency power supply 60 to form a high-frequency plasma on the surface of the target material 50. The positive ions in the plasma then bombard the target material 50 due to a self-biasing effect. Moreover, the positive ions in the plasma are nitrogen ions and argon ions. Furthermore, by bombarding the target material 50 with positive ions, Sc atoms 71 and Al atoms 72 are ejected from the target material 50 and sputtered onto the substrate 10. In this embodiment, a piezoelectric film 20 composed of ScAlN is thus formed on the substrate 10.

[0045] Here, the carbon concentration of the target material 50 is defined as the number of carbon atoms in the target material 50 that is 100 at% of the total number of Sc atoms and Al atoms. In this case, by using a target material 50 with a carbon concentration of 5 at% or less, the carbon concentration of the piezoelectric film 20 can be kept below 2.5 at% when the piezoelectric film 20 is formed by sputtering. Furthermore, regarding the piezoelectric film 20, the lower the carbon concentration of the target material 50, the lower the carbon concentration of the piezoelectric film 20.

[0046] Furthermore, the inventors have conducted a thorough study on the relationship between sputtering and the oxygen concentration of the piezoelectric film 20. Specifically, the oxygen concentration of the piezoelectric film 20 depends on the oxygen pressure within the chamber 40, the oxygen concentration of the target material 50, and the moisture adhering to the substrate 10. Therefore, the inventors investigated the effects of the oxygen pressure within the chamber 40, the oxygen concentration of the target material 50, and the moisture adhering to the substrate 10, and obtained... Figure 6 , Figure 7 , Figure 8 , Figure 9A and Figure 9B The results are shown.

[0047] Furthermore, as components of oxygen within chamber 40, both oxygen and water vapor are present; however, within chamber 40, the water vapor pressure is more than one digit higher than the oxygen pressure. In other words, the water vapor pressure within chamber 40 is the dominant component of oxygen within chamber 40. Therefore, Figure 6 The results show the relationship between the water vapor pressure (hereinafter referred to as water vapor pressure) within the chamber and the oxygen concentration of the piezoelectric film. Furthermore, Figure 8 The values ​​plotted in the figure represent Figure 6 The water vapor pressure within the chamber. Additionally... Figures 6-8 This means as described below. Figure 9B The illustration shows the results of heat treatment of the substrate 10, or more specifically, the results of heat treatment of the substrate 10 at a temperature higher than that during sputtering. Furthermore, Figure 6 The at% oxygen concentration of the target material is the number of oxygen atoms in target material 50 relative to the total number of Sc atoms and Al atoms, expressed as 100 at%. Additionally, Figure 6 The values ​​next to the oxygen concentration of the target material in the text, such as melting, low-oxygen sintering, and normal sintering, indicate the manufacturing method of the target material 50.

[0048] like Figure 6 and Figure 7 As shown, it can be confirmed that the higher the water vapor pressure inside chamber 40, the higher the oxygen concentration in piezoelectric film 20. Furthermore, as... Figures 6-8As shown, it can be confirmed that the higher the oxygen concentration in the target 50, the higher the oxygen concentration in the piezoelectric film 20, thereby reducing the piezoelectric strain constant d33. Therefore, in order to form a piezoelectric film 20 with a low oxygen concentration, it is only necessary to reduce the water vapor pressure in the chamber 40 or reduce the oxygen concentration in the target 50.

[0049] In addition, such as Figure 9A As shown, it was confirmed that when sputtering is performed with moisture adhering to the substrate 10 without heating the substrate 10, the water vapor pressure inside the chamber 40 increases due to the evaporation of the moisture adhering to the substrate 10. On the other hand, as Figure 9B As shown, it was confirmed that when sputtering is performed after removing moisture from the substrate 10 by heating the substrate 10, the water vapor pressure in the chamber remains approximately constant.

[0050] In other words, when the piezoelectric film 20 is formed by sputtering, by creating a state in which moisture does not adhere to the substrate 10, the increase in water vapor pressure within the chamber 40 can be suppressed, and a piezoelectric film 20 with low oxygen concentration can be formed. In this case, regarding the heat treatment of the substrate 10, it is preferable to perform heat treatment at a temperature higher than the temperature during sputtering, which can sufficiently suppress the generation of water vapor from the substrate 10 during sputtering. Furthermore, Figure 9B The results show the effect of using a substrate 10 that has been heat-treated at a temperature higher than that during sputtering.

[0051] As can be seen from the above, for example, when the oxygen concentration of the piezoelectric film 20 is specified to be 0.7 at% or less, the piezoelectric film 20 can be formed by specifying the oxygen concentration of the target material 50 to be 0.0387 at% or less, specifying the water vapor pressure in the chamber 40 to be 14.8 μPa or less, and using the heat-treated substrate 10. In this case, by further reducing the oxygen concentration of the target material 50, the oxygen concentration of the piezoelectric film 20 can be further reduced. Therefore, when it is desired to further reduce the oxygen concentration of the target material 50, even if the water vapor pressure in the chamber 40 is 14.8 μPa or more, the oxygen concentration of the piezoelectric film 20 can sometimes be kept below 0.7 at%. In other words, the water vapor pressure in the chamber 40 and the oxygen concentration of the target material 50 can be appropriately changed as long as the oxygen concentration of the piezoelectric film 20 is at the desired value. However, by heat-treating the substrate 10 as described above, the water vapor pressure in the chamber 40 can be reduced, and the increase in the oxygen concentration of the piezoelectric film 20 can be suppressed.

[0052] In the embodiment described above, the piezoelectric film 20 has a carbon concentration of 2.5 at% or less and an oxygen concentration of 0.35 at% or less. Therefore, the decrease in the piezoelectric strain constant d33 can be suppressed. In this case, by specifying the carbon concentration to 0.7 at% or less and the oxygen concentration to 0.1 at% or less, the decrease in the piezoelectric strain constant d33 can be further suppressed.

[0053] Furthermore, by specifying the carbon concentration to be 2.5 at% or less and the oxygen concentration to be 0.1 at% or less, the piezoelectric film 20 can sufficiently suppress the decrease in the piezoelectric strain constant d33. Similarly, by specifying the carbon concentration to be 1.0 at% or less and the oxygen concentration to be 0.2 at% or less, the piezoelectric film 20 can sufficiently suppress the decrease in the piezoelectric strain constant d33 by specifying the carbon concentration to be 0.3 at% or less and the oxygen concentration to be 0.35 at% or less.

[0054] Furthermore, when the piezoelectric film 20 is formed by sputtering, a heat-treated substrate 10 is used. Therefore, during the formation of the piezoelectric film 20, the increase in water vapor pressure within the chamber 40 can be suppressed, and the increase in oxygen concentration in the piezoelectric film 20 can be suppressed. In this case, by heating the substrate 10 at a temperature higher than that during sputtering, it is difficult for water vapor to be generated from the substrate 10 during sputtering, thus further suppressing the increase in water vapor pressure within the chamber 40.

[0055] (Other implementation methods)

[0056] This disclosure has been described according to embodiments, but it should be understood that this disclosure is not limited to these embodiments and structures. This disclosure also includes various modifications and variations within the same range. Moreover, various combinations and methods, including other combinations and methods of only one element, more than one element, or less than one element, are also included within the scope or concept of this disclosure.

[0057] For example, in the first embodiment described above, the substrate 10 can be heated at a temperature lower than that during sputtering. Even with such heating, the moisture adhering to the substrate 10 can be removed, thus suppressing the rise in water vapor pressure within the chamber 40 during sputtering. In other words, the rise in oxygen concentration in the piezoelectric film 20 can be suppressed.

[0058] In addition, the MEMS sensor of the first embodiment described above can also be used for sensors other than ultrasonic sensors. For example, it can also be used for a pressure sensor formed by having a piezoelectric membrane 20 on the diaphragm portion 11.

Claims

1. A piezoelectric film comprising scandium aluminum nitride, carbon, and oxygen, wherein the concentration of carbon is less than 2.5 at% and the concentration of oxygen is less than 0.35 at%.

2. The piezoelectric film according to claim 1, wherein, The carbon concentration is less than 2.5 at%, and the oxygen concentration is less than 0.1 at%.

3. The piezoelectric film according to claim 1, wherein, The carbon concentration is less than 1.0 at%, and the oxygen concentration is less than 0.2 at%.

4. The piezoelectric film according to claim 1, wherein, The carbon concentration is less than 0.3 at%, and the oxygen concentration is less than 0.35 at%.

5. The piezoelectric film according to claim 1, wherein, The carbon concentration is less than 0.7 at%, and the oxygen concentration is less than 0.1 at%.

6. The piezoelectric film according to claim 1, wherein, The scandium aluminum nitride has the following composition: Sc x Al 1-x N, where 0 < x < 1, and x is greater than or equal to 0.

3.

7. A MEMS sensor comprising the piezoelectric film of claim 1.

8. The MEMS sensor according to claim 7, wherein, It further includes a substrate and a diaphragm portion formed on the substrate.

9. The MEMS sensor according to claim 8, wherein, The piezoelectric film is disposed on the diaphragm portion.

10. A method for manufacturing a piezoelectric film, wherein, The piezoelectric film comprises scandium aluminum nitride, carbon, and oxygen, wherein the concentration of carbon is less than 2.5 at%, and the concentration of oxygen is less than 0.35 at%. The substrate and target are placed in the cavity, and the piezoelectric film is formed by sputtering. The substrate is heated before the configuration is performed.

11. The method for manufacturing a piezoelectric film according to claim 10, wherein, In the heat treatment step, the substrate is heated at a temperature higher than that during the sputtering process.

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