Method for forming silicon nitride film, film formation apparatus, and silicon nitride film

Abstract

The present application provides a method for forming a silicon nitride film capable of forming a silicon nitride film with strong tensile stress by reactive sputtering. The method for forming a silicon nitride film of the present application, wherein a silicon material target (3) and a film formation object (Sw) are arranged opposite to each other in a vacuum chamber (1), a sputtering gas containing nitrogen is introduced into the vacuum chamber in a vacuum atmosphere, a negative potential is applied to the silicon material target, and a silicon nitride film with tensile stress is formed on the surface of the film formation object arranged in an electrically floating state by reactive sputtering; comprising the following steps: setting the film formation object to a non-applied state of bias potential, controlling at least one of the flow ratio of nitrogen to the sputtering gas and the potential applied to the silicon material target to maintain the silicon material target surface in a transition mode between the metal mode and the compound mode, and depositing β-type silicon nitride on the surface of the film formation object.

Description

Technical Field

[0001] This invention relates to a method for forming silicon nitride films, a film forming apparatus, and silicon nitride films. Background Technology

[0002] The aforementioned types of silicon nitride films are used, for example, as hard masks in the manufacturing process of semiconductor devices. Silicon nitride films for this purpose require high tensile stress (above +300 MPa) within a specified refractive index range (e.g., 2.0 ± 0.2), and are typically formed using plasma CVD (see, for example, Patent Document 1). When forming silicon nitride films using plasma CVD, silane-based gases containing hydrogen atoms are typically used as the feed gas. Therefore, introducing hydrogen atoms into the formed silicon nitride film can lead to problems that adversely affect semiconductor devices.

[0003] On the other hand, silicon nitride films can also be formed by reactive sputtering. In this method, a sputtering gas containing rare gases and nitrogen is introduced into a vacuum chamber with a silicon target and the object to be film-formed, and a negative potential is applied to the silicon target. At least one of the nitrogen flow rate ratio relative to the sputtering gas and the potential applied to the silicon target is controlled, and the film is formed while the surface of the silicon target remains in a metallic state. It is well known that most silicon nitride films formed in this way have compressive stress. Even when a silicon nitride film with tensile stress is formed, the tensile stress is not as strong as that obtained by plasma CVD. However, when reactive sputtering is used in the formation of silicon nitride films, it has advantages over plasma CVD, such as the ability to reduce manufacturing costs. Therefore, it is desirable to develop a film-forming method that can form silicon nitride films with the same tensile stress as those formed by plasma CVD.

[0004] Existing technical documents

[0005] Patent documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2009-84639 Summary of the Invention

[0007] The technical problem that the invention aims to solve

[0008] In view of the above, the technical problem to be solved by the present invention is to provide a method, apparatus and silicon nitride film forming method for forming silicon nitride film with strong tensile stress by reactive sputtering.

[0009] means of solving technical problems

[0010] To solve the above-mentioned technical problems, the silicon nitride film deposition method of the present invention involves arranging a silicon target and a film deposition object opposite to each other in a vacuum chamber, introducing a sputtering gas containing nitrogen into the vacuum chamber, applying a negative potential to the silicon target, and forming a silicon nitride film with tensile stress on the surface of the film deposition object, which is set in an electrically floating state, by reactive sputtering. The method is characterized by the following steps: setting the film deposition object to a state without applied bias potential, controlling at least one of the nitrogen gas flow rate ratio relative to the sputtering gas and the applied potential to the silicon target to maintain the surface of the silicon target in a transitional mode between a metallic mode and a compound mode, and depositing β-type silicon nitride on the surface of the film deposition object. In this case, it is sufficient to maintain the state without applied bias potential by applying a positive potential to conductive components arranged around the film deposition object facing the plasma atmosphere generated in the vacuum chamber.

[0011] Here, if a silicon nitride film is formed in a metallic mode as in the existing example described above, the resulting film exhibits an α-type silicon nitride (α-Si3N4) crystal structure. On the other hand, it has been found that if the deposition rate or flow rate ratio is appropriately controlled to maintain the silicon target surface in a transitional mode between the metallic and compound modes, the resulting film exhibits a β-type silicon nitride (β-Si3N4) crystal structure. Such a β-type silicon nitride film possesses tensile stress within a specified refractive index range; in other words, the stress of the silicon nitride film depends on the deposition rate and the flow rate ratio of nitrogen to sputtering gas. Furthermore, it is generally known that tensile stress is easily generated when the formed silicon nitride film has a columnar structure, but it has been understood that maintaining only the transitional mode prevents the silicon nitride film from exhibiting a uniformly spaced columnar structure.

[0012] The inventors of this application conducted repeated and in-depth research and discovered that by placing conductive components around the film-forming object facing the plasma atmosphere generated in the vacuum chamber, and applying a positive potential to the conductive components during reactive sputtering, the resulting film is a columnar β-type silicon nitride (β-Si3N4) exhibiting strong tensile stress (+300MPa). Typically, when the film-forming object is a silicon wafer, or if the surface of the film-forming object is covered with silicon nitride with relatively high resistance, electrons in the plasma atmosphere are charged, applying a so-called self-bias voltage (bias potential) to the film-forming object. In this state, sputtered particles, having higher energy, reach (collide) the film-forming object, damaging the columnar structure. Furthermore, when conductive components are present around the film-forming object, the self-bias voltage applied to the film-forming object is reduced because the electron charge is mitigated (suppressed).

[0013] Therefore, in this invention, a structure is formed by setting the target material to a non-applied bias potential state, controlling at least one of the nitrogen flow rate ratio relative to the sputtering gas and the applied potential to the silicon target, so that the surface of the silicon target is maintained in a transitional mode between the metallic mode and the compound mode, thereby forming a β-type silicon nitride film. The term "non-applied bias potential state" in this invention not only means, for example, the case of actively applying a bias potential using an AC power source, but also includes a state that mitigates the charge of electrons in the plasma atmosphere and minimizes the self-bias voltage applied to the target material. Any reduction in self-bias voltage is acceptable, and it is not limited to applying a positive potential to conductive components disposed around the target material. Furthermore, a DC power source, a high-frequency power source, or an AC power source can be used as the sputtering power source, but a DC power source is preferred. Applying "DC power" from a DC power source also includes applying pulsed DC power. Furthermore, the so-called "controlling the potential applied to the silicon target" includes controlling the potential applied to the silicon target by controlling the power of the sputtering power source connected to the silicon target, and controlling the potential applied to the silicon target by constant current control of the current of the sputtering power source connected to the silicon target, and is not limited to constant voltage control of the potential of the sputtering power source connected to the silicon target. Thus, a silicon nitride film with strong tensile stress can be formed by reactive sputtering. Furthermore, during the film formation process, when silicon nitride adheres to or accumulates on the surface of an anti-adhesion plate or the like in the vacuum chamber, plasma diffusion generated in the vacuum chamber can easily cause unstable discharge. However, in this invention, due to the presence of a conductive component with an applied positive potential, plasma diffusion is suppressed, and the discharge remains stable.

[0014] In this invention, based on regression analysis of inventive experiments regarding the dependence of film formation rate on flow rate ratio, it was confirmed that if the flow rate ratio of nitrogen to sputtering gas is set to x (%), and the film formation rate of the silicon nitride film is set to... If at least one of the nitrogen flow rate ratio and the potential applied to the silicon target satisfies the following formula (1), a silicon nitride film with a tensile stress of +300 MPa or more can be formed.

[0015] y = 0.815x - 7.50……(1)

[0016] Furthermore, in this invention, preferably prior to the process of depositing β-type silicon nitride, the following pre-process is included: controlling at least one of the flow rate ratio of nitrogen gas to sputtering gas and the potential applied to the silicon target, and forming a seed layer of α-type silicon nitride on the surface of the film-forming object while maintaining the surface of the silicon target in a metallic mode. This confirms that by depositing β-type silicon nitride on the α-type silicon nitride seed layer, the formed silicon nitride film exhibits a narrow-gap, uniformly spaced columnar structure and a tensile stress of +400 MPa or more.

[0017] Furthermore, to solve the aforementioned technical problems, the film-forming apparatus of the present invention includes a vacuum chamber in which a silicon target is disposed, comprising: a platform that faces the silicon target within the vacuum chamber and holds the film-forming object in an electrically floating state; a gas introduction mechanism that introduces a sputtering gas containing nitrogen into the vacuum chamber; and a sputtering power supply that applies a negative potential to the silicon target; characterized in that it comprises: conductive components disposed around the platform within the vacuum chamber; and a DC power supply that applies a positive potential to the conductive components. Moreover, the silicon nitride film of the present invention is characterized in that it is composed of columnar β-type silicon nitride and has a tensile stress greater than +300 MPa in the refractive index range of 2.0 ± 0.2. Attached Figure Description

[0018] Figure 1 This is a schematic diagram showing the structure of the sputtering apparatus, which is the film-forming apparatus for silicon nitride film in this embodiment.

[0019] Figure 2 This is a diagram illustrating the method for forming a silicon nitride film according to the first embodiment.

[0020] Figure 3 These are graphs showing the experimental results that confirm the effectiveness of the present invention.

[0021] Figure 4 These are graphs showing the experimental results that confirm the effectiveness of the present invention.

[0022] Figure 5 These are graphs showing the experimental results that confirm the effectiveness of the present invention.

[0023] Figure 6 These are graphs showing the experimental results that confirm the effectiveness of the present invention.

[0024] Figure 7 (a) and (b) are figures illustrating the method for forming a silicon nitride film according to the second embodiment. Detailed Implementation

[0025] Referring to the accompanying drawings, the following description illustrates the method, apparatus, and implementation of the silicon nitride film formation process of the present invention, using a silicon wafer (hereinafter referred to as "substrate Sw") as the film formation object, a target made of silicon material of a specified purity, and argon and nitrogen as sputtering gases to form a silicon nitride film on the surface of the substrate Sw via reactive sputtering. Hereinafter, terms indicating directions such as up and down will be used... Figure 1 Based on.

[0026] Reference Figure 1The film-forming apparatus of this embodiment is a so-called magnetron sputtering apparatus SM, which includes a grounded vacuum chamber 1. A vacuum pump 12 is connected to the vacuum chamber 1 via an exhaust pipe 11, capable of exhausting the vacuum inside the vacuum chamber 1 to a predetermined pressure (vacuum level). Gas pipes 14, equipped with mass flow controllers 13a and 13b, are connected to the side wall of the vacuum chamber 1 and are respectively connected to argon and nitrogen gas sources. Furthermore, by controlling the flow rate through the mass flow controllers 13a and 13b, sputtering gases of argon and nitrogen can be introduced into the vacuum chamber 1 at a predetermined flow rate ratio (nitrogen flow rate ratio relative to the sputtering gas flow rate). In this embodiment, components such as the mass flow controllers 13a and 13b and the gas pipes 14 constitute a gas introduction mechanism.

[0027] A platform 2 is provided inside a vacuum chamber 1. The platform 2 includes: a metal base 21 disposed on the inner side of the bottom surface of the vacuum chamber 1 via an insulator 21a; and a mounting plate 22 made of, for example, aluminum nitride or boron nitride, disposed on the base 21. Electrostatic mounting electrodes 22a are assembled on the mounting plate 22. By energizing the electrodes 22a from a mounting power source (not shown), a substrate Sw placed with its film-forming surface facing upwards can be electrostatically adsorbed (held). At this time, the substrate Sw is in an electrically floating state. Although not specifically illustrated, a heating and cooling mechanism for the substrate Sw is provided on the mounting plate 22, which can adjust the substrate Sw to a specified temperature during the film formation process by reactive sputtering.

[0028] Furthermore, a cathode unit Cu is provided in the vacuum chamber 1. The cathode unit Cu includes a target 3 and a magnet unit 4, which is disposed above the target 3 so that the leakage magnetic field acts in the space between the target 3 and the substrate Sw. A back plate 31 is attached to the side of the target 3 facing away from the sputtering surface 3a. When the periphery of the back plate 31 is mounted on the upper wall of the vacuum chamber 1 by means of an insulating member 32, the target 3 and the substrate Sw are concentrically arranged opposite each other in the vacuum chamber 1 under a vacuum atmosphere. The output from the sputtering power supply Ps is connected to the target 3, and a negatively charged DC power (or pulsed DC power) can be applied. Inside the vacuum chamber 1, a stainless steel or aluminum anti-adhesion plate 5 is also provided, which surrounds the space between the substrate Sw and the target 3 to prevent sputtered particles from adhering to the inner wall of the vacuum chamber 1. The anti-adhesion plate 5 consists of an upper anti-adhesion plate 51 suspended on the upper wall of the vacuum chamber 1 and a lower anti-adhesion plate 52 that moves freely in the vertical direction by means of a lifting mechanism Du equipped with a cylinder or motor.

[0029] A cylindrical block 6 is disposed within the vacuum chamber 1, surrounding the stage 2 and having a truncated conical profile. The block 6 is made of aluminum or copper and constitutes the conductive component of this embodiment, disposed via an insulator 61 disposed inside the bottom surface of the vacuum chamber 1. When the block 6 is disposed, its top is on the same plane as or below the upper surface (film-forming surface) of the substrate Sw held by the stage 2, and at least a portion of its outer cylindrical surface directly faces the plasma atmosphere formed within the vacuum chamber 1. Furthermore, the shape of the block 6 is not limited to this, and it does not need to completely surround the stage 2; for example, it can be configured as multiple plates with arc-shaped profiles arranged on the same circumference. An output 71 from a DC power supply 7 is also connected to the block 6, which applies a positive potential during film formation, functioning as an anode. The film formation method of the first embodiment using the above-described sputtering apparatus SM will be described below.

[0030] After the substrate Sw is placed and electrostatically adsorbed onto the stage 2, vacuum exhaust is performed on the vacuum chamber 1. When the vacuum chamber 1 reaches the specified pressure, sputtering gas is introduced into the vacuum chamber 1 at a specified flow rate ratio through the gas introduction mechanisms 13a, 13b, and 14 while maintaining a certain effective exhaust rate. A negatively charged DC power is applied to the target 3 through the sputtering power supply Ps. At this time, the flow rate ratio of nitrogen to sputtering gas is set to x (%), and the film formation rate of the silicon nitride film is set to... At least one of the nitrogen flow rate ratio and the potential applied to the target 3 is controlled to satisfy the following equation (1) (i.e., the sputtering surface 3a of the target 3 is maintained in a transition mode between the metallic mode and the compound mode). In addition, a positive potential (e.g., in the range of 0V to 100V, preferably 30V) is applied to the bulk 6 through the DC power supply 7.

[0031] y = 0.815x - 7.50……(1)

[0032] In addition, terms such as "metal mode," "compound mode," and "transition mode" are well-known, so detailed explanations are omitted here.

[0033] Thus, a plasma atmosphere is formed in the space between the substrate Sw and the target 3. The target 3 is sputtered using ions of rare gases in the plasma. The sputtered particles disperse from the target 3 according to a predetermined cosine law, such as... Figure 2As shown, columnar β-type silicon nitride, which is a product of the reaction between sputtered particles and nitrogen, is deposited to form a silicon nitride film Fn. Furthermore, to maintain the discharge stability of the plasma atmosphere generated within the vacuum chamber 1, the flow rate ratio is preferably in the range of 25% to 35%, and the negative potential applied to the target 3 is preferably in the range of 300V to 600V (the applied power is, for example, in the range of 3.0kW to 5.0kW). It was confirmed that the silicon nitride film Fn formed as described above has a tensile stress of +300MPa or more within a refractive index range of 2.0 ± 0.2. Moreover, it was confirmed that even with an increase in the cumulative power applied to the target 3, plasma diffusion within the vacuum chamber 1 can be suppressed, ensuring a stable discharge.

[0034] To confirm the above effects, the following experiments were conducted using the aforementioned sputtering apparatus SM. In the first experiment, as sputtering conditions, the negative potential of the DC power applied to the target 3 was set to 550V (DC power of 4.5kW), and the potential applied to the block 6 was set to 30V. Then, with the pressure in the vacuum chamber 1 maintained at 1.0±0.1Pa, the nitrogen flow rate ratio was varied within the range of 27.63% to 29.49%, and the tensile stress and refractive index of the silicon nitride film Fn were measured respectively. The results are as follows: Figure 3 As shown. Furthermore, as a second experiment, following the first experiment, with the pressure maintained at 1.0 ± 0.1 Pa in vacuum chamber 1, the nitrogen flow rate was set to 25.67%, thereby adjusting the potential applied to target 3 and consequently the film formation rate. The tensile stress and refractive index of the silicon nitride film Fn were measured within the range of variation, and the results are as follows: Figure 4 As shown.

[0035] Based on the results of Experiments 1 and 2, the tensile stress of the silicon nitride film Fn depends on the nitrogen flow rate and the film formation rate. At this point, when the nitrogen flow rate is 28.6% and the film formation rate is... At that time, or at the film formation rate of When the nitrogen flow rate ratio is 25.68%, the tensile stress of the silicon nitride film Fn reaches its maximum at a refractive index of 2.03. Based on known crystal structure analysis or SEM images, it was confirmed that the formed product exhibits a β-type silicon nitride (β-Si3N4) crystal structure with a columnar structure. Furthermore, regression analysis of the results of experiments 1 and 2 revealed that... Figure 5 As shown, when the flow rate ratio is set to x (%) and the film formation rate is set to... When y = 0.815x - 7.50 holds true, if at least one of the control flow rate ratio (%) and the potential applied to the target 3 satisfies this condition, a silicon nitride film Fn with a tensile stress of more than +300MPa in the range of refractive index 2.0 ± 0.2 can be formed.

[0036] Next, as the third experiment, starting from the first experiment, the potential applied to target 3 was set to 560V (DC power of 4.5kW), and the flow rate ratio was set to 28.57%. The potential applied to block 6 was varied within the range of 0V to +35V. The current flowing through block 6 (anode current) and the current flowing through target 3 (cathode current) were measured. The results are as follows: Figure 6 As shown, if the positive potential applied to the bulk 6 is increased, the cathode current hardly changes, but the anode current gradually increases, and then hardly changes when it exceeds about 30V. Based on this result, it can be inferred that by applying a positive potential to the bulk 6, electrons in the plasma are attracted to the bulk 6, and the number of charged electrons on the substrate Sw during the film formation process is relatively reduced.

[0037] The embodiments of the present invention have been described above, but various modifications can be made without departing from the technical concept of the present invention. (See also...) Figure 7 In the film formation method of the second embodiment, as described in the first embodiment, a pre-process is performed before the deposition of β-type silicon nitride: controlling at least one of the flow rate ratio of nitrogen gas to sputtering gas and the potential applied to the silicon target 3, and forming a seed layer Ls of α-type silicon nitride on the surface of the substrate Sw while maintaining the surface of the silicon target 3 in a metallic mode. In this case, the film thickness d of the α-type silicon nitride seed layer Ls can be appropriately set according to the range of the α-type silicon nitride core (e.g., 7.5 nm ± 5.0 nm). At this point, simply adjusting the flow rate ratio to 5-15% (preferably 10%) and adjusting the potential applied to the target 3 to maintain the surface of the silicon target 3 in metallic mode is sufficient; for example, setting it to a range of 300V-600V (applied power of 2.0kW-5.0kW (preferably 3.5kW)). Furthermore, the block 6 can be in a potential-applied state or a potential-unapplied state. This confirms that a β-type silicon nitride with a uniformly spaced columnar structure can form a silicon nitride film Fn with a tensile stress of +400MPa or more within a refractive index range of 2.0±0.2. Furthermore, the example described uses rare gas and nitrogen as sputtering gases, but the present invention can also be applied to the case where only nitrogen is used to form a silicon nitride film.

[0038] Explanation of reference numerals in the attached figures

[0039] SM. Sputtering apparatus (silicon nitride film deposition apparatus), Sw. Substrate (film deposition object), Fn. Silicon nitride film, Ls. Seed layer, 1. Vacuum chamber, 13a, 13b. Mass flow controller (component of gas introduction mechanism), 14. Gas pipe (component of gas introduction mechanism), 2. Stand, 3. Silicon target, Ps. Sputtering power supply, 6. Bulk (conductive component), 7. DC power supply.

Claims

1. A method for forming a silicon nitride film, A silicon target and a substrate for film deposition are positioned opposite each other in a vacuum chamber. A sputtering gas containing nitrogen is introduced into the vacuum chamber to apply a negative potential to the silicon target. A silicon nitride film with tensile stress is formed on the surface of the substrate, which is set in an electrically floating state, by reactive sputtering. The characteristic feature is that: The process includes the following steps: setting the target material to a non-applied bias potential state, controlling at least one of the nitrogen flow rate ratio relative to the sputtering gas and the potential applied to the silicon target to maintain the silicon target surface in a transition mode between a metallic mode and a compound mode, and depositing β-type silicon nitride on the surface of the target material. A positive potential is applied to conductive components arranged around the film-forming object facing the plasma atmosphere generated in the vacuum chamber, while maintaining a non-applied bias potential.

2. A method for forming a silicon nitride film, characterized in that: A silicon target and a substrate for film deposition are positioned opposite each other in a vacuum chamber. A sputtering gas containing nitrogen is introduced into the vacuum chamber to apply a negative potential to the silicon target. A silicon nitride film with tensile stress is formed on the surface of the substrate, which is set in an electrically floating state, by reactive sputtering. The characteristic feature is that: The process includes the following steps: setting the target material to a non-applied bias potential state, controlling at least one of the nitrogen flow rate ratio relative to the sputtering gas and the potential applied to the silicon target to maintain the silicon target surface in a transition mode between a metallic mode and a compound mode, and depositing β-type silicon nitride on the surface of the target material. The flow rate ratio of nitrogen to sputtering gas is set as x (%), and the film formation rate of silicon nitride film is set as y (Å / sec). At least one of the flow rate ratio and the potential applied to the silicon target is controlled to satisfy the following equation (1): y=0.815x-7.50……(1).

3. A method for forming a silicon nitride film, characterized in that: A silicon target and a substrate for film deposition are positioned opposite each other in a vacuum chamber. A sputtering gas containing nitrogen is introduced into the vacuum chamber to apply a negative potential to the silicon target. A silicon nitride film with tensile stress is formed on the surface of the substrate, which is set in an electrically floating state, by reactive sputtering. The characteristic feature is that: The process includes the following steps: setting the target material to a non-applied bias potential state, controlling at least one of the nitrogen flow rate ratio relative to the sputtering gas and the potential applied to the silicon target to maintain the silicon target surface in a transition mode between a metallic mode and a compound mode, and depositing β-type silicon nitride on the surface of the target material. Prior to the stacking of β-type silicon nitride, the following pre-process is included: controlling at least one of the flow rate ratio of nitrogen gas to sputtering gas and the potential applied to the silicon target, and forming a seed layer of α-type silicon nitride on the surface of the film-forming object while the surface of the silicon target is maintained in a metallic mode.

4. A silicon nitride film deposition apparatus comprising a vacuum chamber in which a silicon target is disposed, and including: a stand facing the silicon target within the vacuum chamber and holding the film deposition target in an electrically floating state; a gas introduction mechanism for introducing a sputtering gas containing nitrogen into the vacuum chamber; and a sputtering power supply for applying a negative potential to the silicon target; characterized in that: It includes: conductive components arranged around the stage within a vacuum chamber; and a DC power supply that applies a positive potential to the conductive components to maintain a non-applied bias potential on the object to be deposited during film deposition.

5. The silicon nitride film forming apparatus according to claim 4, characterized in that: The top of the conductive component is on the same plane as the upper surface of the platform or is located below the upper surface of the platform.

6. A silicon nitride film, characterized in that: Composed of columnar β-type silicon nitride, it has a tensile stress greater than +300MPa in the refractive index range of 2.0±0.2.

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