Silicon nitride film forming method, film forming apparatus, and silicon nitride film

Abstract

Provided is a silicon nitride film forming method with which a silicon nitride film having a relatively strong tensile stress can be formed by reactive sputtering.　In this silicone nitride film forming method, a silicon target 3 and a film-forming object Sw are disposed opposing each other within a vacuum chamber 1, a sputtering gas containing a nitrogen gas is introduced into the vacuum chamber under a vacuum atmosphere, and a negative potential is applied to the silicon target to form a silicon nitride film having a tensile stress by reactive sputtering on the surface of the film-forming object which is disposed in an electrically floating state, wherein the method includes a step in which the film-forming object is placed in a state in which a bias potential is not being applied, and β-type silicon nitride is deposited on the surface of the film-forming object by controlling the flow amount ratio of the nitrogen gas relative to the sputtering gas and / or the potential to be applied to the silicon target so that the surface of the silicon target is maintained in a transition mode between a metal mode and a compound mode.

Description

[Technical field]

[0001] The present invention relates to a method for forming a silicon nitride film, a film forming apparatus, and a silicon nitride film. [Background technology]

[0002] The above-mentioned silicon nitride films are used, for example, as hard masks in the manufacturing process of semiconductor devices. Silicon nitride films for such applications are required to have a relatively strong tensile stress (+300 MPa or more) within a predetermined refractive index (for example, 2.0±0.2), and are usually formed by a plasma CVD method (see, for example, Patent Document 1). When forming a silicon nitride film by a plasma CVD method, a silane-based gas containing hydrogen atoms is generally used as a raw material gas. For this reason, if hydrogen atoms are incorporated into the formed silicon nitride film, this can cause a problem of adverse effects on the semiconductor device.

[0003] On the other hand, a silicon nitride film can also be formed by reactive sputtering. In this case, a sputtering gas containing a rare gas and nitrogen gas is introduced into a vacuum chamber in which a silicon target and a film-forming object are arranged facing each other, and a negative potential is applied to the silicon target. At this time, the surface of the silicon target is maintained in a metal mode by controlling at least one of the flow rate ratio of the nitrogen gas to the sputtering gas and the potential applied to the silicon target. It is generally known that most of the silicon nitride film formed in this way has compressive stress. Even if a silicon nitride film having tensile stress can be formed, it cannot have a tensile stress as strong as that formed by the plasma CVD method. However, the use of reactive sputtering to form a silicon nitride film has the advantage of reducing manufacturing costs compared to the plasma CVD method. For this reason, there is a demand for the development of a film formation method that can form a silicon nitride film having a tensile stress equivalent to that formed by the plasma CVD method by reactive sputtering. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-84639 A Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above, an object of the present invention is to provide a silicon nitride film forming method, a film forming apparatus, and a silicon nitride film that are capable of forming a silicon nitride film having a relatively strong tensile stress by reactive sputtering. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention provides a method for forming a silicon nitride film, which comprises arranging a silicon target and a film-forming object facing each other in a vacuum chamber, introducing a sputtering gas containing nitrogen gas into the vacuum chamber in a vacuum atmosphere, applying a negative potential to the silicon target, and forming a silicon nitride film having tensile stress on a surface of the film-forming object placed in an electrically floating state by reactive sputtering, the method comprising the steps of: placing the film-forming object in a state in which a bias potential is not applied; and controlling at least one of the flow rate ratio of nitrogen gas to the sputtering gas and the potential applied to the silicon target so that the silicon target surface is maintained in a transition mode between a metal mode and a compound mode, thereby depositing β-type silicon nitride on the surface of the film-forming object. premise Let us assume that. The present invention also relates to A positive potential is applied to a conductive member disposed around the object to be film-formed facing the plasma atmosphere generated in the vacuum chamber, and a state in which a bias potential is not applied is maintained. The present invention is characterized by .

[0007] Here, when a silicon nitride film is formed in the metal mode as in the above conventional example, the formed film has a crystal structure of α-type silicon nitride (α-Si3N4). On the other hand, if the film formation rate and flow rate ratio are appropriately controlled to maintain the silicon target surface in a transition mode between the metal mode and the compound mode, the formed film has a crystal structure of β-type silicon nitride (β-Si3N4), and it was found that such a β-type silicon nitride film has tensile stress in a predetermined range of refractive index, in other words, the stress of the silicon nitride film depends on the film formation rate and the flow rate ratio of nitrogen gas to the sputtering gas. It is generally known that if the formed silicon nitride film has a columnar structure, tensile stress is likely to occur, but it was found that the silicon nitride film does not have a columnar structure with uniform gaps just by maintaining it in the transition mode.

[0008] The inventors of the present application have conducted extensive research and found that when a conductive member is placed around a film-forming target facing a plasma atmosphere generated in a vacuum chamber and a positive potential is applied to the conductive member during reactive sputtering, the film formed becomes a columnar β-type silicon nitride (β-Si3N4) and exhibits a relatively strong tensile stress (+300 MPa). This is because, when the film-forming target is a silicon wafer or when the surface of the film-forming target is covered with silicon nitride, which has a relatively high electrical resistance, electrons in the plasma atmosphere are charged and a so-called self-bias (bias potential) is applied to the film-forming target. In this state, sputter particles scattered from a silicon target reach (collide with) the film-forming target with higher energy, damaging the columnar structure, while when a conductive member is present around the film-forming target, the charging of electrons is alleviated (suppressed) and the self-bias applied to the film-forming target is reduced.

[0009] Therefore, in the present invention, a configuration is adopted in which the film-forming target is placed in a state where no bias potential is applied, and at least one of the flow rate ratio of nitrogen gas to the sputtering gas and the potential applied to the silicon target is controlled so that the surface of the silicon target is maintained in a transition mode between the metal mode and the compound mode, to form a β-type silicon nitride film. The "state where no bias potential is applied" in the present invention does not only mean, for example, excluding the case where a bias potential is actively applied by an AC power source, but also includes a state in which the charging of electrons in the plasma atmosphere can be relaxed and the self-bias applied to the film-forming target can be reduced as much as possible, and is not limited to the application of a positive potential to a conductive member arranged around the film-forming target as long as the self-bias can be reduced. Note that, although a DC power source, a high-frequency power source, and an AC power source can be used as the sputtering power source, it is preferable to use a DC power source. It also includes the case where "DC power" is input from a DC power source and the case where pulsed DC power is input. In addition, "controlling the potential applied to the silicon target" includes controlling the power of a sputtering power supply connected to the silicon target to control the potential applied to the silicon target, and controlling the current of a sputtering power supply connected to the silicon target to a constant current to control the potential applied to the silicon target, and is not limited to controlling the potential of a sputtering power supply connected to the silicon target to a constant voltage. This allows a silicon nitride film having a relatively strong tensile stress to be formed by reactive sputtering. During film formation, if silicon nitride adheres to and accumulates on the surface of a shield plate installed in the vacuum chamber, the plasma generated in the vacuum chamber spreads and the discharge tends to become unstable. However, in the present invention, the conductive member to which a positive potential is applied is present, which suppresses the spread of plasma and allows the discharge to be constantly stable.

[0010] Also, This issue Ming isFrom a regression analysis based on an invention experiment on the dependency of the film formation rate and the flow rate ratio, the flow rate ratio of nitrogen gas to the sputtering gas is x (%), the film formation rate of the silicon nitride film is y (Å / sec), and at least one of the flow rate ratio of nitrogen gas and the potential applied to the silicon target is controlled so as to satisfy the following formula (1). This is characterized by the fact that It was confirmed that a silicon nitride film having a tensile stress of +300 MPa or more can be formed by using this method. y=0.815x-7.50 (1)

[0011] In addition, Ming is and a pre-processing step of forming an α-type silicon nitride seed layer on the surface of the film-forming object in a state where the surface of the silicon target is maintained in a metallic mode by controlling at least one of the flow rate ratio of nitrogen gas to the sputtering gas and the potential applied to the silicon target, prior to the process of depositing the β-type silicon nitride. Characterized by It was confirmed that by depositing β-type silicon nitride on α-type silicon nitride seed crystals, the formed silicon nitride film has a columnar structure with narrow and aligned gaps and a tensile stress of +400 MPa or more.

[0012] In order to solve the above problems, the film forming apparatus of the present invention includes a vacuum chamber in which a silicon target is placed, a stage for holding an object to be film-formed in an electrically floating state facing the silicon target within the vacuum chamber, a gas introducing means for introducing a sputtering gas containing nitrogen gas into the vacuum chamber in a vacuum atmosphere, and a sputtering power supply for applying a negative potential to the silicon target, and a conductive member provided around the stage within the vacuum chamber; The film is formed so that the film is kept in a non-biased state. and a DC power source that applies a positive potential to the conductive member. In the present invention, it is preferable that the top of the conductive member is flush with the upper surface of the stage or is located lower than the upper surface of the stage. The silicon nitride film of the present invention is characterized in that it is composed of β-type silicon nitride having a columnar structure, and has a tensile stress of more than +300 MPa when the refractive index is in the range of 2.0±0.2. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a configuration of a sputtering apparatus as a silicon nitride film forming apparatus according to an embodiment of the present invention. [Diagram 2] 2A to 2C are views for explaining a method for forming a silicon nitride film according to the first embodiment. [Diagram 3] 11 is a graph showing experimental results confirming the effects of the present invention. [Figure 4] 11 is a graph showing experimental results confirming the effects of the present invention. [Diagram 5] 11 is a graph showing experimental results confirming the effects of the present invention. [Figure 6] 11 is a graph showing experimental results confirming the effects of the present invention. [Figure 7] 6A and 6B are diagrams illustrating a method for forming a silicon nitride film according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, with reference to the drawings, an embodiment of the silicon nitride film forming method, film forming apparatus, and silicon nitride film of the present invention will be described, taking as an example a case where a silicon wafer (hereinafter referred to as "substrate Sw") is used as a film forming object, a silicon target of a specified purity is used as a target, and a silicon nitride film is formed on the surface of the substrate Sw by reactive sputtering using argon gas and nitrogen gas as sputtering gas. In the following, terms indicating directions such as up and down are based on FIG. 1.

[0015] 1, the film forming apparatus of this embodiment is a so-called magnetron sputtering apparatus SM, and includes an earthed vacuum chamber 1. A vacuum pump 12 is connected to the vacuum chamber 1 via an exhaust pipe 11, and the inside of the vacuum chamber 1 can be evacuated to a predetermined pressure (vacuum level). A gas pipe 14 is connected to a side wall of the vacuum chamber 1, and communicates with gas sources of argon gas and nitrogen gas, respectively, and has mass flow controllers 13a and 13b interposed therebetween. The mass flow controllers 13a and 13b control the flow rates of the sputtering gases of argon gas and nitrogen gas. ofA predetermined flow rate ratio (flow rate ratio of nitrogen gas to sputtering gas) can be introduced into the vacuum chamber 1. In this embodiment, components such as the mass flow controllers 13a and 13b and the gas pipe 14 constitute the gas introduction means.

[0016] A stage 2 is provided in the vacuum chamber 1. The stage 2 has a metal base 21 arranged on the inside of the bottom surface of the vacuum chamber 1 via an insulator 21a, and a chuck plate 22 made of, for example, aluminum nitride or boron nitride, which is provided on the base 21. An electrode 22a for electrostatic chucking is incorporated in the chuck plate 22, and the substrate Sw placed with its film-forming surface facing upward can be electrostatically attracted (held) by passing electricity to the electrode 22a from a chuck power source (not shown). At this time, the substrate Sw is in an electrically floating state. Although not particularly shown or described, the chuck plate 22 is provided with a heating and cooling mechanism for the substrate Sw, which can adjust the substrate Sw to a predetermined temperature during film formation by reactive sputtering.

[0017] The vacuum chamber 1 is also provided with a cathode unit Cu. The cathode unit Cu includes a target 3 and a magnet unit 4 that is disposed above the target 3 and applies a leakage magnetic field to the space between the target 3 and the substrate Sw. A backing plate 31 is bonded to the side of the target 3 facing the sputtering surface 3a, and when the peripheral portion of the backing plate 31 is attached to the upper wall of the vacuum chamber 1 via an insulating member 32, the target 3 and the substrate Sw are concentrically arranged opposite each other in the vacuum chamber 1 in a vacuum atmosphere. The output from the sputtering power supply Ps is connected to the target 3, and DC power (or pulsed DC power) having a negative potential can be input. In the vacuum chamber 1, a stainless steel or aluminum adhesion prevention plate 5 is also provided to surround the space between the substrate Sw and the target 3 and prevent adhesion of sputter particles to the inner wall of the vacuum chamber 1. The adhesion prevention plate 5 is composed of an upper adhesion prevention plate 51 suspended from the upper wall of the vacuum chamber 1, and a lower adhesion prevention plate 52 that can be moved vertically by a lifting mechanism Du equipped with a cylinder and a motor.

[0018] In the vacuum chamber 1, a cylindrical block body 6 having a truncated cone-shaped outline is provided around the stage 2. The block body 6 is made of aluminum or copper and constitutes the conductive member of this embodiment, and is installed via an insulator 61 installed inside the bottom surface of the vacuum chamber 1. When the block body 6 is installed, the top of the block body 6 is flush with or lower than the upper surface (film formation surface) of the substrate Sw held by the stage 2, and at least a part of the outer cylindrical surface directly faces the plasma atmosphere formed in the vacuum chamber 1. The shape of the block body 6 is not limited to this, and it is not necessary to completely surround the periphery of the stage 2. For example, the block body 6 can be configured by arranging multiple plate materials having an arc-shaped outline on the same circumference. An output 71 from a DC power supply 7 is also connected to the block body 6, and during film formation, a positive potential is applied from the DC power supply 7 to the block body 6 so that the block body 6 functions as an anode. A film formation method of the first embodiment using the above-mentioned sputtering device SM will be described below.

[0019] After the substrate Sw is placed on the stage 2 and electrostatically attracted, the vacuum chamber 1 is evacuated. When the pressure inside the vacuum chamber 1 reaches a predetermined pressure, the gas introduction means 13a, 13b, and 14 introduce sputtering gas into the vacuum chamber 1 at a predetermined flow rate while maintaining a constant effective exhaust speed, and the sputtering power supply Ps applies DC power having a negative potential to the target 3. At this time, the flow rate ratio of the nitrogen gas to the sputtering gas is set to x (%), and the deposition rate of the silicon nitride film is set to y (Å / sec), and at least one of the flow rate ratio of the nitrogen gas and the potential applied to the target 3 is controlled so as to satisfy the following formula (1) (i.e., so that the sputtering surface 3a of the target 3 is maintained in a transition mode between the metal mode and the compound mode). In addition, the DC power supply 7 applies a positive potential (for example, in the range of 0V to 100V, preferably 30V) to the block body 6. y=0.815x-7.50 (1) Incidentally, the terms "metal mode," "compound mode," and "transition mode" are themselves widely known, and therefore detailed explanations thereof will be omitted here.

[0020] As a result, a plasma atmosphere is formed in the space between the substrate Sw and the target 3, the target 3 is sputtered by the rare gas ions in the plasma, the sputtered particles are scattered from the target 3 according to a predetermined cosine law, and as shown in FIG. 2, β-type silicon nitride having a columnar structure, which is a reaction product of the sputtered particles and nitrogen gas, is deposited to form a silicon nitride film Fn. In order to maintain the discharge stability of the plasma atmosphere generated in the vacuum chamber 1, the flow rate ratio is preferably in the range of, for example, 25 to 35%, and the negative potential applied to the target 3 is preferably in the range of, for example, 300 V to 600 V (the input power is, for example, in the range of 3.0 kW to 5.0 kW). It was confirmed that the silicon nitride film Fn formed as described above has a tensile stress of +300 MPa or more in the range of a refractive index of 2.0±0.2. Moreover, it was confirmed that even if the integrated power to the target 3 increases, the spread of the plasma in the vacuum chamber 1 is suppressed, and the discharge can be constantly stabilized.

[0021] In order to confirm the above-mentioned effect, the following experiments were carried out using the sputtering apparatus SM. In the first experiment, the sputtering conditions were set as follows: the negative potential of the DC power input to the target 3 was 550V (DC power was 4.5kW), and the potential applied to the block body 6 was 30V. The flow rate of nitrogen gas was changed in the range of 27.63 to 29.49% while the pressure in the vacuum chamber 1 was maintained at 1.0±0.1 Pa, and the tensile stress and refractive index of the silicon nitride film Fn were measured, and the results are shown in FIG. 3. In the second experiment, the flow rate of nitrogen gas was set to 25.67% while the pressure in the vacuum chamber 1 was maintained at 1.0±0.1 Pa from the first experiment, and the potential applied to the target 3, and thus the film formation rate, were changed in the range of 10.7 to 15.4 Å / sec, and the tensile stress and refractive index of the silicon nitride film Fn were measured, and the results are shown in FIG. 4.

[0022] From the results of the first and second experiments, it is found that the tensile stress of the silicon nitride film Fn depends on the nitrogen gas flow rate and the deposition rate. In this case, it was found that the tensile stress of the silicon nitride film Fn reaches a maximum value at a refractive index of 2.03 when the nitrogen gas flow rate is 28.6% and the deposition rate is 19.5 Å / sec, or when the deposition rate is 13.5 Å / sec and the nitrogen gas flow rate is 25.68%, and it was confirmed from known crystal structure analysis and SEM images that the deposited film has a columnar structure with a β-type silicon nitride (β-Si3N4) crystal structure. In addition, a regression analysis of the results of the first and second experiments revealed that, as shown in FIG. 5, when the flow rate ratio is x (%) and the film formation rate is y (Å / sec), the equation y = 0.815x - 7.50 holds. It was found that by controlling at least one of the flow rate ratio (%) and the potential applied to the target 3 so as to satisfy this equation, a silicon nitride film Fn having a tensile stress of +300 MPa or more and a refractive index in the range of 2.0 ± 0.2 can be formed.

[0023] Next, in the third experiment, the potential applied to the target 3 was set to 560V (DC power 4.5kW) and the flow rate ratio was set to 28.57% from the first experiment, and the potential applied to the block body 6 was changed in the range from 0V to +35V to measure the current value (anode current) flowing through the block body 6 and the current value (cathode current) flowing through the target 3. The results are shown in FIG. 6. According to this, it can be seen that, when the positive potential applied to the block body 6 is increased, the cathode current hardly changes, but the anode current gradually increases, and when it exceeds about 30V, it hardly changes. From this result, it can be inferred that by applying a positive potential to the block body 6, electrons in the plasma are attracted to the block body 6, and the electrons charged to the substrate Sw during film formation are relatively reduced.

[0024] Although the embodiments of the present invention have been described above, various modifications are possible without departing from the scope of the technical concept of the present invention. Referring to Fig. 7, in the film forming method of the second embodiment, as in the first embodiment, a pre-process is provided in which, prior to the process of depositing β-type silicon nitride, a seed layer Ls of α-type silicon nitride is formed on the surface of the substrate Sw in a state in which the surface of the silicon target 3 is maintained in a metal mode by controlling at least one of the flow rate ratio of nitrogen gas to the sputtering gas and the potential applied to the silicon target 3. In this case, the thickness d of the seed layer Ls of α-type silicon nitride may be appropriately set within a range in which nuclei of α-type silicon nitride are formed (for example, 7.5 nm ± 5.0 nm). At this time, the flow rate ratio is 5 to 15% (preferably 10%), the potential applied to the target 3 may be adjusted to a value that maintains the surface of the silicon target 3 in the metal mode, for example, set in the range of 300V to 600V (input power is 2.0kW to 5.0kW (preferably 3.5kW)), and the block body 6 may be in a potential application state or a potential non-application state. According to this, it was confirmed that β-type silicon nitride has a columnar structure with uniform gaps, and a silicon nitride film Fn having a tensile stress of +400 MPa or more with a refractive index in the range of 2.0±0.2 can be formed. In addition, although the case where a rare gas and nitrogen gas are used as the sputtering gas has been described as an example, the present invention can also be applied to the case where a silicon nitride film is formed using only nitrogen gas. [Explanation of symbols]

[0025] SM...sputtering apparatus (silicon nitride film forming apparatus), Sw...substrate (film forming target), Fn...silicon nitride film, Ls...seed layer, 1...vacuum chamber, 13a, 13b...mass flow controllers (components of gas introduction means), 14...gas pipe (component of gas introduction means), 2...stage, 3...silicon target, Ps...sputtering power supply, 6...block body (conductive member), 7...DC power supply.

Claims

1. A method for forming a silicon nitride film, comprising: arranging a silicon target and a film-forming object facing each other in a vacuum chamber; introducing a sputtering gas containing nitrogen gas into the vacuum chamber in a vacuum atmosphere; applying a negative potential to the silicon target; and forming a silicon nitride film having tensile stress on a surface of the film-forming object placed in an electrically floating state by reactive sputtering, comprising: The method includes a step of depositing β-type silicon nitride on a surface of a film-forming object by controlling at least one of a flow rate ratio of nitrogen gas to a sputtering gas and a potential applied to the silicon target so that the surface of the silicon target is maintained in a transition mode between a metal mode and a compound mode while keeping the film-forming object in a state where a bias potential is not applied, A method for forming a silicon nitride film, comprising applying a positive potential to a conductive member arranged around an object to be film-formed facing a plasma atmosphere generated in a vacuum chamber, and maintaining a state in which no bias potential is applied.

2. A method for forming a silicon nitride film, comprising: arranging a silicon target and an object to be film-formed facing each other in a vacuum chamber; introducing a sputtering gas containing nitrogen gas into the vacuum chamber having a vacuum atmosphere; applying a negative potential to the silicon target; and forming a silicon nitride film having tensile stress on a surface of the object to be film-formed, the surface being placed in an electrically floating state by reactive sputtering, The method includes a step of depositing β-type silicon nitride on a surface of a film-forming object by controlling at least one of a flow rate ratio of nitrogen gas to a sputtering gas and a potential applied to the silicon target so that the surface of the silicon target is maintained in a transition mode between a metal mode and a compound mode while keeping the film-forming object in a state where a bias potential is not applied, A method for forming a silicon nitride film, characterized in that, assuming that a flow rate ratio of nitrogen gas to a sputtering gas is x (%) and a film formation rate of a silicon nitride film is y (Å / sec), at least one of the flow rate ratio and a potential applied to a silicon target is controlled so as to satisfy the following formula (1). y=0.815x-7.50...(1)

3. A method for forming a silicon nitride film, comprising: arranging a silicon target and an object to be film-formed facing each other in a vacuum chamber; introducing a sputtering gas containing nitrogen gas into the vacuum chamber having a vacuum atmosphere; applying a negative potential to the silicon target; and forming a silicon nitride film having tensile stress on a surface of the object to be film-formed, the surface being placed in an electrically floating state by reactive sputtering, comprising: The method includes a step of depositing β-type silicon nitride on a surface of a film-forming object by controlling at least one of a flow rate ratio of nitrogen gas to a sputtering gas and a potential applied to the silicon target so that the surface of the silicon target is maintained in a transition mode between a metal mode and a compound mode while keeping the film-forming object in a state where a bias potential is not applied, A method for forming a silicon nitride film, comprising a pre-step of forming an α-type silicon nitride seed layer on a surface of an object to be film-formed, prior to a step of depositing β-type silicon nitride, by controlling at least one of the flow rate ratio of nitrogen gas to the sputtering gas and the potential applied to the silicon target while maintaining the surface of the silicon target in a metallic mode.

4. A silicon nitride film forming apparatus having a vacuum chamber in which a silicon target is placed, the apparatus comprising: a stage for holding an object to be film-formed in an electrically floating state facing the silicon target within the vacuum chamber; a gas introducing means for introducing a sputtering gas containing nitrogen gas into the vacuum chamber in a vacuum atmosphere; and a sputtering power supply for applying a negative potential to the silicon target, A silicon nitride film forming apparatus comprising: a conductive member positioned around a stage within a vacuum chamber; and a DC power supply that applies a positive potential to the conductive member so that a non-bias potential state is maintained on an object to be formed during film formation.

5. A silicon nitride film forming apparatus as described in claim 4, characterized in that the top of the conductive member is flush with the upper surface of the stage or is located lower than the upper surface of the stage.

6. A silicon nitride film which is composed of β-type silicon nitride having a columnar structure and has a tensile stress of more than +300 MPa when the refractive index is in the range of 2.0±0.2.

Images