JPWO2010150610A1 - Thin film actuator and inkjet head - Google Patents

Abstract

A displacement film that expands and contracts in the direction along the film surface by a drive signal, two electrode films formed by sandwiching the displacement film, and formed along one electrode film of the two electrode films A thin film layer comprising: a stress film having a stress in a direction along the film surface; and a substrate having an opening formed therein, the thin film layer covering the opening of the substrate, the thin film The peripheral edge of the layer is fixed to the substrate, and the region of the displacement film corresponding to the opening of the substrate has a convex shape in a direction perpendicular to the film surface due to at least the stress of the stress film when not driven.

Description

  The present invention relates to a thin film actuator and an inkjet head.

  Conventionally, as a microactuator for generating a minute displacement, a thin film actuator having a cantilever structure (single cantilever beam) in which a piezoelectric body, a shape memory alloy, a bimetal, etc. are formed in a thin film (displacement film) on the surface of a substrate is known ing. The thin film actuator can efficiently convert the expansion / contraction deformation along the surface of the displacement film into the displacement in the direction perpendicular to the surface, so that a highly sensitive pressure sensor and various driving elements can be configured.

  On the other hand, a thin film actuator having a cantilever structure (single beam) has a problem that it has a low rigidity because the tip of the beam (displacement film) is not fixed, and is likely to be deformed or twisted by an external force.

  Therefore, in order to cope with such a problem, there has been proposed a method for increasing the rigidity of the displacement film by using a double beam structure in which both ends of the displacement film are fixed or a diaphragm structure in which the peripheral edge is fixed (Patent Document 1). reference).

  A thin film actuator with a diaphragm structure with a fixed beam structure and a peripheral edge as described in Patent Document 1 increases the rigidity of the displacement film, so that the generated pressure can be increased, and it can be stably deformed by an external force. There is an advantage that the central part of the displacement film can be moved in parallel with the substrate, and that it can be used for a pump for transferring gas or liquid by a sealed structure.

  On the other hand, since the thin film actuator having such a structure has both ends or peripheral edges of the displacement film fixed, even if the displacement film is deformed in the contraction direction along the film surface, almost no displacement in the direction perpendicular to the film surface is obtained. I can't.

  Therefore, the thin film actuator described in Patent Document 1 is configured such that a displacement in the vertical direction is obtained by deforming the displacement film in the extending direction along the film surface.

JP-A-8-114408

  However, when the displacement film stretches and deforms from the flat initial state (when not driven), stress concentrates on the low-strength part of the displacement film, and the part buckles. There is a problem that higher-order deformation occurs.

  Furthermore, since the initial state of the displacement film is flat, there is a problem that in principle it is not possible to determine in which direction the upper side or the lower side of the displacement film is displaced. With respect to this deformation direction, the thicknesses of the two electrode films provided on the upper surface and the lower surface of the displacement film are made different to provide a difference in rigidity of the displacement film, thereby determining the displacement direction. However, the stress concentration state is likely to change with a slight imbalance due to dust or scratches, and it is difficult to stabilize the displacement direction of the displacement film in a desired direction.

  The present invention has been made in view of the above problems, and an object thereof is to provide a thin film actuator and an ink jet head capable of stably obtaining a desired displacement shape and displacement direction.

  The above object is achieved by the invention described in any one of 1 to 5 below.

1. A displacement film that expands and contracts in the direction along the film surface by a drive signal;
Two electrode films formed across the displacement film and to which the drive signal is applied;
A thin film layer having a stress film formed along one of the two electrode films and having a stress in a direction along the film surface;
A thin film actuator comprising: a substrate on which an opening is formed;
The thin film layer covers an opening of the substrate, and a peripheral edge of the thin film layer is fixed to the substrate.
The region corresponding to the opening of the substrate of the displacement film has a convex shape in a direction perpendicular to the film surface due to at least the stress of the stress film when not driven.

  2. 2. The thin film actuator according to 1, wherein the direction of the stress of the stress film is an extension direction along a film surface of the stress film.

  3. Of the two electrode films, the electrode film formed on the surface of the displacement film is formed inside the region corresponding to the opening of the substrate of the displacement film. The thin film actuator described.

  4). Of the two electrode films, the electrode film formed on the surface of the displacement film is formed in a region including a region corresponding to the opening of the substrate of the displacement film. The thin film actuator described in 1.

5. The thin film actuator according to any one of 1 to 4,
A nozzle substrate bonded to the back surface of the substrate and having nozzle holes that communicate with the openings of the substrate and discharge ink;
An ink jet head comprising: a pressure chamber that accommodates the ink in the opening and generates pressure by displacement of the displacement film.

  According to the present invention, the region corresponding to the opening of the substrate of the displacement film is configured to be convex in a direction perpendicular to the film surface due to at least the stress of the stress film when not driven. That is, the first initial deformation is applied to the displacement film. As a result, it is possible to prevent the occurrence of higher-order deformation such as buckling or wrinkles due to stress concentration on unnecessary portions. As a result, a desired displacement shape and displacement direction can be obtained stably.

It is a cross-sectional schematic diagram which shows the manufacturing process of the thin film actuator concerning embodiment of this invention, and the aspect of the initial stage deformation | transformation of a thin film actuator. It is a cross-sectional schematic diagram which shows the aspect of a drive deformation | transformation of a thin film actuator. It is a cross-sectional schematic diagram which shows the mode of the shape change at the time of initial deformation by the balance of the bending force of the center part and peripheral part of a thin film actuator. It is a cross-sectional schematic diagram which shows the mode of the shape change at the time of initial deformation by the magnitude | size of the upper electrode film | membrane of a thin film actuator. It is a cross-sectional schematic diagram which shows schematic structure of the inkjet head concerning embodiment of this invention.

  Hereinafter, a thin film actuator and an inkjet head according to an embodiment of the present invention will be described based on the drawings. The present invention is not limited to the embodiment.

  First, a configuration of a thin film actuator according to an embodiment of the present invention and a manufacturing method thereof will be described with reference to FIG. FIG. 1A to FIG. 1H are schematic cross-sectional views showing a manufacturing process of the thin film actuator 1 and a deformation mode (hereinafter, also referred to as initial deformation) when the thin film actuator is not driven.

  First, the substrate 101 is prepared (FIG. 1A). As the material of the substrate 101, silicon, glass, ceramics, or the like can be used. Crystalline silicon (Si) widely used for MEMS (Micro Electro Mechanical Systems) is suitable, and in this embodiment, crystal silicon (Si) is used. An example using silicon will be described.

Next, the substrate 101 is put into a heating furnace and heated at a temperature of about 1500 ° C. for a predetermined time to form a thermal oxide film (quartz: SiO ₂ ) that becomes the stress film 102 on the surface of the substrate 101 (FIG. 1 (b1)). )). Subsequently, the substrate 101 on which the stress film 102 is formed is cooled to room temperature. At this time, due to the difference in thermal expansion coefficient between the substrate (Si) 101 and the stress film (quartz: SiO ₂ ) 102, the stress film 102 generates a stress in the extension direction along the film surface (FIG. 1 (b2); Arrow X1).

  Next, the lower electrode film 103 is formed on the surface of the stress film 102 (FIG. 1C). As a material of the lower electrode film 101, for example, titanium, platinum or the like can be used. As a method for forming the lower electrode film 101, for example, a sputtering method can be used.

  Next, the substrate 101 on which the lower electrode film 103 is formed is again put in a heating furnace, and is heated at a temperature of about 600 ° C. for a predetermined time to form a displacement film 104 on the surface of the lower electrode film 103 (FIG. 1 ( d1)). As a material of the displacement film 104, for example, lead zirconate titanate (PZT) which is a piezoelectric material can be used. As a method of forming the displacement film 104, for example, a sputtering method can be used. Subsequently, the substrate 101 on which the displacement film 104 is formed is cooled to room temperature. At this time, due to the difference in thermal expansion coefficient between the substrate (Si) 101 and the displacement film (PZT) 104, the displacement film 104 generates stress in the contraction direction along the film surface (FIG. 1 (d2); arrow X2). . At this time, the substrate 101 is warped. However, when the thickness of the substrate 101 is sufficiently larger than the thickness of each of the films formed, the amount of warpage is negligible.

The thermal expansion coefficient of Si, which is the material of the substrate 101, is 3 ppm / K, the thermal expansion coefficient of SiO _{2 that} is the stress film 102 is 0.5 ppm / K, and the thermal expansion coefficient of PZT, which is the material of the displacement film 104, is 8 ppm. / K.

  Next, the upper electrode film 105 is formed on the surface of the displacement film 104 (FIG. 1E). As a material of the upper electrode film 105, for example, chromium, gold, or the like can be used. As a method for forming the upper electrode film 105, for example, a sputtering method can be used. In this way, a thin film layer composed of the stress film 102, the lower electrode film 103, the displacement film 104, the upper electrode film 105, and the like is formed.

  Next, a photosensitive resin is applied to the back surface of the substrate 101 using a spin coating method to form a resist film 80 (FIG. 1F). Subsequently, the resist film 80 is patterned by exposure through a photomask (FIG. 1G).

  Next, the substrate 101 is etched using a reactive ion etching method to form an opening 101a (FIG. 1 (h1)), and the thin film actuator 1 is completed. At this time, the periphery of the stress film 102 is fixed to the substrate 101, and the entire film (the stress film 102 and the displacement film 104) has a diaphragm structure. As described above, since the stress in the expansion direction (arrow X1) is applied to the stress film 102 and the stress in the contraction direction (arrow X2) is applied to the displacement film 104, the entire film is perpendicular to the film surface. It becomes a convex shape in the direction (FIG. 1 (h2)). That is, the entire film undergoes a primary initial deformation.

  The magnitude of the stress is determined by the temperature at which the film (stress film 102, displacement film 104) is formed, the difference in thermal expansion coefficient between the film material and the substrate 101 material, and the like. Further, the amount of deformation of the film is determined by the thickness, area, rigidity, etc. of the film in addition to the stress described above. In the case of a diaphragm structure having a circular membrane shape, the center and the periphery of the membrane stress increase. The above variables are adjusted and the stress value is controlled so that the internal stress value is within the range of the fracture stress of the material, including deformation during driving described later.

  Next, deformation during driving of the thin film actuator 1 will be described with reference to FIG. 2A is a schematic cross-sectional view showing a deformation mode when the thin film actuator 1 is not driven, and FIG. 2B is a schematic cross-sectional view showing a deformation mode when the thin film actuator 1 is driven.

  The displacement film 104 made of PZT is dielectrically polarized in a direction perpendicular to the film surface (FIG. 2A; arrow P). When an AC voltage is applied between the upper electrode film 105 and the lower electrode film 103 (FIG. 2B; arrow E), the displacement film 104 expands and contracts in a direction perpendicular to the film surface and a direction along the film surface. It expands and contracts in the opposite phase to (Fig. 2 (b); arrow X). The expansion and contraction of the displacement film 104 in the direction along the film surface is equivalent to the increase or decrease of the internal stress described above, and the displacement film 104 is perpendicular to the film surface around the non-driven position (initial deformation position). (Fig. 2 (b); arrow Y).

  Next, FIG. 3 shows how the shape of the thin film actuator 1 changes during initial deformation due to the balance of bending forces at the center and the periphery.

  As described above, since the stress in the expansion direction of the stress film 102 and the stress in the contraction direction of the displacement film 104 act on the thin film actuator 1 when not driven, the entire film protrudes in a direction perpendicular to the film surface. It becomes a shape.

  In the configuration of the free end where the periphery of the entire film is not fixed, the entire film has a dish shape that is convexly curved in a direction perpendicular to the film surface, but the periphery is fixed to the substrate 101 as in this embodiment. In the case of the diaphragm configuration, there is a possibility that the bending force of the peripheral part of the entire film and the bending force of the central part are antagonized (FIG. 3A).

  When the bending force of the peripheral part is weaker than the bending force of the central part, the central part is large and has a convex shape in the direction perpendicular to the film surface (FIG. 3B). The portion becomes convex in a direction perpendicular to the film surface, and the peripheral portion is raised (FIG. 3C). When the peripheral force is even stronger, the entire film may have a convex shape in the opposite direction perpendicular to the film surface (FIG. 3D).

  In order to increase the amount of displacement when the thin film actuator 1 is driven, it is preferable that the shape of the initial deformation of the entire film be the shape shown in FIG. 3 (b) or FIG. 3 (d).

  FIG. 4 shows how the shape changes during initial deformation depending on the size of the upper electrode film 105 of the thin film actuator 1.

  When the upper electrode film 105 is formed only at the central portion of the entire film (FIG. 4 (a1)), the rigidity of the central portion is increased and it is difficult to deform, so that it approaches the shape shown in FIG. 3 (d). (FIG. 4 (a2)). On the other hand, when the upper electrode film 105 is formed wider than the width W of the opening 101a (FIG. 4 (b1)), the rigidity of the peripheral portion increases and it becomes difficult to deform, so that the shape shown in FIG. (FIG. 4 (b2)).

  As described above, even when parameters such as internal stress, diameter, thickness, and rigidity of the entire film are adjusted, the shape at the time of initial deformation cannot be controlled to the shape shown in FIG. 3B or 3D. The desired shape can be obtained by adjusting the shape of the upper electrode film 105.

  Thus, in the thin film actuator 1 according to the embodiment of the present invention, the region of the displacement film 104 corresponding to the opening 101a of the substrate 101 is the opening 101a perpendicular to the film surface due to at least the stress of the stress film 102 when not driven. Convex shape in the direction of or the opposite direction. That is, the displacement film 104 is subjected to primary initial deformation. As a result, it is possible to prevent the occurrence of higher-order deformation such as buckling or wrinkles due to stress concentration on unnecessary portions. As a result, a desired displacement shape and displacement direction can be obtained stably.

  Next, the configuration of the inkjet head according to the embodiment of the present invention will be described with reference to FIG. FIG. 5 is a schematic cross-sectional view showing a schematic configuration of the inkjet head 1A.

  As shown in FIG. 1, the inkjet head 1A includes a nozzle substrate 2, a body substrate (substrate) 101, a stress film 102, a common electrode film (lower electrode film) 103, a displacement film 104, and a drive electrode film (upper electrode film). The above-described thin film actuator 1 having 105 or the like.

  The nozzle substrate 2 is formed with a plurality of nozzle holes 2a for ejecting ink. As a material for the nozzle substrate 2, silicon, glass, a ceramic material, or the like can be used, but silicon is preferable. The body substrate 101 of the thin film actuator 1 is bonded to the upper surface side of the nozzle substrate 2. Inside the body substrate 101, a pressure chamber (opening) 101 a configured by bonding the nozzle substrate 2, an ink supply path (not shown), and the like are formed. The pressure chamber 101a is formed corresponding to each of the plurality of nozzle holes 2a and stores ink.

  When a drive signal is applied between the drive electrode film 105 and the common electrode film 103 from an external drive circuit (not shown), the displacement film 104 vibrates, and this vibration is formed on the body substrate 101 to store the ink. The pressure in the chamber 101 a is changed, and ink droplets are ejected from the nozzle holes 2 a formed in the nozzle substrate 2.

  By using the above-described thin film actuator 1 that can stably obtain a desired displacement shape and displacement direction as a drive element of the inkjet head 1A, it is possible to eject ink droplets with high accuracy and efficiency.

The present invention has been described above with reference to the embodiments. However, the present invention should not be construed as being limited to the above-described embodiments, and can be changed or improved as appropriate. For example, although the SiO ₂ film is used as the stress film 102 in the above-described embodiment, a SiN film obtained by nitriding Si, another metal film, an insulating film, or the like may be used. Further, although PZT is used as the displacement film 103, a piezoelectric film such as barium titanate, a shape memory alloy film such as titanium nickel alloy, a bimetal such as iron nickel alloy, or the like may be used. Depending on the material of the film, there are differences in rigidity, fracture stress, thermal expansion coefficient, heating temperature at the time of formation, etc., so it is possible to obtain the initial deformation shape, initial deformation amount, drive deformation amount, etc. optimal for the required specifications by combining it can.

  In the above-described embodiment, the film has a diaphragm structure, but a double beam structure in which both ends of the film are fixed may be used. In this case, the same effect as that of the diaphragm structure can be obtained.

  In the above embodiment, the stress film 102, the lower electrode film 103, the displacement film 104, and the upper electrode film 105 are formed in this order on the substrate 101. However, on the substrate 101, The lower electrode film 103, the displacement film 104, the upper electrode film 105, and the stress film 102 may be formed in this order. In this case, the same effect as that of the above-described embodiment can be obtained.

1A Inkjet head 1 Thin film actuator 101 Substrate (body substrate)
101a Opening (pressure chamber)
102 Stress film 103 Lower electrode film (common electrode film)
104 Displacement film 105 Upper electrode film (drive electrode film)
2 Nozzle substrate 2a Nozzle hole

Claims (5)

A displacement film that expands and contracts in the direction along the film surface by a drive signal;
Two electrode films formed across the displacement film and to which the drive signal is applied;
A thin film layer having a stress film formed along one of the two electrode films and having a stress in a direction along the film surface;
A thin film actuator comprising: a substrate on which an opening is formed;
The thin film layer covers an opening of the substrate, and a peripheral edge of the thin film layer is fixed to the substrate.
The region corresponding to the opening of the substrate of the displacement film has a convex shape in a direction perpendicular to the film surface due to at least the stress of the stress film when not driven.   2. The thin film actuator according to claim 1, wherein a direction of the stress of the stress film is an extension direction along a film surface of the stress film.   The electrode film formed on the surface of the displacement film among the two electrode films is formed inside a region corresponding to the opening of the substrate of the displacement film. The thin film actuator described in 1.   The electrode film formed on the surface of the displacement film among the two electrode films is formed in a region including a region corresponding to the opening of the substrate of the displacement film. 2. The thin film actuator according to 2. The thin film actuator according to any one of claims 1 to 4,
A nozzle substrate bonded to the back surface of the substrate and having nozzle holes that communicate with the openings of the substrate and discharge ink;
An ink jet head comprising: a pressure chamber that accommodates the ink in the opening and generates pressure by displacement of the displacement film.