WO2011007645A1

WO2011007645A1 - Thin-film actuator and inkjet head

Info

Publication number: WO2011007645A1
Application number: PCT/JP2010/060513
Authority: WO
Inventors: 松田　伸也
Original assignee: コニカミノルタホールディングス株式会社
Priority date: 2009-07-15
Filing date: 2010-06-22
Publication date: 2011-01-20
Also published as: JPWO2011007645A1

Abstract

Provided is a thin-film actuator, which comprises a thin-film layer having a displacement film that expands and contracts in the direction of the film surface in accordance with drive signals, and a substrate, to which both ends or the edges of the thin-film layer are anchored such that the thin-film layer is displaced in the direction orthogonal to the film surface with the expansion of the displacement film produced by the drive signals. The thin-film layer has initial stress in the direction of expansion along the film surface when not driven.

Description

Thin film actuator and inkjet head

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. Since the thin film actuator can efficiently convert expansion / contraction deformation along the film surface of the displacement film into displacement in a direction perpendicular to the film surface, a highly sensitive pressure sensor and various drive elements can be configured.

On the other hand, the thin-film actuator with a cantilever structure (one-sided beam) has a problem that it has a low rigidity because the tip of the beam (displacement film) is not fixed, and is easily 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).

The thin film actuator having a double beam structure or a diaphragm structure as described in Patent Document 1 has a displacement film having high rigidity, so that the generated pressure can be increased, and the displacement film can be stably deformed by an external force. There is an advantage that the central part of the substrate 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, the displacement in the direction perpendicular to the film surface is almost not. I can't get it.

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

JP-A-8-114408

However, in the thin film actuator as described in Patent Document 1, even if a compressive stress in the extension direction is generated along the film surface on the displacement film by a drive signal (drive voltage), the drive voltage reaches a predetermined voltage. In the initial state, only the thickness of both ends or the periphery of the fixed displacement film increases, and there is a region that does not lead to displacement in the direction perpendicular to the film surface, that is, a dead zone. In this dead zone, the energy input to the displacement film is not converted into the required displacement, so that the driving efficiency is lowered.

Here, the details will be described with reference to FIG. FIG. 5 is a schematic cross-sectional view showing a driving deformation mode of a displacement film in a thin-film actuator having a double-beam structure as described in Patent Document 1.

As shown in FIG. 5, the displacement film 504 is fixed at a portion 504e slightly below the center line 504c in the thickness direction at both ends, and has an asymmetric structure in the thickness direction.

First, in an initial state where a driving voltage is not applied between electrode films (not shown) provided above and below with the displacement film 504 interposed therebetween, the displacement film 504 is in a flat state as shown in FIG. Supported by holdings.

Next, when a driving voltage is applied, the displacement film 504 generates lateral strain due to the piezoelectric lateral effect and deforms in the extending direction along the film surface, but both ends are fixed by the portions 504e. At 504, a compressive stress is generated. When the driving voltage applied to the displacement film 504 is small and the compressive stress generated in the displacement film 504 is smaller than Euler's buckling stress σ _E shown in the following formula (1), as shown in FIG. Even when a driving voltage is applied, the thickness of the displacement film 504 increases around the fixed portion 504e, but displacement in a direction perpendicular to the film surface hardly occurs.

σ _E = C (π ² E) / λ ² (1)
However,
C: constant E: Young's modulus of displacement film λ: ratio of length L and thickness T of displacement film Next, the driving voltage is increased, and the compressive stress of the displacement film 504 becomes Euler's buckling stress σ at a certain voltage value. _{When E} is exceeded, as shown in FIG. 5C, the displacement film 407 starts to buckle and bend, and its central portion is displaced in a direction perpendicular to the film surface. At this time, the portion 504e to which the displacement film 504 is fixed is located slightly below the center line 504c, and the upper thickness of the portion 504e is thicker than the lower thickness. Become. As a result, the displacement film 504 is curved upward, and its central portion is displaced upward and translated.

FIG. 6 shows the relationship between the compressive stress σ of the displacement film 504 and the displacement amount Δy in the direction perpendicular to the film surface.

In the initial region A where the compressive stress σ of the displacement film 504 is smaller than the buckling stress σ _E , almost no displacement occurs. The compression stress sigma exceeds the buckling stress sigma _E region B, the displacement layer 504 begins to bend buckling, displacement Δy increases rapidly in response to compressive stress sigma. As the compressive stress σ further increases, the increase in the displacement amount Δy gradually decreases (region C).

The relationship between the deformation amount Δx in the extending direction along the film surface of the displacement film 504 and the displacement amount Δy in the direction perpendicular to the film surface in the regions B and C is expressed by the following equation (2).

Δy = C (x · Δx) ^0.5 (2)
However,
C: Constant x: Length of the displacement film when no driving voltage is applied As described above, the compressive stress σ and the displacement amount Δy are in a non-linear relationship. In particular, the compressive stress σ is greater than the buckling stress σ _E. In the small initial region A, even if a drive voltage is applied, it hardly contributes to displacement.

The output of the thin film actuator is the product of the displacement in the direction perpendicular to the film surface and the generated force. For this reason, in the initial region A, the output of the thin film actuator becomes almost zero, and the input energy cannot be taken out as an output, and the driving efficiency is greatly reduced.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a thin film actuator and an ink jet head having excellent driving efficiency.

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

1. A thin film layer having a displacement film that expands and contracts in a direction along the film surface by a drive signal;
A thin film actuator comprising: a substrate for fixing a peripheral portion of the thin film layer so as to displace the thin film layer in a direction perpendicular to the film surface by extension of the displacement film by the driving signal;
The thin film actuator, wherein the thin film layer has an initial stress in an extending direction along the film surface when not driven.

2. 2. The thin film actuator according to 1 above, wherein the initial stress is larger than a buckling stress that starts bending the thin film layer in a direction perpendicular to a film surface of the thin film layer.

3. The thin film layer is formed in a lower layer and an upper layer of the displacement film, and has two stress films each having a first initial stress and a second initial stress in the extension direction along the film surface,
The displacement film has a third initial stress in a direction along the film surface;
3. The thin film actuator according to 2, wherein the initial stress is a sum of the first initial stress, the second initial stress, and the third initial stress.

4. 4. The thin film actuator according to item 3, wherein the first initial stress and the second initial stress have different values.

5. The thin film actuator according to any one of 1 to 4,
A nozzle substrate that is bonded to a surface opposite to the surface of the substrate on which the thin film layer is fixed and has nozzle holes that communicate with openings formed in the substrate and eject ink.
The ink jet head according to claim 1, wherein the inside of the opening is a pressure chamber that accommodates the ink and generates pressure by displacement of the displacement film.

According to the present invention, the thin film layer has an initial stress in the extension direction along the film surface when not driven. By setting the initial stress to an appropriate value (for example, greater than or equal to the buckling stress), the initial stress can cover the stress required to exceed the dead band, and the applied drive signal (drive voltage) is small. Even in this case, the thin film layer can be displaced in a direction perpendicular to the film surface in accordance with the driving voltage. Thereby, drive efficiency can be improved.

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 stress of a thin film actuator. It is a figure which shows the relationship between the compressive stress of a thin film layer, and a displacement amount. 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 schematic structure of the inkjet head concerning embodiment of this invention. It is a cross-sectional schematic diagram which shows the aspect of the drive deformation | transformation of the displacement film | membrane of the conventional thin film actuator. It is a figure which shows the relationship between the compressive stress and displacement amount of the displacement film | membrane of the conventional thin film actuator.

Hereinafter, a thin film actuator and an inkjet head according to an embodiment of the present invention will be described with reference to 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. 1I are schematic cross-sectional views showing a manufacturing process of the thin film actuator 1 and a mode of non-driving stress generated in the thin film actuator 1 (hereinafter also referred to as initial stress). .

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

Next, the substrate 101 is put in 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 lower stress film 102 on the surface of the substrate 101 (FIG. 1 ( b1)). Subsequently, the substrate 101 on which the lower stress film 102 is formed is cooled to room temperature. At this time, due to the difference in thermal expansion coefficient between the substrate 101 (Si) and the lower stress film 102 (quartz: SiO ₂ ), a compressive stress in the extension direction is generated in the lower stress film 102 along the film surface (FIG. 1). (B2); Arrow X1).

Next, the lower electrode film 103 is formed on the surface of the lower 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 101 (Si) and the displacement film 104 (PZT), a tensile stress in the contraction direction is generated along the film surface in the displacement film 104 (FIG. 1 (d2); arrow X2).

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.

Next, the substrate 101 on which the upper electrode film 105 is formed is again put into a heating furnace, and is heated by holding at a temperature of about 400 ° C. for a predetermined time, and chemical vapor deposition (CVD) is performed using ethyl silicate (TEOS) as a raw material. Then, quartz (SiO ₂ ) to be the upper stress film 106 is formed (FIG. 1 (f1)). Subsequently, the substrate 101 on which the upper stress film 106 is formed is cooled to room temperature. At this time, due to the difference in thermal expansion coefficient between the substrate 101 (Si) and the upper stress film 106 (quartz: SiO ₂ ), a compressive stress in the extension direction is generated along the film surface in the upper stress film 106 (FIG. 1). (F2); Arrow X3).

In this way, a thin film layer composed of the lower stress film 102, the lower electrode film 103, the displacement film 104, the upper electrode film 105, the upper stress film 106, and the like is formed.

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

Next, the substrate 101 is etched using a reactive ion etching method to form an opening 101a (FIG. 1 (i1)), and the thin film actuator 1 is completed.

At this time, the peripheral edge of the thin film layer is fixed to the substrate 101 to form a diaphragm structure. As described above, the compressive stress in the extension direction (arrow X1) is applied to the lower stress film 102, the tensile stress in the contraction direction (arrow X2) is applied to the displacement film 104, and the extension direction (arrow is applied to the upper stress film 106). Since the compressive stress of X3) acts, for example, the entire film (thin film layer) can be formed in a convex shape in the direction of the opening 101a of the substrate 101 by the stress balance of these films as described later (FIG. 1). (I2)).

At this time, the substrate 101 is warped in a convex shape in the direction of the thin film layer. However, when the thickness of the substrate 101 is sufficiently larger than the thickness of each film, the amount of warpage can be ignored. It is.

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.

When each film is formed with the above setting, a stress (first initial stress) of about −300 MPa is applied to the lower stress film 102 (quartz: SiO ₂ ) due to thermal oxidation, and an about −300 MPa stress is applied to the displacement film 104 (PZT). A stress of +100 MPa (third initial stress) occurs, and a stress of about −100 MPa (second initial stress) is generated in the upper stress film 106 (quartz: SiO ₂ ) by the CVD method. In addition, − sign indicates compressive stress in the direction in which the film extends, and + sign indicates tensile stress in the direction in which the film contracts. Further, the force applied to the film is determined by the product of the stress and the film thickness.

Here, if the Euler's buckling stress σ _E shown in the above-described equation (1) is 100 MPa, for example, the ratio of the thicknesses of the lower stress film 102, the displacement film 104, and the upper stress film 106 is 1: 5. : 3, the sum of the initial stresses generated in each film is 1 × (−300) + 5 × (+100) + 3 × (−100) = − 100, and the buckling stress σ _{E for the} entire thin film layer A compressive stress of 100 MPa can be generated.

That is, by appropriately setting the initial stress and film thickness of each film, the initial stress covers the stress (buckling stress σ _E ) necessary to exceed the dead zone (FIG. 6: region A). Even when a drive signal (drive voltage) to be applied is small, the thin film layer can be displaced in a direction perpendicular to the film surface in accordance with the drive voltage. Thereby, drive efficiency can be improved.

By the way, when a thin-film actuator having a beam structure or a diaphragm structure is used for extension deformation, there is a problem that the displacement direction of the thin-film layer is not determined at the time of deformation. Therefore, in the thin film actuator 1 according to the embodiment of the present invention, the displacement direction is controlled by making the thin film layer an asymmetric structure in the thickness direction. Specifically, as described above, the initial stress (first initial stress and second initial stress) generated in the lower stress film 102 and the upper stress film 106 is made different to provide an asymmetric structure. When the thickness of the upper stress film 106 is increased by several percent with respect to the above-described ratio of the thickness of the lower stress film 102 and the upper stress film 106, the thin film layer is formed on the substrate as shown in FIG. A convex shape can be formed in the direction of the opening 101a. That is, the displacement direction of the thin film layer can be controlled in the direction of the opening 101 a of the substrate 101.

Thus, the larger the difference between the initial stresses (the first initial stress and the second initial stress) between the lower stress film 102 and the upper stress film 106, the more reliably the initial displacement direction can be controlled.

However, on the other hand, as the difference between the initial stresses (the first initial stress and the second initial stress) between the lower stress film 102 and the upper stress film 106 increases, the initial deformation amount increases. As described above, the displacement amount Δy in the direction perpendicular to the film surface at the time of non-driving when no driving voltage is applied (FIG. 2: position S) becomes large. Further, the increase rate of the displacement amount Δy with respect to the stress at the time of driving becomes gentler than in the case of the region B shown in FIG. 6, and the driving efficiency is lowered. 2 shows a direction perpendicular to the compressive stress σ of the thin film layer and the film surface when the difference between the initial stresses (the first initial stress and the second initial stress) between the lower stress film 102 and the upper stress film 106 is large. It is a figure which shows the relationship of displacement amount (DELTA) y.

Therefore, the drive efficiency is maximized by setting the difference between the initial stresses of the lower stress film 102 and the upper stress film 106 to the minimum within a range in which the displacement direction of the thin film layer is stabilized depending on the conditions during manufacture and use. be able to.

The magnitudes of these initial stresses (the first initial stress, the second initial stress, and the third initial stress) are the same as those when the respective films (the lower stress film 102, the upper stress film 106, and the displacement film 104) are formed. It is determined by the temperature, the difference in thermal expansion coefficient between the material of each film and the material of the substrate 101, 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 addition, since the material constituting the film has inherent fracture stress, the above variables are adjusted to control the stress value so that the stress value is within that range, including during driving deformation described later. There is a need to.

Next, deformation during driving of the thin film actuator 1 will be described with reference to FIG. FIG. 3A is a schematic cross-sectional view showing a mode when the thin film actuator 1 is not driven, and FIG. 3B is a schematic cross-sectional view showing a 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. 3A; arrow P). When an AC voltage is applied between the upper electrode film 105 and the lower electrode film 103 (FIG. 3B; 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 a phase opposite to (Fig. 3 (b); arrow X). 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 displaced in a direction perpendicular to the film surface (FIG. 3B; arrow Y ).

As described above, by applying an electric field in the direction in which the displacement film 104 expands and deforms, the region B shown in FIG. 6 can be driven efficiently.

Thus, in the thin film actuator 1 which concerns on embodiment of this invention, the thin film layer was set as the structure which has the initial stress of an expansion | extension direction along a film surface at the time of non-driving. By setting the initial stress to an appropriate value (for example, Euler's buckling stress σ _E shown in equation (1)), the stress necessary to exceed the dead band (FIG. 6: region A) Even when the applied driving signal (driving voltage) is small, the thin film layer can be displaced in a direction perpendicular to the film surface in accordance with the driving voltage. Thereby, drive efficiency can be improved.

Next, the configuration of the inkjet head according to the embodiment of the present invention will be described with reference to FIG. FIG. 4 is a schematic cross-sectional view illustrating a schematic configuration of the inkjet head 1A.

As shown in FIG. 4, the inkjet head 1A includes a nozzle substrate 2, a body substrate (substrate) 101, a lower stress film 102, a common electrode film (lower electrode film) 103, a displacement film 104, a drive electrode film (upper electrode film). ) 105 and the above-described thin film actuator 1 having the upper stress film 106 and the like.

The nozzle substrate 2 has a plurality of nozzle holes 2a for discharging 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 thin film actuator 1 having high driving efficiency as the driving element of the inkjet head 1A, ink droplets can be efficiently discharged.

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, in the above-described embodiment, since the displacement film 104 (PZT) has tensile stress with respect to the substrate 101 (Si), the stress film (lower stress film 102, upper stress film) having compressive stress in the lower layer and upper layer thereof. 103) and configured to provide a SiO ₂ layer is, but if the displacement film itself configured to have a compressive stress, the stress film according is unnecessary.

Specifically, for example, a material such as a metal whose thermal expansion coefficient is larger than that of the displacement film 104 (PZT) is used as a material of the substrate 101, or a thermal expansion coefficient of the substrate 101 (Si) as a material of the displacement film 104. By using a smaller material, the above-described conditions can be satisfied, and the stress film (SiO ₂ ) can be eliminated.

In the above-described embodiment, the SiO ₂ film is used as the stress film (the lower stress film 102 and the upper stress film 106). However, if the material has a smaller thermal expansion coefficient than the substrate 101 (Si), diamond or the like is used. It can also be used.

Further, although PZT is used as the displacement film 104, 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.

Further, as a method of making the thin film layer an asymmetric structure in the thickness direction, the initial stress (the lower stress film 102, the upper stress film 106, the displacement film 104) of the fixed position of the thin film layer and the respective films (the lower stress film 102, the upper stress film 106, and the displacement film 104) The first initial stress, the second initial stress, the third initial stress) and the like can be mentioned, but the same effect can be obtained by changing the rigidity, film thickness, diameter, etc. of each film.

In the above-described embodiment, as a method for generating an initial stress in each film, temperature control at the time of film formation using a thermal expansion coefficient is used. The density of the material element can be changed by changing the gas pressure or the input power. When the gas pressure is lowered, the energy of the material element is converted into film formation energy without being attenuated, so that a denser film is formed. As a result, the compressive stress increases. Similar effects can be obtained when the input power is lowered. In addition, the stress can be controlled by adding a trace amount of impurities to the material.

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

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

Claims

A thin film layer having a displacement film that expands and contracts in a direction along the film surface by a drive signal;
A thin film actuator comprising: a substrate for fixing a peripheral portion of the thin film layer so as to displace the thin film layer in a direction perpendicular to the film surface by extension of the displacement film by the driving signal;
The thin film actuator, wherein the thin film layer has an initial stress in an extending direction along the film surface when not driven.
2. The thin film actuator according to claim 1, wherein the initial stress is larger than a buckling stress that starts bending the thin film layer in a direction perpendicular to a film surface of the thin film layer.
The thin film layer is formed in a lower layer and an upper layer of the displacement film, and has two stress films each having a first initial stress and a second initial stress in the extension direction along the film surface,
The displacement film has a third initial stress in a direction along the film surface;
The thin film actuator according to claim 2, wherein the initial stress is a sum of the first initial stress, the second initial stress, and the third initial stress.
The thin film actuator according to claim 3, wherein the first initial stress and the second initial stress have different values.
The thin film actuator according to any one of claims 1 to 4,
A nozzle substrate that is bonded to a surface opposite to the surface of the substrate on which the thin film layer is fixed and has nozzle holes that communicate with openings formed in the substrate and eject ink.
The ink jet head according to claim 1, wherein the inside of the opening is a pressure chamber that accommodates the ink and generates pressure by displacement of the displacement film.