JP2008155472A - Liquid discharge head and its manufacturing process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid discharge head which makes up the configuration of an individual liquid chamber which gets a sufficient rigidity against the displacement of an oscillating board while assuring the volume required for eliminating the viscous resistance of a liquid as the individual liquid chamber. <P>SOLUTION: The liquid discharge head includes the individual liquid chamber communicating with an outlet port, the oscillating board installed according to the individual liquid chamber and a piezoelectric element which generates a pressure for discharging the liquid while discharging the liquid in the individual liquid chamber from the outlet port. In the cross-section of the individual liquid chamber, the side-wall of the individual liquid chamber has a curvature which makes the inclination of the side-wall decrease gradually in the direction of leaving the oscillating board side. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an increase in density of a liquid discharge head that pressurizes a liquid in an individual liquid chamber and discharges liquid droplets from nozzles using a pressure generated by vibrating a diaphragm, and a manufacturing method thereof.

  A liquid that forms an ink image on recording paper or the like by using a piezoelectric element in which lead zirconate titanate (hereinafter referred to as PZT) is disposed between upper and lower electrodes to displace the diaphragm and eject ink droplets. A discharge head has been put into practical use.

  Patent Document 1 discloses a structure using a thin film PZT with an integrated structure without a bonding process using a silicon substrate, and a precise and high-density liquid discharge head can be produced by a lithography method. In Patent Document 2, a porous silicon part is formed at a desired position on the surface of a silicon wafer, a diaphragm is formed on the upper layer by epitaxial growth, and then the porous structure part is removed by wet etching to manufacture an individual liquid chamber. A simple process is disclosed.

However, if the individual liquid chambers are arranged with high density in these types of heads, the side wall thickness cannot be sufficiently obtained and rigidity cannot be obtained. For this reason, crosstalk occurs and high-resolution image formation cannot be achieved, so there is a conventional technique aimed at improving the rigidity of the side wall. In Patent Document 3, a beam-like portion is provided between the side walls of the individual liquid chamber so that the side walls are not distorted by the displacement of the piezoelectric element. In this publication, impurity ions are added to a desired part to form a beam-like part. Further, in Patent Document 4, the individual liquid chambers are formed by the (111) surface of the single crystal silicon substrate exposed by anisotropic wet etching and the inner surface of the diaphragm, thereby forming a triangular individual liquid chamber as a cross-sectional shape. An example in which the side wall is trapezoidally formed to increase the rigidity is disclosed. Patent Document 5 discloses an example in which an orifice plate is bonded to a diaphragm to form a nozzle.
Japanese Patent No. 2976479 JP 2006-0669152 A JP 10-119267 A JP 2001-353869 A JP 2001-205808 A

  Problem 1: In the conventional technology for countermeasures against crosstalk, the arrangement in which the beam-like portion is arranged between the side walls of the individual liquid chamber cannot efficiently transmit the displacement generated in the diaphragm as pressure to the liquid in the individual liquid chamber. There was a problem. In addition, the individual liquid chamber formed by etching depending on the silicon surface orientation has a problem that it cannot be densified if an attempt is made to secure a necessary volume because the angle of the orientation surface is determined.

  Problem 2: Although a manufacturing method using porous silicon that can form a diaphragm by film formation without using an expensive SOI wafer is simple, the porous silicon portion that can be removed at high speed by hydrofluoric acid is heated. When oxidized, the porous oxide rises from the surface. For this reason, there has been a problem of adversely affecting the performance of the diaphragm formed thereafter.

  A head according to the present invention includes an individual liquid chamber communicating with an ejection port, a diaphragm provided corresponding to the individual liquid chamber, and a piezoelectric element that generates a pressure for ejecting the liquid. A liquid discharge head that discharges liquid in a liquid chamber from the discharge port, wherein in the cross-sectional shape of the individual liquid chamber, the side wall of the individual liquid chamber is inclined toward the direction away from the diaphragm side. It has a curvature that gradually decreases.

  According to the present invention, it is possible to configure the shape of the individual liquid chamber that obtains sufficient rigidity with respect to the displacement of the diaphragm while securing the volume necessary for eliminating the viscous resistance of the liquid as the individual liquid chamber. .

  Further, the swell of the porous silicon in the portion corresponding to the individual liquid chamber can be suppressed, and a thin film to be a vibration plate formed thereon can be formed with high accuracy.

(Head structure)
The structure of the liquid ejection head of this embodiment will be described with reference to the cross-sectional view of FIG. The individual liquid chamber has a long hole shape. A plurality of individual liquid chambers are formed side by side in a direction perpendicular to the direction in which the elongated hole shape extends (long axis direction). FIG. 2 is a sectional view in which the head is cut in a direction perpendicular to the direction in which the individual liquid chamber having a long hole shape extends.

  1 is a single crystal silicon substrate, 2 is an individual liquid chamber, 3 is a side wall between individual liquid chambers, 4 is a diaphragm, 5 is a lower electrode, 6 is a piezoelectric body, 7 is an upper electrode, and 8 is a discharge Indicates an exit. FIG. 2 shows an edge / shooter type head in which an ejection port is formed on a surface intersecting with a surface on which a piezoelectric body is formed.

The case where the individual liquid chambers are arranged at a pitch of 84 μm (corresponding to a density of 300 dpi) is shown. In consideration of the viscous resistance of the ink, the cross-sectional dimensions of the individual liquid chambers in FIG. 2 have a bottom shape in which the diaphragm side width is 44 μm, the bottom side width is 4 μm, and the depth is 30 μm. . The sectional area of the individual liquid chamber is about 1020 μm ² . At this time, the interval between adjacent diaphragms is 40 μm. Thus, since the individual liquid chamber has a portion that narrows with a curvature from the diaphragm side so as to be concave in the individual liquid chamber, the thickness of the side wall becomes smaller in the downward direction (the direction away from the diaphragm). Since it increases, it has a strong structure against crosstalk.

  As a comparative example, FIG. 3 shows a case where an individual liquid chamber having a (111) plane is formed on a single crystal silicon substrate by anisotropic etching. In this case, the etching progress angle is about 54 degrees, and the cross-sectional dimensions of the individual liquid chamber having the same cross-sectional area as in FIG. 2 are inverted triangles having a width of 54 μm on the diaphragm side and a depth of 38 μm. When these are arranged at the same pitch as in FIG. 2, the distance between adjacent diaphragms is 30 μm. As described above, in the comparative example, the interval between the adjacent individual liquid chambers becomes close, and the state is easily affected by crosstalk.

(Head manufacturing method)
An anodizing method is mentioned as a manufacturing method which makes the individual liquid chamber in this embodiment the ship bottom shape. In the anodizing method, when silicon is connected to the anode and platinum is connected to the cathode in a hydrofluoric acid solution and a current is passed, the Si-H bond on the silicon surface is weakened by the supply of holes, and then the electronegativity is the highest. The strong F- ion breaks the Si-H bond, resulting in a Si-F bond on the outermost surface. This is a technique in which many fine holes can be formed on the silicon surface by repeating this process and dissolving it in the solution as SiF4.

  In anodizing, the porosity (porosity in three dimensions) of the porous silicon to be formed can be changed by changing the concentration of hydrofluoric acid constituting the chemical forming solution. As shown in FIG. 1, a silicon wafer partially formed with porous silicon having a different degree of porosity is oxidized in an oxygen atmosphere at 900 ° C. for 1 hour. Then, when the expansion rate of the porous silicon film thickness is measured from (porous silicon film thickness before oxidation) / (film thickness after oxidation), the expansion coefficient is roughly estimated from the condition where the porosity is around 80%. When it becomes zero and exceeds 90%, defects such as a defect in the porous portion are generated. That is, in order to suppress the expansion rate of the porous silicon film thickness, the porosity is preferably in the range of 80% to 90%. In anodization, the porosity depends on the hydrofluoric acid concentration and the current density. Specifically, when the concentration of hydrofluoric acid is low, the porosity increases, and increases even if the current density is increased. However, when the hydrofluoric acid concentration is less than 10%, the setting of the current density satisfying the formation condition of the porosity of about 80% is extremely narrow and difficult. Moreover, since a huge current density is required at a concentration exceeding 30% hydrofluoric acid concentration, a practical range of 10% to 30% hydrofluoric acid concentration is desirable.

  Thus, in this embodiment, a portion of porous silicon corresponding to the individual liquid chamber is formed with a porosity of 80% or more and 90% or less using a P + silicon wafer and then thermally oxidized. Thereby, the swelling of the porous silicon from the wafer surface can be suppressed, and a thin film to be a vibration plate formed thereon can be accurately formed. Then, by removing the porous portion, it is possible to manufacture a diaphragm that efficiently propagates the displacement of the individual liquid chamber constituted by the side wall having the curvature and the piezoelectric element. The side wall having a curvature means that, in the cross-sectional shape of the individual liquid chamber, the side wall of the individual liquid chamber has a curvature such that the inclination of the side wall gradually decreases in a direction away from the diaphragm side.

  In this embodiment, the curvature of the side wall of the individual liquid chamber can be varied by selecting the chemical mask material. Since the spread directly under the mask is determined by the hydrofluoric acid resistance of the chemical liquid, a silicon nitride material having a ratio of the lateral spread in the depth direction of about 0.7 is suitable in this embodiment. Other materials have the same or better resistance to hydrofluoric acid than silicon nitride, but metal materials have problems with adhesion to the silicon substrate, and can be used if this is improved. is there.

(Example 1)
A method for manufacturing the liquid discharge head of this embodiment will be described below. As shown in FIGS. 4 and 5, the main steps of this embodiment are “a formation mask formation step”, “anode formation step”, “diaphragm formation step”, and “piezoelectric body formation step”. , “A step of forming a discharge port”, and “a step of etching the porous portion”. Further, each step will be described in detail.

1. Chemical mask forming process (FIG. 4-a)
As a head substrate, a P + silicon single crystal substrate 1 having a thickness of 625 μm and a specific resistance of 15 mΩ · cm subjected to double-side polishing treatment is prepared. A 400 nm thick silicon nitride film 9 is formed on the head substrate 1 by plasma CVD as a mask material having hydrofluoric acid resistance. Next, a pattern having an opening width of 4 μm is formed by photolithography at a pitch for arranging individual liquid chambers (300 at a pitch of 84 μm), and the silicon nitride film 9 is removed. In the next step, porous silicon is formed from the removed portion of the silicon nitride film. As for the method of forming the silicon nitride 9, there are means such as sputtering and low-pressure CVD, and the hydrofluoric acid resistance can be improved by performing an annealing treatment after the film formation.

2. Step of anodizing (FIG. 4-b)
Next, formation of the porous silicon oxide layer will be described. The part which becomes the flow path of the silicon single crystal substrate 1 is made porous by anodizing. A silicon single crystal substrate 1 in which a chemical conversion mask is formed in an aqueous solution of 15% hydrofluoric acid and 8% isopropyl alcohol as a chemical conversion solution is placed. Platinum electrodes are arranged opposite to each other on both sides of the substrate 1, and current is applied between the platinum electrodes at a current density of 80 mA / cm ² for 15 minutes so that the surface on which the porous silicon is formed becomes a cathode. As a result, silicon is anodized from the opening of the chemical conversion mask, and porous silicon is formed from the surface of the silicon single crystal substrate. Since the anodization proceeds substantially isotropically, the finally formed porous silicon has a cross-sectional shape like a ship bottom with the opening as a center. The porous silicon 10 having a porosity of 85% is formed with a depth of 30 μm in the depth direction from the opening and a width of 20 μm from both sides of the opening. When viewed in a cross section perpendicular to the direction in which the individual liquid chamber extends, the side walls of the individual liquid chamber are formed so as to have zero curvature at a depth of 30 μm while having a curvature in the depth direction. Thereafter, the silicon substrate was sufficiently washed with pure water and dried, and then heated to 900 ° C. in 30 minutes while flowing an oxygen gas at a flow rate of 5 liters / minute into a 600 ° C. furnace, and this was maintained for 1 hour. Thereafter, the temperature is lowered to 400 ° C. in 60 minutes to complete the oxidation of the porous silicon 10.

3. Step of forming a diaphragm (FIG. 4-c)
After the chemical mask 9 is removed, a polysilicon 12 to be the diaphragm 4 is formed on the porous silicon oxide 11 with a thickness of 3 μm by low pressure CVD (Chemical Vapor Deposition). As a material of the diaphragm, an insulating material such as silicon oxide or silicon nitride, a metal material such as Cr, or the like may be formed by sputtering.

4). Step of forming piezoelectric body (FIG. 4-d)
On the vibration plate, a Ti film having a thickness of 50 nm and a Pt film having a thickness of 300 nm are formed by sputtering, and the common lower electrode 5 is formed by patterning using photolithography. Next, PZT is formed to a thickness of 3 μm on the lower electrode by sputtering. This PZT is formed within the width of the individual liquid chamber 2 by photolithography to form a piezoelectric body 6. Similar to the lower electrode 5, the upper electrode 7 is formed on the piezoelectric body 6 by sputtering and photolithography.

5. Step of forming discharge port (FIG. 5-a)
FIG. 5 shows a cross-sectional view of the substrate cut in a direction perpendicular to FIG.
In order to form the discharge port 8 for discharging the liquid, a laser beam having a wavelength of 266 nm and a Q switch frequency of 15 kHz is irradiated to the portion corresponding to the discharge port with an output of 100 mW. The diameter of the discharge port is 30 μm. In FIG. 5, the discharge port is formed by irradiating the side surface of the single crystal silicon substrate 1 (surface perpendicular to the surface on which the piezoelectric body is formed) with laser irradiation. In this embodiment, the edge shooter type is used, but a side shooter type head in which an ejection port is formed on the back surface (the direction opposite to the surface on which the piezoelectric body is formed) may be used. Further, the formation of the discharge port is not limited to laser irradiation, and ICP etching is also possible. Further, in the discharge port forming step, holes for supplying an etching solution, which will be described in the next step, may be formed at the same time.
6). Step of etching the porous portion (FIG. 5-b)
A 10% hydrofluoric acid aqueous solution is infiltrated from the discharge port 8 formed in the previous step, and the porous silicon oxide is removed by etching to form the individual liquid chamber 2 connected to the discharge port 8.

  In order to drive the liquid discharge head according to the present embodiment manufactured as described above, the discharge test was performed by performing flexible mounting. As a result, the discharge of adjacent nozzles was stabilized even at a discharge frequency of 15 KHz by applying a 30 V pulse. There was no problem as a high-density nozzle head.

  Moreover, the SEM photograph in the separate liquid chamber formed in this embodiment is shown in FIG. (A) is a photograph at 3000 times, and (b) is a photograph at 60000 times. As shown in FIG. 6, a large number of recesses having a depth of 10 nm to 50 nm and a width of 100 nm to 500 nm formed by etching of porous silicon oxide are formed in the individual liquid chambers. As a result, the walls in the individual liquid chamber have a fine uneven shape, the surface area is increased, the wettability with the ink is improved, and bubbles can be prevented from adhering.

(Example 2)
In this embodiment, a head in which discharge ports are integrally formed on a diaphragm will be described. A description of the same parts as those in the first embodiment will be omitted. The piezoelectric material is formed by the same method as in the first embodiment. Thereafter, the communication portion 14 communicating with the discharge port is formed in a portion corresponding to the end of the individual liquid chamber where the piezoelectric body is not formed. The communication portion 14 is formed through the diaphragm 4 by ICP or etching (FIG. 7A). In the present embodiment, the diameter of the discharge port is 30 μm, and the communication portion 14 is formed correspondingly. Since the lower surface of the diaphragm 4 is the porous silicon oxide 11, ICP and etching for forming the communication portion 14 can be stopped at the porous silicon oxide 11 and penetrated with high accuracy.

  Next, discharge ports (nozzles) are formed on the diaphragm. The conventional discharge port is difficult to manufacture because it is necessary to join the orifice plate with high precision on a diaphragm that is brittle with a thin film. On the other hand, in this embodiment, since the discharge port is integrally formed on the diaphragm, it is possible to manufacture the discharge port easily and with high accuracy even on a fragile diaphragm with a thin film. Also, the length (depth) of the discharge port can be easily changed in design. The discharge port can be formed by plating or resin. However, the material constituting the discharge port needs to be resistant to the etching of the porous silicon oxide 11.

(Example 2-1: Formation of discharge port by plating)
The discharge port is formed using Ni. First, the seed layer 21 was formed and patterned on the diaphragm 4 in which the communication portion was formed, corresponding to the portion where the discharge port forming member was provided. The seed layer 21 was made of Pt / Ti (FIG. 7-b). Next, a mold (discharge port mold) 22 serving as a discharge port is formed on the communication portion 14 with a resist (FIG. 7C). Since the porous silicon oxide layer 11 exists under the communication portion 14, the discharge port mold 22 can be formed of a resist. The discharge port mold 22 made of resist makes it possible to accurately form the discharge port forming member 23 (orifice plate) by electrolytic plating. Finally, Ni was electroplated at a current density of 1 A / dm ² for 295 minutes to obtain a discharge port forming member 23 made of Ni having a discharge port length of 50 μm (FIG. 7-d). Thereafter, the ejection port mold 22 was removed (FIG. 7E). As a result, the Ni discharge port 23 was formed with an accuracy of 10% with a standard deviation. After forming the discharge port, the porous portion was etched in the same manner as in Example 1 to form the individual liquid chamber 2.

(Example 2-2: Formation of discharge port by resin)
In Example 2-1, the discharge port was formed by plating, but in this example, an example of the discharge port formed by resin is shown. FIG. 7F shows an example in which a discharge port made of a resin layer having a thickness of 50 μm is formed by using a coating means such as spin coating or roll coating. The porous silicon oxide layer 11 under the communication portion 14 has an effect of forming a resin layer with high thickness accuracy.

  On the diaphragm 4 in which the communication portion is formed, a resin layer that is a discharge port forming member is formed by photolithography corresponding to a portion where the discharge port forming member is provided (FIG. 7-). f). In this case, EHPE-3150 (produced by Daicel Chemical Industries, Ltd.) alicyclic epoxy resin, which is an epoxy resin, was used as the photosensitive resin. As the mixed catalyst, 4,4'-di-t-butyldiphenyliodonium hexafluoroantimonate / copper triflate, which is a thermosetting cationic polymerization catalyst, was used. The epoxy resin has sufficient mechanical strength as a structure of the inkjet head, adhesion to the substrate, and ink resistance.

  In addition, after forming the resin layer, a discharge port can be formed in the resin layer by dry etching using a dry etching resistant layer. On the resin layer, a silicon-containing positive resist FH-SP (manufactured by Fuji Hunt Co., Ltd.) was applied as a dry etching resistant layer and patterned by photolithography to open the discharge port. Thereafter, the resin at the discharge port is etched by oxygen plasma etching.

  After forming the discharge port, the porous portion was etched in the same manner as in Example 1 to form the individual liquid chamber 2.

  As described above, since the thin film diaphragm can be formed by the film forming process, the liquid discharge head of this embodiment can be driven at a low voltage. Furthermore, in terms of cost, an expensive SOI wafer is not required, the process time can be shortened by a high selectivity with hydrofluoric acid and high-speed etching, and an inexpensive and high-performance liquid discharge head can be provided.

Graph showing the porosity of porous silicon and the amount of swelling after thermal oxidation Sectional view of a liquid discharge head applicable to the present invention Sectional view of a liquid discharge head created by anisotropic etching as a conventional example Schematic diagram showing a manufacturing process of a liquid discharge head applicable to the present invention (a) formation mask formation process, (b) porous silicon formation process, (c) vibration plate formation process, (d) piezoelectric body Formation process Sectional view cut along a plane perpendicular to FIG. 4 (a) Formation of discharge port, (b) Etching of porous portion SEM image observing inner wall surface of individual liquid chamber applicable to the present invention (a) Observation at 3000 times, (b) Observation at 60000 times Step of forming discharge port on diaphragm (a) Formation of communication portion to discharge port, (b) Formation of plating seed layer, (c) Plating mold formation, (d) Plating discharge port formation, (e ) Plating mold removal, (f) Resin discharge port formation

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Single crystal silicon substrate 2 Individual liquid chamber 3 Side wall 4 Diaphragm 5 Lower electrode 6 Piezoelectric body 7 Upper electrode 8 Discharge port 9 Silicon nitride 10 Porous silicon 11 Porous silicon oxide 12 Polysilicon 14 Communication part to discharge port

Claims (4)

An individual liquid chamber communicating with the discharge port; a diaphragm provided corresponding to the individual liquid chamber; and a piezoelectric element that generates a pressure for discharging the liquid, and the liquid in the individual liquid chamber is A liquid discharge head that discharges from a discharge port,
In the cross-sectional shape of the individual liquid chamber, the side wall of the individual liquid chamber has a curvature such that the inclination of the side wall gradually decreases in a direction away from the diaphragm side.   The liquid discharge head according to claim 2, wherein the individual liquid chamber is made of P + type single crystal silicon.   4. The liquid discharge head according to claim 2, wherein an inner wall of the individual liquid chamber has an uneven shape having a height of 10 nm to 50 nm and a width of 100 nm to 500 nm. It is a manufacturing method of the liquid discharge head according to claim 1,
A step of forming anodized porous silicon having a porosity of 80% or more and 90% or less in a portion to be the individual liquid chamber;
Heating the porous silicon to form porous silicon oxide;
Forming the diaphragm on the porous silicon oxide;
Removing the porous silicon oxide to form the individual liquid chamber;
A method for manufacturing a liquid discharge head, comprising:

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