DE69517720T2 - Ink jet recorder - Google Patents

Description

The present invention relates to an ink jet recording apparatus and, more particularly, to an electrostatically driven ink jet head of the recording apparatus.

Ink jet recording apparatus having an ink jet head for ejecting ink droplets onto a recording medium in response to electrical drive pulses are well known and in widespread use. The ink jet head includes an actuator for converting each electrical drive pulse into a pressure pulse, thereby ejecting an ink droplet from a nozzle of the ink jet head. Ink jet heads using electrostatic attraction for the actuator are disclosed, for example, in JP-A-289351/1990, EP-A-0 479 441, EP-A-0 580 283, as well as in EP-A-0 634 272, EP-A-0 629 502 and EP-A-0 629 503 (the latter three documents constituting prior art under Article 54(3) EPC). An ink jet head according to this prior art comprises one or more nozzles, each having an ink passageway leading to the or each nozzle, a diaphragm forming one plate of a capacitor and provided on a part of the or each ink jet passageway, and a nozzle electrode disposed opposite the diaphragm with an air gap therebetween. The nozzle electrode forms the other plate of the capacitor. An electrical pulse is applied between the diaphragm and the nozzle electrode to deform the diaphragm by means of electrostatic force and thereby eject an ink droplet from the nozzle. EP-A-0 479 441 discloses in particular an ink jet recording apparatus according to the preamble of claim 1.

In these ink jet heads, the actuator includes an oscillating chamber formed between the diaphragm and the nozzle electrode. When the oscillating chamber is exposed to the open air, dust and other airborne particles are attracted to the oscillating chamber of the actuator when the diaphragm is driven. As disclosed in EP-A-0 580 283, this problem can be remedied by sealing the actuator. However, when the actuator is sealed, air trapped within the oscillating chamber resists the electrostatic attraction of the diaphragm and can prevent sufficient electrostatic attraction for normal operation. If the electrostatic attraction of the oscillating chamber decreases, insufficient pressure is generated to adequately eject ink and print quality and reliability drop significantly. The reduction in electrostatic attraction can be compensated for by increasing the drive voltage applied to the actuator. EP-A-0 580 283 discloses a drive voltage of 70 to 100 volts. However, such a high drive voltage is undesirable.

JP-A-59 115 860 discloses an electrostatic pressure sensor included in an ink jet head using piezoelectric actuators. JP-A-2 274 552 discloses an ink jet head with electrostatic actuators in which a pressure chamber communicating with a nozzle is connected to a control chamber via a membrane. The membrane is located on the nozzle Two electrodes arranged perpendicular to the membrane surround the pressure chamber and the control chamber. The control chamber is filled with a medium whose dielectric constant is much higher than that of the ink in the pressure chamber. A voltage applied to the electrodes causes a deflection of the membrane as a result of the difference in dielectric constant between the two media on either side of the membrane. The deflection in turn creates a pressure increase that causes the ejection of ink.

Therefore, the object of the invention is to provide an inkjet head in which sufficient electrostatic attraction can be achieved even with a sealed actuator and a drive voltage of no higher than approximately 45 volts.

This object is achieved with an ink jet recording device as claimed in claim 1.

Preferred embodiments of the invention are the subject of the dependent claims.

In an ink jet head according to the present invention, the oscillation chamber in the actuator is an airtight structure, thereby eliminating the possibility that foreign matter such as airborne particles are attracted to and enter the oscillation chamber when the diaphragm vibrates.

The inventors found that in practice, the pressure increase within the oscillation chamber is small enough so that deformation of the diaphragm by electrostatic attraction is not avoided when the volume V of the sealed oscillation chamber (hereinafter referred to as "actuator volume") is two or more times larger than the volume ΔV "displaced" by the diaphragm deformation necessary to guarantee reliable ink ejection (hereinafter referred to as "displacement volume"). Thus, according to the present invention, sufficient diaphragm deformation can be achieved without requiring an undesirably high drive voltage.

The upper limit of the ratio V/ΔV is set to avoid undesirable enlargement of the inkjet head to achieve high wafer yield (the number of heads produced from one wafer).

It is possible to minimize the volume of the oscillation chamber itself if a volume intended for accommodating a wiring member connecting the nozzle electrode to a terminal member communicates with the oscillation chamber and thus contributes to the actuator volume. In this way, an appropriate actuator volume can be achieved without requiring an increase in the size of the ink jet head.

Sparking and actuator damage caused by sparking can be avoided by sealing an inert gas, nitrogen gas or dry air in the actuator or the oscillation chamber.

The invention is described in more detail below with reference to the drawings, which only show a preferred embodiment, and of which:

Fig. 1 is a partially exploded view of an ink jet head according to a preferred embodiment of the present invention;

Fig. 2 is an enlarged view of detail A of Fig. 1;

Figure 3 is a side cross-section of a complete, assembled ink jet head according to the preferred embodiment of the present invention;

Fig. 4 is a perspective view of the assembled ink jet head;

Fig. 5 is a plan view along the line A-A in Fig. 3; and

Fig. 6 is used to describe the operation of the membrane.

The embodiment of the invention described below is an edge type ink jet head in which ink droplets are ejected from nozzles provided on the edge (front side) of a substrate. Note that the invention is also applicable to a surface type ink jet head in which ink is ejected from nozzles provided on the surface (top side) of the substrate.

The ink jet head 10 of this embodiment is constructed from three substrates 1, 2, 3 which are stacked one on top of the other and structured as described in detail below. A first substrate 1 is enclosed between a second and a third substrate 2 or 3 and is made of a silicon wafer. A plurality of nozzles 4 are formed between the first and third substrates by respective nozzle grooves 11 which are provided on the upper surface of the first substrate 1 so as to run substantially parallel at equal distances from an edge of the substrate. The end of the nozzle groove opposite one edge opens into a recess 12. Each recess is in turn connected to a recess 14 via a narrow groove 13. When assembled, the recess 14 forms a common ink cavity 8 which communicates with the nozzles 4 via openings 7 formed by the narrow grooves 13 and ejection chambers 6 formed by the recesses 12. As shown in Figs. 1 and 3, the recess 14 has an ink supply opening at the rear (the side facing away from the nozzles). The bottom of each ejection chamber 6 includes a diaphragm 5 formed integrally with the substrate 1. As will be understood, the grooves and recesses referred to above can be easily and accurately formed by photolithographic etching of the semiconductor substrate. Note that the diaphragms 5 are formed by first doping the substrate 1 with boron to form an etch stop and then etching to form the diaphragms with a small, uniform thickness.

Electrostatic actuators, each comprising a diaphragm and an associated nozzle electrode, are formed between the first and second substrates. A common electrode 17 of the actuators is provided on the first substrate 1. The magnitude of the work function of the semiconductor constituting the first substrate 1 and the metal used for the common electrode 17 are important factors determining the effect of the electrode 17 on the first substrate 1. The semiconductor material used in this embodiment has a specific resistance of 8 to 12 Ωcm, and the common electrode 17 has a two-layer structure of platinum on a titanium base layer or gold on a chromium base layer. The base layer is essentially provided to increase the bond strength between the substrate and the electrode. However, the present invention is not intended to be limited by this, and various other material combinations can be used depending on the properties of the semiconductor and electrode materials.

As shown in Fig. 2, a thin oxide film 24, about 1 µm thick, is formed on the entire surface of the first substrate 1 except for the common electrode 17. This provides an insulating layer for preventing dielectric breakdown and short circuit during ink jet head operation.

For the second substrate 2 bonded to the lower surface of the first substrate 1, borosilicate glass is used. A recess 15 for accommodating each nozzle electrode 21 is formed on the upper surface of the second substrate 2 below each diaphragm 5. When the second substrate 2 is bonded to the first substrate 1, oscillation chambers 9 are formed at the positions of the recesses 15 between each diaphragm 5 and the opposing nozzle electrode 21. A long, thin support member 35 is arranged in the center of each recess 15. The purpose of the support member 35 is to provide additional support in the case of very thin diaphragms. Support members 35 are not required if sufficient rigidity (elasticity) for ink jet ejection is achieved by molding the diaphragm 5 with sufficient thickness. The height of the support elements 35 can correspond to the depth of the recesses or be smaller.

In this embodiment, recesses 15 formed on the upper surface of the second substrate 2 provide gaps between the membranes and the respective electrodes 21. The length G (see Fig. 3; hereinafter referred to as "gap length") of each gap is equal to the difference between the depth of the recess 15 and the thickness of the electrode 21. Note that this recess may alternatively be formed on the lower surface of the first substrate 1. In this preferred embodiment, the depth of the recess 15 is 0.3 µm, and the pitch and width of the nozzle grooves 11 are 0.2 mm and 80 µm, respectively.

As shown in Fig. 1, the wiring formed on the top of the second substrate 2 comprises the nozzle electrodes 21 and conductive elements 22 which connect each nozzle electrode to a respective terminal element 23. As shown, the conductive elements are arranged in grooves 22a which connect to the recesses 15. The terminal elements 23 are arranged in a corresponding recess formed on an edge of the second substrate 2. All recesses and grooves which accommodate this wiring are preferably of the same depth although this may not always be necessary. The wiring (nozzle electrodes 21, conductive elements 22 and terminal elements 23) are formed by sputtering gold onto the second substrate 2 to a thickness of 0.1 µm. The nozzle electrodes 21 are formed at positions and with shapes that respectively match the diaphragms 5. ITO or other conductive oxide film material may be used instead of gold for forming the electrodes 21, their conductive elements 22 and the terminal elements 23.

Borosilicate glass is also used for the third substrate 3. Nozzles 4, ejection chambers 6, orifices 7 and ink cavity 8 are formed by bonding the third substrate 3 to the upper surface of the first substrate 1. A support member 36 in the ink cavity 8 contributes to reinforcement to prevent collapse of recesses 14 when the first substrate 1 and third substrate 3 are bonded together.

The first substrate 1 and the second substrate 2 are anodically bonded at 270 to 400°C by applying a 500 to 800 volt charge, and the first substrate 1 and the third substrate 3 are then bonded under the same conditions to assemble the ink jet head as shown in Fig. 3. After the anodic bonding, the gap length G between the diaphragm 5 and the nozzle electrode 21 on the second substrate 2 is 0.2 µm in this embodiment.

After the ink jet head is thus assembled, a drive circuit 102 is formed by connecting a flexible printed circuit (FPC) 101 between the common electrode 17 and terminals 23 of the nozzle electrodes 21, as shown in Figs. 3 and 4. An anisotropic conductive film is used in this embodiment to bond the terminals 101 to the electrodes 17 and 23.

In the preferred embodiment, nitrogen gas is introduced into the oscillation chambers 9, which are then hermetically sealed using an insulating sealant 30. The oscillation chambers 9 are sealed close to the terminal elements 23 in this embodiment, thereby enclosing the oscillation chamber 9 and the volume of the vane grooves in the "volume of the actuator" (this will be described in more detail later).

An ink supply pipe 33 and an ink supply vessel 32 are externally attached to the rear of the ink jet head. Ink 103 is supplied from an ink tank (not shown in the figures) into the first substrate 1 via the ink supply pipe 33, the vessel 32 and the ink supply port at the rear of the ink cavity 8 to fill the ink cavity 8 and the ejection chambers 6. The ink in the ejection chambers 6 becomes ink droplets which are ejected from the nozzles 4 and printed on recording paper 105 when the ink jet head 10 is driven, as shown in Fig. 3.

The present invention is characterized in that the oscillation chambers 9 are sealed with the actuator and the volume V of the actuator is set so that the ratio between the actuator volume V and the volume ΔV displaced by a deformation of the membrane 5 is in the range 2 ≤ V/ΔV ≤ 8.

Fig. 6 serves to describe the operation of the membrane 5.

When a drive voltage is applied to an actuator, the capacitor comprising the nozzle electrode 21 and the diaphragm 5 is charged, and the diaphragm 5 is attracted to the electrode 21 by electrostatic force and deformed as shown in Fig. 6. This displacement increases the volume of the ejection chamber 6 and decreases the volume of the oscillation chamber 9 by the displacement volume ΔV. The reduced volume of the oscillation chamber causes the pressure P₀ in the oscillation chamber to increase by a pressure increment ΔP to an increased pressure Pi. When the drive voltage is removed and the capacitor is discharged, the diaphragm 5 returns to its original state (where the diaphragm 5 and the electrode 21 are parallel) in a short time, and part of the displacement volume ΔV is used for ink ejection. Although the deformation of the membrane in response to the drive voltage is a function of time, unless otherwise stated, ΔV and ΔP are used in this description to denote the maximum value, i.e. immediately before the drive voltage is removed.

As can be seen, Fig. 6 shows the case without a support element. If support elements 35 are provided, it could be assumed that Fig. 6 shows only one half of the arrangement (w/2 instead of w), i.e. between one side edge of the membrane and the support element 35, and that the other half is symmetrical between the support element and the other side edge. Although the following discussion assumes that no support elements are used, it applies essentially equally to the case where support elements are used.

As mentioned before, the lower limit of the ratio V/ΔV ensures that the pressure increment ΔP in the oscillation chamber is low enough. The reason for the upper limit of V/ΔV is described below. The values in Table 1 are the design values for inkjet heads with different printing resolutions.

In Table 1, head types (1) and (2) are edge ejection type ink jet heads using a silicon substrate with (100) surface for the first substrate. In head type (1), the actuator volume includes the volume of the oscillation chamber 9 alone and does not include any volume associated with the wiring (leads and terminals) connected to the electrode. In head type (2), the actuator is sealed close to the electrode terminals (see Figs. 3 and 5), and the actuator volume includes the volume of the cavity defined by the lead groove 22a (which serves as a dummy volume for increasing the actuator volume) in addition to the volume of the oscillation chamber 9, thereby reducing the pressure increment ΔP in the oscillation chamber associated with the displacement volume ΔV. The head types (3), (4) and (5) are surface ejection type ink jet heads using a silicon substrate having (110) surface as the first substrate 1, and the actuator volume is similarly maximized by using the dummy volume within the limited head size. Each of the head types (1) to (5) functions sufficiently as an ink jet head and is designed so that the yield of each wafer is maximized. For example, in the case of head type (1), the V/ΔV ratio is 2.31 and the increased pressure in the oscillation chamber is 1.77 kgf/cm² (17.3 x 10⁴ Pa). If a dummy volume is created in this head type without changing the head size, the V/ΔV ratio increases to 4.69 and the increased pressure Pi drops by about 30% to 1.27 kgf/cm² (12.4 x 10⁴ Pa), as shown in head type (2).

It is not possible to further reduce the increased pressure Pi in the oscillation chamber without increasing the head size. This would reduce the yield per wafer and increase the cost per unit.

When the resolution is increased, i.e. the nozzles are arranged closer together, the displacement volume ΔV decreases because the volume of each ejected ink droplet is smaller than in the case of a low resolution head.

In addition, the more nozzles the head has, the larger the dummy volume becomes, and the V/ΔV ratio is increased because the area of the vanes 22 increases relative to the total area of the head.

For example, the area occupied by membranes is about 40% of the total area of the head chip in the case of head types (1) and (2), but is about 25% in head types (3), (4) and (5). The V/ΔV ratio is ≤ 8 when using the largest dummy volume possible in these high resolution inkjet heads without sacrificing yield per wafer or functionality of the inkjet head.

It is not possible to achieve a V/ΔV ratio of more than 8 without increasing the head size, which would reduce the yield per wafer and increase the cost per unit. In addition, the reduction in the pressure increment ΔP in the oscillation chamber achieved with V/ΔV ≤ 8 is sufficient, and a further increase in the V/ΔV ratio does not result in a significant further reduction in the pressure increment: e.g. the increased pressure Pi drops from 1.15 kp/cm² (11.3 x 10⁴ Pa) to only about 1 kp/cm² (9.8 x 10⁴ Pa). The rational range for the V/ΔV ratio when considering inkjet heads with different resolutions is therefore 2 ≤ V/ΔV ≤ 8.

It should also be noted that although the present invention is internally sealed with nitrogen gas as described above, the sealed gas according to the invention is not intended to be limited thereto, and may alternatively be (a) any inert gas (e.g., helium, neon), (b) nitrogen gas, or (c) dry air which is chemically stable and does not chemically react when the ink jet head is driven (during electrical discharge) which could cause a change in gas properties and corrosion or damage to the diaphragm 5 or the individual electrode 21. The preferred order of selection for these sealed gases is (a), (b), and (c) considering these performance requirements, but is (c), (b), (a) considering cost. It follows that (b), nitrogen gas, is the overall preferred selection in terms of both performance and cost. points. These sealed gases also prevent sparking within the oscillation chamber 9 and lead to stable operation.

In order for all actuators of a multi-nozzle inkjet head to have the same characteristics, it is preferable that the respective actuator volumes are unified. As can be understood from Fig. 1, although the volume of the oscillation chambers can easily be made the same for all actuators, the individual guide elements 22 have different lengths. Therefore, if the dummy volume created by the grooves 22a containing the guide elements 22 is included in the total actuator volume, these grooves should preferably be dimensioned so that despite their different lengths, each provides the same dummy volume.

Table 1

V/ΔV ratio of the inkjet head

Head gap G = 0.2 um

Claims (4)

1. Inkjet recording device with: an inkjet head (10) with one or more nozzles (4) and for each nozzle: an ink passage (7, 6) in communication with the nozzle (4), and an electrostatic actuator comprising a diaphragm (5) provided on a part (6) of the ink passage, a nozzle electrode (21) provided opposite the diaphragm (5) with a gap (G) therebetween, the diaphragm forming one plate and the nozzle electrode (21) forming the other plate of a capacitor, and an oscillation chamber (9) having first and second walls, the first wall of which is formed by the diaphragm (5) while the nozzle electrode is fixed to the second wall, the device further comprising drive means (102) for selectively charging and discharging each actuator to deflect its diaphragm (5) by electrostatic force and thereby eject ink jet droplets from the one or more nozzles, characterized in that the oscillation chamber (9) is hermetically sealed with a volume V such that the ratio between the volume V and the volume ΔV displaced by the membrane at maximum deflection of the membrane is in the range 2 ≤ V/ΔV ≤ 8. 2. Device according to claim 1, wherein the volume V includes a volume formed by a cavity (22a) which accommodates a wiring element (22) connected to the electrode (21). 3. Apparatus according to claim 1 or 2, comprising a plurality of nozzles, each of the actuators having substantially the same volume V. 4. Device according to claim 1, 2 or 3, wherein the oscillation chamber (9) or the oscillation chamber (9) and the cavity (22a) for accommodating the guide element (22) are each filled with an inert gas, nitrogen gas or dry air.