Inkjet recording apparatus and method of producing an inkjet head

Abstract: Described is an inkjet recording apparatus having an inkjet head (10) which comprises an ink supply port, a common ink cavity (8), a filter (51) having a plurality of filter channels (13a) communicating with the ink supply port at one end and the common ink cavity (8) at the other end, a plurality of ink ejection nozzles (4) each connected to the common ink cavity by a respective ink passage (6, 7), and a corresponding plurality of pressure generating means (5, 21) respectively associated with said ink passages, said pressure generating means (5, 21) being selectively drivable to eject ink droplets through the respective nozzles (4). The inertance (Mf) of said filter is one-fifth or less of the total inertance (Ma) of all ink passages and nozzles. A high reliable inkjet head for an inkjet recording apparatus enabling uniform ink ejection with no nozzle clogging and virtually no crosstalk is achieved.

Description: The present invention relates to an inkjet recording apparatus and, more particularly, to its inkjet head. The invention also relates to a method of producing an inkjet head. Inkjet recording apparatus having an inkjet head for selectively ejecting ink droplets from a plurality of nozzles towards a recording medium in response to electric drive pulses are well known and commonly used. Generally, the inkjet head has a common ink cavity providing an ink source for the individual nozzles and being connected to each nozzle by a separate ink passage. Each ink passage includes an ejection chamber associated with a respective pressure generating device. The pressure generating devices are responsive to the electric drive pulses for selectively and temporarily increasing the pressure in the associated ejection chamber thereby causing ejection of ink droplets. Various types of pressure generating device are known in the art such as piezoelectric devices, thermal devices and electrostatic devices. The part of the ink passage connecting the ejection chamber to the common ink cavity has a cross-sectional area substantially smaller than that of the ejection chamber itself. This part will be referred to as orifice in the following. The common ink cavity serves as an ink supply buffer and is in turn connected, via an ink supply port, to a larger volume ink supply, i.e. an ink tank etc., typically external to the inkjet head. In manufacturing such inkjet heads it is common practice to etch grooves and recessions respectively corresponding to the common ink cavity, the ink passages and the nozzles into the surface of a first substrate which is then bonded to a second substrate as disclosed in EP-A-0 580 283 and JP-B-62-8316/1987 for instance. The document JP-B-8316/1987 discloses an inkjet recording apparatus according to the precharacterizing portion of claim 1. In this prior art a filter is provided between the ink supply and the common ink cavity to prevent foreign matters from entering the ink passages and possibly clogging the nozzles. The filter comprises a plurality of filter channels provided in parallel between an ink supply opening and the common ink cavity. Grooves for the filter channels are formed simultaneously with the grooves and recesses mentioned above by etching in the vertical direction of a glass substrate using a photoetching method. To fulfill the intended function, the cross sectional area of any filter channel must be smaller than the smallest cross sectional area of the ink passages and that of the nozzles themselves. In the prior art referred to above, however, the filter channels are formed simultaneously with the common ink cavity, the nozzles and the ink passages by an isotropic etching method, and the depth of the filter channels is therefore the same as the depth of the nozzles and the other portions of the ink paths between the filter and the nozzles. As a result, the size of foreign particulate passing through the filter may be the same size as that of the nozzle and orifices. The probability of a nozzle or orifice becoming clogged is therefore high, and the filter function of the prior art not satisfactory. Inkjet heads which employ a silicon substrate allowing use of the more precise anisotropic etching are disclosed in, for example, EP-A-0 479 441, EP-A-0 580 283 and in EP-A-0 634 272, EP-A-0 629 502 and EP-A-0 629 503 (the latter three documents forming prior art according to Art. 54(3) EPC). EP-A-0 479 441 discloses a method according to the precharacterizing portion of claim 9. As mentioned above, the common ink cavity supplies the ink to the ejection chambers through respective orifices, and simultaneously buffers or reduces a pressure increase caused by the backflow of ink from an ink ejection chamber when an ink droplet is ejected from the respective nozzle. The purpose of this buffering effect is to avoid or reduce an interaction, i.e. crosstalk, among the plurality of nozzles. While it would seem possible to enhance the function of the filter by reducing the cross-sectional area of the filter channels compared to that of the orifices and nozzles employing more precise manufacturing methods, it turned out that this is apt to impair the buffering effect thereby increasing crosstalk. The buffering effect of the common ink cavity depends on the compliance of the ink volume contained in it and any contribution by the ink supply system upstream of the common ink cavity. As will be shown later, the compliance is proportional to the square of the ink volume. Because of the general demand for small sized inkjet heads the volume of the common ink cavity should be as small as possible resulting in a correspondingly small buffering effect of the ink within the common ink cavity itself. The smaller the filter channels, the less is the contribution that the supply system upstream of the common ink cavity may have to the total buffering effect. The object of the present invention is to provide an inkjet recording apparatus having an inkjet head with multiple nozzles and a filter, wherein a good filtering function and substantially no crosstalk are obtained at the same time. Another object of the invention is to provide a method of manufacturing such inkjet head. These objects are achieved with an inkjet recording apparatus as claimed in claim 1 and a method as claimed in claim 9, respectively. Preferred embodiments of the invention are subject-matter of dependent claims. According to the invention, the filter, the ink passages and the nozzles are formed simultaneously on a silicon substrate by anisotropic etching. This allows to precisely control the absolute and relative dimensions of the individual cavities and channels in the substrate. It further enables manufacturing of precise small-sized inkjet heads. It has been found out that, despite smaller dimensioned filter channels, the buffering effect of the common ink cavity can be maintained if the filter inertance is at maximum one-fifth the total inertance of all ink passages and nozzles. Thus, the present invention combines the advantages of an excellent filtering function with no or substantially no crosstalk. With the ratio of inertances according to one embodiment of the invention, the supply sytem upstream of the common ink cavity still contributes to the required buffering to an extent that a sufficient total buffering is achieved. Alternatively according to another embodiment of the invention, the buffering effect of the common ink cavity may be increased beyond that of the ink itself by means of a flexible wall or wall portion of the common ink cavity. A combination between these possibilities is also possible. The invention will be described in more detail below with reference to the drawings which illustrate preferred embodiments only and in which: Fig. 1 is a partial exploded perspective view of an inkjet head according to the preferred embodiment of the present invention; Fig. 2 is a perspective view of an inkjet head according to the preferred embodiment of the present invention; Fig. 3 is a lateral cross section of an inkjet head according to the preferred embodiment of the present invention; Fig. 4 is an enlarged partial plan view of the substrate of an inkjet head according to the preferred embodiment of the present invention; Fig. 5(a) to (c) are lateral cross sections showing the ink ejection operation of an inkjet head according to the preferred embodiment of the present invention; Fig. 6(a) to (c) are simplified illustrations of what occurs when a voltage is applied between the diaphragm and electrode of the inkjet head shown in Fig. 5; Fig. 7 is used to describe the various channel constants of the ink path in an inkjet head according to the preferred embodiment of the present invention; Fig. 8 is a plan view similar to Fig. 7 of an inkjet head according to an alternative embodiment of the invention; Fig. 9 is a cross section taken along line D-D in Fig. 8; Fig. 10 is a schematic view of an inkjet recording apparatus according to the present invention; Fig. 11 is used to describe manufacturing steps of a method according to the present invention for forming the channels in substrate 1; Fig. 12 is an enlarged sectional view of a filter groove of an inkjet head according to the preferred embodiment of the present invention; Fig. 13 is an enlarged perspective view of the filter channels in an inkjet head according to the present invention; Fig. 14 is a pattern diagram showing the cutting margins between plural ink path patterns formed by anisotropic etching to a silicon substrate. The embodiment of the invention described below is an edge type inkjet head wherein ink droplets are ejected from nozzles provided at the edge of a substrate. It is to be noted that the invention may also be applied to a face type inkjet head wherein the ink is ejected from nozzles provided on the top surface of the substrate. The inkjet head 10 of this embodiment is made up of three substrates 1, 2, 3 one stacked upon the other and structured as described in detail below. A first substrate 1 is sandwiched between second and third substrates 2 and 3, and is made from a silicon wafer. Multiple nozzles 4 are formed between the first and the third substrate by means of corresponding nozzle grooves 11 provided in the top surface of the first substrate 1 such as to extend substantially in parallel at equal intervals from one edge of the substrate. The end of each nozzle groove opposite said one edge opens into a respective recess 12. Each recess in turn is connected via a respective narrow groove 13 to a recess 14. In the assembled state the recess 14 constitutes a common ink cavity 8 communicating with the nozzles 4 via orifices 7 formed by the narrow grooves 13 and ejection chambers 6 formed by the recesses 12. A filter 51 is formed by a plurality of grooves 13a disposed at the back of recess 14, i.e. the ink supply side. In the assembled state of the substrates the grooves 13a form filter channels (in the following the same reference numeral 13a will be used for the grooves and the channels) The cross sectional area of each filter channel 13a is smaller than that of a nozzle 4, i.e the filter channels provide an effective filtering function preventing the introduction of foreign matter into the ink in the common ink cavity 8, the ink passage (6,7) and the nozzles 4. The bottom of each ejection chamber 6 comprises a diaphragm 5 formed integrally with the substrate 1. As will be understood, the grooves and recesses referred to above can be easily and precisely formed by photolithographic etching of the semiconductor substrate. Diaphragms 5 are preferably formed by first doping substrate 1 with boron to provide for etch stopping followed by etching to form the diaphragms with a thin, uniform thickness. Electrostatic actuators each comprising a diaphragm and an associated nozzle electrode are formed between the first and the second substrate. A common electrode 17 of the actuators is provided on the first substrate 1. A thin oxide film (not shown in figures), approximately 1 µm thick, is formed on the entire surface of first substrate 1 except for the common electrode 17. This creates an insulation layer for preventing dielectric breakdown and shorting during inkjet head drive. Borosilicate glass is used for the second substrate 2 bonded to the bottom surface of first substrate 1. A recess 15 for accommodating a respective nozzle electrode 21 is formed in the top of second substrate 2 below each diaphragm 5. When the second substrate 2 is bonded to the first substrate 1 vibration chambers 9 are formed at the positions of recesses 15 between each diaphragm 5 and the opposing nozzle electrode 21. In this embodiment, recesses 15 formed in the top surface of the second substrate 2 provide for gaps between the diaphragms and the respective electrodes 21. The length G (see Fig. 3; hereinafter the "gap length") of each gap is equal to the difference between the depth of recess 15 and the thickness of the electrode 21. It is to be noted that this recess can alternatively be formed in the bottom surface of the first substrate 1. In this preferred embodiment, the depth of recess 15 is 0.3 µm, and the pitch and width of nozzle grooves 11 are 0.2 mm and 80 µm, respectively. As shown in Fig. 1, the wiring formed in the top surface of second substrate 2 comprises the nozzle electrodes 21 and lead members 22 connecting each nozzle electrode to a respective terminal member 23. As shown, the lead members are located in grooves 22a connecting to the recesses 15. The terminal members 23 are located in a corresponding recess formed at one edge of second substrate 2. 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 third substrate 3 to the top surface of first substrate 1. Support member 36 in ink cavity 8 adds reinforcement to prevent collapsing recess 14 when first substrate 1 and third substrate 3 are bonded together. First substrate 1 and second substrate 2 are anodically bonded at 270 to 400°C by applying a voltage 500 to 800 V, and first substrate 1 and third substrate 3 are then bonded under the same conditions to assemble the inkjet head as shown in Fig. 3. After anodic bonding, the gap length G formed between diaphragm 5 and nozzle electrode 21 on second substrate 2 is 0.2 µm in this embodiment. After the inkjet head is thus assembled, drive circuit 102 is connected by connecting flexible printed circuit (FPC) 101 between common electrode 17 and terminal members 23 of nozzle electrodes 21 as shown in Figs. 3 and 4. An anisotropic conductive film is used in this embodiment to bond leads 101 with electrodes 17 and 23. Ink supply tube 33 and ink supply vessel 32 are fit externally to the back of the inkjet head. Ink 103 is supplied from an ink tank (not shown in the figures) into first substrate 1 via ink supply tube 33, vessel 32, an ink supply port (not shown) and the filter channels 13a at the rear of ink cavity 8 to fill ink cavity 8 and ejection chambers 6. The ink in ejection chambers 6 becomes ink droplets 104 ejected from nozzles 4 and printed to recording paper 105 when inkjet head 10 is driven as shown in Fig. 3. Fig. 4 is an enlarged partial plan view of substrate 1. Substrate 1 of an inkjet head according to the present embodiment is manufactured by anisotropic etching of a single crystal silicon substrate. Anisotropic etching is an etching processing in which the etching speed varies according to the etching direction. The etching speed of crystal face (100) in single crystal silicon is approximately forty times that of crystal face (111), and this is used to form nozzle grooves 11, recesses 12, narrow grooves 13, recess 14, and filter grooves 13a in the present embodiment. Nozzle grooves 11, narrow grooves 13, and filter grooves 13a are formed as V-shaped grooves from crystal faces (111) where the etching speed is slower, resulting in the nozzle grooves 11, narrow grooves 13, and filter grooves 13a having a triangular cross section. Nozzle grooves 11 are 60 µm wide at the base of the triangle. Narrow grooves 13 form three parallel flow channels, each having a base width of 55 µm. Filter grooves 13a are 50 µm wide at the base of the triangle, and 54 parallel filter grooves 13a are formed continuous to recess 14. Recesses 12 and 14 have a trapezoidal cross-sectional shape of which the bottom is crystal face (100) and the sides are crystal face (111). The depth of recesses 12 and 14 is controlled by adjusting the etching time. The V-shaped nozzle grooves 11, narrow grooves 13, and filter grooves 13a are shaped only by crystal face (111), which has the slower etching speed, and the depth is therefore controlled by the groove base width independent of the etching time. These nozzle grooves 11, narrow grooves 13, and filter grooves 13a greatly contribute to the ink ejection volume and speed characteristics of the inkjet head, and require the highest processing precision. In the present embodiment, those parts requiring the highest processing precision are made using the crystal faces with the slowest etching speed by means of anisotropic etching, making it possible to obtain channels of different dimensions with high precision. As described above, the cross sectional area of the filter channels 13a is the smallest cross sectional area of any part of the total ink path. As a result, foreign particulate that could clog the nozzles 4 or orifices 7 is reliably blocked by the filter channels 13a from entering the common ink cavity and the ink passage. A major reason for dropped pixels and other printing defects is thus eliminated, and the reliability of the inkjet head can be assured. Figs. 5 (a) to (c) are lateral cross sections of an inkjet head according to the preferred embodiment of the invention, and are used below to describe the process of deforming the diaphragm from a standby position to cause ink to be ejected from the respective nozzle. Figs. 6 (a) to (c) are simplified diagrams illustrating what happens when a voltage is applied between a diaphragm 5 and the corresponding nozzle electrode 21 in the corresponding states shown in Figs. 5 (a) to (c). An example of the inkjet head operation according to the present invention is described below with reference to Figs. 5 and 6. Fig. 5 (a) shows the inkjet head in the initial state, and Fig. 6 (a) shows the capacitor formed by diaphragm 5 and nozzle electrode 21 at that time is discharged due to the short circuit via resistor 46. In this initial state the ink passage is filled with ink, and the inkjet head is ready to eject ink. When a voltage is applied to an actuator, the capacitor comprising diaphragm 5 and nozzle electrode 21 is charged, and the diaphragm 5 is attracted to electrode 21 by electrostatic force and distorted as shown in Fig. 6 (b). The attraction of diaphragm 5 to nozzle electrode 21 at this time causes the pressure inside ejection chamber 6 to drop as shown in Fig. 5 (b), and ink is supplied in the direction of arrow B from ink cavity 8 to ejection chamber 6. The meniscus 102 formed at nozzle 4 at this time is pulled toward ejection chamber 6. When the drive voltage is removed and the capacitor is discharged, diaphragm 5 returns to its initial state in a short time as shown in Fig. 6 (c). The return of diaphragm 5 increases the pressure in ejection chamber 6, thus causing an ink droplet 104 to be ejected from nozzle 4 while some ink from the ejection chamber 6 is returned in the direction of arrow C through orifice 7 into ink cavity 8 at the same time as shown in Fig 5 (c). The oscillation of ink in the ink path is damped by the orifice 7 having a high flow resistance, and diaphragm 5 returns to the standby position shown in Fig. 5 (a) and is ready for the next eject operation. In the above drive method, the diaphragm is not deformed in the standby state but only deformed when driven. The force applied to the diaphragm is released immediately after the pressure inside the ejection chamber is reduced, which causes the pressure inside the ejection chamber to rise again and eject an ink droplet from the nozzle (a so-called "pull-push-ejection" method). It is to be noted that a so-called "push-ejection" method wherein the diaphragm is constantly deformed in the standby state and released only during inkjet head drive to eject ink may be alternatively used. The "pull-push-ejection" method described in the present embodiment provides a greater ink ejection volume and improved frequency characteristics. It is to be further noted that the action and effect of the present invention are the same even if the drive force and drive method differ. The constants of the inkjet head according to the present embodiment like inertance and flow resistance are described next. As mentioned before, the inertance Mf of the filter is an important factor influencing the crosstalk characteristics of the inkjet head. The inertance Mf of filter 51 is defined as: Mf = ρLf/Sf where ρ is the ink density, Lf is the length of the filter channels 13a, and Sf is the total cross sectional area of all filter grooves 13a. On other hand, the total inertance Ma of all ink passages (6, 7) plus the corresponding nozzles 4 is defined as: n Ma = ρ O l (1/S(x))dx where n is number of nozzles, l is the total length of an ink passage plus the associated nozzle and S(x) is the cross sectional area of the ink passage at coordinate x as defined in Fig. 7. Fig. 7 is a plan view of the preferred embodiment of the invention, and is used to describe the channel constants of ink cavity 8 and filter 51. Inertance is the resistance to volume acceleration of the ink; the greater the inertance, the greater the resistance to acceleration and such forces as the generated pressure. The following description assumes that ink droplets 104 are simultaneously ejected from (n - k) nozzles of an inkjet head comprising n nozzles by driving (n - k) of the associated n actuators; thus, k is the number of "non-driven" nozzles. As has been mentioned, simultaneously to the ejection of an ink droplet 104 from a nozzle 4, some of the ink is returned through orifice 7 to ink cavity 8. The resulting pressure increase ΔP in ink cavity 8 with reference to Fig. 7 is defined as: ΔP = [nα(n-k)/(n+αk)]·Ma·(dUa/dt) where Ua is the volume velocity of ink flowing back from orifice 7 of one "driven nozzle" to ink cavity 8, α = Mf/Ma , the ratio between the inertance Ma (the inertance of the complete eject unit) and the inertance Mf of the filter and t is the time. The ink ejection volume w c from one "non-driven nozzle" at this time which represents the crosstalk resulting from the mutual interference between the ink passages, is the second integral of the pressure increase ΔP in ink cavity 8 divided by nMa, and is therefore: w c = [nα(n-k)/(n+αk)]·∫ Ua dt=[nα(n-k)/(n+αk)]·w o where w o is the volume of the ink flowing back from orifice 7 of one "driven nozzle" to ink cavity 8. Let us consider the worst case for crosstalk. It occurs when all but one nozzle, for example, eleven of twelve nozzles are "driven", i.e. n > > k and α < 1. The crosstalk w c for this case can be expressed by: w c ≈ αw o . The results of tests relating to crosstalk using inkjet heads designed according to the present invention are shown in Table 1. The inkjet head used in these tests had twelve nozzles. When eleven nozzles were driven and one was non-driven, in sample 3 ink ejection from the non-driven nozzle was observed while no crosstalk was observed with samples 1, 2 and 4. Based on these results, the ratio Mf/Ma should be set to about 0.2 or less to prevent crosstalk from occurring in the present embodiment. The flow resistance Rf of the filter 51 in Table 1 is defined as: Rf = (2ηTf 2 Lf)/Sf 3 where η is the ink viscosity, and Tf is the sum of the cross sectional circumferences of the narrow channels 13a. This value indicates the resistance to the volume velocity of the ink; the greater the flow resistance Rf, the greater the resistance to ink flow. If the flow resistance Rf of filter 51 is low enough, no ink supply deficiency resulting from the provision of filter 51 will occur. When all n nozzles are driven at the highest frequency, irregular ink ejection caused by supply deficiencies during high frequency drive were observed with sample 2 in Table 1. Based on this results, the flow resistance Rf should be set to less than about 0.32 x 10 12 Nsec/m 5 , for preventing any ink supply deficiency. The greatest per-ejection ink volume w was observed with sample 4, which yielded the best ink eject characteristics. Sample 4 had 58 filter channels, each 45 µm wide at the base and 50 µm long. The ink compliance C in Table 1 is defined as: C = W/(c 2 ρ) where c is the speed of sound in the ink, and W is the volume of the ink cavity 8. The ink compliance C indicates the deformation resistance of the ink; the greater the ink compliance C, the easier the ink deforms, i.e., the greater the ability of the ink to buffer pressure changes. Fig. 8 is a plan view, similar to Fig. 7, of an alternative embodiment of the invention. Fig. 9 is the cross section at line D-D in Fig. 8. Like that of Fig. 7, the embodiment shown in Fig. 8 comprises plural parallel ink passages of which a few are shown. As shown in Figs. 8 and 9, this embodiment additionally comprises a pressure buffer chamber 53, which is a hollow space formed below the common ink cavity 8. As shown in the figures, the pressure buffer chamber is formed in the same way as the vibration chambers 9 from a recess in the surface of substrate 2 and the bottom of the common ink cavity 8. A transparent oxide conductive film 54 is formed on the bottom of pressure buffer chamber 53 from the same ITO material as nozzle electrodes 21. The bottom of the common ink cavity 8 has substantially the same thickness as diaphragm 5 and constitutes a flexible membrane or buffer wall 55. The pressure increase in ink cavity 8 created when diaphragm(s) 5 in ejection chamber(s) 6 is (are) driven is absorbed, buffered, and effectively cancelled by buffer wall 55, thereby further contributing to prevent the pressure interference or the crosstalk. The primary reason for providing transparent oxide conductive film 54 is to prevent buffer wall 55 from adhering to second substrate 2 and becoming nonfunctional when substrate 1 and second substrate 2 are anodically bonded. Any other material serving this purpose could be used instead. With regard to the manufacturing, however, use of the same material as that of the nozzle electrodes is preferred since then film 54 can be formed simultaneously with the nozzles electrodes by the same manufacturing step. When the ink capacity (compliance) of ink cavity 8 is sufficiently great, the pressure created by the "driven" nozzles and transferred to ink cavity 8 can be buffered by the ink compliance alone. By additionally disposing buffer wall 55 as in this embodiment, sufficient compliance can be obtained even with a small capacity ink cavity 8. Furthermore, with the flexible buffer wall 55 and the chamber 53 below it, crosstalk can even be avoided without caring for the ratio of inertances, unlike the first embodiment described above, provided a sufficiently great total compliance is achieved to suppress any pressure increase in the common ink cavity 8 below that causing the crosstalk. While the invention has been described so far with reference to embodiments using an electrostatic actuator as pressure generating device it will be understood that, as far as crosstalk suppression is concerned, it makes no difference whether the pressure is generated electrostatically, thermally (by means of resistance heating elements provided in each ejection chamber 6) or by means of a piezoelectric element (provided on the side of diaphragm 5 opposite each ejection chamber 6). All kinds of pressure generating device resulting in the same basic function of the inkjet head as that explained above can therefore be employed in the context of the invention. Since such alternative pressure generating devices are known in the art, no further description will be given here. Yet, since the pressure in inkjet heads using electrostatic actuators tends to be higher than with other types of pressure generator, in combination with electrostatic actuators the invention may be particularly useful. In this case it offers the additional advantage that manufacturing steps required for forming the actuators may at the same time be used to provide characteristics of the invention. Fig. 10 shows an overview of a printer as an example of an inkjet recording apparatus that incorporates the inkjet head described above. 300 denotes a platen as a paper transport means that feeds recording paper 105 and is driven by a drive motor (not shown). 301 indicates an ink tank that stores ink in it and supplies ink to the inkjet head 10 through an ink supply tube 306. The inkjet head 10 is mounted on a carriage 302 which is movable by means of carriage drive means (not shown) including a drive motor (not shown) in a direction perpendicular to the direction in which the recording paper 105 is transported. To prevent or recover the nozzles from clogging, in response to a recovery control signal, the inkjet head is moved to a position in front of a cap 304, and then ink discharge operations are performed several times while a pump 303 is used to suction the ink through the cap 304 and a waste ink recovery tube 308 into a waste ink reservoir 305. Inclusion of the filter 51 in inkjet head 10 in the inkjet recording apparatus according to the present invention prevents the penetration of foreign particulate to inkjet head 10, thereby eliminating the need to provide a filter inside ink tank 301 and/or ink supply tube 306, and simplifying the ink supply system. In addition, only inkjet head 10 is disposed on carriage 302 in the present embodiment, but the invention shall not be so limited and the same desirable effects can be obtained whether the ink tank is disposed on the carriage, or whether a disposable inkjet head integrating the ink tank with the print head is used (in which case the complete inkjet head is thrown away when the ink tank is empty). The manufacturing method of an inkjet head according to the present invention is described below with reference to Figs. 11 to 14. Fig. 11 is used to describe the process of this manufacturing method for forming the various grooves and recesses in substrate 1. Figs. 11 (a) to (d) each schematically shows a cross section of only the portion of substrate 1 where the filter grooves 13a are formed (while it is to be understood that the various grooves for the inkjet head are formed simultaneously reference will be mainly to the filter grooves in the following description). A SiO 2 thermal oxidation film 61 has initially been formed to a thickness of 6000 Å (600 nm) by thermal oxidation at 1100°C on the surface of substrate 1, which is single crystal Si in this case. A photoresist film 62 has then been formed by coating the surface of substrate 1 with a photosensitive resin. The resist film 62 has then been exposed via a positive mask describing the line pattern of the filter grooves 13a (and the other grooves and recesses not shown) with ultraviolet light. The resist film 62 has then been developed, rinsed, and dried to form the pattern 63 for the filter grooves 13a that is illustrated in Fig. 11 (a). The line width of the pattern 63 (corresponding to the base width of the triangular filter channels that will finally result) is made narrower than that of the pattern for forming nozzle grooves 11 and narrow grooves 13. The oxide film is then etched using a BHF etching solution of 1:6 (volume ratio) hydrofluoric acid and ammonium fluoride. This etching process removes the oxide film in the pattern 64 for forming the filter grooves 13a. Resist film 62 is then peeled off, resulting in the state shown in Fig. 11 (b). The oxide film in the corresponding pattern regions for the other grooves and recesses is also removed at this time. The single crystal Si of substrate 1 is then etched using an aqueous solution of potassium hydroxide (KOH) and ethanol. As described above, the etching speed of face (100) of single crystal silicon is 40 times faster than that of face (111), and face (111) is therefore exposed by this etching process. Fig. 11 (c) shows the substrate after this etching. At this time, filter grooves 13a are formed by only faces (111) of the single crystal Si. Because filter grooves 13a are formed by the relatively slow etching speed faces (111), there is virtually no etching of these faces (111), and the filter grooves 13a can be formed with a uniform width and depth among the grooves controlled by the line width of the mask pattern. The other grooves and recesses can be similarly formed with high precision. After forming the grooves and recesses, the substrate is washed with hot sulfuric acid, then vapor washed with isopropyl alcohol, and the remaining thermal oxidation film 61 on the surface is removed with BHF. Fig. 11(d) shows the completed filter grooves after removing the thermal oxidation film. A protective thermal oxidation film is then formed again on substrate 1 to complete substrate 1. Fig. 12 is an enlarged partial view of Filter 51 in the direction of arrow A in Fig. 4 and shows one filter channel 13a. Fig. 13 is an enlarged partial perspective view of filter 51 after etching as seen from the recess 14. Filter 51 is formed by etching filter grooves 13a, bonding the first, second, and third substrates 1, 2, and 3 together, and then slicing the substrates to expose the filter. As a result, the filter grooves 13a have a triangular cross section defined by two single crystal Si (111) faces and separated by one (100) face, which is the face used to bond the substrates together. By thus forming the filter grooves 13a with a triangular cross section comprising crystal faces etched at a relatively slow etching speed and a common interconnecting crystal face, the filter can be obtained easily and with high precision. It is to be noted that while single crystal silicon is used for substrate 1 in the present embodiment, germanium, single crystal silicon oxide (quartz), or other materials enabling anisotropic etching can be used. Single crystal silicon is readily obtainable as a semiconductor material, and quartz and germanium are available as high purity crystals enabling high precision processing. A method for mass manufacturing inkjet heads is described below. This method batch processes plural groups of ink path forming grooves and recesses on a single silicon wafer as the substrate 1 using a single pattern; similarly batch processes the second and third substrates with the positions and number of pattern elements coordinated with substrate 1; laminating these three substrates together; and then slicing the laminated wafers into plural inkjet heads. Fig. 14 shows the pattern of the places where the wafer is sliced to separate the individual inkjet heads after anisotropic etching of plural sets of ink path patterns on the single silicon wafer. This slicing pattern is formed as part of the line pattern described above. The patterns for inkjet heads 10 and 10' separated by slicing are formed with the nozzles 4 and filter 51 mutually opposed. After bonding substrates 2 and 3 to substrate 1, the slicing margin ta of adjacent patterns is removed to separate the individual inkjet heads. The filter 51 pattern overlaps the slicing margin ta by margin tb, and the nozzle 4 pattern overlaps the slicing margin ta by margin tc. For example, when the inkjet heads are sliced apart and separated in the dicing process, a grinding stone slightly narrower than the slicing margin ta is used to cut apart the inkjet heads referenced to the filter 51 side. The nozzles 4 are then polished, and post-processed for water repellancy, etc. This manufacturing method enables the batch production of plural inkjet heads, and makes it possible to easily manufacture many inkjet heads at low cost. The manufacturing process includes a cleaning step in which the ink paths are flushed with a cleaning liquid such as pure water after the inkjet heads have been separated. This cleaning process removes any foreign particulate that may have entered during the cutting step. This also reduces manufacturing defects, and thus increases inkjet head production yield. Various means of cutting the inkjet heads apart can be used, including: abrasive grinding by dicing, scribing and then breaking, laser scribing, and cutting by a water jet. Abrasive grinding by dicing enables cutting with relatively good precision. Dicing also makes it possible to assure the length of filter 51 with good precision. Scribing and then breaking is the easiest and quickest method of cutting the inkjet heads apart, and is suited to mass production. Laser scribing does not produce chips from cutting, and has the lowest probability of causing clogging as a result of the manufacturing process. Cutting by a water jet is the most resistant to side effects from heat. It is to be noted that whichever cutting method is used there is no difference in the obtained benefits because the filter 51 is formed by first etching filter grooves, bonding the substrates together, and then cutting to expose the opening of the filter channels forming the filters. Sample 1 2 3 4 Inertance of Filter 51 (Mf) x10 8 kgm -4 0.105 0.608 0.078 0.039 Flow Resistance of Filter 51 (Rf) x10 12 Nsm -5 0.318 0.383 0.021 0.100 Ink Compliance of ink cavity 8 (C) x10 -19 m 5 N -1 7.117 2.312 8.374 2.444 Inertance Ratio α (Mf/Ma) % 17.7 18.1 34.3 12.1 Results Crosstalk (pressure interference between ink passages) o o x o Supply Deficiencies (poor response, irregular ejection) o x o o Amount of Ejected Ink µg/dot 0.093 0.128 0.153 0.165

Claims: An inkjet recording apparatus having an inkjet head (10) which comprises: an ink supply port, a common ink cavity (8), a filter (51) having a plurality of filter channels (13a) communicating with the ink supply port at one end and the common ink cavity (8) at the other end, a plurality of ink ejection nozzles (4) each connected to the common ink cavity by a respective ink passage (6, 7), and a corresponding plurality of pressure generating means (5, 21) respectively associated with said ink passages, said pressure generating means (5, 21) being selectively drivable to eject ink droplets through the respective nozzles (4), characterized in that the cross-sectional area of each filter channel (13a) is smaller than that of a nozzle (4) and the inertance (Mf) of said filter is one-fifth or less of the total inertance (Ma) of all ink passages (6, 7) and nozzles (4). The apparatus according to Claim 1, wherein at least a portion (55) of the walls defining the common ink cavity (8) is flexible. The apparatus according to Claim 2, wherein said wall portion (55) separates the common ink cavity (8) from a hollow chamber (53). The apparatus according to any one of the preceding Claims, wherein said nozzles (4), said ink passages (6, 7), said common ink cavity (8) and said filter (52) are disposed on an anisotropic crystalline substrate (1). The apparatus according to Claim 4 wherein said anisotropic crystalline substrate (1) is made of single crystalline silicon. The apparatus according to any one of the preceding Claims, wherein each pressure generating means (5, 21) is an electrostatic actuator comprising a diaphragm (5) forming a wall portion of the ink passage (6,7) and a nozzle electrode (21) provided opposite to the diaphragm (5) via a gap (G). The apparatus according to any one of Claims 1 to 5, wherein each of said ink passages (6, 7) comprises a wall portion forming a diaphragm and each pressure generating means comprises a piezoelectric element attached to the respective diaphragm. The apparatus according to any one of Claims 1 to 5, wherein said pressure generating means comprises an electrically drivable heating element disposed in the respective ink passage. A method of producing an inkjet head as claimed in Claim 4 comprising the steps of: (a) forming plural sets of grooves and recesses (11, 12, 13, 14, 13a) corresponding to said nozzles (4), ink passages (6, 7) and common ink cavity (8) in a wafer (60) by means of anisotropic etching, (b) forming pressure generating means (5, 21) respectively disposed adjacent to each of said ink passages (6, 7), (c) bonding a cover substrate (3) to the wafer (60) and forming said nozzles (4), ink passages (6, 7), common ink cavity (8) and filter channels by sealing the rims of said grooves and recesses while maintaining the communication therebetween, and (d) separating individual inkjet heads (10) each corresponding to one of said sets from the wafer (60) by cutting a portion at least containing the filter channels (13a), characterized in that step (a) further comprises forming grooves for said filter channels such that the cross-sectional area of each groove formed for a filter channel is smaller than that of each groove formed for a nozzle and, when Lf is the length of each filter channel, Sf is the total cross-sectional area of all filter grooves, n is the number of nozzles, l is the total length of each ink passage and S(x) is the cross-sectional area of each ink passage at a coordinate x counted in the longitudinal direction of the ink passage, with 0≤ x ≤l, the following condition is met: Lf Sf ≤ 1 5 n O l 1 S(x) dx . The method according to Claim 9 wherein the wafer is made of single crystalline silicon, and said grooves and recesses are formed on the (100) face of the silicon wafer. The method according to Claim 9 or 10 wherein said cutting comprises any one of abrasive grinding by dicing, scribing and then breaking, laser scribing and cutting by a water jet.

Inventors: FUJII MASAHIRO, HAGATA TADAAKI, MARUYAMA HIROYUKI, MIYASHITA IKUHIRO, MUKAIYAMA KEIICHI, FUJII, MASAHIRO, HAGATA, TADAAKI, MARUYAMA, HIROYUKI, MIYASHITA, IKUHIRO, MUKAIYAMA, KEIICHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1995
Main classification: B41J2
IPC codes: B41J2