Fluid drop projecting head using taper-shaped chamber for generating a converging surface wave

A fluid drop projecting apparatus which can fly, one by one, fluid drops far smaller than the opening bore from which the drops are projected, and moreover cay readily vary the size of the fluid drops, is provided. To this end, surface waves travelling toward a fluid drop projecting point are applied on the free surface of fluid in a fluid drop projecting chamber having an opening. The surface waves are generated by a surface wave generator including the fluid drop projecting chamber, a diaphragm and a piezo actuator. As the surface waves are generated by the surface wave generator at substantially equal distances from a fluid drop projecting point, the height of the surface waves gradually increases, amplified by their interference, and fluid drops are separated at and projected from the fluid drop projecting point.

BACKGROUND OF THE INVENTION
 The present invention relates to an apparatus for projecting fluid drops,
 and more particularly to an ink jet recording head for causing minute
 fluid drops to fly to a recording medium to record visual images. The
 invention relates to an apparatus for projecting fluid drops, and more
 particularly to an apparatus for causing electroconductive materials,
 which are solid at normal temperature and melted by heating, in a state of
 fluid drops to a circuit substrate or the like and forming bumps thereon
 for connection to LSIs or the like.
 DESCRIPTION OF THE PRIOR ART
 Fluid drop projecting apparatuses according to the prior art for use in ink
 jet printers among others include one disclosed in the U.S. Pat. No.
 3,946,398 in which, as illustrated in FIG. 13A, a piezo element 12 is
 oscillated to expand the volume of an ink chamber 30 thereby to suck a
 fluid 14, such as ink, from an ink tank (not shown) and, afterwards, as
 illustrated in FIG. 13B, the volume of the ink chamber 30 is compressed to
 apply pressure to the fluid 14 thereby to cause fluid drops 20 to fly from
 a nozzle 31 onto a recording medium. They also include another described
 in the Japanese Patent Publication No. 61(1986)-59911 in which a heating
 element is built into an ink chamber, bubbles are instantaneously
 generated by thermal energy in ink, and the ink is projected by the
 expansive force of the bubbles. According to the prior art, many fluid
 drop projecting apparatuses utilizing the principle of pumping have been
 proposed.
 Known fluid drop projecting apparatuses which fly a mist of ink include
 ones disclosed in the Gazettes of the Japanese Patents Laid-open No.
 4(1992)-14455, 4(1992)-299148 and 5(1993)-38810. The one according to the
 Patent Laid-open No. 4(1992)-14455, illustrated in FIGS. 14A and 14B, uses
 as a driver a propagation plate 32 at one end of whose propagation face 33
 a plurality of pairs of comb-shaped electrodes IDT 34 are formed; a
 high-frequency A.C. voltage 35 of about 20 MHz is applied to the driver to
 excite the surface of the propagation face 33 and thereby to generate a
 surface elastic wave A. The surface elastic wave A thereby generated
 travels in the direction of the arrow in the diagram and, when it reaches
 a part where the propagation face 33 is in contact with ink 14, leaks
 therefrom to the ink 14 to become a longitudinal elastic wave (acoustic
 wave), which excites a surface 37 of the ink exposed in a slit 36 to fly a
 mist of fluid drops 20.
 In the apparatus described in the Patent Laid-open No. 4(1992)-299148, as
 shown in FIG. 15, a gap is formed between a slit member 38 and a resonator
 39 to compose an ink chamber 30. The ink chamber 30 is filled with ink 14
 by capillary action; resonant vibration is applied to the resonator 39 in
 the thickness direction; the energy of vibration is propagated to the ink
 eventually to form a random surface wave on an ink interface 41 at an ink
 outlet 40, so that the interference of the surface wave causes particles
 of ink to be projected in a mist form according to the vibrating frequency
 of the resonator.
 According to the Patent Laid-open No. 1993-38810, as illustrated in FIG.
 16, a pair of electrodes 43 are formed on the upper and lower faces of a
 piezoelectric substrate 42, to which a nozzle plate 45 is joined via a gap
 supporter 44, and the gap is filled with ink 14 by capillary force. When a
 voltage displaced by a resonant frequency, which is determined by the
 thickness of the piezoelectric substrate 42, is applied to an intersection
 area 46 formed by the electrodes 43, the piezoelectric substrate 42
 resonates to generate an ultrasonic wave in the ink 14. The ultrasonic
 wave travels through the ink 14 to generate a surface wave on a surface 37
 of ink filling a nozzle 31 immediately above the intersection area 46.
 When the amplitude of this surface wave surpasses a certain level, ink
 drops 20 are projected in a mist form from the nozzle 31.
 According to any one of the above-cited Patents Laid-open Nos. 1992-14455,
 1992-299148 and 1993-38810, though differing in means to generate a
 surface wave on the ink surface, a surface wave is generated at random on
 the free surface of fluid by the same principle as that of mist projection
 by ultrasonic humidifiers, and the interference of the surface wave causes
 the fluid to be projected in a mist form from an indefinite large number
 of projection points.
 A fluid drop projecting apparatus utilizing the sound pressure of acoustic
 streaming is disclosed in the Gazette of the Japanese Patent Laid-open No.
 63(1988)-162253. According to the invention described in this patent, as
 shown in FIG. 17, an ultrasonic acoustic wave is generated by the
 vibration of a piezoelectric transducer 47 and converged by a spherical
 acoustic lens 48 on one point on the free surface 15 of fluid 14, so that
 radiation pressure generated when the acoustic wave hits the free surface
 15 of the fluid 14 works to separate fluid drops 20 from the free surface
 of the fluid and project them.
 Ink jet and various other types of printers are increasingly required to be
 capable of providing pictorial color image outputs. Meeting this
 requirement needs a recording characteristic of continuous and smooth
 shade gradation from the high light to the shadow. In order to achieve
 such a gradation recording characteristic by an ink jet method, it is
 necessary either to modulate the gradation by varying the volume of an ink
 drop from pixel to pixel or to compose each pixel of a plurality of ink
 drops each of which is smaller than a pixel and to vary the number of ink
 drops. By either method, in order to realize smooth shading gradation with
 no tone jump, a technique to form fluid drops sufficiently smaller than
 pixels is indispensable. However, with any of the above-described fluid
 drop projecting apparatuses, it is difficult to form so fine fluid drops
 for the following reasons.
 With the fluid drop projecting apparatuses described in the U.S. Pat. No.
 3,946,398 and the Gazette of the Japanese Patent Publication No.
 1986-59911, illustrated in FIGS. 13A, 13B, the minimum diameter of fluid
 drops that can be projected is about equal to the nozzle bore because both
 project fluid drops by utilizing the principle of pumping, and it is
 extremely difficult to project fluid drops having a diameter equal to,
 say, 1/10 of the nozzle bore. Therefore, in order to enable any such fluid
 drop projecting apparatus to project very fine fluid drops, the nozzle
 bore should be reduced to about the desired diameter of fluid drops.
 However, such a small nozzle bore would make the nozzle more susceptible
 to choking and accordingly less reliable. Therefore, it is extremely
 difficult to form fluid drops as fine as a few .mu.m to 20 .mu.m in
 diameter. Moreover, the smaller nozzle bore means the need for more
 precise machining with the consequence that, where minute fluid drops have
 to be projected from an apparatus based on the principle of pumping, a
 problem arises not only with reliability but also with productivity.
 Next, the fluid drop projecting apparatuses described in the Gazettes of
 the Japanese Patents Laid-open Nos. 1992-14455, 1992-299148 and
 1993-38810, which generate a surface wave on the free surface of fluid and
 project fluid drops in a mist form, can project a mist of fluid drops as
 fine as a few .mu.m in diameter. They further can control the number of
 fluid drops reaching the recording medium by varying the duration of
 projection. However, with these fluid drop projecting apparatuses, as a
 result of using the interference of the surface wave generated at random
 on the free surface of fluid, fluid drops are projected in a mist form
 from an indefinite large number of projection points, inviting
 fluctuations in the diameter of fluid drops projected, and moreover the
 direction and speed of projection also vary from drop to drop. This
 entails a problem in drop-by-drop controllability, which has to be precise
 for ink jet recording heads or bump forming devices. In other words, it is
 difficult to precisely control the positions and volumes of fluid drops
 reaching at the recording medium.
 The utilizing fluid drop projecting apparatus disclosed in the Gazette of
 the Japanese Patent Laid-open No. 1988-162253, which utilizes a sound
 wave, requires large ultrasonic oscillators because of its inefficient
 utilization of the energy of vibration, and accordingly entails a
 correspondingly large overall hardware size. Moreover, as the focal depth
 of the acoustic lens is very shallow, means for precisely controlling the
 position of the free surface of ink is required, and as each individual
 ultrasonic oscillator needs an acoustic lens, the hardware configuration
 is inevitably complex. Furthermore, the apparatus cost is high because the
 circuit configuration requires a band to pass signals of hundreds of MHz,
 involving a high-frequency power amplifying and generating section for
 generating and amplifying high-frequency signals of several MHz to
 hundreds of MHz and a high-frequency power switching section.
 SUMMARY OF THE INVENTION
 An object of the present invention is to solve these problems of the prior
 art and to provide a fluid drop projecting apparatus which can fly one by
 one fluid drops far smaller than the opening from which the drops are
 projected to the desired arrival position of each, and moreover can be
 realized in a simple and inexpensive configuration. Another object of the
 invention is to provide a fluid drop projecting apparatus capable of
 readily varying the drop size.
 According to the invention, there is provided a fluid drop projecting
 apparatus comprising at least a fluid drop projecting chamber having an
 opening involving a fluid drop projecting point, and a surface wave
 generator for forming on the free surface of fluid filling said fluid drop
 projecting chamber, the free surface being formed at the opening of said
 fluid drop projecting chamber, surface waves at substantially equal
 distances from said fluid drop projecting point and travelling toward said
 fluid drop projecting point.
 In the fluid drop projecting apparatus according to the invention, said
 surface waves may have a circular shape centering on said fluid drop
 projecting point.
 In the fluid drop projecting apparatus according to the invention, said
 surface wave generator may have a waveform controller capable of
 controlling the height and length of the surface waves as desired.
 In the fluid drop projecting apparatus according to the invention, said
 surface wave generator comprises at least a fluid drop projecting chamber
 having a circular or polygonal opening whose bore gradually expands from
 the surface in the direction of depth and a fluid stream generator for
 flowing that part of said fluid which is near the bottom of said fluid
 drop projecting chamber in an intermittent stream from the bottom of said
 fluid drop projecting chamber toward the surface, and is configured so as
 to enable the action of said fluid stream to prevent fluid drops from
 being projected from the free surface of said fluid.
 In the fluid drop projecting apparatus according to the invention, said
 fluid stream generator is provided with a fluid stream controller capable
 of controlling as desired the speed and duration of said fluid stream.
 In the fluid drop projecting apparatus according to the invention, said
 fluid stream generator comprises a diaphragm which is connected to the
 bottom of said fluid drop projecting chamber and can be displaced in the
 direction from the bottom of said fluid drop projecting chamber toward the
 surface and an actuator connected to said diaphragm.
 In the fluid drop projecting apparatus according to the invention, said
 fluid stream generator is configured by arranging a heating element near
 the bottom of said fluid drop projecting chamber.
 In the fluid drop projecting apparatus according to the invention, said
 heating element is arranged on the periphery of the bottom of said fluid
 drop projecting chamber.
 In the fluid drop projecting apparatus according to the invention, said
 fluid is a hot melt medium which is solid at normal temperature and melted
 by heating, said apparatus being provided with means to heat said hot melt
 medium.
 In the fluid drop projecting apparatus according to the invention, said hot
 melt medium is electroconductive.
 By the fluid drop projecting method according to the invention, surface
 waves travelling toward a fluid drop projecting point are formed on the
 free surface of fluid at substantially equal distances from said fluid
 drop projecting point.

DETAILED DESCRIPTION OF THE EMBODIMENTS
 With reference to FIGS. 3A, 3B and 3C, how the present invention works will
 be described below. These figures comprise cross sectional views of a
 fluid drop projecting apparatus illustrating the process of fluid drop
 projection; FIGS. 3A, 3B and 3C respectively show a state in which surface
 waves are generated, a state in which a fluid pillar is generated by the
 travel of the surface waves, and a state in which fluid drops are flying.
 In the drawings, reference numeral 10 denotes a fluid drop projecting
 chamber; 11, a diaphragm; 12, a piezo actuator; 21, a surface wave
 generator; and 13, an opening.
 The fluid drop projecting apparatus according to the invention, as shown in
 FIGS. 3, has the fluid drop projecting chamber 10, which has the opening
 13, and the surface wave generator 21 for generating surface waves 16,
 which travel toward a fluid drop projecting point 17 over a free surface
 15 of fluid filling the fluid drop projecting chamber 10.
 As shown in FIG. 3A, the surface wave generator 21 generates the surface
 waves 16 at substantially equal distances from the fluid drop projecting
 point 17. Thus, the surface waves 16 are generated on either the whole or
 part of the periphery of a circle or a polygon around the fluid drop
 projecting point 17. As these surface waves 16 travel toward the fluid
 drop projecting point 7, the surface waves which are in phase interfere
 with one another, and the surface waves 16 gradually increase in height.
 As a result, a fluid pillar 18 is formed in the vicinity of the fluid drop
 projecting point 17 as shown in FIG. 3B. The wave height reaches its
 maximum at the fluid drop projecting point 17, and eventually a fluid drop
 is separated and projected from the top of the fluid pillar 18 as
 illustrated in FIG. 3C.
 The diameter of the projected fluid drop 20, as is evident from FIGS. 3B
 and 3C, varies in proportion to the thickness (diameter) of the fluid
 pillar 18 immediately before the projection. The diameter of the fluid
 pillar 18 in turn varies substantially in proportion to the wave-length of
 the surface waves 16. Here, the wavelength of the surface waves is defined
 by .lambda. shown in FIG. 3A. Whether or not the fluid drop 20 is
 projected depends on the height of the fluid pillar 18, i.e. the height of
 the surface waves 16. Therefore, it is seen that, according to the present
 invention, the diameter of fluid drops does not depend on the size of the
 opening but can be varied with the wavelength of the surface waves 16.
 Furthermore, whether or not a fluid drop is projected can be controlled by
 varying the height of the surface waves 16.
 Such surface waves can be formed by bringing into action an intermittent
 fluid stream 22 from the bottom of the fluid drop projecting chamber 10,
 whose opening gradually expands from the surface toward the bottom as
 illustrated in FIG. 3A, toward the surface. The fluid stream 22, which
 flows from the bottom of the fluid drop projecting chamber 10 toward the
 surface, is subjected to increasing pressure near the wall face of the
 fluid drop projecting chamber 10, as its opening bore narrows toward the
 surface, and increases in speed near the wall face, resulting in the
 generation of surface waves 16, conforming to the shape of the opening 13,
 on the free fluid surface 15. Therefore, if a circular opening is used,
 circular surface waves can be formed or, alternatively, if a polygonal
 opening is used, polygonal surface waves can be formed. It has been
 confirmed by experiment that, here, the wavelength .lambda. of the surface
 waves 16 that are formed can be controlled as desired mainly by varying
 the duration of the generation of the fluid stream 22, and the wave height
 of the surface waves 16 that are formed can be controlled as desired
 mainly by varying the speed of the fluid stream 22. The term "fluid
 stream" as used in describing the present invention is defined as
 collectively denoting both the non-compressive stream of fluid and the
 acoustic stream due to the compression of fluid.
 When formed in a circular shape, the surface waves 16 register the highest
 height amplification rate owing to their interference and, as the surface
 waves which are completely in phase travel toward the fluid drop
 projecting point while interfering with one another, can achieve the most
 efficient, steady and reliable projection of fluid drops.
 Next will be described in detail preferred embodiments of the present
 invention.
 Embodiment 1
 FIGS. 1A and 1B respectively show a plan and a cross section of fluid drop
 projecting apparatuses, which constitute a first preferred embodiment of
 the invention. As illustrated in FIG. 1A, Embodiment 1 comprises a
 plurality of fluid drop projecting apparatuses arranged in parallel for
 application to an ink jet recording head. Each individual fluid drop
 projecting apparatus, as illustrated in FIG. 1B, comprises a fluid drop
 projecting chamber 10 whose opening bore gradually expands in the
 direction of depth, a diaphragm 11 connected to the bottom of the fluid
 drop projecting chamber 10, and a pizeo actuator 12 connected to the
 diaphragm 11. The fluid drop projecting chamber 10 is filled with fluid
 ink 14, and is in continuity to an ink tank 19 via an ink feed path 26.
 Here, an opening 13 and the bottom of the fluid drop projecting chamber 10
 are circularly shaped, respectively measuring 80 .mu.m and 240 .mu.m in
 diameter, and the fluid drop projecting chamber 10 is 100 .mu.m deep. The
 center-to-center pitch between immediately adjoining openings is 254
 .mu.m.
 First, the fluid projecting performance of the fluid projecting apparatus
 was checked. It was confirmed that, when the piezo actuator 12 was given a
 displacement of a single triangular wave-shaped time response of 3 .mu.s
 in time width and 0.2 .mu.m in displacement width as shown in FIG. 2, ink
 drops of about 15 .mu.m could be steadily projected from the center of the
 opening 13. How these fluid drops 20 were projected was observed
 stroboscopically. When the pizeo actuator 12 was driven so as to displace
 the diaphragm 11, first the formation process of circular surface waves
 16, such as shown in FIG. 3A, was witnessed. These circular surface waves
 16, as they travel toward the center, i.e. toward a fluid projecting point
 17, were gradually amplified in height, and formed a fluid pillar 18, such
 as shown in FIG. 3B, in the vicinity of the fluid projecting point 17.
 Immediately after that, as illustrated in FIG. 3C, an ink drop 20 of about
 15 .mu.m in diameter was separated from the fluid pillar 18, and flew
 upward. Thus it was confirmed that the fluid drop projecting apparatus
 according to the invention, as it projects fluid drops by utilizing the
 interference of surface waves, can project ink drops 20 far smaller than
 the bore of the opening 13. Although the drive waveform for the piezo
 actuator 12 in this particular embodiment is triangular as shown in FIG.
 2, it was further confirmed that, if only surface waves 16 such as shown
 in FIG. 3A could be formed on the free surface 15 of fluid, any waveform,
 such as a sine wave, a rectangular wave or a combination thereof, could be
 used to project fluid drops of a diameter smaller than the bore of the
 opening 13 as in Embodiment 1.
 Then, an ink jet recording head was composed of such fluid drop projecting
 apparatuses, and a printing experiment was carried out with it. FIG. 4A
 shows an external perspective view of the printer, and FIG. 4B shows a
 plan of openings 13 in the face opposite to the recording paper of the
 recording head. In the diagram, reference numeral 51 denotes the recording
 paper; 52, the recording head; and 53, a platen. The recording head 52,
 having a plurality of openings 13, was fixed to a carriage 54 so that
 these openings 13, from which ink would be projected, were opposite to the
 platen 53 with the recording paper 51 in-between. Four rows of 32 openings
 13 each, serving as ink projecting points, were arranged in a zigzag form
 as illustrated in FIG. 4B, so that the recording head 52 comprised
 altogether 128 openings 13 arranged at 63.5 .mu.m pitches. Incidentally,
 individual fluid drop projecting apparatuses were enabled to be controlled
 independently of one another by electric recording signals as to whether
 or not to project ink.
 Printing was accomplished in the following manner. First, as illustrated in
 FIG. 4A, the recording head 52 was caused to scan the platen 53 by the
 carriage 54 (main scanning). By controlling the timing of fluid drop
 flying with the 128 fluid drop projecting apparatuses at 15.875 .mu.m
 pitches in the main scanning direction in accordance with image signals,
 four rows of pixels were formed at a pixel density of 1600 dpi in the main
 scanning direction and at 400 dpi in the subscanning direction. Then,
 after advancing the recording paper 51 by 15.875 .mu.m in the subscanning
 direction as shown in FIG. 4A, the recording head 52 was caused to perform
 main scanning in the direction reverse to the first scanning, and another
 four rows of pixels were formed in the same way as in the first scanning.
 By performing altogether four rounds of such scanning, 16 rows of pixels
 were formed at a pixel density of 1600 dpi in both main scanning and
 subscanning directions. Next, after moving the recording paper by 206.375
 .mu.m in the subscanning direction, 16 rows were printed in the same way
 as described above. By repeating the moving of the recording paper 51 by
 206.375 .mu.m in the subscanning direction after every 16 rows of
 printing, an image was formed on an A4 size piece of the recording paper
 51 at a resolution of 1600 dpi in both main scanning and subscanning
 directions.
 Incidentally, when the dot diameters of the ink drops projected from the
 fluid drop projecting apparatuses on the recording paper 51 were measured,
 they were found to be about 21 .mu.m, the right size not to let any
 undesired blank left even when characters were printed closely. Thus, the
 fluid drop projecting apparatuses according to the present invention, in
 spite of the 400 dpi intervals between their openings 13, was confirmed to
 be able to form images of as high a resolution as 1600 dpi because they
 can project fluid drops far smaller than their opening bore.
 In the embodiment described above, the drive conditions for the piezo
 actuator 12 were adjusted not to let fluid drops 20 be projected from the
 free surface 15 of the fluid by the direct action of the fluid stream 22.
 In the embodiments to be described below, for the sake of comparison,
 projection of fluid drops 20 by the direct action of the fluid stream 22
 was attempted. When the displacement of the piezo actuator 12 was
 gradually increased from 0.2 .mu.m eventually to 0.35 .mu.m, a plurality
 of minute fluid drops 20 were projected at random from the leading edges
 of the surface waves 16 simultaneously with the formation of the surface
 waves 16. In this stated, as both diameters and flying directions of the
 fluid drops 20 were random, it was impossible to control the arriving
 position of each of the fluid drops 20. Then, when the displacement of the
 piezo actuator 12 was further increased to 0.5 .mu.m, large fluid drops
 20, about equal to the bore of the opening 13, were projected by the
 conventional mechanism utilizing the principle of pumping. Thus it was
 confirmed that, in order to fly fluid drops 20 smaller than the opening 13
 while controlling the arriving position of each, the fluid stream 22 had
 to be generated so as not to let any fluid drop 20 be flown from the free
 fluid surface 15 by the direct action of the fluid stream 22.
 Embodiments 2 and 3
 FIGS. 6A and 6B show plans of fluid projecting apparatuses which constitute
 respectively second and third preferred embodiments of the present
 invention. FIG. 6A shows a plan of fluid drop projecting apparatuses each
 having an opening 13 of a regular dodecagon circumscribing a circle of 80
 .mu.m in diameter, and FIG. 6B, a plan of fluid drop projecting
 apparatuses each having an opening 13 of a regular hexagon circumscribing
 a circle of 80 .mu.m in diameter. Other aspects than the shape of the
 opening 13 of these embodiments were the same as those of the fluid drop
 projecting apparatus illustrated in FIG. 1B. Under the same driving
 conditions for the piezo actuator 12 as for that of Embodiment 1, no fluid
 drop was projected by either of the apparatuses shown in FIGS. 6A and 6B.
 This state was observed stroboscopically in the same manner as for
 Embodiment 1. As in Embodiment 1, it was witnessed that the driving of the
 actuator resulted in the formation of surface waves in conformity with the
 shape of the polygonal openings, and the height of these surface waves
 gradually increased as they approached the center eventually to form fluid
 pillars. However, it was found that no fluid drop was projected because of
 the lower height amplification rate of the surface waves, and that the
 rate was higher for the dodecagonal openings, which were closer to
 circles. In view of this finding, the displacement of the piezo actuator
 12 was increased to attempt fluid drop projection, and the projection of
 fluid drops 20 became possible at a displacement of 0.24 .mu.m for the
 apparatus of FIG. 6A and at 0.28 .mu.m for that of FIG. 6B.
 Thus it was confirmed that, though the energy input required for projecting
 fluid drops was somewhat greater than with a circular opening, the fluid
 drop projecting apparatus having a polygonal opening in which surface
 waves are generated at substantially equal distances from the fluid drop
 projecting point was also able to project fluid drops smaller than the
 opening bore by the interference of the surface waves. It was further
 confirmed that, like Embodiment 1, these embodiments of the invention,
 when applied to a recording head 52 as illustrated in FIG. 4, could form
 images on recording paper 51 by an ink jet recording process. However,
 since the fluid drop diameter in Embodiments 2 and 3 is 20 .mu.m, greater
 than in Embodiment 1, images were recorded at a resolution of 1200 dpi in
 both main scanning and subscanning directions. It was confirmed that
 images of high quality could be formed thereby. Embodiment 4
 In Embodiment 4, the bore of the circular opening 13 is 1 mm, greater than
 in Embodiment 1. Except for the opening 13, this embodiment has the same
 configuration as Embodiment 1 illustrated in FIG. 1B. When the piezo
 actuator 12 was driven for t=200 .mu.sec and its displacement d was
 gradually increased, steady projection of fluid drops became possible at
 d=4.8 .mu.m, when the drop diameter was about 280 .mu.m. It was confirmed
 that, even when the opening bore was a full millimeter, fluid drops 20 far
 smaller than the bore of the opening 13 could be projected.
 Next, an experiment was carried out to determine the dependence of the
 diameter of projected fluid drops on the drive waveform of the piezo
 actuator 12. While the piezo actuator 12 was driven for t=200 .mu.sec in
 the foregoing example in which fluid drops of 280 .mu.m were projected,
 fluid drop projection was further attempted with different drive
 durations, varied to 145, 100 and 60 .mu.sec. The displacement d of the
 piezo actuator 12 was also adjusted in accordance with the variation in
 drive duration so as to enable fluid drops 20 to be projected steadily. As
 a result, it was found that the fluid drop diameter could be reduced by
 shortening the drive duration. Thus, while the fluid drop diameter was
 about 250 .mu.m at t=145 .mu.sec and d=4.0 .mu.m, it was about 200 .mu.m
 at t=100 .mu.sec and d=3.2 .mu.m and about 140 .mu.m at t=60 .mu.sec and
 d=2.2 .mu.m, the drops being steadily projected in all these cases (see
 Table 1).
 TABLE 1
 Pulse width t Displacement d Fluid drop diameter
 200 .mu.sec 4.8 .mu.m 280 .mu.m
 145 .mu.sec 4.0 .mu.m 250 .mu.m
 100 .mu.sec 3.2 .mu.m 200 .mu.m
 60 .mu.sec 2.2 .mu.m 140 .mu.m
 Thus it was found that this fluid drop projecting apparatus according to
 the invention permits the diameter of fluid drops 20 to be varied by
 controlling the drive duration and displacement of the actuator 12.
 Varying the drive duration and displacement of the actuator corresponds to
 varying the speed of the fluid stream and the duration of fluid stream
 generation. Thus it was confirmed that the fluid drops can be controlled
 as desired by regulating the speed of the fluid stream and the duration of
 its generation.
 Incidentally, although the speed of the fluid stream is controlled with the
 drive waveform of the actuator 12 in Embodiment 4, it was confirmed that,
 even when the actuator 12 was driven under the same conditions, the speed
 distribution of the fluid stream could be varied, and the diameter of
 flying fluid drops could be thereby regulated, by varying the diameter of
 the opening 13 and the shape, i.e. the diameter, depth or the like, of the
 bottom of the fluid drop projecting chamber 10.
 Embodiment 5
 While the foregoing Embodiments 1 through 4 use a fluid stream generator
 consisting of a diaphragm 11 and a piezo actuator 12, Embodiment 5 has, as
 shown in FIG. 7, a fluid stream generator consisting of a heating element
 23 arranged on the bottom of the fluid drop projecting chamber 10. In
 other respects than the fluid stream generator, this embodiment has the
 same configuration as Embodiment 1 illustrated in FIG. 1A. In the fluid
 drop projecting apparatus shown in FIG. 7, rapid heating by the heating
 element 23 generates bubbles 24 in the fluid 14. A variation in pressure
 ensuing from the generation of these bubbles 24 gives rise to a fluid
 stream 22 toward the free surface 15 of the fluid 14 and, as in Embodiment
 1, surface waves 16 travelling toward a fluid drop projecting point 17 are
 generated. The energy input to the heating element 12 was adjusted so that
 the action of the fluid stream 22 ensuing from the generation of the
 bubbles 24 would not let fluid drops 20 generate directly from the free
 surface 15 of the fluid 14.
 As a result, when energy of 135 .mu.J was supplied to a circular heating
 element of 120 .mu.m in diameter at a pulse width of 3 .mu.sec, surface
 waves were formed successfully on the periphery of the opening 13 without
 letting fluid drops directly generate from the opening 13, enabling minute
 fluid drops 20 of about 25 .mu.m in diameter to be projected. However, it
 was found that, with the apparatus of FIG. 7, increasing the energy input
 to the heating element even slightly would readily cause fluid drops 20 to
 be flown by the action of the fluid stream 22 and, accordingly, the
 conditions of energy input to the heating element 23 to ensure steady
 projection had only a narrow margin of allowance.
 Embodiment 6
 Then, a configuration in which the heating element was arranged only on the
 periphery of the bottom of the fluid drop projecting chamber 10, as
 illustrated in FIG. 8, was chosen for Embodiment 6. Namely, it is a
 doughnut-shaped heating element of 240 .mu.m in outer and 200 .mu.m in
 inner diameter. As a result, since a bubble 24 is generated in the
 peripheral part and no bubble is generated in the central part of the
 fluid drop projecting chamber 10 having the configuration shown in FIG. 8,
 it was found that ink drops could be prevented from being directly flown
 by the generation of bubbles, so that the margin of allowance for the
 conditions of energy input to project fluid drops 20 could be
 substantially widened. While Embodiment 5, in order to achieve steady
 projection of fluid drops, the total energy input to the heating element
 23 had to be restrained within an approximate range of 135.+-.7 .mu.J,
 Embodiment 6 was confirmed to permit steady projection within an energy
 input range of 70.+-.20 .mu.J. It was further confirmed that, in the fluid
 drop projecting apparatus configured as shown in FIG. 8, the diameter of
 fluid drops 20 could be varied by regulating the energy input to the
 heating element 23. When the energy input to the heating element 23 was 42
 .mu.J (at a pulse width of 3 .mu.sec), fluid drops of 15 .mu.m in diameter
 were found to be steadily projected. Next, when the energy input was
 varied to 70 .mu.J (at a pulse width of 5 .mu.sec), fluid drops of 18
 .mu.m in diameter could be projected. Further at an energy input level of
 98 .mu.J (at a pulse width of 7 .mu.sec), fluid drops of 22 .mu.m in
 diameter could be projected steadily.
 It was further confirmed that the fluid drop projecting apparatuses which
 are Embodiments 5 and 6, like Embodiment 1, could be successfully applied
 to a recording head 52 having the configuration illustrated in FIGS. 4A-4B
 for ink jet image recording on recording paper 51.
 Embodiment 7
 Next, Embodiment 7 uses as fluid hot melt ink 25 consisting of a blend of
 wax-based resin and carbon black. In this fluid drop projecting apparatus,
 a heater 27 was arranged along the inner wall of the fluid drop projecting
 chamber 10, in which ink was maintained in a molten state. A heater was
 also arranged in an ink tank (not shown) to keep the hot melt ink 25
 molten. The fluid drop projecting chamber 10 is shaped similarly to what
 is shown in FIG. 1. Fluid drop projection was tested with the apparatus
 illustrated in FIG. 9 and, although the hot melt ink 25 required a greater
 energy input to the piezo actuator 12 for ink drop projection than water
 ink, making it necessary for the piezo actuator 12 to be driven for 5
 .mu.sec at a displacement of 0.42 .mu.m, it was confirmed that ink drops
 of around 20 .mu.m in diameter, far smaller than the opening 13, could be
 projected as with Embodiment 1. While this Embodiment uses hot melt ink
 consisting of a blend of wax-based resin and carbon black, other hot melt
 inks can give a similar result as well. It was further confirmed that this
 fluid drop projecting apparatus which is Embodiment 7, like Embodiment 1,
 could be successfully applied to a printer recording head 52 having the
 configuration illustrated in FIG. 4 for ink jet image recording on
 recording paper 51.
 Embodiment 8
 Embodiment 8 is an instance in which fluid drop projecting apparatuses
 according to the present invention are applied to an apparatus for forming
 minute bumps for use in the connection of semiconductors or the like. The
 fluid drop projecting apparatuses used in this embodiment have the same
 configuration as Embodiment 7 shown in FIG. 9, i.e. the configuration in
 which the heater 25 is arranged along the inner wall of each fluid drop
 projecting chamber 10. Embodiment 8 will be described below with reference
 to FIG. 10. Indium, whose melting point is about 110.degree. C., is used
 as electroconductive fluid, and an attempt was made to form indium bumps
 29 of 50 .mu.m in diameter in tip connecting parts formed at 80 .mu.m
 pitches on a flexible substrate 28. The inside of the fluid drop
 projecting number 10 was heated with a heater to about 125.degree. C. to
 give a displacement of 2.4 .mu.m at a pulse width of 20 .mu.sec to the
 actuator 12, and fluid drops were projected toward the flexible substrate
 28, resulting in successful formation of indium bumps 29 of 50 .mu.m in
 diameter in the connecting parts. When the flexible substrate 28 on which
 the indium bumps 29 had been formed were used for connecting a liquid
 crystal panel, the bumps functioned fully satisfactorily for the
 connecting purpose, demonstrating the possibility of highly reliable
 connection. Incidentally, although this particular embodiment of the
 invention uses indium as bump material, a low melting point metal such as
 solder, or some other bump material consisting of electroconductive
 particles of Au, Al, Cu or the like dispersed in a solvent, may be used as
 well.
 Thus, although a fluid drop projecting chamber whose opening bore linearly
 expands in the direction from the fluid surface to the bottom of the fluid
 drop projecting chamber is used in the above-described Embodiments 1
 through 8, it was confirmed that, in order to permit the formation of
 surface waves travelling over the free surface of fluid toward the fluid
 drop projecting point, the opening may as well be bell mouth-shaped as
 illustrated in FIG. 11A or finely step-wise as in FIG. 11B, only if its
 bore gradually expands in the direction of depth, and the same effect
 could be achieved as the foregoing embodiments provide. Furthermore,
 though an actuator using the piezoelectric effect is used in Embodiments 1
 through 4, 7 and 8 of the invention to displace the diaphragm, an
 electromagnetic or a magnetic actuator may be used as well if only it can
 give a desired displacement to the diaphragm. Although the displacement of
 the actuator is transmitted via the diaphragm in Embodiments 1 through 4,
 7 and 8 of the invention, it was confirmed that the same effect could be
 achieved as the foregoing embodiments provide even if the diaphragm was
 dispensed with and a displacement was directly given to the fluid from an
 end of the actuator. Though the diaphragm is arranged immediately below
 the opening to compose a surface wave generator in the embodiments of the
 invention, any other structure in which a fluid stream would generate from
 the bottom of the fluid drop projecting chamber 10 toward the opening 13,
 as illustrated in FIG. 3A, would be acceptable; it was confirmed that a
 configuration in which the piezo actuator 12 and the relevant elements are
 arranged in a position somewhat distant from the bottom opposite to the
 opening 13, as shown in FIGS. 12A through 12C, could provide the same
 effect as the embodiments of the invention do.
 Since a fluid drop projecting apparatus according to the present invention
 causes fluid drops to be projected by the interference of surface waves
 travelling toward the fluid drop projecting point, fluid drops far smaller
 than the opening bore can be flown one by one to the desired arrival point
 for each. The fluid drop projecting apparatus according to the invention
 can also permit the fluid drop diameter to be readily varied by
 controlling the length and height of the surface waves.