Ink-jet head and fabrication method of the same

Abstract: A heater portion including a heating element is arranged between two substrates with a void and a cavity interposed therebetween. When the heater portion is heated by voltage application to the heating element and reaches the bubbling temperature, the ink filling the cavity gives off bubbles so that ink droplets are ejected from the nozzle. The heater portion elastically buckles projected downwards due to thermal stress when the heater portion reaches the buckling temperature. When heater portion reaches the heating temperature, the mid part of the heater portion comes into contact with the top surface of the substrate so as to release heat from the heater portion through the substrate. At this moment, the heater is cooled down rapidly when the voltage application to the heater is stopped.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a thermal type ink-jet head for recording an image on a recording medium by ejecting ink droplets from an ejection nozzle by the pressure rise occurring when bubbles arise in the ink which is heated by a heating element. (2) Description of the Prior Art As the recording head used in a printer for forming an image on a recording medium, thermal type ink-jet heads have been used which eject ink droplets from an ejection nozzle by the pressure rise occurring when bubbles arise in the ink which is heated by a heating element. In a thermal type ink-jet head, a pulse current is supplied to a heating element arranged on the substrate in accordance with the image data, the heating element heats so that the ink being in contact with the heating element is heated giving off bubbles. Generation and expansion of the bubbles causes pressure rise, whereby ink in the ejection nozzle is ejected out as ink droplets. This ink droplet jumps to the recording medium placed in proximity to the ejection nozzle, creating a recording dot in an image as a recording pattern. In the thermal type ink-jet head, part of the heat generated from the heating element dissipates by way of the substrate. Therefore, in order to eject a sufficient amount of ink for image forming on the recording medium, a large amount of current needs to be supplied to the heating element, giving rise to a drawback of the electric power consumption being increased. Japanese Patent Application Laid-Open Hei 7 No.227968 discloses a configuration in which a space as a thermally insulating layer is formed between a heating element (heater) and a substrate so that heat arising from the heating element will not transfer to the substrate. Japanese Patent Application Laid-Open Hei 2 No.3054 discloses a configuration in which a pressure generating means composed of a vibration plate and a heating layer is provided in the form of a cantilever structure or a simple beam supported at both ends and an electric current is supplied to the heating layer so as to cause a heat distortion within the pressure generating means and thereby deform and displace the pressure generating means in the nozzle plate direction, thus causing ink droplets to jump outside. Registered Japanese Patent publication No.2769447 discloses a configuration in which two layers having different thermal conductivities are formed between a heating element and a substrate so as to inhibit thermal transfer from the heating element to the substrate. However, since, with the above ink-jet head, heat arising from the heating element is hard to be released to the exterior, the heat generated while the heat element is energized remains in the heating element after removal of the electric supply. Therefore, even if the current is supplied in pulses to the heating element, the heat builds up in the heating element, elevating the temperature of the heating element, thus causing an overheated state. In this way, the ink-jet head has the drawback of overheating which will lower the response of the heating element, resulting in deficiency in the exact control of the ejected amount of ink. In the configuration disclosed in Japanese Patent Application Laid-Open Hei 2 No.3054, since the pressure generating member is of a cantilever structure or of a beam supported at both ends, the pressure acting on the ink varies depending upon the positions along the beam, presenting poor ink ejection efficiency. The applicant's earlier application, EP 0634273 discloses a casing and a nozzle plate which form a hollow cavity in which ink liquid can be filled. A buckling structure body is disposed within this hollow cavity. A nozzle orifice is provided in a nozzle plate at a position corresponding to the buckling structure body. The buckling structure body has a portion extending in a longitudinal direction. Both ends of the buckling structure body in the longitudinal direction are fixedly attached to the casing via an insulative member. The buckling structure body is formed of a material that is displaced at least in the longitudinal direction by conduction of current from a power source. Thus, an ink jet head of a long lifetime is provided that can provide a great discharge force while maintaining its small dimension. SUMMARY OF THE INVENTION It is thus desirable to provide an ink-jet head which can efficiently heat the ink with heat generated from a heating element while the voltage is applied and hence can reduce the electric power consumption and which can reduce the temperature of heating element rapidly while the voltage is not applied, whereby it is possible to exactly control the ejected amount of ink by making the response frequency of the heating element substantially equal to the frequency of the pulse voltage to be applied to the heating element, thus improving the ink ejection response to the print data. As the means for solving the above drawbacks, the present invention is configured as follows: According to the present invention there is provided an ink-jet head for ejecting ink by heating and bubbling ink, comprising: a heating element to which electric current is supplied so as to boil and bubble the ink; and a head substrate having a void formed between the heating element and the head substrate, characterized in that the heating element is adapted to buckle and consequently move into the void by thermal expansion accompanying the temperature rise thereof Preferably, the heating element comprises: a heater; a protective film for protecting the heater; and an insulation film for insulating the heater and the above components are formed of materials having approximately equal coefficients of linear expansion to each other. Preferably, the heating element is arranged with its movement constrained at both ends with respect to the direction of the thickness and the direction perpendicular to the thickness. Preferably, the heating element is configured so as to come into contact with the head substrate when buckled into the void. Preferably, the void is arranged so as to communicate with a cavity that stores ink. Preferably, the head substrate has a communication hole for establishing communication between the void and the exterior. Preferably, the heating element is provided in a bimetal configuration made up of multiple kinds of metals so as to cause the heating element to deform into the void by the temperature rise thereof Preferably, the heating element is formed with a shape memory alloy layer which will deform into the void when the heating element exceeds a predetermined temperature. Preferably, the apparatus further comprises a piezoelectric actuator which pushes and deforms the heating element toward the void. According to the above first configuration, heat diffusion from the heating element is prevented by the insulating effect of the void when ink is heated, enabling its sharp temperature rise. After ink ejection, the heating element buckles so that heat from the heating element is released through the head substrate to lower the temperature of the heating element rapidly. In the above second configuration, it is possible to prevent occurrence of cracks due to repeated heat cycles. In the above third configuration, it is possible to positively cause the heating element to buckle toward the void. In the above fourth configuration, heat from the heating element can be well dissipated through thermal conduction via its contact with the head substrate. In the above fifth configuration, the ink functions as the heat transfer medium when the heating element release heat thus improving the heat transfer to the head substrate and hence enabling beneficial heat radiation. In the above sixth configuration, heat inside the void can be released outside through the communication hole. In the above seventh, eighth and ninth configurations, it is possible to positively perform deformation of the heating element into the void. In order that the present invention be more readily understood, specific embodiments thereof will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a top view showing the first embodiment of an ink-jet head of the present invention; Fig. 2 is a sectional view showing the first embodiment of an ink-jet head of the present invention; Fig. 3 is an illustrative view showing the first embodiment of an ink-jet head of the present invention, where the heat element is thermally insulated; Fig. 4 is an illustrative view showing the first embodiment of an ink-jet head of the present invention, where the heat element radiates heat; Fig. 5 is a top view showing the second embodiment of an ink-jet head of the present invention; Fig. 6 is a sectional view showing the second embodiment of an ink-jet head of the present invention; Fig. 7 is a sectional view showing the third embodiment of an ink-jet head of the present invention; Figs. 8A and 8B are sectional views showing the fourth embodiment of an ink-jet head of the present invention, where the heat element is thermally insulated and where the heat element radiates heat, respectively; Fig.9 is a sectional view showing the fifth embodiment of an ink-jet head of the present invention; Fig. 10 is a sectional view showing the sixth embodiment of an ink-jet head of the present invention; Figs.11A and 11B are sectional, plan and side views showing a configuration of an ink-jet head in accordance with the seventh embodiment of the present invention; Figs. 12A and 12B are sectional side views schematically showing the deformed state of a heater portion in the above ink-jet head; Fig. 13 is a chart showing the relationship between the temperature of the same heater portion and the amount of displacement; Fig. 14 is a chart showing the relationship between the thinness of the same heater portion and the electric power consumption; Fig. 15 is a chart showing the relationship between the thinness of the same heater portion, the heating temperature and the buckling temperature; Fig .16 is a chart showing the relationship between the thinness of the same heater portion, the buckling temperature, the heating temperature and the electric power consumption; Fig. 17 is a chart showing the relationship between the current density and the internal stress arising in the plated layer formed by a typical Ni-electrolytic plating; Figs.18A and 18B are sectional side views showing the essential components of ink-jet heads according to the eighth embodiment of the present invention; Figs. 19A and 19B are plan and sectional views showing an ink-jet head according to the ninth embodiment of the present invention; Fig.20 is a sectional view showing the ink-jet head according to the ninth embodiment of the present invention, where the heater portion has been elastically deformed; Fig.21 is a sectional view showing a variational configuration of an ink-jet head according to the ninth embodiment of the present invention; Fig.22 is a sectional view showing an ink-jet head according to the ninth embodiment of the present invention with another heater portion configuration; Figs.23A to 23H are views showing a fabrication method of an ink-jet head of the seventh embodiment of the present invention; Figs.24A and 24B are views showing another fabrication method of an ink-jet head of the seventh embodiment of the present invention; and Figs.25A, 25B and 25C are views for illustrating a fabrication method of an ink-jet head of the eighth embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS As an ink-jet head of the present invention, a typical bubble jet type ink-jet head in accordance with the first embodiment will be described with reference to Figs. 1 and 2. Figs.1 and 2 are top and sectional views showing an ink- jet head of the first embodiment. In the ink-jet head in the first embodiment, a heating element 4 is arranged between a nozzle plate 1 and a substrate 2 with spacers 3, 3 disposed between the nozzle plate and heating element 4. Formed between spacers 3 and 3 is a cavity 5 to be filled with ink while a nozzle 6 through which the ink is ejected is formed at the center of nozzle plate 1. A void 7 is formed as a thermally insulating layer under heating element 4. This void 7 is a space which is defined by a depressed portion formed in substrate 2 and between substrate 2 and heating element 4. This void 7 has the function of reducing the heat conduction between heating element 4 and substrate 2. The ink-jet head having the above configuration is constructed so that, upon ink ejection, a pulse of current is supplied from an unillustrated pulse generating power source so as to give off a pulse of heat from heating element 4. This heat vaporizes the ink within cavity 5 in an instant and forms bubbles therein to thereby eject ink from nozzle 6. Next, heating element 4 of this ink-jet head will be described. As shown in Fig. 2, heating element 4 is fixed with both ends fixed between spacer 3 and substrate 2 and is composed of a protective film 11, a heater 12 and an insulating film 13. This protective film 11 is to prevent adherence of ink in cavity 5 to heater 12. Insulation film 13 is to prevent electric leakage of the pulse current applied to heater 12 toward substrate 2. Heater 12 is configured of a metal thin-film having a high coefficient of thermal expansion and a high electric resistance. Heater 12 is adapted to give off heat by the application of a pulse current from the pulse generating power source so that the entire heating element 4 will be heated to a temperature (to be referred to hereinbelow as the bubbling temperature) which will permit the ink in cavity 5 to bubble by film boiling. Arranged at both ends of heater 12 are interconnections 14 for supplying the pulse current from the pulse generating power source to heater 12. Further, when the temperature of heating element 4 exceeds the bubbling temperature, heater 12 buckles due to heat energy (thermal expansion) so that the entire shape of heating element 4 deforms with its part put into contact with substrate 2. Illustratively, as shown in Fig.3, heating element 4 is kept horizontal (i.e., thermally insulated) with respect to substrate 2 at a temperature below the bubbling temperature. With its temperature exceeding the bubbling temperature, heater 12 will buckle, as shown in Fig.4, and heating element 4 will be set flexed to the void 7 side until it comes into contact with substrate 2. That is, this flexure brings part of heating element 4 (heater 12) into contact with substrate 2 (into its heat radiating state). Thus, the heat from heating element 4 (heater 12) is dissipated to substrate 2 and discharged, so that heating element 4 will return to the thermally insulated state shown in Fig.3. Heater 12 is composed of nickel of 6.5 µm thick and 150 µm long, and heats to about 500°C providing the buckling start temperature of 370°C, to perform ink ejection. Nozzle plate 1 may be formed of a stainless steel etching plate, nickel-electroformed plate, photosensitive glass, precision molded plastic, etc. For spacer 3, polyimide, nickel plating, dry film, etc. may be used. For protective film 11, polyimide, SiO 2 , SiN, etc. may be used. For interconnection 14, Ta, Ni, Au, Al, etc. may be used. For heater 12, Ta. Ni, HfB 2 , etc. may be used. For insulation film 13, polyimide, SiO 2 , SiN, etc. may be used. For substrate 2, glass, ceramic, stainless steel, nickel, silicon, etc. can be used. As above, the ink-jet head has void 7 between heating element 4 and substrate 2 so as to be able prevent heat conduction between heating element 4 and substrate 2. Therefore, it is possible to raise the temperature of heating element 4 in an instant to the bubbling temperature. Further, as soon as it has reached the bubbling temperature, heating element 4 buckles together with buckling of heater 12 so that part of the heating element comes into contact with substrate 2 to radiate heat. Therefore, heat from heating element 4 is discharged in a short time and hence the temperature of heating element 4 lowers quickly whereby the heating element returns to the thermally insulated state. According to the ink-jet head of the first embodiment, the response frequency of heater 12 can be set approximately equal to the frequency of the applied pulse current. Therefore, it is possible to markedly precisely control the ejected amount of ink. Here, in the first embodiment where heating element 4 is composed of protective film 11, heater 12 and insulation film 13, it is preferred that their materials should have approximately equal coefficients of thermal expansion. This is to prevent occurrence of cracks due to repeated heat cycles. It is also preferred that a silicone oil should be applied over the contact area between heating element 4 and substrate 2. This will promote heat diffusion from heating element 4 to substrate 2. In the first embodiment, heating element 4 is fixed at both ends with spacer 3 and substrate 2. However, setting method of heating element 4 should not be limited to this. That is, heating element 4 may be set in any manner as long as heating element 4 is inhibited from moving in directions perpendicular to the thickness thereof (in the direction perpendicular to the document and the direction left and right direction in Fig.2). In the first embodiment, heater 12 is adapted to buckle and bring heating element 4 into contact with substrate 2 when the temperature of heating element 4 exceeds the buckling temperature. However, buckling of heater 12 should not occur necessarily with a large deformation. In other wards, heater 12 may and should warp in a degree to reduce the distance between heating element 4 and substrate 2 when the temperature of heating element 4 exceeds the buckling temperature. That is, the reduced distance between heating element 4 and substrate 2 will be able to cool heating element 4 (heater 12) from thermal diffusion to substrate 2. In the first embodiment, though void 7 is formed by a depressed portion in substrate 2, the configuration of void 7 should not be limited to this. For example, as is shown in the second embodiment with reference to Figs. 5 and 6, communication holes 8 may be provided in substrate 2 so that void 7 and external air communicate with each other. This configuration can avoid the pressure rise in void 7 which would occur due to the deformation of heating element 4 so that heating element 4 will easily buckle. Further, since the air in void 7 can be ventilated through communication holes 8, cooling of heating element 4 (heater 12) can be promoted. It is also possible to provide a configuration so that void 7 communicates with cavity 5. In this case, void 7 will also be filled up with ink. In the first embodiment, though heater 12 is composed of a metal membrane having a high coefficient of thermal expansion and a high electric resistance, heater 12 may be formed of a laminated film made up of multiple number of metal membranes. As shown in the third embodiment in Fig. 7, a lamination of two different kinds of metals A and B, having different rates of thermal expansion may be used as heater 12 (insulation film 13 is omitted in Fig.7). In this way, by providing heater 12 with a bimetallic configuration, it is possible to regulate the deforming direction of heater 12, in a controlled manner. In the first embodiment, it is specified that heater 12 should buckle when the temperature of heater 12 exceeds the buckling temperature. However, this will not limit the configuration of heating element 4. For example, as in the fourth embodiment shown in Figs.8A and 8B, a thin film of a shape memory alloy (SMP) 15 may be provided in the lowermost layer of heating element 4. That is, when shape memory alloy 15 is set so that it takes a parallel state with respect to substrate 2 at lower temperature (see Fig. 8A) and takes a deformed state when exceeding the deforming temperature (see Fig.8B), it is possible to realize deformation and cooling of heating element 4 in a beneficial manner. Thus, when such a shape memory alloy 15 is used, there is no need to select a material having a high rate of thermal expansion for heater 12. In the first embodiment, spontaneous shape deformation of heating element 4 is made use of in order to cool heater 12 when placing heating element 4 into contact with substrate 2. That is, the contact between heater 12 and substrate 2 can be attained by selecting a material for heater 12 or shape memory allow 15 so that it deforms into void 7 above the deforming temperature. However, the ink- jet head of the first embodiment should not be limited to this. For example, the ink-jet head may employ a forcible thrusting method by using a piezoelectric actuator which forcibly pushes heating element 4 into void 7. Fig.9 is an illustrative view showing the fifth embodiment of an ink-jet head using this method. As shown in Fig.9, in this configuration, a piezoelectric actuator 21 is arranged on nozzle plate 1 so that it is directed to heating element 4. This piezoelectric actuator 21 is set so that it expand toward heating element 4 when a voltage from an unillustrated power source is applied thereto. Thus, heating element 4 is thrust into void 7 so as to come into contact with substrate 2. This configuration enables contact between heating element 4 and substrate 2 without depending upon the material of heater 12 or without using SMP 15. Since the amount of displacement of heating element 4 and its contact time with substrate 2 can be selected at will, it is possible to control the response frequency of heater 12 at a markedly high accuracy, thus making it possible to adjust the ejected amount of ink with precision. For piezoelectric actuator 21, a laminated piezoelectric actuator having multiple number of layered piezoelectric elements is preferably used. In this case, it is preferred that the displacement of the piezoelectric elements in the direction of their thickness is used to thrust heating element 4. With this configuration, it is possible to set the applied voltage to piezoelectric actuator 21 at 40 V or below while achieving high-speed switching of the shape of piezoelectric actuator 21. Accordingly, it is possible to control the response frequency of heater 21 with a further enhanced accuracy. As piezoelectric actuator 21, thin-film piezoelectric elements can be used. For example, in the sixth embodiment shown in Fig.10, piezoelectric actuator 21 is provided in a laminated configuration (bimorph configuration) consisting of first and second piezoelectric films 22 and 23. This piezoelectric actuator 21 is set so that first piezoelectric film 22 expands along the film surface thereof while second piezoelectric film 23 contracts in the same direction when the voltage is applied. In this arrangement, as the voltage is applied, an abutment point 31 formed at the end of piezoelectric actuator 21 presses down heating element 4 so as to achieve contact between heating element 4 and substrate 2. This configuration simplifies the ink-jet head configuration providing improved assembly workability. Here, as the material of first and second piezoelectric films 22 and 23, PZT (lead zirconium titanate) can be used. In the configuration shown in Fig. 10, instead of first piezoelectric film 22, a metal thin-film such as of stainless steel may be used (uni-morph configuration). Also in this configuration, it is possible to realize beneficial deformation or displacement of piezoelectric actuator 21 while simplifying the configuration of piezoelectric actuator 21 and hence reducing the manufacturing cost. Figs. 11A and 11B are plan and sectional side views, respectively, showing a configuration of an ink-jet head in accordance with the seventh embodiment of the present invention. This ink-jet head 101 is configured so that a heater portion 104 is arranged between a first substrate 102 and a second substrate 103 with a nozzle plate 105 fixed to and along the one end face of substrates 102 and 103. Formed between substrates 102 and 103 is a cavity 106 which will be filled with liquid ink. A nozzle 105a which allows cavity 106 to communicate with the exterior is formed in nozzle plate 105. Heater portion 104 is disposed in cavity 106. Heater portion 104 is composed of a heating element 104a, interconnections 104b, an insulation film 104c and a protective film 104d. Heating element 104a is formed of a thin plate made up of a metal such as Ni, etc., having a high coefficient of thermal expansion and a high electric resistance, and will heat by the electric supply of pulse current from an unillustrated drive circuit via interconnections 104b and bubbles the ink within cavity 106 by film boiling. Insulation film 104c provides electric insulation of heating element 104a and interconnections 104b from substrate 102. Protective film 104d provides watertightness of heating element 104a and interconnections 104b from and against the ink filling cavity 106. A spacing wall 107 is arranged between the top surface of heater portion 104 and substrate 103. This spacing wall 107 forms the space constituting cavity 106 between heater portion 104 and substrate 103. A void 108 is defined by the upper surface of substrate 102 which is disposed below, and opposing, heating element 104a of heater portion 104. This void 108 is formed so as to reduce the contact area of the undersurface of heater portion 104 with substrate 102. Accordingly, heat transfer of the heat generated from heating element 104a of heater portion 104 toward substrate 102 is reduced, while the ink inside cavity 106 is quickly heated as the temperature of heater portion 104 increases rapidly. Therefore, when a pulse of current is applied to heating element 104a, the ink in cavity 106 is heated in an instant, and gives off bubbles when the temperature of heating element 104a reaches the bubbling temperature Tb, to thereby eject ink from nozzle 105a. This bubbling temperature Tb is a temperature unique to the ink used. One end in one direction within the surface, opposing substrates 102 and 103, of heater portion 104 abuts the inner surface of nozzle plate 105 while the other end is held between substrates 102 and 103 with spacing wall 107 interposed between substrate 103 and heater portion 104. Accordingly, movement of heater portion 104 is constrained at both ends with respect to the aforementioned one direction within the surface, opposing substrates 102 and 103, of heater portion 104. Figs .12A and 12B are sectional side views schematically showing the deformation of a heater portion in the above ink-jet head. From heat generated from heating element 104a, heater 104 tries to expand in the directions indicated by the arrows in Fig. 12A. However, since the displacement at both ends in the aforementioned one direction within the surface opposing substrates 102 and 103, a thermal stress occurs in the directions (contracting direction) shown by the arrows in Fig.12A. This thermal stress increases with the temperature rise of heating element 104a, and when the temperature of heating element 104a reaches the predetermined buckling temperature Tc, heater portion 104 will elastically deform (elastically buckle) due to thermal stress. Therefore, the buckling temperature Tc is a temperature at which elastic buckling of heater portion 104 starts. Specifically, when the temperature of heating element 104a is below the buckling temperature Tc, heater portion 104 remains between substrates 102 and 103 and in parallel thereto, as shown in Fig. 12A. When the temperature of heating element 104a reaches the buckling temperature Tc, heater portion 104 buckles with its central part projected to the substrate 102 side. When the temperature of heating element 104a has reached the heating temperature T which is higher than the buckling temperature Tc, heater portion 104 comes in contact with the upper surface of substrate 102 at its middle part as shown in Fig. 12B. Therefore, this heating temperature T is the temperature when heater portion 104 comes into contact with substrate 102 and heating element 104a receives electric supply of pulse current until it reaches the heating temperature T. When the temperature of heating element 104a has reached the heating temperature T and its middle part comes into contact with the upper surface of substrate 102, heat arising in heating element 104a of heater portion 104 is discharged and conducted through substrate 102. If the supply of pulse current to heating element 104a is stopped at this moment, the temperature of heater portion 104 rapidly lowers. With this temperature drop, the thermal stress in heater portion 104 reduces whereby the heater portion 104 reverts back to its original state shown in Fig. 12A by its own elasticity. In connection with the above, if the upper surface of substrate 102, which also defines the bottom surface of void 108 is made curved so that it abuts and fits the undersurface of heating portion 104 after deformation, heater portion 104 having reached at heating temperature T will be able to come into area contact with the upper surface of substrate 102, thus making it possible to efficiently discharge heat from heater portion 104. As an example, heating element 104a is formed of nickel of 6.5 µm thick, 10 µm wide and 150 µm long. Insulation film 104c is formed of SiO 2 of 0.1 µm thick. Protective film 104d is formed of polyimide of 0.1 µm thick. The depth of void 108 is set at 5 µm. In this arrangement, as heater portion 104 is heated up to the heating temperature T (=500°C) for the buckling temperature Tc (= 370°C), the ink inside cavity 106 gives off bubbles and hence ink is ejected from nozzle 105a while the middle part of heater portion 104 moves inside void 108 and comes into contact with the upper surface of substrate 102. The buckling temperature Tc of heater portion 104 is given as follows: Tc = (π 2 /3α) x (H/L) 2 where H, L and α represent the thickness, length and coefficient of linear expansion of heater portion 104. The maximum displacement Ymax of heater portion 4 is given as follows: Ymax = 2qL/π 2 Here, q is determined using the heating temperature T, the buckling temperature Tc and the coefficient of linear expansion α of heater portion 104, from the following relations:- L(q)= π · α · (T - Tc)/ 4 Further, electric power consumption P is given as: P = ρ x H x L x D x C x T where H, L, D and C, ρ and T are the thickness, the length, width and specific heat, density and heating temperature of heater portion 104, respectively. In heater portion 104, if insulation film 104c and protective film 104d are thin enough compared to heating element 104a, the deformed state of heater portion 104 is determined dedicatedly depending upon the material and shape of heating element 104a. For example, Fig.13 shows the relationship between the temperature t of heater portion 104 and the amount of displacement Y for a heating element 104a of Ni with a coefficient of linear expansion α of 1.8 x 10 -5 and of 150 µm long (L) and 10 µm wide (D) when electric power consumption P is changed stepwise through 0.6 W, 0.9 W, 1.2 W, 1.5 W, 1.8 W, 2.1 W to 2.4 W and the thickness H is changed from 1.16 µm to 8.04 µm. As is apparent from Fig.13, the lower the temperature t of heater portion 104, the smaller the amount of displacement Y. In order to provide an amount of displacement equal to or greater than 10 µm, heater portion 104 should be kept at an extremely high temperature, at 600 °C or higher. In order to save electric energy by reducing the power consumption P when heater portion 104 is moved within the void 8 into contact with substrate 102 by virtue of elastic deformation accompanying the temperature rise, the depth of void 8 should be 10 µm or smaller. When the depth of void 8 is set at 4 µm or smaller, the heating temperature T for causing heater portion 104 to come into contact with substrate 102 will not vary too much even if the depth of void 8 fluctuates when manufactured. Therefore, with this condition, it is possible to improve the production yield. Next, Fig.14 shows the relationship between the ratio (L/H: thinness) of the length of heater portion 104 to the thickness and power consumption P for a heater portion 104 of Ni with a coefficient of linear expansion α of 1.8 x 10 -5 and of 150 µm long (L) and 10 µm wide (D) when the amount of displacement Y is changed stepwise as 1.0 µm, 1.5 µm, 2.5 µm, 5.0 µm, 7.5 µm, 8.5 µm, 10.0 µm and 12.5 µm while the heating temperature T is changed from 212°C to 1054°C at each displacement. In particular, Fig.15 shows relationships between the thinness L/H, and heating temperature T, and buckling temperature Tc when the displacement Y of heater portion 104 is at 2.5 µm. Fig.16 shows a relationship between the thinness L/H, heating temperature T, buckling temperature Tc, and power consumption P. As is apparent from Figs. 14 to 16, as the thinness L/H of heater portion 104 becomes greater (as the heater portion 104 becomes thinner), heating temperature T and buckling temperature Tc become lower and hence the power consumption P becomes smaller. For example, with the depth of void 108 (the amount of displacement Y of heater portion 104) set at 2.5 µm, when a heater portion 104 having a thinness L/H of 19.0 (L (length of the heater portion 104) = 150 µm, H (height) = 7.9 µm) is used, buckling temperature Tc is equal to 505°C, heating temperature T is equal to 542°C and power consumption P = 2.4 W. When a heater portion 104 having a thinness L/H of 31.3 (L (length of the heater portion 104) = 150 µm, H (height) = 4.8 µm) is used, buckling temperature Tc is equal to 186°C, heating temperature T is equal to 223°C and power consumption P = 0.6 W. Accordingly, when a type of ink having a bubbling temperature Tb of 186°C is used, the ink will be ejected when the temperature of the heater portion 104 reaches 186°C and then the heater portion 104 will start to be cooled down after only a further temperature rise by 37°C as it comes into contact with substrate 102 at that temperature. With the depth of void 108 (the amount of displacement Y of heater portion 104) set at 10.0 µm, when a heater portion 104 having a thinness L/H of 29.3 (L (length of the heater portion 104) = 150 µm, H(height) =5.1 µm) is used, buckling temperature Tc is equal to 212°C, heating temperature T is equal to 835°C and power consumption P = 2.4 W. Accordingly, when a type of ink having a bubbling temperature Tb of 212°C is used, the ink will be ejected when the temperature of the heater portion 104 reaches 212°C and then the heater portion 104 should be heated by 623°C higher to bring heater portion 104 into contact with substrate 102 to cool it down. From the above consideration, as the depth of void 108, or the amount of displacement Y of heater portion 104 is smaller, the thinness L/H of heating portion 104 can be increased. Thus, the depth of void 108 should be determined so that the thinness L/H value will be equal to 15 or more, preferably 20 or more. This setting makes for reduction in the power consumption P and the temperature variation of heater portion 104, thus improving the durability. From the above formula (1), as the coefficient of linear expansion α becomes greater, heater portion 104 has a lower buckling temperature Tc. Particularly, if a heater portion 104 is formed of a material having a coefficient of linear expansion α of 1x10 -5 /°C or greater, elastic buckling occurs at a lower temperature, making it possible to provide an ink-jet head configuration which has a small power consumption P and hence can provide an excellent durability. When a type of ink having its bubbling temperature Tb equal to the heating temperature T of heater portion 104 is used, heater portion 104 will come into contact with substrate 102 almost at the same time as the ink is ejected. Accordingly, it is possible to provide an ink- jet head configuration which has the heating temperature T lowered and hence can provide an excellent durability. When a type of ink having its bubbling temperature Tb equal to the buckling temperature Tc of heater portion 104 is used, heater portion 104 will start deformation when the ink is ejected. That is, at that moment, the heater portion 104 has not yet come into contact with substrate 102. So, heater portion 104 will radiate a lowered amount of heat so that it is possible to reduce the power consumption for heating heater portion 104. Further, an internal stress in the direction parallel to the top surfaces of substrate 101 and 102 may be previously given to heater portion 104 so that it is possible to lower the buckling temperature Tc of heater portion 104. For example, when heater portion 104 is formed by electrolytic plating, the formed heater portion 104 has an internal stress depending upon the current density as shown in Fig.17. As an example, when heater portion 104 is formed by electrolytically plating in a nickel amidosulfonate bath (sulfamic acid Ni bath) having a pH of 4.4 with the current density set at 1 mA/cm 2 at a temperature of 50°C, the resultant heater 104 has a compressive stress of about 55 MPa acting in the surface direction. This compressive stress corresponds to the stress that would arise by a temperature rise of 14.6°C in the following formula (4). S = E·α·T/(1 + α·T) where S: the stress in the plated layer (N/m 2 ) E: Young's modulus (2.1 x 10 11 N/m 2 for Ni) α: coefficient of linear expansion ((1.8 x 10 -5 /°C for Ni) T: temperature (°C). Therefore, the heater portion 104 prepared by the above electrolytic plating will start to deform at a temperature lower by 14.6°C than that in the case where no internal stress is given. Therefore, lowering of the buckling temperature Tc and heating temperature T of heater portion 104 can improve the device in its durability and reduce the power consumption. Figs. 18A and 18B are sectional side views showing the essential components of ink- jet heads according to the eighth embodiment of the present invention. In the ink-jet heads according to this embodiment, the heater portion is composed of two or three layers. In an ink-jet head 111 shown in Fig.18A, heater portion 114 is configured of two layers, i.e. , an upper layer 114a and a bottom layer 114b, which have been previously given tensile and compressive stresses, respectively. In an ink-jet head 121 shown in Fig.18B, heater portion 124 is configured of three layers, i.e., an upper layer 124a, and a middle layer 124b and a bottom layer 124c, which have been previously given tensile stresses and compressive stresses, respectively. In the case where heater portion 114 or 124 has a multi-layered structure with each layer given a different internal stress as shown in Figs.18A and 18B, when a temperature rise causes compressive stresses to arise, the heater will buckle with its center projected to the side on which the layer, among the multiple layers in heater portion 114 or 124, receiving a stronger compressive stress is disposed. In order for heater portion 114 or 124 to maintain a stabilized state by canceling out the differential stress between the layers, the layer which has a stronger compressive stress acting thereon compared to the other layers should expand greater and hence the heater portion 114 or 124 will curve with the layer having the greater expansion outside. Therefore, heater portion 114 or 124 always buckles with its lower layer 114b or 124c, which has a greater compressive stress acting thereon compared to that on the upper layer 114a or 124a, projected so that heater portion 114 or 124 will positively come into contact with the upper surface of substrate 112 or 122 having void 128 or 128 formed therein. Figs.19A and 19B are a plan view and a sectional side view cut along a plane 401-402, showing an ink-jet head according to the ninth embodiment of the present invention. An ink-jet head 201 has a substrate 206, a spacing wall member 205 and a top plate 204 arranged in layers from the bottom to the top in this order. Provided on one of the end faces defined by substrate 206, spacing wall member 205 and top plate 204, which has an opening communicating with an ink chamber (also referred to as cavity) formed in spacing wall member 205, is a nozzle plate 203. This nozzle plate is attached to the end face as it is oriented perpendicular to the direction of lamination of substrate 206, spacing wall member 205 and top plate 204. Formed on the top of substrate 206 are a heater portion 209, electrodes 208a and 208b together with an unillustrated drive circuit, etc. A space, enclosed by substrate 206, top plate 204 and nozzle plate 203 and heater portion 209, constituting ink chamber 202 is formed inside spacing wall member 205. Top plate 204 is a flat plate made up of a metal such as aluminum, iron, or the like. Nozzle plate 203 has an ejection nozzle 207 through which the ink inside ink chamber 202 is ejected. Ejection nozzle 207 provides communication between ink chamber 202 and ink-jet head 201. Heater portion 209 is composed of a heating element 209a, a protective film 209b and an insulation film 209c. Heating element 209a is a thin plate having an approximately circular shape made up of a metal such as nickel, nickel-chromium alloy, or the like, which has a high coefficient of thermal expansion and a high electric resistance. Heating element 209a is connected with electrodes 208a and 208b and heats when an unillustrated drive circuit supplies pulses of current thereto via electrodes 208a and 208b. When the ink reaches the bubbling temperature, the ink inside ink chamber 202 bubbles by film boiling. Heating element 209a elastically deforms at a temperature approximately equal to the bubbling temperature of the ink so that it slightly projects into ink chamber 202. When the current is supplied to heating element 209a and the ink reaches the bubbling temperature, the ink gives off bubbles. Immediately after bubbling, heater portion 209 including heating element 209a elastically deforms. Protective film 209b is to prevent adherence of ink in ink chamber 202 to heating element 209a and electrodes 208a and 208b. Protective film 209b is preferably formed of polyimide, silicon oxide, silicon nitride, or the like. Insulation film 209c provides electric insulation of heating element 209a and electrodes 208a and 208b from substrate 206. Insulation film 209c is preferably formed of silicon oxide, silicon nitride or the like. Formed in substrate 206 at the position under and opposing heater portion 209 is a void 210. This void 210 communicates with the exterior of ink-jet head 201 via a hole 211 formed in substrate 206 at a position opposing heater portion 209. Provision of void 210 reduces the contact area of the undersurface of heater portion 209 with substrate 206. Accordingly, heat transfer of the heat generated from heater element 209a of heater portion 209 toward substrate 206 is reduced, while the ink inside ink chamber 202 is quickly heated as the temperature of heater portion 209 increases rapidly. Further, provision of hole 11 equalizes the pressure inside void 210 to that outside ink-jet head 201. Therefore, when heater portion 209 elastically deforms, pressure inside void 210 will not lower, so as to permit heater portion 209 to elastically deform smoothly. Heater portion 209 is fixed at the entire periphery of void 210 formed in substrate 206. Therefore, when heater portion 209 is elastically deformed, the ink inside ink chamber 202 will not leak to void 210 or other parts. Next, the operation of the ink-jet head according to the ninth embodiment of the present invention will be described with reference to Figs.19B and 20. Fig.20 is a sectional view showing the state where heater portion 209 has been elastically deformed. In Fig.19B, as a pulse current is supplied to heating element 209a, the ink inside ink chamber 202 heats in an instant. When the temperature of heating element 209a reaches the bubbling temperature of the ink, ink gives off bubbles therein. Then, immediately after this, heating element 209a reaches its buckling temperature, heater portion 209 flexes into ink chamber 202 in an instant by thermal stress, and comes into contact with top plate 204, as shown in Fig. 20. As heater portion 209 abuts top plate 204, heat arising from heating element 209a of heater portion 209 is dissipated and directly conducted through top plate 204. As stated above, top plate 204 is of metal and hence has a good heat radiation performance, so that it discharges heat quickly. When the current supplied to heating element 209a is stopped, the temperature of heater portion 209 sharply lowers. With the reduction in temperature, the thermal stress acting on heater portion 209 decreases, so that heater portion 209 reverts back to its original state shown in Fig.19B by its own elasticity. By the above operation, the pressure produced by the expansion of bubbles arising from the ink heated by heating element 209a and the pressure produced when heater portion 209 elastically deforms act on the ink. Since heating element 209a has a substantially circular shape as stated already, so that with the same area, the heating element 209a of this embodiment is more liable to elastically deform than one having a rectangular shape, for example. Therefore, the configuration of this embodiment provides efficient ejection of ink droplets from ejection nozzle 207. Since heater portion 209 has a so-called diaphragm configuration which is fixed around the entire periphery of void 210, all the pressure arising in ink chamber 202 will act on the ink so as to eject ink droplets out. Therefore, if the bubbles produced by heating element 209a are small in size compared to the conventional configuration, the pressure produced by the expansion of bubbles and the pressure produced when heater portion 209 elastically deforms act on the ink, thus making it possible to eject a required amount of ink from ejection nozzle 207. Resultantly, heating element 209a may be made compact compared to the conventional one and can reduce the power consumption required for heating element 209a to give off bubbles from the ink. In this way, since heater portion 209 is cooled down quickly as the heat arising therefrom is released through top plate 204, this configuration provides a good response to the pulse current supplied to heating element 209a. Therefore, it is possible to control the ejected amount of ink at a high accuracy. Fig.21 is a sectional view showing another configuration of an ink-jet head according to the ninth embodiment of the present invention. As shown in Fig.21, a top plate 204b opposing heater portion 209 having heating element 209a is shaped in a form similar to the deformed shape of heating portion 209 and arranged so that heater portion 209 will come into area contact with top plate 204b when heating portion 209 has elastically deformed by thermal stress. When a pulse current is supplied to heater portion 209 and the temperature reaches the temperature causing elastic deformation, heater portion 209 elastically deforms so that the face opposing top plate 204b comes into area contact with top plate 204b. As a result, heat from heater portion 209 transfers to top plate 204b of metal, and is quickly dissipated through top plate 204b. Accordingly, when the current supplied to heater portion 209 is stopped, the heater portion immediately reverts back to its original state shown in Fig.19B. Fig.22 is a sectional view showing another configuration of a heater portion 209 in an ink-jet head according to the ninth embodiment of the present invention. Since the configurations other than the heater portion are the same as in the ink-jet head 201 shown in Figs.19A and 19B, only the heater portion will be explained. In an ink-jet head 231, in order to regulate the direction of deformation of heater portion 219, the heating element is provided in a multi-layered configuration to adjust the stress therein. Illustratively, heater portion 219 has a two layered structure of a heating element 219a (upper layer) and a heating element 219b (lower layer), which have been previously given compressive stress and tensile stress, respectively. Further, a protective film 219c is formed on the upper side of heating element 219a while an insulation film 219d is formed on the lower side of heating element 219b. Protective film 219c is to prevent adherence of ink in an unillustrated ink chamber to heating element 219a and unillustrated electrodes. Insulation film 219c provides electric insulation of heating element 219b and the unillustrated electrodes from a substrate 216. In the case where heater portion 219 has a multi-layered structure with each layer given a different internal stress, when a temperature rise causes compressive stresses to arise, heater portion 219 will buckle with its center projected to the side on which the layer, among the multiple layers in heater portion 219, receiving a stronger compressive stress is disposed. In order for heater portion 219 to establish a stabilized state by deformation canceling out the differential stress between the layers, the layer which has a stronger compressive stress acting thereon compared to the other layers should expand greater and hence the heater portion 219 will curve with the layer having the greater expansion outside. Therefore, with a temperature rise due to heating of heating elements 219a and 219b, heater portion 219 always elastically deforms with heating element 219a, which has a greater compressive stress acting thereon compared to that on the heating element 219b, projected so that heater portion 219 will positively come into contact with or come closer to top plate 204. In heater portion 219 shown in Fig.22, for example, electrolytic plating in a nickel amidosulfonate bath for plating with the current density of 40 mA/cm 2 is effected over heating element 219b of 3 µm thick, to form a lower layer. Then, electrolytic plating with the current density of 1 mA/cm 2 is effected over heating element 219a of 3 µm thick, to form an upper layer. In this case, a tensile stress of about 55 MPa is given to the lower layer of heating element 219b, while a compressive stress of about 55 MPa is given to the upper layer of heating element 219a. Therefore, when heating to a temperature nearly equal to the bubbling temperature, heater portion 219 deforms toward top plate 204. In the ink-jet head designated at 231 shown in Fig. 22, heater portion 219 is formed in a multi-layered configuration with compressive and tensile stresses given to heating elements 219a and 219b, respectively. Also in ink-jet head 201 shown in Figs.19 and 20, when protective film 209b and insulation film 209c are given compressive and tensile stresses respectively while heating element 209a is adjusted to be free from stress, it is possible to cause heater portion 209 to elastically deform when the heater reaches a temperature nearly equal to the bubbling temperature. Though, in ink-jet head 231 shown in Fig.22, heater portion 219 is formed in a two-layered structure, it is also possible to have the same effects if the heater portion is formed in three or more layers, by adjusting the internal stress to be previously given to each layer. Figs.23A to 23H are views showing a fabrication method of an ink-jet head of the seventh embodiment of the present invention. For each of Figs.23A to 23H, the plan view, the sectional view taken along a plane 303-304 in the plan view and the sectional view taken along a plane 301-302 in each step are shown from the left to the right. When ink-jet head 101 of the seventh embodiment of the present invention is manufactured, a void-forming material 108a of polyimide, A1 or the like is formed with a thickness (e.g., 5 µm) corresponding to the depth of void 108 to be formed, over the top surface of first substrate 102 of glass or metal by a spinner, sputtering or other means. Then, a photoresist corresponding to the plan shape of void 108 is patterned over the top surface. Void-forming material 108a is etched by a 1N NaOH solution or the like leaving the flat shape of void 108. Then the photoresist remaining on the top of the plan shape of void 108 is removed. By this process, a solid shape of void 108 is formed on the top surface of substrate 102 by void-forming material 108a (Fig.23A). Next, SiO 2 or the like is film formed by sputtering or other means so as to provide an insulation film 104c having a predetermined thickness (e.g., 0.1 µm) over the top of substrate 102 having the solid shape of void-forming material 108a for void 108. Then, dry etching such as RIE (reactive ion etching) with CF 4 gas is performed using a photoresist. Then the photoresist is removed. By this process, insulation film 104c is formed over the top surface of substrate 102 except part of the top surface of void-forming material 108a (Fig.23B). Subsequently, Ta of 100 Å thick and Ni of 200 Å thick are film formed by sputtering or other means in this order on the top surface of substrtae 102 including void-forming material 108a and insulation film 104c. Then, etching is performed using a photoresist and then the photoresist is removed. By this process, Ta film for improving the adhesion between insulation film 104a and Ni forming heating element 104a and an Ni film for the surface backing of Ni plating forming heating element 104a are formed over top surface of substrte 102 except part of the top surface of void-forming material 108a (Fig.23C). Here, it should be noted that insulation film 104c, Ta film and Ni film are formed on the top of the void-forming material in such a manner that the area of each layer in the plan view becomes marginally smaller than the area of the lower layer, whereby heating element 104a to be formed on the top of the Ni film will be positively coated by insulation film 104c. Thereafter, a negative type photoresist is applied with the same thickness (e.g., 6.5 µm) as the film thickness of heating element 104a over the top surface of substrate 102 having the void-forming material, insulation film 104c, the Ta film, the Ni film formed thereon, and then the shape correspoidng to the solid shape of heating element 104a to be formed is patterned. Use of a negative type photoresist is by reason of its suitablity of forming tapering shapes for heating element 104a. Next, the Ni film is formed by electroytically plating in a nickel amidosulfonate bath having a pH of 4.4 with the current density set at 20 mA/cm 2 at a temperature of 50°C so as to form an Ni layer having the thikness of heating element 104a in the area other than a photoresist 131 patterned over the top surface of substrate 102 (Fig.23D). As shown in Fig.17, no internal stress will occur in the Ni layer formed by the electroytically plating under the above-mentioned conditions. Further, a photoresist pattern for interconnections are formed on the top surface of the Ni layer. Then, electroytic plating in a nickel amidosulfonate bath is peformed under predetermined conditions with the Ni layer used as the electrode, so as to form interconnections 104b for heating element 104a with a predetermined thickness (e.g., 10 µm) over the top surface of the Ni layer (Fig.23E). The substrate 102 having heating element 104a and interconnections 104b formed thereon by the above process is immersed in a 1N NaOH solution so as to remove photoresist 131 and void-forming material 108a. By this process, void 108 having the depth corresponding to the film thickness of void-forming material 108a is formed in the portion opposing the bottom face of heating element 104a on the top surface of substrate 102 (Fig.23F). Next, a film of SiO 2 , SiN or the like having a predetermined thickness (e.g., 0.1 µm) is formed as protective film 104d by sputtering, vapor deposition or other methods, over the top surface of the Ni layer with photoresist 131 and void-forming material 108a removed, except on the top surface of interconnections 104b. Then the photoresist over interconnections 104b is removed by the lift-off method using acetone etc., or other methods. This completes the formation of heater portion 104 on substrate 102 (Fig.23G). Thereafter, polyimide, dry film etc. are formed with a predetermined shape having a predetermined thickness (e.g., 10 µm) over the top of part of interconnections 104b and part of protective film 104d to form spacing wall 107. Then, second substrate 103 of glass etc., is bonded to the top surface of spacing wall 107. Further, nozzle plate 105 having a nozzle 105a formed therein is bonded to the end face of substrates 102 and 103. This completes ink-jet head 101 in accordance with the seventh embodiment of the present invention (Fig.23H). Figs.24A and 24B are views showing another fabrication method of an ink-jet head of the seventh embodiment of the present invention. For each of Figs. 24A to 24B, the plan view, the sectional view taken along a plane 303-304 in the plan view and the sectional view taken along a plane 301-302 in each step are shown from the left to the right. In this fabrication method, insulation film 104c is formed (Fig.24A) on a first substrate 102 made up of Si, of which the top surface is entirely covered with a thermal oxidation film 102a, in the same manner as shown in Fig. 23B. Then in the same manner as shown in Figs.23C to 23E, Ta film, Ni film, Ni layer and interconnections 104b constituting heater portion 104 are formed in this order. Then, thermal oxidation film 102a on the surface of substrate 102 in the lower part of heater portion 104 is removed by etching with hydrofluoric acid (HF) so as to form void 108 between substrate 102 and heater portion 104 (Fig.24B). Here, insulation film 104c and protective film 104d needs to be composed of SiN so as not to be etched by HF. According to this production method, void 108 can be formed without providing void-forming material 108a, thus making it possible to simplfy the production process of ink-jet head 101 and hence reduce the cost. Since the film thickness of thermal oxidation film 102a formed on the top surface of substrate 102 can be controlled to be 1 µm or below, it is possible to set the depth of void 108 to be relatively shallow, thus making it possible to impove the heat radiation efficiency. Further, since it is easy to uniformly form thermal oxidation film 102a over the top surface of substrate 102, improved production yiled can be obtained. Figs. 25A, 25B and 25C are views showing a fabrication method of an ink-jet head of the eighth embodiment of the present invention. When ink-jet head 111 according to the eighth embodiment wherein heater portion 114 is provided with multiple layers having different internal stresses is produced, the electrolytic plating process for the Ni layer constituting heating element 104a shown in Fig.23D is effected by multiple steps using different current densities from one another, which are controlled based on the relationship shown in Fig.17. When the electrolytic plating process shown in Fig.23D is performed by the combination of the first step for preparing a first layer 141 of 3.5 µm thick with a current density of 1 mA/cm 2 and the second step for preparing a second layer 142 of 3.0 µm thick with a current density of 20 mA/cm 2 . heater portion 114 is formed with the first layer 141 located on the lower part given with a compressive stress of about 55 MPa and the second layer 142 located on the upper part given with no internal stress. By this process, the compressive stress acting on first layer 141 in the heated, heater portion 114 becomes greater than the compressive stress acting on second layer 142, so that heater portion 114 will always deform projected downwards so that the middle part of heater portion 114 will positively come into contact with the top surface of substrate 112 when it is heated. In contrast, when the electrolytic plating process is performed by the combination of the first step for preparing a first layer 141 of 3.5 µm thick with a current density of 20 mA/cm 2 and the second step for preparing a second layer 142 of 3.0 µm thick with a current density of 1 mA/cm 2 , heater portion 114 is formed with the second layer 142 located on the upper part given with a compressive stress of about 55 MPa and the first layer 141 located on the lower part given with no internal stress. By this process, the compressive stress acting on second layer 142 in the heated, heater portion 114 becomes greater than the compressive stress acting on first layer 141, so that heater portion 114 will always elsatically deform projected upwards. In this way, depending upon the structure and the state of attachment of ink-jet head 111, if an unillustrated upper substrate position above heater portion 114 has a greater heat radiation effect than that of substrate 112 located below heater portion 114, the heater portion 114 may be adapted to deform toward the upper substrate. That is, by controlling the current density during the electrolytic plating process for preparation of heating portion 114 , it is possible to make heater portion 114 deform toward either substrate 112 or the upper substrate, taking into account the efficiency of heat radiation. Finally, when the electrolytic plating is performed by the combination of the first step for preparing a first layer 141 of 3.5 µm thick with a current density of 1 mA/cm 2 and the second step for preparing a second layer 142 of 3.0 µm thick with a current density of 40 mA/cm 2 , heater portion 114 is formed with the first layer 141 located on the lower part given with a compressive stress of about 55 MPa and the second layer 142 located on the upper part given with a tensile stress of about 55 MPa. By this process, the compressive stress acting on first layer 141 in the heated, heater portion 114 becomes markedly greater than the compressive stress acting on second layer 142, the middle part of heater portion 114 will positively come into contact with the top surface of substrate 112.

Claims: An ink-jet head for ejecting ink by heating and bubbling ink, comprising: a heating element (4) to which electric current is supplied so as to boil and bubble the ink; a head substrate (2); and a void (7) formed between the heating element and the head substrate (2), characterized in that the heating element (4) is adapted to buckle and consequently move into the void (7) by thermal expansion accompanying the temperature rise thereof. The ink-jet head according to Claim 1, wherein the heating element (4) comprises: a heater (12); a protective film (11) for protecting the heater (12); and an insulation film (13) for insulating the heater (12) and the above components are formed of materials having approximately equal coefficients of linear expansion to each other. The ink-jet head according to Claim 1, wherein the heating element (4) is arranged with its movement constrained at both ends with respect to the direction of the thickness and the direction perpendicular to the thickness. The ink-jet head according to Claim 1, wherein the heating element (4) is configured so as to come into contact with the head substrate (2) when buckled into the void (7). The ink-jet head according to Claim 1, wherein the void (7) is arranged so as to communicate with a cavity (5) that stores ink. The ink-jet head according to Claim 1, wherein the head substrate (2) has a communication hole for establishing communication between the void (7) and the exterior. The ink-jet head according to Claim 1, wherein the heating element (4) is provided in a bimetal configuration made up of multiple kinds of metals so as to cause the heating element (4) to deform into the void (7) by the temperature rise thereof. The ink-jet head according to Claim 1, wherein the heating element (4) is formed with a shape memory alloy layer which will deform into the void (7) when the heating element exceeds a predetermined temperature. An ink-jet head according to Claim 1, further comprising a piezoelectric actuator which pushes and deforms the heating element (4) toward the void (7).

Inventors: HIRATA SUSUMU, HIRATA, SUSUMU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 2000
Main classification: B41J2
IPC codes: B41J2