Manufacturing process of functional film element

Abstract: When green sheets are laminated and integrally fired for a window spacer plate 26 formed with at least a plurality of windows 29, a covering plate 25 for covering the windows 29, and a through hole plate 27 having through holes, 22, 23, communicating with the windows 29, it is arranged to make a ratio t/w of thickness t of the window spacer place 26 to length w of the through holes, 22, 23 in its minor axis direction one or more, and to form a functional film actuator 33 on the outer surface of the covering plate 25 after firing. A gap (separation) is prevented from being generated in the interface between each ceramics plate in producing a ceramics substrate used for the functional film element.

Description: Background of the Invention and Related Art Statement The present invention relates to a manufacturing process of a functional film element and to a functional film element itself, and, more particularly, to a functional film element suitably used for a fluid sensor, a temperature sensor, a fluid pump, a speaker, an actuator, a transducer or the like. In recent years, there has been known a functional film element utilizing an energy conversion function of a functional film, in which a cavity is formed in a substrate, and a functional film actuator is mounted on the outer surface of the cavity, whereby the functional film actuator senses change of physical amount such as pressure or temperature occurring in the cavity, and outputs an electric signal corresponding to the variation of physical amount. A functional film element utilizing such characteristics for converting variation of various physical amount into an electric signal is generally called a functional film sensor or a functional filter. Contrary to the above, when an electric signal (voltage or current) is applied to such functional film element, the functional film actuator generates mechanical stress such as expansion and contraction, bending or vibration in the cavity according to the applied voltage or current, or generates heat so that it acts as a heater or the like for heating inside the cavity. Fig. 4 shows an example of structure of a conventional functional film element 14 or a sensor. A functional film element 14 is constituted by providing through holes 2 and 3 to capture external environment (atmosphere) to be measured, forming in a ceramics substrate 9 a cavity 1 which is a measurement area, and integrally forming a functional film actuator 13 on a covering plate 6 for the external wall positioned opposite to the through holes 2 and 3 in the cavity 1. Here, the ceramics substrate 9 is integrally formed by laminating the covering plate 6 and a through hole plate 8 having the through holes 2 and 3 passing into the cavity 1, each of which is a very thin plate, with a window spacer plate 7 therebetween. The through holes are not limited to the through holes 2 and 3, but a through hole 4 or other through holes are provided depending on an object to increase contact points with the external environment. The window spacer plate 7 is formed with a window 5 in such a manner that three through holes 2, 3 and 4 provided in the through hole plate 8 are formed in a longitudinal extent of the window 5 so that these holes are opened toward the window 5. In addition, a very thin covering plate 6 is laminated on a surface opposite to the side on which the through hole plate 8 of the window spacer plate 7 is laminated to cover and close the window 5, thereby the cavity 1 being formed within the ceramics substrate 9. Then, the ceramics substrate 9 is provided with the functional film actuator 13 on the outer surface of the covering plate 6 at a position corresponding to the cavity 1. Here, the functional film actuator 13 consists of a lower electrode 12, a functional film layer 11, and an upper electrode 10. The conventional functional element 14 is arranged as above, in which, viewing the cavity 1 and the through holes 2, 3 and 4 in a plan view, the cavity 1 overlaps over the through holes 2, 3 and 4 as shown in Fig. 5. A similar structure is shown in EP-A- 572230. On the other hand, in a recent proposal, as seen in a plan view of Fig. 2 showing a positional relationship between a window 29, and through holes 22, 23 and 24, there has been employed, to enhance detection accuracy of the functional film element, a shape in which through holes, for example, 22 and 23 out of those formed in the ceramics substrate are longitudinally elongated in a cavity 21 (window 29). However, in actually manufacturing a functional film element to have the through holes 22 and 23 outwardly elongated from the outer edge of the cavity 21 (window 29), when ceramics green sheets are laminated and integrated under pressure, the through holes 22 and 23 provided through a window spacer plate 26 and a through hole plate 27 prevent transmission of pressure through each green sheet in the laminating direction, making it difficult to apply sufficient pressing pressure, whereby sufficient adhesion cannot be obtained. Consequently, there is a problem such that a covering plate 25 and a window spacer plate 26 are separated at an adhered section between them facing the window 21 to generate a gap 41 after firing as shown in Fig. 3. Generation of such gap 41 is unfavorable because, for example, when the functional film element is used as a pressure measuring sensor, air is left in the cavity when fluid to be measured for pressure is intended to be filled in the cavity to provide a pressure measurement error. In addition, also, when it is used as a temperature sensor, particularly, in measuring temperature of fluid such as liquid, since air left in the gap frequently has thermal capacity different from that of the liquid, the air causes delay of measurement accurately following temperature variation of the liquid. In addition, when it is used for a speaker, mechanical stress is imposed on the covering plate 25 due to vibration of the functional film element, and causes separation of the covering plate 25 or the like to proceed with the gap 41 as a starting point of breakage, thereby the speaker being broken. Summary of the Invention The present invention is made in view of the problems as described above, and the object of the present invention is to provide a process of manufacturing a functional film element in which through holes elongated outward from a cavity, wherein no gap is generated between a covering plate and a window spacer plate. According to the present invention, there is provided a process of manufacturing a functional film element, as set out in claim 1. The invention also provides the ceramics functional film element of claim 4. In the process of manufacturing a functional film element, said through hole is preferably formed with a width of 300 µm or less in the direction of minor axis in the green sheet for the through hole plate. The through hole preferably has a shape of elongated circle, oval or rectangular. According to the process of manufacturing a functional film element according to the present invention, a gap is prevented from being generated at an interface between the covering plate and the window spacer plate in producing the ceramics substrate of the functional film element, thereby a functional film element being provided with excellent detection accuracy and mechanical properties. Here, the functional film herein specifically means a dielectric film, a thermoelectric element film, a piezoelectric/electrostrictive film, a varistor film, or a film consisting of only electrodes. Brief Description of the Drawings Fig. 1 is a sectional view of a functional film element according to the present invention. Fig. 2 is a plan view showing a positional relationship between communicating holes and a cavity of a functional film element according to the present invention. Fig. 3 is a sectional view showing how a gap is generated, the gap being a problem to be solved by the present invention. Fig. 4 is a sectional view of a conventional functional film element. Fig. 5 is a plan view showing a positional relationship between communicating holes and a cavity of a conventional functional film element. Preferred Embodiments of the Invention Now, the present invention is described in detail. Fig. 1 is a sectional view showing an embodiment of a functional film element 34 produced by the manufacturing process of the present invention. The functional film element 34 comprises a ceramics substrate 28, and a functional film actuator 33 integrally formed on the ceramics substrate 28. The ceramics substrate 28 is arranged by integrally laminating a covering plate 25 and a through hole plate 27 formed with through holes 22, 23 and 24 with a window spacer plate 26 therebetween, each these plate being a very thin plate. The window spacer plate 26 is formed with at least one window 29. The window spacer plate 26 is laminated on the through hole plate 27 in such a manner that the through holes 22 and 23 formed in the through hole plate 27 partially overlap and open toward these windows 29 in the window spacer plate 26, and that the through hole 24 completely communicates with the window 29. Therefore, the positional relationship between the window 29 and the through holes 22, 23 and 24 can be shown in a plan view same as Fig. 2. In addition, the covering plate 25 is placed on the surface opposite to the side where the window spacer plate 26 and the through hole plate 27 are laminated to cover and close an opening of the window 29 with the covering plate 25, thereby the cavity 21 being formed inside the ceramics substrate 28. In producing such functional film element 34, it is preferable to make one or more a ratio t/w of thickness t of a green sheet for the window spacer plate 26 to width w of each of the through holes 22 and 23 formed in a green sheet for the through hole plate 27 in its minor axis direction (as shown in Fig. 2), and particularly to make 300 µm or less the width w of each of the through hole 22 and 23. When the green sheets are laminated and integrated under pressure, such conditions suppress prevention of pressure transmission in the laminating direction of the green sheets caused by the window spacer plate 26 and the through holes 22 and 23 formed in the through hole plate 27, whereby a gap 41 is prevented from being generated by peeling in the adhered portion facing the cavity 21 between the covering plate 25 and the window spacer plate 26 after firing. However, if the width w of each of the through holes 22 and 23 in the minor axis direction exceeds 300 m, integration through lamination of green sheets becomes difficult to be maintained at some portion between the covering plate 25 and the window spacer plate 26, so that the gap 41 tends to be generated, even if the t/w value is set within a range of limit of one or more. Therefore, in the present invention, because the width in the minor axis direction is limited, the through hole 22 and 23 preferably have an elongated circle in a plan view as shown in Fig. 2, as well as a rectangular or oval. In addition, the hole may have a shape in which such shape is modified at its ends in the major axis direction to have a polygonal shape, or may be a polygon elongated in one direction. While the conditions for forming the through hole 24 preferably follows those for the through holes 22 and 23, the through hole 24 is need not to entirely communicate with the cavity 21 as shown in Fig. 1, and may be formed in the same manner as the through holes 22 and 23. In addition, the plane shape of the cavity 21 is not limited to the elongated circle shown in Fig. 2, but may have various shapes. In the present invention, the ceramics substrate 28 described above is formed as a one-piece sintered article of ceramics, Specifically, a green sheet is molded from ceramics slurry prepared from a ceramics material, binder and solvent by a conventional device such as a doctor blade or calendar roll. Then, the green sheet is processed through cutting, grinding, or stamping, as necessary, to produce precursors of respective plates formed with the window 29, the through holes 22, 23, and 24. These precursors are laminated, integrated under pressure (while heating, if necessary), and sintered, so that an integrated ceramics substrate 28 is produced. Materials used for constituting the ceramics substrate 28 favorably include alumina and zirconia in view of mechanical properties and moldability, but are not specifically limited. It is also preferred that the produced green sheets have substantially equal firing shrinkage ratio. It is because, if the firing shrinkage ratio significantly differs from one green sheet to another, the ceramic substrate 28 suffers from separation of sheets or deformation in firing. Then, the ceramics substrate 28 is provided with a functional film layer 31 of a functional film actuator 33 on the outer surface of the covering plate 25 to cover a part of the plane shape of the cavity 21. Here, the functional film actuator 33 comprises a lower electrode 32, the functional film layer 31, and the upper electrode 30. These members may be usually formed by various conventional film formation processes such as thick film formation processes including screen printing, spraying, or slurry coating, or thin film formation processes including ion beam, sputtering, or chemical vapor deposition (CVD). Each film of the thus formed functional film actuator 33 (lower electrode 32, functional film layer 31, and upper electrode 30) is then subject to heat treatment (firing). Such heat treatment may be performed every time each film is formed, or in batch after all films are formed. However, in sequential firing, it is preferred that heat treatment temperature in the later process is equal to or lower than that in the previous process. This is because, if the treatment temperature of the later process is higher than that of the previous process, the film burnt in the previous process is also sintered in the later process, leading to possible peeling or aggregation due to over sintering. Materials for the lower and upper electrodes 32 and 30 constituting the functional film actuator 33 are not limited to any particular materials as long as they are conductors which can withstand temperature and atmosphere in forming respective films. Generally used are metals, alloys or conductive ceramics. More specifically, favorable materials are precious metals with high melting point including platinum, gold, palladium, and silver-palladium alloy. In addition, various functional ceramics, functional polymers, or metal materials can be used as materials for the functional film actuator 31 constituting the functional film actuator 33 depending on their applications. For example, materials favorably used are iron (Fe)-silicon (Si) type intermetallic compounds or the like for the thermoelectric element, zinc oxide (ZnO) or the like for the varistor element, or lead zirconium acid titanate (PZT), or plumbum magnesium niobate (PMN) or plumbum nickel niobate (PNN) for the materials exhibiting electric field induced distortion such as piezoelectric or electrostrictive effect. The functional film element 34 thus produced may be provided with only one electrode layer on the covering plate 25. In such case, the ceramics substrate 28 including the covering plate 25 may be produced from various functional materials used for the functional film layer 31. In addition, the thickness of the functional film actuator 33 is typically 100 µm or less, the thickness of the lower and upper electrodes 32 and 30 is typically 20 µm or less, preferably, 5 µm or less, and the thickness of the functional film layer 31 is preferably 50 µm or less, more preferably, 3 µm or more but 40 µm or less to obtain higher potential at a low operating voltage. As described above, the present invention enables production of a functional film element having fluid channel with through holes elongated from a cavity without generating any gap, whereby a functional film element with higher detection accuracy than the prior art can be provided.

Claims: A process of manufacturing a functional film element, which comprises the steps of: placing a covering plate (25) on one side of a window spacer plate (26), which is provided with at least one window (29), to cover and close said window (29); placing a through-hole plate (27) with through-holes (22, 23) on the other side of said window spacer plate (26) so that said through-holes communicate with said window (29), and said through-holes (22, 23) are elongated outward from said window so as to only partially overlap said window; laminating and integrally firing a green sheet of each of said plates (25, 26, 27) in such a manner that the ratio t/w of thickness t of said window spacer plate (26) to width w of each of through-holes (22, 23) in its minor axis direction is one or more, to produce a ceramics substrate formed with a cavity in which said through-holes (22, 23) communicate with said window (29); and forming a functional film (33) on the.outer surface of said covering plate (25). The process of manufacturing a functional film element as set forth in claim 1, wherein each said through-hole (22, 23) has a width of 300 µm or less in the direction of its minor axis. The process of manufacturing a functional film element as set forth in claim 1 or 2, wherein each said through-hole (22, 23) provided in said through-hole plate (27) has the shape of elongated circle, oval or rectangular. A ceramics functional film energy conversion element, having a window spacer plate (26) provided with at least one window (29); a covering plate (25) laminated on one side of said window spacer plate (26) covering and closing said window (29); a through-hole plate (27) laminated on the other side of said window spacer plate (26) and having through-holes (22, 23) which communicate with said window (29); and a functional film (33) on the surface of said covering plate (25) away from said window (29); wherein said covering plate (27), said window spacer plate (26) and said through-hole plate (27) are a ceramics structure integrated by integral firing of respective green sheets; and wherein said through-holes (22, 23) are elongate and have a major axis direction in the direction of their elongation and a minor axis direction transverse thereto, characterised in that said through-holes (22, 23) only partially overlap said window, and the ratio t/w of the thickness t of said window spacer plate (26) to the width w of each said through-hole (22, 23) is one or more. An element according to claim 4, wherein each said through-hole has a width in its minor axis direction of 300 µm or less. An element according to claim 4 or 5 wherein each said through-hole has the shape of elongated circle, oval or rectangular.

Inventors: NANATAKI TSUTOMU, TAKAHASHI NOBUO, NANATAKI, TSUTOMU, TAKAHASHI, NOBUO, Nanataki, Tsutomu, Esupoa Toyoake
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1998
Main classification: B32B18
IPC codes: H01L41, G01L7, G01D21, H01L35