THERMAL PRINT HEAD AND METHOD OF MANUFACTURING THE SAME

Abstract: A thermal printhead 1 according to the present invention includes a multiplicity of heating elements 4a formed in a row on an obverse surface of an elongated substrate 2 at a portion which is offset widthwise toward one longitudinal side of the substrate, and a protective film 8 formed on the obverse surface of the substrate 2 at the widthwise offset portion for covering the heating elements 4a. The protective film 8 is formed to extend on the obverse surface of the substrate 2 continuously from the widthwise offset portion onto one longitudinal side surface 2a of the substrate. A longitudinal edge 8b of the protective film 8 directed toward the other longitudinal side of the substrate 2 is tapered.

Description: TECHNICAL FIELD The present invention relates to a thermal printhead which is designed to perform printing on a recording medium thermosensitively or by thermal transfer. It also relates to a method of making such a thermal printhead. BACKGROUND ART Fig. 7 is a schematic plan view of a prior art thin-film thermal printhead. The thermal printhead 51 includes an elongated rectangular substrate 52 having longitudinal sides 52a and 52b. The substrate 52 has a surface formed with a linear resistor layer 53 extending longitudinally adjacent one longitudinal side 52a. A band-like region between the resistor layer 53 and the longitudinal side 52a of the substrate 52 is provided with a common wiring pattern 54. The common wiring pattern 54 has opposite ends extending to the other longitudinal side 52b of the substrate 52. One of the opposite ends of the common wiring pattern 54 is connected to a common terminal 55. Fig. 8 is an enlarged plan view showing a principal portion of the thermal printhead 51. The common wiring pattern 54 includes a plurality of comb-tooth electrodes 54a extending therefrom. Individual electrodes 56 have respective one end extending between two adjacent comb-tooth electrodes 54a. The other end of each individual electrode 56 extends adjacent to a drive IC 57 mounted on the substrate 52 and is connected, via a non-illustrated wire-bonding pad, to an output terminal of the drive IC 57. As indicated by the chain lines in Fig. 8, the resistor layer 53 is laid over the comb-tooth electrodes 54a and the individual electrodes 56 alternate therewith, thereby defining a heating element 53a between each two adjacent comb-tooth electrodes 54a. Thus, when power is applied to any individual electrode 56, current passes through a portion of the resistor layer 53 defined between two comb-tooth electrodes 54a sandwiching this individual electrode 56, consequently working as a heating element 53a. Fig. 9 is an enlarged sectional view showing a principal portion of the thermal printhead 51. The substrate 52 formed of an insulating material such as alumina ceramic material is provided, on a surface thereof, with a glaze layer 61 extending longitudinally at a portion adjacent to the longitudinal side 52a. The glaze layer 61 is formed with a resistor layer 53 in the form of a thin film for covering the glaze layer. Conductor layers 62a, 62b are formed on the resistor layer 53 in such a manner as to expose the resistor layer 53 at a portion at the top of the glaze layer 61. The exposed portion of the resistor layer 53 serves as heating elements 53a. The conductor layer 62b extending rightward in Fig. 9 serves as the individual electrodes 56, whereas the conductor layer 62a extending leftward in Fig. 9 serves as comb-tooth electrodes 54a. Further, an anti-oxidation film 63 and a protective film 64 are formed to cover the heating elements 53a and the conductor layers 62a, 62b while exposing the wire-bonding pad of each individual electrode 56. An aggregate board divisible into a plurality of substrates 52 may be used for forming the glaze layer 61, the resistor layer 53, the conductor layers 62a, 62b and the anti-oxidation film 63A. A protective film 64 is further formed on the aggregate board thus formed with the anti-oxidation film 63. Specifically, the protective film 64 may be formed in the following manner for example. First, a resist layer 65 is formed to cover the region, including the wire-bonding pads, which is not to be covered with the protective film 64. Then, a Ta 2 O 5 film for example may be formed by chemical vapor deposition or sputtering. Subsequently, the resist layer 65 is etched away. The aggregate board thus formed with the protective film 64 is then divided into a plurality of individual substrates 52 to each of which drive ICs 57 are mounted. The drive ICs 57 and the individual electrodes 56 are connected by wire-bonding for example to provide a thermal printhead 51. However, according to the method described above, the thermal printhead 51 is prepared by dividing the aggregate board after forming the protective film 64. Accordingly, the divisional surface 66, namely the side surface of the substrate 52 along the longitudinal side 52a at longitudinal edge of each layer 61, 53, 62a, 63, are not formed with the protective film 64. Thus, the divisional surface 66 is exposed. Generally, the division of the aggregate board is performed, for example, by providing a nick along a scribing line and then applying stress therealong. This results in irregularities at the divisional surface 66, which is, therefore, in poor condition.. In this way, the divisional surface 66 of the thermal printhead 51 is not only in a poor condition but also is exposed. Accordingly, during handling of the thermal printhead 51 such as incorporation into a casing, the edge of the substrate 52 along the longitudinal side 52a or the edges of the layers 61, 53, 62a, 63 may chip or break if the side surface of the substrate 52 along the longitudinal side 52a comes into contact with the casing or any other object. Further, since the protective film 64 is formed by first forming the resist layer 65 and then removing the resist layer 65 after the growth of the protective film, an edge 64a of the protective film 64 results in a step which is equivalent in height to the thickness to the protective layer 64. When the thermal printhead 51 having such a step at the edge 64a of the protective film 64 is incorporated in an image forming apparatus, an edge of a recording paper 67 transferred in contact with the heating elements 53a may get caught at the step. In such a case, the image forming apparatus recognizes a paper jam because the recording paper 67 does not reach the heating elements 53a, thus resulting in stoppage of the apparatus. Another method of making a thermal printhead is also proposed wherein a plurality of substrates 71 are laminated in such a manner as to expose a film-forming portion of each substrate 71 which is subsequently formed with a protective film by sputtering (See e.g. JP-A-5-92596), as shown in Fig. 10. At the time of forming a glaze layer on the substrate 71, a plurality of projections 72 each made of the same material as the glaze layer are formed in a row on the surface of the substrate. These projections 72 are provided to prevent the laminated substrates 71 from rubbing against each other to avoid damaging of the individual electrodes or other elements due to such rubbing. However, such a method makes it necessary to provide a mounting space on the surface of the substrate 71 for mounting the plurality of projections, which may bar increasing the density of the wiring pattern. Moreover, depending on the location of the projections 72, the edge of the recording paper may get caught at the projections 72 during the recording operation of the thermal printhead. Document EP 0 395 001 A1 discloses a thermal printhead and a method for making the same, whereas multiple layers, including a protective layer, are deposited on an upper surface of a substrate, whereas a longitudinal side surface of the substrate is free of any layers. Document EP 0 654 354 A2 discloses a method for making a thermal printhead, whereas multiple layers are deposited on an upper surface of a substrate, including a protective coating, whereas after depositing of all layers the substrate is subjected to a cutting step, providing a longitudinal side surface of the substrate, which is free of any layers. Document EP 0 398 582 A1 discloses a thermal transfer recording system using a thermal head, whereas the substrate of the thermal head is covered on a top surface, a longitudinal side surface and a rear surface with multiple layers, including a protective layer, so that the top surface and the longitudinal side surface are each covered at least with three layers, whereas the rear surface is covered with at least two layers, all including a protective film. DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a thermal printhead which could be prevented from partially braking due to the bad surface condition and to minimize the likelihood that a recording paper gets caught in during the recording. It is further an object of the present invention to provide a manufacturing method for such a thermal printhead. The object is solved by a thermal printhead according to claim 1 and a method for forming a thermal printhead according to claim 5. Claim 2 to 4 refer to preferred embodiments of the thermal printhead, claims 6 to 9 are related to preferred realizations of the inventive method according to claim 5. Various features and advantages of the present invention will become clear from the embodiment described below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a principal portion of a thermal printhead embodying the present invention. Fig. 2 is a perspective view of the thermal printhead shown in Fig. 1 in a state in which an anti-oxidation film and a protective film are not formed. Fig. 3 illustrates an aggregate board used for providing the thermal printhead shown in Fig. 1. Fig. 4 is a sectional view showing substrates which are laminated for forming a protective film in making the thermal printhead shown in Fig. 1. Fig. 5 is a perspective view showing an example of jig for keeping the laminated state of the substrates in forming the thermal printhead shown in Fig. 1. Fig. 6 is an enlarged sectional view showing a principal portion of the protective film in making the thermal printhead shown in Fig. 1. Fig. 7 is a schematic plan view showing a prior-art thermal printhead. Fig. 8 is an enlarged plan view showing a principal portion of the thermal printhead shown in Fig. 7. Fig. 9 is an enlarged sectional view showing a principal portion of the thermal printhead shown in Fig. 7. Fig. 10 illustrates a method of forming another prior-art thermal printhead. BEST MODE FOR CARRYING OUT THE INVENTION A preferred embodiment of the present invention will be described with reference to Figs. 1 through 6. In this embodiment, a thin-film thermal printhead is employed. As shown in Figs. 1 and 2, a thermal printhead 1 includes an elongated substrate 2 formed of an insulating material such as alumina ceramic material. The substrate 2 is formed, at a widthwise offset portion on an obverse surface thereof, with a glaze layer 3 extending longitudinally (i.e. in the direction of the arrow AB in Fig. 2). The glaze layer 3 is formed by printing and baking a glass paste material for example. The glaze layer has a smoothly arched cross section due to the flow of the glass component during the baking. On the substrate 2 and the glaze layer 3, a resistor layer 4 in the form of a thin film is formed to cover the glaze layer 3. For instance, the resistor layer 4 may be formed by CVD (chemical vapor deposition) or sputtering TaSiO 2 to have a thickness of 500 to 1500 Å. Conductor layers 5a, 5b are formed on the resistor layer 4. The conductor layers 5a, 5b are etched or otherwise processed to expose a predetermined portion of the resistor layer 4 over the top of the glaze layer 3. The exposed portion of the resistor layer 4 serves as heating elements 4a. As clearly shown in Fig. 2, a plurality of slits 6 extending widthwise of the substrate 2 (i.e. in the direction of the arrow CD in Figs. 1 and 2) are formed in the resistor layer 4 and the conductor layers 5a, 5b. The slits 6 may be formed, for example, by etching the resistor layer 4 and the conductor layers 5a, 5b. The provision of the slits 6 allows each of the heating elements 4a to be driven individually. The conductor layer 5b extending rightward from the heating elements 4a in Figs. 1 and 2 serves as individual electrodes. The conductor layer 6a extending leftward from the heating elements 4a in Figs. 1 and 2 serves as a common electrode. The heating elements 4a are covered with an anti-oxidation film 7 which is formed in a manner such as to expose wire-bonding pads of the individual electrodes. The anti-oxidation film 7 may be formed, for example, by depositing SiO 2 into a thickness of 3000 to 6000 Å through CVD or sputtering. A protective film 8 is formed on the anti-oxidation film 7. The protective film 8 extends continuously so that one longitudinal edge 8a thereof lies on a longitudinal side surface 2a of the substrate 2. The protective film 8 on the side surface 2a reaches the boundary between the side surface 2a and the reverse surface of the substrate 2. The other longitudinal edge 8b of the protective film 8 is tapered. Thus, the longitudinal edge 8b of the protective film 8 gradually reduces in thickness toward the end portion. The protective film 8 may be formed by depositing Ta 2 O 5 or Si 3 N 4 into a thickness of from 2 to 4 µm through CVD or sputtering. The method of forming the protective film 8 will be briefly described below with reference to Figs. 3 through 6. As shown in Fig. 3, a glaze layer 3, a resistor layer 4, conductor layers 5a, 5b and an anti-oxidation film 7 are formed on an aggregate board 15 which is divisible into a plurality of substrates 2 each formed subsequently into a thermal printhead 1. However, a protective film 8 is formed on each of the individual substrates 2, instead of the aggregate board 15, after dividing the aggregate board 15 along scribing lines 16. According to this embodiment, a protective film 8 is formed on the plurality of substrates 2 simultaneously instead of forming a protective film 8 on each of the substrates 2 in a separate step. Specifically, as shown in Fig. 4, six substrates 2 obtained from an aggregate board 15 for example are laminated in their thickness direction in widthwise offset relationship. Thus, the portions of the substrates later formed with a protective film 8 are exposed as viewed in plan. In this condition wherein most of the anti-oxidation film 7 of each substrate covering the heating elements 4a as well as the side surface 2a of each substrate 2 next to the anti-oxidation film are exposed, a protective film 8 is formed simultaneously on the respective substrates 2. In Fig. 4, the substrates 2 are depicted as if they are in close contact with each other. In fact, however, since the glaze layer 3 and the other layers are formed on each substrate 2, a small gap is formed between the adjacent substrates 2 when they are laminated together. To keep such a state of the substrates in which the portions later formed with a protective film 8 are exposed, a jig 11 shown in Fig. 5 maybe used for example. The jig 11 may comprise a base member 12 and a pair of mounts 13 provided at longitudinally opposite ends of the base member 12. Each of the mounts 13 is formed with a plurality of retreated steps 14 (six steps in this embodiment) each extending to a side surface 13a. The retreated steps 14 are formed as part of a continuous serration. The pair of mounts 13 are disposed so that the respective serrated portions directed toward each other. The substrates 2 are placed so as to bridge between the retreated steps 14 of the paired mounts 13. With the posture of the substrates 2 kept by the jig 11, a protective film 8 is formed on the exposed portion of each substrate. The protective film may be formed by depositing Ta 2 O 5 or Si 3 N 4 into a thickness of from 2 to 4 µm through CVD or sputtering for example. Thus, the protective film 8 is formed not only on the anti-oxidation film 7 but also continuously onto the side surface 2a of the substrate 2. The sputtering may be continued at least until the protective film 8 reaches the boundary between the side surface 2a and the reverse surface of the substrate 2. Due to such a structure, even if the side surface 2a of the substrate 2 and the edges of the layers 4, 5a, 7 are irregular, the protective film 8 covers these surface and edges. Therefore, even if the side surface 2a of the substrate 2 or the edges come into contact with a casing or any other object during handling of the thermal printhead 1 such as incorporation thereof into the casing, it is possible to prevent the side surface 2a of the substrate 2 and the edges of the layers 4, 5a, 7 from being chipped or broken. Further, according to the above-described method of forming a protective layer 8, the portion to be formed with the protective layer 8 is exposed, whereas the portion not to be formed with such a layer is shielded by another substrate 2 laid thereon. Accordingly, it is unnecessary to form a resist layer on each substrate at a portion not to be formed with a protective layer 8, thereby eliminating the need for etching the resist layer. Unlike a resist layer, each substrate 2 laid over an adjacent substrate at a portion not to be formed with a protective film 8 is not bonded to that adjacent substrate 2. Accordingly, during the growth of the protective film 8, the edge of each substrate 2 laid over the adjacent substrate is somewhat spaced, allowing the protective film to grow at the overlapping portion. However, the film growth is slower at the overlapping portion of the substrate 2 than at the exposed portion of the substrate. As a result, the other longitudinal edge 8b of the protective film 8 is tapered to have a progressively reducing thickness toward its extremity. In the conventional thermal printhead shown in Fig. 9, the edge 64a of the protective film 64 provides a stepped portion. According to this embodiment, by contrast, the longitudinal edge 8b of the protective film 8 is tapered as shown in Fig. 1. Accordingly, with an image forming apparatus provided by incorporating the thermal printhead 1 in a casing or the like, the tip portion of a recording paper 21 as a recording medium is not caught at the longitudinal edge 8b of the protective film 8 during transfer of the recording paper 21. More specifically, in the image forming apparatus incorporating the thermal printhead 1, the protective film 8 is tapered at the longitudinal edge 8b to allow smooth paper transfer, as clearly shown in Fig. 1. Accordingly, a paper jam due to the provision of the protective film 8 is avoided. In the embodiment described above, the protective film 8 is formed simultaneously on six substrates 2. However, the number of substrates 2 to which the protective film 8 is simultaneously formed may be appropriately varied. Further, the design of the jig 11 for keeping the position of the substrates 2 may be modified in various ways. Although the present invention is applied to a so-called thin-film thermal printhead in the above embodiment, it is clear that the present invention may be also applied to a thick-film thermal printhead.

Claims: A thermal printhead comprising: an elongated substrate (2); a multiplicity of heating elements (4a) formed in a row on an obverse surface of the substrate (2) at a portion which is offset widthwise toward one longitudinal side surface (2a) of the substrate (2); a conductor layer (5a, 5b) formed only on the obverse surface of the substrate (2), the conductor layer (5a, 5b) having an edge located in alignment with said one longitudinal side surface (2a) of the substrate (2); a protective film (8) is formed on the obverse surface of the substrate (2) at the widthwise offset portion for covering the heating elements (4a) and the conductor layer (5a, 5b), a longitudinal edge (8b) of the protective film (8) opposite said one longitudinal side surface (2a) of the substrate (2) being tapered; and the protective film (8) extends continuously from the obverse surface of the substrate (2) onto said one longitudinal side surface (2a) of the substrate (2) while also covering said edge of the conductor layer (5a, 5b), located in alignment with said one longitudinal side surface (2a) of the substrate (2). The thermal printhead according to claim 1, wherein the protective film (8) extends to a boundary between said one longitudinal side surface (2a) and a reverse surface of the substrate (2). The thermal printhead according to claim 1 or 2, wherein a resistor layer (4) is interposed between the obverse surface of the substrate (2) and the conductor layer (5a, 5b), the resistor layer (4) having an edge located in alignment with said one longitudinal side surface (2a), the protective film (8) also covering said edge of the resistor layer (4). The thermal printhead according to any one of claims 1 to 3, wherein an anti-oxidation layer (7) is interposed between the conductor layer (5a, 5b) and the protective film (8) on the obverse survace (20) of the substrate (2), the anti-oxidation layer (7) having an edge located in alignment with said one longitudinal side surface (2a), the protective film (8) also covering said edge of the anti-oxidation layer (7). A method of forming a thermal printhead which comprises an elongated substrate (2); a multiplicity of heating elements (4a) formed in a row on an obverse surface of the, substrate (2) at a portion which is offset widthwise toward one longitudinal side surface (2a) of the substrate (2); a conductor layer (5a, 5b) formed on the obverse surface of the substrate (2), the conductor layer (5a, 5b) having an edge located in alignment with said one longitudinal side surface (2a) of the substrate (2); and a protective film (8) formed on the obverse surface of the substrate (2) at the widthwise offset portion for covering the heating elements (4a) and the conductor layer (5a, 5b), a longitudinal edge (8b) of the protective film (8) directed opposite said one longitudinal side surface (2a) of the substrate (2) being tapered; the method comprising the steps of: forming heating elements (4a) and a conductor layer (5a, 5b) on an aggregate board (15) which is divisible into a plurality of substrates (2); dividing the aggregate board (15) into a plurality of substrates (2) each formed with the heating elements (4a); and forming a protective film (8) on each of the substrates (2) in a manner such that the protective film (8) extends continuously from the obverse surface of the substrate (2) onto said one longitudinal side surface (2a) of the substrate (2) while also covering said edge of the conductor layer (5a, 5b), located in alignment with said longitudinal side surface (2a) of the substrate (2). The method of forming a thermal printhead according to claim 5, wherein the protective film (8) is formed to extend to a boundary between said one longitudinal side surface (2a) and a reverse surface of the substrate (2). The method of forming a thermal printhead according to claim 5 or 6, wherein the protective film (8) is formed in a state in which the plurality of substrates (2) are laminated in a thickness direction but shifted widthwise from each other so that portions to be formed with the protective film (8) are exposed. The method of forming a thermal printhead according to any one of claims 5 to 7, wherein a resistor layer (4) is interposed between the obverse surface of the substrate (2) and the conductor layer (5a, 5b), the resistor layer (4) having an edge located in alignment with said one longitudinal side surface (2a); wherein the formation of the resistor layer (4) is performed before the aggregate board (15) is divided; and wherein the formation of the protective film (8) is performed in a manner such that it also covers said edge of the resistor layer (4), located in alignment with said one longitudinal side surface (2a) of the substrate (2). The method of forming a thermal printhead according to any one of claims 5 to 8, wherein an anti-oxidation layer (7) is interposed between the conductor layer (5a, 5b) and the protective film (8) on the above surface (12a) of the substrate (2), the anti-oxidation layer (7) having an edge located in alignment with said one longitudinal side surface (2a); wherein the formation of the anti-oxidation layer (7) is performed before the aggregate board (15) is divided; and wherein the formation of the protective film (8) is performed in a manner such that it also covers said edge of the anti-oxidation layer (7), located in alignment with said one longitudinal side surface (2a) of the substrate (2).

Inventors: HAYASHI HIROAKI, YAMADE TAKUMI, YOKOYAMA EIJI, HAYASHI, HIROAKI, YAMADE, TAKUMI, YOKOYAMA, EIJI, HAYASHI, Hiroaki Rohm Co., Ltd., YAMADE, Takumi Rohm Co., Ltd., YOKOYAMA, Eiji Rohm Co., Ltd.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1998
Main classification: B41J2
IPC codes: B41J2