Patent Publication Number: US-2020298588-A1

Title: Thermal print head and thermal printer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2019-050956 filed in Japan on Mar. 19, 2019; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a thermal print head and a thermal printer. 
     BACKGROUND 
     A thermal print head is an output device that causes a resistor to generate heat and forms an image of a character, a figure, and the like on a thermal print medium and the like using the heat. The thermal print head is widely used in recording equipment, such as a barcode printer, a digital plate maker, a video printer, an imager, and a sticker printer. 
     In the thermal print head, a heat storage layer, a resistor layer, a conductor layer, and a protective film are disposed on a top surface of a ceramic substrate in this order. The resistor layer, the conductor layer, and the protective film are disposed from the heat storage layer over the top surface of the ceramic substrate. The top surface of the porous ceramic substrate includes fine depressed holes, unlike the surface of the heat storage layer formed of a glass and the like. Therefore, for example, when the resistor layer and the like are formed by a sputtering method, a layer formed on the top surface of the ceramic substrate and a layer formed on inner wall surfaces of the holes separately grow, thus possibly forming an interface between the layers and a crack on an edge between the top surface of the ceramic substrate and the inner wall surface of the hole. 
     When the protective film is formed along surfaces of the crack and the like formed on the resistor layer and the conductor layer, a loss, such as cracking, occurs also on the surface of the protective film. When the protective film is formed to cover the crack without being formed inside the gaps of the crack and the like of the resistor layer and the conductor layer, the protective film possibly peels off from the resistor layer and the conductor layer, or a loss, such as cracking, possibly occurs on the surface of the protective film. 
     When the peeling and the loss occur on the protective film, a corrosive substance, such as a sulfur component and a water content, in the air enters from the part, thus possibly causing corrosion and disconnection of the conductor layer forming an electrode. The corrosion and the disconnection of the conductor layer significantly occur at the proximity of the interface between the conductor layers and the crack. 
     The corrosion and the disconnection of the conductor layer occurred in the thermal print head, for example, degrade print quality or decrease a product lifetime of the thermal print head, thus reducing reliability of the thermal print head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a thermal print head according to an embodiment; 
         FIG. 2  is a cross-sectional view of a head substrate according to the embodiment; 
         FIG. 3  is a top view of the head substrate according to the embodiment; 
         FIG. 4  is a cross-sectional view of a circuit substrate according to the embodiment; 
         FIG. 5  is a drawing describing a connection of the head substrate to the circuit substrate according to the embodiment; 
         FIG. 6  is a block diagram of a thermal printer according to the embodiment; 
         FIG. 7  is a cross-sectional view of the head substrate according to Modification 1; 
         FIG. 8  is a drawing describing an effect of a barrier layer; 
         FIG. 9  is a drawing describing the effect of the barrier layer; 
         FIG. 10  is a drawing describing the effect of the barrier layer; and 
         FIG. 11  is a drawing describing the effect of the barrier layer. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments provide a thermal print head including: a first heat storage layer formed on a substrate; a heat generator formed on the first heat storage layer; an electrode formed from the first heat storage layer to the substrate and electrically connected to the heat generator; and a barrier layer that covers the electrode and is formed by a CVD method. 
     The following describes the embodiment of the present invention with reference to the drawings. The embodiment is an example, and the technical scope of the present invention is not limited to this. The drawings are schematically illustrated, and dimensions and the like are different from the actual dimensions and the like. 
       FIG. 1  is a top view of a thermal print head  100  according to the embodiment. As illustrated in  FIG. 1 , the thermal print head  100  includes a heatsink  20 , a head substrate  30 , and a circuit substrate  40 . The head substrate  30  and the circuit substrate  40  are secured adjacent to one another to a principal surface of the heatsink  20  with an adhesive. For the adhesive, a double-sided adhesive tape and a thermosetting resin adhesive, such as a soft silicone resin, can be used. Here, in  FIG. 1 , a direction X is a main-scanning direction and a direction Y is a sub-scanning direction as a moving direction of a print medium. 
     The heatsink  20  is a flat plate formed of a metal having a high thermal conductivity, for example, aluminum. 
     The head substrate  30  has a function of printing on a print medium. As illustrated in  FIG. 1 , the head substrate  30  is a member having the main-scanning direction X in a longitudinal direction. As illustrated in  FIG. 2 , the head substrate  30  includes a supporting substrate  9 , a first heat storage layer  10   a  constituting a glaze layer, a second heat storage layer  10   b , a resistor layer  11 , a conductor layer  12 , a barrier layer  17 , and a protective film  13 . 
     The supporting substrate  9  is formed of an insulator material having a heat resistance, and formed of a ceramic, such as alumina. The supporting substrate  9  may be SiN, SiC, quartz, AlN, or a fine ceramic containing Si, Al, O, N, and/or the like. The supporting substrate  9  is, for example, a rectangular flat plate having a plate thickness of about 0.5 mm to 1.0 mm. Since the supporting substrate  9  is formed of a porous ceramic, fine depressed holes are formed on a top surface  9   a  of the supporting substrate  9 . 
     The first heat storage layer  10   a  and the second heat storage layer  10   b  are formed on the top surface  9   a  of the supporting substrate  9 . The first heat storage layer  10   a  and the second heat storage layer  10   b  are formed of, for example, glass powders containing SiO 2 . The first heat storage layer  10   a  is disposed limited to the proximity of a heat generator  14 . The second heat storage layer  10   b  is disposed separated from the first heat storage layer  10   a  in the Y-axis direction. On top surfaces of the first heat storage layer  10   a  and the second heat storage layer  10   b , fine depressed holes are not formed. 
     The resistor layer  11  is formed on the first heat storage layer  10   a  and the second heat storage layer  10   b  from the first heat storage layer  10   a  to the second heat storage layer  10   b . The resistor layer  11  is formed of, for example, a TaSiO-based, a NbSiO-based, a TaSiNO-based, or a TiSiCO-based electric resistor material. 
     The conductor layer  12  is laminated to be formed on the resistor layer  11 . The conductor layer  12  is formed containing a metal, such as Al, Cu, and an AlCu alloy, as a main material. The resistor layer  11  exposed from the conductor layer  12  functions as the heat generator  14 . 
     The resistor layer  11  and the conductor layer  12  possibly have interfaces between the layers and cracks at the proximity of edges between the top surface  9   a  of the supporting substrate  9  and inner wall surfaces of the depressed holes formed on the top surface  9   a.    
     The barrier layer  17  covers the resistor layer  11  and the conductor layer  12  formed from the first heat storage layer  10   a  to the second heat storage layer  10   b . Gaps of the interfaces between the layers, the cracks, and the like formed on the top surfaces of the resistor layer  11  and the conductor layer  12  are filled with the barrier layer  17 . On a top surface of the barrier layer  17 , the interfaces between the layers and the cracks are not formed. The barrier layer  17  is, for example, formed of SiON by a chemical vapor deposition (CVD) method. 
     The protective film  13  is formed on the barrier layer  17 . The protective film  13  is formed of a hard and fine insulator material having a high thermal conductivity, such as a SiO 2  film, a SiN film, a SiON film, and a SiC film. A material of the surface of the protective film  13  containing at least Si and carbon is preferable because the thermal conductivity increases. 
       FIG. 3  is a top view of the head substrate  30  according to the embodiment. Hatched portions in  FIG. 3  indicate the heat generators  14 . As illustrated in  FIG. 3 , the heat generators  14  are formed in an array shape in the main-scanning direction X with pitches P. A pixel density (dot/inch) in printing on the print medium is determined depending on the areas of the heat generators  14  and the pitches P. The heat generators  14  have one ends connected to individual electrodes  15  and the other ends connected to a common electrode  16 . The conductor layer  12  serves as the individual electrodes  15  and the common electrode  16  illustrated in  FIG. 3 . The individual electrodes  15  have widths G and are arranged having the pitches P. The width of the common electrode  16  may be larger than the widths of the individual electrodes  15 . 
     The circuit substrate  40  illustrated in  FIG. 1  has a function to supply a current to the head substrate  30  based on a control of a control device  80  (not illustrated in  FIG. 1 ). As illustrated in  FIG. 4 , the circuit substrate  40  is disposed on the principal surface of the heatsink  20  so as to be adjacent to the head substrate  30 . The circuit substrate  40  is a substrate whose material is epoxy and the like, and a wiring whose material is copper is performed on the circuit substrate  40 . A predetermined number of driving ICs  41  and connectors  44  are mounted to the circuit substrate  40 . 
     The driving ICs  41  are mounted corresponding to the number of the heat generators  14  of the head substrate  30 . The driving IC  41  is a control element having a switching function configured to control the current supplied to the heat generator  14 . Specifically, the driving IC  41  controls the current supply from a power supply device  90  for each heat generator  14  of the head substrate  30  based on a control signal received from the control device  80  via the connector  44 . 
       FIG. 5  is a drawing describing a connection of the head substrate  30  to the circuit substrate  40 . The driving IC  41  includes output-side terminals electrically connected to the individual electrodes  15  and the common electrode  16  of the head substrate  30  via bonding wires  42 . 
     As illustrated in  FIG. 4 , the driving IC  41  and the bonding wires  42  are sealed by a sealing material  43  formed of an epoxide-based resin. The sealing material  43  protects the bonding wires  42  and the driving IC  41  that connects the head substrate  30  to the circuit substrate  40 . The sealing material  43  may also protect the individual electrodes  15  and the common electrode  16  of the head substrate  30 , and a wiring part of the circuit substrate  40 . While an epoxy resin coating material as a thermosetting resin is mainly used for the sealing material  43 , a silicone resin having flexibility to some extent is used after thermosetting in some cases. 
     Here, a method for manufacturing the head substrate  30  will be described with reference to  FIG. 2 . The supporting substrate  9  in specified dimensions is prepared, and the first heat storage layer  10   a  and the second heat storage layer  10   b  formed of glass is formed on the top surface  9   a  of the supporting substrate  9 . For the first heat storage layer  10   a  and the second heat storage layer  10   b , for example, a glass paste in which a glass powder containing SiO 2  is mixed with an organic solvent is printed on the supporting substrate  9 , and subsequently, sintering is performed to form the first heat storage layer  10   a  and the second heat storage layer  10   b.    
     Subsequently, the resistor layer  11  and the conductor layer  12  are laminated in this order on the first heat storage layer  10   a  and the second heat storage layer  10   b  over the first heat storage layer  10   a  and the second heat storage layer  10   b  with a thin film forming apparatus, such as a sputtering apparatus. 
     Subsequently, etching removal of the conductor layer  12  is performed at the part on which the heat generator  14  is formed. In the example illustrated in  FIG. 3 , the etching removal of the conductor layer  12  is performed with a width G and a length L. The resistor layer  11  at the part on which the etching removal of the conductor layer  12  has been performed serves as the heat generator  14 . 
     Subsequently, the barrier layer  17  that covers the individual electrodes  15 , the common electrode  16 , and the heat generators  14  is formed. For the barrier layer  17 , for example, a film containing SiON is formed by the CVD method. The CVD method is one of the methods for synthesizing materials using a chemical reaction. The CVD method has various variations depending on chemical species to be supplied and required properties. For example, a heat CVD method, a catalyst chemical vapor deposition, a light CVD method, a plasma CVD method, an epitaxial CVD method, an atomic layer deposition, and a metal-organic vapor phase epitaxy are included. The heat CVD method and the plasma method, which use heat for controlling the chemical reaction, are used together in some cases. 
     As a characteristic of the CVD method, an advantage that film formation with a uniform thickness is ensured even on an uneven surface compared with a vacuum evaporation method, such as a Physical Vapor Deposition (PVD) method is provided. Accordingly, the barrier layer  17  is formed by covering the loss, such as the interfaces between the layers and the cracks, formed on the resistor layer  11  and the conductor layer  12 . The loss, such as the interfaces, the cracks, and the like, is not generated on the top surface of the barrier layer  17 . 
     Furthermore, the protective film  13  that covers the barrier layer  17  is formed. To connect the individual electrodes  15  and the common electrode  16  to the circuit substrate  40  by the bonding wires  42 , openings are provided to the barrier layer  17  and the protective film  13  at positions corresponding to the individual electrodes  15  and the common electrode  16 . The head substrate  30  is manufactured as described above. 
     Next, the performance of the head substrate  30  will be described. The head substrate  30  is supplied with a current between the individual electrodes  15  and the common electrode  16  based on the control by the circuit substrate  40 . At a part where the resistor layer  11  contacts the conductor layer  12 , since the current supplied from the circuit substrate  40  flows through the conductor layer  12  where a resistance is low, the resistor layer  11  does not generate heat. However, at a part where the conductor layer  12  has been etched away, since the current flows through the resistor layer  11  where the resistance is high, the resistor layer  11  generates heat. The resistor layer  11  at the part where the conductor layer  12  has been etched away functions as the heat generator  14 . 
     Next, a thermal printer  200  that includes the above-described thermal print head  100  will be described with reference to  FIG. 6 .  FIG. 6  is a schematic block diagram of the thermal printer  200 . As illustrated in  FIG. 6 , the thermal printer  200  includes the above-described thermal print head  100 , a platen roller  50 , and a conveyance mechanism  60 . 
     The conveyance mechanism  60  conveys a print medium  70  and an ink ribbon  71  adhered to the print medium  70  using a conveyance medium  61  in the sub-scanning direction Y. The platen roller  50  presses the print medium  70  and the ink ribbon  71  onto the proximity of the heat generator  14  of the head substrate  30  together with the conveyance medium  61 . The driving IC  41  of the circuit substrate  40  receives a control signal from the control device  80  and supplies a current from the power supply device  90  to the heat generators  14  corresponding to pixels of an image to be printed so as to cause the corresponding heat generators  14  to generate heat. The driving IC  41  controls On/Off of energization to the heat generators  14  at a high speed corresponding to a moving speed of the print medium  70  by the conveyance mechanism  60 . The heat generator  14  generates heat while the current is supplied. An ink of the ink ribbon  71  pressed onto the head substrate  30  by the platen roller  50  melts only at a part positioned on the heat generator  14  during the heat generation, and adheres to the print medium  70 . 
     The heat remaining after melting the ink of the ink ribbon  71  is radiated via the first heat storage layer  10   a  having a small thermal capacity, the supporting substrate  9 , and the heatsink  20 . Since the first heat storage layer  10   a  has the small thermal capacity, the temperature of the first heat storage layer  10   a  changes at high speed following the fast temperature change of the heat generator  14 . When the first heat storage layer  10   a  keeps the high temperature state for a long time after the current supply to the heat generators  14  is stopped, the high temperature state continues also at the periphery of the heat generators  14  to which the current is not supplied. As a result, a blurred image is formed on the print medium  70 . In the thermal print head  100  according to the embodiment, since the temperature of the first heat storage layer  10   a  changes at high speed following the fast temperature change of the heat generator  14 , the thermal printer  200  can form a clear image on the print medium  70  during the fast movement of the print medium  70 . 
     As described above, the thermal print head  100  according to the embodiment includes the barrier layer  17  formed by the CVD method, and the barrier layer  17  covers the electrodes formed on the top surface  9   a  of the supporting substrate  9  formed of a porous ceramic. As illustrated in  FIG. 8 , on the top surface  9   a  of the supporting substrate  9  formed of the porous ceramic, a fine depressed hole  9   f  is provided. Therefore, for example, when the resistor layer  11  and the like are formed by the sputtering method, as illustrated in  FIG. 9 , a layer formed on the top surface  9   a  of the supporting substrate  9  and a layer formed on an inner wall surface of the hole  9   f  separately grow, thus forming a loss  12   g , such as an interface between the layers and a crack, on an edge  9   g  between the top surface  9   a  of the supporting substrate  9  and the inner wall surface of the hole  9   f  in some cases. 
     As illustrated in  FIG. 10 , since the barrier layer  17  formed by the CVD method is formed while filling the loss  12   g , such as an interface between the layers and a crack, the loss does not occur on the barrier layer  17 . Furthermore, the protective film  13  is formed on the barrier layer  17 . Since the barrier layer  17  includes an edge  17   g , the protective film  13  formed on the barrier layer  17  includes a loss  13   g  in some cases. However, a corrosive substance penetrated from the loss  13   g  is blocked by the barrier layer  17 . Therefore, forming the barrier layer  17  ensures avoiding corrosion and disconnection of the conductor layer forming the electrodes due to the corrosive substance penetrated from the loss  13   g . Accordingly, the reliability of the thermal print head can be improved. 
     Here, a case where the barrier layer  17  is not formed will be described. On the top surface  9   a  of the supporting substrate  9 , the fine depressed hole  9   f  is provided. Therefore, as illustrated in  FIG. 9 , for example, when the resistor layer and the like are formed by the sputtering method, a film of the top surface  9   a  of the supporting substrate  9  and a film of the inner wall surface of the hole  9   f  separately grow, thus forming the loss  12   g , such as an interface between the layers and a crack, on the edge  9   g  between the top surface  9   a  of the supporting substrate  9  and the inner wall surface of the hole  9   f  in some cases. When the protective film  13  is formed over the loss  12   g  by, for example, the sputtering method, as illustrated in  FIG. 11 , the loss  13   g  is easily generated also on the protective film  13 . When a sulfur component and a water content in the atmosphere penetrate from the loss  13   g , the conductor layer  12  forming the electrodes is corroded, thus possibly causing disconnection after the elapse of a long time. 
     Since the corrosion of the conductor layer  12  can be suppressed by disposing the barrier layer  17 , the first heat storage layer  10   a  and the second heat storage layer  10   b  can be separately disposed. By disposing the first heat storage layer  10   a  limiting to the proximity of the heat generator  14 , the thermal capacity of the first heat storage layer  10   a  can be decreased. Therefore, even when the heat generator  14  is turned On/Off at high speed, the temperature of the first heat storage layer  10   a  can be changed at high speed corresponding to the On/Off, and the temperature at the proximity of the heat generator  14  also changes at high speed corresponding to the control speed of the heat generator  14 . Accordingly, the thermal responsiveness of the thermal print head  100  can be improved. 
     Modification 1 
     In the above-described description, as illustrated in  FIG. 2 , the case where the first heat storage layer  10   a  and the second heat storage layer  10   b , the resistor layer  11 , the conductor layer  12 , the barrier layer  17 , and the protective film  13  are formed in this order on the top surface  9   a  of the supporting substrate  9  is described. However, the layer configuration of the thermal print head  100  is not required to be limited to this. For example, as illustrated in  FIG. 7 , a second protective film  13   b  may be formed between the conductor layer  12  and the barrier layer  17 . By disposing a first protective film  13   a  and the second protective film  13   b , the durability of the thermal print head  100  can be further improved. 
     While, in the above-described description, the case where the barrier layer  17  is disposed also on the first heat storage layer  10   a  and the second heat storage layer  10   b  is described, the barrier layer  17  may be disposed only between the first heat storage layer  10   a  and the second heat storage layer  10   b.    
     In the above-described description, the case where the head substrate  30  includes the first heat storage layer  10   a  and the second heat storage layer  10   b  is described. However, the number of the heat storage layers included in the head substrate is not required to be limited. For example, the number of the heat storage layers may be one, or three. When the three heat storage layers are disposed, the barrier layer may be disposed on a part other than the respective heat storage layers, or the barrier layer may be disposed over the respective heat storage layers. 
     While, in the above-described description, the case where the ink ribbon  71  is used for printing to the print medium  70  is described, the printing method is not required to be limited to this. For example, the print medium  70  may be a thermal paper. When the print medium  70  is the thermal paper, the pixels of the print medium  70  positioned on the heat generator  14  during the heat generation by the energization are colored through heat sensing. 
     While  FIG. 5  illustrates the state where the driving IC  41  switches every wiring, the wiring pattern illustrated in  FIG. 5  is one example, and it is not necessary to limit to this. For example, the common electrode  16  may be directly connected to the connector  44  without interposing the driving IC  41 . 
     While  FIG. 6  illustrates the state where the print medium  70  is continuously conveyed, the print medium  70  may include a plurality of separated print media. The material of the print medium  70  is various, for example, a paper, a plastic, a film, and a metal, and the material is not required to be limited. 
     According to the thermal print head according to at least one embodiment described above, since the thermal print head includes the barrier layer that covers the electrodes and is formed by the CVD method, the reliability of the thermal print head can be improved. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and sprit of the inventions.