Thermal head

Abstract: In a planar thermal head and a thermal display incorporating it, a plurality of parallel first electrode lines 24 are formed on one side of an electrically resistive layer 25, and a plurality of parallel second electrode lines 26 are formed on the other side of the resistive layer, and oriented to intersect the first parallel electrode lines. A pair of electrodes respectively constituted by part of or by other means connected to one of the first electrode lines 24 and one of the second electrode lines 26 are positioned on opposite sides of the resistive layer 25 for causing a current flow through the resistive layer 25, and a part of the resistive layer 25 through which a current is made to flow by the pair of electrodes 24, 26 forms a thermal dot.

Description: Field of Invention This invention relates to a planar (two-dimensional) thermal head in which the thermal dots are arranged in matrix. Background of Invention Figure 1 is a cross-sectional view showing a known planar thermal head. It comprises a base 1 of ceramic material. Running parallel to the X axis (left to right in the drawing) on the base 1 are a plurality of equally-spaced X wiring lines 2, and over these a thermally insulating layer 3 consisting of a polyimide or other thermal insulating material is applied. A plurality of resistive elements 4 are formed over the thermally insulating layer 3 to act as heat-generating bodies. One side of each resistive element 4 is connected to the X wiring via a through-hole conductor 5. The other side is connected to one of Y wiring lines 6 which are arranged on the thermally insulating layer 3 at equally-spaced intervals running parallel to the Y axis direction (normal to the page). The resistive elements 4 thus form a matrix of thermal dots in the X and Y directions. A thermal display device employing a planar (two dimensional) thermal head similar to that shown in Figure 1 is described in Japanese Laid Open Patent Application No. 208787/1987. Figure 2 shows a cross-sectional view of this planar thermal display which comprises a glass substrate 11, a plurality of X wiring lines 12, a thermally insulating layer 13 of a material such as polyimide, a plurality of through-hole conductors 15, a plurality of Y wiring lines 16, a heat-sensitive, temperature-indicating layer 17, and a plurality of transparent resistive elements 14. To display an image, a single transparent resistive element 14 is selected by selecting one of the X wiring lines 12 and one of the Y wiring lines 16. Specifically, a voltage of 0 is applied to the selected terminal of the X wiring lines 12 and a voltage of 2/3E to the nonselected terminals, and a voltage of E is applied to the selected terminal of the Y wiring lines 16 and a voltage of 1/3E to the nonselected terminals. Thus a voltage of E is applied across selected transparent resistive element 14 while a voltage of 1/3E is applied across the non-selected transparent resistive elements 14. As a result of receiving three times the voltage of the other electrodes, the transparent resistive element 14 selected according to the image data generates nine times as much heat. This local heating induces a colour change in the temperature-indicating layer 17. In the prior art planar thermal heads having the structure shown in Figure 1, the following problems occur: (a) The structure and fabrication process are complex, due to the need to provide one through-hole conductor 5 passing through the thermally insulating layer 3 and one resistive element 4 for each thermal dot. (b) As noted in the Japanese Laid Open Patent Application referred to above, a substantial amount of the heat generated by the resistive elements 4 diffuses to the thermally insulating layer 3. This planar thermal head therefore requires a large driving power. The planar thermal display with the structure shown in Figure 2 suffers from the problem of complex structure and difficult fabrication due to the need to connect the X and Y lines electrically by through-hole conductors passing through the temperature-indicating layer 17. Further complications of structure and fabrication arise from the need to provide the same number of transparent resistive elements as the dots. The cost of these devices is accordingly high. Japanese Patent Application No. JP-A-60109863 discloses a thermal head comprising a heating resistor layer having a set of parallel electrode wires affixed to an upper surface and a perpendicular set of parallel electrode wires affixed to a lower surface. When a given voltage is applied between a pair of wires chosen from the respective sets, electric current flows through the resistor layer at the intersection of the wires, generating a heating element that corresponds to a dot in a matrix. A protective layer is provided above and between the upper electrode wires. This arrangement suffers from the same problem of requiring a large driving power because of the dissipation of heat. Figure 3 illustrates a known thermal head similar to that in JP--A-60109863. Summary of the Invention The invention provides a thermal head having a matrix of thermal dots of electrically resistive material selectively heatable through X and Y electrode lines located on opposite sides of a layer of the resistive material for causing current to flow through the resistive layer, wherein parts of the layer through which current is to flow form the matrix of thermal dots; characterised in that thermally insulating material fills interstices between the X or Y electrode lines. With the structure described above, the electrode lines are selected according to an image signal for printing and a voltage is applied to the selected electrode lines on the two sides of the resistive layer. Current then flows from one electrode line through the resistive layer to another electrode line on the other side. This current produces heat from the region (thermal dot) of the resistive layer located between the two electrode lines. This heat is conducted through the electrode line on the upper surface of the resistive layer to the outside, where it is used for printing. Advantageously the first electrode lines are spaced at equal intervals, and the second electrode lines are spaced at equal intervals, preferably with the first and second electrode lines oriented to intersect at right angles. The insulating material between electrode lines prevents the diffusion of heat to regions adjacent to the thermal dot. DRAWINGS Figure 1 is a cross sectional view of a prior art planar thermal head referred to above; Figure 2 is a cross sectional view of another prior art display device using a planar thermal head referred to above; Figure 3 is a cross sectional view showing part of a further prior art thermal head; Figure 4 is a diagram showing patterns of electrode lines for use in conjunction with the invention; Figure 5A and 5B show diagrammatically examples of electrode lines for use with the invention; Figure 6 is a plan view of a structure of a thermal head and peripheral circuits mounted on the same board; Figure 7 is a cross-sectional view of an embodiment of the invention; Figure 8 is a cross-sectional view of a thermal head incorporated in a display device; Figure 9 is a diagram showing temperature-optical density characteristic of a thermally reversible material used in the device of Figure 8; Figure 10 is a cross sectional view of a further embodiment of display device, similar to that in Figure 8; Figure 11 is an oblique view of a structure in which the display device of Figure 8 or 10 is mounted on the same board as its peripheral circuits; Figure 12a is a perspective view, partially cut away, showing a further embodiment of the invention; Figure 12b through 12f are sectional views along lines B-B, C-C, D-D, E-E and F-F in Figure 12a; Figure 13a is a plan view showing a further embodiment of the invention; Figures 13b 13c and 13d are sectional views along lines B-B, C-C and D-D in Figure 13a; Figure 14 is a schematic diagram showing part of a matrix of thermal dots; Figure 15a is a plan view showing a further embodiment of the invention; Figures 15b, 15c and 15d are sectional views along lines B-B, C-C and D-D in Figure 15a; Figure 16a is a plan view showing a further embodiment of the invention; and Figures 16b and 16c are sectional views along lines B-B and C-C in Figure 16a. Detailed Description of Preferred Embodiments With reference to the prior art illustrated in Figure 3, a substrate 21 is of an electrically insulating material such as a ceramic, glass or plastic, or a metal material, the surface of which has been treated to make it electrically nonconductive. A glazed glass layer 22 is formed on the substrate 21. The substrate 21 and glass layer 22 form the base 23. The glass layer 22 retains heat. On the surface of the glass layer 22 are a plurality of first or X electrode lines 24 spaced at substantially equal intervals running parallel in one direction (in Figure 3 this direction is perpendicular to the page). The first electrode lines 24 are formed on the surface of the glass layer 22, by plating, etching or other means. A continuous electrically resistive layer 25 is applied to the surface of the glazed glass layer 22 and the first electrode lines 24. The electrically resistive layer 25 could be made from tantalum nitride, for example. On the surface of the electrically resistive layer 25 are placed a plurality of second or Y electrode lines 26, spaced at substantially equal intervals and oriented perpendicular to the first electrode lines 24. The second electrode lines 26 can be formed by plating or etching. A region of the electrically resistive layer 25 positioned over one of the first electrode lines 24 and under one of the second electrode lines 26 forms a thermal dot. To the surface of the electrically resistive layer 25 including the second electrode lines 26 is attached a protective layer 27. Preferably, material for the protective layer 27 should be electrically insulating, should have high thermal conductivity, and should adhere tightly to the second electrode lines 26 and the electrically resistive layer 25. For example SiO₂ and Ta₂O₅ are suitable materials. The thickness can be from 2 to 3 micrometers, for example. If the thermal head according to Figure 3 is used in a thermal printer, printing can be accomplished by placing heat-sensitive paper in contact with the protective layer 27, without moving the heat-sensitive paper, or by placing an inked ribbon and paper in contact with the protetive layer 27. Both schemes avoid sliding friction on the surface. The operation of this type of thermal head is described with reference to Figure 4. Electrode lines 24₁ to 24 n are connected through switches A₁, A₂,....,A n to the negative terminal of a power supply E, and the electrode lines 26₁ to 26 n are connected through switches B₁, B₂,...,B n to the positive terminal of the power supply E. The switches A₁, A₂,...,A n and B₁, B₂,...,B n are opened and closed in accordance with an image signal. Assume that in accordance with the image signal switches A₂, A₃, and B₂ are now closed. Current then flows from the positive terminal of the power supply E through switch B₂, the electrode line 26₂, the resistive layer 25, the electrode lines 26₂ and 26₃, the switches A₂ and A₃ to the negative terminal of the power supply E. This current flow generates heat in pixels defined by the thermal dots labelled P₁ and P₂. This heat is conducted to the outside through the second electrode lines 26 and the protective layer 27 in Figure 3. If heat-sensitive paper is in contact with the protective layer 27, the portions of the paper over the points P₁ and P₂ will change colour. This arrangement is structurally simple because there is no need to provide an individual through-hole conductor and resistive element for each dot. Fabrication is simplified. In particular, a wet fabrication process (chemical etching) can be used. The lead wire routing and connections of the electrode lines can also be simplified because the electrode lines are oriented in the X and Y directions and each layer has a thin-film configuration. The density of the thermal dots can be freely altered by changing the spacing of the electrode lines. Moreover, because the electrically resistive layer 25 of the thermal head is located between the electrode lines 24 and 26, the electrode lines can be made to extend to cover most of the layer, so that radiation of residual heat from a thermal dot after selective heat generation is greatly improved, and retention of heat inside the device is reduced. The result is an overall improvement in the thermal efficiency of the planar thermal head. Figures 5a and 5b show examples of patterns for the second electrode lines 26. The second electrode lines 26 may have a constant width, but in the examples illustrated in Figures 5a and 5b the width (surface area) of the second electrode lines 26 is greater in the regions of the thermal dot than in other regions. These lines are accordingly labelled 26a and 26b. In this type of pattern the dissipation of heat generated at one thermal dot toward adjacent thermal dots via the electrode lines 26a or 26b is restricted by the thinned interconnecting portions. Heat generated at the thermal dot is also conducted via the first electrode lines 24, but this heat is uniformly dissipated to the substrate 21 via the heat-retaining glass layer 22. Figure 6 shows a top view of a device in which the planar thermal head of Figures 3 and 4 and its peripheral control circuits are mounted on the same base. Shown in this drawing are several signal terminals 31, shift registers 32, and drivers 33, the matrix wiring 34 connecting the drivers 33 to the electrode lines of the planar thermal head, and the base 35. Due to the simplicity of the structure and fabrication of a planar thermal head as described above, it is easy to fabricate the head and its peripheral control circuits on the same base as shown in Figure 6. Figure 7 is a cross-sectional view of a first embodiment of the invention, with parts identical to the corresponding parts in Figure 3 indicated by the same numbers. In Figure 7 the interstices between adjacent electrode lines 26 on the electrically resistive layer 25 are filled with a thermally insulating material 28 having the same height as the electrode surface of the second electrode lines 26. The purpose of the insulating material 28 is to prevent the diffusion of heat to adjacent second electrode lines 26; that is, to regions adjacent to the thermal dot. This structure improves the conduction of heat toward the exterior. Moreover the provision of the insulation layer reduces leakage current between adjacent electrode lines, thereby preventing generation of heat at non-selected thermal dots. The points noted in relation to Figures 4 through 6 apply also to this embodiment of the invention. The embodiment as described above has the following effects: (a) Because the planar thermal head has a structure in which electrode lines are located on opposite sides of a continuous resistive layer, it is not necessary to provide separate conductors and resistive elements for each thermal dot. The structure is therefore simple and easy to manufacture, and the density of the thermal dots can be increased. The yield of the manufacturing process can also be improved. (b) Due to the above structure, heat generated at the thermal dots is not dissipated into the thermally insulating layer, but is discharged to the outside with very high efficiency. Less driving power is therefore required. (c) The pattern and thickness of the electrode lines can be altered in suitable ways for fine control of the caloric output and the locations in which heat is generated. These features of the above described embodiment make it well suited for use in thermal printers and thermal display devices. By way of background to the invention, Figure 8 is a cross-sectional view showing a display device incorporating the thermal head similar to that shown in Figure 3, which includes a base 23 formed of a substrate 21 and a glazed glass layer 22, a plurality of first electrode lines 24, an electrically resistive layer 25, and a plurality of second electrode lines 26, which are all similar to those shown in Figure 1 with identical reference numerals. In addition, there is provided a thermosheet 29. The thermosheet 29 has its surface coated with a substance having as its principal component a thermally reversible material with a temperature versus optical density characteristic as shown in Figure 9. As can be seen from Figure 9, this material is characterized in that its thermal transition zone is located at a relatively high temperature, and in that the colour change is highly sensitive to temperature variations (i.e. there is small retention of heat). An example of a pigment with these preperities is silver mercury iodide (Ag₂HgI₄). The thermosheet 29 made of this material is secured tightly to the entire surface of the electrically resistive layer 25 and the electrode lines 26, by adhesive or other means. Thermosheets 29 are available in blue, yellow, brown, and other colours, so by stripping off one thermosheet 29 and reattaching another, the display can be modified to make display in different colours to suit particular purposes. In this case the thermosheet 29 should be attached in such a way that it can be removed. When voltage is applied to an electrode line 24 and an electrode line 26, current passes from the electrode line 24 through the electrically resistive layer 25 to the electrode line 26 (or in the reverse direction), heating the region (thermal dot) of the electrically resistive layer 25 located between the electrode line 24 and the electrode line 26. This heat is conducted through the electrode line 26 to the thermosheet 29. The area of the thermosheet 29 thus heated changes colour. Because of the small heat retentivity of the thermally reversible material of the thermosheet 29 noted in Figure 9, the contrast of the colour change on the thermosheet 29 is extremely high. In this embodiment, an excellent display is obtained from natural light incident on the outer surface of the thermosheet 20. Another advantage is that since the colour change takes place at a comparatively high temperature, and with high sensitivity, no cooling is needed to restore the original colour. If necessary the changed colour can be maintained for an extended time by repeated heating of the thermosheet 29 at short intervals. Figure 10 is a cross-sectional view of another thermal display device, with parts identical to the corresponding parts in Figure 8 indicated by the same numbers. In Figure 10 a protective layer 30 is applied to the surface of the electrically resistive layer 25 and the electrode lines 26. The protective layer 30 could be made of Ta₂O₅ or SiO₂, for example. The thickness can be 2 to 3 micrometers, for example. A layer of thermally reversible material 29a is applied directly to the surface of the protective layer 30. The operation of this display device is the same as the operation of the device in Figure 8. Another possible structural addition is a thin, transparent protective layer (not illustrated) applied to the thermosheet 29a in Figure 10. With this arrangement, it is possible to write, by a suitable pen, and erase on the surface of this protective layer, in superposition of the displayed image. The structures shown in Figures 8 and 10 can be combined with peripheral control ciruits in a manner shown in Figure 11. In Figure 11 the peripheral control circuits (shift registers and drivers) 36 and the matrix wiring 34 are mounted on the same base 35 as the display itself. Figures 12a through 12f show a further embodiment of the invention. In this embodiment, the interstices between adjacent electrode lines 26 are filled with a thermally insulating material 28, as shown in Figure 12b, in the same way as the embodiment of Figure 7. Moreover, the interstices between adjacent first electrode lines 24 are filled with a thermally insulating material 41, as is best seen from Figure 12f. The electrically resistive layer 42 of this embodiment comprises three sublayers 43, 44 and 45. The middle layer 44 (Figure 12d) is a continuous layer of an electrically resistive material. The upper and the lower layers 43 and 45 (Figures 12c and 12e) are layers 43a, 45a of an insulating material with spots of resistive material 43b, 45b arranged in matrix, i.e. at positions of the thermal dots. The spots of the resistive material 43b, 45b, are in conductive contact with the resistive layer 44. Thus at the positions of the thermal dots, the resistive material is continuous in a vertical direction to form resistive elements. The provision of the insulating material 43a, 45a, surrounding the spots of the resistive material 43b, 45b reduces diffusion of heat from the selected thermal dot to the neighbouring regions, which reduces the power necessary to heat the selected thermal dot. As a modification, the layer 43 or the layer 45, or both can be eliminated. If both of the layers 43 and 45 are eliminated, the structure is similar to that shown in Figure 7 except for the provision of the insulating material 41. Figures 13a to 13d show a further embodiment of the invention. This embodiment is generally similar to the embodiment of Figures 12a through 12f, except that there is further provided a diode element 51 for each thermal dot. The diode element has one electrode, e.g., anode 51a connected to a first electrode line 24 and has the other electrode, e.g., cathode 51b connected to the spot of resistive material 45b. The diode element 51 can be formed of a polysilicon layer deposited by CVD (chemical vapour deposition) and selectively doped with p-type and n-type impurities, and etched to have the desired pattern. The reverse biased p-n junction is shorted or bypassed by an Al layer 52. The first electrode lines 24 can be formed to be in contact, at one side thereof, with the anodes 51a of the diode elements arranged in line (along the first electrode line) with each other. In the illustrated configuration, each thermal dot is formed at a position where the cathode 51b of each diode element 51 is exposed to and connected with the respective layer 45b rather than at an intersection between a first and a second electrode lines 24 and 26. The insulating layer 45a over the first electrode lines 24 serves to prevent a current from flowing directly from the first electrode lines 24 into the resistive layer 44. The circuitry of the matrix of the thermal dots with diode elements is shown in Figure 14. The function of the diode elements 51 is to prevent heat generation at nonselected thermal dots. If the diode elements 51 are not provided, there can be a small current flowing through the resistive element R of nonselected thermal dots. For instance, if a thermal dot at an intersection between electrode lines B3 and A2 is selected, a part of the current which has passed the resistive element R23 at the intersection between B3 and A2 may then pass through the resistive element R22 at the intersection between A2 and B2 and into the line B2. As a result, heat is generated at the nonselected thermal dot at the intersection between B2 and A2. Provision of the diode elements avoids such undesired heat generation at the nonselected thermal dots. Figures 15a through 15d show a further embodiment of the invention. This embodiment is similar to the embodiment of Figures 13a through 13d except that the diode elements 51 have their anodes 51a connected at the lower surfaces thereof with respective first electrode lines 24. Each first electrode line 24 comprises a wide lower part 24a and a thin upper part 24b continuous with the wide lower part 24a and connected, at its upper surface, with the anode 51a of the diode element 51. Figures 16a through 16c show a further embodiment of the invention. This embodiment is similar to the embodiments of Figure 13a through 13d and 15a through 15d except that each diode element 51 comprises a stack of p-type and n-type layers, with the stack extending in the vertical direction, i.e. perpendicularly to the plane of the planar thermal head. The lower end of the stack forming an anode 51d is connected to a first electrode line 24. The upper end of the stack forming a cathode 51e is exposed to and connected with the resistive layer 44.

Claims: A thermal head having a matrix of thermal dots of electrically resistive material selectively heatable through X and Y electrode lines (24,26) located on opposite sides of a layer (25,42) of the resistive material for causing current to flow through the resistive layer, wherein parts of the layer (25,42) through which current is to flow form the matrix of thermal dots; characterised in that thermally insulating material (28,41) fills interstices between the X or Y electrode lines (24,26). A thermal head according to claim 1, further characterised in that the resistive layer (25,42) is a continuous layer of a resistive material extending throughout the matrix area. A thermal head according to claim 1 or claim 2, further characterised in that the resistive layer (42) further comprises a layer (45,43) having spots (45b,43b) of resistive material formed at the positions of the thermal dots, the spots of resistive material being in electrically conductive contact with a continuous layer (44) of resistive material, and having insulating material (45a,43a) surrounding the spots (45b,43b) of the resistive material. A thermal head according to any of the preceding claims, further characterised in that the X and Y electrode lines (24,26) are in direct contact with the opposite surfaces of the layer of resistive material. A thermal head according to any of the preceding claims, further characterised in that a first set (24) of the X or Y electrode lines is on a surface of an electrically insulating base (21,22). A thermal head according to claim 5, further characterised in that diode elements (51) are provided individually associated with the thermal dots, each having one electrode connected to the electrode lines of the first set on the insulating base (21,22) and having the other electrode connected to the resistive layer (25), with all the diode elements (51) having their electrodes of the same polarity connected to the electrode lines (24) of the first set.

Inventors: ISOBE MINORU OKI ELECTRIC INDU, ISOBE, MINORU OKI ELECTRIC INDUSTRY CO.,LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1987
Main classification: H01L49
IPC codes: B41J2