A thermal head

Abstract

 A thermal head for high speed printing at high temperature comprises a dielectric substrate (10,15), a first thin SiO. layer (20) formed on the substrate, a heater layer (30) on the first SiO₂ layer, conductive layers (40) for coupling the heater layers to an external circuit, a second thin SiO₂ layer (50) on the heater layer, and a protection layer (60) over the second SiO₂ layer. The width of the heater layer is less than 30 µm. The preferable thickness of the SiO₂ layers is less than 2 µm. Preferably, the heater layer is made of tantalum nitride. The presence of the first SiO₂ layer (20) and the narrow width of the heater layer (30) allow the heater to operate at a higher input power, and hence at a higher temperature.

Description

[0001] This invention relates to a thermal head for a thermal printer, and in particular to such a head which operates at higher temperature and at a high power rating, for providing clearer and more rapid printing.

[0002] With the advent of computer technology and advances in the arts of data processing, data communication and/ or facsimile communication, requirements for increased speed of information handling have become more stringent. One known type of rapid printing is a high speed thermal printer, which has a thermal head and a thermal printing paper, and operates on the principle that a thermal head, heated to a high temperature according to the pattern of a desired character to be printed, selectively changes the colour of the thermal paper. A thermal printer has the advantage that it can print not only a predetermined pattern of characters, but also any desired pattern including pictures, Chinese characters, and/or Arabian characters.

[0003] A thermal printer is a kind of dot printer which composes the pattern to be printed as a plurality of dots, and a thermal head has a plurality of heat cells arranged, for example, in a straight line for printing these dots. As the thermal paper moves in a direction perpendicular to the straight line of heat cells, the heat cells are selectively heated, so that the colour of the thermal paper is selectively changed. Thus the desired pattern is printed on the thermal paper.

[0004] We have proposed some thermal heads, such as that shown in United States Patent No. 4,136,274 (British Specification No. 1,524,347).

[0005] Fig.1 of the accompanying drawings shows the cross- section of a prior thermal head disclosed in the above- mentioned US patent. In Fig.1, a glazed alumina substrate 10 has a glazed layer 15 of 40-80,^pm thickness. A heater layer 30 has a thickness of 1000 A to 2000 A and is made of, for example, tantalum nitride (Ta₂N). A conductive layer 40 is attached to the heater layer 30 for providing electrical coupling of the heater line to an external circuit. An S10₂ layer 50, which has a thickness of 1-3pm, prevents oxidation of the heater line. A protection layer 60 reduces wear on the heaters due to friction with the thermal paper. The layer 60 is made of, for example, Ta₂0₅ with a thickness of 3-10µm.

[0006] The structure of Fig.1 has the advantages that fluctuation in the resistance of a heater layer is small, and the life-time of the head is long, provided the power applied to the head is small. However, it has the disadvantage that the power capacity of the head is low. That is to say, the prior thermal head cannot have a high power capacity, and cannot, therefore, provide a high temperature. Operation at a high temperature is essential for high speed printing. For instance, the highest power consumption of a prior thermal head is up to 1.2 watts when the width of the heater layer is 100µm, the length of the heater layer is 215µm, the sheet resistance of the heater layer is 17 ohms/square. The heater layer is heated for 30 minutes with a pulse signal having a pulse width of 1 msec and a period of 50 msec. If that prior thermal head is heated with a power higher than 1.2 watts, the heater is damaged.

[0007] Fig.2 is an explanatory drawing of a sheet resistance, in which a rectangular heater 30 has a side length L, and conductors 100 and 102 have a width L. In that configuration, the resistance between conductors 100 and 102 is independent of the length L, but depends solely upon the thickness of the heater 30 and the material of which the heater 30 is made. Therefore, the sheet resistance of the heater 30 is defined by the resistance between the conductors 100 and 102, and is expressed as R ohms/square, if the resistance appearing between the conductors 100 and 102 is R ohms.

[0008] It is an object of the present invention to provide a new and improved thermal head which can operate at high temperature for high speed printing, and has long life.

[0009] According to the invention a thermal head comprising a dielectric plane substrate; a plurality of heater layers, each having an elongated finger, insulated from one another; conductive layers attached to both extreme ends of the fingers of the heater layers for coupling each heater layer to an external circuit; and an insulation layer on the heater layers; is characterised by an insulation layer made of Si0₂ (silicon dioxide) between the substrate and the heater layers; and in that the width of each finger of the heater layers is less than 30pm.

[0010] Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings wherein:

Fig.1 is a cross-sectional view of a prior thermal head as described above,

Fig.2 is an explanatory drawing of sheet resistance,

Fig.3 is a cross section of one form of thermal head according to the present invention,

Fig.4 is a plan view of a prior thermal head,

Fig.5 is a plan view of the thermal head according to the present invention,

Fig.6 is a plan view of another thermal head according to the present invention,

Fig.7 is a plan view of a further thermal head according to the present invention,

Fig.8 is an experimental curve which shows the effect of the structure of the present invention, and

Fig.9 shows other experimental curves.

[0011] Fig.3 shows the cross section of one form of the present thermal head, in which a glazed alumina substrate 10 has a glazed layer 15 of 40-80pm thickness. An Si0₂ layer 20 has a thickness of 1-6µm and is provided for improving the thermal characteristics of the head. A heater layer 30 has a thickness of 1000 A to 2000 A, and is made of, for instance, tantalum nitride (Ta₂N). A conductive layer 40 is connected to the heater layer 30 for providing electrical coupling of the heater line to an external circuit. An Si0₂ layer 50 with a thickness of 1-3pm prevents oxidation of the heater line. A protection layer 60 reduces wear of the heaters due to friction with a thermal paper. The protection layer 60 is made of, for instance, Ta₂0₅ with a thickness of 3-10µm.

[0012] The feature of the structure of Fig.3 as compared with that of Fig.1 is the presence of the thin Si0₂ layer 20 between the glazed layer 15 and the heater layer 30, so that the heater layer 30 is enclosed between a pair of Si0₂ layers 20 and 50.

[0013] With the presence of the lower Si0₂ layer 20, the heater layer 30 can take more power and can, therefore, provide a high temperature. The effect of the presence of the lower SiO₂ layer 20 depends upon the width of the heater layer, as described later.

[0014] Table 1 below shows experimental results for three samples of thermal heads with the cross section of Fig.3.

[0015] The above experiments are accomplished by applying, for 30 minutes, a pulse signal with a pulse width of 1 msec and a period of 50 msec, and the power consumption shows the power of that pulse signal for which a heater layer is damaged within the 30 minute period. In the above three examples, the density of the heater layer is 8 dots/mm, the sheet resistance of the heater layer is 17 ohms/square, and the thickness of the Si0₂ layers 20 and 50 is 2pm.

[0016] Fig.5 is a plan view of a thermal head of the Examples 1 and 2 above, in which heater layers 31a - 31d are in zigzag fashion or in a meandering or tortuous configuraiton as shown in Fig.5, with a width d₁=20µm, a spacing between adjacent fingers of the meandering pattern d₃=20µm, and a spacing between adjacent heater layers d₂=20pm. Example 2 of the above table is accomplished for a similar heater layer to that of Fig.5, but the width d₁ is 30µm, and the spacings d₂ and d₃ are 10pm.

[0017] It should be appreciated in the above experimentation that the power capacity is considerably increased when the width of the heater layer is less than 30µm and, therefore, high temperature and/or high speed printing operation is accomplished. In Example 1, when a power of 2.4 watts is applied to the heater layer, the heater layer is red-heated, and that red-heated layer is visible through the protection layer 60. Also, in Example 2, a red-heated heater layer is visible. Therefore, it should be noted in Examples 1 and 2 that the power capacity is large. In case of Example 3 in which the width of the heater layer is large, the power consumption is not increased.

[0018] Fig.4 is a plan view of a thermal head of Example 3, in which heater layers 30a-30d are straight,as shown in the figure, and the width d of each heater is 110µm. The spacing between adjacent heaters is 10µm, and the length A of each heater is 215pm. It should be noted that the power consumption in the experiment 3 is only 1.2 watts, which is considerably lower than that of the other Examples. Therefore, the conclusion is reached that it is preferable for the width of the heater line to be less than 30pm.

[0019] Fig.6 is a plan view of another embodiment of the present thermal head, in which a heater layer is in a meandering or tortuous configuration with a slit S in the layer. When the dot density of a thermal head is not so dense, the width of the heater layer is rather wide, and it cannot be less than 30µm. In that case, the slit S is provided in the tortuous pattern. In Fig.6, when the width d₁₀ of a finger of the heater layer is 50pm, a slit S with a width d₁₃=10µm is provided in the finger, so that the rest of the finger is in two parts d₁₁=d₁₂=20µm. Therefore, the effective width of the finger may be less than 30pm, and the high power capacity is obtained as shown in the Table 1.

[0020] Fig.7 is another plan view of a thermal head, or a conductive layer of the thermal head, according to the present invention. In Fig.7, the conductive layer 30a extends as two fingers 30a-l and 30a-4, between which a pair of fingers 30a-2 and 30a-3 are positioned. The layer 30a is coupled to a confronting layer 30b through the fingers 30a-1, 30a-2 and 30a-3, and through the finger 30a-4, and fingers 30b-2 and 30b-1. Thus, the width of each layer 30a or 30b is divided into four spaced-apart. fingers. When the width of each finger is the same as the spacing between the fingers, the width of each finger is only one-seventh of the width of the layer 30a or 30b. Therefore, even when the layer 30a or 30b is wide, the width of a divided finger can be less than 30pm for providing high temperature operation.

[0021] As described above, the important features of the present thermal head are that a thin Si0₂ layer is provided between a heater layer and the substrate, and that the width of a finger of the heater layer is less than 30pm. Figs.8 and 9 show experimental curves which prove the above features.

[0022] Fig.8 shows the curves of a step stress test on a thermal head, in which a pulse signal with a period of 20 msec and a pulse width of 0.5 msec is applied to each finger of a heater layer through a pair of conductive layers, and the structure of the heater layer is such that the width of each finger is 22µm, the spacing between adjacent fingers is 19.5pm, and the length of the tortuous portion of the heater is 230pm, as shown in the figure. The horizontal axis of Fig.8 shows the power of the pulse signal applied to each heater, and the vertical axis of Fig.8 shows the ratio AR/R in which R is the initial resistance of a heater, and ΔR is the change in resistance from the initial value. The test is carried out for 30 minutes for each input power, and each dot in the curve shows the result after a corresponding test of 30 minutes duration. The curve in Fig.8 shows the test results for a thermal head which has an Si0₂ layer between the heater layer and the substrate, the thickness of the Si0₂ layer being 2µm. It should be appreciated from the curve of Fig.8 that the sample being tested is not destroyed by the heat until the input power reaches 3.5 watts.

[0023] Similar tests were carried out by changing the pulse width of the input pulse from 0.5 msec to 2.5 msec, and ' similar results were obtained. The change in the resistance R(orAR/R) in Fig.8 is of no importance in the present test as far as the temperature of the heater, the power consumption of the heater, and/or the life of the heater is concerned. It is the fact that the input power can be high and the life of the sample (2) of the curve is long which is important.

[0024] It should be appreciated from the curve of Fig.8 that a sample which has an Si0₂ layer can accept a high input power, and has long life-time.

[0025] Fig.9 shows another test result, in which the pulse period is 20 msec, the pulse width is 2.5 msec and the width of the heater layer is 110µm. The horizontal axis shows the input power and the vertical axis shows the ratio ΔR/R. The test is carried out for 30 minutes for each input power. The curve (2) in Fig.9 shows the test result where an Si0₂ layer is provided between the heater layer and the substrate, and the curve (1) in Fig.9 shows the test result where no such Si0₂ layer is provided, i.e. the heater layer is located directly on the substrate.

[0026] It should be noted from Fig.9 that the heater layer is broken when an input power of less than 1 watt is reached.

[0027] According to the experimental results of Figs.8 and 9, the conclusion can be reached that two conditions (1) an Si0₂ layer is provided between the heater layer and the substrate, and (2) the heater is narrow, are necessary for applying high input power to the heater.

[0028] We have also carried out an experiment to replace the Si0₂ layer between the heater layer and the substrate by an Si₃N₄ layer, which has the property of preventing diffusion of a molecule and/or an atom. However, it has been found that the life-time of a thermal head provided with such Si₃N₄ layer is worse by 10% than the prior head without the Si0₂ layer.

[0029] Furthermore, we have carried out an experiment to replace the Si0₂ layer between the heater layer and the substrate by a tantalum oxide layer, which has the property that the melting point is high (the melting point of Si0₂ is 1710°C, and the melting point of tantalum oxide is 1870°C). However, it has been found that the life-time of a thermal head with tantalum oxide is worse than that with Si0₂ layer.

[0030] We have also experimented to change the heater layer from tantalum nitride to nickel. However, a thermal head with a nickel heater cannot be heated to red heat even though an SiO₂ layer is provided. Therefore, the tantalum nitride is superior to nickel as the material for the heater layer.

[0031] As described above, it has been proved by experiment that the temperature and the life time of a thermal head are improved by providing an Si0₂ layer between the heater layer and the substrate, and designing the width of a finger of the heater to be less than 30µm. The material of the heater layer is preferably tantalum nitride with a view to improving the operational temperature of the thermal head.

Claims

1. A thermal head comprising a dielectric plane substrate (10,15); a plurality of heater layers (30), each having an elongated finger (31a-31d), insulated from one another; conductive layers (40) attached to both extreme ends of the fingers of the heater layers; and an insulation layer (50) on the heater layers; characterised by an insulation. layer (20) made of Si0₂ (silicon dioxide) between the substrate and the heater layers; and in that the width of each finger of the heater layers at the region between the conductive layers is less than 30pm.

2. A thermal head according to Claim 1, characterised in that the heater layer (30) is made of tantalum nitride.

3. A thermal head according to Claim 1 or Claim 2, . characterised in that each finger (31a-31d) of the heater layers has a tortuous configuration.

4. A thermal head according to any preceding claim, characterised in that the width of each finger of the heater layers is wider than 30pm, and the finger has a longitudinal slit (S) along its centre for separating the finger into two paths (32a,33a---32d,33d), the width of each separated path being less than 30pm.

Drawing