EP0649748A2

EP0649748A2 - Thermal head for printers

Info

Publication number: EP0649748A2
Application number: EP94116818A
Authority: EP
Inventors: Takashi C/O Nec Corporation Ota; Koji C/O Nec Corporation Shigemura
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1993-10-26
Filing date: 1994-10-25
Publication date: 1995-04-26
Anticipated expiration: 2014-10-25
Also published as: DE69417835T2; EP0649748A3; EP0649748B1; DE69417835D1

Abstract

The invention relates to a thermal head for ink-jet printers or thermal printers, which has an array of heat generating zones (20) on a substrate (10) having an insulating surface (10a). Each heat generating zone is defined between two opposite electrodes (12,12') which are formed by patterning a conductor film deposited on the insulating surface (10a) of the substrate. That is, in the heat generating zones (20) the conductor film is absent. A patterned resistor (14) film, which is an anodically oxidizable film such as tantalum film, covers the electrodes and the insulating surface in the heat generating zones, and a surface layer of the resistor film is anodically oxidized to provide a protective layer (16). In this thermal head the anodic oxidation of the resistor film can be performed under optimal conditions since the conductor film is entirely covered with the resistor film. Therefore the thermal head becomes very good in durability, and it is possible to greatly reduce the thickness of the unoxidized part of the resistor film to thereby enhance responsiveness and reduce power consumption.

Description

This invention relates to a thermal head for ink-jet printers or thermal printers including thermal transfer printers.
Thermal printers and ink-jet printers are largely used as output devices for computers and wordprocessors.
Most of conventional thermal printers use a thermal head having an array of heat generating zones formed on an insulated substrate by thin-film techniques. The heat generating zones can be energized selectively individually to print dots in selected patterns. Each heat generating zone is defined between two opposite electrodes formed by patterning a conductor film, and a resistor film exists in each heat generating zone. The electrodes and the resistor film are covered with a protective layer. Heat generated in each heating zone is transferred to a thermographic paper or a heat-sensitive ribbon which makes contact with protective layer above the heat generating zone. Many of conventional ink-jet Printers also use a thermal head of fundamentally the same construction. In ink-jet printers the thermal head is placed in the ink, and each heat generating zone of the thermal head heats a fraction of the ink to boil and produce a bubble which causes a droplet of the ink to spout from a nozzle or orifice above the heat generating zone.
For example, JP 61-260604 A shows a thermal head for thermal printers. The thermal head is produced by the steps of depositing a resistor film such as a tantalum film on an insulated substrate, depositing a conductor film such as an aluminum or copper film on the resistor film, patterning the conductor film so as to form two opposite electrodes for each heat generating zone and expose the resistor film in each heat generating zone, anodically oxidising the surface of the resistor film in each heat generating zone and finally forming a protective layer such as an oxide or nitride layer on both the electrodes and the anodically oxidized resistor surface. In this thermal head the conductor film, resistor film and the protective layer are formed by sputtering methods. That is, the fabrication of the thermal head entails three different sputtering operations. Since sputtering operations are costly due to the expensiveness of sputtering apparatus and targets, the thermal head is relatively high in production cost.
It is possible to use the thermal head construction shown in JP 61-260604 A in ink-jet printers. However, in this case it is important that the protective layer should surely shield the underlying electrodes from the ink. Since the protective layer formed by sputtering is microscopically porous, it is necessary to increase the thickness of the protective layer. The thickened protective layer becomes large in heat capacity, and for this reason it is difficult to enhance the responsiveness and printing speed of the thermal head, and the power consumption of the thermal head increases. By experiments, it was necessary to form the protective layer to a thickness of at least 1.5 µm. By testing samples of the thermal head using a silicon substrate with a thermally oxidized insulating layer (1.33 µm thick) on the surface and having a 1.5 µm thick protective layer of silicon nitride, the responsiveness was about 10 kHz and the minimum input power for each heat generating zone to produce a bubble in the ink was 1000 W/mm².
JP 60-109850 A shows a thermal head for ink-jet printers. The thermal head is produced by the steps of depositing a resistor film (tantalum film) on an insulated substrate by sputtering, depositing a conductor film (aluminum film) on the resistor film, patterning the conductor film so as to form two opposite electrodes for each heat generating zone and exposing the resistor film in the heat generating zones and anodically oxidizing the surfaces of the electrodes and the exposed resistor film to form a protective layer of aluminum oxide on the electrodes and a protective layer of tantalum pentoxide Ta₂O₅ on the resistor surface.
The fabrication of this thermal head entails only two sputtering operations for forming the resistor and conductor films, respectively. This is favorable for a cost reduction. However, there are problems about the anodixally oxidized protective layers. The aluminum oxide layer on the electrodes and the tantalum oxide layer on the resistor film are chemically dissimilar and hence do not chemically bond to each other. Therefore, microscopic slits are liable to appear at the borders between these two layers as a cause of intrusion of the ink. Furthermore, durability of the thermal head may not be sufficient because during operation of the thermal head heat stresses will appear in the border regions of the two different protective layers. Another reason for the insufficient durability is the difficulty of employing optimal conditions for anodic oxidation of the electrodes and the resistor. Optimal conditions for anodic oxidation of the electrodes (aluminum) differ from optimal conditions for anodic oxidation of the resistor (tantalum), but the electrodes and the resistor have to be anodically oxidized simultaneously. Therefore, the actually employed conditions are not optimal for one or both of the electrodes and the resistor. By experiments the durability of this thermal head was at the order of 10⁶ pulses, and failures of the samples were always attributed to a break at the border between the two different protective layers.
It is an object of the present invention to provide an improved thermal head for ink-jet printers or thermal printers, which is enhanced in durability and relatively low in power consumption and can be made high in printing speed.
A thermal head according to the invention comprises a substrate having an insulating surface on which a plurality of heat generating zones are located in a predetermined arrangement, a patterned conductor film which lies on the insulating surface of the substrate and, for each of the heat generating zones, forms two opposite electrodes one of which terminates at an edge of the heat generating zone and the other at the opposite edge, a patterned resistor film which is formed of an anodically oxidizable material and continuously covers the patterned conductor film and the insulating surface in the heat generating zones, and a protective layer which is formed by anodic oxidation of a surface layer of the patterned resistor film.
The principal feature of the invention is that the anodically oxidizable resistor film is deposited after the deposition and patterning of the conductor film. Since the patterned conductor film is entirely covered with the resistor film, the anodic oxidation of the surface of the resistor film can be performed under optimum conditions for the employed resistor material. Therefore, a protective layer of very good quality can be formed to thereby enhance durability of the thermal head. Furthermore, it is possible to greatly reduce the thicknesses of the protective layer and the resistor film to thereby reduce power consumption of the thermal head and enhance responsiveness. A preferred example of anodically oxidizable resistor materials is tantalum.
The invention is useful for both ink-jet printers and thermal printers including thermal transfer printers.
A method of producing a thermal head according to the invention comprises the steps of (a) depositing a conductur film on an insulating surface of a substrate, (b) patterning the conductor film by photolithography to define a plurality of heat generating zones where the conductor film is removed and, for each of the heat generating zones, form two opposite electrodes one of which terminates at an edge of the heat generating zone and the other at the opposite edge, (c) depositing an anodically oxidizable resistor film on the patterned conductor film and exposed areas of the insulating surface of the substrate by a bias sputtering operation using an inert gas as the sputtering gas to cause intrusion of the atoms of the inert gas into the deposited resistor film, (d) patterning the resistor film by photolithography so as to leave the resistor film over the patterned conductor film and on the insulating surface in the heat generating zones, and (e) anodically oxidizing a surface layer of the patterned resistor film.

Fig. 1 is an enlarged top plan view of a part of a thermal head according to the invention;
Fig. 2 is a sectional view taken along the line 2-2 in Fig. 1;
Fig. 3 shows, in a sectional view corresponding to Fig. 2, a minor modification of the thermal head of Figs. 1 and 2;
Fig. 4 shows, in a sectional view corresponding to Fig. 2, another minor modification of the thermal head of Figs. 1 and 2;
Fig. 5 shows a good example of the plane shape of the resistor in and near a heat generating zone (hereinafter "heating zone" for the sake of simplicity) of the thermal head of Figs. 1 and 2;
Fig. 6 is a partial enlargement of Fig. 5 for explanation of the result of a simulation of the distribution of heat generation in the heating zone;
Figs. 7 to 9 are graphs showing variations in the distribution of heat generation with variations in several parameters of the resistor shape;
Fig. 10 shows another example of the plan shape of the resistor;
Fig. 11 is a partial enlargement of Fig. 10 for explanation of the result of a simulation of the distribution of heat generation;
Fig. 12 shows a different example of the plan shape of the resistor; and
Fig. 13 is a partial enlargement of Fig. 12 for explanation of the result of a simulation of the distribution of heat generation.

Figs. 1 and 2 show an essential part of a thermal head embodying the invention. A silicon substrate 10 is used as the body part of the thermal head. A surface layer of the silicon substrate 10 is thermally oxidized to form a silicon oxide layer 10a which serves as an insulating layer. The thermal head has an array of heating zones 20 on the insulated surface 10a of the substrate 10. The heating zones 20 are nearly square zones.
Each heating zone 20 is a gap between two opposite electrodes 12 and 12' which are formed by depositing a conductor film such as aluminum film on the insulating surface 10a of the substrate and removing unnecessary areas of the deposited film by a photolithography process. In this embodiment the conductor film is patterned, as shown in Fig. 1, so as to form a common electrode 12 which has a rectangularly projecting shape in a region adjacent to each heating zone 20 and a set of selective electrodes 12' which are rectangularly elongate and each extend opposite to and in alignment with the rectangular projection of the common electrode 12 for each heating zone 20. The selective electrodes 12' are individually addressable. Preferably, as shown in Fig. 2, each of the electrodes 12 and 12' is tapered in an end region adjacent to the heating zone 20. The tapering can be accomplished in the photolithography process for patterning the conductor film by controlling some factors such as etching time and the strength of adhesion of the photoresist layer to the conductor film.
A resistor film 14 lies on the electrodes 12, 12' and the insulating surface 10a of the substrate 10 in the heating zones 20. Tantalum is a preferred resistor material in view of adequateness of its volume resistivity and excellence in heat resistance. In fabricating the thermal head, the resistor film 14 is deposited after patterning the electrodes 12, 12' over substantially the entire surface area of the thermal head, and by a photolithography process the resistor film is selectively removed in areas widthways between the rectangularly elongate electrodes 12, 12'. As the result, the resistor film 14 continuously covers the electrodes 12, 12' and the insulating surface 10a in the heating zones 20. The resistor film existing in each heating zone 20, indicated at 14a, serves as a heating resistor.
On the entire surface area of the patterned resistor film 14 (inclusive of the heating resistors 14a) there is a protective layer 16 which is formed by anodic oxidation of a surface layer of the resistor film 14.
When a suitable voltage is applied between the common electrode 12 and any of the selective electrodes 12' a current flows in the resistor film 14, and in the heating zone 20 where the electrodes are absent the resistor film 14a generates heat. In the case of using the thermal head in an ink-jet printer, a fraction of ink existing just above the heating zone 20 is heated to boil and produce a bubble, and the bubble forces a droplet of ink to spout from an orifice (not shown) above the heating zone 20 toward a recording paper. In the case of using the thermal head in a thermal printer, the heat generated in the heating zone 20 is directly transferred to a thermographic paper or a heat-sensitive ribbon. The oxide layer 10a on the surface of the substrate 10 provides heat insulation as well as electrical insulation.
In the fabrication of a thermal head according to the invention, both the conductor film (12) and the resistor film (14) are usually deposited by sputtering techniques. In this invention only two sputtering operation are needed since the protective layer 16 is formed by anodic oxidation of the resistor film 14. The patterning of both the conductor film (12) and the resistor film (14) can be performed by conventional photolithography processes.
It is preferable to deposit the resistor film 14 by a bias sputtering method. This is firstly because of fining of the individual grains of the polycrystalline structure of the deposited resistor film. Furthermore, the biasing causes the atoms of the inert gas such as argon gas used as the discharging atmosphere to intrude into the deposited polycrystal of resistor, and in operation of the thermal head, particularly in the case of continuous operation, the inert gas atoms in the resistor film 14 suppress the diffusion of oxygen from the anodically oxidized layer 16 into the resistor 14a in the heating zones 20 and consequently prevent a rise in the resistance of the resistor 14a. Therefore, the thermal head long maintains good printing quality and prolongs its service life. The effect of suppressing the diffusion of oxygen is appreciable when the biasing power is 10%, or above, of the sputtering power. However, if the biasing power exceeds 50% of the sputtering power an excessive increase in the amount of the inert gas atoms in the resistor film 14 augments strains in the resistor film and may cause peeling of the resistor film from the underlying electrodes or the substrate surface during continuous operation of the thermal head.

EXAMPLE 1

This example illustrates the fabrication of the thermal head shown in Figs. 1 and 2.
On the surface of the silicon substrate 10 the thermally oxidized insulating layer 10a had a thickness of 1.35 µm.
The electrodes 12, 12' were formed by depositing an aluminum film on the insulating surface 10a of the substrabe 10 by a sputtering method to a thickness of 0.5 µm and then patterning the aluminum film by a photolithography process. As an etching liquid, an aqueous mixed solution of phosphoric acid, nitric acid, acetic acid and ammonium fluoride was used. Then, by a bias sputtering method a tantalum film (resistor film 14) was deposited to a thickness of 0.25 µm. The sputtering gas was argon gas at 0.5 Pa. The sputtering power was 1000 W, and the biasing power applied to the silicon substrate 10 was 300 W. The deposited tantalum film was selectively removed by photolithography to form the resistor pattern 14 shown in Fig. 1. Next, a surface layer of the patterned resistor film 14 was anodically oxidized in 0.1% aqueous solution of phosphoric acid by application of a voltage of 147 V between the resistor film 14 which was made anode and a counter electrode. The anodic oxidation was performed until the thickness of the resultant Ta₂O₅ layer, viz. protective layer 16, reached 0.3 µm. After the anodic oxidation operation the thickness of the unoxidized tantalum film 14 was 0.1 µm.
The protective layer 16 formed by anodic oxidation is a stoichiometric Ta₂O₅ layer having a very tight structure. Therefore, the thickness of the protective layer 16 can be made very thin to the extent of about 0.3 µm. For this reason the thermal head can be driven at a high speed of 20 kHz or above, and power consumpution becomes low. In the thermal head of this example, the minimum input power to each heating zone 20 for producing a bubble of ink was 650 W/mm², which is a relatively low value.
In a thermal head according to the invention, the object of anodic oxidation is only the resistor material, which is preferably Ta. Therefore, the anodic oxidation can be performed under the conditions optimum for that resistor material. Furthermore, the electrodes 12, 12' and the heating zones 20 are entirely covered by the resistor film 14 without discontinuity in the resistor film itself or the protective layer 16. Therefore, this thermal head is remarkably improved in durability. The thermal head of Example 1 exhibited durability of the order of 10⁷ pulses, which means about 10 times improvement over a known thermal head having two different protective layers formed by anodic oxidation of an electrode material and a resistor material.
In this invention the electrode material is not limited to aluminum. If desired it is possible to use a different material such as copper, aluminum-copper alloy or aluminum-silicon alloy. Tantalum is preferred as the resistor material, but if desired it is possible to make a selection from different materials such as tantalum-aluminum alloys, tantalum nitride, titanium and its alloys and compounds, and niobium and its alloys and compounds.
Referring to Fig. 3, in the case of a thermal head for ink-jet printers, it is preferable to overlay the anodically oxidized protective layer 16 with a cavitation resistant layer 18 for the purpose of further enhancing the durability of the thermal head by protecting the heating zones 20 from damages by repeated cavitation induced by rupture of bubbles in the ink. Tantalum or titanium is preferred as the material of the cavitation resistant layer 18, and this layer 18 can be formed by a sputtering method. The cavitation resistant layer 18 of Ta or Ti is a conductor layer, but there is no possibility of electrical connection of this layer 18 with the electrodes 12, 12' or the resistor film 14 because of the existence of the dielectric oxide layer 16. Therefore, there is no need for patterning of the cavitation-resistant layer 18. It is suitable that the cavitation-resistant layer 18 of Ta or Ti has a thickness of from 0.2 to 0.8 µm. When the cavitation resistant layer (Ta) 18 of such thickness was added to the thermal head of Example 1, the durability improved more than 5 times. If the cavitation resistant layer 18 is made too thick, the strength of adhesion of this layer 18 to the underlying layer lowers because of augmented strains in the layer 18. Considering the durability and production cost of the thermal head, the optimum thickness of the cavitation resistant layer 18 is about 0.5 µm. When the thermal head of Example 1 was provided with the tantalum layer 18 of such a thickness the durability reached the order of 10⁸ pulses.
In the fabrication of a thermal head according to the invention there is a possibility that the surface of the insulating oxide layer 10a of the silicon substrate 10 becomes a somewhat rough surface by the influence of the etching liquid used in the photolithographic process to form the electrodes 12, 12'. The roughening of that surface might result in unevenness of the thickness of the resistor film 14a in each heating zone 20, and unevenness of the thickness will become a cause of failure of the heating zone 20 by concentration of current. Therefore, as shown in Fig. 4, it is also preferable to overlay the insulating oxide layer 10a on the surface of the silicon substrate 10 with another insulating layer 22 which is stable or resistant to the acidic etching liquid. A preferred material of the insulating layer 22 is silicon nitride. A silicon nitride layer can be deposited on the silicon oxide layer 10a by either a chemical vapor deposition (CVD) process or sputtering. For example, the insulating layer 22 of silicon nitride has a thickness of about 1 µm. It is possible to use a different material such as tantalum pentoxide instead of silicon nitride.
In each heating zone 20 of a thermal head according to the invention there is some distribution of heat generation (amounts of heat generated per unit area), and it has been found that the distribution of heat generation depends strongly on the shape of the resistor film 14 in and in the vicinity of the heating zone 20.
In Fig. 5, the following parameters of the resistor film shape are indicated.
W₁: width of the resistor film 14 over a terminal region of each electrode 12, 12' in the vicinity of the heating zone 20;
W₂: width of the terminal region of each electrode 12, 12';
W₃: width of the resistor film 14a in the heating zone 20;
L: length of the heat generating zone 20;
ΔL: length of an edge region where the width of the resistor film 14 continuously changes from W₁ to W₃.
Fig. 5 shows a good example of the resistor film shape. In a representative sample: W₁ was 72 µm. W₂ was 64 µm W₃ was 66 µm, L was 60 µm, and ΔL was 3 µm.
Assuming that the amount of heat generated in the center of the heating zone 20 is P_o and that the amount of heat generated in a region where heat generation peaks is P_max, the ratio of P_max to P_o can be taken as an indication of the distribution of heat generation. If P_max/P_o is close to 1.0, it is presumable that the distribution of heat generation in the heating zone is close to uniformity. Testing of thermal head samples variously different in the resistor shape has proved that the durability of the thermal head is not significantly affected by uneven distributions of heat generation insofar as P_max/P_o is not greater than 2.0. Therefore, it is desirable that P_max/P_o is not greater than 2.0.
When the ratio of W₁ to W₂ was varied while maintaining the ratio W₃/W₁ constantly at 0.92 and the ratio ΔL/L constantly at 0.05, P_max/P_o varied in the manner as shown in Fig. 7. When W₁/W₂ is greater than 1.30, P_max/P_o becomes greater than 2.0. This is because deflection of current occurs by the enlargement of the resistor film widths outside the electrodes (W₁ - W₂) and induces local concentration of current. It is desirable that W₁/W₂ is not greater than 1.30. If no restrictions arise from the fabrication conditions such as the precision of alignment in the patterning of the resistor film, it is preferable to make W₁/W₂ at or below 1.20 because P_max/P_o becomes 1.5 or below.
When the ratio of W₃ to W₁ was varied while maintaining the ratio W₁/W₂ constantly at 1.13 and the ratio ΔL/L constantly at 0.05, P_max/P_o varies in the manner as shown in Fig. 8. When W₃/W₁ exceeds 1.0, P_max/P_o rises to and exceeds 2.0 because of local concentration of current resulting from deflection of current. It is desirable that W₃/W₁ is not greater than 1.00. It is seen that when W₃/W₁ is in the range from 0.89 to 0.96 (W₃ is greater than W₂ since W₁/W₂ is 1.13) P_max/P_o becomes not greater than 1.5 which is indicative of a fairly narrow distribution of heat generation.
When the ratio of ΔL to L was varied while maintaining the ratio W₁/W₂ constantly at 1.13 and the ratio W₃/W₁ constantly at 0.94, P_max/P_o varied in the manner as shown in Fig. 9. It is seen that P_max/P_o exceeds 1.5 and sharply rises when ΔL/L becomes greater than 0.30. This is because deflection of current extends into inner regions near the center of the heating zone. It is desirable that ΔL/L is not greater than 0.30.
In view of the above described experimental results, it is preferable that the resistor film 14 is patterned so as to meet at least the following conditions (1) and (2).

$W₁ ≧ W₃ ≧ W₂ (1)$

$W₁/W₂ ≦ 1.30 (2)$

It is further preferable that the following conditions (3), (4) and (5) are also met.

$W₃/W₁ < 0.96 (3)$

$W₁/W₂ ≦ 1.20 (4)$

$ΔL/L ≦ 0.30 (5)$

In the aforementioned sample having the resistor shape of Fig. 5, W₁/W₂ was 1.13, W₃/W₁ was 0.92, and ΔL/L was 0.05. That is, this sample met the conditions (3), (4) and (5) as well as the conditions (1) and (2). With respect to this sample, the distribution of heat generation was calculated by a simulation based on the Laplace's equation relating to potential distribution in a resistor. As to the conditions employed in the simulation, input voltage was 5.8 V, the thickness of the resistor film (unoxidized part) was 0.1 µm, and the resistivity of the resistor was 1.55 µΩm. The obtained distribution of heat generation is shown in Fig. 6 in which only a lower-left quarter of Fig. 5 is enlarged since the resistor shape in Fig. 5 has symmetry with respect to both a longitudinal center axis and a widthwise center axis. The amounts of generated heat were 600-800 W/mm² in the region A, 400-600 W/mm² in the region B, 200-400 W/mm² in the region C and 0-200 W/mm² in the region D, and P_max/P_o was 1.19. That is, in this case heat generation does not peak in the region around the angled corner T. Therefore, this thermal head sample is fairly high in withstand voltage.
Fig. 10 shows another good example of the resistor shape in and in the vicinity of a heating zone 20. In a typical sample: W₁ was 72 µm, W₂ was 64 µm, W₃ was 72 µm, and L was 60 µm. That is, W₁/W₂ was 1.13, W₃ was equal to w₁, and ΔL was 0. This sample met the conditions (1) and (2) but did not meet the condition (3). With respect to this sample a simulation of the distribution of heat generation gave the result shown in Fig. 11 which is an enlargement of a lower-left quarter of the resistor shape of Fig. 10. The simulation employed the same conditions as in the simulation of the sample of the resistor shape of Fig. 5. The amoutns of generated heat were 600-800 W/mm² in the region A, 200-600 W/mm² in the region E, 800-1400 W/mm² in the region F and 0-200 W/mm² in the region G, and P_max/P_o was 1.75. In this case the resistor film 14 had an entirely uniform width, and hence heat generation peaked in the region F near an edge corner of each electrode 12, 12', but P_max/P_o still remained below 2.0.
Fig. 12 shows an undesirable example of the resister shape. In a sample for a simulation of the dirstibution of heat generation, W₁ was 96 µm, W₂ was 64 µm, W₃ was 64 µm, L was 60 µm, and ΔL was 16 µm. That is, W₁/W₂ was 1.50, which means that the condition (2) was not met. The simulation employed the same conditions as in the simulation with respect to the resistor shape of Fig. 5. The simulation gave the result shown in Fig. 13 which is an enlargement of a lower-left quarter of the resistor shape of Fig. 12. The amounts of generated heat were 600-800 W/mm² in the region A, 800-1400 W/mm² in the regions H, above 1400 W/mm² in the region J, 200-600 W/mm² in the region K and 0-200 W/mm² in the region M, and P_max/P_o was 2.66. In this case, heat generation strongly peaks in the region J around each of the angled corners T (indicated in Fig. 12) of the resistor film 14a in the heating zone. Therefore, the resistor film in the regions J experiences strong and repeated heat shocks during long operation of the thermal head, and consequently breaking of the resistor occurs in these regions J.

In addition to the above described samples, several samples different in the parameters of the resistor shape were evaluated in respect of P_max/P_o and subjected to a test of breaking voltage. In every sample the resistor material was tantalum. The particulars of the samples are shown in the following Table.

Sample No.	Resistor Shape Factors					Breaking Power (W/mm²)
	W₃/W₁	W₃/W₂	W₁/W₂	ΔL/L	P_max/P_o
1 (Fig. 5)	0.92	1.03	1.13	0.05	1.19	1600
2 (Fig. 10)	1.00	1.13	1.13	*)	1.75	1470
3	0.94	1.06	1.13	0.13	1.29	1580
4	0.94	1.06	1.13	0.27	1.32	1570
5	0.89	1.00	1.13	0.07	1.49	1530
6	0.80	1.00	1.25	0.13	1.95	1440
7	1.00	1.31	1.31	*)	2.42	1350
8	0.76	1.00	1.31	0.32	2.15	1380
9 (Fig. 12)	0.67	1.00	1.50	0.27	2.66	1300
*) ΔL = 0

The samples Nos. 1, 3, 4 and 5 met not only the conditions (1) and (2) but also the conditions (3), (4) and (5). In these samples P_max/P_o was fairly close to 1.0, and these samples were relatively high in breaking voltage. The samples Nos. 2 and 6 met the conditions (1) and (2) but did not meet one of the conditions (3), (4) and (5). In these samples P_max/P_o became relatively high but still remained below 2.0. The samples Nos. 7, 8 and 9 did not meet the condition (2). In these samples P_max/P_o exceeded 2.00, and these samples were considerably low in breaking voltage.

Claims

A thermal head for an ink-jet printer or a thermal printer, comprising:
a substrate having an insulating surface on which a plurality of heat generating zones are located in a predetermined arrangement;
a patterned conductor film which lies on said insultating surface of the substrate and, for each of said heat generating zones, forms two opposite electrodes one of which terminates at an edge of the heat generating zone and the other at the opposite edge;
a patterned resistor film which is formed of an anodically oxidizable material and continuously covers said conductor film and said insulating surface in said heat, generating zones; and
a protective layer which is formed by anodic oxidation of a surface layer of said resistor film.
A thermal head according to Claim 1, wherein said anodically oxidizable material of said resistor film is tantalum.
A thermal head according to Claim 1 or 2, wherein said anodically oxidizable material of said resistor film is selected from the group consisting of tantalum alloys, tantalum compounds, titanium, titanium alloys, titanium compounds, niobium, niobium alloys and niobium compounds.
A thermal head according to Claim 1, 2 or 3, further comprising a cavitation resistant layer which covers said protective layer.
A thermal head according to Claim 4, wherein the material of said cavitation resistant layer is selected from the group consisting of tantalum and titanium.
A thermal head according to Claim 5, wherein said cavitation resistant layer has a thickness in the range from 0.2 to 0.8 µm.
A thermal head according to any one of Claims 1 to 6, wherein sad substrate comprises an insulating and acid-resistant layer which provides said insulating surface.
A thermal head according to Claim 7, wherein said insulating and acid-resistant layer is a silicon nitride layer.
A thermal head according to Claim 7 or 8, wherein said substrate has a silicon body and further comprises a silicon oxide layer which underlies said insulating and acid-resistant layer.
A thermal head according to any one of Claims 1 to 9, wherein said resistor film is patterned so as to meet the following conditions (1) and (2) in and in the vicinity of each of said heat generating zones:

$W₁ ≧ W₃ ≧ W₂ (1)$

$W₁/W₂ ≦ 1.30 (2)$

where W₁ is the width of the resistor film over terminal regions of said two opposite electrodes in the vicinity of the heat generating zone, W₂ is the width of the terminal regions of the two opposite electrodes, and W₃ is the width of the resistor film in the heat generating zone.
A thermal head according to Claim 10, wherein said resister film meets the following conditions (3), (4) and (5):

$W₃/W₁ < 0.96 (3)$

$W₁/W₂ ≦ 1.20 (4)$

$ΔL/L ≦ 0.30 (5)$

where L is the length of the heat generating zone, and ΔL is the length of an edge region where the width of the resistor film continuously changes from W₁ to W₃.
A method of producing a thermal head for an ink-jet printer or a thermal printer, comprising the steps of:
depositing a conductor film on an insulating surface of a substrate;
patterning the conductor film by photolithography to define a plurality of heat generating zones where the conductor film is removed and, for each of said heat generating zones, form two opposite electrodes one of which terminates at an edge of the heat generating zone and the other at the opposite edge;
depositing an anodically oxidizable resistor film on the patterned conductor film and exposed areas of said insulating surface by a bias sputtering operation using an inert gas as a sputtering gas to cause intrusion of the atoms of the inert gas into the deposited resistor film;
patterning the resistor film by photolithography so as to leave the resistor film over the patterned conductor film and on said insulating surface in said heat generating zones; and
anodically oxidizing a surface layer of the patterned resistor film.
A method according to Claim 12, wherein in said bias sputtering operation the biasing power is in the range from 10 to 50% of the sputtering power.
A method according to Claim 12 or 13, wherein said inert gas is argon.
A method according to Claim 12, 13 or 14, wherein said resistor film is a tantalum film.