Thermal head

Abstract

A thermal head for a printer in accordance with the present invention comprises: a substrate (11); a heater layer (12) on the substrate; lead wires (13a and 13b) formed on the heater layer for supplying electric power to the heater layer; and a single protective layer (20) for protecting the heater layer and the lead wires by covering them, the protective layer including an oxide or a nitride.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal head for a printer, and particularly to a thermal head suitable for high speed printing, which has a long-lived heater and a long-lived protective layer.

2. Description of the Prior Art

In the prior art, there are printers of an impact type, a thermal printing type, an ink jet type and the like. Among them, the impact type is most popularly utilized. However, a printer of the impact type has limitations in the number of dots printed per unit area and in the size of a single dot and is not suited for printing of fine characters. In addition, a printer of the impact type performs its printing operation mechanically and has the drawback that makes noise during operation.

For a printer of the thermal printing type, heater elements can be made very small since a thermal head can be manufactured by photolithography and therefore fine printing operation is possible. Such a printer of the thermal printing type performs printing operation thermally and does not produce any noise. In view of these merits, a demand for printers of the thermal printing type is rapidly increasing and it is desirable to make further improvements in the lifetime of a thermal head and the printing speed.

The performance of a thermal head depends definitely on the material of the heater and the material of a protective film applied thereon. In order to obtain a thermal head having excellent performance, it is necessary to develop appropriate materials for a heater and a protective film.

FIG. 1 is an enlarged fragmentary sectional view illustrating a conventional thermal head. A heater layer 12 is formed on a substrate 11 and lead wires 13a and 13b are formed on the heater layer 12. The heater layer 12 and the lead wires 13a and 13b are covered with an antioxidant layer 14 and an abrasion resisting layer 15.

In operation, the heater layer 12 generates heat between the lead wires 13a and 13b to which electric power is supplied. A thermosensible paper or an ink ribbon (not shown) is interposed between the thermal head and a platen (not shown) so that characters are printed on the thermosensible paper or transfer paper.

A conventional thermal head, e.g., as disclosed in Japanese Patent Publication No. 8234/1984, comprises a heater layer 12 of TaN, Ta-SiO.sub.2 or the like, an antioxidant layer 14 of SiO.sub.2 and an abrasion resisting layer 15 of Ta.sub.2 O.sub.5. Since the protective film of this thermal head is formed by two layers, namely, the antioxidant layer 14 and the abrasion resisting layer 15, the process of manufacturing the protective film is complicated and takes much time. In addition, although the combination of the SiO.sub.2 antioxidant layer 14 and the Ta.sub.2 O.sub.5 abrasion resisting layer 15 assures a thermal head having relatively long lifetime, further development is desired to obtain a thermal head having a longer lifetime and assuring higher printing speed with a considerable saving of energy.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a thermal head having a long lifetime, the manufacturing process of which is simplified.

A thermal head according to an aspect of the present invention comprises a protective layer containing at least one of the oxides of Ti, Zr, Hf, V, Nb, Cr, Mo, W, B, Mn, Fe, Ni, Co, Th and Ge.

A thermal head according to another aspect of the present invention comprises a protective layer containing at least one of the nitrides of Ti, Zr, Hf, V, Nb, Al, B and Th.

These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged fragmentary sectional view illustrating a conventional thermal head.

FIG. 2 is an enlarged fragmentary sectional view illustrating a thermal head in accordance with the present invention.

FIG. 3 is a diagram showing the resistance change in heaters during a stepped stress test of thermal heads.

FIG. 4 is a diagram showing the resistance change in heaters during a running test for printing operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is an enlarged fragmentary sectional view illustrating a thermal head of an embodiment of the present invention. This thermal head is similar to that of FIG. 1 except that a heater layer 12 and lead wires 13a and 13b are covered with a single layer 20 of a selected oxide or nitride instead of two distinct layers, i.e., the antioxidant layer 14 and the abrasion resisting layer 15.

In the following, thermal heads according to the embodiments of the present invention will be described in comparison with a conventional thermal head.

Sample 1a as a conventional head

This sample 1a was obtained in the following manner. A Ta-SiO.sub.2 heater layer of 3000 to 4000 .ANG. in thickness was formed on a sufficiently clean grazed alumina substrate having a glass coating of 40 to 50 .mu.m in thickness by a double-pole radio frequency sputtering process in an Ar atmosphere at 4.times.10.sup.-3 Pa. The sputtering was performed with input power of 2 KW for 80 minutes. The sheet resistivity of the heater layer 12 thus obtained was 170 .OMEGA./.quadrature.. An Al layer of 1 to 2 .mu.m in thickness for lead wires 13a, 13b, etc. was formed on the heater layer 12 by sputtering and a thermal head pattern of 7/mm was formed by selective etching. Then, an antioxidant layer 14 of SiO.sub.2 having a thickness of 2 .mu.m and an abrasion resisting layer of Ta.sub.2 O.sub.5 having a thickness of 5 .mu.m were formed by sputtering.

Sample 1b for comparison

This sample 1b was formed in the same manner as for the sample 1a, except that an antioxidant layer 14 of SiO.sub.2 as stated above was not provided.

Sample 2a as a first embodiment of the invention

A Ta-SiO.sub.2 heater layer 12 of 3000 to 4000 .ANG. in thickness was formed on a sufficiently clean grazed alumina substrate having a glass coating of 40 to 50 .mu.m in thickness by double-pole radio frequency sputtering in an Ar atmosphere at 4.times.10.sup.-3 Pa. The sputtering was performed with input power of 2 KW for 80 minutes. The sheet resistivity of the heater layer 12 thus obtained was 170 .OMEGA./.quadrature.. An Al layer of 1 to 2 .mu.m in thickness was formed on the heater layer 12 by sputtering and a thermal head pattern of 7/mm was formed by selective etching. Then, a protective layer 20 of Nb.sub.2 O.sub.5 having a thickness of 5 .mu.m was formed by sputtering with input power of 2 KW for 10 hr in an Ar atmosphere at 4.times.10.sup.-3 Pa.

Sample 2b as a second embodiment

This sample 2b was formed in the same manner as for the sample 2a, except that a protective layer 20 was formed of BN instead of Nb.sub.2 O.sub.5.

Sample 3a as a third embodiment

A Mn-SiO.sub.2 heater layer 12 of 3000 to 4000 .ANG. in thickness was formed on a sufficiently clean grazed alumina substrate having a glass coating of 40 to 50 .mu.m in thickness by double-pole radio frequency sputtering in an Ar atmosphere at 4.times.10.sup.-3 Pa. The sputtering was performed with input power of 2 KW for 60 minutes. The sheet resistivity of the heater layer 12 thus obtained was 220 .OMEGA./.quadrature.. An Al lead wires 13a, 13b, etc. of 1 to 2 .mu.m in thickness were formed on the heater layer 12 by sputtering and etching and thereafter a Nb.sub.2 O.sub.5 protective layer 20 of 5 .mu.m in thickness was formed by sputtering in an Ar atmosphere at 4.times.10.sup.-3 Pa.

Sample 3b as a fourth embodiment

This sample 3b was formed in the same manner as for the sample 3a, except that a protective layer 20 of this sample was formed of BN instead of Nb.sub.2 O.sub.5.

Stepped stress test

FIG. 3 is a graph showing the resistance change in the heater during a stepped stress test for the above stated various samples. In the stepped stress test, an acclerated test was conducted by repeating a cycle consisting of: applying pulse voltage of 100 Hz for 3 minutes, stopping the supply of power for 1 minute and then applying again for 3 minutes electric power increased by 0.05 W. Input powers producing a resistance change of 1% in the respective heaters of the above stated samples were compared as permissible input powers. In FIG. 3, the vertical axis represents the resistance change and the horizontal axis represents the input power normalized by the input power which causes the sample 1a of the conventional head to exhibit the resistance change of 1%.

As can be seen from FIG. 3, the sample 2a of the first embodiment is capable of receiving input power higher than that of the conventional head sample 1a by 30% and is capable of receiving input power twice as high as that of the sample 1b for comparison not containing an SiO.sub.2 antioxidant layer 14. Similarly, it can also be seen that the samples 2b, 3a and 3b of the other embodiment are capable of receiving much higher input powers compared with the above stated samples 1a and 1b.

FIG. 4 is a graph showing the resistance change in the heater during the running test of the above stated sample heads. In the running test, each sample head was incorporated in a printer and continuous printing was made with input power of 0.55 W per dot and 30 characters/sec. As to the running distances of the respective sample heads, comparison was made of the running distances by which the respective heaters exhibited a resistance change of 10%. In FIG. 4, the vertical axis indicates the resistance change of the heater and the horizontal axis indicates the normalized running distance, the running distance being normalized by the value of the running distance by which the conventional head 1a exhibits a resistance change of 10%.

As can be seen from FIG. 4, the sample 2a of the first embodiment has the running distance approximately twice as long as that of the conventional head 1a. It can also be seen that the samples 2b, 3a and 3b of the other embodiments have much longer running distances than that of the conventional head 1a. One of the reasons for the longer running distances of the thermal heads in accordance with the present invention is considered to be that the input power in the running test was sufficiently smaller than the permissible input power with respect to the heads of the present invention but substantially attained or exceeded the permissible input power with respect to the sample 1b for comparison or the conventional head 1a. A second reason is considered to be that there was little abrasion of the respective protective layers 20 in the samples of the present invention.

Further embodiments

Besides the above stated samples, sample heads were prepared using various materials and the characteristics thereof were examined.

Sputtering targets of various materials for forming a heater layer 12 were prepared using a vacuum hot press apparatus. An example of the preparing process of those targets will be described in the following.

Mn powder, and SiO.sub.2 powder each being not larger than 350 mesh size, were mixed at a predetermined ratio in a wet manner with ethyl alcohol for 2 hr in an automated mortar. Then, the mixed powder was dried and after that it was placed in a vacuum hot press apparatus at 1500.degree. C. under a pressure of 400 kg/cm.sup.2. Thus, a dense Mn-SiO.sub.2 sputtering target was obtained. The above-described Ta-SiO.sub.2 sputtering target was also prepared in the same manner using Ta powder of 325 mesh size instead of Mn powder. The targets of the other materials were also manufactured in the same manner using a vacuum hot press apparatus.

Table I shows characteristics of the thermal heads having various combinations of heater materials and protective film materials thus obtained. The left end column indicates various heater materials and the top row indicates various oxides as the protective film materials. For example, the characteristics of the thermal head 2a of the first embodiment having the heater layer 12 of Ta-SiO.sub.2 and the protective layer 20 of Nb.sub.2 O.sub.5 are indicated in the box defined by an intersection between the row of Ta-siO.sub.2 and the column of Nb.sub.2 O.sub.5. The value on the upper line in each box indicates a resistance value (.OMEGA./.quadrature.) of a heater layer 12; the value on the middle line indicates normalized permissible input power in the stepped stress test; and the value on the lower line indicates normalized running distance in the running test. The initial resistance value of each heater layer is indicated representatively on the upper line of each box in only the column of Nb.sub.2 O.sub.5. Blanks in the boxes mean that the experiments concerned were not made.

As is understood from the column of Nb.sub.2 O.sub.5 for example, there is a correlation between the result of the stepped stress test and the result of the printing running test. Consequently, although the running test could not be conducted for all the samples because the running distance attains nearly 100 km in the running test, it is believed that a thermal head having a higher permissible input power in the stepped stress test has a longer running distance.

TABLE I__________________________________________________________________________ Protective FilmHeater layer Nb.sub.2 O.sub.5 ThO.sub.2 B.sub.2 O.sub.3 CoO Cr.sub.2 O.sub.3 Fe.sub.3 O.sub.4 GeO.sub.2 HfO.sub.2 MoO.sub.3 MnO.sub.2 NiO TiO.sub.2 .sub.2 O.sub.5 WO.sub.2 Y.sub.2 O.sub.3 ZrO.sub.2__________________________________________________________________________Ta--SiO.sub.2 170 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 130 145 90 130 110120 125 135 100125 130 120 90 120 145 120 200 270 -- 190 ---- -- 220 ---- 195 -- -- -- 230 --Mn--SiO.sub.2 320 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 145 150 100 135 120115 135 145 120130 140 135 110 130 155 130 265 275 -- 220 ---- 200 250 --210 240 215 -- 190 270 190Mo--SiO.sub.2 160 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 125 125 -- 115 105125 125 130 90's,41 115 125 120 -- -- 135 115 170 160 -- -- ---- -- 180 ---- -- -- -- -- 190 --Ti--SiO.sub.2 270 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 145 -- 135 125135 130 130 --125 135 135 -- -- 140 125 225 230 -- 200 --190 200 205 -- -- 210 200 -- -- 230 --Zr--SiO.sub.2 255 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 150 -- 125 115130 135 125 --130 135 130 -- -- 130 110 230 220 -- -- --195 200 -- --190 210 195 -- -- 225 --Hf--SiO.sub.2 240 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 130 -- 120 100125 125 135 --130 135 130 -- -- 135 130 170 200 -- -- ---- -- 190 --185 195 170 -- -- 195 165V--SiO.sub.2 225 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 130 140 -- 135 115120 125 125 --125 130 125 -- -- 135 105 150 195 -- 165 ---- -- -- ---- 160 -- -- -- 175 --Nb--SiO.sub.2 200 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 160 -- 130 115125 135 140 --135 140 135 -- -- 145 130 230 225 -- 190 --170 210 215 --195 220 185 -- -- 230 170Cr--SiO.sub.2 205 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 135 -- 120 100115 125 125 --130 -- -- -- -- --5 150 145 -- -- ---- -- -- --150 -- -- -- -- 160 --W--SiO.sub.2 180 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 120 140 -- 125 100115 120 125 --120 -- -- -- -- 125 -- 170 185 -- -- ---- -- -- ---- -- -- -- -- -- --Fe--SiO.sub.2 175 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 110 125 -- 115 90115 115 105 --110 -- -- -- -- 125 -- 130 -- -- -- ---- -- -- ---- -- -- -- -- -- --Ni--SiO.sub.2 165 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 120 130 -- 120 100120 125 120 --125 -- -- -- -- 130 -- 170 160 -- -- ---- -- -- ---- -- -- -- -- 165 --Co--SiO.sub.2 165 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 130 130 -- 125 120125 125 115 --120 -- -- -- -- 135 -- 180 185 -- -- ---- -- -- ---- -- -- -- -- 175 --Ta--Mo--SiO.sub.2 230 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 150 -- 140 125125 130 130 --135 -- -- -- -- 135 -- 195 200 -- 195 ---- 190 180 --185 -- -- -- -- 200 --Nb--Mo--SiO.sub.2 250 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 150 -- 135 120130 135 130 --185 -- -- -- -- 140 -- 215 210 -- 190 --190 200 195 --190 -- -- -- -- 220 --W--Mo--SiO.sub.2 215 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 135 -- 130 120135 125 130 --125 -- -- -- -- 145 -- 230 200 -- 170 --160 -- 170 ---- -- -- -- -- 210 --Ni--Ti--SiO.sub.2 300 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 145 135 -- 135 115125 125 135 --125 -- -- -- -- 140 -- 175 170 -- 170 ---- -- 180 ---- -- -- -- -- 175 --Ta--Cr--SiO.sub.2 240 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 135 -- 135 125120 130 140 --135 -- -- -- -- 145 -- 180 170 -- 165 ---- 170 190 --175 -- -- -- -- 200 --W--Cr--SiO.sub.2 225 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 130 120 -- 115 95125 120 125 --125 -- -- -- -- 135 -- 170 -- -- -- ---- -- -- ---- -- -- -- -- 175 --Nb--Cr--SiO.sub.2 250 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 145 -- 140 125130 130 125 --135 -- -- -- -- 125 -- 185 200 -- 185 --180 190 -- --195 -- -- -- -- -- --Ta--W--SiO.sub.2 240 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 115 120 -- 125 100125 120 115 --115 -- -- -- -- 125 -- 140 -- -- -- ---- -- -- ---- -- -- -- -- -- --Nb--W--SiO.sub.2 265 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 145 -- 130 105130 125 120 --125 -- -- -- -- 140 -- 165 180 -- 160 --170 -- -- ---- -- -- -- -- 170 --Ta--Cr.sub.2 Ta--SiO.sub.2 260 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 145 165 -- 140 130135 140 125 --140 -- -- -- -- 140 -- 195 215 -- 190 150195 170 -- --195 -- -- -- -- 200 --W--La--SiO.sub.2 220 .rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw..rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 110 130 -- 115 100120 130 105 --120 -- -- -- -- 125 --185 185 -- -- ---- 190 -- ---- -- -- -- -- -- --__________________________________________________________________________

Combinations of a heater and a protective film exhibiting particularly excellent characteristics are as follows: Ta-SiO.sub.2 and Nb.sub.2 O.sub.5 ; Ta-SiO.sub.2 and ThO.sub.2 ; Ta-SiO.sub.2 and HfO.sub.2 ; Ta-SiO.sub.2 and Y.sub.2 O.sub.5 ; Mn-SiO.sub.2 and Nb.sub.2 O.sub.5 ; Mn-SiO.sub.2 and CoO; Mn-SiO.sub.2 and GeO.sub.2 ; Mn-SiO.sub.2 and HfO.sub.2 ; Mn-SiO.sub.2 and MnO.sub.2 ; Mn-SiO.sub.2 and NiO; Mn-SiO.sub.2 and TiO.sub.2 ; Mn-SiO.sub.2 and Y.sub.2 O.sub.5 ; Ti-SiO.sub.2 and Nb.sub.2 O.sub.5 ; Ti-SiO.sub.2 and ThO.sub.2 ; Ti-SiO.sub.2 and CoO; Ti-SiO.sub.2 and GeO.sub.2 ; Ti-SiO.sub.2 and HfO.sub.2 ; Ti-SiO.sub.2 and NiO; Ti-SiO.sub.2 and TiO.sub.2 ; Ti-SiO.sub.2 and Y.sub.2 O.sub.5 ; Zr-SiO.sub.2 and Nb.sub.2 O.sub.5 ; Zr-SiO.sub.2 and ThO.sub.2 ; Nb-SiO.sub.2 and Nb.sub.2 O.sub.5 ; Nb-SiO.sub.2 and ThO.sub.2 ; Nb-SiO.sub.2 and GeO.sub.2 ; Nb-SiO.sub. 2 and HfO.sub.2 ; Nb-SiO.sub.2 and NiO; Nb-SiO.sub.2 and Y.sub.2 O.sub.5 ; Ta-Mo-SiO.sub.2 and ThO.sub.2 ; Ta-Mo-SiO.sub.2 and Y.sub.2 O.sub.5 ; Nb-Mo-SiO.sub.2 and Nb.sub.2 O.sub.5 ; Nb-Mo-SiO.sub.2 and ThO.sub.2 ; Nb-Mo-SiO.sub.2 and GeO.sub.2 ; Nb-Mo-SiO.sub.2 and Y.sub.2 O.sub.5 ; W-Mo-SiO.sub.2 and Nb.sub.2 O.sub.5 ; W-Mo-SiO.sub.2 and ThO.sub.2 ; W-Mo-SiO.sub.2 and Y.sub.2 O.sub.5 ; Ta-Cr-SiO.sub.2 and Y.sub.2 O.sub.5 ; Nb-Cr-SiO.sub.2 and ThO.sub.2 ; Ta-Cr.sub.2 Ta-SiO.sub.2 and ThO.sub.2 ; Ta-Cr.sub.2 Ta-SiO.sub.2 and Y.sub.2 O.sub.5 etc. With those combinations, data obtained show the running distances more than twice that of the conventional head.

In addition, the characteristics of a sample 3c having a thinner Nb.sub.2 O.sub.5 protective layer 20 were examined. The sample 3c was similar to the sample 3a except that the Nb.sub.2 O.sub.5 protective layer of the sample 3c had a thickness of 3 .mu.m. The sample 3c exhibited the normalized input power of 135% in the stepped stress test and the normalized running distance of 170% in the running test. Thus, it is understood that the sample 3c having a thinner protective layer 20 still possesses characteristics superior to those of the conventional head. Furthermore, since the sample 3c had the thinner protective layer, the input power required for printing with it was decreased by approximately 10% as a result of decrease in the thermal capacity of the protective layer.

Table II shows characteristics of thermal heads in the same manner as Table I, except that various nitrides are indicated as the protective film in the top row.

TABLE II__________________________________________________________________________ Protective FilmHeater Layer BN NbN TiN ThN HfN VN ZrN AlN__________________________________________________________________________Ta--SiO.sub.2 170 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 145 110 150 145 155 105 150 120 360 -- 380 370 390 -- 340 --Mn--SiO.sub.2 220 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 175 115 160 165 180 115 175 135 400< -- 360 400< 400 < -- 400< 330Mo--SiO.sub.2 160 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 135 110 135 130 135 100 145 110 305 -- 310 310 300 -- 350 --Ti--SiO.sub.2 270 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 160 120 150 140 160 115 155 125 310 -- 300 270 350 -- 340 200Zr-- SiO.sub.2 255 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 150 115 140 130 145 115 150 110 295 -- 270 240 300 -- 310 --Hf--SiO.sub.2 240 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 105 135 135 145 110 140 115 320 -- 290 310 310 -- 295 --V--SiO.sub.2 225 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 110 165 160 170 105 175 140 295 -- 330 310 330 -- 340 295Nb--SiO.sub.2 200 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 155 115 150 140 160 115 155 120 380 -- 380 340 345 -- 370 --Cr--SiO.sub.2 205 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 105 145 150 155 100 140 125 275 -- 300 310 300 -- 290 240W--SiO.sub.2 180 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 125 100 135 150 150 95 135 105 250 -- 260 300 290 -- 250 --Fe--SiO.sub.2 175 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 120 95 125 135 140 90 130 110 220 -- 230 240 250 -- 250 --Ni--SiO.sub.2 165 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 115 95 110 130 140 95 135 105 220 -- -- 230 225 -- 220 --Co--SiO.sub.2 165 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 120 100 125 130 135 95 125 100 240 -- 230 250 260 -- 240 --Ta--Mo--SiO.sub.2 230 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 150 120 170 165 120 160 135 360 -- 350 360 370 -- 370 320Nb--Mo--SiO.sub.2 250 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 160 115 175 180 175 125 155 130 385 -- 400< 400< 400< 320 370 340W--Mo--SiO.sub.2 215 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 150 115 155 160 170 120 145 140 350 -- 350 365 360 -- 330 330Ni--Ti--SiO.sub.2 300 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 165 125 160 165 175 125 160 135 340 290 330 350 350 300 340 310Ta--Cr--SiO.sub.2 240 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 160 120 175 175 170 110 150 125 375 -- 395 400 390 -- 350 310W--Cr--SiO.sub.2 225 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 140 110 145 150 160 115 140 115 305 -- 330 340 345 -- 320 --Nb--Cr--SiO.sub.2 250 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 150 115 175 160 160 120 150 130 315 -- 370 365 375 -- 310 290Ta--W--SiO.sub.2 240 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 120 100 160 155 150 105 145 120 270 -- 375 360 360 -- 350 --Nb--W--SiO.sub.2 265 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 145 120 165 180 175 115 155 130 300 -- 350 395 380 -- 300 290Ta--Cr.sub.2 Ta--SiO.sub.2 260 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 150 115 180 180 175 105 145 125 340 -- 390 400< 390 -- 340 310W--La--SiO.sub.2 220 .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. 130 105 145 150 140 100 140 105 250 -- 270 290 280 -- 265 --__________________________________________________________________________

Combinations of a heater and a protective film exhibiting particularly excellent characteristics are as follows: Ta-SiO.sub.2 and BN; Ta-SiO.sub.2 and TiN; Ta-SiO.sub.2 and ThN; Ta-SiO.sub.2 and HfN; Ta-SiO.sub.2 and ZrN; Mn-SiO.sub.2 and BN; Mn-SiO.sub.2 and TiN; Mn-SiO.sub.2 and ThN; Mn-SiO.sub.2 and HfN; Mn-SiO.sub.2 and ZrN; Mn-SiO.sub.2 and AlN; Mo-SiO.sub.2 and BN; Mo-SiO.sub.2 and TiN; Mo-SiO.sub.2 and ThN; Mo-SiO.sub.2 and HfN; Mo-SiO.sub.2 and ZrN; Ti-SiO.sub.2 and BN; Ti-SiO.sub.2 and TiN; Ti-SiO.sub.2 and HfN; Ti-SiO.sub.2 and ZrN; Zr-SiO.sub.2 and HfN; Zr-SiO.sub.2 and ZrN; Hf-SiO.sub.2 and BN; Hf-SiO.sub.2 and ThN; Hf-SiO.sub.2 and HfN; V-SiO.sub.2 and TiN; V-SiO.sub.2 and ThN; V-SiO.sub.2 and HfN; V-SiO.sub.2 and ZrN; Nb-SiO.sub.2 and BN; Nb-SiO.sub.2 and TiN; Nb-SiO.sub.2 and ThN; Nb-SiO.sub.2 and HfN; Nb-SiO.sub.2 and ZrN; Cr-SiO.sub.2 and TiN; Cr-SiO.sub.2 and ThN; Cr-SiO.sub.2 and HfN; W-SiO.sub.2 and ThN; Ta-Mo-SiO.sub.2 and BN; Ta-Mo-SiO.sub.2 and TiN; Ta-Mo-SiO.sub.2 and ThN; Ta-Mo-SiO.sub.2 and HfN; Ta-Mo-SiO.sub.2 and ZrN; Ta-Mo-SiO.sub.2 and AlN; Nb-Mo-SiO.sub.2 and BN; Nb-Mo-SiO.sub.2 and TiN; Nb-Mo-SiO.sub.2 and ThN; Nb-Mo-SiO.sub.2 and HfN; Nb-Mo-SiO.sub.2 and VN; Nb-Mo-SiO.sub.2 and ZrN; Nb-Mo-SiO.sub.2 and AlN; W-Mo-SiO.sub.2 and BN; W-Mo-SiO.sub.2 and TiN; W-Mo-SiO.sub.2 and ThN; W-Mo-SiO.sub.2 and HfN; W-Mo-SiO.sub.2 and VN; W-Mo-SiO.sub.2 and ZrN; W-Mo-SiO.sub.2 and AlN; Ta-Cr-SiO.sub.2 and BN; Ta-Cr-SiO.sub.2 and TiN; Ta-Cr-SiO.sub.2 and ThN; Ta-Cr-SiO.sub.2 and HfN; Ta-Cr-SiO.sub.2 and ZrN; Ta-Cr-SiO.sub.2 and AlN; W-Cr-SiO.sub.2 and BN; W-Cr-SiO.sub.2 and TiN; W-Cr-SiO.sub.2 and ThN; W-Cr-SiO.sub.2 and HfN; W-Cr-SiO.sub.2 and ZrN; Nb-Cr-SiO.sub.2 and BN; Nb-Cr-SiO.sub.2 and TiN; Nb-Cr-SiO.sub.2 and ThN; Nb-Cr-SiO.sub.2 and HfN; Nb-Cr-SiO.sub.2 and ZrN; Ta-W-SiO.sub.2 and TiN; Ta-W-SiO.sub.2 and ThN; Ta-W-SiO.sub.2 and HfN; Ta-W-SiO.sub.2 and ZrN; Nb-W-SiO.sub.2 and BN; Nb-W-SiO.sub.2 and TiN; Nb-W-SiO.sub.2 and ThN; Nb-W-SiO.sub.2 and HfN; Nb-W-SiO.sub.2 and ZrN; Ta-Cr-.sub.2 Ta-SiO.sub.2 and BN; Ta-Cr.sub.2 Ta-SiO.sub.2 and TiN; Ta-Cr.sub.2 Ta-SiO.sub.2 and ThN; Ta-Cr.sub.2 Ta-SiO.sub.2 and HfN; Ta-Cr.sub.2 Ta-SiO.sub.2 and ZrN; Ta-Cr.sub.2 Ta-SiO.sub.2 and AlN etc. Those combinations exhibited data of the running distance more than three times that of the conventional head 1a. It is further shown that other combinations in Table II also show the running distance more than twice that of the conventional head 1a.

In addition, the characteristics of a sample 3d having a thinner protective layer of nitride were examined. The sample 3d was similar to the sample 3b, except that the sample 3d had a BN protection layer of 3 .mu.m in thickness. It was found that the sample 3d exhibited normalized input power of 160% in the stepped stress test and running distance of 230% in the running test, those characteristics being considerably superior to those of the conventional head. Also the necessary input power to the heater for printing was decreased by approximately 15%.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims

1. A thermal head for a printer comprising:

a substrate;

a heater layer on said substrate, said heater layer comprising Mn-SiO.sub.2 ;

lead wires connected to said heater layer for supplying electric power thereto; and

a single protective layer for protecting said heater layer and said lead wires, said protective layer comprising Y.sub.2 O.sub.3.

2. A thermal head for a printer comprising:

a substrate;

a heater layer on said substrate, said heater layer comprising Mn-SiO.sub.2 ;

lead wires connected to said heater layer for supplying electric power thereto; and

a single protective layer for protecting said heater layer and said lead wires, said protective layer comprising HfN.