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:
This application is a continuation of application Ser. No. 894,018 filed Aug. 7, 1986, now abandoned. 
    
    
     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 2  or the like, an antioxidant layer 14 of SiO 2  and an abrasion resisting layer 15 of Ta 2  O 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 2  antioxidant layer 14 and the Ta 2  O 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 2  heater layer of 3000 to 4000 Å in thickness was formed on a sufficiently clean grazed alumina substrate having a glass coating of 40 to 50 μm in thickness by a double-pole radio frequency sputtering process in an Ar atmosphere at 4×10 -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 Ω/□. An Al layer of 1 to 2 μm 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 2  having a thickness of 2 μm and an abrasion resisting layer of Ta 2  O 5  having a thickness of 5 μm 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 2  as stated above was not provided. 
     Sample 2a as a first embodiment of the invention 
     A Ta-SiO 2  heater layer 12 of 3000 to 4000 Å in thickness was formed on a sufficiently clean grazed alumina substrate having a glass coating of 40 to 50 μm in thickness by double-pole radio frequency sputtering in an Ar atmosphere at 4×10 -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 Ω/□. An Al layer of 1 to 2 μm 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 2  O 5  having a thickness of 5 μm was formed by sputtering with input power of 2 KW for 10 hr in an Ar atmosphere at 4×10 -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 2  O 5 . 
     Sample 3a as a third embodiment 
     A Mn-SiO 2  heater layer 12 of 3000 to 4000 Å in thickness was formed on a sufficiently clean grazed alumina substrate having a glass coating of 40 to 50 μm in thickness by double-pole radio frequency sputtering in an Ar atmosphere at 4×10 -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 Ω/□. An Al lead wires 13a, 13b, etc. of 1 to 2 μm in thickness were formed on the heater layer 12 by sputtering and etching and thereafter a Nb 2  O 5  protective layer 20 of 5 μm in thickness was formed by sputtering in an Ar atmosphere at 4×10 -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 2  O 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 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 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° C. under a pressure of 400 kg/cm 2 . Thus, a dense Mn-SiO 2  sputtering target was obtained. The above-described Ta-SiO 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 2  and the protective layer 20 of Nb 2  O 5  are indicated in the box defined by an intersection between the row of Ta-siO 2  and the column of Nb 2  O 5 . The value on the upper line in each box indicates a resistance value (Ω/□) 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 2  O 5 . Blanks in the boxes mean that the experiments concerned were not made. 
     As is understood from the column of Nb 2  O 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  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    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  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    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  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    125  125           -- 115                 105125 125 130                                90&#39;s,41 115                                      125                                         120                                            --  -- 135                                                      115    170  160           -- -- ----   --  180                               ----   -- -- --  -- 190                                                      --Ti--SiO.sub.2    270  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    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  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    135  150           -- 125                 115130 135 125                               --130  135                                         130                                            --  -- 130                                                      110    230  220           -- -- --195  200 -- --190  210                                         195                                            --  -- 225                                                      --Hf--SiO.sub.2    240  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    135  130           -- 120                 100125 125 135                               --130  135                                         130                                            --  -- 135                                                      130    170  200           -- -- ----   --  190                               --185  195                                         170                                            --  -- 195                                                      165V--SiO.sub.2    225  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    130  140           -- 135                 115120 125 125                               --125  130                                         125                                            --  -- 135                                                      105    150  195           -- 165                 ----   --  -- ----   160                                         -- --  -- 175                                                      --Nb--SiO.sub.2    200  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    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  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    135  135           -- 120                 100115 125 125                               --130  -- -- --  --                                                      --5    150  145           -- -- ----   --  -- --150  -- -- --  -- 160                                                      --W--SiO.sub.2    180  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    120  140           -- 125                 100115 120 125                               --120  -- -- --  -- 125                                                      --    170  185           -- -- ----   --  -- ----   -- -- --  -- -- --Fe--SiO.sub.2    175  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    110  125           -- 115                  90115 115 105                               --110  -- -- --  -- 125                                                      --    130  --           -- -- ----   --  -- ----   -- -- --  -- -- --Ni--SiO.sub.2    165  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    120  130           -- 120                 100120 125 120                               --125  -- -- --  -- 130                                                      --    170  160           -- -- ----   --  -- ----   -- -- --  -- 165                                                      --Co--SiO.sub.2    165  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    130  130           -- 125                 120125 125 115                               --120  -- -- --  -- 135                                                      --    180  185           -- -- ----   --  -- ----   -- -- --  -- 175                                                      --Ta--Mo--SiO.sub.2    230  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    135  150           -- 140                 125125 130 130                               --135  -- -- --  -- 135                                                      --    195  200           -- 195                 ----   190 180                               --185  -- -- --  -- 200                                                      --Nb--Mo--SiO.sub.2    250  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    140  150           -- 135                 120130 135 130                               --185  -- -- --  -- 140                                                      --    215  210           -- 190                 --190  200 195                               --190  -- -- --  -- 220                                                      --W--Mo--SiO.sub.2    215  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    140  135           -- 130                 120135 125 130                               --125  -- -- --  -- 145                                                      --    230  200           -- 170                 --160  --  170                               ----   -- -- --  -- 210                                                      --Ni--Ti--SiO.sub.2    300  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    145  135           -- 135                 115125 125 135                               --125  -- -- --  -- 140                                                      --    175  170           -- 170                 ----   --  180                               ----   -- -- --  -- 175                                                      --Ta--Cr--SiO.sub.2    240  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    140  135           -- 135                 125120 130 140                               --135  -- -- --  -- 145                                                      --    180  170           -- 165                 ----   170 190                               --175  -- -- --  -- 200                                                      --W--Cr--SiO.sub.2    225  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    130  120           -- 115                  95125 120 125                               --125  -- -- --  -- 135                                                      --    170  --           -- -- ----   --  -- ----   -- -- --  -- 175                                                      --Nb--Cr--SiO.sub.2    250  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    135  145           -- 140                 125130 130 125                               --135  -- -- --  -- 125                                                      --    185  200           -- 185                 --180  190 -- --195  -- -- --  -- -- --Ta--W--SiO.sub.2    240  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    115  120           -- 125                 100125 120 115                               --115  -- -- --  -- 125                                                      --    140  --           -- -- ----   --  -- ----   -- -- --  -- -- --Nb--W--SiO.sub.2    265  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    135  145           -- 130                 105130 125 120                               --125  -- -- --  -- 140                                                      --    165  180           -- 160                 --170  --  -- ----   -- -- --  -- 170                                                      --Ta--Cr.sub.2 Ta--SiO.sub.2    260  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    145  165           -- 140                 130135 140 125                               --140  -- -- --  -- 140                                                      --    195  215           -- 190                 150195 170 -- --195  -- -- --  -- 200                                                      --W--La--SiO.sub.2    220  ←           ←              ←                 ←←                        ←                            ←                               ←←                                      ←                                         ←                                            ←                                                ←                                                   ←                                                      ←    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 2  and Nb 2  O 5  ; Ta-SiO 2  and ThO 2  ; Ta-SiO 2  and HfO 2  ; Ta-SiO 2  and Y 2  O 5  ; Mn-SiO 2  and Nb 2  O 5  ; Mn-SiO 2  and CoO; Mn-SiO 2  and GeO 2  ; Mn-SiO 2  and HfO 2  ; Mn-SiO 2  and MnO 2  ; Mn-SiO 2  and NiO; Mn-SiO 2  and TiO 2  ; Mn-SiO 2  and Y 2  O 5  ; Ti-SiO 2  and Nb 2  O 5  ; Ti-SiO 2  and ThO 2  ; Ti-SiO 2  and CoO; Ti-SiO 2  and GeO 2  ; Ti-SiO 2  and HfO 2  ; Ti-SiO 2  and NiO; Ti-SiO 2  and TiO 2  ; Ti-SiO 2  and Y 2  O 5  ; Zr-SiO 2  and Nb 2  O 5  ; Zr-SiO 2  and ThO 2  ; Nb-SiO 2  and Nb 2  O 5  ; Nb-SiO 2  and ThO 2  ; Nb-SiO 2  and GeO 2  ; Nb-SiO.sub. 2 and HfO 2  ; Nb-SiO 2  and NiO; Nb-SiO 2  and Y 2  O 5  ; Ta-Mo-SiO 2  and ThO 2  ; Ta-Mo-SiO 2  and Y 2  O 5  ; Nb-Mo-SiO 2  and Nb 2  O 5  ; Nb-Mo-SiO 2  and ThO 2  ; Nb-Mo-SiO 2  and GeO 2  ; Nb-Mo-SiO 2  and Y 2  O 5  ; W-Mo-SiO 2  and Nb 2  O 5  ; W-Mo-SiO 2  and ThO 2  ; W-Mo-SiO 2  and Y 2  O 5  ; Ta-Cr-SiO 2  and Y 2  O 5  ; Nb-Cr-SiO 2  and ThO 2  ; Ta-Cr 2  Ta-SiO 2  and ThO 2  ; Ta-Cr 2  Ta-SiO 2  and Y 2  O 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 2  O 5  protective layer 20 were examined. The sample 3c was similar to the sample 3a except that the Nb 2  O 5  protective layer of the sample 3c had a thickness of 3 μm. 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  ←            ←                 ←                      ←                           ←                              ←                                   ←    145  110            150  145  155  105                              150  120    360  -- 380  370  390  -- 340  --Mn--SiO.sub.2    220  ←            ←                 ←                      ←                           ←                              ←                                   ←    175  115            160  165  180  115                              175  135     400&lt;         -- 360   400&lt;                       400 &lt;                           --  400&lt;                                   330Mo--SiO.sub.2    160  ←            ←                 ←                      ←                           ←                              ←                                   ←    135  110            135  130  135  100                              145  110    305  -- 310  310  300  -- 350  --Ti--SiO.sub.2    270  ←            ←                 ←                      ←                           ←                              ←                                   ←    160  120            150  140  160  115                              155  125    310  -- 300  270  350  -- 340  200Zr-- SiO.sub.2    255  ←            ←                 ←                      ←                           ←                              ←                                   ←    150  115            140  130  145  115                              150  110    295  -- 270  240  300  -- 310  --Hf--SiO.sub.2    240  ←            ←                 ←                      ←                           ←                              ←                                   ←    140  105            135  135  145  110                              140  115    320  -- 290  310  310  -- 295  --V--SiO.sub.2    225  ←            ←                 ←                      ←                           ←                              ←                                   ←    140  110            165  160  170  105                              175  140    295  -- 330  310  330  -- 340  295Nb--SiO.sub.2    200  ←            ←                 ←                      ←                           ←                              ←                                   ←    155  115            150  140  160  115                              155  120    380  -- 380  340  345  -- 370  --Cr--SiO.sub.2    205  ←            ←                 ←                      ←                           ←                              ←                                   ←    140  105            145  150  155  100                              140  125    275  -- 300  310  300  -- 290  240W--SiO.sub.2    180  ←            ←                 ←                      ←                           ←                              ←                                   ←    125  100            135  150  150   95                              135  105    250  -- 260  300  290  -- 250  --Fe--SiO.sub.2    175  ←            ←                 ←                      ←                           ←                              ←                                   ←    120   95            125  135  140   90                              130  110    220  -- 230  240  250  -- 250  --Ni--SiO.sub.2    165  ←            ←                 ←                      ←                           ←                              ←                                   ←    115   95            110  130  140   95                              135  105    220  -- --   230  225  -- 220  --Co--SiO.sub.2    165  ←            ←                 ←                      ←                           ←                              ←                                   ←    120  100            125  130  135   95                              125  100    240  -- 230  250  260  -- 240  --Ta--Mo--SiO.sub.2    230  ←            ←                 ←                      ←                           ←                              ←                                   ←    150  120            170  165  120  160                              135    360  -- 350  360  370  -- 370  320Nb--Mo--SiO.sub.2    250  ←            ←                 ←                      ←                           ←                              ←                                   ←    160  115            175  180  175  125                              155  130    385  --  400&lt;                  400&lt;                       400&lt;                           320                              370  340W--Mo--SiO.sub.2    215  ←            ←                 ←                      ←                           ←                              ←                                   ←    150  115            155  160  170  120                              145  140    350  -- 350  365  360  -- 330  330Ni--Ti--SiO.sub.2    300  ←            ←                 ←                      ←                           ←                              ←                                   ←    165  125            160  165  175  125                              160  135    340  290            330  350  350  300                              340  310Ta--Cr--SiO.sub.2    240  ←            ←                 ←                      ←                           ←                              ←                                   ←    160  120            175  175  170  110                              150  125    375  -- 395  400  390  -- 350  310W--Cr--SiO.sub.2    225  ←            ←                 ←                      ←                           ←                              ←                                   ←    140  110            145  150  160  115                              140  115    305  -- 330  340  345  -- 320  --Nb--Cr--SiO.sub.2    250  ←            ←                 ←                      ←                           ←                              ←                                   ←    150  115            175  160  160  120                              150  130    315  -- 370  365  375  -- 310  290Ta--W--SiO.sub.2    240  ←            ←                 ←                      ←                           ←                              ←                                   ←    120  100            160  155  150  105                              145  120    270  -- 375  360  360  -- 350  --Nb--W--SiO.sub.2    265  ←            ←                 ←                      ←                           ←                              ←                                   ←    145  120            165  180  175  115                              155  130    300  -- 350  395  380  -- 300  290Ta--Cr.sub.2 Ta--SiO.sub.2    260  ←            ←                 ←                      ←                           ←                              ←                                   ←    150  115            180  180  175  105                              145  125    340  -- 390   400&lt;                      390  -- 340  310W--La--SiO.sub.2    220  ←            ←                 ←                      ←                           ←                              ←                                   ←    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 2  and BN; Ta-SiO 2  and TiN; Ta-SiO 2  and ThN; Ta-SiO 2  and HfN; Ta-SiO 2  and ZrN; Mn-SiO 2  and BN; Mn-SiO 2  and TiN; Mn-SiO 2  and ThN; Mn-SiO 2  and HfN; Mn-SiO 2  and ZrN; Mn-SiO 2  and AlN; Mo-SiO 2  and BN; Mo-SiO 2  and TiN; Mo-SiO 2  and ThN; Mo-SiO 2  and HfN; Mo-SiO 2  and ZrN; Ti-SiO 2  and BN; Ti-SiO 2  and TiN; Ti-SiO 2  and HfN; Ti-SiO 2  and ZrN; Zr-SiO 2  and HfN; Zr-SiO 2  and ZrN; Hf-SiO 2  and BN; Hf-SiO 2  and ThN; Hf-SiO 2  and HfN; V-SiO 2  and TiN; V-SiO 2  and ThN; V-SiO 2  and HfN; V-SiO 2  and ZrN; Nb-SiO 2  and BN; Nb-SiO 2  and TiN; Nb-SiO 2  and ThN; Nb-SiO 2  and HfN; Nb-SiO 2  and ZrN; Cr-SiO 2  and TiN; Cr-SiO 2  and ThN; Cr-SiO 2   and HfN; W-SiO 2  and ThN; Ta-Mo-SiO 2  and BN; Ta-Mo-SiO 2  and TiN; Ta-Mo-SiO 2  and ThN; Ta-Mo-SiO 2  and HfN; Ta-Mo-SiO 2  and ZrN; Ta-Mo-SiO 2  and AlN; Nb-Mo-SiO 2  and BN; Nb-Mo-SiO 2  and TiN; Nb-Mo-SiO 2  and ThN; Nb-Mo-SiO 2  and HfN; Nb-Mo-SiO 2  and VN; Nb-Mo-SiO 2  and ZrN; Nb-Mo-SiO 2  and AlN; W-Mo-SiO 2  and BN; W-Mo-SiO 2  and TiN; W-Mo-SiO 2  and ThN; W-Mo-SiO 2  and HfN; W-Mo-SiO 2  and VN; W-Mo-SiO 2  and ZrN; W-Mo-SiO 2  and AlN; Ta-Cr-SiO 2  and BN; Ta-Cr-SiO 2  and TiN; Ta-Cr-SiO 2  and ThN; Ta-Cr-SiO 2  and HfN; Ta-Cr-SiO 2  and ZrN; Ta-Cr-SiO 2  and AlN; W-Cr-SiO 2  and BN; W-Cr-SiO 2  and TiN; W-Cr-SiO 2  and ThN; W-Cr-SiO 2  and HfN; W-Cr-SiO 2  and ZrN; Nb-Cr-SiO 2  and BN; Nb-Cr-SiO 2  and TiN; Nb-Cr-SiO 2  and ThN; Nb-Cr-SiO 2  and HfN; Nb-Cr-SiO 2  and ZrN; Ta-W-SiO 2  and TiN; Ta-W-SiO 2  and ThN; Ta-W-SiO 2  and HfN; Ta-W-SiO 2  and ZrN; Nb-W-SiO 2  and BN; Nb-W-SiO 2  and TiN; Nb-W-SiO 2  and ThN; Nb-W-SiO 2  and HfN; Nb-W-SiO 2  and ZrN; Ta-Cr- 2  Ta-SiO 2  and BN; Ta-Cr 2  Ta-SiO 2  and TiN; Ta-Cr 2  Ta-SiO 2  and ThN; Ta-Cr 2  Ta-SiO 2  and HfN; Ta-Cr 2  Ta-SiO 2  and ZrN; Ta-Cr 2  Ta-SiO 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 μm 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.