JPH06227015A - Wear-resistant protective film for thermal head and preparation thereof - Google Patents

Abstract

PURPOSE:To avoid occurrence of cracks by forming a protective film for a thermal head consisting of wear-resistant metal oxide, metal nitride or a mixture of these oxides with different spattering gas content in the thickness direction. CONSTITUTION:A target 12 is fixed on an electrode 13 and a thermal head 15 is fixed on an electrode 14 and after an container 11 is evacuated, an inert gas such as Ar and Kr is introduced so as to bring the pressure at a specified torr. A high frequency bias is applied by making a switch of a high frequency electric source 16a ON and making a switch of another high frequency electric source 16b only for a specified time at an arbitrary time and the place where a protective film layer is formed by means of high frequency spattering and the thickness thereof are controlled thereby. Combination of high frequency bias and no bias or a weak bias is made by adjusting the voltage of this high frequency bias. It is possible to prevent water which is the cause of cracks from penetrating and also to prevent cracks caused by internal stress from being generated by laminating a protective film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wear resistant protective film for a thermal head and a method for producing the same.

[0002]

2. Description of the Related Art Thermal heads are widely used as print heads for computers, word processors, facsimiles and the like. The thermal head is configured such that a large number of dots of a resistance heating element such as polysilicon are arranged, and by selectively energizing them, the printing ribbon is thermally transferred onto the paper for printing. Since the paper is transferred while sliding on the surface of the thermal head, it is necessary to protect the surface of the resistance heating element with a protective film having high abrasion resistance.

As shown in FIG. 1, the spot-shaped printing element in the thermal head is, in order from the bottom, a substrate 1 made of alumina or the like, a heat storage layer 2 made of glaze glass or the like, a heating element layer 3 made of polysilicon or the like, and an electrode 4. 5 and the abrasion-resistant protective film 6. Reference numeral 7 in the figure is a heat generating portion. Generally, the protective film 6 is required to have high hardness, low internal stress due to heat and composition and structure, less likely to be worn, and stable against moisture, alkali, acid, etc. , Si-O-N system, Si-Ti-O-N system, Si-L
Materials such as a-O-N type and Si-Al-O-N type are known.

[0004] Conventional sputtering wear resistant protective films often crack. When such a crack is formed, moisture in the atmosphere easily penetrates into the thermal head through the crack to cause corrosion or peeling. The causes of cracks include internal stress due to heat and composition and structure that develop due to the peening effect, lack of toughness, and other causes. is there. That is, ideally, the abrasion-resistant protective film is formed as shown in FIG. 1, but in reality, as shown by 8 and 8 in FIG. The cause of cracking already exists immediately after the film, and if moisture enters here or heat is repeatedly applied, cracking will occur early.

This problem of step coverage can be solved by forming a wear resistant protective film by using a high frequency bias sputtering method (Japanese Patent Laid-Open No. 63-1352).
61). However, although the high frequency bias sputtering method has an excellent step coverage, it has a peening effect and the sputtering gas (Ar, K) taken into the protective film.
internal stress increases due to (r, etc.). As a result, cracks are likely to occur and the adhesiveness is also reduced. Although the above-mentioned publication describes that the protective film produced by high frequency bias sputtering prevents cracks and peeling, in practice, as described above, cracks easily occur due to internal stress. Further, the above publication does not describe changing the bias to sputter two or more layers.

[0006]

As described above, the conventional wear-resistant protective film causes cracks and corrosion due to poor step coverage of sputtering, while high-frequency bias sputtering causes internal stress and corrosion. Cracks are likely to occur due to low adhesion. Therefore, an object of the present invention is to provide a wear-resistant protective film and a method for producing the same, in which neither cracks caused by internal stress nor cracks caused by step coverage are generated.

[0007]

According to the present invention, a wear-resistant protective film is formed by a sputtering method on a thermal head having a heat-generating layer and a pair of electrodes on a substrate or a heat-storage layer formed on the substrate. The method is characterized in that a part of the wear-resistant protective film is formed under a strong bias and the other part is formed under no bias or under a weak bias, and the protective film thus obtained. Here, the bias may be direct current or alternating current when the abrasion resistant protective film is conductive, and alternating current is used when it is insulating. High frequency bias is usually preferred. According to the present invention, a layer with good step coverage is formed by a strong biasing (preferably high frequency) pattering method on a part of a layer of the wear resistant protective film.
The layer prevents the ingress of moisture, which causes corrosion and cracks. On the other hand, since the layer with less internal stress is formed without bias or under weak bias adjacent to the layer by the strong bias sputtering method, the internal stress of the entire film is reduced. Since the occurrence of cracks is suppressed, cracks due to moisture intrusion and internal stress are totally prevented.

The strong bias sputtering method is preferably used as a bias voltage of -50V to -200.
V, and more preferably sputtering at -60 to -120 V (preferably high frequency sputtering).
Further, the non-biased sputtering or the weak biased sputtering does not use a bias, or a sputtering with a bias voltage of ⅔ or less of the strong bias voltage in sputtering at an arbitrary frequency, preferably with a bias voltage of ½ to 1/10. To tell. To apply a bias, either direct current or alternating current may be used when the abrasion-resistant protective film is conductive, and alternating current is used when the abrasion-resistant protective film is insulating. Further, a high frequency bias is usually used because the electrical property of the abrasion resistant protective film is not limited. According to the present invention, the wear-resistant metal oxide, the metal nitride, or the mixture of the metal oxide and the metal nitride, such as the Si—O—N system or the Si—Ti, in which the content of the sputter gas varies in the thickness direction. -O-
N system, Si-La-O-N system, Si-Al-O-N system,
It is possible to manufacture an excellent wear-resistant protective film for a thermal head, which is made of Si-Sr-O-N type, Si-Mg-O-N type, or a mixture of two or more thereof. Here, the metal is Ti,
Ordinary metals such as Al, Group IIIa B, Group IVa C, Ge,
It means one or more of Si, and preferably one or more containing Si. At least one layer obtained by non-bias sputtering or weak bias sputtering has 0
~ 3 at% sputtering gas (usually Ar or Kr)
Contains less and has less internal stress and does not crack. On the other hand, the content of the sputter gas in at least one other layer obtained by the strong bias is 2 to 10 at%, and the step coverage is improved. The film thickness subjected to strong bias sputtering is preferably 0.1 to 5 μm, more preferably 0.5.
Is about 3 μm. If the thickness is less than 0.1 μm, the step coverage is not sufficient and water penetrates into the film.
This is because if it exceeds m, the internal stress of the entire film becomes too large. On the other hand, the thickness of the film formed under no bias or weak bias is preferably equal to or more than that of the film formed by strong bias sputtering. The two or more "layers" of the wear-resistant protective film of the present invention do not mean layers made of different materials but represent layers having different sputter gas contents produced by changing the bias. Should be noted.

[0009]

According to the present invention, the stress of the protective film is reduced by biasless sputtering of at least one layer,
Adhesion can be improved. In addition, if weak bias sputtering is used instead of no bias, the adhesion is further improved. The reason is that when non-biased sputtering and strong bias sputtering are combined, a tensile stress and a compressive stress are combined to generate shear stress between the two layers.
This is because the combination of the weak bias sputtering and the strong bias sputtering results in a compressive stress and a reduction in the shear stress between the two layers. The publications cited above do not suggest carrying out sputtering with different biases. For the above reasons, the combination of weak bias and strong bias sputtering can provide the most durable protective film. As described above, according to the present invention, it is possible to manufacture a wear-resistant protective film in which neither cracks due to internal stress nor cracks due to step coverage occur. Another advantage of the present invention is that a multilayer film can be easily obtained only by adjusting the bias voltage by using the first sputtering apparatus having only one target, unlike the usual manufacturing of a multilayer film. And is highly productive.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention can be carried out using the sputtering apparatus shown in FIG. The sputtering apparatus includes a sealed vacuum container 11 and a pair of electrodes 13 and 14 arranged to face each other in the container. The electrode 13 supports a material serving as a sputtering source, that is, a target 12, and 14 supports a thermal head 15 on which an abrasion resistant protective film is to be formed. High frequency power supply 16 for electrode 13
a is connected, and a high frequency power source 16b can be connected to the electrode 14. The coil L1 is connected in series on the path from the high frequency power supply 16a to the electrode 13, and the variable capacitors C1 and C2 are connected.
Are connected in parallel. Further, a coil L1 is connected in series and variable capacitors C3 and C4 are connected in parallel in a path from the high frequency power supply 16b to the electrode 14. Switch 1
By opening and closing 7, a high frequency bias can be arbitrarily applied to the thermal head 15.

To carry out the method of the present invention using the above apparatus, first, the target 12 is attached to the electrode, the thermal head 15 is attached to the electrode 14, the container 11 is evacuated, and then the pressure is adjusted to several mtorr. An inert gas such as argon (Ar) or krypton (Kr) gas is introduced. The switch of the high frequency power supply 16a is turned on. on the other hand,
The high-frequency bias power source 16b applies a high-frequency bias by turning on the switch for a predetermined time at an arbitrary time point, thereby controlling the laminated portion and the thickness of the layer by the high-frequency sputtering. By adjusting the voltage of the high frequency bias, the high frequency strong bias and the non-bias or weak bias can be set.

Specific examples will be described below. Example 1 Powders of SiO ₂ and Si ₃ N ₄ were mixed at a molar ratio of 5: 5 and pressed to serve as a target. Input power to the electrode 13 was 4 kw, Ar pressure was 10 mtorr, and high-frequency strong bias voltage of the electrode 14 was −. High-frequency strong sputtering was performed at 100 V and a substrate temperature of 400 ° C. The composition was adjusted by appropriately mixing O ₂ and N ₂ in Ar gas. Unbiased sputtering was 7 μm as the lower layer, and high frequency strong bias was 1 μm as the upper layer. Table 1 shows the results of measuring the internal stress, durability, and the number of defects of the obtained Si-O-N film. Among them, the durability is the number of printable sheets when sublimation color printing is performed with A4 size, and the number of defects is the number of white streak defects in 5 tests.

Example 2 In Example 1, the upper layer and the lower layer were 3 μm without bias,
The intermediate layer was formed with a high frequency strong bias to a thickness of 2 μm. The results are shown in Table 1.

Example 3 In Example 1, the sputtering gas was Kr, the high frequency strong bias sputtering was 1.5 μm as the lower layer, and the bias was 6.5 μm without bias. The results are shown in Table 1.

Comparative Example 1 In Example 1, all layers were formed by high frequency strong bias sputtering to a thickness of 8 μm. The results are shown in Table 1.

Comparative Example 2 In Example 1, all layers were formed in a thickness of 8 μm by biasless sputtering. The results are shown in Table 1.

Example 4 In Example 1, the sputtering gas was Ar, the high-frequency strong bias sputtering was 1.5 μm in the upper layer, and the high-frequency weak bias sputtering (bias −20 V) of the same frequency was 5 μm in the lower layer. The results are shown in Table 1.

Example 5 In Example 4, as a lower layer, a film having a thickness of 6 μm was formed by high frequency weak bias sputtering (bias −10 V), and then the bias voltage was continuously changed to −100 V (−3 V / min). ), And subsequently, a film having a thickness of 1.5 μm was formed by high frequency strong bias sputtering (−100 V) as an upper layer. The results are shown in Table 1.

Example 6 In Example 1, the sputtering gas was Kr, the high frequency bias sputtering was 1.5 μm as the upper layer, and the high frequency weak bias sputtering (bias −10 V) having the same frequency was 6 μm as the lower layer. The results are shown in Table 1.

[0020]

[Table 1]

As is clear from the above examples, it can be seen that the wear-resistant protective film for a thermal head of the present invention has lower internal stress and better durability than conventional protective films. This is because a layer with good step coverage is formed on some part of the wear-resistant protective film by the high frequency sputtering method, so that the entry of water causing cracks is prevented by the layer, while the internal stress is small. Since the layer is formed adjacent to the layer formed by the high frequency sputtering method, the generation of cracks due to the internal stress of the crack formed by the high frequency sputtering method is suppressed. In this way, both intrusion of water and cracks due to internal stress are prevented. Combining weak bias sputtering and strong bias sputtering gives the most durable protective film. Another advantage of the present invention is that a multilayer film can be easily obtained simply by adjusting the bias voltage using one sputtering apparatus equipped with only one target, unlike the usual manufacturing of a multilayer film. And is highly productive.

[Brief description of drawings]

FIG. 1 is a sectional view showing a basic structure of a thermal head.

FIG. 2 is a cross-sectional view showing the structure of a conventional thermal head.

FIG. 3 shows a sputtering apparatus used in the present invention.

[Explanation of symbols]

 1 substrate 2 insulating layer for heat storage (glaze glass) 3 heating element layer (polysilicon) 4, 5 electrode 6 abrasion resistant protective film 7 heating part

Claims (11)

[Claims] 1. A wear-resistant protective film for a thermal head, which is made of a wear-resistant metal oxide, a metal nitride, or a mixture of a metal oxide and a metal nitride, and has different sputter gas contents in the thickness direction. 2. The wear-resistant protective film for a thermal head according to claim 1, wherein the content of the sputter gas in at least one layer is 0 to 3 at%. 3. The wear-resistant protective film for a thermal head according to claim 2, wherein the sputter gas content of at least one other layer is 2 to 10 at%. 4. The sputter gas content is continuously different in the thickness direction, the content of at least a part of the sputter gas is 0 to 3 at%, and the content of another part of the sputter gas is the same. 2 to 10%, which is more than the above. 5. A method of forming a wear-resistant protective film on a thermal head having a heat-generating layer and a pair of electrodes on a substrate or a heat-resistant layer formed on the substrate by a sputtering method. A method for producing a wear-resistant protective film for a thermal head, characterized in that a part of the film is formed under a strong bias and the other part is formed under no bias or a weak bias. 6. The sputter gas content of at least one layer formed under no bias or weak bias is 0 to 3 at.
%, The method for producing a wear-resistant protective film for a thermal head according to claim 5. 7. The method for producing a wear-resistant protective film for a thermal head according to claim 6, wherein the content of the sputter gas in at least one other layer formed under a strong bias is 2 to 10 at%. 8. The thickness of at least one other layer formed under a strong bias is 0.1 to 5 μm.
1. A method for producing an abrasion resistant protective film for a thermal head according to any one of 1. 9. The strong bias voltage is -50 to -200V.
9. The method for producing a wear-resistant protective film for a thermal head according to claim 8, wherein the weak bias or non-bias voltage is 0 to 2/3 of the strong bias. 10. The strong bias voltage is -60 to -120.
The method for producing a wear-resistant protective film for a thermal head according to claim 9, wherein the voltage is V, and the weakly biased or non-biased voltage is 1/10 to 1/2 of the strong bias. 11. The method for producing a wear resistant protective film for a thermal head according to claim 8, wherein the strength of the bias is continuously changed.