JPH08150746A - Thermal head - Google Patents

Abstract

PURPOSE: To provide a thermal head capable of well functioning over a long period of time by effectively preventing the thermal damage of a heating resistor. CONSTITUTION: A heating resistor 12, a pair of electrodes 13 and the protective film covering them are provided on an insulating substrate 11 and a first conductor layer 15 having a Fermi level lower than that of the heating resistor 15 and a pair of electrodes 13, an insulator layer 16 or an N-type conductor layer and the second conductor layer 17 connected to earth potential GND are successively deposited on the protective film 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a thermal head incorporated as a printer mechanism for a word processor, a facsimile or the like.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 15, a thermal head incorporated as a printer mechanism such as a word processor comprises a heat generating resistor 2 made of tantalum nitride or the like and aluminum or the like on an insulating substrate 1 made of alumina ceramics or the like. The heating resistor 2 and the pair of electrodes 3 are adhered to the pair of electrodes 3, and the heating resistor 2 and the pair of electrodes 3 are covered with a protective film 4 made of silicon nitride or the like.
By applying a predetermined electric power based on a print signal in between to selectively heat the heating resistor 2 by Joule heat, the ink of the ink ribbon 5 is melted by the generated heat, and this is transferred to a recording paper. Functions as a thermal head.

The ink ribbon 5 has an ink layer on one main surface of a film made of PET (polyethylene terephthalate) or the like, and a back coating material on the other main surface for reducing friction with a thermal head. And is successively conveyed along with the recording paper onto the heating resistor 2.

[0004]

However, in this conventional thermal head, when printing, when the ink ribbon 5 makes sliding contact with the thermal head, static electricity is generated due to friction between the two. When this static electricity is gradually accumulated and the potential of the surface of the protective film 4 reaches a predetermined value, a discharge occurs between the heating resistor 2 and the surface of the protective film 4, and the protective film 4 is electrostatically destroyed by the discharge. To be done. As a result, it becomes impossible to satisfactorily protect the heat generating resistor 2 and the like with the protective film 4, and a large current flows through the heat generating resistor 2 due to the above-mentioned discharge, so that the heat generating resistor 2 is thermally damaged early. It has a drawback that the function as a thermal head is lost in a short period of time.

Therefore, in order to solve the above-mentioned drawbacks, a conductor layer connected to one of the electrodes 3 is deposited on the protective film 4, and static electricity generated by sliding contact with an ink ribbon or the like is applied on the protective film 4. It has been proposed to escape to the electrode 3 via a conductor layer.

However, the protective film 4 for the thermal head
Even if a conductor layer connected to one of the electrodes 3 is deposited on top of it, if a minute defect (crack or the like) is formed in the protective film 4, the charge due to the sliding contact with the ink ribbon 5 causes the defect. When the area is charged and reaches a predetermined value, discharge starts from the defective area. In order to prevent such discharge from occurring, it is necessary to eliminate the above-mentioned defects, but it is actually extremely difficult to form the protective film 4 having no defects by sputtering or the like, and the thermal head described above is used. Compared with, the life of the heating resistor 2 is slightly extended,
The function as a thermal head was lost in a relatively short period of time.

[0007]

SUMMARY OF THE INVENTION The present invention has been devised in view of the above-mentioned drawbacks, and an object thereof is to effectively prevent thermal damage to a heating resistor,
It is to provide a thermal head that can function well for a long period of time.

[0008]

In the thermal head of the present invention, a heating resistor, a pair of electrodes, and a protective film that covers them are deposited on an insulating substrate, and the protective film is coated on the insulating film. A first conductor layer having a Fermi level lower than that of the heating resistor and the pair of electrodes, an insulator layer or an n-type semiconductor layer, and a second conductor layer connected to a ground potential are sequentially deposited. Characterize.

Further, in the thermal head of the present invention, a heating resistor, a pair of electrodes, and a protective film covering them are adhered on an insulating substrate, and the heating resistor and the heating resistor are provided on the protective film. A first n-type semiconductor layer having a Fermi level lower than that of a pair of electrodes, an insulator layer or a p-type semiconductor layer, and a second n-type semiconductor layer connected to a ground potential are sequentially deposited. Is characterized by.

Further, in the thermal head of the present invention, a heating resistor, a pair of electrodes, and a protective film covering them are adhered on an insulating substrate, and the heating resistor and the heating resistor are provided on the protective film. The n-type semiconductor layer having a Fermi level lower than that of the pair of electrodes and the p-type semiconductor layer connected to the ground potential are sequentially deposited.

[0011]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

(First Embodiment) FIG. 1 is a sectional view showing a first embodiment of a thermal head of the present invention. 11 is an insulating substrate, 12 is a heating resistor, 13 is a pair of electrodes, and 14 is a protective film. , 15 is a first conductor layer, 16 is an insulator layer, and 17 is a second conductor layer.

When the insulating substrate 11 is made of, for example, alumina ceramics, a suitable organic solvent or solvent is added to and mixed with ceramic raw material powder such as alumina, silica, magnesia, etc. to form a sludge, and this is formed by a conventionally known doctor blade. Ceramic green sheet is formed by employing a method such as a calender roll method or a calender roll method, and thereafter, the ceramic green sheet is punched into a predetermined shape and fired at a high temperature (about 1600 ° C.).

Further, a heat storage layer 11a made of glass or the like is deposited on the upper surface of the insulating substrate 11, and the heat storage layer 11 is formed.
“A” accumulates the heat generated by the heat-generating resistor 12 and acts to bring the temperature of the thermal head to a temperature necessary for melting the ink of the ink ribbon in a short time.

The heat storage layer 11a is formed by applying a glass paste obtained by adding and mixing an appropriate organic solvent to glass powder onto the upper surface of the insulating substrate 11 by applying a conventionally known screen printing method or the like. , And is deposited on the upper surface of the insulating substrate 11 by baking it at a high temperature, for example, a temperature of about 700 ° C.

On the upper surface of the heat storage layer 11a, the heating resistor 1
2 and a pair of electrodes 1 connected to both ends of the heating resistor 12
3 and 3 are attached.

The heating resistor 12 is made of, for example, tantalum nitride or the like and has a predetermined electric resistivity, so that when power is applied through the pair of electrodes 13, Joule heat is generated and the ink is discharged. Heat is generated at a temperature necessary to melt the ink on the ribbon, for example, a temperature of 200 ° C to 300 ° C.

The pair of electrodes 13 connected to the heating resistor 12 are made of a metal such as aluminum.
A power source of 0V to 25V is connected through a switching element or the like, and has a function of applying a predetermined electric power necessary for causing Joule heat generation to the heating resistor 12.

The heating resistor 12 and the pair of electrodes 1
3 is deposited with a predetermined thickness on the insulating substrate 11 by using a conventionally known sputtering method or the like, and is processed into a predetermined pattern by using a photolithography technique.

A protective film 14 covering the heating resistor 12 and the pair of electrodes 13 is also applied to the upper surfaces of the heating resistor 12 and the pair of electrodes 13, and the protective film 14 is the heating resistor. 12, the pair of electrodes 13 are protected from abrasion caused by sliding contact of the ink ribbon and corrosion caused by contact of contaminants such as sodium ions contained in the ink ribbon.

The protective film 14 is made of, for example, silicon nitride, sialon, or the like, and is applied to the upper surface of the heating resistor 12 or the like with a predetermined thickness by adopting a conventionally known sputtering method or the like.

On the upper surface of the protective film 14, a first conductor layer 15 is set to a Fermi level lower than that of the heating resistor 12 and the pair of electrodes 13 by being biased to a predetermined potential (for example, 30V to 40V). An insulator layer 16 and a second conductor layer 17 connected to ground potential are successively deposited.

The first conductor layer 15 and the second conductor layer 17
Is made of tantalum nitride, chromium, nickel, TaSiO ₂ or the like, and each is formed with a thickness of 0.1 to 1 μm.

The insulator layer 16 interposed between the first conductor layer 15 and the second conductor layer 17 is made of silicon nitride, sialon, carbon nitride or the like, and has a thickness of about 2 μm.

FIG. 2 is an energy band diagram schematically showing the Fermi level of electrons when the printing operation is not performed.

Since the first conductor layer 15 is biased to a potential of 30V to 40V, the Fermi level ε _F of electrons here is lower than the surface of the heating resistor 12 and the protective film 14.

After starting the printing operation, if the defective portion of the insulator layer 16 is charged with, for example, negative charges, the potential energy of electrons in the second conductor layer 17 rises as shown in FIG. At this time, the first conductor layer 15 and the second conductor layer 15
The band structure between the conductor layer 17 and the conductor layer 17 is largely inclined, and when the degree of inclination exceeds a predetermined value, the electrons of the second conductor layer 17 pass through the insulator layer 16 that has been an electron barrier. A transition is made to the first conductor layer 15, and a current flows.

The electrons transiting to the first conductor layer 15 release thermal energy and are stabilized at the Fermi level of the first conductor layer 15, and the excess electrons are discharged to the power source side.

At this time, since the Fermi level of the first conductor layer 15 is lower than the Fermi level of the heating resistor 12 and the pair of electrodes 13, the Fermi level is stable in the potential well and the heating resistor 12 and the pair of electrodes 13 are stable. The re-transition from the electrode 13 does not occur.

On the other hand, when the defects in the insulating layer 16 are charged with positive charges, the potential energy of electrons in the second conductor layer 17 drops as shown in FIG. At this time, the band structure between the first conductor layer 15 and the second conductor layer 17 is largely inclined, and when the degree of inclination exceeds a predetermined magnitude, the electrons of the first conductor layer 15 act as an electron barrier. After passing through the insulating layer 16 that has been formed, the transition is made to the second conductor layer 17, and a current flows.

The electrons transiting to the first conductor layer 15 release thermal energy and are stabilized at the Fermi level of the second conductor layer 17, and the excess electrons are discharged to the GND side.

At this time, the Fermi level of the first conductive layer 15 is low, and the first conductive layer 15, the heating resistor 12 and the electrode 1 are formed.
Since the potential difference between the first and second electrodes is small, discharge does not occur between the two. Therefore, transition of electrons from the heating resistor 12 and the electrode 13 does not occur, and damage to the heating resistor 12 is effectively prevented.

Therefore, the protective film 14 is hardly broken down, and the heat resistance of the heating resistor 12 is effectively prevented, so that the thermal head can function well for a long period of time.

Further, the heating resistor 12 and the pair of electrodes 1
The potential difference between the third conductor layer 15 and the first conductor layer 15 is set lower than the maximum allowable voltage (A × B) of the protective film 14 obtained by the product of the withstand voltage A of the protective film 14 and the thickness B of the protective film 14. If so, even if the amount of charge on the surface of the second conductive layer 17 is extremely large, discharge does not occur between the first conductive layer 15 and the heating resistor 12 or the like. Therefore, when the withstand voltage of the protective film 14 is A and the thickness thereof is B, the potential of the first conductor layer 15 is
It is preferably smaller than (A × B). For example, the protective film 14 is formed of silicon nitride (withstand voltage: 4
When formed at a voltage of × 10 ⁵ V / mm), the maximum allowable voltage (A × B) of the protective film 14 is 800 V, and the high potential side of the pair of electrodes 3 is 25 V and the low potential side is 0 V.
If it is set to V, the potential of the first conductor layer 15 is set to 30V.
The above relationship is satisfied by biasing to -40V.

Thus, the thermal head of this embodiment applies a predetermined electric power between the pair of electrodes 13 to generate the heating resistor 12.
While selectively generating Joule heat, the generated heat is conducted to the ink ribbon, and the ink of the ink ribbon is melted and transferred to the recording paper to function as a thermal head.

(Second Embodiment) FIG. 5 is a sectional view showing a second embodiment of the thermal head of the present invention. The same parts as those of the thermal head shown in FIG. 1 are designated by the same reference numerals.

The difference between the thermal head shown in FIG. 5 and the thermal head shown in FIG. 1 is that in the first embodiment, the protective film 1 is used.
While the insulator layer 16 is interposed between the first conductor layer 15 and the second conductor layer 17 formed on the second conductor layer 4,
In the embodiment, the first conductor layer 15 formed on the protective film 14
An n-type semiconductor layer 18 in which silicon (Si) is doped with phosphorus (P) is interposed between the semiconductor layer 18 and the first conductor layer 15 and the second conductor layer 17. The point is that Schottky connections were made and a Schottky barrier was formed at these interfaces.

For the n-type semiconductor layer 18, a conventionally well-known CVD method is adopted, silicon is deposited on the first conductor layer 15, and phosphorus is implanted into the deposited silicon by an ion implantation method or the like. The first conductor layer 15
It is formed on the upper surface and is simultaneously Schottky-connected to the first conductor layer 15.

FIG. 6A is a band diagram schematically showing the Fermi level and the energy potential when the printing operation is not performed, and the first conductor layer 15 is 30
Since it is biased to a potential of V to 40 V, the Fermi level ε _F of the electron here is the heating resistor 12 and the protective film 1.
It is lower than the surface of No. 4.

After the printing operation is started, for example, when negative defects are charged in the n-type semiconductor layer 18, the second conductor layer 17 is charged.
The potential energy of the electron at is shown in Fig. 6 (b).
As shown in, rises. At this time, the band structure between the first conductor layer 15 and the second conductor layer 17 is largely inclined, and when the degree of inclination exceeds a predetermined magnitude, the second conductor layer 17 is formed.
Of the electrons pass through the n-type semiconductor layer 18 that has been an electron barrier, transition to the first conductor layer 15, and a current flows.

The electrons transiting to the first conductor layer 15 release thermal energy and are stabilized at the Fermi level of the first conductor layer 15, and the excess electrons are discharged to the power supply side.

At this time, the Fermi level of the first conductor layer 15 is biased to a predetermined potential so as to be lower than the Fermi level of the heating resistor 12 and the pair of electrodes 13, so that the Fermi level is stabilized in the potential well and heat is generated. The re-transition from the resistor 12 and the pair of electrodes 13 does not occur.

On the other hand, when the defects of the n-type semiconductor layer 18 are charged with positive charges, the potential energy of electrons in the second conductor layer 17 is lowered as shown in FIG. 6 (c). At this time, the band structure between the first conductor layer 15 and the second conductor layer 17 is largely inclined, and when the degree of inclination exceeds a predetermined magnitude, the electrons of the first conductor layer 15 act as an electron barrier. The second conductor layer 1 passing through the n-type semiconductor layer 18
Transition to 7 and current flows.

The electrons transiting to the first conductor layer 15 release thermal energy and are stabilized at the Fermi level of the second conductor layer 17, and the excess electrons are discharged to the GND side.

At this time, the Fermi level of the first conductive layer 15 is low, and the first conductive layer 15, the heating resistor 12 and the electrode 1 are formed.
Since the potential difference between the first and second electrodes is small, discharge does not occur between the two. Therefore, transition of electrons from the heating resistor 12 and the electrode 13 does not occur, and damage to the heating resistor 12 is effectively prevented.

Therefore, the protective film 14 is hardly dielectrically damaged, and the thermal resistance of the heating resistor 12 is effectively prevented, so that the thermal head can function well for a long period of time.

(Third Embodiment) FIG. 7 is a sectional view showing a third embodiment of the thermal head according to the present invention. The same parts as those of the thermal head shown in FIG. 1 are designated by the same reference numerals.

The difference between the thermal head shown in FIG. 8 and the thermal head shown in FIG. 1 is that in the first embodiment, the protective film 1 is used.
4, the first conductor layer 15, the insulator layer 16, the second conductor layer 1
7 are sequentially formed, in the third embodiment, the first n-type semiconductor layer 19 in which silicon (Si) is doped with phosphorus (P), the insulating layer 16 and the silicon ( A second n-type semiconductor layer 20 in which Si is doped with phosphorus (P) is sequentially formed, the second n-type semiconductor layer 20 is connected to the ground potential, and the first n-type semiconductor layer 19 generates heat. It is a point biased to a predetermined potential so that the Fermi level is lower than that of the resistor 12 and the pair of electrodes 13.

FIG. 8A is a band diagram schematically showing the Fermi level and energy potential when the printing operation is not performed. The first n-type semiconductor layer 1 is shown in FIG.
Since 9 is biased to a potential of 30V-40V,
The Fermi level ε _F of the electrons here is lower than the surfaces of the heating resistor 12 and the protective film 14.

After the printing operation is started, for example, when the defects of the insulator layer 16 are charged with negative charges, the potential energy of electrons in the second n-type semiconductor layer 20 is as shown in FIG.
As shown in (b), it rises. At this time, the band structure between the first n-type semiconductor layer 19 and the second n-type semiconductor layer 20 is largely inclined, and when the degree of inclination exceeds a predetermined value, the second n-type semiconductor layer is formed. The electrons of the layer 20 pass through the insulator layer 16 that has been an electron barrier, transition to the first n-type semiconductor layer 19, and a current flows.

The electrons transiting to the first n-type semiconductor layer 19 release thermal energy and are stabilized at the Fermi level of the first n-type semiconductor layer 19, and the excess electrons are discharged to the power supply side. At this time, since the Fermi level of the first n-type semiconductor layer 19 is biased to a predetermined potential so as to be lower than the Fermi level of the heating resistor 12 and the pair of electrodes 13, it is stabilized in the potential well and heat is generated. The re-transition from the resistor 12 and the pair of electrodes 13 does not occur.

On the other hand, when the defects in the insulator layer 16 are charged with positive charges, the potential energy of electrons in the second n-type semiconductor layer 20 drops as shown in FIG. 8 (c). At this time, the band structure between the first n-type semiconductor layer 19 and the second n-type semiconductor layer 20 is largely inclined, and when the degree of inclination exceeds a predetermined magnitude, the first n-type semiconductor layer is formed. Insulator layer 16 in which electrons in layer 19 acted as an electron barrier
To transit to the second n-type semiconductor layer 20 and a current flows. The electrons that have transited to the first n-type semiconductor layer 19 release thermal energy and are stabilized at the Fermi level of the second n-type semiconductor layer 20, and the excess electrons are discharged to the GND side.

At this time, since the Fermi level of the first n-type semiconductor layer 19 is low and the potential difference between the first n-type semiconductor layer 19 and the heating resistor 12 and the electrode 13 is small,
No discharge occurs between them. Therefore, the transition of electrons from the heating resistor 12 and the electrode 13 does not occur,
Damage to the heating resistor 12 is effectively prevented.

Therefore, the protective film 14 is almost never dielectrically damaged, and the thermal resistance of the heating resistor 12 is effectively prevented, so that the thermal head can function well for a long period of time.

(Fourth Embodiment) FIG. 9 is a sectional view showing a third embodiment of the thermal head according to the present invention. The same parts as those of the thermal head shown in FIGS. 1 and 7 are designated by the same reference numerals.

The difference between the thermal head shown in FIG. 9 and the thermal head shown in FIG. 1 is that in the first embodiment, the protective film 1 is used.
4, the first conductor layer 15, the insulator layer 16, the second conductor layer 1
7 are sequentially formed, in the fourth embodiment, the first n-type semiconductor layer 19 in which silicon (Si) is doped with phosphorus (P) is formed on the protective film 14, and the silicon (Si) is doped with boron (P). B) doped p-type semiconductor layer 21, silicon (S
Second n-type semiconductor layer 20 in which i) is doped with phosphorus (P)
Are sequentially formed, and the second n-type semiconductor layer 20 is connected to the ground potential, and the first n-type semiconductor layer 19 has a predetermined Fermi level lower than that of the heating resistor 12 and the pair of electrodes 13. It is the point biased to the potential.

FIG. 10A is a band diagram schematically showing the Fermi level and energy potential when the printing operation is not performed, and the first n-type semiconductor layer 19 has a voltage of 30V to 40V, for example. Since the potential is biased, the Fermi level ε _F of electrons here is lower than the surfaces of the heating resistor 12 and the protective film 14.

After the printing operation is started, for example, if the defects of the p-type semiconductor layer 21 are charged with negative charges, the potential energy of electrons in the second n-type semiconductor layer 20 is as shown in FIG. 10 (b). To rise. At this time, the first n
The band structure between the n-type semiconductor layer 19 and the second n-type semiconductor layer 20 is largely tilted, and when the degree of tilt exceeds a predetermined level, electrons in the second n-type semiconductor layer 20 become electrons. The current passes through the first n-type semiconductor layer 19 after passing through the p-type semiconductor layer 21 that has been a barrier.

The electrons transiting to the first n-type semiconductor layer 19 release thermal energy and are stabilized at the Fermi level of the first n-type semiconductor layer 19, and the excess electrons are discharged to the power supply side. At this time, since the Fermi level of the first n-type semiconductor layer 19 is biased to a predetermined potential so as to be lower than the Fermi level of the heating resistor 12 and the pair of electrodes 13, it is stabilized in the potential well and heat is generated. The re-transition from the resistor 12 and the pair of electrodes 13 does not occur.

On the other hand, when the defects of the p-type semiconductor layer 21 are positively charged, the potential energy of the electrons in the second n-type semiconductor layer 20 drops as shown in FIG. 10 (c). At this time, the band structure between the first n-type semiconductor layer 19 and the second n-type semiconductor layer 20 is largely inclined, and when the degree of inclination exceeds a predetermined magnitude, the first n-type semiconductor layer is formed. The electrons in the layer 19 pass through the p-type semiconductor layer 21 that has been an electron barrier, transition to the second n-type semiconductor layer 20, and a current flows.

The electrons transiting to the first n-type semiconductor layer 19 release thermal energy and are stabilized at the Fermi level of the second n-type semiconductor layer 20, and the excess electrons are discharged to the GND side.

At this time, since the Fermi level of the first n-type semiconductor layer 19 is low and the potential difference between the first n-type semiconductor layer 19 and the heating resistor 12 and the electrode 13 is small,
No discharge occurs between them. Therefore, the transition of electrons from the heating resistor 12 and the electrode 13 does not occur,
Damage to the heating resistor 12 is effectively prevented.

Therefore, the protective film 14 is hardly dielectrically damaged, and the thermal resistance of the heating resistor 12 is effectively prevented, so that the thermal head can function well for a long period of time.

(Fifth Embodiment) FIG. 11 is a sectional view showing a fifth embodiment of the thermal head according to the present invention. The same parts as those of the thermal head shown in FIG. 1 are designated by the same reference numerals.

The thermal head shown in FIG. 11 differs from the thermal head shown in FIG. 1 in that the first conductor layer 15, the insulator layer 16, and the second conductor layer 17 are formed on the protective film 14 in the first embodiment. On the other hand, in the third embodiment, the n-type semiconductor layer 22 in which silicon (Si) is doped with phosphorus (P) and the silicon (Si) is doped with boron (B) in the third embodiment. The p-type semiconductor layer 23 is sequentially formed, the p-type semiconductor layer 23 is connected to the ground potential, and the n-type semiconductor layer 2 is formed.
2 is a point biased to a predetermined potential so that the Fermi level is set lower than that of the heating resistor 12 and the pair of electrodes 13.

FIG. 12A is a band diagram schematically showing the Fermi level and energy potential when the printing operation is not performed. The n-type semiconductor layer 22 is biased to a potential of 30V to 40V, for example. Has been done.
Therefore, the Fermi level ε _F of electrons in the n-type semiconductor layer 22 is lower than the surfaces of the heating resistor 12 and the protective film 14.

After the printing operation is started, for example, when the p-type semiconductor layer 23 is negatively charged, the potential energy of electrons in the p-type semiconductor layer 23 rises as shown in FIG. 12 (b). At this time, the p-type semiconductor layer 23 and n
The band structure with the type semiconductor layer 22 is greatly inclined,
When the degree of inclination exceeds a predetermined level, the electrons of the p-type semiconductor layer 23 serve as an electron barrier,
To transit to the n-type semiconductor layer 22 and a current flows.

The electrons transiting to the n-type semiconductor layer 22 release thermal energy and are stabilized at the Fermi level of the n-type semiconductor layer 22, and excess electrons are discharged to the power supply side.

At this time, since the Fermi level of the n-type semiconductor layer 22 is biased to a predetermined potential so as to be lower than the Fermi level of the heating resistor 12 and the pair of electrodes 13, it stabilizes in the potential well, Heating resistor 1
The re-transition from the two electrodes and the pair of electrodes 13 does not occur.

On the other hand, when the p-type semiconductor layer 23 is positively charged, the potential energy of the electrons in the p-type semiconductor layer 23 decreases as shown in FIG. 12 (c).
At this time, the band structure between the p-type semiconductor layer 23 and the n-type semiconductor layer 22 is greatly inclined, and when the degree of inclination exceeds a predetermined level, the electrons of the p-type semiconductor layer 23 act as an electron barrier. After passing through the p-type semiconductor layer 23, the transition to the n-type semiconductor layer 22 occurs and a current flows.

The electrons transiting to the n-type semiconductor layer 22 release thermal energy and are stabilized at the Fermi level of the n-type semiconductor layer 22, and the excess electrons are discharged to the GND side.

At this time, since the Fermi level of the n-type semiconductor layer 22 is low and the potential difference between the n-type semiconductor layer 22 and the heating resistor 12 and the electrode 13 is small, discharge does not occur between them. . Therefore, transition of electrons from the heating resistor 12 and the electrode 13 does not occur, and the heating resistor 1
The damage of 2 is effectively prevented.

Therefore, the protective film 14 is hardly dielectrically damaged, and the thermal resistance of the heating resistor 12 is effectively prevented, so that the thermal head can function well for a long period of time.

(Sixth Embodiment) FIG. 13 is a sectional view showing a sixth embodiment of the thermal head according to the present invention. The same parts as those of the thermal head shown in FIG. 1 are designated by the same reference numerals.

The thermal head shown in FIG. 13 differs from the thermal head shown in FIG. 1 in that the protective film 14 is formed by sputtering silicon nitride or the like in the first embodiment, whereas the sixth embodiment is different. In the example, the protective film 25 is formed to a thickness of about 2 μm by baking glass containing 10 to 80% by weight of an alumina filler (diameter: 0.3 to 0.7 μm) 25a, and the protective film 25 and the heating resistor are formed. The point is that an antioxidant layer 24 made of silicon nitride or the like is interposed between the first and second electrodes 12 and 12 with a thickness of 0.2 to 2 μm.

FIG. 14A is a band diagram schematically showing the Fermi level and energy potential when the printing operation is not performed. The first conductor layer 15 is biased to a potential of 30V to 40V, for example. Has been done. Therefore, the Fermi level ε _F of electrons in the first conductor layer 15
Is lower than the surfaces of the heating resistor 12 and the protective film 25.

After the printing operation is started, for example, when the insulator layer 16 is negatively charged, the potential energy of electrons in the second conductor layer 17 rises as shown in FIG. 14 (b). At this time, the insulator layer 16 and the second conductor layer 1
The band structure between the first conductor layer 17 and the second conductor layer 17 has a large inclination, and when the degree of inclination exceeds a predetermined value, the electrons in the second conductor layer 17 become
The current passes through the insulator layer 16 that has been an electron barrier, transitions to the first conductor layer 15, and a current flows.

The electrons transiting to the first conductor layer 15 release thermal energy and are stabilized at the Fermi level of the first conductor layer 15, and the excess electrons are discharged to the power source side.

At this time, since the Fermi level of the first conductor layer 15 is biased to a predetermined potential so as to be lower than the Fermi level of the heating resistor 12 and the pair of electrodes 13, it stabilizes in the potential well, Heating resistor 12
Also, re-transition from the pair of electrodes 13 does not occur.

On the other hand, when the second conductor layer 17 is positively charged, the potential energy of the electrons in the second conductor layer 17 drops as shown in FIG. 14 (c). At this time, the band structure between the second conductor layer 17 and the insulator layer 16 is largely inclined, and when the degree of inclination exceeds a predetermined level, the electrons of the second conductor layer 17 become an electron barrier. After passing through the insulating layer 16 which has been used, the transition is made to the first conductor layer 15, and a current flows.

The electrons transiting to the first conductor layer 15 release thermal energy and are stabilized at the Fermi level of the first conductor layer 15, and the excess electrons are discharged to the GND side.

At this time, the Fermi level of the first conductor layer 15 is low, and the first conductor layer 15, the heating resistor 12 and the electrode 1 are formed.
Since the potential difference between the first and second electrodes is small, discharge does not occur between the two. Therefore, transition of electrons from the heating resistor 12 and the electrode 13 does not occur, and damage to the heating resistor 12 is effectively prevented.

Therefore, the protective film 25 is hardly dielectrically damaged, and the thermal resistance of the heating resistor 12 is effectively prevented, so that the thermal head can function well for a long period of time.

In this case, the first conductor layer 15 has high adhesiveness to both the protective film 25 made of sintered glass and the insulator layer 16 formed by sputtering silicon nitride or the like. Therefore, the protective film 25 and the first conductor layer 15
Also, peeling or the like does not occur between the respective layers of the insulating layer 16, and the layers 25, 15 and 16 can be firmly adhered onto the insulating substrate 11.

If the insulating layer 16 is made of a hard material (silicon nitride, sialon, silicon carbide) having a Vickers height of 1000 kg / mm ² or more, it is
Even if foreign matter is caught between the thermal head and the thermal paper, the insulating layer 16 and the protective film 25 disposed thereunder are effectively prevented from being deformed.

[0086]

According to the present invention, the dielectric breakdown of the protective film can be effectively prevented, and a thermal head that can function well for a long period of time can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing a thermal head according to a first embodiment of the present invention.

FIG. 2 is an energy band diagram when the thermal head of FIG. 1 is not performing a printing operation.

FIG. 3 is an energy band diagram when the thermal head of FIG. 1 is charged with negative charges.

FIG. 4 is an energy band diagram when the thermal head of FIG. 1 is charged with positive charges.

FIG. 5 is a sectional view showing a thermal head according to a second embodiment of the present invention.

6 is an energy band diagram of the thermal head shown in FIG.

FIG. 7 is a sectional view showing a thermal head according to a third embodiment of the present invention.

8 is an energy band diagram of the thermal head shown in FIG.

FIG. 9 is a sectional view showing a thermal head according to a fourth embodiment of the present invention.

FIG. 10 is an energy band diagram of the thermal head shown in FIG.

FIG. 11 is a sectional view showing a thermal head according to a fifth embodiment of the present invention.

12 is an energy band diagram of the thermal head shown in FIG.

FIG. 13 is a sectional view showing a thermal head according to a sixth embodiment of the present invention.

FIG. 14 is an energy band diagram of the thermal head shown in FIG.

FIG. 15 is a sectional view of a conventional thermal head.

[Explanation of symbols]

 11 ... Insulating substrate 12 ... Heating resistor 13 ... Pair of electrodes 14, 25 ... Protective film 15 ... First conductor layer 16 .... Insulator layer 17 ... Second conductor layers 18, 22 ... n-type semiconductor layer 19 ... First n-type semiconductor layer 20 ... ..Second n-type semiconductor layers 21, 23 ..p-type semiconductor layers 24 ..

Claims (3)

[Claims] 1. A heating resistor, a pair of electrodes, and a protective film covering them are deposited on an insulating substrate, and
On the protective film, a first conductor layer having a Fermi level lower than that of the heating resistor and the pair of electrodes, an insulator layer or an n-type semiconductor layer, and a second conductor layer connected to the ground potential are sequentially formed. A thermal head characterized by being attached. 2. A heating resistor, a pair of electrodes, and a protective film covering them are deposited on an insulating substrate, and
On the protective film, a first n-type semiconductor layer having a Fermi level lower than that of the heating resistor and the pair of electrodes, an insulator layer or a p-type semiconductor layer, and a second n-type connected to the ground potential. A thermal head characterized in that a type semiconductor layer is sequentially deposited. 3. A heating resistor, a pair of electrodes, and a protective film covering them are deposited on an insulating substrate, and
An n-type semiconductor layer having a Fermi level lower than that of the heating resistor and the pair of electrodes and a p-type semiconductor layer connected to the ground potential are sequentially deposited on the protective film. head.