US 3814897 A
A thermal printing head comprising a semiconductor body and a heat-generating resistive layer formed by diffusing an impurity into a surface portion of the semiconductor body by any suitable known method. In the thermal printing head, silicon having a low resistivity is employed to form the semiconductor body and an impurity having a conductivity opposite to that of the semiconductor body is employed to form the heat-generating resistive layer so as to completely concentrate current in the resistive layer and attain remarkable improvements in the heat generating characteristic of the thermal printing head.
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Description (OCR text may contain errors)
United States Patent V [1 1 ()tani et al.
11m. 3,814,897 1 June 4, 1974 THERMAL PRINTING HEAD  Inventors: lliroshi Otani, Shijonawate; Noboru Yukami, Hirakata, both of Japan  Assignee: Matsushita Electric Industrial Co.,
Ltd., Osaka, Japan 221 Filed: Feb. 29, 1972 21 Appl.No.:230,332
 Foreign Application Priority Data Mar. 4, 1971 Japan ..L 46-11675  11.5. C1 219/216, 219/543, 317/234 Q, 346/76 R  Int. Cl. H051) 1/00  Field of Search 219/216, 543; 346/76 R; 1 178/30; 29/569, 590; 317/234 Q  References Cited UNITED STATES PATENTS 1161.457 12/1964 Schroeder et a1 346/76 R 9/1971 Davis et a1. 219/216 X Mosher  ABSTRACT A thermal printing head comprising a semiconductor body and a heat-generating resistive layer formed by diffusing an impurity into a surface portion of the semiconductor body by any suitable known method. In the thermal printing head, silicon having a low resistivity is employed to form the semiconductor body and an impurity having a conductivity opposite to that of the semiconductor body is employed to form the heat-generating resistive layer so as to completely concentr'ate current in the resistive layer and attain remarkable improvements in the heat generating characteristic of the thermal printing head.
5 Claims, 8 Drawing Figures PATENTEnJun 4mm v 381 2.89?
. saw Mr 4 I 1' i THERMAL PRINTING HEAD This invention relates to a thermal printing head for use in a device which is adapted to print information on a heat-sensitive recording medium by means of thermal energy.
Various types of heat-sensitive sheets of paper which produce colors in response to the application of thermal energy are now available on the market. Thermal printing heads of the kind in which head is generated by the concentrated flow of current through a resistor portion have been developed and are now available on the market. The thermal printing heads of this kind can be broadly classified into two types. In one type of thermal printing heads, a plurality of resistor portions are provided on-one end surface of a body of material having a high resistivity as an integral part of the body and current is passed through the resistor bank for generating heat in the resistor bank, while in the other type, a semiconductor such as silicon is used as a heatgenerating resistor and current is supplied to thisresistor through a supporting member supporting the resistor so as to cause generation of heatin the resistor.
In the thermal printing head of the former type, the resistance of the body must be sufficiently higher than that of the heat-generating resistor portions. Practically, an electrical insulator such as glass or ceramics is used as the material for the body, and a compound such as tin oxide is used to form the resistor portions. In the thermal printing head of the latter type in which the body having a high resistivity is formed from a semiconductor such as silicon, the impurity concentration in a surface portion of the semiconductor body is selectively increased by a known method of selective diffusion or the like so as to reduce the resistance of the surface portion to a value extremely lower than that of the semiconductor body. The thermal printing head of the latter type is appreciated in that the greater part of current can be concentrated in the surface portion doped with the impurity thereby generating substantial heat in such portion.
It is a primary object of the present invention to provide a thermal printing head of novel and improved construction showing an improved heat generating characteristic over conventional ones.
The present invention is featured by the fact that, in a thermal printing head comprising a semiconductor body and a heat-generating resistive layerformed by diffusing an impurity into a surface portion of the semiconductor body by any suitable known method, silicon having a low resistivity is employed to form the semiconductor body and an impurity having a conductivity opposite to that of the semiconductor body is employed to form the heat-generatin g resistive layer so as to completely concentrate current in the resistive layer and attain remarkable improvements in the heat generating characteristic of the thermal printing head.
The above and other objects, features and advan-' tages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings, in which: I
FIGS. 1A and 1B are a perspective view and a longitudinal section respectively of a thermal printing head according to the present invention;
FIG. 2 is a perspective view of an array of thermal printing head chips made in accordance with the present invention;
FIGS. 3A to 3D show successive steps for the manufacture of the thermal printing head according to the present invention; and
FIG. 4 is a graph showing variations in the resistivity of a semiconductor relative to temperatures.
Referring to FIGS. IA and IB, a thermal printing head according to the present invention comprises a semiconductor body 11 of p-type or n-type silicon having a low resistivity. A resistive layer I2 is formed in a surface portion including one end of the semiconductor body 11 by a known diffusion method and serves as a heat generating layer. This resistive layer 12 is of the n-type and p-type when the semiconductor body 11 is of the p-type and n-type respectivelythereby forming a pn junction 15 therebetween. A pair of electrodes 13 and 13' are in ohmic contact with the resistive layer I2 so that, when a voltage is applied across these electrodes 13 and 13', concentrated flow of current passes through the resistive layer 12 thereby generating substantial heat in the resistive layer 12. A layer 14 of electrical insulator such as a silicon oxide film is formed on the semiconductor body 11 so as to prevent the electrodes 1 3 and 13 from contacting the surface portions of the semiconductor body 11 which are not doped with the impurity. Colors are produced on a heatsensitive sheet 16 at portions which are engaged by the hot head. It will be understood that such a thermal printing head can be constructed to have any desired size, and a plurality of thermal printing head chips of any suitable shape can also be arranged in an array as seen in FIG. 2. Referring to FIG. 2, a plurality of thermal printing head chips 18 are arranged in a line with an electrical insulator 17 interposed therebetween.
A method of making the thermal printing head of above constructionwill now be described with reference to FIGS. 3A to 3D. i
A semiconductor body 21 of silicon having a low resistivity is prepared at first as shown in FIG. 3A. A perspective view and a side elevation of this semiconductorbody 21 are shown on the left-hand side and righthand side respectively of FIG. 3A. This semiconductor body 21 is pre-formed to have a height h and a thickness w, which conform to the predetermined height and thickness of a thermal printing head to be finally produced. Then, as shown in FIG. 3B, substantial portions of the opposite side surfaces of the semiconductor body 21 are covered with an oxide f lm 23 except the end surface 22 and those surface portions lying within a suitable distance I measured from the end edges of the end surface 22. This can be realized by, for example, thermally oxidizing the entire surfaces of the semiconductor body 211 and then removing the oxide film lying within the above range by photo-etching. After the oxide film has been partly removed, an impurity is diffused into the exposed or non-oxidized surface portions of the semiconductor body 21 as shown in FIG. 3C for forming a diffused layer 24 in the surface portions including the end surface 22 and the side surface portions covering the distance I from the end edges of the end surface 22 of the semiconductor body 21. This diffused layer 24 acts as a resistive layer. A heat generating portion for applying thermal energy to a heatsensitive recording medium is preferably provided on a limited area, and in this respect, the provision of the 3 1 heat generating portion solely on the end surface 22 is essentially required as will be apparent from FlG. 1B. The area of the surface portions covering the length 1 from the end edges of the end surface 22 is preferably as small as possible considering the fact that heat is wastefully lost in such portions. Therefore, the area of these surface portions should be limited to a minimum which is enough for electrical connection with the electrodes. A' known method of vapor phase diffusion may be employed for the diffusion of the impurity. An impurity such as phosphorus belonging to the group V is diffused when the semiconductor body of low resistivity is of the p-type, while an impurity such as boron belonging to the group lll is diffused when the semiconductor body is of the n-type. lt is needless to say that the resistance of the resistive layer can be controlled as desired by suitably diffusing the impurity. Then, as shown in FIG. 3D, a metal such as aluminum or nickel is deposited to provide a pair of electrodes 25 and 25' so as to extend over the oxide film 23 and over substantial portions of the diffused layer 24 on the side surfaces ofthe semiconductor body 21. A method of vac-' uum evaporation may be employed for the deposition of the electrodes 25 and 25'. The vacuum evaporation may be carried out while masking the end surface 22 so that the metal may not be deposited on the end surface 2 2, or after evaporating the metal on the entire surfaces, predetermined portions may be removed by photo-etching I and the electrode portions may be shaped to the desired form. Subsequently, a cutting means such as a diamond cutter or wire saw may be used to cut the head into a size which has a width slightly larger than a predetermined width w After lapping the worked surfaces of the head, chemical etching is applied to the head for reshaping the pn junction 26, which has been impaired during the cutting operation, and adjusting the size of the head so that it has the predetermined width W2 v The thermal printing head may be required to produce a temperature higher than, for example, 400 C depending on the kind of the heat-sensitive recording medium. In the case of conventional thermal printing heads of the type in which silicon having a high resistivity is used as a semiconductor body and an impurity doped layer having a low resistivity is formed in the surface portion of the semiconductor body for obtaining a heat generating layer, the heat generating characteristic of the head is necessarily deteriorated when a high temperature is produced due to the heat generated in the heat generating layer. In the case of the head according to the present invention, however, a high temperature can be satisfactorily produced without deteriorating the heat generating characteristic thereof for the reasons described below.
FIG. 4 shows the temperature characteristic of the lustrated by way of example, and the critical tempera ture becomes remarkably lower at a higher resistivity. An increase in the temperature of the limited portion in contact with a heat-sensitive recording medium is enough for a thermal printing head to act as a heat generator, and the thinner the thickness of the heat generating portion, the thermal energy produced in this portion is more effectively utilized. ln the conventional thermal printing head of the type in which the resistance of the shallow region along the surfaces of a semiconductor body having a high resistivity is reduced by the diffusion of an impurity thereby establishing a sufficiently large difference between the resistance of the doped surface portion and that of the interior portion of the semiconductor body and concentrated flow of current is passed through the doped surface portion for generating a large amount of heat in the doped surface portion, it is desirable to pass the current through the portion which is as near the surface of the semiconductor body as possible, and thus the silicon crystal forming the semiconductor body should have a resistivity as high as possible. In the silicon crystal having such a high resistivity, however, a transition to the intrinsic region takes place readily with an increase in the temperature, and at a temperature higher than the critical point, the resistance of the semiconductor body is abruptly reduced with the result that current flows also through the interior of the semiconductor body and considerable heat is generated in the interior of the semiconductor body. Consequently, a large amount of resistivity of silicon. It will be seen from FIG. 4 that the resistivity of silicon increases with the increase in the temperature within a temperature range higher than room temperature and starts to decrease abruptly beyonda certain critical point. The region in which such abrupt decrease in the resistivity occurs is called an intrinsic region. The critical temperature at which a transition-to this intrinsic region takes place varies depending on the resistivity, and the higher the resistivity, the lower the critical temperature. In FIG. 4, the critical temperatures of silicon crystals having respective resistivities'of 0.7 0cm, 1.3 Qcm, 4 flcm and 10 Om are ilheat is generated not only in the surface portion to be brought into contact with a heat-sensitive recording medium but also in the interior of the semiconductor body, and the heat generating efficiency of the thermal printing head is extremely lowered.
On the other hand, according to the present invention, a resistive layer is formed in the surface portion of a semiconductor body of silicon by diffusing an impurity having a conductivity opposite to that of the semiconductor body so that the path of current is lim ited to one side of the pn junction and substantially all the current flows through the resistive layer. In this case, therefore, the semiconductor need not have a high resistivity. A large amount of heat is generated in the interior of the semiconductor body as is the case with the conventional head only when breakdown of the pn junction occurs to allow for flow of current into the interior of the semiconductor body. However, it is said that the so-called secondary breakdown of a pn junction occurs only when a transition to the intrinsic region takes place in a semiconductor portion having a high resistivity. lnasmch as the present invention employs a semiconductor body of silicon having a low resistivity, the present invention is advantageous over the conventional thermal printing heads in that the path of current is limited to the resistive surface layer up to elevated temperatures and the head is free from any undesirable reduction in the heat generating efficiency up to such elevated temperatures.
What is claimed is:
l. A thermal printing head which generates heat for thermally recording information on a heat sensitive recording medium, comprising:
a body of low resistivity semiconductor material having a critical temperature at which a transition to the intrinsic region takes place which is higher than a color producing temperature of said heat sensitive recording medium;
a layer of opposite conductivity type from said semiconductor body formed on the surface of said body facing said heat sensitive recording medium, said layer extending over and part way down opposite sides of said body to form a semiconductor junction with said body;
an electrically insulating layer covering at least that portion of each side surface of said semiconductor body immediately adjacent to and extending away from said opposite conductivity type layer; and
an electrode formed on each said insulating layer and it extending onto said opposite conductivity type layer in contact therewith, a heat generating conductivity path being formed through said opposite conductivity type layer between each said electrode.
2. A thermal printing head according to claim 1,
wherein each said electrode is disposed in ohmic contact with said opposite conductivity type layer.
' 3. A thermal printing head according to claim 2, wherein said opposite conductivity type layer comprises a doped impurity layer diffused into said semiconductor body.
4. A thermal printing head according to claim 3, further comprising a plurality of said opposite conductivity type layers formed on said surface of said semiconductor body and a plurality of electrodes formed on said insulating layer and in contact with corresponding ones of said plurality of opposite conductivity type layers, each opposite conductivity type layer and its corresponding electrodes being separated from each other opposite conductivity layer and associated electrodes.
5. A printing head according to claim 4, wherein said body of semiconductive material is composed of one of n-type and p-type materials.