Thermal head

The present invention provides a thermal head in which the surface of a heat insulating layer formed by vapor deposition such as sputtering or the like is polished to decrease the rate of defects such as failure in the resistance values of heating elements formed on the heat insulating layer, disconnection and short-circuit of electrodes, apparent foreign materials, etc., and improve the adhesion of the surface of the heat insulating layer. The thermal head includes a heat insulating layer formed on a radiating substrate by sputtering, and heating elements deposited on the surface of the thermal head, wherein the heat insulating layer has columnar crystals composed of silicon, transition metals and oxygen, the surface of the heat insulating layer is polished, and micro irregularity is formed on the polished surface of the heat insulating layer.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a thermal head used for a thermal printer,
 and particularly to a thermal head comprising a heat insulating layer
 formed by vapor deposition such as sputtering in order to improve the
 printing life, wherein the surface of the heat insulating layer is
 polished.
 2. Description of the Related Art
 A conventional thermal head comprises a heat insulating layer formed on a
 heat radiating substrate by vapor deposition such as sputtering or the
 like, and a plurality of heating elements linearly arranged on the heat
 insulating layer so that current is selectively passed through the heating
 elements to record a dot image using heat sensitive recording paper or a
 heat transfer ribbon.
 In the example shown in FIG. 5, the thermal head comprises a heat
 insulating layer 12 formed, by sputtering, in a thickness of about 20
 .mu.m on a substrate 11 of silicon having excellent radiating property,
 composed of silicon, transition metals and oxygen, and having excellent
 heat resistance. In the process for depositing the heat insulating layer
 12, the sputtering pressure is as high as about 1.0 Pa in order to
 intentionally form columnar crystals and deposit the layer with a low
 density, to obtain the heat insulating layer 12 having excellent heat
 insulating property.
 However, since the heat insulating layer 12 comprises columnar crystals,
 the surface thereof exhibits a rough state having initial irregularity
 12a. Also abnormal projections 12b occur due to contaminant particles
 peculiar to the vapor deposition process. The contaminant particles
 represent particles produced by peeling of a film deposited in the vacuum
 container of a vapor deposition apparatus such as a sputtering apparatus
 or the like, and floating as particles having a size of 0.1 to several
 micrometers in the vacuum container. In film deposition, the contaminant
 particles adhere to the substrate surface to produce projections in the
 film formed on the substrate surface with the contaminant particles as
 nuclei.
 FIG. 6 is a drawing showing a three-dimensional image of the surface of the
 heat insulating layer 12, which was output by using an atomic force
 microprobe AFM. As the result of measurement of the surface roughness
 (Rz), Rz=45 nm.
 Although not shown in the drawings, on the heat insulating layer 12 are
 formed a heating resistor, and a common electrode and individual
 electrodes for passing a current through the heating resistor. A
 protecting layer is further coated for protecting the heating resistor and
 each of the electrodes from oxidation and abrasion to form a thermal head.
 The heat insulating layer 12 comprising columnar crystals and formed by
 sputtering as described above has a high degree of defects such as
 variations in the dot resistance value, disconnection and short-circuit of
 the electrode pattern, apparent foreign materials, etc. due to the initial
 irregularity 12a and the abnormal projections 12b formed with the
 contaminant particles peculiar to the vapor deposition process as nuclei,
 and thus has the problem of deteriorating the product quality and
 production yield.
 Therefore, the applicant already proposed that the initial irregularity 12a
 peculiar to the columnar crystals on the surface of the heat insulating
 layer 12, and the macroscopic abnormal projections 12b formed with the
 contaminant particles as nuclei are removed by chemical polishing to form
 substantially a mirror surface, thereby solving the problem of
 deteriorating product quality and production yield. FIG. 3 is a schematic
 drawing showing the surface of the heat insulating layer after chemical
 polishing. In FIG. 3, reference numerals 12a and 12b denote portions
 corresponding to the initial irregularity and abnormal projections,
 respectively, shown in FIG. 5.
 FIG. 4 is a drawing showing a three-dimensional image of the surface of the
 heat insulating layer 12 after polishing, which was output by the atomic
 force microprobe AFM. In this case, the surface is a smooth surface having
 less irregularity and a surface roughness Rz=4.5 nm.
 However, as a result of a printing durability test of the thermal head
 comprising the heating elements formed on the heat insulating layer 12
 chemically polished to substantially a mirror surface, the actual printing
 life was about 20,000,000 to 50,000,000 characters. This was due to a
 trouble mode in which, in printing runs, the protecting layer is cracked
 due to deterioration in adhesion of the films in the upper and lower
 interfaces of the heating resistor formed on the heat insulating layer 12,
 thereby causing dot defects due to oxidation of the heating resistor. This
 was caused by the excessive flatness of the surface of the heat insulating
 layer 12 as a base, and it was thus found that the surface must be
 modified to increase the adhesion.
 In recent years, mass production of thermal heads comprising a silicon
 substrate with excellent heat responsiveness in order to improve printing
 quality has been made, and the contact pressure between a thermal head and
 a printing medium (a heat transfer ribbon or heat sensitive paper) has
 been increased in order to improve printing quality for plain paper.
 Therefore, in a printing operation, high shearing stress is applied to the
 thermal head, as compared with previous thermal heads. The shearing stress
 causes peeling due to fatigue failure in the upper and lower interfaces of
 the heating resistor, thereby interfering with an increase in the printing
 life.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a thermal head
 comprising a heat insulating layer formed by a vapor deposition method
 such as sputtering or the like, and having a polished surface so that the
 heat insulating layer is the best as a base for forming heating elements.
 It is another object of the present invention to provide a thermal head in
 which the adhesion of the polished surface of a heat insulating layer can
 be improved without increases in the rate of defects such as failure in
 the resistance values of heating elements, disconnection and short-circuit
 of electrodes, apparent foreign materials, etc.
 A thermal head of the present invention comprises a heat insulating layer
 formed on a radiating substrate by a vapor deposition method such as
 sputtering or the like, and heating elements deposited on the polished
 surface of the heat insulating layer, wherein the heat insulating layer
 comprises columnar crystals composed of silicon, transition metals and
 oxygen, the surface of the heat insulating layer is polished, and micro
 irregularity is formed on the polished surface of the heat insulating
 layer.
 It is a further object of the present invention to increase the surface
 area of the heat insulating layer by utilizing the above construction
 without causing surface roughness of the heat insulating layer.
 In the thermal head of the present invention, the micro irregularity is
 formed by selectively removing silicon-oxygen bond texture portions which
 are scattered in the heat insulating layer.
 It is a further object of the present invention to uniformly form micro
 irregularity over the entire surface of the heat insulating layer by
 utilizing the above construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 An embodiment of the present invention is described with reference to FIGS.
 1 and 2. FIG. 1 is a schematic drawing showing the state of a heat
 insulating layer of a thermal head of this embodiment, and FIG. 2 is a
 photograph of a three-dimensional image of the heat insulating layer,
 which was output by an atomic force microprobe AFM. In FIGS. 1 and 2, the
 same members as the conventional example are denoted by the same reference
 numerals.
 In the thermal head of this embodiment, in step 1 of forming the heat
 insulating layer, the heat insulating layer 12 is formed by, vapor
 deposition, in a thickness of about 20 .mu.m on the silicon substrate 11
 to form a state equivalent to the heat insulating layer 12 of the
 conventional example shown in FIG. 5.
 The heat insulating layer 12 is composed of silicon, transition metals and
 oxygen, e.g., multiple elements such as Si-Ta-W-Cr-O, or the like, and
 deposited on the silicon substrate 11 by sputtering at a deposition
 pressure of as high as about 1.0 Pa to form columnar crystals at a low
 density. Therefore, the heat insulating layer 12 has excellent heat
 insulating property. In this embodiment, preferred transition metals are
 not limited to Ta, W and Cr. and other transition metals such as Mo, Ti,
 Zr, Nb, Hf, and the like can be used.
 In this state, the heat insulating layer 12 has a surface having the
 initial irregularity 12a and the macroscopic abnormal projections 12b due
 to contaminant particles peculiar to the vapor deposition process, as in
 the conventional example shown in FIG. 5.
 In step 2 of forming the heat insulating layer, the surface of the heat
 insulating layer 12 is polished by a polishing device (not shown) using a
 polishing cloth containing an alkaline chemical polishing solution in
 which amorphous silica (SiO.sub.2) fine powder as a abrasive material is
 dispersed to form a state equivalent to the polished heat insulating layer
 shown in FIG. 3. As an example of the chemical polishing solution, Trade
 Name "48-211 Polish-Ade 0.06 .mu.m" produced by Refine Tec, Ltd. and
 comprising 40 to 41 wt % of amorphous silica fine powder having an average
 particle diameter of 0.06 .mu.m, 0.11 wt % or less of Na.sub.2 O, and
 water as the balance was used.
 In this chemical polishing, the material of the heat insulating layer 12 is
 subjected to the strong chemical polishing actions of the abrasive
 material and the alkaline chemical polishing solution to efficiently
 remove both the initial irregularity 12a due to the columnar crystals and
 the macroscopic abnormal projections 12b due to contaminant particles of
 the vapor deposition process. It is thus possible to easily make the
 surface of the heat insulating layer 12 substantially a mirror surface.
 In step 3 of forming the heat insulating layer, after the surface of the
 heat insulating layer 12 is chemically polished to substantially a mirror
 surface, the surface of the heat insulating layer 12 is immersed in a
 buffered hydrofluoric acid solution for about 30 to 90 seconds to
 selectively dissolve and remove Si-O bond structure portions in the heat
 insulating layer 12 composed of multiple elements such as Si-Ta-W-Cr-O, or
 the like, to form uniform micro irregularity 12c. As an example of the
 buffered hydrofluoric acid solution, Trade Name "Semiconductor BUFFERED
 HYDROFLUORIC ACID 63U1" produced by Daikin Industries, Ltd., and
 comprising 6 wt % of HF, 30 wt % of NH.sub.4 F, and water as the balance
 was used.
 The time of etching with the buffered hydrofluoric acid solution is
 preferably in the range of about 20 to 100 seconds, and an etching time of
 over 100 seconds has the problem of deteriorating mechanical strength due
 to the excessive porosity of the surface of the heat insulating layer 12.
 With an etching time of less than 20 seconds, the micro irregularity 12c
 cannot be effectively formed. Therefore, the etching time is more
 preferably in the range of about 30 to 90 seconds.
 FIG. 2 shows the surface of the heat insulating layer after etching with
 the buffered hydrofluoric acid solution for 60 seconds. FIG. 2 indicates
 that the surface has a surface roughness Rz=25 nm, and uniform micro
 irregularity, as compared with the surface before polishing.
 Table 1 shows comparison between the surface states (the height of
 projections, the diameter of the bottom of projections, and the number of
 projections) of the heat insulating layer 12 in the respective steps.
 TABLE 1
 Comparison of surface states of heat insulating
 layer
 Height Diameter of Number of
 of pro- bottom of projections
 jection projection (per 3.5-.mu.m
 Step Surface (nm) (.mu.m) square)
 Step 1 Abnormal 40-80 0.5-1.0 50-10
 before projection
 polishing
 Step 2 Polished 3-5 0.1 or 1000 or
 after surface less more
 polishing
 Step 3 Micro 10-30 0.2-0.3 300-100
 after irregular-
 etching ity
 Although not shown in the drawings, a heating resistor made of Ta-SiO.sub.2
 or the like is deposited, by sputtering or the like, on the etched surface
 of the heat insulating layer, and then etched by photolithography to form
 a plurality of heating elements.
 On the upper side of these heating elements is deposited a common electrode
 connected to the heating elements, and on the other side of the heating
 elements are deposited independent electrodes for independently passing a
 current through the heating elements. The common electrode and the
 independent electrodes are made of, for example, Al, Cu, or the like, and
 are formed by vapor deposition such as sputtering or the like, and then
 etching in a desired pattern.
 On the heating elements, the common electrode, and the independent
 electrodes is coated, by sputtering or the like, a protecting layer having
 a thickness of about 5 to 10 .mu.m, for protecting the heating elements
 and each of the electrodes.
 In the thermal head of this embodiment produced by the above method, the
 heat insulating layer 12 comprising columnar crystals composed of
 materials of silicon, transition metals and oxygen is formed by vapor
 deposition, and the surface of the heat insulating layer 12 is then
 chemically polished by the alkaline chemical polishing solution containing
 the abrasive material dispersed therein to efficiently remove the initial
 irregularity 12a and the abnormal projections 12b of the surface, to form
 substantially a mirror surface. Then the polished surface of the heat
 insulating layer 12 is etched with the buffered hydrofluoric acid solution
 for about 60 seconds to form micro irregularity 12c having excellent
 uniformity on the surface of the heat insulating layer 12. As a result,
 the adhesion of the heating resistor can be improved by an increase in the
 surface area of the heat insulating layer 12 and the wedge effect of the
 film formed on the heat insulating layer 12 while maintaining the pattern
 formation precision of heating dots.
 As a result of the printing durability test of the thermal head of the
 present invention, the printing life was printing of 50,000,000 to
 80,000,000 characters. It was thus found that the printing life can be
 increased to about twice the life of a conventional thermal head.
 The present invention is not limited to this embodiment, and various
 changes can be made by using, for example, dry etching with carbon
 fluoride gas as an etchant in place of buffered hydrofluoric acid
 according to demand.
 As described above, the thermal head of the present invention has the
 effects below.
 Since the abnormal projections peculiar to vapor deposition produced on the
 surface of the heat insulating layer are removed by chemical polishing,
 and then surface is etched to form micro irregularity, the surface area of
 the heat insulating layer can be increased without causing surface
 roughness of the heat insulating layer. It is thus possible to improve the
 adhesion of the deposited film such as the heating resistor or the like
 formed on the heat insulating layer without increasing the rate of defects
 such as disconnection and short-circuit of the electrodes, and apparent
 foreign materials while decreasing the variation of the resistance value
 and maintaining the pattern formation precision of the heating elements.
 The present invention thus exhibits the effect of improving the printing
 life of the thermal head.
 Since the micro irregularity is formed by selectively removing the
 silicon-oxygen bond portions scattered in the texture of the heat
 insulating layer, the micro irregularity is uniformly formed over the
 entire surface of the heat insulating layer. As a result, the adhesion of
 the deposited film such as the heating resistor or the like formed on the
 heat insulating layer can be improved while keeping down the variations of
 the resistance values of a plurality of heating elements. Therefore, the
 present invention exhibits the effect of forming a thermal head capable of
 improving printing quality, and increasing the printing life of the
 thermal head.
 Furthermore, the heat insulating layer is formed by sputtering, and the
 surface thereof is polished with the chemical polishing solution
 containing the abrasive material dispersed therein to remove the abnormal
 projections produced on the surface of the heat insulating layer, and then
 etched with the buffered hydrofluoric acid solution to form micro
 irregularity. It is thus possible to produce the heat insulating layer
 with high material and thickness precision, and form micro irregularity
 over the entire surface of the heat insulating layer with high
 reproducibility. The present invention thus exhibits the effect of
 improving product quality and production yield.
 Since the time of etching with the buffered hydrofluoric acid solution is
 set to 30 to 90 seconds, it is possible to effectively form micro
 irregularity on the surface of the heat insulating layer, and form
 appropriate micro irregularity over the entire surface of the heat
 insulating layer without deteriorating the mechanical strength of the
 surface of the heat insulating layer. The present invention thus exhibits
 the effect of improving product quality and production yield.