Thermal head

The improved thermal head comprises heating elements which were provided with heating histories to previously change their resistance values by predetermined values; and a carbon-based protective layer which was formed after the heating elements were provided with the heating histories. The invention provides the thermal head in which corrosion and wear of the protective film, and the resistance variation of the heating elements due to thermal recording were significantly reduced, and which has a sufficient durability and stability with the passage of time to perform thermal recording of high-quality images in a consistent manner over an extended period of operation.

BACKGROUND OF THE INVENTION 
This invention relates to the art of thermal heads for thermal recording 
which are used in various types of printers, plotters, facsimile, 
recorders and the like as recording means. 
Thermal materials comprising a thermal recording layer on a substrate of a 
film or the like are commonly used to record images produced in diagnosis 
by ultrasonic scanning (sonography). 
This recording method, also referred to as thermal recording, eliminates 
the need for wet processing and offers several advantages including 
convenience in handling. Hence in recent years, the use of the thermal 
recording system is not limited to small-scale applications such as 
diagnosis by ultrasonic scanning and an extension to those areas of 
medical diagnoses such as CT, MRI and X-ray photography where large and 
high-quality images are required is under review. 
As is well known, thermal recording involves the use of a thermal head 
having a heater (glaze), in which heating elements comprising 
heat-generating resistors and electrodes, used for heating the thermal 
recording layer of a thermal material to record an image are arranged in 
one direction (main scanning direction) and, with the glaze urged at small 
pressure against the thermal material (thermal recording layer), the two 
members are moved relative to each other in the auxiliary scanning 
direction perpendicular to the main scanning direction, and the heating 
elements of the respective pixels on the glaze are heated by energy 
application in accordance with image data to be recorded which were 
supplied from an image data supply source such as MRI or CT in order to 
heat the thermal recording layer of the thermal material and form color, 
thereby accomplishing image reproduction. 
In thermal recording, density unevenness on the recorded image brings about 
a reduction in the quality of the finished image, which may cause an 
important problem in applications requiring high quality image recording. 
Especially the aforementioned medical application requires high quality 
images. Density unevenness also prevents image observation which will 
cause an important problem leading to an erroneous diagnosis. It is thus 
required that the thermal head is capable of recording high quality 
thermal images without density unevenness and having a reduced 
deterioration with the passage of time over an extended period of time. 
Primary causes of the deterioration of the thermal head with the passage of 
time include variation in the properties of heating elements due to heat 
generation, and wear and corrosion (or wear due to corrosion) of the 
glaze. 
A heating element of the thermal head usually comprises a heat-generating 
resistor and a pair of electrodes which energize the heat-generating 
resistor. The resistance value of the heating element varies with the time 
for heat generation and the energy for heat generation. Hence, the more 
the resistance value decreases, the more the amount of heat generation 
increases. The temperature of the heating element associated with the 
supplied energy for heat generation increases by the reduced resistance 
value, which brings about an increase in the image density. 
The heating history which shows the total amount of heat generation, or the 
ratio of heat generation for the image recording performed is inherently 
different in each of the heating elements of the thermal head mounted in 
the thermal recording apparatus. Then, the amount of resistance variation 
is also different in each of the heating elements. Therefore, in the 
course of image recording, differences in the amount of resistance 
variation are produced among the respective heating elements, which gives 
rise to density unevenness on the recorded image in association with the 
differences. 
The thermal head and the thermal material are moved relative to each other 
to perform recording, with the glaze thereon urged at small pressure 
against the thermal material. A protective film is formed on the surface 
of the glaze of the thermal head in order to protect the heating elements 
and other components. It is this protective film that contacts the thermal 
material during thermal recording and the heat-generating resistors heat 
the thermal material through this protective film so as to perform thermal 
recording. 
The protective film is usually made of wear-resistant ceramics; however, 
during thermal recording, the surface of the protective film is heated and 
kept in sliding contact with the thermal material, so it will gradually 
wear and deteriorate upon repeated recording. 
If the wear of the protective film progresses, density unevenness will 
occur on the thermal image or a desired protective strength can not be 
maintained and, hence, the ability of the film to protect the heaters is 
impaired to such an extent that the intended image recording is no longer 
possible (the head has lost its function). 
Particularly in the applications such as the aforementioned medical use 
which require multiple gradation images of high quality, the trend is 
toward adopting thermal films with highly rigid substrates such as 
polyester films and also increasing the setting values of recording 
temperature and of the pressure at which the thermal head is urged against 
the thermal material. Under these circumstances, as compared with the 
conventional thermal recording, a greater force and more heat are exerted 
on the protective film of the thermal head, making wear and corrosion (or 
wear due to corrosion) more likely to progress. 
With a view to preventing the wear of the protective film on the thermal 
head so as to improve its durability, a number of techniques have been 
considered in order to improve the performance of the protective film. 
Among others, a carbon-based protective film (hereinafter referred to as a 
carbon protective layer) is known as a protective film excellent in 
resistance to wear and corrosion. 
Thus, Examined Published Japanese Patent Application (KOKOKU) No. 61-53955 
discloses a thermal head excellent in wear resistance and response 
obtained by forming a very thin carbon protective layer having a Vickers 
hardness of 4500 kg/mm.sup.2 or more as the protective film of the thermal 
head. 
Moreover, Unexamined Published Japanese Patent Application (KOKAI) No. 
7-132628 discloses a thermal head which has a dual protective film 
comprising a lower silicon-based compound layer and an overlying 
diamond-like carbon layer, whereby the potential wear and breakage of the 
protective film are significantly reduced to ensure that high-quality 
image can be recorded over an extended period of time. 
These carbon protective layers have a very high hardness and chemical 
stability, hence they exhibit sufficiently excellent properties to prevent 
wear and corrosion which may be caused by the sliding contact with thermal 
materials. 
However, the carbon protective layers are not enough to resolve the 
aforementioned recording unevenness due to the resistance variation of the 
heating elements with the passage of time. It is also important the carbon 
protective layers have excellent properties in order to record high 
quality thermal images having a reduced deterioration with the passage of 
time over an extended period of time. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished under these circumstances and 
has as an object providing a thermal head of which the variation in the 
resistance values of the heating elements and the wear or deterioration of 
the protective layer were reduced, and which is capable of consistently 
recording high quality thermal images over an extended period of time. 
In order to achieve the above object, the invention provides a thermal head 
comprising: 
heating elements which were provided with heating histories to previously 
change their resistance values by predetermined values; and 
a carbon-based protective layer which was formed after said heating 
elements were provided with said heating histories. 
An amount of the resistance variation to be previously given to said 
heating elements by said heating histories is preferably in the range of 
from 0.1 to 5.0%, more preferably from 0.5 to 2.0%. 
It is preferred that said heating histories are provided in blank recording 
by supplying a specified amount of heat generating energy for a specified 
period of time without recording on a thermal material. 
Said carbon-based protective layer is preferably a carbon protective layer 
containing more than 50 atm % of carbon, more preferably a high-purity 
carbon protective layer. 
Said carbon-based protective layer has preferably a Vickers hardness of 
2000 kg/mm.sup.2 or more, more preferably 2500 kg/mm.sup.2 or more. 
Said carbon-based protective layer has preferably a thickness of from 1 to 
20 .mu.m, more preferably from 2 to 10 .mu.m. 
It is further preferred that said thermal head further comprises at least 
one ceramic-based protective layer as a lower protective layer of said 
carbon-based protective layer on the side of the heating elements. 
Said ceramic-based protective layer is preferably a protective layer made 
of a material selected from the group consisting of silicon nitride 
(Si.sub.3 N.sub.4), silicon carbide (SiC), tantalum oxide (Ta.sub.2 
O.sub.5), aluminum oxide (Al.sub.2 O.sub.3), SIALON (Si--Al--O--N), LASION 
(La--Si--O--N), silicon oxide (SiO.sub.2), aluminum nitride (AlN), boron 
nitride(BN), selenium oxide (SeO), titanium nitride (TiN), titanium 
carbide (TiC), titanium carbide nitride (TiCN), chromium nitride (CrN) and 
mixtures of at least two of these materials, more preferably a protective 
layer made of a material selected from the group consisting of silicon 
nitride, silicon carbide, SIALON and mixtures of at least two of these 
materials. 
Said lower protective layer has preferably a thickness of from 2 to 50 
.mu.m, more preferably from 4 to 20 .mu.m. 
Said carbon-based protective layer has preferably a thickness of from 0.1 
to 5 .mu.m, more preferably from 1 to 3 .mu.m.

DETAILED DESCRIPTION OF THE INVENTION 
The thermal head of the invention will now be described in detail with 
reference to the preferred embodiments shown in the accompanying drawings. 
FIG. 1 shows schematically an exemplary thermal recording apparatus using 
the thermal head of the invention. 
The thermal recording apparatus generally indicated by 10 in FIG. 1 and 
which is hereinafter simply referred to as a "recording apparatus 10" 
performs thermal recording on thermal materials of a given size, say, B4 
(namely, thermal materials in the form of cut sheets, which are 
hereinafter referred to as "thermal materials A"). The apparatus comprises 
a loading section 14 where a magazine 24 containing thermal materials A is 
loaded, a feed/transport section 16, a recording section 20 performing 
thermal recording on thermal materials A by means of a thermal head 66, 
and an ejecting section 22. 
In the thus constructed recording apparatus 10, a thermal material A is 
taken out of the magazine 24 and transported to the recording section 20, 
where the thermal material A against which the thermal head 66 is pressed 
is transported in the auxiliary scanning direction perpendicular to the 
main scanning direction in which the heater (or the glaze) extends (normal 
to the papers of FIGS. 1 and 2) and in the meantime, the individual 
heating elements are actuated in accordance with image data on the image 
to be recorded to perform thermal recording on the thermal material A. 
The thermal material A comprises a substrate of a resin film such as a 
transparent polyethylene terephthalate (PET) film, a paper or the like 
which are overlaid with a thermal recording layer. 
Typically, such thermal materials A are stacked in a specified number, say, 
100 to form a bundle, which is either wrapped in a bag or bound with a 
band to provide a package. As shown, the specified number of thermal 
materials A bundle together with the thermal recording layer side facing 
down are accommodated in the magazine 24 of the recording apparatus 10, 
and they are taken out of the magazine 24 one by one to be used for 
thermal recording. 
The magazine 24 is a case having a cover 26 which can be freely opened. The 
magazine 24 which contains the thermal materials A is loaded in the 
loading section 14 of the recording apparatus 10. 
The loading section 14 has an inlet 30 formed in the housing 28 of the 
recording apparatus 10, a guide plate 32, guide rolls 34 and a stop member 
36; the magazine 24 is inserted into the recording apparatus 10 via the 
inlet 30 in such a way that the portion fitted with the cover 26 is coming 
first; thereafter, the magazine 24 as it is guided by the guide plate 32 
and the guide rolls 34 is pushed until it contacts the stop member 36, 
whereupon it is loaded at a specified position in the recording apparatus 
10. 
The loading section 14 is equipped with a mechanism (not shown) for opening 
or closing the cover 26 of the magazine. 
The feed/transport section 16 has the sheet feeding mechanism using a 
sucker 40 for grabbing the thermal material A by application of suction, 
transport means 42, a transport guide 44 and a regulating roller pair 52 
located in the outlet of the transport guide 44; thermal materials A are 
taken one by one out of the magazine 24 in the loading section 14 and 
transported to the recording section 20. 
The transport means 42 comprises a transport roller 46, a pulley 47a 
coaxial with the roller 46, a pulley 47b coupled to a rotating drive 
source, a tension pulley 47c, an endless belt 48 stretched between the 
three pulleys 47a, 47b and 47c, and a nip roller 50 that pairs with the 
transport roller 46. The forward end of the thermal material A which has 
been sheet-fed by means of the sucker 40 is pinched between the transport 
roller 46 and the nip roller 50 such that the material A is transported. 
When a signal for the start of recording is issued, the cover 26 is opened 
by the OPEN/CLOSE mechanism in the recording apparatus 10. Then, the sheet 
feeding mechanism using the sucker 40 picks up one sheet of thermal 
material A from the magazine 24 and feeds the forward end of the sheet to 
the transport means 42 (to the nip between rollers 46 and 50). At the 
point of time when the thermal material A has been pinched between the 
transport roller pair, the sucker 40 releases the material, and the thus 
fed thermal material A is supplied by the transport means 42 into the 
regulating roller pair 52 as it is guided by the transport guide 44. 
At the point of time when the thermal material A to be used in recording 
has been completely ejected from the magazine 24, the OPEN/CLOSE mechanism 
closes the cover 26. 
The distance between the transport means 42 and the regulating roller pair 
52 which is defined by the transport guide 44 is set to be somewhat 
shorter than the length of the thermal material A in the direction of its 
transport. The forward end of the thermal material A first reaches the 
regulating roller pair 52 as the result of transport by the transport 
means 42. The regulating roller pair 52 are first at rest. The forward end 
of the thermal material A stops here and is subjected to positioning. 
When the forward end of the thermal material A reaches the regulating 
roller pair 52, the temperature of the thermal head 66 (the glaze) is 
checked and if it is at a specified level, the regulating roller pair 52 
starts to transport the thermal material A, which is transported to the 
recording section 20. 
The recording section 20 has the thermal head 66, a platen roller 60, a 
cleaning roller pair 56, a guide 58, a heat sink 67 for cooling the 
thermal head 66, a cooling fan 76 and a guide 62. 
The thermal head 66 is capable of recording on thermal sheets of up to, for 
example, 356.times.432 size at a recording (pixel) density of, say, about 
300 dpi. The head comprises a glaze (heater) in which heating elements 
performing thermal recording on the thermal material A are arranged in one 
direction, that is in the main scanning direction, and the cooling heat 
sink 67 is fixed to the thermal head 66. The thermal head 66 is supported 
on a support member 68 that can pivot about a fulcrum 68a. 
The glaze of the thermal head 66 will be described in detail later. 
It should be noted that the thermal head 66 of the invention is not 
particularly limited in such aspects as the width (in the main scanning 
direction), resolution (recording density) and recording contrast; 
preferably, the head width ranges from 5 cm to 50 cm, the resolution is at 
least 6 dots/mm (ca. 150 dpi), and the recording contrast consists of at 
least 256 levels. 
The platen roller 60 rotates at a specified image recording speed while 
holding the thermal material A in a specified position in the direction 
shown by the arrow in FIG. 1, and transports the thermal material A in the 
auxiliary scanning direction perpendicular to the main scanning direction 
(the direction shown by the arrow X in FIG. 2). 
The cleaning roller pair 56 comprises an adhesive rubber roller made of an 
elastic material (upper side in the drawing) and a non-adhesive roller. 
The adhesive rubber roller picks up dirt and other foreign matter that has 
been deposited on the thermal recording layer of the thermal material A, 
thereby preventing the dirt from being deposited on the glaze or otherwise 
adversely affecting the image recording operation. 
Before the thermal material A is transported to the recording section 20, 
the support member 68 in the illustrated recording apparatus 10 has 
pivoted to UP position so that the glaze of the thermal head 66 is in the 
standby position just before coming into contact with the platen roller 
60. 
When the transport of the thermal material A by the regulating roller pair 
52 starts, said material is subsequently pinched by the cleaning roller 
pair 56 and transported as it is guided by the guide 58. When the forward 
end of the thermal material A has reached the record START position (i.e., 
corresponding to the glaze), the support member 68 pivots to DOWN position 
and the thermal material A becomes pinched between the glaze and the 
platen roller 60 such that the glaze is pressed onto the recording layer 
while the thermal material A is transported in the auxiliary scanning 
direction by means of the platen roller 60 and other parts as it is held 
in a specified position by the platen roller 60. 
During this transport, the respective heating elements on the glaze are 
actuated imagewise to perform thermal recording on the thermal material A. 
After the end of thermal recording, the thermal material A as it is guided 
by the guide 62 is transported by the platen roller 60 and the transport 
roller pair 63 to be ejected into a tray 72 in the ejecting section 22. 
The tray 72 projects exterior to the recording apparatus 10 via the outlet 
74 formed in the housing 28 and the thermal material A carrying the 
recorded image is ejected via the outlet 74 for takeout by the operator. 
FIG. 2 is a schematic cross section of the glaze (heater) of the thermal 
head 66. As shown, to form the glaze, the top of a substrate 80 (which is 
shown to face down in FIG. 2 since the thermal head 66 is pressed downward 
against the thermal material A) is overlaid with a glaze layer (heat 
accumulating layer) 82 which, in turn, is overlaid with a heat-generating 
resistor 84 which, in turn, is overlaid with electrodes 86 which, in turn, 
is overlaid with a protective film and the like. 
FIG. 2 illustrates a preferred embodiment in which the protective film is 
composed of two layers: a ceramic-based lower protective layer 88 
superposed on the heat-generating resistor 84 and the electrodes 86 (or 
the heating element), and a carbon-based upper protective layer, for 
example, carbon protective layer 90 (preferably diamond-like carbon (DLC) 
protective layer) which is formed on the lower protective layer 88. 
The thermal head 66 for use in the invention has essentially the same 
structure as known versions of thermal head except that the heating 
elements are provided with heating histories before the carbon protective 
layer 90 is formed and that the thermal head 66 has the carbon protective 
layer 90. Therefore, the arrangement of other layers and the constituent 
materials of the respective layers are not limited in any particular way 
and various known versions may be employed. Specifically, the substrate 80 
may be formed of various electrical insulating materials including 
heat-resistant glass and ceramics such as alumina, silica and magnesia; 
the glaze layer 82 may be formed of heat resistant glass and heat 
resistant resins including polyimide resin and the like. In addition, the 
heat-generating resistor 84 and electrodes 86 may be formed of various 
materials used in known versions of thermal head and the materials thereof 
are not limited to any particular type. The specific examples of the 
materials for the heat-generating resistor 84 include heat-generating 
resistors such as Nichrome (Ni--Cr), tantalum metal, tantalum nitride, 
ruthenium oxide and polysilicon. The specific examples of the materials 
for the electrodes 86 include electrically conductive materials such as 
aluminum, gold, silver and copper. 
As described above, the resistance value of each heating element of the 
thermal head varies in accordance with the heating history. The heating 
history is inherently different in each of the heating elements of the 
thermal head mounted in the thermal recording apparatus, so the amount of 
resistance variation is also different in each of the heating elements, 
which bring about with the passage of time differences in the amount of 
resistance variation among the heating elements. The recorded image will 
have thus density unevenness in accordance with the differences. 
FIG. 3 shows a graph of an example of the relationship between the 
resistance variation of the heating elements of the thermal head and the 
time for heat generation in blank recording, that is, when a specified 
energy for heat generation (145 mJ/mm.sup.2) was supplied to the thermal 
material A without recording. It should be noted that the rate of 
resistance variation when recording is less than in blank recording, 
because the temperature of the glaze does not increase beyond a certain 
extent by the radiating effect from the glaze to the thermal material A. 
As shown in FIG. 3, the resistance variation of the heating elements in the 
thermal head when a specified energy was supplied is approximately 
proportional to the logarithm of the time for heat generation. In other 
words, the initial resistance variation of the heating elements is large, 
but decreases exponentially with the passage of time, according as the 
total amount of heat generation increases. 
Therefore, using a method in which a specified energy for heat generation 
is supplied to all the heating elements of the thermal head before use for 
a specified period of time, all the heating elements are provided with 
specified uniform heating histories to previously change the resistance 
values thereof by specified amounts, whereupon the subsequent resistance 
variation, hence the differences in the resistance variation among the 
respective heating elements can be significantly reduced. 
In the thermal head 66 of the invention, all of the heating elements 
(including the heat-generating resistors 84 and the electrodes 86) are 
provided with specified heating histories to previously anneal and 
crystallize the heat-generating resistors 84, so that a uniform resistance 
variation is produced in all of the heating elements and resistance values 
of all of the heating elements are stabilized. The thermal head 66 of the 
invention comprises the heating elements having the thus stabilized 
resistance values, whereupon the differences in the resistance variation 
among the respective heating elements which may be caused by thermal 
recording can be reduced to thereby enable thermal recording of 
high-quality images without density unevenness in a consistent manner for 
an extended period of time. It should be noted that the heating histories 
are provided before the carbon protective layer 90 is formed. 
According to the recording apparatus 10 of the invention, the resistance 
variation amount (heating history amount) previously given to the 
respective heating elements in the thermal head 66 is not limited to any 
particular value and can be determined for example using the graph of FIG. 
3 or the like so that the amount of resistance variation which may be 
caused by the user's operation is in a specified range in which images 
having a specified quality can be ensured till the end of service life to 
be attained by the recording apparatus 10. In other words, the resistance 
variation amount to be previously given to the heating elements, that is, 
the energy for heat generation and the time for which the energy is 
supplied can be determined based on the relationship between the 
resistance variation amount which brings about density unevenness and the 
period of time during which maintenance of a specified image quality is 
desired. 
It is necessary to determine the previous variation amount taking account 
of any deterioration of the thermal head which may result from the 
previous change of the resistance values of the respective heating 
elements in the thermal head 66. 
The upper limit of the resistance variation of the heating elements is 
usually from 3 to 5%, hence the resistance variation amount to be 
previously given to the heating elements is preferably in the range of 
from 0.1 to 5%, especially from 0.5 to 2%, taking account of the 
productivity and production efficiency to be described below. 
The temperature (surface temperature) of the thermal head when the heating 
elements of the thermal head 66 are provided with heating histories to 
change the resistance values thereof, hence the energy for heat generation 
supplied in said blank recording to provide the heating elements with the 
heating histories are not limited to any particular values. 
If the temperature of the thermal head in this step is low however, it 
takes much time to sufficiently change the resistance values to exhibit 
the reduction effect on the resistance variation due to thermal recording, 
which adversely affects the productivity, production efficiency and other 
aspects. In addition, this is not practical. 
The upper limit of the temperature when the heating histories are provided 
depend on the heat resisting temperature of the thermal head used. 
The period of time for which the heating histories are provided can be 
determined based on FIG. 3 described above or the like prepared in 
accordance with the temperature of the thermal head (or energy for heat 
generation to attain this temperature) and the resistance variation amount 
to be achieved. 
Heating elements of the thermal head are known to be available usually in 
two types, one being of a thin-film type which is formed by a "thin-film" 
process such as vacuum evaporation, chemical vapor deposition (CVD) or 
sputtering and a photoetching technique, and the other being of a 
thick-film type which is formed by "thick-film" process comprising the 
steps of printing (e.g., screen printing) and firing and an etching 
technique. The thermal head 66 for use in the invention may be formed by 
either method. 
As described above, the illustrated thermal head 66 according to a 
preferred embodiment comprises a protective film composed of the two 
layers: the carbon protective layer 90 and the lower protective layer 88. 
The presence of the lower protective layer enables acquirement of more 
preferred results in various aspects including resistance to wear, 
resistance to corrosion and resistance to corrosion wear. A thermal head 
having a higher durability and a long service life can be thus realized. 
The lower protective layer 88 to be formed on the thermal head 66 of the 
invention may be formed of any known materials as long as they have 
sufficient heat resistance, corrosion resistance and wear resistance to 
serve as the protective film of the thermal head. Preferably, the 
ceramic-based lower protective layer 88 is illustrated. 
Specific materials include silicon nitride (Si.sub.3 N.sub.4), silicon 
carbide (SiC), tantalum oxide (Ta.sub.2 O.sub.5), aluminum oxide (Al.sub.2 
O.sub.3), SIALON (Si--Al--O--N), LASION (La--Si--O--N), silicon oxide 
(SiO.sub.2), aluminum nitride (AlN), boron nitride(BN), selenium oxide 
(SeO), titanium nitride (TiN), titanium carbide (TiC), titanium carbide 
nitride (TiCN), chromium nitride (CrN) and mixtures thereof. Among others, 
silicon nitride, silicon carbide, SIALON are advantageously utilized in 
various aspects such as easy film deposition, reasonability in 
manufacturing including manufacturing cost, balance between mechanical 
wear and chemical wear. Additives such as metals may be incorporated in 
small amounts into the lower protective layer to adjust physical 
properties thereof. 
Methods of forming the lower protective layer 88 are not limited in any 
particular way and known methods of forming ceramic films (layers) may be 
employed by applying the aforementioned thick-film and thin-film processes 
and the like. 
The thickness of the lower protective layer 88 is not limited to any 
particular value but it ranges preferably from about 0.5 .mu.m to about 50 
.mu.m, more preferably from about 2 .mu.m to about 20 .mu.m. If the 
thickness of the lower protective layer 88 is within the stated ranges, 
preferred results are obtained in various aspects such as the balance 
between wear resistance and heat conductivity (that is, recording 
sensitivity). 
The lower protective layer 88 may comprise multiple sub-layers. In this 
case, multiple sub-layers may be formed of different materials or multiple 
sub-layers different in density may be formed of one material. 
Alternatively, the two steps may be combined to obtain sub-layers. 
The thermal head of the invention is not limited to the one having the 
lower protective layer 88, but may have a one-layer protective film 
comprising only the carbon protective layer 90 which will be described 
below. 
The thermal head 66 of the invention has the carbon protective layer 90 
served as the protective film of the heat-generating resistor 84 and other 
parts. 
The illustrated thermal head 66 uses the carbon (DLC) protective layer 90 
as the carbon-based protective layer, but the invention is not limited 
thereto and the carbon-based protective layer is suitably a carbon 
protective layer containing more than 50 atm % of carbon, preferably a 
carbon protective layer comprising carbon and inevitable impurities, more 
preferably a high-purity carbon protective layer having extremely reduced 
or no inevitable impurities, for example the DLC protective layer. The 
inevitable impurities include residual gases in the vacuum chamber 
exemplified by oxygen and gases used during the process such as argon 
(Ar). The content of the gaseous components incorporated into the carbon 
protective layer is suitably as low as possible, preferably not more than 
2 atm %, more preferably not more than 0.5 atm %. According to the 
invention, the components to be incorporated in addition to carbon to form 
the carbon-based protective layer include advantageously elements such as 
hydrogen, nitrogen and fluorine, and semi-metals and metals such as Si, 
Ti, Zr, Hf, V, Nb, Ta, Er, Mo and W. In the case of hydrogen, nitrogen and 
fluorine, the content thereof in the carbon-based protective layer is 
preferably less than 50 atm %, and in the case of the abovementioned 
semi-metals and metals such as Si, Ti and the like, the content thereof is 
preferably not more than 20 atm %. 
We will now describe the carbon protective layer 90 as a typical example of 
the carbon-based protective layer, but it is to be understood that the 
description is also applied to other carbon-based protective layers. 
The carbon protective layer 90 having a high hardness and chemical 
stability provides the thermal head 66 having high reliability over a 
prolonged period of time and of which the protective film is 
advantageously protected from wear and corrosion wear due to thermal 
recording. 
In the thermal head 66 of the invention, the carbon protective layer 90 is 
formed after the heating elements are provided with the aforementioned 
heating histories. 
As described above, the thermal head 66 of the invention comprises heating 
elements of which the resistance variation due to thermal recording was 
reduced by providing all the heating elements with the heating histories 
to thereby previously change the resistance values thereof. 
This operation is usually performed after the thermal head is fabricated. 
According to the considerations by the inventors however, the heating 
history provided in such a high temperature for the fabricated thermal 
head having the protective layer brings about a change in properties or 
partial peeling-off of the carbon protective layer 90, which prevents 
appropriate thermal recording. 
On the other hand, the thermal head 66 of the invention comprises the 
carbon protective layer 90 formed after the heating elements were provided 
with the heating histories. The carbon protective layer 90 having 
favorable and appropriate properties without change in properties or 
partial peeling-off and being excellent in wear resistance and chemical 
stability provides the thermal head 66 having high reliability over a 
prolonged period of time. In addition, density unevenness due to the 
resistance variation is extremely reduced by the heating elements provided 
with the heating histories, as described above. 
It should be noted that the aforementioned lower protective layer 88 may be 
formed before or after providing said heating elements with the heating 
histories. 
The carbon protective layer 90 in the thermal head 66 of the invention 
needs to have a sufficient hardness to serve as the protective film of the 
thermal head, although a higher hardness provides better performance. The 
hardness is preferably more than 2000 kg/mm.sup.2, more preferably more 
than 2500 kg/mm.sup.2, most preferably more than 3000 kg/mm.sup.2 in terms 
of Vickers hardness. 
If the hardness of the carbon protective layer 90 is within the stated 
ranges, preferred results can be obtained in various aspects including 
wear resistance. 
Moreover, the thickness of the carbon protective layer 90 is not limited to 
any particular value but it ranges preferably from 0.1 .mu.m to 5 .mu.m, 
more preferably from 1 .mu.m to 3 .mu.m, in the case of the glaze having 
the lower protective layer 88 as shown in FIG. 2. In the case where the 
lower protective layer 88 is not formed, it ranges preferably from 1 .mu.m 
to 20 .mu.m, more preferably from 2 .mu.m to 10 .mu.m. 
If the thickness of the carbon protective layer 90 is within the stated 
ranges, preferred results can be obtained in various aspects including the 
balance between wear resistance and heat conductivity. 
Methods of forming the carbon protective layer 90 are not limited in any 
particular way and known thick- and thin-film processes may be employed. 
Preferred examples include the plasma-assisted CVD using a hydrocarbon gas 
as a reactive gas to form a hard carbon film and the sputtering of a 
carbonaceous material (e.g., sintered carbon or glassy carbon) as a target 
to form a hard carbon film. 
FIG. 4 shows the concept of a plasma-assisted CVD apparatus to form the 
carbon protective layer 90. The CVD apparatus generally indicated by 100 
comprises a vacuum chamber 102, a gas introducing section 104, plasma 
generating means 106, a substrate holder 108 and a substrate bias source 
110 as the basic components. 
The vacuum chamber 102 is preferably formed of a nonmagnetic material such 
as SUS 304 in order to keep unperturbed the magnetic field generated for 
plasma generation. 
Preferably, the vacuum chamber 102 which is used to form the carbon 
protective layer 90 has pump-down means and presents such a seal property 
that an ultimate pressure of 2.times.10.sup.-5 Torr or below, preferably 
5.times.10.sup.-6 Torr or below, is reached by initial pump-down whereas 
an ultimate pressure between 1.times.10.sup.-4 Torr and 1.times.10.sup.-2 
Torr is reached during film deposition. 
Pump-down means 112 is provided for the vacuum chamber 102 and a preferred 
example is the combination of a rotary pump, a mechanical booster pump and 
a turbomolecular pump; pump-down means using a diffusion pump or a 
cryogenic pump may be suitably used instead of the turbomolecular pump. 
The performance and number of pump-down means 112 may be determined as 
appropriate for various factors including the capacity of the vacuum 
chamber 102 and the nature and flow rate of a gas used during film 
deposition. In order to adjust the pumping speed, various adjustment 
designs may be employed, such as bypass pipes that provide for evacuation 
resistance adjustment and orifice valves which are adjustable in the 
degree of opening. 
Those sites of the vacuum chamber 102 where plasma develops or an arc is 
produced by plasma generating electromagnetic waves may be covered with an 
insulating member, which may be made of insulating materials including MC 
nylon, Teflon (PTFE), polyphenylene sulfide (PPS), polyethylene 
naphthalate (PEN) and polyethylene terephthalate (PET). If PEN or PET is 
used, care must be taken to insure that the degree of vacuum will not 
decrease upon degassing of such insulating materials. 
The CVD apparatus 100 comprises the gas introducing section 104 consisting 
of two parts 104a and 104b, the former being a site for introducing a 
plasma generating gas and the latter for introducing a reactive gas, into 
the vacuum chamber 102 through stainless steel pipes or the like that are 
vacuum sealed with O-rings or the like. The amounts of the gases being 
introduced are controlled by known means such as a mass flow controller. 
Both gas introducing parts 104a and 104b are basically so adapted as to 
displace the introduced gases to the neighborhood of the plasma-generating 
region in the vacuum chamber 102. The blowout position, particularly that 
of the reactive gas introducing part 104b, has a certain effect on the 
thickness profile of the carbon protective layer to be formed and, hence, 
it is preferably optimized in accordance with various factors such as the 
geometry of the substrate (the glaze of the thermal head 66). 
Examples of the plasma generating gas for producing the carbon protective 
layer 90 are inert gases such as helium, neon, argon, krypton and xenon, 
among which argon gas is used with particular advantage because of price 
and easy availability. Examples of the reactive gas for producing the 
carbon protective layer 90 are the gases of hydrocarbon compounds such as 
methane, ethane, propane, ethylene, acetylene and benzene. 
It is required with the gas introducing parts 104a and 104b that the 
sensors in the mass flow controllers be adjusted (calibrated) in 
accordance with the gases to be introduced. 
In plasma-assisted CVD to form the carbon protective layer 90, the plasma 
generating means may utilize various discharges such as direct current 
(DC) glow discharge, radio-frequency (RF) discharge, DC arc discharge and 
microwave ECR discharge, among which DC arc discharge and microwave ECR 
discharge have high enough plasma densities to be particularly 
advantageous for high-speed film deposition. 
The illustrated CVD apparatus 100 utilizes microwave ECR discharge and the 
plasma generating means 106 comprises a microwave source 114, magnets 116, 
a microwave guide 118, a coaxial transformer 120, a dielectric plate 122 
and a radial antenna 124 and the like. 
In DC glow discharge, a plasma is generated by applying a negative DC 
voltage between the substrate and the electrode. The DC power supply for 
use in DC glow discharge has an output of about 1 to 10 kW and a device 
having the necessary and sufficient output to produce the carbon 
protective layer 90 may appropriately be selected. For anti-arc and other 
purposes, a DC power supply pulse-modulated for 2 to 20 kHz is also 
applicable with advantage. 
In RF discharge, a plasma is generated by applying a radio-frequency 
voltage to the electrodes via a matching box, which performs impedance 
matching such that the reflected wave of the radio-frequency voltage is no 
more than 25% of the incident wave. A suitable RF power supply for RF 
discharge may be selected from those in commercial use which produce 
outputs at 13.56 MHz having powers in the range from about 1 kW to about 
10 kW which are necessary and sufficient to produce the carbon protective 
layer 90. A pulse-modulated RF power supply is also useful for RF 
discharge. 
In DC arc discharge, a hot cathode is used to generate a plasma. The hot 
cathode may typically be formed of tungsten or lanthanum boride 
(LaB.sub.6). DC arc discharge using a hollow cathode can also be utilized. 
A suitable DC power supply for use in DC arc discharge may be selected 
from those which produce outputs at about 10 to 200 A having powers in the 
range from about 1 kW to about 10 kW which are necessary and sufficient to 
produce the carbon protective layer 90. 
In microwave ECR discharge, a plasma is generated by the combination of 
microwaves and an ECR magnetic field and, as already mentioned, the 
illustrated CVD apparatus 100 utilizes microwave ECR discharge for plasma 
generation. 
The microwave source 114 may appropriately be selected from those in 
commercial use which produce outputs at 2.45 GHz having powers in the 
range from about 1 kW to 3 kW which are necessary and sufficient to 
produce the carbon protective layer 90. 
To generate an ECR magnetic field, permanent magnets or electromagnets 
which are capable of forming the desired magnetic field may appropriately 
be employed and, in the illustrated case, Sm-Co magnets are used as the 
magnets 116. Consider, for example, the case of using microwaves at 2.45 
GHz; since the ECR magnetic field has a strength of 875 G (gauss), the 
magnets 116 may be those which produce a magnetic field with intensities 
of 500 to 2,000 G in the plasma generating region. 
Microwaves are introduced into the vacuum chamber 102 using the microwave 
guide 118, the coaxial transformer 120, the dielectric plate 122, etc. It 
should be noted that the state of magnetic field formation and the 
microwave introducing path, both affecting the thickness profile of the 
carbon protective layer 90 to be deposited, are preferably optimized to 
provide a uniform thickness for the carbon protective layer 90. 
The substrate holder 108 fixes the thermal head 66 to which the heat sink 
67 is fixed or not fixed, or the glaze and other portions detached from 
the thermal head 66, by known fixing means such as a clamp or a jig in 
such a way that the glaze used as the substrate of film deposition is held 
in a face-to-face relationship with the radial antenna 124. If necessary, 
the glaze may be adapted to be rotatable or otherwise movable relative to 
the plasma generating means 106. 
The distance between the substrate (the surface of the glaze) and the 
radial antenna 124 (the plasma generating section) is not limited to any 
particular value and a distance that provides a uniform thickness profile 
may be set appropriately within the range from about 20 mm to about 200 
mm. 
When forming the carbon protective layer 90, a mask for controlling the 
film deposition area may be used if necessary. Then, a plate-like masking 
member made of a metal such as SUS 304 or aluminum, or a resin such as 
Teflon or the like may be prepared and used for masking the areas to be 
protected from film deposition. 
In order to form the carbon protective layer by plasma-assisted CVD, film 
deposition has to be performed with a negative bias voltage being applied 
to the substrate. The substrate bias source 110 is used to supply the 
required bias voltage. 
The radio-frequency voltage is not limited to the self-bias voltage, but 
the latter is preferably used, since the carbon protective layer 90 has a 
high electrical resistance. The self-bias voltage is a negative DC 
component produced when applying a radio-frequency voltage in the plasma. 
When forming the carbon protective layer, the self-bias voltage in the 
range of -100 to -500 V is usually used. A suitable RF power supply may be 
selected from those in commercial use which produce outputs at 13.56 MHz 
having powers in the range from about 1 kW to about 5 kW. 
When applying a radio-frequency voltage to the substrate, a matching box is 
preferably used for impedance matching between the substrate and the RF 
power supply. The matching box may be of manual control type or automatic 
control type and a variety of commercially available products can be used. 
Instead of the radio-frequency self-bias voltage, a DC power supply 
pulse-modulated for 2 to 20 kHz is also applicable. In this case, the 
voltage to be applied is also in the range of from -100 to -500 V. 
The surface of the substrate (glaze), or the surface of the illustrated 
lower protective layer 88 is preferably etched with a plasma prior to the 
formation of the carbon protective layer 90 in order to improve its 
adhesion to the carbon protective layer 90. 
The etching methods include a method in which a radio-frequency voltage is 
applied via the matching box while generating a plasma by said plasma 
generating means 106, and a method in which a plasma is directly generated 
by a radio-frequency voltage and is used for etching. 
A suitable RF power supply may be selected from those in commercial use 
which produce outputs at 13.56 MHz having powers in the range from about 1 
kW to about 5 kW. The intensity of etching may be determined with the bias 
voltage to the substrate being used as a guide;, an optimal value may be 
selected from the range of -100 to -500 V. 
FIG. 5 shows the concept of a sputtering apparatus to form the carbon 
protective layer 90. 
The sputtering apparatus generally indicated by 130 comprises a vacuum 
chamber 132, a gas introducing section 134, sputter means 136 and a 
substrate holder 138 as the basic components. 
The vacuum chamber 132 in which sputtering is performed to form the carbon 
protective layer, pump-down means 140 provided therefor, and adjusting 
means for pumping speed are advantageously exemplified by those having a 
similar structure to that of said CVD apparatus 100. 
The gas introducing section 134 is a site for introducing a plasma 
generating gas into the vacuum chamber 132 through stainless steel pipes 
or the like that are vacuum sealed with O-rings or the like, as in the gas 
introducing section 104 of said CVD apparatus 100. The amounts of the 
gases being introduced are controlled by known means such as a mass flow 
controller. The gas introducing section 134 is basically so adapted as to 
displace the introduced gas to the neighborhood of the plasma-generating 
region in the vacuum chamber 132. The blowout position is preferably 
optimized to be such that the profile of the generated plasma will not be 
adversely affected. 
To effect sputtering, a target 144 to be sputtered is placed on the cathode 
142, which is rendered at negative potential and a plasma is generated on 
the surface of the target 144, whereby atoms are struck out of the target 
144 and deposit on the surface on the opposed substrate (i.e., on the 
surface of the glaze of the thermal head 66=on the surface of the lower 
protective layer 88) to form the film. 
The sputter means 136 comprises essentially the cathode 142, the area where 
the target 144 is to be placed, a shutter 146 and a DC power supply 152. 
In order to generate a plasma on the surface of the target 144, the 
negative side of the DC power supply 152 is connected directly to the 
cathode 142, which is supplied with a DC voltage of about -300 to -1,000 
V. The DC power supply 152 has an output of about 1 to 10 kW and a device 
having the necessary and sufficient output to produce the carbon 
protective layer 90 may appropriately be selected. The geometry of the 
cathode 142 may be determined as appropriate for various factors such as 
the geometry of the substrate on which the carbon protective layer 90 is 
to be formed. For anti-arc and other purposes, a negative DC power supply 
pulse-modulated for 2 to 20 kHz is also applicable with advantage. 
RF power supplies are also useful to generate plasmas. If an RF power 
supply is to be used, a radio-frequency voltage is applied to the cathode 
142 via a matching box so as to generate a plasma. The matching box 
performs impedance matching such that the reflected wave of the 
radio-frequency voltage is no more than 25% of the incident wave. A 
suitable RF power supply may be selected from those in commercial use 
which produce outputs at 13.56 MHz having powers in the range of from 
about 1 kW to about 10 kW which are necessary and sufficient to produce 
the carbon protective layer 90. 
The target 144 may be secured directly to the cathode 142 with In-based 
solder or by mechanical fixing means but usually a backing plate 154 made 
of oxygen-free copper, stainless steel or the like is first fixed to the 
cathode 142 and the target 144 is then attached to the backing plate 154 
by the methods just described above. The cathode 142 and the backing plate 
154 are adapted to be water-coolable so that the target 144 is indirectly 
cooled with water. 
The target 144 used to form the carbon protective layer 90 is preferably 
made of sintered carbon, glassy carbon or the like. The geometry of the 
target 114 may be determined as appropriate for the geometry of the 
substrate. 
Another method that can advantageously be employed to form the carbon 
protective layer 90 is magnetron sputtering, in which magnets 148 such as 
permanent magnets or electromagnets are placed within the cathode 142 and 
a sputtering plasma is confined within a magnetic field formed on the 
surface of the target 144. Magnetron sputtering is preferred since it 
achieves high deposition rates. 
The shape, position and number of the permanent magnets or electromagnets 
to be used and the strength of the magnetic field to be generated are 
determined as appropriate for various factors such as the thickness and 
its profile of the carbon protective layer 90 to be formed and the 
geometry of the target 144. Using permanent magnets such as Sm-Co and 
Nd-Fe-B magnets which are capable of producing intense magnetic fields is 
preferred for several reasons including the high efficiency of plasma 
confinement. 
The substrate holder 138 is basically the same as the substrate holder 108 
positioned in the CVD apparatus 100 described above and fixes the thermal 
head 66 in position so that the glaze is held in a predetermined 
face-to-face relationship with the cathode 142. 
The distance between the substrate and the target 144 is not limited to any 
particular value and a distance that provides a uniform thickness profile 
may be set appropriately within the range from about 20 mm to about 200 
mm. 
A negative bias voltage is applied to the substrate (the lower protective 
layer 88 in the illustrated case) to obtain the carbon protective layer 
90. A bias source 150 is used to supply the required bias voltage. 
The bias voltage is not limited to any particular type but a 
radio-frequency self-bias voltage is preferably used as in the CVD 
described above. The RF power supply as used in the CVD is applicable and 
the matching box is also preferably used. Instead of the radio-frequency 
self-bias voltage, a DC power supply pulse-modulated for 2 to 20 kHz is 
also applicable with advantage. In this case, the voltage to be applied is 
also in the range of from -100 to -500 V. 
When forming the carbon protective layer 90, the surface of the lower 
protective layer 88 is preferably etched with a plasma prior to the 
formation of the carbon protective layer 90 in order to improve its 
adhesion to the lower layer (lower protective layer 88). 
The etching methods include a method in which a radio-frequency voltage is 
applied to the substrate via the matching box while generating a plasma, 
and a method in which a plasma is directly generated by a radio-frequency 
voltage and is used for etching. The plasma generating means and the RF 
power supply as described above can be used. The intensity of etching may 
be determined with the bias voltage to the substrate being used as a 
guide; usually, an optimal value may be selected from the range of -100 to 
-500 V. 
On the foregoing pages, the thermal head has been described in detail but 
the present invention is in no way limited to the stated embodiments and 
various improvements and modifications can of course be made without 
departing from the spirit and scope of the invention. 
As described above in detail, the present invention provides a thermal head 
in which corrosion and wear of the protective film, and the resistance 
variation of the heating elements due to thermal recording were 
significantly reduced, and which has a sufficient durability and stability 
with the passage of time to perform thermal recording of high-quality 
images in a consistent manner over an extended period of operation. 
The invention will be further illustrated by means of the following 
specific examples. 
EXAMPLE 1 
Supply of Heating History: 
A commercial thermal head (Model KGT-260-12MPH8 of KYOCERA CORP.) was used 
as the base. The thermal head had a silicon nitride (Si.sub.3 N.sub.4) 
film formed in a thickness of 11 .mu.m as a protective layer on the 
surface of the glaze. Therefore, in Example 1, the silicon nitride film 
served as the lower protective layer 88. 
All the heating elements of the thermal head were provided with heating 
histories by continuously supplying heat generating energy of 145 
mJ/mm.sup.2 for 90 minutes. The resistance values of all the heating 
elements in the thermal head were then reduced by about 1.5% on average. 
The carbon protective layer 90 was formed on the surface of the glaze of 
the thermal head which was thus provided with the heating history, by 
means of the plasma-assisted CVD apparatus shown in FIG. 4 to thereby 
fabricate the thermal head 66 having the glaze shown in FIG. 2. 
The plasma-assisted CVD apparatus 100 is now described in detail. 
a. Vacuum Chamber 102 
This vacuum chamber was made of SUS 304 and had a capacity of 0.5 m.sup.3 ; 
pump-down means 112 comprised one unit each of a rotary pump having a 
pumping speed of 1,500 L/min, a mechanical booster pump having a pumping 
speed of 12,000 L/min and a turbomolecular pump having a pumping speed of 
3,000 L/sec. An orifice valve was fitted at the suction inlet of the 
turbomolecular pump to allow for 10 to 100% adjustment of the degree of 
opening. 
b. Gas Introducing Section 104 
This gas introducing section was composed of a mass flow controller 
permitting a maximum flow rate of 100 to 500 sccm and a stainless steel 
pipe having a diameter of 6 mm. The joint between the stainless steel pipe 
and the vacuum chamber 102 was vacuum sealed with an O-ring. 
Argon gas was used as a plasma generating gas. 
c. Plasma Generating Means 106 
A microwave ECR plasma generating apparatus using a microwave source 114 
oscillating at a frequency of 2.45 GHz and producing a maximal output of 
3.0 kW was employed. The generated microwave was guided to the 
neighborhood of the vacuum chamber 102 by means of the microwave guide 
118, passed through the coaxial transformer 120 and directed to the radial 
antenna 124 in the vacuum chamber 102. 
The dielectric plate 122 used was in a rectangular form having a width of 
800 mm and a height of 200 mm. The microwave passing through the microwave 
guide 118 was divided into four on the halfway and introduced into the 
vacuum chamber 102 through 4 portions in the dielectric plate 122. 
A magnetic field for ECR was produced by arranging a plurality of Sm-Co 
magnets used as the magnets 116 in a pattern to conform to the shape of 
the dielectric plate 122. 
d. Substrate Holder 108 
The substrate (that is, the glaze 82 of the thermal head 66; see FIG. 2) 
was held in a face-to-face relationship with the plasma generating section 
and was so adapted that the distance between the substrate and the radial 
antenna 124 could be varied between 50 mm and 150 mm. 
That area of the substrate in which the thermal head was held was set at a 
floating potential in order to enable the application of an etching 
radio-frequency voltage. 
e. Substrate Bias Source 110 
An RF power supply served as the substrate bias source 110 was connected to 
the substrate holder 108 via a matching box. 
The RF power supply had a frequency of 13.56 MHz and could produce a 
maximal output of 3 kW. It was also adapted to be such that by monitoring 
the self-bias voltage, the RF output could be adjusted over the range of 
-100 to -500 V. 
In the CVD apparatus 100, the substrate bias source 110 also serves as the 
substrate etching means. 
Fabrication of Thermal Head 66: 
Thermal head 66 was secured to the substrate holder 108 in the vacuum 
chamber 102 such that the glaze 82 of the thermal head 66 provided with 
the heating history as described above would be in a face-to-face 
relationship with the radial antenna 124. The distance between the 
substrate (surface of the glaze 82) and the radial antenna 124 was set to 
100 mm. All areas of the thermal head other than those where the carbon 
protective layer was to be formed (namely, the non-glaze areas) were 
previously masked. 
After the thermal head was fixed in position, the vacuum chamber 102 was 
pumped down to an internal pressure of 5.times.10.sup.-6 Torr. 
With continued pump-down, argon gas was introduced through the gas 
introducing section 104a and the pressure in the vacuum chamber 102 was 
adjusted to 1.0.times.10.sup.-3 Torr by means of the orifice valve fitted 
on the turbomolecular pump. 
Subsequently, the microwave source 114 was driven to introduce each 
microwave at a power of 400 W through 4 portions in the dielectric plate 
into the vacuum chamber 102 where a microwave ECR plasma was generated. 
The substrate bias source 110 was also driven to apply a radio-frequency 
bias voltage to the substrate and the lower protective layer 88 (silicon 
nitride film) was etched for 2 minutes at a self-bias voltage of -200 V. 
After the end of etching, the plasma-assisted CVD was performed by 
introducing methane gas to adjust the internal pressure in the vacuum 
chamber 102 at 3.0.times.10.sup.-3 Torr, with the radio-frequency voltage 
being kept applied by the self-bias voltage. Thus, the thermal head 66 
having the carbon protective layer 90 formed in a thickness of 1 .mu.m was 
fabricated. The same procedure was repeated to fabricate two additional 
samples of thermal head having the carbon protective layer 90 formed in 
thickness of 2 .mu.m and 3 .mu.m. 
To control the thickness of the carbon protective layer 90 being formed, 
the deposition rate was determined previously and the time required to 
reach a specified film thickness was calculated. 
Evaluation of Performance: 
Using the thus fabricated thermal head and sheets of thermal material (dry 
image recording film CR-AT of Fuji Photo Film Co., Ltd.), a thermal 
recording test was performed. The results showed that normal thermal image 
recording could be performed. 
Comparative Example 1 
The procedure of Example 1 was repeated to fabricate additional three 
samples of thermal head having the carbon protective layer 90 deposited 
thereon in thickness of 1 .mu.m, 2 .mu.m and 3 .mu.m, except that the 
formation of the carbon protective layer and the supply of the heating 
history to each of the heating elements were reversed, that is, the 
formation of the carbon protective layer was followed by the supply of 
heating history to each of the heating elements. 
After the supply of heating history, the resistance values of all the 
heating elements were reduced by about 1.5% on average. 
The thus fabricated samples of thermal head were used to perform an image 
recording test as in Example 1. In this example, extreme density 
unevenness or streaks were confirmed on the image. The surface of the 
glaze in each thermal head was observed by means of a light microscope. 
Change of properties due to heat was confirmed in the carbon protective 
layer. 
These results clearly demonstrate the effectiveness of the present 
invention.