Process for forming thin film, heat treatment process of thin film sheet, and heat treatment apparatus therefor

The invention provides a process for forming a magnetic thin film on a base film, a heat treatment process of a thin film sheet consisting of the base film and the magnetic thin film, and an apparatus for performing heat treatment of the thin film sheet. Tension applied to the thin film sheet is substantially equal to that applied to the base film when the magnetic thin film is formed thereon. Then, the thin film sheet is treated with heat. The thin film sheet is heated with a given temperature gradient to a reactive temperature at which heat shrinkage occurs, while the tension is being applied thereto. Thereafter, the thin film sheet to which the tension is still applied is cooled with substantially the same temperature gradient as applied in heating. The heat treatment apparatus has a film driving unit including a supply reel, a take-up reel, a drive source and guide rollers; a heating unit including heating plates, heater blocks and a temperature controller for heating the sheet to the reactive temperature; and a heat insulating unit including a thermostat and another temperature controller for maintaining the sheet at the nonreactive temperature which is slightly lower than the reactive temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for forming a magnetic thin film 
on a thin plastic base film such as a polyester film, a heat treatment 
process of a sheet of the base film and the magnetic thin film, and an 
apparatus for performing heat treatment therefor. 
A magnetic tape of a metal thin film type has received great attention as a 
high density recording medium. Various processes for fabricating the 
magnetic tape of the metal thin film type have been proposed. Among these 
processes, typical examples are vacuum deposition, electron beam 
deposition, sputtering, and ion-plating. A magnetic tape which is 
fabricated by one of these processes has an extremely thin magnetic layer 
as compared with a conventional magnetic tape on which magnetic particles 
together with a binder are coated on a base film. The thinner the magnetic 
layer is, the better the high frequency response is in recording and 
playback. For this reason, when the magnetic thin film is used, a magnetic 
recording tape which is suitable for high density recording is obtained. 
The base film which is used in a metal thin film type magnetic recording 
tape for long time recording is extremely thin. Assuming that this thin 
film magnetic recording tape is fabricated by vacuum deposition, the base 
film is thermally damaged by radiant heat from a vapor source and latent 
heat of vaporized metal atoms. Then, wrinkles are formed on the film. 
Further, while the vaporized metal atoms are recrystalized and formed as a 
thin film, internal stress corresponding to heat shrinkage of the thin 
film occurs. A base film 10 is curled with a thin film layer 12 facing 
inward due to inner stress, as shown in FIG. 1A. When a magnetic recording 
tape on which a large wrinkle is formed is used, dropout occurs. Further, 
when a curled magnetic recording tape is used, it vertically deviates from 
the normal tape driving path (i.e., it meanders). Thus, the tape is 
misaligned with the head and the tape is irregularly wound around a 
take-up reel. 
In general, a thin film magnetic recording tape is fabricated as follows. A 
magnetic material is deposited on a base film strip of a wide width. 
Various coatings and surface treatments are performed. A thin film sheet 
of a great width is cut into narrow bands of a predetermined width. Thus, 
the magnetic tape is fabricated. The width of the thin film sheet on which 
the magnetic particles are deposited is considerably greater than the 
magnetic recording tape as the final product. In order to completely 
eliminate wrinkles and curls from the thin film magnetic recording tape, 
the thin film sheet must be completely smoothed before it is cut into 
narrow bands. 
For this purpose, the base film on which the thin film is formed may be 
heated. However, when the base film with the thin film thereon as shown in 
FIG. 1A is simply heated, the curled film 10 becomes corrugated, as shown 
in FIG. 1B. Therefore, with a simple heat treatment as described above, 
the wrinkles and curl cannot be uniformly eliminated from the entire 
surface of the film 10. 
As the prior art which is proposed to eliminate the wrinkles and the curls, 
Japanese Patent Disclosure Nos. 53-83706 and 53-104204 disclose measures 
for this purpose. In these patent disclosures, cracks are formed in the 
magnetic particle layer to solve the above problem. However, formation of 
cracks entails another problem. Stiffness of the tape in the longitudinal 
direction of the tape becomes smaller than that in the direction of width 
thereof. When the magnetic recording tape is driven, the tape slidably 
driven past the magnetic head is not brought into contact with the 
magnetic head over its entire width. Thus, spacing loss is increased. 
Further, since stress concentrates especially in the cracks, resistance to 
oblique tearing of the magnetic recording tape may be considerably 
decreased. In other words, the tape tends to be torn obliquely. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above problem 
and has for a first object to provide a process for forming a thin film 
layer without degrading physical strength of a base film and without 
thermal strain such as a wrinkle and a curl. 
In order to achieve the above object of the present invention, there is 
provided a process for forming a thin film wherein predetermined tension 
is applied to a base film while the thin film is being formed thereon, and 
heat treatment of a thin film sheet is performed while the stress is being 
applied. In this heat treatment, the physical strength of the film is not 
degraded and the wrinkles and the curls are eliminated from the fabricated 
thin film sheet. 
It is a second object of the present invention to provide a heat treatment 
process for completely eliminating wrinkles and curls from a sheet with a 
thin film. 
In order to achieve the above object of the present invention, there is 
provided a heat treatment process comprising the steps of heating with a 
predetermined temperature rise gradient a sheet to which constant tension 
is applied and which includes a thin film until heat shrinkage occurs, and 
cooling the heat shrunk sheet, to which the constant tension is applied, 
with a temperature drop gradient corresponding to the temperature rise 
gradient. When this heat treatment is performed, the wrinkles and curls 
which are formed on the sheet when the thin film is formed are completely 
eliminated over the entire surface. 
It is a third object of the present invention to provide a heat treatment 
apparatus for completely eliminating thermal strain such as wrinkles and 
curls from the sheet including the thin film. 
In order to achieve the above objects of the present invention, there is 
provided a heat treatment apparatus comprising means for driving or 
traveling a sheet which includes a thin film and to which constant tension 
is applied, means for heat-insulating to keep the sheet at a temperature 
immediately below a non-reacting temperature at which heat shrinkage 
occurs, and means for heating the sheet traveled at the non-reacting 
temperature to a temperature at which heat shrinkage of the sheet occurs. 
According to the heat treatment apparatus with the above structure, a heat 
shrinking reaction of the thin film sheet is carried out in a thermostat 
at a fixed temperature slightly lower than the temperature of heat 
shrinkage. Therefore, as compared with the case where the sheet is 
heat-treated by a conventional heating apparatus under room temperature, 
variation of temperature during the heat treatment process becomes far 
smaller. Accordingly, the temperature of heat shrinkage of the sheet can 
properly and easily be controlled. Further, since proper tension is 
applied to the sheet, the sheet may not be curled in the opposite 
direction due to heat shrinkage, thus providing a flat sheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described with reference to 
the accompanying drawings. The same or similar reference numerals denote 
the same or similar parts throughout the drawings, and a duplicated 
description thereof will be omitted. 
FIG. 2 shows a reel winding-up type vapor deposition apparatus. A supply 
reel 18 and a take-up reel 20 are disposed inside a housing 16 which is 
coupled to a vacuum pump 14. A base film 10 wound around the supply reel 
18 is supplied to the take-up reel 20 through a tension controller 22, a 
guide roller 24, a cooling can 26 and guide rollers 28 to 34. A pair of 
heaters 36 and 38 are disposed between the guide roller 30 and the guide 
roller 32 so as to put the base film 10 therebetween. 
The base film 10 is driven or traveled by a servo motor (not shown) coupled 
to a rotating shaft of the take-up reel 20. The driving speed of the base 
film 10 is controlled to be constant by the servo motor. Tension applied 
to the base film 10 is preferably kept constant. For this purpose, a back 
tension applying mechanism such as a tension controller 22 is disposed on 
the side of the supply reel 18. A belt slipping mechanism may be used for 
such a mechanism. Alternatively, tension servo mechanisms may respectively 
be disposed on the both sides of the supply reel 18 and the take-up reel 
20. Such a tension servo mechanism is disclosed in Japanese Patent 
Applications Nos. 55-92482 titled "Tension Servo Apparatus" and 55-115385 
titled "Tension Servo Apparatus" both filed by the present applicant 
(OLYMPUS OPTICAL, CO.). These tension servo apparatuses may be utilized in 
this case. The disclosure of the above Japanese applications is now 
combined herewith. 
A vapor source 40 is disposed directly under the cooling can 26. An 
evaporant 42 comprising a magnetic material (particles) of, e.g., Ni 
(nickel), Co (cobalt), Fe (iron) or garnet type magnetic particles is set 
in the vapor source 40. The evaporant 42 may be heated and evaporated by 
resistive heating, RF induction heating, or electron beam heating. 
Vaporized atoms (or molecules) 42.sub.1 are deposited on a surface portion 
of the base film 10 which is not masked by a screen 44, while the base 
film 10 is driven being in contact with the outer circumferential surface 
of the cooling can 26. These atoms 42.sub.1 are then cooled and 
recrystalized. By this recrystalization of the vaporized atoms 42.sub.1, 
internal stress occurs in the film 10, resulting in thermal strain such as 
wrinkles and curls in the film 10. This internal stress may substantially 
be eliminated when the same tension as in formation of the thin film layer 
12 on the film 10 is applied to the film 10 during heat treatment. Thus, a 
thin film sheet which is formed of the base film 10 and a thin film layer 
12 thereon is treated with heat. 
This thin film sheet which consists of the base film 10 and the thin film 
layer 12 thereon is heated by the heaters 36 and 38. The thin film sheet 
(10+12) is heated under the same tension when the vaporized atoms 42.sub.1 
are deposited on the surface of the base film 10. In the structure of FIG. 
2, the cooling can 26 and the rollers 28 to 34 are rotated smoothly. With 
the above structure, tension applied to the base film 10 on the cooling 
can 26 is substantially the same as that applied to the base film 10 
between the heaters 36 and 38. In this condition, the base film 10 with 
the thin film layer 12 thereon is heated by the heaters 36 and 38 to an 
extent that heat shrinkage occurs. If heating is sufficiently performed, 
mechanical strength of the base film 10 is not degraded and the internal 
stress is effectively eliminated. 
The optimal temperature of heat treatment by the heaters 36 and 38 may vary 
in various conditions. A certain temperature cannot be specified. The 
temperature of heat treatment with the heaters 36 and 38 may be changed in 
accordance with the kinds of base film 10 and the evaporant 42, the 
thickness of the base film 10 and a thin film 12 formed thereon, the 
running speed of the film 10 and so on. The actual conditions of heat 
treatment may be determined by a cut-and-try operation in the process of 
fabricating the thin film magnetic tape. For instance, the base film 10 is 
subjected to heat shrinkage in accordance with the degree of shrinkage of 
the thin film which occurs in thin film forming process. Where the 
wrinkles and curls are eliminated without degrading the mechanical 
strength of the base film 10 at a certain temperature, then this 
temperature is defined as the optimal temperature of heat treatment. If 
the tension applied to the base film 10 is too weak, the wrinkles and 
curls cannot be eliminated. On the other hand, if the tension is too 
strong, the base film 10 is elongated. Therefore, when tension applied to 
the thin film sheet during heat treatment is substantially the same as 
that applied during formation of the thin film layer 12 on the base film 
10, no trouble will occur. However, the film tension at the time of thin 
film formation need not strictly be the same as the sheet tension during 
heat treatment. 
FIG. 3 shows a mechanism for treating with heat the thin film sheet which 
consists of the base film 10 and the thin film layer 12. The heaters 36 
and 38 are made of nichrome wires and/or heating elements such as infrared 
ray lamps. Further, the heaters 36 and 38 preferably have a temperature 
sensor such as a thermistor, respectively. The heating elements and the 
temperature sensors are connected to a heater power source (not shown). 
Further, if power supply to the heating element is controlled with NFB 
operation, the film 10 can be heated at a constant temperature. Instead of 
the above automatic temperature control, a bimetal may be used to control 
the temperature. 
FIG. 4 shows a film roll 50 (or thin film sheet roll) of the thin film 
sheet consisting of the base film 10 and the thin film layer 12. According 
to the apparatus of FIG. 2, the film 10 is driven to the side of the 
take-up reel 20 with constant tension and wound around a hub 52. The 
trailing end of the film roll 50 is fixed with an end seal 54. 
Substantially the same tension as applied to the base film 10 in the 
process for forming the thin film layer 12 on the base film 10 is applied 
to the rolled film along the longitudinal direction thereof. Therefore, 
the thin film sheet need not be treated with heat before the thin film 
sheet is rolled. Instead, the rolled sheet may be treated with heat in 
order to obtain the same effect. Thus, the wrinkles and curls can be 
eliminated. In this case, the film roll 50 is housed in a conventional 
thermostat and treated with heat. With the conventional thin film forming 
apparatus, the present invention may provide measures to solve the 
conventional problem. 
FIG. 5 shows a case wherein the thin film sheet which consists of the base 
film 10 and the thin film layer 12 is afresh treated with heat. A thin 
film sheet roll 50.sub.1 is set on a supply reel 18.sub.1. The base film 
10 supplied from the thin film sheet roll 50.sub.1 is wound by a take-up 
reel 20.sub.1 through the guide rollers 28 to 34. At this time, tension of 
the thin film sheet is automatically controlled to be equal to tension 
applied on the film 10 when the thin film layer 12 is formed thereon. With 
such constant tension, the thin film sheet is heated by the heaters 36 and 
38. With this arrangement, the wrinkles and curls can also be eliminated. 
FIG. 6 shows a modification of the mechanism of FIG. 5. The thin film sheet 
is preheated by a first pair of heaters 36.sub.1 and 38.sub.1 and the 
sheet is shrunk with heat by a second pair of heaters 36.sub.2 and 
38.sub.2. With the above structure, the thin film sheet is uniformly 
treated with heat over the entire width of the thin film sheet. 
As described above, according to the present invention, the tension applied 
during heat treatment to the film is equal to that applied during the 
process for forming the thin film layer 12 on the base film 10. Further, 
the base film is properly shrunk with heat in accordance with heat 
shrinkage of the thin film. Therefore, if the proper heat treatment is 
performed according to the process of the present invention, the 
mechanical strength of the base film is not degraded and the wrinkles and 
curls can be eliminated. Therefore, a magnetic recording tape fabricated 
according to the process of the present invention properly comes into 
contact with the magnetic head. Further, dropouts and output level 
variations are minimized. 
Experimental results of a heat treatment process according to the present 
invention will be described below. 
A polyester film of 10 .mu.m was used as the base film 10. A thin film of 
Al-Co (aluminum-cobalt) was deposited on the base film 10 to a thickness 
of 0.3 to 0.5 .mu.m. The results obtained are as shown in Table 1. 
TABLE 1 
______________________________________ 
Heat Treat- 
ment Temp. 
Time 
(.degree.C.) 
(min.) State of Base Film 
______________________________________ 
80 15 No heat shrinkage (curled) 
84 2 Weak curl toward the thin film 12 side 
84 3 Flat (curl removed) 
84 5 Weak curl toward the base film 10 side 
(opposite curl) 
90 0.5 Weak curl toward the thin film 12 side 
90 1 Flat (curl removed) 
90 2 Weak curl toward the base film 10 side 
(opposite curl) 
______________________________________ 
From the experimental results as shown above, the following analysis may be 
made. Since thermal shrinkage does not occur at a temperature of 
80.degree. C. or lower, the curl cannot be removed from the base film at 
such nonreactive temperature (80.degree. C.). On the other hand, when the 
thin film sheet is treated with heat at a temperature of 84.degree. C. for 
3 minutes or 90.degree. C. for one minute, the curl can be removed. 
However, when the temperature of heat treatment is increased to 98.degree. 
C. or higher, the thin film sheet is spontaneously curled toward the base 
film 10 side and a number of wrinkles are formed thereon. Since the heat 
treatment operation is so delicate as mentioned above, the temperature 
range in heat treatment for removing wrinkles from the base film must be 
properly determined. In order to uniformly perform heat shrinkage of the 
base film with heat as well as to prevent excessive heat shrinkage due to 
an abnormally high temperature, heating and cooling must be performed 
slowly to prevent excessive heat shrinkage. Such slow cooling of the thin 
film sheet may be spontaneously performed in a closed thermostat whose 
power supply is cut-off. If the film is heated with substantially the same 
temperature gradient as applied to cooling, the film is not heated to an 
abnormally high temperature as described above. 
FIG. 7 is a graph showing temperature changes in heat treatment as a 
function of time. The film roll 50 wound with constant tension is placed 
in a conventional thermostat (not shown). The temperature in the constant 
temperature bath is increased with a predetermined gradient. This 
temperature gradient is given by b1/a1. The temperature of the thermostat 
is kept at a nonreactive temperature T10 immediately before heat shrinkage 
occurs. Where the temperature T10 is established and this temperature is 
temporarily maintained, then the temperature distribution in the film roll 
50 is made uniform. Further, differences between the temperature before 
heat shrinkage and the temperature after heat shinkage become smaller. 
Therefore, the film roll 50 is treated with heat under excellent 
conditions. When the film roll 50 is entirely maintained at substantially 
the constant temperature, the temperature of the thermostat is then 
increased to a temperature T20 at which heat shrinkage occurs gradually. 
With this heat treatment, thermal strain is eliminated, and the thermostat 
is then cooled to room temperature gradually. 
Assume that the cooling rate of the thermostat, that is, the temperature 
drop gradient is defined as b2/a2. The preferred temperature rise gradient 
b1/a1 is then determined as: 
EQU .vertline.b1/a1.vertline..apprxeq..vertline.b2/a2.vertline. 
The heating rate is selected to be substantially equal to the cooling rate. 
Thus, curls can be removed from the film roll 50 smoothly. Further, 
wrinkles may not be formed. 
The optimal conditions of heat treatment may vary in accordance with 
various factors. Therefore, the fixed optimal conditions cannot be 
determined for all cases. The temperature gradients b1/a1 and b2/a2 and 
temperatures T10 and T20 in heat treatment are changed in accordance with 
the type of base film 10, the kind of evaporant 42, thicknesses of the 
base film 10 and the thin film layer 12 formed thereon, as needed. 
Therefore, the actual optimal conditions may be determined by a 
cut-and-try operation in the process of fabricating the thin film tape. In 
particular, the base film 10 is shrunk with heat in accordance with the 
rate of heat shrinkage of the thin film in the process for forming the 
thin film. If wrinkles and curls are completely eliminated under certain 
conditions, these conditions are defined as optimal conditions. 
As described above, according to the present invention, the base film is 
uniformly shrunk with heat in accordance with heat shrinkage of the thin 
film, so that the mechanical strength of the base film is not degraded and 
the wrinkles and curls of the film are effectively removed. Therefore, the 
magnetic recording tape fabricated according to the process of the present 
invention travels smoothly and comes in contact with the magnetic head 
properly. Further, dropouts and output level variations are decreased. 
FIG. 8 shows a heat treatment apparatus according to one embodiment of the 
present invention. The film roll 50.sub.1 of the thin film sheet is set on 
the supply reel 18.sub.1. The thin film sheet rolled as the film roll 
50.sub.1 is supplied to the take-up reel 20.sub.1 through guide rollers 60 
and 62. The film sheet is driven at a constant speed and with constant 
tension. Heating plates 64 and 66 are disposed between the rollers 60 and 
62. Heater blocks 68 and 70 are mounted on the heating plates 64 and 66, 
respectively. The heating plates 64 and 66 oppose each other and are 
spaced apart from each other. The fim sheet 10 travels between the heating 
plates 64 and 66. The heater blocks 68 and 70 are not illustrated in 
detail. However, they include a heat sensor such as a thermistor. The 
heater blocks 68 and 70 are connected to a heater block temperature 
controller 72. The supply reel 18.sub.1 , the take-up reel 20.sub., the 
film roll 50.sub.1, the guide rollers 60 and 62, the heating plates 64 and 
66, and the heater blocks 68 and 70 are housed or enclosed in a thermostat 
80. The internal temperature of the thermostat 80 is controlled by a 
temperature controller 82 for the thermostat. 
FIG. 9 shows a schematic diagram of the structure of the heater block 
temperature controller 72 of FIG. 8. Assume here that the film 10 is 
shrunk with heat at a temperature of 100.degree. C., an input signal Es 
corresponding to the temperature of 100.degree. C. is supplied to a 
temperature control circuit 72.sub.1. The temperature control circuit 
72.sub.1 supplies the heaters 68 and 70 with power E0 corresponding to the 
level of the input signal Es. The temperature control circuit 72.sub.1 
includes a comparator and a power amplifier. The heaters 68 and 70 are 
heated in accordance with the power E0. Heat generated from the heaters 68 
and 70 is detected by a thermistor 69. The thermistor 69 feeds back to the 
temperature control circuit 72.sub.1 a temperature signal Ed corresponding 
to the temperature of heat generated from the heaters 68 and 70. The 
temperature control circuit 72.sub.1 compares the input signal Es and the 
temperature signal Ed and determines the level of power E0. Thus, the 
temperature of heat generated from the heaters 68 and 70 is kept at 
100.degree. C. 
Referring to FIG. 8, the supply reel 18.sub.1, the take-up reel 20.sub.1, a 
drive source such as a servo motor (not shown) coupled thereto, and the 
guide rollers 60 and 62 constitute film driving means for driving the film 
with constant tension. Further, the heating plates 64 and 66, the heater 
blocks 68 and 70, and the heater block temperature controller 72 
constitute heating means for heating the sheet to a temperature at which 
heat shrinkage of the film 10 occurs. The thermostat 80 and the 
temperature controller 82 for the thermostat constitute heating means for 
heating the film sheet 10 at a temperature immediately before thermal 
shrinkage of the film 10 occurs. The film sheet 10 is preheated to a 
nonreactive temperature before it is heated by the heating plates 64 and 
66. This nonreactive temperature is determined in advance in accordance 
with the type of the film 10 or other related factors. The preheated sheet 
10 passes through between the heating plates 64 and 66 at a constant speed 
and with constant tension. The film 10 is shrunk with heat properly while 
it passes between the heating plates 64 and 66. When heat shrinkage of the 
film 10 is performed properly, thermal strain such as wrinkles and curls 
is completely eliminated from the thin film sheet. 
Heat shrinkage as described above is performed in the thermostat whose 
internal temperature is slightly lower than the temperature of heat 
shrinkage. Therefore, the temperature of the heating plates 64 and 66 does 
not greatly differ from the ambient temperature in the thermostat. 
Accordingly, the temperature of the heater blocks 68 and 70 is controlled 
easily. A necessary time period from the time in which power is supplied 
to the heater blocks 68 and 70 to the time in which they reach a 
predetermined temperature is thus shortened. Further, the temperature of 
the sheet may not change greatly before and after it passes between the 
heating plates 64 and 66. Accordingly, heat shrinkage of the film 10 is 
performed smoothly. Further, since the film 10 is shrunk with heat under 
fixed tension, the thin film sheet may not be curled toward the base film, 
thus fabricating a flat thin film sheet. 
If the tension applied to the thin film sheet is too weak, the wrinkles and 
curls cannot completely be eliminated. On the other hand, if the tension 
applied to the thin film sheet is too strong, the thin film sheet is 
elongated. The tension applied to the thin film sheet during heat 
treatment is therefore selected to be substantially equal to the tension 
applied during manufacture of the thin film 12 on the base film 10. 
However, the tension applied to the thin film sheet during heat treatment 
may not be equal to the tension applied to the base film 10 during 
formation of the thin film 12. 
FIG. 10 shows a modification of the heat treatment apparatus of FIG. 8. In 
the heat treatment apparatus of FIG. 8, a pair of heaters which put the 
film therebetween to heat it is used. On the other hand, in the heat 
treatment apparatus as shown in FIG. 10, a heat roller 74 which smoothly 
rotates and is brought into contact with the sheet 10 is utilized. The 
sheet is, in turn, driven contacting with the circumference of the heat 
roller 74. The heat roller 74 includes a heater which is controlled in 
cooperation with a heat sensor as shown in FIG. 9. The surface temperature 
of the heat roller 74 is kept at a predetermined temperature at which 
shrinkage occurs. With the heat treatment apparatus having the above 
structure, the present inventor conducted an experiment under the 
following conditions: 
Base film 10: a polyester film of 6 .mu.m thickness 
Thin film 12: an Ni-Co alloy which is deposited on the film 10 to a 
thickness of about 0.3 .mu.m 
Heat roller 74: 200 mm in diameter 
Film driving speed: 14 m/min. 
Film tension: 2.5 kg/mm.sup.2 (same as in deposition of the Ni-Co thin 
film) 
Internal Temperature of the thermostat 80: 95.degree. C. 
Temperature of the heat roller 74: 100.degree. C. 
As a result, a thin film sheet without wrinkles and curls in its entire 
width was obtained. 
The present invention is not limited to the above particular embodiment. 
Various changes and modifications may be made within the scope of the 
appended claims. For example, referring to FIG. 3, the amount of heat of 
the heater 36 may be less than that of the heater 38. Thus, a temperature 
gradient may be formed along the direction of thickness of the base film 
or the thin film sheet 10. The thin film forming apparatus disclosed in 
Japanese Patent Application No. 55-176578 filed by the same applicant as 
in the present application may be utilized as needed. With the apparatus 
disclosed in the above patent application, side tension may be applied to 
the sheet which is treated with heat. The disclosure of this patent 
application is also combined herewith. Further, the thin film 12 is not 
restricted to a magnetic thin film. The present invention may be applied 
to fabrication of multilayered tape which has two or more thin films 12. 
Referring to FIG. 8, the temperature of the heating plate 64 may slightly 
differ from that of the heating plate 66. An infrared ray lamp may be used 
as a heater. Further, a plurality of heat rollers 74 as shown in FIG. 10 
may be used to heat the film roll at different temperatures.