JP2000239842A - Vacuum deposition device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vacuum deposition device capable of continuously and uniformly forming a mixed film composed of different elements and having prescribed compositional ratios and objective thickness on the surface of a film in the process of running. SOLUTION: As to this device, a mixed film composed of different elements is formed on a film 17 running in a vacuum tank 6, and a crucible 9 capable of holding a vacuum depositing material 16 of different kinds, an electron gun 4 heating the vapor depositing material 16 and forming a mixed film on the film 17, a thickness measuring means 7 outputting the thickness per component of the mixed film from fluorescent X-ray intensity obtd. by irradiating the mixed film formed on the film 17 by the electron gun 4 with X-rays and a scanning means 21 scanning the thickness measuring means 7 in the direction approximately orthogonal to the film running direction are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum deposition apparatus suitable for producing various film products, and more particularly to a vacuum deposition apparatus for forming a mixed film of different elements on a film running in a vacuum chamber.

[0002]

2. Description of the Related Art As a method of depositing and forming a thin film on a polymer film running in a vacuum chamber, for example, Japanese Patent Application Laid-Open No.
There is a method described in 6273. In this method, a horizontally elongated evaporation source facing and arranged in a direction perpendicular to the moving polymer film and across the width direction of the polymer film is irradiated with an electron beam from a non-scanning electron gun and heated. A thin film is formed on a molecular film. When irradiating an electron beam from an electron beam, the magnetic field is controlled so that the electron beam has the same incident angle at each position of the evaporation source.

However, in this method, since there is no means for monitoring the thickness of the deposited film, for example, when the deposition rate changes due to a change in the degree of vacuum in the vacuum chamber or a change in the surface shape of the deposition material, There is a problem that the thickness of the deposited film varies and is not constant. Further, this method cannot form a mixed film of these vapor-deposited materials on a polymer film by simultaneously vapor-depositing a plurality of materials. As a vacuum evaporation apparatus invented to solve such a problem, for example, a detector that detects a part of the evaporation amount when an evaporation source in a vacuum chamber is heated by an electron gun, and a detection value of the detector And means for controlling the output of the evaporation source based on the above. This type of detector is provided with a crystal oscillator, and utilizes the principle that, when a deposited film adheres to the crystal oscillator, the oscillation frequency varies depending on the film thickness. This vacuum evaporation apparatus indirectly controls the thickness of a thin film formed on a polymer film using a part of the evaporation amount from an evaporation source as a control index.

However, when the above-mentioned detector has a plurality of different types of deposition sources, the detected signal cannot be decomposed into information on each component, and as a result, the control of the chemical composition ratio and the thickness is not performed. There is a problem that the accuracy is significantly reduced. Also, for example, when the direction of the evaporating beam at the time of vapor deposition changes due to indirect control, the measured value of the detector may not match the actually measured value. Furthermore, the above-mentioned detector requires measures such as switching the detector during the measurement when performing long-time continuous measurement due to the limitation of the total deposition amount on the detector, and there is also a problem in the reliability of the measurement. Was.

[0005] An apparatus that solves such a problem is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 1-208465. This apparatus includes an electron gun for exciting an characteristic X-ray by injecting an electron beam at an acute angle into a vapor-deposited film on a substrate after the vapor deposition, a detector for measuring the intensity of the characteristic X-ray, and this detector. Means for controlling the output of each deposition source based on the detected value of
This apparatus can directly measure a vapor-deposited thin film, so that film forming properties are improved as compared with the above-described apparatus.

[0006] However, the characteristic X-ray is RHEED
Since (high-energy electron beam) is excited by being incident on the vapor-deposited film at an acute angle, only information on the very surface layer of the vapor-deposited thin film can be obtained. That is, since information on the entire vapor-deposited thin film cannot be obtained, sufficient information cannot be obtained as an apparatus for producing a vapor-deposited film having a constant composition ratio and total thickness of the mixed film. Further, since the excitation source of the characteristic X-ray is a high energy electron beam, there is also a problem that the irradiated film surface at the irradiated portion is damaged.

[0007]

As described above, in the prior art, a mixed film composed of a plurality of components is uniformly dispersed and formed in the width direction and the running direction of the running film, and has a constant composition ratio and thickness. As a result, it has been difficult to form the film continuously and stably for a long time.

Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and to form a mixed film comprising different elements on a running film surface and having a predetermined composition ratio and a target thickness continuously and in a desired manner. It is an object of the present invention to provide a vacuum deposition apparatus that can be formed uniformly. In the present invention, "film"
The term “collectively” refers to a material having a thin shape with respect to the width and the length, and is used as a concept including not only an original film but also a sheet material.

[0009]

The above object is achieved by the invention described in the claims. That is, the characteristic configuration of the vacuum deposition apparatus according to the present invention is a vacuum deposition apparatus that forms a mixed film made of different elements on a film traveling in a vacuum chamber, and a holding unit that can hold different types of deposition materials, Heating means for heating a deposition material to form a mixed film on the film; and heating the mixed film formed on the film by irradiating the mixed film with X-rays. And a scanning means for scanning the thickness measuring means in a direction substantially orthogonal to the film running method.

[0010] According to this configuration, the vapor deposition components evaporated from the different types of vapor deposition materials held by the holding means directly and in real time from the mixed film formed on the running film and over the entire width of each component. Since the characteristic X-ray intensity can be measured, the measured value can be converted and output as thickness data of each mixed film forming component. Then, processing such as obtaining these deviation values becomes possible. Further, the thickness data can be reliably obtained over the entire width of the film without the necessity of installing a large number of thickness measuring means by the operation of the scanning means, and the manufacturing cost of the entire apparatus can be reduced. Further, since the X-rays are irradiated to detect the thickness data of the deposited mixed film forming component, unlike the irradiation with the high energy electron beam, the mixed film is not damaged. As a result, a vacuum deposition apparatus that can stably form a mixed film having a knitting ratio of a mixed film of different elements and a target thickness on a film surface during running continuously and uniformly in a film width direction and a running direction for a long time. Could be provided. In the present invention, the “direction substantially orthogonal” is used as a concept including not only a strict orthogonal direction but also a slight inclination direction.

It is preferable that the thickness measuring means is arranged at a plurality of locations extending in a direction substantially perpendicular to the running direction of the film, and is scanned by the scanning means in a direction substantially perpendicular to the running direction of the film. With such a configuration, the scanning distance when measuring the thickness while continuously scanning the thickness measuring means in the direction substantially perpendicular to the film running direction can be shortened, and as a result, the thickness measurement over the entire width of the film can be performed. It is convenient that the measurement can be performed in a shorter time and with higher accuracy.

It is preferable that the apparatus further comprises a control means for automatically controlling the heating means based on the thickness data output from the thickness measuring means. With this configuration, it is possible to perform a process of comparing the target value of each component set in advance by the control unit and obtaining a deviation value between them, and perform feedback control of the heating unit based on the process. As a result, thickness control becomes more reliable, and a mixed film having a predetermined chemical composition and a target thickness can be formed into a film with high accuracy, which is convenient.

[0013]

Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a schematic structure of a vacuum evaporation apparatus according to the present embodiment. In this vacuum evaporation apparatus, a polymer film such as polyethylene terephthalate (PET) was used as an example of a film-shaped material to be evaporated. The polymer film 17 set on the unwinding roll 1 of the vacuum chamber 6 travels on the cooling roll 3, passes through the measuring roll 5, and is wound by the winding roll 2. The degree of vacuum in the vacuum chamber 6 is controlled by an exhaust system 10 such as an oil diffusion pump (not shown).
Maintains a predetermined degree of vacuum.

At the bottom of the vacuum chamber 6, a crucible 9 as an example of a holding means for holding the vapor deposition material 16 is arranged. It moves at a low speed while maintaining an equilibrium relationship with the surface to be deposited. That is, the crucible 9 is moved closer to or away from the electron gun 4 in FIG. 1 so that the vapor deposition conditions are kept constant with respect to the moving polymer film. Are arranged so that the irradiation conditions (such as the distance between the electron gun and the electronic material) of the electron beam for irradiating are as constant as possible. Electronic connection 4
Is the electron beam 1 with respect to the vapor deposition material 16 stored in the crucible 9.
Irradiate 8. A part of the evaporation material heated and evaporated by the electron beam 18 is evaporated on the surface of the polymer film 17 running on the cooling roll 3. In the drawing, reference numeral 8 denotes a shielding plate for forming a uniform and good vapor-deposited film on the polymer film 17.

Next, a description will be given of a thickness monitor 7 arranged above the vacuum chamber 6 for measuring the thickness of a thin film deposited on the surface of the polymer film 17. The thickness monitor 7 has a structure capable of moving on the slide rail 21 in a direction orthogonal to the running direction of the polymer film 17. In the thickness monitor 7, an X-ray generator 7a for exciting characteristic (fluorescent) X-rays from the film components is disposed substantially perpendicular to the measurement roll 5. A portion of the characteristic X-rays excited by the X-rays emitted from the X-ray generator 7a to the polymer film 17 that has been deposited and run on the measurement roll 5 in a direction substantially perpendicular to the surface of the measurement roll 5 The light is guided to the spectral crystal plate 7b arranged at an angle of 30 °, and after the wavelength is selected from the spectral crystal plate 7b, the light is guided to the proportional counter 7c. The characteristic X-ray intensity continuously measured at regular intervals by the proportional counter 7c is converted into an analog electric signal at regular intervals. This analog electric signal is amplified by the amplifier 11 and then converted into a digital signal by the analog / digital converter 12. The characteristic X-ray intensity converted into the digital signal is converted into the thickness of each element by the thickness calculator 13 and then input to the control amount calculator 14. Here, the thickness monitor 7, the amplifier 11, the analog / digital converter 12, and the thickness calculator 13 constitute online thickness measuring means.

The control amount calculator 14 compares the preset reference thickness data of each element set in advance with each input measured thickness data to obtain a deviation value. Based on the obtained deviation value information, control data for automatically controlling the electron gun is automatically generated. This control data is sent to the electron gun controller 15. The electron gun control device 15 controls the power supplied to the electron gun 4 and the scanning time of the electron beam according to the input control data. Here, the control amount calculator 14,
The electron gun control unit 15 constitutes control means.

[0017]

(Embodiment 1) An example of actual operation will be described below. As the polymer film 17 to be deposited, a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., E5100; trade name) was used. Other usable polymer films include polypropylene, polyethylene, nylon 6, nylon 66, nylon 12,
Nylon 4, polyvinyl chloride, polyvinylidene chloride and the like can be mentioned, but the polymer film to be applied is not particularly limited to materials.

Aluminum oxide (Al ₂ O) in the form of particles having a size of about 3 to 5 mm is used as an evaporation material (evaporation source).
₃ , purity 99.5%) and silicon oxide (SiO ₂ , purity 9)
9.9%). One crucible 9 holding these materials is made of copper, and has a structure in which a cooling water cooling tube 20 having an outer diameter of 20 mmΦ is provided at the bottom. Cooling water flow rate is about 4m ³ /
Minutes. In order to dispose the vapor deposition material 16 in one row alternately in the crucible 9 so as to face the film width direction, 2 mm
Carbon partition plates 19 having a thickness were arranged at intervals of 100 mm in the width direction, so that a total of eight blocks of material could be stored. This partition plate 19 is used to hold an electron beam 1 of an electron gun 4 described later.
8 are arranged to be inclined at an angle substantially equal to the incident angle on each deposition material. The reason that the partition plate 19 is tilted is that
This is to prevent the electron beam 18 from being obstructed by the partition plate 19 when obliquely incident. In each block secured by the partition plate 19, the two types of vapor deposition materials were alternately and uniformly stored. 2 and 3 show a schematic structure of the crucible 9 used in this embodiment.

As the electron gun 4, a 250 kW electron gun was arranged so as to face a crucible 9 arranged in parallel to the film width direction. The electron gun 4 was designed to vapor-deposit a total of 8 blocks of vapor deposition material, with 4 blocks of SiO _{2 and} 4 blocks of Al ₂ O ₃ alternately arranged in the crucible 9. Although one electron gun was used in this embodiment, a plurality of electron guns are used when the total amount of energy to be charged into the crucible 9 cannot be secured by one unit or when a wide polymer film is deposited. Then, a method of dividing the deposition region may be adopted,
The number of installed electron guns is not particularly limited.

The evacuation system was designed such that the pressure in the vacuum chamber during the vapor deposition could always be kept at 4 × 10 ⁻² Pa or less. In particular,
The structure was such that a 50,000 L / sec oil diffusion pump was directly connected to the bottom of the vacuum chamber. The thickness of the mixed film layer after the deposition is
The thickness was continuously measured by a thickness monitor 7 which was arranged almost directly above the measurement roll and at the center of the polymer film 17 in the width direction.

Next, the thickness monitor 7 will be described in detail. First, using a rhodium X-ray tube as the X-ray generator 7a, a current of 40 kV and 50 mA was passed, and the film 17 running on the measurement roll 5 was irradiated with primary X-rays almost vertically. In this case, the X-ray radiated to the deposited film on the film 17 is a collimated light beam of 30 mmΦ. Some of the characteristic (fluorescent) X-rays excited by the X-rays are located at substantially the same distance from the measurement position of the mixed film, and are arranged at an incident angle of about 30 ° with respect to the measurement roll surface 5. To the thallium hydrogen phthalate spectral crystal plate 7b. The two dispersive crystal plates 7b disperse the characteristic X-ray wavelengths of the elemental components of Si and Al. The characteristic X led to these spectral crystal plates 7b
The lines are split at 8.34 °, the wavelength of Al, and 7.13 °, the wavelength of Si. These selected wavelengths are guided to independent proportional counters 7c, and the characteristic X-ray intensity is measured at regular intervals.

As a detector for measuring the X-ray intensity, a semiconductor detector in which lithium is implanted and diffused in a silicon single crystal can be used in addition to a proportional counter. Not limited. Further, in this embodiment, the wavelength dispersion method using the spectral crystal plate is used. However, in the case where materials of two elements whose energy distributions are clearly different from each other are used,
A non-wavelength dispersion method without using a spectral crystal plate may be used.
However, in order to measure each thickness with high accuracy from a mixed film of two elements having similar energy intensity distributions such as Al and Si, the wavelength dispersion method described in this embodiment is preferable. In this embodiment, the spectral condensed crystal plate 7b and the proportional counter 7
Although c is set to two each time, when the composition of the mixed film further increases, the spectral crystal plate 7b and the proportional counter 7c corresponding to the composition may be added.

The characteristic X measured by the proportional counter 7c
The line intensity was amplified to a level that can be processed by the amplifier 11 from 0 to 5 V, and then converted to a digital signal by the analog / digital converter 12. The resolution is 12 bits. The converted digital signal was converted into thickness data for each element by the thickness calculator 13. As a conversion method, a method of preparing a calibration curve of vapor deposition film thickness and X-ray intensity for a plurality of vapor deposition samples having known thicknesses in advance and converting the data into thickness data based on the calibration curve was adopted.

The thickness monitoring device 7 is disposed substantially directly above the measuring roll with an interval of 5 mm.
6 by a slide rail 21 and a linear motion mechanism (not shown) that changes the rotation of a motor (not shown) into linear motion.
As shown in (a), the width direction of the measurement roll (film 1
In a direction perpendicular to the running direction A of No. 7), the thickness measurement width 1m was intermittently moved (moving distance L = 1m) at intervals of 30mm while maintaining the above height relationship. Here, the slide rail 21, the linear motion mechanism, and the like constitute a scanning unit.

The thickness is measured at each position by the thickness monitor 7.
Was allowed to stand for 20 seconds, and the fluorescence X-ray intensity during this time was continuously measured, and was converted into thickness data based on the average data obtained.
Incidentally, the interval of the movement may be appropriately determined based on the size of the spot area for measuring the thickness and the required quality of the deposited thin film,
There is no particular limitation. The thickness may be measured while the thickness monitor 7 is continuously moved. In this case, the traveling speed is reduced to a speed at which the measurement accuracy can be ensured, or the measurement data and the measurement position are stored, and each time the movement is repeated, the data at the same position coordinates are added and averaged to reduce the thickness. What is necessary is just to measure.

In this embodiment, the thickness monitor is provided inside the vacuum chamber in order to secure the airtightness of the vacuum chamber. However, when the airtightness can be ensured, the thickness monitor is provided outside the vacuum chamber. The scanning structure of the thickness monitor device is not particularly limited.

The thickness data calculated and output by the thickness calculator 13 is sent to the control amount calculator 14. here,
Based on the thickness data, the input power of the electron gun and the stay time of the electron beam are calculated. These control data are used to determine the evaporation rate (corresponding to the evaporation thickness) in each crucible determined in the experiment.
And the input energy.

FIG. 4 shows the relationship between the input energy in the crucible 9 and the film deposition thickness. In the figure, A
1, A2, A3, and A4 indicate the positions of Al ₂ O _{3 in} the crucible, and S1, S2, S3, and S4 indicate the positions of SiO _{2 in} the crucible. , Are sequentially arranged so as to face each other in the width direction of the film. As can be seen from FIG. 4, the target thickness and composition ratio can be controlled by adding or subtracting the energy amount corresponding to the deviation value between the current thickness and the target value from the current value.
However, since the energy supplied to these materials is the source of the electron beam 18 emitted from the same electron gun 4, the energy of each material is actually changed by changing the residence time of the electron beam for each crucible. The input energy can be distributed. These relational expressions are shown below.

[0029] _{ta n = [Pa n / (} ΣPa + ΣPs)] × t0 here ta _n: electron beam scanning time at an aluminum oxide-block n Pa _n: energy put into aluminum oxide block n - amount ShigumaPa: four Total energy input to aluminum oxide of block ΣPs; Total energy input to silicon oxide of 4 blocks in total t0: Time constant depending on hardware (ms: millisecond) Distribution of gas evaporating from each evaporation block is as follows: As shown by 31 (evaporation component from the silicon oxide block) and 32 (evaporation component from the aluminum oxide block) in FIG. 5 showing the evaporation characteristics of each vapor deposition material in the crucible, the strength is highest immediately above, It shows a distribution in which the strength decreases as it spreads laterally. The distribution intensity and shape mainly depend on the intensity of the electron beam, the angle at which the electron beam is incident, the distance between the electron gun and the crucible, and the evaporation area. Therefore, in order to form a film having the same composition ratio in the width direction and the running direction of the film forming the thin film and having a uniform target total thickness, the arrangement of the vapor deposition material is most important. In addition, 30 in FIG. 5 represents the distribution of the thickness in the width direction deposited and formed on the film, and 33 in FIG.
Represents the composition ratio at that time.

The arrangement of materials in this embodiment is shown in FIGS.
And the distance to the surface of the crucible closest to the electron gun.
The separation was 1,000 mm. In the figures, A and S represent Al, respectively.
_Two O _Three , SiO_Two Is stored.

The deposition material is a thin partition plate 19 shown in FIG.
The material was divided and arranged. The reason why the material is divided by the thin partition plate 19 arranged in one crucible 9 without independently arranging a plurality of crucibles in the film width direction is to minimize the undeposited area by the thin partition plate 19. This is because the effect of suppressing the thickness variation in the crossing region of the deposition material is very large.

The polymer film 17 was deposited under the conditions described above. The running speed of the film is 300m / min,
A total of 40,000 m was deposited. The crucible 9 was moved toward the electron gun 4 at a speed of 2 mm / min (the driving device is not shown). And to check the effect of automatic control,
A comparison was made between the case where the automatic control was performed and the case where the control means was cut off with the thickness monitor only operating. The control cycle of the automatic control was set to every 20 seconds, which is the update cycle of the thickness data, and the control was performed using the latest value of the thickness measurement data of the entire film width 1 m. Table 1 shows the results. When the automatic control is not performed, the total thickness variation and the composition ratio change are large, whereas when the automatic control is performed, an extremely stable film with a small total thickness change and the composition ratio change is formed. I understand.

(Embodiment 2) As shown in FIG. 6 (b), two thickness monitor devices having the same mechanism are arranged at an interval of 500 mm, and these two thickness monitor devices 7 are intermittently interposed at an interval of 30 mm. Reciprocated. Moving distance L is approximately 500m
m. The vapor deposition was performed under the same conditions as in Example 1 except for the above conditions. Table 1 shows the results. When the automatic control is not performed, the total thickness variation and the composition ratio variation are large, whereas when the automatic control is performed, the variation in the total thickness variation and the composition ratio is very small as in the case of the first embodiment. It can be seen that a stable film is formed. Further, as compared with Example 1, it can be seen that the thickness information in the flow direction is doubled, and the thickness accuracy is further improved.

[0034]

[Table 1] [Other Embodiments] (1) In the above-described embodiment, an example in which a so-called one-chamber system is used as a vacuum chamber has been described. However, a chamber in which a material to be deposited such as a film travels is different from a chamber in which a deposition material is heated. The present invention is also applicable to a so-called two-chamber type apparatus in which vacuum deposition is performed under reduced pressure.

(2) In the above embodiment, the example in which the unwinding roll and the take-up roll of the material to be vapor-deposited are arranged in the vacuum chamber has been described. However, the present invention can also be applied to a continuous apparatus in which vapor deposition is performed in a high vacuum chamber.

(3) In the above embodiment, the polymer film is taken as an example of the film-like material to be deposited, but the material to be deposited may be paper, cloth, or the like. In addition, other than the above-described aluminum oxide and silicon oxide, a gentle element or compound can be used as a vapor deposition material, and a mixed film composed of two or more elements or components using two or more vapor deposition materials can be used. May be formed.

(4) In the above embodiment, the heating means is an electron gun. However, the present invention can be applied to a vapor deposition apparatus in which a crucible is heated by an induction heating coil.

(5) In the above embodiment, an example is shown in which the slide rail is used as the scanning means and the thickness monitor moves on the slide rail. However, the slide rail is arranged at the side or upper position of the thickness monitor. The thickness monitor may be moved in accordance with the slide rail, or the thickness monitor may be suspended and suspended.

[0039]

As described above, according to the present invention, a mixed film having a composition ratio of a mixed film of different elements and a target thickness is continuously formed on a running film surface for a long time in a film width direction and a running direction. Thus, it was possible to provide a vacuum vapor deposition apparatus capable of forming a film uniformly and stably.

[Brief description of the drawings]

FIG. 1 is a schematic overall configuration diagram of a vacuum evaporation apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a crucible used in a vacuum evaporation apparatus according to an embodiment of the present invention and an arrangement thereof.

FIG. 3 is a diagram illustrating the structure of the crucible of FIG.

FIG. 4 is a graph for explaining a relationship between a crucible input energy amount and a film deposition rate.

FIG. 5 is a graph illustrating the vapor deposition characteristics of each vapor deposition material block.

FIG. 6 is a diagram illustrating an arrangement and a scanning state of the thickness monitoring device.

[Explanation of symbols]

 Reference Signs List 4 heating means 9 holding means (crucible) 14, 15 control means 16 vapor deposition material 17 film 18 electron beam 19 threshold plate 21 scanning means

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsukasa Oshima 2-1-1 Katata, Otsu City, Shiga Prefecture Inside Toyobo Co., Ltd. Research Institute (72) Inventor Kiyoji Iseki 2-1-1 Katata, Otsu City, Shiga Prefecture F-term in Toyobo Co., Ltd. Research Laboratory (reference) 4K029 AA11 AA25 CA01 DB21 EA00 EA01 EA09

Claims (3)

[Claims] 1. A vacuum evaporation apparatus for forming a mixed film made of different elements on a film running in a vacuum chamber, comprising: holding means capable of holding different types of evaporation materials; Heating means for forming a mixed film on the substrate, and a thickness for outputting the thickness of each component of the mixed film from the fluorescent X-ray intensity obtained by irradiating the mixed film formed on the film with X-rays by the heating means. A vacuum evaporation apparatus comprising: a measuring unit; and a scanning unit that scans the thickness measuring unit in a direction substantially orthogonal to the film running method. 2. The apparatus according to claim 1, wherein said thickness measuring means is arranged at a plurality of locations extending in a direction substantially perpendicular to the running direction of said film, and is scanned by said scanning means in a direction substantially perpendicular to the running direction of said film. 1. Vacuum evaporation apparatus. 3. The vacuum deposition apparatus according to claim 1, further comprising control means for automatically controlling said heating means based on the thickness data output from said thickness measuring means.