The present invention relates to a plasma CVD apparatus which are used for preparation of, for example, magnetic recording media and other functional thin films, and more particularly to a continuous plasma CVD apparatus and a continuous plasma CVD method which are suitable for forming, at high speed, a broad and uniform CVD thin film of high quality and free from defects.
In the wide variety of the fields of thin film magnetic recording media and various functional media produced using insulation films such as of polyethylene, polypropylene, polyethylene terephthalate, polyethylene naphthalate, polyaramid and polyimide, it is attempted to further provide on these continuous substrates a plasma CVD film such as a protective film, lubricating film or moistureproof film.
When CVD film formation is carried out on such continuous flexible substrates, mass-production becomes possible and sharp reduction of cost can be expected, and for further improvement in productivity, technique of broad film formation at high speed is important.
The most important requirement for the increase of film forming speed is to produce molecular species such as active ions and radical molecules at a high density by supplying a large quantity of energy to plasma and to effectively introduce them into the substrate. Furthermore, in order to obtain films of high quality, a necessary kinetic energy must be given to the ions introduced into the substrate and for this purpose, a bias voltage is applied to the plasma exciting portion and/or substrate side to perform acceleration of ions. On the other hand, when a large quantity of energy is applied to plasma in this way, energy of the plasma naturally migrates to the substrate to heat the substrate. If the substrate is heated, the film forming speed considerably decreases, causing deterioration of film quality and distortion and breakage of the substrate.
In the case of the substrates having electrical conductivity such as those of thin film magnetic recording media and various functional films, furthermore, ionic current flows through the conductive film whereby the substrates are subjected to further heating due to Joule's heat. It is well known that especially when ion introduction amount is increased for increasing film forming speed and, moreover, ion acceleration voltage is increased for the improvement of film quality, the substrates are considerably damaged.
Furthermore, for increasing the productivity, broadening of the width of substrate is necessary together with increase in film forming speed. An important requirement which governs the width of film is uniformity of plasma density and ion acceleration bias voltage. If the plasma density and the bias voltage are ununiform, film thickness and film quality greatly vary in the width direction.
High-speed formation of broad plasma CVD films is very difficult as mentioned above, and especially when a substrate which has electrical conductivity in at least a part thereof and generates Joule's heat upon passing a current is used, the high-speed formation of broad plasma CVD films becomes more difficult and a breakthrough is required.
Many factors for occurrence of heating of substrate are considered, but main factors are heat generation caused by the striking energy of accelerated ions introduced into the substrate and heat generation caused by Joule's heat of ionic current. Among them, the heat generation caused by the striking energy cannot be avoided because the striking energy is necessary for obtaining a film of high quality. On the other hand, the Joule's heat due to ionic current is unnecessary and depends on the method of bias application for acceleration of ions. How to inhibit the heating by ionic current is the most important point for realization of a stable high-speed process.
Next, results of comparative investigation of conventional examples will be explained. FIG. 28 schematically illustrates the construction of a plasma CVD apparatus which employs ion acceleration method by DC bias as one of conventional examples.
In FIG. 28, the numeral 131 indicates a substrate comprising, for example, a flexible synthetic resin film, 132 indicates a unwind roller for continuously feeding the substrate 131, 133 indicates a DC source connected to the unwind roller 132, 134 indicates an intermediate roller guiding the substrate 131, 135 indicates a rotating drum, 136 indicates a plasma tube, 137 indicates a high-frequency coil wound around the plasma tube 136, 138 indicates a high-frequency electric source which applies a high frequency to the high-frequency coil 137, 139 indicates an anode provided in the plasma tube 136, 140 indicates a DC source connected to the anode 139, 141 indicates a gas inlet formed at the plasma tube 136, and 142 indicates a wind roller which takes up the substrate 131.
According to the method which comprises exciting plasma by the high-frequency coil 137 wound around the plasma tube 136 and applying a bias voltage to the substrate 131 having electrical conductivity by the DC source 133, an ionic current flows through the conductive part of the substrate 131 as shown by arrow 143.
Furthermore, as shown in FIG. 28, in the case of the method of applying a bias voltage from the anode 139 opposite to the side of the substrate 131 with the plasma intervening therebetween, a path for liberating ions from the substrate 131 must be provided in order to prevent the substrate 131 from charging with ion, and an ionic current similarly flows in the direction of arrow 143.
As mentioned above, except for the case of electric resistance of the substrate being very small or very large, when an ionic current flows through the substrate having electric conductivity, a large quantity of Joule's heat is generated by the ionic current to cause decrease in film forming speed and damage of the substrate.
In order to diminish the heating of substrate caused by the ionic current, there was proposed a means according to which one or a plurality of potential rollers are provided on a film of a cooled rotating drum to localize the ionic current onto only the cooled rotating drum and further to divide the current as shown in FIG. 29 (JP-B-7-105037).
In FIG. 29, 151 indicates a substrate comprising a synthetic resin film or the like, 152 indicates a unwind roller for continuously feeding the substrate 151, 153 indicates an intermediate roller for guiding the substrate 151, 154 indicates a rotating drum, 155 indicates a plurality of plasma tubes, 156 indicates a high-frequency coil wound around each of the plasma tubes 155, 157 indicates an anode provided in each of the plasma tube 155, 158 indicates a DC source connected to the anode 157, 159 indicates a gas inlet pipe connected to each of the plasma tubes 155, 160 indicates a wind roller which takes up the substrate 151, and 161 indicates a potential roller for applying a bias voltage.
According to the apparatus shown in FIG. 29, the total quantity of ionic current is the same as in the apparatus shown in FIG. 28 and a sharp reduction of heat which flows into the substrate 151 cannot be attained. Thus, the method of applying a bias voltage using a DC source (same as the method of applying a low-frequency bias which can be regarded to be a direct current as for plasma even though it is alternating current) cannot still solve the defect that the film forming speed is limited by ionic current in the method of film formation with a large ion introduction amount.
In order to increase cooling efficiency of substrate, there is proposed a means of enhancing the close contact between the substrate and the rotating drum utilizing the electrostatic adsorption by applying a high DC voltage between the substrate and the rotating drum.
In this DC biasing method, since the substrate 151 moves, the voltage of potential roller 161 must be adjusted by passing electricity through a slip ring or rotary joint, but when the contact face with the slip ring is soiled or stick slip occurs, the voltage varies and quality of the resulting film is apt to become ununiform.
Furthermore, when the bias applying anode 157 is at high potential, plasma potential increases and abnormal discharging is apt to occur. Moreover, it is difficult to uniformly perform the ion acceleration over a wide area. In addition, owing to charging of the substrate, dusts attach to the substrate and the substrate is apt to be soiled.
As explained above, the method of applying bias voltage using a DC source is high in technical difficulty from the viewpoints of broad and high-speed film formation.
Generation of Joule's heat caused by ionic current can almost be avoided by employing a high-frequency self-biasing method which effects ion acceleration by so-called self-bias voltage by applying a high frequency of 13.56 MHz to the substrate. This is because ionic current flows in the direction of thickness of the substrate and, furthermore, flows into the side of the rotating drum as a displacement current through electrostatic capacity of the insulating film. Moreover, this method has an advantage of easiness in formation of broad film since a uniform bias voltage can be generated all over the substrate.
FIG. 30 is a schematic view of a plasma CVD apparatus in which a high-frequency power is applied to cooling drum, and then excite plasma and simultaneously self-bias voltage is generated (JP-A-8-41645 and JP-A-8-49076). In FIG. 30, 171 indicates a substrate comprising, for example, a flexible synthetic resin film, 172 indicates a unwind roller for continuously feeding the substrate 171, 173 indicates an intermediate roller guiding the substrate 171, 174 indicates a rotating drum, 175 indicates a high-frequency electric source which applies a high frequency to the rotating drum 174, 176 indicates a gas inlet and 177 indicates a wind roller which takes up the substrate 171 formed.
According to this method, substantially no Joule's heat due to ionic current is generated, but a current generated by high-frequency power flows through the electrically conductive film of the substrate and the substrate-carrying system or wall of a vacuum tank to the earth side to generate a large quantity of Joule's heat. In order to prevent this high-frequency current, thorough high-frequency insulation must be performed, but this is very difficult because of the high frequency. If even a slight electrostatic capacity is present between thin film and earth potential, the connection is completed through this electrostatic capacity and, as a result, a high-frequency current flows through the thin film as shown by arrow 78 to generate a large quantity of Joule's heat. Moreover, breakage of the conductive film of the substrate occurs due to the over current generated in this case.
As a result of further detailed investigation, it has been found that even if the high-frequency insulation is lost, when bias voltage is increased, over current flows or abnormal discharging occurs in the vicinity of the portion where the substrate begins to contact with the rotating drum and the portion where the substrate begins to leave the drum to cause breakage of the conductive film or substrate. It is considered that this is because electrostatic capacity is large in the vicinity of the portions. Moreover, dielectric heating of the insulating base film caused by high-frequency bias cannot be ignored.
Furthermore, a method has been proposed which applies a DC bias to the plasma exciting side and further applies a pulse-like high-frequency bias of several hundred kHz to the substrate side (JP-A-8-69622). This method also applies a DC bias voltage, and therefore the ions flowing into the substrate must be liberated by earthing. If the ions are not liberated, the substrate is charged and immediately it becomes impossible to apply the bias voltage. It has been admitted that the bias application by the earth type pulse high frequency to the substrate side has an effect to enhance the plasma density, but substantially no contribution to the bias voltage has been recognized. Therefore, it is considered that the pulse high frequency performs only a secondary part as bias.