referring to fig2 there is shown a plasma cvd apparatus for depositing carbon material on a surface in accordance with the teaching of the present invention . the surface to be coated may , for example , be made of semiconductor , glass , metal , ceramics , organic resins , magnetic substance , and so forth . the apparatus comprises a reaction chamber 18 defining a reaction space 30 therein , first and second electrodes 21 and 22 , a high frequency electric power source 23 for supplying electric power between the electrodes 21 and 22 through a matching transformer 24 , a dc bias source 15 connected in series between the electrodes 21 and 22 , a gas feeding system 11 consisting of four passages 12 to 15 each of which is provided with a flow meter 17 and a valve 16 , a microwave energy supply 11 , a nozzle 19 through which gas excited by the microwave energy supply 20 is introduced into the reaction space 30 , and an exhaust system 26 including a pressure control valve 27 , a turbomolecular pump 28 and a rotary pump 29 . the electrodes are designed such that ( the area of the first electrode 21 )/( the area of the second electrode 22 )& lt ; 1 . a pair of switching means 31 and 32 is provided for inverting the polarities of the electrodes 21 and 22 . in a first position of the switching means , the electrode is grounded while the other electrode 21 is supplied with high frequency electric energy from the power source 23 . in the other second position , the electrode 21 is grounded while the electrode 22 is supplied with high frequency electric energy from the power source 23 . an object having the surface to be coated is mounted on the electrode 21 . in operation of this apparatus , a carrier gas of hydrogen is introduced to the reaction space 30 from the gas feeding passage 12 as well as a reactive gas of a hydrocarbon such as methane or ethylene from the gas feeding passage 13 . the gas introduction rates of hydrogen and the hydrocarbon are 3 : 1 to 1 : 3 , preferably 1 : 1 . in addition to this , a v - group dopant gas such as nh 3 or ph 3 , or a iii - group dopant gas may be inputted to the reaction space 30 through the gas feeding passage 14 or 15 in order to form impurity semiconductors . pre - excitation may be effected by the microwave energy supply 10 . the pressure in the reaction space is maintained within the range between 0 . 001 to 10 torr , preferably 0 . 01 to 0 . 5 torr . high frequency electric energy at a frequency not lower than 1 ghz , e . g . 2 . 45 ghz , is applied to the reactive gas at 0 . 1 to 5 kilo watt for breaking c -- h bonds . when the frequency is selected to be 0 . 1 to 50 mhz , c ═ c bonds can be broken and transformed to -- c -- c -- bonds . by virtue of this reaction , carbon atoms are deposited atoms in the form of a structure in which the diamond structure occurs at least locally . a bias voltage of , for example , - 200 to 600 v is set at the dc bias source 15 . the effective bias voltage level is substantially - 400 to + 400 v when a self bias level in this case of - 200 v is spontaneously applied between the electrodes 21 and 22 with the bias voltage level of the source 15 being zero . generally , the high frequency input power is chosen between 10 watt and 5 kilo watt , preferably between 50 watt and 1 kilo watt . this input power corresponds to 0 . 03 to 3 watt / cm 2 in terms of plasma energy . the substrate temperature is maintained in a range of + 250 ° to - 100 ° c . by means of a temperature control means ( not shown ). when diamond deposition is desired , the substrate temperature has to be elevated further . fig3 shows the resistivity and the vickers hardness of films deposited on a surface , to which high frequency electric energy was applied through the electrode 21 at various power levels . as can be seen from the figure , a harder film was deposited by inputting higher power energy . fig4 shows the resistivity and the vickers hardness of films deposited on a surface which was grounded . comparing fig4 with fig3 it will be apparent that the resistivity of carbon films formed at the anode side ( on the grounded electrode ) becomes lower than that in the cathode side ( supplied with high frequency energy ). in accordance with the teaching of the present invention , a surface is coated with a carbon coating while the deposition condition is changed in order that the hardness of the carbon initially or intermediately deposited on the substrate is relatively low , but the deposition condition is changed such that hardness of the carbon finally deposited becomes very high in order to provide a hard external abrasion - proof surface . this procedure can be carried out in two ways . as seen from fig6 ( a ), the hardness may be changed in steps by stepwise change of the deposition condition in accordance with the above description . alternatively , as seen from fig6 ( b ), the hardness may be changed continuously from the inner surface to the external surface of the carbon coating . the hardness or resistivity of the carbon coating can be changed , rather than monotonically , in order that an intermediate region of the coating is conductive and sandwiched by hard carbon regions . fig5 illustrates such a case including three carbon film regions . the lower and top films 41 and 43 are deposited to have a high degree of hardness while the intermediate film 42 is deposited to have low resistivity . this example can be realized in two ways as illustrated in fig7 ( stepwise change ) and fig8 ( continuous change ). the lower hard film 41 is semi - insulating so that it protects the surface to be coated electrically and mechanically . further the lower hard film has a function as a blocking layer to prevent impurity from entering into the intermediate film 42 and also a function of improving adhesivity to the substrate and the electrical property . the intermediate region 42 has conductivity and functions as a buffer layer to alleviate distortion generated by mechanical stress . a carbon coating was deposited on a transparent polyimide film 35 as shown in fig9 . an amorphous silicon photosensitive semiconductor device 34 was formed on a glass substrate 33 in a conventional manner as well as the polyimide film 35 . a first carbon film 36 of 0 . 6 micron meter thickness was formed on the polyimide film 35 under deposition conditions that the structure was placed on the electrode ( cathode ) supplied with high frequency energy of 260 w , the introduction rate of carbide gas such as methane , ethylene , or ethane diluted by hydrogen ( e . g . methane : hydrogen = 1 : 1 ) was 100 sccm , the pressure of the reactive gas was 0 . 03 torr , and the deposition time was 60 minutes . the hardness and the resistivity were measured to be 1000 kg / mm 2 and 1 × 10 12 ohm centimeter . a second carbon film 37 of 0 . 5 micron meter thickness was formed on the first film 36 under deposition conditions that the electrode supporting the structure was grounded ( as an anode ), the input high frequency energy was 300 w , the introduction rate of carbide gas such as methane , ethylene , or ethane diluted by hydrogen ( e . g . methane : hydrogen = 1 : 1 ) was 100 sccm , the pressure of the reactive gas was 0 . 03 torr , and the deposition time was 40 minutes . the hardness and the resistivity were measured to be 600 kg / mm 2 and 1 × 10 10 ohm centimeter . finally , a third carbon film 38 was deposited in the same deposition conditions as the first film 36 . the first film may be dispensed with . this was carried out in accordance with the diagram shown in fig8 ( a ) and 8 ( b ) rather than fig7 ( a ) and 7 ( b ). that is , the resistivity and the hardness were continuously decreased and increased along with the decrease and the increase of input energy . carbon deposition was started under the deposition conditions that the structure was placed on the electrode ( cathode ) supplied with high frequency energy of 300 w , the introduction rate of carbide gas such as methane , ethylene , or ethane diluted by hydrogen ( e . g . methane : hydrogen = 1 : 1 ) was 100 sccm , and the pressure of the reactive gas was 0 . 03 torr . the input high frequency energy was gradually decreased from 300 w to 200 w at 0 . 5 to 2 . 5 w / min . the hardness and the resistivity were decreased , along with the decrease of the input energy , from 1000 kg / mm 2 to 500 kg / mm 2 and from 1 × 10 12 ohm centimeter to not lower than 1 × 10 8 ohm centimeter respectively . the total thickness of this carbon coating was 0 . 2 micron meter . after the positions of the switch 31 and 32 were reversed ( i . e . the electrode 21 was grounded as an anode ), carbon deposition was resumed while the input power was decreased from 300 w to 200 w and subsequently increased from 200 w to 300 w at 0 . 5 to 2 . 5 w / min . the hardness and the resistivity were changed along with the change of the input energy , that is , the hardness was decreased from 500 kg / mm 2 to 300 kg / mm 2 and subsequently increased from 300 kg / mm 2 to 500 kg / mm 2 and the resistivity was decreased and then increased within the range between 1 × 10 12 ohm centimeter and 1 × 10 8 ohm centimeter . however , the resistivity of this intermediate layer should be lower than that of the underlying hard carbon coating as illustrated in fig8 ( a ) and 8 ( b ). the total thickness of this carbon coating was 0 . 4 to 1 micron meter . after the positions of the switch 31 and 32 were reversed again in the initial positions ( i . e . the electrode 21 was supplied with high frequency energy as a cathode ), carbon deposition was resumed while the input power was increased from 200 w to 300 w at 0 . 5 to 2 . 5 w / min . the hardness and the resistivity were increased , along with the input energy , from 500 kg / mm 2 to 2000 kg / mm 2 and from not lower than 1 × 10 8 ohm centimeter to 1 × 10 12 ohm centimeter . however , the resistivity of this upper carbon film should be higher than that of the intermediate layer . the total thickness of this carbon coating was 0 . 3 to 0 . 7 micron meter . a first carbon film of 0 . 6 micron meter thickness was formed on the polyimide film under deposition conditions that the structure was placed on the electrode ( cathode ) supplied with high frequency energy of 260 w , the introduction rate of carbide gas such as methane , ethylene , or ethane diluted by hydrogen ( e . g . methane : hydrogen = 1 : 1 ) was 100 sccm , the pressure of the reactive gas was 0 . 03 torr , and the deposition time was 60 minutes . the hardness and the resistivity were measured to be 1000 kg / mm 2 and 1 × 10 12 ohm centimeter . a second carbon film of 0 . 5 micron meter thickness was formed on the first film under deposition conditions that the electrode supporting the structure was grounded ( as an anode ), the input high frequency energy was 300 w , the introduction rate of carbide gas such as methane , ethylene , or ethane diluted by hydrogen was 100 sccm , the pressure of the reactive gas was 0 . 03 torr , and the deposition time was 40 minutes . the hardness and the resistivity were measured to be 600 kg / mm 2 and 1 × 10 10 ohm centimeter . on the second film , a third external film was deposited at an input energy of 80 w for 50 min ., at 150 w for 50 min . and at 300 w for 40 min . sequentially . then the third film was formed , having its resistivities of 5 × 10 10 , 2 × 10 12 , and 1 × 10 14 ohm centimeter across its thickness of 1 . 7 micron meters . a first carbon film of 0 . 6 micron meter thickness was formed on the polyimide film under deposition conditions that the structure was placed on the electrode ( cathode ) supplied with high frequency energy of 260 w , the introduction rate of carbide gas such as methane , ethylene , or ethane diluted by hydrogen ( e . g . methane : hydrogen = 1 : 1 ) was 100 sccm , the pressure of the reactive gas was 0 . 03 torr , and the deposition time was 60 minutes . the hardness and the resistivity were measured to be 1000 kg / mm 2 and 1 × 10 12 ohm centimeter . after the positions of the switch 31 and 32 were reversed ( i . e . the electrode 21 was grounded as an anode ), a second carbon film was formed while the input power was decreased from 300 w to 200 w and subsequently increased from 200 w to 300 w at 0 . 5 to 2 . 5 w / min . the hardness and the resistivity were changed along with the change of the input energy , that is , the hardness was decreased from 500 kg / mm 2 to 300 kg / mm 2 and subsequently increased from 300 kg / mm 2 to 500 kg / mm 2 and the resistivity was decreased and then increased within the range between 1 × 10 12 ohm centimeter and 1 × 10 8 ohm centimeter . however , the resistivity of this second carbon film should be lower than that of the first carbon film . the total thickness of this carbon coating was 0 . 4 to 1 micron meter . on the second film , a third external film was deposited at an input energy of 80 w for 50 min ., at 150 w for 50 min . and at 300 w for 40 min . sequentially . then the third film was formed , having its resistivities of 5 × 10 10 , 2 × 10 12 , and 1 × 10 14 ohm centimeter across its thickness of 1 . 7 micron meters . first and third carbon films were deposited in diamond structure . the deposition conditions required to deposited carbon crystals ( diamond ) were 700 ° to 900 ° c . ( substrate temperature ), 1 . 0 to 5 kw ( input high frequency energy ), 12 hours ( deposition time ) and ch 4 / h 2 = 0 . 1 to 4 ( reactive gas ), 3 to 80 torr ( pressure ). the thickness of the first and third films were 0 . 6 micron meter respectively . the vickers hardness was measured to be 10 , 000 kg / mm 2 . the resistivity was 1 × 10 15 ohm centimeter . after the first film deposition , a second carbon film ( i . e . intermediate film ) of 0 . 5 micron meter thickness was formed on the first film under deposition conditions that the electrode supporting the structure was grounded ( as an anode ), the input high frequency energy was 300 w , the introduction rate of methane diluted by hydrogen was 100 sccm , the pressure of the reactive gas was 0 . 03 torr , and the deposition time was 40 minutes . the hardness and the resistivity were measured to be 600 kg / mm 2 and 1 × 10 10 ohm centimeter . subsequently , the third film was formed under the above deposition conditions . while a description has been made for several embodiments , the present invention should be limited only by the appended claims and should not be limited by the particular examples , and there may be caused to artisan some modifications and variation according to the invention . for example , it has been proved effective to add hydrogen , a halogen , boron , nitrogen , phosphorus or the like into the carbon coating . preferably , the proportion of hydrogen or a halogen is not higher than 25 atomic % and the proportion of the other additives are not higher than 5 %. also , though the experiments were carried out for depositing carbon coatings on semiconductor substrates , the carbon coatings can be deposited on a substrate made of an organic resin such as pet ( polyethylenetelephtalene ), pes , pmma , teflon , epoxy and polyimides , metallic meshes , papers , glass , metals , ceramics , parts for magnetic heads , magnetic discs , and others . the types of carbon coatings deposited in accordance with the present invention includes amorphous , polycrystals ( comprising diamond powders ), and diamond films . in the case of a dual film , lower and upper films may be , respectively , amorphous and amorphous ( having different hardnesses ), amorphous and polycrystals , polycrystals and polycrystals , or polycrystals and a diamond film .