Abstract:
Claimed and disclosed is a treatment apparatus for treating a substrate under decompressed atmosphere, comprising: a chamber, an exhausting means for exhausting the chamber, a first electrode provided in the chamber on which the substrate is mounted or held, a second electrode provided in the chamber opposing the first electrode, a liquid supply source containing a liquid material from which a process gas is generated, a housing provided between the liquid supply source and the chamber to be communicated to the liquid supply source and the chamber, a porous heating unit arranged in the housing for generating the process gas by heating the liquid material supplied from the liquid supply source into the housing in order to vaporize the liquid material, a process gas introduction section provided between the housing and the chamber for guiding the process gas from the housing to the chamber and vibrators to vibrate the porous heating unit. The porous heating unit which is capable of generating heat for itself has an element for electrically heating the porous heating unit. This porous heating unit is arranged in the housing for generating the process gas by heating the liquid material supplied from the liquid supply source into the housing in order to vaporize the liquid material. The treatment apparatus further comprises a process gas introduction section disposed between the housing and the chamber for guiding the process gas from the housing to the chamber.

Description:
RELATED APPLICATION 
     This application is a continuation application of Ser. No. 09/556,133, filed Apr. 20, 2000 now U.S. Pat. No. 6,264,788, which is a divisional application of Ser. No. 09/094,451, filed Jun. 10, 1998, issued as U.S. Pat. No. 6,106,737 on Aug. 22, 2000, which is a divisional application of application Ser. No. 08/424,127, filed Apr. 19, 1995, issued as U.S. Pat. No. 5,900,103 on May 4, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a plasma treatment method by which substrates such as semiconductor wafers are etched or sputtered under plasma atmosphere. It also relates to a plasma treatment apparatus for the same. 
     2. Description of the Related Art Recently, semiconductor devices are more and more highly integrated and the plasma treatment is therefore asked to have a finer workability in their making course. In order to achieve such a finer workability, the process chamber must be decompressed to a greater extent, plasma density must be kept higher and the treatment must have a higher selectivity. In the case of the conventional plasma treatment methods, however, high frequency voltage becomes higher as output is made larger, and ion energy, therefore, becomes stronger than needed. The semiconductor wafer becomes susceptible to damage, accordingly. Further, the process chamber is kept about 250 mTorr in the case of the conventional methods and when the degree of vacuum in the process chamber is made higher (or the internal pressure in the chamber is made smaller), plasma cannot be kept stable and its density cannot be made high. 
     SUMMARY OF THE INVENTION 
     When gases are made plasma, the action of ions in the plasma becomes different, depending upon frequencies of high frequency power. In short, ion energy and plasma density can be controlled independently of the other when high frequency power having two different frequencies is applied to process gases. However, ions (loaded particles) easily run from plasma to the wafer at a frequency band, but it becomes difficult for them to run from the plasma sheath to the wafer at another frequency. band (or transit frequency zone). The so-called follow-up of ions becomes unstable. 
     Particularly molecular gases change their dissociation, depending upon various conditions (such as kinds of gas, flow rate, high frequency power applying conditions and internal pressure and temperature in the process chamber), and the follow-up of ions in the plasma sheath changes in response to this changing dissociation. Further, the follow-up of ions at the transit frequency zone also depends upon their volume (or mass). Particularly in the case of molecular gases used in etching and CVD, the dissociation of gas molecules progresses to an extent greater than needed when electron temperature becomes high with a little increase of high frequency power, and the behavior of ions in the plasma sheath changes accordingly. Plasma properties such as ion current density become thus unstable and the plasma treatment becomes uneven, thereby causing the productivity to be lowered. 
     When the frequency of high frequency power is only made high to increase plasma density, the dissociation of gas molecules progresses to the extent greater than needed. It is therefore desirable that the plasma density is raised not to depend upon whether the frequency is high or low. 
     An object of the present invention is therefore to provide plasma treatment method and apparatus capable of controlling both of the dissociation of gas molecules and the follow-up of ions and also capable of promoting the incidence of ions onto a substrate to be treated. 
     Another object of the present invention is to provide plasma treatment method and apparatus capable of raising the plasma density with smaller high frequency power not to damage the substrate to be treated. 
     According to the present invention, there can be provided a plasma treatment method of plasma-treating a substrate to be treated under decompressed atmosphere comprising exhausting a process chamber; mounting the substrate on a lower electrode; supplying plasma generating gas to the substrate on the lower electrode through an upper electrode; applying high frequency power having a first frequency f 1 , lower than the lower limit of ion transit frequencies characteristic of process gas, to the lower electrode; and applying high frequency power having a second frequency, higher than the upper limit of ion transit frequencies characteristic of process gas, to the upper electrode, whereby a plasma generates in the process chamber and activated species influence the substrate to be treated. 
     It is preferable that the first frequency f 1  is set lower than 5 MHz, more preferably in a range of  0 100 kHz-1 MHz. It is also preferable that the second frequency f 2  is set higher than 10 MHz, more preferably in a range of 10 MHz-100 MHz. 
     High frequency power having the frequency lower than the lower limit of ion transit frequencies is applied to the lower electrode. Therefore, the follow-up of ions becomes more excellent and ions can be more efficiently accelerated with a smaller power. In addition, both of ion and electron currents change more smoothly. Further, the follow-up of ions does not depend upon kinds of ion. The plasma treatment can be thus made more stable even when the degree in the process chamber and the rate of gases mixed change. On the other hand, high frequency power having the frequency higher than the upper limit of ion transit frequencies is applied to the upper electrode. Therefore, ions can be left free from frequencies of their transit frequency zone to thereby enable more stable plasma to be generated. 
     Ion transit frequency zones of process gases used by the plasma treatment in the process, such as etching, CVD and sputtering, of making semiconductor devices are almost all in the range of 1 MHz-10 MHz. 
     Impedances including such capacitive components that the impedance relative to high frequency power becomes smaller than several kΩ and that the impedance relative to relatively low frequency power becomes larger than several Ω are arranged in series between the upper electrode and its matching circuit and between them and the ground. Current is thus made easier to flow to raise the plasma density and ion control. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram showing the plasma etching apparatus according to an embodiment of the present invention; 
     FIG. 2 is a flow chart showing the plasma etching method according to an embodiment of the present invention; 
     FIG. 3 shows a waveform of frequency applied to an upper (or second) electrode; 
     FIG. 4 shows a waveform of frequency applied to a lower (or first) electrode (or suscepter); 
     FIG. 5 is a graph showing transit frequency zones of various gases; 
     FIG. 6 is a block diagram showing the plasma etching apparatus according to another embodiment of the present invention; 
     FIG. 7 is a block diagram showing the plasma etching apparatus according to a further embodiment of the present invention; 
     FIG. 8 is a block diagram showing the plasma etching apparatus according to a still further embodiment of the present invention; 
     FIG. 9 is a vertically-sectioned view showing a housing and a ring member of the plasma etching apparatus; 
     FIG. 10 is a vertically-sectioned view showing the ring member being cleaned; 
     FIG. 11 is a vertically-sectioned view showing the ring member being cleaned; 
     FIG. 12 is a perspective view showing an upper shower electrode and a semiconductor wafer dismantled; 
     FIG. 13 is a block diagram showing the plasma etching apparatus according to a still further embodiment of the present invention; 
     FIG. 14 is a vertically-sectioned view showing the plasma etching apparatus when the suscepter is lowered; 
     FIG. 15 is a vertically-sectioned view showing the plasma etching apparatus when the suscepter is lifted; 
     FIG. 16 is a partly-sectioned view showing a wafer carry-in and -out gate and a baffle member; 
     FIG. 17 is a partly-sectioned view showing the wafer carry-in and -out gate and another baffle member; 
     FIG. 18 is a block diagram showing the plasma etching apparatus according to a still further embodiment of the present invention; 
     FIG. 19 is a perspective view showing a cover for the upper shower electrode; 
     FIG. 20 is a perspective view showing another cover for the upper shower electrode; 
     FIG. 21 is a vertically-sectioned view showing the cover for the upper shower electrode; 
     FIG. 22 is a plan view showing the cover for the upper shower electrode; 
     FIG. 23 shows how the cover is attached to the upper shower electrode; 
     FIG. 24 shows how the cover is detached from the upper shower electrode; 
     FIG. 25 is a sectional view showing the cover being cleaned; 
     FIG. 26 is a sectional view showing a further cover; 
     FIG. 27 is a sectional view showing a still further cover; 
     FIG. 28 is a sectional view showing a still further cover; 
     FIG. 29 is a block diagram showing a magnetron plasma etching apparatus in which plasma is being generated; 
     FIG. 30 is a perspective view showing a baffle member arranged on the side of the suscepter; 
     FIG. 31 is a vertically-sectioned view showing a hole formed in the baffle member; 
     FIG. 32 is a vertically-sectioned view showing another hole formed in the another baffle member; 
     FIG. 33 shows plasma generated in the conventional apparatus; 
     FIG. 34 is intended to explain the relation of the process chamber to magnetic field generated by a permanent magnet; 
     FIG. 35 is a block diagram showing the plasma etching apparatus according to a still further embodiment of the present invention; 
     FIG. 36 is a block diagram showing the inside of a vaporizer; 
     FIG. 37 is a sectional view showing another vaporizer; 
     FIG. 38 is a sectional view showing a further vaporizer; 
     FIG. 39 is a perspective view showing a still further vaporizer; 
     FIG. 40 is a sectional view showing a pipe in is which plural kinds of gas are mixed; 
     FIG. 41 is a block diagram showing a plasma CVD apparatus provided with the vaporizer; 
     FIG. 42 is a sectional view showing the inside of the conventional vaporizer; and 
     FIG. 43 is a graph showing the change of gas flow rate at the initial stage of gas supply. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some embodiments of the present invention will be described with reference to the accompanying drawings. Referring to FIGS. 1 through 5, a first embodiment will be described. 
     A process chamber  2  of an etching treatment apparatus  1  is assembled by alumite-processed aluminium plates. It is earthed and a suscepter  5  insulated by an insulating plate  3  is arranged in it. The suscepter  5  is supported by its bottom through the insulating plate  3  and a support  4 . 
     A coolant chamber  6  is formed in the suscepter support  4 . It is communicated with a coolant supply supply (not shown) through inlet and outlet pipes  7  and  8  and coolant such as liquid nitrogen is circulated between it and the coolant supply supply. 
     An internal passage  9  is formed in a suscepter assembly which comprises the insulating plate  3 , the support  4 , the suscepter  5  and an electrostatic chuck  11 , and heat exchanger gas such as helium gas is supplied from a gas supply supply (not shown) to the underside of a wafer W through it. 
     The top center portion of the suscepter  5  is swelled and the electrostatics c chuck  11 , same in shape as the wafer W, is mounted on the swelled portion of the suscepter  5 . A conductive layer  12  of the electrostatic chuck  11  is sandwiched between two sheets of high molecular polyimide film. It is connected to a 1.5 kV DC high voltage power supply  13  arranged outside the process chamber  2 . 
     A focus ring  14  is arranged on the top of the suscepter  5  along the outer rim thereof, enclosing the wafer W. It is made of insulating material not to draw reactive ions. 
     An upper electrode  21  is opposed to the top of the suscepter assembly. Its electrode plate  24  is made of SiC or amorphous carbon and its support member  25  is made by an alumite-process aluminium plate. Its underside is separated from the wafer W an the suscepter assembly by about 15-20 mm. It is supported by the top of the process chamber  2  through an insulating member  22 . A plurality of apertures  23  are formed in its underside. 
     A gas inlet  26  is formed in the center of the support  25  and a gas inlet pipe  27  is connected to it. A gas supply pipe  28  is connected to the gas inlet pipe  27 . The gas supply pipe  28  is divided into three which are communicated with process gas supply sources  35 ,  36  and  37 , respectively. The first one is communicated with the CP 4  gas supply source  35  through a valve  29  and a mass flow controller  32 . The second one with the O 2  gas supply supply  36  through a valve  30  and a mass flow controller  33 . The third one with the N 2  gas supply supply  37  through valve  31  and a mass flow controller  34 . 
     An exhaust pipe  41  is connected to the bottom of the process camber  2 . An exhaust pipe  44  is also connected to the bottom of an adjacent load lock chamber  43 . Both of them are communicated with a common exhaust mechanism  45  which is provided with a turbo molecular pump and the like. The load lock chamber  43  is connected to the process chamber  2  through a gate valve  42 . A carrier arm mechanism  4 o is arranged in the load lock chamber  43  to carry the wafers W ore by one between the process chamber  2  and the load lock chamber  43 . 
     A high frequency power applier means for generating plasma in the process chamber  2  will be described. 
     A first oscillator  51  serves to oscillate high frequency signal having a frequency of 800 kHz. A circuit extending from the oscillator  51  to the lower electrode (or suscepter)  5  includes a phase controller  52 , an amplifier  53 , a matching unit  54 , a switch SW 1  and a feeder rod  55 . The amplifier  53  is an RF generator and the matching unit  54  includes a decoupling capacitor. The switch SW 1  is connected to the feeder rod  55 . A capacitance  56  is arranged on an earthed circuit of the feeder rod  55 . The phase controller  52  houses a bypass circuit (not shown) and a change over switch (not shown) therein to enable signal to be sent from the first oscillator  51  to the amplifier  53  through the bypass circuit. High frequency signal oscillated is applied to the suscepter  5  through the phase controller  52 , the amplifier  53 , the matching unit  54  and feeder rod  55 . 
     On the other hand, a second oscillator  61  serves to oscillate high frequency signal having a frequency of 27 MHz. A circuit extending from the oscillator  61  to the upper (or shower) electrode  21  includes an amplitude modulator  62 , an amplifier  63 , a matching unit  64 , a switch SW 2  and a feeder rod  65 . The amplitude modulator  62  is connected to a signal circuit of the second oscillator  61  and also to that of the first oscillator  51 . It houses a bypass circuit (not shown) and a changeover switch (not shown) in it to enable signal to be sent from it to the amplifier  63  through the bypass circuit. The amplifier  63  is an RP generator and the matching unit  64  includes a decoupling capacitor. The switch SW 2  is connected to the feeder rod  65 . A capacitance  66  and an inductance  67  are arranged on an earthed circuit of the feeder rod  65 . High frequency signal oscillated is applied to the upper electrode  21  through the amplitude modulator  62 , the amplifier  63 , the matching unit  64  and the feeder rod  65 . High frequency signal having the frequency of 800 kHZ can also be applied, as modulated wave, to the amplitude modulator  62 . 
     The reason why the earthed circuit of the feeder rod  55  includes no inductance resides in that the electrostatic chuck II, the gas passage  9 , the coolant chamber  6 , lifter pins (not shown) and the like are included in the lower electrode signal transmission circuit, that the feeder rod  55  itself is long, and that the suscepter  5  itself has large inductance accordingly. 
     The amplifiers  51  and  64  are arranged independently of the other. Therefore, voltages applied to the upper electrode  21  and the suscepter  5  can be changed independently of the other. 
     Referring to FIG. 2, it will be described how silicon oxide film (SiO 2 ) on the silicon wafer W is plasma-etched. 
     Both of the load lock chamber  43  and the process chamber  2  are exhausted to substantially same internal pressure. The gate valve  42  is opened and the wafer w is carried from the load lock chamber  43  into the process chamber  2  (step S 1 ). The gate valve  42  is closed and the process chamber  2  is further exhausted to set its internal pressure in a range of 10-250 mTorr (step S 2 ). 
     The valves  29  and  30  are opened, and CF 4  and O 2  gases are introduced into the process chamber  2 . Their flow rates are controlled and they are mixed at a predetermined rate The (CP 4 +O 2  ) mixed gases are supplied to the wafer W through apertures  23  of the upper shower electrode  21  (step S 3 ). When the internal pressure in the chamber  2  becomes stable at about 1 Pa, high frequency voltages are applied to the upper and lower electrodes  21  and  5  to generate plasma between them. 
     Frequencies of high frequency power applied to the upper and lower electrodes  21  and  5  to generate plasma are controlled as follows (step S 4 ). 
     The Switches SW 1  and SW 2  are opened to disconnect (OFF) the capacitance  56  from the feeder rod  55  and the capacitance  66  and the inductance  67  from the feeder rod  65 . When the oscillators  61 ,  51 , the amplitude modulator  62  and the amplifiers  63 ,  53  are made operative under this state, high frequency power having a certain waveform is applied to the upper electrode  21 . High frequency power having a frequency same as or higher than the higher one of upper ion transit frequencies characteristic of CF 4  and O 2  gases is applied to the upper electrode  21 . High frequency power having a waveform shown in FIG. 3, for example, is applied to the upper electrode  21 . Plasma is thus generated. 
     On the other hand, high frequency power having a certain waveform is applied to the lower electrode  5  by the oscillator  51 . High frequency power having a frequency same as or lower than the lower one of ion transit frequencies characteristic of CF 4  and O 2  gases is applied to the lower electrode  5 . High frequency power having a waveform shown in FIG. 4, for example, is applied to the lower electrode. Ions in plasma are thus accelerated and drawn to the wafer W, passing through the plasma sheath, to thereby act on the wafer W. 
     The high frequency by which plasma is generated has the waveform shown in FIG. 3 in this case. Therefore, the dissociation of gases introduced into the process chamber  2  is not advanced to an extent greater than needed. In addition, the frequency of 800 kHz by which ions in plasma are accelerated and drawn to the wafer W can be controlled in phase by the phase controller  52 . Ions can be thus drawn to the wafer W before the dissociation of gases progresses to the extent greater than needed. when ions most suitable for etching are generated, therefore, they can be made incident onto the wafer W. When they are caused to act on the wafer W while cooling it, therefore, anisotropic etching having a high aspect rate can be realized. 
     The phase control of the high frequency power (frequency: 800 kHz) applied to the lower electrode may be based on a state under which the dissociation of gases does not progress to the extent greater than needed or a state under which the dissociation of gases progresses to the final stage, they are then combined again and become radicals suitable for etching. 
     Further, it may be arranged that a dummy wafer DW is used and that the treatment is carried out while confirming the extent to which the phase of the high frequency 800 kHz is shifted. The timing at which the phase of the high frequency 800 kHz is shifted may be previously set in this case, depending upon kinds of process gases, etching, coating and the like. 
     When the end point of anisotropic etching is detected (step S 5 ), exhaust, process gas introducing and plasma control steps S 6 , S 7  and S 8  are successively carried out to isotropically etch film on the wafer W. The exhaust step S 6  is substantially same as the above-described one S 2 . At the process gas introducing step S 7 , C 4 F 8 , CHF 3 , Ar and CO gases, for example, different from those at the above-dscribed step S 3 , are supplied to the process chamber  2 . 
     At the plasma control step S 8 , plasma is controlled substantially as seen at the above-described step S 4 . When the end point of isotropic etching is detected (step S 9 ), the applying of the high frequency power is stopped and the process chamber  2  is exhausted while supplying nitrogen gas into it (step S 10 ). The gate valve  42  is opened and the wafer W is carried from the process chamber  2  into the load lock chamber  43  (step S 11 ). 
     Referring to FIG. 5 the plasma control steps S 4  and S 8  will be described in more detail. 
     FIG. 5 is a graph showing ion transit frequency zones characteristic of three kinds of gases A, B and C, in which frequencies are plotted on the vertical axis. An ion transit frequency zone Az of gas A extends from an upper end Au to a lower end Al, an ion transit frequency zone Bz of gas B from an upper end Bu to a lower end B 1 , and an ion transit frequency zone Cz of gas C from an upper end Cu to a lower end Cl. CHF 3  or CO gas is cited as gas A. Ar gas is cited as gas B. CF 4 , C 4 F 8  or O 2  gas is cited as gas C. At least one or more gases selected from the group consisting of CF 4 , C 4 F 8 , CHF 3 , Ar, O 2  and CO gases are used as process gas. In short, process gas may be one of them or one of mixed gases (CH 3 +Ar+O 2 ), (CHF 3 +CO+O 2 ), (C 4 F 8 +Ar+O 2 ), (C 4 F 8 +CO+Ar+O 2 ) and (CF 4 +CHF 3 ). 
     When mixed gases of A, B and C are used as process gas, the high frequency power applied to the upper electrode has a frequency higher than the highest one Bu of upper ion transit frequencies Au, Bu and Cu and the high frequency power applied to the lower electrode has a frequency lower than the lowest one Cl of lower ion transit frequencies Al, Bl and Cl. 
     Another etching treatment method conducted using the above- described etching treatment apparatus  1  will be described 
     The switches SW 1  and SW 2  are closed or turned on to connect the signal transmission circuits to their earthed circuits. High frequency signal (frequency: 800 kHz) is amplified directly by the amplifier  53 , bypassing the phase controller  52 , and applied to the suscepter  5  through the matching unit  54 . On the other hand, high frequency signal (frequency: 27 MHz) is amplified directly by the amplifier  63 , bypassing the amplitude modulator  62 , and applied to the upper electrode  21  via the matching unit  64  and the feeder rod  65 . 
     Conventionally, the matching unit arranged on the side of the suscepter is matched relative to the high frequency of 800 kHz but it becomes high in impedance relative to the high frequency of 27 MHz applied from the upper electrode, thereby making it difficult for the high frequency applied from the upper electrode to flow to the suscepter. Plasma is thus scattered, so that the plasma density decreases. 
     In the apparatus  1 , however, the capacitance  56  is arranged between the feeder rod and the ground. A DC resonance circuit can be thus formed relative to the high frequency applied from the upper electrode. when the value of the capacitance  56  is adjusted, considering the constant of a distributed constant circuit, therefore, composite impedance can be made smaller than several Ω to thereby make it easy for the high frequency applied from the upper electrode to flow to the suscepter  5 . Therefore, current density can be raised and plasma density thus attained can also be raised. 
     On the other band, the capacitance  66  and the inductance  67  are attached to the feeder rod  65  arranged on the side of the upper electrode  21 . Therefore, a DC resonance circuit is also provided relative to the high frequency of 800 kHz, thereby making it easy for the high frequency 800 kHz applied to the side of the suscepter  5  to flow to the upper electrode  21 . The incidence of ions in plasma onto the wafer W is promoted accordingly. 
     Although high frequency power having the frequency 27 MHz has been applied to the upper electrode  21  and high frequency power having the frequency 800 kHz to the lower electrode  5  in the above-described embodiment, other frequencies may be set, depending upon kinds of process gas. 
     It is desirable that high frequency power applied to the lower electrode  5  has a frequency lower than the inherent lower ion transit frequency or lower than 1 MHz and that high frequency power applied to the upper electrode  21  has a frequency higher than the inherent upper ion transit frequency or higher than 10 MHz. When so arranged, ions are more efficiently accelerated with a smaller high frequency power and the follow-up of ions in the plasma sheath to bias frequencies becomes stable even when the rate of gases mixed and the degree of vacuum in the process chamber are a little changed. Therefore, ions can be made incident onto the wafer without scattering in the plasma sheath, thereby enabling a finer work to be achieved at high speed. 
     According to the present invention, the follow-up of ions is ore excellent due to the high frequency power applied to he first electrode and they can be more efficiently accelerated with a smaller power. In addition, plasma itself can be kept stable. A more stable treatment can be thus realized even when the degree of vacuum in the process chamber and the rate of gases mixed change. 
     Further, when the dissociation is controlled not to progress to the extent greater than needed and the phase of the high frequency power applied to the first electrode is also controlled, ions or radicals needed for the treatment can be created at a desired timing and they can be made incident onto the wafer. Anisotropic etching treatment having a high aspect rate can be thus attained. In Addition, damage applied to the wafers can be reduced. her, plasma density can be made high without raising the high frequency power and its frequency, and ion control can be made easier. 
     A second embodiment will be described referring to FIG.  6 . Same components as those in the above-described first embodiment will be mentioned only when needed. 
     An etching treatment apparatus  100  has, as high frequency power applier means, two high frequency power supplies  141 ,  151  and a transformer  142 . The primary side of the transformer  142  is connected to the first power supply  141  and then earthed. Its secondary side is connected to both of the upper and lower electrodes  21  and  105 . A first low pass filter  144  is arranged between the secondary side and the upper electrode  21  and a second low pass filter  145  between the secondary side and the lower electrode  105 . The first power supply  141  serves to apply high frequency power having the relatively low frequency such as 380 kHz to the electrodes  105  and  21 . When silicon oxide (SiO 2 ) film is to be etched, it is optimum that a frequency f 0  of high frequency power applied from the first power supply  141  is 380 kHz and when polysilicon (poly-Si) film is to be etched, it is preferably in a range of 10 kHz-5 MHz. 
     The transformer  142  has a controller  143 , by which the power of the first power supply  141  is distributed to both electrodes  105  and  21  at an optional rate. For example, 400 W of full power 100 W can be applied to the suscepter  105  and 600 W to the upper electrode  21 . In addition, high frequency powers whose phases are shifted from each other by 180° are applied Lo the suscepter  105  and the upper electrode  21 . 
     The second power supply  151  serves to apply high frequency power having the high frequency such as 13.56, for example, to the upper electrode  21 . It is connected to the upper electrode  21  via a capacitor  152  and then earthed. This plasma generating circuit is called P mode one. It is optimum that a frequency f 1  of high frequency power applied from it is 13.56 MHz, preferably in a range of 10-100 MHz. 
     It will be described how silicon oxide film (SiO 2 ) on the silicon wafer W is etched by the above-described etching apparatus  100 . 
     The wafer W is mounted on the suscepter  105  and sucked and held there by the electrostatic chuck  11 . The process chamber  102  is exhausted while introducing CF 4  gas into it. After its internal pressure reaches about 10 mTorr, high frequency power of 13.56 MHz is applied from the second power supply  151  to the upper electrode  21  to make CF 4  gas into plasma and dissociate gas molecules between the upper electrode  21  and the suscepter  105 . On the other hand, high frequency power of 380 kHz is applied from the first power supply  141  to the upper and lower electrodes  21  and  105 . Ions and radicals such as fluoric ones in plasma-like gas molecules are thus drawn to the suscepter  105 , thereby enabling silicon oxide film on the wafer to be etched. 
     The generating and keeping of plasma itself are attained in this case by the high frequency power having a higher frequency and applied from the second power supply  151 . Stable and high density plasma can be thus created. In addition, activated species in this plasma are controlled by the high frequency power of 380 kHz applied to the upper and lower electrodes  21  and  105 . Therefore, a more highly selective etching can be applied to the wafer W. Ions cannot follow up to the high frequency power which has the frequency of 13,56 MHz and by which plasma is generated. Even when the output of the power supply  151  is made large to generate high density plasma, however, the wafers W cannot be damaged. 
     The first and second low pass filters  144  and  145  are arranged on the secondary circuit of the transformer  142 . This prevents the high frequency power having the frequency of 13.56 MHz and applied from the second power supply  151  from entering into the secondary circuit of the transformer  142 . Therefore, the high frequency power having the frequency of 13.56 MHz does not interfere with the one having the frequency of 380 kHz, thereby making plasma stable. Blocking capacitors may be used instead of the low pass filters  144  and  145 . Although high frequency powers have been continuously applied to the electrodes in the above case, modulation power which becomes strong and weak periodically may be applied to the electrodes  21  and  105 . 
     A third apparatus  200  will be described with reference to FIG.  7 . Sam components as those in the above-described first and second embodiments will be mentioned only when needed. 
     A high frequency power circuit of this apparatus  200  is different from that of the second embodiment in the following points: A suscepter  205  of the apparatus  200  is not ground, no low pass filter is arranged on the secondary circuit of a transformer  275 ; and a second transformer  282  is arranged on the circuit of a second power supply  281 . 
     The second power supply  281  serves to generate high frequency power of 3 MHz. It is connected to the primary side of the transformer  282 , whose secondary side are connected to upper and lower electrodes  21  and  205 . A controller  293  which controls the distribution of power is also attached to the secondary side of the transformer  282 . 
     It will be described how the etching treatment is carried out by the apparatus  200 . 
     High frequency powers of 3 MHz whose phases are shifted from each other by 180° are applied from the power supply  281  to the suscepter  205  and the upper electrode  21  to generate plasma between them. At the same time, high frequency powers of 380 kHz whose phases are shifted from each other by 180° are applied from a power supply  274  to them. Ions in plasma generated are thus accelerated to enter into the wafer W. 
     Further, the two high frequency power supplies  274  and  281  in the third apparatus are arranged independently of the other. In short, they are of the power split type. Therefore, they do not interfere with each other, thereby enabling a more stable etching treatment to be realized. 
     Furthermore, high frequency powers are supplied from the two power supplies  274  and  281  to both of upper and lower electrodes  21  and  205 , respectively. The flow of current can be thus concentrated on a narrow area between the upper  21  and the lower electrode  205 . As the result, a high density plasma can be generated and the efficiency of controlling ions in plasma can be raised. 
     A fourth embodiment will be described, referring to FIGS. 8 through 12. Same components as those in the above-described embodiments will be mentioned only when needed. 
     As shown in FIG. 8, an etching apparatus  300  has a cylindrical or rectangular column-like air-tight chamber  302 . A top lid  303  is connected to the side wall of the process chamber  302  by hinges  304 . Temperature adjuster means such as a heater  306  is arranged in a suscepter  305  to adjust the treated face of a treated substrate W to a desired temperature. The heater  306  is made, for example, by inserting a conductive resistance heating unit such as tungsten into an insulating sintered body made of aluminium nitride. Current is supplied to this resistant heating unit through a filter  310  to control the temperature of the wafer W in such a way that the treated face of the wafer W is raised to a predetermined temperature. 
     A high frequency power supply  319  is connected to the suscepter  305  through a blocking capacitor  318 . When the wafer W is to be etched, the high frequency power of 13.56 Mz is applied from the power supply  319  to the suscepter  305 . 
     The suscepter  305  is supported by a shaft  321  of a lifter mechanism  320 . When the shaft  321  of the lifter mechanism  320  is extended and retreated, the suscepter  305  is moved up and down. A bellows  322  is attached to the lower end of the suscepter  305  not to leak gases in the process chamber  302  outside. 
     Reaction products deposit in the process chamber  302 . A ring  325  is freely detachably attached to the outer circumference of the suscepter  305 . It is made preferably of PTFE (teflon), PFA, polyimide or PBI (polybenzoimidazole). It may also be made of such a resin that has insulation in a temperature range of common temperature −500° C. or of such a metal like aluminium that has insulating film on its surface. A baffle plate  326  is made integral to it. A plurality of holes  328  are formed in the baffle plate  326 . They are intended to adjust the flow of gases in the process chamber  302 , to make its exhaust uniform, and to make a pressure difference between the treatment space and a space downstream the flow of gases. A top portion  327  of the ring  325  is bent inwards, extending adjacent to the electrostatic chuck  11 , to make the top of the suscepter  305  exposed as small as possible. 
     An upper electrode  330  is arranged above the suscepter  305 . When the etching treatment is to be carried out, the suscepter  305  is lifted to adjust the interval between the suscepter  305  and the upper electrode  330 . The upper electrode  330  is made hollow and a gas supply pipe  332  is connected to this hollow portion  331  to introduce CF 4  gas and others from a process gas supply supply  333  into the hollow portion  331  through a mass flow controller (MPC)  334 . A diffusion plate  335  is arranged in the hollow portion  331  to promote the uniform diffusion or scattering of process gases. Further, a process gas introducing section  337  having a plurality of apertures  336  is arranged under the diffusion plate  335 . An exhaust opening  340  which is communicated with an exhaust system provided with a vacuum pump and others is formed in the side wall of the process chamber  302  at the lower portion thereof to exhaust the process chamber  302  to an internal pressure of 0.5 Torr, for example. 
     When the wafer W is etched in the process chamber  302 , reaction products are caused and they adhere to the ring  325  and the baffle plate  326 , leaving the outer circumference of the suscepter  305  substantially free from them. When the etching treatment is finished, the wafer W is carried out of the process chamber  302  into the load lock chamber  43 . A next new wafer W is then carried from the load lock chamber  43  into the process chamber  302  and etched in it. When this etching treatment is repeated many times, a lot of reaction products adhere to the ring  325 . 
     As shown in FIG. 9, the top lid  303  of the process chamber  302  is opened and the ring  325  is detached from the suscepter  305 . Reaction products are then removed from the ring  325  by cleaning. 
     The time at which the ring  325  must be cleaned is determined as follows: 
     the number of particles adhering to the wafer W which has been treated by the apparatus  300  is counted and when it becomes larger than a predetermined value; 
     the number of particles scattering in the atmosphere exhausted from the apparatus  300  and/or at least in one or more areas in the exhaust pipe is counted and when it becomes larger than a predetermined value; 
     when predetermined sheets of the wafer W have been treated in the apparatus  300 ; and 
     when the total of hours during which plasma has been generated or the plasma treatment has been carried out reaches a predetermined value. 
     Dry or wet cleaning is used. The dry cleaning is carried out in such a way that ClF 3 , CF 4  or NF 3  gas is blown to the ring  325  which is left attached to the suscepter  305  or which is detached from the suscepter  305  and left outside the process chamber  302 , as shown in FIG.  10 . 
     On the other hand, the wet cleaning is carried out in such a way that the ring  325  to which reaction products have adhered is immersed in cleaning liquid  351  in a container  350 , as shown in FIG.  11 . IPA (isopropyl alcohol), water or fluorophosphoric acid is used as cleaning liquid  351 . The ring  325  from which reaction products have been removed by the dry or wet cleaning is again attached to the suscepter  305  and the plasma treatment is then repeated. 
     When the wafers W are to be etched, plural rings  325  are previously prepared relative to one suscepter  305 . If so, cleaned one can be attached to the suscepter  305  while cleaning the other. 
     The dry or wet cleaning can be appropriately used to remove reaction products from the ring  325 . When the dry cleaning is compared with the wet one, however, the former is easier in carrying out it but its cleaning is more incomplete. To the contrary, the latter is more excellent in cleaning the ring  325  but its work is relatively more troublesome. Therefore,it is desirable that the wet cleaning is periodically inserted while regularly carrying out the dry cleaning. 
     The baffle plate will be described referring to FIGS. 12 and 13. 
     As shown in FIG. 12, it is preferable that an effective diameter D 1  is set not larger than a diameter D 2 . The effective diameter D 1  represents a diameter of that area where the process gas jetting apertures  336  are present, and the diameter D 2  denotes that of the wafer W in this case. When the effective diameter D 1  is set in this manner, a high efficient etching can be attained in the process chamber  302 . It is the most preferable that the effective diameter D 1  is set to occupy about 90% of the diameter D 2 . 
     Providing that the underside  338  of the upper electrode has a diameter D 3 , the effective diameter D 1 , the diameter D 2  and the diameter D 3  meet the following inequality (1). 
     
       
           D   1   &lt;D   2   &lt;D   3   (1) 
       
     
     When the ring the whole of which is made of insulating material is used as it is, the effective area of the lower electrode becomes substantially smaller than that of the upper electrode, thereby making plasma uneven. This problem can be solved when the effective area of the lower electrode is made same as that of the upper electrode or when it is made larger than that of the upper electrode. 
     As shown in FIG. 13, the baffle plate  326  is made integral to the ring  325 . It is divided into a portion  360  equal to the diameter D 4  and another portion  361  larger than it, and the inner portion  360  is made of metal such as aluminium and stainless steel while the outer portion  361  of PTPE (teflon), PPA, polyimide, PBI (polybenzoimidazole), other insulating resin or alumite-processed aluminium. 
     The diameter D 4  is made same as or larger than the diameter D 3 . At least the inner portion  360  of the baffle plate  326  is positioned just under the upper electrode  330 . The ring  326  is divided into an upper half  363  and a lower half  364 , sandwiching an insulator  362  between them. The upper half  363  is made of metal such as aluminium and stainless steel and it is made integral to the inner portion  360  of the baffle plate  326 . A power supply  319  which serves to apply high frequency power to the suscepter  305  is connected to these inner portion  360  of the baffle plate  326  and upper half  363  of the ring  325  by a lead  367  via a blocking capacitor  318 . At least those portions (the inner portion  360  of the baffle plate and the upper half  363  of the ring) which are positioned just under the upper electrode  330  are made same in potential. In order to make it easy to exchange the ring  325 , it is preferable that the lead  367  is connected to the upper half  363  of the ring or the inner portion  360  of the baffle plate  326  by an easily-detached socket  368 . A lower suscepter  365  is insulated from the upper one  305  by an insulating layer  366 . The lower half  364  of the ring is also therefore insulated from the upper half  363  thereof by the insulator  362 . 
     When at least that portion of the baffle plate  326  which is positioned just under the upper electrode  330  is made same in potential as the suscepter  305 , as described above, plasma can be made uniform. 
     Referring to FIGS. 14 and 15, it will be described how the side opening  41  of the process chamber  302  through which the wafer W is carried in and out is opened and closed as the suscepter is moved up and down. 
     The ring  325  provided with the baffle plate  326  encloses the suscepter  305 . The lifter means  320  is arranged under the process chamber  302  and the suscepter  305  is supported by the shaft  321  of the lifter means  320 . 
     When the suscepter  305  is moved down, as shown in FIG. 14, the baffle plate  326  is positioned lower than the side opening  41 . When it is moved up, as shown in FIG. 15, the baffle plate  326  is positioned higher than the side opening  41 . 
     When the suscepter  305  is moved down and the baffle plate  326  is positioned lower than the side opening  41 , therefore, the wafer W can be freely carried in and out of the process chamber  302  through the side opening  41 . When the baffle plate  326  is positioned higher than the side opening  41  at the time etching treatment, however, the side opening  41  is shielded from the process space between the upper and the lower electrode, thereby preventing plasma from entering into the side opening  41 . 
     As shown in FIG. 16, it may be arranged that a shielding plate  370  is attached to the outer circumference of the baffle plate  326  and that the side opening  41  is closed by the shielding plate  370  when the suscepter  305  is moved up. Particularly, the side opening  41  is too narrow for hands to be inserted. Therefore, inert gas may be supplied, as purge gas, into a clearance  371  between the shielding plate  370  and the inner face of the process chamber  302  not to cause process gases to enter into the side opening  41 . Similarly, purge gas may also be supplied into a clearance  372  between the wafer-mounted stage  305  and the upper half  363  of the ring  325 . 
     The side opening  41  may be closed by a shielding plate  373  attached to the outer circumference of the baffle plate  326 , as shown in FIG. 17, when the baffle plate  326  is lifted half the side opening  41 . 
     Referring to FIGS. 18 through 28, the cleaning of a fifth CVD apparatus will be described. Same components as those in the above-described embodiments will be mentioned only when needed. 
     A CVD apparatus  500  has a process chamber  502  which can be exhausted vacuum. A top lid  503  is connected to the side wall of the process chamber  502  by hinges  505 . A shower head  506  is formed in the center portion of the top lid  503  at the underside thereof. A process gas supply pipe  507  is connected to the top of the shower head  506  to introduce mixed gases (SiH 4 +H 2 ) from a process gas supply  508  into the shower head  506  through a mass flow controller (MFC)  510 . A plurality of gas jetting apertures  511  are formed in the bottom of the shower head  506  and process gases are supplied to the wafer W through these apertures  511 . 
     An exhaust pipe  516  which is communicated with a vacuum pump  515  is connected to the side wall of the process chamber at the lower portion thereof. A laser counter  517  which serves to count the number of particles contained in the gas exhausted from the process chamber  502  is attached to the exhaust pipe  516 . The process char  502  is decompressed to about 10 −6  Torr by the exhaust means  515 . 
     The process chamber  502  has a bottom plate  521  supported by a substantially cylindrical support  520  and cooling water chambers  522  are formed in the bottom plate  521  to circulate cooling water supplied through a cooling water pipe  523  through them. 
     A suscepter  525  is mounted on the bottom plate  521  through a heater  526  and these heater  526  and the wafer-mounted stage  525  are enclosed by a heat insulating wall  527 . The heat insulating wall  527  has a mirror-finished surface to reflect heat radiated from around. The heater  526  is heated to a predetermined temperature or 400-2000° C. by voltage applied from an AC power supply (not shown). The wafer W on the stage  525  is heated to 800° C. or more by the heater  526 . 
     An electrostatic chuck  530  is arranged on the top of the wafer-mounted stage  525 . It comprises polyimide resin films  531 ,  532  and a conductive film  533 . A variable DC voltage supply (not shown) is connected to the conductive film  533 . 
     A detector section  538  of a temperature sensor  537  is embedded in the suscepter  525  to successively detect temperature in the wafer-mounted stage  525 . The power of the AC power supply which is supplied to the heater  526  is controlled responsive to signal applied from the temperature sensor  537 . A lifter  541  is connected to the suscepter  525  through a member  543  to move it up and down. Those portions of a support plate  546  through which support poles  544  and  545  are passed are provided with bellows  547  and  548  to keep the process chamber  502  air-tight. 
     A cover  560  is freely detachably attached to the shower head  506 . It is made of material of the PTFE (teflon) group, PPA, polyimide, PBI (polybenzoimidazole) or polybenzoazole, which are insulators and heat resistant. In the case of the plasma CVD apparatus, the wafer-mounted stage  525  is heated to about 350-400° C. at the time of plasma process and in the case of the heat CVD apparatus, it is usually heated higher than 650° C. or to about 800° C. The cover  560  is therefore made of such a material that can resist this radiation heat. 
     As shown in FIG. 19, a large-diameter opening  563  is formed in a bottom  561  of the cover  560 . When the cover  560  is attached to the shower head  506 , the gas jetting apertures  511  of the shower head  506  appear in the opening  563 . 
     As shown in FIG. 20, a plurality of apertures  565  may also be formed in the cover  560 . These apertures  565  are aligned with those of the shower head  506  in this case. 
     As shown in FIG. 21, recesses  570  may be formed in the outer circumference of the shower head  506  while claws  571  are for on an inner circumference  562  of the cover.  506 , as shown in FIG.  22 . The claws  571  are. fitted into recesses  570  in this case while elastically deforming the cover  560 . The three claws  571  are arranged on the inner circumference  562  of the cover  560  at a same interval, as shown in FIG.  22 . 
     As shown in FIG. 23, the cover  560  may be attached to the shower head  506  in such a way that bolts  575  are screwed into recesses  573  of the shower head  506  through a cover side  562 . 
     It will be described how upper electrode cover is cleaned. 
     When mixed gases (SiH 4 +H 2 ), for example, are introduced into the process chamber  502  to form film on the wafer W, reaction products adhere to the upper electrode cover  560 . As shown in FIG. 24, the top lid  503  is opened and the cover  560  is detached from the shower head  506 . The cover  560  is then immersed in cleaning liquid  581  in a container  580  (wet cleaning) Or the dry cleaning may be conducted in such a way that cleaning gas such as ClF 3 , CF 4  or NF 3  gas is introduced into the process chamber  502  while keeping the cover  560  attached to the shower head  506 . 
     The time at which the cleaning must be conducted is determined as follows. The number of particles contained in the gas exhausted through the exhaust pipe  516  is counted by the counter  517  and when it becomes larger than a limit value, the cleaning of the cover  560  must be started. 
     As shown in FIG. 26, the underside of the top lid  503  may be covered by a cover  585 , in addition to the shower head  506 . Or the inner face of the process chamber  502  may be covered by a cover  586 , in addition to the shower head  506 , as shown in FIG.  27 . An opening  587  is formed in the cover  586  in this case, corresponding to the side opening  41  of the process chamber  502 . Or a cover  590  having a curved bottom  591  may be used, as shown in FIG.  28 . 
     A sixth embodiment will be described referring to FIGS. 29 through 34. Same components as those in the above-described embodiments will be mentioned only when needed. 
     As shown in FIG. 29, a magnetron type plasma etching apparatus  600  has a rotary magnet  627  above a process chamber  602 . Upper and lower electrodes  624  and  603  are opposed in the process chamber  602 . Process gases are introduced from a gas supply supply  629  to the space between the upper and the lower electrode through an MFC  630 . The rotary magnet  627  serves to stir plasma generated between both of the electrodes  603  and  624 . 
     A suscepter assembly comprises an insulating plate  604 , a cooling block  605 , a heater block  606 , an electrostatic chuck  608  and a focus ring  612 . A conductive film  608   c  of the electrostatic chuck  608  is connected to a filter  610  and a variable DC high voltage supply  611  by a lead  609 . The filter  610  is intended to cut high frequencies. An internal passage  613  is formed in the cooling block  605  and liquid nitrogen is circulated between it and a coolant supply supply (not shown) through pipes  614  and  615 . A gas passage  616  is opened at tops of the suscepter  603 , the heater  617  and the cooling block  605 , passing through the suscepter assembly. The base end of the gas passage  616  is communicated with a heat exchanger gas supply supply (not shown) to supply heat exchanger gas such as helium gas to the underside of the wafer W through it. The heater block  606  is arranged between the suscepter  603  and the cooling block  505 . It is shaped like a band-like ring and it is several mm thick. It is a resistant heating unit. It is connected to a filter  619  and a power supply  620 . 
     Inner and outer pipes  621   a  and  521   b  are connected to the suscepter  603  and the process chamber  602 . They are conductive double pipes, the outer one  621   a  of which is earthed and the inner one  621   b  of which is connected to a high frequency power supply  623  via a blocking capacitor  622 . The high frequency power supply  623  has an oscillator for oscillating the high frequency of 13.56 MHz. Inert gas is introduced from a gas supply supply (not shown) into a clearance between the inner  621   a  and the outer pipe  621   b  and also into the inner pipe  621   b.    
     Except the upper electrode, the inner faces of the top of the process chamber  602  is covered by an insulating protection layer  625 , 3 mm or more thick. Similarly, the inner face of its side wall is covered by an insulating protection layer  626 , 3 mm or more thick. 
     In the conventional magnetron type plasma etching apparatus, the flow of electrons tends to gather near the inner wall of the process chamber, as shown in FIG.  34 . The flow of plasma is thus irradiated in a direction W, that is, to the side wall of the process chamber, thereby damaging it. In the above-described apparatus  600 , however, the side wall of the process chamber  602  is covered by the insulating protection layer  626  so that it can be protected. 
     Process gas supply and exhaust lines or systems of the apparatus  600  will be described. 
     A process gas supply pipe  628  is connected to the side wall of the process chamber  602  at the upper portion thereof and CF 4  gas is introduced from a process gas supply  629  into the process chamber  602  through it. An exhaust pipe  633  is also connected to the side wall of the process chamber  602  at the lower portion thereof to exhaust the process chamber  602  by an exhaust means  631 , which is provided with a vacuum pump. A valve  632  is attached to the exhaust pipe  633 . 
     As shown in FIG. 30, a baffle plate  635  is arranged between the outer circumference of the suscepter  603  and the inner wall of the process chamber  602 . Plural holes  634  are formed in the baffle plate  635  to adjust the flow of exhausted air or gas. 
     As shown in FIG. 31, each hole  634  Is tilted. Therefore, the conductance of gas rises when it passes through the holes  634  and the gradient of electric field becomes gentle accordingly. This prevents discharge from being caused in the holes  634  and plasma from flowing inward under the baffle plate  635 . 
     As shown in FIG. 32, holes  634   a ,  634   b ,  634   c  and  634   d  each having a same pitch may be formed in plural baffle plates  635   a ,  635   b ,  635   c  and  635   d  to form a step like exhaust hole  634 A. This exhaust hole  634 A can be formed when the baffle plates  635   a ,  635   b ,  635   c  and  635   d  are placed one upon the others in such a way that the holes  634   a ,  634   b ,  634   c  and  634   d  are a little shifted from their adjacent ones. When these exhaust holes  634 A are formed, abnormal discharges in plasma generation can be more effectively prevented. 
     In the conventional apparatus, each hole  692  in the baffle plate extends only vertical, as shown in FIG.  33 . Those holes  692  allow plasma to flow inward under the baffle plate and abnormal discharges such as sparkles to be caused in them, thereby causing metal contamination and particles. In the apparatus  600 , however, the holes  634  are directed toward the exhaust opening  633 . The reduction of exhaust speed can be thus prevented. When the direction in which the turbo-pump  631  is driven is made reverse to the flow of exhausted gas, that is, when it is made anticlockwise in a case where exhausted gas flows clockwise, the speed of exhausted gas can be raise to a further extent. 
     A seventh embodiment will be described referring to FIGS. 35 through 43. TEOS gas is used to form film on the wafer W in this seventh plasma CVD apparatus. Same components as those in the above-described embodiments will be mentioned only when needed. 
     The plasma CVD apparatus  700  has a cylindrical or rectangular process chamber  710 , in which a suscepter  712  is arranged to hold a wafer W on it. It is made of conductive material such as aluminium and it is insulated from the wall of the process chamber  710  by an insulating member  714 . A heater  716  which is connected to a power supply  718  is embedded in it. The wafer W on it is heated to about 300° C. (or film forming temperature) by the heater  716 . The process chamber is of the cold wall type in this case, but it may be of the hot wall type. The process chamber of the hot wall type can prevent gas from being condensed and stuck. 
     The electrostatic chuck  11  is arranged on the suscepter  712 . Its conductive film  12  is sandwiched between two sheets of film made of polybensoimidazole resin. A variable DC high voltage power supply  722  is connected to the conductive film  12 . A focus ring  724  is arranged on the suscepter  712  along the outer rim thereof. 
     A high frequency power supply  728  is connected to the suscepter  712  via a matching capacitor  726  to apply high frequency power having a frequency of 13.56 MHz or 40.68 MHz to the suscepter  712 . 
     An upper electrode  730  serves as a plasma generator electrode and also as a process gas introducing passage. It is a hollow aluminium-made electrode and a plurality of apertures  730 a are formed in its bottom. It has a heater (not shown) connected to a power supply  731 . It can be thus heated to about 150° C. by the heater. 
     A process gas supply line or system-provided with a vaporizer (VAPO)  732  will be described referring to FIGS. 35 and 36. 
     Liquid TEOS is stored in a container  734 . At the film forming process, a liquid mass flow controller (LMFC)  736  is controlled by a controller  758  to control the flow rate of liquid TEOS supplied from the container  734  to the vaporizer  732 . 
     As shown in FIG. 36, a porous and conductive heating unit  744  is housed in a housing  742  of the vaporizer  732 . The housing  742  has an inlet  738  and an outlet  740 . The inlet  738  is communicated with the liquid supply side of the container  734 . The outlet  740  is communicated with the hollow portion of the upper electrode  730 . 
     The heating unit  744  is made of sintered ceramics in which conductive material such as carbon is contained, and it is porous. It is preferably excellent in workability and in heat and chemical resistance. Terminals  747  are attached to it and current is supplied from a power supply  746  to it through them. When current is supplied to it, it is resistance-heated to about 150° C. Further, vibrators  748  are embedded in the housing  742 , sandwiching the heating unit  744  between them. It is preferable that they are supersonic ones. The power supply  746  for the heating unit  744  and a power supply (not shown) for the vibrators  748  are controlled by the controller  758 . 
     It will be described how the vaporizer  732  is operated. 
     When liquid TEOS is supplied from the container  734  to the vaporizer  732 , it enters into holes in the porous heating unit  744  and it is heated and vaporized. Because its contact area with the porous heating unit  744  becomes extremely large, its vaporized efficiency becomes remarkably higher, as compared with the conventional vaporizers. 
     Further, vibration is transmitted from vibrators  748  to liquid TEOS caught by the heating unit  744  and in its holes. Heat transfer face and liquid vibrations are thus caused. Therefore, the border layer between the heat transfer face of each hole in the heating unit  744  and liquid TEOS, that is, the heat resistance layer is made thinner. As the result, convection heat transmission is promoted to further raise the vaporized efficiency of liquid TEOS. 
     According to the vaporizer in this case, gas-like TEOS is moved by pressure difference caused between the inlet  738  and the outlet  740  and thus introduced into the process chamber  710  without using any carrier gas. 
     A bypass  750  and a stop valve  752  may be attached to the passage extending from the outlet  740  of the vaporizer, as shown in FIG.  35 . The bypass  750  is communicated with a clean-up unit (not shown) via a bypass valve  754 . The clean-up unit has a burner and others to remove unnecessary gas components. Further, a sensor  756  is also attached to the passage extending from the outlet  740  to detect whether or not liquid TEOS is completely vaporized and whether or not gases are mixed at a correct rate. Detection signal is sent from the sensor  756  to the controller  758 . 
     The operation of the above-described CVD apparatus  700  will be described. 
     The wafer W is carried into the process chamber  710  which has been decompressed to about 1×10 −4  several Torr, and it is mounted on the suscepter  712 . It is then heated to 300° C., for example, by the heater  716 . While preparing the process chamber  710  in this manner, liquid TEOS is vaporized by the vaporizer  732 . High frequency power is applied from the high frequency power supply  728  to the lower electrode  712  to generate reactive plasma in the process chamber. Activated species in plasma reach the treated face of the wafer W to thereby form P-TEOS (plasma-tetraethylorthosilicate) film, for example, on it. 
     Other vaporizers will be described referring to FIGS. 37 through 41. 
     As shown in FIG. 37, a vaporizer  732 A May be made integral to an upper electrode  730 A of a process chamber  710 A. It is attached integral to the upper electrode  730 A at the upper portion thereof with an intermediate chamber  770  ford under it. Its housing  742 A has a gas outlet side  774  in which a plurality of apertures  772  are formed. 
     A gas pipe  776  is communicated with the intermediate chamber  770  in the upper electrode  730 A to introduce second gas such as oxygen and inert gases into it. A bypass  750 A extends from that portion of the upper electrode  730 A which is opposed to the gas pipe  776  to exhaust unnecessary gas from the upper electrode  730 A. Further, plates  780   a ,  780   b  and  780   c  in which a plurality of apertures  778   a ,  778   b  and  778   c  are formed are arranged in the lower portion of the intermediate chamber  770  with an interval interposed between them. 
     As shown in FIGS. 38 and 39, a liquid passage  782  is formed in a heating unit  744 B in the case of a vaporizer  732 B. It includes a center passage  782   a  and passages  782   b  radically branching from the center passage  782   a . When it is formed in the heating unit  744 B in this manner, it enables liquid to be uniformly distributed in the whole of the porous heating unit  744 B, thereby raising gas vaporized efficiency to a further extent. 
     After liquid is vaporized by a vaporizer  738 C, two or more gases may be mixed, as shown in FIG. 40. A second gas supply opening  784  is arranged downstream the vaporizer  738 C and second gas component such as oxygen and inert gases is supplied through it. A gas mixing duct  786  extends downstream it and a bypass  750 C having a bypass valve  754 C, and a stop valve  752 C are further arranged in the lower portion of the gas mixing duct  786 . A strip-like r  788  is housed in the gas mixing duct  786  to form a spiral passage  790  in it. is First and second gas components are fully mixed, while passing through the spiral passage  790 , and they reach a point at which the bypass  750  branches from the passage extending to the side of the process chamber. 
     In addition to TEOS (tetraethylorthosilicate), trichlorsilane (SiHCl 3 ), silicon tetrachloride (SiCl 4 ), pentaethoxytantalum (PEOTa: Ta(OC 2 H h ) 5 ), pentaethoxytantalum (PMOTa: Ta(OCH 3 ) 5 ), tetrasopropoxytitanium (Ti(i-OC 3 H 7 ) 4 ), tetradimethylaminotitanium (TDMAT: Ti(N(CH 3 ) 2 ) 4 ), tetraxisdiethylaminotitanium (TDEAT: Ti(N(C 2 H 5 ) 2 ) 4 ), titanium tetrachloride (TiCl 4 ), Cu(HFA) 2  and Cu(DPM) 2  may be used as liquid material to be vaporized. Further, Ba(DPM) 2 /THF and Sr(DPM) 2 /THF may be used as thin ferroelectric film forming material. Water (H 2 O), ethanol (C 2 H 5 OH), tetrahydofuran (THF: C 4 H 8 O) and dimethylaluminiumhydride (DMAH: (CH 3 ) 2 AlH) may also be used. 
     A vaporizer  819  may be attached to a batch type horizontal plasma CVD apparatus  800 , as shown in FIG.  41 . This CVD apparatus  800  includes a process chamber  814  provided with an exhaust opening  810  and a process gas supply section  812 , a wafer boat  816  and a heater means  818 . Connected to the process gas supply section  812  are a process gas supply line or system having a liquid container  815 , a liquid mass flow controller  817  and a vaporizer  819 . This vaporizer  819  is substantially same in arrangement as the above-described one  732 . 
     As shown in FIG. 42, a conventional vaporizer  701  has a housing  702  which is kept under atmospheric pressure and which is filled with a plurality of heat transmitting balls  703  each being made of material, excellent in heat transmission. These heat transmitting balls  703  are heated higher than the boiling point of liquid material by an external heater means (not shown) to vaporise liquid material introduced from below. Carrier gas is introduced into the vaporizer  701  to carry vaporized process gases. 
     In the conventional vaporizer  701 , however, gas flow rate becomes excessive at the initial stage of gas supply, that is, overshooting is caused. FIG. 43 is a graph showing how gas flow rates attained by the conventional and our vaporizers change at the initial stage of gas supply, in which time lapse is plotted on the horizontal axis and gas flow rates on the vertical axis. A curve P represents results obtained by the conventional vaporizer and another curve Q those obtained by our present vaporizer. As apparent from FIG. 43, gas flow rate overshoots a predetermined one V 1 , in the case of the conventional vaporizer, after the lapse of 10-20 seconds since the supply of gas is started. In the above-described vaporizer used by the present invention, however, it reaches the predetermined flow rate V 1  without overshooting it. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.