Corrosion-resistant system and method for a plasma etching apparatus

A corrosion-resistant system and method for a plasma etching apparatus are provided which are capable of reducing a corrosion or erosion phenomenon of a discharge tube, equipment and/or elements in a chamber of the plasma etching apparatus which is used for localized etching. A micro wave M is oscillated from a micro wave oscillator 20 toward a mixed gas of CF.sub.4 and O.sub.2 in a quartz discharge tube 110 to thereby produce plasma discharge. The micro wave oscillator 20 is controlled in an on-off manner by means of a pulse generator 21, to thereby oscillate a pulsed micro wave M. As a result, it is possible to reduce the erosion of the quartz discharge tube 110 caused by an active species gas G generated by the plasma discharge. Preferably, a corrosion-resistant oil A is filled in the chamber 100 for preventing an X-Y drive mechanism 130, etc., therein from being corroded or eroded by the active species gas G diffusing in the chamber 100.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an corrosion-resistant system and method
 for a plasma etching apparatus in which a relatively thick portion(s) on a
 surface of an object to be etched are locally etched.
 2. Description of the Related Art
 FIG. 5 is a cross section showing an example of a known plasma etching
 apparatus, and FIG. 6 is a diagram showing the state of oscillation of a
 micro wave.
 The plasma etching apparatus performs plasma-discharge using a micro wave.
 A mixed gas containing a halogen-based gas such as CF.sub.4 (carbon
 tetrafluoride) is supplied to a quartz discharge tube 110 which is mounted
 on an upper surface of a chamber 100. As shown in FIG. 6, the micro wave M
 of about 500W is generated or oscillated continuously by a micro wave
 oscillator 120 toward a wave guide 121, so that the mixed gas in the
 quartz discharge tube 110 is made into a plasma state, thus producing
 active species such as fluorine radicals which contribute to etching a
 silicon wafer W.
 On the other hand, the silicon wafer W is fixedly mounted on a stage 101
 which is driven to move in an X-Y direction (i.e., the right and left
 direction as well as the front and rear direction of the sheet of FIG. 5)
 by means of an X-Y drive mechanism 130.
 Specifically, a movable stand 132 carrying thereon a stage 101 is driven to
 move in an X-axis direction by means of a drive motor 131 and in a Y-axis
 direction by means of a drive motor 133 mounted on the movable stand 132.
 With this construction, the active species such as fluorine radicals
 generated by the plasma discharge is jetted from an ejection opening 110a
 of a quartz discharge tube 110 onto a silicon wafer W. At the same time,
 the X-Y drive mechanism 130 moves a relatively thick portion of the
 silicon wafer W (i.e., a portion which forms a surface of the silicon
 wafer W is formed, and which is relatively thicker than a specified
 thickness) right under the ejection opening 110a of the quartz discharge
 tube 110 so that the relatively thick portion can be partially or locally
 etched.
 Here, it is to be noted that during such partial or localized etching, part
 of the active species gas G jetted from the ejection opening 110a might
 diffuse to etch an inner wall of the chamber 100 as well as the X-Y drive
 mechanism 130.
 To avoid this, the plasma etching apparatus employs an corrosion-resistant
 technique.
 That is, the inner wall of the chamber 100 and the X-Y drive mechanism 130
 are subjected to corrosion-resistant coating so that they can be prevented
 from being etched by the active species gas G. Furthermore, even if those
 portions such as threaded portions, rails, bearings of the X-Y drive
 mechanism 130, rotation shafts of the drive motors 131, 133 and so on,
 which are in sliding contact with other elements, are applied with
 corrosion-resistant coatings, such corrosion-resistant coatings would be
 liable to be peeled off during a long period of use. Thus, those
 contacting and sliding portions are coated with a corrosion-resistant oil
 which does not flake off due to repeated sliding actions.
 The above-mentioned plasma etching apparatus and corrosion-resistant
 technique have the following problems.
 A first problem is that since the plasma etching apparatus is constructed
 such that a micro wave M is oscillated or generated continuously so as to
 produce plasma discharge, as illustrated in FIG. 6, the effective period
 or life time of the quartz discharge tube 110 is short and the etching
 rate with respect to the silicon wafer W is low, and the silicon wafer W
 during etching is liable to be contaminated.
 FIG. 7 is a cross section showing the state of corrosion of the quartz
 discharge tube 110.
 If a mixed gas containing a CF.sub.4 (carbon tetrafluoride) gas for
 instance is plasma-discharged, there will be generated an active species
 gas G which contains CF.sub.3 radicals, F radicals, CF.sub.3 cations or
 positive ions, F anions or negative ions, etc. These radials, positive and
 negative ions contribute to the localized etching of the silicon wafer W.
 However, when plasma discharging is continuously conducted, the quartz
 discharge tube 110 continuously absorbs the micro wave, resulting in a
 rapid rise of the heating temperature of the quartz discharge tube 110. As
 a consequence, the reaction of the active species gas G and a SiO.sub.2
 (silicon dioxide) component of the quartz discharge tube 110 is promoted,
 as shown in FIG. 7, so that the inner wall of the quartz discharge tube
 110 is subjected to corrosion, thus forming a hole or holes through the
 quartz discharge tube 110 in a relatively short period of time.
 Furthermore, when the corrosion of the quartz discharge tube 110 has been
 made, the active species gas G reacts with the quartz discharge tube 110
 to turn into a SiF.sub.4 (silicon tetrafluoride) gas, thereby reducing the
 density of the active species gas G ejected to the silicon wafer W to
 lower the etching rate of the silicon wafer W.
 In addition, during the etching of the quartz discharge tube 110, there are
 generated particles of impurities contained in the quartz discharge tube
 110 itself. These particles might be jetted to the surface of the silicon
 wafer W, thus contaminating the silicon wafer W.
 A second problem is that the above-mentioned corrosion-resistant technique
 does not provide a satisfactory corrosion resistant effect.
 Specifically, it is impossible to completely provide all the exposed
 portions of the chamber 100, the X-Y drive mechanism 130 and the like with
 corrosion-resistant coatings. Especially, the X-Y drive mechanism 130 is
 constructed of various members assembled, so it is impossible to coat
 every component member with a corrosion-resistant oil.
 Further, the corrosion-resistant oil applied to the sliding portions
 gradually evaporates so that the base portions underlying the
 corrosion-resistant coatings are exposed during a long period of use. In
 order to avoid this, it is necessary to disassemble the chamber 100 and
 the X-Y drive mechanism 130 regularly or at a predetermined interval and
 re-coat them with the corrosion-resistant oil.
 SUMMARY OF THE INVENTION
 In view of the above, the present invention is intended to obviate the
 above-described problems, and has for its object to provide a
 corrosion-resistant system and a corrosion-resistant method for a plasma
 etching apparatus which are capable of reducing or improving the corrosion
 or erosion phenomena of a discharge tube of the plasma etching apparatus,
 other equipment or elements in a chamber used for localized etching.
 In order to achieve the above object, according to a first aspect of the
 present invention, there is provided a corrosion-resistant system for a
 plasma etching apparatus in which a reactive gas in a discharge tube is
 plasma-discharged by using a micro wave or a high-frequency wave to
 produce an active species gas by means of which a relatively thick portion
 of an object to be etched is locally etched,
 the corrosion-resistant system comprising:
 oscillation means for continuously oscillating the micro wave or the high
 frequency wave; and
 oscillation control means for controlling the oscillation means in an
 on-off manner so that a pulsed micro wave or a pulsed high-frequency wave
 is oscillated from the oscillation means.
 With this arrangement, the micro wave or the high-frequency wave is
 oscillated from the oscillation means under the control of the oscillation
 control means, so that a temperature rise of the discharge tube is
 decreased in comparison with a continuous oscillation, accordingly
 suppressing reactions between active species in a plasma and the discharge
 tube.
 In a preferred form of the corrosion-resistant system for a plasma etching
 apparatus according to the first aspect of the invention, the duty ratio
 of the pulsed micro wave or high-frequency wave is set to 75% or less, and
 the pulse frequency of the pulsed micro wave or high-frequency wave is set
 to 10 kHz or more.
 With this arrangement, the pulsed micro wave or high-frequency wave of the
 duty ratio of 75% or less is oscillated at the pulse frequency of 10 kHz
 or more.
 In another preferred form of the corrosion-resistant system for a plasma
 etching apparatus according to the first aspect of the invention, the
 reactive gas includes a halogen-based gas.
 With this arrangement, the relatively thick portion of the object to be
 etched is locally etched effectively by active species, while suppressing
 the reactions between the discharge tube and the active species of the
 halogen-based gas.
 In a further preferred form of the corrosion-resistant system for a plasma
 etching apparatus according to the first aspect of the invention, the
 halogen-based gas comprises one selected from the group consisting of a
 carbon tetrafluoride gas, a sulfur hexafluoride gas, a nitrogen
 trifluoride gas, and a gas of other fluorocarbon substances.
 With this arrangement, the relatively thick portion of the object to be
 etched is locally etched effectively by active species, while suppressing
 reactions of the discharge tube with the carbon tetrafluoride gas, the
 sulfur hexafluoride gas, the nitrogen trifluoride gas, and the gas of
 other fluorocarbon substances.
 In a still further preferred form of the corrosion-resistant system for a
 plasma etching apparatus according to the first aspect of the invention,
 the discharge tube comprises one selected from the group consisting of a
 quartz discharge tube, a sapphire discharge tube and an alumina discharge
 tube.
 According to a second aspect of the present invention, there is provided an
 corrosion-resistant method for a plasma etching apparatus in which a
 reactive gas in a discharge tube is plasma-discharged to produce an active
 species gas by means of which a relatively thick portion of an object to
 be etched is locally etched, the method comprising the step of producing
 the reactive gas by irradiating a pulsed micro wave or a high-frequency
 wave to the discharge tube of the plasma etching apparatus.
 In a preferred form of the corrosion-resistant method for a plasma etching
 apparatus according to the second aspect of the invention, the duty ratio
 of the pulsed micro wave or high-frequency wave is set to 75% or less, and
 the pulse frequency of the pulsed micro wave or high-frequency wave is set
 to 10 kHz or more.
 In another preferred form of the corrosion-resistant method for a plasma
 etching apparatus according to the second aspect of the invention, the
 reactive gas includes a halogen-based gas.
 In a further preferred form of the corrosion-resistant method for a plasma
 etching apparatus according to the second aspect of the invention, the
 halogen-based gas comprises one selected from the group consisting of a
 carbon tetrafluoride gas, a sulfur hexafluoride gas, a nitrogen
 trifluoride gas, and a gas of other fluorocarbon substances.
 In a still further preferred form of the corrosion-resistant method for a
 plasma etching apparatus according to the second aspect of the invention,
 the discharge tube comprises one selected from the group consisting of a
 quartz discharge tube, a sapphire discharge tube and an alumina discharge
 tube.
 According to a third aspect of the present invention, there is provided an
 corrosion-resistant system for a plasma etching apparatus comprising:
 a chamber in which an object to be etched is received;
 a position control and drive mechanism provided in the chamber for placing
 the object at a predetermined position; and
 a halogen-based gas in the chamber adapted to be plasma discharged to
 produce an active species gas by which the object is etched;
 wherein the position control and drive mechanism is soaked in a
 corrosion-resistant oil which does not react with the active species gas
 and which is received in the chamber.
 With this arrangement, since the position control and drive mechanism is
 soaked in the corrosion-resistant oil which does not react the active
 species gas, even if the diffusing active species gas flows toward the
 position control and drive mechanism, it is possible to prevent the
 position control and drive mechanism from being corroded or eroded by the
 active species gas.
 In a preferred form of the corrosion-resistant system for a plasma etching
 apparatus according to the third aspect of the invention, the
 halogen-based gas comprises one selected from the group consisting of a
 carbon tetrafluoride gas, a sulfur hexafluoride gas, a nitrogen
 trifluoride gas and a gas of other fluorocarbon substances, and the
 corrosion-resistant oil comprises a perfluoropolyether oil which is
 resistant to fluorine.
 With this arrangement, even if the fluorine-containing active species gas
 diffuses which comprises one selected from the group consisting of a
 carbon tetrafluoride gas, a sulfur hexafluoride gas, a nitrogen
 trifluoride gas and a gas of other fluorocarbon substances, such diffusion
 is blocked by the perfluoropolyether oil which is resistant to fluorine,
 thus preventing the position control and drive mechanism from being
 corroded or eroded by the fluorine-containing active species gas.
 In another preferred form of the corrosion-resistant system for a plasma
 etching apparatus according to the third aspect of the invention, the
 system further comprises:
 an oil passage disposed outside the chamber for circulating the
 corrosion-resistant oil in the chamber; and
 filtering means detachably mounted on the oil passage for filtering foreign
 matters which are mixed in the corrosion-resistant oil.
 With this arrangement, the foreign matters mixed in the circulating
 corrosion-resistant oil can be removed by the filtering means, so that the
 cleanliness of the corrosion-resistant oil can be maintained for an
 extended period of time.
 In a further preferred form of the corrosion-resistant system for a plasma
 etching apparatus according to the third aspect of the invention, the
 system further comprises:
 oil feeder means for feeding the corrosion-resistant oil to the chamber;
 oil discharge means for discharging the corrosion-resistant oil from the
 chamber;
 an oil level sensor for detecting a surface level of the
 corrosion-resistant oil and generating a corresponding detection signal;
 and
 first control means for controlling the oil feeder means and the oil
 discharge means based on the detection signal from the oil level sensor in
 such a manner as to maintain the surface level of the corrosion-resistant
 oil at a predetermined reference position.
 With this arrangement, it is possible to always maintain the level or
 surface of the corrosion-resistant oil constant, thereby preventing such a
 situation that the position control and drive mechanism is exposed from
 the corrosion-resistant oil.
 In a still further preferred form of the corrosion-resistant system for a
 plasma etching apparatus according to the third aspect of the invention,
 the system further comprises:
 a pH sensor for detecting the pH of the corrosion-resistant oil and
 generating a corresponding detection signal: and
 second control means for actuating, based on the detection signal from the
 pH sensor, the oil discharge means when the pH of the corrosion-resistant
 oil turns into a value equal to or less than a predetermined reference pH
 value.
 With this arrangement, when the pH of the corrosion-resistant oil decreases
 below the predetermined reference pH value, the corrosion-resistant oil is
 exhausted from the chamber by the oil discharge means.
 In a yet further preferred form of the corrosion-resistant system for a
 plasma etching apparatus according to the third aspect of the invention,
 the system further comprises cooling means for cooling the
 corrosion-resistant oil to a predetermined temperature.
 With this arrangement, the corrosion-resistant oil is always maintained at
 a predetermined temperature by virtue of the cooling means.
 The above and other objects, features and advantages of the present
 invention will be more readily apparent from the following detailed
 description of preferred embodiments of the invention taken in conjunction
 with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS
 Now, preferred embodiments of the present invention will be described in
 detail with reference to the accompanying drawings.
 FIG. 1 schematically illustrates in cross section a corrosion-resistant
 system for a plasma etching apparatus constructed in accordance with a
 first embodiment of the present invention. The same or corresponding
 elements of this embodiment as those of FIG. 5 are identified with the
 same references or symbols.
 In FIG. 1, a reference numeral 1 designates a plasma etching apparatus, and
 a reference numeral 2 a corrosion-resistant system according to a first
 aspect of the present invention. Also, a reference numeral 3 designates a
 corrosion-resistant system according to a third aspect of the invention.
 The plasma etching apparatus 1 is substantially of the same construction as
 that of the plasma etching apparatus shown in FIG. 5. The plasma etching
 apparatus 1 includes a quartz discharge tube 110 mounted on an upper
 portion of a chamber 100, a wave guide 121 disposed outside of the quartz
 discharge tube 110, a micro wave oscillators 20 to be described later in
 detail for oscillating or generating a predetermined micro wave M in the
 wave guide 121, and an X-Y drive mechanism 130 disposed in the chamber
 100.
 The quartz discharge tube 110 is an elongated tubing formed of SiO.sub.2
 and has an ejection opening 110a located inside the chamber 100.
 Gas cylinders 141, 142 are connected to the quartz discharge tube 110
 through supply pipes 140. A CF.sub.4 gas and an O.sub.2 gas are stored in
 the gas cylinder 141, 142, respectively. When valves in the supply tubes
 140 are opened, the CF.sub.4 gas and the O.sub.2 gas in the gas cylinders
 141, 142 are controlled in their flow rates by means of mass flow
 controllers 141a, 142a, respectively, and mixed with each other to form a
 mixed reactive gas, which is then supplied to the quartz discharge tube
 110.
 The wave guide 121 transmits the micro wave M generated by the micro wave
 oscillator 20. Inside one end (i.e., the left-hand side end in FIG. 1) of
 the wave guide 121, there is disposed a reflecting plate 122 in the form
 of a short plunger which serves to reflect the Micro wave M to thereby
 form a standing wave. Furthermore, disposed in the wave guide 121 are
 three stub tuners 123 for matching the phase of the micro wave M, and an
 isolator 124 which bends the reflected micro wave M transmitted to the
 micro wave oscillator 20 at an angle of 90.degree. with respect to the
 transmission direction thereof (i.e., in the direction of the surface of
 FIG. 1).
 On the other hand, the X-Y drive mechanism 130 comprises a movable stand
 132 mounted on a rail 132a on a bottom 100a of the chamber 100 for sliding
 movement relative thereto, a drive motor 131 for driving the movable stand
 132 in an X direction (i.e., in the front and rear direction of FIG. 1 )
 along the rail 132a, and a drive motor 133 mounted on the movable stand
 132 for moving the stage 101 in a Y direction (i.e., in the right and left
 direction of FIG. 1) while supporting the stage 101. The drive motors 131,
 133 are driven to operate under the control of a computer 135.
 The corrosion-resistant system 2 serves to generate a pulsed micro wave M
 in the wave guide 121. The corrosion-resistant system 2 has an oscillation
 means in the form of the micro wave oscillator 20, and an oscillation
 control means in the form of a pulse generator 21.
 The micro wave oscillator 20 can be a well-known device which is turned on
 to generate a continuous micro wave M when it receives a drive signal S of
 the high level from the pulse generator 21. The micro wave oscillator 20
 can generate the continuous micro wave M in the power range of from 100 W
 to 1 kW.
 The pulse generator 21 generates the pulsed drive signal S for driving the
 micro wave oscillator 20. The pulse width and the pulse frequency of the
 drive signal S can be arbitrarily adjusted.
 With this arrangement, when the pulsed drive signal S is input to the micro
 wave oscillator 20, as shown in FIG. 2(a), the micro wave oscillator 20 is
 turned on upon rising of a rectangular wave of the drive signal S, as
 shown in FIG. 2(b). On the other hand, when the drive signal S falls into
 the L level, the micro wave oscillator 20 is turned into an Off state.
 Thus, the micro wave oscillator 20 continuously oscillates the micro wave
 M of the power ranging from 100 W to 1 kW only for the length of time of a
 rectangular wave of the drive signal S. Accordingly, the micro wave
 oscillator 20 oscillates the pulsed micro wave M in synchronization with
 the drive signal S from the pulse generator 21, so that changing the pulse
 width and the frequency of the drive signal S can adjust the duty ratio
 and the pulse frequency of the micro wave M.
 On the other hand, the corrosion-resistant system 3 serves to prevent the
 corrosion or erosion of the chamber 100 and the X-Y drive mechanism 130
 which constitutes a position control and drive mechanism for controlling
 the position of an object to be etched. The corrosion-resistant system 3
 includes a corrosion-resistant oil A in the chamber 100 in which the X-Y
 drive mechanism 130 is soaked or immersed, a fluid passage in the form of
 a circulation pipe 30 for circulation of the corrosion-resistant oil A, a
 filtering means 31, and a valve controller 32 to which an oil level sensor
 33, a pH sensor 34, and a pressure sensor 35 are connected. The valve
 controller 32 constitutes a first controller and a second controller as
 claimed of the present invention.
 The inner wall of the chamber 100 and the surface of the stage 101 are
 subjected to corrosion-resistant coating such as Teflon coating. The
 corrosion-resistant oil A is stored in the chamber 100 thus applied with
 the corrosion-resistant coating.
 The corrosion-resistant oil A can be perfluoropolyether oil such as for
 example FOMBLIN made and sold by the MONTEFLUOS Company, R-95 made and
 sold by the ULVAC Company, or the like.
 The corrosion-resistant oil A has a property to hardly react with the
 active species such as F radicals based on halogen-based gases. The vapor
 pressure of the corrosion-resistant oil A is about 10.sup.13 Torr of which
 the influence on the etching process can be neglected.
 A pump 4 is mounted on the circulation pipe 30 for circulating the
 corrosion-resistant oil A through the circulation pipe 30, as indicated by
 an arrow in FIG. 1.
 Also, the pump 4 is connected with a supply pipe 40 through which the
 corrosion-resistant oil A in the tank 41 is supplied to the chamber 100
 when a valve 50 is opened, the valve 50 cooperating with the pump 4 to
 constitute an oil feeder means.
 The filtering means 31 serves to filter foreign matter in the form of
 particles created or contained in the corrosion-resistant oil A. The
 filtering means 31 has two filters 31a and 31b.
 Specifically, an upstream portion of the circulation pipe 30 is bifurcated
 into two branch pipes 30a, 30b, and a downstream of the circulation pipe
 30 is also bifurcated into two branch pipes 30a', 30b'. The filter 31a is
 disposed between and detachably connected with the pipes 30a, 30a'. The
 filter 31b is disposed between and detachably connected with the pipes
 30b, 30b'.
 With this arrangement, when the valves 51a, 51b and the valves 52a, 52b are
 opened, respectively, the corrosion-resistant oil A flows in the filters
 31a, 31b, respectively, where particles contained therein are filtered. In
 use, only one of the valves 51a, 51b or the valve 52a, 52b is put into an
 open state.
 A discharge pipe 42 is mounted to the upstream portion of such a filtering
 means 31, so that the corrosion-resistant oil A is discharged or exhausted
 to a tank 43 through the discharge pipe 42 upon opening of an oil
 discharge means in the form of a valve 53.
 The valve controller 32 serves to control the opening and closing of the
 valves 50-53 based on the detection signals from the oil level sensor 33,
 the pH sensor 34, and the pressure sensor 35.
 Specifically, the oil level sensor 33 is mounted on a sidewall of the
 chamber 100 for generating a detection output signal S1 representative of
 the detected level or surface position of the corrosion-resistant oil A to
 the valve controller 32. The valve controller 32 determines, upon
 reception of the detection signal S1, whether the oil level indicated by
 the detection signal S is a reference level or position L. Then, the valve
 controller 32 opens the valve 53 when it is determined that the detected
 oil level indicated by the detection signal S1 is lower than the reference
 level L, and closes the valve 50 when it is determined that the detected
 oil level substantially matches or is equal to the reference level L. In
 addition, the valve controller 32 opens the valve 53 when it is determined
 that the detected oil level is higher than the reference level L.
 There may be a case in which during etching, a slight amount of moisture or
 water and the active species gas G would react with each other to generate
 acids such as hydrofluoric acid, etc., which are then dissolved into the
 corrosion-resistant oil A to lower the pH thereof. If a large amount of
 acid is dissolved in the corrosion-resistant oil A, the respective
 elements of the X-Y drive mechanism 130 might be oxidized, which is
 undesirable. To avoid this, the pH sensor 34 is provided at the upstream
 portion of the filtering means 31 for detecting the pH of the
 corrosion-resistant oil A which flows in the circulation pipe 30. When the
 pH of the corrosion-resistant oil A is equal to or less than a
 predetermined reference pH value (e.g., pH 3), the pH sensor 34 outputs a
 detection signal S2 to the valve controller 32. The valve controller 32
 functions to open the valve 53 while closing both the valves 51a, 51band
 the valves 52a, 52b upon receipt of the detection signal S2 from the pH
 sensor 34, and close the valve 53 and open the valve 50 and the valves
 51a, 51b (or the valves 52a, 52b) after the lapse of a predetermined time
 at which the corrosion-resistant oil A is expected to be completely
 discharged.
 In the case where one in use of the filters 31a, 31b is clogged by
 particles admixed in the corrosion-resistant oil A, the pressure of the
 corrosion-resistant oil A at the upstream side of the clogged filter. The
 pressure sensor 35 observes or detects this pressure rise, and generates a
 detection signal S3 to the valve controller 32 when the pressure of the
 corrosion-resistant oil A rises above a predetermined pressure value. The
 valve controller 32 closes the now open ones of the valve 51a, 51b and
 valves 52a, 52b and opens the now closed ones of the valves 51a, 51b and
 valves 52a, 52b upon receipt of the detection signal S3 from the pressure
 sensor 35.
 A cooling device 36 is mounted on a pipe portion at an upstream portion of
 the pump 4 for cooling the corrosion-resistant oil A. Specifically, the
 cooling device 36 observes or monitors the temperature of the
 corrosion-resistant oil A by means of the temperature sensor 36a mounted
 on the circulation pipe 30, and cools the corrosion-resistant oil A so as
 to prevent the vapor pressure of the corrosion-resistant oil A from being
 raised excessively due to a rise in the temperature thereof.
 Next, the operations of the corrosion-resistant systems 2, 3 of this
 embodiment will be described below. Here, it is to be noted that these
 operations are to realize a corrosion-resistant method carried out by the
 plasma etching apparatus in accordance with a second aspect of the present
 invention.
 First of all, the operation of the corrosion-resistant system 2 is
 described.
 The CF.sub.4 gas in the gas cylinder 141 and the O.sub.2 gas in the gas
 cylinder 142 are supplied in a mixed state to the quartz discharge tube
 110 with their flow rates being controlled by means of the mass flow
 controllers 141a, 142a.
 Thereafter, the micro wave oscillator 20 is energized to oscillate a micro
 wave M through which the mixed gas comprising the CF gas and the O.sub.2
 gas is plasma-discharged. As a result, there is generated an active
 species gas G such as CF.sub.3 radicals and F radicals which is ejected to
 an object to be etched in the form of a silicon wafer W from the ejection
 opening 110a of the quartz discharge tube 110.
 Then, the X-Y drive mechanism 130 is controlled by the computer 135 to move
 the silicon wafer W such that the relatively thick portion on the surface
 of the silicon wafer W comes right under the ejection opening 110a of the
 quartz discharge tube 110. In this state, the relatively thick portion of
 the silicon wafer W is etched by the active species gas G.
 At this time, the drive signal S as shown in FIG. 2(a) is input from the
 pulse generator 21 to the micro wave oscillator 20 to thereby generate a
 pulsed micro wave M, as shown in FIG. 2(b), by means of which the mixed
 gas in the quartz discharge tube 110 is plasma-discharged. In this case,
 the drive signal S of the pulse generator 21 is adjusted so as to set the
 duty ratio and the pulse frequency of the micro wave M to respective
 desired values, and at the same time, the phase of the micro wave M is
 adjusted by the three stub tuners 123 so that the maximum power of the
 micro wave M is concentrated in the quartz discharge tube 110.
 FIGS. 3(a) and 3(b) show, in cross section, the different states of the
 plasma generated by the pulsed micro wave M. FIG. 3(a) shows the state of
 a high density plasma generated upon turning on of the micro wave M, and
 FIG. 3(b) shows the state of the plasma remaining upon turning off of the
 micro wave M.
 Under the conditions as set in the above manner, a pulsed micro wave M is
 oscillated, and upon turning on of the pulsed micro wave M, i.e., when the
 pulse M1 shown in FIG. 2(b) is applied to the quartz discharge pipe 110, a
 plasma P is generated in the quartz discharge tube 110, as shown in FIG.
 3(a).
 This plasma P maintains its excited state only for a period of time T1
 corresponding to the width of the pulse M1, during which active species
 gases G such as CF.sub.3 radicals, F radicals, etc., are jetted from the
 ejection opening 110a of the quartz discharge tube 110 toward the silicon
 wafer W.
 Subsequently, when the oscillation of the micro wave M is stopped, the
 plasma-excitation action of the micro wave M is released or stopped so
 that the electron temperature and the plasma density decrease rapidly, but
 the density of the active species gas G is kept almost constant for a
 period of time T2.
 When the following pulse M2 is applied to the quartz discharge tube 110, a
 plasma P shown in FIG. 3(a) is generated again.
 In this manner, the plasma discharge is effected in a pulsed manner.
 FIG. 4 diagrammatically illustrates a rise in the temperature of the quartz
 discharge tube 110.
 With the conventional plasma etching apparatus in which a micro wave M is
 always oscillated continuously, the heating temperature of the quartz
 discharge tube 110 rises rapidly, as shown in a curve C in FIG. 4.
 In contrast to this, according to this embodiment, there is a cooling
 period of time T2 so the heating temperature of the quartz discharge tube
 110 rises and falls in a repeated manner, as indicated at a curve B in
 FIG. 4. Thus, the heating temperature of the quartz discharge tube 110
 does not rise so much even if the apparatus is continuously used for an
 extended period of time.
 As a consequence, the number or frequency of reactions between the active
 species gas G in the plasma P and the quartz discharge tube 110 is quite
 limited in comparison with the conventional apparatus, thus reducing the
 corrosion or erosion degree of the quartz discharge tube 110 due to the
 active species gas G and hence increasing the useful life of the quartz
 discharge tube 110. In addition to this, the generation of particles
 occurring upon erosion or corrosion of the quartz discharge tube 110
 decreases sharply, too, so there will be little or no contamination of the
 silicon wafer W due to particles.
 Furthermore, since active species reacting with the quartz discharge tube
 110 decreases, the active species gas G of a desired density can be
 ejected to the silicon wafer W, thus making it possible to ensure a
 desired etching rate of the silicon wafer W.
 In addition, since the micro wave oscillator 20 is turned on in an
 intermittent manner, it is possible to reduce the electric power
 consumption during oscillation of the micro wave M.
 In order to demonstrate these matters, the inventors conducted the
 following experiment. That is, a pulsed micro wave M having an output
 power of about 500 W is oscillated at the duty ratio of 75% and at the
 pulse frequency of 10 kHz to generate an active species gas G, which is
 then ejected to a silicon wafer W having particles attached thereto, the
 number of the particles having a diameter of less than 1 .mu.m being not
 greater than 100 at the original state of the silicon wafer. The
 corrosion-resistant time of the quartz discharge tube 110 and the number
 of particles attached to the silicon wafer W after the above treatment are
 examined.
 Then, it was found that the quartz discharge tube 110 was destroyed about
 70 hours later from the start, and the number of attached particles after
 the lapse of about 100 hours was less than 1,000.
 In contrast to this, when a micro wave having an output power of about 500
 W is continuously oscillated as in the conventional plasma etching
 apparatus, the quartz discharge tube 110 was destroyed in about 40 hours,
 and the number of attached particles after about 100 hours also exceeded
 more than a predetermined measurement limit.
 In this manner, according to the corrosion-resistant system 2 of this
 embodiment, the corrosion-resistant period of the quartz discharge tube
 110 can be increased, and the contamination of the silicon wafer W can be
 decreased sharply.
 In order to enhance the corrosion-resistant effect of the quartz discharge
 tube 110, it is preferred that the time of the pulse width T1 of the micro
 wave M as indicated in FIG. 2(b) be short, but if it is too short, it is
 impossible to obtain an active species gas G of a desired density.
 Moreover, a plasma P of a high density as indicated in FIG. 3(a) is
 produced in the quartz discharge tube 110 during the on-time or
 oscillation of the micro wave M, i.e., during the time T1 of the pulse
 width of the micro wave M as indicated in FIG. 2(b). On the other hand,
 the plasma P disappears when the oscillation of the micro wave M is turned
 off or stopped, i.e., during the period of time T2 indicated in FIG. 2(b).
 However, the plasma P does not disappear immediately upon turning off of
 the micro wave M, but there remain charged particles such as ions,
 electrons, etc., which constitute the plasma P, for some time. For this
 reason, it is considered that a plasma can be ignited readily by turning
 on the oscillation of the micro wave M before the charged particles
 disappear.
 In view of such a consideration, in order to conduct plasma discharge in a
 smooth manner by applying a pulsed micro wave M to the interior of the
 quartz discharge tube 110, it is desirable to shorten the time T2 or the
 pulse interval as indicated in FIG. 2(b). However, too short the time T2
 not only results in an increased power consumption but also in a shortened
 cooling time for the quartz discharge tube 110, thus making it difficult
 to suppress the rising of the heating temperature of the quartz discharge
 tube 110.
 From the above, the inventors reached the following conclusion. Namely, as
 a result of consideration about the duty ratio and the pulse frequency of
 the micro wave M, which are capable of obtaining a desired density of the
 active species gas while enhancing the corrosion-resistant effect of the
 quartz discharge tube 110, as well as the duty ratio and the pulse
 frequency of the micro wave M, which are capable of expediting smooth
 plasma discharge while suppressing the rising of the heating temperature
 of the quartz discharge tube 110 with a limited amount of power
 consumption, it is preferable to employ a micro wave M having the duty
 ratio of not greater than 75% and the pulse frequency of not less than 10
 kHz.
 Next, the operation of the corrosion-resistant system 3 will be described
 below.
 That portion of the active species gas G of a predetermined density ejected
 from the ejection opening 110aof the quartz discharge tube 110 which
 diffuses without contributing to the etching of the silicon wafer W is
 about to etch the inner wall oft e chamber 100 and the stage 101. However,
 since the inner wall of chamber 100 and the surface of the stage 101 are
 applied with corrosion-resistant coatings, the active species gas G is
 blocked by the corrosion-resistant coatings, so neither the inner wall of
 the chamber.100 nor the stages 101 are subjected to etching.
 Furthermore, the active species gas G flowing toward the X-Y drive
 mechanism 130 is about to etch it due to its activity, but it is blocked
 from advancing toward the X-Y drive mechanism 130 by means of the
 corrosion-resistant oil A in which the X-Y drive mechanism 130 is
 immersed. Thus, the component elements of the X-Y drive mechanism 130 are
 prevented from being etched by the active species gas G.
 Here, it is to be noted that in cases where particles are admixed in the
 corrosion-resistant oil A which prevents the X-Y drive mechanism 130 from
 being etched by the active species gas G as referred to above, the drive
 motors 131, 133 of the X-Y drive mechanism 130 would be caused into
 trouble in operation owing to the particles, thus resulting in a situation
 that the silicon wafer W could not be moved precisely.
 To avoid such a situation, the pump 4 is driven to operate with the valves
 50, 52a, 52b, 53 being closed and the valves 51a and 51b opened, so that
 the corrosion-resistant oil A is thereby forced to circulate through the
 circulation pipe 30.
 With circulation of the corrosion-resistant oil A, the particles contained
 or mixed therein are removed by means of the filter 31a of the filtering
 means 31.
 When a large amount of particles have been accumulated to clog the filter
 31a, the pressure at the upstream side of the filter 31a, e.g., the
 pressure of the corrosion-resistant oil A near the pressure sensor 35
 rises to reach a predetermined pressure value.
 Then, a detection signal S3 is output from the pressure sensor 35 to the
 valve controller 32, so that the valves 51a, 51b are closed and the valves
 52a, 52b are opened under the control of the valve controller 32.
 As a result, the corrosion-resistant oil A circulating through the
 circulation pipe 30 comes to pass through the filter 31b by way of the
 pipe 30b. In this state, the clogged filter 31a is disconnected and
 removed from the pipes 30a, 30a' and replaced with a new filter 31 a which
 is mounted and connected between the pipes 30a and 30a', whereby it is
 possible to always circulate the corrosion-resistant oil A in a clean
 state.
 When acids are generated in the corrosion-resistant oil A to lower the pH
 of the corrosion-resistant oil A to a value of 3 or less, a detection
 signal S2 is input from the pH sensor 34 to the valve controller 32, so
 that the valves 51a, 51b and the valves 52a, 52b are closed, and the valve
 53 is opened, thereby causing the corrosion-resistant oil A to be
 discharged or exhausted from the discharge pipe 42 to the tank 43.
 At this time, the oil level sensor 33 is maintained in an off state, so
 that the valve 53 is closed and the valve 52a, 52b are opened after the
 corrosion-resistant oil A in the chamber 100 and the circulation pipe 30
 has been completely exhausted.
 In this state, the oil level sensor 33 is turned on so that a detection
 signal S1 is output from the oil level sensor 33 to the valve controller
 32, whereby the valve 50 is opened to supply a new corrosion-resistant oil
 A in the tank 41 to the chamber 100 through the supply pipe 40. When the
 level or surface position of the corrosion-resistant oil A comes to match
 the reference level L where the X-Y drive mechanism 130 is completely
 soaked or immersed in the corrosion-resistant oil A, the valve 50 is
 closed.
 When the corrosion-resistant oil A decreases due to an extended period of
 use, the valve controller 32 opens the valve 50 based on a detection
 signal S1 from the oil level sensor 33, thereby supplying the
 corrosion-resistant oil A in the tank 41 to the chamber 100. As a
 consequence, the oil level or surface position of the corrosion-resistant
 oil A in the chamber 100 is always kept to the reference level
 When the temperature of the corrosion-resistant oil A rises due to the
 active species gas G of a high temperature jetted from the chamber 100,
 such a temperature rise is immediately detected by means of the
 temperature sensor 36a, whereby the corrosion-resistant oil A is cooled by
 means of the cooling device 36, thus restoring the temperature of the
 corrosion-resistant oil A to the ordinary or room temperature.
 In this manner, according to the corrosion-resistant system 3, the X-Y
 drive mechanism 130 is subjected to corrosion-resistant treatments or
 coatings, and the cleanliness of the corrosion-resistant oil A is always
 maintained clean, and the pH value thereof is also kept substantially
 constant.
 In order to demonstrate the above effects, the inventors conducted the
 following experiments.
 First of all, the plasma etching apparatus 1 was operated for 23 hours
 without circulating the corrosion-resistant oil A, and then the chamber
 100 was opened or released to the ambient atmosphere for 1 hour. Such a
 severe cyclic operation was carried out for 10 days in a continuous
 manner. After this long-run operation, the X-Y drive mechanism 130 was
 disassembled for examination of the level of corrosion or erosion thereof.
 The X-Y drive mechanism 130 was not corroded at all except for a slight
 change in the color of the corrosion-resistant oil A, and hence a
 satisfactory corrosion-resistant effect was obtained.
 Subsequently, a similar operation was done while circulating the
 corrosion-resistant oil A. After this operation, it was found that a
 satisfactory corrosion-resistant effect on the X-Y drive mechanism 130 was
 obtained and there was no color change in the corrosion-resistant oil A.
 Moreover, the amount of particles in the corrosion-resistant oil A was one
 twentieth as compared with the case in which the corrosion-resistant oil A
 was not circulated.
 Further, water was mixed into the corrosion-resistant oil A at the weight
 ratio (i.e., ratio of water to oil ) of 1%, and a similar operation was
 done with the exception that when the pH of the corrosion-resistant oil A
 decreased to a value of 3 or less, the deteriorated corrosion-resistant
 oil A was exchanged for a new corrosion-resistant oil under the control of
 the pH sensor 34 and the valve controller 32. As a result of this
 operation, it was found that there took place no oxidation phenomenon at
 all.
 It should be noted that the present invention is not limited to the
 above-mentioned embodiments but various modifications and/or changes
 thereof can be made within the spirit and scope of the invention as
 defined in the appended claims.
 For instance, in the above-mentioned embodiments, the CF.sub.4 gas was used
 for the halogen-based gas constituting the reactive gas, but a SF.sub.6
 (sulfur hexafluoride) gas, a NF.sub.3 (nitrogen trifluoride) gas, and
 other fluorocarbon substances can instead be used, while providing the
 same or similar corrosion-resistant effect with respect to the active
 species gas produced from these gases.
 Furthermore, although in the above-mentioned embodiments, the plasma
 etching apparatus oscillating a micro wave has been described, the present
 invention is applicable to a variety of plasma etching apparatuses which
 are capable of oscillating a high-frequency wave to generate plasma
 discharge.
 In addition, in the above-mentioned embodiments, the quartz discharge tube
 110 is subjected to corrosion-resistant treatments or coatings, but every
 discharge tube such as a sapphire discharge tube, an alumina discharge
 tube, etc., which might be etched by active species gases, can be treated
 by corrosion-resistant coatings.
 Moreover, in the above-mentioned embodiments, the corrosion-resistant
 coating has been applied to the inner wall of the chamber 100 and the
 stage 101 alone which are not soaked in the corrosion-resistant oil A, but
 the X-Y drive mechanism 130, the valves 50-53, etc., which are soaked in
 the corrosion-resistant oil A, can also be applied with
 corrosion-resistant coatings, thus improving the overall
 corrosion-resistant effect
 In addition, although in the above-mentioned embodiments, the
 perfluoropolyether oil has been used for the corrosion-resistant oil, any
 corrosion-resistant oil can be employed which has a vapor pressure equal
 to or less than 10.sup.-6 Torr, and hence the corrosion-resistant oil to
 be used is not limited to the perfluoropolyether oil.
 As described above in detail, the present invention provides the following
 remarkable advantages.
 According to the corrosion-resistant system and method for a plasma etching
 apparatus of the first and second aspects of the present invention, the
 discharge tube is only exposed to a plasma for a period of time
 corresponding to a pulse width of a micro wave or a high-frequency wave,
 so the temperature of the discharge tube heated by such a plasma does not
 rise rapidly. As a result, the corrosion or erosion of the discharge tube
 can be reduced, accordingly prolonging the useful life of the discharge
 tube.
 Further, corrosion or erosion of the discharge tube caused by the active
 species gas, that is, reactions between the active species gas and the
 discharge tube decreases, accordingly preventing a decrease in the density
 of the active species gas. As a result, an object to be etched can be
 etched by a desired etching rate.
 Furthermore, the reduced corrosion or erosion of the discharge tube serves
 to suppress the generation of particles of impurities etc., thus
 decreasing the contamination of an object to be etched.
 In addition, the micro wave or the high-frequency wave oscillated in a
 pulsed manner serves to reduce the electric power consumption.
 Moreover, according to the corrosion-resistant system for a plasma etching
 apparatus of the third aspect of the invention, the chamber and the
 position control and drive mechanism are covered with the
 corrosion-resistant coating and the corrosion-resistant oil substantially
 in a complete manner so as to ensure their corrosion resistance, as a
 consequence of which the life time of the plasma etching apparatus can be
 prolonged and the frequency of maintenance can be reduced.
 Still further, due to the provision of the filtering means, foreign matter
 in the corrosion-resistant oil can be removed by the filtering means, so
 that the corrosion-resistant oil can always be kept clean, thus prolonging
 the useful life of the corrosion-resistant oil.
 Furthermore, due to the provision of the oil level sensor and the first
 controller, the level or surface of the corrosion-resistant oil can always
 be maintained constant, thus preventing exposure of the position control
 and drive mechanism. This serves to prevent the position control and drive
 mechanism from being corroded or eroded by the active species gas in a
 reliable manner.
 Further, the corrosion-resistant oil is discharged or exhausted from the
 chamber by an oil discharge means when the pH of the corrosion-resistant
 oil decreases below the predetermined reference pH value, as a consequence
 of which it is possible to avoid such a situation that the position
 control and drive mechanism would be oxidized by acids in the
 corrosion-resistant oil.
 Finally, it is constructed such that the corrosion-resistant oil is at all
 times cooled and kept to a predetermined temperature by means of a cooling
 device. With this construction, it s possible to prevent an increase in
 the vapor pressure of the corrosion-resistant oil due to a temperature
 rise thereof, and hence resultant process contamination as well.