Etching method and apparatus

A reactive ion etching method is described in which a silicon compound film formed on an underlying layer or a substrate is etched through a mask layer by a two-stage procedure. In the two-stage procedure, part of the silicon compound film is first etched with a gas containing a hydrogen-free carbon fluoride gas at a high etching rated and then with a gas containing a hydrogen-containing carbon fluoride gas while reducing the damage on the underlying layer or substrate. The apparatus for carrying out the method is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an etching method and apparatus which is 
utilizable for etching in the manufacture of electronic devices such as 
semiconductor devices. 
2. Description of the Prior Art 
Etching techniques have been employed in various fields, in which there is 
a demand for higher etching rates in order to improve productivity. 
For instance, in the field of electronic materials or the production of 
semiconductor devices, there is a strong demand of increasing the etching 
rate with an increasing size or diameter of semiconductor wafer and an 
increasing degree of fineness of pattern. For in-plane uniform etching of 
a fine pattern in a large-sized wafer, conventional batch systems wherein 
a number of wafers are treated at one time are not appropriate. An etching 
process wherein wafers are treated one by one is desirable. However, the 
treating time in the one-by-one process is only for one wafer with a 
longer time being required than in the batch system. Accordingly, it is 
necessary to increase the etching rate so as to increase a treating 
efficiency. 
Some one-by-one etching techniques have been put into practice but cannot 
be applied to all materials to be treated. For instance, an etching rate 
for SiO.sub.2 is not so high as to obtain the same throughput as in known 
batch systems. 
For high speed etching, problems involved by the high speed of etching have 
to be solved. 
One of the problems is a damage-preventing problem and another is a problem 
on uniformity of the treatment. 
As for the damage prevention, the etching rate may be increased by using a 
great etching energy or a highly reactive gas. However, such use has a 
fear of giving damages on an underlying layer. Although the requirements 
for high speed etching and reduction of the damage are contrary to each 
other, both requirements have to be satisfied for realizing the high 
etching rate. 
With respect to the uniformity of the treatment, it may be sacrificed if 
the energy is increased. For example, if the uniformity is impeded under 
high rate conditions by adoption of the one-by-one treatment, it will be 
meaningless to use such a treatment. In Japanese Patent No. 60-11109, 
there is described a magnetron dry etching method using a high rate 
treatment with reduced damages. However, there is left a problem on the 
uniformity. On the other hand, Japanese Patent Publication No. 57-44749 
discloses an etching technique wherein uniform treatments are possible 
with reduced damages. However, a further improvement is desirable with 
respect to the high speed etching. 
Thus, there is a demand for etching techniques which ensure high speed 
etching and can solve the problems which will be ordinarily involved in 
the high speed etching, e.g. problems on the damages and the uniformity as 
discussed above. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide an etching method 
wherein etching is performed at high rate with a reduced degree of damages 
on underlying layer or layers. 
It is another object of the invention to provide an etching apparatus which 
is suitable for etching at high rate with good uniformity of the etching 
treatment. 
It is a further object of the invention to provide an etching apparatus 
which is suitable for etching at high rate with a reduced degree of 
damages of underlying layers. 
According to one embodiment of the invention, there is provided a reactive 
ion etching method which comprises the steps of: 
forming a mask layer on a silicon compound film in a pattern; 
etching part of the silicon compound film along a depth of the film through 
the mask layer by means of a gas containing a hydrogen-free carbon 
fluoride gas; and 
further etching the remaining portion of the silicon compound film along 
the depth with a gas containing a hydrogen-containing carbon fluoride gas. 
According to another embodiment of the invention, there is provided a 
cathode-coupling parallel plate-type magnetron reactive ion etching (RIE) 
apparatus for carrying out the etching method, the apparatus comprising: 
an anode electrode; 
a cathode electrode spaced in parallel to the anode electrode to form a 
discharge region therebetween and supporting a material to be etched 
thereon; 
a magnet means which is provided behind the anode electrode and which has a 
magnetic field component intersecting with an electric field and is 
movable along a direction intersecting at right angles with the electric 
field; and 
a plurality of magnets provided equally around the cathode electrode and 
having a magnetic field component intersecting with the electric field. 
Preferably, a further electrode having a fluorine-containing resin is 
provided in the apparatus in the discharge region.

PREFERRED EMBODIMENTS OF THE INVENTION 
According to the method of the invention, a silicon compound film formed on 
a suitable support can be etched to a desired depth at a high rate by 
etching it with a gas mainly composed of a hydrogen-free carbon fluoride. 
The portion of the silicon compound film remaining along the depth of the 
film is then etched with a gas mainly composed of a hydrogen-containing 
carbon fluoride gas, so that the etching can be performed without damaging 
an underlying layer or layers. 
More particularly, the etching of the film to a given depth can be effected 
at a high rate. At a stage where the film thickness is small or an 
underlying layer may be influenced by the etching, a gas giving a less 
damage on the underlying layer is used for further etching. By the 
two-stage etching procedure, high rate and low damage etching becomes 
possible according to the invention. 
In the practice of the invention, the gases used in the respective etching 
procedures may comprise, aside from the main gases, other gases such as 
inert gases. The term "main gas" used herein is intended to mean that the 
gas is contained in an etching gas system in such an amount that a desired 
level of etching can be achieved. 
The hydrogen-free carbon fluoride gas used in the present invention 
includes fluorine-containing gases called Flon or Freon gases, of which no 
hydrogen atom is contained in the molecule. Examples of the carbon 
fluorides include those gases of the formulas, C.sub.n F.sub.2n+2 and 
C.sub.n F.sub.2n wherein n is an integer of not less than 1. Preferred 
examples include C.sub.3 F.sub.8, C.sub.2 F.sub.6, C.sub.4 F.sub.8 and the 
like. 
On the other hand, the hydrogen-containing carbon fluoride gases are 
fluorine-containing gases in which hydrogen atom or atoms are contained. 
Examples of such gases include those of the formula, C.sub.n H.sub.2n+ 2- 
m F.sub.m, wherein n and m are, respectively, an integer of not less than 
1. Specific and preferred examples include CHF.sub.3, CH.sub.2 F.sub.2 and 
the like. 
The silicon compound film to be etched in the method of the invention is a 
film made of various silicon compounds such as oxides, nitrides and the 
like of silicon and is not critical provided that it can be etched. 
SiO.sub.2, silicon nitrides such as Si.sub.3 N.sub.4 and the like will be 
effectively etched especially when the underlying layer is a silicon 
substrate which may be greatly damaged by etching. 
The apparatus for carrying out the method of the invention has been defined 
hereinbefore, which is characterized in that one group of magnets are 
provided behind an anode electrode so that they are movable along a 
direction intersecting with an electric field and have a magnetic field 
component intersecting with the electric field and that another group of 
magnets having a magnetic field component intersecting with the electric 
field are provided around a cathode electrode at equal intervals. 
In the apparatus of the invention, when one group of magnets having the 
magnetic field component intersecting with the electric field are moved 
along a direction intersecting at right angles with an electric field 
behind or at a back side of the anode electrode, a magnetic field in a 
main discharge region formed between the anode and cathode electrodes is 
established. In addition, the magnets provided equally around the cathode 
electrode ensures a uniform magnetic field in the main discharge region, 
resulting in uniform etching. 
The "behind or at the back side of" the anode electrode is intended to mean 
a side opposite to the main discharge region where the etching reaction 
proceeds predominantly. The term "provided equally" used herein means not 
only "provided at equal intervals", but also "provided to give a uniform 
magnetic field", resulting in uniform etching. Moreover, the term 
"intersecting" means not only "intersecting at right angles", but also 
intersecting at angles sufficient to show the effect of the magnetic 
field. 
It is preferred that in the main discharge region established between the 
anode and cathode electrodes of the etching apparatus, a third electrode 
having a fluorine-containing resin is provided. 
In this arrangement, a fluorine-based etchant is produced from the 
fluorine-containing resin of the third electrode, by which etching in the 
main discharge region is facilitated with an increasing etching rate. In 
general, fluorine-based ions are supplied by sputtering and function as an 
etchant. 
The electrode used to introduce the etchant is formed as a third electrode 
and can be controlled separately from the cathode with respect to its 
supply power. This makes it possible to independently control an energy 
for the formation of the etchant and an energy for ions implanted into 
materials to be etched, e.g. semiconductive wafers. This is effective in 
achieving low damage and high rate etching. 
The third electrode may be provided with the fluorine-containing resin in a 
manner sufficient to supply the etchant. Thus, it is not necessarily 
required to cover the electrode with the resin although such covering is 
preferred. 
The fluorine-containing resin is preferably so-called Teflon or 
polyethylene tetrafluoride. As a matter of course, other 
fluorine-containing resins capable of releasing a fluorine-containing 
etchant may be likewise used. 
The present invention is more particularly described by way of examples 
wherein reference is made to the accompanying drawings. 
EXAMPLE 1 
In this example, the method of the invention is particularly described, 
which is especially suitable for fabrication of highly integrated 
semiconductor devices. 
In this example, etching of a SiO.sub.2 film is described. The etching of 
SiO.sub.2 is considered to proceed based on a so-called ion-assisted 
reaction where ions are chiefly contributed to etching. However, when a 
material to be etched, such as a silicon wafer, is exposed to a high 
density plasma using a highly dissociating gas capable of producing ions 
such as CF.sub.3 + ions having the capability of etching SiO.sub.2, the 
damage of a substrate with the incident ions is not negligible. If an 
acceleration voltage of the incident ions is suppressed to an extent by 
discharge from a magnetron, application of high RF power for high speed 
etching apparently results in an increase of damages accompanied by an 
increasing ion current density. The suppression of such damages is 
possible only with a sacrifice of the etching rate. This is completely 
overcome by the method of the invention wherein a SiO.sub.2 film can be 
etched at a high rate while suppressing damages of a substrate. 
In the method of the invention, a SiO.sub.2 film formed on an underlying 
layer or a substrate is subjected first to high speed anisotropic etching 
with a gas mainly composed of a H-free flon gas, e.g. C.sub.3 F.sub.8 or 
C.sub.2 F.sub.6, just before the underlying layer is exposed. 
Subsequently, an etching gas mainly composed of a fluorine-containing gas 
containing at least one hydrogen atom in the molecule, e.g. CHF.sub.3, is 
used instead, so that incident ion damages on the underlying layer can be 
suppressed. 
The etching method using a cathode-coupling, parallel plate-type magnetron 
RIE apparatus is particularly described. 
In this example, as shown in FIG. 1(a), a silicon substrate 10 was 
provided, on which a film 1 of a silicon compound such as SiO.sub.2 or SiN 
was formed. On the SiO.sub.2 film 1 was further formed a resist layer 2 
for mask formation, followed by patterning to form a patterned mask layer 
21 as shown in FIG. 1(b). 
The silicon compound film 1 was etched through the patterned mask layer 21 
of FIG. 1(b). Using the cathode-coupling parallel plate-type magnetron RIE 
apparatus, the pattern etching was effected by two stages under the 
following conditions. 
I. Pattern etching step under high rate conditions 
Gas used and its flow rate: C.sub.3 F.sub.8, 46 SCCM 
Pressure of atmosphere: 2 Pa 
RF power: 2.76 W/cm.sup.2 
Applied magnetic field: 100 Gausses 
The above high speed or rate etching under the above conditions was 
continued for a time corresponding to approximately 10% of the film 
thickness left. Sofar as the underlying substrate was not exposed, the 
high speed etching should preferably be continued over the full etching 
time. More particularly, the etching should be continued to a time 
corresponding to not larger than 10%, preferably not larger than 5% of the 
film left along the depth thereof. In this connection, it will be noted 
that the thickness of the silicon compound film 1 is believed to be 
usually scattered at approximately 3% and the scattering has to be taken 
into account for the high speed etching in order not to cause the 
underlying layer to be exposed. 
The high speed etching has a rate as high as about three times that of a 
subsequent low damage etching using the following conditions. 
II. Pattern etching step using the following low damage etching conditions. 
Gas used and its flow rate: CHF.sub.3, 50 SCCM 
Pressure of atmosphere: 2 Pa 
RF power: 1.33 W/cm.sup.2 
Applied magnetic field: 100 Gausses 
The etching should preferably be changed from the high speed etching 
conditions to the low damage etching conditions at the time when the film 
on the underlying layer is as thin as possible provided that the 
underlying layer is not exposed. More particularly, a time at which an 
appropriate remaining thickness of the film being etching is reached 
should be checked and set. The etching is continued for the time checked. 
Alternatively, the time at which the etching conditions are changed may be 
determined by monitoring through visual or other observation. 
As a result of the etching under the different conditions, a fully etched 
structure as shown in FIG. 1(d) is obtained. 
In this example, an etching rate of not lower than 900 nm/minute could be 
attained by carrying out the two-stage pattern etching under the 
conditions of I and II. In addition, the SiO.sub.2 film could be etched 
while suppressing the damage on the substrate. In conventional batchwise 
etching methods, the etching rate was found to be as low as 40 to 50 
nm/minute. According to the procedure of Example 1 of the invention, the 
production efficiency comparable to that of the known batchwise method 
could be achieved even when wafers were etched one by one. 
The reduction of the damage on the substrate will be apparent from the 
following experiment. 
Two gases of CHF.sub.3 and C.sub.3 F.sub.8 used in the above example were, 
respectively, used for etching under conditions of RF power of 1.33 
W/cm.sup.2 in the same manner as in the above example and damages on the 
substrate were evaluated by a thermal wave method. As a result, it was 
found that the damage evaluation was reduced from 465 units for the 
C.sub.3 F.sub.8 gas to 110 units for the CHF.sub.3 gas. It will be noted 
that the value is about 25 units for no damage. Thus, the damage reduction 
was 76%. 
The above results reveal that the method of the invention wherein a 
hydrogen-containing carbon fluoride gas is used for pattern etching at the 
time of exposing the substrate is significantly effective in reducing the 
substrate damages. 
For the etching with a hydrogen-containing carbon fluoride gas used in the 
latter step, activated fluorine is caught with hydrogen, so that the 
selection ratio between SiO.sub.2 and Si can be controlled. 
The damage evaluation of the substrate by the thermal wave method was made 
in the following manner. While an energy was periodically given to an 
object to be measured by means of an Ar ion laser beam modulated at 1 MHz, 
a HeNe laser beam with a beam intensity of approximately 3 MW was focussed 
on the object and a light component only reflected from the object was 
polarized and separated for detection. The above operation was performed 
prior to and after the etching to determine a variation in reflectance or 
a variation in periodic condition as absorption units of the thermal wave. 
This evaluation method is described in detail in "Monthly Semiconductor 
World" by The Press Journal, January, 1987, p. 121-127. 
As described above, according to the method of the invention, a silicon 
compound film such as SiO.sub.2 film or SiN film is dry etched in a 
pattern by a two-stage procedure wherein high speed anisotropic etching is 
first effected using a gas mainly composed of a H-free carbon fluoride gas 
such as C.sub.3 F.sub.8, C.sub.2 F.sub.6 or the like immediately before 
the underlying layer is exposed and low damage etching is then effected 
using a gas mainly composed of a carbon fluoride gas containing at least 
one hydrogen atom in the molecule, e.g. CHF.sub.3. Thus, the high rate and 
low damage etching of the silicon compound film such as a SiO.sub.2 film 
can be achieved. Accordingly, one-by-one etching of wafers may be 
possible. The low damage etching will lead to an improvement of device 
characteristics along with a reduction in the surface treatment after 
etching. 
EXAMPLE 2 
An etching apparatus as shown in FIG. 2 is described in this example. 
As stated before, in order to satisfy the requirement for the high speed 
etching, a microwave plasma etcher utilizing ECR discharge or a RIE 
apparatus utilizing magnetron discharge has been developed as a means for 
forming a high density plasma. With the RIE apparatus of the magnetron 
discharge type, there are involved problems on optimization of an applied 
intensity of magnetic field and in-plane uniformity of materials to be 
treated, e.g. a wafer. To cope with the problems, those apparatus using an 
AC magnetic field or mechanical scanning of permanent magnets have been 
proposed. They have merits and demerits, respectively, and any technique 
of satisfying all the requirements for high speed etching, good uniformity 
and a low damage on underlying layers has not been developed yet. 
Reference is now made to FIGS. 2 (a) and 2(b) wherein a RIE apparatus which 
can meet all the requirements is shown. 
FIG. 2(a) shows a cathode-coupling magnetron discharge-type RIE apparatus 
which includes an anode electrode 3 and magnets 51 to 53 provided behind 
the anode 3. The magnets 51 to 53 are so arranged as to be scanned. The 
apparatus has a cathode electrode 4 in face-to-face relation with the 
anode 3. Magnets 61 to 68 are provided at equal intervals around the 
cathode electrode 4 as shown in FIG. 2(b) to give a magnetic field 
applicator means. The magnets 51 to 53 provided at the back side of the 
anode electrode 3 have a magnetic field component intersecting at right 
angles with an electric field and are movable along directions 
intersecting at right angles with the electric field as shown, for 
example, by opposite arrows in FIG. 2(a). The magnets 61 to 68 provided 
around the cathode electrode 4 have a magnetic field component 
intersecting at right angles with the electric field. In FIGS. 2(a) and 
2(b), broken lines indicate lines of magnetic forces. 
In this embodiment, the uniform magnetic field applicator means consisting 
of the magnets 61 to 68 can prevent a lowering of the etching rate around 
a material 8 to be etched, which will not be compensated with the 
mechanical scanning of the magnets 51 to 53. In FIG. 2(a), the magnets 51 
to 53 provided at the back side of the anode electrode 3 have such an 
arrangement that scanning is made as desired. The magnets 51 to 53 may be 
provided within an etching chamber as shown in FIG. 2(a) or outside the 
chamber. 
In etching of SiO.sub.2, an etching rate is facilitated by utilizing 
sputtering of a cathode material. For instance, as shown in FIG. 2(a), a 
cathode cover 43 may be provided. The cover 43 is constituted of a 
fluorine-containing resin such as Teflon. By this, ion species are 
efficiently produced around the material 8 to be etched (e.g. a 
semiconductor wafer) which contributes to sputtering, thus improving the 
etching rate. The embodiment shown in FIG. 2(a) is one example of an 
arrangement for improving the etching rate by sputtering of such a cathode 
material and is not described for limitation. 
According to this embodiment, in addition to a magnetic field applied to 
the main discharge region between the anode and cathode electrodes, an 
auxiliary magnetic field applicator means is provided around the material 
8 to be etched, thereby ensuring an improved etching rate or speed and an 
improved in-plane uniformity of the material 8 to be etched. Thus, the 
uniformity of the etching can be improved by the use of the high speed 
magnetron RIE etching apparatus. 
Using the above apparatus, high speed etching conditions I as set forth in 
Example 1 may be used singly or in combination with the conditions II for 
high speed etching. Alternatively, the high speed etching of silicon 
materials may be performed without use of such carbon fluoride gases free 
of any hydrogen atoms. 
EXAMPLE 3 
A further embodiment of an etching apparatus of the invention is described 
in this embodiment, wherein a third electrode is used. 
In Example 2, the coverage of the cathode electrode with a 
fluorine-containing resin has been described in order to improve the 
etching rate of SiO.sub.2 in the RIE apparatus. By this, CF.sub.x.sup.+ 
and the like are produced as an etchant by sputtering with incident ions. 
In order to further increase the etching rate, a further embodiment is 
described. FIG. 3 shows such an embodiment. 
In FIG. 3, there is shown a cathode-coupling type RIE apparatus similar to 
one shown in FIG. 2. In FIG. 3, like reference numerals as in FIG. 2 
indicate like parts or members. In a main discharge region established 
between the anode electrode 3 and the cathode electrode 4, a third 
electrode 9 having a fluorine-containing resin cover 91 is provided as 
shown in the figure. The third electrode 9 is applied with high frequency 
power from a high frequency power supply 92. A high frequency power supply 
for the cathode electrode 4 is indicated by 41. 
Application of high frequency power to the third electrode 9 results in an 
increasing amount of an etchant from the fluorine-containing resin cover 
91 by sputtering. 
The third electrode 9 has a ring structure as shown in FIG. 4 which is 
viewed from the direction IV in FIG. 3. An outer diameter, R, should be 
larger than the diameter of the cathode electrode 4. The ring structure 
has an opening at the center thereof with a diameter, r, which is 
substantially equal to a diameter of the material to be etched. In doing 
so, the main discharge is not impeded. 
In this embodiment, a cathode cover 43 may be provided and made of a teflon 
resin. The cathode cover 43 has an opening in which the material to be 
etched is snugly accommodated. 
The feed of a CF.sub.x.sup.+ etchant is increased by sputtering of the 
cathode cover 43 and from the surface of the third electrode 9. This 
results in a high etching rate of SiO.sub.2. If a rare gas such as Ar is 
added to the etching gas, the sputtering can be effectively carried out by 
discharge of the gas. In this condition, the etching may be effected 
without use of any hydrogen-free gas. 
In the apparatus of this embodiment, the power applications to the third 
electrode 9 and the cathode 4 can be separately controlled, so that it 
becomes possible to separately control formation of the etchant and the 
incident ion energy to the material 8 to be etched. Accordingly, the 
material 8 to be etched can be etched at a high rate with a reduced damage 
of the underlying layer. 
More particularly, the power supply 41 for the cathode electrode 4 is able 
to generate a high frequency, for example, of 100 KHz to 13.56 MHz. The 
high frequency with such a relatively low frequency is given to the 
cathode electrode 4, permitting the ions to be readily followed. As a 
consequence, the intensity of the electric field of an ion sheath formed 
on the material 8 to be etched is reduced. On the other hand, the power 
supply 92 for the third electrode 9 generates a high frequency, for 
example, of 13.56 MHz. In this manner, the frequencies and applied power 
intensities can be independently controlled. 
In this example, SiO.sub.2 was used as the material 8 to be etched and 
etched by the used of the apparatus illustrated above under the following 
etching conditions. 
Gas used: C.sub.2 F.sub.6 or C.sub.3 F.sub.8 
Flow rate: 46 SCCM 
Gas pressure: 0.5 to 2 Pa 
Application power: 2.76 W/cm.sup.2 
The reason why a carbon fluoride gas of the formula, C.sub.n F.sub.2n+2 was 
used as an etching gas was to achieve the high speed etching. 
Under the conditions set forth above, efficient etching of SiO.sub.2 could 
be achieved with the damage of the substrate being suppressed. 
In this embodiment, the third electrode 9 covered with a 
fluorine-containing resin 91 is provided in the discharge region and 
applied with high frequency electric power, so that an etchant is 
increased in amount by sputtering. Thus, high speed etching of SiO.sub.2 
becomes possible. The high speed etching leads to an improved throughput 
sufficient for practical application even when wafers are etched one by 
one. 
In the above example illustrated, the carbon fluoride gas free of any 
hydrogen atom in the molecule was used as an etching gas, but an inert gas 
such as argon alone may be used for the etching since fluorine is supplied 
from the fluorine-containing resin 91. Thus, the use of the apparatus as 
shown in FIGS. 3 and 4 ensures etching of silicon compounds such as 
SiO.sub.2 without use of any carbon fluoride gas free of any hydrogen. 
In the apparatus of this embodiment, formation of an etchant and an 
incident ion energy to the material 8 to be etched can be independently 
controlled, making it possible to attain both high speed etching and a 
reduced damage on underlying layer. 
EXAMPLE 4 
In this embodiment, the third electrode 9 is applied to a magnetron RIE 
apparatus and particularly, to a cathode-coupling magnetron RIE apparatus. 
The etching apparatus of this embodiment is shown in FIG. 5 wherein magnets 
51 to 53 are provided behind the anode electrode 3 so that a magnetic 
field is formed as intersecting at right angles with an electric field. By 
this, magnetron etching is performed. 
Like reference numerals as in FIG. 4 indicate like members in FIG. 5. 
When etching was effected under the same conditions as in Example 3, high 
speed etching of SiO.sub.2 as in Example 3 could be achieved. The in-plane 
uniformity inherent to the magnetron RIE technique could be shown. 
EXAMPLE 5 
In this embodiment, a magnetic field generating means is uniformly provided 
around the cathode electrode 5 as shown in FIG. 2 and the third electrode 
9 having the fluorine-containing resin 91 is provided. More particularly, 
the arrangements of FIGS. 2 and 3 are combined together. 
This embodiment is shown in FIG. 6. As will be apparent from the figure, 
auxiliary magnet means 51 to 53 as in FIG. 2 is provided and the third 
electrode 9 covered with the fluorine-containing resin 91 such as teflon 
is placed in the discharge region. The magnet means may further include 
magnets 71 and 72 in order to generate magnetic fields in connection with 
the third electrode 9 as shown in the figure. By the use of the third 
electrode 9, an etchant is increased in amount as stated before, with an 
increasing etching rate of SiO.sub.2. Small electric power for the etching 
was sufficient. For instance, a magnet of 100 gausses was used and a 
desired degree of etching could be achieved at 1.33 W/cm.sup.2. In the 
arrangement of this embodiment, use of only a rare gas such as argon was 
sufficient for etching of a SiO.sub.2 film without use of any 
fluorine-containing gas such as a flon gas.