Surface treatment method and surface treatment apparatus

Etching selectivity is improved in a semiconductor process using a fluorocarbon gas. An energy incident to a substrate is controlled to have a value to cause transition from etching to deposition on a silicon nitride film, ions having (CF.sub.2).sub.n.sup.+ as a major component are guided onto the substrate to perform selective etching of a silicon oxide film against the silicon nitride film.

BACKGROUND OF THE INVENTION 
1. Technology of the Invention 
The present invention relates to a surface treatment method and surface 
treatment apparatus and more particularly, to a method and apparatus for 
selectively etching a silicon oxide film against a silicon nitride film. 
2. Background Art 
Conventionally, for etching a silicon oxide film, a reactive ion etching 
method has been employed because it enables a high accuracy pattern 
formation. In the reactive ion etching method, a substrate to be treated 
(e.g., a substrate on which a thin film to be etched is formed) is placed 
in a vacuum container. A pair of parallel plate electrodes is disposed in 
the container into which a reactive gas is introduced. Electric power of 
high frequency is applied to the electrodes so that gas discharging takes 
place to generate gas plasma, which is used for etching the substrate. 
In addition to the reactive ion etching method, there are other methods 
including plasma etching method, ECR dry etching method, ion beam etching 
method and photo excited etching method. These methods carry out etching 
by chemically and physically react ions in an activated reactive gas onto 
the substrate in the vacuum container. Thus, in this respect, these 
methods are considered to be the same as the reactive ion etching method. 
An example of these etching methods will now be described. 
In performing selective etching to a silicon oxide film against a silicon 
nitride film, for example, the reactive ion etching is carried out by 
electric discharge in a mixture gas of fluorocarbon gas and H.sub.2 or CO 
gas. Since various sorts of ion species generated in the plasma react with 
the substrate, the ion species contributing to the etching and the ion 
species contributing to the deposition co-exist on the substrate, which 
lowers the efficiency of the etching. In addition, the ion species acting 
to raise a selectivity of the silicon oxide film to the silicon nitride 
film have not been confirmed yet and other ion species may be present on 
the substrate, which deteriorates the selectivity. However, as the 
integration density of a semiconductor integrated circuit is increased, 
when performing etching to the silicon oxide film on the silicon nitride 
film to form contact holes or the like, a higher selectivity against the 
underlying silicon nitride is required. Suppose now a case where contact 
holes such as storage node contacts or bit line contacts in a DRAM are 
formed by using a self aligned contact hole (SAC) etching process as shown 
in FIG. 20. On the surface and the side walls of a gate electrode 33 of a 
polycrystalline silicon film which is formed on a silicon substrate 31 via 
a gate insulating film 32, a silicon nitride film 34 is formed. Above the 
silicon nitride film 34, a silicon oxide film 35 having a thickness of 
about 700 .mu.m is formed as an interlayer insulating film. Thereafter, In 
forming a contact hole H in the gate electrode 33 in a self alignment 
manner, a resist pattern R having a somewhat larger size is formed and 
etching is performed by using the silicon nitride film 34 as an etching 
stop layer. At this time, for the purpose of reducing a capacitance 
between wiring, the thickness of the interlayer insulating film cannot be 
made too small and thus must be about 400 nm. The gate electrode is formed 
in a multi-layered structure with the metallic film and is required to 
have a thickness of about 300 nm. The thickness of the silicon nitride 
film 34 is required to be as small as possible since wiring material is 
embedded into the contact hole after the SAC etching step. In this 
example, the thickness of the silicon nitride film 34 is about 50 nm. 
For the SAC etching, the thickest part of the silicon oxide film in the 
opening has a thickness of 700 nm. Since the etch rate of the silicon 
oxide film is unequally distributed on the wafer surface and the thickness 
of the interlayer insulating film is unequally distributed on the wafer 
surface, over-etching is necessary by about 30%. In other words, an 
etching time of about 910 nm (=700 nm .times.1.3) is necessary. 
Thus, the time during which the silicon nitride film 34 is subjected to the 
reactive ion etching (RIE) corresponds to the etching time of the silicon 
oxide film of 510 nm (=910 nm -400 nm). 
At this time, the silicon nitride film must exist at least half of the 
thickness thereof, which is 25 nm. 
Accordingly, the etching selectivity of the silicon oxide film to the 
silicon nitride film is required to be 20.5 (=510/25). 
Thus, by determining the conditions for satisfying the requirements that 
the etching selectivity of the silicon oxide film to the silicon nitride 
film is about 20, the cell size can be remarkably reduced. However, it is 
difficult to cope with such requirements by conventional etching methods. 
For the purpose of improving the selectivity, variety of studies have been 
conducted. 
In summary, conventional selective etching which use a fluorocarbon gas for 
selective etching of the silicon oxide film against the silicon nitride 
film, have a problem that among various sorts of ion species contributing 
to the surface reaction, it is not confirmed which ion species actually 
contributes to the surface reaction. As a result, these methods provides 
low etching efficiency, insufficient selectivity, and therefore 
impractical. 
SUMMARY OF THE INVENTION 
In view of the above circumstances, it is an object of the present 
invention to provide method and apparatus which can improve an etching 
selectivity in a semiconductor process using a fluorocarbon gas. 
In accordance with one aspect of the present invention, there are provided 
a step of supplying a gas containing a fluorocarbon gas, a step of 
dissociating a fluorocarbon gas into ions, and a step of selectively 
etching a silicon oxide film against a silicon nitride film by controlling 
ions having (CF.sub.2).sub.n.sup.+ (n=1, 2, 3, . . . ) as a major 
component among the ions obtained by the ion dissociation step so as to be 
guided onto a semiconductor substrate. As a gas, the fluorocarbon gas may 
contain Ar, Ne, He, Kr or Xe and so on. The term "major component" refers 
to the fact that (CF.sub.2).sub.n.sup.+ is a major component in the 
fluorocarbon ions. 
Preferably, an energy incident to the semiconductor substrate is controlled 
such that a reaction induced on the silicon nitride film by ions including 
(CF.sub.2).sub.n.sup.+ as the major component takes an energy value of 
causing transition from etching to deposition, so as to lead the ions 
including (CF.sub.2).sub.n.sup.+ as the major component onto the 
semiconductor substrate. As a result, selective etching is performed to 
the silicon oxide film against the silicon nitride film. 
More particularly, the energy incident to the semiconductor substrate is 
controlled such that a reaction induced on the silicon nitride film by 
ions including (CF.sub.2).sub.n.sup.+ as the major component takes an 
energy value of causing transition from etching to deposition. For 
example, in the case of using CF.sub.2.sup.+ ions, the incident energy to 
the substrate is set to be 200 eV. In the case of using C.sub.2 
F.sub.4.sup.+ ions, the incident energy to the substrate is set to be 150 
eV. In the case of using such ions having a larger `n` as C.sub.3 
F.sub.6.sup.+ ions or C.sub.4 F.sub.8.sup.+ ions, by reducing the 
energy, etching can be performed with high selectivity. 
Preferably, the etching step includes a step of controlling the energy 
incident to the semiconductor substrate such as to satisfy the following 
relationships where x (=CF.sub.3.sup.+ /(C.sub.2 F.sub.4.sup.+ 
CF.sub.3.sup.+)) is ratio of a CF.sub.3.sup.+ concentration to a (C.sub.2 
F.sub.4.sup.+ +CF.sub.3.sup.+) concentration in the ions obtained by the 
ion dissociation and E is an incident energy directed to the semiconductor 
substrate, whereby the ions are guided onto the semiconductor substrate 
and selective etching is performed to the silicon oxide film against the 
silicon nitride film. 
152 eV &lt; E .ltoreq. 223 eV, and 
x &lt; -0,744 + 19768.7/(214.8E - 21240) 
where x : CF.sub.3.sup.+ /(C.sub.2 F.sub.4.sup.+ +CF.sub.3.sup.+) 
E: ion incident energy substrate. 
The term "ion concentration" is defined as the number of ions per unit 
volume. 
Further, it is preferable that the etching step includes a step of 
selectively etching a silicon oxide film against a silicon nitride film by 
controlling the ions obtained by the ion dissociation such that energy 
incident to the semiconductor substrate is 152 eV or less, and by guiding 
the ions onto the semiconductor substrate. 
Further it is preferable that the etching step is a step of controlling an 
ion beam such as to satisfy the above relationships and guiding the ion 
beam onto the semiconductor substrate. 
Further, it is preferable that the gas supply step is a step of supplying a 
c-C.sub.4 F.sub.8 gas as the fluorocarbon gas, wherein the ion 
dissociation is carried out by generating a plasma from the c-C.sub.4 
F.sub.8. In this case, the c-C.sub.4 F.sub.8 gas is one of fluorocarbon 
gases having 4-ring structure (which applies to the following cases). 
Further, it is preferable that the gas supply step includes a step of 
supplying a c-C.sub.4 F.sub.8 gas as the fluorocarbon gas under a reduced 
pressure into an air-tight reaction container which accommodates the 
semiconductor substrate, the etching step is a step of selectively etching 
a silicon oxide film against a silicon nitride film by changing a 
c-C.sub.4 F.sub.8 gas into plasma to dissociate ions, controlling the 
dissociated ions such that a ratio of CF.sub.3.sup.+ ion concentration to 
a total concentration of (CF.sub.2).sub.n.sup.+ in the ions to be guided 
onto the semiconductor substrate becomes 8% or less, and by guiding the 
controlled ions onto the semiconductor substrate. 
Further, it is preferable that the gas supply step includes a step of 
supplying a c-C.sub.4 F.sub.8 gas and at least one of Kr and Xe gases 
under a reduced pressure into an air-tight reaction container for 
accommodating the semiconductor substrate, and the ion dissociation is 
carried out by generating a plasma from a mixture gas of the c-C.sub.4 
F.sub.8 gas and at least one of the Kr and Xe gases. 
Preferably, the gas supply step includes a step of supplying a c-C.sub.4 
F.sub.8 gas under a reduced pressure into an air-tight reaction container 
which accommodates the semiconductor substrate, and the ion dissociation 
is carried out by exciting the gas by using an electron beam which is 
controlled so that an electron energy is less than 33 eV so as to generate 
plasma. 
Further, it is preferable that the gas supply step includes a step of 
supplying a c-C.sub.4 F.sub.8 gas under a reduced pressure into an 
air-tight reaction container for accommodating the semiconductor 
substrate, and the ion dissociation is achieved by generating a plasma 
from the gas with use of light having wavelengths of between 70.9 nm and 
102.5 nm. 
Preferably, the gas supply step includes a step of supplying a c-C.sub.4 
F.sub.8 gas under a reduced pressure into an air-tight reaction container 
which accommodates the semiconductor substrate, and the ion dissociation 
is carried out by changing the gas into plasma, and by controlling the 
following mathematical formula in which collision coefficient N represents 
collision between c-C.sub.4 F.sub.8 molecules present in the plasma and 
electrons present in the plasma, n.sub.e represents an electron density in 
the plasma, .tau. represents a stay time of the c-C.sub.4 F.sub.8 
molecules in the plasma within the air-tight reaction container, 
N = n.sub.e x .tau. .ltoreq. 7.2E8 
.tau.= P x V x Q.sup.-1 
P: c-C.sub.4 F.sub.8 partial pressure 
V: volume of the reaction container 
Q: c-C.sub.4 F.sub.8 flow rate. 
Preferably, the etching step includes a step of controlling such that an 
incident energy of ions to the semiconductor substrate generated by 
changing the gas into plasma becomes 500 eV or less. 
Further, it is preferable that the ions are generated by generating 
magnetron plasma while controlling such that the stay time .tau. = P x V/Q 
of the c-C.sub.4 F.sub.8 molecules present in the plasma becomes 24 msec 
or less. 
Further, it is preferable that the ions are generated by generating plasma 
by using a parallel plate plasma generation apparatus while controlling 
such that the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules 
present in the plasma becomes 720 msec or less. 
Preferably, the ion dissociation is achieved by exciting the gas by a 
controlled electron beam while controlling such that the stay time .tau. = 
P x V/Q of the c-C.sub.4 F.sub.8 molecules present in plasma becomes 7.2 
msec or less. 
It is preferable that the ion dissociation is achieved by generating plasma 
by using a microwave-excited plasma generation apparatus while controlling 
such that the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules 
present in the plasma becomes 7.2 msec or less. 
Preferably, the ion dissociation is achieved by generating plasma by using 
an inductive coupled plasma generation apparatus while controlling such 
that the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules 
present in the plasma becomes 0.72 msec or less. 
Preferably, the ion dissociation is achieved by generating plasma by using 
a helicon-excited plasma generation apparatus while controlling such that 
the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules present 
in the plasma becomes 0.072 msec or less. 
It is preferable that the gas supply step includes a step of supplying a 
c-C.sub.4 F.sub.8 gas under a reduced pressure into an air-tight reaction 
container which accommodates the semiconductor substrate, and the ion 
dissociation is achieved by changing the gas into plasma by using an 
high-frequency pulse signal modulated with pulses of several milliseconds 
to several hundreds of milliseconds. 
Preferably, the plasma of the gas is generated by a magnetron excited 
plasma generation apparatus. 
Preferably, the gas supply step includes a step of supplying a c-C.sub.4 
F.sub.8 gas under a reduced pressure into an air-tight reaction container 
which accommodates the semiconductor substrate, and the ion dissociation 
is achieved by changing the gas into plasma by using an electron beam 
modulated with pulses of several milliseconds to several tens of 
milliseconds. 
In accordance with second aspect of the present invention, there is 
provided a surface treatment apparatus which comprises gas supply means 
for supplying a fluorocarbon gas onto a semiconductor substrate installed 
within a reaction container, and ion species control means for generating 
a plasma from the fluorocarbon gas and controllably guiding ones 
(CF.sub.2).sub.n.sup.+ of obtained ions as a major component onto the 
semiconductor substrate. 
It is preferable that the gas supply means includes means for controlling 
the gas flow rate Q or the c-C.sub.4 F.sub.8 partial pressure to cause a 
collision coefficient N indicative of collision between c-C.sub.4 F.sub.8 
molecules present in the plasma and electrons present in the plasma to be 
7.2E8 or less to thereby control the c-C.sub.4 F.sub.8 flow rate Q or the 
c-C.sub.4 F.sub.8 partial pressure, where 
N : n.sub.3 x .tau. 
.tau. : stay time in the reaction container (= P x V x Q.sup.-1) 
n.sub.e : electron density in the plasma 
P: c-C.sub.4 F.sub.8 partial pressure 
V: volume of the reaction container 
Q: c-C.sub.4 F.sub.8 flow rate. 
Further, it is preferable that the ion species control means includes means 
for generating a plasma from the gas with use of an electron beam 
modulated with pulses of several milliseconds to several hundreds of 
milliseconds. 
As has been described above, when the ion dissociation is carried out by 
generating the plasma from the gas, in general, ion incident energy (E) 
has a certain distribution. In more detail, the energy has such a 
distribution width .DELTA.E as given below. 
.DELTA.E = (8/3.omega.d) {eV.sub.th).sup.1.5 /(2M).sup.1/2 } 
where .omega.: frequency (rad/sec)= 2.pi.f 
f: frequency of application electric field 
d: sheath width (m) 
M: ion mass 
V.sub.th : V.sub.dc (cathode drop voltage) + V.sub.p (plasma potential) 
e: elementary electric charge. 
Energy distribution f(E) is given as follows. 
f(E)= (4N.sub.0 /.omega..DELTA.E) 1- {2(E - eV.sub.th)/.DELTA.E}.sup.2 
!.sup.0.5 where N.sub.0 : the number of particles (m.sup.-3) (the number 
of ions per 1 m.sup.3) 
In the invention described above, the ion incident energy refers to an 
average energy value of the energy distribution. 
According to the present invention, the silicon oxide film refers to an 
oxide film containing impurities such as boron, phosphorus, arsenic and so 
on. The silicon oxide film may be made of, for example, boron-added 
silicate glass (BPSG) or arsenic-added silicate glass (

DESCRIPTION OF THE EMBODIMENTS 
The embodiments of present invention will be described with reference to 
the accompanying drawings. 
FIG. 1 schematically shows an arrangement of a surface treatment apparatus 
used in an embodiment of the present invention. 
The apparatus comprises a gas supply source 100, an ion source 101 supplied 
with a C.sub.4 F.sub.8 gas as a source gas from the gas supply source 100 
for generating an ion beam 102 by dissociation, a mass separator 103 for 
separating the ion beam 102 for each chemical species, a deceleration 
system 104 for controlling deceleration of the separated ion beam, and a 
surface treatment chamber 107 including a vacuum container 105 for 
performing etching or thin-film depositing operation over a wafer 106 
placed within the vacuum container 105. In the illustrated example, the 
wafer is placed on a susceptor 108 equipped with a temperature control 
mechanism capable of controlling a temperature within a range between 
-50.degree. and 800.degree. C. The vacuum container 105 is connected with 
a gas exhausting system 109. 
Description will next be made as to the surface treatment method used in 
the aforementioned apparatus. 
In the illustrated example, the mass separator is controlled so that the 
radiated ion species becomes (CF.sub.2).sup.+. First of all, description 
will be directed to a step of forming a bit line contact in a DRAM by 
using an SAC (self aligned contact hole) etching process as shown in FIG. 
2(a). A gate electrode 3 made of a polycrystalline silicon film is formed 
on a surface of a silicon substrate 1 via a gate insulating film 2, the 
gate electrode 3 is coated on the surface and side walls with a silicon 
nitride film 4 of 50 nm thick. Thereafter, a diffusing operation is 
performed by using the gate electrode 3 as a mask to form a diffusion 
layer 7 as a source/drain region and to form a MOS FET as a switching 
transistor. Above the MOS FET, a silicon oxide film 5 having a thickness 
of about 700 .mu.m is formed as an interlayer insulating film to which an 
etching is performed by using a resist pattern R as a mask to form a bit 
line contact hole H in a self alignment manner. In this case, the resist 
pattern R is formed to be somewhat large and the etching is carried out by 
using the silicon nitride film 4 as an etching stop layer. 
The substrate thus sequentially formed is used as a wafer to which etching 
is performed by using the resist pattern R as a mask at room temperature 
and by using (CF.sub.2).sup.+ ions irradiated at an incident energy of 
200 eV. 
As a result, as shown in FIG. 2(b), patterning is performed to the silicon 
oxide film 3 with an extremely high selectivity and the bit line contact 
hole H is formed in a self alignment manner with respect to the gate 
electrode 5. 
Thereafter, the polycrystalline silicon film is deposited by a CVD method 
or the like so as to come into contact with the diffusion layer 
(source/drain region) 7 within the contact hole and then subjected to a 
patterning operation to form a bit line 6. 
One part of the diffusion layer 7 is connected to the bit line while on 
another part thereof (not shown) a capacitor is formed through a storage 
node electrode, thus forming a cell. In accordance with the method of the 
present invention, since the bit line contact can be formed in a self 
alignment manner, the cell size can be remarkably reduced and thus a DRAM 
having small size and high reliability is fabricated. 
Next, in order to detect a difference between etching characteristics for 
different ion species, CF.sup.+, CF.sub.2.sup.+ and CF.sub.3.sup.+ are 
separately generated by using a mass separator and then etchings are 
carried out to measure etching amounts. As samples, substrates each having 
a silicon oxide film and a silicon nitride film formed thereon are 
prepared and then subjected to etchings with a radiation energy of 400 eV 
and a substrate temperature being at room temperature. The result is given 
in FIG. 3. As seen from the result, the etch rates of SiO.sub.2 and 
Si.sub.3 N.sub.4 are increased in the order of CF.sup.+, CF.sub.2.sup.+ 
and CF.sub.3.sup.+. In terms of SiO.sub.2 /Si.sub.3 N.sub.4 etch rate 
ratio (selectivity), CF.sub.3.sup.+ is the worst and CF.sup.+ is the 
best. However since CF.sup.+ has a small etch rate, etching to SiO.sub.2 
stops on half way. Accordingly, among these 3 sorts of ions, 
CF.sub.2.sup.+ is most preferable. 
Next, substrates with a silicon oxide film and a silicon nitride film are 
subjected to radiations of CF.sub.2 and C.sub.2 F.sub.4.sup.+ ions with 
varied incident energies, which result is given in FIG. 4. At an incident 
energy of 400 eV, the etch rate of C.sub.2 F.sub.4.sup.+ is twice of that 
of CF.sub.2.sup.+ on the silicon oxide film and silicon nitride film, and 
their selectivity is the same. When the incident energy is reduced down to 
200 eV, the CF.sub.2.sup.+ etching is stopped halfway on the SiO.sub.2 
film and on the Si.sub.3 N.sub.4 film. In the case of C.sub.2 
F.sub.4.sup.+, the etching is stopped on the Si.sub.3 N.sub.4 film but the 
etching proceeded without being stopped on the SiO.sub.2 film, which 
results in a high selectivity of higher than 50. 
Further, ions (CF.sub.2).sub.n.sup.+ and CF.sub.3.sup.+ which are 
generated by dissociation of c-C.sub.4 F.sub.8 are radiated on subjects 
each having a silicon oxide film and a silicon nitride film formed thereon 
with varied incident energies. Their etching characteristics are measured 
and the results are given in FIG. 5. At an incident energy of 300 eV, in 
the case of CF.sub.2.sup.+, etching takes place on the silicon oxide film 
but etching is saturated on the silicon nitride film due to the deposition 
of fluorocarbon polymeric film thereon. At 200 eV, the etching of the 
silicon oxide film by CF.sub.2.sup.+ is also saturated, but the etching of 
the silicon oxide film by C.sub.2 F.sub.4.sup.+ proceeds while the 
etching on the silicon nitride film is saturated. It is appreciated from 
these results that selective etching of the silicon oxide film against the 
silicon nitride film with a high selectivity can be realized at 300 eV for 
CF.sub.2.sup.+ and at 200 eV for C.sub.2 F.sub.4.sup. +. In addition, 
even in the case of C.sub.3 F.sub.6.sup.+ and C.sub.4 F.sub.8.sup.+, when 
an incident energy is set at 150 eV and 75 eV respectively, the selective 
etching of the silicon oxide film against the silicon nitride film can be 
carried out at a high selectivity. In a process of transition from etching 
to deposition, since there exists an energy zone of a certain width within 
which the etching is stopped halfway and the deposition starts, the 
control range of the incident energy value can have an allowance depending 
on the tradeoff with the thickness of the silicon nitride film. 
It is expected from the above result that, even in the case of 
(CF.sub.2).sub.n.sup.+ (n&gt;4), by controlling the energy, the selective 
etching of the silicon oxide film against the silicon nitride film can be 
attained. Even in the case of CF.sub.3.sup.+, as the energy is lowered, 
the etch rates of the silicon oxide film and silicon nitride film are 
reduced. At an incident energy of 125 eV, transition occurs from the 
etching to deposition on the silicon nitride film. 
Incidentally, when attention is directed to C.sub.2 F.sub.4.sup.+, 
CF.sub.2.sup.+ and CF.sub.3.sup.+ ions present in C.sub.4 F.sub.8.sup.+ 
gas plasma, it is found from FIG. 5 that incident ion energies providing a 
selectivity of 20 which is practical in fabricating devices, as mentioned 
above, are 223 eV, 215 eV and 152 eV, respectively. 
In the range of 152 eV to 223 eV, as seen in FIG. 5, it is only C.sub.2 
F.sub.4.sup.+ and CF.sub.3.sup.+ that the etching of the silicon nitride 
film is large enough to cause a problem. Other ions are negligible. 
Accordingly, it is appreciated that, in the above range, when only the 
mixture ratio of these is selected so that the etching selectivity is more 
than 20, the excellent result can be obtained. FIG. 6 shows a curve Q 
representing an upper limit of a mixture ratio x to the incident ion 
energy which is calculated based on the etch rate found in FIG. 5. A zone 
lower Than the curve Q refers to a safe zone in the range of 152 eV to 223 
eV, i.e., in which the etching selectivity can take 20 or more. In other 
words, in the range of 152 eV to 223 eV, only C.sub.2 F.sub.4.sup.+ and 
CF.sub.3.sup.+ contribute to the etching. Considering this fact, the 
relationship between the etch rates of the silicon nitride film and 
silicon oxide film and the incident energy intensity E is calculated. As a 
result of the calculation, assuming that `x` denotes a ratio of 
CF.sub.3.sup.+ to (C.sub.2 F.sub.4.sup.+ +CF.sub.3.sup.+) (i.e., 
x=CF.sub.3.sup.+ /(C.sub.2 F.sub.4.sup.+ +CF.sub.3.sup.+)), if the 
incident energy E directed to the above semiconductor substrate is 
controlled to satisfy the following equation, an excellent etching 
selectivity can be obtained. 
152 eV&lt;E.ltoreq.223 eV 
x&lt;-0.744+ 19768.7/(214.8E -21240) 
where x : CF.sub.3.sup.+ /(C.sub.2 F.sub.4.sup.+ +CF.sub.3.sup.+) 
E: ion incident energy 
When the incident energy is 152 eV or less, the etch rate is reduced. 
However, since the etch rate of the silicon nitride film is small, such 
incident energy may be suitably used as necessary. 
Then, in order to detect an electron energy dependency in the dissociation 
of a C.sub.4 F.sub.8.sup.+ gas, electron energy and the numbers of 
CF.sub.3.sup.+ and C.sub.2 F.sub.4.sup.+ ions generated by the 
dissociation thereof are measured. The result is given in FIG. 7(a). In 
the drawing, abscissa denotes electron energy and ordinate denotes mass 
signal intensity indicative of the above ion numbers. A relationship 
between an existence probability of CF.sub.3.sup.+ ions to C.sub.2 
F.sub.4.sup.+ ions found from FIG. 7(a) and the electron energy is shown 
in FIG. 7(b). When the incident energy below 200 eV is used and attention 
is directed to the etching of the silicon nitride film, ions other than 
the CF.sub.3.sup.+ and C.sub.2 F.sub.4.sup.+ ions are negligible as seen 
in FIG. 5. Accordingly, it is learned that, in order to obtain an etching 
selectivity of 20% or less, it is preferable to set a ratio of 
CF.sub.3.sup.+ ions to C.sub.2 F.sub.4.sup.+ at 20% or less. In this case, 
it is seen from FIG. 7(b) that when the electron energy is suppressed to 
33 eV or less, the ratio of CF.sub.3.sup.+ ions to C.sub.2 F.sub.4.sup.+ 
can be set at 20 % or less. 
As shown in the foregoing embodiment, when the kinetic energy of electrons 
in C.sub.4 F.sub.8.sup.+ plasma is below 33 eV, a ratio of C.sub.2 
F.sub.4.sup.+ ions generated in the plasma becomes high and thus the 
SiO.sub.2 /SiN selective etching can be realized with a selectivity of 20 
or more. However, in magnetron plasma or inductive coupled plasma, E/P (E: 
electric field intensity, P: pressure within the container) is as great as 
100 (V/cm.multidot.Torr) or more, for which reason an average electron 
temperature in the C.sub.2 F.sub.4 plasma is as large as 15 eV and the 
percentage of electrons of the energy below 33 eV is small as shown in 
FIG. 8. In such rare gas plasma as He, Kr or Xe, even when E/P is 100 
(V/cm.multidot.Torr) or more, the electron temperature is low. Based on 
such facts, the inventors of the present application have expected that 
when a mixture gas of the above rare gas and a c-C.sub.4 F.sub.8 gas is 
used in a magnetron, the electron temperature in the plasma drops, the 
percentage of the C.sub.2 F.sub.4 ions increases, and the selectivity in 
the SiO.sub.2 /SiN selective etching increases, and have conducted 
experiments. The experiments have showed that only when Kr and Xe gases 
are used, the SiO.sub.2 /SiN selectivity of 20 or more is obtained by 
mixing the Kr or Xe gas into the c-C.sub.4 F.sub.8 gas at a suitable 
mixture ratio. Further, the mixture ratio of rare gas/C.sub.4 F.sub.8 can 
be reduced in the order of Kr and Xe and in the case of Xe, a small amount 
of addition of Xe enables a selectivity of 20 or more, with great effects. 
This is because, as shown in FIG. 9, the electron temperature of each rare 
gas in a range of E/P&gt;1000 (V/cm.multidot.Torr) is reduced in the order of 
Kr and Xe, which produces great electron cooling effects in the order of 
Xe and Kr. Meanwhile, in the case of He, Ne and Ar, their electron 
temperatures are lower than that of C.sub.4 F.sub.8 but the percentage of 
electrons exceeding 33 eV is relatively large in their distribution 
functions. For this reason, it is considered that sufficient cooling 
effect cannot be obtained and a high selectivity cannot be obtained. In 
this way, when Kr or Xe is added, efficient ionization can be realized 
with a low constant energy suitable for generation of C.sub.2 F.sub.4. 
Embodiment 2: 
FIG. 10 schematically shows an arrangement of a surface treatment apparatus 
used in the method of this embodiment. 
The apparatus utilizes magnetron discharging. In the apparatus, an applying 
power or incident energy to a substrate or a gas pressure, a flow rate and 
a density are adjusted to control the stay time of c-C.sub.4 F.sub.8 
molecules in the plasma to thereby control a component ratio between 
chemical species in the plasma. The apparatus comprises a grounded vacuum 
container 201 which forms a reaction chamber, an upper electrode 201a 
formed as an upper wall of the vacuum container 201, a lower electrode 203 
which is disposed as opposed to the upper electrode 201a within the vacuum 
container 201, which is provided with a temperature control mechanism 
including a cooling piping, and which is used as a sample carrying base, 
an exhaustion system 206 for vacuumizing the interior of the vacuum 
container 201, and a gas supply source 205. The apparatus performs surface 
treating operation over a substrate 207 placed on the lower electrode 203. 
The lower electrode 203 is connected to a high frequency power source 204 
via a blocking capacitor C, while the upper electrode 201a is grounded. 
Applied between these electrodes is a high frequency power of 13.56 Mhz so 
that a plasma 202 is generated within the vacuum container 201. Also 
applied onto the lower electrode 203 is a copper plate interposed between 
polyimide thin films, so that when a voltage of 4 kV is applied from a 
power source (not shown) to the copper plate, the substrate 207 to be 
treated is electrostatically attracted onto the lower electrode. Disposed 
above the upper electrode 201a is a magnetic field generator 208 which is 
made up of a plurality of permanent magnets and its drive mechanism to 
apply a magnetic field to a space defined by the opposing lower and upper 
electrodes 203 and 201a. 
c-C.sub.4 F.sub.8.sup.+ are introduced into the apparatus to generate a 
plasma therein. The flow rate and pressure of the gas and the applying 
power are adjusted in the following manner so as to perform etching. 
The etching conditions are electron density n.sub.e =3E10 cm.sup.-3, 
reaction container volume V=2.7 liters, C.sub.4 F.sub.8 gas flow rate 
Q=4.5 Torr.multidot.liter/sec, process pressure P=40 mTorr, N =n.sub.3 x P 
x V x Q.sup.-1 =7.2E8 (=7.2.times.10.sup.8), where En denotes 
.times.10.sup.n. When electron density n.sub.e =1E9 cm.sup.-3, an etching 
selectivity of 20 is obtained. As a result of analyzing the then ion 
species by using a mass spectrometer, CF.sub.3.sup.+, CF.sub.2.sup.+ and 
C.sub.2 F.sub.4.sup.+ ions are detected and a ratio CF.sub.3.sup.+ 
/(CF.sub.3.sup.+ +C.sub.2 F.sub.4.sup.+) is 6%. 
Only the flow rate Q is changed while the other conditions remain 
unchanged. That is, when the flow rate Q is set to be larger, a sufficient 
selectivity can be obtained. On the other hand, when the flow rate Q is 
set to be smaller, a sufficient selectivity cannot be obtained. This is 
considered to be due to the fact that a stay time is increased and thus 
CF.sub.3.sup.+ ions are increased. Further, when the pressure is increased 
or the container volume is increased, a selectivity is reduced. 
The present invention is not limited to the above C.sub.4 F.sub.8 gas but 
such a gas having a large `n` in (CF.sub.2).sub.n.sup.+ as a C.sub.5 
F.sub.10 gas may be introduced into the apparatus to generate a plasma 
therein. In the latter case, the gas flow rate and pressure, application 
power and magnetic field intensity are adjusted to generate such a plasma 
that contains (CF.sub.2)n.sup.+ having a specific `n` as its major 
component. And when a biasing potential to be applied to the substrate is 
adjusted to control the incident ion energy, the etching of the silicon 
oxide film against the silicon nitride film can be carried out with a very 
excellent selectivity. 
Next, ion species in the magnetron plasma generated by dissociation of the 
C.sub.4 F.sub.8 gas are analyzed and the result of the analysis is shown 
in FIG. 11. FIG. 11(b) shows an example in which the flow rate is set to 
be larger than the case of FIG. 11(a). FIG. 11(a) shows an example in 
which a ratio CF.sub.3.sup.+ /(CF.sub.2.sup.+ +C.sub.2 F.sub.4.sup.+) is 
35% and the etching selectivity of the silicon oxide film against the 
silicon nitride film is 2. FIG. 11(b) shows an example in which the gas 
flow rate is set to be larger than that of FIG. 11(a) and the ratio 
CF.sub.3.sup.+ /(CF.sub.2.sup.+ +C.sub.2 F.sub.4.sup.+) is reduced to 8% 
and the etching selectivity is increased to 20. 
A relationship of the collision coefficient N to the ratio CF.sub.3.sup.+ 
/(CF.sub.2.sup.+ +C.sub.2 F.sub.4.sup.+) is measured and the result of the 
measurement is given in FIG. 12. In the drawing, ordinate denotes the 
ratio CF.sub.3.sup.+ /(CF.sub.2.sup.+ +C.sub.2 F.sub.4.sup.+) and abscissa 
denotes the collision coefficient N expressed by the following equation. 
EQU N=n.sub.e x P x V x Q.sup.-1 
where n.sub.e is electron density in plasma, P process pressure, V the 
volume of a reactor container, Q c-C.sub.4 F.sub.8 flow rate. When N is 
set to be 7.2E8 or less, the ratio CF.sub.3.sup.+ /(CF.sub.2.sup.+ 
+C.sub.4 F.sub.8.sup.+) is 8% or less and the selectivity is 20 or more. 
As a result, it is learned that it is preferable that N is 7.2E8 or less. 
Embodiment 3: 
FIG. 13 is a schematic arrangement of a parallel plate type surface 
treatment apparatus used in a method in accordance with a third embodiment 
of the present invention. 
The apparatus, which comprises a parallel plate type plasma generation 
apparatus, utilizes discharging between electrodes and an applying power 
or gas pressure is adjusted to control a component ratio between chemical 
species in a plasma. The apparatus comprises a vacuum container 301 
forming part of reactor chamber, an upper electrode 301a installed within 
the vacuum container 301, a lower electrode 303 which is installed as 
opposed to the upper electrode 301a within the vacuum container 301, which 
is provided with a temperature control mechanism including a cooling 
piping and which is used also as a sample carrying base, an exhaustion 
system 306 for vaccumizing the interior of the vacuum container 301, and a 
gas supply system 305. The apparatus performs surface treating operation 
over a substrate 307 placed on the lower electrode 303. The lower 
electrode 303 is connected to a high frequency power source 304 through a 
matching circuit (not shown), while the upper electrode 301a is grounded. 
Applied between these electrodes is a high frequency power of 13.56 Mhz to 
generate a plasma 302 within the vacuum container 301. 
A c-C.sub.4 F.sub.8.sup.+ gas is introduced into the apparatus to generate 
the plasma. And a gas flow rate, a pressure and an applying power are 
adjusted in the following manner and then etching is carried out. 
When reactor container volume V=2 liters, C.sub.4 F.sub.8 gas flow rate Q=2 
Torr.multidot.liter/sec, process pressure P=1 torr, N=n.sub.e x P x V x 
Q.sup.-1 =1E9, an etching selectivity of 20 is obtained. As a result of 
analyzing ion species by using a mass spectrometer, a ratio CF.sub.3.sup.+ 
/(CF.sub.2.sup.+ +C.sub.4 F.sub.8.sup.+) is 8% or less. Even in this case, 
the flow rate Q is changed while the other conditions remain unchanged. 
That is, when the flow rate Q is set to be smaller, a sufficient 
selectivity can be obtained, whereas, when the flow rate Q is set to be 
smaller, a sufficient selectivity can be obtained. This is considered to 
be due to the fact that a stay time c-C.sub.4 F.sub.8 is increased and 
thus CF.sub.3.sup.+ ions are increased. Further, when the pressure is 
increased or the container volume is increased, the selectivity is 
reduced. 
Although the above description has been made in connection with the 
parallel plate apparatus based on the magnetron discharging in the present 
embodiment, such another plasma treatment system as a system based on 
electron cyclotron resonance (ECR) or a helicon type reactive ion etching 
(RIE) system using helicon wave may be employed. 
Description will now be made as to a case where, for example, a helicon 
type RIE system is employed. The system, as shown in FIGS. 14 and 15, 
comprises a quartz discharge lamp 402 and a vacuum pump 403 mounted within 
a reaction chamber 401. Disposed within the quartz discharge lamp 402 is a 
double loop type discharging antenna 407 one end of which is connected to 
a high frequency power source 408 and the other end of which is grounded. 
Also disposed within the quartz discharge lamp 402 is an electromagnetic 
coil 409 for generating a uniform magnetic field of 400 gausses within the 
quartz discharge lamp 402, which coil is arranged so that, as the 
electromagnetic coil 409 goes away from the electromagnetic coil 409, a 
magnetic field intensity becomes small within the reaction chamber 401. 
Reference numeral 406 denotes a valve and numeral 410 denotes a substrate 
carrying plate. 
When the system is operated under conditions that electron density n.sub.e 
=1E13 cm.sup.-3, reaction container volume V=2.4 liters, C.sub.4 F.sub.8 
gas flow rate Q=0.88 Torr.multidot.liter/sec, process pressure P=1.7 mtorr 
and N=n.sub.e x P x V x Q.sup.-1, an etching selectivity of 20 is 
obtained. As N is made further smaller, the etching selectivity is 
improved. When ion species are analyzed by using a mass spectrometer, a 
ratio CF.sub.3.sup.+ /(CF.sub.2.sup.+ +C.sub.4 F.sub.8.sup.+) is 8% or 
less. Even in this case, only the flow rate Q is changed while the other 
conditions remain unchanged. That is, when the flow rate Q is large, a 
sufficient selectivity can be obtained ,whereas, when the flow rate Q is 
small, a sufficient selectivity cannot be obtained. This is considered to 
be due to the fact that a stay time increased and CF.sub.3.sup.+ ions 
increased. Further, when the pressure is increased or the container volume 
is increased, the selectivity is reduced. 
Description will further be made as to a case where such an electron beam 
excitation type RIE system (EBEP) as shown in FIG. 18 is used. The system 
comprises a discharging chamber 606 for performing discharging operation, 
and an excitation chamber 605 for exciting electrons generated by the 
discharging and for sending excited electrons to a reaction chamber 601. 
In the reaction chamber, a reactive gas introduced thereinto from a gas 
inlet port 620 is dissociated by the excited electrons and then guided 
onto a substrate 610 to be treated to perform suitable etching. The 
discharging chamber 606 is provided at its rear end with an electrode 616, 
and a discharging gas 614 is introduced into the discharging chamber 606 
via a gas inlet port 613. Reference numerals 612, 616 and 614 denote 
electrodes, respectively. 
When the above system is operated under conditions that electron density 
n.sub.e =1E11 cm.sup.-3, reaction container volume =35 liters, C.sub.4 
F.sub.8 gas flow rate =0.25 Torr.multidot.liter/sec and process pressure 
=0.5 mtorr, N is n.sub.e x P x V x Q.sup.-1 =7E9. 
In this case, an etching selectivity of 20 is obtained. When N is further 
set to be smaller, the etching selectivity is improved. At this time, as a 
result of analyzing ion species by using a mass spectrometer, a ratio 
CF.sub.3.sup.+ /(CF.sub.2.sup.+ +C.sub.4 F.sub.8.sup.+) is 8% or less. 
Description will now be made as to a case where a .mu.-wave plasma etching 
system (ECR) such as shown in FIG. 17 is employed. In this system, a 
microwave of 2.45 GHz generated by a magnetron 704 is guided into a 
reaction container 701 applied with a magnetic field by a magnetic field 
coil 707 via a waveguide 705 and a quartz window 706 so that a reactive 
gas is subjected to an ECR discharging excitation. A RF power of 13.56 Mhz 
is applied to a sample table 703 to control ion radiation energy on a 
wafer 702 placed on the sample table. When this system is operated under 
conditions that electron density n.sub.e =1E11 cm.sup.-3, reaction 
container volume=25 liters, C.sub.4 F.sub.8 gas flow rate =0.38 
Torr.multidot.liter/sec, and process pressure =0.5 mtorr, N is n.sub.e x P 
x V x Q.sup.-1 =3.3E9 and an etching selectivity of the silicon oxide film 
to the silicon nitride film is 20. When N is set to be further smaller, 
the etching selectivity is improved. As a result of analyzing ion species 
by using a mass spectrometer, the ratio CF.sub.3.sup.+ /(CF.sub.2.sup.+ 
+C.sub.4 F.sub.8.sup.+) is 8% or less. 
When an inductive coupled RIE system is operated under such conditions that 
electron density n.sub.e =1E12 cm.sup.-3, reaction container volume V =7 
liters, C.sub.4 F.sub.8 gas flow rate Q=0.5 Torr.multidot.liter/sec and 
process pressure P=3 mtorr, N is n.sub.e x P x V x Q.sup.-1 =4.2E10 and an 
etching selectivity of 20 is obtained. When N is set to be further 
smaller, the etching selectivity is improved. From the result of analyzing 
ion species by using a mass spectrometer, it is found that a ratio 
CF.sub.3.sup.+ /(CF.sub.2.sup.+ +C.sub.4 F.sub.8.sup.+) is 8% or less. 
FIG. 18 shows a result of measuring the variations in the etch rates of the 
silicon oxide film and silicon nitride film when pulse modulation is 
effected with the magnetron plasma. As seen in FIG. 18, the selectivity is 
increased from 14 to 22 with the pulse modulation of 100 msec. 
When a plasma is generated based on photo excitation, a relationship 
between electron energy and the numbers of CF.sub.3.sup.+ and C.sub.2 
F.sub.4.sup.+ ions with respect to different excitation wavelengths is 
measured. FIG. 19 shows the result of the measurement. It is appreciated 
from the result that CF.sub.3.sup.+ ions do not exist in a excitation 
wavelength zone of 70.9 to 102.5 nm, i.e., in an electron energy zone of 
12.1 to 17.5. Accordingly, an excellent etching selectivity can be 
obtained. 
In accordance with the present invention, since the etching selectivity of 
not only the silicon oxide film but also such a silicon oxide film 
containing impurities as BPSG or