 |
Claims  |
|
|
What is claimed is:
1. A surface treatment method for performing treating operation over a surface of a semiconductor substrate by using a fluorocarbon gas, comprising:
a gas supply step of supplying a gas containing a fluorocarbon gas; and
an etching step of generating ions by exciting the fluorocarbon gas, controlling an ion having (CF.sub.2).sub.n.sup.+ (n=1, 2, 3, . . . ) as a major component among the generated ions so as to be guided onto the semiconductor substrate, and
selectively etching a silicon oxide film against a silicon nitride film.
2. A surface treatment method as set forth in claim 1, wherein the etching step includes a step of controlling an energy incident to the semiconductor substrate such that the energy has an energy value capable of causing reaction induced on the
silicon nitride film by the ion having (CF.sub.2).sub.n.sup.+ as the major component to be shifted from etching to deposition.
3. A surface treatment method as set forth in claim 1, wherein 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<E.ltoreq.223 eV, and
x<-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.
4. A surface treatment method as set forth in claim 1, wherein the etching step includes a step of selectively etching a silicon oxide film against a silicon nitride film by controlling the ions such that energy incident to the semiconductor
substrate is 152 eV or less, and by guiding the ions onto the semiconductor substrate.
5. A surface treatment method as set forth in claim 3, wherein the etching step is a step of controlling an ion beam such as to satisfy the relationships and guiding the ion beam onto the semiconductor substrate.
6. A surface treatment method as set forth in claim 4, wherein the etching step is a step of controlling an ion beam such as to satisfy the relationships and guiding the ion beam onto the semiconductor substrate.
7. A surface treatment method as set forth in claim 1, wherein the gas supply step includes a step of supplying a c-C.sub.4 F.sub.8.sup.+ gas as the fluorocarbon gas, and the ions are generated by changing the c-C.sub.4 F.sub.8 gas into plasma.
8. A surface treatment method as set forth in claim 1, wherein 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, and 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 generate ions, controlling the generated 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.
9. A surface treatment method as set forth in claim 1, wherein 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.
10. A surface treatment method as set forth in claim 1, wherein 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.
11. A surface treatment method as set forth in claim 1, wherein 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 is generated by generating a plasma from the gas with use of light having wavelengths of between 70.9 nm and 102.5 nm.
12. A surface treatment method as set forth in claim 1, wherein 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 relation 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.
13. A surface treatment method as set forth in claim 1, wherein 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.
14. A surface treatment method as set forth in claim 12, wherein 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.
15. A surface treatment method as set forth in claim 12, wherein 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.
16. A surface treatment method as set forth in claim 12, wherein the ions are generated 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.
17. A surface treatment method as set forth in claim 12, wherein 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.
18. A surface treatment method as set forth in claim 12, wherein 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.
19. A surface treatment method as set forth in claim 12, wherein 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.
20. A surface treatment method as set forth in claim 3, wherein 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 ions are generated by changing the gas into plasma by using an high-frequency pulse signal modulated with pulses of several milliseconds to several hundreds of milliseconds.
21. A surface treatment method as set forth in claim 4, wherein 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 ions are generated by changing the gas into plasma by using an high-frequency pulse signal modulated with pulses of several milliseconds to several hundreds of milliseconds.
22. A surface treatment method as set forth in claim 20, wherein the plasma of the gas is generated by a magnetron excited plasma generation apparatus.
23. A surface treatment method as set forth in claim 1, wherein 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 ions are generated by changing the gas into plasma by using an electron beam modulated with pulses of several milliseconds to several tens of milliseconds.
24. A surface treatment apparatus comprising:
gas supply means for supplying a fluorocarbon gas onto a semiconductor substrate disposed within a reaction container;
gas exhaustion means for exhausting gas within the reaction container; and
ion species control means for changing the fluorocarbon gas into plasma and controlling such as to guide an ion having (CF.sub.2).sub.n.sup.+ as a major component among the obtained ions onto the semiconductor substrate.
25. A surface treatment apparatus as set forth in claim 24, further comprising means for controlling at least one of the gas supply means and the gas exhaustion means so as to control the c-C.sub.4 F.sub.8 flow rate Q or the c-C.sub.4 F.sub.8
partial pressure such that 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, n.sub.e indicative of an electron density in the plasma and .tau. indicative of a
stay time of the c-C.sub.4 F.sub.8 molecules in the plasma within the air-tight reaction container satisfy the following relationship.
N =n.sub.e x .tau..ltoreq.7.2E8
where .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
26. A surface treatment apparatus as set forth in claim 24, wherein the ion species control means includes means for changing the gas into plasma by using an electron beam modulated with pulses of several milliseconds to several tens of
milliseconds. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
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 < E .ltoreq. 223 eV, and
x < -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 (ASSG).
In the present invention, when (CF.sub.2).sub.n.sup.+ among chemical species generated in the plasma of the fluorocarbon gas are preferentially energy-controlled and then sent onto the substrate, the etching of the silicon oxide film against the
silicon nitride film can be carried out with an excellent selectivity. Thus, when it is desired to perform etching over the silicon oxide film deposited on the silicon nitride film for example, the etching can be stopped on the surface of the silicon
nitride film.
As a result of various experiments, the inventors of the present application has found that when (CF.sub.2).sub.n.sup.+ ions are used in the fluorocarbon gas, the selectivity can be improved and that the energy value at which transition takes
place on the silicon nitride film from etching to deposition is determined by the `n`.
This phenomenon is considered to occur through the following mechanism. That is, as the incident energy of (CF.sub.2)n.sup.+ ions directed to the substrate surface is reduced, the etching product varies, thus reducing an etch rate. When the
incident energy drops to a certain level or less, C remains on the substrate surface to form a substance containing Si-C couplings. Since this substance and the fluorocarbon polymer film formed on the substance function as protective films, when the
ions are continuously directed to the substrate surface at that level, the substrate surface is fully covered with these substances, which results in the etching is stopped halfway and deposition takes place. AS the energy is further decreased,
probability of C residual is increased. When the energy drops to a certain level or less, the etching is stopped and only deposition takes place. The transition energy from etching to deposition on the silicon oxide film is different from that on the
silicon nitride film. By determining the energy value while utilizing the above fact, selective etching can be realized. The present invention is based on the above viewpoint.
In the course of shifting from etching to deposition, there exists an energy zone where the etching is stopped halfway and instead deposition starts to take place, which results in that the incident energy value to be controlled can have an
allowance depending on the tradeoff of the thickness of the silicon nitride film.
It has been further found that, for the purpose of actually realizing this selective etching, when the collision frequency between ions and electrons is controlled at the time of introducing the ions obtained by the plasma dissociation of the
fluorocarbon gas onto the substrate, a high selectivity can be obtained.
Thus, for the purpose of an etching selectivity of 20 which is considered to be necessary for fabricating a device, CF.sup.+, CF.sub.2.sup.+ and CF.sub.3.sup.+ ions are separately generated by using a mass spectrometer, etching is carried out and
etched amounts are measured to detect a difference between etching characteristics based on different ion species. When attention is directed to C.sub.2 F.sub.4.sup.+, CF.sub.2.sup.+ and CF.sub.3.sup.+ ions present in the C.sub.4 F.sub.8 gas plasma,
respective incident ion energies capable of producing the practical selectivity of 20 necessary for the actual device fabrication are 223 eV, 215 eV and 152 eV respectively, as mentioned above (refer to FIG. 5).
In the range of 152 eV to 223 eV, as seen from FIG. 5, it is substantially only C.sub.2 F.sub.4.sup.+ and CF.sub.3.sup.+ that the etching of the silicon nitride film becomes large enough to cause problems. Other ions can be neglected.
Accordingly, it is found that, in this range, only the mixture ratio of these ions should be selected so that the etching selectivity has 20 or more. To this end, an upper limit of a mixture ratio x to the incident ion energy is calculated based on the
etch rate obtained in FIG. 5, and a curve Q indicative of the upper limit is obtained (refer to FIG. 6). A zone below the curve Q is called a safety zone for the incident energy zone of 152 eV to 223 eV, i.e., where the etching selectivity of 20 or more
can be assured. As a result of our experiments, it has been found that, when a relationship between the mixture percentage of CF.sub.3.sup.+ as a cause of reducing the selectivity and the incident energy to the substrate is found and when `x` denotes a
ratio (=CF.sub.3.sup.+ /(C.sub.2 F.sub.4.sup.+ + CF.sub.3.sup.+)) of CF.sub.3.sup.+ to (C.sub.2 F.sub.4 .sup.+ + CF.sub.3.sup.+) in the ions obtained by the dissociation of the fluorocarbon gas, an excellent selectivity can be obtained by controlling the
incident energy E directed to the semiconductor substrate and the ratio `x` to satisfy the following relationships.
152 eV < E .ltoreq. 223 eV
x < -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: incident energy of ions
Further, it has been found from the experiments that, by controlling and introducing onto the substrate the C.sub.2 F.sub.4.sup.+, CF.sub.2.sup.+, and CF.sub.3.sup.+ ions obtained by the dissociation of the fluorocarbon gas so that the energy
directed to the semiconductor substrate become 152 eV or less, the etch rate is reduced but an excellent selectivity can be obtained because the etch rate of the silicon nitride film is small.
It has also been found from our experiments that, even in the case of using plasma dissociation, when the ions introduced onto the substrate are controlled so that a ratio of a CF.sub.3.sup.+ ion concentration to the total (CF.sub.2).sub.n.sup.+
ion concentration is 8% or less, an excellent selectivity can be obtained.
In addition, it has been found from our experiments that when a c-C.sub.4 F.sub.8 gas and at least one of Kr and Xe gases are introduced under a reduced pressure into an air-tight reaction chamber for accommodating a semiconductor substrate and
the gas is changed to a plasma, an excellent selectivity can be obtained.
This is considered to due to the fact that, since the Kr or Xe gas plasma has a low electron temperature, when the gas is mixed with c-C.sub.4 F.sub.8 gas, the further dissociation of C.sub.2 F.sub.4.sup.+ ions generated by the dissociation of
the c-C.sub.4 F.sub.8 gas can be suppressed and C.sub.2 F.sub.4.sup.+ ions to CF.sub.3.sup.+ ions in the plasma can be selectively generated.
Also it has been found from our experiments that the c-C.sub.4 F.sub.8 gas is introduced, excited and ionized with a controlled electron beam to cause the electron energy to be less than 33 eV, an excellent selectivity can be obtained.
Our experiments have showed that when the c-C.sub.4 F.sub.8 gas is introduced and changed to a plasma by using light having wavelengths of 70.9 nm to 102.5 nm, an excellent selectivity can be obtained. This is considered to be due to the fact
that, in this wavelength zone, since C.sub.2 F.sub.4.sup.+ ions can be selectively generated to CF.sub.3.sup.+ ions, an excellent selectivity can be obtained.
Further, our experiments have showed that when a c-C.sub.4 F.sub.8 gas is introduced and changed to a plasma and when an electron density in the plasma and a stay time r of c-C.sub.4 F.sub.8 molecules in the plasma within the air-tight reaction
container are controlled so that a collision coefficient N between the c-C.sub.4 F.sub.8 molecules present in the plasma and electrons present in the plasma becomes 7.2.times.10.sup.8 or less, an excellent selectivity can be obtained, where N is
expressed by n.sub.e x .tau. (=P x V x Q.sup.-1), n.sub.e denotes an electron density in the plasma, P denotes process pressure, V denotes the volume of the reaction container, and Q denotes a c-C.sub.4 F.sub.8 flow rate.
Preferably, the incident energy of ions in the gas plasma directed to the semiconductor substrate is controlled to be 500 eV or less. When the incident energy exceeds 500 eV, the damage of the substrate surface becomes remarkable.
Further, it is preferable that the plasma is generated within a magnetron plasma generation apparatus.
In this case, it has been found that, when the stay time .tau. =P x V/Q of the c-C.sub.4 F.sub.8 molecules present in the plasma is controlled to be 24 sec or less, an excellent sensitivity can be obtained.
It has also been found that, in the case of generating a plasma by using a parallel plate plasma generation apparatus, when the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules present in the plasma is controlled to be 720 msec or
less, an excellent sensitivity can be obtained.
Our experiments have also showed that, in the case of exciting the gas by using a controlled electron beam, when the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules present in the plasma is controlled to be 7.2 msec or less, an
excellent sensitivity can be obtained.
It has also been found from our experimental results that, in the case of generating a plasma by using a .mu.-wave excited plasma generation apparatus, when the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules present in the plasma
is controlled to be 7.2 msec or less, an excellent sensitivity can be obtained.
It has further been found from our experimental results that, in the case of generating a plasma by using an inductive coupled plasma generation apparatus, when the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules present in the
plasma is controlled to be 0.72 msec or less, an excellent sensitivity can be obtained.
It has also been found from our experimental results that, in the case of generating a plasma by using a helicon-excited plasma generation apparatus, when the stay time .tau. = P x V/Q of the c-C.sub.4 F.sub.8 molecules present in the plasma is
controlled to be 0.072 msec or less, an excellent sensitivity can be obtained.
It has furthermore been found from our experimental results that, when the c-C.sub.4 F.sub.8 gas is introduced and changed to a plasma by using a high frequency signal modulated with pulses of several milliseconds to several tens of milliseconds,
an excellent sensitivity can be obtained.
Preferably, the plasma of the gas is a magnetron plasma.
It has been found that, preferably when the c-C.sub.4 F.sub.8 gas is introduced into an air-tight reaction container for accommodating a semiconductor substrate under a reduced pressure and Then changed to a plasma by using an electron beam
modulated with pulses of several milliseconds to several tens of milliseconds, an excellent sensitivity is obtained.
Further, the second aspect of the present invention 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 by means of dissociation of the fluorocarbon gas and controllably guiding ones (CF.sub.2).sub.n.sup.+ of obtained ions as a major component onto the semiconductor substrate.
From the experiment result, it has been found preferable that the gas supply means includes means for controlling the c-C.sub.4 F.sub.8 gas flow rate Q and the c-C.sub.4 F.sub.8 partial pressure P so that the collision coefficient N between the
c-C.sub.4 F.sub.8 molecules present in the plasma and electrons present in the plasma, the electron density n.sub.e and the stay time r of the c-C.sub.4 F.sub.8 molecules in the plasma within the air-tight reaction container satisfy the following
relationship.
N = n.sub.e x .tau. .ltoreq. 7.2E8
where .tau. = P : c-C.sub.4 F.sub.8 partial pressure
V: volume of the reaction container
Q: c-C.sub.4 F.sub.8 gas flow rate
Further, preferably, when the ion species control means includes means for generating a plasma from the gas by using an electron beam modulated with pulses of several milliseconds to several tens of milliseconds, a selectivity can be improved to
a large extent.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic arrangement of a surface treatment apparatus used in a method in accordance with an embodiment of the present invention;
FIGS. 2(a) to 2(c) show surface treatment steps of the embodiment of the present invention;
FIG. 3 shows a relationship between ion species and etch rate;
FIG. 4 shows a relationship between incident energy and etch rate;
FIG. 5 shows a relationship between incident energy and etch rate with respect to various sorts of ion species;
FIG. 6 shows a relationship between incident energy and CF.sub.3.sup.+ /(C.sub.2 F.sub.4.sup.+ + CF.sub.3.sup.+);
FIGS. 7(a) and 7(b) show an electron energy dependency in dissociation of a C.sub.4 F.sub.8 gas, respectively;
FIG. 8 shows a relationship between electron temperature and distribution function;
FIG. 9 shows a relationship between electron temperature and E/P;
FIG. 10 schematically shows a surface treatment apparatus in accordance with the second embodiment of the present invention;
FIG. 11 shows diagrams for explaining flow rate and generated ion species in dissociation of the C.sub.4 F.sub.8 gas;
FIG. 12 shows a relationship between mixture ratio of a CF.sub.3.sup.+ gas and collision coefficient;
FIG. 13 schematically shows a surface treatment apparatus in accordance with the third embodiment of the present invention;
FIG. 14 schematically shows a surface treatment apparatus in accordance with the fourth embodiment of the present invention;
FIG. 15 shows a major part of the same apparatus;
FIG. 16 schematically shows a surface treatment apparatus in accordance with the fifth embodiment of the present invention;
FIG. 17 schematically shows a surface treatment apparatus in accordance with the sixth embodiment of the present invention;
FIG. 18 shows a relationship between pulse width and etching selectivity;
FIG. 19 shows a relationship between electron energy and the numbers of existing CF.sub.3.sup.+ and C.sub.2 F.sub.4.sup.+ ; and
FIG. 20 shows an example of a selective etching.
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>4), by | | |
