METHOD FOR DRY ETCHING INTERLAYER INSULATING FILM

Abstract

A method for dry etching an interlayer insulating film with an ArF resist or KrF resist thereon comprises dry etching fine features into the interlayer insulating film with an etching gas in such a manner as to form a polymer film on the ArF or KrF resist from the etching gas, wherein the etching gas is introduced under a pressure of 0.5 Pa or less, and wherein a Fourier transform infrared spectrum of the polymer film includes a C—F bond peak at about 1200 cm−1, a C—N bond peak at about 1600 cm−1, and a C—H bond peak at about 3300 cm−1.

Description

TECHNICAL FIELD

The present invention relates to a method for dry etching an interlayer insulating film.

BACKGROUND ART

Traditionally, interlayer insulating films were commonly made of SiO₂. Since the 90 nm node, however, low dielectric constant materials (or low-k materials) have been increasingly substituted for SiO₂ as the material for interlayer insulating films in order to solve the problem of wiring delay. It has been proposed that, in order to etch fine grooves or holes in such low dielectric constant films, ArF resist material may be used, instead of conventionally used KrF resist material, since the former is used with a shorter wavelength of light than the latter and hence is suitable for high precision processing (see, e.g., Patent Document 1).

• Patent Document 1: Japanese Laid-Open Patent publication No. 2005-72518, paragraph [0005], etc.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, ArF resist materials generally have poor plasma resistance. Therefore, a fine exposure pattern of an ArF resist material is likely to be distorted due to damage by the plasma etching process. This distortion is directly transferred to the low dielectric constant film under the resist during the etching process, which tends to cause irregularities, such as striations, in the edge surfaces of fine grooves or holes formed in the low dielectric constant film.

Thus, there exists a need in the art to overcome the above problems. It is, therefore, an object of the present invention to provide a method for dry etching an interlayer insulating film in such a manner as to prevent damage to the resist.

Means for Solving the Problems

The present invention provides a method for dry etching an interlayer insulating film with an ArF resist or KrF resist thereon, the method comprising dry etching fine features into the interlayer insulating film with an etching gas in such a manner as to form a polymer film on the ArF or KrF resist from the etching gas, wherein the etching gas is introduced under a pressure of 0.5 Pa or less, and wherein a Fourier transform infrared spectrum of the polymer film includes a C—F bond peak at about 1200 cm⁻¹, a C—N bond peak at about 1600 cm⁻¹, and a C—H bond peak at about 3300 cm⁻¹.

The introduction of the etching gas under a pressure of 0.5 Pa or less prevents the formation of reactive species of the etching gas, thereby reducing damage to the resist. Further, since the interlayer insulating film is etched in such a manner as to form a polymer film on the resist from the etching gas, damage to the resist is further reduced, resulting in a high etch selectivity ratio (i.e., a high ratio of the etch rate of the interlayer insulating film to that of the resist).

The etching gas is preferably a mixture of a CF-based gas, a N-containing gas, and a low molecular weight hydrocarbon gas. The use of these gases allows for the formation of a polymer film on the resist which film exhibits C—F, C—N, and C—H bond peaks in its absorption spectrum. This reduces damage to the resist and allows the low dielectric constant film (or interlayer insulating film) to be etched without premature etch stop.

Further, the etching gas is preferably a mixture of a C_xF_yH_z gas and a N-containing gas. The use of these gases also allows for the formation of a polymer film on the resist which film exhibits C—F, C—N, and C—H bond peaks in its absorption spectrum. This reduces damage to the resist and allows the low dielectric constant film (or interlayer insulating film) to be etched without premature etch stop.

The CF-based gas preferably includes at least one gas selected from the group consisting of CF₄, C₃F₈, C₂F₆, C₄F₈, C₅F₈ and C_xF_yI.

The low molecular weight hydrocarbon is preferably selected from the group consisting of CH₄, C₂H₆, C₃H₈, C₄H₁₀, and C₂H₂.

The C_xF_yH_z gas is preferably CHF₃ gas.

The N-containing gas preferably includes at least one gas selected from the group consisting of nitrogen gas, NO_x, NH₃, methylamine, and dimethylamine.

Further, the C_xF_yI gas is preferably C₃F₇I gas or CF₃I gas, and the interlayer insulating film is preferably an SiOCH-based material.

Effects of the Invention

An advantage of the present invention is that it etches an interlayer insulating film in a low pressure to reduce damage to the resist and hence reduce striations. Another advantage of the invention is that it etches an interlayer insulating film in such a manner as to form a polymer film on the resist from the etching gas to reduce damage to the resist, thereby achieving a high etch selectivity ratio.

BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 shows an etching system 1 for implementing a method for dry etching an interlayer insulating film according to the present invention. The etching system 1 includes a vacuum chamber 11 that allows for etching in low-temperature, high-density plasma. The vacuum chamber 11 includes vacuuming means 12 such as a turbo molecular pump.

The vacuum chamber 11 is made up of a substrate treatment section 13 and a plasma generating section 14 which lies on the substrate treatment section 13. A substrate mounting unit 2 is provided at the bottom center of the substrate treatment section 13. The substrate mounting unit 2 includes a substrate electrode 21 on which a processing substrate S is placed, an insulator 22, and a support base 23. The insulator 22 is interposed between the substrate electrode 21 and the support base 23. Further, the substrate electrode 21 is connected to a first high frequency power supply 25 through a blocking capacitor 24 and acts as a floating potential electrode. This electrode 21 is negatively biased.

A top panel 31 is provided at the top of the plasma generating section 14 and faces the substrate mounting unit 2. It is fixed to the sidewall of the plasma generating section 14 and is connected to a second high frequency power supply 33 through a variable capacitor 32. This top panel 31 is at a floating potential and acts as the opposite electrode.

Further, a gas feed path 41 of gas feeding means 4 is coupled to the top panel 31 to introduce an etching gas into the vacuum chamber 11. This gas feed path 41 is connected to a gas source 43 through gas flow rate control means 42. It should be noted that although only one gas source 43 is shown in FIG. 1, there may be a number of gas sources 43 corresponding to the number of types of gas used in the etching process. In such a case, the gas feed path 41 may be branched into a number of branches corresponding to the number of gas sources 43.

The plasma generating section 14 has a cylindrical dielectric sidewall. A magnetic field coil 51 may be provided around the outside of this sidewall as magnetic field generating means. In such a case, the magnetic field coil 51 produces a circular magnetic neutral line (not shown) within the plasma generating section 14.

A high frequency antenna coil 52 is disposed between the magnetic field coil 51 and the outside of the sidewall of the plasma generating section 14 to generate plasma. The high frequency antenna coil 52 has a parallel antenna structure, and is connected to the junction (or branch point) 34 between the variable capacitor 32 and the second high frequency power supply 33 to receive a voltage from the second high frequency power supply 33. The high frequency antenna coil 52 generates an alternating electric field along the magnetic neutral line produced by the magnetic field coil 51 to produce a plasma along the line.

Although in the present embodiment the antenna coil 52 for generating plasma receives a voltage from the second high frequency power supply 33, it is to be understood that in other embodiments a third high frequency power supply may be provided which is connected to the antenna coil 52. Further, means may be provided to adjust the voltage applied to the antenna coil to a predetermined value.

There will now be described a method of the present invention for dry etching an interlayer insulating film, e.g., in the system shown in FIG. 1.

The present invention is applied to interlayer insulating films composed of a low dielectric constant material (or low-k material) and formed on a substrate S. Examples of such low dielectric constant materials include SiOCH-based materials, such as HSQ and MSQ, which are applied by spin coating, etc. It will be noted that they may be porous.

Examples of suitable SiOCH-based materials are those available under the trade names: “LKD5109r5” from JSR Co., Ltd.; “HSG-7000” from Hitachi Chemical Co., Ltd.; “HOSP” and “Nanoglass” from Honeywell Electric Materials Inc.; “OCD T-12” and “OCD T-32” from Tokyo Ohka Kogyo Co., Ltd.; “IPS 2.4” and “IPS 2.2” from Catalysts & Chemicals Industries Co., Ltd.; “ALCAP-S 5100” from Asahi Kasei Corporation; and “ISM” from ULVAC, Inc.

The method begins by applying a resist material to such an interlayer insulating film and then forming a predetermined (resist) pattern by photolithography. Examples of suitable resist materials include known KrF resist materials (e.g., KrFM78Y from JSR Co., Ltd.) and known ArF resist materials (e.g., UV-II, etc.). It should be noted that when the interlayer insulating film is an SiOCH-based material, a BARC (antireflective coating) may be formed on the interlayer insulating film, and a resist material may be applied to the BARC.

Next, the substrate S with the interlayer insulating film thereon is placed on the substrate electrode 21 within the vacuum chamber 11. The interlayer insulating film on the substrate S is then etched with a high etch selectivity ratio and without striations by introducing an etching gas from the etching gas feeding means 4 and applying RF power from the second high frequency power supply 33 such that a plasma is generated within the plasma generating section 14. Specifically, the etching gas is introduced under an operating pressure of 0.5 Pa or less, preferably 0.1-0.5 Pa, into the vacuum chamber 11 in order to prevent radical reactions.

The etching gas used for the etching method of present invention is of the type that allows the interlayer insulating film to be etched without premature etch stop and in such a manner that a desired polymer film is formed on the resist.

This etching gas may be a mixture of a CF-based gas, a N-containing gas, and a low molecular weight hydrocarbon gas. The CF-based gas serves to etch SiO in the interlayer insulating film, while the N-containing gas and the low molecular weight hydrocarbon gas serve to etch CH in the interlayer insulating film. These gases also contribute to reducing damage to the resist.

The CF-based gas may be composed of at least one gas selected from the group consisting of CF₄, C₃F₈, C₂F₆, C₄F₈, and C₅F₈. Alternatively, it may be a C_xF_yI gas (which contains iodine) such as C₃F₇I or CF₃I, for example. The iodine (I) serves to remove excess fluorine atoms in the gas phase . The low molecular weight hydrocarbon is preferably linear and may be selected from, e.g., CH₄, C₂H₆, C₃H₈, C₄H₁₀, and C₂H₂. Further, examples of suitable N-containing gases include nitrogen gas, NO_x, NH₃, methylamine, and dimethylamine.

Further, the etching gas may be a mixture of a C_xF_yH_z gas and a N-containing gas. Each gas in this gas mixture acts in the same manner as the corresponding gas in the above gas mixture including three gases. The C_xF_yH_z gas may be, e.g., CHF₃ . Further, examples of suitable N-containing gases include nitrogen gas, NO_x, NH₃, methylamine, and dimethylamine.

In order to reduce damage to the resist, the etching gas (mixture) is not mixed with a dilution gas of any of the following noble gases: helium, neon, argon, krypton, and xenon.

The use of an etching gas (mixture) such as described above allows the low dielectric constant interlayer insulating film to be etched in such a manner that a desired polymer film is formed on the resist from the etching gas, thereby preventing damage to the resist. This polymer film exhibits a C—F bond peak at about 1200 cm⁻¹, a C—N bond peak at about 1600 cm⁻¹, and a C—H bond peak at about 3300 cm⁻¹ in its absorption spectrum (measured by a Fourier transform infrared spectrophotometer), although this may vary slightly depending on the measurement method used. This means that the polymer film is a nitrogen-containing CF-based polymer formed as a result of the combining of C from the etching gas with F, N, and H also from the etching gas. When the etching gas is an iodine-containing CF-based gas, a CF-based polymer film containing iodine is further formed during the etching process.

According to the method of the present invention, the interlayer insulating film is etched without premature etch stop by introducing an etching gas such as described above into the vacuum chamber 11 in such a manner that a polymer film such as described above is formed on the resist from the etching gas. To achieve this in the case of the above three-gas mixture, the flow rate of the CF-based gas introduced into the vacuum chamber 11 is preferably approximately 20-40%, more preferably approximately 20-30%, of the total etching gas flow. In the case of the above two-gas mixture, on the other hand, the flow rate of the C_xF_yH_z gas introduced into the vacuum chamber 11 is preferably approximately 20-40%, more preferably approximately 30-40%, of the total etching gas flow.

The present invention will be described in more detail with reference to practical and comparative examples.

Practical Example 1

In this example, a polymer film was formed from the etching gas used for the dry etching method of the present invention, and the infrared absorption spectrum of the formed polymer film was measured by FT-IR.

Specifically, first the parameters of the system shown in FIG. 1 were set as follows: pressure=3 mTorr; antenna power=2200 W; bias power=0 W; setting temperature of substrate Tc=10° C. CF₄ gas (at a flow rate of 60 sccm), N₂ gas (at a flow rate of 90 sccm), and CH₄ gas (at a flow rate of 70 sccm) were then introduced into the vacuum chamber to deposit a polymer film onto a Si substrate. The FT-IR spectrum of this polymer film was obtained by a Fourier transform infrared spectrophotometer.

For comparison, two polymer films were further deposited from a mixture of N₂ gas (at a flow rate of 90 sccm) and CH₄ gas (at a flow rate of 70 sccm) and from a mixture of C₃F₈ gas (at a flow rate of 25 sccm) and Ar gas (at a flow rate of 200 sccm), respectively. It should be noted that all other conditions are the same as described above. The infrared absorption spectra of these polymer films were measured by FT-IR. FIG. 2 shows the measurement results.

A comparison of the three spectra in FIG. 2 reveals the following facts. The spectrum of the polymer film deposited from the etching gas of the present invention (i.e., the CF₄/N₂/CH₄ gas mixture) had a C—N bond beak (at about 1600 cm⁻¹) and a C—H bond peak (at about 3300 cm⁻¹) as did the spectrum of the polymer film deposited from N₂/CH₄ gas mixture, and also had a C—F bond peak (at about 1200 cm⁻¹) as did the spectrum of the polymer film deposited from the C₃F₈/Ar gas mixture. This indicates that the polymer film formed from the etching gas of the present invention contained C—N, C—F, and C—H bonds.

Practical Example 2

In this example, first an SiOCH film serving as an interlayer insulating film was formed on a silicon substrate S by plasma CVD, and an organic film serving as a BARC was formed on the SiOCH film by spin coating. UV-II (an ArF resist material) was then applied to a thickness of 430 nm and patterned into a predetermined pattern by photolithography. The substrate with these films formed thereon was then placed on the substrate electrode 21 of the etching system 1 shown in FIG. 1. Next, the parameters of the etching system 1 were set as follows: antenna side high frequency power supply=2200 W; substrate side high frequency power supply=100 W; setting temperature of substrate=10° C.; pressure=10 mTorr. Then, the BARC was etched by introducing a BARC etching gas mixture of CF₄ gas (at a flow rate of 25 sccm) and CHF₃ gas (at a flow rate of 25 sccm) and generating a plasma therefrom. Next, the parameters of the etching system 1 were set as follows: antenna side high frequency power supply=2200 W; substrate side high frequency power supply=100 W; setting temperature of substrate=10° C.; pressure=3 mTorr. Then the interlayer insulating film was etched by introducing an etching gas mixture of CF₄ gas (at a flow rate of 60 sccm), N₂ gas (at a flow rate of 90 sccm), and CH₄ gas (at a flow rate of 70 sccm) and generating a plasma therefrom. FIG. 3(a) shows an SEM micrograph of the top surface of the etched substrate, and FIG. 3(b) shows a cross-sectional SEM micrograph of the hole enclosed by dotted line A in FIG. 3(a).

FIG. 3(a) indicates that the top surface of the substrate, i.e., the surface of the resist, had no irregularities. Further, the cross-sectional SEM micrograph of FIG. 3(b) indicates that no premature etch stop occurred and a polymer film was formed on the top surface of the substrate and on the interior surface of the inlet end of the hole, indicating that the interlayer insulating film was etched without striations. [The formed polymer film is shown shaded and is indicated by B in FIG. 3(b).] That is, the etching method of the present invention allows an interlayer insulating film to be etched without damage to the resist and hence without striations on the surfaces in the holes formed in the interlayer insulating film.

Practical Example 3

This example examined the relationship between the flow rate ratio of the gases in the etching gas mixture and the selectivity ratio (i.e., the ratio of the etch rate of the interlayer insulating film to that of the resist).

Specifically, an interlayer insulating film was etched in the etching system 1 in the following manner. The parameters of the system were set to the same values as in Practical Example 2 except that the antenna side high frequency power supply was 2000 W and the flow rate ratio of the gases in the etching gas mixture was varied. More specifically, the flow rate of the CH₄ gas was fixed at 70 sccm and the flow rates of the CF₄ gas and N₂ gas were varied as follows:

• (1) CF₄=20 sccm, N₂=30 sccm
• (2) CF₄=32 sccm, N₂=48 sccm
• (3) CF₄=48 sccm, N₂=72 sccm
• (4) CF₄=60 sccm, N₂=90 sccm
• (5) CF₄=80 sccm, N₂=120 sccm
  In this way, the percentages of the gases in the etching gas mixture were varied. It should be noted that flow rate conditions (4) above are identical to those in Practical Example 2. The etch rates of the interlayer insulating film and the resist were measured under each of the flow rate conditions (1) to (5), and the selectivity ratios were calculated from the measured data. FIG. 4 shows the results. FIGS. 5(a), 5(b), 5(c), and 5(d) show cross-sectional SEM micrographs of the substrates that have been etched under the conditions (1), (2), (3), and (5), respectively.

Observation of FIG. 4 reveals the following facts. Under the flow rate conditions (1), i.e., when the flow rates of the CF₄ gas and the N₂ gas were 20 sccm and 30 sccm (i.e., 16% and 25% of the total etching gas flow), respectively, the etch rates of the interlayer insulating film and the resist were 160 nm/min and 12 nm/min, respectively, and hence the selectivity ratio was approximately 13. Under the flow rate conditions (2), i.e., when the flow rates of the CF₄ gas and the N₂ gas were 32 sccm and 48 sccm (i.e., 21% and 32% of the total etching gas flow), respectively, the etch rates of the interlayer insulating film and the resist were 195 nm/min and 3 nm/min, respectively, and hence the selectivity ratio was 65, which is higher than that obtained under the flow rate conditions (1). Under the flow rate conditions (3), i.e., when the flow rates of the CF₄ gas and the N₂ gas were 48 sccm and 72 sccm (i.e., 25% and 37% of the total etching gas flow), respectively, the etch rate of the resist was zero and hence the selectivity ratio was infinite. This resulted from the fact that a polymer film was deposited onto the resist. Further, under the flow rate conditions (5), i.e., when the flow rates of the CF₄ gas and the N₂ gas were 80 sccm and 120 sccm (i.e., 29% and 44% of the total etching gas flow), respectively, the etch rates of the interlayer insulating film and the resist were 200 nm/min and 18 nm/min, respectively, and hence the selectivity ratio was approximately 11.

The above results indicate that the selectivity ratio (of the interlayer insulating film to the resist) can be optimized by adjusting the ratio of the gases in the etching gas mixture. More specifically, when the flow rate of the CF-based gas was 21-28% of the total etching gas flow, the etch rate of the resist was low resulting in a high selectivity ratio.

Observation of FIGS. 5(a) to 5(d) reveals that the etching under the flow rate conditions (1), (2), and (5) above resulted in the formation of irregularities on the surface of the resist and hence the formation of striations. The etching under the flow rate conditions (3), on the other hand, did not cause striations, resulting from reduced irregularities on the resist surface. This means that when the flow rate of the CF-based gas was 25-27% of the total etching gas flow, the etching process not only exhibited a high selectivity ratio, but also did not create irregularities on the resist surface and hence did not create striations.

Comparative Example 1

In this comparative example, interlayer insulating films on substrates were etched in the etching system 1. These interlayer insulating films were similar to those used in Practical Example 2. The etching gas mixture used in this example additionally included Ar gas. The parameters of the system were set as follows: antenna side high frequency power supply=2750 W; substrate side high frequency power supply=450 W; setting temperature of substrate 10° C.; pressure=0.26 Pa. The flow rates (sccm) of the C₃F₈, Ar, N₂, and CH₄ gases of the etching mixture were varied as follows:

• (a) C₃F₈/Ar/N₂/CH₄=16/50/20/26
• (b) C₃F₈/Ar/N₂/CH₄=30/50/20/26
• (c) C₃F₈/Ar/N₂/CH₄=16/100/20/26
• (d) C₃F₈/Ar/N₂/CH₄=16/50/20/40
• (e) C₃F₈/Ar/N₂/CH₄=16/50/50/26
  FIG. 6 shows cross-sectional SEM micrographs of the substrates that have been etched under the above flow rate conditions (a) to (e). The etch rates of the interlayer layer insulating film and the resist were also measured under these conditions, and the selectivity ratios were calculated from the measured data. FIG. 7 shows the results.

Observation of FIGS. 6(a) to 6(e) reveals that the etching under these flow rate conditions (a) to (e) caused the resist surface to be uneven and roughened, thereby creating striations on the sides of the holes. Further, the occurrence of premature etch stop is also observed, meaning that the above conditions (a) to (e) are not practical. Thus the etching under these conditions (a) to (e) resulted in damage to the resist surface. This is the reason why the selectivity ratios obtained were impractically low, as shown in FIG. 7.

INDUSTRIAL USABILITY

The present invention allows an interlayer insulating film to be etched in such a manner as to reduce damage to the resist even if the resist is made of a material having low plasma resistance. Therefore, the invention is particularly advantageously applied to the dry etching of interlayer insulating films of a Low-k material through a resist of an ArF resist material. Thus, the present invention is useful in the semiconductor manufacturing field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of an etching system for implementing a dry etching method according to the present invention.

FIG. 2 is a graph showing measured FT-IR spectra of films that have been etched by the dry etching method of the present invention.

FIG. 3 shows SEM micrographs of a substrate that has been etched by the etching method of the present invention, wherein FIG. 3(a) is atop view and FIG. 3(b) is a cross-sectional view.

FIG. 4 is a graph showing the etch rates (nm/min) of an interlayer insulating film and the resist thereon and the etch selectivity ratio of the interlayer insulating film to the resist, as a function of the ratio of the gases in the etching gas mixture.

FIGS. 5(a) to 5(d) are cross-sectional SEM micrographs of substrates that have been etched with etching gas mixtures containing different ratios of constituents.

FIGS. 6(a) to 6(e) are cross-sectional SEM micrographs of substrates that have been etched by a conventional etching method.

FIG. 7 is a graph showing the etch rate (nm/min) of the interlayer insulating film on each substrate etched by the conventional etching method, the etch rate (nm/min) of the resist on the interlayer insulating film, and the etch selectivity ratio of the interlayer insulating film to the resist.

DESCRIPTION OF REFERENCE NUMERALS

1 . . . etching system

2 . . . substrate mounting unit

4 . . . gas feeding means

11 . . . vacuum chamber

12 . . . vacuuming means

13 . . . substrate treatment section

14 . . . plasma generating section

21 . . . substrate electrode

22 . . . insulator

23 . . . support base

24 . . . blocking capacitor

25 . . . high frequency power supply

31 . . . top panel

32 . . . variable capacitor

33 . . . high frequency power supply

34 . . . junction

41 . . . gas feed path

42 . . . gas flow rate control means

43 . . . gas source

51 . . . magnetic field coil

52 . . . antenna coil

S . . . substrate

Claims

1. A method for dry etching an interlayer insulating film with an ArF resist or KrF resist thereon, comprising:

dry etching fine features into said interlayer insulating film with an etching gas in such a manner as to form a polymer film on said ArF or KrF resist from said etching gas,

wherein said etching gas is introduced under a pressure of 0.5 Pa or less, and

wherein a Fourier transform infrared spectrum of said polymer film includes a C—F bond peak at about 1200 cm−, a C—N bond peak at about 1600 cm−, and a C—H bond peak at about 3300 cm−1.

2-9. (canceled)

10. The method according to claim 1, wherein said interlayer insulating film is an SiOCH-based material.

11. The method according to claim 1, wherein said etching gas includes a CF-based gas, a N-containing gas, and a low molecular weight hydrocarbon gas.

12. The method according to claim 11, wherein said low molecular weight hydrocarbon is selected from the group consisting of CH4, C2H6, C3H8, C4H10, and C2H2.

13. The method according to claim 11, wherein said N-containing gas includes at least one gas selected from the group consisting of nitrogen gas, NOx, NH3, methylamine, and dimethylamine.

14. The method according to claim 1, wherein said etching gas includes a CxFyHz gas and a N-containing gas.

15. The method according to claim 14, wherein said low molecular weight hydrocarbon is selected from the group consisting of CH4, C2H6, C3H8, C4H10, and C2H2.

16. The method according to claim 14, wherein said N-containing gas includes at least one gas selected from the group consisting of nitrogen gas, NOx, NH3, methylamine, and dimethylamine.

17. The method according to claim 11, wherein said CF-based gas includes at least one gas selected from the group consisting of CF4, C3F8, C2F6, C4F8, C5F8 and CxFyI.

18. The method according to claim 17, wherein said low molecular weight hydrocarbon is selected from the group consisting of CH4, C2H6, C3H8, C4H10, and C2H2.

19. The method according to claim 14, wherein said CxFyHz gas is CHF3 gas.

20. The method as claimed in claim 17, wherein said CxFyI gas is C3F7I gas or CF3I gas.