Method for fabrication of semiconductor device

Disclosed herein is a method for fabrication of semiconductor device involving a first step of coating the substrate with a double-layered insulating film in laminate structure having the skeletal structure of inorganic material and a second step of etching the upper layer of the insulating film as far as the lower layer of the insulating film. In the method for fabrication of semiconductor device, the first step is carried out in such a way that the skeletal structure is incorporated with a pore-forming material of hydrocarbon compound so that one layer of the insulating film contains more carbon than the other layer of the insulating film.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP 2006-070272 filed in the Japanese Patent Office on Mar. 15, 2006, the entire contents of which being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabrication of a semiconductor device and, more particularly, to a method for fabrication of a semiconductor device having a porous insulating film.

2. Description of the Related Art

Recent semiconductor devices with a higher degree of integration and scale down than before pose a serious problem with delay of electric signals resulting from the time constant of wiring. This problem is addressed by making the wiring of multi-layer structure from low-resistance copper (Cu) in place of aluminum (Al). Unfortunately, copper hardly undergoes dry etching for patterning unlike any other metallic materials, such as aluminum, which are used for the multi-layer wiring structure in the past. This disadvantage is overcome by employing the damascene method, which consists of forming wiring grooves in an insulating film and embedding a copper film in the wiring groove, thereby forming the wiring pattern.

The foregoing steps can be achieved more efficiently by the dual-damascene method, which consists of forming via holes and wiring grooves and then embedding copper therein simultaneously. This method is attracting attention because it effectively reduces the number of steps.

Highly integrated semiconductor devices are subject to slow-down due to increased capacity between wirings; therefore, they inevitably need fine multi-layer wirings, with the capacity between wirings kept low by interlayer insulating film of so-called low-dielectric material. Examples of such materials include fluorine-containing silicon oxide (FSG) having a dielectric constant of about 3.5, organic polymers typified by polyarylether (PAE), and inorganic silicon compounds typified by carbon-containing silicon oxide (SiOC), hydrogen silsesquioxane (HSQ), and methyl silsesquioxane (MSQ), which have a lower dielectric constant than silicon oxide. Moreover, attempts are being made to further reduce their dielectric constant to about 2.3 by making them porous.

The dual-damascene method mentioned above is applied to the interlayer insulating film having a low dielectric constant by the steps of forming a film of inorganic material (SiOC) on the substrate by chemical vapor deposition (CVD) process, forming a film of organic material (PAE) on the inorganic film by coating, forming wiring grooves in the organic film by etching, and forming via holes in the inorganic film. The thus formed layer structure has an interface between the inorganic film and the organic film and hence permits etching to be performed under good control with a high etching selective ratio of the former to the latter. (See Japanese Patent Laid-Open No. 2004-63859, for instance).

Moreover, CVD process to form the inorganic film can readily adapt itself to varied conditions (gas flow rate and RF power) to cope with fluctuation of film quality, and CVD process usually gives rise to a compact film with high mechanical strength.

SUMMARY OF THE INVENTION

The above-mentioned fabricating method, however, needs an additional step of moving the substrate from one apparatus for CVD process to form the inorganic film to the other apparatus for coating to form the organic film. This step is troublesome and detrimental to productivity. CVD process having such an advantage is hardly applicable to the organic film as the insulating film for the upper layer in which the wiring grooves are formed. Unfortunately, porous SiOC film is the inorganic film comparable to the organic film in dielectric constant, but it does not match the SiOC lower layer in etching selective ratio. This poses a problem with poor processing controllability.

It is desirable to provide a method for fabrication of a semiconductor device, the method giving laminated films by CVD process such that the upper layer undergoes etching more easily than the lower layer.

The gist of the present invention resides in a method for fabrication of semiconductor devices involving a first step of coating the substrate with a double-layered insulating film in laminate structure having the skeletal structure of inorganic material and a second step of etching the upper layer of the insulating film as far as the lower layer of the insulating film. In the method for fabrication of semiconductor device, the first step is carried out in such a way that the skeletal structure is incorporated with a pore-forming material of hydrocarbon compound so that one layer of the insulating film contains more carbon than the other layer of the insulating film.

The foregoing method for fabrication of semiconductor device is characterized in that one of the dual insulating films contains a pore-forming material of hydrocarbon compound in its skeletal structure so that it has a higher carbon content than the other. The insulating film with a higher carbon content functions as a pseudo-organic film, giving rise to an interface between the dual insulating films (or between the pseudo-organic film and the inorganic film). The resulting interface increases the etching selective ratio of the upper layer to the lower layer for etching in the second step that is performed on the upper layer as far as the lower layer. Thus, the lower insulating film functions as a stopper, thereby improving the fabricating controllability. Moreover, the dual insulating films have the skeletal structure of inorganic material and hence both of them can be formed by CVD process.

The method according to the present invention permits etching on the upper insulating layer to be performed under good control, which leads to a dimensionally accurate wiring structure with wiring material embedded in wiring grooves (in the upper insulating film) and via holes (in the lower insulating film). Moreover, it permits the insulating films to be formed by CVD process. CVD process immediately adapts itself to any film-forming conditions for desired film quality, thereby contributing productivity, and it also gives rise to a compact film with high mechanical strength. In addition, it is capable of forming the dual insulating films continuously.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

According to the present invention, the method for fabrication of semiconductor device employs a CVD apparatus and an electron beam irradiating apparatus, which are explained first with reference toFIGS. 1 and 2(which are sectional views).

The CVD apparatus1shown inFIG. 1(which is used to form insulating films) has a reaction chamber2in which films are formed on the surface of the substrate S. The reaction chamber2has an exhaust port (not shown) connected to a vacuum pump so that it can be evacuated.

At the bottom of the reaction chamber2is a substrate holder3on which the substrate S is placed for processing. The substrate holder3functions also as a lower electrode to generate plasma (mentioned later), and it is provided with a heater (not shown) that heats the substrate S.

At the top of the reaction chamber2is an upper electrode4, which functions as a lid, opposite to the substrate holder3. The upper electrode4is provided with pipes (not shown) through which film-forming gas and carrier gas are fed into the reaction chamber2. The upper electrode4also has openings in its entire surface opposite to the substrate holder3through which the gases are supplied to the surface of the substrate S placed on the substrate holder3.

Operation of the foregoing CVD apparatus1to form an insulating film on the surface of the substrate S involves the steps of placing the substrate S on the substrate holder3in the reaction chamber2, supplying the film-forming gas from the gas supplying openings in the upper electrode4, and applying a voltage across the electrodes, thereby generating plasma P above the substrate S. The thus generated plasma P causes film-forming components therein to form the insulating film on the surface of the substrate S.

For decomposition and removal the pore-forming material (mentioned later) dispersed in the insulating film, this embodiment employs the electron beam irradiating apparatus5, which is composed of a reaction chamber6in which the substrate S is irradiated with electron beams E. The reaction chamber6has on its bottom a substrate holder7on which the substrate S is placed and which is provided with a heater (not shown) to heat the substrate S.

At the top of the reaction chamber6is an electron beam irradiating unit8which emanates electron beams E toward the substrate S placed on the substrate holder7.

Operation of the foregoing electron beam irradiating apparatus5to irradiate the surface of the substrate S with electron beams E involves the steps of placing the substrate S with an insulating film formed thereon which contains a pore-forming material on the substrate holder7in the reaction chamber6and irradiating the surface of the substrate S with electron beams E.

According to this embodiment, the semiconductor device is fabricated by the steps illustrated inFIGS. 3A to 3F(which are sectional views). The fabricating steps employ the CVD apparatus and electron beam irradiating apparatus illustrated inFIGS. 1 and 2, respectively.

The first step of fabrication illustrated inFIG. 3Ainvolves coating the substrate11with the first insulating film12of inorganic material containing porogene A′ in its skeleton. Porogene A′ is a pore-forming material composed of a skeletal material and a hydrocarbon compound. This step involves plasma enhanced chemical vapor deposition (PE-CVD) with a film-forming gas containing porogene A′.

To be concrete, the step of forming the first insulating film12starts with introducing the substrate11(identical with the substrate S) into the reaction chamber2of the CVD apparatus1(shown inFIG. 1) to place it on the substrate holder3and then heating the substrate holder3, with the reaction chamber2evacuated below 13 kPa.

Next, the reaction chamber2is supplied with a film-forming gas and a carrier gas of helium (He), with the former containing a skeleton-forming material composed of diethoxymethylsilane (DEMS) and oxygen (O2) and porogene A′, which may be α-terpinene (ATRP), for instance. Then, plasma P is generated by application of RF power across the substrate holder3and the upper electrode4. RF power should be controlled so that the resulting plasma has an energy low enough to prevent porogene A′ from dissociation. This step is carried out typically under the condition that the gas flow ratio is DEMS:O2:ATRP:He=1:1:2:5, the RF power is 500 W, the pressure in the reaction chamber is 7.0 kPa, and the substrate temperature is 250° C.

The foregoing procedure gives rise to the first insulating film12which is composed of a skeleton of inorganic material (or carbon-containing silicon oxide (SiOC)) and porogene A′ (ATRP) dispersed therein in the form of plasma-induced polymer.

The first insulating film12thus formed should have a skeleton of inorganic material (not limited to SiOC) having a low dielectric constant. The porogene A′ used in this step is not limited to α-terpinene (C10H16); it may include any straight or branched hydrocarbon represented by CxHyand oxygen-containing hydrocarbon represented by CxHyOz, where x is 1 to 12. In addition, the porogene A′ should preferably have a cyclic structure like benzene and cyclohexane.

The next step shown inFIG. 3Binvolves irradiation of the substrate11in its heated state with electron beams for decomposition and removal of porogene A′ from the first insulating film12. As the result of this step, the first insulating film12shown inFIG. 3Aturns into the first porous insulating film12A having pores A. This step starts with placing the substrate11(identical with the substrate S), which has the first insulating film12formed thereon, on the substrate holder7in the reaction chamber6of the electron irradiating apparatus5described above with reference toFIG. 2. Then, the surface of the substrate11is irradiated with electron beams E emanating from the electron beam irradiating unit8, with the substrate holder7heated at, say, 400° C.

The third step shown inFIG. 3Cinvolves PE-CVD process which forms on the first porous insulating film12A the second insulating film13of inorganic material containing porogene B′ in its skeleton upon receipt of the film-forming gas containing a skeleton-forming material and porogene B′ of hydrocarbon compound.

The second insulating film13thus formed contains porogene B′ of hydrocarbon compound and hence has a higher carbon content than the first porous insulating film12A. The fact that the content of porogene B′ in the second insulating film13is higher than the content of porogene A′ in the first insulating film12(shown inFIG. 3A) leads to the higher carbon content in the first porous insulating film12A and this is desirable. Porogene B′ may be a hydrocarbon compound as in the case of porogene A′.

The third step employs the CVD apparatus1which has been described above with reference toFIG. 1. It starts with introduction of a film-forming gas and a carrier gas (He) into the reaction chamber2, with the former being composed of a skeleton-forming material of DEMS and porogene B′ of ATRP, which is followed by application of RF power across the substrate holder3and the upper electrode4to generate plasma P. RF power should have an energy low enough to prevent porogene B′ from dissociation. The desirable condition for this step is that the gas flow rate is DEMS:ATRP:He=1:6:5, the RF power is 500 W, the pressure in the reaction chamber2is 13 kPa, and the substrate temperature is 250° C. This condition differs from that for the first insulating film12in that the ratio of ATRP (porogene B′) to DEMS is higher and DEMS is used alone to form the skeleton. The result is that the skeleton has a higher carbon content.

The third step carried out as mentioned above forms the second insulating film13which is composed of a skeleton of inorganic material and porogene B′ dispersed therein. The inorganic material constituting the skeleton is carbon-containing silicon oxide (SiOC) which has a higher carbon content than the first insulating film12. The porogene B′ is ATRP which has been polymerized by plasma. The thus formed second insulating film13contains porogene B′ more than the first insulating film12contains porogene A′. Containing porogene B′, the second insulating film13is a pseudo-organic film.

In the fourth step shown inFIG. 3D, the second insulating film13undergoes etching through the first mask pattern14previously formed thereon. This etching forms the wiring groove15that penetrates the second insulating film13and reaches the first porous insulating film12A. Incidentally, the first mask pattern14should be formed from an inorganic material, such as silicon oxide (SiO2), because the second insulating film13is a pseudo-organic film.

Etching mentioned above should preferably use, as an etching gas, ammonia (NH3) which is used to etch carbonaceous film, because of the high content of porogene B′ of hydrocarbon compound in the second insulating film13. The result of etching in this manner is a high etching selective ratio of the second insulating film13to the first porous insulating film12A. An example of the desirable etching condition is as follows. Etching gas: NH3and O2; gas flow rate: NH3:O2=30:1; bias power: 400 W; and substrate temperature: 0° C. The two-component etching gas may be replaced by NH3alone.

Etching in this manner gives rise to a pseudeo-inorganic-organic interface between the first porous insulating film12A and the second insulating film13(which is a pseudo-organic film). This interface contributes to the high etching selective ratio of the second insulating film13to the first porous insulating film12A. The result is well-controlled etching on the second insulating film13. The fourth step finishes with removal of the first mask pattern14.

The fifth step shown inFIG. 3Estarts with coating the second insulating film13with a photoresist forming the second mask pattern16. Then, the first porous insulating film12A which is exposed at the bottom of the wiring groove15undergoes etching through the second mask pattern16. This etching forms the connecting hole17in the first porous insulating film12A. An example of the etching condition is as follows.

This step may optionally include removal of the second mask pattern16.

The sixth step shown inFIG. 3Fis irradiation with electron beams, with the substrate21heated, for decomposition and removal of porogene B′ contained in the second insulating film13. The result is a conversion of the second insulating film13shown inFIG. 3Einto the second porous insulating film13B which has pores B. This step involves heating in the reaction chamber6of the electron beam irradiating apparatus5the substrate holder7(at, say, 400° C.) supporting thereon the substrate11with the second insulating film13and irradiation of the substrate11with electron beams E emanating from the electron beam irradiating unit8for, say, five minutes.

Incidentally, this embodiment may be modified by replacing irradiation with electron beams by irradiation with ultraviolet rays for decomposition and removal of porogene A′ and porogene B′, respectively in the first insulating film12and the second insulating film13, with the substrate21heated. The same effect will also be achieved by heating only.

The forgoing step is followed by a finishing step (not shown) of filling the wiring groove15and the connecting hole17with an electrically conductive material, thereby forming vias and wirings connected to the substrate11.

Fabrication of semiconductor devices in the way mentioned above causes the second insulating film13to contain more carbon than the first porous insulating film12A because the second insulating film13containing porogene B′ is formed on the first porous insulating film12A. The result is a high etching selective ratio of the second insulating film13to the first porous insulating film12A. Thus the first porous insulating film12A functions as a stopper, allowing the wiring groove15and the connecting hole17to be formed accurately. This, in turn, allows the wiring and via to be formed accurately in the wiring groove15and the connecting hole17, respectively.

The CVD process employed to form both of the first insulating film12and the second insulating film13is flexible to cope with film quality fluctuation. It gives a compact film with high mechanical strength and contributes to improved productivity.

Meanwhile,FIG. 4shows the etching selective ratio of the second insulating film13(or the second porous insulating film13B) to the first porous insulating film12A, which has been explained above with reference toFIGS. 3A to 3F. “Ref” on the abscissa denotes the first porous insulating film12A arising from the first insulating film12after irradiation with electron beams for five minutes under heated condition at 400° C. Likewise, (1) denotes the second porous insulating film13B arising from the second insulating film13after irradiation with electron beams for five minutes under heated condition at 400° C. (2) denotes the second porous insulating film13B arising from the second insulating film13after heat treatment at 400° C. for five minutes. (3) denotes the second insulating film13. These three films vary in carbon content such that (1)<(2)<(3). Etching selective ratio was measured by etching in two ways, one with NH3which is used to etch a carbonaceous film, and the other with a combination of O2and CxFywhich is used to etch an SiOC film. Etching with NH3showed a sign of the etching selective ratio of the second porous insulating film13B (or the second insulating film13) to the first porous insulating film12A increasing in proportion to the carbon content. By contrast, etching with an oxygen-containing gas (such as O2and CO) plus CxFyshowed a sign of the etching selective ratio decreasing in proportion of the carbon content. These results prove that etching with O2and CxFygives a high etching selective ratio of the first porous insulating film12A (with a low carbon content) to the second insulating film13or the second porous insulating film13B (both with a high carbon content).

As compared with the first porous insulating film12A indicated by “Ref”, the second porous insulating film13B has a higher etching selective ratio in (1). A probable reason for this is that the skeleton of the second insulating film12contains more carbon when the first insulating film12and the second insulating film13are formed under certain conditions and that the content of porogene B′ in the second insulating film13is higher than that of porogene A′ in the first insulating film12and hence porogene B′ remains unremoved when the step for removal is carried out under the same condition.

The first embodiment mentioned above is designed to form the wiring groove15in the second insulating film13and then form the connecting hole17in the first porous insulating film12A. However, it is also possible to form the connecting hole17in the first porous insulating film12A and then form the wiring groove15in the second insulating film13.

Although the first embodiment mentioned above assumes that the lower layer under the second insulating film13is a porous insulating film, the lower layer may be replaced by a non-porous SiOC film. In this case, the first insulating film12does not need irradiation with electron beams. This means that CVD process can be used to form the first insulating film12and the second insulating film13consecutively, thereby further improving productivity.

The first embodiment mentioned above involves a step of etching the second insulating film13which contains porogene B′ and hence has a higher carbon content than the first porous insulating film12. However, it may be modified such that etching is performed on the second insulating film13, with the first insulating film12containing porogene A′ and hence having a higher carbon content than the second insulating film13.

The first step, shown inFIG. 5A, starts with forming the first insulating film12containing porogene A′ on the substrate11in the same way as the second insulating film13is formed as explained with reference toFIG. 3Cin the first embodiment. In other words, porogene A′ is ATRP and an adequate film forming condition is selected to prevent porogene A′ from dissociation. Subsequently, the first insulating film12is coated with the second insulating film13, which is a non-porous SiOC film. The first insulating film12has a higher carbon content than the second insulating film13owing to porogene A′ contained therein, and hence it behaves like a pseudo-organic film.

In the next step shown inFIG. 5B, the second insulating film13is coated with the first mask pattern14of resist and etching down to the first insulating film12is performed through the mask pattern14, so that the wiring groove15is formed in the second insulating film13.

The etching gas used in this step is a mixture of oxygen-containing gas (such as O2and CO) and fluorinated carbon (CxFy). As explained above with reference toFIG. 4, this etching gas gives a high etching selective ratio for the inorganic film with a low carbon content. An example of etching condition is as follows.

Etching gas: a mixture of C4F8, CO, N2, and Ar

Etching in this manner has a high etching selective ratio of the second insulating film13to the first insulating film12on account of the interface between the first insulating film12, which is a pseudo-organic film, and the second insulating film13, which is an inorganic film. Therefore, etching on the second insulting film13is carried out under good control. Etching is followed by removal of the first mask pattern14.

The third step show inFIG. 5Cinvolves forming the second mask pattern16of SiO2or the like on the second insulating film13and performing etching through the second mask pattern16on the first porous insulating film12A which has been exposed at the bottom of the wiring groove15, thereby making the connecting hole17in the first insulating film12. An example of etching condition is as follows.

Etching gas: a mixture of NH3and O2

Etching may optionally be followed by removal of the second mask pattern16.

The fourth step shown inFIG. 5Dinvolves irradiation of the substrate21in its heated state with electron beams to make the first insulating film12free of porogene A′ by decomposition. This step converts the first insulating film12(shown inFIG. 5C) into the first porous insulating film12A having pores A.

The foregoing method for fabrication of semiconductor device also produces the effect of increasing the etching selective ratio of the second insulating film13to the first insulating film12because the latter has a higher carbon content than the former in consequence of the second insulating film13of SiOC formed on the first insulating film12containing porogene A′. As the result, the first insulating film12functions as a stopper, allowing for well-controlled etching for the wiring groove15and the connecting hole17. In addition, CVD process can be used to form both the first insulating film12and the second insulating film13. In conclusion, this modified version produces the same effect as the original version of the first embodiment.

Incidentally, this modified embodiment 1 may be so changed as to replace the second insulating film13of SiOC by the second porous insulating film13B having pores B, which is produced when the previously formed second insulating film13containing porogene B′ is freed of porogene B′ by decomposition. It is necessary that porogene B′ have a lower dissociation energy than porogene A′ so that the step of forming the second porous insulating film13B is carried out under the condition that only porogene B′ is selectively removed by decomposition.

The first embodiment (original) mentioned above may have its third step (shown inFIG. 3C) modified such that the second insulating film13is formed by applying an RF power that generates plasma with a higher energy than the dissociation energy of porogene B′ instead of applying an RF power so adjusted as to prevent porogene B′ from dissociation. The thus generated plasma allows partial dissociation of porogene B′, thereby giving rise to skeletons of SiOC having carbon (resulting from dissociation) attached thereto and also containing partially dissociated porogene B′. The RF power should be 800 to 1500 W. The resulting second insulating film13has skeletons with a high carbon content. This leads to a high etching selective ratio with respect to the first porous insulating film12A even in the case where the second insulating film13is freed of porogene B′ by decomposition for conversion into the second porous insulating film13B, which subsequently undergoes etching to form the wiring groove15therein. Moreover, the resulting second insulating film13B has a high strength owing to its skeletons with carbon attached thereto.

The procedure mentioned above is also applicable to the step explained with reference toFIG. 5Ain modified embodiment 1. In other words, the first insulating film12may have the skeletal structure which is composed of SiOC with carbon attached thereto arising from partial dissociation of porogene A′ due to plasma with a higher energy than the dissociation energy of porogene A, and which contains partially dissociated porogene A′.

Second Embodiment

The second embodiment is illustrated inFIGS. 6A to 6F, which are sectional views. It is intended to form an etched porous insulating film of laminate structure.

The first and second steps shown inFIGS. 6A and 6Bare identical with the procedure explained with reference toFIGS. 3A and 3Bin the first embodiment. The first step starts with coating the substrate21with the first insulating film22containing porogene A′ in its skeletal structure. The second step involves irradiating the substrate21in its heated state with electron beams to make the first insulating film22free of porogene A′ by decomposition, for conversion of the first insulating film22(shown inFIG. 6A) into the first porous insulating film22A with pores A (shown inFIG. 6B).

The third step shown inFIG. 6Cinvolves PE-CVD to coat the first porous insulating film22with the second insulating film23which forms from a gas composed of a skeleton-forming material, porogene B′ (as a pore-forming material of hydrocarbon compound), and porogene C′ (as a micropore-forming material of hydrocarbon compound with a lower molecular weight than porogene B′).

Porogene B′ and porogene C′ are hydrocarbons represented by CxHyor oxygen-containing hydrocarbons represented by CxHyOz, with the former having a larger carbon number than the latter. For example, hydrocarbons for porogene B′ may be cyclic ones with a carbon number (x) of 6 to 12, and hydrocarbons for porogene C′ may be those having a carbon number (x) of 1 to 5. In this embodiment, porogene B′ is ATRP and porogene C′ is ethylene (C2H4). Propylene (C3H6) will also find use as porogene C′. An adequate film-forming condition should be selected, with RF power properly controlled to prevent porogene C′ from dissociation, because porogene C′ has a lower dissociation energy than porogene B′.

The film-forming process proceeds in the reaction chamber2of the CVD apparatus1(shown inFIG. 1) which is supplied with DEMS as a skeleton-forming material, ATRP as porogene B′, ethylene (C2H4) as porogene C′, and helium as a carrier gas. The gas in the reaction chamber2is excited by plasma P arising from RF power applied across the substrate holder3and the upper electrode4. The third step proceeds under the following condition.

Pressure in the reaction chamber2: 13 kPa

The reaction in this manner yields the second insulating film23whose skeleton of SiOC contains dispersed therein porogene B′ and porogene C′ which have been polymerized by plasma. The content of porogene B′ has a limit beyond which the film cannot be formed. However, porogene C′, which has a lower molecular weight than porogene B′, can be added even when the content of porogene B′ is at a limit. Consequently, the film-forming gas containing porogene C′ yields the second insulating film23with a higher carbon content than the second insulating film13containing porogene B′ alone, which has been explained with reference toFIG. 3Cin the first embodiment. The result is a higher etching selective ratio of the second insulating film23to the first porous insulating film22A.

The fourth step shown inFIG. 6Dinvolves coating the second insulating film23with the first mask pattern24of SiO2and etching through this mask pattern to form the wiring groove25in the second insulating film23down to the first porous insulating film22A. This etching is performed under the same condition as in the step explained with reference toFIG. 3Din the first embodiment.

The second insulating film23functions as a pseudo-organic film because of porogene B′ and porogene C′ of hydrocarbon compounds contained therein, with the result that an interface occurs between the first porous insulating film22A (as a pseudo inorganic film) and the second insulating film23(as a pseudo organic film). The interface contributes to high etching selective ratio of the second insulating film23to the first porous insulating film22A, and the first porous insulating film22A functions as a stopper allowing for well-controlled etching on the second insulating film23. This etching is followed by removal of the second mask pattern24.

The fifth step shown inFIG. 6Einvolves forming the second mask pattern26of resist on the second insulating film23and etching through this mask pattern on the first porous insulating film22A exposed at the bottom of the wiring groove25. This etching is carried out under the same condition as in the step explained with reference toFIG. 3Ein the first embodiment. This step makes the connecting hole27in the first porous insulating film22A. This etching is followed by removal of the second mask pattern26according to need.

The sixth step shown inFIG. 6Finvolves irradiating the substrate21in its heated state with electron beams to make the second insulating film23free of porogene B′ and porogene C′ by decomposition, for conversion of the second insulating film23(shown inFIG. 6E) into the second porous insulating film23B with pores B and pores C (smaller than pore B in diameter). The second insulating film23has a high pore content because it contains more porogene than the second insulating film13containing porogene B′ alone as explained with reference toFIG. 3Cin the first embodiment, and hence the resulting second porous insulating film23B has a lower dielectric constant than the second porous insulating film13B shown inFIG. 3F.

The foregoing steps are completed by filling the wiring groove25and the connecting hole27with an electrically conductive material and by forming vias and interconnects leading to the substrate21.

The foregoing method for fabrication of semiconductor device also produces the effect of increasing the etching selective ratio of the second insulating film23to the first porous insulating film22A if the latter is coated with the former containing porogene B′ and porogene C′ (and hence containing more carbon than the latter). The second insulating film23in this embodiment contains more carbon than expected in the first embodiment, and this contributes to the higher etching selective ratio for the first porous insulating film22A, which permits the wiring groove25and the connecting hole27to be made by well-controlled etching. Therefore, the wiring of wiring groove25and vias of the connecting hole27are formed with good sizing controllability.

Moreover, this embodiment yields the second porous insulating film23B which contains more pores than the second porous insulating film13B (shown inFIG. 3F) in the first embodiment. The high pore content contributes to further reduction of dielectric constant and hence reduction of wiring capacitance.

Since this embodiment resorts to CVD to form both the first insulating film22and the second insulating film23, it produces the same effect as the first embodiment.

This is a modification of the second embodiment. The first step shown inFIG. 7Astarts with coating the first porous insulating film22A with the second insulting film23. The second step shown inFIG. 7Binvolves heat treatment at 300° C. for 0.5 hours to form the second porous insulating film23B′ which has only porogene C′ removed by decomposition but contains pores B″ resulting from the polymerized porogene B′ with its central part removed by decomposition.

The third step shown inFIG. 7Cinvolves coating the second porous insulating film23B′ with the first mask pattern24and subsequent etching to form the wiring groove25in the second porous insulating film23B′. Due to residual porogene B′, this etching is possible with a high etching selective ratio with respect to the first porous insulating film22A. Since the second porous insulating film23B′ has pores C (which are left after porogene C′ of low-molecular weight hydrocarbons has been removed by decomposition) and pores B″ (which are left after polymerized porogene B′ has been partly removed by decomposition), and pores B″ have a small size and are surrounded by residual porogene B′, etching on it causes less damage than etching on a porous film with porogene B′ and C′ completely removed.

The fourth step shown inFIG. 7Dinvolves making the connecting hole27in the first porous insulating film22A exposed at the bottom of the wiring groove25and additional heat treatment at 400° C. for 0.5 hours for removal of the residual porogene B′ (seeFIG. 7C) by decomposition. This heat treatment converts the second porous insulating film23B′ into the second porous insulating film13B having pores B formed therein. This modified embodiment is designed to remove porogene B′ and porogene C′ in two stages for their complete removal.

The foregoing method for fabrication of semiconductor device also produces the same effect as the first embodiment because it involves etching on the second insulating film23containing more carbon than the first porous insulating film22A and etching in this manner has a high etching selective ratio.

The second embodiment mentioned above, which involves etching on the second insulating film23containing more carbon than the first porous insulting film22A on account of porogene B′ and porogene C′ contained therein, may be modified by application of modified embodiment 1.

According to the modified embodiment 4, the first step shown inFIG. 8Astarts with coating the substrate11with the first insulating film22containing porogene A′ and porogene C′ under the same condition as that for the second insulating film23shown inFIG. 6Cin the second embodiment mentioned above. Porogene A′ is ATRP and porogene C′ is C2H4. The film-forming condition should be adequate to prevent porogene C′ from dissociation. The foregoing step is followed by coating the first insulating film22with the second insulating film23which is a non-porous SiOC film. The thus formed first insulating film22, which contains porogene A′, has a higher carbon content than the second insulating film23, and hence it functions as a pseudo-organic film. The second insulating film23of SiOC may be replaced by the second porous insulating film23B having pores B therein as in the modified embodiment 1.

The second step shown inFIG. 8B, which proceeds in the same way as in the modified embodiment 1 mentioned above, involves coating the second insulating film23with the first mask pattern24and etching through the first mask pattern24to make the wiring groove25(which reaches the first insulating film22) in the second insulating film23. The etching conditions are the same as those in the modified embodiment 1 explained with reference toFIG. 5B. This etching exhibits a high etching selective ratio of the second insulating film23to the first insulating film22owing to an interface between them (or between a pseudo organic film and an inorganic film); the result is well-controlled etching of the second insulating film23. The foregoing step is followed by removal of the first mask pattern24.

The third step shown inFIG. 8Cinvolves coating the second insulating film23with the second mask pattern26and etching through the second mask pattern26to make the connecting hole27in the first insulating film22exposed at the bottom of the wiring groove25. The etching conditions are the same as those in the step explained with reference toFIG. 5C. The foregoing step is followed by removal of the first mask pattern26according to need.

The fourth step shown inFIG. 8Dinvolves irradiation of the substrate21in its heated state with electron beams to make the first insulating film22free of porogene A′ and porogene C′ by decomposition. This step converts the first insulating film22(seeFIG. 8C) into the first porous insulting film22A having pores A.

The foregoing method for fabrication of semiconductor devices also produces the same effect as the second embodiment because of the specific layer structure composed of the first insulating film22containing porogene A′ and porogene C′ and the second insulating film23of SiOC formed thereon. This layer structure results in a higher carbon content in the first insulating film22than in the second insulating film23and hence a high etching selective ratio of the latter to the former. Thus, the first insulating film22functions as a stopper which permits the wiring groove25and the connecting hole27to be made by etching under good control. In addition, CVD process may be employed to form both the first insulating film22and the second insulating film23.

The second embodiment and the modified embodiment 3 both mentioned above are designed such that the second insulating film23is formed with an adequate RF power which prevents porogene C′ from dissociation. However, they may be modified such that the RF power is so controlled as to generate plasma having a higher energy than the dissociation energy of the porogene C′ and a lower energy than the dissociation energy of porogene B′. The resulting second insulating film23has the skeletal structure containing porogene B′ and having carbon attached thereto which arises from dissociated porogene C′. Subsequent removal of porogene B′ by decomposition converts the second insulating film23into the second porous insulating film13B having pores B. The result is that the skeleton has a high carbon content owing to the dissociated carbon attached thereto. This high carbon content is responsible for high strength and pores B contribute to the second porous insulating film13B having a low dielectric constant.

Moreover, the second insulating film23may also be formed by properly adjusting the RF power so that the generated plasma has a higher energy than the dissociation energy of porogene B′ and hence dissociates not only porogene C′ but also part of the porogene B′. Therefore, the resulting second insulating film23has the skeletal structure which contains dissociated carbon and partly dissociated porogene B′. Finally, the second insulating film23is freed of porogene B′ for its conversion into the second porous insulating film23B having pores B. The foregoing procedure produces the skeletal structure containing porogene C′ as well as partially dissociated porogene B′. The resulting second porous insulating film23B has a high strength owing to the high carbon content in its skeletal structure.

The advantage of the foregoing procedure that gives the second insulating film23whose skeletal structure has carbon attached thereto originating from porogene C′ or both porogene C′ and partly dissociated porogene B′ is that even though the second insulating film23is made porous for conversion into the second porous insulating film23B, the resulting second porous insulating film23B keeps the higher carbon content than the first porous insulating film22A, which allows a high etching selective ratio for the first porous insulating film22A.

The foregoing may also be applied to the step explained above with reference toFIG. 7Ain modified embodiment 4. That is, the first insulating film22may be formed by properly adjusting the RF power so that the generated plasma has a higher energy than the dissociation energy of porogene C′ and a lower energy than the dissociation energy of porogene A′, and the resulting first insulating film22has the skeletal structure which contains carbon arising from porogene C′ and porogene A′. Similarly, the second insulating film23may be formed by properly adjusting the RF power so that the generated plasma has a higher energy than the dissociation energy of porogene A′ and its skeletal structure contains carbon arising from porogene C′ as well as partly dissociated porogene A′.

Third embodiment

The third embodiment illustrated inFIGS. 9A to 9L(sectional views) is concerned with the dual damascene method to which the present invention is applied. Incidentally, it is based on the first embodiment explained above with reference toFIGS. 3A to 3F.

The first step shown inFIG. 9Astarts with coating the semiconductor substrate101(having element regions (not shown) formed thereon) with the wiring insulating film (which is a laminate layer consisting of the PAE film102and the SiOC film103). The coating step is followed by making the wiring groove104in the wiring insulating film and forming the embedded copper wiring106in the wiring groove104, with the barrier metal105interposed. The first step is concluded with forming the etch preventing film107of SiC on the copper wiring106and the SiOC film103. What is obtained by the foregoing steps corresponds to the substrate described in the claim.

The second step shown inFIG. 9Binvolves coating the etch preventing film107with the first insulating film108whose skeleton contains porogene A′ (in the same procedure as explained above with reference toFIGS. 3A to 3Cin the first embodiment), irradiating the substrate101in its heated state with electron beams, thereby making the first insulating film108(seeFIG. 9A) free of porogene A′ by decomposition for conversion into the first porous insulating film108A containing pores A, and coating the first porous insulating film108A with the second insulating film109whose skeleton contains porogene B′.

The third step shown inFIG. 9Cinvolves coating the second insulating film109sequentially with the first mask-forming layer201of silicon oxide (SiO2), the second mask-forming layer202of silicon carbide nitride (SiCN), and the third mask-forming layer203of silicon oxide (SiO2).

Of the three mask-forming layers201to203, the first and third mask-forming layers201and203of SiO2are formed by PE-CVD from monosilane (SiH4) as the silicon source and dinitrogen monoxide (N2O) gas as the oxidizing agent, and the second mask-forming layer202of SiCN is formed by PE-CVD. The third mask-forming layer203is coated with the resist mask301having the wiring groove pattern.

The fourth step shown inFIG. 9Dinvolves dry etching through the resist mask301(seeFIG. 9C) on the third mask-forming layer203(seeFIG. 9C) to form the third mask203′ with the wiring groove pattern and ashing with O2plasma and chemical treatment with an organic amine to completely remove the resist mask301and residues arising from etching.

The fifth step shown inFIG. 9Einvolves coating the second mask-forming layer202(having the third mask203′ formed thereon) with the resist mask302having the connecting hole pattern. The resist mask302is patterned in such a way that the connecting hole pattern in it at least partly overlaps the opening in the wiring pattern of the third mask203′.

The sixth step shown inFIG. 9Finvolves dry etching through the resist mask302(seeFIG. 9E) sequentially on the third mask203′, the second mask-forming layer202(seeFIG. 9E), the first mask-forming layer201(seeFIG. 9E), and the second insulating film109. This etching allows a high etching selective ratio of the second insulating film109to the first porous insulating film108A because the former contains porogene B′ dispersed therein in the polymerized form and the latter functions as the stopper. This step gives rise to the connecting hole303through which the first porous insulating film108A is exposed.

Etching in the sixth step also removes the resist mask302, while leaving the third mask203′, which functions as the mask for the wiring groove pattern. Etching on the second mask-forming layer202forms the second mask202′, which functions as the mask for the connecting pattern.

The above-mentioned etching on the third mask (SiO2)203′ to the first mask-forming layer (SiO2)201is accomplished by using the ordinary magnetron etching apparatus under the following condition.

Etching on the lower second insulating layer109is also accomplished by using the ordinary magnetron etching apparatus under the following condition.

The seventh step shown inFIG. 9Ginvolves dry etching through the third mask (SiO2)203′ on the second mask (SiCN)202′. The etched second mask202′ functions as the mask for the wiring groove pattern. The first mask-forming layer201(seeFIG. 9E) turns into the first mask201′ having the connecting hole pattern formed thereon. Etching in this step digs out the first porous insulating film108A (exposed at the bottom of the connecting hole303) to the middle thereof, thereby deepening the connecting hole303.

The eighth step shown inFIG. 9Hinvolves etching through the first mask (SiO2)201′ on the lower layer of the first porous insulating film108A, thereby digging out the connecting hole303further for the etching stopper film107to be exposed. This step also involves etching on the first mask (SiO2)201′ through the third mask (SiO2)203′ (seeFIG. 9G) and the second mask (SiCN)202′, thereby making the wiring groove304in the first mask201′.

The foregoing etching is accomplished by using the ordinary magnetron etching apparatus under the following condition.

The ninth step shown inFIG. 9Iinvolves etching through the second mask (SiCN)202′ (seeFIG. 9H) on the second insulating film109containing porogene B′ which remains at the bottom of the wiring groove304. This etching digs out further the wiring groove304made in the first mask201′, thereby making the wiring groove304in the first mask201′ and the second insulating film109. This etching allows a high etching selective ratio of the second insulating film109to the first porous insulating film108A because the former contains porogene B′ dispersed therein in the polymerized form and the latter functions as the stopper.

The foregoing etching is accomplished by using the ordinary magnetron etching apparatus under the following condition.

Continued etching is performed on the etching stopper film107remaining at the bottom of the connecting hole303so as to permit the connecting hole303opened at the bottom of the wiring groove304to communicate with the lower wiring106. In this way the dual damascene process is completed. This etching is accomplished by using the ordinary magnetron etching apparatus under the following condition.

The tenth step shown inFIG. 9Jinvolves irradiating the substrate101in its heated state with electron beams to make the second insulating film109free of porogene B′ by decomposition, for conversion of the second insulating film109(seeFIG. 9I) into the second porous insulating film109B with pores B.

Then, ordinary damascene process follows. That is, the eleventh step shown inFIG. 9Kinvolves sputtering to form the barrier metal305of tantalum and electrolytic copper plating or sputtering to fill the wiring groove304and the connecting hole303with the copper conductive film306.

The twelfth step shown inFIG. 9Linvolves chemical mechanical polishing (CMP) to remove those parts (remaining above the first mask201′) unnecessary for the wiring pattern out of the conductive part306(seeFIG. 9K) and the barrier metal305, and making the via307in the connecting hole303to form the wiring308in the wiring groove304. This step is concluded by forming the stopper film309of SiC on the first mask201′ and the top of the wiring308.

The foregoing steps illustrated inFIG. 9A to 9Lare repeated to form the multilayer wiring structure of dual damascene structure.

The foregoing method for fabrication of semiconductor devices produces the same effect as the first embodiment because the first porous insulating film108A is coated with the second insulating film109containing porogene B′ in such a way that it contains more carbon than the first porous insulating film108A and the resulting second insulting film109has a high etching selective ratio for the first porous insulating film108A. An additional reason is that CVD process can be employed to form both the first insulating film108and the second insulating film109.