Method and manufacturing a semiconductor device having a ruthenium or a ruthenium oxide

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for fabricating a semiconductor device and more particularly to a method for fabricating a semiconductor device, wherein an ashing of a resist film formed over a ruthenium film or a ruthenium oxide film can be performed at a high selectivity.

[0004] 2. Description of the Prior Art

[0005] In recent years, semiconductor devices including ruthenium or ruthenium oxide have been actively studied and developed in the art. Among them, a capacitor having electrodes made of ruthenium or ruthenium oxide and a dielectric material such as Pb(Zr_xTi_1−x)O₃, (Ba_xSr_1−x)TiO₃, or Ta₂O₅has been employed in some Ferroelectric Random Access Memory (FeRAM) or Dynamic Random Access Memory (DRAM).

[0006] Ruthenium and ruthenium oxide are capable of being processed into a high-anisotropic form by reactive ion etching using oxygen gas (e.g., U.S. Pat. No. 5,254,217 Maniar et al.) and a gas mixture of oxygen gas and halogen gas (e.g., U.S. Pat. No. 5,624,583 Tokashiki et al.). While it become possible to make the size of a capacitor electrode minimized with above etching method, there is a necessity to conduct an ashing of a photo-sensitive material (i.e., photoresist) formed over the electrode made of ruthenium or ruthenium oxide using oxygen plasma.

[0007] FIGS. 1ato 1eare cross sectional views for illustrating substantial steps in an exemplified conventional method for fabricating a semiconductor device having a ruthenium (Ru) film, wherein the step of ashing is performed after the step of contact hole etching.

[0008] First, as shown in FIG. 1a, a Ru film 12 and a silicon dioxide (SiO₂) layer 14 are formed over a silicon (Si) substrate 11. As shown in FIG. 1b, a photoresist is then applied on the SiO₂layer 14 followed by a patterning step using photolithography process to form a resist pattern 13. As shown in FIG. 1c, subsequently, a contact hole 16 for connecting a wiring to the Ru film 12 is formed through the SiO₂layer 14 by a dry etching process using CF₄or the like. After that, as shown in FIG. 1d, the resist pattern 13 is subjected to a plasma ashing process using O₂gas. During the ashing, the Ru film 12 is also gradually eroded because of the formation of volatile RuO₃or RuO₄as a result of reacting Ru with oxygen plasma 22. Consequently, the complete removal of the resist pattern 13 involves a substantial erosion of the Ru film 12. In some cases, the Ru film 12 under the contact hole 16 may be vanished as shown in FIG. 1e.

[0009] In the described conventional example, the ashing is performed after the contact hole etching. In the actual fabrication of the semiconductor device, there may be cases where such an ashing step in which the Ru film 12 may be exposed to oxygen plasma is performed more than one time. Thus, there is a possibility of vanish away the Ru film 12 from the structure after repeating the ashing step several times even though the entire Ru film 12 cannot be removed by the one ashing step.

[0010] For solving such a problem, Yunogami et al. (U.S. Pat. No. 6,326,218) disclose a method for preventing the Ru film 12 from erosion at the time of ashing the resist pattern 13 by forming a platinum (Pt) film 15 on a Ru film 12.

[0011] FIGS. 2ato 2eare cross sectional views for illustrating the steps in such a method disclosed in the above document. In this method, as shown in FIG. 2a, at first, the Ru film 12 is deposited over a Si substrate 11. Then, the platinum (Pt) film 15 is deposited on the Ru film 12. After patterning of the Pt film 15 and Ru film 12, a SiO₂layer 14 is deposited on the Pt film 15. Then, as shown in FIG. 2bresist pattern 13 is formed on the SiO₂layer 14. Furthermore, as shown in FIG. 2c, a contact hole 16 is formed through the SiO₂layer 14 by a dry etching process using CF₄or the like. After that, as shown in FIG. 2d, the resist pattern 13 is subjected to a plasma ashing process using oxygen plasma 22. In this case, as the Ru film 12 is covered with the Pt film 15, the Ru film 12 is not eroded at all, while the resist pattern 13 is completely incinerated by the ashing process as shown in FIG. 2e.

[0012] As described above, in the method for fabricating the conventional semiconductor device, the Ru film 12 may be eroded at the time of ashing. For preventing the Ru film 12 from the erosion, there is a necessity to deposit the Pt film 15 on the Ru film 12 to prevent it from erosion or disappearance. In this case, however, the fabrication becomes more complicated because of the additional steps of forming and patterning the Pt film 15 on the Ru film 12. In addition, the cost of fabricating the semiconductor device becomes increased because of costly Pt. Consequently, a sufficient productivity cannot be attained in the conventional method for fabricating a semiconductor device including Ru or Ru oxide. Furthermore, in some case, the formation of Pt film 15 on the Ru film 12 cannot be done as the need for directly forming a dielectric layer on the Ru film 12 comes out.

[0013] It is an object of the present invention to provide a method for fabricating a semiconductor device with a high productivity, wherein an ashing of a photo-sensitive material formed over ruthenium or ruthenium oxide can be easily realized at a high selectivity.

[0014] In a first aspect of the present invention, a method for fabricating a semiconductor device including at least one of a ruthenium and a ruthenium oxide comprises the step of ashing a photo-sensitive material over the ruthenium or the ruthenium oxide using a gas mixture containing oxygen gas or ozone gas and nitrogen gas, wherein the percentage composition of nitrogen gas is 50% or more.

[0015] Here, the ashing step may be performed by heating a substrate over which the ruthenium or the ruthenium oxide is formed at a temperature of 200° C. or more.

[0016] The ashing step may be performed after etching an interlayer insulation film on the ruthenium or the ruthenium oxide using a photo-sensitive material as a mask. Alternatively, the ashing step may be performed after etching of the ruthenium or the ruthenium oxide using a photo-sensitive material as a mask.

[0017] In a second aspect of the present invention, a method for fabricating a semiconductor device including at least one of a ruthenium and a ruthenium oxide, comprising the steps of: forming a film made of the ruthenium or the ruthenium oxide over a substrate; forming an interlayer insulation film on the ruthenium or the ruthenium oxide film; applying a photo-sensitive material on the interlayer insulation film and patterning the applied photo-sensitive material; etching the interlayer insulation film using the patterned photo-sensitive material as a mask; and ashing the patterned photo-sensitive material using an ashing gas provided as a mixture of a gas that contains oxygen gas or ozone gas and a gas that contains nitrogen gas, wherein the percentage composition of nitrogen gas is 50% or more.

[0018] Here, a contact hole for exposing the ruthenium film or the ruthenium oxide film may be formed in the step of etching the interlayer insulation film.

[0019] According to the above first or second aspect of the invention, there is an advantage of effectively performing the ashing of the photo-sensitive material with a high selectivity and a high ashing rate while preventing a partial disappearance of the ruthenium film or the ruthenium oxide film.

[0020] The reason for such an advantage is as follows. In the prior art method, the ruthenium is etched by oxygen plasma because ruthenium is oxidized to form a volatile compound, i.e., RuO₃or RuO₄. In this case ruthenium may be eroded or disappeared at the time of performing ashing of the photoresist using oxygen plasma.

[0021] On the other hand, according to the present invention the gas for ashing is prepared by mixing O₂gas with a large amount of N₂gas, so that the probability of a collision of oxygen with ruthenium can be decreased. At this time, the ashing rate of photoresist may be slightly decreased with a degree less than the decrease of etching rate of ruthenium. Therefore, the ashing of photoresist with an improved selectivity with respect to ruthenium can be attained.

[0022] The above and other objects, advantages and features of the present invention will be more apparent from accompanying drawings, in which:

[0023] FIGS. 1ato 1eare cross sectional views for illustrating respective steps in the method for fabricating a semiconductor device including a Ru film as a first conventional example in which an ashing of a photoresist is performed after contact hole etching;

[0024] FIGS. 2ato 2eare cross sectional views for illustrating respective steps in the method for fabricating a semiconductor device including a Ru film as a second conventional example in which an ashing of a photoresist is performed after contact hole etching;

[0025] FIGS. 3ato 3eare cross sectional views for illustrating respective steps in the method for fabricating a semiconductor device including a Ru film as one of preferred embodiments of the present invention in which an ashing of a photoresist is performed after contact hole etching;

[0026] FIG. 4 is a graph for illustrating the characteristic features of selectivity and ashing rate with respect to the composition of gas prepared by mixing O₂gas with N₂gas; and

[0027] FIG. 5 is a graph for illustrating the characteristic features of selectivity and ashing rate with respect to substrate temperature in the ashing step.

[0028] The invention will be now described herein with reference to illustrative embodiment. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiment illustrated for explanatory purposes.

[0029] Referring now to FIGS. 3ato 3e, a method for fabricating a semiconductor device will be described as one of preferred embodiments of the present invention.

[0030] As shown in FIG. 3a, at first, a Ru film 12 with a thickness of 100 nm is formed over a silicon (Si) substrate 11 and a silicon dioxide (SiO₂) layer (i.e., an interlayer insulation film) 14 with a thickness of 500 nm is deposited on the Ru film 12. Next, as shown in FIG. 3b, a photo-sensitive material (i.e., photoresist) is applied on the SiO₂layer 14, followed by a patterning step using a photolithography process to obtain a resist pattern 13 with a thickness of 1000 nm. Subsequently, as shown in FIG. 3C, the resist pattern 13 is used as a mask for etching the SiO₂layer 14 by a dry etching process using CF₄or the like to form a contact hole 16 extending through the SiO₂film 14. As shown in the figure, a part of the surface of the Ru film 12 is exposed at the bottom of the contact hole 16, so that the Ru film 12 can be used as an electrode or the like. An etching selectivity of SiO₂with respect to Ru is 20 or more. Thus, for example, the Ru film 12 can be etched about 25 nm in a thickness direction if the amount of over etching is set to 100%. In this case, therefore, the thickness of the remaining Ru film 12 is about 75 nm.

[0031] Furthermore, as shown in FIG. 3d, the resist pattern 13 is removed by ashing using oxygen/nitrogen plasma 33. In this embodiment, the ashing step is performed by an application of 1000 W of 13.56 MHz RF power at a substrate temperature of 250° C. with 360 mTorr of pressure. An ashing gas used in the present embodiment is a mixture gas consisting 220 sccm of O₂and 220 sccm of N₂. In other words, the composition ratio of N₂(i.e., N₂/(N₂+O₂) by volume) is 50%.

[0032] In the present invention, the conditions of the ashing step, especially the composition of the ashing gas and the substrate temperature are studied. FIG. 4 shows the results of the experiments for estimating the characteristic features of selectivity (i.e., etching rate of resist/etching rate of Ru) and resist ashing rate with respect to the composition ratio of N₂in the ashing gas prepared by mixing O₂with N₂in a capacitively coupled plasma ashing apparatus with an application of 1000 W of 13.56 MHz RF power under the conditions of a total gas flow of 440 sccm, a substrate temperature of 250° C., and a pressure of 360 mTorr. As is evident from FIG. 4, the selectivity becomes the maximum when the composition ratio of N₂is 50%, and the selectivity when the composition ratio of N₂is 50% is at least twice as large as the selectivity when the composition ratio of N₂is 10%. As is evident from FIG. 4, the ashing rate of the resist decreases as the composition ratio of N₂increases. The ashing rate of the resist when the composition ratio of N₂is 50% is about 0.7-fold of one when the composition ratio of N₂is 10%. However, a substantial impairment of the productivity cannot be observed because of a resist ashing rate of 1600 nm/min or more.

[0033] Furthermore, additional experiments for estimating the characteristic features of selectivity and ashing rate with respect to the substrate temperature are performed on samples having such a film structure under the following ashing conditions. That is, the ashing is performed using an ashing gas provided as a mixture of 400 sccm O₂gas and 40 sccm N₂(composition ratio of N₂is about 9%) with an application of 1000 W of 13.56 MHz RF power under the conditions of a pressure of 360 mTorr and a substrate temperature of 150, 200, or 250° C. The obtained results are shown in FIG. 5.

[0034] As is evident from FIG. 5, each of the selectivity and the ashing rate rapidly decreases as the substrate temperature decreases. When the substrate temperature is 200° C., the selectivity and the resist ashing rate become almost half of those at a temperature of 250° C. If the substrate temperature further decreases, then the ashing rate further decreases, resulting in no practical applicability.

[0035] In this embodiment, from the above results, the ashing conditions are concluded as described above in which the ashing is performed using an ashing gas provided as a mixture of 220 sccm O₂gas and 220 sccm N₂(composition ratio of N₂is about 50%) with an application of 1000 W of 13.56 MHz RF power under at a pressure of 360 mTorr and a substrate temperature of 250° C. Under such conditions, the selectivity of resist/Ru is about 500. Therefore, the erosion of Ru is only of 20 nm in thickness even if the ashing time is a ten-fold of the time period required for removing the resist of 1000 nm in thickness. The thickness of Ru film before the ashing is 75 nm, so the Ru electrode 12 can be remained without disappearance even if the ashing step is repeatedly performed three times.

[0036] By way of experiment, if the composition ratio of N₂is 10%, the Ru film 12 can be perfectly disappeared by repeating a similar step only two times.

[0037] Considering with the results or the like shown in FIGS. 4 and 5, as described above, the selectivity can be increased by increasing the composition ratio of N₂to 50% or more. In this case, we can find that the Ru film 12 is hardly disappeared even though the ashing step is repeated several times. As shown in FIGS. 4 and 5, furthermore, it is preferable that the substrate temperature may be 200° C. or more in the ashing step so as to prevent the selectivity and the ashing rate from becoming too low.

[0038] As described above, the present invention allows the ashing of the photo-sensitive material over the Ru film at a high selectivity, so that the semiconductor device can be fabricated at a low cost. In addition, at the time of ashing the photo-sensitive material, the Ru film can be prevented from becoming partially disappeared.

[0039] Although the present invention has been described in connection with the preferred embodiment thereof with reference to the accompanying drawings, various changes and modifications are apparent to those skilled in the art.

[0040] In the embodiment, the Ru film 12 is formed over the Si substrate 11. However, it is not limited to such a structure. Similar effects can be obtained by forming a ruthenium oxide film such as RuO₂film over the Si substrate 11. Obviously, same effects can be obtained with a laminated structure of ruthenium film and ruthenium oxide film, too.

[0041] Furthermore, instead of the gas prepared by mixing the O₂-containing gas with the N₂-containing gas, a gas prepared by mixing O₃-containing gas with N₂-containing gas may be used for obtaining similar effects. Thus, the mixture gas may be one prepared by mixing at least one of O₂and O₃with N₂.

[0042] In the embodiment, the ashing after the contact hole etching of the interlayer insulation film (SiO₂film) on the Ru film is described. It may be also applied on the whole steps in which the Ru film is exposed at the time of ashing, for example the ashing step after patterning the Ru film using a resist pattern as a mask.

[0043] It is to be noted that various other changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.