Process for forming metal-filled openings in low dielectric constant dielectric material while inhibiting via poisoning

[0002] 1. Field of the Invention

[0003] This invention relates to a process for forming integrated circuit structures. More particularly, this invention relates to a process for forming metal-filled openings in a layer of low dielectric constant (low k) dielectric material in an integrated circuit structure.

[0004] 2. Description of the Related Art

[0005] The shrinking of integrated circuits has resulted in levels of electrically conductive interconnects being placed closer together vertically, as well as reduction of the horizontal spacing between the electrically conductive interconnects, such as metal lines, on any particular level of such interconnects. As a result, capacitance has increased between such conductive portions, resulting in loss of speed and increased cross-talk. One proposed approach to solving this problem of high capacitance is to replace the conventional silicon oxide (SiO₂) dielectric material, having a dielectric constant (k) of about 4.0, with another insulation material having a lower dielectric constant to thereby lower the capacitance.

[0006] In an article by L. Peters, entitled “Pursuing the Perfect Low-K Dielectric”, published in Semiconductor International, Volume 21, No. 10, September 1998, at pages 64-74, a number of alternate dielectric materials are disclosed and discussed. Included in these dielectric materials is a description of a low k dielectric material having a dielectric constant of about 3.0 formed using a Flowfill chemical vapor deposition (CVD) process developed by Trikon Technologies of Newport, Gwent, U.K. The process is said to react methyl silane (CH₃—SiH₃) with hydrogen peroxide (H₂O₂) to form monosilicic acid which condenses on a cool wafer and is converted into an amorphous methyl-doped silicon oxide which is annealed at 400° C. to remove moisture. The article goes on to state that beyond methyl silane, studies show a possible k of 2.75 using dimethyl silane in the Flowfill process.

[0007] An article by S. McClatchie et al. entitled “Low Dielectric Constant Oxide Films Deposited Using CVD Techniques”, published in the 1998 Proceedings of the Fourth International Dielectrics For ULSI Multilevel Interconnection Conference (Dumic) held on Feb. 16-17, 1998 at Santa Clara, Calif., at pages 311-318, also describes the formation of methyl-doped silicon oxide by the low-k Flowfill process of reacting methyl silane with H₂O₂to achieve a dielectric constant of ˜2.9.

[0008] The incorporation of such carbon-doped silicon oxide dielectric material into interconnect architecture has been very attractive not only because of the low k properties, but also because of the compatibility with conventional silicon process technologies. Generally these materials remain stable upon annealing at temperatures of up to 500° C. The carbon doped silicon oxide materials are characterized by the structure of amorphous silicon oxide with incorporated methyl groups and hydrogen species, and are also characterized by a reduced density in comparison with conventional silicon oxide that can be explained by the formation of microporosity surrounding the incorporated methyl groups. Furthermore, such hydrocarbon-modified silicon oxide dielectric materials deposited by CVD techniques are also characterized by strong adhesion.

[0009] However, when trenches and holes (such as vias and contact openings) are etched into dielectric films or layers, the resulting etched surfaces are exposed to atmospheric contaminants such as, for example, oxygen, nitrogen, rare gases, hydrocarbons, and water vapor which tend to be attracted to such surfaces. In the case of low dielectric constant (low k) materials, which tend to be porous in nature, the adsorption of such contaminants is greater. This is apparently due to a combination of effects including the stripping of weakly bound chemical radicals from the glass matrix that gave rise to the porosity. This is the case, for instance, in low k silicon oxide dielectric materials that achieved their low dielectric constant through the incorporation into the silicon oxide matrix of weakly bonded organic groups such as methyl groups, which effectively lowered the density of the low k dielectric silicon oxide material to achieve the desired low dielectric constant of the material.

[0010] Such organic groups are easily removed from the silicon oxide matrix during plasma etching and cleaning operations such as used, for example, to etch a hole such as a via in the low k dielectric material through a resist mask, and/or to remove the etch mask, and/or to remove etch residues formed either during etching of the hole or removal of the resist mask. If such newly modified surfaces are exposed to atmospheric contamination, e.g., moisture, there is a tendency for this contamination to replace the organic groups that have been removed (i.e., to fill the empty and/or attractive pores). When these now contaminated surfaces are subsequently put into another deposition system for the purpose of creating a thin diffusion barrier, the newly absorbed contaminants tend to diffuse out of the surface of the low k dielectric material, and interfere with the coating of this same surface by the diffusion barrier material. This causes the formation of porous barriers or otherwise defective barriers that do not exhibit normal barrier properties. For instance, in the case of tungsten plug technologies where a titanium film is first deposited by PVD to create a “glue layer”, followed by PVD formation of a TiN diffusion barrier prior to the deposition of the tungsten to fill the hole with a tungsten plug, breakdown of the TiN barrier is frequently observed, as evidenced by the reaction that occurs between the titanium film and the WF₆gas which results in what is commonly referred to as “poisoned vias” or “poisoned plugs”.

[0011] Various approaches have been explored to attempt to solve this problem of via “poisoning”. Zukharev et al. U.S. Pat. No. 6,114,259, assigned to the assignee of this invention, and the subject matter of which is hereby incorporated by reference, teaches treating the etched via sidewalls of the low k carbon-doped silicon oxide dielectric material with a nitrogen plasma, or a nitrogen and oxygen plasma, to densify the exposed low k carbon-doped silicon oxide dielectric material. The Zukharev et al. patent further teaches removal of the photoresist mask used to form the openings with a mild oxidizing agent comprising an H₂O plasma. The H₂O plasma removes the resist mask without damaging the exposed low k carbon-doped silicon oxide dielectric material comprising the sidewalls of the etched via sufficiently to interfere with later filling of the via with an electrically conductive metal filler.

[0012] Wang et al. U.S. Pat. No. 6,028,015, also assigned to the assignee of this invention, and the subject matter of which is also hereby incorporated by reference, teaches treating damaged via sidewalls of low k carbon-doped silicon oxide dielectric material with either a hydrogen plasma or a nitrogen plasma to repair the via sidewall surfaces which have been damaged by prior removal of the photoresist mask with a traditional ashing/oxidation process, i.e., an oxygen plasma. Such a treatment with a hydrogen or nitrogen plasma is said to cause the hydrogen or nitrogen to bond to silicon atoms with dangling bonds left in the damaged surface of the low dielectric constant organo silicon oxide insulation layer to replace organo material severed from such silicon atoms at the damaged surface. Absorption of moisture in the damaged surface of the low dielectric constant organo silicon oxide insulation layer, by bonding of such silicon with moisture, is thereby inhibited.

[0013] John Hu U.S. patent application Ser. No. 09/428,344, also assigned to the assignee of this invention, and the subject matter of which is also hereby incorporated by reference, discloses a process for removing resist mask material from a protective barrier layer formed over a layer of low k silicon oxide dielectric material. The resist removal process comprises exposing the resist mask material to a hydrogen plasma formed from a source of hydrogen such as ammonia, while maintaining the temperature below about 40° C. to inhibit attack of the low k silicon oxide dielectric material by oxygen released from the decomposition of the resist material.

[0014] Catabay et al. U.S. patent application Ser. No. 09/543,412, also assigned to the assignee of this invention, and the subject matter of which is also hereby incorporated by reference, teaches treating damaged surfaces of low k material with carbon-containing gas prior to exposure of the damaged surfaces to atmosphere. Treatment could be after etching an opening such as a via, but prior to removal of resist mask if the substrate will be exposed to atmosphere between the via etch step and the resist removal step.

[0015] The use of composite layers of dielectric material have also been proposed. Catabay et al. U.S. patent application Ser. No. 09/426,056, also assigned to the assignee of this invention, and the subject matter of which is also hereby incorporated by reference, discloses the formation of a composite layer of low k silicon oxide dielectric material on an integrated circuit structure having closely spaced apart metal lines thereon. The composite layer of low k silicon oxide dielectric material is formed by depositing, in high aspect ratio regions between closely spaced apart metal lines, a first layer of low k silicon oxide dielectric material exhibiting void-free deposition properties until the resulting deposition of low k silicon oxide dielectric material reaches the level of the top of the metal lines on the oxide layer. A second layer of low k silicon oxide dielectric material, having a faster deposition rate than the first layer, and less susceptible to etch damage, is then deposited over the first layer up to the desired overall thickness of the low k silicon oxide dielectric layer. A via may then be formed down to the metal line which extends only through the second layer of low k material which is less susceptible to etch damage.

[0016] It has also been proposed to use resist masks to form hard masks which then permits the resist mask to be removed prior to etching of the low k dielectric material. The low k dielectric material is then etched through the hard mask. Subsequent removal of the hard mask does not result in the same damage to exposed surfaces of the low k dielectric material as does the conventional ashing step to remove the resist mask. Such processes are found in Schinella et al. U.S. patent application Ser. No. 09/607,512, and Wang et al. U.S. patent application Ser. No. 09/607,511, both also assigned to the assignee of this invention, and the subject matter of each of which is also hereby incorporated by reference.

[0017] While the above approaches all attempt to remedy the problem of damage to the low k dielectric material, which can result in inferior filling of openings with conductive metal filler materials, i.e., “via poisoning”, it has been found that at least some of the proposed solutions do not completely solve the problem and may unduly complicate the issue and even create further difficulties.

[0018] It would, therefore, be desirable to provide a composite process for filling openings in low k dielectric material with metal filler materials which process is addressed to solution of various portions of the process contributing to the problem of inferior filling of an opening in low k dielectric material with metal filler materials.

[0019] In accordance with the process of the invention, a composite layer of dielectric material is first formed over the integrated circuit structure, comprising a barrier layer of dielectric material, a layer of low k dielectric material over the barrier layer, and a capping layer of dielectric material over the layer of low k dielectric material.

[0020] After formation of the composite dielectric layer, a photoresist mask is photolithographically formed over the capping layer of dielectric material, and the mask is then baked in the presence of UV light to further cure (cross-link) the resist mask material prior to using the resist mask. The composite layer is then etched through the resist mask using an etchant gas mixture which includes a mild oxidizing gas such as CO, rather than O₂, to inhibit damage to the newly exposed surfaces of low k dielectric material.

[0021] The newly exposed surfaces of the low k dielectric material are then subject to an optional densification step to harden the exposed portions of the low k dielectric material. The resist mask is then removed using a plasma formed from a neutral or reducing gas.

[0022] After removal of the resist mask, the exposed surfaces of low k dielectric material are passivated by a low power oxygen plasma to further inhibit damage to the low k dielectric material. In a preferred embodiment, the optional densification step, the resist mask removal step, and the low power oxygen passivation step are all carried out in the same chamber, or at least in separate chamber in the same vacuum system to limit, or preferably eliminate, exposure of the surfaces of low k dielectric material to oxygen or other contaminants such as moisture.

[0023] The substrate is then subject to a solvent clean to remove etch residues and, after rinsing, the substrate is placed in an annealing furnace and annealed to degasify the exposed surfaces of low k dielectric materials.

[0024] The substrate is then placed in a sputtering chamber and RF cleaned with a cleaning plasma following which a thin layer of an electrically conductive material such as PVD titanium is formed over the surfaces of the etched openings. A CVD layer of titanium nitride is then formed over the titanium layer in another chamber in the same vacuum chamber. The coated openings are then filled with a conductive metal filler material capable of filling the openings such as aluminum, tungsten, or copper.

[0025] The sole FIGURE is a flowsheet illustrating the steps of the process of the invention.

[0026] a. Formation of the Composite Dielectric Layer

[0027] In accordance with the process of the invention, a barrier layer of dielectric material is first formed over the integrated circuit structure, e.g., over the raised metal lines of a lower metal interconnect layer. Such a barrier layer, which may comprise a layer of any conventionally formed silicon oxide ranging in thickness from at least greater than 50 nanometers (nm) to as thick as about 100 nm, is described more fully in Catabay et al. U.S. patent application Ser. No. 09/605,380, filed Jun. 27, 2000, assigned to the assignee of this invention, and the subject matter of which is hereby incorporated by reference.

[0028] A layer of low k dielectric material such as low k carbon-doped silicon oxide dielectric material is then formed over the barrier layer. The term “low k”, as used herein, is intended to define a dielectric constant of a dielectric material of 3.5 or less. Preferably, the dielectric constant of a “low k” material will be 3.0 or less, and most preferably will be 2.5 or less. The layer of low k carbon-doped silicon oxide dielectric material may comprise the reaction product of methyl silane (CH₃SiH₃) and hydrogen peroxide (H₂O₂), as described in the previously cited Peters and McClatchie et al. articles, or it may comprise the reaction product of a mild oxidant such as hydrogen peroxide with the carbon-substituted silane materials disclosed in Aronowitz et al. Ser. No. 09/274,457, filed on Mar. 22, 1999, and assigned to the assignee of this application, the subject matter of which is hereby incorporated by reference.

[0029] Also capable of being used as the low k dielectric material is the reaction product of a mild oxidant such as hydrogen peroxide and the organofluoro silanes described in Ser. No. 09/590,310, filed on Jun. 7, 2000, and in Ser. Nos. 09/792,683; 09/792,685; and 09/792,691; all filed on Feb. 23, 2001. All four of these applications relating to the reaction of hydrogen peroxide and organofluoro silanes are assigned to the assignee of this application, and the subject matter of all four applications is hereby incorporated by reference. If desired any of the substituted silanes described above may be blended with other additives to achieve certain properties. For example, one of the substituted silanes may be mixed with unsubstituted silane (SiH₄), if desired.

[0030] A capping layer of dielectric material is then formed over the layer of low k dielectric material. The capping layer of dielectric material may also comprise a layer of any conventionally formed silicon oxide, and is also more fully described in the aforementioned Catabay et al. Ser. No. 09/605,380.

[0031] b. Formation of the Photoresist Mask

[0032] After formation of the composite layer comprising the barrier dielectric layer, the low k dielectric layer, and the capping dielectric layer, a photoresist mask is photolithographically formed over the capping layer of dielectric material, using standard photoresist materials available from commercial sources such as, for example, Sumitomo, Shopley, and TOK, which comprise cross-linkable organic materials. In accordance with the invention, the resist mask, after formation, is subject to a baking step in the presence of UV light such as a mercury light capable of emitting light of UV wavelength to further cross-link or cure the resist mask material prior to using the resist mask to form openings in the composite layer of barrier layer, low K layer, and capping layer. The resist mask is preferably baked for a period of at least about 2 minutes at a temperature of at least about 170° C. during which time the resist mask remains immersed in UV light. Higher temperatures and longer anneal times may be used, but should be avoided to conserve the thermal budget.

[0033] c. Etching of Openings

[0034] The desired openings are then etched in the composite layer through the resist mask using a plasma comprising an etchant gas mixture which includes a mild oxidizing gas such as CO, rather than O₂, to inhibit damage to the newly exposed surfaces of low k dielectric material. Other etchant gases which may be used in combination with CO in the etching of openings through low k dielectric materials while minimizing damage to the low k dielectric material include, C₄F₈, N₂, and Ar. Although the use of oxygen, including O₂and O₃(and any other oxygen-containing gas which will decompose in the plasma to oxygen), is preferably avoided during the etching step to form the openings, a minor amount of such oxygen-containing gases sufficient to passivate the newly etched surfaces could be used, as will be discussed below.

[0035] It should be noted that the term “openings”, as used herein, is intended to include vias used to electrically connect vertically spaced apart layers of metal interconnects, as well as trenches extending horizontally in one or more dielectric layers such as formed in damascene processes. The term “openings” may further include vertical contact openings formed in a dielectric layer down to the contacts of a passive device such as a capacitor or an active device such as a transistor in the integrated circuit structure. Thus, the terms “holes”, “openings”, “vias”, “trenches”, and “contact openings”, as used herein, should all be considered to be interchangeable with respect to the process of the invention.

[0036] d. Optional Densification of Exposed Surfaces of Low K Dielectric Material

[0037] After etching the pattern of openings in the composite layer through the resist mask, the substrate, including the newly exposed surfaces of the layer of low k dielectric material, are optionally subject to a densification step comprising exposure to a nitrogen plasma or a nitrogen plus oxygen plasma to harden the exposed portions of the low k dielectric material. As previously mentioned, such a densification step is more fully described in Zukharev et al. U.S. Pat. No. 6,114,259.

[0038] e. Removal of Photoresist Etch Mask

[0039] The resist mask is then removed using a neutral or reducing gas plasma, such as an H₂O, H₂, or NH₃plasma, or mixtures of reducing or neutral gases, i.e., without using an oxygen plasma, to mitigate or inhibit damage to the exposed low k surfaces. Previously discussed John Hu U.S. patent application Ser. No. 09/428,344, teaches exposing the resist mask material to a hydrogen plasma formed from a source of hydrogen such as ammonia, while maintaining the temperature below about 40° C. to inhibit attack of the low k silicon oxide dielectric material by oxygen released from the decomposition of the resist material. The use of an H₂O plasma to remove the resist mask while mitigating damage to the low k dielectric material is described in the aforementioned Zukharev et al. U.S. Pat. No. 6,114,259.

[0040] f. Passivation of Exposed Surfaces of Low K Dielectric Material

[0041] After removal of the resist mask, the exposed surfaces of low k dielectric material are passivated by a low power (low wattage) oxygen plasma ranging in power from about 200 watts to about 1000 watts. The passivation is carried out for a short period of time of from about 10 seconds to about 2 minutes to further inhibit damage to the low k dielectric material. The pressure in the passivation chamber should be maintained between about 1 millitorr and about 150 millitorr during the passivation step, while the temperature of the substrate during the passivation step should be maintained within the range of from about 10° C. to about 70° C. The desired low concentration of oxygen within the passivation chamber during the passivation step may be maintained by flowing into the passivation chamber a source of oxygen in an amount equivalent to flowing into a 10 liter chamber from about 50 standard cubic centimeters per minute (sccm) of a source of oxygen to about 1000 sccm of a source of oxygen,

[0042] In a preferred embodiment, the optional densification step, the resist mask removal step, and the low power oxygen passivation step are all carried out in the same chamber, or at least in separate chambers in the same vacuum system to limit, or preferably eliminate, exposure of the surfaces of low k dielectric material to oxygen or other contaminants such as moisture.

[0043] g. Cleaning and Degasification of Exposed Surfaces in Etched Openings

[0044] The substrate is then subject to a solvent clean to remove the etch residues remaining from the etching step used to form the openings in the composite layer, and any residues remaining from the removal of the resist mask. The solvent clean step is carried out using a commercially available solvent such as EKC 265, a hydroxylamine type solvent.

[0045] After rinsing, the substrate is placed in an annealing furnace and annealed at from about 350° C. to about 450° C. at atmospheric pressure for a period of from about 30 minutes to about 120 minutes to degasify the materials, including the exposed surfaces of low k dielectric materials.

[0046] h. Filling of Openings Etched in Composite Layer

[0047] The substrate is then placed in a sputtering chamber and RF cleaned with a cleaning plasma such as an argon plasma or a hydrogen plasma for a period of from about 5 seconds to about 60 seconds.

[0048] After this RF cleaning step, a thin layer of an electrically conductive material capable of promoting adherence (a “glue layer”) such as titanium is deposited by PVD (sputtered) over the surfaces of the etched openings in the low k dielectric material. A CVD barrier layer of titanium nitride is then formed over the titanium layer in another chamber in the same vacuum chamber. This is contrast to the prior art practice of sputtering a PVD barrier layer of titanium nitride over the newly deposited layer of PVD titanium. The substitution of a CVD barrier layer of titanium nitride, in accordance with the invention, for the prior art PVD barrier layer of titanium nitride has been found to result in less stress than the PVD layer of titanium nitride, thus increasing the rate of successful filling of the openings.

[0049] The coated openings are then filled with a conductive metal filler material capable of filling the openings such as aluminum, tungsten, or copper. The resultant substrate with metal-filled openings in low k dielectric material has a reduced number of unfilled or unsatisfactorily filled openings because of the practice of the process steps of the invention.