Electrolytic Processing Method and Electrolytic Processing Apparatus

TECHNICAL FIELD

The present invention relates to an electrolytic processing method and an electrolytic processing apparatus, and more particularly to an electrolytic processing method and an electrolytic processing apparatus which, after carrying out damascene plating to fill an interconnect metal into recesses, such as trenches and via holes, formed in a surface of a substrate such as a semiconductor wafer, in processing of metal interconnects of a semiconductor integrated circuit, can be used to remove an excessive interconnect metal layer and a diffusion barrier layer and flatten the surface of the substrate.

BACKGROUND ART

Instead of aluminum or an aluminum alloy which has generally been used as a metal interconnect material for semiconductor integrated circuits, copper, which has low electric resistance and high electromigration resistance, has been put into practical use these days. Copper interconnects are generally formed by a damascene process which comprises filling copper, by plating, into via holes and trenches provided in an insulating film of a substrate, followed by CMP (chemical mechanical polishing) to remove extra copper and a diffusion barrier layer, previously provided for preventing diffusion of copper, and flatten the substrate surface.

An exemplary process for the formation of copper interconnects will now be described with reference to FIGS. 1A through 1D. An insulating film used in this process includes a low-dielectric constant insulating layer. As shown in FIG. 1A, an upper-layer insulating film 22, consisting of an Si—N barrier layer 14, a first low-dielectric constant (low-k) insulating layer 16, a second low-dielectric constant (low-k) insulating layer 18 and a hard mask 20, is deposited on a surface of a substrate W with a lower-layer interconnect 12 of copper formed in a lower-layer insulating film 10, and a via hole 24 and a trench 26 are formed in the upper-layer insulting film 22, e.g., by the lithography/etching technique. Thereafter, a diffusion barrier layer 28, for preventing diffusion of copper, is formed on the upper-layer insulating film 22 and a seed layer 30, which serves as a feeding layer in electroplating, is formed on the diffusion barrier layer 28. A metal material, such as W, Ta/Ta_XN_Y, Ti_XN_Y, W_XN_Y, W_XSi_Y(X and Y are each a numerical value that varies depending on the alloy), Ta_XSi_YN_Z, Ti_XSi_YN_Z(X, Y and Z are each a numerical value that varies depending on the alloy) or Ru, is generally used for the diffusion barrier film 28.

Thereafter, as shown in FIG. 1B, copper 32 is filled into the via hole 24 and the trench 26 of the substrate W while depositing copper 32 over the hard mask 20, for example, by plating. Thereafter, the copper film (copper 32 and seed layer 30) on the outermost surface of the substrate W is removed by chemical mechanical polishing (CMP) using an abrasive slurry. As shown in FIG. 1C, the copper 32 embedded in the trench 26 is removed by the CMP to a depth approximately equal to a thickness of the diffusion barrier layer 28, i.e., until the surface of the copper 32 embedded in the trench 26 reaches a level which is lower than the surface of the diffusion barrier layer 28 by approximately the thickness of the diffusion barrier layer 28. Subsequently, the diffusion barrier layer 28 is polished separately so that the surface of the hard mask 20 becomes approximately flush with the surface of the copper 32 embedded in the via hole 24 and the trench 26, thereby completing the polishing process. An interconnect (upper-layer interconnect) 34 composed of the copper 32 is thus formed in the upper-layer insulating films 22, as shown in FIG. 1D.

A substrate has, in its surface in which metal interconnects are to be processed and formed, a region where recesses for filling of metal interconnects, such as trenches and via holes, are formed and a region where no such recesses are formed (i.e., region without filling of metal interconnects). The expression “outermost surface of the substrate” herein refers to the surface of the region where no such recesses are formed. The expression “recesses of the substrate” refers to the region where such recesses are formed.

It has been proposed to use, as an insulating material (interlevel dielectric material), an organic or inorganic material called low-k material, which has a lower dielectric constant than a conventional CVD (chemical vapor deposition)-SiO₂film. A low-k material has a porous structure with numerous pores so as to lower the dielectric constant, and therefore has a lower mechanical strength than a CVD-SiO₂film. Accordingly, if a low-k material is present underneath a hard mask, the hard mask is likely to peel off from the low-k material, for example when polishing away a copper film and a diffusion barrier layer by CMP. In order to prevent the pee-off of the hard mask, it is necessary to use a lower polishing pressure during the CMP than that for polishing a conventional SiO₂film. Lowering the polishing pressure, however, leads to a lowering of the polishing rate, and therefore is not preferred from the viewpoint of productivity. On the other hand, the cost of an abrasive slurry (polishing liquid comprising a dispersion of abrasive grains in an aqueous solution) accounts for a large proportion of the total cost of a CMP process, and there is a demand for a reduction in the amount of polishing liquid used.

There is, therefore, a proposal to carry out electrolytic processing using an electrolytic solution, instead of an abrasive slurry, to remove excessive copper 32, such as the copper 32 deposited above the hard mask 20, until the surface of the diffusion barrier layer 28 becomes fully exposed, in order to exclusively promote processing of the copper film, i.e., the copper 32 and the seed layer 30. This method is intended to process only the copper film, i.e., the copper 32 and the seed layer 30, by electrolytic processing. At present, no attempt has yet been made to electrolytically process also the diffusion barrier layer 28.

This is because when a positive electric potential is applied to the diffusion barrier layer 28, which is usually of a titanium or tantalum metal, a corrosion-resistant dielectric film (oxide film) having a uniform thickness is formed in the surface of the diffusion barrier layer 28. The dielectric film blocks electric current, making it difficult to carry out electrolytic processing of the diffusion barrier layer 28. Especially in the case of tantalum, an oxide film (Ta₂O₅) is formed in the metal surface in an aqueous solution at any pH. Insofar as the oxide film is dense and has good adhesion to the metal tantalum, the tantalum (Ta) can act like a noble metal and is almost completely resistant to solvents other than hydrofluoric acid and concentrated alkaline solutions, such as HCl, H₂SO₄, H₂PO₄and HNO₃, and even to aqua regia.

Generally-known methods for electrolytic processing of tantalum having such a corrosion resistance include a method which involves the use of an organic electrolytic solution, comprising an anion and an aprotic polar solvent, in electrolytic processing of a surface of a tantalum or niobium material for use in an artificial bone, an artificial dental root or a capacitor so as to roughen the metal surface (see Japanese Patent Laid-Open Publication No. 2003-73900), and a method which involves the sole use of a source of radiation in etching of tantalum pentoxide in a fluorine-containing solution (see Japanese Patent Laid-Open Publication No. H6-49664).

DISCLOSURE OF INVENTION

As shown in FIG. 1B, in order to securely fill an interconnect metal, such as copper 32, into recesses, such as the trenches, 26 by damascene plating, excessive copper 32 is deposited over the hard mask 20 (on the outermost surface of the substrate), while the copper 32 embedded in the recesses, such as the trenches 26 and the via holes 24, is also excessively deposited to a level higher than the level of the outermost surface of the substrate. The copper 32 thus deposited is likely to differ in the surface level between the outermost surface portions of the substrate and the portions corresponding to the recesses of the substrate, i.e., likely to have surface irregularities and unlikely to become flat. In order to remove the copper 32, which has thus been excessively deposited over the entire substrate with the formation of surface irregularities, at a sufficient polishing rate, a high processing pressure is used in conventional CMP. However, it has been difficult to obtain a sufficient flatness of the substrate surface. In particular, the conventional CMP processing may entail defects such as dishing, i.e., depression in a dish-like cross-sectional shape, in interconnects, and erosion, i.e., extra polishing of an insulating film upon polishing of a metal, such as copper.

It has recently been proposed to use, as an insulating material, an organic or inorganic material called low-k material, which has a lower dielectric constant than a conventional CVD-SiO₂film. A low-k material has a porous structure with numerous pores so as to lower the dielectric constant, and therefore has a lower mechanical strength than a conventional SiO₂film. Accordingly, when a CMP process is used to remove a diffusion barrier layer, an underlying layer, e.g., a hard mask, is likely to peel off from a low-k material. It is therefore generally difficult to employ CMP for processing of a substrate having a low-k material layer. If CMP is used for processing of such a substrate, a low polishing pressure must be employed during CMP processing, resulting in low processing rate, i.e., low productivity. Practical use of CMP is thus difficult.

A conceivable method, therefore, is to carry out electrolytic processing instead of CMP, using an electrolytic solution (processing solution) for exclusively promoting processing of copper, to remove excessive copper until a surface of a diffusion barrier layer becomes fully exposed. The diffusion barrier layer, which becomes exposed all over the surface of a substrate during electrolytic processing, because of its hardness and chemical stability, is lower in the polishing rate than copper. Accordingly, if electrolytic processing is continued, copper will be preferentially processed after the exposure of the diffusion barrier layer, which may cause dishing.

It is, therefore, desirable to carry out such electrolytic processing as to make the processing rate for copper and the processing rate for a diffusion barrier layer equal after the diffusion barrier layer has become exposed. However, when electrolytic processing is carried out by using a fluorine ion-containing aqueous solution or a concentrated alkaline solution, which is generally used as a solution system for dissolving a metal usable for a diffusion barrier layer, such as tantalum, titanium, tungsten or ruthenium, as a electrolytic solution, each of the above solutions will also dissolve an underlying insulating film, such as a hard mask, composed of an Si-based material. On the other hand, the cost of a slurry accounts for a large proportion of the total cost of a CMP process, and waste slurry treatment also takes a considerable cost. Therefore, there is a demand for a reduction in the amount of slurry used.

The present invention has been made in view of the above situation. It if therefore an object of the invention to provide an electrolytic processing method and an electrolytic processing apparatus which make it possible to preferentially process a diffusion barrier layer while suppressing processing of an interconnect metal, thereby enabling omission of CMP or a lowering of processing pressure in CMP.

In order to achieve the object, the present invention provides an electrolytic processing method comprising: bringing a surface of a substrate into contact with an electrolytic solution comprising an organic solvent and an electrolyte dissolved into the organic solvent; and applying an electric potential to the surface of the substrate to carry out electrolytic processing of the substrate surface.

When carrying out electrolytic processing (electrolytic polishing) of a diffusion barrier layer by using an electrolytic solution (aqueous solution) which uses water as a solvent, and applying a positive electric potential to the diffusion barrier layer in the aqueous solution, the following problems will arise: For most types of metals usable for a diffusion barrier layer, a passive oxide film will be formed in a surface of the diffusion barrier layer when a positive electric potential is applied to the barrier layer in an aqueous solution. The oxide film precludes the diffusion barrier layer from dissolving as metal ions into the aqueous solution, whereby the electrolytic processing will not proceed. This is due to the oxygen in the water molecules and the oxygen, dissolved oxygen, etc. generated by electrolysis and existing in the aqueous solution. When carrying out electrolytic processing (polishing) of a diffusion barrier layer in an aqueous solution, the processing (polishing) rate can be increased by increasing the electric potential applied to the diffusion barrier layer. However, the theoretical decomposition voltage of water is 1.23V; and application of a higher voltage than 1.23V to a diffusion barrier layer causes decomposition of water, resulting in lowered processing efficiency. Accordingly, the processing (polishing) rate cannot be made so high.

According to the present invention, by using an organic solvent, which is little affected, e.g., by oxygen, as a solvent of an electrolytic solution, electrolytic processing (electrolytic polishing) of a diffusion barrier layer can be carried out in such a manner that a passive oxide film is prevented from being formed in the surface of the diffusion barrier layer upon application of a positive electric potential to the diffusion barrier layer in the electrolytic solution, and the metal of the diffusion barrier layer is allowed to dissolve as metal ions into the electrolytic solution. Organic solvents generally have a high decomposition voltage: For example, the decomposition voltage of propylene carbonate is 6.7V. Accordingly, a high voltage can be applied to a diffusion barrier layer, thereby making the electrolytic processing rate sufficiently high.

The electrolytic processing is preferably carried out in a constant potential-controlled manner by applying a positive electric potential, which is controlled at a predetermined value less than the decomposition voltage of the organic solvent, to the surface of the substrate.

By controlling the electric potential applied to the surface of the substrate, it becomes possible to exclusively process a diffusion barrier layer or to simultaneously process the diffusion barrier layer and an interconnect metal, such as copper. It thus becomes possible to initiate removal of the diffusion barrier layer even when the excessive interconnect metal, such as copper, is not completely removed and partly remains on the substrate.

Though a higher electrolytic processing rate can be obtained by applying a higher voltage to the surface of the substrate, it is necessary to take the decomposition voltage of the organic solvent into consideration, as described above. For example, the decomposition voltage of propylene carbonate is +3.7V (based on silver-silver ion reference electrode). When the solvent is used, the electric potential applied to the substrate surface is preferably less than the solvent decomposition voltage. By the “constant potential” is herein meant to keep the electric potential applied to a substrate surface constant.

According to the present invention, electrolytic processing of a diffusion barrier layer of a semiconductor substrate can be effectively carried out. Further, electrolytic processing of excessive copper deposited on the substrate can also be carried out by controlling the voltage applied to the substrate. It thus becomes possible to efficiently carry out electrolytic processing to remove excessive copper and electrolytic processing to remove a diffusion barrier layer in a successive manner by a single electrolytic cell.

An example of the electrolytic solution contains at least one of fluoride ion, chloride ion, bromide ion, iodide ion, hexafluorophosphate ion, tetrafluoroborate ion, and hexafluoroarsenic ion.

Hexafluorophosphate ion, for example, has a high effect of combining with a metal ion from a diffusion barrier layer and thereby promoting dissolution of the diffusion barrier layer into the solvent, and therefore can be preferably used in electrolytic processing.

Examples of the organic solvent include at least one of propylene carbonate, ethylene carbonate and dimethyl sulfoxide.

Of organic solvents, propylene carbonate, ethylene carbonate and dimethyl sulfoxide have a high dielectric constant and high ability to dissolve an electrolyte, and therefore are preferred. The dielectric constant of an organic solvent is known to be an index of the electrolyte dissolving ability of the organic solvent: A higher electrolytic constant indicates higher ability to dissolve an electrolyte.

Examples of the electrolyte include lithium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, tetramethyl ammonium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate, and lithium tetrafluoroborate.

The electrolyte is a compound having the property of providing an ion which serves to transport electrons in the solution during electrolytic processing, and chemically combining with an object metal of electrolytic processing, i.e., a metal that has dissolved into the solvent. When carrying out electrolytic processing, a metal is dissolved into the solvent. The metal ion is not allowed to exist as it is, but must be combined with another substance and precipitated in the solution. Accordingly, a compound capable of combining with the metal ion is selected as the electrolyte.

Preferably, the electrolytic solution further comprises at least one heterocyclic compound having a triazole ring, a pyrrole ring, a pyrazole ring, a thiazole ring, or an imidazole ring.

When a diffusion barrier layer in a substrate surface is processed by electrolytic processing, an interconnect metal, such as copper, also comes into contact with an electrolytic solution and, depending on the electric potential applied to the substrate surface, the interconnect metal can also dissolve into the electrolytic solution. The dissolution of the interconnect metal into the electrolytic solution can be prevented by adsorbing a heterocyclic compound specifically onto the surface of the interconnect metal. Such a heterocyclic compound is generally called corrosion inhibitor or inhibitor, and contains oxygen, nitrogen and sulfur atom. By adsorbing these atoms specifically onto the surface of the interconnect metal, a substance which will cause corrosion of the interconnect metal, i.e., a substance which will act in such a manner as to dissolve the interconnect metal into the electrolytic solution, can be prevented from directly reacting with the interconnect metal to corrode (dissolve) the metal. Such an inhibitor should be a heterocyclic compound which does not adsorb to the diffusion barrier layer, and may be added in the electrolytic solution or applied in a solution or in a gas phase to the substrate surface before immersing the substrate in the electrolytic solution.

The heterocyclic compound preferably is a nitrogen-containing heterocyclic compound selected from benzotriazole, pyrrole, 3-(2-thienyl)-1-pyrazole, 2-butyl imidazole, 6-thioguanine and trithiocyanuric acid.

These compounds have the property of specifically adsorbing onto a surface of an interconnect metal, such as copper, but not adsorbing onto a diffusion barrier layer. Therefore, the use of such a compound enables selective electrolytic processing of a diffusion barrier layer.

In a preferred embodiment of the present invention, a barrier layer of tantalum, titanium, tungsten, ruthenium or a compound thereof is formed in the surface of the substrate, and the substrate surface is processed by electrolytic processing.

The present invention also provides an electrolytic processing apparatus comprising: a substrate holder for holding a substrate; a processing tool having a processing face for carrying out electrolytic processing of a surface of the substrate; a power source for applying a voltage between the surface of the substrate and the processing tool; and an electrolytic cell for holding an electrolytic solution comprising an organic solvent and an electrolyte dissolved into the organic solvent, and bringing the surface of the substrate and the processing face of the processing tool into contact with the electrolytic solution.

According to this electrolytic processing apparatus, a diffusion barrier layer which appears when an excessive interconnect metal is removed after filling, by damascene plating, the interconnect metal into recesses of a substrate, can be effectively processed (removed).

The electrolyte is, for example, hexafluorophosphate ion.

The use of hexafluorophosphate ion as the electrolyte can more effectively remove a diffusion barrier layer.

In a preferred aspect of the present invention, the electrolytic processing apparatus further comprises a controller for adjusting an electric potential applied to the surface of the substrate.

By controlling the voltage applied to a surface of a substrate, the step of removing an excessive interconnect metal, carried out after the step of filling the interconnect metal into recesses of the substrate by damascene plating, and the step of removing a diffusion barrier layer after the excessive metal removal step can be carried out successively using one electrolytic cell. This enables simplification of the processing process and shortening of the processing time.

According to the present invention, in flatly polishing the surface (surface to be processed) of a substrate on which a diffusion barrier layer and an interconnect metal, such as copper, are exposed, the diffusion barrier layer can be preferentially processed while suppressing processing of the interconnect metal, thereby obtaining a polished surface having excellent flatness. Further according to the present invention, a very low processing pressure can be applied to a surface of a substrate during processing, thereby preventing the occurrence of dishing or erosion and, in addition, a high productivity can be maintained. Furthermore, the present invention makes it possible to eliminate or minimize the use of a slurry, thereby reducing the processing cost.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A through 1D are cross-sectional diagrams illustrating, in a sequence of process steps, a process for forming copper interconnects;

FIG. 2 is a schematic diagram of an electrolytic processing apparatus according to an embodiment of the present invention;

FIG. 3 is a step diagram showing a process for forming copper interconnects;

FIG. 4 is a schematic plan view of an electrolytic processing apparatus according to another embodiment of the present invention;

FIG. 5 is a vertical sectional view of the apparatus of FIG. 4;

FIG. 6 is a photograph of a surface of sample 1 after electrolytic processing in Example 1;

FIG. 7 is a diagram showing a cross-sectional profile of sample 1 after electrolytic processing in Example 1;

FIG. 8 is a photograph of a surface of sample 1 after electrolytic processing in Example 2; and

FIG. 9 is a diagram showing a cross-sectional profile of sample 1 after electrolytic processing in Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described with reference to the drawings. The following description illustrates the case of providing a substrate W, as shown in FIG. 1B, which has been prepared by depositing copper 32 on a diffusion barrier layer 28 on a hard mask 20 while filling copper 32 into via holes 24 and trenches 26; and polishing away the copper film (seed layer 30 and copper 32) as an excessive interconnect metal and the diffusion barrier layer 28, thereby forming interconnects 34 of copper 32 in an upper-layer insulating film 22, as shown in FIG. 1D.

FIG. 2 shows an electrolytic processing apparatus according to an embodiment of the present invention, adapted for use as an electrolytic polishing apparatus. The electrolytic processing apparatus (electrolytic polishing apparatus) is designed to feed electricity from a peripheral portion of the substrate W to conductive materials, i.e., the copper 32 and the diffusion barrier layer 28, provided in the surface (surface to be processed) of the substrate, and has a disk-shaped substrate holder 40 for detachably holding thereon the substrate W with its front surface facing upwardly. The substrate holder 40 rotates (on its axis) by the actuation of a rotating motor 42.

Above the substrate holder 40 is disposed a seal ring 44 which makes pressure contact with a peripheral portion of the substrate W, held by the substrate holder 40, to seal the peripheral portion. With this structure, when the peripheral portion of the substrate W, held by the substrate holder 40, is brought into pressure contact with the seal ring 44, an electrolytic cell 46 circumferentially defined by the seal ring 44 is formed over the upper surface of the substrate W, and an electrolytic solution 48 is held in the electrolytic cell 46.

In the lower surface of the seal ring 44 are provided feeding electrodes 50 which, when the peripheral portion of the substrate W is sealed with the seal ring 44 as described above, come into contact with a peripheral portion of the substrate W outside the seal ring 44 to feed electricity to the surface conductive film, such as the copper 32. A conducting wire 55a extending from the anode of a power source 52 is connected to the feeding electrodes 50.

Above a peripheral portion of the substrate holder 40 is disposed an electrolytic solution supply pipe 54 for supplying the electrolytic solution 48 to the electrolytic cell 46, circumferentially defined by the seal ring 44 and formed over the upper surface of the substrate W. Though not shown diagrammatically, the electrolytic solution after electrolytic processing in the electrolytic cell 46 is sucked and removed through a verticaly-movable suction pipe.

Above the substrate holder 40 is disposed an electrode section 58 which is mounted vertically to the free end of a horizontally-pivotable pivot arm 60. The pivot arm 60 is coupled to an upper end of a pivot shaft 66 which moves vertically by the actuation of a lifting motor 62 and rotates by the actuation of a pivoting motor 64. The electrode section 58 rotates (on its axis) by the actuation of a hollow motor 68 mounted to the free end of the pivot arm 60.

In the interior of the electrode section 58 is provided a counter electrode 70 as a processing tool, having a lower exposed surface which serves as a processing face for electrolytic processing. The counter electrode (processing tool) 70 is, for example, formed of platinum, and to the counter electrode 70 is connected a conducting wire 55b that extends from the cathode of the power source 52, passes through the hollow portion of the pivot shaft 66 and reaches a slip ring 72, and passes through the hollow portion of the hollow motor 68.

In this embodiment, the electrolytic processing apparatus is provided with a controller 74 for adjusting (controlling) a voltage applied between the feeding electrodes 50 and the counter electrode 70.

The electrolytic solution 48 will now be described. A solution comprising an organic solvent and an electrolyte dissolved into the organic solvent is used as the electrolytic solution 48. By thus using an organic solvent instead of water as a solvent, electrolytic processing (electrolytic polishing) of a diffusion barrier layer can be carried out in such a manner that a passive oxide film is prevented from being formed in the surface of the diffusion barrier layer upon application of a positive electric potential to the diffusion barrier layer in the electrolytic solution, and the metal of the diffusion barrier layer is allowed to dissolve as metal ions into the electrolytic solution. Organic solvents generally have a high decomposition voltage: For example, the decomposition voltage of propylene carbonate is 6.7V. Accordingly, a high voltage can be applied to a diffusion barrier layer, thereby making the electrolytic processing rate sufficiently high.

Examples of usable organic solvents include acetal, acetol, 1,2-dichloroethane (10.1), sulfuryl chloride (10), thionyl chloride (9.2), acetyl chloride (15.8), tetrachloroethylene carbonate (9.2), benzyl chloride (23), nitromethane (36), dichloroethylene carbonate (31.6), nitrobenzene (34.8), acetic anhydride (20.7), phosphorus oxychloride (14), benzonitrile (25.2), selenium oxychloride (46), acetonitrile (38), sulfolane (42), propylene carbonate (69), benzyl cyanide (18.4), ethylene sulfide (41), isobutyronitrile (20.4), propionitrile (27.7), ethyl methyl carbonate, ethylene carbonate (89.1), dimethyl carbonate, diethyl carbonate, diphenylphosphonic difluoride (27.9), methyl acetate, γ-butyronitrile(20.3), ethyl alcohol, methyl alcohol, propyl alcohol, butyl alcohol, acetone (20.7), methyl formate, ethyl formate, ethyl acetate, phenylphosphonic dichloride (26), diethyl ether (4.3), tetrahydrofuran (7.6), 2-methyl tetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3-dioxolan, 3-methyl oxazolidinone, 1,2-dimethoxyethane, diphenylphosphonic chloride, trimethyl phosphate (20.6), tributyl phosphate (6.8), N,N-dimethyl formamide (26.6), N,N-dimethyl acetamide (27.8), dimethyl sulfoxide (45), N,N-diethyl formamide, N,N-diethyl acetamide, pyridine (12.3), hexamethylphosphonictriamide (30), hydrazine, ethylenediamine, ethylamine, t-butylamine, ammonia, triethylamine, and γ-butyrolactone. A mixture of two or more of these compounds may also be used. The numerical values in parentheses indicate dielectric constants.

Of these organic solvents, propylene carbonate, ethylene carbonate and dimethyl sulfoxide have a high dielectric constant and high ability to dissolve an electrolyte, and therefore are preferred.

As the electrolyte can be used an alkali metal salt, an ammonium salt, a tetraalkyl ammonium salt, etc. whose ionized anions are fluoride ion, chloride ion, bromide ion, iodide ion, PF₆⁻ (hexafluorophosphate ion), BF₄⁻ (tetrafluoroborate ion), AsF₆⁻ (hexafluoroarsenic ion), etc. Specific examples of such electrolytes include lithium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, tetramethyl ammonium hexafluorophosphate, tetrabutyl ammonium hexafluorophosphate, ammonium tetrafluoroborate and lithium tetrafluoroborate. The concentration of the electrolyte is generally 0.01 to 30% by weight, preferably 0.1 to 10% by weight.

The electrolytic solution 48 may optionally contain as an additive an inhibitor (corrosion inhibitor) composed of a heterocyclic compound containing oxygen, nitrogen, or sulfur atom. By allowing the unshared electron pairs of these elements to be adsorbed on adsorption points on a surface of an interconnect metal, such as copper, dissolution of the interconnect metal as ions into the electrolytic solution can be prevented or retarded. The inhibitor is, for example, a nitrogen-containing heterocyclic compound having a triazole ring, a pyrrole ring, a pyrazole ring, a thiazole ring, or an imidazole ring. Specific examples include benzotriazole, pyrrole, 3-(2-thienyl)-1-pyrazole, 2-butyl imidazole, 6-thioguanine, trithiocyanuric acid, benzotriazole, tolyltriazole, 6-thioguanine, 2-benzothiazolethiol, trithiocyanuric acid, and sodium N,N-diethyldithiocarbamate. Besides nitrogen-containing heterocyclic compounds, alkyl amines, alkanethiols, 1-hexine-3-ols, etc. can be used as an inhibitor.

Electrolytic polishing by this electrolytic polishing apparatus will now be described. First, a substrate W, as shown in FIG. 1C, having the diffusion barrier layer 28 exposed on the surface, is held face up by the substrate holder 40 in a lowered position. Thereafter, the seal ring 44 is brought into pressure contact with a peripheral portion of the substrate W, thereby forming the electrolytic cell 46, circumferentially defined by the seal ring 44, over the surface of the substrate W. At the same time, the feeding electrodes 50 are brought into contact with the surface of the diffusion barrier film 28 at a peripheral portion of the substrate W.

Next, a predetermined amount of the electrolytic solution 48 is supplied from the electrolytic solution supply pipe 54 into the electrolytic cell 46 formed over the surface of the substrate W, and held in it. Thereafter, the electrode section 58 is moved To right above the substrate holder 40, and is then lowered. The lowering of the electrode section 58 is stopped when it has reached a predetermined position at which the lower surface of the counter electrode 70 of the electrode section 58 comes into contact with the electrolytic solution 48 held in the electrolytic cell 46.

In this state, a predetermined voltage is applied from the power source 52 to between the feeding electrodes 50 and the counter electrode 70 with the feeding electrodes 50 as an anode and the counter electrode 70 as a cathode while rotating the substrate holder 40 and the electrode section 58, and pivoting the electrode section 58 horizontally (parallel to the surface to be processed of the substrate W), as necessary, to carry out electrolytic processing (electrolytic polishing) of the diffusion barrier layer 28.

The electrolytic processing is carried out in a constant potential-controlled manner by applying an electric potential, which is controlled at a predetermined value less than the decomposition voltage of the organic solvent of the electrolytic solution 48, between the feeding electrodes 50 and the counter electrode 70. For example, the decomposition voltage of propylene carbonate is +3.7V (based on silver-silver ion reference electrode). Therefore, when propylene carbonate is used as the organic solvent of the electrolytic solution 48, the electric potential applied to the substrate surface is made less than the solvent decomposition voltage so as to prevent decomposition of the organic solvent (propylene carbonate) which would lower the processing efficiency. By the “constant potential” is herein meant to keep the electric potential applied to a substrate surface constant.

By thus controlling the electric potential applied to the surface of the substrate, it becomes possible to exclusively process the diffusion barrier layer 28 or to simultaneously process the diffusion barrier layer 28 and the interconnect metal, i.e., copper. It thus becomes possible to initiate removal of the diffusion barrier layer 28 even when the excessive interconnect metal is not completely removed and the excessive copper film partly remains on the substrate.

The efficiency of electrolytic processing varies depending on the electrolytic solution 48 used and, in addition, the conditions of the power source, such as pulse voltage, pulse period and duty ratio, the distance between the diffusion barrier layer 28 as an anode and the counter electrode 70 as a cathode, the proportion of the surface area of the exposed interconnect metal layer to the overall surface area of the surface to be processed of the substrate W, etc. During electrolytic processing, the entire surface (surface to be processed) of the substrate W acts as an anode. Accordingly, more electric current flows by electrolysis when the diffusion barrier layer 28 covers the surface region of the substrate other than the surface of the interconnect metal layer embedded in the recesses, such as the trenches 26, of the substrate, whereby electrolytic processing of the diffusion barrier layer 28, i.e., dissolution of the layer 28, is promoted. Though the interconnect metal can also dissolve at the same time, the dissolution of the interconnect metal can be prevented and only the diffusion barrier layer 28 can be selectively processed by using the electrolytic solution 48 containing an inhibitor (corrosion inhibitor) (or by applying an inhibitor to the substrate, e.g., by coating, before immersing the substrate in the electrolytic solution).

After completion of the electrolytic processing, the voltage application between the feeding electrodes 50 and the counter electrode 70 is released, and the rotation of the substrate holder 40 and the electrode section 58 is stopped. The electrolytic solution 48 in the electrolytic cell 46 is then removed, and the substrate W after electrolytic processing is transported for the next process.

Besides an electrolytic processing apparatus that employs a direct feeding method in which, as in this embodiment, a positive electric potential is applied directly to a substrate W and a negative electric potential is applied to a counter electrode in carrying out electrolytic processing, it is also possible to use an electrolytic processing apparatus that employs an indirect feeding method. The indirect feeding method is a method of carrying out electrolytic processing of a substrate by disposing an anode and a cathode opposite to and not in contact with a surface (surface to be processed) of the substrate, and applying a voltage between the anode and the cathode. There is a case in which part of the electrolytic solution can decompose by absorbing moisture or oxygen in the air, causing problems such as oxidation of a processing surface of a substrate. Such problems can be avoided by carrying out electrolytic processing in an inert gas atmosphere.

A process for forming copper interconnects will now be described with reference to FIGS. 1A through 1D and FIG. 3.

First, as shown in FIG. 1A, a via hole 24 and a trench 26 are formed in an upper-layer insulting film 22, e.g., by the known lithography/etching technique (step 1). Next, a diffusion barrier layer 28 for preventing diffusion of copper into a first low-dielectric constant insulating layer 16 or a second low-dielectric constant insulating layer 18, is formed on the substrate surface (step 2), and a seed layer 30, which serves as a feeding layer in electroplating, is formed on the diffusion barrier layer 28 (step 3).

The diffusion barrier layer 28 is composed of, for example, a Ta/TaN mixed film or a film of TiN, WN, SiTiN or Ru, deposited, e.g., by a sputtering method or an ALD (atomic laser deposition) method, and the seed layer 30 is composed of a copper film deposited, e.g., by a sputtering method. In the case where the barrier layer 28 is Ru, and a seed layer is not needed, the step 3 is omitted and the step 4 is carried out subsequent to the step 2.

Next, copper 32 as an interconnect metal is deposited on the surface of the substrate W by electroplating (damascene plating) (step 4). Thus, as shown in FIG. 1B, copper 32 is filled into the via hole 24 and the trench 26 and, at the same time, copper 32 is deposited on the outermost surface of the substrate W, thereby covering the low-dielectric constant insulating layers 16, 18, constituting the upper-layer insulating film 22, and a hard mask 20 with the copper 32 as a interconnect metal. The copper 32 has surface irregularities upon completion of the electroplating (damascene plating). It is also possible to use silver or an alloy of copper and silver as an interconnect metal instead of copper. The seed layer 30 and the copper 32 are unified, and functions as an interconnect after completion of a device. Excessive copper 32 is removed by CMP or electrolytic processing (electrolytic polishing). This removal is carrier out for the unified seed layer 30 and copper 32.

In particular, the surface of the substrate W is subjected to CMP or electrolytic processing using an aqueous solution as an electrolytic solution (step 5), thereby removing the excessive copper film (copper 32 and seed layer 30) and making the surface of the copper 32, embedded in the trench 26, flush with the surface of the hard mask 20, as shown in FIG. 1C.

It is also possible to carry out the removal of a copper film by composite electrolytic polishing which comprises allowing a substrate and a polishing member (e.g., polishing pad) in contact with the substrate to move relative to each other while applying a voltage between the substrate and a counter electrode through an electrolytic solution.

The above process can be carried out in a conventional manner. When the removal of the copper film (copper 32 and seed layer 30) is carried out by electrolytic processing, it is also possible to use the electrolytic processing apparatus shown in FIG. 2 which uses an organic solvent. The subsequent removal of the unnecessary diffusion barrier layer 28 lying on the hard mask 20 (step 6) is carried out by using the electrolytic processing apparatus of the present invention, shown in FIG. 2.

In particular, the surface diffusion barrier layer 28 of the substrate W and the counter electrode 70 are brought into contact with above-described electrolytic solution 48, comprising an electrolyte and an organic solvent and optionally an inhibitor (corrosion inhibitor), held in the electrolytic cell 46, and a voltage is applied between the diffusion barrier layer 28 and the counter electrode 70 with the diffusion barrier layer 28 as positive and the counter electrode 70 as negative, thereby selectively polishing away the diffusion barrier layer 28 without removing the copper film (step 6a). An interconnect 34 of copper is thus formed in the upper-layer insulating film 22, as shown in FIG. 1D.

It is also possible to apply an inhibitor in a solution or in a gas phase to the surface diffusion barrier layer 28 of the substrate W, e.g., by coating, before bringing the diffusion barrier layer 28 into contact with an electrolytic solution (step 6b), and then carry out electrolytic processing using the electrolytic solution which does not necessarily contain an inhibitor (step 6c).

Next, according to necessity, CMP processing is carried out at such a very low pressure [e.g., less than 1 psi (about 69 hPa)] as not to cause a defect in the substrate to completely remove the diffusion barrier layer 28 which has not been removed and remains on the hard mask (insulating film) 20 (step 7), thereby exposing the hard mask (insulating film) 20 from which the diffusion barrier layer 28 has been completely removed (step 8). At the same time, the inhibitor covering the interconnect 34 is also removed, and the processing is completed with the copper exposed.

In this embodiment, electrolytic processing of the diffusion barrier layer 28 is carried out by bringing the surface of the substrate W into contact with the electrolytic solution 48. It is also possible to move a substrate and a polishing member, in contact with a surface of the substrate, relative to each other in an electrolytic solution to polish the substrate surface while applying a mechanical action to the substrate surface by the polishing member. When carrying out electrolytic polishing of a diffusion barrier layer until exposure of an insulating film, depending on the material of the diffusion barrier layer and variation in the thickness of the layer, the diffusion barrier layer can remain in a stripe pattern on the surface of the insulating film. Because the processing phenomenon of electrolytic processing generally does not occur unless electrical conduction can be secured, the remaining diffusion barrier layer cannot be removed unless electrical conduction to the diffusion barrier layer can be secured. Accordingly, if the diffusion barrier layer remains in a stripe pattern on the surface of the insulating film, the remaining diffusion barrier layer cannot be removed by electrolytic processing which involves contact of the surface of the substrate W with the electrolytic solution 48. Even in such a case, by carrying out electrolytic processing of the diffusion barrier layer while applying a mechanical action to the diffusion barrier layer by a polishing member, the diffusion barrier layer can be removed without leaving the barrier layer on the insulating film.

FIGS. 4 and 5 show an electrolytic processing apparatus (electrolytic polishing apparatus) according to another Embodiment of the present invention, which is designed to move a substrate and a polishing member, in contact with a surface of the substrate, relative to each other in an electrolytic solution to polish the substrate surface while applying a mechanical action to the substrate surface by the polishing member.

This electrolytic processing apparatus includes a rotatable polishing table 86 having, in an upper surface, a counter electrode 82, to be connected to the cathode of a power source 80, and a polishing member 84; a vertically-movable and rotatable top ring 88 for detachably holding a substrate W; and a dresser 90 and an atomizer 92 both for conditioning a surface of the polishing member 84. Above the polishing table 86 is disposed an electrolytic solution supply section 94 having a large number of electrolytic solution supply orifices 94a for supplying an electrolytic solution to the polishing member 84. The polishing table 86 is also provided with a feeding electrode 96 which is located lateral to the counter electrode 82 and which, when the substrate W held face down by the top ring 88 is lowered and brought into contact with the polishing member 84, comes into contact with the surface of the substrate W to feed electricity to the substrate surface. The feeding electrode 96 is connected to the anode of the power source 80. In the embodiment shown in FIGS. 4 and 5, the dresser 90 is rotatable and vertically movable, and has a number of circular protrusions 90a, for example comprised of hard members, such as diamond pellets, or brushes, attached in a ring-like arrangement to a peripheral region of the lower surface. Though not shown diagrammatically, it is also possible to use a dresser having diamond abrasives attached in a ring-like arrangement to a peripheral region of the lower surface. Though the diameter of the dresser 90 shown in FIG. 5 is approximately the same as the radius of the table, it is also possible to use a disk-shaped dresser having a small diameter of, e.g., about 100 mm and allow it not only to rotate but pivot on the surface of the polishing pad as well. Diamond abrasives may be arranged over the entire pad-contacting surface of such a small-diameter dresser.

The polishing member 84 is comprised of a polishing pad of, e.g., a foamed polyurethane resin, and grooves are formed in a concentric or grid-like arrangement in a polishing surface 84a, i.e., an upper surface, of the polishing member (polishing pad) 84. A number of vertical through-holes are formed in the polishing member 84 so as to secure electrical conduction between the counter electrode 82 and the substrate W via the electrolytic solution supplied to the polishing member 84.

In operation of this embodiment, while keeping the substrate W in contact with the polishing surface 84a of the polishing member 84 and moving the substrate W and the polishing member 84 relative to each other (rotating both of them in this embodiment), the electrolytic solution is supplied to the upper surface of the polishing member 84 and a voltage is applied between the counter electrode 82 and the surface diffusion barrier layer 28 of the substrate to carry out removal of the diffusion barrier layer 28. During the processing, electricity is fed through the feeding electrode 96 to the surface diffusion barrier layer 28 of the substrate. The removal of the diffusion barrier layer 28 is effected by an electrolytic action and a mechanical action. Accordingly, a sufficiently high polishing rate can be obtained by carrying out the polishing even at a lower polishing pressure [e.g., less than 1 psi (about 69 hPa)] than that of common CMP. Conditioning of the polishing surface 84a of the polishing pad 84 may be carried out with the dresser 90 and the atomizer 92 during the processing for the removal of the diffusion barrier layer 28 or during an interval between the removal operations.

As described above, the removal of the diffusion barrier layer 28 by this electrolytic processing apparatus is carried out by supplying the electrolytic solution to the polishing pad and applying a voltage between the counter electrode 82 and the surface diffusion barrier layer 28 of the substrate W while keeping the substrate W in contact with the polishing member (polishing pad) 84 and moving them relative to each other. The voltage applied during the polishing may be changed during the period from the initial stage of polishing of the diffusion barrier layer 28 to near the end of the polishing. When the diffusion barrier layer 28 remains in a stripe pattern on the insulating film 20, it is possible to stop the voltage application so as to remove the remaining diffusion barrier layer 28 only by the mechanical action of the polishing member 84. This can prevent failure of the removal of the diffusion barrier layer 28 due to loss of electrical conduction to the remaining diffusion barrier layer 28. Further, in order to remove, after the removal of the diffusion barrier layer 28, an inhibitor adsorbed on the interconnect metal, it is possible to stop the voltage application and carry out processing only by the mechanical action of the polishing pad 84. In the case of composite electrolytic polishing, it is possible to use an electrolytic liquid containing abrasive grains in an amount of not more than 10% in order to enhance the uniformity of mechanical action.

The present invention can be advantageously used for removing a diffusion barrier layer by dissolving the metal of the diffusion barrier layer with an organic solvent in the process of forming interconnects in a substrate, such as a semiconductor wafer. A metal material selected from tantalum, titanium, tungsten, ruthenium or a compound thereof is generally used for a barrier layer for an interconnect material of a semiconductor device. Specific examples of the compound may include tantalum nitride, titanium nitride, tungsten nitride and silicon tantalum nitride.

EXAMPLE 1

A silicon oxide-film wafer piece with a sputtered Ta/TaN film (film thickness: about 200 nm) having a uniform thickness formed on a surface was prepared as a sample 1. A surface of the Ta/TaN film was polished away by using the electrolytic processing apparatus shown in FIG. 2. An electrolytic solution having the following composition at room temperature was used.

Composition of Electrolytic Solution

Electrolyte: lithium hexafluorophosphate

lithium tetrafluoroborate

tetraethyl ammonium tetrafluoroborate

tetra-n-butyl ammonium tetrafluoroborate

tetraethyl ammonium perchlorate

tetra-n-butyl ammonium bromate

Organic solvent: propylene carbonate

Inhibitor: trithiocyanuric acid

A platinum counter electrode and an Ag/Ag⁺ (silver/silver ion) reference electrode were used. The electrolytic solution was prepared at room temperature by placing into a suitable vessel an appropriate amount of propylene carbonate as an organic solvent and the above six types of electrolytes in such amounts that the molar concentration of each electrolyte becomes 0.5M. Further, 0.1% by weight of the inhibitor was added to the electrolytic solution.

To the sputtered Ta/TaN film at the end of the sample 1 was connected an electrical contact (with two working electrode wires) wired to the positive pole of a power source (HS-501G, Hokuto Denko Corporation), while to the platinum counter electrode was connected an electrical contact (with one counter electrode wire) connected to the negative pole of the power source. The silver/silver ion reference electrode was connected to a reference electrode wire from the power source. A voltage set at an arbitrary value, for example 1V (vs. Ag/Ag⁺), was applied between the sputtered Ta/TaN film and the counter electrode to initiate electrolytic processing. The decomposition voltage of organic solvent must be taken into consideration in selecting a voltage. Thus, the voltage was set at a value not exceeding the decomposition voltage of propylene carbonate, which is 3.7V (vs. Ag/Ag⁺).

FIG. 6 shows a photograph of the surface of the sample 1 After the electrolytic processing of the sputtered Ta/TaN film was carried out at 1V (vs. Ag/Ag⁺) for 10 minutes while masking part of the sputtered Ta/TaN film. As can be seen from FIG. 6, the portion with a masking tape attached to the sample 1 (masked portion) on the right side of the curved boundary line centrally shown in the photograph was not in contact with the electrolytic solution and thus was not processed, whereas in the left-side portion (processed portion), the underlying silicon oxide film was exposed after the electrolytic processing.

FIG. 7 shows a cross-sectional profile of the surface of the sample 1 after the electrolytic processing, as measured with a stylus-type surface irregularity measurement device. The curved line in the Figure shows the cross-sectional profile of sample 1, with the curve on the left side of FIG. 7 showing the profile of the unprocessed portion with the masking tape attached thereto (masked portion) and the curve on the right side of FIG. 7 showing the profile of the portion electrolytically processed (processed portion). The level difference between the two portions is about 2000 angstroms (=200 nm), indicating complete removal of TaN and Ta by the electrolytic processing.

EXAMPLE 2

A silicon wafer piece with a copper plated film (film thickness: about 1.5 μm) having a uniform thickness formed on a surface was prepared as a sample 2. Electrolytic processing was carried out under the same conditions as in Example 1 [at 1V (vs. Ag/Ag⁺) for 10 min], thereby removing away the surface copper plated film of the sample 2.

FIG. 8 shows a photograph of the sample 2 after the electrolytic processing. As can be seen from FIG. 8 the underlying silicon film was not exposed both in the unprocessed portion with a masking tape attached thereto (masked portion) and in the portion which was in contact with the electrolytic solution (processed portion). FIG. 9 shows a cross-sectional profile of the sample 2 after the electrolytic processing. As can be seen from FIG. 9, the portion with the masking tape attached thereto (masked portion), which was not in contact with the electrolytic solution, was approximately flush with the processed surface (processed portion), indicating that processing of the copper plated film was suppressed.

INDUSTRIAL APPLICABILITY

The electrolytic processing method of the present invention can be used for removing an excessive interconnect metal layer and a diffusion barrier layer, formed in a surface of a substrate such as a semiconductor wafer, and flattening the substrate surface after carrying out damascene plating to fill an interconnect metal into recesses, such as trenches and via holes, provided in the substrate surface, in a process for the production of metal interconnects of a semiconductor integrated circuit.

Electrolytic Processing Method and Electrolytic Processing Apparatus

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