Method to fabricate interconnect structures

[0001] 1. Field

[0002] Circuit structures.

[0003] 2. Relevant Art

[0004] Integrated circuits typically use conductive interconnections to connect individual devices on a chip or to send or receive signals external to the chip. A currently popular choice of interconnection material for such interconnections is a copper or copper alloy material.

[0005] One process used to form interconnections, particularly copper (alloy) interconnections, is a damascene process. In a damascene process, a trench is cut in a dielectric and filled with copper to form the interconnection. A via may be in the dielectric beneath the trench with a conductive material in the via to connect the interconnection to underlying integrated circuit devices or underlying interconnections. In one damascene process (a “dual damascene process”), the trench and via are each filled with copper material, by, for example, a single deposition.

[0006] A photoresist is typically used over the dielectric to pattern a via or a trench or both in the dielectric for the interconnection. After patterning, the photoresist is removed. The photoresist is typically removed by oxygen plasma (oxygen ashing). The oxygen used in the oxygen ashing can react with an underlying copper interconnection and oxidize the interconnection. Accordingly, damascene processes typically employ a barrier layer of silicon nitride (Si₃N₄) directly over the copper interconnection to protect the copper from oxidation during oxygen ashing in the formation of a subsequent level interconnection. In interlayer interconnection levels (e.g., beyond a first level over a device substrate), the barrier layer also protects against misguided or unlanded vias extending to an underlying dielectric layer or level (e.g. the barrier layer serves as an etch stop).

[0007] In general, the Si₃N₄ barrier layer is very thin, for example, roughly 10 percent of the thickness of an interlayer dielectric (ILD) layer. A thin barrier layer is preferred primarily because Si₃N₄ has a relatively high dielectric constant (k) on the order of 6 to 7. The dielectric constant of a dielectric material, such as an interlayer dielectric, generally describes the parasitic capacitance of the material. As the parasitic capacitance is reduced, the cross talk (e.g., the characterization of the electric field between adjacent interconnections) is reduced as is the resistance-capacitance (RC) time delay and power consumption. Thus, the effective dielectric constant (k_eff) of an ILD layer is defined by the thin barrier layer and another dielectric material having a lower dielectric constant so that the effect of the dielectric material typically used for the barrier layer (e.g., Si₃N₄) is minimized.

[0008] In prior art integrated circuit structures, a popular dielectric material for use in combination with a barrier layer to form ILD layers was silicon dioxide (SiO₂). Currently, efforts have focused at minimizing the effective dielectric constant of an ILD layer so materials having a dielectric constant lower than SiO₂ have garnered significant consideration. Many of these materials, such as carbon doped oxide (CDO), are porous. The dielectric constant of a dielectric material can be substantially effected by water or liquid absorbed in the pores of the dielectric material.

[0009] A typical part of a damascene process to form an interconnection in an ILD layer is a planarization after a deposition of the interconnect material. A typical planarization is a chemical mechanical polish (CMP). A CMP is a wet process that can introduce water or other liquid into a porous dielectric material. In addition, it can add mechanical stress to a dielectric layer. Stress can damage a dielectric layer and effect circuit performance.

[0016] The features of the described embodiments are specifically set forth in the appended claims. Referring to the following description and accompanying drawings, in which similar parts are identified by like reference numerals, best understand the embodiments.

[0017] FIG. 1 illustrates a cross-sectional, schematic side view of a portion of a circuit substrate structure. Structure 100 include substrate 110 of, for example, a semiconductor material such as silicon or a semiconductor layer on an insulator such as glass. Substrate 110 includes contact point 120 on a surface thereof. In one embodiment, contact point 120 is a device on a substrate (e.g., gate or junction of a transistor, etc.) or a portion of an underlying interconnect line (e.g., a metal trench).

[0018] Overlying a superior surface of substrate 110 (as viewed) is dielectric material 130. In one embodiment, dielectric material 130 is a dielectric material having a dielectric constant less than the dielectric constant of silicon dioxide (k_SiO2=3.9), a “low k dielectric.” A suitable material is, for example, carbon doped oxide (CDO). Dielectric layer 130 is deposited to a desired thickness, such as a thickness suitable to electrically insulate substrate 110 (e.g., devices on or above substrate 110) and to permit an interconnection to be formed therein.

[0019] FIG. 1 shows trench 140 formed in dielectric layer 130. Trench 140 extends, as viewed, into and/or out of the page and indicates a location for an interconnect line. A via would typically be present, though not shown in this cross-section, to contact point 120. Trench 140 and any vias formed to contact points on substrate 110 (e.g., contact point 120, etc.) may be formed, for example, through photo-lithographic patterning techniques.

[0020] FIG. 1 shows barrier layer 150 formed on a surface of dielectric layer 130 (a superior surface as viewed). Barrier layer 150 is conformally deposited on the surface and conforms to the contours of trench 140. In one embodiment, barrier layer 150 is of a material selected to inhibit the diffusion of an interconnection material (e.g., a copper material) formed in trench 140 from diffusing into dielectirc layer 130. A suitable material for barrier layer 150 includes conductive materials such as tantalum (Ta), tantalum nitride (TaN), titanium (Ti), molybdenum (Mo), and niobium (Nb). The thickness of barrier layer 150 may be suitable to inhibit diffusion of, for example, a copper interconnect. A thickness on the order of up to about 20 angstroms is suitable in one embodiment.

[0021] In one embodiment, the structure of FIG. 1 also includes seed layer 160. Seed layer 160 is suitable, for example, where an electroplating process will be used to form the interconnection. For an interconnection material of a copper material, seed layer 160 is a material that will facilitate a copper plating process (e.g., will provide a material to which copper will plate to). Representatively, seed layer 160 is a copper material deposited by chemical or physical deposition techniques. A thickness on the order of less than about 3,000 Å is suitable in one embodiment. FIG. 1 shows seed layer 160 conformally deposited on barrier layer 150 along the sidewalls and bottom of trench 140 and on barrier layer 150 outside trench 140.

[0022] FIG. 1 shows structure 100 after filling trench 140 with interconnect material 170 of, for example, a copper material (e.g., copper or a copper alloy). Suitable copper alloys include, but are not limited to, copper-tin (CuSn), copper-indium (CuIn), copper-cadmium (CuCd), copper-bismuth (CuBi), copper-rutherium (CuRu), copper-rhodium (CuRh), copper-rhenium (CuRe), and copper-tungsten (CuW). A typical introduction technique for a copper interconnection material as noted above is an electroplating process. By way of example, a typical electroplating process involves introducing a substrate (e.g., a wafer) into an aqueous solution containing metal ions, such as a copper sulfate-based solution, and reducing the ions (reducing the oxidation number) to a metallic state by applying current between a substrate with seed material (seed layer 160) and an anode of an electroplating cell in the presence of the solution. Referring to FIG. 1, interconnect material 170 is deposited on seed material 160 to fill trench 140 (and any vias formed between trench 140 and substrate 110) and on a surface of structure 100 outside trench 140 (referred to hereinafter as a field region).

[0023] FIG. 2 shows the structure of FIG. 1 following the removal of interconnect material 170 from the field region (i.e., from areas outside trench 140). In the example where interconnect material 170 is copper or a copper alloy, the material in the field region may be removed by an electropolishing process. In one sense, electropolishing may be thought of as the reverse of plating. In other words, instead of depositing copper or a copper alloy as in a plating process, an electropolishing process electro-chemically dissolves the copper or copper alloy. In the example of copper or a copper alloy, an electropolishing process may be performed by polarizing structure 100 annodically in a phosphoric acid solution (e.g., a concentrated (e.g., 85 percent) phosphoric acid solution). A concentrated phosphoric acid solution is generally highly viscous which aids in the electropolishing process. Other suitable electropolishing solutions include a combination of concentrated phosphoric acid and concentrated sulfuric acid, chromic acid, or acetic acid. The electropolishing solution may also include additional additives. Suitable additives include, but are not limited to, viscosity modifiers such as glycerine; wetting agents such as polyethylene glycol or polypropylene glycol; and film-forming agents such as phosphates or poly-phosphates. In one embodiment, as part of an electropolishing process, structure 100 may be polarized with a voltage of 0.8 to 1.8 volts versus a saturated calomel electrode.

[0024] Referring to FIG. 2, the electropolishing process removes interconnect material 170 (e.g., copper material) from the field region and, in one embodiment, also partially in trench 140 to recess interconnect material 170 in trench 140. One reason to recess trench material 140 at this stage is to ensure that any interconnect material has been removed from the field region. FIG. 2 shows interconnect material 170 recessed in trench 140. FIG. 2 also shows that interconnect material 170 has been removed from the field region as has seed material 160 through the electropolishing process (e.g., particularly where a material for interconnect material 170 and seed layer 160 are similar (e.g., copper)). Thus, FIG. 2 shows barrier layer 150 exposed in the field region.

[0025] FIG. 3 shows the structure of FIG. 2 following the introduction of additional interconnect material in trench 140. In one embodiment, where interconnect material 170 is copper or a copper alloy, supplemental interconnect material 180 is a similar material. One way to deposit supplemental interconnect material 180 is through a chemically-induced oxidation-reduction reaction also referred to herein as electroless plating. Unlike an electroplating process, an electroless plating process is not accomplished by an externally-supplied current, but instead relies on the constituents of the plating process (e.g., constituents of a plating bath) to initiate and carry out the plating process. One technique involves placing structure 100 in a bath containing one or more metal ions to be plated or introduced onto interconnect material 170. In the case of introducing a copper or copper alloy as subsequent interconnect material 180, the copper is in an ionic state having a positive oxidation number. Representatively, copper ions may be present in a bath as copper sulfate in a concentration range of five to 10 grams per liter (g/l). As such, the copper ions are in a sense subsequent interconnect material precursors. Additional precursors to form an alloy may include, but are not limited to, tin, indium, cadmium, etc.

[0026] Without wishing to be bound by theory, it is believed that the exposed conductive surface, in this case a conductive surface of interconnect material 170, on structure 100, when exposed to the components of an electroless plating solution or bath, undergo an oxidation-reduction (REDOX) reaction. The oxidation number of the metal ions of the introduced subsequent interconnect material precursors are reduced while the oxidation number of the reducing agent(s) are increased. Suitable reducing agents therefore are included in an electroless plating bath. In one embodiment, the reducing agents are alkaline metal-free reducing agent such as formaldehyde and a pH adjuster such as tetramethyl ammonium hydroxide (TMAH). In one embodiment, a complexing agent such as ethylene diamine tetra-acetic acid (EDTA) is also present. A representative temperature of a suitable bath to electrolessly deposit copper is on the order of greater than 50° C. and a suitable pH is in the range of 10–13. As shown in FIG. 3, according to this process, the electroless deposition of subsequent interconnect material 180 may be selectively deposited in trench 140 on interconnect material 170.

[0027] Following the introduction of subsequent interconnect material 180, in one embodiment, shunt material 190 is deposited on subsequent interconnect material 180 in trench 140. In one embodiment, shunt material 190 is also deposited by an electroless plating process. Representatively, the shunt material includes cobalt or nickel, or an alloy or cobalt or nickel. Suitable cobalt alloys include, but are not limited to, cobalt-phosphorous (CoP), cobalt-boron (CoB), cobalt-phosphorous-boron CoPB), cobalt-metal-phosphorous (CoMeP), cobalt-metal-boron (CoMeB), and cobalt-metal-phosphorous-boron (CoMePB). As used herein, “Me” includes, but it not limited to, nickel (Ni), copper (Cu), cadmium (Cd), zinc (Zn), gold (Au), silver (Ag), platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), chromium (Cr), molybdenum (Mo), iridium (Ir), rhenium (Re), and tungsten (W). The use of refractory metals (e.g., W, Re, Ru, Rh, Cr, Mo, Ir) tends to improve the adhesive properties of shunt material 190 as well as the mechanical hardness of shunt material 190. Combining Co and/Ni material with a noble metal (e.g., Au, Ag, Pt, Pd, Rh, Ru) allows the noble metals to act as a catalytic surface for the electroless plating on copper interconnect material such as subsequent interconnect material 180. Phosphorous (P) and boron (B) tend to be added to the shunt material as a result of reducing agent oxidation. Phosphorous and boron tend to improve the barrier and corrosion properties of the shunt material.

[0028] In terms of introducing metal ions of shunt material 180 for an electroless plating process, metal ions (shunt material precursors) of cobalt supplied by cobalt chloride, cobalt sulfate, etc., may be introduced in a concentration range, in one embodiment, of about 10–70 grams per liter (g/l), alone or with the addition of compounds containing metal ions of a desired alloy constituent (e.g., Ni, Cu, etc.). Examples of suitable additional compounds include ammonium tungstate (for alloying with W), ammonium perrhenate (for alloying with Re), etc. A suitable concentration range for the addition compound(s) include 0.1 to 10 g/l.

[0029] To reduce the oxidation number of the metal ions, one or more reducing agents are included in an electroless plating bath. In one embodiment, the reducing agents are selected to be alkaline metal-free reducing agents such as ammonium hypophosphite, dimethylamine borate (DMAB), and/or glyoxylic acid in a concentration range of about 2 to 30 g/l. The bath may also include one or more alkaline metal-free chelating agents such as citric acid, ammonium chloride, glycine, acetic acid, and/or malonic acid in a concentration range of about 5 to 70 g/l. Still further, one or more organic additives may also be included to facilitate hydrogen evolution. Suitable organic additives include Rhodafac RE-610™, cystine, Triton X-100™, polypropylene glycol/polyethylene glycol (in a molecular range of approximately 200 to 10,000) and a concentration range of about 0.01 to 5 grams per liter (g/l), an alkaline, metal-free pH adjuster such as ammonium hydroxide, tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, tatrapropyl ammonium hydroxide, and/or tetrabutyl ammonium hydroxide, may further be included in the bath to achieve a suitable pH range, such as a pH range of 3 to 14. A representative process temperature for an electroless plating bath such as described is on the order of 30 to 90° C. The electroless plating process introduces (e.g., plates) shunt material 190 to expose conductive surfaces amenable to the plating reaction. In one embodiment, the conductive surface is limited to subsequent interconnect material 180. Prior to the plating operation, an exposed surface of subsequent conducting material 180 may be treated to improve the uniformity of the electroless plating of shunt material 190. Subsequent interconnect material 180 may be surface treated with an agent such as a 1 to 20 percent by volume hydrofluoric acid (HF), sulfuric acid (H₂SO₄), sulfonic acids such as methane sulfonic acid (MSA), ethane sulfonic acid (ESA), propane sulfonic acid (PSA) and/or benzene sulfonic acid (BSA) for cleaning of the interconnect material.

[0030] FIG. 3 shows an interconnect structure including interconnect material 170, subsequent interconnect material 180, and shunt material 190 as a cap or overlying structure. As a non-limiting example, shunt material 190 has a thickness on the order of 5 to 300 nanometers (nm).

[0031] In the structure described with reference to FIG. 3, barrier layer 150 remained in the field region after the electropolishing of interconnect material 170. One reason the barrier layer remained in the field region is that the electropolishing process was selective for removal of the interconnect material (e.g., selective for copper). In one embodiment, barrier layer 150 is an electrically conductive material, such as tantalum or tantalum nitride. Thus, where barrier layer 150 remains in the field region, it may be desirable to reduce or minimize the electrical conductivity of a material for barrier layer 150.

[0032] FIG. 4 shows the structure of FIG. 3 following the reduction or minimization of electrical conductivity of a material for barrier layer 150 in the field region. In one embodiment, the reduction or minimization of electrical conductivity of a material for barrier layer 150 may be accomplished through an oxidation of the material. Representatively, structure 100 may be placed in an oxygen-containing environment (e.g., an oxygen plasma environment) under temperature conditions greater than, for example, 300° C. FIG. 4 shows structure 100 including barrier layer 250 of oxidized material for the barrier layer in the field region. A subsequent CMP can be introduced to planarize electroless plated shunt layer 190 and avoid topography build up in the next level interconnect structure

[0033] FIG. 5 shows the structure of FIG. 4 following the introduction (e.g., deposition) of a dielectric layer on barrier layer 250 and the composite interconnect (on shunt layer 190). Representatively, dielectric layer 230 is a low k dielectric material (e.g., CDO) formed to a thickness suitable to electrically insulate the composite interconnect and allow the formation of a subsequent interconnect in the dielectric layer. FIG. 5 also shows via 235 and trench 240 formed in dielectric layer 230. Representatively, a mask, such as a photoresist mask, may be used to define an area (e.g., a cross-sectional area) for a via opening and then via 235 may be etched with a suitable chemistry. The mask may then be removed (such as by an oxygen plasma to remove photoresist) and a second mask patterned to define a greater area (e.g., a greater cross-sectional area) for a trench opening. A subsequent etch is introduced to form trench 240 and the second mask is removed leaving the structure shown in FIG. 5.

[0034] FIG. 5 shows via 235 as a partially unlanded via. In that sense, via 235 is formed through dielectric layer 230 and contacts a portion of shunt layer 190 indicated at point 300. A portion of via 235 also contacts barrier layer 250. In the embodiment where barrier layer 250 has had its electrical conductivity reduced or minimized, a partially unlanded via such as via 235 may not adversely effect the circuit (e.g., because the electrical conductivity of barrier layer 250 is reduced or minimized).

[0035] FIG. 6 shows the structure of FIG. 5 following the formation of a, as illustrated, second level interconnect structure of a composite structure including electroplated interconnect material 270 subsequently introduced (electrolessly plated) interconnect material 280 and shunt layer 290. FIG. 6 also shows a subsequent barrier layer 350 with its electrical conductivity minimized or reduced. It is appreciated that the process described with respect to FIGS. 1–6 may be repeated for multiple interconnect levels.

[0036] In the preceding detailed description, specific embodiments were described. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.