NEUTRAL pH COPPER PLATING SOLUTION FOR UNDERCUT REDUCTION

A microelectronic device is formed by forming a seed layer that contains primarily zinc. A plating mask is formed over the seed layer, and a copper strike layer is formed on the seed layer using a neutral pH copper plating bath. A main copper layer is formed on the copper strike layer by plating copper on the copper strike layer. The plating mask is subsequently removed. The main copper layer, the copper strike layer, and the seed layer are heated to diffuse copper and zinc, and form a brass layer under the main copper layer, consuming the seed layer between the main copper layer and the substrate. Remaining portions of the seed layer are removed by a wet etch process. The main copper layer and the underlying brass layer provide a conductor structure.

FIELD

This disclosure relates to the field of microelectronic devices. More particularly, this disclosure relates to plated copper layers in microelectronic devices.

BACKGROUND

Some microelectronic devices have conductor structures to provide low resistance interconnections. The conductor structures are fabricated by depositing a seed layer, forming a plating mask over the seed layer, and electroplating copper on the seed layer where exposed by the plating mask, removing the plating mask, and then removing the seed layer where exposed by the plated copper. Removing the seed layer commonly removes a portion of the plated copper, resulting in undesirable narrowing of the conductor structures. Moreover, removing the seed layer commonly undercuts the plated copper requiring sufficient overlap of the conductor structures over underlying conductive elements such as vias, undesirably increasing design widths of the conductor structures or restricting the number of the underlying conductive elements.

SUMMARY

The present disclosure introduces a method for forming a microelectronic device having a conductor structure by forming a seed layer that contains primarily zinc on a substrate of the microelectronic device. A plating mask is formed over the seed layer, and a copper strike layer is formed on the seed layer where exposed by the plating mask by a strike electroplating process using a neutral pH copper plating bath. A main copper layer is formed on the copper strike layer by plating copper on the copper strike layer. The plating mask is removed after the main copper layer is formed. The main copper layer, the copper strike layer, and the seed layer are heated to diffuse copper from the copper strike layer and the main copper layer, and zinc from the seed layer, to form a brass layer under the main copper layer. The seed layer between the main copper layer and the substrate is consumed by formation of the brass layer. Remaining portions of the seed layer, which are not part of the brass layer, are removed by a wet etch process. The main copper layer and the underlying brass layer provide the conductor structure. The conductor structure has an undercut less than a thickness of the brass layer, and the brass layer does not extend laterally past the main copper layer more than the thickness of the brass layer.

DETAILED DESCRIPTION

In addition, although some of the embodiments illustrated herein are shown in two dimensional views with various regions having depth and width, it should be clearly understood that these regions are illustrations of only a portion of a device that is actually a three dimensional structure. Accordingly, these regions will have three dimensions, including length, width, and depth, when fabricated on an actual device. Moreover, while the present invention is illustrated by embodiments directed to active devices, it is not intended that these illustrations be a limitation on the scope or applicability of the present invention. It is not intended that the active devices of the present invention be limited to the physical structures illustrated. These structures are included to demonstrate the utility and application of the present invention to presently preferred embodiments.

FIG.1AthroughFIG.1Hare cross sections of a microelectronic device having a conductor structure, depicted in stages of an example method of formation. Referring toFIG.1A, the microelectronic device100may be implemented, by way of example, as an integrated circuit, a discrete semiconductor component, a micro electro-optical device, a microelectromechanical system (MEMS) device, or a microfluidics device. The microelectronic device100has a substrate102which may include a dielectric material104extending to a connection surface108of the substrate102, and may include electrically conductive elements106extending to the connection surface108. The dielectric material104may be part of a dielectric layer stack in an interconnect region of the microelectronic device100. The dielectric material104may include, for example, silicon dioxide, silicon oxynitride, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), silicon nitride, or aluminum oxide, extending to the connection surface108. The electrically conductive elements106may be interconnects, vias, or input/output (I/O) pads of the microelectronic device100.

An optional adhesion layer110may be formed on the connection surface108of the substrate102. The adhesion layer110may include titanium and tungsten, for example with 70 weight percent to 95 weight percent tungsten, and 5 weight percent to 30 weight percent titanium. The titanium and tungsten may advantageously provide good adhesion of the adhesion layer110to the substrate102. The adhesion layer110may be formed by a sputter process, and may be, for example, 100 nanometers to 700 nanometers thick.

A seed layer112is formed over the substrate102, on the adhesion layer110, if present, and on the substrate102, if the adhesion layer110is not present. The term “over” should not be construed as limiting the position or orientation of the microelectronic device100, but should be used to provide a spatial relationship between the seed layer112and the substrate102. The seed layer112includes at least 90 weight percent zinc, so that copper may be subsequently electroplated on the seed layer112, and so that the seed layer may be removed after electroplating the copper without significantly degrading the electroplated copper. The seed layer112may be formed by a sputter process. A lower limit of a thickness of the seed layer112may be determined by providing a low sheet resistance for uniform electroplating of the copper across the microelectronic device100. In one aspect, an upper limit of the thickness of the seed layer112may be determined by a heating time to convert all the zinc in the seed layer112to brass, as increasing the thickness of the seed layer112requires increasing the heating time, undesirably reducing throughput. In another aspect, the upper limit of the thickness of the seed layer112may be determined by a criterion of limiting lateral growth of the brass, to avoid electrical shunts or fabrication process complication. By way of example, thicknesses of the seed layer112of 200 nanometers to 2 microns are sufficient to meet the criterion for the lower limit of the thickness and the criterion for the upper limit of the thickness.

The adhesion layer110provides adhesion between the seed layer112and the substrate102. The adhesion layer110may also provide a diffusion barrier by reducing diffusion of the zinc from the seed layer112into the substrate102. Limiting diffusion of the zinc into the substrate102may require a higher thickness for the adhesion layer110than that required for adhesion between the seed layer112and the substrate102. Tungsten in the adhesion layer110may provide an enhanced diffusion barrier against zinc diffusion.

A plating mask114is formed over the seed layer112. The plating mask exposes areas for conductor structures116and covers areas between the areas for the conductor structures116. The plating mask114may include organic polymer material to facilitate subsequent removal without degrading the electroplated copper. In one version of this example, the plating mask114may include photoresist and may be formed by a photolithographic process. In another version, the plating mask114may be formed by an additive process which disposed the organic polymer material on the seed layer112using an inkjet apparatus or a material extrusion apparatus. In a further version, the plating mask114may be formed by a laser ablation process. The plating mask114may be higher than the subsequently-formed electroplated copper.

Referring toFIG.1B, a copper strike layer118is formed on the seed layer112in the areas for the conductor structures116by a strike electroplating process using a neutral pH copper plating bath120. The neutral pH copper plating bath120is formed by adding 1.5 grams/liter to 20 grams/liter of copper, denoted inFIG.1Bas “COPPER”, to water. The copper may be added to the water in the form of an organic copper salt, such as copper acetate or copper citrate. The organic anion of the copper salt may help maintain the copper in solution when the pH of the neutral pH copper plating bath120is adjusted to neutral. In a variation of this example, a portion of the copper may be added to the water in the form of an inorganic copper salt, such as copper sulfate.

A complexing agent, denoted inFIG.1Bas “COMPLEXING AGENT”, is added to the neutral pH copper plating bath120to further maintain the copper in solution when the pH of the neutral pH copper plating bath120is adjusted to neutral. The complexing agent may include an organic acid such as citric acid, ascorbic acid, or acetic acid. Other complexing agents are within the scope of this example. The complexing agent is added in sufficient quantity to maintain the copper in solution while not inhibiting electroplating of the copper onto the seed layer112. The complexing agent may have a molar concentration of about half of a molar concentration of the copper to about twice the molar concentration of the copper. If a portion of the copper is added to the water in the form of an inorganic copper salt, the concentration of the complexing agent may be increased, to maintain the copper in solution.

A grain refining agent, denoted inFIG.1Bas “GRAIN REFINING AGENT”, is added to the neutral pH copper plating bath120to increase uniformity of the thickness of the copper strike layer118. The grain refining agent may be any of several commercially available grain refining agents for copper electroplating, and may be added to the neutral pH copper plating bath120at 0.01 grams/liter to 5 grams/liter. For example polyethyleneimine having a molecular weight of about 600 may be used as the grain refining agent, and be added to the neutral pH copper plating bath120at about 0.2 grams/liter to 2 grams/liter.

An organic alkali reagent, denoted inFIG.1Bas “ORGANIC ALKALI”, is added to the neutral pH copper plating bath120in sufficient quantity to adjust the pH to about neutral, that is a pH value of 5 to 8. Values of the pH below 5 tend to etch the seed layer112before the copper strike layer118can be formed. Values of the pH above 8 tend to degrade the organic polymer in the plating mask114. The organic alkali reagent may include, by way of example, ammonium hydroxide, tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, or ethanol amine. The organic alkali reagent may provide additional complexing functionality to maintain the copper in solution. Organic alkali reagents with amine functional group may provide effective complexing functionality. Ammonium hydroxide or ethanol amine may provide more complexing functionality than tetramethyl ammonium hydroxide or tetraethyl ammonium hydroxide. Tetramethyl ammonium hydroxide or tetraethyl ammonium hydroxide may be less prone to evaporation from the neutral pH copper plating bath120than ammonium hydroxide. A mix of different organic alkali reagents may be used to attain a desired balance between providing complexing functionality and reducing loss by evaporation.

The copper strike layer118is formed by connecting the seed layer112to a cathode connection, denoted “CATHODE” inFIG.1B, of the strike electroplating process, and connecting a strike anode122, which is exposed to the neutral pH copper plating bath120, to an anode connection, denoted “ANODE” inFIG.1B, of the strike electroplating process. The strike anode122may include primarily copper so as to replenish copper in the neutral pH copper plating bath120, or may include a metal such as platinum to reduce erosion of the strike anode122and thus extend the usable lifetime of the strike anode122. During operation of the strike electroplating process, current flows from the strike anode122through the neutral pH copper plating bath120to the seed layer112, electroplating copper from the neutral pH copper plating bath120onto the seed layer112to form the copper strike layer118. The strike electroplating process may provide a current density of 1 amperes/square decimeter (ASD) to 10 ASD to form the copper strike layer118. A temperature of the neutral pH copper plating bath120during the strike electroplating process may be, for example, room temperature, that is, 25° C., to 80° C. The copper strike layer118may be formed more quickly at higher temperature. The neutral pH copper plating bath120may degrade more quickly at higher temperature, for example, by loss of the organic alkali reagent due to evaporation. A temperature of the neutral pH copper plating bath120may be selected to provide a desired balance between a rate of formation of the copper strike layer118and reducing maintenance of the neutral pH copper plating bath120. The copper strike layer118may be formed with a thickness of 0.5 microns to 2 microns, for example. A lower limit of the thickness of the copper strike layer118may be determined by a goal of having no pinholes or discontinuities in the copper strike layer118so as to protect the seed layer112from chemical attack during a subsequent copper plating process. An upper limit of the thickness of the copper strike layer118may be determined by throughput, as the copper strike layer118may be formed at a slower rate than a subsequently formed main copper layer using the subsequent copper plating process.

Having the pH value of the neutral pH copper plating bath120above 5 may advantageously reduce erosion, etching, or other degradation of the seed layer112. Having the pH value of the neutral pH copper plating bath120below 8 may advantageously reduce degradation of the plating mask114.

Referring toFIG.1C, a main copper layer124is formed on the copper strike layer118in the areas for the conductor structures116by a main plating process using a copper plating bath126. The copper plating bath126is formed by adding copper, denoted as “COPPER” inFIG.1C, to water, for example in the form of a copper salt such as copper sulfate. Additives such as accelerators and grain refining agents may be added to the copper plating bath126. A pH value of the copper plating bath126may be below 5, to provide a desired plating rate. The copper strike layer118may advantageously protect the seed layer112from degradation by the copper plating bath126.

The main copper layer124is formed by connecting the seed layer112to a cathode connection, denoted “CATHODE” inFIG.1C, of the main plating process, and connecting a main anode128, which is exposed to the copper plating bath126, to an anode connection, denoted “ANODE” inFIG.1C, of the main plating process. The main anode128may include primarily copper, or may include a metal such as platinum. During operation of the main plating process, current flows from the main anode128through the copper plating bath126to the seed layer112, electroplating copper from the copper plating bath126onto the copper strike layer118to form the main copper layer124. The main plating process is continued to provide a desired thickness for the main copper layer124. The main copper layer124may have a thickness of 1 micron to 100 microns, for example.

Referring toFIG.1D, the plating mask114is removed. The plating mask114may be removed by a wet process using one or more organic solvents130, such as phenol, NMP (1-methyl 2 pyrrolidon), DMSO (dimethyl sulfoxide), or sulfonic acid. Alternatively, the plating mask114may be removed by a dry process using oxygen radicals in a downstream asher or an ozone generator. A combination of a wet process and a dry process may be used to remove the plating mask114.FIG.1Dshows removal of the plating mask114partway to completion.

Referring toFIG.1E, the main copper layer124, the copper strike layer118ofFIG.1D, and the seed layer112are heated by a heating process134, to diffuse copper from the copper strike layer118and the main copper layer124, and to diffuse zinc from the seed layer112, to form a brass layer132between the main copper layer124and the substrate102. The brass layer132extends directly to the main copper layer124. The brass layer132extends to the adhesion layer110, if present. The seed layer112outside of the areas for the conductor structures116may not be significantly depleted by formation of the brass layer132. The brass layer132includes 50 weight percent to 95 weight percent copper and 5 weight percent to 50 weight percent zinc. The brass layer132may be 2 to 10 times thicker than the seed layer112. The brass layer132may be, for example, 1 micron to 5 microns thick.

The heating process134may include a radiant heat operation, as indicated schematically inFIG.1E. Other implementations of the heating process134, such as a hot plate process, an oven process, or a forced air process, are within the scope of this example. The heating process134may heat the main copper layer124, the copper strike layer118, and the seed layer112to a temperature of 250° C. to 350° C., for example. The heating process134is performed for a sufficiently long time period so that the seed layer112between the main copper layer124and the substrate102is consumed by formation of the brass layer132. The heating process134is terminated so that the brass layer132does not extend laterally past the main copper layer124more than a thickness of the brass layer132. For the purposes of this disclosure, the terms “lateral” and “laterally” are understood to refer to a direction parallel to a plane of the connection surface108. By way of example, the heating process134may heat the main copper layer124, the copper strike layer118, and the seed layer112to a temperature of 300° C. for a time period of 10 minutes to 30 minutes. By way of another example, the heating process134may heat the main copper layer124, the copper strike layer118, and the seed layer112to a temperature of 250° C. for a time period of 25 minutes to 75 minutes. By way of a further example, the heating process134may heat the main copper layer124, the copper strike layer118, and the seed layer112to a temperature of 350° C. for a time period of 5 minutes to 10 minutes.

Referring toFIG.1F, remaining portions of the seed layer112, which are not part of the brass layer132, are removed. The remaining portions of the seed layer112are removed by a wet etch process using a zinc etchant136which does not remove a significant amount of the brass layer132or the main copper layer124. The zinc etchant136may include an aqueous solution of sulfuric acid or hydrochloric acid. The zinc etchant136may be implemented as an acidic copper plating bath, for example. Other reagents for removing the remaining portions of the seed layer112are within the scope of this example. During the wet etch process, zinc in the remaining portions of the seed layer112may provide cathodic protection for the brass layer132and the main copper layer124, reducing degradation by the zinc etchant136. Thus, undercut of the main copper layer124may be advantageously reduced compared to having a seed layer without zinc. Undercut of the main copper layer124is less than the thickness of the brass layer132.FIG.1Fshows removal of the seed layer112partway to completion.

Referring toFIG.1G, the adhesion layer110is removed where exposed by the brass layer132. The adhesion layer110may be removed by a wet etch process using an oxidizing etchant138such as hydrogen peroxide. Other reagents for removing the adhesion layer110are within the scope of this example. Removal of the adhesion layer110is terminated before significant undercut of the brass layer132occurs. Undercut of the brass layer132is less than the thickness of the brass layer132.FIG.1Gshows removal of the adhesion layer110partway to completion.

FIG.1Hdepicts the microelectronic device100after formation of the conductor structures116. The main copper layer124and the brass layer132, and the adhesion layer110, if present, under the brass layer132, provide the conductor structures116.

The adhesion layer110or the brass layer132may be laterally recessed from a lateral perimeter of the main copper layer124by an undercut distance140that is less than a thickness142of the brass layer132. The undercut distance140may be less than 10 percent of the thickness142of the brass layer132, advantageously reducing a design overlap of the conductor structures116over the electrically conductive elements106.

The brass layer132may extend laterally past the perimeter of the main copper layer124by an underlap distance144that is less than a thickness142of the brass layer132. The underlap distance144may be less than 10 percent of the thickness142of the brass layer132, advantageously enabling placement of adjacent instances of the conductor structures116within the thickness142of the brass layer132.

FIG.2is a cross section of an example microelectronic device having conductor structures in various configurations. The microelectronic device200may be implemented as an integrated circuit, a discrete semiconductor component, a micro electro-optical device, a MEMS device, or a microfluidics device, for example. The microelectronic device200has a substrate202which may include a first dielectric material204extending to a first surface208of the substrate202, and may include electrically conductive elements206extending to the first surface208. The first dielectric material204may be part of a dielectric layer stack in an interconnect region of the microelectronic device200. The electrically conductive elements206may be electrically conductive vias of an interconnect structure of the microelectronic device200, and may include tungsten, copper, or aluminum.

The microelectronic device200includes a first conductor structure216formed on the first surface208, making electrical contact to one or more of the electrically conductive elements206. The first conductor structure216may provide an interconnect of an interconnect layer for the microelectronic device200. The first conductor structure216includes a first brass layer232on the first surface208, and a first main copper layer224on the first brass layer232. The first conductor structure216may optionally include a first adhesion layer210between the first brass layer232and the first surface208. The first adhesion layer210may include titanium and tungsten, and may be, for example, 100 nanometers to 700 nanometers thick. The first brass layer232includes 70 weight percent to 90 weight percent copper and 10 weight percent to 30 weight percent zinc, and may be, for example, 1 micron to 5 microns thick. The first main copper layer224may be, for example, 3 microns to 30 microns thick. The first conductor structure216may be formed as disclosed in reference toFIG.1AthroughFIG.1H.

A second dielectric layer246is formed over the first conductor structure216. The second dielectric layer246may include, for example, one or more layers of silicon dioxide, PSG, or polyimide. Silicon dioxide and PSG in the second dielectric layer246may be formed by plasma enhanced chemical vapor deposition (PECVD) processes, optionally followed by a planarizing process such as a chemical mechanical polish (CMP) process. Polyimide in the second dielectric layer246may be formed by a photolithographic process.

One or more I/O pads248are formed through the second dielectric layer246to make electrical contact with the first conductor structure216. The I/O pads248may include, for example, one or more layers of titanium, titanium tungsten, nickel, palladium, aluminum alloy, copper, platinum, or gold. The I/O pads248may be formed by removing the second dielectric layer246in areas for the I/O pads248, followed by forming layers of electrically conductive material, and patterning the layers of electrically conductive material, for example using a photolithographically-formed etch mask and a reactive ion etch (RIE) process or a wet etch process. Nickel, palladium or gold in the I/O pads248may be formed by electroless plating processes.

A third dielectric layer250is formed over the second dielectric layer246and the I/O pads248, with openings over the I/O pads248. The third dielectric layer250may include, for example, one or more layers of silicon dioxide, silicon nitride, silicon oxynitride, polyimide, or aluminum oxide.

The microelectronic device200includes a second conductor structure252formed on the third dielectric layer250, making electrical contact to one or more of the I/O pads248. The second conductor structure252may provide a redistribution layer (RDL) for the microelectronic device200. The second conductor structure252includes a second brass layer254on the third dielectric layer250, and a second main copper layer256on the second brass layer254. The second conductor structure252may optionally include a second adhesion layer258between the second brass layer254and the third dielectric layer250. The second adhesion layer258may include titanium and tungsten, and may be, for example, 100 nanometers to 700 nanometers thick. The second brass layer254includes 70 weight percent to 90 weight percent copper and 10 weight percent to 30 weight percent zinc, and may be, for example, 1 micron to 5 microns thick. The second main copper layer256may be, for example, 5 microns to 20 microns thick. The second conductor structure252may be formed as disclosed in reference toFIG.1AthroughFIG.1H.

A fourth dielectric layer260is formed over the second conductor structure252. The fourth dielectric layer260may include, for example, one or more layers of polyimide or polyester. The fourth dielectric layer260may be formed by a photolithographic process, to have one or more openings over the second conductor structure252.

The microelectronic device200includes a third conductor structure262formed on the second conductor structure252, making electrical contact to the second conductor structure252through one of the openings in the fourth dielectric layer260. The third conductor structure262may provide a bump bond pillar for the microelectronic device200. The third conductor structure262includes a third brass layer264on the second main copper layer256, and a third main copper layer266on the third brass layer264. The third brass layer264includes 70 weight percent to 90 weight percent copper and 10 weight percent to 30 weight percent zinc, and may be, for example, 1 micron to 5 microns thick. The third main copper layer266may be, for example, 10 microns to 50 microns thick.

The third conductor structure262may be formed as disclosed in reference toFIG.1AthroughFIG.1H. The seed layer for the third conductor structure262may be formed directly on the second main copper layer256, without an adhesion layer. When the seed layer is heated to form the third brass layer264, zinc from the seed layer and copper from the second conductor structure252will diffuse, so that the third brass layer264extends partway into the second main copper layer256, as indicated inFIG.2.

A diffusion barrier268may be formed on the third main copper layer266, opposite from the third brass layer264, and a solder bump270may be formed on the diffusion barrier268opposite from the third main copper layer266. A combination of the third conductor structure262, the diffusion barrier268, and the solder bump270may provide a bump bond structure272of the microelectronic device200. The solder bump270may be attached to an external lead274by a solder reflow process. The external lead may be implemented as a lead of a lead frame, or a trace on a circuit board, for example.