Patent Application
20070017818
The present invention generally relates to semiconductor integrated circuit technology and, more particularly, to electrolyte compositions for electropolishing or electroetching processes and apparatuses.
Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric layers and conductive paths or interconnects made of conductive materials. Interconnects are usually formed by filling a conductive material in trenches etched into the dielectric layers. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in different layers can be electrically connected using vias or contacts.
The filling of a conductive material into features, such as vias, trenches, pads or contacts, can be carried out by techniques, such as electrochemical deposition or electroless deposition. In an electrodeposition or electroplating method, a conductive material, such as copper, is deposited over the substrate surface, including into such features. Copper is the material of choice for interconnect applications because of its low resistivity and good electromigration properties. After electrodeposition, a material removal technique is employed to planarize and remove the excess metal or overburden from the top surface, leaving conductors only in the features or cavities. This way a network of interconnect structures are formed on the wafer surface.
The standard material removal technique that is most commonly used for the purpose of planarization and overburden removal is chemical mechanical polishing (CMP). During a CMP process, a surface of a substrate is polished by a polishing pad in the presence of a chemical solution or slurry while a force is applied on the substrate to push the surface of the substrate against the polishing pad. Chemical etching and electropolishing (electroetching or electrochemical etching), and electrochemical mechanical polishing or electrochemical mechanical etching are also attractive process options for copper removal.
Electrochemical mechanical polishing (ECMP) planarizes non-planar copper surfaces by electrochemically forming and mechanically removing a surface film on a copper layer. The surface film is typically formed while polishing the substrate with a pad at a reduced force compared to the conventional CMP. A typical force of 0.1-0.6 psi is applied during ECMP, which makes ECMP attractive for polishing metal layers formed on mechanically weak insulators, such as ultra low-k insulators. In contrast, if the force applied during CMP is reduced to 0.1-0.6 psi, the rate of removal of the layers becomes low, which may be undesirable. During electropolishing (including ECMP), the electric current (rather than the force applied) determines the rate of removal. In ECMP technology, a copper-coated wafer is pressed against a polishing pad while feeding an electrolyte that contains a mixture of abrasive particles and chemicals, such as complexing or chelating agents, film forming agents, buffers and surfactants. An anodic potential is applied to the metal layer during ECMP with respect to an electrode, which is also wetted by the electrolyte.
Although much progress has been made in electropolishing approaches and apparatuses, there is still need for lower cost electrochemical removal techniques that uniformly planarize and remove excess conductive films from workpiece surfaces applying low cost and stable electrolytes and low force on the wafer surface and without causing damage and defects, especially on advanced wafers with ultra low-k or extreme low-k materials.
In accordance with an aspect of the invention, a system for electropolishing a conductive surface of a wafer is provided. The system comprises a wafer carrier configured to hold the wafer, a polishing pad, an electrode in proximity to the polishing pad, and an electrolyte. The polishing pad has a polishing surface, wherein the polishing surface is configured to contact the conductive surface. The electrolyte comprises an electrically resistive agent selected from the group consisting of urea and a urea derivative.
In accordance with another aspect of the invention, a method is provided of electropolishing a conductive surface of a wafer using a polishing pad, wherein the conductive surface has features formed therein. The conductive surface is contacted with a polishing surface of the pad and electrolyte is flowed onto the polishing surface while contacting the conductive surface with the polishing surface. The electrolyte comprises an electrically resistive agent selected from the group consisting of urea and urea derivatives.
In accordance with yet another aspect of the invention, a method is provided of forming an electrolyte. An acidic solution is provided and comprises an ion-free agent that is configured for polishing a conductive surface of a wafer using a polishing pad.
According to another embodiment, an electrolyte is provided for polishing a conductive surface of a wafer using a polishing pad. The electrolyte comprises an electrically resistive agent selected from the group consisting of urea and urea derivatives, wherein the electrically resistive agent is selected to act as a chelating agent.
According to yet another embodiment, an electrolyte composition is provided for electropolishing a conductive surface of a wafer using a polishing pad. The electrolyte composition comprises a chelating agent, a surface film forming agent, a pH adjusting agent, and an electrically resistive agent. The chelating agent is selected from the group consisting of citric acid and ammonium oxalate. The surface film forming agent is selected from the group consisting of BTA, methyl benzotriazole, and triazole. The pH adjusting agent is selected from the group consisting of KOH, NH4OH, and trimethyl amine hydroxide, wherein the pH of the electrolyte is between 2 to 6. The electrically resistive agent is selected from the group consisting of urea, urea hydroxide, and urea oxalate.
According to another embodiment, a method is provided for electropolishing a conductive surface of a wafer using a polishing pad. The conductive surface is contacted with an electrolyte and a polishing surface of the polishing pad. The ion-free chelating agent may be selected from the group consisting of urea, urea derivatives, and soluble organic amines.
The preferred embodiments provide an electropolishing electrolyte or electropolishing solution to form a smooth and planar surface during electropolishing of metal layers. The electropolishing electrolyte may be an acidic solution including at least one chelating agent, at least one surface film forming agent, at least one pH adjusting agent and at least one electrically resistive agent.
Use of the preferred electropolishing electrolyte will be exemplified by help of
Electropolishing of the copper layer 24 may be performed using an exemplary electropolishing system 100 shown in
The electropolishing electrolyte 102 may be flowed through the electropolishing pad 104 or delivered onto it by an electrolyte line 110 of an electrode delivery mechanism (not shown). The polishing pad 104 may be rotated, moved in an orbital motion or moved laterally by a moving mechanism during the electropolishing process. Alternatively, the polishing pad may be belt-shaped to be moved linearly, for example bi-linearly or reciprocatingly by a moving mechanism to polish the surface S during the process. The electrode 108 of the system 100 is connected to a negative terminal of a power supply 112, and the conductive surface S is connected to a positive terminal of the power supply 112, using suitable electrical contacts. During the electropolishing process of the conductive surface S, a potential difference is applied between the conductive surface S and the electrode 108 while the electrode 108 and the conductive surface S is wetted by the electropolishing electrolyte 102 and while relative motion is established between the electropolishing pad 104 and the wafer W.
The electropolishing electrolyte 102 is an acidic solution that may include at least one of phosphoric acid, potassium phosphate and ammonium phosphate solutions. The electropolishing electrolyte preferably further comprises additive molecules, such as chelating agents, surface forming agents, complexing agents, pH adjusting agents and conductivity-modulating agents. In some cases, additives can serve dual roles, such as the illustrated conductivity modulating agents. An exemplary cheleating agent may be citric acid, ammonium citrate, oxalic acid or ammonium oxalate or any weak organic acid.
Significantly, the preferred electropolishing electrolyte includes a conductivity-modulating agent, preferably and ion-free agent that makes the solution more electrically resistive by its addition. In the illustrated embodiment, a complexing or chelating agent functions as an electrically resistive agent to modulate the conductivity of the electropolishing electrolyte. The electrolyte conductivity may also be adjusted by changing the acid concentration in the electrolyte. For example, increasing the concentration of the acid by fifty percent (50%) increases the conductivity by almost thirty percent (30%). This increase is particularly advantageous when the electrolyte is used to polish copper plated wafers at high current density. Increasing the conductivity decreases the potential, thus reducing undesirable chemical reactions on the copper surface, such as pitting and corrosion. However, as noted, an electrically resistive agent, such as urea and urea derivatives, can also be used as a complexing or chelating agent to facilitate the removal of the copper ions. In contrast, other chelating agents tend to generate ions and thus disadvantageously contriubte to conductivity.
An exemplary complexing or chelating agent which may function as an electrically resistive agent may be urea, urea oxalate, urea hydroxide, or a soluble organic amine. A surface forming agent in the electropolishing electrolyte 102 aids the planarization of the surface topography of the copper layer 24 shown in
As shown in
An exemplary electropolishing electrolyte composition includes 2-20% H3PO4 by weight; 0.1-2% citric acid by weight; 0.5-2% urea by weight (as electrically resisitive agent); 0.1-0.6% BTA by weight; KOH to adjust the pH to 4-6; 0.1-1.0% colloidal silica by weight; 001-0.5% phytic acid or other surfactants. The size of the colloidal silica is prefrably between 20 nm to 150 nm. The electropolishing electrolyte of this embodiment produces smooth surfaces with high planarization efficiency, such as a planarization efficiency more than 95%, and low cost. The planarization effiency (PE) is defined as:
PE=1−(step-height after polishing/step-height before polishing)×100.
In an experiment, using a profilometer, the step height of an exemplary 100 μm×100 μm square area (or line array) was measured on a copper layer on a patterned wafer surface during electropolishing of the copper layer with the electropolishing electrolyte described. The step height measurements were taken at time intervals corresponding to a copper removal of approximately 100-150 nm. For example, if the step height of the structure (pre-ECMP step) is 500 nm and the step height post-ECMP is 50 nm, then the planarization effiency is about 90%. Curve 200 in graph in
Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.
