1. Introduction

In the process of copper wiring CMP,on the basis of P1 (remove the copper film quickly) realizing the preliminary planarization,the main task of P2 (the elaborate polishing process of removing residual copper) is to remove the residual copper on the wafer surface completely and modify dishing to reduce the extension of it. P2 should guarantee that the polishing rate of the barrier material tends to be zero^[1]. On one hand,a period of over polishing is essential in P2 in order to remove the residual copper completely^[2]. Inescapably,dishing will extend in the process of over polishing. Therefore,dishing has become a worldwide problem that is difficult to solve as well as the key technology of the multi-layered copper routing. On the other hand,copper and the barrier layer have high selectivity,which means that the removal rate of copper is relatively higher than that of Ta/TaN. Distinction of the removal rate will also cause the dishing to extend. Dishing has many defects such as high resistance with growing RC delay and increasing losses of copper wire,which will lead to increasing thermal power consumption and serious electricity migration phenomenon. It can also affect the partial planarization of copper pattern wafer,which cannot meet the industrial requirements of CMP^[3].

At present,almost all of the polishing slurry used on domestic devices of global planarization is imported. A new type of alkaline copper polishing slurry self-developed by the Microelectronics Technology and Material Research Institute in Hebei University of Technology is adopted. It has many advantages such as simple components and environmental protection. The oxidant of the slurry is H$_{2}$O$_{2}$. Being different from benzotriazole (BTA) in the acid slurry with side effects such as being different to clean and forming Cu-BTA film^[4],H$_{2}$O$_{2}$ will not induce ionic soil as the most common oxidizing agent^[5]. On one hand,H$_{2}$O$_{2}$ can make copper be oxidized into ionic to react with the FA/O chelating agent,which generate stable complex. The reaction can accelerate the removal of copper as well as the efficiency of planarization. On the other hand,oxidation film and hydroxide film generating in the H$_{2}$O$_{2}$ oxidation process can act as passivation film to protect the copper wiring in the recess area from being removed,which forms a characteristic self-passivation process. The passive function of H$_{2}$O$_{2}$ can reduce dishing in the process of P2 and optimize the effect of planarization^[6]. This article uses the electrochemical method to investigate the passive character of H$_{2}$O$_{2}$ in the alkaline P2 slurry with different concentrations of H$_{2}$O$_{2}$. In addition,we apply the P2 slurry to the 12 inch copper pattern wafer and integrate its function of modifying dishing. The passivation effect of H$_{2}$O$_{2}$ affecting the planarization of 12 inch copper pattern wafer is examined.

2. Experimental

2.1 Experimental materials and equipment

Experimental materials: a new type of alkaline copper polishing slurry self-developed by the Microelectronics Technology and Material Research Institute in Hebei University of Technology; the FA/O chelating agent and nonionic surface active agent researched independently; oxidizing agent H$_{2}$O$_{2}$ whose mass fraction is 30%; add different concentrations of H$_{2}$O$_{2}$ to confect alkaline P2 slurries before using (PH of all P2 slurries range from 10.3 to 10.7); a 3 inch copper blanket applied to test DRR; a 12 inch copper blanket wafer applied to test MRR; and a 12 inch copper pattern wafer applied to research the planarization of P2 slurry.

Experimental equipment: the E460E polisher produced by France's Alpsitec company; X IC1000 (TM) A2 PAD DH2 24.7''ACAO; *G01 polishing pad; CHI600C electrochemical workstation; 300 step profiler of KLA Tencor/Amhios Technology company; XP-300 Surface Profiler analytical balance with 0.1 mg accuracy (Mettle Toledo AB204-N); and PHB-4 portable PH meter of the Shanghai INESA Scientific Instrument Company.

2.2 Experimental methods and analysis test

2.2.1   Experimental methods

P2 slurry composition: abrasive SiO$_{2}$ whose mass fraction is 40% and FA/O chelating agent and active agent compose P1 slurry. According to 1:6 diluted times dilute P1 slurry directly in order to unify the stock solution of P1 and P2. P2 slurry compound with adding different concentrations of H$_{2}$O$_{2}$ (5,10,15,20,25,30,35,40 ml/L).

Electrochemical electrolyte composition: FA/O chelating agent; different contents of H$_{2}$O$_{2}$ (5,10,15,20,25,30,35,40~ml/L).The process condition of CMP is shown in Table1.

2.2.2   Analysis test

Using the CHI600E electrochemical workstation to observe and analyze the copper polarization curve,$E_{\rm corr}$ and $I_{\rm corr}$ of P2 slurry with different concentrations of H$_{2}$O$_{2}$. Analyze the passivation effect of H$_{2}$O$_{2}$ on controlling the dishing after P2.

Amend the polishing pad for one minute before each P2 experiment. Use the same stock solution,according to 1:6 diluted times to dilute P1 slurry directly into P2 slurry. On the basis of realizing the preliminary planarization of P1 on the copper pattern wafer,use P2 slurry to polish the wafer and an XP 300 step profiler to test dishing.

3. Results and discussion

3.1 Alkaline copper P2 slurry mechanism of action

The main reason that acid polishing slurry of copper CMP is used widely on the international is that copper will form the oxide and hydroxide,which is difficult to dissolve in the alkaline situation resulting in decrease of the copper removal rate. Besides these insoluble particles may cause scratches and defects^[7]. However,acid polishing slurry has many disadvantages with the development of nodes to 45 nm and below. For example,it contains toxic increasing film BTA whose heavy mechanical action will destroy the multiple air brittle low $k$ dielectric,which is influenced by technical parameters including polishing pressure,speed and pad selection^{[8, 9, 10, 11]}. This article uses alkaline slurry developed independently and selects the self-developed complex agent to turn insoluble copper substance into soluble copper amine complexes which is extremely stable. Thus,the problem of insoluble copper formation in the alkaline circumstance is solved effectively^{[12, 13]}.

As an oxidant,H$_{2}$O$_{2}$ oxidizes the surface of copper into copper oxide and hydroxide. Then Cu$^{2+}$ ionized out of Cu(OH)$_{2}$ complex with the FA/O chelating agent quickly under the chemical mechanical impact and form a stable copper-amine complex,which improves the removal rate of copper^[14]. Meanwhile,in order to solve the problem of dishing extending P2,the passivation effect of H$_{2}$O$_{2}$ on recession copper is more focused on. The oxide and hydroxide passivation film adhering to the surface of recession copper prevent recessed copper from dissolving and convex copper wiring has a higher kinetic energy and faster mass transfer. Cu$^{2+}$ ionized out of the Cu(OH)$_{2}$ complex can overcome the potential energy quickly and generate extremely stable resultant. Through this process,distinction of high and low removal rates becomes obvious and dishing decreases after P2,which optimizes the planarization effect. Meanwhile,in order to reduce the extension of dishing,P2 need that the removal rate of the barrier layer tend to zero,which lay a good foundation for barrier layer polishing.

3.2 Potential polarization curve measurement

Different concentrations of H$_{2}$O$_{2}$ alkaline electrolytes are made up. Analyze the corrosion state of copper (Figure2) by testing the potential polarization curve. The corrosion statue includes the corrosion potential ($E_{\rm corr})$ and corrosion current ($I_{\rm corr})$. The changing rule of them in different concentrations of H$_{2}$O$_{2}$ is shown in Figure1.

We test the electrochemical behavior of the copper electrode in different P2 electrolytes respectively; the testing results of polarization curves in different P2 electrolytes are shown in Figure2. Electrochemical anode and cathode electrode reactions: \text{ Anode:}\ \text{Cu}=\text{C}{{\text{u}}^{2+}}+2{{\text{e}}^{-}}. \text{Cathode:}\ {\rm O}_{2}+{\rm H}_{2}{\rm O}_{2}+2{\rm H}_{2}{\rm O}+6{\rm e}^{-}=6{\rm OH}^{-}.

((1))

Through analyzing the change of $E_{\rm corr}$ and $I_{\rm corr}$ and the potential polarization curve,the testing results show the following conclusions.

When the concentration of H$_{2}$O$_{2}$ is 5 ml/L,$E_{\rm corr}$ leaps to a relatively high value. With the increased concentration of H$_{2}$O$_{2}$,$E_{\rm corr}$ continues to increase slowly and $I_{\rm corr}$ augments persistently. According to the data in Table2,when the concentration of H$_{2}$O$_{2}$ is 35 ml/L,the $E_{\rm corr}$ of copper reaches the peak value and $I_{\rm corr}$ continues to decrease. The high $E_{\rm corr}$ shows that the corrosion of copper is slight and the passivation is obvious at this content of P2 electrolyte. Increasing the H$_{2}$O$_{2}$ content to 40 ml/L,$E_{\rm corr}$ changes to decrease and $I_{\rm corr}$ tends to increase.

The polarization curve in Figure2 shows that anode current decreases with ascending H$_{2}$O$_{2}$ content until the H$_{2}$O$_{2}$ content is 35 ml/L. When the concentration of H$_{2}$O$_{2}$ is lower,H$_{2}$O$_{2}$ firstly oxides copper into cuprous ions,which is oxidized into cupric ion in the electrolyte. On the contrary,H$_{2}$O$_{2}$ oxides copper into cupric ions directly. The change of color (changing from green to light blue) in the electrolyte can prove the explanation given above. The passive film attaching to the anode copper electrode prevent copper from further dissolution which increases passivation and descends the anode corrosion current^[15]. At the same time,due to the incrassation of the passive film electron ionized from the anode becoming less,which is unable to provide an electronic supplying the cathode reaction (2) continue to happen. Hence the current density of the cathode also shrinks. Synthesizing the changing rule of anode and cathode,this is thought that diminution of the corrosion current can also explain that the passive effect of H$_{2}$O$_{2}$ on the copper film surface goes up with the augment of H$_{2}$O$_{2}$ concentration.

Therefore,the electrochemical behavior implies that the electrolyte has the strongest inhibitory effect on copper corrosion as well as the best passivation effect at H$_{2}$O$_{2}$ concentration of 35 ml/L. The results can also correspond to the change of the removal rate of copper during the process of P2.

Figure3 shows changes in anodic current over the corrosion time in P2 slurry of 35 mL/L H$_{2}$O$_{2}$. It can be confirmed that the dynamic changes in anode current is beneficial for analyzing the dynamic time-phased reaction occurring in P2 slurry. In the first part of the curve (0-700 s),the anode current descends relatively faster. At this stage,the passivation film that blocks available reaction sites protects recession copper from further reaction. Thus the anode current drops sharply. However,in the rest of curve (700-1200 s),the current persistently eases down. It is suggested that the oxide formed during the early stage of the dissolution is porous,which means continuous dissolution could occur^[16]. Therefore,the passivation character of the H$_{2}$O$_{2}$ changes over time because of the porous oxidization film,which increases the reaction area.

Apply the changing rule of the anode current at H$_{2}$O$_{2}$ content of 35 ml/L to the P2 process. In the convex area,H$_{2}$O$_{2}$ passivation needs to be weak (700-1200 s),which can realize removing residual copper quickly. On the contrary,H$_{2}$O$_{2}$ passivation should be enhanced (0-700 s) to block the recession copper from dissolution to guarantee dishing is not extended.

3.3 Dishing and planarization effect experiment

On the basis of the results of the electrochemical analysis experiment of copper in different H$_{2}$O$_{2}$ content in the electrolyte,prepare the P2 slurry of different contents of 5,15,25,35,40 mL/L respectively. Use these slurries to polish (P2) the 12 inch copper pattern wafer for 30 s.

Choose five points A,B,C,D,E on the 12 inch pattern wafer diameter uniformly as dishing test positions. Figure4 shows the effect of various H$_{2}$O$_{2}$ contents of P2 slurry on dishing. P1 can realize planarization preliminarily and P2 can control dishing to the needs of industry requirements on the basis of removing the residual copper remaining in P1 completely. As it can be seen,when the H$_{2}$O$_{2}$ content is 5 mL/L and 15~mL/L,dishing extended to some extent with respect to it after P1. Dishing was amended with the increase of H$_{2}$O$_{2}$ content and it reaches the minimum at a certain H$_{2}$O$_{2}$ concentration of 3 mL/L. At this concentration,dishing was controlled to the maximum extent.

Figure5 shows the effect of various H$_{2}$O$_{2}$ contents of P2 slurry on the removal rate of Cu/Ta as well as the static corrosion rate of Cu. With the increase,the copper removal rate of P2 reduced gradually. When the H$_{2}$O$_{2}$ content increased to 15~mL/L,the copper removal rate of P2 reduced continuously but not obviously. Meanwhile,dishing was controlled to the maximum extent at an H$_{2}$O$_{2}$ concentration of 35 mL/L. It can be explained that the P1 realized planarization preliminarily and Cu removal rate of P2 just need to meet the standard (about 200 nm/min). When the H$_{2}$O$_{2}$ concentration is 35 mL/L,combined with the minimum static corrosion rate of copper further showing the reinforcement of the passivation of H$_{2}$O$_{2}$,a strong passivation effect of H$_{2}$O$_{2}$ can protect recession copper from being removed and the removal amount of convex copper almost had no difference. At the same time P2 can meet the standard that the barrier layer removal rate is almost zero.

4. Conclusion

The article researched many aspects such as the electrochemical performance of copper wafer in different H$_{2}$O$_{2}$ concentrations in alkaline P2 slurry,the effect of H$_{2}$O$_{2}$ passivation on dishing and also the removal rate of copper film. H$_{2}$O$_{2}$ not only can be used as an oxidant to accelerate the Cu removal rate it can also protect recession regions of copper wafer from being further dissolved due to dense oxide film formation. The changing rule of electrochemical corrosion potential reveals the rule of copper CMP in alkaline P2 slurry that corrosion potential ascends with the augment of the H$_{2}$O$_{2}$ content. The tendency revealed that the passivation effect of H$_{2}$O$_{2}$ is enhanced. Therefore,the passivation function of H$_{2}$O$_{2}$ plays an extremely critical role in P2 on controlling dishing from extending,which is also beneficial for global planarization.