A novel cleaner for colloidal silica abrasive removal in post-cu CMP cleaning

Vol. 36, No. 10 Journal of Semiconductors October 2015 A novel cleaner for colloidal silica abrasive removal in post-cu CMP cleaning Deng Haiwen( 邓海文 ), Tan Baimei( 檀柏梅 ), Gao Baohong( 高宝红 ), Wang Chenwei( 王辰伟 ), Gu Zhangbing( 顾张冰 ), and Zhang Yan( 张燕 ) Institute of Microelectronics, Hebei University of Technology, Tianjin 300130, China Abstract: A novel cleaning solution, named FA/O alkaline cleaner, was proposed and demonstrated in the removal of colloidal silica abrasives. In order to remove both the chemical and physical absorbed colloidal silica abrasives, an FA/OII chelating agent and non-ionic surfactant were added into the cleaner. By varying the concentration of chelating agent and non-ionic surfactant, a series of experiments were performed to determine the best cleaning results. This paper discusses the mechanism of the removal of colloidal silica abrasives with a FA/O alkaline cleaner. Based on the experiment results, it is concluded that both the FA/OII chelating and non-ionic surfactant could benefit the removal of colloidal silica abrasives. When the concentration of FA/OII chelating agent and FA/O non-ionic surfactant reached the optima value, it was demonstrated that silica abrasives could be removed efficiently by this novel cleaning solution. Key words: colloidal silica abrasives removal; FA/O alkaline cleaner; non-ionic surfactant; surface roughness DOI: 10.1088/1674-4926/36/10/106002 EEACC: 2520 1. Introduction According to previous reports, after chemical mechanical polishing (CMP), wafer surface cleanliness is an important factor of electronics products that have a low pass rate. Cleaning quality severely affects the performance, reliability, and stability of electronic products. The cleaning technology of post chemical mechanical polishing, especially for clean Cu wiring polished is in high demand. The chemical and mechanical parts of the CMP process mean that the chemical bonds of the wafer surface are often broken and a polishing liquid of colloidal silica, chelating agent, and other pollutants can be adsorbed on the chip surface; consequently, the post-cmp is very important. At present, acidic and alkaline cleaning fluids are mainly used. Acid cleaning liquid mainly use hydrofluoric acid (HF), nitric acid (HNO 3 /, or another acid to corrode the copper oxide and hydroxide layer, which means that the colloidal particles are adsorbed onto it into the solution, which achieves the cleaning purpose. Chemical etching methods can increase the scratch defects and cause the collapse of fine lines. As in other acid post-cmp cleaning processes, this could not perfectly solve the copper corrosion phenomenon. One of the most widely used alkaline cleaning chemical systems for particle removal in semiconductor manufacturing uses a mixture of ammonium hydroxide (NH 4 OH), hydrogen peroxide (H 2 O 2 /, and de-ionized water, which is known as APM or SC-1 solution. However, APM has a higher etch rate than copper, especially in dense patterned wafers. Tetra methyl ammonium hydroxide (TMAH) has had a wide application in cleaning after polishing; however, as with the amine compounds, it is easy to decompose TMAH under high pressure and it is easily evaporated into the environment. In addition, TMAH can cause some health problems through breathing or adhesion in the skin Œ1. The Cu wiring CMP process generally uses colloidal silica and H 2 O 2 to remove the barrier layer materials. Most of the particles on the copper surface are colloidal silica particles. This happens because after CMP, the new surface has a high energy and needs to adsorb a layer of material to achieve a steady state. At first, the surrounding matter particles are adsorbed on the surface by the Van der Waals force, this force is weak and easy to remove. As the distance becomes closer energy is released and this makes the cleaning chemical adsorption difficult, until in the main key synthesis the traditional cleaning method is difficult to remove Œ2 4. The cleaning solution used in this paper is the FA/O series cleaning solution developed by Hebei University of Technology. Its main ingredients include an FA/OII type chelating agent and an FA/O surfactant. Through experiment, this article analyses the mechanism of FA/O cleaning solution in removing particles. 2. Experiment The polishing process was carried out on a polisher (Alpsitec, E460E) with a colloidal silica based slurry on an embossed pad (Rohmnd Haas, IC 1000 TM /. The slurry used in the experiments for barrier CMP was provided by Institute of Microelectronics, Hebei University of Technology, consisting of 20 wt% colloidal silica without BTA and H 2 O 2. The polishing parameters, such as work pressure, back pressure, platen and carrier rotation speeds, and slurry flow rate were, respectively, set at 1, 1 psi, 65, 60 rpm, 300 ml/min. After polishing, a PVA brush scrubbing process was used to deal with the wafer with the cleaner for 1 min. Then, the test wafers were blow-dried in an atmosphere of nitrogen. When all of the processes were completed, a scanning electron microscopy (SEM, HITACHI * Project supported by the Specific Project Items No. 2 in National Long-Term Technology Development Plan (No. 2009zx02308-003) and the Hebei Province Department of Education Fund (No. QN2014208). Corresponding author. Email: bmtan@hebut.edu.cn Received 6 March 2015, revised manuscript received 14 May 2015 2015 Chinese Institute of Electronics 106002-1

Table 1. Difference between physical adsorption and chemical adsorption. Main features Physical adsorption Chemical adsorption Adsorption properties Van der Waals Bond strength forces Selective No Yes Heat of adsorption 0 20 80 400 (kj/mol) Adsorption rate Fast Slower Figure 1. A SEM image of a colloidal silica abrasive on a copper surface. S-4800) and atom force microscopy (AFM, Agilent 5600LS) were used to determine the cleaning performance. All of the experiments were performed utilizing FA/O cleaner, which consisted of an FA/OII chelating agent and a FA/O non-ionic surfactant. By varying the concentration of the FA/OII chelating agent and FA/O non-ionic surfactant, a series of experiments were performed to determine the optima value. Cu coupons cut from 12 inch Cu blanket and pattern wafers were first used for the experiments. After obtaining the optima value, 12 inch pattern wafers were used to verify the best results. 3. Results and discussion Figure 1 shows an SEM image of the colloidal silica abrasive adsorbed on the surface. The experiment results proved that most of the colloidal silica abrasives residual exists on the copper surface before and after cleaning. Colloidal silica abrasive deposits on the wafer surface by interactions, including the Van der Waals forces and electrostatic forces, among which electrostatic forces are largely predominant. Because the SiO 2 interlayer dielectric surface and the colloidal silica posses same sign of surface zeta potential in aqueous solution, the electronic force existing between the colloidal silica and the hydrophilic SiO 2 surface is repulsive, resulting in the desorption of the silica particles. Particles were adsorbed on the wafer surface mainly by physical adsorption and chemical adsorption. Van der Waals forces played a major role in the physical adsorption while bond strength played a major role for chemical adsorption. Table 1 presents the main differences between physical adsorption and chemisorption. At the end of the multilayer wiring Cu CMP process, the mechanism by which the colloidal silica abrasive is adsorbed onto it may be categorized as follows: (1) The gravitation, such as Van der Waals force and electrostatic force, existing between copper and colloidal abrasives nearby makes these abrasives adsorb rapidly. (2) With H 2 O 2 in the barrier slurry, copper tends to be oxidized to form cupric/cupreous oxides (CuO or Cu 2 O) and hydroxides passivation on the wafer surface, which may result in the silica abrasives being chemisorbed onto this oxide layer by Figure 2. The surface morphology after CMP. means of oxygen bridging bonding. Figure 2 shows the AFM image of the polished copper surface. It is obvious showed that there are large amount of colloidal silica remaining on the copper surface. In order to remove both of the two types of adsorbed colloidal silica abrasives mentioned above at the same time, it is necessary to break the chemical bonding between the colloidal silica and the copper oxides, and lift the silica particles. To achieve this purpose, a combination of FA/OII chelating agent and FA/O non-ionic surfactant was first introduced to remove the colloidal silica abrasives from the polished copper surface. The FA/OII chelating agent is a potent alkaline chelating agent with 13 chelate rings. It can react with metal ions to give a stable and soluble chelate-metal complex, which can be taken away by the liquid. The FA/O non-ionic surfactant is a kind of complex surfactant, consisting of osmotic agent and dispersant. It was chosen because it cannot ionize in water and can significantly reduce the surface tension of the liquid and interfacial tension, which is favorable to the particles removal. What is more, experiments have proven that the adsorption state of particles on the silicon wafers can be controlled and changed with the FA/O non-ionic surfactant Œ5 7. A schematic illustration that the FA/O cleaner removes the colloidal silica abrasives is shown in Figure 3. In the removal of chemical adsorbed colloidal silica particles process, the FA/OII chelating agent plays an important role. As Equation (2) shows, the chelating agent can react with Cu ions to give a stable and soluble copper chelate complex Œ8, making the chemical equilibrium shown in Equation (1) move to the right. CuO 2 CO 2! CuOCH 2 O OH! Cu.OH/ 2, Cu 2C C2OH ; (1) 106002-2

Figure 3. The mechanism of surfactant to remove particles. (a) Particles adsorbed on the surface. (b) Surfactant molecules in the surface layer form a protective layer. (c) Surfactant molecules on the surface of the particles and forces. Figure 4. The surface morphology of post-cmp cleaning with different cleaners. (a) At 1 ppm FA/O chelating agent. (b) At 150 ppm FA/O chelating agent. (c) At 300 ppm FA/O non-ionic surfactant. Cu 2C C 2NH 2 R NH 2! ŒCu.NH 2 R NH 2 / 2 2C : (2) Thus, a thin surface layer of cupric/cupreous oxides is dissolved and the attached colloidal silica is released. According to the preferential adsorption equation: T D KR 2 =F 1 F 2 S; where T is the adsorption time; F 1 is the per unit volume for the adsorbent surface force field; F 2 is the adsorbed species per unit area for the surface force field; R is the adsorption the adsorbent material and spacing; S is the area of adsorbate. Because the FA/O active agent is a non-ionic surfactant macromolecule, the surface area is larger and the adsorption time is short, and it does not react chemically with the Cu surface and so it puts the chemisorption into physisorption, which is easy to clean. Figure 3 shows the mechanism of active agent particle removal. In order to verify the correctness of the theory mentioned above and to determine the efficiency of FA/O cleaner on the removal of colloidal silica abrasives, a cleaner with 1 ppm FA/OII chelating agent and 150 ppm FA/O non-ionic surfactant was used in the experiment. As shown in Figure 4(a), the result indicated that there were still significant amounts of silica particles on the copper surface. There were two ways to enhance the efficiency of the removal of colloidal silica abrasives, namely increase the FA/OII chelating agent concentration or increase the FA/O non-ionic surfactant concentration. A cleaning process with a higher concentration of FA/OII chelating agent was then carried out. It was found that the higher concentration of FA/OII chelating agent was favorable for the removal of colloidal silica abrasives, as shown in Figure 4(b). However, when the concentration of FA/OII chelating agent was higher than 150 ppm, there was no obvious improvement in the cleaning performance. When the concentration of the chelat- 106002-3

Figure 5. The single defect map of wafer cleaned with the optima cleaner. Figure 6. Illustration of the surfactant molecule layer with low concentration. ing agent is too high (above 300 ppm), the interface corrosion phenomenon will occur. With the increased concentration of chelating agents, corrosion is more powerful. This means that it is enough to remove the cupric/cupreous oxides residual on the copper surface at 150 ppm FA/OII chelating agent. Thus, the concentration of FA/O non-ionic surfactant was increased to 50, 100, 300, and 500 ppm. The best results occurred when the concentration of FA/O non-ionic surfactant was 300 ppm or higher, as shown in Figure 4(c). A verification test experiment was then carried out on an Applied Materials 300 mm Reflexion LK tool using 12 inch pattern wafers with the cleaner at 150 ppm FA/O chelating agent and 300 ppm FA/O non-ionic surfactant. The scan tool (EPBFI01 and KLA_PROC_ID) was used to determine the cleaning performance. Figure 5 shows the single defect map of the test wafer. Although there were still 3552 defects on the wafer surface, none was confirmed as a colloidal silica abrasive when one hundred of the total defects were selected randomly to determine what they really were. In the experiment, it was found that the FA/O non-ionic surfactant played a primordial role and at very low concentrations it could not remove the colloidal silica abrasives perfectly. As mentioned above, the FA/O non-ionic surfactant can form a molecule protection layer on the wafer surface and the colloidal silica abrasives. However, when the surfactant concentration is very low, surfactant molecules cannot cover the entire surface of the wafer (see Figure 6), and removal efficiency will decrease. A higher concentration of surfactant implies that more molecules take part in the formation of a protection layer and the lift of the colloidal silica abrasives. What is more, the molecule protection layer itself is also removed during the PVA brush scrubbing process. A higher concentration of the surfactant means that it takes less time to form a new molecule protection layer, achieving the objective of the ongoing protection. The AFM detection results also indicate the efficiency of FA/O cleaner on removing colloidal silica. Figure 7 shows the AFM image of wafer surface treated with FA/O Cleaner. As is shown, a smooth and clean copper surface is obtained. Compared with Figure 2, it can be concluded that a significant removal of colloidal silica is achieved. Figure 8 shows the roughness of Cu surface after cleaning with FA/O cleaner. The roughness result of AFM is the square Figure 7. The surface morphology of post-cmp cleaning. Figure 8. The wafer surface roughness under different surfactant concentration. deviation value (Sq) within 10 10 m 2 area and the roughness is 0.1 nm. Roughness of the pre-cleaning Cu surface is 2.24 m. The roughness of the Cu surface after cleaning with cleaner at 10, 50, 100, 300, and 500 ppm is 1.99, 1.81, 1.20, 1.01, and 1.06 nm, respectively. This shows a tendency for the cleaner with a higher concentration of surfactant to have a better roughness performance. The reasons for this can be divided into two aspects. On one hand, a higher concentration of surfactant means more efficiency in removing the colloidal silica particles, as mentioned above. Because the residual silica particles can deteriorate the roughness, the less silica particles that remain on the 106002-4

Figure 9. The surface morphology of post-cmp cleaning with (a) NH 4 OH and (b) TMAH solutions. wafer surface, the better the surface roughness. In other words, the improvement on the surface roughness also indicates the removal of the colloidal particles. The surfactant may reduce the scratch generating during the brush scrubbing process. Once lifted from the wafer surface, the silica particle will be rolling away from the original place. Since the small particles are moving, they may get aggregated to big ones Œ9. Although the bigger silica particles are much easier to remove, it also increases the chance to make a scratch while the colloidal silica particles roll away on the wafer surface. In order to demonstrate the superiority of FA/O cleaner, the proposed cleaning solutions were compared with the same concentration NH 4 OH solutions and the TMAH solutions. The roughness of the Cu surface after cleaning with NH 4 OH solutions and the TMAH solutions are 10.5 and 8.39 nm, as shown in Figure 9. The experiment results showed that the FA/O cleaner has a better performance than NH 4 OH solutions and TMAH solutions because the FA/O has more than 13 chelate rings Œ10; 11. Compared with other cleaners, it can react with more metal ions and can take the colloidal silica particles away quickly with the cleaner. At the same time, the surfactant can form a protective layer on the surface, which prevents surface corrosion, so it can get a better cleaning effect than the other cleaners. What is more, this kind cleaning fluid composition is simple, environmentally friendly, and does not harm health. 4. Conclusions In this study, a novel cleaning solution, named FA/O alkaline cleaner, was proposed for the removal of colloidal silica abrasives. The experiment results showed that FA/O alkaline cleaner has a good performance on the removal of colloidal silica abrasives through chemical and mechanical mechanism. By adjusting the concentration of FA/OII chelating agent and nonionic surfactant, the optima concentration of surfactant was obtained. When the concentration of the FA/OII chelating agent was 150 ppm and the concentration of non-ionic surfactant was 300 ppm or higher, the colloidal silica began to be removed perfectly. This result is currently being certified on a famous company s production line. It was also found that the higher the concentration of surfactant, the better surface roughness would be obtained, which also indicated the removal of colloidal silica from another aspect. Using this novel cleaner, a clean and smooth copper surface was obtained after the cleaning process. References [1] Manivannan R, Cho B J, Hailin X, et al. Characterization of non-amine based post-copper chemical mechanical planarization cleaning solution. Microelectron Eng, 2014, 122: 33 [2] Steigerwald J M, Murarka S P, Gutmann R J, et al. Chemical processes in the chemical mechanical polishing of copper. Mater Chem Phys, 1995, 41: 217 [3] Yang Fei, Tan Baimei, Gao Baohong, et al. Multilayer copper wiring CMP on the surface of particle removal after research. Micro-Nano Electronic Technology, 2012, 49(12): 829 [4] Chen Haitao, Tan Baimei, Liu Yuling, et al. Study of cleaning method for nanoparticles on wafer surface after CMP. Manufacturing Process Technology, 2011, 1: 4 [5] Yair E E, Starosvetsky M D. Review on copper chemical mechanical polishing (CMP) and post-cmp cleaning in ultra large system integrated (ULSI) an electrochemical perspective. Electrochimica Acta, 2007, 52: 1825 [6] Chen P L, Chen J H, Tsai M S, et al. Post-Cu CMP cleaning for colloidal silica abrasives removal. Microelectron Eng, 2004, 75: 352 [7] Liu Y, Zhang K, Wang F, et al. Study on the cleaning of silicon after CMP in ULSI. Microelectron Eng, 2003, 66: 433 [8] Tan Baimei, Li Weiwei, Niu Xinhuan, et al. Effect of surfactant on removal of particle contamination on Si wafers in ULSI. Trans Nonferrous Met Soc China, 2006, 16: 195 [9] Gao Baohong, Zhu Yadong, Liu Yuling, et al. A new cleaning process combining non-ionic surfactant with diamond film electrochemical oxidation for polished silicon wafers. Journal of Semiconductors, 2010, 31(7): 076002 [10] Li Yan, Sun Ming, Niu Xinhuan, et al. Removal of residual CuO particles on the post CMP wafer surface of multi-layered copper. Journal of Semiconductors, 2014, 35(4): 046001 [11] Huang Y, Guo D, Luo X, et al. Mechanisms for nano particle removal in brush scrubber cleaning. Appl Surf Sci, 2001, 257: 3055 106002-5