Methods and Mechanistic Analysis of Improving the Dispersion of Nano Silica Colloidal Particles in Chemical Mechanical Polishing Slurries

Nengyuan Zeng Hebei University of Technology Hongdong Zhao (  [email protected] ) Hebei University of Technology Yuling Liu Hebei University of Technology Chenwei Wang Hebei University of Technology Chong Luo Hebei University of Technology Wantang Wang Hebei University of Technology Tengda Ma Hebei University of Technology

Research Article

Keywords: dispersant, agglomeration, CMP, surfactant, scratch

Posted Date: July 28th, 2021

DOI: https://doi.org/10.21203/rs.3.rs-739722/v1

License:   This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License

Page 1/19 Abstract

The colloidal silica is usually used as the abrasive for the copper CMP polishing slurry in integrated circuit multilayer copper wiring. The aggregation of colloidal silica in the slurries tend to aggregate spontaneously, resulting in the continuous changes of the polishing effect, such as scratch defects, removal rate, etc. This situation can lead to the potential instability of the polishing slurry, which should be avoided in industrial production. In this paper, the aggregation and dispersion properties of polishing slurries were systematically studied, and an improved scheme was proposed to improve the dispersion of slurries to prevent agglomeration. These affecting factors including slurries’ pH, electrolyte additive

(KNO3) and polyelectrolyte (PAA-Na) were assessed with LPC, Zeta potential and particle size. A better pH range for slurries was determined from the point of view of isoelectric point and dissolution of colloidal silica. And the dissolution of colloidal silica was verifed by UV-vis experiment. Anionic surfactant (SDS), Cationic surfactant (DTAC) and Nonionic surfactant (AEO-15) was analyzed for their dispersion in slurries from the point of view of static electricity force and steric hindrance space force, and the synergistic mechanism of mixed SDS and AEO-15 on slurries dispersion was put forward. In addition, the effect of the improvement of slurry dispersion on reducing scratching defects on the surface of polished copper blanket was also tested in this paper.

1. Introduction

CMP is a technology to achieve global planarization in IC fabrication processes including FEOL and BEOL (front and back end processing of integrated circuit) processes[1, 2]. The reliability of devices is crucial refection of product performance. Therefore, an appropriate slurry is vital and essential for improving production efciency[3]. However,a potential problem in all types of slurry is the particle agglomeration. The interaction between colloidal particles will lead to the aggregation of colloidal particles due to irresistible factors, including: 1. The particles in the colloid collide with each other spontaneously due to Brownian motion, and some of them gather together over the repulsion barrier [4]. 2. The slurry is caked by the shear stress of the pipe wall during transportation of the machine[5, 6]. 3. The grinding particles will also form soft agglomerates during the polishing process, which is completed by the polishing transient[7, 8]. The aggregation of particles lead to the formation of defects. And it has been reported that particle aggregates larger than 0.5um may cause scratches on the surface of the wafer[9–11]. In addition, the material removal rate is affected by the change of mechanical friction stress caused by the uneven loading of large particle aggregates on the wafer surface[8, 12]. Guorui Liu al.[13] showed that the infuence of large particle agglomerates on material removal was also refected in the uniformity in the wafer sheet. Therefore, it is necessary to make the slurry dispersed and stable in to obtain good polishing performance. Moreover, it is necessary to ensure that the slurry can still maintain good dispersion under the polishing conditions of high rotation speed and high stress.

In some reports, Fenzhichang et al.[6] showed that lowering pH or adding ionic salts would reduce the electrostatic repulsive force of SiO2 colloids, leading to slurry agglomeration. Wonseopchoi et al.[12] found that ionic salts would increase the removal rate of dielectric silica, and Jianchao Wang et al.[14]

Page 2/19 found that potassium ion had a higher medium removal rate. In order to improve the dispersion of slurry, some reports suggest using surfactant. Khushnumaasghar et al.[15] found that SDS (sodium dodecyl sulfate) improved the dispersion of slurry and reduced the surface roughness of GaN substrate after polishing. Yanleili et al.[4] found that non-ionic ethoxylated decylalchohol (EDA) can be used as a dispersing agent to reduce scratches in copper interconnection polishing. There are many ways to improve the dispersion of slurry, and surfactants show great potential. However, the dispersion system of slurry still needs to be studied systematically.

For Copper polishing slurry, surface roughness, scratch and polishing rate all affect the polishing effect. Therefore, improving slurries’ dispersion plays an important role in improving surface roughness, reducing scratch and stabilizing polishing rate. In this paper, based on previous studies, the effects of pH, electrolyte (KNO3), Polyelectrolyte (PAA-Na) and surfactants (SDS, DTAC and AEO-15) on the dispersion and aggregation of slurries were studied. And the optimization of slurry dispersion and stability was proved from various parameters such as particle size and LPC, and the improvement of surface quality after polishing was verifed.

2. Experimental

Abrasive of slurries used colloidal silica (40%, average 67.8nm) produced by HB Jingwei company. KNO3 (> 99%) and PAA-Na(Sodium Polyacrylate, > 98%) are supplied by Ron Company. AEO-15(Primary Alcobol Ethoxylate, > 99%), DTAC (Dodecyl trimethyl ammonium chloride > 99%) and SDS (Sodium dodecyl sulfate > 99%) are supplied by Aladdin Company. SDS (Sodium dodecyl sulfate), DTAC (Dodecyl trimethyl ammonium chloride) and AEO (Primary Alcobol Ethoxylate) belong to anionic, cationic and non-ionic surfactants respectively. And PAA-Na is a polyelectrolyte, which was used as a dispersant in this paper. Their structure is shown in Fig. 1.

In order to refect the effect of various additives, the experiment took 5% colloidal silica as the reference(REF) group. Various additives in REF slurries were added as the control group. KOH (reagent grade) and phosphoric acid (reagent grade) were used to regulate pH provided by the company.

The polishing experiment was carried out on French E460 polishing machine with IC1000 (DOW Electronic Materials, American company) as polishing pad, 3-inch Copper blanket wafer (PVD, 1um) as polishing consumables. The polishing process was performed at the fow rate of 300 mL /min, the pressure of 1.5psi, and 87/93 RPM for the thrower and disc speed respectively. Before formal polishing, the polishing machine is padded for 30min (using the process described above) and the polishing pad was trimmed with a C4 diamond dresser (3 M Inc., USA) to remove surface polish residue. The polishing rate was calculated by the flm thickness difference before and after polishing processes. The Zeta potential and average particle size of the slurry were measured using NiCOMP 380ZLS (PSS Corporation, USA). The LPC (large particle size) was measured by the NICOMP Accusizer 780 (PSS, Inc.) to count the large particle sizes (> 0.5µm). The optical inspection microscope (Nikon L300ND) was used to observe

Page 3/19 the surface defects of the wafer. Persee T6 UV-vis (UV/vis) spectrometer was used to qualitatively characterize the dissolution reaction of colloidal silica.

3. Results And Discussion 3.1 Experiment on pH of colloidal silica and dispersion of colloid

+ − In aqueous solution, H or OH will be adsorbed on the surface of hydroxylated SiO2, resulting in charge on the surface of particles[16]. The equation can be expressed as:

Obviously, the surface charge properties of colloidal silica are related to pH. When the colloid is near the isoelectric point(IEP), the surface charge can be zero. The slurry produces high shear stress in the process of pump circulation and pumping, which causes some particles to easily break the repulsion potential barrier Umax and lead to aggregation[17]. The particle size and zeta potential of the colloid can be used as an assessment of the degree and ability of particle aggregation.

Figure 2 showed the change of zeta potential, mean particle size and LPC of colloidal silica with its pH value. Zeta potential was the sliding surface potential of colloidal silica double layer, its absolute value refected the dispersion of colloid, and pH was regulated by KOH and phosphoric acid. Colloidal silica was easy to adsorb H+ and OH− due to its silanol groups[18], which made its surface positively or negatively charged. Increasing the pH of silica colloid will increase the electronegativity of particle surface. As shown in Fig. 2(1), the position of the colloid deviating from its IEP (isoelectric point) gradually made the zeta potential more negative. However, under the strong alkaline condition, the zeta potential will rise back to zero. One possible reason was that a large number of colloidal silica particles dissolved under strong alkaline conditions to form silicate solution. According to relevant literature reports, the dissolution of amorphous SiO2 was caused by the deprotonation of silanol groups and the hydrolysis of Si-O-Si bonds when under alkaline conditions. [19–21]. the salt solution coming from particles dissolve lead weakening electric double layer and lowering the absolute value of zeta potential. Besides, the number of large particles and particle size decreased accordingly due to dissolution, as shown in Fig. 2(2).

The dissolution of colloidal silica particles was not a good thing despite the decline in large particles and particle size, which greatly weakened the role of colloidal silica as an abrasive in slurries. Therefore, the suitable pH of slurries was an important parameter for avoiding colloidal silica particles from dissolving.

Page 4/19 It can be seen from Fig. 2 that the absolute value of zeta potential of pH at 9.6 was the advantage stage in which the colloid stability was the best. 3.2 The effect of electrolytes on the dispersion of the slurry

It is well known that adding a certain amount of electrolyte to the solution will accelerate the aggregation and sedimentation of the colloid [22, 23]. In order to explore the effect of electrolyte concentration on colloid sedimentation, Table 1 showed the effects of a series of concentrations of KNO3 on zeta and LPC in colloidal silica solutions. Trace addition of electrolyte (0.02 wt% KNO3) improved the dispersion of colloid, while the continuous addition of electrolyte reduced its dispersion even to the agglomeration. According to DLVO,whether the colloid is stable or not depends on the balance between electrostatic attraction and electrostatic repulsion between particles[24, 25]. The electrolyte concentration n0 has a great infuence on the repulsion potential energy(UR) according to formula 3. However, n0 has two opposite effects of repulsive potential energy. Through the calculation, it is concluded that UR increases at frst and then decreases with n0[26]. So the electrolyte has a concentration value to reach a relatively high dispersion and stability of colloids. But high concentration of electrolytes will compress the double electrode layer thickness and reduce UR[27].

UR is repulsion potential energy, K1 and K2 are the constants, and n0 is electrolyte concentration

Table 1 Silica with different concentrations of KNO3 Sample Zeta potential LPC(> 0.5um)

5 wt% Silica -26.7mV 174387

5 wt% Silica + 0.02 wt% KNO3 -31.3mV 144756

5 wt% Silica + 0.04 wt% KNO3 -10.1mV 189426

5 wt% Silica + 0.1 wt% KNO3 -2.3mV 284629

5 wt% Silica + 5 wt% KNO3 coagulation coagulation

3.3 Effect of Surfactant on dispersion Stability of Colloid

It is well known that surfactants have the ability to improve colloidal dispersibility[32, 37, 38]. Figure 5 showed the effect of SDS (anionic), DTAC (cation) and AEO (Nonionic) on Zeta potential of slurries. Ionic surfactants had obvious infuence on zeta potential due to electrostatic charge, compared with nonionic

Page 5/19 surfactants. With the increase of solid loading of DTAC in colloidal silica, the zeta potential increased from − 28mv to 18mV. As the solid content of SDS increased, the Zeta potential became more negative (from − 27 to -38 mV). However, AEO had no obvious change. In addition, it can be seen from the data that the cationic surfactant DTAC had a much greater effect on zeta than the anion (SDS).

Since colloidal silica was negatively charged under alkaline conditions, and the cationic surfactant had electrostatic attraction and hydrophobic adsorption effect in the slurry[39], the cationic head group was sufciently close to the surface of colloidal silica particles, thus increasing the surface potential and zeta potential. There was electrostatic repulsion between SDS− ionized by anionic surfactant (SDS) in aqueous solution and colloidal silica with negative surface charge, which prevented SDS− from approaching colloidal silica particles further. However, due to the effect of hydrophobic adsorption, anionic surfactants formed hemispheres or spherical aggregates near negatively charged silica particles, which slightly reduced the surface potential or zeta potential of colloidal silica particles. However, with the addition of higher concentration, the zeta potential was not increased or decreased indefnitely. It was still effective to increase the absolute value of zeta and improve the colloid dispersion by adding SDS, which can be also proved by the decrease of LPC in Fig. 8. However, although AEO failed to change the Zeta potential, its LPC performance was not inferior to that of the former.

It was well known that Nonionic surfactants rely on steric hindrance to realize the dispersion of particles[40, 41]. Due to the existence of charge repulsion, the steric hindrance entropy effect of ionic surfactants was greatly reduced. According to experiment data in Fig. 8–9, the LPC of the slurry mixed with anion SDS and AEO was greatly reduced, and it also had a better performance in terms of zeta and particle size distribution. AEO was adsorbed on the surface of particles by hydrogen bond formed by its hydrophilic chain ether bond and the silicon hydroxyl group, and the other end of the hydrophobic chain was adsorbed with SDS. The mix of anionic surfactant and Nonionic surfactant provided higher electrostatic force and steric hindrance space force to make the slurry more dispersed. The adsorption diagram of surfactant and colloidal silica (Fig. 6) can better explain the dispersion principle described in the above paper.

The diagram of particle size distribution shown in Fig. 7 also illustrated that the dispersion effects of each surfactant that a smaller average particle size and lower P.I value in slurries compared with REF, and it can also be seen from the data that the slurry added with mixed SDS and AEO-15 had a more dispersed particle size than that added alone. In addition, the dispersion effects of SDS and AEO-15 were also shown with the data of zeta and LPC in Fig. 8. Meanwhile, when the confgured size was left at rest for 5 days, the LPC was increased and the LPC stability of slurries with dispersant was better than that of REF. The LPC of the slurry mixed with SDS and AEO-15 was more stable. 3.4 Effect of surfactants on polishing properties

In the polishing process of copper flm of copper interconnect, there was inevitable scratch defect, which will affected the performance of the device[42, 43]. Surface roughness was an indicator for high quality copper flm polishing. Figure 9 showed the surface of the 300mm pattern wafer and the scratch defects

Page 6/19 on the surface of the wafer scanned by SEM. The dispersity of polishing slurries was an associated factor for scratches and roughness on the surface of the polished wafer[44]. So we have prepared a group of relatively dispersed slurries relative to REF according to Fig. 8. Its dispersion can be characterized by zeta potential and LPC. Slurries that REF was used as a reference group, and that the addition of the mix of AEO and SDS was control group was polished in 3inch copper blanket for 60s. And high-speed industrial microscope observed the surface of the copper blanket wafers that had been polished with these slurries. According to some reports, the surfactant was used as the dispersing agent to reduce scratches by reducing particle aggregation[45, 46]. However, the cause of the formation was multifaceted. The dispersant does not completely solve the problem of scratched defects. Figure 10 showed manual statistics of scratches in selected areas on the 3inch diameter wafer. The scratches were assessed according to the microscope image under the optical microscope at a magnifcation of 1000 times. Slurries with dispersant can signifcantly reduce the proportion of scratches on the wafer surface, but there were still scratches. Nevertheless, the dispersion of polishing fuid was an important infuence factor for scratch defects, which was also confrmed by our experimental data.

4. Conclusion

In this study, the factors causing slurry polymerization were studied and explained respectively. In view of the factors of aggregation and sedimentation of colloidal silica particles, an improvement scheme was proposed to improve the dispersion stability. In this paper, the effect of pH on the aggregation level of colloidal silica was studied at frst. By analyzing the relationship between isoelectric point and dissolution, a stable and highly dispersed pH range of colloidal silica was determined that a relatively high zeta potential absolute value was obtained around 9.6. Electrolytes such as KNO3 in slurry can lead to sedimentation at high concentrations and colloid dispersion at lower concentrations. Polyelectrolyte (PAA-Na) can improve the electrostatic repulsion of colloidal silica particles, provided space resistance and enhance colloid dispersion. However, the phenomenon of bridging focculation can be not avoided in spite of low concentration.

In addition, three kinds of surfactants (SDS, DTAC and AEO-15) on the dispersion of colloid were studied and analyzed. The dispersion of ionic surfactant, including SDS and DTAC, was mainly achieved by increasing the electrostatic charge of colloidal particles. Nonionic surfactants (AEO-15) relied on steric hindrance to realize the dispersion of particles. The mix of anionic surfactant (SDS) and Nonionic surfactant (AEO) provided higher electrostatic force and steric hindrance space force to make the slurry more dispersed. The mechanism of enhancing colloidal dispersion by surfactants was explained. The experimental data proved the theoretical feasibility. In addition, the dispersed slurry with the mix of SDS and AEO-15 can improve the surface scratching defects after polishing and were evaluated by high-power micro-image irradiation on the wafer surface. The decrease of scratch defects to some extent proved the improvement of dispersion.

5. Declarations

Page 7/19 Ethics approval and consent to participate: Not applicable

Consent for publication: Not applicable

Availability of data and materials: All data generated or analysed during this study are included in this published article

Competing interests: The authors declare that they have no competing interests

Funding: This paper was supported by the Major National Science and Technology Special Projects (No.2016ZX02301003-004-007)

Authors' contributions:

Nengyuan Zeng: Conceptualization, Writing - original draft.

Hongdong Zhao: Resources, Writing - review & editing.

Yuling Liu: Resources, Supervision, Project administration, Funding acquisition.

Chenwei Wang: Formal analysis, Validation.

Wantang Wang: Investigation.

Tengda Ma: Investigation.

Acknowledgements: Not applicable

Disclosure of potential conficts of interest: the authors declare that they have no competing interests

Research involving Human Participants and/or Animals: Not applicable

Informed consent: informed consent was obtained from all authors.

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Figures

Figure 1

Structural formula of DTAC, SDS, AEO-15 and PAA-Na

Page 12/19 Figure 2

The zeta potential changes with pH(1), and the mean particle size and LPC change with pH(2)

Figure 3

The UV/visible spectra of KOH(a), and the colloidal silica with different pH(b)

Page 13/19 Figure 4

Zeta potential and LPC of colloidal silica with different amounts of PAA-Na.

Page 14/19 Figure 5

Zeta potential of concentration of DTAC, SDS and AEO

Page 15/19 Figure 6

The schematic diagram of SDS, DTAC and AEO-15 dispersed colloidal silica

Page 16/19 Figure 7 the Gaussian distribution of particle size of slurries with different surfactants

Figure 8

Zeta potential and LPC of slurries with different AEO

Page 17/19 Figure 9

The defect diagram that optical microscope scans the 300mm Copper pattern

Figure 10

Page 18/19 Distribution of scratch defects on the surface of 3inc copper wafer was determined by optical inspection microscope scanning. “a” was the surface with the REF slurry polishing and “b” was the REF+AEO+SDS

Page 19/19