알루미나 표면에서의 Poly(acrylic acid) 흡착 특성과

pH가 알루미나 분산액의 안정성에 미치는 영향

전 상 준, 홍 원 희

한국과학기술원 화학공학과

Adsorption Characteristics of Poly(acrylic acid) on Alumina and

the Effect of pH on the Suspension Stability

Sang Jun Jeon, Won Hi Hong

Department of Chemical Engineering

Korea Advanced Institute of Science and Technology

Introduction

Because the planarization of the wafer surface is inevitable for the integrated circuit manufacturing, chemical-mechanical polishing or planarization(CMP) has become an important process technology in semiconductor manufacturing. CMP process is utilized for planarization of insulating and conducting materials used in device wiring. [1]

Alumina slurry is usually used for CMP of conductor. The stability of this slurry is important because the coagulation of the alumina induces some defects on the wafer surfaces.

Polymer adsorption is common method for particle stabilization. They stabilize the particles through the mechanisms such as electrostatic, steric, electrosteric repulsion etc. The dispersion stability of colloidal particles is influenced by the adsorbed amount and the conformation of the polymers adsorbed. In the case of polyelectrolytes,  their adsorption behavior is affected by pH, salt concentration, and other polymers: as salt concentration increases, polyelectrolytes with strongly charged groups behaves like an uncharged polymers. And the weak polyelectrolytes are sensitive pH. [2] Polyelectrolytes also have other conformations with pH in bulk and on particle surface. This influences the interaction between particles and thus their stability.

In this study, the adsorption behavior of PAA on alumina and the effect of pH on this adsorption and suspension stability are reported.

Experiments

(1) Materials

PAA was obtained from Aldrich co. and used without further purification. The average molecular weight of PAA used in adsorption experiment is about 250,000. The aqueous polymer solution was prepared to carry out adsorption experiment.

□-alumina was purchased from EMS. The effective particle diameter after shattering by impeller was about 500nm. And its BET surface area was 28.11 m2/g. The IEP of alumina is about 7.4 . The deionized water was used in this study. The adjustment of pH was carried out using HCl or NH4OH.

(2) Methods and Measurement

Alumina suspension was prepared by shattering the particle in water using impeller during about 5-6 hr. After the polymer solution was added to the suspension, the pH values was adjusted to 4.8, 7.8, 9.1 . The adsorbed amount of polymers on alumina was determined using EA(Elementa Analysis) instrument(EA1110-FISONS). After adsorption during about 24 hr, the solution was centrifuged at 9000rpm for 1 hr and the sediments was dried in oven. This sediment was analyzed using EA instrument by determining the carbon contents.

To estimate the stability of alumina suspension, the solution after adsroption experiment was taken and injected into settling cell. The unstable and coagulated particles falls down with time and the clear region occurs in the top portion of cell. The absorbance of upper region after about 92 hr was measured using UV spectrometer(HP 8452) at 600 nm.

Results and Discussion

The adsorption isotherms of PAA exhibit that the adsorbed amount of PAA sharply increases at low concentration. Adsorption plateau was observed with increasing concentration regardless of pH. But the adsorption density at the plateau decreases with increasing pH. The interaction between PAA and the alumina surface can be described into two ways. One is the electrostatic force between the positively charged alumina surface and  ionized PAA and the other is the hydrogen bonding between the undissociated PAA molecules and hydroxy groups on alumina surface. [3] In acidic range above the pKa of PAA, the carboxyl groups of PAA become carboxylate ion. Thus, PAA and alumina surface have attraction forces. In alkaline range above 9.2, PAA and alumina surface get negative charge and thus they have elecrostatic repulsion forces. [4] Fig.1 illustrates the schematic models describing the interaction between polyacrylic acid and alumina surface in the aqueous suspension. The change of pH also affects the conformation of polymer. Tjipangandjara and coworkers reported that polymer chains have coiled conformation at pH 4 but stretched conformation above the point of zero charge(PZC) due to the intrapolymer electrostatic repulsion. [5]

Fig.2 represents IEP of alumina and IEP change with adding PAA. The IEP of alumina decreases as adsorbed PAA concentration increases. And sharp decreasing occurs even by adding little amount of polymer. Even though it will be mentioned later, the large negative value of zeta potential at high pH is responsible for the high stability of suspension.

Fig.3 shows the stability of alumina suspension after adsorption. The absorbance at 600nm was measured. The high absorbance means a high stability. It is shown that the stability of alumina suspension increases with pH. It is because that the IEP of alumina adsorbing PAA falls below 4 and thus the electrostatic repulsion force between alumina particles increases with pH increasing due to the surface charge. That is, the surface charge of particle becomes more negative as pH increases as shown in Fig.1. It is another reason for the high stability that the stretch conformation of adsorbed polymer at high pH endows more steric hinderance between particles compared to low pH condition. But in the case of pH 7.8 and 9.1, there are little differences in stability. This is because that the PAA in both pH is almost dissociated state and thus they have similar degree of interaction between polymer and particle.

Consequently, it is shown that the adsorbed amount of PAA on alumina and suspension stability increase with pH due to the change in the degree of interaction between particle and polymer and conformation change.

Acknowledgement

This study was supported by KOSEF. The authors wish to thanks for their funding.

References

1. Luo, Q., Ind. Eng. Chem. Res., 39, 3249 (2000).

2. Ishiduki, K. and Esumi, K., Langmuir, 13, 1587 (1997).

3. Ishiduki, K. and Esumi, K., J. Colloid Interface Sci., 185, 274 (1997).

4. Santhiya, D., Subramanian, S., Natarajan, K.A. and Malghan, S.G., J. Colloid Interface Sci., 216, 143 (1999).

5. Tjipangandjara, K.F., Yi-Bin, H., Somasundaran, P. and Turro, N.J., Colloids Surf., 44, 229 (1990).

Figure 1. the schematic models describing the interaction between polyacrylic acid and alumina surface in the aqueous suspension.