Yi-Koan Hong

Interfacial and Electrokinetic Characterization of IPA Solutions Related to Semiconductor Wafer Drying and Cleaning

In this study, the interfacial and electrokinetic phenomena of mixtures of isopropyl alcohol (IPA) and deionized (DI) water in relation to semiconductor wafer drying is investigated. The dielectric constant of an IPA solution linearly decreased from 78 to 18 with the addition of IPA to DI water. The viscosity of IPA solutions increased as the volume percentage of IPA in DI water increased. The zeta potentials of silica particles and silicon wafers were also measured in IPA solutions. The zeta potential approached neutral values as the volume ratio of IPA in DI water increased. A surface tension decrease from 72 to 23 dynes/cm was measured when the IPA concentration increased to 30 vol %. The surface excess of IPA at the air-liquid interface reached a maximum at around 20 vol % IPA. The adhesion forces of silica particles on silicon wafers were measured using atomic force microscopy in IPA solutions. The adhesion force increased as the volume percent of IPA in water increased. Lower particulate contamination was observed when the wafers were immersed and withdrawn from solutions containing less than 25 vol % IPA.

The Effect of Additives in Post-Cu CMP Cleaning on Particle Adhesion and Removal

The adhesion force of silica particles to Cu films and the role of additives on adhesion and removal of particles have been theoretically and experimentally investigated in citric-acid-based post-Cu chemical mechanical planarization (CMP) cleaning solutions. The zeta potential of silica and Cu slightly increases when citric acid is added due to the adsorption of citrates. Citric acid is adsorbed on silica and Cu surfaces, resulting in more negative charges on these surfaces. The adhesion force of silica particles on Cu decreases as the citric acid concentration increases due to more repulsive electrostatic interaction between surfaces. The addition of benzotriazole in the cleaning solution initially decreases adhesion then increases it at high concentrations due to the change in zeta potentials. The addition of tetramethylammonium hydroxide to citric acid increases the particle adhesion force. However, the addition of NH 4 OH results in the lowest adhesion forces. The highest particle removal efficiency is observed when using cleaning solutions that yield the lowest adhesion force.

The Effect of Frictional and Adhesion Forces Attributed to Slurry Particles on the Surface Quality of Polished Copper

The effect of frictional and adhesion forces attributed to slurry particles on the quality of copper surfaces was experimentally investigated during copper chemical mechanical planarization process. The highest frictional force of 9 Kgf and adhesion force of 5.83 nN were observed in a deionized water-based alumina slurry. On the other hand, the smallest frictional force of 4 Kgf and adhesion force of 0.38 nN were measured in an alumina slurry containing citric acid. However, frictional (6 Kgf) and adhesion (1 nN) forces of silica particles in the slurry were not significantly changed regardless of the addition of citric acid. These differences were explained by the strong adsorption of citric ions on alumina but not on silica, which was verified by the charge reversal of alumina in zeta potential measurements. Higher particle adhesion forces resulted in higher friction. A higher magnitude of particle contamination and scratches was observed on polished copper surface in slurry condition with higher adhesion and friction forces.

Effect of Polysilicon Wettability on Polishing and Organic Defects during CMP

This study investigated the wettability effect of polysilicon on the polishing performance and organic defect contamination during polysilicon chemical mechanical polishing (CMP). Contact angle measurement was utilized to understand the nature of polysilicon surfaces. An oxidizer, H 2 O 2 , was added to the silica slurry to modify a hydrophobic polysilicon surface to a hydrophilic surface during polishing. The adhesion force was measured between a polymeric pad particle and a poly-Si wafer surface in KOH solution (pH 11) as a function of H 2 O 2 concentration. The adhesion force of the polymeric pad particle on the polysilicon decreased from 14 to 8 nN as the peroxide concentration increased to 10 vol %, at which the surface became hydrophilic. The hydrophilization of the polysilicon surface during polishing drastically reduced the organic contamination on the polysilicon wafers after polishing. The removal rate, frictional force, and pad temperature during CMP, with and without oxidizing the surface, were measured. They all decreased with the increasing concentrations of the oxidizer. The decrease was attributed to the formation of the lubrication layer of the oxide surface due to the oxidation of polysilicon.

Effect of Poly Silicon Wettability on Polymeric Residue Contamination

The planarization has been implemented on the IC manufacturing such as a DRAM process including the shallow trench isolation (STI) and the self aligned memory cell contact pad (SAC) process. Poly silicon CMP has also been implemented to reduce the step height of gate poly Si in the construction of RCAT (recess channel array transistor) and FinFET three dimension structures. Poly silicon CMP uses either the same or similar pads or slurries as those for oxide CMP. Poly Si CMP is consecutively used in order to form self aligned memory cell contact pad [1]. The adhesion and removal rate of the polymeric residues was investigated as a function of wettability of the poly silicon surface during poly silicon CMP process. An oxidizer makes more hydrophilic poly silicon wafer surface and acts as a function of oxidant on the poly silicon surface in the DI water. Adhesion force between pad particle and poly silicon wafer decreased and saturated as a function of concentration of solution A, as an oxidizer. Figure 1 shows the contact angle of poly surface in the DI (Deionized) water as a function of concentration of solution A as an oxidizer. Contact angles of poly Si surface decreased from 69° to 23° as the concentration of solution A increases to 10 vol% and then reaches a constant value. A high contact angle indicates poor surface wetting while a low angle shows good wetting. The solution A acts as a surface oxidant on the poly Si surface and makes more hydrophilic surface in the DI water. The adhesion force of pad particle on the poly Si wafer surfaces was measured in the KOH solution (pH 11) as a function of solution A concentration. KOH was used for alkaline based slurry of poly silicon polishing and solution A was added in the KOH based solution in order to control the wettability of the poly silicon surface. Adhesion force decreased and saturated as a function of concentration of Solution A. When solution A was added to slurry, the poly silicon surface became hydrophilic due to oxidation reaction of silicon by Solution A. The change of surface wettability affects the order of contamination level on wafer because the interactions between particles and substrates are dependent on the wettability of the surface [2]. Figure 2 shows the changes of the contact angle of poly silicon in the alkaline silica based slurry solution as a function of solution A concentration. Complete wetting behavior was observed below 10° in the mixture solutions of the commercial alkaline based fumed oxide slurry and solution A. However, the only KOH based solution was not sufficient to make complete wetting on the hydrophobic poly silicon surface as shown in Fig 2. The polymeric particle contamination on poly silicon surface wafer in the KOH solution as a function of solution A concentration as show in Fig. 3. In case of KOH solution, polymeric particle was contaminated on poly silicon surface. However, if added the solution A in KOH based solution that not generated the contaminated on poly silicon wafer surface. So, the control of poly silicon wettability during polishing could reduce the attraction force of organic particles thus leads to a lower organic defects after CMP. Much more pad particles with water marks were observed at hydrophobic poly silicon surface than hydrophilic. The mechanism of wettability and high adhesion force of hydrophobic surfaces indicates that the controlling of the wettability of wafer surface played an important role in the adhesion and remove force organic residues on the poly silicon surface. The control of poly silicon wettability during polishing could reduce the attraction force of organic particles thus leads to a lower organic defects after CMP.

The Adsorption Behaviors of Citric Acid on Abrasive Particles in Cu CMP Slurry

The interaction between Cu surface and abrasive particles in slurry solution was characterized. The adsorption behavior of the citrate ions was dependent on the pH of the slurry and the concentration of the citric acid. The adsorption of citrate ions generated a highly negative charge on the alumina surface and shifted isoelectric point (IEP) to lower pH values. The Cu removal rate of alumina slurry was higher than that of colloidal silica based slurry in the investigated pH ranges. Although lower friction forces of Cu were observed in alumina based slurry of pH 4, 6 and 8, a higher friction force was observed at pH 2. This high friction force was attributed to the positive zeta potential and greater adhesion force of particle. It indicates that the magnitudes of particle adhesions on Cu surfaces in slurries can be directly related to the frictional behavior during CMP process.

Effect of Corrosion Inhibitor (BTA) in Citric Acid based Slurry on Cu CMP

In this study, the effect of BTA on polishing behavior was investigated as functions of H 2 O 2 , slurry pH and abrasive particles. The addition of BTA effectively prevented Cu from etching by forming the passivation layer of Cu-BTA regardless of pH and H2O2 concentration in slurry. A thinner passivation layer was grown on Cu in BTA added slurry solutions with a higher contact angle of 60°. The dynamic etch rate, the removal rate with abrasive free slurry, also decreased when BTA was added in slurry at pH 2, 4 and 6. The removal rate of Cu was strongly dependent on types of abrasive particles in slurry. The larger hardness of slurry abrasive particles, the higher removal rates of Cu. The reduction of removal rates in BTA added slurry was determined by the competition between chemical dissolution rate and mechanical abrasion rate.

The Adhesion of Pad Particles on Wafer Surfaces during Cu CMP

The adhesion force of pad and alumina were experimentally and theoretically investigated in slurry solutions of different pHs. The isoelectric point (IEP) of pad particles was measured to be around pH 3. The wafer surfaces showed negative zeta potentials in the investigated pH ranges with exception of FSG and Ta. Cu and Ta showed higher interaction forces than dielectric materials. The lowest adhesion force was measured between pad particle and wafer surfaces in a slurry solution of pH 11. The magnitude of adhesion force of pad particles was lower than alumina particles.