Chemical Reaction

The three potential active species were HF, HF₂^-is and F^- . In this solution, the

dominant species is F^-. F^- reacts with polymer residue on the wafer and reformed.

(TiF₆)^2- is water soluble, so during the water rinse step polymer residue can be removed (Equation 4-7). Meanwhile, the oxide-like polymer residues react with HF and form a water soluble compound H₂[SiF₆] . The water rinse step then removes the soluble compound (Equation 4-8). HF2

is barely involved in the reaction with EKC6800 pH, hence, SiO2 like material loss is minimumal.

NH4F  NH4+

Figure 4.7 Schematic of HF, HF2

and F^- concentration relatives to pH [2].

80 concentration of Titanium (Ti). For Ti rich particles, hydrogen peroxide (H₂O₂) is the only chemical (other than hydroxylamine or fluoride) identified to date that will effectively remove post-etch residues with a high titanium content without damaging the exposed metal or dielectric. The mechanism of dissolution of the Ti-rich residues is not well understood, however, it may proceed according to one of the oxidation reaction above (Equation 4-11) & (Equation 4-12). In chemical reaction (Equation 4-11), a water-soluble-complex [Ti(OH)3O2]^- formed, meanwhile in reaction (Equation 4-12), Ti³⁺ could be Ti(F)₃ or TiN or Ti₂O₃.

As for other types of Si-rich particles, water soluble [SiO2(OH2)]^2- complex formed.

For carbon rich particles or polymers, OH^- and HO2

react with alkyl compound and formed RCOO^- (Equation 4-12). Both [SiO₂(OH₂)]^2- and RCOO^- complex can be removed by the next water rinse step.

Figure 4.8 Cleaning Mechanism of Rezi38.

REZI-38 contains one or more chelating or complexing agents. These agents can sequestrate the dissolved metallic residue to prevents it’s re-deposition on the wafer.

Also, there were one or more metal corrosion inhibitors in REZI-38 that can help prevents excessive attack of the metal by the hydroxide ion.

Hinges Table 4.1 and Graph 4.1 verify that EKC6800 has a better organic residue removal ability than REZI-38 for the single wafer tool chemical category. Meanwhile, there is no difference for Ti rich big sheet particle removal rate.

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4.2.1.3 EKC265

As for EKC265[5] , another semi-aqueous solution from EKC Technology, Inc, Hydroxylamine served as a reducing agent. Metal ions are reduced into lower oxidation states and formed a more stable soluble complex. The attack mechanism is a two step reaction starting with the formation of hydroxyl ions, when the amine component in the stripper is hydrolyzed with water (Equation 4-15).

RNH₂ + H₂O  RNH₃⁺ + OH^- (Equation 4-15) Amine plays several assisting roles in EKC265. First, it can act as a solvent, extremely effective at removing organic materials. Also, amine assists in keeping the soluble residues in solution so that they can be carried away from the water surface. It also creates hydroxide ions, OH^-: these species aid in the dissolution of post-etch residues. HDA removes resist by a process of penetrating, swelling, and reducing Van Der Waal forces. The solvent molecules solvate the polymer molecule and overcome the attractive forces that hold the polymer together.

After the crown capacitor etching, we believe that some metal oxides, e.g. TiO₂, TiO would be formed and left behind. Other stable metal halides, such as TiF3 also remain on the wafer surface. These salts and oxides are insoluble in water, dilute acids, or bases, but they are removed in HDA solutions. Reduction of these metallic species and subsequent formations of chelating complexes play a role in the removal of metal oxides residues. Based on the oxidation/reduction potentials, the metallic species that can be reduced by hydroxylamine are listed in Table 4.5. [8]

The combination of HDA and an organic amine form a strong reducing and

complexing (ligating) solution. The insoluble metal oxide could be reduced to a lower oxidation state and subsequently chelated with the ligand to form a more soluble metal complex which could ultimately end up in the solution. The proposed mechanism of reduction, chelation, and stabilization results in removal of a number of etching residues without attacking the pure metal surfaces.

We postulate the good performance of EKC265 is a result of HDA reaction. In addition, Titanium corrosion is averted due to the basic solution. Hence, EKC265 is chosen as the cleaning treatment chemical for crown wet capacitor post clean.

Table 4.5 Metallic Reduction By Hydroxylamine.

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4.2.1.4 ELM C30

ELM C30[6] consists of 69% of N-N Dimethylformamide (DMF) and 1% of Fluoro-compound. DMF acts as the solvent to organic compounds; meanwhile, fluoride functions as oxygen-silicon, oxygen-metal, carbon-silicon and carbon-halogen bond attacks. The reaction for ELM C30 fluoride-based chemical is basically the same with EKC6800 (Equation 4-4 & Equation 4-5).

By figure 4.9, metal oxides and organic particles react with F^- and HF2

at a pH around 3 to 4 (figure 4.6), the compound formed is dissolved in DMF. The water rinse step is required to rinse away the water soluble complex compound.

Like the results for EKC6800, ELM C30 succeeds in removing most of the organic like residues, but the removal of big sheet particles and small particles is not significant enough.

Figure 4.9 Schematic of ELM C30 reaction of removing fluoro compound and organic carboxylate.

4.2.1.5 EcoPeeler

Ammonium phosphate (NH3H2PO) is the major reactant in EcoPeeler(7). It plays the role of removing metal polymer. Figure 4.10 shows that by controlling NH3, pH value can be controlled. The polymer removal ability of ammonium phosphate is different under different pH values. In Figure 4.11, the cleaning mechanism of EcoPeeler is explained. Ammonium Phosphate breaks the interface between metal and polymer then the polymer reacts with condensed ammonium phosphate and becomes water soluble.

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Figure 4.10 Ammonium phosphate preparation.

Figure 4.11 Cleaning mechanism of EcoPeeler.

4.2.1.6 Summary for Chemical Reaction

Single wafer tool chemicals have poor performance in removing organic type polymer residues; meanwhile batch type tool chemicals EKC 265, ELM C30, and EcoPeeler are rather good in terms of dissolving the organic residues. However, from defect result shown in Figure 4.1, EKC265 has the best removal rate for Titanium rich particles than the others. These kind of particles are more easily removed by a HDA based chemical. As shown in the Pourbaix diagram (Figure 4.11), Ti(OH)3 is easily formed in solutions with high pH.

In conclusion, particle defects removal for the crown capacitor process is more easily achieved in an alkali solution than in a neutral or acid solution. EKC265 is the best particles for the cleaning treatment.

From the result of section 4.1.2, a longer EKC265 treatment time implies a lower particle defect count, but a higher penetration and collapse count. This phenomenon can be infered from the etch rate of Titanium and Titanium oxide in HDA based chemistry. Longer treatment time brings a higher Titanium and Titanium oxide etch rate. Consequently, the capacitor cylinder built from Titanium risks cracking and collapsing. Depending on the bottom titanium cylinder film thickness, EKC265

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treatment time can be adjusted.

On the other hand, IPA treatment time is completely irrelevant to the result of the experiment. We conclude that the IPA treatment time is not a factor that will impact particle defect removal ability.

Figure 4.12 Pourbaix diagram [9] for Titanium in pure water, perchloric acid or sodium hydroxide.