Evaluating Cu/Low-k Cleaning Chemicals
Maria A. Lester, Associate Editor -- Semiconductor International, 2/1/2000
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Maria A. Lester,
Associate Editor
With the emergence of copper and low-k material usage, cleaning criteria are changing. So, in addition to the absence of etching and degradation of the interconnection metals and interlayer dielectrics, high removal efficiency of post-etch residual is also needed for via holes. Cleaning chemicals for copper and low-k vias were evaluated by NEC and ULSI Device Development Laboratories. Results were presented at SEMICON Japan.
Solvents such as amine or DMSO (dimethyl sulfoxide) are used for dissolving post-etch residue, and an inhibitor is used to suppress Cu oxidation and Cu corrosion. Results showed the etch rate of copper films depended on the type of amine chemistries. For example, monoethanolamine and alkylamine etched Cu. On the other hand, DMSO did not. Adding an inhibitor such as BTA (benzotriazole) was shown to suppress copper etching.
The response of low-k films varied. Monoethanolamine and ammonium fluoride were shown to etch HSQ (hydrogen silsesquoxane), an inorganic film. Whereas, methlated-HSQ, an inorganic-organic hybrid film, was moderately etched by the same chemicals. Two low-k films, SiLK (silicon polymer) and BCB (benzocyclobutene), stood up to all chemistries used (Figure). •
Researchers Examine Wet Clean Efficiency of Noble MetalsResults of the etching rates for each low-k film tested are shown by
chemicals. (Source: NEC)
The efficiency of wet chemistry has been challenged constantly by the emergence of new technologies and materials. The conventional RCA cleaning recipe currently used is not effective for many new materials. However, wet chemistries continue to prevail in many arenas due to their effectiveness, economic benefits and familiarity. Continued device geometry shrinks make metal contamination-free silicon surfaces essential for high-performance devices. Understanding adsorption/desorption and cleaning mechanisms is needed to achieve metallic impurities below 1 x 109 at/cm2. Researchers at Tohoku University (Sendai, Japan), Samsung Electronics Corp. (Kyunggi-Do, Korea) and Kurita Water Industries Ltd. (Kanagawa, Japan) evaluated wet cleaning efficiencies for copper (Cu), silver (Ag) and gold (Au) noble metals. Based on experimental results, wet chemistries were found to be more appropriate for Cu and Ag, but not for Au.
Metal adsorption mechanisms were based on measurements of (1) metal particle growth on a silicon surface, (2) metal dissolution characteristics in chemical solutions as a function of pH and redox potential values, (3) the effects of metal-induced oxide (MIO) and chemical oxidation and (4) metal-oxide formation enthalpy. Copper adsorption performance was evaluated after electrochemical metal particle deposition and MIO from the cleaning solution. Silver adsorption performance was studied only in terms of the deposition mechanism because the oxide formation enthalpy of Ag was so high that Ag could not be included in MIO or chemical oxide. The film inclusion mechanism and the metal particle deposition mechanism could not be used to interpret Au adsorption behavior. Results indicated these noble metals are fundamentally different, though they are categorized in the same noble metal group.
Based on the metal adsorption mechanism, research showed the major requirements for Cu cleaning were dissolubility, minimum etching of Si and SiO2, and surface passivation to prevent the readhesion of dissolved Cu ions. For Ag, primary was the dissolution process where a cleaning solution having a higher redox potential than that of Ag was shown to effectively remove the metal. Finally, results from the study indicated it was very difficult to remove Au from the Si wafer surface with current wet cleaning technologies. The research suggested a gettering technique could remove Au contaminants unintentionally induced during the clean process.
The researchers concluded wet cleaning processes are effective for removing Cu and Ag from Si surfaces to contamination levels below the 1.0 x 109 atoms/cm2 level. •
