Modification of DHF for Removal of Metals from Silicon Wafer
Authors: Lee Dong-Hwan , Song Hee-Jin , Jang Sung-Hae , Kim Hyun-Tae , Yi Jae-Hwan , Choi Eun-Suck , Park Jin-Goo 
Published: February 2020

With continued advances in logic devices below 10 nm design rule, the required specification for ultraclean wafer surface becomes very challenging to achieve zero defects in particles below 20 nm and lower than 0.5X1010 atoms/cm2 metal ions on the final substrate surface. Excluding crystal originated defects such as pits, stacking faults and dislocation, an ultraclean wafer surface is characterized as a surface which is particle, organic, metallic contamination, and surface roughness free.

Traditional batch type RCA based wet chemical processing has been used as a major cleaning method in silicon wafer manufacturing. The biggest problem of this RCA based wet cleaning method is its side effect; a target contaminant is being removed, whereas another contaminant gets redeposited on the substrate surface. For example, APM (Ammonia Peroxide Mixture) cleaning is extremely effective in removing particle and organic contaminant, but it allows metallic contaminant to redeposit on substrate surface since the APM solution contains traces of metal ions. HPM (Hydrochloric Acid Peroxide Mixture) cleaning can remove the metal contaminant. However, it causes particle contaminant to redeposit on substrate surface because of the electrostatic attraction. The zeta potential of particles is almost positive, whereas the zeta potential of Si surface is negative in acidic solution. Since the level of a contaminant in particles and metallic concentration on the surface requires an extreme control, these side effects in conventional RCA chemistry could not be tolerated in 10 nm below device substrates. In this study, the possible chemistry modification was attempted to control both particle and metallic contaminants in DHF chemistry at final silicon wafer cleaning step.

We have optimized the metal contamination procedure for Al and Cu with their analysis method with AFM by depositing nano metal particles on a silicon wafer. Al was chemically deposited on Si, whereas Cu was electrochemically deposited. Various additives such as ozone and chelating agents were chosen to add in DHF solutions to control metallic and particulate contaminants. The metal contaminants were removed by Si etching in HF and ozone mixture. The etching rate of Si increased with ozone and HF concentrations increasing. The chelating agent and HF mixture solution did not etch the silicon but oxide. The removal rates of silicon and oxide were measured in HF-based chemicals.

The metal removal efficiency (MRE) has measured in various HF based solutions for Al and Cu. HF and ozone mixture was very effective in removing both metallic contaminants, but it has a certain limitation in controlling particles on silicon. The chelating agents were selected to control both metal and particles. CA 1 (chelating agent 1) and CA2 have chosen for experiments. The concentration was optimized to increase the contaminant removal efficiency. MRE higher than 95% was achieved by adding chelating agents in DHF solutions without causing a particle redeposition.