Superconformal Cu electroless deposition is demonstrated in a CuSO4-EDTA-HCHO (where EDTA is ethylenediaminetetraacetic acid) electrolyte containing 2-mercapto-5-benzimidazolesulfonic acid (MBIS). MBIS reveals a concentration-dependent effect in the deposition rate on planar substrates, whereby acceleration at low concentration and suppression at high concentration are evident. The half-cell reaction experiments show that the acceleration effect of MBIS is mainly associated with the cathodic reaction, while MBIS inhibits the oxidation of HCHO in the anodic reaction. The addition of MBIS offers preferential Cu electroless deposition at the bottom of 500 nm wide trenches. Poly(ethylene glycol) improved the surface roughness, maintaining the shape evolution of superconformal feature filling.