Supercritical Carbon Dioxide Processing of Silicon Wafer Surfaces

Abstract

RC (resistance x capacitance) time constant delay, cross-talk noise, and power dissipation of the interconnect structure become limiting factors for the performance of integrated circuits (IC) as device feature sizes continue to scale down. To address these problems, copper has replaced aluminum for interconnects in leading edge microelectronic devices to satisfy the demand for better performance. Low-k and porous low-k dielectrics are introduced to further reduce the RC delay of interconnects and improve signal transmission. The conventional wet and dry processing approaches, however, face problems with highly porous structures and changes in dielectric constant k due to absorption of chemicals. Supercritical CO₂ (scCO₂) is especially useful for processing porous low-k films since it has solvating properties that are comparable to liquids but mass transfer characteristics comparable to gases and no surface tension. In this work, the removal of copper from silicon surface as well as viable processes for drying, repair and capping of porous methylsilsesquioxane (p-MSQ) films using precursors dissolved in scCO₂ were demonstrated. Copper was etched from a silicon surface using the chelator hexafluoroacetylacetone (hfacH) dissolved in scCO₂ at 40-60°C and 100-250 atm. The Cu(II) shells were removed selectively to the Cu(I)₂O cores by processing with pure scCO₂ and rapidly releasing the system pressure (300 atm/min). Mechanical failure of the Cu(II)O and Cu(II)Cl₂ when CO₂ in stress corrosion cracks quickly expanded delaminated these layers, leaving only Cu(I)₂O on the surface. Etching of both Cu(II) and Cu(I) was achieved when oxidized samples were processed in scCO₂ containing approximately 120 ppm of hfacH for 2 min. The effect of adding 5-7 vol% cosolvents and 0.5-1 vol% Si-bearing precursors to scCO₂ to dry, repair, and cap blanket ashed p-MSQ films (JSR LKD5109) at 160-300 atm and 45-60°C for a 2 min soak was investigated. The drying experimental results showed that all the aliphatic C1-C6 alcohols and acetic acid removed H-bonded silanol (SiO-H) groups but that n-propanol and n-butanol were the most effective and had the lowest vapor pressure at 25°C of the cosolvents studied. Repair and capping results showed that methylsilyl (–O-Si-CH₃) moieties were deposited on the surface by reaction with both isolated/geminal silanol (SiO-H) and H-bonded silanol (SiO-H) groups. As-received ashed p-MSQ had a contact angle of less than 10° and a dielectric constant of 3.5 ± 0.1. After processing in a mixture containing 7% n-propanol and scCO₂, the contact angle was 15° and the dielectric constant decreased to 3.2 ± 0.1. The hydrophobicity of the p-MSQ film was recovered after Si-bearing precursors treatments as shown by contact angles >80°. The dielectric constant of ashed p-MSQ was completely or partially restored after treatments. The bi- or tri-functionality of the molecules with more reactive head groups produced intermolecular linking, and Ti chemical vapor deposition (CVD) showed that the pores of MSQ were capped after bi- or tri-functionality of the molecule processes.