Effective substrate cleaning before deposition Paula Päivike 03.03.2016

Cleaning is necessary! Contaminants cause: defects, delamination, inhibition of film growth… mage from http://www.si2.org/

Cleaning is necessary! HF-solution RCA Standard clean Solvents Contaminants cause: defects, delamination, inhibition of film growth… Wet methods HF-solution RCA Standard clean Solvents Ultra/megasonic baths Mechanical scrubbing Spray cleaning Dry methods Plasma UV-O3 High-velocity air jets Cryogenic aerosol Thermal

Cleaning Si wafers, original RCA SC-1 (75-85 °C) NH4OH : H2O2 : H2O DI water cool/rinse SC-2 (75-85 °C) HCl : H2O2 : H2O Spin dry

Cleaning Si wafers, modified RCA Diluting SC-1 and SC-2 Replacing SC-2 with very dilute room temperature HCl IPA vapour drying HF-last

Information slides

Sources of contaminants Common sources Machining oils Hydraulic & cleaning fluids Adhesives Waxes Human contamination Particles Sources during cleaning process DI water Cleaning jigs Air Cleaning chemicals Plasma etc... (ref. Hattori et al. on slide 9)

Cleaning Si wafers, original RCA Wafers immersed in hot (75-85 °C) cleaning solutions for 10-20 min Rinsing/cooling with overflow running DI water after both treatments SC-1 (organic contaminants, metal particles) NH4OH : H2O2 : H2O 1:1:5 to 1:2:7 SC-2 (alkali residues, residual trace metals, metal hydroxides) HCl : H2O2 : H2O 1:1:6 to 1:2:8 Exact compositions not important Spin dry to avoid water residues and inert gas storage (if not used immediately)

Cleaning Si wafers, original RCA Uses large amounts of chemicals & DI water! Ammonium hydroxide 29% by weight of NH3 30 wt% H2O2 37 wt% HCl SC-1 dissolves native oxide, which is generated back at roughly same rate - > particles removed SC-2 removes Au and Ag residues through oxidative desorption Kern: Handbook of Silicon Wafer Cleaning Technology, 2nd ed. 1993 Hattori et al.: Ultraclean surface processing of silicon wafers: secrets of VLSI manufacturing, 2013

Example: Alternative process by prof. T. Ohmi Ohmi, T. Total Room Temperature Wet Cleaning for Si Substrate Surface J. Electrochem. Soc., Vol. 143, No.9, Sept. 1996

Cleaning Si wafers, modified RCA Hot SC-2 needed to remove residual Au & Ag (ref. Kern) These are no more present in high purity process chemicals -> SC-2 can be substituted with very dilute room temperature HCl IPA vapour drying (ref. Kern) Water is displaced and no residues left when isopropyl alcohol evaporates HF-last Produces an H-terminated Si-surface => no native oxide Must be used immediately because it is easily oxidised/contaminated Concentrated HF results in more residual fluorine on the surface -> Low concentration -> cleaner surface Takahagi et al. The formation of hydrogen passivated silicon single-crystal surfaces using ultraviolet cleaning and HF etching, Journal of Applied Physics 64, 3516 (1988)