1. Effective substrate cleaning method prior to thin film deposition Sami Kinnunen 3.3.2016

2. Substrate cleaning Substrate conditions affect the properties of the deposited thin film Adhesion, grain size… Classes of contamination Particles, metals, organics, native oxide Different techniques Chemical wet, physical wet, and dry cleaning

3. Chemical wet cleaning Acids, bases and solvents Dissolution/decomposition Etching Different chemicals for different contamination Example: RCA clean steps 1.Ammonia/peroxide 2.HF 3.Chloride/peroxide

4. Information Substrate cleaning is about removing unwanted particles from the surface, but also about leaving the surface in known and controlled condition For instance, in RCA clean the ammonia/peroxide step will leave the silicon wafer surface oxidized and hydrophilic whereas HF treatment results in a hydrogen-terminated hydrophobic surface Additionally, etching processes tend to make surfaces rougher, which leads to a larger number of nuclei in deposition, which in turn leads to a smaller grain size

5. Information The RCA steps: The wafers are rinsed in DI water between each step and after the cleaning process. Additionally, a preliminary clean-up treatment with hot H 2 SO 4 -H 2 O mixture (2:1) can be used for wafers having visible residues. StepChemical compositionFunctionTemperatureTime Ammonia/peroxideNH 4 OH:H 2 O 2 :H 2 O (1:1:5) Remove particles and organic contamination70-85 o C10-20 min HFHF:H 2 O (1:100-1000)Remove oxidesRoom30 s Chloride/peroxideHCl:H 2 O 2 :H 2 O Remove metallic/ionic contamination70-85 o C10-20 min

6. References Franssila, S. Introduction to Microfabrication (2nd Edition). 2010. John Wiley & Sons. ISBN: 978-0- 4706-6672-2 Kern, W. The evolution of silicon wafer cleaning technology. 1990. Journal of the Electrochemical Society, Vol. 137:6. P. 1887-1892. ISSN: 0013-4651 Seshan, K. Handbook of Thin-Film Deposition Processes and Techniques (2nd Edition). 2001. William Andrew Publishing. ISBN: 978-0-8155- 1778-8