> But SiO2 surface seems hydrophobic

This is strange. SiO2 is inherently hydrophilic, at least much more than bare-Si. Piranha clean should leave the surface more hydrophilic, not phobic. Once you piranha clean, there is no need to do Acetone/IPA, as piranha is a much better organic contaminant remover. You should typically do a quick HF dip after piranha to remove any redeposited contamination or unwanted native oxidation (however in your case the sample is already oxidized).

You can try spincoating HMDS on the SiO2 samples to improve adhesion. I am not sure however if it will react with your BN solution.

Dear Hasan, tanuj is right, sio2 is already a hydrophilic surafce.

During the cleaning if you did the pirahana and then accetone and ipa, so after pirahana there is no need of ol of this. Yes, you can do organic cleaning by using trylene, accetone and Ipa before the pirahana cleaning.

You just clean your si wafer by using organic and inorganic cleaning and after this dip the wafer into HF solution with water concentration, then surface will be hydrophilic.

Piranha is the strongest oxydation agent which makes the surface hydrophillic. BUT: After you dip the sample into IPA (Propanol) the surface becomes more hydrophobic.

So you have 2 possible solutions: 

1. Don't use IPA or aceton after Piranha (guys above are totally right) or

2. Apply the oxygen plasma treatment after IPA and aceton. Advantage is that you will clean your surface (again), however Piranha was the best. But more importantly, you can make it easily hydrophillic again. Just 10s in the plasma cleaner and you're done.

Good luck!

To make surface more hydrophilic which way is better, I mean piranha treatment or plasma treatment?

thank you

See this on line publication: Hydrophobicity-Hydrophilicity and Silane Surface modification https://www.gelest.com/themencode-pdf-viewer/?file=https://www.gelest.com/wp-content/uploads/Hydrophobicity-Hydrophilicity_and_Silane_Surface_Modification.pdf

Dear Barry, thanks for sharing the publication on surface modification toward Hydrophobicity-Hydrophilicity. I really appreciate your help.

Regards,

nowzesh

It depend which kind of silica , fumed silica or precipitated silica, anyway, originally silica is hydrophilic material and by sillation it become hydrophobic material, so by calcining at temperature about 500-600 C , you can remove sillated group , (usually Si-(CH3))