Utrasonicate the glass slides by immersing in a mixture of pure water and 1% micro-90 soap (available from Cole Parmer) at 60 deg for an hour, then rinse under running tap for minutes. Finally rinse with stream of ethanol and thoroughly dry with a clean air/ nitrogen gun in a dust free environment.

I would add cleaning in acetone for 15 min before the cleaning in ethanol/isopropanol. After the cleaning in acetone, rinse the substrate with deionized water. At the end, dry it with nitrogen gun as it has already been suggested.

Thanks Suresha and Nevena., I will follow those steps.

Use strong basic solution was also an option, at lease from me.

Piranha solution.. but be careful.