In order to prevent the above described stripping of the PMMA layer from the wafer, the use of adhesion promoters is strongly recommended. Helpful is also a tempering of the wafers at maximum temperature (if possible > 200°C) prior to the coating step. The cooling step should be performed quickly and ideally in an exsiccator. The commonly used adhesion promoter HMDS (hexamethyldisilazane) exhibits only a marginal adhesive effect in the case of PMMA, while the adhesion features of PMMA are substantially improved if AR 300-80 is used. This adhesion promoter contains an organic silicon compound, is deposited by spin-coating and then tempered at 170°C before the resist is applied.
Adhesion promoter AR 300-80 is not entirely removed from the substrate surface during aqueous-alkaline development procedures. In some cases, in particular for electroplating applications, residual diphenylsilanediol on the surface may interfere with subsequent processes – which can be prevented using a simple modification. The substrate is initially coated with adhesion promoter AR 300-80 and then tempered at a temperature of at least 180°C. After cooling, the substrate is briefly rinsed with acetone, ethanol, AR 600-71 or AR 300-70 and dried. A measurement of contact angles confirmed that the adhesion-promoting hydrophobic properties of the modified surface remain unaltered.
As a result of the tempering at 180°C, AR 300-80 directly reacts with the substrate surface under formation of highly stable Si-O bonds. Excessive adhesion promoter can be removed by rinsing with organic solvents, while a mono-molecular layer covalently bound to the surface will remain. The modified hydrophobic surface which is only attacked by strongly alkaline solutions (2N NaOH) is on the contrary stable in concentrated HCl and hot organic removers. Even subjecting this modified surface to developer AR 300-44, AR 300-35 and AR 300-26 for 30 minutes does not significantly decrease the contact angle – the hydrophobic surface properties remain unchanged.
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Overview Process Chemicals- Adhesion Promoter