What is the name of adhesion promoter used in case of positive photo resist ?
Hexa Methyl Di Silane
The compound is HMDS — hexamethyldisilazane — and among the four spellings offered only option 1, "Hexa Methyl Di Silane", carries the correct methyl root. The other three substitute "methane" or corrupt the silicon part of the name, so the answer is option 1.
Why an adhesion promoter is needed at all. A silicon wafer's surface is hydrophilic: the native oxide terminates in silanol (Si-OH) groups that readily hold a molecular film of adsorbed water. Photoresist, by contrast, is an organic, hydrophobic polymer. Spinning resist onto a damp hydrophilic surface gives poor wetting and weak bonding, so the pattern lifts or undercuts during development and wet etching — the defect known as resist lifting.
How HMDS fixes it. Applied as a vapour before the resist, it reacts with the surface hydroxyls:
Si-OH + HMDS → Si-O-Si(CH3)3 + NH3
The silanol groups are replaced by trimethylsilyl groups, whose methyl tails point outward. The surface is converted from hydrophilic to hydrophobic and now bonds strongly to the organic resist. The ammonia by-product is carried away, and the treatment is applied in a heated vapour-prime oven rather than by spinning, so the layer is only a molecule or two thick.
| Before HMDS | After HMDS |
|---|---|
| Si-OH terminated | Si-O-Si(CH3)3 terminated |
| Hydrophilic, holds water | Hydrophobic, repels water |
| Poor resist adhesion | Strong resist adhesion |
Where it sits in the process flow : wafer clean → dehydration bake → HMDS prime → resist spin → soft bake → align and expose → develop → hard bake → etch → strip. The dehydration bake immediately before priming matters, since HMDS cannot displace bulk water — only the chemisorbed hydroxyl layer.
Positive resist in particular is more vulnerable because its developer is an aqueous alkaline solution (typically TMAH), which attacks a poorly bonded interface aggressively. The priming step is therefore all but mandatory for positive-resist processes.
Hence, the adhesion promoter is HMDS, option 1.
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