Some common semiconductor substrate surfaces, most notably Si, SiO2, SiN, and some other oxide forming layers, are naturally hydrophilic due to absorbed and surface layer water from ambient humidity. Unless it is removed prior to coating photoresist, this water layer will allow developer and etch chemicals to penetrate the resist/substrate interface and cause extreme etch undercutting or complete delamination of the photoresist film. To ensure adhesion during develop and wet etch, these hydrophilic surfaces must be converted to a hydrophobic state. Applying HMDS (hexamethyldisilazane) to dehydrated wafers is one common method for achieving the surface hydrophobicity required to prevent photoresist delamination.

To prime wafers with HMDS, the surface must first be completely dehydrated. Sufficient desorption of water can usually be achieved by baking the substrate at 140 to 160C (this “dehydration bake” is often performed in a vacuum). Dehydrating leaves surface OH groups hence the surface is still hydrophilic.

 

Next, the surface is exposed to vapor phase HMDS; usually in a priming oven or a single wafer chamber. HMDS is sometimes applied in liquid phase and baked at 110 to 120C but results are less predictable as achieving the optimum surface HMDS mono-layer is difficult.

 

The surface hydrophobicity can be measured (and monitored) by placing a drop of water on the primed surface and measuring the contact angle as shown. An optical goniometer (often attached to an optical microscope) is used to measure this angle with reasonable precision and repeatability.

Optimum water contact angle is typically between 45 and 70 degrees for a wet etch process. It is important to remember however that surfaces may be “over-primed”. If the surface becomes excessively hydrophobic, the photoresist will not uniformly wet the wafer and bubbles or voids in the resist film may result. Often these voids are trapped at the resist/substrate interface and will occasionally “pop” during the soft bake step causing surface contamination and/or missing pattern defects. A defect signature commonly referred to “mouse bites” can be a symptom of over-priming.

 

 

 

REFERENCES

Ralph Dammel, “Diazonaphthoquinone Based Resists”                                                            SPIE Optical Engineering Press, 1993, volume TT 11, 99-100

 

I would like more information or a quotation for HMDS