How HMDS Works
On water-free surfaces, HMDS chemically bonds its Si atom to the oxygen of oxidized surfaces, accompanied by the release of ammonia (NH3). The methyl groups of the HMDS fragment thereby form a hydrophobic surface and thus improve resist wetting and adhesion.
The figure to the right illustrates the mechanism of HMDS bonding (atom colours: C = black, N= blue, O = red, Si = grey, H =white, the arbitrary substratematerial = glaucous).
How to Apply HMDS
The correct application of HMDS is very important in order not to further deteriorate the resist adhesion:
In a so-called bubbler, waterfree nitrogen saturates at room temperature with HMDS vapour. The N2 + HMDS streams onto the heated (75 - 120°C), waterfree substrate thereby forming a thin layer of chemically bonded Si(CH3)3 groups responsible for the desired hydrophobic characteristics.
How NOT to Apply HMDS
In the case of spin-coating the HMDS, a thick (= more than a monolayer) HMDS film forms on the surface without displacing water and without chemically bonding.
After resist coating during the softbake, this excess of HMDS releases ammonia which diffuses into the resist and crosslinks the resin near the substrate. As a consequence, through-development sometimes becomes impossible.
For the same reason, NEVER apply HMDS in a spin coater. HMDS vapour will diffuse into all resist films subsequently coated, and partially crosslink the resist film during softbake, which lowers the development rate and can deteriorate the resist profile and attainable resolution.
We supply HMDS in VLSI- quality in 1 L bottles.
Lithography Application Notes
Our list of application notes with litho-related theoretical and technical background on all stages of micro-structuring can be downloaded from here: application notes.