HMDS (Hexamethyldisilazane) process recipe

HMDS (hexamethyldisilazane) is a vapor-phase adhesion promoter used ahead of photoresist coating: it chemically converts a hydrophilic, oxidized substrate surface (native/thermal SiO2, quartz, glass, most base metals) into a hydrophobic, resist-wetting surface by bonding a monolayer of trimethylsilyl groups, releasing ammonia as a byproduct.

https://nanyte.com/photoresists/hmds · last updated 2026-09-28

At a glance

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Manufacturer: MicroChemicals GmbH (application-note publisher; HMDS itself is a commodity chemical sold by many suppliers, not a single manufacturer's proprietary product)
Chemistry: Ancillary (not photoimageable)
Exposure dose: Not exposed — a vapour-phase prime

Substrate
Prime
75–120 °C
Coat resist

Substrate
Primer
Resist

The prime bonds a monolayer to the bare substrate and the imaging resist is coated over it — nothing in this step is exposed or developed. Schematic cross-sections for HMDS (Hexamethyldisilazane) — feature width, film aspect ratio and sidewall angle are illustrative, not to scale.

—01 / Coating

Spin coating

The manufacturer does not publish a spin curve for HMDS (Hexamethyldisilazane).

HMDS must not be spin-coated as a liquid — the only correct application is vapor-phase, via a nitrogen-carrier 'bubbler' held at room temperature feeding a heated (75-120°C) substrate, where HMDS bonds chemically as a monolayer ("Correct Use of HMDS", Fig. 47-48, p.2).
If liquid HMDS is spun on instead, the resulting layer is only physically (not chemically) bound; ammonia released from it during the resist softbake diffuses into and cross-links the resist near the substrate, suppressing development and degrading resolution and profile ("Incorrect Application of HMDS", Fig. 49, p.2-3).
For the same reason, the note advises against applying HMDS in the same spin coater used for resist coating (slow HMDS evaporation can contaminate later coating runs); if spin-application is unavoidable, it recommends a 100-120°C water-desorption bake beforehand, a 100-120°C thermal-activation bake afterward, and strict spatial separation from resist-coating equipment (p.3).
There is no spin-speed-vs-thickness relationship for a vapor-deposited chemisorbed monolayer, so HMDS has no spin curve to publish.

Adhesion: HMDS is itself the adhesion promoter: it is applied to the bare substrate before the imaging resist is coated.

—02 / Bake

Prime

Prime: 75–120 °C
Notes: Vapor-phase application: HMDS vapor is carried by dry nitrogen from a room-temperature bubbler onto a substrate held at 75-120°C, where it bonds chemically as a monolayer. That 75-120°C is the substrate temperature during the prime itself, not a bake of a coated film — HMDS goes on bare substrate, before any resist. No dwell time is published. If HMDS can only be spin-coated — documented, but explicitly the wrong way to apply it — bake at 100-120°C beforehand to desorb water, then again at 100-120°C to activate after application. Independently of which adhesion promoter is used, a substrate dehydration bake of roughly 120°C for a few minutes (above 140°C on oxidized surfaces, for maximum adhesion) precedes the prime.

SOURCE: Correct Use of HMDS / Incorrect Application of HMDS, p.2-3; Adsorbed Water, p.1

—03 / Exposure

Exposure

HMDS (Hexamethyldisilazane) is not photoimageable. It is a vapour-phase prime applied to the bare substrate before the imaging resist is coated; nothing in this step is exposed or developed.

—04 / Development

Development

Not published for this resist: Developer, Dilution, Time, Method, Rinse — characterize on-tool.

—05 / Post-processing

Hard bake, etch & strip

Not published for this resist: Hard bake, Descum, Etch resistance, Stripper, Storage — characterize on-tool.

—06 / Applications

Where it's used

Practical notes from the datasheet

HMDS (hexamethyldisilazane) is a vapor-phase adhesion promoter, not a photoimageable resist: it converts a hydrophilic, OH-terminated oxide surface (native or thermal SiO2, quartz, glass, most base metals) into a hydrophobic, resist-wetting surface by chemically bonding a monolayer of non-polar trimethylsilyl groups, releasing ammonia as a byproduct (p.2). The only correct application is vapor-phase, via a nitrogen-carrier bubbler at room temperature feeding a substrate heated to 75-120°C; spin-coating liquid HMDS instead yields only a physically bound layer whose ammonia, released during the subsequent resist softbake, diffuses into and cross-links the resist near the substrate, suppressing development and degrading resolution — a failure mode this note documents explicitly as 'Incorrect Application' (p.2-3). On noble metals without native oxide (gold, platinum), HMDS and other organic adhesion promoters show little to no adhesion benefit because they cannot chemically bond to the surface; base metals such as Al, Cr and especially Ti already adhere well without a promoter (p.3-4). The process here follows a general MicroChemicals application-note chapter ('01 Chapter — Basics of Microstructuring: Substrate Preparation') rather than a single manufacturer's technical datasheet for a specific HMDS product or grade — HMDS itself is a commodity chemical sold by many suppliers.

—07 / Troubleshooting

Troubleshooting

Common failure modes for HMDS (Hexamethyldisilazane), answered from the manufacturer's datasheet and application notes. These are starting points — your substrate, tooling and environment shift the specifics, so calibrate on-tool.

After HMDS priming the resist won't clear to the substrate and leaves scum in the openings. Is the HMDS to blame?

It can be, if it was spun on as a liquid. Spin-coated HMDS leaves a comparatively thick, only physically bound film. During the resist softbake that excess releases ammonia, which diffuses into the resist near the substrate and cross-links it, so the development rate drops and through-development can become impossible. The cure is vapour priming: HMDS carried by dry nitrogen from a room-temperature bubbler onto a substrate held at 75-120 °C, where it bonds chemically as a monolayer.