Spin coating your first wafer

1 · Dispense the resist

Coating starts by putting liquid resist onto the substrate, and there are two common ways to do it. A static dispense puddles resist at the center of a stationary (or barely turning) wafer before the spin begins; it is simple and works well for most resists and film thicknesses. A dynamic dispense instead drops the resist while the wafer is already turning at a low speed, spreading it outward as it lands — this uses less resist and can help avoid trapped air bubbles or streaking with some higher-viscosity or fast-drying resists.

Either way, the goal is the same: get enough resist onto the substrate, roughly centered, to fully cover it once the spin spreads it out. Too little and the film can starve at the edges before it's fully coated; too much just spins off as waste. Which dispense style — and roughly how much resist — suits a given resist and wafer size is a process detail on the recipe page, not something to guess at the chuck.

2 · Spread, then spin to final speed

The spin itself is usually a two-stage ramp, not one constant speed. A brief low-speed spread stage first pushes the dispensed puddle out to cover the whole substrate evenly, before the chuck ramps up to the final spin speed that sets the film's thickness. Skipping straight to the final speed can leave the film uneven or incompletely covered near the edges, especially on a larger wafer or a more viscous resist.

Final film thickness falls as spin speed rises, and also depends on the resist's own viscosity and solids content — a more viscous resist coats thicker at the same speed than a thinner one. That relationship is exactly what a resist's spin curve plots, and it's published on every recipe page. Read your target thickness off an actually-plotted point on that curve rather than a remembered number or an interpolation between two — the coating section of the guidelines explains why the curve, not a rule of thumb, is the source of truth.

Sources: Meyerhofer (1978) — resist film spinning; MicroChemicals — Spin-Coating of Photoresists

3 · Remove the edge bead

Spin coating never produces a perfectly flat film: surface tension always piles up a thicker rim of resist at the substrate's outer edge, called the edge bead — it can be many times the thickness of the rest of the film. Left alone, it dries slowly, is prone to cracking or flaking during handling or bake, and can shed particles that contaminate later steps or a downstream tool.

Edge-bead removal (EBR) strips that rim, typically with a ring of solvent dispensed at the wafer's perimeter right after coating, or an edge-only exposure and develop cycle on tools set up for it. It is done immediately, before the film has had time to set, and the details — solvent choice, exposure ring width, timing — are process-specific rather than universal.

Sources: MicroChemicals — Spin-Coating of Photoresists

4 · Soft-bake the film

Straight off the spinner, the film still holds a meaningful amount of the solvent it was dissolved in for coating. The soft bake (also called a prebake) drives that solvent out on a hotplate so the film is dry, dimensionally stable and properly adherent before it ever sees light. It is a balance rather than a maximum to chase: underbaking leaves residual solvent that shows up later as scumming or poor adhesion, while overbaking can degrade the resist's photoactive chemistry and blunt its sensitivity.

Because the right bake scales with how thick the film is, every recipe specifies its own temperature and time — see the soft-bake section of the guidelines for the reasoning, and the glossary entry for the short definition. Don't reuse a bake recipe from a different resist or a different thickness.

5 · Inspect for spin defects

Before moving on to exposure, look at the coated film — under a microscope if you can, but a raking light across the surface catches most of these under the naked eye too. A good coat is uniform, glossy and free of visible texture. Three defects show up often enough on a first coat that it's worth knowing their names and causes.

• —[Striations](/photoresist-glossary#striations) — faint radial ripples across the film, usually from resist that began drying (skinning) before the spin fully spread and leveled it. Dispensing more resist, dispensing faster, or checking the resist hasn't partly dried in its bottle can help.
• —[Comets](/photoresist-glossary#comets) — a trailing streak fanning out from a single point, almost always a particle or gel lump caught under the resist during the spin, dragging a wake behind it as the film spreads outward.
• —[Scumming](/photoresist-glossary#scumming) — a hazy or uneven film, rather than a glossy one, right off the spinner most often means an underbaked or contaminated resist rather than a spin problem at all — it is usually the soft bake, not the spin, worth re-checking first.

None of these are unusual on a first attempt, and each one points back at a specific upstream cause rather than being a mystery. A resist filtered before dispensing (many datasheets specify a filter pore size) rules out most comets, and a resist that hasn't been sitting open or past its shelf life rules out most striations. If the film looks clean, it's ready for the next step — exposure, covered in your first pattern on a maskless tool.

Sources: MicroChemicals — Spin-Coating of Photoresists

Common questions

Where to next

1. Dietrich Meyerhofer. Characteristics of resist films produced by spinning. Journal of Applied Physics (1978). doi:10.1063/1.325357
2. MicroChemicals GmbH. Spin-Coating of Photoresists (application note). https://www.microchemicals.com/dokumente/application_notes/spin_coating_photoresist.pdf