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Dispense
A few drops of solution land on the substrate, still or turning slowly. Thick liquid shows up gray, with color only at its thin edge.
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What is spin coating?
Spin coating is a way to deposit a uniform thin film on a flat substrate by spinning it at high speed.
A small amount of coating material goes on the center of the substrate, either still or turning slowly. The substrate then spins fast, centrifugal force spreads the material outward, and the excess flies off the edge. The machine that does this is a spin coater.
- Main control
- Spin speed
- Typical recipe
- 2 to 5 steps, under a minute
- Rule of thumb
- 4× the speed, ½ the thickness
Watch a film form.
A spin coated film goes from a puddle to a solid layer in about eight seconds. The colors are real: thin films reflect different colors depending on their thickness. Scroll to run it.
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Spread
A slow first step pushes the liquid out to the edge, so the whole surface is wet before thinning starts.
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Spin off
At full speed most of the liquid leaves over the edge. The film thins everywhere at once.
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Evaporate
Solvent leaves and the film sets. Its final thickness depends mostly on spin speed.
Before you coat your first wafer.
Most coating problems start before the spin coater does. Check these first.
Control your environment
Temperature, humidity, and airborne particles all change how a film forms. Coat in the same conditions every time.
Use fresh chemistry
Old or contaminated solutions coat unevenly. Check dates and filter if needed.
Start clean
A contaminated substrate ruins the process before it begins. Clean the substrate, the chuck, and the bowl.
Center the substrate
An accurately centered substrate coats consistently. Center irregular shapes by eye.
For squares and rectangles, a recessed chuck centers the substrate for you and reduces buildup at the edges.
Spin coating, step by step.
There are two ways to start: dispense onto a still substrate, or onto one that's already turning. Pick one to walk through it.
The simplest way to begin, and where most people start. You dispense onto a stationary substrate.
A more advanced method. Dispensing onto a turning substrate can give more consistent coatings and uses less chemistry, which also means less to clean.
With the substrate still, dispense a smooth stream onto the center through the port in the lid, using a pipette or syringe. As a rule, dispense until the puddle covers about half the substrate's diameter.
As soon as you've dispensed the right amount, start the program. Each step of a Laurell recipe sets its own speed and acceleration.
An intermediate step at moderate speed lets the liquid spread smoothly across the whole surface.
As the liquid reaches the edge, a further acceleration throws the excess off. The process bowl keeps it from splashing back onto the substrate.
Accelerate quickly to the final speed, the one that gives your target thickness.
At final speed, solvent evaporates and the film sets. Adjust the exhaust to control how much solvent vapor stays in the chamber, which changes how fast the film dries.
Spin coating can leave a ridge of extra material at the edge of the substrate, called edge bead.
On round wafers, remove edge bead in the spin coater right after coating. Once the film has set, aim a stream of solvent at the edge while spinning slowly, then keep spinning so the remaining solvent dries.
With the edge bead gone, the coating is complete.
Start the spin coater before you dispense, at around 500 rpm.
Dispense a smooth stream onto the center of the turning substrate. It spreads evenly as it lands.
Stop dispensing as the puddle reaches the edge. You'll get the best results if the program moves to the next step as soon as the dispense is done.
Accelerate to throw off the excess. From here, the process is the same as static dispense.
Accelerate quickly to the final speed, the one that gives your target thickness.
At final speed, solvent evaporates and the film sets. Exhaust controls how fast it dries.
Aim solvent at the edge while spinning slowly, then keep spinning to dry.
The coating is complete, usually with less chemistry used than a static dispense.
Dispense
What sets film thickness.
Spin speed is the main lever. Thickness falls with the square root of speed, so spinning four times faster gives a film about half as thick. Try it.
462 nmExpected film, with its color on silicon
Illustrative curve for one solution. To find yours, coat test wafers at a few speeds and measure them.
Other things that change thickness
Solution concentration and viscosity
More solids or a thicker liquid leaves a thicker film at the same speed.
Solvent evaporation
A solvent that evaporates faster sets the film sooner, leaving it thicker. Exhaust and solvent vapor in the chamber change this too.
Spin time
Most thinning happens in the first seconds at speed. After that, extra time mainly finishes drying.
Acceleration
Affects how evenly the film forms, especially over patterned surfaces, more than its final thickness.
Troubleshooting common defects.
Most defects point back to one of a few causes.
| Defect | What you see | Usual cause | What to try |
|---|---|---|---|
| Comets | Streaks fanning out from a point | A particle on the substrate or in the solution | Clean the substrate, filter the solution, coat in cleaner air |
| Striations | Faint radial ridges | Solvent evaporating too fast | Reduce exhaust, keep more solvent vapor in the chamber, or use a slower-drying solvent |
| Edge bead | A thick ring at the edge | Surface tension holding liquid at the edge | Add an edge bead removal step; use a recessed chuck for square substrates |
| Bare patches | Areas with no coating | Too little solution, or a surface that doesn't wet | Dispense more, clean or prime the surface, or try dynamic dispense |
| Pinholes | Tiny holes in the film | Bubbles or particles in the dispensed solution | Dispense slowly and steadily, and let the solution settle before use |
| Chuck marks | A faint circle the size of the chuck | The chuck changes the substrate's temperature, so the film dries unevenly | Let substrate and chuck reach the same temperature, or use a chuck with less contact |
Common questions.
How long does spin coating take?
Most recipes finish in under a minute. The film thins quickly in the first few seconds at speed, then sets as the solvent evaporates.
How do I make a thicker film?
Spin slower, use a more concentrated or more viscous solution, or apply more than one coat. Halving the speed makes the film roughly 40 percent thicker.
Should I use static or dynamic dispense?
Start with static dispense; it's simpler. Move to dynamic dispense when you want more consistent coatings or need to use less chemistry.
How much solution should I dispense?
For a static dispense, enough to cover about half the substrate's diameter. For a dynamic dispense, stop when the puddle reaches the edge.
What is edge bead, and do I need to remove it?
Edge bead is a ridge of extra material at the edge of the substrate. If later steps need a flat surface all the way to the edge, such as contact lithography, remove it. On round wafers, do it in the spin coater right after coating.