The raised rim of resist at a wafer edge can spoil contact exposure and contaminate tools. This guide explains why the edge bead forms and what it does, how automatic edge bead removal and backside rinse work, what to do without them, and how to qualify the step.
In one paragraph: Edge beads form especially with thick films and square substrates [1]. In contact exposure they can make the mask stick and act as a spacer that spoils resolution and dimensions [1]. Automatic EBR and BSR, where fitted, remove the rim and back-side resist on the spin coater; the removal width still has to be qualified. Without them, MicroChemicals describes manual EBR at about 500 rpm on circular substrates, spin program changes and, for non-circular substrates, removing the edge material where handling permits [1]. Qualify the step by measuring thickness near the edge after the softbake.
1. Why the Edge Bead Matters
An edge bead is a raised rim of coating at the edge of a substrate. It forms especially when thick resist films are coated and on square substrates [1]. Low spin speeds make it more pronounced [1]. How the edge bead forms, and the other defects that appear during spinning, are covered in Spin Coating Defects: Comets, Striations, Pinholes and Edge Beads Diagnosed.
For photoresist, the edge bead is more than a cosmetic problem. MicroChemicals names two consequences in contact exposure [1]:
- Sticking. The bead keeps a high residual solvent content even after the softbake, so the mask may stick to the resist.
- Spacing. The bead acts as an unwanted spacer between mask and resist, often causing poor resolution, wrong dimensions or an unintentionally low sidewall angle in the developed pattern.
Resist that wraps around the edge onto the back of the substrate creates a related problem: it can contaminate the softbake hot plate and, through it, later substrates [3]. Edge bead removal (EBR) and backside rinse (BSR) are the two spin-coater steps that deal with the top-side rim and the back side. Why the gap matters for resolution is explained in Mask Aligner Exposure: Contact vs Proximity and the Resolution Limit.
2. How Edge Bead Removal and Backside Rinse Work
Both steps run on the spin coater right after coating, while the substrate is still on the chuck [3]:
| Step | What happens | What it removes |
|---|---|---|
| Edge bead removal (EBR) | A fine stream of solvent is directed at the outer edge of the rotating substrate from above, dissolving a narrow ring of coating | The top-side edge bead and a defined ring of resist |
| Backside rinse (BSR) | Solvent is directed at the underside of the rotating substrate near the edge | Resist that has wrapped around the edge or reached the back side |
General description of the two steps; nozzle position, solvent, flow and timing are set for each process.
MicroChemicals describes a manual version of the first step for circular substrates: dynamic edge bead removal at about 500 rpm with an edge bead removal solvent from a wash bottle with a fine spray nozzle [1]. The note warns against solvents with a high vapour pressure, such as acetone [1]. An automatic EBR arm can control nozzle position, flow and timing, but the actual removal width must still be qualified for the substrate, resist and process conditions.
What to set and record:
- Removal width. How far in from the edge the coating is removed. It should cover the bead and any zone where the film is not usable. Solvent from a backside rinse can also creep onto the front side and leave the rim uncoated [3].
- Speed and time. The speed at which the substrate turns during EBR and BSR, and how long the solvent flows.
- Solvent. One that dissolves the resist and is recommended by its supplier for edge bead removal.
- Timing. EBR before the softbake, while the edge is still soluble; MicroChemicals also notes that a waiting time between coating and softbake helps prevent the existing bead from growing when the viscosity drops at the softbake temperature [1].
3. Options Without Automatic Edge Bead Removal
Where automatic edge bead removal is not available, MicroChemicals lists options that reduce or eliminate the bead [1]:
| Option | Notes [1] |
|---|---|
| Manual dynamic EBR (circular substrates) | About 500 rpm, edge bead removal solvent from a wash bottle with a fine spray nozzle; no high-vapour-pressure solvents such as acetone |
| A resist that reaches the target at high speed | Choose a resist that reaches the target thickness at 3,000–4,000 rpm |
| A short high-speed step | For very thick films, a higher spin speed for a short time |
| Abrupt speed increase at the end | Spins off the bead once the film has dried enough but the bead is still liquid |
| Multiple coatings | Two or more films on top of each other, each at a higher spin speed |
| Waiting before softbake | Prevents the bead from growing when viscosity drops at the softbake temperature; possibly multi-stage drying |
| A close-fitting cavity in the holder | The substrate sits as a close inlay |
| Non-circular substrates | Evaluate an approved edge-removal method for the substrate and process; breaking or wiping edge material is suitable only where the handling procedure permits it |
Summarised from MicroChemicals [1]; the handling caveat in the last row is added here.
Several of these options are spin program changes; how ramps and multi-step programs are written is covered in Spin Coating Acceleration and Multi-Step Recipes: What the Ramp Actually Does. For square substrates and small pieces, where an EBR stream may not follow the corners uniformly, the suitable method depends on the substrate and approved handling procedure; see Spin Coating Small, Square and Fragment Substrates: Chucks, Vacuum and Corners.
4. Qualifying the Edge Bead Removal Step
An EBR step is qualified by measuring the film in the region that matters to the process. Record the coating recipe, bake, measurement zone, number of points and substrates, and acceptance limits. The custom EBR configuration on the AC200 product page (Section 5) illustrates this approach with a specified resist coating and prebake, measurements within 5 mm of the edge, and size-specific acceptance criteria. Those criteria belong to that published custom configuration and are not general tolerances for other models or processes.
Optimising the EBR parameters can make a large difference: in one design-of-experiments study on 40 µm thick negative photoresist coatings, optimising solvent-based EBR reduced the edge bead from approximately 30 µm to less than 1 µm [4]. Such results belong to that resist and process; measure your own.
- Coat and remove. Run the full recipe with EBR and BSR on test substrates.
- Bake. Softbake as in production, then measure; the film loses solvent until then.
- Inspect the edge. Check the removed ring for width and a clean transition, without a new ridge where the solvent stopped; a drying spin after EBR can reflow the resist into a secondary edge bead [3].
- Measure near the edge. Measure thickness at fixed points just inside the removed ring and compare with the centre.
- Check the back side. Look for residues after BSR, especially near the edge.
- Expose a test pattern. In contact exposure, check resolution near the edge, where a remaining bead would show first [1][2].
Coating the resist to a target thickness, and how speed and viscosity set it, is the subject of Photoresist Spin Coating: From Viscosity to Target Thickness and Uniformity.
5. Levon™ Spin Coaters with EBR and BSR
The table quotes the Levon™ product pages; “Not stated” means the page does not give the value.
| Specification | AC200 | AC300-S-CTM | AC300-S-EN |
|---|---|---|---|
| Substrates | Fragments to 8-inch wafers (model-specific limits) | Fragments smaller than 1 cm to 12-inch wafers | Fragments smaller than 1 cm to 12-inch wafers |
| Edge and backside functions | S-PP: integrated backside cleaning and edge debonding; SE: BSR and EBR can be added, rinse/develop module with splash guard optional; PE, SE-PLUS: BSR and EBR and spill-proof cleaning/developing can be added | BSR and EBR, plus splash-proof cover washing and developing functions, can be added | BSR and EBR functions; splash-proof cover washing and developing functions (options) |
| Rotation direction | SE, SE-PLUS: forward and reverse (bidirectional) | Not stated | Not stated |
| Speed (no load where stated) | S-PP, SE: 20–10,000 rpm; PE: max. 10,000 rpm; SE-PLUS: max. 8,000 rpm | Max. 5,000 rpm | Max. 5,000 rpm |
| Acceleration (no load where stated) | S-PP, SE, PE: up to 50,000 rpm/s; SE-PLUS: max. 10,000 rpm/s | Max. 10,000 rpm/s | Max. 1,000 rpm/s |
| Process chamber | PP housing (S-PP) or HDPE chamber/cup (SE, PE, SE-PLUS); PC cover | PTFE-coated stainless steel chamber | Stainless steel alloyed Teflon inner cavity, stainless steel housing |
| Vacuum | S-PP: vacuum pressure display, adjustable | Precision regulation, real-time digital display | Precision regulation, real-time digital display |
| Edge removal width or accuracy (standard models) | Not stated | Not stated | Not stated |
Specifications as published on the Levon™ product pages on 22 October 2026, where “no load” is stated by the page. EBR and BSR are listed as add-on options on several models; the supplied configuration is confirmed in the quotation.
The AC200 product page also lists a custom configuration for edge removal and back washing with published acceptance criteria. It is a custom configuration quoted on request, not a specification of the standard models above:
| AC200-EBR-CTM (custom configuration, quoted on request) | Published value on the AC200 page |
|---|---|
| Process | Automatic desktop spin coater: edge removal + back washing; standard wafers up to 8 inches |
| Edge removal arm | Automatic lifting and swinging; edge removal distance 3–5 mm; accuracy ±0.5 mm; wafers 3 to 8 inches |
| Back washing | Height-adjustable back washing fixture; wafers 4 to 8 inches |
| Pipeline and supply | 2 channels (edge removal liquid, back washing liquid) from two 10 L pressure tanks with low-level sensors; adjustable flow |
| Acceptance coating | A 20 cP photoresist (named on the product page): 500 rpm 5 s at 500 rpm/s, then 2,500 rpm 35 s at 2,000 rpm/s; prebake 100 °C, 100 s; about 1.3 µm |
| Acceptance criterion (thickness within 5 mm of the edge, after prebake) | 4-inch less than ±1.5%; 6-inch less than ±2%; 8-inch less than ±2.5%; 5/9/13 points; 3 wafers per size |
Values as published for the custom configuration on the AC200 product page on 22 October 2026; a custom configuration is defined in the quotation.
How this connects to the guide: the AC200 series lists backside cleaning and edge debonding on the S-PP model and BSR and EBR as add-ons on the SE, PE and SE-PLUS models, so, where these functions are part of the supplied configuration, the steps of Section 2 can be carried out on the spin coater after coating; the configuration is confirmed in the quotation. The AC300-S-CTM and the AC300-S-EN list BSR and EBR functions (an add-on on the AC300-S-CTM) for substrates from fragments to 12-inch wafers. The standard pages do not state the edge removal width; the custom configuration publishes 3–5 mm with ±0.5 mm accuracy. Ask for the width and nozzle arrangement for your configuration. Questions can be sent through the contact form.
6. Keep Exploring the Lithography Knowledge Hub
Start here
- Spin Coating Defects: Comets, Striations, Pinholes and Edge Beads Diagnosed — edge beads, comets and other coating defects
- Spin Coating Small, Square and Fragment Substrates: Chucks, Vacuum and Corners — square substrates and small pieces
- Static vs Dynamic Dispense in Spin Coating: Volume, Coverage and Material Waste — dispense modes and spin programs
Lithography steps
- Photoresist Spin Coating: From Viscosity to Target Thickness and Uniformity — coating the resist to target thickness
- Mask Aligner Exposure: Contact vs Proximity and the Resolution Limit — contact and proximity exposure
- Exposure Dose and Puddle Development: Reading a Resist Contrast Curve — exposure dose and development
Links will be added as each article is published.
Equipment
- AC200 Spin Coater Series — fragments to 8-inch, EBR/BSR options
- AC300-S-CTM Spin Coater — fragments to 12-inch, BSR and EBR options
- AC300-S-EN Spin Coater — fragments to 12-inch, BSR and EBR functions
- Levon™ Spin Coaters — all spin coaters
Questions about a specific spin coater can be sent through the contact form.
7. FAQ
What is an edge bead?
A raised rim of coating at the substrate edge, typical for thick resist films and square substrates [1].
Why remove the edge bead?
In contact exposure it can make the mask stick and act as a spacer, causing poor resolution and wrong dimensions [1].
What is backside rinse?
A solvent stream directed at the underside of the rotating substrate to remove resist that has wrapped around the edge; such resist can otherwise contaminate the softbake hot plate [3].
Can I remove the edge bead manually?
For the circular-substrate procedure described by MicroChemicals, manual dynamic EBR is performed at about 500 rpm with a suitable EBR solvent from a fine nozzle; the note warns against high-vapour-pressure solvents such as acetone [1]. Use this only where the substrate, holder, solvent and equipment operating procedure permit it.
What about square substrates?
MicroChemicals mentions breaking off the edge pieces with the bead or wiping the bead with clean-room wipers [1]; use such methods only where the substrate and the approved handling procedure permit it.
When should EBR happen?
Right after coating, before the softbake, while the edge is still soluble; a short wait before the softbake also keeps the bead from growing [1].